Renata Pereira Lopes

Didier Astruc
email: didier.astruc@u-bordeaux.fr

Zero Valent Zvi

Iron

Biochar as a support for nanocatalysts and other reagents: Recent advances and applications

Keywords: Sustainability, biomass, biochar, catalyst support, metal catalyst, nanocatalysis

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Due to the growing environmental pollution, practices are being encouraged all over the world to minimize the impacts caused by man. One of the bottlenecks to ensure the sustainable use of natural resources is mainly the use of fuels to generate energy. In this sense, the use of biomass from agro industry waste for energy generation has been extensively studied in recent years.

In Brazil, agribusiness in 2019 represented 21.4% of the total Brazilian Gross Domestic Product [START_REF]Agropecuária Brasileira em números[END_REF]. Although it is important to a country's economy, agribusiness generates a large amount of waste. The annual biomass waste generated in the world is in the order of 140 Gt [2]. These residues resulting from agro-industrial activities are generated in the processing inter alia of leather, fibers, food, wood, sugar and alcohol production. The composition of agricultural residues is similar to other lignocellulosic materials: cellulose (40-50%), hemicellulose (20-30%), lignin (20-25%) and ash (1-5%) [3], [START_REF] Santos | Metal catalysts supported on biochars : Part I synthesis and characterization[END_REF]. Many of these residues represent safety and environmental liabilities. In order to guarantee the sustainability of the process, research has been directed towards guaranteeing zero waste. The development of technologies oriented towards a circular economy in agribusiness is one of the strategic directions in the sustainability of these productive chains [START_REF] Montoro | Anaerobic co-digestion of sweet potato and dairy cattle manure : A technical and economic evaluation for energy and biofertilizer production[END_REF]. In this sense, the production of energy from biomass is a practice that has been widely studied.

The waste biomass is a clean, sustainable and renewable energy source and is obtained by residues from agricultural production, processing industries and social activities being available in large quantities [START_REF] Tao | Formation of persistent free radicals in biochar derived from rice straw based on a detailed analysis of pyrolysis kinetics[END_REF], [START_REF] Vamvuka | Combustion behaviour of biomass fuels and their blends with lignite[END_REF], [START_REF] Guo | Evaluation of the catalytic performance of different activated biochar catalysts for removal of tar from biomass pyrolysis[END_REF]. The use of biomass residues has a win-win effect for carbon sequestration and water pollution control [START_REF] Chen | Facile and low-cost fabrication of ZnO/biochar nanocomposites from jute fibers for efficient and stable photodegradation of methylene blue dye[END_REF]. Energy recovery from these residues has become an important alternative to replace conventional fossil fuels like coal [START_REF] Wahab | Thermochemical, kinetic and ash characteristics behaviour of Thar Lignite , agricultural residues and synthetic polymer waste (EVA)[END_REF]. Among the processes for obtaining bio-oil from biomass, pyrolysis is the most commonly used. This occurs, because pyrolysis has increased attention as an acceptable route for waste disposal due to product selectivity, process scalability and versatility [START_REF] Shahbeig | Pyrolysis of municipal sewage sludge for bioenergy production : Thermo-kinetic studies , evolved gas analysis , and techno-socioeconomic assessment[END_REF]. In this process, syngas, bio oil and biochar are generated. Depending on temperature and residence time, maximum gas or liquid yields are produced. Slow-conventional pyrolysis is applied to produce high charcoal yields, while fast or flash pyrolysis is associated with a high reduction in the solid yield [3].

Bio-oil, also called pyrolysis oil or liquid fraction converted from biomass, can replace heavy fuel oil, light fuel oil or natural gas in a wide variety of equipment installations, including boilers, turbines and gas and other heating and power generation devices [START_REF] Gutiérrez | Biofuels from cardoon pyrolysis : Extraction and application of biokerosene / kerosene mixtures in a self-manufactured jet engine[END_REF]. The significant amount of biochar produced during the process of obtaining bio-oil is one of the obstacles to the industrialization of biomass thermal conversion technology [START_REF] Plácido | Microalgae biochar-derived carbon dots and their application in heavy metal sensing in aqueous systems[END_REF]. In general, the biochar is treated inappropriately as an industrial waste; however, its use for immobilization of organic carbon is beneficial for mitigating the greenhouse effect [START_REF] Liu | Pyrolytic production of zerovalent iron nanoparticles supported on rice husk-derived biochar : simple, in situ synthesis and use for remediation of Cr( VI )-polluted soils[END_REF], [START_REF] Benedetti | CO2 Adsorption study on pure and chemically activated chars derived from commercial biomass gasifiers[END_REF]. Thus, the production of new value-added biochar products is very important for economy and management of energy resources.

Although the main element present in the structure of the biochar is carbon (C), its elemental composition tends to vary according to the production conditions of the biochar and the raw material used to produce it [START_REF] Cha | Production and utilization of biochar: A review[END_REF]. In general, biochar contains, in addition to carbon (C), hydrogen (H), oxygen (O) and small amounts of nitrogen (N) and sulfur (S) [START_REF] Liu | Characterization of biochars derived from agriculture wastes and their adsorptive removal of atrazine from aqueous solution: A comparative study[END_REF]. In addition, biochar contains stacks of graphite agglomerates that contain layers of graphene and graphene oxide (GO) whose edge carbons are reactive because they present groups such as -COOH; -OH and -O-, acting as CO2 binding sites [START_REF] Chen | Photochemical and Acoustic Interactions of Biochar with CO2 and H2O : Applications in Power Generation and CO2 Capture[END_REF]. There are two factors that can impact the biochar surface property: (1) the type of biomass feedstock and (2) the carbonization approach that transfers biomass into biochar [START_REF] Zhou | Biochars with excellent Pb (II) adsorption property produced from fresh and dehydrated banana peels via hydrothermal carbonization[END_REF].

Biochar was introduced in daily life with the main function of soil alteration, due to its positive role in carbon sequestration, in reduction in greenhouse gas emissions and improvement of soil fertility [START_REF] Beesley | Effects of biochar and greenwaste compost amendments on mobility, bioavailability and toxicity of inorganic and organic contaminants in a multi-element polluted soil[END_REF]. Its use dates from 100-450 BC by the Amazon tribes making the "terra preta", from Portuguese, which means black earth [START_REF] Boss | Dielectric characterization of white birchactivated biochar composites : A sustainable alternative to radar-absorbing materials[END_REF]. In the soil, the biochar reduces soil acidity, adsorbs and immobilizes nutrients and reduces greenhouse gas emissions (N2O, up to 80%, and CH4) from plants [START_REF] Chen | Photochemical and Acoustic Interactions of Biochar with CO2 and H2O : Applications in Power Generation and CO2 Capture[END_REF], [START_REF] Fang | Mechanism of hydroxyl radical generation from biochar suspensions : Implications to diethyl phthalate degradation[END_REF].

Nowadays, biochar can be used for catalyst production [START_REF] Shen | Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification[END_REF], biodiesel production [START_REF] Konwar | Review on latest developments in biodiesel production using carbon-based catalysts[END_REF], and mainly, as soil additives [START_REF] Lone | Biochar for Sustainable Soil Health : A Review of Prospects and Concerns[END_REF]. The use as an adsorbent material for water and air pollutants is due to its main characteristics that are, as a general rule, large specific surface area, extensive microstructure, porous structure, functional groups of the surface, high nutrient content and great water storage capacity [START_REF] Ahmad | Biochar as a sorbent for contaminant management in soil and water: A review[END_REF], [START_REF] Mohan | Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent -A critical review[END_REF], [START_REF] Liu | Positive impact of biochar amendment on thermal balance during swine manure composting at relatively low ambient temperature[END_REF]. Besides, they have high chemical stability that allows them to operate in corrosive environments and good electrical conductivity [START_REF] Fuertes | A Green Route to High-Surface Area Carbons by Chemical Activation of Biomass-Based Products with Sodium Thiosulfate[END_REF], [START_REF] Okonkwo | The synthesis of micromesoporous carbon derived from nitrogen-rich spirulina extract impregnated castor shell based on biomass self-doping for highly ef fi cient supercapacitor electrodes[END_REF].

Other applications of biochar have also been studied, such as its uses for the production of fuel cells [START_REF] Yu | Utilization of corn cob biochar in a direct carbon fuel cell[END_REF], [START_REF] Elleuch | Experimental investigation of direct carbon fuel cell fueled by almond shell biochar : Part I . Physico-chemical characterization of the biochar fuel and cell performance examination[END_REF], [START_REF] Preuss | Bio-inspired carbon electro-catalysts for the oxygen reduction reaction[END_REF], supercapacitors [START_REF] Jiang | Highly ordered macroporous woody biochar with ultra-high carbon content as supercapacitor electrodes[END_REF], [START_REF] Liu | Rapid microwave activation of waste palm into hierarchical porous carbons for supercapacitors using biochars from di ff erent carbonization temperatures as catalysts †[END_REF] and as support in catalysis for the production of new composites as metallic nanoparticles (NPs) [START_REF] Liu | Removal of 17β-estradiol from aqueous solution by graphene oxide supported activated magnetic biochar: Adsorption behavior and mechanism[END_REF], [START_REF] Qian | Nanoscale zero-valent iron supported by biochars produced at different temperatures : Synthesis mechanism and effect on Cr(VI) removal[END_REF], [START_REF] Yao | Hydrogen production from biomass gasification using biochar as a catalyst/support[END_REF].

Catalysts based on metallic and bimetallic NPs and their oxides are very efficient in reactions of economic interest, such as fine chemistry [START_REF] Zu | Efficient production of the liquid fuel 2,5-dimethylfuran from 5-hydroxymethylfurfural over Ru/Co3O4 catalyst[END_REF], [START_REF] Kong | Switchable synthesis of 2,5-dimethylfuran and 2,5-dihydroxymethyltetrahydrofuran from 5-hydroxymethylfurfural over Raney Ni catalyst[END_REF], [START_REF] Przydacza | Solvothermal hydrodeoxygenation of hydroxymethylfurfural derived from biomass towards added value chemicals on Ni/TiO 2 catalysts[END_REF] or in the removal of contaminants [START_REF] Sousa | Study of Cu NPs reactivity for compounds with different chemical structures: Black reactive dye 5, picric acid and 2,4-D herbicide[END_REF], [START_REF] Ourique | Comparative study of the direct black removal by Fe, Cu, and Fe/Cu nanoparticles[END_REF], [START_REF] Nascimento | Sulfentrazone dechlorination by iron-nickel bimetallic nanoparticles[END_REF], [START_REF] Cruz | Synthesis and characterization of cobalt nanoparticles for application in the removal of textile dye[END_REF]. However, one of their disadvantages is their strong aggregation. Thus, depositing them on a support significantly improves their efficiency.

Studies involving biochar have been reported for decades, and as of 2010, as shown in Fig. 1, the number of publications increased considerably, reaching 3115 publications in 2019, according to data extracted from the Web of Science database in April/2020. Its use as a support in catalysis has increased more modestly, and in 2019 the number of publications was 196 publications as can be seen in the insert in Fig. 1.

Other applications of biochar can also be viewed, such as catalysis, photocatalysis and persulfate activation. It is believed that the number of publications involving the use of biochar as a support in catalysis will increase considerably in the next years. This is because biochar can disperse and stabilize engineered NPs to further enhance their stability and mobility. "Biochar and support"; "Biochar and catalysis", "Biochar and zero valent iron", "Biochar and photocatalysis" and "Biochar and persulfate activation"

The general objective of this review is to provide an overview of the biochar's physical-chemical characteristics, advances in biochar production processes, activation

Number of publication

Year and functionalization of biochar in order to improve its physical and chemical characteristics, and applications as a support for different catalysts and reagents used in different types of reactions and environmental decontaminations. The use of biochar as a support for metallic NPs and their oxides for the remediation of contaminants in aqueous systems and sediments, activation of oxidants such as peroxide and persulfate and photocatalysis are discussed as well as other applications such as the synthesis of fine chemicals and tar reforming. This review will serve as base for new research aiming to add value to this important resource that is produced in large quantities in the world.

Biochar

After oxygen, carbon is the most abundant element in the biosphere, being used by nature as a basis for the storage of renewable energy in the form of carbohydrates [START_REF] Titirici | Sustainable carbon materials[END_REF]. Carbon-based materials have been recognized with the highest scientific awards, including the 1996 Nobel Prize in Chemistry (fullerenes), the 2008 Kavli Prize for Nanoscience (carbon nanotubes) and the 2010 Nobel Prize in Physics (graphene) [START_REF] Titirici | Sustainable carbon materials[END_REF].

Unlike commercial carbon materials (activated carbon, graphene and carbon nanotubes) whose source materials are restricted to some agricultural products, biochar can be produced from practically all biomass, from organic raw materials to organic waste [START_REF] Cho | Preparation of nitrogen-doped Cubiochar and its application into catalytic reduction of p-nitrophenol[END_REF]. As previously mentioned, the production of energy from agribusiness residues leads to three products: syngas, bio oil and biochar. The characteristics and main processes for obtaining the biochar are discussed in the following sections.

Biochar characteristics

Generally, the raw material used to produce biochar from agro-industrial residues consists mainly of hemicellulose, cellulose and lignin as well as some inorganic minerals [START_REF] Tao | Formation of persistent free radicals in biochar derived from rice straw based on a detailed analysis of pyrolysis kinetics[END_REF]. Biomass such as inter alia sugarcane bagasse, coffee straw, soy straw, rice straw, corn cob can be cited as examples. The structure and physicochemical properties of biochar, which are closely dependent on the conditions of the process, affect the way in which it interacts with organic and inorganic compounds [START_REF] Sajjadi | Urea functionalization of ultrasound-treated biochar : A feasible strategy for enhancing heavy metal adsorption capacity[END_REF]. According to [START_REF] Ahmed | Insight into biochar properties and its cost analysis[END_REF] the biochar quality characteristics will change along several dimensions according to final pyrolysis temperature, heating rate and residence time [START_REF] Ahmed | Insight into biochar properties and its cost analysis[END_REF]. In these processes Hierarchical Porous Carbon are generated that refers to carbon materials with two or more different pore sizes, integrating the advantages of micropores, mesopores and macropores [START_REF] Fu | Activation of peroxymonosulfate by graphitized hierarchical porous biochar and MnFe2O4 magnetic nanoarchitecture for organic pollutants degradation: Structure dependence and mechanism[END_REF]. It is important to mention that macropores and mesopores contribute to the rapid transport of ions through the channels of micropores [START_REF] Cuong | Meso/micropore-controlled hierarchical porous carbon derived from activated biochar as a high-performance adsorbent for copper removal[END_REF]. The type of feedstock material is an important factor that determines the final application, because its properties are affected by the nature of the original material [START_REF] Jindo | Physical and chemical characterization of biochars derived from different agricultural residues[END_REF]. Organic matter undergoes thermal decomposition with moisture loss at temperatures up to about 120 °C [START_REF] Yek | Engineered biochar via microwave CO2 and steam pyrolysis to treat carcinogenic Congo red dye[END_REF]. Hemicellulose decomposes at °C, cellulose at 315-400 °C, while lignin gradually decomposes in the temperature range 150-600 °C [START_REF] Wei | Science of the Total Environment Study on reaction mechanism of superior bamboo biochar catalyst production by molten alkali carbonates pyrolysis and its application for cellulose hydrolysis[END_REF], [START_REF] Blasi | Modeling chemical and physical processes of wood and biomass pyrolysis[END_REF].

Biochar is considered as a highly aromatic compound, containing layers of graphene and graphite structures, randomly arranged [START_REF] Pusceddu | Chemical-physical analysis and exfoliation of biochar-carbon matter : from agriculture soil improver to starting material for advanced nanotechnologies[END_REF]. The ordered basal carbons of the aromatic leaf have high concentrations of delocalized π electrons and are therefore considered as Lewis bases [START_REF] Sajjadi | Urea functionalization of ultrasound-treated biochar : A feasible strategy for enhancing heavy metal adsorption capacity[END_REF]. Biochar generally has less aromaticity and hydrophobicity than activated carbon, because it contains more non-carbonized organic matter [START_REF] Wu | Fabrication of bimetallic Ag/Fe immobilized on modified biochar for removal of carbon tetrachloride[END_REF]. The aromaticity of biochar produced depends on the conditions of synthesis. In their work, [START_REF] Park | Impact of oxidative carbonization on structure development of loblolly pine-derived biochar investigated by nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy[END_REF] found that the production of biochar from Debarked loblolly pine (Pinus taeda) at a higher temperature (700 °C) in a nitrogen atmosphere containing 7% oxygen in the pyrolysis stage favored the production of clusters containing a higher number of carbons (C59) [START_REF] Park | Impact of oxidative carbonization on structure development of loblolly pine-derived biochar investigated by nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy[END_REF], as shown in Fig. 2. 3. This quaternary model illustrates biochar structures of the macro to atomic scale [START_REF] Xiao | A Direct Observation of the Fine Aromatic Clusters and Molecular Structures of Biochars[END_REF].

The carbonization promotes the formation of some functional groups on biochar surface such as hydroxyl (OH), carbonyl (CO) and carboxyl ((CO)OH) [START_REF] Chia | Vibrational Spectroscopy Imaging of mineral-enriched biochar by FTIR, Raman and SEM-EDX[END_REF]. Therefore, infrared analysis of the biochar shows characteristic bands of these organic groups, as indicated in Table 1. For C1s X-ray photoelectron spectroscopy (XPS) peaks appear at 284.7 eV, 286.0 eV, and 287.2 eV that are attributed to C-C, CO and CO-= [START_REF] Li | Biocharsupported nanoscale zero-valent iron as an e ffi cient catalyst for organic degradation in groundwater[END_REF], [START_REF] Mandal | Biochar induced modi fi cation of graphene oxide & nZVI and its impact on immobilization of toxic copper in soil *[END_REF].

X-Ray Diffraction (DRX) analysis shows biochar diffraction peaks at 23.3-26.4° and 42.5-44°, that represent (002) and ( 100) diffraction planes of graphite [START_REF] Sun | Activation of persulfate by CO2-activated biochar for improved phenolic pollutant degradation : Performance and mechanism[END_REF], [START_REF] Wang | Metal-free activation of persulfates by corn stalk biochar for the degradation of antibiotic nor fl oxacin : Activation factors and degradation mechanism[END_REF], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF].

Table 1. Main bands of organic groups found in biochar

Wave number (cm -1 )

Vibration type

Ref.

3400-3431

Stretching of O-H [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF], [START_REF] Yan | Nano-magnetite supported by biochar pyrolyzed at different temperatures as hydrogen peroxide activator : Synthesis mechanism and the effects on ethylbenzene removal *[END_REF], [START_REF] Li | Biocharsupported nanoscale zero-valent iron as an e ffi cient catalyst for organic degradation in groundwater[END_REF], [START_REF] Fan | Removal of hexavalent chromium by biochar supported nZVI composite: Batch and fixed-bed column evaluations, mechanisms, and secondary contamination prevention[END_REF], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF] 2930 and 2890 asymmetric and symmetric stretches of aliphatic C-H [START_REF] Yan | Nano-magnetite supported by biochar pyrolyzed at different temperatures as hydrogen peroxide activator : Synthesis mechanism and the effects on ethylbenzene removal *[END_REF], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF] 2372 Stretching of -CH2 [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF]

1590-1703

Carbonyl/carboxyl C=O stretching vibration [START_REF] Yan | Nano-magnetite supported by biochar pyrolyzed at different temperatures as hydrogen peroxide activator : Synthesis mechanism and the effects on ethylbenzene removal *[END_REF], [START_REF] Li | Biocharsupported nanoscale zero-valent iron as an e ffi cient catalyst for organic degradation in groundwater[END_REF], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF] 1621-1643 and 1388-1420

Vibration of C=C [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF], [START_REF] Yan | Nano-magnetite supported by biochar pyrolyzed at different temperatures as hydrogen peroxide activator : Synthesis mechanism and the effects on ethylbenzene removal *[END_REF], [START_REF] Fan | Removal of hexavalent chromium by biochar supported nZVI composite: Batch and fixed-bed column evaluations, mechanisms, and secondary contamination prevention[END_REF]

1033-1103

Vibration of C-O [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF], [START_REF] Yan | Nano-magnetite supported by biochar pyrolyzed at different temperatures as hydrogen peroxide activator : Synthesis mechanism and the effects on ethylbenzene removal *[END_REF], [START_REF] Li | Biocharsupported nanoscale zero-valent iron as an e ffi cient catalyst for organic degradation in groundwater[END_REF], [START_REF] Fan | Removal of hexavalent chromium by biochar supported nZVI composite: Batch and fixed-bed column evaluations, mechanisms, and secondary contamination prevention[END_REF], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF] The Raman spectra of biochar samples present a D band at 1341-1360 cm -1 due stemming from the breathing mode of A1g symmetry that is forbidden in perfect graphite and only becomes active in the presence of disorders often referred to amorphous carbon, and a G band at 1580-1594 cm -1 that represent the in-plane bondstretching motion of pairs of sp 2 carbon atoms in chains and rings demonstrating the existence of graphitic carbon [START_REF] Yu | Magnetic nitrogen-doped sludge-derived biochar catalysts for persulfate activation: Internal electron transfer mechanism[END_REF], [START_REF] Xiao | A Direct Observation of the Fine Aromatic Clusters and Molecular Structures of Biochars[END_REF]. The ratio of the intensity of these two peaks (ID/IG) represents the degree of disorder of carbon materials [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF], [START_REF] Sun | Activation of persulfate by CO2-activated biochar for improved phenolic pollutant degradation : Performance and mechanism[END_REF], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF]. Sun et al.

(2020) observed that the ID/IG ratio of BC500 considerably increased from 0.737 to 1.150 after CO2 activation from 950 °C, Fig. 4 [START_REF] Sun | Activation of persulfate by CO2-activated biochar for improved phenolic pollutant degradation : Performance and mechanism[END_REF]. Cooperative H-Bond (PAHB), Fig. 5b. Therefore, these water molecules and the coronene form a complex with six adsorption sites, which leads to the electronegative oxygen atom (H acceptor), Fig. 5c. This complex is linked to H donor by H bonds of IOPs (Ionizable Organic Products). Then, the hydrogen atom of the IOP considered to be hydrogen donor is linked to the oxygen atom of the complex by polarization-assisted H bond, Fig. 5d and 5e [START_REF] Zhang | Water clusters contributed to molecular interactions of ionizable organic pollutants with aromatized biochar via π-PAHB: Sorption experiments and DFT calculations[END_REF].

Another interesting feature is the presence of free radicals (FRs) in materials obtained through combustion. [START_REF] Fang | Mechanism of hydroxyl radical generation from biochar suspensions : Implications to diethyl phthalate degradation[END_REF] found that suspensions of biochar are capable of generating hydroxyl radicals without the addition of H2O2. According to the authors, (i) the free radicals (FRs) present in biochar suspensions transfer electrons to O2 to form O2 -; (ii) O2 -accepts an electron from FR to produce H2O2; (iii) H2O2 reacts with FRs to form OH radical through a single electron transfer process, as shown in the Eq. 1-4 [START_REF] Fang | Mechanism of hydroxyl radical generation from biochar suspensions : Implications to diethyl phthalate degradation[END_REF].

O2 + e -→ O2 - (Eq. 1)

H + + O2 -→ HO2 (Eq. 2)

2HO2 → H2O2 + O2 (Eq. 3)

H2O2 + e -+ H + → OH + H2O (Eq. 4) Wenmei et al. (2020) investigated the formation of persistent free radicals (PFRs) in rice husk biochar by monitoring the pyrolysis kinetics of the material. These authors found that lignin started to pyrolyze at a lower temperature than cellulose and hemicellulose, and lignin was the main factor for the generation of PFRs. The breakdown of chemical bonds contributed only slightly to the formation of PFRs. The reconfiguration of carbonaceous structures is a more important contributor to the formation of PFRs, whereas the crosslinking between different compositions and the interactions between inorganic chemical and mineral compositions play a significant role in the generation and stabilization of PFR in biochar [START_REF] Tao | Formation of persistent free radicals in biochar derived from rice straw based on a detailed analysis of pyrolysis kinetics[END_REF].

Recently, biochar has been used as electron-transfer catalyst in redox reactions of environmental relevance [START_REF] Cui | Simultaneous sorption and catalytic oxidation of trivalent antimony by Canna indica derived biochars*[END_REF]. The electron-transfer mode of biochar occurs by three types of active redox structures: the quinones that accept electrons (oxidants), the polycondensed aromatic structures of biochar (oxidants) and the phenolic portions (reducing agents) that donate electrons [START_REF] Wei | Simultaneous adsorption and oxidation of antimonite onto nano zero-valent iron sludge-based biochar : Indispensable role of reactive oxygen species and redoxactive moieties[END_REF], [START_REF] Cui | Simultaneous sorption and catalytic oxidation of trivalent antimony by Canna indica derived biochars*[END_REF].

Carbon Quantum Dots

In the syntheses of biochar by hydrothermal carbonization (HTC) from biomass, a great attention has been given to solid products, whereas the pale yellow liquid, a byproduct of the synthesis, has always been considered as a waste and discarded [START_REF] Wu | Pentosan-derived water-soluble carbon nano dots with substantial fluorescence : Properties and application as a photosensitizer[END_REF], [START_REF] Wu | Hydrothermal carbonization of carboxymethylcellulose : One-pot preparation of conductive carbon microspheres and water-soluble fluorescent carbon nanodots[END_REF]. HTC will be discussed in the next items. However, [START_REF] Wu | Pentosan-derived water-soluble carbon nano dots with substantial fluorescence : Properties and application as a photosensitizer[END_REF] found that this liquid contains abundant carbon NPs similar to quantum dots, with excellent photoluminescence and high quantum yield (∼20%) [START_REF] Wu | Pentosan-derived water-soluble carbon nano dots with substantial fluorescence : Properties and application as a photosensitizer[END_REF], [START_REF] Wu | Hydrothermal carbonization of carboxymethylcellulose : One-pot preparation of conductive carbon microspheres and water-soluble fluorescent carbon nanodots[END_REF]. Carbon Quantum Dots (CQDs), a relatively new type of zero-dimensional carbon-based material [START_REF] Zhang | Sustainable Synthesis of Bright Green Fluorescent Nitrogen-Doped Carbon Quantum Dots from Alkali Lignin[END_REF], include carbon NPs with a diameter less than 10 nm, containing several functional groups, such as hydroxyl, epoxy and carboxylic acid on their surface [START_REF] Zahedifar | Nanomagnetic biochar dots coated silver NPs ( BCDs-Ag / MNPs ): A highly efficient catalyst for reduction of organic dyes[END_REF]. CQDs have several applications due to their distinctive thermal, optical, chemical, electronic and mechanical properties, non-blinking luminescence, and magnificent compatibility, flexibility in surface modification and nominal toxicity [START_REF] Sahoo | Carbon dots derived from lychee waste : Application for Fe3+ ions sensing in real water and multicolor cell imaging of skin melanoma cells[END_REF], [START_REF] Liu | Biomass-derived nitrogen self-doped carbon dots via a simple one-pot method : Physicochemical , structural , and luminescence properties[END_REF], [START_REF] Zhu | Green preparation of palm powder-derived carbon dots co-doped with sulfur/chlorine and their application in visible-light photocatalysis[END_REF]. In addition, CQDs have high solubility, excellent photostability, favorable biocompatibility and low toxicity [START_REF] Plácido | Microalgae biochar-derived carbon dots and their application in heavy metal sensing in aqueous systems[END_REF], [START_REF] Liu | Biomass-derived nitrogen self-doped carbon dots via a simple one-pot method : Physicochemical , structural , and luminescence properties[END_REF].

There are several methods for the synthesis of CQDs, such as graphite laser ablation [START_REF] Kang | Pulsed laser ablation based synthetic route for nitrogen-doped graphene quantum dots using graphite flakes[END_REF], microwave heating of carbohydrates [START_REF] Yoshinaga | Particulate, Structural, and Optical Properties of D-Glucose-Derived Carbon Dots Synthesized by Microwave-Assisted Hydrothermal Treatment[END_REF], hydrothermal carbonization of polymers [START_REF] Moradi | Easy synthesis , characterization and cell cytotoxicity of green nano carbon dots using hydrothermal carbonization of Gum Tragacanth and chitosan bio-polymers for bioimaging[END_REF] and plasma albumen pyrolysis [START_REF] Wang | Amphiphilic Egg-Derived Carbon Dots : Rapid Plasma Fabrication, Pyrolysis Process, and Multicolor Printing Patterns*[END_REF], [START_REF] Zhu | Green preparation of palm powder-derived carbon dots co-doped with sulfur/chlorine and their application in visible-light photocatalysis[END_REF]. In addition, CQDs have largely been synthetized from biomass [START_REF] Wei | Economical green and rapid synthesis of CDs-Cu2O/CuO nanotube from the biomass waste reed as sensitive sensing platform for the electrochemical detection of hydrazine[END_REF], [START_REF] Liu | Fabrication of carbon dots@restricted access molecularly imprinted polymers for selective detection of metronidazole in serum[END_REF], [START_REF] Wang | Facile synthesis of novel carbon quantum dots from biomass waste for highly sensitive detection of iron ions[END_REF]. CQDs had been made earlier by other methods. As this is not the focus of the present review, the reading of a recent mini review by Wang and coworkers is therefore recommended [START_REF] Wang | A Mini Review on Carbon Quantum Dots : Preparation, Properties, and Electrocatalytic Application[END_REF].

The CQDs have an absorption band between 210 and 400 nm in the UV-vis.

region that is attributed to the complex electronic transition on the surface, the π conjugated aromatic system and the n-π* transition of carbonyl and other oxygencontaining groups [START_REF] Zhang | Environmentally friendly synthesis of photoluminescent biochar dots from waste soy residues for rapid monitoring of potentially toxic elements[END_REF], [START_REF] Wu | Pentosan-derived water-soluble carbon nano dots with substantial fluorescence : Properties and application as a photosensitizer[END_REF], [START_REF] Liu | Fabrication of carbon dots@restricted access molecularly imprinted polymers for selective detection of metronidazole in serum[END_REF], [START_REF] Wang | Facile synthesis of novel carbon quantum dots from biomass waste for highly sensitive detection of iron ions[END_REF], [START_REF] Zhu | Green preparation of palm powder-derived carbon dots co-doped with sulfur/chlorine and their application in visible-light photocatalysis[END_REF]. The excitation and emission fluorescent peak occurs at approximately 350 and 450 nm, respectively [START_REF] Zhang | Environmentally friendly synthesis of photoluminescent biochar dots from waste soy residues for rapid monitoring of potentially toxic elements[END_REF], [START_REF] Liu | Fabrication of carbon dots@restricted access molecularly imprinted polymers for selective detection of metronidazole in serum[END_REF], [START_REF] Wang | Facile synthesis of novel carbon quantum dots from biomass waste for highly sensitive detection of iron ions[END_REF], [START_REF] Zhu | Green preparation of palm powder-derived carbon dots co-doped with sulfur/chlorine and their application in visible-light photocatalysis[END_REF]. The surface charge of CQDs evaluated by Zeta potential presents negative values, which is assigned to the presence of highly hydrophilic hydroxyl and carboxyl group [START_REF] Sahoo | Carbon dots derived from lychee waste : Application for Fe3+ ions sensing in real water and multicolor cell imaging of skin melanoma cells[END_REF], [START_REF] Zhang | Environmentally friendly synthesis of photoluminescent biochar dots from waste soy residues for rapid monitoring of potentially toxic elements[END_REF].

Synthesized CQDs have relatively stable fluorescence intensity under neutral conditions [START_REF] Zhang | Sustainable Synthesis of Bright Green Fluorescent Nitrogen-Doped Carbon Quantum Dots from Alkali Lignin[END_REF], [START_REF] Sahoo | Carbon dots derived from lychee waste : Application for Fe3+ ions sensing in real water and multicolor cell imaging of skin melanoma cells[END_REF]. The CQD synthesis process involves four stages including dehydration, polymerization, aromatization and carbonization (passivation) [START_REF] Wu | Pentosan-derived water-soluble carbon nano dots with substantial fluorescence : Properties and application as a photosensitizer[END_REF], [START_REF] Liu | Biomass-derived nitrogen self-doped carbon dots via a simple one-pot method : Physicochemical , structural , and luminescence properties[END_REF],

as shown in Fig. 6. In their work, Liu et al. (2020) verified that the optical properties were closely related to sp 2 C, N-doped surface states, and the synergy among them [START_REF] Liu | Biomass-derived nitrogen self-doped carbon dots via a simple one-pot method : Physicochemical , structural , and luminescence properties[END_REF]. showed excellent photocatalytic property in the degradation of rhodamine B and methylene blue [START_REF] Zhu | Green preparation of palm powder-derived carbon dots co-doped with sulfur/chlorine and their application in visible-light photocatalysis[END_REF]. Wu and collaborators (2014) synthesized CQDs/TiO2 composites by the sol-gel method and applied them to the photocatalytic degradation of methylene blue. The oxygen-containing groups of the CQDs act as photosensitizers, absorbing photons that excite electrons from HOMO to LUMO. Free electrons are transferred to the TiO2 conduction band, then react with oxygen (O2) adsorbed on TiO2 to produce active superoxide radical anion (O2 -) [START_REF] Wu | Pentosan-derived water-soluble carbon nano dots with substantial fluorescence : Properties and application as a photosensitizer[END_REF].

Luo and coworkers (2020) also synthesized CQDs from chitosan by the solvothermal method, and they used them as photosensitizers of TiO2 aiming at an application in photoelectrochemistry. According to the authors, the materials exhibited a higher photocurrent compared to the pristine TiO2, with an even greater enhancement under visible-light illumination. This result was attributed to the synergism between CQDs and TiO2, which forms a type II heterojunction, expanding the light response to the visible region [START_REF] Luo | Nitrogen-Doped Carbon Dots/TiO2 Nanoparticle Composites for Photoelectrochemical Water Oxidation[END_REF]. Zhang et al. (2018) also synthesized CQDs from chitosan using the solvothermal method. The CQDs were covalently linked to an indium tin oxide (ITO) surface through a self-assembled silane monolayer, which showed a direct photoelectronic response. The photocurrent was attributed to the improved electron transfer process catalyzed by CQDs under illumination. The materials have potential for application as biocompatible and addressable electrochemical sensors in bioanalytical and bioimaging applications [START_REF] Zhang | Photoelectrochemical response of carbon dots (CDs) derived from chitosan and their use in electrochemical imaging[END_REF].

Biochar synthesis

The carbonization of biomass to produce biochar, among other products, is a complex process that includes many chemical reactions, including inter alia dehydration, decomposition, condensation and polymerization. [START_REF] Okonkwo | The synthesis of micromesoporous carbon derived from nitrogen-rich spirulina extract impregnated castor shell based on biomass self-doping for highly ef fi cient supercapacitor electrodes[END_REF]. This carbonization can occur by processes involving pyrolysis, gasification, flash carbonization and hydrothermal carbonization. Pyrolysis technologies are differentiated by the reaction time of the pyrolysis material (slow and fast) and the heating method (burning of fuels, by electrical heating, or by microwaves) [START_REF] Meyer | Technical, economical and climate related aspects of biochar production technologies: A literature review[END_REF]. The gasification uses high temperatures and some oxygen to produce a non-condensable gas rich in hydrogen and carbon dioxide [START_REF] Brewer | Characterization of biochar from fast pyrolysis and gasification systems[END_REF]. Flash carbonization consists in processes at relatively low temperatures (≤ 400 °C) at short periods of time (between 5 to 10 minutes) in the presence of air [START_REF] Hirst | A simple flash carbonization route for conversion of biomass to porous carbons with high CO2 storage capacity †[END_REF].

Hydrothermal carbonization (HTC), also known as wet pyrolysis, is a thermochemical process performed by submerging biomass in water and then sealed in a confined system to be heated for several hours under saturated pressure [START_REF] Zhou | Biochars with excellent Pb (II) adsorption property produced from fresh and dehydrated banana peels via hydrothermal carbonization[END_REF], [START_REF] White | Porous Hydrothermal Carbons[END_REF].

One of the advantages of HTC is the greater energy efficiency, due to higher solid yields without the need for intensive drying compared to dry pyrolysis [START_REF] Fang | Physically (CO2) activated hydrochars from hickory and peanut hull: preparation, characterization, and sorption of methylene blue, lead, copper, and cadmium †[END_REF]. The heat is transferred to the surface of the material by conduction, convection or radiation, and to the interior of the material by thermal conduction [START_REF] Guiotoku | Molecular and morphological characterization of hydrochar produced by microwave-assisted hydrothermal carbonization of cellulose[END_REF]. During the HTC process, biomass undergoes thermal degradation in water and affects the physical-chemical properties of water, reducing its dielectric constant and making it a good solvent for non-polar substances [START_REF] Zhou | Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels[END_REF]. HTC is carried out under relatively mild temperature (180-250 °C) and pressure conditions in a closed aqueous system, in which the pressure source is mostly related to steam [START_REF] Wu | Hydrothermal carbonization of carboxymethylcellulose : One-pot preparation of conductive carbon microspheres and water-soluble fluorescent carbon nanodots[END_REF], [START_REF] Kubo | Porous Carbohydrate-Based Materials via Hard Templating[END_REF], [START_REF] Titirici | Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage[END_REF], [START_REF] Wang | Sulphur-doped ordered mesoporous carbon with enhance d electrocatalytic activity for the oxygen reduction reaction[END_REF], [START_REF] Yu | Carbohydrate-Derived Hydrothermal Carbons: A Thorough Characterization Study[END_REF]. In 2010, Titirici et al.

published a detailed review of the various synthetic routes to carbon-based materials or composites through the HTC biomass process [START_REF] Hu | Engineering Carbon Materials from the Hydrothermal Carbonization Process of Biomass[END_REF].

The carbonization by pyrolysis microwave has been highlighted [START_REF] Deng | Preparation and characterization of activated carbon from cotton stalk by microwave assisted chemical activation -Application in methylene blue adsorption from aqueous solution[END_REF]. The microwave carbonization process is different from microwave-assisted activation that will be discussed later. Compared to electric heating, microwave heating is advantageous in terms of speed, selectivity, flexibility and ease of control [START_REF] Li | Toluene microwave cracking and reforming over bio-char with in-situ activation and ex-situ impregnation of metal[END_REF].

Microwave pyrolysis is thermal degradation in an inert environment using electromagnetic radiation as a heat source [START_REF] Yek | Engineered biochar via microwave CO2 and steam pyrolysis to treat carcinogenic Congo red dye[END_REF]. In furnace, surface heating of hearth wall does not guarantee a uniform temperature for different particle shapes and sizes of chars, generating a thermal gradient from the hot surface of the coal particle to its interior, preventing the effective removal of gaseous products into the environment, compromising the quality of the prepared activated carbons [START_REF] Li | Preparation of high surface area activated carbons from tobacco stems with K2CO3 activation using microwave radiation[END_REF]. On the other hand, microwave pyrolysis results in materials that receive energy that is converted to heat, at a molecular level through dipolar rotation and ion conduction [START_REF] Yek | Microwave steam activation, an innovative pyrolysis approach to convert waste palm shell into highly microporous activated carbon[END_REF]. Dipole reorientation occurs millions of times in microwave absorbent compounds after exposition to microwave radiation; then, the heating results from friction and collisions between molecules in microwave absorbent compounds [START_REF] Liew | Innovative production of highly porous carbon for industrial effluent remediation via microwave vacuum pyrolysis plus sodium-potassium hydroxide mixture activation[END_REF]. Thus, by microwave, the rapid thermal gradient from the interior of the char particle to its coal surface allows the microwave-induced reaction to proceed more quickly and effectively at a lower temperature, resulting in energy savings and decreasing processing time [START_REF] Li | Preparation of high surface area activated carbons from tobacco stems with K2CO3 activation using microwave radiation[END_REF].

The yield of biochar can be affected by several factors, such as the biomass, carbonization process, and carbonization conditions used. For example, the pyrolysis temperature increase causes the thermal cracking of heavy hydrocarbons, which leads to a decrease in the yield of biochar. Some yields of biochar synthesized by different processes, besides feedstock, the process of obtaining and the general conditions for synthesis of biochar are indicated in Table 2. [

* the low yield was attributed to the amount of water in the sample.

Biochar activation

The adsorption behavior of biochar is the result of the cooperation between physical interaction and chemical reaction. The first depends largely on the surface area and porosity, whereas the second significantly depends on surface functional groups, due to chemical reactions occurring, including ion exchange, complexation, precipitation, electrostatic attraction and π interaction between biochar and the species to be adsorbed [START_REF] Zhou | Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels[END_REF]. The properties of biochar are related to its surface characteristics, due to the oxygen-containing functional groups, mineral fractions and aromatic carbon present in its constitution [START_REF] Zhou | Biochars with excellent Pb (II) adsorption property produced from fresh and dehydrated banana peels via hydrothermal carbonization[END_REF].

However, the direct use of these biochar without any treatment is not straightforward due to their poor textural and physico-chemical properties [START_REF] Santos | Metal catalysts supported on biochars : Part I synthesis and characterization[END_REF].

Hydrophobicity, pore volume and functional groups of biochar can be altered by physical/chemical modifications, such as steam activation, chemical modification, impregnation and heat treatment [START_REF] Wu | Fabrication of bimetallic Ag/Fe immobilized on modified biochar for removal of carbon tetrachloride[END_REF]. The biochar produced from thermal decomposition processes is often activated physically and chemically to increase its specific surface area, porosity, pore size distribution and attach various desired functional groups [START_REF] Yek | Microwave steam activation, an innovative pyrolysis approach to convert waste palm shell into highly microporous activated carbon[END_REF].

The physical activation involves carbonization and activation in a single step or two separate steps at a higher temperature under the gasification of gas atmosphere, such as carbon dioxide or water at 800-900 °C [START_REF] Tsai | Reusing Cow Manure for the Production of Activated Carbon Using Potassium Hydroxide (KOH) Activation Process and Its Liquid-Phase Adsorption Performance[END_REF], [START_REF] Benedetti | CO2 Adsorption study on pure and chemically activated chars derived from commercial biomass gasifiers[END_REF]. The physical activation gives rise to carbons with porosity composed almost exclusively of micropores < 2 nm [START_REF] Fuertes | A Green Route to High-Surface Area Carbons by Chemical Activation of Biomass-Based Products with Sodium Thiosulfate[END_REF],

whereas the chemical activation is more versatile, because it allows the synthesis of activated carbon with an adjustable porosity (made up of micropores and mesopores)

[29] [START_REF] Oginni | Reports Influence of one-step and two-step KOH activation on activated carbon characteristics[END_REF]. Chemical activation is widely used due to its short activation time, low activation temperature and simple control process [START_REF] Ma | Adsorption and regeneration of leafbased biochar for p-nitrophenol adsorption from aqueous solution[END_REF], [START_REF] Tsai | Reusing Cow Manure for the Production of Activated Carbon Using Potassium Hydroxide (KOH) Activation Process and Its Liquid-Phase Adsorption Performance[END_REF], [START_REF] Zhang | Adsorptive and capacitive properties of the activated carbons derived from pig manure residues[END_REF]. In the next items, some of these processes are described.

Chemical activation

Many compounds have been proposed for chemical activation, such acids (H3PO4, H2SO4, HNO3 and HCl), alkaline chemicals (KOH and NaOH) and activating agents (ZnCl2, K2CO3, FeCl3, H2O2, O3) [START_REF] Jang | Adsorption isotherm, kinetic modeling and mechanism of tetracycline on Pinus taeda -derived activated biochar[END_REF], [START_REF] Guo | Evaluation of the catalytic performance of different activated biochar catalysts for removal of tar from biomass pyrolysis[END_REF]. In fact, however, KOH, H3PO4 and ZnCl2 are the most commonly used activators [START_REF] Guo | Evaluation of the catalytic performance of different activated biochar catalysts for removal of tar from biomass pyrolysis[END_REF]. These chemicals are dehydrating agents that influence the pyrolytic decomposition and retard the formation of tars during the carbonization process [START_REF] Hassan | Preparation and characterization of microporous NaOH-activated carbons from hydrofluoric acid leached rice husk and its application for lead(II) adsorption[END_REF]. In the activation process, doping with heteroatom, including inter alia N, P and S introduces functionality to biochar [START_REF] Suo | Rapid removal of triazine pesticides by P doped biochar and the adsorption mechanism[END_REF]. In general, a chemical activation process consists in carbon atoms slightly bound in the carbon structure that reacts with an oxidizing agent (OH -, CO3 2-; PO4 3-; NO3 -; etc.), in which carbon is released as CO or CO2 producing a large number of pores that appears in the residual carbonaceous solid (Eq. 5) [START_REF] Fuertes | A Green Route to High-Surface Area Carbons by Chemical Activation of Biomass-Based Products with Sodium Thiosulfate[END_REF].

CO3 2-+ C → 2CO + O 2- (Eq. 5)
The material activation by chemical compounds can be conducted before (onestep, also called one-pot) [START_REF] Oginni | Reports Influence of one-step and two-step KOH activation on activated carbon characteristics[END_REF] or after (two-step, also called two-pot) the thermal treatment leading to biochar [START_REF] Oginni | Effect of one-step and two-step H3PO4 activation on activated carbon characteristics[END_REF]. Both processes have pros and cons that must be evaluated for the intended purpose. In the biochar activation in two stages, the activating agent facilitates the removal of tarred substances in the pores of the material and acts as a model for the creation of microporosity during activation [START_REF] Oginni | Effect of one-step and two-step H3PO4 activation on activated carbon characteristics[END_REF], [START_REF] Oginni | Reports Influence of one-step and two-step KOH activation on activated carbon characteristics[END_REF]. On the other hand, catalytic pyrolysis of biomass in one-step improves the efficiency of decomposition and the quality of products [START_REF] Fu | Micro-mesoporous carbons from original and pelletized rice husk via one-step catalytic pyrolysis[END_REF].

Activation by H3PO4

Compared to other chemical activators, such as zinc chloride, hydroxide and nitric acid, H3PO4 undergoes less equipment corrosiveness, and the products obtained contain less harmful residues, and relatively low pyrolysis temperature; less pollution for the environment and low costs result thereof [START_REF] Zhou | Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels[END_REF], [START_REF] Chu | Phosphoric acid pretreatment enhances the speci fi c surface areas of biochars by generation of micropores*[END_REF]. The pretreatments using NaOH and HCl remove silica and ash from biochar thoroughly, which leads to cracks and pore opening [START_REF] Luo | The roles of phosphorus species formed in activated biochar from rice husk in the treatment of landfill leachate[END_REF]. The electronegativity of P is smaller compared to those of other heteroatoms, and the covalent radius is greater than that of N and S. Therefore, doping with phosphorus could improve the properties of biochar more effectively, increasing the number of acidic sites on the biochar surface and increasing its porosity [START_REF] Suo | Rapid removal of triazine pesticides by P doped biochar and the adsorption mechanism[END_REF].

Multiple micropores and mesopores form on biochar surface, because phosphoric acid and its derivative (polyphosphoric acid) react with the organic functional groups [START_REF] Yang | Batch interaction of emerging tetracycline contaminant with novel phosphoric acid activated corn straw porous carbon: Adsorption rate and nature of mechanism[END_REF], [START_REF] Suo | Rapid removal of triazine pesticides by P doped biochar and the adsorption mechanism[END_REF]. Oxidation by the catalytic action of H3PO4 in the biochar modification process results in the fixation of oxygen-containing groups on the pore walls, which leads to a decrease in the size of the pores and/or to the transfer of some mesopores to micropores, increasing the biochar surface area [START_REF] Peng | Enhanced adsorption of Cu(II) and Cd(II) by phosphoric acid-modified biochars[END_REF]. The mechanism of formation of these micropores occurs by H + from H3PO4 that contribute to micropore generation via a H + catalysis process. This is due the organic phosphate bridge protection of the carbon skeleton from micropore collapse via the crosslinking of radical phosphate [START_REF] Cho | Preparation of nitrogen-doped Cubiochar and its application into catalytic reduction of p-nitrophenol[END_REF]. In another work, Chu et al (2020) evaluated the activation of biomass with H3PO4 before the pyrolysis stage in which these authors verified the contribution of cellulose and lignin to the formation of micropores. The activated materials have better structural characteristics, and the cellulose is the constituent that made the greatest contribution to the formation of micropores in relation to lignin [START_REF] Chu | Phosphoric acid pretreatment enhances the speci fi c surface areas of biochars by generation of micropores*[END_REF].

The biochar functional group containing oxygen reacts with H3PO4 to generate water vapor to form the structure of the internal pores (Eq. 6) [START_REF] Liu | Preparation of KOH and H3PO4 Modified Biochar and Its Application in Methylene Blue Removal from Aqueous Solution[END_REF]. The H3PO4 catalysis leads to the oxidation and fixation of functional groups containing oxygen in the pore walls, leading to the reduction of the pore size or the transformation of the mesopores into micropores [START_REF] Liu | Preparation of KOH and H3PO4 Modified Biochar and Its Application in Methylene Blue Removal from Aqueous Solution[END_REF]. The reaction with H3PO4 promotes the formation of -COOH, -OH, C=C, aromatic structures and increased the formation of PO or P-OOH functional groups [START_REF] Liu | Preparation of KOH and H3PO4 Modified Biochar and Its Application in Methylene Blue Removal from Aqueous Solution[END_REF].

4H3PO4 + 10C → P4 + 10CO + 6H2O (Eq. 6)

Shi et al. (2019) synthesized activated carbon from sucrose by in situ

hydrothermal impregnation with H3PO4 and subsequent calcination at 500-900 °C.

During the activation process, the decomposition of H3PO4 resulted in the formation of P2O5 and water. The P2O5 sublimation was responsible for the generation of pores in the material. Water molecules also contributed to the development of porosity through carbon gasification (Eq. 7). When a temperature of 900 °C was used, the surface area drastically increased to 1929 m 2 g -1 . At this temperature, P2O5 formed by the decomposition of H3PO4 acts as an oxidizing agent (Eq. 8) [START_REF] Shi | Activated carbons derived from hydrothermal impregnation of sucrose with phosphoric acid : remarkable adsorbents for sulfamethoxazole removal[END_REF].

C + H2O → CO + H2 (Eq. 7)

P2O5 + 5C → 2P + 5CO (Eq. 8) respectively. On the other hand, the biochar-derived activated carbons, formed in twostep, were denominated as Activated Carbon from Kanlow Switchgrass Biochar (KSBC) and Activated Carbon from Public Miscanthus Biochar (PMBC) and had a specific surface area of 697.96 and 161.97 m 2 g -1 , respectively. The adsorption capacity efficiency was evaluated in the caffeine and acetaminophen removal. The KSBM and PMBM were about 3-5 folds higher than KSBC and PMBC [START_REF] Oginni | Effect of one-step and two-step H3PO4 activation on activated carbon characteristics[END_REF].

Zhou et al. (2019) synthesized biochar by Conventional Hydrothermal

Carbonization (HTC). According to the authors, H3PO4 has a multifunctional property, as it serves as a catalyst for the decomposition process of organic matter, creating a liquid-air interface, as well as a phosphorus (P) donor. The biochar surface was further improved by the addition of Na2CO3 whose function was to neutralize biochar, removing the extra H3PO4, breaking the hydrogen bonds between surface functional groups (SFGs). These groups impede surface coordination and introduce carbonate anions that promote precipitation reactions with heavy metal ions [START_REF] Zhou | In situ modification provided by a novel wet pyrolysis system to enhance surface properties of biochar for lead immobilization[END_REF].

Wu and colleagues (2019) synthesized activated biochar prepared by Pomelo Peel using H3PO4. According to the authors, the pores in the material are produced after etching of the biochar framework by H3PO4 oxidizing into carbonate and intercalating the phosphorus compounds [START_REF] Oginni | Effect of one-step and two-step H3PO4 activation on activated carbon characteristics[END_REF].

Activation by alkaline compounds (NaOH and KOH)

NaOH accelerates the dissolution of condensed organic matter, such as lignin and celluloses, during the impregnation step, and it promotes a subsequent chemical reaction during the thermal activation stage [START_REF] Jang | Adsorption isotherm, kinetic modeling and mechanism of tetracycline on Pinus taeda -derived activated biochar[END_REF]. The development of porosity in the material is attributed to activator's dehydration effect of NaOH, which breaks the C-O-C and C-C bonds of hydrochar. Thus, NaOH is reduced to metallic sodium Na, hydrogen gas, and sodium carbonate Na2CO3 according to Eq. 9 [START_REF] Islam | Mesoporous activated coconut shell-derived hydrochar prepared via hydrothermal carbonization-NaOH activation for methylene blue adsorption[END_REF].

6 NaOH + 2 C → 2 Na + 3 H2 + 2 Na2CO3 (Eq. 9)

Na2CO3 produced under thermal decomposition give gases (CO2 and H2O) that is responsible for burst precursors and results in wider holes [START_REF] Hassan | Preparation and characterization of microporous NaOH-activated carbons from hydrofluoric acid leached rice husk and its application for lead(II) adsorption[END_REF].

A similar reaction occurs with KOH activation [START_REF] Castelló | Carbon activation with KOH as explored by temperature programmed techniques , and the effects of hydrogen[END_REF], [START_REF] Yao | Magnetic activated biochar nanocomposites derived from wakame and its application in methylene blue adsorption[END_REF]. Hydrogen, carbon monoxide, potassium vapor, and carbon dioxide are generated during sintering, which results in a large number of pores on the surface of biochar [START_REF] Ma | Adsorption and regeneration of leafbased biochar for p-nitrophenol adsorption from aqueous solution[END_REF]. The addition of KOH first dehydrates cellulose, hemicellulose and lignin. The K + ions enter completely inside the material to generate the pore network (Eqs. 10-12) [START_REF] Zhao | Highly enhanced adsorption performance of tetracycline antibiotics on KOH-activated biochar derived from reed plants †[END_REF], [START_REF] Liu | Preparation of KOH and H3PO4 Modified Biochar and Its Application in Methylene Blue Removal from Aqueous Solution[END_REF].

4 KOH + C → K2CO3 + K2O + 2 H2 (Eq. 10) K2CO3 + 2 C → 2 K + 3 CO (Eq. 11)
K2O + 2 C→2 K + CO (Eq. 12)

In the synthesis of micro-mesoporous carbon derived from castor bean peel the NaOH-activated biochar (132.8 m 2 g -1 ) had a higher specific surface area compared to KOH (117.7 m 2 g -1 ), this last one showed a higher adsorption efficiency.

According to FTIR (Fourier-Transform Infrared Spectroscopy) results obtained by the authors, the KOH-activated biochar has a much higher content of functional groups compared to the NaOH-activated biochar and, consequently, it has greater adsorption capacity [START_REF] Hsu | Adsorption of Ammonium Nitrogen from Aqueous Solution on Chemically Activated Biochar Prepared from Sorghum Distillers Grain[END_REF].

KOH-activated biochar for different biomasses under optimal activation conditions showed different specific surface areas (m 2 g -1 ): (1) biochar from rice husk and rice husk pellet: 1818. [START_REF] Cruz | Synthesis and characterization of cobalt nanoparticles for application in the removal of textile dye[END_REF] The efficiency of KOH-CSCB was mainly related to its porous structure. The efficiency of H3PO4-CSCB was mainly related to the functional groups of biochar containing oxygen and phosphorus, including -OH, -COOH, amino phosphonic acid functional group, and P-OH bond [START_REF] Liu | Preparation of KOH and H3PO4 Modified Biochar and Its Application in Methylene Blue Removal from Aqueous Solution[END_REF].

Activation by ZnCl2

ZnCl2 is a dehydrating agent that influences the thermal decomposition of biomass and inhibits the formation of tar, which results in the degradation of cellulosic material [START_REF] Zhao | Surface characteristics and potential ecological risk evaluation of heavy metals in the bio-char produced by co-pyrolysis from municipal sewage sludge and hazelnut shell with zinc chloride[END_REF], [START_REF] Singh | Highly Efficient Method for the Synthesis of Activated Mesoporous Biocarbons with Extremely High Surface Area for High-Pressure CO2 Adsorption[END_REF], [START_REF] Yan | ZnCl2 modified biochar derived from aerobic granular sludge for developed microporosity and enhanced adsorption to tetracycline[END_REF], [START_REF] Uçar | Preparation and characterization of activated carbon produced from pomegranate seeds by ZnCl2 activation[END_REF]. Dehydration results in carbonization and aromatization of the carbon skeleton enabling the formation of pores in the material structure [START_REF] Zhao | Surface characteristics and potential ecological risk evaluation of heavy metals in the bio-char produced by co-pyrolysis from municipal sewage sludge and hazelnut shell with zinc chloride[END_REF]. ZnCl2 promotes depolymerization processes to produce strong carbonization, a strong breakdown of the liquid phase and, therefore, a greater formation of hydrogen [3]. In the presence of Zn, hydrogen is the largest gas product formed associated with other additives (LiCl, NaCl, KCl, AlCl3), while methane and carbon monoxide production are less important [3]. Hydrogen is released from the aromatic/phenolic structure of the precursor, leaving active sites behind for reactions [START_REF] Uçar | Preparation and characterization of activated carbon produced from pomegranate seeds by ZnCl2 activation[END_REF].

At high ZnCl2 concentrations, some ZnCl2 remains in the external part of the carbon particles, widens the porosity by a localized decomposition of the organic matter, and enhances the meso-and macropore formation [START_REF] Khalili | Production of micro-and mesoporous activated carbon from paper mill sludge: I . Effect of zinc chloride activation[END_REF], [START_REF] Caturla | Preparation of activated carbon by chemical activation with ZnCl2[END_REF]. The increase in porosity and surface area after the process of chemical activation of ZnCl2 may be related to the interstices left by ZnCl2 incorporated in the carbon structure, which governs the porosity of carbon during heat treatment, leading to mesoporous materials [START_REF] Santos | Metal catalysts supported on biochars : Part I synthesis and characterization[END_REF]. ZnCl2 has a relatively low melting and boiling point (263 and 732 °C, respectively), which forms molten salts during the pyrolysis process and distributed in the carbon structure [START_REF] Shi | High-efficiency removal of Cr(VI) by modified biochar derived from glue residue[END_REF]. According to Shi et al. ( 2020) the potential of the pyrolytic process of activation of the biochar derived from glue residue by ZnCl2 can be summarized in Eq.

13-16 [START_REF] Shi | High-efficiency removal of Cr(VI) by modified biochar derived from glue residue[END_REF].

5 ZnCl2 + 10 H2O → Zn5(OH)8Cl2 2H2O + 8 HCl (Eq. 13)

Zn5(OH)8Cl2 2H2O → ZnO + 2HCl + 5 H2O (Eq. 14)

2 ZnO + SiO2 → Zn2SiO4 (Eq. 15)

3 ZnO + 2 C → 3 Zn↑ + CO↑ + CO2↑ (Eq. 16)

Sun and co-workers (2018) evaluated the effect of ZnCl2 activation of biochar from pine wood chips that was used to catalyze the pyrolysis of mixed plastic waste.

According to the authors, ZnCl2 treatment resulted in the formation of abundant acid sites, mainly Lewis acid sites, whose catalytic effect led to an increase in the content of aromatic hydrocarbons and a decrease in the content of alkene in plastic pyrolysis oil, as outlined in Fig. 7 [START_REF] Sun | Effect of ZnCl2-activated biochar on catalytic pyrolysis of mixed waste plastics for producing aromatic-enriched oil[END_REF]. (BC-H3PO4) or ZnCl2 (BC-ZnCl2), which had a high surface area of 680 and 790 m 2 g -1 , respectively. BC-ZnCl2 sample showed more functional groups than the BC-H3PO4 sample, wherein the acidic groups (carbonyl, carboxylic, lactones and phenol groups) content in the BC-ZnCl2 and BC-H3PO4 samples were 0.98 and 0.69 mmol g -1 ,

respectively. The main mechanisms for removing metal ions include porous diffusion, electrostatic interactions and cation-π interactions [START_REF] Wang | Biochars from Lignin-rich Residue of Furfural Manufacturing Process for Heavy Metal[END_REF].

Physical activation

Despite the low cost of physical activation, its effect on porous structures is much less important than that of chemical activation [START_REF] Zhang | Sludgebased biochar activation to enhance Pb(II) adsorption[END_REF]. In the next items, some of these processes are described.

Activation by CO2 and steam atmosphere

Activation by a CO2 atmosphere is one of the most popular methods for producing activated carbon [START_REF] Zhang | Adsorption of acetone and cyclohexane onto CO2 activated hydrochars[END_REF]. The physical activation of carbon materials by a CO2 atmosphere at high temperatures is a simple method that increases the surface area and assists in the graphitization of crude biochar [START_REF] Sun | Activation of persulfate by CO2-activated biochar for improved phenolic pollutant degradation : Performance and mechanism[END_REF]. CO2 is considered as a physical activation agent because it participates in pyrolysis through the reverse reaction of Boudouard, which decreases carbon yield and leads to successive removal of carbon from the pores (Eq. 17) [START_REF] Santos | Metal catalysts supported on biochars : Part I synthesis and characterization[END_REF]. For steam, the process is very similar (Eq. 18) [START_REF] Santos | Metal catalysts supported on biochars : Part I synthesis and characterization[END_REF]. The volatilization and conversion of the organic phase of the primitive hydrocarbons induces the increase of the pore volume, while the carbon skeleton retains the rudimentary porosity and the original structure [START_REF] Zhang | Adsorption of acetone and cyclohexane onto CO2 activated hydrochars[END_REF].

C + CO2 ⇌ 2 CO (Eq. 17 Braghiroli and collaborators (2019) produced biochar physically activated with CO2 atmosphere or steam and chemical activation with KOH impregnation. For biochar from white birch, the specific surface areas were 1700, 881 and 590 m 2 g -1 for KOH, CO2 and steam, respectively. For biochar from black spruce, the specific surface areas were 1662, 735 and 412 m 2 g -1 for KOH, CO2 and steam, respectively. The Cu 2+ adsorption mechanisms involved physical and chemisorption for CO2-activated biochar, while the chemisorption for KOH-activated biochar was probably due to the high proportion of functional groups connected to its surface [START_REF] Braghiroli | Performance of Physically and Chemically Activated Biochars in Copper Removal from Contaminated Mine Effluents[END_REF].

Zhang et al. ( 2019) produced biochar derived from pig manure residues activated by KOH, CO2 and H3PO4, which had a specific surface area of 2335.9, 700.1 and 500.7 m 2 g -1 , respectively. The materials were applied to the removal of methylene blue, obtaining removals of 717.0, 73.9 and 65.1 mg g -1 for KOH, H3PO4 and CO2, respectively [START_REF] Zhang | Adsorptive and capacitive properties of the activated carbons derived from pig manure residues[END_REF].

Yek and coworkers (2020) obtained biochar from orange peel by microwave heating pyrolysis and activation by steam or CO2 in a single step. The total process time was 15 min, which is an advantage over the conventional pyrolysis method, whose time varies between 1 to 7 h. The use of CO2 and steam as an activating agent increases the surface area of the carbonized product, whose results were, respectively, 158.5 and 305.1 m 2 g -1 . The materials were applied to the removal of the Congo red dye, which led to the removal of 136 and 91 mg g -1 for the material activated by steam and CO2, respectively [START_REF] Yek | Engineered biochar via microwave CO2 and steam pyrolysis to treat carcinogenic Congo red dye[END_REF].

Activation by ultrasonic waves and microwave irradiation

Cavitation under ultrasound consists of three steps: nucleation, growth of bubbles and water bubbles with high energy and pressure, which increases the temperature by producing microjets within the liquid [START_REF] Chatterjee | Ultrasound cavitation intensified amine functionalization: A feasible strategy for enhancing CO2 capture capacity of biochar[END_REF]. The generation of microjets, shock waves and highly pressurized overheated regions, that is, hot spots, divides larger particles into smaller particles, cleans smooth surfaces and increases the porosity of solid surfaces [START_REF] Sajjadi | Engineering Lowtemperature acoustic-based activation of biochar for enhanced removal of heavy metals[END_REF]. Ultrasound is able to remove fine mineral particles in the porous biochar and to improve the desired reactions due to increased mass transfer in aqueous solution [START_REF] Chen | Photochemical and Acoustic Interactions of Biochar with CO2 and H2O : Applications in Power Generation and CO2 Capture[END_REF]. The physical effects of ultrasound on plant materials were identified as erosion (of the plant structure), fragmentation (reduction of particle size), sonocapillary effect (increased penetration of solvent into the channels and pores of plant material), local shear stress (flow and micro-flow effects), detexturation (destruction of plant structures) and sonoporation [START_REF] Sajjadi | Urea functionalization of ultrasound-treated biochar : A feasible strategy for enhancing heavy metal adsorption capacity[END_REF].

Sonication increases the porosity of the material due to the exfoliation of the graphical layers of the biochar and the leaching of ash from the pores formed during the thermochemical decomposition of biomass by pyrolysis [START_REF] Chatterjee | Low Frequency Ultrasound Enhanced Dual Amination of Biochar: A Nitrogen-Enriched Sorbent for CO2 Capture[END_REF], [START_REF] Chatterjee | Ultrasound cavitation intensified amine functionalization: A feasible strategy for enhancing CO2 capture capacity of biochar[END_REF]. The cavitation effect of the ultrasound produces a uniform suspension of the biochar, activates the binding sites and leads to a better graft of the functional groups [START_REF] Chatterjee | Ultrasound cavitation intensified amine functionalization: A feasible strategy for enhancing CO2 capture capacity of biochar[END_REF]. Therefore, combined processes using ultrasound aid chemical activation, because ultrasonic waves improve the mass transfer and dispersion of solid particles, allow the nucleation, growth and collapse of cavitation bubbles, thus improving the chemical attack on the biochar surface [START_REF] Wang | Pb(II) sorption by biochar derived from Cinnamomum camphora and its improvement with ultrasound-assisted alkali activation[END_REF]. Microwave activation is considered as a clean and effective modification method that increases the pore structure of the sample [START_REF] Shan | Preparation of microwave-activated magnetic bio-char adsorbent and study on removal of elemental mercury from fl ue gas[END_REF]. In the microwave activation process, the material already carbonized is subjected to microwave heating for a short period of time. In this stage, chemical activating agents are used to obtain a more porous structure and increase of the number of sorption surface sites [START_REF] Paunovic | Ionisable emerging pharmaceutical adsorption onto microwave functionalised biochar derived from novel lignocellulosic waste biomass[END_REF].

Microwave-assisted heating allows for internal and volumetric heating compared to conventional heating, which causes different pore structures and surface areas [START_REF] Liu | The effect of several activated biochars on Cd immobilization and microbial community composition during in-situ remediation of heavy metal contaminated sediment[END_REF]. These results were significantly larger than those obtained for biochar without activation (1.72 m 2 g -1 ). The materials were applied to the in situ remediation of Cd in sediments, which led to excellent results [START_REF] Liu | The effect of several activated biochars on Cd immobilization and microbial community composition during in-situ remediation of heavy metal contaminated sediment[END_REF].

Biochar functionalization

The functionalization of the biochar surface is an important factor that assists in the adsorption performance, improving the soil environment, promoting crop growth and mitigating carbon dioxide, as well as the surface area [START_REF] Tang | Amino-functionalized sewage sludge-derived biochar as sustainable efficient adsorbent for Cu(II) removal[END_REF]. The surface of the biochar is modified to improve the desirable physicochemical characteristics, through an appropriate choice of activation procedures [START_REF] Sajjadi | Urea functionalization of ultrasound-treated biochar : A feasible strategy for enhancing heavy metal adsorption capacity[END_REF]. For example, Narayanan et al.

(2018) synthesized a functionalized catalyst derived from coconut shell, equipped with

CQDs. The treatment with concentrated sulfuric acid led to the anchoring of -SO3H and -COOH groups on the surface of carbon species that reacted to form amidoalkyl naphthols as shown in Fig. 9 [START_REF] Narayanan | Functionalized carbon dot adorned coconut shell char derived green catalysts for the rapid synthesis of amidoalkyl naphthols[END_REF]. Another interesting application was the functionalization of carbon-based materials formed by HTC from glucose performed by Urakami and coworkers (2013). The method consisted of a Diels-Alder cycloaddition reaction of furan fractions present on the surface of an HTC material with the maleimide groups from a maleimide-protected poly (ethylene glycol) (Me-PEG-MI) [START_REF] Urakami | Facile Polymer Functionalization of Hydrothermal-Carbonization-Derived Carbons[END_REF]. Others important functionalization processes are described in the next items.

Amino functionalization of biochar

The incorporation of heteroatoms into the carbon lattice increases the degree of charge delocalization, breaking the structure inertia of the sp 2 -hybridized carbon network [START_REF] Ye | Nitrogen-doped biochar fiber with graphitization from Boehmeria nivea for promoted peroxymonosulfate activation and non-radical degradation pathways with enhancing electron transfer[END_REF]. Nitrogen doping is one of the most promising approaches. Aminofunctionalized adsorbents capture heavy metal ions through the formation of stable chelate complexes and/or strong electrostatic interactions [START_REF] Guo | Synthesis of amino-functionalized biochar / spinel ferrite magnetic composites for low-cost and ef fi cient elimination of Ni(II) from wastewater[END_REF], [START_REF] Tang | Amino-functionalized sewage sludge-derived biochar as sustainable efficient adsorbent for Cu(II) removal[END_REF]. Treatment of biochar with ammonia and amines (both photochemically and non-photochemically) leads to aminolysis of the surface [START_REF] Shahbeig | Pyrolysis of municipal sewage sludge for bioenergy production : Thermo-kinetic studies , evolved gas analysis , and techno-socioeconomic assessment[END_REF]. Guang and Hong (2014) introduced an innovative method for amino-functionalization of biochar. In general, a method for amino-functionalization of biochar consists of a first nitration step involving its impregnation with sulfuric and nitric acid, both concentrated. After washing the material and drying, an amount of sodium hydrosulfite is added to the nitrated beaker, as shown in the scheme of Fig. 10 [166]. Tang and workers (2019) synthesized biochar derived from sewage sludge functionalized using a similar process employing previously pyrolyzed biochar (500 °C) plus hexadecyl trimethyl ammonium chloride and ammonia followed by addition of ethyl tetraethyl orthosilicate (TEOS). The system was stirred in N2 atmosphere, and the gray precipitate, after washing and drying, was calcined under N2. Amino-functionalization was obtained by mixing this solid with APTES whose mixture was refluxed (95 °C, 24 h under N2 flow). This material was used to remove Cu (II), providing satisfactory results. which was used in order to promote partial oxygenation of biochar in order to increase its cation exchange capacity, due to the formation of oxygen functional groups, particularly the groups (-COOH) on the surface of the biochar. The BC-COOH amination was performed by an immersion method, using urea and finalization with the use of diethylenetriamine. The material was applied to remove Hg(II), obtaining satisfactory results [START_REF] Faheem | Adsorption-assisted decontamination of Hg(II) from aqueous solution by multi-functionalized corncobderived biochar[END_REF].

Magnetic functionalization

Biochar is a fine powder that is difficult to separate from the aqueous system.

The magnetization of these material improves its application and re-use because it can be facially separated from the system by an external magnetic field to perform the solid-liquid separation [START_REF] Wang | Preparation of biochar by simultaneous carbonization, magnetization and activation for norfloxacin removal in water[END_REF]. Biochar is magnetized by the introduction of a magnetic material, such as iron, cobalt [START_REF] Yang | CO2 as a reaction medium for pyrolysis of lignin leading to magnetic cobalt-embedded biochar as an enhanced catalyst for Oxone activation[END_REF], [START_REF] Yang | Cobalt-impregnated biochar produced from CO2-mediated pyrolysis of Co/lignin as an enhanced catalyst for activating peroxymonosulfate to degrade acetaminophen[END_REF] or nickel [START_REF] Yao | Magnetic activated biochar nanocomposites derived from wakame and its application in methylene blue adsorption[END_REF]. However, among ferromagnetic materials, iron is most widely used for magnetizing biochar. The principal magnetization process involves chemical co-precipitation, thermal decomposition, hydrothermal synthesis, pyrolysis and microwave assisted heating [START_REF] Mubarak | Plam oil empty fruit bunch based magnetic biochar composite comparison for synthesis by microwave-assisted and conventional heating[END_REF].

In the pyrolysis process, the iron preloading on biomass before the thermal treatment is converted to Fe3O4 according to Equations 19-22 [START_REF] Jiang | Enhancing the Catalytic Activity and Stability of Noble Metal Nanoparticles by the Strong Interaction of Magnetic Biochar Support[END_REF].

Fe 3+ + 3 H2O → Fe(OH)3 + 3 H + (Eq. 19)
Fe(OH)3 → FeO(OH) + H2O (Eq. 20)

2 FeO(OH) → Fe2O3 + H2O (Eq. 21)

9 Fe2O3 + (H2, CO, C) → 6 Fe3O4 + (H2O, CO2, CO)↑ (Eq. 22)

Mubarak and collaborators (2016) synthesized magnetic biochar upon microwave assisted heating. The magnetic biochar showed excellent ferromagnetic property with a saturation magnetization of 8.16 and 4.20 emu/g using microwave and conventional heating, respectively [START_REF] Mubarak | Plam oil empty fruit bunch based magnetic biochar composite comparison for synthesis by microwave-assisted and conventional heating[END_REF].

Han and co-workers (2020) synthesized magnetic bioactivated carbon from lignin. According to the authors, a ferrous salt is incorporated into the carbon matrix during the process of fusing lignin, and it is decomposed and reduced to Hagg iron carbide (Fe5C2) during the carbonization process. Afterwards, the Hagg iron carbide is exposed and oxidized to form magnetite (Fe3O4) during the steam activation process.

Pyrolysis steam is not only an activating agent for the generation and enlargement of pores, but it is also effective for the oxidation of reduced iron species. The best magnetic saturation was 12.05 emu/g [START_REF] Han | Magnetic bio-activated carbon production from lignin via a streamlined process and its use in phosphate removal from aqueous solutions[END_REF].

Spinel ferrite, with the general formula of AFe2O4 (A = Mn, Zn, Ni, Co, Fe, among others) has excellent structural stability and magnetic property due to the variable valence of iron [START_REF] Reddy | Magnetic biochar composite: Facile synthesis, characterization , and application for heavy metal removal[END_REF]. In this sense, Zhai and collaborators (2020) synthesized the photocatalyst biochar@CoFe2O4/Ag3PO4 for the degradation of bisphenol A under irradiation with visible light [START_REF] Zhai | Novel biochar@CoFe2O4/Ag3PO4 photocatalysts for highly efficient degradation of bisphenol a under visible-light irradiation[END_REF]. The material was synthetized via a one-pot method by co-precipitation followed by heating in an autoclave and pyrolysis. The saturation magnetic strength of the biochar@CoFe2O4/Ag3PO4 was 17.30 emu/g, which met the requirements to separate from suspension easily only by the additional magnetic field after being used in the photocatalytic process.

Jiang and coworkers (2019) synthetized magnetic biochar-supported noble-∆ ∆ metal NPs such as Pd@Fe-BC, Pt@Fe-BC and Ag@Fe-BC (BC = biochar, that were applied in p-nitrophenol reduction for recovery of the excessive reductants in the form of H2 [START_REF] Jiang | Biochar-supported magnetic noble metallic nanoparticles for the fast recovery of excessive reductant during pollutant reduction[END_REF]. After pyrolysis, magnetite (Fe3O4) was found in all materials, with mean Pd, Pt and Ag NP size of 2.15, 3.60 and 19.62 nm, respectively. The order of reactivities was Pd@Fe-BC > Pd@Fe-BC > Pd@Fe-BC, which was attributed to the relative sizes of these NPs. The synergic effect was proven by the authors where biochar and Fe3O4 acted as accelerators of the noble metal NP catalysis to produce active hydrogen. The magnetic separation allowed the recuperation and reuse of the material for five times with high efficiency.

In a previously work, Jiang et al. ( 2018) described the study of p-nitrophenol reduction by Pd@Fe3O4/BC, the synthesis of the material being similar to that described above [START_REF] Jiang | Enhancing the Catalytic Activity and Stability of Noble Metal Nanoparticles by the Strong Interaction of Magnetic Biochar Support[END_REF]. Li et al. (2019) [START_REF] Li | Preparation of silver-nanoparticle-loaded magnetic biochar/poly (dopamine) composite as catalyst for reduction of organic dyes[END_REF] synthesized Ag NPs loaded on a polydopamine-coated magnetic biochar catalyst (MC-PDA-Ag). The feedstock used was pine sawdust that was preloaded with iron and pyrolysate under N2. The material was dispersed into dopamine hydrochloride, previously dissolved into Tris-HCl (10 mmol L -1 , pH = 8.5).

The AgNPs were produced using Tollens' reagent (silver ammonia solution). Besides a binder medium to connect the matrix of magnetic biochar and AgNPs, polydopamine served as a reducing agent for the reduction of Ag + to Ag 0 . The synthetic route is shown in Fig. 13. The material was applied to the reduction of organic dyes (methylene blue, rhodamine B and methyl orange) satisfactorily. The material was separated by magnetization and reused in five successive cycles, maintaining cycle stability and high reactivity. Zahedifar et al. [START_REF] Zahedifar | Nanomagnetic biochar dots coated silver NPs ( BCDs-Ag / MNPs ): A highly efficient catalyst for reduction of organic dyes[END_REF] synthesized nanomagnetic biochar dots-coated AgNPs (BCDs-Ag/MNPs) using a green process. The biochar was produced from leaves and stems feedstock that were pyrolyzed at 450 °C. Biochar dots were formed using an acid process involving a mixture of concentrated HNO3 and H2SO4 (1: 3, v/v), followed by sonication for 3h and heating at 100 °C for 24h. The synthesis of aminofunctionalized magnetic NPs (PEI-Fe3O4) was performed by the co-precipitation method. The synthesis of BCDs coated with Fe3O4 magnetic NPs were synthesized by coupling the amino and carboxyl groups on the CQD surface. Finally, the plant extract of Atlantic Pistachio leaves was used for the synthesis of BCDs-Ag/MNPs.

Biochar as a support of nanocatalysts and other reagents

There are two types of catalysts, unsupported and supported. An unsupported catalyst comprises only active material, such as metal complexes and metal compounds such as oxides that are used in the desired reaction [START_REF] Liew | Production of activated carbon as catalyst support by microwave pyrolysis of palm kernel shell : a comparative study of chemical versus physical activation[END_REF]. On the other hand, a supported catalyst consists of small amounts of active material on the surface of a support material called a catalyst support [START_REF] Liew | Production of activated carbon as catalyst support by microwave pyrolysis of palm kernel shell : a comparative study of chemical versus physical activation[END_REF]. The use of heterogeneous catalysts is interest due to their non-toxicity, ease of handling and reuse [START_REF] Narayanan | Functionalized carbon dot adorned coconut shell char derived green catalysts for the rapid synthesis of amidoalkyl naphthols[END_REF].

Although many nanoparticle suspensions are highly efficient, their application in reaction systems is seriously compromised due to their ease of aggregation. Therefore, this support presents advantages in catalysis because it allows to stabilize fragile suspensions of nanoparticles without deactivating them. Some examples of catalyst supported on different supports are: Cu supported on octahedral molecular sieves of manganese oxide [START_REF] Huang | Optimization of Cu catalysts for nitrophenol reduction , click reaction and alkyne coupling †[END_REF]; ruthenium(II) complexes supported on magnetic NPs [START_REF] Wang | A recyclable ruthenium(II) complex supported on magnetic nanoparticles: a regioselective catalyst for alkyne-azide cycloaddition †[END_REF];

Mn-Fe NPs supported on reduced graphene oxide (rGO) nano-sheets [START_REF] Nasser | Mn-Fe nanoparticles on a reduced graphene oxide catalyst for enhanced ole fi n production from syngas in a slurry reactor †[END_REF], among others.

Ali and coworkers (2014) summarized, in their review, some advantages and disadvantages of some main materials (alumina, silica, zeolite and carbon) used as catalyst supports in oxidation reactions [START_REF] Ali | Heterogeneous Metal Catalysts for Oxidation Reactions[END_REF]. There are a few disadvantages for each of these materials and, therefore, the choice of a particular support depends mainly on its availability and ease of obtaining. In particular, the use of biochar has been widely studied as a support for various catalysts and reagents [START_REF] Lee | Biochar as a catalytic material for the production of 1,4-butanediol and tetrahydrofuran from furan[END_REF], [START_REF] Lee | Engineered rice-straw biochar catalysts for the production of value-added chemicals from furan[END_REF], [START_REF] Zhu | Biochar: a new promising catalyst support using methanation as a probe reaction[END_REF]. Biochar has proven to be an effective support due to its high specific surface area, stability and many active sites on the surface [START_REF] Gao | Persulfate activation by sulfide-modified nanoscale iron supported by biochar (S-nZVI/BC) for degradation of ciprofloxacin[END_REF]. Besides, biochar prevents NP agglomeration and improve their reactivity [START_REF] Liu | Pyrolytic production of zerovalent iron nanoparticles supported on rice husk-derived biochar : simple, in situ synthesis and use for remediation of Cr( VI )-polluted soils[END_REF]. Leaching metal ions are adsorbed and immobilized by biochar through complexation or electrostatic adsorption [START_REF] Wu | Excellently reactive Ni/Fe bimetallic catalyst supported by biochar for the remediation of decabromodiphenyl contaminated soil: Reactivity, mechanism, pathways and reducing secondary risks[END_REF]. Another advantage is the ease of obtaining, as it is formed as a by-product of the production of biofuels from biomass.

Recent use of biochar as a stabilizer of metal-organic frameworks (MOFs) has shown its great versatility [START_REF] Navarathna | Rhodamine B Adsorptive Removal and Photocatalytic Degradation on MIL-53-Fe MOF/Magnetic Magnetite/Biochar Composites[END_REF]. Several researchers have integrated transition metals into the carbon template, such as graphene sheet, to produce MOFs, and reported that the carbon template effectively stabilized the metal derivatives [START_REF] Mian | Facile synthesis of sludge-derived MnOx-Nbiochar as an efficient catalyst for peroxymonosulfate activation[END_REF]. MOFs have been applied in several fields due to their unique characteristics, such as large specific surface areas and uniform distribution of active sites [START_REF] Qin | Metal organic framework derivative-TiO2 composite as efficient and durable photocatalyst for the degradation of toluene[END_REF]. In this sense, Navarathna and collaborators (2020) synthesized MIL-53-Fe MOF/Magnetic Magnetite/Biochar composites and evaluated its efficiency in the photocatalytic degradation of Rhodamine B. According to the authors, biochar contributes to the dispersion of MOF, minimizing the aggregation of NPs, provides extra stability to MOF and serves as a secondary adsorption site for heavy metals, oxy anions and organic contaminants [START_REF] Navarathna | Rhodamine B Adsorptive Removal and Photocatalytic Degradation on MIL-53-Fe MOF/Magnetic Magnetite/Biochar Composites[END_REF].

Recent studies have indicated that metal-modified biochar composites provided more active sites and modify the electrical charge, facilitating the removal of contaminants [START_REF] Qiu | Contribution of different iron species in the iron-biochar composites to sorption and degradation of two dyes with varying properties[END_REF]. In the next sections, this theme is discussed in more details.

Metallic NPs and their bimetallic compositions

Zero Valent Iron (ZVI) is an effective reagent for wastewater remediation, due to its good activity, low cost, high surface activity, magnetic recovery property and low toxicity for the environment [START_REF] Xia | One-pot preparation of magnetic Fe@Ag bimetallic catalyst for the catalytic reduction of 4-nitrophenol Desalin[END_REF]. Besides, iron is the fourth most abundant element on earth, and it is a reactive metal with a standard redox potential of -0.44 V, thus also a good reducing agent [START_REF] Pereira | Azo dye degradation by recycled waste zero-valent iron powder[END_REF]. ZVI has been widely used for the removal of environmental contaminants, as azo dye methyl orange [START_REF] Li | Reductive dechlorination of trichloroethene and carbon tetrachloride using iron and palladized-iron cathodes[END_REF], amoxicillin and ampicillin antibiotics [START_REF] Ghauch | Antibiotic removal from water: Elimination of amoxicillin and ampicillin by microscale and nanoscale iron particles[END_REF], and 2,4,6-trinitrotoluene [START_REF] Zhang | 2,4,6-Trinitrotoluene reduction kinetics in aqueous solution using nanoscale zero-valent iron[END_REF], among others. ZVI commonly used for applications is of macroscopic dimension, however, and the action mode involves the large surface of the ZVI sample. On the other hand, at the nanoscale, the lifetime of ZVI does not allow crossing a sufficiently long distance in large water areas to reach the target contaminant, because nano ZVI is too reactive. Let us focus on the nanoscale, for which nano-Zero-valent will now be abbreviated as nZVI. According to Raman and Kanmani (2016), the toxicity of nZVI nanoparticles (NPs) is smaller than that of carbon nanotubes, nanodiamond, nano TiO2, nano ZnO, and also no toxic effect is reported on humans [START_REF] Raman | Textile dye degradation using nano zero valent iron: A review[END_REF]. The nZVI NPs have spherical shape and their diameter size as bare is typically less than 100 nm. In aqueous solutions, all nZVI NPs react with water and oxygen to form an outer iron (hydr)oxide layer [START_REF] O'carroll | Nanoscale zero valent iron and bimetallic particles for contaminated site remediation[END_REF].

Under acidic conditions:

2Fe + 4H + (aq) + O2(aq) → 2Fe 2+ + 2H2O E=+1.67 V (Eq. 23)

2Fe 2+ + 4H + (aq) + O2(aq) → 4Fe 3+ + 2H2O E=+0.46V (Eq. 24)

Under neutral or alkaline conditions: 2Fe + 2H2O → 2Fe 2+ + H2(g) + 2OH - E=-0.39 V (Eq. 25)

2 Fe 2+ + 2H2O → 2Fe 3+ + H2(g) + 2OH - E=-1.60 V (Eq. 26)

Iron has a sufficiently negative redox potential (Eq. 23-26) [START_REF] Liu | Bimetallic Fe nanoparticles: Recent advances in synthesis and application in catalytic elimination of environmental pollutants[END_REF], under acid or alkaline conditions, to reduce various molecules, but efficiency and potential applications depend on the type of size (vide supra).

nZVI have some limitations such as easy precipitation, passivation, poor mass and electron transfer [START_REF] Zhang | Substantially enhanced anaerobic reduction of nitrobenzene by biochar stabilized sulfide-modi fi ed nanoscale zero-valent iron: Process and mechanisms[END_REF], [START_REF] Wang | Aqueous Cr(VI) removal by a novel ball milled Fe 0 -biochar composite : Role of biochar electron transfer capacity under high pyrolysis temperature[END_REF]. In addition, the lack of durability and the mechanical resistance presented by FeNPs also impair their practical applications [START_REF] Wang | Synthesis , characterization and aging study of kaolinite-supported zero-valent iron nanoparticles and its application for Ni(II) adsorption[END_REF]. To overcome these limitations, depositing nZVI on various supports has been widely used, such as clays [START_REF] Ezzatahmadi | Clay-supported nanoscale zero-valent iron composite materials for the remediation of contaminated aqueous solutions : A review[END_REF], [START_REF] Wu | Stability and pH-independence of nano-zero-valent iron intercalated montmorillonite and its application on Cr(VI) removal[END_REF], [START_REF] Wang | Synthesis , characterization and aging study of kaolinite-supported zero-valent iron nanoparticles and its application for Ni(II) adsorption[END_REF], [START_REF] Froehner | Degradation of Organochlorine Compounds using Zero Valent Iron (ZVI) Nanopartles Impregnated in Hydrophobic Modified Bentonite[END_REF], zeolites [START_REF] Kim | Removal of Pb(II) from aqueous solution by a zeolite -nanoscale zero-valent iron composite[END_REF], [START_REF] Li | Zeolite-supported nanoscale zero-valent iron: New findings on simultaneous adsorption of Cd(II), Pb(II), and As(III) in aqueous solution and soil[END_REF], [START_REF] Guo | Removal of tetracycline from aqueous solution by MCM-41-zeolite A loaded nano zero valent iron: Synthesis, characteristic, adsorption performance and mechanism[END_REF], chitosan [START_REF] Ahmadi | Synthesis of chitosan zero-valent iron nanoparticlessupported for cadmium removal: characterization, optimization and modeling approach[END_REF], [START_REF] Su | Three-dimensional honeycomb-like structured zero-valent iron/chitosan composite foams for effective removal of inorganic arsenic in water[END_REF] among others. However, biochar deserves to be highlighted due to the previously mentioned advantages. Furthermore, supporting nZVI in biochar facilitates adsorption and further degradation of the molecule of interest. Wu and co-workers (2019) verified that the π-π interaction, hydrophobic interaction and reduction process contributed to the phenanthrene removal process by modified biochar-supported nZVI [START_REF] Wu | Enhanced phenanthrene removal in aqueous solution using modified biochar supported nano zero-valent iron[END_REF].

According to [START_REF] Lyu | Biochar/iron (BC/Fe ) composites for soil and groundwater remediation : Synthesis, applications, and mechanisms[END_REF], biochar/Fe composites can be divided into three types, based on their different functional applications. The first includes composites for reduction purposes, such as nZVI/BC. The second includes compounds for the purposes of adsorption. The third includes BC/Fe composites used as activators in advanced oxidation processes (AOPs), such as persulfate oxidation [START_REF] Lyu | Biochar/iron (BC/Fe ) composites for soil and groundwater remediation : Synthesis, applications, and mechanisms[END_REF].

The synthesis of nZVI is performed by different methods, but the chemical method using NaBH4 solution is the most widely employed one due its advantages such as speed, simplicity and ease of synthesis, particularly on a laboratory scale [START_REF] Woo | Effects of washing solution and drying condition on reactivity of nano-scale zero valent irons (nZVIs) synthesized by borohydride reduction[END_REF]. However, the toxicity of sodium borohydride and the production of flammable hydrogen gas are some limitations of the synthesis [START_REF] Machado | Green zero-valent iron nanoparticles for the degradation of amoxicillin[END_REF]. Thus, biochar has numerous advantages, because it can function as a support and assist in the iron reduction process to produce nZVI, eliminating the use of auxiliary reducers. In the carbothermic process, ZVI is synthesized from an Fe-rich precursor that is mixed with biomass and subsequently co-melted under low oxygen conditions at high temperature [START_REF] Liu | Removal of Pb(II) and Cr(VI) from aqueous solutions using the flyash-based adsorbent materialsupported zero-valent iron[END_REF], [START_REF] Yu | Pyrolytic behavior of a zero-valent iron biochar composite and its Cu(II) removal mechanism[END_REF].

The synthesis of Fe-biochar composites via a one-step pyrolysis pretreated with an iron precursor at high temperature allows coupling the adsorptive, reducing and complexing characteristics of biochar and Fe in a single material [START_REF] Sun | Multifunctional iron-biochar composites for the removal of potentially toxic elements, inherent cations, and hetero-chloride from hydraulic fracturing wastewater[END_REF]. The biomass, in addition to being able to be transformed for the production of the biochar support, contains groups that can be used in the green iron reduction process to generate Fe 0 , as shown in Equations 27-30 [START_REF] Liu | Pyrolytic production of zerovalent iron nanoparticles supported on rice husk-derived biochar : simple, in situ synthesis and use for remediation of Cr( VI )-polluted soils[END_REF].

Fe3O4 + 2 C → 3 Fe 0 + 2 CO2 (Eq. 27)

2 Fe2O3 + 3 C → 4 Fe 0 + 3 CO2 (Eq. 28)

Fe2O3 + 3 H2 → 2 Fe 0 + 3 H2O (Eq. 29)

Fe2O3 + 3 CO → 2 Fe 0 + 3 CO2 (Eq. 30)

However, the iron source must be carefully selected for a particular purpose. uses FeCl3 as an iron precursor for the production of ZVI supported in biochar by the carbothermic method using different biomasses. They found that different biomasses produce different materials, because, in addition to ZVI, other compounds are formed in the process. For example, the association of silicon (Si) and phosphorus (P) with Fe resulted in the formation of fayalite, phosphates and phosphides of Fe, which limited the efficiency of ZVI production and/or facilitated its corrosion. In addition, chlorine is significantly volatilized in high temperature pyrolysis [START_REF] Lawrinenko | Sustainable Pyrolytic Production of Zerovalent Iron[END_REF]. On the other hand, magnetite (Fe3O4) is available in abundance, and it can be obtained at low cost in large quantities. It reduces the problematic discharge of harmful gases in the pyrolysis effluent [START_REF] Lawrinenko | Macroporous Carbon Supported Zerovalent Iron for Remediation of Trichloroethylene[END_REF]. The synthesis of the Fe-biochar compost, however, is also carried out in two-steps, (1) biomass pyrolysis and (2) reduction of iron precursor ions with a reducing agent [START_REF] Devi | Synthesis of the magnetic biochar composites for use as an adsorbent for the removal of pentachlorophenol from the effluent[END_REF].

Doping a nZVI surface with another metal such as Ni, Pd, Ag, etc., to synthesize bimetallic NPs can increase or modulate its reactivity and reduce the nZVI agglomeration [START_REF] Yi | The key role of biochar in the rapid removal of decabromodiphenyl ether from aqueous solution by biochar-supported Ni/Fe bimetallic nanoparticles[END_REF]. Due to the empty electron orbitals of transition metals, they accept electrons and activate the hydrogen molecule [START_REF] Yi | The key role of biochar in the rapid removal of decabromodiphenyl ether from aqueous solution by biochar-supported Ni/Fe bimetallic nanoparticles[END_REF]. In typical bimetallic ironbased system, nanoscale zero-valent iron serves as a reducing agent, while noble metals or transition metals serve as catalysts [START_REF] Wu | Modified biochar supported Ag/Fe nanoparticles used for removal of cephalexin in solution: Characterization, kinetics and mechanisms[END_REF]. Besides biomass use, that is a waste from agribusiness, alternative materials can be used as an iron source, such as the experiments by Li and collaborators (2017), who synthesized BC@Fe/Ni from steel pickling liquor and cane bagasse [START_REF] Li | Enhanced nitrate removal by novel bimetallic Fe/Ni nanoparticles supported on biochar[END_REF].

Biochar has also been widely used as a support for noble metals for different applications. For example, the use of biochar/gold composites has been used in the development of electrodes for the detection of hydroquinone and catechol isomers [START_REF] Xiang | Biochar decorated with gold nanoparticles for electrochemical sensing application[END_REF], [START_REF] Wang | Gold nanoparticles decorated biochar modi fi ed electrode for the high-performance simultaneous determination of hydroquinone and catechol[END_REF]. Biochar from chicken manure was used as a heterogeneous support for Pd immobilization as a biocatalyst in Suzuki-Miyaura and Heck-Mizoroki reactions Some applications related to different metals and its bimetallic compositions supported on biochar are shown in Table 3. Briefly, in this table are described the reagents that were supported, the feedstock used and biochar process of obtaining, environmental application, material reuse and leaching study.

Although comparing the efficiency of different materials is difficult due to the different conditions used in the experiments, five works with different materials used as a support for ZVI (montmorillonite, porous silica, alginate sphere, cationic resin beads, oak wood biochar) have been selected. All materials were used to remove hexavalent chromium and the main results are briefly described in qe a = 31.65 mg g -1 and removal of 99.75%. 5) The solid particles obtained, including TEOS, were added to an aqueous solution of IPA (429.6 g/L), HCl (79.6 g/L) and DI water, and then a sol-gel reaction continued for a certain time. Calcination at 500 ºC for 5 h. ( 6) Ethanol solution of FeCl3 (90 g/L) was added to porous silica materials. [START_REF] Vamvuka | Combustion behaviour of biomass fuels and their blends with lignite[END_REF] The mixture was heated at 80 ºC for 2 h to ethanol elimination. ( 8) NaBH4 (NaBH4:FeCl3=1:10) was slowly added by 15 minutes.

[Cr(VI)]initial= 25 mg L -1 ; pH = 6.0, reaction time = 60 min, NZVI@QIOS = 1 g L -1

Removal of 96.5%. [START_REF] Jang | Removal of hexavalent chromium ion from aqueous solution using nanoscale zero-valent iron particles immobilized on porous silica support prepared by polymer template method[END_REF] Alginate sphere (alginate-nZVI sphere)

(1) sodium alginate solution (2% (w/v)) was dropped into the CaCl2 solution (5% (w/v)) and the Ca(II)-alginate spheres were formed by rapid gelation. (2) the incorporation of Fe 3+ ions, FeCl3 solution (2% (w/v)), was made over night, and the spheres were filtrated. (3) Addition of NaBH4 (0.2 M) for 10 min under N2 atmosphere. (4) The material was washed with acetone and freeze-dried for 48 h by using a freeze dryer.

[Cr(VI)]initial= 20 mg L -1 ; pH = 5.3, reaction time = 48 h, alginate-nZVI sphere ∼ 5 g L -1 Removal of 98.2%. [START_REF] Huang | Floatable, macroporous structured alginate sphere supporting iron nanoparticles used for emergent Cr(VI) spill treatment[END_REF]

Cationic resin beads (R-nFe)

Resin: Dowex 50WX2 Na + . 20 g of Dowex 50WX2 resin was added to FeCl3 (200 mL, 0.05 M). ( 2) System agitation for 4 h (3) the resin was washed with deionized water (DI); Add the latter material to 150 ml of polyphenol solution (gallic acid) and 100 ml of DI, and stir the suspension for 20 h. The resin containing nZVI was filtered, subjected to washing steps and subjected to

[Cr(VI)]initial= 15 mg L -1 ; pH = 3.2, reaction time = 1 h, R-nFe = 4 g L -1
Removal of 96.1%. [START_REF] Toli | Incorporation of zero valent iron nanoparticles in the matrix of cationic resin beads for the remediation of Cr(VI) contaminated waters[END_REF] lyophilization.

Oak wood biochar (TP-nZVI-OB)

(1) FeCl3 solution (49.25 g L -1 ) was added to an amount of biochar (2.0 g); Fe/C of 2:1, which was subjected to agitation for 2 hours. Tea polyphenols (10 g L -1 ) was added to the suspension. The mixture was kept under stirring for an additional 30 min. The samples obtained were vacuum filtered, washed and vacuum dried at 50 ° C overnight.

[Cr(VI)]initial= 50 mg L -1 ; pH = 2.0, reaction time = 720 min, TP-nZVI-OB = 1 g L -1 Removal of 99.9%.

Activators of oxidizing agents

The main components of biochar are graphene pieces or carbon quantum dots that stabilize transition metal nanoparticles and transition metal oxide nanoparticles by π-bonding to their surface. Peripheral OH and CO2H groups bind the nanoparticles by H-bonding and O-metal site bonding that contribute to this stabilization, but leave a large number of open free sites for interaction with-and catalytic activation of substrates. This situation creates electronic delocalization at the biochar-nanoparticle interacting surface that is favorable for activation by inner-sphere electron transfer processes leading to the formation of free radicals (FRs). Yang and co-workers (2015) reported the degradation of p-nitrophenol by FRs employing biochar from pine. The signs of FRs increased when the biomass was pyrolyzed from 200 to 500 ° C, but decreased at 700 ° C. The authors also found that coating biochar with tamic acid inhibited the generation of OH, but did not inhibit the formation of FRs. The addition of t-butyl alcohol also inhibited the OH generation and decreased p-nitrophenol (PNP) degradation, which was associated with a competition between PNP and OH for the biochar surface. Degradation by-products were determined by GC-MS, such as catechols and hydroquinone [START_REF] Yang | Degradation of p -Nitrophenol on Biochars: Role of Persistent Free Radicals[END_REF].

Advanced Oxidative Processes (AOPs) are characterized by the in-situ generation of free radicals with sufficient potential for instance to degrade various persistent contaminants. The common AOPs mainly include O3/UV, UV/H2O2, H2O2/O3, Fenton, Fenton-like reactions and photocatalytic degradation that are characterized by the production of hydroxyl radicals [START_REF] Jiang | Removal of atrazine by biochar-supported zero-valent iron catalyzed persulfate oxidation: Reactivity, radical production and transformation pathway[END_REF]. Recently, persulfate-based AOPs (PS) has stood out due to its strong oxidizing ability, principally, because sulfate radicals (SO4 -, E° = 2.5-3.1 V) has more positive redox potential than hydroxyl radicals ( OH, E° = 1,8-2,7 V) [START_REF] Zhu | Degradation mechanism of norfloxacin in water using persulfate activated by BC@nZVI/Ni[END_REF]. Compared to H2O2, solid sodium persulfate (Na2S2O8) is more stable, costs less and is easier to transport and store [START_REF] Sun | Activation of persulfate by CO2-activated biochar for improved phenolic pollutant degradation : Performance and mechanism[END_REF]. The SO4 -radical anion preferentially attacks organic compounds through electron transfer reactions, while OH more likely do that through by capturing hydrogen or addition reactions; thus, SO4 - exhibits better selectivity towards organic pollutants than OH [START_REF] Chen | Biochar modification significantly promotes the activity of Co3O4 towards heterogeneous activation of peroxymonosulfate[END_REF].

The persulfate (PS) or peroxymonosulfate (PMS) anion (Fig. 15) contains an O-O bond; it is an oxidizing agent with a relatively strong oxidizing property [START_REF] Du | Efficient removal of acid orange 7 using a porous adsorbent-supported zero-valent iron as a synergistic catalyst in advanced oxidation process[END_REF] [START_REF] Ye | Nitrogen-doped biochar fiber with graphitization from Boehmeria nivea for promoted peroxymonosulfate activation and non-radical degradation pathways with enhancing electron transfer[END_REF]. SO4 -can be produced by activating PMS or PS. PMS is more susceptible to activation due to the asymmetry of its molecular structure [START_REF] Fu | Fabrication of Fe3O4 and graphitized porous biochar composites for activating peroxymonosulfate to degrade p-hydroxybenzoic acid: Insights on the mechanism[END_REF]. The sulfate radical has a high redox potential, high stability and a relatively long half-life of ~ 30-40 µs, which is much longer than that of the hydroxyl radical (10 -3 µs) [START_REF] Gao | Persulfate activation by sulfide-modified nanoscale iron supported by biochar (S-nZVI/BC) for degradation of ciprofloxacin[END_REF]. The conversion of persulfates to SO4 -requires some type of activating agent, including transition metals, high temperatures, UV irradiation, ultrasound and microwave irradiation, among others [START_REF] Magioglou | Activation of Persulfate by Biochars from Valorized Olive Stones for the Degradation of Sulfamethoxazole[END_REF]. Recent studies show that persistent free radicals (PFRs) in biochar activates hydrogen peroxide [START_REF] Fang | Key Role of Persistent Free Radicals in Hydrogen Peroxide Activation by Biochar: Implications to Organic Contaminant Degradation[END_REF] and PS [START_REF] Liu | Activation of persulfate with biochar for degradation of bisphenol A in soil[END_REF] to generate hydroxyl radical and radical sulfate, respectively. -Functional groups such as -COOH and -OH on the surface of biochar act as electron transfer mediators to activate persulfate, (Eqs. 33-34) [START_REF] Zhu | Degradation mechanism of norfloxacin in water using persulfate activated by BC@nZVI/Ni[END_REF], [START_REF] Yao | One-step synthesis of zerovalent-iron-biochar composites to activate persulfate for phenol degradation[END_REF], [START_REF] Wei | Enhanced catalytic degradation of tetracycline antibiotic by persulfate activated with modified sludge bio-hydrochar[END_REF], [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF], [START_REF] Nguyen | Biochar-mediated oxidation of phenol by persulfate activated with zero-valent iron[END_REF].

BC-OOH + S2O8 2-→ BC-OO + SO4 -+ HSO4 - (Eq. 33)

BC-OH + S2O8 2-→ BC-O + SO4 -+ HSO4 - (Eq. 34)

Carbonaceous materials containing sp 2 hybridized graphical structures have a large number of free flowing π electrons (C-π) [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF]. The basic locations on the carbon surface accelerate the decomposition of H2O2 into hydroxyl and perhydroxyl radicals (Eqs. [START_REF] Liu | Rapid microwave activation of waste palm into hierarchical porous carbons for supercapacitors using biochars from di ff erent carbonization temperatures as catalysts †[END_REF][START_REF] Liu | Removal of 17β-estradiol from aqueous solution by graphene oxide supported activated magnetic biochar: Adsorption behavior and mechanism[END_REF] [260], [START_REF] Masaru | Discovery of the Activated-Carbon Radical AC+ and the Novel Oxidation-Reactions Comprising the AC/AC+ Cycle as a Catalyst in an Aqueous Solution[END_REF].

C-?? + H2O2→ C-?? + +OH -+ OH (Eq. 35)

C-?? + + H2O2→C-?? + HO2 + H + (Eq. 36)

Similar mechanisms have been proposed for the biochar-persulfate system (Eq.

37) [START_REF] Nguyen | Biochar-mediated oxidation of phenol by persulfate activated with zero-valent iron[END_REF]. Semiquinone radical anion (SQ -) is generated by reversible reduction of quinone or quinone moieties on humic/fulvic-acid (HA/FA≡Q), which decomposes the PS with the generation of radicals, carrying out the degradation of organics [START_REF] Yu | Magnetic nitrogen-doped sludge-derived biochar catalysts for persulfate activation: Internal electron transfer mechanism[END_REF].

C-?? + S2O8 2-→ C-?? + +SO4 2-+ SO4 - (Eq. 37)

SO4 -is the dominant radical species in acidic and neutral conditions, whereas OH plays an important role under alkaline conditions [START_REF] Zhu | Degradation mechanism of norfloxacin in water using persulfate activated by BC@nZVI/Ni[END_REF]. In alkaline conditions, the generated SO4 -reacted with OH -to produce HSO4 -, according to Eq. 38-39 [START_REF] Yao | One-step synthesis of zerovalent-iron-biochar composites to activate persulfate for phenol degradation[END_REF],

[259], [START_REF] He | Persulfate activation with sawdust biochar in aqueous solution by enhanced electron donor-transfer effect[END_REF], [START_REF] Hussain | Degradation of aniline with zero-valent iron as an activator of persulfate in aqueous solution †[END_REF].

SO4 -+ OH -→ OH + SO4 2- (Eq. 38)
OH + SO4 -→ HSO4 -+ 0.5O2 (Eq. 39)

In addition, • OH radicals can still react with OH -to form H2O, Eq. 40 [START_REF] Wei | Enhanced catalytic degradation of tetracycline antibiotic by persulfate activated with modified sludge bio-hydrochar[END_REF].

OH + OH -+ e -→ O 2-+ H2O (Eq. 40)

In acidic conditions (pH < 2), however, the efficiency decreases because of the electron competition, as can be seen in the Eqs. 41-42 [START_REF] Wei | Enhanced catalytic degradation of tetracycline antibiotic by persulfate activated with modified sludge bio-hydrochar[END_REF].

2H + + 2e -→ H2 (Eq. 41)

S2O8 2-+ e -→ SO4 2-+ SO4 - (Eq. 42)
The efficiency of the process is dependent of the PS concentration. High concentrations, however, limit the efficiency of the activation process, as indicated in Eqs. 43-44 [START_REF] Chen | Removal of tetracycline via the synergistic e ff ect of biochar adsorption and enhanced activation of persulfate[END_REF], [START_REF] Pi | Persulfate activation by oxidation biochar supported magnetite particles for 102 tetracycline removal :Performance and degradation pathway[END_REF].

SO4 -+ SO4 -→ S2O8 2- (Eq. 43) SO4 -+ S2O8 2-→ S2O8 -+ SO4 2- (Eq. 44)
A similar process occurs with excess hydrogen peroxide, as shown in Eq. 45-46 [START_REF] Lai | Fabrication of novel magnetic MnFe2O4/bio-char composite and heterogeneous photo-Fenton degradation of tetracycline in near neutral pH[END_REF].

H2O2 + OH → HO2 + H2O (Eq. 45)

HO2 + OH → O2 + H2O (Eq. 46)

Yua et al. ( 2020) used biochar from shrimps to activate persulfate in the 2,4dichlorophenol removal process. The authors evaluated different pyrolysis temperatures in the synthesis process (400, 600 and 800 °C). The materials at 400 °C (PSS-400) showed a carbon structure with sp 3 hybridization, dominated by high disorder, while at 800 °C (PSS-800) it showed the existence of graphitic carbon (sp 2 hybridization). The catalytic efficiency of the material PSS-800 was 29 times larger than that of PSS-400. For PSS-400, the mechanism was dominated by free radicals, while for PSS-800 the mechanism was non-radical, that is, there was a direct transfer of two electrons. The authors concluded that porosity and carbon configuration were two critical factors in the degradation process [START_REF] Yu | Hierarchical porous biochar from shrimp shell for persulfate activation : A two-electron transfer path and key impact factors[END_REF]. Zhou et al. (2020) also observed this non-radical phenomenon in his work that dealt with persulfate activation by swine bone char-derived hierarchical porous in the degradation of 2,4-dichlorophenol [START_REF] Zhou | Persulfate activation by swine bone char-derived hierarchical porous carbon : Multiple mechanism system for organic pollutant degradation in aqueous media[END_REF]. [START_REF] Pi | Persulfate activation by oxidation biochar supported magnetite particles for 102 tetracycline removal :Performance and degradation pathway[END_REF] found that, in addition to the radicals SO4 -and OH, 1 O2 and O2 - also participate in the process of tetracycline degradation by magnetite supported on biochar [START_REF] Pi | Persulfate activation by oxidation biochar supported magnetite particles for 102 tetracycline removal :Performance and degradation pathway[END_REF]. The species 1 O2 is formed through the recombination of O2 -and the reaction of OH and O2 -based on Eqs. 47-48 [START_REF] Pi | Persulfate activation by oxidation biochar supported magnetite particles for 102 tetracycline removal :Performance and degradation pathway[END_REF].

O2 -+ O2 -+ 2H + → 1 O2 + H2O2 (Eq. 47)

O2 -+ OH → 1 O2 + OH - (Eq. 48)
The use of biochar as a support prevents leaching of free metal ions. This species causes secondary pollution of heavy metals, which represents serious problems inter alia for humans [START_REF] Yu | Efficient removal of several estrogens in water by Fe-hydrochar composite and related interactive effect mechanism of H2O2 and iron with persistent free radicals from hydrochar of pinewood[END_REF]. In addition, leaching of these metals disables the catalyst in successive processes. On the other hand, biochar allows a fast migration of electrons and accelerates electron transfer of the catalyst due to its exclusive electronic structure [START_REF] Xu | Heterogeneous activation of peroxymonosulfate by a biochar-supported Co3O4 composite for efficient degradation of[END_REF]. In this sense, several studies have used biochar as a support for various compounds to activate oxidizing species. For example, Rong et al. ( 2019) used magnetic γ-Fe2O3 supported on biochar from banana peels to activate persulfate in the degradation of bisphenol A in aqueous system. The authors verified that, in addition to SO4 -and OH, O2 -is one of the species responsible for degradation [START_REF] Rong | The magnetic biochar derived from banana peels as a persulfate activator for organic contaminants degradation[END_REF].

Zhu et al. (2020) synthesized BC@nZVI/Ni and applied it to the activation of persulfate for norfloxacin degradation. Under the used conditions, the degradation efficiency of BC@nZVI/Ni was higher than that of nZVI/Ni, by 67.8 and 80.5%, respectively. These results suggested that the degradation of norfloxacin was mainly due to the oxidation of radicals generated by the BC@nZVI/Ni/PS system [START_REF] Zhu | Degradation mechanism of norfloxacin in water using persulfate activated by BC@nZVI/Ni[END_REF].

Mian et al. ( 2019) synthesized MnOx supported on biochar derived from sewage sludge incorporated with agar, activated with NH4OH and pyrolyzed at 800 °C to act as a catalyst for the decomposition of peroxymonosulfate (PMS) for the production of sulfate radicals ( SO4 -). The authors evaluated the efficiency of the catalyst in the degradation of the acid orange 7 and rhodamine B dye. The efficiency of the material was associated with the presence of manganese oxides in different oxidation states, while activation with NH4OH increased the amount of N hetero atoms, area specific surface and microporosity. A non-radical process governed by N-carbon should play the primary role in acid orange 7 degradation instead of radical-induced catalysis [START_REF] Mian | Facile synthesis of sludge-derived MnOx-Nbiochar as an efficient catalyst for peroxymonosulfate activation[END_REF].

Some applications related to metal nanoparticles and oxides supported on biochar as activators of oxidizing agents are described in Table 5. In this table, briefly, the materials that were used, the oxidizing agent used, application, reuse of the material and leaching study are described.

As indicated in Table 6, the biochar showed characteristics similar to graphene oxide (GO) in the sulfadiazine degradation in aqueous systems. As previously reported for other materials, the advantage of biochar over GO is due to its ease of obtaining, which occurs as a by-product of biofuel production from biomass carbonization. a Study performed only for steams; b The optimized condition was applied to the degradation of bisphenol A, ciprofloxacin, orange II and tartrazine; .c For the conditions 0.5 g/L of the catalyst, 10 μM E2 and 5 mM H2O2.; d Study performed only for chloramphenicol; e Rate constants were (k). f Study performed only for methylene blue Table 6. Comparison of biochar and graphene oxide-based supports for Zero-Valent Iron (ZVI) used in sulfadiazine (SDZ) degradation mediated by Fenton-like reaction using hydrogen peroxide Material Syntheses do material General degradation conditions

Main results

Ref.

Threedimensional graphene network

(1) The graphene oxide (GO) was obtained from graphite by Hummer method; (2) A suspension of 0.8 g of GO plus 100 mL of water was subjected to US by 2 h; (3) The system was stirred with KBH4 (3.24 g) for 24 h, under Ar atmosphere. (4) 50 mL of FeSO4•7H2O (50 mM) was added dropwise (1 mL min -1 ) to the system under constant stirring for 1 h, under air protection; [START_REF] Montoro | Anaerobic co-digestion of sweet potato and dairy cattle manure : A technical and economic evaluation for energy and biofertilizer production[END_REF] The material was washed with water and dried in a vacuum freeze dryer.

Material dose: 0.5 g L - 1

; 10 mL of SDZ (10 mg L -1 ), pH = 3.0. When used, the [H2O2] = 3 mM.

(1) 11% removal of SDZ was attributed to surface adsorption; (2) under an argon atmosphere, 44% of SDZ removal was mainly attributed to adsorption and surface reduction; (3) in the presence of dissolved oxygen, 81% removal of SDZ was attributed to oxidation of • OHads; (4) in the presence of H2O2, there was 98% due more generation • OH.

[281]

Biochar (BC) from cornstalk

Biochar was produced by pyrolysis at 700 ºC. ( 2) BC (nZVI:BC 1:7) was added to 100 mL of FeCl3•6H2O (0,05 M) for 24 h (3) purged with N2, and stirred for 30 min (4) 100 mL of NaBH4 (0.2 M), was added dropwise to the suspension, followed by stirring for 30 min. Solid products washed with water and vacuum dried at 60 ºC for 8 h.

Material dose: 1.2 g L - 1 ; 10 mL of SDZ (10 mg L -1 ), pH = 3.0.

[H2O2] = 20 mM. 73% removal, whose main contribution was attributed to •OH. BC allowed a uniform distribution of nZVI, played an important role in SDZ adsorption, H2O2 activation, and relief of nZVI passivation.

[280]

Photocatalysis using semiconductors

Since the discovery of water electrolysis by TiO2 by [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF] [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF], many works have been developed using this semiconductor for different applications including photocatalysis. A photocatalyst is defined as a substance that is activated by adsorbing a photon and is capable of accelerating a reaction without being consumed [START_REF] Umar | Photocatalytic degradation of organic pollutants in water[END_REF]. Semiconductors can be used as photocatalysts that are activated when a photon with energy of hv matches or exceeds the bandgap energy, Eb, of the semiconductor (Fig. 17). This photolytic process generates the electron-hole pair, that is, an electron, e -CB, is promoted from the valence band, VB, into the conduction band, CB, leaving a hole, h + VB behind [START_REF] Hoffmann | Environmental Applications of Semiconductor Photocatalysis[END_REF]. The electron-hole pair allows a series of reductive and oxidative reactions, as shown in Equations 49-51. As can be seen, photocatalysis uses water or oxygen to produce an oxidant/reductant that uses less energy compared to the chemical-consumption process [START_REF] Chen | Facile and low-cost fabrication of ZnO/biochar nanocomposites from jute fibers for efficient and stable photodegradation of methylene blue dye[END_REF].

TiO2 + hv → TiO2(e -CB + h + VB) (Eq. 49)

h + + H2Oads → OH + H + (Eq. 50)

e -+ O2ads → O2 - (Eq. 51)

The scheme of electron-hole pair formation in a semiconductor is shown in the The TiO2 semiconductor has been used in heterogeneous photocatalysis because of its properties such as chemical stability, low cost, and low toxicity [START_REF] Hashimoto | TiO2 Photocatalysis : A Historical Overview and Future Prospects[END_REF].

However, TiO2 undergoes rapid deactivation in practical applications due to low quantum efficiency and fast electron orifice recombination [START_REF] Qin | Metal organic framework derivative-TiO2 composite as efficient and durable photocatalyst for the degradation of toluene[END_REF]. Other semiconductors such as ZnO [START_REF] Wang | Carboxymethyl cellulose stabilized ZnO/biochar nanocomposites : Enhanced adsorption and inhibited photocatalytic degradation of methylene blue[END_REF], [START_REF] Chen | Facile and low-cost fabrication of ZnO/biochar nanocomposites from jute fibers for efficient and stable photodegradation of methylene blue dye[END_REF], Fe2O3, CdS and ZnS are also widely used. In recent years, research on new semiconductors acting as photocatalysts has been extensively explored. The combination of various semiconductors results in different types of heterostructures. Heterostructures, such as the p-n type, Z-scheme and Schottky junction are commonly used in photocatalysis [START_REF] Bueno | Semicondutores heteroestruturados: uma abordagem sobre os principais desafios para a obtenção e aplicação em processos fotoquímicos ambientais e energéticos[END_REF]. The construction of heterostructures has been considered as a promising method, because these structures prominently improve the separation of photoinduced charges and extend the light response range when coupling with narrow bandgap semiconductors [START_REF] Yuan | In-situ synthesis of 3D microsphere-like In2S3/InVO4 heterojunction with efficient photocatalytic activity for tetracycline degradation under visible light irradiation[END_REF]. For further discussion, reviews on this area are available [START_REF] Lv | Structure design, controllable synthesis, and application of metal-semiconductor heterostructure nanoparticles[END_REF][START_REF] Fang | Semiconductor nanoheterostructures for photoconversion applications[END_REF][START_REF] Wang | Recent progress of heterostructure-based photoelectrodes in photoelectrochemical biosensing : a mini review[END_REF].

Powdered semiconductors have some limitations in practical photocatalytic applications mainly due to the difficult separation of the aqueous phase, and they can still cause secondary pollution [START_REF] Peng | Facile fabrication of hollow biochar carbon-doped TiO2/CuO composites for the photocatalytic degradation of ammonia nitrogen from aqueous solution[END_REF]. In addition, other disadvantages include the low usage of light, easy agglomeration and difficulty of separation from the aqueous phase to limit its application under visible irradiation because of its large band gap (3.2 eV for TiO2, for example) and fast electron-hole recombination [START_REF] Xie | Promoting charge separation of biochar-based Zn-TiO2/pBC in the presence of ZnO for ef fi cient sulfamethoxazole photodegradation under visible light irradiation[END_REF]. Biochar plays a vital role in improving electron transport due to the dense carbon layer, electron storage through oxidation and reduction of biochar quinone groups, charge separation through heterojunction between biochar and NPs, and enhancement of active sites by functional biochar surface groups [START_REF] Mian | Sewage sludge-derived TiO2/Fe/Fe3C-biochar composite as an ef fi cient heterogeneous catalyst for degradation of methylene blue[END_REF]. Besides, larger surface area, H bond and electron donor-receptor π-π between the pollutant and biochar promote adsorption reaction [START_REF] Zhang | TiO2 supported on reed straw biochar as an adsorptive and photocatalytic composite for the efficient degradation of sulfamethoxazole in aqueous matrices[END_REF]. Thus, biochar acts as a support to allow semiconductor particles to grow on the surface in a dispersed state, reducing the possibility of particle aggregation [START_REF] Hu | LaMnO3 nanoparticles supported on N doped porous carbon as e ffi cient photocatalyst[END_REF]. [START_REF] Lu | A novel TiO2/biochar composite catalysts for photocatalytic degradation of methyl orange[END_REF] synthetized TiO2 supported on biochar from nut shells. The efficiency of the material was used in the degradation of methyl orange. Methyl orange and intermediates were adsorbed by biochar due to the porous structure as well as the functional biochar groups. Under ultraviolet light, TiO2 is photoexcited generating the electron-hole pair. The presence of biochar was fundamental to inhibit the recombination of the electron-hole pairs [START_REF] Lu | A novel TiO2/biochar composite catalysts for photocatalytic degradation of methyl orange[END_REF]. Others works employing TiO2 supported on biochar for photodegradation are described in the literature, for the degradation of sulfamethoxazole [START_REF] Zhang | TiO2 supported on reed straw biochar as an adsorptive and photocatalytic composite for the efficient degradation of sulfamethoxazole in aqueous matrices[END_REF], [START_REF] Kim | Heterogeneous photocatalytic degradation of sulfamethoxazole in water using a biochar-supported TiO2 photocatalyst[END_REF], amoxicillin [START_REF] Li | Development of a bio-inspired photo-recyclable feather carbon adsorbent towards removal of amoxicillin residue in aqueous solutions[END_REF], safranine [START_REF] Cai | Titanium dioxide-coated biochar composites as adsorptive and photocatalytic degradation materials for the removal of aqueous organic[END_REF] and Reactive Brilliant Blue KN-R [START_REF] Zhang | Treatment of wastewater containing Reactive Brilliant Blue KN-R using TiO2/BC composite as heterogeneous photocatalyst and adsorbent[END_REF]. The doping of TiO2 with noble metals (Schottky junction, Fig. 18) supported on biochar is also described, as Ag/TiO2-biochar that was used in methyl orange removal [START_REF] Shan | Materials Science in Semiconductor Processing Photocatalytic degradation of methyl orange by Ag/TiO2/biochar composite catalysts in aqueous solutions[END_REF]. Heterostructures involving TiO2 and biochar are also gaining prominence, and for instance biochar-doped TiO2/CuO was used in the degradation of ammonia [START_REF] Peng | Facile fabrication of hollow biochar carbon-doped TiO2/CuO composites for the photocatalytic degradation of ammonia nitrogen from aqueous solution[END_REF]. ZnO is one of the most commonly used semiconductors due to its high chemical stability, low cost, low toxicity, and high oxidation properties; however; it has low photocatalytic efficiency [START_REF] El-Bindary | Enhancement of the photocatalytic activity of ZnO nanoparticles by silver doping for the degradation of AY99 contaminants[END_REF]. To overcome these challenges, the construction of ZnO semiconductor heterostructures has been evaluated in order to improve semiconductor photocatalytic activity [START_REF] Guan | Synthesis and characterization of ZnO@RSDBC composites and their Photo-Oxidative degradation of Acid Orange 7 in water[END_REF]. Chen et al. (2019) synthesized the composite ZnO/biochar nanocomposites from jute by a green method, in one pot, in which these authors used Zn(OAc)2 and pyrolysis in an atmosphere of N2 at 700 ° C. Above 250 °C, zinc acetate began to melt, decomposing at 318 °C, to form ZnO (Eq. 52). The material was applied to remove methylene blue in aqueous systems, and under optimized conditions the degradation and mineralization were 99 and 93%, respectively [START_REF] Chen | Facile and low-cost fabrication of ZnO/biochar nanocomposites from jute fibers for efficient and stable photodegradation of methylene blue dye[END_REF]. Other works involving ZnO-biochar has been developed, such as ZnO/biochar from carboxymethyl cellulose for methylene blue degradation [START_REF] Wang | Carboxymethyl cellulose stabilized ZnO/biochar nanocomposites : Enhanced adsorption and inhibited photocatalytic degradation of methylene blue[END_REF]. Zhang et al. (2020) obtained γ-Fe2O3-ZnO-biochar nanocomposites for rhodamine B removal [START_REF] Zhang | Synthesis of γ-Fe2O3-ZnO-biochar nanocomposites for Rhodamine B removal[END_REF].

Zn(OOCCH3)2 + 4 (-O-) → ZnO + 4 CO2 + 3 H2O (Eq. 52)

Lanthanum manganite (LaMnO3) crystallizes in the perovskite structure; it is known for its interesting electrical, magnetic and structural properties [START_REF] Hayat | Processing, device fabrication and electrical characterization of LaMnO3 nanofibers[END_REF], and its use as a photocatalyst is known [START_REF] Shaterian | Synthesis , characterization and photocatalytic activity of LaMnO3 nanoparticles[END_REF]. Phenol degradation in the liquid phase and oxidation of methanol in the gas phase.

There was a loss of only ± 10% of phenol degradation activity after cycle 5. f There was a loss of only ± 10% photo-oxidation of methanol after execution 5. f

Leaching of titanium after photocatalytic assays was not observed in the aqueous solution. (1) oxidized MWCNTs were dispersed in a solution of Milli-Q water (100 mL) and ethanol (100 mL) and subjected to US for 10 min (2) (100 mL) of Ti(OC4H9)4 and ethanol were added slowly to the suspension and stirred for 1 h. (3) Evaporation of the solvent at 90 ºC. (4) The system was calcined in vacuum at 600 ºC for 1 h.

Material dose: 0.5 g L -1 ; [MB] = 10 mg L -1 . The suspension was stirred in the dark for 2 h to ensure the establishment of an adsorption-desorption balance. The system was irradiated with visible light using a cut filter (λ> 450 nm), mercury lamp (125 W). 76% of MB is degraded by MWCNT/TiO2 (1:3) after less 100 min.

[329]

Graphene oxide (GO)

(1) Synthesis of graphene oxide by the Hummers method. (2) TiO2 were added to a GO suspension (1:2 ration), which was previously dispersed for 1 h. After stirring for 30 min under US, the suspension was dried at 293K for 4 h. Finally, the sample was centrifuged at 6000 rpm for 20 min. The supernatant was discarded, while the solids were dried in an oven at 323K overnight.

Material dose: 0.2 g L -1 ; [MB] = 5 mg L -1 ; pH=10. The suspension was stirred for 30 min until it achieved an equilibrium. Time of reaction: 4h. The system was irradiated with xenon lamp (500 W). ∼100% of removal.

[330]

Biochar from furniture industry (MDF)

(1) The MDF biomass was washed with water and dried at 60 ºC for 8 h. (2) The samples were ground; (3) TiO2 was impregnated on the surface of MDF biomass in the proportion of 10% (B10T) (w/w) by immersion in isopropyl alcohol followed by US for 60 min at 50 ºC; (4) The sample was dried in an oven until complete evaporation of the alcohol, followed by pyrolysis at 500 ° C under a nitrogen atmosphere (N2).

Material dose: 1.0 g L -1 ; [MB] = 10 mg L -1 ; pH=5.5. The suspension was stirred for 60 min until it achieved an equilibrium. Time of reaction: 180 min. The system was exposed to a source 10 mW m -2 of UV irradiation (15 W, wavelength range 380-480 nm). ∼80% of removal.

[331]

Synthesis of fine chemicals

Biomass contains low energy density and a complex structural matrix, which can compromise its direct use as a fuel [START_REF] Cao | Phosphoric acid-activated wood biochar for catalytic conversion of starch-rich food waste into glucose and 5-hydroxymethylfurfural[END_REF]. However, alternatives involving the synthesis of fine chemicals from biomass have attracted the attention of researchers.

The valorization of biomass for the synthesis of value-added chemicals is a sustainable process in which produced chemicals can replace petrochemicals as building blocks for various consumer products [START_REF] Yu | Aluminium-biochar composites as sustainable heterogeneous catalysts for glucose isomerisation in a biorefinery[END_REF]. Furfural and hydroxymethylfurfural (HMF) are obtained from lignocellulosic biomass and used as a platform chemical as shown in Acid-catalyzed fructose dehydration is the conventional method of production of HMF [START_REF] Cao | Phosphoric acid-activated wood biochar for catalytic conversion of starch-rich food waste into glucose and 5-hydroxymethylfurfural[END_REF]. The reactions steps of the production of HMF from biomass are shown in Fig. 21.

To accelerate the conversion of biomass, two types of catalysts are used: (a)

Brønsted acids that favor hydrolysis and dehydration through proton attack and (b)

Lewis acids that promote isomerization through displacement of 1.2 hydrides [START_REF] Yu | Catalytic valorization of starch-rich food waste into hydroxymethylfurfural ( HMF ): Controlling relative kinetics for high productivity[END_REF], [START_REF] Yu | Aluminium-biochar composites as sustainable heterogeneous catalysts for glucose isomerisation in a biorefinery[END_REF]. H2SO4, HCl and H3PO4 are the most used homogeneous catalytic reagents for the production of HMF through the dehydration of carbohydrates. They contribute to equipment corrosion and environmental pollution; however, product purification steps are necessary, and it is not possible to recycle the catalysts [START_REF] Cao | Production of 5-hydroxymethylfurfural from starch-rich food waste catalyzed by sulfonated biochar[END_REF]. Heterogeneous catalysis efficiently overcomes these disadvantages of homogeneous catalysis, but generally have low hydrolytic efficiency due to a mass transfer resistance between the solid catalyst and the water-insoluble cellulose [START_REF] Zhang | Effective transformation of cellulose to 5-hydroxymethyl-furfural catalyzed by fluorine anion-containing ionic liquid modified biochar sulfonic acids in water[END_REF]. Studies show that the incorporation of a Lewis acid metal species into porous support materials produces solids with high-performance catalysts for glucose isomerization to fructose [START_REF] Yang | Tin-Functionalized Wood Biochar as a Sustainable Solid Catalyst for Glucose Isomerization in Biorefinery[END_REF]. Fine chemicals obtained from biomass in reactions catalyzed by biochar are described in Table 9. This table describes the catalyst used, the start material, general conditions reactions, products obtained, yield and number of cycles.

Xiong and collaborators (2020) conducted a study of the isomerization from glucose to fructose. This reaction involves a critical intermediate step between many routes of chemical synthesis. The authors used biochar (BC), graphitic oxide (GIO) and graphene oxide (GO) as carbon supports to synthesize heterogeneous catalysts impregnated with Al. The conversions were performed via microwave heating, using a green solvent, in water, and moderate temperature (140 ºC). All materials showed useful catalytic activity in glucose isomerization with yields of 35, 34.3 and 29%, for GIO-Al200, GO-Al200 and BC-Al200, respectively. Comparatively, biochar-based catalysts showed similar catalytic performance and kinetic mode to graphene oxidebased catalysts [START_REF] Xiong | Study of glucose isomerisation to fructose over three heterogeneous carbon-based aluminium-impregnated catalysts[END_REF]. *dimethyl sulfoxide (DMSO) and deionized water (DIW); **AMIMCl: 1-allyl-3-methylimidazolium chloride; ***yield variation obtained for temperatures 500, 600 and 700 °C.

Syngas reforming reactions

Gases are generated in the incomplete degradation (syngas) during pyrolysis of biomass/gasification of biomass, which has several applications such as, for example, power generation using gas turbines or production of liquid fuels through the Fischer-

Tropsch process [START_REF] Buentello-Montoya | Performance of biochar as a catalyst for tar steam reforming : Effect of the porous structure[END_REF]. Biomass gasification has several advantages over fossil fuels such as lower emissions of CO2 and other flue gases, such as H2S, SO2, NOx [START_REF] Shen | In-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasification[END_REF].

However, a major disadvantage that limits the use of syngas is the generation of tars, a complex mixture of aromatic and oxygenated condensed hydrocarbons with molecular weights greater than that of benzene [START_REF] Cheng | Tar elimination from biomass gasification syngas with bauxite residue derived catalysts and gasification char[END_REF].

Tars are produced together with syngas during biomass gasification and their content varies from about 0.5 to 100 g/m 3 , depending on the type and design of the gasifier, the raw material used and the operating conditions [START_REF] Wang | Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning[END_REF]. Tars condense in pipes, filters or heat exchangers of downstream equipment and processes, which causes mechanical damage to the entire system [START_REF] Shen | In-situ catalytic conversion of tar using rice husk char-supported nickel-iron catalysts for biomass pyrolysis/gasification[END_REF]. There are several mechanical/physical gas cleanup systems that exist for removal of both particulates and tar from gases produced by biomass gasification. These processes will not be described here, and a review by Anis and Zainal (2011) is recommended [START_REF] Anis | Tar reduction in biomass producer gas via mechanical , catalytic and thermal methods: A review[END_REF].

Catalytic reforming is considered to be most promising in large scale applications, due to its fast reaction and reliability [START_REF] Wang | Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning[END_REF]. The catalysts for tar reduction are classified in six groups: (i) nickel-based, (ii) non-nickel metal, (iii) alkali metal, (iv) basic, (v) acid, and (vi) activated carbon catalysts [START_REF] Krerkkaiwan | Biomass derived tar decomposition over coal char bed[END_REF], [START_REF] Anis | Tar reduction in biomass producer gas via mechanical , catalytic and thermal methods: A review[END_REF]. Noble metals such as Pt, Rh, Ru, etc. have an effect on tar reforming [START_REF] Zhu | Bismuth impregnated biochar for efficient estrone degradation : The synergistic effect between biochar and Bi/Bi2O3 for a high photocatalytic performance[END_REF]. The nickel-based catalyst is the catalyst indicated for the cracking of tar, because it shows high tar conversion and gas production [START_REF] Mueangta | Catalytic Steam Reforming of Biomass-Derived Tar over the Coal/Biomass Blended Char:Effect of Devolatilization Temperature and Biomass Type[END_REF], [START_REF] Zhu | Bismuth impregnated biochar for efficient estrone degradation : The synergistic effect between biochar and Bi/Bi2O3 for a high photocatalytic performance[END_REF]. However, the deactivation of the catalyst due to carbonaceous deposition, sintering of metals and the high costs of noble metals are still serious problems [START_REF] He | Catalytic Performance of Coal Char for theMethane Reforming Process[END_REF]. Carbon-based catalysts, such as coal char and biomass char, have received attention for tar cracking due to their low cost and good catalytic performance [START_REF] Mueangta | Catalytic Steam Reforming of Biomass-Derived Tar over the Coal/Biomass Blended Char:Effect of Devolatilization Temperature and Biomass Type[END_REF], [START_REF] Burhenne | Benzene removal over a fixed bed of wood char: The effect of pyrolysis temperature and activation with CO2 on the char reactivity[END_REF], [START_REF] El-Rub | Experimental comparison of biomass chars with other catalysts for tar reduction[END_REF].

The thermal cracking of tar in a reactor using char as a catalyst has advantages over common mineral catalysts because: (1) char is produced in the pyrolysis and gasification system; (2) the coal surface is reactivated via gasification, avoiding deactivation; (3) coal is loaded with dispersed metal catalysts, increasing its catalytic capacity [START_REF] Sun | Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char[END_REF]. Besides, co-pyrolysis of coal and biomass is described as a way to increase the catalytic performance of coal in order to reduce tar [START_REF] Mueangta | Catalytic Steam Reforming of Biomass-Derived Tar over the Coal/Biomass Blended Char:Effect of Devolatilization Temperature and Biomass Type[END_REF], [START_REF] Krerkkaiwan | Biomass derived tar decomposition over coal char bed[END_REF]. The catalytic decomposition of tar using biochar from coal pyrolysis and/or biomass has become valuable, because coal production and tar reduction are implemented simultaneously within the gasifier [START_REF] Lai | Influence of Preparation Conditions on Characteristics, Reactivity, and Operational Life of Microsized Fe/Cu Bimetallic Particles[END_REF].

The thermal cracking of tar proposed by Baker and Mudge, using NiO catalyst, is described in Eqs. 53-57 [START_REF] Wang | Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning[END_REF].

tars → C + CnHm + gases (Eq. 53)

CnHm + n H2O → n CO + (n+m/2) H2 (Eq. 54)

CnHm + n CO2 → m/2 H2 + 2n CO (Eq. 55)

CH4 + H2O ⇌ CO + 3 H2 (Eq. 56)

CO2 + H2 ⇌ CO + H2O (Eq. 57)

Other works that described the application of biochar-based materials in reforming reactions are summarized in Table 10. In this table, the catalyst used, reactor specifications and general reaction conditions, start compound, and main results are briefly described.

For comparison, two works employing nickel-catalyzed methane reforming, which was supported on two different supports, hydroxyapatite and biochar are shown in Table 11. It can be seen that the yields of CH4 and CO2 conversion have seen very similar, around 80% for each compound. The biochar acts as a reducer by increasing the catalytic activity of the bauxite residue, presenting high conversion (≅100%) in a humid syngas environment.

[348]

NiO supported on char from wood and coal (15% NiO loading)

Sawdust biomass feed

Reactor tube of quartz: φ: 1-in.; l: 24-in; vacuum-sealing; catalyst bed length: 9 cm; char particle size: 0. The main effect of Cr is the great enhance of the adsorption to CO2. [START_REF] Xu | Catalytic CO2 reforming of CH4 over Cr-promoted Ni/char for H2 production[END_REF] Table 11. Comparison between biochar and other materials used as support for nickel catalysts for dry reforming of methane Support material General conditions for material synthesis

General reaction conditions Overall results

Ref. The process was carried out in a fixed bed reactor (φinternal = 8 mm). The catalyst (300 mg) was diluted twice with inert, non-porous alumina powder. The reactor was filled with the first layers of alumina powder, followed by the catalyst/alumina powder mixture, which was positioned in the center of the reactor, and finally with the second layer of alumina powder. The reactor was fed with a mixture of synthetic gas with the following composition: CH4: 20 mol%, CO2: 20 mol% and N2: 60 mol% with space velocity per hour in weight (WHSV) of 15.9 L h -1 g cat -1 .

Hydroxyapatite

CH4 conversion: 66-78% and CO2 conversion: 73-84%, Time on stream: 90 h

[360]

Biochar produced from cotton stem (SBET = 39 m 2 g -1 )

The catalyst was prepared by impregnation using Ni(NO3)2•6H2O with alumina (γ-Al2O3). The sample was heated under an N2 atmosphere for 120 min, respectively at 800 ºC and 1000 ºC.

Reactor: quartz tube of φinternal=40 mm; l=200 mm; catalyst: 6 g; microwave: f=2450 MHz and P: 0-3000 W; T: 800 °C; volume ratio of CH4/CO2/N2 of 1:1:2 and flow: 120 mL/min. 10% of nickel content shown conversion > 80 and 88% for CH4 and CO2, respectively. The H2/CO ratio are promoted in mixed reforming on Ni/biochar.

[361]

Final considerations

This review illustrates the fact that biochar, a by-product of the biomass carbonization process in energy production, is a promising material as catalyst support with unique characteristics. There are several processes leading to biochar. However, the hydrothermal carbonization has been highlighted due to lower energy consumption.

In this process, quantum carbon dots are obtained with several catalytic applications.

On the other hand, the pyrolysis process allows obtaining different products: biochar, bio-oil and syngas, enabling a sustainable procedure, with zero waste generation. One of the challenges in this regard is to produce the three products with adequate yields and quality for their different applications. In this sense, as seen in this review, the biochar generated in the pyrolysis process can be reused as support for different catalysts toward syngas reforming reactions and synthesis of fine products from biomass. In order to improve the catalytic activity of processes, chemical and/or physical activation can be used, as well as their functionalization. Then, it is believed that biochar use can mitigate environmental impacts, including the generation of greenhouse gases such as CO2 and CH4, being therefore a sustainable process of great economic and social value. The main components of biochar are graphene pieces or carbon quantum dots that stabilize transition metal nanoparticles and transition metal oxide nanoparticles by π-bonding to their surface. Peripheral OH and CO2H groups bind the nanoparticles by H-bonding and O-metal site bonding that contribute to this stabilization, but leave a large number of open free sites for interaction with-and catalytic activation of substrates.

Prospects

As previously discussed, the excellent characteristics of biochar are notorious.

Such characteristics allow applications that extend from its use as a soil additive to support catalysts for various applications. In this sense, new sustainable processes for the production of clean energy will be made possible from the carbonization of biomass and new destinations will be provided to the by-product, biochar. New energy conversion routes with zero waste generation are being implemented, as illustrated in Figure 22. Such practices will allow a significant advance in sustainable agriculture, enabling ecological and income improvements.

Catalyst supported by biochar are finding increasing use towards the production of fine chemicals involving a variety of nanocatalytic processes [START_REF] Astruc | Introduction: Nanoparticles in Catalysis[END_REF] also including the remediation of organic [START_REF] Lu | Nanocatalysts and other nanomaterials for water remediation from organic pollutants[END_REF] and inorganic [START_REF] Lu | Nanomaterials for removal of toxic elements from water[END_REF] pollutants from water. The use of biochar as catalysis support, as that of carbon materials in general, now finds a significant place in the context of hydrothermal carbonization materials that open a spectrum of applications, not only as catalyst supports in heterogeneous catalysis and electrocatalysis, but also in other fields such as batteries and fluorescent nanomaterials [START_REF] Nicolae | Recent advances in hydrothermal carbonisation: from tailored carbon materials and biochemicals to applications[END_REF].

Fig. 1 .

 1 Fig.1. Database: Web of Science, search day: 04/14/2020. Search terms: "Biochar" Insert: Search terms:

Fig. 2 .

 2 Fig. 2. Minimum estimated number of carbons in an aromatic cluster (C59), pyrolysate at 700 °C under an N2 atmosphere (93%) containing 7% CO2. Adapted from: Impact of oxidative carbonization on structure development of loblolly pine-derived biochar investigated by nuclear magnetic resonance spectroscopy and X-ray photoelectron spectroscopy, 96, Park, J. et al. Copyright (2019), with permission from Elsevier, License number: 4856490763912, https://doi.org/10.1016/j.diamond.2019.05.001 [58].

Fig. 3 .

 3 Fig. 3. Quaternary structure of biochar. Reproduced from: A direct observation of the fine aromatic clusters and molecular structures of biochars, 51, Xiao, X. et al. Copyright (2017), with permission from ACS, https://pubs.acs.org/doi/10.1021/acs.est.6b06300. Further permission related to this material excerpted should be directed to the ACS [59].

Fig 4 .

 4 Fig 4. Raman spectra of the five biochar (BC) activated by CO2 atmosphere at different temperatures. Reproduced from: Activation of persulfate by CO2-activated biochar for improved phenolic pollutant degradation: performance and mechanism, 380, Sun, C. et al. Copyright (2020), with permission from Elsevier, License number: 4843540842806, https://doi.org/10.1016/j.cej.2019.122519 [63]. Zhang et al. (2018) proposed the contribution of the ice-type adlayer (agglomerates of water) to the aromatic surface of the biochar as acceptors of H of ionizable organic pollutants (IOPs) in the adsorption process. According to the authors, when biochar is in water, each water molecule turns a hydrogen atom pointing to an aromatic ring (H ••• π interactions), Fig. 5a. Another hydrogen atom bonds to the oxygen atom of a lateral water molecule due to Polarization-Assisted or σ-Bond

Fig. 5 .

 5 Fig. 5. Structural details of the complex of coronene with a cyclic chair water clusters (a, b, and c), phenol adsorbed on a coronene (d), and phenol adsorbed on the complex (e). Reproduced from: Water clusters contributed to molecular interactions of ionizable organic pollutants with aromatized biochar via π-PAHB: Sorption experiments and DFT calculations, 240, Zhang, K. et al. Copyright (2018), with permission from Elsevier, License number: 4843540657075, https://doi.org/10.1016/j.envpol.2018.04.083 [70].

Fig. 6 .

 6 Fig. 6. Scheme of Carbon Quantum Dots synthesis. Reproduced from: Pentosan-derived water-soluble carbon nano dots with substantial fluorescence: properties and application as a photosensitizer, 315, Qiong, W. et al. Copyright (2014), with permission from Elsevier, License number: 4843550159542, https://doi.org/10.1016/j.apsusc.2014.06.127 [73]. Ziqi et al. (2020) synthesized CQDs from waste palm powders in the presence of thionyl chloride as a dopant through a one-stage hydrothermal treatment, which

  proposed by[START_REF] Okonkwo | The synthesis of micromesoporous carbon derived from nitrogen-rich spirulina extract impregnated castor shell based on biomass self-doping for highly ef fi cient supercapacitor electrodes[END_REF], the nitrogen-rich spirulina extract was used to mediate KOH activation. The structural characteristics were greatly improved, which led to a specific surface area of 1527 m 2 g -1[START_REF] Okonkwo | The synthesis of micromesoporous carbon derived from nitrogen-rich spirulina extract impregnated castor shell based on biomass self-doping for highly ef fi cient supercapacitor electrodes[END_REF].[START_REF] Ismail | Oxygen functionalized porous activated biocarbons with high surface area derived from grape marc for enhanced capture of CO2 at elevated-pressure[END_REF] synthesized biochar from marc grape with KOH activation in two steps. The KOH/grape marc biochar ratio of 3 presented the highest specific surface area, i.e. 2473 m 2 g -1 , high micropore volume (0.72 cm 3 g -1 ) and a pore diameter of 0.74 nm. The performance of the material was evaluated in the capture of CO2, the excellent result being associated with the coexistence of high specific surface area, high microporosity and oxygenated functional groups on the surface[START_REF] Ismail | Oxygen functionalized porous activated biocarbons with high surface area derived from grape marc for enhanced capture of CO2 at elevated-pressure[END_REF]. Hsu et al. (2019) compared the efficiency of biochar from sorghum distiller grains activated by KOH and NaOH in the adsorption of ammoniacal nitrogen. Although

Fig. 7 .

 7 Fig. 7. Lewis acid scheme on the surface of the ZnCl2 activated biochar. Reproduced from: Effect of ZnCl2activated biochar on catalytic pyrolysis of mixed waste plastics for producing aromatic-enriched oil, 81, Sun, K. et al. Copyright (2018), with permission from Elsevier, License number: 4844661221741, https://doi.org/10.1016/j.wasman.2018.09.054 [149].

  )C + H2O ⇌ CO + H2 (Eq. 18)[START_REF] Fang | Physically (CO2) activated hydrochars from hickory and peanut hull: preparation, characterization, and sorption of methylene blue, lead, copper, and cadmium †[END_REF] synthesized biochar from walnut and peanut hulls physically activated by CO2. The specific surface areas for the material activated at 900 °C with 2 h of activation were, respectively, 928 and 1308 m 2 g -1 . The materials were applied in the removal of methylene blue, lead, copper and cadmium by those who showed high removal rates[START_REF] Fang | Physically (CO2) activated hydrochars from hickory and peanut hull: preparation, characterization, and sorption of methylene blue, lead, copper, and cadmium †[END_REF].Cuong et al. (2019) synthesized biochar from rice husks through alkalineleaching and physico-chemical activation processes. The authors used a KOH: biochar ratio of 3 and activated at 800 °C for 0.5 h under a CO2 atmosphere. The biochar had a specific surface area of 2330 m 2 g -1 , and it was used to remove Cu(II) with an adsorption capacity greater than 265 mg g -1[START_REF] Cuong | Meso/micropore-controlled hierarchical porous carbon derived from activated biochar as a high-performance adsorbent for copper removal[END_REF].

  [START_REF] Sajjadi | Engineering Lowtemperature acoustic-based activation of biochar for enhanced removal of heavy metals[END_REF] found that physical activation combined with acid treatment and functionalization with urea grafted more functional groups containing nitrogen onto the surface of the biochar, leading to complete adsorption of heavy metal ions. Overall, a 40% increase (from 110.13 to 154.96 m 2 g -1 ) in the microporous area and a 23% reduction (from 13.50 to 9.89 m 2 g -1 ) in the mesoporous surface area were obtained after 0.5 min of ultrasound. The microporous increase is attributed to the formation of microjets and shock waves induced by ultrasound, which causes the generation of new pores, the opening of blocked pores and the exfoliation of the graphical structure of the biochar, as shown in Fig.8[START_REF] Sajjadi | Engineering Lowtemperature acoustic-based activation of biochar for enhanced removal of heavy metals[END_REF].

Fig. 8 .

 8 Fig. 8. The impact of short-term ultrasound activation on micro-porosity of biochar. Reproduced from: Engineering Low-temperature acoustic-based activation of biochar for enhanced removal of heavy metals, 34, Sajjadi B. et al. Copyright (2020), with permission from Elsevier, License number: 4896510793009, https://doi.org/10.1016/j.jwpe.2020.101166 [155].

Fig. 9 .

 9 Fig. 9. Proposed mechanism for the synthesis of amidoalkyl naphthols over biochar-based catalyst. Reproduced from: Functionalized carbon dot adorned coconut shell char derived green catalysts for the rapid synthesis of amidoalkyl naphthols, 520, Divya P. et al. Copyright (2018), with permission from Elsevier, License number: 4843560656508, https://doi.org/10.1016/j.jcis.2018.02.077 [162].

Fig. 10 .

 10 Fig. 10. Schematic amino-modification of biochar. Reproduced from: Amino modification of biochar for enhanced adsorption of copper ions from synthetic wastewater, 48, Guang-Xi Yang and Hong Jiang, Copyright (2014), with permission from Elsevier, License number: 4843560847487, https://doi.org/10.1016/j.watres.2013.09.050 [166]. Bamdad et al. (2018) functionalized biochar with 3-aminopropyltrimethoxysilane (APTES) [167]. Therefore, the authors prepared a suspension of biochar in distilled water, with pH between 3 and 4, and slowly added APTES. After sonication, the reaction was carried out under reflux at 70 °C, as shown in the diagram in Fig. 11.

Fig. 11 .

 11 Fig. 11. Modification of the biochar surface with APTES under reflux at 70 °C. Reproduced from: Nitrogen Functionalized Biochar as a Renewable Adsorbent for Efficient CO2 Removal, 32, Bamdad et al. Copyright (2018), with permission from American Chemical Society, https://doi.org/10.1021/acs.energyfuels.8b03056 [167].

  The great contribution of this work consisted in the model of reduction by the material, which attributed the excellent catalytic activity to the distinct electronic structure of the Pd modified by Fe species, mediated by the key functions present in the biochar (C-O, C=O, COOH and OH). Metallic Pd, Pd oxides, Fe(II), and Fe(III) coexist in Pd@Fe3O4/BC, which has been proven by XPS analysis. The reaction model proposes: (1) the adsorption on the catalyst surface of BH4 -that loses an electron forming hydrogen atoms on Pd NPs; (2) the 4-NP, in the form of nitrophenolate ion, is reversible adsorbed on the catalyst surface, and (3) the hydrogen atoms and electrons are transferred from hydrogen for 4-NP, followed by any steps of dehydro-deoxygenation until the formation of 4-aminophenol, as shown in Fig. 12.

Fig. 12 .

 12 Fig. 12. Possible pathway of 4-nitrophenol reduction by Pd@Fe3O4/biochar Reproduced from: Enhancing the catalytic activity and stability of noble metal nanoparticles by the strong interaction of magnetic biochar support, 57, Jiang, S. et al. Copyright (2018), with permission from American Chemical Society, https://doi.org/10.1021/acs.iecr.8b02777 [174].

Fig. 13 .

 13 Fig. 13. Synthetic route to NPs loaded on a polydopamine-coated magnetic biochar catalyst (MC-PDA-Ag). Reproduced from: Preparation of silver-nanoparticle-loaded magnetic biochar/poly(dopamine) composite as catalyst for reduction of organic dyes, 555, Li, H. et al. Copyright (2019), with permission from Elsevier, License number: 4843650301201, https://doi.org/10.1016/j.jcis.2019.08.013 [179].

For example,

  [START_REF] Kong | Synthesis of FC-supported Fe through a carbothermal process for immobilizing uranium[END_REF] synthesized flour carbon (FC)-supported nano-flake Fe (Fe-FC). The authors found that FeS was formed by carbothermic reduction from ferrous sulfate (FeSO4) under a reducing atmosphere[START_REF] Kong | Synthesis of FC-supported Fe through a carbothermal process for immobilizing uranium[END_REF].Lawrinenko et al. (2017)

[ 229 ]Fig. 14 .

 22914 scheme of reactions is shown in Fig.14.HCOOH → H2O + CO (Eq. 31)2CrO4 2-+ 10H + + 3CO → 2Cr 3+ + 3CO2 + 5H2O (Eq. 32)

  b Preparation of aqueous solution of PVAL (15 g/L) and n-hexane solution of LPO (42.2 g/L). (2) Addition of MMA (0.448 M), TEOS (0.124 M) and PDMS (22.7 g/L) in LPO solution. (3) Addition of this last mix in PVAL solution. (4) System agitation by 3h at 70 ºC, under N2 atmosphere. (

  amount of Cr(VI) adsorbed at equilibrium. b PVAL (polyvinyl alcohol); LPO (lauroyl peroxide); MMA (Methyl methacrylate); TEOS (tetraethyl 1 orthosilicate) PDMS (methacryloxypropyl-terminated polydimethylsiloxane) and IPA (isopropyl alcohol).

2

 2

Fig. 15 .

 15 Fig. 15. Structural formula (a) persulfate (PS) (b) peroxymonosulfate (PMS) anions

  According to Fang et al., for the PRFs formation, (1) the phenolic lignin in the biomass is pyrolyzed to form condensed structures with abundant portions of phenol or quinone, (2) a phenol group transfers one electron to transition metals, pre-existing in the biomass, (3) this is accompanied by the formation of PFRs linked to the surface of biochar, as shown in Fig.16. Finally, the PFRs in the biochar react directly with H2O2 by transferring a single electron forming • OH[START_REF] Fang | Key Role of Persistent Free Radicals in Hydrogen Peroxide Activation by Biochar: Implications to Organic Contaminant Degradation[END_REF].

Fig. 16 .

 16 Fig. 16. Proposed Mechanisms of PFR formation and H2O2 activation by biochar. Reproduced from: Key Role of Persistent Free Radicals in Hydrogen Peroxide Activation by Biochar: Implications to Organic Contaminant Degradation, 48, Guodong, F. et al. Copyright (2014), with permission from American Chemical Society, https://doi.org/10.1021/es4048126 [256].

Fig. 17

 17 Fig. 17.

  Fig. 17.

Fig. 17 .

 17 Fig. 17. Scheme of electron-hole pair formation in a semiconductor. Adapted from: Recent developments in photocatalytic water treatment technology : A review, Water Res. 44, Nan, M. et al. Copyright (2010), with permission from Elsevier, License number: 4897180681151, https://doi.org/10.1016/j.watres.2010.02.039. [285].

Fig. 18 .

 18 Fig. 18. Scheme of catalytic degradation of methyl orange (MO) by Ag/TiO2-biochar. Reproduced from: Photocatalytic degradation of methyl orange by Ag/TiO2/biochar composite catalysts in aqueous solutions, 114, Shan, R. et al. Copyright (2020), with permission from Elsevier, License number: 4843651310688, https://doi.org/10.1016/j.mssp.2020.105088 [303].

  [START_REF] Hu | LaMnO3 nanoparticles supported on N doped porous carbon as e ffi cient photocatalyst[END_REF] synthesized LaMnO3 NPs (LMO) supported on N-doped biochar from soybean waste (PCB). The presence of PBC increases the oxygen vacancy in LaMnO3, increasing the level of donors and decreasing the band difference in such a way that it can use sunlight more efficiently.

[ 328 ]a

 328 Not determined; b Study done only for ciprofloxacin; c Study done only for paraquat; d Study done only for methyl paraben; e Study done only for Ti-KBC; f Study done only for TiO2/SWP700.

Fig. 20 [

 20 Fig. 20[START_REF] Lee | Biochar as a catalytic material for the production of 1,4-butanediol and tetrahydrofuran from furan[END_REF],[START_REF] Fuente-Hernández | Reduction of Furfural to Furfuryl Alcohol in Liquid Phase over a Biochar-Supported Platinum Catalyst[END_REF],[START_REF] Chen | Acid-base synergistic catalysis of biochar sulfonic acid bearing polyamide for microwave-assisted hydrolysis of cellulose in water[END_REF].[START_REF] Xia | Efficient conversion of 5-hydroxymethylfurfural to high-value chemicals by chemo-and bio-catalysis[END_REF] published a review that highlighted the latest advances in catalytic conversion of HMF into value-added chemicals by some important reactions[START_REF] Xia | Efficient conversion of 5-hydroxymethylfurfural to high-value chemicals by chemo-and bio-catalysis[END_REF]. Another important review was written by[START_REF] Pileidis | Levulinic Acid Biorefineries: New Challenges for Efficient Utilization of Biomass[END_REF] in which they describe the production of levulinic acid from biomass. This molecule is also used as a platform in the synthesis of fine chemicals[START_REF] Pileidis | Levulinic Acid Biorefineries: New Challenges for Efficient Utilization of Biomass[END_REF].

Fig. 20 .

 20 Fig. 20. HMF as a platform compound for diverse reactions. Reproduced from: Efficient conversion of 5hydroxymethylfurfural to high-value chemicals by chemo-and bio-catalysis, 8, Xia, H. et al. Copyright (2018), with permission from Royal Society of Chemistry, https://doi.org/10.1039/c8ra05308a [336].

Fig. 21 .

 21 Fig. 21. Reaction steps and catalysts involved in the production of HMF from biomass. Reproduced from: Catalytic valorization of starch-rich food waste into hydroxymethylfurfural (HMF): Controlling relative kinetics for high productivity, 237, Iris, K. M. et al. Copyright (2019), with permission from Elsevier, License number: 4844661159641, https://doi.org/10.1016/j.biortech.2017.01.017 [338].

  fixed-bed reactor: φinternal: 3 mm; atmospheric pressure, T: 850 °C; CH4/CO2 ration (3:2, 1:1 and 2:3); flow: 20 mL/ min

( 1 )

 1 The material was doped with Ni by impregnation using Ni(NO3)2. (2) Drying at 105 ºC overnight. (3) The theoretical nickel content was 5.7% w/w.

Fig. 22 .

 22 Fig. 22. Scheme illustrating the use of biomass in the conversion of bioenergy and value-added products: sustainable agriculture

Table 2 .

 2 Yield of biochar synthesized by different processes

	Feedstock	Process	General condition	Yield	Ref.
	Arabica coffee husks	Pyrolysis Heating ramp: 10 ºC min -1 ; residence time: 4 h; Temperature: 350 and 600 ºC	36 and 33% for respectively. 350 and 600 ºC,	[108]
	Pine nut shell Pyrolysis Temperature (400-700 ºC) and pressure (0.1-2.0 MPa) for 20 min under N2 flow (80 mL min -1 ).	0.1 to 2.0 MPa. ∼35 to 45% from 400 to 700 ºC. ∼45 to 35% from	[109]
	Tea waste	Catalytic pyrolysis	Flow rates of CO2: 200 mL catalyst: 1.00 g. from 200 to 720 °C. Ni/SiO2 Heating rate of 10 °C min -1 min -1 . Biomass: 1.50 g;	34-35%	[110]
	Coconut fiber (CF) and eucalyptus leave (EL)	HTC	10 g of biomass + 100 mL of deionized water; heating of autoclave 150-375 ºC.	90% (150 ºC) to respectively. to 375 ºC, for CF and EL up 35.3 and 28.1%	[111]
	Eucalyptus sawdust	HTC	80 g of sawdust: distilled water (ratio 1: 7). Purge the 160 -250 ° C for 1 h. stirring at 100 rpm. Heating reactor with N2 for 5 min,	160 to 250 ºC. ∼70 to 35% from	[112]
	Fresh (FBPB) and dehydrated banana peel (DBPB)	HTC	5 g FBPB or 2.5 g DBPB + 50 mL of H3PO4 solution 20% (soaked for 2 h). The mixture was transferred to an ° C for 2 h. autoclave and heated to 230	15.6* and 35.75 for FBPB and DBPB, respectively.

  [START_REF] Guo | Evaluation of the catalytic performance of different activated biochar catalysts for removal of tar from biomass pyrolysis[END_REF] compared biochar activation from rice husks (RHC) using different activating agents, H3PO4, KOH and ZnCl2. The specific surface area (m 2 g -1 )was RHC-ZnCl2 (394.50)> RHC-KOH (390.59)> RHC-H3PO4 (346.30)> RHC(89.93).

	The catalytic performance of the materials was evaluated in the tar decomposition, with
	an efficiency obtained of 83.4, 91.6 and 94.2% for, respectively, RHC-ZnCl2, RHC-
	H3PO4 and RHC-KOH [8].
	Wang et al. (2020) synthesized biochar from corn cob activated with H3PO4

Table 4 .

 4 As can be seen,

		nZVI	Starch	Carbothermic reduction	Removal of uranium in aqueous system.	Unrealized	Unrealized	[235]
		Magnetic ZVI	Peanut hull	Carbothermic reduction	Removal of Cr(VI) and trichloroethylene in aqueous system.	Unrealized	Unrealized	[236]
		ZVI	Red oak (RO) and switchgrass (SG)	Carbothermic reduction	Removal arsenic in aqueous system.	RO: 1°(81.8) and 16°(44.8) and 16°(36.3) * SG: 1°(50.97)	[Fe]≅0 (Data not shown)	[237]
		ZVI	Rice straw	Carbothermic reduction	Removal of 2,4,6-trinitrotoluene; hexahydro-1,3,5-trinitro-1,3,5-triazine; 2,4-system. difluorophenol in aqueous dibromophenol and 2,4-	Unrealized	Unrealized	[238]
		Sulfidated-Fe 0	Seaweed (Ulva prolifera)	Hydrothermal Carbonization	Removal of tetrabromobisphenol A in aqueous system.	1° (1.45) and 6° (1.17 mg g -1 )**	Unrealized	[239]
		Fe/Pd	Rice straw/HCl, NaOH, HF, H2O2, HNO3 and KMnO4	Chemical reduction with KBH4	Removal of 1,2,4-trichlorobenzene in aqueous system.	Unrealized	Unrealized	[240]
		nZVI	Sewage sludge	Carbothermic reduction	Removal of Cr(VI) in aqueous system.	Unrealized	Unrealized	[241]
		Pd/Fe	Corn straw	Chemical reduction with NaBH4	Removal of trichlorobenzene in aqueous system.	Unrealized	Unrealized	[242]
	1	nZVI-Si/Pd Rice straw *Column leaching experiment; **Adsorption capacity Chemical reduction with KBH4	Removal of Cr(VI) in aqueous system.	Unrealized	Unrealized	[243]

biochar showed similar results to the other supports. The great advantage over other supports is that biochars are easy to obtain.

Table 4 .

 4 Comparison of Zero-Valent Iron (ZVI) supports for Cr(VI) removal in aqueous system reactions. 1

	Support	General synthesis conditions of the material	General conditions	Efficiency	Ref.
	material		of reaction
	Montmorillonite	(1) ZVI were synthesized via chemical reduction by NaBH4. (2) ZVI and	[Cr(VI)]initial= 20 mg L -1 ;
	(Fe in Mt-nZVI)	montmorillonite suspensions were, separately, sonicated to ensure dispersion	pH = 5.5, reaction time
		of the particles (15 min). (3) Mixing both suspensions, stirring for 12 h.	= 240 min, Fe in Mt-
			nZVI= 2.75 g L -1

Table 5 .

 5 Various materials supported in biochar to activate persulfate (PS) or peroxymonosulfate (PMS) or hydrogen peroxide (HP) 1

	Material	Activated	Application	Number of		Ref.
		agent		material cycles	Leaching study
				(% of removal)
	MnFe2O4-Biochar from stems, leaves and corn cores	PMS	Removal of orange II in aqueous system.	1° (100); 2° (50) and 3° (46) a	[Fe]<40 and [Mn]<70 cycles. a μg/L for the three	[50]
	Co3O4-Biochar	PMS	Degradation of ofloxacin in aqueous system.	1-3° (100); The decreased. rate has been degradation	[Co]≅1.5 mg/L	[252]
	Fe3O4-Biochar	PMS	Degradation of p-hydroxybenzoic acid in aqueous system b .	1° (100); 2° (71) and 3° (65)	Unrealized	[254]
	ZVI supported on biochar from rice straw.	PS	Oxidation of phenol in aqueous system.	Unrealized	Unrealized	[260]
	Cu supported on biochar from corn stalks.	PS	Removal of tetracycline in aqueous system.	Unrealized	Unrealized	[263]
	Magnetic MnFe2O4/biochar (photo-Fenton system)	HP	Degradation of tetracycline in aqueous system.	1° (95); 2° (94); 3° (92) and 4° (90)	[Fe] and [Mn]<0.2 mg/L.	[265]
	Fe-hydrochar	HP	Removal of several estrogens (E1, E2 and E3) in aqueous system.	1° (94.6) and 5° (81.5) c	[Fe]=0.017 (pH 3.5), detected (pH ≥ 5.1) c . 4.1) and [Fe] not [Fe]=0.011 mg/L (pH	[267]
	Co3O4-Biochar	PMS	Degradation of chloramphenicols (chloramphenicol, florfenicol and thiamphenicol) in aqueous system.	1° (97.8) and 10° (85) d	Unrealized	[268]
	ZVI-sludge derived biochar.	PS	Degradation of acid orange and landfill leachate as target pollutants in aqueous system.	1° (0.0718); 2° (0.0655) and 3° (0.0502 min -1 ) e	[Fe]total=4.43 ppm (pH (pH 2.01). [Fe]total=22.12 ppm 9.13) and	[270]

Table 7 .

 7 Use of various photocatalysts supported on biochar

	Semiconductor	Bangap energy (eV) and electromagnetic spectrum range used in catalysis	Application	cycles (% of removal) Number of material	Leaching study	Ref.
	Biochar@ZnFe2O4/BiOBr Z-scheme heterojunction	Bg: 2.02; Visible light irradiation (λ > 420 nm)	Degradation of ciprofloxacin in aqueous system.	4 cycles with removal >70%.	Unrealized.	[311]
	BiOBr supported on biochar from wood flour waste	Bg≅ 2.70; Visible light was a 200 W LED lamp	Rhodamine B dye and Cr(VI) removal in aqueous system.	Unrealized.	Unrealized.	[312]
	Biochar-based heterojunction photocatalyst Bi2S3/BiOBr/BC	Bg≅ 1.69; 50 W energy-saving wavelength: 475 nm) visible LED light (dominant	Diclofenac removal in aqueous system.	4 cycles with removal >82.64%.	Unrealized.	[313]
	Hematite/biochar from Salacca skin waste Biochar-TiO2 composite	Bg: 2.21; 20-W UVB lamp Bg: 2.60-2.70; 500 W visible light.	Degradation methylene blue in aqueous system. Degradation of methylene wastewater. blue (MB) in simulated	The k slightly decreasing 1-2° and remains constant until 10th cycle. Unrealized.	1° cycle ≅ 5.5% of Fe (% in relation to FeTotal in the catalyst; 5° cycle <0.5% and 6-10° ≅ 0 Unrealized.	[314] [315]
	Bi2WO6 supported on biochar from pine modified with urea (n-biochar).	Bg: 2.28; (λ> 420 nm) -visible-light irradiation (500 W Xe lamp).	Rhodamine B dye and Cr(VI) removal in aqueous system.	1° (100) and 4° (98.4)	Unrealized.	[316]
	Bi/Bi2O3 supported on biochar	Bg a ; UV/Vis irradiation (500 W Xe lamp (λ> 350-700 nm)	Degradation of estrone in aqueous system.	1° (94.7) and 4° (82.9)	The bismuth concentration μg/L in the 5 cycles. varies between 9.2-6.1	[317]
	CuWO4 supported on biochar	Bg: 1.86; (λ > 400 nm) visible-light irradiation from tungsten lamp (150 mW/cm 2 )	Photocatalytic degradation of of methanol. system and electro-oxidation and rhodamine-B in aqueous ciprofloxacin, methylene blue	1° (97), 2° (93), 3° (90), 4° (86) e 5° (79) b .	Unrealized.	[318]
	Zn-Co-LDH supported on biochar Bg: 1.83; 10 W UV-B light irradiation source	Degradation of gemifloxacin in aqueous system.	5 cycles with removal >85%.	Unrealized.	[319]
	Fe@Zn@biochar from corn stalks	Bg a ; 260 ± 10 Lux of light intensity	Cr(VI) removal in aqueous and soil system.	Unrealized.	Unrealized.	[320]
	Cu2O-CuO@biochar	Bg: 1.70 and 2.10 for CuO and Cu2O, respectively; 10 W UV-B light irradiation.	Degradation of reactive Orange in aqueous system.	5 cycles with removal >85%.	Unrealized.	[321]

Table 8 .

 8 Comparison between biochar and other carbon materials in the synthesis of TiO2-based composites for photocatalytic degradation of methylene blue (MB)

	Material	Syntheses do material

Table 9 .

 9 Fine chemicals obtained from biomass in reactions catalyzed by biochar.

	Catalyst	Compound of origin Conditions	Product obtained	Yields	Number of cycles (Yield %)	Ref
	Ru-Re supported on biochar from rice straw	Furan	T=160 °C; P=30 bar H2; weight ratio of feed/catalyst = 8; furan conversion = 80%.	Main products were: 1,4-butanediol (1,4-BD); tetrahydrofuran (THF),	1,4-BD ≅ 30% THF ≅ 50%	The catalyst showed loss of activity after the reaction.	[185]
	Ru-ReOx supported on biochar from rice straw (RSB) by pyrolysis in an N2 or CO2 environment	Furan	T=160 °C; P= 30 bar de H2; furan conversion = 80%.	1,4-butanediol (1,4-BD); tetrahydrofuran (THF)	Ru-ReOx/RSB-N2: 1,4-BD ≅ 25% THF ≅ 55% THF ≅ 70% 1,4-BD ≅ 15% Ru-ReOx/RSB-CO2:	Unrealized.	[186]
	Phosphoric acid-activated wood biochar	Bread waste (BW); Rice waste (RW); Spaghetti waste (SW)	Food waste 0.5 g, catalyst 0.2 g, 180 °C, 20 min, DMSO/DIW * 10 mL (3/1, v/v)	5-hydroxymethylfurfural (HMF)	BW: 24.5 ; RW: 22.2 and SW:14.7 Cmol%	Unrealized.	[332]
	Al supported on biochar from wood waste by N2 (10%Al-700N2) or CO2 (10%Al-700CO2) pyrolysis atmosphere	Glucose	Catalyst: 0.25 g, 0.5 g glucose in 10 mL water; T=160 °C; t=20 min; (microwave: 1900 W).	Fructose	10%Al-700N2: 14-16 Cmol% *** 10%Al-700CO2: 16-18 Cmol% ***	1° (21.5 mol%); 2° (13.5) and 3° (7.8 mol%). Conditions: 5 20% Al-500N2 min, acetone/H2O and	[333]
	Biochar-Supported Platinum Catalyst	Furfural	Catalyst: 0.025 mmol; T=210 °C for 2 h and 1500 psi, solvent: toluene; 3% w/w of Pt.	Furfuryl alcohol;	60.8%	≈ Same conversion and selectivity in two decrease 20%. the conversion cycles. In the third,	[334]
	Bamboo-derived biochar sulfonic acid bearing polyamide	Cellulose	Catalyst: 0.1 g, cellulose: 0.2 g, water: 1.5 mL, microwave: 350 W, T:90 °C, t: 2 h.	5-hydroxymethylfurfural (HMF)	23%	Efficiency (20%) has been proven in 4 different cycles.	[335]

Table 10 .

 10 Application of biochar-based materials in reforming reactions

	Catalyst	Starting compound	Reactor

specifications and some conditions Principal results Ref Biochar from the pyrolysis (RC) of hardwood and activated carbon (AC)

  Tar concentration = 20 g/Nm3 ; benzene(61.34), toluene(23.13) and naphthalene (15.53%).Reactor: l=47 cm; φ=2.66 cm. The bed area: h:7 cm. Gas permanence time in the bed: 0.5 s. P=atmospheric; T= 850 °C; t = 3 h. The % of CO; CO2; CH4 and H2, were respectively: RC: 32.7, 6.70; 27.6 and 33.0 AC: 33.6; 6.38; 33.0 and 27.0

					[346]
	Rice husk char-supported nickel-iron	Rice husk (0.125-0.5 mm)	Fast pyrolysis; T 800 °C; 5g of rice husk; carrier gas N2; flow rate 1.0 L/min; catalyst 5 g	Conversion of 92.3 and 93% using Ni-Ni char (with calcination), respectively. Fe char (without calcination step) and	[347]
	Bauxite residue-biochar
	mixed catalyst led. Biochar
	from sawdust by gasification
	(GBC)

and bauxite activated with HCl and calcined at 600 °C Naphthalene

  : 1.6 g/Nm3 Reactor: quartz tube of φinternal=15 mm; l=600 mm; catalyst bed: l=50-60 mm; bed volume:5.3 cm 3 ; Catalyst particle size:180-210 μm; T 700; P=101.3 kPa; Space velocity: 4500 h -1

  3-0.45 mm; catalysts: 18 g; gas residence time: 0.3 s; gas flow rate: 9.12 L/min; gasification agent: air; reforming time: 15 min; T= 800°C.

				Conversion efficiency: 97% in the best wood char > no catalyst. char > Ni/coal char > Ni/Wood char > coal conditions. Catalyst performance:	[349]
	Fe-rich char (with 10% Fe2O3 addition in the one-step method with microwave heating)	Methane reforming	Microwave reforming; quartz reactor; φ:4 cm; l=16 cm; catalyst: 3g; T: 800 °C; CH4:CO2:N2 1:1:2, flow: 120 t=160 min mL/min. Volumetric hourly space velocity: 2.4 L/(g h);	Conversion > 95%. High material stability, evidenced by decrease in conversion <5% in 160 min.

Acknowledgment Financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior -Brazil (CAPES) for Process: 88881.337360/2019-01, CNPq/FAPEMIG (agreement recorded in SICONV: 793988/2013), the University of Bordeaux and the Centre National de la Recherche Scientifique (CNRS) and helpful suggestions of the referees are gratefully acknowledged. 69 7.

LMO-PCB was applied in the degradation of Direct Green. Some photo-generated electrons are removed by the N-graphitic mediator, which suppresses the recombination of pair electron-hole, while some electrons react with oxygen to produce O2 •-in the conduction band. Pyridinic-N and pyrrolic-N act as active sites, in which oxygen is reduced to O2 •-when it reacts on the plane. Finally, h + react with H2O or OH - to form • OH radicals [START_REF] Hu | LaMnO3 nanoparticles supported on N doped porous carbon as e ffi cient photocatalyst[END_REF] (Fig. 19). Low-cost graphitic carbon nitride (gC3N4) is gaining great prominence due to the rapid response to visible light, low bandgap energy (2.8 eV) and low cost [START_REF] Meng | Photocatalytic behavior of biochar-modified carbon nitride with enriched visiblelight reactivity[END_REF]. This material is a new organic semiconductor induced by visible light, free of metals, with good chemical stability and suitable band positions [START_REF] Li | Synthesis of Biochar-Supported K-doped g-C3N4 Photocatalyst for Enhancing the Polycyclic Aromatic Hydrocarbon Degradation Activity[END_REF]. However, the practical photocatalytic applications of gC3N4 are limited by the rapid recombination of photoinduced electron-hole pairs and the poor response of visible light [START_REF] Meng | Photocatalytic behavior of biochar-modified carbon nitride with enriched visiblelight reactivity[END_REF]. Li and Lin (2020) obtained K-doped g-C3N4 supported on biochar from corn straw. The introduction of potassium effectively inhibited the growth of gC3N4 grains, reduced the energy of the gap band and increased the range of response to visible light [START_REF] Li | Synthesis of Biochar-Supported K-doped g-C3N4 Photocatalyst for Enhancing the Polycyclic Aromatic Hydrocarbon Degradation Activity[END_REF].

Other recent works are described in Table 7. In this table the semiconductors used, the bandgap energy of the material and the range of the electromagnetic spectrum used, and application and reuse of the material and leaching study are briefly described.

As shown in Table 8, biochar showed characteristics similar to multiwalled carbon nanotubes (MWCNTs) and graphene oxide (GO) used as support for the semiconductor TiO2 in the photocatalytic degradation of methylene blue (MB) in aqueous system. It is important to mention that for these materials with adsorbent characteristics, an MB adsorption-desorption equilibrium step is necessary.