Hicham Zeghioud
email: hicham.zeghioud@unilasalle.fr

Lydia Fryda

Hayet Djelal

Aymen Assadi

Abdoulaye Kane

A comprehensive review of biochar in removal of organic pollutants from wastewater: characterization, toxicity, activation/functionalization and influencing treatment factors

Keywords: Biochar, Characterization, Toxicity, Contaminant removal, Functionalization, Magnetization

The current climate and environmental challenges demand novel and cost-effective sustainable materials with a high efficacy and eco-friendly properties. Biochar (BC) has gathered a great interest in the last years across different research fields, thanks to its special physicochemical characteristics such as high specific surface area, high porosity, ion exchange capacity and because of scientific evidence in removing various contaminants from soil, water, and air. This review aims to give the state of the art on the removal of toxic organic pollutants from wastewater using biochar, activated biochar, functionalized biochar and metal oxide or clay base biochar composites, commenting on the substrates' physiochemical properties, the role of the most used characterization techniques toxicity, activation/functionalization and influencing treatment factors in organic pollutant removal from wastewater.

The effect of different operating parameters of the water treatment process is summarized and thoroughly discussed. The toxicity of biochar components such as heavy metals, dioxins and polycyclic aromatic hydrocarbons (PAHs) is addressed. The various application processes of biochar in water treatment at both laboratory and pilot scale is also reviewed. The activation techniques, functionalization pathways and biochar basednanocomposite synthesis methods for enhancing the pollutant elimination efficiency are also extensively discussed. Finally, the paper suggests research directions in the field of biochar application in wastewater treatment.

Introduction

The world population increase over the past two decades led to an increasing demand in water volume, both for human consumption and for the industry sector, which results to increased wastewater volumes released in the aquatic environment. A variety of pollutants are detected in wastewater streams and have been reported in literature, such as; carcinogenic heavy metals [START_REF] Park | Competitive adsorption of heavy metals onto sesame straw biochar in aqueous solutions[END_REF], petroleum hydrocarbons [START_REF] Li | Offshore oil spill response practices and emerging challenges[END_REF], polycyclic aromatic hydrocarbons (PAHs) [START_REF] Flesch | Characterization and determination of the toxicological risk of biochar using invertebrate toxicity tests in the state of Aguascalientes,México[END_REF], organic dyes [START_REF] Alves De Lima | Mutagenic and carcinogenic potential of a textile azo dye processing plant effluent that impacts a drinking water source[END_REF], phenols, pesticides, pharmaceuticals and veterinary antibiotics [START_REF] Zeghioud | Review on discharge Plasma for water treatment: mechanism, reactor geometries, active species and combined processes[END_REF]. A diversity of pollutant treatment techniques has been extensively discussed in literature [START_REF] Liu | Comparison and performance assessment of ozone-based AOPs in view of trace organic contaminants abatement in water and wastewater: A review[END_REF][7] [START_REF] Zhao | Application of coagulation/flocculation in oily wastewater treatment: A review[END_REF]. However, the design of optimal, efficient, and operational treatment methods is still considered a challenge. Among different techniques which have been the subject of research, we identified the most promising: chemical oxidation, electrocoagulation, membrane separation, reverse osmosis, filtration, adsorption and biological treatment [START_REF] Luo | A critical review on the application of biochar in environmental pollution remediation: Role of persistent free radicals (PFRs)[END_REF][10] [START_REF] Aboudalle | A novel system coupling an electro-Fenton process and an advanced biological process to remove a pharmaceutical compound, metronidazole[END_REF].

Adsorption is one of the most efficient methods for the removal of a wide variety of pollutants from gaseous and wastewater streams [START_REF] Osagie | Dyes adsorption from aqueous media through the nanotechnology: A review[END_REF] [START_REF] Kim | Review of adsorption-membrane hybrid systems for water and wastewater treatment[END_REF]. During the adsorption process the pollutants are removed from the effluent by being adsorbed on the active sites on the surface of a solid substrate (adsorbent). Certain parameters such as temperature, pH or the concentration of adsorbate of the liquid phase influence the adsorption capacity and their fluctuations can result to the desorption of the adsorbed species from the surface back into the liquid phase [START_REF] Nguyen | Metal salt-modified biochars derived from agro-waste for effective congo red dye removal[END_REF].

Common adsorbents include clays, zeolites and active carbon, the latter being widely used for efficient micropollutant removal especially in the potabilization of water.

Active carbon used in Europe is usually produced from biomass transported over long distances, such as coco coir or coco shells, as well as coal. Local residual biomass and well controlled pyrolysis conditions offer the advantage of sustainably produced substrates, biochars, as an alternative to currently used active carbon [START_REF] Joseph | Sustainability assessment of activated carbon from residual biomass used for micropollutant removal at a full-scale wastewater treatment plant[END_REF].

The initial interest in biochar focused on the improvement of agricultural productivity via enhancement of soil fertility, increasing soil nutrient levels and water retaining capacity, while decreasing greenhouse gas emissions and carbon sequestration are drivers that gain a lot of attention currently [START_REF] Lehmann | Biochar for Environmental Management[END_REF] [START_REF] Mandal | Designing advanced biochar products for maximizing greenhouse gas mitigation potential[END_REF] [START_REF] Yaashikaa | A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy[END_REF]. Biochar is considered a cost-effective and environmentally friendly material [START_REF] Luo | A critical review on the application of biochar in environmental pollution remediation: Role of persistent free radicals (PFRs)[END_REF] [START_REF] Othmani | Biochar and activated carbon derivatives of lignocellulosic fibers towards adsorptive removal of pollutants from aqueous systems: Critical study and future insight[END_REF], with large surface area, high porosity, functional groups, high cation exchange capacity, stability, reusability [START_REF] Yaashikaa | A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy[END_REF] [START_REF] Li | A critical review on the application and recent developments of post-modified biochar in supercapacitors[END_REF]. These characteristics make it a suitable substrate in environmental applications which have also gained a lot of attention the last years. Some very recently published reviews demonstrate the effective application of biochar for both heavy metals and organic pollutants (pesticides, phenols) removal from contaminated soil [START_REF] Li | Application of co-pyrolysis biochar for the adsorption and immobilization of heavy metals in contaminated environmental substrates[END_REF] [START_REF] Jeyasubramanian | A complete review on biochar: Production, property, multifaceted applications, interaction mechanism and computational approach[END_REF] [START_REF] Guo | Biochar-Facilitated Soil Remediation: Mechanisms and Efficacy Variations[END_REF]. Regarding the produced biochar properties, these are related to the nature of the feedstock (lignocellulosic biomass: green waste, organic residues, energy crops) and the preparation conditions.

Biochar is a high carbon containing material with high specific surface area and a large porosity which is produced via the carbonization of biomass under a limited oxygen environment [START_REF] Lehmann | Biochar for Environmental Management[END_REF]. The carbonization process allows the valorization of diverse biomass sources like lignocellulosic residues from agro-industrial and forestry activities, animal manure, dewatered sludges, even micro algae, marine and aquatic species [START_REF] Ghodake | Review on biomass feedstocks, pyrolysis mechanism and physicochemical properties of biochar: State-of-the-art framework to speed up vision of circular bioeconomy[END_REF] [START_REF] Mandal | Designing advanced biochar products for maximizing greenhouse gas mitigation potential[END_REF].

Lignocellulosic biomass pyrolyzed at high temperature (>600 o C) produces biochars with high aromatic carbon content, high stability and therefore resistance to microbial decomposition. Animal manure conversion produces biochar with high ash content and therefore a very high pH value, due to the nutrients present in the feedstock. As a consequence, biochar may act as a slow-release fertilizer and alkalinity regulator for acidic soils. The high nutrients content of biochar is linked to its potential for soil cation exchange capacity (CEC), water holding capacity and plays a role in the nitrogen cycle in soil.

The term carbonization includes the technologies of pyrolysis, hydrothermal carbonization, flash carbonization, gasification and torrefaction [START_REF] Pang | Advances in thermochemical conversion of woody biomass to energy, fuels and chemicals[END_REF] [START_REF] Yaashikaa | A critical review on the biochar production techniques, characterization, stability and applications for circular bioeconomy[END_REF] [START_REF] Li | A critical review of the production and advanced utilization of biochar via selective pyrolysis of lignocellulosic biomass[END_REF]. These technologies vary with respect to their relative co-products and their maximum yields which are linked to the operating conditions such as pressure and temperature, its heating rate and its residence time in the reactors [START_REF] Jeyasubramanian | A complete review on biochar: Production, property, multifaceted applications, interaction mechanism and computational approach[END_REF] [START_REF] Tomczyk | Biochar physicochemical properties: pyrolysis temperature and feedstock kind effects[END_REF]. Low carbonization temperatures (<300-400 • C) result in low porosity biochars. Medium carbonization temperatures (400-700 •C) yield biochars with the highest porous development and a higher aromaticity, the latter promoting electron donor interaction [START_REF] Osagie | Dyes adsorption from aqueous media through the nanotechnology: A review[END_REF]. At moderate pyrolysis temperature, the biochar surface may contain oxygen and nitrogen surface groups, which add functionalities and could act as electron acceptors. Biochar prepared at temperatures above 700 °C are hydrophobic and recalcitrant, and have no apparent functionality anymore due to the loss of almost all O and H in their structure [START_REF] Tomczyk | Biochar physicochemical properties: pyrolysis temperature and feedstock kind effects[END_REF].

The term functional biochar refers to biochars (a) either produced at low temperature and thus still containing some O and H which allow the formation of surface functional groups, but with the drawback of low porosity and surface area, or (b) produced at usual pyrolysis conditions as presented earlier, and further treated either chemically or physically in order to enhance its functional groups while keeping its high surface area porosity and recalcitrance.

By further post treating biochars it is possible to obtain functional materials for environmental applications, in water and air treatment of industrial effluents and off gases.

In specific, the application of biochar in wastewater treatment is believed to overcome the problem of cytotoxic, genotoxic and mutagenic activities in living organisms resulting from the application of ozonation, hydrogen peroxide, chlorine dioxide for organics pollutants removal [START_REF] Shin | Reactions of Ferrate(VI) with Iodide and Hypoiodous Acid: Kinetics, Pathways, and Implications for the Fate of Iodine during Water Treatment[END_REF].

The adsorption efficiency of organic compounds on biochar is based mainly on electrostatic interaction, partitioning, hydrophobic interaction, pore-filling, hydrogen bonding formation, π-π interaction between biochar and organic pollutants [START_REF] Abbas | A critical review of mechanisms involved in the adsorption of organic and inorganic contaminants through biochar[END_REF].

The wide range of biomass feedstock used in the production of biochar in combination with the production conditions applied, lead to a range of products with variable physicochemical composition and properties. There is a strong interest in linking biochar physicochemical characteristics and their potential application.

Concerted efforts are being done to regulate and define quality requirements related to biochar, develop guidelines for biochar and substrates used for its production. At this moment, certain organizations are developing quality standards, namely the European biochar certificates (EBC) in Europe, and the International Biochar Initiative (IBI) at international level. The EBC indeed provides good suggestions to produce biochar that can potentially be used in various applications but it is not their objective to make an elaborate correlation between physicochemical characteristics and potential application. This review aims to give the state of the art on the removal of toxic organic pollutants from wastewater using biochar, activated biochar, functionalized biochar and metal oxide or clay base biochar composites, commenting on the substrates' physicochemical characteristics, toxicity, activation/functionalization techniques and the link with the adsorption process parameters.

The impact of adsorption process parameters such as temperature, biochar loading, pollutants initial concentration, initial pH, presence of ions and contact time is discussed, to evaluate the activation/modification of biochars and their effect on the treatment efficiency. This review paper also addresses recent biochar characterization techniques giving information on the interpretation of analysis results mainly for FTIR, XPS, XRD, TGA and Raman. The role of each technique on evaluating biochar properties is underlined. Furthermore, the modification of certain biochar characteristics via activation, functionalization and nanocomposite incorporation is discussed. The link between biochar treatment/post-modification process and biochar proprieties is highlighted. This review aims to contribute to the ongoing discussion and research efforts in EU and globally on biochar quality standards, that may complement and elaborate the existing EBC and IBI standards (European Biochar Certificate and International Biochar Initiative). This review focuses on the applications of biochar in physicochemical treatments and does not address biological processes that may include biochar at some stage.

Biochar characterization techniques

Recent studies prove the importance of thorough biochar characterizations to help identify agronomic and environmental applications and to distinguish biochar from other organic matter that provide comparable functions, either as soil improver or environmental applications [30][31]. The application area of biochar is large, depending on its characteristics such as specific surface area, surface functional groups, stability and structure and, to a lesser degree, elemental composition [START_REF] Zhou | Production and beneficial impact of biochar for environmental application: A comprehensive review[END_REF]. These characteristics make the choice of appropriate characterization techniques for biochar of a great interest and a difficult task at the same time. A judicious and complete characterization of biochar preceding to any target applications requires a high level of understanding the correlations between biochar production conditions and the physiochemical properties of the biochar [33][34]. The properties of biochar can be classified in four main categories: i) chemical properties (chemical composition, functional groups, pH, ion exchange capacity; ii) physical properties (porosity, pore size distribution, specific surface area); iii) structure and surface and morphology); iv) and thermal stability properties; each category requires specific characterization techniques as shown in figure 1. We would like to comment that the stability of biochar may be defined differently if one considers focusing on the evolution of physicochemical properties during applications as a stability indicator. Stability of biochar in the soil for example, is evaluated in several ways that include protocols of accelerated aging using chemical reagents, the objective of which tests is to estimate the long-term carbon storage capacity of biochars in the carbon credit context, or in GHG emission tests during incubation experiments to estimate the effect of biochar/hydro char in compost. These applications deviate from the objective of this review and will therefore not be further analyzed.

The characterization techniques reported in literature include Fourier Transform Infrared Spectrometer (FTIR), Cation Exchange Capacity (CEC) [START_REF] Askeland | Comparative characterization of biochars produced at three selected pyrolysis temperatures from common woody and herbaceous waste streams[END_REF], Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), X-Ray photoelectron spectrometry (XPS) [START_REF] Meng | Removal of sulfadiazine from aqueous solution by in-situ activated biochar derived from cotton shell[END_REF], Thermo Gravimetric Analysis (TGA), Differential scanning calorimetry (DSC), Nuclear Magnetic Resonance (NMR), Raman spectroscopy, Brunauer Emmett Teller (BET) [START_REF] Jindo | Physical and chemical characterization of biochars derived from different agricultural residues[END_REF].

Emerging pollutants appear in very low concentrations, in the range from ng•L -1 to µg Table 2 shows a variety of functionalized novel biochar -based materials, with the potential to eliminate a variety of pollutants (antibiotics, dye, pesticides and steroides) at different treatment conditions. The categorization of table 2 is based on pollutant rather than biochar materials because at this stage of research it is not yet possible to discuss categories of novel functional materials and link "products" to pollutants to be removed. In the reported literature the efficiency of biochar in eliminating a pollutant is expressed both, as a removal efficiency (%) or as an adsorption capacity (mg/g). We observed that the acidic treatment is the most applied one for biochar modification/activation.

Physical proprieties.

Physical properties include the specific surface area (SSA), particle size distribution, the bulk density, pore size and pore volume distribution [START_REF] Ding | Activation of persulfate by nanoscale zero-valent iron loaded porous graphitized biochar for the removal of 17β-estradiol: Synthesis, performance and mechanism[END_REF]. These parameters are directly linked to the biochar production conditions such as reactor temperature and residence time, addition of oxygen containing media in the process (air, pure oxygen, CO2 and steam) and/or post production processing to modify the product [START_REF] Hu | Cobalt-gadolinium modified biochar as an adsorbent for antibiotics in single and binary systems[END_REF]. In the work of Xiong et al., five different biochars from various feedstocks were applied on the adsorption of epoxiconazole (EPC) fungicide. The results showed that the adsorption capacity of these five biochars towards EPC is closely related to the total pore volume and specific surface area [START_REF] Xiong | Factors study for the removal of epoxiconazole in water by common biochars[END_REF]. Patel et al. also reported a positive correlation between the biochar sorptive potential toward ciprofloxacin/acetaminophen, the biochar surface area, and the total pore volume [START_REF] Patel | Ciprofloxacin and acetaminophen 63 sorption onto banana peel biochars: Environmental and process parameter influences[END_REF].

Beside total surface area also biochar surface heterogeneity plays an important role in the sorption behavior. The BET analysis is applied to study the effect of biochar modification on SSA as discussed in Figure 5 or to evaluate the effect of pyrolysis temperature on SSA of produced biochar. In their work Liu et al. reported the increase of SSA and total pore volume with increasing pyrolysis temperature. In the presence of acid the accumulation of inorganic content and the formation of new minerals at the biochar surface further increased the SSA [START_REF] Liu | Mechanism of sulfamic acid modified biochar for highly efficient removal of tetracycline[END_REF]. In addition to an increase in total pore volume, Cai et al. reported a reduction in pore size [START_REF] Cai | Pyrrolic N-rich biochar without exogenous nitrogen doping as a functional material for bisphenol A removal: Performance and mechanism[END_REF].

Chemical properties.

Chemical properties of biochar include the point of zero charge (PZC), Cation Exchange Capacity (CEC), surface functional groups and electrical conductivity. The structural and elemental analysis are important to predict the impact of biochar on the environment [START_REF] Xiang | Potential hazards of biochar: The negative environmental impacts of biochar applications[END_REF].

The SEM technique is coupled with energy-dispersive X-ray spectroscopy (EDX or EDS)

to obtain insight to the O/C atomic ratio, elemental components composition and distribution at the surface, and further on to explain the relationship between biochar preparation parameters and its resulting properties. For example, the O/C atomic ratio reflects the carbonization degree of the biochar, and indicates the degree of aromaticity of biochars, which is linked to their stability and therefore to its carbon sequestration potential [START_REF] Ma | Study of Biochar Properties by Scanning Electron Microscope -Energy Dispersive X-Ray Spectroscopy (SEM-EDX)[END_REF].

On the other hand, EDX is reported as a potential tool for the direct O/C atomic ratio measurement rather than applying the CHNS elemental measurement [58][59]. Some works reported that the H/C and O/C atomic ratios in biochar have a positive correlation with the concentration of hydroxyl, carboxyl, and carbonyl groups and thereby to CEC [60][61]. The elemental ratios of H/C refer to the aromatization degree of carbonization [START_REF] Chen | Transitional adsorption and partition of nonpolar and polar aromatic contaminants by biochars of pine needles with different pyrolytic temperatures[END_REF]. However, the value of O/C also represents the hydrophilicity of biochar [START_REF] Chun | Compositions and sorptive properties of crop residue-derived chars[END_REF].

XPS analysis is a very valuable characterization technique to identify the components functional groups on the biochar surface and their metallic state of compounds, before and after biochar composite synthesis [START_REF] Shen | Removal of tetracycline from an aqueous solution using manganese dioxide modified biochar derived from Chinese herbal medicine residues[END_REF]. XPS spectral lines (C 1s, O 1s and N 1s photoelectron lines) are rich in chemical information (Table 3). Their precise positions on the energy axis reflect the local electronic environment of the photoemitting atom, a clear shift for C-O, C = O, COO-and CO3 2-carbons can be seen [START_REF] Smith | Spectroscopy Analysis of Biochar, Biochar A Guid[END_REF]. Zhang et al confirmed the presence of Fe-O in biochar/FexOy composite by XPS at binding energies of 711.3 and 725.0 eV for the Fe 2p3/2 and Fe 2p1/2 core levels, respectively. Moreover, the adsorption of methylene blue leads to a decrease in peaks intensities for the majority of C 1s and O 1s spectrum [START_REF] Zhang | A green biochar/iron oxide composite for methylene blue removal[END_REF]. On the other hand, the appearance of N 1s in the XPS spectrum of biochar/FexOy-MB suggested that the adsorption involves N electrostatic attraction [66][67]. The incorporation of Mn on Nano-zerovalent manganese/biochar composite resulted to the detection of Mn 2+ and Mn 3+ present at binding energy values of 642.5 eV (Mn 2p3/2), 653.5 eV (Mn 2p1/2) for Mn 2+ and 645.3 eV that corresponds to Mn 3+ [START_REF] Iqbal | Nano-zerovalent manganese/biochar composite for the adsorptive and oxidative removal of Congo-red dye from aqueous solutions[END_REF]. Other works reported the presence of; i) O-Fe (530.12-530.26 eV) in O1 s XPS spectra of biochar/Fe composites [START_REF] Zhao | Chlortetracycline hydrochloride removal by different biochar/Fe composites: A comparative study[END_REF]; ii) Fe2p and Co2p at 711.3 eV and 779.8 eV, respectively [START_REF] Ma | A novel, efficient and sustainable magnetic sludge biochar modified by graphene oxide for environmental concentration imidacloprid removal[END_REF]; iii) Cr 3+ at 584.20 and 575.04 eV [START_REF] Cheng | Adsorption performance and mechanism of iron-loaded biochar to methyl orange in the presence of Cr6+ from dye wastewater[END_REF]. XPS can also be used to evaluate the elemental O/C molar ratio that may be considered as an indication for biochar stability [START_REF] Spokas | Review of the stability of biochar in soils: Predictability of O:C molar ratios[END_REF].

The use of Fourier Transform Infrared spectroscopy (FTIR) in biochar surface characterization is also very common. This technique gives detailed information on the oxygen-containing functional groups present on the surface of biochar (Table 4), evaluates the success of metal binding during surface treatment and/or the surface functional groups transformation after pollutants adsorption [START_REF] Liu | Removal of 17βestradiol from aqueous solution by graphene oxide supported activated magnetic biochar: Adsorption behavior and mechanism[END_REF]. Al-Wabel et al reported the increase of the broad absorption band attributed to O-H stretching vibrations (at around 3320 cm -1 ) in biochar with the moisture increase, however the presence of C-O-C stretching vibrations band indicated the occurrence of non-decomposed oxygenated groups [START_REF] Al-Wabel | Designing chitosan based magnetic beads with conocarpus waste-derived biochar for efficient sulfathiazole removal from contaminated water[END_REF]. FTIR can analyze organic functional groups commonly observed in biochars, as carbon-hydrogen (C-H), C = C, C=O, C-O, and silicon-oxygen (Si-O) [80][81]. Table 4 shows the functional groups and their wavenumber in published literature on biochar and biochar composites. Nano-zerovalent manganese/biochar 3320 1580 [START_REF] Iqbal | Nano-zerovalent manganese/biochar composite for the adsorptive and oxidative removal of Congo-red dye from aqueous solutions[END_REF] Graphene oxide /CoFe2O

Surface structure and morphology

The surface morphology and chemical composition are important attributes of biochar.

Scanning electron microscope (SEM) and transmission electron microscope (TEM) are valuable tools to evaluate the development of surface structure, porosity and pores distribution of biochar before and after modification [START_REF] Hu | Cobalt-gadolinium modified biochar as an adsorbent for antibiotics in single and binary systems[END_REF].

Raman spectroscopy is considered a powerful technique for exploring the graphitization or defects on carbonaceous materials. In biochar engineering and development, the interest focuses on two bands; i) the D band which represents the disorder and defect degree caused by vacancies, edges and functional groups, ii) the G band which is related to the crystalline and graphitic structures [START_REF] Yun | Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation Process: Singlet Oxygenation Versus Mediated Electron Transfer[END_REF]. Meanwhile, the estimated intensity ratio of the D band relative to the G band (ID/IG) reflects the disorder and level of defect of the analyzed biochar (Table 5). It is suggested that an increase in ID/IG ratio due to the addition of iron suggests an increase of defective sites [START_REF] Rong | The magnetic biochar derived from banana peels as a persulfate activator for organic contaminants degradation[END_REF]. It is also reported that the increase in pyrolysis temperature until a certain value increases the defected sites formed on biochar [START_REF] Bai | A new application pattern for sludgederived biochar adsorbent: Ideal persulfate activator for the high-efficiency mineralization of pollutants[END_REF]. Also a low ID/IG ratio meant a relatively high degree of graphitization [START_REF] Zhang | Novel sodium bicarbonate activation of cassava ethanol sludge derived biochar for removing tetracycline from aqueous solution: Performance assessment and mechanism insight[END_REF].

Generally, the ID/IG value increases with increasing pyrolysis temperatures, indicating that high pyrolysis temperatures are beneficial for the growth of defect structures in produced biochar [96][97]. The X-Ray Diffraction (XRD) technique is applied to determine the crystal structures and phase composition in a solid material. It can also evaluate the ratio of crystalline and amorphous phase and define the material modification such as metal oxide loading [START_REF] Shen | Removal of tetracycline from an aqueous solution using manganese dioxide modified biochar derived from Chinese herbal medicine residues[END_REF], magnetic biochar composite synthesis [START_REF] Ma | A novel, efficient and sustainable magnetic sludge biochar modified by graphene oxide for environmental concentration imidacloprid removal[END_REF]. The technique can evaluate the stability of biochar components after modifications, for example SiO2 modification [START_REF] Zhang | A green biochar/iron oxide composite for methylene blue removal[END_REF]. The use of XRD spectra is reported to evaluate the interlayer spacing of the activated biochar according to Bragg's formula and/or the average crystalline size with the help of the Scherrer equation [START_REF] Prasannamedha | Enhanced adsorptive removal of sulfamethoxazole from water using biochar derived from hydrothermal carbonization of sugarcane bagasse[END_REF].

Bragg's equation:

𝑑 = 𝜆 2𝑠𝑖𝑛𝛳

In which λ is the X-ray wavelength whose value is 0.154 nm generally, and θ is the angle of the peak position.

Scherrer equation

This equation is used to evaluate the average crystalline size along c-axis (Lc) and the size of the layer planes (La).

𝐿 = 𝑘𝜆 𝐵𝑐𝑜𝑠𝛳

In which k is the value of the shape factor (k = 0.9 and 1.84) which are used frequently for the purpose of calculation of Lc and La respectively. B is the value of line broadening at half width of the peak known as FWHM (Full width half maximum) in radians.

Biochar stability

Thermogravimetry analysis (TGA) is applied to evaluate the thermal stability of biochar [START_REF] Chen | Low-temperature constructing N-doped graphite-like mesoporous structure biochar from furfural residue with urea for removal of chlortetracycline from wastewater and hydrothermal catalytic degradation mechanism[END_REF] and characterize its structure, as it gives information about the pyrolysis yield, the content of moisture and the different structural components such as hemicellulose, cellulose, and lignin [START_REF] Lee | Iron-impregnated spent coffee ground biochar for enhanced degradation of methylene blue during cold plasma application[END_REF]. Generally, the weight loss in TG analysis can be divided into three stages shown in Table 6 below; the first stage represents the loss due the water evaporation, and decomposition of bonded hydrated compounds, during the second stage the major loss observed is explained by the thermal decomposition of polar organic compounds derived from most cellulose, hemicellulose and holocellulosic precursors [START_REF] Kim | Adsorptive removal of tetracycline from aqueous solution by maple leaf-derived biochar[END_REF]. Finally the last weight loss stage was attributed to the slow and gradual thermal decomposition of high molecular weight components present in the biomass [START_REF] Singh | Biochars for the removal of naphthenic acids from water: A prospective approach towards remediation of petroleum refinery wastewater[END_REF].

Toxicity of biochar

Biochar has an active role in environmental remediation as an adsorbent, filter medium or active catalytic composite [START_REF] Deng | Application of biochar as an innovative substrate in constructed wetlands/biofilters for wastewater treatment: Performance and ecological benefits[END_REF], nonetheless it is imperative to systematically discuss its potentially negative environmental effects and prevent risks during application. Some studies focus on the evaluation of harmful components of biochar such as dioxins [START_REF] Lyu | Effect of pyrolysis temperature on potential toxicity of biochar if applied to the environment[END_REF],

heavy metals [START_REF] Oleszczuk | Biochar properties regarding to contaminants content and ecotoxicological assessment[END_REF], polycyclic aromatic hydrocarbons (PAHs) [START_REF] Visioli | Assessing biochar ecotoxicology for soil amendment by root phytotoxicity bioassays[END_REF][107], environmentally persistent free radicals (EPFRs) [START_REF] Pan | Environmentally persistent free radicals: Occurrence, formation mechanisms and implications[END_REF], volatile organic compounds (VOCs) and perfluorochemicals (PFCs) [START_REF] Zhang | Biochar for environmental management: Mitigating greenhouse gas emissions, contaminant treatment, and potential negative impacts[END_REF].

The content of heavy metals in biochar differs with biomass type and source [START_REF] Oleszczuk | Biochar properties regarding to contaminants content and ecotoxicological assessment[END_REF][110],

for example sludge biochar, which has a high heavy metal content [START_REF] Chagas | The residual effect of sewage sludge biochar on soil availability and bioaccumulation of heavy metals: Evidence from a three-year field experiment[END_REF]. The concentration of heavy metals in biochar is therefore linked to the origin biomass and is retained during pyrolysis [START_REF] Devi | Risk analysis of pyrolyzed biochar made from paper mill effluent treatment plant sludge for bioavailability and eco-toxicity of heavy metals[END_REF]. In addition, PAHs in biochar are reported to have high biotoxicity effect on plants and microorganisms in different environmental media. The content of PAHs in biochar depends on the process itself, namely residence time, temperature and special attention should be given to avoiding the condensation of tar loaded gases on the biochar. In specific, PAH formation i) varies with the feedstock type or nature [START_REF] Xiang | Potential hazards of biochar: The negative environmental impacts of biochar applications[END_REF], ii) is reduced at sufficient heating rate during pyrolysis [START_REF] Hale | Quantifying the total and bioavailable polycyclic aromatic hydrocarbons and dioxins in biochars[END_REF], iii) decreases while pyrolysis temperature and/or residence time in reactor increase [START_REF] Chen | Leaching of polycyclic aromatic hydrocarbons (PAHs) from sewage sludge-derived biochar[END_REF].

Dioxins also form during the biochar production process, the factors that influence are; i) Chloride (Cl) content in feedstock, as reported by Wiedner et al. [START_REF] Wiedner | Chemical evaluation of chars produced by thermochemical conversion (gasification, pyrolysis and hydrothermal carbonization) of agro-industrial biomass on a commercial scale[END_REF], ii) reactor temperature, with dioxins forming as temperature reduces, usually when the flue gas cools down [START_REF] Hale | Quantifying the total and bioavailable polycyclic aromatic hydrocarbons and dioxins in biochars[END_REF]. EPFRs can be produced during pyrolysis process with lignin, cellulose and hemicellulose in biomass fibers acting as a precursor [START_REF] Odinga | Occurrence, formation, environmental fate and risks of environmentally persistent free radicals in biochars[END_REF], [START_REF] Lei | Comprehensive understanding the chemical structure evolution and crucial intermediate radical in situ observation in enzymatic hydrolysis/mild acidolysis lignin pyrolysis[END_REF]. In addition, the content of these free radicals increases with the pyrolysis temperature [START_REF] Liao | Detecting free radicals in biochars and determining their ability to inhibit the germination and growth of corn, wheat and rice seedlings[END_REF]. It may pose a potential environmental risk since they can induce the formation of reactive oxygen species with high cytotoxicity and phytotoxicity within environmental media [START_REF] Dellinger | The role of combustion-generated radicals in the toxicity of PM2.5[END_REF].

Other potential risks of biochar are due to adsorbed pollutants on the biochar surface, the presence of residual chemicals in biochar originating from the plant residues persisting the pyrolysis process, and finally the leaching out of various components from the biodegradation under natural conditions of aged biochar [START_REF] Cui | Drywet and freeze-thaw aging activate endogenous copper and cadmium in biochar[END_REF].

The voluntary biochar quality standards that apply in Europe with the European Biochar Certificate (EBC) [START_REF] Ebc | European Biochar Certificate -Guidelines for a Sustainable Production of Biochar[END_REF], in the UK with the biochar quality mandate (BQM) and in the USA with the International Biochar Initiative-Biochar Standard (IBI-BS) propose limits for certain harmful components in biochar such as; PAH, benzo (a) pyrene, polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans and heavy metals [120][121]. However, these standards are safeguarding biochar production rather than final applications, therefore prior to applying novel adsorbents with potential ecotoxic effects the national safety regulations for air, soil and water need to be secured at all times.

Factors influencing pollutants removal

The elimination of pollutants and the adsorption capacity of biochar are greatly affected by its physicochemical properties as well as of the target pollutants nature, and the operating conditions of the treatment, as reported in Tables 1 and2

Effect of biochar dose

The impact of biochar dose on pollutant removal efficiency is reported in detail in most of the adsorption studies, because of the high correlation observed between adsorbent dosage and adsorption capacity. However there is an optimum value of biochar dose, beyond this threshold there is no enhancement in the pollutant elimination anymore [START_REF] Singh | Self-engineered iron oxide nanoparticle incorporated on mesoporous biochar derived from textile mill sludge for the removal of an emerging pharmaceutical pollutant[END_REF].

This tendency is explained by the optimum use of the active adsorption sites for pollutant adsorption [START_REF] Du | Activation of persulfate by graphitized biochar for sulfamethoxazole removal: The roles of graphitic carbon structure and carbonyl group[END_REF]. Beyond the optimum point, the decrease in the adsorption capacity may be due to the inadequate availability of pollutant molecules for spare active sites [START_REF] Wang | Waste printed circuit boards as novel potential engineered catalyst for catalytic degradation of orange II[END_REF]. In another study the increased biochar quantity results to a faster achievement of the reaction equilibrium, indicating the increased availability of vacant reactive sites on the adsorbent surface, which leads to a high adsorption rate at the beginning of the process [START_REF] Vigneshwaran | Facile synthesis of sulfur-doped chitosan/biochar derived from tapioca peel for the removal of organic dyes: Isotherm, kinetics and mechanisms[END_REF]. However the increase in biochar dosage decreases the adsorption capacity exponentially but moderately at lower dosages and slightly at higher dosages [START_REF] Lawal | Adsorption mechanism and effectiveness of phenol and tannic acid removal by biochar produced from oil palm frond using steam pyrolysis[END_REF].

Effect of initial solution pH

The pH of pollutant solution is a factor which influences the removal efficiency because of the predominant role on the charge characteristics through the protonation and deprotonation of oxygen-containing groups on the biochar or biochar composite surface [START_REF] Tan | Removal of Pb(II) ions from aqueous solution by manganese oxide coated rice straw biochar -A low-cost and highly effective sorbent[END_REF]. The pH value also influences the degree of ionization of the adsorbate [START_REF] Kassimi | Selective and competitive removal of three basic dyes from single, binary and ternary systems in aqueous solutions: A combined experimental and theoretical study[END_REF]. Wang et al reported that anion adsorption is favored at pH less than the point of zero charge (PZC) because the adsorbent surface is positively charged. However, when the surface is negatively charged at pH values above the PZC the cation adsorption is favored [START_REF] Wang | Phosphogypsum as a novel modifier for distillers grains biochar removal of phosphate from water[END_REF].

In the work of Chaukura et al, a maximum adsorption of methyl orange was recorded at pH value lower than the PZC of biochar, this was explained by possible electrostatic attraction of the dye molecules to biochar surface since the dye molecules existed as anions at the reported conditions [START_REF] Chaukura | Synthesis, characterisation and methyl orange adsorption capacity of ferric oxide-biochar nano-composites derived from pulp and paper sludge[END_REF]. Generally, the organic dyes adsorption capacity by biochar or nano-metal biochar composites decrease with increasing solution pH values [START_REF] Zhao | Formation and mechanisms of nano-metal oxide-biochar composites for pollutants removal: A review[END_REF].

Effect of pollutant concentration

The initial concentration of the adsorbate plays a crucial role in the efficiency of the adsorption process. It is reported that the adsorption capacity decreases with the increase in the initial concentration of pollutants [START_REF] Zhou | Recent advances for dyes removal using novel adsorbents: A review[END_REF]. This is attributed to the occupation of active sites on the adsorbent material surface by the targeted pollutants [START_REF] Vigneshwaran | Facile synthesis of sulfur-doped chitosan/biochar derived from tapioca peel for the removal of organic dyes: Isotherm, kinetics and mechanisms[END_REF]. Some works reported a proportional relationship between pollutant initial concentration and adsorption capacity of biochar; however, the removal yield of pollutant decreases [135][136]. An explanation of this can be that at higher concentration, the mass transfer driving force increased [START_REF] Chakraborty | Mechanistic insight into sorptive elimination of ibuprofen employing bi-directional activated biochar from sugarcane bagasse: Performance evaluation and cost estimation[END_REF], which enhanced the diffusion rate and the probability that molecules reached the material surface increases [START_REF] Iqbal | Kinetic study of Cr(III) and Cr(VI) biosorption using rosa damascena phytomass: A rose waste biomass[END_REF]. We would like to comment that if the diffusion rate is enhanced, one may observe positive correlation between concentration and rate of adsorption, instead of adsorption capacity [START_REF] Mu | NaOH-modified mesoporous biochar derived from tea residue for methylene Blue and Orange II removal[END_REF][136], as the latter should be dependent of the total available sorption site amount on biochar.

Sayin et al. reported a decrease of adsorption efficiency with increasing the ciprofloxacin

initial concentration from 50 to 150 mg/L, and this was explained by the gradual saturation of specific active site for antibiotic molecules adsorption [START_REF] Sayin | From green biowaste to water treatment applications: Utilization of modified new biochar for the efficient removal of ciprofloxacin[END_REF]. Nnadozie and P.A. Ajibade studied the adsorption of Indigo Carmine (IC) dye using C. odorata biochar, the result showed a decrease in the removal efficiency with increasing dye concentration from 10 to 100 mg/L with the same biochar dose [START_REF] Nnadozie | Isotherm, kinetics, thermodynamics studies and effects of carbonization temperature on adsorption of Indigo Carmine (IC) dye using C. odorata biochar[END_REF].

Effect of anions presence

The pollutant molecules may compete with various coexisting anions such as NO3 -, Cl -, SO4 2-and PO4 3-during adsorption in aqueous environment. Wang et al reported a decrease in the removal efficiency of p-arsanilic acid in the presence of PO4 3-because of the stronger competitiveness for the adsorptive sites on the surface of biochar [START_REF] Wang | Hierarchically structured twodimensional magnetic microporous biochar derived from hazelnut shell toward effective removal of p-arsanilic acid[END_REF]. The increase in NaCl content decrease the adsorption capacity of biochar [START_REF] Liu | Removal of 17βestradiol from aqueous solution by graphene oxide supported activated magnetic biochar: Adsorption behavior and mechanism[END_REF], which was explained by the reduction of repulsion force between adsorbent particles and hence the increase of aggregation of nanomaterials [START_REF] Zhou | Sorption performance and mechanisms of arsenic(V) removal by magnetic gelatin-modified biochar[END_REF]. However, beyond a certain dose of NaCl increase, the adsorption capacity increases, which was attributed to the increase of ionic strength which improved the activity coefficient of hydrophobic organic compounds, resulting in a decrease in solubility (i.e. salting-out effect), thus was conducive to pollutant adsorption [START_REF] Li | Tetracycline absorbed onto nitrilotriacetic acidfunctionalized magnetic graphene oxide: Influencing factors and uptake mechanism[END_REF]. It can also be explained by surface charge neutralization of the carbonaceous adsorbents by the double layer compression and the reduced solubility of the micropollutants in the presence of high Na + concentration [START_REF] Shin | Single and competitive adsorptions of micropollutants using pristine and alkali-modified biochars from spent coffee grounds[END_REF].

Effect of contact time

The effect of contact time needs to be considered in the evaluation of the biochar adsorption system. Published research reports a fast increase in the adsorption capacity at the beginning of the treatment and a gradual slow-down of the adsorption capacity until adsorption equilibrium is reached, when the sorption sites were fully occupied [135][143].

This can be explained by the gradual adsorption saturation of active sites on biochar and the gradual decrease in the differential concentration between solution bulk phase and the surface of the adsorbent [START_REF] Trinh | Phosphate Adsorption by Silver Nanoparticles-Loaded Activated Carbon derived from Tea Residue[END_REF]. Similar results were observed by Velusamy et al. [START_REF] Velusamy | Analysis on the removal of emerging contaminant from aqueous solution using biochar derived from soap nut seeds[END_REF] and

Effect of temperature

The adsorption temperature can also affect the pollutants removal capacity by biochar and its composites. However, the majority of water treatment experiments are conducted at ambient temperature (25 °C) to simulate the temperature predominant in the environments [START_REF] Xu | Optimizing the modification of wood waste biochar via metal oxides to remove and recover phosphate from human urine[END_REF]. It is reported that the adsorption increases with increasing temperature in the range of 15-35°C [START_REF] Cheng | Adsorption performance and mechanism of iron-loaded biochar to methyl orange in the presence of Cr6+ from dye wastewater[END_REF], which reflects an endothermic process [START_REF] Jung | Hydrothermal synthesis of hierarchically structured birnessite-type MnO2/biochar composites for the adsorptive removal of Cu(II) from aqueous media[END_REF].

Similar results on the endothermicity of : i) p-nitrophenol adsorption by pine sawdust biochar was reported in the work of Liu et al [START_REF] Liu | Adsorption characteristics and mechanism of p-nitrophenol by pine sawdust biochar samples produced at different pyrolysis temperatures[END_REF] and ii) diclofenac adsorption by pine wood biochar [START_REF] Lonappan | An insight into the adsorption of diclofenac on different biochars: Mechanisms, surface chemistry, and thermodynamics[END_REF]. The increase in temperature increased the probability of pollutant molecules contacting the active sites [START_REF] Jung | Hydrothermal synthesis of hierarchically structured birnessite-type MnO2/biochar composites for the adsorptive removal of Cu(II) from aqueous media[END_REF]. However, the increase of temperature in the same range can reduce the adsorption capacity in an exothermic process [136][145]. On the other hand there is a proportional relationship between temperature and the molecular movement acceleration, which promotes adsorbent-ion interaction and reduction of the Gibbs free energy [151][152]. The increase of temperature can enhance the formation of aromatic carbon present on the surface of biochar resulting to improved surface characteristics for adsorbing an extensive range of pollutants [START_REF] Wang | Sorption of four hydrophobic organic contaminants by biochars derived from maize straw, wood dust and swine manure at different pyrolytic temperatures[END_REF].

Effect of chemical impregnation ratio on the removal of pollutants

The ratio of the non-processed to magnetic material was studied in several works to evaluate its effect on pollutant removal efficiency. It is found that the increase of this ratio increases the adsorption capacity of magnetic biochar until an optimum value but further increase leads to a decrease of the adsorption capacity [START_REF] Yap | Microwave induced synthesis of magnetic biochar from agricultural biomass for removal of lead and cadmium from wastewater[END_REF]. However, Son et al report that copper removal capacities decrease with decreasing biochar/Iron ratio and Fe recovery efficiency, explained by the plugging of biochar's surface pores with iron oxide particles [START_REF] Son | Heavy metal removal from aqueous solutions using engineered magnetic biochars derived from waste marine macroalgal biomass[END_REF]. In addition, Yang et al. found that the increase in FeCl3/sawdust impregnation mass ratio increases the removal efficiency of mercury which was attributed to oxygen-rich functional groups formation at biochar surface and especially C=O group.

However, an excess of FeCl3 can lead to; i) the aggregation of Fe3O4 particles on the magnetic biochar surface, and/or ii) the variation of textural properties and therefore a decrease in mercury removal yield [START_REF] Yang | Mercury Removal by Magnetic Biochar Derived from Simultaneous Activation and Magnetization of Sawdust[END_REF].

Post-processing modification of biochar: Activation and functionalization

The thermal decomposition of biomass in the absence of oxygen, or at very limited oxygen environment, produces biochars that still contain various refractory oxides, depending on the biomass feedstock, such as Fe2O3, SiO2, KCl, Al2O3, CaSO4, CaCO3 [START_REF] Vijayaraghavan | The importance of mineral ingredients in biochar production, properties and applications[END_REF].

Furthermore, the surface of most pristine biochars are often negatively charged in association with its abundant oxygen-containing function groups, thus exhibiting specific sorption to cations (e.g., heavy metals ions) [START_REF] Yang | Surface functional groups of carbon-based adsorbents and their roles in the removal of heavy metals from aqueous solutions: A critical review[END_REF]. However, sorption toward anionic species (i.e., oxyanion, anionic dyes and organics) is limited [START_REF] Tan | Biochar-based nano-composites for the decontamination of wastewater: A review[END_REF]. This limits the applications of biochar which drives researchers to introduce new and additional metal oxides with different methods to modify biochar for targeted applications [START_REF] Dai | Tuning oxygenated functional groups on biochar for water pollution control: A critical review[END_REF].

In addition, biochar prepared by conventional pyrolysis of biomass may have poor physicochemical properties such as surface oxygenated groups, surface area, pore volume, and pore width. These properties fluctuate considerably and depend on feedstock types and production conditions [START_REF] Wang | An overview of carbothermal synthesis of metalbiochar composites for the removal of oxyanion contaminants from aqueous solution An overview of carbothermal synthesis of metal e biochar composites for the removal of oxyanion contaminants from aqueous so[END_REF]. Nevertheless, the available functional groups (carboxyl (-COOH), hydroxyl (-OH), and carbonyl (C=O)), are quite important in water treatment [START_REF] Liu | Modification of bio-char derived from fast pyrolysis of biomass and its application in removal of tetracycline from aqueous solution[END_REF]. The hydrophilic (polarity index) or hydrophobic property (aromaticity) of biochar highly depends upon the type and nature of existing functional groups on the surface. Different techniques available have been reported to boost the abundance of this last, such as steam activation, impregnation method, chemical and heat treatments [START_REF] Shen | Surface Chemical Functional Groups Modification of Porous Carbon[END_REF].

Biochar can also act as an excellent photocatalyst when combined with other catalysts, because of the resulting favorable physicochemical properties such as; high surface area with high active site, enhanced charge separation, higher availability of functional groups, high porous volume, better catalyzing ability, stability and recoverability [START_REF] Ahmaruzzaman | Biochar based nanocomposites for photocatalytic degradation of emerging organic pollutants from water and wastewater[END_REF]. However, the synthesis of photocatalyst-based biochar must be optimized [START_REF] Sun | Facile synthesis of novel ternary g-C3N4/ferrite/biochar hybrid photocatalyst for efficient degradation of methylene blue under visible-light irradiation[END_REF]. As reported by Yu et al., the addition of an appropriate amount of ZnO into biochar enhanced both the adsorption capacity and the photocatalytic ability of the nanocomposites [START_REF] Yu | ZnO/biochar nanocomposites via solvent free ball milling for enhanced adsorption and photocatalytic degradation of methylene blue[END_REF]. Similar results were reported in the application of Mn-loaded and Fe-loaded biochar for the atrazine elimination in aqueous solution with heterogeneous catalytic ozonation [START_REF] Tian | Heterogeneous catalytic ozonation of atrazine with Mn-loaded and Fe-loaded biochar[END_REF].

Ahmaruzzaman reported a different synthesis technique of biochar supported photocatalyst like Sol-gel method, Ultrasound assisted synthesis, Thermal polycondensation technique, Solvothermal process, Hydrothermal process [START_REF] Ahmaruzzaman | Biochar based nanocomposites for photocatalytic degradation of emerging organic pollutants from water and wastewater[END_REF].

As seen in the Figure 4, the different post-modification methods of biochar are classified into four main categories; 1) modifications to increase the surface area and porosity; 2) modifications to render the biochar surface positively charged; 3) Increasing surface oxygen-containing functional groups and 4) magnetization of biochar in order to facilitate/enhance the biochar particles recovery. It may be that one treatment can enhance two or more proprieties at the same time, for example the case of H2SO4 treatment which enhances the specific surface area and increases the oxygen-containing functional groups. There are different classifications for these modifications reported in literature [START_REF] Cheng | Adsorption of emerging contaminants from water and wastewater by modified biochar: A review[END_REF][18] such as the classification according to physicochemical characteristics of biochar targeted by the modification [START_REF] Wang | An overview of carbothermal synthesis of metalbiochar composites for the removal of oxyanion contaminants from aqueous solution An overview of carbothermal synthesis of metal e biochar composites for the removal of oxyanion contaminants from aqueous so[END_REF], or according to the nature of process (chemical, physical or composite) [START_REF] Benis | Treatment of aqueous arsenic -A review of biochar modification methods[END_REF]. Figure 5 shows the effect of different modification treatments on the biochars' specific surface area (SSA) as reported in literature. We observe that: i) the result for a treatment technique varies from one biochar to another (case of Mn-BC modification and H3PO4 treatment for example); ii) some treatments lead to a decrease in SSA (case of FeCl3-BC and AlCl3-BC); iii) some techniques presented a considerable increase in the SSA such as CO2 activation and K2CO3 treatment. The Increase of biochar SSA resulting from different posttreatment techniques does not always mean the improvement of its performance towards the removal of organic pollutants.

Figure 5:

The evolution of specific surface area (SSA) of biochar according to different modification techniques in some reported works ( [START_REF] Nguyen | Metal salt-modified biochars derived from agro-waste for effective congo red dye removal[END_REF], [START_REF] Cao | Investigation into adsorption characteristics and mechanism of atrazine on nano-MgO modified fallen leaf biochar[END_REF], [START_REF] Wang | Synergistic removal of copper and tetracycline from aqueous solution by steam-activated bamboo-derived biochar[END_REF], [START_REF] Sewu | Decolorization of cationic and anionic dye-laden wastewater by steam-activated biochar produced at an industrialscale from spent mushroom substrate[END_REF], [START_REF] You | Magnetic cobalt ferrite biochar composite as peroxymonosulfate activator for removal of lomefloxacin hydrochloride[END_REF], [START_REF] Chen | A novel porous biocharsupported Fe-Mn composite as a persulfate activator for the removal of acid red 88[END_REF],

[77], [START_REF] Azalok | UV-light-induced photocatalytic performance of reusable MnFe-LDO-biochar for tetracycline removal in water[END_REF], [START_REF] Xiao | Graphitic carbon nitride/biochar composite synthesized by a facile ball-milling method for the adsorption and photocatalytic degradation of enrofloxacin[END_REF], [START_REF] Zheng | Efficient adsorption of ciprofloxacin using Ga2S3/S-modified biochar via the high-temperature sulfurization[END_REF], [START_REF] Ma | Hydrothermal synthesis of magnetic sludge biochar for tetracycline and ciprofloxacin adsorptive removal[END_REF], [START_REF] Wang | Influence of feedstocks and modification methods on biochar's capacity to activate hydrogen peroxide for tetracycline removal[END_REF], [START_REF] Wang | Influence of feedstocks and modification methods on biochar's capacity to activate hydrogen peroxide for tetracycline removal[END_REF], [START_REF] Cai | Pyrrolic N-rich biochar without exogenous nitrogen doping as a functional material for bisphenol A removal: Performance and mechanism[END_REF], [START_REF] Yang | Mechanistic insights into removal of norfloxacin from water using different natural iron orebiochar composites: more rich free radicals derived from natural pyrite-biochar composites than hematite-biochar composites[END_REF], [START_REF] Zhao | Chlortetracycline hydrochloride removal by different biochar/Fe composites: A comparative study[END_REF], [START_REF] Wang | Sludge-derived biochar as efficient persulfate activators: Sulfurization-induced electronic structure modulation and disparate nonradical mechanisms[END_REF], [START_REF] Borthakur | Computational and experimental assessment of pH and specific ions on the solute solvent interactions of clay-biochar composites towards tetracycline adsorption: Implications on wastewater treatment[END_REF], [START_REF] Liu | Mechanism of sulfamic acid modified biochar for highly efficient removal of tetracycline[END_REF], [START_REF] Ma | Iron/zinc and phosphoric acid modified sludge biochar as an efficient adsorbent for fluoroquinolones antibiotics removal[END_REF], [START_REF] Zheng | MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants[END_REF], [START_REF] Zhang | Ball-milled biochar for galaxolide removal: Sorption performance and governing mechanisms[END_REF], [START_REF] Sun | Activation of persulfate by CO2activated biochar for improved phenolic pollutant degradation: Performance and mechanism[END_REF], [START_REF] Shen | Removal of tetracycline from an aqueous solution using manganese dioxide modified biochar derived from Chinese herbal medicine residues[END_REF]).

Moreover, as reported in detail by Barquilha and Braga, organic and inorganic pollutants adsorption on biochars can involve different mechanisms, including physical and chemical interactions. The interaction between biochars and adsorbate species is not simple and may involve different adsorption mechanisms, such as van der Waals forces, electrostatic interactions, surface complexation, ion-exchange, hydrophobic interactions, πinteractions, co-precipitation, partition, and pore-filling [START_REF] Barquilha | Adsorption of organic and inorganic pollutants onto biochars: Challenges, operating conditions, and mechanisms[END_REF]. Other studies report on the mechanisms of dye adsorption by sewage sludge-biochar [START_REF] Gopinath | Conversion of sewage sludge into biochar: A potential resource in water and wastewater treatment[END_REF] and adsorption mechanisms of modified biochar on typical emerging contaminants [START_REF] Cheng | Adsorption of emerging contaminants from water and wastewater by modified biochar: A review[END_REF].

Acid-base activation/decoration

Generally, biochar modification by acidic treatment occurs in two main ways: activation and decoration. Activation is to develop porous structure and increase specific surface area by using chemical activating agents with highly etching ability. Whereas, decoration emphases on enhancing surface activity, by using zero-valent iron nanoparticles [START_REF] Dong | Factors influencing degradation of trichloroethylene by sulfide-modified nanoscale zero-valent iron in aqueous solution[END_REF],

Fe3O4 and FeOOH as additives [START_REF] Yang | Preparation of ferric-activated sludge-based adsorbent from biological sludge for tetracycline removal[END_REF]. Another work reported the enhancement of sorbent hydrophobicity by decoration with different acid and fatty acids such as; pure lauric acid, acetic anhydride, phosphoric acid and citric acid [START_REF] Navarathna | Biochar Adsorbents with Enhanced Hydrophobicity for Oil Spill Removal[END_REF][186] [START_REF] Osemeahon | Removal of Crude Oil from Aqueous Medium by Sorption on Sterculis setigera[END_REF].

Gurav et al., reported the application of pinewood biochar decorated with coconut oilderived fatty acids for adsorptive removal of crude petroleum oil from water. Fatty acids composition attached to the resulted biochar (mg/g of biochar) was reported as: lauric acid (9.024), myristic acid (5.065), palmitic acid (2.769), capric acid (1.639), oleic acid (1.362), stearic acid (1.114), and linoleic acid (0.130). Decorated biochar presented a higher hydrophobicity and a good reusability with stable adsorption efficiency at 51.40% after fifth adsorption-desorption cycles [START_REF] Gurav | Adsorptive removal of crude petroleum oil from water using floating pinewood biochar decorated with coconut oil-derived fatty acids[END_REF]. The sulfamic acid modified biochar reported in the work of Liu et al., presented a specific surface area with 2.6 times larger than that of the original biochar introducing more functional groups on the biochar. Furthermore, the resulted biochar shows an improved performance for tetracycline adsorption [START_REF] Liu | Mechanism of sulfamic acid modified biochar for highly efficient removal of tetracycline[END_REF].

Many studies have proved that biochar treated with a strong oxidizing agent (e.g., HNO3/H2SO4, HNO3, HF/HNO3, KMnO4, H2O2 and NaClO) enhance its sorption capacity, creates charged and hydrophilic surface functional groups, and increases their colloidal stability and mobility [START_REF] Cho | Sorption of aqueous Zn[II] and Cd[II] by multiwall carbon nanotubes: The relative roles of oxygen-containing functional groups and graphenic carbon[END_REF]. The activation of biochar in one step and two steps by KOH and KMnO4 as a green activator was found to give a large pore volume, pore channel and proportion of aromatized structure, it revealed to have a high adsorption capacity for methylene blue removal [START_REF] Zheng | Insight into the KOH/KMnO4 activation mechanism of oxygen-enriched hierarchical porous biochar derived from biomass waste by in-situ pyrolysis for methylene blue enhanced adsorption[END_REF]. The oxidation of corncob biochar by of HNO3-H2SO4 mixture was found to boost the adsorption capacity of biochar, decrease hydrophobicity and increase the amounts of oxygen-containing groups [START_REF] Abdoul Magid | Enhanced adsorption of polystyrene nanoplastics (PSNPs) onto oxidized corncob biochar with high pyrolysis temperature[END_REF]. The treatment of sludge biochar modified by Fe/Zn + H3PO4 presented an adsorption capacity for ciprofloxacin (CIP), norfloxacin (NOR) and ofloxacin (OFL) from water up to 20 times of those of sludge biochar only or H3PO4 modified biochar [START_REF] Ma | Iron/zinc and phosphoric acid modified sludge biochar as an efficient adsorbent for fluoroquinolones antibiotics removal[END_REF].

Some works reported the NaOH activation of biochar or biomass before pyrolysis in order to well-developed mesoporous structure [START_REF] Córdova | Simultaneous adsorption of a ternary mixture of brilliant green, rhodamine B and methyl orange as artificial wastewater onto biochar from cocoa pod husk waste. Quantification of dyes using the derivative spectrophotometry method[END_REF], enhance the higher specific surface area, total pore volume, aromaticity [START_REF] Shin | Competitive adsorption of pharmaceuticals in lake water and wastewater effluent by pristine and NaOH-activated biochars from spent coffee wastes: Contribution of hydrophobic and π-π interactions[END_REF], and graphitized orderly carbon layer in biochar [START_REF] Mu | NaOH-modified mesoporous biochar derived from tea residue for methylene Blue and Orange II removal[END_REF].

The KOH treatment was also reported to increase the surface area and pore volume [START_REF] Piloni | Biochars from Spirulina as an alternative material in the purification of lactic acid from a fermentation broth[END_REF].

Apparently, biochar modification with a strong inorganic acid is one of a simple and effective processes for biochar activation. However, the scaling-up into an industrial scale will produce a large amount of acidic or basic wastewater, with negative impact to the aquatic environment.

Persulfate activation

Persulfate (PS) activated biochar was reported in many studies in the last years [173][195], due to the increased pollutant removal efficiency from water by combining the advantage of PS radicals effect and the high specific surface area of biochar [196][197].

It is noticed that, compared with the traditional hydroxyl radical-based AOP, Sulfate radicals-AOP has a higher redox potential (2.5-3.1 V (SO4 •-) vs. 2.7 V ( • OH)), but longer half-life time (30-40 μs (SO4 •-) vs. <1 μs ( • OH)) and wide pH adaptability [START_REF] Wang | Activation of persulfate (PS) and peroxymonosulfate (PMS) and application for the degradation of emerging contaminants[END_REF]. Fan et al.

reported the slight adsorption of bisphenol A (8.1%) by sludge-derived biochar (SBC) alone and limited direct oxidation (2.4%) by sole peroxymonosulfate (PMS), nevertheless, the removal efficiency of bisphenol A was boosted to 94.5% within 60 min in the presence of both SBC and PMS. that the explanation is that PMS is bound to SBC and forming a surface reactive complex (SBC-PMS*), which would abstract the electrons from the adsorbed pollutant through the conductive carbon tunnel [START_REF] Fan | Activation of peroxymonosulfate by sewage sludge biochar-based catalyst for efficient removal of bisphenol A: Performance and mechanism[END_REF].

Furthermore, different biochar modification options can be combined to promote the elimination of aqueous pollutants, such as magnetization and peroxymonosulfate (PMS) activation, reported by You et al. [START_REF] You | Magnetic cobalt ferrite biochar composite as peroxymonosulfate activator for removal of lomefloxacin hydrochloride[END_REF].

Physical activation

Physical activation of biochar is the partial oxidation of the material to increase its porosity, under low to medium heat treatment (<250 o C) under air conditions or high temperature (>750 • C) treatment with CO2, steam, blends thereof. At high temperatures H2O and CO2 become reactive and therefore allow for chemical reactions with the biochar matrix [START_REF] Hagemann | Activated carbon, biochar and charcoal: Linkages and synergies across pyrogenic carbon's ABCs[END_REF].

Jellali et al. reported the increase of the surface area and total pore volume after CO2 activation by about 5.9 and 1.5 times, respectively [START_REF] Jellali | Sludge-derived biochars: A review on the influence of synthesis conditions on pollutants removal efficiency from wastewaters[END_REF]. Meanwhile the activation process with air at a low temperature (≈275 °C) for 4 h significantly enhanced the structure and texture of the biochar generated from different sewage sludge types [START_REF] Méndez | Preparation of carbon-based adsorbents from pyrolysis and air activation of sewage sludges[END_REF]. Sewu et al.

reported that the steam activation of biochar enhances the porosity and increases the specific surface area by 5 times and the adsorption capacity of crystal violet by 4.1 time [START_REF] Sewu | Decolorization of cationic and anionic dye-laden wastewater by steam-activated biochar produced at an industrialscale from spent mushroom substrate[END_REF]. Similar results were reported by Wang et al. for the synergistic removal of Cu 2+ and tetracycline (TC) with steam-activated bamboo-derived biochar [START_REF] Wang | Synergistic removal of copper and tetracycline from aqueous solution by steam-activated bamboo-derived biochar[END_REF]. Ibrahim et al reported that steam activation of carbonized Oil Palm Mesocarp Fiber presents a high removal of the chemical oxygen demand and suspended solids from 395 mg/L and 117 mg/L to 122 mg/L and 7 mg/L, respectively in a treated wastewater after 6 consecutive treatment cycles [START_REF] Ibrahim | Reduction of residual pollutants from biologically treated palm oil mill effluent final discharge by steam activated bioadsorbent from oil palm biomass[END_REF]. Some works reported that alkali and steam treatments can be used to increase porosity and clean blocked pores [203][204].

Biochar-based composites

It is reported that producing biochar-based composites allows to combine the advantages of biochar with nano-materials, where the resulting composites usually exhibit great improvement in functional groups, pore properties, surface active sites, adjusted net surface charges, catalytic degradation ability and easy for recovery [204][180]. These composites can be classified into three categories based on their synthesis process towards biochar-based nano-materials; i) nano-metal oxide/hydroxide-biochar composites, ii) magnetic biochar composites [START_REF] Tan | Biochar-based nano-composites for the decontamination of wastewater: A review[END_REF], and iii) clay mineral based-biochar [START_REF] Arif | Synthesis, characteristics and mechanistic insight into the clays and clay minerals-biochar surface interactions for contaminants removal-A review[END_REF].

Nano-metal oxide/hydroxide-biochar composites

Several chemical additives have been used in the synthesis of these composites, like H3PO4, NaCl, K2CO3, MgCl2, ZnCl2, KOH, CaCl2 and FeCl3, which may be added before or after biomass carbonization [START_REF] Wang | An overview of carbothermal synthesis of metalbiochar composites for the removal of oxyanion contaminants from aqueous solution An overview of carbothermal synthesis of metal e biochar composites for the removal of oxyanion contaminants from aqueous so[END_REF], in powder forms or dissolved, as shown in figure 5.

Table 6 summarizes some works reported on nano-metal oxide-biochar composite synthesized to treat organic pollutants, where it can be seen that there are different preparation methods for these composites under diverse operating conditions. So, for example we find co-precipitation, pyrolysis, impregnation, sol-gel method and chemical reduction. Using a different classification method, in their review Zhao et al classified the nano-metal oxide-biochar composites preparation into four methods as presented in the figure 6; i) Impregnation which is characterized by its simplicity and a large capacity of obtained composites, ii) Chemical coprecipitation characterized by low cost, high purity, homogeneous nanoparticles, both these methods have the disadvantage of causing chemical pollution, iii) direct pyrolysis characterized by the easy preparation and the prior biomass enriched with target heavy metals but it is difficult to control the nano-metal oxides/biochar ratio in the composites, iv) Ball milling with various advantages like Low cost, easy to operate, no chemical pollution, effective reduction of particle size of metal oxides. In the downside, during the ball milling preparation particles are easily dispersed in water and move to surface runoff, pollutants can migrate out of the contaminated site, posing potential risks to groundwater [START_REF] Zhao | Formation and mechanisms of nano-metal oxide-biochar composites for pollutants removal: A review[END_REF]. Special attention is given to the ball milling technique, during which samples including biochars are crushed to nanometer size in the presence of metal oxide particles [START_REF] Wang | Recent advances in engineered biochar productions and applications[END_REF].

Ball milling has the capacity to increase the SSA, probably by revealing pores [START_REF] Peterson | Increasing biochar surface area: Optimization of ball milling parameters[END_REF], and to increase the oxygen-containing functional groups (e.g., carboxyl, lactone, and hydroxyl) [START_REF] Lyu | Effects of ball milling on the physicochemical and sorptive properties of biochar: Experimental observations and governing mechanisms[END_REF]. Zheng et al. developed a new ball-milling process to synthesize MgO/biochar nanocomposites with a dual-functional adsorbent to remove both cationic and anionic pollutants. The developed materials enhanced the adsorption of methylene blue (MB) for about 8.4 times, likely due to increased surface area and pore volume [START_REF] Zheng | MgO modified biochar produced through ball milling: A dual-functional adsorbent for removal of different contaminants[END_REF]. was added drip by drop to the mixture, and then sonicated, followed by water remove and drying.

[68]

Finally Sewu and his co-workers reported that using non-magnetic goethite mineral (α-FeOOH) in co-pyrolysis with firwood biomass to produce biochar proved as a green process when compared to the ubiquitous and conventionally used FeCl3, which has a negative impact due to toxicity [START_REF] Sewu | Facile magnetic biochar production route with new goethite nanoparticle precursor[END_REF].

Magnetic biochar composites

Significant research efforts have been carried out to improve the adsorption capacity of biochar. However, due to the small particle sizes and lower density, the separation and reuse of biochar after water pollutants elimination remain one of the main problems to be resolved [START_REF] Li | Preparation and application of magnetic biochar in water treatment: A critical review[END_REF]. Magnetic biochar has been successfully produced by various methods as reported in literature [START_REF] Feng | Preparation of magnetic biochar and its application in catalytic degradation of organic pollutants: A review[END_REF], presented in figure 7, showing the impregnation-pyrolysis, chemical co-precipitation, solvothermal and reductive co-precipitation methods. It is shown that these biochar composites provide effective absorptivity, ready separation by using external magnets and easy recycling [START_REF] Kumar Reddy | Magnetic biochar composite: Facile synthesis, characterization, and application for heavy metal removal, Colloids Surfaces A Physicochem[END_REF]. Nevertheless, the traditional loadingprocess of magnetic medium increases the cost of the sorbent [START_REF] Qu | Magnetic removal of dyes from aqueous solution using multi-walled carbon nanotubes filled with Fe2O3 particles[END_REF]. Thines et al reported three main common techniques employed to produce magnetic material such as pyrolysis, co-precipitation and calcination method [START_REF] Thines | Synthesis of magnetic biochar from agricultural waste biomass to enhancing route for waste water and polymer application: A review[END_REF]. An additional common production method was reported in literature such as, conventional heating in an electrical furnace [START_REF] Theydan | Adsorption of methylene blue onto biomass-based activated carbon by FeCl3 activation: Equilibrium, kinetics, and thermodynamic studies[END_REF],

microwave heating in a modified furnace [216][154]. The co-precipitation method was found to be the simplest due to its easy step of mixing and heating. This process focuses on the molar ratio of transition metal's ion being used during the mixing for improving the characteristics of the magnetic biochar produced, in terms of magnetism and porosity [START_REF] Thines | Synthesis of magnetic biochar from agricultural waste biomass to enhancing route for waste water and polymer application: A review[END_REF]. Co-precipitation methods result to coating the surface of biochar with different iron oxides such as, Fe3O4, c-Fe2O3, and CoFe2O4 particles, which give magnetic proprieties for active sites of iron oxide for pollutant elimination [START_REF] Kumar Reddy | Magnetic biochar composite: Facile synthesis, characterization, and application for heavy metal removal, Colloids Surfaces A Physicochem[END_REF][217] [START_REF] Baig | Effect of synthesis methods on magnetic Kans grass biochar for enhanced As(III, V) adsorption from aqueous solutions[END_REF].

It has been reported that additives with a high decomposition capacity (e.g., AlCl3, FeCl2

and MgCl2) exhibits catalytic activity, higher sorption capability, increasing porosity and creating positively charged adsorption sites [START_REF] Wang | An overview of carbothermal synthesis of metalbiochar composites for the removal of oxyanion contaminants from aqueous solution An overview of carbothermal synthesis of metal e biochar composites for the removal of oxyanion contaminants from aqueous so[END_REF][219] [START_REF] Han | Adsorption kinetics of magnetic biochar derived from peanut hull on removal of Cr (VI) from aqueous solution: Effects of production conditions and particle size[END_REF]. Table 7 summarizes some results on the preparation of magnetic biochars following different synthesis pathways, different metal reagents and different operating conditions.

Table 7: Literature on magnetic biochar composites synthesis for organic pollutants removal.

Magnetic biochar

Clay mineral modification

Clay-biochar composite is the result of pre-mixing the raw biomass before pyrolysis or the post mixing of produced biochar with clay minerals [START_REF] Cheng | Adsorption of emerging contaminants from water and wastewater by modified biochar: A review[END_REF]. The synthesis methods, characteristics of clay minerals-biochar and the surface interactions with contaminants during treatment has been reviewed by Arif et al [START_REF] Arif | Synthesis, characteristics and mechanistic insight into the clays and clay minerals-biochar surface interactions for contaminants removal-A review[END_REF]. This composite is characterized by high porosity and a great compatibility with water pollutants [START_REF] Fu | The single/co-adsorption characteristics and microscopic adsorption mechanism of biochar-montmorillonite composite adsorbent for pharmaceutical emerging organic contaminant atenolol and lead ions[END_REF]. Zhao and Zhou revealed that clay-mineral modification enhanced the micro-pore area of biochar by more than 200% while only a slight growth in the mesopore area was observed [START_REF] Zhao | Insight into interaction between biochar and soil minerals in changing biochar properties and adsorption capacities for sulfamethoxazole[END_REF]. It is also reported that organo-mineral layer of clay increases the biochar stability and its ability to resist the process of decomposition when biochar is applied to soil [START_REF] Lu | Ironmontmorillonite treated corn straw biochar: Interfacial chemical behavior and stability[END_REF].

From a mechanistic point of view, interactions of BC-clay/mineral composites with inorganic and organic contaminants can be established according to different mechanisms including ion exchange, physical adsorption, precipitation, complexation, electrostatic interaction, H-bonding, partitioning, electron-donor-acceptor (EDA) interaction, hydrophobic interaction and pore-filling [START_REF] Han | A critical review of clay-based composites with enhanced adsorption performance for metal and organic pollutants[END_REF].

Biochar applications for organic pollutants removal from water

Adsorption

There is a high research interest in the adsorption as a technique for mineral and/or organic contaminated water treatment, reflected by the high number of publications in the area [START_REF] Sewu | Biochar from the co-pyrolysis of Saccharina japonica and goethite as an adsorbent for basic blue 41 removal from aqueous solution[END_REF] water and wastewater effluent. The results revealed that the competitive adsorption of NPX, DCF, and IBU for the NaOH-activated SCW biochar was more spontaneously endothermic than that for the pristine SCW biochar [START_REF] Shin | Competitive adsorption of pharmaceuticals in lake water and wastewater effluent by pristine and NaOH-activated biochars from spent coffee wastes: Contribution of hydrophobic and π-π interactions[END_REF].

Filtration

The filtration technique was also studied and tested for the removal of different pollutants from wastewater. Wang et al., reported the removal of microplastic spheres by different biochar filters in a column system. In their study, biochar was packed in a 7 cm height bed between two sand layers of 3 cm height each. The results show that the biochar filters provide significant capacity for the removal and immobilization of 10 μm diameter microplastic spheres (more than 95% removal efficiency) which is much higher than that of similar grain-sized sand filter studied [START_REF] Wang | Filtration of microplastic spheres by biochar: removal efficiency and immobilisation mechanisms[END_REF]. Biochar can also be used for the pathogens removal (such as Escherichia coli and Enterococci) of as a common sand filter [START_REF] Werner | Agronomic benefits of biochar as a soil amendment after its use as waste water filtration medium[END_REF].

Kaetzl et al, studied the elimination of organic pollutant from raw sewage by using Miscanthus-biochar based filter media. Some works reported that the filtration removal is more dominant than adsorption where the result showed 74 ± 18% of chemical oxygen demand (COD) removal by biochar filters when only 61 ± 12% of COD reduction was recorded for sand filters [START_REF] Kaetzl | Slow sand filtration of raw wastewater using biochar as an alternative filtration media[END_REF]. Reddy et al., have used a filtration column with an inner diameter of 7 cm and a length of 61 cm for the elimination of different contaminants from urban storm water runoff. The results proved that biochar-filter reduced the total suspended solids (TSS), nitrate, phosphate and polycyclic aromatic hydrocarbons (PAHs) by an average of 86%, 86%, 47% and 68%, respectively [START_REF] Reddy | Evaluation of Biochar as a Potential Filter Media for the Removal of Mixed Contaminants from Urban Storm Water Runoff[END_REF].

Coupled Adsorption and advanced oxidation processes (AOPs)

Recently, the application of biochar (BC)-based composite materials in catalytic oxidation treatment has been proposed for organic contaminants elimination and degradation [245][246]. The introduction of these composites in the AOPs is due to the dual benefits of adsorption and catalytic degradation [START_REF] Faheem | Application of biochar in advanced oxidation processes: supportive, adsorptive, and catalytic role[END_REF]. light with a material dosage of 1 mg/mL, respectively [START_REF] Xiao | Graphitic carbon nitride/biochar composite synthesized by a facile ball-milling method for the adsorption and photocatalytic degradation of enrofloxacin[END_REF]. It is worth mentioning that about 98 % of tetracycline removal was achieved by MnFe-LDO-biochar catalyst within 240 min upon exposure to a UV light [START_REF] Azalok | UV-light-induced photocatalytic performance of reusable MnFe-LDO-biochar for tetracycline removal in water[END_REF]. On the other hand, Luo et al., reviewed the formation mechanisms of persistent free radicals in biochar and their interactions with organic and inorganic contaminants. They reported the combination of biochar with different oxidation processes such as; H2O2, photocatalysis, peroxydisulfate, persulfates, peroxymonosulfate [START_REF] Luo | A critical review on the application of biochar in environmental pollution remediation: Role of persistent free radicals (PFRs)[END_REF]. It is also reported that the effect of some typical anions such as; SO4 2-, H2PO4 -, HCO3 -and Cl -can act as inhibitors by trapping the radicals and therefore compete with the adsorption [START_REF] He | Sugarcane biochar as novel catalyst for highly efficient oxidative removal of organic compounds in water[END_REF].

The electrical discharge plasma was also coupled with biochar as reported by Lou et al., for the degradation of tetracycline hydrochloride in aqueous medium [START_REF] Lou | Removal of tetracycline hydrochloride in aqueous by coupling dielectric barrier discharge plasma with biochar[END_REF]. The result showed that during discharge plasma process, the specific surface area and the amount of oxygen-containing functional groups of biochar increased due to the interaction of biochar and reactive species generated by the plasma. The amount of generated • OH radicals was higher in the combined system due to the decomposition of O3 and H2O2.

Furthermore, this process leads to the elimination of tetracycline hydrochloride (C0=50 mg/L) at 89.36% in only 5 min [START_REF] Lou | Removal of tetracycline hydrochloride in aqueous by coupling dielectric barrier discharge plasma with biochar[END_REF].

Water treatment by biochar at pilot scale

Nowadays, the development of biochar based pilot or industrial plant reactors for contaminated water treatment is of great importance, because of the large amount of wastewater daily generated in the world [START_REF] Kakade | World eutrophic pollution of lake and river: Biotreatment potential and future perspectives[END_REF]. There are only very few works reported on the organic/inorganic pollutants removal at pilot scale biochar reactors. Ashoori et al.

reported the removal of nitrate, metals, and trace organic contaminants from urban stormwater runoff by pilot-scale biochar-amended woodchip bioreactor, composed of 9 parallel columns of 10 cm in diameter and 50 cm length, where every three columns are filled with the same adsorbent (Woodchips, Woodchips + biochar and Woodchips + straw).

The results showed a high capacity of removing nitrate and four of the five metals tested (i.e., Cd, Cu, Ni and Pb but not Zn), however a weak adsorption capacity for organic pollutant was recorded [START_REF] Ashoori | Evaluation of pilot-scale biochar-amended woodchip bioreactors to remove nitrate, metals, and trace organic contaminants from urban stormwater runoff[END_REF]. Finally, scientific advances on the extrapolation of results present a significant challenge requiring more investigations to be conducted within the framework of a reusability over several cycles.

Conclusion and future research directions

Biochar has been recommended as a promising adsorbent material for organic contaminants removal from wastewater. Some works estimated the costs incurred in the production of biochar based adsorbents; a cheaper process and a reusability ability with multiple cycles of biochar is possible [START_REF] Chakraborty | Mechanistic insight into sorptive elimination of ibuprofen employing bi-directional activated biochar from sugarcane bagasse: Performance evaluation and cost estimation[END_REF]. Nevertheless, supplementary research in several perspectives is necessary to ensure the efficacy and cost-effectiveness of biochar to scale-up it into a large-scale application, mostly in the following areas:

1) Some of the reported biochar sorbents (such as Coir pith, Coconut coir dust, Pandanus leaves and paper waste) showed low efficiency and elimination capacity to organic pollutants. It is expected that surface modifications will help increase their capacity.

2) To achieve optimal environmental remediation activity for biochars it is essential to further explore the relationships between certain factors; production process, activation, functionalization, and treatment all in an environmental ecofriendly manner.

3) It is important to keep in mind that a promising bio based sorbent successfully applicable at industrial scale should be economically attractive and raw materials should be extensively available in large quantities in nature or as a by-product (bioresidues) [START_REF] Park | The past, present, and future trends of biosorption[END_REF]. Regenerating and reusing using these sorbents for several cycles at an industrial scale can reduce the cost and energy consumption for production and offer a sustainable product. learning (ML) is a subdivision of artificial intelligence (AI) that aims to reduce explicit computers programming by learning from experience and prediction [START_REF] Lakshmi | Artificial intelligence (AI) applications in adsorption of heavy metals using modified biochar[END_REF]. Predictive models that use machine learning can reduce the workload, cost, space requirement and time consumption for remediation of effluents [START_REF] Bhagat | Development of artificial intelligence for modeling wastewater heavy metal removal: State of the art, application assessment and possible future research[END_REF]. For example ML can be applied to develop prediction models for optimizing the pyrolysis process and predict biochar yield and C-char according to biomass characteristics and pyrolysis conditions [START_REF] Zhu | Machine learning prediction of biochar yield and carbon contents in biochar based on biomass characteristics and pyrolysis conditions[END_REF]. As stated in [START_REF] Zhu | Insights into the adsorption of pharmaceuticals and personal care products (PPCPs) on biochar and activated carbon with the aid of machine learning[END_REF], science-informed design of biochar substrates with high removal capacity for organic contaminants can prove a valuable tool for designing sustainable wastewater treatment systems. The developed models based on data from advanced microscopic and spectroscopic techniques (biochar surface functionality and porous structure) gave accurate predictions on the adsorption capacity of the considered materials.

The conception of more compact reactors that can accommodate biochar supported catalysts are necessary for real scale applications. Future research must be devoted to implement biochar-based catalysts in pilot scale reactors that can be used also at industrial level.

Optimizing biochar properties and activation techniques and parameters is important for obtaining maximum efficiency for organic pollutant elimination, less post-treatment byproducts and less energy consumption.

Well-coordinated actions currently going on, guided by the European biochar certificate (EBC) in Europe, and the IBI at international level, to regulate and standardize quality requirements and develop guidelines or good practices concerning the biomass supply, biochar production and biochar applications which mainly are limited to agriculture and organic farming. One needs to keep in mind that the first concern of the EBC has been to

  This further elaboration of quality standards is the objective of ongoing research and demonstration projects at EU level (ThreeC, HortiBlueC), to name a few. Recent reviews aim to categorize the large amount of information of biochar physicochemical properties and their effect on removal mechanisms by attributing preparation technologies, functionalization techniques, to certain biochar characteristics and propose specific applications, as for example in Wang et al and Patel et al.[28][29]. The knowledge of biochar characteristics and their possible effect on adsorption mechanisms towards various types of pollutants in aqueous media is necessary to clarify what steps to take towards improving biochar characteristics by means of various treatments like activation and functionalization.

Figure 1 :

 1 Figure 1: Classification of characterization techniques versus biochar properties

Figure 2 :

 2 Figure 2: Frequency of applying certain techniques for biochar characterization (data

  [START_REF] Jellali | Sludge-derived biochars: A review on the influence of synthesis conditions on pollutants removal efficiency from wastewaters[END_REF]. Several works report the effect of different parameters and their interaction on the removal efficiency of pollutants with the help of multivariate optimization approach[START_REF] Salehi | Phosphoric acid-treated Spent Tea Residue Biochar for Wastewater Decoloring: Batch Adsorption Study and Process Intensification using Multivariate Data-based Optimization[END_REF][START_REF] Alameri | Sorption and removal of crude oil spills from seawater using peat-derived biochar: An optimization study[END_REF]. Figure3shows the factors influencing the removal of organic pollutants by biochar. In general, biochar concentration and initial pH of the solution are the most discussed.

Figure 3 :

 3 Figure 3: The main influencing parameters in organic pollutants removal by biochar.

  Li et al., in the removal of ciprofloxacin[START_REF] Li | Study of ciprofloxacin removal by biochar obtained from used tea leaves[END_REF].Sayin et al., in their work on the removal of ciprofloxacin by H3PO4 modified biochar, reported the decrease of the adsorption rate of antibiotic molecules with increasing time of treatment and that until the equilibrium point of time[START_REF] Sayin | From green biowaste to water treatment applications: Utilization of modified new biochar for the efficient removal of ciprofloxacin[END_REF]. Nguyen et al. in their study tested diverse Metal salt-modified biochars derived from different agro-waste for the removal of Congo red dye. The results revealed the different equilibrium times needed for the different biochars applied at the same initial concentration of the dye[START_REF] Nguyen | Metal salt-modified biochars derived from agro-waste for effective congo red dye removal[END_REF].

Figure 4 :

 4 Figure 4: Typical engineered biochar modifications and its classification.

Figure 6 :

 6 Figure 6: Different pathways for Nano-metal oxide/hydroxide-biochar composites

Figure 7 :

 7 Figure 7: Schematic diagram of common Magnetic biochar preparation methods.

  [START_REF] Liu | Efficient adsorptive removal of short-chain perfluoroalkyl acids using reed straw-derived biochar (RESCA)[END_REF][START_REF] Liu | Elucidating dominant factors of PO43-, Cd2+ and nitrobenzene removal by biochar: A comparative investigation based on distinguishable biochars[END_REF].Lu et al. reported the simultaneous elimination of metal ions and organic compounds from aqueous solutions by rice bran, bamboo and their derived biochars. These biochars presented a high sorption capacity for acenaphthene removal compared to the original biomass whereas the coexisting Cu 2+ and Cr +6 both inhibited the sorption of acenaphthene by the biochar[START_REF] Lu | Removal of acenaphthene by biochar and raw biomass with coexisting heavy metal and phenanthrene, Colloids Surfaces A Physicochem[END_REF]. Similar results were reported by Cheng et al. for the adsorption of methyl orange in the presence of Cr 6+ from dye wastewater[START_REF] Cheng | Adsorption performance and mechanism of iron-loaded biochar to methyl orange in the presence of Cr6+ from dye wastewater[END_REF].The simultaneous removal of toxic Pb(II) ions, poly(acrylic acid) and Triton X-100 from their mixed solution was achieved by using engineered acid activated biochars obtained from horsetail herb precursor[START_REF] Wiśniewska | Simultaneous removal of toxic Pb(II) ions, poly(acrylic acid) and Triton X-100 from their mixed solution using engineered biochars obtained from horsetail herb precursor -Impact of post-activation treatment[END_REF]. Shin et al. investigated the competitive adsorption of three pharmaceuticals; naproxen (NPX), diclofenac (DCF), and ibuprofen (IBU) from lake

Table 1

 1 gives a global overview of biochar properties, their function, their role in pollutant adsorption and relevant production parameter of biochar

Table 1 :

 1 Global overview of biochar properties, their function, their role in pollutant adsorption and relevant production parameter of biochar

	Biochar Property	Function	Application/	Biochar production
			mechanism involved	parameter
	SSA, porosity	sorption capacity	Immobilization of	Residence time, high
			contamination from	temperatures
			solid, liquid and
			gaseous media
	Ion-exchange	electrostatic	adsorption of organic	post functionalization
	properties on surface	interactions with polar	contaminants	(acidic or base
		or non-polar groups		treatment)
	high mechanical and			high temperatures
	chemical stability			.
	Surface functional	Chemical bonding with	Absorption and	Low temperature
	groups	certain molecules	immobilization of	production, post
			certain toxins and drugs	treatment
				(functionalization)
		Negatively or positively	Attraction/repulsion of	Feedstock (high ash
	pH and pHpzc	charged surface	ionically charged	content biomass leads
			molecules	to high pH biochars),
				post functionalization
				(acidic or base
				treatment)

Table 2 :

 2 Biochars applied in the treatment of organic pollutants.

	Biochar

material Treatment of biochar Pollutant (initial Concentration) Removal yield or adsorption capacity (qmax) References Medicine (antibiotics)

	Sawdust-coal biochar		Epoxiconazole (10 mg/L)	97 % (400 min)	[50]
	Grape pomace-derived biochar		Cymoxanil pesticide (100 mg/L)	qmax =161 mg /g biochar	[51]
	CO2 activated biochar		Phenol (0.5 mM) Chlorophenol (0.5 mM)	50 -60% (60 min)	[52]
	Cobalt-gadolinium Activated magnetic loofah sponge biochar modified biochar	KOH treatment	Ciprofloxacin (20 mg/L) Tetracycline (20 Imidacloprid mg/L) Steroides	99.55% (6 h) 99.23% (6 h) qmax =738 mg.g -1	[38] [53]
	Sulfamic acid modified biochar Graphene oxide Activated biochar derived from biochar supported on activated magnetic		Tetracycline (100 mg/L) Sulfadiazine (10 mg/L) 17 β -estradiol (6 mg/L)	412.95 mg/g biochar ≈95% (1400 min) qmax =46.22 mg/g biochar	[39] [36] [54]
	cotton shell Iron loaded
	Rice husk Carbonization at 500 •C biochar porous graphitized	H2SO4 or KOH treatments activation biochar	Tetracycline (20 mg/L) 17b-estradiol (3 mg/L)	min) 93.1% (180 100 % (45 min)	[40] [55]
	Rice straw and		Tetracycline (C0 =	T = 25 •C
	swine manure Carbonization at	H3PO4 modified biochar	30-200 mg•L -1)	pH = 5.0-9.0 qmax = 167.5	[41]
	700 •C			mg•g-1
	Iron oxide nanoparticle loaded biochar		Ofloxacin (30 mg/L)	96% (5 h)	[42]
				T = 21-30 •C
				pH = 1.0-10.0
	Bamboo Carbonization at 380 •C	H3PO4 treatment	Sulfamethoxazole, sulfathiazole and sulfamethazine	qmax SMX = 237 mg•g-1 88.1 mg•g-1 qmax STZ =	[43]
				qmax SMZ =
				65.7 mg•g -1
	Anthriscus			qmax = 392.94
	sylvestris-derived activated	NaOH treatment	Diclofenac (DF) Cephalexin (CPX)	mg g -1 for DF qmax = 724.54	[44]
	biochar			mg g -1 for CPX
			Dyes
			Rhodamine (0.01
	CaWO4-biochar nanocomposites		mM) Methyl orange	71% (30 min) 73% (30 min)	[45]
			(0.05 mM)
	ZnO/biochar nanocomposites		Methylene blue (160 mg/L)	95.19% (225 min)	[46]
	Fe, N codoped biochar		Acid orange (20 mg/L)	93.4% (20 min)	[47]
	Pinecone biochar		Black-3	346.856 mg g-1	[48]
			Pesticides
	P doped biochar		Triazine (2 mg/L)	>96% (20 min)	[49]

Table 3 :

 3 Binding energy in XPS spectra for the main surface functional groups on biochars

	306
	Biochar			C 1s Line		O 1s Line	N 1s	Refere
									Line	nce
			C-C	C = C C-O	O-C-O	C=O	Organic	Organic	C-N
							C=O	C-O
	Biochar/FexO	284.8	284.8 286.1	288.3		533.3	531.62	399.6	[66]
	y composite							and
									402.0
	Biochar		284.4-	286.3	289.1-	287.8-	531.3-	533.3-	399-	[65]
			284.7	-	289.3	288.0	532.3	533.9	401
			285.0-	286.6
			285.2
	Biochar/Fe					531.44-	532.23-		[69]
	composites					531.57	533.08
	Graphene	284.8	284.8 286.5						[70]
	oxide
	/CoFe2O4 -
	sludge
	biochar
	composite
	Iron-loaded	284.8	284.8					400.2	[71]
	biochar
	Biochar-		284.4	285.6		288.5				[72]
	based	iron
	oxide

305

reported in literature.

Table 4 :

 4 Peak positions of the main surface functions in FTIR spectra of biochar and biochar-based composites reported in literature.

Table 5 :

 5 The position of D band and G band in Raman spectra of biochar reported in literature.

	Biochar	D band position	G band position	ID/IG	Reference
		(cm -1 )		(cm -1 )
	Sludge-derived	1350	1580	0.84 and 1.06 [73]
	biochar
	Magnetic		1338	1589	0.94 and 0.99 [95]
	biochar derived
	from	banana
	peels
	Pyrrolic	N-rich	1343	1588	0.79 and 1.07 [56]
	biochar

Table 6 :

 6 The main weight loss stages of biochars in thermogravimetry analysis reported in literature.

Table 6 :

 6 Reported work on Nano-metal oxide-biochar composites synthesis for organic pollutants removal.

	Biochar	Synthesis	Reagent	Operating Conditions	Reference
		method
	MnOx-coated rice	Co-	KMnO4	The mixed solution was
	straw biochar	precipitation		vibrating continuously at
				30 °C for 8 h and then
				oven-dried at 80 °C for 24
				h after washing.

  Premarathna, et al.

	reported the synthesis of a novel clay-biochar composite from montmorillonite (MMT) and
	red earth (RE) clay materials with municipal solid waste (MSW) biochar in pyrolysis. The
	obtained composites exhibit a high adsorption capacity towards removing tetracycline

antibiotic in aqueous media

[START_REF] Premarathna | Clay-biochar composites for sorptive removal of tetracycline antibiotic in aqueous media[END_REF]

. Similar results were reported by Borthakur et al. for tetracycline removal from wastewater

[START_REF] Borthakur | Computational and experimental assessment of pH and specific ions on the solute solvent interactions of clay-biochar composites towards tetracycline adsorption: Implications on wastewater treatment[END_REF]

. It is noticed that the surface properties of the biochar samples are considerably affected by the incorporation of different clay material.

  Huang et al. revealed that morphological and chemical features of biochar played an important role in hydroxyl radical (°OH) generation[START_REF] Huang | Influence of morphological and chemical features of biochar on hydrogen peroxide activation: Implications on sulfamethazine degradation[END_REF]. Various methods have been tested and validated by researchers to develop biochar supported catalyst for the activation of hydrogen peroxide and persulfate such as; co-precipitation, impregnation, liquid phase reduction, hydrothermal and pyrolysis[START_REF] Pan | Preparation of biochar and biochar composites and their application in a Fenton-like process for wastewater decontamination: A review[END_REF][START_REF] Kumar | Advances in design strategies for preparation of biochar based catalytic system for production of high value chemicals[END_REF]. Meanwhile, the role of biochar in hydroxyl radical based AOPs can be classified in three categories; biochar as a catalyst, biochar as supporting material and biochar as cathode in electro-Fenton process[START_REF] Nidheesh | Potential role of biochar in advanced oxidation processes: A sustainable approach[END_REF]. Some works reported the synthesis of biochar composite with a double function of adsorption and photocatalytic degradation. Leichtweis et al., reported the preparation of biochar derived from pecan nutshell with ZnO (biochar-ZnO) for the treatment of acid red 97 in aqueous solution. The obtained material degraded 100% of acid red 97 in only 67 min of treatment (30 min with biosorption + 37 min with photocatalysis)[START_REF] Leichtweis | New composite of pecan nutshells biochar-ZnO for sequential removal of acid red 97 by adsorption and photocatalysis[END_REF]. Similar results reported by Xiao et al., on the treatment of enrofloxacin with 50% g-C3N4/biochar, report a removal of 45.2% and 81.1% of enrofloxacin (C0=10 mg/L) under darkness and

Acknowledgments

"The authors would like to acknowledge the financial support of the INTERREG North-West Europe ThreeC with the project number NWE 1010, under the umbrella of the European Regional Development Fund (ERDF)."

Combined modified-clay/biochar composites revealed noteworthy advantages over single-modified-clays, due to their high adsorption capacity, low cost and more suitable for removing anionic dyes and phenolic compounds.

There are a very few studies on pilot/large-scale application of biochar for organic pollutants removal in literature.

The progress of research on magnetic biochar with catalytic degradation activity for organic contaminants elimination from aqueous medium is an important and a new research direction and will become a research focus soon. Studies on the toxic components produced during magnetic biochar synthesis should be carefully considered.

The implementation of a sustainable management plan for pollutant-loaded Sewage derived biochars (SDBs) is critical. This strategy should follow the circular economy concept and would permit: i) an eco-friendly and low-cost regeneration and reuse of these SDBs in upcoming adsorption cycles, ii) the recovery of the adsorbed chemicals and it reprocess in an industry as resources, and iii) the preservation of the environment counter to further pollution.

Two strategies concepts should be explored to achieve functional and cost-effective modified biochars for the efficient elimination of organic compounds from wastewater: i)

The first one is optimizing the biochar physicochemical activation process using a variety of eco-friendly reagents, ii) The second option concerns the exploitation of the high specific surface areas of prepared biochar to impregnate nano-composites. The application of an experience plan software to optimize the influencing parameter is a powerful and innovative tool in the experiment design and analysis phase. Machine safeguard health and safety while producing and using biochar in a sustainable way.

Biochar based substrates for environmental applications are a promising alternative to currently applied substrates for wastewater treatment and organic pollutant elimination, but there is yet no set of criteria on the physicochemical properties of the substates that guarantee satisfactory adsorption efficiencies. Such well-coordinated actions are currently still in the form of literature reviews that try to draw conclusions and offer a suggestions and guidelines for researchers that need to build upon the existing but very dispersed knowledge on the potential applications of biochar and biochar composites in environmental applications.