Ana S Mestre
email: asmestre@fc.ul.pt

Fabian Hesse

Cristina Freire

Conchi O Ania

Ana P Carvalho
email: ana.carvalho@fc.ul.pt

Chemically activated high grade nanoporous carbons from low density renewable biomass (Agave sisalana

Keywords: sisal, biomass thermochemical conversion, K 2 CO 3 and KOH activation, nanoporous carbons, nitrogen adsorption isotherms, kinetic assays, equilibrium adsorption data, pharmaceutical compounds removal, ibuprofen, iopamidol

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Introduction

The use of residues for the production of valuable products has become a common practice in several economic sectors, as it contributes to the implementation of a circular economic strategy adopted by European Union and aligned with United Nations 2030 Agenda for Sustainable Development.

In the manufacture of activated carbons at industrial scale, the choice of the precursors is an important issue due to various aspects as price fluctuations, availability and variability of sources, that can compromise the quality and price of the final product.

Coconut-shell is a key precursor in the fabrication of activated carbons [START_REF]The Economics of Activated Carbon[END_REF] due to its low ash content which leads to activated carbons with low metals percentage, very good attrition resistance and generally high surface areas [START_REF] Marsh | Activated Carbon[END_REF][START_REF] Schaeffer | Activated carbon 2013 market update or, the carbon convolution[END_REF]. Coal and wood are also widely used precursors; however coal-derived materials usually have large ash contents, while wood-derived granular activated carbons display poor attrition resistance. Other renewable sources (e.g. fruit shells and stones) are also employed at industrial scale for the manufacture of activated carbons, with less expression. From an academic point of view, the list of precursors for the preparation of activated carbons is quite large, as shown in various reviews [START_REF] Ioannidou | Agricultural residues as precursors for activated carbon production -A review[END_REF][START_REF] Yahya | Agricultural bio-waste materials as potential sustainable precursors used for activated carbon production: A review[END_REF][START_REF] Jain | Hydrothermal conversion of biomass waste to activated carbon with high porosity: A review[END_REF][START_REF] Gupta | A Critical Analysis on the Efficiency of Activated Carbons from Low-Cost Precursors for Heavy Metals Remediation[END_REF][START_REF] Abioye | Recent development in the production of activated carbon electrodes from agricultural waste biomass for supercapacitors: A review[END_REF].

The search for alternative synthetic routes that allow the preparation of high grade activated carbons from biomass residues (particularly low density biomass), is an issue of academic and industrial relevance. The use of renewable feedstocks is expected to have important environmental and economic impacts, since it allows to close the loop of many biomass wastes through their use in the production of valuable tradeable goods.

Conventional activation methods usually starts either by the carbonization of the biomass -to remove volatile matter -and further thermal treatment of the char in the presence of an oxidizing agent to develop the pore network (physical activation); or by the activation of the precursor mixed with an activating agent (chemical activation) [START_REF] Marsh | Activated Carbon[END_REF].

Besides porosity, the density and hardness of the activated carbons strongly depend on the properties of the precursor, with low density ones typically leading to low density and soft (low attrition resistance) carbons [START_REF] Mestre | Nanoporous carbons synthesis: an old story with exciting new chapters[END_REF].

Alternative carbonization and activation approaches have been largely explored for the past decades aiming at reducing the energy consumption, while producing highly porous activated carbons with controlled pore architectures. Among them, the hydrothermal carbonization of polysaccharides followed by the activation of the hydrochar has emerged as an interesting method that allows the preparation of superactivated carbons [START_REF] Falco | Tailoring the porosity of chemically activated hydrothermal carbons: Influence of the precursor and hydrothermal carbonization temperature[END_REF][START_REF] Sevilla | High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials[END_REF][START_REF] Romero-Anaya | Spherical carbons: Synthesis, characterization and activation processes[END_REF][START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: remarkable adsorbents for pharmaceutical compounds[END_REF] with microspherical morphology [START_REF] Romero-Anaya | Spherical carbons: Synthesis, characterization and activation processes[END_REF][START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: remarkable adsorbents for pharmaceutical compounds[END_REF][START_REF] Mestre | High performance microspherical activated carbons for methane storage and landfill gas or biogas upgrade[END_REF] and improved performance for various applications (e.g., energy storage/production, gas adsorption) [START_REF] Sevilla | High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials[END_REF][START_REF] Mestre | High performance microspherical activated carbons for methane storage and landfill gas or biogas upgrade[END_REF][START_REF] Nunes | Sucrose-derived activated carbons: electron transfer properties and application as oxygen reduction electrocatalysts[END_REF][START_REF] Titirici | Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage[END_REF]. On the other hand, two of the main constituents of lignocellulosic biomass, i.e. cellulose and hemicellulose, are complex structures composed by saccharic units (pentoses and hexoses). Therefore, following a similar approach, it is possible to obtain a char after the acid digestion and polycondensation of biomass [START_REF] Wang | A new route for preparation of hydrochars from rice husk[END_REF]. Such acid-char may be used as carbon rich precursor for the synthesis of activated carbons.

In this study, a series of high grade nanoporous carbons were prepared from the acid digestion and polycondensation of a biomass residue (Agave sisalana), and evaluated for the removal of emerging pollutants from aqueous solution (i.e. ibuprofen and iopamidol). The acid-chars obtained after the acid digestion and polycondensation of the biomass were chemically activated using KOH and K 2 CO 3 . The effect of the acid digestion conditions in the apparent density, porosity and composition of both the acidchars and their corresponding nanoporous carbons after chemical activation was investigated. The resulting nanoporous carbons displayed moderate to high surface area with some samples outperforming a commercial benchmark carbon for the removal of pharmaceutical compoundsibuprofen and iopamidolfrom aqueous solution.

Experimental section

Synthesis of the acid-chars

Sisal (Agave sisalana) residues received from a rope industry (Cordex, Portugal) were used as raw precursor. This biomass is composed by (wt./wt.) 65.8% of cellulose, 12.0% of hemicellulose, 9.9% of lignin, 0.8% of pectin, and 0.3% of wax and water soluble compounds [START_REF] Andrade | Characterization of the different fractions obtained from the pyrolysis of rope industry waste[END_REF]; therefore about 78% of its structure is constituted by saccharic units. The synthesis of the sisal-derived acid-chars was inspired in the work of Wang et al. [START_REF] Wang | High surface area porous carbons prepared from hydrochars by phosphoric acid activation[END_REF] on other biomass precursor (i.e. rice husk). Briefly, sisal fibers of ca. 1 cm long were digested under stirring in H 2 SO 4 solutions (Sigma-Aldrich, 95-98%) for 15 min in the case of 12 and 13.5 M solutions, and for 30 min in the case of 9 M solutions. In all cases a 1:10 wt./v ratio (g of sisal to cm 3 of H 2 SO 4 solution) was used and the treatment was carried out at 50 ºC, controlled by a water bath (VWR Scientific Model 1201).

After the acid digestion, the solid residue (undigested sisal fibres) was removed, and the dark acidic liquor was then heated at 90 ºC under reflux during 6 h, to allow the polycondensation reactions rendering a solid product (acid-char). The polycondensation was carried out using acid concentrations equal or lower than those used in the digestion step. With exception of the acid-char prepared with H 2 SO 4 13.5 M in both digestion and polycondensation steps additional 5 cm 3 of acid (same concentration as in the digestion step) or water were added per gram of biomass used. The acid-char was recovered by filtration, washed with distilled water until neutral pH, dried at 100 ºC overnight and crushed in order to get a fine powder (particles with dimensions < 0.297 mm). The sisalderived acid-chars were labelled as SX/Y, where X and Y correspond to the H 2 SO 4 concentration (M) used in the digestion and the polycondensation step, respectively (e.g. S9/9).

Chemical activation of the acid-chars

The acid-chars were activated with KOH or K 2 CO 3 , using various contacting methods, acid-char:activating agent ratios and activation temperatures. Two methods were used to allow the contact between the acid-char and the chemical activating agent: physical mixing of both powders in a mortar, and solution impregnation. In the former, the activation was carried out by mixing 1 g of the acid-char with K 2 CO 3 (Aldrich, 99 %) or KOH (Panreac, 85 %) in a weight ratio of 1:3 (acid-char:activating agent). In the case of solution impregnation, the activating agent is dissolved in water, mixed with the acidchar and stirred for 2 h at room temperature; the suspension was then dried at 80 ºC to allow the evaporation of the excess water and crushed before the carbonization. The activation was performed at 700 or 800 ºC for 1 h in a horizontal furnace (Thermolyne, model 21100) under a N 2 flow of 5 cm 3 s -1 and using a heating rate of 10 ºC min -1 . After cooling down to room temperature under N 2 flow, the materials were washed with distilled water until neutral pH, dried overnight at 100 ºC, crushed to a fine powder (particles with dimensions < 0.297 mm). Activated carbons were labelled after the corresponding acid-chars followed by the activating agent (C for K 2 CO 3 , and H for KOH) and its amount per gram of acid-char (in g). Unless otherwise stated, samples were prepared by physical mixture and activated at 800 ºC. The activated carbons prepared by solution impregnation were labelled with (s). As an example, sample S13.5/13.5/C3(s) corresponds to the carbon prepared by activation of acid-char S13.5/13.5 at 800 ºC using 3 g of K 2 CO 3 and solution impregnation.

Characterization of the materials

Apparent -tapped-densities of the carbons were determined through a methodology adapted from the literature for powdered materials with 90 % of particles with dimensions lower than 0.177 mm (80 mesh) [START_REF] Jorda-Beneyto | Advanced activated carbon monoliths and activated carbons for hydrogen storage[END_REF][START_REF]AWWA Standard for Powdered Activated Carbon[END_REF][START_REF] Cefic | Test methods for activated carbon[END_REF]. Briefly, a graduated cylinder was filled with a known amount (typically 0.5 g) of the powdered carbon (Mettler Toledo AB204-S/FACT), tapped in a rubber pad for several minutes until no further settling was observed, and the volume was recorded. The densities presented correspond to the mean value of at least three assays. The apparent densities were determined on a dry basis according to equation [START_REF]The Economics of Activated Carbon[END_REF].

𝑑𝑒𝑛𝑠𝑖𝑡𝑦 = 1000 × 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒,𝑖𝑛 𝑔 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒,𝑖𝑛 𝑐𝑚 3 × (100-% 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒) 100
Eq. 1

The morphology of selected samples was evaluated by Scanning Electron Microscopy (SEM) performed at a Zeiss Supra 55 VP apparatus using 5 kV as accelerating voltage.

Scanning was performed in situ on a sample powder without coating. The crystallinity of selected samples was assessed by X-ray powder diffraction (XRD) data obtained in a Pan Analytical PW3050/60X´Pert PRO apparatus equipped with a X´Celerator detector and with automatic data acquisition (X´Pert Data Collector (v2.0b) software). The diffractograms were collected at room temperature using monochromatized CuK radiation as incident beam (40 kV-30 mA), the 2 range of 5º to 60º was scanned with a step size of 0.017º and a time per step of 0.6 s.

The chemical composition of selected acid-chars was obtained by elemental analysis (CHNSO) using LECO CHNS-932 and LECO VTF-900 (for direct oxygen quantification) automatic analysers. Data is expressed in dry basis since the samples were previously dried at 105 ºC. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was recorded in a Nicolet Magna-IR560 spectrometer provided with a high sensitivity mercury cadmium telluride detector (MCT-A) that operates at cryogenic temperature. Spectra were recorded on powder samples without KBr addition. Each spectrum was obtained from the accumulation of 256 scans, recording with a spectral resolution of 2 cm -1 in the mid-IR (4000-650 cm -1 ) spectral range. The pH at the point of zero charge (pH PZC ) of the samples was measured following the reverse mass titration method [START_REF] Noh | Estimation of the point of zero charge of simple oxides by mass titration[END_REF] and using a Symphony SP70P pH meter. The thermal analysis (Setaram Labsys) was carried out on ca. 10-25 mg of sample under a nitrogen flow rate of 50 cm 3 min -1 , a heating rate of 15 ºC min -1 and up to a final temperature of 900 ºC. The ash content was determined after exposure of the samples at 600 ºC in air, until constant mass.

The porosity of the samples was characterized by equilibrium adsorption/desorption isotherms of N 2 at -196 ºC and CO 2 at 0 ºC measured in volumetric analysers from Micromeritics (ASAP 2010 and Tristar 3000, respectively). The samples (60-100 mg)

were previously outgassed at 120 ºC for 17 h under vacuum (pressure < 10 -2 Pa). Data from the N 2 isotherms were used to estimate the apparent surface area, A BET , by the Brunauer-Emmett-Teller (BET) method, following the IUPAC [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF][START_REF] Rouquerol | Is the bet equation applicable to microporous adsorbents?[END_REF] and ISO 9277 [START_REF] Iso | Determination of Specific Surface Area of Solids by Gas Adsorption -BET Method[END_REF] recommendations for the determination of this parameter in microporous solids.

The relative pressure range for the determination of the BET area was selected according with two criteria: (i) positive line intersect of multipoint BET fitting (C > 0) and (ii) maximum p/p 0 value limited to the pressure range where n ads (1-p/p 0 ) continuously increases with p/p 0 [START_REF] Rouquerol | Is the bet equation applicable to microporous adsorbents?[END_REF]. The total pore volume, V total , was assessed using the Gurvich rule [START_REF] Rouquerol | Adsorption by Powders and Porous Solids -Principles, Methodology and Applications[END_REF], that is, the number of N 2 moles adsorbed at relative pressure close to unity (i.e. p/p 0 = 0.975) converted in total pore volume assuming that the pores are filled with the condensed N 2 in the normal liquid state. The  s method was also applied to the N 2 adsorption data, taking as reference the isotherm reported by Rodríguez-Reinoso et al. [START_REF] Rodriguez-Reinoso | A standard adsorption isotherm for the characterization of activated carbons[END_REF], enabling to quantify the total micropore volume, V  total , and also the ultra, V  ultra , and supermicropore, V  super , volumes. The total micropore volume (width < 2.0 nm) was obtained by back extrapolation of the  s plots linear region at high relative pressure ( s > 1) while the ultramicropore volume (width < 0.7 nm) was evaluated by the interception of the linear range defined by the experimental data between p/p 0 of 0.02 and, about, 0.4. The supermicropore volume (0.7 < width < 2.0 nm) corresponds to the difference V  total -V  ultra . The microporosity was also quantified by applying the Dubinin-Radushkevich (DR) equation to the N 2 and CO 2 adsorption data (W 0 N2 and W 0 CO2 , respectively). The pore size distribution (PSD) analysis in the micro-mesopore range was calculated from the N 2 adsorption data using the 2D-NLDFT-HS model that assumes surface heterogeneity and slit geometry of pores in activated carbons [START_REF] Jagiello | 2D-NLDFT adsorption models for carbon slit-shaped pores with surface energetical heterogeneity and geometrical corrugation[END_REF]. The micropore size distributions (MPSD) were further assessed from CO 2 adsorption data according to the method proposed by Pinto et al. [START_REF] Pinto | Comparison of Methods to Obtain Micropore Size Distributions of Carbonaceous Materials from CO 2 Adsorption Based on the Dubinin-Radushkevich Isotherm[END_REF] that is a variation of the DR equation which does not imposes constrains to the shape of the distribution.

Liquid phase adsorption assays

Selected acid-char derived activated carbons were tested along with a commercial benchmark carbon for the removal of two pharmaceutical compounds (PhCs) -ibuprofen and iopamidol-from aqueous solution. The commercial activated carbon -Norit SAE Super (NS) -from Cabot is a powdered material specially developed for wastewater treatment. According to the general characteristics reported in the datasheet provided by the supplier, carbon NS has an apparent (tapped) density of 425 kg m -3 , ca. 97 % of the particles have sizes lower than 150 µm, and displays a basic character. Ibuprofen (sodium salt, Sigma-Aldrich -Lot BCBC9914V) and iopamidol (Hovione -Lot 163926HQ01324) solutions were prepared with ultra-pure water without pH adjustment. The solutions have a pH value around 5. About 6 mg of activated carbon and a given volume of ibuprofen or iopamidol solution were mixed in glass vials maintained at 30 ºC (water bath, Grant GD100 controller) and stirred at 700 rpm (multipoint agitation plate Poly, Variomag). Samples were collected after the desired contact time (see details in Table 1). After removing the activated carbon by filtration, the amount of ibuprofen or iopamidol remaining in solution was determined by UV-vis spectrophotometry (Genesys 10S) at the wavelength of maximum absorbance (ca. 221 and 242 nm for ibuprofen and iopamidol, respectively). The PhC uptake was calculated using the equation:

q t = C 0 -C t W × V Eq. 2
where q t is the amount (mg g -1 ) of PhC adsorbed at time t, C 0 is the initial and C t is the PhC concentration at time t (mg dm -3 ), V is the volume of PhC solution (dm 3 ) and W is the weight (g) of dried carbon.

Screening adsorption tests, as well as kinetic and equilibrium assays were performed in the experimental conditions summarized in Table 1 (mass, contact time, initial concentration, volume). Given data correspond to the average value obtained in three independent assays. Kinetic data were fitted to the linear forms of the pseudo-first and pseudo-second order kinetic models [START_REF] Ho | Review of second-order models for adsorption systems[END_REF] while the equilibrium adsorption data were analysed considering the non-linear equations of the Langmuir [START_REF] Langmuir | The adsorption of gases on plane surfaces of glass, mica and platinum[END_REF] and Freundlich [START_REF] Freundlich | Over the adsorption in solution[END_REF] isotherm models.

Results and discussion

Characteristics of the acid-chars

The use of different concentrations of H 2 SO 4 in the digestion and polycondensation steps allowed to attain yields between 35-15 % (Figure 1(a)), being the highest value slightly higher than that reported by Wang et al. [START_REF] Wang | A new route for preparation of hydrochars from rice husk[END_REF] for rice husks in similar experimental conditions. As seen, the concentration of sulphuric acid must be of at least 12 M in the digestion step, and 9 M in the polycondensation to reach yields above 30 %;

otherwise the yield dropped abruptly to 15 %. The elemental analysis data (Table S1) confirmed the enrichment in carbon content and the decrease in the oxygen content of the acid-chars, compared to the precursor. Despite the differences in the yields, the composition of the acid-chars was quite similar (i.e., 63 wt.% C, 4 wt.% H, 0.1 wt.% N, 0.23-0.41 wt.% S, and 32 wt.% O) and close to that of the rice husk-derived chars reported in the literature using a similar procedure [START_REF] Wang | High surface area porous carbons prepared from hydrochars by phosphoric acid activation[END_REF].

The use of different H 2 SO 4 concentrations seemed to have a small influence in the thermal stability of the samples, as inferred from the thermogravimetric profiles (Figure 1(b)). The TG/DTG curves of the acid-chars present two weight loss steps: the first peak appearing below 150 ºC is attributed to moisture loss, while the second one between 200 and 800 ºC corresponds to the main mass loss (ca. 50 %), linked to evolution of the volatile matter remaining in the acid-chars. The pattern of the second peak features a shoulder at around 280 ºC corresponding to the decomposition of labile (acidic) groups;

the main loss at about 400 ºC can be assigned to the decomposition of more stable oxygen containing functionalities and also some sulphur groups [START_REF] Bandosz | Surface chemistry of activated carbons and its characterization[END_REF][START_REF] Ruiz | Surface heterogeneity effects of activated carbons on the kinetics of paracetamol removal from aqueous solution[END_REF]. These profiles are different from those of the pristine sisal fibres (S); the sharp peaks corresponding of the depolymerization of hemicelluloses, the cleavage of glucosidic linkages of cellulose and decomposition of cellulose oligomers into tars [START_REF] Andrade | Characterization of the different fractions obtained from the pyrolysis of rope industry waste[END_REF] are not observed in the acidchars. This confirms the polycondensation of these monomers during the acid treatment at 90 ºC.

Regarding surface chemistry, the DRIFT spectra of samples S13.5/13.5, S13.5/9 and S9/9 (Figure 1(c)) revealed similar profiles. The acid-chars present a broad band at 3500-3200 cm -1 , assigned to O-H stretching vibrations in surface hydroxylic groups [START_REF] Biniak | The characterization of activated carbons with oxygen and nitrogen surface groups[END_REF][START_REF] Fanning | A DRIFTS study of the formation of surface groups on carbon by oxidation[END_REF]. The peaks ~2920 cm -1 and ~2850 cm -1 are assigned, respectively, to symmetric and asymmetric C-H stretching vibrations of aliphatic moieties of the acid-chars [START_REF] Biniak | The characterization of activated carbons with oxygen and nitrogen surface groups[END_REF][START_REF] Yu | Sulfonated coal-based solid acid catalyst synthesis and esterification intensification under ultrasound irradiation[END_REF].

The peaks centred at 1755-1734 cm -1 and 1637-1624 cm -1 can be assigned to stretching vibrations of C=O moieties in various configurations (e.g., lactones, anhydrides, ester or carboxylic acid, and quinones) [START_REF] Biniak | The characterization of activated carbons with oxygen and nitrogen surface groups[END_REF][START_REF] Fanning | A DRIFTS study of the formation of surface groups on carbon by oxidation[END_REF]. The shoulder around 1580-1560 cm -1 and the peak at 1510 cm -1 are assigned to aromatic ring C=C stretching; this peak is red-shifted likely due to the high density of oxygen in the materials (~32 %, Table S1) [START_REF] Fanning | A DRIFTS study of the formation of surface groups on carbon by oxidation[END_REF]. The presence of aromatic ring C=C bonds points out the increase in the aromatization degree after the acid treatment. The peak at 1473-1454 cm -1 can be attributed to carboxylcarbonate structures [START_REF] Biniak | The characterization of activated carbons with oxygen and nitrogen surface groups[END_REF][START_REF] Fanning | A DRIFTS study of the formation of surface groups on carbon by oxidation[END_REF], and the band around 1250 cm -1 to ether structures (ether bridges between rings and C-O stretch in ether moieties) [START_REF] Fanning | A DRIFTS study of the formation of surface groups on carbon by oxidation[END_REF]. The peak at 955 cm -1 can also be assigned to C-O-C vibrations in cyclic anhydrides [START_REF] Fanning | A DRIFTS study of the formation of surface groups on carbon by oxidation[END_REF], while bands below 950 cm -1 (i.e. 905 cm -1 and 802 cm -1 ) are characteristic of out-of-plane deformation vibrations of C-H groups in aromatic structures [START_REF] Biniak | The characterization of activated carbons with oxygen and nitrogen surface groups[END_REF]. )

Infrared spectra also revealed the presence of sulphur moieties, in agreement with the sulphur content detected by elemental analysis. The band at 1119 cm -1 is assigned to stretching vibration in -SO 3 H and the one between 1034-1028 cm -1 to S=O stretching vibration. Both were more clearly observed in acid-char S9/9, and would contribute to the acidic nature of the acid-chars [START_REF] Yu | Sulfonated coal-based solid acid catalyst synthesis and esterification intensification under ultrasound irradiation[END_REF].

The acidic nature of the chars was confirmed by the low values of the pH PZC (about 2 pH units), regardless the H 2 SO 4 concentration used in the synthesis (Table S1). It should be highlighted that the presence of acidic oxygen groups is an important feature when envisaging further activation, as it is usually linked to a higher reactivity of the chars [START_REF] Jain | Hydrothermal conversion of biomass waste to activated carbon with high porosity: A review[END_REF][START_REF] Titirici | Chemistry and materials options of sustainable carbon materials made by hydrothermal carbonization[END_REF]. Additionally, it will allow a good dispersion and wettability of the materials in water, favouring the contact with the activating agent in the case of solution impregnation; this will be further discussed below.

According with the XRD patterns the acid-chars have an amorphous-like carbon structure, as evidenced by the broad and low-intensity diffraction peaks (Figure 1(d)).

The concentration of sulphuric acid has a low impact on the crystallinity of the samples, although, the samples prepared with the lower concentrations of acid presented lower structural order: (002) plane of graphite-like structures at ca. 20 º for S13.5/13.5 while at ca. 24º and with lower intensity for S9/9. The diffraction peak at 40 º is attributed to (101) crystalline planes of graphite in the microcrystalline regions of the carbon matrix.

The shift of both peaks compared to the values of graphite (ca. ~27 º (002) and ~45 º (101) ICDD 25-284 [START_REF]International Center for Diffraction Data. Power Diffraction File Alphabetical Index; International Center for Diffraction Data[END_REF]) is indicative of increased distance between crystalline plans.

This could be due to the presence of oxygen and sulphur groups, as evidenced by elemental analysis, DRIFT spectra data, and acidic pH PZC values of these samples (Table S1).

On the other hand, the concentration of sulfuric acid had a dramatic influence on synthesis yields, the morphology and apparent densities. In fact, digestion in H 2 SO 4

13.5 M rendered high density acid-chars, regardless the concentration of acid used in the polycondensation step. In contrast, the density of the samples digested in H 2 SO [START_REF] Ioannidou | Agricultural residues as precursors for activated carbon production -A review[END_REF] 12 M was found to follow a different trend with the concentration of acid used in the polycondensation step. High density materials were obtained when the polycondensation was carried out in H 2 SO 4 12 M, with the density decreasing sharply for lower concentration of sulfuric acid (from 600 to 150 kg m -3 , Figure 1(a)). This behavior is certainly related with the morphological changes observed by SEM analysis (Figure 1(e) and (f)). While sample S13.5/13.5 has a compact morphology formed of small primary particles fused onto a dense and rough material, sample S9/9 is composed of interconnected spheres with diameters of around 500 nm, that originate an aerogellike structure with an extended pore transport systems.

Summarizing, the selection of the concentration of H 2 SO 4 in both digestion and polycondensation steps controls the density of the acid-char, especially for values lower than 12 M. The higher yields obtained for the synthesis carried out in 12 M (or higher) H 2 SO 4 in both steps are related with a more effective extraction of the precursor's saccharic units, and a more extensive polycondensation reaction.

The N 2 adsorption data shows that the concentration of the acid does not influence the porosity of the acid-chars. All of them displayed type II isotherms (not shown) characteristic of materials with an incipient porosity [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]; accordingly, low surface area values (< 10 m 2 g -1 ) were obtained for all the acid-chars. However, important differences were observed in the porosity of the nanoporous carbons obtained after activation of the acid-chars (see discussion below).

Characteristics of the nanoporous carbons

The textural properties and apparent densities of the nanoporous carbons prepared by activation of the acid-chars are clearly dependent on the H 2 SO 4 concentrations used in the synthesis of the latter. Figure 2(a) shows the trends corresponding to samples prepared by activation using K 2 CO 3 at 800 ºC of various acid-chars.

As seen in Figure 2(b) all the carbons displayed type I isotherms according to the IUPAC classification [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF], indicating that they are essentially microporous materials.

Despite the similarities in chemical composition and stability of the acid-chars (Table S1 and Figure 1 To evaluate the effect of the contact method between the precursor and the activating agent, two samples S12/12 and S13.5/13.5 were prepared by activation using K 2 CO 3 by physical mixture and solution impregnation (series s). Whereas physical mixing rendered carbons with a moderate porosity development, impregnation from solution allowed the preparation of micro-/mesoporous solids, with mesopore volumes accounting for ca. 50 % of the total pore volume (Fig. 3(b) and Fig. S1). The isotherms of the carbons obtained from solution impregnation present H2(a) type hysteresis loops [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF][START_REF] Thommes | Physical adsorption characterization of nanoporous materials: progress and challenges[END_REF], indicating complex pore structures; the desorption branch of the hysteresis loop of carbon S13.5/13.5/C3(s) is steeper than the adsorption branch, pointing out to ink-bottle shape pores and pore-blocking effects. The relative pressure of the closing end of the hysteresis loop suggests cavitation-induced evaporation effects, characteristic of samples with narrow pore necks connecting the main mesopore cavities [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF][START_REF] Thommes | Physical adsorption characterization of nanoporous materials: progress and challenges[END_REF].

It should be pointed out that this methodology (activation of acid-chars) allowed to prepare carbons with different textural features than those obtained by chemical activation of sisal wastes themselves [START_REF] Mestre | Activated carbons from sisal waste by chemical activation with K 2 CO 3 : Kinetics of paracetamol and ibuprofen removal from aqueous solution[END_REF]. These are mainly microporous carbons [START_REF] Mestre | Activated carbons from sisal waste by chemical activation with K 2 CO 3 : Kinetics of paracetamol and ibuprofen removal from aqueous solution[END_REF][START_REF] Andrade | Visible light driven photooxidation of phenol on TiO 2 /Cu-loaded carbon catalysts[END_REF] while herein described materials displayed a higher porosity development in the full micro-mesopore range, despite presenting similar global preparation yields (ca. 16-18 % for two-step and 17 % for optimized direct activation [44]).

Regarding the effect of the activating agent, the material obtained by KOH solution impregnation displayed the highest total pore volume, and a mainly micropore network as inferred from the shape of the nitrogen adsorption isotherm (type I and absence of hysteresis loop). The analysis of the pore size distribution obtained by applying the 2D-NLDFT-HS method to the N 2 adsorption data (Fig. 3(c)) confirms the dependence of the pore network characteristics with the contacting method and the activating agent.

Physical mixing with K 2 CO 3 and solution impregnation with KOH rendered mainly microporous carbons, whereas the carbons prepared upon solution impregnation with K 2 CO 3 present a well-developed mesopore structure centered at about 7 nm. Regarding morphology, images in Fig. S2 reveal that the roughness of the acid-char S13.5/13.5 particles changed after activation, leading to particles with smooth and exfoliated sections certainly due to the matrix consumption during activation (activation yields around 50 %, Table 2). The XRD patterns of the activated materials (Fig. S3) reveal the expected patterns of amorphous carbons with a low structural order.

Compared to the acid-chars the XRD patterns of the nanoporous carbons present slightly more intense peaks at 2θ values of ca. 25 and 44º, corresponding to the reflections of carbonaceous materials. This is associated to an increase in the (cm 3 g -1 )

Pore width (nm) S13.5/13.5/H3(s) S13.5/13.5/C3(s) S13.5/13.5/C3 aromatization during the activation, and a low surface functionalization. In fact, both thermogravimetric analysis and infrared spectroscopy (Fig. S4) indicate that activation leads to a decrease in the surface functionalization (mass loss of ca. 15 wt.% for the activated carbon, compared to 56 wt.% for acid-char). Also pH PZC values of samples S13.5/13.5/C3(s) and S13.5/13.5/H3(s) are less acidic than that of pristine acid-char (respectively, 3.8 and 4.9 versus 1.7).

Figure 4 shows smooth surfaces for the carbons obtained by K 2 CO 3 activation using physical mixing of high density acid-chars (samples S13.5/13.5, S13.5/9 and S12/12).

For the low density acid-char (sample S9/9), the activation has less effect on the morphology (Figure 4). SEM images show that the impregnation in solution with K 2 CO 3 seems to originate less compact particles; this is in line with the 50 % decrease in the density in sample S13.5/13.5/C3(s) compared to S13.5/13.5/C3 (Fig. 2(a) and 3(a)), despite their similar activation yields (51 versus 45 %, Table 2). Activation with KOH leads to a pronounced morphological change, originating sponge-like particles that justify the very low apparent density of the final material (< 50 kg m -3 ). Major morphology changes upon KOH activation of chars have also been reported in the literature [START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: remarkable adsorbents for pharmaceutical compounds[END_REF][START_REF] Mestre | High performance microspherical activated carbons for methane storage and landfill gas or biogas upgrade[END_REF][START_REF] Marques | Apple tree branches derived activated carbons for the removal of β-blocker atenolol[END_REF], and can be justified by the distinct activation mechanisms between K 2 CO 3 and KOH. The KOH activation reactions start at 400 ºC and lead to the formation of gases and K-compounds, which together activate the precursor; at converse, the decomposition of K 2 CO 3 starts at 700-800 ºC, allowing a less extensive consumption of the matrix [START_REF] Wang | KOH activation of carbon-based materials for energy storage[END_REF]. This fact also justifies the lower activation and global yields obtained upon KOH activation (Table 2).

The main textural parameters obtained from the analysis of N 2 isotherms at -196 ºC of the nanoporous carbons are compiled in Table 2. With exception of sample S9/9/C3, all the synthesized carbons present a wide distribution of micropores. By means of KOH activation of high density acid-chars, or K 2 CO 3 activation of low density acid-chars, it is possible to attain surface areas higher than 1900 m 2 g -1 and total pore volumes of 1.0 cm 3 g -1 . Such carbons are mainly composed of supermicropores; this is likely associated to the low activation and global yields (ca. 20 % and 3 %, respectively) and the low densities (< 100 kg m -3 ).

The activation with KOH at 700 ºC allows higher yields but lower supermicropore volumes and still low density carbons. Compared to the carbons prepared by physical mixing, higher pore volumes and a well-developed network of mesopores are obtained upon solution impregnation with K 2 CO 3. Furthermore, the activation and global yields are similar in both methods, pointing out that solution impregnation allows a more uniform and deeper penetration of the activating agent on the particles.

The microporosity was further analysed through CO 2 adsorption isotherms. The micropore size distributions presented in Figure 5 reveal that the carbons prepared by physical mixing of high density acid-chars with K 2 CO 3 have narrow micropore distributions in the ultramicropore range, thus showing molecular sieve properties. The carbons prepared by solution impregnation with K 2 CO 3 or KOH present monomodal and continuous distributions in the range from 0.4 to 2.0 nm. Additionally, sample S9/9/C3 have bimodal distribution of micropores. It is important to note that the commercial carbon used as benchmark for the liquid adsorption assays also presented a bimodal distribution of micropores.

Interestingly, the surface area values follow an inverse correlation with the apparent densities of the nanoporous carbons; this allows to identity three major groups of samples linked with the properties of the acid-char, the contacting methodology and the activating agent. As seen in Figure 6, a fine control of the synthesis parameters allows to prepare low density superactivated carbons (> 1800 m 2 g -1 ) and high density materials with surface areas close to 800 m 2 g -1 . V α total (cm 3 g -1 )

V α ultra (cm 3 g -1 )

V α super (cm 3 g -1 )

W DR N2 (cm 3 g -1 )

W DR CO2 (cm 3 g -1 )

S13.5/13. The activation of high density acid-chars with K 2 CO 3 by solution impregnation also rendered materials with high surface areas ( 1400 m 2 g-1 ), a well-developed micromesopore network, high activation and global yields (ca. 46-51 wt.% and 16-18 wt.%, respectively) and apparent densities close to those of commercial samples. It must be highlighted that such combination of porous features is not common by K 2 CO 3 activation of low-ash content precursors.

Apparent density (kg m -3

)

Ibuprofen and iopamidol adsorption onto selected nanoporous carbons

Based on screening assays, four nanoporous carbons were selected for the removal of ibuprofen and iopamidol (Figure 7), two active pharmaceutical ingredients commonly detected in wastewater effluents and drinking water. The removal efficiencies after 18 h of contact time are presented in Fig. S5. Samples S13.5/13.5/C3(s), S13.5/13.5/H3(s) and S9/9/C3 attained similar ibuprofen removal efficiencies in the range 40-45 %, as opposed to the low uptake of carbon S13.5/13.5/C3 (ca. 25 %). For iopamidol, the removal efficiencies of the selected carbons ranged between 10-74 %, following the sequence: S13.5/13.5/H(s) > S9/9/C3 > S13.5/13.5/C3(s) > S13.5/13.5/C3. The lower uptake of carbon S13.5/13.5/C3 for both compounds can be explained by its textural features (Table 2), therefore no additional studies were performed for this sample.

Ibuprofen [START_REF] Mestre | Activated carbons prepared from industrial pre-treated cork: Sustainable adsorbents for pharmaceutical compounds removal[END_REF] Iopamidol [START_REF] Mestre | Influence of activated carbons porous structure on iopamidol adsorption[END_REF] 1. Structure and dimensions of ibuprofen sodium salt [START_REF] Mestre | Activated carbons prepared from industrial pre-treated cork: Sustainable adsorbents for pharmaceutical compounds removal[END_REF] and iopamidol species [START_REF] Mestre | Influence of activated carbons porous structure on iopamidol adsorption[END_REF] (critical dimensions highlighted in bold).

Based on their distinct porous features, samples S13.5/13.5/C3(s) and S13.5/13.5/H3(s)

were selected for the kinetic and equilibrium assays, along with the commercial activated carbon (NS). Data presented in Figure 8 show that sample S13.5/13.5/H3(s) outperformed the commercial carbon (both in kinetic and equilibrium assays) for both compounds, whereas the comparative balance for carbon S13.5/13.5/C3(s) vs the commercial adsorbent was favourable only for ibuprofen. These results will be discussed in terms of the combined effect of porous features and surface chemistry of the materials.

Ibuprofen Iopamidol

C

e (mg dm -3

)

Kinetic data was fitted to the pseudo-second order kinetic equation, with coefficients of determination higher than 0.98 (Fig. 8). With the exception of the adsorption of iopamidol on sample S13.5/13.5/H3(s), all the systems reached the equilibrium in about 3 h. According to the pseudo-second order kinetic parameters for ibuprofen adsorption (Table 3) the three carbons show global adsorption rates of the same order of magnitude, although slightly lower k 2 values (fast rate) were obtained for the carbons containing mesopores (samples S13.5/13.5/C3(s) and NS), despite the similar uptakes.

The initial adsorption rate is 4.5 or 6.5 times faster in sample S13.5/13.5/H3(s) than in S13.5/13.5/C3(s) and NS (respectively), pointing out the key role of the larger micropores. These results are in agreement with previous data reported in the literature for the adsorption of small pharmaceutical compounds (i.e. atenolol, caffeine, ibuprofen acid form) on activated carbons [START_REF] Marques | Apple tree branches derived activated carbons for the removal of β-blocker atenolol[END_REF][START_REF] Galhetas | Chars from gasification of coal and pine activated with K 2 CO 3 : Acetaminophen and caffeine adsorption from aqueous solutions[END_REF][START_REF] Mestre | Activated carbons for the adsorption of ibuprofen[END_REF].

In the case of iopamidol, the highest global adsorption rate was also attained by sample S13.5/13.5/H3(s). The different iopamidol uptakes at kinetic equilibrium attained by the three adsorbents limits the comparison and analysis of their initial adsorption rates since this kinetic parameter depends on both the global adsorption rate and the uptake.

The adsorption rate of S13.5/13.5/C3(s) and NS for both pharmaceutical compounds is similar. Sample S13.5/13.5/H3(s) has the highest uptake for both pollutants, in mmol per gram the uptake of ibuprofen being almost twice than that of iopamidol. Compared to the other two samples, carbon S13.5/13.5/H3(s) has at least twice the volume of supermicropores, and a continuous micropore size distribution (Figure 5). This seems to favour both the adsorption rate and the uptake of ibuprofen and iopamidol. Similar results have been reported for the adsorption of paracetamol and iopamidol in sucrosederived carbons in similar experimental conditions [START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: remarkable adsorbents for pharmaceutical compounds[END_REF].

Table 3. Pseudo-second order ibuprofen and iopamidol adsorption parameters at 30 ºC for carbons S13.5/13.5/C3(s), S13.5/13.5/H3(s) and commercial sample (NS): k 2 is the pseudosecond order rate constant; h is the initial adsorption rate; t 1/2 is the half-life time; q e,calc and C e,calc are respectively, the PhC uptake and that remaining in solution at equilibrium both calculated by the pseudo-second order kinetic model.

Sample k 2 (g mg

-1 h -1 ) R 2 h (mg g -1 h -1 ) t 1/2 (h) q e,calc
C e,calc (mg dm -3 ) (mg g -1 ) (mmol g -1 ) a Ibuprofen S13. [START_REF] Yahya | Agricultural bio-waste materials as potential sustainable precursors used for activated carbon production: A review[END_REF] Confronting the micropore size distributions (Figure 5) with the critical dimensions of the pollutants (smallest section of the most stable configuration computed in aqueous solution, highlighted in bold in Figure 7), it can be concluded that ibuprofen molecule (critical dimension -0.72 nm) [START_REF] Mestre | Activated carbons prepared from industrial pre-treated cork: Sustainable adsorbents for pharmaceutical compounds removal[END_REF] can be easily adsorbed on the three carbons, while iopamidol (critical dimensions of 0.6, 1.2 or 1.5 nm, for the monomer, dimer or trimer, respectively) [START_REF] Mestre | Influence of activated carbons porous structure on iopamidol adsorption[END_REF] presents diffusional constrains for most of them, with the exception of sample S13.5/13.5/H3(s) due to its continuous micropore size distribution. It is interesting to note that carbon S13.5/13.5/C3(s) presents the lowest iopamidol uptake;

this could be explained by the negligible contribution of micropores wider than 1.2 nm, that would allow the adsorption of iopamidol monomers but not dimers and trimers.

Additionally, the mesoporosity of this carbon is not capable of accommodating the bulky iopamidol aggregates (dimers, trimers or even bigger aggregates), due to the average mesopore size (Fig. S6) and the above-mentioned pore-blocking effects (Fig.

3(b)

). As a result, iopamidol removal on carbon S13.5/13.5/C3(s) does not follow the expected correlation with V  super + V meso reported in the literature [START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: remarkable adsorbents for pharmaceutical compounds[END_REF][START_REF] Mestre | Influence of activated carbons porous structure on iopamidol adsorption[END_REF]. It should also be pointed out that in the case of carbon S13.5/13.5/C3(s) the concentration of iopamidol remaining in solution at equilibrium is higher than 100 mg dm -3 ; according to molar conductivities measurements [START_REF] Mestre | Influence of activated carbons porous structure on iopamidol adsorption[END_REF], this is indicative of the presence of iopamidol molecular aggregates that would not accommodate in the micropores of this sample (Fig. 5). Bearing this in mind, it can be inferred that the high iopamidol uptake on samples S13.5/13.5/H3(s) and NS is favoured due to the presence of wider micropores that allow to achieve lower equilibrium concentration in solution (ca. 23 and 77 mg dm - 3 , respectively) preventing the formation of molecular aggregates in solution.

The equilibrium adsorption isotherms presented in Figure 8 are characterized by an initial rise followed by a slight continuous increase or a plateau depending on the system. The experimental data were fitted to the non-linear forms of the Langmuir [START_REF] Langmuir | The adsorption of gases on plane surfaces of glass, mica and platinum[END_REF] and Freundlich [START_REF] Freundlich | Over the adsorption in solution[END_REF] equations; the fitting parameters, the adjusted coefficients of determination and reduced chi-square values are presented in Table 4.

In the case of ibuprofen, the three carbons present similar F-type curves, with ibuprofen uptake of sample S13.5/13.5/H3(s) being twice as big as that of carbon NS in the range of assayed concentrations. This is an agreement with the total micropore volumes of the adsorbents. F-type isotherms are indicative of multilayer adsorption in the sample, which would allow higher adsorption capacities with increasing the ibuprofen concentration in solution [START_REF] Freundlich | Over the adsorption in solution[END_REF].

For iopamidol all the carbons show a steep initial rise in the isotherm, characteristic of a high affinity between the adsorbate and adsorbent. As in the case of ibuprofen, sample S13.5/13.5/H3(s) stands out with a remarkably high adsorption capacity whereas sample S13.5/13.5/C3(s) attains half the adsorption capacity of the commercial carbon NS.

Comparing the isotherms obtained for both PhCs, it is clear that iopamidol has higher affinity for tested adsorbents than ibuprofen.

Regarding equilibrium data (Table 4) the adjusted coefficients of determination (R

Reduced . Langmuir parameters: q m -monolayer adsorption capacity, K L -Langmuir constant.

Freundlich parameters: K F -Freundlich constant (mg 1-1/n (dm 3 ) 1/n g -1 ), n -Freundlich exponent. It should be mentioned that the water solubilities of ibuprofen sodium salt (ca. 100 000 mg dm -3 ) and iopamidol (ca. > 200 000 mg dm -3 ) [START_REF] Mestre | Activated carbons prepared from industrial pre-treated cork: Sustainable adsorbents for pharmaceutical compounds removal[END_REF]) are much higher than the range of concentrations used in this study. Thus, this parameter is not expected to affect the different adsorption trends in the adsorption of these compounds.

Sample

Adsorption capacities are also expressed in mmol of PhC per gram of carbon (Table 5), to facilitate data comparison due to the differences in the molecular weights of ibuprofen and iopamidol. Using these units, it can be observed that carbon S13.5/13.5/H3(s) presents similar uptake of ibuprofen and iopamidol, despite the lower dimension of the former. This has to be attributed to the anionic character of ibuprofen at neutral pH, which would favor the interactions with water while decreasing affinity for adsorption.

Table 5. Amount of pharmaceutical compounds adsorbed for an equilibrium concentration in solution (C e ) of 90 mg dm -3 . Values calculated from the fitting of the equilibrium adsorption data to Freundlich equation.

Sample q 90 mg dm-3 Ibuprofen q 90 mg dm-3 Iopamidol (mg g -1 )

(mmol g -1 ) a (mg g -1 ) (mmol g -1 ) a S13. Ibuprofen uptake (in mmol g -1 ) of S13.5/13.5/C3(s) is 3-times larger than that of iopamidol, most certainly due to diffusional constrains in the monomodal micropore network of this carbon centered at around 0.6-0.7 nm, which only allows the adsorption of iopamidol monomers and ibuprofen ions. The low iopamidol adsorption capacity (in mmol g -1 ) of S13.5/13.5/C3(s) compared to S13.5/13.5/H3(s) and carbon NS demonstrates that when dealing with the adsorption of bulky molecules, it is essential to characterize the mesopore network. In fact, the complex mesopore network of sample S13.5/13.5/C3(s) would hinder the adsorption of iopamidol species, due to the sieving effects in the ink-bottle shape mesopores.

The effect of surface chemistry in the adsorption process must also be considered, particularly considering the different ionic state of ibuprofen and iopamidol. At the conditions of herein reported adsorption studies (aqueous solutions at pH 5) the surface of carbon S13.5/13.5/C3(s) is slightly negatively charged and ibuprofen is an anion, thus the results seem to point out that - interactions predominate over electrostatic ones. Similarly, iopamidol is a neutral molecule thus the role of electrostatic interactions can be disregarded. In the case of the commercial carbon, the surface is slightly positively charged (pH PZC of 8.4) and it presents a larger fraction of micropores compared to carbon S13.5/13.5/C3(s). Its slightly lower ibuprofen adsorption capacity thus confirms the hypothesis of a higher contribution of - interactions over electrostatic ones in the adsorption of ibuprofen.

Conclusion

We herein propose a two-step method for the valorization of a low density biomass waste (sisal, Agave sisalana) through the synthesis of acid-chars with varied density and morphology. The oxygen-rich surface and acidic nature of the acid-chars contributes to increase their reactivity during the activation step, making them excellent precursors for the preparation of high density nanoporous carbons with tailored pore structure in the full micro-mesopore range. This approach rendered carbon materials with better porous development when compared to the chemical activation of the same biomass precursor by traditional methods [START_REF] Mestre | Activated carbons from sisal waste by chemical activation with K 2 CO 3 : Kinetics of paracetamol and ibuprofen removal from aqueous solution[END_REF][START_REF] Andrade | Visible light driven photooxidation of phenol on TiO 2 /Cu-loaded carbon catalysts[END_REF]. Furthermore, these adsorbents outperformed nanoporous carbons commercialized for water remediation purposes, in terms of adsorption capacity and rate of two pharmaceutical compounds (ibuprofen and iopamidol). Both the acid-chars and their corresponding activated carbons display interesting textural, morphological and surface properties that make them promising materials for other applications in the fields of energy storage, catalysis, and as electrode materials or supports of (bulky) molecules. In this regard, so far acid-chars have proven to be effective supports for the immobilization molybdenum catalysts for solventless olefins epoxidation [START_REF] Petit | Sisal derived acid-char as support of Mo(II) complex for solventless olefin epoxidation[END_REF].

Figure 1 .

 1 Figure 1. (a) Effect of H 2 SO 4 concentration in the digestion and polycondensation steps on the yields and tapped densities of the acid-chars; (b) thermogravimetric profiles (TG) and derivative (DTG) curves of selected samples; (c) DRIFT spectra and (d) XRD patterns of selected acidchars. SEM images of samples (e) S13.5/13.5 and (f) S9/9 (a colour version of this figure can be viewed online).

Figure 2 .

 2 Figure 2. Properties of the activated carbons prepared by physical mixing with K 2 CO 3 (3 g K 2 CO 3 per gram of acid-char) followed by activation at 800 ºC during 1 h (a) micro and mesopore volumes and the influence of acid-char precursor in apparent density of the activated carbons is also displayed, (b) N 2 adsorption isotherms (a colour version of this figure can be viewed online).

Figure 3 .

 3 Figure 3. (a) Micro and mesopore volumes of the activated carbons prepared by solution impregnation of acid-chars S12/12 and S13.5/13.5 with K 2 CO 3 or KOH (3 g activating agent per gram of acid-char) followed by activation at 800 ºC during 1 h. The influence of acid-char precursor in apparent density of the activated carbon is also displayed; (b) N 2 adsorption isotherms of activated carbons prepared from acid-char S13.5/13.5; (c) Cumulative pore size distributions obtained from the 2D-NLDFT-HS method applied to the N 2 adsorption isotherms (A colour version of this figure can be viewed online).

Figure 4 .

 4 Figure 4. SEM images of the nanoporous carbons. The insets correspond to higher amplifications to better illustrate the surface of the materials.

a

  Activation Yield = (m nanoporous carbon / m acid-char )100 b Global Yield = (m nanoporous carbon / m sisal )100 c Evaluated at p/p 0 = 0.975 in the N 2 adsorption isotherms at -196 ºC d Difference between V total and V α total

Figure 6 .

 6 Figure 6. Correlation between A BET and apparent density of the synthesized nanoporous carbons (squares) and the commercial sample (circle).

  32 (length) × 0.72 (width) × 0.77 (thickness) in nm Monomer -1.5 (length) × 1.5 (width) × 0.6 (thickness) in nm Dimer -1.5 (length) × 1.5 (width) × 1.2 (thickness) in nm Trimer -1.5 (length) × 1.5 (width) × 1.8 (thickness) in nm

Figure 7 .

 7 Figure 7. Structure and dimensions of ibuprofen sodium salt[START_REF] Mestre | Activated carbons prepared from industrial pre-treated cork: Sustainable adsorbents for pharmaceutical compounds removal[END_REF] and iopamidol species[START_REF] Mestre | Influence of activated carbons porous structure on iopamidol adsorption[END_REF]

Figure 8 .

 8 Figure 8. Kinetic results (top) and adsorption isotherms (down) of ibuprofen (left) and iopamidol (right) adsorption at 30 ºC. Symbols correspond to the experimental data while lines represent the fitting to the pseudo-second order kinetic model (top) and the fitting to the Freundlich equation (bottom). Error bars are included. Experimental conditions of the kinetic assays 6 mg carbon/30 cm 3 of pharmaceutical solution with 180 mg dm -3 . (A colour version of this figure can be viewed online).

  a Calculated considering the molecular weight of ibuprofen anion and iopamidol monomers.

Table 1 .

 1 Experimental conditions and activated carbons used in the screening, kinetic and equilibrium adsorption assays for the removal ibuprofen and iopamidol.

	Assay	Activated carbons	m AC (mg)	V PhC (cm 3 )	Conditions [PhC] 0 (mg dm -3 )	Contact time

Table 2 .

 2 Textural properties of the nanoporous carbons, activation and global yields.

  Micropore size distributions of the physically (left) and solution impregnated as well as commercial sample (right) obtained by fitting the CO 2 adsorption isotherms at 0 ºC to the method described by Pinto et al.[START_REF] Pinto | Comparison of Methods to Obtain Micropore Size Distributions of Carbonaceous Materials from CO 2 Adsorption Based on the Dubinin-Radushkevich Isotherm[END_REF]. (A colour version of this figure can be viewed online).

	Figure 5. Acid-char	Contacting	Activating
						density	methodology	agent
						High		Solution	KOH
						Low		Physical	K 2 CO 3
						High		Solution	K 2 CO 3
						Low		Physical	K 2 CO 3
						High		Physical	K 2 CO 3
	5/C3	45	15	781	0.33	0.02	0.31	0.10	0.21	0.32	0.34
	S13.3/13.5/C3(s)	51	18	1419	0.89	0.45	0.44	0.22	0.22	0.56	0.45
	S13.5/13.5/H3(s)	20	7	2309	1.03	0.07	0.96	0.17	0.79	0.87	0.50
	S13.5/13.5/H3(s/700)	33	11	1906	0.82	0.07	0.75	0.29	0.46	0.75	-
	S13.5/9/C3	45	14	602	0.25	0.01	0.24	0.13	0.11	0.24	0.33
	S12/12/C3	50	18	818	0.34	0.01	0.33	0.19	0.14	0.33	0.21
	S12/12/C3(s)	46	16	1372	0.72	0.25	0.47	0.26	0.21	0.52	-
	S12/9/C3	43	13	1391	0.63	0.09	0.54	0.33	0.21	0.53	-
	S9/9/C3	20	3	1938	1.00	0.13	0.87	0.00	0.87	0.70	0.51
	NS	-	-	1297	0.70	0.30	0.40	0.02	0.38	0.39	0.27

Table 4 .

 4 Fitting parameters to the Langmuir and Freundlich models and chi-square test analysis,

	

Acknowledgements

This work was developed under the financial support of projects UID/MULTI/00612/2013 (CQB) and UID/ QUI/50006/2013 -POCI/01/0145/FERDER/007265 (REQUIMTE) from FCT/MEC through national funds and co-financed by FEDER, under the Partnership Agreement PT2020. ASM thanks FCT for a Post-doctoral grant SFRH/BPD/86693/2012. The authors thank Cordex for supply of the sisal residues, Salmon & Cia for the supply of carbon NS, and Hovione for the supply of iopamidol.