María C Fernández De Cordoba

Juan Matos
email: j.matos@udt.cl

Ricmary Montaña

Po S Poon

Silvania Lanfredi

Fabiano R Praxedes

Juan C Hernández-Garrido

José J Calvino

Elena Rodríguez- Aguado

Enrique Rodriguez-Castellón

Conchi O Ania
email: conchi.ania@cnrs-orleans.fr

Sunlight photoactivity of rice husks-derived biogenic silica

Keywords: Biogenic silica, rice husks, photocatalysis, rhodamine B, artificial solar light

Carbon-containing SiO 2 -based photocatalysts were prepared by the solvothermal treatment of rice husk as biogenic precursor in the presence and absence of TiO 2 and used for the photocatalytic degradation of rhodamine-B under simulated solar light. Data showed that the prepared catalysts are mainly composed of biogenic silica and displayed mesoporous character with surface areas ranging from 65 to 174 m 2 g -1 . The obtained materials showed photocatalytic activity comparable to that of commercial TiO 2 -P25 powders for the degradation of rhodamine-B under sunlight. The calcination at 350 ºC improved the photocatalytic activity of the biogenic precursor by three times in the absence of TiO 2 . At converse, calcination of sample SiO 2 -TiO 2 decreased the photoactivity due to the appearance of non-photoactive TiSiO 4 crystalline phases in the catalyst, as inferred by XRD and XPS. A reaction mechanism for rhodamine-B degradation excluding the deethylation pathway has been proposed, based on the evolution of the absorbance spectra of rhodamine-B upon the photocatalytic tests.

Introduction

TiO 2 is considered the best semiconductor photocatalyst for the degradation of aromatic molecules in water due to several advantages such as a high photostability under UV irradiation, resistance to strong acid/bases, inertness and relatively low cost [START_REF] Serpone | Photocatalysis: fundamentals and applications[END_REF][START_REF] Herrmann | [END_REF]. However, it has important scale-up limitations such as the high and pH-dependent recombination rate, a low absorption of solar light, and the low to moderate surface area of most commercial TiO 2 -based photocatalysts, which limits their application to low effluent volumes and low pollutants concentration [START_REF] Linsebigler | Photocatalysis on TiO 2 surfaces: Principles, mechanisms and selected results[END_REF][START_REF] Kaneko | Photocatalysis: science and technology[END_REF]. It has been extensively reported than most of these limitations can be overcome by using various co-supports such as alumina [START_REF] Gómez | [END_REF], silica [6,7], and activated carbon [8][9][10].

Among them, it been reported that the photocatalytic activity of TiO 2 can be enhanced by introducing SiO 2 , as the titania-silica (TiO 2 -SiO 2 ) mixed oxides benefit from the photocatalytic activity of titania, and the high mechanical and thermal stability of silica [11][12][13][14]. The photocatalytic activity of TiO 2 /SiO 2 catalysts, being strongly dependent on many synthetic parameters, may be more or less photoactive than titanium dioxide powders, but they usually display better sedimentability properties, thus enabling the separation of the catalyst after the photocatalytic treatment [11][12][13][14]. Furthermore, as the production of silica can be cheaper than that of titania, the costs of the photocatalysts can be significantly reduced.

In this context, biogenic (i.e., natural-resources-based) silica is receiving an increasing attention in different fields covering wastewater treatment [15], biomedicine [16], energy storage in batteries [17] and catalysis [18], due to the availability, low cost and sustainability of these materials. However, scarce studies have focused on its potentialities in solar photocatalysis due to the low sunlight absorption coefficient of most silica-based materials [19,20].

Keeping this in mind, the main objective of this work was to prepare biogenic silica and silica-TiO 2 hybrid catalysts from an abundant biomass waste, and to evaluate their photocatalytic activity for the degradation of a recalcitrant dye in aqueous medium under artificial solar light.

Rice husk, an agro-waste product largely generated around the world has been used as natural source for the fabrication of the silica-based materials, as is contains ca. 15-20% of inorganic (SiO 2 ) matter. On the other hand, Rhodamine B was chosen as target molecule, as it is a toxic dye, one of the mostly used fluorophores in biotechnology applications [21], and also widely used in cosmetic products. Being a toxic and largely used compound its photocatalytic degradation on various semiconductor-based photocatalysts has been extensively reported [22][23][24][25][26].

Experimental

Synthesis of silica-based materials

Analytical grade titanium (IV) isopropoxide and Rhodamine B (RhB) were purchased from Sigma-Aldrich. Absolute ethanol was purchased from Merck. Rice husk was provided by a Chilean agroindustry company, and it was used as received. The catalysts were prepared by solvothermal treatment of ca. 0.5 g of crushed and sieved rice husks (sample RH) in 9 mL of absolute ethanol in a Teflon-lined autoclave heated at 180 ºC for 16 h. The obtained solid was filtered and washed several times in absolute ethanol and dried under static-air at 100 ºC for 2 h.

The resulting catalyst was denoted as RH-Si. A hybrid catalyst was prepared by adding ca. 0.5 g titanium isopropoxide to the RH suspension in ethanol before the solvothermal treatment (sample RH-Si-Ti). Additionally, a TiO 2 catalyst was synthesized following the solvothermal procedure in the absence of RH (sample TiO 2 synth). Calcination of the catalysts was carried out in a muffle at 350 °C for 2 h under static air; calcined samples were denoted RH-Si-350 and RH-Si-Ti-350.

Commercial TiO 2 (P25, Evonik) and amorphous SiO 2 (Sigma-Aldrich) were also used for comparative purposes.

Characterization of materials

The texture of the materials was characterized by measuring the N 2 adsorption-desorption isotherms at -196 ºC (Micromeritics equipment). The samples were previously outgassed under dynamic vacuum (ca. 10 -5 Torr) at 200ºC for 17 h. Ultrahigh purity gas (i.e., 99.9992 for N 2 ) was supplied by Air Products. The isotherms were used to evaluate the specific surface area (BET equation), the total pore volume (evaluated at a relative pressure of 0.99) and the micropore volume from the Dubinin-Radushkevich formulism [START_REF] Rouquerol | Adsorption by Powders and Porous Solids: Principles, Methodology and Applications[END_REF].

The surface pH of the materials was measured as the pH of a suspension of the catalysts in distilled water (ca. 50-5 mg of solids in 20 mL) under stirring at 800 rpm [START_REF] Matos | [END_REF]. The pH of the suspension was measured until a constant value (ca. 1-2 h) and plotted as a function of the amount of catalyst in the suspension. The surface pH was considered as the value at the plateau of the representation.

The carbon content in the catalysts was estimated from gravimetric analysis. Chemical bonds were studied by Fourier transformed infrared spectroscopy (FTIR). The sample was diluted in KBr (ratio of 1:100) and the measurements were carried out in the range of 4000-400 cm -1 with a resolution of 4 cm -1 for 100 scans (Shimadzu, IR Affinity-1).

X-ray diffractograms on powder samples were obtained using a Shimadzu (model XRD-6000) diffractometer, with Cu-K1 radiation (k = 1.54056 Ǻ) provided with a graphite monochromator and operating at 40 kV and a current of 30 mA. Measurements were carried out over an angular range of 5-80° with a scanning step of 0.02° and a fixed counting time of 10 s. Divergence, scattered and receiving radiation slits were 1 o , 1 o and 0.2 mm respectively. From XRD measurements, a Crystallinity Index (CI) was calculated as proposed by Focher and coworkers [29] using the equation ( 1):

Crystallinity index (%) = I 111 -I am I 111 x100 ( 1
)
where I 111 is the maximum intensity value for the scattering angle of silica phase at 2 of 22º and I am is the minimum intensity value of the amorphous diffraction peak at 17º.

The optical features were evaluated from UV-Vis diffuse reflectance spectroscopy, recorded on a spectrophotometer equipped with an integrating sphere, and using BaSO 4 as a reference.

Absorbance and reflectance measurements were carried out between 220 and 700 nm and transformed to a magnitude proportional to the extinction coefficient through the Kubelka-Munk function, F(R∞). The energy band gaps (E g ) were estimated from the Tauc representation using a double linear fitting.

The morphology and microstructure of the samples was studied by scanning electron microscopy (SEM) and energy dispersed X-ray (EDX). SEM images were acquired in a (FEG) SEM Nova NanoSEM 450, operated at 24 kV, with integrated EDX system (EDAX TEAM TM ) to perform elemental composition analysis on the targeted region of interest.

The surface atomic composition of samples was analysed by X-ray photoelectronic spectroscopy

Adsorption studies and photocatalytic tests

Rhodamine-B (RhB) was selected as recalcitrant dye to carry out the adsorption and photocatalytic degradation tests, using UV-visible spectroscopy (554 nm) to follow the extent of the reaction. Initially, kinetics studies were performed to evaluate the adsorption capacity of the catalysts in the dark. For this, solutions of RhB of various initial concentrations ranging from 2 to 20 mg L -1 were put in contact with ca. photons m -2 s -1 ). The radiation intensity in the UV region was ca. 20 W m -2 measured with a radiant power meter, corresponding to a total radiation intensity of 250 W m -2 . The amount of photocatalyst in the suspension was constant (ca. catalyst loading of 1 g L -1 ) for all the samples.

However, for a proper comparison, equivalent amounts of commercial silica or titania matching the composition of the hybrid catalysts were also measured. The initial concentration of RhB in solution in the photocatalytic tests was adjusted based on the dark adsorption assays, to assure the same concentration in solution for all the catalysts (ca. 8.5 mg L -1 ) when the illumination was applied. All the experiments were performed at least in duplicates, and the accuracy of the experiments was found to be below 2%. Here, average data is presented.

Results and discussion

Characterization of the materials

Figure 1 shows a series of SEM images of the rice husk raw material after crushing and sieving.

The typical morphology of the biomass precursor is observed, characterized by an aligned cellular structure. Interestingly, the raw biomass displayed a well-ordered mountain-valleys microstructure (Fig. 1c), similar to that commonly reported for cross-linked polymerized hydrogels [30]. Figure 2 shows the SEM images corresponding to the catalysts prepared by the solvothermal treatment of the rice husks, so as to investigate the evolution of the morphology and microstructure of the precursor upon the modification. After the solvothermal treatment at 180°C (sample RH-Si), the morphology of the rice husks is preserved, with the mountain-valley morphology evolving to a flat surface (Fig. 2a,2b). After calcination at 350°C (sample RH-Si-350), the sample displayed an amorphous morphology (Fig. 2c,2d). For the case of Ti-containing samples (Fig. 2e,2f), the mountain-valley morphology of the precursor is preserved after the solvothermal treatment, indicating that the Ti-species do not have a strong influence in the morphology of SiO 2 -based material (Fig. 2e). A small cracking of the husks particles is also observed during the solvothermal treatment (Fig. 2f); for the Ti-containing sample after calcination (RH-Si-Ti-350) spherical (globular) particles are clearly observed (Fig. 2g,2h), likely suggesting some particles coalescence during calcination.

For a better understanding of the interaction between the biogenic-silica matrix and the Ticontaining particles, a SEM-EDX analysis was performed to sample RH-Si-Ti-350 (Fig. 3) in different regions (marked as 1 and 2). As seen, the spherical particles are embedded within the planar surface of the matrix (Fig. 3a,3b). The EDX spectrum of the different regions (Fig. 3c, 3d, 3g, 3h) shows the different composition: rich in Si and rich in Ti atoms. For instance, the planar particles in Figures 3b and3e (region 1) are mainly composed by Si atoms (ca. 19.6 and 3.3 at.% for Si, and Ti, respectively), while the spherical ones (region 2) are predominantly composed by Ti atoms (ca. 2.4 and 22.6 at.% for Si and Ti, respectively). In both regions, the almost similar and high oxygen content (ca. 77 and 75 at.%) is consistent with the presence of mixtures of SiO 2 and TiO 2 . Furthermore, the high Si composition in region 2 suggests the existence of a more complex crystalline phase such a TiSiO 4 (see discussion below). The chemical composition of the catalysts was further studied by XPS. Figure 4 shows the C 1s core level spectra of all four samples, while data corresponding to the raw rice husks is show in

Figure S1

in the Supplementary Material File. For all the samples, the C 1s core level spectra were deconvoluted to three main contributions: the intense peak at 284.9 eV was assigned to graphitic carbon in sp 2 configurations; the peak at 286.5 eV to C-O, C-H bonds (phenolic, alkoxy, ether), and the contribution featuring at 288.2 eV was assigned to O-C=O bond (carboxylic, ester groups). These functional groups are generally found in lignocellulosic materials [31] such as cellulose and biomass wastes.

Interestingly, important modifications were obtained upon the solvothermal and calcination treatments. RH-Si sample showed a marked contribution of the signal at 286.5 eV (Fig. 4a) assigned to carbonyl groups. This is in agreement with previous works from the literature reporting the large proportion of carbonyl groups for nanostructured materials prepared by solvothermal synthesis of biomass [32,33]. The effect was not so pronounced for sample RH-Si-Ti sample (Fig. 4c), suggesting that the incorporation of Ti avoids the formation of these groups and/or promotes its decomposition. The contribution of both carbonyls and carboxylic groups clearly decreases after calcination (Fig. 4b,4d). Table 1 shows a summary of the atomic concentration of the elements detected by XPS for the rice husks and the synthesized catalysts. Besides the expected elements such as C, N, O, Si and Ti, other elements such as N, Mg, and K were detected in some samples at very low atomic concentrations. Thus, no significant influence on the adsorption or the photocatalytic activity would be expected. After calcination, a remarkable decrease in the amount of carbon was observed for the samples regardless the presence of Ti. This is in agreement with the increase in oxygen. The effect was more pronounced for the non Ti-containing samples, which could be due to a higher interaction between carbon groups and Ti atoms. The increase in Si after calcination is also remarkable, with values 4 and 13 times higher for RH-Si-350 and RH-Si-Ti-350, respectively, than their corresponding non-calcined counterparts. The low Si amount of sample RH-Si-Ti should be pointed out; this suggests the presence of silica particles embedded in the matrix, that would diffuse to the surface upon calcination. In addition, the ratio Ti/Si is ca. 19.8 for RH-Si-Ti and ca. 2.2 for RH-Si-Ti-350, suggesting a strong interaction between Si and Ti atoms during the calcination.

This was confirmed by the analysis of the O 1s core level spectra of the samples (Fig. 5). As seen, the spectra of samples RH-Si and RH-Si-350 (Fig. 5a,5b) are practically similar, with a main peak at ca. 533.0 eV assigned to O-Si bonding in a SiO 2 structure [34]. In this case, calcination of sample (RH-Si-350) caused an increase in the intensity of the spectrum, indicating a better annealing of SiO 2 . In the presence of Ti, the O 1s core level spectra were deconvoluted to In sum, calcination of RH-Si-Ti sample yields a thermally stable material with nanocrystalline domains of various inorganic oxide frameworks. This complex structure and the binding energy assignments for the O 1s region of XPS spectra can be figure out by a super-cell Si 3 Ti 3 O 12

showed in the Fig. S2 (supplementary material). The formation of this ordered SiO 2 /TiO 2 complex structure suggest a kind of assembly mechanism similar than for the case of inorganic oxides/blocks copolymer assemblies reported by Yang and coworkers [35].

Figure 6a shows the N 2 adsorption-desorption isotherms at -196 ºC of the synthesized catalysts and the rice husk precursor (RH) before the solvothermal treatment. A summary of textural properties and physicochemical parameters of the samples is compiled in Table 2.

All the catalysts displayed a moderate porosity dominated by mesopores, with surface areas ranging from 65 to 174 m 2 g -1 (Table 1). The calcination treatment increased the surface area of the catalysts, with sample RH-Si-350 showing a BET surface area one order magnitude higher than that of RH-Si. In the presence of titania, the effect of the calcination on the surface area was less pronounced, accounting for an increase of ca. 2.7. The samples also displayed different carbon content, following the order: RH-Si-Ti-350 < RH-Si-350 < RH-Si-Ti ~ RH-Si. The carbon content was quite similar for the non-calcined samples regardless the presence of titania, suggesting that TiO 2 does not modify the decomposition of the RH in the solvothermal process.

The slight difference in the carbon content between RH-Si-Ti-350 and RH-Si-350 could be attributed to a more acidic pH in the Ti-containing sample, being responsible of a higher consumption of carbon atoms in the calcination. After calcination the surface pH of RH-Si became more basic, likely due to the elimination of functional groups of acidic nature [32] -typically observed after the solvothermal synthesis of biomass-and/or to the formation of metallic silicates. Figure 6b shows the FTIR spectra for the studied samples; a summary of the band positions and characteristics observed in the FTIR spectra are listed in Table S1 in the Supplementary File. The absorption bands in the rice husks spectra are attributed to physically adsorbed water, functional groups of cellulose, hemicellulose and lignin, as well as the inorganic components in their composition [36]. Samples RH, RH-Si and RH-Si-350 present an absorption band at 804 cm -1 attributed to the symmetric vibrations of the Si-O bonds in the silicon-oxygen tetrahedrons (SiO 4 ) [37,[START_REF] Sócrates | Infrared Characteristic Group Frequencies[END_REF]; the bands at around 465 cm -1 are assigned to Si-O-Si bending vibrations [START_REF] Ahmed | [END_REF]. The latter band was not clearly observed in the Ti-containing samples. The variations in the intensity of the peak at 1650 cm -1 -attributed to δ-H 2 O deformation and the presence of C=C bonds in organic components-are remarkable, with samples RH-Si and RH-Si-350 showing a lower intensity than RH-Si-Ti and RH-Si-Ti-350. This is indicative of changes in the polarity of the samples after the incorporation of Ti [START_REF] Ahmed | [END_REF], in agreement with their more basic (hydrophobic) character as inferred from the surface pH values (Table 2).

Figure 6c shows the XRD patterns of the studied samples; RH and RH-Si showed a diffraction line at 22 o related to amorphous SiO 2 , and another peak at 17 o associated with the remnant carbon matrix. RH-Si-350 sample showed a remarkable decrease in the crystallinity, represented by the broadening of the peak at 22 o . In the case of RH-Si-Ti, the diffraction bands related to the crystalline phase anatase of TiO 2 were observed, with an increase in the crystallinity after the calcination.

For RH-Si-Ti-350, the phase associated to SiO 2 was no longer observed, suggesting that the silica phase is embedded within the crystalline framework of TiO 2 [40]. This phenomenon would be associated with a redispersion of SiO 2 nanoparticles, since a similar behavior (i.e., decrease in the average crystallite size) was obtained after calcination of RH-Si (Table 2). Indeed, it seems reasonable that the particles of SiO 2 located nearby the TiO 2 ones would promote the formation of the TiSiO 4 phase observed by SEM, XPS and FTIR data as discussed above. The XRD pattern of the titanium silicate would overlap with that TiO 2 anatase phase (JCPDS card # 21-1272), the only phase detected in RH-Si-Ti-350.

The different crystallinity of the samples was quantified using the Crystallinity index (CI), calculated as the ratio between the intensity at 22 o and that at 17 o of silica bands, as proposed by Focher et al [29]. The obtained values (Table 2) indicate no modification of this parameter for RH and RH-Si, and a decrease for RH-Si-Ti, suggesting that the change of crystallinity is dependent on the presence of titanium. Data also showed that sample RH-Si-350 is an amorphous material, while for RH-Si-Ti-350, the CI cannot be evaluated since the silica bands were no longer observed.

Figure 6d shows the UV-visible absorbance spectra of the prepared catalysts along with commercial TiO 2 and SiO 2 . The spectrum of TiO 2 -P25 presented the characteristic absorption sharp edge of the anatase form of TiO 2 (predominant phase) in the UV region rising above 400 nm, rendering a band gap value of 3.09 eV, in agreement with the values reported for this material [START_REF] Serpone | Photocatalysis: fundamentals and applications[END_REF][START_REF] Herrmann | [END_REF][START_REF] Linsebigler | Photocatalysis on TiO 2 surfaces: Principles, mechanisms and selected results[END_REF][START_REF] Kaneko | Photocatalysis: science and technology[END_REF]. A remarkably high intensity in the absorbance spectra was observed in the visible range for all the biogenic silica-based samples compared to TiO 2 -P25 and SiO 2, which absorbance were negligible in the visible range. This can be attributed to the carbon content of the samples.

After calcination, RH-Si-350 absorbed more photons in the visible range than RH-Si. This could be associated to the larger surface area and lower average crystallite size of this sample, since both parameters have been associated with low light scattering features [41,42]. RH-Si-Ti-350 showed a different trend with a UV-vis spectrum with two-components of absorbance; the one in the UV region corresponds to the absorption cut-off of TiO 2 , with a slight red-shift compared to TiO 2 -P25. The second region in the visible light is associated to the absorption of the carbon phase itself. The corresponding energy band gap (E g ) value of and RH-Si-Ti-350 was 3.4 eV agrees with the value reported in the literature for TiSiO 4 materials based [43].

Adsorption studies

The equilibrium adsorption isotherms of RhB on all the studied catalysts are shown in Figure 7.

Previous kinetics studies showed that a 90 min of contact time was required for all the catalysts to attain the equilibrium (Fig. S3). The main adsorption parameters obtained from fitting the experimental data to Langmuir and Freundlich models are summarized in Table 3.

With the exception of sample RH-Si, data plotted in Figure 7 showed a good fitting to Freundlich model, with regression coefficients higher than 0.99 (Table 3). Fitting of experimental data to Langmuir model (Fig. S4) was not so good, with regression coefficients lower than 0.97 in most of cases. Given the better fitting and accuracy of Freundlich model to describe the experimental data, only the adsorption parameters derived from this model will be further discussed (Table 3).

As seen in Figure 7, the samples displayed quite different uptakes of the dye, with the highest uptake for RH-Si-350, about 15 times higher than that of RH-Si (the least adsorbing sample).

This could be attributed to either the higher surface area of the former, or the increase in the surface pH upon calcination (Table 2). No direct correlation between the RhB adsorption and the surface area was found for the Ticontaining samples, indicating the role of the specific interactions between RhB and the surface of the catalysts on the adsorption capacity. Despite the surface area of RH-Si-Ti-350 is higher than that of RH-Si-350, the adsorption capacity of the latter was clearly higher (Table 3),

suggesting that RhB is adsorbed on the basic sample via interactions between the electron-donor hydroxyl groups in the surface of SiO 2 with the cationic species formed in the diethylamino group in the zwitterion (RhB is a zwitterionic cationic dye, as can be seen from Figure S5 in the supplementary material, with a pKa of 3.1 units, being the zwitterion form predominant under our experimental conditions). A similar adsorption mode has been suggested by Chen and coworkers [44] on a negatively charged SiO 2 -TiO 2 composite. The higher K F in RH-Si-350 indicates that the adsorption is favored in basic conditions, where the zwitterionic form of the dye is predominant in solution. In contrast, RhB uptake in the TiO 2 -containing samples was practically the same after calcination, despite their differences in surface area and pH (Table 2). Freundlich heterogeneity parameter is higher than unity for all the samples, indicating a high surface heterogeneity. The effect is more pronounced after calcination, likely due to a higher exposition of electron-donor groups from biogenic silica and to the increase in surface area [45]. f linear regression coefficient for fitting to Freundlich model.

Photocatalytic tests

Figure 8 shows the photocatalytic degradation of RhB on the different photocatalysts, compared to the photolysis of the dye under simulated solar light. The performance of commercial SiO 2 and TiO 2 (P25 and TiO 2 -synth) has also been included. Table 4 compiles the first-order apparent rateconstants (k app ) obtained from the fitting of the photodegradation kinetic data after 180 min.

Above 180 min the response of the kinetic data was not linear, suggesting the presence of additional reactions (degradation or accumulation of intermediates). The apparent rate-constant was selected to compare the photocatalytic activity of the different materials [46]. Table 4. Kinetic parameters for the RhB photodegradation. The conversion of RhB for the non-containing TiO 2 catalysts (Figure 8a) was higher than that of the photolytic reaction. The performance of RH-Si (ca. 25 % RhB conversion) was enhanced after calcination (sample RH-Si-350) with ca. 60 % of conversion after 5 h. The kinetics of the reaction was also enhanced after the calcination, as seen in the k app value of RH-Si-350 (Table 3).

Sample

This high photocatalytic activity for RH-Si-350 can be attributed to the adequate surface area (Table 2), of the material that is responsible for a higher uptake (Table 3), and/or the presence of a small but not negligible carbon phase remnant after calcination. Interestingly, the conversion of RhB with the commercial silica powders for a similar catalyst loading was almost negligible.

The incorporation of TiO 2 in the silica-based catalysts improved the photodegradation of the dye (Fig. 8b) compared to RH-Si, although different trends were observed for the calcined samples.

The conversion of RhB over RH-Si-Ti was ca. 70 %, comparable to that obtained for the commercial benchmark TiO 2 -P25, and with a kinetic constant also close to that of the latter. On the contrary, the photocatalytic activity of RH-Si-Ti-350 was quite poor, in terms of RhB conversion (ca. 10 % after 5 h of irradiation) and kinetic constant. This contrasts with the optical features of this sample (Fig. 6d), that presented an absorbance profile similar to that of TiO 2 , with a cut-off at around 400 nm. This result suggests that the titanium silicate crystalline phase formed during calcination (Fig. 6b and6c, respectively) is most likely non-photoactive for the degradation of the dye. The decreased photocatalytic activity of RH-Si-Ti-350 (compared to RH-Si-Ti) does not follow the trend of the surface pH, adsorption uptake or porosity. We also attribute this fact to the formation of non-photoactive titanium silicates. Indeed, according to literature [43] different TiSiO 4 structures can be commonly found in ABO 4 oxides (e.g., CrVO 4type, zircon, and scheelite) and SiO 2 -TiO 2 composites. This polymorph is comprised of regular edge-sharing octahedra of TiO 6 [43] where the silicon atoms occupy the tetrahedral sites constructed by cubic-packed oxygen atom arrays. Thus, the poor photocatalytic activity of sample RH-Si-Ti-350 could be explained by the lack of coordination sites for the adsorption of hydroxyl moieties and oxygen; this would inhibit the capability of the catalyst to generate oxygen-radical species (i.e., hydroxyl or superoxide radicals) in the compact arrangements of Ti and Si atoms. It should be noted that the experiments using TiO 2 -P25 were performed with an equivalent amount of TiO 2 than that of the hybrid catalysts (ca. 0.134 g L -1 ). Thus, in terms of kinetic constant (k app ), TiO 2 -P25 is only 1.3 times more active than herein synthesized materials.

It is also important to correlate the photocatalytic results with the adsorption parameters obtained from Freundlich model (Table 3). The higher Freundlich adsorption constant (K F ) of RH-Si-350 points out a high affinity towards the retention of RhB. Then, the superior photoactivity of RH-Si-350 compared to RH-Si (as discussed below) can be attributed to the increased uptake.

Furthermore, RhB is a zwitterionic cationic dye with a pK a of 3.1 units, being predominant the zwitterionic form under our experimental conditions. The high uptake of sample RH-Si-350

indicates that the adsorption is favored in basic conditions, where the zwitterionic form of the dye is predominant in solution. In contrast, RhB uptake in the TiO 2 -containing samples was practically the same after calcination, despite their differences in surface area and pH (Table 2).

Mechanism for RhB photodegradation

Considering all the above, we propose a mechanism for the photodegradation of RhB under simulated solar light on the silica-derived catalysts. Rhodamine B has a permanent positive charge (diethylamino group) and a negative charge depending on the ionization state of the carboxylic group for which adsorption is governed by the electrostatic interactions between the surface of the catalysts and the charges in RhB. On negative surfaces RhB is adsorbed by the diethylamino group (positively charged), while for positively charged surfaces the preferred adsorption mode is through the carboxylic moiety. Under our experimental conditions, RhB is in its zwitterionic form in solution (structure C in Fig. S5), while the catalysts are neutral (ca. RH-Si, RH-Si-Ti and RH-Si-Ti-350) or negatively charged (ca. RH-Si-350). Based on the adsorption studies, the uptake is favored in RH-Si-350 (negatively charged according to its surface pH (Table 2). Thus, the preferred adsorption mode is through the diethylamino group. This is important since the interactions between the catalyst and the pollutant define the possible photocatalytic degradation pathway.

In the case of RhB, two photodegradation mechanisms have been reported [25,44,[47][48][START_REF] Saison | New insights into Bi 2 WO 6 properties as a visible-light photocatalyst[END_REF], either through successive deethylation steps [25,44,47], or by direct chromophore cleavage [47][48][START_REF] Saison | New insights into Bi 2 WO 6 properties as a visible-light photocatalyst[END_REF] followed by consecutive oxidations of the aromatic rings. When RhB is adsorbed through the carboxylic moiety in acidic photocatalysts (not the case of the herein studied materials) or at acidic solution pH, the dominant photodegradation mechanism is the deethylation route [25,44,47]. The deethylation route shows a characteristic hypsochromic shift in the UV-vis spectra of the solution, due to the formation of the intermediates (ca. 15-60 nm shift for each deethylation step) [25,44,47]. We did not observe any shift in the UV-visible spectra as a function of the irradiation time for any of the synthesized silica-based photocatalysts (Fig. 9) nor for TiO 2 -P25 (Fig. S6). Furthermore, most performing catalysts showed a basic surface pH. All this points out that the degradation of RhB would proceed through the oxidation of the aromatic ring. On the other hand and given the low degradation yield for the non-catalyzed reaction, the oxidation of RhB is most likely promoted by reactive oxygen species (ROS) formed upon irradiation of the catalysts, as widely reported in the literature for other photocatalytic materials [25,[50][51][52][53][54] such as TiO 2 [START_REF] Linsebigler | Photocatalysis on TiO 2 surfaces: Principles, mechanisms and selected results[END_REF][START_REF] Kaneko | Photocatalysis: science and technology[END_REF], Pb 3 Nb 4 O 13 /SiO 2 [25], Bi-based nanomaterials [47,[START_REF] Saison | New insights into Bi 2 WO 6 properties as a visible-light photocatalyst[END_REF][50][51][52][53][54] and even TiO 2 -SiO 2 composites [44].

Considering all this, the most plausible mechanism for the photodegradation of RhB using biogenic silica-based photocatalysts under simulated solar light would consist on the ring-cleavage of the dye induced by the reaction with ROS (Figure S7). This would lead to the formation of hydroxylated intermediates (structure V in Fig. S7) upon the photooxidation of twosimilar ortho positions of one diethylamino group. The degradation of smaller aromatic fragments of RhB formed from the hydroxylation (such as hydroquinone/benzoquinone) can also be inferred from the decrease in the UV region of the spectra (Fig. 9).

Finally, it should also be mentioned that the prepared catalysts were well dispersed in the solutions, and allowed an easy and efficient separation of liquid and solid phases during the experiments (compared to TiO 2 nanopowders used as benchmark catalyst). This is most important for photocatalytic degradation purposes where phase separation/filtration steps are involved and it could be related to the adequate surface pH of the catalysts (from almost neutral to a slightly, Table 2), Further studies on sedimentability should be carried out to confirm and quantify this behavior.

Conclusions

We have prepared hybrid silica-titania-carbon catalysts from the solvothermal treatment of a biomass waste (i.e., rice husks) as biogenic precursor. The prepared materials were used as photocatalysts for the degradation of a recalcitrant dye under solar irradiation. The catalysts are mainly composed of biogenic silica and carbon, depending on the final synthesis step. Indeed, the calcination at 350 ºC of the silica-based catalyst improved its photocatalytic activity of the by three times; at converse, calcination of the silica-titania material rendered a material with lower photocatalytic activity, due to the formation of a non-photoactive crystalline phase (e.g. TiSiO 4 ).

Figure 1 .

 1 Figure 1. SEM images of the raw rice husk precursor at various magnifications.

Figure 2 .

 2 Figure 2. SEM images of the prepared catalysts. (a-b): RH-Si; (c-d): RH-Si-350; (e-f): RH-Si-Ti;

Figure 3 .

 3 Figure 3. SEM-EDX analysis of sample RH-Si-Ti-350. (a-e): SEM images; (b-f): SEM images

Figure 4 .

 4 Figure 4. C 1s core level spectra for the synthesized catalysts: (a): RH-Si; (b): RH-Si-350; (c):

3- 4 Figure 5 .

 45 Figure 5. O 1s core level spectra of the synthesized catalysts: (a): RH-Si; (b): RH-Si-350; (c):

Figure 6 .

 6 Figure 6. Characterization of the prepared catalysts: (a) N 2 adsorption-desorption isotherms

Figure 7 .

 7 Figure 7. Adsorption isotherms of RhB on the synthesized SiO 2 -based catalysts. Symbols

5 e

 5 RhB adsorbed after 90 min in dark conditions; f First-order apparent rate-constant evaluated in the range 0-180 min irradiation time.

Figure 8 .

 8 Figure 8. RhB photocatalytic degradation in (a) SiO 2 -based and (b): Ti-containing photocatalysts.

Figure 9 .

 9 Figure 9. Changes in the absorbance of the UV-visible spectra of the solution as a function of the

Figure 1 .

 1 Figure 1. SEM images of the raw rice husk precursor at various magnifications.

Figure 2 .

 2 Figure 2. SEM images of the prepared catalysts. (a-b): RH-Si; (c-d): RH-Si-350; (e-f): RH-Si-Ti;

Figure 3 .

 3 Figure 3. SEM-EDX analysis of sample RH-Si-Ti-350. (a-e): SEM images; (b-f): SEM images

Figure 4 .

 4 Figure 4. C 1s core level spectra for the synthesized catalysts: (a): RH-Si; (b): RH-Si-350; (c):

Figure 5 .

 5 Figure 5. O 1s core level spectra of the synthesized catalysts: (a): RH-Si; (b): RH-Si-350; (c):

Figure 6 .

 6 Figure 6. Characterization of the prepared catalysts: (a) N 2 adsorption-desorption isotherms

Figure 7 .

 7 Figure 7. Adsorption isotherms of RhB on the synthesized SiO 2 -based catalysts. Symbols

Figure 8 .

 8 Figure 8. RhB photocatalytic degradation in (a) SiO 2 -based and (b): Ti-containing photocatalysts.

Figure 9 .

 9 Figure 9. Changes in the absorbance of the UV-visible spectra of the solution as a function of the

Table 1 .

 1 Summary of atomic concentration (at. %) of the elements detected by XPS analysis.

	Sample	C	N	O	Si	Ti	Mg	K
	RH	65.13	1.87	28.51	4.48	nd	nd	nd
	RH-Si	51.63	1.78	40.25	6.34	nd	nd	nd
	RH-Si-350	7.39	0.77	63.80	26.60	nd	0.83	0.62
	RH-Si-Ti	46.19	nd	41.95	0.56	11.07	nd	0.23
	RH-Si-Ti-350	17.84	nd	57.92	7.48	16.38	nd	0.38

a nd: not detected.

Table 2 .

 2 Selected textural, chemical and structural properties of the studied catalysts.

	Sample	S BET a 2 -1		V	total 3	b -1		Wo 3 DR -1 c	Carbon (wt.%)	d	pH	e	CI f (%)	D g (nm)
		(m	g	)	(cm	g	)	(cm	g	)
	RH		5		0.01		0.00	40.8	--	50.6	1.47 (SiO 2 111)
	RH-Si	10		0.02		0.01	18.5	6.7	50.3	1.58 (SiO 2 111)
	RH-Si-350	101		0.13		0.03	5.0	8.2	--	0.77 (SiO 2 111)
	SiO 2 comm	250		0.40		--		--	6.5	--	--
	RH-Si-Ti	65		0.07		0.02	17.2	6.0	46.2	2.11 (SiO 2 111)
	RH-Si-Ti-350	174		0.20		0.05	3.0	5.9	--	4.79 (TiO 2 101)
	TiO 2 -P25	45		0.08		0.00	< 0.1	6.4	--	--
	TiO 2 -synth	--			--				--		--	5.9	--	--

a BET specific surface area. b Total pore volume evaluated at at P/Po 0.99. c Micropore volume evaluated using the Dubinin-Raduskevich method. d Carbon content obtained from gravimetric analysis. e Surface pH. f Crystallinity index defined by eq. (1). g Average crystallite size obtained by Scherrer's equation.

Table 3 .

 3 Adsorption parameters for RhB obtained from fitting the experimental equilibrium adsorption data to the Langmuir and Freundlich model. Langmuir adsorption constant; b maximum adsorption capacity obtained from Langmuir model; c linear regression coefficient for fitting to Langmuir model; d Freundlich adsorption constant; e Freundlich heterogeneity parameter;

	Sample	K L	a (L mg -1 )	q max	b (mg g -1 )	R L 2 c	K F	b (mg g -1 )	n F	c	R F	2 d
	RH-Si		0.07		9.2	0.992		0.5	1.2	0.968
	RH-Si-350		1.43		15.7	0.967		7.5	2.4	0.997
	RH-Si-Ti		0.23		10.5	0.958		2.1	1.9	0.999
	RH-Si-Ti-350		0.78		10.2	0.955		4.3	3.0	0.995

a

Table Captions Table 1 .

 Captions1 Summary of atomic concentration (at. %) of the elements detected by XPS analysis.

Table 2 .

 2 Selected textural, chemical and structural properties of the studied catalysts.

Table 3 .

 3 Adsorption parameters for RhB obtained from fitting the experimental equilibrium adsorption data to the Langmuir and Freundlich model.

Table 4 .

 4 Kinetic parameters for the RhB photodegradation.

Acknowledgements

Matos and Ania thank the financial support of the Franco-Chilean network BIOCval2E (REDES-170004 project). Matos also thanks the Funding from CONICYT PIA/APOYO CCTE AFB170007, and from Millennium Science Initiative of the Ministry of Economy, Development and Tourism, in Chile (grant Nuclei on Catalytic Processes towards Sustainable Chemistry).

Calvino thanks the Funding from MINECO/FEDER (Project MAT2017-87579-R).

Data also showed that the adsorption mode of rhodamine B defines the photodegradation pathway favoring the degradation through the chromophore cleavage, excluding the deethylation pathway. Furthermore, some of the prepared photocatalyst showed catalytic activity comparable to that of commercial TiO 2 -P25 powders for the degradation of a recalcitrant dye under simulated solar light. We believe that the present results open new perspectives for the use of low-cost biogenicderived materials as low-cost precursors for the synthesis of materials with a large potential as photocatalysts for environmental remediation.


Adriana I Moral-Rodríguez

Conchi Roberto Leyva-Ramos

Maria Concepcion

Ovin Ania

Raul Ocampo-Pérez

Elizabeth D Isaacs-Páez

Damarys H Carrales-Alvarado

José Parra

Conchi O Ania

Jose B Parra

Tailoring the textural properties of an activated carbon for enhancing its adsorption capacity towards diclofenac from aqueous solution

Keywords: Activated carbon, Adsorption, CO 2 activation, Diclofenac, Microporosity, Tailoring porosity

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

INTRODUCTION

Potential environmental and health problem due to water contamination caused by the presence of emerging pollutants is attracting attention to the society. Due to their worldwide consumption, pharmaceuticals and their metabolites represent an overwhelming portion of trace emerging contaminants detected in surface and drinking water, groundwater, and wastewaters [START_REF] Houeto | Assessment of the health risks related to the presence of drug residues in water for human consumption: Application to carbamazepine[END_REF]). Most commonly found pharmaceutical pollutants in effluents from wastewater treatment plants (WWTP) are antibiotics, antacids, antidepressants, analgesics, antipyretics, steroids, anti-inflammatory, -blockers, tranquilizers and stimulants [START_REF] Ternes | Occurrence of drugs in German sewage treatment plants and rivers[END_REF][START_REF] Daughton | Pharmaceuticals and personal care products in the environment: Agents of subtle change?[END_REF][START_REF] Rivera-Utrilla | Removal of nitroimidazole antibiotics from aqueous solution by adsorption/bioadsorption on activated carbon[END_REF].

Nowadays, the non-steroidal anti-inflammatory drugs (NSAIDs) are extensively prescribed to cure many diseases and are the most widely used therapeutic group worldwide, which has led to its presence in different groundwater or surface water sources. Diclofenac (DCF) is an NSAID drug and is employed to decrease swelling and alleviate pain in pathologies such as rheumatoid arthritis. DCF has the highest critical toxicity within the NSAIDs, with a half maximal effective concentration (EC 50 ) less than 100 mg/L [START_REF] Cleuvers | Aquatic ecotoxicity of pharmaceuticals including the assessment of combination effects[END_REF], and can be found in drinking water, groundwater, WWTP effluents and surface water supplies in several countries at concentration levels varying from ng/L to g/L [START_REF] Houeto | Assessment of the health risks related to the presence of drug residues in water for human consumption: Application to carbamazepine[END_REF].

The concentrations of pharmaceuticals in water resources seldom exceed the curative doses, but they are steadily released to the environment; so the concentrations remain almost constant, posing a critical hazzard to human health and environment [START_REF] Richards | Effects of pharmaceutical mixtures in aquatic microcosms[END_REF]). Most of them are hardly biodegradable and can be poorly eliminated and biodegraded by traditional biological treatment. Therefore, elimination of pharmaceuticals in WWTPs is a crucial topic of interest [START_REF] Ternes | Removal of pharmaceuticals during drinking water treatment[END_REF][START_REF] Clara | Removal of selected pharmaceuticals, fragrances and endocrine disrupting compounds in a membrane bioreactor and conventional wastewater treatment plants[END_REF][START_REF] Watkinson | Removal of antibiotics in conventional and advanced wastewater treatment: Implications for environmental discharge and wastewater recycling[END_REF][START_REF] Kasprzyk-Hordern | The removal of pharmaceuticals, personal care products, endocrine disruptors and illicit drugs during wastewater treatment and its impact on the quality of receiving waters[END_REF][START_REF] Rúa-Gómez | Occurrence and removal of lidocaine, tramadol, venlafaxine, and their metabolites in German wastewater treatment plants[END_REF], and there is a pressing need for improving current treatment technologies. The efficiency of activated carbon adsorption technology as an end-ofpipe technology to upgrade existing WWTP to remove the emerging micropollutants has long been investigated [START_REF] Carvalho | Advanced methods for the removal of acetaminophen from water in A[END_REF][START_REF] Carvalho | Ibuprofen in the aquatic environment: Occurrence, ecotoxicity and water remediation technologies[END_REF].

The modification of activated carbons (ACs) for improving their capacities for adsorbing DCF has been studied in few works. For instance, [START_REF] Torrellas | Chemical-activated carbons from peach stones for the adsorption of emerging contaminants in aqueous solutions[END_REF] investigated the adsorption of diclofenac on chemical-activated carbon synthesized from peach stones. This AC was chemically modified with an HNO 3 solution to increase the acidic sites and was thermally modified to decrease the acidic groups. The pristine AC and the two modified ACs showed no adsorption capacity towards DCF for concentrations less than 20 mg/L, attributing this behavior to weak DCF-adsorbent interactions. In contrast, for concentrations above 20 mg/L, the uptake of DCF was continually increased by raising the concentration, and the capacity of pristine AC reached 200 mg/g for a concentration of DCF at equilibrium of 70 mg/L. Furthermore, the thermal modification of AC did not substantially influence the adsorption capacity, whereas the chemical modification reduced the adsorption capacity. [START_REF] Saucier | Microwave-assisted activated carbon from cocoa shell as adsorbent for removal of sodium diclofenac and nimesulide from aqueous effluents[END_REF] analyzed the pH dependence of the adsorption equilibrium and kinetics of DCF from aqueous solution on an AC produced by microwave-assisted chemical activation of coconut shell. The findings showed that the maximum uptake of DCF adsorbed was 63.47 mg/g at an optimal pH = 7. Álvarez et al. (2015) analyzed the adsorption of DCF on chemically modified carbon xerogels, attaining uptakes of 182.5 and 80 mg/g for the materials modified with urea and sulfuric acid, respectively. Lastly, the competitive adsorption of diclofenac in the presence of salicylic acid, acetylsalicylic acid and atenolol on three commercial ACs was examined by [START_REF] Rakić | The adsorption of pharmaceutically active compounds from aqueous solutions onto activated carbons[END_REF], with adsorption capacities ranging from 13.8 to 31 mg/g for all four pollutants.

In previous studies, it has been reported that the adsorption capacities of the ACs towards DCF varied from 13.8 to 347 mg/g; this is an essential issue for screening the adequate carbon adsorbent to be integrated into current water treatment technologies for removing DCF. The relation between the adsorption capacity of AC towards DCF and the surface chemistry of AC has been addressed; nevertheless, the role of the porous structure (surface area, pore diameter and pore volume) on the adsorption capacity has not been investigated completely.

Consequently, the purpose of this work was to find out the relationship between the adsorption capacity of ACs towards DCF and their textural properties and to substantiate the optimal characteristics of an AC to maximize the capacity for adsorbing DCF. This work was aimed at preparing a series of nanoporous carbons by modifying a commercial AC by physical activation with CO 2 and varying the activation time, to develop further the porous structure and improve the adsorption capacity towards DCF from water solution. Additionally, the porous structure of ACs considerably affected the adsorption capacity, and the mechanism of DCF adsorption on AC was elucidated by conducting adsorption experiments at different pH.

EXPERIMENTAL METHODS

Materials

A commercial microporous carbon (sample F) obtained by steam activation of coconut shell (CPL, Carbon Link) was selected for this study. The particle size of carbon F ranged from 0.40 to 0.75 mm. Three nanoporous carbons having increasing burn-off degree were prepared by activating the carbon F under a CO 2 atmosphere (5 ºC/min up to 850 ºC in 100 mL/min N 2 , afterward change to CO 2 , 10 mL/min) [START_REF] Gomis-Berenguer | Boosting visible light conversion in the confined pore space of nanoporous carbons[END_REF]) during times of 12, 24 and 40 h. The carbon samples were designated as F12, F24 and F40, respectively.

Chemical and textural properties

The elemental composition of the ACs was quantified using elemental analyzers. The carbon, hydrogen and nitrogen were determined by a LECO CHNS-932 (ASTM D-5373), sulfur was quantified by a LECO S-144DR (ASTM D-4239) analyzer, and oxygen was evaluated with a LECO VTF-900 CHNS-932 microanalyzer. All the analysis have been done at least in triplicates with a standard deviation below 2 %. Besides, the pH at the point of zero charge (pH PZC ) was determined using a modification of the mass-titration method detailed elsewhere [START_REF] Co | Importance of the hydrophobic character of activated carbons on the removal of naphthalene from the aqueous phase[END_REF]).

The porous structure of the samples was analyzed using the N 2 adsorption-desorption isotherms at 77 K. The measurements were performed in a high-resolution physisorption analyzer, Micromeritics, ASAP 2020, equipped with three pressure transducers and a molecular pump, allowing to improve the sensitiveness at low pressures. Strict conditions were programmed to ensure N 2 adsorption equilibrium so that the determination of each isotherm lasted on average between 90 and 120 h. Prior to the measurement, the outgassing of the carbon samples was carried out at 120 ºC overnight under vacuum. The surface area (S BET ) and total pore volume (V P ) were calculated from the N 2 adsorption isotherm. The micropore volume (V Micro-NLDFT ) and pore size distribution (PSD) were evaluated with the 2D-NLDFT-HS model, and considering that the surface of the carbon pores was heterogeneous (Jagiello and Olivier 2013b). The adsorption isotherms of CO 2 were measured in a volumetric analyzer (Tristar 3020, Micromeritics) at 273 K. The narrow microporosity (W 0 ) was evaluated by fitting the Dubinin-Radushkevich equation to the adsorption isotherm data of CO 2 , and the [START_REF] Stoeckli | Evolution of microporosity during activation of carbon[END_REF] equation was applied to estimate the average micropore width (L 0 ). The experimental isotherms were recorded in duplicates with an accuracy of ca. 2 %. The standard deviations associated to the calculation of the textural parameters from the experimental data were below 2 %. Here, average data is presented.

Raman spectroscopy was performed by excitation with green laser light (532 nm) in the range between 800-3550 cm -1 . The spectra were acquired in a Jobin-Yvon Horiba HR800 spectrometer equipped with a Leica microscope. The acquisition time for each spectrum was 300 s; to guarantee a representative sampling, spectra were recorded in 4 different spots for each sample. The temperature programmed desorption experiments (TPD) were carried out in a chemisorption analyzer (Micromeritics, Autochem II) under and argon flow rate of 50 mL min -1 , at a heating rate of 10 ºC min -1 up to a final temperature of 1000 ºC. CO and CO 2 evolution was monitored by mass spectrometry (OmniStar 3000). Data was performed in duplicates (average date is presented), with an with an accuracy of ca. 5 %.

Adsorbate

Diclofenac sodium salt was provided by Sigma-Aldrich. The molecular structure and physicochemical properties of this anti-inflammatory are shown in Table 1. The density functional theory (DFT) was applied to obtain the molecular structure of DCF [START_REF] Moral-Rodríguez | Removal of ronidazole and sulfamethoxazole from water solutions by adsorption on granular activated carbon: equilibrium and intraparticle diffusion mechanisms[END_REF].

Experimental adsorption equilibrium data

In this work, the DCF solutions were fixed by adding a given aliquot of a DCF stock solution into a volumetric flask (50 mL) and then, diluting using a solution having a constant ionic strength of 0.01 N and pH of 7, 9 or 11. The latter was made by mixing suitable volumes of 0.01 N HCl and NaOH solutions. The DCF solution was sampled, and the sample (10 mL) was later analyzed to confirm the initial concentration of DCF, which varied from 400 to 2000 mg/L. The concentration of DCF in water samples was quantified by a UV-Visible spectrophotometric method, and the absorbance of DCF in an aqueous solution was measured in a spectrophotometer, Shimadzu UV 2600, at a wavelength of 276 nm.

An experimental batch adsorber was used to obtain the adsorption equilibrium data of DCF on ACs. A specific mass of the AC (25 mg) and volume of DCF solution (40 mL) were added in a centrifugal vial (50 mL), which was the batch adsorber. Thereupon, the vial was partially immersed in a thermostatic bath. The adsorbent and the solution were allowed to equilibrate during 7 days. Once the experiments reached equilibrium, an aliquot (10 mL) was sampled, and the concentration of DCF at equilibrium was quantified by the method explained previously. The pH of the solution in the adsorber was determined periodically during the adsorption experiment, and was maintained constant at a given value by the addition of few drops of 0.01 M HCl y NaOH solutions. Changes in the adsorber solution volume due to pH adjustment were considered negligible because they were less than 1.0 %.

The uptake of DCF adsorbed on the ACs was computed by carrying out a mass balance, which is represented by the subsequent mathematical expression:

q = V(C 0 -C e ) m ( 1
)
Where q is the mass of DCF adsorbed on AC, mg/g; C 0 is the initial concentration of DCF, mg/L; C e is the concentrattion of DCF at equilibrium, mg/L; m is the mass of AC, g; and V is the solution volume in the adsorber, L. The error associated to the determination of DCF was found to be lower than 1.5 %, assuring that the deviations in the estimation of the uptake are lower than 4 % in all cases.

The adsorption of DCF in the effluent from a WWTP was carried out to study the actual application of the F and F40 for removing DCF. The adsorption of DFC was determined by contacting the ACs with a solution prepared by spiking DCF in a sample of an effluent from a WWTP situated in the city of San Luis Potosí, SLP, Mexico. The initial and equilibrium concentrations of DCF, TC and TOC in the solution were quantified as explained earlier. The pH of the wastewater solution was not adjusted nor maintained constant during the adsorption of DCF.

Quantification of Total Organic Carbon

The total carbon (TC), the inorganic carbon (IC) and the total organic carbon (TOC) of water samples were determined in a TOC analyzer, Shimadzu TOC-L, equipped with an autosampler ASI-L. The TC was measured by injecting a water sample into the reaction chamber packed with an oxidizing catalyst, and the oxidation of TC (organic and inorganic carbon)

generated CO 2 at T = 680 °C. Then, the IC was evaluated by injecting the sample into the chamber containing phosphoric acid. Under acidic conditions, the organic carbon was not oxidized, and the IC was converted into CO 2 . The difference between the TC and IC resulted in the TOC. The experimental error in TOC analysis was lower than 5% (average data is presented).

RESULTS AND DISCUSSION

Variation of the ACs properties with the activation time

The primary purpose of this work was to ascertain the optimal porous properties of ACs for removing DCF from aqueous solution. The pristine AC was modified to enhance its porosity and enable access to the internal surface area, to maximize its adsorption capacity towards DCF.

The pristine AC was subjected to physical activation with CO 2 during varying activation times.

The adsorption isotherms of N 2 of the four ACs at 77 K are depicted in Fig. 1(a), and based upon IUPAC classification [START_REF] Rouquerol | Adsorption by powders and porous solids: Principles, methodology and applications[END_REF], the shapes of the adsorption isotherm were representative of type Ia for F, F12 and F24, that is typical of greatly microporous ACs with a distribution of micropore sizes mainly below 1 nm. In contrast, sample F40 showed an isotherm of type Ib featuring a wide knee at low relative pressures, typical of materials having larger micropores and narrow mesopores. The opening of the isotherm knee and the total volume of N 2 adsorbed increased gradually with the activation time, which was attributed to the widening of pore size and the enlargement of the micropore volume during the activation.

The principal textural properties of the ACs are listed in Table 2, and all these properties increased with the activation time. It can be noted that the S BET of F40 carbon increased almost 2.3 times compared to the S BET of F carbon. Additionally, the values of V P , W 0 , V Micro and L 0 for F carbon were more than doubled after 40 hours of activation.

The N 2 adsorption-desorption isotherm was interpreted with the 2D-NLDFT-HS model (Jagiello and Olivier 2013a) to evaluate the PSD of all the ACs (See Fig. 1(b)). This figure corroborates the formation and widening of the large micropores (0.7 nm > d p > 2 nm) for F12, F24 and F40 carbons as the activation time progressed. Notably, the increase in the micropores in the range of 1-2 nm was more pronounced for F40 carbon. The micropore volume of F40 carbon (V Micro , W o, N2 and W o, CO2 ) was almost twice that of F carbon (See Table 2), confirming the development of microporosity along the activation time.

Additionally, the adsorption isotherms of CO 2 were determined to evaluate the narrow microporosity of the ACs [START_REF] Garrido | Use of nitrogen vs. carbon dioxide in the characterization of activated carbons[END_REF][START_REF] Contreras | A comparison of physical activation of carbon xerogels with carbon dioxide with chemical activation using hydroxides[END_REF]) at 273 K, and are graphed in Fig. 2, showing the development of micropores as the activation time increased. At low relative pressures less than 0.010, the volume of CO 2 adsorbed on F12 and F40 carbons was lower than that on the pristine carbon, confirming the enlargement of the narrow micropores with the activation time. Moreover, the volume of CO 2 adsorbed on F40 was less than that of F24, indicating that the narrow microporosity decreased at the expense of increasing the total micropore volume. The adsorption isotherm of CO 2 on the pristine carbon intersects those on the modified carbons, demonstrating that the activation favors the enlargement of the micropores.

The changes in the porosity did not have an impact in the structure of the series of carbons, as inferred from Raman spectroscopy (Figure Sx in the Supporting File).

Regarding the chemical composition, the successive activation of the carbons barely modified their composition, as can be inferred from the values of the elemental analysis listed in Table 3. The carbon content in all the ACs is higher than 95 wt.%, whereas that of oxygen is less than 4 wt.%. The oxygen content increases slightly with the activation time; however, the low oxygen contents reveal that all ACs have a minimal quantity of O-surface groups. This was quantification of the CO and CO 2 -evolving groups determined by TPD-MS (Table 3), and the evolution of the pH of the point of zero charge upon activatin, rendering carbons with a slightly more basic character.

In sum, the characterization results confirmed that the activation time with CO 2 only provoked changes in the porosity of thecarbons, with no signification influence on the structure and chemical nature of the carbon surface.

Diclofenac adsorption isotherms

The experimental adsorption equilibrium data of DCF on the ACs were interpreted by the Radke-Prausnitz (R-P) isotherm model. This isotherm model was selected because, as argued later, the isotherm shape of F carbon was different from the isotherm shapes of the modified carbons. Besides, the R-P isotherm combines the characteristics of the Freundlich and Langmuir isotherms. The R-P model can be expressed by the subsequent equation:

q = aC 1 + bC β (2)
where a (L g -1 ), b (L  mg - ) and  are the isotherm constants.

The R-P isotherm model was fitted to the experimental data, and the isotherm constants were computed by a regression method employing the nonlinear algorithm of Rosenbrock-Newton. The isotherm parameters and the values of the average percentage deviations (%D) are

given in Table 4. The subsequent equation was applied to estimate %D:

%D= ( 1 N ∑ | q exp -q pred q exp | N i=1 ) ×100 % ( 3
)
Where N is the number of experimental data; q exp is the experimental uptake of DCF adsorbed, mg/g; and q pred is the uptake of DCF adsorbed estimated by the R-P isotherm model, mg/g. The R-P model fitted the experimental data satisfactorily because the %D varied from 0.18 to 9.34 %.

The adsorption isotherms of DCF on F, F12, F24 and F40 carbons at 25 °C and pH of 7 are depicted in Fig. 3, and significant differences can be noticed on both the isotherm shapes and adsorption capacities of four carbons. The adsorption isotherms on all modified carbons show a Langmuir-type isotherm with a steep rise at low concentrations of DCF and leveling off at equilibrium concentrations of DCF greater than 1200 mg/L. This behavior is characteristic of adsorbents having energetically homogeneous surfaces. For carbon F, the trend of adsorption equilibrium data of DCF was different, and the uptake of DCF increases by raising the concentration of DCF and does not level off or reach a plateau even at concentrations of DCF at equilibrium as high as 1500 mg/g. This behavior is commonly observed in materials showing energetically heterogeneous surface. Thus, the surfaces of the modified carbons are more energetically homogenous than the surface of the pristine carbon.

Influence of the activation time on the AC capacity for adsorbing DCF

As illustrated in Fig. 3, the capacity of the ACs for adsorbing DCF from aqueous solution was considerably affected by the activation time and the decreasing order of the adsorption capacities was as follows: F40 > F24 > F12 > F. As shown in this figure, the maximum experimental adsorption capacities of F40, F24, F12, and F carbons were 1033, 821, 522 and 271 mg/g, correspondingly. A comparison of the adsorption capacities of all ACs can be performed by estimating the uptake of DCF adsorbed at an equilibrium concentration of DCF of 1200 mg/L, and these values are referred to as Q 1200 . The concentration of 1200 mg/L was chosen as the value closest to the maximum uptake where experimental data is available for all the carbons.

The values of Q 1200 were predicted from the R-P adsorption models and are listed in Table 4.

Analyzing the vales of Q 1200 , the adsorption capacities of F12, F24, and F40 carbons were enhanced 2.15, 2.85 and 3.85 times as a result of the activation time. This enhancement can be primarily attributed to the modification of the textural properties that favored the adsorption capacity by increasing the surface area and widening of the pore size. Comparable findings have been previously presented throughout the literature for the adsorption of organic compounds on ACs [START_REF] Mestre | Waste-derived activated carbons for removal of ibuprofen from solution: Role of surface chemistry and pore structure[END_REF][START_REF] Nowicki | Comparison of physicochemical and sorption properties of activated carbons prepared by physical and chemical activation of cherry stones[END_REF].

The relationship between the adsorption capacity of ACs towards DCF and the textural properties was further studied by plotting the maximum adsorption capacity (Q 1200 ) versus the surface area, S BET , and the results are shown in Fig. 4(a). As expected, Q 1200 increased almost linearly concerning S BET , and this tendency can be due to that the capacity of ACs for adsorbing organic compounds is highly affected by the magnitude of S BET . The effect of S BET on Q 1200 can be incorporated by recalculating the uptake per unit of surface area using the following equation:

U Norm = Q 1200 S BET (4)
where, U Norm is the normalized uptake of DCF per unit of surface area, mg/m 2 .

The dependence of U Norm on the pore volumes (V P and V Micro ) and average micropore width (L 0 ) is also illustrated in Fig. 4(b-d), and as seen in these figures, U Norm varied similarly concerning the textural properties V p , V Micro and L 0 . Furthermore, it can be observed that U Norm increases slightly and almost linearly for the F, F12 and F24 carbons, U Norm reached a maximum value for F24 carbon, and then U Norm was reduced for F40 carbon. It is essential to mention that the value of U Norm increased with the activation time for the ACs with average micropore width ranging between 1.4 and 1.6 nm, but U Norm for F40 carbon dropped when the micropores are widened over this range. This trend reveals that the longer activation times do not favor the adsorption capacity and the critical role of the pore dimensions on the adsorption capacity.

Although Q 1200 raised as a result of increasing the pore volume and surface area of ACs, the decrease in U Norm for F40 carbon demonstrated that the porosity of F24 carbon was more effectively utilized than that of F40 carbon (see Fig. 4(b-d)). For application purposes, the F24 carbon represents the most viable option, taking into account the costs associated with the additional activation time required for synthesizing F40 carbon. This result indicates that F24 carbon is the material whose activation conditions optimized the textural properties for removing DCF efficiently from water solutions.

Dependence of the adsorption capacity on the solution pH

The solution pH can dramatically influence the capacity of ACs for adsorbing ionizable compounds, as it may affect the charge of the AC surface and the species of DCF existing in aqueous solution. The effect of the solution pH on the capacity of F40 carbon for adsorbing DCF was examined by determining the adsorption isotherms at pH of 7, 9 and 11. Experimental adsorption equilibrium data was not procured at pH below 6 because of the low solubility of DCF. A white solid was formed in a DCF solution when the solution pH was reduced below 6.

Carbon F40 was chosen because it presented the largest uptake and the highest concentration of functional groups containing oxygen (Table 2). The corresponding adsorption isotherms are depicted in Fig. 5, showing a remarkable reduction in the uptake of DCF adsorbed by raising the solution pH from 7 to 11.

In accordance to the speciation diagram of DCF in aqueous solution (Fig. 6), the DCF can predominantly be as a non-ionizable molecule at pH < 2, and as an anionic or deprotonated species at pH > 6. In this work, the anionic molecule (DCF -) was the only species present because all the experiments were conducted at pH > 6. Furthermore, the solution pH also affected the surface charge of the nanoporous ACs. The pH PZC of the ACs varied within the range 8.5-9.2 (Table 2), so that the surface of the ACs had a positive charge at pH = 7, and negative at pH = 11. At pH = 7, the DCF -species were attracted to the surface of ACs whereas there was repulsion between the DCF -species and the surface of ACs at pH = 11. Thus, the adsorption capacity was lessened by increasing the solution pH. This result can be explained recalling that the adsorption capacity was favored by the electrostatic attraction at pH = 7 and was diminished by the electrostatic repulsion at pH = 11.

The reduction of the adsorption capacity for F40 caused by increasing the solution pH was evaluated by comparing Q 1200 estimated at pH of 7, 9 and 11. The values of Q 1200 for F40 carbon at pH 7, 9 and 11 are given in Table 4. Thus, the adsorption capacity of F40 was reduced approximately 1.37 and 1.78 times by increasing the solution pH from 7 to 9, and from 7 to 11, correspondingly. The magnitude of this decrease in adsorption capacity indicates that the electrostatic interactions contributed significantly to the adsorption mechanism of the anionic species of DCF.

Adsorption mechanism of DCF

It is very well documented that the aromatic compounds can be predominantly adsorbed on ACs by the dispersive interactions due to π-electrons of the aromatic ring of the organic compound and π-electrons of the graphene layers of the ACs [START_REF] Coughlin | Role of surface acidity in the adsorption of organic pollutants on the surface of carbon[END_REF]. The π-π dispersive interactions are highly affected by the surface area and accessibility to surface area.

Indeed, the activation of a carbon adsorbent increases the number of graphene layers available, while the widening of the micropore width ensures the accessibility to the surface area. The adsorption capacity of the ACs raised almost linearly by increasing the surface area, corroborating that the dispersive interactions significantly influenced the adsorption of DCF on ACs. Furthermore, the adsorption capacity of ACs may be favored or disfavored by the electrostatic interactions between the ACs surface charge and the DCF species present in water solution.

Comparison of adsorption capacities of carbon materials

The maximum adsorption capacities of F carbon series were compared to those of other carbonaceous adsorbents published throughout the literature, and the maximum capacities for adsorbing DCF are listed in Table 5. The comparison was only among carbon-based materials because these materials usually exhibit high adsorption capacities towards organic compounds from water solution. Many different types of carbon materials have been investigated for adsorbing DCF from aqueous solutions, among these materials are activated carbons and biochars from various precursors, graphene oxide, expanded graphite, xerogels and carbon nanotubes. As shown in Table 5, the adsorption capacities of herein modified carbons are larger than those for the other carbon-based materials reported in previous works. The adsorption capacity of F40 was twice those of a porous carbon synthesized by pyrolyzing of metal azolate framework-6 (MAF-6) [START_REF] An | Adsorptive removal of wide range of pharmaceutical and personal care products from water by using metal azolate framework-6 derived porous carbon[END_REF], and graphene oxide [START_REF] Nam | Adsorption characteristics of diclofenac and sulfamethoxazole to graphene oxide in aqueous solution[END_REF], and about three times that of an AC commercially known as F400 [START_REF] Moral-Rodríguez | Removal of ronidazole and sulfamethoxazole from water solutions by adsorption on granular activated carbon: equilibrium and intraparticle diffusion mechanisms[END_REF]). Thus, the F40 presented the highest capacity of all carbonaceous adsorbents reported throughout the literature so far.

The capacity of each AC was normalized by dividing it by its surface area, and the adsorption capacity per unit of surface area was also listed in Table 5. Again, the DCF adsorption capacities of F40, F24 and F12 per unit surface area outperformed those of other carbon materials reported in the literature. The normalized adsorption capacity of F24 was nearly 4 times that of MAF-6 carbon, and the normalized capacity of the commercial activated carbon F400 is slightly greater than that of carbon F. The adsorption capacity per unit surface area is highly dependent on the accessibility of DCF to the pores of the carbon materials. Greater normalized adsorption capacity means that the surface area of the adsorbent is more efficiently used for adsorbing DCF. All the above findings show the paramount importance of tailoring the textural properties of the ACs for achieving an elevated uptake of DCF.

Removal of DCF from a sample of water effluent from a WWTP

The effect of the water matrix upon the capacity of the F and F40 carbons for adsorbing DCF was investigated by removing DCF from the effluent of a WWTP. The solution of DCF was prepared by spiking DCF in an effluent sample from a municipal WWTP, and the dosage of ACs was 1.25 g/L. The adsorption isotherms of DCF on the F and F40 carbons from the water sample of WWTP are depicted in Fig. 7. Besides, the Radke-Prausnitz isotherm modeled the adsorption equilibrium data of DCF from WWTP, and the values of %D and adsorption isotherm constants are included in Table 4.

As seen in Fig. 7, the capacities of F and F40 carbons for adsorbing DCF were reduced in the WWTP solution. The values of Q 1200 for carbons F and F40 in the WWTP solution were estimated to be 129 and 679 mg/g, respectively (See Table 4), and comparing these results with those in the buffer solution at pH = 7, it can be noticed that the Q 1200 of the F and F40 carbons were decreased 47.1 % and 27.8 %. This result was somewhat expected because the effluent of a WWTP contains several inorganic and organic compounds, which may compete with the DCF for the adsorption sites of F and F40 ACs. The higher decrease of the adsorption capacity of carbon F than that of F40 could be attributed the narrow porosity of carbon F, which would be clogged up more easily upon adsorption of other organic pollutants present in the WWTP.

The application of the F40 carbon in WWTP was studied by evaluating the removal of DCF, TC and OC by adsorption on F40 from the WWTP effluent sample for a DCF initial concentration of 674 mg/L. The concentrations of DCF, TC and OC in the WWTP samples are given in Table 6, and the progress of other relevant water quality parameters (IC, pH, conductivity and others) were also determined but were not reported in this work because their variation during adsorption was not crucial for removing of DCF. The percentage removal of DCF, TOC and OC were 77.7, 79.8 and 88.1 %, correspondingly. Consequently, the removal of DCF from WWTP solution can be effectively performed by adsorption on F40 carbon.

CONCLUSIONS

The adsorption capacity of ACs towards DCF from aqueous solutions was significantly affected by the porous structure of the ACs. The textural properties of the ACs were considerably modified by physical activation with CO 2 during different times, and the surface area was raised, and the micropores were widened by increasing the activation time. The surface chemistry of the ACs was slightly modified by the physical activation with CO 2 , so differences in the adsorption capacity were mainly attributed to change in the textural properties of the ACs.

The adsorption capacities of ACs were enhanced by increasing the activation time. The adsorption capacities of the ACs increased almost linearly with the S BET , V P , W 0 and L 0 .

Furthermore, the maximum adsorption capacities of the modified ACs were well above those reported in the literature. The F24 carbon presented the highest normalized uptake of DCF adsorbed so that the F24 carbon had the optimal textural characteristics to remove DFC efficiently from aqueous solution. Consequently, the capacity of AC for adsorbing DCF can be maximized by the proper modification of its textural properties.

The capacity of ACs for adsorbing emerging pollutants can be considerably enhanced by tailoring the textural properties of the ACs. The concentration of DCF in a WWTP effluent was effectively reduced by adsorption on F40, attaining a percentage removal of 77.7 % of this pollutant, along with a large removal of TOC and TC.

Fig. 1 .

 1 Fig. 1. Adsorption-desorption isotherms of N 2 on the nanoporous carbons at 77 K (a), and pore size distributions calculated using 2D-NLDFT-HS method (b).

Fig. 2 .

 2 Fig. 2. Adsorption isotherms of CO2 at 273 K on the nanoporous carbons.

Fig. 3 .

 3 Fig. 3. Adsorption isotherms of DCF on the F carbon series at T = 25 ºC and pH = 7. The symbols represent the experimental data and the lines depict the prediction of the Radke-Prausnitz isotherm.

Fig. 4 .Fig. 5 .Fig. 6 .

 456 Fig. 4. Effect of selected textural properties on the adsorption capacities of ACs towards DCF. a) Q 1200 vs. S BET , b) U Norm vs. V P , c) U Norm vs. V micro , d) U Norm vs. L 0 . Fig. 5. Effect of solution pH on the adsorption isotherm of DCF on F40 at T = 25 °C. The symbols represent the experimental data and the lines depict the prediction of the Radke-Prausnitz isotherm.Fig. 6. Speciation diagram of DCF in aqueous solution as a function of the solution pH.

Fig. 7 .

 7 Fig. 7. Effect of water matrix on the adsorption of DCF on F and F40 carbons at T = 25 °C. The symbols represent the experimental data and the lines depict the prediction of the Radke-Prausnitz isotherm.

Table 1 .

 1 Molecular structure and physicochemical properties of DCF.

	Drug	Molecular structure	Molecular weight (g/mol)	Size X,Y,Z (Å)	Log K ow a	pKa
	Diclofenac sodium (DCF) C 14 H 10 C l2 NNaO 2		318.13	X: 0.960 Z: 0.472 Y: 0.708	3.91 b	4.15

a Octanol-water partition coefficient. b

[START_REF] Nam | Adsorption characteristics of selected hydrophilic and hydrophobic micropollutants in water using activated carbon[END_REF]

.

Table 2 .

 2 Textural properties of the F, F12, F24 and F40 carbons.

	Activated	S BET	V P a	V Meso b,c	V Micro b	W o, N2 c	L 0	d	W o, CO2	c
	Carbon	(m 2 /g)	(cm 3 /g)	(cm 3 /g)	(cm 3 /g)	(cm 3 /g)	(nm)	(cm 3 /g)
	F	799	0.35	0.03	0.31	0.37	0.91	0.25
	F12	1146	0.50	0.04	0.45	0.55	1.37	0.33
	F24	1279	0.58	0.06	0.50	0.64	1.56	0.37
	F40	1801	0.86	0.15	0.66	0.80	2.00	0.36
	a Total pore volume determined at P/P 0 ≈ 0.99.
	b Calculated from 2D-NLDFT-HS model (Jagiello and Olivier 2013b)

c Micropore volume calculated by the Dubinin-Radushkevich method.

d Average micropore size evaluated by the Stoeckli-Ballerini equation.

Table 3 .

 3 Selected chemical paramters of the studied nanoporous carbons: Elemental analysis (wt.%) on dry ash free basis, point of zero charge, and amount of CO and CO 2 evolved from the materials upon TPD-MS.

	Activated Carbon	C (wt.%) H (wt.%) O (wt.%) pH PZC	CO (mmol/g)	CO 2 (mmol/g)
	F	96.40	0.70	2.90	8.7	698	335
	F12	96.12	0.45	3.43	8.5	517	238
	F24	95.96	0.39	3.65	8.8	549	205
	F40	96.04	0.22	3.74	9.2	589	158

Table 4 .

 4 Parameter values of the Radke-Prausnitz adsorption isotherms for adsorption of DCF from aqueous solution on the activated carbons. T = 25 °C.

	Activated Carbon	pH	Q 1200 (mg/g)	a (L/g)	b (L β mg -β )	β	%D
	F	7 WWTP	244 129	1.4 0.5	0.1 0.001	0.54 1.08	8.82 8.86
	F12	7	524	93.4	0.6	0.84	2.26
	F24	7	696	768.7	3.4	0.84	8.13
		7	940	836.7	2.71	0.84	9.34
	F40	9 11	686 528	371.4 625.7	0.8 2.3	0.95 0.91	3.90 3.96
		WWTP	679	103.5	0.31	0.90	0.18

* WWTP = Water sample from a WasteWater Treatment Plant.

Table 5 .

 5 Maximum adsorption capacities of carbon materials towards DCF from aqueous solutions.

		Maximum	Adsorption
	Adsorbent	adsorption capacity	capacity per surface area	S BET (m 2 /g)	Experimental conditions	Reference
		(mg/g)	(mg/m 2 )
	F40	1033	0.574	1801
	F24 F12	821 522	0.642 0.455	1279 1146	pH = 7 T = 25 °C	This work
	F	271	0.339	799
	Porous Carbon
	(Metal azolate	503	0.161	3123	pH = 6.5	(An et al. 2018)
	framework-6)
	Graphene oxide (GO)	500	--	NA*	pH = 7 T = 20 °C	(Nam et al. 2015)
	Activated biochar (pine chip)	373	0.274	1360	pH = 7 T = 25 ºC	(Jung et al. 2015)
	Granular AC (F-400)	347	0.378	919	pH = 6 T = 25 ºC	(Moral -Rodríguez 2014)
	Expanded graphite	330	--	NA*	No specified	(Vedenyapina et al. 2013)
	Activated carbon (pine sawdust)	263.7	0.331	796	pH = 6.3 T = 30 ºC	(Álvarez-Torrellas et al. 2016)
	Granular AC (F-400)	234	0.235	997	pH = 6.3 T = 23 ºC	(Sotelo et al. 2014)
	Carbon xerogel treated with sulfuric acid	80.0	0.278	288	pH = 6.1 T = 30 ºC	(Álvarez et al. 2015)
	AC (cocoa shell)	63.47	0.103	619	pH = 7 T = 25 ºC	(Saucier et al. 2015)
	AC (Tea wate)	62	0.072	865	T ≈ 30 °C	(Malhotra et al. 2018)
	Granular AC	36.23	0.078	462.96	pH = 5.5 T = 25 °C	(De Franco et al. 2018)
	CNTs/Al 2 O 3	29	0.122	237	pH = 6	(Wei et al. 2013)
	AC (Cyclamen persicum tubers)	22	0.036	606.78	pH = 2 T = 25 ºC	(Jodeh et al. 2015)

* NA = No available.

Table 6 .

 6 Concentrations and removal percentages of DCF, TC and TOC before and after contacting the WWTP solution containing DCF with the F40 carbon.

		DCF	TC	TOC
	Sample
		(mg/L)	(mg/L)	(mg/L)
	WWTP without spiking DCF	0	31.9	2.41
	WWTP spiked with DCF	674	289	267
	WWTP spiked with DCF after adsorption on F40	150	58.5	31.8
	Removal Percentage (%)	78	80	88

ACKNOWLEDGMENTS

This work was supported by Consejo Nacional de Ciencia y Tecnologia, CONACyT, Mexico (grants numbers INFR-2012-01-188381 and CB-2012-02-182779). COA and JBP thank the support of the Spanish MINECO (grant number CTM2014/56770-R).