A M Borrero-López

E Masson

A Celzard

V Fierro

Modelling the production of solid and liquid products from the hydrothermal carbonisation of two biomasses

Keywords: olive stones, beechwood, modelling, hydrothermal carbonization, furfural, phenolics

 regarding the cellulose, hemicellulose and lignin contents of the two bioresources. Adequate fits were obtained for the pH of the liquid phase (9-11% overestimation for OS and BW, respectively), the yields of solid (7-8% error for OS and BW, respectively) and liquid (6-16% error for OS and BW, respectively) and the production of 5-hydroxymethylfurfural and several phenolic compounds, while some discrepancies for the production of furfural and a few phenolic compounds were observed but were properly analysed and discussed.

Introduction

Hydrothermal carbonisation (HTC) is attracting increasing interest because of the mild pressure and temperature conditions involved in this process [START_REF] Liu | Hydrothermal treatment of grass: A low-cost, green route to nitrogen-doped, carbon-rich, photoluminescent polymer nanodots as an effective fluorescent sensing platform for label-free detection of Cu(II) ions[END_REF], the production of valuable products in the aqueous phase [START_REF] Guardia | Apple Waste: A Sustainable Source of Carbon Materials and Valuable Compounds[END_REF], and the possibility of using the hydrochars obtained as precursors of new carbonaceous [START_REF] Yahia | Activated Carbons from Hydrothermal Carbons Prepared in Milk[END_REF] or oxide materials [START_REF] Kavil | Multifunctional nanohybrid material from discarded razor blades as cost-effective supercapacitor electrodes and oil-spill cleaners[END_REF]. Hydrothermal carbonisation of biomass is indeed a simple process in which bioresources are treated in hot pressurised water in an autoclave at moderate temperatures, within the typical range of 140-240ºC.

Then, hydrothermal reactions occur at the medium pressures generated by water evaporation in the hermetically sealed autoclave [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF]. The hydrochars produced when biomass was subjected to HTC have been used for soil conditioning [START_REF] Bento | Release of nutrients and organic carbon in different soil types from hydrochar obtained using sugarcane bagasse and vinasse[END_REF], development of lubricants [START_REF] Huang | Preparation and tribological properties of core-shell Fe3O4@C microspheres[END_REF], stabilisation of Pickering emulsions [START_REF] Xie | Pickering emulsions stabilized by amphiphilic carbonaceous materials derived from wheat straw[END_REF], conversion to solid fuels [START_REF] Lucian | Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste[END_REF][START_REF] Ma | Effect of water-washing of wheat straw and hydrothermal temperature on its hydrochar evolution and combustion properties[END_REF][START_REF] Zhao | Characterization of hydrothermal carbonization products (hydrochars and spent liquor) and their biomethane production performance[END_REF], adsorbents [START_REF] Lei | A Novel Nitrogen Enriched Hydrochar Adsorbents Derived from Salix Biomass for Cr (VI) Adsorption[END_REF], and metal detection [START_REF] Liu | Hydrothermal treatment of grass: A low-cost, green route to nitrogen-doped, carbon-rich, photoluminescent polymer nanodots as an effective fluorescent sensing platform for label-free detection of Cu(II) ions[END_REF].

In addition to hydrochar, the release of many valuable products from the main components of biomass, i.e., from hemicellulose, cellulose and lignin, is achieved through bond cleavage and self-catalysed reactions due to the presence of water [START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF]. Materials scientists often neglect the liquid phase although various phenolic compounds [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF], as well as furfural (FU) and 5-hydroxymethyl furfural (5-HMF) [START_REF] Grote | Molecular Dynamics Simulations of Furfural and 5-Hydroxymethylfurfural at Ambient and Hydrothermal Conditions[END_REF][START_REF] Wataniyakul | Preparation of hydrothermal carbon acid catalyst from defatted rice bran[END_REF], can be obtained, making this phase a high-added value, cheap and safe source of chemicals [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF][START_REF] Nakason | Hydrothermal carbonization of unwanted biomass materials: Effect of process temperature and retention time on hydrochar and liquid fraction[END_REF][START_REF] Zhao | Characterization of hydrothermal carbonization products (hydrochars and spent liquor) and their biomethane production performance[END_REF]. However, products separation is still difficult and therefore, restricting production to a limited number of product by catalysis [START_REF] Sun | Bright Side of Lignin Depolymerization: Toward New Platform Chemicals[END_REF] seems a very interesting strategy. Moreover, the importance and applications of FU and 5-HMF have been ubiquitously highlighted [START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF]. Among the phenolic compounds, some interesting chemicals such as vanillin and syringaldehyde have also been found [START_REF] Rodrigues Pinto | Lignin as Source of Fine Chemicals: Vanillin and Syringaldehyde[END_REF][START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF]. Others, such as acetosyringone, guaiacylacetone, guaiacol or syringol, have also been produced, all of them having noteworthy antioxidant and antimicrobial activity [START_REF] Conde | Production of antioxidants by non-isothermal autohydrolysis of lignocellulosic wastes[END_REF][START_REF] Gullón | Antioxidant and antimicrobial activities of extracts obtained from the refining of autohydrolysis liquors of vine shoots[END_REF]. Finally, the production of gases at higher temperatures, usually catalysed by acids and/or metal, has also been extensively studied [START_REF] Salimi | Influence of promoted bimetallic Ni-based catalysts and Micro/Mesopores carbonaceous supports for biomass hydrothermal conversion to H2-rich gas[END_REF]. In conclusion, HTC undoubtedly is an outstanding biorefinery tool for converting lignocellulosic resources into valuable, useful solid, liquid and gaseous products.

On the other hand, efforts have focused on understanding the kinetics of the involved transformation mechanisms. Thus, [START_REF] Keiller | Biochemical Compositional Analysis and Kinetic Modeling of Hydrothermal Carbonization of Australian Saltbush[END_REF] stated that hydrothermal carbonisation may occur through direct reactions, often of first order, whereas others have discussed the possibility of intermediates and side products [START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF]Gallifuoco et al., 2018;Gallifuoco and Di Giacomo, 2018;[START_REF] Zhao | Characterization of hydrothermal carbonization products (hydrochars and spent liquor) and their biomethane production performance[END_REF]. The modelling of HTC products was carried out by applied statistical methods (Gallifuoco and Di Giacomo, 2018), response surface methodology [START_REF] Heidari | Prediction of Hydrothermal Carbonization with Respect to the Biomass Components and Severity Factor[END_REF][START_REF] Zhao | Characterization of hydrothermal carbonization products (hydrochars and spent liquor) and their biomethane production performance[END_REF] and process simulators [START_REF] Gómez | Modeling and simulation of a continuous biomass hydrothermal carbonization process[END_REF]). Overall, these studies require powerful calculation tools in addition to the development of sophisticated models, due to the complexity of kinetics [START_REF] Marcotullio | The Chemistry and Technology of Furfural Production in Modern Lignocellulose-Feedstock Biorefineries[END_REF], as well as accurate experimental and analytical studies. In a former work, the solid and liquid products obtained when cellulose, hemicellulose and lignin were subjected to HTC have been meticulously studied. Various models were presented that were able to predict solid and liquid yields, the final pH of the liquid phase and the concentrations of FU, 5-HMF and several phenolic products [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF].

The objective of the present study is to test the validity of these models when the entire lignocellulosic biomass, instead of its independent components, is submitted to HTC. To this end, olive stones (OS) (Olea europaea) and beechwood (BW) (Fagus sylvatica) have been subjected to HTC at temperatures ranging from 140 to 240ºC and durations ranging from 1 to 24 hours. The severity factor, which is a useful tool for understanding HTC, was used to simultaneously account for the effects of time and temperature. The experimental data were finally compared to the predictions of the models.

Materials and Methods

Raw materials

The cellulose, hemicellulose and lignin contents of OS and BW were determined according to the Tappi standards (T 264, T 203 and T 222, respectively). Each analysis was repeated three times and the average error was lower than 3%. More details can be found elsewhere [START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF].

Olive stones (OS) were purchased at a local factory in Gabès (Tunisia) and ground at a size of less than 2 mm. The cellulose, hemicellulose and lignin contents were 40.5, 21.7 and 29.9 wt.%, respectively [START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF]. Beechwood (BW) was cut and milled and the resultant chips of an average size of less than 1 mm were recovered by using a powder collector. The corresponding cellulose, hemicellulose and lignin contents were 43.6, 33.5 and 22.1 wt.%, respectively.

All compounds used for identification and quantification of phenolic and furan compounds using HPLC equipment were analytical grade and purchased from Sigma-Aldrich (Steinheim, Germany).

Hydrothermal carbonisation

Hydrothermal carbonisation tests were performed according to the conditions established elsewhere [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF]. In brief, 2 g of the selected biomass were introduced into a glass vial with 16 g of distilled water. The vial was then placed in a 100 mL Teflon-lined autoclave, sealed and placed in a preheated oven at the desired temperature (140-240ºC) for times ranging from 1 to 24h.

The combined effect of time and temperature on the process was taken into account through the severity factor, logR 0 , [START_REF] Overend | Fractionation of Lignocellulosics by Steam-Aqueous Pretreatments[END_REF], as follows:

(1) where the higher importance of temperature, T (°C), with respect to time, t (min), can be observed in the exponential dependence of R 0 on T and its linear dependence on t.

Regarding the empirical parameter ω, a value of 14.75 has been traditionally used with lignocellulosic materials. However, it has been modified as needed to accommodate a variety of bioresources and temperatures [START_REF] Kim | Severity Factor Coefficients for Subcritical Liquid Hot Water Pretreatment of Hardwood Chips[END_REF][START_REF] Ko | Effect of liquid hot water pretreatment severity on properties of hardwood lignin and enzymatic hydrolysis of cellulose[END_REF]. Table 1 shows the experimental conditions and consequent severity factors used for OS and BW.

Liquid/Solid separation

Upon completion of the HTC, the brownish solid, or hydrochar, was separated from the liquid by vacuum filtration and completely dried in a vacuum oven, where it was held for 6h at 60ºC before being weighed and stored. After separation, the liquid was also weighed and refrigerated to avoid further reactions until analysis.

Chromatographic analysis

The quantitative analysis of the main compounds in the liquid phase was carried out using an Ultimate 3000 high-performance liquid chromatograph (Thermo Fisher Scientific, Germany), coupled to an automatic sampler and with both diode array and fluorescence detectors. Two different columns were used for the appropriate separation and quantification of furan and phenolic compounds: Hypersyl Green PAH (Thermo Fisher Scientific, Germany) and Pinnacle DB BiPh 5 μm (Restek, USA), respectively.

The quantification of furans was performed at UV absorption wavelengths of 220, 276, 284 and 291 nm by selecting water and acetonitrile as mobile phases. For the determination of phenolic compounds, the same mobile phases were used, but 195, 201, 231 and 300 nm were the selected UV absorption wavelengths. More information on the analysis protocol can be found elsewhere [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF]. The limits of detection for HPLC measurements were around 0.0004 g/L.

Yield analysis

The hydrochar and liquid-phase yields, i.e., Y HC and Y LC , respectively, were calculated according to the following equations ( 2) and ( 3):

(2)

(3)

where W HC expresses the dry weight of the resultant hydrochar, W BC is the initial weight of the biomass compound (BC) fed into the autoclave, L is the resultant liquid content after HTC, and L 0 is the initial amount of distilled water added.

Procedure to estimate HTC products

The present procedure is based on recent models [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF] initially developed to describe the behaviour of the three main biomass components:

cellulose, hemicellulose and lignin, submitted to HTC separately, in terms of final pH, hydrochar and liquid yields, and concentrations in FU, 5-HMF and phenolic molecules.

Based on these former models, we tried to assess whether it was possible to predict the final composition and yields of HTC products or the possible synergistic effects when the real biomass is submitted to HTC. For that purpose, we used two rather different biomasses, OS and BW, the former being much richer in lignin than the latter. The final pH of the liquid phase reads as follows:

(4)

where p1, p2 and p3 are experimental parameters. A polynomial expression was then determined for each biopolymer: cellulose, hemicellulose and lignin.

The hydrochar yield from each biomass compound is as follows:

(5)

where the hydrochar yield of each lignocellulosic polymer (Y HC,BC ) depends on the constant initial and final hydrochar yields (Y 0 and Y ∞ , respectively), and on two parameters, k and m, which account for the slope of the yield drop and the severity value at which the drop starts, respectively.

The concentrations of FU and 5-HMF produced when submitting biomass to HTC read, respectively:

(6) (7)
Both models were applied because the latter takes into account an intermediate in the production of FU from xylose. C BC was the initial concentration of the BC (cellulose, hemicellulose or lignin) while k 1 , k 2 and k 3 were the constants corresponding to the various reactions taking place.

The concentrations of the following phenolic molecules produced from lignin read as follows:

Vanillin, syringaldehyde and acetosyringone:

(8) Guaiacyalcetone:

(9)

Syringol, guaiacol and phenol production:

(10)

where a determined set of parameters has been developed for each compound.

Using the corresponding model and taking into account the lignocellulosic composition of both OS and BW, the aforementioned yields and concentrations of the main HTC products were estimated.

Relative error analysis

To determine to what extent the various models match the real data, the following expression was used to calculate the relative error:

(11)

where V exp and V th are respectively the experimental and predicted data, whether for pH value, hydrochar or liquid yields, or concentration of a given product obtained during the HTC of OS or BW.

Results

Final pH of the liquid phase

The final pH after HTC gives information on the severity of the process and on the reactions that have taken place. Fig. 1a shows the pH of the liquid fraction after HTC of OS and BW along with the predicted data obtained.

Figure 1

As can be seen, the pH of the liquid fraction after HTC was generally higher for BW than for OS within the studied range of severity, which is in good agreement with their lignocellulosic composition. At low severity, the higher the cellulose and hemicellulose contents, the higher the pH final values [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF]. In the former paper it was shown that, as severity increases, lignin produces a slight increase in the final pH, cellulose decreases it considerably, and hemicellulose gives a minimum of about pH 5. Therefore, the experimental results observed represent a clear combination of the degradations of the three biopolymers. Although a slight overestimation of pH was observed (about +9% and +11% for OS and BW, respectively), the predicted evolution of pH with severity followed the same trend as the experimental one.

Hydrochar yield

Fig. 1b shows the values of for OS and BW. At low severity, was higher than due to the higher cellulose content of BW, a biopolymer that was found to lead to the highest yields under these conditions [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF].

Nevertheless, at the highest severities, lignin began to play a more significant role in the hydrochar yield, leading to values of closer to those of .

The differences between the predicted and experimental were as low as about 7% and 8% for OS and BW, which shows the relevance of the proposed equation for predicting the yield of hydrochar of two very different biomasses submitted to HTC.

Liquid yield

Fig. 2a shows the liquid yield for both biomasses, that of OS being higher than that of BW due to its higher lignin and lower cellulose contents, respectively. Nevertheless, due to the high hemicellulose content of BW, the two datasets were very similar. In our former work [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF], no model could be obtained for the liquid yields from the HTC of hemicellulose, cellulose and lignin, due to the scattering of the results. However, the experimental liquid yields of lignin, cellulose and hemicellulose and the corresponding compositions were used here to predict the results. Thus, for each severity value and for each bioresource, the liquid yields previously obtained from raw cellulose, hemicellulose and lignin were weighed with the lignocellulose concentration, generating the predicted values. In spite of that impossibility to previously obtain a proper model [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF], the predicted data were found to be in very good agreement with the OS liquid yield, with an average error of about 6%; however, the BW liquid yield was slightly overestimated, with an average error of 16%.

Fig. 2b shows the differences between experimental and predicted values from the model. The general overestimation of the liquid yield, especially for BW, can be explained by some synergistic effect that may occur when the three biopolymers are linked together. As a result, some reactions can be hindered and the extraction of water from the raw biomass can be limited. In addition, it can be concluded that both cellulose and hemicellulose must be responsible for this higher resistance to HTC, because OS, with a higher lignin content, does not present such an overestimation of liquid yield [START_REF] Xiao | Hydrothermal carbonization of lignocellulosic biomass[END_REF].

Figure 2

Concentrations of FU and 5-HMF and estimates

Fig. 3 shows the concentrations of FU and 5-HMF experimentally produced from OS and BW. Unexpectedly, the FU concentration from OS was significantly higher than that from BW. Such a finding was surprising because BW contains a much higher fraction of hemicellulose, which is directly responsible for the production of FU.

Moreover, the latter from both materials was much higher than expected, based on their hemicellulose content and our previous study [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF], as clearly seen in Fig. 3a and 3b, which also show the predictions of models A and B. This can be explained because higher concentrations of pentoses are known to be responsible for side reactions, so the lower hemicellulose concentration of both biomasses may actually increase the yields [START_REF] Marcotullio | The Chemistry and Technology of Furfural Production in Modern Lignocellulose-Feedstock Biorefineries[END_REF]. In addition, since FU is generally produced under acid-catalysed conditions [START_REF] Dashtban | Production of furfural: overview and challenges[END_REF][START_REF] Matsagar | Direct Production of Furfural in One-pot Fashion from Raw Biomass Using Brønsted Acidic Ionic Liquids[END_REF], cellulose degradation, which produces a substantial decrease in pH, can potentially enhance the production of FU from OS and BW compared to hemicellulose itself. Moreover, the lower pH reported for OS can also explain the difference between the two biomasses.

The FU yields obtained are in the range of other studies, as already shown in a previous work [START_REF] Borrero-López | High added-value products from the hydrothermal carbonisation of olive stones[END_REF].

Figure 3

Regarding 5-HMF production, overall, the predictions were closer to the experimental data than for FU. Nonetheless, it is worth mentioning the extremely high overestimation shown by model A when considering a constant calculated from the equations (data not shown). The production of 5-HMF was higher for BW for almost all the considered severity factors, which was consistent with its higher cellulose content when compared to OS. Nevertheless, the highest 5-HMF production was obtained from OS at the highest tested severity factor. In summary, the most reliable predictions obtained were those considering equations developed from the application of model B.

Concentrations of phenolic compounds and estimates

It is well known that lignin can produce a wide range of phenolic compounds from the decomposition of its structure into its constituent units [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF][START_REF] Cesari | Antioxidant properties of phenolic surrogates of lignin depolymerisation[END_REF][START_REF] Yang | Analysis of phenolic compounds obtained from bamboo microwave liquefaction for fast-curing phenol-formaldehyde resin preparation[END_REF]. In the present work, nine different types of phenolic compounds have been identified and quantified for OS and BW, the concentrations of which have also been estimated by application of the previous models developed from HTC tests on lignin [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF]. These phenolic compounds are vanillin, guaiacylacetone, syringol, syringaldehyde, guaiacol, acetosyringone, phenol, acetovanillone and creosol. Only the first seven compounds were plotted in Fig. 4 for OS and BW, because the concentrations of acetovanillone and creosol were below the limits of detection (0.0005 and 0.0004 mg/L, respectively) throughout the whole range of conditions studied. The absence of creosol was somehow expected, since previous studies on the HTC of lignin only produced it at severities greater than 6 [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF]. However, the concentration of acetovanillone had comparable values to syringaldehyde [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF], thus the production of acetovanillone should be somewhat limited by the synergistic action of both cellulose and hemicellulose and the corresponding reduction of pH.

Figure 4

Since the concentration of lignin in OS is the highest, one would think that the concentrations of phenolic compounds from this bioresource would be the highest.

However, similar to what occurred with FU production, this was true only for vanillin, guaiacylacetone and final phenol concentration, while the remaining phenolic compounds concentrations from OS were similar to or less than from BW. This fact highlights the extreme importance of the lignin content, which may vary significantly from one bioresource to another. Since the concentration of monolignols and other units may be very different, different concentrations of the resulting products can be obtained [START_REF] Anderson | Differences in S/G ratio in natural poplar variants do not predict catalytic depolymerization monomer yields[END_REF]Santos et al., 2015a;Santos et al., 2015b). Both OS and BW derive from hardwoods, thus close syringyl(S) / guaiacyl(G) (p-hydroxyphenylpropane) ratios are expected. Some authors [START_REF] Simon | Optimization of a formic/acetic acid delignification treatment on beech wood and its influence on the structural characteristics of the extracted lignins[END_REF] have reported that BW has a S/G ratio close to 1, while others suggest lower ratios that depend on the extraction process [START_REF] Choi | Characterization of Residual lignins from chemical pulps of spruce (Picea abies L.) and beech (Fagus sylvatica L.) by analytical pyrolysis-gas chromatography/mass spectrometry[END_REF]. In contrast, the OS S/G ratio was postulated at 1.08 [START_REF] Harman-Ware | Characterization of Endocarp Biomass and Extracted Lignin Using Pyrolysis and Spectroscopic Methods[END_REF]. On the contrary, the lignin used for modelling was PROTOBIND 1075, which comes from softwood, so that the S/G ratio drops dramatically to very low values depending on the initial biomass [START_REF] Anderson | Differences in S/G ratio in natural poplar variants do not predict catalytic depolymerization monomer yields[END_REF], which makes the structure of lignin completely different. Therefore, this fact may compromise the accuracy of the model [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF][START_REF] Li | Study on structure and thermal stability properties of lignin during thermostabilization and carbonization[END_REF][START_REF] Sameni | Physico-Chemical Characterization of Lignin Isolated from Industrial Sources for Advanced Applications[END_REF].

Nevertheless, the predicted data showed adequate fits for the vast majority of them, highlighting the validity and adaptability of the model. However, here again, the influence of the simultaneous HTC of the remaining lignocellulosic biopolymers affects the prediction of the results by modifying the interactions and modifying the composition of the liquid phase. Thus, in our previous work, it has been shown how the final pH from lignin HTC increased with severity [START_REF] Borrero-López | Modelling the reactions of cellulose, hemicellulose and lignin submitted to hydrothermal treatment[END_REF], although, for the bioresources studied in this work, the final pH has decreased as a result of the presence of both cellulose and hemicellulose. Thus, the compounds that depend on acidic or basic catalysis can have a completely different behaviour, as shown for vanillin, acetosyringone, acetovanillone and syringaldehyde, whose concentration was generally overestimated as a result of low pH.

In addition, the general underestimation observed for syringyl-derived units may also result from the higher S/G ratio of hardwoods, with respect to softwoods, which is known to facilitate the effectiveness of delignification [START_REF] Anderson | Differences in S/G ratio in natural poplar variants do not predict catalytic depolymerization monomer yields[END_REF][START_REF] Schmetz | Versatility of a Dilute Acid/Butanol Pretreatment Investigated on Various Lignocellulosic Biomasses to Produce Lignin, Monosaccharides and Cellulose in Distinct Phases[END_REF].

Conclusions

The hydrothermal carbonisation of OS and BW was successfully performed in a wide range of severity factor values. pH, hydrochar and liquid yields as well as some furan and phenolic compounds were determined, and the experimental results were compared to estimates. These estimates were obtained taking into account the hemicellulose, cellulose and lignin contents of the biomasses as well as the results of the fits obtained for the same parameters when these components of biomass were separately submitted to HTC. The following general trends could be derived.

-The final pH of liquids derived from BW was generally higher than that measured in OS-derived liquids, which is in good agreement with their lignocellulosic composition. An average overestimate of 10% of the pH value was obtained because of the synergistic effect between the three lignocellulosic compounds, but the trends were well predicted.

-A similar trend was observed in the hydrochar yield although the agreement between predicted and experimental values was much better (around 7%).

-The liquid yield was higher for the OS because of its higher lignin and lower cellulose contents. A good fit was obtained, reaching about 6 and 16% average error for OS and BW, respectively.

-Extremely high concentrations were obtained for FU, which is attributable to lower rates of side reactions and the influence of the final pH. The concentrations of 5-HMF were lower, and a better approximation was obtained with model B. The concentrations of phenolic compounds were of the same order as those predicted, but highly dependent on the selected biomass and pH.

Although further analysis is needed to explain the unexpected concentrations of FU, the models predicted trends similar to those determined experimentally. A systematic study in which mixtures of biomass components would be submitted to HTC would be necessary to understand the undergoing reaction mechanisms and synergistic effects. The dashed lines represent the predicted values from the models.

Figure Captions Fig. 1 .

 Captions1 Figure Captions

Fig. 3 .

 3 Fig. 3. Concentrations of FU and 5-HMF produced from OS (a & c) and BW (b & d).

Fig. 4 .

 4 Fig. 4. Phenolic compounds concentration from OS (left) and BW (right). The dashed

  Figure 1

  Figure 2

Table 1 .

 1 Performed tests and corresponding values of severity (logR 0 ).

	Olive Stones		Beechwood
	Test conditions	logR 0	Test conditions	logR 0
	160ºC, 6h	4.32	160ºC, 1h	3.54
	170ºC, 2h	4.14	160ºC, 2h	3.85
	170ºC, 4h	4.44	160ºC, 3h	4.02
	170ºC, 6h	4.62	160ºC, 6h	4.32
	180ºC, 2h	4.43	160ºC, 12h	4.62
	180ºC, 4h	4.74	160ºC, 24h	4.92
	180ºC, 6h	4.91	180ºC, 1h	4.13
	190ºC, 4h	5.03	180ºC, 2h	4.43
	190ºC, 6h	5.21	180ºC, 3h	4.61
	190ºC, 8h	5.33	180ºC, 6h	4.91
	200ºC, 2h	5.02	180ºC, 12h	5.21
	200ºC, 4h	5.32	180ºC, 24h	5.51
	200ºC, 6h	5.50	200ºC, 1h	4.72
	220ºC, 2h	5.61	200ºC, 2h	5.02
	240ºC, 1h	5.90	200ºC, 3h	5.20
	240ºC, 2h	6.20	200ºC, 6h	5.50
			200ºC, 12h	5.80
			200ºC, 24h	6.10

Acknowledgements

This work was supported by a grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" program (ANR-11-LABX-0002-01, Lab of Excellence ARBRE) and TALiSMAN project, funded by FEDER (2019-000214). A.M.B-L. acknowledges the Ph.D. Research Grant received from the Ministerio de Educación, Cultura y Deporte (FPU16/03697).

Declaration of interest

The authors declare no competing financial interest.


A L Bursztyn Fuentes

R L S Canevesi

P Gadonneix

S Mathieu

A Celzard

V Fierro

Paracetamol removal by Kon-Tiki kiln-derived biochar and activated carbons

Keywords: Kon-Tiki kiln, activated carbon, biochar, CO 2 activation, paracetamol adsorption

Biochar was obtained from Eucalyptus pruning residues with a non-conventional device named Kon-Tiki kiln. The average heat of combustion of the biochar, 27.3 MJ kg -1 , was higher than that of Eucalyptus wood, 17.8 MJ kg -1 . Activation with CO 2 was performed by varying the activation time from 0 to 60 minutes. The activated carbons (ACs) and the carbon precursor have been characterised and tested for paracetamol removal in the liquid phase, studied in both kinetic and equilibrium aspects. ACs presented an increase in BET area (up to 845 m 2 /g), total pore volume and microporosity with the activation time. The pseudo-second order model was the one that best fitted the experimental data. Elimination of paracetamol was much faster when using ACs, 5h, than when using the biochar, 3 days. However, pollutant removal was greater than 95% for all materials, which is a promising result for lowcost biochars in a difficult economic context. All the adsorption equilibrium experiments exhibited multilayer behaviour, showing values up to 98 mg g -1 for the maximum monolayeradsorption capacity.

Introduction

Pharmaceutical and Personal Care Products (PPCPs) are emerging pollutants, which are increasingly found in treated urban and industrial wastewaters, as well as in watercourses [START_REF] Kolpin | Pharmaceuticals, hormones, and other organic wastewater contaminants in U.S. streams, 1999-2000: A national reconnaissance[END_REF]. PPCPs are not listed as pollutants in the World Health Organization guidelines for drinking water quality [START_REF] Who | Guidelines for Drinking-water Quality[END_REF] and, therefore, they are not regulated. For this reason, studies on their effects on human and environmental health are still rare [START_REF] Daughton | Pharmaceuticals and Personal Care Products in the Environment : Agents of Subtle Change?[END_REF]. Although PCPCs are present in low concentrations in aquatic ecosystems, typically at trace levels (ng L -1 to μg L -1 ), they can still induce adverse effects due to cumulative effects and continuous exposure [START_REF] Boudrahem | Single and mixture adsorption of clofibric acid, tetracycline and paracetamol onto Activated carbon developed from cotton cloth residue[END_REF][START_REF] Dordio | Preliminary media screening for application in the removal of clofibric acid, carbamazepine and ibuprofen by SSF-constructed wetlands[END_REF].

Paracetamol (N-4-hydroxyphenylacetamide or acetaminophen), a common analgesic and anti-inflammatory for humans and animals, is usually chosen as a model molecule [START_REF] García-Mateos | Removal of paracetamol on biomass-derived activated carbon: Modeling the fixed bed breakthrough curves using batch adsorption experiments[END_REF][START_REF] Lladó | Role of activated carbon properties in atrazine and paracetamol adsorption equilibrium and kinetics[END_REF][START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: Remarkable adsorbents for pharmaceutical compounds[END_REF][START_REF] Mestre | Activated carbons from sisal waste by chemical activation with K 2CO 3: Kinetics of paracetamol and ibuprofen removal from aqueous solution[END_REF][START_REF] Spessato | Synthesis of superparamagnetic activated carbon for paracetamol removal from aqueous solution[END_REF][START_REF] Terzyk | The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro. Part II. TG, FTIR, and XPS analysis of carbons and the temperature dependence of adsorption kinetics at the neutral pH[END_REF]Terzyk et al., 2003;[START_REF] Terzyk | The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: The temperature dependence of adsorption at the neutral pH[END_REF] because it is a pharmaceutical compound used worldwide and does not require medical prescription. Depending on the source, 58 to 90% of the ingested paracetamol and its metabolites are excreted by the human body without being metabolised [START_REF] Correia | Oxidative effects of the pharmaceutical drug paracetamol on the edible clam Ruditapes philippinarum under different salinities[END_REF][START_REF] Wu | Paracetamol in the environment and its degradation by microorganisms[END_REF]. In surface waters, the median concentration of paracetamol detected was 0.055 ± 0.051 μg L -1 [START_REF] Bound | Predicted and measured concentrations for selected pharmaceuticals in UK rivers: Implications for risk assessment[END_REF][START_REF] Gros | Development of a multi-residue analytical methodology based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) for screening and trace level determination of pharmaceuticals in surface and wastewaters[END_REF][START_REF] Wiegel | Pharmaceuticals in the river Elbe and its tributaries[END_REF]) although higher concentrations have been measured. Paracetamol has been found in concentrations up to 6 µg L -1 in effluents from European sewage treatment plants, up to 10 µg L -1 in USA natural waters, and higher than 65 μg L -1 in the Tyne River in the UK [START_REF] Wu | Paracetamol in the environment and its degradation by microorganisms[END_REF]. In raw wastewater, paracetamol was detected at a median concentration of 48 ± 75 μg L -1 but in raw hospital effluents and some wastewater effluents, paracetamol concentrations can even exceed 150 μg L -1 [START_REF] Wu | Paracetamol in the environment and its degradation by microorganisms[END_REF]. Activated carbons (ACs) have been widely studied to remove contaminants from water, and PPCPs are no exception [START_REF] Acosta | Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char[END_REF][START_REF] Baccar | Removal of pharmaceutical compounds by activated carbon prepared from agricultural by-product[END_REF][START_REF] Marques | Pharmaceuticals removal by activated carbons: Role of morphology on cyclic thermal regeneration[END_REF][START_REF] Mestre | Wastederived activated carbons for removal of ibuprofen from solution: Role of surface chemistry and pore structure[END_REF][START_REF] Mestre | Activated carbons for the adsorption of ibuprofen[END_REF][START_REF] Biochar | Tetracycline removal with activated carbons produced by hydrothermal carbonisation of Agave americana fibres and mimosa tannin[END_REF]. Pyrolysis of biomass at the laboratory scale for the production of ACs requires specific and expensive equipment both for carbonisation and activation steps. Commercial carbons are usually physically activated with steam, CO 2 , air or mixtures, or are chemically activated by adding zinc salts, KOH or H 3 PO 4 to the carbon precursors [START_REF] Zhang | Preparation of activated carbon from forest and agricultural residues through CO2 activation[END_REF]. After chemical activation, an extensive washing step with water and sometimes HCl is needed [START_REF] Fierro | Kraft lignin as a precursor for microporous activated carbons prepared by impregnation with ortho-phosphoric acid: Synthesis and textural characterisation[END_REF][START_REF] Schaefer | Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials[END_REF]. Despite the desired chemical and textural properties are successfully achieved by chemical or physical activation, the overall yield of the process is sometimes quite low, especially by KOH activation [START_REF] Basta | 2-Steps KOH activation of rice straw: An efficient method for preparing high-performance activated carbons[END_REF][START_REF] Szczurek | Chemical activation of tannin-based hydrogels by soaking in KOH and NaOH solutions[END_REF].

This study focuses on the development of low-cost adsorbent production strategies. In this sense, Schmidt and Taylor designed a non-conventional carbonisation device named Kon-Tiki kiln, which allows the production of large amounts of carbonaceous material with minimal infrastructure and without external energy supply, promoting the reuse of agricultural byproducts [START_REF] Schmidt | Kon-Tiki -the democratization of biochar production[END_REF]. It follows the principle of pyrolysis of biomass layer after layer in an open metal kiln, conical in shape, easy to use, fast and resulting in low greenhouse gas emissions [START_REF] Cornelissen | Emissions and char quality of flame-curtain "Kon iki" kilns for farmer-scale charcoal/biochar production[END_REF]. The process of carbonisation using this device resembles the carbon production by ancient civilisations and small contemporary communities, which proves that it is well known, easy to reproduce and cheap. Kon-Tiki kilnderived biochar has been used for soil amendment as it possesses chemical properties and agronomic effects similar to those seen for biochars produced in other types of kilns [START_REF] Pandit | Biochar from "Kon iki" flame curtain and other kilns: Effects of nutrient enrichment and kiln type on crop yield and soil chemistry[END_REF].

The aim of this study was to produce biochar from Eucalyptus pruning residues in a Kon-Tiki kiln and to use them as such or more carbonised and activated with CO 2 , for the removal of paracetamol removal in water.

Materials and Methods

Biochar and activated carbons (ACs) preparation

Biochar was obtained from the urban pruning of Eucalyptus branches from La Plata, Argentina. Charring was performed in a Kon-Tiki kiln according to the procedure presented by [START_REF] Schmidt | Kon-Tiki -the democratization of biochar production[END_REF]. In short, fire was first produced inside the kiln with small branches (diameter d about 1 cm) and, as soon as a layer of ash was perceived, new larger branches were added to the kiln (d < 4 cm). The kiln was fed constantly using the same criteria until all the selected biomass was consumed. Apart from the addition of more feedstock when the flames are low, the temperature in the Kon-Tiki kiln cannot be controlled, but the operating and investment costs are much lower than those of traditional carbonisation kilns. The maximum temperature recorded before quenching was 700ºC and was measured by using a copper-aluminium K-type thermocouple sheathed in an alumina tube and coupled to a digital thermometer. Figure SI 1 shows an image of the device in operation. Then, distilled water was poured to put out the fire. The biochar thus obtained was then filtered and dried at 105°C for 48h. The biochar was ground and sieved, and the particle fraction used in this study was 600-250 µm (mesh 30-60, ASTM). The material was labelled as CV.

Physical activation with CO 2 was carried out in a quartz tube reactor installed in a horizontal furnace. The inner diameter of the quartz tube was 3.5 cm and its length was 100 cm. A quartz boat containing between 0.5 and 0.6 g of CV was located in the centre of the quartz tube and the system was purged for 1 h with a 100 mL min -1 nitrogen flow to eliminate any trace of oxygen before heating. Then, the oven was heated at 5°C min -1 to 900°C, and the latter temperature was held for 1h. Then, the nitrogen flow was switched to a 25 mL min -1 CO 2 flow, which was maintained for a given time of 15, 30 or 60 min. After this time, the sample was cooled under N 2 flow. The resultant activated carbons (ACs) were labelled as CV15, CV30 and CV60, respectively. The ACs obtained were weighed and the yield of the process (Y) was calculated according to equation ( 1):

(1) where W CH is the initial weight of biochar and W C is the weight of the sample after activation.

A reference carbon material was prepared by heating CV up to 900°C for 1h without CO 2 activation and it was labelled CV0.

Materials characterisation

Carbon, hydrogen, nitrogen and sulphur contents in all carbonaceous materials were determined using an elemental analyser (Vario El Cube, Elementar). Oxygen was also determined in the same equipment in a second step. The measurements were performed in duplicate. The ash content was estimated by difference.

The morphology of the samples was studied with a Scanning Electron Microscope (FEG-SEM Hitachi S 4800) equipped with an EDX (Energy Dispersion of X-rays) instrument. The latter was used for semi-quantitative analysis and element mapping.

Calorific tests were carried out in a 6200 Isoperibolic Calorimeter (Parr) in order to determine the enthalpy of combustion of the raw biomass (Eucalyptus wood) and the biochar (CV). For that purpose, sample weights between 0.6 and 0.9 g were used and the variation of temperature while burning in an excess of oxygen was recorded. The measurements were performed with a resolution of 0.0001°C over a working range of 20 to 40°C.

The porous texture was characterised by N 2 adsorption-desorption at -196°C and by CO 2 adsorption at 0°C, using an automatic equipment (ASAP 2020, Micromeritics). First, the samples were outgassed at 110°C under secondary vacuum for at least 48h. The N 2 isotherms were used to estimate the apparent surface area, A BET (m 2 g -1 ), by applying the BET equation in the adequate range of relative pressures (p/p 0 ), and the total pore volume, V T (cm 3 g -1 ), at p/p 0 = 0.97. The p/p 0 range was chosen by plotting V ads  (1 -p/p 0 ) as a function of p/p 0 , V ads being the STP adsorbed volume, and we took the range of values of p/p 0 for which V ads  (1p/p 0 ) increased with p/p 0 . The micropore volume, V DR (cm 3 g -1 ), was determined from the application of the Dubinin-Radushkevich (DR) equation to the N 2 adsorption, V DR,N2 , and to the CO 2 adsorption isotherms, V DR,CO2 [START_REF] Dubinin | Fundamentals of the theory of adsorption in micropores of carbon adsorbents: Characteristics of their adsorption properties and microporous structures[END_REF]. The mesopore volume, V MESO (cm 3 g - 1 ), was calculated as the difference between V 0.97 and V DR,N2 . The average pore size, L 0 (nm), was calculated using the following equation:

(2) where E 0 (kJ mol -1 ) is the characteristic adsorption energy derived from the application of the DR method to the N 2 adsorption isotherms [START_REF] Stoeckli | Pore size distributions of active carbons assessed by different techniques[END_REF]. The 2D NLDFT-HS method was applied to obtain the pore size distributions (PSD) from both the N 2 and CO 2 adsorption isotherms. Surface areas, S NLDFT (m 2 g -1 ), micropore volumes, V , NLDFT (cm 3 g -1 ), and total pore volumes, V T, NLDFT (cm 3 g -1 ), were also obtained from the PSDs.

The textural information was supplemented by mercury porosimetry (Autopore IV, Micromeritics) to obtain macro and mesopore size distributions. The skeletal density of the solids was obtained by helium pycnometry (AccuPyc II 1340, Micromeritics), using samples previously degassed at 100°C under vacuum.

Given that the material yield is as important as the surface area, the total surface area, A TOTAL (m 2 g -1 ), can be used as a criterion for selecting materials for further testing [START_REF] Acosta | Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char[END_REF][START_REF] Fierro | Rice straw as precursor of activated carbons: Activation with ortho-phosphoric acid[END_REF]. It reads:

L (3)
The pH at the point of zero charge, pH PZC , which is the pH where the net charge of the surface is zero, was determined according to Carrott et al. [START_REF] Carrott | Thermal treatments of activated carbon fibres using a microwave furnace[END_REF]. Briefly, 15 mg of material was added to 15 mL of NaNO 3 solution (0.1 mol L -1 ), bubbled with N 2 for 2 min, sealed and placed in a stirrer. The initial pH of the solution was recorded, and, after 48h of stirring, the suspension was filtered and the equilibrium pH was measured.

The amount of proton-binding groups was also measured. For that purpose, 0.1g of activated carbon was suspended in 50 mL of NaNO 3 solution (0.01 mol L -1 ) as support electrolyte and was stirred overnight to equilibrate. The suspension was then titrated with NaOH (0.1 mol L -1 ) under saturation with N 2 [START_REF] Jagiello | Stable Numerical Solution of the Adsorption Integral Equation Using Splines[END_REF] using an automatic titrator (905 Titrando, Metrohm controlled by Tiamo® V2.2 software). The titration curves, pH vs volume, were transformed to proton-binding isotherms, Q vs pH, using the proton balance equation.

Here, Q is equal to the total amount of protonated sites on the material. The relationship between the experimental proton-binding isotherms and pK a values is given by an integral equation [START_REF] Jagiello | Determination of Proton Affinity Distributions for Chemical Systems in Aqueous Environments Using a Stable Numerical Solution of the Adsorption Integral Equation[END_REF]. The integration of this function over finite intervals, arbitrarily defined, gives the amount of sites whose pK a values belong to these intervals. For the evaluation of the unknown distribution, f (pK a ), the numerical SAIEUS-pK-Dist program was used.

Paracetamol adsorption studies 2.3.1 Kinetic studies

The materials were tested as paracetamol adsorbents in liquid phase. A paracetamol stock solution of concentration 180 mg L -1 was prepared without adjusting the initial pH, which was equal to 6. 98% pure paracetamol (Acros Organics) was used. The 2D and3D For kinetic studies, 15 mg of carbon and 15 mL of paracetamol solution were mixed in a glass flask that was introduced into a water bath at 30°C and stirred at 500 rpm in a 15position stirring plate (Mutistirrer 15, VELP Scientifica). After the desired contact time, an aliquot was removed and the amount of paracetamol remaining in the solution was determined by UV spectroscopy at 243 nm (Lambda 35 UV/VIS Spectrometer, Perkin Elmer). All tests have been carried out in duplicate. The effect of the initial paracetamol concentration, 10, 20 and 40 mg L -1 was studied.

The paracetamol uptake was calculated according to the following equation:

(4) where (mg g -1 ) is the amount of paracetamol removed at time , (mg L -1 ) is the initial concentration of paracetamol, (mg L -1 ) is the concentration of paracetamol at time , (L) is the volume of the solution and (g) is the weight of dry carbon.

Kinetic modelling

Two different kinetics models were applied to describe the experimental data. The first was the pseudo-first order (PFO) kinetic model proposed by Lagergren [START_REF] Lagergren | Zur theorie der sogenannten adsorption gelöster stoffe, Kungliga Svenska Vetenskapsakademiens[END_REF], mathematically represented by equation ( 5):

(5) where q t and q e are the adsorbed amounts (mg g -1 ) at time t and at equilibrium, respectively, and k 1 (min -1 ) is the kinetic rate constant of pseudo-first order.

The second kinetic model applied was the pseudo-second order (PSO) kinetic model proposed by [START_REF] Ho | Pseudo-second order model for sorption processes[END_REF], and mathematically represented by equation ( 6):

(6)
where k 2 (g mg -1 min -1 ) is the pseudo-second order rate constant.

The product represents the initial adsorption rate, h. The half-life time, t½, which is the time required for the adsorbent to uptake half of the adsorbate at equilibrium and which is often used as a measure of adsorption rate was also calculated. For PFO kinetic model, it is calculated according to equation ( 7) and, it is calculated according to equation ( 8) for the PSO:

(7) (8)

Equilibrium tests

Adsorption studies at equilibrium were carried out at 20, 30 and 40°C by varying the paracetamol concentration (5 -180 mg L -1 ) for the materials CV and CV15. After 5 days and 24h of stirring, respectively, to guarantee equilibrium, the sample suspensions were filtered and diluted if necessary. Then, the concentration of paracetamol remaining in solution at equilibrium (C e ) was determined, and the corresponding uptake (q e ) was calculated using equation (4). Experiments were done in duplicate.

The isotherms were modelled with the Langmuir, Freundlich and Sips equations ( 9), ( 10) and ( 11), respectively:

(9)

where and have the same meaning as before, K L (L mg -1 ) is the Langmuir constant, and

(mg g -1 ) is the monolayer adsorption capacity.

(

) 10
where K F (mg 1-1/n L 1/n g -1 ) is the Freundlich constant and n (dimensionless) is the Freundlich exponent.

(

) 11
where K S (L mg -1 ) is the Sips constant, and n S (dimensionless) is the heterogeneity factor.

The BET liquid adsorption equilibrium model was also fitted to the experimental data, since this model considers the possibility of multilayer adsorption. The model is mathematically represented by equation ( 12).

(

) 12
where K BET (L mg -1 ) is the BET constant, and C s (mg g -1 ) is the saturation concentration of paracetamol in water.

Parameter estimation

All the parameters were estimated using as an objective function the sum of the square of errors, obtained through equation ( 13):

(13)

where F OBJ is the objective function value, n dat is the number of experimental points and Y i,exp and Y i,mod are respectively the experimental and predicted value for the experimental point i.

The method of Nelder and Mead was applied to such optimisation, using the mathematic software Maple®.

Results and discussion

Characterization of the carbon materials

Calorific value and elemental composition

The average heat of combustion of CV (27.3 MJ kg -1 ) was higher than that of wood (17.8

MJ kg -1 ). These calorific values are consistent with what has been reported in the literature for different species of Eucalyptus [START_REF] Gaqa | The Properties and Suitability of Various Biomass/Coal Blends for Co-Gasification Purposes[END_REF][START_REF] Heidari | Effect of process conditions on product yield and composition of fast pyrolysis of Eucalyptus grandis in fluidized bed reactor[END_REF][START_REF] Khider | Heat Value of Four Hardwood Species from[END_REF][START_REF] Kumar | Effect of tree-age on calorific value and other fuel properties of Eucalyptus hybrid[END_REF] and are slightly lower than for Eucalyptus-derived chars produced by fast pyrolysis using a pilot-scale fluidised bed reactor [START_REF] Heidari | Effect of process conditions on product yield and composition of fast pyrolysis of Eucalyptus grandis in fluidized bed reactor[END_REF]. These trends have also been reported for other agroforestry tree crops [START_REF] Fuwape | Charcoal and fuel value of agroforestry tree crops[END_REF][START_REF] Khider | Heat Value of Four Hardwood Species from[END_REF] and for other types of biomass such as corn crop residues [START_REF] Mullen | Bio-oil and bio-char production from corn cobs and stover by fast pyrolysis[END_REF]. The calorific value obtained here indicates that CV can be potentially used as fuel.

Table 1 presents the elemental analysis of the different materials studied. The carbon content in the original sample (CV) was 81.8 wt. % and it slightly decreased after carbonisation at 900°C to 80.6 wt. %. Activation with CO 2 further decreased the carbon content due to evolution of volatiles and enrichment in ashes of the activated carbons. Thus, the ash content of the CV sample was 4.4 wt. %, and increased up to 10.7 wt. % after carbonisation at 900°C and even to 46.4 wt. % after CO 2 activation for 60 min. The hydrogen and nitrogen contents were low, and there was no significant difference between the materials. The SEM images in Figure 1a show that the wood structure was preserved after carbonisation and activation, and an increase in the ash content can be observed with activation, see Figure 1b. Thus, CV60 contained nearly 50 wt .% of ashes, clearly seen in the photo. A basic characterisation of this material was done just to illustrate trends, but no further paracetamol adsorption tests was performed on it. EDX analysis allowed to identifying and mapping inorganic components in the ashes, see Figure 1c. Among these, calcium was the major element identified ranging from 6 wt. % in CV to 30 wt. % in CV30. Mg, K, Fe and Al contents ranged between 2 wt. % and 6 wt. %, with no clear trend. Other elements such as Si, P, Na, Cu and Mn were identified but at lower percentages (<1 wt. %). These results are consistent with already reported ash composition of Eucalyptus wood [START_REF] Girón | Fly ash from the combustion of forest biomass (Eucalyptus globulus bark): Composition and physicochemical properties[END_REF][START_REF] Khanna | Chemical properties of ash derived from Eucalyptus litter and its effects on forest soils[END_REF].

Figure 1

Textural properties

Figure 2a shows the N 2 adsorption/desorption isotherms of the materials. The original sample (CV) presents, according to the IUPAC classification, a type I isotherm with a marked knee in the region of low relative pressure and a slight slope at higher relative pressures [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. This is characteristic of essentially microporous materials with a narrow micropore size distribution and an absence of significant mesoporosity, as also shown by the PSD (Figure 2d). The longer the activation time, the wider the knee and the higher the slope, indicating a widening of the micropore size distribution and the development of mesoporosity. The hysteresis loop appears and becomes more marked with the activation time, which is attributed to capillary condensation in the mesopores [START_REF] Lin | Mesoporous carbons from waste tire char and their application in wastewater discoloration[END_REF].

Figure 2 It is well known that following the activation step, which is a controlled gasification of the carbon, part of the material is lost (called burn-off, BO (%) = 100 -Y (%). The extent of the activation influences the morphology and the textural properties of the activated carbons (Al [START_REF] Bahri | Diuron Multilayer Adsorption on Activated Carbon from CO2Activation of Grape Seeds[END_REF]. As shown in Table 1, the carbon yield decreased with the activation time:

from 81.7% for the reference material (CV0) to 10.8% after 60 minutes of exposure to CO 2 .

Concerning the BET area (A BET ), see Figure 3, activation with CO 2 increased it from 392 to 845 m 2 g -1 from CV to CV30, respectively. CO 2 developed the pore texture over the entire range of pore diameters, in particular the microporosity by opening closed pores as well as widening existing ones, in agreement with previous studies [START_REF] Rodríguez-Reinoso | Surface heterogeneity effects of activated carbons on the kinetics of paracetamol removal from aqueous solution[END_REF].

The pore texture parameters of all carbon materials are presented in Figure 3 and in Table SI 4. By comparing CV to CV0, A BET increased from 392 to 498 m 2 g -1 , due to the usual volatile evolution when increasing the temperature from 700°C (typical Kon-Tiki charring temperature) to 900°C. The tars produced during pyrolysis at 700°C could partially block the micropores that would be opened at 900°C [START_REF] Ibarra | Catalytic effects of zinc chloride in the pyrolysis of Spanish high sulphur coals[END_REF], as suggested by the values of L 0 that did not change when increasing the pyrolysis temperature from 700 to 900°C (i.e., from CV to CV0, see Table SI 4). A BET reached a maximum value of 845 m 2 g -1 after 30 min of CO 2 activation but decreased by around 10% when the activation time was 60 min (Figure 3 and Table SI 4). Despite the fact that the ash-free fraction of CV60 may have a very high surface area, this material also has a high ash content and ashes do not have a significant surface area. Except for CV60, all the materials had higher V DR, CO2 values than V DR, N2 , suggesting that a significant part of the microporosity was not accessible to N 2 at the equilibrium times given for this analysis. This also means that, to give an accurate overview of the textural properties of the materials, both N 2 and CO 2 adsorption isotherms must be taken into account, as this has been shown in Figure 2c. Therefore, V DR, N2 , V DR, CO2 and V 0.97 are given for comparison with textural data reported in the open literature, but V , NLDFT and V T, NLDFT give a more precise indication of the microporosity and total porosity, respectively (see Figure SI 3 and Table SI 4).

Figure 3 shows the evolution of the A BET , A TOTAL , S NLDFT , the total and micropore volume and the average pore size with the burn-off (BO, wt. %). While A BET and S NLDFT showed a marked increase from 392 to 845 m 2 g -1 and from 600 to 1000 m 2 g -1 at 30 min of activation (58.9 % BO), respectively, A TOTAL only slightly increased from 392 to 415 m 2 g -1 at 15 min of activation (39.6% BO) and markedly decreased afterwards. The microporosity fraction decreased with the increase in the burn-off. Based on previous studies [START_REF] Acosta | Tetracycline adsorption onto activated carbons produced by KOH activation of tyre pyrolysis char[END_REF][START_REF] Basta | 2-Steps KOH activation of rice straw: An efficient method for preparing high-performance activated carbons[END_REF][START_REF] Biochar | Tetracycline removal with activated carbons produced by hydrothermal carbonisation of Agave americana fibres and mimosa tannin[END_REF], we selected CV15 for further paracetamol adsorption studies because a good compromise between A BET and Y was reached.

Figure 3 Figure 2d shows that CV presented a bimodal PSD, with a narrow peak centred at 0.4 nm and some pores with a diameter of 0.75 nm. The PSD widened with the activation time up to CV30 at the same time as the pores narrower than 1 nm increased. CV60 showed a continuous PSD with an enlargement of the pore size. Besides, mercury porosimetry suggests a higher volume of mesopores in the activated carbons (Figure 4).

Figure 4

Figure 4 shows the PSD in the mesopore (2-50 nm) and macropore (> 50 nm) ranges.

However, the equipment used for mercury intrusion, with a maximum intrusion pressure of 414 MPa, only allows access to pores with diameters greater than 3.6 nm. CV and CV0

showed very similar PSDs, but the amount of pores narrower than 100 nm decreased compared to those of larger diameter, in agreement with the higher ash content of ACs produced at longer activation times. The bulk density of the materials calculated at a mercury intrusion pressure of 0.0036 MPa decreased from 0.21 g cm -3 for CV to 0.16 g cm -3 for CV60, which suggests the increase of porosity of the materials with the activation time, not only in the micropore range already evidenced by N 2 + CO 2 adsorption, but also in the mesopore and macropore ranges.

Surface chemistry

With respect to surface chemistry, the values of pH PZC shown in Table 1 suggest that CV and derived ACs have a basic nature. Although the ash contents were only quantified for raw eucalyptus biomass and CV, a simple visual analysis of Figure 1b and the estimation presented from the elemental analysis revealed that ashes became noticeable as the activation time increased, and were evident in CV60. Then, a basic nature of the ACs was expected and it should be greatly influenced by the ash content.

A more detailed analysis was performed on samples CV and CV15 to study the modification of surface functionalities with the activation process. Figure 5 shows the results of the potentiometric titration of these materials. Activation increased the density of functional groups from 0.63 to 1.04 mmol g -1 (see Table 2). The number of peaks, representing different types of functional groups, was 4 and 5 for CV and CV15, respectively.

The pK a distributions showed the predominance of strongly basic species such as lactol-or hydroxyl-containing functional groups with pK a > 8 [START_REF] Seredych | Oxygen reduction on chemically heterogeneous iron-containing nanoporous carbon: The effects of specific surface functionalities[END_REF]. The basicity of CV and CV15 materials was confirmed by the high value of pH PZC , around 8 and 11, respectively (see again Table 1).

Figure 5 Table 2. Results of potentiometric titration measurements: peak position and number of types of functional groups (mmol/g, in brackets); the pH PZC is also recalled. This analysis is relevant given that changes in the surface chemistry of activated carbons can also affect the adsorption process of pollutants by dissociation of functional groups on the adsorbent surface (Ruiz et al., 2010).

Removal of paracetamol in the liquid phase

Paracetamol adsorption kinetics

Figure 6 shows the kinetics of paracetamol adsorption at 30°C on CV, CV0, CV15 and CV30 for initial paracetamol concentrations of 10, 20 and 40 mg L -1 . While CV took almost 3 days to equilibrate, CV15 and CV30 reached equilibrium in 1h at C 0 = 10 mg L -1 and 5 h at C 0 = 40 mg L -1 . CV0 reached equilibrium in about 5h at C 0 = 10 mg L -1 and 12h at C 0 = 40 mg L -1 . For the three ACs (CV0, CV15 and CV30), the removal of paracetamol at equilibrium was complete for the three concentrations tested. The experimental kinetic data were best fitted by the pseudo-second order kinetic model.

Figure 6

Table SI 5 shows the kinetic parameters derived from the application of PFO and PSO models, and the corresponding determination coefficients R 2 . For the three concentrations of paracetamol tested, a longer activation time had a positive effect on the initial (h) and global (k 1 and k 2 ) adsorption rates, and reduction of the adsorption half-time (t 1/2 ), i.e., the time required to reach half of the paracetamol uptake at equilibrium. For example, t 1/2 at the highest initial concentration (40 mg L -1 ) for CV15 and CV30 was 6 minutes while it was 282 min for CV. Then, it can be concluded that the adsorption process of paracetamol is faster with activation. Given the dimensions of the paracetamol molecule (0.46 nm and 0.66 nm for monomer and dimer, respectively), micropores are known to be the active sites with the highest adsorption potential for its removal from the liquid phase [START_REF] Mestre | Wastederived activated carbons for removal of ibuprofen from solution: Role of surface chemistry and pore structure[END_REF]. Thus, a higher volume of micro-and mesopores and a wider micropore size distribution should allow a faster diffusion of paracetamol molecules in the materials.

The initial concentration of paracetamol did not show a marked effect on the PFO constant.

However, the parameter k 2 decreased with the increase in C 0 , in agreement with previous studies of adsorption of diuron [START_REF] Bahri | Diuron Multilayer Adsorption on Activated Carbon from CO2Activation of Grape Seeds[END_REF] or ibuprofen [START_REF] Mestre | Activated carbons for the adsorption of ibuprofen[END_REF].

Regarding the removal efficiency, there was no significant difference between the ACs for all the concentrations tested (removal between 96 and 99%). For low initial concentrations of paracetamol (10 mg L -1 ), the CV removed as much paracetamol as ACs at equilibrium, but its performance decreased with the increase in C 0 . This is an interesting result, given that this type of pollutant is present at low concentrations in the environment. Since the production of CV is an easy and reproducible process, which does not require any exceptional equipment, it is a promising material to continue exploring in difficult economic contexts.

Paracetamol equilibrium adsorption studies

The paracetamol adsorption isotherms displayed in Figure 7 suggest that the adsorption mechanisms of the original carbon (CV) and the activated carbon CV15 were the same, as both were type II [START_REF] Brunauer | On a Theory of the van der Waals Adsorption of Gases[END_REF]. The isotherm obtained for CV15 has two defined regions: in the first, a steep initial rise that approaches a plateau is observed and, in the second region, an upward curvature is recorded for higher equilibrium paracetamol concentrations.

The former phenomenon is related to the formation of a complete monolayer, while the latter is explained by multilayer or aggregate formation at the interface at high coverage [START_REF] Girods | Activated carbons prepared from wood particleboard wastes: Characterisation and phenol adsorption capacities[END_REF]. The formation of multilayers could be related to the characteristics of the adsorbent surface and to the pH of the medium, which could affect the interactions between the solvent, the adsorbate and the adsorbent (Al [START_REF] Bahri | Diuron Multilayer Adsorption on Activated Carbon from CO2Activation of Grape Seeds[END_REF]. This type of isotherm has also been reported by [START_REF] Terzyk | The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: The temperature dependence of adsorption at the neutral pH[END_REF] for some chemically ACs when removing paracetamol at similar concentrations, but this is not the usual behaviour reported in the literature concerning paracetamol adsorption. However, it has been also observed for other pollutants such as phenol [START_REF] Girods | Activated carbons prepared from wood particleboard wastes: Characterisation and phenol adsorption capacities[END_REF], cationic dyes [START_REF] Cotoruelo | Adsorption of oxygen-containing aromatics used in petrochemical, pharmaceutical and food industries by means of lignin based active carbons[END_REF] and diuron [START_REF] Bahri | Diuron Multilayer Adsorption on Activated Carbon from CO2Activation of Grape Seeds[END_REF]. Figure 7a shows a higher paracetamol uptake at 20°C compared to that determined at 30°C, which could be related to the experimental uncertainty (the error bars of the two temperatures indeed overlap), as well as to small uptake differences between these two fairly close temperatures.

Figure 7 Langmuir, Freundlich and Sips models were fitted to the data corresponding to CV and CV15 samples (see Table SI 6 and Figures SI 10 and SI 11). In addition, the Langmuir and Freundlich models were applied to the monolayer region only (see Table SI 7 andFigure SI 8). This approach gave results different from those obtained when applied to the entire range of equilibrium experimental data. Since the isotherms were type II, multilayer adsorption may be better described by applying the BET equation in the liquid phase [START_REF] Brunauer | Adsorption of Gases in Multimolecular Layers[END_REF].

Table SI 9 summarises the parameters derived from this model.

The monolayer adsorption capacities q m derived from the application of the BET model were equal to the values obtained when the Langmuir model was applied to the monolayer region. The values obtained were in the range of 14.4 to 14.9 mg g -1 and 88.1 to 98.2 mg g -1

for CV and CV15, respectively. In addition, the ratios of the K BET and C s parameters were very close to the values of K L of the monolayer region. This proximity of the parameters means that at low concentration, the two models converge towards a very similar value of Henry's constant.

As far as the effect of temperature is concerned, there is no clear trend. The calculated parameters, such as the equilibrium constant K L and the monolayer capacity q m of the Langmuir equation, do not change gradually with increasing temperature. Contrasting results have been reported in literature. Consistent with our study, some authors ( [START_REF] Baccar | Removal of pharmaceutical compounds by activated carbon prepared from agricultural by-product[END_REF] and [START_REF] Mestre | Activated carbons for the adsorption of ibuprofen[END_REF]) did not notice any effect of temperature on the removal of pharmaceutical products, among them ibuprofen and diclofenac. More specifically on paracetamol, while García-Mateos et al. [START_REF] García-Mateos | Removal of paracetamol on biomass-derived activated carbon: Modeling the fixed bed breakthrough curves using batch adsorption experiments[END_REF] reported an exothermic nature of the adsorption process (i.e., the uptake decreased with temperature), others [START_REF] Terzyk | The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: The temperature dependence of adsorption at the neutral pH[END_REF] and [START_REF] Cotoruelo | Adsorption of oxygen-containing aromatics used in petrochemical, pharmaceutical and food industries by means of lignin based active carbons[END_REF]) observed an endothermic behaviour (i.e., the uptake increased with temperature ). Moreover, Villaescusa et al. [START_REF] Villaescusa | Mechanism of paracetamol removal by vegetable wastes: The contribution of Π-Π interactions, hydrogen bonding and hydrophobic effect[END_REF] reported similar sorption capacity when the temperature was maintained in the range of 5-20°C, but a slight increase in sorption was observed when the temperature was increased to 25-30°C.

Comparison with data from the literature

Figure 8 collects information related to paracetamol removal with different types of activated carbons and modified carbons reported in the literature [START_REF] Cabrita | Removal of an Analgesic Using Activated Carbons Prepared From[END_REF][START_REF] Cotoruelo | Adsorption of oxygen-containing aromatics used in petrochemical, pharmaceutical and food industries by means of lignin based active carbons[END_REF]Galhetas et al., 2014aGalhetas et al., , 2014b;;[START_REF] García-Mateos | Removal of paracetamol on biomass-derived activated carbon: Modeling the fixed bed breakthrough curves using batch adsorption experiments[END_REF][START_REF] Mestre | Sustainable activated carbons prepared from a sucrose-derived hydrochar: Remarkable adsorbents for pharmaceutical compounds[END_REF] and in the present study. Comparison with the literature is sometimes difficult because experimental variables chosen for materials synthesis and removal assays differ considerably from one study to another. For instance, the paracetamol concentrations tested here are significantly lower, but more realistic, than those used in other articles, whereas they have a very high influence on the adsorption capacities obtained in this work.

Figure 8 There is a trend (R 2 = 0.78) that suggests that the higher the surface area, the higher the maximum paracetamol removal capacity (q m ). As mentioned in many articles, surface chemistry also influences the removal capacity, and different activation processes and/or heteroatom doping can introduce changes in the surface functionalities of the ACs. Hence, the surface functional groups and the net surface charge might explain the variability remaining unexplained in the material performance.

Conclusion

Four microporous activated carbons were obtained from a biochar precursor, called CV, produced with Eucalyptus pruning residues charred in a simple and low-cost device, a Kon-Tiki kiln. Additional CO 2 activation significantly enhanced the specific surface area, the total pore volume and the micropore volume.

The equilibrium paracetamol (PC) adsorption was reached at 5h for the activated carbons and at 3 days for their precursor at the highest initial paracetamol concentration tested, and a pseudo-second order model suitably fitted the experimental data. The textural characterisation suggested an opening and widening of micropores and a development of mesopores with CO 2 activation, which favoured the diffusion of paracetamol molecules in the material.

At low concentrations, which are the most realistic for this type of contaminants, CV had a very high removal efficiency, similar to those of the activated carbons tested (> 95 %). A simple adjustment (i.e., an increase) in the solid / liquid ratio might be enough to reduce the time for the efficient treatment of water in vulnerable communities with such inexpensive carbon material.

Concerning paracetamol uptake at equilibrium, CV and CV15 showed a favourable adsorption and the corresponding isotherms suggested the formation of paracetamol multilayers. The equilibrium adsorption capacities of CV15, whether for monolayer or multilayer adsorption, were about 5 times higher than for CV. This has been attributed to the more developed textural properties of CV15. 2015;2-Galhetas et al, 2014a;3-Galhetas et al, 2014b;4-Cotoruelo et al, 2011;5-Cabrita et al, 2010;6-Spessato et al., 2020;7-Lladó et al., 2015 and8-Marques et al., 2017).

Captions of the

Paracetamol removal by Kon-Tiki kiln-derived biochar and activated carbons

Bursztyn Fuentes et al.

  structures of paracetamol are shown in Figure SI 2.

FiguresFigure 1 .

 1 Figure 1. (a) SEM images of CV (500 ×, left) and CV30 (200 ×, right); (b) Photos of the

  Figure 1. (a) SEM images of CV (500 ×, left) and CV30 (200 ×, right); (b) Photos of the

Figure 2 .

 2 Figure 2. Adsorption-desorption isotherms of (a) N 2 at -196°C and (b) CO 2 at 0°C; (c) fits of

Figure 3 .

 3 Figure 3. Changes of pore texture parameters as function of the burn-off: (a) surface areas

Figure 4 .

 4 Figure 4. Pore size distribution in the ranges of mesopores and macropores obtained by

Figure 5 .

 5 Figure 5. Distribution of functional groups as a function of pK a , for CV (blue) and CV15

Figure 6 .

 6 Figure 6. Fits of the PFO and PSO models to the experimental data of paracetamol adsorption

Figure 7 .

 7 Figure 7. Experimental paracetamol adsorption isotherms at 20°C (red), 30°C (blue) and

Figure 8 .

 8 Figure 8. Maximum paracetamol adsorption capacities of activated carbons (obtained in

Figure 2

 2 Figure 2

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6 40

 6 Figure 6

Figure 7

 7 Figure 7

Table 1 .

 1 Elemental composition (wt. %), yield Y (%) and pH PZC of the materials.

	Sample	C	H	N	O	S	Ash*	Y (%)	pH PZC
	CV	81.8	1.8	0.4	11.6	0.0	4.4	100	8.3
	CV0	80.6	0.8	0.4	7.5	0.0	10.7	81.7	10.3

* Calculated as 100all other element weight fractions.

Acknowledgements

Amalia Bursztyn gratefully acknowledges CONICET for the PhD scholarship and Martiniano Picicco who helped with the carbonisation. The IJL members gratefully acknowledge the financial support of TALiSMAN project, funded by FEDER (2019-000214).

Paracetamol removal by Kon-Tiki kiln-derived biochar and activated carbons

Bursztyn Fuentes et al.