Treatment of effluents containing 2-chlorophenol by adsorption onto chemically and physically activated biochars

The application of adsorption using biochars for the remediation of effluents containing emerging contaminants, including chlorophenols, is a hotspot and trend development in the literature. This treatment is more interesting when using readily available wastes and at no cost, such as malt bagasse, for example. Here, the biochars were produced from malt bagasse, by physical and chemical activation (with CO2 and ZnCl2, respectively) and employed as adsorbents in the remediation of effluents containing 2-chlorophenol. Results revealed that the activated biochars have mesoporous structures and surface areas of 161 m2 g (CO2) and 545 m2 g (ZnCl2). For both activated biochars, adsorption of 2-chlorophenol was favored under acid conditions, with the highest adsorption capacities found using ZnCl2-activated biochar. The maximum adsorption capacity using ZnCl2-activated biochar was 150 mg g. The process was endothermic and spontaneous. ZnCl2-activated biochar exhibited an efficiency of 98% (using a dosage of 10 g L) in the treatment of industrial effluents containing 2-chlorophenol.


Introduction
Aromatic organic compounds, such as chlorophenols (CPs), are an essential group of emerging contaminants (ECs), extremely difficult to biodegrade [1]. Since they are high toxicity components, carcinogenic and mutagenic, CPs are priority and hazardous contaminants [2]. CPs are widely used in industrial applications, manufacturing herbicides [3], insecticides, pesticides, wood preservation, and as a result, are commonly found dispersed in aquatic resources, soil, and industrial effluents [4]. Based on the environmental risks caused by CPs, its removal from effluents is required. The removal of chlorophenols from effluents can be performed by chemical, biological, and physicochemical treatments [5]. On the one hand, some of these techniques have complex processes, high energy consumption, low removability, the addition of chemical agents, thus increasing the cost of the process involved. On the other hand, the adsorption technique presents high efficiency, simplicity in the application process, resulting in a low operation cost [6]. In the adsorption process, contaminants present in aqueous solution are transferred to a solid (adsorbent) phase [7,8]. Subsequently, the adsorbent can be regenerated [7,9] or stored in a moisture-free place and, the polished effluents can be released or reused in processes that do not require high purity water [7].
Activated carbon is commonly employed as an adsorbent for contaminants removal from liquid streams. This material has high pore volume and upper surface area, favoring the efficiency of the adsorption process. However, the use of this material on a large scale becomes expensive [5]. As a result, biochars derived from no-cost alternative wastes, and prepared by alternative routes have gained attention [9] to make the adsorption process more attractive. In this context, malt bagasse (a by-product J o u r n a l P r e -p r o o f The biomass employed for the development of activated biochars was barley malt bagasse, which was furnished by a beer industry, located in southern Brazil. The biomass was naturally dried in the sun for 48 h, followed by oven drying at 50 °C for 6 h, and milled to generate a material with particle size smaller than 0.5 mm. For the adsorption experiments, 2-chlorophenol solution (128.56 g mol -1 and 99% purity) was supplied by Sigma-Aldrich. For chemical activation, CaO (56.08 g mol -1 , 95% purity) and ZnCl2 (136.30 g mol -1 , 97% purity) were obtained from Dinamica (Brazil). CO2 (purity 95%) was supplied by White Martins (Brazil).

Preparation of biochar adsorbents
During the development of this work, three materials were prepared, characterized, and used for adsorption purposes (biochar and two activated biochars).
The first material produced (biochar without activation) was pyrolyzed from an initial investigation of different operational parameters of temperature and holding time.
For this, the following temperatures were investigated: 500, 600, and 700 ºC. Besides, the following holding times were evaluated: 10, 30, and 50 min. The characteristics of the different biochars produced were presented in a recent publication by our group [16]. Through these preliminary results, an optimal experimental condition was obtained (isotherm time of 10 min and a temperature of 500 °C). This condition was then adopted for this work.
The biochars activation procedure was subsequently conducted. The methods adopted for biochar activation with ZnCl2 and CO2, as well as the parameters evaluated in the adsorption experiments, are presented in Fig. 1.

J o u r n a l P r e -p r o o f
The methodology used for biochar activation with ZnCl2 was conducted, according to Streit et al. [17]. The procedures adopted are presented in Fig. 1, including the pyrolysis and chemical activation step. In a simplified way, the procedure involves preparing a precursor by mixing CaO (10 % wt.), ZnCl2 (40 % wt.), distilled water, and 40 g of malt bagasse. This mixture was conducted under agitation for one hour and then dried at a temperature of 353 K for 24 h. The precursor was finally inserted in the reactor to be pyrolyzed under the following conditions: final temperature (900 ºC), heating rate (5 ºC min -1 ), holding time (50 min), and nitrogen flow rate of 150 mL min -1 . The reactor used was previously described by Zazycki et al. [9]. The final step to obtain activated biochar was the leaching with HCl 6 mol L -1 for 24 h in an orbital shaker (with a ratio of 10 mL HCl:g material -1 ), to remove Zn, and subsequently, washing procedure with deionized water until the neutral pH solution. The material was dried in an oven for 12 h at 50 °C.
For CO2 physical activation, the biochar was initially prepared by pyrolysis with N2 (500 °C and 10 min). Subsequently, N2 flow was replaced by CO2. The biochar was activated until 900 °C and remained at this temperature for 50 min. The detail of the activation process was reported in another work [11].

Characterization of activated biochars
ZnCl2 and CO2 activated biochars were characterized by different techniques as follows: scanning electronic microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), specific surface area (BET), pore distribution by BJH (Barret-Joyner-Halenda), X-ray diffraction (XRD) and zero charge point (pHPZC). The characteristics of morphology, functional groups, structure, pore properties, and their J o u r n a l P r e -p r o o f distribution, load potential as (H + and OH -), and surface charge were evaluated. More details of the characterizations used in the materials are presented in the supplementary material (Table 1S).

2-chlorophenol adsorption experiments
All tests were conducted in a thermostatic stirrer (Solab, Brazil) at 150 rpm, using 0.01 g of adsorbent and 20 mL of 2-chlorophenol aqueous solution (the vessels with solutions were closed to avoid volatilization of the adsorbate). The adsorption experiments were divided into three steps: evaluation of pH effect and biochar type (biochar, CO2 activated biochar, ZnCl2 activated biochar), kinetic study and, equilibrium isotherms.
The tests to evaluate the impact of pH and adsorbent type were performed using the three adsorbents produced (biochar, CO2 activated biochar, ZnCl2 activated biochar). The initial concentration of 2-chlorophenol was 50 mg L -1 , 120 min of contact time, and pH from 2 to 12. The pH was adjusted by NaOH and HCl solutions; Kinetic studies were conducted with 2-chlorophenol solutions at pH 6 and initial

2-chlorophenol adsorption modeling
The adsorption kinetic data of 2-chlorophenol were adjusted with the pseudo-first order, pseudo-second order, and Avrami models [7,18]. Langmuir [19] and Freundlich [7] models were fitted to the experimental data to verify the behavior of equilibrium isotherms. The initial sorption rate was also calculated [20].

J o u r n a l P r e -p r o o f
The statistical indicators used to prove the fit quality are presented in the supplementary material (S.3).

Desorption and reuse tests
Adsorption/desorption cycles were conducted to verify the possibility of reusing the ZnCl2 activated biochar. Adsorption was performed using 200 mL of a 50 mg L -1 2chlorophenol solution, which was mixed with 0.1 g of adsorbent and stirred at 150 rpm (Solab, SL 222, Brazil) for 120 min and 298 K. The adsorbent was then separated from the liquid phase. Subsequently, the loaded adsorbent was submitted to thermal desorption in an oven for 240 min at 373 K. This process was performed for ten cycles.
Thermal desorption was selected due to the volatile character of 2-chlorophenol.

Treatment of effluents containing 2-chlorophenol
A real effluent was simulated to verify the efficiency of ZnCl2-activated biochar for the treatment of a mixture of aromatics in a medium with high salt concentration.
The compounds used are generally found in industrial effluents, specifically from the petrochemical area. The simulated effluent composition is listed in Table 1.
The experiments were conducted using adsorbent dosages of 1, 5, and 10 g L -1 .
The adsorbent was contacted with 200 mL of the effluent, which was agitated for 2 h at 150 rpm and 298 K. The treatment efficiency was evaluated by acquiring spectra of the simulated effluent before and after the treatments, in the range 200-600 nm, using UV-Vis spectroscopy (Shimadzu, UVmini-1240, Japan).

Characterization of activated biochars
N2 adsorption/desorption isotherms and pore distribution by BJH (Barret-Joyner-Halenda) desorption method of both activated biochars are depicted in Fig. 2. It was found that the isotherms are very similar, being of type IV accompanied by hysteresis [23]. Although isotherms are the same type (IV for the two adsorbents), the hysteresis is different (type H3 for ZnCl2-activated biochar and type H4 for CO2-activated biochar).
The pore size distribution for both samples indicated the presence of mesopores. The specific surface area (BET) presented values close to 545 m² g -1 and 161 m² g -1 for ZnCl2-activated biochar and CO2-activated biochar, respectively. These values were extremely higher in relation to the biochar without activation (6.5 m² g -1 ), and confirm that the activation efficiently performed. Therefore, the improvement of the structural characteristics of the material was achieved. The ZnCl2-activated biochar sample presented a high area value when compared to the CO2-activated biochar. This trend corroborates that the textural properties were improved after HCl leaching. According to Streit et al. [17], this occurs as a result of the removal of ZnCl2 and CaO, which were initially added. In summary, both materials were mesoporous and presented interesting values of surface area. However, ZnCl2-activated biochar presented superior characteristics than CO2-activated biochar.
The XRD patterns of both activated biochars produced are shown in Fig. 2 (c).
The diffractograms present a broad peak at approximately 2θ = 23º, corresponding to the plane (002), which is a feature of amorphous carbon. In general, an amorphous J o u r n a l P r e -p r o o f structure is suitable for adsorption since an amorphous material has more empty spaces that allow the accommodation of the adsorbate molecules.
The FT-IR spectra presented in Fig. 2 (d) was obtained to identify the main functional groups in the activated biochars. It was possible to observe that both activated biochars presented the same bands at 3454, 1635, and 1091 cm -1 . The band at 3454 cm -1 is due to the stretching of O-H bonds of hydroxyl groups. This band is less pronounced in the CO2-activated biochar spectrum when compared with the ZnCl2activated biochar spectrum. The band at 1635 cm -1 is related to the C=C stretching bond of the aromatic rings resulting from the dehydration and cyclization of carbohydrates during the pyrolysis process. The band at 1091 cm -1 can be ascribed to the C-O stretching of carboxyl groups and bending vibration band of hydroxyl groups [24]. It was also possible to observe that the bands at 2959 and 2855 cm -1 are present only in the CO2 biochar spectrum. These two bands are related to sp 3 C-H stretching of the lignin structure, suggesting that lignin was not completely decomposed. This observation suggests that ZnCl2 activation leads to higher decomposition of the precursor material. Indeed, ZnCl2 can react with the lignocellulosic components increasing the aromatic condensation reactions. ZnCl2 assists the evolution of H2 from the hydroaromatic structure of the precursor material, leaving the sites on the adjacent molecules, which will undergo aromatization. These interspaces present between the carbon layers are responsible for the high porosity of the activated biochar produced [25]. Besides that, ZnCl2 impregnated in the precursor prevents the shrinkage of the particle during the pyrolysis [8]. On the contrary, in physical activation, carbon dioxide acts as an oxidant agent, which reacts with the decomposition products formed in the pyrolysis. Thus, the pores of the biochar are unobstructed, improving the porosity of the material [26]. In this way, CO2 activation does not lead to the formation of new pores. The SEM results corroborate the textural properties previously described, where CO2-activated biochar has a lower surface area and lower total pore volume.
The characterization techniques like BET/BJH, XRD, FT-IR and SEM, revealed that the activation with ZnCl2 was more consistent than the activation with CO2, generating a biochar with a well-developed porous structure, higher pore volume and surface area. This behavior is an indication that the material prepared by chemical activation with ZnCl2 will have a better adsorption potential.

Effects of pH and adsorbent type on 2-chlorophenol adsorption
The pH of the solution affects the adsorbate molecules and, consequently, affects the adsorption capacity. The surface charge of the adsorbent also depends on the pH.
The pH value in which the adsorbent surface charge is zero is defined as the point of zero charge (pHPZC) [27]. When the pH of the solution is lower than pHPZC, the J o u r n a l P r e -p r o o f functional groups of the adsorbent surface are protonated. In this way, the adsorbent surface charge becomes positive. On the contrary, if the pH of the solution is higher than the pHPZC, some surface functional groups of the adsorbent release H + , and the adsorbent surface acquires negative charge [28]. In this way, when pH<pHPZC, adsorption of anionic species is favored, whereas, when pH>pHPZC, adsorption of cationic species is favored [29]. pHPZC can be determined by the plot of pHinitial × pHfinal plot. Fig. 3 (a) shows the pHinitial × pHfinal plot of the biochars developed in this work. It can be observed that the pHPZC values were 6.28, 3.36, and 6.42 for the non-activated biochar, ZnCl2-activated biochar, and CO2-activated biochar, respectively. Adsorbents with more acidic surface groups and few basic surface groups present lower pHPZC [30]. Indeed, from the FTIR analysis Fig. 2 (d), it is possible to see that the band assigned to stretching of O-H of hydroxyl groups is more pronounced in the ZnCl2-activated biochar spectrum, indicating that this adsorbent material has more acidic hydroxyls groups on the surface.
The effect of the initial pH on 2-chlorophenol adsorption was investigated by varying the pH from 2 to 12. Fig. 3 (b) report the effect of initial pH using the nonactivated biochar, ZnCl2-activated biochar, and CO2-activated biochar. Firstly, it is possible to observe that the higher adsorption capacities were achieved by the ZnCl2activated biochar. This behavior can be explained due to the textural properties exhibited by the adsorbents produced. As discussed in the previous section, the activation step provided an increase in specific surface area and total pores volume, respectively, from 6.51 m 2 g -1 and 0.009 cm 3 g -1 (non-activated biochar) to 161.70 m 2 g -It was also verified that the 2-chlorophenol adsorption increased with the pH decrease. The higher adsorption capacity was obtained at pH 2 for the three biochars investigated, reaching the values of 7.00 mg g -1 , 26.68 mg g -1, and 45.00 mg g -1 for the non-activated biochar, CO2-activated biochar and ZnCl2-activated biochar, respectively.
In basic pH values, a pronounced decrease of the adsorption capacity was noticed.
These observations can be explained considering the ionization aspects of 2chlorophenol molecules and the surface charge of the biochars. 2-chlorophenol is a week acid (pKa=8.85) and exists as a neutral molecule in acid pH [31]. Similarly, the adsorbent surface becomes highly protonated and positively charged below the pHPZC, which favors the adsorption due to the electrostatic attraction with the 2-chlorophenol neutral molecules [32]. As the pH of the solution increases, the surface of the adsorbent becomes less positively charged, weakening the electrostatic attraction between the adsorbate and the adsorbent, reducing the adsorption capacity. The increase of the pH above the pHPZC makes the biochar surface negatively charged, and when pH>pKa (8.85), 2-chlorophenol molecules start to undergo to an anionic form. Thus, an electrostatic repulsion interaction can take place between the adsorbate and the adsorbent. Furthermore, the competitive adsorption of OHions can also result in a decrease in the adsorption capacity [32].
From the behavior of the adsorption of 2-chlorophenol using ZnCl2-activated biochar in different initial pH values, it is possible to observe that the adsorption capacity remained practically constant in the pH range from 2 to 6. Above pH 8, the adsorption capacity decreases. For the non-activated biochar and CO2-activated biochar, the adsorption capacity remains constant in the range of pH from 2 to 4 and after begins to decrease. These specificities can be explained since ZnCl2-activated biochar has more hydroxyl groups. Although ZnCl2-activated biochar has net negative charge above pH 3.36, which only occurs at pH 6.28 and 6.42 for the non-activated biochar and CO2activated biochar, respectively, the number of positively charged hydroxyl sites can be greater than in the non-activated biochar and CO2-activated biochar, thus possessing greater electrostatic attraction with the 2-chlorophenol molecules.
Since ZnCl2-activated biochar exhibited the higher adsorption capacities, the subsequent adsorption studies were conducted using this adsorbent. Besides that, the subsequent experiments were carried out at pH 6, where the adsorbent efficiency was suitable.

Adsorption kinetic profile
The adsorption kinetics express how much of the adsorbate is removed from the liquid phase as a function of time. The kinetic study of 2-chlorophenol adsorption on ZnCl2-activated biochar was carried out at pH 6.0, varying the initial 2-chlorophenol concentration from 25 to 300 mg L -1 . Fig 4 shows the results It can be reported the increase in adsorption capacity with the initial concentration of 2-chlorophenol, as also presented in the literature [33]. Besides, it was possible to evaluate that, regardless of the initial concentration of 2-chlorophenol, adsorption was fast in the first 20 min of the process, reaching its equilibrium in approximately 60 min. This trend occurs because at the beginning of the process, the adsorbent sites are fully available, and these sites are progressively occupied along the time.
The kinetic parameters are shown in Table 2. In Table 2, it is possible to observe that all models used were adequate to describe the kinetic data. However, the pseudosecond-order model stands out since the determination coefficient (R²), and adjusted J o u r n a l P r e -p r o o f determination coefficient (R²adj) were the highest presented and the average relative error (ARE) was the lower. In summary, 2-chlorophenol adsorption on ZnCl2-activated biochar followed the pseudo-second-order model. The q2 values increased with the initial concentration of 2-chlorophenol, agreeing with the experimental behavior.
Furthermore, q2 values closed with the experimental values (qe). The initial sorption rate parameter (h0) was directly proportional to C0, indicating that, at the initial stages, 2chlorophenol was faster adsorbed at higher C0 values.

Fig. 5 shows the equilibrium isotherm curves of 2-chlorophenol adsorption on
ZnCl2-activated biochar at different temperatures. According to Giles et al. [34], the isotherm curves are L1 type. In general, they indicate that the adsorbed molecules present strong intermolecular attractions. The isotherms were favorable, but it is possible to see that the plateau was not reached in the concentration ranges used, which may be an indication that the adsorption sites were not completely occupied [34]. It is also noticeable that the isotherm curves were favored with an increase in temperature.
The behavior of isotherms suggests an endothermic adsorption process. This trend is normally attributed to the thermal collision phenomenon.
The isotherm parameters are depicted in Table 3. According to the values of (R²), (R 2 adj), and (ARE), the equation that best fits the experimental data was the Freundlich model. This trend suggests that sites with different energies are present on the biochar surface, which was confirmed by the FT-IR spectrum. The Freundlich constant (kF), J o u r n a l P r e -p r o o f which is associated with the adsorption capacity, increased with the temperature confirming high adsorption capacities at 328 K. Since the Freundlich model has not a specific parameter for adsorption capacity, the experimental value was used for comparison. Here, ZnCl2-activated biochar presented an adsorption capacity of 150 mg g -1 for 2-chlorophenol (Fig. 5). This value can be considered adequate and promising for adsorption of the 2-chlorophenol compound, when compared to the literature. The following 2-chlorophenol adsorption capacities have been reported: 25.94 [1], 70.52 [35] and 225 mg g -1 [2].

Thermodynamic results
Freundlich parameters were used to estimate the values of ΔG 0 , ΔH 0 and ΔS 0 .
The thermodynamic parameters are shown in Table 4. The results obtained for ΔG 0 represent that the increase in temperature provides more energy available in the system; the negative value confirms the spontaneity of the process. The positive value obtained for ΔH 0 confirms that the adsorption of 2-chlorophenol on the ZnCl2-activated biochar is an endothermic process. The value of ΔH 0 obtained in this work indicates that the interaction between the adsorbent and adsorbate occurs by physisorption, as it presents energy below 40 kJ mol -1 [36]. The positive value of ΔS 0 suggests that there is an increase in structural disorder after the adsorption process.

Regeneration study
The reuse of the produced adsorbent (ZnCl2-activated biochar) was tested to verify the regeneration capacity. Fig. 6 shows the cycles used to test the adsorption J o u r n a l P r e -p r o o f capacity. The first cycle presented an adsorption capacity of 52.7 mg g -1 . This capacity was maintained for five cycles. After the fifth cycle, the capacity decreased to 27.5 mg g -1, and the other cycles showed a drop below 20 mg g -1 . This decrease in adsorption capacity after consecutive cycles can be related to the thermal treatment used for desorption. The thermal treatment under the air atmosphere can have destroyed the pores and, consequently, the reduction of porosity and surface area. Furthermore, the adsorption sites were possibly damaged by the temperature. Studies using carbon-based materials to uptake organic compounds from aqueous solutions also reported that your adsorbents could be reused for consecutive cycles. Zazycki et al. [37] prepared a char from MDF wastes to remove a food dye from solutions. They found that the char could be used for 8 adsorption/desorption cycles. Lütke et al. [22] developed an activated carbon from black wattle bark to remove phenol from effluents. They found that the material can be used in two cycles, with a slight decrease in adsorption capacity. In this work, it is suggested the reuse of ZnCl2-activated biochar for 5 times. After 5 cycles, adsorption capacity is not suitable.

Efficiency in the treatment of simulated effluent
An effluent was prepared to verify the efficiency of ZnCl2-activated biochar to remove several phenolic compounds. The simulated effluent was treated using different dosages of the activated biochar, and the spectra of the simulated effluent before and after the treatments were acquired from 200 to 600 nm using a UV-Vis spectrophotometer. These spectra are shown in Fig. 7. The areas under the absorption bands from 200 and 600 nm were used to measure the amount of phenolic compounds removed from the synthetic effluent. The removal percentages were calculated from the areas under the absorption bands. At first, it can be seen that the bands in the spectra after the treatment were amortized, indicating that the compounds were removed from the liquid. ZnCl2-activated biochar presented efficiencies of 50.2%, 81.0%, and 98.2% for the effluent treatment, using dosages of 1 g L -1 , 5 g L -1, and 10 g L -1 , respectively.
These results indicate that ZnCl2-activated biochar is an efficient adsorbent for the treatment of industrial effluents containing phenolic compounds.

Conclusion
Novel activated biochars were produced from malt bagasse by chemical and physical activations, and were applied in the remediation of effluents containing 2chlorophenol. The specific surface areas of CO2 activated biochar, and ZnCl2-activated biochar were 161 m² g -1 and 545 m² g -1 , respectively. Therefore, the most efficient activation was with ZnCl2. The CO2 and ZnCl2 activated biochars were characterized as mesoporous. Both activated biochars exhibited an amorphous structure and the presence of hydroxyl functional groups. ZnCl2-activated biochar presented more cavities on the surface when compared with CO2 activated biochar.
The adsorption of 2-chlorophenol on the biochars was favored under acid conditions. ZnCl2-activated biochar was more efficient in removing 2-chlorophenol from aqueous solutions than CO2 activated biochar. 2-chlorophenol adsorption on ZnCl2-activated biochar followed the pseudo-second order model, being the equilibrium attained within 60 min. Freundlich model showed a better fit with the isotherm curves.
The maximum adsorption capacity was 150 mg g -1 . Adsorption process was endothermic and spontaneous. The reuse of ZnCl2-activated biochar can be performed by 5 times. ZnCl2-activated biochar is an excellent adsorbent to polish industrial       J o u r n a l P r e -p r o o f