The Removal of Crystal Violet from Textile Wastewater Using Palm Kernel Shell-Derived Biochar

: In this study, we explored the adsorption potential of biochar derived from palm kernel shell (BC-PKS) as an a ﬀ ordable adsorbent for the removal of crystal violet from wastewater. Kinetics, equilibrium, and thermodynamics studies were carried out to evaluate the adsorption of crystal violet onto BC-PKS. The kinetics adsorption process followed the pseudo-second-order model, indicating that the rate of adsorption is principally controlled by chemisorption. The adsorption equilibrium data were better ﬁtted by the Langmuir isotherm model with a determination coe ﬃ cient of 0.954 and a maximum adsorption of 24.45 mg / g. Thermodynamics studies found the adsorption of crystal violet by BC-PKS to be endothermic with increasing randomness at the BC-PKS / crystal violet interface. The percentage removal and adsorption capacity increased with the pH of the solution, as the negative charges on the biochar surface at high pH enhance the electrostatic attraction between crystal violet molecules and BC-PKS. Increasing the BC-PKS dosage from 0.1 to 1.0 g increased percent removal and decreased the adsorption capacity of crystal violet onto BC-PKS. Therefore, biochar from agricultural by-products, i.e., palm kernel shell, can be cost-e ﬀ ective adsorbents for the removal of crystal violet from textile wastewater.


Introduction
Wastewater from paper, textile, cosmetics, food processing, and plastic producing industries contain various kinds of organic and harmful substances such as dyes [1]. Dyes such as crystal violet (CV) are composed of many functional groups that are stable and difficult to decompose because of their aromatic nature [2,3]. CV is used for making black and blue inks in ball-point pens and in printer ink jet manufacturing industrials [4]. Color paints, pharmaceuticals, leather, detergents, fertilizers, varnish, and waxes are also manufactured from CV [5][6][7]. However, CV, similar to most dyes, is a toxic carcinogenic with a recalcitrant classification because of its non-biodegradability, persistence in various environments, and poor microbial metabolization [7][8][9]. Furthermore, CV causes undesirable colorations of water bodies, resulting in low light penetration for photosynthetic activities, which affects aquatic life such as the growth of tumors in fish [10,11]. Consequently, the removal of

Data Analysis
The kinetic data were analyzed using the pseudo-first-and second-order-models [55,56] where q t is the amount of adsorbed CV (mg/g) per unit mass of BC-PKS at time t, q e is the amount of adsorbed CV (mg/g) per unit mass of BC-PKS when equilibrium is reached, k 1 is a pseudo-first-order rate constant (1/h), and k 2 is a pseudo-second-order rate constant (g/mg/h). The results of isothermal adsorption experiments were analyzed using the Langmuir (Equation (3)) and Freundlich (Equation (4)) models [55][56][57] q e = Q m K L C 1 + K L C , where C e is the concentration of CV in the aqueous solution at equilibrium (mg/L), K L is the Langmuir adsorption constant (L/mg) related to the binding energy, Q m is the maximum adsorption amount (adsorption capacity in mg/g), K F is the distribution coefficient (L/g), and n is the Freundlich constant. All parameters of the models were estimated using Sigma-Plot 10.0. The thermodynamic characteristics of the adsorption experiments were analyzed using the following equations [23,55,56]: where ∆G 0 is the change in Gibb's free energy (kJ/mol), ∆S 0 is the change in entropy (J/mol K), ∆H 0 is the change in enthalpy (kJ/mol), R is the ideal gas constant (J/mol K), K e is the equilibrium constant, and α is the amount of BC-PKS (g/L). ∆S 0 and ∆H 0 are obtained from the relationship between lnK e and 1/T in Equation (7).  Figure 1 shows the results of the adsorption kinetic experiments carried out by varying the contact time from 0.5 h to 24 h using 400 mg/L initial concentration. The sorption of CV increased precipitously during the first 30 min of the adsorption process and later slowed until equilibrium was reached. The adsorption rate controlled by intra-particle diffusion involves pore and surface diffusion, which occur coherently within the adsorbent particles [58]. The rapid removal may be attributed to the diffusion of CV to mesopores on the surface of BC-PKS. In our previous study [59], the average pore diameter of BC-PKS was 4.1 nm, which corresponds to mesopores (2-50 nm) as per the classification of the International Union of Pure and Applied Chemistry. The dispersion of CV molecules through the mesopores toward the BC-PKS intra particle matrix caused the removal rate to disincline because of low dye concentration and fewer adsorption sites [59][60][61].

Kinetic Studies
Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 14 and α is the amount of BC-PKS (g/L). ΔS 0 and ΔH 0 are obtained from the relationship between lnKe and 1/T in Equation (7). Figure 1 shows the results of the adsorption kinetic experiments carried out by varying the contact time from 0.5 h to 24 h using 400 mg/L initial concentration. The sorption of CV increased precipitously during the first 30 min of the adsorption process and later slowed until equilibrium was reached. The adsorption rate controlled by intra-particle diffusion involves pore and surface diffusion, which occur coherently within the adsorbent particles [58]. The rapid removal may be attributed to the diffusion of CV to mesopores on the surface of BC-PKS. In our previous study [59], the average pore diameter of BC-PKS was 4.1 nm, which corresponds to mesopores (2-50 nm) as per the classification of the International Union of Pure and Applied Chemistry. The dispersion of CV molecules through the mesopores toward the BC-PKS intra particle matrix caused the removal rate to disincline because of low dye concentration and fewer adsorption sites [59][60][61]. The pseudo-first-and second-order models were employed to explore the mechanism involved in the adsorption process, and the results are summarized in Table 1. Comparatively, a higher determination coefficient (R 2 ) value was obtained for the pseudo-second-order than for the pseudofirst-order. This implies that the experimental data is a superior fit with pseudo-second-order model for CV adsorption onto BC-PKS and the adsorption process is principally governed by chemisorption through electron sharing between BC-PKS and CV [62][63][64]. The adsorption of CV to BC-PKS can be explained by ion exchange between deprotonated carboxyl and hydroxyl groups on the surface of BC-PKS and the quaternary amine groups of cationic CV molecule [64]. Similar results have been reported for the sorption of dye onto rice husk [65] and the adsorption of basic dye onto BC-PKS [66].

Adsorption Isotherm
The equilibrium adsorption isotherm experiment was studied under different initial concentrations (50,100,200, 300, 400, and 500) mg/L using 0.5 g of BC-PKS per 30 mL of CV solutions The pseudo-first-and second-order models were employed to explore the mechanism involved in the adsorption process, and the results are summarized in Table 1. Comparatively, a higher determination coefficient (R 2 ) value was obtained for the pseudo-second-order than for the pseudo-first-order. This implies that the experimental data is a superior fit with pseudo-second-order model for CV adsorption onto BC-PKS and the adsorption process is principally governed by chemisorption through electron sharing between BC-PKS and CV [62][63][64]. The adsorption of CV to BC-PKS can be explained by ion exchange between deprotonated carboxyl and hydroxyl groups on the surface of BC-PKS and the quaternary amine groups of cationic CV molecule [64]. Similar results have been reported for the sorption of dye onto rice husk [65] and the adsorption of basic dye onto BC-PKS [66].

Adsorption Isotherm
The equilibrium adsorption isotherm experiment was studied under different initial concentrations (50,100,200, 300, 400, and 500) mg/L using 0.5 g of BC-PKS per 30 mL of CV solutions at a temperature of 25 • C. The equilibrium adsorption isotherm determines the interrelation between the CV concentration in solution and adsorbed CV on the solid phase in terms of surface property and adsorption affinity [67].
Langmuir and Freundlich isotherms were employed to analyze the equilibrium data, and each isotherm parameter was calculated, as presented in Figure 2 and Table 2. The selection of the better isotherm model fit for adsorption data was based on the determination coefficient value (R 2 ). Fitting the experimental results to Freundlich and Langmuir adsorption isotherms for BC-PKS yielded R 2 values of 0.924 and 0.954, respectively [68]. The Langmuir isotherm model provided a better fit for CV adsorption than the Freundlich isotherm model. The Langmuir isotherm fit indicates a monolayer coverage of CV onto the outer surface of the BC-PKS through uniform CV molecules adsorption onto the surface active sites [69].
Additionally, the Langmuir isotherm could be used to predict the favorability or unfavorability of the adsorption process when expressed in terms of dimensionless separation parameters RL [70]. RL is defined as follows: where KL is the Langmuir constant, and C0 is the initial concentration. Therefore, the calculated RL values for initial CV concentrations (50-500 mg/L) ranged from 0.081 to 0.468. Values between 0 and 1 indicate a favorable adsorption of CV onto BC-PKS [71]. A similar phenomenon was reported for the adsorption of direct dye onto rice husk [72] and the adsorption of basic dye onto BC-PKS [66]. Table 2 lists the obtained values of 1/n (0.1 < 1/n < 1), indicating strong bonding between BC-PKS and CV.
The maximum CV adsorption capacity was determined to be 24.45 mg/g for BC-PKS, which is comparable to other adsorbents, as shown in Table 3. The low adsorption of BC-PKS may be because of poor surface chemistry resulting from undeveloped porosities and fewer functional groups [73]. Therefore, various modification techniques such as metal oxide impregnation and functionalization can be explored to improve the adsorption capacity [74]. However, millions of tons of PKS is produced annually from oil palm plantations [75]; its use as an adsorbent helps solve the issue of disposal with no environmental impact. Additionally, the particle size and granular nature of BC-PKS allow easy separation in the filtration or precipitation processes when compared to sugar bagasse fiber that can form sludge.

Adsorbents
Langmuir Isotherm Freundlich Isotherm  Additionally, the Langmuir isotherm could be used to predict the favorability or unfavorability of the adsorption process when expressed in terms of dimensionless separation parameters R L [70]. R L is defined as follows: where K L is the Langmuir constant, and C 0 is the initial concentration. Therefore, the calculated R L values for initial CV concentrations (50-500 mg/L) ranged from 0.081 to 0.468. Values between 0 and 1 indicate a favorable adsorption of CV onto BC-PKS [71]. A similar phenomenon was reported for the adsorption of direct dye onto rice husk [72] and the adsorption of basic dye onto BC-PKS [66]. Table 2 lists the obtained values of 1/n (0.1 < 1/n < 1), indicating strong bonding between BC-PKS and CV.
The maximum CV adsorption capacity was determined to be 24.45 mg/g for BC-PKS, which is comparable to other adsorbents, as shown in Table 3. The low adsorption of BC-PKS may be because of poor surface chemistry resulting from undeveloped porosities and fewer functional groups [73]. Therefore, various modification techniques such as metal oxide impregnation and functionalization can be explored to improve the adsorption capacity [74]. However, millions of tons of PKS is produced annually from oil palm plantations [75]; its use as an adsorbent helps solve the issue of disposal with no environmental impact. Additionally, the particle size and granular nature of BC-PKS allow easy separation in the filtration or precipitation processes when compared to sugar bagasse fiber that can form sludge.

Thermodynamics
The influence of thermal temperatures on CV removal by BC-PKS was examined using 30 mL of solution with a 400 mg/L initial concentration, and 0.5 g of BC-PKS at temperatures of 15, 25, and 35 • C. The thermodynamic parameters (Table 4) show negative values for Gibb's free energy change (∆G ), indicating that CV adsorption onto BC-PKS is feasibly spontaneous. The decrease in the negative values of ∆G with increasing temperature implies a favorable CV adsorption at higher temperatures. This is primarily due to the increased mobility of the CV molecules in solution [87]. The values of ∆H and ∆S are estimated from the slope and intercept of the plot of InK e versus 1/T for CV adsorption onto BC-PKS (Table 4). The positive values of ∆H • confirm that the adsorption reactions of CV onto BC-PKS are endothermic, suggesting the adsorption of CV was more favorable with increasing temperature [10,88]. In addition, ∆H 0 was 60.5 kJ/mol, indicating that CV adsorption onto BC-PKS was achieved via chemisorption, which occurs when ∆H 0 > 40 kJ/mol [89,90]. The positive values of ∆S for BC-PKS signify an increase in the disorder at the interfaces between CV and BC-PKS during the CV adsorption on BC-PKS that is accompanied with changes in the CV and BC-PKS structures [10,91].

Effects of the pH of the Solution
The adsorption efficiency under different solution pH was investigated because variations in pH affect the degree of ionization of the adsorptive molecule and the BC-PKS surface properties [92]. Thus, adsorption of CV onto BC-PKS at 25 • C with initial dye concentration of 400 mg/L and a BC-PKS dose of 0.5 g was examined by adjusting the solution pH from 2 to 10. The adsorption of CV on BC-PKS increased from 12.4 to 16.4 mg/g when the pH increased from 2 to 4 ( Figure 3). Additionally, the percentage removal of CV on BC-PKS increased from 51.8% at pH 2.0 to 68.7% at pH 6.0. CV adsorption on BC-PKS showed a rapid increase in the adsorption capacity and high percentage removal from pH 4 to pH 6, primarily due to the change of surface charge of BC-PKS. The zero point charge of BC-PKS was reported to be 5.50 [93] and 3.94 [94]. As the pH of the CV dye solution increases, the surface of BC-PKS become negatively charged, enhancing the adsorption of the positively charged cationic dye through electrostatic attraction [92,95]. Similar observations were reported in the use of rice husk for the removal of basic dyes from wastewater [96].
reactions of CV onto BC-PKS are endothermic, suggesting the adsorption of CV was more favorable with increasing temperature [10,88]. In addition, ΔH 0 was 60.5 kJ/mol, indicating that CV adsorption onto BC-PKS was achieved via chemisorption, which occurs when ΔH 0 > 40 kJ/mol [89,90]. The positive values of ∆S ○ for BC-PKS signify an increase in the disorder at the interfaces between CV and BC-PKS during the CV adsorption on BC-PKS that is accompanied with changes in the CV and BC-PKS structures [10,91].

Effects of the pH of the Solution
The adsorption efficiency under different solution pH was investigated because variations in pH affect the degree of ionization of the adsorptive molecule and the BC-PKS surface properties [92]. Thus, adsorption of CV onto BC-PKS at 25 °C with initial dye concentration of 400 mg/L and a BC-PKS dose of 0.5 g was examined by adjusting the solution pH from 2 to 10. The adsorption of CV on BC-PKS increased from 12.4 to 16.4 mg/g when the pH increased from 2 to 4 ( Figure 3). Additionally, the percentage removal of CV on BC-PKS increased from 51.8% at pH 2.0 to 68.7% at pH 6.0. CV adsorption on BC-PKS showed a rapid increase in the adsorption capacity and high percentage removal from pH 4 to pH 6, primarily due to the change of surface charge of BC-PKS. The zero point charge of BC-PKS was reported to be 5.50 [93] and 3.94 [94]. As the pH of the CV dye solution increases, the surface of BC-PKS become negatively charged, enhancing the adsorption of the positively charged cationic dye through electrostatic attraction [92,95]. Similar observations were reported in the use of rice husk for the removal of basic dyes from wastewater [96].

Effect of BC-PKS Dosage
The removal of CV by BC-PKS was studied using six different BC-PKS dosages (0.1−1.0 g) with 30 mL of CV with an initial concentration of 400 mg/L; the results are plotted in Figure 4. The percentage removal of CV increases steadily as the BC-PKS dosage increases because of the increase in the number of adsorption sites [97]. As the BC-PKS dosage increases from 0.1 to 1.0 g, the percentage removal increases from 50.8% to 86.4%. Therefore, 30.9 g/L of BC-PKS dosage is required to remove 80% of the CV. Conversely, an increased BC-PKS dose reduced the amount of CV adsorption to unit mass of BC-PKS from 60.4 mg/g to 10.4 mg/g due to the split in the flux or the concentration gradient between the solute concentration in solution and the solute concentration at the BC-PKS surface [79,98]. percentage removal increases from 50.8% to 86.4%. Therefore, 30.9 g/L of BC-PKS dosage is required to remove 80% of the CV. Conversely, an increased BC-PKS dose reduced the amount of CV adsorption to unit mass of BC-PKS from 60.4 mg/g to 10.4 mg/g due to the split in the flux or the concentration gradient between the solute concentration in solution and the solute concentration at the BC-PKS surface [79,98].

Effect of Competitive Cations
Wastewater from industries contains various contaminants, such as, acids, bases, salts, metal ions, and other inorganic compounds. The existence of cations in the wastewater contributes to a high ionic strength solution [99] that may significantly impact the adsorption capacity of BC-PKS [100]. Therefore, the effect of cations including monovalent (Na + ), divalent (Ca 2+ ), and trivalent (Al 3+ ) ions on the adsorption of CV by BC-PKS were studied. The results as illustrated in Figure 5 demonstrates that BC-PKS adsorption of CV decreased in the presence of Na + , Ca 2+ , and Al 3+ , indicating the impact of the cations on the adsorption of CV by BC-PKS through a competitive sorption mechanism caused by the reduction in the attraction between active sorption sites and CV molecules [101]. However, the differences in the adsorption amount were more significant with the rise of cationic concentrations because of increased competition for adsorption sites [102]. Additionally, the impact of the co-existing cations on the CV adsorption followed the decreasing order: Al 3+ -> Ca 2+ > Na + because Al 3+ has a higher valence charge than the monovalent (Na + ) and divalent (Ca 2+ ) cations [103].

Effect of Competitive Cations
Wastewater from industries contains various contaminants, such as, acids, bases, salts, metal ions, and other inorganic compounds. The existence of cations in the wastewater contributes to a high ionic strength solution [99] that may significantly impact the adsorption capacity of BC-PKS [100]. Therefore, the effect of cations including monovalent (Na + ), divalent (Ca 2+ ), and trivalent (Al 3+ ) ions on the adsorption of CV by BC-PKS were studied. The results as illustrated in Figure 5 demonstrates that BC-PKS adsorption of CV decreased in the presence of Na + , Ca 2+ , and Al 3+ , indicating the impact of the cations on the adsorption of CV by BC-PKS through a competitive sorption mechanism caused by the reduction in the attraction between active sorption sites and CV molecules [101]. However, the differences in the adsorption amount were more significant with the rise of cationic concentrations because of increased competition for adsorption sites [102]. Additionally, the impact of the co-existing cations on the CV adsorption followed the decreasing order: Al 3+ -> Ca 2+ > Na + because Al 3+ has a higher valence charge than the monovalent (Na + ) and divalent (Ca 2+ ) cations [103].

Conclusion
The feasibility of the agricultural by-product PKS for removing CV from textile wastewater was investigated in this study. Kinetic studies revealed that chemisorption is the key limiting factor for the rate of CV adsorption. The equilibrium data were better fitted to the Langmuir model, indicating

Conclusions
The feasibility of the agricultural by-product PKS for removing CV from textile wastewater was investigated in this study. Kinetic studies revealed that chemisorption is the key limiting factor for the rate of CV adsorption. The equilibrium data were better fitted to the Langmuir model, indicating that CV adsorbed on BC-PKS as a monolayer with uniform CV molecules adsorption. Thermodynamic studies revealed that CV adsorption to BC-PKS was a spontaneous, endothermic process with increasing CV adsorption to BC-PKS at higher temperatures. The increase of solution pH changed the surface charge of BC-PKS, which served favorable adsorption sites for cationic CV dyes via electrostatic attraction. When the BC-PKS loading rate was increased, the percentage of CV removal was found to increase due to the increase of available adsorption sites, but the adsorption capacity began to decrease because of the decline of the flux gradient. The presence of cations in the wastewater decreased the CV adsorption to BC-PKS because of increased competition for adsorption sites. It can be concluded that biochar prepared from the agricultural byproduct has adequate adsorption capacity and can be used as an efficient and cost-effective adsorbent for the removal of CV from textile wastewater.

Conflicts of Interest:
The authors declare no conflict of interest.