# The Removal of Crystal Violet from Textile Wastewater Using Palm Kernel Shell-Derived Biochar
## 3 Results and Discussion
### 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 \({}^{\circ}\)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 \(R_{L}\)[70]. \(R_{L}\) is defined as follows:  
\[R_{L}=1/[1+(K_{L}\times C_{0})], \tag{9}\]  
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].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Adsorbents**} & \multicolumn{3}{c}{**Langmuir Isotherm**} & \multicolumn{3}{c}{**Freundlich Isotherm**} \\ \cline{2-7}  & **Qn (mg/g)** & \(K_{L}\) (L/mg) & \(R^{2}\) & \(K_{F}\) & 1/\(n\) & \(R^{2}\) \\ \hline BC-PKS & 24.45 & 0.023 & 0.954 & 2.07 & 0.447 & 0.924 \\ \hline \end{tabular}
\end{table}
Table 2: Langmuir and Freundlich constants for CV on BC-PKS.  
Figure 2: CV adsorption to BC-PKS as a function of equilibrium concentration of CV with the Langmuir and Freundlich isotherm model fits (initial concentration: 50-500 mg/L; volume: 30 mL; adsorbent dosage: 0.5 g; reaction time: 24 h; temperature: \({}^{\circ}\)C).  
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.