# Effective Adsorption of Phenoxyacetic Herbicides by Tomato Stem-Derived Activated Carbons
## 3 Results and Discussion
### Adsorption Study
#### 3.2.2 Adsorption Isotherms  
The correlations between the adsorbed amount (\(q_{\rm e}\)) and the adsorbate concentration at equilibrium (\(C_{\rm e}\)) are shown in Figure 6.  
Langmuir [32] and Freundlich [33] models, the two most commonly used theoretical approaches [34], were applied to describe the experimental adsorption isotherms. The equation of the Langmuir isotherm has the following form:  
\[\frac{C_{\rm e}}{q_{\rm e}}=\frac{1}{q_{\rm m}}C_{\rm e}+\frac{1}{q_{\rm m}K_ {\rm L}} \tag{13}\]  
where \(q_{\rm m}\) is [PERSON]'s maximum adsorption capacity (mmol/g) and \(K_{\rm L}\) is the [PERSON] isotherm constant (L/mmol). These isotherm parameters were calculated from the slope and intercept obtained for the linear relationship of \(C_{\rm e}/q_{\rm e}\) vs. \(C_{\rm e}\). Figure 5: The Weber-Morris diffusion model plots for adsorption of 2,4-D and MCPA on tomato stem activated carbon (TS-AC). The Freundlich isotherm equation has the following form:  
\[\ln\ q_{e}=\ln\ K_{\rm F}+\frac{1}{\rm n}\ln\ C_{\rm e} \tag{14}\]  
where \(K_{\rm F}\) is the Freundlich isotherm constant related to adsorption capacity (mmol/g) (L/mmol)\({}^{1/n}\) and \(n\) is the Freundlich isotherm constant (dimensionless). The \(K_{\rm F}\) and \(1/n\) values were calculated from the slope and intercept obtained for the linear plot \(\ln\ q_{\rm e}=f(\ln\ C_{\rm e})\). As in the case of the kinetic models, the evaluation of the fit of the experimental data to the theoretical models was based on the analysis of the \(R^{2}\) and \(\chi^{2}\) values obtained. Lower \(\chi^{2}\) values and \(R^{2}\) values closer to unity indicated a better model fit. The parameters of the Langmuir and Freundlich isotherms and the \(R^{2}\) and \(\chi^{2}\) values describing the adsorption of 2,4-D and MCPA on activated carbons derived from tomato stem (TS-AC) are listed in Table 7. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**TS-AC**} & \multicolumn{6}{c}{**2.4-D**} & \multicolumn{6}{c}{**MCPA**} \\ \cline{2-9}  & \multicolumn{4}{c}{**Langmuir Isotherm Model**} \\ \cline{2-9}  & \(q_{\rm m}\) & \(K_{\rm L}\) & \(R^{2}\) & \(\chi^{2}\) & \(q_{\rm m}\) & \(K_{\rm L}\) & \(R^{2}\) & \(\chi^{2}\) \\ \hline AC-KOH-550 & 0.840 & 1.530 & 0.996 & 0.0013 & 0.830 & 6.136 & 0.996 & 0.0024 \\ AC-KOH-650 & 1.417 & 5.468 & 0.999 & 0.0012 & 1.302 & 8.972 & 0.995 & 0.0068 \\ AC-KOH-750 & 1.806 & 8.201 & 0.998 & 0.0025 & 1.721 & 11.08 & 0.999 & 0.0045 \\ AC-NaOH-550 & 0.904 & 2.398 & 0.993 & 0.0011 & 1.056 & 3.244 & 0.993 & 0.0016 \\ AC-NaOH-650 & 1.637 & 3.030 & 0.997 & 0.0012 & 1.513 & 7.466 & 0.994 & 0.0062 \\ AC-NaOH-750 & 1.960 & 9.129 & 0.999 & 0.0029 & 1.902 & 11.78 & 0.998 & 0.0034 \\ \hline \multicolumn{9}{c}{**Freundlich isotherm model**} \\ \cline{2-9}  & \(K_{\rm F}\) & \(1/n\) & \(R^{2}\) & \(\chi^{2}\) & \(K_{\rm F}\) & \(1/n\) & \(R^{2}\) & \(\chi^{2}\) \\ \hline AC-KOH-550 & 0.534 & 0.584 & 0.990 & 0.0033 & 0.766 & 0.314 & 0.914 & 0.0079 \\ AC-KOH-650 & 1.311 & 0.357 & 0.990 & 0.0281 & 1.247 & 0.251 & 0.992 & 0.0091 \\ AC-KOH-750 & 1.831 & 0.326 & 0.979 & 0.0058 & 1.778 & 0.276 & 0.982 & 0.0075 \\ AC-NaOH-550 & 0.682 & 0.501 & 0.972 & 0.0063 & 0.857 & 0.424 & 0.990 & 0.0049 \\ AC-NaOH-650 & 1.377 & 0.497 & 0.973 & 0.0088 & 1.451 & 0.300 & 0.987 & 0.0090 \\ AC-NaOH-750 & 2.057 & 0.329 & 0.980 & 0.0079 & 2.002 & 0.280 & 0.990 & 0.0071 \\ \hline \end{tabular} \(q_{\rm m}\) (mmol/g): \(K_{\rm L}\) (L/mmol): \(K_{\rm F}\) (mmol/g)(L/mmol)\({}^{1/n}\)).  
\end{table} 
Table 7: Langmuir and Freundlich isotherm parameters for the adsorption of 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenylohexacetic acid (MCPA) on activated carbon derived from tomato stems (TS-AC). Figure 6: Adsorption isotherms of 2,4-D and MCPA on tomato stem-derived activated carbon (TS-AC) (line: fitting of Langmuir isotherm) (experimental conditions: herbicides initial concentrations from 0.3 to 1 mmol/L, adsorbent dosage = 0.25 g/L, pH = original (-3.3), temperature = 23 °C).