# Effective Adsorption of Phenoxyacetic Herbicides by Tomato Stem-Derived Activated Carbons
## 3 Results and Discussion
### Adsorption Study
#### 3.2.3 Effects of Solution pH and Ionic Strength  
The adsorption of organic compounds on activated carbon depends on the adsorbate, adsorbent, and solution physicochemical properties. Both the pH of the solution and its ionic strength can influence the dissociation of the adsorbate molecules and the ionization of the adsorbent surface, which can affect adsorption. Depending on the conditions, this can either increase or decrease the efficiency of the adsorption process. Accordingly, a comprehensive understanding of the impact of pH and ionic strength on the adsorption mechanism is indispensable for enhancing the overall process and achieving maximal efficiency in the remediation of these contaminants from aqueous matrices. Figure 7 demonstrates the strong pH dependence of herbicide adsorption (pH 2.5-10). Both 2,4-D and MCPA were adsorbed most efficiently at the lowest pH (2.5); adsorption decreased drastically with further increases in pH up to about 4. Thereafter, adsorption stabilized at a more or less constant level, and further increases in solution pH from about 4-5 to 10.5 did not result in any noticeable changes in this efficiency. The most drastic changes in adsorption with an increase in solution pH from 2.5 to 10 were observed for AC-KOH-750. In this case, the adsorption efficiency of 2,4-D decreased by more than 57 pp (from 79.4% at pH 2.5 to 22.0% at pH 10), while the MCPA adsorption decreased from 77.5% (pH 2.5) to 16.8% at pH 10. Relatively the least sensitive to pH changes was AC-NaOH-550, for which a decrease in adsorption of 32-33 ppm was observed when the pH of the solution was changed from 2.5 to 10 (from 40% to 7% for 2,4-D and from 41% to 8% for MCPA). Figure 7: Effect of initial pH on 2,4-D and MCPA adsorption onto activated carbon produced from tomato stems (TS-AC) (experimental conditions: 1 mmol/L herbicide, 0.25 g/L TS-AC, at 23 °C). \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**adsorbent**} & \multirow{2}{*}{**S\({}_{\text{SET}}\) (m\({}^{2}\)/g)**} & \multicolumn{2}{c}{**Adsorption Capacity, \(q_{\text{m}}\) (mmol/g)**} & \multirow{2}{*}{**Reference**} \\ \cline{3-3} \cline{5-5}  & & **2,4-D** & & **MCPA** \\ \hline AC-NaOH-550 & 791 & 0.904 & 1.056 & This paper \\ AC-NaOH-650 & 1565 & 1.637 & 1.513 & This paper \\ AC-NaOH-750 & 2087 & 1.960 & 1.902 & This paper \\ AC from pistachie shell/CD & 556 & 2.051 & 1.565 & [42] \\ AC from rice straw/H\({}_{3}\)PO\({}_{4}\) & 771 & 1.270 & 1.640 & [48] \\ AC from rice straw/H\({}_{3}\)PO\({}_{4}\) & 613 & 1.230 & 1.640 & [48] \\ AC from particleboard & 1211 & 1.370 & 1.870 & [46] \\ F-400 & 800 & 1.860 & 1.940 & [44] \\ Sorbo Norit AC & 1225 & 1.490 & 2.080 & [45] \\ Ceca AC40 AC & 1201 & 1.560 & 2.599 & [45] \\ AC from PET (K\({}_{2}\)CO) & 1206 & 1.949 & 2.450 & [49] \\ AC from willow & 1280 & 2.310 & 2.413 & [47] \\ AC from hemp shives & 1324 & 2.446 & 2.460 & [47] \\ AC from miscanthus & 1420 & 2.577 & 2.677 & [47] \\ AC from flax shives & 1587 & 2.682 & 2.725 & [47] \\ AC from PAN (KOH) & 2828 & 2.490 & 2.630 & [49] \\ \hline \end{tabular}
\end{table} 
Table 9: \(Cont.\)The adsorption behavior of 2,4-D and MCPA exhibited high similarity, a predictable outcome given their comparable physicochemical properties (Table 1). In evaluating the influence of pH on adsorption, the acid dissociation constant (pK\({}_{\text{a}}\)) emerges as a critical parameter. The pKa values for 2,4-D and MCPA are 2.98 and 3.14, respectively (Table 1). Consequently, adsorbate molecules are predominantly protonated at pH values below the pKa, while at pH values exceeding the pKa, they exist primarily in anionic form. As the solution pH increases, a progressive dissociation of adsorbate molecules occurs. According to [PERSON] et al. [45], at approximately pH 5, 2,4-D molecules in solution are nearly fully ionized. This phenomenon is significant, as the interaction of a neutral molecule with the adsorbent surface differs substantially from that of an ionized molecule.