# 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
Furthermore, the surface charge of the adsorbent, whether positive or negative, constitutes a pivotal factor in the adsorption process. The pH\({}_{\text{pzc}}\) values given in Table 2 for the studied activated carbons make it possible to predict how the charge on their surface can change as the pH of the solution is changed. This knowledge can help to explain the regularities observed in Figure 7. The point of zero charge (pH\({}_{\text{pzc}}\)) is the pH at which the adsorbent's surface charge is neutral. The adsorbent surface exhibits a positive charge when pH < pH\({}_{\text{pzc}}\) and a negative charge when pH > pH\({}_{\text{pzc}}\). The pH\({}_{\text{pzc}}\) values for these activated carbons range from 6 to 7 (Table 2). Such close pH\({}_{\text{pzc}}\) values between the different AC explain the similar pH dependence of herbicide adsorption observed on all these activated carbons (Figure 7). Both 2,4-D and MCPA were adsorbed most efficiently at pH 2.5. This suggests that the interaction of the non-dissociated adsorbate molecules (pH below pK\({}_{\text{a}}\)) with the positively charged adsorbent surface (pH below pH\({}_{\text{pzc}}\)) is most favorable. The observed attenuation of adsorption efficiency under alkaline conditions (pH exceeding both pKa and pH\({}_{\text{pzc}}\)) is plausibly a consequence of repulsive electrostatic forces between phenoxylate anions and the negatively charged surface of the activated carbons. Similar solution pH-dependent adsorption behavior of phenoxy-acetic herbicides has been reported on lignite [50], carbon black, or various activated carbons [44; 47; 51; 52], among others. The influence of ionic strength on herbicide adsorption onto AC was assessed using data from water and NaCl solutions (0.01, 0.1, and 0.5 mol/L), with the results displayed in Figure 8.  
Figure 8: The effect of ionic strength on the adsorption of 2,4-D and MCPA on activated carbon produced from tomato stems (TS-AC) (experimental conditions: 1 mmol/L herbicide, 0.25 g/L TS-AC, original pH, at 23 \({}^{\circ}\)C). Herbicide adsorption remained unaffected by increasing NaCl concentration (Figure 8) across all six activated carbons. Thus, 2,4-D and MCPA adsorption onto tomato stem-derived activated carbons is independent of ionic strength. A similar finding (no effect of ionic strength on adsorption) was also observed for the adsorption of 2,4-D and MCPA on various activated carbons [48; 53]. On the other hand, a few papers reported enhanced adsorption with increasing concentration of the inorganic salt in solution. This was the case, for example, for the adsorption of 2,4-D on commercial activated carbons [45] or the adsorption of 2,4-D and MCPA on lignite [50]. A slight adsorption decrease in MCPA in the presence of NaCl in solution was also reported [45]. Therefore, it is crucial to recognize that the influence of solution ionic strength on adsorption is system-specific. It depends on the physicochemical properties of both the adsorbate and adsorbent, necessitating individual consideration for each adsorbate-adsorbent system.