# Effective Adsorption of Phenoxyacetic Herbicides by Tomato Stem-Derived Activated Carbons
## 3 Results and Discussion
### Adsorption Study
#### 3.2.2 Adsorption Isotherms
650 < AC-NaOH-650 < AC-KOH-750 < AC-NaOH-750) appears to correlate strongly with the porous structure of the TS-AC. In addition to the surface chemistry, textural properties, including micropore volume and specific surface area, significantly influence the adsorption characteristics of TS-AC. Generally, the adsorption capacity of an adsorbent is directly proportional to its micropore volume (\(V_{\text{mi}}\)) and specific surface area (\(S_{\text{BET}}\)), owing to the increased availability of active sites for adsorbate molecule interaction. Thus, the lowest adsorption capacity of AC-KOH-550 results from its most poorly developed porous structure (\(V_{\text{mi}}\) = 0.268 cm\({}^{3}\)/g, \(S_{\text{BET}}\) = 575 m\({}^{2}\)/g). AC-NaOH-750 appeared to be the most effective adsorbent, having the highest micropore volume and specific surface area (\(V_{\text{mi}}\) = 0.928 cm\({}^{3}\)/g and \(S_{\text{BET}}\) = 2085 m\({}^{2}\)/g, respectively). The adsorption properties of the other ACs are also correlated with their microporous structure and BET surface area. This strong correlation observed for both herbicides suggests that the textural properties of the tomato stem-derived activated carbon (TS-AC) have a more significant influence on herbicide adsorption than their surface chemistry. In our earlier works [37; 38], it was also found that the effect of activated carbon surface chemistry on the sorption capacity of chloroorganic compounds is smaller than the effect of porosity development. This was demonstrated in the granular activated carbon samples with very little differentiation of porous structure parameters and enormously different surface chemistry. In the case of activated carbon samples with enormously different porous structure parameters and almost identical surface chemistry, a correlation was found between the adsorption capacity of chloroorganic compounds (e.g., chlorophenol) and their BET surface area and micropore volume [39]. A consistent trend was observed across all six TS-AC: 2,4-D exhibited a slightly higher adsorption tendency than MCPA. This finding is supported by the literature, which has documented similar results for these herbicides on various adsorbents, including switchgrass biochar [40], Filtrasorb 300 activated carbon [41], AC from pistachio shells prepared via physical activation [42], or some carbon blacks [43]. However, this is not a definite trend, as there are just as many reports of better adsorption of MCPA compared to 2,4-D [44; 45; 46; 47; 48; 49]. Therefore, the observed variability in the relative adsorption of 2,4-D and MCPA is more accurately described due to the distinct physicochemical attributes inherent to each adsorbent. These properties influence the mechanism of herbicide molecule attachment to the adsorbent surface. The adsorption mechanism of phenoxyacetic herbicides, focusing on 2,4-D, has been thoroughly investigated and summarized in recent reviews [3; 7; 8]. The adsorption of 2,4-D on activated carbons results from hydrophobic and \(\pi\)-\(\pi\) interactions, attractive and/or repulsive electrostatic interactions, and hydrogen bonding interactions [3; 7; 8]. As anticipated, the adsorption of 2,4-D on TS-AC proceeded via these mechanisms. Given the structural similarity of MCPA to 2,4-D, it is reasonable to expect that MCPA adsorbs through the same mechanisms. Table 9 compares the adsorption capacities of the various activated carbons described in the literature towards 2,4-D and MCPA. The TS-AC described herein exhibits satisfactory adsorption capacities comparable to other activated carbons. A more detailed comparison of the different adsorbents can be found in the review papers [3; 7; 8].