# Adsorption of Metolachlor and Its Transformation Products, ESA and OXA, on Activated Carbons
## 3 Results
### Adsorption
#### 3.2.2 Transformation Products ESA and OXA Adsorption
Regarding the activated carbon R-PO, the adsorption of ESA and OXA was found to be very low, revealing values of metabolite adsorbed of below 5%; this fact could be associated with several factors: (i) the lower porous development compared to the other adsorbents (Table 2), as was previously assessed to the low adsorption of MET; (ii) the competitive adsorption of water molecules could have occurred due to the hydrophilic character of this adsorbent (Table 2), with the oxygen groups being favorable for the creation of water clusters, hindering the accessibility of the metabolite to the inner porous structure [37], and (iii) the oxidized groups located on the structure of the carbon material can cause a withdrawing effect of the electrons, reducing the \(\pi\)-electron density on the aromatic rings and decreasing the dispersive interaction between the metabolite and carbon material [37]. Similar effects have been previously reported for the adsorption of other aromatic herbicides [38; 39; 40]. The influence of solution pH on the adsorption capacity was analyzed for the adsorption of OXA on the four adsorbents. Figure 4 shows the comparison of the adsorption evolution for a solution of the compound at natural pH (4.7-5.2, being the pH measured for OXA solution) and at pH 2 (adjusted with some drops of HCl). In all cases, independently of the nature of the adsorbent, the adsorption is increased for lower pH, reaching a removal efficiency of close to 100% for the S-KC and R-KC materials at equilibrium time. In aqueous solutions, the surface of the adsorbent and the molecules coexist in a complex system with different superficial charges than can generate repulsive or attractive electrostatic interactions. In the case of the OXA metabolite, the anionic form is predominant in a solution at pH above its pKa of 4.8 (Table S1). In a solution at pH 2, OXA's anionic form becomes minor in the solution but is still present. All of the employed adsorbents are positively charged at pH 2 (solution pH < superficial pH); therefore, the adsorption capacity is increased due to the decrease in repulsive force between the OXA and the surface charges along the graphitic layers of the adsorbents. This effect is more remarkable in the case of AQ630 (adsorption at pH 2 increases to two times the adsorption at natural pH), as a consequence of its higher negatively charged character at pH 2 (adsorbent with the higher superficial pH, Table 2). Figure 5 shows the adsorption isotherms obtained for ESA and OXA on S-KC, R-KC and AQ630 (the adsorbent R-PO was excluded due to its low adsorption observed above). The higher adsorption of both TPs was obtained for S-KC and R-KC with an experimental adsorption capacity, \(q_{max,~{}exp}\), of 218 and 273 mg/g for ESA and 439 and 554 mg/g for OXA, respectively (Table 5). As already observed in the kinetic study, adsorption is greater for OXA in all studied adsorbents. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**adsorbent**} & \multicolumn{2}{c}{**ESA**} & \multicolumn{2}{c}{**OXA**} \\ \cline{2-5}  & \(\mathbf{q_{e,~{}exp}}\) & \(\mathbf{q_{e,~{}exp}}\) & \(\mathbf{q_{e,~{}exp}}\) & \(\mathbf{q_{e,~{}exp}}\) \\  & **(mg/g)** & **(Removal \%)** & **(mg/g)** & **(Removal \%)** \\ \hline S-KC & 120 \(\pm\) 6 & 57 \(\pm\) 3 & 160 \(\pm\) 10 & 82 \(\pm\) 5 \\ R-KC & 137 \(\pm\) 1 & 68 \(\pm\) 4 & 166 \(\pm\) 6 & 82 \(\pm\) 2 \\ R-PO & 10 \(\pm\) 7 & 4 \(\pm\) 2 & 10 \(\pm\) 6 & 5 \(\pm\) 2 \\ AQ630 & 76 \(\pm\) 3 & 38 \(\pm\) 2 & 60 \(\pm\) 8 & 32 \(\pm\) 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Experimental adsorption parameters obtained by the kinetics of ESA and OXA adsorption for the studied adsorbents. Figure 4: Influence of the solution pH (closed symbols for natural pH and opened symbols for pH 2) on the adsorption of OXA on (**a**) S-KC, (**b**) R-KC, (**c**) R-PO and (**d**) AQ630 (0.2 g OXA/g adsorbent). The dashed line corresponds to the saturation. The error bars represent the standard deviation.