# 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
Figure 5: Adsorption isotherms on the S-KC, R-KC and AQ630 adsorbents of (**a**) ESA and (**b**) OXA in the range of 0.05–1.5 g compound/g adsorbent. The error bars represent the standard deviation. The ESA and OXA isotherms were fitted to the classical Langmuir and [PERSON] theoretical models, being better fitted for the Langmuir model for all adsorbents (with higher R\({}^{2}\) values), which indicates this model's adequateness to evaluate the adsorption capacity. The maximum adsorption calculated by [PERSON]'s model, qmax--which corresponds to the coverage of the monolayer [25]--suggests that the adsorption mainly occurs in a molecular monolayer, in agreement with the shape of the experimental isotherms (Figure 5). The value obtained for the Langmuir adsorption constant, \(b\), was higher for the carbon material R-KC, for both metabolites, indicating greater adsorption energy for this material, which is in accordance with their upper adsorption capacity. This fact is probably associated with the higher presence of mesopores--compared with S-KC material (0.13 vs. 0.04 cm\({}^{3}\)/g, respectively) (Table 2)--which provide transport pores to reach sorption sites located in the micropores. Despite the higher mesoporous development of AQ630 compared to R-KC (0.20 vs. 0.13 cm\({}^{3}\)/g, respectively), the ESA and OXA adsorption is lower in AQ630 because of its minor microporosity (more than two times lower than in R-KC). According to the literature, the ESA and OXA metabolites are commonly quantified at concentrations between 0.0001 and 0.005 mg/L in groundwater, depending on the sample collection area [11; 12; 13; 14; 15]. The adsorption of ESA and OXA was studied employing low concentrations, close to those quantified in groundwater (0.001 and 0.01 mg/L), since the initial concentration may impact the value of adsorption coefficients. At equilibrium, the adsorption of ESA and OXA on the adsorbent R-KC followed the same trend, showing a decrease in the adsorption capacity, \(q_{e}\), when the initial concentration went from 8 to 0.01 mg/L, whereas no remarkable effect was observed when the concentration decreased from 0.01 to 0.001 mg/L (Table 6). On the contrary, for commercial activated carbon, AQ630, no significant changes in the adsorption were noticed when the initial concentration was reduced, indicating that the adsorption process on this material is not influenced by the initial concentration. The observed decrease in the R-KC at lower initial concentration could be mainly associated with the smaller concentration gradient between the amount adsorbed inside \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Adsorbent**} & \multicolumn{3}{c}{**Initial Conditions**} & \multicolumn{2}{c}{**qe (\%)**} \\ \cline{2-5}  & **C\({}_{\text{initial}}\) (mg/L)** & **mg Metabolite/mg Adsorbent** & **ESA** & **OXA** \\ \hline \multirow{3}{*}{R-KC} & 8 & 0.2 & 68 \(\pm\) 4 & 82 \(\pm\) 2 \\  & 0.01 & 0.005 & 48 \(\pm\) 15 & 48 \(\pm\) 7 \\  & 0.001 & 0.0005 & 48 \(\pm\) 11 & 47 \(\pm\) 6 \\ \hline \multirow{3}{*}{AQ630} & 8 & 0.2 & 38 \(\pm\) 2 & 32 \(\pm\) 3 \\  & 0.01 & 0.005 & 32 \(\pm\) 5 & 43 \(\pm\) 4 \\ \cline{1-1}  & 0.001 & 0.0005 & 35 \(\pm\) 2 & 41 \(\pm\) 5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Experimental adsorption capacities obtained for different initial concentration of ESA and OXA solutions for the R-KC and AQ630.