# 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  
The kinetic study was performed on ESA and OXA compounds separately for the four activated carbons. A fast and high adsorption was observed for both TPs on R-KC, S-KC and AQ630 in short timeframes, reaching a plateau at ca. 3 h (Figure S3). Table 4 shows the results of the adsorption capacity at equilibrium time for both compounds. A similar trend was observed for both metabolites, and the highest removal efficiency was obtained for the carbons activated with potassium carbonate (samples R-KC and S-KC). This fact was more remarkable in the case of OXA, for which the adsorption was found to higher than 80% on the S-KC and R-KC samples at equilibrium time. For both OXA and ESA, adsorption efficiencies were less or equal to 5% on R-PO and between 32% and 38% for the commercial activated carbon, AQ630. These differences could be associated with the microporous development of the adsorbent, pointing out a higher adsorption of the metabolites on those materials which present greater microporous volume (Table 2). The S-KC and R-KC samples with adsorbed OXA were thermally decomposed under inert atmosphere. The materials displayed a mass loss of 4.4 and 2.8%, respectively, above ca. 100 \({}^{\circ}\)C (Figure S2b,c), confirming the metabolite confinement on the adsorption sites of the materials. Comparing the adsorption of both TPs on the same adsorbents and in identical adsorption conditions, only the intrinsic properties of the molecules should be evaluated to explain the differences in the adsorption capacities. In this sense, since the water solubility is quite similar for OXA and ESA (Table S1), both compounds differ in molecular weight and the nature of the substituent groups influencing their adsorptivity. The greater adsorption of OXA compared to ESA on the adsorbents S-KC and R-KC could be explained by its lower molecular weight (279.33 vs. 329.70 g/mol, respectively, Table S1). Additionally, it \begin{table}
\begin{tabular}{c|c c c c c|c c c c c} \hline \hline \multirow{2}{*}{**adsorbent**} & \multicolumn{5}{c|}{**Kinetics**} & \multicolumn{5}{c}{**Isotherms**} \\ \cline{2-10}  & **q\({}_{\text{\tiny{exp}}}\)** & **q\({}_{\text{\tiny{exp}}}\)** & **q\({}_{\text{\tiny{re}}}\)** & **q\({}_{\text{\tiny{e}}}\)** & **k\({}_{\text{2}}\)** & **[PERSON]** & & **[PERSON]** & \\  & **(mg/g)** & **(removal \%)** & **(mg/g)** & **(g/mg h)** & **R\({}^{2}\)** & **q\({}_{\text{max}}\)** & **b** & **R\({}^{2}\)** & **k\({}_{\text{f}}\)** & **n\({}_{\text{f}}\)** & **R\({}^{2}\)** \\ \hline S-KC & 486 \(\pm\) 3 & 98 \(\pm\) 2 & 493 & 0.051 & 0.999 & 991 & 0.40 & 0.969 & 599 & 8.7 & 0.958 \\ R-KC & 465 \(\pm\) 2 & 99 \(\pm\) 1 & 467 & 0.047 & 0.995 & 1023 & 40.40 & 0.710 & 760 & 8.3 & 0.983 \\ R-PO & 208 \(\pm\) 6 & 40 \(\pm\) 2 & 204 & 0.240 & 0.970 & 580 & 0.02 & 0.901 & 19 & 1.4 & 0.940 \\ AQ630 & 340 \(\pm\) 5 & 77 \(\pm\) 2 & 330 & 0.034 & 0.983 & 390 & 3.50 & 0.824 & 281 & 10.3 & 0.961 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experimental adsorption capacity, pseudo-second order kinetics parameters, and isotherms non-linear fitting to the Langmuir and Freundlich models parameters, obtained by a non-linear fitting of MET adsorption. The coefficients of determination (R\({}^{2}\)) are also included. seems that the shape of OXA fits better in the pores of those adsorbents. As a consequence, a largest part of microporosity could be accessible to OXA than to ESA for sorption sites.