# Adsorption of Metolachlor and Its Transformation Products, ESA and OXA, on Activated Carbons
## 3 Results
### Adsorption  
#### 3.2.1 Metolachlor Adsorption  
Figure 3a shows the adsorption kinetics of MET for the four adsorbents. As we observed in our previous studies [19], in all cases, an increase in the adsorption time implies a fast increase in the removal of MET before equilibrium was reached (ca. 3-4 h). However, the amount adsorbed in equilibrium conditions was very different for each adsorbent, with the highest adsorption for the S-KC material, followed by R-KC, AQ630 (the commercial activated carbon) and R-PO. The greater adsorption of carbonate activated carbons, R-KC and S-KC, (as it was already reported [19]) is mainly associated with their larger surface area, higher than 2000 m\({}^{2}\)/g (Table 2). The experimental data were well-fitted to the pseudo-second order model (correlation coefficients between 0.979-0.999) showing a calculated \(q_{e}\) values closed to those experimentally obtained (Table 3). The highest value of the rate constant, \(k_{2}\), was obtained for the adsorption on R-PO, while AQ630 showed the slowest adsorption.  
Figure 3: Evolution of MET adsorption as a function of contact time employing 0.5 g MET/g adsorbent (**a**), and MET isotherms, 0.05–1.5 g MET/g adsorbent (**b**), obtained at 25 \({}^{\circ}\)C for the four studied adsorbents. The error bars represent the standard deviation.  
Figure 2: DTG curves obtained from 25 to 800 \({}^{\circ}\)C (10 \({}^{\circ}\)C/min) under Ar atmosphere of (**a**) raw rape and rape impregnated with K\({}_{2}\)CO\({}_{3}\), and (**b**) raw sunflower and sunflower impregnated with K\({}_{2}\)CO\({}_{3}\). The profile of K\({}_{2}\)CO\({}_{3}\) is included for comparison.  
The MET adsorption isotherms were realized for a contact duration of 3 h. Figure 3b reveals the following trend of adsorption capacity values: R-KC > S-KC > AQ630 > R-PO. The experimental isotherms were fitted to the [PERSON] and Freundlich models (Table 3), showing the best adequateness to the Freundlich equation. Despite the different chemical composition of the adsorbents (Table 2), the variation of the adsorption capacity is mainly associated with the textural parameters, since the electrostatic interactions are discarded due to the non-polar character of MET [17; 19]. Accordingly, the highest value (above 1000 mg/g) was observed for the material with the greater \(S_{BET}\) and higher mesoporous development (R-KC); in contrast, the lowest adsorption capacities (\(ca\). 280 and 435 mg/g for R-PO and AQ630, respectively) were obtained for the samples with lower surface area. The acidic character of the R-PO sample also showed an important effect, it being the adsorbent with the lowest adsorption capacity, probably associated with the adsorption competition with water molecules.  
Additionally, after the adsorption of MET on R-KC and S-KC, the samples were tested using thermogravimetry (Figure S2a,b), revealing a total mass loss of 27% and 26% for R-KC and S-KC, respectively, from ca. 230 \({}^{\circ}\)C (while the pristine samples presented 8 and 13%). This increase in the mass loss is associated with the thermal decomposition of MET adsorbed on the porosity of the material.