Low Sorption and High Desorption of Pretilachlor Metabolites Indicated Greater Mobility Potential than Parent Compound.

Abstract

Pretilachlor herbicide is extensively used in rice cultivation. In open environment, it degrades into different metabolites. Information on the environmental mobility of these metabolites is limited. This study aimed to assess the potential movement risks associated with two principal metabolites of pretilachlor. To achieve the objectives, the sorption-desorption behaviour and the thermodynamic properties of pretilachlor (P), metabolite 2',6'-diethyl-N-(2-hydroxyethyl)aniline (M1) and 2-chloro-2',6'-diethylacetanilide (M2) were studied in different soils governing their mobility. Sorption kinetics followed a pseudo-second-order model (R² > 0.98) across all the soils, while the equilibrium data fitted well to the Freundlich isotherm (R² > 0.99). The sorption capacity decreased in the order P > M1 > M2, with Freundlich adsorption coefficients (K_F-ads) ranging from 3.96 to 23.09 for P, 2.17-21.64 for M1 and 2.05-7.53 µg^1-1/n g^-1 mL^1/n for M2. Sorption correlated positively with soil organic matter (OM) content; however, appreciable retention was also observed in soils with low OM (0.32%) where sorption coefficients (K_D) ranged from 4.20 to 5.44 mL g^-1 for P, 1.56 to 3.22 mL g^-1 for M1 and 0.91 to 1.47 mL g^-1 for M2. It highlighted about the contribution of clay and mineral fractions in addition to OM. PCA revealed two principal components, with PC1 (87.8%) and PC2 (9.2%) explaining 97.0% of the total variance, with the highest variation contributed by OM, clay and CEC. The optimum temperature for sorption was found to be 30 °C, beyond which a decline was observed. Desorption was low for P and moderate to high for M1 and M2 across different soils with hysteresis index (H) ranging from 0.003 to 0.758, indicating the partial irreversibility of the process. Thermodynamic analysis revealed that sorption was endothermic (ΔH = 1.64-7.49 kJ mol^-1), spontaneous (ΔG = - 1.75 to - 7.90 kJ mol^-1), entropy driven (ΔS = 0.05-0.30 J mol^-1 K^-1) and primarily physical in nature. Overall, this study revealed that P exhibited moderate sorption, whereas M1 and M2 showed low sorption and high desorption than P, indicating their greater mobility potential.