Assessment of molecular descriptors involved in the sorptive interactions of phenoxy-carboxylic herbicides in soil treated with different concentrations of humic acid

Abstract

Functional relationships in the sorption of 15 phenoxy-carboxylic herbicides (carboxyl, methyl-, methylethyl-, butyl- or octyl ester forms) on soil (Calcic Luvisol) samples treated with concentrations of humic acid ranging from 1 to 104 mg kg-1 have been examined. Multivariate data treatments were applied to a matrix including sorption values and herbicide parameters calculated with molecular mechanics software. The sorption patterns depended on the herbicide structure and form, as well as on humic acid concentration in the soil. In general, application rates higher than 5 g kg-1 were required for systematic enhancement of herbicide sorption compared to untreated soil. Whereas solvation energies and energies due to van der Waals, bond stretching and dipole interactions were molecular descriptors useful to forecast the sorption of phenoxy-carboxylics on the soil, we found that herbicide sorption was chiefly governed by molecular parameters such as the molecular volume, the alkylation degree, the O/C atomic ratio, the polar surface area and the heat formation of the herbicide molecule.