Sorption of Bromacil, Chlortoluron, and Diuron by Soils

Abstract

Abstract Sorption of bromacil (5‐bromo‐3‐ sec ‐butyl‐6‐methyluracil), chlortoluron [3‐(3‐chloro‐4‐methylphenyl),1,1‐dimethylurea], and diuron [3‐(3,4‐dichlorophenyl)1,1‐dimethylurea] by soils containing contrasting organic carbon (OC) content was studied. The sorptive capacity of the soils was assessed at 4 and 25°C by the batch equilibrium procedure over a broad range of aqueous phase herbicide concentrations. The herbicides showed the following order of increasing tendency to be sorbed: diuron > chlortoluron > bromacil. Sorption constants ( K f ) were from 50‐ to 120‐fold higher for the organic soil than for the loamy sand soil, reflecting sorption dependence on soil OC. This variation in K f was reduced to less than twofold variation in K oc by normalizing sorption to OC. Although sorption was higher at 4 than at 25°C, the difference was only minor at this temperature range. The isosteric heats of sorption (δ H s ) were relatively small and exothermic, suggesting the involvement of entropy driven mechanism. Water solubility ( S ) and octanol‐water partition coefficient ( K ow ) of the herbicides were determined and used to derive equations for estimating K oc with r 2 ≥0.989. The practical utility of these correlations is that they allow reasonable estimates of K oc from more easily determined molecular parameters.