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Sorption of Bromacil, Chlortoluron, and Diuron by Soils¹

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 (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 r² 0.989. The practical utility of these correlations is that they allow reasonable estimates of K_oc from more easily determined molecular parameters.

¹ Contribution of the Oregon Agricultural Experiment Station. Technical Paper no. 7337. Departments of Agricultural Chemistry and Soil Science, Oregon State Univ., Corvallis, OR 97331.

² Research Associate; Professor of Chemistry; Research Chemist, ARS-USDA; Professor of Soil Chemistry; and Professor of Agricultural Chemistry, respectively.

Received for publication November 1, 1985.

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