THE INTERACTION OF SOME PESTICIDES AND HERBICIDES WITH HUMIC SUBSTANCES

The interaction of the pesticides, chlordimeform and lindane and the h erbicides paraquat, 2,4-dichlorophenoxyacetic acid and atrazine with h umic substances (humic and fulvic acids) has been studied. Binding iso therms were measured by equilibrium dialysis and used to derive the Gi bbs energies of interaction of the biocide ligands with the humic subs tances. Detection of binding at very low ligand concentrations (50 mu M) was demonstrated by ultracentrifugation. Microcalorimetry was used to measure the enthalpies of interaction as a function of ligand conce ntration. The data were interpreted using a Langmuir-type model to obt ain the enthalpies of interaction at saturation and the association co nstants. Both equilibrium dialysis and microcalorimetry grave comparab le specific Gibbs energies (Delta g, J g(-1)) of interaction for those systems (paraquat and chlordimeform) where a complete thermodynamic a nalysis was possible. The specific Gibbs energies of binding of paraqu at and chlordimeform to aquatic fulvic acid were of the ol der of -5 J g(-1);md an order of magnitude larger than for binding to peat humic acid.