The interaction of some pesticides and herbicides with humic substances

The interaction of the pesticides, chlordimeform and lindane and the herbicides paraquat, 2,4-dichlorophenoxyacetic acid and atrazine with humic substances (humic and fulvic acids) has been studied. Binding isotherms were measured by equilibrium dialysis and used to derive the Gibbs energies of interaction of the biocide ligands with the humic substances. Detection of binding at very low ligand concentrations (50 μM) was demonstrated by ultracentrifugation. Microcalorimetry was used to measure the enthalpies of interaction as a function of ligand concentration. The data were interpreted using a Langmuir-type model to obtain the enthalpies of interaction at saturation and the association constants. Both equilibrium dialysis and microcalorimetry gave comparable specific Gibbs energies (Δg, Jg−1) of interaction for those systems (paraquat and chlordimeform) where a complete thermodynamic analysis was possible. The specific Gibbs energies of binding of paraquat and chlordimeform to aquatic fulvic acid were of the order of −5 Jg−1 and an order of magnitude larger than for binding to peat humic acid.