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.