Theoretical aspects and experimental evidence of the capacity of humic substances to bind herbicides by charge-transfer mechanisms (electron donor-acceptor processes)

Abstract

Theoretical aspects of the possible occurrence of electron donor-acceptor processes involving free radical intermediates and leading to stable charge-transfer complexes between electron donor herbicides and acceptor quinone-like units of humic acids were discussed on the basis of similar mechanisms occurring on biological scale in the chloroplasts. Experimental evidence of the former hypotheses was given by the analyses of IR and electron spin resonance data obtained from interaction complexes of a number of s-triazines and substituted ureas (photosynthesis inhibitor herbicides) with soil humic acids. In the cases of s-triazines it was shown that with decrease of the capacity of humic molecules to form ionic and hydrogen bonds, the tendency to act as electron acceptor increased, as shown by the higher free radical generation observed after the interaction process. The existence of correlations between the bioactivity of substituted ureas, expressed as inhibitory potency in the Hill reaction (pI50 indexes), and free radical concentrations in the humic acid - herbicide complexes was also demonstrated.