The Environmental Fate of Herbicides: The Role of Humic Substances

The long-term fate of herbicides in the environment is strongly influenced by adsorption phenomena operated by soil colloids (clay and humus). Persistence and leachability, bioactivity and biodegradability of herbicides are mainly dependent upon the bonding mechanisms involved in the adsorption processes; qualitative information was obtained applying analytical (DTA, elementary and functional group analysis) and spectroscopic (ESR and IR) techniques to study the products of interaction of selected typical s-triazines and substituted ureas with humic acids of different origins. The significant increase in nitrogen percentage and decrease in C/N ratios observed in obtained complexes appeared to be strictly related to acidic (carboxylic and phenolic) functional groups' content of humic acids. The shifting to higher temperatures of the characteristic exothermic peaks due to decarboxylation and phenolic OH-dehydration in the interaction products revealed a higher thermostability owing to the formation of stable bonds. The existence of ionic bonds between carboxylic and phenolic OH-groups of humic acids and the most basic herbicides was demonstrated by the apparent reduction of the characteristic infrared absorption bands and the contemporaneous increase of the carboxylate ions bands. Infrared analysis showed that weaker hydrogen bonds with less basic herbicides (i.e., substituted ureas) can be formed by these groups, as well as by carbonyl and alcoholic OH. Electron donor-acceptor bonds (n - ji interactions) between activated triazine rings and de activated aromatic nuclei of humic acids were hypothesized on the basis of the observed shifting of specific CH wagging absorption of s-triazine in the infrared spectra of interaction products. The generally observed increase of the free radicals' content and the broadening of linewidths in the single-line ESR spectra obtained following adsorption, confirmed the occurrence of electron donor-acceptor processes involving free radical intermediates. ESR results also indicated the occurrence of extensive molecular association, which resulted in an increased molecular complexity in the adsorption products which offered additional delocalization sites by mesomerism to free electrons of the humic molecule. Therefore the feasible formation of covalent bonds between quinonoid structures of humic acids and nitrogenated groups of herbicides was suggested. Oxidative mechanisms involving an attack on the herbicide molecule by humic free radicals might also be hypothesized to explain the strong enhancement of paramagnetism. Different binding mechanisms will act in the interaction of herbicides with humic materials, vary ing with chemical reactivity of the herbicide and active functional groups and free radical contents of humic molecules.