Combined effects of an oxidative enzyme and dissolved humic substances on (13)C-labelled 2,4-D herbicide as revealed by high-resolution (13)C NMR spectroscopy.

Phenoxyalkanoic acids are a widely used class of herbicides. This work employed high-resolution (13)C NMR to study the structural changes induced by humic substances and horseradish perodixase on 2,4-dichorophenoxyacetic acid (2,4-D) (13)C-labelled in the side chain. NMR spectra showed that humic substances chemically catalyze abiotic splitting of [(13)C]2,4-D into 2,4-dichlorophenol and [(13)C]acetic acid at pH 7 but not at pH 4.7. Peroxidase did not catalyze the oxidative degradation of [(13)C]2,4-D at any pH tested and inhibited the effect of humic substances. Catalytic degradation by humic substances was attributed to free-radical reactions enhanced by the stereochemical contribution of large conformational structures formed by heterogeneous humic molecules at neutral pHs. Inhibition of 2,4-D degradation when humic substances were combined with peroxidase was explained by modification of both chemical and conformational humic structure due to peroxidase-promoted oxidative cross-coupling among humic molecules. Our findings show for the first time that the abiotic degradation of 2,4-D is catalyzed by dissolved humic substances at neutral pH. 70-76.