Adsorption mechanisms of s-triazine and bipyridylium herbicides on humic acids from hop field soils

Abstract

Humic acids (HAs) have been isolated according to conventional extraction, fractionation and purification methods from soil samples collected in a hop field in Bavaria (Germany), under the plant rows (HAa), between the rows (HAb) and in an adjacent unexploited soil location (control, HAc). Interaction products between each HA sample and each of the herbicides atrazine, simazine, diquat and paraquat were laboratory prepared in aqueous medium and under ambient conditions, according to a previously tested procedure. Both the original, unreacted HAs and their herbicide interaction products were analyzed for elemental and functional group composition, and by Fourier-transform infrared (FT-IR), fluorescence and electron-spin resonance (ESR) spectroscopies. Minor differences are measured for properties of the three HAs studied, thus suggesting that the hop cultivation has affected only secondarily the compositional and functional chemical properties of HA in these soils. The three HAs also exhibit a similar behaviour in the interaction with each herbicide examined. The water-stable association between the N-rich herbicide molecules and HA is firstly supported by the N% increase measured in all the interaction products. FT-IR and ESR spectroscopic results suggest that multiple binding mechanisms may occur in the adsorption process with the formation of (a) ionic bonds between carboxylate groups of HA and protonated heterocyclic and amine N-atoms of the triazine or positively charged N-atoms of the bipyridylium compound; (b) charge-transfer (π-π) bonds between the electron-donor triazine ring or the electron-acceptor deactivated bipyridylium ring and complementary electron-donor or -acceptor structural moieties of HAs; (c) hydrogen bonds within suitable complementary groups of the HA and the triazine. Fluorescence data suggest that fluorophore groups may also be involved to various extent in the interaction mechanisms.