ADSORPTION MECHANISMS OF S-TRIAZINE AND BIPYRIDYLIUM HERBICIDES ON HUMIC ACIDS FROM HOP FIELD SOILS

Humic acids (HAs) have been isolated according to conventional extract ion, fractionation and purification methods from soil samples collecte d in a hop field in Bavaria (Germany), under the plant rows (HAa),betw een the rows (HAb) and in an adjacent unexploited soil location (contr ol, HAc). Interaction products between each HA sample and each of the herbicides atrazine, simazine, diquat and paraquat were laboratory pre pared in aqueous medium and under ambient conditions, according to a p reviously tested procedure. Both the original, unreacted HAs and their herbicide interaction products were analyzed for elemental and functi onal group composition, and by Fourier-transform infrared (FT-IR), flu orescence and electron-spin resonance (ESR) spectroscopies. Minor diff erences are measured for properties of the three HAs studied, thus sug gesting that the hop cultivation has affected only secondarily the com positional and functional chemical properties of HA in these soils. Th e three HAs also exhibit a similar behaviour in the interaction with e ach herbicide examined. The water-stable association between the N-ric h herbicide molecules and HA is firstly supported by the N% increase m easured in all the interaction products. FT-IR and ESR spectroscopic r esults suggest that multiple binding mechanisms may occur in the adsor ption process with the formation of (a) ionic bonds between carboxylat e groups of HA and protonated heterocyclic and amine N-atoms of the tr iazine or positively charged N-atoms of the bipyridylium compound; (b) charge-transfer (pi-pi) bonds between the electron-donor triazine rin g or the electron-acceptor deactivated bipyridylium ring and complemen tary electron-donor or -acceptor structural moieties of HAs; (c) hydro gen bonds within suitable complementary groups of the HA and the triaz ine. Fluorescence data suggest that fluorophore groups may also be inv olved to various extent in the interaction mechanisms.