ADSORPTION OF ALACHLOR BY HUMIC ACIDS FROM SEWAGE-SLUDGE AND AMENDED AND NON-AMENDED SOILS

Organic matter content is ascertained to be the major factor for alach lor adsorption in soils. Limited knowledge exists, however, as to the effects of structural and chemical properties of soil organic matter o n the mechanisms and extent of alachlor adsorption. The binding mechan isms and adsorption isotherms for alachlor have been measured onto thr ee humic acids (HA) originating from a municipal sewage sludge, a slud ge-amended soil, and a control soil. Spectroscopic techniques, includi ng Fourier-transform infrared, fluorescence and electron spin resonanc e analyses, and high-pressure liquid chromatography have been used. Th e magnitude of alachlor adsorption and the type of binding mechanism i nvolved depended on the nature and chemical properties of the HA. Soil HA exhibited linear isotherms and higher alachlor adsorption at low c oncentration, whereas sludge humic acid featured an S-shaped isotherm and increasing adsorption at higher alachlor concentration. Multifunct ional hydrogen bonds and charge-transfer bonds were preferentially inv olved in the adsorption of alachlor at low concentration, especially o nto well humidified, highly aromatic soil humic acids rich in 0-contai ning groups. Hydrophobic bonding appeared to predominate at higher ala chlor concentration, especially onto low-humified, highly aliphatic sl udge HA. A reliable quantitative evaluation of the extent of adsorptio n of alachlor to HA in the experimental range of concentration used co uld be provided by the use of both the Freundlich K-values and the dis tribution coefficients, K(d).