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 alachlor adsorption in soils. Limited knowledge exists, however, as to the effects of structural and chemical properties of soil organic matter on the mechanisms and extent of alachlor adsorption. The binding mechanisms and adsorption isotherms for alachlor have been measured onto three humic acids (HA) originating from a municipal sewage sludge, a sludge-amended soil, and a control soil. Spectroscopic techniques, including Fourier-transform infrared, fluorescence and electron spin resonance analyses, and high-pressure liquid chromatography have been used. The magnitude of alachlor adsorption and the type of binding mechanism involved depended on the nature and chemical properties of the HA. Soil HA exhibited linear isotherms and higher alachlor adsorption at low concentration, whereas sludge humic acid featured an S-shaped isotherm and increasing adsorption at higher alachlor concentration. Multifunctional hydrogen bonds and charge-transfer bonds were preferentially involved in the adsorption of alachlor at low concentration, especially onto well humidified, highly aromatic soil humic acids rich in 0-containing groups. Hydrophobic bonding appeared to predominate at higher alachlor concentration, especially onto low-humified, highly aliphatic sludge HA. A reliable quantitative evaluation of the extent of adsorption of alachlor to HA in the experimental range of concentration used could be provided by the use of both the Freundlich K-values and the distribution coefficients, K(d).