Adsorption of imazethapyr to amended and nonamended soils and humic acids

Soil amendment with municipal sewage sludge affects the total amount and chemical nature of soil organic matter, including its humic acid (HA) component. These effects are expected to modify the quantitative and mechanistic aspects of herbicide adsorption to soil. The magnitude and mechanisms of adsorption of the imidazolinone herbicide, imazethapyr, to sewage-sludge amended and nonamended soils, and their HA components, were studied using a batch equilibrium method coupled with high-pressure liquid chromatography, and by Fourier-transform infrared (FT-lR), fluorescence and electron spin resonance (ESR) spectroscopic techniques. Sludge-amended soils exhibited nonlinear, L-shaped adsorption isotherms, and much higher adsorption capacity than nonamended soils, which had linear isotherms. Soil-isolated HAs had nonlinear, L-shaped isotherms and adsorbed imazethapyr in amounts much higher than the corresponding whole soils. Spectroscopic data indicate that imazethapyr may interact with HAs through multiple-binding mechanisms including ionic, charge-transfer, and hydrogen bonds. Results of this work indicate that organic matter applied as sludge amendment to soil is able to increase the magnitude and modify the mechanism of imazethapyr adsorption to soil, and confirms the prominent role played by HAs in this process.