Butachlor Sorption in Organically Rich Soil Particles

Abstract

The herbicide butachlor [N‐(butoxymethyl)‐2‐chloro‐N‐(2,6‐diethylphenyl)acetamide] is a pre‐emergence herbicide belonging to the chloroacetanilide group used widely to control grasses in Asian countries. Butachlor has toxicity to aquatic organisms and earthworms and can induce apoptosis in mammalian cells. This study used a batch equilibration method to investigate butachlor sorption by different size organomineral fractions of a black soil. About 60 to 70% of butachlor was adsorbed on the clay size fraction, 30 to 40% on the silt fraction, and <0.3% by the fine sand fraction. For the clay fraction, the nonlinearity of adsorption was significant in the low equilibrium concentration range. For the silt and fine sand fractions, the isotherms were near linear across the concentration range studied. Butachlor adsorption on a clay fraction treated with H2O2 showed that the degree of nonlinearity became smaller, and the value of the normalized sorption coefficient for organic C (Koc) was much higher than that of the untreated clay fraction. Mineral colloids might have a dual function in the butachlor adsorption process. For an adsorbent with a very low soil organic matter (SOM) content, mineral colloids could enhance butachlor adsorption because of their high exposed mineral surface, which would result in higher Koc values. For an adsorbent with a high SOM content, mineral colloids might weaken the SOM partition function because of their strong association with and coating of SOM, which would result in lower Koc values. These results are useful to better understand the adsorption mechanism of anthropogenic organics in different components and size fractions of soils.