Can assessing for potential contribution of soil organic and inorganic components for butachlor sorption be improved?

Abstract

Sorption of butachlor to various types of common soil components was investigated. Six pure minerals (montmorillonite [Mont], kaolinite [Kaol], Ca homoionic montmorillonite [Ca-Mont] and kaolinite [Ca-Kaol], amorphous hydrated Al and Fe oxides [AHOs-Al, AHOs-Fe]), four soil alkali-extractable pure humic acids (HAs), and the four corresponding HAs originated real unmodified and HO-treated soils were selected as the representative sorbents. Results showed that the HAs played a crucial role, and clay minerals (especially Mont) also showed an important effect in butachlor sorption. The AHOs may likely influence only in a mediator way by enhancing the availability of sorption domains of HAs. By removing 78% (on average) of the total organic carbon (TOC) from the soils with HO, the content ratio of clay to TOC (RCO) increased by an average of 367% and became >60. This change simultaneously decreased the sorption capacity of soils (40%, on average). Considering that the surface sorption domain on clay minerals may be highly exposed and more competitive after the partial removal of soil organic matter (SOM), this reaffirmed the potential contribution from clay minerals. It can thus be inferred that in the real soil where SOM and clay minerals are associated, the coating of clay minerals may have weakened the partition function of SOM or blocked some sorption domain within SOM, resulting in a decreased sorption of butachlor. Therefore, clay minerals, especially 2:1 type expanding minerals, may play a dual function vs. SOM content for the sorption of butachlor in soil.