This project aimed to develop X-ray absorption spectroscopic (XAS) method coupled with sequential extraction method to determine the chemical speciation of chromium (Cr) in contaminated farmland soils before and after the soils were remediated using the tillage method. The information was further used for evaluating the potential risk of Cr in the soils after the remediation. Soil samples were collected from different locations in two contaminated farmlands in Taoyuan and Changhua, respectively, before and after remediation. The XAS results of the soils showed that the predominant Cr species in the soils are Cr hydroxides and oxides. The chemical speciation and fractionation, obtained using XAS and sequential extraction methods, showed similar trends in terms of their spatial distributions in the contaminated sites. The relative contents of Cr oxides or residual phase increased with increasing the distance from the irrigation water inlets of the farmlands or increasing soil depth, while the other species and phases exhibited an opposite trend. The highest contents of the hydroxide species or reducible/oxidizable phases in the soils near the irrigation water inlets revealed the input of Cr from anthropogenic sources. After remediation, the total content and the chemical speciation/fractionation in the soils at different locations became similar, and the solubility of Cr in the soils decreased. These results indicated the positive effects of the tillage remediation method on decreasing the potential risk of soil Cr contamination. However, further investigation is required to consider the potential release of Cr from the reducible Cr phase in groundwater after tillage remediation. The complementary information from combining the XAS and sequential extraction method allows the effective identification of the predominant Cr-associated phases in soils and understanding their chemical transformation under certain conditions in the environment.

Heavy metals, Soil chromium contamination, X-ray absorption spectroscopy, Sequential extraction, Chemical speciation, Spatial distribution