The key forms of Chromium influencing the long-term stability of Cr in Cr-contaminated soils and chemical stabilized soils

The toxicity and persistence of chromium in soils challenge the ecosystem and human health. Various remediation strategies have been developed to eliminate soil Cr contamination, and the most popular one is chemical stabilization. However, chemical stabilization only changes the form of Cr and does not change the concentration of Cr, so the long-term stability of Cr has been controversial. On the other hand, some researches found that the concentration of Cr(VI) in the stabilized soil after remediation has increased. We collected Cr-contaminated soils and one-year-stabilized soils from four research sites in northern, central, and southwestern China, trying to understand the difference of Cr species and structure in soils with various soil properties. Results showed despite the different clay content and mineral composition, all contaminated and stabilized soils are alkaline (pH 7.36 ~ 10.5). In addition, there are differences in the pollution levels of Cr and Cr (VI) in soils. In northern China, Cr(VI) was the main state of Cr-contaminated soils; however, Cr is mainly present in Cr-contaminated soil in the form of Cr(III) in southern China. For chemical stabilized soils, Cr concentrations remained similar to Cr-contaminated soils (1500~9000 mg/kg), but the concentration of Cr(VI) (5~55 mg/kg) was reduced through commercial remediation materials. The speciation of Cr in Cr-contaminated soils transformed from exchangeable Cr and Cr bound to carbonates into Cr bound to Fe-oxides and residuals in stabilized soils. SEM-EDS, XAFS and μ-XRF results revealed the main forms and structure of Cr, and showed Cr unevenly distributed on the surface or edge of the mineral. The acid leaching test revealed that Cr(VI) could be released from Cr-contaminated soils by acid, and soils can release Cr(VI) under different acidity conditions. Cr(VI) from soils collected from northern and southern China was released from acidity of [H⁺]=0.1M and [H⁺]=0.5M, respectively. This was due to erosion of coating minerals or Cr(VI)-bearing minerals. Our study suggested that stabilization technology not only reduces the toxicity of chromium, improves the stability of chromium, but also partially recovers the physical and chemical properties of soil. Meanwhile, in future remediation projects, it is necessary to consider the existing forms of chromium in different soils to develop remediation strategies.