Transformation of exogenous hexavalent chromium in soil: Factors and modelling

There is a great need, but not yet available, for quantitative transformation models and influencing factors of exogenous hexavalent chromium (Cr(VI)) in soil for environmental risk assessment and regulation. Therefore, the transformation processes of exogenous Cr(VI) in 13 soils across China were investigated. The changes in Cr forms in soils spiked with 100 mg kg^-1 of Cr(VI) over 90-120 days indicate that both the reduction and immobilization of Cr(VI) occurred with the decrease in availability of Cr(VI) and its reduced counterpart Cr(III). In these processes, soil pH is the controlling factor and pH 6.5 is a critical inflection, where pH < 6.5 promoted the Cr(VI) transformation and decreased the Cr availability. A two-parameter model with a complementary error function for the reduction of Cr(VI) to Cr(III) using soil pH and incubation time was developed with the regression coefficient (R²) of 0.98 and root-mean-square-error (RMSE) of 7.94 %, which was validated using data from other independent literature. This semi-mechanistic model suggests that diffusion process of electrons controlled the Cr(VI) reduction. These results are helpful for understanding the Cr(VI) evolution in soil in a long term and complementing the risk assessment of redox-sensitive metal contaminated soils.