Kinetic process of Cr(III) in contaminated soils characterized by diffusive gradients in thin films technique

Trivalent chromium has historically been considered as an environmentally benign micronutrient due to its low mobility; however, its kinetic process in soil remains poorly understood. Here, the labile fraction and kinetics of Cr(III) in contaminated soils were explored using diffusive gradients in thin films (DGT) and the DGT-induced fluxes model. In contrast to the low mobility of Cr(III) in soils reported by the classic equilibrium partitioning method, we observed steady resupply from the solid phase through a dynamic process, wherein Cr(III) in the soils were maintained by an intermediate resupply rate with the R values at their maximum (R_max). The resupply of Cr(III) was influenced by the kinetic parameters and soil properties: (i) the resupply ability (R-R_diff) was influenced by pH and response time (T_c); (ii) R_max, was controlled by pH, T_c, and the desorption rate (k_-1); (iii) k_-1 was influenced by pH and soil texture. This study presents the new information regarding the kinetics of Cr(III) in soils and demonstrates that Cr(III) is steadily resupplied by soil, which is not captured by equilibrium-based methods, furthering our insight of the geochemical behavior of Cr(III). This information was essential for understanding the toxicity of Cr and improving remediation.