Mechanism and modeling of hexavalent chromium interaction with a typical black soil: the importance of the relationship between adsorption and reduction

摘要

Black soils have a significant retention effect on the migration of Cr( VI ) towards groundwater, and Cr( VI ) adsorption and reduction are both involved in this process. However, the adsorption and reduction of Cr( VI ) were always investigated separately in previous studies resulting in an unclear relationship between them. In this study, the adsorption and reduction kinetic processes of Cr( VI ) by a typical black soil were separately investigated under different initial Cr( VI ) concentrations (40–400 mg L ~(?1) ) and pH conditions (3.5–7.0) by the means of desorption treatment, and the equilibrium relationship between aqueous and adsorbed Cr( VI ) was innovatively established based on the kinetic data. It was found that under pH 5.7 the adsorbed Cr( VI ) content on soil particles was linearly correlated with the remaining Cr( VI ) concentration in solution with time ( R ~(2) = 0.98), and the reduction rate of Cr( VI ) in the reaction system was linearly correlated with the adsorbed Cr( VI ) content on soil particles with time ( R ~(2) = 0.99). With pH decreasing from 7.0 to 3.5, the partition of Cr( VI ) between solid and aqueous phases turned out to be of a non-linear nature, which can be fitted better by the Freundlich model. The retention of Cr( VI ) by black soil was determined to follow the “adsorption–reduction” mechanism, where the Cr( VI ) was first rapidly adsorbed onto the soil particles by a reversible adsorption reaction, and then the adsorbed Cr( VI ) was gradually reduced into Cr( III ). A two-step kinetic model was developed accordingly, and the experimental data were fitted much better by the two-step adsorption–reduction kinetic model ( R ~(2) = 0.89 on average) compared with the traditional first-order and second-order kinetic models ( R ~(2) = 0.66 and 0.76 on average respectively). This paper highlights the novel two step kinetic model developed based on the proposed “adsorption–reduction” mechanism of Cr( VI ) retention by a typical black soil.