The Adsorption and Aging Process of Cadmium and Chromium in Soil Micro-aggregates

The particle size and components of soil aggregates have played important roles in the migration and transformation of heavy metals. The present study focused on the adsorption behavior and aging characteristics of cadmium (Cd) and chromium (Cr) in various granular aggregates and soil components, which were studied by analyzing the adsorption isotherm, adsorption kinetics, and heavy metal speciation distribution. The results showed that, compared with other aggregates, clay aggregates (0-0.002 mm) had the strongest adsorption effect on Cd and Cr and that there was no significant positive correlation between the adsorption amount and the particle size of aggregates for Cd and Cr. In general, the influence of three components on Cd was organic matter > amorphous iron > free iron oxide, and the influence on Cr was free iron oxide > amorphous iron > organic matter. The adsorption isotherm showed that the correlation coefficient of the Langmuir model (R² ) was higher than that of the Freundlich model (R² ), indicating that the adsorption of Cd and Cr by soil aggregates can be well described by the Langmuir model with monolayer adsorption behavior. Kinetic adsorption studies showed that quasi-first-order kinetics and quasi-second-order kinetics were more consistent with the actual adsorption amounts of Cd and Cr in soil aggregates, respectively. At the same time, the forms of Cd and Cr gradually transformed from unstable to stable after entering the soil. After 60 days, Cd was mainly oxidized, and residual, and Cr was mainly reduced and residual. These results provide a theoretical basis for assessing the environmental risks of Cd and Cr and providing prevention and treatment methods. Environ Toxicol Chem 2022;41:975-990. © 2022 SETAC.