Research on the Transformation Mechanism of Soil Cadmium Fractions under the Conditions of Freeze-thaw and Crude Oil Contamination in Wetlands in the Songnen Plain, China

Environmental changes can significantly affect heavy metals behave in soil. Currently, we know little about the impact of crude oil contamination on heavy metal fractions in soil, with few studies available on heavy metal fraction transformation under the composite action of freeze-thaw and crude oil contamination. In this study, soil samples were collected from the Momoge Wetland in the Songnen Plain. Properties including pH, cation exchange capacity, calcium carbonate, free iron oxide, dissolved organic carbon, microbial biomass carbon, and cadmium (Cd) fractions under freeze-thaw, crude oil contamination, and the two combined (composite action) were analysed. The results show that under freeze-thaw, crude oil contamination, and composite action, soil properties changed significantly; soil water-soluble Cd content significantly increased by 54.17%, 62.50%, and 33.33% from control levels, respectively; soil ion-exchangeable Cd content decreased significantly by 10.90%, 23.73%, and 18.64% from control levels, respectively; soil residual Cd content significantly increased by 80.36%, 94.64%, and 132.14% from control levels, respectively; carbonate-bound, humic acid-bound, Fe-Mn oxide-bound, and strongly organic-bound Cd did not change significantly. Soil dissolved organic carbon, pH, CaCO3, and free iron oxide were significantly correlated with water-soluble, ion-exchangeable, carbonate-bound, and Fe-Mn oxide-bound Cd, respectively. This study provides a theoretical basis for evaluating soil Cd behaves in crude oil contaminated wetlands in cold regions.