Effect of redox processes on soil Cd activity under submerged conditions

Cadmium activity in soils may vary considerably under submerged conditions, yet the mechanisms are not well understood. In this study, a red soil (pH=4.46) and a fluvo-aquic soil (pH=7.10) were submerged and incubated under a nitrogen atmosphere at constant temperature ((25.0±0.5)℃) to study the variation of Cd activity. The pH increased to 5.92 after 60 days for the red soil and decreased to 6.62 after 3 days and then rose to 6.89 for the fluvo-aquic soil. The pe+pH for both soils decreased from 14.16 to 5.38. The variation of pH and pe+pH caused the Fe redistribution in exchangeable (Exc-), carbonate-bound (Carb-), oxide-bound (Oxide-) and organic matter-bound (Org-) fractions, and transformation of Exc-Cd to Carb-, Oxide-and Org-Cd. The negative correlations were significant or highly significant between Exc-Cd and Exc-Fe, as well as between Exc-Cd and (Carb-Cd+Oxide-Cd+Org-Cd) or (Carb-Fe+Oxide-Fe+Org-Fe), indicating that Fe redistribution controlled Cd redistribution in the soil solid phase and also affected the concentrations of Fe and Cd in soil solution. During the first 7 days following flooding in the red soil, Exc-Cd was the major fraction in the solid phase and the Cd concentration in the solution phase increased as the pe+pH decreased. From days 15 to 60 after submergence for the red soil and over the whole incubation period for the fluvo-aquic soil, Carb-Cd and Oxide-Cd were the dominant forms in the solid phase. The soluble Cd concentration declined as pe+pH decreased. Dissolved Organic Matter (DOM) did not significantly affect Cd solubility in the submerged soils.