Mechanisms for high Cd activity in a red soil from southern China undergoing gradual reduction

Cadmium (Cd) activity may vary in flooded soils but the mechanisms underlying these variations have remained conflicting and ambiguous. In this study, a continuous N2 bubbling experiment was conducted using a red soil from the subtropical region of southern China to identify factors controlling Cd activity under reductive conditions. The results showed that Cd solubility, although very low, increased gradually with time during the experiment. From 0 to 14 h, the pH and pe+pH of soil suspension decreased, respectively, from 5.6 to 5.5 and from 11.1 to 9.6. The solubility of Cd was enhanced within this period due to the kinetics of solubilisation of salts or dispersion of colloids upon rewetting, the pH decrease resulting from the hydrolysis of soluble Al and Fe(III) and the competition for soil surface sites from dissolved organic carbon (DOC) and cations, including Al3+, K+, and Mg2+ in the soil solution. From 14 to 114 h, however, the reductive dissolution of Fe and Mn oxides, the DOC consumption by microbial respiration, and the competition of Cd2+ with K+ and Mg2+ for the soil surface sites were the major mechanisms promoting the release of Cd into the soil solution, as the pH of soil suspension increased from 5.5 to 5.8 and the pe+pH continued to decrease from 9.6 to 7.5. On the other hand, more than half of the total Cd was present in the exchangeable fraction during the experiment, indicating that Cd activity remained high. Further, it was observed that Cd transformed from the Fe-Mn oxide-bound fraction to the carbonate-bound fraction and Fe from the oxide- and carbonate-bound fractions to the exchangeable fraction during the experiment. Cadmium redistribution had a close relationship with Fe redistribution, suggesting that the chemical behaviours of Fe oxides affected the physicochemical properties of soil solid phases and further controlled Cd redistribution among soil solid phases. The above results implied that the mechanisms controlling the solubility of Cd were different within the different ranges of pH and pe+pH after rewetting of the air-dried soil, and Fe chemistry might be an important factor inducing the transformation of Cd among several soil solid phases when soil became reducing.