Mobilization of Cadmium and Arsenic During Anoxic-Oxic Alteration in Paddy Soils: A Vital Role of Manganese

Abstract

ABSTRACT Flooding and drainage of paddy fields can induce the activation of soil cadmium (Cd)/arsenic (As), which will accumulate in rice grains and threaten human health. Mn oxides influence soil Cd and As availability, but few detailed studies have been reported on how Mn redox simultaneously affects Cd and As mobility during flooding and drainage. Two paddy soils with various Mn contents were incubated under three redox conditions, namely 30-d reduction, 30-d oxidation and 20-d reduction subsequent with 10-d oxidation, to investigate Cd and As migration. During reduction, the soil abundant in native Mn oxides possessed more high-valence Mn oxides, which promoted As oxidation and retarded the reductive dissolution of Fe (oxyhydro)oxides, thereby decreasing As mobility. Cd mobility mainly depended on the conversion between Fe-Mn oxides-bound Cd and exchangeable Cd. The high Mn content was conducive to decreasing Cd mobilization. Furthermore, the oxidation of As in soils was linked with a Fenton-like effect. The generated hydroxyl radicals (•OH) decreased with oxidation and the oxidation of Mn(II) by •OH inhibited As migration but promoted Cd activation. In summary, the mobility of Cd and As was lower in Mn rich soils, which may involve the immobilization, oxidation and catalytic effects of Mn oxides.