Role of redox potential in chemical transformations of selenium in soils

Irrigated agricultural areas with high levels of soil Se face the dilemma of elevated Se levels in drainage and groundwater systems, which will adversely affect the environment and the wildlife. The release of Se from soils to water systems is dependent on the speciation of Se, which is primarily governed by the redox potential. This study describes the dynamics of redox transformations of naturally occurring Se in a soil matrix as a continuous function of time during a sequence of oxic-anoxic-oxic transition. An experimental setup where a solution stream is continuously passed through a soil column in a closed loop to reach an equilibrium between the soil and the solution was used. The pH and redox potential (Eh) were recorded and the solution was analyzed over time for various Se forms and Mn2+. During soil reduction, the total soluble Se and SeO42- decreased, while the SeO32- and other Se forms (organic Se, elemental Se, and selenide) increased initially, then decreased. Both soluble Se and SeO42- decreased during the anoxic phase, slowly at first, then more rapidly, and were characterized by first-order rate constants. During reoxidation, the total soluble Se and SeO42- increased, SeO32- initially increased followed by a decrease, and other Se forms decreased. Decrease in seO(3)(2-) during reduction may be partly due to the precipitation of MnSeO3. This study showed that the soil Eh plays an important role in mobilizing Se into water systems.