Selenium chemistry in amorphous iron (hydr)oxide-applied soil as influenced by redox potential (Eh) and pH

Abstract

A laboratory study was carried out that focused on the stability of immobilized Se in amorphous iron (hydr)oxide-applied soils in relation to the redox potential (Eh) and pH. Applied soils were incubated at 28°C (± 0.1) under submerged conditions. Without the addition of a microbial nutrient source, no significant changes in the incubated soil Eh and, hence, in the amount of soluble Se were observed. On the other hand, the Eh changed within a few days of incubation with the addition of 1% glucose to the samples. Under oxidative conditions (Eh > 200 mV), the amount of soluble Se in the incubated samples ranged from 2 to 22 μg L−1. Under mildly oxidative conditions (200 to 0 mV), the Eh decreased and pH increased simultaneously, resulting in a subsequent increase in the amount of soluble Se. After 28 d of incubation, the Eh drastically decreased to a strongly reductive state in both soil and supernatant phase, and the amount of soluble Se increased to a maximum value followed by a decrease to a low level. Reductive dissolution of applied amorphous iron (hydr)oxide occurred during this transition period that led to the subsequent desorption of immobilized Se into the solution phase. The amount of desorbed Se further reduced to insoluble Se forms in a strongly reductive condition. Se species in total soluble Se in soils with mixed [Se(IV) + Se(VI)] contamination were determined under different Eh-pH conditions. The amount of soluble Se(VI) inversely followed the changes in the Eh values in an oxidative to mildly reductive condition and that closely followed the changes in the Eh values in a reductive to strongly reductive condition in this experiment. The amount of soluble Se(VI) increased followed by a decrease in the amount of total soluble Se; however, Se(VI) in soluble Se was the major species throughout the incubation period. The increase in the amount of soluble Se(IV) and organic Se species was inversely related to the decrease in the Eh values throughout the incubation period. These results suggest that immobilized Se in amorphous iron (hydr)oxide-applied soil is stable either under oxidative or strongly reductive environment.