Influence of inherent iron and oxygen concentrations on selenite sorption process using bentonite

Abstract

The oxygen concentration and inherent Fe in bentonite have a significant influence on the Se(IV) sorption process. In this study, the sorption of selenite on natural bentonite was investigated using a batch experiment method, and the distribution coefficient ( K d) values were obtained in the pH range from 2.0 to 10.0 under oxic/anoxic conditions. The K d values always reached a maximum value at a pH of 4 under oxic conditions and at a pH of 8 under anoxic conditions; meanwhile, the K d value under anoxic conditions was larger than the value under oxic conditions, especially in regard to the maximum K d values. The oxygen conditions have a significant influence on the ratio of redox-sensitive Fe2+/Fe3+, which was closely related to the difference in the K d values under both oxic/anoxic conditions. Ferric selenite and green rust could be responsible for the maximum K d values under oxic/anoxic conditions. In the leaching experiments, we found that the Fe2+ in bentonite could replace Mg2+ and Al3+ in the octahedral sheet. Spectroscopy methods, such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) were used to characterize the surface properties of the samples after reaction. Overall, this study shows that the addition of Fe2+-containing materials into backfill/buffer materials under anoxic condition could enhance the sorption of 79Se(IV).