Sorption of Se(IV) and Se(VI) to coal fly ash/cement composite: Effect of Ca2 + and high ionic strength

Abstract

The sorption behavior and mechanisms of Se from wastewater with high salinity are unclear, a scenario that can be encountered in the flue-gas-desulfurization (FGD) blowdown at coal-fired power plants. In this study, we investigated the uptake behavior and mechanisms of Se(IV) and Se(VI) by a coal fly ash/cement composite at high ionic strengths (IS) of up to 3.0M NaCl or CaCl2 using batch sorption experiments, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and X-ray absorption spectroscopy. NaCl, regardless of IS, only caused slight changes in Se(IV) and Se(VI) uptake. Presence of CaCl2 significantly enhanced Se(IV) uptake, but had minor effect at low IS (≤0.3M) and no effect at high IS (>0.3M) on Se(VI) uptake. Both Se(IV) and Se(VI) mainly formed outer-sphere surface complexes in the presence of NaCl. The enhanced Se(IV) uptake by CaCl2 is likely due to the formation of Ca-SeO3 precipitate(s). Outer-sphere complexation is the main mechanism for Se(VI) sorption in the presence of CaCl2, with additional formation of minor amounts of inner-sphere complex at IS >0.3M. The results added new mechanistic insights for Se removal from industrial wastewaters with high salinity and will be useful for immobilization of Se in the co-management of FGD blowdown and coal fly ash wastes.