Enhancing the effect of bisulfite on sequestration of selenite by zerovalent iron

The enhancing effect of bisulfite (HSO₃^-) on the kinetics of Se(iv) sequestration by zerovalent iron (ZVI) was systematically investigated as a function of headspace volume, HSO₃^- concentration and initial pH (pH_ini). To exclude the role of HSO₃^- as an electrolyte, the kinetics of Se(iv) removal by ZVI with the presence of SO₄^2- was determined as a control. With increasing headspace volume from 0 to 2.0 mL, the rate of Se(iv) removal by ZVI experienced a considerable enhancement whereas the further increase in the headspace volume resulted in a drop in Se(iv) removal rate. Se(iv) was always removed by ZVI with a higher rate in the presence of HSO₃^- than that in the presence of SO₄^2- at various headspace volumes, which was mainly ascribed to the release of H⁺ and the depletion of O₂ from the oxidation of HSO₃^- (i.e., 2HSO₃^- + O₂ → 2SO₄^2- + 2H⁺). Furthermore, HSO₃^- accelerated the reduction of ferric oxides and hydroxides to a Fe(ii)-containing solid intermediate, which was beneficial to the reductive removal of Se(iv). The SEM, Fe K-edge XAFS and Se XANES analysis for Se(iv)-treated ZVI samples confirmed that HSO₃^- facilitated the transformation of ZVI to iron(oxyhydr)oxides (e.g., magnetite and lepidocrocite) and the reduction of Se(iv) to Se(0) compared to SO₄^2-. The enhancing effect of HSO₃^- on Se(iv) sequestration varied with the concentration of HSO₃^- and initial pH, with the greatest effect achieved at 2.0 mM of Se(iv) and pH_ini 5.0. Since bisulfite is inexpensive and its final product is sulfate, a common anion existing in water, taking advantage of bisulfite to enhance the ZVI's reactivity under limited oxygenated conditions is a promising method.