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

Abstract

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 SO₄²⁻ 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 SO₄²⁻ 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 → 2SO₄²⁻ + 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 SO₄²⁻. 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.