Heterogeneous selenite reduction by zero valent iron steel wool
Mine drainage from the low-sulfur surface coal mines in southern West Virginia, USA, is circumneutral (pH > 6) but contains elevated selenium (Se) concentrations. Removal of selenite ions from aqueous solutions under anoxic condition at pH 6–8.5 by zero valent iron steel wool (ZVI-SW) was investigated in bench-scale kinetic experiments using wet chemical, microscopic and spectroscopic techniques (X-ray photoelectron spectroscopy). ZVI-SW could effectively and efficiently remove SeIV from solution with pH 6–8.5. A two-step removal mechanism was identified for SeIV reduction by ZVI-SW. The proposed mechanism was electrochemical reduction of SeIV by Fe0 in an initial lag stage, followed by a faster heterogeneous reduction, mediated by an FeII-bearing phase (hydroxide or green rust). Solution pH was a critical factor for the kinetic rate in the lag stage (0.33 h−1 for pH > 8 and 0.10 h−1 for pH 6–8). The length of lag stage was 20–30 min as determined by the time for dissolved FeII concentration to reach 0.30 ± 0.04 mg L−1 which was critical for induction of the faster stage. About 65% of the initial SeIV was reduced to Se0, the primary reductive product in both stages.