Role of dissolved oxygen in metal(loid) removal by zerovalent iron at different pH: its dependence on the removal mechanisms

Although the influence of oxygen on contaminant sequestration by zerovalent iron (ZVI) has been investigated in quite a few studies, there are many conflicting results in the literature. The kinetics of the removal of five metal(loid)s (arsenate, copper ion, lead ion, selenite, and chromate) by ZVI was investigated under N-2, air, or O-2 purging conditions in well-mixed batch systems at pH buffered at either 4.0 or 6.0. We found that oxygen had very different influences on different metal(loid)s removal by ZVI at different pH. Cr(VI) was mainly reduced by Fe2+ rather than Fe-0 to Cr(III) at pH 4.0 and 6.0, thus its removal was favored under oxic conditions. The atmosphere favoring the transformation of Fe-0 to iron (hydr)oxides benefited the sequestration of As(V) because As(V) was removed by adsorption/coprecipitation. Pb(II) was removed via both direct reduction by Fe-0 and precipitation as PbO and thus the removal of Pb(II) by ZVI was most favored under N-2-purging conditions at pH 4.0 and 6.0. Cu(II) and Se(IV) were removed via direct reduction by Fe-0 at pH 4.0 but their removal mechanisms partially shifted from reduction to adsorption with increasing oxygen concentration at pH 6.0, resulting in a complex influence of oxygen on their removal. In summary, the role of oxygen in metal(loid)s removal by ZVI is strongly dependent on the removal mechanisms of metal(loid)s at different pH.