Ferrous iron oxidation in the presence of antimonate at neutral pH: Mutual effects on iron mineral products and antimony sequestration

Antimony is a priority pollutant, whose mobility in redox-dynamic environments may be controlled by interactions with Fe(III) hydroxide minerals that form via Fe(II) oxidation. In this study, we examined the Fe(III) hydroxide precipitates and associated mechanisms of Sb(V) sequestration that result from Fe(II) oxidation in the presence of Sb(V) under neutral pH conditions. To achieve this aim, oxidation experiments were carried out in O2-saturated, Fe(II)-bearing solutions (buffered at pH 7) over a range of environmentally relevant Sb(V) concentrations (equivalent to Sb(V):Fe(II) molar ratios of 0, 0.01, 0.04, 0.1 and 0.25). Under these experimental conditions, Fe(II) oxidation occurred rapidly (within 20 minutes) causing associated removal of Sb(V) from solution via coprecipitation with the resulting Fe(III) hydroxides. At low Sb(V):Fe(II) ratios (< 0.1), lepidocrocite was the only Fe(III) mineral product of Fe(II) oxidation, whereas higher ratios resulted in formation of feroxyhyte. Both lepidocrocite and feroxyhyte retained Sb(V) within their crystal structure via Sb(V)-for-Fe(III) substitution. This mechanism of Sb(V) retention largely protected the solid-phase Sb(V) from release processes. Collectively, these results highlight the coupled role that interactions between Sb geochemistry and the Earth's near-surface Fe cycle can play in controlling both Fe(III) hydroxide mineralogy and Sb mobility.