Impact of silica on the reductive transformation of schwertmannite and the mobilization of arsenic

Schwertmannite is an important Fe(III) mineral in acid–sulfate soil and acid–mine drainage environments because it is both widespread and highly reactive towards trace elements, such as As. Transformation of schwertmannite to more crystalline phases, such as goethite, may strongly influence As mobility. However, previous research suggests that the rate and extent of schwertmannite transformation can be strongly retarded by the presence of Si – a ubiquitous species in natural waters. The present study examines the impact of Si on reductive transformation of schwertmannite and the associated behavior of Fe and coprecipitated As. Synthetic As(V)-coprecipitated schwertmannite (Fe₈O₈(OH)_4.2(SO₄)_1.9(AsO₄)_0.0005) was subjected to microbially-mediated reducing conditions for 126 days in the presence of three environmentally-relevant Si concentrations (0, 1.9 and 9.5 mM Si). In addition, complementary sorption experiments and short-term abiotic mineral transformation experiments were conducted to examine the interactive impacts of Si and Fe²⁺ on schwertmannite stability. Sorption experiments revealed negligible Si sorption to schwertmannite under acidic conditions, with Si sorption only being important towards near-neutral pH. In the 126 day biotic incubations, the onset of reducing conditions in the initially acidic schwertmannite suspensions stimulated dissimilatory Fe(III) reduction, producing Fe²⁺ and simultaneously causing pH to increase to ∼6.5. Sorption of Si to the schwertmannite surface at this near-neutral pH partially retarded the rate of Fe²⁺-catalyzed transformation of schwertmannite to goethite. However, the effect of Si was minor under microbially-reducing conditions, with Fe²⁺ catalyzing rapid schwertmannite transformation even in the presence of abundant Si. Our short-term abiotic experiments demonstrate that the limited effect of Si is a consequence of Fe²⁺ being produced concurrently with increases in pH. This allows Fe²⁺-schwertmannite interactions to proceed prior to the formation of surface-passivating Si species. The Fe²⁺ catalyzed transformation of As(V)-coprecipitated schwertmannite to goethite caused a major increase in -extractable As, but had little effect on aqueous As concentrations. The reduction of Fe(III) and the subsequent onset of dissimilatory reduction led to formation of siderite (FeCO₃) and mackinawite (FeS), respectively. The reduction of As(V) to As(III) was associated with the Si-dependent mobilization of As into the aqueous-phase. There was a concurrent decrease over time in the concentrations of -extractable As, which occurred independent of Si concentrations and appeared to be related to formation of siderite and mackinawite. The findings from this study provide new insights into the evolution of iron mineralogy and associated arsenic mobility following the establishment of reducing conditions in schwertmannite- and Si-rich environments.