Influence of Dissolved Silicate on Rates of Fe(II) Oxidation

• Published in: Environmental science & technology Vol. 50; no. 21; pp. 11663 - 11671
• Main Authors: Kinsela, Andrew S, Jones, Adele M, Bligh, Mark W, Pham, An Ninh, Collins, Richard N, Harrison, Jennifer J, Wilsher, Kerry L, Payne, Timothy E, Waite, T. David
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01-11-2016
• Subjects: Acidity; Ferric Compounds - chemistry; Ferrous Compounds; Iron; Iron - chemistry; Metal oxides; Oxidation; Oxidation-Reduction; Reaction kinetics; Silica; Silicates - chemistry; Australia
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.6b03015

Increasing concentrations of dissolved silicate progressively retard Fe­(II) oxidation kinetics in the circum-neutral pH range 6.0–7.0. As Si:Fe molar ratios increase from 0 to 2, the primary Fe­(III) oxidation product transitions from lepidocrocite to a ferrihydrite/silica-ferrihydrite composite. Empirical results, supported by chemical kinetic modeling, indicated that the decreased heterogeneous oxidation rate was not due to differences in absolute Fe­(II) sorption between the two solids types or competition for adsorption sites in the presence of silicate. Rather, competitive desorption experiments suggest Fe­(II) was associated with more weakly bound, outer-sphere complexes on silica-ferrihydrite compared to lepidocrocite. A reduction in extent of inner-sphere Fe­(II) complexation on silica-ferrihydrite confers a decreased ability for Fe­(II) to undergo surface-induced hydrolysis via electronic configuration alterations, thereby inhibiting the heterogeneous Fe­(II) oxidation mechanism. Water samples from a legacy radioactive waste site (Little Forest, Australia) were shown to exhibit a similar pattern of Fe­(II) oxidation retardation derived from elevated silicate concentrations. These findings have important implications for contaminant migration at this site as well as a variety of other groundwater/high silicate containing natural and engineered sites that might undergo iron redox fluctuations.