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

Abstract

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.

Colloidal Dynamics of Freshly Formed Iron Oxides under the Influence of Silicic Acid

“…In aqueous solution, the formation of Fe oxides (i.e., lepidocrocite and maghemite; Fig. 2a and 2b) in the presence of silicic acid can be a consequence of increased pH (Kinsela et al, 2016; Kanematsu et al, 2018). The freshly formed Fe oxides may be involved in the crystallization process by intensifying the production of Fe(III) solids via catalyzing the oxidation reaction and participating in colloidal interactions (i.e., aggregation or steric stabilization; Kinsela et al, 2016; Nguyen and Picardal, 2016).…”

“…2a and 2b) in the presence of silicic acid can be a consequence of increased pH (Kinsela et al, 2016; Kanematsu et al, 2018). The freshly formed Fe oxides may be involved in the crystallization process by intensifying the production of Fe(III) solids via catalyzing the oxidation reaction and participating in colloidal interactions (i.e., aggregation or steric stabilization; Kinsela et al, 2016; Nguyen and Picardal, 2016). The stability of Fe oxide suspensions depends strongly on their surface charge and particle size (Walsch and Dultz, 2010; Nguyen and Picardal, 2016; Nguyen et al, 2017b).…”

“…The stability of Fe oxide suspensions depends strongly on their surface charge and particle size (Walsch and Dultz, 2010; Nguyen and Picardal, 2016; Nguyen et al, 2017b). Since silicic acid may adsorb or incorporate into the structure of Fe oxides (Carlson and Schwertmann, 1981; Hiemstra et al, 2007; Kaegi et al, 2010; Voegelin et al, 2010), it appears to be a factor affecting both the particle size growth (Doelsch et al, 2000; Pokrovski et al, 2003; Kinsela et al, 2016) and surface charge development of freshly formed Fe oxides, thereby changing their colloidal stability (Fig. 6).…”

“…There is the possibility that silicic acid is incorporated into the structure of Fe oxides (Carlson and Schwertmann, 1981; Kaegi et al, 2010; Voegelin et al, 2010), negating possible electrostatic effects. Consequently, this adsorbed and incorporated silicic acid is likely responsible for modifying the oxidation degree (Kinsela et al, 2016) and colloidal stability of the freshly formed Fe oxides.…”

“…3a, might be directly related to the presence of silicic acid. Silicic acid can compete with Fe(II) to be adsorbed on Fe(III) oxides (Kinsela et al, 2016; Huang et al, 2018), which leads to the prevention of particle size growth, and oxidation is, therefore, reduced (Vempati and Loeppert, 1989; Pokrovski et al, 2003). The effect of silicic acid on the retardation of Fe oxide formation was most clearly observed in the pH range from 5 to 9.…”