Cr(VI) reduction by Fe(II) sorbed to silica surfaces

The reaction kinetics of groundwater contaminants are integral to evaluating the fate and transport of toxic metals in the environment. For redox sensitive contaminants, such as chromium, the kinetics of different reaction pathways can vary by orders of magnitude. Species-specific rate constants for iron-chromium oxidation-reduction reactions are unknown for many systems, especially in the presence of sorbing surfaces. We investigate the role of quartz and amorphous silica (SiO_2(am)) surfaces in mediating abiotic reduction of Cr(VI)_aq by aqueous and sorbed Fe(II) using batch sorption and redox experiments. Sorption edges indicate outer-sphere (Fe(II)_ads,OS) and inner-sphere (Fe(II)_ads,IS) complexes are present on both silica surfaces, and their abundance depends on pH, ionic strength, and surface disorder. The rate constants for Cr(VI)_aq reduction by Fe(II) species increase in the following order: k_aq ≪ k_{ads,OS,quartz} < k_{ads,OS,SiO2(am)} < k_{ads,IS,quartz} < k_{ads,IS,SiO2(am)}, suggesting that increasing proximity of Fe(II) to the negatively charged silica surface enhances the rate of reduction of Cr(VI)_aq. However, we observe that experiments with larger amounts of sorbed Fe(II) reduce less total Cr(VI)_aq over time, which we attribute to a portion of the sorbed Fe(II) being sequestered into the Cr(III)-Fe(III)-oxyhydroxide precipitates forming on the silica surface. Therefore, the balance between increases in the rate and decreases in the total amount of Cr(VI)_aq reduction by various sorbed Fe(II) species must be considered when devising remediation strategies.