Production of Cr(VI) from CrxFe1–x(OH)3 precipitates and NOM-Cr(III) colloids upon reaction with H2O2 under oxic conditions

Abstract

Cr(III)-Fe(III) hydroxides (CrxFe1–x(OH)3 precipitates) and NOM-Cr(III) colloids are common products of Cr(VI) reduction during remediation and natural processes. However, re-oxidation of Cr(III) to Cr(VI) can undermine remediation efforts. Nevertheless, until now, less is known about the oxidation of Cr(III) from naturally occurring Cr(III) (i.e., CrxFe1–x(OH)3 precipitates and NOM-Cr(III) colloids) by H2O2. Here, we examined the oxidation of Cr(III) from Cr0.5Fe0.5(OH)3 and NOM-Cr(III) colloids by H2O2 under oxic conditions. Batch experiments demonstrated that Cr(VI) generation via Cr(III) oxidation from both Cr0.5Fe0.5(OH)3 and NOM-Cr(III) colloids increased with increasing H2O2 concentration. Increasing pH and addition of Fe2+ promoted Cr(III) oxidation, but the promoting effect of pH was more significant on Cr0.5Fe0.5(OH)3, whereas the promoting effect of Fe2+ was significant on NOM-Cr(III) colloids. By evaluating the effects of Fe species on Cr(VI) generation from Cr0.5Fe0.5(OH)3 and NOM-Cr(III) colloids, we proposed that an intermediate reactive Fe species formed during the reaction with H2O2 activated Cr(III) oxidation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) analyses collectively supported that the surface structural Fe in CrxFe1–x(OH)3 precipitates might contribute to the formation of reactive Fe species that promoted Cr(III) oxidation. In contrast, the decomposition of complexed NOM from NOM-Cr(III) colloids enabled the formation of Cr(III)-H2O2 complex that is favorable for subsequent Cr(III) oxidation. Results gained from this study provide a complete understanding of the long-term stability of naturally occurring Cr(III) under environmentally relevant conditions.