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.

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