A new pathway for hexavalent chromium formation in soil: Fire-induced alteration of iron oxides

Iron oxides are important pedogenic Cr(III)-bearing phases which experience high-temperature alteration via fire-induced heating of surface soil. In this study, we examine if heating-induced alteration of Cr(III)-substituted Fe oxides can potentially facilitate rapid high-temperature oxidation of solid-phase Cr(III) to hazardous Cr(VI). Synthetic Cr(III)-substituted ferrihydrite, goethite and hematite were heated up to 800 °C for 2 h. Corresponding heating experiments were also conducted on an unpolluted Ferrosol-type soil, which had a total Cr content of 220 mg kg^-1, initially undetectable Cr(VI) and Fe speciation comprising a mixture of hematite, goethite and ferrihydrite (according to Fe K-edge EXAFS spectroscopy). Up to ∼50% of the initial Cr(III) was oxidised to Cr(VI) during heating of Cr(III)-substituted ferrihydrite and hematite, with the greatest extent of Cr(VI) formation occurring at 200-400 °C. In contrast, heating of Cr(III)-substituted goethite resulted in up to ∼100% of Cr(III) oxidizing to Cr(VI) as the temperature approached 800 °C. In the Ferrosol-type soil, heating at ≥400 °C also resulted in large amounts of Cr(VI) formation, with a maximum total Cr(VI) concentration of 77 mg kg^-1 forming at 600 °C (equating to oxidation of ∼35% of the soil's total Cr content). A relatively large portion (31-42%) of the total Cr(VI) which formed during heating of the soil was exchangeable, implying a high level of potential mobility and bioaccessibility. Overall, the results show that Cr(VI) forms rapidly via the oxidation of Fe oxide-bound Cr(III) at temperatures which occur in surface soils during fires. On this basis and given the frequency and extent of wild-fires around the world, we propose that fire-induced oxidation of Fe oxide-bound Cr(III) may represent a globally-significant pathway for the natural formation of hazardous Cr(VI) in surface soil.