Nonlinear Redox Transformations of Chromium in Soil during Wildfire Heating: The Critical Role of Iron Mineralogy

Fire activity, including wildfires and urban fires, is increasing in frequency and severity, significantly impacting soil-borne metals such as chromium (Cr), which can be transformed from benign Cr(III) to toxic Cr(VI) during heating. However, the reaction pathway of Cr(VI) formation during wildfires remains unclear. We investigated the impacts of Fe-bearing minerals on the fire-induced formation of Cr(VI). Magnetite (Fe₃O₄) synthesized and doped with Cr(III), and Fe and Cr rich soils were heated up to 800 °C to investigate temperature-dependent transformations. For the synthetic system, Cr(III) oxyhydroxide (CrOOH) was oxidized to metastable Cr(VI) trioxide (CrO₃) up to 600 °C, which spontaneously converted to Cr(III) oxide (Cr₂O₃) with increasing temperature to 800 °C. In the soil samples, Fe-bearing minerals reacted with Cr(III) hydroxide [Cr(OH)₃] and chromite [FeCr₂O₄; Cr(III)] to form Cr(VI) and magnetite up to 600 °C, which react with each other with increasing temperature and reduce Cr(VI) to form chromite and hematite (α-Fe₂O₃). These findings highlight the role of Fe-bearing minerals in controlling the Cr(VI) formation and reduction pathway during fires. Our results have implications for understanding how wildfires contribute to the formation of toxic metals in soils, providing valuable insights for predicting the risks posed by wildfires.