Fire-induced oxidation of Cr(III) in serpentine soils

Ophiolite rocks and their weathering products are the major hosts of Cr(III)-bearing phases in the Earth’s crust. Trivalent Cr is rather benign, but risks arise because of its potential for oxidation to the toxic and carcinogenic hexavalent form. Manganese oxides are the most effective natural oxidants, whereas oxidation by atmospheric oxygen is, luckily, very slow. However, oxidation reactions under the highly oxidizing environment of wildfires is largely unexplored. Under this context, we examined the genesis of Cr(VI) by fire-induced heating of ultramafic rocks and serpentine soils. Total Cr(VI) contents of unheated soils ranged from 1.3 to 24 mg kg–1, while the water soluble fraction accounted for less than 2% of total Cr(VI). After heating the soil samples at 600°C for 1 h, total Cr(VI) contents increased by factors 2-41. The water soluble fraction accounted for 33–52% of total Cr(VI). Our results suggest that in fire-impacted areas, there is an increased risk of geogenic Cr(VI) contamination. Even though soil temperatures could exceed 700 °C, our preliminary results showed that Cr(VI) contents in a fire-impacted site did not change as dramatically as under laboratory conditions.