Cr(VI) Formation Related to Cr(III)-Muscovite and Birnessite Interactions in Ultramafic Environments

Abstract

Chromium is abundantly and primarily present as Cr­(III) in ultramafic rocks and serpentine soils. Chromium­(III) oxidation involving chromite (FeCr₂O₄) via interactions with birnessite has been shown to be a major pathway of Cr­(VI) production in serpentine soils. Alternatively, Cr­(III)-bearing silicates with less Cr­(III) may provide higher Cr­(VI) production rates compared to relatively insoluble chromite. Of the potential Cr­(III)-bearing silicates, Cr­(III)-muscovite (i.e., fuchsite) commonly occurs in metamorphosed ultramafic rocks and dissolution rates may be comparable to other common Cr­(III)-bearing phyllosilicates and clays. Here, we examine the formation of Cr­(VI) related to Cr­(III)-muscovite and birnessite (i.e., acid birnessite) interactions with and without humic matter (HM) via batch experiments. Experimentally, the fastest rate of Cr­(VI) production involving Cr­(III)-muscovite was 3.8 × 10^–1 μM h^–1 (pH 3 without HM). Kinetically, Cr­(III)-muscovite provides a major pathway for Cr­(VI) formation and Cr­(VI) production rates may exceed those involving chromite depending on pH, available mineral surface areas in solution, and the abundance of Cr­(III) present. However, when HM is introduced to the system, Cr­(VI) production rates decrease by as much as 80%. This highlights that HM strongly decreases but may not completely suppress the formation and mobilization of Cr­(VI). A Sri Lankan serpentine soil was utilized to provide context with regards to the experimental results. Despite Cr­(VI) in the soil solids and Cr­(VI) formation being favorable from Cr­(III)-bearing minerals, no detectable Cr­(VI) was released into soil solutions potentially due to the abundance of HM. Overall, the dynamic interactions of Cr­(III)-bearing silicates and birnessite provide a kinetically favorable route of Cr­(VI) formation which is tempered by humic matter