Cr(VI) formation related to Cr(III)-muscovite and birnessite interactions in ultramafic environments

Chromium is abundantly and primarily present as Cr(III) in ultramafic rocks and serpentine soils. Chromium(III) oxidation involving chromite (FeCr ₂O₄) 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.