FORMATION OF CR(VI) RELATED TO CR(III)-MUSCOVITE IN SOIL AND GROUNDWATER SYSTEMS

Chromium release and oxidation from Cr(III)-bearing minerals is an environmental hazard in sediments, soils and groundwater related to ultramafic rocks and their metamorphic derivatives. Birnessite has been shown to promote Cr(III) oxidation from Cr(III)-enriched oxides (i.e., chromite). However, Cr(III)-bearing silicates with overall less total Cr(III) are hypothesized to result in faster rates of Cr(VI) production due to lower mineral solubilities. Here, we investigate Cr(III) release and oxidation related to geochemical interactions between Cr(III)-muscovite (i.e., fuchsite) and birnessite in subsurface aqueous environments with and without humic matter (HM). The initial rates of Cr(VI) production are fast (1.52 to 3.39 × 10-4 mg kg-1 h-1 at pH values of 6 and 3, respectively, without HM) and thereafter rates are decreasing, where 20 µg of Cr(VI) per kilogram of fuchsite can be achieved in 45 days. Rates of Cr(VI) formation are dependent on both birnessite and Cr-muscovite suspension densities (m2 L-1) and on solution pH where higher Cr(VI) production rates are achieved with higher suspension densitites and at lower pH values, agreeing with past studies. When HM is added to the system, Cr(VI) production decreases significantly, supported by observations made on serpentine soils in Sri Lanka. Overall, Cr-muscovite and potentially other Cr(III)-bearing silicates (e.g., chlorite and clays) are major inputs of Cr(VI) to soils, sediments and groundwater capable of being moderated by the abundance and availability of HM.