The role of organics in the oxidative release of Cr(VI) from Cr(OH)3 by δ-MnO2

Abstract:

The environmental release of the toxic and mobile Cr(VI) from poorly soluble and non-toxic Cr(III) mineral phases primarily occurs via oxidation by Mn(IV) oxides. This process accounts for the contamination of a number of natural and anthropogenically perturbed environments. The mechanism and controls upon the reaction of Mn(IV) oxides with insoluble Cr(III) minerals remain poorly understood. Further to this, despite near ubiquity in nature and a number of studies investigating Mn(IV) oxides reactivity towards natural organic compounds, few studies have assessed the impact of organics upon the oxidation of Cr(III) minerals by Mn(IV) oxides.

This study aimed to determine the changes in redox state of the Cr and Mn during reactions between the environmentally dominant Cr(OH)₃ and δ-MnO₂ minerals, under circum-neutral conditions, in the presence and absence of organic and humic acids. We used a combination of aqueous analytical techniques, alongside cryo-TEM-EELS which enabled mapping of the Cr and Mn redox state of the solid phases at the nanometre scale.

In the absence of organic compounds, a steady but significant release of Cr(VI) occurred prior to cessation of the reaction, despite the presence of a large pool of un-reacted Cr(III) and Mn(IV). Analysis using cryo-TEM-EELS suggests a passivation mechanism, through the detection of Mn(II) at the Cr and Mn mineral surface. The presence of oxalate and citrate variably promoted and inhibited Cr(VI) release, depending upon their concentrations relative to the mineral phases. The presence of humic acids, however, invariably promoted Cr(VI) release under the conditions tested. The complex impacts of organics on Cr(VI) release by δ-MnO₂-mediated oxidation of Cr(OH)₃ result from the various roles that organics can play in the reaction, acting as complexants, reductants and adsorbing species.