A review of the formation of Cr(VI) via Cr(III) oxidation in soils and groundwater

Chromium fluxes and speciation in ultramafic catchments and global rivers

Manganese (Mn) oxides, which are generally considered biogenic in origin within natural systems, are the only oxidants of Cr(III) under typical environmental conditions. Yet the influence of Mn biooxide mineral structural evolution on Cr(III) oxidation under varying geochemical conditions is unknown. In this study we examined the role of light, organic carbon, pH, and the structure of biogenic Mn oxides on Cr(III) oxidation. Aging of Mn oxides produced by a marine bacterium within the widespread Roseobacter clade resulted in structural ripening from a colloidal hexagonal to a particulate triclinic birnessite phase. The structurally diverse Mn oxides were then reacted with aqueous Cr(III) within artificial seawater in the presence or absence of carbon and light. Here we found that Cr(III) oxidation capacity was highest at near neutral pH and in the combined presence of carbon and light. Mn oxide ripening from a hexagonal to a triclinic birnessite phase led to decreased Cr(III) oxidation in the presence of carbon and light, whereas no change in reactivity was observed in the absence of carbon and/or in the dark. As only minimal Cr(III) oxidation was observed in the absence of Mn oxides, these results strongly point to coupled Mn oxide- and photo-induced generation of organic and/or oxygen radicals involved in Cr(III) oxidation. Based on Mn oxide concentration and structural trends, we postulate that Mn(II) produced from the oxidation of Cr(III) by the primary Mn oxide is recycled in the presence of organics and light conditions, (re)generating secondary hexagonal birnessite and thereby allowing for continuous oxidation of Cr(III). In the absence of this Mn oxide regeneration, Cr(III) induced structural ripening of the hexagonal birnessite precludes further Cr(III) oxidation. These results highlight the complexity of reactions involved in Mn oxide mediated Cr(III) oxidation and suggest that photochemical carbon reactions are requisite for sustained Cr(III) oxidation and persistence of reactive Mn oxides.

/pdf/chromium-iii-oxidation-by-biogenic-manganese-oxides-with-gc237san37.pdf

Chromium(iii) oxidation by biogenic manganese oxides with varying structural ripening.

Chromate reduction by zero-valent Al metal as catalyzed by polyoxometalate

Manganese chemistry in soils is a function of complex, competing biotic and abiotic reactions. The role of soil-borne fungi in mediating these reactions is poorly understood. The objective of this article is to document direct observation of fungal Mn oxidation in soil under near in situ conditions, and to isolate, describe and confirm the role of fungi in the observed Mn oxidation, and present a model to explain our observations. We incubated soil under different moisture contents in sample cells designed to allow us to use synchrotron microspectroscopic techniques to analyse areas as small as 38x40 microm2. Mn was redistributed and accumulated in distinct small circular shapes or in dendritic patterns near the air-soil interface when water-saturated soil was incubated for >or=7 days. Mn oxidation did not occur at 3 or 52 degrees C indicating that oxidation was caused by microbial activity. Mn-oxidizing fungi were isolated from the sample cells and cultured on agar. Reinoculation of sterile soil with the Mn-oxidizing isolates resulted in the formation of Mn oxides around fungal hyphae. A model to describe the distinct zonal distribution of Mn oxides in the sample cells is presented. We believe that our data are the first direct observation of Mn oxidation by soil-inhabiting fungi under in situ conditions. Mn-oxidizing fungi may play an underappreciated role in the cycling of Mn in soils.

Fungal manganese oxidation in a reduced soil

Although Cr(III) is considered a Cr species with low mobility and toxicity, the potential oxidation of Cr(III) to Cr(VI) in soils containing Mn requires further scientific attention because of the acute toxicity of Cr(VI). This study evaluates the inhibition or enhancement of Cr(III) oxidation by Mn oxide in the presence of various organic ligands and concentrations. Batch experiments indicate that 200 μM of organic ligands inhibited 20 μM Cr(III) oxidation by Mn oxide at pH 4 because of the occurrence of reductive dissolution of Mn oxide. Although oxalate did not show strong inhibition of Cr(III) oxidation at pH 10, nevertheless, citrate and N-hydroxyethyl-ethylenediamine-triacetic acid (HEDTA) ligands did retard Cr(III) oxidation. It is possible that the co-precipitation of Cr(III) and organic ligand, which has a low dissociation constant, results in low release of Cr(III) and subsequent oxidation. The current results imply that the management of Cr(III)-containing waste is very important to prevent potential oxidation of Cr(III) by Mn oxides, particularly at low pH. However, the presence of organic ligands in the field may lead to low Cr(III) oxidation where Cr(III) waste is disposed.

Effect of organic complexing ligands on Cr(III) oxidation by MNOx

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Effect of organic complexing ligands on Cr(III) oxidation by MNOx