XANES Evidence for Oxidation of Cr(III) to Cr(VI) by Mn-Oxides in a Lateritic Regolith Developed on Serpentinized Ultramafic Rocks of New Caledonia

Abstract

Although several laboratory studies showed that Mn-oxides are capable of oxidizing Cr(II) to Cr(VI), very few have reported evidence for such a reaction in natural systems. This study presents new evidence for this redox reaction between Cr(III) and Mn-oxides in a lateritic regolith developed on ultramafic rocks in New Caledonia. The studied lateritic regolith presents several units with contrasting amounts of major (Fe, Al, Si, and Mg) and trace (Mn, Cr, Ni, Co) elements, which are related to varying mineralogical compositions. Bulk XANES analyses show the occurrence of Cr(VI) (up to 20 wt % of total chromium) in the unit of the regolith which is also enriched in Mn (up to 21.7 wt % MnO), whereas almost no Cr(VI) is detected elsewhere. X-ray powder diffraction indicates that the large amounts of Mn in this unit of the regolith are due to the occurrence of Mn-oxides (identified as a mixture of asbolane, lithiophorite and birnessite) and Mn K-edge XANES data indicate that Mn occurs mainly as Mn(IV) in this unit, although small amounts of Mn(III) could also be detected. These results strongly suggest a direct role of the Mn-oxides on the occurrence of Cr(VI) through a redox reaction between Cr(III) and Mn(IV) and/or Mn(III). Owing to the much larger toxicity and solubility of Cr(VI), such a co-occurrence of Cr and Mn-oxides in these soils could then represent an important risk for the environment. However, the significant amounts of Cr(VI) released after reacting the samples from the studied sequence with a 0.1 M (NH)4H2PO4 solution, designed to remove tightly sorbed chromate species, suggest that Cr(VI) mainly occurs as sorption complexes. This hypothesis is reinforced by spatially resolved XANES analyses, which show that Cr(VI) is associated with both Mn- and Fe-oxides, and especially at the boundary between these two mineral species. Such a distribution of Cr(VI) suggests a possible readsorption of Cr(VI) onto surrounding Fe-oxyhydroxides (mainly goethite) after oxidation by the Mn(IV)-oxides. These results, added to leaching tests with a 0.01 M CaCl2 solution indicative of low exchangeability of Cr in the investigated samples, suggest that secondary sorption reactions onto Fe-oxides might significantly decrease the environmental impact of the oxidation of Cr(III) to Cr(VI) by Mn-oxides.

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

“…However, the kinetics of Cr(III) oxidation by O 2 are extremely slow in natural settings (55,56). As a result, the most common oxidant for Cr(III) in Earth surface environments is Mn, in the form of Mn(III,IV) (hydr)oxides (15,(57)(58)(59)(60)(61). The kinetics of this process can be generally formulated as (62)(63)(64)(65):…”

Geochemistry of serpentine agricultural soil and associated groundwater chemistry and vegetation in the area of Atalanti, Greece

“…Bearing in mind the geogenic source of Cr in the area the data indicate that the releasing process of this PHE within the shallow alluvial aquifer is conditioned by the geometry of the basin and subsequently by the groundwater flow and residence time. Chromium mobilization and groundwater enrichment in ophiolite related aquifers has been attributed by several authors to the oxidative action of Mn oxides as well as the alkaline water pH (Chardot et al, 2007;Cheng et al, 2011;Dermatas et al, 2014;Fandeur et al, 2009;Hseu and Iizuka, 2013). The process requires oxidation of Cr(III) present in primary minerals to Cr(VI) by Mn-oxides.…”

“…The process requires oxidation of Cr(III) present in primary minerals to Cr(VI) by Mn-oxides. The subsequent release of Cr into the groundwater is controlled by adsorption reactions on the surface of Mn and Fe-oxides (Fandeur et al, 2009). Other authors have suggested a potential role of different electron acceptors and complex mineral phases such as layered double hydroxides and hydroxycarbonates in geogenic Cr oxidation and release from the solid phase under Mediterranean climate conditions (Langone et al, 2013).…”

Evaluation of trace element availability from secondary metallurgical slag generated in steelmaking by sequential chemical extraction

“…In the case of Cr, environmental concern is generally placed on the potential existence of Cr(VI), or possible oxidation of Cr(III) to Cr(VI), which mainly stipulate Cr toxicity (Fandeur et al 2009). In addition to the discussion in ''Pseudo-total element concentrations of weekly subsamples'', a routine analysis combining water extraction according to the two-stage batch test SFS-EN 12457-3 (L/S 10) and subsequent supernatant analysis by (1,5-)diphenylcarbatzide addition followed by absorbance quantification with a PerkinElmer (PerkinElmer, Norwalk, USA) Lambda 25 spectrophotometer was performed on two combined three-week ladle slag samples representative of the equivalent sampling period.…”