RESUMO
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.