Search

During the last decade, the world faced record-breaking giant fires as observed in Australia and California, a trend that is expected to increase in the forthcoming years due to climate change (Palinkas, 2020; Sharples et al., 2016; van Oldenborgh et al., 2021). In addition to their large ecological impacts, wildfires are more and more regarded for their potential threat to human health through air pollution (Xu et al., 2020). However, water pollution resulting from wildfires represents an underestimated pathway for wildfires-induced health risk (Abraham et al., 2017). This latter impact is related to the heat generated by wildfires that can propagate towards several centimeters in the soil and transform/destroy soil components. In addition to weakening soil physical stability, such transformation/destruction can change the speciation of potentially toxic elements (PTEs) that are associated with these soil components, leading to enhanced mobility towards waterways (Abraham et al., 2017; Terzano et al., 2021). One notable PTE is chromium, which is naturally present in soils mostly as trivalent Cr(III), but can represent an environmental and health issue when occurring as hexavalent Cr(VI). Recent studies reported Cr(III) oxidation to Cr(VI) upon laboratory-heating of Cr(III)-doped Fe-oxyhydroxides (Burton et al., 2019a; 2019b). Besides, Cr(III) oxidation to Cr(VI) upon controlled heating was also demonstrated for different types of soils (Burton et al., 2019b; Rascio et al., 2022; Thery et al., 2023). All these considerations suggest a significant effect of wildfires on Cr(III) oxidation to Cr(VI) in soils, with a possible influence on Cr mobility that could further impact freshwater quality. This risk of freshwater Cr(VI) pollution is expected to particularly concern ultramafic catchments because of the related occurrence of Cr-rich soils.We have tried to address this question by performing laboratory-heating of several soils types (Ferralsols, Cambisols and Vertisols) developed on various geological settings (ultramafic, mafic and volcano-sedimentary) in New Caledonia, a French overseas territory which is a good representative of wildfires-threatened tropical ultramafic catchments (Toussaint, 2020). The results obtained revealed a significant influence of soil heating on Cr(III) oxidation to Cr(VI), followed by an enhanced Cr(VI) mobility, in all soil types. However, the magnitude of Cr(III) to Cr(VI) oxidation and Cr mobility depended on the actual nature of the soil, Ferralsols showing the highest Cr(VI) release compared to Cambisols and Vertisols. These differences were further interpreted on the basis of the changes in Cr speciation (including redox) induced by laboratory-heating of the investigated soils, as revealed by synchrotron-based X-ray absorption spectroscopy analyses. Finally, a simple risk assessment relying on the hypothesized concentration of suspended particulate matter (SPM) issued from burned soils in the related waterways allowed to emphasize a risk of wildfires-induced freshwater Cr(VI) pollution for ultramafic catchments composed of Ferralsols (Thery et al., 2023). Beyond the single case of New Caledonia, the results of this study point to the need to foster collaborative studies in order to further evaluate this risk of wildfires-induced freshwater Cr(VI) pollution at tropical ultramafic catchments on a global scale.

The systematic mineralogy of niobium (Nb) is complex, with more than one hundred species dominated by multicomponent oxides of similar chemistry. The determination of Nb speciation in solids (i.e., the distribution between the phases and the crystal-chemical environment of Nb) is thus a challenge in geological contexts. Here, we present the first Nb L2,3-edges X-ray absorption near-edge structure (XANES) measurements on various Nb minerals and synthetic oxides with geological relevance. The interpretation of Nb L2,3-edges XANES spectra in the light of crystal-field theory shows the sensitivity of spectra to local site symmetry and electronic environment around Nb atoms. Crystal-field multiplet simulations give estimates of the 10Dq crystal-field parameter values for Nb5+, which range from 2.8 to 3.9 eV depending on Nb coordination and Nb—O distances. Rather than a 10Dq vs. R–5 relationship (where R represents the average Nb-O bond distance) expected in a point-charge model, we find a R–3 dependence with the crystal-field splitting for reference materials with octahedrally coordinated Nb. Complementary ligand-field multiplet simulations provide evidence of charge transfer between Nb and O. The contribution of the ionic and covalent characters to the Nb-O bonds is equivalent, unlike more ionic 3d metal–O bonds. This systematic characterization of the L2,3-edges XANES spectral properties of Nb provides information on the mechanisms by which Nb5+ substitutes for Fe3+, Ti4+, or Ce4+ in oxides common in geological contexts. Whereas the substitution of Nb5+ for Ce4+ does not modify the local structure of the cation site in cerianite, the substitution of Nb5+ for Ti4+ in rutile and anatase results in an increase of the cation-ligand distance and a decrease in the symmetry of the cation site. Conversely, the substitution of Nb5+ for Fe3+ in hematite and goethite results in a smaller cation site distortion. Our study demonstrates the usefulness of L2,3-edges XANES spectroscopy to determine Nb speciation in minerals to understand the processes of enrichment of this critical metal.