Chromium speciation and distribution in vertical redox gradients of Ni laterite profiles

Tropical weathering of ultramafic rocks leads to the formation of nickel (Ni) laterites in which iron (Fe), Ni and chromium (Cr) are enriched across a weathering profile. In the case of Cr, the hexavalent species, Cr(VI), is a highly mobile and toxic pollutant. Cr(VI) can often leach into groundwaters, making it crucial to understand the processes that control its mobility. Fe (oxyhydr)oxides are usually the main mineral phase in the most weathered zones of Ni laterites, and are critical hosts for Cr species. However, their role in the redox behavior and partitioning of Cr species, as well as the mechanisms involved in sequestration or release into (ground)waters is unknown. To address this, we will probe the speciation and local bonding environment of Cr in both natural Ni laterite samples and synthetic analogues using Cr K-edge XAS. Thus, we can quantify speciation changes along a vertical redox gradient in Ni laterite profiles, and evaluate Cr leaching potential into groundwaters.