Optimized sequential extraction for chromium from laterite deposits

Chromium is naturally enriched in nickel laterite deposits, the world’s main source of nickel. Both of its main redox species (III and VI) are important, but Cr(VI) is the more mobile, toxic and carcinogenic pollutant species. It is often leached from nickel laterites, potentially contaminating ground- and drinking waters. Thus, it is crucial to quantify the partitioning for chromium in the laterite host phases because these associations dictate the potential mobility, bioavailability and toxicity of chromium. Although such assessments are usually evaluated through sequential extraction procedures (SEP), so far, no SEP has been designed, tested and validated for chromium partitioning from laterites that are typically dominated by iron (oxyhydr)oxides.

Here we evaluated existing SEPs and optimized and validated a new scheme. For our optimized SEP, we tested the efficiency of various extractants on Cr- and Fe-bearing phases (e.g., goethite, chromite) commonly present in nickel laterites. We then applied our SEP on nickel laterites from the Philippines. Our results revealed that Cr recovery was up to 10x higher compared to non-Ni laterite optimized standard SEPs. The developed SEP efficiently dissolved crystalline Fe phases recovering 25-34% of the total chromium. Existing SEP schemes only recovered 2-18% because they only partially dissolved the host goethite and hematite. Our SEP also prevented the overestimation of organic matter bound Cr. It could also quantify strongly adsorbed chromium oxyanions. These improvements provide a better quantification of different chromium hosts and thus improve our understanding of the potential environmental impacts of chromium in mining-impacted lateritic areas.