Critical evaluation of chromium partitioning in iron (oxyhydr)oxide-rich laterites

Chromium (Cr) leached from iron (Fe) (oxyhydr)oxide-rich tropical laterites can substantially impact downstream groundwater, ecosystems and human health. However, its partitioning into mineral hosts, its binding, oxidation state and also potential release are poorly defined. This is in part due to the current lack of well-designed and validated Cr-specific sequential extraction procedures (SEPs) for laterites. To fill this gap, we have (i) first optimized a Cr SEP for Fe (oxyhydr)oxide-rich laterites using synthetic and natural Cr-bearing minerals and laterite references, (ii) we used a complementary suite of techniques and critically evaluated existing non-laterite and non-Cr optimized SEPs, compared to our optimized SEP and (iii) confirmed the efficiency of our new SEP through analyses of laterites from the Philippines. Our results show that other SEPs inadequately leach Cr host phases and underestimated the Cr fractions. Our SEP recovered up to seven times higher Cr contents because it (a) more efficiently dissolves metal-substituted Fe phases, (b) quantitatively extracts adsorbed Cr, and (c) prevents overestimation of organic Cr in laterites. With this new SEP, we can estimate the mineral specific Cr fractionation in Fe-rich tropical soils more quantitatively, and thus improve our knowledge of the potential environmental impacts of Cr from lateritic areas.