Enrichment and speciation of chromium during basalt weathering: Insights from variably weathered profiles in the Leizhou Peninsula, South China

Basalt-derived soils are widespread worldwide. Such soils contain high levels of heavy metals like chromium (Cr), which is a serious environmental concern. However, little is known regarding the enrichment and speciation of Cr during the basalt weathering process. Therefore, two basalt-derived soil profiles (Nitisol and Ferralsol) in the Leizhou Peninsula, south tropical China, were investigated to explore the redistribution and transformation of Cr during basalt weathering. All profiles could be divided into three layers: rocks, saprolites, and soils. The Nitisol and Ferralsol profiles exhibited strong (kaolinization) and extreme (laterization) degrees of weathering, respectively. Results showed that Cr concentrations in the saprolites (234 to 315 mg.kg(-1)) were higher than those in basalt rocks (139 to 159 mg.kg(-1)), indicating that Cr was enriched with the continuous loss of Si and other mobile macro-elements. While high levels of Cr were also enriched in the soils (178 to 430 mg.kg(-1)) accompanied with Fe. However, in the upper soils of the Ferralsol profile, the acidity and organic matter could promote the leaching of Cr. Geochemical fractions and EPMA mapping showed that chromite and olivine were the main Cr-bearing minerals in basalt, but Fe-oxides (e.g., goethite and hematite) contained the highest portion of Cr in weathered saprolites and soils. The availability of Cr in the soil was extremely low due to the high stability of Cr bound to Fe-oxides. However, the decreasing contents of Cr bound to Fe-oxides in the upper soils of the Ferralsol profile indicated that Cr could also be released during Fe leaching. In conclusion, the weathering of basalt can lead to the enrichment of Cr in Fe-(hydro)oxides, which are the main controlling minerals for Cr mobility in basalt-derived soils. Further research is needed to evaluate the effect of Fe-(hydro)oxide formation and dissolution on the release of soil Cr.