Insights into the evolution of Cr(VI) species in long-term hexavalentchromium contaminated soil

Compare to the content of Cr(VI), thedistribution ofspecific Cr(VI) speciesin soil is rarelypaid attention to, which maylead to an inaccurate environmental risk assessment of Cr(VI) contaminated soil or inability to meet stringent require-ment for soil remediation. Herein, to reveal the primary mechanisms and factors controlling the evolution of Cr(VI)species in soil, the distribution of Cr(VI) and Cr(III) species in soils with different particle sizes and textures was syste-matically investigated by using a modified sequential extraction procedure and spectroscopy characterizations(e.g., SEM-EDS mapping). The results show that a significant proportion of Cr(VI) can be captured by minerals contain-ing exchangeable calcium ions and metal oxide hydrates in the soil, forming a relatively stable adsorbed Cr(VI). Also, asmall fraction of Cr(VI) can precipitate as calcium chromate with free calcium ion which is the most stable Cr(VI) spe-cies in the soil. The majority of Cr(VI) discharged into soil tends to be reduced by ferrous ions or minerals containingferrous ions with a productof Fe(III)-Cr(III)coprecipitate.Therefore,the speciation of Cr in the soil isclosely correlatedto Fe and Ca. After the equilibrium of adsorption, precipitation, and reduction reactions of Cr(VI), the rest of Cr(VI)retains as the form of its original water-soluble state in soil. The evolution of Cr(VI) species and the content of specificCr species in soil are mainly determined by the contents of iron, exchangeable calcium ions and metal oxide hydrates,which effect the Cr(VI) reduction, precipitation and adsorption, respectively.