Chromium Speciation Associated with Iron and Manganese Oxides in Serpentine Mine Tailings

Chromium is easily trapped by Fe-oxyhydroxides when the two elements are weathered from serpentinites. Additionally, manganese oxides are the only known naturally occurring oxidant for Cr(III). To elucidate the effects of Fe and Mn oxides on Cr speciation from serpentine mine tailing, this study explored Cr(VI) generation from lithiogenic Cr that increases as a result of tailing weathering, which was caused by partitioning changes of this element with pedogenic Fe and Mn oxides. Twenty-nine tailing samples (0-5 cm) were obtained from an abandoned site of serpentine mining on Wan-Ron Hill in eastern Taiwan. Samples were characterized using chemical extraction, as well as mineralogical and statistical approaches. Based on the experimental results, exchangeable Cr(VI) was observed in all samples ranging from 34.8 to 183 mu g/kg. Chromite was the main Cr-bearing mineral examined in this study. Total labile Cr release and subsequent Cr(VI) generation led to the hypothesis that the labile Cr resulted initially and mainly from chromite weathering. Dithionite-citratebicarbonate and oxalate extractions facilitated identifying the incongruent dissolution between Cr and Fe (and/or Mn). The Cr(VI) increased with concentrations of various forms of Fe and Mn, except for total Fe, revealing that Cr(VI) could be released from the surface of the Mn oxides, where it had been oxidized, and subsequently re-adsorbed onto the surface of the surrounding Fe oxides.