Reduction of Cr(VI) to Cr(III) in artificial, contaminated soil using ferrous sulfate heptahydrate and sodium thiosulfate

Due to widespread use in industrial applications, chromium compounds are often considered one of the most serious heavy metal pollutants found at environmental sites. Stabilization/solidification has been studied as a means of immobilizing chromium within a solid matrix. Reduction to a lower valence state is an effective treatment option when coupled with a stabilization/solidification treatment, so that any chromium subsequently leached from the treated soil into groundwater is the less toxic and less mobile trivalent form of chromium. Some studies also indicate that reduction from Cr(VI) to Cr(III) is a necessary preliminary step to effective immobilization. Recent studies have reported the use of ferrous sulfate heptahydrate as an effective reducing agent. However, the possibility of chromium release after such treatment has also been called into question. This technical note presents an alternative to ferrous sulfate heptahydrate, the less-expensive sodium thiosulfate, to reduce hexavalent-chromium contaminated soil and compares the results with the use of ferrous sulfate heptahydrate. The most effective ratio of chemical reagent to hexavalent chromium is reported for the purpose of reducing the chromium to the trivalent form in an artificial, contaminated soil for both reductants. The effectiveness of reduction to trivalent chromium was measured using a modified TCLP test. Based on the results reported in this technical note and possible cost savings over the use of ferrous sulfate heptahydrate, further study in the use of sodium thiosulfate for treatment of Cr(VI) contaminated soil is suggested. © 2009 ASCE.