In Situ Chemical Reduction of Cr(VI) in Groundwater Using a Combination of Ferrous Sulfate and Sodium Dithionite: A Field Investigation

A field study was conducted to evaluate the performance of a ferrous iron based in situ redox zone for the treatment of a dissolved phase Cr(VI) plume at a former industrial site. The ferrous iron based in situ redox zone was created by injecting a blend of 0.2 M ferrous sulfate and 0.2 M sodium dithionite into the path of a dissolved Cr(VI) plume within a shallow medium to fine sand unconfined aquifer formation. Monitoring data collected over a period of 1020 days after more than 100 m of linear groundwater flow through the treatment zone indicated sustained treatment of dissolved phase Cr(VI) from initial concentrations between 4 and 8 mg/L to less than 0.015 mg/L. Sustained treatment is assumed to be primarily due to the reduction of Cr(VI) to Cr(III) by ferrous iron adsorbed to, precipitated on, and/or incorporated into aquifer iron (hydr)oxide solid surfaces within the treatment zone. Precipitated phases likely include FeCO3 and FeS based on saturation index considerations and SEM/EDS analysis. The detection of solid phase sulfites and thiosulfates in aquifer sediments collected from the treatment zone more than 2 years following injection suggests dithionite decomposition products may also play a significant role in the long-term treatment of the dissolved phase Cr(VI)

Treatment of Arsenic, Heavy Metals, and Acidity Using a Mixed ZVI-Compost PRB

A 30-month performance evaluation of a pilot permeable reactive barrier (PRB) consisting of a mixture of leaf compost, zero-valent iron (ZVI), limestone, and pea gravel was conducted at a former phosphate fertilizer manufacturing facility in Charleston, SC. The PRB is designed to remove heavy metals and arsenic from groundwater by promoting microbially mediated sulfate reduction and sulfide-mineral precipitation and arsenic and heavy metal sorption. Performance monitoring showed effective treatment of As, Pb, Cd, Zn, and Ni from concentrations as high as 206 mg L−1, 2.02 mg L−1, 0.324 mg L−1, 1060 mg L−1, and 2.12 mg L−1, respectively, entering the PRB, to average concentrations of −1, −1, −1, −1, and −1, respectively, within the PRB. Both As(III) and As(V) were effectively removed from solution with X-ray absorption near edge structure (XANES) analysis of core samples indicating the presence of As(V) in oxygen-bound form and As(III) in both oxygen- and sulfur-bound forms. XANES solid phase sulfur analysis indicated decreases in the peak amplitude of intermediate oxidized sulfur species and sulfate components with increasing distance and depth within the PRB