Abstract
Permeable reactive barrier (PRB) technology is commonly placed in the list of options for groundwater remediation at hazardous waste sites, where Cr(VI) treatment in groundwater has been a concern more than any other inorganic contaminants. The objective of this study was to investigate the effectiveness and longevity of Cr(VI) removal with zero-valent iron based PRBs. The main focus was on two parameters affecting the performance of Cr(VI) removal with PRBs: (1) amount of reactive media and (2) groundwater flux. Laboratory scale columns packed with different amounts of iron powder and quartz sand mixture were fed with an aqueous solution containing 20 mg/L chromium under different groundwater fluxes. When chromium treatment efficiencies of the columns were compared with respect to iron powder/quartz sand ratio, the amount of iron powder was found to be an important parameter for treatment efficiency of PRBs. When the same experiments were conducted at higher fluxes, an increase was observed in the treatment efficiency in the column containing 50% iron. This suggested that the precipitates may not be accumulating at higher fluxes, which in turn, create available surface area for reduction. Extraction experiments were also performed to determine the fraction of chromium that sorbed onto iron hydroxides or iron oxyhydroxides. Analyses showed that chromium was not removed by sorption and that reduction is the only removal mechanism in the laboratory experiments. Cr(VI) removal rate constant and complete removal efficiency values were determined for each reactive mixture. These values are considered to be important design parameters for the field scale permeable reactive barrier applications.