Reductive precipitation of uranium(VI) by zero-valent iron

This study was undertaken to (letermine the effectiveness of zero- valent iron (Fe⁰) and several adsorbent materials in removing uranium (U) from contaminated groundwater and to investigate the rates and mechanisms that are involved in the reactions. Fe⁰ filings were used as reductants, and the adsorbents included peat materials, iron oxides, and a carbon-based sorbent (Cercona Bone-Char). Results indicate that Fe⁰ filings are much more effective than the adsorbents in removing uranyl (U0₂²⁺) from the aqueous solution. Nearly 100% of U was removed through reactions with Fe⁰ at an initial concentration up to 76 mM (or 18 000 mg of U/L). Results from the batch adsorption and desorption and from spectroscopic studies indicate that reductive precipitation of U on Fe⁰ is the major reaction pathway. Only a small percentage (<4%) of UO₂²⁺ appeared to be adsorbed on the corrosion products of Fe⁰ and could be desorbed by leaching with a carbonate solution. The study also showed that the reduced U(IV) species on Fe⁰ surfaces could be reoxidized and potentially remobilized when the reduced system becomes more oxidized. Results of this research support the application of the permeable reactive barrier technology using Fe⁰ as a reactive media to intercept U and other groundwater contaminants migrating to the tributaries of Bear Creek at the U.S. Department of Energy's Y-12 Plant located in Oak Ridge, TN.