Uranium is one of the most commonly detected radionuclides in the environment. Of the two most predominant oxidation states, U(VI) is much more soluble, mobile and toxic than (IV). Consequently, converting U(VI) to its insoluble form U(IV) can facilitate removal of U from water and greatly lower its mobility and biological/human exposure. We prepared a class of highly reactive zero-valent iron (ZVI) nanoparticles using starch or carboxymethyl cellulose (CMC) as a stabilizer, and tested the effectiveness of the nanoparticles for reductive removal/immobilization of U(VI) from simulated groundwater. This work focused on effects of the stabilizers and water chemistry (pH, bicarbonate, and DOM) on the reductive removal effectiveness and possible re-oxidation of U(IV) under various conditions. At a groundwater pH of 7.0, 35 mg/L of the nanoparticles were able to precipitate 99.0% of 25 mg/L of U(VI) within 12 hours. The bicarbonate extraction tests indicated that 84% of precipitated U was in the form of U(IV) and 16% as U(VI) that was adsorbed and co-precipitated. Both starch and CMC-stabilized ZVI offered comparable removal effectiveness with that of non-stabilized ZVI particles (>96% removal in the pH range of 5.6-8.0). However, the stabilization technique facilitates soil deliverability and in situ remediation of U in soil. More effective removal was observed at lower pH. The presence of 1 and 2.5 mM of NaHCO3 decreased the removal efficiency from 99% to 91% and 68% respectively. And the presence of 10 mg/L of humic acid lowered the removal from 99% to 94%. When kept under anoxic conditions, no re-dissolution of U was detected. However, when subjected to atmospheric oxidation conditions, 54% of the removed U was re-dissolved in the solution after 6 months. The results indicated that stabilized ZVI nanoparticles are highly effective for reductive immobilization of U(VI) in contaminated soil under natural groundwater conditions.

Uranium (VI) removal by stabilized zero-valent iron in simulated groundwater: Effects of stabilizers and water chemistry

247th National Meeting (2014)

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