Efficient recovery of U(VI) from strongly alkaline solution using nanoscale zero-valent iron

Abstract Although nanoscale zero-valent iron (nZVI) has been intensively investigated for the remediation of uranium contaminated water, few efforts have been dedicated to U(VI) removal and recovery by nZVI from strongly alkaline solutions (e.g., pH 10.7) in the presence of carbonate. This work demonstrates that U(VI) can be removed by nZVI from strongly alkaline solutions in the anaerobic atmosphere. Results show that reductive precipitation is responsible for U(VI) removal with an effective reduction efficiency of nZVI up to 95.5 ± 3.3% (6.09 ± 0.21 g of U/g of Fe). In the aerobic atmosphere, however, the efficiency is no more than 8.3% (0.53 g of U/ g of Fe) due to relatively rapid corrosion reactions of nZVI as well as the re-oxidation of U(IV). The difference in the efficiency originates not only from the variation in the kinetics of corrosion reactions, but also from U(VI) redox and coordinate chemistry. Under the aerobic condition, uranium removal reaches the maximum at 1 h of reaction and then decreases, because uranium delivery from solid phase occurs due to successive processes of U(IV) oxidation, UO22+ complexation with carbonate, and [UO2(CO3)3]4- desorption from surfaces. The uranium delivery limits the application of nZVI for uranium removal under the aerobic condition, but provides a candidate approach for the uranium recovery from the spent nZVI. A recovery of 100% can be obtained by sodium carbonate solution in the presence of O2. Both the oxidation and desorption processes control the uranium recovery rate. This study not only offers new insight into the mechanisms of interactions between U(VI) and nZVI especially in strongly alkaline media with high concentration of carbonate, but also provides a reference approach to the issue of U(VI) removal and recover from the wastewater generated in the nuclear industry.