Journal of the American Chemical Society, Vol.137, No.10, 3670-3677, 2015
Efficient Uranium Capture by Polysulfide/Layered Double Hydroxide Composites
There is a need to develop highly selective and efficient materials for capturing uranium (normally as UO22+) from nuclear waste and from seawater. We demonstrate the promising adsorption performance of S-x-LDH composites (LDH is Mg/Al layered double hydroxide, [S-x](2-) is polysulfide with x = 2, 4) for uranyl ions from a variety of aqueous solutions including seawater. We report high removal capacities (q(m) = 330 mg/g), large K-d(U) values (10 (4)-10 (6) mL/g at 1-300 ppm U concentration), and high % removals (>95% at 1-100 ppm, or similar to 80% for ppb level seawater) for UO22+ species. The S-x-LDHs are exceptionally efficient for selectively and rapidly capturing UO22+ both at high (ppm) and trace (ppb) quantities from the U-containing water including seawater. The maximum adsorption coeffcient value K-d(U) of 3.4 x 10 (6)mL/g (using a V/m ratio of 1000 mL/g) observed is among the highest reported for U adsorbents. In the presence of very high concentrations of competitive ions such as Ca2+/Na+, S-x-LDH exhibits superior selectivity for UO22+, over previously reported sorbents. Under low U concentrations, (S-4)(2-) coordinates to UO22+ forming anionic complexes retaining in the LDH gallery. At high U concentrations, (S-4)(2-) binds to UO22+ to generate neutral UO2S4 salts outside the gallery, with NO3 entering the interlayer to form NO3-LDH. In the presence of high Cl concentration, Cl- preferentially replaces [S-4](2) and intercalates into LDH. Detailed comparison of U removal efficiency of S-x-LDH with various known sorbents is reported. The excellent uranium adsorption ability along with the environmentally safe, low-cost constituents points to the high potential of Sx-LDH materials for selective uranium capture.