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 (10⁴-10⁶ mL/g at 1-300 ppm U concentration), and high % removals (>95% at 1-100 ppm, or ∼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 × 10⁶ 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, (S₄)^2- coordinates to UO₂²⁺ forming anionic complexes retaining in the LDH gallery. At high U concentrations, (S₄)^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 [S₄]^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 S_x-LDH materials for selective uranium capture.