Intercalation of salicylaldoxime into layered double hydroxide: Ultrafast and highly selective uptake of uranium from different water systems via versatile binding modes

We report the first example of MgAl layered double hydroxide intercalated with salicylaldoxime (SALDH) which exhibits excellent uranium (U(VI)) capture performance. In U(VI) aqueous solutions, the SA-LDH shows a tremendous maximum U(VI) sorption capacity (q(m)(U)) of 502 mg.g(-1), surpassing most known sorbents. For the aqueous solution with an initial U(VI) concentration (C-0(U)) of similar to 10 ppm, similar to 99.99 % uptake is achieved in a wide pH range of 3-10. At C-0(U) similar to 20 ppm, >99 % uptake is reached within only 5 min, and pseudo-second-order kinetics rate constant (k(2)) of 44.9 g.mg(-1).min(-1) reaches the record value, placing the SA-LDH amongst the fastest U adsorbing materials reported to date. In contaminated seawater with 35 ppm of U while highly concentrated metal ions of Na+, Mg2+, Ca2+, and K+, the SALDH still displays exceptionally high selectivity and ultrafast extraction for UO22+, giving >95 % uptake of U(VI) within 5 min, and the k(2) value of 0.308 g.mg(-1).min(-1) for seawater surpasses most reported values for aqueous solutions. Versatile binding modes toward U by SA-LDH, including complexation (UO22+ with SA(-) and/or CO32-), ion exchange and precipitation, contribute to the preferable uptake of U at different concentrations. X-ray absorption fine structure (XAFS) analyses demonstrate that one uranyl ion (UO22+) binds to two SA(-) anions and two H2O molecules forming 8-coordinated configuration. The U coordinates with O atom of the phenolic hydroxyl group and N atom of the -C=N-O- group of SA(-), forming a stable six-membered ring motif, which endows the fast and robust capture of U. The wonderful uranium trapping ability makes the SA-LDH among the best adsorbent used for uranium extraction from various solution systems including seawater. (C) 2023 Elsevier Inc. All rights reserved.