Scavenging of U(VI) from Impregnated Water at Uranium Tailings Repository by Tripolyphosphate Intercalated Layered Double Hydroxides

The impregnated water at a uranium tailings repository has characteristics that its uranium concentration is low and it contains many other interfering ions. In order to develop an efficient adsorbent for such wastewater, Mg-Al layered double hydroxide (Mg-Al-NO3-LDH) and tripolyphosphate intercalated LDH (TPP-LDH-1 and TPP-LDH-2) were synthesized by an anion exchange method and characterized by using multiple analysis techniques, and their behavior and mechanism for U(VI) removal from aqueous solutions were investigated. It is found that their adsorption capacities for U(VI) follow the order Mg-Al-NO3-LDH < TPP-LDH-1 < TPP-LDH-2. The maximum sorption capacities of Mg-Al-NO3-LDH, TPP-LDH-1, and TPP-LDH-2 for U(VI) at pH 5.0 and 298.15 K derived from the Langmuir model are 201.27, 399.01, and 501.76 mg/g, respectively. In addition, tripolyphosphate intercalated LDH show a higher sorption selectivity toward U(VI) over other metal cations and anions. X-ray photoelectron spectroscopy (XPS) and Fourier transform-infrared spectroscopy (FT-IR) characterizations suggest that the higher adsorption capacity and sorption selectivity of TPP-LDH-2 are probably due to the inner-sphere surface complexes resulting from the phosphate groups (P=O, -PO3) with U(VI). The applicability of TPP-LDH-2 for scavenging of U(VI) from the impregnated water at a uranium tailings repository in South China was further evaluated. It is found that TPP-LDH-2 shows excellent removal efficiency for U(VI). The results indicate that tripolyphosphate intercalated LDH is a promising adsorbent for scavenging of U(VI) from the impregnated water at the uranium tailings repository.