Efficient removal of uranium (VI) by 3D hierarchical Mg/Fe-LDH supported nanoscale hydroxyapatite: A synthetic experimental and mechanism studies

Abstract 3D hierarchical Mg/Fe-LDH supported nanoscale hydroxyapatite materials (Mg/Fe-LDHs@nHAP) were facilely synthesized by the ultrasound-assisted synthesis method. Various techniques for characterization, such as X-ray diffraction (XRD), Fourier transformed infrared (FT-IR), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area measurements, disclosed that Mg/Fe-LDHs@nHAP composites had hierarchical layered structure with large BET surface area (231.4 m2/g) as well as plentiful phosphate-containing and hydroxyl-containing groups. The results of batch adsorption experiments showed that U(VI) removal by Mg/Fe-LDHs@nHAP composites was less influenced by other coexisting ions, suggesting the high selectivity of U(VI) by Mg/Fe-LDHs@nHAP composites. The pseudo-second-order models and intra-particle diffusion model can well represent the removal kinetics of U(VI) on Mg/Fe-LDHs@nHAP, and the maximum U(VI) capture capacity of Mg/Fe-LDHs@nHAP reached 845.16 mg/g calculated by Langmuir model at pH = 6.0 and 298 K. The regeneration experiment demonstrated that Mg/Fe-LDHs@nHAP composites held good stability and reusability for extraction of U(VI). The removal mechanisms of U(VI) on Mg/Fe-LDHs@nHAP involved ion exchange, surface complexation and dissolution-precipitation by XPS, EDX and FT-IR spectra investigation. According to the XRD analysis, the absorbed U(VI) was finally formed into a chernikovite precipitate with a flower-like shape induced by phosphate-containing groups. The work indicated that the as-synthesized Mg/Fe-LDHs@nHAP composites are effective adsorbents for the removal of U(IV) in wastewater.