Synthesis of a Fe/Ca-based phosphate material and its application for adsorption of uranium ions from aqueous solution

Abstract

Uranium (U) ion contamination in aquatic systems has received considerable attention worldwide. In this paper, an adsorption material was synthesized with a Fe/Ca-based phosphate (CFB-PM) by a sol–gel method. The effect of pH, reaction time and initial concentration of U ions on its capacity to remove U ions from aqueous solution was investigated via static batch experiments. Comparative studies of U ion removal by CFB-PM with four sorbents, namely: nano zero-valent iron (nZVI), hydroxyapatite (HAP), hydroxyapatite-loaded nano zero-valent iron (HAP@nZVI) and high basicity steel slag-loaded hydroxyapatite (HBSS@HAP), were performed. Results showed that U ion adsorption capacity of CFB-PM was better than that of all four. The adsorption capacity showed a decreasing order as: CFB-PM (643.34 mg g–1) > HAP (549.86  mg g–1) > HBSS@HAP (321.82  mg g–1) > HAP@nZVI (153.62  mg g–1) > nZVI (102.65  mg g–1). Scanning electron microscopy energy-dispersive spectrometry examination suggested that the adsorbed U ions were mainly in the form of spheres, sheets or petals on the surfaces of CFB-PM. X-ray diffraction revealed several U-bearing mineral phases (i.e. Ca(UO2)2(PO4)2·3H2O, HPUO6·4H2O and (UO2)3PO4·4H2O). The U ion adsorption behaviours were further explored by Fourier transform IR spectroscopy. The U ion adsorption process of CFB-PM could be well described by a quasi-second-order adsorption kinetics model and the Langmuir adsorption isotherm model. The separation coefficient (RL) was close to zero, indicating that U ion adsorption was dominated by single-layer chemisorption. The findings reported in this study have implications for applying the synthesized material for remediation of U ion-contaminated groundwater.