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

Uranium (U) ion contamination in aquatic systems has received considerable attention worldwide. In this paper, an adsorption material was synthesized with 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 of removing U ions from aqueous solution was investigated by 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 nZVI, HAP, HAP@nZVI and HBSS@HAP. The adsorption capacity showed a decreasing order as: CFB-PM (643.34 mg/g) > HAP (549.86 mg/g) > HBSS@HAP (321.82 mg/g) > HAP@nZVI (153.62 mg/g) > nZVI (102.65 mg/g). SEM-EDS examination suggested that the adsorbed U ions were mainly in the form of spheres, sheets or petals on the surfaces of CFB-PM. XRD detection revealed several U-bearing mineral phases (i.e. Ca(UO2)2(PO4)2·3H2O, HPUO6·4H2O and (UO2)3PO4·4H2O). The U ion adsorption behaviors were further explored by FTIR spectroscopy. The uranium ion adsorption process of CFB-PM could be well described by 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.