Ultra-high capacity and selectivity for uranium fixation by carbon nanosphere supported hydroxyapatite nanorod adsorbent

Uranium (U(VI)) has chemical and radiological toxicity, so the effective treatment of uranium-containing wastewater is crucial for both environmental safety and human health. Here, a carbon nanosphere (CNS) supported hydroxyapatite (HAP) nanorod (HAP/CNS) adsorbent was prepared using a simple glucose-assisted hydrothermal method toeffectively immobilize U(VI). Glucose not only derived CNS, but also facilitated HAP crystallization, prohibited HAP aggregation, and introduced oxygen-containing functional groups (i.e., COOH). The optimized HAP/CNS possessed a fantastic adsorption capability of 3080.3 mg/g for U(VI), nearly three times that of HAP and much higher than many reported HAP-based adsorbents. Notably, HAP/CNS was less affected by coexisting ions (distribution coefficient, Kd, researched 6.0 × 104 mL/g) and humic acid, and maintained good capability for real wastewater. The pseudo-second-order kinetic model and Langmuir isotherm model could better explain U(VI) removal behavior by HAP/CNS. Results showed that HAP/CNS and UO22+ combined to form a new uranium-containing compound, i.e., calcium-uranium mica (Ca(UO2)2(PO4)2·3H2O) via ion exchange and dissolution-precipitation, which should be the main reason for the ultra-high capacity and selectivity of HAP/CNS. Additionally, the hydrophilic oxygen-containing functional groups synergistically facilitated U(VI) fixation through complexation. This work introduces a superior adsorbent for purifying uranium-contaminated wastewater and elucidates its synergetic mechanism in uranium fixation.