Intensive study on the ultra-high capacity, selectivity and stability in uranium immobilization by hydroxyapatite

Hydroxyapatite (HAP) has been recognized as an effective absorbent for uranium. Recognizing and revealing its fate in coordinating with uranyl ions (U(VI)) under various conditions are vital for its practical application. In this work, HAP nanoflowers were synthesized by a simple wet chemical method, and the immobilization properties of HAP on U(VI) and U(VI) phase transformation during immobilization process were investigated systematically using a host of experimental methods and advanced characterization techniques. The results show that HAP can effectively immobilize U(VI) at high concentrations (50–3000 mg/L), with a maximum U(VI) adsorption capacity (q) of 10,200 mg/g and a distribution ratio (K d ) higher than 1.16 × 10 5  mL/g. Under the presence of sufficient U(VI) and a pH around 1.5 ∼ 3.0, HAP can rapidly react with U(VI) to form Autunite (Ca(UO 2 ) 2 (PO 4 ) 2 ·3H 2 O) with a nanoflake morphology, which is demonstrated excellently long-term stability. A moderately low pH that enables form adequately free calcium ions and phosphate groups via hydrolysis of HAP is essential to the mineralization of U(VI). This work is expected to pave the way for the use of HAP to develop highly efficient immobilization agents for the remediation of uranium-contaminated scenes and presents a comprehensive insight into the mechanisms of long-term U(VI) stabilization.