Uranium removal from mining water using Cu substituted hydroxyapatite

In this study, synthetic copper substituted hydroxyapatite (Cu-Hap), Cu_xCa_10-x(PO₄)₆(OH)₂ were prepared by co-precipitation method and were used as reactive materials in batch experiments to immobilize uranyl. The limit of incorporation of Cu into a single-phased Cu-Hap reached x(Cu) ≤ 1.59. The synthetic Cu-Hap samples obtained with various Cu contents were contacted with synthetic uranyl doped solutions and with real mining waters showing various pH and chemical compositions. A fast and strong decrease of the uranium concentration was observed, followed by the establishment of an equilibrium after 1-4 days of contact with the solutions. Examination of the solid phase after uranium uptake was performed using a combination of techniques. Depending on the composition of the solution and the copper content of the Cu-Hap, various mechanisms of uranium removal were observed. Based on the experimental results and geochemical simulations, it appeared that the main interest for using Cu-Hap is to enlarge the domain of water compositions for which the precipitation of meta-torbernite (H₃O)_0.4Cu_0.8(UO₂)₂(PO₄)₂.7.6 H₂O is the predominant mechanism associated to the uranium removal, especially for pH > 6.7 where carbonate uranium species are predominant. (authors)