The physical chemistry of uranium (VI) immobilization on manganese oxides

Manganese oxides show strong affinity towards uranium, and have a promising application in uranium immobilization in environmental protection. We successfully synthesized a series of Mn oxide materials of different structures and investigated their U(VI) immobilization performances. The results showed that all Mn oxides share similar sorption capacities per unit surface area, implying similar physical chemistry during immobilization. Among these Mn oxides, α-MnO₂ shows the most outstanding performance for uranium uptake (280 mg/g). More detailed studies on interfacial properties of U(VI) on α-MnO₂ were performed to elucidate the binding mechanism. The uptake was largely influenced by acidity, but less impacted by ionic strength, indicative of an inner-sphere binding mode. The selectivity for uranium is much higher than other selected metal ions, i.e. Co²⁺, Ni²⁺, Eu³⁺, etc. ATR-FTIR, and EXAFS results showed that in both mild acidic and neutral conditions, U(VI) formed bidentate binuclear structure on α-MnO₂, as evidenced by υ_as(O = U=O) at 912 cm^-1 and the number of Mn in U coordination shell. UO₂(OH)₂ precipitate was found at the molecular level in neutral condition (pH 7-8). The results reveal the physical chemistry in uranium immobilization process on manganese oxide surfaces and helps to better understand the uranium environmental migration. Furthermore, it provides an alternative approach for radioactive water purification.