Efficient removal of uranium from aqueous solution by zero-valent iron nanoparticle and its graphene composite

Zero-valent iron nanoparticle (ZVI-np) and its graphene composites were prepared and applied in the removal of uranium under anoxic conditions. It was found that solutions containing 24ppm U(VI) could be completely cleaned up by ZVI-nps, regardless of the presence of NaHCO₃, humic acid, mimic groundwater constituents or the change of solution pH from 5 to 9, manifesting the promising potential of this reactive material in permeable reactive barrier (PRB) to remediate uranium-contaminated groundwater. In the measurement of maximum sorption capacity, removal efficiency of uranium kept at 100% until C₀(U)=643ppm, and the saturation sorption of 8173mgU/g ZVI-nps was achieved at C₀(U)=714ppm. In addition, reaction mechanisms were clarified based on the results of SEM, XRD, XANES, and chemical leaching in (NH₄)₂CO₃ solution. Partially reductive precipitation of U(VI) as U₃O₇ was prevalent when sufficient iron was available; nevertheless, hydrolysis precipitation of U(VI) on surface would be predominant as iron got insufficient, characterized by releases of Fe²⁺ ions. The dissolution of Fe⁰ cores was assigned to be the driving force of continuous formation of U(VI) (hydr)oxide. The incorporation of graphene supporting matrix was found to facilitate faster removal rate and higher U(VI) reduction ratio, thus benefitting the long-term immobilization of uranium in geochemical environment.