Mechanistic investigation of U(VI) sequestration by zero-valent iron/activated carbon composites

Mechanistic investigation of U(VI) removal on zero-valent iron/activated carbon (ZVI/AC) was elucidated by batch, X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS) techniques. Batch experiments showed that U(VI) removal on ZVI/AC was significantly higher than that of ZVI, whereas the reduction of U(VI) on ZVI was slightly higher than that of ZVI/AC under reaction time of 12 h. No influence of carbonate on U(VI) removal by ZVI/AC was observed at pH < 5.0, whereas U(VI) removal on ZVI at pH < 5.0 was significantly enhanced with increasing carbonate concentration due to the electrostatic attraction of positive charge of ZVI/AC and negative uranyl-carbonate complexes such as UO2CO3(aq)/UO2(CO3)(2)(2-) species. The ZVI/AC composites displayed the high adsorption capacity and good recycle performance. The high adsorption performance of AC and strong redox capacity of ZVI were responsible for U(VI) removal by XPS and XANES analysis. The corrosion products of ZVI/AC were transferred from magnetite into goethite with increasing reaction time by analysis of XRD pattern. According to EXAFS analysis, the bonding distance of U-U shell for ZVI was slightly shorter than that of ZVI/AC, indicating that redox and adsorption was mainly interaction mechanism of U(VI) on ZVI and ZVI/AC, respectively. These findings are crucial for the application of ZVI-based composites for U(VI) removal in actual environmental remediation.