Synergistic removal and reduction of U(VI) and Cr(VI) by Fe3S4 micro-crystal

In this study, the simultaneous removal of U(VI) cations and Cr(VI) anions on flower-like greigite (Fe3S4) microcrystal was investigated by batch experiments and characterized by spectroscopy techniques. Fe3S4 micro-crystal has good removal potentials for both positive U(VI) ions (423.0 mg/g) and negative Cr(VI) ions (231.3 mg/g) at pH 5.0, and T = 298 K. Furthermore, it is interesting to note that simultaneous U(VI) and Cr(VI) removal resulted in a better removal performance (505.4 mg/g for U(VI) and 274.1 mg/g for Cr(VI) when U(VI) and Cr(VI) were removed simultaneously) than their single removal system under the same condition. The XPS, Raman, and XRD lattice analyses indicated that the synergistic removal mechanisms of U(VI) and Cr(VI) by Fe3S4 were attributed to: (1) Fe3S4 micro-crystal could adsorb U(VI) and Cr(VI) ions via its different functional groups, i.e., U(VI) ions mainly complexed with sulfide functional groups, while Cr(VI) ions mainly complexed with oxygen functional groups; (2) The simultaneous sequestration of U(VI) cations and Cr(VI) anions by Fe3S4 could enhance electrostatic attraction for each other. The higher U(VI) than Cr(VI) sorption performance is owing to the electrostatic cation-anion interaction and complexation of uranium to chromium; (3) U(VI) and Cr(VI) ions could be reduced to less toxic U(IV) and Cr(III) species with the help of reducing Fe(II) and S(-II) components in Fe3S4. The electron-accepting ability of Cr(VI) was stronger than that of U(VI) on Fe3S4. This work is of great importance for the understanding of the synergistic removal mechanisms of uranium cations and chromium anions on Fe3S4, and the usage of Fe3S4 microcrystal in toxic metal ions removal, especially the efficient elimination of both cations and anions from wastewater.