Synergistic effect of MIL-101(Cr) and nanoscale zero-valent iron (nZVI) for efficient removal of U(VI) and assessment of this composite to inactivate Escherichia coli

nZVI was immobilized on MIL-101(Cr), a metal-organic framework (MOF), and subsequently utilized for the removal of U(VI) from aqueous solution. This composite was also tested for its toxicity to Escherichia coli (E. coli). The results demonstrated that MIL-101(Cr) and nZVI had a significant synergistic effect. This composite material is excellent for removing U(VI) and the optimal ratio of MIL-101(Cr) to nZVI was found to be 1.2:1 on mass basis. Adsorption data exhibited excellent fitting to the Freundlich and pseudo-second-order kinetic models, and a maximum removal capacity of 526.32 mg/g of U(VI) was achieved by M-Fe at 298 K (pH = 6.0). The removal mechanisms were clarified by the X-ray absorption near edge structure (XANES), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). Results reveled that U(VI) was mainly removed through chemisorption, forming a bidentate mononuclear edge-sharing linkage between UO3 pyramid and iron(hydro)oxide. Through free radical quenching experiments and EPR analysis, it was indicated that the existence of reactive oxygen species (ROS) accelerates the oxidation of nZVI, generating more iron oxides or hydroxides for U(VI) adsorption, while inhibiting the reduction of U(VI) to U(IV). Furthermore, immobilized nZVI on MIL-101(Cr) mitigated the biotoxicity of Escherichia coli (E. coli). Direct contact played a crucial role in inactivating E. coli cells by nZVI/E. coli system. Extracellular reactive oxygen species (ROS) played a vital role in inactivating E. coli in the M-Fe/E. coli system where center dot OH is the main ROS. These findings enhance our understanding of the mechanism of U(VI) removal and bactericidal effect on organisms using ZVI-based composites, which will broaden the application scope of the two materials in pollution removal, especially in radioactive decontamination.