Enhanced immobilization of U(VI) using a new type of FeS-modified Fe0 core-shell particles

Sulfur-modified zero valent iron (S-ZVI) particles have been reported to show improved reactivity and selectivity than conventional ZVI. However, current methods for ZVI sulfidation do not fully utilize the advantages of the material, and S-ZVI has not been tested for U(VI) immobilization. In this work, we synthesized a new type of FeSmodified ZVI core-shell particles (FeS@Fe0) through a facile two-step reaction approach, and then tested for reductive sequestration of U(VI) in water. X-ray diffraction, Scanning transmission electron microscopy, and physical property analyses confirmed the formation of the core-shell structure, surface compositions and magnetic properties. Batch kinetic tests showed that FeS@Fe-0 with an Fe-0/FeS molar ratio of 1: 1 offered the highest U(VI) reduction rate, prolonged reactive life than pristine ZVI, and the reduced uranium was most resistant to reoxidation when exposed to oxygen. The retarded first-order kinetic model was able to adequately interpret the experimental rate data. FeS@Fe-0 performed well over the pH range 5.5-9.0, with higher pH more favoring the reaction. High concentrations (5-10 mg/L) of humic acid, bicarbonate (1-5 mM) and Ca2+ (1 mM) showed only modest inhibition to the U(VI) reduction. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and extraction studies indicated that U(VI) was immobilized via both direct adsorption and reductive precipitation, where Fe-0 was the main electron source, with Fe-0, sorbed Fe(II) and structural Fe(II) acting as the electron donors. FeS@Fe-0 may serve as an improved material for efficient immobilization of U(VI) and other redox-active contaminants in water.