Enhanced As(III) Sequestration Using Sulfide-Modified Nano-Scale Zero-Valent Iron with a Characteristic Core-Shell Structure: Sulfidation and As Distribution

Sulfide-modified nano-scale zero-valent iron (S-nZVI) was synthesized and employed for the removal of aqueous As(III). The structure and removal performance of S-nZVI was investigated and compared with that of pristine nZVI. S-nZVI has an optimal As removal capacity of 240 mg/g, which is much higher than that of nZVI. The sulfidation of nZVI also enhanced the As(III) removal rate, and the optimum pH for As(III) removal with S-nZVI was in a broad enhancement largely depended on the S/Fe molar ratio. The range from 3 to 8. Transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and X-ray photo electron spectroscopy (XPS) were employed to characterize the S-nZVI before and after reacting with As(III). The results demonstrated that S-nZVI had a unique core-shell structure. Sulfur was incorporated into the shell of S-nZVI, and the thickness of the surface layer increased from 5 nm to approximately 30 nm, which suggested that more As(III) could be sequestered by the nanoparticles. Therefore, better As(III) removal was observed with the increased S/Fe molar ratio. The As distribution in solid phase was used to describe the As(III) removal mechanism, and the results revealed that As(III) removal is a complex process that includes surface adsorption and coprecipitation. Sulfidation promoted the precipitation process and inhibited outer-sphere complexation, which led to enhanced As removal. Oxygen impaired the structure of S-nZVI and generated oxidants via iron sulfide transformations, which drove theAs(III) oxidation and contributed to the total As removal. The large As(III) removal capability and chemical stability of S-nZVI show its potential as an effective and environmentally friendly material for As(III) removal from aqueous solutions.