Removal of selenium from water with nanoscale zero-valent iron: Mechanisms of intraparticle reduction of Se(IV)

Increasing evidences suggest that nanoscale zero-valent iron (nZVI) is an effective agent for treatment and removal of selenium from water. For example, 1.3 mM selenite was quickly removed from water within 3 min with 5 g/L nZVI. In this work, reaction mechanisms of selenite [Se(IV)] in a single core–shell structured nanoscale zero-valent iron (nZVI) particle were studied with the method of spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) integrated with X-ray energy dispersive spectroscopy (XEDS). This method was utilized to visualize solid phase translocation and transformation of Se(IV) such as diffusion, reduction, deposition and the effect of surface defects in a single nanoparticle. Se(IV) was reduced to Se(-II) and Se(0), which then formed a 0.5 nm layer of selenium at the iron oxide-Fe(0) interface at a depth of 6 nm from the surface. The results provided near atomic-resolution proof on the intraparticle diffusion-reduction of Se(IV) induced by nZVI. The STEM mapping also discovered that defects on the surface layer accelerate the diffusion of selenium and increase the capacity of nZVI for selenium sequestration. Mechanisms of selenium (IV) sequestration in nanoscale zero-valent iron (nZVI). [Display omitted] •Se(IV) can be separated from water, reduced and encapsulated by nZVI.•Near atomic-resolution imaging on the Se diffusion-reduction in nZVI is obtained.•Defects on the nZVI nanoparticles apparently can enhance selenium removal.•Reduction and sequestration fully explain rapid and large capacity Se(IV) removal.•STEM-XEDS is usable for visualizing solid phase reaction at near atomic resolution.