Visualizing Arsenate Reactions and Encapsulation in a Single Zero-Valent Iron Nanoparticle

A nanostructure-based mechanism is presented on the enrichment, separation, and immobilization of arsenic with nanoscale zero-valent iron (nZVI). The As–Fe reactions are studied with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). Near-atomic resolution (<1 nm 3 ) electron tomography discovers a thin continuous layer (23 ± 3 Å) of elemental arsenic sandwiched between the iron oxide shell and the zerovalent iron core. This points to a unique mechanism of nanoencapsulation and proves that the outer layer, especially the Fe(0)–oxide interface, is the edge of the As–Fe reactions. Atomic-resolution imaging on the grain boundary provides strong evidence that arsenic atoms diffuse preferably along the nonequilibrium, high-energy, and defective polycrystalline grain boundary of iron oxides. Results also offer direct evidence on the surface sorption or surface complex formation of arsenate on ferric hydroxide (FeOOH). The core–shell structure and unique properties of nZVI clearly underline rapid separation, large capacity, and stability for the treatment of toxic heavy metals such as cadmium, chromium, arsenic, and uranium.