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 nm3) 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.