Nanoencapsulation of arsenate with nanoscale zero-valent iron (nZVI): A 3D perspective

The principal forces driving the efficient enrichment and encapsulation of arsenic (As) into nanoscale zero-valent iron (nZVI) are the disordered arrangement of the atoms and the gradient chemical potentials within the core-shell interface. The chemical compositions and the fine structure of nZVI are characterized with a combination of spherical aberration corrected scanning transmission electron microscopy (Cs-STEM), X-ray energy-dispersive spectroscopy (XEDS), electron energy loss spectroscopy (EELS), and high-resolution X-ray photoelectron spectroscopy (HR-XPS). Atomically resolved EELS at the oxygen K-edge unfolds that the Fe species in nZVI are well stratified from Fe(III) oxides in the outermost periphery to a mixed Fe(III)/Fe(II) interlayer, then Fe(II) oxide and the pure Fe(0) phase. Reactions between As(V) and nZVI suggest that a well-structured local redox gradient exists within the shell layer, which serves as a thermodynamically favorable conduit for electron transfer from the iron core to the surface-bound As(V). HR-XPS with ion sputtering shows that arsenic species shift from As(V), As(III)/As(V) to As(V)/As(III)/As(0) from the iron oxide shell-water interface to the Fe(0) core. Results reinforce previous work on the efficacy of nZVI for removing and remediating arsenic while the analytical TEM methods are also applicable to the study of environmental interfaces and surface chemistry.