Ions removal by iron nanoparticles: a study on solid–water interface with zeta potential

In this study, nanosclae zero-valent iron (nZVI) reacted with three heavy metal cations and one anion in aqueous solutions. In different times of reaction and in particle concentrations, zeta potentials and iso-electric point (IEP) of reacted iron nanoparticles were measured. X-ray photoelectron spectroscopy (XPS) confirmed that nZVI particles had core–shell structure. nZVI particles also exhibited that FeOOH as sorbent and metallic iron as reductant. When nZVI particles were dosed into deionized (DI) water, IEP of nZVI shortly shifted to 7.3 and had kept stable during the next 3 h. The conclusion is that after contacting with water molecular, the nZVI surface presented more negatively than the surface of dry fresh nZVI (IEP = 8.3). Reactions which involved ions, water and iron nanoparticles resulted the change of IEP. Specifically, the peak-like trend of IEP in nZVI–Cu(II) pair indicated a possible reaction process as Cu2+ → Cu+ → Cu0, while Zn(II) removal was achieved via sorption on iron oxide shell followed by zinc hydroxide precipitation. Unlike nZVI–Cu(II) and nZVI–Zn(II), IEP of nZVI–Ni(II) closed to IEP of nZVI–water initially and finally up to 12. In the nZVI–PO4(III) solution, PO bonds on solid surface tended to adsorb protons so IEP remained increasing at whole reaction period. It was constantly lower than IEP of nZVI–water, though.