Sequestration of Metal Cations with Zerovalent Iron NanoparticlesA Study with High Resolution X-ray Photoelectron Spectroscopy (HR-XPS)

Abstract

Applications of nanoscale zerovalent iron (nZVI) for removal of metal cations in water are investigated with the result that nZVI has much larger capacity than conventional materials for the sequestration of Zn(II), Cd(II), Pb(II), Ni(II), Cu(II), and Ag(I). Characterizations with high-resolution X-ray photoelectron spectroscopy (HR-XPS) confirm that the iron nanoparticles have a core−shell structure, which leads to exceptional properties for concurrent sorption and reductive precipitation of metal ions. For metal ions such as Zn(II) and Cd(II) with standard potential E0 very close to or more negative than that of iron (−0.41 V), the removal mechanism is sorption/ surface complex formation. For metals with E0 greatly more positive than iron, for instance Cu(II), Ag(I), and Hg(II), the removal mechanism is predominantly reduction. Meanwhile, metals with E0 slightly more positive than iron for example Ni(II) and Pb(II) can be immobilized at the nanoparticle surface by both sorption and reduction. The dual sorption and reduction mechanisms on top of the large surface of nanosized particles produce rapid reaction and high removal efficiency, and offer nZVI as an efficient material for treatment and immobilization of toxic heavy metals.