Xiaoqin Li

Zero-valent iron nanoparticle technology is becoming an increasingly popular choice for treatment of hazardous and toxic wastes, and for remediation of contaminated sites. In the U.S. alone, more than 20 projects have been completed since 2001. More are planned or ongoing in North America, Europe, and Asia. The diminutive size of the iron nanoparticles helps to foster effective subsurface dispersion whereas their large specific surface area corresponds to enhanced reactivity for rapid contaminant transformation. Recent innovations in nanoparticle synthesis and production have resulted in substantial cost reductions and increased availability of nanoscale zero-valent iron (nZVI) for large scale applications. In this work, methods of nZVI synthesis and characterization are highlighted. Applications of nZVI for treatment of both organic and inorganic contaminants are reviewed. Key issues related to field applications such as fate/transport and potential environmental impact are also explored.

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.

Palladized zero-valent iron nanoparticles have been frequently employed to achieve enhanced treatment of halogenated organic compounds; however, no detailed study has been published on their structures, especially the location and distribution of palladium within the nanoparticles. In this work, the structural evolution of palladized nanoscale iron particles (Pd-nZVI, with 1.5 wt % Pd) was examined using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS) techniques. The STEM-XEDS technique enables direct visualization of the nanoscale structural and compositional changes of the bimetallic particles. For a freshly made Pd-nZVI sample, the particles consist of a metallic iron core and a thin amorphous oxide shell, and Pd is observed to form 2-5 nm islands decorating the outer surface of the nanoparticles. Upon exposure to water, Pd-nZVI undergoes substantial morphological and structural changes. STEM-XEDS elemental maps show that Pd infiltrates through the oxide layer to the metallic iron interface, which is accompanied by oxidation and outward diffusion of the iron species. Within a 24 h period, Pd is completely buried underneath an extensive iron oxide matrix, and a fraction of the nanoparticles exhibits a hollowed-out morphology with no metallic iron remaining. The microstructural variations observed concur with the reactivity data, which shows that the aged bimetallic particles display an 80% decrease in dechlorination rate of trichloroethene (TCE) compared to that of the fresh particles. These findings shed new light on the function of palladium in hydrodechlorination reactions, nZVI aging and deactivation, and the longevity of Pd-nZVI nanoparticles for in situ remediation.