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 distribution of palladium within the nanoparticles. In this work, structural evolution of palladized nanoscale iron particles (Pd-nZVI, with Pd at 1.5 wt%) was examined using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS) techniques. For a freshly made Pd-nZVI sample, the particles adopt a core-shell structure consisting of a metallic iron core and a thin amorphous oxide shell, and that 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 compositional changes, with STEM-XEDS elemental maps showing Pd infiltrates down through the oxide layer to the metallic iron interface accompanied by oxidation and outward diffusion of the iron species. Within 24 hour aging period, Pd is completely buried underneath an extensive iron oxide matrix, and a fraction of the nanoparticle exhibits a hollowed-out morphology with no metallic iron remaining. The microstructural variations observed concur with the reactivity data, which shows the aged bimetallic particles display an 80% decrease in dechlorination rate of trichloroethene (TCE) compared to the fresh particles. These findings also shed new insight on the function of palladium in hydrodechlorination reactions, nZVI particle aging and deactivation, and the longevity of Pd-nZVI nanoparticles for in-situ remediation applications.
Citation: Environmental Science & Technology
Pub Type: Journals
"zero-valent iron", "nanoparticles", "STEM", "XEDS"