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Article

Characterization and Properties of Metallic Iron Nanoparticles: Spectroscopy, Electrochemistry, and Kinetics

Supporting Info

James T. Nurmi,^† Paul G. Tratnyek,*^† Vaishnavi Sarathy,^† Donald R. Baer,^‡ James E. Amonette,^‡ Klaus Pecher,^‡ Chongmin Wang,^‡ John C. Linehan,^‡ Dean W. Matson,^‡ R. Lee Penn,^§ and Michelle D. Driessen^§

Department of Environmental and Biomolecular Systems, Oregon Health & Science University, 20000 NW Walker Road, Portland, Oregon 97006, Pacific Northwest National Laboratory, P.O, Box 999, Richland, Washington 99352, and Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455

Environ. Sci. Technol., 2005, 39 (5), pp 1221–1230

DOI: 10.1021/es049190u

Publication Date (Web): December 16, 2004

†

Oregon Health & Science University.

Corresponding author phone: (503)748-1023; fax: (503)748-1273; e-mail: tratnyek@ebs.ogi.edu.

‡

Pacific Northwest National Laboratory.

University of Minnesota.

Abstract

There are reports that nano-sized zero-valent iron (Fe⁰) exhibits greater reactivity than micro-sized particles of Fe⁰, and it has been suggested that the higher reactivity of nano-Fe⁰ may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. In an effort to better understand the reactivity of nano-Fe⁰, we have used a variety of complementary techniques to characterize two widely studied nano-Fe⁰ preparations: one synthesized by reduction of goethite with heat and H₂ (Fe^H2) and the other by reductive precipitation with borohydride (Fe^BH). Fe^H2 is a two-phase material consisting of 40 nm α-Fe⁰ (made up of crystals approximately the size of the particles) and Fe₃O₄ particles of similar size or larger containing reduced sulfur; whereas Fe^BH is mostly 20−80 nm metallic Fe particles (aggregates of <1.5 nm grains) with an oxide shell/coating that is high in oxidized boron. The Fe^BH particles further aggregate into chains. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe₂O₃, Fe₃O₄, micro-sized Fe⁰, or a solid Fe⁰ disk, which is consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinonewhich presumably probes inner-sphere surface reactionsreacts more rapidly with Fe^BH than Fe^H2, whereas carbon tetrachloride reacts at similar rates with Fe^BH and Fe^H2, presumably by outer-sphere electron transfer. Both types of nano-Fe⁰ react more rapidly than micro-sized Fe⁰ based on mass-normalized rate constants, but surface area-normalized rate constants do not show a significant nano-size effect. The distribution of products from reduction of carbon tetrachloride is more favorable with Fe^H2, which produces less chloroform than reaction with Fe^BH.