The Reactivity and Characterization of Arsenic Doped Iron Oxide
Nanoparticles

Jasmine Erbs (Chem) , advisors: Lee Penn (Chem), Subir Banerjee (Geo)

Ferrihydrite (Fe5HO8 · 4H2O) is a poorly crystalline mineral commonly formed as a result of low-temperature geochemical processes at and near the Earth's surface.1 Both natural and synthetic ferrihydrite occurs in nanoparticulate form, with surface areas of 200 m2/g or greater.2 It is an important adsorbent of minor elements in surface systems because of its high reactivity and large surface area.1 These particles are the precursors to more stable iron oxide minerals, including goethite and hematite, and the presence of dopants can substantially alter the phase transformations between ferrihydrite and these more stable iron oxides. In aqueous systems, arsenic is often associated with iron oxide minerals and has a high affinity for these minerals' surfaces.3 Recently, high concentrations of arsenic in groundwater around the world have generated interest in the mechanisms controlling the geochemical cycling of arsenic. However, the mechanisms of its adsorption and release are not well understood, although it is known that arsenic can be released by desorption from and reductive dissolution of metal oxides.3

The goals of my research are to quantify the relative reactivity and characterize growth and phase transformations of arsenic-doped iron oxide nanoparticles. In preliminary studies, three types of ferrihydrite particles were prepared: (1) particles with 0% arsenic doping, (2) particles with 1 wt.% or 10 wt.% arsenic doping, and (3) particles equilibrated with 1 wt.% or 10 wt.% arsenic loading. The reductive dissolution of the five samples was examined using a quinone assay. Arsenic doping increases the reactivity in comparison to undoped particles.

Future work will include the synthesis of ferrihydrite materials with a greater range of arsenic doping and loading. Growth studies will be completed with each sample at several different temperatures. The reactivity of these samples will be measured by
quantifying the redox activity using the quinone assay, in which both forms of the quinone and aqueous Fe2+ are measured as a function of time. The materials will be characterized before and after reduction using atomic absorption spectroscopy, inductively coupled plasma-mass spectrometry, powder X-ray diffraction, transmission electron microscopy, and X-ray absorption spectroscopy in order to track differences in composition, crystal structure (long-range and short-range), and particle size and shape.

1) Waychunas, G. A.; Rea, B. A.; Fuller, C. C.; Davis, J. A., Surface chemistry of ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate. Geochimica et Cosmochimica Acta 1993, 57, 2251-2269.

2) Schwertmann, U.; Cornell, R. M., Iron Oxides in the Laboratory. 2nd ed.; Wiley-VCH: Weinheim, 2000.

3) Smedley, P. L.; Kinniburgh, D. G., A review of the source, behavior and distribution of arsenic in natural waters. Applied Geochemistry 2002, 17, 517-568.