U51C-01 INVITED
Concerning the ferrous-ferric charge distribution in magnetite nanoparticles
Nanoparticles of iron oxides are abundant environmental constituents that, when bearing structural Fe(II), can serve as a source of electrons for reducing metals such as U(VI) or Cr(VI). Iron oxides such as magnetite possess the characteristic of facile charge redistribution by thermally activated Fe(II/III) valence interchange reactions, in this case by transfer of minority spins within the octahedral sublattice. This characteristic in combination with the finite dimensions of nanoparticles leads to the possibility of a ferrous-ferric charge distribution that differs substantially from bulk magnetite. I will present the results of density functional theory calculations focused on determining the sensitivity of the ferrous-ferric charge distribution to the reduction of periodicity from three dimensions, to two, to one, and finally nanoparticles. In addition, we will discuss the effects of surface hydration on the charge distribution. Hydrated two-dimensional (100) surfaces show a charge distribution that is different than bulk magnetite. Nanoparticles in particular partition the trivalent charge to the surface and the divalent charge to the core to reduce the total electrostatic energy. This redistribution affects the magnetic structure of the nanoparticles as well. The findings imply that electron availability for surface chemistry on magnetite nanoparticles is strongly size and shape dependent.
U51C-02
Kinetics of Reductive Dissolution: Influence of Nanoparticle Size and Aluminum Doping on Ferrihydrite Reactivity
The composition and size of iron oxide nanoparticles in natural systems varies considerably. Here, we present results examining the influence of size, and aluminum and arsenic doping on the redox reactivity of iron oxide (ferrihydrite and magnetite) nanoparticles as determined from kinetic studies using either hydroquinone as a reductant or benzoquinone as an oxidant. Variable-temperature experiments are used to determine activation energies and frequency factors so as to quantitatively compare reactivities of the nanoparticles as a function of varying composition and size. Results show that surface-area normalized rate constants for reductive dissolution of ferrihydrite and for oxidation of magnetite decrease with increasing size. Interestingly, aluminum doping results in increased reactivity in the ferrihydrite system whereas aluminum doping in the magnetite system results in substantially depressed reactivity. Finally, variations in reactivity as a result of arsenic doping are strongly dependent upon the preparation method. Ferrihydrite prepared by coprecipitation with arsenate exhibit enhanced reactivity, while particles equilibrated with solutions containing arsenate exhibit decreased reactivity. The impact of small changes in synthetic and storage procedures will also be specifically addressed.
U51C-03
Adsorption of Arsenate Anions and Divalent Cations as a Probe for Surface Reactivity of Environmentally-Relevant Birnessite Nanoparticles
Layer-type Mn oxides of the birnessite family are widespread minerals in natural aqueous environments that may be found in the nanometer size range, as individual particles, or in diverse aggregated forms or films. They are believed to be originally formed by microbial catalysis and are extremely reactive towards cation adsorption. In particular, certain heavy metals preferentially bind to these Mn oxides versus to other natural metal oxides. These metals include lead, zinc, and cobalt cations, among others. This high affinity is due to the presence of both highly-negatively charged cation vacant sites within the Mn oxide layer structure and to unsatisfied oxigen bonds at the crystal surfaces. Both types of sites contribute approximately equivalently to total cation adsorption, but as specific surface area increases above values of 40 m2/g (i.e., particle size decreases correspondingly), the contribution of the latter type of sites is presumed to be predominant. The magnitude of the charge on both types of sites is also presumed to be pH dependent, and thus variable, mostly of negative nature, and thus explaining its high affinity towards cations. However, several literature reports exist on the binding of anions such as arsenate, phosphate, and borate, by birnessite-type Mn oxides. The present work investigates the chemical conditions upon which binding of anions occurs in layer-type Mn oxides, and their stability. For this, arsenate ions were chosen as probes because they have been reported in the literature to bind in inner-sphere mode at the surface of birnessite-type minerals, through X-ray absorption spectroscopy work. Two synthetic birnessite minerals, analogs to biogenically produced Mn oxide nanoparticles, were synthesized and characterized for BET specific surface area, X-ray diffraction, and average Mn oxidation state. Suspensions of these oxides were equilibrated in the presence of arsenate ions to investigate the chemical conditions and stability of arsenate sorption. The results advance further understanding of the surface structure and reactivity of birnessite mineral nanoparticles of environmental relevance.
U51C-04
Removal of Toxic Metals and Radionuclides from Soils and Waters Using Nanosize Hydroxyapatite
Many inorganic and organic ion filters or media have been investigated as potential candidates for removing the hazardous ions from contaminated water solutions but the processing costs, including manufacture of the filters, are high. Furthermore, the problem of permanently disposing of the "spent (used)" ion filters (which contain the radionuclides) remains an unsolved problem. One material that could satisfy this need is hydroxyapatite (HA), the mineral component in human bone. High surface area (> 150 m2/g) HA particles were prepared by a unique and innovative method that consists of reacting (24 hours at 37°C) particles (45-75 μm) of a calcium borate glass with an aqueous solution containing phosphate ions. The effectiveness of this HA in removing the radionuclides and the hazardous metals from contaminated water was determined by chemically analyzing the feed and effluent solutions using an inductively coupled plasma-mass spectrometer (ICP-MS). The ion removal effectiveness was not affected by the filtration methods, batch or column, and very high (e.g., Kd = 2×106 mL/g for U) ion removal was achieved for the ions tested. Calcium phosphate powder generated from industrial by-products was also tested for the ion removal as an attractive source of HA.
U51C-05
Band-edge electronic structure modification in iron oxide and oxyhydroxide nanoparticles
Transition metal oxide and oxyhydroxide nanoparticles are the focus of considerable current interest in geochemistry because of their roles in biogeochemical redox cycles. Much progress has been made in understanding the structure and phase relationships in mineral nanoparticles, but the effects of small size and modified surface structure on reactivity remains an outstanding problem. Common environmental nanoparticles have been shown to exhibit enhanced chemical reactivity relative to bulk mineral surfaces, but the origin of this behavior is not well established. We studied the electronic structure component of mineral reactivity by comparing oxygen K-edge soft x-ray absorption and emission spectra of three phases of iron oxide and oxyhydroxide nanoparticles with spectra obtained from bulk minerals of the equivalent phases. These spectroscopies probe the electronic states in the vicinity of the semiconductor band gap that are important for interfacial (photo)electrochemical processes. Both the conduction and valence band states of goethite (α-FeOOH) nanoparticles exhibit considerable broadening that is likely associated with surface and interior disorder and which leads to an effective reduction in the band gap. By contrast, the band gaps of hematite (α- Fe2O3) and maghemite (γ-Fe2O3) nanoparticles are increased as a consequence of shifts in the threshold positions of the valence and conduction band, respectively. The origins of these electronic structure modifications are presently uncertain. Nevertheless, the data directly predict that the reducing potential of conduction band electrons in maghemite nanoparticles and the oxidizing potential of hematite valence band electrons are enhanced relative to the bulk minerals, and that simple photochemical studies can be used to validate these predictions.