HR: 08:45h
AN: U51C-03    [Abstracts]
TI: Adsorption of Arsenate Anions and Divalent Cations as a Probe for Surface Reactivity of Environmentally-Relevant Birnessite Nanoparticles
AU: * Olivos-Suarez, A I
EM: genneth9@gmail.com
AF: UNAM, LAFQA, Instituto de Geografia, MEXICO, DF 04510, Mexico
AU: Villalobos, M
EM: marvilla@igg.unam.mx
AF: UNAM, LAFQA, Instituto de Geografia, MEXICO, DF 04510, Mexico
AB: 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.
DE: 0461 Metals
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
SC: Union [U]
MN: 2007 Joint Assembly