Birnessite minerals (layered-type MnO2) have a high sorption capacity towards metals and thus strongly influence the distribution and bioavailability of metals in the environment. Traditionally, the mechanisms of metal sorption on mineral surfaces have been probed using bulk X-ray absorption measurements of samples pre-equilibrated for hours to days. However, the evolution of distinct surface species may occur in the second-to-minute time scale. In this study, we applied quick X-ray absorption spectroscopy (QXAS) to gather time-resolved XAS spectra of Co or Ni sorbed at the surface of d-MnO2 particles at different surface loadings and pH values. Previously, Co(II) has been shown to sorb at the vacancy sites of Mn oxides as a triple-corner sharing (TCS) complex, which can be oxidized and become incorporated (INC) into the MnO2 sheet.1 Nickel can adsorb as a mixture of INC and TCS species at pH 8 at the surface of both biogenic and abiotic (d-MnO2) birnessite, while at pH 6 it occurs as a TCS complex on biogenic birnessite and as a mixture of TCS, double-corner sharing (DCS) and double-edge sharing (DES) complexes at vacancy and particle edge sites of d-MnO2.2,3 Here, we probed the evolution of different Co/Ni surface species at d-MnO2surface with time using Co and Ni K-edge QXAS spectra to follow changes in the bonding environment of Co and Ni. We also collected data at the Mn K-edge to monitor changes in Mn oxidation number and birnessite structure. Time-resolved information on metal surface speciation will help us better understand the reactivity of different surface sites and the mechanisms of metal-mineral interactions that can be used to predict the fate of heavy metals in aquatic and terrestrial environments.1Manceau, A., et al, 1997, American Mineralogist 82, 1150-1175.2Pe�a, J. et al, 2010, Geochim.Cosmochim.Acta 74, 3076-3089.3Simanova A. et al, 2013, in preparation.

Time-resolved investigation of Co and Ni sorption mechanisms at the surface of d-MnO<sub>2</sub>

247th National Meeting (2014)

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Time-resolved investigation of Co and Ni sorption mechanisms at the surface of d-MnO2

Time-resolved investigation of Co and Ni sorption mechanisms at the surface of d-MnO₂