HR: 0830h
AN: B41D-0926    [PDF]
TI: Insight From EXAFS and the Multi Site Complexation Model (MUSIC) Into the Discrepancy Between SCM-Predicted and Experimental pH-Dependent Ni Sorption to Hydrous Manganese Oxyhydroxide
AU: * Haack, E A
EM: haackea@mcmaster.ca
AF: School of Geography and Geology, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1 Canada
AU: Smith, D S
EM: ssmith@wlu.ca
AF: Department of Chemistry, Wilfrid Laurier University, 75 University Avenue W., Waterloo, ON N2L 3C5 Canada
AU: Trainor, T P
EM: fftpt@uaf.edu
AF: Department of Chemistry and Biochemistry, University of Alaska Fairbanks, P.O. Box 756160, Fairbanks, AK 99775 United States
AU: Warren, L A
EM: warrenl@mcmaster.ca
AF: School of Geography and Geology, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1 Canada
AB: Hydrous manganese oxyhydroxides (HMO) are receiving increasing attention from both geomicrobiologists, due to the importance of microbial catalysis in their formation, and trace element geochemists, due to their importance as trace metal sorbents in a wide number of environments. Accurate models of the mechanisms of trace metal sequestration in association with HMO are thus required for an overall understanding of metal reactive transport. We have examined the utility of a surface complexation model to describe pH-dependent Ni sorption to synthetic HMO, a layered oxide comparable in structure to microbially-catalyzed Mn oxide. Surface complexation models (SCM) have successfully reproduced metal and proton binding curves to a number of geochemically relevant solids, including bacteria and mineral surfaces. However, because these models are derived from potentiometric titrations, they have intrinsic limitations; very high and very low proton affinity sites cannot be characterized. Results from our study indicated that Ni sorption to HMO could not be described by assuming Ni sorption to the SCM-derived proton binding sites. Ni sorption capacity of the solid far exceeded proton binding capacity at low pH values. Examination of the Ni-sorbed HMO samples with EXAFS indicated that the discrepancy between Ni sorption and proton binding could not be ascribed to the precipitation of a separate Ni solid phase. Rather, all at pH values examined (2.0, 4.0 and 6.5) Ni sorbed above/below layer vacancies in the MnO$_{6}$ octahedral layers of HMO, as interlayer tridentate corner-sharing complexes. At circumneutral pH (6.5), Ni also substituted directly for structural Mn. A theoretical pK$_{a}$ spectrum was derived for HMO using the multi site complexation model (MUSIC) and proton affinity constants for surface functional groups relevant for Ni sorption, as constrained by EXAFS, were identified. Ni sorption as an interlayer complex occurred at sites with very high proton affinity, thus accounting for the under-prediction of Ni sequestration at low pH by the SCM. These results indicate that prediction of Ni sequestration to HMO requires the integration of molecular level speciation of Ni with theoretical and experimentally derived surface modeling approaches. Further, the results of this study are likely to have far reaching implications for modeling trace metal interactions with a number of important sorbent surfaces.
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2003 Fall Meeting