Combined EXAFS and FTIR investigation of sulfate and carbonate effects on Pb(II) sorption to goethite (#alpha#-FeOOH)

摘要

Understanding the effects of carbonate and sulfate on Pb(II) sorption to iron (hydr) oxides is critically important in developing accurate models of Pb fate and transport in natural environments. Macroscopic measurements show that Pb(II) uqtake on iron (hydr) oxides can be altered significantly by dissolved sulfate and carbonate. Our investigation elucidates the molecular-scale processes giving rise to these macroscopic effects by characterizing the structures of Pb(II) sorption complexes forming on goethite (#alpha#-FeOOH) in the presence of carbonate and, in separate experiments, sulfate using in-situ Pb L_(III)-EXAFS and ATR-FTIR spectroscopies. Under all conditions studied, Pb(II) forms predominantly bidentate inner-sphere complexes to Fe(O,OH)_6 octahedra. Both corner- and edge-sharing complexes are observed by EXAFS spectroscopy, The relative fraction of corner-sharing complexes increases at low pH (5-6) for carbonate concentrations up to latm (i.e., we see the same behavior in samples free of added ligands and in those with carbonate present up to latm), and is enhanced in the presence of sulfate. The formation of trrnary surface complexes is evaluated as a possible explanation for enhanced Pb uptake in the presence of high CO_2 (latm) and in the presence of sulfate.