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

Understanding the effects of carbonate and sulfate on Pb(II) sorption to ir on (hydr)oxides is critically important in developing accurate models of Pb fate and transport in natural environments. Macroscopic measurements show that Pb(II) uptake on iron (hydr)oxides can be altered significantly by dis solved sulfate and carbonate. Our investigation elucidates the molecular-sc ale processes giving rise to these macroscopic effects by characterizing th e structures of Pb(II) sorption complexes forming on goethite (alpha-FeOOH) in the presence of carbonate and, in separate experiments, sulfate using i n-situ Pb L-III-EXAFS and ATR-FTIR spectroscopies. Under all conditions stu died, Pb(II) forms predominantly bidentate inner-sphere complexes to Fe(O,O H)(6) octahedra. Both corner- and edge-sharing complexes are observed by EX AFS spectroscopy. The relative fraction of corner-sharing complexes increas es at low pH (5-6) for carbonate concentrations up to 1 atm (i.e., we see t he same behavior in samples free of added ligands and in those with carbona te present up to 1 atm), and is enhanced in the presence of sulfate. The fo rmation of ternary surface complexes is evaluated as a possible explanation for enhanced Pb uptake in the presence of high CO2 (1 atm) and in the pres ence of sulfate.