Abstract
The effects of sulfate anions on the uptake of Pb(II) onto goethite were investigated at the molecular level using in situ Pb LIII-EXAFS and ATR-FTIR spectroscopies. Macroscopic uptake data show that Pb uptake can be enhanced by at least 30% at pH 5 in the presence of 3.16 mM sulfate and that sulfate uptake at pH 7 can be enhanced by more than a factor of 3 in the presence of 1.0 mM Pb. Consistent with behavior in sulfate-free systems, Pb(II) forms inner-sphere complexes sharing either corners or edges with Fe(O,OH)6 octahedra under all conditions studied. The relative fraction of corner-sharing complexes is, however, significantly enhanced in the presence of sulfate at pH 5, 6, and 7 (all conditions studied) and additional sulfate species with C3v or lower point symmetry were noted in the presence of Pb by ATR-FTIR. Drawing on bond valence and structural constraints developed in J. D. Ostergren et al. (2000, J. Colloid Interface Science224, 000–000), these results indicate formation of Type A ternary complexes bonded to the surface through Pb that is bound as a bridging bidenate complex to two adjacent A-type (singly coordinated) surface oxygens ((≡Fe–O)2–Pb–OSO3).
Published Version
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