The effects of sulfate anions on the uptake of Pb(II) onto goethite were investigated at the molecular level using in situ Pb L-III-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 Ca 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 ef al. (2000, J. Colloid Interface Science 224, 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). (C) 2000 Academic Press.