Outer-sphere adsorption of Pb(II)EDTA on goethite

Fourier transform infrared (FTIR) and extended X-ray absorption fine struct ure (EXAPS) spectroscopic measurements were performed on Pb(II)ethylenediam inetetraacetic (EDTA) adsorbed on goethite as a function of pH (4-6), Pb(II )EDTA concentration (0.11-72 mu M), and ionic strength (16 mu M-0.5 M). FTI R measurements show no evidence for carboxylate-Fe(III) bonding or protonat ion of EDTA at Pb:EDTA = 1:1. Both FTIR and EXAFS spectroscopic measurement s suggest that EDTA acts as a hexadentate ligand, with all four of its carb oxylate and both of its amine groups bonded to Pb(II). No evidence was obse rved for inner-sphere Pb(II)-goethite bonding at Pb:EDTA = 1:1. Hence, the adsorbed complexes should have composition Pb(II)EDTA(2-). Because substant ial uptake of PbEDTA(II)(2-) occurred in the samples, we interpret that Pb( II)EDTA(2-) adsorbed as outer-sphere complexes and/or as complexes that los e part of their solvation shells and hydrogen bond directly to goethite sur face sites. We propose the term "hydration-sphere" for the latter type of c omplexes because they should occupy space in the primary hydration spheres of goethite surface functional groups and to distinguish this mode of sorpt ion from common structural definitions of inner- and outer-sphere complexes . The lack of evidence for inner-sphere EDTA-Fe(III) bonding suggests that previously proposed metal/ligand-promoted dissolution mechanisms should be modified, specifically to account for the presence of outer-sphere precurso r species. Copyright (C) 1999 Elsevier Science Ltd.