Spectroscopic studies of Pb(II)-sulfate interactions at the goethite-waterinterface

We used a combination of in situ attenuated total reflectance Fourier trans form infrared (ATR-FTIR) spectroscopy and X-ray absorption fine structure ( XAFS) spectroscopy to conduct molecular scale studies on Pb(II)-sulfate int eractions at the solid-water interface of goethite at pH 4.5, 5.0, and 6.0. Both the ATR-FTIR studies (probing sorbed SO4 in a Row cell setup as a fun ction of the Pb concentration) and the EXAFS studies (probing sorbed Pb at high levels of co-adsorbing SO4) indicated the formation Pb-SO4 ternary com plexes at the goethite surface. Based on the combined information from the IR and XAFS studies, possible Pb-SO4 ternary complex configurations were pr esented and discussed by comparison to a set of reference sulfate FTIR spec tra. In addition to forming ternary complexes with SO4, adsorption of Pb al so promoted SO4 sorption to the goethite surface by changing the surface ch arge, leading to additional formation of inner- and outer-sphere SO4 sorpti on complexes not coordinated by Pb. The relative impacts of these mechanism s (i.e., ternary complex formation versus electrostatic effects) appeared t o be a function of pH and the level of Pb addition. Formation of ternary co mplexes was promoted (relative to the importance of electrostatic effects) at low pH values and high Pb concentrations, whereas electrostatic effects were more pronounced at high pH values and low Pb concentrations. In additi on, it was found that part of the SO4 initially sorbed at the goethite surf ace as inner-sphere complexes without being coordinated by Pb was transform ed into SO4-Pb ternary complexes as the Pb concentration was increased, an effect most pronounced at low pH. This study shows that co-adsorption of SO , and Pb may lead to changes in both the extent and mechanisms of the adsor ption of these contaminants to the goethite surface relative to binary Pb/g oethite and SO4/goethite systems. The presence of co-adsorbing metals or an ions may therefore significantly impact the behavior of contaminants in env ironmental settings. Copyright (C) 2001 Elsevier Science Ltd.