Surface complexation of Pb(II) at oxide-water interfaces: III. XAFS determination of Pb(II) and Pb(II)-chloro adsorption complexes on goethite and alumina

摘要

Pb(II) sorption on goethite and γ-alumina was studied in aqueous 0.1 M NaCl solutions as a function of pH (5-7), sorption density (1.2-5 μmoles/m~2), and [Pb]_T (0.24-4.53 mM) using XAFS (X-ray absorption fine structure) spectroscopy. At pH 7, the presence of Cl~- has little effect on the Pb(II)/goethite and Pb(II)/γ-alumina adsorbate spectra. However, at pH ≤ 6, the XAFS spectra indicate the presence of Pb(II)-chloro ternary complexes on goethite. XAFS-derived Pb-Fe distances suggest that these complexes are attached to goethite surfaces through Pb-O_(sfc) and Cl~--Fe_(sfc) bonds simultaneously, i.e., both Pb(II) and Cl~- ions bond as an inner-sphere species. In contrast, Pb-Cl~--Al_(sfc) ternary complexes were not observed on γ-alumina. These results suggest that the stability trends of Cl~--Me-oxide bonds are similar to the relative stabilities of aqueous Me-chloride complexes, where Me is an oxide-forming cation. Cl~--Fe_(sfc) and adsorbate-Cl~--Fe~(sfc) bonding should be taken into account when describing surface complexation of Pb(II) and other soft Lewis acid metal ions on iron oxides and iron oxyhydroxides in Cl~--bearing waters.