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

Abstract

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/m2), 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Osfc and Cl−Fesfc bonds simultaneously, i.e., both Pb(II) and Cl− ions bond as an inner-sphere species. In contrast, PbCl−Alsfc 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−Fesfc and adsorbate-Cl−Fesfc 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.