SURFACE COMPLEXATION OF PB(II) AT OXIDE-WATER INTERFACES - III - XAFSDETERMINATION OF PB(II) AND PB(II)-CHLORO ADSORPTION COMPLEXES ON GOETHITE AND ALUMINA

Pb(II) sorption on goethite and gamma-alumina was studied in aqueous 0 .1 M NaCl solutions as a function of pH (5-7), sorption density (1.2-5 mu moles/m(2)), and [Pb](T) (0.24-4.53 mM) using XAFS (X-ray absorpti on fine structure) spectroscopy. At pH 7, the presence of Cl- has litt le effect on the Pb(II)/goethite and Pb(II)/gamma-alumina adsorbate sp ectra. However, at pH less than or equal to 6, the XAFS spectra indica te the presence of Pb(II)-chloro ternary complexes on goethite. XAFS-d erived Pb-Fe distances suggest that these complexes are attached to go ethite 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 con trast, Pb-Cl--Al-sfc ternary complexes were not observed on gamma-alum ina. These results suggest that the stability trends of Cl--Me-oxide b onds are similar to the relative stabilities of aqueous Me-chloride co mplexes, where Me is an oxide-forming cation. Cl--Fe-sfc and adsorbate -Cl--Fe-sfc bonding should be taken into account when describing surfa ce complexation of Pb(II) and other soft Lewis acid metal ions on iron oxides and iron oxyhydroxides in Cl--bearing waters. Copyright (C) 19 98 Elsevier Science Ltd.