In situ structural study of the surface complexation of lead(II) on the chemically mechanically polished hematite (1(1)over-bar02) surface

A structural study of the surface complexation of Pb(II) on the (1 (1) over bar 02) surface of hematite was undertaken using crystal truncation rod (CTR) X-ray diffraction measurements under in situ conditions. The sorbed Pb was found to form inner sphere (IS) complexes at two types of edge-sharing sites on the half layer termination of the hematite (1 (1) over bar 02) surface. The best fit model contains Pb in distorted trigonal pyramids with an average Pb-O bond length of 2.27(4) angstrom and two characteristic Pb-Fe distances of 3.19(1) angstrom and 3.59(1) angstrom. In addition, a site coverage model was developed to simulate coverage as a function of sorbate-sorbate distance. The simulation results suggest a plausible Pb-Pb distance of 5.42 angstrom, which is slightly larger than the diameter of Pb's first hydration shell. This relates the best fit surface coverage of 0.59(4) Pb per unit cell at monolayer saturation to steric constraints as well as electrostatic repulsion imposed by the hydrated Pb complex. Based on the structural results we propose a stoichiometry of the surface complexation reaction of Pb(II) on the hematite (1 (1) over bar 02) surface and use bond valence analysis to assign the protonation schemes of surface oxygens. Surface reaction stoichiometry suggests that the proton release in the course of surface complexation occurs from the Pb-bound surface O atoms at pH 5.5. (C) 2018 Elsevier Inc. All rights reserved.