Selective Adsorption of Pb(II) on an Annealed Hematite (1102) Surface: Evidence from Crystal Truncation Rod X-ray Diffraction and Density Functional Theory

The Pb(II)-binding mechanism on an annealed hematite (1 (1) over bar 02) surface was studied using crystal truncation rod (CTR) X-ray diffraction coupled with density functional theory (DFT) calculations. The best fit CTR model suggested that Pb(II) sorbed selectively to one type of edge-sharing surface site (ES2) over two other potential surface sites. From the best fit model structure, it was found that the Pb surface complex species forms a trigonal pyramid geometry. The base consists of three oxygen groups, two of which are associated with the substrate surface (O-I and O-III) and one that is a distal O extending toward solution. The trigonal pyramid geometry is slightly distorted with Pb-O bond lengths ranging from 2.21 to 2.31 angstrom and O-Pb-O bond angles ranging from 72 degrees to 75 degrees. Under this structural distortion, the nearest distance between Pb and Fe is found to be 3.39(1) angstrom. Consistent with the CTR results, DFT calculations indicate the Pb binding energy at the ES2 site is at least 0.16 eV more favorable than that at the other two potential binding sites considered. Using bond-valence rules we propose a stoichiometry of Pb(II) binding on the hematite (1 (1) over bar 02) surface which indicates proton release through the deprotonation of all oxygen groups bonding to Pb.