Selective Adsorption of Pb(II) on an Annealed Hematite (1-102) Surface: Evidence from Crystal Truncation Rod X-ray Diffraction and Density Functional Theory

The Pb(II) binding mechanism on an annealed hematite (11 ̅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 Pb(II) exhibited selective sorption to one specific type of edge-sharing surface site (ES2) over the other two types of potential surface sites. From the best fit model structure, it is found that the Pb surface complex species form a trigonal pyramid local geometry with the base consisting of three oxygen groups that include two surface O groups (IO and IIIO) and one distal O extending toward solution. The trigonal pyramid geometry is slightly distorted with Pb-O bond lengths ranging from 2.21 to 2.31 Å and O-Pb-O bond angles ranging from 72° to 75°. Under this structural distortion the nearest distance between Pb and Fe is found to be 3.39(1) Å. Consistent with the CTR results, DFT calculations indicate the Pb binding energy at the ES2 site is at least 0.06 eV stronger compared to the other two types of potential sites. Using bond-valence rules we propose a stoichiometry of Pb(II) binding on the hematite (11 ̅02) surface, which indicates proton release through the deprotonation of all oxygen groups bonding to Pb.