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

Abstract

The Pb­(II)-binding mechanism on an annealed hematite (102) 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 (ES 2 ) 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 (IO and IIIO) 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 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 ES 2 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 (102) surface which indicates proton release through the deprotonation of all oxygen groups bonding to Pb.

Toward Informed Design of Nanomaterials: A Mechanistic Analysis of Structure–Property–Function Relationships for Faceted Nanoscale Metal Oxides

“…The importance of integrating surface science with direct sorption experiments was underscored by a recent publication by Qiu et al, who studied lead sorption on the full-termination {012} facet of cut and polished hematite. 74 In a prior study, the same research group found that, on the half-layer termination, Pb 2+ bound at two distinct FeO 6 octahedral edges to form two different bidentate, edge-sharing structures. 73 However, on the full-layer termination, only one of these structures was found because the reorientation of Fe and O in the half-layer termination meant that the binding geometry (i.e., bond angles and lengths) of the second structure were too energetically unfavorable.…”

“…72 While both terminations contain singly and triply coordinated terminal oxygens, the half-layer termination also contains doubly coordinated oxygens, known to be less exchangeable. 73,74 Comparison of the reflectivity profiles suggests that the halflayer termination is the dominant termination in water but that some full-layer termination exists. 69,73,75,76 The clean {001} facet has been reported to have three different terminations: a single Fe termination, a double Fe termination, and an O termination.…”

“…73 However, on the full-layer termination, only one of these structures was found because the reorientation of Fe and O in the half-layer termination meant that the binding geometry (i.e., bond angles and lengths) of the second structure were too energetically unfavorable. 74 That surface structures with subtle differences influence the availability of sorbate complexation sites highlights the importance of integrating an understanding of NMO surface structure with the thermodynamics of binding geometries and the sorption behavior of contaminants across different NMO facets.…”

“…1,30,233,271,272 Crystal truncation rod (CTR) X-ray diffraction can distinguish similar binding structures on the same surface, though this technique utilizes cut and polished single-crystal samples, which may not be wholly representative of in situ sorption behavior of NMOs. 73,74 Raman spectroscopy is also a versatile technique successful at elucidating the defect chemistry of metal oxides, with in situ Raman spectroscopy probing these defects under varying environmental conditions. 30,273,274 Additionally, probe-molecule-assisted nuclear magnetic resonance (NMR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) have demonstrated their ability to discern surface structures and features of various facets.…”

“…The full-layer ridge-and-valley topography has singly coordinated hydroxyl groups on the ridges and triply coordinated hydroxyl groups in the valleys (no doubly coordinated hydroxyl groups). , The full-layer termination is dominated by singly undercoordinated iron sites, but the half-layer termination has a reduced density of these sites . While both terminations contain singly and triply coordinated terminal oxygens, the half-layer termination also contains doubly coordinated oxygens, known to be less exchangeable. , Comparison of the reflectivity profiles suggests that the half-layer termination is the dominant termination in water but that some full-layer termination exists. ,,, …”