X-ray Analyses of Lead Adsorption on the (001), (110), and (012) Hematite Surfaces

Abstract

Predicting the environmental fate of lead relies on a detailed understanding of its coordination to mineral surfaces, which in turn reflects the innate reactivity of the mineral surface. In this research, we investigated fundamental dependencies in lead adsorption to hematite by coupling extended X-ray absorption fine structure (EXAFS) spectroscopy on hematite particles (10 and 50 nm) with resonant anomalous X-ray reflectivity (RAXR) to single crystals expressing the (001), (012), or (110) crystallographic face. The EXAFS showed that lead adsorbed in a bidentate inner-sphere manner in both edge and corner sharing arrangements on the FeO octahedra for both particle sizes. The RAXR measurements confirmed these inner-sphere adsorption modes for all three hematite surfaces and additionally revealed outer-sphere adsorption modes not seen in the EXAFS. Lead uptake was larger and pH dependence was greater for the (012) and (110) surfaces, than the (001) surface, due to their expressing singly- and triply coordinated oxygen atoms the (001) surface lacks. In coupling these two techniques we provide a more detailed and nuanced picture of the coordination of lead to hematite while also providing fundamental insight into the reactivity of hematite.

Molecular-scale study of Cr(vi) adsorption onto lepidocrocite facets by EXAFS, in situ ATR-FTIR, theoretical frequency calculations and DFT+U techniques

“…Adsorption of arsenate on hematite surfaces also showed facet-dependence, the bridging bidentate arsenate complexes were formed on the {012} hematite facet, while the edgesharing complexes appeared on the {012} and {102} facets. 12,13 In addition, Noerpel et al 14 revealed that both outer-sphere and bidentate lead complexes were formed on the {001}, {012} and {110} hematite facets, wherein the hematite with exposed {110} and {012} facets exhibited better adsorption performance than that with {001} facets. These coordination structures commonly have a strong correlation with the surface coverage; for example, bidentate binuclear complexes appeared to have higher surface coverage than monodentate ones.…”

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

“…69,247 Noerpel et al used resonant anomalous X-ray reflectivity (RAXR) to determine the surface coverages of Pb 2+ sorbed on various hematite facets and similarly showed that the {012} and {110} facets (with higher densities of singly and triply coordinated hydroxyl groups than {001}) had a higher surface coverage of Pb 2+ per surface area. 76 The higher density of singly coordinated hydroxyl groups on the {100} surface of hematite was also shown to be responsible for the greater sorption capacity (per surface area) of humic substances compared with the {001} facet, 235 with analogous conclusions for enhanced sorption and subsequent hydrolysis of 4-nitrophenyl phosphate on the {100} vs. {001} facet of nanohematite. 238 In addition to relative differences in sorption capacities, recent research has demonstrated that different facets of nanoscale hematite also promote different complexation geometries for both inorganic and organic contaminants.…”

“…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. 77 The Fe single termination and O termination have been observed as coexisting and are variably stable depending on the oxygen partial pressure, oxidation conditions, annealing temperature, and whether the measurements are done in a dry or humid/aqueous environment.…”

“…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. ,,, …”

“…Accordingly, the {012} and {110} facets of hematite have much stronger adsorption affinities and higher capacities for contaminants of concern (Table ). For example, Catalano et al have reported the preference of arsenate (As­(VI)) and selenite (Se­(IV)) to bind to singly and triply coordinated oxygen functional groups because of favorable sterics, kinetics, and acidities. , Noerpel et al used resonant anomalous X-ray reflectivity (RAXR) to determine the surface coverages of Pb 2+ sorbed on various hematite facets and similarly showed that the {012} and {110} facets (with higher densities of singly and triply coordinated hydroxyl groups than {001}) had a higher surface coverage of Pb 2+ per surface area . The higher density of singly coordinated hydroxyl groups on the {100} surface of hematite was also shown to be responsible for the greater sorption capacity (per surface area) of humic substances compared with the {001} facet, with analogous conclusions for enhanced sorption and subsequent hydrolysis of 4-nitrophenyl phosphate on the {100} vs. {001} facet of nanohematite …”