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Layered double hydroxides (LDH) containing various exchangeable anions were studied to show how X-ray Photoelectron Spectroscopy (XPS) can provide information on the local environments of the different elements within the interlayer anionic groups and their possible influence on the LDH interlayer hydroxide surfaces. As such, XPS can potentially provide additional information about these systems that cannot be obtained by other common spectroscopic methods, such as infrared and Raman spectroscopy. A Mg₆Al₂X(OH)₁₆. 4H₂O with X representing interlayer anions CO₃²⁻, PO₄³⁻, SO₄²⁻, MoO₄²⁻, CrO₄³⁻, Fe(CN)₆⁴⁻, and Fe(CN)₆³⁻ was studied. The hydroxide layer structure is characterized by the Mg 2p and Al 2p with a binding energy of around 50.1 and 74.5 eV for the normal CO₃²⁻ containing LDH. The O 1s contained three peaks related to the layer OH-groups at 531.6 eV, interlayer CO₃²⁻ at 530.5 eV and interlayer water at 532.4 eV. Similar observations were made for the other interlayer anions showing characteristic P 2p, S 2p, and Mo 3d peaks. Intercalation with CrO₄³⁻ shows that a significant amount of the Cr⁶⁺ has been reduced to Cr³⁺. Finally, the intercalation of hexacyanoferrate in hydrotalcite showed the potential of XPS in detecting changes in the oxidation state of Fe upon intercalation in the LDH with a change in the Fe 2p peaks with a shift in binding energy and the possibility of determining the amount of reduction of Fe(III) to Fe(II). In general, the XPS high-resolution scans of P 2p, S 2p, Mo 3d, and Cr 2p show that slightly lower binding energies are observed compared to the binding energy values for the corresponding anionic groups as part of a rigid crystal structure, such as in minerals. Overall, the influence of the nature of the interlayer anion on the binding energy of the elements (Mg, Al, O) in the layered double hydroxide structure is minimal and considered to be within the experimental error of XPS. A detailed analysis of XPS data in combination with infrared and Raman spectroscopy shows how XPS can provide additional information that is not readily available via vibrational spectroscopy. XPS can simultaneously account for both surface and bulk properties of LDH that are not available through common vibrational spectroscopic methods. Full article

FeMgAl−MoS₄ LDH was successfully synthesized by a one-pot hydrothermal process followed by ion-exchange methods, and this novel adsorbent was first conducted for aqueous selenite and selenate elimination. The Fe as a component for metal cation layers of LDHs could modulate the layer charge density, leading to more functional groups inserted into layers, and more importantly, this heterogeneous Fe can catalyze the surface reactions between Se(IV) or Se(VI) with S(-II) for oxoanions sequestration. The mechanisms are ion exchange between functional groups with HSeO₃⁻ and SeO₃²⁻ for Se(IV) or SeO₄²⁻ for Se(VI), followed by reduction by S(-II) from MoS₄²⁻ groups. The existence of Fe in LDH cation layers, obviously enhanced the reactions (almost two times more for Se(IV) and three times more for Se(VI), respectively), resulting in satisfying adsorption capacities of 483.9 mg/g and 167.2 mg/g for Se(IV) and Se(VI), respectively. Mechanisms were further revealed by elementary analysis, XRD, FT−IR, SEM−EDX, and XPS, as well as the quantitative study. For sorption kinetics, the calculated values of capacities from the pseudo-second-order model are much closer to the experimental values. For sorption isotherms, Langmuir is better than the Freundlich isotherms model for closer capacities (505 mg/g for selenite and 172 mg/g for selenate). All these results demonstrated that the presence of heterogeneous Fe could catalyze the reduction of Se (IV/VI) for the aqueous system, and maybe other high oxidative states hazardous ions. So FeMgAl−MoS₄ is a kind of novel adsorbent that offers a promising multi-functional and highly efficient solution for water selenium purification. Full article