Molybdenum(VI) Adsorption onto Lepidocrocite (γ-FeOOH): In Situ Vibrational Spectroscopy and DFT+U Theoretical Study

Abstract

In situ polarized ATR-FTIR spectroscopy and the DFT+U calculations were used to investigate the Mo(VI) ions surface speciation on lepidocrocite. Adsorption of molybdate ions is found to be tetrahedral monodentate binuclear (C 3v ) on the (010) face. DFT+U calculations were performed to help characterizing the geometries of the complexes adsorbed to assign the vibrational frequencies and get insight into the energies of adsorption. The (010) surface was chosen as a model of lepidocrocite surface. Calculations are consistent with the sorption of MoO 4 2− ions to form a monodentate complex at basic pH. This complex is protonated when the surface coverage increases, and the pH decreases. At the full coverage, calculations show alternate monodentate protonated binuclear complexes and surface hydroxyl groups. Moreoever, the formation of hydrogen bonds with the surface hydroxyls and between adsorbed complexes plays an important role in the structure stability.

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

“…11 In addition, Davantès et al observed that Mo(VI) adsorption on {010} lepidocrocite facets via a monodentate binuclear complex produced 3 to 4 peaks based on the vibrational spectra calculations using DFT calculations. 28 Therefore, these Cr-O stretching bands in our study also indicated that more than one inner-sphere complex formed Environmental Science: Nano Paper on the LEP surface. This speculation was also verified by the above EXAFS analysis.…”

“…Compared with the nonhydrogenated monodentate complex (E ads = 2.16 eV), the protonated 2HMB complex formed a network of hydrogen bonds on the hydroxyl surface and stabilized the subsurface structure of the {010} facet. 28 On the fully deprotonated {010} facet, the surface coordination structures were endothermic in the MB case: E ads = 9.62 eV. The corresponding protonated structure (HMB) was slightly stabilized: E ads = 6.73 eV.…”

“…35,38 Considering on-site Coulomb repulsion, the DFT+U method has been employed to investigate iron oxides, where the parameter (Hubbard U) varies from 2 to 5 eV. 28 According to a previous report, U = 5 eV is an appropriate selection for reproducing the experimental band gap of ∼2.0 eV of the bulk FeOOH. 39 Additionally, the antiferromagnetic arrangement was also considered with antiparallel spin into and between layers.…”

“…In addition, there are few studies focused on the interfacial structures of HMIs on the {010} lepidocrocite facet. For instance, Davantès et al 28 observed that Mo(VI) ions were adsorbed on plate-like lepidocrocite with an exposed {010} facet via the formation of a protonated monodentate binuclear complex. Otte et al 29 also found that arsenate coordinated to the {010} lepidocrocite facet by alternating protonated monodentate binuclear complexes.…”

Development of hollow δ-FeOOH structures for mercury removal from water

“…Iron oxyhydroxide (FeOOH) nanoparticles are cheap, harmless and eco-friendly (Patra & Kim 2017), and used widely for efficient removal of various metals from aqueous media (Kokkinos et al 2015). Iron hydroxides have been used widely as decontaminants to remove oxyanions like arsenite, arsenate, chromate, vanadate, phosphate, etc (Davantès et al 2016). Recently, δ-FeOOH, magnetic iron oxyhydroxide, has been reported as an efficient adsorbent for Hg(II) from contaminated water (Maia et al 2019).…”