L-edge X-ray Absorption Spectroscopy Research Articles

Exceptional Hydrogen Uptake in Crystalline InxGa1-xN Semiconductors.

The irradiation of InN and InxGa1-xN samples with low-energy H ions results in exceptionally high hydrogen uptake in a crystalline semiconductor. This phenomenon is attributed to specific In-H complex formation. By exploiting spectral fingerprints of the In-H complexes observable in In L3-edge X-ray absorption spectroscopy, we provide direct evidence of complex formation. Density functional theory calculations assist in interpreting the X-ray absorption spectra and offer insights into the energetics of complex formation. We quantify the total amount of reversibly incorporated hydrogen in these semiconductors and discuss their strengths and weaknesses as innovative materials for hydrogen storage.

X-ray and Nuclear Spectroscopies to Reveal the Element-Specific Oxidation States and Electronic Spin States for Nanoparticulated Manganese Cyanidoferrates and Analogs

In this publication, the potential non-gadolinium magnetic resonant imaging agent—nanoparticulate K2Mn[Fe(CN)6]—its comparison sample KFe[Co(CN)6], as well as their reference samples were measured and analyzed using Mn, Co and Fe L-edge X-ray absorption spectroscopy (L XAS). From the information obtained, we conclude that K2Mn[Fe (CN)6] has a high spin (hs)-Mn(II) and a low spin (ls)-Fe(II), while KFe[Co(CN)6] has an hs-Fe(II) and an ls-Co(III). In these Prussian blue (PB) analog structures, the L XAS analysis also led to the conclusion that the hs-Mn(II) in K2Mn[Fe(CN)6] or the hs-Fe(II) in KFe[Co(CN)6] bonds to the N in the [M(CN)6]4−/3− ions (where M = Fe(II) or Co(III)), while the ls-Fe(II) in K2Mn[Fe(CN)6] or the ls-Co(III) in KFe[Co(CN)6] bonds to the C in the [M(CN)6]4−/3− ion, suggesting the complexed metalloligand [Mn(II) or Fe(II)] occupies the N-bound site in PB. Then, nuclear resonant vibrational spectroscopy (NRVS) was used to confirm the results from the L XAS measurements: the Mn(II), Eu(III), Gd(III), Fe(II) cations complexed by [M(CN)6]n−-metalloligand all take the N-bound site in PB-like structures. Our NRVS studies also prove that iron in the K2Mn[Fe(CN)6] compound has a 2+ oxidation state and is surrounded by the C donor atoms in the [M(CN)6]n− ions.

Hydrous Iridium Oxide Thin Films: Revealing the Structure of a Highly Active Model Catalyst By in-Situ Ir L 3-Edge XANES and EXAFS

Currently, only iridium oxide shows favourable activity and stability under the harsh oxygen evolution reaction (OER) conditions for application in proton exchange membrane water electrolysis (PEM-WE) [1-3]. There is a well established inverse relationship between OER activity and stability of iridium oxide-based OER catalysts given its degree of crystallinity and hydration [4-6]. Crystalline, anhydrous iridium oxides are more stable but have diminished OER activity compared to amorphous, hydrous iridium oxides, which are more active. Different OER mechanisms have been theorized to occur on these materials, explaining their difference in performance [6]. Several spectroscopic investigations have been performed to explore the properties of such different iridium oxides, in particular to identify the chemical and electronic state of their active sites. However, a consensus regarding the active site structure and OER mechanism has yet to be reached [7-10].Using scanning electron microscopy, x-ray photoelectron spectroscopy, and combining in-situ Ir L 3-edge x-ray absorption spectroscopy with density functional theory (DFT) calculations and ab initio thermodynamics, we have investigated in-situ electrochemically grown porous hydrous iridium oxide thin films (HIROF) as a model system to examine the chemical and electronic structure of the highly active, hydrous iridium oxide species. In-situ extended x-ray absorption fine structure(EXAFS) results show that HIROF grows preferentially in a form most often associated with the OER catalytically active site of hydrous iridium oxides. Calculations over different possible structures allowed us to identify a unique structural group with enhanced hydrogenation that best fits the EXAFS data. In-situ x-ray absorption near edge structure (XANES) results reveal a lower onset potential for the redox behaviour of HIROF compared to rutile IrO2. Based on this study, we propose a new structural model explaining the high activity and poorer stability of hydrous iridium oxides compared to crystalline IrO2.

Real-time observation of intermediate species in electrochemical oxygen evolution reactions on CoOx through soft X-ray absorption spectroscopy

Real-time monitoring of oxygen species during the oxygen evolution reaction (OER) on cobalt oxide, one of the major OER electrocatalysts, was achieved through operando observation using wavelength-dispersive X-ray absorption spectroscopy (XAS) at the oxygen K-edge. The XAS spectra were collected every 3 s during electrode potential sweeping for the OER, and oxygen intermediate species related to the rate-determining step of the OER were detected just below the onset potential. Moreover, the cobalt L-edge XAS results demonstrate that the Co3O4 species dominates the surface of the electrode at a relatively high potential region, where the OER occurs. The present technique has significant potential for application in the analysis of solid–liquid interfaces, whose fundamental understanding is useful for further improvement of electrocatalysis.

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate.

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Speciation of toxic pollutants in Pb/Zn smelter slags by X-ray Absorption Spectroscopy in the context of the literature

Pb/Zn smelter slag is a hazardous industrial waste from the Imperial Smelting Process (ISP). The speciation of zinc, lead, copper and arsenic in the slag controls their recovery or fate in the environment but has been little investigated. X-ray Absorption Spectroscopy (XAS) was applied to this complex poorly crystalline material for the first time to gain new insights about speciation of elements at low concentration. Zn, Cu, As K-edge and Pb L3-edge XAS was carried out for a Pb/Zn slag from a closed ISP facility in England, supported by Fe, S and P K-edge XAS. Results are presented in the context of a full review of the literature. X-ray fluorescence showed that concentrations of Zn, Pb, Cu and As were 8.4, 1.6, 0.48 and 0.45 wt%, respectively. Wüstite (FeO) was the only crystalline phase identified by X-ray diffraction, but XAS provided a more complete understanding of the matrix. Zn was found to be mainly present in glass, ZnS, and possibly solid solutions with Fe oxides; Pb was mainly present in glass and apatite minerals (e.g., Pb5(PO4)3OH); Cu was mainly speciated as Cu2S, with some metallic Cu and a weathering product, Cu(OH)2; As speciation was likely dominated by arsenic (III) and (V) oxides and sulfides.

X-ray Spectroscopic Study of the Electronic Structure of a Trigonal High-Spin Fe(IV)═O Complex Modeling Non-Heme Enzyme Intermediates and Their Reactivity.

Fe K-edge X-ray absorption spectroscopy (XAS) has long been used for the study of high-valent iron intermediates in biological and artificial catalysts. 4p-mixing into the 3d orbitals complicates the pre-edge analysis but when correctly understood via 1s2p resonant inelastic X-ray scattering and Fe L-edge XAS, it enables deeper insight into the geometric structure and correlates with the electronic structure and reactivity. This study shows that in addition to the 4p-mixing into the 3dz2 orbital due to the short iron-oxo bond, the loss of inversion in the equatorial plane leads to 4p mixing into the 3dx2-y2,xy, providing structural insight and allowing the distinction of 6- vs 5-coordinate active sites as shown through application to the Fe(IV)═O intermediate of taurine dioxygenase. Combined with O K-edge XAS, this study gives an unprecedented experimental insight into the electronic structure of Fe(IV)═O active sites and their selectivity for reactivity enabled by the π-pathway involving the 3dxz/yz orbitals. Finally, the large effect of spin polarization is experimentally assigned in the pre-edge (i.e., the α/β splitting) and found to be better modeled by multiplet simulations rather than by commonly used time-dependent density functional theory.

Visualization of air-induced oxidation in single crystalline LiFe0.6Mn0.4PO4 nanowires with carbon sheath using soft X-ray spectromicroscopy

Single crystalline LiFe0.6Mn0.4PO4 (LFMP) nanowires with carbon sheath were analyzed by scanning transmission X-ray microscopy (STXM) with a spatial resolution of around 130 nm. The pinpoint Fe L-edge X-ray absorption spectroscopy (XAS) spectra in the STXM images revealed the natural oxidation of Fe2+ to Fe3+ near the tip of the nanowire by air exposure. The pinpoint O K-edge XAS spectra simultaneously changed with the Fe L-edge XAS in the STXM mapping because of the hybridization between the O 2p and Fe 3d orbitals. On the other hand, the Mn L-edge XAS spectra showed an almost uniform Mn2+ state all over the nanowires, suggesting that the Mn2+ state is stable against the natural oxidation in contrast to the Fe2+ state. We have thus demonstrated that STXM is a useful technique to visualize oxidized areas of materials and to reveal detailed information about their electronic structure.

Silica-magnesium-titanium Ziegler-Natta catalysts. Part 1: Structure of the pre-catalyst at a molecular level

In this paper, which is the first part of a more extended work, we elucidate the molecular level structure of a highly active SiO2-supported Ziegler-Natta precatalyst obtained by reacting a dehydroxylated silica and a solution of an organomagnesium compound with TiCl4. The synergetic combination of Ti K-edge and Ti L3-edge X-ray Absorption spectroscopy (XAS) and diffuse reflectance UV–Vis spectroscopies, complemented by Density Functional Theory (DFT) simulations, indicate that small TiCl3 clusters similar to β-TiCl3 coexist with isolated monomeric Ti(IV) species. Ti K-edge Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy allows the quantification of these two phases and demonstrates that the Ti(IV) sites are 6-fold coordinated (either by six chlorine ligands or by five chlorine and one oxygen ligands), but highly distorted, similar to what is modelled for TiCl4-capped MgCl2 nanoplatelets. Finally, IR spectroscopy suggests that the MgCl2 phase has a molecular character (Far-IR) and that the only accessible Mg2+ sites are uncoordinated cations acting as Lewis acid sites (IR of CO adsorbed at 100 K). Based on these experimental findings, we propose the co-existence in the precatalyst of small TiCl3 clusters and of mixed oxo-chloride magnesium-titanium structures deposited at the silica surface. The evolution of the precatalyst in the presence of the activator and of the monomer is discussed in the second part of this work.

Detailed Fe 3d electronic structures of Prussian blue analogs studied by X-ray absorption spectroscopy

Herein, we studied the detailed electronic structures of Prussian blue analogs with hexacyanoferrate around Fe L- and K absorption edges using room temperature X-ray absorption spectroscopy (XAS). Fe is trivalent for all the synthesized samples. By L3-edge XAS measurements, the excitation from Fe 2p3/2 core level to the 3d (t2g↓), 3d (eg↓), 3d (eg↑) states, and the π*, and σ* states of the Fe–CN molecular orbitals are identified with 3d (t2g↑)3 3d (t2g↓)2 configuration. The same excitation from the 1 s core level is identified using Fe K-edge XAS spectra. The relationship between lattice constants and the energy difference between 3d (t2g↓) to π* and σ* transitions from the Fe 2p3/2 core level suggests that the change in the lattice constant influences the electronic structure near the Fe atom.

Chemical-state distributions in charged LiCoO2 cathode particles visualized by soft X-ray spectromicroscopy

Lithium-ion deintercalation/intercalation during charge/discharge processes is one of the essential reactions that occur in the layered cathodes of lithium-ion batteries, and the performance of the cathode can be expressed as the sum of the reactions that occur in the local area of the individual cathode particles. In this study, the spatial distributions of the chemical states present in prototypical layered LiCoO2 cathode particles were determined at different charging conditions using scanning transmission X-ray microscopy (STXM) with a spatial resolution of approximately 100 nm. The Co L3- and O K-edge X-ray absorption spectroscopy (XAS) spectra, extracted from the same area of the corresponding STXM images, at the initial state as well as after charging to 4.5 V demonstrate the spatial distribution of the chemical state changes depending on individual particles. In addition to the Co L3-edge XAS spectra, the O K-edge XAS spectra of the initial and charged LiCoO2 particles are different, indicating that both the Co and O sites participate in charge compensation during the charging process possibly through the hybridization between the Co 3d and O 2p orbitals. Furthermore, the element maps of both the Co and O sites, derived from the STXM stack images, reveal the spatial distribution of the chemical states inside individual particles after charging to 4.5 V. The element mapping analysis suggests that inhomogeneous reactions occur on the active particles and confirm the existence of non-active particles. The results of this study demonstrate that an STXM-based spatially resolved electronic structural analysis method is useful for understanding the charging and discharging of battery materials.

Investigation of the Effect of Milling Duration on a Ce-Gd Doped Zirconolite Phase Assemblage Synthesised by Hot Isostatic Pressing

Zirconolite is a candidate ceramic wasteform under consideration for the immobilisation of the UK civil PuO2 inventory. In the present work, a baseline dual-substituted zirconolite with the target composition (Ca0.783Gd0.017Ce0.2)(Zr0.883Gd0.017Ce0.1)(Ti1.6Al0.4)O7 was fabricated by hot isostatic pressing (HIPing). In order to optimise the microstructure properties and improve the obtained yield of the zirconolite phase, a range of planetary ball milling parameters were investigated prior to consolidation by HIP. This included milling the batched oxide precursors at 400 rpm for up to 120 min, the pre-milling of CeO2 (PuO2 surrogate) to reduce the particle size and using a CeO2 source with finer particle size (<5 µm). The HIPed zirconolite product consisted of both zirconolite-2M and zirconolite-3T polytypes in varying proportions; however, an additional perovskite phase was obtained in varying quantities as a secondary phase. Ce L3-edge X-ray absorption spectroscopy was utilised to determine the Ce oxidation state. In this study, the ideal milling parameter for the fabrication of zirconolite waste forms was defined as 60 min at 400 rpm.

In situ x-ray absorption investigations of a heterogenized molecular catalyst and its interaction with a carbon nanotube support.

A highly active heterogenized molecular CO2 reduction catalyst on a conductive carbon support is investigated to identify if its improved catalytic activity can be attributed to strong electronic interactions between catalyst and support. The molecular structure and electronic character of a [Re+1(tBu-bpy)(CO)3Cl] (tBu-bpy = 4,4'-tert-butyl-2,2'-bipyridine) catalyst deposited on multiwalled carbon nanotubes are characterized using ReL3-edge x-ray absorption spectroscopy under electrochemical conditions and compared to the homogeneous catalyst. The Reoxidation state is characterized from the near-edge absorption region, while structural changes of the catalyst are assessed from the extended x-ray absorption fine structure under reducing conditions. Chloride ligand dissociation and a Re-centered reduction are both observed under applied reducing potential. The results confirm weak coupling of [Re(tBu-bpy)(CO)3Cl] with the support, since the supported catalyst exhibits the same oxidation changes as the homogeneous case. However, these results do not preclude strong interactions between a reduced catalyst intermediate and the support, preliminarily investigated here using quantum mechanical calculations. Thus, our results suggest that complicated linkage schemes and strong electronic interactions with the initial catalyst species are not required to improve the activity of heterogenized molecular catalysts.

Solvation structure of the Hg(NO3)2 and Hg(TfO)2 salts in dilute aqueous and methanol solutions: An insight into the Hg2+ coordination chemistry

A synergic approach combining X-ray absorption spectroscopy (XAS) and Molecular Dynamics (MD) has been used to investigate the solvation properties of dilute aqueous and methanol solutions of the Hg(NO3)2 and Hg(TfO)2 (where TfO−=trifluoromethanesulfonate or triflate) salts. A new Lennard-Jones potential has been developed to be used in classical MD simulations of Hg2+-containing systems by refining the force-field parameters to reproduce the Hg2+ ion coordination in water in agreement with the XAS data. A different behavior of the Hg2+ ion in water and methanol has been highlighted by the analysis of the MD simulations carried out with the newly developed interaction potential: in aqueous solutions of the Hg(NO3)2 and Hg(TfO)2 salts, the Hg2+ first coordination shell is composed only of water molecules, while in methanol solutions of the same salts the Hg2+ cation coordinates both counterions and methanol molecules in its first solvation sphere. The MD results have been confirmed by comparison with the Hg L3-edge XAS experimental data. Independently of the formation of ion pairs with the counterions, the Hg2+ ion in solution tends to form heptacoordinated first shell complexes, with an arrangement of the ligands depending on the nature of the solvent: in water a 7-fold cluster of water molecules in a C2 symmetry is found, while in methanol the Hg2+ solvation shell is composed of both counterions and methanol molecules, with oxygen atoms coordinating Hg2+ arranged in a distorted pentagonal bipyramid geometry. These findings represent a significant step forward in the understanding of the solvation chemistry of the Hg2+ ion, which is fundamental to improve the efficiency of mercury removal procedures that are crucial to the safety of water resources.

Remediation of Pb-contaminated soil using modified bauxite refinery residue

This study examines amendment of Pb-contaminated soil with modified bauxite refinery residue (MBRR) to decrease soil Pb mobility and bioaccessibility. Amendment experiments were conducted using four soils contaminated with Pb from various sources, including smelting, shooting-range activities and Pb-based paint waste. Lead L3-edge X-ray absorption spectroscopy (XAS) indicated that Pb speciation in these soils was a mixture of Pb sorbed to Fe (hydr)oxide and clay minerals, along with Pb bound to organic matter. Amendment with MBRR decreased water-soluble Pb and/or Toxicity Characteristic Leachate Procedure (TCLP) Pb concentrations. Lead L3-edge XAS and X-ray diffraction (XRD) indicated that Pb retention by MBRR occurred via sorption to Fe- and Al-(hydr)oxides at low Pb loadings, in addition to formation of hydrocerussite (Pb3(CO3)2(OH)2) at high loadings. Soil amendment with MBRR had relatively little effect on gastric-phase Pb bioaccessibility; as quantified via the Solubility/Bioavailability Research Consortium, SBRC, in vitro assay. In contrast, amendment with MBRR caused substantial decreases in relative intestinal-phase Pb bioaccessibility (Rel-SBRC-I) due to increased Pb sorption by MBRR’s Fe- and Al-hydr(oxide) minerals as simulated GI tract conditions shifted from the gastric- to the intestinal-phase. These decreases in Rel-SBRC-I point to the potential efficacy of using amendment with MBRR to decrease soil Pb bioavailability.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Evaluation of 134Ce as a PET imaging surrogate for antibody drug conjugates incorporating 225Ac

IntroductionThe in vivo generator 134Ce/134La has the potential to serve as a PET imaging surrogate for both alpha-emitting 225Ac and 227Th radionuclides due to the unique CeIII/CeIV redox couple and the relatively long half-life of 134Ce. The purpose of this study was to demonstrate the compatibility of 134Ce with DOTA-based antibody drug conjugates, which would act as therapeutic agents when incorporating 225Ac. MethodsThe in vivo biodistributions of [134Ce]Ce-DOTA and [134Ce]Ce-citrate were assayed by microPET imaging over 25 h in Swiss Webster mice to determine the in vivo stability of the [134Ce]Ce-DOTA complex. L3-edge X-ray absorption spectroscopy measurements were used to confirm the Ce oxidation state and the formation of a fully coordinated Ce-DOTA complex. The in vivo biodistribution of [134Ce]Ce-DOTA-Trastuzumab was assayed over 147 h by microPET imaging in SK-OV-3 tumor-bearing NOD SCID mice to evaluate tumor uptake and in vivo stability. Mice were euthanized at 214 h after administration of the radiolabeled antibody conjugate, and imaged 1 h later. An ex vivo biodistribution experiment was then performed in order to corroborate the PET images. Results[134Ce]Ce-DOTA displayed rapid renal elimination and high in vivo stability over 25 h, with negligible bone and liver uptake, in comparison to [134Ce]Ce-citrate. L3-edge X-ray absorption spectroscopy experiments confirmed the 3+ oxidation state within the stable Ce-DOTA complex. MicroPET images of [134Ce]Ce-DOTA-Trastuzumab displayed elevated tumor uptake over 214 h, with minimal bone and liver uptake analogous to previously reported [225Ac]Ac-DOTA-Trastuzumab biodistribution results, and the ex vivo biodistribution of [134Ce]Ce-DOTA-Trastuzumab corroborated the final PET images. ConclusionThese results demonstrate that 134Ce allows for long-term tumor targeting with DOTA-based antibody drug conjugates and may therefore be used to trace antibody drug conjugates incorporating 225Ac.

In situ time-resolved XAS study on metal-support-interaction-induced morphology change of PtO2 nanoparticles supported on γ-Al2O3 under H2 reduction

Through in situ time-resolved X-ray absorption spectroscopy (XAS), we observed the dynamic behavior of the formation of two-dimensional low-coordinated Pt particles through reduction of PtOx particles by H2, which were under metal-support interaction (MSI) with the γ-Al2O3 support. The relatively stabilized PtOx species were reduced below 373 K, and the interfacial PtOx species on γ-Al2O3 surface were reduced above 373 K. The time-resolved Pt L3-edge XAS revealed that the morphology changed from three-dimensional PtOx particles to two-dimensional low-coordinated Pt particles during the in situ reduction of PtOx/Al2O3 at 473 K, whereas at 373 K, the interfacial PtOx species remained after its reduction. This study demonstrated that in situ time-resolved XAS is one of the most definitive techniques to unveil the MSI effect during the reductive formation process of Pt nanoparticle catalysts loaded on γ-Al2O3.

Local structure analysis of Sb, Bi, and Ag dopant atoms in Mg2Si semiconductor by x-ray absorption spectroscopy and first-principles calculation

The local structures around Sb, Bi, and Ag dopant atoms in the environmentally friendly semiconductor Mg2Si were investigated by Sb K-edge, Bi L3-edge, and Ag K-edge x-ray absorption spectroscopy performed at 10 K. Fourier transforms (FTs) of the k3-weighted extended x-ray absorption fine structure (EXAFS) were analyzed. The experimental FTs of k3-weighted EXAFS were compared with the results of calculations using model clusters with Sb, Bi, and Ag atoms at the 8c, 4a, and 4b sites. The inverse FT of the χ(R) spectrum was calculated to refine the local structures for neighboring atoms around the Sb, Bi, and Ag atoms, and the interatomic distances and Debye–Waller factors were determined from the fit of the inverse FTs. The occupation of the 4a site by Sb and Bi atoms was demonstrated and that of the 8c site was investigated for Ag atoms. First-principles calculations were performed to clarify the characteristic change in the second-neighbor distances around the Ag atoms. The evaluation of the crystal orbital Hamilton population clarified that the change in the second-neighbor distances is caused by the bonding character formed between the Ag and Mg atoms. These results suggest that the Ag atoms mainly occupy the 8c site, while the large value of the Debye–Waller factor for the second neighboring atoms implies the possibility of the partial occupation of Ag atoms at the 4b sites. These findings provide an explanation for limiting the p-type conductivity in Mg2Si semiconductors.

Observation of surface precipitation of ferric molybdate on ferrihydrite: Implication for the mobility and fate of molybdate in natural and hydrometallurgical environments

Adsorption of molybdate (Mo(VI)) on the surfaces of ferrihydrite is one of the most critical processes that control its mobility and fate in the environment. However, the sorption mechanism and the effect of pH on the speciation of Mo(VI) on ferrihydrite surfaces are not well understood. In this study, X-ray diffraction (XRD), Raman, Fourier transform infrared (FTIR), and Mo K-edge and L3-edge X-ray absorption spectroscopy (XAS) have been utilized to characterize the Mo(VI) species sorbed on ferrihydrite under various pH conditions. XRD, Raman, and FTIR results show that at acidic pH, surface precipitation of poorly crystalline ferric molybdate (PCFM) occurs under apparently undersaturated conditions (theoretical log IAP < log Ksp) and is enhanced by the aging process, whereas Mo(VI) is mainly present as surface adsorbed species at circum-neutral pH. The Mo K-edge and L3-edge X-ray absorption near edge structure (XANES) analyses show that a mixture of tetrahedrally and octahedrally coordinated Mo(VI) simultaneously exists at pH 3–7 and the octahedral Mo(VI) species decreases with increasing pH. The Mo-Fe interatomic distances (3.52–3.56 Å) derived from EXAFS fittings suggest the corner-sharing complexation of both MoO4 and MoO6 with FeO6 octahedra. As the pH decreases from 7 to 3, the coordination number of the Mo-Fe shell (CNMo-Fe) increases from 0.6(3) to 1.9(3), possibly due to the gradual transformation of surface adsorbed Mo(VI) to PCFM. These findings on the observation of Mo(VI) complexation, surface precipitation, and their marked pH dependence during the Mo(VI) adsorption on ferrihydrite have important implications for both understanding the mobility and fate of Mo(VI) in natural and hydrometallurgical industry impacted environments and developing optimal applications for the remediation of Mo contamination in aqueous environments.