Vibrational spectroscopy at the IRE
FOURIER-TRANSFORM INFRARED (FT-IR) AND RAMAN SPECTROCOPY
Vibrational spectroscopic experiments of actinides in aqueous solutions and at mineral surfaces. Identification of the dominant aqueous species and surface complexes formed under the prevailing conditions comprising the molecular complexes formed with bioligands.
- Bruker Vertex 80/v vacuum FT-IR spectrometer
Detectors: MCT, FIR-DTGS and Bolometer; total spectral region: 12,000 – 5 cm−1 (0.8 – 2000 µm)
Max. spectral resolution: 0.2 cm−1
Implemented Rapid Scan technology
- Bruker Vertex 70/v vacuum FT-IR spectrometer
Detectors: MCT and MIR-DTGS; total spectral region: 12,000 – 370 cm−1 (0.8 – 27 µm)
Max. spectral resolution: 0.5 cm−1,
Implemented Rapid Scan technology
FT-Raman modul (Nd-YAG Laser λexc.= 1064 nm, P = 1000 mW)
FT-IR Analysis Techniques:
- Attenuated Total Reflection (ATR) technique:
Suitable for wet pastes and liquid samples.
Several ATR units (diamond/ZnSe, diamond/KRS5) combinable with flow cells.
Experiments can be performed in ambient atmosphere, under anoxic conditions and at elevated temperatures (≤ 70 °C).
Solid (KBr or PE pellets, Nujol) or liquid samples
FT-Raman Analysis Techniques:
- Cuvette for liquid samples available
Picture: Laboratory work
In situ Attenuated Total Reflection (ATR) FT-IR spectroscopy –
A valuable tool for the identification of aquoeus (surface) species of heavy metals
Aqueous samples containing low concentrations of actinides require the detection of very low absorption changes in front of a strong absorbing background mainly due to the solvent. This can be accomplished by in situ ATR FT-IR spectroscopy using a flow cell. This setup allows the investigation of aqueous species in the micromolar concentration range.[1–3] Furthermore, the formation and release of surface complexes at the mineral-water interface can be monitored in real time.[4–8] Moreover, complexation of heavy metal ions with biomolecules can be studied under physiological relevant conditions.[9–11]
The setup of an in situ sorption experiment:
Fig. 1: Scheme of the setup for in situ ATR FT-IR spectroscopy. The IR beam propagates through the ATR crystal under total reflection and the evanescent waves probes the interface of a thin mineral film – directly prepared onto the crystal's surface – and the aquoeus phase of the flow cell. The aquoeus solutions from the reservoirs usually contain the sorbate or pure background electrolyte for the study of the sorption and desorption processes, repsectively.
- Aqueous speciation of Np(VI) and U(VI)
A comparison of the IR spectra and the calculated speciation based on the thermodynamic NEA data base show discrepancies for both actinide systems.
Already at pH < 4 significant contributions from hydrolysis products – most likely monomeric species – are observed.The assignment of the Np-carbonato species is supported by the bands above 1300 cm−1 representing modes of the COO− group.
Fig. 2: Calculated speciation of the Np(VI) and U(VI) systems (0.1 M NaCl; ambient atmosphere). Below: Mid-IR spectra of the respective solutions and the (tentative) assignments to the major species present at the given pH values.
- Sorption complexes of Np(V) on hematite
The deposition of a mineral film directly on the ATR crystal allows the on-line monitoring of the formation of actinidic sorption complexes. During the entire course of the experiment, IR spectra are recorded continuously providing a time resolution of the surface processes in the sub-minute time range. A typical in situ sorption IR experiment consists of three subsequently performed steps: (1) Conditioning/Equilibration, (2) Sorption, (3) Flushing/Desorption.
(1) Conditioning/Equilibration: After preparation of the stationary mineral film on the ATR crystal's surface, its stability is verified by flushing the film with the respective background electrolyte for a certain time. The spectra obtained represent potential contributions of the solid phase to the spectra of the subsequent sorption process.
(2) Sorption: During exposure time of the sorbing agent, the spectral properties of the solid phase changes due to occurring surface processes. This change is reflected by the spectra denoted as "Sorption" and generally represents the formation of sorption complexes.
(3) Flushing/Desorption: After a steady state is obtained, i.e. the spectra of the "Sorption" step do not further change with time, the reversibility of the sorption processes can be verified by subsequently flushing the flow cell with background electrolyte. The spectra obtained often exhibit negative bands representing the release of surfaces species formed during the "Sorption" process. In case of a high reversibility, the same spectral characteristics are observed as during the previous sorption processes
From these spectra, a bidendate inner sphere sorption complex of NpO2+ on hematite is strongly suggested.The “Flushing” – i.e. subsequent flushing of the mineral film with blank solution – reveals the removal of species (predominantly outer sphere complexes) which are only weakly bound during the time of sorption.
Fig. 3: Mid-IR spectra of the Np(V) sorption onto hematite. The spectra of the three experimental steps as described above (A). The "Sorption" spectra of the same sorption system performed at different ionic strengths (B) and initial Np(V) concentrations.
- Complexation of U(VI) with biomolecules
The highly phosphorylated protein phosvitin serves as a model system for the elucidation of the molecular interaction of uranyl with native proteins in aqueous solution.
Fig. 4: Mid-IR spectra of phosvitin in aqueous solution. From top to below: Spectra recorded at pH 4 and 8 and calculated difference. Spectrum of phosvitin−UO22+ complex ([UO22+]: 10 µM.
Fig. 5: Proposed phosvitin−UO22+ complexes.
 Kretzschmar, J., Jordan, N., Brendler, E., Tsushima, S., Franzen, C., Foerstendorf, H., Stockmann, M., Heim, K. and Brendler, V. (2015) "Spectroscopic evidence for selenium(IV) dimerization in aqueous solution." Dalton Transactions 44, 10508-10515.
 Gückel, K., Tsushima, S. and Foerstendorf, H. (2013) "Structural characterization of the aqueous dimeric uranium(VI) species: (UO2)2CO3(OH)3−." Dalton Transactions 42, 10172-10178.
 Müller, K., Foerstendorf, H., Tsushima, S., Brendler, V. and Bernhard, G. (2009) "Direct spectroscopic characterization of aqueous actinyl(VI) species: A comparative study of Np and U." Journal of Physical Chemistry A 113, 6626-6632.
 Comarmond, M. J., Steudtner, R., Stockmann, M., Heim, K., Müller, K., Brendler, V., Payne, T. E. and Foerstendorf, H. (2016) "The Sorption Processes of U(VI) onto SiO2 in the Presence of Phosphate: from Binary Surface Species to Precipitation." Environmental Science & Technology 50, 11610-11618.
 Müller, K., Gröschel, A., Rossberg, A., Bok, F., Franzen, C., Brendler, V. and Foerstendorf, H. (2015) "In situ Spectroscopic Identification of Neptunium(V) Inner-Sphere Complexes on the Hematite–Water Interface." Environmental Science & Technology 49, 2560-2567.
 Foerstendorf, H., Jordan, N. and Heim, K. (2014) "Probing the surface speciation of uranium (VI) on iron (hydr)oxides by in situ ATR FT-IR spectroscopy." Journal of Colloid and Interface Science 416, 133-138.
 Müller, K., Foerstendorf, H., Brendler, V., Rossberg, A., Stolze, K. and Gröschel, A. (2013) "The surface reactions of U(VI) on γ-Al2O3 - In situ spectroscopic evaluation of the transition from sorption complexation to surface precipitation." Chemical Geology 357, 75-84.
 Müller, K., Foerstendorf, H., Brendler, V. and Bernhard, G. (2009) "Sorption of Np(V) onto TiO2, SiO2, and ZnO: An in situ ATR FT-IR spectroscopic study." Environmental Science & Technology 43, 7665-7670.
 Heller, A., Barkleit, A., Foerstendorf, H., Tsushima, S., Heim, K. and Bernhard, G. (2012) "Curium(III) citrate speciation in biological systems: a europium(III) assisted spectroscopic and quantum chemical study." Dalton Transactions 41, 13969-13983.
 Barkleit, A., Foerstendorf, H., Li, B., Rossberg, A., Moll, H. and Bernhard, G. (2011) "Coordination of uranium(VI) with functional groups of bacterial liposaccharide studied by EXAFS and FT-IR spectroscopy." Dalton Transactions 40, 9868-9876.
 Li, B., Barkleit, A., Raff, J., Bernhard, G. and Foerstendorf, H. (2010) "Complexation of U(VI) with highly phosphorylated protein, phosvitin. A vibrational spectroscopic approach." Journal of Inorganic Biochemistry 104, 718-725.