Vibrational spectroscopy at the IRE
Fourier-transform infrared (FT-IR) and Raman spectroscopy
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.
Different attenuated total reflection (ATR) units (diamond and ZnSe), with flow cells. Experiments can be performed in ambient atmosphere, under anoxic conditions and at elevated temperatures (≤ 70 °C).
for solid (KBr or PE pellets, Nujol) or liquid samples
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 heavy metal’s surface speciation
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. Vibrational spectroscopic experiments can be performed not only on aqueous solutions but also on mineral-water interfaces. Moreover, complexation of heavy metal ions with biomolecules can be studied under physiological relevant conditions.
The principle of an in situ sorption experiment:
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: FT-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 TiO2
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 induced sorption, IR spectra are re-corded continuously providing a time resolution of the sorption process in the sub-minute time range. From these spectra, a bidendate inner sphere sorption complex of NpO2+ on TiO2 is strongly suggested.The “2nd conditioning” – i.e. subsequent flushing of the mineral film with blank solution – reveals the removal of species (outer sphere complex) which are only weakly bound during the time of sorption.
Fig. 3: FT-IR spectra of the Np(V) sorption onto TiO2 (50 µM, pH 7.6, 2H2O, 0.1 M NaCl, N2 atmosphere).
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: FT-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.
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