Katja Schmeide

Interaction of uranium(VI) with various modified and unmodified natural and synthetic humic substances studied by EXAFS and FTIR spectroscopy

Abstract The complexation of uranium(VI) by humic acids (HAs) and fulvic acids (FAs) was studied to obtain information on the binding of uranium(VI) onto functional groups of humic substances. For this, various natural and synthetic HAs were chemically modified resulting in HAs with blocked phenolic OH groups. Both from the original and from the modified humic substances, solid uranyl humate complexes were prepared at pH 2. FTIR and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to study the chemical modification process of humic substances, to study the structure of uranyl humate complexes and to evaluate the effect of individual functional groups of humic substances (carboxylic and phenolic OH groups) on the complexation of uranyl ions. The results confirmed the predominant blocking of phenolic OH groups in the modified HAs. These modified HAs are suitable model substances to study the role of phenolic OH groups of HAs in dependence on pH. By EXAFS spectroscopy, identical structural parameters were determined for all uranyl humates. Axial UO bond distances of 1.78 A were determined. In the equatorial plane approximately five oxygen atoms were found at a mean distance of 2.39 A. The blocking of phenolic OH groups of HAs did not change the near-neighbor surrounding of uranium(VI) in uranyl humate complexes. Thus, the results confirmed that predominantly HA carboxylate groups are responsible for binding of uranyl ions and that the influence of phenolic OH groups is insignificant under the applied experimental conditions. The carboxylate groups are monodentate coordinated to uranyl ions.

Investigation of humic acid complexation behavior with uranyl ions using modified synthetic and natural humic acids

We investigated the influence of phenolic OH groups on the complexation behavior of humic acid (HA) with UO22+ ions at pH 4. Starting from synthetic HA type M1, natural HA Aldrich, and natural HA Kranichsee, we synthesized modified HAs with blocked phenolic OH groups by derivatization with diazomethane. The partial blocking of phenolic OH groups was confirmed by a radiometric method which quantitatively determined the functional groups and by FTIR spectroscopy. The complexation behavior of the chemically modified and unmodified HAs with UO22+ ions was investigated by time-resolved laser-induced fluorescence spectroscopy. The experimental data were evaluated with the metal ion charge neutralization model. We determined comparable complexation constants for all HAs. Two modified HAs (type M1 and Aldrich) had significantly lower loading capacities for UO22+ ions (10.5 ± 0.9% and 9.7 ± 1.6%, respectively) than the corresponding unmodified HAs 18.0 ± 2.0% and 17.5 ± 1.6%, respectively). This indicates that the blocking of the phenolic OH groups changes the complexation behavior of HAs.

Uranium(VI) sorption onto phyllite and selected minerals in the presence of humic acid

We studied the influence of humic acid (HA) on the uranium(VI) sorption onto the rock material phyllite and onto its main mineral constituents quartz, muscovite, chlorite, and albite at an ionic strength of 0.1 M in the pH range of 3.5 to 9.5 under aerobic conditions. The uranium(VI) concentration was 1 × 10-6 M and the HA concentration was 5 and 60 mg/L, respectively. The solid/solution ratio was 12.5 g/L. Furthermore, we studied the uranium and HA sorption on ferrihydrite (3 × 10-4 M Fe) and compared the results to the sorption behavior of phyllite. The study showed that the uranium sorption onto phyllite and onto its mineral constituents is influenced by the pH-dependent sorption behavior of the HA. Due to high HA sorption onto the solids in the acidic pH range the uranium uptake is enhanced compared to the uranium uptake in the absence of HA. A high concentration of dissolved HA in the near-neutral pH range reduces the uranium sorption due to formation of aqueous uranyl humate complexes. Furthermore, we could show that the high uranium and HA sorption on phyllite is primarily caused by minor amounts of the secondary mineral ferrihydrite that is formed due to weathering of phyllite. Thus, the ferrihydrite predominates the contributions of the main minerals quartz, muscovite, chlorite, and albite, that are naturally present in the rock material phyllite.

EXAFS study on the neptunium(V) complexation by various humic acids under neutral pH conditions

Abstract The structure of Np(V) humic acid (HA) complexes at pH 7 was studied by extended X-ray absorption fine structure analysis (EXAFS). For the first time, the influence of phenolic OH groups on the complexation of HA and Np(V) in the neutral pH range was investigated using modified HAs with blocked phenolic OH groups and Bio-Rex70, a cation exchange resin having only carboxyl groups as proton exchanging sites. The formation of Np(V) humate complexes was verified by near-infrared (NIR) spectroscopy. Axial Np-O bond distances of 1.84–1.85 Å were determined for the studied Np(V) humate complexes and the Np(V)-Bio-Rex70 sorbate. In the equatorial plane Np(V) is surrounded by about 3 oxygen atoms with bond lengths of 2.48–2.49 Å. The comparison of the structural parameters of the Np(V) humates with those of Np(V)-Bio-Rex70 points to the fact that the interaction between HA and Np(V) in the neutral pH range is dominated by carboxylate groups. However, up to now a contribution of phenolic OH groups to the interaction process cannot be excluded completely. The comparison of the obtained structural data for the Np(V) humates to those of Np(V) carboxylates and Np(V) aquo ions reported in the literature indicates that humic acid carboxylate groups predominantly act as monodentate ligands. A differentiation between equatorial coordinated carboxylate groups and water molecules using EXAFS spectroscopy is impossible.

Neptunium(IV) complexation by humic substances studied by X-ray absorption fine structure spectroscopy

Summary We studied the coordination environment of neptunium(IV) in complexes with various natural and synthetic humic and fulvic acids at pH 1 by X-ray absorption fine structure (XAFS) spectroscopy. The results were compared to those obtained for the interaction of neptunium(IV) with Bio-Rex70, a cation exchange resin having solely carboxylic groups as metal binding functional groups. In both neptunium humate complexes and neptunium Bio-Rex70 sorbates, Np4+ is surrounded by about 10 oxygen atoms at an average distance of 2.36±0.02 Å. This verifies that the carboxylic groups are the main complexing sites of the humic substances responsible for binding neptunium(IV) in the acidic pH range. The data suggest a predominant monodentate coordination of the carboxylate groups to neptunium(IV) ions.

The relationship of monodentate and bidentate coordinated uranium(VI) sulfate in aqueous solution

Abstract The coordination of U(VI) sulfate complexes has been investigated by uranium LIII-edge EXAFS and HEXS measurements with the aim to distinguish monodentate and bidentate coordinated sulfate in aqueous solution. UV-vis absorption spectroscopy has been used to differentiate the species and to determine the species distribution as a function of the [SO42−]/[UO22+] ratio. A monodentate coordination prevails in solutions with [SO42−]/[UO22+] ratio of 1, where UO2SO4 is the dominant species. Besides the dominating monodentate sulfate a small amount of bidentate sulfate could be observed, indicating that two isomers may exist for UO2SO4. With increasing [SO42−]/[UO22+] ratio the UO2(SO4)22− species becomes the main species. The uranium atom of this species is coordinated by two bidentate sulfate groups.

Redox stability of neptunium(V) and neptunium(IV) in the presence of humic substances of varying functionality

Abstract The reducing properties of humic substances (humic acid (HA) and fulvic acid (FA)) of varying functionality towards Np(V) have been studied under anaerobic conditions between pH 3.5 and pH 9.0 in batch experiments. For Np redox speciation in solution solvent extraction, NIR absorption spectroscopy and ultrafiltration were applied. The reduction rate varied with type of humic substances, solution pH, HA to Np concentration ratio, and equilibration time. In comparison to natural humic substances, synthetic HA with designed redox properties lead to a stronger reduction of Np(V) to Np(IV). The reducing capacities of humic substances towards Np(V) could be correlated to their phenolic/acidic OH group content, which includes both hydroquinone-like moieties and non-quinoid phenols. By applying a synthetic HA with blocked phenolic/acidic OH groups, the dominance of phenolic/acidic OH groups as the redox-active moieties of humic substances was verified. The Np(IV) formed in the course of the experiments is predominantly humic colloid-bound. Np(IV) oxo/hydroxide colloids, that might be formed in addition, are stabilized by adsorbed humic substances. The remaining Np(V) occurs as NpO2+ ion or Np(V) humate depending on pH. The ability of synthetic HA to effectively maintain Np in the tetravalent state during humate complexation experiments could be shown.

Ternary uranium(VI) carbonato humate complex studied by cryo-TRLFS

Abstract The complex formation of U(VI) with humic acid (HA) in the presence of carbonate was studied by time-resolved laser-induced fluorescence spectroscopy at low temperature (cryo-TRLFS) at pH 8.5. In the presence of HA, a decrease of the luminescence intensity of U(VI) and no shift of the emission band maxima in comparison to the luminescence spectrum of the UO2(CO3)34− complex, the dominating U(VI) species under the applied experimental conditions in the absence of HA, was observed. The formation of a ternary U(VI) carbonato humate complex of the type UO2(CO3)2HA(II)4− starting from UO2(CO3)34− was concluded from the luminescence data. For this complex a complex stability constant of log K=2.83 ± 0.17 was determined. Slope analysis resulted in a slope of 1.12 ± 0.11, which verifies the postulated complexation reaction. The results agree very well with literature data. Speciation calculations show that the formation of the ternary U(VI) carbonato humate complex can significantly influence the U(VI) speciation under environmental conditions.

Luminescence properties of uranium(VI) citrate and uranium(VI) oxalate species and their application in the determination of complex formation constants

Abstract For the first time, the interaction of uranium(VI) with citric acid and oxalic acid in aqueous solution was investigated using time-resolved laser-induced fluorescence spectroscopy (TRLFS) between pH 2 and 4. The complex species UO2cit− and (UO2)2(cit)22− formed in citrate medium as well as UO2ox and UO2(ox)22− formed in oxalate medium show no luminescence emissions at room temperature. However, by coupling the fluorescence measurement technique with a low temperature system (−120 ºC, cryo-TRLFS), emission signals of the various complex species could be detected. The emission signals are bathochromic shifted in comparison to the emission maxima of the uncomplexed uranium(VI) cation. Using the spectroscopic data, the corresponding complex formation constants were calculated, which corroborate literature data.

Binary and ternary uranium(VI) humate complexes studied by attenuated total reflection Fourier-transform infrared spectroscopy

The complexation of U(VI) with humic acid (HA) in aqueous solution has been investigated at an ionic strength of 0.1 M (NaCl) in the pH range between pH 2 and 10 at different carbonate concentrations by attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. For the first time, the formation of binary and ternary U(VI) humate complexes was directly verified by in situ spectroscopic measurements. The complex formation constants for the binary U(VI) humate complex (UO(2)HA(II)) and for the ternary U(VI) mono hydroxo humate complex (UO(2)(OH)HA(I)) as well as the ternary U(VI) dicarbonato humate complex (UO(2)(CO(3))(2)HA(II)(4-)) determined from the spectroscopic data amount to log β(0.1 M) = 6.70 ± 0.25, log β(0.1 M) = 15.14 ± 0.25 and log β(0.1 M) = 24.47 ± 0.70, respectively, and verify literature data.