[show abstract][hide abstract] ABSTRACT: The influence of initial pH and energy input during suspension homogenization on the stabilizing performance and coordination type of commercial available polyacrylate dispersant were studied. Additionally to widely used rheology and electroacoustic measurement techniques the alumina suspensions were analysed with centrifugal separation and in situ ATR-FTIR to study the impact of varied powder processing in detail. In contrast to zeta potential analysis and viscosity measurements only the determination of sedimentation properties by centrifugal separation shows the effect of macroscopic changes in powder processing. A combination of positively charged alumina surface and a high shear homogenization leads to the most stable suspension. Accordingly ATR-FTIR results show a correlation between improved suspension stability and inner-sphere coordination of polyacrylate. Moreover it was possible to determine an optimal pH range for inner-sphere adsorption. It can be shown that macroscopic changes in powder processing influence the coordination of dispersant and thus the suspension stability.
Journal of the European Ceramic Society 01/2011; 32:363-370. · 2.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: The bis(salicylhydroxamato) and bis(benzohydroxamato) complexes of UO(2)(2+) in aqueous solution have been investigated in a combined experimental and computational effort using extended X-ray absorption fine structure and UV-vis spectroscopy and density functional theory (DFT) techniques, respectively. The experimentally unknown bis(benzoate) complex of UO(2)(2+) was investigated computationally for comparison. Experimental data indicate 5-fold UO(2)(2+) coordination with mean equatorial U-O distances of 2.42 and 2.40 A for the salicyl- and benzohydroxamate systems, respectively. DFT calculations on microsolvated model systems [UO(2)L(2)OH(2)] indicate UO(2)(2+) eta(2)-chelation via the hydroxamate oxygen atoms in excellent agreement with experimental data; calculated complex stabilities support that UO(2)(2+) prefers hydroxamate over carboxylate coordination. The 414 nm absorption band of UO(2)(2+) in aqueous solution is blue-shifted to 390 and 386 nm upon complexation by salicyl- and benzohydroxamate, respectively. Calculated time-dependent DFT excitation energies of [UO(2)L(2)OH(2)], however, occasionally fail to reproduce accurately experimental UV-vis spectra, which are dominated by UO(2)(2+) <-- L(-) charge-transfer contributions. We additionally show that the U(VI) large-core pseudopotential approximation recently developed by some of the authors can routinely be applied for electronic structure calculations not involving uranium 5f occupations significantly different from U(VI).
[show abstract][hide abstract] ABSTRACT: The groundwater bacterium Pseudomonas fluorescens (CCUG 32456) isolated at a depth of 70 m in the Äspö Hard Rock Laboratory secretes a pyoverdin-mixture with four main components (two pyoverdins and two ferribactins). The dominant influence of the pyoverdins of this mixture could be demonstrated by an absorption spectroscopy study. The comparison of the stability constants of U(VI), Cm(III), and Np(V) species with ligands simulating the functional groups of the pyoverdins results in the following order of complex strength: pyoverdins (PYO) > trihydroxamate (DFO) > catecholates (NAP, 6HQ) > simple hydroxamates (SHA, BHA). The pyoverdin chromophore functionality shows a large affinity to bind actinides. As a result, pyoverdins are also able to complex and to mobilize elements other than Fe(III) at a considerably high efficiency. It is known that EDTA may form the strongest actinide complexes among the various organic components in nuclear wastes. The stability constants of 1:1 species formed between Cm(III) and U(VI) and pyoverdins are by a factor of 1.05 and 1.3, respectively, larger compared to the corresponding EDTA stability constants. The Np(V)-PYO stability constant is even by a factor of 1.83 greater than the EDTA stability constant. The identified Np(V)-PYO species belong to the strongest Np(V) species with organic material reported so far. All identified species influence the actinide speciation within the biologically relevant pH range. The metal binding properties of microbes are mainly determined by functional groups of their cell wall (LPS: Gram-negative bacteria and PG: Gram-positive bacteria). On the basis of the determined stability constants raw estimates are possible, if actinides prefer to interact with the microbial cell wall components or with the secreted pyoverdin bioligands. By taking pH 5 as an example, U(VI)-PYO interactions are slightly stronger than those observed with LPS and PG. For Cm(III) we found a much stronger affinity to aqueous pyoverdin species than to functional groups of the cell wall compartments. A similar behavior was observed for Np(V). This shows the importance of indirect interaction processes between actinides and bioligands secreted by resident microbes.
[show abstract][hide abstract] ABSTRACT: The complexes of uranium(VI) with salicylhydroxamate, benzohydroxamate, and benzoate have been investigated in a combined computational and experimental study using density functional theory methods and extended X-ray absorption fine structure spectroscopy, respectively. The calculated molecular structures, relative stabilities, as well as excitation spectra from time-dependent density functional theory calculations are in good agreement with experimental data. Furthermore, these calculations allow the identification of the coordinating atoms in the uranium(VI)-salicylhydroxamate complex, i.e. salicylhydroxamate binds to the uranyl ion via the hydroxamic acid oxygen atoms and not via the phenolic oxygen and the nitrogen atom. Carefully addressing solvation effects has been found to be necessary to bring in line computational and experimental structures, as well as excitation spectra.
[show abstract][hide abstract] ABSTRACT: The unknown complex formation of Cm(III) with two hydroxamic acids, salicylhydroxamic (SHA) and benzohydroxamic acid (BHA) was studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Hydroxamate containing chelating substances have the potential to enhance the solubility and mobility of metals and radionuclides by forming complexes. We explored the fluorescence properties, lifetimes and individual fluorescence emission spectra of the formed Cm(III) hydroxamate species. In both Cm(III)–hydroxamic acid systems a 1:1 and a 1:2 complex of the type MpLqHr could be identified from the fluorescence emission spectra having peak maxima at 600 and 609nm, respectively. An indirect excitation mechanism of the Cm(III) fluorescence was observed in the presence of the hydroxamic acids. Consistent stability constants were determined by using either indirect or direct excitation mode of the Cm(III) fluorescence. In the Cm(III)–SHA system, the stability constants are logβ111=16.52±0.14 and logβ121=24.09±0.62. The complex formation constants of the Cm(III)–BHA species result to logβ110=6.52±0.19 and logβ120=11.60±0.50. The stability constants were compared to those of natural pyoverdins.
[show abstract][hide abstract] ABSTRACT: Fluorescent Pseudomonas species secrete pyoverdin-type siderophores with a high potential to dissolve, bind, and thus transport uranium in the environment. The formation of complexes of UO2 with pyoverdins released by the groundwater bacterium Pseudomonas fluorescens (CCUG 32456) isolated at a depth of 70 m in the Äspö Hard Rock Laboratory, Sweden, was studied. Mass spectrometry indicated that the cells produce a pyoverdin–mixture with four main components: pyoverdin with a succinamide side chain, pyoverdin with a succinic acid side chain, ferribactin with a succinamide side chain, and ferribactin with a glutamic acid side chain. Three pK values could be determined from the pH-dependent changes in the absorption spectra of the pyoverdin mixture: log ß012 = 22.67 ± 0.15 (pK1 = 4.40), log ß013 = 29.15 ± 0.05 (pK2 = 6.48), and log ß014 = 33.55 ± 0.05 (pK3 = 10.47). The fluorescence properties of the pyoverdin mixture were pH-dependent. The emission maximum changed from 448 nm at pH = 2.1 to 466 nm in the pH 3.8–8.9 range. At pH > 4 a mono-exponential fluorescence decay dominates with a decay time of 5865 ± 640 ps. A drastic change in the intrinsic fluorescence properties, e.g., static fluorescence quenching, occurred due to the complex formation with UO2 . Species containing UO2 of the type MpLqHr were identified from the dependencies observed in the ultraviolet visible and time-resolved laser-induced fluorescence spectroscopy spectra at pyoverdin concentrations below 0.1 mM. The following average formation constants were determined: log ß112 = 30.00 ± 0.64 and log ß111 = 26.00 ± 0.85 at ionic strength I = 0.1 M (NaClO4). The determined stability constants can be used directly in safety calculations of the mobilizing effect of released pyoverdins on uranium, in uranium-contaminated environments such as mine waste disposal sites.
Geomicrobiology Journal - GEOMICROBIOL J. 01/2008; 25:157-166.
[show abstract][hide abstract] ABSTRACT: The complex formation of uranium(VI) with salicylhydroxamic, benzohydroxamic, and benzoic acid was investigated by time-resolved
laser-induced fluorescence spectroscopy (TRLFS). We observed in all three systems a decrease in the fluorescence intensity
with increasing ligand concentration. All identified complexed uranyl species are of the type MpLqHr. In the uranium(VI)-salicylhydroxamate system a 1: 1 complex with a stability constant of log β
111 = 17.34±0.06 and a 1: 2 complex with a stability constant of log β
122 = 35.0±0.11 was identified. Also in the uranium(VI)-benzohydroxamate system the stability constants are determined to be
110 = 7.92±0.11 and log β
120 = 16.88±0.49. In the uranium(VI)-benzoate system only a 1: 1 complex is existent with a stability constant of log β
110 = 3.56±0.05.
Journal of Radioanalytical and Nuclear Chemistry 01/2008; 277(2):371-377. · 1.47 Impact Factor
[show abstract][hide abstract] ABSTRACT: Prevention of blood coagulation is very often a prerequisite for successful medical devices. For that purpose, passivation of the key coagulation enzyme thrombin through the derivatization of the material's surface with an amidine-based molecule has been found to be promising. To further enhance the efficiency of this approach, thin layers of maleic anhydride copolymers offering different physico-chemical characteristics were tethered with carboxyl terminated polyethylene glycol to covalently immobilize a benzamidine-type derivative. The free carboxyl surface groups produced by the attachment of polyethylene glycol (PEG) were quantified by Ag(+) labeling and subsequent XPS detection. The film thickness as well as the carboxyl group content were found to be clearly dependent on the copolymer hydrophobicity and the nature of the PEG molecule. For the assessment of the anchorage of the thrombin to the benzamidine-derivative functionalized surfaces, the substrates were immersed in a buffered thrombin solution and the enzyme adsorption was studied using immunostaining/confocal laser scanning microscopy. Higher degrees of thrombin binding were observed for substrates configured with the hydrophilic compared to the more hydrophobic copolymer. Moreover, surface-bound spacers based on alpha,omega-heterobifunctional PEG amino acids (alphaAm,omegaAc-PEG) also enhanced the benzamidine surface density in comparison to homofunctional PEG diacids (alphaAc,omegaAc-PEG) because of a lower degree of carboxyl inactivation due to PEG 'bridging'. Altogether, the choice of copolymer coatings and the type of PEG spacers were demonstrated to enhance the efficiency of the thrombin scavenging by the covalently immobilized coagulation inhibitor.
[show abstract][hide abstract] ABSTRACT: One of the urgent tasks in the field of nuclear technology is the final storage of radioactive substances. As a part of the safety requirements the protection of humans and the environment from the danger of radioactive substances in case of the release from the final storage is
essential. For performing long-term safety calculations the detailed understanding of the physico-chemical effects and influences which cause the mobilisation and transport of actinides are necessary. The presented work was a discrete part of a project, which was focused on the clarification of the influence of microorganisms on the migration of actinides in case of the release of actinides from a final storage. The influence of microbial produced substances on the mobilisation of selected actinides was studied thereby. The microbial produced substances studied in this project were synthesized by bacteria from the Pseudomonas genus under special conditions. Fluorescent Pseudomonads secrete bacterial pyoverdin-type siderophores with a high potential to complex and transport metals, especially iron(III). The aim of the project was to determine how and under which conditions the bioligands are able to complex also radioactive substances and therefore to transport them. For this work the alpha-emitting actinides uranium, curium and neptunium were chosen because their long-life cycle and their radiotoxicity are a matter of particular interest.
This work dealed with the interaction of the actinides U(VI), Np(V) and Cm(III) with model ligands simulating the functionality of the pyoverdins. The functional groups that participate in the metal binding of the pyoverdins are the catechol group of the chromophore and the
ligand sites in the peptide chain, i.e. the hydroxamate groups and the α-hydroxy acid moieties. For the simulation of the hydroxamate functionality the monohydroxamates
salicylhydroxamic acid (SHA) and benzohydroxamic acid (BHA) and the natural trihydroxamate desferrioxamine B (DFO) and for the simulation of the catechol groups 6-hydroxyquinoline (6HQ) and 2,3-dihydroxynaphthalene (NAP) were used. A further ligand with carboxyl functionality, benzoic acid (BA), was used as a comparison. Absorption spectroscopy, laser fluorescence spectroscopy, X-ray absorption spectroscopy and vibrational spectroscopy were applied for the determination of the stability constants to assess the strength of the formed actinide-model ligand-complexes, for the clarification of the structures of the formed complexes and to observe the variation of the speciation of the actinides during the interaction with the ligands. Furthermore, for the first time density functional theory (DFT) calculations were performed to determine the molecular structure of the actinide-modelligand-complexes. Thus, the objectives of this work were the determination of the spectroscopic properties, speciation and stability constants of the model ligands and the formed actinide-model ligand-complexes, the clarification of the complex structures and a
comparison of the results with those of the pyoverdins.
The comparison of the stability constants of the studied ligands with the three actinides U(VI), Cm(III) and Np(V) systems results mainly in the following order of complex strength:
PYO ≥ DFO > NAP > 6HQ > SHA ≥ BHA > BA.
Benzoic acid, the ligand with the carboxyl functionality, has the lowest stability constant of 103. Both monohydroxamates, SHA and BHA, form 1:1 complexes with similar stability. The stability constants of the 1:2 complexes of SHA with Cm(III) and Np(V) are slightly higher than those of BHA, which is probably caused by a stabilizing effect of the additional phenolic OH-group of SHA. This behaviour was also found in the theoretical calculations of the U(VI)-complexes. The natural siderophores DFO and PYO have the highest stability constants with
U(VI) and form the strongest complexes (constants from 1012 to 1034). The reason therefore is the structure and high number of functional groups of these ligands; DFO has three hydroxamate groups, the pyoverdin molecule has the catechol groups of the chromophore functionality in addition to the hydroxamate groups. The model ligands for the chromophore functionality, NAP and 6HQ, form stronger complexes than SHA and BHA, but weaker complexes than DFO and PYO. From this it can be reasoned that the chromophore
functionality probably plays an important role for the coordination of the actinides to the pyoverdins.
The comparison of the stability constants of the complexes of the ligands SHA, BHA and 6HQ with the studied actinides U(VI), Cm(III) and Np(V) shows that the strength of the complex formation decreases from U(VI) via Cm(III) to Np(V). The reason therefore is the different charge density of the actinide ions. The UO22+-ion has an effective charge of + 3.3 (with a coordination number of 5 and an ionic radius of ~ 0.6), the Cm3+-ion of + 2.6 and the NpO2+-ion of + 2.3. Therefore, the neptunyl ion has the lowest charge density of the studied actinide ions and on account of this it forms the weakest complexes with the lowest stability constants. The strength of the complex formation of the ligands NAP, DFO and PYO decreases from Cm(III) via U(VI) to Np(V). Cm(III) forms stronger complexes than U(VI) although Cm(III) has a lower effective charge. The reason therefore could be a possible
structural hampering of the coordination through the linear O=U=O unit.
The structure of the aqueous U(VI)-complexes was studied using EXAFS spectroscopy and FTIR spectroscopy.
From the results of the EXAFS spectra one can conclude that the coordination of the uranyl ion to the hydroxamic acid groups of the SHA, BHA and DFO ligands results in a shortening of the distance of the equatorial oxygen atoms. In contrast to this the coordination of the
uranyl ion to the carboxyl group of BA yields in a longer U-Oeq bond length. From the findings of the EXAFS studies with NAP and pyoverdin one can conclude a strong affinity of U(VI) to the catechol functionality of the pyoverdin molecule. For the observation of the complexation in the ATR-FTIR spectra the region around the vibration band of the uranyl ion (916 cm-1) is interesting to observe. In the spectra of the
U(VI)-BHA- and U(VI)-SHA-system a mixture of two complexes with 1:1 and 1:2 stoichiometry was observed, which was also existing in the speciation. Furthermore, on the basis of the vibration bands of the ligands it could be ascertained that the hydroxamate groups of SHA and BHA are deprotonated and directly involved in the complexation. Also, in case of SHA it could be verified that the phenolic OH-group is protonated at the investigated pH values. At pH 3 the pH dependent spectra of the U(VI)-DFO-system showed the formation of a 1:1 complex similar to those of the monohydroxamates. With increasing pH up to 4 the formation of a 1:1 complex was observed, in which the uranyl ion is bound to two hydroxamic acid groups. This underlines the assumption that the complex had a 112-stoichiometry, which was concluded on the basis of the other used experimental methods.
Solid phases of U(VI) complexes were assembled by precipitation from the aqueous U(VI)-SHA and U(VI)-BHA solutions. The structure of these powder solids was analyzed using EXAFS, XRD and FTIR. The analysis of the solid phases showed that the solid complexes are most likely consistent with the complexes in aqueous solution with 1:2 stoichiometrie. The comparison of the uranium and carbon percentage of the solids with those of the uranium compounds described in the literature (for the gravimetric estimation of uranium contents) results in analogue values. In the FTIR spectra of the solids vibration bands at 916 cm-1 were observed according to the bands of the 1:2 complexes in aqueous solution. The results of the EXAFS measurements indicated a different short-range order of the U(VI) in solid phases and solutions. The comparison of the structural parameters of the solid phases with those of the aqueous U(VI)-hydroxamate complex species points to strong differences. Thus, in aqueous solution the distance of the equatorial oxygen atoms of 2.41 Å is significant shorter than those
of the solid complexes with 2.47 Å (SHA) and 2.44 Å (BHA). The XRD measurements showed spectra high in reflexes and with significant peaks which could not be assigned to known U(VI) solid phases.
In a cooperation with the Institute of Theoretical Chemistry at the University of Cologne density functional theory (DFT) calculations were performed to determine the molecular structure of 1:1 and 1:2 U(VI)-complexes with SHA, BHA and BA. The precise molecular
structures of the complexes in gas phase have been calculated as well as the relative stabilities and the time-dependent DFT excitation spectra with consideration of the solvation effects. The relative stabilities calculated with DFT confirm the order of strength of the complexes determined using the stability constants log β (SHA ≥ BHA > BA). Furthermore, the higher binding energies of the 1:2 complexes point to a higher complex stability of these complexes in comparison to the corresponding 1:1 complexes. This could be also demonstrated by means of the stability constants determined by the experimental studies. The peak maxima of the TD-DFT excitation spectra deviate at 0.4 ± 0.2 eV from the absorption maxima of the experimental UV-vis spectra. Thus, calculated and experimental spectra show a good qualitative agreement. For the 1:1 complex of the U(VI)-SHA-system the structurally coordination of the uranium ion to the hydroxamate group could be clarified with the help of the theoretical modelling.
The comparison of the calculated structures, binding energies, bond lengths and excitation spectra of the two possible coordination modes [O,O] and [N,O’] showed clearly that the uranyl ion is bound preferable to the two oxygen atoms of the hydroxamate group ([O,O]-mode). Therefore, the method of DFT could contribute to eliminate shortcomings in the experimental determination of the complex structure in case of the U(VI)-SHA-system.
The model ligands and their complexes with U(VI), Cm(III) and Np(V) were characterized spectroscopically and their widely unknown stability constants were determined for the first time. Furthermore, the structures of the U(VI)-hydroxamate-complexes were clarified using
ATR-FTIR spectroscopy and theoretical calculations. The comparison of the results of the model ligands with those of the pyoverdins showed that the chromophore functionality of the pyoverdins probably plays an important role for the coordination of the actinides to the pyoverdins. Furthermore, conclusions to the strength of the formed actinide-model ligandand actinide-pyoverdin-complexes could be drawn from those results. The pyoverdins formed U(VI)-complexes with stability constants up to 1030, Cm(III)-complexes with constants up to 1032 and Np(V)-complexes with values up to 1020. The hydroxide ion OH- and the carbonato ion CO32- are the most important inorganic complexing agents in natural aquatic systems. They are highly concentrated and have great complexing ability. With the three studied actinides U(VI), Cm(III) and Np(V) complexes with stability constants from 102 to 1020 were formed. The comparison of the constants of OH- and CO3
2- with those of the organic microbial ligands showed that the pyoverdins complexes the actinides with similar and particularly higher strength than the inorganic complexing agents. Thus, it appears that the pyoverdins have a high potential to bind actinides and transport them in natural aquatic systems even though the pyoverdins exist in lower concentrations. Therefore, the studied bioligands are able to mobilize the actinides in natural aquatic systems, for example through
dissolving them from solid phases, if they are present in the nature in specific concentrations. So, such bioligands can essentially influence the behaviour of actinides in the environment. The results of this work contribute to a better understanding and assessment of the influence
of the microbial ligands to the mobilisation and migration of the radionuclides. The outcomes could be used to quantify the actinide-mobilising effect of the bioligands, which are released, for example, in the vicinity of a nuclear waste disposal site.