Search

During the past we have studied the complex formation behavior of uranium(VI) towards several aromatic carboxylic acids. These studies were continued, so that the formation constants of the monohydroxo- and dihydroxo- as well as some trihydroxobenzoic acids are available. A short review of these formation constants will be given. Besides this we have compared the complex formation constants to the protonation constants of the carboxyl group. A linear relationship between these two properties was found. An decreasing protonation constant is connected to an increasing complex formation constant. Nevertheless some exceptions of this rule were observed. These deviations can be explained by the different type of complex formation between uranium(VI) and the organic ligand. This concerns especially the number and position of the hydroxo groups in phenolic carboxylic acids. In the case of 2,3-dihydroxybenzoic the two OH-groups are involved in the complex formation already at relative low pH values. During the complex formation two protons are released from the organic ligand. The formed complex does not include the carboxylic group. The two phenolic hydroxo groups form a stable five atom ring with the uranylion. This result could be confirmed by EXAFS measurements.

Due to the sensitive detection of light fluorescence properties often are used to study the interaction of metal ions with various ligands. Several actinides as UO2++, Am3+ and Cm3+ show intense fluorescence properties and can be detected at very low concentrations. However, for neptunium and plutonium fluorescence properties are not known up to now. Nevertheless many organic ligands present in the environment as humic substances and wood degradation products fluorescence after excitation. To study the interaction of non-fluorescent actinides with these organic ligands fs-laser pulses were used for excitation. The light emitted from the ligand is observed by ICCD-cameras in a picosecond to nanosecond time scale. Using this TRLFS system the interaction of Np(V) with several small organic ligands was studied. As examples results will be presented for the interaction of Np(V) with 2,3-dihydroxybenzoic acid and 4-hydroxy-3-methoxybenzoic acid. For the interaction of uranyl with 2,3-dihydroxybenzoic acid a complex formation with the phenolic OH groups was found. In the case of Np(V) the interaction is very different. At pH values below 5.0 only the interaction with the carboxylic group can be observed and the interaction with the phenolic group starts at higher pH. This results in a 2:1 complex. Studies of the complex formation of Np(V) with 4-hydroxy-3-methoxybenzoic acid at pH 6 showed also a 2:1 complex, demonstrating the Np(V) interaction with the carboxylic group and the phenolic OH group. Nevertheless the fluorescence spectra did not show in any case a single component spectrum. This may be connected to other reactions of the non-complexed ligand. Therefore the influence of excited state reactions of the ligand will be discussed.

In a former study we analyzed the complex formation between uranium and 2,3-dihydroxybenzoic acid using the fluorescence properties of the ligand. The formation constant was found to be log K11 = -3.11 ± 0.16.We have validated this complex formation using the fluorescence properties of uranium. The formed complex has no fluorescence properties. This is confirmd by a monoexponetial fluorescence decay and also the fluorescence spectra show no shift in the emission maxima.However the fluorescence lifetime of the non-complexed uranium in the solution depends on the concentration of the added ligand. This is due to the dynamic quench effect of the ligand molecule. This quench effect is strongly correlated to the pH of the solution. This leads to the conclusion that only the protonated ligand effects this quench process. Taking this effect into account we are able to calculate the fluorescence intensities of the non-complexed uranium and by use of these data the complex formation between uranium and 2,3-dihydroxybenzoic acid. We obtained a one to one complex formation accompanied by the release of two protons from the ligand, as also found in the study of the fluorescence properties of the ligand. However the derived complex formation constants differ between both methods. The formation constant obtained from the uranium study was found to be log K = -3.99 ± 0.44. This is about 0.9 orders of magnitude lower compared to the constant derived from the study of the ligand. The confidence limits of the two data sets overlap nevertheless the deviation of the both constants leads to the conclusion that other reactions may be involved. From the uranium fluorescence we do not expect such reactions. But the non-complexed ligand may show separate reactions in the excited state, which lead to a change of the fluorescence intensity of the ligand and resulting in an increase of the calculated formation constant. The consequence of the inclusion of these reactions in the examination leads to an exact determination of the ligand concentration accompanied by a much better agreement of the assigned formation constants.