Bernhard Kuczewski1,2,*, Hana Tatjana Dittrich2, Corinna Dully2, Markus Lagos3, Edit Marosits2, Christian M. Marquardt3
1 Division of Nuclear Chemistry, University of Cologne (DE) 2 Institute of Analytical Chemistry and Food Chemistry,
Graz University of Technology (AT)
3 Institute of Nuclear Waste Disposal, Forschungszentrum Karlsruhe (DE)
* Corresponding author: [email protected]
Abstract
Speciation methods for iodine and uranium were developed, to obtain additional data for the determination of groundwater’s redox state. By capillary electrophoresis iodide and iodate as well as U(VI) and U(IV) were separated. The developed methods are described as well as the application for iodine species to nearly natural samples.
Introduction
The measurement of redox parameters seems to be easy but especially in systems like groundwater it can become difficult. The determination of redox potential requires equilibrium, which is seldom fulfilled due to slow kinetics of important redox reactions that have great influence on the potential. In many cases only electrochemical and optical sensors are used. Applying speciation methods additional data can be obtained. This will help to improve the reliability of the sensors data.
The use of separation techniques combined with sensitive detectors allows the determination of several redox ion pairs like FeII/III, UVI/IV or I-/IO3-/I2. Ion
chromatography (IC) and capillary electrophoresis (CE) was verified for the determination of the iodine species. However, for uranium only CE was applied which separates the ions in an electrical field by their charge to radius ratio. In this case both -
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diode array (DAD) and inductively coupled plasma mass spectrometry (ICPMS) - could be used for detection.
Method development
Iodine speciation
For the method development we have prepared solutions of the sodium salts of iodide and iodate that were stored at a dark cold place (refrigerator at +6°C). The stock solutions were diluted by different media, to prove the effects of synthetic and nearly natural sample media (e.g. high saline synthetic groundwater). While the development of the ion chromatography was up to now not completed, the CE-DAD method works well. Table 1 gives the final conditions for the separation of the iodine species by the CE-DAD system. Figure 1 shows the electropherogram of a sample containing iodide and iodate.
Table 1: Separation conditions for iodine species with CE-DAD
instrument Agilent 3D CE
DAD 190-400 nm
capillary fused silica,
50µm ID, 60 cm length electrolyte 100 mM acetic acid
injection 30 mbar, 10 s separation -15 kV, 0 mbar complete in 15 min ‐1 0 1 2 3 4 5 6 7 8 9 0 2 4 6 8 10 time [min] in te n sit y [a .u .] iodide iodate iodine and neutral species (EOF)
173 calibration of iodate in 1M AcOH 0 0,5 1 1,5 2 2,5 3 0 10 20 30 40 50 60 concentration (mg/L) pe ak ar ea calibration of iodide in 1M AcOH 0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0 10 20 30 40 50 60 concentration (mg/L) pe ak ar ea
Figure 2: Calibration of iodide and iodate obtained by CE-DAD
The obtained calibrations for iodide and iodate in 1 M acetic acid are plotted in figure 2. The calibration has to be fitted individually, because in some cases the peak shape is strongly modified by the media. Also we have to consider that the linearity of the calibration for iodide is limited. The CE-DAD method provides information between 0.5 and 20 mg/L. Above a concentration of 20 mg/L the peak runs over into the signal of the neutral species.
Uranium speciation
The U(IV) solution was prepared by electrochemical reduction of uranyl nitrate dissolved in hydrochloric acid (~0.8 M). The U(VI) solution (25mg/L) was reduced at - 10 V during 45 minutes Delécaut (2004). The electrolysis cell was cooled in an ice bath. As electrodes simple platinum coils were used. To prevent the precipitation of the U(IV) the pH was kept at 0.7. The solution was checked several times by UV/Vis spectroscopy during the reduction. 5 minutes after the spectra showed a pure U(IV) solution the electrolysis was stopped and the stock solution was stored under oxygen free argon atmosphere.
Table 2: Separation conditions for uranium species with CE-ICPMS
CE instrument home made
ICPMS instrument Agilent 7500 ce
capillary fused silica,
50µm ID, 65 cm length electrolyte/buffer 100 mM acetic acid
10 mM Na2EDTA
pH ~ 3.5 injection 100 mbar, 10 s separation -25 kV, 150 mbar
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The CE was a home made system coupled to an Agilent 7500 ICPMS. The conditions are given in table 2. Figure 3 depicts the electropherogram of a sample containing U(VI) and U(IV).
Figure 3: Electropherogram of U(VI) and U(IV), separated by CE-ICPMS, Cs used as internal standard
Several mixtures of U(VI) and U(IV) with different ratios were tested, to ensure the reliability of this separation. Therefore the stock solutions were mixed and instantly diluted in the buffer. To ensure that the complexation prevents the fast oxidation of U(IV) a mixture of both uranium redox species was measured after 10 and 80 minutes (storage at normal atmosphere). The effect of 1% change is within the error of the method which is around ± 5%. Details are given in table 3.
Table 3: Time dependence of uranium species at normal atmosphere, determined with CE-ICPMS
time U(IV) U(VI)
expected 83 % 17 %
10 min 81 % 19 %
80 min 80 % 20 %
Method application
While testing the calibration we have recognised that some redox reactions took place in the standard solutions. In a 0.05 M HClO4 stock solution iodide and iodate were mixed
and measured regularly by our method to achieve better statistics. Iodide though was reduced significantly to iodine, while iodate seemed to be more stable. The time dependence is shown in figure 4. This observation indicates the problem with the determination of the redox state. The redox equilibrium can be disturbed very easily. All
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solutions have to be stored under stable conditions (inertgas box, oxygen free, dark to prevent iodine photochemistry, at constant temperature …) in order to avoid redox measurement artefacts. The method opens the possibility to determine redox reactions, but for standard measurements like calibration samples, it is necessary to avoid the contact of the different iodine species.
0 10 20 30 40 50 60 70 80 90 100 110 0 10 20 30 40 50 time [min] re la ti ve sp e ci e s co n te n d [% ] iodide iodate
Figure 4: Redox effects in standard solutions (mixture of iodine and iodate in 0.05 M perchloric acid
The method was also applied to several other systems, in which the iodine species had contact with clay minerals (Opalinus clay, Wyoming kaolinite KGa1b) in a highly saline synthetic pore water Pearson (1998). The details are given in table 4. The time dependent sorption of iodate is shown in figure 5. The iodine species which were not absorbed on the clay were determined by CE-DAD. In these systems no conversion of the species into each other was observed. The iodide concentrations were below the limit of detection of the CE-DAD. The determination of iodine was not possible because in the nearly natural samples too many other neutral species were present. But they don’t have characteristic spectra. Therefore the DAD cannot differentiate between them.
Table 4: Conditions for the near nature sorption experiments
synthenic pore water analog Pearson (1998), except SrCl
mass clay 80.5 ± 0.5 mg
volume 20 ml
initial iodine concentration (iodine in
iodide and iodate)
100 mg/L
pH value (opalinus clay) 7.08 ± 0.5 pH value (KGa1b) 5.85 ± 0.7
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iodate / synth porewater
0 5 10 15 20 25 30 0 50 100 150 200time [h]
co
n
cen
tr
at
io
n
[m
g/
L]
opalinus clay kaolinite KGa1bFigure 5: Time dependent sorption of iodate onto kaolinite and Opalinus clay, determined by CE-DAD
Summary and Conclusions
In principle the method for iodine works well, but with the sensitivity of the diode array detector is mostly insufficient for environmental conditions. Furthermore iodine can not be separated from other neutral analytes. Therefore transferring the iodine method to the CE-ICPMS would be useful, but it was not available during these experiments.
Also the samples from the Interlaboratory Comparison Exercise have to be measured by the CE-ICPMS in order to show the comparability and reliability of this method. The method for uranium works well for standard samples and has to be tested with synthetic and natural samples. For plutonium and neptunium the method is in routine operation
Bürger (2007), Ambard (2005), Kuczewski (2003). In the future a speciation method for
iron will be developed and compared to other measurement methods, too.
Acknowledgement
The authors thank R. A. Buda, E. Gromm, T. Wunderlich and J. V. Kratz from the Institute for Nuclear Chemistry of Johannes Gutenberg-University of Mainz for the collaboration and hospitality during the development for the uranium speciation method.
177 References
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Bürger S, Banik N L, Buda R A, Kratz J V, Kuczewski B, Trautmann N, (2007), Speciation of the oxidation states of plutonium in aqueous solutions by UV/Vis spectroscopy, CE-ICP-MS and CE-RIMS Radiochim. Acta 95, 433–438.
Delécaut G, Maes N, De Cannière P, Wang L, (2004), Effect of reducing agents on the uranium concentration above uranium(IV) amorphous precipitate in Boom Clay pore water Radiochim. Acta 92, 545–550.
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Pearson F J, (1998), Opalinus Clay experimental water: A1 Type, Version 980318. PSI Internal report TM-44-98-07, Paul Scherrer Institut, Villigen PSI, Switzerland.
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