TRATAMIENTO DE INDUCCIÓN A LA REMISIÓN
II. A.5.4.2.TIPOS DE TPH
Edit Marosits1*, Bernhard Kuczewski1,2, Kerstin Wenzl1
1 Institute of Analytical Chemistry and Food Chemistry, Graz University of
Technology, Technikerstr. 4, 8010 Graz (AT)
2 Division of Nuclear Chemistry, University Cologne, Zülpicher Str. 45, 50674
Köln (DE)
* Corresponding author: [email protected]
Abstract
The sorption of iodine species by the well known KG1a-b kaolinite and humic acid (HS, Sigma Aldrich) was studied in NaClO4-solution within the framework of batch
experiments. The sorption-behaviour of two species was examined in the pH range of 2- 10. Both iodide and iodate had low sorption capacity on kaolinite: below 8% and 21%, respectively. The pH of the solutions affected the adsorbability of the species only at pH<4. The ternary systems were examined by adding HS to the already equilibrated iodine species - kaolinite systems. Compared to the binary systems, an additional sorption was observed for both species, which was higher for iodide (up to 26%) than for iodate (<14%).
Introduction
Iodine, an essential element, can be found all over in the biosphere and the environment. In order to assess the risks and long-term effects of the long-lived isotope 129I produced in reactors and deposited with the nuclear waste, it is important to understand the environmental behaviour of iodine. It is a redox sensitive element with several possible oxidation states (from -1 to +7). The migration and retention behaviour of the iodine is highly dependent on its redox form. The dominating species in aqueous phases is iodide, which has a higher mobility compared to iodate.
Within this framework the sorption behaviour of iodide and iodate by kaolinite and humic acid was investigated. Several studies reported already that though iodate has a
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much lower mobility than iodide, both of them have a low sorption capacity on kaolinite (Couture and Seitz (1983)), (Muramatsu et al. (1990)), Neal and Truesdale (1976)). On the contrary, organic matter seems to have an important role on iodine retention (Sheppard and Thibault (1992)). The studies about the iodine species and humic substances are very contrary. Muramatsu et al. observed considerable sorption by humic acid for both iodide and iodate (Muramatsu et al. (1990)). Other authors reported that there was no interaction between iodide and HS, while molecular I2 reacted with humic
acid without linear kinetics being observed (Ashworth et al. (2003)). The mechanism of retardation of iodine by humic substances is a hydrophilic substitution of hydrogen by iodine on a phenolic ring (Reiller et al. (2006)).
In this study the results of new batch experiments were compared to already existing data for binary systems and additionally ternary systems (iodine species – kaolinite – humic acid) were examined.
Materials and Methods
Samples
The iodide and iodate stock-solutions with a concentration of 10 g/L were prepared from NaI and NaIO3, respectively. They were stored in a dark place (normal
atmosphere, 22 °C).
The well-characterised KGa1-b kaolinite with a surface area of 11.7 m2/g (Pruett and
Webb (1993)) was used for the experiments. 0.1 M NaClO4 solution was prepared in
one batch and used for all samples in this work. The experiments were carried out under atmospheric conditions.
For ternary systems a 10 g/L HS solution was prepared using humic acid from Sigma Aldrich cleaned and treated according to Kim and Buckau (1988).
Sorption experiments
54 samples in dark glass bottles were prepared with 20 ml 0.1 M NaClO4 and a kaolinite
concentration of 4 g/L. These samples were divided into 6 test series each with pH values 2, 3, 4, 5, 6, 7, 8, 9, 10. The pH values were adjusted with HClO4 and NaOH for
two weeks until they were nearly stable. In order to have a concentration of 10 mg/L I-, IO3-, respectively,the half of the solutions were spiked with 20 µl of iodide solution and
the other half with 20 µl of iodate solution.
In order to calculate the loss from the pristine iodine species amount - the adsorbed quantity - “control standards” for iodide and iodate were prepared. For this reason the same bottles and same NaClO4 solution spiked with the iodine species were used
without the clay mineral. All samples were kept in a dark place (normal atmosphere, 22 °C) and were shaken 3 times a week by hand. After 14 days the pH of all samples was measured and 100 µl liquid near to the surface was pipetted off from each sample. These samples were diluted and the iodine content was measured by ICPMS. Then the binary systems and the “control standards” were spiked with different amounts of HS (50 mg/L, 100 mg/L, 200 mg/L) for each test series. The samples were shaken every second day and stored in the dark for another 14 days (normal atmosphere, 22 °C), after
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which the pH of all samples was determined and the iodine concentration measured the same way as before.
Analytical techniques
The pH value was determined by using an Orion 3 Star pH-meter (Thermo) with a refillable Mettler Toledo In®Lab Routine pH electrode. The iodine concentration in the samples was measured by the Agilent 7500 ICPMS and the adsorbed amount was calculated as the concentration difference between the “control standards” and the particular sample.
Results
Binary systems
The sorption of iodide and iodate by kaolinite was very low (Figure 1). In the case of iodide the adsorbed amount was between 0-8%. For iodate, in most cases it was around 10% but with higher values in the acidic area (up to 21%).
Figure 1: Binary systems: iodine species - kaolinite. The adsorbed amount of iodine species (in %) is plotted against the pH value of the solution at the time of measurement.
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Ternary systems
Adding humic acid to the already equilibrated binary system resulted in a further loss of iodine species from the solutions attributed to additional sorption, or other chemical reactions caused by humic acid.
In the ternary systems with iodide only the samples with pH<3.5 show a correlation between the adsorbed amount and the HS concentration in the solutions. On the contrary, the iodate – kaolinite – humic acid systems seem to have a clearly recognizable positive correlation between the adsorbed iodate amount and the HS concentration of the samples.
Figure 2: Ternary systems: iodine species - kaolinite - humic acid. All test series with different iodine species and HS concentrations are presented. The iodine species totally adsorbed by kaolinite and humic acid (in %) is plotted against the pH value of the solution at the time of measurement.
Summary and Conclusions
The results for the binary systems with little or no sorption of iodide and a low sorption capacity for iodate reinforced the data already reported by other authors (Couture and
Seitz (1983)). The low sorption can be explained with the fact that clay minerals
carrying negative charges push away anions (Muramatsu et al. (1990)), (Whitehead
(1984)). This also explains the slightly higher values for iodate in the acidic area. These
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(2006) reported that the low iodide uptake on argillaceous rocks was dependent of the exposure time of the samples with atmospheric oxygen during batch experiments. The ternary systems show an additional sorption attributed to humic acid, which is contrary to some earlier results (Reiller and Moulin (2003)). It may occur that molecular iodine was formed in the solutions and the iodine was bound on HS in these experiments. Further investigations with methods for structure examination like XPS or EXAFS can provide more detailed information about the sorption mechanism of iodine species in ternary systems.
Acknowledgement
We thank Prof. Dr. Martin Dietzel and Daniel Höllen for their help with the ICPMS measurements. We also express our gratitude to Laura Felgitsch who helped to complete the laboratory work.
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