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Abstract
Anthropogenic $^{129}$I release increased the $^{129}$I/$^{127}$I ratio by several orders of magnitude. In the 1960s, detonations of atmospheric nuclear weapons contributed to the $^{129}$I release; today the main sources are the reprocessing plants in La Hague and Sellafield. 90 % of $^{129}$I is released into the ocean, gaseous emissions account for only 10 %. Today, this continuous release has increased the $^{129}$I/$^{127}$I isotopic ratios from some 10$^{-12}$ in the prenuclear age to 10$^{-7}$ or higher in wide parts of Europe.
Numerous measurements of water samples from the English Channel to the North Sea were published. Total activity concentrations of $^{129}$I and isotopic ratios in sea water are well known. However, findings by Hou et. al.[1, 2] indicate disequilibria of the chemical species containing $^{129}$I compared to those containing $^{127}$I. The present work aims at understanding chemical speciation of the released $^{129}$I. Chemical reactions and kinetic aspects of iodine chemistry in sea water are modelled.
Water samples were taken in the vicinity of La Hague. In addition to total $^{129}$I and $^{127}$I, chemical speciation was investigated at trace concentrations, differentiating iodide, iodate and organo-iodine species.
The results show higher contents of $^{129}$I in sea water around La Hague compared to measurements from the North Sea. In addition, the $^{129}$I content was higher at the southwest coast of La Hague as compared to the northeast coast. $^{129}$I/$^{127}$I ratios of 10$^{-6}$ up to 10$^{-5}$ were found for total iodine and iodate, while $^{129}$I/$^{127}$I ratios of 10$^{-7}$ to 10$^{-6}$ were measured for the iodide fraction. Iodate/iodide ratios show lager variation for $^{127}$I with values between 2.6 to 7.4 compared to $^{129}$I with a range from 1.6 to 4.5. In addition, iodate/iodide ratio of $^{129}$I decreased with the distance to La Hague, while the iodate/iodide ratio of $^{127}$I did not show any correlation with the distance. Further indication for organic bound iodine co-eluting with iodide was found.
In addition, laboratory batch experiments were performed to simulate the behavior of radioactive $^{129}$I. Different iodine chemistry in sea water compared to deionized water was studied with $^{125}$I as iodide in a first experiment. A second experiment was performed with iodate and iodide in synthetic sea water to compare the behavior of the two species. In a third step, organic molecules were added as possible reaction partners for the iodine species.
An equilibrium between iodate and iodide was obtained. Iodate spiked solutions contained 10 to 20 % of iodide. Similarly, iodide spiked solutions contained 10 to 20 % iodate as measured by $^{125}$I gamma ray spectrometry.
References
[1] X. Hou, Chemical Speziation of Long-Live Radionuclides in the Environment. Technical University of Denmark, Roskilde, 2008.
[2] X. Hou, A. Aldahan, S. P. Nielsen, G. Possnert, H. Nies, J. Hedfors, Speciation of $^{129}$I and $^{127}$I in Seawater and Implications for Sources and Transport Pathways in the North Sea, Journal of Environmental Science and Technology 2007, 41, 5993-5999.
