RadChem 2018

I-129 and I-127 speciation in sea water by IC-ICP-MS and AMX

15 May 2018, 11:30

15m

Marble Hall (Casino Conference Centre)

Verbal Radionuclides in the Environment, Radioecology RER 1

Speaker

Mr Alex Hölzer (Institute for Radioecology and Radiation Protection, Leibniz Universität Hannover)

Description

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.