Speciation Analysis and Environmental Tracer Studies of ¹²⁹I

Iodine is well known as an essential nutrient element for mammals. It has evoked extensive interests in investigation of its behaviours and transportation processes in various environmental components. 129I, a long-lived radioisotope of iodine (1.57×107 years), is predominantly discharged to the environment from nuclear reprocessing plants (especially Sellafield in United Kingdom and La Hague in France), providing a unique point source for environmental tracing studies.

Speciation analysis of 129I can be used not only for studying the geochemical cycle of iodine, but also for investigating various environmental processes. This thesis presents the methods developed for chemical speciation analysis of 129I in environmental and biological samples (i.e. seawater, aerosols, and seaweed), as well as its applications for tracing environmental processes and investigating geochemical cycle of stable 127I in ocean, atmosphere and seaweed.

Based on the previous work, a persulfate oxidation decomposition method was further investigated for converting organic iodine to inorganic forms in natural water samples. This method was proved to be efficient and reliable for decomposing organic iodine, and has been successfully applied for determination of 129I in natural waters and leachates containing organic iodine species.

A method has been established for speciation analysis of 129I and 127I in aerosol samples collected on polypropylene filter. Sequential extraction using water and NaOH solution was employed to extract water-soluble iodine (WSI) and NaOH soluble iodine (NSI), and alkaline ashing for residue insoluble aerosol iodine (RII).

WSI was further partitioned to iodide and iodate using anion exchange chromatography. Parameters, such as amount of leaching agents used, leaching time, ashing time and temperature, and addition of iodine protecting reagent that might influence stability of iodine species and chemical yield of iodine species during chemical separation, were investigated in order to obtained accurate results of 129I species in aerosol.

The results on the analysis of total and different species of 129I in real samples demonstrate that the developed method is reliable for accurate determination of 129I and 127I species in aerosol samples. Distributions of chemical species (iodide and iodate) of 129I and 127I in the seawater from the central Arctic, Greenland coast, Danish coast and offshore Fukushima, Japan were investigated.

Iodate for both 129I and 127I is the predominant form for the seawater in the central Arctic and Greenland coast, whereas iodide is the major specie of 129I and 127I in Danish coast. A distinct distribution pattern of iodine species was observed in Fukushima offshore seawater, where the I-/IO3 ratios for 127I were in the range of 0.07-0.27 reflecting a dominant iodate in the seawater, but the I-/IO3 - ratios for 129I indicated that iodide is the major iodine specie.

These investigations demonstrate that variation of iodine species is dependent on origins of iodine (nuclear reprocessing plants and nuclear accident), transportation pathways (along the coast and open area of ocean) and biogeochemical processes. The remarkably increased 129I inventory in the Arctic by 2011 suggests that it may acts as a secondary source of 129I that outflows to the downstream seas, such as the Greenland Sea, and the western Atlantic Ocean, as well as to the atmosphere and the Arctic ecosystem.

Aerosol samples collected at two locations (Risø, Denmark, a coastal site, and Tsukuba Japan, about 170 km southwest of the Fukushima Dai-ichi nuclear power plant) during spring 2011 (shortly after the Fukushima nuclear accident) and winter 2014 were analyzed for iodine species of 129I and 127I including soluble and insoluble iodine.

The results indicate that the concentrations and species of 129I and 127I in aerosols are strongly related to the sources and pathways of air masses, and that Fukushima-derived 129I only contributed less than 6% of total 129I in the northern Europe during the accident period. Chemical species of 129I and 127I in seaweed (Fucus Serratus and Vesiculosus) collected in Danish coastal areas in 2014 were determined including water-soluble iodine (iodide, iodate and soluble organic iodine) and water insoluble iodine.

In combination of iodine species in seawater simultaneously collected with seaweed, possible mechanism of iodine uptake by seaweed was explored in natural marine system. - atomic