I-129 is produced in the lithosphere primarily by spontaneous fission of U-238. As the uranium content of lignite can be quite high (up to 2500 ppm), a considerable amount of fissiogenic I-129 may have accumulated since the formation of the deposit, provided that it can be regarded as a closed system. In this case, the concentrations of I-129 and U-238 could be used for dating the lignite layers. However, as iodine adsorbs very well to coal, the lignite may represent a sink for iodine carried by groundwater. Also anthropogenic I-129 (released in large quantities from nuclear fuel reprocessing plants) may have contaminated old lignite layers. The aim of our investigation was to establish a procedure for the determination of the I-129/I-127 isotopic ratio in lignite and, in a first step, to decide whether or not lignite is polluted by anthropogenic I-129. We used fresh material from surface mining, and also archived material from sub-surface mining in 1967.
Since laboratory intercomparison efforts on I-129 have led to unsatisfactory results in organic samples [1], we investigated various methods of iodine extraction. After combustion of the crushed lignite, inorganic iodine was collected in KOH, with Na2SO3 or hydrazine as reducing agent, and then precipitated as AgI; organic iodine was trapped on charcoal and eluted with chloroform. A few samples showed rather high contents of sulfur; it had to be separated as NiS prior to the AgI precipitation. The iodine content of solutions from different stages of the procedure was determined with an ion-specific electrode. The uranium content of the samples was determined by alpha counting. Finally, the I-129/I-127 ratio was determined by AMS at the Vienna Environmental Research Accelerator (VERA).
Compared to the I-129/I-127 isotope ratio of 10-13 predicted from a geochemical model [2] for a closed system, our values from samples stemming from two different sites in Austria are in the order of 10-11, which suggests that substantial pollution by anthropogenic I-129 is present. From systematic investigations on laboratory blanks, it seems likely that the high I-129/I-127 contamination happened already in the natural environment.
[1] M.L. Roberts, M.W.Caffee. I-129 interlaboratory comparison: Phase II results. Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 388-394.
[2] J.T. Fabryka-Martin, H.Bentley, D. Elmore, P.L. Airey. Geochimica et Cosmochimica Acta 49 (1985) 337-347.
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