Speciation of radiocesium and radioiodine in aerosols from Tsukuba after the Fukushima nuclear accident.

Aerosol samples were collected from Tsukuba, Japan, soon after the 2011 Fukushima nuclear accident and analyzed for speciation of radiocesium and radioiodine to explore their chemical behavior and isotopic ratios after the release. Most (134)Cs and (137)Cs were bound in organic matter (53–91%) and some in water-soluble fractions (5–15%), whereas a negligible proportion of radiocesium remained in minerals. This pattern suggests that sulfate salts and organic matter may be the main carrier of Cs-bearing particles. The (129)I in aerosol samples is contained in various proportions as soluble inorganic iodine (I(–) and IO3(–)), soluble organic iodine, and unextractable iodine. The measured mean (129)I/(131)I atomic ratio of 16.0 ± 2.2 is in good agreement with that measured from rainwater and consistent with ratios measured in surface soil samples. Together with other aerosols and seawater samples, an initial (129)I/(137)Cs activity ratio of ∼4 × 10(–7) was obtained. In contrast to the effectively constant (134)Cs/(137)Cs activity ratios (1.04 ± 0.04) and (129)I/(131)I atomic ratios (16.0 ± 2.2), the (129)I/(137)Cs activity ratios scattered from 3.5 × 10(–7) to 5 × 10(–6) and showed temporally and spatially different dispersion and deposition patterns between radiocesium and radioiodine. These findings confirm that (129)I, instead of (137)Cs, should be considered as a proxy for (131)I reconstruction.