Determination of ¹²⁹I in aerosols using pyrolysis and AgI-AgCl coprecipitation separation and accelerator mass spectrometry measurements

Airborne radioactive iodine is a key concern for the transport and dispersion of radioactive contamination and radiation exposure evaluation during nuclear accidents and nuclear emergency preparedness. Information about long-lived ¹²⁹I in aerosols is vital for the reconstruction of the level and distribution of short-lived and highly radiotoxic ¹³¹I, as well as for understanding the atmospheric cycling of iodine. However, aerosol ¹²⁹I concentration is difficult to measure due to its low concentration in remote areas, away from nuclear pollution sources. In this study, a novel method for the determination of ¹²⁹I in aerosols collected on a glass fiber filter was developed using high-temperature pyrolysis and AgI-AgCl coprecipitation for separation coupled with highly sensitive accelerator mass spectrometry (AMS) measurements. It is worth noting that even though the pyrolysis behaviors of various iodine species were investigated and found to be different, all of the iodine can be quantitatively recovered. Iodate was released from the aerosols through its decomposition to iodine at temperatures over 500 °C. The chemical yield of iodine during pyrolysis was 81.5 ± 5.8%. The detection limit for ¹²⁹I in the aerosol samples was 1.3 × 10⁴ atoms per m³, at least reducing the required aerosol sample size by a factor of three, in contrast to the method using alkaline-ashing separation coupled to solvent extraction. For aerosol samples collected in Asia with ¹²⁹I/¹²⁷I ratios of (0.1-10) × 10⁻⁹, a volume of 1000 m³ air is sufficient for the determination of ¹²⁹I. The developed method was used to analyse aerosol samples collected in Xi'an, an inland Chinese city. It is observed that ¹²⁹I concentrations ranged within (0.38-5.19) × 10⁵ atoms per m³, with ¹²⁹I/¹²⁷I ratios of (21.7-252) × 10⁻¹⁰, which are comparable to those collected in Japan before the Fukushima nuclear accident and Spain, while much lower than those observed in Northern Europe.