Interaction process between gaseous CH3I and NaCl particles: implication for iodine dispersion in the atmosphere.

Hanaa Houjeij,Anne-Cécile Gregoire,Laurent Cantrel,Gwenaëlle Le Bourdon,Sophie Sobanska

doi:10.1039/d1em00266j

Abstract

Gaseous iodomethane (CH3I) is naturally emitted into the atmosphere by biological activity in oceans and during severe accidents (SAs) in nuclear power plants. In this latter case, a part of radioactive iodine such as 131I may be released. Improving the knowledge of CH3I transport and reactivity in the atmosphere is important since they are strongly linked to first the cycle of ozone and second to the dispersion of radioactive CH3I with potential radiological consequences on both the environment and human health. Here, the interaction process of CH3I with NaCl as a surrogate of atmospheric aerosols was investigated under ambient air conditions by using Diffuse Reflectance Fourier Transform Spectroscopy (DRIFTS). The DRIFTS spectra of NaCl clearly evidenced CH3I adsorption on the NaCl particle surface. A part of CH3I ((1.68 ± 0.85) × 1014 molecule per mgNaCl) was found to be strongly bonded to NaCl since no desorption was observed. The CH3I adsorption on the NaCl surface presented a 1st order kinetics relative to its gas phase concentration. The uptake coefficient was determined to be in the order of 10-11. These results show a low probability of CH3I to be taken up by halide-containing aerosols. These data are crucial for completing the iodine atmospheric chemical scheme.

Highlights

Gaseous organic iodine species (CH3I, 127I) are naturally emitted at trace level into the atmosphere over oceans through the algae and phytoplankton activities [1,2]
CH3I adsorption was clearly observed at low humidity level NaCl (%relative humidity (RH)=20% at 23°C)
In addition to the adsorbed CH3I bands already referenced in the literature, the Diffuse Reflectance Fourier Transform Spectroscopy (DRIFTS) spectra revealed the presence of new strong vibrational bands at 1073 and 1024 cm-1

Summary

Introduction

Gaseous organic iodine species (CH3I, 127I) are naturally emitted at trace level (maximum at 2000 ppt) into the atmosphere over oceans through the algae and phytoplankton activities [1,2]. It should be noted that local meteorological conditions can cause a great variability in activity concentrations in environmental media and can result in locations further away being higher in concentration than closer locations [17-20] With this intention, the French Institute of Radioprotection and Nuclear Safety (IRSN) has developed spécifie simulation software's (C3X plateform) which is capable of reproducing the release and dispersion of the radionuclides in the atmosphere [21-23]. One potential problem with such simulation tool is that the transport of 131-Iodine products following an accident is currently modelled without considering the physical/chemical evolution of iodine in the atmosphere, i.e. the gas phase evolution or interaction with atmospheric aerosols even if recent works 24 focused on organic reactivity in the field of nuclear accident. This is worrisome because the simulated atmospheric dispersion of radioactive isotopes of iodine [20,25,26] may be quite different from the actual one due to the different possible fates of iodine within the Earth's atmosphere

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Results

Conclusion