Experimental and theoretical study on the capture/desorption of gaseous methyl iodide on sea salt aerosols

Iodine-131, when released into the environment during severe nuclear power plant accident can have a high radiological impact on the population at short term [1]. Interaction between gaseous Iodine compounds and aerosols was not considered by the current post-accident management. In this context, this work was focused on investigating the influence of sea salt aerosols on the transport of gaseous methyl iodide (CH₃I). The identification of uptake processes as well as the formation of new products at the particle surfaces was the main objectives.

We have studied the interaction between NaCl particles as surrogate of sea salt particles and CH₃I in various humidity conditions to reproduce the atmospheric conditions.

The nature of this interaction was investigated by Infrared Spectroscopy (DRIFTS, Diffuse Reflectance Infrared Fourier Spectroscopy). Solid NaCl was exposed to CH₃I (1000 and 500 ppm) with a relative humidity (RH) ranging between 0 and 80%.

DRIFTS results clearly evidenced adsorbed CH₃I on NaCl particles surface under both dry and humid conditions. The adsorption process can be fitted with First-order Langmuir adsorption isotherm model and exhibited very low uptake coefficients in all the experimental conditions.

Additionally, to the CH₃I absorption bands, the DRIFT spectrum evidenced typical absorption bands that could be assigned either to the CH₂ deformation of CH₂I₂ or to CH₃ degenerate rocking of CH₃Cl. The formation of new bands appears only when CH₃I is in presence of halogenated salts. However, at RH = 80%, the water layer at the particle surface inhibits the interaction between gaseous CH₃I and NaCl surface due to the low solubility of CH₃I in water.

Theoretical calculations are carried out to complement the experimental results. Isolated hydrated clusters of CH₃I are characterized by means of electronic structure calculations and ab initio molecular dynamics is used to mimic the CH₃I / salt system at various humidities.

Although the uptake and accommodation coefficients of CH₃I are quite low, a coverage of particle surface with CH₃I-derived compounds may affect the reactivity of the particles and in term the cycling life of Iodine in the atmosphere.

Reference

[1] Lebel, L. S.; Dickson, R. S.; Glowa, G. A. J. Environ. Radioact. 2016, 151, 82–93.

We acknowledge support by the French government through the Program “Investissement d'avenir” through the Labex CaPPA (contract ANR-11-LABX-0005-01) and I-SITE ULNE project OVERSEE (contract ANR-16-IDEX-0004), CPER CLIMIBIO (European Regional Development Fund, Hauts de France council, French Ministry of Higher Education and Research) and French national supercomputing facilities (grants DARI x2016081859 and A0050801859).