Experimental and theoretical study on the capture/desorption of gaseous methyl iodide on sea salt aerosols
Authors: Toubin Céline , Houjeij Hanaa , Infuso Maxime , Anne-Cécile Gregoire , Gwenaelle Le Bourdon , Laurent Cantrel , Sonia Taamalli , Florent Louis , Denis Duflot , Sophie Sobanska 
Published: March 2020

&lt;p&gt;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 (CH&lt;sub&gt;3&lt;/sub&gt;I). The identification of uptake processes as well as the formation of new products at the particle surfaces was the main objectives.&lt;/p&gt;&lt;p&gt;We have studied the interaction between NaCl particles as surrogate of sea salt particles and CH&lt;sub&gt;3&lt;/sub&gt;I in various humidity conditions to reproduce the atmospheric conditions.&lt;/p&gt;&lt;p&gt;The nature of this interaction was investigated by Infrared Spectroscopy (DRIFTS, Diffuse Reflectance Infrared Fourier Spectroscopy). Solid NaCl was exposed to CH&lt;sub&gt;3&lt;/sub&gt;I (1000 and 500 ppm) with a relative humidity (RH) ranging between 0 and 80%.&lt;/p&gt;&lt;p&gt;DRIFTS results clearly evidenced adsorbed CH&lt;sub&gt;3&lt;/sub&gt;I 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.&lt;/p&gt;&lt;p&gt;Additionally, to the CH&lt;sub&gt;3&lt;/sub&gt;I absorption bands, the DRIFT spectrum evidenced typical absorption bands that could be assigned either to the CH&lt;sub&gt;2&lt;/sub&gt; deformation of CH&lt;sub&gt;2&lt;/sub&gt;I&lt;sub&gt;2&lt;/sub&gt; or to CH&lt;sub&gt;3&lt;/sub&gt; degenerate rocking of CH&lt;sub&gt;3&lt;/sub&gt;Cl. The formation of new bands appears only when CH&lt;sub&gt;3&lt;/sub&gt;I is in presence of halogenated salts. However, at RH = 80%, the water layer at the particle surface inhibits the interaction between gaseous CH&lt;sub&gt;3&lt;/sub&gt;I and NaCl surface due to the low solubility of CH&lt;sub&gt;3&lt;/sub&gt;I in water.&lt;/p&gt;&lt;p&gt;Theoretical calculations are carried out to complement the experimental results. Isolated hydrated clusters of CH&lt;sub&gt;3&lt;/sub&gt;I are characterized by means of electronic structure calculations and ab initio molecular dynamics is used to mimic the CH&lt;sub&gt;3&lt;/sub&gt;I / salt system at various humidities.&lt;/p&gt;&lt;p&gt;Although the uptake and accommodation coefficients of CH&lt;sub&gt;3&lt;/sub&gt;I are quite low, a coverage of particle surface with CH&lt;sub&gt;3&lt;/sub&gt;I-derived compounds may affect the reactivity of the particles and in term the cycling life of Iodine in the atmosphere.&lt;/p&gt;&lt;p&gt;Reference&lt;/p&gt;&lt;p&gt;[1] Lebel, L. S.; Dickson, R. S.; Glowa, G. A. J. Environ. Radioact. 2016, 151, 82&amp;#8211;93.&lt;/p&gt;&lt;p&gt;We acknowledge support by the French government through the Program &amp;#8220;Investissement d'avenir&amp;#8221; 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).&lt;/p&gt;