Atmospheric iodine formation

As part of the biogeochemical cycle, the injection of iodine-containing gases into the atmosphere, and their subsequent chemical transformation therein, play a crucial role in environmental and health aspects associated with iodine - most importandy, in determining the quantity of the element available to the mammalian diet. This chapter focuses on these processes and the variety of gas- and aerosol-phase species that constitute the terrestrial iodine cycle, through discussion of the origin and measurement of atmospheric iodine in its various forms ( Sources and Measurements of Atmospheric Iodine ), the principal photo-chemical pathways in the gas phase ( Photolysis and Gas-Phase Iodine Chemistry ), and the role of aerosol uptake and chemistry and new particle production ( Aerosol Chemistry and Particle Formation ). Potential health and environmental issues related to atmospheric iodine are also reviewed ( Health and Environment Impacts ), along with discussion of the consequences of the release of radioactive iodine (1-131) into the air from nuclear reactor accidents and weapons tests that have occurred over the past half-century or so ( Radioactive Iodine Atmospheric Sources and Consequences ). 

The major impact of atmospheric iodine chemistry is the resultant depletion of O3, while other consequences, such as enhanced cloud formation, remain to be established. 

As has been discussed in the preceding sections, the containment of a nuclear power plant represents a very effective passive barrier, at least for a certain period of time, for confining the radionuclides released from the reactor core and from the primary system in a severe accident. This renders possible their plate-out from the containment atmosphere by natural processes such as deposition of aerosols or, as regards iodine, formation of non-volatile compounds, as well as by the action of engineered safety features (e. g. sprays). Nevertheless, in safety considerations it has to be assumed that the containment is not a permanent and absolutly tight confinement, but that there would be several fundamental mechanisms by which a certain fraction of the radionuclides could escape from the containment. The most important of these mechanisms are... 

The sulphites, both normal and acid, are easily oxidised, and in solution readily undergo atmospheric oxidation with the formation of sulphates. The oxidation proceeds more readily in neutral than in acid solution,2 and is accelerated by warming. The change in SOa-content of a solution of potassium metabisulphite (0-1 per cent.) kept in a stoppered bottle, observed by titration at intervals of aliquot portions with standard iodine solution, has been observed to be as follows 3... 

The rather fast reaction rate of halomethanes with Cl atoms suggests that this process may play a primary role in the removal of halomethanes from the troposphere and results in the formation of HC1 or 1C1 molecules. These degradation pathways do not lead to bromine or iodine atoms but to relatively stable molecules, which may initiate a different bromine and iodine cycles in the marine boundary layer. The atmospheric lifetime of IC1 is probably controlled by its sunlight photodissociation to iodine and chlorine atoms. Another possible degradation pathway of IC1 may be the hydrolysis to hypoiodous acid IOH, which may further be dissolved in seawater. 

A solution of 10 mL of borane-methyl sulfide complex (10 M BH3 in methyl sulfide) in 75 mL THF was placed in a He atmosphere, cooled to 0 °C, treated with 21 mL of 2-methylbutene, and stirred for 1 h while returning to room temperature. This was added directly to the crude 4-ethoxy-3-ethylthio-5-methoxystyrene. The slightly exothermic reaction was allowed to stir for 1 h, and then the excess horane was destroyed with a few mL of MeOH (in the absence of air to avoid the formation of the dialkylboric acid). There was then added 19 g of elemental iodine followed,... 

A volatile iodine species, neither elemental nor organic, which has been found in steam/air atmospheres, has been identified as hypoiodous acid (HOI) (Cartan et al., 1968). In water-cooled power reactors, any fission products released from fuel will pass into hot alkaline water and thence to a steam-air mixture. These conditions are thought to favour the formation of HOI (Keller et al., 1970), but the evidence is indirect. For example, tests for elemental iodine or iodine with an oxidation state higher than that of HOI gave negative results. 

In Nature there is an iodine cycle and indeed thousands of tonnes of iodine escape from the oceans every year as iodide in sea spray or as molecules containing iodine, which are produced by marine organisms. Marine algae emit volatile iodine as iodomethane (CH3I) and diiodomethane (CH2I2) and these may even moderate the world s climate by helping cloud formation. Some of this iodine is deposited on land where it may become part of the bio-cycle. Recently it has been shown that rice plants also emit iodomethane and that this accounts for about 4% of that which is present in the atmosphere. 

In another industrial process, iodine is heated above its melting point (113°C) and reacted with elemental fluorine under three atmosphere of pressure. A precaution is taken not to heat iodine above 150°C to avoid the formation of another product, IF7. Fluorine is added continuously until all iodine has reacted and converted to IF5 [66],... 

When 5,6-anhydro-l,2-0-isopropylidene-a-I>-glucofuianose (67) in ether was allowed to react with carbon monoxide (12 atmospheres, at room temperature) in the presence of sodium cobalt tetracarbonyl for 3 days, the stoichiometric amount of carbon monoxide was absorbed. The mixture was cooled to —5 , and subsequent treatment with methanol and iodine by the procedure of Heck and Breslow resulted in the formation (in 80 % yield) of the methyl uronate (70) and, in a yield of about 10%, the 6-deoxy-hexos-5-ulose (69). Reduction of the methyl uronate (70) and of the dialdose derivative (68) with lithium aluminum hydride yielded identical sugars. 

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