Iodine

Endpoint summary

Additional information

Hydrolysis in the aquatic compartment and photolysis in the atmosphere play the decisive role for the environmental abiotic degradation.

When iodine gets in contact with water it rapidly degrades to a two-step disproportionation in iodide and iodate. Hypoiodous acid is assumed to be a key intermediate in this process (Truesdale, 1994). While the first step is a quasi-instantaneous concentration independent pseudo first-order reaction (Eigen, 1962) which can be assumed to be promoted by environmental pH values (by removing formed H^+-ions), findings on the second step indicate a pH and concentration dependent second-order process. Although the second step is significantly slower than the first equilibrium reaction a transformation of the major partition of molecular iodine can be assumed within the first hour of the degradation process (Truesdale, 1994). The relevant transformation products for the aquatic compartment are iodide (up to 90 % of total dissolved iodine, FOREGS database) and iodate as well as rapidly formed species of iodine and dissolved organic matter, like humic acid etc.. From kinetic and thermodynamic modelling there are indications that hypoiodous acid is the main iodine species reacting with organic matter at seawater pH of 8.2 (Truesdale, 1995).

Iodine in the gas phase is rapidly photolyzed by sunlight due to the strong absorption of visible wavelength. The lifetime of iodine is less then 30 seconds for overhead sun conditions (Saiz-Lopez, 2004; Jenkin, 1985). In absence of sunlight iodine transformation by reaction with free atmospheric radicals (OH, ozone, NOx etc) is assumed to be negligible (McFiggans, 2004). Although numerous studies have been conducted the full geochemical lifecycle of iodine in the atmosphere, particularly the conversion processes between gas phase and particle phase, is still not fully understood. However, iodine in aerosols is dominated by inorganic iodine oxides and soluble organic iodine species (Baker, 2005). In the gas phase basically methyl iodide (CH₃I) and diiodomethane (CH₂I₂) can be found. Beside of this major substances in the gas phase several other organic iodine species are known (Carpenter, 1999). Anyhow, particularly molecular iodine and to a minor degree diiodomethane are considered to be the key species for particle formation in the marine boundary layer (McFiggans, 2004). By gas-to-particle conversion the lifetime of atmospheric iodine is extended to several days before iodine is removed through dry and wet deposition.

Miyake and Tsunogai (Miyake, 1963) found indications that in marine surface waters photo oxidation of iodide ions to iodine occur. But nevertheless, as iodine is rapidly hydrolyzed to dissolved ionic species the degradation route of photo transformation of iodine in seawater is regarded as negligible. In soil are no information available on the occurrence of the photolysis process on soil surface, but due to the binding of iodine to organic soil compounds and the fact that iodine in soil is basically available as iodide this also can be considered as minor pathway (EU-DAR, public version, Potassium Iodide, 2008).

 

References:

Baker AR (2005). Marine Aerosol Iodine Chemistry: The Importance of Soluble Organic Iodine, Environ Chem, 2, 295-298.

Carpenter LJ, Sturges WT et al. (1999). Short-lived alkyl iodides and bromides at Mace Head, Ireland: Links to biogenic sources and halogen oxide production, J Geophys Res, 104(D1), 1679–1689, doi:10.1029/98JD02746.

European Commission, (2008) Draft Assessment Report (DAR) – public version – Initial risk assessment provided by the rapporteur Member State The Netherlands for the new active substance Potassium Iodide (of the review programme referred to in Article 8(1) of Council Directive 91/414/EEC.

Jenkin ME, Cox RA (1985). Kinetics study of the Reaction IO + NO₂ + M -> IONO₂ + M, IO + IO -> Products, and I + O₃ -> IO + O₂, J Phys Chem, 89(1), 192-199.

McFiggans G et al. (2004). Direct evidence for coastal iodine particles from Laminaria macroalgae – linkage to emissions of molecular iodine, Atmos. Chem. Phys., 4, 701–713.

Miyake Y, Tsunogai S (1963). Evaporation of iodine from the ocean, J. Geophys. Res., 68, 3989 -3993.

Saiz-Lopez A et al. (2004). Absolute absorption cross-section and photolysis rate of I₂, Atmos. Chem. Phys. 4, 1443-1450.

Truesdale VW, Canosa-Mas C, Luther GW (1994). Kinetics of Disproportionation of Hypoiodous Acid, J Chem Soc Faraday Trans, 90(24), 3639-3643.

Truesdale VW, Luther GW (1995). Molecular iodine reduction by natural and model organic substances in seawater, Aq. Geochem., 1, 89 -104.