Iodine

Endpoint summary

Description of key information

Iodine is a naturally occurring inorganic element which is found in all compartments of the environment. Muramatsu and Yoshida reported that the valence states of the major iodine species in the environment are: iodide, I^-(-1), iodate, IO₃^-(+5), elemental iodine, I₂(+0), and methyl iodide, CH₃I (+1) (Muramatsu, 1999). Furthermore, several inorganic or organic iodine containing species are known depending on the compartment and prevalent conditions. Without consideration of the influence of organic substances and the biosphere on the speciation of iodine the dominant species in the hydrosphere, atmosphere and pedosphere can be estimated on basis of the Eh- and pH- values with molecular iodine and iodide playing the major role under common environmental conditions (Rucklidge, 1994).

Most of the elemental iodine (I₂) is produced synthetically from iodate and/or iodide salts. Aerobic and anaerobic biodegradation can be considered as negligible, and in principle only abiotic degradation processes are relevant for the environmental fate and transportation processes. Thus, hydrolysis in the aquatic compartment and photolysis in the atmosphere are key transformations.

Elemental iodine in contact with water is rapidly disproportionated into iodide and hypoiodite (IO^-) with the latter being further disproportionated to iodide and iodate. Although the second step is significantly slower a transformation of the major partition of molecular iodine can be assumed within the first hour of the degradation process (Truesdale, 1994). In addition to this inorganic transformation products with iodide being the major component (up to 90 % of total dissolved iodine, FOREGS database) iodine also rapidly forms organic species by reaction with dissolved organic matter, like humic acid etc.. Hypoiodous acid is considered as a key intermediate in the hydrolysis of iodine (Truesdale, 1994), and to be an important species for the reaction with organic matter in aqueous solutions due to observations in the marine environment (Truesdale, 1995).

In the ocean iodide and iodate are the dominant species with varying ratios depending on the depth and the geographic location. Although iodate is thermodynamically more stable under oxic conditions in a slightly basic solution like seawater, there seems to be a kinetic barrier which prevents a direct oxidation from iodide to iodate so that iodide persists in seawater (Truesdale, 1974, Sugawara, 1958). Aside from this kinetic barrier the speciation is affected by adsorption/scavenging onto metal oxides/hydroxides (Price, 1973; Price, 1977; Neal, 1976), and since iodine is a biophilic element also biologically mediated reduction and oxidation processes as well as incorporation into biogenic particles play a major role. Finally, exchange processes between bottom waters and sediments are very important for the geochemical fate of iodine (Wakefield, 1985; Bojanowski, 1970; Pedersen, 1980; Kennedy 1987a; Kennedy, 1987b; de Luca Rebello, 1990; Malcolm, 1984; Neal, 1976).

Castledine and Davis analyzed the iodine concentration in several seaweed species observing a high bioaccumulation which can be assumed to contribute to the iodine content of marine sediments (Castledine, 1973; Brehler, 1974). Further findings give indications that iodine is also incorporated by algae and transformed inter alia into iodinated volatile organic compounds (VOCs) (Burreson, 1976; Carpenter, 1999; McFiggans, 2002). The exact physiological function as well as the mechanism of uptake and metabolism is still unsolved. However, the release of this iodinated VOCs is one of the key processess in the geochemical lifecycle of iodine as the ocean is the major reservoir of available iodine and therefore being the major supplier of atmospheric iodine (Lovelock, 1973; Fuge, 1986; Ullman, 1990).

In addition, Amachi et al. (Amachi, 2001) found indications that a wide variety of terrestrial and marine bacteria are also capable of methylating iodine at environmental level of iodide (0.1 µM).

Released iodinated VOCs or molecular iodine are rapidly photolysed in the marine atmosphere. While the lifetime of molecular iodine is less than 10 seconds for overhead sun conditions (Saiz-Lopez, 2004; Jenkin, 1985), the lifetime of iodinated VOCs varies from 200 seconds to 90 hours (Roehl, 1997). Furthermore Duce et al. estimated that the mean lifetime of gaseous iodine before adsorption to a surface is about 30 minutes (Duce, 1965). In the marine boundary layer the transformation products of iodine and iodinated VOCs play a key role in the particle formation and the ozone depletion (McFiggans, 2000; Mäkelä, 2002; O'Dowd, 2002). Principally in these aerosols inorganic iodine oxides as well as soluble iodocarbons can be observed (Baker, 2005). Pechtl et al. found that iodate is increasingly depleted in acidic aerosols by inorganic reactions with decreasing pH value. Furthermore he assumed that HOI is formed in aqueous chemistry and then reacts with dissolved organic matter originated from the ocean. By combination of these organic and inorganic reaction cycles he was able to reproduce field observations, but constrained that each aerosol particle seems to be an "individual laboratory" with unique properties that may constantly change during its lifetime (Pechtl, 2007).

Gas-to-particle conversion is estimated to extend the atmospheric lifetime of iodine by about two weeks. Important parameters for the atmospheric aerosol lifetime are inter alia particle size, coagulation and scavenging processes, sedimentation, winds, etc.. In general, dry and wet deposition are the major loss processes of atmospheric iodine, and accordingly, these are main sources of iodine in soil. Another source are the decomposition processes of organisms. From a vast number of studies it is apparent that in general the iodine content varies between soil type and locality as it is influenced by a great number of factors (Ernst, 2003; FOREGS database; Muramatsu, 1999; Sheppard, 1992; Johnson, 1980), e.g. it can be observed that while in acidic soils and prevalent oxidizing redox potential iodide is being transformed in iodine which is readily volatile, in more basic soils iodine exists as non-volatile iodate. Based on the estimation of Rucklidge iodide is the major existent species of iodine in soil which is available for interactions with soil components.

Johnson analyzed about 200 soil samples for organic content and total iodine, and found a good correlation of high organic matter content and enrichment of total iodine for topsoil samples (Johnson, 1980). These findings are also supported by a great many of other studies concluding that soils rich in organic matter/humus are rich in iodine (Gallego, 1959a; Gallego, 1959b; Sinitskaya, 1969; Irinevich, 1970; Sazonov, 1970).

In addition, studies give evidence that soil iodine also binds to hydrous oxides of iron and aluminium. The extent of this process is depending on the pH of the soil and decreases at pH values above 7. Hence, it is assumed that hydroxyl ions are blocking the corresponding binding positions in soil and iodide could not be adsorbed (Whitehead, 1973; Whitehead, 1974). In general, findings indicate that iodine in soil will predominantly exist as soluble species and the content of organic matter as well as the presence of iron and aluminium hydroxides play the decisive role for the retention of iodine. In this context Muramatsu analyzed the influence of soil microorganisms on the behavior of iodine. It can be observed that microorganisms or their products play an important role in the accumulation of iodine as well as in the loss process from soil by evaporation. In saturated soils microorganisms support the development of reducing soil conditions. Within this process iodine is transformed into methyliodide which is readily volatile and barely soluble in water (Muramatsu, 1999; Amachi, 2001).

Ashworth et al. found evidence in a long-term adsorption/desorption experiment indicating that iodine within oxic environments is less mobile and, presumably, less bio-available than in anoxic environments (Ashworth, 2006).

These findings are supported by results of a one-year long-term laboratory migration-test in which an accumulation of iodine in the transition zone between anoxic and oxic soil conditions indicating a mobility of iodine only in the saturated/ low redox zone was observable. In contrast to the marine environment uptake by terrestrial plants can be considered as low, since only low uptake by ryegrass was found (Ashworth, 2003).

Newton and Shacklette also could not find a relationship between iodine concentration in plants and iodine content of the soils on which they grew (Newton, 1951; Shacklette, 1967).

Further loss mechanisms from soil are still quite unclear. Beside from evaporation it is assumed that already absorbed iodine is lost from soil by leaching of the binding material (organic matter, iron/aluminium hydroxides) into the groundwater or by decomposition of the organic matter which enables transformation and evaporation of iodine. Additionally, it is supposed that in case of soil saturation with iodine additional iodine will not be retained and will readily pass the soil compartment (Fuge, 1986).

 

  

References:

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