The marine hydrosphere has high concentrations of halogens, with iodine being the least common and chlorine the most. Halogens, including iodine, are concentrated by various species of marine organisms such as macroalgae and certain seaweeds. Release from these organisms makes a major contribution to the atmospheric concentration of the halogens. Iodine is present as the least abundant halogen in the Earth's crust. It is likely that in primordial times the concentration in surface soils was higher, but today the iodine content of soils varies and most has been leached out in areas of high rainfall or by previous glaciation. Environmental degradation caused by massive deforestation and soil erosion is accelerating this process. This variability in soil and water iodine concentration is quite marked, with some valleys in China having relatively high iodine concentrations in water, and other parts of China with negligible amounts in soil and water. Table 1
shows the relative abundance of various halogens in the natural environment, while Figure 1 illustrates the cycle of iodine in nature.
Commercial production of iodine occurs almost exclusively in Japan and Chile, with iodine extracted from concentrated salt brine from underground wells, seaweed, or from Chilean saltpetre deposits.
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