TRANSPORT OF IODINE IN THE ENVIRONMENT AND PATHWAYS INTO THE BIOSPHERE

Iodine is an essential element for humans being an important component of the thyroid hormone thyroxine; deficiency of dietary iodine leads to a series of diseases collectively known as iodine deficiency disorders or IDD. It is vital, therefore, to understand its geochemistry and more particularly its pathways into the biosphere. The oceans represent the largest reservoir of iodine on the Earth and virtually all iodine in the terrestrial environment derives from the oceans by way of the atmosphere. Iodine is volatilised from the sea in several forms with CH3I probably being the most important of these. This volatilised iodine is deposited on land by wet and dry precipitation and consequently soils from near coastal environments are enriched in iodine with soils remote from the sea being depleted. The traditional view of the distribution of IDD is coloured by this generalised view of the transport of iodine from the marine to terrestrial environments. However, the direct marine influence on the terrestrial environment may well be limited with concentrations of iodine in soils around 80 to 100 km from the coast being fairly similar to those from central continental regions. It is also assumed that iodine in soils is transferred to plants and that these, in turn, represent a major pathway of iodine into animals and humans. However, the soil to plant concentration factor for iodine has been shown to be low, due to the strong sorption of iodine by various soil components. Little iodine in soils has been found to be easily leachable and no correlation of soil and plant iodine has been demonstrated. In addition it has been shown that little iodine is translocated from the roots of plants to the aerial parts and it is likely that most is taken in from the atmosphere through the stomata, which would require iodine to be volatilised from soils in areas remote from marine influence. Such volatilisation may be limited due to iodine being strongly bound in soil. With IDD recently re-occurring in many affluent western European countries it is time to re-assess the environmental cycle of iodine.