The volatilisation and sorption of 129I in coniferous forest, grassland and frozen soils

129I is a potentially important radionuclide in safety assessments of proposed deep geological radioactive waste repositories due to its radiotoxicity, high mobility and long physical half-life (15.7 million years). In soils, iodine is present both in an inorganic form and in organohalide complexes, some of which are volatile under natural environmental conditions. This study has examined volatilisation, sorption and the effect of freezing on sorption and loss of 125I (physical half-life 60.2 days), as a surrogate for 129I, within coniferous forest and grassland soils. The results do not suggest that volatilisation from these soils is a significant pathway for the transport of 129I. Strong and specific sorption of iodine to humic substances has been demonstrated, which is reduced at freezing temperatures. It is hypothesised that rapid sorption to soil humic substances can significantly reduce volatilisation rates. The effect of freezing conditions on iodine extractability from soils suggests a microbially mediated sorption process.