Volatilization: a soil degassing coefficient for iodine

Abstract

Iodine, an element essential to some animals, is ubiquitous in the biosphere. Unlike other metallic elements, molecular I is volatile, and other inorganic species present in aerated soils, such as I − and IO 3 −, may also volatize as hydrides, hydrogen iodide (HI), or hydrogen iodates (HIO 3, HIO 4). Methyl iodide has been measured in soils, and it is likely evolved from soils and plants. The long-lived radioisotope 129I is abundant in nuclear wastes, and its high solubility in groundwater makes it an important element in the performance assessment of underground disposal facilities. Overestimates of soil I residence half-times by traditional foodchain models may be due to underestimation of volatilization. Field and lysimeter experiments over a 3-year period, and direct trapping experiments in the laboratory are reported. The results, combined with values from the literature, indicate the soil I degassing coefficient for a wide range of soil types, vegetated and bare, wet and dry, is lognormally distributed with a geometric mean of 2·1 x 10 −2 year −1, a range of 1·8 x 10 −4 to 1 year −1 and a geometric standard deviation of 3·0. The results of a biosphere model simulation including degassing reduces soil I concentrations fivefold and increases air concentrations 25-fold at steady state, compared to simulations without degassing.