FACTORS AFFECTING THE SOIL SORPTION OF IODINE

Iodine-129 is an important radionuclide released from nuclear faciliti es because of its long radioactive half-life and its environmental mob ility. Its retention in surface soils has been linked to pH, organic m atter, and Fe and Al oxides. Its inorganic solution chemistry indicate s I will most likely exist as an anion. Three investigations were carr ied out to provide information on the role of the inorganic and organi c chemistry during sorption off by soil. Anion competition using Cl- s howed that anion exchange plays a role in I sorption in both mineral a nd organic soils. The presence of Cl decreased the loss of I- from sol ution by 30 and 50% for an organic and a carbonated sandy soil respect ively. The I remaining in solution was associated primarily with disso lved organic carbon (DOC). The loss rate from solution appears to depe nd on two reactions of I with the soil solids (both mineral and organi c) creating both a release to and a loss from solution, and the reacti on of I with the DOC (from very low to high molecular weight). Composi tion analyses of the pore water and the geochemical modelling indicate that I sorption affects the double-charged anion species in solution the most, particularly SO4--. Iodide introduced to natural bog groundw ater at three concentrations (10(-3), 10(-1) and 10 meg L(-1)) remaine d as I- and was not lost from solution quickly, indicating that the as sociation of I with DOC is slow and does not depend on the DOC or I co ncentration. If sorption of I to soil solids or DOC is not sensitive t o concentration, then stable I studies, which by necessity must be car ried out at high environmental concentrations, can be linearly extrapo lated to radioactive I at much lower molar concentrations.