Organo-Iodine Formation in Soils and Aquifer Sediments at Ambient Concentrations

One of the key risk drivers at radioactive waste disposal facilities is radioiodine, especially I-121. As iodine mobility varies greatly with iodine speciation, experiments with I-129-contaminated aquifer sediments from the Savannah River Site located in Aiken, SC, were carried out to test iodine interactions with soils and aquifer sediments. Using tracer I-125(-) and stable I-127(-) additions, it was shown that such interactions were highly dependent on I- concentrations added to sediment suspensions, contact time with the sediment, and organic carbon (OC) content, resulting in an empirical particle-water partition coefficient (K-d) that was an inverse power function of the added I- concentration. However, K-d values of organically bound I-127 were 3 orders of magnitude higher than those determined after 1-2 weeks of tracer equilibration, approaching those of OC. Under ambient conditions, organo-iodine (OI)was a major fraction (67%) of the total iodine in the dissolved phase and by implication of the particulate phase. As the total concentration of amended I- increased, the fraction of detectable dissolved OI decreased. This trend, attributed to OC becoming the limiting factor in the aquifer sediment, explains why at elevated I- concentrations OI is often not detected.