Iodine sorption by highly weathered soils

Radioactive iodine (I) isotopes such as I-129 are widely distributed throughout Department of Energy (DOE) and other nuclear sites as a result of past enrichment activities. While natural-abundance I serves an important role in thyroid function in human beings, anthropogenic I can cause thyroid cancer. Predominant I species are anionic and generally assumed to move conservatively through typical negatively-charged subsurface material with the mobile water phase. However, many of the 129I contaminated soils and subsurface materials on the DOE Savannah River Site (SRS) near Aiken, SC, contain significant positive charge, which could retain I-129. A series of batch and column experiments were performed in order to determine sorption of three different I species (iodide, iodate, and an organic I compound, 4-iodaniline) over a range of aqueous concentrations (10^-4 to 10^-8 M) onto surface and subsurface soils from SRS. Total aqueous I in solution after equilibration was measured using ICP-MS and used to calculate the partitioning coefficient (Kd). Kd values for all three species were much higher at lower dissolved I concentrations (i.e., non-linear partitioning), and correlated with CDB-extractable Fe and Al and well as soil organic matter content. For the species and concentrations that exhibited high Kd values (>5), we conducted column experiments to evaluate partitioning under dynamic advective systems, using tritium for comparison as a conservative hydrologic tracer. We tracked pH, EC, and hydraulic head in real-time, and the effluent was collected for I analysis as described above. The results indicate a complex pattern of I movement in the natural environment affected by isotope concentration, speciation, and soil characteristics.