Concentration-dependent mobility, retardation, and speciation of iodine in surface sediment from the Savannah River Site.

Iodine occurs in multiple oxidation states in aquatic systems in the form of organic and inorganic species. This feature leads to complex biogeochemical cycling of stable iodine and its long-lived isotope, (129)I. In this study, we investigated the sorption, transport, and interconversion of iodine species by comparing their mobility in groundwaters at ambient concentrations of iodine species (10(-8) to 10(-7) M) to those at artificially elevated concentrations (78.7 µM), which often are used in laboratory analyses. Results demonstrate that the mobility of iodine species greatly depends on, in addition to the type of species, the iodine concentration used, presumably limited by the number of surface organic carbon binding sites to form covalent bonds. At ambient concentrations, iodide and iodate were significantly retarded (K(d) values as high as 49 mL g(-1)), whereas at concentrations of 78.7 µM, iodide traveled along with the water without retardation. Appreciable amounts of iodide during transport were retained in soils due to iodination of organic carbon, specifically retained by aromatic carbon. At high input concentration of iodate (78.7 µM), iodate was found to be reduced to iodide and subsequently followed the transport behavior of iodide. These experiments underscore the importance of studying iodine geochemistry at ambient concentrations and demonstrate the dynamic nature of their speciation during transport conditions.

A novel approach for the simultaneous determination of iodide, iodate and organo-iodide for 127I and 129I in environmental samples using gas chromatography-mass spectrometry.

In aquatic environments, iodine mainly exists as iodide, iodate, and organic iodine. The high mobility of iodine in aquatic systems has led to (129)I contamination problems at sites where nuclear fuel has been reprocessed, such as the F-area of Savannah River Site. In order to assess the distribution of (129)I and stable (127)I in environmental systems, a sensitive and rapid method was developed which enables determination of isotopic ratios of speciated iodine. Iodide concentrations were quantified using gas chromatography-mass spectrometry (GC-MS) after derivatization to 4-iodo-N,N-dimethylaniline. Iodate concentrations were quantified by measuring the difference of iodide concentrations in the solution before and after reduction by Na(2)S(2)O(5). Total iodine, including inorganic and organic iodine, was determined after conversion to iodate by combustion at 900 °C. Organo-iodine was calculated as the difference between the total iodine and total inorganic iodine (iodide and iodate). The detection limits of iodide-127 and iodate-127 were 0.34 nM and 1.11 nM, respectively, whereas the detection limits for both iodide-129 and iodate-129 was 0.08 nM (i.e., 2pCi (129)I/L). This method was successfully applied to water samples from the contaminated Savannah River Site, South Carolina, and more pristine Galveston Bay, Texas.

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 129I. As iodine mobility varies greatly with iodine speciation, experiments with 129I-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 125I- and stable 127I- 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 (Kd) that was an inverse power function of the added I- concentration. However, Kd values of organically bound 127I 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.