Speciation of iodine isotopes inside and outside of a contaminant plume at the Savannah River Site

摘要

A primary obstacle in understanding the fate and transport of the toxic radionuclide ~(129)I (a thyroid seeker) is an accurate method to distinguish it from the stable isotope, ~(127)I, and to quantify the various species at environmentally relevant concentrations (～10~(-8) M). A pH-dependent solvent extraction and combustion method was paired with accelerator mass spectrometry (AMS) to measure ambient levels of ~(129)I/~(127)I isotope ratios and iodine speciation (iodide (I~-), iodate (IO_3~-), and organo-I (OI)) in aquatic systems. The method exhibited an overall uncertainty of 10% or less for I~- and IO_3~-, and less than 30% for OI species concentrations and enabled ~(129)I measurements as low as 0.001 Bq/L (1 Bq/L = 10~(13) M). The method was used to analyze groundwater from the Savannah River Site (SRS), South Carolina, USA, along a pH, redox potential (Eh), and organic carbon gradient (8-60 μM DOC). The data confirmed that the ~(129)I/~(127)I ratios and species distribution were strongly pH dependent and varied in a systematic manner from the strongly acidic source. While ~(129)I speciation in plume samples containing total I concentrations ＞1.7 Bq/L was similar whether measured by AMS or GC-MS ([I~-] » [IO_3~-] = [OI]), AMS enabled ~(129)I speciation measurements at much lower concentrations than what was possible with GC-MS. AMS analyses demonstrated that groundwater samples minimally impacted by the plume were still orders of magnitude higher than ambient ~(129)I concentrations typically found elsewhere in the USA groundwaters and rivers. This is likely due to past atmospheric releases of volatile ~(129)I species by SRS nuclear reprocessing facilities near the study site. Furthermore, the results confirmed the existence of ~(129)I not only as I~-, but also as OI and IO_3~- species.