Importance of Microbial Activity On Groundwater Iodate and Organo-Iodine Speciation and Mobility At Two DOE Sites

Santschi, Peter H.¹  Xu, Chen¹  Schwehr, Kathleen A.¹  Kaplan, Daniel I.²  Yeager, Chris M.² 

[1] Texas A & M Univ., College Station, TX (United States);Savannah River Site (SRS), Aiken, SC (United States). Savannah River National Lab. (SRNL)

关键词: Iodine-129;    Iodine Speciation;    Iodine Isotope Ratios;    Role of Natural Organic Matter;    Mobility;    Savannah River Site;    Hanford Site;    Fukushima Site;   

DOI  :  10.2172/1240154
RP-ID  :  DOE-TAMU--0006823
PID  :  OSTI ID: 1240154

美国|英语

来源: SciTech Connect

【 摘 要 】

Iodine (I) occurs in multiple oxidation states in aquatic systems in the form of organic and inorganic species (iodide and iodate). This fact leads to complex biogeochemical cycling of Iodine and its long-lived isotope, ¹²⁹I, a major by-product of nuclear fission. Results from our newly developed, sensitive and rapid method for speciated isotopic ratios (¹²⁹I/¹²⁷I) via GC-MS, which compare favorably with Accelerator Mass Spectroscopy, demonstrate that the mobility of ¹²⁹I species greatly depends on the type of I species and its concentration, pH, and sediment redox state. At ambient concentrations (~10⁷ M), I- and IO^-3 are significantly retarded by sorption to mineral surfaces and covalent binding to aromatic moieties in natural organic matter (NOM), even when NOM is present at low concentrations such as occur at Hanford. At concentrations traditionally examined in sorption studies (??? 10^-4 M), I- travels along with the water. Iodate removal can also occur through incorporation into CaCO₃ crystal lattice, e.g., at the Hanford Site. Removal of iodine from the groundwater through interaction with NOM is complicated by the release of mobile organo-I species, as was observed at SRS and Hanford. A small fraction of NOM that is bound to iodine can behave as a mobile organo-I source, a process that we were able to numerically simulate. Field and laboratory studies evaluating the cause for steady increases in ¹²⁹I concentrations (up to 1000 pCi L-1) emanating from radiological basins at SRS indicate that an increase of 0.7 pH units in groundwater over 17 years can account for the observed increased groundwater ¹²⁹I concentrations. Bacteria from a ¹²⁹I-contaminated aerobic aquifer at the F-area of SRS can accumulate I- at environmentally relevant concentrations (10^-7 M), and enzymatically oxidize I-, which together with microbially produced MnO₂ and superoxide or organic acids can significantly contribute to organo-iodine formation.