Radioiodine Biogeochemistry and Prevalence in Groundwater

Abstract

129I is commonly either the top or among the top risk drivers, along with 99Tc, at radiological waste disposal sites and contaminated groundwater sites where nuclear material fabrication or reprocessing has occurred. The risk stems largely from 129I having a high toxicity, a high bioaccumulation factor (90% of all the body's iodine concentrates in the thyroid), a high inventory at source terms (due to its high fission yield), an extremely long half-life (16M years), and rapid mobility in the subsurface environment. Another important reason that 129I is a key risk driver is that there is uncertainty regarding its biogeochemical fate and transport in the environment. We typically can define 129I mass balance and flux at sites, but cannot predict accurately its response to changes in the environment. As a consequence of some of these characteristics, 129I has a very low drinking water standard, which is set at 1 pCi/L, the lowest of all radionuclides in the Federal Register. Recently, significant advancements have been made in detecting iodine species at ambient groundwater concentrations, defining the nature of the organic matter and iodine bond, and quantifying the role of naturally occurring sediment microbes to promote iodine oxidation and reduction. These recent studies have led to a more mechanistic understanding of radioiodine biogeochemistry. The objective of this review is to describe these advances and to provide a state of the science of radioiodine biogeochemistry relevant to its fate and transport in the terrestrial environment and provide information useful for making decisions regarding the stewardship and remediation of 129I contaminated sites. As part of this review, knowledge gaps were identified that would significantly advance the goals of basic and applied research programs for accelerating 129I environmental remediation and reducing uncertainty associated with disposal of 129I waste. Together the information gained from addressing these knowledge gaps will not alter the observation that 129I is primarily mobile, but it will likely permit demonstration that the entire 129I pool in the source term is not moving at the same rate and some may be tightly bound to the sediment, thereby smearing the modeled 129I peak and reducing maximum calculated risk.

Use of Iodine to Biofortify and Promote Growth and Stress Tolerance in Crops

“…However, because the I − /${\text{IO}}_3^{-}$ ratio depends on biological activity, it is not limited strictly to a thermodynamic balance (Kaplan et al, 2014), as shown in Figure 4. This fact makes it difficult to predict the pattern of iodine speciation in a particular soil.…”

Application of hydrotalcite in soil immobilization of iodate (IO3−)

“…7,8 In some cases, IO 3 À is the predominant iodine form, which accounts for up to approximately 70% of total iodine, with iodide and organoiodine being minor components. 9,10 The fate and mobility of iodine in soils depend largely on its interactions with soil components. Several recent studies reported that natural organic matter (NOM), and especially its aromatic components, played an important role in the sorption of iodine to soil and/or sediment.…”

Superoxide Production by a Manganese-Oxidizing Bacterium Facilitates Iodide Oxidation

“…Iodide (I Ϫ ) (oxidation state Ϫ1) is considered the main iodine species that is released from nuclear power reprocessing facilities because of the redox properties of the reprocessing wastewater (8). When I Ϫ is released into highly oxic environments (oxidation reduction potential of Ͼ0.5 to 0.8 V, depending on the pH), it will spontaneously transform into elemental iodine (I 2 ) (oxidation state 0).…”

“…Changes in redox potential control the environmental behavior of iodine, including its mobility and biogeochemical cycling (4)(5)(6)(7)(8). Iodide (I Ϫ ) (oxidation state Ϫ1) is considered the main iodine species that is released from nuclear power reprocessing facilities because of the redox properties of the reprocessing wastewater (8).…”