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

Metal-Encapsulated, Polymer-Containing Halide Salt Composites as Potential Long-Term Hosts for Radioiodine: Evaluating Halmets, Polyhalmets, and Halcermets

This paper presents composite waste form concepts for radioiodine immobilization including halide-metal (halmet) composites, polymer-halide-metal (polyhalmet) composites, and halide-ceramic-metal (halcermet) composites with data from experiments to evaluate these ideas. The encapsulant metal of choice for the pellets was Bi0, and a cold-press-and-sinter approach was used for creating the pellets. A polymer (i.e., polyacrylonitrile or PAN) phase was included in some composite forms because this porous, passive polymer is used as a host matrix for active chemisorption-based getters. Metals of Ag0, Bi0, and Cu0 were separately embedded into PAN beads, which were loaded with iodine in static tests. Included are details of experiments where PAN removal from the iodine-loaded composite beads was evaluated to reduce the overall volume of the final waste requiring immobilization and to improve the thermal stability of the final composite form. While these experiments demonstrate new concepts for radioiodine immobilization, more work is needed to fully understand the limitations of these approaches and further optimizations are needed before implementation at larger scales is feasible.

Longevity prediction of reactive media in permeable reactive barriers considering the contamination level and groundwater velocity at the planning site, with a focus on cadmium removal by zeolite

Zeolite is a versatile and effective reactive material used in permeable reactive barriers (PRBs) for remediating groundwater contaminated with heavy metals. In this study, we evaluated the influence of subsurface environmental conditions, namely contamination level (C0) and groundwater velocity (v), on predicting the longevity of zeolite for cadmium (Cd) removal. Batch experiments were performed to investigate the effect of C0 on Cd removal, and column experiments were performed to examine how Cd transportation through zeolite varies at different C0 and v. Breakthrough curves (BTCs) were analyzed with an advection-dispersion equation (ADE) coupled with nonequilibrium sorption rate models. The reaction parameters indicating the performance metrics of zeolite were determined using an iterative fitting approach-retardation factor (R), partitioning coefficient (β), and mass transfer coefficient (ω). R exhibited dependence on C0, but was unrelated to v; its rapid increase at lower C0 was explained by Langmuir sorption isotherms. β and ω, integral to sorption dynamics and mass transfer, respectively, showcased functional relationships with v. β decreased gradually as v increased, described by the nonequilibrium sorption model, whereas ω increased steadily with v, guided by the Monod function. Using the relationship of these parameters, the fate and transport of Cd within zeolite was simulated under various subsurface environmental conditions to construct the longevity prediction function. Thus, this study introduces a method for predicting the longevity of reactive materials, which can be valuable for designing PRBs with high longevity in the future.

Co-Occurrence of Bromine and Iodine Species in US Drinking Water Sources That Can Impact Disinfection Byproduct Formation

Bromine and iodine species are precursors for forming disinfection byproducts in finished drinking waters. Our study incorporates spatial and temporal data to quantify concentrations of inorganic (bromide (Br-), iodide (I-), and iodate (IO3-)), organic, and total bromine (BrT) and iodine (IT) species from 286 drinking water sources and 7 wastewater effluents across the United States. Br- ranged from <5-7800 μg/L (median of 62 μg/L in surface water (SW) and 95 μg/L in groundwater (GW)). I- was detected in 41% of SW (1-72 μg/L, median = <1 μg/L) and 62% of GW (<1-250 μg/L, median = 3 μg/L) samples. The median Br-/I- ratio in SW and GW was 22 μg/μg and 16 μg/μg, respectively, in paired samples with detect Br- and I-. BrT existed primarily as Br-, while IT was present as I-, IO3-, and/or total organic iodine (TOI). Inorganic iodine species (I- and IO3-) were predominant in GW samples, accounting for 60-100% of IT; however, they contributed to only 20-50% of IT in SW samples. The unknown fraction of IT was attributed to TOI. In lakes, seasonal cycling of I-species was observed and was presumably due to algal productivity. Finally, Spearman Rank Correlation tests revealed a strong correlation between Br- and IT in SW (RBr-,IT = 0.83) following the log10 (Br-, μg/L) = 0.65 × log10 (IT, μg/L) - 0.17 relationship. Br- and I- in treated wastewater effluents (median Br- = 234 μg/L, median I- = 5 μg/L) were higher than drinking water sources.

Structural contributions of different manganese oxide minerals on the redox transformations and proliferation of iodine

The redox capabilities of birnessite minerals are contingent upon the physical characteristics of the solid, indicating that different allotropes have various reactivities. Here, the role of these structural differences on the oxidation of iodine, a risk driving environmental contaminant in several federal complexes, was investigated. The mechanism of which can be seen here, with one of the minerals of study, acid birnessite. The pH range chosen for this study was pH 5-6. Throughout the experiments it was seen that the average oxidation state (AOS) had the greatest contribution to the differences in redox capabilities of the various birnessite minerals. Several trends were observed throughout this study: as AOS decreased, oxidation of iodide (I-) increased; as specific surface area (SSA) increased, the sorption of iodate (IO3-) increased. Additional experiments were conducted at trace levels of iodine, to better model environmental conditions. In that case, a one-step conversion of I- to IO3- occurred, to a greater extent than under artificially elevated concentrations.

A review of iodine in plants with biofortification: Uptake, accumulation, transportation, function, and toxicity

Iodine deficiency can cause thyroid disease, a serious health problem that has been affecting humans since several years. The biofortification of plants with iodine is an effective strategy for regulating iodine content in humans. In addition, radioiodine released into the atmosphere may contaminate terrestrial ecosystem along with dry or wet deposition and its accumulation in plants may cause exposure risks to humans via food chain. Recent progress in understanding the mechanisms related to iodine uptake, elementary speciation, dynamic transportation, nutritional role, and toxicity in plants is reviewed here. First, we introduced the iodine cycle in a marine-atmosphere-land system. The content and speciation of iodine in plants under natural conditions and biofortification backgrounds were also analyzed. We then discussed the mechanisms of iodine uptake and efflux by plants. The promotion or inhibition effects of iodine on plant growth were also investigated. Finally, the participation of radioiodine in plant growth and its safety risks along the food chain were evaluated. Furthermore, future challenges and opportunities for understanding the participation of iodine in plants have been outlined.

Adsorption and Detection of Iodine Species by a Thorium-Based Metal-Organic Framework

Actinide-bearing metal-organic frameworks (MOFs) encompass intriguing structures and properties, but the radioactivity of actinide cripples their applications. Herein, we have constructed a new thorium-based MOF (Th-BDAT) as a bifunctional platform for the adsorption and detection of radioiodine, a more radioactive fission product that can readily spread through the atmosphere in its molecular form or via solution as anionic species. The iodine capture within the framework of Th-BDAT from both the vapor phase and the cyclohexane solution has been verified, showing that Th-BDAT features maximum I₂ adsorption capacities (Q_max) of 959 and 1046 mg/g, respectively. Notably, the Q_max of Th-BDAT toward I₂ from cyclohexane solution ranks among the highest value for Th-MOFs reported to date. Furthermore, incorporating highly extended and π-electron-rich BDAT^4- ligands renders Th-BDAT as a luminescent chemosensor whose emission can be selectively quenched by iodate with a detection limit of 1.367 μM. Our findings thus foreshadow promising directions that might unlock the full potential of actinide-based MOFs from the point of view of practical application.

Recent advances in the removal of radioactive iodine by bismuth-based materials

Nowadays, the demand for nuclear power is continue increasing due to its safety, cleanliness, and high economic benefits. Radioactive iodine from nuclear accidents and nuclear waste treatment processes poses a threat to humans and the environment. Therefore, the capture and storage of radioactive iodine are vital. Bismuth-based (Bi-based) materials have drawn much attention as low-toxicity and economical materials for removing and immobilizing iodine. Recent advances in adsorption and immobilization of vapor iodine by the Bi-based materials are discussed in this review, in addition with the removal of iodine from solution. It points out the neglected areas in this research topic and provides suggestions for further development and application of Bi-based materials in the removal of radioactive iodine.

Spatiotemporal Variability of Groundwater Iodine in the Northern Arid Basins: Significance for Safe Water Supply

The genesis of geogenic iodine (I)-contaminated groundwater poses a significant threat to long-term water exploitation. Safe and sustainable water supply, particularly in the northern arid basins, demands a quantitative prediction of the high variability of I distribution over hydrogeological timescales. Here, bioenergetics-informed reactive transport modeling was combined with high-resolution molecular characterization of fueling organic matter to decipher the time-controlled interactions between vertical flow and (bio)geochemical processes in I transport within the Datong aquifers. The declining reactivities of I-bearing organic matter and Fe oxides in the 15-40 m depth decreased the rate of I release, while a growing number of pore volumes flushed through the aquifers to leach out I^- and organic I. This removal effect is compensated by the desorption of I^- from Fe oxides and secondary FeS generated from the concurrent reduction of Fe oxides and SO₄^2-. Consequently, peak concentrations of groundwater I^- may have appeared, depending upon the vertical recharge rate, at the first several pore volumes flushed through the aquifers. The current vertical distributions of the various I species likely represent a quasi-steady state between I mobilization and leaching. These new mechanistic insights into the dynamic hydrogeological-(bio)geochemical processes support secure groundwater use in the I-affected northern arid basins.

Large seasonal fluctuations of groundwater radioiodine speciation and concentrations in a riparian wetland in South Carolina

Recent studies evaluating multiple years of groundwater radioiodine (¹²⁹I) concentration in a riparian wetland located in South Carolina, USA identified strong seasonal concentration fluctuations, such that summer concentrations were much greater than winter concentrations. These fluctuations were observed only in the wetlands but not in the upland portion of the plume and only with ¹²⁹I, and not with other contaminants of anthropogenic origin: nitrate/nitrite, strontium-90, technecium-99, tritium, or uranium. This unexplained observation was hypothesized to be the result of strongly coupled processes involving hydrology, water temperature, microbiology, and chemistry. To test this hypothesis, an extensive historical groundwater database was evaluated, and additional measurements of total iodine and iodine speciation were made from recently collected samples. During the summer, the water table decreased by as much as 0.7 m, surface water temperature increased by as much as 15 °C, and total iodine concentrations were consistently greater (up to 680%) than the following winter months. Most of the additional iodine observed in the summer could be attributed to proportional gains in organo-iodine, and not iodide or iodate. Furthermore, ¹²⁹I concentrations were observed to be two-orders-of-magnitude greater at the bottom of the upland aquifer than at the top. A coupled hydrological and biogeochemical conceptual model is proposed to tie these observations together. First, as the surface water temperature increased during the summer, microbial activity was enhanced, which in turn stimulated the formation of mobile organo-I. Hydrological processes were also likely involved in the observed iodine seasonal changes: (1) as the water table decreased in summer, the remaining upland water entering the wetland was comprised of a greater proportion of water containing elevated iodine concentrations from the low depths, and (2) water flow paths in summer changed such that the wells intercepted more of the contaminant plume and less of the diluting rainwater (due to evapotranspiration) and streamwater (as the lower levels promote a predominantly recharging system). These results underscore the importance of coupled processes influencing contaminant concentrations, and the need to assess seasonal contaminant variations to optimize long-term monitoring programs of wetlands.

A cationic thorium-organic framework with triple single-crystal-to-single-crystal transformation peculiarities for ultrasensitive anion recognition

Single-crystal-to-single-crystal transformation of metal-organic frameworks has been met with great interest, as it allows for the creation of new materials in a stepwise manner and direct visualization of structural transitions when subjected to external stimuli. However, it remains a peculiarity among numerous metal-organic frameworks, particularly for the ones constructed from tetravalent metal cations. Herein, we present a cationic thorium-organic framework displaying unprecedented triple single-crystal-to-single-crystal transformations in organic solvents, water, and NaIO3 solution. Notably, both the interpenetration conversion and topological change driven by the SC-SC transformation have remained elusive for thorium-organic frameworks. Moreover, the single-crystal-to-single-crystal transition in NaIO3 solution can efficiently and selectively turn the ligand-based emission off, leading to the lowest limit of detection (0.107 μg kg-1) of iodate, one of the primary species of long-lived fission product 129I in aqueous medium, among all luminescent sensors.

Hybrid Sorbents for 129I Capture from Contaminated Groundwater

Radioiodine (¹²⁹I) poses a risk to the environment due to its long half-life, toxicity, and mobility. It is found at the U.S. Department of Energy Hanford Site due to legacy releases of nuclear wastes to the subsurface where ¹²⁹I is predominantly present as iodate (IO₃^-). To date, a cost-effective and scalable cleanup technology for ¹²⁹I has not been identified, with hydraulic containment implemented as the remedial approach. Here, novel high-performing sorbents for ¹²⁹I remediation with the capacity to reduce ¹²⁹I concentrations to or below the US Environmental Protection Agency (EPA) drinking water standard and procedures to deploy them in an ex-situ pump and treat (P&T) system are introduced. This includes implementation of hybridized polyacrylonitrile (PAN) beads for ex-situ remediation of IO₃^--contaminated groundwater for the first time. Iron (Fe) oxyhydroxide and bismuth (Bi) oxyhydroxide sorbents were deployed on silica substrates or encapsulated in porous PAN beads. In addition, Fe-, cerium (Ce)-, and Bi-oxyhydroxides were encapsulated with anion-exchange resins. The PAN-bismuth oxyhydroxide and PAN-ferrihydrite composites along with Fe- and Ce-based hybrid anion-exchange resins performed well in batch sorption experiments with distribution coefficients for IO₃^- of >1000 mL/g and rapid removal kinetics. Of the tested materials, the Ce-based hybrid anion-exchange resin was the most efficient for removal of IO₃^- from Hanford groundwater in a column system, with 50% breakthrough occurring at 324 pore volumes. The functional amine groups on the parent resin and amount of active sorbent in the resin can be customized to improve the iodine loading capacity. These results highlight the potential for IO₃^- remediation by hybrid sorbents and represent a benchmark for the implementation of commercially available materials to meet EPA standards for cleanup of ¹²⁹I in a large-scale P&T system.

Iodine immobilization by materials through sorption and redox-driven processes: A literature review

Radioiodine-129 (¹²⁹I) in the subsurface is mobile and limited information is available on treatment technologies. Scientific literature was reviewed to compile information on materials that could potentially be used to immobilize ¹²⁹I through sorption and redox-driven processes, with an emphasis on ex-situ processes. Candidate materials to immobilize ¹²⁹I include iron minerals, sulfur-based materials, silver-based materials, bismuth-based materials, ion exchange resins, activated carbon, modified clays, and tailored materials (metal organic frameworks (MOFS), layered double hydroxides (LDHs) and aerogels). Where available, compiled information includes material performance in terms of (i) capacity for ¹²⁹I uptake; (ii) long-term performance (i.e., solubility of a precipitated phase); (iii) technology maturity; (iv) cost; (v) available quantity; (vi) environmental impact; (vii) ability to emplace the technology for in situ use at the field-scale; and (viii) ex situ treatment (for media extracted from the subsurface or secondary waste streams). Because it can be difficult to compare materials due to differences in experimental conditions applied in the literature, materials will be selected for subsequent standardized batch loading tests.

Short-Term Iodine Dynamics in Soil Solution

Assessing the reactions of iodine (I) in soil is critical to evaluate radioiodine exposure and understand soil-to-crop transfer rates. Our mechanistic understanding has been constrained by method limitations in assessing the dynamic interactions of iodine between soil solution and soil solid phase over short periods (hours). We use microdialysis to passively extract soil solution spiked with radioiodine (¹²⁹I^- and ¹²⁹IO₃^-) to monitor short-term (≤40 h) in situ fixation and speciation changes. We observed greater instantaneous adsorption of ¹²⁹IO₃^- compared to ¹²⁹I^- in all soils and the complete reduction of ¹²⁹IO₃^- to ¹²⁹I^- within 5 h of addition. Loss of ¹²⁹I from solution was extremely rapid; the average half-lives of ¹²⁹I^- and ¹²⁹IO₃^- in soil solution were 4.06 and 10.03 h, respectively. We detected the presence of soluble organically bound iodine (org-¹²⁹I) with a low molecular weight (MW) range (0.5-5 kDa) in all soils and a slower (20-40 h) time-dependent formation of larger MW org-I compounds (12-18 kDa) in some samples. This study highlights the very short window of immediate availability in which I from rainfall or irrigation remains in soil solution and available to crops, thus presenting significant challenges to phytofortification strategies in soil-based production systems.

A review of the behavior of radioiodine in the subsurface at two DOE sites

Multiple processes affect the fate of the radioactive isotope ¹²⁹I in the environment. Primary categories of these processes include electron transfer reactions mediated by minerals and microbes, adsorption to sediments, interactions with organic matter, co-precipitation, and volatilization. A description of dominant biogeochemical processes is provided to describe the interrelationship of these processes and the associated iodine chemical species. The majority of the subsurface iodine fate and transport studies in the United States have been conducted at U.S. Department of Energy (DOE) sites where radioisotopes of iodine are present in the environment and stored waste. The DOE Hanford Site and Savannah River Site (SRS) are used to illustrate how the iodine species and dominant processes at a site are controlled by the prevailing site biogeochemical conditions. These sites differ in terms of climate (arid vs. sub-tropical), major geochemical parameters (e.g., pH ~7.5 vs. 4), and mineralogy (carbonate vs. Fe/Al oxide dominated). The iodine speciation and dominant processes at a site also have implications for selection and implementation of suitable remedy approaches for ¹²⁹I.

Role of fungal laccase in iodide oxidation in soils

Previously, we hypothesized that microbial laccase oxidizes iodide (I^-) in soils to molecular iodine (I₂) or hypoiodous acid (HIO), both of which are easily incorporated into natural soil organic matter, and thus plays a role in iodine sorption on soils. In this study, soil iodide oxidase activity was determined by a colorimetric assay to evaluate if laccase is responsible for iodide oxidation in soils. Three types of Japanese soil showed significant iodide oxidase activities (0.751-2.87 mU g soil^-1) at pH 4.0, which decreased with increasing pH, until it was no longer detected at pH 5.5. The activity was inhibited strongly by autoclaving or by the addition of common laccase inhibitors. Similar tendency of inhibition was observed in soil laccase activity, which was determined with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a substrate. Significant positive correlations (R² values of 0.855-0.896) between iodide oxidase activity and laccase activity were observed in two of three soils. Commercially available fungal laccases showed only very low iodide oxidase activities (4.68-18.0 mU mg^-1), but enhanced activities of 102-739 mU mg^-1 were observed in the presence of redox mediators. Finally, we successfully isolated fungal strains with iodide-oxidizing phenotype in the presence of redox mediators. Polyacrylamide gel electrophoresis of the culture supernatant of Scytalidium sp. strain UMS and subsequent active stain revealed that the fungal laccase actually oxidized iodide in the presence of redox mediators. These results suggest that at least part of iodide in soils is oxidized by fungal laccase through the laccase-mediator system.

Organic and inorganic colloids impacting total iodine behavior in groundwater from the Datong Basin, China

The geochemical behaviors of colloids in aquifers played an important role in determining the fate of iodine in groundwater system. To decipher the impact of colloids on iodine mobilization in aquifers, three successive pore-sized colloids filtration (0.45μm, 30kDa and 5kDa) were conducted on in-situ during groundwater sampling. The results showed that the distribution ratios (f) of total iodine (I_T) and iron in the dissolved solution (i.e., 5kDa ultrafiltered) were from 0.78 to 0.99 and from 0.56 to 0.94, respectively. Natural organic matter (NOM) in the colloidal fractions obtained the f values ranging from 0.14 to 1.00. The decreased Eh values from recharge area to discharge area indicated redox potential of groundwater system changed from oxidizing to reducing along groundwater flowpath, and interestingly, the corresponding iodine fractions in groundwater were decreasing in dissolved solution and increasing in colloidal fractions. Inverse correlation between Fe and DOC and positive correlation between iodine and DOC suggested the occurrence of reductive dissolution of iron oxyhydroxides and degradation of organic iodine in groundwater system. Iodine distribution increased in dissolved solution and decreased in colloids with pH increase. Moreover, as pH increased, f (Fe) and f (DOC) decreased in dissolved solution and increased in colloids. Relatively weak correlation of f (I_T) with f (Fe) and strong relationship between f (I_T) and f (DOC) in the large grain size fractions suggested the Fe-OM complexes controlled iodine distribution in groundwater, which depends on the presence of Fe bridges. Negligible association of iodine with Fe and NOM in the small grain size fractions might be attributed to the effects of abundant OH^- content in groundwater.

The physicochemical distribution of 131I in a municipal wastewater treatment plant

As a consequence of therapeutic and diagnostic treatment of patients with thyroid diseases, ¹³¹I is introduced into the sewage system on a regular basis. This presents an opportunity to use the ¹³¹I as a tracer to study its partitioning and transport within a wastewater treatment plant (WWTP). In the case of nuclear accidents where ¹³¹I is one of the most prominent nuclides, an understanding of iodine partitioning and transport will be valuable for developing models that may prognosticate the activity concentrations in sludge and outflow, especially after an accidental input. In this study, samples from various locations inside a municipal WWTP were taken and for each sample, three different fractions were separated by a chemical extraction process. These fractions were analysed for their ¹³¹I activity concentrations by gamma-ray spectroscopy. While about 30% of the radioiodine activity in the inflow is associated with organic molecules, this amounts to about 90% after biological treatment. This is caused by the accumulation of ¹³¹I bound to organic matter in the return sludge and by a transfer of ¹³¹I from the inorganic to the organic fractions, most likely mediated by microbial action. In the outflow, inorganic and low-molecular ¹³¹I is dominant, but the overall activity concentration is reduced to about 50-75%.

Sorption and speciation of iodine in groundwater system: The roles of organic matter and organic-mineral complexes

Characterizing the properties of main host of iodine in soil/sediment and the geochemical behaviors of iodine species are critical to understand the mechanisms of iodine mobilization in groundwater systems. Four surface soil and six subsurface sediment samples were collected from the iodine-affected area of Datong basin in northern China to conduct batch experiments and to evaluate the effects of NOM and/or organic-mineral complexes on iodide/iodate geochemical behaviors. The results showed that both iodine contents and k_f-iodate values had positive correlations with solid TOC contents, implying the potential host of NOM for iodine in soil/sediment samples. The results of chemical removal of easily extracted NOM indicated that the NOM of surface soils is mainly composed of surface embedded organic matter, while sediment NOM mainly occurs in the form of organic-mineral complexes. After the removal of surface sorbed NOM, the decrease in k_f-iodate value of treated surface soils indicates that surface sorbed NOM enhances iodate adsorption onto surface soil. By contrast, k_f-iodate value increases in several H₂O₂-treated sediment samples, which was considered to result from exposed rod-like minerals rich in Fe/Al oxyhydroxide/oxides. After chemical removal of organic-mineral complexes, the lowest k_f-iodate value for both treated surface soils and sediments suggests the dominant role of organic-mineral complexes on controlling the iodate geochemical behavior. In comparison with iodate, iodide exhibited lower affinities on all (un)treated soil/sediment samples. The understanding of different geochemical behaviors of iodine species helps to explain the occurrence of high iodine groundwater with iodate and iodide as the main species in shallow (oxidizing conditions) and deep (reducing conditions) groundwater.

Recent advances in the detection of specific natural organic compounds as carriers for radionuclides in soil and water environments, with examples of radioiodine and plutonium

Among the key environmental factors influencing the fate and transport of radionuclides in the environment is natural organic matter (NOM). While this has been known for decades, there still remains great uncertainty in predicting NOM-radionuclide interactions because of lack of understanding of radionuclide interactions with the specific organic moieties within NOM. Furthermore, radionuclide-NOM studies conducted using modelled organic compounds or elevated radionuclide concentrations provide compromised information related to true environmental conditions. Thus, sensitive techniques are required not only for the detection of radionuclides, and their different species, at ambient and/or far-field concentrations, but also for potential trace organic compounds that are chemically binding these radionuclides. GC-MS and AMS techniques developed in our lab are reviewed here that aim to assess how two radionuclides, iodine and plutonium, form strong bonds with NOM by entirely different mechanisms; iodine tends to bind to aromatic functionalities, whereas plutonium binds to N-containing hydroxamate siderophores at ambient concentrations. While low-level measurements are a prerequisite for assessing iodine and plutonium migration at nuclear waste sites and as environmental tracers, it is necessary to determine their in-situ speciation, which ultimately controls their mobility and transport in natural environments. More importantly, advanced molecular-level instrumentation (e.g., nuclear magnetic resonance (NMR) and Fourier-transform ion cyclotron resonance coupled with electrospray ionization (ESI-FTICRMS) were applied to resolve either directly or indirectly the molecular environments in which the radionuclides are associated with the NOM.

Microbial Transformation of Iodine: From Radioisotopes to Iodine Deficiency

Iodine is a biophilic element that is important for human health, both as an essential component of several thyroid hormones and, on the other hand, as a potential carcinogen in the form of radioiodine generated by anthropogenic nuclear activity. Iodine exists in multiple oxidation states (-1, 0, +1, +3, +5, and +7), primarily as molecular iodine (I₂), iodide (I^-), iodate [Formula: see text] , or organic iodine (org-I). The mobility of iodine in the environment is dependent on its speciation and a series of redox, complexation, sorption, precipitation, and microbial reactions. Over the last 15years, there have been significant advances in iodine biogeochemistry, largely spurred by renewed interest in the fate of radioiodine in the environment. We review the biogeochemistry of iodine, with particular emphasis on the microbial processes responsible for volatilization, accumulation, oxidation, and reduction of iodine, as well as the exciting technological potential of these fascinating microorganisms and enzymes.

Role of natural organic matter on iodine and (239)(,240)Pu distribution and mobility in environmental samples from the northwestern Fukushima Prefecture, Japan

In order to assess how environmental factors are affecting the distribution and migration of radioiodine and plutonium that were emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, we quantified iodine and (239,240)Pu concentration changes in soil samples with different land uses (urban, paddy, deciduous forest and coniferous forest), as well as iodine speciation in surface water and rainwater. Sampling locations were 53-63 km northwest of the FDNPP within a 75-km radius, in close proximity of each other. A ranking of the land uses by their surface soil (<4 cm) stable (127)I concentrations was coniferous forest > deciduous forest > urban > paddy, and (239,240)Pu concentrations ranked as deciduous forest > coniferous forest > paddy ≥ urban. Both were quite distinct from that of (134)Cs and (137)Cs: urban > coniferous forest > deciduous forest > paddy, indicating differences in their sources, deposition phases, and biogeochemical behavior in these soil systems. Although stable (127)I might not have fully equilibrated with Fukushima-derived (129)I, it likely still works as a proxy for the long-term fate of (129)I. Surficial soil (127)I content was well correlated to soil organic matter (SOM) content, regardless of land use type, suggesting that SOM might be an important factor affecting iodine biogeochemistry. Other soil chemical properties, such as Eh and pH, had strong correlations to soil (127)I content, but only within a given land use (e.g., within urban soils). Organic carbon (OC) concentrations and Eh were positively, and pH was negatively correlated to (127)I concentrations in surface water and rain samples. It is also noticeable that (127)I in the wet deposition was concentrated in both the deciduous and coniferous forest throughfall and stemfall water, respectively, comparing to the bulk rainwater. Further, both forest throughfall and stemflow water consisted exclusively of organo-iodine, suggesting all inorganic iodine in the original bulk deposition (∼ 28.6% of total iodine) have been completely converted to organo-iodine. Fukushima-derived (239,240)Pu was detectable at a distance ∼ 61 km away, NW of FDNPP. However, it is confined to the litter layer, even three years after the FDNPP accident-derived emissions. Plutonium-239,240 activities were significantly correlated with soil OC and nitrogen contents, indicating Pu may be associated with nitrogen-containing SOM, similar to what has been observed at other locations in the United States. Together, these finding suggest that natural organic matter (NOM) plays a key role in affecting the fate and transport of I and Pu and may warrant greater consideration for predicting long-term stewardship of contaminated areas and evaluating various remediation options in Japan.

Radioiodine sorption/desorption and speciation transformation by subsurface sediments from the Hanford Site

During the last few decades, considerable research efforts have been extended to identify more effective remediation treatment technologies to lower the (129)I concentrations to below federal drinking water standards at the Hanford Site (Richland, USA). Few studies have taken iodate into consideration, though recently iodate, instead of iodide, was identified as the major species in the groundwater of 200-West Area within the Hanford Site. The objective of this study was thus to quantify and understand aqueous radioiodine species transformations and uptake by three sediments collected from the semi-arid, carbonate-rich environment of the Hanford subsurface. All three sediments reduced iodate (IO3(-)) to iodide (I(-)), but the loamy-sand sediment reduced more IO3(-) (100% reduced within 7 days) than the two sand-textured sediments (∼20% reduced after 28 days). No dissolved organo-iodine species were observed in any of these studies. Iodate uptake Kd values ([Isolid]/[Iaq]; 0.8-7.6 L/kg) were consistently and appreciably greater than iodide Kd values (0-5.6 L/kg). Furthermore, desorption Kd values (11.9-29.8 L/kg) for both iodate and iodide were consistently and appreciably greater than uptake Kd values (0-7.6 L/kg). Major fractions of iodine associated with the sediments were unexpectedly strongly bound, such that only 0.4-6.6 % of the total sedimentary iodine could be exchanged from the surface with KCl solution, and 0-1.2% was associated with Fe or Mn oxides (weak NH2HCl/HNO3 extractable fraction). Iodine incorporated into calcite accounted for 2.9-39.4% of the total sedimentary iodine, whereas organic carbon (OC) is likely responsible for the residual iodine (57.1-90.6%) in sediments. The OC, even at low concentrations, appeared to be controlling iodine binding to the sediments, as it was found that the greater the OC concentrations in the sediments, the greater the values of uptake Kd, desorption Kd, and the greater residual iodine concentrations (non-exchangeable, non-calcite-incorporated and non-Mn, Fe-oxide associated). This finding is of particular interest because it suggests that even very low OC concentrations, <0.2%, may have an impact on iodine geochemistry. The findings that these sediments can readily reduce IO3(-), and that IO3(-) sorbs to a greater extent than I(-), sheds light into earlier unexplained Hanford field data that demonstrated increases in groundwater (127)I(-)/(127)IO3(-) ratios and a decrease groundwater (129)IO3(-) concentrations along a transect away from the point sources, where iodine was primarily introduced as IO3(-). While a majority of the radioiodine does not bind to these alkaline sediments, there is likely a second smaller iodine fraction in the Hanford subsurface that is strongly bound, presumably to the sediment OC (and carbonate) phases. This second fraction may have an impact on establishing remediation goals and performance assessment calculations.

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

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 (IO3(-)), and organo-I (OI)) in aquatic systems. The method exhibited an overall uncertainty of 10% or less for I(-) and IO3(-), 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(-)]≫[IO3(-)]=[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 IO3(-) species.

Radioiodine Biogeochemistry and Prevalence in Groundwater

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.

Temporal variation of iodine concentration and speciation (127I and 129I) in wetland groundwater from the Savannah River Site, USA

(129)I derived from a former radionuclide disposal basin located on the Savannah River Site (SRS) has concentrated in a wetland 600 m downstream. To evaluate temporal environmental influences on iodine speciation and mobility in this subtropical wetland environment, groundwater was collected over a three-year period (2010-2012) from a single location. Total (127)I and (129)I showed significant temporal variations, ranging from 68-196 nM for (127)I and <5-133 pCi/L for (129)I. These iodine isotopes were significantly correlated with groundwater acidity and nitrate, two parameters elevated within the contaminant plume. Additionally, (129)I levels were significantly correlated with those of (127)I, suggesting that biogeochemical controls on (127)I and (129)I are similar within the SRS aquifer/wetland system. Iodine speciation demonstrates temporal variations as well, reflecting effects from surface recharges followed by acidification of groundwater and subsequent formation of anaerobic conditions. Our results reveal a complex system where few single ancillary parameters changed in a systematic manner with iodine speciation. Instead, changes in groundwater chemistry and microbial activity, driven by surface hydrological events, interact to control iodine speciation and mobility. Future radiological risk models should consider the flux of (129)I in response to temporal changes in wetland hydrologic and chemical conditions.

Effects of radiation and temperature on iodide sorption by surfactant-modified bentonite

Bentonite, which is used as an engineered barrier in geological repositories, is ineffective for sorbing anionic radionuclides because of its negatively charged surface. This study modified raw bentonite using a cationic surfactant (i.e., hexadecyltrimethylammonium [HDTMA]-Br) to improve its sorption capability for radioactive iodide. The effects of temperature and radiation on the iodide sorption of surfactant-modified bentonite (SMB) were also evaluated under alkaline pH condition similar to that found in repository environments. Different amounts of surfactant, equivalent to the 50, 100, and 200% cation-exchange capacity of the bentonite, were used to produce the HDTMA-SMB for iodide sorption. The sorption reaction of the SMB with iodide reached equilibrium rapidly within 10 min regardless of temperature and radiation conditions. The rate of iodide sorption increased as the amount of the added surfactant was increased and nonlinear sorption behavior was exhibited. However, high temperature and γ-irradiation ((60)Co) resulted in significantly (∼2-10 times) lower iodide Kd values for the SMB. The results of FTIR, NMR, and XANES spectroscopy analysis suggested that the decrease in iodide sorption may be caused by weakened physical electrostatic force between the HDTMA and iodide, and by the surfactant becoming detached from the SMB during the heating and irradiation processes.

Radioiodine concentrated in a wetland

Most subsurface environmental radioactivity contamination is expected to eventually resurface in riparian zones, or wetlands. There are a number of extremely sharp biogeochemical interfaces in wetlands that could alter radionuclide speciation and promote accumulation. The objective of this study was to determine if a wetland concentrated (129)I emanating from a former waste disposal basin located on the Savannah River Site (SRS) in South Carolina, USA. Additionally, studies were conducted to evaluate the role of sediment organic matter in immobilizing the radioiodine. Groundwater samples were collected along a 0.7-km transect away from the seepage basin and in the downstream wetlands. The samples were analyzed for (129)I speciation (iodide (I(-)), iodate (IO3(-)), and organo-I). Groundwater (129)I concentrations in many locations in the wetlands (as high as 59.9 Bq L(-1)(129)I) were greatly elevated with respect to the source term (5.9 Bq L(-1)(129)I). (129)I concentration profiles in sediment cores were closely correlated to organic matter concentrations (r(2) = 0.992; n = 5). While the sediment organic matter promoted the uptake of (129)I to the wetland sediment, it also promoted the formation of a soluble organic fraction: 74% of the wetland groundwater (129)I could pass through a 1 kDa (<1 nm) membrane and only 26% of the (129)I was colloidal. Of that fraction that could pass through a 1 kDa membrane, 39% of the (129)I was organo-I. Therefore, while wetlands may be highly effective at immobilizing aqueous (129)I, they may also promote the formation of a low-molecular-weight organic species that does not partition to sediments. This study provides a rare example of radioactivity concentrations increasing rather than decreasing as it migrates from a point source and brings into question assumptions in risk models regarding continuous dilution of released contaminants.

Membrane-based assay for iodide ions based on anti-leaching of gold nanoparticles

We report a label-free colorimetric strategy for the highly selective and sensitive detection of iodide (I(-)) ions in human urine sample, seawater and edible salt. A poly(N-vinyl-2-pyrrolidone)-stabilized Au nanoparticle (34.2-nm) was prepared to detect I(-) ions using silver (Ag(+)) and cyanide (CN(-)) ions as leaching agents in a glycine-NaOH (pH 9.0) solution. For the visual detection of the I(-) ions by naked eye, and for long time stability of the probe, Au nanoparticles (NPs) decorated mixed cellulose ester membrane (MCEM) was prepared (Au NPs/MCEM). The Au NPs-based probe (CN(-)/Ag(+)-Au NPs/MCEM) operates on the principle that Ag(+) ions form a monolyar silver atoms/ions by aurophilic/argentophilic interactions on the Au NPs and it accelerates the leaching rate of Au atoms in presence of CN(-) ions. However, when I(-) is introduced into this system, it inhibits the leaching of Au atoms because of the strong interactions between Ag/Au ions and I(-) ions. Inductively coupled plasma mass spectrometry, surface-assisted laser desorption/ionization time-of-flight mass spectrometry were used to characterize the surface properties of the Au NPs in the presence of Ag(+) and I(-). Under optimal solution conditions, the CN(-)/Ag(+)-Au NPs/MCEM probe enabled the detection of I(-) by the naked eye at nanomolar concentrations with high selectivity (at least 1000-fold over other anions). In addition, this cost-effective probe allowed the determination of I(-) ions in complex samples, such as urine, seawater, and edible salt samples.

Uptake mechanism for iodine species to black carbon

Natural organic matter (NOM) plays an important role in determining the fate and transport of iodine species such as iodide (I(-)) and iodate (IO3(-)) in groundwater system. Although NOM exists as diverse forms in environments, prior iodine studies have mainly focused on uptake processes of iodide and iodate to humic materials. This study was conducted to determine the iodide and iodate uptake potential for a particulate NOM (i.e., black carbon [BC]). A laboratory-produced BC and commercial humic acid were used for batch experiments to compare their iodine uptake properties. The BC exhibited >100 times greater uptake capability for iodide than iodate at low pH of ~3, while iodide uptake was negligible for the humic acid. The uptake properties of both solids strongly depend on the initial iodine aqueous concentrations. After uptake reaction of iodide to the BC, X-ray absorption fine structure spectroscopy results indicated that the iodide was converted to electrophilic species, and iodine was covalently bound to carbon atom in polycyclic aromatic hydrocarbons present in the BC. The computed distribution coefficients (i.e., Kd values) suggest that the BC materials retard significantly the transport of iodide at low pH in environmental systems containing even a small amount of BC.

Speciation analysis of 129I and its applications in environmental research

129I, a long-lived radionuclide, is important in view of geological repository of nuclear waste, and environmental tracing applications related to diverse natural processes of iodine. The environmental behaviors and bioavailability of 129I are highly related to its species. A number of methods have been reported for speciation analysis of 129I in a variety of environmental samples. These methods have been applied in many researches, including conversion processes of iodine species in marine and terrestrial systems, migration and retention of iodine in soil and sediment, geochemical cycling of iodine, as well as studies on atmospheric chemistry of iodine. This article aims to review these methods and their applications in environmental research.

Iodine-129 and iodine-127 speciation in groundwater at the Hanford site, US: iodate incorporation into calcite

The geochemical transport and fate of radioiodine depends largely on its chemical speciation that is greatly affected by environmental factors. This study reports, for the first time, the speciation of stable and radioactive iodine in the groundwater from the Hanford Site. Iodate was the dominant species and accounted for up to 84% of the total iodine present. The alkaline pH (pH ∼ 8) and predominantly oxidizing environment may have prevented reduction of the iodate. In addition, groundwater samples were found to have large amounts of calcite precipitate which were likely formed as a result of CO2 degassing during removal from the deep subsurface (>70m depth). Further analyses indicated that between 7 and 40% of the dissolved (127)I and (129)I that was originally in the groundwater had coprecipitated in the calcite. Iodate was the main species incorporated into calcite and this incorporation process could be impeded by elevating the pH and decreasing ionic strength in groundwater. This study provides critical information for predicting the long-term fate and transport of (129)I. Furthermore, the common sampling artifact resulting in the precipitation of calcite by degassing CO2, had the unintended consequence of providing insight into a potential solution for the in situ remediation of groundwater (129)I.

Distribution coefficients (K(d)) of stable iodine in estuarine and coastal regions, Japan, and their relationship to salinity and organic carbon in sediments

The sediment-water distribution coefficient, (K(d)), is one of the most important parameters in radionuclide assessment models. In this study, we determined K ds of stable iodine (I) in estuarine and coastal regions. We studied 16 estuarine and coastal regions of Japan and obtained I data on water and sediments. Data on salinity, pH, dissolved organic carbon and dissolved oxygen in water, and organic carbon (OC) in sediments were also obtained as estuarine variables. Determined (K(d))S of I in the Sagami River estuary decreased along the salinity gradient (salinity range, 0.1-33.8), indicating that salinity is one of the important factors controlling the (K(d)) values; however, when the (K(d)) values were compared among all the estuaries, the difference between minimum and maximum (K(d)) values varied by about two orders of magnitude in a narrow salinity range of 30.0-34.4. A significant correlation between (K(d)) value and OC content in sediments was observed in all the stations with a salinity of ≥ 30 except for stations in the Ishikari and Onga River estuaries. The exceptions are probably due to different sources of the sediments, which are explained by the results of relatively low I/OC ratios in sediments in those two estuaries, compared to the other estuaries. Thus, OC in sediments as well as salinity may be responsible for the variation of (K(d))S of I in the estuarine and coastal regions.

Laccase-catalyzed oxidation of iodide and formation of organically bound iodine in soils

Laccase oxidizes iodide to molecular iodine or hypoiodous acid, both of which are easily incorporated into natural soil organic matter. In this study, iodide sorption and laccase activity in 2 types of Japanese soil were determined under various experimental conditions to evaluate possible involvement of this enzyme in the sorption of iodide. Batch sorption experiment using radioactive iodide tracer ((125)I(-)) revealed that the sorption was significantly inhibited by autoclaving (121 °C, 40 min), heat treatment (80 and 100 °C, 10 min), γ-irradiation (30 kGy), N(2) gas flushing, and addition of reducing agents and general laccase inhibitors (KCN and NaN(3)). Interestingly, very similar tendency of inhibition was observed in soil laccase activity, which was determined using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) as a substrate. The partition coefficient (K(d): mL g(-1)) for iodide and specific activity of laccase in soils (Unit g(-1)) showed significant positive correlation in both soil samples. Addition of a bacterial laccase with an iodide-oxidizing activity to the soils strongly enhanced the sorption of iodide. Furthermore, the enzyme addition partially restored iodide sorption capacity of the autoclaved soil samples. These results suggest that microbial laccase is involved in iodide sorption on soils through the oxidation of iodide.

How mobile is iodide in the Callovo-Oxfordian claystones under experimental conditions close to the in situ ones?

The iodide behaviour towards the Callovo-Oxfordian claystone was studied using batch and diffusion experiments under conditions which limited the artefacts cited in the literature to be responsible for the iodide uptake (i.e. the experiments were carried out under anoxic conditions with N(2)/CO(2) atmosphere with a monitoring of the iodine redox-state). The results show that all the radioactive iodine was (125)I(-), with no measurable activity for (125)IO(3)(-), which is known to have a higher affinity for the rock than iodide. Moreover, the batch experiments revealed no sorption, independently of the initial iodide concentration (from 10(-6) to 10(-3) mol L(-1)) and the contact time (up to 106 days). Conversely, the diffusion experiments indicated a weak but measurable retention. The through-diffusion experiments led to distribution ratio values only significant (R(D)~0.05 mL g(-1)) for initial iodide concentration ≤ 10(-4) mol L(-1). Higher R(D) values were estimated from out-diffusion experiments, ranging from about 0.05 mL g(-1) for an initial concentration of 10(-3) mol L(-1) to 0.14 mL g(-1) for the lowest one. A retention phenomenon that could be reversible and kinetically-controlled was proposed to explain the differences in the extent of the iodide retardation of the two types of diffusion experiments.

Persistent source influences on the trailing edge of a groundwater plume, and natural attenuation timeframes: the F-Area Savannah River Site

At the Savannah River Site's F-Area, wastewaters containing radionuclides were disposed into seepage basins for decades. After closure and capping in 1991, the U.S. Department of Energy (DOE) has being monitoring and remediating the groundwater plume. Despite numerous studies of the plume, its persistence for over 20 years has not been well understood. To better understand the plume dynamics, a limited number of deep boreholes were drilled to determine the current plume characteristics. A mixing model was developed to predict plume tritium and nitrate concentrations. We found that the plume trailing edges have emerged for some contaminants, and that contaminant recharge from the basin's vadose zone is still important. The model's estimated time-dependent basin drainage rates combined with dilution from natural recharge successfully predicted plume tritium and nitrate concentrations. This new understanding of source zone influences can help guide science-based remediation, and improve predictions of the natural attenuation timeframes.

Sequestration and remobilization of radioiodine (129I) by soil organic matter and possible consequences of the remedial action at Savannah River Site

In order to investigate the distributions and speciation of (129)I (and (127)I) in a contaminated F-Area groundwater plume of the Savannah River Site that cannot be explained by simple transport models, soil resuspension experiments simulating surface runoff or stormflow and erosion events were conducted. Results showed that 72-77% of the newly introduced I(-) or IO(3)(-) were irreversibly sequestered into the organic-rich riparian soil, while the rest was transformed by the soil into colloidal and truly dissolved organo-iodine, resulting in (129)I remobilization from the soil greatly exceeding the 1 pCi/L drinking water permit. This contradicts the conventional view that only considers I(-) or IO(3)(-) as the mobile forms. Laboratory iodination experiments indicate that iodine likely covalently binds to aromatic structures of the soil organic matter (SOM). Under very acidic conditions, abiotic iodination of SOM was predominant, whereas under less acidic conditions (pH ≥5), microbial enzymatically assisted iodination of SOM was predominant. The organic-rich soil in the vadose zone of F-Area thus acts primarily as a "sink," but may also behave as a potentially important vector for mobile radioiodine in an on-off carrying mechanism. Generally the riparian zone provides as a natural attenuation zone that greatly reduces radioiodine release.