Residence time effects on technetium reduction in slag-based cementitious materials.

A long-term disposal of technetium-99 (99Tc) has been considered in a type of cementitious formulation, slag-based grout, at the U.S. Department of Energy, Savannah River Site, Aiken SC, U.S.A. Blast furnace slag, which contains S and Fe electron donors, has been used in a mixture with fly ash, and Portland cement to immobilize 99Tc(VII)O4-(aq) in low level radioactive waste via reductive precipitation reaction. However the long-term stability of Tc(IV) species is not clearly understood as oxygen gradually diffuses into the solid structure. In this study, aging effects of Tc speciation were investigated as a function of depth (<2.5cm) in slag-based grout using X-ray absorption spectroscopy. All of Fe(II) in solids was oxidized to Fe(III) after 117d. However, elemental S, sulfide, and sulfoxide persists at the 0-8mm depths even after 485d, suggesting the presence of a reduced zone below the surface few millimeters. Pertechnetate was successfully reduced to Tc(IV) after 29d. Distorted hydrolyzed Tc(IV) octahedral molecules were partially sulfidized and or polymerized at all depths (0-8mm) and were stable in 485d aged sample. The results of this study suggest that variable S species contribute to stabilize the partially sulfidized Tc(IV) species in aged slag-based grout.

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.

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.

Physiological and genetic analysis of multiple sodium channel variants in a model of genetic absence epilepsy.

In excitatory neurons, SCN2A (NaV1.2) and SCN8A (NaV1.6) sodium channels are enriched at the axon initial segment. NaV1.6 is implicated in several mouse models of absence epilepsy, including a missense mutation identified in a chemical mutagenesis screen (Scn8a(V929F)). Here, we confirmed the prior suggestion that Scn8a(V929F) exhibits a striking genetic background-dependent difference in phenotypic severity, observing that spike-wave discharge (SWD) incidence and severity are significantly diminished when Scn8a(V929F) is fully placed onto the C57BL/6J strain compared with C3H. Examination of sequence differences in NaV subunits between these two inbred strains suggested NaV1.2(V752F) as a potential source of this modifier effect. Recognising that the spatial co-localisation of the NaV channels at the axon initial segment (AIS) provides a plausible mechanism for functional interaction, we tested this idea by undertaking biophysical characterisation of the variant NaV channels and by computer modelling. NaV1.2(V752F) functional analysis revealed an overall gain-of-function and for NaV1.6(V929F) revealed an overall loss-of-function. A biophysically realistic computer model was used to test the idea that interaction between these variant channels at the AIS contributes to the strain background effect. Surprisingly this modelling showed that neuronal excitability is dominated by the properties of NaV1.2(V752F) due to "functional silencing" of NaV1.6(V929F) suggesting that these variants do not directly interact. Consequent genetic mapping of the major strain modifier to Chr 7, and not Chr 2 where Scn2a maps, supported this biophysical prediction. While a NaV1.6(V929F) loss of function clearly underlies absence seizures in this mouse model, the strain background effect is apparently not due to an otherwise tempting Scn2a variant, highlighting the value of combining physiology and genetics to inform and direct each other when interrogating genetic complex traits such as absence epilepsy.

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.

Upward movement of plutonium to surface sediments during an 11-year field study.

An 11-year lysimeter study was established to monitor the movement of Pu through vadose zone sediments. Sediment Pu concentrations as a function of depth indicated that some Pu moved upward from the buried source material. Subsequent numerical modeling suggested that the upward movement was largely the result of invading grasses taking up the Pu and translocating it upward. The objective of this study was to determine if the Pu of surface sediments originated from atmosphere fallout or from the buried lysimeter source material (weapons-grade Pu), providing additional evidence that plants were involved in the upward migration of Pu. The (240)Pu/(239)Pu and (242)Pu/(239)Pu atomic fraction ratios of the lysimeter surface sediments, as determined by Thermal Ionization Mass Spectroscopy (TIMS), were 0.063 and 0.00045, respectively; consistent with the signatures of the weapons-grade Pu. Our numerical simulations indicate that because plants create a large water flux, small concentrations over multiple years may result in a measurable accumulation of Pu on the ground surface. These results may have implications on the conceptual model for calculating risk associated with long-term stewardship and monitored natural attenuation management of Pu contaminated subsurface and surface sediments.

Interactions among phosphate amendments, microbes and uranium mobility in contaminated sediments.

The use of sequestering agents for the transformation of radionuclides in low concentrations in contaminated soils/sediments offers considerable potential for environmental cleanup. This study evaluated the influence of three types of phosphate (rock phosphate, biological phosphate, and calcium phytate) and two microbial amendments (Alcaligenes piechaudii and Pseudomonas putida) on U mobility. All tested phosphate amendments reduced aqueous U concentrations more than 90%, likely due to formation of insoluble phosphate precipitates. The addition of A. piechaudii and P. putida alone were found to reduce U concentrations 63% and 31%, respectively. Uranium removal in phosphate treatments was significantly reduced in the presence of the two microbes. Two sediments were evaluated in experiments on the effects of phosphate amendments on U mobility, one from a stream on the Department of Energy's Savannah River Site near Aiken, SC and the other from the Hanford Site, a Department of Energy facility in Washington state. Increased microbial activity in the treated sediment led to a reduction in phosphate effectiveness. The average U concentration in 1 M MgCl(2) extract from U contaminated sediment was 437 microg/kg, but in the same sediment without microbes (autoclaved), the extractable U concentration was only 103 microg/kg. The U concentration in the 1 M MgCl(2) extract was approximately 0 microg/kg in autoclaved amended sediment treated with autoclaved biological apatite. These results suggest that microbes may reduce phosphate amendment remedial effectiveness.

Phosphate sources and their suitability for remediation of contaminated soils.

Phosphate minerals and specifically apatite show promise for environmental cleanup because they can form stable compounds with a wide range of cationic contaminants. However, phosphate minerals naturally accumulate some heavy metals that may cause additional contamination of the environment if used improperly. Nine commercially available phosphate materials were evaluated for remediation of contaminated soil based on solubility, concentration of metal/metalloid impurities, and leachability of impurity metal/metalloids. The phosphate materials consisted of three groups: processed (i.e., fertilizers), mined (rock phosphates from different formations), and biogenic (ground fish bone). Processed and mined rock phosphates contained relatively high total concentrations of As, Co, Cr, and Cu but did not exceed the RCRA toxicity characteristic leaching procedure (TCLP) limits. Biogenic apatite contained much lower metal concentrations than processed and mined rock phosphate and was appreciably more soluble. By combining biogenic and mined phosphate it is possible to obtain a wide range of phosphate release rates, permitting rapid immobilization of contaminants while providing a slow release of phosphate for continued long-term treatment.

Apatite and phillipsite as sequestering agents for metals and radionuclides.

Laboratory and greenhouse studies were conducted to quantify apatite and phillipsite (zeolite) sequestration of selected metal contaminants. The laboratory batch study measured the sorption of aqueous Co2+, Ba2+, Pb2+, Eu3+, and UO2(2+). Apatite sorbed more Co2+, Pb2+, Eu3+, and UO2(2+) from the spike solution than phillipsite, resulting in distribution coefficients (Kd values) of >200,000 L kg(-1). Phillipsite was more effective than apatite at sorbing aqueous Ba2+. Results from the laboratory study were used to design the greenhouse study that used a soil affected by a Zn-Pb smelter from Pribram, Czech Republic. Two application rates (25 and 50 g kg(-1)) of phillipsite and apatite and two plant species, maize (Zea mays L.) and oat (Avena sativa L.), were evaluated in this study. There was little (maize) to no (oat) plant growth in the unamended contaminated soil. Apatite and, to a slightly lesser extent, phillipsite additions significantly enhanced plant growth and reduced Cd, Pb, and Zn concentrations in all analyzed tissues (grain, leaves, and roots). The sequestering agents also affected some essential elements (Ca, Fe, and Mg). Phillipsite reduced Fe and apatite reduced P and Fe concentrations in oat tissues; however, the level of these elements in oat leaves and grains remained sufficient. Sequential extractions of the soil indicated that the Cd, Pb, and Zn were much more strongly sorbed onto the amended soil, making the contaminants less phytoavailable.

Colloid transport and deposition in water-saturated Yucca Mountain tuff as determined by ionic strength.

Colloid mobility and deposition were determined in model systems consisting of quartz sand or crushed Yucca Mountain tuff, latex microspheres (colloidal particles), and simulated groundwater. Ionic strength (I) was manipulated as a first step in defining limiting conditions for colloid transport in a system modeled after geochemical conditions at the Yucca Mountain site. Solutions of deionized water (DI), 0.1x, 1x, and 10x (the ionic strength of simulated groundwater) (I = 0.0116 M) were used in saturated columns under steady-state flow conditions. Separate experiments with conservative tracers indicated stable hydrodynamic conditions that were independent of I. Colloids were completely mobile (no deposition) in the DI and 0.1x solutions; deposition increased to 11-13% for 1x and to 89-97% for 10x treatments with similar results for sand and tuff. Deposition was described as a pseudo-first-order process; however, a decreasing rate of deposition was apparent for colloid transport at the 10x condition through the tuff. A linear dependence of colloid removal (extent and deposition rate coefficient) on I is illustrated for the model Yucca Mountain system and for a glass-KCl system reported in the literature. This simple relationship for saturated systems may be useful for predicting deposition efficiencies under conditions of varying ionic strength.

Physical and chemical determinants of colloid transport and deposition in water-unsaturated sand and Yucca Mountain tuff material.

Colloid mobility was determined in a system consisting of quartz sand or crushed Yucca Mountain tuff, simulated groundwater (J-13), and hydrophilic latex particles. Water content (theta) and ionic strength (I; DI water, 0.1x, 1x, 10x groundwater dilution) were manipulated to define limiting conditions for colloid transport atthe Yucca Mountain site. Colloid transport, measured with a centrifuge method at relatively high theta (saturation >36% for sand, >62% for crushed tuff) in DI water, was equivalent to transport at 100% saturation measured with conventional columns. When variables were isolated, increasing I and decreasing theta resulted in a greater extent of colloid deposition; I was more important at higher theta; physical properties were more important at lower theta. I and theta had an interactive effect on colloid deposition whereby synergism was generally observed, especially for simulated groundwater (1x); antagonism was observed at 10x groundwater dilution. At 19% moisture saturation on the crushed tuff, a decreasing rate of colloid deposition was observed. This corresponded to a hydrodynamic condition of 79% immobile water where solute tracers were excluded from a fraction of the pore volume. This suggests that a portion of the favorable sites for deposition were associated with the excluded or immobile water domain and were not accessible to colloids.