Site- and species-specific metal concentrations, mobility, and bioavailability in sediment, flora, and fauna of a southeastern United States salt marsh

Salt marshes are highly productive and valuable coastal ecosystems that act as filters for nutrients and pollutants at the land-sea interface. The salt marshes of the mid-Atlantic United States often exhibit geochemical behavior that varies significantly from other estuaries around the world, but our understanding of metal mobility and bioavailability remains incomplete for these systems. We sampled abiotic (water and sediment) and native biotic (three halophyte and two bivalve species) compartments of a southeastern United States salt marsh to understand the site- and species-specific metal concentrations, fractionation, and bioavailability for 16 metals and metalloids, including two naturally occurring radionuclides. Location on the marsh platform greatly influenced metal concentrations in sediment and metal bioaccumulation in halophytes, with sites above the mean high-water mark (i.e., high marsh zone) having lower concentrations in sediment but plants exhibiting greater biota sediment accumulation factors (BSAFs). Transition metal concentrations in the sediment were an average of 6× higher in the low marsh zone compared to the high marsh zone and heavy metals were on average 2× higher. Tissue- and species-specific preferential accumulation in bivalves provide opportunities for tailored biomonitoring programs. For example, mussel byssal threads accumulated ten of the sixteen studied elements to significantly greater concentrations compared to soft tissues and oysters had remarkably high soft tissue zinc concentrations (~5000 mg/kg) compared to all other species and element combinations studied. Additionally, some of our results have important implications for understanding metal mobility and implementing effective remediation (specifically phytoremediation) strategies, including observations that (1) heavy metals exhibit distinct concentration spatial distributions and metal fractionation patterns which vary from the transition metals and (2) sediment organic matter fraction appears to play an important role in controlling sediment metal concentrations, fractionation, and plant bioavailability.

Nuclear waste Educator's workshop: What and how do we teach about nuclear waste?

A workshop was held at the Massachusetts Institute of Technology (MIT) on July 25th and 26th, 2022. The objective was to develop a blueprint for educating next-generation engineers and scientists about nuclear waste management and disposal, which requires knowledge from diverse disciplines, including nuclear, chemical, civil, environmental, and geological science and engineering. The 49 participants included university professors, researchers, industry experts, and government officials from different areas. First, we have developed a list of key fundamental knowledge on waste management and disposal across the nuclear fuel cycle. In addition, we discussed strategies on how to teach students with diverse backgrounds through innovative teaching strategies as well as how to attract students into this area. Through the workshop, we identified the critical needs to (1) develop community resources for nuclear waste education; (2) synthesize historical perspectives, including past contamination and the management of general hazardous waste; (3) emphasize a complete life-cycle perspective, including proper waste management as the key component for energy sustainability; (4) teach students how to communicate about the key facts and risks to technical and non-technical audiences; and (5) accelerate the use of the state-of-art-technologies to attract and retain a young workforce. Furthermore, we aim to build a diverse, inclusive community that supports students in developing their own narratives about nuclear waste, particularly in recognizing that antagonistic views have been important to improving safety and protecting public health and the environment.

Microbial community dynamics and cycling of plutonium and iron in a seasonally stratified and radiologically contaminated pond

Plutonium (Pu) cycling and mobility in the environment can be impacted by the iron cycle and microbial community dynamics. We investigated the spatial and temporal changes of the microbiome in an iron (Fe)-rich, plutonium-contaminated, monomictic reservoir (Pond B, Savannah River Site, South Carolina, USA). The microbial community composition varied with depth during seasonal thermal stratification and was strongly correlated with redox. During stratification, Fe(II) oxidizers (e.g., Ferrovum, Rhodoferax, Chlorobium) were most abundant in the hypoxic/anoxic zones, while Fe(III) reducers (e.g., Geothrix, Geobacter) dominated the deep, anoxic zone. Sulfate reducers and methanogens were present in the anoxic layer, likely contributing to iron and plutonium cycling. Multinomial regression of predicted functions/pathways identified metabolisms highly associated with stratification (within the top 5%), including iron reduction, methanogenesis, C1 compound utilization, fermentation, and aromatic compound degradation. Two sediment cores collected at the Inlet and Outlet of the pond were dominated by putative fermenters and organic matter (OM) degraders. Overall, microbiome analyses revealed the potential for three microbial impacts on the plutonium and iron biogeochemical cycles: (1) plutonium bioaccumulation throughout the water column, (2) Pu-Fe-OM-aggregate formation by Fe(II) oxidizers under microaerophilic/aerobic conditions, and (3) Pu-Fe-OM-aggregate or sediment reductive dissolution and organic matter degradation in the deep, anoxic waters.

Experimental measurements and numerical simulations of the transport and retention of nanocrystal CdSe/ZnS quantum dots in saturated porous media: Effects of electrolytes, organic ligand, and natural organic matter

This study explores the transport and retention of CdSe/ZnS quantum dot (QD) nanoparticles in water-saturated sand columns as a function of electrolytes (Na+ and Ca2+), ionic strength, organic ligand citrate, and Suwannee River natural organic matter (SRNOM). Numerical simulations were carried out to understand the mechanisms that govern the transport and interactions of QDs in porous media and to assess how environmental parameters impact these mechanisms. An increase in the ionic strength of NaCl and CaCl2 increased QDs retention in porous media. The reduction of the electrostatic interactions screened by dissolved electrolyte ions and the increase of divalent bridging effect are the causes for this enhanced retention behavior. Citrate or SRNOM enhanced QDs transport in NaCl and CaCl2 systems by either increasing the repulsion energy barrier or inducing the steric interactions between QDs and the quartz sand collectors. A non-exponential decay characterized the retention profiles of QDs along the distance to the inlet. The modeling results indicated the four models containing the attachment, detachment, and straining terms - Model 1: M1-attachment, Model 2: M2-attachment and detachment, Model 3: M3-straining, and Model 4: M4-attachment, detachment, and straining - closely simulated the observed breakthrough curves (BTCs) but inadequately described the retention profiles.

Sources, seasonal cycling, and fate of plutonium in a seasonally stratified and radiologically contaminated pond

Unlike short-term laboratory experiments, studies at sites historically contaminated with radionuclides can provide insight into contaminant migration behavior at environmentally-relevant decadal timescales. One such site is Pond B, a seasonally stratified reservoir within Savannah River Site (SC, USA) has low levels (μBq L^-1) of plutonium in the water column. Here, we evaluate the origin of plutonium using high-precision isotope measurements, investigate the impact of water column geochemistry on plutonium cycling during different stratification periods, and re-evaluate long-term mass balance of plutonium in the pond. New isotopic data confirm that reactor-derived plutonium overwhelms input from Northern Hemisphere fallout at this site. Two suggested mechanisms for observed plutonium cycling in the water column include: (1) reductive dissolution of sediment-derived Fe(III)-(oxyhydr)oxides during seasonal stratification and (2) plutonium stabilization complexed strongly to Fe(III)-particulate organic matter (POM) complexes. While plutonium may be mobilized to a limited extent by stratification and reductive dissolution, peak plutonium concentrations are in shallow waters and associated with Fe(III)-POM at the inception of stratification. This suggests that plutonium release from sediments during stratification is not the dominant mechanism driving plutonium cycling in the pond. Importantly, our analysis suggests that the majority is retained in shallow sediments and may become increasingly recalcitrant.

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters

Uranium dioxide (UO₂) and metaschoepite (UO₃•nH₂O) particles have been identified as contaminants at nuclear sites. Understanding their behavior and impact is crucial for safe management of radioactively contaminated land and to fully understand U biogeochemistry. The Savannah River Site (SRS) (South Carolina, USA), is one such contaminated site, following historical releases of U-containing wastes to the vadose zone. Here, we present an insight into the behavior of these two particle types under dynamic conditions representative of the SRS, using field lysimeters (15 cm D x 72 cm L). Discrete horizons containing the different particle types were placed at two depths in each lysimeter (25 cm and 50 cm) and exposed to ambient rainfall for 1 year, with an aim of understanding the impact of dynamic, shallow subsurface conditions on U particle behavior and U migration. The dissolution and migration of U from the particle sources and the speciation of U throughout the lysimeters was assessed after 1 year using a combination of sediment digests, sequential extractions, and bulk and μ-focus X-ray spectroscopy. In the UO₂ lysimeter, oxidative dissolution of UO₂ and subsequent migration of U was observed over 1-2 cm in the direction of waterflow and against it. Sequential extractions of the UO₂ sources suggest they were significantly altered over 1 year. The metaschoepite particles also showed significant dissolution with marginally enhanced U migration (several cm) from the sources. However, in both particle systems the released U was quantitively retained in sediment as a range of different U(IV) and U(VI) phases, and no detectable U was measured in the lysimeter effluent. The study provides a useful insight into U particle behavior in representative, real-world conditions relevant to the SRS, and highlights limited U migration from particle sources due to secondary reactions with vadose zone sediments over 1 year.

Tissue-Specific Toxicokinetics of Aqueous Radium-226 in an Estuarine Mussel, Geukensia demissa

Radiological contamination of coastal habitats poses potential risk for native fauna, but the bioavailability of aqueous radium (Ra) and other dissolved metals to marine bivalves remains unclear. This study was the first to examine the tissue-specific disposition of aqueous ²²⁶Ra in a coastal mussel, specifically the Atlantic ribbed mussel Geukensia demissa. Most organ groups reached steady-state concentrations within 7 days during experimental exposure, with an average uptake rate constant of 0.0013 mL g^-1 d^-1. When moved to Ra-free synthetic seawater, mussels rapidly eliminated aqueous ²²⁶Ra (average elimination rate constant 1.56 d^-1). The biological half-life for aqueous ²²⁶Ra ranged from 8.9 h for the gills and labial palps to 15.4 h for the muscle. Although previous field studies have demonstrated notable ²²⁶Ra accumulation in the soft tissues of marine mussels and that, for freshwater mussels, tissue-incorporated ²²⁶Ra derives primarily from the aqueous phase, our tissue-specific bioconcentration factors (BCFs) were on the order of (8.3 ± 1.5) × 10^-4 indicating low accumulation potential of aqueous ²²⁶Ra in estuarine mussels. This suggests marine and estuarine mussels obtain ²²⁶Ra from an alternate route, such as particulate-sorbed Ra ingested during filter-feeding or from a contaminated food source.

The Influence of Iron and Ligand Type on Plutonium Uptake in Two Strains of Hydroponically Grown Corn ( Zea Mays )

This work investigates the uptake and root-shoot transport of plutonium (Pu) and iron (Fe) in corn ( Zea mays ) to gain insight into the Pu uptake pathway. Plutonium has no known biological function in plants yet may feasibly enter plants through the uptake pathway used by Fe (an essential nutrient), as these two elements have similar chemical properties. A series of experiments was conducted in which two hydroponically grown corn strains (one normal and one deficient in the transporter protein for Fe) were exposed to varying concentrations of complexed Pu and Fe. Results suggest that while Fe did inhibit Pu uptake to a certain extent, Pu was able to use alternative uptake pathways. In a 10 ppb Pu:1 ppb Fe hydroponic solution, all shoots had detectable shoot Pu concentrations compared to only 22% of plants when the Fe concentration was raised to 10 ppb. While root Pu accumulation was reduced for the corn strain deficient in the Fe transporter protein at lower Pu media concentrations, there were no differences at higher Pu concentrations, signifying the existence of substitute transport routes. A comparison of citrate and deferoxamine B (DFOB) ligand influence found that Pu complexed with DFOB remained in the roots of the plant, while movement of Pu into the shoots of the plant was more prevalent with the Pu-citrate complex. This study advances understanding of the behavior and mobility of Pu in the terrestrial environment and specifically the interactions between Pu and an essential nutrient in a common crop species.

Comparative uptake, translocation, and plant mediated transport of Tc-99, Cs-133, Np-237, and U-238 in Savannah River Site soil columns for the grass species Andropogon virginicus

This study examines the ability of the grass species Andropogon virginicus to alter the subsurface transport and redistribution of a suite of radionuclides (⁹⁹Tc, ¹³³Cs (stable analog for ¹³⁵Cs and ¹³⁷Cs), ²³⁷Np, ²³⁸U) with varying chemical behaviors in a Savannah River Site soil via the use of vegetated and unvegetated soil columns. After an acclimation period, a small volume of solution containing all radionuclides was introduced into the columns via Rhizon© pore water sampling tubes. Plants were grown for an additional 4 weeks before shoots were harvested, and columns were prepared for sampling. Plant presence led to decreased radionuclide release from the columns, mainly due to radionuclide specific combinations of system hydrology differences resulting from plant transpiration as well as plant uptake. For the most mobile radionuclides, ⁹⁹Tc followed by ²³⁷Np, plant presence resulted in significantly different soil concentration profiles between vegetated and unvegetated columns, including notable upward migration for ²³⁷Np in columns with plants. Additionally, plant uptake of ⁹⁹Tc was the greatest of all the radionuclides, with plant tissues containing an average of 44 % of the ⁹⁹Tc, while plant uptake only accounted for <2 % of ²³⁷Np and <0.5 % of ¹³³Cs and ²³⁸U in the system. Although overall plant uptake of ¹³³Cs and ²³⁸U were similar, the majority of ¹³³Cs taken up by plants was associated with ¹³³Cs already available in the aqueous phase while ²³⁸U uptake was mainly associated with the solid phase, meaning that plant activity resulted in a fraction of the native ²³⁸U being mobilized and thus, made available for plant uptake. Overall, this study quantified the influence of several plant-mediated physical and biogeochemical factors that have significant influence on radionuclide mobility and transport in this complex system which can be further utilized in future system or site-specific environmental transport and risk assessment models.

Temporal evolution of Pu and Cs sediment contamination in a seasonally stratified pond

There remains a lack of knowledge regarding ecosystem transfer, transport processes, and mechanisms, which influence the long-term mobility of Pu-239 and Cs-137 in natural environments. Monitoring the distribution and migration of trace radioisotopes as ecosystem tracers has the potential to provide insight into the underlying mechanisms of geochemical cycles. This study investigated the distribution of anthropogenic radionuclides Pu-239 and Cs-137 along with total organic carbon, iron, and trace element in contaminated sediments of Pond B at the Savannah River Site (SRS). Pond B received reactor cooling water from 1961 to 1964, and trace amounts of Pu-239 and Cs-137 during operations. Our study collected sediment cores to determine concentrations of Pu-239, Cs-137, and major and minor elements in solid phase, pore water and an electrochemical method was used on wet cores to determine dissolved elemental concentrations. More than 50 years after deposition, Pu-239 and Cs-137 in sediments are primarily located in the upper 5 cm in area where deposition of particulate-bound contaminants was prevalent and located between 5 and 10 cm in areas of high sedimentation, showing a limited migration of Pu-239 and Cs-137. A Factor analysis demonstrated different sediment facies across the pond resulting in a range of geochemical processes controlling accumulation of Pu and Cs. Highest concentrations appear to be controlled by particulate input from the influent canal, dominated by clay, silt, and sand minerals bearing Fe. Elevated Pu-239 in the sediments were observed in areas with high organic matter and higher deposition rate relative to the Pond B system near the outlet indicating strong association of Pu with OM and particulates. Therefore, organic matter cycling likely plays a role in Pu redistribution between sediment and overlying pond water, and deposition in organic rich sediments accumulating near the outlet. Though Pu appears to have been distributed throughout the pond, Cs-137 concentrations remained the highest near the influent canal.

Hybrid extractive scintillator resin for simultaneous concentration and detection of plutonium from aqueous solutions

A scoping study of a commercially available resin selective for aqueous plutonium (Pu), AnaLig® Pu-02, modified with scintillator was investigated as a scheme to simultaneously concentrate and detect Pu in aquatic matrices. The extractive scintillating resin was comprised of a silica base, functionalized for plutonium extraction, grafted with plastic scintillator of polyvinyl toluene (PVT) and 2-(1-naphthyl)-4-vinyl-5- phenyloxazole (vNPO) fluor. Scintillator was incorporated onto the AnaLig® Pu-02 resin in a two-step process of silanization followed by surface-polymerization. Successful modification was facilitated by grinding the resin beads prior to silanization to expose cleaved silica surface sites appropriate for scintillator grafting. The modified resin was subjected to initial characterization of batch uptake and radioluminosity measurements where a total detection efficiency of 32.5% was observed. The modified resin was then subjected to pH 1 simulants containing environmental relevant groundwater constituents of varying concentration. Concentrations of 0.001M Fe(III) interfered with Pu uptake, while concentrations of up to 0.01M Ca(II) and 0.001 mM concentration of natural uranium and thorium had minimal influence on plutonium uptake. A translucent column packed with the modified AnaLig® Pu-02 was placed in a commercial flow-cell radiation detector for real-time detection of plutonium; a total detection efficiency of 20.4% was achieved for on-line measurements. The modification of AnaLig® Pu-02 results in a minimum detection limit capable of meeting the EPA limit for gross alpha activity in drinking water given a sufficient counting time of 15 min and approximately 300 mL of solution volume.

Leaching and transport of technetium from reducing cementitious waste forms in field lysimeters

Field lysimeters tests examined leaching of technetium-99 (⁹⁹Tc) from two types of cementitious waste forms and found that the presence of blast furnace slag reduced the overall leaching of ⁹⁹Tc from the waste form. The two cementitious waste forms were a slag-grout 45%/45%/10% mixture of fly ash, blast furnace slag, and cement, respectively, referred to as slag-grout or a 55%/45% mixture of cement and fly ash, respectively, referred to here simply as cement. Duplicate sources of each composition were buried in four lysimeters for approximately 10 months to evaluate leaching characteristics under natural meteorological conditions in South Carolina, USA. Effluent samples were collected four times during the experiment, and the distribution of ⁹⁹Tc in the sediment was determined by destructively segmenting the lysimeters at the end of the experiment. The transport of Tc within the lysimeter was simulated by assuming advection, dispersion, and sorption in partially saturated porous media, and by using a shrinking-core type approximation for the release of Tc from the source. The shrinking-core model predicted that the oxidation front created by the oxygenated infiltrating groundwater moved into the cementitious source at a rate of 14 μm/day. As this front moved through the source, Tc(IV) was oxidized to the highly mobile Tc(VII) (as TcO₄^-) species, which then was transported through the sediment primarily via advection due to a small partitioning coefficient (C_solid/C_aq; K_d = 0.14 mL/g). The simulations predicted a cycle of accumulation of Tc in sediment at the source between rainfall events, followed by downward advection due to infiltration during rainfall events. The anomalous upward movement of Tc peak was predicted to be due to upward flux caused by evaporation after the experiment was terminated by capping the lysimeter. These experiments demonstrate that Tc leaching from cementitious waste forms under simulated vadose zone oxidizing conditions can be reasonably approximated by the shrinking core model, and the migration of Tc through the sediment is profoundly influenced by the presence of slag in the grout formulation and hydraulic conditions due to the low sorption affinity of TcO₄^-.

Experimental measurements and numerical simulations of the transport and retention of nanocrystal CdSe/ZnS quantum dots in saturated porous media: effects of pH, organic ligand, and natural organic matter

The risks of environmental exposures of quantum dot (QD) nanoparticles are increasing, but these risks are difficult to assess because fundamental questions remain about factors affecting the mobility of QDs. The objective of this study is to help address this shortcoming by evaluating the physico-chemical mechanisms controlling the transport and retention of CdSe/ZnS QDs under various environmental conditions. The approach was to run a series of laboratory-scale column experiments where QDs were transported through saturated porous media with different pH values and concentrations of citrate and Suwannee River natural organic matter (SRNOM). Numerical simulations were then conducted and compared with the laboratory data in order to evaluate parameters controlling transport. QD suspensions were injected into the column in an upward direction and ICP-MS used to analyze Cd²⁺ concentrations (C) in column effluent and sand porous media samples. The increase in the background solution pH values enhanced the QD transport and decreased the QD retention. QD transport recovery percentages obtained from the column effluent samples were 2.6%, 83.2%, 101.7%, 96.5%, and 98.9%, at pH levels of 1.5, 3.5, 5, 7, and 9, respectively. The effects of citrate and SRNOM on the transport and retention of QDs were pH dependent as reflected in the influence of the electrostatic and steric interactions between QDs and sand surfaces. QDs were mobile under unfavorable deposition conditions at environmentally relevant pHs (i.e., 5, 7, and 9). Under favorable pH conditions for deposition (i.e., 1.5), QDs were completely retained within the porous media. The retention profiles of QDs showed a non-exponential decay with distance to the inlet, attributed to multiple deposition rates caused by the QD particles and surface heterogeneities of the quartz silica sand. Results of the diameter ratios of QDs to the median sand grains, in suspensions of DI water at pH 1.5, of citrate at pH 1.5, and of citrate at pH 3.5 indicate straining as the dominating mechanism for QD retention in porous media. The blocking effect and straining were significant under favorable deposition conditions and the detachment effect was non-negligible under unfavorable deposition conditions. Physico-chemical attachment and straining are the governing mechanisms that control the retention of QDs. Overall, experimental results indicate that aggregation, deposition, straining, blocking, and DLVO-type interactions affect the advective transport and retention of QDs in saturated porous media. The simulations were conducted using models that include terms describing attachment, detachment, and straining terms-model 1: M1-attachment, model 2: M2-attachment and detachment, model 3: M3-straining, and model 4: M4-attachment, detachment, and straining. The results from simulations with M2-attachment and detachment and M4-attachment, detachment, and straining matched best the observed breakthrough curves, but all four models inadequately described the retention profiles. Our findings demonstrate that QDs are mobile in porous media under a wide range of physico-chemical conditions representative of the natural environment. The mobility behavior of QDs in porous media indicated the potential risk of soil and groundwater contamination.

Enthalpy of Uranium Adsorption onto Hematite

The influence of temperature on the adsorption of metal ions at the solid-water interface is often overlooked, despite the important role that adsorption plays in metal-ion fate and transport in the natural environment where temperatures vary widely. Herein, we examine the temperature-dependent adsorption of uranium, a widespread radioactive contaminant, onto the ubiquitous iron oxide, hematite. The multitemperature batch adsorption data and surface complexation models indicate that the adsorption of uranium, as the hexavalent uranyl (UO₂²⁺) ion, increases significantly with increasing temperature, with an adsorption enthalpy (ΔH_ads) of +71 kJ mol^-1. We suggest that this endothermic, entropically driven adsorption behavior is linked to reorganization of the uranyl-ion hydration and interfacial water structures upon U^VI adsorption at the hematite surface. Overall, this work provides fundamental insight into the thermodynamics driving metal-ion adsorption reactions and provides the specific enthalpy value necessary for improved predictive geochemical modeling of U^VI adsorption in the environment.

Dissolution and Vertical Transport of Uranium from Stable Mineral Forms by Plants as Influenced by the Co-occurrence of Uranium with Phosphorus

Plants could mobilize (dissolution followed by vertical transport) uranium (U) from mineral forms that are otherwise stable. However, the variability of this plant-mediated mobilization of U as a function of the presence of various essential plant nutrients contained in these minerals remains unknown. A series of column experiments were conducted using Andropogon virginicus to quantify the vertical transport of U from stable mineral forms as influenced by the chemical and physical coexistence of U with the essential nutrient, phosphorus (P). The presence of plants significantly increased the vertical migration of U only when U was precipitated with P (UO₂HPO₄·4H₂O; chernikovite) but not from UO₂ (uraninite) that lacks any essential plant nutrient. The U dissolution was further increased when chernikovite co-occurred with a sparingly available form of P (FePO₄) under P-limited growing conditions. Similarly, A. virginicus accumulated the highest amount of U from chernikovite (0.05 mg/g) in the presence of FePO₄ compared to that of uraninite (no-P) and chernikovite supplemented with KH₂PO_4. These results signify an increased plant-mediated dissolution, uptake, and leaching of radioactive contaminants in soils that are nutrient deficient, a key factor that should be considered in management at legacy contamination sites.

Functionalized Polymer Thin Films for Plutonium Capture and Isotopic Screening from Aqueous Sources

The rapid screening of plutonium from aqueous sources remains a critical challenge for nuclear nonproliferation efforts. The determination of trace-level Pu isotopes in water requires offsite sample preparation and analysis; therefore, new methods that combine plutonium purification, concentration, and isotopic screening in a fieldable detection system will provide an invaluable tool for nuclear safeguards. This contribution describes the development and characterization of thin polymer-ligand films for the isolation and concentration of waterborne Pu for direct spectroscopic analyses. Submicron thin films were prepared through spin coating onto Si wafers and consisted of combinations of polystyrene (PS) with dibenzoylmethane, thenoyltrifluoroacetone, and di(2-ethylhexyl)phosphoric acid (HDEHP). Pu uptake studies from solutions at pH from 2.3 to 6.3 indicated that only films containing HDEHP exhibited significant recovery of Pu. High alpha spectroscopy peak energy resolutions were achieved for PS-HDEHP films over a range of film thicknesses from 30 to 250 nm. A separate study was performed to evaluate uptake from a primarily Pu(V) solution where it was observed that doubling the HDEHP loading in the film increased uptake of Pu by an order of magnitude. X-ray photoelectron spectroscopy (XPS) analysis revealed that HDEHP was highly concentrated within the first few nanometers of the film at the higher loading. XPS analysis also revealed that, in the presence of water, HDEHP was stripped from the surface layer of the film at circumneutral pH. While significant losses of ligand were seen in all samples, higher loadings of HDEHP resulted in measurable amounts of ligand retained after a 12-h soak in water. Findings of this study are being used to guide the development of thin-film composite membrane-based detection methods for the rapid, fieldable analysis of Pu in water.

Impact of Natural Organic Matter on Plutonium Vadose Zone Migration from an NH4Pu(V)O2CO3(s) Source

We investigated the influence of natural organic matter (NOM) on the behavior of Pu(V) in the vadose zone through a combination of the field lysimeter and laboratory studies. Well-defined solid sources of NH₄Pu(V)O₂CO₃(s) were placed in two 5-L lysimeters containing NOM-amended soil collected from the Savannah River Site (SRS) or unamended vadose zone soil and exposed to 3 years of natural South Carolina, USA, meteorological conditions. Lysimeter soil cores were removed from the field, used in desorption experiments, and characterized using wet chemistry methods and X-ray absorption spectroscopy. For both lysimeters, Pu migrated slowly with the majority (>95%) remaining within 2 cm of the source. However, without the NOM amendment, Pu was transported significantly farther than in the presence of NOM. Downward Pu migration appears to be influenced by the initial source oxidation state and composition. These Pu(V) sources exhibited significantly greater migration than previous studies using Pu(IV) or Pu(III) sources. However, batch laboratory experiments demonstrated that Pu(V) is reduced by the lysimeter soil in the order of hours, indicating that downward migration of Pu may be due to cycling between Pu(V) and Pu(IV). Under the conditions of these experiments, NOM appeared to both enhance reduction of the Pu(V) source as well as Pu sorption to soils. This indicates that NOM will tend to have a stabilizing effect on Pu migration under SRS vadose zone field conditions.

Sorption and desorption of radiocesium by muscovite separated from the Georgia kaolin

Radiocesium (¹³⁷Cs) sorption by moderately weathered, sand-sized muscovite, obtained as a byproduct of kaolin ore processing, was observed at various concentrations of added stable Cs (0-100 μmol/L) over a 130 d period. After 18 h of batch sorption with 1 mmol/L NaCl as background electrolyte, conditional ¹³⁷Cs K_d values were near 2000 L/kg across the entire range of added stable Cs. Over four succeeding months, the K_d values increased by large factors for suspensions with little added Cs but increased only slightly for the suspensions with the most added Cs. The large grains of muscovite used in this study behaved distinctly differently than previously studied, much finer illite in that highly Cs-selective but low-abundance cation exchange sites appeared to be unavailable to the aqueous Cs during the first few days of the experiment. Diffusion pathways to highly Cs-selective sites were thought to be much longer in the muscovite than in frayed edges of illite, causing the highly Cs-selective sites to be isolated from the bulk solution. The longer diffusion pathways may be due to much greater stiffness of the material bounding interlayer wedges in the muscovite than in illite. This isolation from solution led to slow uptake at trace levels of Cs though the final K_d values (after 130 d) at those levels were comparable to those found for some illite. After 130 days, the original solutions were replaced by new electrolyte solutions containing no Cs, to observe ¹³⁷Cs desorption over another 130 d period. There was no indication of desorption of ¹³⁷Cs from the slowly accessible, highly Cs-selective sites apparently reached by most of the ¹³⁷Cs during sorption at the low Cs levels. The byproduct mica from kaolin processing might serve effectively as a chemically stable sorbent to isolate accidently released radiocesium and to hold it until the ¹³⁷Cs is virtually gone.

Malicious Overtones

A method for detecting electronic data theft from computer networks is described, capable of recognizing patterns of remote exfiltration occurring over days to weeks. Normal traffic flow data, in the form of a host’s ingress and egress bytes over time, is used to train an ensemble of one-class learners. The detection ensemble is modular, with individual classifiers trained on different traffic features thought to characterize malicious data transfers. We select features that model the egress to ingress byte balance over time, periodicity, short timescale irregularity, and density of the traffic. The features are most efficiently modeled in the frequency domain, which has the added benefit that variable duration flows are transformed to a fixed-size feature vector, and by sampling the frequency space appropriately, long-duration flows can be tested. When trained on days or weeks worth of traffic from individual hosts, our ensemble achieves a low false-positive rate (<2%) on a range of different system types. Simulated exfiltration samples with a variety of different timing and data characteristics were generated and used to test ensemble performance on different kinds of systems: When trained on a client workstation’s external traffic, the ensemble was generally successful at detecting exfiltration that is not simultaneously ingress-heavy, connection-sparse, and of short duration—a combination that is not optimal for attackers seeking to transfer large amounts of data. Remote exfiltration is more difficult to detect from egress-heavy systems, like web servers, with normal traffic exhibiting timing characteristics similar to a wide range of exfiltration types.

High effectiveness of pure polydopamine in extraction of uranium and plutonium from groundwater and seawater

Sorption properties of polydopamine (PDA) for uranium and plutonium from an aqueous environment are reported at three different pH values (2, 4 and 6.5–7). In addition to deionized (DI) water, artificial groundwater (GW) and seawater (SW) were used with U uptake close to 100% in each case. PDA polymer has been identified as a material with extremely high sorption capacity Q_max ∼500 mg g⁻¹ of the polymer at pH 6.5 and high selectivity for uranium. Similar high sorption properties are revealed for plutonium uptake. PDA-uranyl and PDA-plutonium interactions responsible for the observed adsorption processes have been addressed with a set of experimental techniques including FTIR spectroscopy, electron microscopy and cyclic voltammetry.

We demonstrate that complexation of pure polydopamine with uranium and plutonium species allows efficient retention of these actinides from groundwater and seawater.

Interdisciplinary Round-Robin Test on Molecular Spectroscopy of the U(VI) Acetate System

A comprehensive molecular analysis of a simple aqueous complexing system-U(VI) acetate-selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

Removal capacity and chemical speciation of groundwater iodide (I-) and iodate (IO3-) sequestered by organoclays and granular activated carbon

Radioiodine (present mostly as ¹²⁹I) is difficult to remove from waste streams or contaminated groundwater because it tends to exist as multiple anionic species (i.e., iodide (I^-), iodate (IO₃^-) and organo-iodide) that do not bind to minerals or synthetic materials. In this work, the efficacy of organoclay OCB and OCM, and granular activated carbon (GAC) as sorbents to bind I^- and IO₃^- from artificial groundwater (AGW) was examined. These sorbents were highly effective at removing I^- and IO₃^- from AGW under oxic condition, with the adsorption capacity up to 30 mg I/g sorbent. Based on X-ray spectroscopy measurements, I^- was bound to organic ligands in organoclays OCB and OCM, but when GAC was exposed to I^- in groundwater, the sequestered I species was molecular I₂. For IO₃^- interacting with organoclay OCB and GAC, the adsorbed I species remained being IO₃^-, but when organoclay OCM that contains both quaternary amine and sulfur was exposed to IO₃^-, the sulfur compound would reduce IO₃^- to I^- that was then bound to organic ligands. Thus, the inexpensive and high-capacity organoclays and GAC may provide a practical solution for removing ¹²⁹I contaminant from environmental systems and liquid nuclear wastes.

The Uptake and Translocation of 99Tc, 133Cs, 237Np, and 238U Into Andropogon Virginicus With Consideration of Plant Life Stage

Hydroponic uptake studies were conducted to evaluate the uptake and translocation of Tc, Cs (stable analog for Cs), Np, and U into established and seedling Andropogon virginicus specimens under controlled laboratory conditions. Plant specimens were grown in analyte-spiked Hoagland nutrient solution for 24 h, 3 d, and 5 d. Translocation to shoots was greatest for Tc and Cs, likely due to their analogous nature to plant nutrients, while U (and Np to a lesser extent) predominantly partitioned to root tissue with less extensive translocation to the shoots. Plant age contributed significantly to differences in concentration ratios for all nuclides in shoot tissues (p ≤ 0.024), with higher concentration ratios for seedling specimens. Additionally, duration of exposure was associated with significant differences in concentration ratios of Cs and Tc for seedlings (p = 0.007 and p = 0.030, respectively) while plant part (root or shoot) was associated with significant differences in concentration ratios of established plants (p < 0.001 for both nuclides). Statistically significant increases in radionuclide uptake in seedling specimens relative to established plants under controlled conditions suggests that, in addition to geochemical factors, plant life stage of wild grasses may also be an important factor influencing radionuclide transport in the natural environment.

Anion-exchange polymer filament coating for ultra-trace isotopic analysis of plutonium by thermal ionization mass spectrometry

A new sample loading procedure was developed for isotope measurements of ultra-trace amounts of Pu with thermal ionization mass spectrometry (TIMS) that is based on a polymer thin film architecture. The goals were to simplify single filament TIMS sample preparation for Pu, while preserving the sensitivity and accuracy of the resin bead loading method, and to eliminate sample losses experienced with the bead loading method. Rhenium filaments were degassed, dip-coated with a thin (~ 120 nm) hydrophobic base layer of poly(vinylbenzyl chloride) (PVBC), and spotted with an aqueous solution comprising triethylamine-quaternized PVBC and diazabicyclo[2.2.2]octane crosslinker. This procedure formed a toroidal, hydrophilic anion-exchange polymer spot surrounded by the hydrophobic base polymer. The thin film-coated filaments were direct loaded with 10 pg of New Brunswick Laboratory certified reference material (NBL CRM) 128 from a 9 M HCl matrix. Aqueous sample droplets adhered to the anion-exchange polymer spot, facilitating sample loading. Toroidal spots with a thickness of 20-30 µm generated the highest sample utilization, surpassing the sample utilization of the standard bead loading method by 175%. Measured isotopic ratios were in good agreement with the certified value of the ²³⁹Pu/²⁴²Pu ratio for NBL CRM 128. The use of dimpled filaments further aided sample loading by providing a well-shaped substrate to deposit the sample droplet. No sample losses were experienced with the thin film loading method over 65 sample analyses. Finally, polymer coatings suppressed filament aging under atmospheric conditions, enabling the bulk production of filaments with adequate shelf life for future analyses.

Plutonium environmental chemistry: mechanisms for the surface-mediated reduction of Pu(v/vi)

In recent decades, interest in plutonium mobility has increased significantly due to the need of the United States, as well as other nations, to deal with commercial spent nuclear fuel, nuclear weapons disarmament, and the remediation of locations contaminated by nuclear weapons testing and production. Although there is a global consensus that geologic disposal is the safest existing approach to dealing with spent nuclear fuel and high-level nuclear waste, only a few nations are moving towards implementing a geologic repository due to technical and political barriers. Understanding the factors that affect the mobility of plutonium in the subsurface environment is critical to support the development of such repositories. The importance of redox chemistry in determining plutonium mobility cannot be understated. While Pu(iv) is generally assumed to be immobile in the subsurface environment due to sorption or precipitation, Pu(v) tends to be mobile due to its relatively low effective charge and weak complex formation. This review highlights one particularly important aspect of plutonium behaviour at the mineral-water interface-the concept of surface-mediated reduction, which describes the reduction of plutonium on a mineral surface. It provides a conceptual model for and evidence supporting or refuting each proposed mechanism for surface-mediated reduction including (i) radiolysis at the mineral surface, (ii) electron transfer via ferrous iron or manganese in the mineral structure, (iii) electron shuttling due to the semiconducting properties of the mineral, (iv) disproportionation of Pu(v), (v) facilitation by proton exchange sites, (vi) stabilisation of Pu(iv) due to the increased concentration gradient within the electrical double layer, and (vii) a Nernstian favourability of Pu(iv) surface complexes and colloids. It also provides new perspectives on future research directions.

Linear Free Energy Relationship for Actinide Sorption to Graphene Oxide

Th(IV) and Np(V) sorption to graphene oxide (GO) was studied as a function of pH from 0-7.5 and analyte concentrations (0.01-1 mg/L for Th(IV) at pH 3 and 0.005-10 mg/L for Np(V) at pH 7). Starting at pH 1, greater than 90% Th(IV) sorption to GO occurred while significant Np(V) sorption to GO started at pH 5. Surface complexation modeling (SCM) using an electrostatic double layer model simultaneously modeled Th(IV) and Np(V) sorption to GO over the pH and the analyte concentration ranges. The SCM indicated that Th(IV) complexation to sulfonate sites dominated at a low pH 0-3 and its complexation to carboxylate sites dominated at a higher pH 3-7.5. In contrast, Np(V) showed a stronger affinity for sulfonate sites than carboxylate sites over the pH and concentration ranges examined in this work. Combining the results from a previous study on Eu(III) and U(VI) sorption to GO, the affinities of actinide/lanthanide sorption to GO was found to follow the trend in actinide/lanthanide ion effective charges (e.g., Th⁴⁺ (+4) > UO₂²⁺ (+3.2) > Eu³⁺ (+3) > NpO₂⁺ (+2.2)), which is similar to actinide sorption to iron oxides and clay minerals. Moreover, a linear free energy relationship was observed between the stability constants for actinides and Eu(III) (with the oxidation state from III to VI) complexation to carboxylate sites on GO and the stability constants for their complexation to carbonate.

Phosphorus Stress-Induced Changes in Plant Root Exudation Could Potentially Facilitate Uranium Mobilization from Stable Mineral Forms

Apparent deficiency of soil mineral nutrients often triggers specific physio-morphological changes in plants, and some of these changes could also inadvertently increase the ability of plants to mobilize radionuclides from stable mineral forms. This work, through a series of sand-culture, hydroponics, and batch-equilibration experiments, investigated the differential ability of root exudates of Andropogon virginicus grown under conditions with variable phosphorus (P) availability (KH₂PO₄, FePO₄, Ca₃(PO₄)₂, and no P) to solubilize uranium (U) from the uranyl phosphate mineral Chernikovite. The mineral form of P, and hence the bioavailability of P, affected the overall composition of the root exudates. The lower bioavailable forms of P (FePO₄ and Ca₃(PO₄)₂), but not the complete absence of P, resulted in a higher abundance of root metabolites with chelating capacity at 72 hrs after treatment application. In treatments with lower P-bioavailability, the physiological amino acid concentration inside of the roots increased, whereas the concentration of organic acids in the roots decreased due to the active exudation. In batch dissolution experiments, the organic acids, but not amino acids, increase the dissolution U from Chernikovite. The root exudate matrix of plants exposed to low available forms of P induced a >60% increase in U dissolution from Chernikovite due to 5-16 times greater abundance of organic acids in these treatments. However, this was ca. 70% of the theoretical dissolution achievable by this exudate matrix. These results highlight the potential of using active management of soil P as an effective tool to alter the plant-mediated mobilization of U in contaminated soil.

One-dimensional Spatial Distributions of Gamma-ray Emitting Contaminants in Field Lysimeters Using a Collimated Gamma-ray Spectroscopy System

One-dimensional scans of gamma-ray emitting contaminants were conducted on lysimeters from the RadFLEX facility at the Savannah River Nationals Laboratory (SRNL). The lysimeters each contained a contamination source that was buried in SRNL soil. A source consisted of Cs, Co, Ba, and Eu incorporated either into a solid waste form (Portland cement and reducing grout) or applied to a filter paper for direct soil exposure. The lysimeters were exposed to natural environmental conditions for 3 to 4 y. The initial contaminant activities range from 4.0 to 9.0 MBq for the solid wasteforms and 0.25 to 0.47 MBq for the soil-incorporated source. The measurements were performed using a collimated high-purity germanium gamma-ray spectrometer with a spatial resolution of 2.5 mm. These scans showed downward mobility of Co and Ba when the radionuclides were incorporated directly into the SRNL soil. When radionuclides were incorporated into the solid waste forms positioned in the SRNL soil, Cs exhibited both upward and downward dispersion while the other radionuclides showed no movement. This dispersion was more significant for the Portland cement than the reducing grout wasteform. Europium-152 was the only radionuclide of those studied that showed no movement within the spatial resolution of the scanner from the original placement within the lysimeter. Understanding radionuclide movement in the environment is important for developing strategies for waste management and disposal.

Mobility of Aqueous and Colloidal Neptunium Species in Field Lysimeter Experiments

Due to its radiotoxicity, long half-life, and potentially high environmental mobility, neptunium transport is of paramount importance for risk assessment and safety. Environmental transport of neptunium through field lysimeters at the Savannah River Site was observed from both oxidized (Np(V)) and reduced (Np(IV)) source materials. While transport from oxidized neptunium sources was expected, the unexpected transport from reduced neptunium sources spurred further investigation into transport mechanisms. Partial oxidation of the reduced neptunium source resulted in significant release and transport into the mobile aqueous phase, though a reduced colloidal neptunium species appears to have also been present, enhancing neptunium mobility over shorter distances. These field and laboratory experiments demonstrate the multiple controls on neptunium vadose zone transport and chemical behavior, as well as the need for thorough understanding of radionuclide source terms for long-term risk prediction.

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.

Sulfur speciation in untreated and alkali treated ground-granulated blast furnace slag

Reduced sulfur species in ground-granulated blast furnace slag (GGBFS) play an important role in immobilizing radionuclide contaminants in caustic cement-GGBFS mixtures via reductive precipitation reaction. However, sulfur (S) speciation and its stability in GGBFS have not been clearly understood. In this study, S speciation of GGBSF in alkaline radionuclide liquid waste simulant solutions was investigated using S K-edge X-ray absorption near edge structure spectroscopy (XANES) and powder X-ray diffraction (XRD) measurements. Although S mineralogy was not detectable by XRD due to the amorphous nature in GGBFS, XANES analysis revealed that GGBSF contained high concentration of sulfoxide (~57%), followed by S(0) (~37%), sulfate (~3.81%), and sulfonate (~2.33%). When GGBFS was reacted with anoxic or oxygenated alkali solutions, it retained most of sulfoxide with some changes in the fraction of elemental S, sulfonate and sulfate, indicating the involvement of reduced S species in the reductive precipitation of radionuclides. This study shows the presence of intermediate S valence species in GGBFS.

High-Resolution 4D Preclinical Single-Photon Emission Computed Tomography/X-ray Computed Tomography Imaging of Technetium Transport within a Heterogeneous Porous Media

A dynamic ^99mTc tracer experiment was performed to investigate the capabilities of combined preclinical single photon emission computed tomography (SPECT) and X-ray computed tomography (CT) for investigating transport in a heterogeneous porous medium. The experiment was conducted by continuously injecting a ^99mTc solution into a column packed with eight layers (i.e., soil, silica gel, and 0.2-4 mm glass beads). Within the imaging results it was possible to correlate observed features with objects as small as 2 mm for the SPECT and 0.2 mm for the CT. Time-lapse SPECT imaging results illustrated both local and global nonuniform transport phenomena and the high-resolution CT data were found to be useful for interpreting the cause of variations in the ^99mTc concentration associated with structural features within the materials, such as macropores. The results of this study demonstrate SPECT/CT as a novel tool for 4D (i.e., transient three-dimensional) noninvasive imaging of fate and transport processes in porous media. Despite its small scale, an experiment with such high resolution data allows us to better understand the pore scale transport which can then be used to inform larger scale studies.

Ambient aging of rhenium filaments used in thermal ionization mass spectrometry: Growth of oxo-rhenium crystallites and anti-aging strategies

Degassing is a common preparation technique for rhenium filaments used for thermal ionization mass spectrometric analysis of actinides, including plutonium. Although optimization studies regarding degassing conditions have been reported, little work has been done to characterize filament aging after degassing. In this study, the effects of filament aging after degassing were explored to determine a “shelf-life” for degassed rhenium filaments, and methods to limit filament aging were investigated. Zone-refined rhenium filaments were degassed by resistance heating under high vacuum before exposure to ambient atmosphere for up to 2 months. After degassing the nucleation and preferential growth of oxo-rhenium crystallites on the surface of polycrystalline rhenium filaments was observed by atomic force microscopy and scanning electron microscopy (SEM). Compositional analysis of the crystallites was conducted using SEM-Raman spectroscopy and SEM energy dispersive X-ray spectroscopy, and grain orientation at the metal surface was investigated by electron back-scatter diffraction mapping. Spectra collected by SEM-Raman suggest crystallites are composed primarily of perrhenic acid. The relative extent of growth and crystallite morphology were found to be grain dependent and affected by the dissolution of carbon into filaments during annealing (often referred to as carbonization or carburization). Crystallites were observed to nucleate in region specific modes and grow over time through transfer of material from the surface. Factors most likely to affect the rates of crystallite growth include rhenium substrate properties such as grain size, orientation, levels of dissolved carbon, and relative abundance of defect sites; as well as environmental factors such as length of exposure to oxygen and relative humidity. Thin (∼180 nm) hydrophobic films of poly(vinylbenzyl chloride) were found to slow the growth of oxo-rhenium crystallites on the filament surfaces and may serve as an alternative carbon source for filament carburization.

Effect of Natural Organic Matter on Plutonium Sorption to Goethite

The effect of citric acid (CA), desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on plutonium (Pu) sorption to goethite was studied as a function of organic carbon concentration and pH using batch sorption experiments at 5 mg_C·L^-1 and 50 mg_C·L^-1 natural organic matter (NOM), 10^-9-10^-10 M ²³⁸Pu, and 0.1 g·L^-1 goethite concentrations, at pH 3, 5, 7, and 9. Low sorption of ligands coupled with strong Pu complexation decreased Pu sorption at pH 5 and 7, relative to a ligand-free system. Conversely, CA, FA, and HA increased Pu sorption to goethite at pH 3, suggesting ternary complex formation or, in the case of humic acid, incorporation into HA aggregates. Mechanisms for ternary complex formation were characterized by Fourier transform infrared spectroscopy in the absence of Pu. CA and FA demonstrated clear surface interactions at pH 3, HA appeared unchanged suggesting HA aggregates had formed, and no DFOB interactions were observed. Plutonium sorption decreased in the presence of DFOB (relative to a ligand free system) at all pH values examined. Thus, DFOB does not appear to facilitate formation of ternary Pu-DFOB-goethite complexes. At pH 9, Pu sorption in the presence of all NOM increased relative to pH 5 and 7; speciation models attributed this to Pu(IV) hydrolysis competing with ligand complexation, increasing sorption. The results indicate that in simple Pu-NOM-goethite ternary batch systems, NOM will decrease Pu sorption to goethite at all but particularly low pH conditions.

Quantitative Measurement of Ligand Exchange with Small-Molecule Ligands on Iron Oxide Nanoparticles via Radioanalytical Techniques

Ligand exchange on the surface of hydrophobic iron oxide nanoparticles is a common method for controlling surface chemistry for a desired application. Furthermore, ligand exchange with small-molecule ligands may be necessary to obtain particles with a specific size or functionality. Understanding to what extent ligand exchange occurs and what factors affect it is important for the optimization of this critical procedure. However, quantifying the amount of exchange may be difficult because of the limitations of commonly used characterization techniques. Therefore, we utilized a radiotracer technique to track the exchange of a radiolabeled ¹⁴C-oleic acid ligand with hydrophilic small-molecule ligands on the surface of iron oxide nanoparticles. Iron oxide nanoparticles functionalized with ¹⁴C-oleic acid were modified with small-molecule ligands with terminal functional groups including catechols, phosphonates, sulfonates, thiols, carboxylic acids, and silanes. These moieties were selected because they represent the most commonly used ligands for this procedure. The effectiveness of these molecules was compared using both procedures widely found in the literature and using a standardized procedure. After ligand exchange, the nanoparticles were analyzed using liquid scintillation counting (LSC) and inductively coupled plasma-mass spectrometry. The labeled and unlabeled particles were further characterized by transmission electron microscopy (TEM) and dynamic light scattering (DLS) to determine the particle size, hydrodynamic diameter, and zeta potential. The unlabeled particles were characterized via attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and vibrating sample magnetometry (VSM) to confirm the presence of the small molecules on the particles and verify the magnetic properties, respectively. Radioanalytical determination of ¹⁴C-oleic acid was used to calculate the total amount of oleic acid remaining on the surface of the particles after ligand exchange. The results revealed that the ligand-exchange reactions performed using widely cited procedures did not go to completion. Residual oleic acid remained on the particles after these reactions and the reactions using a standardized protocol. A comparison of the ligand-exchange procedures indicated that the binding moiety, multidenticity, reaction time, temperature, and presence of a catalyst impacted the extent of exchange. Quantification of the oleic acid remaining after ligand exchange revealed a binding hierarchy in which catechol-derived anchor groups displace the most oleic acid on the surface of the nanoparticles and the thiol group displaces the least amount of oleic acid. Thorough characterization of ligand exchange is required to develop nanoparticles suitable for their intended application.

Experimental evidence for ternary colloid-facilitated transport of Th(IV) with hematite (α-Fe2O3) colloids and Suwannee River fulvic acid

Previous field experiments have suggested colloid-facilitated transport via inorganic and organic colloids as the primary mechanism of enhanced actinide transport in the subsurface at former nuclear weapons facilities. In this work, research was guided by the hypothesis that humic substances can enhance tetravalent actinide (An(IV)) migration by coating and mobilizing natural colloids in environmental systems and increasing An(IV) sorption to colloids. This mechanism is expected to occur under relatively acidic conditions where organic matter can sorb and coat colloid surfaces and facilitate formation of ternary colloid-ligand-actinide complexes. The objective of this work was to examine Th transport through packed columns in the presence of hematite colloids and/or Suwannee River fulvic acid (SRFA). In the presence of SRFA, with or without hematite colloids, significant transport (>60% recovery within the effluent) of thorium occurred through quartz columns. It is notable that the SRFA contributed to increased transport of both Th and hematite colloids, while insignificant transport occurred in the absence of fulvic acid. Further, in the presence of a natural sandy sediment (as opposed to pure quartz), transport is negligible in the presence of SRFA due to interactions with natural, clay-sized sediment coatings. Moreover, this data shows that the transport of Th through quartz columns is enhanced in ternary Th-colloid-SRFA and binary Th-SRFA systems as compared to a system containing only Th.

Influence of the goethite (α-FeOOH) surface on the stability of distorted PuO2 and PuO2–x phases

Experiments by [Powell, B. A., Dai, Z. R., Zavarin, M., Zhao, P. H., Kersting, A. B.: Stabilization of plutonium nano-colloids by epitaxial distortion on mineral surfaces. Environ. Sci. Technol. 45, 2698 (2011).] deduced the heteroepitaxial growth of a bcc Pu4O7 phase when sorbed onto goethite from d-spacing measurements obtained from selected-area electron diffraction (SAED) patterns. The structural and/or chemical modification of Pu(IV) oxide (PO) nanocolloids upon sorption to goethite, in turn, affects colloidal-transport of Pu in the subsurface. In this study, molecular simulations were applied to investigate mechanisms affecting the formation of non-fcc PO phases and to understand the influence of goethite in stabilizing the non-fcc PO phase. Analyses of the structure, chemistry, and formation energetics for several bulk PuO2 and PuO2–x phases, using ab initio methods, show that the formation of a non-fcc PO can occur from the lattice distortion (LD) of fcc PuO2 upon sorption and formation of a PO–goethite interface. To strain and non-uniformly distort the PuO2 lattice to match that of the goethite substrate at ambient conditions would require 88 kJ/mol Pu4O8. The formation of a hypostoichiometric PuO2–x phase, such as the experimentally-deduced bcc, Ia3̅ Pu4O7 phase, requires more O-poor conditions and/or high energetic inputs (> +365 kJ/mol Pu4O7 at O-rich conditions). Empirical methods were also applied to study the effect of lattice distortion on sorption energetics and adsorbate particle growth using simple heterointerfaces between cubic salts, where KCl clusters (notated as KClLD) of varying size and lattice mismatch (LM) were sorbed to a NaCl cluster. When the lattice of a KClLD cluster has <15% LM with that of a NaCl substrate, the sorption of KClLD onto NaCl is exothermic (<–80 kJ/mol) and the KClLD cluster can reach sizes of ~2–5 nm on the NaCl substrate. These models suggest that the lattice of a fcc PuO2 particle can distort upon formation of a heterointerface with goethite to lower LM, in turn better enabling the growth of the PO adsorbates and yielding more exothermic adsorption energies. A more detailed understanding of the interfacial environment between PO and goethite is obtained through structural, chemical, and energetic analyses on modeled PuO2 (110)– and PuO2–x (110)–goethite (001) heterointerfaces. Structural analyses of the heterointerfaces continue to support that the lattice of PO is strained to better match that of goethite and thus lead to the formation of a non-fcc PO phase. When the lattice of the PO (110) surface is distorted to match that of the goethite (001) surface, the alignment and d-spacings from simulated electron diffraction patterns for the PO–goethite heterointerfaces reproduce experimental observations. Non-fcc PO thin-films are also found to be stabilized through the formation of an interface with goethite, as the work of adhesion for the PuO2– and PuO2–x–goethite interfaces are 1.4 J/m2 and 2.0 J/m2, respectively. Analyses of electron and charge density of the heterointerfaces also show that covalent- to polar-covalent bonding at the interface promotes the stabilization of a PO–goethite interface. The results from these models contribute to experimental observations, providing further understanding of how the goethite substrate influences the formation and stabilization of a non-fcc PO phase. Furthermore, the information from this study aids in better understanding processes at mineral–water interfaces that influence actinide transport.

A Novel Method for Tracer Concentration Plutonium(V) Solution Preparation

Preparation of relatively pure low concentration Pu(V) solutions for environmental studies is nontrivial due to the complex redox chemistry of Pu. Ozone gas generated by an inexpensive unit designed for household-use was used to oxidize a 2 × 10(-8) M Pu(IV) solution to predominantly Pu(VI) with some Pu(V) present. Over several days, the Pu(VI) in the solution reduced to Pu(V) without further reducing to Pu(IV). The reduction from Pu(VI) to Pu(V) could be accelerated by raising the pH of the solution, which led to an immediate conversion without substantial conversion to Pu(IV). The aqueous Pu was found to be stable as predominately Pu(V) for greater than one month from pH 3-7; however, at circumneutral pH, a sizable fraction of Pu was lost from solution by either precipitation or sorption to the vial walls. This method provides a fast means of preparing Pu(V) solutions for tracer concentration studies without numerous extraction or cleanup steps.

Surface Complexation Modeling of Eu(III) and U(VI) Interactions with Graphene Oxide

Graphene oxide (GO) has great potential for actinide removal due to its extremely high sorption capacity, but the mechanism of sorption remains unclear. In this study, the carboxylic functional group and an unexpected sulfonate functional group on GO were characterized as the reactive surface sites and quantified via diffuse layer modeling of the GO acid/base titrations. The presence of sulfonate functional group on GO was confirmed using elemental analysis and X-ray photoelectron spectroscopy. Batch experiments of Eu(III) and U(VI) sorption to GO as the function of pH (1-8) and as the function of analyte concentration (10-100, 000 ppb) at a constant pH ≈ 5 were conducted; the batch sorption results were modeled simultaneously using surface complexation modeling (SCM). The SCM indicated that Eu(III) and U(VI) complexation to carboxylate functional group is the main mechanism for their sorption to GO; their complexation to the sulfonate site occurred at the lower pH range and the complexation of Eu(III) to sulfonate site are more significant than that of U(VI). Eu(III) and U(VI) facilitated GO aggregation was observed with high Eu(III) and U(VI) concentration and may be caused by surface charge neutralization of GO after sorption.

Effects of Titanium Doping in Titanomagnetite on Neptunium Sorption and Speciation

Neptunium-237 is a radionuclide of great interest owing to its long half-life (2.14 × 10(6) years) and relative mobility as the neptunyl ion (NpO2(+)) under many surface and groundwater conditions. Reduction to tetravalent neptunium (Np(IV)) effectively immobilizes the actinide in many instances due to its low solubility and strong interactions with natural minerals. One such mineral that may facilitate the reduction of neptunium is magnetite (Fe(2+)Fe(3+)2O4). Natural magnetites often contain titanium impurities which have been shown to enhance radionuclide sorption via titanium's influence on the Fe(2+)/Fe(3+) ratio (R) in the absence of oxidation. Here, we provide evidence that Ti-substituted magnetite reduces neptunyl species to Np(IV). Titanium-substituted magnetite nanoparticles were synthesized and reacted with NpO2(+) under reducing conditions. Batch sorption experiments indicate that increasing Ti concentration results in higher Np sorption/reduction values at low pH. High-resolution transmission electron microscopy of the Ti-magnetite particles provides no evidence of NpO2 nanoparticle precipitation. Additionally, X-ray absorption spectroscopy confirms the nearly exclusive presence of Np(IV) on the titanomagnetite surface and provides supporting data indicating preferential binding of Np to terminal Ti-O sites as opposed to Fe-O sites.

Chemical stabilization of chromate in blast furnace slag mixed cementitious materials

Cement waste form (CWF) technology is among the leading approaches to disposing of metals and liquid low-level nuclear waste in the United States. One such material, saltstone, includes slag, fly ash and Portland cement to enhance the immobilization of contaminants (e.g., Cr, (99)Tc) in alkaline liquid wastes. To evaluate the stability of such redox sensitive contaminants in saltstone, the effects of slag as a source of reductant on Cr immobilization was evaluated in aged (<300 d) saltstone monoliths. Specifically, we investigated the effects of artificial cement pore waters on the Cr release and the spatially resolved Cr chemical state analysis using synchrotron based microfocused X-ray microprobe analysis. The microprobe analysis indicated the heterogeneous distribution of insoluble Cr(III)-species in saltstone. Although at most of 20% Crtotal was leached at the top few (2-3) millimeter depth, the release of Cr(VI) was small (<5%) at 5-30 mm with slight changes, indirectly suggesting that Cr is likely present as insoluble Cr(III) species throughout the depths. The study suggests that this saltstone formulation can effectively retain/immobilize Cr under the oxic field condition after ⩽300 d of aging time.