Noble gas adsorption to tuff.

A method was developed to measure trace noble gas element adsorption to the surfaces of geologic materials in the presence of a background gas that could potentially compete for surface adsorption sites. Adsorption of four noble gas elements (Ne, Ar, Kr, and Xe) at a concentration of 100 ppm in helium and nitrogen were measured on a sample of crushed tuff at 0, 15, 30, and 45 °C. In addition, Ne, Ar, Kr, and Xe at 250 ppm and 500 ppm in nitrogen at 15 °C were measured. Noble gas adsorption was found to increase with increasing atomic mass and decreasing temperature. It was also observed that the relative increase in noble gas element adsorption with decreasing temperature tends to increase with increasing atomic mass. As the noble gas concentrations in nitrogen increased, adsorption increased in a slightly non-linear fashion which could be modeled using a Freundlich isotherm. For noble gas concentrations that were ≤100 ppm Henry's Law constant were calculated.

Hybrid Sorbents for 129I Capture from Contaminated Groundwater.

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

Evaluation of materials for iodine and technetium immobilization through sorption and redox-driven processes.

Radioactive iodine-129 (129I) and technetium-99 (99Tc) pose a risk to groundwater due to their long half-lives, toxicity, and high environmental mobility. Based on literature reviewed in Moore et al. (2019) and Pearce et al. (2019), natural and engineered materials, including iron oxides, low-solubility sulfides, tin-based materials, bismuth-based materials, organoclays, and metal organic frameworks, were tested for potential use as a deployed technology for the treatment of 129I and 99Tc to reduce environmental mobility. Materials were evaluated with metrics including capacity for IO3- and TcO4- uptake, selectivity and long-term immobilization potential. Batch testing was used to determine IO3- and TcO4- sorption under aerobic conditions for each material in synthetic groundwater at different solution to solid ratios. Material association with IO3- and TcO4- was spatially resolved using scanning electron microscopy and X-ray microprobe mapping. The potential for redox reactions was assessed using X-ray absorption near edge structure spectroscopy. Of the materials tested, bismuth oxy(hydroxide) and ferrihydrite performed the best for IO3-. The commercial Purolite A530E anion-exchange resin outperformed all materials in its sorption capacity for TcO4-. Tin-based materials had high capacity for TcO4-, but immobilized TcO4- via reductive precipitation. Bismuth-based materials had high capacity for TcO4-, though slightly lower than the tin-based materials, but did not immobilize TcO4- by a redox-drive process, mitigating potential negative re-oxidation effects over longer time periods under oxic conditions. Cationic metal organic frameworks and polymer networks had high Tc removal capacity, with TcO4- trapped within the framework of the sorbent material. Although organoclays did not have the highest capacity for IO3- and TcO4- removal in batch experiments, they are available commercially in large quantities, are relatively low cost and have low environmental impact, so were investigated in column experiments, demonstrating scale-up and removal of IO3- and TcO4- via sorption, and reductive immobilization with iron- and sulfur-based species.

Effects of hydrated lime on radionuclides stabilization of Hanford tank residual waste.

Chemical stabilization of tank residual waste is part of a Hanford Site tank closure strategy to reduce overall risk levels to human health and the environment. In this study, a set of column leaching experiments using tank C-104 residual waste were conducted to evaluate the leachability of uranium (U) and technetium (Tc) where grout and hydrated lime were applied as chemical stabilizing agents. The experiments were designed to simulate future scenarios where meteoric water infiltrates through the vadose zones into the interior of the tank filled with layers of grout or hydrated lime, and then contacts the residual waste. Effluent concentrations of U and Tc were monitored and compared among three different packing columns (waste only, waste + grout, and waste + grout + hydrated lime). Geochemical modeling of the effluent compositions was conducted to determine saturation indices of uranium solid phases that could control the solubility of uranium. The results indicate that addition of hydrated lime strongly stabilized the uranium through transforming uranium to a highly insoluble calcium uranate (CaUO4) or similar phase, whereas no significant stabilization effect of grout or hydrated lime was observed on Tc leachability. The result implies that hydrated lime could be a great candidate for stabilizing Hanford tank residual wastes where uranium is one of the main concerns.

Determination of Organic Partitioning Coefficients in Water-Supercritical CO2 Systems by Simultaneous in Situ UV and Near-Infrared Spectroscopies.

CO2 injected into depleted oil or gas reservoirs for long-term storage has the potential to mobilize organic compounds and distribute them between sediments and reservoir brines. Understanding this process is important when considering health and environmental risks, but little quantitative data currently exists on the partitioning of organics between supercritical CO2 and water. In this work, a high-pressure, in situ measurement capability was developed to assess the distribution of organics between CO2 and water at conditions relevant to deep underground storage of CO2. The apparatus consists of a titanium reactor with quartz windows, near-infrared and UV spectroscopic detectors, and switching valves that facilitate quantitative injection of organic reagents into the pressurized reactor. To demonstrate the utility of the system, partitioning coefficients were determined for benzene in water/supercritical CO2 over the range 35-65 °C and approximately 25-150 bar. Density changes in the CO2 phase with increasing pressure were shown to have dramatic impacts on benzene's partitioning behavior. Our partitioning coefficients were approximately 5-15 times lower than values previously determined by ex situ techniques that are prone to sampling losses. The in situ methodology reported here could be applied to quantify the distribution behavior of a wide range of organic compounds that may be present in geologic CO2 storage scenarios.

In situ spectrophotometric determination of pH under geologic CO2 sequestration conditions: method development and application.

CO(2) injection into deep geologic formations for long-term storage will cause a decrease in aqueous pH due to CO(2) dissolution into reservoir water/brine. Current studies seeking to assess chemical changes under geological CO(2) sequestration (GCS) conditions rely largely on thermodynamic modeling due to the lack of reliable experimental methods. In this work, a spectrophotometric method utilizing bromophenol blue to measure pH in laboratory experiments under GCS-relevant conditions was developed. The method was tested in simulated reservoir fluids (CO(2)-NaCl-H(2)O) at different temperatures, pressures, and ionic strengths, and the results were compared with those from other experimental studies and geochemical models. Measured pH values were generally in agreement with the models, but inconsistencies were present between the models. In situ pH measurements for a basalt rock-CO(2)-brine system were conducted under GCS conditions. The pH increased to 3.52 during a 10-day period due to rock dissolution, compared to pH 2.95 for the CO(2)-brine system without rock. The calculated pH values from geochemical models were 0.22-0.25 units higher than the measured values (assuming all iron in the system was in the form of Fe(2+)). This work demonstrates the use of in situ spectrophotometry for pH measurement under GCS-relevant conditions.

Geochemical implications of gas leakage associated with geologic CO2 storage--a qualitative review.

Gas leakage from deep storage reservoirs is a major risk factor associated with geologic carbon sequestration (GCS). A systematic understanding of how such leakage would impact the geochemistry of potable aquifers and the vadose zone is crucial to the maintenance of environmental quality and the widespread acceptance of GCS. This paper reviews the current literature and discusses current knowledge gaps on how elevated CO(2) levels could influence geochemical processes (e.g., adsorption/desorption and dissolution/precipitation) in potable aquifers and the vadose zone. The review revealed that despite an increase in research and evidence for both beneficial and deleterious consequences of CO(2) migration into potable aquifers and the vadose zone, significant knowledge gaps still exist. Primary among these knowledge gaps is the role/influence of pertinent geochemical factors such as redox condition, CO(2) influx rate, gas stream composition, microbial activity, and mineralogy in CO(2)-induced reactions. Although these factors by no means represent an exhaustive list of knowledge gaps we believe that addressing them is pivotal in advancing current scientific knowledge on how leakage from GCS may impact the environment, improving predictions of CO(2)-induced geochemical changes in the subsurface, and facilitating science-based decision- and policy-making on risk associated with geologic carbon sequestration.

Trace metal source terms in carbon sequestration environments.

Carbon dioxide sequestration in deep saline and depleted oil geologic formations is feasible and promising; however, possible CO(2) or CO(2)-saturated brine leakage to overlying aquifers may pose environmental and health impacts. The purpose of this study was to experimentally define a range of concentrations that can be used as the trace element source term for reservoirs and leakage pathways in risk simulations. Storage source terms for trace metals are needed to evaluate the impact of brines leaking into overlying drinking water aquifers. The trace metal release was measured from cements and sandstones, shales, carbonates, evaporites, and basalts from the Frio, In Salah, Illinois Basin, Decatur, Lower Tuscaloosa, Weyburn-Midale, Bass Islands, and Grand Ronde carbon sequestration geologic formations. Trace metal dissolution was tracked by measuring solution concentrations over time under conditions (e.g., pressures, temperatures, and initial brine compositions) specific to the sequestration projects. Existing metrics for maximum contaminant levels (MCLs) for drinking water as defined by the U.S. Environmental Protection Agency (U.S. EPA) were used to categorize the relative significance of metal concentration changes in storage environments because of the presence of CO(2). Results indicate that Cr and Pb released from sandstone reservoir and shale cap rocks exceed the MCLs by an order of magnitude, while Cd and Cu were at or below drinking water thresholds. In carbonate reservoirs As exceeds the MCLs by an order of magnitude, while Cd, Cu, and Pb were at or below drinking water standards. Results from this study can be used as a reasonable estimate of the trace element source term for reservoirs and leakage pathways in risk simulations to further evaluate the impact of leakage on groundwater quality.

Review: Technical and policy challenges in deep vadose zone remediation of metals and radionuclides.

Contamination in deep vadose zone environments is isolated from exposure so direct contact is not a factor in its risk to human health and the environment. Instead, movement of contamination to the groundwater creates the potential for exposure and risk to receptors. Limiting flux from contaminated vadose zone is key for protection of groundwater resources, thus the deep vadose zone is not necessarily considered a resource requiring restoration. Contaminant discharge to the groundwater must be maintained low enough by natural attenuation (e.g., adsorption processes or radioactive decay) or through remedial actions (e.g., contaminant mass reduction or mobility reduction) to meet the groundwater concentration goals. This paper reviews the major processes for deep vadose zone metal and radionuclide remediation that form the practical constraints on remedial actions. Remediation of metal and radionuclide contamination in the deep vadose zone is complicated by heterogeneous contaminant distribution and the saturation-dependent preferential flow in heterogeneous sediments. Thus, efforts to remove contaminants have generally been unsuccessful although partial removal may reduce downward flux. Contaminant mobility may be reduced through abiotic and biotic reactions or through physical encapsulation. Hydraulic controls may limit aqueous transport. Delivering amendments to the contaminated zone and verifying performance are challenges for remediation.

Thermodynamic model for uranium release from hanford site tank residual waste.

A thermodynamic model of U solid-phase solubility and paragenesis was developed for Hanford Site tank residual waste that will remain in place after tank closure. The model was developed using a combination of waste composition data, waste leach test data, and thermodynamic modeling of the leach test data. The testing and analyses were conducted using actual Hanford Site tank residual waste. Positive identification of U phases by X-ray diffraction was generally not possible either because solids in the waste were amorphous or their concentrations were not detectable by XRD for both as-received and leached residual waste. Three leachant solutions were used in the studies: deionized water, CaCO3 saturated solution, and Ca(OH)2 saturated solution. Analysis of calculated saturation indices indicate that NaUO2PO4·xH2O and Na2U2O7(am) are present in the residual wastes initially. Leaching of the residual wastes with deionized water or CaCO3 saturated solution results in preferential dissolution Na2U2O7(am) and formation of schoepite. Leaching of the residual wastes with Ca(OH)2 saturated solution appears to result in transformation of both NaUO2PO4·xH2O and Na2U2O7(am) to CaUO4. Upon the basis of these results, the paragenetic sequence of secondary phases expected to occur as leaching of residual waste progresses for two tank closure scenarios was identified.

In situ long-term reductive bioimmobilization of Cr(VI) in groundwater using hydrogen release compound.

The results of a field experiment designed to test the effectiveness of a novel approach for long-term, in situ bioimmobilization of toxic and soluble Cr(VI) in groundwater using a hydrogen release compound (HRC)--a slow release glycerol polylactate--are described. The field experiment was conducted at the Hanford Site (Washington), a U.S. Department of Energy nuclear production facility, using a combination of hydrogeological, geophysical, geochemical, and microbiological measurements and analyses of water samples and sediments. The results of this experiment show that a single HRC injection into groundwater stimulates an increase in biomass, a depletion of terminal electron acceptors O2, NO3-, and SO4(2-), and an increase in Fe2+, resulting in a significant decrease in soluble Cr(VI). The Cr(VI) concentration has remained below the background concentration in the downgradient pumping/ monitoring well, and below the detection limit in the injection well for more than 3 years after the HRC injection. The degree of sustainability of Cr(VI) reductive bioimmobilization under different redox conditions at this and other contaminated sites is currently under study.

Residual waste from Hanford tanks 241-C-203 and 241-C-204. 1. Solids characterization.

Bulk X-ray diffraction (XRD), synchrotron X-ray microdiffraction (microXRD), and scanning electron microscopy/ energy-dispersive X-ray spectroscopy (SEM/EDS) were used to characterize solids in residual sludge from single-shell underground waste tanks C-203 and C-204 at the U.S. Department of Energy's Hanford Site in southeastern Washington state. Cejkaite [Na4(UO2)(CO3)3] was the dominant crystalline phase in the C-203 and C-204 sludges. This is one of the few occurrences of cejkaite reported in the literature and may be the first documented occurrence of this phase in radioactive wastes from DOE sites. Characterization of residual solids from water leach and selective extraction tests indicates that cejkaite has a high solubility and a rapid rate of dissolution in water at ambient temperature and that these sludges may also contain poorly crystalline Na2U207 [or clarkeite Na[(UO2)O(OH)](H2O)0-1] as well as nitratine (soda niter, NaNO3), goethite [alpha-FeO(OH)], and maghemite (gamma-Fe2O3). Results of the SEM/EDS analyses indicate that the C-204 sludge also contains a solid that lacks crystalline form and is composed of Na, Al, P, O, and possibly C. Other identified solids include Fe oxides that often also contain Cr and Ni and occur as individual particles, coatings on particles, and botryoidal aggregates; a porous-looking material (or an aggregate of submicrometer particles) that typically contain Al, Cr, Fe, Na, Ni, Si, U, P, O, and C; Si oxide (probably quartz); and Na-Al silicate(s). The latter two solids probably represent minerals from the Hanford sediment, which were introduced into the tank during prior sampling campaigns or other tank operation activities. The surfaces of some Fe-oxide particles in residual solids from the water leach and selective extraction tests appear to have preferential dissolution cavities. If these Fe oxides contain contaminants of concern, then the release of these contaminants into infiltrating water would be limited by the dissolution rates of these Fe oxides, which in general have lowto very low solubilities and slow dissolution rates at near neutral to basic pH values under oxic conditions.

Residual waste from Hanford tanks 241-C-203 and 241-C-204. 2. Contaminant release model.

Release of U and 99Tc from residual sludge in Hanford waste tanks 241-C-203 and 241-C-204 atthe U.S. Department of Energy's (DOE) Hanford Site in southeastern Washington state was quantified by water-leaching, selective extractions, empirical solubility measurements, and thermodynamic modeling. A contaminant release model was developed based on these experimental results and solid-phase characterization results presented elsewhere. Uranium release was determined to be controlled by two phases and occurred in three stages. In the first stage, U release is controlled by the solubility of tejkaite, which is suppressed by high concentrations of sodium released from the dissolution of NaNO3 in the residual sludges. Equilibrium solubility calculations indicate the U released during this stage will have a maximum concentration of 0.021 M. When all the NaNO3 has dissolved from the sludge, the solubility of the remaining cejkaite will increase to 0.28 M. After cejkaite has completely dissolved, the majority of the remaining U is in the form of poorly crystalline Na2U2O7 [or clarkeite Na[(UO2)O(OH)](H20)0-1]. In contact with Hanford groundwater this phase is not stable, and becquerelite becomes the U solubility controlling phase, with a calculated equilibrium concentration of 1.2 x 10(-4) M. For Tc, a significant fraction of its concentration in the residual sludge was determined to be relatively insoluble (20 wt % for C-203 and 80 wt % for C-204). Because of the low concentrations of Tc in these sludge materials, the characterization studies did not identify any discrete Tc solids phases. Release of the soluble fraction of Tc was found to occur concomitantly with NO3-. It was postulated that a NaNO3-NaTcO4 solid solution could be responsible for this behavior. The Tc release concentrations for the soluble fraction were estimated to be 2.4 x 10-6 M for C-203 and 2.7 x 10(-5) M for C-204. Selective extraction results indicated that the recalcitrant fraction of Tc was associated with Fe oxides. Release of the recalcitrant fraction of Tc was assumed to be controlled by dissolution of Fe oxide in the form of ferrihydrite. Based on this assumption and measured values for the ratio of recalcitrant Tc to total Fe in each bulk sludge, the release concentration of the recalcitrant fraction of Tc was calculated to be 3.9 x 10(-12) M for C-203 and 10.0 x 10(-12) M for C-204.

Oxidation of H2S by iron oxides in unsaturated conditions.

Previous studies have demonstrated that gas-phase H2S can immobilize certain redox-sensitive contaminants (e.g., Cr, U, Tc) in vadose zone environments. A key issue for effective and efficient delivery of H2S in these environments is the reactivity of the gas with indigenous iron oxides. To elucidate the factors that control the transport of H2S in the vadose zone, laboratory column experiments were conducted to identify reaction mechanisms and measure rates of H2S oxidation by iron oxide-coated sands using several carrier gas compositions (N2, air, and O2) and flow rates. Most experiments were conducted using ferrihydrite-coated sand. Additional studies were conducted with goethite- and hematite-coated sand and a natural sediment. Selective extractions were conducted at the end of each column experiment to determine the mass balance of the reaction products. XPS was used to confirm the presence of the reaction products. For column experiments in which ferrihydrite-coated sand was the substrate and N2 was the carrier gas, the major H2S oxidation products were FeS and elemental sulfur (mostly S8(0), represented as S(0) for simplicity) at ratios that were consistent with the stoichiometry of the postulated reactions. When air or O2 were used as the carrier gas, S(0) became the dominant reaction product along with FeS2 and smaller amounts of FeS, sulfate, and thiosulfate. A mathematical model of reactive transport was used to test the hypothesis that S(0) forming on the iron oxide surfaces reduces access of H2S to the reactive surface. Several conceptual models were assessed in the context of the postulated reactions with the final model based on a linear surface poisoning model and fitted reaction rates. These results indicate that carrier gas selection is a critical consideration with significant tradeoffs for remediation objectives.