Tank Waste Solutions Research Articles

Immobilization and exchange of perrhenate in sodalite and cancrinite

Highly alkaline nuclear tank wastes containing key anionic contaminants have leaked into the subsurface at the U.S. Department of Energy's Hanford Site. Laboratory studies showed that when simulated caustic tank wastes contact subsurface sediments, mineral dissolution and precipitation result in feldspathoid formation. Feldspathoids are environmentally important for waste management and disposal purposes because of their purported potential to sequester contaminants of interest (e.g., 99TcO4−, 137Cs+, 90Sr2+) into their structure. We investigated the incorporation of perrhenate (ReO4−), a surrogate for TcO4−, in the presence of competing anions (X) including OH−, NO2−, NO3− and Cl− on feldspathoid formation and under conditions mimicking tank waste solution compositions. The resulting solids were characterized by their chemical composition, structure and morphology. Regardless of solution pH, sodalite formed in the presence of Cl− and NO2− whereas NO3− promoted either cancrinite formation in 16-mol OH−/kg (16 m) or mixed sodalite/cancrinite phases in 1-mol OH−/kg (1 m) solutions. In the presence of Cl−, NO2−, and NO3− less than 0.02 mol fraction of ReO4− was incorporated into the feldspathoid phase(s). Although, the NO2-sodalite and mixed NO3-cancrinite/sodalite phases incorporated significantly more ReO4− than NO3-cancrinite or Cl-sodalite phases, the total ReO4− fraction was ∼1% or less of the total sites. The ReO4− immobilized in ReO4-sodalite, NO2-sodalite, mixed NO3-cancrinite/sodalite and NO3-cancrinite was resistant to ion exchange with either NO2− or NO3−. The results imply that ReO4−, and thus by analogy 99TcO4−, does not compete well with smaller ions for incorporation into feldspathoids, but, once sequestered, is difficult to exchange.

Spatially Resolved U(VI) Partitioning and Speciation: Implications for Plume Scale Behavior of Contaminant U in the Hanford Vadose Zone

A saline-alkaline brine containing high concentration of U(VI) was accidentally spilled at the Hanford Site in 1951, introducing 10 tons of U into sediments under storage tank BX-102. U concentrations in the deep vadose zone and groundwater plumes increase with time, yet how the U has been migrating is not fully understood. We simulated the spill event in laboratory soil columns, followed by aging, and obtained spatially resolved U partitioning and speciation along simulated plumes. We found after aging, at apparent steady state, that the pore aqueous phase U concentrations remained surprisingly high (up to 0.022 M), in close agreement with the recently reported high U concentrations (up to 0.027 M) in the vadose zone plume (1). The pH values of aged pore liquids varying from 10 to 7, consistent with the measured pH of the field borehole sediments varying from 9.5 to 7.4 (2), from near the plume source to the plume front. The direct measurements of aged pore liquids together with thermodynamic calculations using a Pitzer approach revealed that UO2(CO3)3(4-) is the dominant aqueous U species within the plume body (pH 8-10), whereas Ca2UO2(CO3)3 and CaUO2(CO3)32- are also significant in the plume frontvicinity (pH 7-8), consistent with that measured from field borehole pore-waters (3). U solid phase speciation varies at different locations along the plume flow path and even within single sediment grains, because of location dependent pore and micropore solution chemistry. Our results suggest that continuous gravity-driven migration of the highly stable U02(CO3)34 in the residual carbonate and sodium rich tank waste solution is likely responsible for the detected growing U concentrations in the vadose zone and groundwater.

Caustic Recycle from Hanford Tank Waste Using NaSICON Ceramic Membranes

A sodium (Na) Super Ion Conductor (NaSICON), has been studied at Pacific Northwest National Laboratory (PNNL) to investigate its ability to separate sodium from radioactively contaminated sodium salt solutions for treating U.S. Department of Energy (DOE) tank wastes. Ceramatec Inc. developed and fabricated a membrane disk containing a proprietary NAS-GY material formulation that was electrochemically tested in a bench-scale apparatus with both a simulant and a radioactive tank-waste solution to determine the membrane performance when removing sodium from DOE tank wastes. Implementing this sodium separation process can result in significant cost savings by reducing the disposal volume of low-activity wastes and by producing a 19 M NaOH feedstock product for recycle into waste treatment processes such as sludge leaching, regenerating ion exchange resins, inhibiting corrosion in carbon-steel tanks, or retrieving tank wastes. In actual waste tests, average sodium transport rates of 10.3 kg/day/m2 were achieved at average sodium transport efficiencies of 99%. The membrane was found to be highly selective to sodium ions resulting in no detectable cation transport except Na and a small quantity (0.04% to 0.06%) of 137Cs. An average decontamination factor of 2000 was observed with respect to 137Cs. As expected, Gibbsite precipitation was observed as OH− ions were depleted from the tank waste.

Reactive transport modeling of column experiments on the evolution of saline–alkaline waste solutions

Leakage of saline–alkaline tank waste solutions often creates a serious environmental contamination problem. To better understand the mechanisms controlling the fate of such waste solutions in the Hanford vadose zone, we simulated reactive transport in columns designed to represent local site conditions. The Pitzer ion interaction module was used, with principal geochemical processes considered in the simulation including quartz dissolution, precipitation of brucite, calcite, and portlandite, multi-component cation exchange, and aqueous complexation reactions. Good matches were observed between the simulated and measured column data at ambient temperature (∼ 21 °C). Relatively good agreement was also obtained at high temperature (∼ 70 °C). The decrease of pH at the plume front is examined through formation of secondary mineral phases and/or quartz dissolution. Substantial formation of secondary mineral phases resulting from multi-component cation exchange suggests that these phases are responsible for a decrease in pH within the plume front. In addition, a sensitivity analysis was conducted with respect to cation exchange capacity, selectivity coefficient, mineral assemblage, temperature, and ionic strength. This study could serve as a useful guide to subsequent experimental work, to thermodynamic models developed for the concentrated solutions at high ionic strength and to other types of waste plume studies.

Recent studies of uranium and plutonium chemistry in alkaline radioactive waste solutions

Solubility studies of uranium and plutonium in a caustic, radioactive Savannah River Site tank waste solution revealed the existence of uranium supersaturation in the as-received sample. Comparison of the results to predictions generated from previously published models for solubility in these waste types revealed that the U model poorly predicts solubility while Pu model predictions are quite consistent with experimental observations. Separate studies using simulated Savannah River Site evaporator feed solution revealed that the known formation of sodium aluminosilicate solids in waste evaporators can promote rapid precipitation of uranium from supersaturated solutions.

Microscopic reactive diffusion of uranium in the contaminated sediments at Hanford, United States

Microscopic and spectroscopic analyses of uranium‐contaminated sediments from select locations at the U.S. Department of Energy (DOE) Hanford site have revealed that sorbed uranium (U) often exists as uranyl precipitates associated with intragrain fractures of granitic clasts. The release of U to contacting fluids appears to be controlled by intragrain ion diffusion coupled with the dissolution kinetics of the precipitates that exist in the form of Na‐boltwoodite. Here we present a coupled microscopic reactive diffusion model for the contaminated sediment on the basis of experimental measurements of intragrain diffusivity in the granitic lithic fragments and the dissolution kinetics of synthetic Na‐boltwoodite. Nuclear magnetic resonance, pulse gradient spin echo measurements showed that the intragrain fractures of the granitic clasts isolated from the sediment contained two domains with distinct diffusivities. The fast diffusion domain had an apparent tortuosity of 1.5, while that of the slow region was two orders of magnitude larger. A two‐domain diffusion model was assembled and used to infer the geochemical conditions that led to intragrain uranyl precipitation during waste‐sediment interaction. Rapid precipitation of Na‐boltwoodite was simulated with an alkaline U‐containing, high‐carbonate tank waste solution that diffused into intragrain fractures, which originally contained Si‐rich pore water in equilibrium with feldspar grains in the lithic fragments. The model was also used to simulate uranyl dissolution and release from contaminated sediment to recharge waters. With independently characterized parameters for Na‐boltwoodite dissolution, the model simulations demonstrated that diffusion could significantly decrease the rates of intragrain uranyl mineral dissolution due to diffusion‐induced local solubility limitation with respect to Na‐boltwoodite.

Cancrinite and sodalite formation in the presence of cesium, potassium, magnesium, calcium and strontium in Hanford tank waste simulants

High-level radioactive tank waste solutions that have leaked into the subsurface at the US Department of Energy Hanford Site, Washington, are chemically complex. Here, the effect of five cations, Cs +, K +, Sr 2+, Ca 2+ and Mg 2+, on mineral formation and transformation pathways under conditions mimicking Hanford tank leaks is investigated. Sodium silicate was used to represent the dissolved silicate from sediments. The silicate was added into a series of simulants that contained 0.5 M aluminate, 1 M or 16 M NaOH, and the NO 3 - salts of the cations. The precipitates were monitored by X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectroscopy. In the 1 M NaOH simulants, low concentration of Cs + (<100 mM) did not affect the formation of lepispheric cancrinite and sodalite, whereas only highly crystalline cancrinite formed when Cs + concentration was ⩾250 mM. An unidentified feldspathoid or zeolite intermediate phase was observed in the presence of high concentrations of Cs + (500 mM). The presence of K + did not alter, but slowed, the formation of cancrinite and sodalite. The presence of divalent cations led to the formation of metastable or stable silicates, aluminates, hydroxides, or aluminosilicates. The formation of these intermediate phases slowed the formation of cancrinite and sodalite by consuming OH −, silicate, or aluminate. Compared with the concentrations used in this study, the concentrations of radioactive Cs + and Sr 2+ in the tank solutions are much lower and divalent cations (Ca 2+ and Mg 2+) released from sediments likely precipitate out as hydroxides, silicates or aluminates; therefore, the authors do not expect that the presence of these monovalent and divalent cations significantly affect the formation of cancrinite and sodalite in the sediments underneath the leaking waste tanks.

Enhanced Radionuclide Immobilization and Flow Path Modifications by Dissolution and Secondary Precipitates

Caustic radioactive wastes that have leaked at Hanford Site (Richland, WA) induce mineral dissolution and subsequent secondary precipitation that influence the fate and transport of contaminants present in the waste solutions. The effects of secondary mineral precipitates, formed after contacting solids with simulated caustic wastes, on the flow path changes and radionuclide immobilization were investigated by reacting quartz, a mixture of quartz and biotite, and a Hanford sediment (Warden soil: coarse-silty, mixed, superactive, mesic Xeric Haplocambids) with simulated caustic tank waste solution. Continuous Si dissolution and concomitant secondary mineral precipitation were the principal reactions observed in both batch and flow-through tests. Nitrate-cancrinite was the dominant secondary precipitate on mineral surfaces after 3- to 10-d reaction times in batch experiments. X-ray microtomography images of a reacted quartz column revealed that secondary precipitates cemented quartz grains together and modified pore geometry in the center of the column. Along the circumference of the packed column, however, quartz dissolution continuously occurred, suggesting that wastes that leaked from buried tanks in the past likely did not migrate vertically as modeled in risk assessments but rather the pathways likely changed to be dominantly horizontal on precipitation of secondary precipitate phases in the Hanford vadose zone. Based on batch equilibrium sorption results on the reacted sediments, the dominant secondary precipitates (cancrinites) on the mineral surfaces enhanced the sorption capacity of typical Hanford sediment for radionuclides 129I(-I), 79Se(VI), 99Tc(VII), and 90Sr(II), all of which are of major concern at the Hanford Site.

A Comparison of Crystalline Silicotitanate and Ammonium Molybdophosphate-Polyacrylonitrile Composite Sorbent for the Separation of Cesium from Acidic Waste

Crystalline silicotitanate manufactured and marketed by UOP as IONSIV IE-911 and an ammonium molybdophosphate-polyacrylonitrile (AMP-PAN) composite sorbent have been evaluated for the removal of cesium from Idaho National Engineering and Environmental Laboratory (INEEL) concentrated acidic tank waste. Equilibrium isotherms were generated with simulated concentrated tank waste solutions and fit to the Langmuir equation. Nonequilibrium sorption was evaluated in column tests employing 1.5 cm3 columns operating at 5, 10 and 20 bed volumes of flow per hour. Results of the equilibrium and nonequilibrium testing for both sorbents are compared.

Colloid Formation at Waste Plume Fronts

Highly saline and caustic tank waste solutions containing radionuclides and toxic metals have leaked into sediments at U.S. Department of Energy (DOE) facilities such as the Hanford Site (Washington state). Colloid transport is frequently invoked to explain migration of radionuclides and metals in the subsurface. To understand colloid formation during interactions between highly reactive fluids and sediments and its impact on contaminant transport, we simulated tank waste solution (TWS) leakage processes in laboratory columns at ambient and elevated (70 degrees C) temperatures. We found that maximum formation of mobile colloids occurred at the plume fronts (hundreds to thousands times higher than within the plume bodies or during later leaching). Concentrations of suspended solids were as high as 3 mass %, and their particle sizes ranged from tens of nanometers to a few micrometers. Calcium carbonate is always one of the dominant phases of the plume front colloids, while the other phases varied with solution pH and temperature. During infiltration of the leaked high-Na+ waste solution, rapid and completed Na+ replacement of exchangeable Ca2+ and Mg2+ from the sediment caused accumulation of these divalent cations at the moving plume front. Precipitation of supersaturated Ca2+/Mg2+-bearing minerals caused dramatic pH reduction atthe plume front. In turn, the reduced pH caused precipitation of other minerals. This understanding can help predict the behavior of contaminant trace elements carried by the tank waste solutions and could not have been obtained through conventional batch studies.

Colloid Formation in Hanford Sediments Reacted with Simulated Tank Waste

Solutions of high pH, ionic strength, and aluminum concentration have leaked into the subsurface from underground waste storage tanks atthe Hanford Reservation in Washington State. Here, we test the hypothesis that these waste solutions alter and dissolve the native minerals present in the sediments and that colloidal (diameter < 2 microm) feldspathoids form. We reacted Hanford sediments with simulated solutions representative of Hanford waste tanks. The solutions consisted of 1.4 or 2.8 mol/kg NaOH, 0.125 or 0.25 mol/kg NaAlO4, and 3.7 mol/kg NaNO3 and were contacted with the sediments for a period of 25 or 40 days at 50 degrees C. The colloidal size fraction was separated from the sediments and characterized in terms of mineralogy, morphology, chemical composition, and electrophoretic mobility. Upon reaction with tank waste solutions, native minerals released Si and other elements into the solution phase. This Si precipitated with the Al present in the waste solutions to form secondary minerals, identified as the feldspathoids cancrinite and sodalite. The solution phase was modeled with the chemical equilibrium model GMIN for solution speciation and saturation indices with respect to sodalite and cancrinite. The amount of colloidal material in the sediments increased upon reaction with waste solutions. At the natural pH found in Hanford sediments (pH 8) the newly formed minerals are negatively charged, similar to the unreacted colloidal material present in the sediments. The formation of colloidal material in Hanford sediments upon reaction with tank waste solutions is an important aspect to consider in the characterization of Hanford tank leaks and may affect the fate of hazardous radionuclides present in the tank waste.

Comparison of Hanford Colloids and Kaolinite Transport in Porous Media

Understanding colloid transport at the Hanford site in Washington State is critical in assessing migration of radionuclides because colloid transport is a potential means for facilitated off‐site migration of radioactive wastes. In this study, eight saturated column experiments were conducted to investigate transport of Hanford colloids and a model colloid (kaolinite) through two types of porous media (Hanford sediments characteristic of 2:1 clay minerals and silica Accusand). Experiments were conducted at a pH value of 10 to mimic the conditions at the Hanford site. The Hanford colloids used were obtained by reacting Hanford sediments with simulated tank waste solutions. The results show that factors influencing transport of Hanford colloids and kaolinite include flow velocity, solution ionic strength, medium type, and colloid properties. Hanford colloids exhibited higher deposition rates than kaolinite in both Hanford sediments and Accusand. Likewise, Hanford sediments retained more colloids than did the silica Accusand. Comparison of transport behaviors of the two colloids through two sands supports the assumption that chemical heterogeneity is important in controlling particle–particle and particle–collector interactions in colloid retention and transport.

Alteration of Kaolinite to Cancrinite and Sodalite by Simulated Hanford Tank Waste and its Impact on Cesium Retention

AbstractCaustic nuclear wastes have leaked from tanks at the US Department of Energy’s Hanford site in Washington State (USA) causing hundreds of thousands of gallons of waste fluids to migrate into the underlying sediments. In this study, four simulant tank waste (STW) solutions, which are high in NaOH (1.4 and 2.8 mol/kg), NaNO3 (3.7 mol/kg) and NaAlO2 (0.125 and 0.25 mol/kg), were prepared and reacted with reference kaolinite KGa-1 and KGa-2 at 50 and 80°C for up to 2 months. The structure and morphology of the resulting products were characterized using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. The products were also examined for cation exchange and Cs+ sorption as a function of ionic strength and types of cations in the background solutions. Cancrinite and sodalite were the only new minerals observed in all of the conditions tested in this experiment. Two major chemical processes were involved in the reactions: dissolution of kaolinite and precipitation of cancrinite and sodalite. Increasing NaOH concentration and temperature, and decreasing NaAlO2 concentration increased the transformation rate. Both cancrinite and sodalite appeared stable thermodynamically under the experimental conditions. The newly formed feldspathoids were vulnerable to acid attack and pronounced dissolution occurred at pH below 5.5. Cancrinite and sodalite can incorporate NaNO3 ion pairs in their cages or channels. Sodium in cancrinite and sodalite was readily exchangeable by K+, but less easily by Cs+ or Ca2+. The feldspathoid products sorb nearly an order of magnitude more Cs+ than the unaltered kaolinite. The Cs adsorption is reduced by competing cations in the background solutions. At low ionic strength (0.01 M NaNO3 or 0.005 M Ca(NO3)2), Ca2+ was more competitive than Na+. When the concentration of the background solution was increased 10 times, Na+ was more competitive than Ca2+.

Evaluation of 235U Isotopic Dilution with the Addition of Depleted Uranium to Supernatant Liquid Waste

Laboratory-scale results on experiments performed to examine the feasibility of isotopic dilution of 235U in supernatant liquid storage tanks at the Savannah River Site are described. The isotopic dilution tests were accomplished by adding an alkaline depleted uranium solution to small portions of simulated and actual storage tank waste solutions with enriched 235U compositions. Based on the laboratory observations, recommendations were made, which involved the addition of significant quantities of uranyl carbonate solution to more than 4 million l of 235U enriched waste stored in tank 43H at the site to reduce the risk for criticality.A post-uranyl carbonate addition analysis on the tank supernate confirmed the effectiveness of depleted uranium in isotopic dilution of 235U. The 235U enrichment in tank 43H was isotopicaly diluted from an original high of >4 wt% down to <0.5 wt%, as predicted from the laboratory investigations.

Geochemical evolution of highly alkaline and saline tank waste plumes during seepage through vadose zone sediments

Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.

Colloid-facilitated Cs transport through water-saturated Hanford sediment and Ottawa sand.

In this study, a series of saturated column experiments were conducted to investigate effects of colloids on Cs transport in two types of porous media (Hanford sediment characteristic of 2:1 clay minerals and silica Ottawa sand). The colloids used were obtained by reacting Hanford sediment with simulated tank waste solutions. Because of the highly nonlinear nature of Cs sorption found in batch experiments, we used two different concentrations of Cs (7.5 x 10(-5) M and 1.4 x 10(-8) M) for the transport experiments. The presence of colloids facilitated the transport of Cs through both Hanford sediment and Ottawa sand via association of Cs with mobile colloidal particles. Due to the nonlinearity of the Cs sorption, the colloid-facilitated Cs transportwas more pronounced atthe low Cs concentration (1.4 x 10(-8) M) than at the high concentration (7.5 x 10(-5) M) when expressed relative to the inflow Cs concentration. In the absence of colloids, no Cs moved through the 10-cm long columns during the experiment within about 20 pore volumes, exceptfor the high Cs concentration in the Ottawa sand where a complete Cs breakthrough was obtained. Also, it was found that colloid-associated Cs could be partially stripped off from colloids during the transport. The stripping effect was controlled by both Cs concentration and sorption capacity of the transport matrix.

Cesium sorption from concentrated acidic tank wastes using ammonium molybdophosphate-polyacrylonitrile composite sorbents

Ammonium molybdophosphate-polyacrylonitrile (AMP-PAN) composite sorbents have been evaluated for the removal of cesium from Idaho National Engineering and Environmental Laboratory (INEEL) concentrated acidic tank waste. Batch contacts were performed to qualitatively evaluate the effects of increased nitric acid, sodium and potassium. An equilibrium isotherm was generated with simulated concentrated tank waste solutions and fit to the Langmuir equation. Additional batch contact experiments were performed to determine if mercury, plutonium and americium would sorb onto AMP-PAN. Dynamic sorption was evaluated in column tests employing 1.5 cm3 columns operating at 5, 10 and 20 bed volumes of flow per hour. Results indicate, as expected, that dynamic cesium sorption capacity is reduced as the flowrate is increased. Calculated dynamic capacities for cesium were 22.5, 19.8 and 19.6 mg Cs/g sorbent, for 5, 10 and 20 bed volume per hour flows, respectively. The thermal stability of loaded AMP-PAN was evaluated by performing thermogrovimetric analysis (TGA) on samples of AMP, PAN (polymer), and AMP-PAN. Results indicate that AMP-PAN is stable to 400 °C, with less than a 10% loss of weight, which is at least partially due to loss of water of hydration. The evaluation of AMP-PAN indicates that it will effectively remove cesium from concentrated acidic tank waste solutions.

Trapping of inert gases in sodalite and cancrinite crystals

Highly alkaline nuclear tank wastes containing key anionic contaminants have leaked into the subsurface at the U.S. Department of Energy's Hanford Site. Laboratory studies showed that when simulated caustic tank wastes contact subsurface sediments, mineral dissolution and precipitation result in feldspathoid formation. Feldspathoids are environmentally important for waste management and disposal purposes because of their purported potential to sequester contaminants of interest (e.g., 99TcO4−, 137Cs+, 90Sr2+) into their structure. We investigated the incorporation of perrhenate (ReO4−), a surrogate for TcO4−, in the presence of competing anions (X) including OH−, NO2−, NO3− and Cl− on feldspathoid formation and under conditions mimicking tank waste solution compositions. The resulting solids were characterized by their chemical composition, structure and morphology. Regardless of solution pH, sodalite formed in the presence of Cl− and NO2− whereas NO3− promoted either cancrinite formation in 16-mol OH−/kg (16 m) or mixed sodalite/cancrinite phases in 1-mol OH−/kg (1 m) solutions. In the presence of Cl−, NO2−, and NO3− less than 0.02 mol fraction of ReO4− was incorporated into the feldspathoid phase(s). Although, the NO2-sodalite and mixed NO3-cancrinite/sodalite phases incorporated significantly more ReO4− than NO3-cancrinite or Cl-sodalite phases, the total ReO4− fraction was ∼1% or less of the total sites. The ReO4− immobilized in ReO4-sodalite, NO2-sodalite, mixed NO3-cancrinite/sodalite and NO3-cancrinite was resistant to ion exchange with either NO2− or NO3−. The results imply that ReO4−, and thus by analogy 99TcO4−, does not compete well with smaller ions for incorporation into feldspathoids, but, once sequestered, is difficult to exchange.