Effect of Si/Al molar ratio and curing temperatures on the immobilization of radioactive borate waste in metakaolin-based geopolymer waste form.

Immobilization of radioactive borate waste (RBW) using a geopolymer with a high Si/Al ratio has been challenging because boron-silicon networks lower the compressive strength and delay the setting time. In this study, metakaolin-based geopolymer waste form to immobilize simulant RBW was fabricated using different Si/Al ratios (1.0-1.4) and curing temperatures (26 and 60 â„ƒ). The 7-day compressive strength results revealed that a certain amount of silicon and an elevated curing temperature are required to achieve high compressive strength and waste loading. Following waste acceptance criteria tests, all geopolymers exhibited compressive strengths higher than 3.445 MPa. The leachability index of boron was higher than 6.0, and the leaching mechanism was identified as diffusion. No significant structural changes in the geopolymer were observed after thermal cycling and gamma irradiation tests. The physically bound or unincorporated RBW was leached out of the geopolymer during water immersion and leaching tests; however, boron, which was chemically connected with silicon, was present as an inert phase together with a geopolymer binder. Consequently, immobilizing RBW using a geopolymer with a low Si/Al ratio (1.4) is beneficial in terms of RBW loading and structural durability.

Design and Application of Materials for Sequestration and Immobilization of 99Tc.

99Technetium (99Tc) is a hazardous radionuclide that poses a serious environmental threat. The wide variation and complex chemistries of liquid nuclear waste streams containing 99Tc often create unique, site specific challenges when sequestering and immobilizing the waste in a matrix suitable for long-term storage and disposal. Therefore, an effective management plan for 99Tc containing liquid radioactive wastes (such as storage (tanks) and decommissioned wastes) will likely require a variety of suitable materials/matrixes capable of adapting to and addressing these challenges. In this review, we discuss and highlight the key developments for effective removal and immobilization of 99Tc liquid waste in inorganic waste forms. Specifically, we review the synthesis, characterization, and application of materials for the targeted removal of 99Tc from (simulated) waste solutions under various experimental conditions. These materials include (i) layered double hydroxides (LDHs), (ii) metal-organic frameworks (MOFs), (iii) ion-exchange resins (IERs) as well as cationic organic polymers (COPs), (iv) surface modified natural clay materials (SMCMs), and (v) graphene-based materials (GBMs). Second, we discuss some of the major and recent developments toward 99Tc immobilization in (i) glass, (ii) cement, and (iii) iron mineral waste forms. Finally, we present future challenges that need to be addressed for the design, synthesis, and selection of suitable matrixes for the efficient sequestration and immobilization of 99Tc from targeted wastes. The purpose of this review is to inspire research on the design and application of various suitable materials/matrixes for selective removal of 99Tc present globally in different radioactive wastes and its immobilization in stable/durable waste forms.

Evaluating the impact of drying on leaching from a solidified/stabilized waste using a monolithic diffusion model.

The release rates of constituents of potential concern from solidified/stabilized cementitious waste forms are potentially impacted by drying, which, however, is not well understood. This study aimed to identify the impacts of drying on subsequent leaching from Cast Stone as an example of a solidified cementitious waste form. The release fluxes of constituents from monoliths after aging under 100, 68, 40, and 15 % relative humidity for 16, 32, and 48 weeks, respectively, were derived from mass transfer tank leaching tests following EPA Method 1315. A monolithic diffusion model was calibrated based on the leaching test results to simulate the leaching of major and redox-sensitive constituents from monoliths after drying. The reduction in physical retention of constituents (tortuosity-factor) in the unsaturated zone was identified as the primary impact from drying on subsequent leaching. Fluxes of both major (i.e., OH-, Na, K, Ca, Si, and Al) and redox-sensitive constituents (i.e., Tc, Cr, Fe, and S) from monoliths during leaching were well described by the model. The drying-induced reduction of tortuosity-factor and concomitant changes in porewater pH and redox conditions can significantly change the subsequent release fluxes of pH- and redox- sensitive constituents.

Potential of indigenous bacteria driven U(VI) reduction under relevant deep geological repository (DGR) conditions.

Understanding the biogeochemical U redox processes is crucial for controlling U mobility and toxicity under conditions relevant to deep geological repositories (DGRs). In this study, we examined the microbial reduction of aqueous hexavalent uranium U(VI) [U(VI)aq] by indigenous bacteria in U-contaminated groundwater. Three indigenous bacteria obtained from granitic groundwater at depths of 44-60 m (S1), 92-116 m (S2), and 234-244 m (S3) were used in U(VI)aq bioreduction experiments. The concentration of U(VI)aq was monitored to evaluate its removal efficiency for 24 weeks under anaerobic conditions with the addition of 20 mM sodium acetate. During the anaerobic reaction, U(VI)aq was precipitated in the form of U(IV)-silicate with a particle size >100 nm. The final U(VI)aq removal efficiencies were 37.7%, 43.1%, and 57.8% in S1, S2, and S3 sample, respectively. Incomplete U(VI)aq removal was attributed to the presence of a thermodynamically stable calcium uranyl carbonate complex in the U-contaminated groundwater. High-throughput 16S rRNA gene sequencing analysis revealed the differences in indigenous bacterial communities in response to the depth, which affected to the U(VI)aq removal efficiency. Pseudomonas peli was found to be a common bacterium related to U(VI)aq bioreduction in S1 and S2 samples, while two SRB species, Thermodesulfovibrio yellowstonii and Desulfatirhabdium butyrativorans, played key roles in the bioreduction of U(VI)aq in S3 sample. These results indicate that remediation of U(VI)aq is possible by stimulating the activity of indigenous bacteria in the DGR environment.

Prediction of stable radon fluoride molecules and geometry optimization using first-principles calculations.

Noble gases possess extremely low reactivity because their valence shells are closed. However, previous studies have suggested that these gases can form molecules when they combine with other elements with high electron affinity, such as fluorine. Radon is a naturally occurring radioactive noble gas, and the formation of radon-fluorine molecules is of significant interest owing to its potential application in future technologies that address environmental radioactivity. Nevertheless, because all isotopes of radon are radioactive and the longest radon half-life is only 3.82 days, experiments on radon chemistry have been limited. Here, we study the formation of radon molecules using first-principles calculations; additionally, possible compositions of radon fluorides are predicted using a crystal structure prediction approach. Similar to xenon fluorides, di-, tetra-, and hexafluorides are found to be stabilized. Coupled-cluster calculations reveal that RnF6 stabilizes with Oh point symmetry, unlike XeF6 with C3v symmetry. Moreover, we provide the vibrational spectra of our predicted radon fluorides as a reference. The molecular stability of radon di-, tetra-, and hexafluoride obtained through calculations may lead to advances in radon chemistry.

Decontamination of neutron-activated radioactive concrete waste by separating Eu, Co, Fe, and Mn-containing sand particles using dense medium separation.

Neutron-activated concrete waste is one of the most challenging radioactive wastes to decontaminate because the radionuclides exist in a chemically stable binding state, and it is very difficult to break those bindings with the conventional acid decontamination method. Here, we suggest a new dense medium separation (DMS) of felsic and mafic minerals from simulated neutron-activated concrete waste using sodium-polytungstate (SPT) solution because most elements (Eu, Co, Fe, and Mn) that can be activated by neutrons are concentrated in mafic minerals. We also determined the optimal density of the SPT solution as ∼ 2.70 g/cm3, and a high degree of decontamination was achieved for sand particles ranging from 75 to 500 µm in size. Under these optimized conditions, DMS (80.02%) exhibits much higher radionuclide removal efficiency (RRE) than 5 M acid decontaminations (23.27-31.29%) for Eu. Furthermore, DMS (59.38-63.36%) shows similar RRE to 5 M acid decontaminations (41.67-73.94%) for Fe, Mn, and Co. We believe this DMS process could be useful and applicable to the decontamination of neutron-activated concrete wastes because it is possible to perform a large-scale process compared to conventional acid decontamination methods, which is also advantageous in reducing secondary waste generation and facile radionuclide recovery.

The evolution of hydrated lime-based cementitious waste forms during leach testing leading to enhanced technetium retention.

The interaction between radionuclides and cementitious material phases is crucial in the prediction of the long-term disposal behavior of cementitious waste forms. This work focuses on the behavior of technetium-99 (Tc) within a hydrated-lime based waste form developed as a candidate to immobilize high-sulphate containing liquid wastes known to inhibit cement solidification when using a fly ash based formulation. In leach testing, the hydrated-lime based formulation demonstrated improvement in Tc retention over a fly ash containing formulation beginning after 14 d leaching. The mineralogical evolution of the hydrated-lime samples during leach testing showed a decrease in portlandite content and reduction capacity at the onset of the Tc retention improvement. Leach testing upwards of 400 days showed the improved Tc retention was sustained. Samples cured for different lengths of time (28 days vs 60 days) confirmed that the improved Tc retention and mineralogic change was caused by cement - leachant interactions and not the sample curing time. The Tc observed diffusivities in the hydrated-lime samples are amongst the lowest measured in a cement waste form tested for development at the US Department of Energy Hanford site, leading to a possible pathway to improved cement conditioning where contaminants can be retained for long disposal times.

Comparative study of PMS oxidation with Fenton oxidation as an advanced oxidation process for Co-EDTA decomplexation.

In nuclear industry, Co-EDTA complex is generated due to the decontamination activities of nuclear power plants (NPPs). This complex is extremely refractory to the convention methods and can escalate the mobility of Co radionuclide in the environment. Due to its hazardous impact on human and environment, the effective treatments of Co-EDTA complexes are highly recommended. In this study, for the first time, we applied both hydroxyl (OH) and sulfate radical (SO4-) based advanced oxidation processes (AOPs) namely Fenton and peroxymonosulfate (PMS) reactions for the Co-EDTA decomplexation. Both reactions exhibited higher Co-EDTA decomplexation at pH = 3, however, the PMS based reaction was found to be superior, which showed highest decomplexation efficiency (without pH adjustment) over Fenton reaction (pH = 1-13). Moreover, PMS based system was found to be more suitable than Fenton reaction, because PMS showed best Co-EDTA decomplexation efficiency without any additional catalyst dosages at the shorter reaction time. XRD data confirmed the presence of both CoO and Co(OH)2 in the precipitates after treatment. The electron spin resonance spectroscopy (ESR) analysis identified OH and SO4- in Fenton and PMS system, respectively. From this study, we believe that PMS based reaction is a superior alternative of Fenton reaction for the Co-EDTA decomplexation.

Co2+/PMS based sulfate-radical treatment for effective mineralization of spent ion exchange resin.

Sulfate radical advance oxidation processes (SR-AOPs) have attracted a greater attention as a suitable alternative of the hydroxyl radical based advance oxidation process (HR-AOPs). In this study, for the first time we report liquid phase mineralization of nuclear grade cationic IRN-77 resin in Co2+/peroxymonosulfate (PMS) based SR-AOPs. After the dissolution of cationic IRN-77 resin, 30 volatile and 15 semi-volatile organic compounds were analyzed/detected using non-targeted GC-MS analysis. The optimal reaction parameters for the highest chemical oxygen demand (COD) removal (%) of IRN-77 resin were determined, and the initial pH, PMS dosage, and reaction temperature were found to be the most influential parameters for the resin degradation. We successfully achieved ∼90% COD removal (1000 mg/L; 1000 ppm) of dissolved spent resin for SR-AOPs by optimizing the reaction parameters as initial pH = 9, Co2+ = 4 mM (catalyst), PMS = 60 mM (as oxidant) at 60 °C temperature for 60 min reaction. The electron spin resonance spectroscopy (ESR) spectra confirmed the presence of SO4∙- and OH∙ as main reactive species in the Co2+/PMS resin system. In addition, Fourier transform infrared spectroscopy (FT-IR) analyses were used for structural characterization of solid and liquid phase resin samples. We believe that this work will offer a robust approach for the effective treatment of spent resin generated from nuclear industry.

Metallic technetium sequestration in nickel core/shell microstructure during Fe(OH)2 transformation with Ni doping.

This study investigates the impacts of Ni doping on technetium-99 (Tc) sequestration in aqueous solutions through transformation of Fe(OH)2(s) to iron spinel (magnetite) under alkaline conditions. Extensive solid characterization was performed for the mineral phases produced, as well as the Tc/Ni speciation and distribution within these phases. X-ray diffraction results show that iron spinel was the dominant mineral product without detectable Ni incorporation. The doped Ni ions mainly precipitated as fine Fe/Ni oxide/hydroxide particles, including strongly reduced nanometer-sized spheroidal Ni-rich and metallic Ni phases. High-resolution analytical scanning transmission electron microscopy using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy on the produced solid samples (focused ion beam-prepared specimens) revealed three Tc distribution domains dominated by nanocrystals and, especially, a Tc-rich metallic phase. Instances of metallic Tc were specifically found in spheroidal, Ni-rich and metallic nanoparticles exhibiting a core/shell microstructure that suggests strong reduction and sequential precipitation of Ni-Tc-Ni. Mass balance analysis showed nearly 100% Tc removal from the 4.8 × 10-4 M Tc solutions. The finding of the metallic Tc encapsulation indicates that Tc sequestration through Ni-doped Fe(OH)2(s)-to-iron spinel transformation process likely provides an alternative treatment pathway for Tc removal and could be combined into further waste treatment approaches.

Development of a Geochemical Speciation Model for Use in Evaluating Leaching from a Cementitious Radioactive Waste Form.

Cast Stone has been developed to immobilize a fraction of radioactive waste at the Hanford Site; however, constituents of potential concern (COPCs) can be released when in contact with water during disposal. Herein, a representative mineral and parameter set for geochemical speciation modeling was developed for Cast Stone aged in inert and oxic environments, to simulate leaching concentrations of major and trace constituents. The geochemical speciation model was verified using a monolithic diffusion model in conjunction with independent monolithic diffusion test results. Eskolaite (Cr2O3) was confirmed as the dominant mineral retaining Cr in Cast Stone doped with 0.1 or 0.2 wt % Cr. The immobilization of Tc as a primary COPC in Cast Stone was evaluated, and the redox states of porewater within monolithic Cast Stone indicated by Cr are insufficient for the reduction of Tc. However, redox states provided by blast furnace slag (BFS) within the interior of Cast Stone are capable of reducing Tc for immobilization, with the immobilization reaction rate postulated to be controlled by the diffusive migration of soluble Tc in porewater to the surface of reducing BFS particles. Aging in oxic conditions increased the flux of Cr and Tc from monolithic Cast Stone.

Development of geopolymer waste form for immobilization of radioactive borate waste.

Radioactive borate waste containing a high concentration of boron (B) is problematic to be solidified using cement because soluble borate such as boric acid hinders the hydration reaction. In this study, borate waste was used as a raw material for metakaolin-based geopolymer according to the characteristic that B replaces a part of Si. Geopolymers using KOH alkaline activator (K-geopolymers) showed higher compressive strength than geopolymers using NaOH alkaline activator (Na-geopolymer). In addition, the compressive strength increased proportionally to the Si/(Al+B) ratio regardless of the alkaline cation species. These variations in compressive strength might be due to the viscosity of the geopolymer mixture, atomic size of alkaline cations, and the increase in Si content. The characteristic analyses (XRD, FT-IR, and solid state 11B MAS NMR) indicated that B was incorporated into the geopolymer structure. Thus, the K-geopolymer has a dense and homogeneous microstructure. In a semi-dynamic leaching test, less B leached from the geopolymers compared to the cement waste form. Consequently, borate waste can be solidified using metakaolin-based geopolymer, and the use of a KOH alkaline activator is advantageous in terms of mechanical property and structural durability.

Efficient radon removal using fluorine-functionalized natural zeolite.

Radon (Rn) can easily leak into the environment through groundwater owing to its high water solubility. Therefore, studying the chemical factors influencing the content and removal of Rn from groundwater is crucial for the evaluation and mitigation of its radiological risks to public health. In this study, we conducted a redundancy analysis (RDA) of Rn in groundwater and performed batch sorption experiments for efficient Rn removal from the groundwater collected from Daejeon using natural zeolite (NZ) and fluorine-functionalized natural zeolite (FFNZ) sorbents. The redundancy analysis revealed a positive correlation between the concentrations of Rn and fluorine (F) in groundwater, indicating that F can support the long-term retention of Rn in groundwater. NZ and FFNZ achieved ~40% and ~70% removal of Rn, respectively, following 24 h of treatment, indicating a significant impact of F (in FFNZ) toward Rn removal from groundwater. Based on the results, Rn is considered to interact with F through the van der Waals force, which limits the volatilization of Rn from the solution. Similarly, the fluorine-functionalized sorbent would interact preferentially with Rn, thereby enhancing its sorption and removal from groundwater.

Effect of ion exchange resin particle size on homogeneity and leachability of Cs and Co in polymer waste form.

We report the size effect of ion exchange resins (IERs) on Cs and Co distribution in polymer waste forms. Ball mill ground IERs (BG) waste form resulted in relatively better homogeneous waste distribution and displayed superior Cs and Co leachability indexes compared with the same polymer waste form prepared with non-ground IERs (NG).

Transport of Colloidal Particles in Microscopic Porous Medium Analogues with Surface Charge Heterogeneity: Experiments and the Fundamental Role of Single-Bead Deposition.

Understanding colloid transport in subsurface environments is challenging because of complex interactions among colloids, groundwater, and porous media over several length scales. Here, we report a versatile method to assemble bead-based microfluidic porous media analogues with chemical heterogeneities of different configurations. We further study the transport of colloidal particles through a family of porous media analogues that are randomly packed with oppositely charged beads with different mixing ratios. We recorded the dynamics of colloidal particle deposition at the level of single grains. From these, the maximum surface coverage (θmax = 0.051) was measured directly. The surface-blocking function and the deposition coefficient (kpore = 3.56 s-1) were obtained. Using these pore-scale parameters, the transport of colloidal particles was modeled using a one-dimensional advection-dispersion-deposition equation under the assumption of irreversible adsorption between oppositely charged beads and colloids, showing very good agreement with experimental breakthrough curves and retention profiles at the scale of the entire porous medium analogue. This work presents a new approach to fabricate chemically heterogeneous porous media in a microfluidic device that enables the direct measurement of pore-scale colloidal deposition. Compared with the conventional curve-fitting method for deposition constant, our approach allows quantitative prediction of colloidal breakthrough and retention via coupling of direct pore-scale measurements and an advection-dispersion-deposition model.

Dissolved Carbonate and pH Control the Dissolution of Uranyl Phosphate Minerals in Flow-Through Porous Media.

Uranyl phosphate minerals represent an important secondary source of uranium release at contaminated sites. In flow-through column experiments with background porewater (BPW) of typical freshwater aquifer composition (pH 7.0, ∼0.2 mM total carbonate (TC)), dissolution of K-ankoleite (KUO2PO4·3H2O), Na-autunite (NaUO2PO4·3H2O), and Ca-autunite (Ca(UO2)2(PO4)2·6H2O) was controlled by mineral solubility at steady-state U release. Effluent concentrations indicated exchange with BPW cations, and postreaction characterization showed alteration of the initial mineral composition, changes in structure (decreased crystallinity, increased disorder, and distortion of U-P mineral sheets) and possible neoformation of phases of similar structure. Increasing the BPW pH and TC to 8.1-8.2 and 2.2-3.7 mM, respectively, resulted in mineral undersaturation and produced ca. 2 orders-of-magnitude higher U and P release without reaching steady state. Minerals incorporated less BPW cations into their structures compared to low carbonate BPW experiments but showed structural disorder and distortion. Faster dissolution rates were attributed to the formation of binary and ternary uranyl carbonate complexes that accelerate the rate-determining step of uranyl detachment from the uranyl-phosphate layered structure. Calculated dissolution rates (log Rs between -8.95 and -10.32 mol m-2 s-1), accounting for reaction and transport in porous media, were similar to dissolution rates of other classes of uranyl minerals. In undersaturated solutions, dissolution rates for uranyl phosphate, oxyhydroxide, and silicate minerals can be predicted within 1-2 orders-of-magnitude from pH ∼5-10 on the basis of pH/carbonate concentration.

Impact of Cr and Co on 99Tc retention in magnetite: A combined study of ab initio molecular dynamics and experiments.

The effect of co-mingled dopants, Co(II) and Cr(III), on Tc(IV) incorporation and retention in magnetite under varying temperatures (75-700 °C) was explored using ab initio molecular dynamics simulations, batch experiments, and solid phase characterization. Tc(IV) stabilization was achieved with a magnetite surface oversaturated with or containing an equal number of Tc and Cr. Under oversaturation conditions, the forced formation of a Cr2O3 phase on the magnetite surface may help prevent Tc release. Upon Co addition, and depending on the relative concentration of Tc, Cr, and Co at the magnetite surface, Co was found to preferentially stabilize Cr rather than Tc and suppress the formation of the protective Cr2O3 surface phase. Only systems with similar Cr/Co concentrations or relatively high Cr concentrations stabilized Tc within magnetite. As such, the relative concentration of Tc, Cr, and Co was identified as a critical parameter for maximizing dopant efficacy towards Tc stabilization in magnetite.

Relative permeability for water and gas through fractures in cement.

Relative permeability is an important attribute influencing subsurface multiphase flow. Characterization of relative permeability is necessary to support activities such as carbon sequestration, geothermal energy production, and oil and gas exploration. Previous research efforts have largely neglected the relative permeability of wellbore cement used to seal well bores where risks of leak are significant. Therefore this study was performed to evaluate fracturing on permeability and relative permeability of wellbore cement. Studies of relative permeability of water and air were conducted using ordinary Portland cement paste cylinders having fracture networks that exhibited a range of permeability values. The measured relative permeability was compared with three models, 1) Corey-curve, often used for modeling relative permeability in porous media, 2) X-curve, commonly used to represent relative permeability of fractures, and 3) Burdine model based on fitting the Brooks-Corey function to fracture saturation-pressure data inferred from x-ray computed tomography (XCT) derived aperture distribution results. Experimentally-determined aqueous relative permeability was best described by the Burdine model. Though water phase tended to follow the Corey-curve for the simple fracture system while air relative permeability was best described by the X-curve.

Investigation of 3H, 99Tc, and 90Sr transport in fractured rock and the effects of fracture-filling/coating material at LILW disposal facility.

Batch adsorption, batch diffusion, and flow-through column experiments were conducted using groundwater and fractured rock collected in unsaturated zone to increase our understanding of sorption and transport behavior of radionuclides. Increasing Kd values were observed in the sequence 90Sr, 99Tc, and 3H regardless of the geological media tested. For all sorbing radionuclides, Kd values for the fracture-filling/coating material were observed to be higher than those for without fracture-filling/coating material regardless of the groundwater. These higher Kd values are the result of zeolite mineral in filling/coating material of fractured rock. The batch diffusion and flow-through column experiments were also conducted using the same fractured rock sample, and the results of diffusion and column experiments showed similar trend of radionuclide sorption and transport to sorption experiment. In this study, sorption Kd of radionuclide was determined and used to increase our understanding of radionuclide retardation through fracture-filling/coating materials.

99Tc immobilization from off-gas waste streams using nickel-doped iron spinel.

Technetium-99 (99Tc) incorporation within stable spinel phases is a novel method for 99Tc removal and immobilization from waste streams. In this study, transformation of Ni-doped Fe(OH)2(s) to spinel minerals, e.g. trevorite (NiFe2O4), is explored as a method for removing 99Tc from Hanford Waste Treatment and Immobilization Plant (WTP) primary off-gas waste stream simulant. The Fe(OH)2(s) transformation process was found to reduce 99Tc(VII) to 99Tc(IV) and incorporate reduced Tc(VI) into the produced spinel simultaneously. Nickel doping was applied in the mineral transformation to inhibit potential reoxidation of 99Tc(IV). Solid phase characterization by XRD and XANES confirmed the formation of nickel substituted ferric-spinel, and suggest incorporation of 99Tc(IV) in the final spinel. Furthermore, in the primary off-gas stream, which contains both redox-sensitive contaminants Cr(VI) and 99Tc(VII), results from solution analysis and solid digestion indicate that nearly 100% Cr and over 80% 99Tc can be simultaneously removed by adding Fe(OH)2(s) to solution with a solid to solution ratio of 5 g/L under near neutral and alkaline conditions. The 99Tc removal approach developed herein provides an alternative treatment method to eliminate the proposed recycle process of the off-gas waste stream, which ultimately can reduce WTP mission cost and operation time.