Isotope Separation Factor Research Articles

Investigations on Hydrogen Isotope Separation Factor Employing Palladium-Based Solid Metallic Membranes

At present, there are several methods for hydrogen isotope separation (in elemental form), the most important being cryogenic distillation, thermal diffusion, and gas chromatography. However, these methods have a series of drawbacks, namely, high complexity, high energy consumption, and associated costs. Taking into account these disadvantages, a promising separation method is the one based on solid metallic membranes because of its advantages like low energy consumption and reduced complexity. This method uses the difference between some of the isotopes’ properties, namely, solubility; diffusivity; and, implicitly, permeability. This work envisages the integration of an isotopic separation module, based on membrane permeation, on the exhaust gas line from the current experimental rigs employed at ICSI, to recover and store the hydrogen isotopes. We obtained a maximum separation factor of 5.66 for the lowest studied concentration of deuterium in the hydrogen isotopic mixture (0.05 atomic fraction). The results show that hydrogen isotope separation is possible using palladium/silver membranes. Still, the throughput of the permeated gas is very low, and a significant number of stages will be necessary to obtain the desired purity (above 99.5%).

Applicability of self-consistent reaction field (SCRF) method to DFT estimation of hydrogen isotope separation factors in reversible processes

ABSTRACT In earlier quantum chemical calculations of isotope effects, chemical species in the liquid phase were generally treated as existing in the gas phase. In recent years, however, advances in computational programs have made it easier for the self-consistent reaction field (SCRF) method to handle chemical species in the liquid phase, and as a result, it has become easier to apply the SCRF method to isotope effect calculations. This paper concerns the scope of application of the DFT-SCRF method to reversible processes for hydrogen isotope enrichment. It is found that the applicability of the method depends on the type of the intermolecular interaction in the liquid phase and the degree of hydrogen isotope effect (separation factor) on which the process is based. When the magnitude of the isotope effect of the separation system is greater than 10–1, the simple SCRF method is fully applicable; when the magnitude is around 10–2, SCRF with a dimer model, in which the monomer is replaced by a dimer, is applicable for the analysis of the liquid phase with relatively strong intermolecular interactions. Anharmonic correction to the separation factor calculated based on harmonic frequencies may be effective to systems with the liquid phase with weak intermolecular interactions.

Hydrogenotrophic pathway dominates methanogenesis along the river-estuary continuum of the Yangtze River

Rivers are considered as an important source of methane (CH4) to the atmosphere, but our understanding for the methanogenic pathway in rivers and its linkage with CH4 emission is very limited. Here, we investigated the diffusive flux of CH4 and its stable carbon isotope signature (δ13C-CH4) along the river-estuary continuum of the Yangtze River. The diffusive CH4 flux was estimated to 27.9 ± 11.4 μmol/m2/d and 36.5 ± 24.4 μmol/m2/d in wet season and dry season, respectively. The δ13C-CH4 values were generally lower than -60‰, with the fractionation factor (αc) higher than 1.055 and the isotope separation factor (εc) ranged from 55 to 100. In situ microbial composition showed that hydrogenotrophic methanogens accounts for over 70% of the total reads. Moreover, the incubation test showed that the headspace CH4 concentration by adding CO2/H2 to the sediment was orders of magnitude higher than that by adding trimethylamine and sodium acetate. These results jointly verified the river-estuary continuum is a minor CH4 source and dominated by hydrogenotrophic pathway. Based on the methanogenic pathway here and previous reported data in the same region, the historical variation of diffusive CH4 flux was calculated and results showed that CH4 emission has reduced 82.5% since the construction of Three Gorges Dam (TGD). Our study verified the dominant methanogenic pathway in river ecosystems and clarified the effect and mechanism of dam construction on riverine CH4 emission.

Tritium separation from radioactive wastewater by hydrogen isotope-selective exchange of hydrogen-bonded fluorine

The separation of tritium (3H) from radioactive wastewater is a persistent challenging problem in the nuclear industry from an environmental perspective. This study provides new insight into the tritium separation by investigating hydrogen isotope-selective exchange reaction on hydrogen-bonded fluorine (F····H····O) site. In this study, fluorinated mesoporous silica, F-MCM-41_NCs were prepared as a novel tritium sorbent that showed a significantly enhanced tritium isotope separation factor (α) compared to the pristine MCM-41 (α = 1.2). The optimal α of 3.3 is achieved using 0.5F-MCM-41_NC prepared with 0.5 g of NH4F per gram of MCM-41. This optimal α is attributed to an excellent compromise between the amount and the stability of (NH4)2SiF6 on the pore channel with the integrity of the MCM-41 framework. Experimental, theoretical, and analytical characterization results consistently support chemical affinity quantum sieving effect at F····H····O hydrogen-bonding sites via the hydrogen isotope-selective exchange reaction as the dominant tritiumseparation mechanism.

Boron adsorption and isotopic separation from water by isostructural metal-organic frameworks MIL-100(M)

Adsorption has been expected to be promising for boron-containing water treatment, boron resources recovery and boron isotopic separation. Although metal-organic frameworks have been demonstrated to be potential adsorbent candidates, in-depth investigation of the structure-performance relationship remains insufficient. Herein, the performance and mechanism of boron adsorption and isotopic separation were systematically investigated with isostructural MOFs. Six isomorphic MIL-100(M) materials with metal species from the third main group (Al, Ga and In) and the fourth period (Sc, Cr and Fe) were solvothermally synthesized as boron adsorbents. Superior boron isotopic separation factors (S) with 10B preference and comparable boron adsorption capacity (Q) were observed through batch boron adsorption experiments, where MIL-100(Al) exhibited the highest S of 1.618 and MIL-100(Sc) achieved the largest Q of 8.74 g-B/mol-MOF. A high correlation between metal atomic radius and performance was revealed and a plausible mechanism was proposed for boron adsorption and isotopic separation involving the important role of the metal center through post-adsorption characterization. This study will pave the way for designing boron adsorbents with improved properties and promote the efficient utilization of boron resources in water.

DFT, MD simulations and experimental analysis of adsorptive complexation and isotope separation of gadolinium ion with macrocyclic crown ether embedded polymeric resin

Di-benzo-18-crown-6 ether (DB18C6) functionalized polymethyl acrylic acid (PMA) resin was synthesized after screening by employing density functional theory (DFT) and molecular dynamics (MD) simulations for the enrichment of gadolinium isotopes, Gd-155 and Gd-157. Batch adsorption and column chromatography studies were performed at various pH, gadolinium ion concentration and temperatures to check its efficacy and validation of theoretical approach. The adsorption capacity of gadolinium ion with the PMADB18C6 resin was found to be quite modest with a value of 1.128 mg/g. Both MD simulations and adsorption experiments confirm the Temkin type of adsorption isotherm. The positive entropy change along with the positive enthalpy change indicates that sorption of the Gd3+ ion with PMADB18C6 is entropy driven process. From the chromatography experiments, the lighter isotope (Gd-155) is seen to be depleted in the breakthrough volume samples which specify that the resin phase is depleted with heavier isotope (e.g Gd-160) contrast to natural Gd composition. The separation coefficient (ε) for Gd155/158, Gd156/158, Gd157/158, Gd155/160 and Gd157/160 isotopic pairs were found to be 1.67 × 10−3, 2.65 × 10−3, 5.40 × 10−4, 3.18 × 10−3 and 1.87 × 10−3 respectively. The experimental isotopic separation factor corroborates the DFT findings. Further, both DFT and MD simulations confirm the experimentally observed endothermic complexation of Gd3+ ion with PMADB18C6 resin. The present findings employing combined DFT, MD simulations and experimental approaches are expected to benefit in the fundamental understanding of molecular complexation based isotope separation and design and development of future gadolinium isotopic enrichment technologies using chemical exchange methods.

Inversion of boron isotope effect during chemical exchange in extraction systems

In the processes of boron isotopic exchange in liquid-liquid systems using boric acid as the main working substance, the lighter boron isotope is concentrating in the organic phase, which takes place both in the case of phase exchange (tributyl phosphate, TBP) and at the chemical (trioctylamine, TOA) isotope exchange. That is, boron-10, as a rule, predominantly passes into the organic phase. In contrast to the previously observed effects, our studies for several extraction systems using secondary amines (diethyl, dipropyl, and dibutylamine as the organic phase) demonstrated the inversion of the boron isotope effect when a more light isotope 10B is concentrated in the aqueous phase. At the same time, the level of the values of the single stage separation factor of boron isotopes turned out to be quite high and amounted to 1.020 - 1.028. The paper discusses the possible causes of the discovered phenomenon, and also considers the potential for practical application of such an effect.

Effect of Applied Voltage on Isotope Separation Factor By Electrodialysis Using Lithium Ion Conductive Electrolyte La0.57Li0.29TiO3

Tritium, which is used as a fuel in a thermonuclear fusion reactor, is produced in the fusion reactor blanket by the reaction of neutrons with lithium whose 6Li-isotope-ratio is 90%. Other than the mercury amalgam method, which has a large environmental impact, no practical lithium isotope enrichment technology has been established. Electrodialysis using the lithium ion conductive electrolyte La0.57Li0.29TiO3 (LLTO) is promising, as reported to achieve high isotope enrichment [1]. In this study, we investigate the effect of applied voltage in electrodialysis on the isotope separation factor.An anode (primary solution side) and a cathode (secondary solution side) were formed on both front and back surfaces of LLTO. One reference electrode was also formed on each surface of the LLTO. DC voltage of various magnitudes was applied between the anode and the cathode. The electrochemical impedance between the reference electrode on the primary solution side and the anode during the application of this voltage was measured by AC impedance spectroscopy. At the same time, the electrochemical impedance between each of the secondary solution side reference electrode and the cathode was also measured. The amount of transferred lithium and the isotope ratio of lithium were measured by an inductively coupled plasma mass spectrometer (ICP-MS).The lithium isotope separation factor increased linearly with decreasing applied voltage. Here we assume the following: ① The existence probability of such atoms with respect to the minimum value of the energy required for lithium in the crystal lattice to move to the adjacent site has a normal distribution, ② In 6Li and 7Li, the standard deviation of the normal distribution is the same and the energy average the values are different. Then, in a narrow energy region near the activation energy of 6Li or 7Li transfer, it is expected that the ratio of 6Li in the energy transfer lithium decreases linearly with the increase in energy. Our experimental results seem to follow this prediction.[1] S. Honda, K. Shin-mura and K. Sasaki, J. Ceram. Soc. Japan, 126, [5], 331-335 (2018).

Mechanistic study on boron adsorption and isotopic separation with magnetic magnetite nanoparticles

Good boron removal and isotopic separation behavior was observed on magnetic magnetite nanoparticles (MMNs) in a previous study, but the mechanism of boron adsorption and isotopic separation remains unclear. Here, experimental studies accompanied by quantum chemistry calculations were implemented for the first time to reveal the nature of boron adsorption and isotopic separation on MMN. First, the fitted peaks of Fe2p of MMN by X-ray photoelectron spectrometry (XPS) before and after adsorption showed decrease in Fe(II)/Fe(III) ratio, which validated the formation of Fe–O–B interaction. Fe–O–B bond was also observed through fitted peaks of O1s and dihydroxy complexation where MMNs prefer to adsorb [B(OH)4]−(H3BO3) at pH pHzpc) identified by the ratio of integrated peaks. ATR-FTIR revealed the hydroxyl moiety as the main adsorptive group. The selectivity of MMN toward [B(OH)4]− at pH pHzpc was figured out by calculating the ratio between the integrated areas of B–O bands at different pH values. As a result, the trend of adsorption capacity as well as the isotopic separation factor with pH was well illustrated. The concluded adsorption mechanism from the experiment was further verified with the simulated adsorption energies (ΔE) and isotopic separation factors (S) calculated by DFT quantum simulation.

Hydrogen isotope effect of nanoporous palladium

Materials used in hydrogen isotopes separation are crucial in modern hydrogen energy field. In this article, a promising material nanoporous palladium was prepared and its relative properties were studied. Nanoporous palladium with pore scale of about 5 nm was fabricated by free dealloying corrosion. Its kinetic and PCT curves of hydrogen/deuterium adsorption at room temperature were tested using a Sievert-type volumetric apparatus. Microstructures were observed with Scanning Electron Microscopy and Atomic Force Microscopy. Crystalline structure was characterized with X-Ray Diffractometer before and after deuterium adsorption. Comparative experiments with spongy palladium that is commonly used in relative industry were also carried out. According to the results, nanoporous palladium shows faster hydrogen/deuterium adsorption rate than spongy palladium, which is due to its nanoporous structure that supplies a large amount of specific surface area. PCT curves of hydrogen/deuterium adsorption in nanoporous palladium among 298–338 K were tested and plateau pressures at different temperatures were obtained. Deuterium/hydrogen isotopes separation factors were calculated using plateau pressures above and were plotted with temperature. It's found that nanoporous palladium shows larger hydrogen isotope separation factors compared with spongy palladium at temperatures no higher than 323 K. These findings illustrate that nanoporous palladium would be an ideal material for hydrogen isotopes separation applications.

Enhancing the selectivity of hydrogen isotopic water in membrane distillation by using graphene oxide

Isotopes of hydrogen are playing an increasingly important role in applications such as controllable nuclear fusion and other nuclear processes, yet directly separating isotopic water from its mixture is still energy-intensive and largely limited by the moderate separation factor values. In this study, Graphene oxide (GO) and reduced graphene oxide (rGO) nanosheets, combining optimized hydrophobicity by grafting perfluoroalkylsilanes (PFAS), have been used for improving the selectivity of isotopic water in membrane distillation. By utilizing heavy/light water mixture as a model, permeation flux and isotopic selectivity of the membranes are systematically evaluated. The graphene oxide membranes, particularly for the rGO grafted by PFAS, exhibit enhanced selectivity with an increased isotopic separation factor, as compared with that of commercial membranes, which is probably due to the efficient synergy between unimpeded nanochannels and near-super hydrophobic surface. A maximum mean separation factor value of 1.067, combined with a permeation flux of 0.47 kg m−2 h−1 have been achieved for the grafted rGO -membrane reduced at 750 °C. Furthermore, our study demonstrates utilizing two-dimensional materials, like GO and rGO, can be an effective avenue for promoting the isotopic selectivity in membrane distillation, based on increasing hydrophobic water vapor channels with higher selectivity.

Boron removal and reclamation by magnetic magnetite (Fe3O4) nanoparticle: An adsorption and isotopic separation study

Magnetic magnetite nanoparticle (MMN) was studied as an adsorbent for effective boron removal and isotopic separation in aqueous solution for the first time. The adsorption equilibrium was reached rapidly in 1.5 h, and a high capacity of 4.57 mmol/g was achieved at pH = 7 at 45 °C. Quasi second-order kinetic model and Henry isotherm suitably described boron adsorption on magnetite nanoparticles. Furthermore, magnetite nanoparticles showed better affinity for 10B, and a high boron isotopic separation factor (S) of 1.332 was observed at pH = 6 and 15 °C. The adsorption was a spontaneous chemisorption process where the entropy effect is the main driving force instead of enthalpy change. The FTIR and XPS spectra of boron-encapsulated MMN indicated the formation of new FeOB bonds. High boron adsorption capacity, excellent boron isotope separation factor, low cost and easy regeneration empower MMN as an adsorbent of great practical value for boron removal and isotope reclamation.

Novel Functionalized Cellulose Microspheres for Efficient Separation of Lithium Ion and Its Isotopes: Synthesis and Adsorption Performance.

The adsorption of lithium ions(Li+) and the separation of lithium isotopes have attracted interests due to their important role in energy storage and nuclear energy, respectively. However, it is still challenging to separate the Li+ and its isotopes with high efficiency and selectivity. A novel cellulose-based microsphere containing crown ethers groups (named as MCM-g-AB15C5) was successfully synthesized by pre-irradiation-induced emulsion grafting of glycidyl methacrylate (GMA) and followed by the chemical reaction between the epoxy group of grafted polymer and 4′-aminobenzo-15-crown-5 (AB15C5). By using MCM-g-AB15C5 as adsorbent, the effects of solvent, metal ions, and adsorption temperature on the adsorption uptake of Li+ and separation factor of 6Li/7Li were investigated in detail. Solvent with low polarity, high adsorption temperature in acetonitrile could improve the uptake of Li+ and separation factor of lithium isotopes. The MCM-g-AB15C5 exhibited the strongest adsorption affinity to Li+ with a separation factor of 1.022 ± 0.002 for 6Li/7Li in acetonitrile. The adsorption isotherms in acetonitrile is fitted well with the Langmuir model with an ultrahigh adsorption capacity up to 12.9 mg·g−1, indicating the unexpected complexation ratio of 1:2 between MCM-g-AB15C5 and Li+. The thermodynamics study confirmed the adsorption process is the endothermic, spontaneous, and chemisorption adsorption. As-prepared novel cellulose-based adsorbents are promising materials for the efficient and selective separation of Li+ and its isotopes.

Calculation of Lithium Isotope Effects in Extraction Systems with Benzo-15-crown-5 and Its Derivatives

With taking into account the β-factor value of the lithium ion aqua complex found earlier, the single stage extraction coefficient of isotope separation (α) of the 6Li–7Li pair has been calculated for the extraction system containing the benzo-15-crown-5–Li+ (H2O)Cl– complex in the organic phase, which corresponds to a real extraction process. Similar calculations have been carried out for other anions, as well as for various benzo-15-crown-5 derivatives with varying the number and type of donor atoms and different substituents in the benzene ring. Quantum-chemical calculations of the vibrational frequencies of the isotopic forms of the crown ether complex were performed using the Firefly program with the RHF/6-311++G** basis set. It has been shown that the isotope separation factor in this case is lower than in the case of the anhydrous benzo-15-crown-5–Li+Cl– complex and the model benzo-15-crown-5–Li+ (H2O) complex.

Observation of 18O/16O isotope effects at the cathode of a polymer electrolyte membrane fuel cell

ABSTRACT18O/16O isotope effects were observed at the cathode of a polymer electrolyte membrane fuel cell at 25 and 35°C. Results of experiments in which the 18O/16O isotope ratios of the oxygen gases supplied to and exhausted from the cell were measured revealed that the lighter isotope 16O reacted more preferentially to form water molecules at the cathode than the heavier one, 18O. The value of the oxygen isotope separation factor, S1, defined as the ratio of the 18O/16O isotope ratios of the oxygen gases supplied to and exhausted from the cell, ranged from 1.0030 to 1.0139, and tended to decrease with decreasing rate of oxygen utilisation (θ) and with increasing flow rate of the feed oxygen gas (DF). The value of another separation factor, S2, defined as the ratio of the 18O/16O isotope ratios of the exhausted oxygen gas and oxygen having reacted to form water molecules at the cathode, ranged from 1.0049 to 1.0304. The S2 value was much less affected by the change in θ and DF than the S1 value with the majority of the S2 value being in the range of 1.0240–1.0304.

Molybdenum stable isotope fractionation during the precipitation of powellite (CaMoO4) and wulfenite (PbMoO4)

Laboratory batch experiments were conducted to study Mo isotope fractionation during the precipitation of powellite (CaMoO4) and wulfenite (PbMoO4). These minerals act as important sinks of dissolved Mo, especially in alkaline-pH mine drainage. Their precipitation might be traceable through Mo isotope analyses, provided that it induces Mo isotope fractionation. Experiments involved the presence/absence of dissolved sulfate and inorganic carbon (DIC) to better mimic the aqueous chemistry of mine drainage, as these constituents are characteristic of mine waters.Molybdate minerals preferentially incorporated light Mo isotopes from aqueous solution with fractionation patterns that are linked to the saturation state. The Rayleigh fractionation model was most suitable to describe powellite precipitation under supersaturated conditions. The average Rayleigh isotope separation factor for powellite precipitation was −0.82 ± 0.12‰, with no significant effect from the presence/absence of sulfate and DIC. Upon approaching chemical equilibrium, bidirectional exchange of molybdate between dissolved Mo and powellite led to a pronounced release of isotopically light Mo to solution, which overrode the Rayleigh fractionation pattern formed at early time. Molybdenum isotope data during wulfenite precipitation could reasonably be fitted using the Rayleigh and equilibrium fractionation models, which yielded isotope separation factors of −0.25 ± 0.10‰ and −0.75 ± 0.07‰, respectively. These results contribute to a growing knowledge of Mo isotope fractionation during processes of dissolved Mo removal (e.g., adsorption and precipitation), thus far all of which show that an enrichment in heavy Mo isotopes in solution is indicative of Mo attenuation.

Separation of boron isotopes by chemical exchange in liquid-liquid systems

The currently used technologies for the production of boron isotopes for nuclear power and nuclear medicine are based on the application of toxic boron halides. The report is devoted to alternative processes for enrichment of 10B and 11B by chemical exchange in extraction systems using boric acid, that is, without of boron halides. The results of studying phase and isotope equilibrium in two liquid-liquid systems are considered: an aqueous solution of boric acid – an organic phase in the form of tributyl phosphate or a solution of trioctylamine in o-xylene. The obtained values of the separation factor for boron isotopes characterize the predominantly phase isotope effect in the first system, and in the second system the effect of chemical isotope exchange. Accordingly, the values of the single stage separation factor are 1.0067 ± 0.0035 and 1.025 ± 0.005 and are close to similar values for rectification of boron trifluoride and chemical exchange in the anisole process of boron isotope separation. The practical use of the studied extraction systems may allow the separation of boron isotopes in the horizontal cascades of centrifugal extractors in the absence of an environmental hazard.

Influence of stratiform clouds on δD and δ18O of monsoon water vapour and rain at two tropical coastal stations

Studies of moisture dynamics associated with major tropical rain activities with the aid of stable oxygen and hydrogen isotopic (δ18O and δD) measurements in precipitation and water vapour are of current research interest. Here we present such measurements of rain and water vapour samples collected at two tropical coastal hill stations of southern peninsular India during April–October, 2012. An apparent seasonal cycle is observed both in rain and vapour isotopes. Despite of the isotopic equilibrium observed between rain and vapour during the whole sampling period, it often deviates in sub seasonal time scales. Rain-vapour isotopic separation factors are lower compared to the equilibrium separation values during stratiform rain events of Indian summer monsoon, while the separation factors are close to the equilibrium values during convective events. This is possibly due to the decreased vertical mixing of moisture during stratiform rain events.

Boron isotopic separation with pyrocatechol-modified resin by chromatography technology: Experiment and numerical simulation

Boron isotopic separation was investigated experimentally and theoretically on boron-specific adsorbent, namely pyrocatechol-modified resin (CL-RESIN). The static adsorption results with initial boron concentration of 8.648gL−1 at 25°C were well fitted by the pseudo-second-order kinetic model and Henry isotherm model, from which relevant parameters were used for subsequent isotopic separation simulation by Aspen Chromatograph. The maximum static boron isotopic separation factor with CL-RESIN was 1.13 at 25°C, thus adsorption-based boron isotopic separation in a chromatographic column packing with CL-RESIN was further studied by experiments and simulations. 11B had greater affinity with CL-RESIN in concentrated boric acid solution with dynamic boron isotopic separation factor of 1.15 and enriched 10B was collected in effluent. Simulation of boron isotopic separation was completed, and simulated results were in good agreement with experimental breakthrough curves under the same conditions. Furthermore, the dynamic behaviors of boron isotopic separation were predicted by simulation under different operating conditions. Response surface methodology (RSM) was employed to determine optimized operating conditions in a chromatographic column. Consequently, the optimized operating conditions were achieved with flow rate of 1.24mLmin−1, 44cm column length, 0.46cm column diameter and feed concentration of 8.648gL−1.

Pyrocatechol-modified resins for boron recovery from water: Synthesis, adsorption and isotopic separation studies

Two kinds of pyrocatechols, pyrocatechol (CL) and nitropyrocatechol (NCL), were chosen by the conductive value change (Δ) they cause in the boric acid-polyol solutions, and corresponding modified resins (CL-RESIN and NCL-RESIN) were synthesized by new methodology for effective boron removal and isotopic separation. The optimized boron adsorption occurs at pH=9.06 for CL-RESIN, and pH=6.70 for NCL-RESIN, with the maximum adsorption capacity 0.7886mmol·g−1 and 0.7931mmol·g−1, which were comparable to commercial IRA 743. Boron adsorption on prepared resins was saturated within 12h and can be well described by pseudo-second-order kinetic model. Freundlich isotherm model fits well at low boron concentration while Langmuir isotherm model fits better at high concentration. Furthermore, the boron isotopic separation factors S on two prepared resins are 1.080 for CL-RESIN and 1.140 for NCL-RESIN, which are far higher than all previous results. Boron removal and isotopic separation capacities make it possible that three problems, boron removal, isotopic separation and boron reusability, can be addressed in single adsorption process.