Why hydrogen isotope separation matters
Hydrogen isotope separation is important for future fusion technologies. Deuterium and tritium are used as fusion fuels, but only about two percent of the fuel is consumed in each operating cycle. The exhaust stream therefore contains unreacted deuterium and tritium, together with protium formed in secondary reactions or released from structural materials. Efficient recovery and recycling require methods capable of separating mixtures containing all three hydrogen isotopes.
This is difficult because protium, deuterium and tritium have nearly identical chemical properties. Their molecular forms - H2, D2 and T2 - differ mainly in mass, so their separation depends on small isotope-dependent quantum effects.
In our study, published in Nature Communications, we demonstrate the selective separation of an initially equimolar H2/D2/T2 mixture using Ag(I)-exchanged zeolite type Y. The work directly demonstrates tritium-containing isotope separation and provides a platform for evaluating microporous materials for tritium enrichment and recycling.
Why tritium changes the experiment
Many porous materials have been investigated for separating H2 and D2. Tritium is more difficult to study directly because gaseous T2 requires specialized containment, controlled preparation, radiological monitoring and reliable recovery after each measurement.
Consequently, most experimental studies have focused on stable H2/D2 mixtures, while separation factors involving decaying T2 have often been predicted theoretically or inferred from experiments with stable isotopes. Direct measurements remain necessary because multicomponent mixtures cannot always be understood from single-gas adsorption alone. H2, D2 and T2 compete for the same adsorption sites of the crystal structure, and heavier isotopologues may replace lighter molecules during adsorption.
The challenge was therefore not only to introduce T2 into an adsorption experiment, but also to prepare, analyse and recover a controlled ternary H2/D2/T2 mixture safely.
A material that amplifies isotope effects
For the experiments, we used Ag(I)-exchanged zeolite type Y, or AgY. Exposed undercoordinated Ag+ sites in the microporous structure act as strong adsorption sites for hydrogen molecules.
Although H2, D2 and T2 have the same electronic structure, their different masses lead to different zero-point energies within the adsorption potential. The heavier isotopologues interact more strongly with the Ag+ sites. This mechanism, known as chemical affinity quantum sieving, converts small quantum-mechanical differences into measurable adsorption selectivity.
A further advantage is that the separation can be performed above liquid-nitrogen temperature, offering a potentially less energy-intensive alternative to conventional cryogenic distillation at lower temperatures.
A tritium-capable platform in Leipzig
The experiments were conducted in the radiochemistry laboratory of the Department of Reactive Transport at the Leipzig site of the Helmholtz-Zentrum Dresden-Rossendorf.
The facility combines tritium handling, vacuum gas preparation, cryogenic adsorption, thermal desorption spectroscopy, quadrupole mass spectrometry and radiometric analysis. T2 was supplied from a uranium tritide source, and the vacuum system was used to prepare defined binary and ternary isotope mixtures.
For each experiment, activated AgY was cooled to approximately 82–85 K and exposed to the selected gas mixture. After equilibration, the gas phase was removed. The sample was then heated at a controlled rate while the desorbing isotopologues were monitored by mass spectrometry.
Tritium required an additional analytical pathway. Gas leaving the mass spectrometer was transferred to a heated reactor, converted into tritiated water over a copper oxide catalyst, collected and quantified by liquid scintillation counting. Each measurement therefore combined adsorption, isotope analysis and safe tritium recovery in one controlled workflow.
Separating the ternary mixture
The central experiment used an initially equimolar H2/D2/T2 mixture. After exposure to AgY at approximately 83 K, the adsorbed isotopologues were analysed during controlled heating.
The desorption spectra showed a clear preference for the heavier isotopologues. T2 was strongly enriched, D2 was retained to a lesser extent, and H2 adsorption was almost completely suppressed. Considering the homonuclear isotopologues, the adsorbed molar ratio was: H2:D2:T2 = 1:41:175.
To our knowledge, this is the first TDS study to experimentally demonstrate the selective separation of an equimolar ternary H2/D2/T2 mixture.
Binary-mixture experiments clarified the behaviour of the ternary system. For equimolar mixtures, the measured selectivities were 29 for D2 over H2, 244 for T2 over H2 and 4.2 for T2 over D2. The high T2/H2 value demonstrates the strong discrimination between the lightest and heaviest isotopologues at the Ag+ sites.
The study also addressed two questions specific to tritium. First, isotope exchange formed small amounts of HD, HT and DT, but these species were minor relative to the total adsorbed amounts and did not dominate the separation. Second, comparison measurements before and after T2 exposure showed no significant change in D2 uptake before and after T2 exposure, indicating that the adsorption capacity and the number of accessible Ag+ adsorption sites remained unchanged under the investigated conditions.
Continuing the research
The study formed an important part of Dr Alexandra Becker’s doctoral dissertation. Her work established the methodology for preparing, separating and analysing hydrogen isotope mixtures containing tritium.
The Reactive Transport Group, led by Cornelius Fischer, is contributing to the DFG Research Training Group RTG 2721 (Hydrogen Isotopes), at Leipzig University with this work. The RTG provides an interdisciplinary research environment in which doctoral researchers can address fundamental questions concerning the separation, detection and application of hydrogen isotopes.
Following the completion of Alexandra’s doctorate, Jing Liu is continuing this line of research as one component of her own doctoral dissertation. She is extending the investigation to additional microporous materials to examine how different material properties influence hydrogen isotope adsorption and separation. The aim is to determine which material characteristics control competitive adsorption and desorption in H2/D2/T2 mixtures. A practically useful adsorbent must combine high selectivity with sufficient capacity, accessible operating temperatures, regeneration ability, cycling stability and resistance to tritium exposure.
The findings reveal that tailored adsorption sites in microporous materials can utilize the intrinsically small differences between hydrogen isotopologues, providing a promising strategy for isotope-selective adsorption. Although further work is needed to evaluate adsorption capacity, cycling stability and process-scale implementation, the study establishes an experimental basis for developing adsorption-driven technologies for tritium enrichment and recycling.