Salt offers a faster route to heavy water

Natural water contains minute concentrations of ‘heavy’ oxygen and hydrogen. Water enriched in these heavy isotopes is critical for a range of specific applications in medicine and nuclear science, yet the process of concentrating the heavy water component remains stubbornly inefficient.

Reporting in the journal Theoretical Foundations of Chemical Engineering, a research team in Russia has demonstrated that it is possible to increase the concentration efficiency by adding certain salts to the conventional distillation process^1,2.

Heavy water composed of H₂O molecules having the heavy deuterium isotope of hydrogen (²H) is used as a diagnostic tracer in medicine, as well as an effective moderator of nuclear processes. The heavy oxygen isotope ¹⁸O is also in high demand in medical applications for its use in positron emission tomography (PET), a powerful medical diagnostic technique for visualizing metabolic processes and physiological activity.

These heavy isotopes are present in natural waters at only very low concentrations, typically 1 part in 6,000 for ²H and 1 in 500 for ¹⁸O. They also tend to be slightly enriched during evaporation and condensation. However, the isotope separation factor is so marginal that it can take hundreds of evaporation and condensation cycles to significantly enrich a heavy water by conventional distillation processes.

With a view to improving the efficiency of this process, Nikolai Kulov and colleagues from the Kurnakov Institute of General and Inorganic Chemistry and Mendeleev University of Chemical Technology of Russia in Moscow developed and tested an automated distillation column modified to allow for salt irrigation, which had been reported to increase isotopic separation.

“The introduction of water-soluble salts leads to the formation of hydrate complexes, which would change the equilibrium composition of the vapor and liquid in the column,” says Kulov.

In the experimental distillation column, water is heated in a boiler, and the evaporated water vapour rises through a chamber packed with steel spring-like spiral prisms where it is sprayed with a salt solution. When the vapour condenses near the top of the column, it is collected and recirculated, resulting in a gradual enrichment in heavy isotopes.

With the right salts, in this case carbamide for deuterium and magnesium chloride for ¹⁸O, the researchers found that the isotope separation factor increased by a factor of ten for ²H and by 50% for ¹⁸O.

“Our method for improving the fractionation of water isotopes reduces energy consumption per unit output and allows the dimensions of the equipment to be minimized,” says Kulov. “We plan to investigate more effective salt agents and the possibility of simultaneous production of heavy hydrogen and oxygen products.”