Heavy water, or deuterium oxide (D₂O), is chemically identical to ordinary water (H₂O) but is slightly heavier due to its atomic structure. The difference lies in the hydrogen isotopes. Ordinary water contains protium, an isotope with only a proton. Heavy water contains deuterium, which possesses both a proton and a neutron, making it roughly twice as massive as protium. This small change in mass imparts different physical and nuclear properties to D₂O.
Natural Presence of Deuterium
Heavy water is a naturally occurring isotopic variant found in all natural bodies of water. Water molecules contain a mixture of protium and deuterium. The concentration of deuterium is extremely low, but it is present in a remarkably constant ratio throughout the world’s water supply.
Scientists use a reference standard called Vienna Standard Mean Ocean Water (VSMOW) to define this natural abundance. In this standard, the ratio is approximately one deuterium atom for every 6,420 protium atoms, translating to about 155.76 parts per million (ppm) of deuterium in natural water.
While the concentration is low, it is not perfectly uniform across all natural water sources. For example, water trapped in Antarctic ice sheets tends to have a slightly lower concentration of deuterium compared to ocean water. This is because the lighter H₂O molecules evaporate more easily and condense less readily than the heavier D₂O molecules, leading to minor isotopic fractionation across the water cycle. The fundamental source material for all heavy water production is the vast, naturally occurring, low concentration of deuterium found in rivers, lakes, and the sea.
Industrial Methods for Heavy Water Production
Obtaining usable quantities of heavy water requires highly specialized industrial separation processes. These methods must exploit the minute physical and chemical differences between water molecules containing protium and those containing deuterium. Because these isotopes are chemically nearly identical, achieving high-purity enrichment requires an enormous energy input.
The Girdler Sulfide (GS) process was the dominant method for large-scale production from the 1950s to the 1980s. This process relies on a chemical exchange reaction between water (H₂O) and hydrogen sulfide gas (H₂S). The reaction is reversible, and the equilibrium constant changes significantly with temperature.
In the GS process, water and hydrogen sulfide circulate through countercurrent towers operating at two different temperatures. In the cold towers (around 30°C), deuterium atoms preferentially migrate from the hydrogen sulfide gas to the water, enriching the liquid. Conversely, in the hot towers (about 130°C), the equilibrium shifts, causing deuterium to move preferentially from the water back into the hydrogen sulfide gas. This dual-temperature exchange system, set up in a cascade, gradually concentrates the deuterium in the water stream.
The GS process is effective but energy-intensive and requires careful handling of the toxic and corrosive hydrogen sulfide gas. Initial stages typically enrich the water to about 15% to 20% deuterium oxide. To achieve the high purity required for nuclear applications (often greater than 99% D₂O), the pre-enriched water must be further purified using secondary methods.
Secondary purification methods include vacuum distillation, which separates molecules based on the slight difference in boiling points between light and heavy water. Electrolysis is another method, where continuous electric current breaks down water molecules. Since the bond in light water (H-O-H) breaks more easily than the bond in heavy water (D-O-D), ordinary water is consumed faster, leaving the heavy water concentrated.
Essential Roles in Science and Industry
The unique properties of high-purity heavy water justify its use in advanced industrial and scientific applications. The most significant use is in the nuclear power industry, where heavy water acts as a neutron moderator and coolant in specific reactor designs, such as the Canadian Deuterium Uranium (CANDU) reactor. A moderator slows down high-energy neutrons released during nuclear fission, making them more likely to cause further fission events and sustain the chain reaction.
Deuterium is far less likely to absorb these moderated neutrons than protium, which is a strong neutron absorber. This low neutron absorption cross-section allows heavy water reactors to operate using natural, unenriched uranium fuel. This eliminates the complex and costly uranium enrichment process necessary for most other types of commercial nuclear reactors.
Beyond the nuclear sector, heavy water is an invaluable tool in scientific research, particularly in biochemistry and medicine. It is used as a tracer in metabolic studies, such as the doubly labeled water method, to measure energy expenditure in humans and animals. Researchers track how quickly the body turns over hydrogen and oxygen, providing precise measurements of caloric burn.
In chemistry, heavy water is a standard solvent for Nuclear Magnetic Resonance (NMR) spectroscopy, a technique used to determine the structure of organic molecules. The D₂O atoms do not interfere with the hydrogen signals being measured, providing a blank background for analysis. The pharmaceutical industry also uses deuterium to create “deuterated drugs,” which are formulated to have a slower metabolism, potentially extending the drug’s effectiveness and reducing the required dosage.