Xenon - Occurrence and Production

Occurrence and Production

Xenon is a trace gas in Earth's atmosphere, occurring at 87±1 parts per billion (nL/L), or approximately 1 part per 11.5 million, and is also found in gases emitted from some mineral springs.

Xenon is obtained commercially as a byproduct of the separation of air into oxygen and nitrogen. After this separation, generally performed by fractional distillation in a double-column plant, the liquid oxygen produced will contain small quantities of krypton and xenon. By additional fractional distillation steps, the liquid oxygen may be enriched to contain 0.1–0.2% of a krypton/xenon mixture, which is extracted either via adsorption onto silica gel or by distillation. Finally, the krypton/xenon mixture may be separated into krypton and xenon via distillation. Extraction of a liter of xenon from the atmosphere requires 220 watt-hours of energy. Worldwide production of xenon in 1998 was estimated at 5,000–7,000 m3. Because of its low abundance, xenon is much more expensive than the lighter noble gases—approximate prices for the purchase of small quantities in Europe in 1999 were 10 €/L for xenon, 1 €/L for krypton, and 0.20 €/L for neon; the much more plentiful argon costs less than a cent per liter.

Within the Solar System, the nucleon fraction of xenon is 1.56 × 10−8, for an abundance of one part in 64 million of the total mass. Xenon is relatively rare in the Sun's atmosphere, on Earth, and in asteroids and comets. The planet Jupiter has an unusually high abundance of xenon in its atmosphere; about 2.6 times as much as the Sun. This high abundance remains unexplained and may have been caused by an early and rapid buildup of planetesimals—small, subplanetary bodies—before the presolar disk began to heat up. (Otherwise, xenon would not have been trapped in the planetesimal ices.) The problem of the low terrestrial xenon may potentially be explained by covalent bonding of xenon to oxygen within quartz, hence reducing the outgassing of xenon into the atmosphere.

Unlike the lower mass noble gases, the normal stellar nucleosynthesis process inside a star does not form xenon. Elements more massive than iron-56 have a net energy cost to produce through fusion, so there is no energy gain for a star when creating xenon. Instead, xenon is formed during supernova explosions, by the slow neutron capture process (s-process) of red giant stars that have exhausted the hydrogen at their cores and entered the asymptotic giant branch, in classical nova explosions and from the radioactive decay of elements such as iodine, uranium and plutonium.