Bromine

Once So Essential, Now Mired In Controversy ... But Why?

Fifty years ago bromine was produced on a massive scale and turned into lots of useful compounds. Photography relied on the light-sensitivity of silver bromide, doctors prescribed potassium bromide as a tranquiliser, leaded petrol needed dibromomethane to ensure the lead was removed via the exhaust gases, bromomethane was widely used to fumigate soil and storage facilities, and fire extinguishers contained volatile organobromine compounds. Today these uses have either disappeared or are in sharp decline.

World production of liquid bromine once exceeded 300,000 tonnes per year, of which a significant part was produced by a plant on the coast of Anglesey, Wales, which closed in 2004. This extracted the element from sea water, which contains 65 p.p.m. of bromide, and was done by using chlorine gas to convert the bromide to bromine which was then removed by blowing air through the water.

The bromine story began with 24-year-old student Antoine-Jérôme Balard (1802–1876) who found that the salt residues left by evaporating brine from Montpellier, France, gave an oily red liquid when treated with acid. He realised this was a new element and reported it to the French Academy, who confirmed his discovery. When they realised it was chemically similar to chlorine and iodine they proposed the name bromine, based on the Greek word bromos meaning stench.

While some uses of bromine have declined because the products made from it are no longer needed, others have been discouraged because of the damage this element could cause to the ozone layer. Volatile organobromine compounds are capable of surviving in the atmosphere long enough to reach the upper ozone layer where their bromine atoms are 50 times more damaging than the chlorine atoms which nevertheless are the main threat, coming as they did from the widely used chlorofluorocarbons, the CFCs. The Montreal Protocol which outlawed the CFCs sought also to ban the use of all volatile organobromines by 2010, and this restriction especially applied to the fumigant bromomethane and compounds such as CBrClF₂ which were in fire extinguishers for electrical fires or those in confined spaces.

Bromomethane was a particular cause for concern but banning has been problematical because it had some uses for which alternatives have yet to be found. Often referred to as methyl bromide, CH₃Br (boiling point 3.5°C), it was widely used to kill pests in the soil, in storage facilities, and to treat wood before it is exported. In the soil it killed nematodes, insects, bacteria, mites and fungi which threaten crops such as seed crops, lettuce, strawberries, grapes, and flowers such as carnations and chrysanthemums. (Iodomethane, CH₃I, is a possible replacement as a soil fumigant.)

In fact bromomethane is not quite so threatening as it first appears. Environmental research uncovered the unexpected result that half the bromomethane sprayed on soil never evaporates into the air because it is consumed by bacteria. Nor are man-made organobromines the main source of these compounds in the atmosphere. Marine plankton and algae release around half a million tonnes of various bromomethanes a year and in particularly tribromomethane (aka bromoform, CHBr₃).

Even more surprising has been the discovery that something in the oceans is making pentabromodiphenyl ether. This has been used as a fire-retardant, and when in 2005 it was found to be present in whale blubber it was at first thought to be the man-made variety. However, the carbon atoms it contained had detectable amounts of ¹⁴C meaning that they were of recent origin, whereas the fire retardant is made entirely from fossil resources and contains no ¹⁴C.

Another complex bromine compound from the sea is the purple dye once used for clothes worn by the Roman Emperors. Tyrian purple as it was called was extracted from the Mediterranean mollusc Murex brandaris and this molecule contains two bromine atoms and is 6,6'-dibromoindigo.

Even when it appears benign as bromide ions in water, this element can still pose a threat to health. Ozonising drinking water in order to sterilise it converts any bromide to bromate (BrO^3-) which is a suspected carcinogen and so must not exceed 10 p.p.b. Some reservoirs in California where this has been exceeded have had to be drained because of it.

Once so beneficial, bromine now appears to cause nothing but trouble and yet in ways unseen, such as in the pharmaceutical industries, it still continues to be used to provide intermediates in the manufacture of live-saving drugs.