Ancients called mercury the “first matter” from which all other metals were formed. For centuries, mercury, a heavy metal, was also used in medicine. Yet mercury is now in such disfavour that an international treaty exists to curb its use…
Mercury fascinates. It shines like silver. And it is the only metal to be liquid at room temperature. Formerly known as hydrargyrum – from the Greek for ‘water’ and ‘silver’, its chemical symbol is Hg.
Mercury has atomic number 80. It is used for making industrial chemicals or for electrical and electronic applications, and its biggest producers are China and Kyrgyzstan.
Mercury’s most common bright red ore, cinnabar (mercury sulphide), has been used as a pigment since Neolithic times. Some 10,000 years ago, the earliest artists used it to daub pictures of aurochs, the now extinct giant wild cattle they hunted, on the walls of caves in Turkey.
The Romans used it as a form of rouge make-up, and the Chinese to colour their exquisite lacquerware. In the Middle Ages, the pigment was mixed with wax to provide the seals placed on formal documents.
Above all, it is mercury’s unique relationship with gold that always fascinated the alchemists. Mercury is one of the few metals that react with that most sought-after and alluring of all the metals: gold.
Once the Philosophical Stone of Alchemy…
The chemical reaction is extraordinary to watch. In his laboratory at University College London, chemistry professor Andrea Sella peels off a fragile leaf of gold and drops a shimmering ball of mercury on it. Before your eyes, you can see the gold is gradually vanishing, folding itself around the silver-coloured blob like a sheet, before dissolving it away. Amazingly, the lump of gold/mercury amalgam is still silver in colour. It looks as if the lump of mercury amalgam is “eating” gold leaf. Neat party trick, isn’t it?
Almost all metals can form amalgams with mercury, at the exception of iron, platinum, tungsten and tantalum.
Now, if you boil off the mercury, it will evaporate and you will be left with a residue of pure gold. Another really neat and interesting party trick! In ancient times, you can imagine how people would think this is magical!
Mercury is the bad boy of the periodic table – exquisitely beautiful, but also deadly.
Impact on Health
Most middle-age or older adults will have encountered mercury thermometers at some point. Silver-mercury amalgams are important in dentistry. Many of us have amalgam fillings in our teeth. Even fairly recently, mercury was still used in antiseptics, laxatives, anti-depressants, and drugs to combat syphilis.
But there is no way of getting around it…
Mercury is a neuro-toxin. It affects the cerebellum, which is the part of the brain that helps you move properly, and co-ordinate your movements. Mercury also harms the kidneys and other organs. The neurological damage it does is irreversible.
The neurological damage that mercury causes is irreversible.
“Mercury is a profound, systemic and long-term poison for humans, but also for other organisms,” says Andrea Sella. “So getting mercury into the environment is a very serious issue.”
About half the mercury that enters the environment every year comes from volcanic eruptions and other geological processes. Nothing can be done about it. The other half the mercury that enters the environment every year is released by mankind.
The mercury in medicines and tooth fillings will eventually find its way into the atmosphere. And tiny amounts of mercury vapour are the light source in fluorescent bulbs – that’s why they need to be disposed of very carefully. It is involved in the production of industrial chemicals, and has electrical and electronic applications.
Tooth fillings and smashed light bulbs, however, only account for a fraction of the 2,000 tonnes of mercury released by humans into the environment each year.
About a quarter is a by-product of power generation. There are traces of mercury in coal, so coal-fired power stations pump mercury vapour into the atmosphere. Even more, over a third is a consequence of our lust for gold.
The Elementary Business of Gold Extraction
Gold-mercury amalgam is used in the extraction of gold from ore. The process has proved effective where gold fines (“flour gold”) would not be extractable from ore using hydro-mechanical methods.
Large amounts of mercury were used in placer mining, where deposits composed largely of decomposed granite slurry were separated in long runs of “riffle boxes”, with mercury dumped in at the head of the run. The amalgam formed is a heavy solid mass of dull grey colour.
Where stamp mills were used to crush gold-bearing ore to fines, part of the extraction process involved the use of mercury-wetted copper plates, over which the crushed fines were washed. Periodic scraping and re-mercurizing of the plate resulted in amalgam for further processing.
Amalgam obtained by either process was then heated in a distillation retort, recovering the mercury for reuse and leaving behind the gold. As this released mercury vapours into the atmosphere, the process could induce adverse health effects and long-term environmental pollution.
The use of mercury in 19th century placer mining in California, now prohibited, has caused extensive pollution problems in rivers and estuaries, which have had environmental repercussion to this day. Sometimes substantial slugs of amalgam are found in downstream river and creek bottoms by amateur wet-suited miners seeking gold nuggets with the aid of an engine-powered water vacuum mounted on a float.
Today, mercury amalgamation has been replaced by other methods to reclaim gold and silver from ore in developed nations. Hazards of mercurial toxic waste have played a major role in the phasing out of the mercury amalgamation processes. However, mercury amalgamation is still regularly used by small-scale gold placer miners (often illegally), particularly in developing countries.
The chemical forms an alloy with gold that can be quickly and easily filtered out in lumps from mined and ground rock. These lumps are heated to evaporate the mercury and leave purified gold.
Toxic mercury vapours escape into the environment and workers’ lungs as a result – the convention’s goal is to cut these emissions.
An estimated 10-15 million small-scale miners dig, dredge, sluice and pan for gold worldwide. Many of the world’s poorest people make their living from gold-mining and most of them use mercury to separate the pure gold metal from the silt.
Harvesting a kilogram of gold requires 1.3 kg of mercury.
According to official estimates CNRS, this means there is at least 12 tonnes of mercury released per year.
Methyl Mercury and the Environment
Compared to mercury metal and amalgam, mercury salts are highly toxic due to their solubility in water.
In water, mercury transforms into a highly toxic organic molecule – methyl mercury (CH3Hg+) – which is readily absorbed into the bodies of algae and plankton. In freshwater, it tends to become attached to dissolved organic matter. As such, it is readily broken down by sunlight (into inorganic mercury). In sea-water it tends to become attached onto chloride (Cl– – remember that salt is sodium chloride, NaCl). When it is tightly bonded to chloride in this way, methyl mercury is not so easily degraded by sunlight. Thus, the lifetime of methyl mercury is much longer in the marine environment.
Algae and planktons are eaten by larger animals, which are in turn eaten by larger creatures, which in turn are often eaten by us. As it goes up the food chain, the toxic chemical becomes increasingly concentrated in the process, making it a particular threat to the developing brains of young and unborn children.
“What we are concerned about is fish at the top of the food chain, fish like swordfish, the predator fish”, says Dr Kate Spencer, an environmental geo-chemist. “By the time you get to the top of the food chain, we are talking thousands of times more mercury in the flesh of our fish.”
Investigating Mercury Pollution
The Redox Reaction
The redox reaction involved where mercury oxidises the copper is:
- Hg2+ + Cu → Hg + Cu2+.
A nitric acid solution of salts under investigation is applied to a piece of copper foil, and any mercury ions present will leave spots of silvery-coloured amalgam. Silver ions leave similar spots but are easily washed away, making this a means of distinguishing silver from mercury.
The worst case of mass mercury poisoning the world has ever seen happened in Japan in the first half of the 20th Century. Symptoms appeared only gradually in the fishing village of Minamata.
At first, nobody could explain why people began to slur their speech, or stumbled when they walked. They would have trouble swallowing, or tremble uncontrollably. Children were born with disabilities. Thousands would die with what became known as Minamata Disease.
It took 30 years – until the 1960s – to identify the cause of the suffering: a local plastics factory that was dumping mercury into the bay. The mercury was contaminating fish, the staple food of the local population.
And that is the problem with mercury… In the short-term, its effects are not dramatic enough to act as a viable deterrent.
Breaking the Link between Mercury and Gold
The good news here – the world has come together to beat its mercury habit. There is hope we can deal with some of the even bigger environmental challenges we face…