Lead in the environment

Lead in the environment

Interest in lead pollution has tended to focus on the environmental consequences of the use of tetraethyl lead in petrol and of lead compounds in paint. However, this element and its compounds have been trickling into the environment for thousands of years. Simon Cotton

Lead has been known and used since antiquity. Because of its ease of extraction, its malleability and low melting point, the ancient Romans used the metal and its compounds extensively – in pots and pans, in plates and cups, in paints and cosmetics and even in the building of aqueducts. The Romans also mined lead ores, mainly galena (PbS) and lead carbonate, for their silver content, the basis of coinage. After reducing the ore to a mixture of lead and silver, they passed air over the hot mixture, which oxidised the lead to PbO. While most of the PbO was absorbed into the bone-ash pot in which the reaction was done, around one per cent was lost to the atmosphere, and spread by air currents.

In short

●● Environmental lead levels have fluctuated throughout history ●● Ice cores and peat bogs reveal historical trends in lead contamination where the lead came from

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(IDTIMS), Laser Excited Atomic Fluorescence Spectrometry (LEAFS) and Inductively Coupled Plasma Sector Field Mass Spectrometry

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Lead analysis

Lead poisoning

There are other ways in which lead and its compounds have, over thousands of years, entered the environment (and humans). For example, lead acetate, which like many Pb(ii) salts has a sweet taste, was formed from the reaction of leaden vessels with wine. Concentrated, the Romans used this sapa as a sweetener and the lead poisoning that ensued is thought, albeit controversially, to be in part responsible for the ‘decline and fall of the Roman Empire’. Traditionally known as ‘sugar of lead’, sapa was replaced by lead oxide in the 14th and 15th centuries to sweeten wine, which caused colic, symptomatic of lead poisoning. More recently, in the 1980s lead poisoning was linked with moonshine whiskey made using lead vessels, pipes and stills. There have been many other incidents of

lead poisoning.1 In the mid-1990s lead oxide was illicitly added to Hungarian paprika as a colourant and in China in 2009 lead smelters were closed because more than 2000 children in the local vicinity had been found to have high levels of lead in their blood. One eight-year old boy’s blood contained 239 µg of lead per litre, which is ca five times the safe level.

Lead with your wine or paprika?

The measurement of lead in the environment, deposited over thousands of years on lake sediments, peat, soils and even Greenland snow was pioneered by the US scientist Clair Cameron Patterson (1922–1995). Before man started to work lead ores, the source of lead in the environment was simply wind-blown dust from soil. Very small particles (below 5 mm in diameter) could be transported thousands of miles from Saharan Africa to Greenland ice. Patterson developed ultraclean methods for sampling and handling samples of ice and snow. He used tools made of Teflon, polycarbonate and acidcleaned stainless steel, and in the laboratory scientists, clad in special ‘clean-room overalls’ handled the samples in ultra-clean cold rooms.2 Ice drilled from deep down also had to be decontaminated, done simply by disregarding the outer part which had been in contact with the atmosphere. To measure the very low lead concentrations, the scientists used Isotope Dilution Thermal Ionisation Mass Spectrometry September 2010 | Education in Chemistry | 153

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Fig 1

World lead production since 3000 BC. Note the logarithmic scale on the y-axis.

Production worldwide/tonnes per year 106

105

104 103 102

Fig 2

Analysis of samples from Lindow bog.12

90 60 30 10

Rise and fall of Athens

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Exhaustion of Roman lead mines

Industrial Silver revolution production in Germany Spanish production of silver in New World

0 Calendar years

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206Pb/207Pb

1.19

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Lead isotope studies

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reflecting the extensive mining of lead ores in the Greek and Roman Empires. Scientists estimated that the total lead fallout on the Greenland ice sheet from Greek and Roman mining during this 800 year period was about 15 per cent of that from the use of lead alkyls in gasoline during the 20th century. Ice gives a historical snapshot of lead pollution

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1.18 1.17 1.16

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0 −2000

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Roman Republic and Empire

Excess Pb/mg m-2 per year

Introduction of coinage

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Discovery of cupellation

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SIMON FRASER / SCIENCE PHOTO LIBRARY

(ICP–SFMS). Isotope ratios were measured by mass spectroscopy, while the ages of samples were established by 14C and 210Pb dating. As early as the 1960s Patterson realised that much more lead was entering the environment than in pre-industrial times. Many scientists, he discovered, had grossly underestimated the problem, because they had used contaminated ‘blank’ background samples. In 1969 he published the results of an investigation of the lead in Greenland ice,3 which revealed that lead concentrations increased by a factor of at least 200 between 800 BC and AD 1969, with the sharpest rise after 1940. He concluded that the increase before 1940 was from lead smelting, and since 1940 attributable to the use of lead alkyls in petrol. In a later study of albacore tuna published in 1980,4 Patterson and his co-worker Dorothy Settle showed that commercial packed tuna contained around 10 000 times as much lead as sea-water dwelling fish. They attributed much of the increase to lead from the soldered cans entering the food chain. Consequently, just as a programme began in 1973 to remove lead from US petrol, subsequent campaigns were launched to take lead out of paint and canning. Further investigation of Greenland ice5 covered the period up to 7760 years before the present time, with samples taken at depths up to 1286.5 metres below the surface. The results showed that even in the period covering the peak of ancient Greek and Roman civilisations between 1700 and 2500 years before the present, lead levels in the ice were much greater than natural values,

Study of the lead isotopes present (see Box) in samples in the late 1990s pointed to the likely source of the lead.6 The 206Pb/207Pb ratio in Greenland ice samples showed a decrease in the ratio from above 1.20 until 962 BC to values of the 206Pb/207Pb ratio as low as 1.1828 in 43 BC. These values could be attributed to lead mining from the Rio Tinto mines in south western Spain (with some coming from the Cartagena mine in south east Spain), which were extensively worked for their silver-rich lead ores. Fig 1 shows that the high level of lead mining during the Roman Empire fell during the Dark Ages, to be revived in the Middle Ages and with the coming of the Industrial

1000

E 1500

G

1.15

1.14 2000

Revolution, production intensified. Similar patterns have been found in other sources, such as peat bogs and lake sediments, in other parts of the world, including one from Lindow bog, south of Manchester. Fig 2 shows lead contamination predating the Roman occupation of Britain (indicating the existence of a mining industry). Extraction of lead ceased with the onset of the Dark Ages, but production resumed in the mediaeval period and rose sharply during the Industrial Revolution. The 208Pb/206Pb and 206Pb/207Pb isotopic ratios in the peat during the period 200 BC–200 AD indicates that the contamination was caused by English lead ores, not Spanish ones. The top part of the peat was missing, so that no study of lead levels post-1850 was possible. Samples from a Swiss peat bog7 from the present day to over 10 000 BC led to similar findings. Additionally, data for the 20th century showed a significant rise in lead levels following the introduction of leaded petrol, and a corresponding fall since the coming of unleaded petrol in the late 1970s. At the same

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time, the use of leaded fuel caused a drop in the 206Pb/207Pb ratio from ca 1.16 to ca 1.12, from 1936–1979, corresponding to the lead content in the petrol then in use in Switzerland. Moreover, over the years 1976 to 1991, mean blood lead levels of persons aged 1–74 dropped by 78 per cent, from 0.62 to 0.14 µmol l–1(12.8– 2.8 µg decilitre). Similar patterns were seen in childrens’ lead levels.8 Concern continues9, 10, however, about the effect upon children’s IQ of even concentrations of 10 μg per decilitre or less. (In the 1990s Herbert Needleman found high bone lead levels were associated with lower IQ, poor attention, aggression, and delinquency.)11 There is also concern, especially in the US, over the number of old buildings that still contain lead paint. Flakes of paint falling off the buildings are a potential danger for children, and there is also the possibility of toxic lead-containing dust being generated when such old buildings are renovated. n Acknowledgement: I am very grateful to

Professor Claude Boutron for supplying several reprints.

Simon Cotton is a chemistry teacher at Uppingham School ,Rutland LE15 9QE

References 1. J. V. Allcott, R. A. Barnhart and L. A. Mooney, J. Am. Med. Assoc., 1987, 258, 510. 2. C. F. Boutron et al, C. R. Geosci.., 2004, 336, 847. 3. M. Murozumi, T. J. Chow, and C. C. Patterson, Geochim. Cosmochim. Acta., 1969, 33, 1247. 4. D. M. Settle and C. C. Patterson, Science, 1980, 207, 1167. 5. S. Hong et al, Science, 1994, 265, 1841. 6. K. J. R. Rosman et al, Environ. Sci. Technol., 1997, 31, 3413. 7. W. Shotyk et al, Science, 1998, 281, 1635. 8. J. L. Pirkle et al, J. Am. Med. Assoc., 1994, 272, 284. 9. R. L. Canfield et al, New Engl. J. Med., 2003, 348, 1517. 10. M. L. Miranda, et al, NeuroToxicology , 2009, 30, 1019. 11. H. L. Needleman et al, J. Am. Med. Assoc., 1996, 275, 363. 12. G. Le Roux et al, J. Environ. Monit., 2004, 6,502

Further reading J. O. Nriagu, Lead & lead poisoning in Antiquity. New York: John Wiley, 1983. R. P. Weeden, Poison in the pot. Illinois: South Illinois University, 1984. S. C. Gilfillan, Rome’s ruin by lead poison. Long beach, Ca: Wenzel, 1990. G. Markowitz and D. Rosner, Deceit and denial. Ca: University of California, 2002. J. Emsley, The elements of murder, pp261–317. Oxford: OUP, 2005, . W. Troesken, The great lead water pipe disaster. Boston MA: MIT, 2006. L. Denworth, Toxic truth: a scientist, a doctor, and the battle over lead. Boston: Beacon, 2009.

Box Lead isotopes Lead has four stable isotopes. Of these, 204Pb does not arise from a radioactive precursor, but 206Pb is the final element in the decay chain of 238U, 207Pb is the final element in the decay chain of 232Th and 208Pb is the final element in the decay chain of 235U. The isotopic composition of lead in nature is variable because different lead ores have received different contributions of different isotopes from uranium and thorium decay. The 206Pb/204Pb and 206Pb/207Pb ratios commonly found in lead ores throughout the world generally range between 16.0–18.5 and 1.19–1.25, respectively. However, the lead ore from Broken Hill, Australia, has a very low 206Pb/207Pb ratio (1.03–1.10), and this ore has been widely used to make tetraethyllead (and tetramethyl lead) used in European petrol. On the other hand, lead from the Mississippi Valley ore deposit, US, has a greater radiogenic lead isotopic composition (206Pb/204Pb ~ 20.0; 206Pb/207Pb = 1.31–1.35), so that US leaded gasoline typically had significantly higher 206Pb/207Pb ratios than in European petrol.

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didn’t, then excuses were made for their non-action. In addition, another treatment was gaining momentum. Robert Koch cultured the tuberculosis bacilli in a broth with glycerol as an additive, evaporated the mixture at 100 oC and filtered it to give a product he named ‘Tuberculin’. This was hailed as a revolutionary treatment and, despite no firm evidence that it was effective, it enjoyed a 20-year cruel popularity from 1890. You can still buy hypophosphite remedies over the counter. The Anglo–French Drugs Industry (an Indian company) markets Maximin Forte Syrup. Each teaspoonful contains 82 mg of calcium hypophosphite. But there is no mention of its use against tuberculosis but rather: As a supplement to prevent deficiency states, to strengthen immunity and prevent recurrent infections in both paediatric and geriatric populations.

A glowing element

Phosphorus is unique among elements in first being discovered in human urine. The glowing substance must have been an extraordinary sight for Brandt and the early finding of high levels of phosphorus in brain tissue would have reinforced its mystery. No wonder that it was later promoted as a ‘brain food’. Indeed, Coca Cola was initially marketed as a brain tonic, owing to its phosphoric acid content. However, the complexity of phosphorus’ role in the brain remained little explored until the work of another German, Johann Thudichum, in the late 19th century. And his first major treatise was on the pathology of urine. Further reading

We recommend John Emsley’s text as an excellent overview of the chemistry of phosphorus in a general, medical and historical context.1 All the pre-1900 references listed may be downloaded, free of charge, from the Internet. Wikipedia has a useful article on Brandt and the extraction of phosphorus from urine.12 n

References 1. J. Emsley, The shocking history of phosphorus. London: Macmillan, 2000. 2. R. Hooper, A new medical dictionary. Philadelphia: M. Carey & Son, 1817. 3. A. T. Dronsfield, P. M. Ellis and A. Battistini, Ed. Chem., 2004, 41(6), 162. 4. F. Magendie, A formulary for the preparation and medical administration of certain new remedies, 2nd Edn. London: John Churchill, 1836. 5. S. L. Mitchill and E. Miller, The medical repository and reviews of American publications in medicine and surgery. New York: T & J Swords, 1802. 6. J. W. Howe, Excessive venery, masturbation and continence. New York: E. B. Trent, 1887. 7. E. Shorter, Women’s bodies: a social history of women’s encounter with health, ill-health, and medicine. New Brunswick: Transaction Publishers, 1991. 8. A. T. Dronsfield, T. M. Brown and P. M. Ellis, Ed. Chem., 2004, 41(6), 151. 9. J. F. Churchill, Dublin Hospital Gazette, 1857, 4, 252. 10. J. A McArthur, Consumption and tuberculosis: notes on their treatment by hypophosphites. Boston: Alfred Mudge & Son, 1880. 11. Anon, Chemical News, 1875, 31, 24. 12. text: http://en.wikipedia.org/wiki/Hennig_Brandt

Alan Dronsfield is emeritus professor of the history of science in the school of education, health and sciences, at the University of Derby, Derby DE22 1GB. Pete Ellis is professor of psychological medicine at the School of Medicine and Health Sciences, University of Otago, Wellington, PO Box 7343, Wellington South, New Zealand. September 2010 | Education in Chemistry | 155


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