S280 Science Matters
"The Rise and Fall of Leaded Petrol"
SUMMARY
Metallic lead is made by roasting lead sulphide, PbS, in air. Lead oxide is first formed and yields molten lead when heated with carbon.
Lead oxide, PbO, is usesd to make leaded glass, and some pottery glazes. It is made by blowing air through molten lead. The oxide dissolves in dilute nitric or acetic acid to give the aqueous lead ion, Pb2+(aq).
Lead sulphate, lead carbonate and lead hydroxide have very low solubilities in water. Thin films of such compounds protect lead from corrosion in moist air and hard water.
White lead, [2PbCO3, Pb(Oh)2], was once an important pigments in white paint. Children with pica who are exposed to old peeling paint are at risk from poisoning by white lead and other lead pigments.
Other groups at risk from lead poisoning include workers exposed to lead in the smelting, glazing, pottery and battery industries, and imbibers of alcoholic or fruit drinks stored in, or prepared in contact with, metallic lead or lead glazes.
Symptoms of lead poisoning include colic, anaemia, pallor, wrist and foot drop, and encephalopathy, a mental and nervous disorder.
Lead concentrations in a solution such as blood can be determined by atomic absorption spectroscopy. A sample is atomized in the path of radiation emitted by excited lead atoms in a lamp. The amount of radiation absorbed by the sample atoms is a measure of the lead concentration in the sample solution.
Most ingested or inhaled lead is excreted. The absorbed part passes to the skeleton via the blood and soft tissues. Over 90% of body lead is in the bones, and bone lead levels are a measure of long-term exposure. Blood lead is a measure of relatively recent exposure (a matter of months).
There is evidence that symptoms of frank lead poisoning have sometimes been observed at blood lead concentrations in the 60-100µg dl-1 range. In the UK, male and female employees who work with lead must be suspended at blood lead levels of 70µg dl-1 and 40µg dl-1respectively. As for the general population, the DHSS recommended that steps should be taken to reduce the exposure of anyone whose blood lead level exceeds 25µg dl-1.
EC Directives specify upper limites of 50µg l-1 for lead concentrations in tap-water, and 2µg m-1 for the average annual lead concentration in air.
In the four-stroke petrol engine, a mixture of air and hydrocarbons is drawn into the engine cylinders, compressed and ignited by a spark. The gaseous products of the combusion reaction expand, pushing the piston back, before being expelled from the cylinder in an exhaust stroke.
Engine knock results from premature explosive chemical reactions which occur in front of the advancing flame front durin gthe ignition stroke. Unfortunately, it occurs more readily at higher compression ratios, where engine power and fuel economy are greater.
The research octane number (RON) is a measure of the resistance of a fuel to engine knock. It is the percentage of TMP by volume in a heptane-TMP mixture which has the same susceptibility to knocking as the fuel.
Straight-chain alkanes have very low RON values. The values from branched-chain alkanes, cycloalkanes and alkenes are much higher, and those for aromatic hydrocarbons are higher still.
When crude oil is fractionally distilled, the light and heavy gasoline fractions in the petrol boiling temperature range are usually only about 20% of the oil volume, and have low RON values of about 60-70. They are called straight-run petrols.
Catalytic reforming of straight-run petrol increases the RON value to 95-100, principally by destroying straight-chain alkanes and creating aromatics.
Catalytic cracking breaks up the hydrocarbon molecules in gas oil and other higher boiling temperature fractions, producing smaller high-octane hydrocarbon molecules with boiling temperatures in teh petrol range. The product can then be blended with catalytically reformed straight-run petrol, greatly increasing the volume of petrol obtainable from crude oil.
In the 1960s and 1970s, petrol made in this way was 85-90 RON. The final lift to 97 4-star petrol was provided by addition of tetraethyl and tetramethyl lead whose combustion products were emitted through the exhaust.
Possible alleviative measures include the fitting of lead filters, lower compression ratios, more intensive chemical refining of petrol and substitution of high-volume oxygenates for the low-volume lead alkyls.
In the 1970s, the average blood lead concentration in the population was about one-quarter of that at which symptoms of frank lead poisoning have sometimes been observed.
Isotope dilution mass spectrometry has been used to reveal increases in the lead content of Arctic snow since 800BC, and in the lead content of bones since prehistoric times. The results are widely held to show that as a result of lead technologies, such as plumbing, smelting and the use of leaded petrol, the modern world exposes people to very much greater concentrations of lead than did a primitive prehistoric existence.
There is evidence that lead levels well below those at which symptoms of frank lead poisoning occur, may have small, adverse effects on child attainment and behaviour. This evidence, however, is not unequivocal, mainly because of uncertainties caused by the possible influence of confounding variables, one of which is social class.
Changes in isotope ratios when leaded petrol with an unusually low 206Pb/207Pb ratio was introduced in Italy suggested that, in Turin, petrol was responsible for nearly all airborne lead, and for about 25% of adult blood lead.
Metallic platinum and rhodium are used as catalysts in three-way catalytic converters to destroy hydrocarbons, carbon monoxide and nitrogen oxides. They are poisoned by lead compounds in leaded petrol exhaust streams.
Eight releveant points
1. Lead is a well established poison: that is, there are many well documented cases of deaths caused by the inadvertent ingestion of quite small quantities of lead compounds.
2. Blood lead levels in the population as a whole are closer to officially recognized danger levels than for any other toxin.
3. Once there was official recognition of levels at which obvious symptoms of lead poisoning might appear (eg blood lead concentrations of 70µg dl-1), it was a plausible hypothesis that levels well below this might have subltel effects on bahaviour which, although hard to detect, are nevertheless real.
4. It was possible to present children as being particularly at risk, and children have a huge emotional claim on the nation's consideration.
5. Background lead in the environment is primarily anthropogenic.This offered greater opportunities for generating public guilt about environmental lead, and guilt is a spur to action. There has, for example, been no comparable agitation (until recently) about the protection of the population from emissions of background, natural radon gas which are believed to cause some 2 500 UK deaths per year.
6. In the UK, lead alkyls were made by Associated Octel, which was owned by a consortium of multinational oil companies. The major car manufacturers which built vehicles to run on leaded petrol are also multinationals. The environmentalist campaign could therefore be staged as a contest with big business, in which the oppositions' arguments were prejudiced by a desire for profit. This attracted support from important sections of political opinion in the UK that are hostile to large companies. Multinational companies are peculiarly vulnerable in this respect although, or perhaps because, attitudes towards them are ambivalent. Both Labour and Conversative governments have been anxious to draw multinational investment into the UK, and offered generous concessions as bait. Yet, once established, multinationals can be branded as unpatriotic or immoral because their interests transcend national boundaries, and they can mitigate the effects of a disagreeable national policy in one country by moving resources to another.
7. The removal of lead from petrol makes possible the reduction of other unpleasant pollutants, such as nitrogen oxides, carbon monoxide and hydrocarbons, through catalytic conversion.
8. There were obvious solutions to the problem, and the social costs, such as unemployment and higher prices, were not enormous.
Back to S280
|