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= Lead =
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Introduction
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Lead () is a chemical element; it has symbol Pb (from Latin ) and
atomic number 82. It is a heavy metal that is denser than most common
materials. Lead is soft and malleable, and also has a relatively low
melting point. When freshly cut, lead is a shiny gray with a hint of
blue. It tarnishes to a dull gray color when exposed to air. Lead has
the highest atomic number of any stable element and three of its
isotopes are endpoints of major nuclear decay chains of heavier
elements.
Lead is a relatively unreactive post-transition metal. Its weak
metallic character is illustrated by its amphoteric nature; lead and
lead oxides react with acids and bases, and it tends to form covalent
bonds. Compounds of lead are usually found in the +2 oxidation state
rather than the +4 state common with lighter members of the carbon
group. Exceptions are mostly limited to organolead compounds. Like the
lighter members of the group, lead tends to bond with itself; it can
form chains and polyhedral structures.
Since lead is easily extracted from its ores, prehistoric people in
the Near East were aware of it. Galena is a principal ore of lead
which often bears silver. Interest in silver helped initiate
widespread extraction and use of lead in ancient Rome. Lead production
declined after the fall of Rome and did not reach comparable levels
until the Industrial Revolution. Lead played a crucial role in the
development of the printing press, as movable type could be relatively
easily cast from lead alloys. In 2014, the annual global production of
lead was about ten million tonnes, over half of which was from
recycling. Lead's high density, low melting point, ductility and
relative inertness to oxidation make it useful. These properties,
combined with its relative abundance and low cost, resulted in its
extensive use in construction, plumbing, batteries, bullets, shots,
weights, solders, pewters, fusible alloys, lead paints, leaded
gasoline, and radiation shielding.
Lead is a neurotoxin that accumulates in soft tissues and bones. It
damages the nervous system and interferes with the function of
biological enzymes, causing neurological disorders ranging from
behavioral problems to brain damage, and also affects general health,
cardiovascular, and renal systems. Lead's toxicity was first
documented by ancient Greek and Roman writers, who noted some of the
symptoms of lead poisoning, but became widely recognized in Europe in
the late 19th century.
Atomic
========
A lead atom has 82 electrons, arranged in an electron configuration of
[Xe]4f145d106s26p2. The sum of lead's first and second ionization
energies--the total energy required to remove the two 6p electrons--is
close to that of tin, lead's upper neighbor in the carbon group. This
is unusual; ionization energies generally fall going down a group, as
an element's outer electrons become more distant from the nucleus, and
more shielded by smaller orbitals. The sum of the first four
ionization energies of lead exceeds that of tin, contrary to what
periodic trends would predict. This is explained by relativistic
effects, which become significant in heavier atoms, which contract s
and p orbitals such that lead's 6s electrons have larger binding
energies than its 5s electrons. A consequence is the so-called inert
pair effect: the 6s electrons of lead become reluctant to participate
in bonding, stabilising the +2 oxidation state and making the distance
between nearest atoms in crystalline lead unusually long.
Lead's lighter carbon group congeners form stable or metastable
allotropes with the tetrahedrally coordinated and covalently bonded
diamond cubic structure. The energy levels of their outer s- and
p-orbitals are close enough to allow mixing into four hybrid sp3
orbitals. In lead, the inert pair effect increases the separation
between its s- and p-orbitals, and the gap cannot be overcome by the
energy that would be released by extra bonds following hybridization.
Rather than having a diamond cubic structure, lead forms metallic
bonds in which only the p-electrons are delocalized and shared between
the Pb2+ ions. Lead consequently has a face-centered cubic structure
like the similarly sized divalent metals calcium and strontium.
Bulk
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Pure lead has a bright, shiny gray appearance with a hint of blue. It
tarnishes on contact with moist air and takes on a dull appearance,
the hue of which depends on the prevailing conditions. Characteristic
properties of lead include high density, malleability, ductility, and
high resistance to corrosion due to passivation.
Lead fishing weights
Lead's close-packed face-centered cubic structure and high atomic
weight result in a density of 11.34 g/cm3, which is greater than that
of common metals such as iron (7.87 g/cm3), copper (8.93 g/cm3), and
zinc (7.14 g/cm3). This density is the origin of the idiom 'to go over
like a lead balloon'. Some rarer metals are denser: tungsten and gold
are both at 19.3 g/cm3, and osmium--the densest metal known--has a
density of 22.59 g/cm3, almost twice that of lead.
Lead is a very soft metal with a Mohs hardness of 1.5; it can be
scratched with a fingernail. It is quite malleable and somewhat
ductile. The bulk modulus of lead--a measure of its ease of
compressibility--is 45.8 GPa. In comparison, that of aluminium is 75.2
GPa; copper 137.8 GPa; and mild steel 160-169 GPa. Lead's tensile
strength, at 12-17 MPa, is low (that of aluminium is 6 times higher,
copper 10 times, and mild steel 15 times higher); it can be
strengthened by adding small amounts of copper or antimony.
The melting point of lead--at 327.5 °C (621.5 °F)--is very low
compared to most metals. Its boiling point of 1749 °C (3180 °F) is the
lowest among the carbon-group elements. The electrical resistivity of
lead at 20 °C is 192 nanoohm-meters, almost an order of magnitude
higher than those of other industrial metals (copper at ; gold ; and
aluminium at ). Lead is a superconductor at temperatures lower than
7.19 K; this is the highest critical temperature of all type-I
superconductors and the third highest of the elemental
superconductors.
Isotopes
==========
Natural lead consists of four stable isotopes with mass numbers of
204, 206, 207, and 208, and traces of six short-lived radioisotopes
with mass numbers 209-214 inclusive. The high number of isotopes is
consistent with lead's atomic number being even. Lead has a magic
number of protons (82), for which the nuclear shell model accurately
predicts an especially stable nucleus. Lead-208 has 126 neutrons,
another magic number, which may explain why lead-208 is
extraordinarily stable.
With its high atomic number, lead is the heaviest element whose
natural isotopes are regarded as stable; lead-208 is the heaviest
stable nucleus. (This distinction formerly fell to bismuth, with an
atomic number of 83, until its only primordial isotope, bismuth-209,
was found in 2003 to decay very slowly.) The four stable isotopes of
lead could theoretically undergo alpha decay to isotopes of mercury
with a release of energy, but this has not been observed for any of
them; their predicted half-lives range from 1035 to 10189 years (at
least 1025 times the current age of the universe).
Three of the stable isotopes are found as the end-points of the three
major decay chains: lead-206, lead-207, and lead-208 from the
primordial uranium-238, uranium-235, and thorium-232, respectively.
These decay chains are called the uranium chain, the actinium chain,
and the thorium chain. Their isotopic concentrations in a natural rock
sample depends greatly on the presence of these three parent uranium
and thorium isotopes. For example, the relative abundance of lead-208
can range from 52% in normal samples to 90% in thorium ores; for this
reason, the standard atomic weight of lead is given to only one
decimal place. As time passes, the ratio of those isotopes to lead-204
increases, as they are continually added to by radioactive decay of;
this change in their relative amounts allows for lead-lead dating and
uranium-lead dating. Lead-207 exhibits nuclear magnetic resonance, a
property that has been used to study its compounds in solution and
solid state, including in the human body.
Chemistry
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Bulk lead exposed to moist air forms a protective layer of varying
composition. Lead(II) carbonate is a common constituent; the sulfate
or chloride may also be present in urban or maritime settings. This
layer makes bulk lead effectively chemically inert in the air. Finely
powdered lead, as with many metals, is pyrophoric, and burns with a
bluish-white flame.
Fluorine reacts with lead at room temperature, forming lead(II)
fluoride. The reaction with chlorine is similar but requires heating,
as the resulting chloride layer diminishes the reactivity of the
elements. Molten lead reacts with the chalcogens to give lead(II)
chalcogenides.
Lead metal resists sulfuric and phosphoric acid but not hydrochloric
or nitric acid; the outcome depends on insolubility and subsequent
passivation of the product salt. Organic acids, such as acetic acid,
dissolve lead in the presence of oxygen. Concentrated alkalis dissolve
lead and form plumbites.
Inorganic compounds
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Lead shows two main oxidation states: +4 and +2. The tetravalent state
is common for the carbon group. The divalent state is rare for carbon
and silicon, minor for germanium, important (but not prevailing) for
tin, and is the more important of the two oxidation states for lead.
This is attributable to relativistic effects, specifically the inert
pair effect, which manifests itself when there is a large difference
in electronegativity between lead and oxide, halide, or nitride
anions, leading to a significant partial positive charge on lead. The
result is a stronger contraction of the lead 6s orbital than is the
case for the 6p orbital, making it rather inert in ionic compounds.
The inert pair effect is less applicable to compounds in which lead
forms covalent bonds with elements of similar electronegativity, such
as carbon in organolead compounds. In these, the 6s and 6p orbitals
remain similarly sized and sp3 hybridization is still energetically
favorable. Lead, like carbon, is predominantly tetravalent in such
compounds.
There is a relatively large difference in the electronegativity of
lead(II) at 1.87 and lead(IV) at 2.33. This difference marks the
reversal in the trend of increasing stability of the +4 oxidation
state going down the carbon group; tin, by comparison, has values of
1.80 in the +2 oxidation state and 1.96 in the +4 state.
Lead(II)
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Lead monoxide exists in two polymorphs, litharge α-PbO (red) and
massicot β-PbO (yellow), the latter being stable only above around 488
°C. Litharge is the most commonly used inorganic compound of lead.
There is no lead(II) hydroxide; increasing the pH of solutions of
lead(II) salts leads to hydrolysis and condensation. Lead commonly
reacts with heavier chalcogens. Lead sulfide is a semiconductor, a
photoconductor, and an extremely sensitive infrared radiation
detector. The other two chalcogenides, lead selenide and lead
telluride, are likewise photoconducting. They are unusual in that
their color becomes lighter going down the group.
Lead dihalides are well-characterized; this includes the diastatide
and mixed halides, such as PbFCl. The relative insolubility of the
latter forms a useful basis for the gravimetric determination of
fluorine. The difluoride was the first solid ionically conducting
compound to be discovered (in 1834, by Michael Faraday). The other
dihalides decompose on exposure to ultraviolet or visible light,
especially the diiodide. Many lead(II) pseudohalides are known, such
as the cyanide, cyanate, and thiocyanate. Lead(II) forms an extensive
variety of halide coordination complexes, such as [PbCl4]2−,
[PbCl6]4−, and the [Pb2Cl9]'n'5'n'− chain anion.
Lead(II) sulfate is insoluble in water, like the sulfates of other
heavy divalent cations. Lead(II) nitrate and lead(II) acetate are very
soluble, and this is exploited in the synthesis of other lead
compounds.
Lead(IV)
==========
Few inorganic lead(IV) compounds are known. They are only formed in
highly oxidizing solutions and do not normally exist under standard
conditions. Lead(II) oxide gives a mixed oxide on further oxidation,
Pb3O4. It is described as lead(II,IV) oxide, or structurally
2PbO·PbO2, and is the best-known mixed valence lead compound. Lead
dioxide is a strong oxidizing agent, capable of oxidizing hydrochloric
acid to chlorine gas. This is because the expected PbCl4 that would be
produced is unstable and spontaneously decomposes to PbCl2 and Cl2.
Analogously to lead monoxide, lead dioxide is capable of forming
plumbate anions. Lead disulfide and lead diselenide are only stable at
high pressures. Lead tetrafluoride, a yellow crystalline powder, is
stable, but less so than the difluoride. Lead tetrachloride (a yellow
oil) decomposes at room temperature, lead tetrabromide is less stable
still, and the existence of lead tetraiodide is questionable.
Other oxidation states
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Some lead compounds exist in formal oxidation states other than +4 or
+2. Lead(III) may be obtained, as an intermediate between lead(II) and
lead(IV), in larger organolead complexes; this oxidation state is not
stable, as both the lead(III) ion and the larger complexes containing
it are radicals. The same applies for lead(I), which can be found in
such radical species.
Numerous mixed lead(II,IV) oxides are known. When PbO2 is heated in
air, it becomes Pb12O19 at 293 °C, Pb12O17 at 351 °C, Pb3O4 at 374 °C,
and finally PbO at 605 °C. A further sesquioxide, Pb2O3, can be
obtained at high pressure, along with several non-stoichiometric
phases. Many of them show defective fluorite structures in which some
oxygen atoms are replaced by vacancies: PbO can be considered as
having such a structure, with every alternate layer of oxygen atoms
absent.
Negative oxidation states can occur as Zintl phases, as either free
lead anions, as in Ba2Pb, with lead formally being lead(−IV), or in
oxygen-sensitive ring-shaped or polyhedral cluster ions such as the
trigonal bipyramidal Pb52− ion, where two lead atoms are lead(−I) and
three are lead(0). In such anions, each atom is at a polyhedral vertex
and contributes two electrons to each covalent bond along an edge from
their sp3 hybrid orbitals, the other two being an external lone pair.
They may be made in liquid ammonia via the reduction of lead by
sodium.
Organolead
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Carbon
Hydrogen
Lead can form multiply-bonded chains, a property it shares with its
lighter homologs in the carbon group. Its capacity to do so is much
less because the Pb-Pb bond energy is over three and a half times
lower than that of the C-C bond. With itself, lead can build
metal-metal bonds of an order up to three. With carbon, lead forms
organolead compounds similar to, but generally less stable than,
typical organic compounds (due to the Pb-C bond being rather weak).
This makes the organometallic chemistry of lead far less wide-ranging
than that of tin. Lead predominantly forms organolead(IV) compounds,
even when starting with inorganic lead(II) reactants; very few
organolead(II) compounds are known. The most well-characterized
exceptions are Pb[CH(SiMe3)2]2 and plumbocene.
The lead analog of the simplest organic compound, methane, is
plumbane. Plumbane may be obtained in a reaction between metallic lead
and atomic hydrogen. Two simple derivatives, tetramethyllead and
tetraethyllead, are the best-known organolead compounds. These
compounds are relatively stable: tetraethyllead only starts to
decompose if heated or if exposed to sunlight or ultraviolet light.
With sodium metal, lead readily forms an equimolar alloy that reacts
with alkyl halides to form organometallic compounds such as
tetraethyllead. The oxidizing nature of many organolead compounds is
usefully exploited: lead tetraacetate is an important laboratory
reagent for oxidation in organic synthesis. Tetraethyllead, once added
to automotive gasoline, was produced in larger quantities than any
other organometallic compound, and is still widely used in fuel for
small aircraft.
Other organolead compounds are less chemically stable. For many
organic compounds, a lead analog does not exist.
Origin and occurrence
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Solar System abundances Atomic number Element Relative amount
42 Molybdenum |0.798
46 Palladium |0.440
50 Tin |1.146
78 Platinum |0.417
80 Mercury |0.127 '82' 'Lead' |'1'
90 Thorium 0.011
92 Uranium 0.003
In space
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Lead's per-particle abundance in the Solar System is 0.121 ppb (parts
per billion). This figure is two and a half times higher than that of
platinum, eight times more than mercury, and seventeen times more than
gold. The amount of lead in the universe is slowly increasing as most
heavier atoms (all of which are unstable) gradually decay to lead. The
abundance of lead in the Solar System since its formation 4.5 billion
years ago has increased by about 0.75%. The Solar System abundances
table shows that lead, despite its relatively high atomic number, is
more prevalent than most other elements with atomic numbers greater
than 40.
Primordial lead--which comprises the isotopes lead-204, lead-206,
lead-207, and lead-208--was mostly created as a result of repetitive
neutron capture processes occurring in stars. The two main modes of
capture are the s- and r-processes.
In the s-process (s is for "slow"), captures are separated by years or
decades, allowing less stable nuclei to undergo beta decay. A stable
thallium-203 nucleus can capture a neutron and become thallium-204;
this undergoes beta decay to give stable lead-204; on capturing
another neutron, it becomes lead-205, which has a half-life of around
17 million years. Further captures result in lead-206, lead-207, and
lead-208. On capturing another neutron, lead-208 becomes lead-209,
which quickly decays into bismuth-209. On capturing another neutron,
bismuth-209 becomes bismuth-210, and this beta decays to polonium-210,
which alpha decays to lead-206. The cycle hence ends at lead-206,
lead-207, lead-208, and bismuth-209.
In the r-process (r is for "rapid"), captures happen faster than
nuclei can decay. This occurs in environments with a high neutron
density, such as a supernova or the merger of two neutron stars. The
neutron flux involved may be on the order of 1022 neutrons per square
centimeter per second. The r-process does not form as much lead as the
s-process. It tends to stop once neutron-rich nuclei reach 126
neutrons. At this point, the neutrons are arranged in complete shells
in the atomic nucleus, and it becomes harder to energetically
accommodate more of them. When the neutron flux subsides, these nuclei
beta decay into stable isotopes of osmium, iridium, platinum.
On Earth
==========
Lead is classified as a chalcophile under the Goldschmidt
classification, meaning it is generally found combined with sulfur. It
rarely occurs in its native, metallic form. Many lead minerals are
relatively light and, over the course of the Earth's history, have
remained in the crust instead of sinking deeper into the Earth's
interior. This accounts for lead's relatively high crustal abundance
of 14 ppm; it is the 36th most abundant element in the crust.
The main lead-bearing mineral is galena (PbS), which is mostly found
with zinc ores. Most other lead minerals are related to galena in some
way; boulangerite, Pb5Sb4S11, is a mixed sulfide derived from galena;
anglesite, PbSO4, is a product of galena oxidation; and cerussite or
white lead ore, PbCO3, is a decomposition product of galena. Arsenic,
tin, antimony, silver, gold, copper, bismuth are common impurities in
lead minerals.
World lead resources exceed two billion tons. Significant deposits are
located in Australia, China, Ireland, Mexico, Peru, Portugal, Russia,
United States. Global reserves--resources that are economically
feasible to extract--totaled 88 million tons in 2016, of which
Australia had 35 million, China 17 million, Russia 6.4 million.
Typical background concentrations of lead do not exceed 0.1 μg/m3 in
the atmosphere; 100 mg/kg in soil; 4 mg/kg in vegetation, 5 μg/L in
fresh water and seawater.
Etymology
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The modern English word 'lead' is of Germanic origin; it comes from
the Middle English and Old English (with the macron above the "e"
signifying that the vowel sound of that letter is long). The Old
English word is derived from the hypothetical reconstructed
Proto-Germanic ('lead'). According to linguistic theory, this word
bore descendants in multiple Germanic languages of exactly the same
meaning.
There is no consensus on the origin of the Proto-Germanic . One
hypothesis suggests it is derived from Proto-Indo-European ('lead';
capitalization of the vowel is equivalent to the macron). Another
hypothesis suggests it is borrowed from Proto-Celtic ('lead'). This
word is related to the Latin , which gave the element its chemical
symbol 'Pb'. The word is thought to be the origin of Proto-Germanic
(which also means 'lead'), from which stemmed the German .
The name of the chemical element is not related to the verb of the
same spelling, which is derived from Proto-Germanic ('to lead').
Classical era
===============
Because silver was extensively used as a decorative material and an
exchange medium, lead deposits came to be worked in Asia Minor from
3000 BC; later, lead deposits were developed in the Aegean and
Laurion. These three regions collectively dominated production of
mined lead until . Beginning c. 2000 BC, the Phoenicians worked
deposits in the Iberian peninsula; by 1600 BC, lead mining existed in
Cyprus, Greece, and Sardinia.
Rome's territorial expansion in Europe and across the Mediterranean,
and its development of mining, led to it becoming the greatest
producer of lead during the classical era, with an estimated annual
output peaking at 80,000 tonnes. Like their predecessors, the Romans
obtained lead mostly as a by-product of silver smelting. Lead mining
occurred in central Europe, Britain, Balkans, Greece, Anatolia,
Hispania, the latter accounting for 40% of world production.
Lead tablets were commonly used as a material for letters. Lead
coffins, cast in flat sand forms and with interchangeable motifs to
suit the faith of the deceased, were used in ancient Judea. Lead was
used to make sling bullets from the 5th century BC. In Roman times,
lead sling bullets were amply used, and were effective at a distance
of between 100 and 150 meters. The Balearic slingers, used as
mercenaries in Carthaginian and Roman armies, were famous for their
shooting distance and accuracy.
Lead was used for making water pipes in the Roman Empire; the Latin
word for the metal, , is the origin of the English word "plumbing".
Its ease of working, its low melting point enabling the easy
fabrication of completely waterproof welded joints, and its resistance
to corrosion ensured its widespread use in other applications,
including pharmaceuticals, roofing, currency, warfare. Writers of the
time, such as Cato the Elder, Columella, and Pliny the Elder,
recommended lead (and lead-coated) vessels for the preparation of
sweeteners and preservatives added to wine and food. The lead
conferred an agreeable taste due to the formation of "sugar of lead"
(lead(II) acetate), whereas copper vessels imparted a bitter flavor
through verdigris formation.
The Roman author Vitruvius reported the health dangers of lead and
modern writers have suggested that lead poisoning played a major role
in the decline of the Roman Empire. Other researchers have criticized
such claims, pointing out, for instance, that not all abdominal pain
is caused by lead poisoning. According to archaeological research,
Roman lead pipes increased lead levels in tap water but such an effect
was "unlikely to have been truly harmful". When lead poisoning did
occur, victims were called "saturnine", dark and cynical, after the
ghoulish father of the gods, Saturn. By association, lead was
considered the father of all metals. Its status in Roman society was
low as it was readily available and cheap.
Confusion with tin and antimony
=================================
Since the Bronze Age, metallurgists and engineers have understood the
difference between rare and valuable tin, essential for alloying with
copper to produce tough and corrosion resistant bronze, and 'cheap and
cheerful' lead. However, the nomenclature in some languages is
similar. Romans called lead ("black lead"), and tin ("bright
lead"). The association of lead and tin can be seen in other
languages: the word in Czech translates to "lead", but in Russian,
its cognate () means "tin". To add to the confusion, lead bore a
close relation to antimony: both elements commonly occur as sulfides
(galena and stibnite), often together. Pliny incorrectly wrote that
stibnite would give lead on heating, instead of antimony. In countries
such as Turkey and India, the originally Persian name came to refer
to either antimony sulfide or lead sulfide, and in some languages,
such as Russian, gave its name to antimony ().
Middle Ages and the Renaissance
=================================
Lead mining in Western Europe declined after the fall of the Western
Roman Empire, with Arabian Iberia being the only region having a
significant output. The largest production of lead occurred in South
Asia and East Asia, especially China and India, where lead mining grew
rapidly.
In Europe, lead production began to increase in the 11th and 12th
centuries, when it was again used for roofing and piping. Starting in
the 13th century, lead was used to create stained glass. In the
European and Arabian traditions of alchemy, lead (symbol ♄ in the
European tradition) was considered an impure base metal which, by the
separation, purification and balancing of its constituent essences,
could be transformed to pure and incorruptible gold. During the
period, lead was used increasingly for adulterating wine. The use of
such wine was forbidden for use in Christian rites by a papal bull in
1498, but it continued to be imbibed and resulted in mass poisonings
up to the late 18th century. Lead was a key material in parts of the
printing press, and lead dust was commonly inhaled by print workers,
causing lead poisoning. Lead also became the chief material for making
bullets for firearms: it was cheap, less damaging to iron gun barrels,
had a higher density (which allowed for better retention of velocity),
and its lower melting point made the production of bullets easier as
they could be made using a wood fire. Lead, in the form of Venetian
ceruse, was extensively used in cosmetics by Western European
aristocracy as whitened faces were regarded as a sign of modesty. This
practice later expanded to white wigs and eyeliners, and only faded
out with the French Revolution in the late 18th century. A similar
fashion appeared in Japan in the 18th century with the emergence of
the geishas, a practice that continued long into the 20th century. The
white faces of women "came to represent their feminine virtue as
Japanese women", with lead commonly used in the whitener.
Outside Europe and Asia
=========================
In the New World, lead production was recorded soon after the arrival
of European settlers. The earliest record dates to 1621 in the English
Colony of Virginia, fourteen years after its foundation. In Australia,
the first mine opened by colonists on the continent was a lead mine,
in 1841. In Africa, lead mining and smelting were known in the Benue
Trough and the lower Congo Basin, where lead was used for trade with
Europeans, and as a currency by the 17th century, well before the
scramble for Africa.
Industrial Revolution
=======================
In the second half of the 18th century, Britain, and later continental
Europe and the United States, experienced the Industrial Revolution.
This was the first time during which lead production rates exceeded
those of Rome. Britain was the leading producer, losing this status by
the mid-19th century with the depletion of its mines and the
development of lead mining in Germany, Spain, and the United States.
By 1900, the United States was the leader in global lead production,
and other non-European nations--Canada, Mexico, and Australia--had
begun significant production; production outside Europe exceeded that
within. A great share of the demand for lead came from plumbing and
painting--lead paints were in regular use. At this time, more (working
class) people were exposed to the metal and lead poisoning cases
escalated. This led to research into the effects of lead intake. Lead
was proven to be more dangerous in its fume form than as a solid
metal. Lead poisoning and gout were linked; British physician Alfred
Baring Garrod noted a third of his gout patients were plumbers and
painters. The effects of chronic ingestion of lead, including mental
disorders, were also studied in the 19th century. The first laws aimed
at decreasing lead poisoning in factories were enacted during the
1870s and 1880s in the United Kingdom.
Modern era
============
The last major human exposure to lead was the addition of
tetraethyllead to gasoline as an antiknock agent, a practice that
originated in the United States in 1921. It was phased out in the
United States and the European Union by 2000.
In the 1970s, the United States and Western European countries
introduced legislation to reduce lead air pollution. The impact was
significant: while a study conducted by the Centers for Disease
Control and Prevention in the United States in 1976-1980 showed that
77.8% of the population had elevated blood lead levels, in 1991-1994,
a study by the same institute showed the share of people with such
high levels dropped to 2.2%. The main product made of lead by the end
of the 20th century was the lead-acid battery.
From 1960 to 1990, lead output in the Western Bloc grew by about 31%.
The share of the world's lead production by the Eastern Bloc increased
from 10% to 30%, from 1950 to 1990, with the Soviet Union being the
world's largest producer during the mid-1970s and the 1980s, and China
starting major lead production in the late 20th century. Unlike the
European communist countries, China was largely unindustrialized by
the mid-20th century; in 2004, China surpassed Australia as the
largest producer of lead. As was the case during European
industrialization, lead has had a negative effect on health in China.
Production
======================================================================
As of 2014, production of lead is increasing worldwide due to its use
in lead-acid batteries. There are two major categories of production:
primary from mined ores, and secondary from scrap. In 2014, 4.58
million metric tons came from primary production and 5.64 million from
secondary production. The top three producers of mined lead
concentrate in that year were China, Australia, and United States. The
top three producers of refined lead were China, United States, and
India. According to the Metal Stocks in Society report of 2010, the
total amount of lead in use, stockpiled, discarded, or dissipated into
the environment, on a global basis, is 8 kg per capita. Much of this
is in more developed countries (20-150 kg per capita) rather than less
developed ones (1-4 kg per capita).
The primary and secondary lead production processes are similar. Some
primary production plants now supplement their operations with scrap
lead, and this trend is likely to increase in the future. Given
adequate techniques, lead obtained via secondary processes is
indistinguishable from lead obtained via primary processes. Scrap lead
from the building trade is usually fairly clean and is re-melted
without the need for smelting, though refining is sometimes needed.
Secondary lead production is therefore cheaper, in terms of energy
requirements, than is primary production, often by 50% or more.
Primary
=========
Most lead ores contain a low percentage of lead (rich ores have a
typical content of 3-8%) which must be concentrated for extraction.
During initial processing, ores typically undergo crushing,
dense-medium separation, grinding, froth flotation, drying. The
resulting concentrate, which has a lead content of 30-80% by mass
(regularly 50-60%), is then turned into (impure) lead metal.
There are two main ways of doing this: a two-stage process involving
roasting followed by blast furnace extraction, carried out in separate
vessels; or a direct process in which the extraction of the
concentrate occurs in a single vessel. The latter has become the most
common route, though the former is still significant.
(thousand tons)
align="right"|2,400
align="right"|500
align="right"|335
align="right"|310
align="right"|250
align="right"|225
align="right"|135
align="right"|80
align="right"|76
align="right"|75
align="right"|41
align="right"|41
align="right"|40
align="right"|40
align="right"|35
align="right"|33
align="right"|33
Other countries align="right"|170
Two-stage process
===================
First, the sulfide concentrate is roasted in air to oxidize the lead
sulfide:
: 2 PbS(s) + 3 O2(g) → 2 PbO(s) + 2 SO2(g)↑
As the original concentrate was not pure lead sulfide, roasting yields
not only the desired lead(II) oxide, but a mixture of oxides,
sulfates, and silicates of lead and of the other metals contained in
the ore. This impure lead oxide is reduced in a coke-fired blast
furnace to the (again, impure) metal:
: 2 PbO(s) + C(s) → 2 Pb(s) + CO2(g)↑
Impurities are mostly arsenic, antimony, bismuth, zinc, copper,
silver, and gold. Typically they are removed in a series of
pyrometallurgical processes. The melt is treated in a reverberatory
furnace with air, steam, sulfur, which oxidizes the impurities except
for silver, gold, bismuth. Oxidized contaminants float to the top of
the melt and are skimmed off. Metallic silver and gold are removed and
recovered economically by means of the Parkes process, in which zinc
is added to lead. Zinc, which is immiscible in lead, dissolves the
silver and gold. The zinc solution can be separated from the lead, and
the silver and gold retrieved. De-silvered lead is freed of bismuth by
the Betterton-Kroll process, treating it with metallic calcium and
magnesium. The resulting bismuth dross can be skimmed off.
Alternatively to the pyrometallurgical processes, very pure lead can
be obtained by processing smelted lead electrolytically using the
Betts process. Anodes of impure lead and cathodes of pure lead are
placed in an electrolyte of lead fluorosilicate (PbSiF6). Once
electrical potential is applied, impure lead at the anode dissolves
and plates onto the cathode, leaving the majority of the impurities in
solution. This is a high-cost process and thus mostly reserved for
refining bullion containing high percentages of impurities.
Direct process
================
In this process, lead bullion and slag is obtained directly from lead
concentrates. The lead sulfide concentrate is melted in a furnace and
oxidized, forming lead monoxide. Carbon (as coke or coal gas) is added
to the molten charge along with fluxing agents. The lead monoxide is
thereby reduced to metallic lead, in the midst of a slag rich in lead
monoxide.
If the input is rich in lead, as much as 80% of the original lead can
be obtained as bullion; the remaining 20% forms a slag rich in lead
monoxide. For a low-grade feed, all of the lead can be oxidized to a
high-lead slag. Metallic lead is further obtained from the high-lead
(25-40%) slags via submerged fuel combustion or injection, reduction
assisted by an electric furnace, or a combination of both.
Alternatives
==============
Research on a cleaner, less energy-intensive lead extraction process
continues; a major drawback is that either too much lead is lost as
waste, or the alternatives result in a high sulfur content in the
resulting lead metal. Hydrometallurgical extraction, in which anodes
of impure lead are immersed into an electrolyte and pure lead is
deposited (electrowound) onto cathodes, is a technique that may have
potential, but is not currently economical except in cases where
electricity is very cheap.
Secondary
===========
Smelting, which is an essential part of the primary production, is
often skipped during secondary production. It is only performed when
metallic lead has undergone significant oxidation. The process is
similar to that of primary production in either a blast furnace or a
rotary furnace, with the essential difference being the greater
variability of yields: blast furnaces produce hard lead (10% antimony)
while reverberatory and rotary kiln furnaces produce semisoft lead
(3-4% antimony).
The ISASMELT process is a more recent smelting method that may act as
an extension to primary production; battery paste from spent lead-acid
batteries (containing lead sulfate and lead oxides) has its sulfate
removed by treating it with alkali, and is then treated in a
coal-fueled furnace in the presence of oxygen, which yields impure
lead, with antimony the most common impurity. Refining of secondary
lead is similar to that of primary lead; some refining processes may
be skipped depending on the material recycled and its potential
contamination.
Of the sources of lead for recycling, lead-acid batteries are the most
important; lead pipe, sheet, and cable sheathing are also significant.
Applications
======================================================================
Contrary to popular belief, pencil leads in wooden pencils have never
been made from lead. When the pencil originated as a wrapped graphite
writing tool, the particular type of graphite used was named
'plumbago' (literally, 'lead mockup').
Elemental form
================
Lead metal has several useful mechanical properties, including high
density, low melting point, ductility, and relative inertness. Many
metals are superior to lead in some of these aspects but are generally
less common and more difficult to extract from parent ores. Lead's
toxicity has led to its phasing out for some uses.
Lead has been used for bullets since their invention in the Middle
Ages. It is inexpensive; its low melting point means small arms
ammunition and shotgun pellets can be cast with minimal technical
equipment; and it is denser than other common metals, which allows for
better retention of velocity. It remains the main material for
bullets, alloyed with other metals as hardeners. Concerns have been
raised that lead bullets used for hunting can damage the environment.
Shotgun cartridges used for waterfowl hunting must today be lead-free
in the United States, Canada, and in Europe.
Lead's high density and resistance to corrosion have been exploited in
a number of related applications. It is used as ballast in sailboat
keels; its density allows it to take up a small volume and minimize
water resistance, thus counterbalancing the heeling effect of wind on
the sails. It is used in scuba diving weight belts to counteract the
diver's buoyancy. In 1993, the base of the Leaning Tower of Pisa was
stabilized with 600 tonnes of lead. Because of its corrosion
resistance, lead is used as a protective sheath for underwater cables.
Lead has many uses in the construction industry; lead sheets are used
as architectural metals in roofing material, cladding, flashing,
gutters and gutter joints, roof parapets. Lead is still used in
statues and sculptures, including for armatures. In the past it was
often used to balance the wheels of cars; for environmental reasons
this use is being phased out in favor of other materials.
Lead is added to copper alloys, such as brass and bronze, to improve
machinability and for its lubricating qualities. Being practically
insoluble in copper, the lead forms solid globules in imperfections
throughout the alloy, such as grain boundaries. In low concentrations,
as well as acting as a lubricant, the globules hinder the formation of
swarf as the alloy is worked, thereby improving machinability. Copper
alloys with larger concentrations of lead are used in bearings. The
lead provides lubrication, and the copper provides the load-bearing
support.
Lead's high density, atomic number, and formability form the basis for
use of lead as a barrier that absorbs sound, vibration, and radiation.
Lead has no natural resonance frequencies; as a result, sheet-lead is
used as a sound deadening layer in the walls, floors, and ceilings of
sound studios. Organ pipes are often made from a lead alloy, mixed
with various amounts of tin to control the tone of each pipe. Lead is
an established shielding material from radiation in nuclear science
and in X-ray rooms due to its denseness and high attenuation
coefficient. Molten lead has been used as a coolant for lead-cooled
fast reactors.
Batteries
===========
The largest use of lead in the early 21st century is in lead-acid
batteries. The lead in batteries undergoes no direct contact with
humans, so there are fewer toxicity concerns. People who work in lead
battery production or recycling plants may be exposed to lead dust and
inhale it. The reactions in the battery between lead, lead dioxide,
and sulfuric acid provide a reliable source of voltage.
Supercapacitors incorporating lead-acid batteries have been installed
in kilowatt and megawatt scale applications in Australia, Japan, and
the United States in frequency regulation, solar smoothing and
shifting, wind smoothing, and other applications. These batteries have
lower energy density and charge-discharge efficiency than lithium-ion
batteries, but are significantly cheaper.
Coating for cables
====================
Lead is used in high voltage power cables as shell material to prevent
water diffusion into insulation; this use is decreasing as lead is
being phased out. Its use in solder for electronics is also being
phased out by some countries to reduce the amount of environmentally
hazardous waste. Lead is one of three metals used in the Oddy test for
museum materials, helping detect organic acids, aldehydes, acidic
gases.
Compounds
===========
In addition to being the main application for lead metal, lead-acid
batteries are also the main consumer of lead compounds. The energy
storage/release reaction used in these devices involves lead sulfate
and lead dioxide:
:(s) + (s) + 2(aq) → 2(s) + 2(l)
Other applications of lead compounds are very specialized and often
fading. Lead-based coloring agents are used in ceramic glazes and
glass, especially for red and yellow shades. While lead paints are
phased out in Europe and North America, they remain in use in less
developed countries such as China, India, or Indonesia. Lead
tetraacetate and lead dioxide are used as oxidizing agents in organic
chemistry. Lead is frequently used in the polyvinyl chloride coating
of electrical cords. It can be used to treat candle wicks to ensure a
longer, more even burn. Because of its toxicity, European and North
American manufacturers use alternatives such as zinc. Lead glass is
composed of 12-28% lead oxide, changing its optical characteristics
and reducing the transmission of ionizing radiation, a property used
in old TVs and computer monitors with cathode-ray tubes. Lead-based
semiconductors such as lead telluride and lead selenide are used in
photovoltaic cells and infrared detectors.
Biological effects
======================================================================
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Lead has no confirmed biological role, and there is no confirmed safe
level of lead exposure. A 2009 Canadian-American study concluded that
even at levels that are considered to pose little to no risk, lead may
cause "adverse mental health outcomes". Its prevalence in the human
body--at an adult average of 120 mg--is nevertheless exceeded only by
zinc (2500 mg) and iron (4000 mg) among the heavy metals. Lead salts
are very efficiently absorbed by the body. A small amount of lead (1%)
is stored in bones; the rest is excreted in urine and feces within a
few weeks of exposure. Only about a third of lead is excreted by a
child. Continual exposure may result in the bioaccumulation of lead.
Toxicity
==========
Lead is a highly poisonous metal (whether inhaled or swallowed),
affecting almost every organ and system in the human body. At
airborne levels of 100 mg/m3, it is immediately dangerous to life and
health. Most ingested lead is absorbed into the bloodstream. The
primary cause of its toxicity is its predilection for interfering with
the proper functioning of enzymes. It does so by binding to the
sulfhydryl groups found on many enzymes, or mimicking and displacing
other metals which act as cofactors in many enzymatic reactions. The
essential metals that lead interacts with include calcium, iron, and
zinc. High levels of calcium and iron tend to provide some protection
from lead poisoning; low levels cause increased susceptibility.
Effects
=========
Lead can cause severe damage to the brain and kidneys and, ultimately,
death. By mimicking calcium, lead can cross the blood-brain barrier.
It degrades the myelin sheaths of neurons, reduces their numbers,
interferes with neurotransmission routes, and decreases neuronal
growth. In the human body, lead inhibits porphobilinogen synthase and
ferrochelatase, preventing both porphobilinogen formation and the
incorporation of iron into protoporphyrin IX, the final step in heme
synthesis. This causes ineffective heme synthesis and microcytic
anemia.
Symptoms of lead poisoning include nephropathy, colic-like abdominal
pains, and possibly weakness in the fingers, wrists, or ankles. Small
blood pressure increases, particularly in middle-aged and older
people, may be apparent and can cause anemia. Several studies, mostly
cross-sectional, found an association between increased lead exposure
and decreased heart rate variability. In pregnant women, high levels
of exposure to lead may cause miscarriage. Chronic, high-level
exposure has been shown to reduce fertility in males.
In a child's developing brain, lead interferes with synapse formation
in the cerebral cortex, neurochemical development (including that of
neurotransmitters), and the organization of ion channels. Early
childhood exposure has been linked with an increased risk of sleep
disturbances and excessive daytime drowsiness in later childhood. High
blood levels are associated with delayed puberty in girls. The rise
and fall in exposure to airborne lead from the combustion of
tetraethyl lead in gasoline during the 20th century has been linked
with historical increases and decreases in crime levels.
Exposure sources
==================
Lead exposure is a global issue since lead mining and smelting, and
battery manufacturing, disposal, and recycling, are common in many
countries. Lead enters the body via inhalation, ingestion, or skin
absorption. Almost all inhaled lead is absorbed into the body; for
ingestion, the rate is 20-70%, with children absorbing a higher
percentage than adults.
Poisoning typically results from ingestion of food or water
contaminated with lead, and less commonly after accidental ingestion
of contaminated soil, dust, or lead-based paint. Seawater products can
contain lead if affected by nearby industrial waters. Fruit and
vegetables can be contaminated by high levels of lead in the soils
they were grown in. Soil can be contaminated through particulate
accumulation from lead in pipes, lead paint, residual emissions from
leaded gasoline.
The use of lead for water pipes is a problem in areas with soft or
acidic water. Hard water forms insoluble protective layers on the
inner surface of the pipes, whereas soft and acidic water dissolves
the lead pipes. Dissolved carbon dioxide in the carried water may
result in the formation of soluble lead bicarbonate; oxygenated water
may similarly dissolve lead as lead(II) hydroxide. Drinking such
water, over time, can cause health problems due to the toxicity of the
dissolved lead. The harder the water the more calcium bicarbonate and
sulfate it contains, and the more the inside of the pipes are coated
with a protective layer of lead carbonate or lead sulfate.
Ingestion of applied lead-based paint is the major source of exposure
for children: a direct source is chewing on old painted window sills.
Additionally, as lead paint on a surface deteriorates, it peels and is
pulverized into dust. The dust then enters the body through
hand-to-mouth contact or contaminated food or drink. Ingesting certain
home remedies may result in exposure to lead or its compounds.
Inhalation is the second major exposure pathway, affecting smokers and
especially workers in lead-related occupations. Cigarette smoke
contains, among other toxic substances, radioactive lead-210. "As a
result of EPA's regulatory efforts, levels of lead in the air [in the
United States] decreased by 86 percent between 2010 and 2020." The
concentration of lead in the air in the United States fell below the
national standard of 0.15 μg/m3 in 2014.
Skin exposure may be significant for people working with organic lead
compounds. The rate of skin absorption is lower for inorganic lead.
Lead in foods
===============
Lead may be found in food when food is grown in soil that is high in
lead, airborne lead contaminates the crops, animals eat lead in their
diet, or lead enters the food either from what it was stored or cooked
in. Ingestion of lead paint and batteries is also a route of exposure
for livestock, which can subsequently affect humans. Milk produced by
contaminated cattle can be diluted to a lower lead concentration and
sold for consumption.
In Bangladesh, lead compounds have been added to turmeric to make it
more yellow. This is believed to have started in the 1980s and
continues . It is believed to be one of the main sources of high lead
levels in the country. In Hong Kong the maximum allowed lead level in
food is 6 parts per million in solids and 1 part per million in
liquids.
Lead-containing dust can settle on drying cocoa beans when they are
set outside near polluting industrial plants. In December 2022,
Consumer Reports tested 28 dark chocolate brands and found that 23 of
them contained potentially harmful levels of lead, cadmium or both.
They have urged the chocolate makers to reduce the level of lead which
could be harmful, especially to a developing fetus.
In March 2024, the US Food and Drug Administration recommended a
voluntary recall on 6 brands of cinnamon due to contamination with
lead, after 500 reports of child lead poisoning. The FDA determined
that cinnamon was adulterated with lead chromate.
Lead in plastic toys
======================
According to the United States Center for Disease Control, the use of
lead in plastics has not been banned as of 2024. Lead softens the
plastic and makes it more flexible so that it can go back to its
original shape. Habitual chewing on colored plastic insulation from
stripped electrical wires was found to cause elevated lead levels in a
46-year-old man. Lead may be used in plastic toys to stabilize
molecules from heat. Lead dust can be formed when plastic is exposed
to sunlight, air, and detergents that break down the chemical bond
between the lead and plastics.
Treatment
===========
Treatment for lead poisoning normally involves the administration of
dimercaprol and succimer. Acute cases may require the use of disodium
calcium edetate, the calcium chelate, and the disodium salt of
ethylenediaminetetraacetic acid (EDTA). It has a greater affinity for
lead than calcium, with the result that lead chelate is formed by
exchange and excreted in the urine, leaving behind harmless calcium.
Environmental effects
======================================================================
The extraction, production, use, and disposal of lead and its products
have caused significant contamination of the Earth's soils and waters.
Atmospheric emissions of lead were at their peak during the Industrial
Revolution, and the leaded gasoline period in the second half of the
twentieth century.
Lead releases originate from natural sources (i.e., concentration of
the naturally occurring lead), industrial production, incineration and
recycling, and mobilization of previously buried lead. In particular,
as lead has been phased out from other uses, in the Global South, lead
recycling operations designed to extract cheap lead used for global
manufacturing have become a well documented source of exposure.
Elevated concentrations of lead persist in soils and sediments in
post-industrial and urban areas; industrial emissions, including those
arising from coal burning, continue in many parts of the world,
particularly in the developing countries.
Lead can accumulate in soils, especially those with a high organic
content, where it remains for hundreds to thousands of years.
Environmental lead can compete with other metals found in and on plant
surfaces potentially inhibiting photosynthesis and at high enough
concentrations, negatively affecting plant growth and survival.
Contamination of soils and plants can allow lead to ascend the food
chain affecting microorganisms and animals. In animals, lead exhibits
toxicity in many organs, damaging the nervous, renal, reproductive,
hematopoietic, and cardiovascular systems after ingestion, inhalation,
or skin absorption. Fish uptake lead from both water and sediment;
bioaccumulation in the food chain poses a hazard to fish, birds, and
sea mammals.
Anthropogenic lead includes lead from shot and sinkers. These are
among the most potent sources of lead contamination along with lead
production sites. Lead was banned for shot and sinkers in the United
States in 2017, although that ban was only effective for a month, and
a similar ban is being considered in the European Union.
Analytical methods for the determination of lead in the environment
include spectrophotometry, X-ray fluorescence, atomic spectroscopy,
and electrochemical methods. A specific ion-selective electrode has
been developed based on the ionophore
'S','S-methylenebis('N','N'-diisobutyldithiocarbamate). An important
biomarker assay for lead poisoning is δ-aminolevulinic acid levels in
plasma, serum, and urine.
Restriction and remediation
======================================================================
By the mid-1980s, there was significant decline in the use of lead in
industry. In the United States, environmental regulations reduced or
eliminated the use of lead in non-battery products, including
gasoline, paints, solders, and water systems. Particulate control
devices were installed in coal-fired power plants to capture lead
emissions. In 1992, U.S. Congress required the Environmental
Protection Agency to reduce the blood lead levels of the country's
children. Lead use was further curtailed by the European Union's 2003
Restriction of Hazardous Substances Directive. A large drop in lead
deposition occurred in the Netherlands after the 1993 national ban on
use of lead shot for hunting and sport shooting: from 230 tonnes in
1990 to 47.5 tonnes in 1995. The usage of lead in Avgas 100LL for
general aviation is allowed in the EU as of 2022.
In the United States, the permissible exposure limit for lead in the
workplace, comprising metallic lead, inorganic lead compounds, and
lead soaps, was set at 50 μg/m3 over an 8-hour workday, and the blood
lead level limit at 5 μg per 100 g of blood in 2012. Lead may still be
found in harmful quantities in stoneware, vinyl (such as that used for
tubing and the insulation of electrical cords), and Chinese brass. Old
houses may still contain lead paint. White lead paint has been
withdrawn from sale in industrialized countries, but specialized uses
of other pigments such as yellow lead chromate remain, especially in
road pavement marking paint.
Stripping old paint by sanding produces dust which can be inhaled.
Lead abatement programs have been mandated by some authorities in
properties where young children live. The usage of lead in Avgas 100LL
for general aviation is generally allowed in United States as of 2023.
Lead waste, depending on the jurisdiction and the nature of the waste,
may be treated as household waste (to facilitate lead abatement
activities), or potentially hazardous waste requiring specialized
treatment or storage. Lead is released into the environment in
shooting places and a number of lead management practices have been
developed to counter the lead contamination. Lead migration can be
enhanced in acidic soils; to counter that, it is advised soils be
treated with lime to neutralize the soils and prevent leaching of
lead.
Research has been conducted on how to remove lead from biosystems by
biological means: Fish bones are being researched for their ability to
bioremediate lead in contaminated soil. The fungus 'Aspergillus
versicolor' is effective at absorbing lead ions from industrial waste
before being released to water bodies. Several bacteria have been
researched for their ability to remove lead from the environment,
including the sulfate-reducing bacteria 'Desulfovibrio' and
'Desulfotomaculum', both of which are highly effective in aqueous
solutions. Millet grass 'Urochloa ramosa' has the ability to
accumulate significant amounts of metals such as lead and zinc in its
shoot and root tissues making it an important plant for remediation of
contaminated soils.
See also
======================================================================
* Derek Bryce-Smith - one of the earliest campaigners against lead in
petrol in the UK
* Thomas Midgley Jr. - discovered that the addition of tetraethyllead
to gasoline prevented "knocking" in internal combustion engines
* Clair Cameron Patterson - instrumental in the banning of
tetraethyllead in gasoline in the US and lead solder in food cans.
* Robert A. Kehoe - foremost medical advocate for the use of
tetraethyllead as an additive in gasoline.
Further reading
======================================================================
* [
https://www.degruyter.com/viewbooktoc/product/460569 Table of
contents]
*
*
* Ingalls, Walter Renton (1865-),
[
https://www.gutenberg.org/ebooks/63784 Lead Smelting and Refining,
With Some Notes on Lead Mining]
*
External links
======================================================================
*[
https://www.researchgate.net/profile/Catherine_Hammett-Stabler/publication/288272623_The_Toxicology_of_Heavy_Metals_Getting_the_Lead_Out/links/572748fd08ae586b21e28b65.pdf
The Toxicology of Heavy Metals: Getting the Lead Out], American
Society for Clinical Pathology
*[
https://www.crown.co.za/modern-mining/industry-news/13897-covid-19-to-reduce-global-lead-production-by-5-2-in-2020
COVID-19 to reduce global lead production by 5,2% in 2020 (with a
figure showing global lead production, 2010-2024)]
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Lead
