Tin and Lead

Tin and lead are the last two members of the carbon family but, in keeping with the general tendency for metallic character to increase with atomic number in a family, they show typical metallic properties. However, they have comparatively weak electropositive characteristics and strong electron acceptor properties. Bonds to nonmetals such as carbon have considerable covalent character and make up an important aspect of their chemistry.

Both tin and lead form compounds in which they have the oxidation states (IV) and (II). The former state is found in the organometallic compounds that have considerable environmental significance. Overall, lead compounds show much greater toxicity than those of tin, and environmental lead poses a more significant hazard than does tin.

Organotin compounds are used for a variety of purposes, such as wood preservatives, marine antifouling paints (especially tri-n-butyltin compounds), fungicides, and stabilizers for poly(vinyl chloride). The organotin compounds have the general formula RnSnX4-n, where X is a suitable anion; the most important systems are those for which n = 2 or 3. The toxicity of these organotin compounds is greatest for trialkyl compounds with short carbon chains. Dioctyltin compounds are sufficiently nontoxic to be used in plastics employed for food packaging. However, extraction of mono- and dialkyltin compounds into drinking water from plastic pipes is of some concern, as is accumulation of organotin compounds in some marine environments; health effects are not well established at this point. Microbial and photochemical reactions are known to degrade some organotin compounds now in use fairly rapidly, although the behavior with respect to degradation of many of these compounds has not been established fully. On the other hand, microbial processes can also form methylated tin species.

Tributyl tin has been widely used in marine anti-fouling paints, which are designed to kill marine organisms that attach to and foul boat hulls. The tributyl tin group is attached to an organic polymer coating through an Sn-O linkage to an ester group. Slow hydrolysis by sea-water releases the tributyl tin as the chloride (n-Bu3SnCl) or oxide [(n-Bu3Sn)2O], killing the organisms that cause fouling but also allowing the compound to accumulate in the ocean where it is toxic to a variety of marine life in heavily used locations such as harbors. It is particularly toxic to shellfish, crustaceans, and juvenile fish, and is bioaccumulated. Tributyl tin can survive in an aerobic medium for months, and for much longer in anaerobic sediments. Many nations, including the United States, have imposed severe restrictions on the use of this substance.

The final degradation product of organotin compounds is an inorganic tin oxide. Unlike the case with lead, inorganic forms of tin are relatively nontoxic. Tin metal is used to coat steel to make cans used for packaging food and other materials. It also finds use in lead-free solders and pewter.

The toxicity of lead in the environment has caused extensive concern in recent years. The current U.S. limit for lead in water is 0.015 mg/liter. Like mercury, Pb(II) forms comparatively covalent bonds with appropriate donor groups in complexes, generally favoring sulfur and nitrogen over oxygen donors, and it may owe some of its physiological action to replacement of other metals in some enzymes. Low levels have subtle effects on the nervous system, while higher levels can lead to many symptoms, such as severe effects on the nervous system, including loss of sight and hearing, as well as symptoms of gout, headache, insomnia, anemia, kidney damage, diarrhea, stomach pains, intestinal paralysis, and eventually death.

A great deal of the environmental lead, and most of the airborne lead, has come from organolead compounds widely used as the tetraalkyls added to gasolines (Section 6.6). These were adopted in the 1920s through largely U.S. developments to allow an increase in automotive engine compression ratios, even though they were known to be toxic and an alternative, methanol, was widely proposed as an effective alternative.26 Upon combustion the organolead compounds are converted to elemental lead, lead oxide, or a lead halide. These products are not volatile, and atmospheric lead is essentially all particulate. Much of this will settle near the source, and soils near areas with heavy vehicular traffic often have elevated lead levels. As discussed in Section 6.7.4, lead additives have been removed from gasoline in much of the world, in part because of concerns about lead toxicity, but also to permit the use of catalytic converters.

Although atmospheric lead is primarily particulate, it is still distributed worldwide. Analysis of Greenland ice cores show a relatively constant, low lead content up to about 1750, but with a period of increased concentration at levels corresponding to the Roman period, when it is known that mining and use of lead increased (Figure 10-16). Values increased after 1750 as the Industrial Revolution encouraged continued increases in lead use. Lead concentrations in the ice then went up much more rapidly after about 1940, corresponding to widespread use of lead in gasoline.

26An extensive "expose" was published by J. L. Kitman in The Nation, March 20, 2000; available at http://www.thenation.com.

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Corrected 14C (solar) years before present

FIGURE 10-16 Estimates of world lead production over time. Note that cupellation is a process for purifying precious metals by melting the impure metal in a cupel (a flat, porous dish) and then directing a blast of hot air onto it. The unwanted metals are oxidized and partly vaporized and partly absorbed into the pores of the cupel. Redrawn with permission from D. M. Settle and C. C. Patterson, Science, 207 1167-1175. Copyright © 1980 American Association for the Advancement of Science.

Corrected 14C (solar) years before present

FIGURE 10-16 Estimates of world lead production over time. Note that cupellation is a process for purifying precious metals by melting the impure metal in a cupel (a flat, porous dish) and then directing a blast of hot air onto it. The unwanted metals are oxidized and partly vaporized and partly absorbed into the pores of the cupel. Redrawn with permission from D. M. Settle and C. C. Patterson, Science, 207 1167-1175. Copyright © 1980 American Association for the Advancement of Science.

Many common lead salts, such as the carbonate, hydroxide, and sulfide, are extremely insoluble in water; this normally limits the dissolved lead content in a body of water, although in principle complexing agents in the water may considerably increase the effective solubility.

Various technological sources of lead (Table 10-8) can serve as origins of ingested lead in humans and consequent lead poisoning. As with other heavy metals strongly bound by biological complexing agents, lead is a cumulative poison and can act through long-term ingestion of relatively small quantities. Lead can be retained in the body for long periods, especially in bones, where it can replace some of the calcium. Lead poisoning in children from ingestion of paint fragments is well known. This is primarily a hazard with older paints, in which lead pigments were widely used. Basic lead carbonate, (PbCO3)2 Pb(OH)2, known as white lead, is an example of a white pigment, while lead chromate is an orange or yellow one. The oxides, sulfate, and a variety of other lead compounds also were employed. Large amounts of white pigment is used even in colored paints to give opacity; lead content does not relate to color. These lead materials are largely absent in modern paint formulations, because of both the availability of superior materials (TiO2) and legal restrictions arising from recognition of the hazard. It is interesting that the problem of lead poisoning in children from the lead in paint was recognized as early as 1900, and white lead in indoor paint was prohibited in France in 1909; international treaties to eliminate it were proposed in 1922, but the United States did not ban lead in interior paints until 1971. Even then, lead remained legal in paints for some exterior uses.

In spite of the toxicity of lead, and occasional calls for its use to be decreased, annual use in the United States has increased, from 1230 metric tons in 1970 to about 1600 metric tons in 1998, even though its use in gasoline decreased. Most of the increase (to well over 80% of the total, most of which is recycled) has been in batteries.

Less common now, but of possible significance in the past, is the extraction of lead into drinking water from lead pipes used in plumbing systems. (One may note the similarity between "plumbing'' and plumbum, the Latin word for lead.) Lead pipes are not now used in drinking water systems (although they

TABLE 10-8

Some Applications and Uses of Lead

As metal

Batteries Water pipes Solder

Structural use Radiation shielding Shot

Antifriction metal Type metal

As inorganic lead

Pottery glazes Glass

Insecticides Paints

Stabilizers in plastics Putty, caulk, etc. Matches

As organolead

Gasoline additives

Lead-acid storage batteries are most widely used.

No longer used; some may still exist in old construction

As an alloy; also now removed from drinking water applications

Has been used for roofing, glass mounting, etc.

High density makes it a good absorber of radiation

An alloy, also contains antimony and arsenic

Some bearing compositions

Increases index of refraction; gives sparkle to crystal No longer used

White pigment and base, also colored pigments; now removed from indoor and most outdoor paints Heat stabilizer in poly(vinyl chloride) (often as organic acid salt)

Generally phased out except in undeveloped nations may remain in some old constructions), but lead-containing solder has been common in copper piping systems, and in some older water system fixtures, including drinking fountains, one can find both solder and brass components that contain lead. Hot water, and water that has been in contact with the plumbing for some time, can dissolve significant quantities of lead from these sources. Therefore, it has been recommended that water be allowed to run for a few minutes before being used for drinking, and that water from hot water lines not be consumed. Many measurements have shown higher (sometimes much higher) levels of lead in first-draw water in the morning from household supplies than are found after the water has been allowed to run for a few minutes. In addition, the low-solubility lead salts that form as coatings on such plumbing surfaces can be further solubilized if the acidity of the water increases. Although lead-based solders and other components are not now used in drinking water systems in the United States, only recent installations are likely to have lead-free solder.

A more subtle source of ingested lead is pottery utensils. Lead salts have been a common constituent of some glazes used since antiquity to decorate pottery, and while apparently inert, some lead may in fact be leached out under acidic conditions, particularly if the glaze is improperly fired. Acidic foods (e.g., orange juice) can provide these conditions. One theory for the decline of the Roman Empire proposes that much of the population was functionally impaired by chronic lead poisoning originating from wine stored in lead-glazed vessels. Another theory attributes the same effect to the use of lead plumbing. Lead is also found in crystal glass, and can be extracted into wine stored in crystal decanters. Actually, lead contamination of alcoholic beverages of various kinds has occurred frequently through the use of lead components in presses, stills, or other equipment, and may still be a problem with illegal production such as "moonshine" whiskey.

From at least Roman times up to the eighteenth century, lead-containing materials were common additives used to sweeten sour wine,27 and although the toxicity of lead in these applications had been recognized very early, this information was not generally understood. Lead compounds have also been used as food adulterants. White lead was added to flour to increase weight, and as late as 1994 many people in Hungary were poisoned when red lead, Pb3O4, was added to paprika. Even into the twentieth century, lead compounds were used in medicines.

Lead compounds (e.g., lead stearate, lead phthalate), have seen use as heat stabilizers in poly (vinyl chloride) materials (e.g., electrical wiring insulation,

27When wine is boiled down to a reduced volume with litharge, PbO, or grape juice is boiled down in a lead vessel, a sweet syrup containing lead is produced which was used in antiquity to sweeten and preserve wines. (Lead tetraacetate was called "sugar of lead.") Other alcoholic beverages have sometimes been produced with stills having lead components. For an interesting review, see J. Eisinger, Nat. Hist., 105 (7), 48 (1996).

water pipes). In the United States, current use is restricted to the first of these, and European use is expected to decrease as less toxic metal compounds such as organotin or calcium/zinc formulations take over. It is materials such as these that contribute to heavy metal release when "organic" materials such as plastics are incinerated.

Other sources of lead in the human environment have been lead arsenate and lead acetate used as insecticides. Lead also has widespread use in batteries, (discussed in Section 15.8.5.2), and solder in electrical equipment, and a significant amount enters the environment as spent ammunition from hunting. The problem here arises from the shotgun pellets used in duck hunting, which are ingested by the birds as they feed in sediments. Combustion of coal and oil releases trace amounts of lead. Castellino gives a concise history of the uses and toxicity of lead.28

There has been considerable controversy regarding the hazards associated with lead emissions into the environment. Various evaluations suggest that on the whole, body lead concentrations have not been increased very greatly over what is assumed to be a "normal" level by industrial and automotive emissions. On the other hand, there also are indications that symptoms of lead poisoning can be detected after only small increases. Children are particularly sensitive, and relatively small elevations in lead exposure can lead to learning disabilities.

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