Mercury Mercury Vapor

Elemental mercury was employed in hundreds of applications, many of which (e.g., electrical switches) took advantage of the unusual property that it is a liquid that conducts electricity well. In automobiles built before 2000, electrical switches that operate convenience and trunk lighting contained mercury, as did instrument panels and antilock brakes; all of this mercury is lost to the environment when the cars are recycled for their steel unless the element is specifically collected, as is required in some locales.

Elemental mercury is still used in fluorescent light bulbs, including the small ones now used domestically to replace incandescent bulbs, and in the mercury lamps employed for street lighting. Although energized mercury atoms emit light in the ultraviolet rather than the visible wavelength region of the spectrum (Chapter 1), the bulbs are coated with a material that absorbs the UV and re-emits it from the bulb as visible light. The metal is released into the environment if such light bulbs are broken, though the mercury content of fluorescent lamps has been reduced by about 80% since the mid' 1980s, down to 5—10 mg each today. This amount of mercury is less than the additional quantity of the metal that would have been emitted into the air by a coal-fired power plant if an incandescent light bulb, with its much lower efficiency in converting electricity to light, had been used instead. Fluorescent bulks are virtually the only use of mercury for which a suitable alternative has not been found. For street lighting, there has been a shift toward the use of sodium vapor lamps, since such bulbs present a lower toxicity hazard and are even more efficient light sources than mercury bulbs.

Mercury is the most volatile of metals, and its vapor is highly toxic. Adequate ventilation is required whenever mercury is used in closed quarters, since the equilibrium vapor pressure of mercury is hundreds of times the maximum recommended exposure. Mercury vapor consists of free, neutral atoms. If inhaled, the atoms diffuse from the lungs into the bloodstream; then, because they are electrically neutral, they readily cross the blood-brain barrier to enter the brain. The result is serious damage to the central nervous system, manifested by difficulties with coordination, eyesight, and tactile senses. Liquid mercury itself is not highly toxic, and most of what is ingested is excreted. Nevertheless, children should not be allowed to play with droplets of the metal because of the danger from breathing the vapor.

The latter part of the twentieth century saw a substantial decline in anthropogenic emissions of mercury into the water and land environments from many sources in developed countries, resulting from governmental attempts to reduce its uses and emissions. Emissions of mercury from large industrial operations in developed countries have been successfully curtailed. The overall use of mercury has decreased in the United States by more than 95% in the last three decades. For example, mercury has been eliminated from batteries. In the United States, the reduction of mercury emissions arising from disposal of mercury-containing products has resulted mainly from

• emission controls on municipal and medical waste incinerators, and

• removal of batteries and paint from the waste stream.

In Canada, emission reductions have come from controls on metal smelting, the near-complete closure of the chlor-alkali industry (which had used mercury electrodes), and controls on waste incineration.

Over the 1990s, the global concentration of airborne mercury increased by about 1.5% per year, notwithstanding the decline in industrial emissions. Large amounts of mercury vapor are released into the air as a result of the unregulated burning of coal and fuel oil, both of which always contain trace amounts of the element, and of incinerating municipal waste that contains mercury in products such as batteries. Currently, coal-fired power plants and municipal and medical waste incinerators are the biggest sources of mercury emissions to the atmosphere in North America. The vaporized mercury is eventually oxidized and returns in rain, often falling far from the site of the original emissions. The issue of Mercury Emissions from Power Plants is discussed in detail in the online Case Study associated with this chapter.

In air, the great majority of elemental mercury is in the vapor (gaseous) state, with only a fraction of it bound to airborne particles. Airborne gaseous mercury can travel long distances before being oxidized and then dissolving in rain and subsequently being deposited on land or in water bodies. This global cycling of mercury results in its being distributed even to remote parts of the planet.

U.S. Emission Controls on Power Plants

The 1100 coal-burning power plants in the United States are its last unregulated major emitters of mercury into the environment. As a result of legal pressure from environmental groups, by court order the U.S. EPA was forced in 2005 to issue regulations that will reduce these emissions. The reductions are scheduled to occur in two phases:

• By 2010, total mercury emissions are to be reduced from the current 48 tons a year to 38 tons. This will be accomplished as a "co-benefit" of the new reductions in sulfur dioxide and nitrogen oxide emissions from the plants that must be implemented by that date, as discussed in Chapters 3 and 4.

• By 2018, annual mercury emissions must be reduced to 15 tons.

Each state has been assigned an emissions budget and is required to develop a plan for meeting its assigned emission reduction. States and power plants can trade reductions among themselves, as long as overall goals are achieved. Critics of the new program point out that averaging of reductions between years is allowed, so that higher emissions could occur in later years if greater-than-regulated reductions occur early. They also point out that "hot spots" of mercury deposition may be perpetuated because of the trading provisions and that the overall reduction in emissions will be quite small for many years to come.

Mercury Amalgams

Mercury readily forms amalgams, which are solutions or alloys with almost any other metal or combination of metals. The dental amalgam that has been used to fill cavities in teeth for more than 150 years initially has a putty-like consistency. It is prepared by combining approximately equal proportions of liquid mercury and a solid mixture that is mainly silver with variable amounts of copper, tin, and zinc. The slight expansion of volume that accompanies its solidification ensures that the final amalgam fills the cavity. When a filling is first placed in a tooth, and whenever the filling is involved in the chewing of food, a tiny amount of the mercury is vaporized. Some scientists believe that mercury exposure from this source causes long-term health problems in some individuals, but an expert panel of the U.S. National Institutes of Health concluded that dental amalgams do not pose a health risk. A recent study of adults found that no measure of exposure to dental mercury—neither the concentration of the element in the urine nor the number of dental fillings—correlated with any measure of mental functioning or fine-motor control; another study found that no 1Q or other neurological problems correlated with the use of amalgams for filling teeth in children.

Some countries in Europe such as Germany have banned the use of mercury in fillings, at least for pregnant women and small children. Mercury-free "amalgams" for use in dentistry are under development; porcelain fillings are already common, though expensive. Some fears have been expressed about the release of elemental mercury vapor into the atmosphere when cremating deceased persons who had amalgam-filled teeth, since the amalgam decomposes at high temperatures. In countries such as Sweden, crematoria are fitted with selenium filters that remove most mercury from emissions by forming mercury selenide crystals.

In some areas, dentists are now required to install a separator to capture mercury from their wastewater rather than have it flow down drains and become part of municipal sewage. On average, each dentist produces about one kilogram of mercury waste per year. Dentists collectively release about the same amount of the metal as is emitted by coal-fired power plants. Because of the control of emissions by dentists, the sewage sludge used by farmers as fertilizer (Chapter 16) will have a much lower concentration of mercury in the future.

In working some ore deposits, tiny amounts of elemental gold or silver are extracted from much larger amounts of the denser particles of soil or sediment by adding elemental liquid mercury to the mixture. The mercury extracts the gold or silver by forming an amalgam, which is then roasted to distill off the mercury. From 1570 until about 1900, this process was used to extract silver from ores in Central and South America. About one gram of mercury was lost to the environment for every gram of silver so produced, resulting in the release of almost 200,000 tonnes of mercury. The mercury was shipped to these regions from Almaden, Spain, and from Peru. Until recently, the corresponding process of extracting gold by amalgamation with mercury was carried out in China in both large-scale and small-scale operations. The ratio of mercury to gold in such workshops, some of which continue illegally today in remote regions, averaged 15 to 1.

Today, the gold extraction procedure using mercury is carried out on a large scale in Brazil to obtain gold from muddy sediments; it results in substantial mercury pollution both in the air and, because of careless handling practices, in the Amazon River itself. The health hazards to those whose work involves the vaporization of mercury are significant, since the element is so toxic in its gaseous form. Indeed, mercury vaporized from such operations currently makes up more than 10% of the anthrogogenic emissions of mercury in air. People who live in mining regions often inhale air in which the concentration of elemental mercury exceeds 50 /xg/m5, which is 50 times the public exposure guideline of the World Health Organization (WHO). As a consequence, many "amalgam burner" workers exhibit tremors and other signs of mercury poisoning. In addition, mercury in surface sediments disturbed by slash-and-burn deforestation and agriculture in the region enters the aquatic environment, where some of it enters the food chain. The European Union has undertaken initiatives to incorporate inexpensive technology into the process to prevent the massive release of mercury into the air and to the Amazon river during the extraction of gold.

Mercury and the Industrial Chlor-Alkali Process

An amalgam of sodium and mercury is used in some industrial chlor-alkali plants in the process that converts aqueous sodium chloride into the commercial products chlorine, Cl2, and sodium hydroxide, NaOH, (and gaseous hydrogen) by electrolysis. In order to form a concentrated, pure solution of NaOH, flowing mercury is used as the negative electrode (cathode) of the electrochemical cell. The metallic sodium that is produced by reduction in the electrolysis immediately combines with the mercury and is removed from the NaCl solution without having reacted with the aqueous medium:

When metals such as sodium are dissolved in amalgams, their reactivity is greatly lessened compared to that for the free state, so that the otherwise highly reactive elemental sodium in the Na-Hg amalgam does not react with the water in the original solution. Instead, the amalgam is removed and later induced by the application of a small electrical current to react with water in a separate chamber, thereby producing sodium hydroxide that is free of salt.

The mercury is recovered after NaOH production and is recycled back to the original cell. The recycling of mercury is not complete, however, and some finds its way into the air and into the water body from which the plant's cooling water is obtained and to which it is returned. Although liquid mercury is not soluble in water or in dilute acid, it can be oxidized to soluble form by the intervention of bacteria that are present in natural waters. By this means, the mercury becomes accessible to fish.

The mass of mercury lost to the environment from the average chloralkali plant has decreased enormously since the problem was identified in the 1960s. Nevertheless, installations in North America that use mercury electrodes have largely been phased out. They were replaced by ones that use a fluorocarbon membrane that separates the NaCl solution from the chloride-free solution at the negative electrode. The membrane is designed such that Na+, but not anions, can pass through it. In both types of cells, the overall reaction is

2 NaCl(aq) + HzO(l)-*• 2 NaOH(aq) + Cl2(g) + Hz(g)

The 2+ Ion of Mercury

Like its partners zinc and cadmium in the same subgroup of the periodic table, the common ion of mercury is the 2+ species, Hg2+, the mercuric or mercury(II) ion. An example of a compound containing the mercuric ion is the red ore cinnabar, HgS, i.e., Hg2+S2~. Like most sulfides, this salt is very insoluble in water; indeed, the wastewater at chlor-alkali plants is sometimes treated by adding a soluble salt such as Na2S that contains the sulfide ion, since this action precipitates ionic mercury as HgS.

PROBLEM 15-2

The solubility product, Ksp, for HgS is 3.0 X 10 53. Calculate the solubility of HgS in water in moles per liter and transform your answer into the number of mercuric ions per liter. According to this calculation, what volume of water in equilibrium with solid HgS contains a single Hg2+ ion?

Most of the mercury in the environment is inorganic, in the form of the Hg2+ ion. The levels of ionic mercury even in remote areas are two to five times as great as preindustrial values, with local polluted sites having levels 10 or more times greater. In natural waters, much of the Hg2+ is attached to suspended particulates, so it is eventually deposited in sediments—a topic considered in further detail when soil and sediment chemistry is discussed (Chapter 16).

The nitrate salt of Hg2+ is water soluble and was at one time used to treat the fur used to make felt for hats. The fur was immersed in a hot solution of mercuric nitrate, which made the fibers rough and twisted so they would then mat together easily. As a consequence of this constant exposure to mercury, workers in the felt trade often displayed nervous disorders: muscle tremors, depression, memory loss, paralysis, and insanity (giving rise to the expression "mad as a hatter," a concept familiar to fans of Lewis Carroll's Alice in Wonderland). Mercury vapor and, to a lesser extent, mercury salts attack the central nervous system, but the main target organs for Hg2+ are the kidney and the liver, where it can cause extensive damage.

Mercuric oxide, HgO, is present in a paste in mercury cell batteries such as those used in hearing aids. If the discarded spent batteries are subsequently incinerated as garbage, the volatile mercury can be released into the air. The amount of mercury used in ordinary flashlight batteries, added as a minor constituent in the zinc electrode to prevent its corrosion and thereby extend the shelf life of the product, was first drastically curtailed—typically from about 10,000 ppm to about 300 ppm in alkaline batteries—and in many cases has been completely eliminated, thereby halving the mercury in domestic garbage. In North America, only some "button batteries" used in watches, calculators, hearing aids, etc. still have significant mercury content.

The other inorganic ion of mercury, Hg22+, is not very toxic since it combines in the stomach with chloride ion to produce insoluble Hg2Cl2.

Methylmercury Toxicity

When in combination with anions that are capable of forming covalent bonds, the mercuric ion Hg2+ forms covalent molecules rather than an ionic solid. For example, HgCl2 is a molecular compound, not a salt of Hg~+ and Cl~. As chloride ion forms a covalent compound with Hg2+, so does the methyl anion, CH? , yielding the volatile molecular liquid dimethylmercury, Hg(CH3)2. The process of dimethylmercury formation occurs in the muddy sediments of rivers and lakes, especially under anaerobic conditions, when anaerobic bacteria and microorganisms convert Hg2+ into Hg{CH})2. The active agent in the biomethylation process is a common constituent of microorganisms; it is a derivative of vitamin BJ2 with a CH3 anion bound to cobalt and is called methylcohalamin.

The less volatile mixed compounds CH3HgCl and CH3HgOH, collectively called methylmercury (or monomethylmercury), are often written as CH3HgX, or somewhat misleadingly as CH3Hg+X , since these substances, like most of those written as Hg2+, consist of covalent molecules, not ionic lattices. In fact, the methylmercury ion CH3Hg^ exists as such only in compounds with anions such as nitrate or sulfate. Methylmercury compounds are even more readily formed in the same way as dimethylmercury at the surface of sediments in anaerobic water. Methylmercury production predominates over dimethylmercury formation in acidic or neutral aqueous systems. Sulfate ion, S042-, stimulates the sulfate-reducing bacteria that methylate mercury; in contrast, the presence of sulfide ion results in formation of mercury sulfide complexes that do not undergo methylation.

Due to its volatility, dimethylmercury evaporates from water relatively quickly unless it is transformed by acidic conditions into the monomethyl form. The pathways for the production and fate of dimethylmercury and other mercury species in a body of water are illustrated in Figure 15-1. Methylation of inorganic mercury does occur in anaerobic regions of lakes, especially near the interface of the epilimnion and the hypolimnion, and at the interface of the latter with sediments, but not in aerobic water. Organic-rich sediments at the bottom of warm, shallow lakes are important sites of methylmercury production. Wetlands are also active sites of methylmercury production. Methylmercury in surface water is photodegraded (to as yet unknown products) and is the most important sink for this substance in some lakes.

I Air Hg° -> Hg(Il) CH3HgCH3 |

I Air Hg° -> Hg(Il) CH3HgCH3 |

Environment Conversion Low Drowning

FIGURE 15-1 The cycling of mercury in fresh-water lakes. [Source: Adapted from M. R. Winfrey and J. W. M. Rudd, "Environmental Factors Affecting the Formation of Methylmercury in Low pH Lakes/' Environmental Toxicology and Chemistry 9 (1990): 853-869.J

Mercuric ion itself is not readily directly transported across biological membranes. Methylmercury is a more potent toxin than are salts of Hg2+ because it is soluble in fatty tissue in animals, bioaccumulates and biomagni-fies there, and is more mobile. Once ingested, the original covalent CH3HgX compound is converted to substances in which X is a sulfur-containing amino acid; in some of these forms, it is soluble in biological tissue and can cross both the blood-brain barrier and the human placental barrier, presenting a twofold hazard. Methylmercury is, in fact, the most hazardous form of mercury, followed by the vapor of the element. The main toxicity of methylmercury occurs in the central nervous system. In the brain, methylmercury is converted to Hg2+, which is probably responsible for the brain damage. Mercury vapor is also oxidized to this ion once it has entered the cell. Thus the usual barriers in the cell to Hg2+ are circumvented by Hg° and CH3HgX, which by their electric neutrality can penetrate through the defenses and which later can be converted to the highly toxic +2 ionic form.

Most of the mercury present in humans is in the form of methylmercury. Almost all methylmercury originates from the fish in our food supply: Mercury in fish is usually at least 80% methylmercury. Mercury contamination is the reason behind about 97% of the advisories against eating fish caught in various regions of North America. In contrast to organochlorines, which predominate in the fatty portions of fish, methylmercury can bind to the sulfhydryl group in proteins and so is distributed throughout the fish. Consequently, the mercury-containing part cannot be cut away before the fish is eaten.

Fish absorb methylmercury that is dissolved in water as it passes across their gills (bioconcentration); they also absorb it from their food supply

(biomagnification). The ratio between methylmercury in fish muscle and that dissolved in the water in which the fish swims is often about 1 million to 1, and can exceed 10 million to 1. The highest methylmercury concentrations (over 1 ppm) are usually found in large, long-lived predatory marine species such as shark, king mackerel, tilefish, swordfish, and large tuna (sold as steaks and sushi), as well as in fresh-water species such as bass, trout, and pike. Indeed, the U.S. Food and Drug Administration warns women of childbearing age not to eat the first four types of marine fish in the list above. On average, the older the fish, the more methylmercury it will have bioaccumulated. Noncarnivorous species such as whitefish do not accumulate very much mercury since hiomagnification in their food chain operates to a much lesser extent than in carnivorous fish. On average, most Americans take in almost half their methylmercury from tuna (mostly of the canned variety), followed by swordfish, pollock, shrimp, and cod. Canada has recently limited the maximum concentration of mercury in six species of ocean fish to one part per million; this concentration is not uncommon in swordfish, although most fish have levels of 0.10-0.15 ppm. The U.S. EPA has set a criterion of 0.3 ppm maximum for methylmercury in fish tissue.

In lakes, the mercury content in fish is generally greater in acidic water, probably because both the solubility of mercury is greater and the methyla-tion of mercury is faster at lower pH. In this way, the acidification of natural waters indirectly increases the exposure of fish-eaters to methylmercury. The relationship between mercury levels in small fish and the pi 1 of the water is illustrated in Figure 15-2. The data in this figure are from a collection of lakes in Wisconsin, eastern Ontario, and Nova Scotia; most of the acidic lakes are in Nova Scotia. ;

FIGURE 15-2 The relationship between pH and mercury concentration in a fish for a standard fish length for 48 lakes from Ontario, Nova Scotia, and Wisconsin. [Source: D. tean, "Mercury Pollution," Canadian Chemical News (January 2003): 23.1

FIGURE 15-2 The relationship between pH and mercury concentration in a fish for a standard fish length for 48 lakes from Ontario, Nova Scotia, and Wisconsin. [Source: D. tean, "Mercury Pollution," Canadian Chemical News (January 2003): 23.1

Mehg Solubility Kinetic

Methylmercury Accumulation in the Environment and in the Human Body

The half-life of methylmercury compounds in humans, about 70 days, is much longer than that for Hg"'_ salts, due in part to the compounds' greater solubility in a lipid environment. Consequently, methylmercury can accumulate in the body to a much higher steady-state concentration, even if on a daily basis a person consumes amounts that individually would not be harmful.

PROBLEM 15-3

If the half-life of methylmercury in the human body is 70 days, what is its steady-state accumulation in a person who consumes daily 1.0 kg of fish containing 0.5 ppm methylmercury? [Hint: Recall the discussion of steady-state concentrations in Chapter 6 J

Most of the well-publicized environmental problems involving mercury have arisen from the fact that in the methylated form it is a cumulative poison. However, in high enough concentration it can be acutely fatal. In 1997, cancer researcher Karen Wetterhahn of Dartmouth College died from mercury poisoning several months after a drop or two of pure dimethylmercury apparently seeped through latex gloves she was wearing while using the compound in experiments. Dialkylmercury compounds, including dimethylmercury, are sometimes called supertoxic because they are lethal even in small amounts.

At the fishing village of Minamata, Japan, a chemical plant employing Hg"+ as a catalyst in a process that produced polyvinyl chloride discharged mercury-containing residues into Minamata Bay. The methylmercury compounds, mainly CH3Hg—SCH3, that subsequently formed from the inorganic mercury by biomethylation by microorganisms in the bay's sediments then bioaccumulated. Concentrations were as high as 100 ppm in the fish, which were the main component of the diet for many local residents, (By way of contrast, the current U.S. recommended limit for methylmercury in fish to be consumed by humans is 0.3 ppm.) Thousands of people in Minamata were affected in the 1950s by mercury poisoning from this source, and hundreds of them died from it. Because the onset of symptoms in humans is delayed, the first signs of Minamata disease were observed in cats who ate discarded fish: They began jumping around and twitching, ran in circles, and finally threw themselves into the water and drowned. Symptoms in humans arise from dysfunctions of the central nervous system, since the target organ for methylmercury is the brain; they include numbness in arms and legs, blurring and even loss of vision, loss of hearing and muscle coordination, and lethargy and irritability.

Since methylmercury can be passed to the fetus, children born to Minamata mothers poisoned even slightly by mercury showed severe brain damage, some to a fatal extent. The infants showed symptoms similar to those of cerebral

palsy; mental retardation, seizures, motor disturbance, and even paralysis. Just as in the case of high PCB levels, discussed in Chapter 11, the developing fetuses were much more affected by methylmercury than were the mothers themselves. The poisonings at Minamata must surely rank as one of the major environmental disasters of modern times.

Other Sources of Methylmercury

Organic compounds of mercury have been used as fungicides in agriculture and in industry and enter the environment as a side effect of these applications. However, as a result of contact with soil, the compounds are eventually broken down and the mercury becomes trapped as insoluble compounds by attachment to sulfur ions present in clays and organic matter.

Hundreds of deaths in Iraq in 1956, 1960, and 1972, and a few in China and the United States, resulted from the consumption of bread made from seed grain (intended for planting) that had been treated with mercury-based fungicides to reduce seedling losses from fungus attack. The fungicides contained compounds of ethylmercurv, CHjC^Hg^ the toxicity of which is presumed to be similar to thar of methylmercury. In Sweden and Canada, the use of mercury compounds to treat seeds led to a significant reduction in the number of birds of prey that consumed the smaller birds and mammals that fed on the scattered seed. The use of ethylmercury products in agriculture has now been curtailed in North America and western Europe.

Mercury is leached from rocks and soil into water systems by natural processes, some of which are accelerated by human activities. Flooding of vegetated areas can release mercury into water. For example, after the flooding of huge areas of northern Quebec and Manitoba in constructing hydroelectric power dams, the newly submerged surface soils (and to a lesser extent the vegetation) released a considerable quantity of soluble methylmercury, formed from the "natural" mercury content of these media. The additional methylmercury resulted from contact of soil-bound Hg2+ with anaerobic bacteria produced by the decomposition of the immersed organic matter. In this way, previously insoluble inorganic mercury was converted to methylmercury, which readily dissolved in the water. The methylmercury subsequently entered the food chain through its absorption by fish, and native persons who ate fish from these flooded areas now have substantially elevated levels of mercury in their bodies. Indeed, the methylmercury concentration in fish from these areas, 5 ppm or more, approaches that previously associated only with regions of industrial mercury pollution.

In 1999, the safety of using a mercury-based preservative to prevent microbial growth in many vaccine preparations administered to infants was questioned by the American Academy of Pediatrics and the U.S. Public Health Service. The preservative, Thimerosal, is CH3CH2—Hg—S—C6H4COOH; it is sometimes said to contain the ethylmercury ion, but presumably, like most methylmercury systems, it is actually a covalent compound. This substance has now been removed by its manufacturer in all vaccines destined for use in young children in the United States, with the exception of inactivated influenza vaccine. The preservative was also widely used as a topical disinfectant.

The Use of Mercury in Preservatives and as Medications

Compounds of the phenylmercury ion, CgH^Hg"1", with acetate or nitrate as the anion, have been used to preserve paint while in the can and to prevent mildew after application of latex paint, particularly in humid areas. The phenylmercury salts are not as toxic to humans as are methylmercury compounds, since they break down quickly into compounds of the less toxic Hg"+. However, mercury compounds have been banned from indoor latex paints in North America for more than a decade because some ingestion of the element from this source is inevitable. Phenylmercury compounds were also formerly used as slimicides in the pulp-and-paper industry in order to prevent the growth of slime on wet pulp; because this practice has now been curtailed and because mercury-containing wastes are now usually treated, mercury releases from such sources have greatly decreased.

Because of their antiseptic and preservative qualities, however, some mercury compounds are still used in pharmaceuticals (especially topical antiseptics) and cosmetics. Elemental mercury was also used in some pharmaceuticals in the old days. Indeed, the antidepressant pills that Abraham Lincoln took, mainly in the years before becoming president, contained the element; indeed, some medical historians think the leaching of mercury into his bloodstream can account for his often bizarre behavior in that period. The mercury ore cinnabar is Still used today in China, as a drug, pigment, and preservative.

PROBLEM 15-4

A quantity of a mercury-chlorine compound is included in a shipment of waste to a toxic waste disposal dump. Before it can be disposed of properly, the owners of the dump need to know whether it is HgCl2, or Hg2Cl2, or some other compound. They send a sample of it for analysis and find that it contains 26.1% chlorine by mass. What is the empirical formula of the compound?

Safe Level of Mercury in the Body

It is somewhat reassuring that both the direct effects of methylmercury on humans and the prenatal effects probably havg thresholds below which no effects are observed. Currently the daily methylmercury intake of 99.9% of Americans lies below the WHO's "safe limit." Nonetheless, some effects of methylmercury consumption on human vision are observed even when the concentration of (total) mercury in hair lies below the generally recognized threshold of 50 ppm.

However, if prenatal health is the main consideration and if a safety factor of 10 is applied, a substantial fraction of the population of the United States would exceed the safe limit. The WHO has concluded that levels of 10-20 ppm of methylmercury in hair indicate that a pregnant woman has sufficient methylmercury in her blood to represent a threat to a developing fetus. This places at risk the developing fetus of more than 30% of the women in some native communities in northern Canada, for example, in which fish play a large part in the diet. Although it is clear that high levels of methylmercury can result in developmental disabilities, there is continuing controversy over whether methylmercury acquired through a diet high in fish and marine mammals can cause significant neurological damage to an adult or a developing fetus. The epidemiological studies of this question have, to this point, produced inconsistent results.

PROBLEM 15-5

What is the mass, in milligrams, of mercury in a 1.00-kg lake trout which just meets the Northern American standard of 0.50 ppm Hg? What mass of fish, each at the 0.50-ppm Hg level, would you have to eat in order to ingest a total of 100 mg of mercury?

PROBLEM 15-6

The new U.S. EPA oral reference dose for methylmercury is 0.1 micrograms per kilogram body weight per day. What mass offish can a 60-kg woman safely eat each week if the average methylmercury level in the fish is 0.30 ppm? Approximately how many average servings offish does this correspond to?

International Controls on Mercury

Although atmospheric emissions now dominate mercury concerns in most countries, especially developed ones, other sources also contribute significantly elsewhere. Mercury is still used extensively in the extraction of gold and silver, as well as in the production of chlorine in chlor-alkali plants, in developing countries.

In 2005 the United Nations Environment Programme considered devising a global treaty to curb the production of mercury and to ban completely the export of mercury between countries. However, the United States led a movement, which ultimately was successful, that instead proposed voluntary partnerships between countries to improve their management of mercury.

Lead

Although the environmental concentration of lead, Pb, is still increasing in some parts of the world, the uses that result in its uncontrolled dispersion have been greatly reduced in the last few decades in many developed countries. Consequently, its concentration in soil, water, and air has decreased substantially.

Lead's relatively low melting point of 327°C allows it to be readily worked—it was the first metal to be extracted from its ores—and shaped. Lead was used as a structural metal in ancient times as well as for weather-proofing buildings, in water pipes and ducts, and for cooking vessels. Lead is still used for roofing and flashing and for soundproofing in buildings. When combined with tin, it forms solder, the low-melting alloy used in electronics and in other applications (e.g., tin cans) to connect solid metals.

Analysis of ice-core samples from Greenland indicates that atmospheric lead concentration reached a peak in Roman times that was not equaled again until the Renaissance. The history of lead s presence in the environment can be seen in Figure 15-3, in which the ratio of two stable lead isotopes

Mid-Holocene, pre-anthropogenic background

Early Holocene, pre-anthropogenic background

1.12

1.20

1.12

1.20

Introduction of unleaded gasoline Introduction of leaded gasoline industrial Revolution/ Australian Pb imports German silver mining of Pb ores

Post-Roman decline in European Pb mining

Roman Pb mining

Pb mining begins (Phoenicians, Greeks)

Soil dust flux increases (deforestation, agriculture)

Mid-Holocene, pre-anthropogenic background

Early Holocene, pre-anthropogenic background

FIGURE 15-3 Isotopic composition of lead in a Swiss peat bog and the chronology of atmospheric lead deposition. Notice the change in depth scale at 100 cm. [Source: W. Shotyk et al., "History of Atmospheric Lead Deposition Since MC BP from a Peat Bog, Jura Mountain, Switzerland," Science 281 (1998): 1635, Figure 3B.J

in samples taken from a peat bog in Switzerland is plotted against the depth at which the sample was taken. The layers of the bog were laid down gradually over millennia, and each layer incorporated lead-containing dust particles deposited from the air at the time. Lead originating in different geographic locales has different isotope ratios, so we can tell the origin of atmospheric lead at different times in the past from the graph. The 206Pb/207Pb ratio is close to 1.20 for depths exceeding 145 cm, or 3000 years ago (sediment ages being determined by 14C dating of the peat); the variations in the ratio below that depth reflect changes in the dominance of weathering of soils and rocks in different areas over time. Beginning about 3000 years ago, the ratio fell to 1.18, reflecting the isotopic composition of European lead ores mined during Roman times and thereafter. Previous to that, in Greek times, silver was first mass-produced for use in coins; apparently the substantial amount of lead contaminant in the crude silver escaped into the air during the refining of the metal. In about 1860, the isotope ratio of the lead deposited in Europe began a continual decrease, with the rate of change increasing with the introduction of leaded gasoline in about 1940, probably as a consequence of the extensive use of lead in it, first from Australia (ratio of 1.04) and then also from Canada. Recently the ratio has begun to increase due to the decreased use of lead in European gasoline.

Elemental Lead as an Environmental Risk

Elemental lead is also found in ammunition ("lead shot") used in huge amounts by hunters, especially of waterfowl. Many ducks and geese are injured or die from chronic lead poisoning after ingestion of lead shot, which dissolves in the acidic environment inside them. In addition, ducks consume the pellets left lying on the ground or at the bottom of ponds, since they look like food or grit. Birds (such as bald eagles) sometimes prey on ducks and other waterfowl that have been shot by hunters but not harvested by them, or they sometimes eat lead shot to help grind food in their gizzard; these predators become victims of lead poisoning. For these reasons, lead shot has been banned in the United States, Canada, the Netherlands, Norway, and Denmark. However, in North America, many loons die because they swallow and are subsequently poisoned by lead sinkers and jigs still used in sport fishing.

Lead ammunition in the form of bullets and shotgun shells (used for shooting wild game) also poses an environmental problem. Condors in California suffer from lead poisoning, sometimes fatally, when they eat deer that have been shot and then abandoned by hunters; the lead bullets explode into many fragments on impact and contaminate the meat.

Ionic 2+ Lead in Water and Food as an Environmental Hazard to Humans

Although elemental lead is not 3n environmental problem to most life forms, it does become a real concern when it dissolves to yield an ionic species.

The stable ion of lead is the 2+ species, Pb(ll) as Pb2+. For example, lead forms the ionic lead sulfide, PbS, Pbz+S2~, which is the metal-bearing component of the highly insoluble ore galena, from which almost all lead is extracted.

Lead does not react on its own with dilute acids. Indeed, elemental lead is stable as an electrode in the lead storage battery, even though it is in contact with fairly concentrated sulfuric acid, H2S04. However, some lead in the solder that was commonly used in the past to seal tin cans will dissolve in the dilute acid of fruit juices and other acidic foods if air is present—that is, once the can has been opened—since lead is oxidized by oxygen in acidic environments;

2 Pb(s) + 02 + 4 H+-> 2 Pb2+(aq) + 2 HzO

The Pb2+ produced by this half-reaction contaminates the contents of the can; for this reason, lead solder is not usually used any more for food containers in North America. Partially as a result of this change, the average daily intake of lead for two-year-old children dropped from about 30 fig in 1982 to about 2 fig in 1991.

The 1845 Franklin Expedition to find a Northwest Passage across the Arctic is thought to have failed because the members all died from lead poisoning from the solder in the tin cans that held their food. Canadian writer Margaret Atwood has written eloquently about the incident in her short story "The Age of Lead":

It was the tin cans that did it, a new invention back then, a new technology, the ultimate defence against starvation and scurvy. The Franklin Expedition was excellently provisioned with tin cans, stuffed full of meat and soup and soldered together with lead. The whole expedition got lead poisoning. Nobody knew it. Nobody could taste it. It invaded their bones, their lungs, their brains, weakening them and confusing their thinking, so that at the end those that had not yet died in the ships set out in an idiotic trek across the stony, icy ground, pulling a lifeboat laden down with toothbrushes, soap, handkerchiefs and slippers, useless pieces of junk. When they were found (ten years later, skeletons in tattered coats, lying where they'd collapsed) they were headed back toward the ships. It was what they'd been eating that had killed them.

[Margaret Atwood, "The Age of Lead," in Wilderness Tips, copyright 1991 by O. W. Toad Limited]

The recommended maximum levels for important heavy metal ions in drinking water are summarized in Table 15-2. The limit for lead, 10-15 ppb, is sometimes exceeded in water delivered to the consumer even though it was sufficiently pure when it left the water treatment plant. Lead used in the solder in the joints of domestic copper water pipes, and lead used in previous decades and centuries to construct the pipes themselves, can dissolve in drinking water during its transport to the point of consumption, particularly if the water is quite acidic or particularly soft. This problem of contamination of

IHriBHHflSI

IHriBHHflSI

Metal

U.S. EPA Maximum Contaminant Level (ppb)

Canadian Maximum

Acceptable Concentration (ppb)

World Health Organization Guideline (ppb)

As Cd Cr

Hg (inorganic)

10 5

50 1 10

10 3

50 6 10

water by lead during transit became a controversial issue in 2007 in the hometown (London, Ontario) of one of the authors of this book, with the concern quickly spreading to older homes in other cities in Ontario as well. In general, it is a good idea not to drink water that has been standing overnight in older drinking fountains or in the pipes of older dwellings; water in such plumbing systems should be allowed to run for a minute or so.

The contamination of water by lead is less of a problem in areas of calcareous water, since an insoluble layer containing compounds such as PbC03 forms on the surface of the lead by reaction of the metal with dissolved oxygen and the carbonate ion, C03'^, in the water (Chapter 13). This layer prevents the metal underneath from dissolving in the water that passes over it. In some regions of England and in some cities in the northeast United States that have soft water and networks of old lead pipes, phosphates are added to drinking water in order to form a similar insoluble protective coating of lead phosphate on the inside of lead pipes and so reduce the concentration of dissolved lead.

Lead in water is more fully absorbed by the body than is lead in food. Now that many other sources of lead have been phased out, drinking water accounts for about one-fifth of the collective lead intake of Americans, whose major source is from food. Many domestic water treatment systems successfully remove the great majority of lead from drinking water. Bottled water sold in plastic containers usually has very low levels of lead, averaging 16 ppt in one recent survey, which is not much higher than those in groundwater taken from pristine deep aquifers. Bottled water in glass containers has more lead, up to about 1 ppb, since tiny amounts of the metal are leached from the glass.

PROBLEM 15-7

According to an informal 1992 survey, the drinking water in about one-third of the homes in Chicago had lead levels of about 10 ppb. Assuming that an adult drinks about 2 L of water a day, calculate the total lead that residents of these Chicago homes obtain daily from their drinking water.

Continue reading here: Lead Salts as Glazes and Pigments

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  • Lennox
    What is steady state accumulation of a person who consumes daily one kilogramme of fish contain 9ppt?
    7 years ago