Organotin Compounds

Although inorganic compounds of tin (Sn) are relatively nontoxic, the bonding of one or more carbon chains to the metal results in substances that are toxic. Such organotin compounds have some common uses, such as additives to stabilize PVC plastics and fungicides to preserve wood, and therefore are of environmental concern.

Tin forms a series of compounds of general formula R3SnX, which are molecular substances though often shown in formulas as if they were ionic, e.g., (R3Sn+) (X~), where R a hydrocarbon group and X is a monatom: anion; corresponding compounds such as (R3Sn)zO also occur. All these compounds are toxic to mammals when R is a very short alkyl chain; maximum toxicity occurs when R is the ethyl group, C2H5, and decreases progressively with increasing chain length.

For fungi the greatest toxic activity is attained when each hydrocarbon chain has four carbons in an unbranched chain, i.e., when R is the n-butyl group,—CH2CH2CH2CH3 (or simply n-QjHg). T'ributykin oxide, (R3Sn)20 where R = n-C4H9, and the corresponding fluoride have both been used as fungicides; commonly they are incorporated as antifouling agents in the paint applied to docks, to the hulls of boats, to lobster pots, and to fishing nets, etc. to prevent the accumulation of slimy marine organisms such as the larvae of barnacles. In recent years tributyltin has been incorporated into polymeric coatings for boat hulls; a thin layer of the compound subsequently forms around the hull. The tin compounds replaced copper(I)

oxide, Cu20, in such applications since their effectiveness lasts longer than a single season.

Unfortunately, some of the tributyltin compound leaches into the surface waters in contact with the coatings or paint, particularly in harbors where the boats are moored, and subsequently enters the food chain via the microorganisms that live near the surface. This can lead to sterility or death for fish and some types of oysters and clams that feed on these microorganisms. Some countries have restricted the use of tributyltin compounds to large ships. Thus, although the concentration of tributyltin has decreased in the waters of small harbors and marinas, the pollutant still tends to concentrate in marine coastal regions due to its use on large vessels. Scientists are worried that the presence of tributyltin compounds in these waters could affect fish reproduction.

For this reason, the International Maritime Organization banned new applications of tributyltin to ships of any size effective 2003 and required that this material be removed from all old applications by 2008. Ironically, the triazine herbicide added to copper-based antifoulant paints that were introduced to replace those based upon tributyltin degrades only slowly in water and has now begun to accumulate there.

Higher organisms have enzymes that break down tributyltin fairly rapidly, so it is not very toxic to humans. However, most humans now have detectable levels of tributyltin in their blood.

Since the 1970s, arsenic has been used in the form of the compound chro-mated copper arsenate, CCA, to pressure-treat lumber in order to prevent rot and termite damage. Unfortunately, some of the arsenic leaches out of the wood over time. U.S. and Canadian producers of CCA-treated wood voluntarily phased out use of the arsenic compound at the end of 2003 for wood destined for residential structures such as decks, picnic tables, fences, and playground equipment. CCA is discussed in further detail in the section on chromium. The U.S. EPA has already banned arsenic in all other pesticides.

Arsenic in Drinking Water

Arsenic—much of it from natural sources—is one of the most serious environmental health hazards. The presence of significant levels of arsenic in drinking-water supplies is a significant and controversial environmental issue. Natural levels of arsenic in water can be quite high, and it is more common for health problems to arise from this source than from anthropogenic arsenic. Although arsenic has been used for millennia as a poison, the major health problem stemming from its presence at low levels in drinking water is cancer. Drinking arsenic-contaminated water has also been linked to diabetes and cardiovascular disease, perhaps by disrupting a hormonal process associated with both conditions.

Arsenic is carcinogenic in humans. Lung cancer results from the inhalation of arsenic and probably also from its ingestion. Cancers of the lung, bladder, and skin, and perhaps also of the kidney, arise from ingested arsenic, including that in water. The mechanism by which arsenic causes cancer is not clear. Evidence suggests that it acts as a cocarcinogen, inhibiting the DNA repair mechanism and thereby enhancing the cancer-causing abilities of other carcinogens. There is evidence from Chile that smoking and simultaneous exposure to high levels of arsenic in drinking water act synergistically in causing lung cancer; i.e., their effect taken together is greater than the sum of their individual effects if each acted independently, as discussed in Chapter 4. Other data from Chile show that exposure to arsenic during early childhood or even in utero increases subsequent mortality in young adulthood from both malignant and nonmalignant lung diseases. Indeed, arsenic seems to act synergistically with several cofactors—i.e., factors whose presence negatively affects the health of an individual to an extent greater than if it or the arsenic operated independently. Exposure to excessive levels of UV from sunlight and a lack of selenium in the diet (stemming from malnutrition and/or low selenium levels in local foods) are other cofactors with arsenic. The protective effect of selenium in reducing the amount of active arsenic in the body may arise from the formation of a biomolecule containing an As = Se bond. Research is under way to determine whether selenium supplementation of the diet would be effective in countering the negative health effects of excess arsenic in the drinking water of Bangladesh and the Bengal region of India.

Drinking water, especially that derived from groundwater, is a major source of arsenic for many people. Although anthropogenic uses of arsenic can result in its contamination of water, by far the greatest problems occur with that produced by natural processes. Groundwater in several parts of the world is highly contaminated with inorganic arsenic. Unfortunately, the arsenic is tasteless, odorless, and invisible, so its presence is not easily detected.

Major problems from high arsenic levels occur in the Bengal Delta, with the result that tens of millions of people in Bangladesh and in the West Bengal region of India drink arsenic-laced water. The World Health Organization has called this the "largest mass poisoning of a population in history." The problem arose from the creation of tens of millions of tube wells, which mine groundwater that was previously inaccessible. The concrete tube wells extend 20 m (60 ft) or more into the ground. Ironically, the wells were constructed by UNICEF in the 1970s and early 1980s in an otherwise highly successful project to eliminate epidemics of diarrhea, cholera, and other waterborne diseases and to reduce the high child-mortality rate caused by use of microbially unsafe water drawn from streams, ponds, and shallow wells used in the past. About half the tube wells—affecting about 50 million people in Bangladesh— produce water with arsenic levels as high as 500-1000 ppb, greatly exceeding the 10 ppb WHO guideline for drinking water (Table 15-2). The sediments through which the groundwater travels contain the arsenic. Generally, the deeper the well beyond about 20 m, the lower the concentration of arsenic.

Several million people living in the Bengal Delta region will probably contract skin disorders from drinking arsenic-laced groundwater if remedial action is not taken; a fraction of them will also suffer from the more serious ailment of arsenicosis, which can cause cancer of the skin, bladder, kidneys, and lungs. Skin lesions appear after 5-15 years of exposure to high levels of arsenic in drinking water. A large number of residents of West Bengal, India, have already developed such lesions—the usual outward sign of chronic exposure to arsenic—that may develop into skin cancer because they consumed arsenic-laced groundwater from underground wells. The main cause of arsenic-related deaths among these people is lung cancer. It has also been established that rice and vegetables grown in Bangladesh using irrigation water from tube wells are also contaminated by arsenic, and this may be the dominant source of the element for some people. Grains and beans absorb additional arsenic from the water they absorb when they are cooked.

Recent research from Bangladesh indicates that increasing levels of arsenic and/or of manganese in drinking water confer progressively more and more negative effects on the intellectual levels of six- and ten-year-old children. The Mn levels in one such study averaged 1.4 ppb, compared to the WHO standard of 0.5 ppb. Elevated manganese levels are also present even in the United States: Approximately 6% of domestic wells exceed the U.S. EPA lifetime health advisory concentration of 0.3 ppb Mn in drinking water.

The origin of the dissolved arsenic in the water in Bangladesh and India is controversial. Normally the element, as arsenate ion, is coprecipitated with and adsorbed on the surface of iron oxides in the soil, as would have occurred in ancient times when sediments were being laid down. However, the arsenic, along with the iron, dissolves when insoluble Fe(III) is reduced by natural organic carbon to the more soluble Fe(II) state. Indeed, the higher the concentration of dissolved iron, the higher the arsenic concentration found in the water. The controversy centers around whether the dominant process is the natural one, by which buried peat acts as the reducing agent and has been doing so for millennia, or whether the release has been greatly accelerated in recent years as an indirect effect of annually lowering the water table by extracting massive amounts of water for crop irrigation. In the latter mechanism, the subsequent recharge of the depleted aquifer transports carbon in the water drawn down from the surface, resulting in further reduction of iron oxides and solubilization of the arsenic. Reduction of arsenic from As(V), as it exists when adsorbed to the iron mineral, to the more soluble As(lII) form is also believed to be a factor in solubilizing the element. The water obtained from adjacent wells separated even by only tens of meters from each other can differ enormously in arsenic content, apparently as a result of being drawn from sediments initially laid down in ancient times by different streams that had different sources of organic carbon being deposited simultaneously. Widespread testing in 1999 of tube wells in Bangladesh identified those delivering high arsenic levels, and the handles on such wells were painted red to warn people of the danger. Thousands of larger, deeper wells that draw water from less contaminated aquifers have subsequently been installed as centralized facilities in many villages.

Arsenic-contaminated drinking water is also a major problem in Chile, Argentina, Mexico, Nepal, Taiwan, Cambodia, Vietnam, and large areas of China. Indeed, 8% of the deaths of Chilean adults over 30 are attributable to arsenic poisoning. In a study of residents of Taiwan who were exposed to high levels of the element in their well water, a relationship between arsenic exposure and skin cancer incidence has been established. As in Bangladesh, arsenic only became a problem when people began to drink groundwater, which was touted as being purer than surface water, since the latter is often contaminated by sewage.

Drinking-Water Standards for Arsenic

Drinking water, especially groundwater, is a major source of arsenic for most people. The global average inorganic arsenic content of drinking water is about 2.5 ppb. The World Health Organization has set 10 ppb as the acceptable limit for arsenic in drinking water, and the European Union adopted this standard in 2003 (Table 15-2). The standard in many developing countries is still 50 ppb, which is no longer considered to be protective.

In the last days (2000) of the Clinton administration, the maximum contaminant level for arsenic in U.S. drinking water was lowered from 50 ppb to 10 ppb. Although the Bush administration at first withdrew this regulation, it later concluded that the reduction was warranted. As a result, the 10-ppb limit became law in February 2002, and the compliance date was set for 2006.

The shape of the dose-response curve (see Chapter 10) at such low concentrations of arsenic is unknown. Assuming that no threshold exists, linear extrapolations of human cancer incidence from populations that were exposed to high levels of arsenic leads to the conclusion that there is a l-in-1000 lifetime risk of dying from cancer induced by normal background levels of arsenic. This estimate makes arsenic almost equivalent to environmental tobacco smoke and radon exposure as an environmental carcinogen. Drinking water over a lifetime at the 50-ppb level, the old U.S. standard, would cause bladder or lung cancer in about 1% of the population, a much greater risk than continuously consuming any other water-based contaminant at its MCL. Some environmentalists argue that the arsenic standard should be lowered still further, to 3 ppb, at which the risk is 1 in 1000, whereas at 10 ppb it is about 3 per thousand. About 57 million Americans currently drink water containing more than 1 ppb of arsenic; areas of the contiguous United States whose groundwater sometimes contains more than 10 ppb As are shown in dark green in Figure 15-7. Most affected systems lie in the West, Midwest, Southwest, and New England and use groundwater having naturally occurring arsenic.

IB Counties with arsenic concentrations exceeding 10 ppb in 10% or more of samples.

IB Counties with arsenic concentrations exceeding 5 ppb in 10% or more of samples.

Counties with arsenic concentrations exceeding 3 ppb in 10% or more of samples.

Counties with fewer than 10% of samples exceeding 3 ppb, representing areas of lowest concentration.

Counties with insufficient data in the USGS database to make estimates.

FIGURE 15-7 Average arsenic concentrations in U.S. drinking water. (Source: "Pressure to Set Controversial Arsenic Standard Increases/' Environmental Science and Technology 34 (2000): 208A.1

One of the difficulties in setting a standard for arsenic levels in drinking water is deciding the manner in which the element operates as a carcinogen. For carcinogens that induce cancer directly—by damaging DNA—the assumption is made that no amount of exposure to the substance is safe, since the risk from it rises from zero in proportion to exposure. However, as mentioned previously, there is evidence that arsenic does not act directly but indirectly, by inducing cell damage and regrowth or by inhibiting repair of DNA damage caused by other carcinogens such as UV light or tobacco smoke. For carcinogens that act indirectly, there can be a threshold, a level below which the substance can be considered safe and not cause damage.

Some scientists are not convinced that the estimates of cancer risk quoted above are at all realistic, since the extrapolation of the cancer incidence from high arsenic levels to the low environmental concentrations may not be valid if arsenic acts indirectly as a carcinogen. It will be difficult to resolve this issue by analyzing cancer trends in different parts of the United States, however, since the predicted fraction of bladder and lung cancers caused by arsenic is still a small percentage of the total for these diseases.

One argument that was advanced against making the arsenic standard even as low as 10 ppb in the United States is that it forces some small-scale suppliers of drinking water to shut down since they cannot afford the cleanup costs associated with introducing equipment to remove the element. Such shutdowns may lead consumers to turn to water supplies that are even more unsafe in other respects. Indeed, lowering the standard to 10 ppb is estimated to cost users of small water utilities, i.e., many of those in rural areas, several hundred dollars per year, whereas it will cost users of large facilities only a few dollars annually.

Removal of Arsenic from Water

The most widely used process for removing arsenic is to flow the drinking water over activated alumina (aluminum oxide), onto the surface of which the arsenic is adsorbed. The surface requires periodic cleaning of adsorbed species to remain effective. Reverse osmosis can also be used to remove arsenic, although, as previously discussed (Chapter 14), the process is expensive.

Because arsenic readily adsorbs onto iron oxide, water can be passed through a bed of ferric oxide to remove most of the arsenic. Alternatively, arsenic can be captured when iron hydroxide is precipitated from water, in a technique similar to the removal of colloids described in Chapter 14. Some of the other techniques used in villages in India and Bangladesh use the alumina method described above or filtration of the water through sand. All removal techniques require regular maintenance of the equipment and the proper periodic disposal of the arsenic-laden wastes. Some analysts believe that none of the arsenic-removal techniques work reliably in many areas, partly due to poor maintenance; instead, people should be directed to deeper wells with low arsenic contamination rather than trying to clean arsenic out of water from shallower wells that contains high levels of the element. Centralized water treatment plants using surface water are being constructed in some areas to overcome dependence on groundwater.

Like calcium and magnesium, arsenic can be removed from drinking water at large treatment facilities by precipitating it in the form of one of its insoluble salts. The arsenic in surface water normally exists as As(V). Since the salt formed between the ferric ion, Fe3+, and arsenate ion is insoluble, the soluble salt ferric chloride, FeCl3, can be dissolved in the water and the precipitated ferric arsenate, FeAs04, can be filtered from the resulting mixture:

Fe3+ + As043^-* FeAs04(s)

Arsenic in groundwater often exists as As(III) since reducing conditions occur underground. Such arsenic must be oxidized to As(V) before this removal process can occur.

Arsenic cannot be removed from water by cation exchange, since it occurs as an anion, not a cation. However, anion exchange can be used to remove arsenic from drinking water. Anion exchange also works better for As(V) than As(lll), since the latter exists partially as the neutral H3P03, rather than an anionic form at normal water pH values (6.5-8.5), whereas As(V) is completely ionic in that range (see Additional Problem 3). Anion exchange is problematic if appreciable amounts of sulfate ion, S04~, are also present in the water, as these are exchanged preferentially to arsenate, thereby tying up many sites and leaving fewer at which arsenic can exchange.

Continue reading here: Steady State of Arsenic Levels in Natural Waters

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