Arsenic

Arsenic is a member of the same family as phosphorus. Although a classical poison, it is also an essential element. It occurs in the same phosphate rocks from which phosphorus chemicals are obtained, and in many industrial phosphates, arsenic remains as an impurity, and thus is found in small amounts in phosphate fertilizers and also among combustion products, mine tailings, and by-products from the metallurgical processing of copper and other metals. It is also used in pesticides—for example, sodium methanarsonate salts, dimethy-larsinic acid, and inorganic arsenates and arsinites—although use of inorganic arsenical pesticides in agriculture is now prohibited in the United States. Earlier applications included pigments. Arsenic compounds have had considerable use as medicinals, especially against blood parasites, while other arsenic compounds, particularly salts of arsanilic acid [3-aminophenylarsonic acid, NH2C6H4AsO(OH)2], have been used as feed additives for poultry and pigs. The main current use of arsenic compounds is as a wood preservative, most frequently as "chromated copper arsenate," a mixture of chromic oxide,

28N. Castellino, in Inorganic Lead Exposure, N. Castellino, P. Castellino, and N. Sannolo, eds., Lewis Publishers, Boca Raton, FL, 1995, Chapter 2.

copper oxide, and arsenic pentoxide. Close to 20,000 metric tons of arsenic was used for this purpose in the United States in 1995, far more than in the rest of the world combined. There is growing concern about the release of arsenic from treated wood in sensitive areas such as playgrounds.

Arsenic, like phosphorus, is generally encountered as an oxoanion species, that is, arsenic acid, H3AsO4, an arsenate (AsO^-), or an arsenite (AsOj~) salt. The latter is considered to be more active biologically. In these forms its toxicity is well known, especially for soluble compounds from which the arsenic can readily be adsorbed. A lethal adult human dose of an arsenate is about 100 mg, but lower doses over a period of time can produce chronic poisoning. Effects include skin lesions and hardening of the skin (hyperkeratoses), which may lead to cancer. As of 1998, it was estimated that 200,000 people in West Bengal, India, were suffering toxic effects from arsenic-containing drinking water obtained from "tube wells" drilled to tap underground aquifers primarily to provide irrigation water, but also to eliminate the need to use contaminated surface water for drinking. Many more, potentially up to 70 million, are similarly exposed in Bangladesh.29 The mechanism for what appears to be increasing arsenic release to the water is unclear at this point. The sediments that make up the aquifer are rich in iron sulfide, and the most probable explanation is that the arsenic (as arsenate) is largely adsorbed on FeO(OH) particles produced from these sulfides. Anoxic conditions caused by a high organic content reduces the iron and releases the arsenic. Correlation of the arsenic concentrations with the anoxic character of the well and with the dissolved iron content in the water are evidence for this explanation, which suggests that the process might be reversed by aeration of the water and letting the contaminated ferric hydroxide produced settle. Less likely explanations propose an oxidative process connected to seasonal fluctuation of the water table that brings oxygen into the soils, along with flushing action from the water withdrawal, or displacement of arsenic by high levels of phosphorus from fertilizers.

In the United States, there is concern about low levels of arsenic occurring naturally in drinking water supplies. The limit has been 50 ppb, but there is evidence that at this level arsenic can contribute to bladder and lung cancer, and may also contribute to other health problems such as diabetes. The maximum level recommended by the World Health Organization is 10ppb (also the level recommended in the United States by the EPA), but in 2001 a regulation setting this limit was withdrawn pending further stydy, largely for economic reasons connected to the costs of achieving it. As of this writing the 10 ppb limit had been reconfirmed.

29 W. Lepkowski, Arsenic crisis in Bangladesh, Chem. Eng. News, p. 27; Nov. 16, 1998; also P. Bagla and J. Kaiser, Science, 274 (1996), and R. Nickson, J. McArthur, and W. Burgess, Nature, 395, 338 (1998).

Aliphatic organic arsenic compounds have higher toxicity, and there is evidence that they can be produced from arsenates by the action of anaerobic bacteria, much as methylmercury is formed from inorganic Hg. The following sequence of steps can take place in sediments under the action of the appropriate bacteria:

OH H CH3 CH3

II II II II

where (a) is arsenic acid, arsenates; (b) is arsenious acid, arsenites; (c) is methylarsonic acid; and (d) is dimethylarsinic acid.

All these can be extracted into water. Further, additional biological processes can convert the methyl compounds to di- and trimethylarsine, (CH3)2 AsH and (CH3)3As, which are extremely toxic, volatile compounds. These are readily oxidized, and there is an arsine-arsenic acid biological cycle. As with methylmercury, these methylated arsenic compounds are fat soluble and may concentrate in the food chain. There are reports of illness and fatalities resulting from release of volatile organoarsenicals from the pigments in wallpaper under conditions of dampness and mildew; for example, it is claimed that Napoleon's death was due to this cause, and that an American ambassador to Italy became very ill for the same reason. Aromatic arsenic derivatives are less toxic than the aliphatic ones.

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