Arsenic is not actually a metal; it is a metalloid—its properties are intermediate between those of metals and nonmetals. However, for convenience we discuss it in this chapter.

Arsenic compounds such as the oxide As203, white arsenic, were common poisons used for murder and suicide from Roman times through the Middle Ages. In the seventeenth century, arsenic was believed in some European societies to be not only a poison but also a magical substance that was a cure for certain ailments, including i mpotence, and to be a prophylactic against the plague. Indeed, arsenic compounds have been used therapeutically for 2000 years, and even today about 50 Chinese drugs contain the element. There are small background levels of arsenic in many foods, and a trace amount of this element apparently is essential to good human health.

Arsenic(III) Versus Arsenic(V) Toxicity

Arsenic occurs in the same group of the periodic table as phosphorus and so also has an s2|)3 electron configuration in its valence shell. Loss of all three p electrons gives the 3 + ion, whereas sharing of the three electrons gives triva-lent arsenic; collectively, these two forms are designated As(III). Arscnic(III) commonly exists in aqueous solution and in solids as the arsenite ion, As033 (which can be considered to be As'+ bonded to three surrounding O2^ ions), or one of its successively protonated forms: HAs032~, H2As03~, or H3As03.

Alternatively, loss of all five valence shell electrons gives the 5 + ion, and sharing them all gives pentavalent arsenic; collectively, these two forms are designated As(V). Arsenic(V) also commonly exists as an oxyanion, the arsenate ion, As043" (equivalent to As5+ bound to four O2- ions), or one of its successively protonated forms: HAsC^2-, H2As04 , or H3As04.


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Overall, arsenic acts much like phosphorus, which commonly exists in the analogous oxyanion forms P033" and P043~, called phosphite and phosphate, respectively. However, arsenic has more of a tendency than phosphorus to form ionic rather than covalent bonds, since it is more metal-like. Due to the similarity in properties, arsenic compounds coexist with those of phosphorus in nature. Consequently, arsenic often contaminates phosphate deposits and commercial phosphates.

Arsenic's lethal effect when consumed in an acute dose is due to gastrointestinal damage, resulting in severe vomiting and diarrhea. Inorganic As(III) is more toxic than As(V), although some of the latter is converted by reduction to the former in the human body. It is thought that the greater toxicity of As(III) is due to its ability to be retained in the body longer since it becomes bound to sulfhydryl groups in one of several enzymes. Due to the subsequent inactivity of the enzymes, energy production in the cell declines and the cell is damaged. Once the arsenic becomes methylated in the liver, it does not bind tightly to enzymes and hence is largely detoxified.

Continue reading here: Anthropogenic Sources of Arsenic to the Environment

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