Arsenic in Organic and Other Molecular Forms

The common environmental organic forms of arsenic are not simple methyl derivatives, as with mercury and lead. Rather, they are water-soluble oxyacid derivatives that can be excreted by the body and thus are less toxic than some

FIGURE 15-8 Steady-state model mass-balance diagram for arsenic in Lake Ontario. [Source: Adapted from S. Thompson et al., "A Modeling Strategy for Planning the Virtual Elimination of Persistent Toxic Chemicals irom the Creat Lakes," journal of Great Lakes Research 25 (1999): 814.1

Atmospheric deposition

based sources

Outflow

Water Column

465 ng/L

49 A

Deposition

Resuspension

Surface Sediment

(Rates are in tonnes/year)

inorganic forms. As previously mentioned, in water arsenic occurs most commonly as the As(V) acid H3As04, i.e., (0H)3As = 0, or one of its deproto-nated forms. Biological methylation in the environment by methylcobalamin initially involves the replacement of one or more —OH groups of the acid by —CH3 groups. Monomethylation by the human liver and kidneys converts most but not all ingested inorganic arsenic to (CH3)(0H)2As = 0 and then to the corresponding dimethyl acid, which is then readily excreted.

Although most daily exposure to arsenic by North American adults is due to food intake, especially meat and seafood, much of the arsenic present in food sources occurs in the organic form and is readily excreted. In seafood, the common forms of arsenic are either the (CH3)4As+ ion, a form of As(III) itself, or this ion with one methyl group replaced by —CH2CH2OH or —CH2COOH. The organic forms of arsenic found in seafood are probably noncarcinogenic and are much less toxic than inorganic forms, as illustrated in dramatic fashion by their high LD50 values, which lie in the thousands of milligrams per kilogram, compared to those for inorganic arsenic, whose LD5qS are about 1% of these values (see Table 15-3).

In contrast to the compounds discussed above, neutral As(III) compounds such as arsine, AsH3, and trimethylarsine, As(CH3)3, are the most toxic forms of arsenic. Curiously, the trimethyl compound is produced by the reaction, under humid conditions, of molds in wallpaper paste with the arsenic-containing green pigment CuHAs03 in wallpaper. Instances of mysterious illnesses and even of human "death by wallpaper" due to chronic exposure to the As(CH3)3 gas released into rooms by this mechanism have been reported. Some historians believe Napoleon was fatally poisoned by the

TABLF 15-3

LD50 Values for Some Common Forms of Arsenic

Name

Formula

LDS0 (mg/kg)

Arsenous acid

Arsenic acid

Methylarsonic acid

Dimethylarsonic acid

Arsenocholine

Arsenobetaine

H3AS03 H3As04 CH3ASO(OH)2 (CHj)2AsO(OH)

(CH3)3As+CH2CH2OH (CH3)3As+CH2ŒXr

14 20

700-1800 700-1800

6500 >10,000

Source: X. C. Le, "Arsenic Speciation in the Environment," Canadian Chemical News (September 1999): 18.

Source: X. C. Le, "Arsenic Speciation in the Environment," Canadian Chemical News (September 1999): 18.

trimethylarsine emitted from the wallpaper in his chronically damp house on the island of St. Helena, where he had been exiled. There have also been episodes of human poisoning from gaseous arsine that was accidentally generated and released when aqueous solutions of As(IIl) in the form of HAs02 came into contact with an easily oxidized metal such as aluminum or zinc and the arsenic was further reduced to As( —III):

2 Al(s) + HAs02.+ 6 H+-» 2 Al3+ + AsH3 + 2 H20

Continue reading here: Chromium

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