Speciation and the Toxicity of Heavy Metals

Although mercury vapor is highly toxic, the heavy metals Hg, Pb, Cd, Cr, and As are not particularly toxic as the condensed free elements. However, all are dangerous in the form of their cations and most are also highly toxic when bonded to short chains of carbon atoms. Biochemically, the mechanism of the toxic action usually arises from the strong affinity of the cations for sulfur. Thus, sulfhydryl groups, —SH, which occur commonly in the enzymes that control the speed of critical metabolic reactions in the human body, readily attach themselves to ingested heavy-metal cations or to molecules that contain the metals. Because the resultant metal-sulfur bonding affects the entire enzyme, it cannot act normally and, as a result, human health is adversely affected, sometimes fatally. The reaction of heavy-metal cations M2+ (where M is Hg, Pb, or Cd) with the sulfhydryl units of enzymes R—S—H to produce stable systems such as R—S—M—S—R is analogous to their reaction with the simple inorganic chemical hydrogen sulfide, H,S, with which they yield the insoluble solid MS.


Write the balanced chemical reactions that correspond to the reaction of an M2+ ion (a) with H2S and (b) with R—S—H to produce hydrogen ions and the products mentioned above.

A common medicinal treatment for acute heavy-metal poisoning is the administration of a compound that binds to the metal even more strongly than does the enzyme; subsequently the metal-compound combination is sol-ubilized and excreted from the body. One compound used to treat mercury and lead poisoning is British Anti-Lewisite (BAL); its molecules contain two —SH groups that together capture the metal. Also useful for this purpose is the calcium salt of ethylenediaminetetraacetic acid (EDTA), a well-known compound that extracts and solubilizes most metal ions. The metal ions are complexed by the two nitrogens and the charged oxygens to form a chelate (Chapter 14) which is subsequently excreted from the body.

British Anti-Lewisite

Treatment of heavy-metal poisoning by chelation therapy is best begun early, before neurological damage has occurred. The calcium rather than the sodium salt is used in order that calcium ion is not inadvertently leached from the body by the EDTA.

The toxicity for all four heavy metals depends very much on the chemical form of the element, i.e., upon its speciation. Substances that are almost totally insoluble pass through the human body without doing much harm. The most devastating forms of the metals

• cause immediate sickness or death (e.g., a sufficiently large dose of arsenic oxide) so that therapy cannot exert its effects in time, and

• can pass through the membrane protecting the brain—the blood-brain barrier—or the placental barrier that protects the developing fetus.

For mercury and lead, the forms that have alkyl groups attached to the metal are highly toxic. Because such compounds are covalent molecules, they are soluble in animal tissue and can pass through biological membranes, whereas charged ions are less able to do so; e.g., the toxicities of lead as the ion Pb2^ and in covalent molecules differ substantially.

The toxicity of a given concentration of a heavy metal present in a natural waterway depends not only on its speciation but also on the water's pH and on the amounts of dissolved and suspended carbon in it, since interactions such as complexation and adsorption may well remove some of the metal ions from potential biological activity.

Continue reading here: Bioaccumulation of Heavy Metals

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