Page 35 - Microsoft Word - Lund BioinorgChem 08.doc

35

Interlude: Cadmium

Cadmium is contained in small amounts in zinc ores (zinc blende, e.g., contains 0.1-0.5%
Cd). Small amounts are also present in shales, in black coal and phosphate minerals such
as phosphorite. Since phosphate minerals are used in fertilisers, Cd becomes introduced
into farmlands and thus into farming products und nutrients. Cd is used in cadmium-based
accus, as a protective for iron against rusting, in CdS-based yellow to orange pigments,
cadmium soaps, and neutron absorbers in nuclear power stations

Coastal sea water can contain up to 30 µg/l Cd. Cd is enriched in sea weeds, sea shells,
squids and, occasionally, fish, as well as in mushrooms and leafy vegetables. Other
vegetables, wheat and grass accumulate Cd only when grown on contaminated farm land
stemming from fertilisers and industrial emissions.

According to the WHO, 70 µg Cd per day are tolerable. About 35 µg are taken up daily
via non-contaminated food. Smokers are subjected to an additional uptake of ca. 35 µg per
day. Acute syndromes of poisoning can emerge with a singular uptake of 15 mg; about
3
500 mg are lethal. The threshold limit value (TLV) is 0.1 mg/m , the biological tolerance
value (BTV) 15 µg/l (urine) and 1.5 µg/l (blood). Cd is carcinogenic (A2), mutagenic
(C2) and teratogenic.

Interlude: Mercury

In nature, Hg occurs mainly in elemental form and in the form of cinnabar (HgS).
Metallic mercury does not corrode in air. Earlier applications were fillings of
thermometers, barometers and the like. Nowadays it is used in chloralkali electrolysis,
dental fillings, and batteries. Coal fired power plants and volcanic exhalations are
important sources for mercury pollution

3
The TLV is 0.1 (Hg and HgCl 2) and 0.01 (MeHgCl) mg/m , respectively, the BTV 50
(blood) and 200 µg/l (urine) for inorganic Hg compounds and 100 µg/l for organic
compounds. The LD 50 value (Rat, oral) amounts to 57 mg/kg (LD 50: lethal dose for 50%
of the test animal).

Paracelsus employed mercury preparations against skin diseases, syphilis and
inflammation. Hg preparations have also a long standing tradition as desinfectants
(sublimate = HgCl 2; mercurochrome). Contrasting its lighter homologues, Hg also
forms monovalent compounds. An example is calomel = Hg 2Cl 2.

Mercury and its compound are liable to speciation in the atmosphere, aquasphere and
siderosphere. The speciation is partly abiotic (chemical and photochemical speciation), partly
biotic (such as the methylation of Hg). Some of the more important paths of mutual
interconversion are depicted in Fig. 28. The methylation of mercury under physiological
conditions is carried out by methylcobalamin, a close relative to vitamin B 12
(adenosylcobalamin).

Figure 28. (following page). Speciation of mercury. Anthropogenic sources are framed,
microbial sources are in red. Su = substrate