27
Mercury [Hg, 80]
27.1 INTRODUCTION
Mercury (Hg), a metal of the group 12 in the periodic table of elements, is the only
metal that exists in the liquid state in natural conditions. Its average content in the
Earth’s crust is 0.07 mg/kg, and is much lower in igneous rocks (0.004–0.008 mg/kg)
than in sedimentary rocks (0.01–0.4 mg/kg). It is likely to be concentrated in argillaceous sediments. Contents of Hg in coal vary within the range of 0.2–10 mg/kg, and
in fly ash, the content is around 0.01 mg/kg.
Mercury occurs mainly at +2 oxidation stage, but may also have valences from
+1 to +3. It easily forms amalgams with noble metals. It may also be methylated by
microorganisms in various natural environments and reveals a chalcophillic character, and thus combine readily with sulfur. Its main mineral, of commercial importance, is cinnabar (metacinnabar), HgS. Calomel (mercurous chloride), Hg 2 Cl 2 , is
rarely found in nature. Sulfide (e.g., schuetteite, Hg 3 SO 4 O 2 ), selenide, telluride, and so
on of Hg may occur in various ore deposits. There are several Hg host minerals such
as amphiboles, sphere, sphalerite, and other sulfides. The most important geochemical features of Hg are as follows: (1) affinity to form strong bonds with S; (2) formation of organomercury compounds; (3) formation of gaseous compounds; (4) volatility
of metallic form; and (5) low affinity for oxygen bound to carbon. The characteristics
of Hg in the environment are its number of chemical and physical forms, such as
Hg 0 , HgCl 2 , HgO, HgS, CH 3 HgCl, and (CH 3 ) 2 Hg. All these compounds behave differently in various ecosystems. Some of the mercuric salts are soluble in water, whereas
organomercuries are neither soluble nor do they react with weak acid and bases.
World total (rounded) production (without the United States) of Hg in 2010 was
1960 t, of which (in t) China produced 1400; Kyrgyzstan produced 250; and Chile
produced 150 (by-product) (USGS 2011). Its production has not changed much during the last 10 years. However, there is some information that due to recent regulations and restrictions in the primary Hg production, real data are not available
(Kabata-Pendias 2011). In some countries its production was lowered or stopped, and
Hg is obtained as a by-product. At the small-scale, it is obtained at the gold mining.
Major uses of Hg are still in (1) chloralkali production; (2) vinyl, chloride monometer production; (3) artisanal gold mining; (4) batteries and some control instruments;
(5) pharmacology; (6) stomatology; (7) wood impregnates; and (8) paints. Its use,
especially in agriculture (pesticides and seeds treatments), in electric lamps has been
recently forbidden. Thimerosal (merthiolate; C 3 H 3 HgNaO 2 S) is used since the 1980
as preservative in vaccines and in various cosmetics and medical preparations. Also,
calomel (mercurous chloride, Hg 2 Cl 2 ) was used in medicine as a diuretic, as well
as in some cosmetics. There is a lot of disagreement about the dangers of Hg in the
medical and dental professions, and some recent decisions may forbid Hg addition
to medical and pharmaceutical products. However, WHO and American Academy
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