*4.3.1 Physico-chemical properties*

Mercury (Hg) is a heavy metal with a density of 13.6 and an atomic weight of 200.6 g mol−1. It is rare in the earth's crust and occurs in several forms of ore. *Cinnabar* HgS is the principal mercurial mineral [1, 5]. Mercury is a unique metal, as it is the only metal liquid at room temperature. In addition, it exists in nature in three forms (elemental, inorganic, and organic). In the atmosphere, most mercury is present in the form of elemental mercury vapor, while inorganic and organic species predominate in soil, water, plants and animals. Methylmercury, an organic compound of mercury, is formed as a result of the methylation of inorganic forms of mercury by microorganisms in soil and water [29].
