Mercury (Hg) is in the third series of transition metals beneath zinc (Zn) and cadmium (Cd).
The major source of mercury is the natural degassing of the Earth's crust.
Man made sources include
The fate of mercury released into the environment depends on the conditions in which it finds itself.
Under anaerobic conditions (deficiency of oxygen), mercury ions tend to be converted into insoluble mercury(II) sulphide, since there is usually a significant concentration of sulphide ions present due to the action of reducing bacteria.
Hg2+(aq) + S2-(aq)HgS(s)
As mercury(II) sulphide (HgS) is involatile and insoluble it does not pose a significant danger.
Under aerobic conditions, mecury ions tend to be methylated to produce the methylmercury cation.
As the methylmercury cation is soluble in water and not easily eliminated from body tissues, it tends to accumulate and can pose a significant danger.
Further methylation produces dimethylmercury.
This is insoluble in water and therefore is not easily absorbed from solution, however, it is volatile and can enter the food chain via the atmosphere.
The danger posed by mercury depends on its precise chemical form.
The methylmercury cation disrupts the blood-brain barrier, leading to
Inorganic mercury (mainly Hg2+) disrupts the function of the liver and kidneys.
Both forms of mercury interact with sulphur in proteins and enzymes disrupting their function, but the precise structure of the complexes formed differ for Hg2+ and CH3Hg+ hence their different effects on different parts of the body.