Environmental geochemistry of mercury

Mercury occupies a unique and infamous place in environmental biogeochemistry. It was the first chemical species for which a direct connection was proven between relatively low concentrations in a natural water system, bioaccumulation up the biogeochemical food webs, and a serious health impact on a human population at the top of the food chain. 

The usual form of mercury in aqueous solution is the ion. Mercury has two oxidation states, Hg(l) and Hg(ll), but the first of these, which contains the unusual ion Hg - Hg+, is stable only as insoluble salts such as Hg2Cl2, Itdisproportionates in solution as follows from reaction (1) 

We can see from the reaction that reduction of under anaerobic conditions, for example in bottom sediments, gives the metal in liquid form. 

Mercury(ll) is a very soft Lewis acid, which forms stable complexes preferentially with soft Lewis bases such as sulfur ligands. You should remember here that the major natural form of mercury is sulfides. Increasing the pH of the aqueous solution due to pollution or river water discharge to marine water leads to precipitation of HgO. We know that HgO has finite solubility in water, and the solution may be described in terms of mercury(II) hydroxide as the following reactions (2)-(4) 

The major complicating factor in environmental biogeochemistry of mercury and its speciation is the biological methylation of Hg to CHaHg and (CHv)2Hg. This 

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