HgO Hg2 H2Oeqn 523

This ionic mercury (HgII) adheres to aerosols and thus has a short (days to weeks) residence time in the atmosphere; rainfall delivers it to the local soils and rivers. Ionic mercury is readily methylated (eqn. 5.24) by both abiotic and biotic pathways. However, most scientists now agree that methylation by anaerobic sulphate reducing bacteria (SRB) is most important.

(methylcobalamin)

In equation 5.24 the Hg2+ ion forms a covalent compound dimethylmercury (Hg(CH3)2) by bonding with the methyl anion (CH-). The methyl anion is a derivative of vitamin B12 called methylcobalamin, a common constituent of the bacteria themselves. At low to neutral pH, methyl mercury (CH3Hg+) forms in a similar way to dimethylmercury (Fig. 5.15). Methyl mercury is a potent toxin because it is soluble in the fatty tissues (lipophylic, see Box 4.14) of animals, ultimately attacking the central nervous system. It enters the food chain either directly absorbed from the water (e.g. into fish) or when plankton, a food source for fish, feed on the methyl mercury-rich bacteria. Each step in the food chain increases the concentration of mercury in the organic tissues (biomagnification) because organisms cannot eliminate mercury as fast as they ingest it (Fig. 5.15). As a result, mercury-contaminated fish are the main cause of mercury poisoning in humans. This is a particularly alarming situation for the Amazon region where fish are the staple food of the poor. Unfortunately, this pattern of mercury poisoning is very well known. Between 1953 and 1975 in the now infamous Japanese fishing village of Minamata, over 600 people were poisoned (115

Fig. 5.15 Cycling of mercury in lakes. Ionic mercury (Hg2+) is readily methylated by bacteria, especially sulphate-reducing bacteria in reducing sediments. Methyl mercury (CH3Hg+) enters the food chain through phytoplankton and is biomagnified in fish. Elemental mercury (Hg0) is quite insoluble except when complexed by dissolved organic ligands (Hg0 = org), making it more mobile and susceptible to oxidation in the water column. Pathways labelled S represent sedimentation, pathways labelled V represent volatilization.

Fig. 5.15 Cycling of mercury in lakes. Ionic mercury (Hg2+) is readily methylated by bacteria, especially sulphate-reducing bacteria in reducing sediments. Methyl mercury (CH3Hg+) enters the food chain through phytoplankton and is biomagnified in fish. Elemental mercury (Hg0) is quite insoluble except when complexed by dissolved organic ligands (Hg0 = org), making it more mobile and susceptible to oxidation in the water column. Pathways labelled S represent sedimentation, pathways labelled V represent volatilization.

a i a a a ta vi w fatally), by methylated mercury in fish. The methyl mercury, a waste product of the plastics industry, had been discharged into coastal waters near the village.

The bioavailability of mercury is complex, controlled by fundamental biogeo-chemical parameters such as type of microbial communities and Eh/pH (as discussed above), but also by its partitioning between solid and liquid phases and its complexation particularly with dissolved organic ligands (see Box 6.4). Laboratory studies show that humic acids acting as ligands (see Box 6.4) drastically increase the solubility of otherwise rather insoluble Hg0 at near-neutral pH. Moreover, the soluble complex formed is much less reactive than Hg0 to mineral surfaces in river sediments. This finding is highly significant for tropical rivers like the Amazon that contain high levels of dissolved organic matter and humic acids. It implies that mercury will remain mobile rather than fixed to mineral surfaces in river sediments, and may be transported to sites where oxidation and methylation is occurring, increasing the risk of toxicity. Once in local soils and water, mercury is also difficult to remove. In the Sierra Nevada region of the western USA, millions of kilograms of mercury were lost to the environment during the late 19th century gold mining. Much of this legacy remains today, slowly releasing mercury into surface and groundwaters.

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