BIOLOGICAL TREATMENT OF MERCURY WASTES Introduct

Introduction. It would seem paradoxical that biological systems could be put to useful work treating as toxic a substance as mercury. But in limited ways and degrees this has occurred, and the successes are reviewed here.

Theory of Mercury Transformation Process(es). "Perhaps the most toxic metals are the nonessential metals such as cadmium, lead, and mercury" (7:318). Some microorganisms that encounter these metals have developed strategies to prevent entry of them into their cells or literally to pump them back out again: sequestration, active transport, or chemical transformation (7:319). The last of these (oxidation-reduction reactions) is used to reduce the mercuric ion, Hg+2 [also written as Hg(II)] to elemental, metallic mercury, Hg0, which is volatile and can thus diffuse away from a microbial cell through surrounding air or water (7:323).

Methylation of mercuric ion forms [mono]methyl mercury, CH3Hg+1, or dimethylmercury, (CH3)2Hg--10 to 100 times more toxic to organisms than inorganic mercury; but these forms can also be diffused away, by the organism (7:323). (Other metals similarly methylated are tin, lead, arsenic and selenium (7:323)).

Summers has described more fully than all others referenced here the enzymatic details of mercury "biotransformations" from one valence state to another or from one organic state to another or to inorganic, elemental mercury (9). She suggests that bacterial ability to convert both

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157 g (about 1/3-lb) of total Hg were released from the plant per day, more than 90% of which was in the sludge (4:801). So, again, microbes were taking up metal mercury. Only about 20% of what was released was from the influent; 80% was generated within the plant from the historically leaking seals (4:801). Compared to 157 g/day of total Hg release, only 0.4 g/day of methylmercury was released, three-quarters of that generated from within the plant. But in this case, only 30% of the methylmercury was removed as sludge, and 70% was released to the river in the effluent (4:801). The microbes, then, appear to have rejected, or "diffused away" the (more toxic) methylmercury. In yet another study of mercury in sewage treatment plants, Goldstone et al. studied an English wastewater plant (in Norwich)(5). This study was far more concerned with the mechanisms of removal than the previously described plant reviews. The English plant was also smaller than the others, roughly 4.5 MGD. The study and its results, for all their intended academic rigor, were complicated by "sampling campaigns" in May 1986 and October 1987; predictably a number of factors changed in the interim (5:273). In 1986, soluble and insoluble mercury removals
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Approximate Word count = 1886
Approximate Pages = 8 (250 words per page)