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Dimethylmercury or CO2

Dimethylmercury, (CH3)2Hg, is one of the most powerful neurotoxins known with a toxic threshold estimated at > 200 μg/L. It is a colouress liquid that has the capacity to rapidly pass through materials like latex, polyvinylchloride and neoprene.As such, protective gloves worn in most laboratories provide an inadequate barrier. In 1997, a well-known American ecotoxicologist, Professor Karen Wetterhahn, died after spilling a few drops of dimethylmercury on her latex-gloved hand while using the chemical in a fume-chamber. Methylcobalamin, a form of Vitamin B12, methylates mercury compounds to form CH3Hg+. Confirmation that methylcolabamin occurs in anaerobic bacteria such as Methanosarcia barkeri was made in Yeliseev et al (1993). M. bakeri occurs in a variety of anaerobic conditions including the rumen of cows, mud at the bottom of lakes and landfills.

In 2007, as part of its initiatives to reduce CO2 emissions, the Australian Federal Government put forward a bill to replace all incandescent light bulbs in use in Australia by compact fluorescent bulbs. The rational for this decision was the longer life and greater energy efficiency of fluorescent bulbs. Federal Environment Minister, Malcolm Turnbull, stated: ‘globally, electric lighting generates emissions equal to 70 per cent of those from all the world’s passenger vehicles’. Replacement of incandescent bulbs with fluorescent alternatives will be mandatory by 2010 – and Australia will be the first country to have undertaken this transformation.

The positive aspects of this initiative are clear: fluorescent tubes and bulbs are about 20 per cent more efficient than incandescent bulbs and last more than five times longer. However, there are negative aspects as well: the fluorescent systems are more expensive to make, and they contain small amounts of mercury. Herein lies the conundrum… for in Australia, most of the spent fluorescent tubes will be destined for landfills.

Landfills contain anaerobic bacteria like M. bakeri, and these are capable of converting mercury and mercuric compounds into dimethylmercury. If most consumers simply dispose of spent tubes in standard waste collection systems, they will end up in landfills, and when broken, they will leak mercury into an anaerobic environment. In a few large Australian industries, there are recycling systems for fluorescent tubes, but these are expensive. Although many developed countries have systems to recycle fluorescent tubes, there is currently no Australian Government initiative to prevent tubes from being dumped in landfills. (Indeed, suggestions have even been made that spent fluorescent tubes could be stored in cupboards by domestic householders until recycling systems are established.) While the level of mercury per tube is not high (Hg levels range from 3.5 mg to about 12 mg – depending on the specifications of the tube), from 2010 millions of spent fluorescent tubes will be broken and dumped in Australian landfills per year.

Note that in Washington State, where incandescent bulbs are still widely used, Thurston County Public Health & Services Department’s 2007 data sheet estimates that in Washington alone, some 225 kilogrammes of mercury are released from fluorescent tubes into landfill per annum.

Considerations

  1. What are the likely consequences of the Australian Federal Government’s initiatives to make the sole use of compact fluorescent bulbs mandatory?
  2. What, if anything, should the government do to mitigate contamination of landfills by mercury?
  3. Through what moral perspective could the motives of the Federal Government be seen in a favourable light?

References

Yeliseev, A., Gärtner, P., Harms, U., Linder, D. and Thauer, R.K. 1993. ‘Function of methylcobalamin: coenzyme M methyltransferase isoenzyme II’ in Methanosarcina barkeri. Archives of microbiology 159(6): 530–536.


Article taken with permission from: Buckeridge, J.St,J.S., 2008. 4 E's: Ethics, Engineering, Economics and Environment. RMIT University Press, Melbourne, Australia, pp 97-100.