MadSci Network: Earth Sciences
Query:

Re: Why is there so much mercury in coal compared to modern plants?

Date: Tue Jan 5 13:10:33 2010
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton University
Area of science: Earth Sciences
ID: 1257970447.Es
Message:

Coal combustion is one of the largest sources of Hg in the environment because of the Hg in coal. Coal formed from plants, but burning wood today does not release significant Hg. Were the plants that formed coal higher in Hg than modern plants? And, if so, why?


You have forgotten that coal seams sit in the earth for many millions of years. During that time, water seeps through them, and whatever is dissolved in the water is carried along. Some of it gets left behind in the coal seam.

This means that you get naturally-occurring mercury compounds in coal, among other things. Since coal is not processed before it's burned to remove these impurities, they either stay behind, concentrated, in the ash -- natural uranium residues make some coal ash low-level radioactive waste -- or go up the chimney with the other volatiles like carbon and sulfur oxides. Since metallic mercury is volatile, it goes up the chimney and spreads downwind of the smokestack.

But again, the mercury didn't come from the original plant material (which is the source of the sulfur). Mercury was leached into the coal by the action of ground water on natural mercury-containing minerals.

Dan Berger


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