MERCURY IN COAL: WHERE DOES IT COME FROM?

Mercury released from coal during burning for energy production has become an issue of significant concern worldwide. Mercury is a natural component of coal that is released to the flue gas during combustion. Plant species that make up coal have varied over geologic time; however, dominant species were vascular plants with some structure that functioned similarly to leaves of today with stomata for gas exchange. Based on our work with present day species it is hypothesized that mercury present in coal was derived by plant assimilation from the atmosphere. This paper summarizes mercury concentrations measured in forest, wetland, desert, and grassland plant species as a function of air and soil mercury exposures and time. Foliage of plants exposed to air mercury concentrations representative of ambient conditions today and low mercury containing soils (background) exhibited values ranging from 0.02 to 0.2 ppm (based on dry weight). These are within the range reported for coal in the COALQUAL database (0.17 +/- 0.17 ppm). Foliar mercury concentrations were found to vary as function of plant species with deciduous species tending to accumulate more than others. Foliage of plants sampled in the field exhibited lower mercury concentrations than those used in laboratory experiments most likely due to the lower air exposure concentrations over time. At ambient exposure conditions we cannot be certain of the proportion of mercury derived from the soil versus the atmosphere; however, based on data derived using multiple soil and air mercury treatments, plant foliage mercury concentrations were more statistically significantly influenced by air concentrations than by soil concentrations. Measurement of the flux of elemental mercury between foliage and the atmosphere for a variety of plant species demonstrated that at low air and soil mercury exposures (comparable to ambient conditions) exchange consisted of both deposition and emission (-3 to 3 ng m-2 h-1). Deposition of Hg from the atmosphere to plant foliage dominated for grassland, wetland and coniferous species.