Content, association and potential modes of occurrence of mercury in diverse biomass types and their ecological significance

Toxic Hg species pose a global ecological problem due to the significant anthropogenic emissions of Hg into the atmosphere, especially during coal combustion. A study about the content, association and potential modes of occurrence of mercury and their ecological significance in eight biomass varieties belonging to woody, herbaceous, agricultural and aquatic biomass groups was conducted based on a combination of different chemical and mineralogical analyses and leaching procedures. The Hg contents in these biomasses vary between 0.003 ppm and 0.043 ppm (mean 0.013 ppm), which are about an order of magnitude lower than the Clarke value of coal (0.10 ppm). It seems that Hg occurs in both inorganic and organic matter of biomass, as the preferable association and potential modes of occurrence of this element include inorganic matter, hemicellulose, and water-soluble Cl-, S-, N-, and Na-bearing chlorides, sulphates, and nitrates such as halite, sylvite, arcanite, Ca nitrate, and nitrocalcite in the biomass system studied. Alternative renewable and C-neutral solid fuels poor in Hg such as appropriate sustainable biomasses are suggested to substitute partially or completely the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants to minimize the ecological problems related to this element.