Mass-dependent and mass-independent variations in the isotope composition of mercury in a sediment core from a lake polluted by emissions from the combustion of coal

Abstract

A dated sediment core from a lake polluted with mercury (Hg), other heavy metals, and arsenic (As) from coal-burning power plants was analysed to test the hypothesis that power plant emissions have distinctive Hg isotope signatures which may be preserved in sediments but are altered by natural processes. Coal and fly ash were also analysed. The research yielded evidence for mass-dependent and mass-independent fractionation of Hg isotopes (MDF and MIF, respectively) by combustion and flue gas reactions in the power plants and natural processes in the lake. Power plant pollution and earlier pollution attributable to domestic coal burning produced a characteristic isotope signature indicative of depletion in lighter isotopes by MDF and enrichment in 199Hg and 201Hg by MIF, suggesting loss of isotopically light gaseous Hg(0) and reactions of Hg with free radicals at the sources of pollution; but coal and fly ash data showed that combustion imparted a different signature to the ash, corroborating chemical evidence that reactive gaseous Hg(II), not particulate Hg(II), was the principal Hg fraction deposited in the lake. Moreover, the core data imply alteration of the anthropogenic isotope signature by microbially mediated MDF and MIF, with alteration of the microbial activities themselves by toxic effects of As and metals from the emissions. Effects of metals on isotope fractionation increased with the stability constants and ligand field stabilisation energies of metal complexes, suggesting inhibition of microbial enzymes and metal binding by microbial carrier molecules. The importance of fractionation by natural (possibly microbial) processes is also indicated by depletion in 199Hg and 201Hg owing to MIF in sediments predating local pollution. In brief, the isotope signature of the polluted sediment is probably the net result of abiotic reactions at the sources of pollution, microbial activities in the lake, and effects of toxic pollutants on the microflora.