Isotope signatures of atmospheric mercury emitted from residential coal combustion

Abstract

Residential coal combustion (RCC) is a major source of atmospheric mercury (Hg) in rural areas of China, but little is known about the isotope signatures of Hg from this source. In the present study, the isotope compositions of speciated Hg (Hg0, Hg2+, and Hgp) in flue gas emitted from RCC were investigated in two important coal-producing areas (Xingren (XR) and Jinsha (JS)) of Guizhou Province, Southwest China. The total Hg concentration in discharged flue gas is in the range of 1.80–9.65 μg/m3 at the two sites, and the emission ratio reaches 99.87%. Isotope signatures of total Hg in flue gas are similar to those of Hg in feed coal, with near-zero Δ199Hg and negative δ202Hg (−1.47‰ for XR and −3.00‰ for JS) in discharged flue gas. Such isotope signatures are very different from those of Hg emissions from modern coal-fired power plants with much higher δ202Hg signals. Negative shifts from Hg0 to Hg2+ in flue gas for δ202Hg (−0.94‰) and Δ199Hg (−0.51‰) were observed, suggesting that both mass-dependent fractionation and mass-independent fractionation occurred during RCC. The nuclear volume effect produced by chlorine oxidation of Hg0 to Hg2+ may play an important role in the observed mass-independent fractionation in Hg2+. Without any air pollution control devices, RCC potentially increases the atmospheric Hg levels and has a negative-shifting impact on the δ202Hg of atmospheric Hg.