Interactions of atmospheric elemental mercury with natural, synthetic, and anthropogenically derived substrates.

摘要

This study focused on investigating substrate---air exchange of Hg with two types of materials: natural soils (Chapter 2 & 3) and coal combustion products (CCPs) (Chapter 4 & 5). Laboratory studies presented in Chapter 2 and 3 indicated that natural background soil may be a source or sink of atmospheric Hg0 depending on environmental parameters (air Hg0 concentration, UV-light and visible light, precipitation) and soil properties (Hg concentration, pH, organic matter content). At low air concentrations (2.8 +/- 0.8 ng/m3) soils emitted Hg to the air in light conditions (mean flux: 1.3 +/- 1.0 ng/m 2hr) and adsorbed Hg0 in dark conditions (mean flux: -1.1 +/- 1.2 ng/m2hr); while at elevated air Hg 0 concentration (5.8 +/- 1.0 ng/m3) deposition was the dominant flux (mean flux of -2.1 +/- 1.6 and -4.6 +/- 1.25 ng/m2hr in the light and dark, respectively). Exposure to UV-light (UV-B, C) significantly increased Hg emission from dry and water---saturated soils. Mercury newly deposited to the soils as HgCl2 was not readily released.;Laboratory studies using CCPs presented in Chapter 4 indicated that most dry samples acted as sinks for atmospheric Hg in the dark at 25°C. When exposed to light or increased temperature (45°C), deposition of Hg to the fly ash substrates in most cases continued but decreased. Wet FGD (flue gas desulfurization) samples emitted Hg. However, they became a sink for atmospheric Hg or exhibited low Hg emission rates when dried. In situ data (Chapter 5) indicated that fly ash and water saturated FGD materials emitted Hg to atmosphere at low rates (-0.1 to 1.2 ng/m2hr) while FGD material mixed with fly ash and pyrite exhibited higher emission rates (∼10 ng/m 2hr), but were still comparable with natural background soils (-2 to 13 ng/m2hr). Air temperature, solar radiation, and relative humidity were important factors correlated with measured Hg fluxes.;Information gained from Chapter 2 & 3 is anticipated to be useful for regional/global scale modeling studies estimating air---soil exchange of Hg0. Data obtained in Chapter 4 and 5 can be applied for the assessment of Hg release from CCPs that are placed in landfill, and to help make regulatory and management decision for CCPs.