Homogeneous and Heterogeneous Oxidation of Gaseous Elemental Mercury

Mercury is an atmospheric global pollutant with complex cycling behavior. Two-thirds of the mercury present in our atmosphere is anthropogenic in origin. Chemical oxidation of gaseous elemental mercury governs the deposition rate of mercury over most lakes, land, and oceans. A major uncertainty comes from the effect of atmospheric surfaces such as aerosols. Much research is devoted to mercury capture technologies to be used in coal fire power plants, which are the major source of anthropogenic emissions.;This thesis is a report on oxidation kinetics and mechanistic studies relevant to mercury-scavenging reactions. It provides an overview of the mechanisms of mercury oxidation by ozone, nitrogen dioxide, and titanium dioxide (exposed to ultra-violet light). The role of surfaces was quantified, as appropriate for each system. Crossover effects between gaseous co-pollutants (e.g. CO, SO2) and surfaces (SiO2, TiO2) are discussed. Rate constants were measured for each process and product studies were performed and compared with the available literature.;The effects of different surfaces and gases on the oxidation of mercury by ozone were measured. This reaction was confirmed to be a surface-enhanced gas phase initiated reaction with a second-order rate constant for pure gas-phase (kgas = (5.40 +/- 0.56) x 10-19 cm3 molec-1 s-1) and an enhanced surface component (ksur = (2.91 +/- 0.12) x 10-15 cm7 molec-1 s-1 ), or knet = (6.1 +/- 1.1) x 10 -19 cm3 molec-1 s-1. Water vapor had no effect on the rate but liquid water and gaseous carbon monoxide both rapidly accelerated the reaction. Mechanisms were placed in context with atmospheric oxidative scavenging processes. Future work may combine aerosols (soot, acid, silica) in ozone oxidation reactions and/or addition of SO2 gas.;The feasibility of removing mercury from a coal flue gas via titanium dioxide and ultra violet light was investigated. Discussed are some of the possible surface chemistry models of oxidation. The uptake rates of mercury over photosensitized titanium dioxide films was described by the Langmuir-Hinshelwood rate equation, where KHg = (5.1 +/- 2.4) x 10-14 cm3 molec-1 and k = (7.4 +/- 2.5) x 1014 molec cm-2 min-1. Effects of sulfur dioxide and water were evaluated but neither was found to impede the reaction. By contrast low oxygen level strongly impeded oxidation rates. Deposits of HgO on titania surfaces were widely dispersed in concentrated clusters. Currently there is no explanation to this pattern. Future experiments may use light emitting diodes to capture Hg0(g) over TiO 2;The oxidation of mercury by nitrogen dioxide was found to be a pure gas phase reaction, second order with respect to NO2, where k = (3.5 +/- 0.5) x 10-35 cm6 molec-2 s-1. The mechanism was conjectured to be a two-step addition of NO2 to Hg0; at higher NO2 concentrations the reaction may be first order with respect to NO2 but further experiments would be required for validation. The rate constant was also found in agreement with a previous study. Rates were unaffected by changes in pressure, available surfaces, presence of SO 2, and water. It was discovered that TiO2 surfaces saturated in HgO deposits, when exposed to NO2 were 'revived' in Hg uptake activity. It is suspected the reaction between HgO and NO2 re-disperses the deposits.