Oxidation Of Elemental Mercury Research Articles

Behavior of mercury emitted from the combustion of coal and dried sewage sludge: The effect of unburned carbon, Cl, Cu and Fe

As the behavior of mercury generated from the combustion of mercury-containing fuels is highly dependent upon mercury speciation, many studies have investigated mercury speciation following combustion of various fuels. Unburned carbon and the content of chlorine (Cl), copper (Cu), and iron (Fe) compounds in ash are reported to affect the behavior of mercury. This study was conducted to understand the important factors impacting behavior of mercury from single coal combustion and co-combustion of dried sewage sludge with coal. Factors reported in previous studies were selected as potential factors affecting mercury oxidation and retention. Six coal and six dried sewage sludge samples were used to conduct single coal combustion and co-combustion tests. Each fuel sample was combusted in a lab-scale drop tube combustion system. Concentrations of elemental and oxidized mercury were determined using the Ontario Hydro Method. Ash samples collected on the bottom of the combustion system were analyzed to determine unburned carbon and mercury retention percentages. Unburned carbon and Cl were found to be important in both oxidation and retention of mercury for single coal combustion. Cl, Cu, and Fe content increased the oxidation of elemental mercury generated from co-combustion of dried sewage sludge with coal.

Non-thermal plasma injection-CeO2-WO3/TiO2 catalytic method for high-efficiency oxidation of elemental mercury in coal-fired flue gas

The combined method of non-thermal plasma injection and 3%CeO2-WO3/TiO2 catalysis system was proposed to enhance Hg0 oxidation in flue gas. Compared with the plasma system, the combined system had excellent oxidation performance. Approximately 86.6% of Hg0 was oxidized and energy yield reached 32.9µgkJ−1 at 2.6JL−1 and 110°C . The combined system widened the temperature windows of CeO2-WO3/TiO2 catalyst (80–300°C), showed good resistance to H2O and SO2, and also overcame the inhibitory effect of NO on Hg0 oxidation in the plasma system·NH3 inhibited the Hg0 oxidation; however, the coexistence of NO with NH3 significantly mitigated the inhibition. In the combined system, the CeO2-WO3/TiO2 catalyst could catalytically decompose the ozone generated by gas discharge, and increased the production of high adsorbed active oxygen, which was favorable for Hg0 oxidation process. The Hg0 oxidation process followed the homogeneous reaction mechanism and the Mars-Maessen mechanism in NTP injection system and CeO2-WO3/TiO2 system, respectively.

DFT studies of Hg0 oxidation by gaseous advanced oxidation method: H2O2/Fe3O4 (1 1 1) and hydroxyl pre-adsorbed Fe3O4 (1 1 1) surface

Elemental mercury oxidation mechanism by gaseous advanced oxidation method was studied using density functional theory on Fe3O4 (111) surface containing H2O2 molecule. Fetet1- and Feoct2-terminated Fe3O4 (111) surfaces have been simultaneously considered both in H2O2 decomposition and hydroxyl pre-adsorbed Fe3O4 (111) interfaces. It is found that the Feoct2-terminated surface was more favored for H2O2 decomposition, and H2O2 was easier to decompose and generate two hydroxyls than Fetet1-terminated surface. Through the discussion of Fetet1- and Feoct2-terminal mechanisms, the Mulliken charge population, and the partial density of states, we found that OH had different reaction activity generated on different Fe-terminal. Hg strongly interacted with the free state OH mainly due to the highly reactive and strong electrophilic ability of OH radical. The oxidation of Hg formed stable oxidized mercury species on Fe-terminated surface and most of the lost electron transferred from Hg to unbonded hydroxyl during Hg oxidation. The result showed that the combination of Hg and hydroxyl was exothermic reaction, which was favorable to spontaneous processes of Hg oxidation. The OH–Hg–OH and Hg–OH intermediates had a lower desorption energy when they detached from the surface and was the major reaction pathway.

Ag-Mo modified SCR catalyst for a co-beneficial oxidation of elemental mercury at wide temperature range

V2O5-MoO3-TiO2 (V-Mo-Ti) is often used as a selective catalytic reduction catalyst for NOx from coal-fired flue gas. The performance of a V-Mo-Ti catalyst for the oxidation of elemental mercury (Hg0) was investigated. It was found that Mo was resistant toward sulfur dioxide and can enhance the Hg0 adsorption capacity. Ag was employed to enhance the Hg0 oxidation reaction and can enlarge reaction temperature window. Doping with Ag can significantly enhance the oxidation of Hg0, and adding only 0.5% Ag can keep Hg0 oxidation efficiency to approximately 90% with 5ppmHCl, with an increase of 20–40% compared to that of V-Mo-Ti catalyst. Besides, the reaction temperature window of catalyst was enlarged from 150 to 400°C. TEM and XPS characterization data indicated that Ag nanoparticles were loaded on the Mo/V-Ti carrier, maintaining Ag-Mo/V-Ti at a higher oxidation state. Furthermore, the TPR and Deacon reaction tests suggested that the Ag dopant might enhance the redox behavior, which facilitates the Deacon or semi-Deacon reactions for HCl activation. In addition, Hg0 desorption and breakthrough experiments and mercury valence state change experiments were carried out to investigate the Hg0 catalytic oxidation mechanisms at various temperature ranges.

Comparative study of Co/TiO2, Co–Mn/TiO2 and Co–Mn/Ti–Ce catalysts for oxidation of elemental mercury in flue gas

The Co–Mn/Ti–Ce catalyst prepared by sol–gel and impregnation method was evaluated for catalytic oxidation of Hg0 in the simulated flue gas compared with Co/TiO2 and Co–Mn/TiO2. The results showed that Co–Mn/Ti–Ce catalyst exhibited higher catalytic activity (around 93% Hg0 removal efficiency in the temperature of 150 °C with 6% O2, 400 ppm NO, 200 ppm SO2 and 3% H2O) than Co/TiO2 and Co–Mn/TiO2. Based on the characterization results of N2 adsorption–desorption, XRD, UV–Vis, XPS, H2-TPR and Hg-TPD, it could be concluded that the lower band gap, better reducibility and mercury adsorption capability and the presence of Co3+/Co2+, Mn4+/Mn3+ and Ce4+/Ce3+ redox couples as well as surface oxygen species contributed to the excellent Hg0 oxidation removal performance. In addition, well dispersion of active components and a synergetic effect among Co, Mn and Ce species might improve the activity further. A Mars–Maessen mechanism is thought to be involved in the Hg0 oxidation. The lattice oxygen derived from MnOx or CoOx would react with adsorbed Hg0 to form HgO and the consumption of lattice oxygen could be replenished by O2. For Co–Mn/Ti–Ce, MnOx−1 could be alternatively reoxidized by the lattice oxygen derived from adjacent CoOx and CeOx which is beneficial to the Hg0 oxidation.

Oxidation of elemental mercury with non-thermal plasma coupled with a wet process

The oxidation of the elemental mercury (Hg0) by non-thermal plasma (NTP) coupled with a wet process has been studied. Effects of oxygen content, supplied voltage, gas temperature, wet process and NO/SO2 concentration on Hg0 oxidation were investigated using a dielectric barrier discharge reactor. Results indicate that both O and O3 generated by O2 during the discharge process contribute to the oxidation of Hg0. The increase of O2 content improves the Hg oxidation efficiency. It was noted that the Hg oxidation efficiency declines when the supplied voltage is too high, there was an optimal voltage level for Hg0 oxidation. High temperatures promote oxidation of Hg0. The main reason for this is that the high temperature increases the generation of O atoms and the reaction rate of O3 with Hg0. Compared with the NTP process only, the Hg oxidation efficiency increases from 43 to 82% after adding a wet process when the supplied voltage was 8kV at room temperature. The presence of NO hindered mercury oxidation due to competitive consumption of O3. However, at the level of 100ppm NO, more than 50% Hg oxidation efficiency could be achieved in the NTP combined with wet process system. It is found SO2 enhance oxidation of Hg0, because of the positive effect of SO3 generated in the NTP environment.

Heterogeneous oxidation of elemental mercury vapor over RuO2/rutile TiO2 catalyst for mercury emissions control

The catalytic oxidation of elemental mercury (Hg(0)) vapor is an effective way to enhance mercury removal from coal-fired power plants. RuO2 catalyst was found to be an excellent Hg(0) oxidation catalyst. When rutile TiO2 was used as the catalyst support, RuO2 formed well dispersed nano-layers due to the very similar crystal structures of RuO2 and rutile TiO2, giving higher Hg(0) oxidation activity over anatase TiO2 support. The RuO2/rutile TiO2 catalyst can be used at the tail end section of the selective catalytic reduction (SCR) unit for Hg(0) oxidation. It showed good Hg(0) oxidation performance under sub-bituminous and lignite coal simulated flue gas conditions with low concentration of HCl or HBr gas. The RuO2/rutile TiO2 catalyst also showed excellent resistance to SO2 under bituminous coal simulated flue gas, maintaining higher than 90% Hg(0) oxidation with up to 2,000ppmv SO2 present. The oxidized mercury in a form of HgCl2 has a high solubility in water and can be easily captured by other air pollution control systems such as wet scrubbers in coal-fired power plants.

Elemental Mercury Oxidation over Fe–Ti–Mn Spinel: Performance, Mechanism, and Reaction Kinetics

The design of a high-performance catalyst for Hg0 oxidation and predicting the extent of Hg0 oxidation are both extremely limited due to the uncertainties of the reaction mechanism and the reaction kinetics. In this work, Fe-Ti-Mn spinel was developed as a high-performance catalyst for Hg0 oxidation, and the reaction mechanism and the reaction kinetics of Hg0 oxidation over Fe-Ti-Mn spinel were studied. The reaction orders of Hg0 oxidation over Fe-Ti-Mn spinel with respect to gaseous Hg0 concentration and gaseous HCl concentration were approximately 1 and 0, respectively. Therefore, Hg0 oxidation over Fe-Ti-Mn spinel mainly followed the Eley-Rideal mechanism (i.e., the reaction of gaseous Hg0 with adsorbed HCl), and the rate of Hg0 oxidation mainly depended on Cl• concentration on the surface. As H2O, SO2, and NO not only inhibited Cl• formation on the surface but also interfered with the interface reaction between gaseous Hg0 and Cl• on the surface, Hg0 oxidation over Fe-Ti-Mn spinel was obviously inhibited in the presence of H2O, SO2, and NO. Furthermore, the extent of Hg0 oxidation over Fe-Ti-Mn spinel can be predicted according to the kinetic parameter kE-R, and the predicted result was consistent with the experimental result.

The effect of pH on Synthesis of BiOCl and its photocatalytic oxidization performance

To explore the effect of pH on the photocatalytic oxidation activities of BiOCl catalysts, the BiOCl catalysts under different pH values were prepared via a facile hydrothermal method. The as-prepared catalysts were characterized by BET, XRD, HRSEM, SEM and UV–vis to find out the impact of pH values. Photocatalytic ability of BiOCl catalysts were evaluated by oxidation of gaseous elemental mercury under UV light irradiation. It was found that BiOCl catalysts prepared under alkaline condition exhibited the best photocatalytic oxidation activities. The difference of photocatalytic activity among the as-prepared catalysts can be attribute to the growth orientation of the crystal BiOCl catalyst and the adsorption capacity of elemental mercury.

XANES study of elemental mercury oxidation over RuO2/TiO2 and selective catalytic reduction catalysts for mercury emissions control

Linear combination fitting of the X-ray Absorption Near Edge Spectroscopy (XANES) was used to quantify oxidized mercury species over RuO2/TiO2 and Selective Catalytic Reduction (SCR) catalysts under different simulated flue gas conditions. Halogen gases play a major role in mercury oxidation. In the absence of halogen gas, elemental mercury can react with sulfur that is contained in both the RuO2/TiO2 and SCR catalysts to form HgS and HgSO4. In the presence of HCl or HBr gas, HgCl2 or HgBr2 is the main oxidized mercury species. When both HCl and HBr gases are present, HgBr2 is the preferred oxidation product and no HgCl2 can be found. The formation of HgO and HgS cannot be neglected with or without halogen gas. Other simulated flue gas components such as NO, NH3, SO2 and CO2 do not have significant effect on oxidized mercury speciation when halogen gas is present.

Research on the effect of additives on mercury speciation in coal‐fired derived flue gas

In order to study the effect of additives on mercury speciation in coal‐fired derived flue gases, the additives experiment was conducted in one 110MW coal‐fired power plant, the additives including calcium bromide and iron oxide. Taking advantage of the Ontario Water Act (OHM) and sorbent tube method, mercury at the denitration (SCR) entrance and export and desulfurization (WFGD) entrance was sampled and the variation of mercury before and during adding the additives was analyzed. The results show that the additives of calcium bromide can oxidize elemental mercury and contribute to increasing the proportion of divalent mercury, and in denitration device, the ratio of divalent mercury in flue gas shows an increasing trend along with the increasing of calcium bromide. However, there is no such trend appearing at desulfurization entrance. In addition, the additives of iron oxide have no obvious effect on the oxidation of elemental mercury in flue gas. Adding a small amount of iron oxide has little effect on the form of mercury, and may even reduce the proportion of divalent mercury. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1566–1574, 2016

Investigation of processes controlling summertime gaseous elemental mercury oxidation at midlatitudinal marine, coastal, and inland sites

Abstract. A box model incorporating a state-of-the-art chemical mechanism for atmospheric mercury (Hg) cycling was developed to investigate the oxidation of gaseous elemental mercury (GEM) at three locations in the northeastern United States: Appledore Island (AI; marine), Thompson Farm (TF; coastal, rural), and Pack Monadnock (PM; inland, rural, elevated). The chemical mechanism in this box model included the most up-to-date Hg and halogen chemistry. As a result, the box model was able to simulate reasonably the observed diurnal cycles of gaseous oxidized mercury (GOM) and chemical speciation bearing distinct differences between the three sites. In agreement with observations, simulated GOM diurnal cycles at AI and TF showed significant daytime peaks in the afternoon and nighttime minimums compared to flat GOM diurnal cycles at PM. Moreover, significant differences in the magnitude of GOM diurnal amplitude (AI > TF > PM) were captured in modeled results. At the coastal and inland sites, GEM oxidation was predominated by O3 and OH, contributing 80–99 % of total GOM production during daytime. H2O2-initiated GEM oxidation was significant (∼ 33 % of the total GOM) at the inland site during nighttime. In the marine boundary layer (MBL) atmosphere, Br and BrO became dominant GEM oxidants, with mixing ratios reaching 0.1 and 1 pptv, respectively, and contributing ∼ 70 % of the total GOM production during midday, while O3 dominated GEM oxidation (50–90 % of GOM production) over the remaining day when Br and BrO mixing ratios were diminished. The majority of HgBr produced from GEM+Br was oxidized by NO2 and HO2 to form brominated GOM species. Relative humidity and products of the CH3O2+BrO reaction possibly significantly affected the mixing ratios of Br or BrO radicals and subsequently GOM formation. Gas–particle partitioning could potentially be important in the production of GOM as well as Br and BrO at the marine site.

Hydrothermal synthesis of carbon spheres – BiOI/BiOIO3 heterojunctions for photocatalytic removal of gaseous Hg0 under visible light

The carbon modified BiOI/BiOIO3 (CSs-BiOI/BiOIO3) composites with heterostructures have been prepared via a facile hydrothermal method, using carbon spheres, bismuth nitrate and potassium iodate as precursors. Moreover, a new synthesis mechanism has been proposed. The photocatalytic activities of as-prepared catalysts are evaluated by oxidation of gaseous elemental mercury under visible light irradiation. It is found that the CSs-BiOI/BiOIO3 exhibits highly enhanced visible photocatalytic activity, compared with that of pure BiOIO3. The results show that the Hg0 removal efficiency of CSs-BiOI/BiOIO3 reach up to 92.9% and 83.0% under LED light irradiation and simulated sunlight irradiation respectively. The improved photocatalytic activity can be ascribed to the increased the absorption of visible light, enhanced charge separation and transfer and the surface properties. This work points to CSs-BiOI/BiOIO3 nanocomposites as new and efficient visible light photocatalysts for environmental remediation applications.

Manganese doped CeO2-ZrO2 catalyst for elemental mercury oxidation at low temperature

To develop an efficient catalyst used for elemental mercury (Hg0) oxidation at low temperature in coal-fired power plant, Mn doped CeO2-ZrO2 catalysts were synthesized and studied concerning their performance on Hg0 oxidation. Under the atmosphere of simulated coal-fired flue gas, Ce0.47Zr0.23Mn0.3O2 (CZM0.3) exhibited the best activity at 150°C. It was found that Hg0 oxidation performance over CZM0.3 was determined by the flue gas compositions. Gaseous O2 was important for the oxidation process, because it regenerated the lattice oxygen and replenished the chemisorbed oxygen, which boosted Hg0 oxidation. HCl and NO could improve the Hg0 oxidation efficiency slightly, respectively. In N2+HCl+O2 atmosphere, the Hg0 oxidation efficiency observed was significantly higher than those in N2+O2 or N2+HCl atmosphere. Compared with the effects of HCl on the Hg0 oxidation activity, the similar results were obtained for effects of NO on the performance Hg0 oxidation. NO could facilitate the Hg0 oxidation activity, especially in the presence of O2. In N2+SO2 atmosphere, SO2 inhibited Hg0 oxidation. In N2+O2+SO2 atmosphere, SO2 promoted Hg0 oxidation probably due to the generation of SO3. Water vapor inhibited Hg0 oxidation, because it diminished the HCl and Hg0 adsorption. The characterization of the CZM0.3 indicated that the higher activity of the catalyst was most likely attributed to the presence of more oxygen vacancies, enhanced Mn4+/(Mn4++Mn3+) ratio and more surface adsorbed oxygen on the catalyst surface.

Catalytic oxidation removal of gaseous elemental mercury in flue gas over niobium‐loaded catalyst

Nb‐Co‐Ce/Al2O3 catalysts prepared by impregnation, sol‐gel method, and co‐precipitation were examined for elemental mercury (Hg0) oxidation in a simulated coal combustion flue gas. The catalysts were characterized by SEM‐EDS, BET, XRD, FTIR, and XPS techniques. The performances of different niobium‐loaded catalysts on Hg0 oxidation efficiency with regard to preparation method, assistant, reaction temperature, and typical individual flue gas were investigated. The results showed that the Hg0 oxidation efficiency performed at the highest value for niobium‐loaded catalysts at 523 K. SO2 was observed to have a negative effect on Hg0 oxidation. The niobium‐loaded catalysts prepared by the sol‐gel method and co‐precipitation exhibited higher Hg0 oxidation efficiency than that prepared by impregnation, due to higher specific surface area and more exposure of active sites. However, it also results in these catalysts being affected by SO2 more easily. An enhancing effect of O2 was observed and an addition of HCl would promote the Hg0 oxidation further. Results also indicated that synergistic effects between NbOx, CoOx, and CeOx could promote the sustainable capacity of Hg0 oxidation for Nb‐Co‐Ce/Al2O3 catalysts. Hg0 oxidation over Nb‐Co‐Ce/Al2O3 catalysts is thought to follow a Mars‐Maessen mechanism in which lattice oxygen derived from NbOx would react with absorbed Hg0.

Mechanism of Heterogeneous Mercury Oxidation by HBr over V2O5/TiO2 Catalyst.

Catalytic oxidation of elemental mercury (Hg(0)) through a selective catalytic reduction (SCR) system is a promising method to reduce mercury emissions from coal-burning power plants. The density functional theory (DFT) and periodic slab models were used to study the reaction mechanism of Hg(0) oxidation by HBr on V2O5/TiO2 SCR catalyst surface. The interaction mechanisms of Hg(0), HBr, HgBr, and HgBr2 on V2O5/TiO2(001) were investigated. The oxidation reaction energy profiles and the corresponding geometries of the intermediates, final states, and transition states were researched. The results indicate that Hg(0) and HgBr2 are weakly adsorbed on the oxygen sites of the V2O5/TiO2(001) surface with physisorption. HgBr is chemically adsorbed on the surface. HBr is dissociatively adsorbed on the surface with an energy barrier of 85.59 kJ/mol. The reaction of Hg(0) oxidation by HBr follows the Eley-Rideal mechanism: Hg(0) interacts with a surface Br from HBr dissociation to form HgBr, and surface HgBr further interacts with HBr to form HgBr2, last HgBr2 desorbs from the surface. Comparing the energy pathway of Hg(0) oxidation over V2O5/TiO2(001) surface by HBr to that of HCl, it is found that the dissociation energy barrier of HBr is lower than that of HCl, the formation and desorption energy barriers of HgBr2 are also lower than that of HgCl2, which explains why HBr is much more effective than HCl in promoting Hg(0) oxidation.

Significance of Fe2O3 modified SCR catalyst for gas-phase elemental mercury oxidation in coal-fired flue gas

To improve the ability of commercial selective catalytic reduction (SCR) catalyst to catalyze the oxidation of gaseous elemental mercury, Fe2O3 was introduced. Modifying with Fe2O3 can significantly enhance the elemental mercury oxidation ability of SCR catalyst. Fe2O3/SCR prepared by an impregnation method was employed as mercury oxidation catalysts in the simulated flue gas, and the role of Fe2O3 was investigated. The temperature window was from 150 to 450°C. In this study, Fe2O3/SCR (1% Fe, wt.) was found to be an optimal catalyst with a mercury oxidation efficiency of higher than 90% at 350°C using a simulated flue gas. The catalysts were characterized by X-ray diffraction (XRD), Brunauer Emmet Teller (BET) measurements, and X-ray photoelectron spectroscopy (XPS). The results indicated that the Fe2O3 was well-dispersed on the surface of SCR. The surface areas and catalytic oxidation activity were not consistent patterns, and the diameter of the mercury atom was much smaller than the pore diameter of the Fe2O3/SCR catalysts. Loading content of Fe2O3 was a very important factor for the removal of mercury. HCl was the most effective flue gas component responsible for the Hg0 oxidation. However, SO2 had a slight inhibition effect on Hg0 oxidation. Furthermore, change experiment of a mercury valence state was performed. And the mechanism of Hg0 oxidation was also discussed.

Catalytic oxidation and capture of elemental mercury from simulated flue gas using Mn-doped titanium dioxide

Titanium dioxide (TiO2) and Mn-doped TiO2 (Mn(x)-TiO2) were synthesized in a sol-gel method and characterized by BET surface area analysis, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Gasphase elemental mercury (Hg0) oxidation and capture by the Mn-doped TiO2 catalyst was studied in the simulated flue gas in a fixed-bed reactor. The investigation of the influence of Mn loading, flue gas components (SO2, NO, O2, and H2O) showed that the Hg0 capture capability of Mn(x)-TiO2 was much higher than that of pure TiO2. The addition of Mn inhibits the grain growth of TiO2 and improves the porous structure parameters of Mn(x)-TiO2. Excellent Hg0 oxidation performance was observed with the catalyst with 10% of Mn loading ratio and 97% of Hg0 oxidation was achieved under the test condition (120 °C, N2/6%O2). The presence of O2 and NO had positive effect on the Hg0 removal efficiency, while mercury capture capacity was reduced in the presence of SO2 and H2O. XPS spectra results reveal that the mercury is mainly present in its oxidized form (HgO) in the spent catalyst and Mn4+ doped on the surface of TiO2 is partially converted into Mn3+ which indicates Mn and the lattice oxygen are involved in Hg0 oxidation reactions.

Mercury oxidation from bromine chemistry in the free troposphere over the southeastern US

Abstract. The elevated deposition of atmospheric mercury over the southeastern United States is currently not well understood. Here we measure partial columns and vertical profiles of bromine monoxide (BrO) radicals, a key component of mercury oxidation chemistry, to better understand the processes and altitudes at which mercury is being oxidized in the atmosphere. We use data from a ground-based MAX-DOAS instrument located at a coastal site ∼ 1 km from the Gulf of Mexico in Gulf Breeze, FL, where we had previously detected tropospheric BrO (Coburn et al., 2011). Our profile retrieval assimilates information about stratospheric BrO from the WACCM chemical transport model (CTM), and uses only measurements at moderately low solar zenith angles (SZAs) to estimate the BrO slant column density contained in the reference spectrum (SCDRef). The approach has 2.6 degrees of freedom, and avoids spectroscopic complications that arise at high SZA; knowledge about SCDRef further helps to maximize sensitivity in the free troposphere (FT). A cloud-free case study day with low aerosol load (9 April 2010) provided optimal conditions for distinguishing marine boundary layer (MBL: 0–1 km) and free-tropospheric (FT: 1–15 km) BrO from the ground. The average daytime tropospheric BrO vertical column density (VCD) of ∼ 2.3 × 1013 molec cm−2 (SZA < 70°) is consistent with our earlier reports on other days. The vertical profile locates essentially all tropospheric BrO above 4 km, and shows no evidence for BrO inside the MBL (detection limit < 0.5 pptv). BrO increases to ∼ 3.5 pptv at 10–15 km altitude, consistent with recent aircraft observations. Our case study day is consistent with recent aircraft studies, in that the oxidation of gaseous elemental mercury (GEM) by bromine radicals to form gaseous oxidized mercury (GOM) is the dominant pathway for GEM oxidation throughout the troposphere above Gulf Breeze. The column integral oxidation rates are about 3.6 × 105 molec cm−2 s−1 for bromine, while the contribution from ozone (O3) is 0.8 × 105 molec cm−2 s−1. Chlorine-induced oxidation is estimated to add < 5 % to these mercury oxidation rates. The GOM formation rate is sensitive to recently proposed atmospheric scavenging reactions of the HgBr adduct by nitrogen dioxide (NO2), and to a lesser extent also HO2 radicals. Using a 3-D CTM, we find that surface GOM variations are also typical of other days, and are mainly derived from the FT. Bromine chemistry is active in the FT over Gulf Breeze, where it forms water-soluble GOM that is subsequently available for wet scavenging by thunderstorms or transport to the boundary layer.

Novel Effective Catalyst for Elemental Mercury Removal from Coal-Fired Flue Gas and the Mechanism Investigation.

Mercury pollution from coal-fired power plants has drawn attention worldwide. To achieve efficient catalytic oxidation of Hg(0) at both high and low temperatures, we prepared and tested novel IrO2 modified Ce-Zr solid solution catalysts under various conditions. It was found that the IrO2/Ce0.6Zr0.4O2 catalyst, which was prepared using the polyvinylpyrrolidone-assisted sol-gel method, displayed significantly higher catalytic activity for Hg(0) oxidation. The mechanism of Hg(0) removal over IrO2/Ce0.6Zr0.4O2 was studied using various methods, and the Hg(0) oxidation reaction was found to follow two possible pathways. For the new chemisorption-regeneration mechanism proposed in this study, the adsorbed Hg(0) was first oxidized with surface chemisorbed oxygen species to form HgO; the HgO could desorb from the surface of catalysts by itself or react with adsorbed HCl to be release in the form of gaseous HgCl2. O2 is indispensable for the chemisorption process, and the doping of IrO2 could facilitate the chemisorption process. In addition, the Deacon reaction mechanism was also feasible for Hg(0) oxidation: this reaction would involve first oxidizing the adsorbed HCl to active Cl species, after which the Hg(0) could react with Cl to form HgCl2. Additionally, doping IrO2 could significantly improve the Cl yield process. In summary, the novel IrO2 modified catalyst displayed excellent catalytic activity for elemental mercury oxidation, and the proposed reaction mechanisms were determined reasonably.