Oxidation Of Elemental Mercury Research Articles

Deeply insight into the inhibition mechanism of SO2 on mercury oxidation over Mn/CNT: A DFT study

Mn/CNT is a kind of low temperature catalyst, which can effectively remove elemental mercury from flue gas at low temperature, but its mercury removal ability was significantly inhibited by SO2. In this work, the inhibition mechanism of SO2 was investigated by first-principle calculation based on density functional theory. Three different surfaces of Mn/CNT were constructed: MnO/CNT, MnO2/CNT and Mn2O3/CNT, and the adsorption of Hg0 and SO2 on these three surfaces were studied. It suggests that adsorption energy of Hg0 on MnO/CNT is the highest, which is −2.42 eV. Combined with the electron partial density of states (PDOS) after adsorption, SO2 can compete with Hg0 for the same Mn active site on the catalyst surface during the adsorption process, and the adsorption capacity of Hg0 is weaker than that of SO2. Oxidation path of Hg0 to HgO on the surface of Mn/CNT indicates that the generation of HgO needs to cross at least two energy barriers, with energies of 0.552 eV and 1.25 eV, respectively. However, when Hg0 was oxidized to HgO and adsorbed on the surface of Mn/CNT, SO2 could reduce it to Hg0 again, generating SO3 to block elemental mercury oxidation.

Near surface oxidation of elemental mercury leads to mercury exposure in the Arctic Ocean biota

Atmospheric mercury (Hg(0), Hg(II)) and riverine exported Hg (Hg(II)) are proposed as important Hg sources to the Arctic Ocean. As plankton cannot passively uptake Hg(0), gaseous Hg(0) has to be oxidized to be bioavailable. Here, we measured Hg isotope ratios in zooplankton, Arctic cod, total gaseous Hg, sediment, seawater, and snowpack from the Bering Strait, the Chukchi Sea, and the Beaufort Sea. The Δ200Hg, used to differentiate between Hg(0) and Hg(II), shows, on average, 70% of Hg(0) in all biota and differs with seawater Δ200Hg (Hg(II)). Since Δ200Hg anomalies occur via tropospheric Hg(0) oxidation, we propose that near-surface Hg(0) oxidation via terrestrial vegetation, coastally evaded halogens, and sea salt aerosols, which preserve Δ200Hg of Hg(0) upon oxidation, supply bioavailable Hg(II) pools in seawater. Our study highlights sources and pathways in which Hg(0) poses potential ecological risks to the Arctic Ocean biota.

Deactivation effect of elemental mercury oxidation over a V2O5-WO3/TiO2 catalyst by Pb in simulated coal-fired flue gas

Lead (Pb) is a typical heavy metal in the flue gas of coal-fired power plants, which can cause the deactivation of SCR catalysts. In this study, Pb-doped vanadium-based catalysts were prepared and investigated for the deactivation effect and mechanism of elemental Hg oxidation caused by Pb. The results showed that doping Pb on vanadium-based catalysts led to severe deactivation, which was more pronounced with increased Pb content. Further investigation illustrated that the introduction of Pb not only weakened the BET surface properties of vanadium-based catalysts but also increased the aggregation of surface species on the catalysts. After Pb introduction, the adsorption capacity toward elemental Hg, proportion of highly oxidized V5+, and surface Oβ ratio significantly decreased severely weakening the activity of the catalyst. In addition, we demonstrated that the reaction between Pb and active VO/V−OH groups of the catalyst formed inactive V−O−Pb species, which was the direct cause of catalyst deactivation.

Review on Mercury Control during Co-Firing Coal and Biomass under O2/CO2 Atmosphere

Combining biomass co-firing with oxy-fuel combustion is a promising Bioenergy with Carbon Capture and Storage (BECCS) technology. It has the potential to achieve a large-scale reduction in carbon emissions from traditional power plants, making it a powerful tool for addressing global climate change. However, mercury in the fuel can be released into the flue gas during combustion, posing a significant threat to the environment and human health. More importantly, mercury can also cause the fracture of metal equipment via amalgamation, which is a major risk for the system. Therefore, compared to conventional coal-fired power plants, the requirements for the mercury concentration in BECCS systems are much stricter. This article reviews the latest progress in mercury control under oxy-fuel biomass co-firing conditions, clarifies the impact of biomass co-firing on mercury species transformation, reveals the influence mechanisms of various flue gas components on elemental mercury oxidation under oxy-fuel combustion conditions, evaluates the advantages and disadvantages of various mercury removal methods, and finally provides an outlook for mercury control in BECCS systems. Research shows that after biomass co-firing, the concentrations of chlorine and alkali metals in the flue gas increase, which is beneficial for homogeneous and heterogeneous mercury oxidation. The changes in the particulate matter content could affect the transformation of gaseous mercury to particulate mercury. The high concentrations of CO2 and H2O in oxy-fuel flue gas inhibit mercury oxidation, while the effects of NOx and SO2 are dual-sided. Higher concentrations of fly ash in oxy-fuel flue gas are conducive to the removal of Hg0. Additionally, under oxy-fuel conditions, CO2 and metal ions such as Fe2+ can inhibit the re-emission of mercury in WFGD systems. The development of efficient adsorbents and catalysts is the key to achieving deep mercury removal. Fully utilizing the advantages of chlorine, alkali metals, and CO2 in oxy-fuel biomass co-firing flue gas will be the future focus of deep mercury removal from BECCS systems.

Unveiling the electron-acceptor effect in selenium-doped Cu-N-C catalyst with atomically dispersed active sites for boosting Hg0 oxidation

M-N-C catalysts have considerable potential in the fields of catalysis, such as highly polluting gaseous elemental mercury (Hg0) oxidation. However, the direct application of these kinds of catalysts for mercury removal is stymied by their low catalytic efficiency. Heteroatom-doping in M-N-C catalysts has been proposed as a powerful strategy to the promote catalytic of Hg0, but the origin of boosting catalytic activity is still elusive. Herein, it is disclosed that selenium doping induces an obvious electron-acceptor effect for accelerating the Hg0 catalytic oxidation. The model selenium-doped Cu-N-C catalyst with atomically dispersed active sites was verified by various characterization methods, such as SEM-EDS and scanning transmission electron microscopy. The optimal Cu-NC-Se with Cu/Se ratio of 1:1 exhibited the best Hg0 removal performance, over 8-fold enhancement in their Hg0 oxidation capacities than raw selenium-free Cu-NC. At the optimal reaction temperature of 90 °C, Cu-NC-Se achieved an average Hg0 removal rate of 92.44% within 60 min under single pure N2 atmosphere. As for the influence of flue gas components, the presence of O2 promoted the removal of mercury in Cu-NC-Se, whereas CO2, NO and SO2 served as slight inhibitors. Even after five cycles of the experiment, the average mercury oxidization efficiency was still remained at 88.5%, showing that Cu-NC-Se can be reused with good maintenance of the catalytic activity and atomic dispersion of Cu and Se in the structure. The unsaturated Cu sites connected with pyridinic N on the carbon matrix can serve preferential adsorption to Hg0 and doped selenium. Meanwhile, the selenium doping accelerated Hg0 adsorption and accept sufficient electrons for promoting Hg0 oxidation to forming stable HgSe, which is also corroborated by the theoretical analysis results based on density functional theory. This work not only provides an alternative facile synthetic strategy for developing heteroatom-doping atomically dispersed M-N-C catalysts, but also represents a significant advance for deepening the understanding of the Hg0 catalytic oxidation mechanism on M-N-C catalyst.

Amorphous TiO2 doped with carbon for visible light photocatalytic oxidation of elemental mercury

A novel simple sol–gel method, flame assisted fabrication of amorphous TiO2 doped with carbon, was demonstrated. The as-prepared samples were identified by TGA, XRD, SEM and BET. And the optical property of the as-prepared samples was measured by the UV–vis and FTIR. The obtained amorphous C-TiO2 (carbon-doped TiO2) photocatalysts were used to remove gaseous elemental mercury under visible light (LED) and the optimum doping levels of C-TiO2 were determined. The results show that all the samples had a good mercury removal efficiency (>50 %). When the volume ratio of TBOT/ethanol reached 1:13, the sample has the best mercury removal efficiency 76 %. The incorporation of carbon and the microspherical structure enhanced the ability of light absorption, which led to the improvement of photocatalytic activity. This work has proposed the probable mechanism of removing gaseous elemental mercury by amorphous C-TiO2.

Fabrication of BiOIO3/g-C3N4/Er3+ containing oxygen vacancies for photocatalytic oxidation of gaseous elemental mercury

Construction of heterojunction and formation of oxygen vacancy are effective methods to improve the performance of photocatalysts. In this work, Er3+-doped BiOIO3/g-C3N4 photocatalyst with oxygen-rich vacancies was prepared by a simple hydrothermal and calcination method for the photocatalytic oxidation of elemental mercury. The structural properties of BiOIO3/g-C3N4/Er3+ were analyzed by XRD, SEM and BET. The results showed that all samples exhibited good photocatalytic oxidation ability for Hg0 under visible light irradiation. Among them, the photocatalyst with Er3+-doped BiOIO3/g-C3N4 and then calcined has the highest removal efficiency of Hg0, which is 68.83%. This may be due to the increase of oxygen vacancy caused by Er3+ doping, which enables the photocatalyst to have higher electron transfer rate and wider visible light absorption range. Finally, the possible mechanism of photocatalytic oxidation of gaseous elemental mercury with BiOIO3/g-C3N4/Er3+ photocatalyst was proposed. This study provides some reference for the design and application of photocatalysts containing oxygen vacancies.

Modelling the coupled mercury-halogen-ozone cycle in the central Arctic during spring

Near-surface mercury and ozone depletion events occur in the lowest part of the atmosphere during Arctic spring. Mercury depletion is the first step in a process that transforms long-lived elemental mercury to more reactive forms within the Arctic that are deposited to the cryosphere, ocean, and other surfaces, which can ultimately get integrated into the Arctic food web. Depletion of both mercury and ozone occur due to the presence of reactive halogen radicals that are released from snow, ice, and aerosols. In this work, we added a detailed description of the Arctic atmospheric mercury cycle to our recently published version of the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem 4.3.3) that includes Arctic bromine and chlorine chemistry and activation/recycling on snow and aerosols. The major advantage of our modelling approach is the online calculation of bromine concentrations and emission/recycling that is required to simulate the hourly and daily variability of Arctic mercury depletion. We used this model to study coupling between reactive cycling of mercury, ozone, and bromine during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) spring season in 2020 and evaluated results compared to land-based, ship-based, and remote sensing observations. The model predicts that elemental mercury oxidation is driven largely by bromine chemistry and that particulate mercury is the major form of oxidized mercury. The model predicts that the majority (74%) of oxidized mercury deposited to land-based snow is re-emitted to the atmosphere as gaseous elemental mercury, while a minor fraction (4%) of oxidized mercury that is deposited to sea ice is re-emitted during spring. Our work demonstrates that hourly differences in bromine/ozone chemistry in the atmosphere must be considered to capture the springtime Arctic mercury cycle, including its integration into the cryosphere and ocean.

Theoretical insight into the influence of SO2 on the adsorption and oxidation of mercury over the MnO2 surface

The inferior sulfur resistance is a major technical bottleneck of Mn-based catalysts to be employed as low-temperature selective catalytic reduction (SCR) catalyst for elemental mercury (Hg0) oxidation. The first-principle calculation based on density functional theory was adopted to investigate the influence of SO2 on mercury adsorption and oxidation over the MnO2 surface. The results indicated that Hg0 preferred to adsorb at the hollow and Obr bridge sites on the surface, and the adsorption of HgO was a strong chemisorption behavior. SO2 could be retained on the MnO2 surface through chemisorption. The adsorbed SO2 generated inhibition on Hg0 adsorption by the competitive and repulsive effects, while the effect of SO2 on HgO adsorption was negligible. SO2 showed almost no impact on the adsorption and dissociation of O2. Hg0 adsorption and oxidation at the active sites adjacent to the adsorbed SO2 was apparently restrained on the SO2-covered surface, and the desorption of generated HgO became easier compared to that on the clean surface. By contrast, Hg0 oxidation at the nonadjacent sites to SO2 was little influenced by the adsorbed SO2. The study results would offer guidance for impairing the inhibitory effect of SO2 and improving the SO2 resistance of Mn-based catalysts.

Source mechanisms and transport patterns of tropospheric bromine monoxide: findings from long-term multi-axis differential optical absorption spectroscopy measurements at two Antarctic stations

Abstract. The presence of reactive bromine in polar regions is a widespread phenomenon that plays an important role in the photochemistry of the Arctic and Antarctic lower troposphere, including the destruction of ozone, the disturbance of radical cycles, and the oxidation of gaseous elemental mercury. The chemical mechanisms leading to the heterogeneous release of gaseous bromine compounds from saline surfaces are in principle well understood. There are, however, substantial uncertainties about the contribution of different potential sources to the release of reactive bromine, such as sea ice, brine, aerosols, and the snow surface, as well as about the seasonal and diurnal variation and the vertical distribution of reactive bromine. Here we use continuous long-term measurements of the vertical distribution of bromine monoxide (BrO) and aerosols at the two Antarctic sites Neumayer (NM) and Arrival Heights (AH), covering the periods of 2003–2021 and 2012–2021, respectively, to investigate how chemical and physical parameters affect the abundance of BrO. We find the strongest correlation between BrO and aerosol extinction (R=0.56 for NM and R=0.28 for AH during spring), suggesting that the heterogeneous release of Br2 from saline airborne particles (blowing snow and aerosols) is a dominant source for reactive bromine. Positive correlations between BrO and contact time of air masses, both with sea ice and the Antarctic ice sheet, suggest that reactive bromine is not only emitted by the sea ice surface but by the snowpack on the ice shelf and in the coastal regions of Antarctica. In addition, the open ocean appears to represent a source for reactive bromine during late summer and autumn when the sea ice extent is at its minimum. A source–receptor analysis based on back trajectories and sea ice maps shows that main source regions for BrO at NM are the Weddell Sea and the Filchner–Ronne Ice Shelf, as well as coastal polynyas where sea ice is newly formed. A strong morning peak in BrO frequently occurring during summer and that is particularly strong during autumn suggests a night-time build-up of Br2 by heterogeneous reaction of ozone on the saline snowpack in the vicinity of the measurement sites. We furthermore show that BrO can be sustained for at least 3 d while travelling across the Antarctic continent in the absence of any saline surfaces that could serve as a source for reactive bromine.

Spontaneous adsorption–oxidation of gaseous elemental mercury via a conjugated unit –NH+˙–Cl*: creation and mechanisms

A Cl-doped protonated polyaniline-coated multiwall carbon nanotube (Cl-PANI+@MWCNT) composite provides a new method for the spontaneous adsorption–oxidation of gaseous elemental mercury.

Cu-CeO2 nanorings with abundant oxygen vacancies for superior catalytic oxidation

Ceria (CeO2) catalysts with different structures and shapes usually exhibit significant variations in catalytic activity. In this work, copper-doped CeO2 (Cu-CeO2) nanorings with abundant oxygen vacancies were synthesized by a Cu induced self-assemble method via hydrothermal process. The formation process of Cu-CeO2 investigated by TEM and XRD showed that the introduction of Cu dopants was critical to the formation of unique nanorings structure. The catalytic oxidation activity of Cu-CeO2 was evaluated by elemental mercury oxidation in the flue gas. Specifically, the Cu-CeO2 nanorings had a superior mercury oxidation efficiency of about 100% at 200 ℃ due to the abundance of oxygen vacancies.

Sonochemical oxidation and stabilization of liquid elemental mercury in water and soil

Over 3000 mercury (Hg)-contaminated sites worldwide contain liquid metallic Hg [Hg(0)l] representing a continuous source of elemental Hg(0) in the environment through volatilization and solubilization in water. Currently, there are few effective treatment technologies available to remove or sequester Hg(0)l in situ. We investigated sonochemical treatments coupled with complexing agents, polysulfide and sulfide, in oxidizing Hg(0)l and stabilizing Hg in water, soil and quartz sand. Results indicate that sonication is highly effective in breaking up and oxidizing liquid Hg(0)l beads via acoustic cavitation, particularly in the presence of polysulfide. Without complexing agents, sonication caused only minor oxidation of Hg(0)l but increased headspace gaseous Hg(0)g and dissolved Hg(0)aq in water. However, the presence of polysulfide essentially stopped Hg(0) volatilization and solubilization. As a charged polymer, polysulfide was more effective than sulfide in oxidizing Hg(0)l and subsequently stabilizing the precipitated metacinnabar (β-HgS) nanocrystals. Sonochemical treatments with sulfide yielded incomplete oxidation of Hg(0)l, likely resulting from the formation of HgS coatings on the dispersed µm-size Hg(0)l bead surfaces. Sonication with polysulfide also resulted in rapid oxidation of Hg(0)l and precipitation of HgS in quartz sand and in the Hg(0)l-contaminated soil. This research indicates that sonochemical treatment with polysulfide could be an effective means in rapidly converting Hg(0)l to insoluble HgS precipitates in water and sediments, thereby preventing its further emission and release to the environment. We suggest that future studies are performed to confirm its technical feasibility and treatment efficacy for remediation applications.

Role of CuFe2O4 in elemental mercury adsorption and oxidation on modified bentonite for coal gasification

• Roles of CuFe 2 O 4 in Hg 0 adsorption and oxidation on CuFe 2 O 4 -modified bentonite. • Hg 0 adsorption and oxidation mechanism of CuFe 2 O 4 established. • The adsorption of Hg 0 on the surface of CuFe 2 O 4 (1 1 0) have the stronger interaction. Taking spinel CuFe 2 O 4 -modified bentonite as the research object, the experimental and thermodynamic techniques were integrated to study the effect of CuFe 2 O 4 active components on removing Hg 0 in coal gasification process. Meanwhile, the density functional theory was adopted to identify the CuFe 2 O 4 ’s adsorption and oxidation mechanisms for Hg 0 . The results showed that the doping of CuFe 2 O 4 increased the active sites for adsorbing Hg 0 on the surface of bentonite and enabled bentonite with effective magnetic properties. Compared with pure bentonite, the CuFe 2 O 4 -modified one was able to convert more Hg 0 into Hg 2+ and particulate mercury[Hg(P)], which exhibited its better capacities of adsorbing and catalyzing Hg 0 . The thermodynamic simulation outcomes further verified that the bentonite adsorbent modified by CuFe 2 O 4 became more conducive to the conversion of Hg 0 to Hg 2+ . Moreover, DFT results also presented that Hg 0 was under stronger adsorption effect at the Cu-top active sites of CuFe 2 O 4 (1 1 0), which was closely related to the hybridization of orbitals between Hg and Cu atoms. The reaction of Hg 0 with O atoms followed the Markov-Mason mechanism, meaning that Hg diffusion at Fe/Cu-top site was the controlling step, HgO desorption at O-top site and hollow was the control step. The reaction of Hg 0 with active S atoms followed the two mechanisms of Eley−Riddel(E−R) and Langmuir−Hinshlwood (L−H). The energy barrier of L-H mechanism was lower than that of E-R mechanism, which was dominant in the reaction process. The excellent performance suggested that the CuFe 2 O 4 -modified bentonite was a promising sorbent for mercury removal in coal gasification.

A novel bionic flower-like Z-scheme Bi4O5I2/g-C3N5 heterojunction with [formula omitted] active sites and π-conjugated structure for increasing photocatalytic oxidation of elemental mercury

The carrier life regulation and redox media are key factors in photocatalytic purification of the environment. Herein, based on the specific π-conjugation network structure of g-C3N5, a novel Bi4O5I2/g-C3N5 Z-scheme heterojunction enriching I−/I3− active sites photocatalyst was creatively prepared, which properties were confirmed by X-ray photoelectron spectroscopy and photoelectrochemical analysis. Benefiting from the π conjugate network structure and I−/I3−redox medium, the photocatalytic activity of the as-prepared Bi4O5I2/g-C3N5 photocatalyst has been significantly improved by more than 3.1 times, and the highest photocatalytic mercury removal arrives up to 92.6%. This research proposes new enlightenment for the accelerated transport of electrons in heterojunction photocatalyst, which in turn contributes a highly efficient photocatalyst to control heavy metal pollutants in the flue gas at coal combustion power stations.

Enhancing the oxidation efficiency of elemental mercury in the presence of NO and SO2 by using Cu(II)O doped TiO2 photothermal catalysts: Kinetics and mechanisms

This study aimed to apply sol–gel synthesized Cu(II)O doped TiO2 to enhance the oxidation of gaseous elemental mercury (Hg0) in the flue gases containing NO and SO2 at the temperatures of 120-180℃ under the irradiation of near-ultraviolet (near-UV). The influences of SO2 and NO on the oxidation efficiency of Hg0 were investigated by in-situ diffuse reflection infrared spectroscopy (DRIFT) and density functional theory (DFT). Experimental results showed that an optimal 5 %Cu(II)O/TiO2 had the oxidation efficiencies of Hg0 from 40 to 100 % at 120-180℃. The oxidation efficiencies of Hg0 were 85 and 70 % at 120℃, and 40 and 5 % at 180 °C, in the presence of solely SO2 and NO, respectively. Surface characterization showed that Cu(II)O was evenly dispersed over the surface of TiO2 catalysts. NO could inhibit the oxidation of Hg0 since it consumed the chemisorbed oxygen (Oα) and compete with Hg0 at the surface active sites. In contrast, SO2 could promote the oxidation efficiency of Hg0 due to the formation of HgSO4 on the surface of Cu(II)O/TiO2 catalysts. In the exposure of NO/SO2, NO and SO2 can be adsorbed on the catalyst surface reacting with ̇OH. The photothermal oxidation reactivity of Hg0 was inhibited due to the competitive adsorption between NO/SO2 and Hg0. The Langmuir-Hinshelwood (L-H) kinetic model successfully simulated the oxidation rate of Hg0. Moreover, DFT was further applied to estimate the binding energies of different gas molecules confirming the competence for active sites on the surface of Cu(II)O/TiO2 between NO/SO2 and Hg0.

A kinetic study on mercury oxidation by HCl over typical Mn-based SCR catalysts

A kinetic approach was adopted to analyze the reaction mechanism of elemental mercury (Hg0) oxidation by HCl over three Mn-based low-temperature SCR catalysts, MnOx/TiO2, Fe-MnOx/TiO2 and CeMnO3. Experimental results validated that Hg0 oxidation efficiencies of the catalysts were promoted by HCl at different temperatures. The kinetic models for the heterogeneous Hg0 oxidation were established and verified based on the experimental data. The verification results demonstrated that Hg0 oxidation over the Mn-based catalysts follows Langmuir-Hinshelwood mechanism. The kinetic parameters of reaction rate constant and HCl adsorptionconstant were calculated from the model fitting equations. The reaction rate constant was raised with the increase of temperature, while the HCl adsorptionconstant presented the opposite trend. The Mn-based catalysts with the reaction rate constant of 84.17–335.77 s−1 showed the advantage over some other catalysts such as commercial V2O5-(WO3)/TiO2 and CeO2/TiO2. The kinetic parameters were further improved in the presence of O2. The apparent activation energy for Hg0 oxidation over the catalysts that was derived from the kinetic parameters was 4.13–12.53 kJ/mol, which was much lower than that of the other approaches. The advantageous kinetic parameters and apparent activation energy were favorable for the Mn-based catalysts to act as low-temperature SCR catalyst for synergistic Hg0 removal. The kinetic study results were of fundamental significance for designing the reactor according to the known flue gas conditions, used catalyst and required Hg0 oxidation efficiency.

Oxygen-Induced Elemental Mercury Oxidation in Chemical Looping Combustion of Coal.

Mercury emission is an important issue during chemical looping combustion (CLC) of coal. The aim of this work is to explore the effects of different flue gas components (e.g., HCl, NO, SO2, and CO2) on mercury transformation in the flue gas cooling process. A two-stage simulation method is used to reveal the reaction mechanism of these gases affecting elemental mercury (Hg0) oxidation. Furthermore, using this method, Hg0 oxidation by eight oxygen carriers (Co3O4, CaSO4, CeO2, Fe2O3, Al2O3, Mn2O3, SiO2, and CuO) commonly used in CLC are investigated and their Hg0 oxidation efficiencies were compared with the existing experimental results. The results show that HCl, NO, and CO2 promote Hg0 oxidation during flue gas cooling, while SO2 inhibits Hg0 oxidation. The stronger the oxygen release capacity of oxygen carriers, the higher the oxidation efficiency of Hg0 becomes. The order of Hg0 removal efficiency from high to low is Co3O4, CuO, Mn2O3, CaSO4, Fe2O3, CeO2, Al2O3, and SiO2, and this sequence is in good agreement with the existing experimental results. Different flue gas components directly or indirectly affect the O2 content, thus affecting the content of gaseous oxidized mercury (Hg2+). Different oxygen carriers have different oxygen release capacities and different Hg0 oxidation efficiencies. Therefore, O2 is the core species affecting the mercury transformation in CLC.

Calibration Approach for Gaseous Oxidized Mercury Based on Nonthermal Plasma Oxidation of Elemental Mercury.

Atmospheric mercury measurements carried out in the recent decades have been a subject of bias largely due to insufficient consideration of metrological traceability and associated measurement uncertainty, which are ultimately needed for the demonstration of comparability of the measurement results. This is particularly challenging for gaseous HgII species, which are reactive and their ambient concentrations are very low, causing difficulties in proper sampling and calibration. Calibration for atmospheric HgII exists, but barriers to reliable calibration are most evident at ambient HgII concentration levels. We present a calibration of HgII species based on nonthermal plasma oxidation of Hg0 to HgII. Hg0 was produced by quantitative reduction of HgII in aqueous solution by SnCl2 and aeration. The generated Hg0 in a stream of He and traces of reaction gas (O2, Cl2, or Br2) was then oxidized to different HgII species by nonthermal plasma. A highly sensitive 197Hg radiotracer was used to evaluate the oxidation efficiency. Nonthermal plasma oxidation efficiencies with corresponding expanded standard uncertainty values were 100.5 ± 4.7% (k = 2) for 100 pg of HgO, 96.8 ± 7.3% (k = 2) for 250 pg of HgCl2, and 77.3 ± 9.4% (k = 2) for 250 pg of HgBr2. The presence of HgO, HgCl2, and HgBr2 was confirmed by temperature-programmed desorption quadrupole mass spectrometry (TPD-QMS). This work demonstrates the potential for nonthermal plasma oxidation to generate reliable and repeatable amounts of HgII compounds for routine calibration of ambient air measurement instrumentation.

Graphitic Carbon Nitride for Gaseous Mercury Emission Control: A Review

Mercury emissions from combustion of coal, biomass, and solid waste threaten human life and the environment. As the principal mercury species in flue gas, elemental mercury (Hg0) can easily escape from available air pollution control instruments because of its chemical inertness, high volatility, and aqueous insolubility. Recently, graphitic carbon nitride (g-C3N4) seems to be an intriguing candidate for Hg0 emission control. This review highlights the latest advances in adsorptive and photocatalytic removal of Hg0 by g-C3N4-based composites. Metal oxide (MeOx)- and halide-modified g-C3N4 can effectively capture Hg0 from simulated flue gas. Nonetheless, acidic flue gas components can degrade the performances of MeOx/g-C3N4 adsorbents, while halide-modified g-C3N4 suffers from release of halogen and potential leaching risk of mercury. Metal sulfide (MeSx)-modified g-C3N4 appears to be the best candidate for Hg0 capture owing to its superior tolerance to sulfur dioxide, fast adsorption rate, and high mercury capacity. Combining bismuth oxyhalides (BiOX) with g-C3N4 can promote photocatalytic oxidation of elemental mercury under visible light irradiation; nonetheless, the Hg0 removal efficiency is still unsatisfactory, and the underlying photocatalysis mechanism is not yet clear.