Cost-Effective Manganese Ore Sorbent for Elemental Mercury Removal from Flue Gas

Tailored bacteria tackling with environmental mercury: Inspired by natural mercuric detoxification operons

Mercury (Hg) and its inorganic and organic compounds significantly threaten the ecosystem and human health. However, the natural and anthropogenic Hg environmental inputs exceed 5000 metric tons annually. Hg is usually discharged in elemental or ionic forms, accumulating in surface water and sediments where Hg-methylating microbes-mediated biotransformation occurs. Microbial genetic factors such as the mer operon play a significant role in the complex Hg biogeochemical cycle. Previous reviews summarize the fate of environmental Hg, its biogeochemistry, and the mechanism of bacterial Hg resistance. This review mainly focuses on the mer operon and its components in detecting, absorbing, bioaccumulating, and detoxifying environmental Hg. Four components of the mer operon, including the MerR regulator, divergent mer promoter, and detoxification factors MerA and MerB, are rare bio-parts for assembling synthetic bacteria, which tackle pollutant Hg. Bacteria are designed to integrate synthetic biology, protein engineering, and metabolic engineering. In summary, this review highlights that designed bacteria based on the mer operon can potentially sense and bioremediate pollutant Hg in a green and low-cost manner.

Efficient immobilization and detoxification of gaseous elemental mercury by nanoflower/rod WSe2/halloysite composite: Performance and mechanisms

Gaseous mercury pollution control technologies with low stability and high releasing risks always face with great challenges. Herein, we developed one halloysite nanotubes (HNTs)-supported tungsten diselenide (WSe2) composite (WSe2/HNTs) by one-pot solvothermal approach, curing Hg0 from complicated flue gas (CFG) and reducing second environment risks. WSe2 as a monolayer with nano-flower structure and HNTs with rod shapes in the as-prepared sorbent exhibited outstanding synergy efficiency, resulting in exceptional performance for Hg0 removal with high capture capacity of 30.6 mg·g-1 and rate of 9.09 μg·g-1·min-1, which benefited from the high affinity of selenium and mercury (1 ×1045) and the adequate exposure of Se-terminated. The adsorbent showed beneficial tolerance to high amount of NOx and SOx. An online lab-built thermal decomposition system (TPD-AFS) was employed to explore Hg species on the used-sorbent, finding that the adsorbed-mercury species were principally mercury selenide (HgSe). Density functional theory calculations indicated that the hollow-sites were the major adsorption sites and exhibited excellent selectivity for Hg0, as well as HgSe generation needed to overcome the 0.32 eV energy barrier. The adsorbed mercury displayed high environmental stability after the leaching toxicity test, which significantly decreased its secondary environmental risks. With these advantages, WSe2/HNTs possess enormous potential to achieve the effective and permanent immobilization of gaseous mercury from CFG in the future.

To be support or promoter: the mode of introducing ceria into commercial V2O5/TiO2 catalyst for enhanced Hg0 oxidation

Ce-modified commercial vanadium-based catalysts are still in a rapid development stage in terms of optimizing Hg⁰ oxidation performance. Due to the universal property of ceria, it can act as either support or promoter to supported vanadium-based catalysts. However, the introduction mode of Ce on the Hg⁰ oxidation is still unclarified. Herein, introducing Ce to vanadium-based catalysts as a promoter (VCe/Ti) plays a more effective role in the Hg⁰ oxidation than only doping Ce into TiO₂ support (V/CeTi). It is revealed that the strong interaction between V and Ce increases the orbital hybridization, and reduces the lowest unoccupied molecular orbital (LUMO) of V, which is conducive to adsorbing and activating HCl. The excellent performance of the VCe/Ti catalyst can be ascribed to its superior redox ability, stronger HCl adsorption capacity, abundant surface oxygen vacancies, and the redox equilibrium (Ce³⁺ + V⁵⁺ ↔ Ce⁴⁺ + V⁴⁺), which improves electron transfer, and thus the catalytic activity. This work provides the potential application of Ce-modified V-based catalysts for the simultaneous control of NO_x and Hg⁰ in stationary sources.

Effecting pattern study of SO2 on Hg0 removal over α-MnO2 in-situ supported magnetic composite

α-MnO₂ was in-situ supported onto silica coated magnetite nanoparticles (MagS-Mn) to study the adsorption and oxidation of Hg⁰ as well as the effecting patterns of SO₂ and O₂ on Hg⁰ removal. MagS-Mn showed Hg⁰ removal capacity of 1122.6 μg/g at 150 °C with the presence of SO₂. Hg⁰ adsorption and oxidation efficiencies were 2.4% and 90.6%, respectively. Hg⁰ removal capability deteriorated at elevated temperatures. Surface oxygen and manganese chemistry analysis indicated that SO₂ inhibited the Hg⁰ removal through consumption of adsorbed oxygen and reduction of high valence manganese. This inhibiting effect was observed to be counteracted by O₂ at lower temperatures. O₂ tended to compete with SO₂ for active sites and further create additional adsorbed oxygen sites for Hg⁰ surface reaction via surface dissociative adsorption rather than replenish the active sites consumed by SO₂. The high valence manganese was also preserved by O₂ which was essential to Hg⁰ oxidation. The intervention of O₂ in the inhibition of SO₂ on Hg⁰ removal was weakened at temperatures higher than 250 °C. Aa a result, Hg⁰ tends to be catalytic oxidized in the condition of low reaction temperatures and with the presence of O₂ over α-MnO₂ oriented composites.

Structure-Directing Role of Support on Hg0 Oxidation over V2O5/TiO2 Catalyst Revealed for NOx and Hg0 Simultaneous Control in an SCR Reactor

The crystal structure of TiO₂ strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO₂ with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg⁰ oxidation. The Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B was the best, followed by V₂O₅/TiO₂-A and V₂O₅/TiO₂-R. Further experimental and theoretical results indicate that gaseous Hg⁰ reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with respect to HCl and Hg⁰ concentration were approximately 0 and 1, respectively. The excellent Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO_x and Hg⁰ on state-of-the-art V₂O₅/TiO₂ catalyst, a combination of anatase and brookite TiO₂-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH₃ on Hg⁰ oxidation.

Mercury adsorption characteristics of carbon sorbent with low surface area

Several studies have been conducted to decrease the cost of sorbents used for the control of mercury emissions. Thus far, several sorbents with low surface areas have been reported to exhibit promising mercury removal capacities. However, based on the results reported, it is difficult to understand the mechanisms of adsorption and oxidization of elemental mercury on sorbents with low surface areas compared to those with higher surface areas. Three types of materials with different surface areas were evaluated herein for use as carbon sorbents for the adsorption of elemental mercury: (1) coal, (2) sewage sludge, and (3) unburned carbon. The respective raw sorbents and FeCl₃-impreganted congeners were evaluated. Each sorbent was tested in a fixed-bed reactor system under two simulated flue gas conditions (1) without and (2) with 20 ppm hydrogen chloride (HCl). The injection of HCl increased the mercury adsorption efficiency of all tested sorbents by decreasing the emission of elemental mercury. Doping the sorbent with FeCl₃ increased the mercury adsorption efficiency during the earlier test period under both simulated flue gas conditions (without and with HCl). FeCl₃-impregnated activated carbon and FeCl₃-impregnated unburned carbon emitted large amounts of oxidized mercury during the later test periods.Implications: We tested three types of sorbents to investigate the mercury adsorption characteristics of sorbents with low surface area. The mercury adsorption test was conducted by varying the raw material of the sorbent, chemical impregnation of the sorbent and the simulated flue gas composition. We found that HCl in simulated flue gas increased the mercury adsorption efficiency of both the raw and FeCl₃-impregnated sorbents by decreasing the emission of elemental mercury.

Nanosized ZnIn2S4 supported on facet-engineered CeO2 nanorods for efficient gaseous elemental mercury immobilization

Nanosized ZnIn₂S₄ supported on facet-engineered CeO₂ nanorods were prepared by solvothermal method to effectively capture gaseous elemental mercury from flue gas. The CeO₂/ZnIn₂S₄ sorbent exhibited excellent mercury removal performance (>90%) in a wide temperature range from 60 to 240 ℃ and showed much higher mercury adsorption capacity than pure CeO₂ due to the enlarged specific surface area and abundant active oxygen and sulfur sites on the surface. It was found that CeO₂/ZnIn₂S₄ has good resistance to SO₂, NO and H₂O. At the optimal 120 ℃, the equilibrium Hg⁰ adsorption capacity of CeO₂/ZnIn₂S₄ can reach 19.172 mg/g, which is superior to the reported series of benchmark materials. X-ray photoelectron spectroscopy and temperature programmed desorption of mercury confirmed that the adsorbed mercury existed on the surface as HgO and HgS, indicating that catalytic oxidation and chemisorption occurred on the surface of the adsorbent. The adsorption energy of Hg⁰ on the CeO₂ (110) and ZnIn₂S₄ (110) surfaces calculated with density functional theory (DFT), further confirms that the surface activated oxygen and sulfur sites are the most stable adsorption sites. Furthermore, the good regeneration capability of CeO₂/ZnIn₂S₄ makes it more promising for Hg⁰ capture in practical applications.

A Critical Review on Removal of Gaseous Pollutants Using Sulfate Radical-based Advanced Oxidation Technologies

Excessive emissions of gaseous pollutants such as SO₂, NO_x, heavy metals (Hg, As, etc.), H₂S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO₄•^-)-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO₄•^--based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO₂, NO_x, Hg, As, H₂S, and VOCs) using SO₄•^--based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO₄•^--based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO₄•^--based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO₄•^--based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.

Potential hazards of novel waste-derived sorbents for efficient removal of mercury from coal combustion flue gas

Novel waste-derived sorbents synthesized through one-step co-pyrolysis of wood and PVC (or brominated flame retarded plastic) were demonstrated as cost-effective sorbents for mercury (Hg) removal in our previous studies. To introduce magnetism and improve porosity, Fe species were further doped into such waste-derived sorbents. The ultimate fate of Hg-laden sorbents after their service is mainly disposed in landfill. Therefore, the stability of Hg/halogens on the spent sorbents is an important topic. In this work, the leachability of Hg/Cl/Br from four waste-derived sorbents was evaluated using toxicity characteristic leaching procedure (TCLP). Three traditional sorbents (Cl-impregnated activated carbon, Br-impregnated activated carbon and commercial activated carbon) were also tested for comparison. Experimental results suggested that the stability of Hg/Cl/Br on four waste-derived sorbents was far higher than that prepared by chemical impregnation. For four waste-derived sorbents, little Hg was leached out whereas certain amounts of Cl/Br escaped into the leachate. Interestingly, Fe-doping effectively improved the stability of Hg/Cl/Br on the waste-derived sorbents. Kinetic analysis revealed that diffusion process and surface chemical reaction were respectively the rate-limiting step for waste-derived sorbents before and after Fe-doping. Water-washing pretreatment could remove loosely-bonded Cl/Br from the waste-derived sorbents, while the Cl/Br essential for Hg removal was retained.

Co9S8 nanoparticles-embedded porous carbon: A highly efficient sorbent for mercury capture from nonferrous smelting flue gas

In this study, a novel Co₉S₈ nanoparticles-embedded porous carbon (Co₉S₈-PC) was designed as an effective reusable sorbent for Hg⁰ capture from smelting flue gas. Some flue gas components can create more active sites on Co₉S₈-PC for Hg⁰ adsorption, but compete with Hg⁰ for the same sulfur sites over nano Co_1-xS/Co₃S₄ (CoS) and Co_1-xS/Co₃S₄ embedded porous carbon (CoS-PC), which can be ascribed to the difference in crystal structure between Co₉S₈ and Co_1-xS/Co₃S₄. Therefore, Co₉S₈-PC shows much better Hg⁰ capture ability than CoS and CoS-PC under smelting flue gas. O₂, SO₂ and HCl improve Hg⁰ adsorption on Co₉S₈-PC mainly through creating Co³⁺ site, but H₂O has neglectable effect on Hg⁰ capture. Co₉S₈-PC shows a remarkably large Hg⁰ adsorption capacity of 43.18 mg/g, which is greatly higher than the representative metal sulfides for Hg⁰ removal from smelting flue gas. During Hg⁰ adsorption, Co³⁺ is the primary site to directly interact with Hg⁰, and the adsorbed mercury exists as HgS. Co₉S₈-PC exhibits an excellent recyclability for capturing Hg⁰, which is mainly assigned to the replenishment of consumed Co³⁺ site by O₂, SO₂ and HCl. Therefore, Co₉S₈ nanoparticles-embedded porous carbon is an efficient, sustainable and highly recyclable sorbent for Hg⁰ recovery from smelting flue gas.

Self-grown oxygen vacancies-rich CeO2/BiOBr Z-scheme heterojunction decorated with rGO as charge transfer channel for enhanced photocatalytic oxidation of elemental mercury

Oxygen vacancy-rich CeO₂/BiOBr was prepared via solvothermal method combined with rGO to design a Z-scheme heterojunction, which was used for photocatalytic oxidation of gaseous elemental mercury. The Z-scheme heterojunction constructed by interface engineering significantly promotes charge carriers transfer at the interface. Moreover, the surface oxygen vacancies and Ce³⁺/Ce⁴⁺ redox centers tend to capture electrons to accelerate the Z-scheme path of charge transfer to maintain efficient redox performance and facilitate molecular oxygen activation to boost photocatalytic removal of Hg⁰. The collaboration of oxygen vacancies, Ce³⁺/Ce⁴⁺ and heterojunction enhances the photocatalytic oxidation activity, which achieves a removal efficiency of 76.53%, which is 1.29 times that of BiOBr and 1.91 times that of CeO₂. The effect of actual flue gas components (SO₂, NO and HCl) on the performance of photocatalytic Hg⁰ removal was further investigated. Combined with DFT theoretical calculations, the photocatalytic reaction mechanism of Z-scheme heterojunction with oxygen vacancies-rich was proposed. It provides a feasible strategy for the development of high-efficiency Z-scheme heterojunction photocatalytic system for environmental purification.