Dual Roles of Nano-Sulfide in Efficient Removal of Elemental Mercury from Coal Combustion Flue Gas within a Wide Temperature Range

Ultra-high adsorption of Hg0 using impregnated activated carbon by selenium

Activated carbon was one of the main adsorptions utilized in elemental mercury (Hg⁰) removal from coal combustion flue gas. However, the high cost and low physical adsorption efficiency of activated carbon injection (ACI) limited its application. In this study, an ultra-high efficiency (nearly 100%) catalyst sorbent-Se_x/Activated carbon (Se_x/AC) was synthesized and applied to remove Hg⁰ in the simulated flue gas, which exhibited 120 times outstanding adsorption performance versus the conventional activated carbon. The Se_x/AC reached 17.98 mg/g Hg⁰ adsorption capacity at 160 °C under the pure nitrogen atmosphere. Moreover, it maintained an excellent mercury adsorption tolerance, reaching the efficiency of Hg⁰ removal above 85% at the NO and SO₂ conditions in a bench-scale fixed-bed reactor. Characterized by the multiple methods, including BET, XRD, XPS, kinetic and thermodynamic analysis, and the DFT calculation, we demonstrated that the ultrahigh mercury removal performance originated from the activated Se species in Se_x/AC. Chemical adsorption plays a dominant role in Hg⁰ removal: Selenium anchored on the surface of AC would capture Hg⁰ in the flue gas to form an extremely stable substance-HgSe, avoiding subsequent Hg⁰ released. Additionally, the oxygen-containing functional groups in AC and the higher BET areas promote the conversion of Hg⁰ to HgO. This work provided a novel and highly efficient carbon-based sorbent -Se_x/AC to capture the mercury in coal combustion flue gas. Graphical abstract Selenium-modified porous activated carbon and the interface functional group promotes the synergistic effect of physical adsorption and chemical adsorption to promote the adsorption capacity of Hg⁰.

Nanosized Zn-In spinel-type sulfides loaded on facet-oriented CeO2 nanorods heterostructures as Z-scheme photocatalysts for efficient elemental mercury removal

Developing of effective photocatalysts is of great significance for realizing photocatalytic environment purification. Herein, an interfacial bent bands and internal electric field modulated CeO₂/ZnIn₂S₄ Z-scheme heterojunction for photocatalytic Hg⁰ oxidation. It is found that the charge transfer mechanism of Z-scheme was driven by the interfacial bent bands and internal electric field, which was confirmed by electrochemical measurements, electron spin paramagnetic resonance spectroscopy and density functional theory calculations. Moreover, the (110) dominant CeO₂ nanorods partially converted Ce⁴⁺ to Ce³⁺ and formed oxygen vacancies, and as an electron mediator in Z-scheme systems to further facilitate charge transfer process and molecular oxygen activation. Under the strong synergistic effect between the large specific surface area, Z-scheme heterojunction and oxygen vacancies, the optimized photocatalyst exhibits 86.7% of photocatalytic removal efficiency. This work provides Z-scheme heterojunction photocatalyst design perspective for photocatalytic air purification.

Removal and Recovery of Gaseous Elemental Mercury Using a Cl-Doped Protonated Polypyrrole@MWCNTs Composite Membrane

Due to the restrictions on mercury mining, recovering the mercury from mercury-containing waste is attracting increasing attention. This study successfully achieved the removal and recovery of gaseous elemental mercury (Hg⁰) by using membrane technology. A novel composite membrane of Cl-doped protonated polypyrrole-coated multiwall carbon nanotubes (Cl-PPy@MWCNTs) was fabricated in which MWCNTs acted as the framework to support the active component Cl-PPy. The morphology, structure, and composition of the prepared membranes were determined by field emission scanning electron microcopy, energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, etc. The composite membrane exhibited an excellent performance in Hg⁰ removal (97.3%) at a high space velocity of 200,000 h^-1. The dynamical adsorption capacity of Hg⁰ was 3.87 mg/g when the Hg⁰ breakthrough reached 10%. The adsorbed Hg⁰ could be recovered/enriched via a leaching process using acidic NaCl solution; meanwhile, the membrane was regenerated. The recovered mercury was identified in the form of Hg²⁺, with a recovery efficiency of over 99%. Density functional theory calculations and mechanism analysis clarified that the electrons of Hg⁰ transported to the delocalized electron orbits of protonated PPy and then combined with Cl^- to form Hg₂Cl₂/HgCl₂. Finally, we first demonstrated that the analogous protonated conductive polymers (e.g., polyaniline) also possessed good Hg⁰ removal ability, implying that such species may offer more outstanding answers and attract attention in future.

Different Crystal Forms of ZnS Nanomaterials for the Adsorption of Elemental Mercury

ZnS is a promising sorbent in recovering Hg⁰ from industrial flue gas due to its excellent Hg⁰ adsorption capacity. However, the internal structure-activity relationship still needs to be further clarified. In this work, ZnS sorbents with different structures were synthesized with the hydrothermal method by tuning the temperature. The samples had significant differences in the crystallinity, morphology, particle size, and sulfur (S) active sites. The results indicated that Hg⁰ removal performance was determined by the specific surface area and S active sites. ZnS synthesized at low temperatures (80-ZnS and 120-ZnS) had a larger surface area, while the S sites on the high-temperature-synthesized sample (160-ZnS) were more active for Hg⁰ adsorption. The 160-ZnS sample exhibited a much higher Hg⁰ adsorption amount per unit surface area. Further characterization revealed that S₂^2- and S_x were the main active sites for Hg⁰ adsorption. S_x existed in the form of long-chain polysulfur (L-S_x) on 80-ZnS and 120-ZnS, while it exhibited in the form of short-chain polysulfur (S-S_x) on 160-ZnS. L-S_x had negligible adsorption ability, while S-S_x had a high affinity for Hg⁰. Hg⁰ can react with S₂^2- and S-S_x, forming α-HgS and β-HgS, respectively. The new insight in this work can provide theoretical guidance for the design and structure optimization of ZnS, facilitating its practical industrial application.

Binary mineral sulfides sorbent with wide temperature range for rapid elemental mercury uptake from coal combustion flue gas

Developing efficient sorbents with rapid kinetics is the main challenge encountered for Hg⁰ capture from coal combustion flue gas in a sorbent injection scenario. Binary mineral sulfide-based materials combining copper sulfide (CuS) and zinc sulfide (ZnS) to exert their capabilities for Hg⁰ capture at the low- and high-temperature was for the first time reported for Hg⁰ removal to realize a wide temperature range sorbents. When the molar ratio between CuS and ZnS was 10%, the as-synthesized 10Cu-Zn nanocomposite exhibited excellent Hg⁰ uptake rate at 150°C that could degrade 40 μg/m³ of Hg⁰ to undetectable level at the end of a 60-s experiment with the dosage of only 1 mg. This Hg⁰ uptake rate is folds higher compared to that when bare CuS or ZnS was adopted alone at this specific temperature. The typical flue gas atmospheres had negligible effect on Hg⁰ removal over 10Cu-Zn in a short contact time, which further suggests that the binary sorbents were proper to be injected before the electrostatic precipitator system. Moreover, it is found that, by adjusting the ratio between CuS and ZnS, it is potential to develop binary sorbent suiting any temperature conditions that may achieve an exceedingly high Hg⁰ capture performance. Thus, this work not only justified the candidature of 10Cu-Zn as a promising alternative to traditional activated carbon for Hg⁰ capture from coal combustion flue gas but also guided the future development of multi-component mineral sulfide-based sorbents for Hg⁰ pollution remediation from various industrial flue gases.

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.

Insight into the interfacial stability and reaction mechanism between gaseous mercury and chalcogen-based sorbents in SO2-containing flue gas

Chalcogen-based materials have been confirmed to possess large adsorption capacities for gaseous elemental mercury (Hg⁰) from SO₂-containing flue gas. However, the interface reaction mechanisms and the interfacial stability are still ambiguous. Here, we selected some commonly used chalcogen-based sorbents (e.g., X, ZnX, CuX. X = S, Se) to investigate the in-depth reaction mechanisms. The adsorption capacities, structure effect on thermal and surface mercury stability, and interfacial reaction mechanism in the absence/presence of SO₂ were evaluated. The experimental results indicated that Cu-chalcogenide had higher Hg⁰ adsorption capacity and surface Hg-X bonding stability compared with zinc one, while they exhibited an opposite degree of thermal stability. Moreover, all the chalcogenides showed well SO₂ tolerance but with a slight difference. Chalcogenides with the same crystal structures, like ZnX or CuX, exhibited similar properties in stability and interfacial Hg⁰ and SO₂ reaction mechanism. X^- in chalcogenides have a better affinity to mercury, while in the Hg⁰ capture process, the existence of multivalent metal elements (like Cu²⁺ and Cu⁺) can faster the Hg⁰ oxidation for the further chemical-adsorption. This work provides a basic understanding of the application for efficiently enriching and recycling gaseous Hg⁰ from industrial SO₂-containing flue gas.

Multifunctional Binding Strategy on Nonconjugated Polymer Nanoparticles for Ratiometric Detection and Effective Removal of Mercury Ions

Developing a multifunctional platform for the selective detection and effective removal of toxic ions is a major challenge when addressing heavy metal contamination in environmental science. Herein, novel nonconjugated polymer nanoparticles (PNPs) called mercaptosuccinic acid-thiosemicarbazide PNPs (MT-PNPs) with appealing fluorescence and stability are synthesized via facile one-step hydrothermal treatment for attractive sensing and simultaneous removal of mercury(II). Interestingly, aggregation-induced fluorescence switch-off and scattering enhancement are found upon the addition of Hg²⁺, rendering MT-PNPs as a ratiometric sensor for selective and accurate Hg²⁺ monitoring. A wide linear range (0.1-1471 μM) and a low detection limit (95 nM) are obtained. This dual-signal opposite responses triggered by Hg²⁺ originate from the formation of MT-PNP-Hg²⁺ congeries via the multisite binding between S,N,O-containing groups of MT-PNPs and mercury. Meanwhile, target-induced aggregation renders an effective Hg²⁺ separation from contaminative aqueous media by MT-PNPs, which exhibits a satisfactory absorption efficiency of 90.42% within 50 min. Upon the simple Na₂S treatment, the MT-PNPs can be regenerated and reused. This work thus delivers an applicable method for the ratiometric detection and effective removal of mercury with the novel nonconjugated PNPs, offering potential in tackling the problem of heavy metal ion pollution for environmental monitoring and remediation.

Enhancing the catalytic oxidation of elemental mercury and suppressing sulfur-toxic adsorption sites from SO2-containing gas in Mn-SnS2

It is difficult to stabilize gaseous elemental mercury (Hg°) on a sorbent from SO₂-containing industrial flue gas. Enhancing Hg° oxidation and activating surface-active sulfur (S*) can benefit the chemical mercury adsorption process. A Mn-SnS₂ composite was prepared using the Mn modification of SnS₂ nanosheets to expose more Mn oxidation and sulfur adsorption sites. The results indicate that Mn-Sn₂ exhibits better Hg° removal performances at a wide temperature range of 100-250 °C. A sufficient amount of surface Mn with a valance state of Mn⁴⁺ is favorable for Hg° oxidation, while the electron transfer properties of Sn can accelerate this oxidation process. Oxidized mercury primary exists as HgS with surface S*. A larger surface area, stable crystal structure, and active valance state of each element are favorable for Hg° oxidation and adsorption. The Mn-SnS₂ exhibits an excellent SO₂ resistance when the SO₂ concentration is lower than 1500 ppm. The effects of H₂O and O₂ were also evaluated. The results show that O₂ has no influence, while H₂O and SO₂ coexisting in the flue gas have a toxic effect on the Hg° removal performance. The Mn-SnS₂ has a great potential for the Hg° removal from SO₂-containing flue gas such as non-ferrous smelting gas.

AMn2O4 (A=Cu, Ni and Zn) sorbents coupling high adsorption and regeneration performance for elemental mercury removal from syngas

AMn₂O₄ (A= Cu, Ni and Zn) spinel sorbents synthesized by a low-temperature sol-gel auto-combustion method were for the first time used to eliminate elemental mercury (Hg⁰) from syngas. CuMn₂O₄ sorbent exhibits the highest Hg⁰ adsorption performance under simulated syngas, higher than 95 % Hg⁰ capture efficiency is obtained at 200 °C. Adsorption-regeneration experiments demonstrate that the regenerability of CuMn₂O₄ is excellent. The influences of syngas compositions on Hg⁰ elimination over CuMn₂O₄ were examined. H₂S plays the most important role in Hg⁰ removal from syngas, which can be adsorbed on CuMn₂O₄ surface and transformed into active sulfur species to react with Hg⁰ to form surface-bonded HgS. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) experiments prove that HgS is formed on the spent sorbent. The surface Cu²⁺ cations and chemisorbed/lattice oxygens participate in Hg⁰ adsorption and transformation. HCl promotes Hg⁰ removal by forming surface active chlorine species. H₂, CO, and H₂O inhibit Hg⁰ removal under N₂ atmosphere. However, they exhibit no obvious effect on Hg⁰ removal with the assistance of H₂S. The excellent Hg⁰ capture performance, good regenerability and H₂O resistance of CuMn₂O₄ make it to be a very promising sorbent for Hg⁰ removal from syngas at higher temperature.

Significant Enhancement of Gaseous Elemental Mercury Recovery from Coal-Fired Flue Gas by Phosphomolybdic Acid Grafting on Sulfurated γ-Fe2O3: Performance and Mechanism

The existing technologies to control Hg emissions from coal-fired power plants can be improved to achieve the centralized control of Hg⁰ emissions, which continue to pose a risk of Hg exposure to human populations. In this work, MoS_x@γ-Fe₂O₃, formed by the sulfuration of phosphomolybdic acid (HPMo)-grafted γ-Fe₂O₃, was developed as a magnetic and regenerable sorbent to recover gaseous Hg⁰ from coal-fired flue gas as a cobenefit to the use of wet electrostatic precipitators. The thermal stability of γ-Fe₂O₃ was notably enhanced by HPMo grafting; thus, the magnetization of MoS_x@γ-Fe₂O₃ hardly decreased during the application. The kinetic analysis indicates that the chemical adsorption of gaseous Hg⁰ was mainly dependent on the amounts of surface S₂^2- and surface adsorption sites. Although the amount of S₂^2- on sulfurated γ-Fe₂O₃ decreased after HPMo grafting, the amount of surface adsorption sites significantly increased due to the formation of a layered MoS_x structure on the surface. Therefore, the ability of sulfurated γ-Fe₂O₃ to capture Hg⁰ was improved considerably after HPMo grafting. Furthermore, low concentrations of gaseous Hg⁰ in coal-fired flue gas can be gradually enriched by at least 1000 times by MoS_x@γ-Fe₂O₃, which facilitates the recovery and centralized control of gaseous Hg⁰ in flue gas.

Mercury in natural gas streams: A review of materials and processes for abatement and remediation

The role of natural gas in mitigating greenhouse gas emissions and advancing renewable energy resource integration is undoubtedly critical. With the progress of hydrocarbons exploration and production, the target zones become deeper and the possibility of mercury contamination increases. This impacts on the industry from health and safety risks, due to corrosion and contamination of equipment, to catalyst poisoning and toxicity through emissions to the environment. Especially mercury embrittlement, being a significant problem in LNG plants using aluminum cryogenic heat exchangers, has led to catastrophic plant incidents worldwide. The aim of this review is to critically discuss the conventional and alternative materials as well as the processes employed for mercury removal during gas processing. Moreover, comments on studies examining the geological occurrence of mercury species are included, the latest developments regarding the detection, sampling and measurement are presented and updated information with respect to mercury speciation and solubility is displayed. Clean up and passivation techniques as well as disposal methods for mercury-containing waste are also explained. Most importantly, the environmental as well as the health and safety implications are addressed, and areas that require further research are pinpointed.

A Review on Adsorption Technologies for Mercury Emission Control

This study summarized existing adsorption technologies for the removal of elemental mercury in the flue gas. Both carriers (e.g., active carbon (AC), pyrolyzed char, inorganic adsorbents and fly ash) and various modification methods (pore structure improvement, oxygen-containing functional groups addition and new active reagents impregnation) were compared to shed light on the development of future adsorption technology. AC and char possibly performed more mercury adsorption capacity (MAC) compared with fly ash and inorganic adsorbents since carbon atom existence was easier to form the active halogen groups (C-X) and oxygen containing groups. Though both pore structure improvement and chemical group formation improved the MAC of adsorbents, the chemical modification methods (oxygen-containing functional groups addition and new active reagents impregnation) were more effective. The impregnation of halogen, sulfur and metal chloride could distinctly form lots of active sites on the adsorbents and developed high effective mercury adsorbents. In the future, the adsorption researches possibly focus on SO₂ and H₂O resistance of adsorbents, separable adsorbents, low-cost chemical modification methods, and utilization potential of fly ash.

CeO2-Decorated ?-MnO2 Nanotubes: A Highly Efficient and Regenerable Sorbent for Elemental Mercury Removal from Natural Gas

CeO₂ nanoparticle-decorated ?-MnO₂ nanotubes (NTs) were prepared and tested for elemental mercury (Hg⁰) vapor removal in simulated natural gas mixtures at ambient conditions. The composition which had the largest surface area and a relative Ce/Mn atomic weight ratio of around 35% exhibited a maximum Hg⁰ uptake capacity exceeding 20 mg?g^?1 (2 wt %), as determined from measurements of mercury breakthrough which corresponded to 99.5% Hg⁰ removal efficiency over 96 h of exposure. This represents a significant improvement in the activity of pure metal oxides. Most importantly, the composite nanosorbent was repeatedly regenerated at 350 ?C and retained the 0.5% Hg⁰ breakthrough threshold. It was projected to be able to sustain 20 regeneration cycles, with the presence of acid gases, CO₂, and H₂S, not affecting its performance. This result is particularly important, considering that pure CeO₂ manifests rather poor activity for Hg⁰ removal at ambient conditions, and hence, a synergistic effect in the composite nanomaterial was observed. This possibly results from the addition of facile oxygen vacancy formation at ?-MnO₂ NTs and the increased amount of surface-adsorbed oxygen species.

Role of Sulfur Trioxide (SO3) in Gas-Phase Elemental Mercury Immobilization by Mineral Sulfide

Mineral sulfide based sorbents were superior alternatives to traditional activated carbons for elemental mercury (Hg⁰) immobilization in industrial flue gas. A systematical study concerning the influence of sulfur trioxide (SO₃) on Hg⁰ adsorption over a nanosized copper sulfide (Nano-CuS) was for the first time conducted. SO₃ was found to significantly inhibit the Hg⁰ removal over Nano-CuS partially because SO₃ oxidized the reduced sulfur species (sulfide) with high affinity to mercury to its oxidized sulfur species (sulfate). Moreover, a brand new "oxidation-reduction" mechanism that led to a simultaneous oxidation of sulfide and reduction of mercury on the immobilized mercury sulfide (HgS) was responsible for the inhibitory effect. Even though the released Hg⁰ from the reduction of mercury in HgS could be oxidized by SO₃ into its sulfate form (HgSO₄) and recaptured by the sorbent, the "oxidation-reduction" mechanism still compromised the Hg⁰ capture performance of the Nano-CuS because HgSO₄ deposited on the sorbent surface could be easily leached out when environmentally exposed. These new insights into the role of SO₃ in Hg⁰ capture over Nano-CuS can help to determine possible solutions and facilitate the application of mineral sulfide sorbents as outstanding alternatives to activated carbons for Hg⁰ immobilization in industrial flue gas.