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

Review on magnetic adsorbents for removal of elemental mercury from coal combustion flue gas

Under the influence of human activities, atmospheric mercury (Hg) concentrations have increased by 450% compared with natural levels. In the context of the Minamata Convention on Mercury, which came into effect in August 2017, it is imperative to strengthen Hg emission controls. Existing Air Pollution Control Devices (APCDs) combined with collaborative control technology can effectively remove Hg2+ and Hgp; however, Hg0 removal is substandard. Compared with the catalytic oxidation method, Hg0 removal through adsorbent injection carries the risk of secondary release and is uneconomical. Magnetic adsorbents exhibit excellent recycling and Hg0 recovery performance and have recently attracted the attention of researchers. This review summarizes the existing magnetic materials for Hg0 adsorption and discusses the removal performances and mechanisms of iron, carbon, mineral-based, and magnetosphere materials. The effects of temperature and different flue gas components, including O2, NO, SO2, H2O, and HCl, on the adsorption performance of Hg0 are also summarized. Finally, different regeneration methods are discussed in detail. Although the research and development of magnetic adsorbents has progressed, significant challenges remain regarding their application. This review provides theoretical guidance for the improvement of existing and development of new magnetic adsorbents.

Self-Constructing 100% Water-Resistant Metal Sulfides through In Situ Acid Etching for Effective Elemental Mercury (Hg0) Capture

Metal sulfides (MSs) can efficiently entrap thiophilic components, such as elemental mercury (Hg0), and realize environmental remediation. However, there is still a critical problem challenging the extensive application of MSs in related areas, i.e., how to self-regulate their water (H2O) resistance without complexing the sorbent preparation procedure. This work for the first time developed an in situ acid-etching method that self-engineered the water affinity of MSs through changing the interfacial interaction between MSs and Hg0/H2O. The introduction of abundant, undercoordinated sulfur onto the sorbent surface was the primary reason accounting for the significantly improved H2O resistance. The high surface coverage of undercoordinated sulfur induced the formation of polysulfur chains (Sx2-) that stabilized Hg0 via a bridging bond and repelled H2O, attributed to the favorable electron configurations. These properties made the surface of MSs highly hydrophobic and increased the adsorption selectivity toward Hg0 over H2O. The MSs exhibited 100% H2O resistance even in the presence of 20% H2O, which is much higher than the H2O concentration under most practical scenarios. From these perspectives, this work for the first time overcame the detrimental effects of H2O on MSs through a self-regulating way that is scalable and negligibly complexes the sorbent preparation pathway. The highly water-resistant and cost-effective MSs as prepared can serve as efficient Hg0 removal from industrial flue gas in the future.

Highly efficient Cr (VI) removal from electroplating wastewater by regenerable copper sulfides: Mechanism and magical induction effect for Cr resource recovery

The current sorbents used to remove Cr (VI) from electroplating wastewater are faced with some challenges including the difficulty in separating, regenerating, and safely disposing of adsorbed Cr species. To address these challenges, CuSx/TiO2 was developed to recover Cr (VI) from electroplating wastewater. CuSx/TiO2 had superior performance in removing Cr (VI), with the rate and capacity of approximately 9.36 mg g-1 h-1 and 68.8 mg g-1 at initial pH 4.0, respectively. Additionally, Cu2+ released from CuSx/TiO2 during Cr (VI) removal would come back to its external surface as the Cu(OH)2 precipitate at initial pH 4.0, which helped to prevent the generation of secondary pollution. The Cu(OH)2 precipitate would be decomposed into CuOx after calcination, which would then be transformed back into CuSx by re-sulfuration for regeneration. Hence, CuSx showed a magical induction effect on Cr (VI) recovery, and Cr (VI) from electroplating wastewater might be gradually enriched as Cr2O3 in the sandwich between CuSx and TiO2 through multiple regenerations and removals, which could be considered as a chromium ore resource for industrial applications when the amount of enriched Cr2O3 reached more than 30 wt%. Overall, CuSx/TiO2 showed great potential as a promising sorbent for Cr (VI) removal from electroplating wastewater.

Construction of a novel heteropoly molybdophosphate/graphitized carbon nitride s-scheme heterostructure with enhanced photocatalytic H2O2 evolution activity

The preparation of hydrogen peroxide using a photochemical energy conversion process is a promising green pathway. Herein, a series of graphite-phase carbon nitride/Polyoxometalate (CN-HMoP) composite photocatalysts were prepared by interlayer coordination and segregation strategies. During the synthesis process, the presence of -NH2 or H ions at the CN terminus makes it display a positive charge, which is capable of attracting HMoP with a polyanionic structure. Subsequently, the N atoms on the triazine ring preferentially coordinate with the Mo ions in HMoP to form Mo-N bonds, and the HMoP nanoparticles are able to be inserted layer by layer into the CN nanosheets by interlayer coordination to make the sheet exfoliation, which contributes to the enhancement of the transmission of photogenerated carriers. Notably, the establishment of the S-Scheme heterostructure facilitates the spatial separation of electron-hole pairs and maintains the strongest redox potential. More importantly, under solar/visible light irradiation, the H2O2 generation rate of CN-HMoP reached 137.1 μmol L-1h-1 and 113.1 μmol L-1h-1, which was 17.7 and 16.8 times higher than pristine CN. Therefore, this work can provide guidance for the synthesis of semiconductor photocatalysts for more efficient and green generation of H2O2.

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.

Hg0 Conversion over Sulfureted HPMo/γ-Fe2O3 with HCl at Low Temperatures: Mechanism, Kinetics, and Application in Hg0 Removal from Coal-Fired Flue Gas

Recently, sulfureted metal oxides have been developed for the catalytic oxidation of Hg⁰ to HgCl₂ using HCl as an oxidant at low temperatures, and they exhibit excellent Hg⁰ removal performance. Owing to the lack of reaction mechanisms and kinetics, further improvement in their performance for Hg⁰ conversion is extremely restricted. In this study, the reaction mechanism of Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ with HCl at low temperatures was investigated using Hg balance analysis and transient reaction. The chemical adsorption of Hg⁰ as HgS and the catalytic oxidation of Hg⁰ to HgCl₂ both contributed to Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃. Meanwhile, the formed HgCl₂ can adsorb onto sulfureted HPMo/γ-Fe₂O₃. Then, the kinetics of Hg⁰ conversion, Hg^t adsorption, and HgCl₂ desorption were developed, and the kinetic parameters were gained by fitting the Hg balance curves. Subsequently, the inhibition mechanism of H₂O and SO₂ on Hg⁰ conversion over sulfureted HPMo/γ-Fe₂O₃ was determined by comparing the kinetic parameters. The kinetic model suggested that both HgCl₂ resulting from Hg⁰ oxidation and unoxidized Hg⁰ can be completely adsorbed on sulfureted HPMo/γ-Fe₂O₃ with a moderate mass hourly space velocity. Therefore, sulfureted HPMo/γ-Fe₂O₃ can be developed as a reproducible sorbent for recovering Hg⁰ emitted from coal-fired power plants.

Simultaneous Adsorption of Gaseous Hg0 and Hg(II) by Regenerable Monolithic FeMoSx/TiO2: Mechanism and its Application in the Centralized Control of Hg Pollution in Coal-Fired Flue Gas

FeMoS_x/TiO₂ was investigated as a regenerable sorbent to simultaneously adsorb Hg⁰ and Hg(II) from coal-fired flue gas for the centralized control of Hg pollution discharged from coal-fired power plants. The performance of FeMoS_x/TiO₂ for Hg(II) and/or Hg⁰ adsorption was evaluated on a fixed-bed reactor at 80 ^oC, and the mutual interference between Hg⁰ adsorption and Hg(II) adsorption was analyzed using individual adsorption, simultaneous adsorption, and two-stage adsorption. FeMoS_x/TiO₂ displayed an excellent capacity for individual Hg⁰ adsorption (41.8 mg g^-1) and a moderate capacity for individual Hg(II) adsorption (0.48 mg g^-1). Two types of adsorption sites were present on FeMoS_x/TiO₂ for gaseous Hg adsorption (S⁰ and FeS₂/MoS₃ sites). X-ray photoelectron spectroscope and kinetic analyses demonstrated that Hg⁰ and Hg(II) could adsorb onto S⁰ sites, whereas only Hg⁰ was adsorbed onto FeS₂/MoS₃ sites. As Hg⁰ competed with Hg(II) for the S⁰ sites, the amount of Hg(II) adsorbed slightly decreased by 16% in the presence of Hg⁰. However, Hg⁰ adsorption onto the FeS₂/MoS₃ sites predominated over the Hg⁰ adsorption onto FeMoS_x/TiO₂ and it was not inhibited in the presence of Hg(II). Therefore, the amount of Hg⁰ adsorbed on FeMoS_x/TiO₂ was only decreased by 2% in the presence of Hg(II).

Comparison of pyrite-phase transition metal sulfides for capturing leaked high concentrations of gaseous elemental mercury in indoor air: Mechanism and adsorption/desorption kinetics

Understanding the characteristics of pyrite-phase transition metal sulfides for the adsorption and desorption of gaseous elemental mercury (Hg⁰) is of vital significance for their applications in gaseous Hg⁰ capture. In this study, the adsorption and desorption of gaseous Hg⁰ onto pyrite-phase transition metal sulfides (i.e., FeS₂/TiO₂, CoS₂/TiO₂, and NiS₂/TiO₂) were compared, and the mechanisms of their differences were revealed by the kinetic analysis. The Co/NiS and SS bonds in dumbbell-shaped CoS₂ and NiS₂ were not entirely broken after oxidizing physically adsorbed Hg⁰, whereas the FeS and SS bonds in dumbbell-shaped FeS₂ were. Thus, the activation energies of CoS₂/TiO₂ and NiS₂/TiO₂ for oxidizing physically adsorbed Hg⁰ were smaller than that of FeS₂/TiO₂, causing the stronger abilities of CoS₂/TiO₂ and NiS₂/TiO₂ to oxidize physically adsorbed Hg⁰ than that of FeS₂/TiO₂. However, the bonding strengths of Hg-S in HgS adsorbed on dumbbell-shaped CoS₂ and NiS₂ were relatively weaker because of the sharing of S^2- in HgS with S^- and Co²⁺/Ni²⁺, causing the decreases in heat stabilities of HgS adsorbed on CoS₂/TiO₂ and NiS₂/TiO₂. Therefore, HgS adsorbed on CoS₂/TiO₂ and NiS₂/TiO₂ can be voluntarily decomposed to release gaseous Hg⁰, which should be combined with FeS₂/TiO₂ for the emergency treatment of liquid Hg⁰ leakage indoors.

Coordinatively Unsaturated Selenides over CuFeSe2 toward Highly Efficient Mercury Immobilization

Metal selenides have been demonstrated as promising Hg⁰ remediators, while their inadequate adsorption rate primarily impedes their application feasibility. Based on the critical role of coordinatively unsaturated selenide ligands in immobilizing Hg⁰, this work proposed a novel strategy to enhance the Hg⁰ adsorption rate of metal selenides by magnitudes by purposefully adjusting the selenide saturation. Copper iron diselenide (CuFeSe₂), in which the surface reconstruction tended to occur at ambient temperature, was adopted as the concentrator of unsaturated selenides. The adsorption rate of CuFeSe₂ reached as high as 900.71 μg·g^-1·min^-1, far exceeding those of the previously reported metal selenides by at least 1 magnitude. The excellent resistance of CuFeSe₂ to flue gas interference and temperature fluctuation warrants its applicability in real-world conditions. The theoretical investigations and mechanistic interpretations based on density functional theory (DFT) calculation further confirmed the indispensable role of unsaturated selenides in Hg⁰ adsorption. This work aims not only to develop a Hg⁰ remediator with extensive applicability in coal combustion flue gas but also to take a step toward the rational design of selenide-based sorbents for diverse environmental remediation by the facile surface functionalization of coordinatively adjustable ligands.

Novel Counteraction Effect of H2O and SO2 toward HCl on the Chemical Adsorption of Gaseous Hg0 onto Sulfureted HPW/γ-Fe2O3 at Low Temperatures: Mechanism and Its Application in Hg0 Recovery from Coal-Fired Flue Gas

In this work, sulfureted phosphotungstic acid-grafted γ-Fe₂O₃ (HPW/γ-Fe₂O₃) was investigated as a regenerable monolithic sorbent to recover gaseous Hg⁰ upstream of wet flue gas desulfurizations (FGDs), and the effects of HCl, SO₂, and H₂O on the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ were investigated with Hg balance analysis and kinetic analysis. Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was remarkably promoted in the presence of HCl, and most Hg⁰ was catalytically oxidized to HgCl₂. Moreover, the chemical adsorption of Hg⁰ was notably restrained as the key species for Hg⁰ transformation to HgS (i.e., S₂^2-) was rapidly oxidized by Cl*. However, the effect of HCl on Hg⁰ conversion over sulfureted HPW/γ-Fe₂O₃ was almost counteracted by H₂O and SO₂ as they competed with physically adsorbed Hg⁰ and S₂^2- for the consumption of Cl*. Therefore, the chemical adsorption of Hg⁰ onto sulfureted HPW/γ-Fe₂O₃ in the presence of SO₂ and H₂O was slightly inhibited by HCl, and only a small amount of HgCl₂ was formed. Moreover, sulfureted HPW/γ-Fe₂O₃ exhibited a moderate ability for gaseous HgCl₂ adsorption. As a result, sulfureted HPW/γ-Fe₂O₃ showed excellent performance in recovering Hg⁰ from the flue gas upstream of the FGDs for the centralized control of Hg⁰ emitted from coal-fired plants.

Photo- and thermo-catalytic mechanisms for elemental mercury removal by Ce doped commercial selective catalytic reduction catalyst (V2O5/TiO2)

The elemental mercury was catalytically removed by V₂O₅/TiO₂ and Ce doped V₂O₅/TiO₂ catalysts under the UV irradiation at 30-160 °C to determine whether the catalysts could simultaneously have both thermo- and photo-catalytic activities. The physicochemical properties of catalysts were characterized by XRD, SEM, EDX, BET, XPS, UV-visible, PER and EIS. The experimental results demonstrated that V₂O₅/TiO₂ and Ce-doped catalysts possessed both thermo- and photo-catalytic reactivities. A suitable reaction temperature (120 °C) and UV light had promoting effects on mercury removal efficiency. In addition, owing to the high oxidation capability as well good oxygen storage performance of Ce⁴⁺, Ce doping could greatly improve the mercury removal properties of the catalyst, reduce the inhibition of SO₂ and make NO the component with enhanced effect. Ce doping also had the capability of enhancing the light absorption intensity in the UV region as well as the separation rate of photoinduced carriers. Finally, DFT calculations of V-Ti and Ce-V-Ti for Hg⁰ removal were investigated to further verify the experimental conclusion.

Enhanced photocatalytic Hg0 oxidation activity of iodine doped bismuth molybdate (Bi2MoO6) under visible light

The application of photocatalytic Hg⁰ oxidation under visible light is an up-and-coming method to solve the problem of energy shortage and environmental pollution. In this work, iodine doped Bi₂MoO₆ nanomaterials were prepared by one-step solvothermal method. The photocatalytic oxidation efficiency was greatly improved by iodine doping from 35.5% to 95.2% in the N₂ + 4% O₂ atmosphere under visible light. The main reason was that iodine doping decreased the band gap of the catalyst, expanded the optical response range and intensity, sped up the separation rate of photoinduced carriers and reduced the recombination rate. In addition, the flue gas components of SO₂ and NO played a promoting role in mercury removal. Iodine doped Bi₂MoO₆ had good stability and still maintained high mercury removal efficiency after 5 cycles. Density functional theory (DFT) calculations and experiments demonstrated that iodine doping changed the valence band and conduction band of the catalyst, making superoxide ions, hydroxyl radicals and photoinduced hole become the active species of the catalytic reaction.

Hollow Core-Shell potassium Phosphomolybdate@Cadmium Sulfide@Bismuth sulfide Z-Scheme tandem heterojunctions toward optimized Photothermal-Photocatalytic performance

A hollow core-shell potassium phosphomolybdate (KMoP)@cadmium sulfide (CdS)@bismuth sulfide (Bi₂S₃) Z-scheme tandem heterojunction is fabricated by a simple hydrothermal strategy and kept in a water bath to continue the reaction. At the same time, the ternary structure combined Keggin-type polyoxometalate with two photosensitive sulfide semiconductors to form a stable hollow core-shell heterojunction. KMoP@CdS@Bi₂S₃ with a narrow band gap of ∼ 1.2 eV also has excellent photothermal performance, which may further promote photocatalytic efficiency. The hollow core-shell KMoP@CdS@Bi₂S₃ tandem heterojunction shows excellent H₂ production performance, Cr^VI reduction ability and photocatalytic degradation performance of highly toxic tetracycline (TC). Under visible light irradiation, the photocatalytic H₂ generation rate of the KMoP@CdS@Bi₂S₃ tandem heterojunction reaches 831 μmol h^-1, which is 103 times higher than that of pristine KMoP. The photocatalytic reduction efficiency of Cr^VI and degradation efficiency of TC are as high as 95.5 and 97.51%, ∼4 times higher than that of KMoP. The boosted photocatalytic performance can be ascribed to the formation of core-shell Z-scheme tandem heterojunctions favoring spatial charge separation and the narrow band gap, which extends the photoresponse to visible light/NIR regions. When TC and Cr^VI exist at the same time, the reduction efficiency of Cr^VI can be as high as 99.64% because the intermediate of TC degradation can promote the reduction of Cr^VI. In addition, the photocatalytic performance of the KMoP@CdS@Bi₂S₃ heterojunction remains nearly constant after 4 recycles, which indicates high stability. The design strategy may provide new insights for preparing other high-performance core-shell tandem heterojunction photocatalysts for solar energy conversion.

Different Design Strategies for Metal Sulfide Sorbents to Capture Low Concentrations of Gaseous Hg0 in Coal-Fired Flue Gas and High Concentrations of Gaseous Hg0 in Smelting Flue Gas

Capturing gaseous Hg⁰ using regenerable metal sulfides is a promising technology to recover gaseous Hg⁰ from both coal-fired flue gas (CFG) and smelting flue gas (SFG) for the centralized control. Gaseous Hg⁰ concentration in SFG is 2-3 orders of magnitude higher than that in CFG; therefore, the design strategy of metal sulfides for capturing gaseous Hg⁰ from CFG is quite different from that from SGF. In this work, the structure-activity relationship of metal sulfides to capture Hg⁰ was investigated according to the remarkable difference in MoO₃ loading on sulfureted FeTiO_x to capture low/high concentrations of gaseous Hg⁰. The rate of Hg⁰ adsorption onto metal sulfides was mainly related to the amounts of adsorption sites and S₂^2- on the surface, the affinity of adsorption sites to gaseous Hg⁰, and the gaseous Hg⁰ concentration. Meanwhile, the capacity for Hg⁰ adsorption was approximately equal to the less of the amount of adsorption sites and S₂^2- on the surface. Furthermore, capturing low concentrations of gaseous Hg⁰ from CFG required the metal sulfide sorbents having more adsorption sites with strong affinity to gaseous Hg⁰, while capturing high concentrations of gaseous Hg⁰ from SFG required the sorbents with enough adsorption sites.

Recovering gaseous Hg0 using sulfureted phosphotungstic acid modified γ-Fe2O3 from power plants burning Hg-rich coal for centralized control

Conversion of gaseous elemental mercury (Hg⁰) to particulate-bound mercury (Hg^P) by the injection of disposable sorbents or to gaseous oxidized mercury (Hg²⁺) by catalysts is not suitable to control Hg⁰ emissions from power plants burning Hg-rich coal because removed Hg may be released as secondary pollution, particularly during the employment of fly ash and desulfurization gypsum. In this study, sulfureted phosphotungstic acid modified γ-Fe₂O₃ (HPW/γ-Fe₂O₃) was employed as a magnetic and reproducible sorbent to recover gaseous Hg⁰ in coal-fired flue gas for the centralized control. Sulfureted HPW/γ-Fe₂O₃ showed an excellent capacity to enrich gaseous Hg⁰ from low concentrations to ultra-high concentrations (>10 mg m^-3), which benefited to condensing it into liquid Hg. Moreover, sulfureted HPW/γ-Fe₂O₃ exhibited excellent para-magnetism, enabling it to be magnetically reclaimed after Hg⁰ capture; this magnetization did not disappear after multiple thermal desorption of Hg⁰ due to its excellent thermal stability. Furthermore, sulfureted HPW/γ-Fe₂O₃ was regenerated by re-sulfuration without decreasing the Hg⁰ capture performance. Therefore, gaseous Hg⁰ recovery using sulfureted HPW/γ-Fe₂O₃ is a promising, economical-effective, and eco-friendly technology for the centralized control of Hg pollution emitted from power plants that burn coal with a high Hg content.

Outstanding performance of CuO/Fe–Ti spinel for Hg0 oxidation as a co-benefit of NO abatement: significant promotion of Hg0 oxidation by CuO loading

Conversion of gaseous Hg⁰ to soluble Hg²⁺ using selective catalytic reduction (SCR) catalysts with gaseous HCl as an oxidant as a co-benefit of NO abatement is widely used for resolving Hg pollution from coal-burning power plants.

Application of magnetic nanomaterials in magnetic in-tube solid-phase microextraction

Development of magnetic nanomaterials has greatly promoted the innovation of in-tube solid-phase microextraction. This review article gives an insight into recent advances in the modifications and applications of magnetic nanomaterials for in-tube solid-phase microextraction. Also, different magnetic nanomaterials which have recently been utilized as in-tube solid-phase microextraction sorbents are classified. This study shows that magnetic nanomaterials have gained significant attention owing to large specific surface area, selective absorption, and surface modification. Magnetic in-tube solid-phase microextraction has been applied for the analysis of food samples, biological, and environmental. However, for full development of magnetic in-tube SPME, effort is still needed to overcome limitations, such as mechanical stability, selectivity and low extraction efficiency. To achieve these objectives, research on magnetic in-tube SPME is mainly focused in the preparation of new extractive phases.