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

Biomimetic mercury immobilization by selenium functionalized polyphenylene sulfide fabric

Highly efficient decontamination of elemental mercury (Hg0) remains an enormous challenge for public health and ecosystem protection. The artificial conversion of Hg0 into mercury chalcogenides could achieve Hg0 detoxification and close the global mercury cycle. Herein, taking inspiration from the bio-detoxification of mercury, in which selenium preferentially converts mercury from sulfoproteins to HgSe, we propose a biomimetic approach to enhance the conversion of Hg0 into mercury chalcogenides. In this proof-of-concept design, we use sulfur-rich polyphenylene sulfide (PPS) as the Hg0 transporter. The relatively stable, sulfur-linked aromatic rings result in weak adsorption of Hg0 on the PPS rather than the formation of metastable HgS. The weakly adsorbed mercury subsequently migrates to the adjacent selenium sites for permanent immobilization. The sulfur-selenium pair affords an unprecedented Hg0 adsorption capacity and uptake rate of 1621.9 mg g-1 and 1005.6 μg g-1 min-1, respectively, which are the highest recorded values among various benchmark materials. This work presents an intriguing concept for preparing Hg0 adsorbents and could pave the way for the biomimetic remediation of diverse pollutants.

In situ acid etching boosts mercury accommodation capacities of transition metal sulfides

Transition Metal sulfides (TMSs) are effective sorbents for entrapment of highly polluting thiophiles such as elemental mercury (Hg⁰). However, the application of these sorbents for mercury removal is stymied by their low accommodation capacities. Among the transition metal sulfides, only CuS has demonstrated industrially relevant accommodation capacity. The rest of the transition metal sulfides have 100-fold lower capacities than CuS. In this work, we overcome these limitations and develop a simple and scalable process to enhance Hg⁰ accommodation capacities of TMSs. We achieve this by introducing structural motifs in TMSs by in situ etching. We demonstrate that in situ acid etching produces TMSs with defective surface and pore structure. These structural motifs promote Hg⁰ surface adsorption and diffusion across the entire TMSs architecture. The process is highly versatile and the in situ etched transition metal sulfides show over 100-fold enhancement in their Hg⁰ accommodation capacities. The generality and the scalability of the process provides a framework to develop TMSs for a broad range of applications.

Formation of sulfur oxide groups by SO2 and their roles in mercury adsorption on carbon-based materials

The presence of SO₂ display significant effect on the mercury (Hg) adsorption ability of carbon-based sorbent. Yet the adsorption and oxidation of SO₂ on carbon with oxygen group, as well as the roles of different sulfur oxide groups in Hg adsorption have heretofore been unclear. The formation of sulfur oxide groups by SO₂ and their effects on Hg adsorption on carbon was detailed examined by the density functional theory. The results show that SO₂ can be oxidized into SO₃ by oxygen group on carbon surface. Both C-SO₂ and C-SO₃ can improve Hg adsorption on carbon site, while the promotive effect of C-SO₂ is stronger than C-SO₃. Electron density difference analyses reveal that sulfur oxide groups enhance the charge transfer ability of surface unsaturated carbon atom, thereby improving Hg adsorption. The experimental results confirm that surface active groups formed by SO₂ adsorption is more active for Hg adsorption than the groups generated by SO₃.

Reverse Conversion Treatment of Gaseous Sulfur Trioxide Using Metastable Sulfides from Sulfur-Rich Flue Gas

Sulfur trioxide (SO₃) is an unstable pollutant, and its removal from the gas phase of industrial flue gas remains a significant challenge. Herein, we propose a reverse conversion treatment (RCT) strategy to reduce S(VI) in SO₃ to S(IV) by combining bench-scale experiments and theoretical studies. We first demonstrated that metastable sulfides can break the S-O bond in SO₃, leading to the re-formation of sulfur dioxide (SO₂). The RCT performance varied between mono- and binary-metal sulfides, and metastable CuS had a high SO₃ conversion efficiency in the temperature range of 200-300 °C. Accordingly, the introduction of selenium (Se) lowered the electronegativity of the CuS host and enhanced its reducibility to SO₃. Among the CuSe_1-xS_x composites, CuSe_0.3S_0.7 was the optimal RCT material and reached a SO₂ yield of 6.25 mmol/g in 120 min. The low-valence state of selenium (Se^2-/Se^1-) exhibited a higher reduction activity for SO₃ than did S^2-/S^1-; however, excessive Se doping degraded the SO₃ conversion owing to the re-oxidation of SO₂ by the generated SeO₃^2-. The density functional theory calculations verified the stronger SO₃ adsorption performance (E_ads = -2.76 eV) and lower S-O bond breaking energy (E_a = 1.34 eV) over CuSe_0.3S_0.7 compared to those over CuS and CuSe. Thus, CuSe_1-xS_x can serve as a model material and the RCT strategy can make use of field temperature conditions in nonferrous smelters for SO₃ emission control.

Influence of SO3 on the MnOx/TiO2 SCR catalyst for elemental mercury removal and the function of Fe modification

Elemental mercury (Hg⁰) is a highly hazardous pollutant of coal combustion. The low-temperature SCR catalyst of MnO_x/TiO₂ can efficiently remove Hg⁰ in coal-burning flue gas. Considering its sulfur sensitivity, the effect of SO₃ on the catalytic efficiency of MnO_x/TiO₂ and Fe modified MnO_x/TiO₂ for Hg⁰ removal was investigated comprehensively for the first time. Characterizations of Hg-TPD and XPS were conducted to explore the catalytic mechanisms of Hg⁰ removal processes under different conditions. Hg⁰ removal efficiency of MnO_x/TiO₂ was inhibited irreversibly from 92% to approximately 60% with the addition of 50 ppm SO₃ at 150 ℃, which resulted from the transformation of Mn⁴⁺ and chemisorbed oxygen to MnSO₄. The existence of H₂O would intensify the inhibitory effect. The inhibition almost disappeared and even converted to promotion as the temperature increased to 250 ℃ and above. Fe modification on MnO_x/TiO₂ improved the Hg⁰ removal performance in the presence of SO₃. The addition of SO₃ caused only a slight inhibition of 1.9% on Hg⁰ removal efficiency of Fe modified MnO_x/TiO₂ in simulated coal-fired flue gas, and the efficiency maintained good stability during a 12 h experimental period. This work would be conducive to the future application of MnO_x/TiO₂ for synergistic Hg⁰ removal.

MXenes-A New Class of Two-Dimensional Materials: Structure, Properties and Potential Applications

A new class of two-dimensional nanomaterials, MXenes, which are carbides/nitrides/carbonitrides of transition and refractory metals, has been critically analyzed. Since the synthesis of the first family member in 2011 by Yury Gogotsi and colleagues, MXenes have quickly become attractive for a variety of research fields due to their exceptional properties. Despite the fact that this new family of 2D materials was discovered only about ten years ago, the number of scientific publications related to MXene almost doubles every year. Thus, in 2021 alone, more than 2000 papers are expected to be published, which indicates the relevance and prospects of MXenes. The current paper critically analyzes the structural features, properties, and methods of synthesis of MXenes based on recent available research data. We demonstrate the recent trends of MXene applications in various fields, such as environmental pollution removal and water desalination, energy storage and harvesting, quantum dots, sensors, electrodes, and optical devices. We focus on the most important medical applications: photo-thermal cancer therapy, diagnostics, and antibacterial treatment. The first results on obtaining and studying the structure of high-entropy MXenes are also presented.

Spherical-shaped CuS modified carbon nitride nanosheet for efficient capture of elemental mercury from flue gas at low temperature

Mercury (Hg⁰) pollution poses a huge threat to human health and the environment due to its high toxicity, long persistence and bioaccumulation in the environment. Most of the traditional Hg⁰ adsorbents have a low reaction rate, high operating cost, especially poor resistance to SO₂, which limited their practical application. In this work, nanosheet g-C₃N₄ was used as the support and modified by CuS to capture flue gas mercury. Take advantage of the large specific surface area of g-C₃N₄ to increase the BET of the composite and decrease the use of CuS. The effects of CuS loading, reaction temperature, and common components in the coal-fired flue gas on the mercury removal performance were studied respectively. The experimental outcomes showed that the 10CuS/g-C₃N₄ (10CuS/CN) reaches as high as almost 100% Hg⁰ removal efficiency under the temperature of 40-120 ℃. Meanwhile the common components like SO₂, NO, HCl and H₂O have no obvious inhibition effects on Hg⁰ removal efficiency of the 10CuS/CN adsorbent. S_x^2- and Cu²⁺ as the primary bonding sites shows a synergy effect on Hg⁰ removal. 10CuS/CN is a promising material for Hg⁰ removal under various flue gas conditions, which is expected to be a substitute for traditional adsorbents.

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.

Highly Efficient and Selective Hg(II) Removal from Water Using Multilayered Ti3C2Ox MXene via Adsorption Coupled with Catalytic Reduction Mechanism

Mercury (Hg) removal is crucial to the safety of water resources, yet it lacks an effective removal technology, especially for emergency on-site remediation. Herein, multilayered oxygen-functionalized Ti₃C₂ (Ti₃C₂O_x) (abbreviated as M-Ti₃C₂) nanosheets were prepared to remove Hg(II) from water. The M-Ti₃C₂ has demonstrated ultrafast adsorption kinetics (the concentration decreased from 10 400 to 33 μg L^-1 in 10 s), impressively high capacity (4806 mg g^-1), high selectivity, and broad working pH range (3-12). The density functional theory (DFT) calculations and experimental characterizations unveil that this exceptional Hg(II) removal is owing to the distinct interaction (e.g., adsorption coupled with catalytic reduction). Specifically, Ti atoms on the {001} facets of M-Ti₃C₂ prefer to adsorb Hg(II) in the form of HgClOH, which subsequently undergoes homolytic cleavage to form radical species (e.g., ^•OH and ^•HgCl). Immediately, the ^•HgCl radicals dimerize and form crystalline Hg₂Cl₂ on the edges of M-Ti₃C₂. Up to ∼95% of dimeric Hg₂Cl₂ can be efficiently recovered via facile thermal treatment. Notably, owing to the adsorbed ^•OH and energy released during the distinct interaction, M-Ti₃C₂ has been oxidized to TiO₂/C nanocomposites. And the TiO₂/C nanocomposites have shown to have better performance on the photocatalytic degradation of organic pollutants than Degussa P25. These exceptional features coupled with mercuric recyclable nature make M-Ti₃C₂ an outstanding candidate for rapid/urgent Hg(II) removal and recovery.

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.

Acceleration of Hg0 Adsorption onto Natural Sphalerite by Cu2+ Activation during Flotation: Mechanism and Applications in Hg0 Recovery

The rate of gaseous Hg⁰ adsorption onto natural sphalerite increased by approximately 1.9-7.7 times after Cu²⁺ activation during flotation of the natural sphalerite to remove impurities. Via a new pathway involving CuS, physically adsorbed Hg⁰ was oxidized by CuS to HgS on natural sphalerite after Cu²⁺ activation. In a similar intrinsic ZnS pathway, physically adsorbed Hg⁰ was oxidized by ZnS to HgS. The rate of the CuS pathway for Hg⁰ capture was generally significantly larger than that of the intrinsic ZnS pathway. Thus, Hg⁰ adsorption onto natural sphalerite was notably accelerated after Cu²⁺ activation. However, the kinetic analysis indicated that the capacity of natural sphalerite for Hg⁰ capture did not vary. Because the properties of the activated sphalerite for Zn smelting were barely degraded after Hg⁰ capture, the spent activated sphalerite for Hg⁰ capture can be reused for Zn smelting. Moreover, most of the gaseous Hg⁰ captured by activated sphalerite can be recovered eventually as liquid Hg⁰ in the condenser unit of Zn smelters. Thus, Hg⁰ recovery by activated sphalerite is a cost-effective and environmentally friendly technology to recover Hg⁰ from Zn smelting flue gas, thus replacing the complex and dangerous Boliden-Norzink process.

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.

Amorphous Molybdenum Selenide Nanosheet as an Efficient Trap for the Permanent Sequestration of Vapor-Phase Elemental Mercury

The key challenge of vapor-phase elemental mercury (Hg0) sequestration is the rational design of a sorbent with abundantly available ligands that exhibit excellent affinity toward Hg0 to simultaneously achieve a high uptake capacity and rapid capture rate. In this work, it is demonstrated how the correct combination of functional ligands and structural properties can form an ideal remediator for permanent Hg0 immobilization. The adsorption capacity of an amorphous molybdenum triselenide (MoSe3) nanosheet greater than 1000 mg g-1 is the highest recorded value compared to previously reported sorbents tested in a fixed-bed reactor. Meanwhile, the uptake rate of 240 µg g-1 min-1 is also the highest recorded rate value. Mercury selenide as formed exhibits extremely low leachability when environmentally exposed. This impressive performance is primarily attributed to the appropriate layer space between the nanosheets that permeated Hg0 and the existence of diselenide (Se2 2-) ligands that exhibit excellent affinity toward Hg0. Thus, this work not only provides a promising trap for permanent Hg0 sequestration from industrial and domestic sources with minimum hazard but also provides a detailed illustration of using structural advantages to obtain an ideal sorbent as well as guidance for the further development of Hg0 decontamination techniques.

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

Mercury capture from flue gas remains a challenge for environmental protection due to the lack of cost-effective sorbents. Natural manganese ore (NMO) was developed as a cost-effective sorbent for elemental mercury removal from flue gas. NMO sorbent showed excellent Hg⁰ removal efficiency (>90%) in a wide temperature window (100-250 °C) under the conditions of simulated flue gas. O₂, NO, and HCl promoted Hg⁰ removal due to the surface reactions of Hg⁰ with these species. SO₂ and H₂O slightly inhibited Hg⁰ removal under the conditions of simulated flue gas. O₂ addition could also weaken the inhibitory effect of SO₂. NMO sorbent exhibited superior regeneration performance for Hg⁰ removal during ten-cycle experiments. Quantum chemistry calculations were used to identify the active components of NMO sorbent and to understand the atomic-level interaction between Hg⁰ and sorbent surface. Theoretical results indicated that Mn₃O₄ is the most active component of NMO sorbent for Hg⁰ removal. The atomic orbital hybridization and electrons sharing led to the stronger interaction between Hg⁰ and Mn₃O₄ surface. Finally, a chemical looping process based on NMO sorbent was proposed for the green recovery of Hg⁰ from flue gas. The low cost, excellent performance, superior regenerable properties suggest that the natural manganese ore is a promising sorbent for mercury removal from flue gas.