Metastable Facet-Controlled Cu2WS4 Single Crystals with Enhanced Adsorption Activity for Gaseous Elemental Mercury

Adsorption removal of mercury from flue gas by metal selenide: A review

Mercury (Hg) pollution has been a global concern in recent decades, posing a significant threat to entire ecosystems and human health due to its cumulative toxicity, persistence, and transport in the atmosphere. The intense interaction between mercury and selenium has opened up a new field for studying mercury removal from industrial flue gas pollutants. Besides the advantages of good Hg° capture performance and low secondary pollution of the mineral selenium compounds, the most noteworthy is the relatively low regeneration temperature, allowing adsorbent regeneration with low energy consumption, thus reducing the utilization cost and enabling recovery of mercury resources. This paper reviews the recent progress of mineral selenium compounds in flue gas mercury removal, introduces in detail the different types of mineral selenium compounds studied in the field of mercury removal, reviews the adsorption performance of various mineral selenium compounds adsorbents on mercury and the influence of flue gas components, such as reaction temperature, air velocity, and other factors, and summarizes the adsorption mechanism of different fugitive forms of selenium species. Based on the current research progress, future studies should focus on the economic performance and the performance of different carriers and sizes of adsorbents for the removal of Hg0 and the correlation between the gas-particle flow characteristics and gas phase mass transfer with the performance of Hg0 removal in practical industrial applications. In addition, it remains a challenge to distinguish the oxidation and adsorption of Hg0 quantitatively.

Current status of advancement in remediation technologies for the toxic metal mercury in the environment: A critical review

Currently, pollution due to heavy metals, in particular dissolved mercury, is a major concern for society and the environment. This work aims to evaluate the current scenario regarding the removal/elimination of mercury. Mercury removal through adsorption is mainly done through artificial resins and metallic-organic frameworks. In the case of the zinc organic framework, it was able to adsorb Hg2+, reaching an adsorption capacity of 802 mg g-1. As for the Hg(0) the coconut husk was found to have the lowest equilibrium time, 30 min, and the highest adsorption capacity of 956.2 mg g-1. Experimental reports and molecular simulation indicate that the adsorption of mercury and other chemical forms occurs due to electrostatic interactions, ion exchange, precipitation, complexation, chelation, and covalent bonds, according to the material nature. The reported thermodynamic results show that, in most cases, the mercury adsorption has an endothermic nature with enthalpy levels below 40 kJ mol-1. Thermal and chemical regeneration methods lead to a similar number of 5 cycles for different materials. The presence of other ions, in particular cadmium, lead, and copper, generates an antagonistic effect for mercury adsorption. Regarding the other current technologies, it was found that mercury removal is feasible through precipitation, phytoremediation, and marine microalgae; all these methods require constant chemicals or a slow rate of removal according to the conditions. Advanced oxidative processes have noteworthy removal of Hg(0); however, Fenton processes lead to mineralization, which leads to Fe2+ and Fe3+ in solution; sonochemical processes are impossible to scale up at the current technology level; and electrochemical processes consume more energy and require constant changes of the anode and cathode. Overall, it is possible to conclude that the adsorption process remains a more friendly, economical, and greener process in comparison with other processes.

Selenium-modified activated coke: a high-capacity and facile designed Hg0 adsorbent for coal-fired flue gas

The substantial amount of mercury emissions from coal-fired flue gas causes severe environmental contamination. With the Minamata Convention now officially in force, it is critical to strengthen mercury pollution control. Existing activated carbon injection technologies suffer from poor desulfurization performance and risk secondary-release risks. Therefore, considering the potential industrial application of adsorbents, we selected cost-effective and readily available activated coke (AC) as the carrier in this study. Four metal selenides-copper, iron, manganese, and tin-were loaded onto the AC to overcome the application problems of existing technologies. After 120 min of adsorption, the CuSe/AC exhibited the highest efficiency in removing Hg0, surpassing 80% according to the experimental findings. In addition, the optimal adsorption temperature window was 30-120 °C, the maximum adsorption rate was 2.9 × 10-2 mg·g-1·h-1, and the effectiveness of CuSe/AC in capturing Hg0 only dropped by 5.2% in the sulfur-containing atmosphere. The physicochemical characterization results indicated that the AC surface had a uniform loading of CuSe with a nanosheet structure resembling polygon and that the Cu-to-Se atomic ratio was close to 1:1. Finally, two possible Hg0 reaction pathways on CuSe/AC were proposed. Moreover, it was elucidated that the highly selective binding of Hg0 with ligand-unsaturated Se- was the key factor in achieving high adsorption efficiency and sulfur resistance in the selenium-functionalized AC adsorbent. This finding offers substantial theoretical support for the industrial application of this adsorbent.

Edge-Enriched Molybdenum Disulfide Ultrathin Nanosheets with a Widened Interlayer Spacing for Highly Efficient Gaseous Elemental Mercury Capture

Transition metal sulfides have exhibited remarkable advantages in gaseous elemental mercury (Hg⁰) capture under high SO₂ atmosphere, whereas the weak thermal stability significantly inhibits their practical application. Herein, a novel N,N-dimethylformamide (DMF) insertion strategy via crystal growth engineering was developed to successfully enhance the Hg⁰ capture ability of MoS₂ at an elevated temperature for the first time. The DMF-inserted MoS₂ possesses an edge-enriched structure and an expanded interlayer spacing (9.8 Å) and can maintain structural stability at a temperature as high as 272 °C. The saturated Hg⁰ adsorption capacities of the DMF-inserted MoS₂ were measured to be 46.91 mg·g^-1 at 80 °C and 27.40 mg·g^-1 at 160 °C under high SO₂ atmosphere. The inserted DMF molecules chemically bond with MoS₂, which prevents possible structural collapse at a high temperature. The strong interaction of DMF with MoS₂ nanosheets facilitates the growth of abundant defects and edge sites and enhances the formation of Mo⁵⁺/Mo⁶⁺ and S₂^2- species, thereby improving the Hg⁰ capture activity at a wide temperature range. Particularly, Mo atoms on the (100) plane represent the strongest active sites for Hg⁰ oxidation and adsorption. The molecule insertion strategy developed in this work provides new insights into the engineering of advanced environmental materials.

Mercury removal from flue gas by a MoS2/H2O heterogeneous system based on its absorption kinetics

An enhanced MoS₂/C₁₀TAB/H₂O system was built and investigated for Hg⁰ removal based on strengthening the Hg⁰ gas-liquid mass transfer. The results showed that adding 7 mg/L C₁₀TAB can improve the Hg⁰ removal efficiency from 76.5 to 88.7% as decrease of the solution surface tension. Keeping 2000 rpm of stirring rate accelerated the renewal rate of gas-liquid interface, thereby enhancing Hg⁰ removal. SO₂ slightly promoted the Hg⁰ removal efficiency to 91% because of the absorption of SO₂ causing a decrease in the solution pH from 6.9 to 4.3. NO participated in Hg⁰ removal reactions but not removed in this system which visibly enhanced the Hg⁰ removal efficiency to 94%. The Hg mass transfer kinetics were analyzed to determine how C₁₀TAB promoted Hg⁰ removal. The Hg-TPD, Hg fate, and species results revealed that Hg⁰ was first oxidized to Hg²⁺, then bonded with S to generate HgS and enrich on the MoS₂. Therefore, improving the Hg⁰ gas-liquid mass transfer can enhance Hg⁰ removal in MoS₂/H₂O system, which can provide reference for purification of other insoluble pollutants in absorption system.

Engineering of Defect-Rich Cu2WS4 Nano-homojunctions Anchored on Covalent Organic Frameworks for Enhanced Gaseous Elemental Mercury Removal

Fabricating two-dimensional transition-metal dichalcogenide (TMD)-based unique composites is an effective way to boost the overall physical and chemical properties, which will be helpful for the efficient and fast capture of elemental mercury (Hg⁰) over a wide temperature range. Herein, we constructed a defect-rich Cu₂WS₄ nano-homojunction decorated on covalent organic frameworks (COFs) with abundant S vacancies. Highly well-dispersed and uniform Cu₂WS₄ nanoparticles were immobilized on COFs strongly via an ion pre-anchored strategy, consequently exhibiting enhanced Hg⁰ removal performance. The saturation adsorption capacity of Cu₂WS₄@COF composites (21.60 mg·g^-1) was 9 times larger than that of Cu₂WS₄ crystals, which may be ascribed to more active S sites exposed in hybrid interfaces formed in the Cu₂WS₄ nano-homojunction and between Cu₂WS₄ nanoparticles and COFs. More importantly, such hybrid materials reduced adsorption deactivation at high temperatures, having a wide operating temperature range (from 40 to 200 °C) owing to the thermostability of active S species immobilized by both physical confined and chemical interactions in COFs. Accordingly, this work not only provides an effective method to construct uniform TMD-based sorbents for mercury capture but also opens a new realm of advanced COF hybrid materials with designed functionalities.

Controllable Disordered Copper Sulfide with a Sulfur-Rich Interface for High-Performance Gaseous Elemental Mercury Capture

Copper sulfide (CuS) has received increasing attention as a promising material in gaseous elemental mercury (Hg⁰) capture, yet how to enhance its activity at elevated temperature remains a great challenge for practical application. Herein, simultaneous improvement in the activity and thermal stability of CuS toward Hg⁰ capture was successfully achieved for the first time by controlling the crystal growth. CuS with a moderate crystallinity degree of 68.8% showed a disordered structure yet high thermal stability up to 180 °C. Such disordered CuS can maintain its Hg⁰ capture activity stable during longtime test at a wide temperature range from 60 to 180 °C and displayed strong resistance to SO₂ (6%) and H₂O (8%). The significant improvement can be attributed to the synergistic effect of a moderately crystalline nature and a unique sulfur-rich interface. Moderate crystallinity guarantees the thermal stability of CuS and the presence of abundant defects, in which copper vacancy enhances significantly the Hg⁰ capture activity. The sulfur-rich interface enables CuS to provide plentiful highly active S_x^2- sites for Hg⁰ adsorption. The interrelation between structure, reactivity, and thermal stability clarified in this work broadens the understanding toward Hg⁰ oxidation and adsorption over CuS and provides new insights into the rational design and engineering of advanced environmental materials.

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.

MXene-supported copper-molybdenum sulfide nanostructures as catalysts for hydrogen evolution

The synthesis and enhanced HER performance of Cu2MoS4/Ti3C2Tx-30% was demonstrated.