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

FeSe2 and Its Composites for Pollutants Removal: Synthesis, Mechanisms, and Application Potential

In recent years, the research of FeSe2 and its composites in environmental remediation has been gradually carried out. And the FeSe2 materials show great catalytic performance in photocatalysis, electrocatalysis, and Fenton-like reactions for pollutants removal. Therefore, the studies and applications of FeSe2 materials are reviewed in this work, including the common synthesis methods, the role of Fe and Se species as well as the catalyst structure, and the potential for practical environmental applications. Hereinto, it is worth noting in particular that the lower-valent Se (Se2- ), unsaturated Se (Se- ), and Se vacancies (VSe ) can play different roles in promoting pollutants removal. In addition, the FeSe2 material also demonstrates high stability, reusability, and adaptability over a wider pH range as well as universality to different pollutants. In view of the overall great properties and performance of FeSe2 materials compared with other typical Fe-based materials, it deserves and needs further research. And finally, this paper presents some challenges and perspectives in future development, looking forward to providing helpful guidance for the subsequent research of FeSe2 and its composites for environmental application.

Highly dispersed amorphous nano-selenium functionalized carbon nanofiber aerogels for high-efficient uptake and immobilization of Hg(II) ions

Owing to the strong Hg-Se interaction, Se-containing materials are promising for the uptake and immobilization of Hg(II) ions; compared with metal selenides or selenized compounds, elemental Se contains the highest ratio of Se. However, it remains a challenge to fully expose all the potential Se binding sites and achieve high utilization efficiency of elemental Se. Through rational design on the structure, dispersity, and size of materials, Se/CNF aerogels composed of abundant well-dispersed and amorphous nano-Se have been prepared and applied for the high-efficient uptake and immobilization of Hg(II) ions. The well-dispersion of nano-Se increases the exposure of Se sites, the amorphous structure benefits the easy cleavage of Se-Se bonds, the 3D porous networks of aerogels permit fast ions transport and easy operation. Benefiting from the combination effect of strong Hg-Se interaction and sufficient exposure of Se-enriched sites, the Se/CNF aerogels demonstrate strong binding ability (Kd = 3.8 ×105 mL·g-1), high capacity (943.4 mg·g-1), and preeminent selectivity (αMHg > 100) towards highly toxic Hg(II) ions. Notably, the utilization efficiency of Se in Se/CNF aerogels is as high as 99.5%. Moreover, the strong Hg-Se interaction and extraordinary stability of HgSe could minimize the environmental impact of the spent Se/CNF adsorbents after its disposal.

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.

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.

SO2-Driven In Situ Formation of Superstable Hg3Se2Cl2 for Effective Flue Gas Mercury Removal

Flue gas mercury removal is mandatory for decreasing global mercury background concentration and ecosystem protection, but it severely suffers from the instability of traditional demercury products (e.g., HgCl₂, HgO, HgS, and HgSe). Herein, we demonstrate a superstable Hg₃Se₂Cl₂ compound, which offers a promising next-generation flue gas mercury removal strategy. Theoretical calculations revealed a superstable Hg bonding structure in Hg₃Se₂Cl₂, with the highest mercury dissociation energy (4.71 eV) among all known mercury compounds. Experiments demonstrate its unprecedentedly high thermal stability (>400 °C) and strong acid resistance (5% H₂SO₄). The Hg₃Se₂Cl₂ compound could be produced via the reduction of SeO₃^2- to nascent active Se⁰ by the flue gas component SO₂ and the subsequent combination of Se⁰ with Hg⁰ and Cl^- ions or HgCl₂. During a laboratory-simulated experiment, this Hg₃Se₂Cl₂-based strategy achieves >96% removal efficiencies of both Hg⁰ and HgCl₂ enabling nearly zero Hg⁰ re-emission. As expected, real mercury removal efficiency under Se-rich industrial flue gas conditions is much more efficient than Se-poor counterparts, confirming the feasibility of this Hg₃Se₂Cl₂-based strategy for practical applications. This study sheds light on the importance of stable demercury products in flue gas mercury treatment and also provides a highly efficient and safe flue gas demercury strategy.

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.

Covalent and Non-covalent Functionalized Nanomaterials for Environmental Restoration

Nanotechnology has emerged as an extraordinary and rapidly developing discipline of science. It has remolded the fate of the whole world by providing diverse horizons in different fields. Nanomaterials are appealing because of their incredibly small size and large surface area. Apart from the naturally occurring nanomaterials, synthetic nanomaterials are being prepared on large scales with different sizes and properties. Such nanomaterials are being utilized as an innovative and green approach in multiple fields. To expand the applications and enhance the properties of the nanomaterials, their functionalization and engineering are being performed on a massive scale. The functionalization helps to add to the existing useful properties of the nanomaterials, hence broadening the scope of their utilization. A large class of covalent and non-covalent functionalized nanomaterials (FNMs) including carbons, metal oxides, quantum dots, and composites of these materials with other organic or inorganic materials are being synthesized and used for environmental remediation applications including wastewater treatment. This review summarizes recent advances in the synthesis, reporting techniques, and applications of FNMs in adsorptive and photocatalytic removal of pollutants from wastewater. Future prospects are also examined, along with suggestions for attaining massive benefits in the areas of FNMs.

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.

Strengthen the Affinity of Element Mercury on the Carbon-Based Material by Adjusting the Coordination Environment of Single-Site Manganese

Mercury, as a highly poisonous pollutant, poses a severe threat to the global population. However, the removal of Hg⁰ can only be carried out at below 100 °C due to the weak binding of the adsorbent. Herein, a series of carbon-based materials with different coordination environments and atomic dispersion of single-site manganese were prepared, and their elemental mercury removal performance was systematically investigated. It was demonstrated that the coordination environment around manganese determines its electronic structure and size, thus affecting its affinity with mercury. The obtained best adsorbents atomically dispersed Mn with atom size near 0.2 nm, achieves high Hg⁰ removal efficiency and over 13 mg/g Hg⁰ adsorption capacity at 200 °C. And the SO₂ resistance performance of single atoms (∼0.2 nm) is much better than clusters (∼1-2 nm) because of its high selectivity, that the effect of SO₂ is only 3%. Density functional theory (DFT) reveals that Mn with four-nitrogen atoms (Mn-N₄-C═O) is more active than other number nitrogen coordination materials. Moreover, the presence of carboxyl groups around manganese also promotes affinity for Hg⁰. This work might shed new light on the enhancement of Hg⁰ affinity in carbon-based materials and the rational design of the coordination structure of the tunable Hg⁰ activities.

Molecular-level insights into the immobilization of vapor-phase mercury on Fe/Co/Ni-doped hierarchical molybdenum selenide

A novel and efficient adsorbent (TM-MoSe₂, TM = Fe, Co, Ni) for mercury removal was developed and studied. The adsorption of mercury species (Hg⁰, HgCl, and HgCl₂) and the oxidation of Hg⁰ by HCl on TM-MoSe₂ (001) surface were explored at molecular level by density functional theory (DFT). The results shown that the Hg⁰ adsorption capacity of MoSe₂ was improved by the doping of Fe/Co/Ni, which was also confirmed by experiments. The initial Hg⁰ removal efficiency of MoSe₂-based adsorbents reached 96.4-100.0%. In addition, HgCl was mainly adsorbed on TM-MoSe₂ (001) surface in the form of dissociation. The escape of Hg atom from HgCl resulted in the release of Hg⁰ again. However, HgCl₂ could be fixed well on the surface of adsorbent through molecular adsorption or dissociative adsorption. For the oxidation process of Hg⁰ by HCl, it abided with the Langmuir-Hinshelwood mechanism. In comparison with direct oxidation (Hg → HgCl₂), two-step pathway (Hg → HgCl → HgCl₂) was an achievable reaction route with lower energy. Furthermore, the Hg → HgCl process was the rate-limiting step of the two-step pathway. The proposed adsorption and oxidation mechanism of mercury species on TM-MoSe₂ (001) provide advanced strategies on the development of adsorbents for industrial mercury removal.

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

Purposively designing environmental advanced materials and elucidating the underlying reactivity mechanism at the atomic level allows for the further optimization of the removal performance for contaminants. Herein, using well facet-controlled I-Cu₂WS₄ single crystals as a model transition metal chalcogenide sorbent, we investigated the adsorption performance of the exposed facets toward gaseous elemental mercury (Hg⁰). We discovered that the decahedron exhibited not only facet-dependent adsorption properties for Hg⁰ but also recrystallization along the preferential [001] growth direction from a metastable state to the steady state. Besides, the metastable crystals with a predominant exposure of {101} facets dominated the promising adsorption efficiency (about 99% at 75 °C) while the saturated adsorption capacity was evaluated to be 2.35 mg·g^-1. Subsequently, comprehensive characterizations and X-ray adsorption fine structure (XAFS), accompanied by density functional theory (DFT) calculations, revealed that it might be owing to the coordinatively unsaturated local environment of W atoms with S defects and the surface relative stability of different facets, which could be affected by the change in surface atom configuration. Hence, the new insight into the facet-dependent adsorption property of transition metal chalcogenide for Hg⁰ may have important implications, and the atomic-level study directly provides instructions for development and design of highly efficient functional materials.

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.

Surface-Engineered Sponge Decorated with Copper Selenide for Highly Efficient Gas-Phase Mercury Immobilization

The paramount challenge in design and synthesis of materials for vapor-phase elemental mercury (Hg⁰) immobilization is to achieve a balance between performance and economy for practical applications. Herein, a newly designed electroless plating coupled with an in situ selenization method was developed to construct a copper selenide (Cu₂Se)-functionalized commercial polyurethane sponge (PUS) as an efficient Hg⁰ trap. Intrinsic features such as easy availability of the raw material, facile preparation, and excellent performance guarantee the Cu₂Se/PUS to be applicable in industrial uses. The Cu₂Se/PUS exhibits a maximum adsorption capacity (Q_m,Cu2Se/PUS) of 25.90 mg·g^-1, while this value is 758.80 mg·g^-1 when normalized to the Cu₂Se coating amount. This value of Q_m,Cu2Se is equal to 79.7% of its corresponding theoretical value (Q_t,Cu2Se), far exceeding the availability of Cu₂Se anchored on other supports. Meanwhile, the Cu₂Se/PUS exhibited a quick response for Hg⁰, with an extremely high uptake rate of 1275.84 μg·g^-1·min^-1. Even under harsh conditions, the Cu₂Se/PUS still immobilizes Hg⁰ effectively, which is crucial for real-world applications. This work not only provides a promising trap for permanent Hg⁰ sequestration from industrial sources but also illustrates a versatile platform for the economic fabrication and practical application of advanced functional sponges in diverse environmental remediation.

Toward an Understanding of Fundamentals Governing the Elemental Mercury Sequestration by Metal Chalcogenides

The lack of fundamental understanding of the chemistry governing elemental mercury (Hg⁰) immobilization over metal chalcogenides (MChals) is the key challenge impeding the interpretations of Hg⁰ behaviors in global cycles. This work therefore made the first endeavor toward the establishment of a roadmap capable of describing and depicting Hg⁰ sequestrations by various MChals. The results suggest that the binding energy between the metal cations and chalcogen anions is a proper descriptor that could predict the immobilization behaviors of Hg⁰ over zinc chalcogenides (ZnS and ZnSe) that exhibit an identical molecular structure, i.e., the lower the binding energy was, the higher the Hg⁰ sequestration performance that was obtained. The validity of this descriptor was further demonstrated over a series of MChals sharing structural similarities. A scaling relationship was thus established, which further proved the Hg⁰ immobilization performance of MChals was generally in reverse proportion to the above-mentioned binding energy. Although there is still a long way toward the proposal of a full roadmap that can predict and depict the Hg⁰ immobilization behaviors over all MChals, this work marks the first step on this road and provides guides for further studies by understanding the fundamentals governing Hg⁰ sequestration over MChals with structural similarities.