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

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.

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.

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.

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.

[Formula: see text] modified AC for removing gaseous elemental mercury from flue gas

Mercury pollution from coal-fired power plants has always been an environmental concern, and adsorption technology is an effective method for Hg⁰ removal. In this study, copper-iron binary metal sulfide modified activated carbon ([Formula: see text]) adsorbent was synthesized. The performance and mechanism of mercury removal were tested by adsorption experiments and characterization methods. The results showed that the mercury removal efficiency of [Formula: see text] under simulated flue gas (SFG) conditions at 150 °C could reach 91%, which was higher than [Formula: see text] and [Formula: see text]. Compared with [Formula: see text], the introduction of Fe increased the proportion of [Formula: see text], which was usually linked to [Formula: see text], resulting in the generation of more [Formula: see text](Cu^IS^I) species. Meanwhile, the generation of active sulfur sites such as [Formula: see text] was generated, which had a facilitative effect on the oxidation of Hg⁰. Stable [Formula: see text] was the predominant product on the adsorbent surface. The 2% sulfur loadings had significantly improved the sulfur resistance of conventional AC.

Gaseous Arsenic Capture in Flue Gas by CuCl2-Modified Halloysite Nanotube Composites with High-Temperature NOx and SOx Resistance

Gaseous arsenic emitted from coal combustion flue gas (CCFG) causes not only severe contamination of the environment but also the failure of selective catalytic reduction (SCR) catalysts in power plants. Development of inexpensive and effective adsorbents or techniques for the removal of arsenic from high-temperature CCFG is crucial. In this study, halloysite nanotubes (HNTs) at low price were modified with CuCl₂ (CuCl₂-HNTs) through ultrasound assistance and applied for capturing As₂O₃(g) in simulated flue gas (SFG). Experiments on arsenic adsorption performance, adsorption mechanism, and adsorption energy based on density functional theory were performed. Modification with CuCl₂ clearly enhanced the arsenic uptake capacity (approximately 12.3 mg/g) at 600 °C for SFG. The adsorbent exhibited favorable tolerance to high concentrations of NO_x and SO_x. The As₂O₃(III) was oxidized and transformed into As₂O₅(V) on the CuCl₂-HNTs. The Al-O bridge had the highest adsorption energy for the O end of the As-O group (-2.986 eV), and the combination formed between arsenic-containing groups and aluminum was stable. In addition, the captured arsenic could be stabilized in the sorbent at high temperature, making it possible to use the sorbent before the SCR system. This demonstrates that CuCl₂-HNTs is a promising sorbent for arsenic oxidation and removal from CCFG.

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.