Preparation of halloysite nanotubes-encapsulated magnetic microspheres for elemental mercury removal from coal-fired flue gas

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.

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.

Insights into synergistic oxidation mechanism of Hg0 and chlorobenzene over MnCo2O4 microsphere with oxygen vacancy and acidic site

The simultaneous control of Hg⁰ and chlorinated organics has become the frontier of environmental engineering but still lacks the understanding of synergistic oxidation mechanism. Herein, we designed a Mn-Co catalyst with abundant oxygen vacancies and acidities, which delivered more than 90 % oxidation performance of Hg⁰ within 100-325 °C and achieved 90 % conversion of chlorobenzene at 220 °C. A synergistic effect was observed in the oxidation of Hg⁰ and chlorobenzene. Experimental and computational results revealed that Lewis acid over Mn site weakened C-Cl bands of chlorobenzene by electronic traction. The strong interaction between adsorbed mercury and Cl further promoted dechlorination process to generate HgCl₂ gas, while accelerating the nucleophilic substitution of Brønsted acid attacking the benzene ring over Co site, consequently triggering synergistic oxidation of Hg⁰ and chlorobenzene. Oxygen vacancies enhanced the initial adsorption of Hg⁰ and chlorobenzene. Meanwhile, the interfacial charge-transfer from Hg-d to Cl-p orbitals alleviated deactivation of Lewis acid and slowed down the consumption of Brønsted acid, which accelerated the conversion of intermediates to CO₂/H₂O and promoted deep oxidation of chlorobenzene. This work provides a unique insight into the promotion of the synergistic oxidation of Hg⁰ and chlorobenzene and is expected to guide the industrial applications.

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.

Excellent adsorption performance and capacity of modified layered ITQ-2 zeolites for elemental mercury removal and recycling from flue gas

Adsorption is a superior method for removing and recycling high concentration of mercury from nonferrous metal smelting flue gas, especially adsorbents with good sulfur resistance and large adsorption capacity. In this study, Co and Mn oxide-modified layered ITQ-2 zeolites were designed to capture and recycle elemental mercury (Hg⁰). The physicochemical characteristics of the adsorbents were characterized using BET, XRD, FESEM, TEM, and XPS, and the results showed that Mn/ITQ-2 zeolite has a large specific surface area, and MnO_x was highly dispersed on ITQ-2 zeolite. The Hg⁰ removal efficiency and adsorption capacity of the 5%Mn/ITQ-2 zeolite at 300 °C were 97% and 2.04 mg/g in 600 min, respectively, much higher than those of the previously reported 5%Mn/MCM-22 zeolite. The 2%Co-2%Mn/ITQ-2 zeolite exhibited a higher SO₂ resistance performance. The mechanism of Hg⁰ removal was concluded to be driven by the primary catalytic oxidation of MnO_x, secondary oxidation of active chlorine, and concurrent chemisorption. However, the Hg⁰ adsorption capacity was determined by the specific surface area and pore structure of ITQ-2. The 2%Co-2%Mn/ITQ-2 zeolite exhibited a high SO₂ resistance performance. The Mn/ITQ-2 and Co-Mn/ITQ-2 zeolites have excellent regenerability and reusability, which can realize mercury recycling from flue gas.

Amidoxime modified Fe3O4@TiO2 particles for antibacterial and efficient uranium extraction from seawater

Uranium extraction and recovery play a critical role in guaranteeing the sustainable nuclear energy supply and protecting the environmental safety. The ideal uranium sorbents possess high adsorption capacity, excellent selectivity and reusability, as well as outstanding antimicrobial property, which are greatly desired for the real application of uranium extraction from seawater. To address this challenge, a novel magnetic core-shell adsorbent was designed and fabricated by a facile method. The obtained amidoximed Fe₃O₄@TiO₂ particles (Fe₃O₄@TiO₂-AO) achieved equilibrium in 2 h and the maximum adsorption capacity calculated from Langmuir model is 217.0 mg/g. The adsorption kinetics followed the pseudo-second-order model. Meanwhile, the Fe₃O₄@TiO₂-AO exhibited great selectivity when competitive metal ions and anions coexisted. In addition, the magnetic Fe₃O₄@TiO₂-AO could be conveniently separated and collected by an external magnetic field, the regeneration efficiency maintained at 78.5% even after ten adsorption-desorption cycles. In natural seawater, the uranium uptake reached 87.5 μg/g in 33 days. Furthermore, the TiO₂ contained adsorbent showed effective photo induced bactericidal properties against both E. coli and S. aureus. The Fe₃O₄@TiO₂-AO with great U(VI) adsorption performance is highly promising in uranium extraction and reclamation.

Electrospun metal-organic frameworks hybrid nanofiber membrane for efficient removal of As(III) and As(V) from water

Metal-organic frameworks (MOFs) have been widely applied for pollutants removal in water. However, the powdered MOFs are always suffered from aggregation during use and difficult collection after use. These problems discount their efficiency and inhibit their reusability. In this work, Zr-based MOF (UiO-66) was successfully imprisoned into a water-stable polyacrylonitrile (PAN) substrate by electrospinning. The containing UiO-66 hybrid membrane was confirmed by instrumental characterizations and its stability was also investigated by ICP-OES analysis. The obtained composite membrane can efficiently remove both arsenite (AsIII) and arsenate (AsV) from water under natural pH conditions. The adsorption kinetic fitted well with pseudo-second-order model and was dominated by chemisorption. Its adsorption isotherm can be described by Langmuir model. The maximal adsorption capacities of the hybrid membrane for As(V) and As(III) were 42.17 mg/g and 32.90 mg/g, respectively. Our results demonstrated that the MOFs-dispersed electrospun nanofiber membrane can greatly inherit the MOFs' original adsorption properties and exhibits good regenerability without loss of MOFs. Electrospinning is an effective and practical method for the preparation of MOFs hybrid membrane, which makes the composite very easy to be collected after use.