Novel Simultaneous Removal Technology of NO and SO2 Using a Semi-Dry Microwave Activation Persulfate System

Effect of relative humidity on the desulfurization performance of calcium-based desulfurizer

Low desulfurization efficiency impedes the wide application of dry desulfurization technology, which is a low-cost and simple process, and one significant solution is the development and manufacture of high-performance desulfurizers. In this study, firstly, a steam jet mill was used to digest quicklime; then, we utilized numerical simulation to study the flow field distribution and analyze the driving factors of quicklime digestion; and lastly, the desulfurization performance of the desulfurizer was evaluated under different relative humidities. The results show that the desulfurizer prepared via the steam jet mill had better apparent activity than traditional desulfurizers. Also, the entire jet flow field of the steam jet mill is in a supersonic and highly turbulent flow state, with high crushing intensity and good particle acceleration performance. Sufficient contact with the nascent surface maximizes the formation of slaked lime. The experiments demonstrated that the operating time with 100% desulfurization efficiency and the "break-through" time for the desulfurizer prepared via the steam jet mill is longer than that of traditional desulfurizers, and has significant advantages, especially at low flue gas relative humidity. Compared with traditional desulfurizers, the desulfurizer prepared via steam jet mill expands the range of acceptable flue gas temperature, and the failure temperature is 1.625 times that of traditional desulfurizers. This work breaks through the technical bottleneck of low dry desulfurization efficiency, which is an important step in pushing forward the application of dry desulfurization.

Mechanistic understanding of green synthesized cerium nanoparticles for the photocatalytic degradation of dyes and antibiotics from aqueous media and antimicrobial efficacy: A review

In recent years, extensive research endeavors are being undertaken for synthesis of an efficient, economic and eco-friendly cerium oxide nanoparticles (CeO2 NPs) using plant extract mediated greener approach. A number of medicinal plants and their specific parts (flowers, bark, seeds, fruits, seeds and leaves) have been found to be capable of synthesizing CeO2 NPs. The specific key phytochemical constituents of plants such as alkaloids, terpenoids, phenolic acids, flavones and tannins can play significant role as a reducing, stabilizing and capping agents in the synthesis of CeO2 NPs from their respective precursor solution of metal ions. The CeO2 NPs are frequently using in diverse fields of science and technology including photocatalytic degradation of dyes, antibiotics as well as antimicrobial applications. In this review, the mechanism behind the green synthesis CeO2 NPs using plant entities are summarized along with discussion of analytical results from characterization techniques. An overview of CeO2 NPs for water remediation application via photocatalytic degradation of dyes and antibiotics are discussed. In addition, the mechanisms of antimicrobial efficacy of CeO2 NPs and current challenges for their sustainable application at large scale in real environmental conditions are discussed.

Efficient degradation of chlorpyrifos and intermediate in soil by a novel microwave induced advanced oxidation process: A two-stage reaction

The application of microwave/peroxymonosulfate (MW/PMS) in soil remediation has been limited by some shortages including low utilization efficiency of oxidants, low MW absorption capacity of soil particles and incomplete degradation of intermediate. In this study, heating pad waste (HPW) was added in the MW/PMS system to increase the ability of absorbing MW and degradation efficiency of toxic intermediate. A two-stage method for degradation of chlorpyrifos (CPF) and its intermediate 3,5,6-trichloro-2-pyridinol (TCP) by MW/PMS assisted with HPW was proposed. In the first stage, more than 90% of CPF was degraded within 15 min before the addition of HPW, and most of the CPF was converted into TCP through direct or indirect pathways under the action of 1O2. In the second stage, more than 70% of the generated TCP was rapidly degraded through SO4•- oxidation and electron transfer. The TCP was further degraded with the assistance of HPW through methylation, hydroxylation and dechlorination etc., and the toxicity of degradation products was decreased significantly. pH and soil organic matter had little influences on CPF and TCP degradation. Therefore, a new strategy for remediation of CPF contaminated-soil was provided based on MW/PMS technology and the concept of "treating waste with waste".

Removal of NO in flue gas simulated by the Fe2+/Cu2+-activated double oxidant system

⋅ O H The wet denitrification technology has a good development prospect due to its simple system and mild reaction conditions, and related research has become a hot topic in the field of flue gas purification. In this work, a novel simultaneous removal technology of NO from flue gas using Fe2+/Cu2+-catalytic H2O2/(NH4)2S2O8 system was developed for the first time. The feasibility of this new flue gas cleaning technology was explored through a series of experiments and performance analyses. The mechanism of oxidation products, free radicals and simultaneous removal of NO was revealed. The effects of the main process parameters on the removal of NO were investigated. Relevant results demonstrated that the removal efficiency of NO was elevated when the concentration of (NH4)2S2O8 or reacting temperature increased, while it was decreased after increasing the raising of Fe2+, Cu2+ and H2O2 concentrations. The main radicals were and·S O 4 - , using the electron spin resonance technique in the solution, and played a very important role in NO removal. The main products were carried out by ion chromatography and elemental N material accountancy, and the results showed that it was sulfate and nitrate in the solution, which provided theoretical guidance for the subsequent treatment and resource utilization of the absorption solution. The results of the study provided a theoretical basis for the industrial application of wet denitrification.Highlights A new green process of NO removal by a wet process with Fe2+/Cu2+ activated (NH4)2S2O8 system is proposed in this paper;Elimination mechanisms and paths of NO are elucidated;The synergistic role produced by Cu2+ and Fe2+ is beneficial to the purification of NO;The synergistic role produced by (NH4)2S2O8 and H2O2 increased the concentration of free radicals in the solution;This process jointly considers the enhanced removal of NO and recycling of transition metal ions.

Mitigation approaches and techniques for combustion power plants flue gas emissions: A comprehensive review

Population growth and urbanization are driving energy demand. Despite the development of renewable energy technologies, most of this demand is still met by fossil fuels. Flue gases are the main air pollutants from combustion power plants. These pollutants include particulate matter (PM), sulfur oxides (SOx), nitrogen oxides (NOx), and carbon oxides (COx). The release of these pollutants has adverse effects on human health and the environment, including serious damage to the human respiratory system, acid rain, climate change, and global warming. In this review, a wide range of conventional and new technologies that have the potential to be used in the combustion power plant sector to manage and reduce flue gas pollutants have been examined. Nowadays, conventional approaches to emissions control and management, which focus primarily on post-combustion techniques, face several challenges despite their widespread use and commendable effectiveness. Therefore, studies that have proposed alternative approaches to achieve improved and more efficient methods are reviewed. The results show that new advances such as novel PM collectors, attaining an efficiency of nearly 100 % for submicron particles, microwave systems, boasting an efficiency of nearly 90 % for NO and over 95 % for SO2, electrochemical systems achieving above 90 % efficiency for NOx reduction, non-thermal plasma processes demonstrating an efficiency close to 90 % for NOx, microalgae-based methods with efficiency ranging from 80 % to 99 % for CO2, and wet scrubbing, exhibit considerable potential in addressing the shortcomings of conventional systems. Furthermore, the integration of hybrid methods, particularly in regions prioritizing environmental concerns over economic considerations, holds promise for enhanced control and removal of flue gas pollutants with superior efficiency.

Chromium removal from chromium gypsum through microwave hydrothermal crystal phase regulation

Chromium gypsum (CG) is a common hazardous waste formed in chromium salt or electroplating industries. The trapped or lattice-doped CrO42- in gypsum crystals are difficult to be reduced or removed by traditional methods, which will be re-oxidized or slowly released during long-term hypaethral storage. In this study, microwave hydrothermal treatment was applied to remove chromium in CG. Under optimal conditions (solid-liquid ratio of 1:5, 0.1 M sulfuric acid as liquid media, and 110 °C), over 99% of the chromium in CG can be removed within 10 min. XRD spectra indicated that 59.8% gypsum was transformed to from dihydrate gypsum to hemihydrate gypsum. The toxicity leaching test shows that chromium in CG is 377.0 mg/L before detoxification and 0.55 mg/L after detoxification, which proves that chromium in CG lattice can be efficiently removed. This work enables to significantly advance the dehydration phase transformation process of gypsum and release the heavy metal impurities within it more quickly and provides new possibilities to treat similar solid waste containing gypsum or minerals with hydration water.

Continuous NO Upcycling into Ammonia Promoted by SO2 in Flue Gas: Poison Can Be a Gift

Although ammonia (NH3) synthesis efficiency from the NO reduction reaction (NORR) is significantly promoted in recent years, one should note that NO is one of the major air pollutants in the flue gas. The limited NO conversion ratio is still the key challenge for the sustainable development of the NORR route, which potentially contributes more to contaminant emissions rather than its upcycling. Herein, we provide a simple but effective approach for continuous NO reduction into NH3, promoted by coexisting SO2 poison as a gift in the flue gas. It is significant to discover that SO2 plays a decisive role in elevating the capacity of NO absorption and reduction. A unique redox pair of SO2-NO is constructed, which contributes to the exceptionally high conversion ratio for both NO (97.59 ± 1.42%) and SO2 (99.24 ± 0.49%) in a continuous flow. The ultrahigh selectivity for both NO-to-NH3 upcycling (97.14 ± 0.55%) and SO2-to-SO42- purification (92.44 ± 0.71%) is achieved synchronously, demonstrating strong practicability for the value-added conversion of air contaminants. The molecular mechanism is revealed by comprehensive in situ technologies to identify the essential contribution of SO2 to NO upcycling. Besides, realistic practicality is realized by the efficient product recovery and resistance ability against various poisoning effects. The proposed strategy in this work not only achieves a milestone efficiency for NH3 synthesis from the NORR but also raises great concerns about contaminant resourcing in realistic conditions.

Application of Ca-based adsorbents in fixed-bed dry flue gas desulfurization (FGD): a critical review

Sulfur dioxide, which comes from the flue gas emitted by the steel and coal power industries, is extremely harmful to humans and the natural environment. Due to its high efficiency and economy, dry fixed-bed desulfurization technology and Ca-based adsorbents have attracted wide attention. In this paper, a detailed outline of the process of the fixed-bed reactor, performance indexes, economic value, recent research, and industrial applications of the dry fixed-bed desulfurization process was summarized. The classification and properties, preparation method, desulfurization mechanism, and influencing factors of Ca-based adsorbents were discussed. This review indicated the challenges in the commercialization of dry Ca-based fixed-bed desulfurization and demonstrated the possible solutions. It is beneficial to promote industrial application by improving the utilization efficiency of Ca-based adsorbent, reducing the amount of adsorbent and operation cost, and developing ideal regeneration methods.

A critical review on the technique and mechanism of microwave-based denitrification in flue gas

Microwave radiation has received extensive attention due to its significant thermal and non-thermal effects, and the development of MW-based denitrification in flue gas has become one of the most promising methods to avoid the defects of ammonia escape, high temperature and cost in traditional SCR. This review introduces the thermal and non-thermal effects of microwaves and divides MW-based denitrification methods into MW reduction and oxidation denitrification, systematically summarizes these denitrification methods, including MW discharge reduction, MW-induced catalytic reduction using active carbon, molecular sieves, metal oxides (transition metals, perovskites, etc.), MW-induced oxidation denitrification with and without additional oxidant, and discusses their removal pathway and mechanism. Finally, several research prospects and directions regarding the development of microwave-based denitrification methods are provided.

3D-printed heterogeneous Cu2O monoliths: Reusable supports for Antibiotic Treatmentantibiotic treatment of wastewater

In this study, surfactant stabilized dispersions of the Cu₂O microparticles in a commercially available photocurable resin were 3D printed into both porous and non-porous monoliths, and the heterogeneous Cu₂O catalytic monolith with improved mass transfer characteristics was applied for antibiotic wastewater treatment. The physicochemical properties of catalytic monoliths were characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and thermogravimetric. Ten intermediates were analyzed and identified by GC-MS, and the corresponding degradation pathways were proposed. Both numerical simulation and degradation experiments were used to explore the mass transfer mechanism and catalytic performance of the monoliths. The results showed that the 3D-printed monolith with a well-defined porous network exhibited a high ofloxacin degradation efficiency (100%) based on the sulfate radical-based advanced oxidation processes. In addition, the catalytic monolith showed sustained high activity over 7 reusable cycles demonstrating its feasibility in removal of antibiotics from wastewater.

Insight into the role of Fe in the synergetic effect of persulfate/sulfite and Fe2O3@g-C3N4 for carbamazepine degradation

In this work, the role of Fe in the synergetic effect of persulfate/sulfite and Fe₂O₃@g-C₃N₄ (FCN) for carbamazepine (CBZ) degradation was studied. Unexpectedly, Fe₂O₃ in FCN plays very different roles for sulfite [S(IV)] and persulfate (PS) activation. Specifically, since photo-generated holes (h⁺) can transform S(IV) into SO₄^-, and photo-generated electrons (e^-) can accelerate Fe(III) reduction which promotes transition metal based S(IV) activation, a synergetic effect of photocatalysis and Fe is observed in FCN/S(IV)/vis system. In contrast, in FCN/PS/vis system, both Fe(III)/Fe(II) cycle and PS activation compete for e^-. Since PS is a stronger electron acceptor, Fe(III) reduction by e^- is limited. Therefore, the contribution of Fe₂O₃ in FCN/S(IV)/vis system is 3 times higher than that in FCN/PS/vis system. Initial pH affects CBZ removal by changing surface charge of catalysts and oxidants species, while the effect varies for different catalysts and oxidants. This study provides new insight into the synergetic effect of photocatalysis and transition metal for SO₄^- generation, which contributes to catalyst design for environmental application.

Reaction Behavior and Influencing Mechanisms of Different Fly Ashes on the NO Removal by Using the Ultraviolet Irradiating Chlorite Method

Our previous work had demonstrated that UV/NaClO₂ was the best advanced oxidation method in terms of nitric oxide (NO) removal, but we have not studied the impact of the fly ash on NO removal under such conditions. For this, this paper selected six kinds of fly ashes and studied their effects on NO removal. The micromorphology, elemental composition, and the elemental oxidation states of these six fly ashes were characterized by scanning electron microscopy-energy-dispersive X-ray spectra, X-ray photoelectron spectroscopy, and inductively coupled plasma methods. The main inorganic components in the six fly ashes are metal oxides (Fe₂O₃/Fe₃O₄, SiO₂, Al₂O₃, ZnO, MgO, and TiO₂), carbonates (Na₂CO₃ and CaCO₃), and chlorides (NaCl, KCl, and MgCl₂). The experimental results suggested that high solubility was the premise condition for the fly ashes exhibiting an inhibitory effect on NO removal. Among all of the metal compounds, Fe₂O₃/Fe₃O₄ exhibited the highest inhibitory contribution rate to the NO removal (22.9-45.7%). The anions of Cl^- and CO₃ ^2- acted as scavengers for the free radicals which greatly impaired the oxidation of NO. Based on the simulation experimental results and the UV-vis analysis, the order of inhibitory contribution rates of various metal compounds to the NO removal was determined as Fe₂O₃/Fe₃O₄ > TiO₂ ≈ Na₂CO₃ > Al₂O₃ ≈ ZnO ≈ MnO₂ > CaCO₃ > NaCl > KCl ≈ SiO₂ ≈ MgCl₂.

Efficient catalyst prepared from water treatment residuals and industrial glucose using hydrothermal treatment: Preparation, characterization and its catalytic performance for activating peroxymonosulfate to degrade imidacloprid

Water treatment residuals (WTRs), as by-products of drinking water treatment plant, were used as catalyst for persulfate activation to degrade organic pollutants. In this study, G-HWTRs were successfully prepared by hydrothermal treatment, which combined WTRs and a hydrothermal reducing agent (industrial glucose) in different ratios. These materials manifested upgraded performance compared with raw WTRs and HWTRs (prepared only with WTRs under hydrothermal condition) in imidacloprid (IMD) degradation. The elemental composition, structure, morphological and magnetic properties of the G-HWTRs were investigated. And the influences of peroxymonosulfate (PMS) concentration, G-HWTRs dosage, initial pH, water matrix on IMD degradation were determined. The results demonstrated that G-HWTRs-3 had the best catalytic performance, 10 μM IMD was almost completely degraded in the system of G-HWTRs (0.2 g L^-1) and PMS (0.1 mM) within 2 h without pH adjustment. Based on the results of the electron spin-resonance spectroscopy (ESR) tests and radicals scavenging experiments, all of SO₄^-, OH, ¹O₂ and O₂- were the reactive oxygen species driving the IMD degradation, and OH was regarded as the main role of IMD degradation. The possible degradation pathways of IMD were further proposed based on the degradation intermediates that identified by LC-MS. Besides, further experiments indicated G-HWTRs has degradation potential for various pollutants, the degradation rate of atrazine (ATZ), acetochlor (ACE) and simazine (SMX) within 2 h achieved 92.54%, 83.88% and 90.25%, respectively. These results confirmed G-HWTRs has good catalytic performance and activation potential on PMS, providing an effective method for remediating organic polluted wastewater.

Abatement of NO/SO2/Hg0 from flue gas by advanced oxidation processes (AOPs): Tech-category, status quo and prospects

This review article provides a state-of-art insight into the removal of NO, SO₂ and elemental mercury (Hg⁰) from flue gas by using advanced oxidation processes (AOPs) method. Firstly, the main flue gas purification strategies based on AOPs would be classified as gas-gas, gas-liquid and gas-solid systems preliminarily, and the primary chemistry/mechanism of the above homogeneous/heterogeneous reaction systems were presented as the oxidation of NO, SO₂ and Hg⁰ by the oxidative free radicals (OH, O₂ and SO₄^-etc.). Secondly, the research progress and reaction pathways for separately or simultaneously removing NO, SO₂ and Hg⁰ from flue gas by AOPs has been reviewed elaborated and analyzed in more details. Notably, the wet/dry oxidation coupled with efficient absorption process would be a promising method of efficient removal of above gaseous pollutants. Subsequently, four types of assumed layout modes were described graphically. The application prospects of AOPs for the purification of flue gas from coal-fired boiler or industrial furnace were evaluated and found that the operation cost and utilization of oxidants must be reduced and improved respectively. Finally, the limitations in the current removal technologies based on AOPs are highlighted, meanwhile the future research directions are suggested, such as cut down the cost of oxidants and catalysts, improve the yield and valid utilization of highly reactive radicals and enhance the reactivity, resistance and stability of catalysts. Significantly, it is also envisaged that the review could enrich the knowledge repository to function as a scientific reference for the sustainable development of economical, effective and environment-friendly technologies for the abatement of a wide variety of emissions from flue gas, and further improve the feasibility and reliability of the strategies for moving from laboratory studies to large-scale development and industrial application.

Targeted degradation of refractory organic compounds in wastewaters based on molecular imprinting catalysts

Efficient removal of low-concentration refractory pollutants is a crucial problem to ensuring water safety. The use of heterogeneous catalysis of molecular imprinting technology combined with traditional catalysts is a promising method to improve removal efficiency. Presently, the research into molecular imprinting targeting catalysts focuses mainly on material preparation and performance optimization. However, more researchers are investigating other applications of imprinting materials. This review provides recent progress in photocatalyst preparation, electrocatalyst, and Fenton-like catalysts synthesized by molecular imprinting. The principle and control points of target catalysts prepared by precipitation polymerization (PP) and surface molecular imprinting (S-MIP) are introduced. Also, the application of imprinted catalysts in targeted degradation of drugs, pesticides, environmental hormones, and other refractory pollutants is summarized. In addition, the reusability and stability of imprinted catalyst in water treatment are discussed, and the possible ecotoxicity risk is analyzed. Finally, we appraised the prospects, challenges, and opportunities of imprinted catalysts in the advanced oxidation process. This paper provides a reference for the targeted degradation of refractory pollutants and the preparation of targeted catalysts.

Satellite Observation of Spatio-temporal Variations in Nitrogen Dioxide over West Africa and Implications for Regional Air Quality

BACKGROUND

Nitrogen dioxide (NO₂) is known to affect human health, causing heart and cardiovascular diseases, and it has been shown that locations with long term NO₂ pollution recorded a high number of fatalities due to the COVID-19 pandemic. There are no ground stations monitoring emissions of NO₂ over West Africa. The present study aimed to use satellite observations to examine pollutant trends over this region.

OBJECTIVE

To examine the trend of NO₂ over the entire West Africa sub region in relationship to contributions to environmental emissions using satellite-derived data. This enables the assessment of West Africa regional air pollution hot spots in relationship to enhancing atmospheric factors. The results from this study will also be useful guidance for setting air quality standards for air pollution controls to minimize health hazards.

METHODS

The present study examined thirteen years of average monthly values of nitrogen dioxide (NO₂) to determine the spatio-temporal variation of this pollutant over West Africa. Satellite data for NO₂ between 2005 and 2017 were used to determine the variation in pollution levels over West Africa. Correlations between NO₂ and meteorological variables (wind speed, rainfall and air temperature) were obtained to explain the influence of West African weather on the region's pollution accumulation.

RESULTS

The present study observed that NO₂ concentrations varied from place to place and from season to season. Nitrogen dioxide concentrations during the dry season were higher (sometimes 200% higher) than values observed in the wet season which ranged between 0.5 and 6×10¹⁵ molec/cm². Nitrogen dioxide north-south oscillation during the course of a year is largely controlled by the inter-tropical discontinuity (ITD) zone as high concentrations of NO₂ are found in the vicinity of the ITD where wind speeds and horizontal vorticity approaches zero. Correlation analysis between NO₂ and some atmospheric variables indicated NO₂ concentrations are well influenced by atmospheric variables showing bipolar signals depending on the season. An increasing trend of NO₂ was also found over selected cities of the region. This indicated that regional air quality is gradually deteriorating.

CONCLUSIONS

The implications of worsening regional air quality were examined in the light of the prevailing COVID-19 pandemic. The dominant atmospheric factor determining pollution episodes in the region is the inter-tropical discontinuity line which marks the meeting point between the two wind regimes over the region. Densely populated areas are characteristically prone to elevated pollution and have experienced high fatalities during the COVID-19 pandemic.

COMPETING INTERESTS

The authors declare no competing financial interests.

A Critical Review on Removal of Gaseous Pollutants Using Sulfate Radical-based Advanced Oxidation Technologies

Excessive emissions of gaseous pollutants such as SO₂, NO_x, heavy metals (Hg, As, etc.), H₂S, VOCs, etc. have triggered a series of environmental pollution incidents. Sulfate radical (SO₄•^-)-based advanced oxidation technologies (AOTs) are one of the most promising gaseous pollutants removal technologies because they can not only produce active free radicals with strong oxidation ability to simultaneously degrade most of gaseous pollutants, but also their reaction processes are environmentally friendly. However, so far, the special review focusing on gaseous pollutants removal using SO₄•^--based AOTs is not reported. This review reports the latest advances in removal of gaseous pollutants (e.g., SO₂, NO_x, Hg, As, H₂S, and VOCs) using SO₄•^--based AOTs. The performance, mechanism, active species identification and advantages/disadvantages of these removal technologies using SO₄•^--based AOTs are reviewed. The existing challenges and further research suggestions are also commented. Results show that SO₄•^--based AOTs possess good development potential in gaseous pollutant control field due to simple reagent transportation and storage, low product post-treatment requirements and strong degradation ability of refractory pollutants. Each SO₄•^--based AOT possesses its own advantages and disadvantages in terms of removal performance, cost, reliability, and product post-treatment. Low free radical yield, poor removal capacity, unclear removal mechanism/contribution of active species, unreliable technology and high cost are still the main problems in this field. The combined use of multiactivation technologies is one of the promising strategies to overcome these defects since it may make up for the shortcomings of independent technology. In order to improve free radical yield and pollutant removal capacity, enhancement of mass transfer and optimization design of reactor are critical issues. Comprehensive consideration of catalytic materials, removal chemistry, mass transfer and reactor is the promising route to solve these problems. In order to clarify removal mechanism, it is essential to select suitable free radical sacrificial agents, probes and spin trapping agents, which possess high selectivity for target specie, high solubility in water, and little effect on activity of catalyst itself and mass transfer/diffusion parameters. In order to further reduce investment and operating costs, it is necessary to carry out the related studies on simultaneous removal of more gaseous pollutants.

Recent advances in hybrid wet scrubbing techniques for NOx and SO2 removal: State of the art and future research

Recently, the discharge of flue gas has become a global issue due to the rapid development in industrial and anthropogenic activities. Various dry and wet treatment approaches including conventional and hybrid hybrid wet scrubbing have been employing to combat against these toxic exhaust emissions. However, certain issues i.e., large energy consumption, generation of secondary pollutants, low regeneration of scrubbing liquid and high efficieny are hindering their practical applications on industrial level. Despite this, the hybrid wet scrubbing technique (advanced oxidation, ionic-liquids and solid engineered interface hybrid materials based techniques) is gaining great attention because of its low installation costs, simultaneous removal of multi-air pollutants and low energy requirements. However, the lack of understanding about the basic principles and fundamental requirements are great hurdles for its commercial scale application, which is aim of this review article. This review article highlights the recent developments, minimization of GHG, sustainable improvements for the regeneration of used catalyst via green and electron rich donors. It explains, various hybrid wet scrubbing techniques can perform well under mild condition with possible improvements such as development of stable, heterogeneous catalysts, fast and in-situ regeneration for large scale applications. Finally, it discussed recovery of resources i.e., N₂O, NH₃ and N₂, the key challenges about several competitive side products and loss of catalytic activity over time to treat toxic gases via feasible solutions by hybrid wet scrubbing techniques.

Application Prospect of K Used for Catalytic Removal of NOx, COx, and VOCs from Industrial Flue Gas: A Review

NOx, COx, and volatile organic compounds (VOCs) widely exist in motor vehicle exhaust, coke oven flue gas, sintering flue gas, and pelletizing flue gas. Potassium species have an excellent promotion effect on various catalytic reactions for the treatment of these pollutants. This work reviews the promotion effects of potassium species on the reaction processes, including adsorption, desorption, the pathway and selectivity of reaction, recovery of active center, and effects on the properties of catalysts, including basicity, electron donor characteristics, redox property, active center, stability, and strong metal-to support interaction. The suggestions about how to improve the promotion effects of potassium species in various catalytic reactions are put forward, which involve controlling carriers, content, preparation methods and reaction conditions. The promotion effects of different alkali metals are also compared. The article number about commonly used active metals and promotion ways are also analyzed by bibliometric on NOx, COx, and VOCs. The promotion mechanism of potassium species on various reactions is similar; therefore, the application prospect of potassium species for the coupling control of multi-pollutants in industrial flue gas at low-temperature is described.

Enhancement of Removal of VOCs and Odors from Wood by Microwave-Activated Persulfate

Since traditional methods for removing volatile organic compounds (VOCs) from wood consume large amounts of energy and generate environmental pollution, it is desired to develop a convenient and green treatment method. Oxidation by microwave-activated persulfate (MW-PS) is a promising alternative method that has been used to eliminate VOCs from wood. The penetration of microwave energy can destroy the wood pit membranes and increase VOC emissions. The VOCs are further degraded by ·OH and SO₄ ^•-, which are generated via the activation of microwaves. This phenomenon can be detected by the electron paramagnetic resonance spectrometry. The 35 types of main VOCs of natural wood were determined, including alkanes/terpenes, alcohols/ethers, esters, aldehydes/ketones, and others. In the MW-PS system, 23 compounds were removed with an efficiency of 100%. Specifically, as one of the major compounds, the content of alkanes/terpenes was sharply decreased, and no alcohols/ethers and esters were detected. It was found that the optimal conditions of the MW-PS system for the minimum release of VOCs from wood were the microwave power of 462 W, irradiation time of 30 min, and PS dosage of 0.5 mmol/L.

Scale-up experiments of SO2 removal and the promoting behavior of NO in moving beds at medium temperatures

The dry flue gas desulfurization (FGD) method was studied, which is a part of the integrated removal of multi-pollutants at medium temperatures. Although dry flue gas treatment is a simple and effective method, it is still a highly empirical-led application technology. A superior desulfurization adsorbent, fine powder of NaHCO₃ (hereinafter called fine NaHCO₃), was selected by scale-up experiments. A deep understanding of the reaction process and mechanism is then explored, which helps the further optimization of dry desulfurization. Based on the multi-factor experiments for NaHCO₃, the effect mechanism of NO on desulfurization using NaHCO₃ is also proposed. The conversion of SO₃²⁻ → SO₄²⁻ is promoted by the existence of NO. Therefore, a slight decline can be found. According to the influences of the SO₂ concentration and the residence time, it is concluded that the diffusion of SO₂ into the channel of NaHCO₃ is the rate-limiting step. Impressively, the reaction process of reactants was clearly studied by in situ FTIR spectroscopy to determine the whole process. Moreover, the recycling of NaHCO₃ is the main direction for reducing adsorbent consumption in the next step. The predictable insights are beneficial for profoundly understanding the gas composition synergetic interaction for the SO₂ removal by the dry treatment using NaHCO₃.

A superior desulfurizer, fine NaHCO₃ was selected by scale-up experiments. A deep understanding of the reaction process and mechanism was explored. The effect mechanism of NO on desulfurization using NaHCO₃ was proposed by in situ FTIR results.

Utilization of electrochemical treatment and surface reconstruction to achieve long lasting catalyst for NOx removal

The development of catalysts has seen tremendous growth recently but most strategies only report utilization of catalysts for a few initial cycles without taking into account the influence of oxygen poisoning. Here, the magnetic Fe₃O₄@EDTA-Fe (MEFe, having a core Fe₃O₄ particle with EDTA-Fe coating) was investigated as a model catalyst for long-term recycling for the removal of nitrogen oxide (NO_x) from NO/O₂ mixture, followed by N₂O recovery. The concentration of oxygen in the flue gas was found to have a strong impact on NO_x absorption and catalytic response. To circumvent the oxygen poisoning, the MEFe was subjected to electrochemical treatment in the presence of neutral red (N.R.) and NO removal efficiency was ∼95 % noted. Furthermore, the surface of the catalyst degraded significantly (p < 0.05) after 6-7 repetitive cycling due to surface catalytic reactions, surface poisoning, oxidation of metallic species as well as residual stresses. The MEFe surface was reconstructed after 7 cycles using EDTA solution and Fe source to achieve similar surface coating as the fresh MEFe catalyst. The reconstructed MEFe exhibited similar NO_x absorption capability as the fresh MEFe and the reconstruction loop was repeated several times to achieve long term cycling, which make the catalyst cost-effective. Hence, it is proposed that a successful regeneration process can be employed for promising, sustainable and long-lasting catalytic treatment of air pollutants.

NOx attenuation in flue gas by •OH/SO4•--based advanced oxidation processes

The combustion of fossil fuels has resulted in rapidly increasing emissions of nitrogen oxide (NO_x), which has caused serious human health and environmental problems. NO capture has become a research focus in gas purification because NO accounts for more than 90% of NO_x and is difficult to remove. Advanced oxidation processes (AOPs), features the little secondary pollution and the broad-spectrum strong oxidation of hydroxyl radicals (^•OH), are effective and promising strategies for NO removal from coal-fired flue gas. This review provides the state of the art of NO removal by AOPs, highlighting several methods for producing ^•OH and SO₄^•-. According to the main radicals responsible for NO removal, these processes are classified into two categories: hydroxyl radical-based AOPs (HR-AOPs) and sulfate radical-based AOPs (SR-AOPs). This paper also reviews the mechanisms of NO capture by reactive oxygen species (ROS) and SO₄^•- in various AOPs. A HiGee (high-gravity) enhanced AOP process for improving NO removal, characterized by intensified gas-liquid mass transfer and efficient micro-mixing, is then proposed and discussed in brief. We believe that this review will be useful for workers in this field. Graphical abstract.

Sulfur dioxide removal: An overview of regenerative flue gas desulfurization and factors affecting desulfurization capacity and sorbent regeneration

Numerous mitigation techniques have been incorporated to capture or remove SO₂ with flue gas desulfurization (FGD) being the most common method. Regenerative FGD method is advantageous over other methods due to high desulfurization efficiency, sorbent regenerability, and reduction in waste handling. The capital costs of regenerative methods are higher than those of commonly used once-through methods simply due to the inclusion of sorbent regeneration while operational and management costs depend on the operating hours and fuel composition. Regenerable sorbents like ionic liquids, deep eutectic solvents, ammonium halide solutions, alkyl-aniline solutions, amino acid solutions, activated carbons, mesoporous silica, zeolite, and metal-organic frameworks have been reported to successfully achieve high SO₂ removal. The presence of other gases in flue gas, e.g., O₂, CO₂, NOx, and water vapor, and the reaction temperature critically affect the sorption capacity and sorbent regenerability. To obtain optimal SO₂ removal performance, other parameters such as pH, inlet SO₂ concentration, and additives need to be adequately governed. Due to its high removal capacity, easy preparation, non-toxicity, and low regeneration temperature, the use of deep eutectic solvents is highly feasible for upscale utilization. Metal-organic frameworks demonstrated highest reported SO₂ removal capacity; however, it is not yet applicable at industrial level due to its high price, weak stability, and robust formulation.

MnOx-CuOx cordierite catalyst for selective catalytic oxidation of the NO at low temperature

Low-value solid waste cordierite honeycomb ceramics were used as carrier of SCO denitration catalyst, and the active component was supported by the impregnation method to improve the performance of the catalyst. Firstly, the effect of calcination conditions on the denitration performance of the Mn-loaded cordierite catalyst was studied for the cordierite-loaded active component MnO_X. Secondly, the preferred catalyst was reloaded with another active component to further improve its denitration performance; the bimetal ratios were affected by the denitration performance, which was, finally, characterized by XRD, XPS, and SEM. The result shows the following: (1) Mn-loaded cordierite prepared at 450 °C for 3 h has a good denitration effect; (2) the MnO_X-CuO_X/CR catalyst is superior to MnO_X-FeO_X/CR, MnO_X-CoO_X/CR, and MnO_X-CeO_X/CR; (3) the MnO₂ crystal form in the single metal-supported catalyst plays a major role, and Cu₂Mn₃O₈ in the bimetallic catalyst affects the performance and activity of the catalyst. Graphical abstract.