Absorption and Oxidation of Nitric Oxide (NO) by Aqueous Solutions of Sodium Persulfate in a Bubble Column Reactor

Removal of nitric oxide in bioreactors: a review on the pathways, governing factors and mathematical modelling

The constant surge in nitric oxide in the atmosphere results in severe environmental degradation, negatively impacting human health and ecosystems, and is presently a global concern. Widely used physicochemical technologies for nitric oxide (NO) removal comes with high installation and operational costs and the production of secondary pollutants. Thus, biological treatment has been emphasized over the last two decades, but the poor solubility of NO in water makes it a challenging issue. The present article reviews the various technical aspects of biological treatment of nitric oxide, including the removal pathways and reactor configurations involved in the process. The most widely used technologies in this regard are chemical adsorption processes followed by biological reactors like biofilters, biotrickling filters and membrane bioreactors that enhance NO solubility and offer the flexibility and scope of further improvement in process design. The effect of various experimental and operational parameters on NO removal, including pH, carbon source, gas flow rate, gas residence time and presence of inhibitory components in the flue gas, is also discussed along with the developed mathematical models for predicting NO removal in a biological treatment system. There is an extensive scope of investigation regarding the development of an economical system to remove NO, and an exhaustive model that would optimize the process considering maximum practical parameters encountered during such operation. A detailed discussion made in this article gives a proper insight into all these areas.

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.

Sodium ascorbate as additive in red mud slurry for simultaneous desulfurization and denitrification: Insights into the multiple influence factors and reaction mechanism

Based on the ultra-low emission demand of SO₂ and NO_x in flue gas, a new absorption method was proposed to improve the desulfurization and denitrification efficiency and reduce the amount of ozone by using sodium ascorbate as an additive in red mud slurry. Compared with pure red mud slurry, the red mud (RM) + sodium ascorbate (SA) slurry significantly improved the denitrification efficiency from 24% to 84% and the desulfurization efficiency to 98%. Meanwhile, the effects of RM, SA concentration, reaction time and O₃/NO molar ratio on desulfurization and denitrification efficiencies were studied. The results showed that the RM + SA composite slurry maintained high efficiencies of desulfurization and denitrification for 240 min under the optimized conditions. As an antioxidant, the introduction of SA inhibited the excessive oxidation of sulfite, and itself could easily react with NO₂ through the redox reaction, greatly promoting the absorption of NO₂. In addition, the reaction mechanism of the simultaneous removal of SO₂ and NO₂ by red mud and sodium ascorbic mixed slurry combined was proposed.

A brand new two-phase wet oxidation absorption system for the simultaneous removal of SO2 and NOX from simulated marine exhaust gas

Marine engine exhaust emissions are increasingly harmful to the natural environment and human health and must be controlled. A self-synthesized amide (BAD, C₁₂H₂₅NO) in the laboratory shows a strong absorption capacity of nitric acid and nitrous acid, which may solve the problem that only using chlorine-based oxidant as an absorbent cannot completely absorb or retain NO₂ produced by NO oxidation in previous studies. Based on Multiwfn and VMD (Visual Molecular Dynamics) program calculation, the formation mechanism of hydrogen bonds between BAD with nitric acid and nitrous acid was revealed by electrostatic potential (ESP) analysis and further confirmed by FT-IR (Fourier transform infrared spectroscopy) spectra research. Subsequently, simultaneous removal of SO₂ and NO_X from simulated flue gas was carried out by using NaClO/BAD as a two-phase composite absorbent, and the maximum removal efficiencies of SO₂ and NO_X were 98.9% and 86.6%, respectively. The recycling experiments and the engineering experiments showing that NaClO/BAD can solve the problem of absorption of NO₂, and it can be a promising composite absorbent in wet desulfurization and denitrification of marine engine exhaust gas in practical applications.

Degradation of phenol using Fe(II)-activated CaO2: effect of ball-milled activated carbon (ACBM) addition

In-situ chemical oxidation (ISCO) based on peroxide activation is one of the most promising technologies for removing organic contaminants from natural groundwater (NGW). However, use of the most common form of hydrogen peroxide (H₂O₂) is limited owing to its significantly rapid reaction rate and heat generation. Therefore, in the present study, the activation of calcium peroxide (CaO₂), a slow H₂O₂ releasing agent, by Fe(II) was proposed (CaO₂/Fe(II)), and the phenol degradation mechanisms and feasibility of NGW remediation were investigated. The optimum molar ratio of [phenol]/[CaO₂]/[Fe(II)] (phenol = 0.5 mM) was 1/10/10, resulting in 87.0-92.5% phenol removal within 120 min under a broad initial pH range of 3-9. HCO₃^-, PO₄^3-, and humic acid significantly inhibited degradation, whereas the effects of Cl^-, NO₃^-, and SO₄^2- were negligible. Reactive oxygen species (ROS) were identified based on the results of phenol degradation in the presence of scavengers and electron spin resonance (ESR) spectroscopy, which demonstrated that ¹O₂ played the dominant role, supported by ^•OH, in CaO₂/Fe(II). Phenol removal in NGW (67.81%) was less than that in distilled and deionized water (DIW, 92.5%) at a [phenol]/[CaO₂]/[Fe(II)] ratio of 1/10/10. However, phenol removal was significantly improved (∼100%) by increasing the CaO₂ and Fe(II) doses to 1/20/20-40. Furthermore, when 125-250 mg L^-1 of ball-milled activated carbon (AC_BM) was added (CaO₂/Fe(II)-AC_BM), phenol removal was enhanced from 67.81% to 90.94-100% in the NGW. CaO₂/Fe(II)-AC_BM exhibited higher total organic carbon (TOC) removal than CaO₂/Fe(II). In addition, no notable by-products were detected using CaO₂/Fe(II)-AC_BM, whereas the polymerisation products of hydroxylated and/or ring-cleaved compounds, that is, aconitic acid, gallocatechin, and 10-hydroxyaloin, were found in the reaction with CaO₂/Fe(II). These results strongly suggest that CaO₂/Fe(II)-AC_BM is highly promising for groundwater remediation, minimizing degradation byproducts and the adverse effects caused by the NGW components.

Kinetic study on degradation of micro-organics by different UV-based advanced oxidation processes in EfOM matrix

Effluent organic matter (EfOM) contains a large number of substances that are harmful to both the environment and human health. To avoid the negative effects of organic matter in EfOM, advanced treatment of organic matter is an urgent task. Four typical oxidants (H₂O₂, PS, PMS, NaClO) and UV-combined treatments were used to treat micro-contaminants in the presence or absence of EfOM, because the active radical species produced in these UV-AOPs are highly reactive with organic contaminants. However, the removal efficiency of trace contaminants was greatly affected by the presence of EfOM. The degradation kinetics of two representative micro-contaminants (benzoic acid (BA) and para chlorobenzoic acid (pCBA)) was significantly reduced in the presence of EfOM, compared to the degradation kinetics in its absence. Using the method of competitive kinetics, with BA, pCBA, and 1,4-dimethoxybenzene (DMOB) as probes, the radicals (HO^·, SO₄^-·, ClO^·) proved to be the key to reaction species in advanced oxidation processes. UV irradiation on EfOM was not primarily responsible for the degradation of micro-contaminants. The second-order rate constants of the EfOM with radicals were determined to be (5.027 ± 0.643) × 10² (SO₄^-·), (3.192 ± 0.153) × 10⁴ (HO^·), and 1.35 × 10⁶ (ClO^·) (mg C/L)^-1 s^-1. In addition, this study evaluated the production of three radicals based on the concept of R_ct, which can better analyze its reaction mechanism.

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.

A novel combined system using Na2S2O8/urea to simultaneously remove SO2 and NO in marine diesel engine exhaust

The novel combined system using Na₂S₂O₈/urea was used to simultaneously absorb nitric oxide and sulfur dioxide emissions from marine diesel engines as well as inhibit the formation of nitrate in cleaning wastewater to meet the increasingly stringent requirements of regulations. The influences of reaction temperature, Na₂S₂O₈ concentration, urea concentration, SO₂ concentration, NO concentration and pH value on SO₂ removal efficiency, NO removal efficiency and nitrate concentration were investigated. The experimental results showed that different reaction temperatures had different influences on SO₂ removal efficiency, NO removal efficiency and nitrate concentration. An increase in Na₂S₂O₈ could improve the absorption of NO. The addition of urea could effectively improve the removal efficiency of NO and reduce the nitrate concentration. The removal efficiencies of 1000 ppm NO and 1000 ppm SO₂ achieved 100 % with 0.2 mol/L Na₂S₂O₈ and 2 mol/L urea at 70℃, and the nitrate content was 8.56 mg/L which was far lower than the regulatory requirement of 60 mg/L. The acidic condition (pH ≤ 5.5) not only facilitated the absorption of NO but also reduced the generation of nitrate. According to the experimental results, the novel combined system was promising to be applied to the control technology of marine diesel engine exhaust.

Simultaneous removal of NO and SO2 with a micro-bubble gas-liquid dispersion system based on air/H2O2/Na2S2O8

A novel environment-friendly process was proposed to conduct the simultaneous removal of NO and SO₂. In this process, a micro-bubble generator was utilized to generate the micro-bubble gas-liquid dispersion system (MBGLS) by inhaling mixed gas (NO, SO₂ and/or air) and absorption liquid. The MBGLS was then sprayed into an oxidation absorption column reactor, in which NO and SO₂ were oxidized and absorbed. As the additives, air, H₂O₂ and/or Na₂S₂O₈ were brought into the MBGLS to investigate their effects on the simultaneous removal of NO and SO₂. In addition, the effects of initial pH and temperature of the absorption liquid on the simultaneous removal of NO and SO₂ were also explored. The performance of the MBGLS in removing NO and SO₂ was excellent. Even if the MBGLS was composed of tap water, NO and SO₂, the removal efficiencies of NO and SO₂ respectively reached 78% and 94.4%. The additives significantly improved the removal performance of the MBGLS. Under the conditions of pH = 8 and room temperature and the addition of air, SO₂ was removed completely and the NO removal efficiency reached 99.5% when Na₂S₂O₈ to H₂O₂ molar ratio was 0.005/0.05. The effect of the absorbent temperature on the removal of NO and SO₂ was insignificant. With the increase in pH, the removal of NO in both H₂O₂ aqueous solution and Na₂S₂O₈ aqueous solution firstly increased and then decreased, but pH had no effect on the removal of SO₂.

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.

Bioconversion of Hg0 into HA-Hg for simultaneous removal of Hg0 and NO in a denitrifying membrane biofilm reactor

Bacterial mercury oxidation coupled to denitrification offers great potential for simultaneous removal of elemental mercury (Hg⁰) and nitric oxide (NO) in a denitrifying membrane biofilm reactor (MBfR). Four potentially contributory mechanisms tested separately, namely, membrane gas separation, medium absorption, biosorption and biotransformation, which contributed 4.9%/7.2%, 8.1%/8.9%, 38.8%/9.5% and 48.2%/84.9% of overall Hg⁰/NO removal in MBfR. Herein, Hg⁰ bio-oxidation, oxidative Hg⁰ biosorption and denitrification played leading roles in simultaneous removal of Hg⁰ and NO. Living microbes performed simultaneous Hg⁰ bio-oxidation and denitrification, in which Hg⁰ as electron donor was biologically oxidized to oxidized mercury (Hg²⁺), while NO as the terminal electron acceptor was denitrified to N₂. The Hg²⁺ further complexed with humic acids in extracellular polymeric substances via functional groups (-SH, -OH, -NH^- and -COO^-) and formed humic acids bound mercury (HA-Hg). Non-living microbial matrix performed oxidative Hg⁰ biosorption, in which Hg⁰ may be physically adsorbed by cellular matrix, then non-metabolically oxidized to Hg²⁺ via oxidative complexation with -SH in humic acids and finally cleavage of S-H bond and surface charge transfer led to formation of HA-Hg. Therefore, bioconversion of Hg⁰ to HA-Hg by Hg⁰ bio-oxidation and oxidative Hg⁰ biosorption coupled with NO denitrification to N₂ dynamically cooperated to accomplish simultaneous removal of Hg⁰ and NO in MBfR.

Simultaneous Removal of SO2 and NO by O3 Oxidation Combined with Wet Absorption

The effects of ozone concentration, NaOH concentration, type and concentration of additives, initial pH, temperature, and NO and SO₂ concentration on simultaneous removal of NO and SO₂ were studied through ozone oxidation combined with wet absorption. Results indicated that ozone concentration and the type and concentration of additives had the most significant effect on NO removal. The optimal ozone concentration was 250 ppm (NO/NO₂ = 1), and the best additive was KMnO₄. The removal efficiency of NO _x was as high as 97.86% when NO/NO₂ = 1, and the concentration of KMnO₄ was 0.025 mol/L. Considering economic and other factors, the KMnO₄ concentration was selected to be 0.006 mol/L. At this time, the removal efficiencies of NO _x and SO₂ were 81.35 and 100%, respectively. This method has potential application prospects for simultaneous removal of SO₂ and NO in the industrial flue gas.

Radical-induced oxidation removal of multi-air-pollutant: A critical review

Sulfur dioxide (SO₂), nitric oxide (NO) and elemental mercury (Hg⁰) are three common air pollutants in flue gas. SO₂ and NO are the main precursors for chemical smog and Hg⁰ is a bio-toxicant for human. Cooperative removal of multi-air-pollutant in flue gas using radical-induced oxidation reaction is considered as one of the most promising methods due to the high removal efficiency, low cost and less secondary environmental impact. The common radicals used in air pollution control can be classified into four types: (1) hydroxyl radical (OH), (2) sulfate radical (SO₄^-), (3) chlorine-containing radicals (Cl, ClO₂, ClO, HOCl^-, etc.) and (4) ozone. This review summarizes the generation methods and mechanism of the four kinds of radicals, as well as their applications in the removal of multi-air-pollutant in flue gas. The reactivity, selectivity and reaction mechanism of the four kinds of radicals in multi-air-pollutant removal were comprehensively described. Finally, some future research suggestions on the development of new technique for cooperative removal of multi-air-pollutant in flue gas were provided.

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

As it has a simple system and a small floor area, flue gas simultaneous desulfurization and denitrification technology has a good development prospect, 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 and SO₂ from flue gas using a semi-dry microwave activation persulfate system was developed for the first time. A series of experiments and characterization analyses had been implemented to research the feasibility of this new flue gas purification technology. The oxidation products, free radicals, and mechanism of NO and SO₂ simultaneous removal were revealed. The effect of the main technological parameters on NO and SO₂ simultaneous removal was also studied. Relevant results demonstrated that an increase in the microwave radiation power, persulfate concentration, and O₂ concentration enhanced NO and SO₂ simultaneous removal. The increase of NO and SO₂ concentrations weakened NO and SO₂ simultaneous removal. The reagent dosage, pH value of the solution, and reaction temperature showed a dual influence on NO and SO₂ simultaneous removal. Free-radical capture experiments revealed that both SO₄^-• and ^•OH that were produced by microwave activation of persulfate were the major active species and played very key roles in NO and SO₂ simultaneous removal. The main products (sulfate and nitrate) and byproducts (NO₂) in the tail gas were found. The process application and product post-treatment routes were also proposed. The result may provide the necessary inspiration and guidance for the development and application of microwave-activated advanced oxidation technology in the flue gas treatment area.

Simultaneous NO and SO2 removal by aqueous persulfate activated by combined heat and Fe2+: experimental and kinetic mass transfer model studies

This study evaluates the chemistry, kinetics, and mass transfer aspects of the removal of NO and SO₂ simultaneously from flue gas induced by the combined heat and Fe²⁺ activation of aqueous persulfate. The work involves experimental studies and the development of a mathematical model utilizing a comprehensive reaction scheme for detailed process evaluation, and to validate the results of an experimental study at 30-70 °C, which demonstrated that both SO₂ and Fe²⁺ improved NO removal, while the SO₂ is almost completely removed. The model was used to correlate experimental data, predict reaction species and nitrogen-sulfur (N-S) product concentrations, to obtain new kinetic data, and to estimate mass transfer coefficient (K_La) for NO and SO₂ at different temperatures. The model percent conversion results appear to fit the data remarkably well for both NO and SO₂ in the temperature range of 30-70 °C. The conversions ranged from 43.2 to 76.5% and 98.9 to 98.1% for NO and SO₂, respectively, in the 30-70 °C range. The model predictions at the higher temperature of 90 °C were 90.0 and 97.4% for NO and SO₂, respectively. The model also predicted decrease in K_La for SO₂ of 1.097 × 10^-4 to 8.88 × 10^-5 s^-1 (30-90 °C) and decrease in K_La for NO of 4.79 × 10^-2 to 3.67 × 10^-2 s^-1 (30-50 °C) but increase of 4.36 × 10^-2 to 4.90 × 10^-2 s^-1 at higher temperatures (70-90 °C). This emerging sulfate-radical-based process could be applied to the treatment of flue gases from combustion sources. Graphical abstract.

Direct catalytic oxidation and removal of NO in flue gas by the micro bubbles gas–liquid dispersion system

Abstract

The method of micro bubbles is widely applied in the fields of water and soil treatment. A novel treatment method of NO in flue gas through a gas–liquid two-phase system formed by micro bubbles is proposed in this study. The system depends on the generation of hydroxyl radicals. The NO removal performance of the micro gas–liquid dispersion system induced by catalysts and O3 was explored and the reaction pathways were elucidated. Micro bubbles, Fe2+, and Mn2+ in solution improved NO removal performance significantly. Salinity and surfactants affected the removal performance of NO by altering micro bubbles. In the presence of Fe2+, the NO removal rate reached 65.2% at pH 5, 75.8% under 0.5 g/L NaCl and 82.1% under 6 mg/L sodium dodecyl sulfate. In the presence of Mn2+, the NO removal rate reached 69.2% at pH 5, 83.2% under 0.5 g/L NaCl and 92.3% under 6 mg/L sodium dodecyl sulfate. However, in the presence of both Mn2+ and Fe2+, NO conversion rate was 93.2%. The NO removal rate in the presence of O3 was further improved under the same conditions. The study provides the basis for the application and development of micro bubbles in flue gas treatments for NO removal. The results can help to solve the problems of high operating cost, large oxidant consumption, secondary pollution, and high energy consumption in traditional NO removal methods.

Graphic abstract

Methanol-Promoted Oxidation of Nitrogen Oxide (NOx) by Encapsulated Ionic Liquids

The removal of nitrogen oxides (NO_x) has been extensively studied due to their harmful effects to health and environment. In this work, encapsulated ionic liquids (ENILs) are used as catalysts for the NO oxidation at humid conditions and low temperatures. Hollow carbon capsules (C_Cap) were first synthesized to contain different amounts of 1-butyl-3-methylimidazolium nitrate IL ([bmim][NO₃]), responsible for the catalytic oxidation. Then, the materials were characterized using different techniques, by analyzing microstructure, porosity, elemental composition, and thermal stability. The catalytic performance of ENIL materials was tested for NO conversion at different conditions. Thus, NO concentration was fixed at 2000 ppm at dry and humid conditions. Then, the methanol promotion of the reaction was demonstrated, increasing the NO conversion values in all cases, and the alcohol/water ratio was optimized. The temperature effect was studied as well, using the optimal conditions based on the previous measurements. The results reflect that humid conditions do not have a negative effect in terms of NO conversion when using ENILs, opposite behavior as observed for C_Cap and traditional catalysts studied before. The low amount of IL inside the material (40% in mass) was found to be the optimum for the task, reaching conversions of almost 45% in near industrial conditions of temperature and O₂ and H₂O concentrations in the flue gas with a GHSV of 10,000 h^-1.

Simultaneous Removal of SO2 and NO Using a Novel Method of Ultraviolet Irradiating Chlorite-Ammonia Complex

A novel advanced oxidation process (AOP) using ultraviolet/sodium chlorite (UV/NaClO₂) is developed for simultaneous removal of SO₂ and NO. NH₄OH, as an additive, was used to inhibit the generation of ClO₂ and NO₂. The removal efficiencies of SO₂ and NO reached 98.7 and 99.1%. NO removal was enhanced by greater UV light intensity and shorter wavelengths but was insensitive to changes in pH and temperature. SO₂ at 500-1000 mg/m³ improved NO removal, especially in the absence of UV. The coexistence of SO₂ and O₂ facilitated the removal of NO by ClO₂^-. HCO₃^-, Cl^-, and Br^- enhanced NO removal, but their roles were negligible when UV was added. The generation of ClO₂ and ClO^•/HO^• was verified by an UV-vis spectrometer, electron spin resonance (ESR), and radical-quenching tests. The mechanisms responsible for the removal of SO₂ and NO were attributed to the synergism between acid-base neutralization and radical-induced oxidation. The ClO₂^- evolution and product composition were demonstrated by UV-vis and X-ray photoelectron spectroscopy (XPS). Kinetics analyses showed that the Hatta numbers were 329-798 and 747-1000 without and with UV. Thus, the gas-film resistance mainly controlled the mass-transfer process.

Influence of chloride ions on organic contaminants decolorization through the Fe0-activated persulfate oxidation process: efficiency and intermediates

This study was conducted to evaluate the influence of chloride ions (Cl^-) on organic contaminants decolorization by the Fe⁰-activated persulfate process (PS/Fe⁰), as well as the generation of transformation products. Orange II (OII) was chosen as the target pollution. The results indicated that Cl^- influenced the OII decolorization by PS/Fe⁰ system, resulting in the generation of chlorine-containing by-products. OII containing Cl^- solution can be efficiently decolorized by PS/Fe⁰ process, and the decolorization efficiencies changed depending on Cl^- concentration due to the reaction between Cl^- and sulfate radicals (SO₄ ^-•). The operating cost for 94% color and 64% chemical oxygen demand (COD) removal of the OII dye was estimated at 0.73 USD/m³. The chlorine-containing by-products, such as chlorobenzene, 3,5-dichloro-benzene-1,2-diol, and 2,3-dichloro-2,3-dihydro-1,4-naphthoquinone, were generated during the reaction. The results further indicated that increasing both PS concentration and temperature enhanced OII decolorization and reduced the generation of chlorine-containing intermediates. The addition of ultrasound can further decrease the generation of chlorine-containing intermediates under high-temperature conditions. The proposed pathways of decolorization of OII containing Cl^- also indicated that SO₄ ^-• dominated the OII degradation, while the presence of Cl^- led to the generation of chlorine-containing intermediates.

Removal of multi-pollutant from flue gas utilizing ammonium persulfate solution catalyzed by Fe/ZSM-5

A nano-sized iron loaded ZSM-5 zeolite (Fe/ZSM-5) catalyst was firstly used to activate (NH₄)₂S₂O₈ solution for the simultaneous removal of multi-pollutant from flue gas. The simultaneous removal efficiencies 100% of SO₂, 72.6% of NO and 93.4% of Hg° were achieved under the condition that the catalyst dose was 0.8 g/L, concentration, pH and temperature of (NH₄)₂S₂O₈ solution were 0.03 mol/L, 5 and 65 °C, respectively. The stability of catalyst was checked by a continuous test, proving that the catalytic activity was maintained for 4 h and the leached iron reached low levels. Based on the catalyst characterizations, product analysis and literatures, the removal mechanism was speculated preliminarily, during which, OH and SO₄^- played key roles for oxidizing NO and Hg° into NO₃^- and Hg²⁺.

Simultaneous Removal of NOx and SO2 through a Simple Process Using a Composite Absorbent

In this work, the feasibility of the simultaneous removal of NOx and SO2 through a simple process using a composite absorbent (NaClO2/Na2S2O8) was evaluated. Factors affecting the removal of NOx and SO2, such as NaClO2 and Na2S2O8 concentrations, solution temperature, the initial pH of solution, gas flow rate, and SO2, NO, and O2 concentrations were studied, with a special attention to NOx removal. Results indicate that a synergistic effect on NOx removal has been obtained through combination of NaClO2 and Na2S2O8. NaClO2 in the solution played a more important role than did Na2S2O8 for the removal of NOx. The above factors had an important impact on the removal of NOx, especially the solution temperature, the initial pH of the solution, and the oxidant concentrations. The optimum experimental conditions were established, and a highest efficiency of NOx removal of more than 80% was obtained. Meanwhile, tandem double column absorption experiments were conducted, and a NOx removal efficiency of more than 90% was reached, using NaOH solution as an absorbant in the second reactor. A preliminary reaction mechanism for NOx and SO2 removal was deduced, based on experimental results. The composite absorbent has the potential to be used in the wet desulfurization and denitration process, to realize the synergistic removal of multi-pollutants.

Simultaneous removal of NO and SO2 from flue gas by combined heat and Fe2+ activated aqueous persulfate solutions

The use of advanced oxidation processes (AOPs) to integrate flue gas treatments for SO₂, NO_x and Hg⁰ into a single process unit is rapidly gaining research attention. AOPs are processes that rely on the generation of mainly the hydroxyl radical. This work evaluates the effectiveness of the simultaneous removal of NO and SO₂ from flue gas utilizing AOP induced by the combined heat and Fe²⁺ activation of aqueous persulfate, and elucidates the reaction pathways. The results indicated that both SO₂ in the flue gas and Fe²⁺ in solution improved NO removal, while the SO₂ is almost completely removed. Increased temperature led to increase in NO removal in the absence and presence of both Fe²⁺ and SO₂, and in the absence of either SO₂ or Fe²⁺, but the enhanced NO removal due to the presence of SO₂ alone dominated at all temperatures. The removal of NO increased from 77.5% at 30 °C to 80.5% and 82.3% at 50 °C and 70 °C in the presence of SO₂ alone, and from 35.3% to 62.7% and 81.2%, respectively, in the presence of Fe²⁺ alone. However, in the presence of both SO₂ and Fe²⁺, NO conversion is 46.2% at 30 °C, increased only slightly to 48.2% at 50 °C; but sharply increased to 78.7% at 70 °C compared to 63.9% for persulfate-only activation. Results suggest NO removal in the presence of SO₂ is equally effective by heat-only or heat-Fe²⁺ activation as the temperature increases. The results should be useful for future developments of advanced oxidation processes for flue gas treatments.

Enhanced effect of HAH on citric acid-chelated Fe(II)-catalyzed percarbonate for trichloroethene degradation

This work demonstrates the impact of hydroxylamine hydrochloride (HAH) addition on enhancing the degradation of trichloroethene (TCE) by the citric acid (CA)-chelated Fe(II)-catalyzed percarbonate (SPC) system. The results of a series of batch-reactor experiments show that TCE removal with HAH addition was increased from approximately 57 to 79% for a CA concentration of 0.1 mM and from 89 to 99.6% for a 0.5 mM concentration. Free-radical probe tests elucidated the existence of hydroxyl radical (HO^•) and superoxide anion radical (O₂^•-) in both CA/Fe(II)/SPC and HAH/CA/Fe(II)/SPC systems. However, higher removal rates of radical probe compounds were observed in the HAH/CA/Fe(II)/SPC system, indicating that HAH addition enhanced the generation of both free radicals. In addition, increased contribution of O₂^•- in the HAH/CA/Fe(II)/SPC system compared to the CA/Fe(II)/SPC system was verified by free-radical scavengers tests. Complete TCE dechlorination was confirmed based on the total mass balance of the released Cl^- species. Lower concentrations of formic acid were produced in the later stages of the reaction for the HAH/CA/Fe(II)/SPC system, suggesting that HAH addition favors complete TCE mineralization. Studies of the impact of selected groundwater matrix constituents indicate that TCE removal in the HAH/CA/Fe(II)/SPC system is slightly affected by initial solution pH, with higher removal rates under acidic and near neutral conditions. Although HCO₃^- was observed to have an adverse impact on TCE removal for the HAH/CA/Fe(II)/SPC system, the addition of HAH reduced its inhibitory effect compared to the CA/Fe(II)/SPC system. Finally, TCE removal in actual groundwater was much significant with the addition of HAH to the CA/Fe(II)/SPC system. The study results indicate that HAH amendment has potential to enhance effective remediation of TCE-contaminated groundwater.

Benzene oxidation by Fe(III)-catalyzed sodium percarbonate: matrix constituent effects and degradation pathways

Complete degradation of benzene by the Fe(III)-activated sodium percarbonate (SPC) system is demonstrated. Removal of benzene at 1.0 mM was seen within 160 min, depending on the molar ratios of SPC to Fe(III). A mechanism of benzene degradation was elaborated by free-radical probe-compound tests, free-radical scavengers tests, electron paramagnetic resonance (EPR) analysis, and determination of Fe(II) and H2O2 concentrations. The degradation products were also identified using gas chromatography-mass spectrometry method. The hydroxyl radical (HO.) was the leading species in charge of benzene degradation. The formation of HO. was strongly dependent on the generation of the organic compound radical (R.) and superoxide anion radical (O.). Benzene degradation products included hydroxylated derivatives of benzene (phenol, hydroquinone, benzoquinone, and catechol) and aliphatic acids (oxalic and fumaric acids). The proposed degradation pathways are consistent with radical formation and identified products. The investigation of selected matrix constituents showed that the Cl and HCO₃ had inhibitory effects on benzene degradation. Natural organic matter (NOM) had accelerating influence in degrading benzene. The developed system was tested with groundwater samples and it was found that the Fe(III)-activated SPC has a great potential in effective remediation of benzene-contaminated groundwater while more further studies should be done for its practical application in the future because of the complex subsurface environment.

An investigation of mass transfer-reaction kinetics of NO absorption by wet scrubbing using an electrolyzed seawater solution

The mass transfer-reaction kinetics of NO absorption by wet scrubbing using electrolyzed seawater was studied in a bench-scale bubbling reactor.

Enhanced degradation of benzene by percarbonate activated with Fe(II)-glutamate complex

Effective degradation of benzene was achieved in sodium percarbonate (SPC)/Fe(II)-Glu system. The presence of glutamate (Glu) could enhance the regeneration of Fe(III) to Fe(II), which ensures the benzene degradation efficiency at wider pH range and eliminate the influence of HCO3 (-) in low concentration. Meanwhile, the significant scavenging effects of high HCO3 (-) concentration could also be overcome by increasing the Glu/SPC/Fe(II)/benzene molar ratio. Free radical probe compound tests, free radical scavenger tests, and electron paramagnetic resonance (EPR) analysis were conducted to explore the reaction mechanism for benzene degradation, in which hydroxyl radical (HO•) and superoxide anion radical (O2 (•-)) were confirmed as the predominant species responsible for benzene degradation. In addition, the results obtained in actual groundwater test strongly indicated that SPC/Fe(II)-Glu system is applicable for the remediation of benzene-contaminated groundwater in practice.

UV-Enhanced NaClO Oxidation of Nitric Oxide from Simulated Flue Gas

A wet de-NOxtechnique based on an UV-enhanced NaClO oxidation process was investigated for simulated flue gas of a diesel engine using a bench-scale reaction chamber. The effects of UV irradiation time, initial pH value, and available chlorine concentration of NaClO solution were studied, respectively. The results showed that when the UV irradiation time was 17.5 min and the initial pH value of NaClO solution was 6, NO removal efficiency of UV/NaClO solution was increased by 19.6% compared with that of NaClO solution. Meanwhile, when the available chlorine concentration of NaClO solution decreased from 0.1 wt% to 0.05 wt%, the enhancement in NO removal efficiency of UV/NaClO solution increased from 19.6% to 24%, compared with that of NaClO solution. The reaction pathways of NaClO solution photolysis and NO removal by UV/NaClO process were preliminarily discussed. The results suggested that HOCl might be the most active species that released many UV-induced photooxidants through photolysis reactions, which played an important role in NO removal process.

Removal of NO from flue gas using heat-activated ammonium persulfate aqueous solution in a bubbling reactor

Ammonium persulfate aqueous solution was used for the first time for NO removal from flue gas in a bubbling bed due to its low cost compared with other persulfate salts (sodium persulfate, potassium persulfate and ammonium persulfate).