A novel method of ultraviolet/NaClO2-NH4OH for NO removal: Mechanism and kinetics

Activity and recyclability enhancement of pH-dependent Fe0@BC-mediated heterogeneous sodium percarbonate (SPC)-reducing agents (RA) system

Dyes pose great threats to the aquatic environment and human health. Fe0-based Fenton-like systems have been widely employed for the degradation of organic dyes. However, the regulation of degradability and recyclability was still unclear. In this study, Rhodamine B (RhB) was served as the model pollutant, hydroxylamine hydrochloride was selected as the RA, the natural photocatalysis system demonstrated stable operation. RA, as performance enhancement agent, was firstly reported in micro/nano-Zero-Valent Iron@Biochar (m/nZVI@BC) based SPC-RA system. Carrier size-fractionated m/nZVI@BC was fabricated by one-step carbothermal method. As a result, RA synergistically interacted with SPC, and the reaction time reduced from 15 min to 4 min. In the 0.010 g m/nZVI@BC-mediated SPC-RA system, over 95% of RhB (100 mg·L-1, 1041.667 mg·g-1) was successfully degraded. The maximum degradation ability could still exceed 1g·g-1 via 5 times repeated applications. Meanwhile, the loss of degradability, caused by halving SPC concentration could be compensated by RA dosage measurement. The entire degradation process was predominantly dominated by free radicals (•OH> 1O2> •O2-> •CO3-). Reactive oxidizing species (ROSs) were primarily excited by α-Fe0, Fe3C and N sites of biochar (BC). Light and BC carrier dedicated slight influence. These discoveries shed a light on the activity and recyclability regulation of catalytic material, aligning with the principles of green chemistry and cleaner production. This study demonstrates a novel approach to efficient management of solid waste disposal, reuse of waste biomass, advanced treatment of dye-containing wastewater, pollution control in aquatic environments.

Enhanced NOx absorption in flue gas by wet oxidation of red mud and phosphorus sludge

The environmental problem caused by industrial emissions of NOx has been studied in the past dacades. In this study, red mud coupling with phosphorus sludge were used to enhance the solution to absorb NOx from the flue gas. Firstly, red mud reacted with the binder silicic acid in the phosphorus sludge, destroying the emulsion structure of the phosphorus sludge. Then, the P4 in the phosphorus sludge is completely released, and the P4 reacted with O2 in the flue gas to produce O3 and O. NO and NO2 contained in the flue gas reacted with the active O and O3 to produce high-valent NOx, such as NO3, N2O5. At last, the mixed slurry of red mud and phosphorus sludge absorbed the high-valent NOx, resulting in the formation of Ca5(PO4)3F along with HNO3. Using phosphorus sludge to produce O3 in the reaction process can reduce the production cost of O3 and achieve waste utilization. Meanwhile, the interaction between red mud and phosphorus sludge can promote phosphorus sludge to produce O3 and remove F- from phosphorus sludge, as well as avoid the problem of secondary pollution. This study should be helpful for red mud and phosphorus sludge utilization and flue gas denitration.

Chlorine-mediated electrochemical advanced oxidation process for ammonia removal: Mechanisms, characteristics and expectation

Chlorine-Mediated Electrochemical Advanced Oxidation (Cl-EAO) technology is a promising approach for ammonia removal from wastewater due to its numerous advantages, including small infrastructure, short processing time, easy operation, high security, and high nitrogen selectivity. This paper provides a review of the ammonia oxidation mechanisms, characteristics, and anticipated applications of Cl-EAO technology. The mechanisms of ammonia oxidation encompass breakpoint chlorination and chlorine radical oxidation, although the contributions of active chlorine, Cl, and ClO remain uncertain. This study critically examines the limitations of existing research and suggests that a combination of determining free radical concentration and simulating a kinetic model would help elucidate the contributions of active chlorine, Cl, and ClO to ammonia oxidation. Furthermore, this review comprehensively summarizes the characteristics of ammonia oxidation, including kinetic properties, influencing factors, products, and electrodes. The amalgamation of Cl-EAO technology with photocatalytic and concentration technologies has the potential to enhance ammonia oxidation efficiency. Future research should concentrate on clarifying the contributions of active chlorine, Cl, and ClO to ammonia oxidation, the production of chloramines and other byproducts, and the development of more efficient anodes for the Cl-EAO process. The main objective of this review is to enhance the understanding of the Cl-EAO process. The findings presented herein contribute to the advancement of Cl-EAO technology and provide a foundation for future studies in this field.

Performance and Mechanism of Chlorine Dioxide on BTEX Removal in Liquid and Indoor Air

With the development of the chemical industry, benzene, toluene, ethylbenzene, and xylene (BTEX) have gradually become the major indoor air pollutants. Various gas treatment techniques are widely used to prevent the physical and mental health hazards of BTEX in semi-enclosed spaces. Chlorine dioxide (ClO₂) is an alternative to chlorine as a secondary disinfectant with a strong oxidation ability, a wide range of action, and no carcinogenic effects. In addition, ClO₂ has a unique permeability which allows it to eliminate volatile contaminants from the source. However, little attention has been paid to the removal of BTEX by ClO₂, due to the difficulty of removing BTEX in semi-enclosed areas and the lack of testing methods for the reaction intermediates. Therefore, this study explored the performance of ClO₂ advanced oxidation technology on both liquid and gaseous benzene, toluene, o-xylene, and m-xylene. The results showed that ClO₂ was efficient in the removal of BTEX. The byproducts were detected by gas chromatography-mass spectrometry (GC-MS) and the reaction mechanism was speculated using the ab initio molecular orbital calculations method. The results demonstrated that ClO₂ could remove the BTEX from the water and the air without causing secondary pollution.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

In situ doping lignin-derived carbon quantum dots on magnetic hydrotalcite for enhanced degradation of Congo Red over a wide pH range and simultaneous removal of heavy metal ions

A new N, S-CQDs@Fe₃O₄@HTC composite was prepared by loading N, S carbon quantum dots (N, S-CQDs) derived from lignin on magnetic hydrotalcite (HTC) via an in-situ growth method. The characterization results showed that the catalyst had a mesoporous structure. These pores facilitate the diffusion and mass transfer of pollutant molecules inside the catalyst, allowing them to approach the active site smoothly. The catalyst performed well in the UV degradation of Congo red (CR) over a wide pH range (3-11), with efficiencies over 95.43 % in all cases. Even at a high NaCl content (100 g/L), the catalyst showed extraordinary CR degradation (99.30 %). ESR analysis and free radical quenching experiments demonstrated that OH and O₂^- were the main active species governing CR degradation. Besides, the composite had outstanding removal efficiency for Cu²⁺ (99.90 %) and Cd²⁺ (85.08 %) simultaneously due to the electrostatic attraction between the HTC and metal ions. Moreover, the N, S-CQDs@Fe₃O₄@HTC had excellent stability and recyclability during five cycles, making it free of secondary contamination. This work provides a new environment-friendly catalyst for the simultaneous removal of multiple pollutants and a waste-to-waste strategy for the value-added utilization of lignin.

A Novel Method Based on Hydrodynamic Cavitation for Improving Nitric Oxide Removal Performance of NaClO2

In the removal of nitric oxide (NO) by sodium chlorite (NaClO2), the NaClO2 concentration is usually increased, and an alkaline absorbent is added to improve the NO removal efficiency. However, this increases the cost of denitrification. This study is the first to use hydrodynamic cavitation (HC) combined with NaClO2 for wet denitrification. Under optimal experimental conditions, when 3.0 L of NaClO2 with a concentration of 1.00 mmol/L was used to treat NO (concentration: 1000 ppmv and flow rate: 1.0 L/min), 100% of nitrogen oxides (NOx) could be removed in 8.22 min. Furthermore, the NO removal efficiency remained at 100% over the next 6.92 min. Furthermore, the formation of ClO2 by NaClO2 is affected by pH. The initial NOx removal efficiency was 84.8-54.8% for initial pH = 4.00-7.00. The initial NOx removal efficiency increases as the initial pH decreases. When the initial pH was 3.50, the initial NOx removal efficiency reached 100% under the synergistic effect of HC. Therefore, this method enhances the oxidation capacity of NaClO2 through HC, realizes high-efficiency denitrification with low NaClO2 concentration (1.00 mmol/L), and has better practicability for the treatment of NOx from ships.

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.

Study on NO Removal Characteristics of the Fe(II)EDTA and Fe(II)PBTCA Composite System

Fe²⁺ complexation wet denitrification technology has become a research hotspot. It is very important to achieve efficient regeneration of the absorbent and increase NO absorption in the Fe²⁺ complexation system. They are the key to the industrial application of the Fe²⁺ complexation absorption process. In this paper, 2-phosphonate-butane-1,2,4-tricarboxylic acid and ethylenediamine tetraacetic acid were used as ligands to prepare a composite system for the first time. The characteristics of NO removal were investigated under different temperatures, pHs, Fe²⁺ concentrations, O₂ contents, NO concentrations, CO₂ contents, and SO₂ concentrations. Compared with the single ligand, the results show that the denitrification performance of the solution with a complex ligand is significantly improved. In this system, pH 9, 40 °C temperature, and 20 mmol/L Fe²⁺ concentration are the economic ideal conditions for NO removal. The system can realize simultaneous removal of NO and SO₂, but SO₂ in flue gas has a dual effect on the NO removal reaction.

Simultaneous Removal of SO2 and NO by O3 Oxidation Combined with Seawater as Absorbent

Aiming at NOx (NO 90%, NO2 10%) and SO2 in simulated vessel emissions, denitration and desulfurization were studied through ozone oxidation combined with seawater as absorbent. Specifically, the different influencing factors of denitration and desulfurization were analyzed. The results indicated that the oxidation efficiency of NO can reach over 90% when the molar ratio of O3/NO is 1.2. Ozone oxidation and seawater washing in the same unit can decrease the temperature of ozone oxidation of NO, avoid high temperature ozone decomposition, and enhance the oxidation efficiency of NO. When NO inlet initial concentration is lower than 800 ppm, the NOx removal efficiency can be improved by increasing NO inlet concentration, and when NO inlet initial concentration is greater than 800 ppm, increasing the concentration of NO would decrease the NOx removal efficiency. Increasing the inlet concentration of SO2 has minor effect on desulfurization, but slightly reduces the absorption efficiency of NOx due to the competition of SO2 and NOx in the absorption solution. Besides, final products (NO2−, NO3−, SO32−, and SO42−) were analyzed by the ion chromatography.

Visible-Light Photocatalytic Chlorite Activation Mediated by Oxygen Vacancy Abundant Nd-Doped BiVO4 for Efficient Chlorine Dioxide Generation and Pollutant Degradation

Visible-light photocatalytic chlorite activation has emerged as an efficient oxidation process for micropollutant elimination. However, the in-depth mechanism of chlorite activation is not understood. In this study, using neodymium-doped bismuth vanadate (Nd_xBi_1-xVO_4-δ) as a model catalyst, we describe the oxygen vacancy (OV)-mediated chlorite activation process for efficient ClO₂ generation and cephalexin (CPX) degradation. DFT calculations and in situ DRIFTS suggest that the OV-introduced surface -OH serves as the Brønsted acidic center for chlorite adsorption. The OV-mediated chlorite activation involves multistep reactions that surface hydroxylation and proton transfer from the surface -OH to chlorite, forming metastable chlorous acid (HClO₂) and further disproportionating to ClO₂. As compared with vis-photocatalysis, the vis-photocatalysis coupled with chlorite activation (vis/chlorite) technique exhibits superior performance in antibiotic degradation and achieves efficient microorganism inactivation. This work uncovers the role of OVs on chlorite activation and provides a rational strategy for designing visible-light-driven oxidation techniques in water and wastewater treatment.

Insights into the wavelength-dependent photolysis of chlorite: Elimination of carbamazepine and formation of chlorate

Simultaneous removal of chlorite (ClO₂^-) and organic micro-pollutants (OMPs) by the irradiation of UV is a novel process. In this study we used UV light emitting diode (UV-LED) as a new UV source to systematically investigate the effect of UV wavelength (255, 285, 365 nm) on the simultaneous removal of carbamazepine (CBZ) and ClO₂^-. Removal of both CBZ and ClO₂^- followed the order of 255, 285, and 365 nm. Formation of hydroxyl radical and reactive chlorine species (RCS) were confirmed during the photolysis of ClO₂^- using probe compounds. RCS were always the predominant contributor to the degradation of CBZ in UV-LED/ClO₂^- system. The impacts of ClO₂^- dosage, pH, bicarbonate/carbonate (HCO₃^-/CO₃^2-), and the effect of natural organic matter (NOM) on CBZ degradation were also evaluated. This study identified the products produced by CBZ through possible degradation pathways during the transformation process. Further, the amount of ClO₂^- may affect the amount of chlorate produced in UV/ClO₂^- system. Overall, our research provides an in-depth analysis of the effects of UV wavelength on the simultaneous removal of ClO₂^- and OMPs in water.

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.

Paracetamol degradation via electrocatalysis with B and N co-doped reduced graphene oxide: Insight into the mechanism on catalyst surface and in solution

This paper presents the use of B and N co-doped reduced graphene oxide (BN-GN) as an electrode for paracetamol electrochemical degradation. The reaction mechanism, focused on active sites in the atom level and dominant radical species generated through the reaction, was analyzed by characterization, density functional theory (DFT) calculation, quenching experiments, and electron paramagnetic resonance analysis. The characterization results indicated that the introduction of N and B functionalities into GN improved catalytic activity due to the generation of new surface defects, active sites, and improvement of conductivity. Results of experiments and DFT showed that co-doping of B and N greatly improved the catalytic activity, and the B atoms in C-N-B groups were identified as main active sites. The main active substances of BN-GN generated in the electrocatalytic oxidation of paracetamol in the solution were O₂^•- and active chlorine. The influence of O₂^•- and active chlorine on the efficiency/path of catalytic oxidation and the proposed mechanism were also determined for paracetamol degradation. This study provides an in-depth understanding of the mechanism of BN-GN catalysis and suggests possibilities for practical applications.

Simultaneous Removal of SO2 and NO x Using Steel Slag Slurry Combined with Ozone Oxidation

In this work, steel slag slurry was used in combination with O3 oxidation for the simultaneous removal of SO2 and NO x in a laboratory-scale wet flue gas desulfurization process. The effects of the oxidation temperature, steel slag concentration, initial SO2 concentration, and pH value on the desulfurization and denitrification efficiencies were studied. The results showed that the highest NO x removal efficiency occurred at an oxidation temperature of 90 °C. With an increase of the oxidation temperature above 90 °C, the denitrification efficiency decreased due to the decomposition of N2O5. The effect of the SO2 concentration on denitrification was complicated. When the concentration of SO2 was 500 ppm, generation of SO3 2- promoted the absorption of NO2. However, higher SO2 concentrations strengthened the competitive absorption of SO2 and NO x . In the pH range of 8.5-4.5, the denitrification efficiency was maintained at about 96%. The component analyses of the aqueous solution and the solid residue were conducted to investigate the compositions of the absorption products. The results showed that NO3 - and SO4 2- were the major anions in the aqueous solution. The nitrogen balance was analyzed to be 95.8%, clearly illustrating the migration and transformation path of nitrogen. In the solid residue, most alkaline substances were consumed, and the final products were mainly CaSO4 and FeO. Accordingly, the reaction mechanism of simultaneous desulfurization and denitrification using steel slag combined with ozone oxidation was proposed.

Performance and mechanism of oxidation, and removal of trace SeO2 in flue gas utilizing a H2O2, NaClO2, and Ca2+ slurry

Selenium (Se) is one of the toxic trace elements in flue gas emitted from coal-fired power plants. A method for oxidation and absorption of selenium dioxide (SeO₂) by hydrogen peroxide (H₂O₂) and sodium chlorite (NaClO₂) in a calcium-based slurry is proposed, in which hydrated lime (Ca(OH)₂) and calcium carbonate (CaCO₃) were selected to be absorbents, and H₂O₂ and NaClO₂ were used to oxidize SeO₂ and selenite ion to selenate ion to reduce their toxicities. The effects of reaction temperature, H₂O₂ and NaClO₂ concentrations, solution pH, and SeO₂ concentration were investigated, from which the optimal reaction conditions were established. The results indicated that the oxidation efficiency was affected significantly by the solution pH, and H₂O₂ and NaClO₂ concentrations. The oxidation efficiency of SeO₂ and selenite ion to selenate ion was below 40% using H₂O₂ and that increased to 92% with NaClO₂ at the optimal conditions. For removal, Ca(OH)₂ and CaCO₃ slurries were used as absorbents and the efficiencies were affected by the slurry temperature. Oxidation behavior and mechanism were analyzed based on the characterization of removal products using hydride generation-atomic fluorescence spectrometry and X-ray photoelectron spectroscopy. SeO₂ and selenite ion can be oxidized to selenate ion, which is less toxic, by active species such as hydroxyl radicals (HO·) and chlorine dioxide (ClO₂), which are released during the decomposed processes of H₂O₂ and NaClO₂ in acid conditions.

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.

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.

Simultaneous removal of chlorite and contaminants of emerging concern under UV photolysis: Hydroxyl radicals vs. chlorate formation

It is well known that using chlorine dioxide (ClO₂) as a disinfectant inevitably produces a common disinfection byproducts chlorite (ClO₂^‒). In this study, we found that UV photolysis after ClO₂ disinfection can effectively eliminate both ClO₂^‒ and contaminants of emerging concern (CECs). However, the kinetic mechanisms of UV/ClO₂^‒ process destructing CECs, as well as transformation of ClO₂^‒ in UV/ClO₂^‒ system are not clear yet. Therefore, we systematically investigated the UV/ClO₂^‒ system to assist us appropriately design this process under optimal operational conditions. In this work, we first investigated the impact of water matrix conditions (i.e., pH, bicarbonate and natural organic matter (NOM)) and ClO₂^‒ dosage on the UV/ClO₂^‒ process. We found that bicarbonate and NOM have inhibition effects, while lower pH and higher ClO₂^‒ dosage have enhancement effects. Besides, hydroxyl radical (HO^•) and reactive chlorine species (RCS) are generated from UV/ClO₂^‒ system, and RCS are main contributors to CBZ degradation. Then we proposed a possible degradation pathway of CBZ based on the determined products from experiments. Additionally, we found that photolysis of ClO₂^‒ resulted in the generation of chloride (Cl^‒) and chlorate (ClO₃^‒). As the ClO₂^‒ dosage increases, the yield of ClO₃^‒ increased while that of Cl^‒ decreased. Finally, we elucidated the second order rate constant of the target organic compound with HO• has a strong correlation with the formation of ClO₃^‒.

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.

Aging retardation strategy of PVDF membranes: evaluation of free radical scavenging effect of nano-particles

ESR and spectrophotometry proved that nano-particles can effectively remove free radicals produced by NaClO, and analyzed the mechanism of delaying aging.

A ClO-mediated photoelectrochemical filtration system for highly-efficient and complete ammonia conversion

The ability to convert excess ammonia in water into harmless N₂ is highly desirable for environmental remediation. We present a chlorine-oxygen radical (ClO)-mediated photoelectrochemical filtration system for highly efficient and complete ammonia removal from water. The customized photochemical device comprised a Ag-functionalized TiO₂nanotube array mesh photoanode and a Pd-Cu co-modified nickel foam (Pd-Cu/NF) cathode. Under illumination, holes generated at the anode catalyzed the conversion of H₂O and Cl^- to HOand Cl, respectively. In turn, these radicals then reacted further, yielding ClO, which selectively decomposed ammonia. The cathode enabled further reduction of anodic byproducts such as NO₃^- to N₂. The complete oxidation of all dissolved ammonia was achieved within 15 min reaction under neutral conditions, where N₂ was the dominant product. The impact of key parameters was assessed, which enabled the discovery of optimal reaction conditions and the proposal of the underlying working mechanism. The flow-through configuration demonstrated a 5-fold increase of ammonia oxidation rate compared to the conventional batch reactor. The role of ClO in the oxidation of ammonia was verified with electron paramagnetic resonance and scavenger studies. This study provided greater mechanistic insights into photoelectrochemical filtration technology and demonstrated the potential of future nanotechnology for removing ammonia.

Simultaneous removal of SO2, NO and Hg0 using an enhanced gas phase UV-AOP method

The core for simultaneous removal of SO₂, NO and Hg⁰ is the oxidation of NO and Hg⁰. Radical induced oxidation of NO and Hg⁰ is considered to be the most efficient method. We develop a novel gas phase advanced oxidation process (AOP) of UV-Heat/H₂O₂-NaClO₂ to simultaneously remove SO₂, NO and Hg⁰ due to a great synergism between H₂O₂ and NaClO₂ under thermal and ultraviolet (UV) co-catalysis. The results indicated that the SO₂ removal was always good, while the removal of NO and Hg⁰ was affected by NaClO₂ and UV. Higher catalytic temperature and longer flue gas residence time favored the removal of NO and Hg⁰. The presence of SO₂ and NO facilitated Hg⁰ removal. Kinetics analyses were conducted to provide the reaction rate of removal of NO and Hg⁰ under different conditions. X-ray photoelectron spectroscopy (XPS) revealed the product composition as Cl^-, Hg²⁺, NO₃^- and SO₄^2-. Electron spin resonance (ESR) tests confirmed the generation of HO. Cost analyses demonstrated the better cost performance of the proposed method compared to SCR-ACI combined method. HO and ClO₂ were proved to be the main oxidant. The reaction mechanism for removal of NO and Hg⁰ by using UV-Heat/H₂O₂-NaClO₂ were proposed finally.

Simultaneous removal of SO2 and NO using a novel method with red mud as absorbent combined with O3 oxidation

Aiming at SO₂ and NO_x in industrial flue gas, the simultaneous removal of SO₂ and NO_x was conducted using red mud as absorbent with O₃ oxidation. The effects of different factors on the desulfurization and denitration efficiency were investigated. The results show that the increase of oxidation temperature is not conducive to the absorption of NO_x (O₃/NO molar ratio >1). Low concentration of SO₂ promotes the absorption of NO_x, however high concentration of SO₂ inhibits absorption of NO_x. The main product after desulfurization and denitration of red mud solid residue is bassanite (CaSO₄·0.5H₂O). In addition, when the pH is greater than 5, calcium carbonate and sodium alkali play major roles in red mud, and when 3.5< pH < 5, sodium aluminosilicate hydrate (1.08Na₂O·Al₂O₃·1.68SiO₂·1.8H₂O), calcium nepheline (Na₆CaAl₆Si₆(CO₃)O₂₄·2H₂O) and garnet (Ca₃Al₂(SiO₄)(OH)₈) take part in the reaction. According to typically practical conditions with oxidation temperature at 130 °C and the O₃/NO molar ratio being 1.8, the desulfurization efficiency of red mud can reach 93 % in the first hour, and the denitration efficiency can be maintained at about 87 %. Besides, the reaction mechanism with multi oxidation absorption steps was also proposed.

A novel method of pH-buffered NaClO2-NaCl system for NO removal from marine diesel engine

Marine diesel engines produce a lot of exhaust gas (NO, SO₂). Based on the situation that wet scrubbing methods have been already applied to ship desulfurization and seawater is easily accessible around the ships, this paper proposed a novel AOP (advanced oxidation process) of NaClO₂ (sodium chlorite) with Cl^- (abundant Cl^- exist in seawater) to remove NO from the flue gases of marine engines. The buffer capacity of NaAC (sodium acetate), the effect of Cl^- concentration, and Cl^- promotion mechanism on NO removal were investigated. The result showed that the existence of NaAC in solution could inhibit the rapid decline of the solution pH. The addition of Cl^- achieved a remarkable promotion to NO removal at lower NaClO₂ concentration, which was due to the fast generation of ClO₂ from the promotion decomposition of NaClO₂ by Cl^- in acidic condition. Then, the thermodynamic and dynamic mechanism of the generation of ClO₂ was intensively analyzed. And the mechanism of NO removal was discussed finally.

A Bifunctional Electroactive Ti4O7-Based Membrane System for Highly Efficient Ammonia Decontamination

Herein, an electroactive filtration system, consisting of a Ti4O7 anode and a Pd-Cu co-modified nickel foam cathode, was developed and applied for the decontamination of ammonia from water. When assisted with an external electrical field, ClO• was generated on the surface of the Ti4O7 anode, which then reacted selectively with ammonia to generate N2. The anodic byproduct, NO3−, could also be reduced efficiently at the functional cathode to produce N2 as well. Electron paramagnetic resonance technique and radical scavenging tests synergistically verified the essential role of ClO• during the highly efficient ammonia conversion process. Relative to conventional batch systems, the developed flow-through design demonstrated enhanced ammonia conversion kinetics, thanks to the convection-enhanced mass transport. The proposed technology also showed desirable stability across a wide environmental matrix. This work provides new insights for the development of advanced and affordable continuous-flow systems towards effective decontamination of ammonia.

Multi-air-pollutant removal by using an integrated system: Key parameters assessment and reaction mechanism

How to cost-efficiently and cooperatively remove SO₂, NO and Hg⁰ in flue gas is a hot topic in the field of air pollution control. This work developed an integrated system that consists of a dual-absorption system and a vapor oxidation system, in which Na₂CO₃ and H₂O₂/Na₂S₂O₈ were used as the absorbent and oxidant. The results indicated that the efficiencies of SO₂ removal and NO conversion reached 99.5% and 93% respectively. Rising the vaporization temperature and decreasing the pH of H₂O₂/Na₂S₂O₈ could facilitate the NO conversion. The spent Na₂CO₃ after desulfurization was demonstrated to be a good absorbent for NO₂ removal. The best conditions of pH and temperatures for the dual-absorber were determined as 10/8 and 60/60 °C, respectively. The presence of 1000 mg/m³ SO₂ and 300 mg/m³ NO favored the Hg⁰ removal. TMT-15, an organic sulfur compound, was demonstrated to be useful in retaining Hg²⁺, with an efficiency of 92%. According to the analyses of electron spin resonance (ESR), ion chromatography (IC), atom fluorescence spectrometry (AFS) and X-ray photoelectron spectroscopy (XPS), SO₄^- and HO were proved to be the key radicals, and the existing forms of N- and Hg- species in the product were identified as NaNO₂/NaNO₃ and HgCl₂.

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.

Photocatalytic removal of NO and Hg0 using microwave induced ultraviolet irradiating H2O/O2 mixture

We developed a novel method, microwave (MW) induced ultraviolet (UV) irradiating H₂O/O₂, to cooperatively remove NO and Hg⁰, with the efficiencies of 89.3% and 99.5%. It also can remove 97% SO₂. O₂ at a content of 2-8% was sufficient to conduct a good removal of NO and Hg⁰. Ozone (O₃) and hydroxyl radical (HO•) were proved to be the major oxidants for the removal of Hg⁰ and NO, respectively. High temperature facilitated NO removal but impaired Hg⁰ removal. SO₂ greatly promoted the removal of NO and Hg⁰ due to the formation of SO₄•^-. The presence of Cl^{- and Br-}suppressed NO removal but promoted Hg⁰ removal, because Cl^- and Br^-quenched HO• to produce Cl- and Br-radicals. The produced NO₂ could be totally absorbed by the Na₂SO₃ solution that followed the main reactor. The O₃ yield and the formation of HO• under different conditions were determined using iodine quantity method and electron spin resonance (ESR). The distributions of anion concentration and mercury proportion were obtained using ion chromatography (IC) and cold atom fluorescence spectrometry (AFS), and the main products were identified to be SO₄^2-, NO₃^- and HgO. The mechanisms of removal of SO₂, NO and Hg⁰ were speculated.

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.

A Critical Review of Recent Progress and Perspective in Practical Denitration Application

Nitrogen oxides (NOx) represent one of the main sources of haze and pollution of the atmosphere as well as the causes of photochemical smog and acid rain. Furthermore, it poses a serious threat to human health. With the increasing emission of NOx, it is urgent to control NOx. According to the different mechanisms of NOx removal methods, this paper elaborated on the adsorption method represented by activated carbon adsorption, analyzed the oxidation method represented by Fenton oxidation, discussed the reduction method represented by selective catalytic reduction, and summarized the plasma method represented by plasma-modified catalyst to remove NOx. At the same time, the current research status and existing problems of different NOx removal technologies were revealed and the future development prospects were forecasted.

Promoting Effect of H+ and Other Factors on NO Removal by Using Acidic NaClO2 Solution

In this study, NaClO2 was selected as a denitration oxidant. In order to clarify the mechanism of NaClO2 as an oxidation agent for NO removal efficiency, the effects of H+ and other factors (NaClO2 concentration, temperature, and the other gas) on the NO removal efficiency were investigated. NaClO2 showed a promotional ability on NO removal, whose efficiency increased with the increase of NaClO2 concentration. One hundred percent removal efficiency of NO could be achieved when the NaClO2 concentration was 0.014 mol/L. Furthermore, raising the reaction temperature benefited the removal of NO. The lower the pH, the better the NO removal efficiency. The promoting effect of H+ on the NO removal was studied by the Nernst equation, ionic polarization, and the generation of ClO2. Under the optimal conditions, the best removal efficiency of NO was 100%. Based on the experimental results, the reaction mechanism was finally speculated.

Elemental mercury removal by a novel advanced oxidation process of ultraviolet/chlorite-ammonia: Mechanism and kinetics

A novel advanced oxidation process (AOP) of ultraviolet/chlorite-ammonia (UV/NaClO₂-NH₄OH) was developed to remove Hg⁰ from flue gas. The distribution of mercury concentration in three solutions of NaClO₂-NH₄OH, KCl, and H₂SO₄-KMnO₄ was determined by cold atom fluorescence spectrometry (AFS). The role of NH₄OH was to help NaClO₂ preserving and/or stabilizing Hg²⁺ meanwhile inhibiting the photo-production of ClO₂. In the absence of UV, decreasing pH promoted the release of Hg²⁺ from NaClO₂-NH₄OH; introducing NO, SO₂, O₂, Br^-, Cl^-, and HCO₃^- suppressed Hg⁰ oxidation. In the presence of UV, rising temperature accelerated the release of Hg²⁺ from NaClO₂-NH₄OH; while SO₂, Br^- and HCO₃^- facilitated Hg⁰ oxidation. In the absence and presence of UV, Hg⁰ oxidation was controlled by ClO₂^- and by ClO/Cl₂O₂/HO/ClO₂, respectively. The formations of ClO/HO/ClO₂ were confirmed by electron spin resonance (ESR). X-ray photoelectron spectroscopy (XPS) revealed that the products of Hg⁰ and ClO₂^- were HgCl₂, and ClO₂, Cl-, ClO₃-, Cl₂, and ClO₄-, respectively. Analysis of kinetics showed that the Hatta numbers were 23-133 and 69-305 without and with UV, respectively, thus, the gas-film mass transfer was the rate-determining step. This paper gives a new insight in radical behavior in Hg⁰ oxidation.

Extremely Efficient Decomposition of Ammonia N to N2 Using ClO• from Reactions of HO• and HOCl Generated in Situ on a Novel Bifacial Photoelectroanode

The conversion of excess ammonia N into harmless N₂ is a primary challenge for wastewater treatment. We present here a method to generate ClO^• directionally for quick and efficient decomposition of NH₄⁺ N to N₂. ClO^• was produced and enhanced by a bifacial anode, a front WO₃ photoanode and a rear Sb-SnO₂ anode, in which HO^• generated on WO₃ reacts with HClO generated on Sb-SnO₂ to form ClO^•. Results show that the ammonia decomposition rate of Sb-SnO₂/WO₃ is 4.4 times than that of WO₃ and 3.3 times than that of Sb-SnO₂, with achievement of the removal of NH₄⁺ N on Sb-SnO₂/WO₃ and WO₃ being 99.2 and 58.3% in 90 min, respectively. This enhancement is attributed to the high rate constant of ClO^• with NH₄⁺ N, which is 2.8 and 34.8 times than those of Cl^• and HO^•, respectively. The steady-state concentration of ClO^• (2.5 × 10^-13 M) is 10² times those of HO^• and Cl^•, and this is further confirmed by kinetic simulations. In combination with the Pd-Cu/NF cathode to form a denitrification exhaustion system, Sb-SnO₂/WO₃ shows excellent total nitrogen removal (98.4%), which is more effective than WO₃ (47.1%) in 90 min. This study provides new insight on the directed ClO^• generation and its application on ammonia wastewater treatment.