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

Unveiling the activity difference cause and ring-opening reaction routes of typical radicals induced degradation of toluene

This study employs five UV-AOPs (PMS, PDS, H2O2, NaClO and NaClO2) to produce radicals (•OH, SO4•-, ClO•, O2•- and 1O2) and further comparatively studies their activity sequence and activity difference cause in toluene degradation. The toluene mineralization efficiency as a descending order is 73 % (UV-PMS) > 71 % (UV-PDS) > 70 % (acidified-UV-NaClO) > 55 % (UV-H2O2) > 36 % (UV-NaClO) > 35 % (UV-NaClO2); that of conversion efficiency is 99 % (acidified-UV-NaClO) > 95 % (UV-PMS) > 90 % (UV-PDS) > 74 % (UV-H2O2) > 44 % (UV-NaClO) > 41 % (UV-NaClO2). Acidic pretreatment significantly boosts the reactivity of UV-NaClO. ESR combined with radical quenching tests reveals the radicals' generation and evolution, and their contribution rates to toluene conversion, i.e. ClO• > SO4•- > O2•- > 1O2 > •OH. Theoretical calculations further unveil the ring-opening reaction routes and the nature of the activity difference of different radicals. The minimum energy required for ring-opening reaction is 116.77, 150.63, 168.29 and 191.92 kJ/mol with respect to ClO•, SO4•-, 1O2 and •OH, and finding that the ClO•-HO• pair is the best for toluene mineralization. The difficulty for eliminating typical VOCs by using UV-AOPs method is determined as toluene > chlorobenzene > benzene > ethyl acetate.

Progress on mechanism and efficacy of heterogeneous photocatalysis coupled oxidant activation as an advanced oxidation process for water decontamination

The rising debate on the dilemma of photocatalytic water treatment technologies has driven researchers to revisit its prospects in water decontamination. Nowadays, heterogeneous photocatalysis coupled oxidant activation techniques are intensively studied due to their dual advantages of high mineralization and high oxidation efficiency in pollutant degradation. This paved a new way for the development of solar-driven oxidation technologies. Previous reviews focused on the advances in one specific coupling technique, such as photocatalytic persulfate activation and photocatalytic ozonation, but lack a consolidated understanding of the synergy between photocatalytic oxidation and oxidant activation. The synergy involves the migration of photogenerated carriers, radical reaction, and the increase in oxidation rate and mineralization. This review systematically summarizes the fundamentals of activation mechanism, advanced characterization techniques and synergistic effects of coupling techniques for water decontamination. Besides, specific cases that lead researchers astray in revealing mechanisms and assessing synergy are critically discussed. Finally, the prospects and challenges are put forward to further deepen the research on heterogeneous photocatalytic activation of oxidants. This work provides a consolidated view of the existing heterogeneous photocatalysis coupled oxidant activation techniques and inspires researchers to develop more promising solar-driven technologies for water decontamination.

Photolysis of chlorite by solar light: An overlooked mitigation pathway for chlorite and micropollutants

Chlorite (ClO₂^-) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO₂^- but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO₂^- by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO₂^- could be efficiently decomposed to chloride (Cl^-) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl^- yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O₃), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO₂^- system with the steady-state concentrations following the order of O₃ (≈ 0.8 μΜ) > ClO• (≈ 4.4 × 10^-6 μΜ)> •OH (≈ 1.1 × 10^-7 μΜ)> Cl• (≈ 6.8 × 10^-8 μΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO₂^- system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min^-1 at pH 7.0, while most of them were negligibly degraded by SSL or ClO₂^- treatment alone. Kinetic modeling of BZF degradation by SSL/ClO₂^- at pHs 6.0 - 8.0 suggested that •OH contributed the most, followed by Cl•, O₃, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO₂^- system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO₂^- and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO₂^- and micropollutants, which has significant implications for understanding their fate in natural environments.

Kinetic Role of Reactive Intermediates in Controlling the Formation of Chlorine Dioxide in the Hypochlorous Acid-Chlorite Ion Reaction

An advanced experimental protocol is reported for studying the kinetics and mechanism of the complex redox reaction between chlorite ion and hypochlorous acid under acidic condition. The formation of ClO₂ is followed directly by the classical two-component stopped-flow method. In sequential stopped-flow experiments, the target reaction is chemically quenched using NaI solution and the concentration of each reactant and product is monitored as a function of time by utilizing the principles of kinetic discrimination. Thus, in contrast to earlier studies, not only the formation of one of the products but the decay of the reactants was also directly followed. This approach provides a firm basis for postulating a detailed mechanism for the interpretation of the experimental results under a variety of conditions. The intimate details of the reaction are explored by simultaneously fitting 78 kinetic traces, i.e., the concentration vs. time profiles of ClO₂^-, HOCl, and ClO₂, to an 11-step kinetic model. The most important reaction steps were identified, and it was shown that two reactive intermediates have a pivotal role in the mechanism. While chlorate ion predominantly forms via the reaction of Cl₂O, chlorine dioxide is exclusively produced in reaction steps involving Cl₂O₂. This study leads to clear conclusions on how to control the stoichiometry of the reaction and achieve optimum conditions to produce chlorine dioxide and to reduce the formation of the toxic chlorate ion in practical applications.

Chlorine dioxide-based oxidation processes for water purification：A review

Chlorine dioxide (ClO₂) has emerged as a broad-spectrum, safe, and effective disinfectant due to its high oxidation efficiency and reduced formation of organochlorinated by-products during application. This article provides an updated overview of ClO₂-based oxidation processes used in water treatment. A systematic review of scientific information and experimental data on ClO₂-based water purification procedures is presented. Concerning ClO₂-based oxidation derivative problems, the pros and cons of ClO₂-based combined processes are assessed and disinfection by-product (DBP) control approaches are proposed. The kinetic and mechanistic data on ClO₂ reactivity towards micropollutants are discussed. ClO₂ selectively reacts with electron-rich moieties (anilines, phenols, olefins, and amines) and eliminates certain inorganic ions and microorganisms with high efficiency. The formation of chlorite and chlorate during the oxidation process is a crucial concern when utilizing ClO₂. Future applications include the combination of ClO₂ with ferrous ions, activated carbon, ozone, UV, visible light, or persulfate processes. The combined process can reduce by-product generation while still ensuring ClO₂ sterilization and disinfection. Overall, this research could provide useful information and new insights into the application of ClO₂-based technologies.

A computational study on the adsorption of arsenic pollutants on graphene-based single-atom iron adsorbents

Integrated gasification combined cycle (IGCC) is a promising clean technology for coal power generation; however, the high volatility and toxicity of arsenic pollutants (As₂, As₄, AsO and AsH₃) released from an IGCC coal plant cause serious damage to human health and the ecological environment. Therefore, highly efficient adsorbents for simultaneous treatment of multiple arsenic pollutants are urgently needed. In this work, the adsorption characteristics and competitive adsorption behaviors of As₂, As₄, AsO, and AsH₃ on four kinds of graphene-based single-atom iron adsorbents (Fe/GA) were systematically investigated through density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The results suggest that single-vacancy Fe/GA doped with three nitrogen atoms has the largest adsorption ability for As₂, As₄, AsO and AsH₃. The adsorption energies of As₂, AsO and As₄ on Fe/GA depend on both charge transfer and orbital hybridization, while the adsorption energy of AsH₃ is mainly decided by electronic transfer. The adsorption differences of As₂, As₄, AsO and AsH₃ on four Fe/GA adsorbents can be explained through the obvious linear relationship between the adsorption energy and Fermi softness. As₂, As₄, AsO and AsH₃ will compete for adsorption sites when they exist on the same adsorbent surface simultaneously, and the adsorption capacities of AsO and As₂ are relatively stronger. After the competitive adsorption between AsO and As₂, AsO occupies the adsorption site at 300-900 K. This theoretical work suggests that Fe/GA is a promising adsorbent for the simultaneous removal of multiple arsenic pollutants with high adsorption capacity and low cost.

Theoretical insight into mercury species adsorption on graphene-based Pt single-atom catalysts

Mercury emission from coal-fired flue gases is environmentally crucial. Revealing the interaction between mercury (Hg) and functional materials is significant to controlling emission. We conducted an investigation into the adsorption mechanism of mercury species onto graphene-based Platinum (Pt) single-atom catalysts (SACs). Single-atom Pt is the active center for Hg species chemisorption, with an adsorption energy range of 0.555–3.792 eV. In addition, Hg species adsorbed preferentially at lower temperatures. Pt/3N-GN exhibits a higher adsorption ability than Pt/SV-GN. The strong interaction of Hg⁰ with Pt SACs contributed to atomic-orbital hybridization between them. Further analysis revealed that s, p orbitals of Hg contribute significantly to orbital hybridization with Pt SACs. Moreover, the charge decomposition analysis confirmed that s, p orbitals of Hg hybridized with d, s orbitals of Pt SACs. The net charge transfer from Hg⁰ to Pt/SV-GN and Pt/3N-GN are 0.059 and 0.097 e⁻, respectively. The higher the charge transfers, the more intense the electron and orbital interaction between Hg and the surface. Consequently, Pt/3N-GN is a highly effective catalyst for Hg adsorption.

Adsorption mechanism: all mercury species can chemically adsorb on Pt/SV-GN and Pt/3N-GN. The charge decomposition analysis confirmed that s, p orbitals of Hg hybridized with d, s orbitals of Pt SACs. Pt/3N-GN is more superior for mercury removal.

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.

The multiple roles of chlorite on the concentrations of radicals and ozone and formation of chlorate during UV photolysis of free chlorine

Chlorine dioxide (ClO₂) has emerged as a promising alternative to free chlorine for water disinfection and/or pre-oxidation due to its reduced yields of chlorinated disinfection byproducts. ClO₂ decomposes to form chlorite (ClO₂^-), which influences the following advanced oxidation processes (AOPs) for micropollutant abatement in drinking water. This study aims at investigating the effects of ClO₂^- on the concentrations of reactive species (e.g., radicals and ozone) and on the formation of chlorate in the UV/chlorine AOP. Results showed that the concentration of ClO^· in the UV/chlorine process remarkably decreased by 98.20-100.00% in the presence of ClO₂^- at concentration of 0.1-1.0 mg·L^-1 as NaClO₂. The concentrations of HO^· and ozone decreased by 42.71-65.42% and by 22.02-64.31%, respectively, while the concentration of Cl^· was less affected (i.e., 31.00-36.21% reduction). The overall concentrations of the reactive species were differentially impacted by ClO₂^-'s multiple roles in the process. UV photolysis of ClO₂^- generated HO^· but not Cl^·, ClO^· or ozone under the drinking water relevant conditions. ClO₂^- also competed with chlorine for UV photons but this effect was minor (< 1.0%). The radicals/ozone scavenging by ClO₂^- outcompeted the above two to lead to the overall decreasing concentrations of the reactive species, in consistency with the kinetic model predicted trends. ClO₂^- reacted with radicals and ozone to form chlorate (ClO₃^-) but not perchlorate (ClO₄^-). HO^· played a dominant role in ClO₃^- formation. The findings improved the fundamental understanding on micropollutant abatement and inorganic byproduct formation by the UV/chlorine process and other AOPs in ClO₂^--containing water.

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 Hg0 by Composite Oxidant NaClO/NaClO2 in a Packed Tower

Based on the implementation of the global Minamata Convention, developing an efficient and economical technology for mercury reduction in coal-fired flue gas becomes a hotspot in the field of air pollution control. The composite oxidant NaClO/NaClO₂ combined with limestone was used in the simultaneous removal of SO₂ and Hg⁰ in this study, and the three-factor and four-level orthogonal experiments were performed in a packed tower. The influential sequence of various factors on SO₂ and Hg⁰ removals was investigated through range analysis of the orthogonal experiments. Results showed that factors affecting desulfurization was C > A > B (liquid-gas ratio > oxidant concentration ratio > initial pH of absorption liquid), while factors affecting Hg⁰ removal was A > C > B (oxidant concentration ratio > liquid-gas ratio > initial pH of absorption liquid). Optimum conditions of simultaneous desulfurization and demercuration by NaClO/NaClO₂ were A4B1C4; that is, the oxidant concentration ratio was 10/4 (mmol/L:mmol/L), the initial pH was 5, and the liquid-gas ratio was 18 (L/m³). The simultaneous removal efficiencies of SO₂ and Hg⁰ reached 99.5 and 85.4% under these optimum conditions, respectively. Analysis of the characteristics of the solid products showed that the main products of the wet oxidation were CaSO₄ and CaSO₃. Analysis of the existing form of oxidized mercury showed that 23% of mercury was in the gypsum, while 77% was in the supernatant. Results of this research would provide a practical reference for promoting the simultaneous removal of SO₂ and Hg⁰ by NaClO/NaClO₂ with limestone in industrial application.

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.