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

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.

Mn(II) oxidation by the UV/chlorine system under near-neutral pH conditions: The important role of ClO· and ClO2

The oxidation kinetics of Mn(II) by free chlorine is relatively low under near-neutral pH conditions which limits the Mn removal efficiency in drinking water treatment. Therefore, this study investigated the oxidation efficiency of Mn(II) by the UV-enhanced chlorination (UV/chlorine) system and identified the responsible reactive radical species. The results show that the oxidation kinetic of Mn(II) was greatly enhanced by the UV/chlorine system under near-neutral pH or even acidic conditions. The pseudo-first-order reaction rate of Mn(II) at pH 8.0 (within the first 20 min) increased from 2.60 × 10-5 s-1 to 3.41 × 10-4 s-1. Based on the scavenging experiments and the steady-state kinetic modeling, ClO· and ClO2, whose steady-state concentration (∼10-10 M and ∼10-9 M, respectively at pH 8.0) was at least 4 orders of magnitude higher than that of HO· and Cl·, were recognized as the dominant reactive species contributing to the oxidation of Mn(II). Kinetic model calculations indicate that the contribution of ClO· to the oxidation of Mn(II) was consistently maintained above 70 %, and ClO2 also played an important role in the oxidation of Mn(II) especially under acidic and alkaline conditions. In addition, the background components of HCO3- and Cl- had negligible influence on the oxidation efficiency because they barely changed the concentration of the ClO· and ClO2. This study first demonstrates the important role of ClO2 in the oxidation of Mn(II) in the UV/chlorine system, and the possible role of ClO2 in the degradation of some organic pollutants needs to be carefully evaluated in the future.

Performance evaluation of the UV activated chlorite process on trimethoprim: Degradation efficiency, energy consumption and disinfection by-products formation

As the primary inorganic by-product species of ClO₂, chlorite is believed to have negative toxicological effects on human health and therefrom greatly limits the wide application of ClO₂ in water treatment. The synergistic trimethoprim (TMP) removal concerning degradation efficiency, energy consumption and disinfection by-products (DBPs) formation in the UV activated chlorite process accompanied by the simultaneously elimination of chlorite was comprehensively evaluated. UV/chlorite integrated process removed TMP far more rapidly than UV (1.52%) or chlorite (3.20%) alone due to the endogenous radicals (Cl•, ClO• and •OH), the contributing proportions of which were 31.96%, 19.20% and 44.12%. The second-order rate constants of TMP reaction with Cl•, ClO• and •OH were determined to be 1.75 × 10¹⁰, 1.30 × 10⁹ and 8.66 × 10⁹ M^-1 s^-1. The effects of main water parameters including chlorite dosage, UV intensity, pH as well as water matrixes (nature organic matter, Cl^- and HCO₃^-) were examined. k_obs obeyed the order as UV/Cl₂>UV/H₂O₂≈UV/chlorite>UV, and the cost ranking via electrical energy per order (EE/O, kWh m^-3 order^-1) parameter was UV/chlorite (3.7034) > UV/H₂O₂ (1.1625) >UV/Cl₂ (0.1631). The operational scenarios can be optimized to achieve the maximum removal efficiencies and the minimum energy costs. The destruction mechanisms of TMP were proposed by LC-ESI-MS analysis. The overall weighted toxicity in subsequent disinfection was assessed as UV/Cl₂>UV/chlorite > UV, the values of which in post-chlorination were 6.2947, 2.5806 and 1.6267, respectively. Owing to the vital roles of reactive chlorine species (RCS), UV/chlorite displayed far higher TMP degradation efficiency than UV, and concurrently presented much less toxicity than UV/Cl₂. In an effort to determine the viability of the promising combination technology, this study was devoted to reduce and reuse chlorite and synchronously realize the contaminants degradation efficiently.

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.

Iron species activating chlorite: Neglected selective oxidation for water treatment

Chlorite (ClO₂ ^-) is the by-product of the water treatment process carried out using chlorine dioxide (ClO₂) as an effective disinfectant and oxidant; however, the reactivation of ClO₂ ^- has commonly been overlooked. Herein, it was unprecedentedly found that ClO₂ ^- could be activated by iron species (Fe_b: Fe⁰, Fe^II, or Fe^III), which contributed to the synchronous removal of ClO₂ ^- and selective oxidative treatment of organic contaminants. However, the above-mentioned activation process presented intensive H⁺-dependent reactivity. The introduction of Fe_b significantly shortened the autocatalysis process via the accumulation of Cl^- or ClO^- during the protonation of ClO₂ ^- driven by ultrasonic field. Furthermore, it was found that the interdependent high-valent-Fe-oxo and ClO₂, after identification, were the dominant active species for accelerating the oxidation process. Accordingly, the unified mechanisms based on coordination catalysis ([Fe ^N (H₂O) _a (ClO _x ^m-) _b ] ^{n +}-P) were putative, and this process was thus used to account for the pollutant removal by the Fe_b-activated protonated ClO₂ ^-. This study pioneers the activation of ClO₂ ^- for water treatment and provides a novel strategy for "waste treating waste". Derivatively, this activation process further provides the preparation methods for sulfones and ClO₂, including the oriented oxidation of sulfoxides to sulfones and the production of ClO₂ for on-site use.

Natural manganese sand activates sodium hypochlorite to enhance ionic organic contaminants removal: Optimization, modeling, and mechanism

Although sodium hypochlorite acting as an oxidant has been investigated for the role it plays in the degradation of organic contaminants, little attention has been paid to its activation and efficient utilization. In this study, natural manganese sand (NMS) was verified to be effective for activation of sodium hypochlorite (NaClO). Due to the generation of O₂^-, the removal efficiency of ionic organic contaminants in NMS/NaClO system was 1.9-4.1 times higher than that in NMS or NaClO alone. Hence, NMS activated NaClO system performed ~96.6 % contaminants removal efficiency at a wide pH range (pH 5-9). Kinetic modeling yielded that the NMS dosage was more important than NaClO dosage. Long-term stability was observed in the presence of various salts (bicarbonate, sulfate, phosphate, and chloride). Characterization results revealed that electron transfer among NMS, NaClO, and organic contaminants was responsible for NaClO activation. Then NaClO-based Fenton-like process was proposed by tracing the degradation intermediates of methyl orange (MO) and generations of reactive oxygen species in the MO/NMS/NaClO system. This study presents the potential of NMS to activate NaClO and enhance ionic organic contaminants removal from aquatic environments.

Evaluating UV254 absorbance reductions in landfill leachate for municipal sewage co-treatment through timed UV/electrooxidation

Landfill leachate contains dissolved organic matter (DOM) exhibiting high ultraviolet absorbance at 254 nm (UVA₂₅₄). The UVA₂₅₄ limits leachate co-treatment with municipal sewage by hindering the downstream UV disinfection efficiency at wastewater treatment plants. Here, we alleviated the UVA₂₅₄ by timing the radiation in a UV/electrooxidation (UV/EO) process to accelerate reactive species formation. At 200 A·m^-2, the UV radiation was delayed by 10 min to accumulate 21 mg·L^-1 as Cl₂, which enhanced the initial radical formation rate by 5.25 times compared with a simultaneous UV/EO. The timed operation increased the steady-state concentrations of ClO^• by 700 times to 4.11 × 10^-14 M and reduced the leachate UVA₂₅₄ by 78.2% after 60 min. We identified that aromatic formulas with low oxygen content were susceptible to UV/EO from Fourier-transform ion cyclotron resonance mass spectrometry analysis. The toxicity of the treated leachate and generated byproducts was assessed through specific oxygen uptake rates (SOUR) and developmental assays with Platynereis dumerilii. After quenching the residual chlorine, leachate co-treatment at 3.5% v/v presented minimal toxicological risk. Our findings provide operational insights for applying UV/EO in high UVA₂₅₄ matrices such as landfill leachate.

The removal of NO from flue gas by NaOH-catalyzed H2O2 system: Mechanism exploration and primary experiment

Currently, most advanced oxidation denitrification technologies require long flue gas residence time to obtain ideal NO removal efficiency. The NaOH-catalyzed H₂O₂ system proposed in this paper can obtain 98% NO removal efficiency under the condition of flue gas residence time of 3 s. The mechanism of NO removal and H₂O₂ decomposition to O₂ were proposed. It was confirmed with ESR (Electron-spin-resonance), inhibitor experiments and UV-Vis spectrophotometer that the main group in the reaction process was·O₂^- radicals, which reacted with NO to form ONOO^-, and ONOO^- would be gradually transformed into NO₃^- and NO₂^- in the air. The effect of some primary factors on the NO removal efficiency and the percentage of H₂O₂ decomposition to O₂ were also investigated. The increase of initial pH has a positive effect on NO removal, while the promotion of NO removal by increasing H₂O₂ concentration and reaction temperature is limited and the increase of NO has a negative effect on NO removal. Initial pH has a dual impact on the percentage of H₂O₂ decomposition to O₂, H₂O₂ concentration and reaction temperature promote the decomposition of H₂O₂ to O₂, while NO concentration has an inhibiting effect on it.

Surface, Interface and Structure Optimization of Metal-Organic Frameworks: Towards Efficient Resourceful Conversion of Industrial Waste Gases

Industrial waste gas emissions from fossil fuel over-exploitation have aroused great attention in modern society. Recently, metal-organic frameworks (MOFs) have been developed in the capture and catalytic conversion of industrial exhaust gases such as SO₂ , H₂ S, NO_x , CO₂ , CO, etc. Based on these resourceful conversion applications, in this review, we summarize the crucial role of the surface, interface, and structure optimization of MOFs for performance enhancement. The main points include (1) adsorption enhancement of target molecules by surface functional modification, (2) promotion of catalytic reaction kinetics through enhanced coupling in interfaces, and (3) adaptive matching of guest molecules by structural and pore size modulation. We expect that this review will provide valuable references and illumination for the design and development of MOF and related materials with excellent exhaust gas treatment performance.

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.

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.

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.

Microwave-Induced Deep Catalytic Oxidation of NO Using Molecular-Sieve-Supported Oxygen-Vacancy-Enriched Fe-Mn Bimetal Oxides

A novel microwave (MW) catalytic oxidation denitrification method was developed, which can deeply oxidize NO into nitrate/nitrite with little NO₂ yield. A molecular-sieve-supported oxygen-vacancy-enriched Fe₂O₃-MnO₂ catalyst (Ov-Fe-Mn@MOS) was fabricated. Physicochemical properties of the catalyst were revealed by various characterization methods. MW irradiation was superior to the conventional heating method in NO oxidation (90.5 vs 70.6%), and MW empowered the catalyst with excellent low-temperature activity (100-200 °C) and good resistance to H₂O and SO₂. Ion chromatography analysis demonstrated that the amount of nitrate/nitrite accounted for over 90.0% of the N products, but the main product gradually varied from nitrate to nitrite as the reaction proceeded because of the switching of the main reaction path of NO removal. Mechanism analyses clarified that NO oxidation was a non-radical catalytic reaction: (i) the chemisorbed NO on ≡Mn(IV) reacted with O₂* to produce nitrate and (ii) the excited NO* due to MW irradiation reacted with the active O* generated from Ov···O₂ to form nitrite. Density functional theory calculations combined with electron paramagnetic resonance tests revealed the promotional effects of Fe₂O₃ in (i) boosting the Ov's quantity; (ii) facilitating O₂ adsorption; (iii) increasing the nitrite formation; and (iv) alleviating the suppression of SO₂.

New advanced oxidation progress with chemiluminescence behavior based on NaClO triggered by WS2 nanosheets

As one integral part of coping strategies for addressing water pollution, advanced oxidation progresses (AOPs) get enormous attentions in recent years. However, the complex synthesis and high cost of H₂O₂ and K₂S₂O₈ hampered their developments. Herein, a novel AOP with the chemiluminescence (CL) property based on economic NaClO and WS₂ nanosheets was proposed to achieve efficient decomposition of organic pollutants. In this AOP, WS₂ nanosheets exhibited a dual-function feature of the catalyst and energy acceptor. It demonstrated that the reaction order of WS₂ nanosheets was equal to 0.8271 and enormous singlet oxygen (¹O₂),·ClO and hydroxyl radical (·OH) were generated in rhodamine B (RhB) degradation process. Interestingly, a strong CL emission was observed and reflected the relative concentration of ¹O₂ and·OH for adjusting the oxidizing capability in WS₂ nanosheets-NaClO system. Through a series of degradation tests, RhB, methylene blue (MB), p-nitrophenol and phenol were decomposed and the degradation efficiency of over 90% was achieved. Therefore, this study not only builds a chemiluminescent AOPs to eliminate organic pollutants, but also broadens the applications of WS₂ nanomaterials and CL in environmental field.

Simultaneous Nitrite Resourcing and Mercury Ion Removal Using MXene-Anchored Goethite Heterogeneous Fenton Composite

The integrated system of gas-phase advanced oxidation process combined with sulfite-based wet absorption process is a desirable method for simultaneous removal of SO₂, NO, and Hg⁰, but due to the enrichment of nitrite and Hg²⁺, resourcing harmless wastewater is still a challenge. To tackle this problem, this study fabricated a bifunctional β-FeOOH@MXene heterogeneous Fenton material, of which the crystalline phase, morphology, structure, and composition were revealed by using X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy-energy dispersive x-ray spectroscopy, and transmission electron microscopy. It exhibits excellent performance on nitrite oxidation (99.5%) and Hg²⁺ removal (99.7%) and can maintain stable outstanding ability after 13 cycles, with superior Hg²⁺ adsorption capacity (395 mg/g) and ultralow Fe leaching loss (<0.018 wt %). The synergism between MXene and β-FeOOH appears as follows: (i) MXene, as an inductive agent, directionally converted Fe₂O₃ into β-FeOOH in the hydrothermal method and greatly reduced its monomer size; (ii) the introduced ≡Ti(III)/≡Ti(II) accelerated the regeneration of ≡Fe(II) via rapid electron transfer, thereby improving the heterogeneous Fenton reaction; and (iii) MXene strongly immobilized β-FeOOH to greatly inhibit Fe-leaching. HO^•, ^•O₂^--, and ¹O₂ were the main radicals identified by electron spin resonance. Radical quenching tests showed their contributions to NO₂^- oxidation in the descending order HO^• > ¹O₂ > ^•O₂^-. Quantum chemical calculations revealed that ^•OH-induced oxidation of NO₂^- or HNO₂ was the primary reaction path. Density functional theory calculations combined with X-ray photoelectron spectroscopy and Raman characterizations displayed the Hg²⁺ removal mechanism, with Hg₂Cl₂, HgCl₂, and HgO as the main byproducts. This novel material provides a new strategy for resourcing harmless wastewater containing nitrite and Hg²⁺.

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

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

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.

A novel co-catalyzed system between persulfate and chlorite by sonolysis for removing triphenylmethane derivative

Triphenylmethane (tpm) derivatives (e.g. tpm_CV) have threatened the safety of the aquatic environment due to the potential toxicity and carcinogenicity. In this study, the novel ultrasonic/persulfate/chlorite (US/S₂O₈^2-/ClO₂^-) oxidation process was developed for the effective removal of tpm_CV in wastewater. The apparent non-integer kinetics (n around 1.20) of tpm_CV degradation under different factors (R²_Adj > 0.990) were investigated, respectively. Inhibiting effects of anions were greater than those of cations (except Fe(II/III)). The adding of micromolecule organic acids could regulate degradation towards positive direction. The double response surface methodology (RSM) was designed to optimize tpm_CV removal process, and the acoustic-piezoelectric interaction was simulated to determine the propagation process of acoustic wave in the reactor. The possible degradation pathway was explored to mainly include carbonylation, carboxylation, and demethylation. The estimated effective-mean temperature at the bubble-water interface was calculated from 721 to 566 K after introducing the ClO₂^-, however, the adsorption or partitioning capacity of tpm_CV in the reactive zone was widened from 0.0218 to 0.0982. The proposed co-catalysis of US/S₂O₈^2-/ClO₂^- was based on the determined active species mainly including ClO₂, SO₄⋅^-, and ⋅OH. Compared with other US-based processes, the operating cost (3.97 $/m³) of US/S₂O₈^2-/ClO₂^- with the EE/O value (16.8 kWh/m³) was relatively reduced.

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.

Ultrasonic role to activate persulfate/chlorite with foamed zero-valent-iron: Sonochemical applications and induced mechanisms

The novel system, consisting of composite oxidants (persulfate/chlorite, S2O82-/ClO2-) and stationary phase activator (zero-valent-iron foam, Fe0f) driven by ultrasonic (US) field, was applied to treat the triphenylmethane derivative effectively even at low temperature (≈ 289 K). By comparisons of sub-systems, the US roles to S2O82-, ClO2-, and Fe0f were seriatim analyzed. US made the reaction order of multi-component system tend to within 1 (leading to de-order reaction), and widened pH activating range of the Fe0f by sonicate-polishing during the process of ClO2- co-activating S2O82-. US and Fe0f were affected by fluid eddy on activating S2O82-/ClO2-. The Fe0f had slight effect on the temperature of US bubble-water interface but the addition of ClO2- lowered it. The partitioning capacity of the above US reactive zone increased during the reaction. US and ClO2- could enrich the kinds of degradation intermediates. The contributions of free radicals (ClOx-based radicals, sulfate radicals (SO4-), and hydroxyl radicals (OH)) and non-free radicals (ClO2, and O = FeIV/V from ionic Fe under "-O-O-" of S2O82- and cyclic adjustment reaction of ClO2-) processes by sonochemical induction were equally important by corresponding detection means. Especially, real-time and online high-resolution mass spectrum by self-developing further confirmed the chain transfers of different free radicals due to US role. The findings expanded the application of sono-persulfate-based systems and improved understanding on activation mechanism.

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^‒.

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 and NO using a spray dryer absorption (SDA) method combined with O3 oxidation for sintering/pelleting flue gas

Based on the demand of sintering/pelleting flue gas ultra-low emission, a semi-dry method using a spray dryer absorber (SDA) combined with O₃ oxidation was proposed for simultaneous removal of SO₂ and NO. Effects of O₃ injection site, O₃/NO molar ratio, and spray tower temperature on the removal efficiencies were investigated. It was revealed that both desulfurization and denitrification efficiencies could reach to 85% under the conditions of setting O₃ injection site inside of tower, O₃/NO molar ratio 1.8, spray tower temperature 85°C, Ca/(S + 2 N) molar ratio 2.5 and slurry flow rate 300 mL/hr. CaSO₃/Ca(OH)₂ mixture slurry was used as absorbent to simulate operating conditions in iron and steel industry. The result shows that the addition of CaSO₃ weakens both removal efficiencies. In addition, the reaction mechanism of simultaneous removal of SO₂ and NO using SDA combined with O₃ oxidation was proposed.

Efficient SO2 Removal and Highly Synergistic H2O2 Production Based on a Novel Dual-Function Photoelectrocatalytic System

The direct conversion of SO₂ to SO₃ is rather difficult for flue gas desulfurization due to its inert dynamic with high reaction activation energy, and the absorption by wet limestone-gypsum also needs the forced oxidation of O₂ to oxidize sulfite to sulfate, which is necessary for additional aeration. Here, we propose a method to remove SO₂ with highly synergistic H₂O₂ production based on a novel dual-function photoelectrocatalytic (PEC) system in which the jointed spontaneous reaction of desulfurization and H₂O₂ production was integrated instead of nonspontaneous reaction of O₂ to H₂O₂. SO₂ was absorbed by alkali liquor then oxidized quickly into SO₄^2- by a nanorod α-Fe₂O₃ photoanode, which possessed high alkali corrosion resistance and electron transport properties. H₂O₂ was produced simultaneously in the cathode chamber on a gas diffusion electrode and was remarkably boosted by the conversion reaction of SO₃^2- to SO₄^2- in the anode chamber in which the released chemical energy was effectively used to increase H₂O₂. The photocurrent density increased by 40% up to 1.2 mA·cm^-2, and the H₂O₂ evolution rate achieved 58.8 μmol·L^-1·h^-1·cm^-2 with the synergistic treatment of SO₂, which is about five times than that without SO₂. This proposed PEC cell system offers a cost-effective and environmental-benign approach for dual purpose of flue gas desulfurization and simultaneous high-valued H₂O₂ production.

Discovering the Importance of ClO• in a Coupled Electrochemical System for the Simultaneous Removal of Carbon and Nitrogen from Secondary Coking Wastewater Effluent

Inorganic constituents in real wastewater, such as halides and carbonates/bicarbonates, may have negative effects on the performance of electrochemical systems because of their capability of quenching HO^•. However, we discovered that the presence of Cl^- and HCO₃^- in an electrochemical system is conducive to the formation of ClO^•, which plays an important role in promoting the simultaneous elimination of biorefractory organics and nitrogen in secondary coking wastewater effluent. The 6-h operation of the coupled electrochemical system (an undivided electrolytic cell with a PbO₂/Ti anode and a Cu/Zn cathode) at a current density of 37.5 mA cm^-2 allowed the removal of 87.8% of chemical oxygen demand (COD) and 86.5% of total nitrogen. The electron paramagnetic resonance results suggested the formation of ClO^• in the system, and the probe experiments confirmed the predominance of ClO^•, whose steady-state concentrations (8.08 × 10^-13 M) were 16.4, 26.5, and 1609.5 times those of Cl₂^•- (4.92 × 10^-14 M), HO^• (3.05 × 10^-14 M), and Cl^• (5.02 × 10^-16 M), respectively. The rate constant of COD removal and the Faradaic efficiency of anodic oxidation obtained with Cl^- and HCO₃^- was linearly proportional to the natural logarithm of the ClO^• concentration, and the specific energy consumption was inversely correlated to it, demonstrating the crucial role of ClO^• in pollutant removal.

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.

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.

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.