Recyclable MFe₂O₄ (M = Mn, Zn, Cu, Ni, Co) coupled micro–nano bubbles for simultaneous catalytic oxidation to remove NO_x and SO₂ in flue gas

Abstract

NO_x can be efficiently removed by micro–nano bubbles coupling with Fe³⁺ and Mn²⁺, but the catalyst cannot be reused and the adsorption wastewater should be treated. This work developed a new technology that uses micro–nano bubbles and recyclable MFe₂O₄ to simultaneously remove NO_x and SO₂ from flue gas, and clarified the effectiveness and reaction mechanism. MFe₂O₄ (M = Mn, Zn, Cu, Ni and Co) prepared by a hydrothermal method was characterized. The results show that MFe₂O₄ can be activated to produce ˙OH which can accelerate the oxidation absorption of NO_x. Compared with no catalyst, the NO_x conversion rate increased from 32.85% to 83.88% in the NO_x–SO₂–MFe₂O₄-micro–nano bubble system, while the removal rate of SO₂ can reach 100% at room temperature. The catalytic activities of MFe₂O₄ showed the following trend: CuFe₂O₄ > ZnFe₂O₄ > MnFe₂O₄ > CoFe₂O₄ > NiFe₂O₄. The results provide a new idea for the application of advanced oxidation processes in flue gas treatment.

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1. Introduction

With the development of industry, nitrogen oxides (NO_x) and sulfur dioxide (SO₂) have become the most common atmospheric pollutants.^1,2 The emission of these pollutants causes various environmentally harmful effects such as acid rain, photochemical smog, ozone depletion and climate warming.³ At present, the main methods to control NOx in industry are dry denitration technology and wet denitration technology. Dry denitrification techniques include selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR).^4–6 Wet denitrification techniques include oxidation, complexation absorption and acid–base absorption.^7–9 There are many kinds of pollutants in the actual flue gas, which make it difficult to deal with. It is a new trend to study the oxidation and absorption of various pollutants in flue gas by simple devices.¹⁰ Flue gas desulfurization and denitrification integration refers to the process of simultaneous desulfurization and denitrification in the same equipment. Although this method can effectively improve the flue gas treatment efficiency and reduce the energy consumption, the main research is still in the research and development stage, there is still a long way to go before the large-scale industrialization. Micro–nano bubbles (MNB) coupled oxygen carrier (OCs) technology is a wet flue gas integrated removal technology, which can simplify the system, reduce the cost, and ensure a high removal efficiency of NO_x and SO₂,^11,12 and has a strong application potential. It is a new direction of the comprehensive removal of multi-pollutant flue gas.¹³

Micro–nano bubble technology refers to the bubble with the diameter between tens of microns to hundreds of nanometers when the bubble occurs.¹⁴ Compared with ordinary bubbles, it has the following characteristics: small size leads to large surface area, low rising speed, high internal pressure, large gas solubility, and free radicals formed when crushed.^13,15 Hydroxyl radicals generated by the breakdown of micro–nano bubbles are an important reactive oxygen species with an oxidation potential of 2.8V.^16–18 Xiao¹⁹ et al. used micro–nano bubbles to remove SO₂ and NO (1000 mg m⁻³), and NO removal efficiency reached 87.7%. It is well known that Fe²⁺ ions can promote the interaction between NO_x and S(IV) in aqueous solution to form nitrosyl complexes, which lead to enhanced NO_x removal.^20,21 Although the activation of transition metals can accelerate the oxidative absorption of NO_x, it is not suitable for recycling, and the product is heavy metal wastewater that causes secondary pollution. Therefore, it is necessary to use magnetic solid catalyst instead of ionic catalyst.

Previous studies have shown that spinel ferrite (chemical formula: M²⁺Fe₂³⁺O₄) has better mechanical stability, stronger mechanical strength, reoxidation ability and environmental friendliness than single metal oxide.^22–24 In actual use, MFe₂O₄ have acted as an oxygen supply and a catalyst.^25,26 The standard spinel structure is a lattice composed of 32 divalent oxygen ions that form a closed-pack, face-centered cubic arrangement with 64 tetrahedral interstitial sites and 32 octahedral interstitial sites.²⁷ In the spinel structure, M²⁺ and Fe³⁺ are respectively located in the sites of tetrahedron or octahedron composed of oxygen and have metal synergistic effects which determine good oxygen transferability and catalysis properties.^28,29 Therefore, spinel ferrites were attractive candidates because of their rich crystal chemistry, low cost, nontoxicity and environmentally friendly properties.^30,31 Xu et al.³² found that a small quantity of palladium-doped ferrite spinel could achieve high NO conversion and the catalyst cost reduction. However, the simultaneous use of transition metal oxide and MNB for NO_x removal in a system has rarely been reported. Furthermore, the synergistic effects of the different metals in ferrites for NO and SO₂ removal have been rarely mentioned.

In this study, the recyclable catalyst MFe₂O₄ (M = Mn, Co, Ni, Cu, Zn) synthesized with hydrothermal method and MNB were used to form the NO_x–SO₂–MFe₂O₄–MNB system for simultaneous removal of NO_x and SO₂ from flue gas. The structural, physical, and chemical properties of the as-prepared MFe₂O₄ before and after the reaction were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), the Brunauer–Emmett–Teller (BET) method and X-ray photoelectron spectroscopy (XPS). The removal rates of NO and SO₂ were monitored with the flue gas analyzer and ion chromatography to explore the actions of MNB and the effects of various divalent cations contained in the ferrite. The goal of this study was to evaluate the simultaneous NO and SO₂ removal efficiencies, and elucidate the chemistry and reaction pathways of the NO_x–SO₂–MFe₂O₄–MNB system.

2. Experimental

2.1. Materials

All the materials used for synthesizing catalysts, including ethylene glycol, poly(ethylene glycol) (PEG), sodium acetate (CH₃COONa), ferric chloride (FeCl₃·6H₂O), manganese chloride (MnCl₂·H₂O), zinc chloride (ZnCl₂), cupric chloride (CuCl₂·2H₂O), nickel chloride (NiCl₂·6H₂O), cobalt chloride (CoCl₂·6H₂O), ethyl alcohol (C₂H₆O), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification.

2.2. Synthesis of MFe₂O₄ (M = Mn, Co, Ni, Cu and Zn) by the hydrothermal method

The MFe₂O₄ (M = Mn, Co, Ni, Cu and Zn) microspheres were synthesized based on the hydrothermal method. Briefly, polyethylene glycol and sodium acetate were added into ethylene glycol under vigorous stirring conditions. Then partial chloride was added to reach the metal cation (Fe³⁺ and M²⁺) molar ratio of 2 : 1. The resultant homogeneous solution was obtained and then placed into the 500 mL Teflon-lined autoclave, reacted at 200 °C for 10 h. The black product was collected with a magnet, washed several times by ethanol and distilled water, and then dried at 80 °C for 6 h under vacuum. Finally, the resulting product was sufficiently grinded to obtain fresh MFe₂O₄ (M = Mn, Co, Ni, Cu and Zn).

2.3. Characterization

A scanning electron microscope (SEM, Hitachi S4800) was adopted to observe the morphologies. Energy dispersive spectrometer (EDS) with Horiba 7593-H was used to confirm the proportion of each element in the catalysts. Powder X-ray diffraction meter (XRD, D/max 2550 PC, Japan Rigaku) operation was completed with Cu K_α radiation in a 2θ angle between 10° and 80° at a rate of 2° min⁻¹ to monitor crystal structure. The surface properties and valence state of the catalysts were determined through X-ray photoelectron spectroscopy (XPS) experiments by using the Escalab 250Xi spectrometer with an Al-Kα radiation source at an energy step size of 1 eV to obtain high-resolution XPS spectra. The spectra were calibrated with respect to C 1s at a binding energy of 284.8 eV. The specific surface area of the samples was determined by using the Brunauer–Emmett–Teller (BET) method on an automated area and pore size analyzer (Autosorb-iQ) based on nitrogen adsorption desorption isotherm. The samples were outgassed at 200 °C for 2 h to remove remaining moisture and then analyzed with N₂ gas as an adsorbent at the temperature of liquid nitrogen. ˙OH and ˙O₂⁻ in the NO_x–SO₂–MFe₂O₄–MNB system were detected via electron paramagnetic resonance spectroscopy (EPR, JEOL-FA200), and DMPO was used for capturing ˙OH and ˙O₂⁻.

2.4. Reactivity measurement in the NO_x–SO₂–MFe₂O₄–MNB system

Fig. 1 shows the NO_x–SO₂–MFe₂O₄–MNB system installation diagram. Firstly, NO_x and SO₂ in the cylinder are fed into the mixer in a certain proportion to mix evenly. Gas flow rate is controlled by the rotameter. The flue gas analyzer (Testo350) was adopted to observe the intake gas concentration. Then a certain amount of MFe₂O₄ is uniformly dispersed in the absorption solution by ultrasonic oscillation. Finally, the configured gas and absorption solution are passed through the micro–nano bubble generator together to form the NO_x–SO₂–MFe₂O₄–MNB system in the reaction tower, so as to complete the catalytic oxidation and absorption process of NO_x and SO₂. The removal efficiency of NO_x was calculated by the concentrations of NO₂⁻ and NO₃⁻ in the absorption liquid measured with ion chromatography (ICS-1100 AS11-HC, Thermo Fisher Scientific).

Fig. 1 The NO_x–SO₂–MFe₂O₄–MNB system installation diagram.

The rated parameters of the micro–nano bubble generator were set as follows: the effluent pressure (0.5 MPa), the inlet water flow (300 mL min⁻¹) and the intake air amount (48 mL min⁻¹). The volume of the oxidation absorption reactor is 1.5 L. MFe₂O₄ was uniformly dispersed in a tubular reactor as a catalyst. A gas mixture containing 5113 mg m⁻³ NO_x and 3371 mg m⁻³ SO₂ was used as the intake gas. NO_x is mainly composed of 3795 mg m⁻³ NO and 2021 mg m⁻³ NO₂. In order to explore the practical application performance of the NO_x–SO₂–MFe₂O₄–MNB system, the effects of the addition of different recyclable MFe₂O₄ catalysts on the integrated desulfurization and denitrification efficiency were discussed by analyzing the IC test results of the absorption solution.

2.5. Analysis methods

The removal efficiency of NO_x can be calculated aswhere η_NOx represents the removal efficiency of NO_x, N_out is the amount of NO₂⁻ and NO₃⁻ in the absorption liquid, N_in is the amount of NO_x entering the system, c_NO and c_NO2 are the concentrations of NO and NO₂ in the intake air respectively, are the concentrations of NO₂⁻ and NO₃⁻ in the absorption liquid respectively, q is the air inflow, V is the volume of water in the rection tower, M is the molecular weight, t is the system running time.

The removal efficiency of SO₂ can be calculated as

where η_SO2 represents the removal efficiency of SO₂, is the amount of SO₄²⁻ in the absorption liquid, is the amount of SO₂ entering the system, c_SO2 are the concentrations of SO₂ in the intake air, are the concentrations of SO₄²⁻ in the absorption liquid.

3. Results and discussion

3.1. Physicochemical characterization

The SEM images of MFe₂O₄ (M = Mn, Zn, Cu, Ni and Co) are shown in Fig. 2. The five spinel ferrites are similar to each other, exhibiting a spherical structure with a dense surface and abundant fine particles. Among the five OCs, CuFe₂O₄ and CoFe₂O₄ exhibited more agglomeration and sintering, possibly due to high temperature calcination and the small-dimension effect. Compared with CuFe₂O₄ and CoFe₂O₄, MnFe₂O₄, NiFe₂O₄ and ZnFe₂O₄ are spherical with the uniform shape and distribution. Fig. 2 gives a detailed description of MFe₂O₄. MFe₂O₄ with the grain diameter of 100–350 nm. Among them, CoFe₂O₄ and CuFe₂O₄ had the smallest average particle sizes (127 nm for CoFe₂O₄ and 151 nm for CuFe₂O₄), which might be the cause for their relatively serious agglomeration. The particle sizes of various MFe₂O₄ showed the following decreasing sequence: MnFe₂O₄ > ZnFe₂O₄ > NiFe₂O₄ > CuFe₂O₄ > CoFe₂O₄.

Fig. 2 SEM images of MFe₂O₄: (a) MnFe₂O₄, (b) CoFe₂O₄, (c) NiFe₂O₄, (d) CuFe₂O₄ and (e) ZnFe₂O₄ (f) size distributions of MFe₂O₄ nanoparticles.

EDS analysis results (shown in Table 1) confirmed that the component contents of the samples are in reasonable agreement with the atomic percentages deduced from their molecular formulas. The elemental composition in Table 1 indicates that there are only three elements (O, M and Fe) in the composite. Furthermore, the M/Fe molar ratios of samples were approximately 0.5. The contents of the three elements in each complex MFe₂O₄ were 10.48 ± 0.99% (M, at%) and 22.18 ± 0.42% (Fe, at%), respectively.

Table 1 Physical and chemical properties of the investigated spinel catalysts

Catalysts	SBET (m^2 g^−1)	Elemental composition (atomic percentage, %)			Crystallite size from XRD (nm)
		M cation (Mn/Zn/Cu/Ni/Co)	Fe	M/Fe
MnFe2O4	22.243	10.83	22.26	0.487	15.94
CoFe2O4	121.352	11.94	22.72	0.458	11.61
NiFe2O4	72.727	10.21	21.63	0.472	10.07
CuFe2O4	30.009	9.24	21.92	0.422	12.78
ZnFe2O4	20.326	10.20	22.37	0.456	14.11

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Table 1 shows the specific surface areas of the five spinel catalysts. The five ferrites exhibited different specific surface areas due to their different metal compositions. Among the five OCs, CoFe₂O₄ had the largest specific surface area (121.352 m² g⁻¹), followed by NiFe₂O₄ (72.727 m² g⁻¹). The ferrites of MnFe₂O₄, CuFe₂O₄ and ZnFe₂O₄ had the smallest specific surface areas (less than 30 m² g⁻¹). The smaller the diameter of MFe₂O₄ nanoparticles was, the larger the specific surface area was (Fig. 2f and Table 1).

XRD measurements were conducted to identify the crystal structures of the samples. The XRD patterns of MFe₂O₄ (M = Mn, Zn, Cu, Ni and Co) are shown in Fig. 3. X-ray powder diffraction patterns of MnFe₂O₄, CoFe₂O₄, NiFe₂O₄ and ZnFe₂O₄ samples were quite similar to those of cubic spinel-type structures belonging to the Fd3̄m (227) space group. The main diffraction reflection peaks included (220), (311), (400), (422), (511) and (440) crystal planes, which well matched with the standard XRD pattern representing the cubic spinel structure (JCPDS no. 38-0430,22-1086, 10-0325, 34-0425 and 10-1108), indicating the good crystallinity of these MFe₂O₄.³³ Unlike these samples, however, CuFe₂O₄ exhibited a well-defined spinel ferrite structure with a strongest peak reflection peak at the plane (211) that matched well with the standard XRD pattern (JCPDS no. 34-0425) representing a tetragonal system (space group: I4₁/amd).³⁴

Fig. 3 XRD patterns of the investigated spinel catalysts MFe₂O₄ and their standard profiles.

The crystallite size D for each sample was estimated according to the most intense reflection peak (311) with Scherrer's equation (eqn (7)):³⁵

where D is the average crystallite size; K is Scherrer constant, 0.89; corresponds to the wavelength of Cu-Kα radiation, 0.154 nm; β is the full width at half maximum intensity for a reflection maximum; θ is the diffraction angle.

With Scherrer's Equation, crystallite sizes of MFe₂O₄ nanocomposites were calculated to be 13 nm (Table 1). The crystallite sizes of these OCs ranged from 10.07 to 15.94 nm. The crystallite sizes of the five MFe₂O₄ showed the following decreasing sequence: MnFe₂O₄ > ZnFe₂O₄ > CuFe₂O₄ > CoFe₂O₄ > NiFe₂O₄. As the crystal grains of OCs increased, the crystallinity became better, but the probability of agglomeration also increased. CoFe₂O₄ and NiFe₂O₄ were most prone to agglomerate because of their smallest grain sizes, which were consistent with the SEM results (Fig. 2).

In order to give an insight into the elemental chemical valence and electronic properties on the surface layers of MFe₂O₄, XPS measurements of MFe₂O₄ before and after the reactions were conducted to examine the chemical compositions and electronic structures of the samples. Fig. 4 shows the XPS spectra of MFe₂O₄, respectively. In full-scale XPS spectra (Fig. 4a), the apparent peaks of Fe 2p, C 1s, O 1s, Mn 2p, Co 2p, Ni 2p, Cu 2p and Zn 2p illustrated that iron, manganese, cobalt, nickel, copper and zinc were the main elements of MFe₂O₄. The comparison of the peak differentiation-imitating analysis results of the nano-MFe₂O₄ before and after the reaction (Fig. 4b–e) indicated that the binding energy of M 2p_3/2 was slightly reduced. After the reaction, the M(II) content in MFe₂O₄ decreased, whereas the M(III) content increased with a variation range of 8.5 ± 4.03%. Fe 2p_3/2 was found to have characteristic peaks of Fe(II) and Fe(III) before the reaction, and the contents Fe(II) and Fe(III) were 72.46% and 27.53%, respectively. After the reaction, the contents of Fe(II) and Fe(III) were changed to 66.67% and 33.33% (Fig. 4f). The chemical states of Fe were changed in the catalytic process, so the catalyst always maintained a certain activity.³⁶ These results demonstrated that Fe and M elements were chemically cyclically converted during the catalytic oxidation of NOx, while generating active sites for free radicals.

Fig. 4 XPS patterns of MFe₂O₄ in the NO_x–SO₂–MFe₂O₄–MNB system before and after reactions: full-range scan of the samples (a), Ni 2p_3/2 (b), Cu 2p_3/2 (c), and Fe 2p_3/2 of ZnFe₂O₄ (d).

3.2. Catalytic performance

The catalytic activities of the prepared samples of ZnFe₂O₄ nanocomposites against NO_x and SO₂ are shown in Fig. 5a. The ZnFe₂O₄ addition amount of 0.02 g promoted the removal of NO_x by the NO_x–SO₂–MFe₂O₄–MNB system, whereas the addition amount of 0.05 g inhibited the removal of NO_x. The addition of trace catalysts can promote the oxidation absorption of NO_x. However, excessive catalysts need to consume reactive oxygen species in the process of transformation to high price state, which forms a competitive relationship with the oxidation of NO_x and SO₂, resulting in the reduction of NO_x removal rate. In addition, with the increase of the amount of additives, the particle catalyst and micro nano bubbles collide with each other, leading to a part of bubbles in advance rupture, unable to produce hydroxyl radicals, resulting in a reduction of NO_x removal rate. Therefore, 0.02 g of iron-based spinel samples were separately added to the NO_x–SO₂–MFe₂O₄–MNB system.

Fig. 5 Effect of spinel catalyst on NO_x and SO₂ removal rate: ZnFe₂O₄ (a), MFe₂O₄ (b).

The NO_x removal efficiency is shown in Fig. 5b. The NO_x removal efficiency of the NO_x–SO₂–MFe₂O₄–MNB system was improved after the addition of MFe₂O₄. The combined efficiency of MNB and MFe₂O₄ in removing NO_x reached 70.23 ± 10.63%, while the efficiency of SO₂ removal reached 100% (eqn (17)–(21)). Oxidative absorption of NO_x involves two steps. NO_x is firstly oxidized into NO₂⁻, which is then oxidized to NO₃⁻ (eqn (13)–(16)). MnFe₂O₄, CuFe₂O₄ and ZnFe₂O₄ significantly improved the efficiency of the second step. After adding CuFe₂O₄, the efficiency of converting NO_x into NO₂⁻ increased by 10.16%, and the conversion into NO₃⁻ was 18.09%. Therefore, the comprehensive efficiency rate of NOx treatment by the NO_x–SO₂–CuFe₂O₄–MNB system reaches 83.88%. The catalytic activities showed the following decreasing sequence: CuFe₂O₄ > ZnFe₂O₄ > MnFe₂O₄ > CoFe₂O₄ > NiFe₂O₄.

3.3. Free radical identification

EPR test was conducted to detect the radicals and analyze the oxygen species of the NO_x–SO₂–MFe₂O₄–MNB system. As shown in Fig. 6a, weak signals were observed in MNB solution alone, and an intensive symmetrical four-line peak was detected in the NO_x–SO₂–MFe₂O₄–MNB system. The four-line peak with the intensity ratio of 1 : 2 : 2 : 1 in the spectrum is the typical spectral shape of the DMPO–˙OH adducts,³⁷ which indicates the existence of ˙OH in the system. However, the characteristic 1 : 1 : 1 triplet assigned to DMPO–˙O₂⁻ adducts³⁸ formed by ˙O₂⁻ and DMPO in Fig. 6b was very weak, indicating that the system mainly removes NO_x through ˙OH instead of ˙O₂⁻. The intensity of the peak represents the concentration of ˙OH in the system, indicating that the concentration of ˙OH decreases in turn: MNB/CuFe₂O₄ ≫ MNB > H₂O. This result was consistent with the removal rate of NO_x, which directly proved that ˙OH oxidized NO_x is the key to denitrification of flue gas. It can also be proved that MFe₂O₄ can be activated by MNB to produce ˙OH, which is dominant in the process of NO_x removal.

Fig. 6 EPR spectra of the NO_x–SO₂–MFe₂O₄–MNB system: DMPO–˙OH (a), and DMPO–˙O₂⁻ (b).

3.4. Proposed reaction pathways

It can be found that the NO_x–SO₂–MFe₂O₄–MNB system has a high catalytic efficiency as a heterogeneous catalytic system, based on characterizing the physical and chemical properties of MFe₂O₄ (M = Mn, Zn, Cu, Ni and Co) before and after the reaction and analyzing the SO₂ and NO_x removal efficiency. Hydroxyl radical is the main oxidant during the chemical oxidation of SO₂ and NO_x in aqueous solution. In this system, the sources of hydroxyl radicals are divided into two parts. Some hydroxyl radicals are derived from the rupture of MNB,^39,40 and the other part is derived from the activation of recyclable MFe₂O₄. The chemistry of simultaneous oxidation of NO_x and SO₂ by micro–nano bubbles breakdown and MFe₂O₄ activation is complex, since it takes place through electron transfer from the lattice oxygen, free radical oxidation reactions via ˙OH and ˙O₂⁻, and the direct reaction of the NO_x with HSO₃⁻.^41,42

The catalytic oxidation mechanism of NO_x–SO₂–MFe₂O₄–MNB system is described visually in the reaction scheme shown in Fig. 7. As the MNB slowly rise and shrink in the water, the charge ions (H⁺, OH⁻) rapidly concentrate at the very narrow gas–liquid interface. At the moment of MNB explosion, due to the drastic changes caused by the disappearance of the gas–liquid interface, the concentrated ions on the interface will immediately release the stored chemical energy, thus producing a large number of ˙OH. MFe₂O₄ is a magnetic semiconductor material with spinel structure. Oxygen in molecular state (O₂) is adsorbed on the metal surface and then dissociated into adsorbed atoms (O ad) because MFe₂O₄ is magnetic.⁴³ The type I adsorbed oxygen atom (O ad(I)) is first formed during dissociation, which is equivalent to a locally adsorbed oxygen atom on the metal surface. The oxygen atom in this state has extremely high reactivity to NOx. In addition, as a semiconductor material, MFe₂O₄ consists of a low energy valence band full of electrons and an empty high energy conduction band. In the excited state, the valence band electron jumps to the valence band, which is called the conduction band electron (e_CB⁻). At the same time, a highly active hole in the valence band is generated, which is called the valence band electron hole (h_VB⁺). The electron hole pair produces a large number of reactive oxygen species such as hydroxyl free radicals and hydrogen peroxide free radicals on the material surface, as shown in eqn (8)–(12). Reactive oxygen species is the main oxidant in the process of removing NO_x and SO₂. According to the double reduction oxidation mechanism, a kind of cations (Fe³⁺) perform the function of activating and oxidizing NO_x and SO₂, and their reoxidation depends on the lattice oxygen (O²⁻) transferred along the lattice. Another metal cation (M²⁺ (M = Mn, Co, Ni, Cu and Zn)) in the reduced state assumes the role of accepting gas-phase oxygen.

Fig. 7 Schematic diagram of the NO_x–SO₂–MFe₂O₄–MNB reacting system.

The main reactions involved in spinel catalytic oxidation are:

MFe₂O₄ → e_CB⁻ + h_VB⁺ (8)

e_CB⁻ + O₂ → ˙O₂⁻ (9)

˙O₂⁻ + H₂O → HOO˙ + OH⁻ (10)

OH⁻ + h_VB⁺ → ˙OH (11)

2HOO˙ + e_CB⁻→ ˙OH + O₂ + OH⁻ (12)

The main reactions involved in the simultaneous oxidation of NO_x and SO₂ are:

2NO + O₂ → 2NO₂ (13)

2NO₂ + H₂O → 2H⁺ + NO₃⁻ + NO₂⁻ (14)

NO + ˙OH → H⁺ + NO₂⁻ (15)

NO₂⁻ + ˙OH → NO₂ + OH⁻ (16)

NO₂ + ˙OH → H⁺ + NO₃⁻ (17)

NO + ˙O₂⁻ → NO₃⁻ (18)

SO₂ + H₂O ↔ HSO₃⁻ + H⁺ (19)

HSO₃⁻ ↔ SO₃²⁻ + H⁺ (20)

HSO₃⁻ + ˙OH → H₂O + SO₃˙⁻ (21)

SO₃²⁻ + ˙OH → SO₃˙⁻ + OH⁻ (22)

SO₃˙⁻ + 2˙OH → H₂O + SO₄²⁻ (23)

2NO₂ + SO₃²⁻ + H₂O → 2NO₂⁻ + SO₄²⁻ + 2H⁺ (24)

2NO₂ + 2SO₃²⁻ + H₂O → NO₂⁻ + NO₃⁻ + 2HSO₃⁻ (25)

4. Conclusion

This work developed a new technology that uses micro–nano bubbles and MFe₂O₄ to simultaneously remove NO_x and SO₂ from flue gas. The physicochemical properties of MFe₂O₄ prepared by the hydrothermal method were characterized. The results indicated that both micro–nano bubbles and MFe₂O₄ improved NO_x removal, while and the removal rate of SO₂ can reached 100% at room temperature. NO_x removal rate reached 83.88% in NO_x–SO₂–CuFe₂O₄–MNB system. At the moment when the MNB burst, the concentrated ions on the gas–water interface will release the stored chemical energy at once, thus producing a large number of hydroxyl radicals. Recyclable MFe₂O₄ (M = Mn, Zn, Cu, Ni and Co) can produce ˙OH and ˙O₂⁻ by being activated, thus accelerating the oxidation and absorption process of NO_x and SO₂. However, this research also points out many areas that require further research, such as using the system for organic gas and wastewater treatment, which is beyond the scope of current research, but is still the subject of future research.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant number U1660107], the Fundamental Research Funds for the Central Universities [grant number 2232020A-10].

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