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.

Introduction

Nitric oxide (NO) is a terrible air pollutant that can form acid rain, photochemical smog, and haze.[1] Stationary sources such as power plants and industrial boilers/furnaces[2,3] and mobile sources such as automobiles and ships are the main sources of NO emissions.[4,5] Selective non-catalytic reduction and selective catalytic reduction are mature flue gas denitrification technologies that have been widely applied in power plants and industrial boilers/furnaces, with ammonia and urea as the reductants.[6,7] As for the treatment of mobile source pollution, noble metal-induced three-way catalysis, such as Pd/Pt-based zeolite catalysts, is popularized in gasoline cars and diesel cars, with unburnt alkane and urea as the main reductants.[8,9] Intrinsically speaking, the aforementioned technologies belong to the harmless disposal of NO (NO → N2), rather than the recycling method. As an important N-containing substance, converting NO to high value-added products such as nitrate-based compounds (ammonium nitrate or calcium ammonium nitrate) is more in accordance with the circular economy concept. Taking full use of the advanced oxidation process (AOP) is one of the good methods to realize this target.

Integrating AOP oxidation with wet absorption is considered to be a promising method for recovering NO.[10] The hydroxyl radical (HO•), singlet oxygen (1O2), ozone (O3), sulfate radicals (SO4•–), oxychloride radical (ClO•), and chlorine dioxide (ClO2) have been used to oxidize NO into nitrate. These radicals usually originated from common oxidants such as H2O2,[11] persulfate (PS),[12] oxone (PMS),[13] NaClO2,[14] and NaClO,[15] and so on, and catalytic means including ultraviolet/vacuum ultraviolet (UV/VUV),[16] transition metals,[13,17,18] thermal,[12] microwave,[19] and sonic[20] have been used to yield these radicals. Compared with other catalysis methods, the UV- or VUV-induced AOP method is better from the view of practical application, which is due to the following advantages: (1) insensitive to the solution pH, (2) less energy consumption, (3) cost-efficient, and (4) less secondary environmental impact. UV-AOP methods mainly include UV/H2O2,[16] UV/PS and UV/PMS,[21,22] UV/NaClO,[23] and UV/NaClO2,14 and so forth.

In previous work,[24] we had demonstrated that UV/ClO2– showed an excellent performance in the removal of NO, with the efficiency of 99.1%. Moreover, the ClO2 generated from the photodecomposition of ClO2– (eqs and 2) played a leading role in the NO removal process, but the yield of NO2 was high since the yield of ClO• was low. To deal with this problem, NH4OH was selected as an additive to suppress the formation of ClO2, meanwhile increasing the ClO• yield (eq ). Furthermore, we also systematically compared the performance and cost-effectiveness of the five typical UV-AOP methods (UV/H2O2, UV/PS, UV/PMS, UV/NaClO, and UV/NaClO2) in the removal of NO and demonstrated that NaClO2 possessed the highest absorbance at 254 nm (124.3 abs/mol), implying that it had the highest quantum yield. The experimental results indicated that UV/NaClO2 was far superior to other UV–AOP methods in terms of the NO removal. The evolution of the radicals in the five UV–AOP systems under different pH and different SO2 concentrations was unveiled. However, the reaction behavior of the fly ash during the NO removal process was not studied; thus, further exploring that the reaction behavior as well as its influencing mechanism in the NO removal is of great significance.

As we all know, fly ash is a common component present in industrial flue gas, and it may affect the UV light penetration or quench the radicals to affect the oxidation process. However up to now, few studies have reported its influence on the UV-AOP method’s ability to remove NO, especially the latest UV/NaClO2. As well, the influencing mechanisms of different kinds of fly ashes on NO removal are also not clear now. Hence, the objective of this paper is to investigate the effects of different kinds of fly ashes on NO removal using the UV/NaClO2 AOP method. The investigated fly ashes were sampled from some coal-fired power plants and steel mills. Based on the characterization analyses of different fly ashes and the experimental results on the NO removal in the presence of different fly ashes, the main inhibitors (metal oxides or non-metal oxides) in fly ashes were identified and their influencing mechanisms on the NO removal were revealed. The information provided in this paper may be of great significance for the application of UV/NaClO2 in flue gas denitrification.

Materials and Methods

Chemicals

All chemicals used in this study are of analytical grade. The NaClO2 solution (0.5 mM) was prepared with NaClO2 powders (80% wt) and deionized water. According to the characterization analysis results of different fly ashes, we used pure metal oxides, that is Fe2O3 (96.5% wt); Fe3O4 (99.0% wt); SiO2 (99.0% wt); Al2O3 (99.0% wt); ZnO (99.0% wt); MgO (98.0% wt); TiO2 (99.0%); carbonates, that is Na2CO3 (99.5% wt) and CaCO3 (99.0% wt); and chlorides, that is NaCl (99.5% wt), KCl (99.5% wt), and MgCl2 (99.0% wt) to simulate the components in fly ashes. These chemicals were purchased from the Aladdin Company. We used six kinds of fly ashes to conduct the NO removal experiments. They were sampled from the steelmaking factory, coal-fired power plant, blast furnace, and thermal power plant. Specifically, the first fly ash was sampled from a bag filter in some steelmaking factory. The second and third fly ashes are fine and coarse fly ashes, respectively, which were sampled from the fourth electric field and the first electric field of an electrostatic precipitator in some coal-fired power plant. The fourth fly ash was sampled from a bag filter in some blast furnace. The fifth fly ash was sampled from an electrostatic precipitator in some thermal power plant. The sixth fly ash is a kind of fly ash with high alkalinity, which was sampled from some coal-fired power plant in the northwest of China.

Experimental Apparatus

Figure shows the experimental flow chart, which consists of simulated flue gas generation, a UV photolysis reactor, and tail gas detection. The core part is a cylinder and a jacketed quartz-wall UV-photolysis reactor, which is heated with a thermostat water bath. The diameter and height of the inner and outer cylinders are 60/96 mm and 140/200 mm, respectively. The low-pressure lamp (TUV PS-S, Philips Co., Beijing, 12 W, and the light intensities are 2.54 × 10–4 E·s–1) is placed inside the cylinder. The temperature and pH of the composite solution are detected online using an inside thermocouple and a pH meter. During the experiments, the NaClO2 solution will be first mixed with the fly ash and then be used to conduct the NO removal experiments. After drying the tail gas, the NO concentration of the inlet and outlet flue gases are detected by using an infrared flue gas analyzer (Photon, Madur Co, Austria). The efficiencies of NO conversion were calculated by eq .where η is the removal efficiency after an appropriate reaction time; Cin and Cout are the inlet and outlet concentrations of NO, mg/m3, respectively.

Figure 1

Experimental flow chart.

Analytical Methods

Scanning electron microscopy (SEM) (Zeiss Ultra 60, Carl Zeiss NTS, LLC North America) was used for imaging the surface morphologies of different fly ashes. The fly ashes were fixed on stubs with carbon dots and then sputter-coated with a 2 nm gold layer. Coated samples were examined under an accelerating voltage of 5 kV at different magnifications. Energy-dispersive X-ray spectra (EDS) of different fly ashes were also obtained using an energy dispersive spectroscopy analyzer (XF lash 5060FQ Annular EDX detector, Bruker, Germany). The binding energies of Na 1s, Mg 1s, Al 2p, K 2p, Ca 2p, Fe 2p, and Zn 2p in different fly ashes were analyzed by using X-ray photoelectron spectroscopy (XPS) (ESCALAB250 spectrometer with an Al Kα source (1486.6 eV)). The detection conditions are 10 kV voltage with a base pressure of 2 × 10–9 Mbar. The XPS characterization was conducted after drying and grinding the fly ashes. The contents of the metals in the six fly ashes were measured using an inductively coupled plasma source mass spectrometer (ICP–MS, 7700, Agilent Technology Co., USA).

Results and Discussion

Fly Ashes Characterizations

Figure a–f show the SEM images and photographs of the six kinds of fly ashes. It can be found that the first and second fly ashes are presented in the form of adhesive superfine particles (<10 μm), and parts of them are agglomerated into blocks (the diameter ranges from tens to hundreds of microns). However, their colors are totally different: the first fly ash is reddish brown and the second fly ash is yellowish white. By contrast, the third and fourth fly ashes are presented in the form of incompact large particles (the diameter ranges from tens to hundreds of microns), and they look like stones with sharp edges and corners. It can be found in the photographs that the third fly ash is more like sand with a tan color, while the color of the fourth fly ash is black brown. Also, the fifth and sixth fly ashes are presented in the form of fine powders which are like flour and mainly consist of spheroidal particles. The color of the fifth fly ash is light gray and that of the sixth fly ash is pure white. According to the photographs, we could conclude that the fly ashes sampled from power plants were presented in light colors, while those sampled from steelmaking and blast furnaces were presented in deep colors. The results suggest that after thorough combustion, most of the combustible substances will be burned out by power plant boilers, and only a large number of inorganic salts will be remained; however, due to the high contents of metal oxides, the fly ashes sampled from steelmaking and the blast furnace will appear in the color of metal compounds.

Figure 2

SEM–EDS analyses of the six fly ashes. (a) First fly ash; (b) second fly ash; (c) third fly ash; (d) fourth fly ash; (e) fifth fly ash; and (f) sixth fly ash.

Figure also provides the EDS analysis results by which the elemental proportions are obtained, and the detailed data are available in Table S1. For the first fly ash, O and Fe are the main elements which account for 37.0% wt and 36.0% wt, respectively, suggesting that Fe is the most abundant metal in the first fly ash. The proportions of the other elements are 17.7% wt for Ca, 3.0% wt for C, 1.3% wt for Na, 0.6% wt for Mg, 1.2% wt for Al, 1.2% wt for K, and 0.7% wt for Mn, so the first fly ash possesses an abundant variety of metals. The blast furnace is also one of the steelmaking links, thus the fourth fly ash may also have abundant metals. However, it can be found in the fourth fly ash that the contents of Fe and O account for only 9.6 and 22.0% wt, respectively, while the contents of Zn and C are as high as 15.0 and 39.8%, respectively, so the fourth fly ash has a large amount of unburnt carbon and a high-value metal, Zn. The resource for the unburnt carbon mainly originated from the reductant coke. The second and third fly ashes are fine and coarse fly ashes sampled from the same coal-fired power plant. It can be found that compared with the coarse fly ash, the fine fly ash has more Cl, K, and Na but less O, Si, Ca, and Fe, so NaCl and KCl are easily enriched on fine particles, but Fe2O3, CaO, and SiO2 are inclined to attaching onto the coarse fly ash. The fifth and sixth fly ashes are all sampled from the power plants in the northwest of China, and they are famous for their high alkalinity. The EDS analysis results of the fifth and sixth fly ashes verify this point: the contents of Al, Si, O, and C are as high as 10.9% wt/14.0wt, 12.0% wt/10.9% wt, 41.3% wt/44.2%, and 32.2% wt/27.5% wt, respectively. The results suggested that the alkaline Al2O3 and SiO2 might suppress the coal’s thorough combustion, resulting in a high content of unburnt carbon. One thing should be noted that the third, fifth, and sixth fly ashes also have little Ti, so these fly ashes may also contain TiO2. In addition, the reddish brown color of the first fly ash and the black brown color of the fourth fly ash may be due to the presence of the high content of Fe and Zn, respectively.

As a result of the EDS analysis results, which can only give the elemental proportion of the fly ash surface, we further performed the ICP analysis to accurately unveil the metal constitution of the six kinds of fly ashes. Table lists the ICP analysis results, including the metal content and metal proportion. The main metals in the first fly ash are Fe ≫ Ca ≫ Mg > Mn > K > Na > Al, in which the proportions of Fe and Ca are 46.43 and 6.76%, respectively. The main metals in the second fly ash are Fe > K > Zn > Ca > Na ≫ Mg > Al > Ti > Mn, in which the proportions of Fe, K, and Zn are 21.74, 12.62, and 6.57%, respectively. The main metals in the third fly ash are Fe ≫ Ca > K > Mg > Al > Ti > Na, in which the proportion of Fe is 34.15%. The main metals in the fourth fly ash are Fe ≫ Zn > Ca ≫ Al > Na > K, in which the proportions of Fe and Zn are 14.15 and 3.55%, respectively. The main metals in the fifth fly ash are Al > Fe > Ti > K > Ca, in which the proportions of Al and Fe are only 3.84 and 2.33%, respectively. The main metals in the sixth fly ash are Al > Fe > Ti > Ca > K, in which the proportions of Al and Fe are only 7.12 and 2.19%, respectively. From the view of elemental content, the Fe content in the first four fly ashes is hundreds of g per kg of fly ash, so Fe compounds should be the primary inorganic salt in the first four samples; also, the contents of Ca, K, Mg, Zn, and Na are high in different levels; the contents of Al and Fe in the last two fly ashes are tens of g per kg of fly ash. Thus, Al and Fe compounds should be the primary inorganic salt in the last two samples.

Table 1

ICP Analysis Results of the Six fly Ashes

sample	element	elemental content (mg/kg)	elemental proportion (%)
the first fly ash	Fe	464312.9	46.43
	Ca	67636.9	6.76
	Mg	13306.8	1.33
	Mn	9001.1	0.90
	K	8469.3	0.85
	Na	7768.3	0.78
	Al	2613.8	0.26
the second fly ash	Fe	217434.3	21.74
	K	126217.2	12.62
	Zn	65720.2	6.57
	Ca	45294.4	4.53
	Na	32200.1	3.22
	Mg	7201.2	0.72
	Al	4949.5	0.49
	Ti	3082.0	0.31
	Mn	763.1	0.08
the third fly ash	Fe	341506.5	34.15
	Ca	52074.7	5.21
	K	11016.8	1.10
	Mg	10532.4	1.05
	Al	7140.4	0.71
	Ti	6463.5	0.65
	Na	4212.5	0.42
the fourth fly ash	Fe	141503.6	14.15
	Zn	35530.3	3.55
	Ca	23149.6	2.31
	Al	8569.2	0.86
	Na	2268.0	0.23
	K	1636.8	0.16
the fifth fly ash	Al	38443.2	3.84
	Fe	23257.4	2.33
	Ti	5436.6	0.54
	K	4380.4	0.44
	Ca	3512.7	0.35
the sixth fly ash	Al	71199.4	7.12
	Fe	21944.5	2.19
	Ti	6951.1	0.70
	Ca	5551.2	0.56
	K	1182.3	0.12

Furthermore, X-ray photoelectron spectroscopy was performed to determine the elemental oxidation states of the six fly ashes. All the data concerning the peak separation refer to the NIST X-ray Photoelectron Spectroscopy Database.[25]Figure shows the spectra of Na 1s, Mg 1s, Al 2p, K 2p, Ca 2p, and Fe 2p of the first fly ash, in which the peak at 1071.0 eV is ascribed to NaCl or Na2SO4; the peaks at 1302.7 and 1303.9 eV are due to the presence of Mg(OH)2 and MgO, respectively; the peak at 74.9 eV is assigned to Al2O3; the peaks at 292.7 and 295.2 eV are all assigned to KCl; the peaks at 346.5 and 350.3 eV are ascribed to CaO and CaCO3, respectively; and the peaks at 710.9 and 724.0 eV are assigned to Fe2O3 and Fe3O4, respectively. Figures –8 represent the XPS spectra of the other five fly ashes, and Table S2 lists all the binding energies and the corresponding compounds. For the second fly ash, only Na 1s, Al 2p, K 2p, Ca 2p, and Fe 2p are detected. The peak at 1072.1 eV is attributed to NaCl or Na2CO3, the peaks at 74.6 and 77.3 eV are all ascribed to Al2O3, the peaks at 293.1 and 295.8 eV are all assigned to KCl, the peaks at 347.6 and 351.2 eV are ascribed to CaO and CaCO3, respectively, and the peaks at 710.8 and 723.9 eV are attributed to Fe2O3 and Fe3O4, respectively. The constitution of the third fly ash is the same as that of the second fly ash, namely NaCl/Na2CO3, Al2O3, KCl, CaO/CaCO3, and Fe2O3/Fe3O4 because they are generated under the same combustion conditions. As for the fourth fly ash, the XPS spectra of Na 1s, Al 2p, K 2p, Ca 2p, Fe 2p, and Zn 2p were observed, and the peak separation process is the same as mentioned above. A new peak at 1022.5 eV corresponding to Zn 2p is assigned to the formation of ZnO. Hence, the constitution of the fourth fly ash is NaCl/Na2CO3, Al2O3, KCl, CaO/CaCO3, Fe2O3/Fe3O4, and ZnO. The fifth and sixth samples are highly alkaline fly ashes, and only the XPS spectra of Al 2p, Ca 2p, and Fe 2p are detected. The fifth fly ash possesses the metal compounds of Al2O3, CaO/CaCO3, and Fe2O3/Fe3O4, and the sixth fly ash has the metal compounds of Al2O3, CaCO3, and Fe2O3.

Figure 3

XPS analysis of the fly ash sampled from a steelmaking factory (the first fly ash). Na 1s (A); Mg 1s (B); Al 2p (C); K 2p (D); Ca 2p (E); and Fe 2p (F).

Figure 4

XPS analysis of the fine fly ash sampled from some coal-fired power plant (the second fly ash). Na 1s (A); Al 2p (B); K 2p (C); Ca 2p (D); and Fe 2p (E).

Figure 8

XPS analysis of the high alkaline fly ash sampled from some coal-fired power plant (the sixth fly ash). Al 2p (A); Ca 2p (B); and Fe 2p (C).

XPS analysis of the coarse fly ash sampled from some coal-fired power plant (the third fly ash). Na 1s (A); Al 2p (B); K 2p (C); Ca 2p (D); and Fe 2p (E).

XPS analysis of the fly ash sampled from some blast furnace (the fourth fly ash). Na 1s (A); Al 2p (B); K 2p (C); Ca 2p (D); Fe 2p (E); and Zn 2p (F).

XPS analysis of the fly ash sampled from some thermal power plant (the fifth fly ash). Al 2p (A); Ca 2p (B); and Fe 2p (C).

Effects of Fly Ashes and the Corresponding Inorganic Salts on the NO Removal

Based on the above ICP and XPS analysis results, we can carry out the following investigating experiments more effectively. Then, we studied the effects of the six fly ashes on the NO removal by using UV/NaClO2. As shown in Figure A, without fly ash, the NO removal efficiency obtained by using UV/NaClO2 alone is 85.6%. After adding the six fly ashes, the NO removal efficiency is sharply decreased from 85.6 to 45, 33, 66, 60, 52, and 66%, respectively, which demonstrates that fly ash is a powerful inhibitor which can greatly suppress the radical-induced oxidation of NO. Furthermore, the inhibition order is 2 > 1 > 5 > 4 > 3 = 6. The inhibitory effect of the fly ashes may be due to the following reasons: (1) the inorganic salts in the fly ash may quench the radical species such as •OH, ClO•, and Cl2O2,[26−28] resulting in the decrease in the radical yield and radical activity, so the oxidation efficiency of NO is declined; (2) on the other hand, adding fly ash will make the solution turbid and further impact UV light penetrating, so as a result, the ClO2– molecules are difficult to effectively absorb the UV254 photons, resulting in the decrease in the oxidation ability of the reaction system. In order to verify the above speculation and find out which factor is the main reason causing the decrease in NO removal efficiency, the following experiments were carried out.

Figure 9

Effect of the six fly ashes on the NO removal using the UV/NaClO2 method (A); effect of chloride on the NO removal (B); effect of carbonate on the NO removal (C); effects of Al2O3 and SiO2 on the NO removal (D); effects of Fe2O3 and Fe3O4 on the NO removal (E); and effects of TiO2, MnO2, and ZnO on the NO removal (F). The NaClO2 concentration is 5 mM, the UV254 light power is 14 W, the volume of reaction solution is 500 mL, and the reaction temperature is 50 °C.

In previous studies, it had been demonstrated that the free radicals could react with the chloride ions and the carbonate ions to form various secondary free radicals, such as ClOH•–, Cl2•–, and CO3•–.[26,27,29,30] According to the EDS and XPS analysis results, the six fly ashes have chloride and carbonate, so we first studied their effects on the NO removal using NaCl, MgCl2, and KCl as the chemicals to simulate the chlorides. As shown in Figure B, when NaCl is used as the chloride and with the Cl concentration rising from 1.1 to 7.4 mM, a slight decrease in the NO removal efficiency from 85.6 to 82.5% appears. When NaCl is substituted with MgCl2, the NO removal efficiency is constant at around 85%. If KCl is used and the concentration is adjusted from 0.2 to 12.2 mM, the NO removal efficiency is stable in the range of 82–85%. Hence, from the aforementioned experimental results, chlorides exhibited a slight inhibitory effect on UV/NaClO2 removing NO, and the excess chloride did not significantly suppress the NO removal process. According to the studies,[29,30] although Cl– could quench radicals such as HO• to decline the oxidation capacity of the reaction system, it could also produce a number of secondary free radicals such as Cl• and ClOH•– (eqs and 6), and the in situ-produced Cl• and ClOH•– were reported to be capable of being useful for the removal of NO. Hence, adding chloride will not significantly affect the NO removal process.

Figure C shows the impact of the carbonate on UV/NaClO2 in removing NO. When we used Na2CO3 as the carbonate and increased the CO32– concentration to 0.6–3.7 mM, the NO removal efficiency decreased from 85.6 to 73.3–74.7%; but if we used CaCO3 as the carbonate and increased its concentration to 0.9–8.9 mM, the NO removal efficiency just decreased from 85.6 to 79.5–82.1%. It could be found that both of the two carbonates had an inhibitory effect on UV/NaClO2 in removing NO, but the inhibition resulting from Na2CO3 was larger than that of CaCO3. The difference in the inhibitory effect is mainly due to the difference in the solubility of the two carbonates: Na2CO3 is freely soluble while CaCO3 is slightly soluble. According to the previous studies,[26,27] CO32– could quench HO• to form the secondary radical CO3•– (eq ), but the oxidation capacity of CO3•– is weaker than that of HO•, so the NO removal efficiency was decreased in the presence of carbonate. We could also find that the variation in the carbonate content would not greatly impact the NO removal process, implying that CO32– only affected the yield of HO• and not the yield of Cl radicals. Moreover, Cl radicals such as ClO2, ClO•, and Cl2O2 contributed more to the NO removal, which had been demonstrated by the previous studies.[14] Thus, the NO removal efficiency was not significantly changed with the variation in carbonate concentrations.

Furthermore, we studied the effect of the aluminum salt and silicon salt on the NO removal. According to the XPS analysis results, Al2O3 and SiO2 are the main existing forms of Al and Si in the fly ashes, so we used Al2O3 and SiO2 as the chemicals to study their effects on NO removal. As shown in Figure D, as the Al2O3 concentration increases to 0.4–5.0 mM, the removal efficiency of NO significantly decreases from 85.6 to 63–81%. Thus, Al2O3 was demonstrated to be a strong radical inhibitor for the UV/NaClO2 denitrification system. As for the effect of SiO2, it can be found that the NO removal process will be unaffected by SiO2, and the removal efficiency of NO is constant at 85%, so SiO2 is harmless for the radical-induced oxidation of NO. As we all know, the light transmittance of Al2O3 is less than that of SiO2; thus, the light propagation in Al2O3 is more difficult than that of SiO2. Hence, due to the decrease in the UV luminous flux, the radical yield would be greatly decreased after Al2O3 addition. As a result, the NO removal process was suppressed.

According to the EDS, XPS, and ICP analysis results, Fe is the most abundant metal in the six fly ashes and the main existing forms of Fe are Fe2O3 and Fe3O4. Therefore, the effects of Fe2O3 and Fe3O4 on the removal of NO were further studied. As shown in Figure E, when the concentrations of Fe2O3 and Fe3O4 increase to 0.13–6.4 and 0.08–4.3 mM, respectively, the NO removal efficiency significantly decreases to 41.5–79.5 and 53.0–73.3%, respectively. Consequently, the inhibitory impacts of Fe2O3 and Fe3O4 would be intensified with the increase in Fe contents, and their inhibitory impacts are comparable. Fe2O3 and Fe3O4 have deep colors, which can reduce the luminous flux of UV light, and the photon utilization rate would be decreased and the oxidation of NO would be inhibited. Besides, Fe3O4 and Fe2O3 would be decomposed into Fe2+ under UV light irradiation (eq ),[18,20] and the produced Fe2+ would then consume OH• (eq ), ClO2, ClO•, and Cl2O2, causing the decline of the NO removal efficiency.

The contents of Ti, Mn, and Zn in fly ash are relatively small, but they may also affect NO removal, so we studied the effects of TiO2, MnO2, and ZnO on the NO removal. As shown in Figure F, when 0.3 mM TiO2, 0.3 mM MnO2, and 4.6 mM ZnO are used, the NO removal efficiencies decrease to 72, 68, and 67%, respectively. TiO2, MnO2, and ZnO have strong shielding effects in UV light, so they can reduce the luminous flux, resulting in the decrease in radical yield and inhibiting the oxidation of NO. Besides, the addition of fly ash will also increase the turbidity of the solution. In addition, the turbidity will hinder the passage of light in the solution. Thus, we investigated the turbidity on the NO removal, and the results are shown in Figure S1. It can be seen that when the turbidity rises from 1 to 900, the removal efficiency of NO is almost unchanged, suggesting that the turbidity of the solution is not the main inhibitory factor for the radical-induced oxidation of NO.

Mechanism Analysis

In order to reveal the influencing mechanism of the aforementioned fly ashes as well as their metal oxides on the removal of NO, we used pure metal compounds including Fe2O3/Fe3O4, NaCl/Na2CO3, CaO/CaCO3, KCl, SiO2, Al2O3, MgCl2, TiO2, ZnO, and MnO2 to simulate the fly ash slurries to conduct the NO removal experiments. Figure A provides comparable photographs of the real and simulated fly ash solutions. It can be found that the first, the fifth, and the sixth simulated fly ash solutions are presented in a similar form and color to those of the real ones; the third and the fourth simulated fly ash solutions are different from the real ones, which is mainly due to the fact that the third and the fourth real fly ashes are more like stones (as illuminated in SEM images). Thus, they cannot be evenly dispersed in the solution without violent agitation. On the other hand, the simulated samples are totally prepared with pure chemicals, while the real ones definitely contain lots of impurities, so the simulated fly ash solution is more transparent and brighter, therefore they cannot be totally the same in color.

Figure 10

Comparable photographs of the real fly ash and simulated fly ash solutions (A); effect of the real and simulated fly ashes on the NO removal (B); inhibitory contribution rates of different metal compounds to the NO removal (C); absorbance of the six fly ashes under UV254 light (D); absorbance of different metal oxides and anions under UV254 light (E). The NaClO2 concentration is 5 mM, the UV254 light power is 14 W, the volume of reaction solution is 500 mL, and the reaction temperature is 50 °C.

Accordingly, we performed comparing experiments on NO removal in the presence of the simulated and real fly ashes. The corresponding experimental results are shown in Figure B. It can be found that the NO removal efficiencies obtained in the presence of simulated fly ashes are close to those obtained in the presence of real fly ash, and the differences in the NO removal efficiencies are only 3.74, 27.12, 10.65, 2.81, 1.07, and 3.15%, respectively. In consideration of the allowable error bars, the inhibitory effect caused by the simulated fly ashes can be considered basically consistent with the real ones. Therefore, the harmful impact resulting from the six fly ashes on the NO removal is mainly due to the synergistic function of different types of metal or non-metal compounds.

In order to determine the inhibitory contribution rates of different metal compounds to the NO removal, we got the suppression sum of the different metal oxides and, respectively, calculated their inhibitory contribution rates to the NO removal. As shown in Figure C, for the first fly ash, the order of the inhibitory contribution rate on the NO removal is Fe2O3/Fe3O4 (45.7%) > MnO2 (19.0%) > NaCl/Na2CO3 (12.6%) > CaCO3 (7.1%) > Al2O3 (6.4%) > KCl (3.8%) ≈ SiO2 (3.0%) ≈ MgCl2 (2.4%). For the second fly ash, the order of the inhibitory contribution rate is Fe2O3/Fe3O4 (35.4%) > CaCO3 (18.0%) > Al2O3 (16.5%) > NaCl/Na2CO3 (12.6%) > KCl (9.3%) ≈ SiO2 (8.1%). For the third fly ash, the order of the inhibitory contribution rate is Fe2O3/Fe3O4 (37.4%) > NaCl/Na2CO3 (18.9%) > TiO2 (14.1%) > CaCO3 (9.0%) = Al2O3 (9.0%) > SiO2 (4.2%) > KCl (3.4%) ≈ MgCl2 (3.3%). For the fourth fly ash, the order of the inhibitory contribution rate is ZnO (28.3%) > Fe2O3/Fe3O4 (25.5%) > NaCl/Na2CO3 (18.1%) > CaCO3 (10.1%) > Al2O3 (9.2%) > KCl (4.6%) ≈ SiO2 (4.2%). For the fifth fly ash, the order of the inhibitory contribution rate is Fe2O3/Fe3O4 (29.2%) > TiO2 (25.5%) > Al2O3 (24.1%) > CaCO3 (9.0%) > SiO2 (6.8%) ≈ KCl (5.5%). For the sixth fly ash, the order of the inhibitory contribution rate is Al2O3 (39.6%) > Fe2O3/Fe3O4 (22.9%) > TiO2 (17.0%) > CaCO3 (10.7%) > KCl (5.2%) ≈ SiO2 (4.7%). It can be found that the inhibitory contribution rate of Fe2O3/Fe3O4 is the highest among all of the metal compounds. Additionally, the inhibitory contribution rates of NaCl/Na2CO3, Al2O3, TiO2, ZnO, and MnO2 are relatively high in consideration of their contents in the fly ashes. Specifically, the content of Ti in fly ash is much smaller than that of Zn, inferring that the inhibitory effect of Ti is stronger than Zn under equal content. The inhibitory contribution rate of Al is affected by its content in fly ash. If the contents of Al and Zn are close in fly ash, their inhibitory effects are very close, which indicates that their inhibitory effect on the NO removal is comparable. The inhibitory contribution rates of MnO2, CaCO3, and NaCl/Na2CO3 are also very close. However, the content of Mn in fly ash is much smaller than that of Ca and Na. Therefore, the inhibitory effect of MnO2 is higher than that of CaCO3 and NaCl/Na2CO3. As illuminated by the ICP results, the content of Ca in fly ash is much higher than that of Na; thus, the inhibitory contribution rate of Na2CO3 is significantly higher than that of CaCO3. Combining with the experimental results in Figure B,C, it can be found that the inhibitory effect of NaCl is far less than that of NaCO3. Hence, the order of the inhibitory contribution rate to the NO removal of CaCO3, NaCl, and Na2CO3 is Na2CO3>CaCO3>NaCl. As for the inhibitory effects of KCl, MgCl2, and SiO2, their inhibitory contribution rates are similar and will not be significantly changed with their content variation. Finally, according to the above analyses combined with the EDS, XPS, and ICP characterization analyses and the experimental results, the order of the overall inhibitory contribution rates of all of the metal compounds to the NO removal can be concluded as follows: Fe2O3/Fe3O4 > TiO2 ≈ Na2CO3 > Al2O3 ≈ ZnO ≈ MnO2 > CaCO3 > NaCl > KCl ≈ SiO2 ≈ MgCl2.

How do these metal compounds affect the radical-induced oxidation of NO? We need to illuminate this issue. As mentioned in Section , anions such as Cl– and CO32– can quench the free radicals to form various secondary free radicals such as ClOH•–, Cl2•–, and CO3•– and then further impair the oxidation capacity of the UV/NaClO2 system. The other metal compounds may not be capable to rapidly scavenge the free radicals, but they can impact the UV light penetration and absorb the high-energy photons to suppress the radical formation. Therefore, we further tested the absorbance of the six fly ashes as well as their metal oxides under UV254 light, and the results are shown in Figure D,E. It can be found that adding the first, second, fifth, and sixth fly ashes can greatly increase the absorbance of the NaClO2 solution, suggesting that these four fly ashes can effectively absorb the UV254 light. However, it can also be found that adding the third and fourth fly ashes does not increase the absorbance, which is mainly because their particle sizes are so big and they cannot be dissolved in the solution, as demonstrated in Figure A. By contrast, the turbidities of the first, second, fifth, and sixth fly ashes are higher, indicating that the metal compounds in these four fly ashes were solved or at least suspended in the solution. Hence, the relatively high solubility is the premise condition to compare the inhibitory contributions of different fly ashes to NO removal. We further studied the UV254 light absorbance of different metal oxides and anions at equivalent levels to compare their shading effects on the UV254 light. As shown in Figure E, their light shading effects in a descending order are TiO2 (799.9 Abs/mol) > MnO2 (558.5 Abs/mol) > Fe3O4 (398.8 Abs/mol) > Fe2O3 (358.1 Abs/mol) > CO32- (232.8 Abs/mol) > SiO2 (144.5 Abs/mol) > Al2O3 (131.9 Abs/mol) > ZnO (106.2 Abs/mol) ≈ Cl– (78.2 Abs/mol). Hence, TiO2, MnO2, Fe3O4, Fe2O3, and CO32– could greatly suppress the UV light utilization and decrease the radical yield.

Based on the aforementioned analyses, it can be concluded that if the fly ashes can be highly dissolved in the solution, the following metal oxides will greatly affect the UV light utilization. (1) Because of the high content, Fe2O3/Fe3O4 will act as the primary inhibitor for the radical formation; (2) soluble anions such as Cl– and CO32– will change the solution constitution to change the radical species to decline the oxidation capacity of the reaction system; and (3) during the aforementioned process, TiO2, MnO2, SiO2, Al2O3, and ZnO with low contents will also participate in the inhibitory reaction to affect the radical-induced oxidation of NO from two aspects: one is the absorption of high-energy photons, and the other is inducing radical quenching or making the free radicals inactive.

Conclusions

In this paper, the influencing mechanisms of six typical fly ashes on the NO removal by using UV/NaClO2 were studied. The micromorphology, elemental composition, and the elemental oxidation states of the six fly ashes were revealed. The inhibitory effect of the six fly ashes on the NO removal was illuminated by changing the composition and content of different metal compounds. The mechanism analysis suggested that high solubility is the premise condition for the fly ashes exhibiting an inhibitory effect on the NO removal. Fe2O3/Fe3O4 exhibited the highest inhibitory contribution rate to NO removal. The anions of Cl– and CO32– acted as scavengers to quench the free radicals and impair the oxidation capacity of the UV/NaClO2 system. The order of the overall inhibitory contribution rate of metal compounds in fly ash on the NO removal was determined as Fe2O3/Fe3O4 > TiO2 ≈ Na2CO3 > Al2O3 ≈ ZnO ≈ MnO2 > CaCO3 > NaCl > KCl ≈ SiO2 ≈ MgCl2.