Synthesis of zeolites from coal fly ash for removal of harmful gaseous pollutants: a review

SOx, NOx, COx, volatile organic compounds, and Hg vapor cause direct and indirect harm to the atmospheric environment and human health because they contribute to the formation of photochemical smog, acid rain, and haze and possess potential toxicity (e.g., carcinogenicity). Therefore, effectively controlling and reducing the pollution caused by these chemicals are crucial. Several methods have been developed to avoid such pollution. Among them, adsorption is one of the most common and effective techniques. As adsorption materials, zeolites have great potential in reducing air pollution. Moreover, coal fly ash (CFA)-based zeolite synthesis provides an alternative for the simultaneous treatment of air and solid waste pollution. In this study, the progress of research on zeolite synthesis by CFA in recent years is reviewed, and the challenges faced by CFA during zeolite synthesis are discussed. In addition, the applications of CFA-based zeolites in harmful gas removal and control are reviewed. Improving the synthesis of high-purity, high-performance fly ash zeolite materials may be necessary to develop efficient and low-cost materials for the removal of harmful gases because we are entering a new era where wastes for disposal are being utilized.


INTRODUCTION
Coal is the second largest fuel in the global energy mix, and it accounted for 27% of the global energy consumption in 2018 ( Fig. 1(a)) (BP, 2019). China remains to be the world's largest energy consumer, and it contributed 34% to the growth of the global energy demand in 2018 ( Fig. 1(b)) (BP, 2019). In addition, coal is still China's dominant fuel. Its share of total energy consumption was 58% in 2018 ( Fig. 2(a)). Fig. 2(b) shows the trend of coal consumption, which increased slightly in 2018 compared with the value in 2017 (BP, 2019).
Coal fly ash (CFA), a by-product of coal combustion, is one of the most abundant industrial solid wastes, and the emission of CFA increases annually. CFA contains heavy metals, polycyclic aromatic hydrocarbons, silica, and other toxic substances (Tanaka et al., 2008b;Pandey et al., 2011;Bartoňová, 2015). High CFA emissions can take over land resources, pollute the air, and damage human health (Delkash et al., 2015). Therefore, proper disposal of CFA is urgently required. At present, CFA is mainly used as building materials in the form of concrete (Dilmore and Neufeld, 2001), cement (Bouzoubaa and Lachemi, 2001), and hollow blocks (Kumar, 2003) (Fig. 3). This material is also utilized in backfilling treatments (Yao et al., 2015). However, these applications have low utilization and low added value. Therefore, utilizing the advantages of CFA and transforming this material into a high-value-added product have become research hotspots. The main components of CFA are Si-and Al-containing compounds, which are similar to zeolite. Hence, CFA is conducive to the synthesis of zeolite (Cardoso et al., 2015).
Zeolite has broad application prospects due to its potential molecular sieving, high specific surface area, and good thermal and chemical stability (Wang et al., 2010;Jiang et al., 2011;Charkhi et al., 2012). Zeolite is a kind of aluminosilicate molecular sieve crystal with uniform pores, and its skeleton contains Al, Si, and O (Andaç et al., 2005). Cations and water exist in the pores (Vuong and Do, 2009). As depicted in Fig. 4, tetrahedral TO 4 (T = Si, Al) is the most basic structural unit of the zeolite framework, and interconnected tetrahedral TO 4 requires additional cations to balance the negative charge generated by Al ions (Belviso, 2018).
The polyhedral rings that are formed by sharing O atoms and connected in different ways are called secondary building unit (SBU). Several common SBU are illustrated in Fig. 5. Composite building unit (CBU) is formed by the connection of SBU, and they form different kinds of zeolite skeleton structures through additional connections (Fig. 6) (Smith, 1988). Several common zeolite structures are illustrated in      Table 1 presents their framework structures and related materials. Different zeolite structures result in distinct channels (Fig. 8). Benefitting from these structural features, zeolites have the characteristics of adsorption selectivity, high specific surface, and high ion exchange capacity (Hasan et al., 2012;Mousavi et al., 2013). Consequently, zeolites have enormous potential for adsorption and catalytic applications. Sustained and rapid economic development and industrialization would inevitably increase the consumption of energy and other materials and could cause a series of environmental pollution problems, such as pollution by SO x (Wang et al., 2018a;Carter et al., 2018;Yang et al., 2019b), NO x (Gaskin et al., 2019), and volatile organic compounds (VOCs) (Aziz and Kim, 2017;Difallah et al., 2017;Wang et al., 2018b). Atmospheric aerosol is one of the main pollutants that affect the urban environment and human health, and it directly or indirectly causes haze, photochemical smog, acid rain, and other climate changes (Li et al., 2016). This pollutant contains many toxic and harmful substances and serves as a carrier of other pollutants, which cause great harm on human health and the ecological environment. Depending on the formation mechanism, atmospheric aerosols can be divided into primary and secondary aerosols (Seinfeld and Pankow, 2003). Primary aerosols refer to aerosol particles that are directly discharged into the atmosphere, and secondary aerosols are particles produced by the oxidation of certain gaseous pollutants (e.g., SO x , NO x , and hydrocarbons) in the atmosphere (e.g., from SO 2 to SO 4 2-) (Pöschl, 2005).
Zeolites have been widely used for air pollution control due to their excellent performance. In recent years, the synthesis of zeolites with considerable properties (e.g., wide availability and low cost) from CFA has elicited much interest from researchers. Bukhari et al. (2015) introduced the progress of research on and the application of converting CFA into zeolites through microwave and ultrasonic energies. Mallapur and Oubagaranadin (2017) reviewed the progress of research on synthetic zeolites from fly ash, municipal solid waste, agricultural waste, industrial kaolin waste, and other hazardous inorganic waste. Belviso (2018) comprehensively analyzed the characteristics and application prospects of coal ash and biomass ash and discussed the key factors affecting the synthesis of zeolites from coal ash and the shortcomings of various synthetic methods. The present work reviews the synthesis methods of various zeolites from CFA and highlights the application progress of CFA-based zeolites in gas pollution control.  (Lok et al., 1984;Corma et al., 2004;Lewis et al., 2004;Bouizi et al., 2007 (Newsam et al., 1989;Occelli et al., 2000;Greiser et al., 2016;Schroeder et al., 2018 (Baerlocher and Meier, 1972;Helliwell et al., 1993;Artioli and Marchi, 1999;Zhang et al., 2001 (Sankar et al., 1998;Kongshaug et al., 2000;Zhang et al., 2008;Itakura et al., 2011) Fig. 8. Channel structure of (1) MCM-41 (Jabbari et al., 2015), (2) ZSM-5 (Flanigen et al., 1978), (3) faujasite (Johnson et al., 1999).

SYNTHESIS OF ZEOLITES FROM CFA
The research on and understanding of the crystallization mechanism of zeolites provide theoretical guiding significance for the synthesis of zeolites with specific structures and functions and have far-reaching importance for the process and technology of industrial production. Throughout the development history of zeolites, the discussion of their synthesis mechanism has consistently been a controversial topic. The formation of zeolites is a complicated process that involves the transformation of silicates and aluminates, the formation and dissolution of aluminosilicate, the continuous change in the gel's solid and solution phases, and the formation and growth of zeolite nucleation. On the basis of the changes in solid and liquid phases during synthesis, solid-phase, liquid-phase, and bidirectional transformation mechanisms have been proposed (Cundy and Cox, 2003). With the incessant progress of experimental and characterization methods, the understanding of the crystallization mechanism of zeolites has been constantly improved.
The principle of the solid-phase transformation mechanism is that no solid-phase dissolution should exist in the crystallization process, nor should the liquid phase be directly involved in the nucleation and crystal growth of zeolites. Only the structural rearrangement of the aluminosilicate skeleton generated by the gel solid under certain conditions is involved, resulting in the nucleation and crystal growth of zeolites (Tsapatsis et al., 1996). Breck (1964) investigated the relationship between zeolite crystallization and time through X-ray diffraction. The crystallization process was divided into two stages, namely, induction and growth. The solid-phase transformation mechanism was comprehensively examined via electron microscope analysis. Guth and Caullet (1986) verified that in extreme cases where only a large number of amorphous gels is present, the liquid phase does not participate in the crystallization process. Instead, the gel is directly rearranged in situ to synthesize zeolites. Kerr (1966) proposed the liquid-phase transformation mechanism of zeolite crystallization. They found that the gel was initially dissolved in NaOH solution and produced dissolving derivatives, which in turn yielded the zeolite in the solution. The liquid-phase transformation mechanism also considers an equilibrium state between the solid and liquid phases. The growth of the crystal nucleus consumes the silicate and aluminate ions in the solution, and the consumption of the liquid-phase components in the crystallization process leads to the continued dissolution of the solid phase (Grizzetti and Artioli, 2002).
Improvements in characterization methods gave way to the development of the bidirectional transformation mechanism. Gabelica et al. (1984) studied the crystallization process of ZSM-5 through a dual system and found that the crystallization process of one system has many similarities with the liquidphase mechanism, whereas the other high-silicon system belongs to the solid-phase mechanism. Using the small-angle neutron scattering technique, Iton et al. (1992) confirmed that the crystallization mechanism of ZSM-5 varies with different silicon sources. Van Grieken et al. (2000) reported that the crystallization process of ZSM-5 zeolite is due to the simultaneous existence of solid-and liquid-phase mechanisms.
Theoretically, given that the main components of CFA are Si and Al, zeolite can be synthesized from CFA. Many domestic and foreign scholars have conducted studies on this field (Yaping et al., 2008). The synthesis methods for zeolite include direct hydrothermal, two-step hydrothermal, alkaline fusion-assisted hydrothermal, and microwaveassisted methods. This section focuses on the analysis of the process and parameters of each method for the formation of zeolite types from CFA and the advantages and limitations of each approach. Fig. 9 displays the preparation process of zeolites with CFA as the raw material, and Table 2 shows information on the preparation methods and their products.

Direct Hydrothermal Method
The direct hydrothermal method involves dissolving CFA in an alkaline solution and subsequently synthesizing zeolite products through thermal treatment. Querol et al. (1999) investigated the influence of 14 kinds of Spanish CFA with different physical and chemical properties on zeolite synthesis. The results showed that CFA with the highest content of aluminum-silicate glass has the greatest potential for industrial application. The researchers also noted that in the process of synthesizing zeolites from CFA, the main active substances are glass and quartz, whereas mullite has low activity (Querol et al., 1995). Izidoro et al. (2012) discussed the synthesis of zeolites from five kinds of Brazilian CFA using the hydrothermal method and discovered that zeolite made from CFA containing more than 70% SiO 2 and Al 2 O 3 has a high ion exchange capacity. The results also suggest that the element composition of CFA is one of the important factors that influence the synthesis of zeolites. Inada et al. (2005a) analyzed the effect of the Si/Al molar ratio and alkaline condition on the type of synthetic zeolite. The authors showed that it is beneficial to the formation of Zeolite P with silica-rich CFA at a low alkali concentration, whereas the addition of a Si source can promote the formation of Zeolite P at a high alkali concentration. This finding indicates that the addition of Si or Al sources can control the type of  synthetic zeolites. Wałek et al. (2008) studied the influence of different solid/liquid (S/L) ratios on the synthesis of zeolites from CFA. They discovered that a low S/L ratio is favorable to the dissolution of CFA. A single-phase NaP1 zeolite was obtained at conditions of 2 M NaOH solution, 104°C solution temperature, and 80°C crystallization temperature. The relationship between the synthesis conditions and the products was clearly described at a low S/L ratio. The S/L ratio usually used is 50-150 g dm -3 , which greatly limits the dissolution of CFA. But the low S/L ratio (4 g dm -3 ) reached 85 wt% in 4 hours. Murayama et al. (2002) described the effect of alkali on the mechanism for the synthesis of fly ashbased zeolites. They discovered that the OHin the alkaline solution greatly promotes the dissolution of Si and Al ions, and Na ions promote the formation of Zeolite P, which can selectively capture K + . Using biomass fly ash as the raw material, De Rossi et al. (2019) synthesized Zeolite P with a specific surface area of 56.35 m 2 g -1 through an in situ method at 60°C. The direct hydrothermal method has a simple operation and low production cost. However, the method still has several disadvantages, including long hydrothermal time, high temperature, high energy consumption, low product purity, low yield, and many by-products. Hollman et al. (1999) were the first to extend the one-step method to two-step synthesis of pure zeolites from CFA. The Si in CFA can be extracted by adding NaOH solution to synthesize pure zeolite (> 99%). In addition, the solid residue from the reaction can be converted into zeolite via the traditional method. The cationic exchange capacity of pure zeolite is approximately two times that of the blend zeolite/ residual CFA, which renders the former more suitable for wastewater treatment than the latter. Moriyama et al. (2005) described the effect of high-temperature processing and drainage during operation on the synthesis of zeolites from CFA. They found that the appropriate synthesis condition involves 2.5-3.5 mol L -1 of NaOH and 0.88-1.10 L kg -1 of liquid/solid. The zeolite obtained by a pilot plant has a higher cation exchange capacity than that produced by the traditional method. Wang et al. (2008) discussed the effects of NaOH concentration on the dissolution of silica in CFA and synthesized different types of zeolites through the twostep method. The findings revealed that due to the increase in NaOH concentration, Zeolite A with a narrow particle size distribution can be obtained within a short crystallization time. Submicron Zeolite A was synthesized from CFA for the first time. Musyoka et al. (2012) optimized the synthesis parameters of pure-phase Zeolite P synthesized from three kinds of CFA by using the two-step method. They reported that the quality of synthetic zeolites is largely affected by the CFA source and can be improved by adjusting the experimental conditions. Moreover, the cation exchange capacity of purephase Zeolite P can reach 4.11 meg g -1 .

Two-step Hydrothermal Method
The Si and Al ions in the waste liquor of the direct hydrothermal synthesis process can be fully utilized in the two-step synthesis process. Zeolites with high purity can be further obtained by adding a Si or Al source. However, this method entails a complex operation, long reaction cycle, low yield of pure zeolite, and high cost.

Alkaline Fusion-assisted Hydrothermal Method
The alkaline fusion-assisted hydrothermal method was introduced to improve the yield and quality of zeolite synthesis. This method comprises two main parts. The first part involves melting the mixture of CFA and alkali at a high temperature. In the second part, the alkali melt product is processed through hydrothermal treatment. Shigemoto et al. (1993) were the first to introduce the alkaline fusion-assisted hydrothermal process for the synthesis of Zeolite X with a crystallinity of 62%. Mullite was proven to be an inert substance that limits the quality and yield of zeolite synthesized from CFA. The mechanism of this method was also analyzed by Berkgaut and Singer (1996), who proved that the formation of sodium aluminosilicates occurs through an endothermic reaction at 170-180°C. Aluminosilicate gel can be formed and further converted into zeolite by mixing with water. The researchers also discovered that mullite can be fully hydrolyzed and converted into zeolite by adding a small amount of water to the mixture of CFA and NaOH before melting. Rayalu et al. (2000) analyzed the effects of NaOH concentration on the conversion of CFA and types of zeolites. They showed that a low NaOH concentration leads to a small extraction amount of aluminum silicate in CFA, which is not conducive to the formation of zeolite. However, excessive NaOH enrichment results in the conversion of zeolite to hydroxysodalite. Zeolites synthesized through the alkali fusion method usually have a higher specific surface area, cation exchange capacity, and crystallinity than those synthesized using the traditional hydrothermal method (Molina and Poole, 2004). Wajima and Sugawara (2010) reported that the amount of calcium in CFA has an important effect on the type of zeolite, and a low calcium content is conducive to the synthesis of zeolite. Yang et al. (2019a) proved that the synthesis of zeolite with the double alkali method is more effective than that with the single alkali approach. Using response surface methodology, Sivalingam and Sen (2018) synthesized Zeolite X with a maximum crystallinity of 76% and a maximum yield of 88%. The crystal violet adsorption capacity is 99.62%, which is better than that of commercial Zeolite X. Feng et al. (2018) quantified the transfer of heavy metals from fly ash to produce zeolite and wastewater and realized high conversion of the main elements (98.2% Al and 96.5% Si) to synthesize zeolite and zero secondary solid waste.
Many studies have proven that zeolites synthesized through the alkaline fusion-assisted hydrothermal method have high conversion and quality. This method can be successfully applied to different kinds of CFA, but it requires calcination, which increases the cost of preparation and needs further study. Querol et al. (1997a) proposed the synthesis of zeolites from CFA through the microwave-assisted method, which is a synthesis process that involves changing the heating mode from electric heating to microwave heating. Compared with the traditional hydrothermal method, the microwave method can obtain zeolites with the same quality but within a shorter time. Tanaka et al. (2008a) proved that the microwave method can shorten the crystallization time and obtained singlephase Zeolite A from CFA through this technique. Inada et al. (2005b) described the effect of microwave irradiation on the yield of zeolite synthesized from CFA and analyzed the crystallization mechanism of zeolite. They demonstrated that microwave irradiation can effectively improve zeolitization, continuous microwave radiation can inhibit the formation of zeolite crystals, and early microwave heating is beneficial to the formation of zeolite. Behin et al. (2015) conducted an experimental study on the microwave synthesis of Zeolite A. Zeolite Linde Type A with high crystallinity and adsorption capacity were synthesized from CFA and industrial wastewater within a short time (10-30 min) at a relatively low-power microwave irradiation (100-300 W).

Microwave-assisted Method
Related studies indicate that the major advantages of the microwave-assisted method include increasing the crystallization rate, shortening the crystallization time, and reducing production costs, all of which provide the possibility for the industrialized production of zeolites from CFA. However, due to disadvantages, such as low zeolite conversion rate, numerous by-products, and certain difficulties in amplification engineering equipment, this method has been investigated only at the laboratory scale. Further studies are necessary before applying this method at the industrial scale.
In addition to these usual methods, other techniques, including molten salt method (Park et al., 2000;Choi et al., 2001), dialysis method (Tanaka et al., 2006), ultrasoundassisted method (Musyoka et al., 2011;Ojumu et al., 2016), and supercritical water technology (Jin et al., 2010), have also been used for the synthesis of zeolites from CFA. Different preparation methods and conditions can form different kinds of zeolites. Fig. 10 depicts the morphology of CFA and several typical zeolites obtained from CFA.

Iron works, fertilizer plants and petrochemical industry
Harmful to the brain, cardiovascular system and even fatal NO x Fossil fuel combustion Photochemical smog, acid rain (Zhang et al., 2019b) SO 2 Coal combustion Acid rain, asthma (Zhang et al., 2019a) Hg Volcanic activity, fossil fuel combustion Air and water pollution, insanity, pneumonia (Qi et al., 2015) VOCs Benzene Petroleum, adhesives, lacquers, pesticides, gasoline, paints, combustion sources Carcinogen, damage the ozone layer, photochemical smog (Lee et al., 2004;Bhatia et al., 2009;Cosseron et al., 2013) Toluene Ethylbenzene Many achievements in the removal of gaseous pollutants have been obtained through the combination of the abovementioned methods and mechanisms. Table 4 presents a summary of the research results on removing gaseous pollutants using zeolites synthesized by CFA.

Removal of S Compounds
SO 2 and SO 3 are the major components of acid rain, and they can form sub-micron acid particles that are absorbed by the human body Hui et al., 2015). Srinivasan and Grutzeck (1999) used Class F CFA as a raw material for the synthesis of zeolites and investigated the influencing factors of SO 2 adsorption. The results showed that the amount of removed SO 2 depends on the type of zeolite, degree of drying, and zeolite content. Regardless of the CFA source, the sample (150°C) containing analcime and sodalite could adsorb 6-7 mg SO 2 per gram, whereas the sample containing X, Y, and Na-P1 had significantly low adsorption efficiency. Suchecki et al. (2004) proved that fly ash zeolite (FAZ) is an effective SO 2 adsorbent and has a higher adsorption capacity than CFA (Fig. 8). In addition, thermal activation increases the SO 2 adsorption capacity of FAZs by two to three times, reaching up to 38 mg SO 2 per gram of zeolite. Izquierdo et al. (2013) prepared Ca-exchanged Zeolite Y from CFA and performed corresponding SO 2 adsorption experiments. They reported that the adsorption ability of SO 2 is strongly associated with the presence or absence of water in the flue gas. In the presence of water, H 2 SO 4 and CaSO 4 are formed in the adsorption process, which seriously changes the structure of zeolite and reduces its activity. Czuma et al. (2016) investigated the effect of different CFAbased zeolites obtained by changing the melting reaction parameters on the adsorption capacity of SO 2 . The authors stated that the yield and type of zeolites can be influenced by the alkali/ash ratio, alkali melting temperature, and adsorption capacity of SO 2 . They also observed that the adsorption capacity decreases after the first cycle, whereas it remains approximately constant during the second and third cycles.

Removal of CO2
The rapid increase in the concentration of CO 2 in the atmosphere has elicited people's concern about global climate change and CO 2 emission reduction. Liu et al. (2011) used the alkaline fusion hydrothermal method to form Zeolite A and A + X mixtures from CFA, which were then used in a CO 2 adsorption test. Compared with Zeolite X synthesized at a high temperature of 90°C, the two zeolites have better performance in capturing CO 2 from flue gas and are more selective for CO 2 than N 2 . Lee and Jo (2010) studied the adsorption behavior of Ca ion-exchanged Na-P1 and Na-A zeolites prepared from CFA at a low CO 2 concentration. Their results indicated that the best CO 2 adsorption can be mainly attributed to the electrostatic behavior of Ca ions and the enhancement of acid-base interactions. Soe et al. (2016) and Muvumbu (2015) emphasized that Ca ion-exchanged zeolites have excellent CO 2 adsorbability. Sarmah et al. (2013) performed CO 2 adsorption experiments on CFA-based zeolite modified by monoethanolamine/N,N-dimethylaniline and diethylamine/N,N-dimethylaniline, and the adsorption capacity reached 6.89 and 5.99 mmol g -1 respectively. In addition, the modified zeolite had a lower cost and higher adsorption efficiency than commercial zeolite. Kalvachev et al. (2016) investigated the influence of preparation conditions on the conversion of CFA into zeolite and CO 2 adsorption. Table   4.

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They reported that Zeolite X can be obtained when the alkali/ash ratio is 2, the roasting temperature is 550°C, and the hydrothermal reaction occurs at 90°C for 2 h. The adsorption capacity of CO 2 can reach 60 mg g -1 at 22°C. Dindi et al. (2017) studied the effects of different polyethylenimine (PEI: MW-600 and MW-1200) loadings (15%, 20%, and 25%) on CO 2 adsorption. The results showed that PEI with a low molecular weight is better than that with a high molecular weight. The adsorption performance improves as the PEI load increases, but an excessive load hinders dispersion and degrades performance. At a loading of 25%, PEI-600 achieves the maximum CO 2 adsorption energy (26 mg g -1 ) at 70°C.

Removal of N Compounds
NO x are common air contaminants that cause environmental issues, such as acid rain. López-Salinas et al. (1997) formed Zeolite P or faujasite-type zeolites from CFA and investigated the catalytic performance of 3.6 wt.% Cu-impregnated zeolites in the CO reduction of NO. At 505 K, the conversions of NO for Cu-loaded Zeolite P and faujasite-type zeolite were 47% and 36%, respectively, whereas the untreated CFA exhibited almost no activity in the reaction. Bopaiah and Grutzeck (2000) examined the synthesis of zeolites from CFA, ground glass particles, and cement kiln dust and their adsorption of NO 2 . The hydrothermal reaction was executed at 90°C for 56 days. The zeolite yield was about 30%, and it was able to capture 88 mg of NO 2 per gram of zeolite. Karthikeya and Saravanan (2013) compared the catalytic removal of NO x using ion-exchange CFA-based X-zeolite and commercial 13-X zeolite. The authors stated that the catalytic performance of Ag-X, Fe-X, and Ni-X catalysts prepared from CFA is better than that of the commercial 13-X zeolite catalyst. Furthermore, Ag and Fe zeolite catalysts have the best NO x reduction performance at high and low temperatures, respectively, and no obvious deactivation occurs after 50 h of reaction. Izquierdo et al. (2016) investigated the selective catalytic reduction of NO with fly ash-derived Zeolite Y exchanged with Cu 2+ or Fe 2+ . At temperatures below 200°C and in the presence of water, the difference in NO reduction is caused by the different temperatures of active substances Cu 2+ and Fe 3+ in the ion-exchanged zeolites and Zeolite LY-Cu with a large surface area and high reactivity. The entire equation can be expressed as 4NO + 4NH 3 + O 2 ↔ 4N 2 + 6H 2 O, where water is a product of the reaction. The presence of water vapor in the airflow can upset the equilibrium, increase the concentration of NO + O 2 , further promote the oxidation of Cu + , and increase a large number of active sites because the oxidation of Cu + to Cu 2+ below 325°C is due to NO + O 2 . Maiti et al. (2018) applied X-ray photoelectron spectroscopy to examine the adsorption of harmful gaseous pollutants in CFA-based zeolite containing Na-P1 and analcime phases. Their findings revealed that synthetic zeolite has acid and thermal resistivity and can be used for internal and external environmental conditions. The highest adsorption of N2O, NO, CO, and NO2 on the zeolite surface reached 64.77%, 24.60%, 12.94%, and 10.63%, respectively. Therefore, CFAbased zeolite could be used to purify the most common air contaminants in indoor and outdoor environments.

Removal of Hg Vapor
Hg is one of the most prominent pollutants in the environment due to its high volatility, high toxicity, and biological enrichment. Wdowin et al. (2014) investigated the adsorption of Hg from flue gases by activated carbon with bromine (AC/Br), a natural zeolite, and zeolites (Na-X and Na-P1) synthesized from CFA. The authors stated that untreated synthetic zeolite has almost no adsorption for Hg. The Hg removal efficiency of Na-X/Ag obtained through the ion exchange method was higher than that obtained through the melting method and superior to AC/Br and natural zeolite. The increased Hg adsorption efficiency of Na-X/Ag was attributed to the highest specific surface area of the zeolite treated with Ag and the highest combination of microporosity and medium porosity. In a subsequent study, Wdowin et al. (2015) indicated that the removal efficiency of Hg is related to the structure of the adsorbent (powder or particle), flue gas flow, contact time, and experimental temperature. According to the experimental results, Agimpregnated CFA-based Zeolite X reduces the Hg content in flue gas and obtains a long-term Hg breakthrough of 15-40%. The effect of granular materials on Hg capture is preferable to that of powder materials. Wang et al. (2015) studied the adsorption performance of Hg by zeolite achieved from CFA by using the supercritical hydrothermal method. The CFA-based zeolite obtained at an S/L ratio of 15, NaOH concentration of 1 mol L -1 , supercritical water temperature of 400°C, and reaction time of 5 min exhibited a satisfactory Hg removal performance. The authors emphasized that a low Si/Al ratio and low Ca and Fe content are beneficial to the synthesis of zeolite. The sodalite and cancrinite in the product were the main active crystals, and at 100°C reaction for least 450 min, the removal efficiency of Hg exceeded 75% and 60%, respectively. Ma et al. (2019) used fly ash and Fe-containing waste as raw materials to synthesize magnetite through the one-step supercritical hydrothermal method. After modification using HCl, the removal rate of the elemental Hg (Hg 0 ) in the flue gas by zeolite exceeded 80% within the temperature range of 50-300°C.

Removal of VOCs
VOCs have attracted increasing attention due to their contribution to the creation of photochemical smog and harmful effects on the atmosphere and human health. Zhang et al. (2012) reported the adsorption of benzene on CFAbased Zeolite X modified using 20 wt.% HCl and 10 M NaOH. During their tests, they observed that the maximum adsorption capacity of Zeolite X on benzene is 151 mg g -1 , which is higher than that of commercial activated carbon. The results showed that the adsorption capacity is influenced by the preparation temperature and time. In a subsequent study, the researchers discovered that Zeolite X produced from CFA at a hydrothermal temperature of 140°C for 8 h has an Si/Al molar ratio of 7.9 and an adsorption rate of 66.51% on benzene gas (Wei et al., 2013). Zhou et al. (2014) generated CFA-based zeolite by alkali mixture (NaOH and KOH) modification and used it for benzene vapor adsorption. The results showed that single-phase zeolite NaP1 can be obtained in the 8 M mixed alkali solution, and the adsorption energy of benzene vapor can reach 69.2%, which is similar to that of commercial benzene vapor adsorbents. Rayalu et al. (2006) utilized surface-modified Zeolite Y formed from CFA as an adsorbent for VOCs. Their results indicated that Zeolite Y modified by hexadecyltrimethylammonium bromide increases the hydrophobicity and adsorption capacity of VOCs. This material is stable at 250°C, which is higher than the desorption temperature. In addition, the product has strong adsorption selectivity toward VOCs, good stability, and regeneration possibility. Bandura et al. (2016) demonstrated that Na-X zeolite made from CFA has a higher adsorption capacity (383.67-582.91 µg g -1 ) for benzene, toluene, and xylene in flue gas compared with the CFA-based zeolite Na-P1 (1.44-20.50 µg g -1 ), natural clinoptilolite (0.68-2.84 µg g -1 ), and diatomite (0.20-0.96 µg g -1 ). This was because micropores play a major role in the structure of faujasite with large-scale channels. The adsorption capacity of the experimental materials was in the order of xylene > toluene > benzene. Another study experimented on the use of synthetic zeolites from high-alumina CFA in the adsorption of formaldehyde, benzene, and toluene (Yuan et al., 2015). The products were amorphous silicate materials, crystalline calcite, and some impurities. In addition, the adsorption performance of the products on VOCs was similar to that of commercial activated carbon and stronger than that of commercial diatomite. The order of adsorption was toluene > benzene > formaldehyde. The Fourier transform infrared spectroscopy results indicated that a weak hydrogen bond interaction exists between the products and the three volatile organic compounds.

CHALLENGES AND PERSPECTIVES
Coal fly ash, the solid waste generated by coal-fired power plants, is one of the most innovative renewable energy sources. However, due to the annual increase in its cumulative emission, the resource utilization of CFA has become a serious issue. At present, the primary method of comprehensively utilizing this substance is to produce cement, concrete, and other building materials. However, numerous studies have emphasized that transforming CFA into zeolite products is an effective way to utilize and add high value to it. Our review summarizes the four major ways of synthesizing zeolites from CFA, namely, the direct hydrothermal, two-step hydrothermal, alkaline fusion-assisted hydrothermal, and microwave-assisted methods. Each method exhibits major advantages and disadvantages, so further research is required. Exploring novel synthetic methods or combining them with other techniques should also be considered. Because only a few types of zeolites can be synthesized currently from CFA, additional types should be developed and synthesized, and their purity, crystallinity, and conversion rates should be improved. Accelerating the industrialization of CFA-based zeolite synthesis is also an important research goal. Related literature suggests that the application of CFA-based zeolites in removing harmful gases mainly focuses on ion exchange and adsorption; Therefore, there is still much room for research on catalytic oxidation of these zeolites to remove harmful gases. Furthermore, research should be conducted to increase their efficiency in filtering exhaust gas and regenerating.