Fabrication of Multifunctional Hybrid Composites Using Fly Ash to Control Environmental Pollutants : A Review

Air pollutants such as volatile organic compounds (VOCs), nitrogen oxides (NOx), sulfur dioxide (SO2), as well as water pollutants including heavy metal, are harmful to human and environment. Effective control and reduction of their pollution is therefore an important topic for today’s scientists. Fly ash (FA) is a type of industrial waste that can cause multiple environmental problems if discharged into the air. On the other hand, because of its high porosity, large specific surface area, and other unique characteristics, the FA can also be used as a low-cost and high efficient adsorbent with some simple modifications. This paper reviews the effects of FA on treatment of the above air and water pollution based on our research experience over many years, including to the current status of global FA utilization, physicochemical properties, principle of adsorption, and the application direction of FA in the future. It focuses on the use of nanocomposite technology to fabricate functional FA fibrous membranes to adsorb VOCs from air, and treat heavy metal wastewater. This present review first describes the fabrication technology of FA nanocomposites and their mechanism of adsorption VOCs from air. Utilization of nanofiber technology to fabricate multi-functional FA emerging composite materials to mitigate air and water pollution has great potential in the future, especially use of pollutant material to control other pollutants.


Introduction
Nanotechnology, nanocomposites, smart-technology, smart-living and others are buzzwords that represent the development of modern science and technology.Humans currently live in a time of rapid development, with daily life constantly becoming more high-tech and intelligent.However, in recent years, the threat posed by environmental pollution to human health and the environment has also increased.Such as air and water pollutions including nitrogen oxides (NOx) [1], volatile organic compounds (VOCs) [2], sulfur dioxide (SO2) [3], heavy metal pollution [4].Fly ash (FA), a type of industrial waste, is a coal combustion product composed of particulates [5].With rapid industrial development, large demand for electricity has led to hundreds of millions of tons of FA (around 80% of which is fly ash) being discharged every year worldwide.The annual coal consumption of China in 2012 was about 50.2% of the global coal consumption [6].Power generation using coal provides energy for daily life and industry, but it also produces atmospheric pollution, water pollution, all of which disrupt ecological cycles.FA pollutes the atmosphere and threatens human health if it is discharged directly from power plant chimneys into the atmosphere without treatment [7,8].This is a significant contributor to China's recent air quality and haze problems [9,10].
However, with developing science and technology, FA from power plants is changing from industrial waste into an industrial raw material [11][12][13].FA particles are porous and honeycombed, with large specific surface areas and high adsorption activities.FA particle sizes range from 0.5 µm to 300 µm, and their bead walls have porosities of 50%-80% with high water absorption [14].In addition, FA contains valuable oxide components, such as SiO2, Al2O3, CaO, MgO, Na2O, TiO2, and essential elements, including P, K, Mg, Zn, Fe, Mn, and others [5,7].Therefore, FA has attracted the attention of many researchers.FA is a pozzolanic mixture that is formed by high temperature combustion of pulverized coal.Its chemical composition is similar to clay, with a good lime binding capacity.Thus, it is primarily used for production of cement [15], clay bricks [16], hollow blocks [17], aerated concrete [18], and other building materials [19][20][21].It can be also be used as an agricultural fertilizer and soil conditioner due to its low bulk density, high water retention capacity, favorable pH, and other properties [6,22,23].Ram et al. [24] studied the effects of FA on rice crop production from 1996 to 2000 and found that FA improved texture, fertility, and crop productivity of mine spoil.Figure 1 shows the value of conversion of industrial waste FA into various useful products.
FA can also be used as a low-cost adsorbent because of its excellent specific surface area, porosity, particle size [25], water retention capacity [14], and other natural characteristics [26].Ram et al. [27] showed that FA could leach trace metals with efficiencies that vary with FA concentration, degree of association with ash particles, leaching duration, and pH of the leachate.Bhattacharyya et al. [28] found that FA could be used as an adsorbent in a cascade with a plasma reactor to NOx emission from biodiesel engines.Hower et al. [29] showed that FA carbon captures Hg efficiently.Other researchers have found that, after simple modification, FA is a good adsorbent of SO2 [30] and VOCs [31].Furthermore, one-step fabrication of FA into a fibrous membrane by electrospun technology to adsorb VOCs from air has been successfully reported by our research group [32,33].On the other hand, FA can also be combined with TiO2 [34] or AgNO3 [35] to produce a multifunctional fibrous membrane under electrospinning for purifying water, such as adsorption of heavy metals (Hg, Pb), removal of dyes (methylene blue), antibacterial activity, carcinogenic arsenic (As) and toxic organic dyes.Fabrication of multifunctional hybrid composites using pollutant material (fly ash) to control other pollutants that plays an important role in reducing and controlling environmental pollution.In addition, the perfect combination of nanotechnology and FA can reduce environmental pollution while producing low-cost and high-performance nanocomposites.

China
China is a populous country with high electricity consumption per capita.Coal is one of the main energy sources in China, accounting for 70% of the total energy consumption from 1978 to 2009 [36].China is the largest coal mining and consuming country in the world, and it has surpassed Japan to become the world's largest coal-importing country since 2008 [36].China consumed more than 3 billion tons of coals in 2010 [37], and most cities in northern China use coal-fired heating in the winter.Figure 2 shows the coal consumption of China compared with other countries and its relationship with GDP.China clearly is dependent on energy from coal.However, burning coal produces FA in the air that causes environmental problems such as acid rain, fog and haze, and heavy metal pollution [38][39][40].Solving the problem of FA pollutant from burning coal would be a great contribution to environmental and human health.Figure 3 shows the production and utilization rate of FA from 2001 to 2015 and the degree of utilization of FA in different cities of China in 2010.The annual output of FA is huge in China.At the end of 2015, about 580 million tons of FA were produced, accounting for about 77% of the global FA production [41].FA utilization in China increased after 2004 and reached about 70% in 2015 (Figure 3a), which is closely related to the rapid development of China's economy and technology.However, different statistical offices have different estimates of the utilization rate of fly ash in China.For example, Greenpeace [41] estimated the utilization rate to be only 30%.Regardless of the value, the utilization rate of FA in China is very high.According to statistics, the global average utilization rate of FA is about 25% [42].Other researchers [43][44][45] reported that the utilization rate of FA is about 67%.The utilization rate of FA in different cities in China is shown in Figure 3b.The utilization rate of fly ash in eastern China is clearly higher than in western China, due to the large market for FA in the east [37].The specific utilization of FA is shown in Table 1.Although there are many ways to utilize FA in China, few technologically advanced products use FA.If processes involving FA are not strictly controlled, they can cause secondary pollution [46].[41]; comprehensive utilization rate of fly ash in Chinese cities in 2010 (b) [37].
Table 1.Specific utilization of fly ash in China.

Item Specific utilization
Building materials Cement, fly ash bricks, fly ash ceramics, fly ash blocks, concrete, mortar Road construction Embankments, pavement bases and pavement

Backfill
Structure backfill, construction backfill, filling in low-lying areas and wastelands, filling mines, filling coal mining subsidence areas, building materials for pits, tidal marshes Agriculture Soil improvement, fertilizer production, land reclamation Recycling useful raw materials Hollow microspheres, Al2O3, Fe2O3, SiO2, carbon granules

India
About 65% of the Indian population uses coal as the main source of electricity [47].Power generation in India has increased from 1360 MW in 1947 to about 200 GW in 2012 and is expected to increase to 290 GW in 2047 [48].Annual production of FA in India is therefore also increasing rapidly.From 2015 to 2016, India consumed about 640 million tons of coal (about 25% of the total coal consumption) for power generation [48].In addition, the quality of coal in India is lower than in most advanced countries, with about 40% ash.This means that plants produce 0.74 Kg/KWh of power generation, which is 41% higher than the global average [48].That is, the poorer is the quality of coal, the more pollutants are produced, such as PM, NOx, and SOx.India production of FA from 1996 to 2011 is shown in Figure 4a.Production of FA during 2010 and 2011 increased to about 130 million tons, which was about 85.7% higher than during 1996 to 1997.The FA utilization rate also increased from 9.63% in 1996 to 1997 to 54.53% in 2010 to 2011 (Figure 4a), an increase of 466% over a 15 year period.This shows that people are aware of the value of FA utilization with technological advances and increased incomes.Figure 4b shows uses of FA in India [49]: 42% for cement, 13% for bricks and tiles, 11% for land reclamation, 11% for mine filling, 8% for ash dyke raising, 5% for roads and flyover, 2% for agriculture, 1% for concrete, and 7% for other uses [49].In addition, with the promulgation of the new policy in India, the Ministry of Environment, Forests, and Climate Change (MoEFCC) has revised standards for FA usage and disposal by granting permission to use it for agriculture.The ministry has also made it mandatory for power plants to provide FA free of charge to users within a 300-kilometre-radius, and cement industries are required to be setup within 300 kilometers of a coal power plant to increase utilization of FA and reduce costs [48].Parab et al. [50] and other researchers [51][52][53][54][55] found that FA of India can improve soil structure and be used to reclaim dumps and degraded soils by making soil more fertile and increasing the yield of agricultural products.Given the advantages of FA and the policies in place in India, it is expected that the utilization rate of FA in India will continue to increase in the near future.

USA
The United States has multiple energy sources and high energy efficiency due to their advanced technology and equipment.Americans use many types of energy, including petroleum, natural gas, coal, renewable energy, and nuclear electric power.The energy consumption of the United States in 2016 is displayed in Figure 5a.The total United States primary energy consumption was about 97.4 quadrillion Btu, only 15% of which was coal.Natural gas and renewable energy accounted for 39% of total energy.Although renewable energy only accounted for 10%, it includes many sources such as biomass waste, biofuel, and hydroelectric.It can be inferred that the United States has a strong sense of environmental protection, which makes it particularly favor use of clean energy based on advanced technology.Because of these factors, coal-fired power generation in the United States is not substantial.The energy consumption for power stations in 2013 is shown in Figure 5b.It can be clearly seen that the United States relies mainly on natural gas to generate electricity, which accounts for about 50.8% of energy production.Coal accounts for only 21.9% of all power generation.The growth rate of FA in the United States is very slow, as shown in Figure 6.FA production grew from 53.4 million short tons in 2013 to 54.6 million short tons in 2033, an increase of just under 3% [56].Basu et al. [23] reported that utilization of FA in the United States is as high as 65%.FA is mainly used in mine fill, cement, wallboard, snow and ice control, agriculture, and cosmetics in the United States.The use of FA in cement kilns in the United States has grown from about 1 million tons in 2001 to more than 4 million tons in 2006.FA is also used in a number of consumer products, including bowling balls and carpeting.In muddy feedlots, FA is used to absorb excess water.Coal ash is also used along with salt to help clear snowy and icy roads and to provide tire traction [57,58].In addition, housing projects and rising demand in the ready-mixed concrete market are expected to be major drivers for future FA utilization in the United States [56].

Other countries
Other countries produce a small amount of FA from coal-fired power.For example, Germany produces 40 million tons per year, the UK produces 15 million tons, and Australia produces 10 million tons.The annual production of FA has not reached 10 million tons per year in Canada, France, Denmark, Italy, or the Netherlands.These countries produce very little FA every year compared to China and India, but their FA utilization is high.The average utilization rate of Germany, the UK, Australia, Canada, and France is 76%.The average of Denmark, Italy, and the Netherlands is up to 100%.FA is used in the same ways in these countries as in China, India, and USA [23,50,59].

Physical and chemical properties of fly ash
The physical and chemical properties of FA play an important role in its study, including its morphology, its chemical properties and its mineralogical properties.FA properties may vary depending on combustion conditions and collector setup.The average size of FA fine particles is less than 20 microns, with a bulk density of 0.54-0.86g/cm 3 , a surface area of 300-500 m 2 /kg, and a pH value of 1.2-12.5.FA can be classified into three main groups of acidic ash, mildly alkaline ash (pH 8-9), and strongly alkaline ash (pH 11-13) [6].

Morphological characteristics
Figure 7 and Figure 8 show scanning electron microscopy (SEM) images of FA.Figures 7a and 7b show the morphological characteristics of non-carbonated FA, including different sized spherical particles, cenospheres, and plerospheres.Most of the small particles are well-rounded and solid spheres.Some large particles are vesicular cenospheres due to the presence of gas bubbles or vapor [60].Figures 7c and 7d show morphologies after FA carbonization, which has the "cubic-like" structures of calcite (Figure 7c) and "needle-like" structures of aragonite (Figure 7d).In addition, FA particles are reported to be globular and irregular with high porosity [61], glassy cenospheres [62][63][64], spherical-shaped and flake-like particles [65,66], spherical particles of varying sizes and particles of unburned coal [62,67,68], and predominantly spherical in shape and consisting of solid spheres, cenospheres, irregular-shaped debris and porous unburnt carbon [6].The morphological characteristics of FA depend on particle size, but cenospheres are an exception.FA precipitators are sized between 20-53 μm and 75-106 μm, while cenospheres are 20-45 μm and 106-150 μm [62].FA particles smaller than 20 μm consist of mostly smooth spherical particles, while a large number of irregularly shaped grains are between 75 and 106 μm, and a small number of irregularly shaped vesicular grains are more than 150 μm (see Figure 8 for details) [69].

Chemical and mineralogical composition
The chemical and mineralogical composition of FA plays an important role in the analysis of the application and disposal of FA, including its use as a cement replacement material.The chemical composition of FA can be determined by X-ray fluorescence (XRF) and spectrometry techniques.Researchers [70][71][72][73][74][75][76] have reported that FA is composed of SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, TiO2, P2O5, and small amounts of MnO, BaO, and SO3.The detailed chemical composition of fly ash is given in Table 2. Some heavy metals are also found in FA, such as Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn [76,77], as shown in Table 3.The chemical composition of FA may differ based on geographical origin and combustion conditions.However, the chemical composition is generally dominated by SiO2, followed by Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, TiO2, and P2O5.FA can also be divided into high calcium ash (≥10% CaO) and low calcium ash (<10% CaO), according to the content of CaO.High calcium ash can remove acidic gases and fix hazardous heavy metals, such as Cd and Cu [76].CaO is present in FA as free lime and embedded in glassy spheres.Usually, the CaO content increases with decreasing particle size [60].Loss on ignition (LOI) has long served as a standard method to measure unburned carbon in FA [77][78][79].All types of FA have a certain LOI value indicating their unburnt carbon content (Table 2).Unburnt carbon in FA powder has properties similar to activated carbon, such as good adsorption capacity.
Some minerals in FA can be measured by X-ray diffraction (XRD) and infrared spectroscopy techniques.Nyambura et al. [60] analyzed the XRD spectra of non-carbonated and carbonated FA, as shown in Figure 9. Minerals in non-carbonated FA were found to mainly consist of amorphous, CaO, hematite, mullite, and quartz (see Fig. 9a).The minerals in carbonated FA are mainly composed of amorphous CaCO3, magnetite, mullite, plagioclase, quartz, bassanite, and anhydrite (see Fig. 9a).The major crystalline phases are mullite and quartz for most ash [80][81][82][83].The CaO in non-carbonated FA is converted into calcite (main component: CaCO3) when it is carbonized.These results have been described in SEM spectrum analysis, and the "cubic-like" structures in Figure 9c are calcites [60].Many other researchers have also confirmed these results [84,85].Among the many oxides in FA, recovery of alumina has attracted interest in recent decades.Therefore, as science and technology progresses, the recovery and utilization of industrial waste to benefit humans and the environment will help reduce environmental impacts.

Removing air pollutants
As a cheap adsorbent, FA can effectively reduce environmental pollution through simple processing because it has a high porosity and specific surface area, in addition to other useful characteristics.Pollutants that can be adsorbed by FA include volatile organic compounds (VOCs), nitrogen oxides (NOx) and sulfur dioxide (SO2).

Volatile organic compounds
Volatile organic compounds (VOCs) are one of air pollutants mainly produced by fossil fuel combustion [87,88], painting [89], refining [90], building [91], and other industries [92,93].In recent decades, rapid development of industry has led to a dramatic increase in anthropogenic VOCs.There are more than about 300 chemical structures of VOCs, which can be classified as alkanes, aromatics, esters, and aldehydes, among others [94,95].Many VOCs are harmful to humans in even small amounts, such as benzenes and formaldehyde, which are suspected carcinogens [96].Xylene and aliphatic hydrocarbons are mainly produced by the paint industry and are harmful to the environment and humans [97,98].Polycyclicaromatic hydrocarbons (PAHs) are a group of VOCs containing several benzene rings, have been identified as carcinogenic VOCs [99].In sunlight, photochemical reactions of VOCs and NOx will generate ozone, peroxide, nitro-aldehydes, and other photochemical smog compounds that cause secondary pollution and affect people's eyes and respiratory systems, endangering human health [100][101][102][103]. Table 4 shows some VOCs harmful to human health [104].Thus, the reduction and control of VOCs from air are important research topics for air purification.
Table 4. Sources and health effects of major volatile organic compounds [104].Seo et al. [105] evaluated adsorption capacity of 7 building materials (activated carbon, gypsum board mixed with a given quantity of activated carbon, board made out of activated carbon, humidity-controlling porous ceramic material, humidity-controlling silicate calcium, ceramic tile and ordinary gypsum board) for adsorption of 4 kinds of VOCs in a small test chamber.They showed that activated carbon has the highest VOCs adsorption capacity compared with other building materials.Chmielewski et al. [106] studied the reduction of VOCs from coal combustion using electron beam treatment and found that the concentration of polycyclic aromatic hydrocarbons (VOCs) was greatly reduced from 40% to 98%.Other researchers have also found that other materials effectively reduce VOCs, including cashew nut shell liquid [107,108], Sol-Gel [109], dairy lagoon [110], CuO-CeO2 mixed oxides [111], Mn-Cu mixed oxide [112], manganese oxides [113], and other metal oxide catalysts [114,115].Among them, activated carbon is one of the most common adsorbents because of its low-cost, high availability, excellent specific surface area, pore size, porosity, chemical functional group, and other properties [116][117][118].Nevertheless, activated carbon adsorption of macromolecules and hydrophilic VOCs is not good because it has a multi-microporous structure and is a nonpolar adsorbent [104].On the other hand, activated carbon fibers adsorb VOCs better than activated carbon because of their short and straight thin-fiber shaped micropores that increase intraparticle adsorption kinetics compared to activated carbon [119,120].In addition, activated carbon fibers also play an important role during adsorption or desorption, they have large adsorption capacity and high mass transfer rates [121,122].

Dangerous
Similar to adsorption on activated carbon fibers, FA can be used to fabricate a functional adsorption fibrous membrane through simple electrospinning.Electrospinning is an efficient method for fabricating fibers from nanometer to micrometer, with advantages of high efficiency, low cost, easy operation [123].Figure 10 shows a schematic of an electrospun fibrous membrane and a VOCs adsorption experiment.Kim et al. [33] successfully synthesized polyurethane fibers containing different amounts of FA and analyzed their adsorption of chloroform, benzene, toluene, xylene, and styrene (Figure 11).They found that polyurethane fibers containing 30 wt% FA had the smallest fiber diameter and the maximum specific surface area, it results in the highest VOCs adsorption capacity compared to other fibrous membranes.Furthermore, the adsorption of VOCs by fibrous membranes was in the following order: styrene > xylene > toluene > benzene > chloroform.This is because adsorption of VOCs by membranes not only depends on the surface area and fiber diameter, but also on other characteristics, including molecular polarity, molecular structure, molecular weight, pore size, porosity, chemical functional groups electronic and steric effects, π-complexes, adsorption kinetics, ionization potential, dipole moment, boiling point, temperature and humidity [104,[124][125][126][127]. Other researchers have reported similar results [128][129][130].In general, the less ionization potential aromatic compounds are easier to be absorbed by FA fibrous membranes due to their instability [131].And the adsorbent with high C/O ratio has the high adsorption capacity for adsorption of nonpolar VOCs [132].Use of FA fibrous membrane to adsorb VOCs from air is an innovative research, which provides a new idea for using FA fibrous membrane to filter air pollutants.As an emerging material, FA fibrous membrane can be modified into a variety of air filters with great development potential in the field of environmental pollution management.

Nitrogen oxides
A small amount of NOx can be adsorbed by FA because of the unburned carbon in FA and the physical characteristics of FA such as high porosity and specific surface area [133,134].In fact, this carbon is a precursor to activated carbon, but it has gone through devolatilization during combustion in the furnace of a power station [135].Therefore, the internal carbon can be activated by some simple modifications to the FA to increase its adsorption capacity.The general method used is steam activation and physical separation, but because the ash content of carbon-enriched FA is too high, using chemical demineralization to activate carbon is the best method [136].Rubio et al [136] studied adsorption of NOx by unburnt carbon from FA and found that the conversion curves of NO on different FA samples reached steady state after 10 h.In addition, the enriched-carbon FA showed high NO abatement capacity due to increased carbon content, surface area, and porosity.Lzquierdo et al. [30] synthesized Cu and Fe exchange type Y zeolites from FA for removal of NOx from flue gas.Rubel et al. [137] also reported that carbon-rich products from combustion ash could decrease NOx emissions and Hg due to ion-pair interactions between NO + and O2 -at the surface of carbon, with subsequent condensation of NO2 in micropores.In addition, the pore size on the surface of the adsorbent can be changed after adsorption of a certain amount of Hg, producing the optimum pore size (micropores, < 2nm) for adsorbed NOx [138].Rubel et al [137] also showed that the surface area of adsorbents increased due to adsorption of NOx, as shown in Figure 12.From the above literatures, it can be concluded that unburned carbon concentration in FA plays a decisive role for reducing NOx from air, and the high surface area of FA will further improve the NOx adsorption capacity.

Sulfur dioxide
Coal power generation plays an important role in electricity generation worldwide, accounting for about 41% of the world's power generation in 2006.In some countries, this percentage is even higher, such as Poland (92%), China (79%), and India (69%) [139,140].The large amount of coal used causes an increase in SO2 and a small amount of SO3, which pollute the air.SO2 is the main chemical responsible for formation of acid rain [141][142][143].In addition, SO2 and SO3 are harmful gases that contribute to formation of submicron acid particles that can penetrate human lungs and be absorbed into the bloodstream [144].Many researchers have reported that blending FA with Ca(OH)2 or CaO can produce an adsorbent via a hydration process with a better SO2 reduction than that of hydrated lime [145][146][147].Lee et al. [148] found similar results, as shown in Figure 13.An adsorbent composed of a mixture of FA/CaO/CaSO4 was successfully synthesized with a higher SO2 adsorption capacity than that of pure FA, CaSO4, and CaO, due to its larger specific surface area (64.5 m 2 /g).In addition, all adsorbents did not exhibit any significant desulfurization activity.SEM images of adsorbent before and after SO2 adsorption are shown in Figure 14.The high density of pore structures on the surface of the adsorbent before adsorbing SO2 can be clearly seen.However, the pores disappear after adsorption of SO2, due to SO2 being converted into sulfate salts (CaSO4) and covering the surface of the adsorbent.From the above results, it can be concluded that the ability of FA to capture SO2 is mainly related to its surface area, pore structures and calcium ion content.Specific surface area is increased after the FA is hydrated, leading to more calcium ions that can react with SO2 to produce CaSO4 [149,150].

Removing water pollutants
Industrial development is likely to cause water pollution, especially heavy metal pollution, because of its inherent toxicity, large sources, persistence, non-degradability, and other factors and can cause serious harm to humans, animals, plants, and the environment [151,152].Some of the common sources of anthropogenic heavy metals are displayed in Table 5 [153].The treatment of waste water containing heavy metal can be divided into three categories: physical methods, chemical methods, and biological methods [5,154].Physical methods include adsorption (activated carbon, carbon nanotubes, kaolinite and montmorillonite, FA, and other low-cost adsorbents), membrane filtration (ultrafiltration, reverse osmosis, nanofiltration, and electrodialysis), coagulation and flocculation, flotation, and other methods [5,155].Chemical methods include chemical precipitation (hydroxide precipitation, sulfide precipitation, chelating precipitation, and chemical precipitation combined with other methods), ion-exchange, electrochemical treatment technologies, and others [5,154].Biological methods include bio-adsorbents and use of microorganism, as well as others [156][157][158].Among them, ion-exchange, adsorption and membrane filtration are the most commonly used method, and use of modified FA is an economical and efficient method for treatment of heavy metal wastewater because FA is derived from industrial waste.
FA can be used directly or indirectly (synthesized as zeolites) to adsorb heavy metal from polluted water [159,160].And it is a potential adsorbent that can remove Cu, Pb, Zn, Mn, Cd, Cr, and Ni from wastewater [161,162].On the other hand, the pH of the aqueous solution directly affects adsorption of metal ions due to changes in surface charge of the adsorbent and the degree of ionization [159,163].FA has a high adsorption capacity for Cu 2+ and Zn 2+ ions at pH 8.In addition, using FA to adsorb Cu 2+ and Zn 2+ ions was faster than that of natural zeolite and peanut husk [164].The SEM images of the FA before and after the adsorption of heavy metals are shown in Figure 15.As shown in Fig. 15b, it can be seen that the porosity between the FA particles is significantly reduced.This could be attributed to that the surface of FA is covered by complexes formed by heavy metal ions [163].Moreover, FA has a stronger adsorption capacity for Pb 2+ and Cd 2+ than other heavy metal ions.Adsorption of Pb 2+ and Cd 2+ can be attributed to chemical sorption or chemisorption involving valence forces through sharing or exchange of electrons between the sorbent and sorbate, because their adsorption kinetics followed second-order reaction kinetics [163,165].With the development of nanotechnology, FA can be fabricated into a functional fibrous membrane with a high capacity for adsorbing heavy metals [34].FA particles can be perfectly combined with polyurethane fibers via electrospinning, and the FA fibrous membrane has higher adsorption capacity and faster adsorption rate for adsorption of Pb than that of Hg.This is attributed to that the affinity of formation of PbOH + is higher compared to HgOH + [166,167].The results are encouraging for the removal of heavy metals using FA fibrous membrane from industrial wastewater.FA fibrous membrane can be used as a variety of filters with low-cost and high performance.These are a great help to improve the utilization of FA and reduce the pressure of environmental pollution.The mechanism of modified FA for treatment of heavy metal wastewater can be summarized as follows: most FA is alkaline with a high adsorption capacity and high surface area, and negative charge accumulates at the FA surface in alkaline solutions, FA can be expected to remove some metal ions from wastewater by electrostatic interactions or precipitation-adsorption; among them, the ion exchange capacity, high surface area, and unique pore characteristics of modified FA play an important role for treatment of heavy metal wastewater [6,154].

Conclusions
Fly ash (FA) can be used as the base material of some adsorbents to treat air and water pollution due to its unique porous properties.In recent years, many experts in the field of environmental pollution control have studied FA.In this paper, the mechanism of FA for adsorption of air pollutants and water pollutants has been studied, in addition to a comparison of FA production of different countries, as well as the physical and chemical properties of FA.Detailed results are as follows: i.
Fly ash can be used in building materials, building works, roads construction, agriculture, and other fields.ii.
Fly ash can be converted to inexpensive and high performance adsorbents by simple modifications due to its unique porous properties.For example, is can be made into various kinds of zeolites.iii.
Modified fly ash has the ability to adsorb VOCs, NOx and SO2 emissions in the air and can also remove some heavy metals from wastewater, due to its high porosity, high surface area, appropriate pore size, high porosity, alkalinity, negative charge and other unique characteristics.iv.
Pristine fly ash is a powder, which limits its scope of use.Using nanotechnology, it can be synthesized into low-cost, multi-functional and multi-purpose porous hybrid composites for adsorption of air pollutants and water pollutants.Fly ash porous hybrid composites as an emerging material, which has great potential in the future.

Figure 1 .
Figure 1.Various application values of the fly ash.

Figure 2 .
Figure 2. Comparison of coal consumption in China with other important coal consuming countries (a); relationship between coal consumption and GDP in China (b) [36].

Figure 3 .
Figure 3. Production and utilization of fly ash from 2001 to 2015 (a)[41]; comprehensive utilization rate of fly ash in Chinese cities in 2010 (b)[37].

Figure 4 .
Figure 4. Production and utilization of fly ash (a) and uses of fly ash in India (b) [49].

Figure 5 .
Figure 5. Energy consumption in the United States in 2016 (a) [57] and energy consumption of power stations in the United States in 2013 (b) [58].

Figure 7 .
Figure 7. SEM images of non-carbonated (a and b) and carbonated (c and d) fly ash [60].

Figure 8 .
Figure 8. SEM images of fly ash particles; a-c and d-f are before and after removal of unburned carbon by combustion, respectively [69].

Figure 13 .
Figure 13.(a) Desulfurization activity of different adsorbents and (b) relationship between desulphurization capacity and specific surface area [148].