Advances in Methods for Recovery of Ferrous, Alumina, and Silica Nanoparticles from Fly Ash Waste

: Fly ash or coal ﬂy ash causes major global pollution in the form of solid waste and is classiﬁed as a “hazardous waste”, which is a by-product of thermal power plants produced during electricity production. Si, Al, Fe Ca, and Mg alone form more than 85% of the chemical compounds and glasses of most ﬂy ashes. Fly ash has a chemical composition of 70–90%, as well as glasses of ferrous, alumina, silica, and CaO. Therefore, ﬂy ash could act as a reliable and alternative source for ferrous, alumina, and silica. The ferrous fractions can be recovered by a simple magnetic separation method, while alumina and silica can be extracted by chemical or biological approaches. Alumina extraction is possible using both alkali- and acid-based methods, while silica is extracted by strong alkali, such as NaOH. Chemical extraction has a higher yield than the biological approaches, but the bio-based approaches are more environmentally friendly. Fly ash can also be used for the synthesis of zeolites by NaOH treatment of variable types, as ﬂy ash is rich in alumino-silicates. The present review work deals with the recent advances in the ﬁeld of the recovery and synthesis of ferrous, alumina, and silica micro and nanoparticles from ﬂy ash.


Introduction
Fly ash or coal fly ash (CFA) is a spherical, glass-like, heterogeneous particle produced as a by-product from the combustion of pulverized coal during electricity production in thermal power plants (TPPs). Morphologically, fly ash particles are spherical in shape, with sizes varying from 200 nm to several microns, and structurally have ferrospheres, cenospheres, aluminosilicate spheres, or plerospheres, and irregular-shaped carbonaceous particles [1]. Fly ash has almost all the elements present in geological samples-that is, metals, heavy metals, and organic contents. Though the major composition of fly ash almost remains same throughout the world, the composition still varies based on the source of coal, their geographical origin, furnace temperature, and the operating conditions of the boiler [2]. As fly ash is derived from coal, which is rich in minerals, fly ash is also rich in silica, alumina, and ferrous [3], which are the three major contents of fly ash. Besides this, CFA also has minor oxides, such as rutile, K 2 O, CaO, Na 2 O, and phosphorous oxides, as well as traces of Cu, Cr, Zn, Ni, and Mo oxides [4]. In addition to this, fly ash is also loaded with several toxic heavy metals, such as Al, Ni, Co, Cr, Cd, Zn, Mo, As, and Hg, which categorizes fly ash into "hazardous materials" [5], and poses a potential threat to the flora, fauna, and the environment.
Every year, a million tonnes (MTs) of fly ash are produced around the globe, especially in the USA, China, France, and India [6]. Fly ash is not a serious concern for developed countries, but it carbon [9,46,47]. The class F fly ash has a higher amount of alkali and sulfate than the class C fly ash. While, for cementing agent, class F requires Portland cement, hydrated lime and quicklime, whereas class C has self-cementing properties. Class F generally requires an addition of air entrainer, which is not required by the class C fly ash. When it comes to the application, class F is used in high SO 4 3− exposure conditions, has high fly ash content concrete mixes and is explored for the structural and HP concretes. Whereas class C fly ash is not suitable for high sulfate conditions, limited to low fly ash content concrete mixes are mainly used for the residential construction. Goodzari and Sanei [40] and Olga Sharonova et al. [40].

Elemental Properties of Fly Ash
The mineralogy and composition of fly ash is not constant, rather it varies from place to place, parent coal source, operating parameters and temperature of TPPs [42], the extent of coal preparation and cleaning, furnace design, usual climate storage [43] and handling. The mineralogical properties determine the crystalline phases of the fly ash, and their composition varies from 15-45% in the fly  [40] and Olga Sharonova et al. [40]. Goodzari and Sanei [40] and Olga Sharonova et al. [40].

Elemental Properties of Fly Ash
The mineralogy and composition of fly ash is not constant, rather it varies from place to place, parent coal source, operating parameters and temperature of TPPs [42], the extent of coal preparation and cleaning, furnace design, usual climate storage [43] and handling. The mineralogical properties determine the crystalline phases of the fly ash, and their composition varies from 15-45% in the fly ash. Generally, fly ash has silica 40-60%, alumina 20-40% and ferrous 5-15% by weight fractions [44],

Chemical Properties of Fly Ash
The pH of the fly ash tends to vary from acidic to alkaline (4.5 to 12.0), depending on the source of coal and the number of trace elements in them [48]. Fly ash produced from bituminous coal, is mostly acidic even though it has higher sulfur content, while alkaline fly ash is produced from the sub-bituminous coal, which has lower sulfur content, and has higher Ca and Mg content than that derived from bituminous coal [49]. Similarly the electrical conductivity (EC) of fly ash varies between 0.177 to 14 S/m, which directly corresponds to the quantitative concentration of soluble cations and anions in the fly ash [2,50]. Likewise, mineralogy and chemical composition too depend on the various parameters of coal combustion. Chemically, about 90-99% of the fly ash fraction constitutes oxides of silicon, aluminum, iron, calcium and titanium, (~0.5% to 3.5%), which are made up of oxides of sodium, potassium, phosphorus, manganese and sulfur [51], and the remaining fractions are the trace elements, including rare earth and radioactive elements. As per the universal rule, smaller particles with higher surface areas than the larger ones are also applicable to the fly ash particles-hence, smaller fly ash particles tend to accumulate a higher concentration of elements (As, Cd, Cu, Ga, Mo, Pb, S, Sb, Se, Ti and Zn) on their surface in comparison to the larger fly ash particles [52]. Fly ash particles have both crystalline and glassy amorphous materials. Silicates are present in crystalline form-i.e., sillimanite and mullite, while most of the silicates are present in the glass form. The average glass content in U.S. fly ash is 90%, while in Indian fly ash it varies from 49-69% by weight. This indicates that Indian fly ash has more crystalline content than the U.S. fly ash. Table 1. Normal range of fly ash chemical composition produced from different coal types (in wt. %) [7,53,54]. The chemical composition of the core or interior part of the fly ash is almost masked by the depositions of elements on the surface layer of fly ash particles [2]. Moreover, these surface layers get depositions of various elements during volatilization and condensation of molten slag in the furnace [55]. It has been reported that the concentration of some of the elements on the surface layer has many more folds than that of parent coal [56]. All fly ashes derived from different coal types, have oxides of Fe, Al, Si and varying carbon content. The chemical composition of fly ashes derived from different coal sources is given above in Table 1.

Physical Properties of Fly Ash
Based on the percentage of unburned carbon, fly ash color may vary from tan to grey or black [7]. The darker the color of fly ash, the higher the carbon content [57]. Based on the above fact, it is obvious that lower grades of coal (lignite, sub-bituminous) having a lesser amount of carbon, will produce light -i.e., tan to buff-colored fly ash [58]-while the higher grades of coal (anthracite and bituminous), being rich in carbon, will produce dark colored fly ash-i.e., grey to black. Moreover, calcium oxide content too contributes in the color of fly ash, as lower grades of coal have higher calcium content than the higher grades of coal, and provide white shade to the fly ash [55]. The specific surface area and the specific gravity of fly ash tend to vary in the range of 2000 to 6800 cm 2 per gram and 2.1 to 3.0 g/cm 3 , respectively [9]. Regarding the particle sizes of fly ash, their composition varies from one geographical area to other, and for instance, the size of sandy particles is 2-0.5 mm and 4.75-0.075 mm in the U.S. and Indian fly ash, respectively, while the size of silt particles in U. S. fly ash vary from 0.05-0.002 mm and 0.075-0.002 mm in Indian fly ashes. However, the size of clay particles in both U.S. and Indian fly ashes are less than 0.002 mm. Sandy particles in U.S. fly ash are sub-divided into very coarse, coarse, medium, fine and very fine, and their total composition in fly ash is 32.4%, whereas in Indian, fly ash total composition of sandy particles is 35.69%, which indicates that the Indian fly ashes have 2-4% more sandy particles than the U.S. fly ashes. The percentage of silty particles in both U.S. fly and Indian fly ash are more than 60%; however, the U.S fly ash have marginally higher content of silty particles than the Indian fly ash. The average silty content in U.S. Fly ash is 63.2%, whereas in Indian fly ash it is 62.39%. Clay particles in U.S. fly ash are 4.3% in comparison to Indian fly ash having 1.91% of the clay. Hence, the U.S. fly ash has a 2-3% higher amount of clay particles than the Indian fly ash [7]. Variation in fly ash is also seen due to the different structural properties of the particles-i.e., cenospheres [59,60], plerospheres [61], ferrospheres [62] and irregular-or angular-shaped carbon particles [63], which are already briefly described in the introduction section. Cenospheres have a bulk density in the range of 0.4-0.6 ton/m 3 and constitute up to 5% of the total weight of fly ash [32].

Applications of Fly Ash
Fly ash has great importance and numerous advantages either in the bulk form or in their separate natural nanostructured particles, which is depicted in the Figure 3. Besides, the fly ash also has a higher amount of Si, Al, and Fe that can be used in hydrometallurgy using the environmentally-friendly approach for the recovery of minerals at an economical cost [64,65]. The bulk form of fly ash can be potentially used as a biofertilizer, as it contains a rich source of plant nutrients such as, Na, Ca, K, P, Zn, Mg, Mn, Mo, etc. Moreover, the zeolites synthesized from fly ash can also be used for the sustained and controlled release of the N, P, K and other minerals to the plants [23,66]. In the field of agriculture, [67,68] the bulk fly ash can be used for resource conservation, reclamation of the contaminated sites and restoration of industrial sites [69]. Besides agriculture, the fly ash also finds application in civil engineering [70] (bricks, tiles, cements, blocks), tiles [71,72], brick making, cements, geopolymer [73], landfills [74], mining [75], agriculture river embankments [76], fillers [77,78], panels and composite materials [79] and in metallurgy for the recovery of value-added minerals. The natural nanostructured form of fly ash-i.e., cenospheres, ferrospheres, carbonaceous particles and plerospheres, finds applications in nano-ceramics, mechanical engineering, construction of lightweight materials [80] and wastewater treatment. Besides, individual microspheres are also used for making thermoset plastics, concrete materials, nylon, material for coating [81], high-density polyethylene (HDPE), and others. In, hydrometallurgy, the high content of ferrous, alumina and silica in the fly ash, which is a waste, can possibly be considered as one of the most reliable materials for the recovery of ferrous, alumina and silica and their derivatives [82]. There are several reports where fly ash has been used for the synthesis of highly pure alumina and SiNPs, which are discussed below in Section 4. The recovery of such value-added minerals opens new horizons, as it not only reduces the global pollution in the form of solid waste but also acts as an alternative material for Si, aluminum and ferrous [83]. Ceramics2020, 3 FOR PEER REVIEW 8 horizons, as it not only reduces the global pollution in the form of solid waste but also acts as an alternative material for Si, aluminum and ferrous [83].

Fly Ash Production and Utilization at a Glance in India
The major fly ash-producing countries are the USA, China, India, Russia [84] and, out of these, most of the developed countries have started the ~100% utilization of fly ash in the forms of cement, bricks, tiles [85], roads, embankments, etc. [6]. Nevertheless, fly ash utilization has become a major challenge for developing countries, which have a long way to go in order to achieve this effort. Thermal power stations that use pulverized coal/lignite as a fuel produce a huge amount of fly ash as a by-product [86]. In a developing country such as India, fly ash utilization was only 40-60% in the year 2017-2018, and the remaining was disposed in the fly ash ponds that, again, pose a serious threat to the living organism and the environment. In India, during 2016-2017, the total fly ash generated was about 169.25 MTs and, out of which, about 107.10 MTs was utilized, and the remaining 63 MTs was disposed nearby the TPPs [7]. By the end of 2017, there were about 155 thermal power plants that used 600 MTs of coal producing 170 MTs of fly ash and out of which 107 MTs of fly ash was used in different forms by the various industries [7]. This value increased in 2018, where the total TPPs reached to 167 generating an enormous amount of fly ash. In the year 2019, 129 MTs of fly ash was generated, out of which 70% of the fly ash was utilized and the remaining 30% was dumped in the local vicinity of the TPPs. In comparison to several European countries and the U.S.A, India is lagging in fly ash utilization, as these countries have achieved up to 100% utilization of fly ash. Moreover, India still needs a lot of time for the implementation of bulk utilization of the ash. Despite conducting various research studies and the development of numerous technologies for fly ash utilization and proper disposal, a consistent visible trend for utilization has not been obtained. Hence, fly ash management is likely to remain an important area of national concern.

Fly Ash Production and Utilization at a Glance in India
The major fly ash-producing countries are the USA, China, India, Russia [84] and, out of these, most of the developed countries have started the~100% utilization of fly ash in the forms of cement, bricks, tiles [85], roads, embankments, etc. [6]. Nevertheless, fly ash utilization has become a major challenge for developing countries, which have a long way to go in order to achieve this effort. Thermal power stations that use pulverized coal/lignite as a fuel produce a huge amount of fly ash as a by-product [86]. In a developing country such as India, fly ash utilization was only 40-60% in the year 2017-2018, and the remaining was disposed in the fly ash ponds that, again, pose a serious threat to the living organism and the environment. In India, during 2016-2017, the total fly ash generated was about 169.25 MTs and, out of which, about 107.10 MTs was utilized, and the remaining 63 MTs was disposed nearby the TPPs [7]. By the end of 2017, there were about 155 thermal power plants that used 600 MTs of coal producing 170 MTs of fly ash and out of which 107 MTs of fly ash was used in different forms by the various industries [7]. This value increased in 2018, where the total TPPs reached to 167 generating an enormous amount of fly ash. In the year 2019, 129 MTs of fly ash was generated, out of which 70% of the fly ash was utilized and the remaining 30% was dumped in the local vicinity of the TPPs. In comparison to several European countries and the U.S.A, India is lagging in fly ash utilization, as these countries have achieved up to 100% utilization of fly ash. Moreover, India still needs a lot of time for the implementation of bulk utilization of the ash. Despite conducting various research studies and the development of numerous technologies for fly ash utilization and proper disposal, a consistent visible trend for utilization has not been obtained. Hence, fly ash management is likely to remain an important area of national concern.

Fly Ash as a Source of Ferrous, Alumina and Silica
Although the qualitative compositions of fly ash generated in different parts of the globe are almost identical, they differ in their chemical and physical properties [87]. The utilization of major fly ash fractions such as magnetic and non-magnetic (alumina and silica-rich) and several other narrow fractions can significantly increase the scope of the utilization of the fly ash from the thermal power plant [88]. However, almost all the fly ash, irrespective of their origin, has almost all the above-mentioned minerals in variable amounts. Based on the mineralogy of fly ash, it can be a reliable and valuable alternative resource for the generation of ferrous, alumina and silica-based micro and nanoparticles [23]. Ferrous, alumina and silica generally co-exist in the fly ash (shown in Figure 4) as either ferro-alumino-silicates (FAS) or alumina, and silica can be present in the form of crystalline mullite, sillimanite or quartz [89,90]. No doubt, fly ash is a rich source of FAS, CaO and rutile (TiO 2 ), which is primarily composed of amorphous alumino-silicates and other crystalline minerals such as mullite, quartz, hematite and magnetite [91]. The recovery of ferrous particles from fly ash is possible by a simple magnetic separation method, while the residual nonmagnetic fractions rich in alumino-silicates can be used for the recovery of alumina and silica. Once the ferrous fractions are separated from fly ash, then the silica and alumina can be recovered by multidisciplinary approaches-i.e., chemical route (sol-gel) [92], chemical coprecipitation [93], fungal synthesis and bacterial synthesis [94]. The alumina, being amphoteric in nature, can be extracted from the fly ash by chemical approaches such as acidic treatment and NaOH treatment [95]. It can be further converted to nano-alumina powders by several chemical methods, such as thermal decomposition [96], co-precipitation. The major fractions of fly ash are silica that can be extracted by both chemical (NaOH or KOH treatment) and microbial (bacterial and fungal) approaches [4].

Fly Ash as a Source of Ferrous, Alumina and Silica
Although the qualitative compositions of fly ash generated in different parts of the globe are almost identical, they differ in their chemical and physical properties [87]. The utilization of major fly ash fractions such as magnetic and non-magnetic (alumina and silica-rich) and several other narrow fractions can significantly increase the scope of the utilization of the fly ash from the thermal power plant [88]. However, almost all the fly ash, irrespective of their origin, has almost all the abovementioned minerals in variable amounts. Based on the mineralogy of fly ash, it can be a reliable and valuable alternative resource for the generation of ferrous, alumina and silica-based micro and nanoparticles [23]. Ferrous, alumina and silica generally co-exist in the fly ash (shown in Figure 4) as either ferro-alumino-silicates (FAS) or alumina, and silica can be present in the form of crystalline mullite, sillimanite or quartz [89,90]. No doubt, fly ash is a rich source of FAS, CaO and rutile (TiO2), which is primarily composed of amorphous alumino-silicates and other crystalline minerals such as mullite, quartz, hematite and magnetite [91]. The recovery of ferrous particles from fly ash is possible by a simple magnetic separation method, while the residual nonmagnetic fractions rich in aluminosilicates can be used for the recovery of alumina and silica. Once the ferrous fractions are separated from fly ash, then the silica and alumina can be recovered by multidisciplinary approaches-i.e., chemical route (sol-gel) [92], chemical coprecipitation [93], fungal synthesis and bacterial synthesis [94]. The alumina, being amphoteric in nature, can be extracted from the fly ash by chemical approaches such as acidic treatment and NaOH treatment [95]. It can be further converted to nanoalumina powders by several chemical methods, such as thermal decomposition [96], co-precipitation. The major fractions of fly ash are silica that can be extracted by both chemical (NaOH or KOH treatment) and microbial (bacterial and fungal) approaches [4]. Silica is extracted from the fly ash in the form of sodium silicate using chemical methods. Further sodium silicate leachate can be neutralized with dilute HCl, and silica gel can be formed by the solgel method [97]. After extracting ferrous fractions, one can extract alumina and silica as the residue is rich in crystalline alumino-silicates that are unreacted and non-extractable. It can be processed and modified further for the synthesis of different classes of zeolites. Ultimately, fly ash may serve as an important alternative material for the recovery of FAS and zeolites [97], which would consequently reduce global pollution in the form of solid waste. Silica is extracted from the fly ash in the form of sodium silicate using chemical methods. Further sodium silicate leachate can be neutralized with dilute HCl, and silica gel can be formed by the sol-gel method [97]. After extracting ferrous fractions, one can extract alumina and silica as the residue is rich in crystalline alumino-silicates that are unreacted and non-extractable. It can be processed and modified further for the synthesis of different classes of zeolites. Ultimately, fly ash may serve as an important alternative material for the recovery of FAS and zeolites [97], which would consequently reduce global pollution in the form of solid waste.

Properties of Ferrous Particles Extracted from Fly Ash
Iron is one of the major elements present in the earth crust and is found in all the earthy materials such as rocks, soils, clay, volcanic ash and coal. The major fractions of iron in coal are present either as sulfur-bearing or non-sulphur-bearing minerals [98]. The sulphur-containing iron minerals in coal are pyrite (FeS 2 ) [99], jarosite [KFe 3 (SO 4 ) 2 (OH) 6 ] [100], troilite [101] and pyrrhotite [102] (both FeS) [103,104], while the non sulphur-bearing iron minerals [105] are ankerite (CaFe(CO 3 ) 2 ) [106][107][108], illite [109] and siderite (FeCO 3 ) [110][111][112]. The generation of ferrospheres occurs in the carbon matrix, which provides a reducing medium when iron-containing minerals from coal interact with other elements of coal in the molten form and transform into ferrous oxides. These ferrous oxides are, especially, magnetite, hematite and maghemite, along with Al, Si, Ca, etc. [62,113]. The extraction and utilization of ferrous materials from waste fly ash not only makes the whole process greener but, at the same time, it also reduces the burden from the environment in the form of pollution. The fly ash extracted ferrous particles are mainly spherical in shape, along with few elliptical, molten-drop, dendritic-shaped particles [55]. The deposition of ferrous particles is responsible for the magnetic properties of ferrospheres. The deposited ferrous particles are either angular or spherules shaped. The deposited iron oxide phases vary from magnetite, maghemite, hematite and goethite. The Fe is always associated with other elements, such as Ca, Al, Si, Ca, Mg, Na, O, P, etc., and ferrous particles are always associated with elemental oxides of these elements, and find applications in iron-based industries, which are described below in detail, under Section 6.1.

Advances in the Recovery of Ferrous Particles from Fly Ash
The ferrous particles extracted from fly ash can be used as the precursor material for the synthesis of various types of pure IONPs [114]. Moreover, they also act as an alternative source for different iron oxide-based industries [115], such as steel and steel-based industries. Conventionally, ferrous particles are extracted either by wet magnetic separation method or by dry magnetic separation method [116]. Both the methods apply an external magnetic field, but one is extracted in the slurry form, while the other is extracted in their dried form, as shown in Figure 5. In the dry form, it can be extracted by using conveyor belts with a magnetic effect at one end, while in wet slurry it can be extracted by using a strong external magnet [82]. There have been several advances in the extraction of ferrous particles from fly ash in the last three decades and these are highlighted here in chronological order.

Properties of Ferrous Particles Extracted from Fly Ash
Iron is one of the major elements present in the earth crust and is found in all the earthy materials such as rocks, soils, clay, volcanic ash and coal. The major fractions of iron in coal are present either as sulfur-bearing or non-sulphur-bearing minerals [98]. The sulphur-containing iron minerals in coal are pyrite (FeS2) [99], jarosite [KFe3(SO4)2(OH)6] [100], troilite [101] and pyrrhotite [102] (both FeS) [103,104], while the non sulphur-bearing iron minerals [105] are ankerite (CaFe(CO3)2) [106][107][108], illite [109] and siderite (FeCO3) [110][111][112]. The generation of ferrospheres occurs in the carbon matrix, which provides a reducing medium when iron-containing minerals from coal interact with other elements of coal in the molten form and transform into ferrous oxides. These ferrous oxides are, especially, magnetite, hematite and maghemite, along with Al, Si, Ca, etc. [62,113]. The extraction and utilization of ferrous materials from waste fly ash not only makes the whole process greener but, at the same time, it also reduces the burden from the environment in the form of pollution. The fly ash extracted ferrous particles are mainly spherical in shape, along with few elliptical, molten-drop, dendritic-shaped particles [55]. The deposition of ferrous particles is responsible for the magnetic properties of ferrospheres. The deposited ferrous particles are either angular or spherules shaped. The deposited iron oxide phases vary from magnetite, maghemite, hematite and goethite. The Fe is always associated with other elements, such as Ca, Al, Si, Ca, Mg, Na, O, P, etc., and ferrous particles are always associated with elemental oxides of these elements, and find applications in iron-based industries, which are described below in detail, under Section 6.1.

Advances in the Recovery of Ferrous Particles from Fly Ash
The ferrous particles extracted from fly ash can be used as the precursor material for the synthesis of various types of pure IONPs [114]. Moreover, they also act as an alternative source for different iron oxide-based industries [115], such as steel and steel-based industries. Conventionally, ferrous particles are extracted either by wet magnetic separation method or by dry magnetic separation method [116]. Both the methods apply an external magnetic field, but one is extracted in the slurry form, while the other is extracted in their dried form, as shown in Figure 5. In the dry form, it can be extracted by using conveyor belts with a magnetic effect at one end, while in wet slurry it can be extracted by using a strong external magnet [82]. There have been several advances in the extraction of ferrous particles from fly ash in the last three decades and these are highlighted here in chronological order. Olga et al. [117] isolated and studied the composition and morphology of the magnetic microspheres from two different fly ash samples from Russian thermal power plants. About eight Olga et al. [117] isolated and studied the composition and morphology of the magnetic microspheres from two different fly ash samples from Russian thermal power plants. About eight fractions of ferrospheres in a range of sizes from 0.4 to 0.02 mm were recovered from high-calcium fly ash and studied using the scanning electron microscope-Electron diffraction spectroscopy (SEM-EDS) analysis of ferrospheres and ferrospheres composition. Shoumkova et al. [118] reported the physicochemical characterization and magnetic separation of coal fly ashes from the various regions of Russian TPPs [119]. The fly ash magnetic concentrates were isolated by wet high gradient magnetic separation using a laboratory-scale solenoid separator of SALA type. Besides this, they also studied the comparative properties of magnetic and non-magnetic fractions. Xue and Lu, [120] studied the microstructures of ferrospheres in fly ashes with their detailed SEM, Energy Dispersive X-Ray (EDX) and Environmental Scanning electron microscope (ESEM) analyses. Here, the magnetic ferrospheres were isolated by magnetic separation technique and the various phases found in the fly ash were magnetite and hematite. The extracted ferrospheres contained Al, Si, S and Ca and were reported as having morphological structures in the form of smooth, polygonal, dendritic, granular and molten drop characteristics. Oliveira et al. [121] reported the extraction and characterization of spinel magnetites from the silico-aluminous fly ash from TPPs of a French region using SEM-EDS, X-ray Diffraction (XRD), TEM, Mossbauer spectroscopy and Variable sample magnetometer (VSM). It was concluded that Mg substituted Fe in the magnetite structure and the formula was closer to MgFe 2 O 4 than Fe 3 O 4 . Apart from this, there was Mn, Ca and Si at lower percentages. Fomenko et al. carried out the phase composition of magnetic microspheres isolated from Russian TPP and performed a Mossbauer study in six narrow size ranges of magnetic microspheres isolated from power plant fly ash. It was found that Al, Mg and Ti are the major phases in the isolated magnetic microspheres. They reported that the magnetic properties of magnetic microspheres depend on the Fe content and the distribution pattern of the cation over the spinel sites [122]. Fulekar and Yadav [50] extracted ferrous particles from the fly ash slurry using an external magnet in conjunction with an ultrasonicator, where slurry was fed to the ultrasonicator fitted with a strong neodymium magnet. The sonication causes the lysis of larger particles into several smaller particles, which in turn releases the trapped ferrous particles, and consequently has an enhanced effect on the recovery of ferrous particles. It was further purified by stirring on a magnetic stirrer without using a magnetic bead. The ferrous particles respond to the magnetic field of the stirrer where ferrous particles adhere to the center, while non-ferrous particles adhere at the periphery of the Petri plate. Finally, the dried ferrous particles were analyzed using UV-Visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), VSM, dynamic light scattering (DLS), particle size analysis (PSA), XRD, Field Emission Scanning electron microscopy (FESEM) and TEM. The analyses revealed that the particles were spherical in shape, whose sizes varied from 200 nm to 7 microns, and had mixtures of different iron oxide phases. Fulekar and Yadav [50] treated ferrospheres with acid and used the ferrous rich solution to synthesize ferrous carbonate, maghemite, magnetite and hematite of high purity at nanoscale, using a chemical precipitation and calcination method by optimizing environmental parameters. The purity of transformed IONPs varied from 93-97% [50]. Further, the synthesized IONPs were used for the heavy metal removal from fly ash-based simulated wastewater.
From the above-mentioned work it can be concluded that, to date, investigators have only recovered ferrous fractions from fly ash by applying either wet magnetic separation or dry separation methods. Further, almost all of them have characterized the extracted ferrous fractions from fly ash by using FTIR, VSM, DLS, PSA, XRD, FESEM and TEM. The investigators concluded that the extracted ferrous particles are rich in Fe, and spherical-shaped ferrospheres have either rough or smooth surfaces. Rough surfaced ferrospheres are due to the deposition of large spherules or angular deposition on the surface of preformed cenospheres, whereas the smooth-surfaced ferrospheres have even distribution of ferrous on their surface. The shape of the deposited iron oxide particles on the aluminisilicate spheres are angular, rectangular, and mostly present in the mixed phases of hematite, magnetite and maghemite. Rough surfaced ferrospheres have a strong magnetic property in comparison to the smooth-surfaced ferrospheres. Smooth-surfaced ferrospheres have fewer depositions of ferrous particles on their surface so they have weak magnetic properties. Besides rough and smooth-surfaced ferrospheres, there were few of elliptical, molten-drop, or dendritic shape [55].
From, all the reported works, it was found that Fe was always associated with other elements, such as Ca, Al, Si, Ca, Mg, Na, O, P, etc. [39], and hence the ferrospheres were never free from such elemental oxides. Such impure fly ash ferrospheres have been used as fillers in nanocomposites, coke, smelting [12], steel production and other iron and iron-based industries [123]. Additionally, the ferrospheres also find their application in catalysis-i.e., for profound oxidation and oxidative coupling of methane (OCM) [124], thermolysis of heavy oil and petroleum residue [114], magnetic carriers for the separation of recombinant proteins [125], and composite sorbents.
Specifically, it was shown that small amounts of ferrospheres containing 87.5% Fe 2 O 3 and 2.0 wt. % MnO are catalysts for the oxidative coupling of methane [126], while narrow fractions of ferrospheres with a lower iron content are effective catalysts for the CH 4 deep oxidation [127,128]. The purified ferrous particles can be applied in the field of in medicine and drug delivery-i.e., in MRI, radionuclide therapy, medical diagnosis, cancer-hyperthermia, magnetic storage, magnetic ink printing, a biosensors and for bioseparation for portable devices [21].
Thus, here in our ferrous recovery we have reported a method for the elimination of elemental impurities from ferrospheres by acidic treatment-i.e., concentrated HCl treatment of ferrospheres in 1:20 ratio, under sonication at 70-80 • C for one hour. Further, the ferrous rich acidic leachate was used for the synthesis of four different types of IONPs (ferrous carbonate, magnetite, hematite and maghemite) under optimized conditions. The purity of such single phased IONPs varied from 86-97% and their purity was cross-checked with the commercially available nanoparticles in the market. This method has suggested the transformation of ferrous particles into highly pure IONPs. The summarized tabulated form of various ferrous recovery approaches is highlighted below in Table 2.

Extraction and Synthesis of Alumina Nanoparticles from Fly Ash
Aluminum is the third largest element in the earth crust, and is a constituent of soil, rocks and minerals, such as bauxites and clay. Aluminum, being amphoteric in nature [131], can leach out from the source material either by strong alkali hydroxides or strong mineral acids. Generally, aluminum is present in the crystalline form, such as alumina in fly ash [132]. Mullite is one such mineral in fly ash, which has a high composition of alumina [133]. Mullites are mostly present in the matrix part of fly ash [134,135], which makes it a very difficult task to leach out alumina in higher concentrations into the solution. Until now, numerous investigators have successfully extracted the alumina by means of both acidic treatment and NaOH treatment. High-alumina coal fly ash is a potential starting material for the preparation of Al(OH) 3 [136]. Techniques of the aluminum extraction from coal fly ash can be classified into acidic [136], alkali [136] and acidic-alkali method [137,138]. The acidic method requires acid-resistant and airtight processing equipment. The acidic-alkali method is a complicated process [24] with a series of procedures [136], including sintering [139], silica-alumina separation, purification [140], precipitation [141,142] etc. The alkali dissolution method is a promising method for avoiding calcinations of the coal fly ash at a higher temperature (more than 800 • C) and lower energy cost [136,143]. Several investigators previously reported alumina extraction methods, out of which a few of them are described below.

Alkali-Based Extraction
Park et al. reported that the synthesis of alumina from fly ash derived highly pure alum using NH 2 Al (SO4) 2 [96] . It was achieved by mixing CFA with ammonia in the water at controlled pH followed by the successive crystallization. The effect of heating (conventional and microwave) was observed on the decomposition of the alum. Alumina extracted from the microwave-assisted-derived alum was a fine powder with a high surface area. Fly ash-based ultrafine Al(OH) 3 was synthesized by the NaOH dissolution method by Su, Yang [138]. It was reported that silica was extracted by 8 M NaOH at 90-95 • C for 150 min and the extraction efficiency of silica was 40%. In the next step, alumina was extracted at 260 • C for 60 min by mixing fly ash with white lime and 20 M NaOH. The alumina extraction efficiency was about 89% and the final product was aluminum hydroxide instead of alumina, which can be further converted into alumina by calcination at a high temperature. Li et al. extracted Al 2 O 3 from fly ash by the mixed-alkaline hydrothermal process [137]. Here, alumina leaching was done using a solution of NaOH and calcium hydroxides by a hydrothermal process, and further alumina leaching in accordance with temperature, solid to liquid ratio and Ca/Si ratio was observed. The Ca effect on the alumina leaching was seen with increased temperature, calcium-silicon ratio and solid-liquid ratio. Under optimal conditions, alumina extraction reached to 91.3%. Wang et al. reported a technique "NH 4 HSO 4 roasting technology" for the recovery of aluminum and iron from fly ash [144]. It is a two-step procedure, wherein Al and Fe were leached out from fly ash and the leached Al, and Fe were precipitated with the NH 4 HCO 3 solution. Again, it was leached with a NaOH solution, and finally there was the carbonation-based decomposition of the NaAl(OH) 4 solution. Hence, it is a novel method for the extraction of alumina from fly ash. The study comprised of thermodynamics and kinetics of Al recovery from fly ash and reported Al extraction efficiency up to 90.5% by optimizing conditions at CFA: ammonium hydrogen sulfate, 1:8 mole at 400 • C for 60 min.
Virendra Yadav [50] reported a method where the fly ash was added to concentrated sulphuric acid of 2-8 M, (but most preferably at 8 M) by keeping a solid-liquid ratio of 1:5 in a 100-200-mL round-bottom flask in a reflux condenser. The reaction was carried out for 60-90 min at 90-260 • C with continuous stirring at 400-500 rpm. After the completion of the reaction, the leachate was collected by centrifugation at 5000-7000 rpm for 5-10 min. The alumina leachate obtained in the form of aluminum sulfate was directly calcinated in a muffle furnace at 800 • C for 6 h in a quartz crucible. The properties of the synthesized nanoparticles were analyzed by sophisticated instruments, such as FTIR, Raman, PSA, XRD, FESEM and TEM. Further, the ferrous-free fly ash, was separately mixed with 8 M concentrated sulphuric acid, nitric acid and hydrochloric acids by maintaining a solid-liquid ratio of 1:5 at 125 • C with continuous stirring at 400-500 rpm for 90 min in a 100-mL round-bottom flask fitted with a reflux condenser. All three acids were analyzed for their leaching efficiency of Al. It can be concluded that HCl had maximum leaching efficiency. Further, the alumina leachate was converted directly to solid alumina by calcination at 600-700 • C for 56 h in a quartz crucible. The summarized form of extraction of Al and synthesis of alumina and its derivatives by alkali-based dissolution from fly ash are given below in Table 3. Table 3. Acid-based dissolution of Alumina from fly ash.

Acid-Based Extraction
Matjie et al. [138] reported the recovery of Al 2 O 3 from bituminous coal-derived fly ash of South African TPPs where the calcium aluminate was obtained by mixing CFA with CaO and then calcinated at 1000-1200 • C. It was further treated with sulphuric acid and about 85% alumina extraction was reported using this approach. Nayak and Panda (2010) reported sulphuric acid-based extraction of alumina from Talcher (Odisha) TPPs fly ash and proposed that lower sulphuric acid molarity and ambient heat is not suitable for the recovery of a higher amount of alumina and the same can be achieved only at a higher solid-to-liquid ratio [147]. Shi et al. prepared coarse alumina nanoparticles from fly ash using sulphuric acid as a leaching agent [148,149] and the extraction efficiency of alumina was 87%. Bai et al. achieved alumina extraction of up to 85% by concentrated sulphuric acid-based thermal decomposition, where calcination at 300 • C converted most of the alumina into aluminum sulfate [150]. Wu et al. reported the leaching of Al from the fly ash using concentrated H 2 SO 4 along with pressure. They also reported the effect of coal size, experiment timing and temperature on Al leaching from fly ash. They concluded that pressure and the smaller size of fly ash particles have positive effects on the Al extraction from fly ash. Al extraction efficiency reached up to 82.4% under optimal conditions [151]. Shemi et al. reported several processes for aluminum recovery and, in one of the methods, they used 6M sulphuric acid at 250 • C for 6 h along with acetylacetone in the gas phase for the facilitation of the alumina extraction [152].
From all the above-reported methods for alumina extraction, it was found that sulphuric acid was employed for the leaching of Al or alumina extraction from fly ash. The Al extraction efficiency varied from 82.5% to 92.2%. It was observed that the molarity of acids, base and temperature also play an important role. The high molarity of either acids or base at high temperatures can increase the yield of alumina extraction from the fly ash. Here, Al was extracted using sulphuric, nitric and hydrochloric acids, but alumina was synthesized by only sulphuric acid-treated leachate. The Al leachate obtained was directly converted to alumina powder by the calcination of leachate at a temperature above 600 • C. The summarized form of extraction of Al and synthesis of alumina and its derivatives from fly ash by the acid-based treatment method is given below in Table 4.

Acid-Alkali Based Extraction
Several studies report the use of both acidic and alkaline treatment applied subsequently in order to increase the yield of alumina leachate and ultimately the extraction of alumina particles from fly ash. Aluminum, being amphoteric in nature, dissolves in both acids and bases, but the extraction of Al with strong mineral acids alone is not that effective as the alumina (mullite and sillimanite) in fly ash is mainly crystalline and inert. Both of these sources of alumina can readily react with a strong alkali, such as NaOH. The application of both the strong acids and bases along with heating may facilitate the conversion of the retractile, crystalline and inert material to react and ultimately make them available into the solution. To date only a countable number of attempts have been made in this field, which necessitates further extensive research work.
Valeev et al. reported a method where Al-chloride solution obtained by leaching coal fly ash can be further processed to extract sandy grade alumina, which is essentially suitable for metallic aluminum production. They reported the formation of amorphous alumina via the calcination of aluminum chloride hexahydrate obtained by salting-out from acidic Al-Cl liquor. This step was followed by the alkali treatment with further Al 2 O 3 dissolution and recrystallization as Al(OH) 3 particles, and a final calcination step was employed to obtain sandy grade alumina with minimum impurities. The major advantage of this novel approach is that the process does not require expensive high-pressure equipment and also reutilizes the alkaline liquor and gibbsite particles from the Bayer process, which significantly reduces the production cost of the sandy grade alumina [153].

Microbial Leaching of Alumina from Fly Ash
Certain fungi and bacteria have a capability of Al or aluminum leaching from bauxite, fly ash, kaolin and red mud, as they produce several mineral and organic acids along with numerous carbohydrates, which has been reported earlier by numerous investigators. Bacteria, such as Thiobacillus spps [154], Acidithiobacillus bacteria [155][156][157] sulfate reducing bacteria (SRB) [158] and others, produce dilute sulphuric acid, that acts as an extractant for the aluminum and other metals from fly ash and other similar sources of alumina. As acids are produced in milli mols quantities, the Al leaching efficiency with these bacteria is not as effective as it is with the chemical method.
While fungi, such as Aspergillus niger [159], Penicillium notatum [160] and Penicillium chrysogenum [161] produce citric acids, gluconic acids and several other weak organic acids that act as the main lixiviants for Al leaching from the fly ash. These weak organic acids are also responsible for various metabolic activities in the fungi [162]. A. niger is ubiquitous in nature and thrives in the air, soil and indoor environments. The citric acids also take part in the leaching of metals from several metallic samples. A. niger strains are used in the industries for the commercial production of citric acids [163]. Previously this strain was used to leach out Al and Fe from the bauxite, kaolin and fly ash, but no attempt has yet been made for the synthesis of alumina. This fungus produces citric acid as a major metabolite in the sucrose growth medium that leaches out alumina and other metals such as Fe. Nevertheless, the yield is very low due to the lower citric acid generation in the medium. Few investigators have used Penicillium and A. niger for the alumina extraction from bauxite, red mud, kaolin and fly ash [164]. Some of the previous work done in the field of bioleaching and the biosynthesis of alumina is highlighted below.
Xu and Ting described the effect of bioleaching on the fly ash by A. niger [165]. They conducted a study to find optimal parameters for metal bioleaching by varying fly ash pulp density, spore and sucrose quantity and the timing of fly ash addition [166]. The Al and Fe leaching was reported to be 12.3 ppm, which was far lower than the Zn, which was reported as 77.6 ppm. The acids responsible for the leaching were citric acid and gluconic acid. Again, the same group of investigators in 2009 reported the A. niger-based bioleaching of Al, including other metals from fly ash [167]. They showed that the leaching concentration of metals was directly related to citric acid productions. Krishna et al. reported alumina extraction from the bauxite red mud by using numerous extremophiles bacteria [168]. A. niger was used for the initial bioleaching of Al from ferrous free fly ash residue and then for the further synthesis of alumina from the leachate. Fulekar and Yadav [50] carried out the Al leaching from fly ash using Asp. niger (grown in sucrose medium) supernatant, and wet mycelia in an incubator shaker at 200 rpm and at 28 • C for 48-72 h. Fly ash was mixed with supernatant and 10 g of wet mycelia and the solid-to-liquid ratio 1:5 was maintained. The leachate was obtained by filtration using Whatman filter paper no. 42 under sterile conditions in both of the experiments. The leachate was dried in a rotary evaporator; the obtained powder was analyzed by UV-Vis, PSA and FESEM-EDS. The summarized form of fungi mediated the extraction and synthesis of alumina and its derivatives from fly ash, which are given below in Table 5. Table 5. Aluminum leaching by from fly ash by fungi.

Operating Conditions Fungus Used Product Findings
Xu and Ting [165] Citric acid, Gluconic acid Aspergillus niger Al in the medium Reported that the optimal parameters for bioleaching of metals by varying CFA pulp density, spore concentration, sucrose concentration and time of addition of CFA. Leaching of Al and Fe was 12.3 ppm, which was far lower than the Zn, which was 77.6 ppm. The responsible acids for the leaching were citric acid and gluconic acid.
Xu and Ting [166] Aspergillusniger Al in the medium Showed that the leaching concentration of metals was directly related to the citric acid productions.

Fulekar and
Yadav [50] Citric acid Aspergillus niger Al, Al 2 (SO 4 ) 3 Showed that the lesser yield of alumina present in mixtures of alumina, aluminum sulfate and Al(OH) 3

Alumina occurs in numerous meta-stable phases, which include gamma [γ]-, epsilon [η]-, delta [d]-, theta [θ], kappa [κ]-and χ-alumina.
Among all these phases of alumina, the [a] phase is the most stable, while the gamma phase is the most important and widely used nanosized alumina material. Gamma alumina is widely used in petroleum and automobile industries as a catalyst and catalyst substrate; in ceramics [169] and glasses. It is used as an adsorbent, for spacecraft materials, microelectronics, thermal resistant materials, biomedical purposes [170], as a coating material for thermal wear and abrasive optoelectronics and in metallurgy [171]. Ultrafine aluminum hydroxide is an important green flame retardant inorganic material with multi-functions of retarding, suppressing smoke and filling [172]. The various important properties of alumina and alumina nanoparticles [173][174][175] are given below in Figure 6. The fly ash based extraction of alumina can be considered as an environmentally-friendly approach and is important from a scientific point of view as it focuses not only on the disposal of waste but also acts as a substitute for aluminum source [138].

Synthesis of SiNPs from Fly Ash
The synthesis of SiNPs from fly ash has been reported using chemical and biological methods. The chemical methods initially involve the extraction of sodium silicate from the fly ash using strong hydroxides followed by neutralization by sol-gel technique, whereas microbial synthesis has been carried out using Fusarium oxysporum under optimized conditions. Both approaches are mentioned

Synthesis of SiNPs from Fly Ash
The synthesis of SiNPs from fly ash has been reported using chemical and biological methods. The chemical methods initially involve the extraction of sodium silicate from the fly ash using strong hydroxides followed by neutralization by sol-gel technique, whereas microbial synthesis has been carried out using Fusarium oxysporum under optimized conditions. Both approaches are mentioned below for the recovery and synthesis of SiNPs from fly ash.

Silica Extraction from Fly Ash by Chemical Method
It is possible to extract silica from fly ash either by alkali-dissolution [176] or by the alkali-fusion method [177]. The alkali-dissolution based silica extraction is done at 90-100 • C, for a duration of 90-150 min with a variable stirring speed and molarity of NaOH, shown in Figure 7. In the alkali dissolution method, the silicates from fly ash react with the NaOH in the aqueous phase to form sodium silicate. Further, the sodium silicate is treated with dilute HCl or sulphuric acid to form a silica gel by sol-gel technique. In the alkali fusion method [178] (shown in Figure 8), fly ash is mixed with NaOH or KOH and calcinated at a higher temperature to obtain a new fused silicate product. Further, the recovery of silicate from the silicate material is done by acidic treatment. However, fly ash has several acid-soluble elements such as Na, K, P, Mg, Ca, and Fe [18,179], which can be present in the obtained silicate in a minute quantity [20].
Therefore, these alkali and other metals can be eliminated by dilute HCl treatment at above 100 • C for 2-3 h. It is a conventional Bayer's process for the silica extraction from the silicate-rich material. Previously, numerous researchers have reported silica extraction from fly ash by alkali-dissolution methods. A few of them are highlighted below in chronological order.
Falayi et al. leached out silica from fly ash by KOH by varying the stirring rpm, size of fly ash, extraction temperature and solid-to-liquid ratio [180]. The optimum extraction parameters include time 6 h; 3M KOH; solid-to-liquid ratio 1:25 temperature 100 • C; rpm 500. However, there was no attempt made for the synthesis of SiNPs from the leachate further. Similar, work was also carried out by Wang et al. who studied the kinetics of silica and alumina extraction from fly ash by concentrated NaOH and observed the effect of temperature, stirring speed and mass ratio of sodium hydroxide to silica, on silica extraction rate [144]. The silica extraction was found to be 95.6% under the optimized conditions. Piekos and Paslawska (1998) investigated the leaching of Si element from fly ash, which was carried using distilled water, seawater and synthetic seawater with variable ratios of water and fly ash [181]. Virendra Yadav (2019) extracted sodium silicate from ferrous free fly ash using 8 M NaOH by keeping the solid-to-liquid ratio at 1:5 at 95 • C for 90 min along with stirring at 400-500 rpm. Silica gel was obtained by neutralization with 1M HCl and, finally, dried powder was treated with 1M HCl at 110 • C for 3 h to remove the trace elements in the form of impurities. The obtained precipitate was initially oven dried at 40-60 • C, followed by calcination at 400 • C for two hours. The purity of the obtained SiNPs was more than 92%, as confirmed by the EDS [50]. Virendra Yadav (2019) reported the synthesis of amorphous SiNPs from Gandhinagar (Gujarat, India), thermal power plant using the alkali dissolution method. The ferrous and alumina extracted fly ash residue was treated with 8 M NaOH, at 95 • C, in a round-bottom flask under reflux systems for 90 min by maintaining a solid-to-liquid ratio of 1:5. The leachate of sodium silicate was obtained by centrifugation at 7000 rpm for 10 min. The residue was discarded while the sodium silicate was used for the synthesis of SiNPs by sol-gel method. Further, about 40 mL of sodium silicate was titrated with dilute 2N HCl at room temperature. A white gel formed near pH 10, which ceased to form on the further addition of dilute HCl. The gel was left undisturbed for 24 h for ageing. Further, the mixture was centrifuged at 5000-7000 rpm for 10 min to obtain the white precipitate. The white precipitate was dried in an oven at 60 • C for 4-6 h and was finally calcinated at 400 • C for 4 h in a muffle furnace by slowly increasing the temperature by 10 • C/minute until the temperature reached 400 • C. The final synthesized SiNPs were characterized using FTIR, PSA, FESEM-EDS, TEM and XRD. It was found that the sizes of SiNPs ranged between 40-80 nm, they wre spherical in shape, and were fused together to form a floral-shaped aggregated structure. The SiNPs were amorphous in nature and a broad hump was observed in the 2-theta region at 15-30 • , having peaks centered at [21][22]. FTIR also reveals the three characteristic bands of SiNPs in the region of 400-1200 cm −1 . The EDS revealed the purity of SiNPs, which was 80-95%, and had impurity in the form of Na, Al and C due to the improper washing of SiNPs [50]. Therefore, these alkali and other metals can be eliminated by dilute HCl treatment at above 100 °C for 2-3 h. It is a conventional Bayer's process for the silica extraction from the silicate-rich material. Previously, numerous researchers have reported silica extraction from fly ash by alkali-dissolution methods. A few of them are highlighted below in chronological order.
Falayi et al. leached out silica from fly ash by KOH by varying the stirring rpm, size of fly ash, extraction temperature and solid-to-liquid ratio [180]. The optimum extraction parameters include time 6 h; 3M KOH; solid-to-liquid ratio 1:25 temperature 100 °C; rpm 500. However, there was no attempt made for the synthesis of SiNPs from the leachate further. Similar, work was also carried out by Wang et al. who studied the kinetics of silica and alumina extraction from fly ash by concentrated NaOH and observed the effect of temperature, stirring speed and mass ratio of sodium hydroxide to silica, on silica extraction rate [144]. The silica extraction was found to be 95.6% under the optimized conditions. Piekos and Paslawska (1998) investigated the leaching of Si element from fly ash, which was carried using distilled water, seawater and synthetic seawater with variable ratios of water and fly ash [181]. Here the tiles were crushed into powder using a mortar pestle and it was mixed with sodium bicarbonate in the ratio of 1:3 in a platinum crucible and calcinated at 1000 • C for 30 min. After the completion of fusion, the mixture, along with crucible, was dipped in a boiling mixture of dilute HCl and H 2 SO 4 in a beaker. Heating was done until the complete dissolution of the lumps. Finally, the mixture was filtered through Whatman filter paper no. 42, where the supernatant was discarded and the solid powder was retained. The filter paper along with the powder was calcinated at 400 • C for two hours. After the analysis of SiNPs by instruments, it was found that the particles were spherical shaped, fused and highly aggregated together to form large particles. The EDS revealed the purity, as there were only peaks for Si and O, mainly along with minor peaks of trace elements. The FTIR revealed the three characteristic bands in the region of 400-1200 cm −1 and XRD revealed broad humps with a peak centered at 2-theta 22 • , which confirms the amorphous nature of the SiNPs [182]. The summarized form of the chemically-mediated extraction and synthesis of silica from fly ash are given in Table 6. Virendra Yadav (2019) extracted sodium silicate from ferrous free fly ash using 8 M NaOH by keeping the solid-to-liquid ratio at 1:5 at 95 °C for 90 min along with stirring at 400-500 rpm. Silica gel was obtained by neutralization with 1M HCl and, finally, dried powder was treated with 1M HCl at 110 °C for 3 h to remove the trace elements in the form of impurities. The obtained precipitate was initially oven dried at 40-60 °C, followed by calcination at 400 °C for two hours. The purity of the obtained SiNPs was more than 92%, as confirmed by the EDS [50]. Virendra Yadav (2019) reported the synthesis of amorphous SiNPs from Gandhinagar (Gujarat, India), thermal power plant using the alkali dissolution method. The ferrous and alumina extracted fly ash residue was treated with 8 M NaOH, at 95 °C, in a round-bottom flask under reflux systems for 90 min by maintaining a solid-toliquid ratio of 1:5. The leachate of sodium silicate was obtained by centrifugation at 7000 rpm for 10 min. The residue was discarded while the sodium silicate was used for the synthesis of SiNPs by solgel method. Further, about 40 mL of sodium silicate was titrated with dilute 2N HCl at room temperature. A white gel formed near pH 10, which ceased to form on the further addition of dilute HCl. The gel was left undisturbed for 24 h for ageing. Further, the mixture was centrifuged at 5000-7000 rpm for 10 min to obtain the white precipitate. The white precipitate was dried in an oven at 60 °C for 4-6 h and was finally calcinated at 400 °C for 4 h in a muffle furnace by slowly increasing the temperature by 10 °C/minute until the temperature reached 400 °C. The final synthesized SiNPs were

Microbial Leaching of Silicon and Silica Syntheses from Fly Ash
Microbial methods can be used for the leaching of silica from the fly ash by fungi and bacteria. These microbes produce different acids, enzymes and other metabolites that initially leach out Si from the fly ash and then transforms the leached Si from the medium into SiNPs. Microbial-based silica synthesis has advantages over chemical methods, as there is no need for toxic chemicals. However, microbial methods are time-consuming in comparison to chemical methods. Bacteria also have a tendency to dissolve the silica from various minerals. A group of bacteria called "Phosphate solubilizing bacteria" [183] has a tendency to leach out silica from the silicates [184]. Bacteria belonging to this group are Bacillus circulans, Bacillus mucilaginous, Bacillus edaphics [185]. These bacteria show a significant effect in mineral solubilization, and are mainly phosphates, as well as silicates in the soil for the proper uptake of the nutrition by the plant. These bacteria have certain enzymes-polysaccharides-that have a valuable role in silica leaching wherein mucopolysaccharide [186] is the main lixiviant for the silica. Previously, Zhan et al. used the three different Bacillus strains and studied the leaching of silica from the bauxite ores by individual bacteria as well as in co-operation [187]. The mixed culture leached more silica from the solution in comparison to the individual microbe. Among fungi, only Fusarium oxysporum has been used for the synthesis of SiNPs from fly ash, where the purity of silica was up to 40% and impurities in the form of carbon 50% were present [188].
Bacillus circulans is Gram-positive, rod-shaped, endospore-forming bacteria that has the potential to desilicate the silica-enriched media [189]. They are a chemo-organoheterotrophic bacterium that leaches out silica from silicates by attacking the alumino-silicate bonds [190]. It is generally used as a bio-fertilizer as they have a tendency to dissolve the K and Si from soil [191]. They are also called "siliceous bacteria" as they can destroy the silica-rich minerals [192]. These heterotrophic bacteria can grow at temperature, ranging between 5-20 • C or 30-37 • C, but most optimally at 30-37 • C. They produce an extra-cellular polysaccharide, capsular slime and other metabolites such as organic acids 2-Keto gluconic acids, ammonia and various other amino acids which play a role in silica leaching [193]. Silica removal from fly ash by silicate bacteria B. circulans is due to the production of mucilaginous capsules containing exopolysaccharides (EPS) [194]. Here, in our approach, it produces mucopolysaccharides (MPS) that attacks the Na-silicate bonds and leach out soluble silica in the medium. Virendra Yadav [50] synthesized porous silica nanosheets of size 80-120 nm by using B. circulans MTCC 6811 supernatant and fly ash-extracted sodium silicate. The silica was synthesized after the incubation of the fungal supernatant and sodium silicate mixture at optimized conditions. The optimum ratio was 4:1 of supernatant to the extracted sodium silicate by volume. Fulekar and Yadav synthesized spherical shaped aggregated clusters of SiNPs of size 40-80 nm by incubating the Fusarium oxysporum supernatant and sodium silicate at optimized ratio-i.e., 3:2 in an incubator shaker at 28 • C for 48-72 h [50]. Here the Si was leached from the sillimanite and mullite of fly ash by the hydrolytic enzymes of F. oxysporum. The leached Si from the fly ash forms water-soluble SiNPs which was further dried and silica powder was obtained using the rotary evaporator. Khan et al. [188] also reported the leaching of Si from these two minerals of fly ash and the formation of water-soluble SiNPs which were recovered using the rotary evaporator. The summarized form of fungi-mediated leaching and synthesis of silica/silicon from fly ash are given below in Table 7. Reactions:

Properties and Applications of SiNPs
SiNPs have always been light for their use in the field of research, industries and medicine owing to their innumerable technological and biomedical applications. SiNPs are specifically used for resins, silica-based catalysts, molecular sieves and several other materials [195]. Until now, their application has been most exploited in biology and medicine as drug carriers [196]. Recently, biocompatible [197], bio conjugated [198] and doped SiNPs [199] have gained huge attention in cancer therapy-i.e., cancer cell imaging [200,201], DNA and microarray detection [202], barcoding tags separation [203], drug delivery [204] medicine [205] and the purification of biological molecules and cells [196]. Bulk silica is a conventional adsorbent that has been used for a very long time to remove odor. However, nowadays SiNPs have replaced conventional bulk silica and is used as a conventional adsorbent for wastewater treatment-i.e., purification of wastewater [206], removal of pollutants inorganic (heavy metals), organic pollutants (pesticides) and ultrasensitive single bacterium detection [207]. Their compounds and composites in the form of zeolites are also used for water purification.

Role of Nanotechnology: Nano adsorbents for Heavy Metal Removal
Nanotechnology involves the understanding and control of objects with dimensions ranging between 1-100 nm [208]. Nanotechnology is the science and art of matter that is manipulated on an atomic and molecular scale. According to the national nanotechnology initiative (NNI), nanotechnology was defined as "anything smaller than 100 nm with novel properties" [209]. The important features of nanotechnology and nanoparticles are attributed to their shape, size, surface characteristics and surface energy. As the nanoparticles have infinite size, the surface area is relatively large and consequently has higher reactivity and sorption capacity in comparison to several heavy metals [210]. These nanotechnology-based features allow the nanoparticles to become chemically more reactive by changing its strength and other properties. Hence, concerning the fortification of the environment, nanotechnology embraces the assurance of providing new and inimitable improvements [211]. Nanoparticles are used in several fields viz. wastewater treatment [212], medicine for drug delivery, electronics [213], etc. In wastewater treatment, nanoparticles are used as an adsorbents (activated carbon, silica gel, and alumina), which are generally obtained from the commercial precursors of respective nanoparticles that make them expensive, thereby making the whole adsorption mechanism cost intensive.
However, the production of the nanoadsorbents from waste materials, such as red mud [214], fly ash [215] and rice husk [216,217], makes them significantly cheaper, greener and environmentallyfriendly. However, cost analysis, easy availability, non-toxicity and recyclable nature are important factors for the selection of an adsorbent for the wastewater treatment and heavy metal removal that control the total expenditure of the adsorption process. If adsorbents are derived from the waste, such as fly ash, then the adsorbent cost will definitely be low and, ultimately, so will the cost of the adsorption process. Thus, nanoparticles synthesized from waste, such as fly ash, bauxite rice husk, etc., may act as potential candidates as precursors for nanoadsorbents. Nanoparticles may serve as an efficient adsorbent for the removal of heavy metals from wastewater due to their high surface area, enhanced adsorption sites and the functional groups that are present on their surface [213]. Nanomaterials have a wide range of applications regarding the technological and environmental challenges for wastewater treatments. Numerous works have been reported based on metal oxides, as they are very effective and efficient adsorbents in the clean-up of environmental contaminants [218][219][220] owing to their small size, which permits them to penetrate into the contamination zone [221,222] where other bulk adsorbents and micro particles cannot.
Though the fly ash derived nanoparticles have immense potential as nanosorbents for the remediation of heavy metals from wastewater, very few works are available where such nanoparticles derived from fly ash have been used for the remediation of pollutants or heavy metals. Thus, here several examples have been provided where IONPs and SiNPs have been used for the remediation of heavy metals. Out of both IONPs and SiNPs, the former has numerous advantages over SiNPs, as it can be easily manipulated under the influence of an external magnetic field and can exhibit superparamagnetism [223][224][225][226]. Moreover, due to the presence of unpaired electrons in their 3D shell, they have Fe 2+ , Fe 3+ ions, and form either ferrimagnetic or ferromagnetic particles [227,228].
The magnetic properties make their recovery very easy at the end of the reaction, and they are recyclable and lower in cost. In general, IONPs exists in various forms (magnetite, maghemite, and hematite) [229][230][231] which are commonly used for the remediation of heavy metals from wastewater. These IONPs remediate heavy metals by adsorption techniques, which is one of the most effective techniques in comparison to precipitation [232,233], coagulation [232][233][234], chemical reduction [235,236], ion exchange [235,237,238] and low expenditure. Some of the examples where IONPs have been applied, for the wastewater-based remediation of heavy metals, are cited below.
Chang et al. synthesized monodisperse chitosan-coated magnetite nanoparticles with the average size of 13.5 nm and utilized them for the remediation of Cu 2+ ions. Cu 2+ removal efficiency was above pH 2 [235]. Yantasee et al. [239] synthesized magnetite, functionalized with thiol, with a size of 40 nm, and applied it for the remediation of Hg, Ag, Pb, and Cd. Similarly, Song et al. [240] used (γ-Fe 2 O 3 ) encapsulated with polyrhodamine for the removal of Mn 2+ , Cr 2+ , Cd 2+ , Hg 2+ -here heavy metals were removed from the aqueous solution and it was concluded that the solution pH, initial metal ion concentration, contact time and species of metal ions influence the uptake of heavy metal ions. Chou and Lien., (2010) applied dendrimer-conjugated magnetite nanoparticles for the remediation of Zn at acidic and alkaline pHs [241]. Predescu and Nicole., (2012) synthesized and used 10 nm maghemite nanoparticles for the remediation of Zn, Cu and Cd at 2.54 pH from wastewater by adsorption [242]. Better adsorption capacity was obtained for metal ions, especially in the case of maghemite, into the cationic resin (γ-Fe 2 O 3 -R-H), and a higher adsorption tendency for hexavalent chromium in comparison to other metal ions. From the above work, it was concluded that, in all the cases, capped magnetic nanoparticles were synthesized from the iron precursors and further applied for the heavy metal removal.
In comparison to IONPs, SiNPs are used to a much lesser extent for the remediation of heavy metals from wastewater. They are mainly used for providing support to the nanoadsorbents or as a carrier for them. A few examples are given below where the investigators used silica either in bulk form or powder form for the removal of heavy metals, dyes, and pesticides from wastewater. Some of the selective works, cited below, focuses on the remediation of heavy metals from wastewater using SiNPs. Sheet et al. performed a study where nanosized graphite oxide, silica/graphite oxide composites and SiNPs were applied for heavy metal ion remediation from aqueous solutions using a batch adsorption method [243]. The nanosized graphene oxide-based remediation of heavy metals was in the following order: nickel>zinc>lead>cadmium>chromium. The results suggested that the optimum efficiency for heavy metal remediation is when the adsorbents (silica/graphite oxide composite) are in the ratio of (2:3). Karnib et al. comparatively studied the remediation of Cd, Pb, Zn, Cr, and Ni in a batch experiment by using activated [A c ] carbon, SiNPs and a composite of silica-Ac carbon. Ni exhibited the maximum removal percentage by A c -Carbon at all concentrations, whereas the percentage removal reduced with an increase in the concentration of heavy metals [244]. Kong et al. reported the selective removal of heavy metals (Pb 2+ , Cu 2+ , Hg 2+ , Cd 2+ , and Zn 2+ ) from the aqueous solutions using silica fume waste-derived SiNPs with different functional groups on their surface [245].
Yadav and Fulekar 2018 reported the biogenic synthesis of IONPs from iron precursors (salts of ferrous sulphate heptahydrate and ferric chloride), by the co-precipitation method. Here, all the solutions were prepared in the aqueous leaf extract of Tridax spps, which was used as a capping agent for IONPs. The synthesized IONPs were further characterized by sophisticated instruments. The synthesized IONPs ranged between 15-60 nm and were spherical in shape, but they also showed higher aggregation. The XRD and Raman revealed the mixed phases-i.e., magnetite and maghemite in the IONPs. Finally, the synthesized IONPs were used for the remediation of Pb and Cr from the 20% fly ash aqueous solutions in a batch experiment at fixed, temperature, pH, and dosage. The removal efficiency of Pb and Cr was up to 85 and 96%, respectively [21].
Yadav et al., 2020, reported the synthesis of 18-60 nm amorphous IONPs by using sonochemical methods, which were characterized by the sophisticated instruments and assessed their potential for the remediation of fly ash heavy metals-mainly Pb and Cd in a batch adsorption study. The Pb and Cr ions were remediated up to 97.96% and 82.8%, respectively, from 20% fly ash aqueous solutions [246].

Conclusions
Fly ash is considered as one of the major pollutants and one million tonnes is produced every year around the globe. The disposal of fly ash into ash ponds requires a huge amount of land that, in turn, leads to water and soil pollution. Additionally, fly ash is loaded with several toxic heavy metals, which impose a potential threat to living organisms. At present, in developing countries, such as India, where the utilization rate of fly ash is only 40-50% in the form of bricks, types of cement, tiles, etc., half of the fly ash remains unutilized every year. Fly ash is rich in ferrous, alumina and silica and can be successfully used in the recovery of valuable minerals which not only reduces the pollution but also provides an alternative source for ferrous, alumina and silica for the industries. Therefore, in the present review, emphasis has been given to the advancement in the methods for the recovery of ferrous, silica and alumina nanoparticles with the latest technology and instruments. Fly ash is a spherical-shaped particle constituting elements like Al, Si, and have either Mg or Ca, S on their surface, as revealed by the EDS. The ferrous particles are generally spherical in shape, whose sizes vary between nanometers to several microns. The magnetic fractions are easily extracted by the magnetic separation method. Among non-ferrous materials-i.e., silica and alumina-they comprise major fractions of fly ash. Silica, being insoluble in acids, was preferably extracted with alkali by chemical method and microbial synthesis using F. oxysporum and Bacillus spps. These microbes produce specific proteins and enzymes that initially leach the Si into the aqueous medium and bio transform the SiNPs. Alumina in fly ash is present in crystalline form, so it is relatively inert. Al, being amphoteric, reacts with both strong alkali and mineral acids and leaches out Al into the solution, which is processed further for the synthesis of alumina nanoparticles by chemical and microbial methods. From all the above-reported methods, the Al extraction efficiency varied between 82.5%-92.2%; it was observed that the molarity of the acids and base and temperature play an important role. The high molarity of either acids or base at high temperature may increase the yield from the fly ash. Fly ash may serve as a potential, reliable and alternative material for the ferrous-, alumina-and silica-based industries. Fly ash, deriving such nanoparticles, find reliable applications in wastewater treatment and environmental clean-up. In the future, fly ash derived nanoparticles can be used in the fields of electronics, ceramics, glass foundries, drug delivery, and agriculture, and in food industries, based on their purity.