Experimental and Computational Approaches for the Structural Study of Novel Ca-Rich Zeolites from Incense Stick Ash and Their Application for Wastewater Treatment

At present, chemical Si/Al sources are mainly used as precursor materials for the manufacturing of zeolites. Such precursor materials are quite expensive for commercial synthesis. Here, we have reported the synthesis of Ca-based zeolite from incense stick ash waste by the alkali-treatment method for the first time. Incense stick ash (ISA) was used as a precursor material for the synthesis of low Si zeolites by the alkali-treatment method. The as-synthesized zeolites were characterized by various instruments like particle size analyzer (PSA), Fourier transform infrared (FTIR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), electron diffraction spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray fluorescence (XRF). FTIR and XRD helped in the identification of the microstructure and crystalline nature of the zeolites and also confirmed the synthesis of Ca-based zeolite with two thetas at 25.7°. The microscopic analysis by FESEM and TEM exhibited that the size of synthesized Ca-rich zeolites varies from 200 to 700 nm and they are aggregated and cuboidal in shape. Additionally, structural, electronic, and density of states’ characteristics of gismondine (Ca2Al4Si4O16·9H2O) structures were evaluated by computational simulations (first principle, density functional theorem). The structural optimization of structures was carried out in the first stage under the lowest condition of total energy and forces acting on atoms for the lattice constant, as well as the available experimental and theoretical findings. The present research approach predicted the transformation of ISA waste into a value-added mineral, i.e., zeolite, which was further used for the removal of both heavy metals and alkali metals from fly ash-based wastewater using inductively coupled plasma-optical emission spectroscopy (ICP-OES).


Introduction
Zeolites are crystalline aluminosilicates having elements either from group I or II as counter ions [1]. Zeolites mainly consist of Al, Si, and O, which forms their basic framework [2]. The structure of zeolites comprises the framework of [SiO 4 ] 4_ and [AlO 4 ] 5 tetrahedra linked to each other at the corners by sharing their oxygen [3,4]. The tetrahedral 3D network of zeolites has a lot of voids and spaces in them [5], and these voids supplement the characteristics to the zeolites, for instance, the adsorption of molecules in the huge internal channels [6,7]. Zeolites can be readily dehydrated and rehydrated [8] and are used as cation exchangers [9], soil decontamination [9,10], and molecular sieves [11]. The unique and remarkable properties of zeolites viz. porous nature, negative charge, water-insoluble, higher melting point, and water trapping property [12][13][14] make them a potential material as an adsorbent [15] for wastewater treatment [16], as fertilizers in agriculture [17], and as an ion exchanger [18][19][20][21][22][23][24][25][26]. Based on their origin, zeolites can be of two types, i.e., natural or synthetic [27,28]. Synthetic zeolites are synthesized in laboratory conditions by optimizing the conditions.
Presently, for commercial production, synthetic zeolites are preferred over natural zeolites, as they can be modified as per the need of the industry. Moreover, synthetic zeolites are more precise, ordered, crystalline, and uniform in size of the particles. However, the synthesis of zeolites from Al-and Si-rich chemical sources is not economical [29]. On the other hand, natural zeolites are present in nature in the form of volcanic ashes [30,31], sedimentary rocks [32], clay and kaolins [33,34], fly ash [28,35], bauxite minerals [36], and other earthy materials which may reduce the cost of the zeolites but purity remains a major concern. Incense stick ash (ISA) being a waste material of household and religious places can act as a source of zeolites owing to its similar composition to the natural zeolites [37,38]. Yet to date, no attempt has been made by the scientific community for the synthesis of zeolites from overlooked waste like ISA [39]. Every day, tons of incense sticks are consumed at religious places and homes in countries like China, Thailand, Japan, Burma, and India for deity purposes which in turn produces ISA. The burning of incense stick leaves behind ashes as a residue that has to be disposed of in the river or other water sources especially in India where it holds religious values and is considered holy [40,41]. Indian incense stick ash has high Ca, Mg, Fe, Si, and Al content where Ca and Mg alone constitute 50-60% total weight of the ash [42][43][44], while silica is 20% and alumina is 5-10% forming 25-30% of the total weight. All these elements are essential for the synthesis of zeolites [35]. So, instead of disposing the ashes into the river, it can be utilized for the synthesis of zeolites which may have potential to treat wastewater and protect the environment.
ISA was employed in the synthesis of zeolites, by the alkali-treatment method under laboratory conditions. As per our knowledge, ISA-based synthesis of zeolites has been reported for the first time over here. The synthesized zeolites were characterized by particle size analyzer (PSA), Fourier transform infrared (FTIR), field emission scanning electron microscope (FESEM), electron diffraction spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). The synthesized zeolites belong to the sodalite class and are crystalline, aggregated, and cuboidal in shape with size ranging from 200 nm to 700 nm. Further, the potential of synthesized zeolites was explored for the removal of heavy metals and alkali metals from fly ash aqueous solutions with respect to time using inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis. In conclusion, the utilization of ISA for zeolites will reduce the disposal problem of ISA and water pollution. 2.2. Method. The incense stick ash was transferred in a 100 ml beaker and washed 2-3 times with distilled water to eliminate the carbon and other unburnt particles. Further, from the ISA slurry, ferrous fractions were extracted using an external neodymium magnet [44]. Removal of ferrous from ash enhances the zeolitic property as it interferes with the transformation of zeolites [45]. The nonferrous fractions were dried either at room temperature or in an oven at 60°C till complete dryness. Thereafter, the nonferrous part was treated with 2 N H 2 SO 4 in a round-bottomed (RB) flask in solid to liquid ratio of 1 : 10 at room temperature (RT). The mixture was continuously stirred using a magnetic stirrer under reflux. It removes the excess of Ca in the form of chlorides from the ash. The residue was collected by centrifuging the mixture at 7000 rpm for 5 minutes and discarding the supernatant.

Materials and Methods
The dried residue was further treated with 2 N-6 N H 2 SO 4 at 95°C under stirring in a reflux condenser which removes Al from the ash by dissolving it at higher temperature. Further, the residue was collected by centrifuging the mixture at 7000 rpm for 5 minutes. The supernatant was removed, and the residue was washed 2-3 times with distilled water to eliminate any acidic moieties.
Further, in a RB flask, the dried residue was treated with 4-8 M NaOH in a ratio of 1 : 5 at 95°C along with continuous stirring for 90 minutes under reflux. The residue was collected by centrifuging the mixture at 7000 rpm for 10 minutes. The residue was collected, and the supernatant was discarded. The final residue was washed 2-3 times using distilled water to eliminate any NaOH or NaCl particles from the surface and dried in a hot air oven at 60°C. The complete schematic process for synthesis of zeolites from ISA is shown in Figure 1.

Preparation of 20%
Fly Ash Aqueous Solutions. 50gram fly ash was weighed and mixed with the 250 ml distilled water in a plastic bottle, and the mixture was placed inside a horizontal shaker at 200 rpm for 24-48 hours. After the time interval, the mixture was allowed to settle down at RT and filtered using Whatman filter paper no. 42 where the aqueous solution was retained while the residue was discarded. The aqueous solution was stored in a glass bottle which was used as a source of heavy and alkali metals.

Batch Adsorption Study of Heavy and Alkali
Metals from CFA Aqueous Solutions. Batch adsorption study was conducted for the remediation of heavy and alkali metals from aqueous solutions of fly ash using ISA synthesized zeolites. For this, 150 ml of CFA aqueous solution was transferred in a 250 ml Erlenmeyer flask. To this aqueous solution, 9 mg synthesized zeolites were added. The mixture was kept in an incubator shaker at 150 rpm and 25°C. An aliquot of the sample was collected initially at time 0 minute while the final sample was collected after 120 minutes. Both the samples were analyzed by the ICP-OES for the detection of the concentration of various heavy and alkali metals after the specified time.

Characterization of Ca-Rich
Zeolites. X-ray fluorescence spectroscopy (XRF) was used for the analysis of major elemental oxides of Ca-rich zeolites. The analysis was done to detect the chemical content of the zeolite sample, and for that, 5 grams of Ca-rich zeolite powder sample was taken and measured with PANalytical Epsilon 5, The Netherlands, fitted with a 4 KW with 60 kV, 125 mA, Rh anode X-ray tube. FTIR analysis was carried out using PerkinElmer, "Spectrum 6500" (USA) by solidifying the samples with KBr pellets at a resolution of 2 cm -1 . The mid-IR region, i.e., from 400-4000 cm -1 , provided the transmission measurement. The particle size measurement and distribution of the ISA sample were analyzed by Malvern Zetasizer, Z-90 (UK), particle size analyzer with a detection range of 0.1 nm to 300 microns, and it was analyzed by three average runs. The FESEM-EDS helps in revealing the size and eternal features, porosity, and shape of the synthesized calcium-rich zeolites. The surface structural analysis of zeo-lites was analyzed by the FESEM, Nova, NANOSEM 450 (FEI, USA), besides trace and minor elements by the EDS analysis using Oxford elemental analyzer attached with it. The XRD patterns of the calcium-rich zeolites were obtained using a D-8 Advance, Bruker (Germany), instrument that analyzed the sample in powdered form to confirm the nature and class of the synthesized zeolites. The XRD patterns were recorded in the 2-theta range of 5-70, with a step size of 0.02 and a time of 2 seconds/step at 30 kV and a current of 30 mA. The HRTEM was used for the identification of morphological details of zeolites, d-spacing, and diffraction pattern. The zeolite was dispersed into the double-distilled water and sonicated for 10 minutes, and using the drop cast method, a thin layer was deposited on carbon-coated copper grids which were analyzed by FEI Model Tecnai G2 20 Twin (200 kV) (USA).

Results and Discussion
The basic chemical composition of ISA comprises oxides such as CaO (49.6%), SiO 2 (20.3%), Al 2 O 3 (4.7%), Fe 2 O 3 (4.28%), MgO (3.9%), and K 2 O (8.2%), making it a suitable material for the synthesis of the zeolite [44]. The Si/Al ratio of ISA was four which indicates a higher amount of silica in the ISA. Through alkali activation and simultaneous heating, the silica phase (15-20%) existing in ISA participates in the zeolitization process by dissolving into the solution whereas quartz or mullite remains in the nonreactive mineral phase [46]. Materials with higher Ca content form calcium hydroxide in aqueous media, and owing to its lower water solubility, Ca forms calcium carbonate during zeolitization [46]. During this step, there is a rise in the pH of the solution, due to calcium hydroxide along with NaOH; moreover, Ca substitutes the native Na + ions in the sodium zeolites with Ca 2+ ions; hence, it may generate some calcium-exchanged zeolites [46,47]. The ISA treatment with the NaOH hydrothermal method synthesizes zeolites by the following mechanism or chemical reactions: 3.1. FTIR for Identification of Functional Groups of Zeolites. FTIR spectroscopy is an important analytical technique that is used for the identification of the functional groups present in the synthesized zeolites. On the basis of functional group, a zeolite can be classified into its specific class along with XRD and EDS data. Figure 2 shows a typical FTIR spectrum of ISA-based Ca-rich zeolite products like calcium carbonate which shows polymorphism [48]. The bands around 460 cm -1 are attributed to the interstitial pore formation in the synthesized zeolites [49][50][51][52]. The bands around 1135 cm -1 are ascribed to the silicate bond Si-O-Si [53,54], while sharper and deeper nature of the bands indicates enhanced crystallinity of the synthesized zeolites [55]. The bands near 1600 cm -1 are due to the -OH molecule, and another band near 3435 cm -1 is because of the bending vibration of the water molecule in the samples [56]. Incidentally, broader transmittance bands are detected between 3450 and  3 Adsorption Science & Technology 3440 cm -1 and centered at 3435 cm -1 which can be assigned to an asymmetrical O-H-stretching linked to Na + , Ca 2+ , Si 4+ , and/or Al 3+ [57].
This indicates that there is bridging between Si and Al tetrahedra by the -OH ion. However, hydrated aluminum silicates comprised of such OH-bonding in the end products also exist. The stretching vibrations near 3450 cm -1 and bending vibrations at 1640-1620 cm -1 in the end products suggest zeolite crystallization and higher cation exchange capacity (CEC) [58]. Figure 3 shows the particle size distribution of calcium zeolite particles. The average particle size is 1568 nm while the graph displays two peaks, a major peak at 1694 nm and a minor peak near 4851 nm. The PSA shows a larger particle size as compared to the TEM (200-700 nm) due to the fact that in PSA there is a formation of hydrodynamic layer (includes core and molecules adsorbed over the surface) around the nanoparticle which gives overall larger particle size [59]. Moreover, TEM analysis requires a dry sample; however, PSA uses a sample in a solvated state where solvent molecules associate with the nanoparticle surface to form a hydration layer [60], while calculating the particle size using TEM, this hydration layer is absent over the nanoparticle surface; hence, the obtained particle size was comparatively    Adsorption Science & Technology lower [61]. The PDI of the particle is 0.210 which proves that the particles are monodispersed in the solvent.

XRD for Phase Identification and Confirmation of Zeolite
Synthesis. The XRD diffractogram in Figure 4 revealed the minerals and crystalline nature of the Ca-zeolite which shows a major intensity peak at 25°and several minor intensity peaks at 31.6, 38.9, 41.7, and 48.95°. The formation of zeolites was determined based on the peak at 25°a nd d-spacing values. The most common Ca-based zeolites are gismondine (Ca 2 Al 4 Si 4 O 16 ·9H 2 O) which have the characteristic peaks at 21.7°, 26.6°, and 27.9°and the peak at 32.6°could be due to calcite [62].

XRF Analysis of Zeolites for Chemical Composition of
Zeolites. XRF technique is the most reliable technique for the detection of major elemental oxides present in a sample. Figure 5 shows a typical XRF spectrum of an ISA-based zeolite material, whose elemental composition is provided in Table 1. The spectra clearly show that the zeolite has the highest amount of Ca, followed by potassium, Fe, Silica, Mg, and Al. Higher Ca content confirms the presence of Ca-based zeolites. But the lower value of Al indicates that the zeolite needs to be supplemented with Al from outsource to maintain the Si/Al ratio. Lower Al content, i.e., 5-8%, in ISA can be maintained during the zeolitization step [62]. The resultant zeolite is low Si/Al-based zeolites [63]. The Ca-based zeolite nature is also supported by the FTIR and FESEM-EDS.   [64]. C and S are present as impurities which can be attributed due to the improper washing of the zeolite sample. As ISA has high Ca and C, so the synthesized Ca-based zeolite has both the elements in higher amount.
The absence of ferrous from EDS elemental composition analysis indicates the removal of most of the ferrous particles, which have a positive effect on the property of zeolites [65] since ferrous may interfere with the purity of the zeolites.
3.6. TEM Analysis for Morphology of Synthesized Calcium-Rich Zeolites. At the time of exposure of zeolites with the electron beams, there is a possibility of damage and charging of zeolites, so there is a requirement of sophisticated techniques such as high-resolution TEM for visualization [66]. The TEM micrographs shown in Figures 7(a)-7(d) reveal the morphology similar to the FESEM, where the cuboidalshaped particles have lengths varying between 200 nm and 700 nm and a width ranging from 100 nm to 300 nm. Besides, there are several aggregations of smaller particles which are visible as brighter particles under the TEM field. The first three images depict Ca-rich zeolite particles in groups, while Figures 7(d) and 7(e) show two and single particle, respectively. In both, the image particles are cuboidal in shape, while in Figure 7(e), the dimension of a single   6 Adsorption Science & Technology particle is 250 nm wide and 700 nm long. The crystal images of zeolite are overlapping to some extent; the edges of the crystal are obvious. Moreover, the scattering area electron diffraction (SAED) pattern in Figure 7(f) shows that the particles have polycrystalline nature. The Ca-based or Ca-rich zeolites have several advantages over Na-based zeolites. The application of Na-based zeolite is restricted in soil and water and is not an appropriate material to be used in agricultural-and aqua-based products as   [67]. The challenging problems in such situations are (a) to reduce the harsh conditions of Na-based ISA zeolites during its application in soil and water and (ii) to increase the Ca-exchanged zeolites. So the synthesized zeolites from the ISA will solve the problem associated with these issues.

Remediation of Heavy Metals and Alkali Metals by Ca-Rich Zeolites.
Here, the dosage of the zeolite particles was fixed, i.e., 10 mg in 200 ml solution of the wastewater. The remediation of heavy (Cd, Mn, Zn, Pb, Al, Cu, Co, Cr, and Ni) and alkali metals (Ca, Mg, Ba, and Al) by adsorption process in a multicomponent system was performed by shake flask method in an incubator shaker. The percent removal of heavy metals was calculated by the following formula: where C o and C f are the initial and equilibrium concentration (ppm) of metal ions in solution, respectively.
The removal efficiency of all the seven heavy metals (Cu, Co, Ni, Pb, Cr, Zn, and Cd) decreased, in the solution after 2 hours as shown in Table 2, while the value of Pb reached below the detection level of ICP-OES as the Pb concentration was already less in the wastewater solution in comparison to the other heavy metals. Moreover, higher electronegativity of Pb can also be a contributing factor [68]. In the process of metal ion adsorption, the elemental property of electronegativity becomes highly significant as higher electronegativity is responsible for the stronger covalent formation of metals with O atoms situated on the surface of zeolites [69].
As far as the remediation of the nonheavy metals (Ba, Ca, Mg, Mn, and Al) is concerned, a reduction in the value of Ba, Ca, Mg, Mn, and Al was noticed after 2 hours as shown in Table 3. All these are alkali metals that are present significantly in fly ash, and their disposal into the river or other water bodies will increase the alkalinity or hardness of water [70]. So zeolites can be an efficient and economical adsorbent for the removal of alkali metals from wastewater [70]. Here, the adsorption of both heavy metals and alkali metals takes place on the surface of zeolite particles due to the electrostatic attraction and coordination. Several investigators have also reported the remediation of numerous heavy metals from wastewater by nanoadsorbents due to electrostatic attraction and coordination [71]. The zeolite surface is positively charged at neutral or slightly acidic pH, which gives rise to PZC repulsion to cations. The coor-dination between CH on the zeolite surface and the bivalent metal ions is responsible for the adsorption of cations on the zeolites. Such phenomenon of adsorption and deposition was observed because the fly ash aqueous solutions are a multicomponent system where several heavy metals and nonheavy metals compete for the limited adsorption sites. Once these binding sites are occupied, there is a healthy competition among ions and nonmetallic ions, which results in desorption of the heavy metals.

Computation Approach
A theoretical simulation to investigate the structural, electronic, and density of states' characteristics of gismondine (Ca 2 Al 4 Si 4 O 16 ·9H 2 O) structures was carried out. Figure 8 shows the estimated theoretical models using first principles, density functional theory calculations [72] as implemented in the Quantum Espresso program [73]. Interaction of atomic cores and valence electrons utilizing the extremely soft pseudopotential technique is proposed by Vanderbil [73]. Perdew-Burke-Ernzerhof (PBE) [74] exchange correlational functional was used to solve the Kohn-Sham equation [75], using the generalized gradient approximation [76]. It has shown its ability to replicate ground state energy characteristics across a broad range of a variety of crystals that are similar to the experimental findings. An electronic wave function was added. 70 Ry energy cutoff values at particular k-points were produced by the 18 × 18 × 18 Monkhorst-Pack mesh in the Brillouin zone for gismondine (Ca 2 Al 4-Si 4 O 16 ·9H 2 O) structures. The energy convergence threshold between two successive steps was maintained at 10-6 eV, and the maximal Hellmann-Feynman force acting on each atom was kept to a minimum [77] less than 0.001 eV. Finally, the optimized structures were utilized to compute the electronic and density of states' properties calculated in the present study. Figure 9, the structural optimization of gismondine (Ca 2 Al 4 Si 4 O 16 · 9H2O) structures was carried out in the first stage under the lowest condition of total energy and forces acting on atoms for the lattice constant, as well as the available experimental [78] and theoretical findings. The computed value of the lattice constant agrees well with observed values [78] and differs by less than 1% from other theoretical work [79]. The differences can be attributed to the differences in calculating methods.

Electronic and PDOS Structure Calculation. In
To better comprehend the electronic characteristics, the density of states (DOS) and projected DOS (PDOS) are computed, since they may explain the cause for bandgap modulation and the gap of gismondine (Ca 2 Al 4 Si 4 O 16 · 9H 2 O) structures was in 4.734 eV. The DOS and PDOS of Ca, Al, Si, and O atoms are shown in Figure 9. The picture clearly shows that s, p, and d are the most contributed energy in the formation of gismondine type zeolite; the Ca and Al d orbital is the strong peak at Fermi level just above the conduction band maxima (CBM) in the range 0 to -8 eV, as well as some density contribution near the valence band minima (VBM) in the region 4 eV to 18 eV. This additional

Conclusion
Increasing global awareness towards environmental issues acts as a driving force towards the recovery of value-added minerals from waste materials. The present paper focuses on the green synthesis of zeolites using incense stick ash waste. The synthesis of zeolites from the household waste incense stick ash is justified as the waste, whose disposal causes environmental problem especially in water, is utilized. The obtained Ca-rich zeolites were cuboidal in shape with size between 200 and 700 nm as revealed by TEM analysis. The zeolites were microporous and crystalline in nature suitable as an adsorbent material for the remediation of contaminants present in water. The other analysis results such as FTIR, PSA, XRD, and XRF also confirmed the successful synthesis of zeolites from ISA. The present work effectively draws possibility to utilize the ISA for the synthesis of Carich zeolites. Besides, theoretical calculations were also established for the band structure and density of states'  9 Adsorption Science & Technology characteristics of gismondine (Ca 2 A l4 Si 4 O 16 ·9H 2 O) structures. The structural optimization was carried out in the first stage under the lowest condition of total energy and forces acting on atoms for the lattice constant. The bandgap of gismondine structures was in 4.734 eV. The quality of the fabricated zeolite product strongly relies on environmental treatments and the strength of the raw materials [80]. The ISA-based zeolites have the potential to remediate both heavy and alkali metals from wastewater. Hence, the ISAbased synthesis of zeolites is an efficient, low-cost, and green chemistry-based method that opens a new possibility in the field of material science and catalysis.

Data Availability
The data sets used and analyzed during the current study are available within the article only.

Conflicts of Interest
The authors declare that there is no conflict of interest. A., G.K., and N.C. were responsible for formal analysis; V.K.Y., G.G., and N.C. were responsible for investigation; V.K.Y., D.A., and N.C. were responsible for resources; G.K., D.A., M.H.A., G.G., and N.C. were responsible for data curation; D.A., V.K.Y., G.K., G.G., and N.C. were responsible for writing-original draft preparation; D.A. and V.K.Y. were responsible for writing-review and editing; V.K.Y., M.H.A., G.G., and N.C. were responsible for visualization; V.K.Y. and N.C. were responsible for supervision; V.K.Y., D.A., and N.C. were responsible for project administration; D.A. was responsible for funding acquisition. Reviewing, editing, and computational approach simulated by G.K.I. and S.H.K. and Mitesh B. Solanki. All authors have read and agree to the published version of the manuscript.