Hydrothermal synthesis of zeolites from residual waste generated via indirect carbonation of coal fly ash

Indirect carbonation, a technology to store CO 2 and produce stable CaCO 3 and MgCO 3 , involves elution of Ca and Mg from industrial waste and subsequent carbonation. Although substantial residual waste is generated after the elution of Ca and Mg, its recycling attributes have not been adequately scrutinized. The residual waste has lower Ca and Mg contents and higher Si and Al contents than those of the raw material (i.e., industrial waste). This study involves the hydrothermal synthesis of zeolite‑P using residual waste from indirect carbonation, conducted at both 100 and 180 ℃ . The properties of these zeolites are compared with those synthesized from coal fly ash (CFA). The synthe‑ sized zeolites were characterized by X‑ray diffraction, field emission scanning electron microscope, thermogravimetric analyzer, and Brunauer–Emmett–Teller (BET). A high zeolite conversion efficiency was achieved through a hydro‑ thermal reaction (up to 87%), even though Si and Al were not added to the residual waste. Additionally, the cation exchange capacity and BET specific surface area of the synthesized zeolites were high (200 cmol kg −1 and 73 m 2 g −1 , respectively). These findings highlight the possibility of synthesizing zeolites using the residual waste from indirect carbonation as an alternative to conventional zeolite synthesis using industrial waste such as CFA. The synthesized zeolite‑P is expected to be effective in wastewater treatment, detergent manufacturing, and water softening.


Introduction
Zeolites have a three-dimensional structure comprising Al, O, Si, other alkali metals (Na and K), and H 2 O molecules [1].They are porous materials with a large surface area and have cavities arranged in an aluminosilicate hydrate framework with a negative charge.Over the past few decades, the demand for zeolites has significantly increased owing to their emerging applications in various fields including catalysis, adsorption, water softening, wastewater treatment, and agriculture.Approximately 150 different synthetic zeolites are currently synthesized worldwide, which account for a total annual zeolite production of approximately 3 Mt [2].The global zeolite market, which was valued at USD 4326 million in 2019, has been projected to be worth USD 6190 million in 2027 [2].However, significant challenges such as the production cost of synthetic zeolites and logistical constraints exist in this arena [2,3].Commercially, zeolites are produced from chemical precursors such as NaAlO 2 and Na 2 SiO 3 , making them considerably expensive.Therefore, alternative zeolite synthesis routes featuring reliable and economical source materials such as coal fly ash (CFA) [1,4], red mud [5], and kaolin [6,7] have been studied.The synthesis of zeolites using industrial waste containing abundant Si and Al instead of pure chemical reagents can assist in improving economic efficiency and reducing environmental pollution.
In this study, experiments were conducted to synthesize zeolites using the residual waste obtained via indirect carbonation of CFA-a coal combustion waste.Indirect carbonation is a technology for producing stable CaCO 3 and MgCO 3 , which involves extracting Ca and Mg from industrial waste or natural minerals using solvents, such as acids and ammonium salts, and then reacting them with CO 2 [6][7][8].Although indirect carbonation shows promise as a carbon-neutral technology for CO 2 storage, it is disadvantageous in terms of its questionable economic feasibility and landfill utilization for residual waste.Substantial residual waste is generated after the solvent-based extraction of Ca and Mg from industrial waste.Valuable components of the residual waste (i.e., Si and Al), which are present in industrial waste, remain intact but are landfilled.Notably, although the Ca and Mg contents of the residual waste are low, the contents of valuable elements such as Si and Al are often high.To the best of our knowledge, few studies have been conducted to date on recycling the residual waste from indirect carbonation.
CFA has previously been used as a material for indirect carbonation owing to its high Ca content as well as zeolite synthesis because of its high Si and Al contents [2,[9][10][11][12].CFA is a highly abundant industrial solid waste used for zeolite synthesis.Moreover, it is an excellent waste material for synthesizing meso-and microporous materials because of its similar chemical composition and physicochemical properties to those of zeolite [2,9,10].Although zeolite and CFA have similar elemental compositions, they differ in terms of their crystalline structure.
In terms of chemical composition, CFA primarily contains silica, alumina, ferrous oxide, and calcium oxide, along with different amounts of residual carbon and trace elements.However, significant inter-and intra-regional differences exist with respect to the composition [13].The chemical composition of CFA varies with location, the type of coal burned, burning conditions, and the origin of the parent coal.The composition and mineralogical phase content of CFA are principally influenced by the origin and type of coal burned in power plants.Generally, the CFA derived from sub-bituminous and lignite coals is characterized by higher CaO, MgO, and SO 3 contents and lower SiO 2 and Al 2 O 3 amounts than those of higher-grade fuels such as bituminous and anthracite coals [14].
The hydrothermal technique is considered the primary route for synthesizing zeolites using CFA.In this technique, water and a base are used as a solvent and mineralizer, respectively, at various temperatures and pressures.
In this study, we synthesized crystalline zeolite-P through a hydrothermal reaction using the residual waste produced after indirect carbonation to improve the economic feasibility of indirect carbonation technology and to implement a zero-discharge system.Zeolites were synthesized via the hydrothermal reaction of CFA-derived residual waste at low and high temperatures (100 and 180 C), and their properties were compared with those of zeolites obtained via the hydrothermal reaction of CFA.Additionally, the conversion efficiencies of the synthesized zeolites were determined by comparing the dehydration weight loss of synthesized zeolite and pure zeolite [15].

Materials
CFA obtained from a thermal power plant in Korea was used as an indirect carbonation material.The CFA particles were ≤ 425 μm in size.The following analytical-grade chemicals were used in the experiments: ammonium chloride (98.5%), hydrochloric acid (36.0%), and sodium hydroxide (97.0%).X-ray diffraction (XRD; SmartLab, Rigaku, Japan) and X-ray fluorescence (XRF; XRF-1700, Shimadzu, Japan) analyses were conducted to determine the components and properties of solid samples.Field emission scanning electron microscopy (FE-SEM; MIRA-3, Tescan, Czech Republic) and thermogravimetric analysis (TGA; TGA 7, Perkin Elmer, USA) were performed to examine the surface morphology of the solids and determine the temperature-dependent weight loss, respectively.For the TGA, each sample was placed on a Pt plate and heated from 50 to 900 C at 20°C min −1 to measure its weight change.The specific surface area of the solids was measured using the Brunauer-Emmett-Teller (BET) method (Autosorb-iQ, Quantachrome, USA).

Zeolite synthesis using residual waste
First, the residual waste from indirect carbonation was prepared as a raw material to synthesize zeolites.
CFA and an indirect carbonation solvent (0.3 M HCl or 0.3 M NH 4 Cl) were mixed at a ratio of 1:50 (g:mL) and stirred at 250 rpm for 1 h.The solvents have been commonly used in previous studies on indirect carbonation [8][9][10][16][17][18][19][20].The solids obtained by filtering the resulting suspension were dried.The products obtained using HCl and NH 4 Cl-referred to as 'residual waste' herein-are denoted as RW-HCl and RW-NH 4 Cl, respectively.
The zeolite synthesis is described henceforth.The two residual waste specimens were separately mixed with a 2 M NaOH solution at a ratio of 1:5 (g:mL) and then aged by stirring at 250 rpm at room temperature for 4 h.The resulting suspensions were hydrothermally reacted at 100 C for 24 h while stirring at 250 rpm and then filtered.The obtained solids were dried at 90 C for 12 h.The hydrothermal synthesis was also performed at 180 C. Additionally, zeolites were hydrothermally synthesized using CFA at 100 and 180 ℃ in the same manner.The synthesized zeolites-denoted as ZE-CFA-100, ZE-HCl-100, ZE-NH 4 Cl-100, ZE-CFA-180, ZE-HCl-180, and ZE-NH 4 Cl-180-were analyzed by XRD, XRF, TGA, and FE-SEM.

Adsorption properties of the synthesized zeolites
The cation-exchange capacity (CEC) and BET specific surface area were measured to evaluate the adsorption capacity of the zeolites hydrothermally synthesized at 100 and 180 C, and these values were compared with those of the raw materials (RW-NH 4 Cl, RW-HCl, and CFA).The CEC was determined using the ammonium method (EPA test method 9081).

Zeolite synthesis using residual waste
The Si and Al contents of the CFA were 58 and 18%, respectively, which were considerably higher than those of the other components; the Si/Al ratio was 3.2.The Si and Al contents of the residual waste were 58-60% and 21-24%, respectively, which were higher than those of the CFA; the Si/Al ratio was 2.3-2.8.The Si/Al ratios of both the CFA and residual waste were appropriate; therefore, additional aluminum or silicate was not required to synthesize zeolite P [19][20][21].
XRD analysis indicated that the primary components of both the residual waste and CFA were quartz (SiO 2 ) and mullite [3(Al 2 O 3 )•2(SiO 2 )] (Fig. 1a-c), which are precursors of hydrothermally synthesized zeolites and can act as sources of Si and Al, respectively.FE-SEM images of the residual waste and CFA revealed their spherical shape and smooth surfaces that hardly had any pores (Fig. 2a-c).
The solids obtained via hydrothermal synthesis at 100 and 180 C were confirmed to be zeolite P through their XRD patterns (Fig. 1d-i).P-type zeolites are known to belong to the gismondine family and exhibit unique characteristics [13].In particular, zeolite P can be used for wastewater treatment and agriculture.To construct the zeolite structure during the hydrothermal process, the Si and Al components of the residual waste and CFA must be dissolved in a solution first; the aging process plays a vital role in this regard [22].During the aging process conducted for more than 4 h in the present study, the Si and Al from quartz and mullite were dissolved in NaOH solution to form water-soluble sodium aluminosilicate.As a result, the aging conducted in the alkaline solution prior to the hydrothermal reaction promoted the zeolite synthesis [23,24].
The XRD peak intensities of the zeolites synthesized at 180 C (54,000-60,000 counts; Fig. 1g-i) were approximately 1.5 times higher than those of the zeolites synthesized at 100 C (34,000-53,000 counts; Fig. 1d-f ).The overall crystallinity and content of synthetic zeolites increase with increasing hydrothermal temperature [25].The differences between the zeolites formed at different temperatures can be ascribed to the properties of mullite-a mixture of silicon and aluminum oxide-which is more stable than quartz-a silicon oxide.Because of the low solubility and inertness of mullite, the zeolite formation could be limited depending on the synthesis conditions [14].In other words, the hydrothermal synthesis of zeolites using residual waste and CFA could have disadvantages such as the partial dissolution of Si and a lower dissolution rate of Al than that of Si at 100 C. FE-SEM images of the synthesized zeolites (Fig. 2d-i) revealed a significantly higher number of pores in the synthesized zeolites than those in the raw materials (i.e., the residual waste and CFA).The zeolites hydrothermally synthesized at 100 C had a cactus-like spherical shape with many flat parts on their surfaces (Fig. 2d-f ).In contrast, the zeolites synthesized at 180 C were irregularly shaped and had a coral-like morphology with extremely rough surfaces (Fig. 2g-i).Notably, the zeolites synthesized at 180 C had larger surface areas than those of the zeolites synthesized at 100 C. Depending on the SiO 2 / Al 2 O 3 ratio and other synthesis conditions, zeolite P exhibits different morphologies such as cylindrical, rectangular-prism-shaped, donut-like, fiber-like, diamondlike, cactus-like, nut-like, cabbage-like, and wool-ball-like morphologies [20].
Table 1 lists the XRF results of the zeolites hydrothermally synthesized at 180 C. Compared to the Na 2 O contents of the raw materials (0.98-1.13%), those of the synthesized zeolites increased significantly to 9.8-11.5%.The molecular formula of the synthesized zeolite-P calculated using the XRF results was Na 6 Al 6.9-7.9Si 12.3-16.9, whose Na:Al:Si molar ratio is relatively close to that of the theoretical formula (Na 6 Al 6 Si 10 O 32 •12H 2 O; identified by XRD) [20].Impurities other than the zeolites possibly existed in the hydrothermally synthesized solids because the suspension (raw materials and NaOH solution) was hydrothermalized without removing any impurities after the suspension was aged.
The Si/Al ratio of zeolites influences their adsorption properties.Low-silica zeolites (Si/Al < 2) have high ionexchange capacity and can be used to adsorb ammonium and heavy metals [2,26].This is applicable to the zeolites synthesized using residual waste at 180 C in this study, as their Si/Al ratio was 1.8-1.9(Table 1).Note that the Si/ Al ratio of the zeolites synthesized using residual waste is lower than that of ZE-CFA-180 and seemingly within the desirable limit, indicating superior applicability.

Zeolite conversion efficiency
Figure 3 shows the TGA results of the six zeolites synthesized using the residual waste and CFA and that of pure zeolite-P synthesized using Na 2 SiO 3 and NaAlO 2 [27].The TGA results were used to calculate the conversion efficiencies of the six zeolites.According to [15], the conversion efficiency of synthesized zeolites can be reliably calculated by TGA compared to other analytical methods.When a zeolite is heated, weight loss occurs owing to moisture loss at 40-400 C. Specifically, the loss of physically adsorbed moisture present in the zeolite pores occurs at 40-200 C, and the loss of moisture in the hydration complex formed using exchangeable cations occurs at 200-400°C [28].In the temperature range 40-400 C, the zeolites hydrothermally synthesized at 100 and 180 C exhibited weight losses of 12.0-15.5% and 16.0-16.5%,respectively; pure zeolite-P showed a weight loss of 18.5% (Table S1).
The conversion efficiency (%) was calculated by dividing the weight loss of the synthesized zeolite by that of pure zeolite P and multiplying by 100.The conversion efficiencies of the zeolites hydrothermally synthesized at 100 and 180 C were 65-84% and 87-89%, respectively; essentially, the zeolites exhibited a higher conversion efficiency at higher hydrothermal temperatures (Table 2).Notably, the zeolites hydrothermally synthesized at 180 C using only the residual waste and CFA without additional Si and Al exhibited high conversion efficiencies of up to 89%, with little difference observed in the zeolite conversion efficiency between the residual waste and CFA.These results suggest that both the residual waste from indirect carbonation as well as CFA, which has been extensively used to synthesize zeolites can be used to synthesize high-performance zeolites.

Adsorption properties of zeolites
Table 3 lists the CECs of the raw materials and synthesized zeolites.The CECs of the residual waste and CFA were similar and considerably low (4.8-5.3 cmol kg −1 ).These values are similar to those of previously investigated CFA [29,30].Moreover, the CECs of the zeolites synthesized at 180 C (178-200 cmol kg −1 ) were approximately three times higher than those of their counterparts synthesized at 100 C (46-57 cmol kg −1 ).Despite using the same aging process to synthesize the zeolites, the CEC of the zeolites increased with increasing hydrothermal temperature.The CEC of the zeolite P synthesized at 180 °C is similar to those reported previously (Table S2) [31][32][33][34].The significantly high CEC of the zeolites synthesized at 180 °C indicates their potential effectiveness in fields such as pollutant removal and water softening.
Table 4 lists the BET specific surface areas of the raw materials and synthesized zeolites; those of the zeolites synthesized at 100 and 180 C were 30-72 m 2 g −1 and 72-87 m 2 g −1 , respectively.The BET specific surface area of the zeolites increased with increasing hydrothermal temperature, similar to the CEC-temperature trend.The BET specific surface areas of the zeolites synthesized at 180 C (72-87 m 2 g −1 ) were 12-26 times higher than those of the residual waste and CFA.These results are consistent with the surface characteristics observed through FE-SEM (Fig. 2) in terms of the surface-related differences between the smooth raw materials (residual waste and CFA) and the pore-rich and pointed synthetic zeolites.The specific surface areas of the zeolite P synthesized using residual waste and CFA were similar to or higher than those previously reported.For instance, in previous studies, the BET specific surface area of zeolite P synthesized using CFA via hydrothermal curing and aging is 19-88 m 2 g −1 (Table S3) [19,20,30,[32][33][34][35].

Conclusions
In this study, zeolite P was hydrothermally synthesized at 100 and 180 C using the residual waste from indirect carbonation without adding Si or Al.The zeolites hydrothermally synthesized at 180 C had higher CECs and larger specific surface areas than those of their counterparts synthesized at 100 C.Moreover, the CECs of the zeolites synthesized at 180 C (189-200 cmol kg −1 ) were 36-39 times higher than those of the residual waste, while their BET specific surface areas (72-73 m 2 g −1 ) were 12-16 times higher.The conversion efficiency of the zeolites synthesized at 180 C was remarkably high (87%), emphasizing the suitability of the residual waste as a material for zeolite synthesis.This approach on utilizing residual waste offers several advantages such as mitigating the disposal of substantial waste in landfills, which can lead to significant environmental and financial benefits.Additionally, materials with superior ion exchange and adsorption properties can be obtained through residual-wastebased zeolite synthesis.The zeolite-P hydrothermally synthesized using indirect-carbonation-derived residual waste could be used to remove pollutants, including heavy metals, ammonium ions, and toxic and radioactive waste species; extract potassium from seawater; and soften water.The use of residual waste from indirect carbonation can open new horizons for enabling the economical synthesis of zeolites, being promising for use in many applications and fields.Future work will involve extensive research on the production of calcium carbonate through indirect carbonation, while synthesizing zeolite using the residual waste generated during this process.An economic assessment of the entire process will be also conducted together with quantitative determination of the CO 2 stored and energy consumed.

Fig. 3
Fig.3TGA results of pure zeolite-P and the zeolites synthesized at 100 and 180 C

Table 1
Molecular formulae of the synthesized zeolites calculated using the XRF results

Table 2
Conversion efficiencies of zeolites synthesized using CFA and residual waste at 100 and 180 C a Hydrothermal reaction temperature

Table 3
CECs of the raw materials and synthesized zeolites

Table 4
BET specific surface areas of the raw materials and synthesized zeolites a Hydrothermal reaction temperature BET specific surface area (m 2 g −1 )