Synthesis of 4A Zeolite Molecular Sieves by Modifying Fly Ash with Water Treatment Residue to Remove Ammonia Nitrogen from Water

Abstract

The widespread presence of ammonia nitrogen (NH₄⁺–N) pollutants poses a serious threat to water environment health. In this study, a novel zeolite (WTR–CFA zeolite) with excellent adsorption performance is synthesized using CFA as the raw material and water treatment residue (WTR) as the aluminum source through an ultrasonic–assisted alkali melt hydrothermal method. Compared with traditional CFA–zeolite, WTR–CFA zeolite only generates 4A zeolite with a single crystal phase, and the peak shape is sharp, which results in better crystallization. WTR–CFA zeolite perfectly solves the technical problems of the low utilization rate and poor controllability of the crystal form in traditional artificially synthesized zeolites. The maximum NH₄⁺–N adsorption capacity of WTR–CFA zeolite is 29.80 mg/g, which is higher than that of most adsorbents reported in previous studies. After five cycles of adsorption regeneration, the regeneration efficiency of WTR–CFA zeolite only decreased from 98.84% to 97.12%, which demonstrates excellent environmental value. The adsorption isotherms and kinetics of NH₄⁺–N conform to the Langmuir model and quasi–second order kinetic model, respectively, which indicates that ion–exchange–dominant chemical adsorption plays a major role in the adsorption mechanism. In summary, this study combines the use of CFA and WTR resources with the treatment of aquatic pollution to reduce material synthesis costs, control the crystal structure of WTR–CFA zeolite, and increase adsorption capacity. This approach achieves the goals of “waste treatment and turning waste into treasure”.

1. Introduction

Nitrogen is a necessary chemical element for the metabolic growth of organisms and a key nutrient in agricultural production [1]. However, the presence of excessive nitrogen can consume a large amount of dissolved oxygen in water [2]. Even if the concentration is as low as 3 mg/L, it can have toxic effects on aquatic organisms, including fish [3]. In addition, high levels of NH₄⁺–N in water can cause significant harm to the human body, such as carcinogenesis, teratogenicity, and weakened blood oxygen supply [4,5,6]. Reducing the entry of NH₄⁺–N into natural water ecosystems is an urgent need. Among various NH₄⁺–N removal methods, adsorption has shown good economic and practical advantages due to its simplicity, high efficiency, low investment, and reusable adsorbent [7,8]. The main challenge is to find adsorbent materials that are resource–rich, low–cost, highly efficient, and stable.

Zeolite is a potential environmental adsorption material that meets the application expectations. Among the commonly synthesized new zeolites, there are four crystal forms with high adsorption performance: 4A–type, 13X–type, NaY–type, and NaP–type. The 4A–type zeolites are more widely used for pollutant adsorption due to their excellent ion exchange ability [9], such as for iron and manganese [10], Hg²⁺ [11], Cs⁺ [12], and ciprofloxacin in wastewater [13]. In addition, the 4A zeolite has been synthesized by many researchers to treat NH₄⁺–N wastewater due to its high adsorption performance [14,15]. The current preparation of 4A zeolite mainly relies on pure chemical substances, which increases the economic cost of synthesis [16,17,18]. Therefore, inexpensive mineral raw materials and solid waste are in high demand to replace these synthetic chemicals.

As one of the most common types of industrial solid waste emissions, the worldwide amount of coal fly ash (CFA) has reached 750 million tons/year in recent years [19]. Because CFA has various toxic substances such as heavy metals and polycyclic aromatic hydrocarbons, improper CFA handling can seriously endanger human health and pose significant challenges to the atmospheric environment and land resources [20,21]. Because CFA has a similar composition to the 4A zeolite, CFA can provide a basis to synthesize zeolite, recycle waste, and achieve environmental remediation [22,23]. Researchers investigated the effect of different reaction conditions on the synthesis of zeolite using CFA as raw material and found that the molar ratio of Si–Al was the most important influencing factor [24,25]. Specifically, different Si–Al ratios will produce different types of zeolites. For example, the theoretical Si–Al ratio for the complete crystal cell of A–type zeolite is 1, the skeletal Si–Al ratio of X–type zeolite is between 1.0 and 1.5, and the skeletal Si–Al ratio of Y–type zeolite is between 1.5 and 3.0 [9]. Considering that CFA contains metal elements, the doping of some metal elements is beneficial for adjusting the ideal state of 4A zeolite synthesis [26]. For example, the Si–Al ratio of CFA can be adjusted by adding Al–containing exogenous substances to improve its zeolite synthesis [27]. To avoid the additional economic costs of directly adding chemical reagents, water treatment residue (WTR) with severe pollution and abundant aluminum element exhibits significant advantages. WTR is a sludge byproduct generated during the water purification process in a water supply plant [28,29,30]. Currently, the coagulants of water supply plants widely adopt aluminum salts, such as poly–aluminum chloride. Impurities in the raw water precipitate into mud with the coagulant, causing a large amount of aluminum ions to eventually deposit in the WTR [31,32]. Researchers have found that there is a certain risk of the release of PAC used by water supply plants when discharged into natural water bodies. Especially when the pH of natural water bodies is less than 4, there is a risk of aluminum leaching [33]. Therefore, using WTR as an aluminum source to modify CFA is conducive to the formation of the 4A zeolite, and subsequently alleviating the environmental risk of CFA and WTR.

Because a large amount of iron and aluminum coagulants is added during the treatment process, WTR contains tens to hundreds of times more metals (such as Al) than common solid waste [32]. To improve global emission standards, WTR cannot be directly emitted into the environment, land use costs are high, and its potential value cannot be realized. By using WTR as an Al source to modify CFA, it can make CFA and 4A zeolite have similar silicon–aluminum ratios, enhance the formation of the 4A zeolite, and reduce the environmental risk of CFA and WTR.

The purpose of this study is to (1) successfully synthesize pure phase, single–phase, and high crystallinity 4A zeolite (WTR–CFA zeolite) using WTR modified CFA; (2) investigate the effects of various influencing factors on NH₄⁺–N adsorption and evaluate the NH₄⁺–N removal capacity and cyclic regeneration ability of WTR–CFA zeolite; and (3) reveal the adsorption mechanism of WTR–CFA zeolite. The research results are expected to provide novel solutions to turn CFA and WTR into treasures and generate economic benefits as new resources. At the same time, 4A zeolite with a good crystal structure will be prepared to solve the environmental pollution caused by NH₄⁺–N wastewater.

2. Materials and Methods

2.1. Raw Materials and Reagents/Chemicals

The original CFA sample was collected from Zhejiang Sci–Tech University. Table S1 shows the chemical composition of CFA. CFA was mainly composed of 48.21 wt.% SiO₂ and 33.18 wt.% Al₂O₃. Its Si–Al ratio was 2.47, and the remaining components were small amounts of metal oxides. WTR was formed by adding PAC and PAM to campus lake water to coagulate and precipitate, and it was calcined at 800 °C before use. The chemical reagents (NH₄Cl, NaOH, HCl, KCl, CaCl₂, MgO, KNaC₄H₄O₆·4H₂O, and Nessler’s reagent) were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All aqueous solutions were prepared using deionized (DI) water.

2.2. Preparation of WTR–CFA Zeolite

Figure S1 shows the flowchart of the WTR–CFA zeolite preparation. First, the original CFA was ground, sieved, calcined and decarbonized at 800 °C. Then it was mixed with HCl solution at a liquid–solid ratio of 25 mL/g. After heating and refluxing for 1 h, the acid leached fly ash was then mixed evenly with NaOH at a mass ratio of 1.2:1 and underwent alkali melting at 650 °C. The obtained sample was represented as CFA. Through experiments on preparing samples with different proportions of WTR doping, it was determined that CFA and WTR were mixed in a 7:3 mass ratio (Figure S2). The mixture and distilled water were added to a polytetrafluoroethylene bottle at a liquid–to–solid ratio of 25 mL/g, aged at 90 °C for 2 h, and ultrasonically treated. Subsequently, the homogenized gel mixture was transferred to a hydrothermal reactor and crystallized at 105 °C for 8 h. Finally, the reactants were filtered, washed, and dried to obtain the product WTR–CFA zeolite. The product prepared by using CFA as the raw material without doping WTR was named CFA–zeolite. Physical and chemical analysis was separately conducted on the above two types of materials.

2.3. Physical and Chemical Measurements

The physical and chemical properties of the prepared samples were analyzed using the following techniques: (1) X-ray diffraction (XRD, D8 advance, Bruker Corporation, Billlerica, MA, USA) using Cu K radiation, operating at 40 kV and 80 mA, and with the samples scanned over the 5–60° interval; (2) scanning electron microscopy (SEM, GeminiSEM300, Zeiss, Oberkochen, Germany) with an accelerating voltage of 5 kV; (3) X-ray photoelectron spectroscopy (EDS, AZtecOne X–Max N 50, Oxford Instruments PLC, Abingdon, UK) for element mapping imaging with an accelerating voltage of 15 kV; and (4) thermogravimetric analysis (Netzsch STA449 F5 thermal analyzer, Netzsch, Selb, Germany) in the temperature range of 45–1000 °C.

2.4. Batch Adsorption Experiment

By adding DI water to dilute the NH₄⁺–N stock solution (1000 mg/L), simulated NH₄⁺–N wastewater with different concentrations was prepared for experiments. At room temperature (25 °C), a certain concentration of NH₄⁺–N solution was prepared in a stoppered Erlenmeyer flask for batch adsorption studying. The WTR–CFA zeolite samples were added, and 0.1 mol/L HCl and NaOH were used to adjust the NH₄⁺–N solution to the desired pH. Then, the sample was stirred on a constant–temperature magnetic stirrer and immediately centrifuged (4000 rpm for 10 min). The supernatant was passed through a 0.45 μm filter. The remaining NH₄⁺–N concentration was measured using Nessler’s reagent spectrophotometry. The NH₄⁺–N adsorption rate ($\eta$, %) and adsorption capacity ($q_e$ mg/g) were calculated as follows:

$$q_e=\left(C_0-C_e\right)V/m$$

(1)

$$\mathsf{\eta }=(C_0-C_e)/C_0$$

(2)

where q_e (mg/g) is the equilibrium adsorption capacity; $\eta$ (%) is the NH₄⁺–N adsorption rate; C₀ (mg/L) and C_e (mg/L) are the initial NH₄⁺–N concentration and equilibrium NH₄⁺–N concentration, respectively; and V (mL) and m (g) are the volume of the water sample and mass of the adsorbent, respectively.

The optimal adsorption parameters of WTR–CFA zeolite to adsorb NH₄⁺–N were studied using the solid–liquid ratio (1–16 g/L), solution pH (5–8), and cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺).

To measure the adsorption isotherm, nine initial concentrations of NH₄⁺–N of 4–400 mg/L were used (pH was 7, and the adsorbent dosage level was 5 g/L). Adsorption data (equilibrium NH₄⁺–N concentration versus quantity adsorbed) were fitted using the Langmuir and Freundlich models.

The Freundlich equation explains the adsorption onto a heterogeneous surface with uniform energy [33]:

$$q_e=K_F{C}_e^{1/n}$$

(3)

where 1/n is the Freundlich constant that represents the adsorption capacity, and K_F (mg/g) is the Freundlich constant that represents the adsorption intensity. The Freundlich adsorption constant n should be 1–10 for beneficial adsorption.

The Langmuir equation is based on the monolayer adsorption on the active sites of the adsorbent [34]:

$$q_e=\frac{q_m{K}_L{C}_e}{1+K_L{C}_e}$$

(4)

where q_m (mg/g) is the maximum adsorption capacity; K_L (L/mg) is the Langmuir isotherm coefficient.

The effect of the contact time, which varied from 0 to 90 min., was studied at pH 7, a dosage level of 5 g/L, and an initial concentration of 50 mg/L. The data were fitted to pseudo–first order and pseudo–second order kinetic models.

The pseudo–first order equation is

$$q_t=q_e(1-e^{-k_{1}t})$$

(5)

where k₁ (min⁻¹) is the pseudo–first order adsorption rate constant; q_t is the amount adsorbed at time t (min). The pseudo–second order equation is

$$q_t=\frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}$$

(6)

where k₂ [g/(mg·min)] is the adsorption rate constant of the pseudo–second order.

2.5. Desorption and Regeneration

The initial concentration of NH₄⁺–N was 400 mg/L, the solid–liquid ratio was 5 g/L, and the adsorption was performed for 1 h under pH 7 conditions. After 30 min, the supernatant was obtained, and its equilibrium concentration was measured to determine its saturated adsorption capacity. Then, 0.1 mg/L NaCl solution was used as the regeneration solution to regenerate the saturated WTR–CFA zeolite: 3 g of the saturated WTR–CFA zeolite was weighed, added to a conical flask containing 50 mL of regeneration solution, and stirred on a magnetic stirrer for 1 h. Afterward, the desorbed residue was removed, washed, and added to 400 mg/L NH₄⁺–N wastewater again to fully react for 1 h for re–adsorption. Finally, it was used in the next adsorption/desorption cycle as described earlier. The adsorption/desorption cycle was repeated five times under identical conditions. The regeneration rate was calculated using the following equation:

$$\eta =(400-C_e^{\prime })/(400-C_e)\times 100\%$$

(7)

where C_e and C_e’ (mg/L) are the equilibrium concentrations before and after regeneration, respectively.

3. Results and Discussion

3.1. Characterization of the Physical and Chemical Properties

Figure 1 shows the XRD profiles of CFA, CFA–zeolite, and WTR–CFA zeolite. The XRD pattern of the sample was compared with the diffraction pattern of the single–phase pattern in the data file prepared by the International Diffraction Data Center. The characteristic diffraction peaks of the 4A zeolite (PDF#43–0142) and 13X zeolite (PDF#38–2037) were observed in CFA–zeolite (Figure 1b). Meanwhile, the WTR–CFA zeolite only had the characteristic diffraction peaks of the 4A zeolite (PDF#43–0142) (Figure 1a) with sharp peak patterns and good crystallization. Thus, the crystal structure of the WTR–doped molecular sieve was changed, which made the crystal structure of the CFA–zeolite transition from a 13X zeolite to a 4A zeolite and ultimately form a single crystal structure.

Figure 2 shows the SEM images of CFA, CFA–zeolite, and WTR–CFA zeolite. The SEM observation of CFA showed amorphous spherical glass beads with varying particle sizes (Figure 2a1,a2). The maximum diameter of the particle was 27.0 μm. After the pre–treatment, aging, and crystallization of CFA, spherical CFA particles transformed into cubic 4A zeolite and octahedral 13X zeolite (Figure 2b1,b2), and the particle size also decreased to 4.4 μm. Finally, from Figure 2c1,c2, it was observed that by adding WTR to change the Si–Al ratio of CFA, the octahedral 13X zeolite gradually disappeared and eventually became a single cubic 4a zeolite crystal form, with a smaller particle size than CFA zeolite, reaching 2.7 μm. The XRD data in this study (Figure 1) also confirmed this result.

Figure 3 shows the elemental analysis results of CFA–zeolite and WTR–CFA zeolite with changes in the element content of the samples before and after the WTR modification. Figure 3a suggests that the silicon–aluminum ratio of the CFA–zeolite was 1.59:1. After WTR was used as the aluminum source, the silicon–aluminum ratio of WTR–CFA zeolite was 1.02:1, which was closer to the silicon–aluminum ratio of the 4A zeolite.

Additionally, Figure 4 shows the thermogravimetry–differential thermal analysis of WTR–CFA zeolite. Below 500 °C, a significant decrease in weight to approximately 80% was observed. This may be due to the water loss in WTR–CFA zeolite caused by heating. Between 500 and 1000 °C, the curve showed a basically stable state, indicating that there was almost no mass loss. It demonstrated that WTR–CFA zeolite had good thermal stability and satisfied the temperature requirements during the adsorption process.

3.2. Effect of NH₄⁺–N Adsorption Conditions

3.2.1. WTR–CFA Zeolite Dosage

Figure 5a shows the effect of the WTR–CFA zeolite on the NH₄⁺–N adsorption performance. With the increase in WTR–CFA zeolite dosage, the NH₄⁺–N adsorption rate also increased. When the WTR–CFA zeolite dosage was 5 g/L, a turning point occurred, and the NH₄⁺–N adsorption rate reached the maximum value of 72.33%. When the dosage was below 5 g/L, increasing the zeolite dosage could increase the loading number of ion exchange sites and the NH₄⁺–N adsorption rate [35]. When the dosage was greater than 5 g/L, the removal effect did not significantly change. The reason is that the effective adsorption capacity of NH₄⁺–N per unit mass of the adsorbent is limited. In addition, the agglomeration and sedimentation phenomena between zeolite particles were intensified, which decreased the available surface area during the adsorption process [36,37]. Therefore, 5 g/L is the optimal dosage for WTR–CFA zeolite.

3.2.2. Effect of pH

Figure 5b shows the effect of pH on the NH₄⁺–N adsorption performance. When the pH is 5–8, the highest NH₄⁺–N adsorption rate is 76.48%. When the pH is below 6, there is H⁺ in the system, H⁺ has a smaller particle size than NH⁴⁺, and there is a competition between the two [38]. In addition, the low pH may make 4A zeolite dissolve, which reduces the adsorption rate [39]. When the pH exceeds 7, NH⁴⁺ easily combines with OH⁻ to form NH₃·H₂O, which is not conducive to the NH₄⁺–N removal [40]. Therefore, pH 7 is reasonable for using WTR–CFA zeolite to treat NH₄⁺–N.

3.2.3. Effect of Coexisting Cations

Figure 6 shows that the presence of Na⁺, K⁺, and Ca²⁺ significantly reduced the NH₄⁺–N removal rate. The reason is that coexisting cations can change the pore size of the molecular sieve and compete with NH⁴⁺ for adsorption sites. When the competing ion is Mg²⁺, the removal rate is identical to that of NH⁴⁺ alone, which indicates that there was almost no competition for adsorption vacancies between Mg²⁺ and NH⁴⁺. This phenomenon is mainly related to the adsorption affinity of cations. The adsorption affinity of cations affects the exchange ability. A larger ionic radius corresponds to greater adsorption affinity and an easier exchange to the molecular sieve skeleton. The radius of Na⁺, K⁺, Ca²⁺, and Mg²⁺ in descending order are Ca²⁺>K⁺>Na⁺>Mg²⁺. Therefore, the order of effects of the same concentration of cations on the adsorption of NH₄⁺–N by WTR–CFA zeolite is Ca²⁺ > K⁺ > Na⁺ > Mg²⁺.

3.3. Adsorption Isotherms

Figure 6 shows the relationship between the Langmuir and Freundlich models, and

Table 1. Parameters of the adsorption isotherm of NH₄⁺–N on CFA–zeolite and WTR–CFA zeolite.

Material	Langmuir Model			Freundlich Model
	R2	qm (mg/g)	KL (L/mg)	R2	n	KF (mg/g)
CFA–zeolite	0.9959	27.08	0.0629	0.8458	2.418	3.162
WTR–CFA zeolite	0.9954	31.03	0.0628	0.8237	2.495	4.007

lists the calculated isotherm parameters and regression coefficients. Within the studied range of equilibrium NH₄⁺–N concentration, the Langmuir adsorption isotherm model (R² = 0.9954) better described the adsorption behavior of WTR–CFA zeolite than the Freundlich adsorption isotherm model (R² = 0.8237). Thus, the adsorption process after WTR modification was a uniform single-layer adsorption process. In addition, when 0.1 < 1/n < 0.5, the adsorption process proceeded easily. One obtained 1/n = 0.4 in the present isothermal equation, which indicates that NH₄⁺–N was readily adsorbed into WTR–CFA zeolite. According to the Langmuir adsorption isotherm, the maximum adsorption capacity (q_m) of WTR–CFA zeolite for NH₄⁺–N is 31.03 mg/g. The WTR–CFA zeolite synthesized in the present experiment has better adsorption capacity for NH₄⁺–N than many reported NH₄⁺–N adsorption materials in

Table 2. Adsorption capacities of NH₄⁺–N on other adsorbents compared to CFA–zeolite and WTR–CFA zeolite.

Adsorbents	Raw Materials	Adsorption Capacity (mg/g)	References
Naturally occurring clinoptilolite	/	14.72	[41]
Naturally occurring mordenite	/	15.13	[42]
Zeolite P1	CFA and Na2SiO3·9H2O	25.13	[43]
HAlO–modified Zeolite	Hydrated aluminum oxide and natural zeolite	30.00	[44]
Faujasite	Low–calcium fly ash	28.65	[36]
NaA zeolite	CFA	27.50	[45]
CFA–zeolite	CFA	27.08	This study
WTR–CFA zeolite	CFA and WTR	31.03	This study

. The reason may be that WTR provides sufficient aluminum sources to prepare 4A zeolite with higher purity, better structure, and higher cation exchange capacity. Overall, WTR–CFA zeolite has high application value in the field of NH₄⁺–N adsorption and is a promising adsorbent.

3.4. Adsorption Kinetics

To evaluate the kinetic mechanisms that control the adsorption process, pseudo–first order and pseudo–second order models were employed to interpret the experimental data. According to the relevant data in Figure 7, the adsorption capacity of WTR–CFA zeolite for NH₄⁺–N in wastewater significantly increased within 0–15 min of the reaction; after 15 min, the NH₄⁺–N adsorption amount remained relatively balanced. For the WTR–CFA zeolite synthesized in this study, within 0–15 min of the NH₄⁺–N adsorption reaction, the adsorption speed was mainly controlled by external diffusion. At this time, there was a relatively large concentration difference of NH₄⁺–N at the solid–liquid interface, a large hydraulic mass transfer force was generated, and a large amount of NH₄⁺–N was adsorbed in a short time. Within 15–30 min of the reaction, NH₄⁺–N had to be transferred from the surface of the adsorbent to the pores, which resulted in high mass transfer resistance and slow adsorption speed. After 30 min of reaction, the reaction reached equilibrium, the adsorption almost stopped, and there was no significant change in NH₄⁺–N removal.

According to the relevant kinetic coefficients in

Table 3. Parameters of the kinetic models for the adsorption of NH₄⁺–N on CFA–zeolite and WTR–CFA zeolite.

Material	Pseudo–First Order Model			Pseudo–Second Order Model
	R2	qm (mg/g)	K1 (min−1)	R2	qe (mg/g)	K2 (g/(mg·min))
CFA–zeolite	0.7609	19.24	0.00340	0.9997	18.98	0.0538
WTR–CFA Zeolite	0.7530	23.08	0.00223	0.9996	22.72	0.0448

, the quasi–second order model (Figure 7b) can better fit the kinetic data than the quasi–first order model (Figure 7a), and the calculated q_e is consistent with the experimental value. Thus, chemical adsorption is the rate–limiting step of adsorption [46].

3.5. Desorption/Regeneration

The study of adsorbent recycling is important to improve the economic practicability because the NH₄⁺–N saturated material is no longer effective and must be disposed of in a landfill. Figure 8 shows the WTR–CFA zeolite adsorption–regeneration cycle test chart. It can be seen that the NH₄⁺–N adsorption capacity of the WTR–CFA zeolite is 16.05 mg/g and 14.47 mg/g after five adsorptions–regeneration cycles, indicating that the recovered WTR–CFA zeolite still has good adsorption capacity. Therefore, the WTR–modified zeolite is a kind of adsorbent with good cycle regeneration ability. The decrease in adsorption ability may be due to the binding of some surface functional groups on the surface of WTR–CFA zeolite and the weak leaching force of NH₄⁺–N in neutral medium, causing the active site in the WTR–CFA zeolite channel to still be occupied by NH₄⁺.

3.6. Adsorption Mechanism

The material structure, physical adsorption, and ion exchange were involved in the adsorption of NH₄⁺–N onto WTR–CFA zeolite (Figure 9). The unique crystal structure and pore distribution of WTR–CFA zeolite facilitate the pollutant adsorption. Silicon oxygen tetrahedra and aluminum oxygen tetrahedra can construct an infinitely expanding three–dimensional spatial framework to form many holes and channels in the zeolite structure [47]. XRD and SEM analysis results showed that WTR–CFA zeolite formed a cubic crystal structure with only 4A–type zeolite. Moreover, the average pore size of the 4A zeolite is 0.4 nm, which is larger than the ion diameter of NH⁴⁺ (0.286 nm), so NH⁴⁺ can enter the hole. Thus, the effect of adsorbing NH₄⁺–N is achieved.

The physical adsorption of NH₄⁺–N can be promoted by the interfacial bonding force and surface charge of WTR–CFA zeolite. Mainly due to the combined action of the dispersion force and electrostatic force, the zeolite structure has a specific surface area that can generate an interfacial bonding force to absorb and remove NH₄⁺–N. In addition, zeolite has a significant electrostatic force due to its negative charged structure. This force forms an electric field around zeolite and enables WTR–CFA zeolite e to selectively absorb polar ammonia molecules [48].

The ion–exchange property of WTR–CFA zeolite is the main reason that NH₄⁺–N can be removed from water. The EDS analysis results indicated that the silicon–aluminum ratio changed from 1.59:1 to 1.02:1 after the modification with WTR as the Al source. This result suggests that Al³⁺ in WTR–CFA zeolite replaced Si⁴⁺ and that the tetrahedral structure of silicon oxide generated a large negative charge, for which exchangeable cations (such as K⁺, Ca²⁺, and Na⁺) in aqueous solutions compensated. However, the interaction force between the lattice and the cation in the lattice is relatively weak. Within a certain limit, it has reversible ion exchange performance [49], which makes it easy to exchange ions with NH⁴⁺ in aqueous solutions. In addition, the adsorption of NH₄⁺–N by WTR–CFA zeolite follows a quasi–second order kinetic equation, which indicates that the NH₄⁺–N removal by WTR–CFA zeolite mainly results from the combined action of ion exchange and physical adsorption. Here, the ion exchange plays a dominant role in the NH₄⁺–N adsorption.

4. Conclusions

This study successfully synthesized a 4A zeolite with a single intact cubic crystal structure using CFA and WTR through an ultrasonic–assisted alkali melt hydrothermal method. The batch adsorption results suggested that adding 5 g/L WTR–CFA zeolite to an NH₄⁺–N solution at pH = 7 maximized the adsorption capacity to 29.80 mg/g. Na⁺, K⁺, and Ca²⁺, which coexist in the external environment, compete with NH⁴⁺ for adsorption to the WTR–CFA zeolite, whereas Mg²⁺ has no significant effect. In addition, the experimental data fitting results conformed to the Langmuir model and quasi–second order kinetic equation, which indicates that the single–layer chemical adsorption dominated by ion exchange is the main adsorption mechanism. After five cycles of adsorption regeneration, the regeneration efficiency of WTR–CFA zeolite only decreased from 98.84% to 97.12%, which demonstrates excellent economic benefits and environmental value. Therefore, WTR–CFA zeolite is a promising adsorbent for removing NH₄⁺–N pollutants from wastewater and helping to achieve the resource utilization and “waste treatment” of CFA and WTR.