A Study on the Removal of Phosphate from Water Environments by Synthesizing New Sodium-Type Zeolite from Coal Gangue

Abstract

Excessive phosphorus emissions are a significant driver of severe eutrophication in water bodies, and developing an efficient and cost-effective adsorbent for phosphorus removal is imperative. In this study, a Na-type zeolite was synthesized from coal gangue sourced from an open-pit mine in Xinjiang province, China. The synthesis process involved drying, crushing, alkali activation, aging, hydrothermal crystallization, and Na⁺ ion exchange. Orthogonal design identified the optimal synthesis parameters: an alkali-to-ash ratio of 1:1, aging at 20 °C for 12 h, and crystallization at 130 °C for 12 h. Aging time exerted the greatest influence on the phosphate removal efficiency. The optimized zeolite exhibited excellent phosphate adsorption performance, achieving a removal efficiency of up to 96% and a capacity of 16 mg/g. The adsorption kinetics followed both pseudo-first-order and pseudo-second-order models, indicating processes governed by combined physical and chemical mechanisms. Isotherm data fitting with Freundlich and Langmuir models suggested the presence of both homogeneous and heterogeneous active sites. Thermodynamic studies confirmed a spontaneous and endothermic process, increasingly favorable at higher temperatures. Characterizations via scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy confirmed the formation of Na-type zeolite and revealed structural and compositional changes following phosphate adsorption. Aluminum and calcium binding played key roles in the chemical adsorption mechanisms. This work not only offers a high-efficiency, low-cost solution for phosphorus removal from wastewater but also provides a sustainable pathway for the valorization of coal gangue in the Zhundong area of Xinjiang, China.

1. Introduction

With the increasing exploitation of environmental resources, large volumes of phosphorus-containing wastewater are being discharged into rivers and lakes, making phosphorus one of the primary pollutants in surface water and a key contributor to eutrophication [1]. Currently, phosphorus removal technologies mainly include biological, chemical, and adsorption methods [2,3,4]. Among these techniques, adsorption has attracted growing attention due to its high efficiency, low energy consumption, operational simplicity, broad material availability, low cost, and potential for phosphorus recovery [5]. Zeolites, crystalline hydrated aluminosilicates with uniform microporous structures and high contents of silicon and aluminum, have demonstrated excellent adsorption capacities, and their thermal stability and structural resilience make them particularly suitable for wastewater treatment [6]. However, the cost of preparing synthetic zeolites using conventional chemical reagents is quite high [7]. So, searching for efficient, cost-effective, safe, and environmentally friendly alternative raw materials for preparing zeolites is imperative.

Coal gangue, low-carbon, hard industrial solid waste generated during coal mining and processing, includes excavation gangue, naturally occurring gangue, and waste separated during coal washing [8]. Currently, over 70% of China’s coal production comes from the ecologically fragile western provinces, leading to the accumulation of large quantities of coal gangue around mining areas [9]. Due to economic, technological, and logistical constraints, large volumes of coal gangue are stockpiled near mining areas in western China, leading to severe environmental degradation, including vegetation loss and land desertification. Therefore, there is an urgent need to develop effective and high-value utilization strategies for coal gangue to transform it from waste into a valuable resource and to promote sustainable, eco-friendly development in resource-rich regions.

Zeolites are generally classified into natural and modified types. The synthesis of modified zeolites typically involves controlling factors such as the SiO₂/Al₂O₃ ratio in the raw materials, impurity content, type of structure-directing agents, and crystallization time [10]. When the SiO₂/Al₂O₃ ratio exceeds 1.5 [11,12], the material is considered suitable for zeolite synthesis [12]. Initial studies in our study indicate that coal gangue in the Zhundong area of Xinjiang, China, contains high concentrations of SiO₂ and Al₂O₃ with a favorable ratio of 2–5. This ratio falls within the optimal range for zeolite synthesis. Although the syntheses and characterizations of zeolites from coal gangue have been extensively studied [7,13,14], there are no studies concerning the preparation of Na-type zeolite from the optimal range of SiO₂/Al₂O₃ in the Zhundong area of Xinjiang, China, and research on its performance and mechanism in phosphate adsorption is still lacking.

Herein, this study investigates the synthesis of Na-type zeolite using coal gangue from Xinjiang’s Zhundong open-pit mine as the primary raw material while optimizing parameters to achieve high crystallinity and superior phosphorus removal performance through orthogonal experiments. The performance and mechanisms for phosphate removal were systematically evaluated. Key influencing factors, including the adsorbent dosage, pH, and coexisting anions, were investigated to elucidate their regulatory effects on the adsorption process. Adsorption kinetics, isotherms, and phosphate speciation analyses were conducted to reveal the underlying removal mechanisms. This work establishes a sustainable “waste-to-resource” paradigm, simultaneously enabling high-value utilization of coal gangue (SiO₂/Al₂O₃ = 2–5) and effective remediation of phosphorus-contaminated waters while advancing circular economy principles in environmental management.

2. Materials and Methods

2.1. Materials

Coal gangue was collected from an open-pit coal mine located in the Junggar Basin, Xinjiang, China. Analytical-grade reagents were used throughout the study unless otherwise specified. Potassium dihydrogen phosphate (KH₂PO₄), ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), potassium persulfate (K₂S₂O₈), sodium chloride (NaCl), sodium nitrate (NaNO₃), sodium sulfate (Na₂SO₄), sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), and sodium dithionite (Na₂S₂O₄) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Ascorbic acid (C₆H₈O₆) and ammonium chloride (NH₄Cl) were obtained from China National Pharmaceutical Group Corporation (Sinopharm Chemical Reagent Co., Ltd.) (Shanghai, China). Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), ammonium fluoride (NH₄F), and hydrogen peroxide (H₂O₂) were purchased from their respective suppliers, as listed. Sodium citrate (C₆H₅Na₃O₇·2H₂O) was provided by Tianjin Zhiyuan Chemical Reagent Co., Ltd., (Tianjin, China) and sodium hydroxide (NaOH) was supplied by Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China) Ultrapure water used throughout the experiments was provided by Hangzhou Wahaha Group Co., Ltd. (Hangzhou, China) Unless otherwise specified, all reagents were of analytical grade and used without further purification.

2.2. Characterization

Scanning electron microscopy (SEM) was employed to observe the surface morphology variations in the coal-gangue-based Na-type zeolite prior to and following phosphate adsorption (SEM; ZEISS Sigma 360, Carl Zeiss AG, Oberkochen, Germany). The changes in the crystal structure and phase composition of the ceramic matrix before and after phosphate adsorption were analyzed using X-ray diffraction (XRD; Rigaku-2038, Rigaku Corporation, Tokyo, Japan), a Brunauer–Emmett–Teller (BET) surface area and pore size analyzer (ASAP 2460, Micromeritics, Shanghai, China), X-ray photoelectron spectroscopy (XPS; K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA), Fourier-transform infrared (FTIR) spectroscopy (Nicolet iS20, Thermo Fisher Scientific, USA), and X-ray fluorescence (XRF) spectroscopy (ZSX Primus IV, Rigaku Corporation, Japan).

2.3. Preparation of Coal-Gangue-Based Na-Type Zeolite

Coal gangue obtained from an open-pit coal mine in the Zhundong region of Xinjiang, China, was used as the primary raw material. Detailed information on the coal gangue can be seen in Figure 1. Sodium hydroxide (NaOH) served as the main alkali activator to reconstruct the aluminosilicate framework of the gangue through alkali fusion and subsequent hydrothermal crystallization. During this process, native cations in the material were replaced with Na⁺ to enhance the ion exchange performance of the resulting zeolite.

The collected coal gangue was air-dried for 3 days to minimize moisture content, then ground into powder using a roller crusher. The dried material was further milled using a planetary ball mill at 300 rpm for 2 h. The resulting powder was sieved through a 200-mesh screen, and the fraction passing through the sieve was collected as qualified mineral powder. The coal gangue powder was then mixed with a NaOH solution at a predetermined mass ratio in a thermostatic water bath. The slurry was stirred under controlled temperature for a set duration. The homogeneous mixture was subsequently transferred to a Teflon-lined autoclave and subjected to hydrothermal treatment in an oven with a specified temperature and time. After crystallization, the product was cooled to room temperature, centrifuged, filtered, and oven-dried to yield coal-gangue-based Na-type zeolite. The synthesis process is illustrated in Figure 2.

2.4. Phosphate Adsorption Experiments

All adsorption experiments were conducted in brown conical flasks (effective volume 150 mL) as reactors. Each flask was filled with 100 mL of 50 mg/L KH₂PO₄ solution, followed by the addition of the coal-gangue-based Na-type zeolite adsorbent. The sealed reactors were placed in a thermostatic shaker operating at 150 rpm and maintained at 25 ± 0.5 °C under dark conditions for dynamic adsorption tests.

Effect of Adsorbent Dosage: To investigate the influence of the adsorbent dosage on the phosphate removal efficiency, the coal-gangue-based Na-type zeolite was pre-dried to a constant weight at 105 °C for 12 h and added to the flasks in dosages ranging from 0.1 to 0.4 g with increments of 0.05 g. Each flask contained 100 mL of 50 mg/L KH₂PO₄ solution. The mixtures were shaken at 150 rpm and 25 ± 0.5 °C for 36 h before sampling.

Effect of Initial Solution pH: To evaluate the impact of the initial solution pH on phosphate adsorption, the pH of the 50 mg/L KH₂PO₄ solution (100 mL) was adjusted to 3.0, 5.0, 7.0, 9.0, and 11.0 using 0.1 mol/L NaOH or HCl solutions. A fixed dosage of 0.3 g adsorbent was added to each flask, which was then shaken at 150 rpm and 25 ± 0.5 °C for 36 h before sampling.

Effect of Coexisting Anions: The competitive effects of common anions (Cl⁻, NO₃⁻, SO₄²⁻, and CO₃²⁻) on phosphate adsorption were studied by adding sodium salts (NaCl, NaNO₃, Na₂SO₄, Na₂CO₃) to the 50 mg/L KH₂PO₄ solution (100 mL) at varying concentrations (0 to 300 mg/L, increments of 50 mg/L). Subsequently, 0.3 g of adsorbent was added. The mixtures were shaken at 150 rpm and 25 ± 0.5 °C for 36 h before sampling.

Adsorption Kinetics: For kinetic studies, 0.3 g of adsorbent (pre-dried at 105 °C for 12 h) was added to 100 mL of 50 mg/L KH₂PO₄ solution in 150 mL conical flasks. The flasks were shaken at 150 rpm and 25 ± 0.5 °C. Samples were collected at predetermined time intervals (0, 0.5, 1, 2, 4, 8, 12, 24, 36, and 48 h) for analysis.

Adsorption Isotherms and Thermodynamics: KH₂PO₄ standard solutions with concentrations ranging from 25 to 150 mg/L (increments of 25 mg/L) were prepared by serial dilution. Each 100 mL solution was adjusted to pH 6.8 ± 0.2 using 0.1 mol/L HCl or NaOH. A fixed adsorbent dosage of 0.3 g was added, and the mixtures were shaken at 150 rpm for 36 h at three different temperatures (15 °C, 30 °C, and 45 °C) before sampling.

To assess the reusability and stability of the gangue-based Na-type zeolite, five successive adsorption/desorption cycles were conducted. First, 0.30 g of the adsorbent was put into contact with 100 mL of 50 mg/L phosphate solution for 36 h to reach equilibrium. Desorption was carried out with 100 mL of 1 mol/L NaOH, followed by thorough washing to neutrality and vacuum drying at 60 °C prior to reuse.

To investigate the phosphate adsorption performance of the new sodium-type zeolite in real wastewater, 0.3 g of the adsorbent was added to 100 mL of domestic sewage (TP~3.1 mg/L) and 100 mL of municipal effluent (TP~0.9 mg/L). The mixtures were shaken at 150 rpm and 25 ± 0.5 °C for 24 h before sampling.

2.5. Phosphorus Fractionation Analysis of Phosphate-Saturated Na-Type Zeolite

To further investigate the phosphate adsorption mechanism of coal-gangue-based Na-type zeolite, sequential extraction was employed to fractionate the adsorbed phosphorus into five forms: soluble adsorbed phosphorus (S-P), aluminum-bound phosphorus (Al–P), iron-bound phosphorus (Fe–P), calcium-bound phosphorus (Ca–P), and occluded phosphorus (O–P) [15]. The extraction steps are described below.

Soluble adsorbed phosphorus (S-P): A 0.5 g sample of phosphate-saturated zeolite was mixed with 50 mL of 1 mol/L NH₄Cl solution and shaken for 0.5 h. After centrifugation and filtration, the supernatant was collected for phosphorus analysis.

Aluminum-bound phosphorus (Al–P): The residue from step (1) was washed twice with 25 mL saturated NaCl solution and centrifuged. Then, 50 mL of 0.5 mol/L ammonium fluoride solution was added for extraction under constant shaking at 25 °C for 1 h. The supernatant was separated by centrifugation, and 15 mL of 0.8 mol/L boric acid was added to adjust the pH to 3.0 before phosphorus determination.

Iron-bound phosphorus (Fe–P): The solid residue from step (2) was again washed twice with 25 mL saturated NaCl, centrifuged, and extracted with 50 mL of 0.1 mol/L NaOH. The mixture was shaken at 25 °C for 2 h, allowed to stand for 16 h, then shaken again for 2 h before centrifugation. The supernatant was diluted to 50 mL, followed by the addition of 1.5 mL concentrated H₂SO₄, and centrifuged again. Finally, 2.5 mL of the filtrate was used for phosphorus analysis.

Calcium-bound phosphorus (Ca–P): The residue from step (3) was washed twice with 25 mL saturated NaCl and extracted with 50 mL of 0.5 mol/L H₂SO₄. After shaking for 1 h and centrifugation, the supernatant was analyzed for phosphorus content.

Occluded phosphorus (O–P): The remaining residue was washed twice with 25 mL saturated NaCl and treated with a mixture of 40 mL of 0.3 mol/L sodium citrate and 5 mL of 1 mol/L NaHCO₃ in an 80 °C water bath. Then, 1 g of sodium thiosulfate was added and reacted for 15 min. The extract was diluted to 100 mL. A 5 mL aliquot was subjected to hydrogen peroxide oxidation and alkaline hydrolysis, followed by drying and redissolution to determine the phosphorus concentration.

Before total phosphorus analysis, all samples were centrifuged at 4000 rpm for 5 min to remove suspended solids. The supernatants were filtered through a 0.45 μm membrane prior to analysis.

Phosphorus concentrations (initial and residual) were measured using the potassium persulfate oxidation–molybdenum/antimony anti-spectrophotometric method [16]. The phosphate removal efficiency, adsorption capacity, and equilibrium adsorption capacity were calculated using the following equations:

The removal efficiency:

$$\mathrm{R}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})}{\mathrm{C}0}}\ast 100$$

(1)

Adsorption capacity calculation:

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}})\mathrm{V}}{\mathrm{m}}}$$

(2)

Equilibrium adsorption capacity:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})\mathrm{V}}{\mathrm{m}}}$$

(3)

where q_t (mg/g) is the adsorption capacity at time t; q_e (mg/g) is the adsorption capacity at equilibrium; C₀, C_t, and C_e (mg/L) are the initial, time-t, and equilibrium concentrations of total phosphorus in the solution, respectively; V (L) is the volume of the phosphate solution; m (g) is the mass of the zeolite used; and R (%) is the phosphate removal efficiency.

3. Results and Discussion

3.1. Optimization of Synthesis Parameters for Na-Type Zeolite

To optimize the preparation conditions for coal-gangue-based Na-type zeolite, an orthogonal experimental design using an L₁₆ (5³) orthogonal array was conducted. Five factors were selected for evaluation: the alkali-to-gangue ratio, aging time, aging temperature, crystallization temperature, and crystallization time. Each factor was tested at three levels. The phosphate adsorption capacity of the synthesized zeolite was used as the performance evaluation index.

Range analysis was employed to determine the relative influence of each factor on the phosphate adsorption capacity [15]. A detailed factor-level table is provided (

Table 1. Level and factor table.

Factors/Levels	Alkali-to-Gangue Ratio	Aging Time (h)	Aging Temperature (°C)	Crystallization Temperature (°C)	Crystallization Time (h)
Level one	1:0.5	12	20	90	6
Level two	1:1	24	50	110	12
Level three	1:1.5	36	80	130	24

). During the experiments, 0.3 g of the synthesized zeolite was added to 50 mL of phosphate solution with an initial concentration of 50 mg/L. The mixture was shaken at 150 rpm for 36 h in a thermostatic shaker at 25 °C. Afterward, samples were collected to measure the residual phosphate concentration, and the adsorption capacity was calculated. The experimental data were analyzed using the range analysis method to identify the most significant factors affecting phosphorus adsorption.

As shown in

Table 2. L₁₆(5³) orthogonal experimental design for the synthesis of coal-gangue-based Na zeolite.

Number	Alkali-to-Gangue Ratio	Aging Time (h)	Aging Temperature (°C)	Crystallization Temperature (°C)	Crystallization Time (h)	Adsorption Capacity (mg/g)
1	1:1	24	50	110	6	10.92
2	1:1	12	80	90	24	14.79
3	1:0.5	24	80	90	12	9.43
4	1:1.5	36	80	130	6	0.98
5	1:0.5	12	20	90	6	13.4
6	1:0.5	12	50	130	6	12.95
7	1:1.5	12	50	90	24	15.7
8	1:0.5	36	20	110	24	1.17
9	1:0.5	36	50	90	12	15.54
10	1:0.5	36	20	90	6	1.22
11	1:1.5	12	20	110	12	15.55
12	1:1	12	20	130	12	15.98
13	1:0.5	12	80	110	6	13.53
14	1:1.5	24	20	90	6	1.09
15	1:0.5	12	20	90	6	13.4
16	1:0.5	24	20	130	24	9.44
K1	90.08	115.3	71.25	84.57	67.49
K2	41.69	30.88	55.11	41.17	56.5
K3	33.32	18.91	38.73	39.35	41.1
R	56.76	96.39	32.52	45.22	26.39

, among the selected synthesis parameters for coal-gangue-based Na-type zeolite, aging time had the most significant influence on the phosphorus adsorption performance, while hydrothermal crystallization time exhibited the least impact. This suggests that adequate aging is critical for enhancing the reactivity and homogeneity of raw materials during the alkali fusion and zeolite crystallization processes.

The optimal synthesis conditions were determined to be an alkali-to-gangue ratio of 1:1, an aging time of 12 h, an aging temperature of 20 °C, a crystallization time of 12 h, and a crystallization temperature of 130 °C. Under these conditions, the synthesized zeolite achieved a maximum phosphate removal efficiency of 96%, with an adsorption capacity of 15.98 mg/g. These findings demonstrate that coal gangue can be effectively converted into high-performance zeolitic adsorbents through controlled alkali fusion–hydrothermal treatment, offering a promising strategy for both solid waste valorization and phosphorus pollution mitigation.

3.2. Adsorption Behavior and Mechanism of Phosphorus

3.2.1. The Effects of Environmental Operational Parameters

The dosage of the adsorbent significantly impacts the adsorption behavior and performance. So, the influence of the coal-gangue-based Na-type zeolite dosage on phosphate removal was investigated, as illustrated in Figure 3a. As the adsorbent dosage increased from 1 g/L to 4 g/L, the phosphate removal efficiency improved markedly from 51% to 96%. However, when the dosage increased from 3 g/L to 4 g/L, the removal efficiency showed negligible change.

Considering both process cost (minimal dosage) and treatment efficiency (removal rate ≥90%), we determined the optimal dosage to be 3 g/L. Under this condition, the adsorbent exhibited excellent performance, achieving a removal efficiency of 96% and a corresponding adsorption capacity of 15.98 mg/g. These results indicate that an appropriate dosage of coal-gangue-derived Na-type zeolite can effectively balance treatment efficacy and material economy in phosphate adsorption applications.

The initial pH of the solution significantly influences the phosphate adsorption performance of coal-gangue-based Na-type zeolite [17]. As the pH increased from 3 to 11 (Figure 3b), a clear decline in the adsorption capacity was observed, suggesting a relatively stable interaction between phosphate species and the zeolite surface in acidic-to-near-neutral conditions. However, when the pH increased beyond 7, the adsorption capacity dropped sharply. This decline can be explained by the increased concentration of hydroxide ions (OH⁻) on the zeolite surface at higher pH levels, which leads to intensified competition with phosphate species (HPO₄²⁻ and PO₄³⁻) for active adsorption sites, primarily attributed to changes in cation solubility, the surface charge of the adsorbent, and the speciation of phosphate in solution [18]. As a result, the phosphate removal efficiency of the zeolite was significantly reduced under alkaline conditions [19]. These results indicate that in wastewater treatment applications, optimizing the pH value is of vital importance for enhancing the adsorption efficiency of phosphorus removal by coal-gangue-based sodium-type zeolites.

In real aquatic environments, coexisting anions may compete with phosphate for active adsorption sites on coal-gangue-based Na-type zeolites. In this study, the influence of four common anions—Cl⁻, NO₃⁻, SO₄²⁻, and CO₃²⁻—on phosphate removal was systematically investigated. As shown in Figure 3c, all tested anions exhibited varying degrees of inhibitory effects on phosphate adsorption, following the order of CO₃²⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻.

The observed competition can be interpreted using the Hard and Soft Acids and Bases (HSABs) theory, which posits that hard acids preferentially coordinate with hard bases and soft acids with soft bases [20]. Phosphate (PO₄³⁻), a highly charged, low-polarizability tetrahedral anion, tends to form stable coordination bonds with hard acid sites (e.g., Fe³⁺, Al³⁺, Ca²⁺) present on the zeolite surface, such as ≡Fe–O–PO₃ [21]. Among the competing anions, SO₄²⁻ shares similar physicochemical characteristics with PO₄³⁻—a high charge and low polarizability—allowing it to compete effectively for the same hard acid sites and form ≡Fe–O–SO₃ bonds. However, due to its lower charge density, the binding strength of SO₄²⁻ is slightly weaker than that of PO₄³⁻ [22].

CO₃²⁻, another hard base, not only competes for binding sites but also precipitates with Ca²⁺ as CaCO₃, thereby reducing the availability of free Ca²⁺ for calcium phosphate precipitation. This indirect effect further inhibits the phosphate removal efficiency [23]. In contrast, Cl⁻, characterized by a low charge and high polarizability, is classified as a soft base with weak affinity for hard acid sites. Although Cl⁻ cannot effectively displace phosphate from binding with Fe³⁺ or Al³⁺, it may slightly reduce electrostatic attraction by compressing the electrical double layer via ionic strength effects [24]. NO₃⁻, with lower charge density and slightly higher polarizability than PO₄³⁻, is also a hard base but shows limited competitive interaction due to its weaker coordination ability with hard acid sites [25]. Overall, the inhibitory effects of coexisting anions on phosphate adsorption followed a descending trend of CO₃²⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻, highlighting the importance of considering ionic composition in practical applications of zeolite-based phosphate removal systems.

3.2.2. Adsorption Kinetics

As shown in Figure 4, the phosphate adsorption efficiency of the coal-gangue-based Na-type zeolite increased rapidly during the initial 24 h, which can be attributed to the abundance of available active sites on the zeolite surface. After 24 h, the adsorption process gradually approached equilibrium, indicating saturation of the adsorption sites. This two-stage behavior—an initial rapid uptake followed by a slower equilibrium phase—is typical of solid–liquid adsorption systems. The equilibrium adsorption capacity was determined to be 15.98 mg/g, with a phosphate removal efficiency exceeding 95%.

To further elucidate the adsorption mechanism, the kinetic data were fitted using several commonly employed models: the pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models [26]. These models help evaluate the rate-controlling steps and potential adsorption mechanisms involved in the phosphate removal process. The corresponding mathematical expressions for these models are provided below:

The pseudo-first-order equation:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}(1-{\mathrm{e}}^{-{\mathrm{tk}}_1})$$

(4)

The pseudo-second-order equation:

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{{{\mathrm{q}}_{\mathrm{e}}}^2{\mathrm{tk}}_2}{(1+{\mathrm{q}}_{\mathrm{e}}{\mathrm{tk}}_2)}}$$

(5)

The Elovich equation:

$${\mathrm{q}}_{\mathrm{t}}=\mathrm{a}+\mathrm{blnt}$$

(6)

The intraparticle diffusion model:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{p}}{\mathrm{t}}^{1/2}+\mathrm{c}$$

(7)

The adsorption capacity at time t is denoted as q_t (mg/g), while q_e (mg/g) represents the equilibrium adsorption capacity. The constant K₁ (min⁻¹) corresponds to the rate constant of the pseudo-first-order kinetic model, and K₂ (g·mg⁻¹·min⁻¹) is the rate constant of the pseudo-second-order model. The constant k_p (mg·g⁻¹·min^1/2) in the intraparticle diffusion model reflects the diffusion rate within the adsorbent particles. C is the surface characteristic constant of the adsorbent.

The pseudo-first-order kinetic model (R² = 0.98) provided a better fit to the experimental data compared to the pseudo-second-order (R² = 0.96) and Elovich models (R² = 0.95), indicating that physical adsorption was the predominant mechanism during the initial stage of phosphate uptake. This suggests that phosphate ions were primarily adsorbed onto the surface of the Na-type zeolite through van der Waals forces or electrostatic interactions [27]. However, the pseudo-second-order model yielded a calculated equilibrium adsorption capacity (15.898 mg/g) that closely matched the experimental value, implying that chemisorption may also contribute to the overall adsorption process [28].

The adsorption process of phosphate onto coal-gangue-based Na-type zeolite can be divided into four primary stages: bulk diffusion, film diffusion, intraparticle diffusion, and surface adsorption [29]. According to the intraparticle diffusion model, the adsorption process involves two distinct phases. The initial linear portion of the plot corresponds to external film diffusion, while the subsequent linear region is associated with intraparticle diffusion within the porous structure of the adsorbent [30]. Notably, the multilinear plot does not pass through the origin, suggesting that phosphate adsorption onto the zeolite is a complex, multi-mechanism process involving both surface interaction and pore diffusion.

Moreover, the comparison of the two intraparticle diffusion rate constants (Kp₁ and Kp₂) in

Table 3. The kinetic parameters of equilibrium phosphorus adsorption onto coal-gangue-based Na-type zeolite.

Models	Parameters	Values
Pseudo-first-order equation	q1 (mg/g)	16.013
	K1 (1/h)	0.076
	R12	0.976
Pseudo-second-order equation	q2 (mg/g)	15.898
	K2 (1/h)	0.002
	R22	0.963
Elovich equation	a (mg/g)	2.369
	b (mg/g·h)	0.211
	R22	0.953
Intraparticle diffusion equation	Kp1 (mg/g·h0.5)	2.866
	C1 (mg/g)	0.971
	R12	0.969
	Kp2 (mg/g·h0.5)	0.385
	C2 (mg/g)	13.109
	R22	0.784

reveals that Kp₁ is approximately 7.5 times greater than Kp₂, indicating that intraparticle diffusion is likely the rate-limiting step in the overall adsorption process.

3.2.3. Adsorption Isotherms

At adsorption equilibrium, isotherm models are employed to quantitatively describe the relationship between the amount of adsorbate retained on the solid phase and its equilibrium concentration in the liquid phase, thereby providing insight into the underlying adsorption mechanisms. Among the classical models, the Langmuir isotherm assumes monolayer adsorption onto a homogeneous surface with identical binding sites and no interaction between adsorbed molecules. In contrast, the Freundlich model is empirical and accounts for multilayer adsorption onto heterogeneous surfaces.

These models are fundamental for understanding the surface characteristics and adsorption behavior of the materials. The Langmuir isotherm, in particular, is based on a theoretical framework that presumes uniform adsorption sites and single-layer coverage without lateral interactions among adsorbed molecules [31]. The corresponding equation is expressed as follows:

$${\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}={\displaystyle \frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{q}}_{\mathrm{m}}}}+{\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{m}}}}$$

(8)

The Freundlich adsorption isotherm is an empirical model that describes multilayer adsorption onto heterogeneous surfaces. Unlike the Langmuir model, it assumes that the adsorption sites are not equivalent and that the adsorption energy decreases exponentially with surface coverage. This model is particularly useful for characterizing adsorption processes involving surface heterogeneity and variable affinity. The mathematical expression of the Freundlich isotherm is given as follows:

$${\mathrm{lnq}}_{\mathrm{e}}={\mathrm{lnK}}_{\mathrm{F}}+{\displaystyle \frac{1}{\mathrm{n}}}{\mathrm{lnC}}_{\mathrm{e}}$$

(9)

In the above equations, q_e (mg/g) and C_e (mg/L) represent the equilibrium adsorption capacity and the residual phosphorus concentration, respectively. In the Langmuir model, q_m (mg/g) denotes the maximum adsorption capacity predicted under monolayer adsorption, while K_F (L/mg) is the Langmuir constant related to the affinity of the binding sites. In the Freundlich model, K_F (L/mg) is a constant indicative of adsorption capacity, and n is the heterogeneity factor reflecting adsorption intensity. A higher n value indicates stronger adsorption affinity and greater adsorption efficiency of the material.

As shown in Figure 5, increasing the initial phosphate concentration from 50 mg/L to 150 mg/L, along with elevating the reaction temperature from 15 °C to 45 °C, significantly enhanced the phosphate adsorption performance of the coal-gangue-based Na-type zeolite. Higher temperatures shortened the diffusion path of phosphate ions within the mesoporous structure and enhanced mass transfer efficiency at the solid–liquid interface.

The Langmuir and Freundlich constants and their corresponding correlation coefficients for coal-gangue-based Na-type zeolite are presented in

Table 4. Langmuir and Freundlich isotherm parameters of phosphorus adsorption onto coal-gangue-based Na-zeolite.

Temperatures	Langmuir			Freundlich
	qm (mg/g)	KL (L/mg)	RL2	KF (L/mg)	n	RF2
15 °C	31.238	0.078	0.9829	5.328	2.542	0.9830
30 °C	34.621	0.090	0.9773	6.061	2.486	0.9845
45 °C	34.870	0.143	0.9723	8.221	2.857	0.9738

. Both isotherm models demonstrated an excellent fit to the adsorption data, with R² values exceeding 0.95. This indicates that within the tested temperature range, phosphate adsorption onto the coal-gangue-based Na-type zeolite involves a combination of chemisorption and physisorption mechanisms. The adsorbent provides both homogeneous monolayer adsorption sites, as described by the Langmuir model, and heterogeneous active sites, as reflected by the Freundlich model. Notably, the heterogeneous sites exhibit a slightly stronger affinity for phosphate adsorption than other sites.

Furthermore, the maximum adsorption capacity (q_m) increased with temperature, rising from 31.24 mg/g at 15 °C to 34.87 mg/g at 45 °C. This trend, alongside the continuous increase in the Langmuir constant (K_L), suggests that the adsorption process is endothermic. Higher temperatures promote phosphate diffusion and enhance the exposure of active sites, although a slight decrease in the surface affinity of the adsorbent was observed.

In the Freundlich isotherm model, the parameter n reflects the adsorption affinity between the adsorbate and the adsorbent. Values of n within the range of 2 to 10 indicate favorable adsorption; n = 1 corresponds to a linear adsorption process, while n < 0.5 suggests poor adsorption capacity [31]. As shown in Table 4, the n values for phosphate adsorption by coal-gangue-based Na-type zeolite are all greater than 2, indicating that the material has a strong affinity for phosphate uptake.

3.2.4. Adsorption Thermodynamics

Temperature is a critical factor influencing both the adsorption efficiency and equilibrium capacity of an adsorbent. In this study, the effect of varying temperatures (15 °C, 30 °C, and 45 °C) on the phosphate adsorption performance of coal-gangue-based Na-type zeolite was systematically investigated. The results demonstrated that increasing temperature enhanced both the adsorption efficiency and the maximum adsorption capacity.

Previous studies have shown that in a solid–liquid adsorption system, two concurrent processes occur: solute adsorption and solvent desorption. The adsorption of solute molecules by the adsorbent leads to a decrease in system entropy due to reduced molecular freedom, whereas solvent desorption contributes to an increase in entropy. The interplay between these opposing entropy changes is a key factor governing the adsorption behavior [32].

The thermodynamic parameters related to phosphate adsorption were calculated using the following equations:

$${\mathrm{K}}_{\mathrm{d}}={\displaystyle \frac{{\mathrm{q}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{e}}}}$$

(10)

$$\text{△}{\mathrm{G}}^0=-{\mathrm{RTlnK}}_{\mathrm{d}}$$

(11)

$${\mathrm{lnK}}_{\mathrm{d}}=-{\displaystyle \frac{\text{△}{\mathrm{H}}^0}{\mathrm{RT}}}+{\displaystyle \frac{\text{△}{\mathrm{S}}^0}{\mathrm{R}}}$$

(12)

The thermodynamic parameters—Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°)—were employed to evaluate the influence of temperature on the adsorption mechanism and spontaneity of phosphate removal by coal-gangue-based Na-type zeolite. The results are illustrated in Figure 6, and the corresponding equilibrium constants and thermodynamic values are summarized in

Table 5. Equilibrium adsorption partition coefficient and thermodynamic function of coal-gangue-based Na-type zeolite.

Temperatures/K	Kd	ΔG/kJ·mol−1	ΔH/kJ·mol−1	ΔS/kJ·mol−1
288	1.752	−1.347	19.623	0.072
303	2.443	−2.255
318	3.831	−3.551

.

The calculated ΔG° values were negative at all studied temperatures, indicating that the phosphate adsorption process is spontaneous. Furthermore, the magnitude of ΔG° decreased with increasing temperature, suggesting that higher temperatures enhance the adsorption rate and favor the overall adsorption process. A positive ΔH° value confirms that the adsorption is endothermic, meaning that heat input promotes greater phosphate uptake on the zeolite surface. This is consistent with the observed increase in adsorption capacity at elevated temperatures.

The positive ΔS° value indicates an increase in system disorder during adsorption, which can be attributed to the enhanced mobility of phosphate ions and the release of water molecules from hydration shells as adsorption proceeds. This increase in entropy further supports the favorable nature of the adsorption process at higher temperatures. Together, these thermodynamic findings suggest that phosphate adsorption onto coal-gangue-based Na-type zeolite is a spontaneous and endothermic process, with increased temperature enhancing both the adsorption capacity and molecular disorder at the solid–liquid interface.

3.2.5. Analysis of Saturated Phosphorus Forms in Coal-Gangue-Based Na-Type Zeolite

To further elucidate the phosphate removal mechanism of coal-gangue-based Na-type zeolite, a sequential extraction method was employed to differentiate the adsorbed phosphorus into five fractions: soluble phosphate (S-P, extracted with NH₄Cl solution), aluminum-bound phosphate (Al-P, extracted with NH₄F and H₃BO₃ solutions), iron-bound phosphate (Fe-P, extracted with NaOH solution), calcium-bound phosphate (Ca-P, extracted with H₂SO₄ solution), and occluded phosphate (O-P, extracted with a mixture of Na₃C₆H₅O₇·2H₂O, NaHCO₃, and Na₂S₂O₄) [33].

As shown in Figure 7, the dominant phosphorus species after adsorption were Ca-P (37%) and Al-P (35%), accounting for over 72% of the total adsorbed phosphate. Occluded phosphate represented approximately 21%, while Fe-P and soluble phosphate contributed only 1.8% and 5.2%, respectively. These results suggest that the adsorption of phosphate onto the modified zeolite is governed by a synergistic mechanism involving both physical and chemical adsorption. This finding is consistent with the earlier kinetic and isotherm analyses, further supporting the material’s high affinity and specificity for phosphate binding through ligand exchange and surface precipitation pathways.

3.3. Mechanism of Phosphate Adsorption onto Coal-Gangue-Based Na-Type Zeolite

The phosphate adsorption mechanism of coal-gangue-based Na-type zeolite was further elucidated through surface morphology analysis using scanning electron microscopy (SEM). As shown in Figure 8a, prior to adsorption, the zeolite exhibited a well-developed porous structure characterized by microcracks and a loose surface morphology, indicative of typical zeolite crystallinity. This rough and irregular surface facilitates enhanced contact with phosphate ions and increases the number of exposed active sites, thereby promoting chemisorption.

After phosphate adsorption, the SEM image in Figure 8b reveals that the zeolite surface becomes coated with a layer of crystalline material, appearing relatively smooth and with fewer visible pores. This morphological transformation supports the hypothesis that ligand exchange primarily occurs at the surface active sites of the zeolite, where phosphate ions bind through chemical interactions [34].

These findings, consistent with previous adsorption kinetic and isotherm results, confirm that both surface structural characteristics and active site availability play crucial roles in phosphate removal.

X-ray fluorescence (XRF) analysis was employed to investigate changes in the elemental composition of the coal-gangue-based Na-type zeolite before and after phosphate adsorption. As shown in

Table 6. Elemental composition of coal-gangue-based Na-type zeolite before and after phosphorus adsorption.

Elements	Before Adsorption	After Adsorption	Units
O	42.847	51.128	mass%
Si	17.759	13.243	mass%
Ca	12.361	9.736	mass%
Na	11.984	8.239	mass%
Al	7.223	5.398	mass%
Fe	3.305	3.402	mass%
C	1.917	2.892	mass%
K	0.887	1.284	mass%
Ti	0.717	0.733	mass%
Sr	0.662	1.767	mass%
S	0.194	0.191	mass%
Ba	0.082	0.072	mass%
Mn	0.055	0.052	mass%
P	0.009	1.863	mass%

, the zeolite predominantly contains Al, Si, and Ca, indicating that these elements play important roles in the phosphate removal process. Following phosphate adsorption, a marked increase in phosphorus (P) content was observed, confirming the effective adsorption capacity of the zeolite. Concurrently, a decrease in the relative intensities of the Al and Ca peaks was also noted, which may be attributed to partial hydrolysis or the involvement of these elements in surface complexation or precipitation reactions during adsorption [15]. These findings suggest that the phosphate removal mechanism involves not only surface adsorption but also possible chemical interactions with Al- and Ca-containing functional groups within the zeolite framework.

The specific surface area and pore structure of the coal-gangue-based Na-type zeolite were characterized using a fully automated BET surface area analyzer. As shown in Figure 9a, the material exhibited a specific surface area of 8.82 m²/g, a total pore volume of 0.0009 cm³/g, and an average pore diameter of 22.20 nm. The mesoporous fraction displayed an average pore size of 29.83 nm.

These results indicate that the material possesses a mesoporous structure, which facilitates the diffusion of phosphate ions and provides accessible pathways to active adsorption sites, thereby contributing to its overall phosphate removal efficiency.

Figure 9b presents the X-ray diffraction (XRD) patterns of the raw coal gangue and the coal-gangue-based Na-type zeolite synthesized via alkali fusion–hydrothermal crystallization. Phase identification was conducted using the Jade 6.5 software with standard reference cards. The raw coal gangue primarily consisted of SiO₂, CaCO₃, and CaAl₂Si₂O₈, with no detectable peaks corresponding to typical zeolite structures. In contrast, the synthesized material exhibited distinct characteristic diffraction peaks corresponding to Na-type zeolite phases such as Na₃.₆Al₃.₆Si₁₂.₄O₃₂·14H₂O and Na₆Al₆Si₁₀O₃₂(H₂O)₁₂, confirming the successful formation of crystalline zeolite. These results demonstrate that the alkali fusion–hydrothermal method is an effective and feasible approach for converting coal gangue into Na-type zeolite, which exhibits strong potential for phosphate removal applications [35].

As shown in Figure 9c, the raw coal gangue exhibited a broad absorption band at 3427 cm⁻¹, attributed to the O–H stretching vibrations of hydroxyl groups or adsorbed water. After phosphate adsorption, the absorption peak at 980.3 cm⁻¹ was enhanced and slightly shifted, which can be attributed to the substitution of surface hydroxyl groups (e.g., ≡Si–OH or ≡Al–OH) by phosphate ions, resulting in the formation of ≡Si–O–PO₃⁻ or ≡Al–O–PO₃⁻ bonds. This structural transformation alters the vibrational modes of Si–O and Al–O bonds, indicating that chemical bonding plays a dominant role in the adsorption process [36].

Moreover, the disappearance of the characteristic peak at 1448 cm⁻¹ after adsorption suggests that phosphate binding occurred primarily through newly exposed surface functionalities—such as mesopores, hydroxyl groups, and metal oxide active sites—rather than involving regeneration of the original functional groups. These findings confirm that the alkali fusion–hydrothermal crystallization process effectively removes inert components from raw coal gangue, exposing active sites (e.g., Al–OH and Si–O⁻) and thereby significantly enhancing the material’s phosphate adsorption capacity [37].

Based on adsorption experiments and kinetic, isotherm, and thermodynamic models, combined with comprehensive characterization techniques including SEM, XRD, XRF, and FTIR, the phosphate adsorption mechanism of coal-gangue-based Na-type zeolite (Figure 10) is summarized below.

Ion exchange: Ion exchange is a mass transfer process that involves the exchange of ions between a solid adsorbent and the liquid phase. During phosphate adsorption, phosphate ions (PO₄³⁻) replace exchangeable metal cations such as Na⁺ and Ca²⁺ present in the zeolite structure. The alkali fusion–hydrothermal modification significantly increases the Na⁺ content in the zeolite, thereby enhancing its ion exchange capacity and improving the phosphate removal efficiency.

Surface complexation: Surface complexation involves the formation of chemical bonds between phosphate ions and active functional groups on the zeolite surface. This process releases H⁺ ions and consumes hydroxyl groups. The binding affinity and stability of metal–phosphate complexes on the zeolite surface vary depending on factors such as the ionic radius, valence state, and electronic configuration of the metal ions involved [38].

Physical adsorption: Physical adsorption is driven by weak intermolecular forces, including van der Waals interactions, dispersion forces, induction forces, and electrostatic attractions. Phosphate ions are adsorbed onto the zeolite surface through these reversible interactions, establishing a dynamic equilibrium between adsorption and desorption [39,40]. Both van der Waals forces and electrostatic attractions contribute to the physical adsorption of phosphate onto the coal-gangue-based Na-type zeolite.

3.4. Comparison of Phosphate Removal Performance Among Different Materials

As shown in

Table 7. Comparison of phosphate adsorption amount of different materials.

Name	Form	Adsorption Capacity (mg/g)	References
Steel slag	Powder	2.20	[41]
Coal cinder	Powder	4.47	[42]
Shale	Blocky	4.37	[42]
Biological filter material	Blocky	2.79	[43]
Phosphorus removal filter material	Blocky	4.24	[43]
Volcanic rock	Granule	1.16	[43]
Vermiculite	Blocky	1.63	[43]
Artificial biochar	Blocky	6.79	[44]
Rice-husk-ash-derived zeolite NaP1	Powder	19.3	[45]
Lanthanum hydroxide-modified zeolite	Powder	9.14	[46]
Lanthanum-Modified Sludge Biochar	Powder	30.77	[47]
Coal-gangue-based Na-type zeolite	Powder	34.87	This study

, the maximum phosphate adsorption capacity of coal-gangue-based Na-type zeolite, calculated from Langmuir isotherm parameters, is 34.87 mg/g. To evaluate its phosphate removal efficiency, this value was compared with several commonly used adsorbents.

The results demonstrate that the coal-gangue-based Na-type zeolite exhibits adsorption capacities multiple times higher than those of other materials, including steel slag, coal slag, shale (8 times), biofilter media (12 times), conventional phosphate removal media (8 times), volcanic rock (30 times), vermiculite (21 times), and synthetic biochar (5 times). These comparisons highlight the superior phosphate adsorption performance of the modified zeolite, indicating its strong potential for practical wastewater treatment applications.

3.5. Regeneration Study and Actual Water Body Effects

In order to evaluate the reusability and stability of the gangue-based Na-type zeolite, five successive adsorption–desorption cycles were conducted (Figure 11). The results demonstrate that the material maintains high stability during the first four cycles. The initial adsorption capacity was 15.98 mg/g with a phosphate removal efficiency above 95%. After the second and fourth cycles, the adsorption capacities were 15.64 mg/g (2.1% decrease) and 15.55 mg/g (2.7% decrease), respectively, indicating negligible performance loss. In the fifth cycle, the adsorption capacity decreased to 10.99 mg/g (31.2% reduction), which is likely due to partial pore blockage and exhaustion of active sites after repeated regeneration.

Importantly, these results confirm that the gangue-based Na-type zeolite exhibits excellent reusability and structural stability over at least four cycles, with only moderate loss observed in the fifth cycle. Moreover, alkaline regeneration not only enables material reuse but also facilitates phosphate recovery from the regenerant solution, thereby reducing the risk of secondary pollution. The phosphate-rich alkaline eluate can be further processed to precipitate calcium phosphate, enabling nutrient recovery and minimizing solid waste generation.

The phosphorus removal efficiency in real water samples was then evaluated to assess the material’s practical purification effect. As shown in Figure 12, the new sodium-type zeolite efficiently reduced the phosphate concentrations from 3.1 mg/L and 0.9 mg/L to below 0.015 mg/L. The phosphorus removal rates in both domestic sewage and municipal effluent exceeded 98% within 2 h, demonstrating strong phosphate retention and minimal desorption. These results confirm the excellent performance and practical applicability of the gangue-based Na-type zeolite in real wastewater systems.

3.6. Economic and Environmental Analysis

The gangue-based Na-type zeolite was synthesized under the optimal condition of an alkali-to-gangue ratio of 1:1. Based on current market prices, the cost of NaOH is about 0.30–0.80 USD/kg, while coal gangue is industrial solid waste that is abundantly available at negligible or near-zero cost. Accordingly, the estimated preparation cost of the adsorbent is 0.15–0.40 USD/kg, which is substantially lower than that of conventional commercial zeolites (typically 1–3 USD/kg) or specialty modified sorbents [48,49,50,51].

In addition to the low raw material cost, the adsorbent exhibited good regeneration ability, retaining more than 97% of its phosphate adsorption capacity after four reuse cycles. This recyclability further reduces the effective treatment cost per cycle.

From an environmental perspective, the use of coal gangue not only diverts large volumes of industrial solid waste from landfill but also converts it into a value-added material for water remediation. Furthermore, the spent zeolite, enriched with phosphate, shows potential as a soil amendment. As shown in

Table 8. Heavy metal content of the used new sodium-type zeolite.

Name	Units	Cd	Cr	Cu	Pb	Zn
Used new sodium-type zeolite	mg/kg	0.01	10.22	5.82	2.19	4.32
Soil pollution risk screening value in China (6.5 < pH ≤ 7.5)	mg/kg	0.3	200	100	120	250

, we analyzed the contents of conventional heavy metals (Cd, Cu, Pb, Cr, and Zn) in the used zeolite and compared them with the soil pollution risk screening values in China (6.5 < pH ≤ 7.5, GB 15618-2018) [52]. The measured concentrations were far below the threshold values, indicating low ecological risk and supporting its potential application as a nutrient-rich soil amendment.

Compared with engineering alternatives such as sediment dredging or large-scale ecological restoration, which are labor-intensive and costly, the coal-gangue-based Na-type zeolite provides a more convenient, efficient, and economically viable solution for phosphorus control in eutrophic waters. This dual advantage of industrial waste valorization and low-cost pollutant removal highlights the strong potential of the proposed adsorbent for large-scale environmental applications.

4. Conclusions

This study synthesized coal-gangue-based Na-type zeolite from Xinjiang coal gangue via alkali fusion–hydrothermal treatment and Na⁺ ion exchange and investigated its phosphate removal performance and mechanism. Using orthogonal design and range analysis, the optimal synthesis parameters were identified as an alkali-to-ash ratio of 1:1, an aging time of 12 h, an aging temperature of 20 °C, a crystallization time of 12 h, and a crystallization temperature of 130 °C. Among these, aging time had the greatest impact on the phosphate removal efficiency. Phosphate removal improved with adsorbent dosage, optimal at 3 g/L (96% removal, 16 mg/g capacity). Acidic pH favored adsorption; increasing pH reduced efficiency. Coexisting anions inhibited adsorption in the order CO₃²⁻ > SO₄²⁻ > NO₃⁻ > Cl⁻, yet zeolite maintained good capacity even at high competing ion concentrations. The kinetics fitted pseudo-first- and pseudo-second-order models, indicating combined physical and chemical control. The Langmuir and Freundlich isotherms fit well; the max capacity was 34.62 mg/g at 30 °C. Adsorption was spontaneous and endothermic, with entropy increasing at higher temperatures. Phosphorus mainly existed as Ca-P (37%), Al-P (35%), and occluded P (21%). SEM showed that the surface presented a porous structure before adsorption and a crystalline coating after adsorption. XRF spectroscopy confirmed that the phosphorus content increased after adsorption. FTIR spectroscopy revealed that phosphate replaced hydroxyl groups, forming chemical bonds (≡Si–O–PO₃⁻ and ≡Al–O–PO₃⁻), highlighting that chemical adsorption is the main process.