AbstractThis study utilized a straightforward method to fabricate zeolite X, demonstrating high adsorption performance for the removal of NH3-N from wastewater. The adsorption behavior of zeolite X towards NH3-N was investigated through various methods including adsorption kinetics, isothermal adsorption, and thermodynamic models. The mechanism of zeolite X for removing NH3-N were analyzed using techniques such as SEM, EDS, XRD, BET, and XPS. The results indicated that when t = 30 min, pH = 6, zeolite dosage was 20 g/L and initial NH3-N concentration was 100 mg/L, the removal efficiency of NH3-N could reach 95.5%. The adsorption process followed Freundlich model and were endothermic. The enthalpy change (ΔH), entropy change (ΔS) and Gibbs function (ΔG(308.15 K)) were 10.66 kJ/mol, 28.81 J/(mol·K), and 1.78 kJ/mol, respectively. SEM and XRD analysis show that the crystal shape of zeolite X is octahedral, which is a typical morphology of FAU-type zeolite. The specific surface area of zeolite X was as high as 643 m2/g. The results of XPS analysis show that after Na+ exchange with NH4+, hydrogen bonds were formed between ammonia nitrogen and oxygen in zeolite X. This study introduces a novel adsorbent, offering a fresh approach to tackling ammonia nitrogen pollution in wastewater.
Graphical Abstract1. IntroductionAmmonia Nitrogen (NH3-N) wastewater primarily originates from industrial production and domestic activities. This includes the fertilizer, pharmaceutical, coking, and petrochemical industries, in addition to landfill leachate and domestic sewage. If this wastewater is released into water bodies without adequate treatment, it can lead to environmental issues such as eutrophication [1]. Consequently, the efficient management of NH3-N wastewater is crucial for safeguarding the aquatic environment [2]. Currently, the primary methods for NH3-N removal encompass biological denitrification [3], electrochemical processes [4], photocatalytic techniques [5], and adsorption approaches [6]. Although the biological treatment method has been mature and cost-effective, its relatively long treatment cycle and strict demand for constant operating conditions limit its wide application in the treatment of high-concentration NH3-N wastewater [7]. The electrochemical method remains unsuitable for large-scale industrial applications due to its suboptimal current efficiency and elevated energy consumption [8]. The application of photocatalytic treatment to NH3-N wastewater presents several challenges, including the ease with which the catalyst can be lost, its difficult recovery process, and its high cost. These factors significantly limit the practical implementation of this technology. Adsorption, in comparison to other methods, is characterized by its high efficiency, straightforward process and no need to add any chemical reagents [9, 10].
Among myriads of adsorbent materials, zeolite has garnered widespread use due to its notable adsorption efficiency for NH3-N and its ready accessibility [11–13]. Natural zeolite, a porous silicate mineral, is characterized by the formula Am[(AlO2)p(SiO2)q]·n(H2O), where A represents an alkali metal or alkaline earth metal. The zeolite structure is composed of the smallest constituent monomer, which is a silicon-oxygen and aluminum-oxygen tetrahedral skeleton [14]. The diverse connection modes of these silicon (aluminum) oxygen tetrahedra result in the formation of numerous pores and channels within the zeolite structure [15]. These features endow the zeolite with adsorption separation, ion exchange, diffusion, and catalytic properties [16, 17]. However, the adsorption efficacy of natural zeolite is constrained, necessitating the addition of substantial quantities to achieve the pertinent effluent NH3-N concentration in accordance with water quality standards [18]. Indeed, numerous studies have been conducted on the synthesis of zeolite powder from solid waste materials [19]. Yang et al. [20] synthesized zeolite 4A utilizing fly ash as the primary raw material through a NaOH/Na2CO3 melt fusion process. The adsorption capacity of the resultant 0.18 g zeolite for Cu2+ at a concentration of 100 mg/L was found to be 55.5 mg/g, achieved within a timeframe of 60 minutes. Cardoso et al. [21] successfully synthesized zeolite Na-P1 from fly ash for the purpose of wastewater treatment, and the research demonstrated that the application of zeolite Na-P1 to pig manure wastewater could yield a high removal efficiency of total NH3-N, with an impressive rate of 31 mg/g. To enhance the adsorption efficacy and utilization rate of NH3-N by zeolite, it is imperative to artificially synthesize a cost-effective and efficient zeolite specifically designed for NH3-N adsorption.
Halloysite, a naturally occurring aluminosilicate mineral with a unique tubular structure, has excellent adsorption performance and a large specific surface area, which enables it to efficiently adsorb ammonia nitrogen in wastewater. The halloysite crystal is a monoclinic-systematic aqueous layered silicate mineral [22], characterized by its dioctahedral type structure and a 1:1 structural unit layer. Where Si and Al exist in the amorphous phase, α-quartz and mullite constitute the crystalline phase. The alkaline activator has the capacity to weaken the bonding between SiO2 and Al2O3 in the crystalline phases of a-quartz and mullite. This process disrupts the bond structure, generates a significant number of broken point defects and free ends, and induces particle movement. The process can instigate the formation of thermodynamically unstable states, leading to the production of soluble amorphous Si, Al, and hydroxyl sodium aluminate. It is known that Na+ has a good promotional effect on the nucleation and crystallization of the zeolite synthesis process [20]. In addition, halloysite has abundant reserves and low cost, with a market price of 800–1500 yuan/ton, while ordinary industrial biochar is 4000–8000 yuan/ton. At present, the treatment of NH3-N wastewater faces problems such as limited adsorption efficiency of natural zeolite, and traditional treatment technologies are limited in terms of efficiency, cost or operating conditions. In particular, there is a lack of artificially synthesized zeolite materials that are highly optimized for NH3-N adsorption. In addition, although the economic and structural advantages of halloysite as a potential adsorbent material have been recognized, its performance improvement and mechanism exploration in NH3-N adsorption applications are not deep enough.
In this study, zeolite X was synthesized using lower-cost raw materials (mainly halloysite, sodium hydroxide, and sodium silicate), and the synthesis process is simple and can be prepared on a large scale by one-pot method. The effects of zeolite X dosage, adsorption time, pH, initial concentration of NH3-N, and temperature on NH3-N adsorption were investigated, and the optimal process parameters were determined. The adsorption rate was analyzed by kinetic model, and the adsorption isotherm was fitted by Langmuir, Freundlich and Dubinin-Radushkevich model to quantify the adsorption capacity. In addition, the thermodynamic parameters were calculated (such as Gibbs free energy change ΔG, enthalpy change ΔH, entropy change ΔS), revealing the adsorption mechanism and providing data support for the establishment of theoretical models. Advanced characterization techniques (such as SEM, EDS, XRD, BET, TGA, etc.) were used to explore in depth the interaction between the zeolite and ammonia nitrogen, elucidating the adsorption mechanism from a microscopic level.
2. Materials and Methods2.1. Chemical ReagentHigh purity halloysite (Al2[Si2O5](OH)4·(1 ~ 2)H2O) was provided by Power China Zhongnan Engineering Corporation Limited. Sodium silicate (Na2SiO3·9H2O), ammonium chloride (NH4Cl), sulfuric acid (H2SO4), sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. NH3-N liquid consumables (LH-YN2: sodium hydroxide (NaOH), mercury chloride (HgCl2), LH-YN3: zinc sulfate (ZnSO4), potassium sodium tartrate (NaKC4H4O6)) were purchased from Beijing Lianhua Yongxing Science and Technology Development Co., Ltd. All experiments were conducted in ultrapure water equipment (ZC1-A10Y, Changsha Zhichuang, China) to prepare deionized water with a resistivity of 18.0 MΩ·cm.
2.2. Concentration Determination and Synthesis Process2.2.1. Concentration determinationThe concentration of ammonia nitrogen was determined using the Lianhua Technology Multi-Parameter Detector 5B-3B(V11). 1 mL of the special reagent LH-YN3 and 1 mL of the special reagent LH-YN2 were added to each reaction tube in sequence, shaken thoroughly, and then left to stand for 10 minutes. Subsequently, the reaction solution was poured into a 10 mm colorimetric dish. Before measurement, blank calibration needs to be performed. The colorimetric dish was placed in the colorimetric groove and the cover was closed. Once the displayed value on the screen stabilized, the value displayed on the screen at this time represented the ammonia nitrogen concentration of the sample.
2.2.2. Synthesis processThe adsorbent zeolite X was obtained from Power China Zhongnan Engineering Corporation Limited. The specific preparation processes were as follows (Fig. 1): (1) The halloysite was heat treated in a muffle furnace for 2 h at a temperature of 773.15 K. (2) A mixture of high purity halloysite (5 g) and NaOH (10 g), in a mass ratio of 1:2, was prepared, with the addition of a specific quantity of deionized water. The mixture was heated to boiling under stirring for about 1 h. (3) During the boiling process, a solution of Na2SiO3 was gradually introduced. Sodium silicate served as the silica source, while sodium aluminate interacted with it to yield sodium aluminosilicate gel. (4) The sodium silicate gel, once obtained, was aged at room temperature for a duration of 8 hours and subsequently washed until it reached neutrality. (5) The aged gel was subsequently filtered at a temperature of 323.15 K and allowed to dry for a duration of 24 hours, resulting in the formation of a white powder. (6) The dried powder underwent calcination at a temperature of 373.15 K for a duration of 4 hours, was subsequently ground into a fine powder, and the resultant product was identified as zeolite X.
2.3. Material CharacterizationThe surface morphology and structure of zeolite X were observed by scanning electron microscope-energy dispersive spectrometer (SEM, JSM-7900F, Japan), and the surface elements of the sample were determined by an energy dispersive spectroscopy (EDS) attachment. The phase structure of zeolite X was determined by X-ray diffractometer (XRD, Shimadzu 6100, Japan) with copper target (Kαλ = 0.154178 nm). The experimental voltage was 50 kV, and the scanning speed was 2°/min. The chemical composition and valence state of the floc surface were characterized by X-ray photoelectron spectroscopy (XPS, PHI 5000, USA) using a monochromatic Al-Kα light source with a power of 300 W. The Zeta potential of zeolite X at a specific pH value was measured and analyzed by a nano-particle size and Zeta potential analyzer (DLS, Malvern Zetasizer Nano ZS90, UK). Nitrogen adsorption/desorption isotherms were measured at 77 K using a fully automated specific surface and porosity analyzer (BET, Micromeritics ASAP 2460, USA) to analyze pore and surface structure. Thermogravimetric (TG) measurements were performed using a thermogravimetric analyzer (TGA, Netzsch TG 209 F3, Germany). The sample was heated to 1073.15 K in a dynamic nitrogen atmosphere at a heating rate of 283.15 K/min to evaluate its thermal stability and mass change.
2.4. NH3-N Adsorption ExperimentThe analytically pure NH4Cl, weighing precisely 3.819 g, was meticulously dissolved in an adequate quantity of distilled water. The solution was then diluted in a 1000 mL volumetric flask to create an NH3-N stock solution with a concentration of 1000 mg/L. The NH3-N removal efficiency (Re) and adsorption capacity (Q) were calculated as follows:
where Re(NH3-N) is the removal efficiency of NH3-N, %; C0 is the initial concentration of NH3-N, mg/L; Ce is the concentration of NH3-N at adsorption equilibrium, mg/L; Q is the adsorption capacity, the amount of NH3-N adsorbed per unit mass of zeolite X, mg/g; m is the mass of zeolite, g; V is the volume of NH3-N solution, L.
The 0.2 g zeolite X was added to the conical flask with stopper, and then quickly poured into 250 mL of NH4Cl solution with initial concentrations of 30, 40, 50, 80 and 90 mg/L, respectively. The conical flask with stopper was placed in a speed-regulating multi-purpose oscillator and adsorbed at room temperature for 30 min. After the oscillation was completed, the supernatant was taken and filtered with a 0.45 μm membrane. The residual NH3-N concentration after adsorption was determined by Nessler’s reagent spectrophotometry. The equilibrium adsorption isotherms were obtained by Langmuir, Freundlich and Dubinin-Radushkevich isotherm models. A 250 mL solution of NH3-N, with a concentration of 100 mg/L, was extracted from a conical bottle equipped with a plug. The zeolite X dosage was set to 0.2 g, the solution pH was 6, at room temperature, and the adsorption time was 0.5, 1, 5, 20 and 30 min, respectively. The pseudo-first-order kinetics, pseudo-second-order kinetics and intraparticle diffusion diffusion model were used to describe the adsorption mechanism and adsorption performance. Thermodynamics of NH3-N adsorption on zeolite X by setting the temperature at 308.15 K, 318.15 K, 328.15 K, and 338.15 K, respectively.
2.5. Adsorption Behavior2.5.1. Isothermal adsorption modelThe Langmuir model posits that adsorption transpires uniformly across the adsorbent’s surface, where each adsorbed molecule is attached to the adsorbent with equivalent adsorption energy. Furthermore, it presumes no interaction exists between the molecules at adjacent sites [23]. The Freundlich model posits that adsorption on the adsorbent surface is non-uniform, occurring through multi-layer adsorption. This model suggests that the capacity for adsorption increases infinitely as the initial concentration of the adsorbate rises [24]. The (D-R) isotherm model is predicated on the theory of micropore volume filling. This model posits that the process of adsorbate molecules occupying micropores adheres to an orderly filling mechanism [25]. The relevant Eq. (3) and Eq. (4) are as follows:
Langmuir model:
Freundlich model:
Dubinin-Radushkevich model:
where Qe is the equilibrium adsorption capacity, mg/g; Qm is the maximum adsorption capacity, mg/g; b is the adsorption coefficient, which is related to the adsorption performance, L/mg; RL is a dimensionless separation factor, reaction adsorption characteristics; Kf is Freundlich affinity coefficient, which represents the binding strength of adsorbent and adsorbate; n is the Freundlich constant. K is the adsorption isotherm constant of D-R model, mol2/kJ2; ɛ is Polanyi potential, kJ/mol; E is the average free energy, kJ/mol; T is the Kelvin temperature, K; R is a universal gas constant, 8.314 J/(mol·K).
2.5.2. Adsorption kinetics modelThe pseudo-first-order kinetic model is usually used to describe the external mass transfer process and predict the equilibrium adsorption capacity. The pseudo-second-order kinetic model is mainly used to describe the chemical adsorption at the activation point. The intraparticle diffusion model can be used to predict whether the intraparticle diffusion is the rate-determining step [26]. The relevant Eq. (8), Eq. (9), Eq. (10) are as follows:
Pseudo-first-order kinetic model:
Pseudo-second-order kinetic model:
Intraparticle diffusion model:
where Qt is the adsorption capacity of the adsorbent at time t, mg/g; t is the adsorption time, min; k1 is the adsorption rate constant of the pseudo-first-order kinetic model, min−1; k2 is the adsorption rate constant of the pseudo-second-order kinetic model, g/(mg·min); id is different adsorption stages; Kid is the internal diffusion rate constant at different stages; I is the constant of boundary layer thickness.
2.5.3. Adsorption thermodynamics modelAdsorption thermodynamics reflects the reaction direction and reaction degree in the reverse adsorption process. Through the Gibbs equation, we can calculate ΔH, ΔS and ΔG, and then analyze the effect of temperature on the reaction [27, 28]. The Eq. (11), Eq. (12), Eq. (13), Eq. (14) are applied for analysis:
where Kd is the partition constant; ΔG is the Gibbs free energy of adsorption, kJ/mol; ΔH is the adsorption enthalpy change, kJ/mol; ΔS is the adsorption entropy change, J/(mol·K).
3. Results and Discussion3.1. The Effects of Zeolite X Dosage and pHWhen the dosage escalated from 0.4 g/L to 20 g/L, the removal efficiency rose from 15.4% to an impressive 95.5% (20 min). This indicates that the removal efficiency of NH3-N by zeolite X increased with the increase of adsorbent dosage [29]. The adsorption capacity of zeolite X for NH3-N diminishes with an increase in zeolite addition. Specifically, when the dosage of zeolite escalates from 0.4 g/L to 20 g/L, the adsorption capacity of zeolite X for NH3-N drops from 39.3 mg/g to 4.2 mg/g. This decline is attributed to the fact that an increased dosage results in a reduction in the activity of the adsorbent on the specific surface area per unit volume [30]. Furthermore, at higher solid-liquid ratios, the adsorbent tends to agglomerate, which is detrimental to the adsorption of NH3-N by zeolite X.
Alterations in pH can modify the existing form of NH3-N within the solution and alter the charge properties of the zeolite surface, thereby affecting the adsorption effect [31]. NH3 (ammonia) is partially protonated to form NH4+ in water, with a pKa value of 9.25. This means that most of the ammonia will exist as ammonium ions (NH4+) in environments with pH values lower than 9.25. As illustrated in Fig. 2(b), the removal efficiency falls below 22% when the pH value is either less than 4 or greater than 8. When the pH falls below 4, the concentration of H+ in the solution increases. This elevated concentration can compete with NH4+ for adsorption sites on the zeolite X. Given that the radius of H+ (0.240 nm) is smaller than that of NH4+ (0.535 nm), it is more likely to exchange with Na+ on the zeolite. This exchange process leads to a reduction in the removal efficiency of NH3-N [32]. In addition, under strong acidic conditions, zeolites X could dissolve and the surface negative charge decreases. The electrostatic attraction between zeolites X and positively charged NH4+ is reduced, which is not conducive to the adsorption of NH3-N. By measuring the zeta potential distribution of zeolite X at pH values of 6 and 9, as shown in Fig. 2(c), it was found that the surface charge increased with increasing alkalinity of the solution. However, when the pH was greater than 8, the number of OH− in the solution increased, and the form of NH3-N gradually changed from positively charged NH4+ to uncharged NH3, so the removal efficiency of NH3-N decreased in an alkaline environment. In summary, the pH selected in this study was 6, at which time the maximum adsorption capacity could reach 36.9 mg/g.
3.2. Adsorption Kinetics AnalysisThe removal efficiency of NH3-N by zeolite X was 26.54%, and the adsorption capacity was 34.35 mg/g (1 min). After 1 min, the removal efficiency and adsorption capacity increased slowly. When the adsorption time was from 5 minutes to 30 minutes, the adsorption capacity only increased from 35.25 mg/g to 36.90 mg/g, and the removal efficiency only increased from 27.23% to 28.52%. In the initial stage of the adsorption process, the initial concentration of NH3-N is high, and there are a large number of adsorption sites on zeolite X, which can adsorb a large amount of NH3-N. In the later stage of the adsorption process, as the concentration of NH3-N decreases, the number of adsorption sites that zeolite X can provide significantly reduces, therefore, the removal efficiency and adsorption capacity increase slowly [29].
When t = 30 min, pH = 6, T = 298.15 K±2 K, m = 0.8 g/L, C0 = 100 mg/L, the adsorption capacity of zeolite X for NH3-N reached as high as 36.90 mg/g. Table 1 provides a comparative study of different zeolite materials for the removal of NH3-N. By literature comparison, we found that this study has very good experimental results. This study synthesizes highly efficient zeolite X for NH3-N adsorption by using low-cost and abundantly available halloysite as raw material. This provides an innovative and widely applicable solution for the field of wastewater treatment.
Electrostatic adsorption usually has a fast initial rate of adsorption, followed by a slower rate as the adsorption sites become saturated until an equilibrium is reached [35]. The findings are depicted in Fig. 3(b), (c) and Table S1. The R2 values for both pseudo-first-order and pseudo-second-order kinetics fittings are proximate to 1, suggesting that the adsorption of NH3-N by zeolite X is a result of the combined effect of physical electrostatic adsorption and chemical adsorption (ion exchange) [36]. The Qe values derived from both models closely align with the experimental data.
As illustrated in Fig. 3(d) and Table S2, the intraparticle diffusion model demonstrates that I is not zero, and the resultant straight lines do not intersect the origin. This suggests that intraparticle diffusion is not the sole rate-controlling step in the adsorption of NH3-N by zeolite X [37]. The adsorption of NH3-N by zeolite X is mainly divided into two stages: surface diffusion and intraparticle diffusion [38]. Both K1d and K2d showed a decreasing trend with the increase of zeolite dosage. This phenomenon is attributed to the fact that an increase in zeolite X dosage diminishes the likelihood of a single site being occupied, consequently reducing both the surface diffusion rate and the adsorption capacity for NH3-N. The coefficient of determination (R2) during the surface diffusion phase is notably higher than that during the intraparticle diffusion phase, suggesting that surface diffusion has a more significant influence on the overall adsorption process.
3.3. Adsorption Isotherm AnalysisWhen the initial concentration of NH3-N was set at 30, 40, 50, 80, and 90 mg/L, the adsorption capacity of zeolite X for NH3-N was observed to be 26.1, 27.8, 34.2, 37.5, and 36.2 mg/g respectively after a period of 30 minutes. When the initial concentration of NH3-N was low (C0 = 30, 40 mg/L), the removal efficiency of NH3-N by zeolite was great, and the corresponding removal rates were 64.44% and 53.62%, respectively. As the concentration of NH3-N increased, the adsorption sites on zeolite X were progressively filled by NH4+ until they reached saturation. As the concentration of the initial solution continued to increase, the adsorption capacity for NH3-N exhibited a very slow rate of increase.
The Langmuir, Freundlich, and Dubinin-Radushkevich models were employed for the isothermal adsorption analysis. The findings are presented in Fig. 4(b) and Table S3. The correlation coefficient R2 of the Freundlich equation is higher than that of the Langmuir equation, which indicates that the adsorption process is mainly multi-layer adsorption. The Freundlich constant (n = 3.84) falls within the range of 1 to 10, suggesting that the zeolite X surface is heterogeneous. This heterogeneity favors the adsorption of NH3-N by zeolite X [30, 39]. The correlation coefficient (R2) of the D-R equation is 0.5457, indicating that the model does not adequately fit the experimental results. According to the D-R isotherm adsorption mechanism [40], when the adsorption energy (|E|) is between 1 ~ 8 kJ/mol, the process exhibits physical adsorption. When |E| is within the range of 8 ~ 16 kJ/mol, the adsorption mechanism is dominated by surface adsorption, driven by ion exchange. When |E| exceeds 16 kJ/mol, adsorption is controlled by chemical reactions. In Fig. 4(c), the specific surface area of zeolite X was as high as 643 m2/g. The pore volume determined by the t-plot method was 186 cm3/kg. The average pore size of zeolite X is 1.658 nm. Zeolite X exhibits typical type I adsorption isotherm characteristics, and its adsorption rate at low relative pressure is extremely fast, reaching the saturation point quickly.
3.4. Thermodynamic AnalysisWhen the water temperature increased from 308.15 K to 328.15 K, the removal efficiency of NH3-N by zeolite X increased from 28.5% to 34%. The increase in temperature can increase the collision frequency and energy between molecules, which is conducive to ammonia nitrogen molecules overcoming the potential barrier with the zeolite surface and adsorbing more easily on the zeolite. When the temperature exceeded 328.15 K, the removal efficiency began to decrease significantly. This phenomenon is attributed to the fact that as temperature increases, the strength of the van der Waals force between the adsorbent and the adsorbate diminishes. The average kinetic energy of the adsorbate rises, thereby increasing the desorption driving force. Consequently, NH4+ is more likely to escape from the surface of the adsorbent, leading to a reduction in the adsorption efficiency of zeolite X for NH3-N.
To delve deeper into the thermodynamic behavior of adsorption, experiments were conducted at temperatures of 308.15 K, 318.15 K, and 328.15 K. The results are shown in Fig. 4(e) and Table S4. The enthalpy change (ΔH = 10.66 kJ/mol) is positive, indicating that the adsorption process is an endothermic reaction [41]. The entropy change ΔS(28.81 J/(mol·K)) is positive, indicating that the adsorption process is an entropy increase process. The Gibbs function (ΔG) is positive, suggesting that the adsorption behavior is not spontaneous and necessitates specific conditions for its occurrence. TGA characterization, as shown in Fig. 4(f), revealed the initial mass loss of zeolite X in the low-temperature region (room temperature to 473.15 K) mainly due to the evaporation of adsorbed water, reflecting the activity of its surface hydroxyl groups. The limited mass change in the high-temperature stage (up to 1073.15 K) indicated good thermal stability of the zeolite structure and no significant release of organic impurities or structural water in this range.
3.5. Adsorption Mechanism3.5.1. SEM, EDS and XRD characterization
Fig. 5(a–d) presents the morphology of zeolite X, both pre and post adsorption of NH3-N, accompanied by EDS and XRD images for further analysis. The SEM analysis reveals that the crystalline structure of zeolite X is octahedral, indicative of a typical FAU-type zeolite morphology. Comparing Fig. 5(a) and Fig. 5(b), it can be observed that the morphology of zeolite X does not change significantly before and after the adsorption of NH3-N. Meanwhile, as shown in Fig. 5(c), the mass fractions of O, Si, and Al elements in zeolite X do not change significantly after adsorbing NH3-N. However, the mass fractions of Na and N elements change noticeably, with their proportions before adsorption being 13.73% and 0.1%, respectively, and after adsorption also being 7.6% and 6.87%, respectively. The XRD analysis, as depicted in Fig. 5(d), with a 2θ value of 6.1°, corresponds to the (111) crystal plane of FAU-type zeolite, which is consistent with the standard card PDF # 38–0237. In summary, the process of zeolite X adsorbing NH3-N in this study did not change the original structure of the zeolite X. There is an ion exchange process between Na+ and NH4+ in the adsorption of NH3-N by zeolite X.
3.5.2. XPS characterization
Fig. 6(a–d) shows the XPS of the zeolite X before and after adsorption of NH3-N. As shown in Fig. 6(a), the elements present in the zeolite X prior to adsorption were Na, Si, Ca, Al, and O. After adsorption of NH3-N, the production of N elements can be found, notably the valence band spectrum of N1s shows a significant difference in peak height in Fig. 6(b), Fig. 6(c) and Fig. 6(d) present the valence band spectra of O. Despite the consistent structure, the adsorbed O displays a positive shift of 0.5 eV in the valence band spectrum. This shift is attributed to the formation of hydrogen bonds between NH3-N and O in zeolite X following the ion exchange of Na+ and NH4+. These hydrogen bonds increase the electron cloud density of oxygen, leading to a positive shift in the binding energy of oxygen.
3.5.3. Possible removal pathway for NH3-NBased on the analysis results of adsorption kinetics and characterization results such as EDS and XPS, it can be concluded that the possible pathway for zeolite X to remove NH3-N is shown in Fig. 7. At the initial stage of weakly acidic condition, NH3-N molecules are attracted to the zeolite X due to electrostatic interactions. Subsequently, these molecules adhere to the zeolite X surface in a multi-layered adsorption configuration. The NH3-N molecules that are adsorbed subsequently diffuse towards the interior of zeolite X, where they undergo an ion exchange with Na+. This process of ion exchange is a critical step in enabling zeolite X to effectively adsorb NH3-N molecules. The NH3-N molecules adsorbed onto the zeolite X form hydrogen bonds with the highly polar oxygen elements within the zeolite X. This interaction enhances the stability of the molecular structure of the adsorbed zeolite X, thereby reducing the likelihood of desorption for the NH3-N molecules.
4. ConclusionsIn this study, we successfully synthesized a novel zeolite X with an enhanced adsorption capacity for NH3-N. This was accomplished using ultra-economical raw materials and yielded an octahedral crystal form, indicative of the FAU-type zeolite morphology. When pH = 6, zeolite X dosage was 20 g/L and initial NH3-N concentration was 100 mg/L, the removal efficiency of NH3-N could reach 95.5% (20 min). Both too high and too low pH are unfavorable for zeolite adsorption of ammonia nitrogen. The adsorption kinetics showed that the adsorption process is the result of a synergistic effect of physical electrostatic adsorption and chemical adsorption (ion exchange). The Freundlich isotherm adsorption model aptly describes the adsorption process, suggesting that it predominantly involves multilayer adsorption. The specific surface area of zeolite X was as high as 643 m2/g. The pore volume determined by the t-plot method was 186 cm3/kg. The average pore size of zeolite X is 1.658 nm. The adsorption process was endothermic and non-spontaneous. EDS and XPS characterization indicated that after Na+ exchange with NH4+, hydrogen bonds were formed between NH3-N and oxygen in zeolite X. As a new type of low-cost adsorbent, the material has broad application prospects in removing NH3-N from wastewater. Despite the significant progress in adsorption performance and mechanism analysis, the study has not fully covered the complexity of industrial-scale applications, the details of regeneration and recycling of adsorbent materials, and the potential impact of the inherent variability of raw halloysite, which are directions that need to be further explored in future research.
AcknowledgementsThis work is jointly funded by the Scientific Research Foundation of Education Office of Hunan Province (No. 22C0111).
NotesConflict-of-Interests Statement The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Author Contributions Y.H. (Professor) developed the conceptualization, methodology, and wrote the manuscript. S.M. (Bachelors Student) developed the conceptualization, methodology, wrote the manuscript and completed the experiment. K. (Bachelors Student) helped in developing the conceptualization and methodology of the study. Y. (Bachelors Student), Y.L. (Bachelors Student), Y.S. (Bachelors Student) provided valuable research insights into the study and helped to review the manuscript. X.F. (Bachelors Student), Q.R. (Bachelors Student) and J.Y. (Bachelors Student) contributed to the writing and provided valuable research insights. S. (Associate Professor) reviewed the manuscript and provided valuable insights. References1. Fan J, Wu H, Liu R, et al. Non-thermal plasma combined with zeolites to remove ammonia nitrogen from wastewater. J. Hazard. Mater. 2021;401:123627. doi:https://doi.org/10.1016/j.jhazmat.2020.123627
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