Green synthesis of zeolite and its regeneration for adsorption of ammonia nitrogen in water

doi:10.21203/rs.3.rs-4841210/v1

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Green synthesis of zeolite and its regeneration for adsorption of ammonia nitrogen in water

https://doi.org/10.21203/rs.3.rs-4841210/v1

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This study presents a novel green approach for the preparation of zeolite material from fly ash using a combination of mechanochemical method. Compared to traditional methods, this approach reduces energy consumption, minimizes reagent usage, and facilitates the efficient recycling of fly ash. The physicochemical properties of the synthesized zeolite, including crystal structure and porosity, were systematically investigated. The synthesized zeolite was employed for the adsorption of ammonia nitrogen from aqueous solutions, and their adsorption kinetics and thermodynamics were comprehensively studied. The results revealed that the adsorption of ammonia nitrogen onto the zeolite follows the Langmuir adsorption model. Additionally, the zeolite exhibited strong selective adsorption and remarkable resistance to interference from coexisting cations in the aqueous solution. Finally, regeneration experiments were conducted using NaCl, NaClO, and their mixtures to desorb ammonia nitrogen from the spent zeolite. A total of 17 regeneration cycles were performed until the adsorption capacity of the zeolites was exhausted. The adsorption performance of the regenerated zeolite was evaluated to assess the impact of different reagents and regeneration cycles on adsorption efficiency. The optimal regeneration method was identified, leading to the successful valorization of fly ash into zeolite for ammonia adsorption and the development of effective regeneration strategies for spent zeolite.

zeolite

waste reuse

ammonia adsorption

regeneration

selective adsorption

Ammonia nitrogen is one of the common pollutants, which exists in water in the form of NH₃ or NH₄⁺, and its main sources are wastewater produced by textile dyeing and finishing, organic compounds manufacturing, inorganic compounds manufacturing, petrochemicals, tannery, iron and steel industries, and livestock and poultry farming, urban wastewater treatment plants, and other domestic farm wastewater (Mazloomi and Jalali 2016; Meng et al. 2020). Ammonia nitrogen is a nutrient in the water body, is the main form of nitrogen in the water-phase environment, is an important pollutant in the eutrophication of water bodies, is an important indicator of eutrophication of water bodies (Ren et al. 2022).

Adsorption is one of the most popular methods for ammonia nitrogen removal due to its simple operation, low energy consumption, high removal rate and renewability (Liu et al. 2024). While zeolite molecular sieve is a porous material, its structure consists of specific zeolite minerals, showing an empty skeleton structure (Neilsen et al. 2022). This porous structure makes the zeolite molecular sieve has a large specific surface area, can adsorb substances smaller than its pore size, and exclude the larger size of the material, play the role of material "sieving". In addition, the polarity of the internal pore channel gives zeolite molecular sieve strong ammonia and nitrogen adsorption performance (Liu et al. 2024). At present, zeolite molecular sieve has been used as an irreplaceable consumable in downstream application industry, with the development of downstream application industry, the demand also continues to rise (Smith et al. 2018). However, due to the further increase in environmental protection, enterprises have to increase environmental protection investment while ensuring the production quality, thus increasing the production cost. Therefore, in order to reduce the production cost, finding cheap raw materials to synthesize zeolite molecular sieves and applying them to adsorb ammonia nitrogen pollutants in water bodies is an effective way to realize green chemistry.

Fly ash is an industrial by-product produced in the process of thermal power generation, metal smelting and heating, etc. Its large amount of stockpiling not only occupies land resources, but also may cause pollution to the environment (Xu and Shi 2018). However, fly ash is also a resource with development potential, which can be resourcefully utilized in a variety of ways. These pathways include replacing traditional materials in the field of building materials to produce bricks, concrete, and cement (Krishnaraj and Ravichandran 2021); in the field of environmental protection for flue gas treatment and adsorption of hazardous substances (Zhuang et al. 2016); in the field of agriculture as a fertilizer or soil conditioner to provide nutrients needed for plant growth; in the field of chemical engineering for the synthesis of products such as molecular sieves; and through the extraction of valuable components of fly ash, such as hollow beads, magnetic beads, and residual charcoal, alumina, etc., for high value-added utilization (Yao et al. 2015). Fly ash has a large amount of silica-alumina elements in the form of mullite and quartz (Matjie et al. 2005, Boycheva et al. 2020), and such substances are less active and cannot be directly involved in zeolite crystallization. The principle of zeolite preparation using fly ash is to utilize the abundant silicon and aluminum elements in fly ash, which are converted into zeolite by hydrothermal synthesis under alkaline conditions (Yang et al. 2019, Panitchakarn et al. 2014). Fly ash is first activated by using alkaline solutions such as sodium hydroxide to induce dissolution and activation of silica-aluminate (El-Naggar et al., 2008, Huber et al. 2018);

Then, under high temperature and pressure hydrothermal conditions, silica-aluminate ions polymerize to form the primary structural units of zeolite, which undergoes a crystallization process to generate zeolite; finally, the high value-added conversion of fly ash is achieved by removing impurities and enhancing the thermal stability and pore structure of zeolite through washing and roasting (HO 2006, Murukutti and Jena 2022). However, this process generates a large amount of wastewater and has high energy consumption. Mechanical force chemical processing of tailings is based on the principle of applying high-energy mechanical forces, such as mechanical grinding, to disrupt the crystal structure of tailings and form new surfaces and defects, thus increasing the active sites and specific surface area (Ye et al. 2025). This process may be accompanied by the breaking and rearrangement of chemical bonds, altering the chemical composition of the tailings, inducing phase transitions, promoting redox reactions, increasing the solubility of the solids, and possibly changing the surface properties of the particles (Sun et al. 2024a, Qiu et al. 2024). These changes help to improve the leaching efficiency of the metals in the tailings and the ability to participate in subsequent reactions, making tailings resource utilization more efficient. Moreover, these processes do not require high-temperature heating and do not produce waste water, waste gas, and waste residue, which can effectively realize the recycling of waste (Luo et al. 2022).

In this study, we propose to prepare zeolite materials by processing fly ash using mechanochemical method for adsorption of ammonia nitrogen in water. At the same time, we investigated the effect of NaCl, NaClO and their mixtures on the desorption and regeneration of artificial zeolites. Through the regenerated artificial zeolite's adsorption capacity of ammonia nitrogen, we evaluated the effects of different adsorbents and regeneration times on the zeolite's adsorption effect, and explored the most suitable adsorbents and regeneration times. Finally, we realized the regeneration and utilization of fly ash to prepare zeolite for ammonia nitrogen adsorption, and studied the regeneration method of zeolite after adsorption.

2.1 Materials

Fly ash from a thermal power plant in Guangzhou, sodium meta-aluminate (NaAlO₂), sodium hydroxide (NaOH), hydrochloric acid (HCl), ammonium chloride (NH₄Cl) sodium chloride (NaCl) sodium hypochlorite solution (NaClO), potassium sodium tartrate (KNaC₄H₄O₆-4H₂O), potassium iodide (KI) mercuric iodide (HgI) were purchased from Guangzhou Chemical Reagent Factory.

2.2 Instruments and Methods

UV spectrophotometer (SHIMADZU UV-2600), Shimadzu Corporation, Japan; Electronic balance (JJ-300), Changshu Shuangjie Testing Instrument Factory; Electric blast drying oven (DHG-9023A), Shanghai Yiheng Scientific Instrument Co. Shanghai Anting Scientific Instrument Factory; Ball Mill (KE-0.4L), Qidong Honghong Instrument Equipment Factory; Muffle Furnace (MF-1200C), Beytec Corporation. The concentrations of ammonium nitrogen (NH₄⁺-N) were determined spectrophotometrically using Nessler's reagent.

2.3 Zeolite synthesis method

Zeolite material was synthesized by ball milling method + hydrothermal crystallization. The first step is to weigh 171.5 g of ball milling media and put it into the ball milling jar, then press 3.0 g of fly ash and 0.6 g of sodium metaaluminate into the ball milling jar. Then the ball milling jar was put into the ball mill, fixed and started to run, set the running time is 5.0 h, the running speed is 900 rpm. after the ball mill running is finished, the ball milling media and the material will be separated, and the material will be transferred to the Teflon-lined stainless steel autoclave at 110 ℃ for 5 hours of crystallization, and finally the material will be dried and milled to obtain zeolite products.

2.4 Adsorption experiment

For the adsorption experiments, 8.0 g was cast into 200 mL of prepared 500 mg/L NH₄Cl solution was added to the above stirring cup and stirred with a hexagonal stirrer. The concentration of ammonia nitrogen is determined by the Nessler's reagent method (Liu et al. 2024). Adsorption kinetics is a dynamic process that describes the rate of adsorption. In order to better study the adsorption process and adsorption mechanism, three kinetic models were used in this experiment to describe the dynamic process of adsorption rate as follows:

(1) Pseudo-first-order kinetic equation:

$$\:\text{ln}\left({q}_{e}-{q}_{t}\right)=\text{ln}{q}_{e}-{k}_{a}t$$

where k_a (min^− 1) denotes the adsorption rate constant for the quasi-primary model, and qe (mg/g) and qt (mg/g) denote the amount of ammonia adsorbed by the zeolite at equilibrium and time.

(2) Pseudo-second-order kinetic equations

$$\:\frac{t}{{q}_{t}}=\frac{1}{\left({k}_{b}{q}_{e}^{2}\right)}+\frac{t}{{q}_{e}}$$

where k_b (g/(mg·h)) represents the adsorption rate constant for the quasi-secondary model.

(3) Internal diffusion modeling:

$$\:{q}_{t}={k}_{c}{t}^{1∕2}+\text{H}$$

where H is related to the thickness of the diffusion layer and k_c (mg/(g·h^1/2) is the diffusion rate constant.

The adsorption isotherm can be analyzed using three equations as follows:

(1) Freundlich isotherm:

The Freundlich model is an empirical adsorption model generally used to represent the adsorption process of a target on a non-homogeneous surface, which assumes that the amount of adsorption grows indefinitely with the concentration of adsorbate in the solution, and is expressed as follows:

$$\:{Q}_{e}={K}_{F}{{C}_{e}}^{1/n}$$

where n and K_F are constants related to adsorption capacity and adsorption strength under the Freundlich model, Qe (mg·g^− 1) is the adsorbed amount of the composite material, and Ce (mg·L^− 1) is the concentration at solution adsorption equilibrium.

(2) Langmuir isothermal adsorption model:

Langmuir isothermal adsorption model assumes that the adsorption sites on the surface of the adsorbent are uniformly distributed, the adsorption of adsorbent is a monomolecular layer adsorption and there is no interaction between adsorbate molecules.The Langmuir isothermal model is expressed as follows:

$$\:{Q}_{e}=\frac{{Q}_{m}{K}_{L}{C}_{e}}{1+{K}_{L}{C}_{e}}$$

where Q_e (mg·g^− 1) is the maximum adsorption capacity of the composite, Q_m (mg·g^− 1) is the theoretical saturated adsorption capacity of the monolayer adsorption, C_e (mg·L^− 1) is the concentration at the equilibrium of adsorption of the solution, and K_L (L·mg^− 1) is a constant related to the affinity of the adsorption site.

2.5 Zeolite regeneration methods

Zeolites after adsorption of ammonia nitrogen were regenerated by the following three methods and the effects of these three regeneration methods were compared. Regeneration stage: three different regeneration agents were numbered separately.

Treatment 1: Weigh 5.0 g of NaCl solid and prepare a 1000 mL NaCl volumetric flask;

Treatment 2: Measure 36.8 mL of NaClO solution and prepare 1000 mL of NaClO solution;

Treatment 3: 5.0 g NaCl solid and 36.8 mL NaClO solution, prepared into 1000 mL mixed solution. Regeneration process: the dried artificial zeolite was transferred to stirring cups 1–6, 1–3 cups were added with 200 mL of the corresponding regeneration agent, and 4–6 cups were added with 400 mL of the corresponding regeneration agent. 6 stirring cups were placed in a coagulation test mixer, and stirred for 60 min at a rotational speed of 200 r·min^− 1. At the end of the stirring process, the artificial zeolites were centrifugally washed for more than 3 times, and then dried. The regeneration of artificial zeolite was completed.

2.6 Characterization

X-ray diffraction (XRD) analysis was carried out using the Rigaku X, taLAB Sunergy instrument from Japan, with an angle range of 5–80°. FT-IR spectroscopy was performed on a Tensor 27 FT-IR spectrometer (Bruker, Germany) and X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical composition of elements using ESCALAB 250Xi (Thermo Fisher, American). The surface properties and pore size of adsorbents were estimated by N₂ adsorption-desorption experiments performed at 77 K (BET, JW-BK122W, China).

3.1 Synthesis of zeolite

The XRD spectra (Fig. 1) exhibits a significant match with the zeolite's crystallographic planes as documented in PDF#43–0147, with distinct peaks observed at 2θ values of 7.2° and 10.1°, aligning with the (100) and (110) planes of the zeolite structure. The peaks in 2θ values of 17.6° and 30.8° appear particularly prominent and are attributed to the (211) and (411) planes, respectively. Additional peaks at 2θ = 23.9°, 27.1°, 34.1°, 42.1°, 49.5°, and 58.5° correspond to the (311), (321), (332), (522), (542), and (643) crystal planes. These findings are generally consistent with the characteristic diffraction peaks reported for the zeolite (Sun et al. 2024b).

In Fig. 2, the absorption peaks of Zeolite near 3430 cm^− 1 and 1650 cm^− 1 were related to the tensile vibrations of its surface coordination water and adsorbed water (Sun et al. 2024b). The absorption at 717 cm^− 1 and 565cm^− 1 were attributed to tetrahedral stretching vibration and Si-O bending vibration, respectively (Kabadayi et al. 2024), Meanwhile, P-O tensile vibrations appeared at 1000 cm^− 1 (Mu et al. 2021).

The presence of O,C, Si and Al elements can be clearly found in the zeolite as shown in Fig. 3(a), in Fig. 3(b), the peaks at 292 eV and 289 eV correspond to C = O and O-C = O, respectively, and the peaks at 284 ~ 285 eV correspond to C-C/C = C (San et al. 2025), and in Fig. 3(c) the peaks at 530 ~ 532 eV are mainly C-O and Al-O, and the weak peaks in 537 eV are considered to be O-C = O (Chen et al. 2020). As shown in Fig. 3(d) the asymmetry of the Al 2p spectral line is explained by using a double-peak fit. Similarly, the Al³⁺ion in the [AlO₄] unit has a small coordination number, corresponding to a low binding energy (~ 74.26 eV) in the system, also the aluminum oxide in the system is an intermediate oxide (Lin et al. 2023). The peak around 102 eV in Fig. 3e corresponds to SiO₂, which indicates that the Si element in the system exists mainly as an oxide (Yi et al. 2023).

Table 1

Textural parameters for the sample.
Sample	BET surface area (m² g^− 1)	Pore volume (cm³ g^− 1)	Pore size (nm)
Zeolite	23.84	0.054	5.59

As shown in Fig. 4a, the composites all exhibit strong N₂ adsorption in the region of P/P₀ > 0.45, which mainly occurs on the outer surface of the mesoporous zeolite composites coupled with increased basal spacingand. In Fig. 4b, the pore size distribution maps are all relatively wide, ranging from 2 to 30 nm (Huang et al. 2013), showing the excellent adsorption capacity of zeolite. The specific surface area, pore volumes and average poresize are listed in Table 1. Zeolite composites possess specific surface areas of 23.84 m² g^− 1, pore volume of 0.054 cm³ g^− 1 and pore size of 5.59 nm.

3.2 Adsorption kinetics

As shown in Fig. 5 it is important to achieve efficient adsorption as effectively as possible in the context of practical cost and operational feasibility. Therefore, we evaluated ammonia nitrogen adsorption under the adsorption kinetic conditions of zeolite and performed a kinetic analysis of the adsorption process using pseudo-first-order kinetics as well as pseudo-second-order kinetics, describing the diffusive behavior of adsorbent and the movement of adsorbent molecules, reflecting the dynamic interactions between adsorbent and adsorbent. It was observed that the adsorption process was mainly divided into three stages: fast adsorption (0–60 min), slow adsorption (60–120 min) and adsorption equilibrium (after 120 min). qt up to 12.03 mg/g has good adsorption capacity (Sun et al. 2021).

3.3 Adsorption isotherms

As shown in Fig. 6, the adsorption kinetics experiments determined the time at which the zeolite reaches adsorption equilibrium, so adsorption isotherm experiments were used to describe the effects of different adsorption temperatures and initial concentrations on the adsorption process at this time The experimental results for the effects of different initial ammonia nitrogen concentrations were fitted by the Langmuir adsorption isotherm and Freundlich adsorption isotherm, which were used to describe the effects of different adsorption temperatures and initial concentrations on the adsorption process at this time (Luo et al. 2020).

3.4 Effect of different treatments on zeolite adsorption

Since the coexisting ions may affect the adsorption of ammonia nitrogen by zeolite, we tested the effect of Na⁺, K⁺, Mg²⁺ and Ca²⁺ interference (Fig. 7), and we were able to find that the coexisting ions all have some effect on the adsorption capacity of ammonia nitrogen, in which the effect of Na⁺ and K⁺ on its adsorption capacity is significantly less than that on the effect of Mg²⁺ and Ca²⁺ ions.

In addition, we tested the regeneration capacity of zeolite in Fig. 8, and the experimental data were measured as the adsorption amount of ammonia nitrogen adsorbed by artificial zeolite for 1.0 h. We compared the effects of different adsorbents and the number of regeneration times on the adsorption effect of artificial zeolite. From the adsorption amount, the regeneration effect was kept at a high level in the first 4 times, and the adsorption efficiency was increased again in the 6th time, and the regeneration effect of the regeneration agent on the artificial zeolite was not obvious after carrying out regeneration for 10 times.There was no obvious difference in the regeneration effect of NaClO and NaCl + NaClO mixed regeneration agent on the artificial zeolite with comparable regeneration capacity, and the content of Na⁺ of the three kinds of regeneration agents was analyzed, and the Na⁺ content of the regeneration agent was found to be higher than the Na⁺ content of the three kinds. The Na⁺ content of regeneration agent is more than that of Treatment 1 and Treatment 2 regeneration agent, and then compare the adsorption effect of Treatment 3 regeneration agent, and found that the regeneration effect of artificial zeolite adsorption amount of the first few times is slightly higher than that of Treatment 2 regeneration agent NaClO solution, and significantly higher than that of Treatment 1 regeneration agent NaCl solution.

Fly ash and sodium meta-aluminate were used as raw materials to synthesize zeolite materials by ball milling method with hydrothermal crystallization method, which can adsorb ammonia nitrogen efficiently and explored the effect of three regeneration treatments on reuse of adsorbed ammonia nitrogen was studied. The XRD spectra exhibits a significant match with the zeolite's crystallographic planes as documented in PDF#43–0147 and the IR spectra shows that the absorption at 717 cm^− 1 and 565 cm^− 1 is attributed to tetrahedral stretching vibrations and Si-O bending vibrations. The presence of O,C, Si and Al elements can be clearly found in the zeolite. Additionally, the composites all exhibit strong N₂ adsorption in the region of P/P₀ > 0.45, which mainly occurs on the outer surface of the mesoporous zeolite composites coupled with increased basal spacingand. Zeolite composites possess specific surface areas of 23.84 m² g^− 1, pore volume of 0.054 cm³ g^− 1 and pore size of 5.59 nm. Adsorption kinetics and adsorption isotherms reflect the excellent adsorption capacity of zeolites and the effect of Na⁺ and K⁺ on its adsorption capacity is significantly less than that on the effect of Mg²⁺ and Ca²⁺ ions. Compared the adsorption effect of Treatment 3 (NaClO + NaCl) regeneration agent, and found that the regeneration effect of artificial zeolite adsorption amount of the first few times is slightly higher than that of Treatment 2 regeneration agent NaClO solution, and significantly higher than that of Treatment 1 regeneration agent NaCl solution.

Author’s contributions JC and ZH designed this study. JC and ZZ performed lab analyses. JL, ZZ, JW, SB and XT performed data analyses and all authors were involved in data interpretation

Funding The study was supported by the Key-Area Research and Development Program of Guangdong Province (2020B0202080001).

Data availability All data obtained have been included into the manuscript and available from the corresponding author upon reasonable request.

Declarations

Ethical approval This article neither contained any human participants nor animals which require ethical approval.

Consent to participate Not applicable.

Consent to publish The final manuscript was read and approved by all authors.

Competing interests The authors declare that they have no financial or non-financial conflict of interest.

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Green synthesis of zeolite and its regeneration for adsorption of ammonia nitrogen in water

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Abstract

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1. Introduction

2. Materials and methods