Synthesis and Characterization of Magnetic Fe<sub>3</sub>O<sub>4</sub>/Zeolite NaA Nanocomposite for the Adsorption Removal of Methylene Blue Potential in Wastewater Treatment

Tran, Ngoc Bich Thi; Duong, Ngoc Bich; Le, Ngoc Long

doi:https://doi.org/10.1155/2021/6678588

Journal of Chemistry

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Research Article | Open Access

Volume 2021 | Article ID 6678588 | https://doi.org/10.1155/2021/6678588

Synthesis and Characterization of Magnetic Fe₃O₄/Zeolite NaA Nanocomposite for the Adsorption Removal of Methylene Blue Potential in Wastewater Treatment

Ngoc Bich Thi Tran,¹Ngoc Bich Duong,²and Ngoc Long Le^3,4

Academic Editor: Liviu Mitu

Received11 Nov 2020

Revised21 Jan 2021

Accepted23 Jan 2021

Published01 Feb 2021

Abstract

In this research, the magnetic Fe₃O₄/zeolite NaA nanocomposite (Fe₃O₄/ZA), Fe₃O₄ nanoparticles, and zeolite NaA have been synthesized by facile hydrothermal methods for adsorption removal of methylene blue from aqueous solution. The as-synthesized Fe₃O₄/ZA nanocomposite was characterized by X-ray diffraction (XRD), MicroRaman analysis, Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray fluorescence (XRF), N₂ adsorption isotherms (BET), and UV-VIS analysis. The results show that with a small weight loading of Fe₃O₄, the ∼3.3% Fe₃O₄/ZA sample exhibits a high adsorption capacity (∼40.36 mg·g⁻¹) and removal efficiency (∼96.8%) compared to that of the zeolite NaA (∼32.99 mg·g⁻¹ and 79.11%, respectively). Interestingly, the removal efficiency and the adsorption capacity increase rapidly with the increase of adsorption time (10–60 minutes) and Fe₃O₄ loading (∼3.3–9.3% wt.) in the Fe₃O₄/ZA composition. The adsorption mechanism of MB molecules of the Fe₃O₄/ZA can be addressed at the combination of the interaction between active sites on the surfaces and edges of the invert spinel ferrite Fe₃O₄ nanoparticles and zeolite NaA with MB molecules. Our approach provides a simple, efficient, and scalable synthesis process that render practical applications of the magnetic Fe₃O₄/ZA nanocomposite as a lower-cost adsorbent for wastewater treatment.

1. Introduction

Water is one of the most important resources essential for all living organisms. However, due to the fast growing in production of textile and plastics industries, methylene blue became the byproduct of the fabrication processes and is considered a contaminant that remains untreated in wastewater. Methylene blue (MB) (3, 7-bis (dimethylamino)-phenothiazin-5-iumchloride) is a thiazine cationic dye which is commonly used for biological staining also and coloring paper, hair, cottons, and wools [1]. The accumulation of MB in wastewater may cause many health effects in breathing, vomiting, eye burns, diarrhea, and nausea [2] and negative impacts to the environment. In the recent years, such methods as flocculation [3], membrane filtration [4], oxidation [5], photocatalytic degradation [6], and adsorption [7] have been developed for removing MB from abovementioned wastewater. Among these techniques, adsorption stands out to be a suitable approach that is capable of separating organic molecules by using adsorbent materials. Previous reports showed that MB can be adsorptive and removed from aqueous solution by using activated carbon obtained from Pea Shells [8] or corn husk [9], biomass fabricated from Algae D. Antarctica [10], agricultural residue walnut shell [11], and zeolite-based materials [12–14].

Zeolites are aluminosilicate minerals which are classified as porous materials due to their crystalline microstructures possess cages, channels, and open void spaces within their frameworks [15]. Zeolite frameworks are constructed by [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedral, linked together to form cages connected by pore openings of defined size. The presence of [AlO₄]⁻ in the zeolite framework introduces negative charges which is balanced by cations such as Ca²⁺, K⁺, and Na⁺ [16]. Previous studies show that zeolite materials possess good adsorption efficiency for removal of heavy metals like arsenic [17], mercury [18], fluoride [19], and organic dyes [20] as adsorbents. Zeolite NaA [21], with chemical composition [Na₁₂ [(AlO₂)₁₂ (SiO₂)₁₂]. 27 H₂O, is identified by the Al/Si ratio of ∼1.0. Zeolite NaA is currently a commercially important zeolite used in the industry for catalysis, adsorption, and industrial gas separations. However, difficulty in separating the adsorbed zeolite from the aqueous solution remains a considerable problem. In industries, magnetic separation is desirable due to its advances in operation compare to filtration, centrifugation, or gravitational separation.

On the other hand, magnetic iron oxide-based materials have attracted much attention in many environmental applications. Among them, invert spinel-structured Fe₃O₄ oxide possesses high chemical stability, high coercively, low Curie temperature, and low toxicity [22]. Due to their magnetic property, the Fe₃O₄ solid phase can be separated easily from adsorbed liquid medium by using magnetic separation. Several synthesis methods such as the microemulsion technique [23], electrochemical synthesis [24], hydrothermal method [25], sol-gel process [26], and coprecipitation synthesis [27] exist for synthesizing magnetic Fe₃O₄ materials. Among the abovementioned methods, the hydrothermal approach is an effective method to fabricate nanoparticle size materials within a short time [28].

In this research, we employed a facile hydrothermal synthesis method to synthesize magnetite Fe₃O₄/zeolite NaA nanocomposite (denoted as Fe₃O₄/ZA) as low-cost adsorbents for the removal of MB from aqueous solutions. The advantages of magnetic adsorbents are magnetic separation capability that can be easily separated from the solution by applying an external magnet. The as-synthesized Fe₃O₄/ZA nanocomposite was characterized using XRD, Micro-Raman, FTIR, EDX, XRF, and FESEM to investigate the formation of magnetite Fe₃O₄/ZA nanocomposite samples. Their adsorption capacity for removal of MB organic molecules was conducted by bath adsorption experiments and investigated using UV-VIS absorbance analysis. Our approach provides a simple, efficient, and scalable synthesis process that renders practical applications of the as-synthesized magnetic Fe₃O₄/ZA nanocomposite for wastewater treatment.

2. Materials and Methods

2.1. Chemicals and Materials

The analytical graded chemicals were used without purification. The main chemicals used include sodium silicate solution (Na₃SiO₃ 30% SiO₂), aluminum hydroxide (Al (OH)₃ 99%), sodium hydroxide (NaOH, 99%), hydrochloric acid (HCl, 99%), iron (II) chloride tetrahydrate (FeCl₂.4H₂O, 99%), iron (III) chloride hexahydrate (FeCl₃.6H₂O, 98%), methylene blue (MB) (C₁₆H₁₈ClN₃S.xH₂O, >82%), ammonia (NH₄OH, 32%), and hydrochloric acid (HCl, 0.1 M) solutions obtained from Sigma Aldrich (Singapore).

2.2. Preparation of Zeolite NaA

Zeolite NaA was prepared by hydrothermal method as described in [29] with modification. In brief, sodium silicate (SiO₂ source) and aluminum hydroxide (Al₂O₃ source) were dissolved separately in deionized water and sodium hydroxide solution. The SiO₂ and Al₂O₃ sols were formed when the sodium silicate and aluminum hydroxide was dissolved in sodium hydroxide solution and followed by filtration to remove the insoluble residues. The molar ratios of SiO₂/Al₂O₃, Na₂O/SiO₂, and H₂O/Na₂O are controlled at ∼1.3, 1.5, and 160, respectively. After that, these as-prepared solutions were ultrasonic-assisted mixed together and transferred to a Teflon-lined stainless steel autoclave reactor and sealed and heated at ∼120 ± 5°C for 6 hours. The obtained solid phase was collected and washed with distillated water until the pH value reached 8.0 and dried overnight at ∼105°C.

2.3. Preparation of Magnetic Fe₃O₄ Nanoparticles

Magnetic Fe₃O₄ nanoparticles (denoted as Fe₃O₄ NPs) were synthesized by the hydrothermal method [25] from an alkaline solution of Fe²⁺ and Fe³⁺ salts. In brief, a solution of FeCl₂ and FeCl₃ with Fe²⁺/Fe³⁺ molar ratio of ∼1/2 was mixed at room temperature under ultrasonication condition. After that, ammonia solution (∼30%) was added dropwise to control pH of the as-prepared solution in range of ∼8.0–9.0. The final solution was transferred to a Teflon-lined stainless steel autoclave reactor and sealed and crystallized at ∼120 ± 5°C for 3 hours. After the reaction process, the black precipitation was collected and washed several times with deionized water.

2.4. Synthesis of Fe₃O₄/ZA Nanocomposite

For the synthesis of Fe₃O₄/ZA nanocomposite, an amount of ∼6.0 g of as-prepared zeolites A was dispersed into 100 mL of distilled water and sonicated for ∼30 minutes. Next, a volume ∼20 mL of FeCl₃ and FeCl₂ solutions (molar ratio of Fe²⁺/Fe³⁺ is 1/2) was added into the as-prepared suspension. Then, about ∼10 mL of a 1.0 M NaOH solution was added dropwise under ultrasonication condition in other to control thevpH of this solution in range of ∼8.0–9.0. After that, these solutions were ultrasonic-assisted mixed together and transferred to a 150 mL Teflon-lined stainless steel autoclave reactor (∼80 % volume filled) and sealed and heated at ∼120 ± 5°C for 6 hours. The obtained solid phase was collected by centrifugation for characterization and washed with distillated water until the pH value reached 8.0 and dried overnight at ∼105°C.

To investigate the effect of Fe₃O₄ NP loading to the adsorption capability of the as-synthesized Fe₃O₄/ZA nanocomposite, different molar ratios of (Fe²⁺/Fe³⁺) in the precursors were varied while the amounts of zeolites A in the precursor were kept constant. The Fe₃O₄ NP loading in the Fe₃O₄/ZA nanocomposite samples were designed to be about ∼3.3, 6.6, and 9.3 wt. %. The percentage of Fe₃O₄ loading p_F (%) is calculated by the following equation:whereas denotes for the weight of Fe₃O₄ solid phase forming from the synthesis process obtained from XRF data and is the weight of zeolite A. The final products were used for the structural characterization and bath adsorption experiment investigation.

2.5. Structural Characterization

The X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance diffractometer with Cu radiation ( = 1.5406 Å). The morphology of the samples was observed by field-emission scanning electron microscopy (FESEM) (S-4800, Hitachi). Elemental compositions were estimated using energy-dispersive X-ray (EDX) spectroscopy and Zeiss EVO LS10 scanning electron microscopy (SEM) equipped with the Oxford INCA ADAX detector. Chemical composition was determined by employing the X-ray fluorescence (XRF) method on a S6 JaAGUAR instrument, Bruker. Micro-Raman measurements were carried out using Horiba XploRA ONE spectrometer equipped with Olympus BX50 microscope attachment to focus the laser beam on a 180 × 120 μm² selected area of the sample. A green argon laser ( = 532 nm, 15 mW) was used as an excitation source with exposition time of 15 seconds and 900 lines per mm grating monochromator with liquid nitrogen-cooled CCD. Fourier-transform infrared spectra (FTIR) in range of 400–4000 cm⁻¹ were recorded using a Frontier FT-IR/NIR spectrophotometer, PerkinElmer. UV-VIS absorbance spectra in range of 200–800 nm were obtained by employing Cary 60 UV-VIS spectrophotometer, Agilent Technologies. Low-temperature adsorption isotherms were employed to determine the specific surface area (m²·g⁻¹) of the as-synthesized samples using NOVA 1000e Autosorb-1 (Quanta-Chrome Instruments) equipped with NovaWin data acquisition software. The pH value of all the investigated solution was measured by HI2020 edge® Multiparameter pH Meter, Hanna Instruments.

2.6. Batch Adsorption Experiments

In this research, we investigate the potential of the as-synthesized Fe₃O₄/ZA nanocomposite as an adsorbent for removing methylene blue (MB) molecules from solution. In our experiments, the MB solution was prepared by dissolving ∼0.300 g·MB powder in ∼1.0 L double-distilled water and used as pollutant for bath adsorption experiments. The initial concentration and of MB solution are = 300 mg·L⁻¹ and 3.8, respectively.

In the typical bath adsorption experiment, a volume ∼50.0 mL of the MB solution was added into 50 mL polypropylene graduated centrifuge tubes with screw cap. Then, an amount ∼0.360 g of the Fe₃O₄/ZA nanocomposite with different loading (3.3%, 6.6%, and 9.3%) of Fe₃O₄ NPs was added to this solution. The pH of solutions was kept at ∼8.60 by adding NH₄OH and HCl (0.1 M) solutions. After that, the tubes were placed on an isothermal vibrator (HY-4A Cycling Vibrator, Sinosource Ltd.) with a rate of 300 rpm and temperature of ∼25°C and kept for 30 minutes. These solutions were centrifuged at 6300 rpm (using an EBA 200S Centrifuge, Hettich) to separate the solid phase from solutions, and the upper (80% vol.) supernatant were collected. The remained concentration of MB left in the collected supernatants was monitored by evaluating the maximum absorption in UV-VIS spectra of the corresponding samples. The MB removal efficiency R (%) and the adsorption capacity (mg·g⁻¹) of the Fe₃O₄/ZA nanocomposite and ZA were calculated using the following equations:where (mg·L⁻¹) is the initial concentration of MB; (mg·L⁻¹) is the MB concentration at equilibrium condition of each experiment, V (L) is the volume of MB solution, and m (g) is the weight of the Fe₃O₄/ZA nanocomposite and ZA samples used in experiments. In addition, the absorbance A (a.u.) and the concentration (mol·L⁻¹) of MB solution are linearly related by the BeerLambert law, according to the following equation:where (L·mol⁻¹·cm⁻¹) is the molar absorptivity of MB ( = 95000 L·mol⁻¹·cm⁻¹) [30], l (cm) is the optical path length of solution. In addition, the initial and equilibrium adsorption of MB and can be calculated by sum of area under the fitted peaks in the UV-VIS absorbance spectra of the supernatants collected from bath adsorption experiments, as described in the following equation:where are the areas under the characteristic absorbed peaks centered at maximum wavelength (nm). Under our experimental conditions, by applying the Voigt fitting function (using OriginPro® 8.0 software), the absorbance spectrum of MB exhibits six main characteristic peaks centered at monochromatic wavelengths of ∼246, 292, 498, 572, 617, and 664 nm, which is the one employed in most literatures [1, 2, 20], as seen in Figure 1. Therefore, the MB removal efficiency R (%) was calculated using equation (4).

The bath adsorption experiment was first conducted using the as-synthesized zeolite A for comparison. Simultaneously, the as-synthesized Fe₃O₄/ZA nanocomposite with different Fe₃O₄ NP loadings was conducted in the same condition for evaluating the effect of Fe₃O₄ NPs (wt. %) loading on MB removal. For investigating the removal efficiency and adsorption capacity of the Fe₃O₄/ZA nanocomposite, two separate groups of samples 3.3% Fe₃O₄/ZA and 6.6% Fe₃O₄/ZA were used for the bath adsorption experiments. The experiments were conducted with a series of samples by applying the same experimental parameters, and the supernatant of each sample was collected at different adsorption time (interaction time) from 10 to 240 minutes. Each adsorption experiment was conducted with three replications. The average data were used to plot in the UV-VIS graphs.

3. Results and Discussion

3.1. Microstructure of Magnetic Fe₃O₄/ZA Nanocomposite

The formation and crystalline structures of the as-synthesized magnetic Fe₃O₄/ZA nanocomposite, magnetic Fe₃O₄ NPs, and ZA can be well characterized by X-ray diffraction (XRD) analysis. The XRD analysis is based on Bragg’s equation [31], where n is the order of diffraction, is the wavelength of incident X-ray (Cu , 0.15406 nm), is the interplanar (hkl) spacing, and is the angle between the incident X-ray and the scattering planes. XRD patterns of the Fe₃O₄/ZA nanocomposite, Fe₃O₄ NPs, and ZA samples are shown in Figure 2. For the as-synthesized ZA, the presence of characteristic peaks located at ∼7.3°, 10.3°, 16.3°, 21.9°, 24.2°, 27.4°, 29.9°, and 34.3° matched well with standard diffraction peaks of (100), (110), (111), (210), (300), (311), (321), (330), and (332) planes of zeolite NaA (zeolite A, (Na) JCPDS # 00-038-0241). These observed peaks indicate the existence of zeolite A (Linde Type A) [32] crystalline phase in the sample and well agreed with previous reports [25, 33].

The XRD pattern of the as-prepared Fe₃O₄ NPs shows the diffraction peaks located at ∼18.7°, 31.3°, 35.6°, 43.3°, 45.1°, 53.6°, 57.3°, and 62.9° correspond to diffraction peaks of (111), (200), (311), (400), (422), (511), and (440) planes of Fe₃O₄ crystalline phase (Fe₃O₄ magnetic, JCPDS #00-065-0731). These characteristic peaks confirm the dominance of Fe₃O₄ magnetic phase [25, 27] in the sample. The formation mechanism of Fe₃O₄ NPs can be explained by the reaction of Fe²⁺/Fe³⁺ in the hydrothermal reaction as follows:

For the Fe₃O₄/ZA nanocomposite samples with different Fe₃O₄ loadings (3.3, 6.6, and 9.3 wt.%), the presence of characteristic peaks centered at ∼18.8°, 30.3°, 35.6°, 43.5°, 57.3°, and 63.2° corresponds to (111), (200), (311), (400), (511), and (440) planes of Fe₃O₄ crystalline phase (JCPDS #00-065-0731) indicating that Fe₃O₄ magnetic phase has been successfully incorporated on the ZA matrix and forming Fe₃O₄/ZA nanocomposite. The crystallite size along a specific (hkl) direction of Fe₃O₄/ZA nanocomposite, ZA, as well as magnetic Fe₃O₄ samples can be calculated using Scherrer’s equation [34] (equation (8)) with accepted shape factor of K = 0.94. Scherrer’s equation is given as follows:where is the wavelength of X-ray radiation (Cu = 1.5406 nm), is the full width at half maximum (FWHM), is the diffraction angle, and is the average crystallite size. By applying Scherrer’s equation, the crystallite size for (311) peak of Fe₃O₄ NPs was calculated to be ∼19.1 nm while the crystallite size of ZA calculated for (100) is ∼96.5 nm. The crystallite size of the Fe₃O₄/ZA nanocomposite and magnetic Fe₃O₄ phases of the as-synthesized composite samples is listed in Table 1.

As a powerful characterization technique, Raman analysis offers us more information about the structure of aluminosilicate zeolitic materials [35] and the magnetic phase of iron oxides [36]. As seen in Figure 3, Lorentz fitting of the micro-Raman spectrum of ZA sample exhibits Raman vibration modes located at ∼109.5, 164.4, 279.4, 341.6, 475.7, 716.9, 970.2, 1041.4, and 1099.8 cm⁻¹. The strongest bands located at ∼475.7 cm⁻¹ are assigned to the bending mode of 4-membered Si–O–Si rings (4MR) [37, 38]. While the Raman bands at ∼341.6 and ∼408.3 cm⁻¹ are attributed to the bending mode of 6-membered Si–O–Si rings (6MR), the Raman bands at ∼970.2, 1041.4, and 1099.8 cm⁻¹ are attributed to the asymmetric T–O–T (T ≡ Si or Al) stretching motions [37, 38]. Interestingly, the band at ∼109.5 cm⁻¹ is attributed to the bending mode of higher membered rings, such as 8-membered rings (8MR) of zeolite A that also observed in previously reports [39]. In case of Fe₃O₄ NPs sample, Raman spectrum reveal the dominant bands centered at ∼221.6, 270.2, 379.6, 476.9, 647.3, and 1256.9 cm⁻¹ of the Fe₃O₄ NPs. These peaks are well agreed with previous reports [37, 38] that confirm the magnetic phase of the as-synthesized Fe₃O₄ NPs. Careful investigation the Raman spectra of the Fe₃O₄/ZA nanocomposite samples with different Fe₃O₄ NP mass loadings show that all the spectra exhibit three dominant Raman bands at ∼270, 565 and ∼637–648 cm⁻¹, which correspond to the modes of magnetic Fe₃O₄ phase [40], respectively. These observations help confirm the Fe₃O₄/ZA nanocomposite has been successfully synthesized.

3.2. Surface Morphology of Fe₃O₄/ZA Nanocomposite

The surface morphologies of the as-synthesized Fe₃O₄/ZA nanocomposite, Fe₃O₄ NPs and ZA samples were observed using FESEM, as shown in Figure 4. As seen in Figures 4(a)–4(c), the surface morphology of the ZA sample is dominated with characteristic cubic shape and relatively shiny edges. The crystallite sizes in range of ∼5–10 μm with well-defined and smooth edges display the typical cubic morphology of LTA zeolites that can be clearly observed.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

In Figures 4(g)–4(i), particles that are more spherical in shape and significantly smaller in dimension are evident in the FESEM images and defined the typical morphology of Fe₃O₄ nanoparticles. The magnetic crystalline phase of the Fe₃O₄ nanoparticles has been clearly confirmed with XRD results (Figure 2) and Raman analysis (Figure 3). The introduction of Fe₃O₄ NPs into the zeolite A structures lead to noticeable changes in the surface morphology, as shown in Figures 4(d)–4(f). After decorated with Fe₃O₄ NPs, the surface of 3.3% Fe₃O₄/ZA nanocomposite sample is covered with nanoparticles that occupy porous surface structure. The porous surface morphology may lead to the increasing of specific surface area (SSA) (Table 1) and facilitating the adsorption capability of the as-synthesized (∼3.3–9.3%) Fe₃O₄/ZA nanocomposite samples (SSA ∼63.18–117.04 m²·g⁻¹) when compared with that of bare zeolite A (∼56.49 m²·g⁻¹) and Fe₃O₄ NPs (∼5.97 m²·g⁻¹).

3.3. Chemical Composition

Energy-dispersive X-ray (EDX) was employed to analyze the chemical composition of the as-synthesized 3.3% Fe₃O₄/ZA nanocomposite and Fe₃O₄ NP samples. The EDX spectra (Figures 5(b) and 5(d)) were recorded in the area shown in Figures 5(a) and 5(c) (SEM images).

(a)

(b)

(c)

(d)

EDX spectra in Figure 5(d) show that the main chemical composition of Fe₃O₄ NPs sample is Fe (41.67%) and O (57.33%) characterized by two peaks located at binding energy (Fe ) ∼6.46 and ∼7.08 keV, whereas the main elemental compositions of the Fe₃O₄/ZA nanocomposite sample are include O (53.38%), Na (8.8%), Al (6.82%), Si (7.40%), and Fe (6.96%). In addition, the appeared peaks in regions of ∼6.46 and ∼7.08 keV are originated from the binding energies of Fe Kα indicating the existence of iron in the sample. The calculated Fe/O ratio is ∼0.73 that is matched well with the stoichiometric ratio (Fe/O = 0.75) of Fe₃O₄ oxide which indicates the presence of magnetic iron oxide in the as-synthesized sample. Also, from the EDX results, the calculated Si/Al ratio of the as-synthesized Fe₃O₄/ZA nanocomposite is ∼0.940 that is close to the stoichiometric Si/Al ratio of zeolite A (Si/Al = 0.925). The presence of iron and oxygen further confirmed the dominance of magnetite nanoparticles in the nanocomposite.

3.4. Fourier-Transform Infrared Spectroscopy (FTIR) Analysis

FTIR analysis provides more information about the structures of the as-synthesized Fe₃O₄/ZA nanocomposite because it reveals the characteristic vibrational bands of the functional groups dominating in their structures, as shown in Figure 6. The FTIR absorption peaks located at ∼3360–3510 cm⁻¹ indicate the presence of the characteristic hydroxyl group (–OH) of the water on the as-synthesized Fe₃O₄/ZA composite zeolite A and Fe₃O₄ NPs [41].

In the FTIR spectra of the Fe₃O₄/ZA nanocomposite and ZA samples, the peaks centered at ∼1630 cm⁻¹ can be attributed to the vibration bending modes of coordinated (–OH) groups attached on the surfaces of zeolite A crystals [41]. The vibration band occuring at ∼1090 cm⁻¹ and 986 cm⁻¹ represents the internal vibrations due to the asymmetric stretching of Si–O–Al tetrahedral [41]. In addition, the spectra at ∼660 cm⁻¹ of the zeolite A samples can be attributed to internal vibrations due to symmetric stretching within the zeolite structure. The presence of the external T–O linkage, caused by the double ring [29], is also confirmed by the band at the wavenumber of ∼550 cm⁻¹. The band at position ∼466 cm⁻¹ indicates the presence of internal tetrahedron vibrations of Si–O and Al–O with bending modes of the sodalite cages found in zeolites [29].

For the as-synthesized Fe₃O₄/ZA nanocomposite, the presence of Fe₃O₄ NPs leads to the variation of vibration band of the zeolite A. The absorption peaks centered at ∼760 cm⁻¹ (in sample 3.3% Fe₃O₄/ZA), ∼730 cm⁻¹ (in sample 6.6% Fe₃O₄/ZA), and ∼660 cm⁻¹ (in sample 9.3% Fe₃O₄/ZA) correspond to Fe²⁺–O–Fe³⁺, Fe³⁺–O, and Fe²⁺–O bonds from Fe₃O₄ which also reported previously [42–44]. Those peaks did not appear in the zeolite A spectrum but presented the spectrum of Fe₃O₄ sample. This observation suggests that the Fe₃O₄ NPs have been introduced to the zeolite A structure.

3.5. Selective Adsorption Removal of Methylene Blue

3.5.1. Effect of Fe₃O₄ Loading on Fe₃O₄/ZA Nanocomposite on the Removal of MB

The adsorption capacity and the effect of Fe₃O₄ NPs mass loading on the Fe₃O₄/ZA nanocomposite to the removal of MB were evaluated by determining the UV-VIS absorbance spectra of the as-synthesized Fe₃O₄/ZA and zeolite A samples. As seen in Figure 7(a), the absorbance spectrum of the as-synthesized ZA shows a significant decrease in the intensity and total sum of area under the fitted peaks at wavelength 664 nm compared to that of the initial MB solution. The calculated adsorption capacity (, mg·g⁻¹) and the MB removal efficiency (R, %) of ZA sample are ∼32.99 mg·g⁻¹ and ∼79.11%, respectively, which were close to those of the previously reports [20] (Table 2).

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7

UV-VIS absorbance spectra of supernatants collected from batch adsorption experiments. (a) The absorbance spectra of the MB solution treated by Fe₃O₄/ZA nanocomposite with different Fe₃O₄ NP loadings; (b, c) absorbance spectra of 3.3% Fe₃O₄/ZA and 6.6% Fe₃O₄/ZA samples at different interaction (adsorption) times; (d) removal efficiency (R, %) and adsorption capacity (, mg·g⁻¹) of 3.3% Fe₃O₄/ZA and 6.6% Fe₃O₄/ZA samples, respectively; linear fitting results of removal efficiency and adsorption capacity in adsorption ranges of 0–60 minutes (e) and 60–240 minutes (f) in (d); the experiments data are presented as solid dots with error bars, while solid lines are linear fitting results of experiments data in Figures 7d, 7e, and 7f.

For the Fe₃O₄/ZA nanocomposite samples, the corresponding UV-VIS spectra indicate the adsorption capacity of Fe₃O₄/ZA nanocomposites through the decrease in intensity of characteristic absorbed peaks of MB (Figures 1 and 7(a)) and total sum of area under fitted peaks (Table 2). The adsorption capacity and the removal efficiency increase from ∼81.90 to ∼94.93% and ∼34.12 to ∼39.56 mg·g⁻¹, respectively, when loading of Fe₃O₄ NP increases from ∼3.3 to ∼6.6% wt. These calculated values are significantly high when compared to that of the ZA sample (this work) and magnetic iron (Fe₃O₄) mud previously reported by Liu et al. (∼27.4 mg·g⁻¹ [27]). The high adsorption capacity and the removal efficiency indicate the strong effect of Fe₃O₄ NP loading on the as-synthesized Fe₃O₄/ZA nanocomposite [45]. The maximum adsorption capacity of the as-synthesized Fe₃O₄/ZA nanocomposite (∼40.46 mg·g⁻¹) is rather high than that reported with Z-Fe₃O₄ NCs adsorbent (∼1.15 mg·g⁻¹) [13], magnetic zeolite HY-Fe₃O₄ (∼28.41 mg·g⁻¹), zeolite 13X-Fe₃O₄ (∼28.41 mg·g⁻¹) [46], and magnetic NaY zeolite composite (∼2.045 mg·g⁻¹) [47]. The maximum adsorption capacity of various adsorbents reported for MB removal from aqueous solution is listed in Table 3.

It can also be observed that the Fe₃O₄/ZA nanocomposite samples exhibit absorbance peaks centered at ∼378 nm and large absorbed background compared to that of ZA sample. The absorbance peaks of Fe₃O₄/ZA nanocomposite samples can be originated from FeCl₄⁻ [48] and ≡FeOH₂⁺, ≡FeOH, and ≡FeO⁻ complexes forming from electrostatic interaction between ferric (III) surface ions in Fe₃O₄ NPs and Cl^–ions in MB molecules as well as OH^– ions in alkaline condition. In addition, functional groups such as ≡FeOH₂⁺, ≡FeOH, and ≡FeO⁻ interact with Na⁺ on ZA surfaces to form (≡FeO–Na⁺) through electrostatic forces that could be favourably replaced by MB. Therefore, the possible adsorption mechanism for the removal of MB can be explained by the following equations:

This observation provides strong evidence that support for the adsorption removal capability of the as-synthesized Fe₃O₄/ZA nanocomposite.

3.5.2. Adsorption Capability versus the Contact Time

The effect of interaction time (or adsorption time) on removal of MB by the as-synthesized Fe₃O₄/ZA nanocomposite is shown in Figures 7(b), and 7(c). The batch adsorption experiments were conducted with two groups of samples including 3.3% Fe₃O₄/ZA and 6.6% Fe₃O₄/ZA nanocomposite for comparison. As seen in Figure 7(c), the absorbance spectra of sample 3.3% Fe₃O₄/ZA nanocomposite show that the absorbance intensity of MB remains in the supernatants were decreased significantly. The decrease in the absorbance indicates that a large concentration of MB molecules was removed from the initial solutions. The experimental data of removal efficiency R (%) and adsorption capacity (mg·g⁻¹) of samples 3.3% Fe₃O₄/ZA and 6.6% Fe₃O₄/ZA nanocomposite were plotted with errors bars as seen in Figure 7(d). The R and values were obtained to be ∼84.76% and ∼35.3 mg·g⁻¹ after 10 minutes and increased to ∼92.47% and ∼38.5 mg·g⁻¹, after 60 minutes, respectively. The R and values increase with an increasing of interaction time and reach to ∼96.87% and ∼40.36 mg·g⁻¹ after 240 minutes, as seen in Figure 7(e). When the amount of Fe₃O₄ loading increased to ∼6.6% wt. as in samples 6.6% Fe₃O₄/ZA nanocomposite, the R and values of these samples were observed to increase significantly to ∼92.35% and ∼38.48 mg·g⁻¹ at about 10 minutes of adsorption. At 60 minutes of interaction, these values were calculated to be about ∼92.73% and ∼38.64 mg·g⁻¹ and reached to ∼97.89% and ∼40.46 mg·g⁻¹ at 240 minutes, respectively. Experiments data are provided in Table 4.

Careful analysis of the variation of the obtained experimental data are in Figure 7(d) with the linear fitting results presented in Figures 7(e) and 7(f); one can observe that the removal efficiency and adsorption capacity of the two samples versus the interaction time change significantly and can be separated in two stages of about 0–60 minutes and 60–240 minutes. The R and values of the as-synthesized Fe₃O₄/ZA nanocomposite samples in the first stages of adsorptions (lower than 60 minutes) increase nonlinearly with higher rate (slope of ∼0.05–0.16; correlation factor = 0.42–0.86), while in the second stage, these values increase linearly with lower rate (slope of ∼0.01–0.12 and = 0.87–0.98). The R and values obtained in second stage suggests that the adsorption removal of MB follows a kinetic adsorption model of pseudo-second-order [20] which involves both physical and chemical interactions. The phenomenon can be explained by considering the surface structure and morphology of the as-synthesized Fe₃O₄/ZA nanocomposite (Figure 4). The incorporation of Fe₃O₄ particles in to zeolite NaA structure leads to the increase in a large number of active sites that can interact with MB molecules. At the first stage of adsorption, with a large number of available active sites, the adsorption process occurred simultaneously in the MB solution leading to the increase in adsorption capacity. It was also noticed that the rate of adsorption (slopes of the fitted lines) decreased after 60 minutes indicating that the availability of active sites on Fe₃O₄/ZA nanocomposite surfaces for the MB molecules to occupy had been reduced at equilibrium adsorption condition. The kinetic adsorption model of pseudo-second-order is defined by the following equations [27]:

The solution of equation (11) can be obtained by direct integration under the conditions: t = 0 and = 0 at t = t and = as follows:where (mg·L⁻¹) and (mg L⁻¹) are the equilibrium and at (t) min adsorption capacity and (g mg⁻¹·min⁻¹) is the pseudo-second-order diffusion rate constant that describes the amount of MB molecules adsorbed on the Fe₃O₄/ZA NCs at interaction time (t) and at equilibrium, respectively. In this regard, the values and are obtained from the plot of versus time (t) as listed in Table 5.

The uptake of the MB molecules by the Fe₃O₄/ZA nanocomposite can be addressed at the combination of the interaction between active sites the surfaces and edges of the Fe₃O₄ nanoparticles and zeolite NaA with the MB molecules. In this case, the invert spinel structure of the as-synthesized Fe₃O₄ NPs provides a large number of active sites (O²⁻ and Fe²⁺/Fe³⁺) that simultaneously electrostatically adsorbs the positive (C₆H₁₈SN₃⁺) and negative (Cl⁻) polarized poles of MB molecules, whereas the removal of MB in case of the zeolite NaA resulted from the electrostatic attraction between the cationic MB molecule and the negatively surface-charged zeolite A. Furthermore, due to the magnetic property of the as-synthesized Fe₃O₄/ZA nanocomposite, the materials can be recovered separated conveniently from aqueous solution for reuse (as seen in Figure 1(c)). Therefore, the magnetic Fe₃O₄/ZA nanocomposite is a suitable adsorbent for removal of MB from aqueous solution especially for purifying wastewater.

4. Conclusions

In conclusion, we have successfully synthesized and characterized the Fe₃O₄/zeolite NaA nanocomposite for potential adsorption removal of methylene blue in term of wastewater treatment. The as-synthesized Fe₃O₄/ZA nanocomposite exhibited a high adsorption capacity and removal efficiency compared to those of the as-synthesized zeolite NaA in the same experimental condition. The adsorption mechanism of MB molecules by the Fe₃O₄/ZA nanocomposite can be addressed at the combination of the interaction between active sites the surfaces and edges of the Fe₃O₄ spinel-structured nanoparticles and zeolite NaA with the MB molecules. With a small loading of ∼3.3% Fe₃O₄, the removal efficiency reaches up to ∼96.8% and ∼40.36 mg·g⁻¹ of adsorption capacity. The removal efficiency and the adsorption capacity were observed to increase linearly with the slightly increasing of adsorption time and Fe₃O₄ NP loading in the as-synthesized nanocomposites. Our results underscore the feasibility of magnetic zeolite nanocomposite materials as a lower cost adsorbent for the application of wastewater purification.

Data Availability

Some data used to support the findings of this study are included in the tables and graphs within the article. More data (Excel files and unedited images) are available from the corresponding author upon reasonable request.

Disclosure

This research received no external funding.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors would like to thank Ho Chi Minh City University of Technology (HCMUT), VNU-HCM, and Tra Vinh University for the support of time and facilities for this study.

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Copyright © 2021 Ngoc Bich Thi Tran et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Chemistry

Synthesis and Characterization of Magnetic Fe3O4/Zeolite NaA Nanocomposite for the Adsorption Removal of Methylene Blue Potential in Wastewater Treatment

Abstract

1. Introduction

2. Materials and Methods

2.1. Chemicals and Materials

2.2. Preparation of Zeolite NaA

2.3. Preparation of Magnetic Fe3O4 Nanoparticles

2.4. Synthesis of Fe3O4/ZA Nanocomposite

2.5. Structural Characterization

2.6. Batch Adsorption Experiments

3. Results and Discussion

3.1. Microstructure of Magnetic Fe3O4/ZA Nanocomposite

3.2. Surface Morphology of Fe3O4/ZA Nanocomposite

3.3. Chemical Composition

3.4. Fourier-Transform Infrared Spectroscopy (FTIR) Analysis

3.5. Selective Adsorption Removal of Methylene Blue

3.5.1. Effect of Fe3O4 Loading on Fe3O4/ZA Nanocomposite on the Removal of MB

3.5.2. Adsorption Capability versus the Contact Time

4. Conclusions

Data Availability

Disclosure

Conflicts of Interest

Acknowledgments

References

Copyright

Synthesis and Characterization of Magnetic Fe₃O₄/Zeolite NaA Nanocomposite for the Adsorption Removal of Methylene Blue Potential in Wastewater Treatment

2.3. Preparation of Magnetic Fe₃O₄ Nanoparticles

2.4. Synthesis of Fe₃O₄/ZA Nanocomposite

3.1. Microstructure of Magnetic Fe₃O₄/ZA Nanocomposite

3.2. Surface Morphology of Fe₃O₄/ZA Nanocomposite

3.5.1. Effect of Fe₃O₄ Loading on Fe₃O₄/ZA Nanocomposite on the Removal of MB