Coal fly ash/CoFe₂O₄ composites: a magnetic adsorbent for the removal of malachite green from aqueous solution

Abstract

A coal fly ash/CoFe₂O₄ (CFA/CFO) magnetic composite was synthesized by a facile hydrothermal synthesis method, and then used as an adsorbent for the removal of malachite green (MG) dye from water. The structure, morphology, and properties of the composite were characterized by XRF, SEM, XRD, FT-IR, BET and VSM techniques. A batch adsorption experiment on the removal of MG was performed with CFA/CFO, and the effect of various experimental parameters, such as the adsorbent dosage and contact time of MG at 25 °C, were investigated. Three isotherm models, namely the Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich (DKR) models, were applied to describe the adsorption process. The results indicated that the Freundlich and DKR models fit quite well with the experimental data. Thermal and microwave methods were used and compared in the regeneration experiment. All the results indicated that the as-prepared composite should be considered as a potential low-cost adsorbent for easy and efficient removal of MG from water.

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1. Introduction

Dyes and pigments, which are widely used in textiles, printing, dyestuff manufacturing, and food processing, have always been a major problem for wastewater treatment.¹ Disposal of synthetic dyes into waters without proper treatment gives rise to severe problems and concerns, which may affect human and marine life adversely due to the toxic nature of the dyes with suspected carcinogenic, neurotoxic, and mutagenic effects.^2,3 Moreover, the complex aromatic structures of the dyes give them complex composition, stable chemical properties, and high chemical oxygen consumption, making them much more difficult to treat.⁴

Malachite green (MG) is an organic compound that is classified as a triarylmethane dye used in the dyestuff industry and has been used traditionally as a dye for materials such as silk, leather, and paper as well as in distilleries.⁵ In addition, MG is also used as a fungicide and antibacterial in the aquaculture industry to control fish parasites and disease.⁶ However, MG has emerged as a controversial antimicrobial and has been banned for aquaculture use because it is highly cytotoxic to mammalian cells and acts as a tumor-enhancing substance, thereby posing significant risk to human health.⁷ Therefore, measures must be taken to treat effluents containing MG prior to their discharge into receiving waters to prevent environmental pollution in aquatic ecosystems.

Various treatment processes have been investigated widely to remove dyes from wastewaters, including physical separation, chemical oxidation, and biological degradation.^8–10 However, many traditional methods do not show significant effectiveness because most of the synthetic dyes exhibit anti-oxidation, anti-photodecomposition, and anti-biodegradation properties as well as other prohibitory characteristics.¹¹ Adsorption is considered an attractive alternative for treating such wastewaters due to its advantages of ease of operation, efficiency, and simplicity of the equipment used.¹² Many adsorbents have been reported to decolorize wastewater.^13,14 Activated carbon (AC), for example, offers an attractive option for the efficient removal of various organic contaminants from water due to its large specific surface area and highly developed pore structure.¹⁵ Powdered AC, in particular, is one of the most commonly used adsorbents in the effluent treatment field with the advantage of high adsorption speed. However, filtration, the traditional method for separating powdered AC, could cause the blockage of filters and lead to the secondary pollution.¹⁶ Furthermore, the relatively high cost and poor regeneration capability have prohibited its large-scale application.

Coal fly ash (CFA) is a highly investigated adsorbent that is inexpensive because it is produced in great amounts as an industrial solid waste from the electricity generation process in coal-fired power stations.^17,18 The disposal of CFA has become an increasing economic and environmental burden, as it is generally dumped in landfills.¹⁹ It has been utilized as a by-product in many applications including in cement and concrete products, as structural fill and cover material, in roadway and pavement construction, which decreases CFA disposal and replaces some scarce or expensive resources.^20,21 However, a large fraction of the produced CFA is still unutilized; therefore, research is needed to develop new alternative environmentally friendly applications that can further exploit CFA.

Another application for CFA is as a low-cost adsorbent for the removal of heavy metals,²² organics,²³ and dyes^24,25 from wastewater. Raw fly ash usually exhibits low adsorption capacity, thus modification of CFA with physical and chemical methods has been investigated to enhance its adsorption capacity for dye containing wastewaters. Wang et al.²⁶ studied the removal efficiency of methylene blue and rhodamine B from aqueous solution with CFA after hydrothermal treatment with NaOH. Gao et al.²⁷ investigated a combined treatment of CFA and Ca(OH)₂/Na₂FeO₄ for the removal of methyl orange. Banerjee et al.²⁸ examined the efficiency of CFA activated by HCl for adsorption of methylene blue from its aqueous solution. Cao et al.²⁹ evaluated the adsorption performance of CFA after modification by polydimethydiallylammonium chloride (PDMDAAC) surfactant for decoloring dyeing wastewater. These modified CFAs have been shown to be efficient adsorbents to remove dyes with enhanced adsorption efficiencies. However, applications of modified CFA for the removal of dyes in wastewater are still limited.

Compared with traditional filtration or centrifugation separation, magnetic particles such as Fe₃O₄ and ferrites have received considerable attention in recent years, because they can be separated easily from the medium after adsorption.^30–32 However, most of these pure magnetic materials are disadvantageous because of their low adsorption capacity, which limit their application to environmental problems. Thus, many types of magnetic composites have been synthesized and used as adsorbents for the removal of dyes. Jiang et al.³³ synthesized an AC/NiFe₂O₄ magnetic composite for the removal of methyl orange using a facile hydrothermal method. Ai et al.³⁴ prepared a composite of montmorillonite and CoFe₂O₄ (CFO) using a facile one-step low-temperature refluxing route and evaluated the adsorption performance of methylene blue on the composite. Yang et al.³⁵ successfully synthesized a magnetic Fe₃O₄/AC nanocomposite for the first time with a rice-husk-based AC as the adsorbent, for the removal of methylene blue.

CFO, as one of the most important spinel ferrites, has been widely used in various technological fields,^36–38 due to its moderate saturation magnetization, excellent chemical stability, and mechanical hardness.^39,40 However, little attention has been paid to CFO based magnetic adsorbents combined with CFA. In this work, a new kind of magnetic composite of CFA/CoFe₂O₄ (CFA/CFO) was synthesized via a hydrothermal method to remove MG dye in this work. The composite adsorbent synthesized, combining the advantages of CFA and magnetic materials, is considered an attractive alternative for treating dye wastewaters due to its advantages of ease of operation, low cost, high efficiency, and magnetic separation convenience. The effect of various experimental parameters, such as the adsorbent dosage and contact time of MG at 25 °C, were investigated. The adsorption isotherms and regeneration of the CFA/CFO composite are also discussed.

2. Experimental materials and methods

2.1. Materials and reagents

The CFAs used in this work, classified to F category according to the percentage of calcium oxide (CaO), were taken from a bituminous coal-burning power plant in Jinan, Shandong, China. As a rapidly developing clean combustion technology, circulating fluidized bed boiler is adopted for the combustion of coal in the plant. It makes no difference to the CFAs properties in the desulphurization method, because the CFAs were obtained from the electrostatic precipitator which is prior to the desulphurization by wet processes.

All chemicals and reagents used in the experiments and analyses were of analytical reagent grade and were used as received without further purification. Concentrated ammonium hydroxide (NH₃·H₂O), sodium chloride (NaCl), ferric nitrate nonahydrate [Fe(NO₃)₃·9H₂O], and cobalt nitrate hexahydrate [Co(NO₃)₂·6H₂O] were purchased from Shanghai Chemical Reagents Company, Shanghai, China. Malachite green (MG), with a molecular formula of C₂₃H₂₅ClN₂, was obtained from Zhiyuan Chemical Reagents Company, Tianjin, China, and used in this study as the target contaminant. The chemical structure of the studied MG molecule is shown in Fig. 1.

Fig. 1 Molecular structure of malachite green.

All solutions were prepared using 18 MΩ cm ultrapure Milli-Q (MQ) water and stored in the dark at 4 °C when not in use. All glassware and plasticware were soaked in 5% HCl for several days and rinsed thoroughly with MQ water before use. A background solution containing 0.1 mol L⁻¹ NaCl was prepared one week prior to the commencement of each experiment to ensure that equilibrium between the background solution and the atmosphere was reached. An accurately weighed quantity of MG was dissolved in MQ water to prepare a stock solution (1000 mg L⁻¹). Experimental solutions of the desired concentration were obtained by successive dilutions with MQ water.

2.2. Modification of fly ash

CFA was washed with sufficient distilled water to remove surface dust and the soluble inorganic materials that were present in the pores from the sedimentation process. The sample was oven-dried at 105 °C for 24 h before use.

The CFA/CFO composite was synthesized using the hydrothermal method. In a typical synthesis process, stoichiometric amounts of Fe(NO₃)₃·9H₂O (9 mmol) and Co(NO₃)₂·6H₂O (4.5 mmol) salts were dissolved in 30 mL MQ water, and then 3 g CFA was added. A few drops of NH₃·H₂O were added gently to the solutions to adjust their pH to 11, and then the solution was stirred for 30 min at room temperature. After this, the mixture was transferred into a sealed Teflon-lined stainless-steel autoclave vessel (50 mL capacity), which was heated at 180 °C for 18 h in an oven. After cooling to room temperature, the obtained brown product was filtrated and washed several times with MQ water and anhydrous ethanol. The remaining solid was then dried at 105 °C for 2 h. Finally, the dried substance was calcined in a muffle furnace for 2 h at 650 °C, and the as-prepared CFA/CFO composite was ready for use.

Porous CFO powder was prepared in the absence of CFA, while other conditions were kept constant. In addition, we prepared CFA without using metal nitrates while other conditions were kept constant, to compare its adsorption performance to that of the CFA/CFO composite and CFO powder.

2.3. Characterization of the adsorbents

To determine the chemical compositions of the products, their X-ray diffraction (XRD) patterns were measured on a D/MAX2500 VXRD diffractometer. The diffractometer was operated at 40 kV and 40 mA and scanned from 10° to 80° (2θ) at room temperature.

The surface morphology of the CFA, CFO, and CFA/CFO were characterized by a scanning electron microscope (SEM) (Supra 55 Zeiss).

The actual chemical composition of the initial and modified CFAs was determined by X-ray fluorescence (XRF) using an energy dispersive X-ray fluorescence analyzer (ARL QUANT'X, Thermo Scientific, USA).

The specific surface area (BET surface area), pore diameter, and pore volume of the samples were determined by N₂ adsorption under 77 K using an automated gas sorption analyzer (Autosorb-iQ, Quantachrome Co., USA). All samples were degassed at 150 °C prior to the adsorption experiments. The BET surface area was obtained by applying the BET equation to the adsorption data.

Magnetic measurements were performed with a cryogen-free vibrating sample (VSM) magnetometer operating using a closed-cycle He cryostat. The Physical Properties Measurement System (PPMS DynaCool, Quantum Design, USA) in VSM option was used with a sensitivity of 0.016 mT, operating magnetic field up to 9 T, and temperature range of 1.8–300 K.

2.4. Adsorption studies

2.4.1. Adsorption experiments. Batch adsorption studies were conducted to evaluate the effect of various parameters, such as kind of adsorbent (CFA, CFO, or CFA/CFO), adsorbent dose, and contact time, on the removal of MG at 25 °C. Typically, a certain amount of adsorbent sample (CFA, CFO, or CFA/CFO) was introduced into 150 mL of MG solution (25 mg L⁻¹) under mechanical stirring (500 rpm) at neutral pH (∼7) for various intervals (1–5 min). At the predetermined time interval, the adsorbent was removed from the dye solution after the reaction, and then the MG concentration in the solution was determined by measuring the absorbance of the solution using a double beam UV-Vis spectrophotometer (TU-1901, Persee, China). From full wavelength scanning, it was found that the absorption maxima in the UV-Vis spectra of MG solution occur at 617 nm. The concentrations of the solutions were determined by using the linear regression equation (y = −0.012 + 0.051x, R² = 0.999) obtained by plotting a calibration curve for the dye over a range of concentrations.

The MG removal efficiency (η, %) was determined as a percentage with eqn (1):

where C₀ (mg L⁻¹) is the initial MG concentration, and C_t (mg L⁻¹) is the residual MG concentration at time t (min).

2.4.2. MG adsorption capacity of CFA/CFO. Experiments were performed using a batch equilibrium technique by placing a certain amount of CFA/CFO adsorbent (0.60 g) in 250 mL Erlenmeyer flasks containing 150 mL of MG dye solution of various concentrations (25–200 mg L⁻¹) under constant stirring until equilibrium was achieved. The obtained data were plotted and adjusted with isotherm adsorption models to analyze the dye adsorption onto the adsorbents.

The amount of MG adsorbed per unit mass of adsorbent (q, mg g⁻¹) was calculated using eqn (2):

where C₀ and C_e are the initial and equilibrium concentrations of dye (mg L⁻¹), respectively, m (g) is the mass of the CFA/CFO, and V (L) is the initial volume of the solution.

2.5. Regeneration experiments

Regenerating and reusing exhausted adsorbents plays an important role in wastewater treatment. With this in mind, many methods have been investigated to regenerate dye or heavy metal ions adsorbed to adsorbents, such as acid, alkali, and organic solvents. In this study, regeneration experiments of the CFA/CFO were carried out using two different methods: microwave and thermal treatment. Thermal treatment is presently the main regeneration method for some solid adsorbents, but it needs high temperature, and its weight loss and burned-loss are very high. Microwave regeneration is a new method for microwave-absorbing materials that possesses such advantages as high regeneration efficiency, low energy consumption, low regeneration cost, and simple operation.

The CFA/CFO composite, saturated by MG dye solution that had been added to 150 mL of 500 mg L⁻¹ MG solution with a stirring speed of 250 rpm for 60 min, was separated from the aqueous solution and dried in an oven at 105 °C before the regeneration process. The as-obtained adsorbent was treated either with 900 W microwave irradiation for 30 min or at 500 °C in the muffle furnace for 60 min. It was then added to 25 mg L⁻¹ MG again, to reassess the adsorption capacity of the CFA/CFO adsorbent at an adsorbent dosage of 4 g L⁻¹.

3. Results and discussion

3.1. Characterization of the adsorbents

3.1.1. XRF studies. The chemical composition analysis of the samples by XRF is shown in Table 1. The major components of the samples were oxides of Si, Al, and Fe, with various other oxides occupying the remainder of the samples. A portion of cobalt oxide and iron oxide increased from 0.09% and 8.99% in the CFA powder to 10.21% and 26.66% in the CFA/CFO composite, decreasing the portion of all the other oxides after modification of the CFA. Although XRF could not be used to determine the presence of oxygen due to the low fluorescence quantum yield of light elements, this is not important because oxides make up a large majority of the products.

Table 1 Chemical composition (% by weight) of the coal fly ash (CFA) and CFA/CoFe₂O₄ (CFA/CFO) samples

Samples	SiO2	Al2O3	Fe2O3	Co3O4	CaO	K2O	TiO2	SO3	ZrO2	ZnO
CFA	60.6	18.77	8.99	0.09	4.03	3.27	1.91	1.68	0.103	0.088
CFA/CFO	43.03	13.56	26.66	10.21	2.84	1.17	0.94	1.27	0.069	0.042

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3.1.2. SEM studies. SEM was used to investigate the morphology of the samples. In Fig. 2(a), a majority of the CFO particles are irregular spheres with a relatively homogeneous crystal-size distribution of approximately 30 nm. The image in Fig. 2(b) shows the characteristic morphology of the original CFA, which consisted of spherical vitreous particles of different sizes with relatively smooth surfaces. As for the hydrothermal-treated CFA, the surfaces of the spherical dust granules become more rugged, compared with the original CFA, as a result of hydrothermal crystallization and elimination of volatile materials during the thermal treatment. A significant change in the physical structure of the CFA occurred after the modification of the CFA, as shown in Fig. 2(d). When CoFe₂O₄ was loaded on the CFA, the CFA/CFO composite exhibited a dispersed foliated structure. A majority of the large spherical surfaces disappeared, replaced by many flaky clusters and numerous undefined irregular shapes, which makes the CFA/CFO exhibit a higher surface area and pore volume, as confirmed by the BET data of this study.

Fig. 2 SEM images of CoFe₂O₄ (CFO) (a), original coal fly ash (CFA) (b), hydrothermal-treated CFA (c), and CFA/CFO (d).

3.1.3. XRD studies. XRD measurements were employed to investigate the phase and structure of the synthesized samples and the XRD patterns are shown in Fig. 3. Based on the obtained results, it could be observed that the original CFA is a fully anamorphic material, as evidenced by the lack of clear reflections on the diffractogram in Fig. 3(a). A broad peak appearing at 2θ = 22.3° indicated the existence of silicon dioxide in the original CFA. Compared with the original CFA, two diffraction peaks at 2θ = 25.5° and 35.5° can be observed more clearly in Fig. 3(b) for the CFA after hydrothermal-treatment, which may be related to the presence of alumina and ferric oxide. In Fig. 3(c), it was found that CFO has high crystallinity, and the diffraction peaks can be indexed perfectly to the cubic spinel structure (JCPDS card no. 22-1086). In addition, there is a characteristic peak of impurities appearing at 2θ = 33.2°, corresponding to Fe₂O₃, that is detected in the XRD pattern, implying that the formation of the spinel is not single phase. As shown in Fig. 3(d), the weak diffraction peaks at 2θ = 30.1°, 35.4°, 43.0°, 56.9°, and 62.5° correspond to the (220), (311), (400), (511), and (440) crystal planes of CFO (JCPDS no. 22-1086) suggesting the formation of CFO nanocrystals in the obtained CFA/CFO composite. Furthermore, the observed diffraction peaks are broad and less sharp, indicating the as-prepared CFA/CFO particles with small dimensions.⁴¹

Fig. 3 X-ray diffraction patterns of original coal fly ash (CFA) (a), hydrothermal-treated CFA (b), CoFe₂O₄ (CFO) (c), and CFA/CFO (d).

3.1.4. FTIR studies. To get supplementary information on the interactions/bonds present on the new substrate, FTIR spectra analysis of the samples was conducted (Fig. 4) to identify the functional groups present.

Fig. 4 FTIR spectra of original coal fly ash (CFA) (a), hydrothermal-treated CFA (b), CoFe₂O₄ (CFO) (c), and CFA/CFO (d).

It is noted that almost no difference between the original and hydrothermal-treated CFA is found. The peaks at 1080 and 787 cm⁻¹ are related to the antisymmetric and symmetric Si–O–Si stretching vibrations, respectively, while 470 cm⁻¹ is assigned to O–Si–O bending vibration.⁴² The presence of these adsorption bands confirms the existence of the amorphous SiO₂, which is the main constituent of the CFA. The FTIR spectra of the samples showed two peaks characteristic of ferrites being positioned at approximately 455 and 590 cm⁻¹, as shown in Fig. 4(c). The absorption band at 590 cm⁻¹ was attributed to the stretching vibrations of the metal ion at the tetrahedral A-site and that at 455 cm⁻¹ to the octahedral group complexes.^43,44 An intense peak at 455 cm⁻¹ was also observed for the CFA/CFO (Fig. 4(d)), which further confirms the existence of CFO; whereas the peak at 590 cm⁻¹ becomes weak, which may result from the superposition of the samples. Moreover, the water molecules associated with the cations and hydrogen bonded to the oxygen ions of the framework explains the peak recorded at 1636 cm⁻¹, characteristic of the bended mode of water molecules.⁴⁵ The results indicated that the CFO was well deposited onto the CFA surface.

3.1.5. N₂ adsorption studies. Fig. 5 shows the N₂ adsorption–desorption isotherms for the CFA, CFO and CFA/CFO composite. The samples exhibit type IV isotherms according to the International Union of Pure and Applied Chemistry (IUPAC) classification of adsorption isotherms, which are typical of mesoporous structures. An H3-type hysteresis of the isotherms, especially for the CFA/CFO, is seen at the relative pressure (P/P₀) between 0.6 and 1.0, which is typical for aggregates of plate-like particles giving rise to slit-shaped pores.⁴⁶ This kind of hysteresis is typical for the presence of open large pores, which allows an easy diffusion of the reactants through the materials. The presence of adsorption hysteresis, due to a different behavior in adsorption and desorption, was determined by the pore shape and volume.

Fig. 5 The N₂ adsorption–desorption isotherms of coal fly ash (CFA) (a), CoFe₂O₄ (CFO) (b), and the CFA/CFO composite (c).

In Table 2, an increase is observed when it comes to the average pore width and pore volume, ranging from 4.52 nm and 0.036 cm³ g⁻¹ for the CFA to 15.60 nm and 0.360 cm³ g⁻¹ for the CFA/CFO composite, respectively. The CFA/CFO composite has a much greater surface area (92.609 m² g⁻¹) than that of the CFA (11.648 m² g⁻¹), attributing to the formation of plate-like particles after the modification of the CFA. In general, for adsorbent, a large surface area can offer more active adsorption sites. Thus, the relatively higher surface area of the CFA/CFO is considered to be favorable for the adsorption capacity of MG.

Table 2 Textural parameters of coal fly ash (CFA), CoFe₂O₄ (CFO), and the CFA/CFO composite obtained from the N₂ adsorption isotherms

Samples	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore width (nm)
CFA	11.648	0.036	4.52
CFO	11.055	0.058	27.04
CFA/CFO	92.609	0.360	15.60

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3.1.6. Magnetism studies. The magnetization measurements for the as-prepared CFA/CFO were carried out using a vibrating sample magnetometer (VSM) at room temperature with an applied magnetic field of 20 kOe and the plot of magnetization (M) versus magnetic field (H) and its enlargement near the origin are depicted in Fig. 6(a). From the magnetization curves, the very weak hysteresis revealed the resultant magnetic particles were nearly superparamagnetic with a saturation magnetization (M_s) of 3.14 emu g⁻¹ at room temperature. The results confirm that most of the magnetic particles remain nanoscale in the composite CFA/CFO.

Fig. 6 The magnetization curves for CFA/CFO at 300 K (the inset figure shows the zoom of the curve with data near to the zero) (a), and the magnetic separation of the absorbent from the well-dispersed solution under an external magnetic field (b).

As displayed in Fig. 6(b), the magnetic separation performance of the CFA/CFO composite was tested by placing a conventional laboratory magnet near the glass bottle after dispersion of CFA/CFO in aqueous solution. The synthetic product was attracted toward the magnet in a few seconds, demonstrating its high magnetic sensitivity. There was a significant change in color of the aqueous solution from black to colorless which indicated that the solution could be easily decanted to remove the composite.

These results showed that CFA/CFO possessed magnetism and could be potentially used as a magnetic adsorbent to remove dye contaminants from aqueous solution. It also proved an easy and efficient way of separating the adsorbent after adsorption experiments, which is an attractive alternative to filtration or centrifugation.

3.2. Adsorption studies

3.2.1. Effect of different kinds of adsorbents. The effect of different kinds of adsorbents (CFA, CFO, or CFA/CFO) on the removal of MG is shown in Fig. 7. It is evident that the MG removal efficiency of CFA/CFO is much higher than that of CFA or CFO. The adsorption enhancement in the removal of dyes could be attributed to the specific surface area improvement of the CFA/CFO after the modification of the CFA as is mentioned above, which resulted in an increased diffusion path length and a promotion for the interaction of the MG onto the surface of CFA.

Fig. 7 The removal efficiencies of coal fly ash (CFA) (a), CoFe₂O₄ (CFO) (b), and CFA/CFO (c) for the removal of malachite green (25 mg L⁻¹), at 25 °C, pH = 7, agitation rate of 500 rpm, and the adsorbent dosage of 4 g L⁻¹.

3.2.2. Effect of adsorbent dosage. Optimum dosage of adsorbent is one of the key parameters for an economic and efficient dye adsorption process. For this study, a weighed amount of a different mass (0.03–0.75 g) of adsorbent was added in the 250 mL Erlenmeyer flasks containing 150 mL of dye solution (25 mg L⁻¹) under mechanical stirring (500 rpm) at neutral pH. The effect of CFA/CFO dosage on the removal of MG is shown in Fig. 8. Dye removal efficiency increases from 33 to 93%, with an increase in the dosage of CFA/CFO from 0.2 to 4 g L⁻¹, while the adsorption amount per unit mass of adsorbent decreases rapidly with increasing the adsorbent dosage. The results were due to an increase in the available adsorption surface area and adsorption active sites.⁴⁷ However, the increase was not significant for the removal efficiency at the adsorbent dosage of 4 g L⁻¹ because most of the MG molecules in the aqueous solution were adsorbed. It was observed that increasing adsorbent dose than 4 g L⁻¹ produces ineffective increase in adsorption, and hence, 4 g L⁻¹ could be considered as the optimal dosage.

Fig. 8 The effect of the coal fly ash/CoFe₂O₄ dosage on the adsorption amount per unit mass of adsorbent (a) and removal efficiency (b) for the removal of malachite green (25 mg L⁻¹), at 25 °C, pH = 7, and agitation rate of 500 rpm.

3.2.3. Effect of contact time. Evaluation of the effect of contact time is essential because it provides vital information on how fast the adsorption process reaches equilibrium. How contact time carried out at different adsorbent dosages for the adsorption of MG dye affected adsorption is also shown in Fig. 9. The MG removal efficiency increases dramatically as the contact time increases within the first minute of contact time, suggesting rapid external diffusion and surface adsorption which is due to a number of free active sites present on the surface of the adsorbent. The color removal of MG by CFA/CFO was rapid at first. Then, the adsorption of MG slowed down and finally reached equilibrium after a contact time of 5 min, which can be ascribed to saturation of the active sites on the adsorbent by dye molecules. Based on these results, 5 min was used as the equilibrium time in the kinetic adsorption experiments to ensure complete equilibrium in the adsorption process.

Fig. 9 The effect of contact time on the removal of malachite green (25 mg L⁻¹), with the coal fly ash/CoFe₂O₄ dosages of 0.2 g L⁻¹ (a), 1 g L⁻¹ (b), 2 g L⁻¹ (c), and 4 g L⁻¹ (d) at 25 °C, pH = 7, and agitation rate of 500 rpm.

3.2.4. Adsorption isotherms. The adsorption isotherm was evaluated using the three sorption isotherm models from Langmuir,⁴⁸ Freundlich,⁴⁹ and Dubinin–Kaganer–Radushkevich (DKR)⁵⁰ to analyze the adsorption process of MG onto CFA/CFO.

The Langmuir isotherm has been applied successfully for different sorption processes and is used often to explain the sorption of dyes onto adsorbents. The Langmuir theory is based on the assumptions that the adsorption is a monolayer and takes place at specific homogeneous sites on the adsorbent. The Langmuir equation is shown in eqn (3):

where q_e (mg g⁻¹) is the amount of adsorbed dye on the adsorbent at the adsorption equilibrium time, C_e (mg L⁻¹) is the equilibrium concentration of the dye solution, q_m (mg g⁻¹) is the maximum adsorption capacity of the adsorbent, and K_L (L mg⁻¹) is the Langmuir isotherm constant.

The linear form of the Langmuir equation is shown in eqn (4):

The Langmuir constants, K_L and q_m, can be obtained by the linear plot of C_e versus C_e/q_e.

A dimensionless constant called the equilibrium parameter, R_L, is used to express the essential characteristics of the Langmuir isotherm; R_L is determined by eqn (5):

where C₀ (mg L⁻¹) is the initial dye concentration, and R_L indicates whether the type of adsorption is unfavorable (R_L > 1), linear (R_L = 1), favorable (0 < R_L < 1), or irreversible (R_L = 0).⁵¹

The Freundlich equation assumes a heterogeneous surface with a non-uniform distribution of adsorption heat over the surface. The Freundlich equation is expressed by eqn (6):

where K_F and n are the Freundlich constants indicating adsorption capacity and intensity, respectively.

The linear form of the Freundlich equation is shown in eqn (7):

The value of 1/n is indicative of isotherm type in that 1/n = 0, 0 < 1/n < 1, and 1/n > 1 are related to irreversible, favorable, and unfavorable isotherms, respectively.⁵² Then, K_F and 1/n can be determined from the intercept and slope of the linear plot between ln C_e and ln q_e.

The DKR isotherm is more general than the Langmuir or Freundlich isotherms because it does not assume a homogeneous surface or constant adsorption potential. The DKR isotherm model, which is based on the Polanyi theory, was applied to estimate the type of dye adsorption onto the adsorbent. The DKR equation is shown in eqn (8):

where q_m(DKR) (mg g⁻¹) is the theoretical saturation capacity of the DKR equation, β (mol² J⁻²) is a constant related to the sorption energy, and ε is the Polanyi potential, which is related to the equilibrium concentration through eqn (9):where T is the temperature (K), R is the gas constant (kJ mol⁻¹ K⁻¹), and C_e is the equilibrium concentration of the dye in solution.

The DKR equation can also be rearranged to a linear form, as shown in eqn (10):

ln q_e = ln q_m(DKR) − βε² (10)

When ln q_e was plotted against ε², the value of q_m(DKR) and β were obtained. The sorption energy, E (kJ mol⁻¹), which is related to the β value, is shown in eqn (11):

The magnitude of E could be related to the reaction mechanism, governed by ion-exchange (8 kJ mol⁻¹ < E < 16 kJ mol⁻¹) or physical forces (E < 8 kJ mol⁻¹) to estimate the type of adsorption.⁵³

Comparisons were made to choose the isotherm model that best describes the experimental data. The isotherm parameters and linear regressions of the three isotherm models are shown in Table 3 and Fig. 10, respectively.

Table 3 Adsorption isotherm constants for the adsorption of malachite green onto coal fly ash/CoFe₂O₄

Langmuir		Freundlich		Dubinin–Kaganer–Radushkevich
qm (mg g^−1)	90.9	KF	4.16	qm(DKR) (mg g^−1)	89.3
KL (L mg^−1)	0.3793	n	1.34	β  (mol^2 kJ^−2)	10.68
RL	0.095			E (kJ mol^−1)	0.22
R^2	0.879	R^2	0.998	R^2	0.992

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Fig. 10 Langmuir (a), Freundlich (b), and Dubinin–Kaganer–Radushkevich (c) adsorption isotherms for the adsorption of malachite green onto coal fly ash/CoFe₂O₄.

The R² values show that the Freundlich model, with R² = 0.998, has a higher value than that of the Langmuir model, which suggests that the Freundlich model yielded a better fit to the experimental data. The value of n = 1.34 indicates that the adsorption of MG onto CFA/CFO is a favorable process.

The obtained data also fit quite well to the DKR isotherm model with R² = 0.992. In the DKR model, the value of E was less than 8 kJ mol⁻¹ in this study, suggesting that the MG adsorption onto the CFA/CFO was dominated by physical adsorption and a predominance of van der Waals forces.

Comparison of the adsorption capacities using CFA/CFO as adsorbent with that of the other previously reported adsorbents has been done and presented in Table 4. It shows that the obtained CFA/CFO possesses higher adsorption capacity for MG than that of other reported adsorbents, indicating that the as-prepared CFA/CFO has great potential for application in MG dye removal from water. It can be concluded that the CFA/CFO is a good adsorbent for the removal of MG dye from aqueous solution.

Table 4 Comparison of the adsorption capacities of MG onto various adsorbents

Adsorbents	q (mg g^−1)	T (K)	pH	References
CFA/CFO	89.3	298	7	This study
Coconut coir activated carbon	27.4	323	7	54
Iron humate	19.2	295	6.5	55
Bagasse fly ash	170.3	303	7	56
AC/CFO	89.3	303	5	57
Mango seed husks	47.9	303	5	58
Lignite activated carbon	149.0	298	Natural	59
Rattan sawdust	62.7	303	10	60

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3.3. Regeneration

The removal efficiency of regenerated CFA/CFO for MG was calculated, to assess the practical utility of the CFA/CFO composite. The results are shown in Fig. 11. The thermal treatment resulted in slightly higher removal efficiency than the microwave irradiation for the MG-adsorbed CFA/CFO composite, with efficiencies of 92% and 90%, respectively. Microwave and thermal regeneration displayed an overall decrease in removal efficiency as observed compared to the original CFA/CFO composite, with a removal efficiency of 93%. The CFA/CFO composite adsorbent kept its adsorption efficiency after regeneration with negligible changes, indicating that there are almost no irreversible sites on the surface of the CFA/CFO nanocomposite. Although the thermal method proved to have a high regeneration capacity, microwave regeneration is a new method that has demonstrated good regeneration efficiency for microwave absorbing materials with low energy consumption, low regeneration cost, and simple operation. These results indicated that CFA/CFO could potentially be used as an efficient adsorbent in wastewater treatment to avoid secondary pollution.

Fig. 11 Removal efficiency of the thermal (a) and microwave (b) regeneration methods.

4. Conclusions

In this study, a CFA/CFO magnetic composite was successfully synthesized via a facile hydrothermal synthesis method. MG was selected as a model of various dyes to investigate the adsorption capacity of the CFA/CFO composite. XRF, XRD, SEM, FTIR, BET surface area, and VSM were used to characterize the as-prepared composite. A significant increase in the surface area of the CFA was caused by the modification of the cobalt ferrite, which resulted in a higher adsorption capacity of MG onto the CFA/CFO composite. The magnetic properties of CFA/CFO indicated it could be separated and retrieved easily by an outer magnet after adsorption. The adsorption of MG onto the CFA/CFO fits quite well to the Freundlich and DKR isotherm models. Regeneration experiments showed that CFA/CFO composite has the potential to be regenerated and reused by both of the thermal and microwave methods. These results indicated that the as-prepared composite could be considered as a potential low-cost adsorbent for easy and efficient removal of MG from water.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21307075).

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Footnote

† These authors contributed equally to this work.

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