Synthesis of a novel magnetic nano-zeolite and its application as an efficient heavy metal adsorbent

A novel magnetic nano-zeolite (MNZ@MS) is successfully synthesized, characterized and applied to adsorp heavy metals from solution. In the synthesis of MNZ@MS, the fly ash magnetic sphere (MS) and [C18H37 (CH3)2-N+-(CH2)3 -N+-(CH3)2C18H37] Cl2 is used as carrier and directing agent, respectively. The characterization results of XRD, XRF, XPS and SEM demonstrate that the nano-scale (200–600 nm) Linde F(K) zeolite completely wraps the magnetic spherical fly ash particle, and the saturation magnetization value of MNZ@MS is around 17.7 emu g−1. MNZ@MS exhibits a favorable and efficient adsorption performances on heavy metals, and the maximum adsorption capacity of Cu, Cd and Pb on MNZ@MS is 59.9 mg g−1, 188 .6 mg g−1 and 909.1 mg g−1, respectively. The higher pH value in solution is more conducive to the adsorption process of heavy metals on MNZ@MS. The adsorption is a fast process, well represented by the pseudo-second-order model. Concerning the equilibrium behavior, Langmuir isotherm model are more suitable for describing the adsorption. Furthermore, in competitive adsorption system, the adsorption process of Pb is the most difficult to be interfered, and the order of adsorption advantage is Pb>Cu>Cd. MNZ@MS may be applied as a low-cost and efficient magnetic adsorbent for wastewater treatment to remove heavy metals.


Introduction
With the development of chemical industries, a large amount of pollutants are discharged into the environment, and governments pay more attention to problems of various pollution, including water pollution. Heavy metals are common water pollutants which are harmful to environment and human health [1][2][3][4][5]. Wastewater containing heavy metal mainly comes from some modern industries such as car manufacturing, battery production, metal plating, mining and tannery industries [6,7]. Through transformation and accumulation of food chain, heavy metal elements contained in wastewater will eventually be transferred to human body, thus causing harm to human health [8,9]. The long-term intake of heavy metals are detrimental to human body including bladder, brain, liver, kidney damage and other various neurodegenerative diseases [10,11]. According to China's sewage discharge standards (GB8978/1996), the concentration of Cu, Cd and Pb in effluent must be lower than 0.5 mg l −1 , 0.1 mg l −1 and 1.0 mg l −1 , respectively [12]. Therefore, it is necessary to remove heavy metals from wastewater before discharge.
Nowadays, main methods for removing heavy metals from wastewater are as follows: chemical precipitation, ion exchange, membrane filtration, electrolysis, adsorption and etc [13][14][15]. Among these methods, the adsorption has become one of hot research methods due to its relatively cheap and simple operation. In addition, the adsorbent is the core of adsorption method [16][17][18]. Zeolite material is a general term for a class of alumino-silicate crystalline materials. Many studies have shown that zeolite materials obtain better adsorption effect on heavy metals [19]. For example, the needle-like nano-crystalline zeolites was applied for Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. efficient removal of Cu and Pb from solutions [20]. The modified synthetic clinoptilolite was used for the removal of Zn, Pb, Cd and Cu from aqueous solutions, and the maximum adsorption capacity of modified synthetic clinoptilolite for heavy metals was higher than that of unmodified natural clinoptilolite [21]. The Linde F (K) zeolite was synthesized and its efficient adsorption for heavy metals was proved [22][23][24]. Moreover, the adsorption mechanism of Cu, Ni and Cd on zeolite was studied [25].
In recent years, to improve the adsorption efficiency of zeolite as much as possible, more attention has been paid to the synthesis of nano-zeolite materials. For instance, different diquaternary ammonium-type surfactants were utilized as structure directing surfactants to the synthesis of nano-zeolites [26][27][28]; An ultrasound assisted hydrothermal method that could convert fly ash to nanozeolite X was developed, and the products was used for heavy metals adsorption [29]. Furthermore, the zeolite nanoparticles could also be synthesized through a hydrothermal method [30].
On the other hand, the decreasing size of zeolite materials makes it difficult to be separated from solutions, which leads to nano-zeolite material could not be widely used for wastewater treatment. In order to overcome these difficulties, many scholars have studied ways to make magnetic zeolite materials. For example, The silicacoated magnetic nanocomposites were synthesized and applied for Pb 2+ removal from aqueous solution [31]; the magnetic zeolite NaA was prepared from loading Fe 3 O 4 and applies for adsorption of Cu and Pb from solutions [32]. Then, through in situ co-precipitation method, zeolite/Fe 3 O 4 magnetic nanocomposit was synthesized [33,34]. In above studies, Fe 3 O 4 is used as magnetic source, which increases the cost of magnetic zeolite materials. Thus, an economical and effective magnetic zeolite material needs to be developed.
Fly ash is a kind of solid waste produced by coal combustion power plants. The main component of fly ash is glass spheres mainly composed of silica-alumina oxides. The glass spheres with high iron content (magnetic spheres) often have relatively strong magnetism, which is proved in related literatures [35][36][37][38]. For example, a novel magnetic fly ash/acrylic acid composite microgel was prepared and utilized for selective adsorption of Pb [39]. However, studies on the preparation of magnetic adsorbents using fly ash magnetic spheres as carriers are very limited at present.
Hence, based on previous research, this paper focuses on the following points: (1) A nano-zeolite adsorbent with obvious magnetic properties (MNZ@MS) is synthesized using fly ash magnetic spheres (MS) as carrier and The adsorption behaviors of heavy metals in competitive adsorption system are also carried out. The aims of this paper are to provide an efficient and inexpensive magnetic treatment material for heavy metal wastewater and promote the comprehensive utilization of fly ash.

Adsorbent synthesis
The fly ash magnetic spheres used in this experiment were purchased from a company in Kunming, Yunnan Province, China. The samples of fly ash from power plants were treated by magnetic separation method in the company. Potassium silicate (Analytically pure) and potassium hydroxide (Analytically pure) used in the experiment are purchased from China national pharmaceutical chemical reagent Co. Ltd. Partial potassium aluminate (Chemical purity) is purchased from Shandong Luke chemical industry Co. Ltd, China.
[C 18 H 37 (CH 3 ) 2 -N + -(CH 2 ) 3 -N + -(CH 3 ) 2 C 18 H 37 ]Cl 2 (98%) is purchased from Henan Daochun Chemical Technology Co., Ltd, China. The synthesis system of MNZ@MS is as follow: 6.1087 g potassium silicate, 2.744 g partial potassium aluminate and 0.1819 g directing agent are added into a Teflon bottle which contains potassium hydroxide (240 ml, 10 mol·l −1 ) solution. A water bath reaction device installed with an electric blender keep the reaction temperature around 75°C. During the synthesis process, magnetic stirrer maintains stirring the reaction system, and the reaction time is 10 h. When the reaction lasts for 5 h, 3 grams of MS is added to the synthetic system. Then, the reaction continues for another 5 h. The electric blender keeps reaction mixtures in suspension. After the synthesis reaction is finished, deionized water is used to wash the reaction products until the pH value of filter liquor is around 7. Ultimately, the reaction product is dried under 105°C and the final product is MNZ@MS.

Adsorption experiments
The Copper nitrate (Analytically pure), Cadmium nitrate (Analytically pure), Lead nitrate (Analytically pure), sodium hydroxide (Analytically pure) and hydrochloric acid (37.5%) used in the experiment are purchased from China national pharmaceutical chemical reagent Co. Ltd. The adsorption experiments are conducted as typical batch trials in single solute systems. In each adsorption trial, a quantity of MNZ@MS is dispersed in 10 ml heavy metal solution in a 20-mL Teflon bottle. The bottles are subsequently immersed in a water bath and agitated at 200 rpm. Following the adsorption, each dispersion is filtered through a 0.45 μm membrane and the concentration of heavy metals in the filtered solution are determined by an Atomic Absorption Spectrometer. For studying the influence of solution pH, the initial concentration of Cu, Cd and Pb is 60 mg l −1 , 18 0 mg l −1 and 500 mg l −1 , respectively. The different initial concentration for Cu, Cd and Pb is determined by exploratory experiment for evaluating the adsorption capacity of MNZ@MS. The solution pH ranges from 3 to 9. The adsorption dosage of MNZ@MS is 1 g l −1 , the adsorption temperature is 25°C, and the adsorption time is 60 min. For investigating the influence of adsorption time and adsorption kinetics, the initial concentration of Cu, Cd and Pb is also 60 mg l −1 , 18 0 mg l −1 and 500 mg l −1 , respectively. The dosage of MNZ@MS is 1 g l −1 , the adsorption temperature is 25°C, and the adsorption time ranges from 20 min to 480 min. For studying the influence of initial concentration and adsorption isotherm, the initial concentration of Cu and Cd ranges from 20 mg l −1 to 280 mg l −1 , and the initial concentration of Pb ranges from 100 mg l −1 to 600 mg l −1 . The dosage of MNZ@MS is 1 g l −1 , the adsorption temperature is 25°C, and the adsorption time is 60 min. For studying the interactions between heavy metals, adsorption experiments are conducted as typical batch trials in single and competitive solution systems. The initial concentration of three heavy metals ranges from 20 mg l −1 to 280 mg l −1 , the dosage of MNZ@MS is 1 g l −1 , the adsorption temperature is 25°C, and the adsorption time is 60 min.
The calculation of the mass of heavy metals adsorbed per unit mass of MNZ@MS at time t and the equilibrium are as equations (1) and (2).
Where, V(ml) is the solution volume, m (g) is the mass of the adsorbent, Q t (mg g −1 ) is the mass of heavy metals adsorbed per unit mass of zeolite at time t, Q e (mg g −1 ) is the mass of heavy metals adsorbed per unit mass of zeolite at adsorption equilibrium, C 0 (mg l −1 ) is the initial concentration of heavy metals, C t (mg l −1 ) is the concentration of heavy metals at time t, C e (mg l −1 ) is the concentration of heavy metals at adsorption equilibrium.

Materials characterization
The MS and MNZ@MS are characterized by x-ray diffraction (XRD), X-ray fluorescence, Scanning electron microscopy-energy dispersive spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, Microiontophoresis apparatus and x-ray photoelectron spectroscopy, and liquid specific surface area measuring instrument.
X-ray fluorescence is analyzed using ARL-9800 X-ray fluorescence analyzer from Swiss ARL. The XRD patterns of powder samples are acquired using a Shimadzu XD-3A diffractometer, employing Cu-Kα radiation (λ=1.540 56 Å). The morphologies of the composites are observed with a HITACHI(S-3400 N) scanning electron microscope, and FT-IR spectra are recorded on a Nicolet iS5 FT-IR spectrometer using pressed KBr discs. The surface zeta potential is determined by JS94H microiontophoresis apparatus (POWEREACH). The x-ray photoelectron spectroscopy spectra are recorded on a PHI 5000 VersaProbe XPS equipment. The liquid specific surface area is determined by Xigo liquid specific surface area measuring instrument.

Results and discussions
3.1. Liquid specific surface area The liquid specific surface area results of MS (a) and MNZ@MS was deducted based on the fitting results of computer, which is shown in figure 1. The liquid specific surface area of MS and MNZ@MS is 699.4 m 2 g −1 and 2123.6 m 2 g −1 , respectively. The liquid specific surface area of MNZ@MS is larger than that of pure nano zeolite [40]. This demonstrates that the liquid specific surface area of composite material can be increased through coating nano zeolite on MS.

SEM
The micromorphology and micro-composition of MNZ@MS and MS are characterized by SEM and EDX. Figure 3 shows the SEM images and EDS results of MS. The spherical particles with abundant micro crystal on the surface can be observed in the figures 3(a) and (b). The detail of abundant micro crystal could be seen with large magnification (figures 3(c) and (d)). The micromorphology of MS is typical magnetic iron-rich fly ash sphere, and the crystal on its surface might be the magnetite [35,36]. The results of EDS suggest that, the main compositions (oxide form) of crystal are as follow: Fe 2 O 3 (70.68%), SiO 2 (21.71%), Al 2 O 3 (6.44%), which are basically in accord with our speculation. Figure 4 shows the SEM images and EDS results of MNZ@MS. Contrast with figure 3, the spherical particle is changed to spheroid shape (figures 4(a) and (b)) and the surface is completely encapsulated by reaction products (figures 4(c) and (d)). The details of reaction products could be observed in figures 4(c) and (d), which dipicts the microstructures of nano-zeolite. The nano-zeolite has tetragonal crystal structure and the size ranges from 200 nm to 600 nm. The results indicate that the addition of [C 18 H 37 (CH 3 ) 2 -N + -(CH 2 ) 3 -N + -(CH 3 ) 2 C 18 H 37 ] Cl 2 effectively reduces the size of zeolite crystals [26][27][28]. Moreover, the results of EDS indicates the main compositions (oxide form) of nano-zeolite are as follow: K 2 O (22.41%), SiO 2 (35.2 7%), Al 2 O 3 (39.77%), which are corresponded to Linde F(K) zeolite/KAlSiO 4 .1.5H 2 O. Therefore, the nano-zeolite is successfully coated on the surface of MS through synthetic reaction.   Figure 6 shows the magnetic properties of MNZ@MS and MS. The saturation magnetization value (SMV) of MS and MNZ@MS is about 27.5 emu g −1 and 17.7 emu g −1 , respectively. Such magnetism is comparable to many magnetic materials [44][45][46][47][48]. The coercivities of MS and MNZ@MS are around −70.1 Oe and −60.2 Oe, respectively, which indicates that the magnetic sphere has weak residual magnetism. In addition, the SMV and coercivity subtraction of MNZ@MS should be attributed to the coating of some non-magnetic zeolite on the surface of MS. Figure 7 shows the magnetic separation performance of MNZ@MS. In the absence of environmental magnetic force, MNZ@MS can be stably suspended in the solution. However, after adding external magnet, the MNZ@MS could be well separated from solution, which proves the excellent magnetic ability of MNZ@MS.

XPS
The surface properties of MNZ@MS and MS are analyzed by x-ray photoelectron spectroscopy (XPS), and the results are shown in figures 8 and 9. Survey scan of MS depicts two strong and sharp peaks at 284 eV and 531 eV, which could be attributed to C 1s and O 1s, respectively. Four low peaks are found at 73 eV, 100 eV, 118 eVand 149 eV, which are corresponded to Al 2p, Si 2p, Al 2s and Si 2s. Figure 8

Solution pH
The solution pH is one of important factors affecting the adsorption process. Figure 10 shows the effect of solution pH on the adsorption capacity of Cu, Pb and Cd on MNZ@MS. From figure 10(a), the adsorption capacity of three heavy metals is obviously improved with the increase of solution pH. This phenomenon is often reported by other literatures related to heavy metals adsorption process [49][50][51][52][53][54]. In addition, the maximum adsorption capacity of Cu, Pb and Cd reaches around 60 mg g −1 , 18 0 mg g −1 and 500 mg g −1 , respectively. This experimental results could be explained as follows: a. The measurement results of Zeta potential of MNZ@MS. Figure 10(b) shows the Zeta potential of MNZ@MS. It could be observed that the Zeta potential of MNZ@MS continuous decreases with the increase of solution pH, and the point of zero charge (PZC) of MNZ@MS is about 3.3. Because of the positive charge of hydrated heavy metal ions, the adsorption process would become difficult when solution pH is lower than 3.3. However, when solution pH is higher than 3.3, the continuously decreasing Zeta potential would accelerate the adsorption process. b. The competitive effect of H + in solution. When solution pH is lower than 7, the competitive adsorption process would happen between H + and heavy metals, resulting in the decrease of adsorption capacity. c. The precipitation reaction in the system. As we know, heavy  metals reacts with OH − and forms hydroxide precipitation at high solution pH, which might enhance the apparent adsorption efficiency.

Adsorption time and kinetics
The kinetics equations used to investigate the adsorption kinetics are showed as follows [55][56][57]: The pseudo-first-order model: is the mass of heavy metals adsorbed per unit mass of the zeolite at time t. Q e-exp (mg g −1 ) and Q e-cal (mg g −1 ) are the mass of heavy metals adsorbed per unit mass of the zeolite at adsorption equilibrium obtained from experimental work and model calculations. k 1 (min −1 ) is the rate constant of the first-order model. The pseudo-second-order model [15,18]: is the mass of heavy metals adsorbed per unit mass of the zeolite at time t. Q e-cal (mg g −1 ) is the mass of heavy metals adsorbed per unit mass of the adsorbent at adsorption equilibrium obtained from model calculations. k 2 (g.mg −1 .min −1 ) is the rate constant of the second-order model. Figure 11(a) shows the effect of adsorption time on the adsorption capacity of heavy metals. The initial concentration of Cu, Cd and Pb is 60 mg l −1 , 18 0 mg l −1 and 500 mg l −1 , respectively; the adsorption time ranges from 20 min to 480 min; the dosage of MNZ@MS is 1 g l −1 . From figure 11(a), the adsorption process of heavy metals on MNZ@MS is very fast, and it almost reaches adsorption equilibrium in 20 min. These results may be ascribed to the abundant active adsorption sites on MNZ@MS. The adsorption kinetics profiles of heavy metals on MNZ@MS are depicted in figures 11(b) and (c), which shows the fitting results of pseudo-first-order model and pseudo-second-order model on adsorption process, respectively. Compared figures 11(b) with figure 11(c), it can be seen that the pseudo-second-order model better describes the adsorption reaction process of heavy metals on MNZ@MS. Furthermore, we calculate the kinetics parameters of pseudo-first-order model and pseudo-second-order model, and the results are shown in table 1. From the results of calculated adsorption capacity (q cal ) and R 2 value, we also believe that the pseudo-secondorder model presents suitable fitting results, based on the higher value of R 2 and the closer value between q exp (experimental adsorption capacity) and q cal .

Adsorption isotherm
The equation of Langmuir isotherm is as follow [55,58,59]; Where Q e and C e are the amount adsorbed (mg g −1 ) and the concentration of heavy metals in solution (mg l −1 ), both at equilibrium; k F and n are constants for Freundlich isotherm, and they are indicative of the adsorption capacity (mg g −1 ) and adsorption intensity. Figure 12 and table 2 presents the adsorption isotherm (Langmuir isotherm and Freundlich isotherm) fitting results of heavy metals on MNZ@MS, with the initial concentration of Cu and Cd ranging from 20 mg l −1 to 280 mg l −1 , the initial concentration of Pb ranging from 100 mg l −1 to 600 mg l −1 , the dosage of MNZ@MS is 1 g l −1 . Compared the fitting curve of Langmuir isotherm(figure 12(a)) with that of the Freundlich isotherm, the Langmuir adsorption isotherm is more suitable than the Freundlich adsorption isotherm to describe the adsorption process of heavy metals on MNZ@MS. The R 2 value in table 2 also provides us similar results. As the basic assumptions of Langmuir adsorption isotherm, the adsorption process of heavy metals on MNZ@MS Table 1. Pseudo-first-order and pseudo-second-order rate constants calculated from experiment data.
Pseudo-first-order  could be considered as a monolayer adsorption process. In addition, according to the computational results on table 2, the maximum adsorption capacity of heavy metals for Cu, Cd and Pb on MNZ@MS is 59.9 mg g −1 , 188 .6 mg g −1 and 476.1 mg g −1 , respectively.

Competitive adsorption system
In industrial wastewater, multiple heavy metals often exist at the same time. Due to the interaction of heavy metals, the adsorption efficiency will be different from that of a single adsorption system.
Q 0 is the equilibrium adsorption capacity under a single adsorption system. Q 1 is the equilibrium adsorption capacity under a competitive adsorption system. Figure 13 depicts the effect of initial concentrations on the reduction rates of heavy metals in competitive adsorption system, with the initial concentrations of heavy metals ranging from 20 to 280 mg l −1 , and the dosage of MNZ@MS is 1 g l −1 . The reduction rate of Cd is the highest among three heavy metals, and it maintains around 70% regardless of the changs of initial concentration. In addition, the reduction rate of Pb is the lowest, and it improves with the increase of initial concentrations. Besides, the reduction rate of Cu appears between Pb and Cd, and the variation trend is similar with that of Pb. This experiment results suggest that, under competitive adsorption system, the adsorption process of Pb is the most difficult to be interfered, while the adsorption process of Cd is the most susceptible to the interference from other heavy metals. Under low initial concentration system, the adsorption advantages of Pb and Cu appear more obviously at low initial concentrations.  Figure 13. The effect of initial concentration on the reduction rate of heavy metals in competitive adsorption system.

Conclusions
In this paper, the MNZ@MS is successfully prepared using MS as carrier and [C 18 H 37 (CH 3 ) 2 -N + -(CH 2 ) 3 -N + -(CH 3 ) 2 C 18 H 37 ]Cl 2 as directing agent. The characterization results of materials exhibit that the surface of spherical MS particles are completely wrapped by nano-scale Linde F(K)zeolite. The MNZ@MS presents favorable magnetic properties, and the saturation magnetization value of MNZ@MS is around 17.7 emu g −1 . The Zeta potential result indicates that MNZ@MS is suitable to the adsorption of heavy metals from solution. In addition, the higher solution pH is beneficial to the adsorption process of heavy metals on MNZ@MS. The pseudo-second-order model are more suitable for fitting the adsorption process of heavy metals on MNZ@MS. Isotherm study suggests that the adsorption process follows the Langmuir isotherm model. Moreover, the maximum adsorption capacity of heavy metals on MNZ@MS is 59.9 mg g −1 , 188 .6 mg g −1 and 476.1 mg g −1 , respectively. Under competitive adsorption system, the adsorption process of Pb is most difficult to be interfered, while the adsorption process of Cd is most susceptible to the interference from other heavy metals. The results proved that MNZ@MS is a low-cost and efficient magnetic adsorbent, and it has great potential for the application in heavy metal wastewater treatment.