Plant-mediated synthesis of Cu0.5Zn0.5Fe2O4 nanoparticles using Minidium leavigatum and their applications as an adsorbent for removal of reactive blue 222 dye

Cu0.5Zn0.5Fe2O4 magnetic nanoparticles were synthesized using Minidium leavigatum extract and required metal salts by a green method. This method has some benefits such as nontoxic, economic viability, ease to scale up, less time consuming and environmental friendly approach for the synthesis of Cu0.5Zn0.5Fe2O4 magnetic nanoparticles without using any hazardous organic chemicals. The sample was characterized by powder x-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), energy dispersive x-ray analysis (EDX), and Brunauer–Emmett–Teller (BET) surface area analysis. The environmentally friendly synthesized nanoparticles were used as adsorbent for removal of reactive blue 222 (RB 222) dye from aqueous solutions. The effects of initial dye concentration and nanoparticle dosage on dye adsorption were assessed. Adsorption equilibrium studies were investigated to determine the adsorption capacity of the adsorbents by using the Langmuir and Freundlich isotherm models. The Langmuir model yielded more suitable than the Freundlich model for the adsorption of RB 222 on Cu0.5Zn0.5Fe2O4 magnetic nanoparticles. Adsorption kinetics obeyed a pseudo second-order kinetic model.


Introduction
Nowadays, synthetic dyes are widely used in the textile, rubber, printing, leather, cosmetics, and plastics industries. One of the basic environmental problems of these industries is the pollution of wastewater with hazardous organic dyes. The presence of organic dyes in water resources causes a critical damage to aquatic species and prevents sunlight exposure for aquatic plant and animal species, thus it makes changes in aquatic ecosystem. In addition, these dyes can be very toxic and even carcinogenic to human and mammalian animals; therefore, the removal of these contaminations from wastewater is essential [1][2][3][4][5]. There are various techniques for dye removal from wastewater such as ozonation [6], electrochemical destruction [7], photodegradation [8], biological methods include aerobic degradation, anaerobic degradation, biosorption, etc [9][10][11][12], adsorption [13], coagulation [14], membrane filtration [15], and ion exchange [16] methods. Adsorption process is a simple, low cost, and quite effective method compared to the other mentioned procedures. Since the nanoparticles have high surface area, they can act as an efficient adsorbent for the removal of dyes from wastewater [17]. The removal of organic dyes by using magnetic nanoparticles as adsorbents is an interesting field for wastewater treatment, because the magnetic nanoparticles that adsorbed the organic dye are easily separated from the solution by using a magnet at the end of process [18].
In recent years, the spinel ferrite nanoparticles have received a lot of attention due to their unique magnetic and electrical properties. These nanoparticles have been used as catalysts for the synthesis of organic compounds [19], sensor [20], permanent magnet [21], wastewater treatment [22][23][24], microwave devices [25,26], electronic devices [27], and biomedical agents [28]. There are various ways to synthesize these nanoparticles [29][30][31][32][33][34][35], but plant-mediated synthesis of them has been attracted by chemists, because of their cost-effective, environmental friendly synthesis, milder conditions, feasibility, easier and faster procedure respect to the other chemical and physical methods. In addition, the plant-mediated nanoparticle synthesis is proper for the large scale synthesis of nanoparticles and due to its biocompatibility, it is suitable for the medical applications. In this method, the aqueous extract is used to make nanoparticles at normal temperatures and pressures, thus saving a lot of energy is obtained. The phytoconstituents which existed in the plants such as alkaloids, flavonoids, saponins, steroids, tannins and other nutritional compounds have two essential roles as reducing and stabilizing agents. [36][37][38].
Mindium laevigatum (Vent.) Rech. f. & Schiman-Czeika is one of the useful medicinal plants that belong to Campanulaveae family and it is endemic plant of Iran and Turkey. It is widely used as blood purifier, antiasthma, anti-dyspnea, and especially as an anti-hemorrhoid drug in traditional medicine [39,40]. The Persian name of the plant is Gholeshekafteh and in Zanjan province, Iran is called Gengedasi. Narnar is its former scientific name: Michauxia laevigata Vent [40]. These families have important medicinal activities for treatment of various diseases such as tonsillitis, laryngitis, bronchitis, and warts because of their strong gel and flavonoids that obtained from its seed, flowers and stem gum [41]. There have been no reports on the use of Mindium laevigatum in the plant-mediated synthesis of nanoparticles in literature.
Reactive blue 222 (RB 222) is an azo dye and it is used for coloring cotton, wool, silk, polyamide textiles, and ink color [42]. The structure and characteristics of RB 222 are shown in scheme 1.
In this research, we discuss the plant-mediated synthesis of Cu 0.5 Zn 0.5 Fe 2 O 4 NPs using Minidium leavigatum as an economy, simple and eco-friendly approach and its application as an adsorbent for removal of RB 222 dye. This is the first report on the plant-mediated synthesis of nanoparticles using Minidium leavigatum.  nanoparticles were identified by x-ray powder diffraction (XRD) with a Philips PW1730 x-ray diffractometer using Cu (Kα) radiation (wavelength: 1.5406 Å), operated at 40 kV and 30 mA at room temperature in the range of 2θ from 10 to 80. The particle size and morphology of the sample surfaces was studied by a Tescan Mira III FESEM scanning electron microscope equipped with an energy dispersive x-ray spectrometer (EDX). EDX analysis was performed to further confirmation of the nanoparticles composition. A Philips EM208s-100 Kv transmission electron microscopy (TEM) was used. Atomic force microscopy (AFM) were performed by a Bruker ICON AFM. The magnetic properties of sample were measured at room temperature using vibrating sample magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran).

Preparation of aqueous extract
The stems of Minidium leavigatum were washed, crushed and powdered. For preparation of the extract, 20 grams of the powdered plant material is loaded into the thimble, which is placed inside the soxhlet extractor. Then 300 ml of distilled water is added to a 500 ml round-bottom flask which is attached to a 250 ml soxhlet extractor and a condenser on a heating mantle. The solvent is heated to reflux and the extraction process is continued for 12 h.  . For studying the progress of the adsorption, 5 ml of solution were removed from the reaction medium at regular time intervals. The nanoparticles were separated from solution using a magnet and the solution was centrifuged. The change on the absorbance at a maximum wavelength (λ max =612 nm) of dye was monitored by UV-vis spectrophotometer. The adsorption performance of the process was defined as % adsorption=(A 0 − A)/A 0 ×100, where A 0 is the initial absorbance and A is the final absorbance at λ max =612 nm. The crystal structure analysis was carried out by the x-ray diffraction patterns. The XRD pattern of Cu 0.5 Zn 0.5 Fe 2 O 4 NPs was shown in figure 2. The XRD analysis confirmed the cubic structure of the compound. The average crystallite size of nanoparticles was determined by the full width at half maximum of the XRD patterns using the Scherrer formula (D=0.9λ/βcos θ). In the formula D is the crystallite size (nm), λ is the x-ray wavelength of Cu Kα=0.154 nm, β is the full width at half maximum of the peak and θ is the Bragg angle [45]. By using the formula, the average size of Cu 0.5 Zn 0.5 Fe 2 O 4 NPs was 23.5 nm.

Results and discussion
In   From the BET results, the pore volume, BET surface area, and average pore diameter for Cu 0.5 Zn 0.5 Fe 2 O 4 NPs were calculated 0.068 47 (cm 3 /g), 5.0189 m 2 g −1 , and 54.57 nm, respectively.      can be shown in figure 8, the removal efficiency of RB 222 is increased by increment of adsorbent dosage. It is because of raising the number of vacant sites toward the RB 222 molecule at constant dye concentration.

Effect of initial dye concentration
The effect of initial concentration of RB 222 (20, 30, and 40 mg l −1 ) in the presence of 0.04 g of Cu 0.5 Zn 0.5 Fe 2 O 4 NPs at room temperature was investigated as shown in figure 9. According to these results, the removal efficiency of RB 222 is decreased by increasing the initial concentration of RB 222.

Adsorption isotherms
The adsorption isotherms show the quantity of solute adsorbance per unit weight of adsorbent as a function of concentration in the bulk solution. There are various isotherm models such as Freundlinh [46], Langmuir [47], Dubinin-Radushkevich [48], Harkins-Jura and Temkin [49]. The Freundlich isotherm is based on multilayer adsorption on the surface of the adsorbent and is depicted in equation (1).
Where q e (mg g −1 ) is the amount of RB 222 adsorbed per unit mass of Cu 0.5 Zn 0.5 Fe 2 O 4 NPs at equilibrium, K f (mg (1−1/n) .l 1/n−1 .g −1 ) is the Freundlich constant, n (g l −1 ) is the heterogenicity factor, and C e (mg l −1 ) is the equilibrium liquid concentration of RB 222. A plot of Lnq e versus LnC e yields a straight line with slope=1/n and y-intercept=LnK f . The Langmuir equation is based on monolayer adsorption on the adsorbent surface to estimate the maximum adsorption capacity and is depicted in equation (2).

Kinetic study
The adsorption kinetics of RB 222 on Cu 0.5 Zn 0.5 Fe 2 O 4 NPs was investigated by using the pseudo first-order, the pseudo second-order, and the intraparticle diffusion models. The equations of investigated kinetic models were shown in table 3 [50].
The pseudo first-order, pseudo second-order, and intraparticle diffusion model plots for adsorption of RB 222 on Cu 0.5 Zn 0.5 Fe 2 O 4 NPs were shown in figures 13-15, respectively. The adsorption reached equilibrium after 45 min without remarkable increment in RB 222 adsorption after that time. The kinetic parameters for the models were tabulated in table 4. The correlation values (R 2 ) for the kinetic models showed that pseudo secondorder model yields more suitable than the other models for the adsorption of RB 222 on Cu 0.5 Zn 0.5 Fe 2 O 4 NPs.

Model Equation Parameters
Pseudo first-order

Conclusion
Cu 0.5 Zn 0.5 Fe 2 O 4 magnetic nanoparticles have been prepared using Minidium leavigatum extract and appropriate metal salts by a green method. These magnetic nanoparticles have excellent adsorption capacity for removal RB 222 from water. Equilibrium data were investigated by Freundlich and Langmuir models. The results showed that the Langmuir model yields more suitable than the Freundlich model for the adsorption of RB 222 on Cu 0.5 Zn 0.5 Fe 2 O 4 NPs. The maximum adsorption capacity was determined as 32.26 mg g −1 . Adsorption kinetic data for pseudo first-order, pseudo second-order, and intraparticle diffusion models were obtained and the results showed that pseudo second-order kinetic model is more suitable than the other two models.

Acknowledgments
This work was supported by the 'Zanjan Branch, Islamic Azad University'.