Characterization and photocatalytic degradation of methyl orange in water by Ni0.1Cu0.1Zn0.8Fe2O4 nanoparticles synthesized by gel-auto combustion and microwave- assisted methods

Over the past few decades, magnetic nanoparticles have been considered by many researchers due to their unique physicochemical, magnetic and optical properties and extensive applications in the field of biomedical (such as magnetic resonance imaging (MRI), drug delivery system and magnetic hyperthermia), catalysis, information technology, telecommunication and environmental remediation, destruction of organic pollutants such as dyes, etc. In this work, it has been attempted to synthesize Ni0.1Cu0.1Zn0.8Fe2O4 by both gel-auto combustion and microwave-assisted methods and the resulting powders were characterized by different analysis involved, XRD, SEM, IR, EDS, VSM and UV-Vis spectra. The effect of these two ferrites on the absorption and degradation of methylene orange dye was studied at different concentrations (100, 160 and 200 ppm). The average crystalline size of MNPs obtained by gel-auto combustion and MW-assisted, were 77.9 and 44.1 nm, respectively. The specific magnetization curves exhibit ferromagnetism of soft magnetic materials and the (Ms) values of samples were 20.64 and 52.89 (emu/g) for MNPs obtained by gel-auto combustion and MW-assisted, respectively. Finally, both ferrites showed high absorption of methylene orange dye in water under UV-Vis irradiation (λ> 400 nm).


Introduction
Dyes are important pollutants that lead to serious hazards for humans, the other animals and creatures come alive. Dyes can't be decomposed by aerobic treatment processes. Therefore, removing them from industrial wastewater before discharging into the environment requires a great deal of attention. In industries, different types of commercially dyes are used frequently.
Factors in the production of these pollutants include the textile, plastics, cosmetics and waste water 2 [5]. Photocatalysts and their structures are widely contaminated by water resulting from the exposure of light to decomposition of hazardous compounds. Recently, several physical, chemical, and biological methods used to treat water include reversal, adsorption, ultrafiltration, ozonation and biodegradation [6]. Magnetic nanoparticles (MNPs) are another compound capable of absorbing organic pollutants. Nanoparticles and especially ferrite spinel have advantages such as easy preparation, easy changes in nanoparticle size, surface morphology [5]. Except for the above mentioned ferrites are able to be easily separated by magnet due to their good magnetic properties.
These advantages have led to the use of ferrites in the water and wastewater pollutants removal industry. In addition, spinel ferrites have been widely used in biomedicine due to their non-toxicity, reduced side effects of cancer drugs and long-distance magnetic guidance, including: imaging contrast probes, hyperthermia agents, magnetic-guided vectors and drug-delivery carriers and etc [7][8][9][10][11]. Spinel ferrites have the general formula MFe2O4 known for their magnetic, optical, electronic, catalytic and photoelectric properties [12].  [14]. There are different physical and chemical methods for preparing MNPs, including: hydrothermal, co-precipitation, ultrasonic cavitation, thermal decomposition, sol-gel, combustion, microwave, sonochemical, microemulsion etc, each of which has its own advantages and disadvantages [15][16][17][18][19][20][21][22][23]. In this work, ferrites were synthesized in two self-combustion and microwave methods, and their differences in magnetic, optical and photocatalytic properties were investigated for the absorption of methylene orange dye. The microwave-assisted method was attractive due to its effective thermal energy. On the other hand, this method resulted in increased process speed (short reaction time), improved magnetic properties, improved size distribution of small nanoparticles, and increased contaminant uptake and degradation [24]. deionized water (H2O) were used and were supplied from Merck Millipore.

Synthesis of Ni0.1Cu0.1Zn0.8Fe2O4 NPs by gel-auto combustion method
The stoichiometric amounts of the nitrate salts of zinc, nickel, copper and iron with ammonium nitrate and glycine (as fuel) were dissolved in distilled water and heated to 80 ° C until the excess water evaporated and were obtained a reddish-brown gel. Subsequently, the heater temperature was increased to 300 ° C and after 10 minutes the combustion reaction was carried out. The brown powder of the nanoparticles obtained was ball milled for 12 min at a frequency of 30 W (Retsch-MM400, 60 Hz with maximum power 150 W). MNPs stored and kept at room temperature for different analyses. Finally, the ball milling to ensure nanoscale particles was carried out for 12 min at the same frequency. So, MNPs stored and kept at room temperature for different analyses.

Preparation of methylene orange solution and photocatalytic activities
Initially  Fig. 1 shows the X-ray diffraction pattern of the single-phase spinel ferrite cubic structure for both samples prepared by gel-auto combustion and microwave-assisted methods, which means that neither of the two methods makes any changes to the spinel ferrite structure. Also, the average size of NPs prepared by microwave irradiation and gel-auto combustion method were 77.9 and 44.1 nm, respectively. Therefore, microwave waves improve the size of nanoparticles that are more suitable for the applications mentioned.

FT-IR spectroscopy
As shown in Fig. 2

DRS analysis
To investigate the optical properties of the nanoparticles, DRS analysis was applied (Fig. 3). The optical properties of the samples appeared at 462 and 464 nm, respectively. So again, through his analysis, we came to the conclusion that microwave waves improve optical properties.

EDS analysis
EDS analysis shows the purity of the samples at room temperature (Fig. 4). According to this analysis, only the Ni, Cu, Zn, Fe and O elements were found in both samples and no secondary phase was observed. and under microwave irradiation (B).

Vibrating sample magnetometer (VSM) analysis
The specific magnetization curves exhibit ferromagnetism of soft magnetic materials and the saturation magnetization (Ms) values of samples were 20.6 and 52.9 emu/g for Ni0.1Cu0.1Zn0.8Fe2O4 obtained by gel-auto combustion (Fig. 5A) and microwave irradiation (Fig.   5B), respectively. Therefore, microwaves improve the magnetic properties of the NPs.

Conclusion
The samples were synthesized by microwave and gel-auto combustion methods and investigated by XRD, IR, DRS, EDS, VSM and UV-Vis spectra. Studies have shown that micro-waves improve the size of nanoparticles, their magnetic and optical properties, and high absorption of pollutants. Ni0.1Cu0.1Zn0.8Fe2O4 synthesized by MW-assisted with 92% uptake of pollutants can be used as a good sorbent for removal of organic pollutants in water and wastewater.