Preparation of Nickel Ferrite/Carbon Nanotubes Composite by Microwave Irradiation Technique for Use as Catalyst in Photo-Fenton Reaction

Nickel ferrite/multi-walled carbon nanotubes (NiFe2O4/MWCNTs) composite has been rapidly synthesized via microwave irradiation technique. The structural properties of the product was investigated by X-ray diffraction (XRD), N2 adsorption/desorption isotherms, thermogravimetric analysis (TGA), Raman spectroscopy and, scanning electron microscopy (SEM). Catalytic behavior of the composite material on the advanced photo-Fenton degradation of Amaranth dye was evaluated. The synthesis conditions employed on the microwave system were: temperature (235 °C), power (500 W), pressure (600 psi) and irradiation time (30 min). Characterization results showed the formation of hybrid material, containing a predominantly microporous structure, with surface area and total pore volume of 54 m2 g-1 and 0.2249 cm3 g-1, respectively. The composite exhibited higher catalytic activity compared to the pure NiFe2O4, reaching 100% of decolorization at 60 min of reaction, which can be attributed to a synergism between NiFe2O4 and MWCNTs. Therefore, NiFe2O4/MWCNTs composite can be used as a promising photo-Fenton catalyst to degrade Amaranth dye from aqueous solutions.


Introduction
Advanced oxidation processes (AOPs) are alternative emerging techniques for the degradation of organic pollutants in wastewater [1][2][3][4][5][6] . AOPs are divided in a variety of methods, and among them, heterogeneous Fenton reaction is one of the most interests, which to use iron-based solid catalyst, whose major advantage is its easy recovery from the solution by a field magnetic for further reutilization [7][8][9] . In the presence of a light source, known as photo-Fenton reaction, the pollutant degradation rate substantially increases 10,11 . The photo-Fenton process applies the combination of hydrogen peroxide, iron ions and light irradiation in an acidic aqueous medium (pH ≤ 3) 12 , producing highly oxidative radicals (HO • ) 10 , leading to degradation of pollutant molecules. Therefore, a simplified mechanism for the heterogeneous photo-Fenton degradation of organic pollutant under light irradiation can be depicted as follows (Equations 1-3): (1) where, ≡ Fe III and ≡ Fe II corresponds to iron species on the surface of a heterogeneous catalyst.
Recently, coupling of multi-walled carbon nanotubes (MWCNTs) with ferrite have been reported as a potential catalyst for degradation of organic pollutant. This coupling may favor the separation of electron-hole pairs on the catalyst, avoiding their recombination and generating more oxidative radicals (HO • ), leading to a high catalytic performance 13 .
Several ferrite/carbon nanotubes composites have been used for different applications [14][15][16][17][18][19] , but very few them have been used for application in OAPs 13,[20][21][22][23] 24,25 . NiFe 2 O 4 is a cubic oxide with a typical inverse spinel structure and has attracted much interest because of its fascinating magnetic and electromagnetic properties 26 , while the MWCNTs have attracted increasing research interest as dye adsorbent 27 , support for enzyme immobilization 28 and catalyst support 29 .
In this work, nickel ferrite (NiFe 2 O 4 )/carbon nanotubes (MWCNTs) composite was prepared via a rapid alternative method (microwave route) for application as catalyst on degradation of amaranth dye using heterogeneous photo-Fenton process under visible light irradiation. A modified procedure for the preparation of NiFe 2 O 4 / MWCNTs composite was employed in this work, which was based on a previously reported work 13 , where a hydrothermal conventional method has been employed for the synthesis process 13 . From hydrothermal route, 20 h of reaction time has been necessary for the production of the respective composite 13 . Therefore, this present work aims to use microwave irradiation as heat source in order to accelerate the formation of material. For the obtaining the composite sample containing 25 wt% of MWCNTs, nickel nitrate (1.45 g) and iron nitrate (4.04 g) were firstly dissolved in 100 mL of ethyl alcohol. Then, 0.40 g of MWCNTs was dispersed in 600 mL of ethyl alcohol. After, the ethyl alcohol/MWCNTs suspension was added into the saline solution under stirring for 30 min at room temperature (25 o C). This suspension was adjusted to a pH value of 14 using 10 M NaOH solution, and kept under stirring for 15 min. Then, 100 ml of deionized water was added to previous suspension, and kept under vigorous stirring for 30 min. Posteriorly, the final suspension was transferred to several high-pressure reaction vessels and submitted to microwave irradiation (MARS 6 Microwave equipment, ESP 1500 plus, USA), under the following conditions: temperature (235 °C), power (500 W), pressure (600 psi) and irradiation time (30 min). The obtained composite was collected and washed with deionized water for several times, and then, dried at 110 o C for 12 h. For comparison purposes of the catalytic activity, pure NiFe 2 O 4 particles were prepared using the same previous mentioned procedure without the addition of MWCNTs. The concentration of free Fe ions in the solution after irradiation was measured by atomic absorption spectroscopy (Agilent Technologies, 200 series AA) to monitor their leaching from the catalysts.

Materials, procedures and characterization techniques
Characterization of the materials was identified using an X-ray diffractometer (Rigaku Miniflex 300), with Cu-Kα radiation, powered at 30 kV and 10 mA. Scans were performed over 2θ angles ranging from 15 to 65 o . Thermogravimetric analysis was carried out on a TGA-50 Shimadzu analyzer at a heating rate of 10 o C min -1 in presence of an air flow rate of 50 mL min -1 , in the temperature range from 25 to 900 °C. Nitrogen adsorption-desorption isotherms were obtained at 77 K carried out on an ASAP 2020 apparatus at relative pressure (P/P 0 ) ranging from 0 to 0.99. Specific surface areas were calculated according to the Brunauer-Emmett-Teller (BET) method and, the pore-size distributions were obtained according to the Barret-Joyner-Halenda (BJH) method. Raman spectroscopy measurements were performed at room temperature using a micro-positioning system B&WTek and an Andor Shamrock 303i monochromator. The morphology of the composite was examined by a scanning electron microscope (SEM, JEOL JSM-6610LV) at 15 kV, and its chemical composition was obtained by energy dispersive X-ray spectroscopy (EDS), which is coupled to the SEM equipment.

Photo-Fenton experiment
For the degradation tests of 50 mL Amaranth dye solution at room temperature (50 mg L -1 ) and pH 2.5 (adjusted using 0.1 M H 2 SO 4 ), the catalyst amount (NiFe 2 O 4 and NiFe 2 O 4 / MWCNTs composite) used was 0.05 g and the H 2 O 2 (30% v/v) volume was 50 µL. Prior to illumination, the aqueous suspension containing catalyst and dye was magnetically stirred in the dark until to achieve the adsorption equilibrium. In order to avoid adherence of the magnetic catalyst on the magnetic bar, a vigorous agitation (150 rpm) was employed. It was found that an agitation rate above this value is adequate for to obtain a homogeneous suspension during the stirring step. Then the suspension was exposed to visible light irradiation under stirring. The visible-light source was commercial fluorescent lamp (85 W, Empalux) positioned 10 cm above the liquid surface. Samples were taken at set intervals using a syringe and, filtered immediately through a PVDF membrane (0.45 µm). The dye concentration in the filtered suspension was determined by the absorbance reading on an UV-vis spectrophotometer (Shimadzu, UV-2600), at a maximum absorption wavelength of 520 nm.    Representation of the nitrogen adsorption-desorption isotherms and pore-size distributions for MWCNTs, pure NiFe 2 O 4 and NiFe 2 O 4 /MWCNTs samples are shown in Figure 4. The isotherms for the all the samples shown in Figure 4a are similar and can be classified as type II. The shape of these isotherms indicates that all the samples possess predominantly microporous structure. In addition, the microporous structure was confirmed by the analysis of pore-size distribution (Figure 4b), which shows spectra of pore-size distributed on the microporous region (pore-size less than 2 nm). Pore properties of the samples are shown in Table 1. Values of surface area and total pore volume of NiFe 2 O 4 /MWCNTs composite are between those of MWCNTs and pure NiFe 2 O 4 . Table 1. Pore properties of the samples.

Sample
Surface area (m 2 g -1 ) Total pore volume (cm 3 g -1 )    (Figure (5c)) and MWCNTs (Figure (5d)). In Figure  (5e), it is showed the chemical composition of composite obtained from EDS analysis. Particles with irregular shape can be observed on the respective images of the composite (Figure (5a)) and NiFe 2 O 4 (Figure (5c)), whereas nanotubes well adhered on the surface of the NiFe 2 O 4 particle can be observed in Figure (5b). In addition, it is possible to observe that the nanotubes maintained their morphology after the microwave process. From Figure (  and without H 2 O 2 (catalyst/light) showed negligible decolorization results, whereas the Fenton process (catalyst/ H 2 O 2 /dark) exhibited about 7.0% of decolorization at 60 min of reaction time. Therefore, the effective dye degradation is attributed to the synergetic effect of the combination among catalyst/H 2 O 2 /visible light (photo-Fenton process). As shown in Figure 6, the linear relationship of C/C 0 versus reaction time shows that the dye decolorization via photo-Fenton process followed the zero-order kinetics 3,8,33 for both the catalysts. The slopes of lines correspond to the reaction rate constants (k composite = 0.017 mg L -1 min -1 and k ferrite = 0.001 mg L -1 min -1 ). This result indicates that the composite shows higher activity compared to pure ferrite. The dye was substantially degraded from the aqueous solution, reaching 100% at 60 min of reaction time. On the other hand, 60% of decolorization was obtained at 60 min using pure ferrite. The significant enhancement in catalytic activity by the NiFe 2 O 4 / MWCNTs composite can be attributed to the synergistic effect between NiFe 2 O 4 and MWNTs that reduce the rate of recombination of photoinduced electrons and holes, leading to high catalytic performance 13,30 . In addition, the higher surface area and pore volume of NiFe 2 O 4 /MWCNTs composite compared to the pure NiFe 2 O 4 could offer a larger contact and diffusion of dye molecules within the pores of its particles, contributing to the close contact between the HO• radicals and dye molecules, which leads to an increasing reaction rate.   From leaching essays for both the catalysts, very low amount of Fe (about 2 mg L -1 ) was detected by atomic absorption spectroscopy in the solution after 60 min of irradiation, indicating that the dye degradation was due to the heterogeneous Fenton reaction. According to Brazilian environmental legislation (CONAMA) 34 , a maximum of 15 mgFe L -1 is allowed for discharge into a body of water. Therefore, this result indicates a good stability of the catalyst in photo-Fenton reaction.

Conclusions
NiFe 2 O 4 /MWCNTs composite was successfully prepared using microwave irradiation technique. The studies showed the formation of single phase particles of NiFe 2 O 4 on the composite. Hybrid material with surface area and total pore volume of 54 m 2 g -1 and 0.2249 cm 3 g -1 , respectively, was obtained using a microwave irradiation time of 30 min. NiFe 2 O 4 /MWCNTs composite was found to be a more efficient catalyst than the pure NiFe 2 O 4 for the Amaranth degradation by the photo-Fenton reaction under visible light, reaching total degradation at 60 min of reaction time. Therefore, the combined effect between NiFe 2 O 4 and MWNTs promoted a significant improvement in catalytic activity. The results indicated that the NiFe 2 O 4 /MWCNTs composite could be employed as an efficient catalyst for the treatment of dye-containing wastewater.

Acknowledgements
The authors would like to thank to FAPERGS (Foundation for Research of the State of Rio Grande do Sul/Brazil) by the scholarship for the author C. R.