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

Abstract

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.

Keywords:
Multi-walled carbon nanotubes; nickel ferrite; composite; photo-Fenton

1. Introduction

Advanced oxidation processes (AOPs) are alternative emerging techniques for the degradation of organic pollutants in wastewater^1-6. 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^7-9. 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) $\equiv {\mathrm{Fe}}^{\mathrm{III}}\mathit{+}{H}_2{O}_2\mathit{+}\mathit{light}\mathit{irradiati}\mathit{on}\mathit{\to }\equiv {\mathrm{Fe}}^{\mathrm{II}}\mathit{+}\mathrm{HO}_2{}^{•}\mathit{+}{H}^{+}$

(2) $\equiv {\mathrm{Fe}}^{\mathrm{II}}\mathit{+}{H}_2{O}_2\mathit{+}\mathit{light}\mathit{irradiati}\mathit{o}\mathit{n}\mathit{\to }\equiv {\mathrm{Fe}}^{\mathrm{III}}\mathit{+}{\mathrm{HO}}^{•}\mathit{+}{H}^{-}$

(3) $\begin{array}{c}\mathrm{Organic}\mathrm{pollutant}+{\mathrm{HO}}^{•}\mathit{\to }\mathrm{degradation}\mathrm{products}\hfill \\ \left({\mathrm{CO}}_2+H_{2}O+\mathrm{intermediates}\right)\hfill \end{array}$

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 performance¹³.

Several ferrite/carbon nanotubes composites have been used for different applications^14-19, but very few them have been used for application in OAPs^13,20-23. Recently, nickel ferrite (NiFe₂O₄)/carbon nanotubes (MWCNTs) composite has been prepared via a conventional route using teflon-lined stainless autoclave at 180 ºC for 20 h, being applied as photocatalyst for the degradation of phenol under UV irradiation¹³. NiFe₂O₄/MWCNTs hybrids were prepared via one-step hydrothermal method at 180 ºC for 20 h, and their photocatalytic activity was investigated for the decolorization of Congo Red dye in aqueous solution under simulated solar light irradiation²³. However, no work has yet been developed using NiFe₂O₄/MWCNTs composite prepared via microwave route for application as a photo-Fenton catalyst under visible light irradiation. It is well known that the use of microwave route for the preparation of powdered catalysts presents as great advantages short synthesis time and production of high surface area particles, being this last characteristic of great importance for catalytic purposes^24,25.

NiFe₂O₄ is a cubic oxide with a typical inverse spinel structure and has attracted much interest because of its fascinating magnetic and electromagnetic properties²⁶, while the MWCNTs have attracted increasing research interest as dye adsorbent²⁷, support for enzyme immobilization²⁸ and catalyst support²⁹.

In this work, nickel ferrite (NiFe₂O₄)/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.

2. Materials and Methods

2.1 Materials, procedures and characterization techniques

Multi-walled carbon nanotubes (MWNTs-COOH functionalized) (Purity ˃ 95 wt%, outside diameter: 10-20 nm, inside diameter: 5-10 nm, Length: 10-30 mm) were purchased from Nanostructured & Amorphous Materials, Inc., USA. Amaranth dye (CAS number: 915-67-3; chemical formula: C₂₀H₁₁N₂Na₃O₁₀S₃; molecular weight: 604.47 g mol^-1) was used as a model pollutant. Nickel nitrate [Ni(NO₃)₂.6H₂O], iron nitrate [Fe(NO₃)₃.9H₂O] and anhydrous ethyl alcohol were utilized without any further treatment.

A modified procedure for the preparation of NiFe₂O₄/MWCNTs composite was employed in this work, which was based on a previously reported work¹³, where a hydrothermal conventional method has been employed for the synthesis process¹³. From hydrothermal route, 20 h of reaction time has been necessary for the production of the respective composite¹³. 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 ºC). 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 ºC for 12 h. For comparison purposes of the catalytic activity, pure NiFe₂O₄ 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.

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 65º. Thermogravimetric analysis was carried out on a TGA-50 Shimadzu analyzer at a heating rate of 10 ºC 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/P₀) 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.

2.2 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 H₂SO₄), the catalyst amount (NiFe₂O₄ and NiFe₂O₄/MWCNTs composite) used was 0.05 g and the H₂O₂ (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.

3. Results and Discussion

XRD diffraction patterns of MWCNTs, pure NiFe₂O₄ and NiFe₂O₄/MWCNTs composite are shown in Figure 1. The major peaks corresponding to the planes (0 0 2) and (1 0 1) at 2θ positions of 26.5 and 43.4º are characteristic peaks of MWCNTs³⁰ (Figure 1a). In Figure 1b, the peaks located at 18.4, 30.3, 35.7, 37.3, 43.3, 53.8, 57.3, and 63.0° can be indexed to the (111), (220), (311), (222), (400), (422), (511) and (440) crystal planes of NiFe₂O₄ spinel, respectively, according to the JCPDS Card No. 54-0964 (Bars inset to Figure 1 correspond to reference NiFe₂O₄). From Figure 1c, the presence of the peak to 26.5° (which corresponds to MWCNTs material) on the XRD pattern of the NiFe₂O₄/MWCNTs composite, indicates the successful formation of respective hybrid material.

Figure 1
X-ray diffractograms of (a) MWCNTs, (b) pure NiFe₂O₄ and, (c) NiFe₂O₄/MWCNTs composite. Bars inset correspond to reference NiFe₂O₄, according to the JCPDS Card No. 54-0964.

Raman spectra of MWNTs, NiFe₂O₄/MWNTs composite and pure NiFe₂O₄ are shown in Figure 2. The peaks at 1350 cm^-1 (band D) and 1580 cm^-1 (band D) are typical of MWCNTs³¹ (Figure 2a), while the peaks to 470 cm^-1 and 690 cm^-1 correspond to typical peaks of NiFe₂O₄³² (Figure 2b). All peaks above mentioned are presents on the Raman spectrum of the NiFe₂O₄/MWCNTs composite (Figure 2c), demonstrating the successful preparation of the NiFe₂O₄/MWNTs hybrid composite.

Figure 2
Raman spectra of (a) MWCNTs, (b) pure NiFe₂O₄ and, (c) NiFe₂O₄/MWCNTs composite.

Thermogravimetric analysis (TGA) was used to evaluate the amount of MWCNTs incorporated on the prepared NiFe₂O₄/MWCNTs composite. Figure 3 shows the TGA of the pure NiFe₂O₄ (Figure 3a), NiFe₂O₄/MWCNTs composite (Figure 3b) and MWCNTs (Figure 3c). According to Figure 3, the MWCNTs have total mass loss at 800 °C (Figure 3c), while the pure NiFe₂O₄ remains stable up to 900 °C (Figure 3a). From Figure 3b, it is possible to observe a mass loss about 25%, corresponding to thermal oxidation of MWNTs presents in the NiFe₂O₄/MWCNTs composite. This result indicates that the procedure used in this work for the composite preparation was done successfully.

Figure 3
Thermogravimetric analysis of (a) pure NiFe₂O₄, (b) NiFe₂O₄/MWCNTs composite, and (c) MWCNTs.

Representation of the nitrogen adsorption-desorption isotherms and pore-size distributions for MWCNTs, pure NiFe₂O₄ and NiFe₂O₄/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 NiFe₂O₄/MWCNTs composite are between those of MWCNTs and pure NiFe₂O₄.

Figure 4
(a) Nitrogen adsorption/desorption isotherms of MWCNTs, NiFe₂O₄/MWCNTs and NiFe₂O₄ and, their (b) pore-size distributions.

Table 1
Pore properties of the samples.

Sample	Surface area (m2 g-1)	Total pore volume (cm3 g-1)
NiFe2O4	32	0.1684
MWCNTs	103	0.4881
NiFe2O4/MWCNTs	54	0.2249

Figure 5 shows SEM images of NiFe₂O₄/MWCNTs composite (Figures (5a) and (5b) under different magnifications), NiFe₂O₄ (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 NiFe₂O₄ (Figure (5c)), whereas nanotubes well adhered on the surface of the NiFe₂O₄ 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 (5e), it was found an atomic ratio of 1:2 (Ni:Fe), which confirms the formation of NiFe₂O₄ spinel.

Figure 5
SEM images of NiFe₂O₄/MWCNTs composite [Magnification: 500 X (a); 20,000 X (b)], NiFe₂O₄ (Magnification: 500 X) (c) and MWCNTs (Magnification: 20,000 X) (d), and EDS analysis (e) of NiFe₂O₄/MWCNTs composite.

Preliminary tests on the presence of catalyst in the dark without H₂O₂ (catalyst/dark), without catalyst (H₂O₂/light) and without H₂O₂ (catalyst/light) showed negligible decolorization results, whereas the Fenton process (catalyst/H₂O₂/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/H₂O₂/visible light (photo-Fenton process). As shown in Figure 6, the linear relationship of C/C₀ 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 NiFe₂O₄/MWCNTs composite can be attributed to the synergistic effect between NiFe₂O₄ 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 NiFe₂O₄/MWCNTs composite compared to the pure NiFe₂O₄ 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.

Figure 6
Catalytic activity of NiFe₂O₄ and NiFe₂O₄/MWCNTs for the degradation of Amaranth dye by photo-Fenton process.

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.

4. Conclusions

NiFe₂O₄/MWCNTs composite was successfully prepared using microwave irradiation technique. The studies showed the formation of single phase particles of NiFe₂O₄ on the composite. Hybrid material with surface area and total pore volume of 54 m² g^-1 and 0.2249 cm³ g^-1, respectively, was obtained using a microwave irradiation time of 30 min. NiFe₂O₄/MWCNTs composite was found to be a more efficient catalyst than the pure NiFe₂O₄ 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 NiFe₂O₄ and MWNTs promoted a significant improvement in catalytic activity. The results indicated that the NiFe₂O₄/MWCNTs composite could be employed as an efficient catalyst for the treatment of dye-containing wastewater.

5. 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.

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Publication Dates

• Publication in this collection
  03 Aug 2017
• Date of issue
  2017

History

• Received
  15 Sept 2016
• Reviewed
  04 July 2017
• Accepted
  07 July 2017