rGO/MnO₂ nanowires for ultrasonic-combined Fenton assisted efficient degradation of Reactive Black 5

Large amounts of synthetic/reactive dyes including Reactive Blue, Reactive Red and Reactive Yellow, are used in industry for dying and colouring materials. Reactive Black 5 (RB5) dye is much used in textile colouring due to its special features such as solubility, high shade of the colour and easy attachment to fibre (Vijayaraghavan et al. 2013). During the colouring/dying process, a significant proportion of dyes are released into the environment which affects aquatic life, drinking water and the health of human beings (Gupta et al. 2012; Karatas et al. 2012). Hence the removal of organic pollutants from waste water is a major environmental challenge. In this regard, a wide variety of methods, such as biological, physical and chemical methods have been developed to remove RB5 molecules from waste water (Siboni et al. 2011; Kalkan et al. 2014; Sadeghi et al. 2014; Amin et al. 2015). All of these approaches have some drawbacks, such as formation of by-products, slow reaction rate for the degradation of RB5 molecules, the need for dissolved oxygen and long retention time, low efficiency, and some of them are very expensive. Therefore, development of a suitable catalyst and a simple, efficient and environmentally friendly method is essential.

Metal oxides (Poulios & Tsachpinis, 1999; Aguedach et al. 2008; Kansal et al. 2009; Chong et al. 2015) have applications as catalysts for the degradation of RB5 owing to their structural and optical properties which play a crucial role in the catalytic activity. Among these, MnO₂ draws much attention due to its relatively low crystallite size and large surface area (Kumar et al. 2014). In particular, MnO₂ has revealed a superior catalytic activity and adsorption capability for dye degradation and heavy metal ion removal from wastewater. For instance, single crystalline β-MnO₂ nanowires (NWs) fabricated by a simple hydrothermal method showed a maximum degradation rate of 95% for methylene blue in the presence of hydrogen peroxide (Zhang et al. 2015). Up to now, many methods like redox-precipitation (Wu et al. 2013), microwave irradiation (Ai et al. 2008), and vapor–liquid–solid (Li et al. 2005) have been proposed to synthesize nanocrystalline MnO₂. However, these methods suffer from drawbacks such as agglomeration, stability, heat control, pressure issues, requirement of high temperatures, low reaction rate and production of secondary nanostructures. Recently, a graphene/MnO₂ nanocomposite has proved to have superior electrode performance in super capacitor applications, using graphene oxide and manganese sulphate as the precursors (Feng et al. 2013).

The Fenton reaction suffers from the drawback of the constant loss of the catalyst (Zhang et al. 2006; Harichandran & Prasad 2016). In this regard, ultra-sonication can be used as a support, during which the propagation of the sound wave through the Fenton reaction causes extremely high temperatures and pressures in a microscopic region of the sonicated liquid, thereby resulting in chemical excitation, resulting in the initiation or enhancement of the catalytic activity in the solution through the formation of new, relatively stable chemical species that can diffuse further into the solution to create chemical effects. Hence, an ultrasonic-combined Fenton reaction in an aqueous solution not only enhances the amount of free radicals, but also alters the reactor conditions favouring a good interaction of the created free radicals with the organic effluents for efficient removal (Namkung et al. 2008; Song et al. 2009). Recently it has been used in the improvement of degradation of several reactive dyes, such as Rhodamine B (Chen et al. 2016), Reactive Blue 181 (Basturk & Karatas 2014) and Reactive Blue 49 (Siddique et al. 2014), but the use of an ultrasonic-assisted Fenton reaction for the decoloration of RB5 dye using reduced graphene oxide (rGO)/MnO₂ NWs has not yet been reported.

Herein, we report the synthesis and physico-chemical properties of rGO coated MnO₂ NWs for enhanced catalytic decoloration of RB5 dye with an ultrasonic-combined Fenton process. Further, both rGO coating and ultrasonication of the NWs reveal an increase in decoloration of RB5 by 30%. Also, a higher production rate of organic fragments in the ultrasonic-combined Fenton process is witnessed.

Graphite powder and RB5 dye were purchased from Sigma-Aldrich and used as received. Manganese sulfate, potassium permanganate, sodium nitrate, sodium hydroxide, sulfuric acid (98%), hydrogen chloride, and hydrogen peroxide (30%) from Merck were used to produce the NWs. All reagents were of analytical grade. The crystalline phases of the products were identified using powder X-ray diffraction (Rigaku miniflex 600, Japan) using CuKα radiation (λ = 1.5406 Å). Scanning electron microscopy (JEOL/JSM-6380LA, Japan) was employed to observe the morphologies and sizes of the NWs. The Raman spectra was registered in the spectral range of 400–2,000 cm⁻¹ using a Raman spectrometer LabRam HR800 (Horiba Jobin Yvon Inc. USA). The specific surface area of the prepared samples was estimated using the Brunauer–Emmett–Teller (BET) method from the nitrogen adsorption–desorption isotherms obtained using a BELSORP Mini II instrument (BEL Japan Co., Ltd). The concentration of the RB5 dye after degradation was recorded using a UV-Vis spectrometer (ocean optics USB4000-UV-VIS, India). All reactions were conducted with stirring, using a magnetic stirrer (Remi MLH Plus, India) at 700 rpm and centrifugation was carried out using a centrifuge (Remi, R-8C, India). The aqueous solutions were sonicated using a Le-120UPP (20 KHz, 120 W, Leela Electronics Company, India) sonicator. To evaluate the by-products, gas chromatography-mass spectrometry (GC–MS) was used (GCMS-QP 2010 Plus Shimadzu mass spectrometer). An HP-5 ms column with dimensions of 0.25 μm, 30 m × 0.25 mm (i.d.) was used. The temperature was ramped as follows: 36 °C for 1 min, 5 °C min⁻¹ up to 300 °C and hold time 10 min. The temperature of the inlet, source and transfer line was 250, 230 and 280 °C, respectively. The analyses were done using splitless (0.7 min) injection.

A modified Hummer's method was used to prepare rGO (Sreejesh et al. 2015). To prepare graphene oxide, 0.5 g of sodium nitrate, 1 g of graphite flakes and 23 mL of concentrated sulfuric acid, were stirred on a magnetic stirrer at room temperature to form a homogeneous solution. 3 mg of potassium permanganate was added to the above solution within 1 h to avoid the rise of temperature of the mixture and the suspension was continuously stirred at 35 °C for 12 h. Finally, the solution was oxidized by adding 5 mL 30% hydrogen peroxide to the above suspension. To remove the organic residues and other metal ions in the mixture as a result of reduction of potassium permanganate, the brown solid product was centrifuged at 2,000 rpm for 30 min at each time by soaking with deionized water and 5% hydrogen chloride for several times, then dried at 60 °C in a vacuum oven for 12 h. The obtained product was dispersed using ethanol and calcined at 150 °C for 4 h to reduce GO into rGO.

rGO-MnO₂ NWs were prepared using the same method as described above by adding 0.025 g of rGO to the above solution.

Evidently, rGO/MnO₂ NWs consist of more dislocations due to heterogeneous nucleation, as well as interfaces between the MnO₂ lattice and rGO nanosheets (Abdolhosseinzadeh et al. 2015). The dislocation density is 1.96 × 10¹⁵ m⁻² and 4.63 × 10¹⁵ m⁻² for the NWs of MnO₂ and rGO/MnO₂, respectively. The results are summarized in Table 1.

To gain more structural information, the Raman spectra of MnO₂ and rGO/MnO₂ NWs are shown in Figure 1(b). Three core peaks at 506 cm⁻¹, 565 cm⁻¹, and 648 cm⁻¹ are identified for the MnO₂ NWs. The Raman band at 648 cm⁻¹ is due to the symmetric ν₂ (Mn–O) stretching vibration of MnO₆ groups, and the band at 565 cm⁻¹ is attributed to the ν₃ (Mn–O) stretching vibration in the basal plane of MnO₆ sheets (Li et al. 2012). The low-frequency Raman band at 506 cm⁻¹ refers to an external vibration that is due to the translational motion of the MnO₆ octahedra, thus revealing good crystallinity of the α-MnO₂. The rGO is characterized using the Raman spectrum by two specific peaks, G band at 1,599.4 cm⁻¹ which is a characteristic of sp²-hybridized C–C bonds in a two-dimensional hexagonal lattice, and D band at 1,345.8 cm⁻¹ corresponding to the defects in the graphene structure. As shown in Figure 1(b), the I_D/I_G ratio is reduced slightly from 0.99 for rGO to 0.97 for rGO/MnO₂ NWs, due to the presence of defects during hydrothermal reduction. In addition, the characteristic band observed at 572.2 cm⁻¹ in rGO/MnO₂ NWs belongs to A_g spectroscopic species originating from breathing vibrations of MnO₆ octahedral, suggesting the successful integration of MnO₂ into rGO (Yao et al. 2013).

The morphology of the rGO sheet is shown in the inset of Figure 2(B-a), revealing a crumpled structure after exfoliation (Kakaei & Hasanpour 2014). Therefore, from Raman and scanning electron microscopy studies, it is evident that MnO₂ NWs are wrapped with rGO sheets, which prevents the agglomeration of MnO₂ NWs. Figure 2(B-b) shows the rippled morphology of the rGO sheets, bundling MnO₂ NWs at lower magnification.

The catalytic activity of the products was performed in a 250 mL borosil glass beaker. Initially, the reactants consisted of a 10 μM RB5 compound with 100 mL H₂O and pH was set at 5 using sulfuric acid or a sodium hydroxide solution. Subsequently, a 20 mg catalyst MnO₂ or rGO/MnO₂ was added to the solution. After mixing 6 mL of 30 wt% hydrogen peroxide solutions, O₂ bubbles were produced simultaneously. The resulting suspension was stirred on a magnetic stirrer for 30 min to attain the adsorption/desorption equilibrium between dye RB5 and suspended solid particles. In addition, an rGO/MnO₂ stock solution was dispersed with probe sonication using an ultrasonic power of 120 W and a frequency of 20 + 2 kHz. The tip of the probe was 6 cm in length, and 1 cm in diameter. pH was again tuned to a desired value at a reaction temperature of 40 ± 2 °C. The ultrasonication facilitates a direct contact of the reaction matrix with the mechanical vibrations without significant loss of energy. Ultrasonic-combined Fenton experiments were carried out in a 50 mL polypropylene beaker at a regular time interval from 0 to 60 min. The samples designated as MnO₂, rGO/MnO₂, and US/rGO/MnO₂ were used for catalysis. After achieving the complete adsorption/desorption equilibrium state, the analytical samples (5 mL) were extracted from the solution at the regular time intervals. Subsequently, the catalyst was separated from the solution by centrifugation at 2,000 rpm for 10 min and the remaining solution was analyzed by an ultraviolet visible spectrophotometer to measure the absorbance of the dye solution.

To study the catalytic oxidation ability of the above α-MnO₂ NWs, the decomposition of RB5 with the assistance of hydrogen peroxide was preferred as a probe reaction. A blank cuvette filled with ultrapure water was used prior to each measurement.

The optical spectra of the RB5 solution based on the absorbance vs wavelength along with the decolorization rate at different reaction periods is shown in Figure 3(a)–3(c).

The intensity of the maximum absorption peak of the RB5 reduced gradually with time. It can also be noted that the maximum wavelength of absorption was changed from 600 nm to 596 nm, 590.5 nm, and 582.5 nm after reaction durations of 10 min, 20 min, and 60 min, respectively. The blue shifts of the absorption wavelength revealed the catalytic decay of the RB5 (Weng et al. 2013). Eventually, the unique absorption peak became wide and weak in intensity, indicating the decolorization of RB5.

Figure 3(a)–3(c) show the absorption curves of RB5 solution. The plot at t = 0 is the initial dye solution (100 mL, 20 mg L⁻¹) after stirring magnetically for 30 min to achieve adsorption/desorption equilibrium. Initially, a huge amount of dye is removed because of the larger generation of hydroxyl radicals accompanied by the fast disintegration of hydrogen peroxide. The concentration of dye in all samples decreases with increasing reaction time until the almost complete degradation of the dye.

The enhanced performance of rGO/MnO₂ towards the degradation of RB5 could be ascribed to the synergistic effect between MnO₂ and rGO in the presence of hydrogen peroxide. Graphene with 2D layers of sp²-bonded structures can provide strong chemical and mechanical interactions, as well as increased electron transport between MnO₂ NWs and the rGO matrix (Qu et al. 2014). The RB5 molecules are easily adsorbed to the surfaces of GO through π–π conjugation based on its giant π-conjugation frame and two-dimensional planar configuration until the adsorption–desorption equilibrium. This accounts for the high concentration of hydroxyl radicals on the surface of rGO/MnO₂ NWs compared to bare MnO₂. Also, the structural and morphological properties of rGO/MnO₂ NWs reveal the superior catalytic activity.

Firstly, based on the XRD results, the average crystallite size of rGO/MnO₂ NWs is smaller with more dislocation density compare to the α-MnO₂ NWs. As a result of the lower particle size, the surface area increases, resulting in the adsorption of more H₂O₂ and dye molecules onto the surface of rGO/MnO₂. It is also evident from the BET data that the rGO/MnO₂ NWs reveals a higher surface area than the pristine MnO₂ NWs. This promotes the amount of reactive ^•OH and O₂^•− free primitives attacking the dye molecules, leading to an enhancement in the degradation of RB5 molecules (Videla et al. 2015). Meanwhile, increased surface defects such as dislocations in the nanocomposite, as given in Table 1, play a crucial role in increasing the internal surface area of the catalyst, enhancing species diffusivity and chemical reactivity.

Secondly, it is evident from scanning electron microscopy analysis, that MnO₂ NWs are wrapped with rGO, which prevents the aggregation of MnO₂ NWs as well as serving as electron transfer channels to generate free radicals ^•OH, HOO^•, or O₂^•, which cause disintegration of RB5 molecules. As a result, the MnO₂/rGO NWs shows enhanced catalytic performance for degradation of RB5 dye.

As seen in Figure 3(d), after 60 min, the degradation efficiency in Fenton's process is 84.5%. However, during the same period, the ultrasonic-combined Fenton process results in about 95% decolorization. The results clearly indicate the significant enhancement in the ultrasonic-combined Fenton process compared with Fenton's alone could be due to the synergistic effects. The maximum color removal efficiency of RB5 in the present work is relatively more than the reported values, as shown in Table 2. rGO/MnO₂ NWs catalyst exhibits better performance in terms of low dosage and short reaction time and efficiency.

It is evident from Figure 4 that the intensity of the corresponding peaks increases consistently from RB5/MnO₂, RB5/rGO/MnO₂, to US/RB5/rGO/MnO_2, indicating the higher decomposition rate of the dye by the ultrasonic-combined Fenton process. Particularly, the selected peaks at m/z ratio value 78, 106, 143, 198 and 300 match very well with the organic by-products as given in Table 3.

Table 3

Possible degradation products of RB5 after catalytic reaction as determined using GC–MS

Intermediates formed during degradation of RB-5 .	Name of the intermediate .
	(4-ethylsulfonyl)phenyl)diazene
	Naphthalene
	Benzene
	Sodium 5-aminonaphthalene-1,6-disulfinate
	Sodium 5-aminonaphthalene-1-sulfinate
	Naphthalen-1-amine
	Phenyldiazene
	Sodium 1-[(4-sulfamoylphenyl)amino] ethanesulfonate

Intermediates formed during degradation of RB-5 .	Name of the intermediate .
	(4-ethylsulfonyl)phenyl)diazene
	Naphthalene
	Benzene
	Sodium 5-aminonaphthalene-1,6-disulfinate
	Sodium 5-aminonaphthalene-1-sulfinate
	Naphthalen-1-amine
	Phenyldiazene
	Sodium 1-[(4-sulfamoylphenyl)amino] ethanesulfonate

The various organic species after the disintegration of RB5 may have been formed through the following steps: (i) cleavage of the benzene ring; (ii) cleavage of the S–C bond between the aromatic ring and the sulfonate groups by the hydroxyl radicals attack; (iii) breaking of various N–C and C–C bonds of the chromophore compound, loss of sodium ions plus the gain of hydrogen; and (iv) –N = N– double bond split (Sharma & Roy 2015).

In order to determine the effect of other parameters on the decoloration of the pollutant, experiments were performed at constant pH = 5 for 60 min, using the ultrasonic-combined Fenton catalysis, with different concentrations of dye, rGO/MnO₂, and H₂O₂.

The amount of catalyst in the ultrasonic-combined Fenton reaction is another factor taken into consideration. Generally it is observed that the percentage degradation increases when increasing the amount of catalyst, while keeping other factors fixed. As the catalyst amount increases, a larger quantity of dye is adsorbed on the surface of the rGO/MnO₂ NWs. This increases the probability of a reaction between dye molecules and oxidizing species, thus causing an enhancement in the degradation percentage (Akpan & Hameed 2009). Figure 5(b) shows the affect of catalyst amount on the dye decolorization percentage.

Thus, the presence of excess H₂O₂ can lower the treatment efficiency of the Fenton reaction.

MnO₂ NWs and rGO/MnO₂ NWs were synthesized by a simple hydrothermal method, and catalytic performance was tested using RB5 as a model pollutant. rGO/MnO₂ NWs reveal superior catalytic performance towards the decomposition of the RB5, because the large surface area of sp²-bonded rGO structure provides good electron transfer channels and interface dislocations. Besides, as has been shown by X-ray diffraction and scanning electron microscopy studies, the structural and morphological properties of NWs, such as defect density and agglomeration, are well correlated with the catalytic performance. The rGO coating of MnO₂ NWs results in 20% enhancement in catalytic activity compared to pristine MnO₂ NWs towards degradation of RB5 dye with hydrogen peroxide based on the Fenton catalysis. The application of a coupled ultrasonic-combined Fenton process using an rGO/MnO₂ catalyst, further enhances the degradation rate of RB5 by 30% due to synergic effects. Additionally, the as-grown rGO/MnO₂ retained its catalytic stability for several cycles due to the wrapping of NWs with rGO. The various intermediate byproducts formed during the catalytic reaction were successfully detected with higher production rates in the ultrasonic-combined Fenton reaction compared to the Fenton reaction. Therefore, rGO/MnO₂ NWs exhibit good catalytic activity and can be used for environmental protection.

H. S. Nagaraja acknowledges the Government of India for financial support through the DST-SERB project (No. SB/S2/CMP-105/2013).