Efficient photocatalytic degradation of aniline blue under solar irradiation by ternary cobalt ferrite/graphitic carbon nitride/bentonite nanocomposite

The current research describes the synthesis, characterization and application of CoFe2O4/g-C3N4/bentonite as a novel nanocomposite for the efficient degradation of aniline blue under solar irradiation. Powder XRD, TIR, SEM, TEM, VSM and UV-DRS were used to describe the formation and morphology of the composite. The composite has been used as a heterogeneous photocatalyst to degrade aniline blue in the presence of H2O2. In the presence of H2O2 in solar radiation, it was possible to degrade 88.5% of 10 ppm aniline blue solution just in 50 min using 50 mg of the composite. The improvement in photodegradation rate in the existence of H2O2 was attributed to the advanced oxidation process (AOP) mechanism of photo-Fenton involving the production of reactive hydroxyl and perhydroxyl radicals. The degradation was found to follow first-order kinetics with high regression coefficient with elevated rate constant.


Introduction
Aniline blue (AB) is an acidic dye that belongs to the triphenylmethane class of dye (Egzar et al. 2013). This dye is readily soluble (Lorenc-Grabowska and Gryglewicz 2007) in water and is extensively used in different textile industries for dying of nylon, wool, silk and cotton for which its presence in the industrial effluents may contribute aquatic environmental contamination (Eykholt and Davenport 1998). Besides, aniline blue is exceptionally steady for which its occupancy time in the aquatic environment is extremely high (Pare et al. 2008). This dye is also identified as China blue, marine blue and soluble blue 3 M. Aniline blue has a characteristic of two benzene rings attached to the central carbon atom in addition to a p-quinoid group which acts as the chromophore along with -NH 2 and -NR 2 groups as auxochromes (Sirés et al. 2008). Usually, aniline blue is considered as the xenobiotic compounds, recalcitrant molecules and noxious substances to microorganisms, animals and plants. Due to the poor competence of conventional supervision, this dye sustains more in industrial wastewaters and dirt (Azmi et al.1998;Culp and Beland 1996).
Effluents from the textile industry containing different toxic organic dyes pose a serious threat to our aquatic environments (Daneshvar et al. 2003;Ledakowicz and Gonera 1999). These dyes have a very complex structure and generate carcinogenicity in the course of hydrolysis, oxidation and other chemical reactions (Ooi et al. 2012;Hayati and Mahmoodi 2012;Man et al. 2012). Different conventional techniques such as biological treatment, reverse osmosis and activated carbon adsorption are in force to eliminate such toxicity from the aquatic environment. However, in modern research advanced oxidation processes (AOPs) which account for photocatalysis (Rauf and Ashraf 2009;Liu et al. 2011) and Fenton's processes (Alnuaimi et al. 2007;Bouasla et al. 2010) are considered as the most encouraging method for organic dye degradation from the aquatic environment. The advanced oxidation process utilizes the in situ generations of hydroxyl and superoxide radicals as an oxidizing agent which initiate the sequence of a chemical reaction and smash down the complex molecules in less toxic components (Rauf and Ashraf 2012). Photocatalyst having band gap in the range of 1.1 -3.0 eV makes use of visible light to degrade organic dyes competently (Vijayaraghavan et al. 2016). Spinel metal ferrites with distinctive structural features and two or more cations along with imperfect oxygen sublattice along with a narrow band gap play a significant role in the photodegradation (Dom et al. 2011). Metal ferrites have recently received a lot of attention for their use as visible light photocatalysts for the degradation of organic dyes in water. One more benefit of using ferrites is their magnetic nature. Iron (III) oxides are especially much magnetic materials (Machala et al. 2011), and hence, ferrites are used alone as photocatalysts in the motive that they can be easily detached from the reaction medium easily (Ao et al. 2008). The Pbs quantum dots decorated ZnO/TiO 2 photocatalyst exhibit a superior aniline blue degradation efficiency and photocatalytic activity (Lee et al. 2018). In UV/solar radiation, 3d series transition metal ions doped in TiO 2 show enhanced photodegradation potential of aniline blue. (Devi et al. 2010). Different semiconducting nanomaterials like ZnO, ZnS and SnO 2 have been investigated for efficient degradation of aniline blue in visible light (Egzar et al. 2013). Even though literature assessment supports some efficient mode of degradation of aniline blue by different nanomaterials, the application of metal ferrite-based ternary composite is still unexplored in this category of effort. Considering this aspect, the present paper discusses the novel ternary cobalt ferrite-based nanocomposite with narrow band gap of 1.59 eV for efficient degradation of aniline blue in visible light radiation. The physical and optical properties were documented by FTIR, powdered XRD, VSM, SEM, TEM and UV-Vis-DRS techniques. O] and urea, including sodium hydroxide (NaOH). Sigma-Aldrich provides hydrogen peroxide (6%), bentonite and aniline blue. Without any further purification, all chemicals were utilized for synthesis. Cinnamon is collected from local market.

Preparation of cinnamon extract assisted cobalt ferrite
Cobalt ferrite (CoFe 2 O 4 ) nanoparticles were synthesized via self-combustion using aqueous extract of cinnamon followed by calcinations in muffle furnace (Deraz 2010;Kooti et al. 2013). 5 mM ferric nitrate [Fe (NO 3 ) 3 .9H 2 O] and 2.5 mM cobalt nitrate [Co (NO 3 ) 2 .6H 2 O] were added slowly in the aqueous cinnamon extract. This mixture (pH = 2) is heated at 80° C to get a gel like substance. The reddish-brown gel-like substance was again heated on a heater at 250 °C to decompose completely by self-combustion. The ample powder so obtained is finally calcined at 600 °C for 3 h to get fine crystals of cobalt ferrites.

Preparation of graphitic carbon nitride
Graphitic carbon nitride (g-C 3 N 4 ) was prepared by heat treatment of urea under ambient pressure in a muffle furnace (Isotemp Programmable Muffle Furnace 650-750 Series, Fisher Scientific) for 3 h at 600 o C to conclude the reaction. The resulting yellowish powder was washed with distilled water to eliminate any remaining matter adsorbed on it and lastly dried up at 80° C.

Fabrication of ternary composite
Two stages have been used to develop the ternary nanocomposite. First, a binary composite of nickel ferrite and graphitic nitride was developed, and this binary composite was then converted into a ternary composite by introducing bentonite. In a distinctive procedure, 100 mg of cobalt ferrite and 80 mg of graphitic carbon nitride mixed homogeneously by using mortar pestle for 30 min. The solid mixture is calcined at 400 0 C for 2 h. To achieve uniform dispersion, the binary mixture was dissolved in distilled water and sonicated for 30 min. Centrifugation was used to separate the solid binary mixture. In another experiment, 50 mg of bentonite was dissolved in 50 mL deionized water and magnetically stirred. The solid binary composite was introduced to this solution and stirred for 12 h. After that, centrifugation has been used to extract the ternary composite.

Study of photocatalytic degradation of aniline blue (AB)
The photocatalytic activity of the as-synthesized CoFe 2 O 4 /g-C 3 N 4 /bentonite ternary composite was investigated by degrading aniline blue (AB) under solar irradiation, which has an average solar insolation of 4.29 kwh/m2/day in this geographical area at a latitude of 24.81°N when compared to National Renewable Energy Laboratory (NREL) satellite data. In a typical experiment, 10 mg, 30 mg and 50 mg of composite was introduced into 60 mL of 10 ppm aniline blue solution with 1 mL of 10% hydrogen peroxide (H 2 O 2 ) and stirred in the absence of light for 30 min to reach adsorption-desorption equilibrium. Under constant stirring, the mixture was exposed to solar radiation. The degradation of the dye was monitored periodically with the help of spectrophotometer (Shimazu UV-1900i) by withdrawing 4 mL of the mixture and centrifuging immediately (5 min, 3000 rmp). The absorbance was recorded over a wavelength range of 200 to 800 nm. Equation (1) has been used to evaluate the catalytic degradation efficiency.
where A 0 is the absorbance of AB before degradation and A is the absorbance of AB after degradation.

FTIR analysis
Careful observation on the FTIR spectra of cobalt ferrite in Fig. 1(a) shows intense peaks at 455 cm −1 and 535 cm −1 corresponding to intrinsic stretching vibrations of Fe-O at the tetrahedral site and Co-O at the octahedral site (Das and Dhar 2020a, b;Topkaya et al. 2013). Co 2+ ions prefer to be in the octahedral region, whereas Fe 3+ ions prefer to be in both the octahedral and tetrahedral sites. Further observations show strong absorptions at 1104 cm −1 which is the same as phenolic hydroxyl group of flavonoids type of compound in the cinnamon extract (De Souza et al. 2018). The standard stretching modes of CN heterocycles were assigned to the different characteristic bands in the FTIR spectra of g-C 3 N 4 in the range 1200 to 1700 cm-1 ( Fig. 1(b)) which is in good agreement with the literature (Shi et al. 2015). The FTIR spectra of the ternary composite Fig. 1(c) assign all the characteristic band of individual components present in it. The sharp peaks in the range 460-532 cm −1 are due to M-O bond in the tetrahedral and octahedral sites. Distinct peak on 811 cm −1 signifies bending vibration of heptazine of g-C 3 N 4 . Separate peak at 1030 cm −1 is same as to the presence of bentonite in the composite. Three peaks in the range 1430-1620 correspond to heptazine derived repeating unit of g-C 3 N 4 . The broad band in the range 3188-3370 cm −1 is due to partial condensation and absorption of water molecule.

XRD analysis
The XRD bluepr int of CoFe 2 O 4 , g-C 3 N 4 and CoFe 2 O 4 /g-C 3 N 4 /bentonite is presented in Fig. 2(a, b and c). In Fig. 2 (100) and (002) (110), (210) and (144) planes as reported in literature (Hebbar et al. 2018) that is well matched, which confirms the composite formation. The average crystallite diameter (Dc) of CoFe 2 O 4 particles and CoFe 2 O 4 /g-C 3 N 4 / bentonite nanocomposite was deliberated to be 26.62 nm and 30.92 nm, respectively, by the Scherrer formula (Bunaciu et al. 2015). The increase in the value of crystallite size of the composite compared to cobalt ferrite confirms the encapsulation of the ferrite core by graphitic carbon nitride and bentonite.

VSM analysis
Vibrating sample magnetometers were used to implement magnetic studies upon this synthesized cobalt ferrite nanoparticles and ternary composites. Magnetic parameters such as remnant magnetization (Mr), saturation magnetization (Ms) and coercivity (Hc) are shown in Table 1. The hysteresis loop (Fig. 3) obtained indicates the ferromagnetic behaviors of the cobalt ferrite nanoparticles with saturation magnetization value 4.9348 emu / g. The abrupt drop in the saturation magnetization value of the ternary composite is due to incorporation of nonmagnetic graphitic carbon nitride and bentonite in the composite. Also some impurity phases may be present in the composite, which declines the magnetic property. Similar drop in Ms value is observed in some other metal ferrite composite with nonmagnetic components (Paul and Dhar 2020;Huang et al. 2015).

SEM analysis
SEM micrographs of cobalt ferrite, graphitic carbon nitride, bentonite and the ternary nanocomposite are shown in Fig. 4. The micrograph of the composite (F) shows that the semiconducting graphitic carbon nitride and bentonite segments are covered over cobalt ferrite consistently. The spherical encapsulated shape of the particle confirms the composite formation within the size range of 50 µm.

TEM analysis
The phase compositions of the ternary composite were studied using transmission electron microscopy. Figure 5 depicts a TEM representation of the composite. TEM picture of ternary composite in Fig. 5(A, B, C, D) indicates that the bentonite-and graphitic carbon nitride-encapsulated composite particles are almost nearly spherical and the average sizes of particles were found to be 49.29 nm. The crystalline nature of the nanocomposite was confirmed SAED analysis

UV-DRS analysis
The plot of reflectance versus wavelength of the component material and composite confirms that the absorption of light will take place in the solar radiation's visible spectrum. In addition, the measured band gap in the material and in the composite as obtained from the Tauc's plot reveals that the composite can efficiently degrade the dye in the solar radiation (Fig. 6).

Investigation of aniline blue (AB) degradation by nanocomposite
The photocatalytic performance of the ternary composite was assessed by degrading aniline blue (AB) in the presence of 10% H 2 O 2 in the acidic medium under solar irradiation.  The degradation of aniline blue (AB) with different composite amounts is shown in Fig. 7a, b and c. When 10 mg of composite is used along with 1 ml of 10 percent H 2 O 2 , 50% degradation of AB is achieved in 50 min. As soon as 30 mg and 50 mg composites are used concurrently in the same condition, the degradation percentage rises to 77.5% and 88.5%.

The kinetic studies
The degradation of aniline blue was found to follow first-order kinetics. The degradation of AB can be formulated as; ln(C 0 ∕C) = kt where 'C 0 ' is the initial concentration of AB and 'C' is the concentration of AB after certain time 't' with rate constant 'k' of the reaction. Since Beer-Lambert's law states that both absorbance and concentration are directly proportional, the concentration should be used to replace the absorbance. Plots of ln C 0 /C versus time (t) at 600 nm for three different composite quantities revealed that they were all linear as shown in Fig. 8(a, b and c).
From this linear plot, the calculated rate constants are found to be 0.0132, 0.0323 and 0.0443, respectively. In the kinetics plots, the regression correlation coefficient (R 2 ) variables are found to be 0.9198, 0.9840 and 0.9849, respectively. These values suggest that the reaction rate seems to be very moderate in the presence of H 2 O 2 . Also C/C 0 decreases exponentially with time, confirming that the degradation follows first-order kinetics.

Effect of pH
The catalytic performance of the composite was optimum at a pH 3.5. The performance was monitored up to a pH of 7, and it was observed that beyond this pH range the degradation percentage declines. This observation points to a faster generation of hydroxyl radicals at this pH, which promotes Fenton degradation.

Effect of catalyst amount
The photocatalytic degradation of aniline blue by ternary cobalt ferrite/graphitic carbon nitride/bentonite nanocomposite in the presence of H 2 O 2 was studied using different amounts of catalyst ranging from 10 to 50 mg. For a 50 mg catalyst, the degradation performance was obtained to be as significant as 88.5%. The catalytic performance decreases when the catalyst concentration is increased.

A plausible mechanistic approach to the degradation of Aniline Blue (AB) under solar irradiation in the presence of H 2 O 2
Hydrogen peroxide-assisted photodegradation of AB in solar radiation by cobalt ferrite composite having g-C 3 N 4 as one of the components may take place through photo-Fenton mechanism of advanced oxidation process (AOP). Here Fe (II) of cobalt ferrite and H 2 O 2 efficiently generate hydroxyl radical which degrade the dye successfully. Moreover, the inclusion of g-C 3 N 4 in the composite significantly reduces the composite's band gap and increases the composite's potential for charge transfer and also facilitates the generation of electron-hole pairs, resulting in hydroxyl radical generation (Mushtaqet al. 2020;Zhang et al. 2007;Sun et al. 2007). The reactions behind the degradation of aniline blue by photo-Fenton process can be depicted as follows (Das and Dhar 2020a, b) and is shown in Fig. 9.

Comparison with other Composite/ Materials for aniline blue degradation
Heterojunction nanocomposite dependent on cobalt ferrite has still not been thoroughly investigated, particularly in the area of toxic aniline blue degradation in aqueous solution. Some reported works are tabulated as under (Table 2).

Conclusion
In a stepwise protocol, a novel type of ternary nanocomposite CoFe 2 O 4 /g-C 3 N 4 /bentonite was productively fabricated. The composite was thoroughly examined by powder FTIR, XRD, VSM, SEM, TEM and UV-DRS studies. The composite's photodegradation ability to degrade aniline blue in the existence of H 2 O 2 in solar radiation is well documented through spectrochemical observations. The composite can degrade aniline blue up to the extent of 88.5% in 50 min. The cobalt ferrite-based nanocomposite for such a degradation strategy is not endorsed by any such literature. In conclusion, the current finding can be said to be novel and widely recognized at the industrial scale to mitigate aquatic contamination from a hazardous dye such as aniline blue. This finding expands further scope to extrapolate the current work on environmental contamination mitigation in the future.