Sonochemical synthesis of mesoporous ZrFe₂O₅ and its application for degradation of recalcitrant pollutants

Abstract

In this paper, we have reported the sonochemical synthesis and characterization of zirconium ferrite (ZrFe₂O₅), and its use as a catalyst in advanced oxidation processes (AOPs) using decolorization/degradation of azo and non-azo dyes as model processes. The efficacy of ZrFe₂O₅ for dye decolorization has been compared with the conventional catalyst TiO₂ synthesized using sonochemical sol–gel method. Sonochemically synthesized ZrFe₂O₅ and TiO₂ have been characterized using PSD, XRD, FESEM, TEM, DRS, BET and TGA. ZrFe₂O₅ is revealed to have a lower band gap energy than TiO₂. However, the α-Fe₂O₃ (hematite) phase in ZrFe₂O₅ acts as a recombination center of photogenerated electrons and holes, which adversely affects the photo-activity of ZrFe₂O₅. In the context of degradation of recalcitrant pollutants, this adverse effect is compensated by Fenton activity of the α-Fe₂O₃ (hematite) phase which is stimulated by addition of H₂O₂. Sonochemically synthesized ZrFe₂O₅ also has high adsorption capacity for the textile dyes that assists their effective degradation through Fenton and photocatalytic mechanisms. The decolorization experiments have employed individual AOPs of sonolysis, photocatalysis, (heterogeneous) Fenton and various combinations of these as hybrid AOPs. Due to combined facets of photo- and Fenton-activity, ZrFe₂O₅ is a better catalyst for hybrid AOPs than TiO₂. Nonetheless, the Fenton-activity of ZrFe₂O₅ overwhelms the photo-activity in dye decolorization. The most efficient hybrid AOP is revealed to be sono-photo-Fenton, in which both photo- and Fenton- activities of ZrFe₂O₅ are utilized. Analysis of decolorization experiments has also provided interesting mechanistic insight into interactions and synergies between different competing mechanisms of hybrid AOPs.

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1. Introduction

Wastewater discharge from process industries contains numerous recalcitrant organic pollutants, which are not degradable with conventional biological treatment of anaerobic digestion. Effective degradation of these pollutants requires advanced oxidation processes (AOPs), which are based on the production of oxidizing radicals.^1–5 Among the various AOPs, photocatalysis has been studied extensively by several researchers to enhance the photocatalysis activity by modification of catalysts with transition metals (Fe-, Au-, Ag-, Mg-, Ni-, Au-)^6,7,8–13 and rare-earth elements (Ce-, Er-, Yb-)^14,15 as well as non-metallic elements (N- and F-)^16,17 and 2D layered materials (such as graphene, MoS₂ etc.).¹⁸ The more recent AOP of sonolysis has been used for degradation of wide variety of recalcitrant pollutants.^6,7,19–21 In this process, the oxidizing radicals are produced through transient cavitation, which is growth and implosive collapse of tiny gas/vapor bubbles that create intense energy concentration on extremely small spatial and temporal scale.²² Sonolysis has also been applied in presence of catalyst possessing adsorption capacity and/or photo-activity and/or Fenton activity. The processes combining these techniques are called sonocatalysis, sono-photocatalysis and sono-photo-Fenton process, respectively.^22–27 These hybrid AOPs have been reported to give enhanced degradation of the recalcitrant pollutants than individual sonolysis.

In this study, we have reported sonochemical synthesis and characterization of zirconium ferrite (ZrFe₂O₅). Synthesis of metal ferrites (e.g. Zn and Mn-ferrite) through sonochemical route has been reported by earlier authors.^28–30 Some distinct merits of sonochemical route for ferrite synthesis are: one-pot synthesis, mild operating conditions, low energy requirement, and use of safe chemicals.^31,32 This catalyst has been used with sonolysis for degradation/mineralization of recalcitrant pollutants such as textile dyes. ZrFe₂O₅ has been found to possess both photo activity and Fenton activity; and hence, we have also explored the additional contributions of an external UV-source and H₂O₂ addition on enhancement in degradation/mineralization of azo and non-azo textile dyes. The efficacy of ZrFe₂O₅ in degradation/mineralization of the textile dyes has been compared with conventional photocatalyst of TiO₂. The results of experiments with sonication have been compared with experiments with mechanical agitation. Combination of these techniques gives the hybrid AOPs of sonocatalysis, photocatalysis, sono-photocatalysis and heterogeneous sono-Fenton and sono-photo-Fenton. Another facet of this study is investigation into the physical mechanism of these hybrid AOPs and identification of the relative roles of individual phenomena of adsorption, Fenton reaction, photocatalysis and physical/chemical effects of transient cavitation. Thus, this study not only reports synthesis of metal ferrite catalyst, which can be coupled with sonolysis for the hybrid AOPs mentioned earlier, but also gives a mechanistic insight into these hybrid AOPs. To the best of our knowledge, this paper is the first report on sonochemical synthesis of ZrFe₂O₅ with dual (photo- and Fenton-) activity, and its application for degradation of the textile dyes (azo and non-azo).

2. Experimental section

2.1 Materials

The following chemicals have been used for the present study: titanium(IV) isopropoxide (Alfa Aesar), zirconium oxychloride (Merck), glacial acetic acid (Merck) and ammonium solution (30%, Merck), hydrogen peroxide (30%, Merck), ethanol (Merck) and iron(II) acetate (Sigma-Aldrich), ethylenediaminetetraacetic acid (Merck), benzoquinone (Merck), isopropyl alcohol (Merck). The model dye of Methylene Blue (C₁₆H₁₈N₃SCl) was purchased from Loba Chemie, while Acid Red B or Acid Red 14 (C₂₀H₁₂N₂Na₂O₇S₂) was procured from A.B. Chemicals and Instrument. Dye solutions in all experiments were prepared using ultrapure water (≥18 MΩ cm resistivity at 25 °C).

2.2 Synthesis of ZrFe₂O₅ and TiO₂ particles

Protocol for sonochemical synthesis of Zr-ferrite (ZrFe₂O₅). In this protocol, an aqueous solution of 3.223 g (0.2 M) of zirconium oxychloride and 3.479 g (0.4 M) of iron(II) acetate was prepared in 50 mL of water. The molar ratio of zirconium chloride to iron acetate in this solution was 1 : 2. The pH of the solution was maintained at 6.5 using 30% ammonium solution. The reaction mixture was sonicated for 2 h with 5 s ON and 5 s OFF pulse mode. The temperature of the reaction mixture during sonication was maintained at 30 °C using an ice bath. 2 h of sonication of this mixture resulted in brown colored solid suspension. The solid phase in the suspension was separated by centrifugation at 5000 rpm for 30 min (Remi Equipment, model: R-8C). Solid particles were further dried in a hot air oven (Navyug, model: 101) at 100 °C for 12 h. The solid powder obtained after drying was ground and calcined at different temperatures in the range of 500° to 900 °C.

Protocol for sonochemical synthesis of TiO₂. An ultrasound-assisted sol–gel method was used for synthesis of TiO₂ particles. In this protocol, 2 mL of titanium (IV) isopropoxide was added to 30 mL of ethanol and 1 mL of acetic acid mixture. The mixture was then sonicated for 10 min using 20 kHz ultrasonic probe with 40% amplitude (Sonics, model: VCX500, 500 W), which corresponded to theoretical power input of 200 W. Actual power input to the system and the pressure amplitude of the ultrasound waves generated by the probe was determined as 1.5 bar using calorimetric technique.²⁶ After 10 min sonication of the mixture, 30 mL of water was added to the reaction mixture followed by sonication for another 30 min, which resulted in gel formation. The gel was dried in a hot air oven (Navyug, model: 101) at 100 °C for 12 h. White powder obtained after drying was calcined at 300 °C and 500 °C to obtain TiO₂ in anatase form.

2.3 Assessment of catalytic activity of ZrFe₂O₅ and TiO₂ using dye decolorization

The catalytic activity of the synthesized TiO₂ and ZrFe₂O₅ was assessed using protocols of sonocatalysis, photocatalysis, sono-photocatalysis and (heterogeneous) sono-Fenton. Decolorization/degradation of textile dyes was used as the model reaction. Two textile dyes, viz. Acid Red B (an azo dye, abbreviated hereafter as ARB), and Methylene Blue (non-azo dye, abbreviated hereafter as MB) were chosen as model recalcitrant organic pollutants. All experiments were carried out using 50 mL dye solutions with initial concentration of 20 ppm (corresponding molar concentration of 0.04 mM for ARB and 0.063 mM for MB). In each experiment, 25 mg of catalyst was added to the dye solution prior to experiment. All the experiments were conducted at neutral pH.

Adsorption of dye onto catalyst. Both catalysts, viz. TiO₂ and ZrFe₂O₅ show tendency to adsorb dye molecules. The adsorbed dye molecules may be mistakenly counted as degraded molecules. In order to isolate this artifact, the adsorptive capacity of TiO₂ and ZrFe₂O₅ for both model dyes was assessed prior to main experiments. The protocol followed was as follows: 25 mg of catalyst was added in 50 mL of dye solution with initial concentration of 20 ppm. The progress of adsorption study was evaluated by withdrawing 1 mL sample of the dye solution at regular time interval. The residual concentration of the dyes in these samples was determined using UV-visible spectrophotometer.

Decolorization experiments. Ultrasound-assisted experiments (viz. protocols of sonocatalysis, sono-Fenton, sono-photocatalysis and sono-photo-Fenton) were carried out using an ultrasound bath (Jeio Tech, Korea, model: UC-10), which operates at a frequency of 40 kHz and a theoretical power of 200 W. For the photo-assisted experiments (viz. protocols of sono-photocatalysis, sono-photo-Fenton and photocatalysis with mechanical stirring (MS)), two UV sources, viz. a UVA lamp (11 W, Phillips) having the light emission in the range of 300–400 nm with maximum peak emission at 365 nm and a UVC lamp (11 W, Phillips) having the light emission in the range of 200–300 nm with maximum peak emission at 254 nm, were used. The experiments were conducted with either a single UV source (UVA) or a combination of two UV sources (UVA + UVC). The UV source was placed above the reaction beaker at a distance of 14 cm from the surface of dye solution in the beaker (for greater details along with schematic of the experimental set up, we refer reader to our earlier paper²⁶).

In ultrasound-assisted experiments, the reaction beaker was placed exactly in the center of the bath. The UVA lamp was switched on for 15 min before the experiment so that the diffusion of UV light in the reaction solution is uniform. During the reaction, the temperature of the dye solution was maintained at 25 ± 1 °C. In ultrasound-assisted experiments, the temperature of the reaction mixture increases during sonication. This temperature rise was controlled by circulating cooling water in the bath using a temperature controlled bath (Amkette Analytics, model: WB2000). The decolorization reaction was monitored by withdrawing aliquots of dye solution at regular intervals. These aliquots were centrifuged at 5000 rpm (Remi Equipment, model: R-8C) to separate the solid catalyst particles and the residual dye concentration in these samples was analyzed at 512 nm (for ARB dye) and 663 nm (for MB dye) using UV-visible spectrophotometer (Perkin Elmer, model: Lambda 35). In addition, the removal of total organic carbon (TOC) in the dye solution with mineralization was analyzed using a TOC analyzer (O-I-Analytical, model: Aurora 1030).

In view of experimental results of adsorption experiments (described in the next section), ZrFe₂O₅ catalyst was added to the ARB dye solution just before the commencement of main treatment. For consistency, the same protocol was followed for the other dye of MB as well. On the basis of results of characterizations of catalysts, the decolorization experiments were devised in 7 different categories on the basis of the prevalent mechanism of decolorization. Each of these categories used either TiO₂ or ZrFe₂O₅ as the catalyst. In view of the Fenton active α-Fe₂O₃ phase present in ZrFe₂O₅ (as described in greater detail in the next section), the experiments using ZrFe₂O₅ were also carried out with external addition of H₂O₂ (7.83 mM). Moreover, for experimental categories employing ultrasound, the saturation level of the liquid medium was also varied. In one category, liquid medium saturated with dissolved oxygen (9 ppm) was used, while in another category, the dissolved oxygen content of the medium was lowered to 2 ppm by application of vacuum and heating. With permutation–combination of different techniques, the following experimental protocols employing either individual AOP or hybrid AOP have been devised.

Individual AOPs. Sonolysis, photocatalysis (with either TiO₂ or ZrFe₂O₅) and heterogeneous Fenton (with ZrFe₂O₅ + H₂O₂).
Hybrid AOPs. Sonocatalysis (with either TiO₂ or ZrFe₂O₅), sono-photocatalysis (with either TiO₂ or ZrFe₂O₅ and UV source), sono-Fenton (with ZrFe₂O₅ + H₂O₂) and sono-photo-Fenton (with ZrFe₂O₅ + H₂O₂ + UV source).

3. Characterization of ZrFe₂O₅ and TiO₂ particles: results and discussion

3.1 Particle size distribution (PSD) analysis

The particles size distribution of ZrFe₂O₅ and TiO₂ powder was performed using Delsa™ Nano C (Beckman Coulter, model: A53878) particle size analyzer. The result of particle size distribution is shown in Fig. 1. The particle distribution of synthesized catalysts was as follows:

Fig. 1 Particle size distribution of Zr-ferrite (ZrFe₂O₅) and TiO₂ powders.

TiO₂: D (10%): 308.60 nm, D (50%): 692.80 nm and D (90%): 1736.50 nm.

ZrFe₂O₅: D (10%): 159.60 nm, D (50%): 272.60 nm and D (90%): 560.40 nm.

3.2 X-Ray diffraction (XRD) analysis

The structural characteristics of the catalysts were determined using X-ray powder diffractometer (Bruker, model: D8 Advance) with monochromatic Cu-K_α (λ = 1.5406 Å) radiation. Fig. 2 and 3 show the XRD spectrum of the catalyst particles in the range 2θ = 10–80°. The strong peak in Fig. 2 at 2θ = 25.4° corresponds to the predominant (101) anatase phase of TiO₂.²⁴ The other peaks corresponding to lattice plane (004), (200), (105), (211), (204), (311), (112) and (103) also represented anatase phase of TiO₂.^13,33,34 As seen from Fig. 2, the presence of above peaks in the XRD spectrum of TiO₂ prior to calcination is an evidence that titanium dioxide was formed during the sonochemical treatment. Sonochemically synthesized TiO₂ has anatase phase, which remains unaltered even after calcination of the particles at a temperature of 500 °C.

Fig. 2 Powder X-ray diffraction results of TiO₂ particles.

Fig. 3 Powder X-ray diffraction results of ZrFe₂O₅ particles calcined at different temperatures.

In case of ZrFe₂O₅, the solid particles obtained after sonochemical treatment did not show any peak corresponding spinel structure. This essentially means that synthesis of ZrFe₂O₅ is not complete after sonochemical treatment, and external calcination of the solid product is essential for formation of ferrite. As seen in Fig. 3A, calcination of solid product from sonochemical treatment did not yield ferrite till calcination temperature of 600 °C. Above this threshold temperature, peaks corresponding to the ferrite phase appeared in the XRD spectrum of calcined solids. The samples calcined at 700 °C showed peaks at (220), (311) and (440) along with α-Fe₂O₃ phase at (104) and tetragonal ZrO₂ at (022). The diffractograms of samples calcined at 800 and 900 °C (shown in Fig. 3B) showed high intensity peaks at (111), (220), (311), (422), (440) and (533), which correspond to the spinel structure of ferrite particles.^14,35 Some peaks at (012), (104), (113), (024) and (300) were also seen in Fig. 3B, which correspond to the hematite phase (α-Fe₂O₃).^36,37 Solids calcined at 900 °C also showed the presence of monoclinic phase (corresponding to peak at (−111, 111)) and tetragonal phase (corresponding to peak at (022, 140)) of ZrO₂.^37,38 Reduction in the intensity of peak at (220) for solids calcined at 900 °C (as compared to 800 °C) could be attributed to agglomeration of particles (as observed in our previous study by Goswami et al.³²). The increase in intensity of peak (104) for solids calcined at 900 °C could be attributed to the formation of α-Fe₂O₃ phase. It is noteworthy that this phase has Fenton activity, and this forms the basis of the experimental category with external H₂O₂ addition, as noted earlier.

3.3 FESEM and TEM analysis

The surface morphologies of the prepared ZrFe₂O₅ and TiO₂ particles were performed using Field Emission Scanning Electron Microscopy (Carl Zeiss GmbH, model: SIGMA VP, Germany) and Transmission Electron Microscope (JEOL, model: JEM 2100, USA). Fig. 4 and 5 represent the FESEM and TEM images of as-synthesized TiO₂ and ZrFe₂O₅ particles. It could be seen from these pictures that both ZrFe₂O₅ and TiO₂ particles are isotropic in shape and uniform in terms of size distribution.

Fig. 4 FESEM images for surface morphology of (A) ZrFe₂O₅ and (B) TiO₂ at 30k×.

Fig. 5 TEM images of sonochemically synthesized (A) ZrFe₂O₅ and (B) TiO₂.

3.4 Diffuse reflectance spectra (DRS) analysis

Fig. 6 shows the diffuse reflectance spectra (DRS) of ZrFe₂O₅ and TiO₂ powder. The band gap energies for both the catalysts have been determined by plotting [F(R)hv]^0.5 against photon energy hv (eV); and extrapolating the linear portion of [F(R)hv]^0.5 to 0 (zero). Here, F(R) is the Kubelka–Munk function, which is defined as:^22,39 F(R) = (1 − R)²/2R, where R is the reflectance. The DRS results showed that sonochemically synthesized ZrFe₂O₅ has lower band gap energy than TiO₂. With this analysis, the band gap energies for ZrFe₂O₅ and TiO₂ have been estimated as 2.05 and 3.1 eV, respectively.

Fig. 6 (A) UV-visible diffuse reflectance spectra of ZrFe₂O₅ and TiO₂, and (B) estimated band gap energy of ZrFe₂O₅ and TiO₂.

3.5 BET surface area analysis

The surface area of ZrFe₂O₅ and TiO₂ was analyzed using a fully automated BET surface area analyzer (Beckman Coulter, model: SA3100) by nitrogen multilayer adsorption measured as a function of relative pressure (P_s/P₀) in the range of 0.05–0.2. The N₂ adsorption–desorption isotherms and the corresponding pore size distribution curve (inset figure) for ZrFe₂O₅ and TiO₂ are shown in Fig. 7. The pore size distribution and pore volume analysis were determined from desorption data using Barrett–Joyner–Halenda (BJH) method. As per this analysis, the predominant pore diameter for ZrFe₂O₅ was in the range of 8–11 nm (shown in inset of Fig. 7A) and 5–7 nm for TiO₂ (shown in inset of Fig. 7B). This range of pore diameter indicates the mesoporous structure of the material.^40–42 Also, the adsorption–desorption isotherms showed a large hysteresis loop of type VI characteristic that indicates the presence of mesoporosity due to the capillary condensation in the open mesoporous channels.^43,44 The BET surface areas for ZrFe₂O₅ and TiO₂ were 97.13 and 152.39 m² g⁻¹, respectively; while the pore volumes of ZrFe₂O₅ and TiO₂ were 0.2808 and 0.3465 cm³ g⁻¹, respectively.

Fig. 7 Results of BET surface area analysis. N₂ adsorption–desorption isotherms of catalysts and the corresponding BJH pore size distribution curves (inset). (A) ZrFe₂O₅ and (B) TiO₂.

3.6 Thermogravimetric analysis (TGA)

In order to determine the thermal characteristics of ZrFe₂O₅ and TiO₂, TG/DTG analysis was performed using thermo-gravimetric analyzer (TG 209 F1 Libra, NETZSCH) under N₂ environment with a flow rate of 40 mL min⁻¹. The analyses were conducted using heating rates of 10 °C min⁻¹ and the temperature range from 30 °C to 900 °C. The thermo-gravimetric analysis results of ZrFe₂O₅ calcined at 800 °C and TiO₂ calcined at 500 °C are shown in Fig. 8. The TGA results showed two stages of weight loss. The major weight loss of 2.20% and 0.41% for TiO₂ and ZrFe₂O₅, respectively, was observed from room temperature to 150 °C, which corresponds to dehydration of physically adsorbed water. The second and minor weight loss of 1.8% and 0.04% for TiO₂ and ZrFe₂O₅, respectively, was observed in the range of 150–900 °C. This could be attributed to the decomposition of residual contaminants from the reaction mixture present in the solid products after synthesis.

Fig. 8 Thermo-gravimetric analysis (TGA) results of (A) ZrFe₂O₅ and (B) TiO₂.

4. Dye decolorization: results and discussion

Before presenting the results of this study and related discussion, we give a brief preamble about the chemical mechanism of the degradation of ARB and MB dyes, which has been well documented in published literature. Gao et al.⁴⁵ and Xia et al.⁴⁶ have postulated detail of the degradation pathways of ARB dye with identification of several intermediates during photocatalytic degradation. The degradation of ARB dye is initiated by attacking of ˙OH radicals produced through photocatalysis process. The main decolorization reactions occur during treatment are the breakage of the chromophoric azo (–N=N–) bonds, hydroxylation and oxidation. Some of the intermediates of ARB dye degradation that have been reported by Gao et al.⁴⁵ and Xia et al.⁴⁶ are 1,4-dihydroxynaphthalene, 1,2- and 1,4-naphthoquinone, 1-naphthol, 2-hydroxybenzoic acid, 1,2-benzene dicarboxylic acid, benzoic acid, carboxylic acids (maleic acid, oxalic acid, lactic acid, acetic acid and formic acid), ketones and aldehydes. Yu and Chuang;⁴⁷ Houas et al.⁴⁸ and Wang et al.⁴⁹ have studied the degradation of MB dye and have reported that the main chemical mechanism of degradation is due to hydroxylation/oxidation induced by ˙OH and HO₂˙ radicals formed by AOPs. The main degradation products of MB dye are C₁₄H₁₃N₂OS, C₁₃H₁₁N₂OS, C₁₂H₈NO₂S, and C₁₂H₉N₂OS.

4.1 Dye adsorption

The results of dye adsorption on ZrFe₂O₅ and TiO₂ catalyst particles are given in Fig. 9. As stated earlier, the BET surface area analysis revealed much higher surface area for TiO₂ than ZrFe₂O₅. The adsorption of MB dye on both catalysts was ∼5.5%, while the adsorption percentage of ARB dye was 6.6 and 19.4% on TiO₂ and ZrFe₂O₅, respectively. An explanation for higher adsorption of ARB on ZrFe₂O₅ could be given in terms of presence of α-Fe₂O₃ phase on as-synthesized ZrFe₂O₅. This phase helps to produce cationic sites around the particles of ZrFe₂O₅ after addition to water. ARB being an acidic dye, it always has the tendency for ionic bonding with the cationic sites of the particles. The attraction of anionic dye molecule to a cationic site on the ZrFe₂O₅ essentially results in higher extent of adsorption of ARB. In view of large adsorption of ARB dye, ZrFe₂O₅ particles were added to dye solution just before the treatment so as to reduce the artifacts due to dye adsorption.

Fig. 9 Adsorption study of ARB and MB dyes onto ZrFe₂O₅ and TiO₂ powders.

4.2 Decolorization experiments

The summary of the experimental results of dye decolorization is given in Table 1 (for the ARB dye) and Table 2 (for the MB dye). The time trends of dye decolorization in different experimental categories employing either individual or hybrid AOP are shown in Fig. 10 and 11 for ARB and MB dye, respectively. The TOC removal in experiments employing with both ARB and MB dye is shown in Fig. 12 (individual as well as hybrid AOPs). Tables 1 and 2 also depict the pseudo 1^st order kinetic constant fitted to the decolorization data. It should be noted, however, that the decolorization is non-uniform in time. Hence, the kinetic constant has been determined on the basis of decolorization obtained in first 30 min of treatment (although the total treatment time was 60 min), during which almost of the total decolorization was obtained. The degradation of the dye with the two catalysts can have different mechanisms on the basis of the properties of the catalysts. For TiO₂, the possible mechanisms could be adsorption and photocatalysis. For ZrFe₂O₅, the possible mechanisms are adsorption, photocatalysis and Fenton reactions due to the presence of α-Fe₂O₃ phase. The net decolorization obtained in any protocol is a manifestation of the above mechanisms that can occur simultaneously as shown in Scheme 1. Some peculiar trends of decolorization could be identified from Tables 1 and 2 and Fig. 10 and 11. Given below are these trends along with discussion of possible underlying mechanism.

Table 1 Summary of experimental results on decolorization/degradation of Acid Red B (ARB)^a

Experimental category	Decolorization of ARB (η%)
a Note: US – ultrasound assisted, MS – assisted with mechanical stirring, η – decolorization efficiency (%), k – pseudo 1^st order kinetic constant (s^−1) calculated on the basis of initial 30 min of decolorization reaction data.
1. Sonolysis (US)	13.74 ± 2.23	k = 3.94 × 10^−5

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	TiO2		ZrFe2O5		(ZrFe2O5 + H2O2)
	60 min	k (s^−1)	60 min	k (s^−1)	60 min	k (s^−1)
2. Fenton (MS)	—	—	—	—	60.19 ± 4.16	3.12 × 10^−4
3. Photocatalysis (MS + UVA)	55.38 ± 2.47	2.28 × 10^−4	32.80 ± 1.91	1.58 × 10^−4	—	—
4. US + Sat.	34.83 ± 0.99	1.34 × 10^−4	51.86 ± 1.35	2.28 × 10^−4	67.57 ± 0.48	6.14 × 10^−4
5. US + Unsat.	37.52 ± 1.23	1.59 × 10^−4	46.53 ± 1.66	1.56 × 10^−4	70.46 ± 0.67	4.25 × 10^−4
6. US + UVA + Sat.	80.58 ± 0.69	4.80 × 10^−4	59.10 ± 1.70	3.36 × 10^−4	90.54 ± 2.28	1.40 × 10^−3
7. US + UVA + Unsat.	78.15 ± 0.94	4.01 × 10^−4	53.73 ± 1.68	2.27 × 10^−4	82.07 ± 0.59	5.46 × 10^−4

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Table 2 Summary of experimental results on decolorization/degradation of Methylene Blue (MB)^a

Experimental category	Decolorization of MB (η%)
a Note: US – ultrasound assisted, MS – assisted with mechanical stirring, η – decolorization efficiency (%), k – pseudo 1^st order kinetic constant (s^−1) calculated on the basis of initial 30 min of decolorization reaction data.
1. Sonolysis (US)	8.35 ± 1.24	k = 2.39 × 10^−5

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	TiO2		ZrFe2O5		(ZrFe2O5 + H2O2)
	60 min	k (s^−1)	60 min	k (s^−1)	60 min	k (s^−1)
2. Fenton (MS)	—	—	—	—	40.76 ± 2.01	4.73 × 10^−4
3. Photocatalysis (MS + UVA)	34.07 ± 3.44	1.98 × 10^−4	31.30 ± 3.61	2.77 × 10^−4	—	—
4. US + Sat.	32.12 ± 2.83	2.16 × 10^−4	50.34 ± 1.35	4.65 × 10^−4	53.19 ± 1.51	5.35 × 10^−4
5. US + Unsat.	32.49 ± 2.82	1.16 × 10^−4	51.11 ± 1.16	4.68 × 10^−4	51.69 ± 2.95	7.18 × 10^−4
6. US + UVA + Sat.	60.96 ± 2.14	2.31 × 10^−4	57.78 ± 1.08	6.03 × 10^−4	87.62 ± 1.21	6.75 × 10^−4
7. US + UVA + Unsat.	49.45 ± 2.79	1.85 × 10^−4	49.30 ± 1.50	6.02 × 10^−4	62.17 ± 2.52	5.88 × 10^−4

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Fig. 10 Time history of decolorization of ARB with various operating conditions: (A) individual AOPs, (B) hybrid AOPs (sonocatalysis and sono-Fenton), and (C) hybrid AOPs (sono-photocatalysis and sono-photo-Fenton process). Note: US – ultrasound, MS – mechanical stirring, ZrF – zirconium ferrite.

Fig. 11 Time history of decolorization of MB with various operating conditions: (A) individual AOPs, (B) hybrid AOP (sonocatalysis and sono-Fenton), and (C) hybrid AOPs (sono-photocatalysis and sono-photo-Fenton process). Note: US – ultrasound, MS – mechanical stirring, ZrF – zirconium ferrite.

Fig. 12 Total organic carbon (TOC) removal after 60 min of treatment of ARB and MB dyes with various operating conditions: (A) individual AOPs, (B) hybrid AOPs (sonocatalysis and sono-Fenton), and (C) hybrid AOPs (sono-photocatalysis and sono-photo-Fenton process). Note: US – ultrasound, MS – mechanical stirring, ZrF – zirconium ferrite.

Scheme 1 Schematic depicting the decolorization/degradation mechanisms with TiO₂ and ZrFe₂O₅ catalysts.

(1) Least decolorization for both dyes is seen for the AOP of sonolysis. In this case, the dye decolorization occurs through the ˙OH radicals generated by cavitation bubbles. Due to rather low concentration of the dye solution, the probability of interaction between dye molecules and radicals is low, which is essentially reflected in lower decolorization. Comparing among the two dyes, sonolysis yields higher decolorization for the azo ARB dye. We attribute this result to the chemical structure of the ARB dye, in which the azo group (–N=N– linking the azo dye), which is susceptible to ˙OH radicals attack.

(2) ZrFe₂O₅ under Fenton treatment yields much higher decolorization for the ARB dye than MB dye. This result is a consequence of greater adsorption of the ARB dye on ZrFe₂O₅. Radicals generated through Fenton reaction of α-Fe₂O₃ phase in ferrite have high probability of interaction with the adsorbed dye molecules. Being extremely unstable, these radicals do not diffuse into the medium. Hence, the extent of adsorption of dye onto the catalyst surface is also a crucial mechanism in overall decolorization. Consequently, higher decolorization was seen for the ARB dye that has higher extent of adsorption.

(3) Under the individual AOP of photocatalysis, TiO₂ yields higher decolorization for both dyes than ZrFe₂O₅. This is an anomaly as the band gap for ZrFe₂O₅ is much lower than TiO₂, as revealed in the DRS analysis, and hence, the photoactivity of ZrFe₂O₅ is expected to be higher. We explain this anomaly as follows on the basis of a similar study by Xia and Yin on photocatalytic activity of core–shell structured α-Fe₂O₃/TiO₂ nanocomposites:⁵⁰ Under the UV light irradiation, electron–hole pairs are produced in the conduction and valence bands of ZrFe₂O₅ and as a result, a heterojunction is formed between ZrFe₂O₅ and α-Fe₂O₃. The α-Fe₂O₃ present in the ZrFe₂O₅ powders could act as a recombination center of the photogenerated electrons and holes. Also, α-Fe₂O₃ present in ZrFe₂O₅ may absorb most of the electromagnetic irradiation and promote favorable conditions for the presence of hole–electron recombination centers, so that ZrFe₂O₅ catalyst exhibits lower photocatalytic activity even after having the lower band gap energy than the TiO₂.

The temperature of calcination determines the concentration of α-Fe₂O₃ phase. The concentration of α-Fe₂O₃ phase in ZrFe₂O₅ grows with temperature of calcination. Therefore, additional decolorization experiments were performed using ZrFe₂O₅ particles calcined at lower temperature of 650 °C, with techniques of photocatalysis (MS + UVA) and Fenton process (MS + Fe²⁺ + H₂O₂). These results were compared with decolorization obtained with ZrFe₂O₅ calcined at 900 °C. The photo-activity of the two catalysts was revealed to be almost equal. However, the Fenton activity of ZrFe₂O₅ calcined at 650 °C was significantly lower. The details of these experiments are given in ESI† provided with the manuscript.

It should be noted that photo-chemically generated radicals are utilized mainly for the decolorization of adsorbed dye molecules. Thus, the extent of dye adsorption on catalyst surface also contributes to overall mechanism of decolorization. Although ZrFe₂O₅ is a stronger adsorbent than TiO₂, the net decolorization with ZrFe₂O₅ is lower as a consequence of its inferior photocatalytic activity.

(4) The saturation level of the medium has no effect on dye decolorization for both dyes under the protocol of sonocatalysis, as almost similar decolorization has been obtained for both dyes, with saturated as well as unsaturated medium.

(5) Under the protocol of sonocatalysis (in which the catalyst is added to the dye solution but no external UV source is provided), an interesting manifestation of Fenton active α-Fe₂O₃ phase of ZrFe₂O₅ is seen. In this case, the role of catalyst is merely that of an adsorbent that creates higher local concentration of dye on its surface. Although the adsorption of MB dye on both TiO₂ & ZrFe₂O₅ is similar, higher decolorization is seen for ZrFe₂O₅. We attribute this result to Fenton-reactions occurring in the solution due to the H₂O₂ generated in situ by the recombination of ˙OH radicals generated from transient cavitation. A similar trend is also seen for the ARB dye, however, contribution of higher adsorption of the ARB dye to higher extent of decolorization cannot be ruled out. Addition of external H₂O₂ to the medium further enhances the Fenton reactions and extent of dye decolorization. We would like to point out that there are many possible routes through which H₂O₂ can degrade the dyes. One mechanism is evaporation of H₂O₂ in cavitation bubbles and decomposition of this H₂O₂ to produce higher ˙OH radicals during transient collapse of bubbles, which can degrade the dye. Alternately, H₂O₂ itself is an oxidant which can degrade the dyes. However, results of our previous study have clearly distinguished the relative contributions of these mechanisms in comparison to the Fenton-mechanism.⁵¹ Our previous results confirm that contribution of first two mechanisms is relatively trivial and the oxidation of the textile dye in presence of H₂O₂ is predominantly affected through the Fenton mechanism. Therefore, rise in extent of decolorization of dyes with addition of H₂O₂ is attributed to Fenton type reactions induced by the α-Fe₂O₃ (or hematite phase) of the ferrite catalyst.

The contribution of adsorption in overall decolorization is more pronounced in this protocol. Significantly higher decolorization with external addition of H₂O₂ is seen for the ARB dye than MB dye. Due to higher adsorption, the probability of interaction of the ARB dye molecules with the ˙OH radicals generated through heterogeneous Fenton reaction is higher, which essentially results in greater decolorization.

(6) Under the protocol of sono-photocatalysis, where an external UV source is employed, marked enhancement in decolorization is seen. TiO₂ yields high decolorization (∼80%) for ARB, while relatively lesser decolorization is seen for MB. The decolorization for the ZrFe₂O₅ catalyst is relatively lesser as a result of reduced photoactivity due to presence of α-Fe₂O₃ phase. However, with addition of H₂O₂ initiates the Fenton activity of the α-Fe₂O₃ phase of the catalyst to give the hybrid AOP of sono-photo-Fenton, and the extent of decolorization shows marked rise.

In the protocols of sono-photocatalysis and sono-photo-Fenton, the negative effect of unsaturation of the medium is evident. For both dyes, the decolorization reduces by ∼10–15% with unsaturation. We attribute this result to intensification of the transient cavitation with unsaturation of the medium.⁵² As a result, the intensity of the shock waves generated by bubbles also increases these shock waves are responsible for desorption of the dye molecules from the surface of ZrFe₂O₅.²² This phenomenon reduces the probability of interaction of dye molecules with ˙OH radicals generated either through heterogeneous Fenton reaction between H₂O₂ and α-Fe₂O₃ phase of ZrFe₂O₅ or through the photocatalysis.

The reduction in total organic carbon (TOC) of the reaction solution with individual as well as hybrid AOPs employed in this study is depicted in Fig. 12. The typical trends in TOC removal are essentially same as that for the decolorization. Among the individual AOPs, highest TOC removal is seen for sono-Fenton (US + ZrFe₂O₅ + H₂O₂), while among the hybrid AOPs, highest TOC removal is seen for sono-photo-Fenton with ZrFe₂O₅. If we compare the performance of two catalysts for TOC removal in individual and hybrid AOPs, we find that TiO₂ gives better performance in AOPs where external UV source was used (sono-photocatalysis). ZrFe₂O₅, on the other hand, gives better performance in protocols where H₂O₂ was added to the medium (viz. sono-Fenton or sono-photo-Fenton). The former result is a consequence of higher photo-activity of TiO₂ (which overwhelms the moderate adsorption capacity), while the latter result is a consequence of utilization both photo as well as Fenton activity of the ZrFe₂O₅ catalyst.

4.3 Effect of UV source for mineralization

Wu et al.⁵³ and Le et al.⁵⁴ have given a general mechanism of the photo-degradation and mineralization of textile dyes. As per hypothesis of Wu et al.⁵³ and Le et al.,⁵⁴ the dye molecule is excited in presence of UV light irradiation and injects/transfers an electron to the conduction band of the photocatalyst and get converted itself to a cationic dye radical. This radical has different fates in presence of a photocatalyst or Fenton reagent (Fe³⁺). In presence of photocatalyst (either ZrFe₂O₅ or TiO₂), the dye radical can generate cation radical species Dye˙⁺ and an electron in the conduction band.⁵⁴ The electron can combine with dissolved oxygen to generate superoxide anion radical (O₂˙⁻), while the Dye˙⁺ species can react with OH⁻ ions to generate ˙OH radicals. Both ˙OH and O₂˙⁻ species can cause degradation of the dye molecules leading to their mineralization. In presence of Fenton reagent, the dye cation radical species can react with Fe³⁺ to generate Fe²⁺,⁵³ which further reacts with H₂O₂ through Fenton reactions to generate ˙OH radical species, which can degrade the dye molecules into numerous intermediate species, finally leading to mineralization. In the context of present study, the former mechanism applies for the hybrid AOP sono-photocatalysis, while the latter mechanism applies for sono-photo-Fenton process.

In order to assess the effect of UV light sources, we have conducted experiments under with either UVA or combined with UVA + UVC light. Fig. 13 shows the result of this study. Comparison of the decolorization results with single UV source of UVA and combined source UVA + UVC revealed that extent of dye decolorization does not change appreciably with application of UVC source in addition of UVA. However, the extent of TOC removal shows marked enhancement with simultaneous application of UVC with UVA (∼40% enhancement for ARB dye and ∼65% enhancement for MB dye). This essentially means that UVC photo emission (with peak emission at 254 nm which assists photolysis of H₂O₂ to generate ˙OH radicals) mainly contributes to further degradation of the intermediates/byproducts of dye degradation, which results in higher mineralization of the organic carbon.

Fig. 13 (A) Time history of decolorization under hybrid AOP (sono-photo-Fenton reaction) assisted with different UV light sources. (B) Effect of UV irradiation sources for TOC removal with hybrid AOP (sono-photo-Fenton reaction).

4.4 Distinguishing among contributions of oxidizing species

The degradation of textile dye is effected by three oxidizing species, viz. h⁺ (hole), O₂˙⁻ (superoxide radical) and ˙OH (hydroxyl radical). These species are generated through different mechanisms in either individual or hybrid AOPs as discussed in previous sections.

In order to determine the individual contributions of these oxidizing species to degradation of dyes, experiments were conducted with radical scavengers. The experiments were performed in presence of 2 mM ethylenediaminetetraacetic acid (EDTA), 2 mM benzoquinone (BQ), and 5 mM isopropyl alcohol (i-PrOH) that act as the scavengers for h⁺, O₂˙⁻ and ˙OH, respectively. The results of decolorization/degradation of ARB and MB dyes in photo-catalysis process with different scavengers are shown in Fig. 14. The decolorization profiles with addition of scavengers have been compared against the decolorization profile for either (ZrFe₂O₅ + UVA) or (TiO₂ + UVA) with mechanical agitation. The experimental results for both dyes showed that the addition of EDTA in the reaction solution in presence of ZrFe₂O₅ or TiO₂ significantly lowers degradation efficiency of ARB (32.7% with ZrFe₂O₅ and 25% with TiO₂) and MB (32.5% with ZrFe₂O₅ and ∼34.5% with TiO₂). Addition of BQ and i-PrOH in the reaction mixture results in moderate reduction in degradation efficiency as follows: (1) MB + i-PrOH: 17.2% and 25.6% reduction, for ZrFe₂O₅ and TiO₂ respectively; (2) ARB + i-PrOH: 6% and 15.9% reduction, for ZrFe₂O₅ and TiO₂ respectively; (3) MB + BQ: 27.3% and 7.7% reduction, for ZrFe₂O₅ and TiO₂ respectively; and (4) ARB + i-PrOH: 17.2% and 25.6% reduction, for ZrFe₂O₅ and TiO₂ respectively. These numbers clearly indicate that the reduction in degradation with addition of either BQ or i-PrOH is not as marked as that for EDTA addition. Recently, Xiang et al.⁴¹ have studied the photo-catalysis process with Ag₃PO₄ for degradation of MB and reported that MB degradation is greatly affected in presence of EDTA as hole (h⁺) scavenger, which is the consequence of the present study of decolorization. In another study by Su et al.,⁴² it has been reported that hole (h⁺) does not play any important role in degradation pathway of Acid Orange-II dye, and the degradation occurs mainly through ˙OH radicals generated from photo-catalysis process on the surface of mesoporous Zn-ferrite (ZnFe₂O₄) nanoparticles. Moreover, Xiang et al.⁴¹ have not observed any effect of ˙OH radical in degradation of MB with graphene-modified Ag₃PO₄. This result is in contradiction to the results by Su et al.⁴² This essentially indicates that the predominant degradation pathway of dye is specific to the reaction conditions, and no general conclusions could be drawn. However, in the context of present study, the experimental results reported in Fig. 14 reveal that all oxidizing species, viz. h⁺, ˙OH and O₂˙⁻ contribute to degradation of ARB and MB dyes. On a relative basis, the significant contribution to oxidation is by h⁺ and O₂˙⁻ species.

Fig. 14 Photocatalytic activity of ZrFe₂O₅ and TiO₂ in presence of different scavengers. (A) ARB dye and (B) MB dye.

4.5 Synergy effect

As noted earlier, for any particular protocol, several mechanisms occur simultaneously which contribute to the overall decolorization. The positive/negative synergies between these mechanisms result in enhanced/reduced decolorization in any hybrid AOP. Fig. 15 depicts the synergies in the hybrid AOPs. In this section, we have attempted to determine the synergy between these mechanisms. The synergy effect in hybrid advanced oxidation processes (HAOPs) has been calculated with following formula:

Fig. 15 Synergism of hybrid AOPs: (A) synergistic effect for decolorization of ARB (azo dye), and (B) synergistic effect for decolorization of MB (non-azo dye). Note: while calculation of the synergies, the extent of adsorption of the dye onto the photocatalyst (after total reaction time of the advanced oxidation treatment) has been accounted.

The highest synergies are seen for both catalysts and both dyes in the hybrid AOP of sonocatalysis. As noted earlier, in sonocatalysis protocols, the catalyst mainly acts as an adsorbent that creates high localized concentration of dye molecules, which essentially increases the probability of interaction between dye molecules and oxidizing radicals generated through transient cavitation bubbles. Small negative synergies are seen for ZrFe₂O₅ catalyst in protocol of sono-photocatalysis and sono-photo-Fenton. In both of these protocols, the oxidizing radicals generated at the surface of catalyst either photo-chemically or through heterogeneous Fenton reaction (of α-Fe₂O₃ phase) interaction with dye molecules adsorbed on the catalyst surface. The acoustic waves generated by transient cavitation cause desorption of the dye molecules which hampers their interaction probability with radicals. This phenomenon renders negative synergy between individual mechanism of sonolysis, photocatalysis and Fenton reactions.

5. Conclusions

In this paper, we have reported sonochemical synthesis and characterization of zirconium ferrite (ZrFe₂O₅), which has potential application as catalyst in AOPs. The efficacy of ZrFe₂O₅ in degradation of the recalcitrant organic pollutants has been assessed with decolorization/degradation of azo and non-azo textile dyes as yardstick using various individual and hybrid AOPs. The results have been compared against the conventional photocatalyst of TiO₂. The characterization of ZrFe₂O₅ reveals that it has lower band gap energy than TiO₂. However, it also possesses α-Fe₂O₃ (hematite) phase. This phase has adverse effect on the photo-activity of the ZrFe₂O₅ due to which the effective photo-activity of ZrFe₂O₅ reduces despite the lower band gap energy. However, in the context of degradation of recalcitrant pollutants, this adverse effect is compensated by the Fenton activity of α-Fe₂O₃ (hematite) phase. Moreover, sonochemically synthesized ZrFe₂O₅ also has high adsorption capacity that promotes the Fenton- and photo-activity. As a result of these facets, ZrFe₂O₅ is revealed to be a better catalyst for hybrid AOPs than TiO₂. Experimental results of dye decolorization have revealed that Fenton-activity of ZrFe₂O₅ has higher contribution to decolorization than photo-activity. The most efficient protocol for dye decolorization is revealed to be sono-photo-Fenton, in which both photo- and Fenton- activities of ZrFe₂O₅ are utilized. The analysis of decolorization experiments has also provided interesting mechanistic insight into interactions between different competing mechanisms of hybrid AOPs, which could be useful for further research in hybrid advanced oxidation processes.

Acknowledgements

Authors acknowledge the analytical facilities provided by Central Instruments Facility (CIF), I.I.T. Guwahati. The use of XRD facility (procured through FIST grant no. SR/FST/ETII-028/2010 from Department of Science and Technology, Government of India) at Department of Chemical Engineering, I.I.T. Guwahati is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: The details experimental results of decolorization with ZrFe₂O₅ calcined at lower (650 °C) and higher temperature (900 °C) is given with this manuscript. See DOI: 10.1039/c5ra06148b

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