Sonophotocatalytic treatment of Bismarck Brown G dye and real textile effluent using synthesized novel Fe(0)-doped TiO₂ catalyst

Abstract

Using the novel Fe(0)-TiO₂-doped catalyst, the degradation of Bismarck Brown G dye was compared by means of advanced oxidation processes, such as sonolysis (13 mm and 25 mm probe tip diameters), photolysis (UV light) and sonophotolysis. The effect of the initial dye solution pH, H₂O₂ concentration, gas bubbling (argon, oxygen, air and nitrogen), dye concentration and reaction volume were studied. Understanding the degradation mechanism from these studies, the Bismarck Brown G dye treatment was further intensified by catalytic treatments, such as commercial TiO₂, synthesized Fe(0) and synthesized Fe(0)-doped TiO₂ under sonolytic, photolytic and sonophotolytic irradiations. SEM, TEM, XRD and DRS characterization studies of the sonolytically synthesized catalyst shows that they were of uniform shape, nanoscale in size and had good absorption properties. Among the processes studied, sonophotocatalytic treatment of Bismarck Brown G dye in the presence of Fe(0)-doped TiO₂ showed the highest colour removal with the smallest amount of catalyst addition and for larger reaction volume. The practical applicability of the synergistic effects with real textile effluent signifies that the studied process is highly efficient for a safer environment.

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1. Introduction

The textile industry is one of the largest water consuming industries as there are different processes involved, which ultimately discharges wastewater with unconsumed dyes and other chemicals. Among textile dyes, azo dyes are well-known for carcinogenic organic substances, wherein the reductive cleavage of its azo linkage produces aromatic amines that lead to human liver cancer. The control and efficient removal of these dye pollutants from textile wastewater is an important measure of environmental protection.¹ Electrocoagulation, adsorption, biodegradation, chlorination and ozonation are the frequently used primary methods for the removal of dye compounds from wastewater.^2–4 However, these treatment processes are non-destructive as the pollutants are only transferred from one phase to another, thus requiring an additional method to achieve the complete the detoxification of the environment. Moreover, in general, the primary degradation products of azo dyes are aromatic amines, which are toxic, carcinogenic, and teratogenic.^5,6 Advanced oxidation processes (AOPs) emerge as an alternative that quickly, directly and non-selectively oxidizes a broad range of organic pollutants based on the generation of very reactive species for example, hydroxyl radicals (˙OH).^7,8

In recent years, the application of ultrasonic energy for an AOP has drawn attention toward solving the problems associated with conventional non-destructive wastewater treatment. Cavitation science and engineering is a field involving the application of ultrasonic waves to chemical processing despite its high cost of electrical consumption.⁹ Ultrasound (US) irradiation causes acoustic cavitation through which micro sized bubbles grow and collapse, causing intense local heating, high pressures, and very short bubble lifetimes; these transient, localized hot spots drive high-energy chemical reactions.¹⁰ The result of it is the splitting of water molecules into homogeneous radicals (H˙ and ˙OH) within or out of the bubbles through cavitation.^11–13

The efficiency of the AOP treatment methods depends on the rate of generation of free radicals and the degree of contact with the contaminants. When these two parameters are maximized, the treatment efficiency would attain its maximum. In this view, process integration is conceptually advantageous in wastewater treatment because it can eliminate the disadvantages associated with each individual process.^14,15 Although photocatalysis and sonolysis have been extensively employed individually for the degradation of several organic species in water, studies on their combined use (i.e. sonophotocatalysis) have been limited. The coupling of ultrasonic irradiation with other AOPs, such as TiO₂ photocatalysis, has produced a synergistic effect in those systems on the formation of the active species.^16,17 Researchers have used sonophotocatalysis in a variety of investigations, i.e. from water decontamination to direct pollutant degradation. This process provides an excellent opportunity to reduce reaction time and the amount of reagents used without the need for extreme physical conditions. Given its advantages, the sonophotocatalytic process has a futuristic application from an engineering and fundamental aspect in commercial applications.¹⁸ Several studies involving high frequency ultrasound have focused on the combinative or hybrid techniques (i.e., the methods of using ultrasonic irradiations in combination with other advanced oxidation methods and/or biological treatment) because such novel techniques have proven to be more advantageous than ultrasound alone in effectively degrading recalcitrant contaminants.

The mechanism of this sonophotocatalysis effect is attributed to (a) activation of the photocatalyst surface, (b) enhancement of the mass transport of organic compounds, and (c) aggregate breakage.¹⁹ The degradation of azo dye in an aqueous solution using zero-valent iron represents a new generation of environmental remediation technologies that could provide cost-effective solutions to some of the most challenging environmental cleanup problems.^20–22 The mineralization and recovery of catalysts employed are more desirable than mere decolourisation in view of environmental safety. Unfortunately, most discoloration methods are unable to efficiently remove total organic carbon (TOC).

In this work, commercial TiO₂, green synthesized Fe(0) and Fe(0)-doped TiO₂ has been used as catalysts for the sonophotocatalytic degradation of an azo dye, Bismarck Brown G (or Basic Brown 1). Various studies reveal that the addition of nanoparticles has enhanced the degradation of pollutants through the oxidation path.^23–27 When employing a catalyst, both oxidation and reduction of the pollutant take place (H˙ mediated reduction pathway, sonolysis of noble metal ions to form noble metal nanoparticles).^10,28 Initially, a series of experiments were carried out by varying initial dye solution pH, H₂O₂ concentration, gas bubbling (argon, nitrogen, oxygen and air), dye concentration and reaction volume to understand the mechanism of degradation of the dye under ultrasonic and ultraviolet light irradiations. With the analysis of these experimental results, the dye treatment was performed with the said catalyst, which could demonstrate that enhanced ˙OH mediated oxidation as well as H˙ mediated reduction play a significant role in the acceleration (sonophotolysis) of dye degradation. This study provides a way to increase the efficiency of sonophotocatalysis and also helps to understand the mechanism of degradation of the dye using the catalyst. Further, the sonophotocatalytic study is extended with the application of the synthesized catalysts with real textile effluent obtained from the textile industry after preliminary physical treatment.

2. Experimental

2.1 Materials

TiO₂ (Degussa P25, Germany) having a specific surface area of 57 m² g⁻¹ was used as a starting material to prepare Fe(0)-doped TiO₂. Bismarck Brown G (BBG) was obtained from Sigma-Aldrich and used as supplied. BBG is a certified biological stain, for microscopy, histology and cytology and also used in textile industries. High purity analytical reagents were used for the sample solution preparation: hydrochloric acid (Merck), sodium hydroxide (Merck), hydrogen peroxide solution (30% w/w, Merck), ferrous sulphate FeSO₄·7H₂O (Merck). All reagents used were of analytical grade, and the solutions were prepared using Millipore water.

2.2 Synthesis of sonocatalysts

2.2.1 Zero valent iron. The green synthesis of zero valent iron using polyphenols as the reducing agent has been illustrated by numerous research works.²⁹ In this study, the reaction of ferrous sulphate with green tea extract as the polyphenol source forms the basis for the preparation of zero valent iron. Green tea extract was brewed by heating green tea leaves to 80 °C followed by vacuum filtration. Green tea synthesized nanoscale zero-valent iron (ZVI) was then prepared by mixing 0.5 M FeSO₄·7H₂O and green tea in a 2 : 1 volume ratio, resulting in the final ZVI solution. Later, the solution was centrifuged for 10 min at 6000 RPM after which the remaining pellet was washed twice with deionised water. The resultant ZVI was then dried in a hot air oven for 24 h at 105 °C.²²

2.2.2 Zero valent iron doped TiO₂. ZVI doped TiO₂ catalyst was prepared by the sonication method. TiO₂ (P25 degussa) was mixed with the aqueous solution of FeSO₄·7H₂O (5 atomic weight%). The mixture was sonicated for 30 min to allow for the penetration of ZVI (Fe(0)) ions into the titanium dioxide crystal matrix. The supernatant liquid (water) is evaporated by heating at 105 °C over a period of 24 h.³⁰

2.2.3 Characterization of synthesized catalysts. The morphological characteristics of synthesized ZVI and TiO₂ doped ZVI were studied using scanning electron micrograph (SEM) and transmission electron micrograph (TEM). The images were obtained using SEM with an accelerating voltage of 3 kV. The magnification was adjusted so as to visualize the shape of ZVI, in addition to the effect of TiO₂ doping on ZVI. UV-visible diffuse reflectance spectra (DRS) for the catalysts, TiO₂, Fe(0) and Fe(0)-doped TiO₂ were recorded to understand the absorption capacity of the catalysts studied. The crystallite sizes of prepared catalysts were determined with the help of an X-ray diffractometer. X-ray diffractometer (XRD) data were recorded by the radiation source of Cu Kα (40 kV, 30 mA) and NaI as the detector. The powder samples were analyzed in 2θ range from 10° to 80°. The particle size (D) of ZVI and TiO₂ doped ZVI were found using the Debye–Scherrer formula,where, λ is the X-ray wavelength and β represents full-width half-maximum (FWHM) obtained at a corresponding 2θ angle.

2.3 Degradation study procedure

The experimental setup consists of a 100 ml glass reactor to hold the BBG dye solution with a water cooling jacket to keep the reactor contents at constant temperature (27 ± 0.5 °C). A sonication probe (SONICS Vibra-cell, VCX 500) with a titanium tip (13 and 25 mm in diameter), which emits ultrasound waves at 20 kHz and delivers a set power output of 100 W, is used for all the experiments. For photolytic experiments, 4 W UV lamps emitting light at 256 nm were used.

1000 ppm of the BBG dye stock solution was prepared by dissolving 1 g of dye in 1000 ml of Millipore water, and from the stock solution, the required concentration of BBG was prepared. The initial pH of the BBG solution was altered as 2, 3, 4, 5, 6, 7, 8, 9 and 10 by adding either 0.1 N hydrochloric acid solution or 0.1 N sodium hydroxide solution. For the gas bubbling study, the BBG solution was purged with various gases, such as oxygen, argon, nitrogen and air for a duration of 5 min in order to make the solution saturated with the respective gas nuclei prior to sonolysis or photolysis.³¹ The initial dye concentration was varied as 10, 50 and 100 ppm. The reaction volume variation study was also performed to identify the effectiveness of the sonolysis, photolysis and sonophotocatalysis processes so that the studied process could be employed for field applications. Three different jacketed reactors with capacities of 100, 500 and 1000 ml were used. The treatment of dye with catalysts was done by varying the load from 0.1 to 1 g. It is noteworthy to mention that each of the experiments were done thrice, and the mean of them has been reported.

Based on the experimental results of the model dye (i.e. BBG), efforts were made to employ the sonophotocatalytic process to real textile effluent treatment. For this, the textile effluent was collected from the textile industry after preliminary treatment. The physical and chemical characteristics of the collected textile effluent were studied as per standard methods (APHA 1998). The decolourization of BBG and the textile effluent were monitored every 15 min with intermittent sampling.

2.3.1 Analytical procedure. The optical absorption spectra of the BBG dye solutions were recorded by a JASCO UV-visible spectrophotometer with a quartz cuvette 10 mm in path length. The maximum peak for the absorption spectra of BBG dye is 447 nm. In order to quantify the degree of degradation, COD (closed reflux method as per standard methods: APHA 1998) analysis of the treated textile effluent was performed.

3. Results and discussion

3.1 Effect of pH

Decolourization of the BBG dye aqueous solution was carried out at various initial pH values (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0). Fig. 1 shows that the initial solution pH has a remarkable influence on the decolourization efficiency (BBG: 10 ppm, reaction volume: 100 ml, treatment time: 60 min.). The UV-vis spectral variation of BBG dye at different time intervals under ultrasound irradiation (pH 4, 13 mm probe) is also shown in the figure. The decolourization of the dye is highest when the initial pH of the aqueous solution is 4.0 and lowest at an initial pH of 10 for all conditions studied (13 mm, 25 mm and UV irradiation). Among all, the 25 mm probe has resulted in giving highest colour removal of 66% followed by the 13 mm probe, and it is lower for UV irradiation for all pH variations when compared to the US-treated dye solution. The factor that had influenced higher colour removal with US-treated dye is the ultrasonic power, which is set as 100 W, whereas in the case of UV, it is only 4 W. A sonication probe with a larger diameter (25 mm) has a larger irradiating area and provides better energy dissipation than a smaller diameter probe (13 mm). In addition, for the same power output, the ultrasonic intensity produced with a 25 mm probe would be higher than with a 13 mm probe. The number of cavitational events would be higher with larger areas of irradiating surfaces, leading to increased ˙OH radical production for the same conditions.³² This helps in effective treatment of the BB1 dye solution as the 25 mm probe makes it cavitationally more active volume for the given input power than the 13 mm probe. In acidic conditions, the lone pairs of electrons present on the nitrogen atoms of amino groups of BBG dye would be protonated. In addition, it was reported that under this state the oxidation reaction was highly favoured at lower pH values, and the oxidation reaction rate decreases at higher pH values.¹ It is observed that the decolourization rate had decreased with an increased initial solution pH (beyond pH 4) for all of the cases, which is due to the deprotonation of lone pairs of electrons present on the nitrogen atoms of amino groups.

Fig. 1 Decolourization of BBG dye solution with initial pH variation.

3.2 Effect of hydrogen peroxide dosage

The addition of hydrogen peroxide, which can act as an activator and strong oxidant having an oxidation potential of 1.78 eV, would result in the additional production of hydroxyl radicals upon ultrasonic and UV irradiation. The resulting hydroxyl radicals (oxidation potential: 2.80 eV) are the major cause of the increasing oxidation reaction. The effect of hydrogen peroxide concentration on the removal of the BBG is shown in Fig. 2. Experiments are conducted by varying H₂O₂ concentration from 300 to 2100 ppm (BBG: 10 ppm, pH: 6.04, reaction volume: 100 ml, treatment time: 60 min). When the concentration of H₂O₂ is increased from 300 to 900 ppm, the decolourization efficiency increased from 28 to 59% for the 13 mm probe and from 33 to 70% for the 25 mm probe. For UV treated aqueous solution, the decolourization efficiency varied from 15% to 42% from 300 to 2100 ppm, respectively. Maximum decolourization is achieved at 900 ppm of H₂O₂ for US irradiation, irrespective of the probe tip diameter and at 2100 ppm for UV irradiation. The addition of H₂O₂ concentrations exceeding 900 ppm and 2100 ppm reduced the decolourization efficiency for US and UV irradiation. This may be attributed to the auto decomposition of H₂O₂ to oxygen and water, and the recombination of ˙OH radicals produced.³³ Moreover, higher concentrations of H₂O act as free-radical scavengers itself, thereby decreasing the concentration of ˙OH radicals and reducing compound elimination efficiency.²⁷ For UV treated aqueous solution, optimum H₂O₂ concentration is on the higher side (2100 ppm) than a US treated aqueous solution (900 ppm). US has the advantage over UV in that mere sonication itself produces ˙OH radicals in addition to the ˙OH radicals produced through H₂O₂ splitting, whereas UV's only resource for producing ˙OH radicals is H₂O₂.

Fig. 2 Decolourization of BBG dye solution with the variation of H₂O₂: (a) 13 mm probe, (b) 25 mm probe and (c) UV light.

3.3 Effect of gas bubbling

The gases oxygen, argon, nitrogen and air are bubbled one at a time for 5 min in the BBG aqueous solution (BBG – 10 ppm, pH – 6.04, reaction volume – 100 ml, treatment time: 60 min) prior to irradiation so as to saturate the solution with the respective gas nuclei. The maximum decolourization was observed for argon-bubbled dye solution for both sonolysis and photolysis treatment, and the smallest degree of decolourization occurs for nitrogen-bubbled dye solution (Fig. 3). In the sonolysis process, dissolved gases form the nuclei for cavitation. Argon-bubbled gas gave higher colour removal due to its inert nature and higher specific heat capacity ratio. At the time of cavitation bubble collapse, higher temperatures and pressures are generated⁹ with monoatomic gases with higher specific heat capacity ratios (γ) than those with polyatomic gases with lower heat capacity ratios (γ). Unlike other gases of study, argon-bubbled nuclei doesn't undergo any radical scavenging reactions. In addition to the thermal dissociation of oxygen molecules leading to the generation of hydroxyl radicals in the oxygen-bubbled dye solution, inside the cavitation, bubble oxygen scavenges hydrogen atoms to form a hydroperoxyl radical, which is an oxidizing agent. This radical causes a number of other reactions to occur, resulting in the formation of H₂O₂, O₂, O and H₂ as products.³⁴ The formation of atomic nitrogen and oxygen, nitrogen fixation can occur in the cavity bubbled with an air bubbled dye solution. However, in nitrogen-bubbled dye solution, nitrogen molecules inside the cavitation bubble may react at high temperatures with hydroxyl radicals and oxygen atoms to give nitrous and nitrogen oxides,³⁵ thus reducing the activity of ˙OH radicals with BBG molecules. This leads to the reduced colour removal with nitrogen nuclei, compared with other gas nuclei. It is also observed that gas bubbling has minimal effect on the simple photolysis process, which is evident that in sonolysis process the gas content is the major drive for the production of ˙OH radicals.

Fig. 3 Decolourization of BBG dye solution with purging of various gases: (a) 13 mm probe, (b) 25 mm probe and (c) UV irradiation.

3.4 Effect of initial dye concentration and reaction volume variation

The decolourization efficiency for various initial dye concentrations shows (Fig. 4A) that the efficiency gradually decreases with an increase in initial dye concentration (pH – 6.04, reaction volume – 100 ml, treatment Time: 60 min.), which shows that the amount of ˙OH radicals produced by individual processes were insufficient for interaction with the increasing concentration of dye molecules. Sonolysis treatment has resulted in a higher degree of colour removal than photolysis treatment, irrespective of dye concentration. In the photolysis process, when the dye concentration was increased, the dye starts acting as an internal filter and as a result, the rate of decolourization of the dye decreases. The increasing dye concentration leads to shield the entering photons in solution and as a result, the rate of decolourization decreases due to a reduction in hydroxyl radical (˙OH) formation.³⁶

Fig. 4 Decolourization of BBG dye solution: (A) varying BBG concentration and (B) varying volume.

The study with varying reaction volumes of the dye solution is performed to understand the intensity of the treatment methods undertaken because for real time application of these systems needs to handle larger volumes of textile effluents. The decolourization efficiency for different volume of dye solution for the treatment time of 60 min is shown in Fig. 4B (BBG – 10 ppm, pH – 6.04, treatment time: 60 min). For mere sonolysis and photolysis treatments, the increase in the reaction volume of the dye had reduced the rate of decolourization. This shows that both sonolysis and photolysis were not effective when it comes to larger volumes. In the case of sonolysis, whether it is 13 mm or 25 mm probe, the effective irradiation area is restricted near to the surface of the probe tip. In addition, the frequency used in this case is 20 kHz; the length of the frequency and its intensity is only 6 cm. This means that for the rest of the volume, mere mixing occurs and that the volume of the dye solution does not undergo a cavitation reaction. This indicates that with the increase in distance from the irradiation source, the intensity of ultrasound decreases. In the case of photolysis, the same concept holds well that the UV radiation can't reach the entire volume of the solution unless the solution is vigorously stirred to bring in effective contact of the solution with UV light. These reasons limit the effectiveness of the production of ˙OH radicals required to act on the dye molecules and hence, these systems resulted in reduced decolourization efficiency for larger volume reactors.

3.4.1 Summary of the results. The pseudo first-order rate constant values for the studies performed are listed in Table 1. From the aforementioned studies, the following points were to be considered for the efficient treatment of BBG dye:

Table 1 Pseudo first order rate constant for BBG dye solution with varying parameters

SI. no.	Parameter	Experimental conditions	Rate constant × 10^−2/min
			Photolysis	Sonolysis
				13 mm	25 mm
1	pH	4	1.032	1.651	2.011
2	H2O2	2100 ppm for UV and 900 ppm for US	0.778	1.601	1.939
3	Gases	Argon	0.245	0.543	0.569
		Oxygen	0.170	0.362	0.379
		Air	0.153	0.324	0.339
		Nitrogen	0.084	0.174	0.182
4	BBG concentration	10 ppm	0.295	0.327	0.356
		50 ppm	0.080	0.235	0.249
		100 ppm	0.055	0.115	0.118
5	Reaction volume	100 ml	0.250	0.325	0.338
		500 ml	0.100	0.164	0.165
		1000 ml	0.077	0.131	0.136

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• From the pH study, it is understood that the BBG dye should be in hydrophobic form (or the BBG dye is more hydrophilic) to have an effective interaction between the BBG molecule with the produced ˙OH radicals at the point of the cavitation event or photolytic reaction.

• From the H₂O₂ study, it is understood that when the number of ˙OH radicals are increased in bulk liquid medium upon H₂O₂ splitting (either through US or UV irradiation), efficient colour removal is achieved. The colour removal achieved by this method is the highest among the parameters studied wherein the production of ˙OH in the bulk liquid medium is instantaneous than compared with that of gas bubbling.

• From gas bubbling study, when the gas nuclei is monatomic (argon gas producing more ˙OH radicals), there exists a good colour removal followed by other gas nucleii (oxygen, air and N₂). Again, this study shows the importance of ˙OH radical production (or the availability of ˙OH radicals in the bulk liquid medium) for improved treatment.

• From the initial concentration study, with an increase in initial dye concentration, the available ˙OH radicals for the oxidation reaction has significantly reduced providing a lower degree of colour removal under the same experimental conditions. This is a further indication that the chosen dye is highly hydrophilic.

• From the volume variation study, there is a reduced colour removal rate with an increased volume of the dye solution showing the limitations of the US and UV treatment processes that individual processes can't hold well for larger volumes.

For the efficient colour removal of BBG dye, it can be said that there should more ˙OH radicals in the bulk liquid medium, and with the amount of energy spent and the chemicals used for both the processes, a better method is required to simultaneously reduce the overall cost and to achieve better efficiency in a shorter duration. This is the motivation behind catalyst preparation for the processes studied for BBG dye colour removal.

3.5 Catalyst characterization

3.5.1 SEM and TEM analysis. The SEM and TEM images of ZVI showed clustered aggregates representing their crystalline cubic indices as visualised in Fig. 5A and B. The doping of Fe(0) on the TiO₂ crystal matrix has been clearly identified by the homogenous distribution of TiO₂ with Fe(0). The agglomeration observed in both the images (ZVI doped TiO₂ and ZVI) is due to the van der Waals forces among the magnetic Fe(0) particles and higher band gap energy of the TiO₂ particles. The high rate of Fe(0) doping is evident as the doping reaction takes place efficiently in a solution rather than in a solid state of Fe(0). The influence of doping on the particle size has been identified by XRD spectra, and it is in good agreement with the SEM and TEM images.

Fig. 5 (A) SEM images: (a) synthesized Fe(0) and (b) Fe(0)-doped TiO_2.; (B) TEM images: (a) synthesized Fe(0) and (b) Fe(0)-doped TiO₂; (C) XRD spectra of Fe(0) and Fe(0)-doped TiO₂; D. UV-visible diffuse reflectance spectra: (a) TiO₂, (b) Fe(0) and (c) Fe–TiO₂.

3.5.2 XRD analysis. The XRD spectra of Fe(0) and Fe(0)-doped TiO₂ as shown in Fig. 5C represents that the doping of Fe(0) with TiO₂ has changed the crystalline behaviour as well as the size. It was found that the doping of TiO₂ has caused an increase in the particle size of the doped catalyst. From the Scherrer equation, the grain size of synthesized Fe(0) was 13.9 nm, which has been determined to be increased to 21.4 nm for Fe(0)-doped TiO₂, owing to the doping effect. The peaks of Fe(0) representing the Miller indices of (110) and (211) are characteristics of Fe(0) that are found at the 2θ angles of 28.75° and 45.76°, respectively. However, TiO₂ doping has been clearly confirmed by the reduced Fe(0) characteristic peaks and also by high intensity anatase peaks (2θ = 25.36°, 48.73°) with lesser rutile peaks (2θ = 27.5°, 37.7°). The pure TiO₂ constitutes the most active anatase and also some of the other crystalline rutile phases.³⁷ The replacement of Fe(0) in the crystal framework of TiO₂ has been attributed to the reduction of the rutile phases, thereby suggesting the active doping. It is also due to the reduction in the oxygen vacancies on the TiO₂ surface that inhibit the crystallization of other phases.³⁸ However, the formed Fe ions were not of complete metallic form (i.e. Fe(0)) based on XPS data (see the ESI†), and much of the Fe were in an oxidized state (i.e. Fe₂O₃). The grain size of Fe(0) and Fe(0)-doped TiO₂ from the XRD spectra were calculated using the Debye–Scherrer equation and were found to be 13.9 nm and 21.4 nm, respectively.

3.5.3 UV-vis DRS analysis. The UV-vis diffuse reflectance spectra of the TiO₂, Fe(0) and Fe(0)-doped TiO₂ is shown in Fig. 5D. The DRS analysis shows that the % transmittance is decreasing in the order of TiO₂ > Fe(0) > Fe(0)-doped TiO₂. It can be inferred from the spectra that Fe(0)-doped TiO₂ exhibits a higher light absorption both in the visible and ultraviolet ranges than TiO₂ and Fe(0). Similar types of results were reported by several authors.^39–42

3.6 Decolourization study with catalyst

The commercial TiO₂ and the synthesized catalyst (Fe(0) and Fe(0)-doped TiO₂) were subjected to BBG dye colour removal under ultrasonic and UV irradiations. The results are presented in the following sections.

3.6.1 TiO₂. The TiO₂ catalyst has been widely used as a semiconductor photocatalyst, which, under UV irradiation, excites the creation of electron hole pairs, splitting the water molecules into H˙ and ˙OH radicals. The adsorption of the pollutant also occurs with the addition of TiO₂. Hence, the combined effect of adsorption of the pollutant on TiO₂ surface and the ˙OH radical production leads to efficient degradation.⁴³ Fig. 6 shows the decolourization of BBG dye upon US and UV irradiation with varying TiO₂ concentration (BBG – 10 ppm, pH – 6.04, reaction volume – 100 ml, reaction time – 60 min). When compared with other treatment methods (i.e. initial pH variation, gas bubbling, H₂O₂ concentration variation and volume variation), the decolourization rate of the UV irradiated solution showed fewer differences with US irradiation. However, the decolourization efficiency showed a larger increase than with all the previous treatment methods studied. The decolourization efficiency has reached maximum with the addition of 0.75 g TiO₂. With a further increase in TiO₂, the decolourization efficiency was nearly constant, and it had slowly started to reduce with an increase in the TiO₂ amount. After a certain concentration, the TiO₂ would form a cluster obstructing UV irradiation from passing through the entire liquid medium; with US irradiation, it reduces the ultrasonic intensity, thereby reducing the cavitation activity. Apart from this, increased catalyst dosage can act as a hydroxyl radical scavenger (28). Again, a maximum colour removal of 86% was achieved for a 25 mm probe, followed by a 13 mm probe (81%) and UV irradiation (80%).

Fig. 6 Decolorization of BBG dye solution with TiO₂ catalyst.

3.6.2 n-Fe(0). The synthesized zero valent iron powder (Fe(0)) is used as a catalyst, and its effectiveness on colour removal upon US and UV irradiation (BBG: 10 ppm, pH: 6.04, reaction volume: 100 ml, treatment time: 60 min) is shown in Fig. 7. There is a slight increase in the decolourization rate with US irradiation (87% for 25 mm and 82% for 13 mm–1 g catalyst), and there is a slight decrease in decolourization efficiency with UV irradiation (78%–1 g catalyst). The cavitation reaction is known to produce H₂O₂ as well upon recombination of ˙OH radicals, and the H₂O₂ in reaction with the zero valent iron powder (Fe(0)) is corroded.⁴⁴ The corroded Fe(0) oxidizes from Fe(0) to Fe²⁺, which then reacts with H₂O₂ in a Fenton's like process to generate ˙OH radicals and Fe³⁺. The Fe(0) then reduces the Fe³⁺ back to Fe²⁺, and the cycle continues. The reactions are presented below, expressed as

Fe(0) + H₂O₂ → Fe²⁺ + 2OH⁻

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH

Fig. 7 Decolorization of BBG dye solution with Fe(0) catalyst.

However, UV-treated samples lack this Fenton's reaction and hence only catalyst activity prevails for dye colour removal.

3.6.3 n-Fe(0)-doped TiO₂. It has been reported by several researchers^37,45 that iron-doped TiO₂ results in higher UV light absorption.^4,22,46 In addition, it is proved that the doping of metals with TiO₂ narrows the band gap energy and enhances the separation of electrons and holes, which is essential for photocatalytic activity.³⁷ This means that ZVI-TiO₂-doped catalyst could decompose the dye molecule at a faster rate. The effect of the doped catalyst under different treatments (Fig. 8) shows that (BBG: 10 ppm, pH: 6.04, reaction volume: 100 ml, catalyst: 0.1 g, treatment time: 60 min) doped catalyst activity is the higher than that of the undoped, irrespective of the treatment methods with a lesser catalytic addition. In the presence of a doped catalyst, the US- and UV-treated dye solutions resulted in ∼99% and 92%, respectively.

Fig. 8 Decolorization of BBG dye solution with Fe(0)-TiO₂ catalyst.

3.7 Sonophotocatalytic treatment of BBG

The synergistic effects of sonolysis and photocatalysis are well understood from their basic mechanisms of chemical effect (production of radicals by sonolysis and photolysis) for degrading pollutants and physical effects (microjets, shockwaves and microturbulence by sonolysis) for providing effective contact between pollutant and radicals. Apart from these, the combined processes have several other advantages, including the regeneration of the catalyst surface, maintaining uniform dispersion of the catalyst, mass transfer enhancement and disintegration of the solid catalyst into smaller particles, thus increasing the surface area. Because it was found that for 10 ppm, BBG concentration had resulted in nearly complete colour removal with the doped catalyst under the conditions studied (Section 3.7.3), the chosen dye concentration was 25 ppm (system: 25 mm probe + UV, reaction volume: 100 ml, pH: 6.04, treatment time: 60 min) for various studies of the sonophotocatalytic treatment to have its visible effect. The results are shown in Fig. 9 and their respective rate constants were presented in Table 2. The synergistic effect of the sonophotocatalytic process is clearly observed as the colour removal efficiency is greater, compared with the individual sonolysis and photolysis processes. In order to validate the synergistic effect, simple sonolysis and photolysis experiments (system: 25 mm probe, UV, reaction volume: 100 ml, pH: 6.04, treatment time: 60 min) were performed with a 25 ppm dye solution. It was observed that simple sonolysis and photolysis could result in ∼15% and ∼5% decolorization, whereas sonophotolytic treatment had resulted in ∼36% decolourization. From the studies (3.1–3.3), it is understood that the degradation of BBG dye occurs through the hydroxylation reaction in the bulk liquid medium. Because BBG dye is hydrophilic in nature, it will remain in the bulk aqueous medium, and the degradation of these pollutants depends on how far the produced ˙OH radicals move to reach the pollutant molecules.⁴⁷ However, these ˙OH radicals are highly reactive, short lived, and they try to recombine to form water molecules or hydrogen peroxide without reaching the pollutant molecule under normal conditions. These uncertainties would be eliminated when the photolysis (which can produce a greater number of radicals through UV light illumination in the presence of a doped catalyst) and sonolysis (which can yield a greater number of ˙OH radicals through cavitation) were applied together. UV light shielding by the catalyst and agglomeration of the catalyst were overthrown by the cavitational activity through microjets and microturbulence actions (6). These combined effects bring in an effective ˙OH radical production and have effective oxidation of the BBG dye. These effects were reflected in experimental results that sonophotocatalysis of BBG dye in the presence of a doped catalyst was highly efficient (98%), compared with the other processes studied. The synergistic mechanism of ultrasound, ultraviolet irradiation and catalytic activities of Fe(0)-doped TiO₂ are pictorially illustrated in Fig. 10.

Fig. 9 Sonophotolytic degradation of BBG with various parameters.

Table 2 Pseudo first order rate constant for a BBG dye solution in the presence of a catalyst

SI. no.	Catalyst	Experimental conditions	Rate constant × 10^−2/min
			Photolysis	Sonolysis		Sonophotolysis
				13 mm	25 mm
1	TiO2, 0.75 g	10 ppm, pH = 6.04 for US and UV, 25 ppm, pH = 6.04 for sonophotolysis	2.092	2.129	2.921	1.293
2	Fe (0), 1 g		2.229	2.655	2.881	1.290
3	Fe(0)-doped TiO2, 0.1 g		3.732	7.358	7.582	5.665

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Fig. 10 Synergistic effect of the sonophotocatalytic process.

3.8 Sonophotocatalytic treatment of textile effluent

The textile effluent was collected from the textile industry (Clothing Company, Tirupur, Tamilnadu, India) after preliminary treatment. The physico-chemical characteristics of the textile effluent were tested⁴⁸ under the same conditions as it was received, and these results are listed in Table 3. The sonophotocatalytic degradation studies were performed for the effluent with the following conditions: reaction volume – 1000 ml, catalyst (Fe(0)-doped TiO₂) – 0.1 g, H₂O₂ – 900 ppm, gas bubbling – air. A larger reaction volume of 1000 ml is chosen, while considering its practical applicability. It needs to be mentioned here that air is chosen as the bubbling gas as it is less costly, and for practical application, air is the most appropriate, readily available gas. Further, the catalyst and H₂O₂ concentrations were based on the studies detailed elsewhere in this report. The UV-vis spectrum (Fig. 11) of the sonophotocatalytically treated effluent at various time intervals shows that there is a drastic reduction in the colour removal of the effluent, and the colour removal itself was evident after 15 min of treatment time. The spectrum of raw textile effluent exhibits characteristic peaks in the visible and UV regions, indicating the presence of dyes as well as other organic compounds. In accordance with the spectrum results, it is easily understood the present treatment methodology could degrade both dyes and other organic compounds present in the textile effluent. The COD analysis was also performed for every 15 min time interval, and the results are presented in Fig. 11. Again, there was a drastic reduction in the COD value during the first 15 min of treatment time, and it is in good agreement with the UV-vis spectrum results. The decrease in COD value of the textile effluent from 2400 to 293.5 mg l⁻¹ is further evident that the sonophotocatalytic treatment not only helps in complete colour removal but also degrades the complex compounds present in the textile effluent, leading to efficient treatment.

Table 3 Physico-chemical characteristics of the textile effluent

SI. no.	Characteristics	Value
1	Color	Dark-blue color liquid
2	pH	9.06
3	Total hardness as CaCO3 (mg l^−1)	960
4	Turbidity (NTU)	64
5	Total dissolved solids (mg l^−1)	209 031.0
6	Conductivity (micromhos cm^−1)	35 600
7	BOD5 at 20 °C (mg l^−1)	128 (VFL)
8	COD (mg l^−1)	2400
9	Total alkalinity as CaCO3 (mg l^−1)	4124

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Fig. 11 COD variation and UV-visible spectrum of sonophotocatalytically treated textile effluent.

4. Conclusions

In this experimental study, the decolourization of 10 ppm of Bismarck Brown G was investigated by varying several parameters with the application of ultrasound and UV light. Initial studies of Bismarck Brown G dye with pH, H₂O₂, gases (oxygen, argon, nitrogen and air), catalysts, dye concentration and variable volume proved to indicate that ˙OH radical production is the key for effective colour removal. The oxidation reaction is through hydrolysis in the bulk liquid medium either through US or UV. From the individual parametric study, pH 4.0 (US and UV), 900 ppm H₂O₂ (US), 2100 ppm H₂O₂ (UV), argon gas bubbling (US and UV), lower BBG dye concentration (US and UV) and lower reaction volume (US and UV) gave maximum decolourization efficiency. Further intense study with the nanocatalyst (commercial TiO₂, green synthesized Fe(0) and Fe(0)-doped TiO₂) proved effective for higher BBG dye colour removal with lesser catalyst quantity. Among the methodology adopted, sonophotocatalysis is the best treatment method for the BBG dye solution in the presence of Fe(0)-doped TiO₂. The sonophotocatalytic treatment of real textile wastewater further supports the claim that for practical applicability, synergistic effects are more important that individual parametric effects to handle larger volumes of textile effluents.

Acknowledgements

The authors are grateful to the Ministry of Environment and Forests (MoEF), Government of India, for financial support of the project. The authors further extend their gratitude to the Environmental Engineering Laboratory, Department of Civil Engineering, NIT Tiruchirappalli, for assisting in performing the effluent characterization and COD analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra07685k

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