mpg-C3N4): Preparation, characterization and catalytic performance

Journal of Molecular Liquids 290 (2019) 111208

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Strontium titanate nanocubes assembled on mesoporous graphitic carbon nitride (SrTiO3/mpg-C3N4): Preparation, characterization and catalytic performance Paria Eghbali a,b, Aydin Hassani c,d,e,⁎, Buse Sündü a, Önder Metin f,⁎⁎ a

Department of Chemistry, Faculty of Science, Atatürk University, 25240 Erzurum, Turkey Department of Analytical Chemistry, Faculty of Pharmacy, Girne American University, 99428 Kyrenia, North Cyprus, Mersin 10, Turkey c Nanomaterials based Water Treatment Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam d Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam e Department of Materials Science and Nanotechnology Engineering, Faculty of Engineering, Near East University, 99138 Nicosia, North Cyprus, Mersin 10, Turkey f Department of Chemistry, College of Sciences, Koç University, 34450 Sariyer, Istanbul, Turkey b

a r t i c l e

i n f o

Article history: Received 25 February 2019 Received in revised form 6 June 2019 Accepted 17 June 2019 Available online xxxx Keywords: Sonocatalysis Graphitic carbon nitride SrTiO3 nanocubes Nanocomposites Wastewater treatment Dye removal

a b s t r a c t This study is a report on successful synthesis of strontium titanate (SrTiO3) nanocubes using a conventional hydrothermal method, their assembly on mesoporous graphitic carbon nitride (mpg-C3N4) to yield novel SrTiO3/ mpg-C3N4 nanocomposites, as well as sonocatalyst performance of SrTiO3/mpg-C3N4 to eliminate basic violet 10 (BV10) as a model pollutant from aqueous solutions. To this end, the structure of as-synthesized SrTiO3/ mpg-C3N4 nanocomposites was characterized through TEM, HR-SEM, EDS, XRD, FTIR, BET, PL, and UV–vis DRS analyses. To manifest the sonocatalytic performance of SrTiO3/mpg-C3N4 nanocomposites, the main operating parameters such as sonocatalyst dosage, solute concentration, ultrasonic power, initial pH, and reaction time span for the elimination of BV10 were also examined. The best sonocatalytic performance of 80% was further achieved using 0.3 g L−1 sonocatalyst, 10 mg L−1 BV10, and an ultrasonic power of 240 W at pH = 5 (natural) in 120 min of reaction time. The removal rate constant of BV10 via US/SrTiO3/mpg-C3N4 system was 0.0146 min−1, almost 12 times higher than that achieved by the only ultrasonic process. According to these results, the kinetics of dye elimination system could be determined by the use of pseudo-first-order kinetic model and scavenging compounds also exerted the following adverse effects on the sonocatalytic efficiency in the order of EDTA-2Na b BQ b t-BuOH. Moreover, a mechanism was proposed for the elimination of BV10 in the presence of SrTiO3/mpg-C3N4 nanocomposites under the sonocatalytic system. Finally, the reusability test of SrTiO3/mpg-C3N4 nanocomposites in the BV10 removal revealed that nearly 10% drop occurred after the five successive cycles. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Major challenges encountered by scientists in the 21st century are the ones related to environmental needs preserved in a secure, clean, and sanitary condition for the viability of human beings. In this respect, water can significantly contribute to life protection of living cells to the

⁎ Correspondence to: A. Hassani, Nanomaterials based Water Treatment Research Group, Ton Duc Thang University, Ho Chi Minh City, Vietnam. ⁎⁎ Correspondence to: Ö. Metin, Department of Chemistry, College of Sciences, Koç University, 34450 Sariyer, Istanbul, Turkey. E-mail addresses: [email protected], [email protected] (A. Hassani), [email protected], [email protected] (Ö. Metin).

https://doi.org/10.1016/j.molliq.2019.111208 0167-7322/© 2019 Elsevier B.V. All rights reserved.

entire ecosystem. Unfortunately, huge quantities of hazardous wastes discharged into aquatic media without convenient treatment are increasingly threatening the environment [1,2]. Of the worst water contaminants are synthetic dyes utilized in papers, textiles, cosmetics, etc. [3]. Majority of synthetic dyes are also harmful, toxic, carcinogenic and their poor biodegradability may result from high stability leading to terrible diseases and other health problems in living organisms [4–6]. As an example; Basic Violet 10 (BV10), commonly known as Rhodamine B and also a hazardous dye, is broadly used in dyeing industries [7,8], which necessitates waste treatment through a hygienic, eco-friendly, and affordable method prior to flowing into natural streams and water bodies [9]. The experienced and reported strategies in this domain include biological treatment, precipitation, membrane filtration, reverse osmosis, adsorption, coagulation, etc. [1,10–12], with their

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own benefits and restrictions. The established physical approaches are rather costly and also incapable of complete removal of pollutants [1,13]. As a research topic in the last few decades, environmental pollutant removal as an economically striking procedure has been exercised by alleviation of contaminants through the use of semiconductor sonocatalysts and photocatalyst with Advanced Oxidation Processes (AOPs) [14,15]. Such processes using semiconductor sonocatalysts have been reported to be operative options for the removal of organic dyestuffs [7,16,17]. During this process, ultrasound waves result in a rapid growth as well as collapse of bubbles in water, giving rise to a tremendous increase in heat and pressure. Exposure of the bubbles to high temperatures also brings about thermal dissociation of water and consequently produces hydroxyl radicals (•OH) as a robust oxidant for oxidation of organic contaminants [16]. As a novel AOPs, sonochemical degradation of organic contaminants in water solution is similarly associated with the formation of reactive species via cavitation phenomena [18–20]. However, large amounts of energy are consumed by the use of ultrasound for the elimination of dye pollutants. Additionally, the use of sonolysis rarely yields a complete decomposition of target pollutants by itself. These limitations are surmounted via the utilization of a convenient catalyst with the sonolysis, namely sonocatalytic process, which has gained a great interest in this field [16]. The sonocatalytic effectiveness of dye-pollutant disintegration can noticeably be improved through being exposed to an appropriate catalyst, which is likely to arise from a synergistic effect between the ultrasonic function and the catalyst [21]. In this regard, a variety of catalysts have been tested as sonocatalysts for the elimination of dyes including CeO2-biochar [22], ZnFe-Cl NLDH [23], Sm-ZnO [24], CoFe2O4/rGO [25], Ni-ZnO [26], LaFeO3 [27], KNbO3 [28], Fe3O4 [29], WO3 [30], β-Bi2O3 [31], WS2 [32], CoFe2O4/mpg-C3N4 [33], and Au-Fe3O4-AC [34]. However, it is still required to develop new sonocatalysts with a low cost, high catalytic activity, and reusability. Researchers have also focused on the graphitic carbon nitride (gC3N4), a metal-free polymeric semiconductor with useful features such as great chemical stability, adjustable electron configuration and low cost [1,9,35,36]. Nevertheless, it is not possible to make use of gC3N4 alone as a sonocatalyst because of its several restrictions such as having low bandgap resulting in rapid recombination of electron-hole (e− − h+) pairs and limited surface area. These disadvantages can be properly resolved by the preparation of mesoporous g-C3N4 (mpgC3N4) with a much larger surface area enabling the combination of mpg-C3N4 with other semiconductors owing to ideal band gaps in order to develop the absorption range of mpg-C3N4 [37]. On the other hand; SrTiO3, as a semiconductor oxide having perovskite structure with a band gap of 3.2 eV, has gained a substantial attention in the field of photocatalysis due to its favorable band edge potentials and interesting physico-chemical and structural properties [38,39]. According to the authors' experiences in the preparation of mpg-C3N4-based nanocomposites for various sono/photocatalytic applications [9,33], the sonocatalytic performance of mpg-C3N4 might be further enhanced through its combination with SrTiO3 nanostructures because of the synergistic effects in the hybrid structure of two different semiconductors enabling possible separation of charge carriers. Considering the scope summarized above, the present research describes the synthesis of SrTiO3/mpg-C3N4 nanocomposites as operative, low-cost, stable and reusable sonocatalysts for the elimination of BV10 from the aquatic environment. To manifest the sonocatalytic performance of SrTiO3/mpg-C3N4 nanocomposites, major operating parameters such as sonocatalyst dosage, solute concentration, ultrasonic power, initial pH, and reaction time span, as well as radical scavengers for the removal of BV10 were examined. Moreover, a mechanism was proposed in the present study to elucidate the significant enhancement of sonocatalytic activity for the dissociation of BV10 from aquatic phase under ultrasonic irradiation. To the best of authors' knowledge, this study was the first example of sonocatalytic dye removal by SrTiO3/ mpg-C3N4 nanocomposites from aqueous solution.

2. Experimental 2.1. Materials Strontium hydroxide octahydrate (Sr(OH)2·8H2O, 95%), titanium tetrachloride (TiCl4, ≥99 %), guanidine hydrochloride (CH6ClN3, ≥99%), Ludox® HS40 colloidal silica (40 wt% suspension in H2O), ammonium hydrogen difluoride (NH4HF2, 95%), ethanol (C2H5OH, 99%), edetate disodium (EDTA-2Na), tert-butyl alcohol (t-BuOH), and benzoquinone (BQ) were obtained from Sigma-Aldrich Co. and used in their original form. The basic violet 10 (BV10) was also acquired from Alvan Sabet Co. (Iran). The characteristics of BV10, as a target pollutant, were presented in Table 1. All the reaction solutions were prepared in distilled water. 2.2. Synthesis of catalysts 2.2.1. Synthesis of mpg-C3N4 The detailed process for the synthesis of mpg-C3N4 through a silica templating method can be found elsewhere [37]. 2.2.2. Synthesis of SrTiO3 nanocubes (NCs) SrTiO3 NCs was synthesized by a well-established sol-precipitationhydrothermal treatment method [40,41]. The typical protocol included preparation of an aqueous solution (80 mL of Sr(OH)2.8H2O, 9.15 mmol and 1 mL of TiCl4) as the metal precursors. NaOH pellets (5 g) were added to the above bimetallic solution to induce coprecipitation of SrTiO3. A very viscous suspension was produced after the dissolution of NaOH pellets, which was then conveyed to a Teflonlined autoclave (125 mL) and heated at 240 °C for 36 h. The solution was filtered followed by complete washing of the product by distilled water and dried at 80 °C for 24 h. 2.2.3. Synthesis of SrTiO3/mpg-C3N4 nanocomposites SrTiO3 NCs were assembled on mpg-C3N4 through the liquid selfassembly technique to yield SrTiO3/mpg-C3N4 nanocomposites [42]. The typical procedure comprised the dispersion of mpg-C3N4 (200 mg) in ethanol (10 mL) by the help of ultrasonication. Afterward, 100 mg of SrTiO3 NCs was added dropwise into the mpg-C3N4 dispersion. The obtained mixture was then sonicated for 3 h for the complete adhering of SrTiO3 NCs by mpg-C3N4. To isolate the resultant SrTiO3/ mpg-C3N4 nanocomposites from the solution, the mixture was centrifuged at 7000 rpm for 10 min after the addition of ethanol and drying overnight at natural room temperature. Corresponding to an adjustable SrTiO3/mpg-C3N4 molar ratio of 1/1, 1/2, and 1/3 were obtained according to the initial amount of raw materials. 2.3. Instrumentation The crystallinity of the as-synthesized SrTiO3/mpg-C3N4 nanocomposites was studied by X-ray powder diffraction with Cu-Kα radiation (XRD, Panalytical Empyrean diffractometer, 40 kV, 15 mA, 1.54051 Å). The morphologies of the samples were characterized by highresolution scanning electron microscope (HR-SEM, Zeiss Sigma 300) equipped with EDAX analyzer and transmission electron microscopy Table 1 Characteristics of basic violet 10 (BV10) dye. Chemical structure

Molecular formula C28H31ClN2O3

Mw (g mol−1) 479.01

λmax (nm) 554

Chemical class Basic

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(TEM, Hitachi HT7700 with EXALENS, 120 kV) working at highresolution (HR) mode. The Fourier transform infrared (FTIR) spectra were measured by Perkin Elmer spectrometer (Model: 1600, USA) from 400 to 4000 cm−1. The ultraviolet-visible diffuse reflectance spectrometry (UV–vis DRS) of the samples was recorded by UV–vis-NIR spectrophotometer (Shimadzu UV-3600 plus). BET surface area and average particle size of the samples were analyzed by Brunauer-EmmettTeller (BET) method using Micromeritics ASAP 2020 HD accelerated surface area and porosimetry analyzer. The photoluminescence (PL) spectra were measured by an Agilent Cary Eclipse PL spectrometer. 2.4. Sonocatalytic test Traditionally, a certain amount of the sonocatalyst and 100 mL of BV10 was added into a 250 mL Erlenmeyer flask. No pH adjustment was considered for the suspension throughout the trials, however, the pH was adjusted by NaOH and HCl with specific molarity and recorded with a pH meter (Mettler Toledo, Germany) to examine its impact on sonocatalytic process. The solution was shaken in dark for 10 min to ascertain the adsorption-desorption equilibrium. The suspension was stirred by a magnet to find out the contribution of adsorption alone to the process of elimination. The flask was then placed in an ultrasonic bath (VWR, 45 kHz, 240 W, USC 1700 TH) to initiate the sonocatalytic process. The bottom of the flask was located at 2 cm above from the ultrasonic irradiation source where the maximum turbulence of the solution was observed to provide identical conditions for the experiments. The position of the reactor in the middle of the bath was kept constant. The ultrasonic system was placed in a dark place to prevent any possible photocatalytic process. The ultrasonic irradiation was enough to ensure the appropriate mixing of the solution containing SrTiO3/mpg-C3N4 nanocomposites. The temperature increases in the ultrasonic bath were controlled by the addition of ice-water cubes into the water bath. To find out the role of adsorption alone in the dye elimination system, the suspension was stirred magnetically. At given reaction time intervals, 4 mL of sample were collected periodically, centrifuged (Allegra X-30R, US) at 10,000 rpm for 10 min, and then dye absorbance measured by UV–vis spectrophotometer (Shimadzu 1800, Japan) at λmax = 554 nm. The removal efficiency (RE) was examined using the RE = [(A0 − At)/A0] × 100 formula, where A0 and At are the initial absorbance of BV10 and its absorbance after the sonocatalytic process, respectively. The sonocatalyst was concentrated through centrifugation, rinsed by distilled water, and then dried to examine its reusability. The recovered sonocatalyst was added to the fresh BV10 solution for the next run of the sonocatalytic experiment under similar conditions. The process was repeated for five times. 2.5. Detection of hydroxyl radicals The •OH radicals were detected in the reaction of photocatalysis using a terephthalic acid (TA) photoluminescence approach. To this end, 5 × 10−4 M of TA was poured into 2 × 10−3 M of NaOH and the dye solution was exchanged by TA solution. Then, 0.3 g L−1 of SrTiO3/ mpg-C3N4 nanocomposites were added to 100 mL of TA solution, followed by exposing to ultrasonic irradiation. After certain intervals, the samples were gathered to be analyzed by fluorescence spectrophotometer at the excitation wavelength of 350 nm. 3. Results and discussion 3.1. Characterization The XRD patterns of pristine mpg-C3N4, pristine SrTiO3, and SrTiO3/ mpg-C3N4 nanocomposites at different ratios were shown in Fig. 1. In the case of pristine mpg-C3N4 (Fig. 1A), the peak arose at the 2θ of 13.2° and 27.6° are readily indexed to (100) and (002) which were attributed to the intralayer/in-plane and interplanar graphitic stacking

Fig. 1. XRD pattern of samples. (A) mpg-C3N4, (B) SrTiO3, (C) SrTiO3/mpg-C3N4 (1/1), (D) SrTiO3/mpg-C3N4 (1/2), (E) SrTiO3/mpg-C3N4 (1/3).

plane of mpg-C3N4, respectively (JCPDS no. 01–087-1526) In Fig. 1B, the diffraction peaks at 2θ = 22.7°, 32.3°, 39.9°, 46.4°, 52.2°, 57.7°, 67.6°, 72.4°, 77.1°, 81.5°, and 86.1° were readily indexed to the (100), (110), (111), (200), (210), (211), (220), (300), (310), (311), and (222) planes of perovskite SrTiO3 structure with a good crystallinity (JCPDS no. 00-035-0734) [43,44]. The XRD patterns of nanocomposites (Fig. 1C–E) revealed the reflection planes of both SrTiO3 and mpg-C3N4, confirming the nanocomposites were prepared successfully. Using the Debye-Scherrer equation and the reflection at 32.3°, the average crystallite size of SrTiO3 NCs was estimated and the results are given in Table 2. The obtained results revealed that the addition of mpg-C3N4 into the SrTiO3 remarkably hindered crystalline growth. Additionally, the (110) plane was applied to SrTiO3 NCs to find out lattice values as well as cell volume (cubic phase, a = b = c). The lattice parameter and the cell volume of the SrTiO3 NCs were also reported in Table 2. The lattice constants of SrTiO3 in the nanocomposites are slightly higher than those of pure SrTiO3. Both of these values could be matched with those presented in JCPDS no. 00-035-0734 standard for SrTiO3 (i.e. a = b = c = 3.905 Å, and v = 59.55 Å3). According to the XRD patterns, it could be concluded that the nanocomposites consisted of SrTiO3 and mpg-C3N4 with no impurity phases in addition of SrTiO3 to mpg-C3N4 which rendered no changes in the crystal-phase of neither SrTiO3 nor mpg-C3N4 nanocomposites. The HR-SEM images of SrTiO3/mpg-C3N4 nanocomposites, in which different sizes of SrTiO3 cubes were observed in HR-SEM images; was also illustrated in Fig. 2 (A, B). Both cubic and sheet-like structures were presented by SrTiO3/mpg-C3N4 nanocomposites suggesting them as the intimate mixture of both SrTiO3 and mpg-C3N4 and indicating successful deposition of SrTiO3 NCs over mpg-C3N4 sheets. As shown in Fig. 2C, the EDS spectra of the SrTiO3/mpg-C3N4 (2/1) nanocomposites represented all the expected elements (C, N, O, Sr, and Ti), suggesting a successful synthesis of SrTiO3/mpg-C3N4 nanocomposites. It should be noted that no characteristic peaks of any impurities were present in the EDS spectrum, revealing a high purity of SrTiO3/mpgC3N4 nanocomposites. To get more insights into the morphology of SrTiO3/mpg-C3N4 nanocomposites, TEM analysis was also performed on it. As shown in Fig. 3 (A, B), the TEM images of SrTiO3/mpg-C3N4 (1/2) indicated successful assembly of the SrTiO3 NCs on mpg-C3N4 sheets. Moreover, the TEM images clearly illustrated the cubic structure of SrTiO3 with a particle diameter of 30 nm and the sheet-like structure of mpg-C3N4, which closely adjoin each other.

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Table 2 Cell parameters and crystallite size of the samples. Sample

d110 (Å)

Lattice parameters (Å)

Unit cell volume (Å3)

Average crystallite size (nm)

59.45 60.09 60.09 60.09

32 20 19 16

a=b=c SrTiO3 SrTiO3/mpg-C3N4 (1/1) SrTiO3/mpg-C3N4 (1/2) SrTiO3/mpg-C3N4 (1/3)

2.76 2.77 2.77 2.77

3.903 3.917 3.917 3.917

The FTIR spectra of mpg-C3N4, SrTiO3 and the SrTiO3/mpg-C3N4 (1/ 2) nanocomposites were depicted in Fig. 4. For SrTiO3 (Fig. 4A), the peaks observed at 424 cm−1, 458 cm−1, and 616 cm−1 were attributed to the Ti\\O stretching vibrations and bending vibration of O\\Ti\\O present in the TiO6 octahedron, while the ones located at 1459 cm −1, 3145 cm−1, and 3742 cm−1 could be assigned to the OH groups of hydrated oxide surface and the adsorbed water [45]. The band at 877 cm−1 was associated with the stretching vibration of Sr\\O [46]. For mpg-C3N4 (Fig. 4B), the peak at 809 cm−1 corresponded to the striazine ring vibrations while the bands located at 1255 cm−1, 1325 cm−1, 1400 cm−1, 1574 cm−1, and 1624 cm−1 matched to the

typical stretching vibration of C\\N heterocycles, and strong band around 3145 cm−1 conforming to the N\\H stretching or NH2 groups of the aromatic rings; respectively [1,38]. The FTIR spectrum of the SrTiO3/mpg-C3N4 (1/2) (Fig. 4C) was comprised of all the peaks of mpg-C3N4 and SrTiO3, indicating that the combination of SrTiO3 and mpg-C3N4 had no effects on the structural characteristics of two components. The optical characteristics of mpg-C3N4, SrTiO3, SrTiO3/mpg-C3N4 (1/1), SrTiO3/mpg-C3N4 (1/2), and SrTiO3/mpg-C3N4 (1/3) nanocomposites were investigated by the UV–vis DRS (Fig. 5). The pure mpgC3N4, SrTiO3, SrTiO3/mpg-C3N4 (1/1), SrTiO3/mpg-C3N4 (1/2), and

Fig. 2. (A, B) HR-SEM images and (C) the associated EDS spectrum of SrTiO3/mpg-C3N4 (1/2) nanocomposites.

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Fig. 3. The representative TEM images of SrTiO3/mpg-C3N4 (1/2) nanocomposites at two different magnifications.

SrTiO3/mpg-C3N4 (1/3) samples were highlighted in Fig. 5A, showing absorption bands with absorption edges at ca. 440, 381, 425, 442, and 439 nm; respectively. As it can be observed in Fig. 5A, mpg-C3N4 absorbs the light from UV to visible region with a maximum at 450nm owing to the charge transfer from the valence band to the conduction band with the band gap of (2.8 eV). On the other hand, SrTiO3 can only be excited by UVA light with the wavelength b390 nm due to its relatively wide band gap (3.2 eV). However, since the UV–vis diffuse reflectance spectra of the nanocomposites is red shifted compared with that of pristine SrTiO3, it is obvious that the visible light absorption of SrTiO3/mpgC3N4 nanocomposites is higher than that of pristine SrTiO3. It can be assumed that the red shift observed for the nanocomposites is due to the introduction of mgp-C3N4 and the formation of heterojunction between two distinct materials. Therefore, the light harvesting efficiency of the SrTiO3/mpg-C3N4 nanocomposites is higher than that of pristine SrTiO3 in the visible light region, which is beneficial for the catalytic activity. Comparison with the SrTiO3, absorption edges of the SrTiO3/mpgC3N4 nanocomposites with various mass ratios showed a red shift, implying that SrTiO3/mpg-C3N4 nanocomposites could improve utilization efficiency for light from sonoluminescence [47]. The band gap energies (Eg) of the samples were also estimated with the formula (αhν)2 = K (hν-Eg), wherein hν refers to the energy of a photon (eV), α represents the absorption coefficient, K is a constant, and Eg stands for the band gap

[48]. As depicted in Fig. 5B, the band gap energy values of 2.82, 3.25, 2.91, 2.80, and 2.82 eV were respectively obtained for the pristine mpg-C3N4, SrTiO3, SrTiO3/mpg-C3N4 (1/1), SrTiO3/mpg-C3N4 (1/2),

Fig. 4. FTIR spectra of (A) SrTiO3, (B) mpg-C3N4, and (C) SrTiO3/mpg-C3N4 (1/2) samples.

Fig. 5. (A) UV–vis DRS spectra and (B) plots to determine the band gap of the samples.

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and SrTiO3/mpg-C3N4 (1/3); indicating a slightly lower Eg value as a result of the coupling between mpg-C3N4 and SrTiO3 at 1/2 mass ratio. The increasing mass ratio of mpg-C3N4 in the nanocomposite results in decrease on the bandgap energy of the nanocomposite compared to pristine SrTiO3, implying that electronic transitions are easily assisted. [47]. It should be mentioned that the lower band gap energies facilitate electronic transitions [1]. An easier occurrence of the ultrasound production of e− − h+pairs in short band gap semiconductors also led to better performance of SrTiO3/mpg-C3N4 (1/2) sample compared with other hybrid samples for sonocatalytic removal of BV10, as discussed in Section 3.2. To prove the mesoporous structure of mpg-C3N4 and to demonstrate the change in the surface area with respect to different mpg-C3N4/ SrTiO3 ratios, N2 adsorption-desorption isotherms and pore size distribution (inset) analysis were performed on mpg-C3N4, SrTiO3/mpgC3N4 (1/1), SrTiO3/mpg-C3N4 (1/2), and SrTiO3/mpg-C3N4 (1/3) nanocomposites. As can be seen from Fig. 6, mpg-C3N4 and all nanocomposites show Type IV isotherm with H3 hysteresis loop confirming their mesoporous structure [1]. The surface area and pore volume of the samples were calculated by BET and BJH models, respectively, and the results are listed in Table 3. The surface area and pore volume of all nanocomposites are less than pristine mpg-C3N4 and the increasing mass ratio of SrTiO3 results in the decrease in the surface area and pore volume. Although surface area of catalysts generally have considerable influence in the sonocatalytic processes, in the case of the

SrTiO3/mpg-C3N4 (1/2) nanocomposites samples, it was observed that the improved catalytic performance is mainly related to suppression of the sonogenerated charge carriers from recombination, rather than the surface area. In order to investigate the sonocatalytic activity as well as exhibit the formation of •OH radicals during the sonocatalytic degradation process, photoluminescence (PL) analysis were conducted on the mpg-C3N4/ SrTiO3 (1/2) nanocomposites assisted degradation of BV10. The PL spectra of the samples were recorded at an excitation wavelength of 350 nm to investigate the sonocatalytic activities (Fig. 7). It should be noted that, the higher PL intensity indicates the higher recombination rate of charge carriers and thus lower catalytic activity [15,49]. Upon the assessment of Fig. 7, it is concluded that the mpg-C3N4 fastest rate of e− − h+ recombination rate as it possessed the highest PL intensity while SrTiO3 has the slowest e− − h+ rate of recombination as it showed the lowest PL intensity owing to its high crystalline nature and less intrinsic defects in the ordered cubic structure [1,50]. As illustrated in Fig. 7A, the SrTiO3/mpg-C3N4 (1/2) nanocomposites possessed a weaker intensity than pristine mpg-C3N4, indicating the possible reduction in e− − h+ recombination rate and thus provided the best sonocatalytic degradation performance. It should be noted that our findings are consistent with the results reported in the literature [1,50]. Fig. 7B indicates the PL spectra changes through the sonocatalytic process with terephthalic acid (TA) solution for the SrTiO3/mpg-C3N4 (1/2) nanocomposites at different time intervals. As can be concluded

Fig. 6. N2 adsorption-desorption isotherms and pore size distribution (inset) plots of samples. (A) mpg-C3N4, (B) SrTiO3/mpg-C3N4 (1/1), (C) SrTiO3/mpg-C3N4 (1/2), and (D) SrTiO3/mpgC3N4 (1/3).

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Table 3 Porous structure parameters of the samples. Sample

BET surface area (m2 g−1)

Pore volume (cm3 g−1)

Average pore width (nm)

mpg-C3N4 SrTiO3/mpg-C3N4 (1/1) SrTiO3/mpg-C3N4 (1/2) SrTiO3/mpg-C3N4 (1/3)

192.34 91.57 102.20 150.27

0.68 0.35 0.40 0.54

11.14 17.01 18.33 16.23

by Fig. 7B, the formation of •OH radicals was enhanced by increasing the ultrasonic irradiation time. 3.2. Effects of mass ratios of SrTiO3 and mpg-C3N4 on sonocatalytic activity of SrTiO3/mpg-C3N4 The sonocatalytic activity of SrTiO3/mpg-C3N4 nanocomposites can be dramatically affected by tuning the SrTiO3/mpg-C3N4 mass ratio. Therefore, three different SrTiO3 and mpg-C3N4 mass ratios (1/1, 1/2, and 1/3) were examined to find out their impacts on BV10 sonocatalytic elimination at constant sonocatalyst dosage (0.3 g L−1), initial dye concentration (10 mg L−1), ultrasonic power (240 W), pH (5), and time span (120 min) (Fig. 8). According to Fig. 8, changing mass ratio of SrTiO3 and mpg-C3N4 from 1/1 to 1/2 had raised the removal efficiency

Fig. 8. Effect of mass ratio of SrTiO3 and mpg-C3N4 on the removal of BV10. Experimental condition: [SrTiO3/mpg-C3N4] = 0.3 g L−1, [BV10]0 = 10 mg L−1, ultrasonic power = 240 W, and pH = 5.

from 70.3% to 80%; respectively. However, a mass ratio of 1/3 had reduced the removal efficiency to 73.4%. It seemed that the sonocatalytic activity of as-synthesized sonocatalysts was linked to the interaction of SrTiO3 NCs with mpg-C3N4 in the hybrid structure. This observation might be also ascribed to the ability of mpg-C3N4 to produce electrons following ultrasonic irradiation, and to the ability of SrTiO3 to accept electrons. Supposedly, further addition of mpg-C3N4 up to 1/2 could increase the trapping sites of sonogenerated carriers leading to their prolonged lifetime, which could improve sonocatalytic activity. However, a superfluous mpg-C3N4 addition N1/2 could work as a recombination center of electrons and holes, and consequently reduce the sonocatalytic activity [45]. Furthermore, it should be noted that the SrTiO3/mpg-C3N4 (1/2) nanocomposites perform better among the samples under the ultrasonic irradiation; hence, of which those with a mass ratio of 1/2 were used as the most active sonocatalysts to continue the rest of trials. 3.3. Comparison of various processes for BV10 removal

Fig. 7. (A) PL spectra of mpg-C3N4, SrTiO3, and SrTiO3/mpg-C3N4 (1/2) samples, (B) PL spectra changes during sonocatalytic process with terephthalic acid solution for the SrTiO3/mpg-C3N4 (1/2) nanocomposites at different time intervals.

The results of a comparative study on BV10 removal were displayed in Fig. 9 via various processes to discover the most effective system. A low (15.1%) removal efficiency was also obtained as BV10 was exposed to sonication, which might be caused by the low production rate of •OH radicals through the only sonolysis. The next experiments were also run by the removal of ultrasonic irradiation. No considerable removal of BV10 was further observed following adsorption of BV10 on the SrTiO3 and mpg-C3N4 after 120 min of reaction time. It is appealing that the adsorption of BV10 on SrTiO3/mpg-C3N4 (38.8%) inhibits those of SrTiO3 (15.6%) and mpg-C3N4 (18.1%), suggesting a synergistic effect between SrTiO3 NCs and mpg-C3N4. However, sonocatalysis with the bare mpg-C3N4, SrTiO3, and SrTiO3/mpg-C3N4 nanocomposites presented removal efficiencies of 52.5%, 54.5%, and 80%; respectively. This improvement could be attributed to the developed cavitation phenomena occurred on the sonocatalyst surface giving rise to more H2O dissociation and further •OH radical generation [21,32]. Furthermore, the solid particles could enhance the mass transfer rate of pollutants between the reactants [32]. Therefore, the US/SrTiO3/mpg-C3N4 system could be regarded as an effective process for BV10 elimination because of the synergy effect of the hybrid process [32]. As different processes and conditions including the aspects of sonolysis, adsorption, and sonocatalytic were tested in the present study, it was possible to evaluate the effectiveness of the processes through specifying their

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P. Eghbali et al. / Journal of Molecular Liquids 290 (2019) 111208

3.4. Studies on the effects of important parameters on sonocatalytic removal of BV10

Fig. 9. Comparison between various processes involved in the sonocatalysis of BV10. Experimental condition: [catalyst] = 0.3 g L−1, [BV10]0 = 10 mg L−1, ultrasonic power = 240 W, and pH = 5.

synergistic factor, which was estimated for US/mpg-C3N4, US/SrTiO3, and US/SrTiO3/mpg-C3N4 systems using Eq. (1). Accordingly, pseudofirst-order kinetic model (Eq. (2)) was considered to assess the rate of BV10 removal through sonolysis, adsorption, sonocatalysis processes, and corresponding rate constants (kapp). The effects of various processes on the apparent pseudo-first-order constant of BV10 removal were shown in Table 4. The rate constant value of US/SrTiO3/mpg-C3N4 system was similarly detected to be the uppermost for the elimination of BV10, explaining that it was the most active system among all the examined processes. The obtained regression coefficients (R2) in all the processes approximated to 1, indicating the ability of pseudo-firstorder kinetic equation in proper description of the BV10 removal process.

Synergy factor ¼

ln

ksonocatalysis ksonolysis þ kadsorption

ð1Þ

  A0 ¼ kapp t A

ð2Þ

where, kapp refers to rate constant (min−1) and A0 and A are initial and final absorbance of BV10 at 0 and t min of reaction. Values of 2.5, 2.8, and 2.9 were further calculated for the synergy factor of US/mpg-C3N4, US/ SrTiO3, and US/SrTiO3/mpg-C3N4 system processes, respectively; clarifying the positive interaction of ultrasound irradiation and catalysts for effective removal of BV10.

Table 4 Determination of the removal efficiency, pseudo-first-order kapp (min−1) removal rate constants and coefficient of determination (R2) within 120 min of reaction time. Processes Only US SrTiO3 mpg-C3N4 SrTiO3/mpg-C3N4 US/mpg-C3N4 US/SrTiO3 US/SrTiO3/mpg-C3N4

Removal efficiency (%)

kapp (min−1)

15.04 15.57 18.07 38.84 52.48 54.45 80

0.0012 0.0013 0.0014 0.0039 0.0065 0.0071 0.0146

R2 0.9258 0.8864 0.9119 0.9683 0.9861 0.9613 0.9838

3.4.1. SrTiO3/mpg-C3N4 dosage Using an ultrasonic power of 240 W, the influence of the SrTiO3/ mpg-C3N4 dosage on the sonocatalytic elimination of BV10 from aquatic environment (10 mg L−1) at pH 5 was inspected. According to Fig. 10A, the sonocatalyst dosage reached an optimum value that resulted in the uppermost removal efficiency; showing an enhancement from 69.2% to 80% during 120 min of reaction time by raising the sonocatalyst dosage from 0.1 g L−1 to 0.3 g L−1. Obviously, high quantities of •OH radicals and reactive sites are supplied for the elimination of target pollutant by increasing the dosage of the sonocatalyst, giving rise to the elevated removal efficiency [25]. The removal efficiency, however, revealed a significant decline at the sonocatalyst dosage above 0.3 g L−1. Such a decrease with the extra sonocatalyst could be explained by the accumulation of the sonocatalyst in the solution, which consequently reduced the surface reactive sites for the generation of •OH radicals. Moreover, the ultrasound might be disseminated by the additional catalyst in the dye solution, which could inhibit heat and energy conduction near the surface of the catalyst [51]. Accordingly, further experiments were conducted with an optimum SrTiO3/mpg-C3N4 dosage of 0.3 g L−1. 3.4.2. BV10 concentration The impact of initial solute concentration on the removal efficiency of BV10 at pH 5 in the presence of SrTiO3/mpg-C3N4 nanocomposites (0.3 g L−1) was studied by an ultrasonic power of 240 W. The results (Fig. 10B) highlighted that the removal efficiency had gradually dropped from 88.7% to 54.0% by increasing the dye concentration from 5 mg L−1 to 20 mg L−1, respectively, within a reaction time of 120 min. This observation might be described by the occupation of more active sites on the sonocatalyst due to the higher concentration of BV10 molecules hindering the absorption of heat and energy released from the collapse of the cavitation bubbles. Thus, the reduced energy fairly decreased the production of oxidizing agents; thereby, reducing the removal efficiency of the process [52,53]. Additionally, increasing BV10 concentration and fixed values for other operational parameters generated similar numbers of •OH radicals, which needed elimination of more BV10 molecules and also intermediates from their disintegration. Consequently, an ascending trend in solute concentration significantly decreased the removal efficiency [23]. Moreover, the formation of reactive •OH on the sonocatalyst surface was reduced by highlyconcentrated BV10 solutions because the dye molecules screened active sites on the surface, which resulted in the deficient production of •OH radicals and low removal levels [54]. 3.4.3. Ultrasonic power Depending on the extent of ultrasound power used, it is possible to significantly alter the number of generated active cavitation bubbles [55,56]. Therefore, the influence of ultrasonic power on the elimination of the BV10 was assessed at pH value of 5 using SrTiO3/mpg-C3N4 nanocomposites (0.3 g L−1) and BV10 (10 mg L−1) (Fig. 10C). Clearly, boosting ultrasonic power could raise the removal efficiency. Ultrasonic powers of 150, 240, 300, and 350 W correspondingly yielded BV10 removal efficiencies of 57.9%, 80%, 81.8%, and 84.7%, respectively, during the course of 120 min reaction time. Such an improved efficiency could be resulted by the elevated generation of •OH radicals caused by the accelerated cavitation phenomenon at high ultrasonic powers. It should be noted that the cavitation at lower ultrasonic power at higher intensities is more drastic than cavitation [57]. Accordingly, higher localized temperatures and pressures are produced at the cavitation sites formed under low powers. However, the optimum ultrasonic power is specific to each system and it depends on both photon energy and photon number generated due to the hotspot and sonoluminescence phenomena [57]. On the other hand, at lower ultrasonic powers, photon energy is enhanced; while the number of photons

P. Eghbali et al. / Journal of Molecular Liquids 290 (2019) 111208

9

Fig. 10. Sonocatalytic removal of BV10 at (A) SrTiO3/mpg-C3N4 dosage. Experimental condition: [BV10]0 = 10 mg L−1, pH = 5, and ultrasonic power = 240 W; (B) dye concentration. Experimental condition: [SrTiO3/mpg-C3N4] = 0.3 g L−1, pH = 5, and ultrasonic power = 240 W; (C) ultrasonic power. Experimental condition: [SrTiO3/mpg-C3N4] = 0.3 g L−1, [BV10]0 = 10 mg L−1, pH = 5; (D) pH of solution. Experimental condition: [SrTiO3/mpg-C3N4] = 0.3 g L−1, [BV10]0 = 10 mg L−1, and ultrasonic power = 240 W.

is increased at higher ultrasonic powers [57]. Moreover, the system turbulence rises as a result of using high ultrasonic powers, ultimately leading to enhanced mass transfer rate of the pollutant from liquid to the surface of SrTiO3/mpg-C3N4 nanocomposites [32]. Finally, another possible explanation could be that free and accessible reactive sites on the catalyst surface are maintained with the cleaning action of the ultrasonic irradiation [54,58]. Even so, no significant differences were recorded for the decomposition rate using ultrasonic powers of 240, 300, and 350 W. An ultrasonic power of 240 W, therefore, was chosen for the subsequent experiments because of its cost-effective energy consumption. 3.4.4. Initial pH Industrial wastewater typically displays a wide range of pH. Besides, the pH of dye aqueous solution plays a substantial role in various systems [59,60]. Therefore, the influence of pH on BV10 elimination was examined at pH ranging from 4 to 10 by the inclusion of the BV10 (10 mg L−1), SrTiO3/mpg-C3N4 nanocomposites (0.3 g L−1), and an ultrasonic power of 240 W. The results revealed that a higher removal efficiency of BV10 was obtained at acidic medium rather than that of basic one (Fig. 10D). The removal efficiency enhancement of the pollutant in acidic phase was also accompanied by the BV10 protonation leading to enriched hydrophobicity of the molecules [61] which brought about a

better accessibility of the molecules to the bubble-liquid interface, whereby it yielded maximum concentration of •OH radicals. Other relevant studies in this domain reported that hydrophobicity of BV10 had not been altered at acidic conditions [55,61,62]. The carboxyl group could be also easily deprotonated whereas pH of solution had exceeded the acid dissociation constant (pKa = 3.7) of BV10, which could alter the cationic form of BV10 into zwitterionic one [55,61]. Accordingly, the dye molecule was inhibited through the change of hydrophobic property from approximating the negatively charged cavitation bubbles and also the surface of sonocatalyst [61]. It would diminish the concentration of •OH radicals; thereby, influencing the removal efficiency of BV10. As a different perspective, it is also believed that falling pH levels raises the oxidation potential of •OH radicals, which could further account for the elevated removal efficiency [25,62]. On the other hand, the high pH values lead to scavenged •OH radicals and negate their reaction with dye molecules [63]. Apparently, a higher accessibility of BV10 to active sites of sonocatalyst resulted in a greater removal efficiency of BV10 dye at acidic and natural pH levels (85.6% and 80%, respectively). This outcome normally denotes a much-applied sense as the reaction system needs no constant supplementation of an acid or provision of the reaction process with a buffer solution to preserve acidic condition in the reaction system. Consequently, subsequent experiments were carried out at the natural pH (pH = 5) of the BV10 solution.

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3.5. Kinetics studies on the sonocatalytic removal of BV10 The corresponding data obtained on the kinetics of sonocatalytic removal of the BV10 under ultrasonic irradiation over the operational parameters were presented in Table 5. According to the authors' review of the related literature, the Langmuir-Hinshelwood kinetic expression could be applied to the sonocatalytic process (Eq. (2)) [64]. The data from the experiments were then inserted into a pseudo-first-order kinetic formula, whose results such as regression coefficient and the kapp values were presented in Table 5. It could be observed from Table 5 that the R2 value was over 0.90 in any individual case, demonstrating a good description of the BV10 removal process by the selected kinetic equation. 3.6. Effect of scavengers on BV10 removal The mechanism for the sonocatalytic removal of BV10 using SrTiO3/ mpg-C3N4 nanocomposites was determined through trapping experiments with active species using a sonocatalytic process. A series of removal experiments were also performed here through various scavengers to assess the roles of different active species in BV10 elimination. The employed scavengers were EDTA-2Na, t-BuOH, and BQ for h+, •OH, and O−· 2 ; respectively [9]. A removal efficiency of 80% was also obtained, since the reaction could be completed in the absence of any scavenger. The removal efficiency, however, declined to 42.12%, 32.10%, and 26.74% in the presence of EDTA-2Na, BQ, and t-BuOH; respectively (Fig. 11). It was observed that various impacts of the decomposition rate by addition of scavengers had followed the order of EDTA2Na b BQ b t-BuOH. It should be noted that sonocatalytic removal is af− − fected by ample quantities of ions such as Cl−, SO2− 4 , NO3 , NO2 , etc. in real water samples. The influence of matrix was also examined with some well water to assess the effectiveness of a sonocatalytic process in BV10 dissociation. The presence of several typical anions could have resulted in the observed drop in the BV10 removal efficiency in the well water [65]. Based on the above observations, it was concluded that •OH radicals had an influential contribution to the sonocatalytic removal of BV10. 3.7. Proposed reaction mechanism for sonocatalytic removal of BV10 There are two standpoints, viz. “hot spot” and “sonoluminescence” explaining the mechanism of sonocatalytic BV10 removal in the presence of SrTiO3/mpg-C3N4. The first mechanism includes the cavitation phenomenon in the aquatic phase forming “hot spots” at high temperatures [66]. Such hot spots can thus stimulate the pyrolysis of water molecules to generate •OH radicals and H• radicals as shown in Eqs. (3) and (4) [32,67]. Oxygen molecules are also disintegrated to Table 5 Effect of the operational parameters on the apparent pseudo-first-order constant of removal for sonocatalytic process. [BV10]0 (mg L−1)

[SrTiO3/mpg-C3N4] (g L−1)

pH

Ultrasonic power (W)

kapp (min−1)

10 10 10 10 10 5 15 20 10 10 10 10 10 10 10

0.1 0.2 0.3 0.4 0.5 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

5 5 5 5 5 5 5 5 5 5 5 4 7 8 10

240 240 240 240 240 240 240 240 150 300 350 240 240 240 240

0.0097 0.0128 0.0146 0.0129 0.0091 0.0177 0.0101 0.0063 0.0075 0.0151 0.0158 0.0167 0.0121 0.0113 0.0102

R2 0.9588 0.9499 0.9838 0.9785 0.9346 0.9909 0.9750 0.9094 0.9524 0.9803 0.9881 0.9928 0.9889 0.9573 0.9893

Fig. 11. Effect of different scavengers on the removal efficiency of BV10. Experimental condition: [SrTiO3/mpg-C3N4] = 0.3 g L−1, [BV10]0 = 10 mg L−1, [scavenger] = 2 mM, pH = 5, and ultrasonic power = 240 W.

yield oxygen atoms, generating •OH radical after reacting with water molecules (Eqs. (5), (6)) [32]. ÞÞÞ

H2 O →  OH þ H

ð3Þ



ð4Þ

OH þ  OH→H2 O2 ÞÞÞ

O2 → 2O

ð5Þ

H2 O þ O →2HO

ð6Þ

It is also worth pointing out that the local high temperature and its heat energy can possibly stimulate the thermal excitation of the semiconductor; thereby, leading to the generation of e− − h+ pairs like the light [68]. In this respect, the light generated by the cavitation effect can create a wide range of wavelength lights, and the use of such wavelengths can be utilized to improve the sonocatalytic system. In this study, a novel sonocatalyst was synthesized, composed of a wideband gap semiconductor (SrTiO3) and a narrow-band one (mpgC3N4), whose band gaps were 3.25 eV and 2.82 eV; respectively. It should be noted that the combination of both wide- and narrow-band gap semiconductors can effectively extend the range of the optical response [69]. Besides, utilization of semiconductor catalysts in the sonocatalytic system can improve the efficiency of the sonocatalytic system through the formation of e− − h+ pairs following the excitation of e− from the valence band (VB) to the conduction band (CB). The sonoluminescence mechanism can thus account for such an improvement in the presence of sonocatalyst. In sonoluminescence, light is emitted through the recombination of the free radicals created within cavitation bubbles [24]. The considerable development of the sonocatalytic activity of sonocatalysts was attained by an important contribution of the highly efficient charge separation during a sonocatalytic reaction. To understand the separation of the sonogenerated charge carriers in the SrTiO3/mpg-C3N4 nanocomposites, the conduction of CB and VB potentials of the SrTiO3 and mpg-C3N4 were determined by the following equations [70]: 1 ECB ¼ χ−Ee − Eg 2

ð7Þ

EVB ¼ ECB þ Eg

ð8Þ

P. Eghbali et al. / Journal of Molecular Liquids 290 (2019) 111208

where EVB and ECB show the VB and CB band potentials, respectively; χ refers to the absolute electronegativity of the semiconductor, Ee is the energy of free electrons vs. hydrogen (4.5 eV vs. NHE), and Eg is the band gap energy of the semiconductor [70,71]. The calculated χ values were 5.29 and 4.73 for SrTiO3 and mpg-C3N4; respectively [72]. According to these data, ECB values of −0.83 and −1.30 eV vs. NHE were estimated for SrTiO3 and mpg-C3N4, respectively, and the corresponding EVB values were 2.41 and 1.52 eV vs. NHE. The fast recombination of e− − h+ pairs was also inhibited by the suitable band edge positions and close interfacial connection between mpg-C3N4 and SrTiO3 NCs, leading to an increased sonocatalytic activity to eliminate the BV10. This evidence reflected a rational mechanism for the boosted sonocatalytic activity of the SrTiO3/mpg-C3N4 nanocomposites under ultrasonic irradiation presented schematically (Fig. 12), comprised of the advancement of the charge transfer efficiency at the SrTiO3/mpgC3N4 interface. According to Fig. 12, mpg-C3N4 is excited upon irradiating the SrTiO3/mpg-C3N4 nanocomposites by ultrasonic waves to produce e− − h+ pairs because of its low band gap (2.82 eV). It should be noted that the sonoluminescence could result in the formation of the light flash with an average photon energy of 6 eV as that in a photocatalytic process [73]. The sonosensitization of BV10 molecules adsorbed on the catalyst should be also considered under ultrasonic irradiation. According to the review of the related literature [1,73], the BV10 molecules could inject the excited electrons into the CB of SrTiO3, and they could also become BV10 cationic radicals (BV10·+). The oxidized dye might have been reduced by the oxidation of BV10 or other reactions. On the other hand, the electrons could migrate from the CB of mpg-C3N4 to the CB of SrTiO3 because the CB potential of mpg-C3N4 is more negative than that of SrTiO3. Additionally, VB potential of mpg-C3N4 is less positive than that of SrTiO3 and the sonogenerated holes over the SrTiO3 consequently transfer to the VB of mpg-C3N4. Accordingly, SrTiO3/mpg-C3N4 nanocomposites efficiently reduce the recombination of sonogenerated e− − h+ pairs, hence, boosting the sonocatalytic removal efficiency. On the other hand, it is well known that the abundant mesoporous structure of mpg-C3N4 would contribute to generating more active species and adsorb vast

11

contaminants on the sonocatalyst surface [74]. Thereby, efficient separation of sonogenerated carriers is supposed to render the improved sonocatalytic activity under ultrasonic irradiation for SrTiO3/mpg-C3N4 nanocomposites. Thus, the dissolved O2 to O−· 2 are declined by the electrons gathered on the CB of SrTiO3 because of more negative CB edge po+ tential than E0 (O2/O−· 2 ) (−0.33 eV vs. NHE) [72]. The h found in VB of mpg-C3N4 is not also able to oxidize OH− into •OH because the VB edge potential of mpg-C3N4 (1.52 V) is less positive than the standard redox potential of OH−/·OH (2.38 eV vs. NHE) [75]. Therefore, the h+ present in mpg-C3N4 oxidizes the BV10 molecules into removal products. However, the h+ on the VB of SrTiO3 (2.41 eV) is capable of reacting with OH− ions to produce •OH radicals because of more positive potentials in comparison with OH−/·OH (2.38 eV vs. NHE); though, it cannot directly react with H2O on account of the band potential values (•OH/ + H2O = 2.72 eV vs. NHE). Thus, it can be concluded that O−· and 2 , h • OH species contribute to the pollutant elimination. The following equations represent a summary of the entire sonocatalytic reaction mechanism under ultrasonic irradiation: Ultrasonic irradiation ðcavitation effectÞ→light ðsonoluminescenceÞ þ heat ðhot spotÞ ð9Þ ÞÞÞ

BV10 → BV10

ð10Þ ÞÞÞ

SrTiO3 =mpg‐C3 N4 þ BV10 → BV10þ þ SrTiO3 e− CB ÞÞÞ

þ

SrTiO3 =mpg‐C3 N4 → SrTiO3 =mpg‐C3 N4 e− CB þ hVB

ð11Þ


ð12Þ



− mpg‐C3 N4 e− CB →SrTiO3 eCB

ð13Þ

þ
þ
SrTiO3 hVB →mpg‐C3 N4 hVB

ð14Þ


− SrTiO3 e− CB þ O2 →O2 þ
OH− þ SrTiO3 hCB → OH

Fig. 12. A proposed mechanism for sonocatalytic removal of BV10 over SrTiO3/mpg-C3N4 nanocomposites under ultrasonic irradiation.

ð15Þ ð16Þ

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P. Eghbali et al. / Journal of Molecular Liquids 290 (2019) 111208 þ

  þ O− 2 þ h þ OH þ BV10=BV10 =BV10 →removal products

ð17Þ

3.8. Reusability and stability of SrTiO3/mpg-C3N4 nanocomposites The SrTiO3/mpg-C3N4 sonocatalyst was evaluated in terms of reusability in order to prove the durability and cost-effectiveness of the system (Fig. 13A); in which, constant values were considered for all factors including reaction time, pH, solute concentration, dosage of sonocatalyst, and ultrasonic power. To this end, the sonocatalyst was collected and rinsed three times with deionized water, followed by drying in an oven at 80 °C. Then, the recovered catalyst was consumed anew for the subsequent cycles in the removal process. The sonocatalyst

was also evaluated for reusability up to five times and its catalytic activity was tested under ultrasound irradiation. The recovered sonocatalyst exhibited to be reusable for five rounds with no marked activity loss. Every five cycles of sonocatalysis mediated by SrTiO3/mpg-C3N4 also yielded removal efficiencies of 80%, 79.6%, 78.7%, 74.8%, and 70.2%, respectively, that clearly demonstrated durability of SrTiO3/mpg-C3N4 nanocomposites for the removal of target pollutants from aqueous solution. An assessment was also made on the nature of the recovered SrTiO3/mpg-C3N4 (1/2) nanocomposites. As depicted in Fig. 13 (B, C) the XRD and HR-SEM of the nanocomposites resembled those of the fresh nanocomposites after the five cycles. Such an outcome confirmed no significant structural changes in the SrTiO3/mpg-C3N4 (1/2) nanocomposites with a suitable stability throughout the sonocatalytic process.

Fig. 13. (A) Reusability of SrTiO3/mpg-C3N4 (1/2), (B) XRD pattern, and (C) HR-SEM of SrTiO3/mpg-C3N4 (1/2) nanocomposites before and after five consecutive experimental runs.

P. Eghbali et al. / Journal of Molecular Liquids 290 (2019) 111208

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Table 6 Sonocatalytic removal of BV10 by various catalysts in aqueous solution. Catalyst Fe3O4 MGZ@SiO2 TiO2 nanotube LuFeO3 TiO2 nanoflake Sr(OH)28H2O AgBr ZnO/CNTs Graphene-TiO2 Bi12O17Cl2 SrTiO3/mpg-C3N4

Initial BV10 concentration

Catalyst dosage

pH

30 mg L−1 25 mg L−1 50 mg L−1 5 mg L−1 10 mg L−1 10 mg L−1 10 mg L−1 20 mg L−1 2 × 10−5 mol L−1 8 mg L−1 10 mg L−1

1.5 g L−1 0.5 g L−1 2 g L−1 4 g L−1 0.5 g L−1 1 g L−1 1 g L−1 30 mg/30 mL 0.02 g/100 mL 2 g L−1 0.3 g L−1

3 6 Neutral 6.5 6.9 7 7 No data No data 7.5 5 (Natural)

3.9. Comparison of efficiency of SrTiO3/mpg-C3N4 sonocatalyst with literature In order to show the advantages of the present US/SrTiO3/mpg-C3N4 system, the obtained results for the removal of BV10 over the SrTiO3/ mpg-C3N4 (1/2) nanocomposites were compared with several reported sonocatalysts in the viewpoint of consumed catalyst dosage, ultrasonic power, as well as removal efficiency. The removal efficiency values of various sonocatalysts for BV10 removal were shown in Table 6, as noted in previous reports [7,16,61,76–82]. This comparison indicated the superiority of present sonocatalyst for the removal of BV10 from water solution compared with other sonocatalysts. Particularly, the removal rate in the presence of most reported sonocatalysts took longer times and required a high catalyst to dye ratio. 4. Conclusion To sum up, a successful synthesis of a novel and effective sonocatalyst SrTiO3/mpg-C3N4 was developed via a self-assembly method and its sonocatalytic performance was investigated for the elimination of BV10. The assembly of SrTiO3 NCs on mpg-C3N4 was also confirmed through advanced analytical techniques. A conspicuous sonocatalytic performance was also exhibited by the as-synthesized SrTiO3/mpg-C3N4 (1/2) nanocomposites as opposed to pure SrTiO3 and mpg-C3N4 samples, suggesting a synergistic effect between SrTiO3 NCs and mpg-C3N4. Furthermore, it was experimentally reported that mass ratio, sonocatalyst dosage, initial pollutant concentration, ultrasonic power, and pH of the solution could alter the removal efficiency. The highest removal efficiency of 80% was also obtained when 0.3 g L−1 of SrTiO3/mpg-C3N4 nanocomposites was used with 1/2 mass ratio and 10 mg L−1 dye at pH 5 and an ultrasonic power of 240 W during a 120 min of time span. Moreover, the presence of radical scavengers reduced the removal efficiency of BV10, implying the dominant role of free radicals for BV10 removal in the sonocatalytic system. A possible mechanism was further presented for the elimination of BV10 in the sonocatalytic system. Based on the results of reusability experiments, effective wastewater treatment would be plausible through the reusable synthesized SrTiO3/mpg-C3N4 nanocomposites. Acknowledgments The financial support by the Science Academy in the context of “Young Scientists Award Program (BAGEP)” is highly acknowledged. Paria Eghbali as a post-doc researcher gratefully acknowledges the support of Atatürk University. References [1] T. Sureshkumar, S. Thiripuranthagan, S.M.K. Paskalis, S. Kumaravel, K. Kannan, A. Devarajan, Synthesis, characterization and photodegradation activity of graphitic C3N4-SrTiO3 nanocomposites, J. Photochem. Photobiol. A 356 (2018) 425–439.

Ultrasonic power 450 W 650 W 50 W 40 kHz 80 W 80 W 80 W 200 W 750 W 300 W 240 W

Time 120 min 60 min 120 min 90 min 180 min 180 min 180 min 60 min 150 min 30 min 120 min

Removal efficiency 75.94% 84% 85% ~89% 74% ~75% 72% 49% 88.66% 86% 80%

Refs. [7] [16] [61] [76] [77] [78] [79] [80] [81] [82] This study

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