GO nanocomposites: An affordable photocatalyst for the mitigation of carcinogenic organic pollutants

Colloids and Surfaces A 596 (2020) 124721

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Rational design of novel ternary Sm2WO6/ZnO/GO nanocomposites: An affordable photocatalyst for the mitigation of carcinogenic organic pollutants

T

M. Arunpandiana, K. Selvakumarb, A. Rajac, P. Rajasekarand, C. Ramalingana, E.R. Nagarajana, A. Pandikumare,f, S. Arunachalama,* a Nanomaterials Laboratory, Department of Chemistry, School of Advanced Sciences, International Research Centre, Kalasalingam Academy of Research and Education (Deemed to be University), Krishnankoil, 626126, Tamil Nadu, India b Institude of Microstructure and Property of Advanced Materials, Beijing University of Technology, 100 Ping Le Yuan, Chaoyang, Beijing, 100124, PR China c Multifunctional Materials Research Laboratory, Department of Physics, International Research Centre, Kalasalingam Academy of Research and Education (Deemed to be University), Krishnankoil, 626126, Tamil Nadu, India d Graduate School of Science and Technology, Shizuoka University, 3-5-1 Joho-ku, Naka-Ku, Hamamatsu, 432-8011, Japan e Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi, 630003, Tamil Nadu, India f Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, 201002, India

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Photodegradation UV-light irradiation Graphene oxide Sm2WO6-ZnO/GO Degradation pathway Organic pollutants

In this scenario, a ternary type of novel Sm2WO6/ZnO incorporated on GO (SWZG) nanocomposites were synthesized by the ultrasonic-assisted hydrothermal route and confirmed by various characterization techniques. The diffraction peaks obtained from Powder-X-ray analysis reveals good crystalline nature of as preparedSWZG composite. Scanning Electron Microscopereveals the red algae like the hexagonal structure of SWZ composition an elongated form on the graphene oxide sheet. The Transmission electron spectroscopy reveals the confirmation of algae like structure of Sm2WO6 and ZnO in elongated form on the surface of the graphene oxide sheet. The elemental composition and the oxidation state of C, Zn, O, Sm and W were confirmed by X-ray Photoelectron Spectroscopy analysis. The charge separation efficiency of the photogenerated electron-hole pairs in SmW, SmWZ and SmWZG was confirmed by photoelectrochemical measurements. SWZG catalyst shows superior photocatalytic activity towards the degradation of Ciprofloxacin (CIP) and Methylene blue (MB) with the efficiency of 96.92 & 98.24 % within 90 and 25 min. In Reactive Oxidative Species study, photogenerated holes and

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Corresponding author. E-mail address: [email protected] (A. S.).

https://doi.org/10.1016/j.colsurfa.2020.124721 Received 27 January 2020; Received in revised form 11 March 2020; Accepted 16 March 2020 Available online 18 March 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved.

Colloids and Surfaces A 596 (2020) 124721

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superoxide radicals are the main reactive species in the CIP and MB degradation process. Besides, the stability and reusability of the catalysts were confirmed by the recycle test. In the CIP degradation pathway, the reaction intermediate (7-amino-1-cyclopropyl-6-fluoro-2-hydroxy-4-oxo-1,4-dihydro quinolone-3-carboxylic acid (m/z 278.07)) were elucidated by Gas Chromatography–Mass Spectroscopy analysis and a feasible degradation pathway was also proposed. Finally, the ternary SWZG composite would be a promising candidate for the degradation of organic pollutants.

1. Introduction

charge-transfer characteristics. It has been considered as an excellent platform for the preparation of nanocomposites, which enhance the photocatalytic efficiency [42]. In this present work, the SWZG nanocomposite is prepared by hydrothermal route. Towards the degradation process, the as-prepared SWZG nanocomposites having highly efficient degradation performance in the degradation of ciprofloxacin (CIP) and methylene blue (MB) under UV light illumination. The pollutant CIP is a crucial material for the treatment of bacterial diseases. CIP is a second-generation fluoroquinolone (FQ) antibiotic [43]. The textile dyes like MB is an abundant source of a coloured organic compound and is becoming an increasingly common environmental risk. MB is used in the dye industry and printing textiles industry and is the main water pollutant to the aquatic environment [44,45]. The degradation method is the main technique for the decomposition of pollutant. In recent years, the semiconductor photocatalyst studies have been reported for the degradation of pollutants because it is a highly efficient material [46–49]. From this work, the CIP and MB are degraded by SWZG catalyst from water to minimize the environmental risks. By this semiconductor photocatalysis process, the harmful organic pollutants are completely mineralized into harmless substances like CO2, H2O and some mineral acids. The structure of ciprofloxacin and methylene blue were represented in Fig. 1.

Environmental pollution is the major problem for human survival; mainly water is one of the most serious problems to threaten human existence [1]. In every year, the level of pollution is increasing by industrialization to damage the environment. The industrial wastewater discharged into the environment without proper treatment and it is very dangerous to both aquatic species and environment [2,3]. Industrial wastewater treatment before discharge is a very complicated process. Because, some of the wastewater treatment processes are very expensive, recently the researchers developed advanced oxidation process (AOPs), it has been developed the wastewater treatment. Among the AOPs process, the photocatalysis is used for environmental remediation including, water purification. Because these methods having more efficient and very low-cost techniques, mainly it is a "green approach" method to mineralized the hazardous organic pollutants [4–6]. Recent studies semiconductor photocatalysts are used to destroy the organic pollutants in the photocatalysis process [7–11]. These photocatalytic processes are having highly effective and Eco-friendly to the environment. The highly efficient and novel catalytic materials syntheses are developed by researchers and it has much importance in recently. The semiconductor photocatalysts are used to degrade the organic pollutants and it has been the beneficial technology for refinery of the environment [12–14] and is also a potential solution to the environmental polluting problem. The variety of semiconductor metal oxides are used in the photocatalytic process including TiO2, ZnO, WO3, ZnS, SnO2 etc, [15–20]. Compared to others, the high efficacy semiconducting materials like TiO2 and ZnO is the best choice for photocatalytic degradation because it having large bandgap and good photosensitivity [21,22]. Among this two semiconductor photocatalyst, the high efficient ZnOhaving a good photocatalytic activity, Because of their unique potential in environmental detoxification [23–33] and it has many advantages, such as low cost, stability, highly active and size-dependent properties [34–36] compared to other catalysts. In recent, the ZnO photocatalysts are developed by coupling with other semiconductor oxide materials [37]. The coupling of two or more semiconductors photocatalytic materials is increasing the photocatalytic efficiency due to the photogenerated electron-hole pairs separation and interfacial charge transfer efficiency having more effective [38,39]. The rare earth metal tungstate products are attracting the researchers, because of their fascinating structure, interesting Physicochemical behaviour and their vast potential for the industrial applications, including paramagnetic behaviour and catalysts,etc., [40,41]. Besides, the carbon-based materials (GO), having very attractive properties like effective surface area, unique structures and outstanding

2. Materials and synthesis methods 2.1. Required of raw materials Samarium acetate (Sigma Aldrich, India), Oxalic acid, Zinc acetate, Sodium hydroxide, Sodium tungstate, Graphite powder, Organic pollutants (MB and CIP)(Merck Chemicals, India) are used as main raw materials. All the reaction solutions are made by De-ionized water with purity.

2.2. Synthesize of Sm2WO6 (SW) In 250 ml beaker, 100 mL of (3 mmol) Na2WO4.2H2Oand 20 mL of (6 mmol) Samarium tungstate were taken and stirred for 30 min. Sodium hydroxide was used to adjust the pH solution by the precipitate formation and then 0.5 g urea was added and stirred for 60 min. The precipitate suspension was involved in the hydrothermal reaction with 180 °C for 24 h. After the hydrothermal reaction, the solid precipitate was washed with water and ethanol and dried at 60 °C for overnight. Finally, it annealed at 550 °C for 8 h in a muffle furnace.

Fig. 1. Structure of (a) ciprofloxacin, (b) methylene blue organic pollutants. 2

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2.3. Synthesize of Sm2WO6–ZnO (SWZ)

Thermo ESCALAB 250 instrument. Shimadzu UV-2450 spectrometer was used to analyze UV–vis diffuse reflection spectroscopy (DRS). UV–vis spectroscopy (Shimadzu UV-1800) was used to predict the absorption peak values for the degradation samples.

In the above Sm2WO6 suspension mixture, 0.4 M Zn (CH3CO2)2·2H2O (100 mL) solution was added drop wisely and stirred for 30 min. Then 0.6 M oxalic acid (100 mL) was mixed drop wisely until stirred for 4 h to the formation of a precipitate. After that, the solution was involved in a hydrothermal reaction using Teflon lined stainless steel autoclave at 180 °C for 12 h. Besides, the solid precipitate was washed with water and ethanol and dried at 60 °C for overnight and treated at 550 °C for 8 h in a muffle furnace.

2.6. Evaluation of catalytic degradation of pollutants The Heber multi-lamp photoreactor (model HML-MP 88) with eight parallel mercury lamps emitting light (wavelength of 365 nm) was used to degrade the organic pollutants (CIP and MB) under UV light illumination. For the degradation process, about 0.04 g of the sample was mixed in 40μM concentration CIP solution (100 mL). The above mixture was stirred at dark within 30 min to assure the adsorption-desorption equilibrium between the SWZG sample and CIP solution. After stirring, the dispersed solution was poured in an open borosilicate glass tube (40 cm height and 20 mm diameter) photo reactor vessel. During the degradation reaction, 5 mL of degradation sample was collected for each 15 min time intervals and analyzed the absorption peak of CIP at 276 nm using UV visible spectrometer. After the irradiation, the remaining catalysts were involved in the reusability test and it reveals the stability of the sample. The aforesaid process is the same as the MB decomposition but the concentration is differing. The photocatalytic reaction chamber (reactor) and the degraded process were shown in Scheme 2.

2.4. Synthesize of Sm2WO6/ZnO-GO (SWZG) Hummer’s method was used to prepare the Graphene oxide [50,51]. GO (1 mg/ml)and the catalyst Sm2WO6/ZnO (Sm2WO6/ZnO are synthesized by the above two following procedure) (100 mg/ml) is ultrasonically mixed and involved in sonication for 1 h. After that, the dispersed solution was stirred for 2 h. Finally, the samples are collected via ultracentrifuged and washed with water. Finally, it dried at 60 °C for 12 h to obtain Sm2WO6/ZnO-GO photocatalysts. The schematic representation of the synthetic route is portrayed in Scheme 1(a) and the synthesis mechanism of Sm2WO6/ZnO-GO nanocomposite was illustrated in Scheme 1(b). 2.5. Characterization techniques

2.7. Formation of by-product procedures and analysis The Scanning electron microscope (SEM) (EVO-80, CARL ZEISS) is used to reveals the morphology structure of the material with the (EDS) spectroscopy (AMETEK-EDAX (Z2e Analyzer)). The purity of phase and the crystalline structure of the samples were determined by X-ray diffraction (XRD) (Bruker instrument of modelD8 advance ECO XRD systems with SSD160 1D Detector) in the range between 10-80° with 0.02′ step size. X-ray photoelectron spectroscopy analysis was examined by

The photocatalytic degradation of the CIP process, the identification of CIP degradation intermediates in SWZG suspension samples after irradiation for 45 min were collected and centrifuged to remove powders for analysis. After that, the clear samples were withdrawn and involved in a separation method using 150 mL of chloroform. The organic moiety was collected separately and evaporated under heating

Scheme 1. a) Schematic representation of hydrothermal preparation of Sm2WO6/ZnO/GO (SWZG) nanocomposite and b) Synthesis mechanism of Sm2WO6/ZnO/GO nanocomposite. 3

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is having a red algae morphology structure. The ZnO has an irregular hexagonal morphology structure and having a good number of cavities in the surface of the sample (Fig. 3c). In Fig. 3d–f presents SEM images of Sm2WO6-ZnO composite indicating the formation of combined red algae like hexagonal structure. In Fig. 3g, h reveals SEM image of Sm2WO6-ZnO/GO nanocomposite containing Sm2WO6, ZnO and GO nanosheets. The composite Sm2WO6ZnO is well deposited on the surface of GO layer, which can improve their photocatalytic performance. The corresponding EDS elemental images of Sm2WO6-ZnO/GO nanocomposite are shown in Fig. S1 with the homogeneously distributed C, O, Zn, Sm and W respectively. The TEM image of the SWZG nanocomposite is portrayed in Fig. S2 and it reveals the confirmation of algae like structure of Sm2WO6 and ZnO in elongated form on the surface of the graphene oxide sheet. It is one of the evidence for the morphology structure of the material. The Sm2WO6 and the ZnO materials are well dispersed in the surface of GO layer. Fig. S2 (e) reveals that the fringe pattern of ZnO and Sm2WO6 and it is confirmed by the crystalline nature of the as synthesized material.

3.3. XPS analysis To investigate the factual information about the element composition and their valance state of host and dopant elements of the prepared nanocomposite were accomplished by X-ray photoelectron spectroscopy measurements. In Fig. 4 the formation of SWZG nanocomposite which confirmed by the XPS spectroscopy with exposed the corresponding signals of elements like, samarium (Sm), Oxygen (O), Carbon (C), Tungsten (W) and Zinc (Zn). The formation of presenting elements in XPS spectroscopy is well under the EDX analysis. Fig. 4(b–f) views the XPS spectra for carbon (C) 1 s, samarium (Sm) 3d, tungsten (W) 4f, oxygen (O) 1 s and zinc (Zn) 2p respectively. For Fig. 4b the Samarium having the valence state of 3+ and shows the core spectrum peak for Sm 3d3/2 and Sm 3d5/2 with the binding energies 1083.7 eV and 1110.1 eV respectively. The value for spin energy separation between Sm 3d3/2 and Sm 3d5/2 are 26.4 eV. It has a good agreement compared to the previous reports [55]. The binding energy value 284.7eV corresponds to the C 1s spectrum peak. For Zn, the XPS spectra are displayed in Fig. 4f and the centred peaks Zn 2p3/2 and Zn 2p1/2 having binding energy at 1022.2ev and 1045.2 eV, which indicates Zn2+ valence state. In O1 s, the high-intensity peaks at 532.1 eV having an adsorbed oxygen species and the other two values 530.3ev and 533.8 eV are the deconvolution peaks and it relates to the lattice and chemisorbed oxygen. The W 4f5/2 and W 4f7/2 peaks are shown in fig. and it locates at the range 34.6 eV and 36.9 eV [56] respectively. The results conclude that the binding energy values for the presenting elements are confirmed by

Scheme 2. Schematic representation of photocatalytic reactor and the degradation process.

and then reconstituted with 2 mL of chloroform mixture. From the collecting samples, the removal of main intermediates product of CIP was predicted by gas chromatography/mass spectrometry (GC/MS) at optimum condition for 45 min. to execute this chromatographic analysis, gas chromatography (Agilent technologies 7820A) system equipped with series mass selective detector (5977E MSD), 7673 series autosampler were used with a DB-5MS capillary column. By this study, the main intermediate product of CIP degradation was reported. 3. Results and discussions 3.1. Powder X-ray diffraction (PXRD) analysis Powder X-ray diffraction was employed to analyze the crystalline structure and phase purity of the synthesized materials. The well-defined X-ray diffraction peaks of GO, ZnO, Sm2WO6, Sm2WO6/ZnO-GO materials are represented in Fig. 2. The obtained sharp peaks at the scattering angles, 2θ = 18.21, 21.22, 24.13, 28.06, 28.78, 29.66, 31.55, 32.56, 34.16, 46.09, 47.98, 49.02, 54.38° and the miller indices planes are affirmed by a body-centred monoclinic structure of Sm2WO6 (JCPDS – 23-1401). In Sm2WO6, there is no impurity peaks (Sm2O3, WO3) were observed because as-synthesized samples having high purity in nature. For GO, the intense peak present at 11.3° attributes to (001) plane and it is shown in Fig. 2a. The diffraction peaks of ZnO (JCPDS 36-1451) [52–54] can be attributed to the peaks at 31.6, 34.23, 36.18, 47.38, 56.54, 62.81, 66.22, 67.89, 69.08° respectively. The X-ray diffraction peaks of Sm2WO6, ZnO, and GO are confirmed the SWZG nanocomposites material. The GO plane having very low intensity, because the mass of loading of GO was very little [42]. 3.2. Morphology analysis The SEM revealed the surface morphology of the SWZG nanocomposites. The morphologies of the as-obtained photocatalysts a) Sm2WO6 (a, b), b) ZnO (c), c) Sm2WO6-ZnO (d, e, f) and d) Sm2WO6ZnO/GO (g, h) were displayed in Fig. 3. The undoped Sm2WO6 catalyst

Fig. 2. X-ray powder diffraction patterns of a) Graphene oxide, b) ZnO, c) Sm2WO6 and d) 5% Sm2WO6 doped Sm2WO6/ZnO/GO (SWZG) nanocomposite. 4

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Fig. 3. SEM images of a), b) Sm2WO6, c) ZnO, d-f) SWZ Nanocomposite and g, h) SWZG Nanocomposite at different magnification.

the successful synthesize of SWZG nanocomposite and exposed to the valance state of elements such as Sm, W, O, C and Zn are 1, 1, -2, +4 and +2 respectively.

3.4. Optical properties by DRS-UV spectroscopy The UV- diffused reflectance spectroscopy (DRS-UV) is used to evaluate the energy gap value for the samples and detect the applicable light for the pollutant decomposition via the bandgap determination.

Fig. 4. XPS analysis of SWZG nanocomposite, (a) Survey spectrum, (b) C 1s, (c) Sm 3d, (d) W 4f, (e) O 1s and (f) Zn 2p. 5

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recombination behaviour of the SW, SWZ and SWZG nanocomposites and it is portrayed in Fig. S4. In general, the photocatalytic activity is mainly related to the recombination rate of photogenerated electron – hole pairs. The highest intensity results are indicates in the fast e− - h+ recombination rate and the lower Pl intensity may be due to the slow. In this present study, the Pl spectra of SW, SWZ and SWZG shows the similar peak positions but the intensity is much differ. By loading of GO in SWZG, the recombination charge carrier is effectively reduced due to the Pl intensity of SWZG also decreased. This low recombination rate of photogenerated electron – hole pairs are indicates the SWZG nanocomposite having higher photocatalytic activity.

The band energy value determination has a leading role in the degradation process. From the valance band to conduction band the electrons are excited due to the illumination of light and the pollutant is degrading by the generation of e− - h+pairs. In SWZG, the absorption spectra of nanocomposite are increasing the intensity than that of SW, ZnO and SWZ. The absorption was shown in the range at 415 nm; therefore the degradation was carried out under UV region. The photon energy (h©) and (〈h©) 2are plotted by a Tauc’s plot and the energy values are SW is 3.54 eV, ZnO is 3.23 eV [57], SWZ is 3.17 eV and the band energy value 3.13 eV for SWZG composite at 600 °C (Fig. 5). The low bandgap value has boosted the reaction and increase the efficiency of the photocatalytic degradation reaction in the visible region.

3.7. Photocatalytic degradation 3.5. Photocurrent measurement 3.7.1. The efficiency of the catalyst loading The optimum parameters like catalyst dosage, variety of catalyst and the pollutant concentrations for the degradation process were analyzed due to find the suitable conditions for the degradation process. Initially, the degradation efficiency of various catalysts was analyzed for CIP and MB degradation under UV light irradiation. The photodegradation of various catalysts like, Sm2WO6, ZnO, Sm2WO6-ZnO and Sm2WO6-ZnO/GO were shown in Fig. 6(a, b). From the results, the Sm2WO6-ZnO/GO photocatalyst having a higher degradation efficiency of CIP and MB degradation under UV light irradiation than that of Sm2WO6, ZnO and Sm2WO6-ZnO photocatalysts. Besides it, the optimization condition of catalyst dosage supporting, concentration CIP& MB and the reactive oxidative species are to be analyzed. From Fig. 6a, b the efficiency of SWZG composite is much more than other catalysts and the C/C0 values neared zero. The optimized condition for the required amount of GO for the catalytic degradation of pollutants was represented in Fig. 6(c, d). Fig. 7(a,b) represents the kinetics study of the degradation reaction, from this plot the degradation rate is plotted between Time and ln (C0/ C) and the values are mentioned in Table 1. The corresponding rate constant value of various catalysts are shown in Table 1, From the results, SWZG composite having higher rate constant value compared to others, due to the pollutant decomposition depends on photoelectron separation behaviour before recombination. The photocatalytic efficacy of SWZG is much more due to its high optical absorbance property, the effective separation and large surface area. Fig. S5 corresponds to the dark experiment for the degradation of CIP and MB as a function of SWZG nanocomposite. In the absence of light the adsorption process takes place. The adsorptions percentage of CIP and MB increased linearly with the addition of impurities in the Sm2WO6 material. Therefore, the ternary type of SWZG nanocomposite having the higher adsorption percentage value compared to SW, ZnO and SWZ nano materials. The photodegradation efficiency is strongly related to the optimum

The charge separation efficiency of the photogenerated electronhole pairs in SmW, SmWZ and SmWZG was further confirmed by photoelectrochemical measurements. LSV response of SmW, SmWZ and SmWZG modified electrodes in dark and illuminated condition are shown in Fig. S3(A). Fig. S3(B) shows the LSV measurements of SmW, SmWZ and SmWZG modified electrodes in the chopped condition. Among these pure SmW showed a comparatively low photocurrent than SmWZ and SmWZG, suggesting higher electron-holerecombination rate. However, the photocurrent was significantly increased in SmWZ and SmWZG, indicating less recombination rate than SmW; due to formation of heterostructure between the SmW and ZnO. Moreover, the SmWZG shows highest photocurrents, representing the incorporation of G further reduce the electron-hole recombination rate and boost up the photoelectrocatalytic activity. Fig. S3(C) shows the LSV response SmWZG modified electrodes in dark, light and chopped condition 1 M KOH. It is evident that there is no appreciable current is observed but in other side upon illumination there is an enhancement in the current was observed due to the high photo-response of the materials. While chopping the illumination, the rise and fall in the current which matches to the dark and under illumination response of the electrode clearly indicate the good photoesponse of the prepared electrode. EIS measurements were also carried out to understand the electron transfer processes (Fig. S3(D)). In the Nyquist plot of EIS, the smaller semicircle radius indicates smaller electron transfer resistance, which enhances the electron transfer in the photocatalytic degradation process. The SmWZG composite possesses the smallest semicircle radius when compared to SmW and SmWZ, which implies the higher photoelectrocatalytic activity. 3.6. Photoluminescence spectroscopy The photoluminescence (PL) spectroscopy is used to determine the charge carrier transfer and to find the separation and electron – hole

Fig. 5. a) DRS-UV absorbance spectra of SWZG nanocomposite and b) Tacu’s plot of SW, SWZ and SWZG nanocomposites. 6

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Fig. 6. Photodegradation of CIP and MB solution under different conditions a) Different catalyst under CIP (catalysts: 40 mg, CIP conc.: 4 × 10−5, SW-5%) b) Different catalyst under MB (catalysts: 40 mg, MB conc.: 1 × 10−5, SW-5%) c) & d) weight ratio percentage for GO in CIP degradation and MB degradation.

photocatalyst dosage. Therefore, we have varied SWZG catalyst dosage from 0.2 g to 0.5 g for CIP and MB photodegradation and the results are portrayed in Fig. 8(c, d). It observed that the photodegradation efficacy was gradually increased while the catalyst dosage also increased up to 0.2 g to 0.4 g; in above 0.4 g the efficiency was decreased. The efficient degradation performances were achieved at 0.4 g. However, higher loading of catalyst dosage (above 0.4 g) might generate the penetration of light sources throughout the reaction suspension which diminishes the production of photogenerated charge carriers, thereby; the degradation efficiency was decreased [57]. Therefore, we have used 0.4 g dosage for further studies in both CIP and MB. The photodegradation of CIP (4 × 10−5 M) and MB (1 × 10−5 M) pollutants in the presence of SWZG (40 mg) nanocomposite are the analyzed absorption spectra values by various time intervals. Thus, the absorption value272 nm and 660 nm correspond to CIP and MB

Table 1 The Kinetics parameters for the photocatalytic degradation of (CIP & MB) Organic pollutants under UV light irradiation with various catalysts. S. No

1. 2. 3. 4.

Catalysts

Sm2WO6 ZnO Sm2WO6/ZnO Sm2WO6/ZnOGO

Degradation efficiency (%)

Apparent rate constants k (min−1)

CIP

MB

CIP

MB

40.545 68.299 87.258 96.92

57.225 79.894 91.765 98.243

0.0519 0.1148 0.206 0.3479

0.0849 0.1604 0.2497 0.4042

Fig. 7. Kinetics parameters for the photocatalytic degradation of a) CIP and b) MB under various catalysts. 7

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Fig. 8. Photodegradation of CIP & MB solution under different conditions a) Different concentration of CIP (SWZG: 40 mg) b) Different concentration of MB (SWZG: 40 mg) Different dosage of SWZG (CIP conc.: 4 × 10−5) and d) Different dosage of SWZG (MB conc.: 1 × 10−5).

Fig. 9. a) Absorption spectrum for photodegradation of (4 × 10−5 conc.) aqueous CIP solution under UV light irradiation in the presence of 40 mg SWZG catalyst, (b) Absorption spectrum for photodegradation of 1 × 10−5 conc. methylene blue under UV light irradiation.

contaminants does not change. The gradually decreased absorption peak was observed until 90 min (Fig. 10a). Nevertheless, after the degradation time of 90 min, CIP was not completely degraded. For the individuals of CIP and MB was degraded within only 90 and 25 min. The degradation efficiency is very low for the contaminant than that of their individuals (Fig.10b).

respectively at an irradiation time of 90 min and 25 min (Fig. 9a, b). 3.7.2. Optimization of pollutants concentration The optimized concentration is a crucial role in the degradation of pollutants. The CIP and MB were optimized by the corresponding concentration between 20 μM to 60 μM and 5 μM to 20 μM. From CIP degradation the optimized concentration was achieved in 40 μM and in MB 10 μM was achieved and it is shown in (Fig. 8a, b). The concentration of pollutant increased then decreases the rate of degradation, due to the adsorption of surface molecules of the catalyst decrease the efficacy of catalyst.

3.7.4. Detection of reactive oxidative species (ROS) The ROS are found the degradation mechanism and also determined the effect of scavengers. The various scavengers like holes, %O2−electron and •OH are investigating the main active species [58–60] and it is shown in Table S1. As from Fig. 11(a, b), after the incorporation of TEOA and BQ, the photodegradation efficiency of CIP and MB greatly prohibited. In contrarily, there was a slight effect was obtained on the degradation of CIP and MB while the addition of ethanol and BQ (CIP) and TEOA and IPA (MB). The results evidenced that the major

3.7.3. Degradation of mixed pollutants The industrial wastewater having a mixing of organic pollutants due to this reason the organic contaminants (mixing of CIP & MB)was analyzed.For this result, the contaminants absorption peak for the 8

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Fig. 10. (a) Absorption spectrum for mixing of 4 × 10−5 conc. aqueous CIP solution & 1 × 10−5 conc. Methylene blue solution under UV light irradiation in the presence of 40 mg SWZG catalyst (b) Photodegradation of individual CIP, individual MB and mixing of CIP&MB.

Fig. 11. a) Photodegradation of CIP and MB solution under scavengers (CIP conc.: 4 × 10−5, MB conc.: 1 × 10−5) & (SWZG-40 mg); BQ-0.1 mmol-1, TEOA-1 mmol-1, IPA-0.1 mmol-1, ethanol −50 mL, irradiation time – CIP - 90 min. & MB - 25 min.).

Fig. 12. Effect of solution pH on CIP degradation (CIP conc.: 4 × 10−5 & SWZG: 40 mg).

the positive holes are leaves. For Sm2WO6in CB, the electron is conducted with atmospheric oxygen to produced superoxide radical anion (%O2−) and occurred the electron-hole recombination. In VB the hydroxyl radical (%OH) are formed by the reaction of holes and water molecule. These two main species (ROS) facilitate CIP and MB degradation.

Scheme 3. Schematic representation for photocatalytic mechanism of SWZG nanocomposite.

involvement of h+(CIP) and O2%− (MB) radicals and the minor role of % OH & e− (CIP) and h+&%OH(MB) for the photodegradation by the SWZG catalyst under UV light illumination. From the schematic representation (Scheme 3), upon illumination of light, in ZnO/GO the electron was gets excited and it migrated from valence bond (VB) to conduction band (CB), while in valence bond (VB)

3.7.5. Effect of inorganic ions The wastewater not only contains organic contaminants but also 9

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Fig. 13. Recycle test for SWZG Photocatalyst under UV light illumination (a) Recycle efficiency of CIP and (b) MB.

Table 2 Comparative report of photo catalytic activity of Sm2WO6/ZnO-GO nano composites with various nanocomposites for the photo catalytic degradation of organic pollutants. S. No

Catalyst

Weight of catalyst (g/L)

Organic pollutant

Irradiation source

% of degradation

Degradation time (min)

Reference

1

Sm2WO6/ZnO-GO

0.04

UV light

Nd2WO6/ZnO-GO

0.04

UV light

96.92 98.24 96.66 98.14

3 4 5

0.02 0.2 0.05

Visible light UV light Visible light

92.8 97 77.6 100

6 7

Fe3O4/ZnO-GO Bi2O3-ZnO Bi2WO6/CeVO4/ Allophone Fe3O4/g-C3N4 Gd2WO6 - ZnO/ bentonite

90 25 120 50 150 90 240 420

This Work

2

CIP MB CIP MB MO AB1 Gaseous acetalde-hyde

Ag@PCNS/ BiVO4

RhB CIP BPA CIP

Visible light Visible light

8

0.05 0.03 0.025 0.04

97.83 97.9 98.3 92.6

140 60 60 120

Visible light

[42] [65] [66] [67] [68] [54] [69]

table, the result clearly explains that the inorganic ions on the degradation process decrease the efficiency of removal rate. On the other hand, these negatively charge ions can react with hydroxyl radical (·OH) and holes (h+) to make the weak reactive species and difficult to degradation. From the degradation reaction, the inorganic ions could react with h+ and ·OH to generate weak oxidizers through the following equations, (Eqs. (1)–(8))

H2 PO−4 +• OH → H2 PO•4 + OH−

(1)

H2 PO−4 + h+ → H2 PO•4

(2)

CO32 −

(3)

+• OH →

CO•3−

+ OH−

Cl− +• OH → Cl• + OH−

(4)

Fig. 14. X-ray powder diffraction patterns of SWZG nanocomposite for before and after degradation of CIP.

Cl− + h+ → Cl•

(5)

Cl• + Cl• → Cl2

(6)

contains inorganic anion like, sulphate, dihydrophosphate, carbonate and chloride [61]. Therefore, it is very important to analyze the impact of these ions on the degradation process. The inhibitive influences of inorganic ions on the degradation reaction could be attributed to two possible reasons. ie),

SO32 − +• OH → SO•3− + OH−

(7)

SO32 − + h+ → SO•3−

(8)

Finally, the effect of inorganic ions in the degradation process was found to rank in the order of without ions > Cl− > SO32− > CO32− > H2PO4- respectively.

i) Inhibition of catalytic activity of the reactive material. ii) Ionic strength changes of reaction medium.

3.7.6. Effect of solution pH For the degradation process, the effect of solution pH is having a very important role because it affects the photodegradation percentage and it explains Wastewater can be acidic or basic. Generally, the surface of the catalyst is mainly related to the adsorption of pollutants, for this reason, it caused by the solution pH. The photodegradation efficiency at different pH ranges from 5 to 11 has evaluated by the addition HCl

In Fig. S6 shows the effect of added inorganic ions on the removal of CIP and MB. From the results, the presence of ions in the suspension solution causes the inhibiting effect on the efficiency of CIP and MB degradation. The presence of inorganic ions H2PO4−, CO32-, SO32- and Cl− with the degradation efficiency are listed in Table S1. From the 10

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Scheme 4. Schematic representation for degradation pathway of CIP intermediates.

degradation and after degradation of CIP in presence of SWZG nanocomposite was shown in Fig. 14. After the irradiation reaction the structure morphology of the SWZG photocatalyst has also been investigated using SEM analysis and it is portrayed in Fig. S7. After the fifth irradiation run the photocatalyst loses its original (red algae like hexagonal structure in the surface of GO sheet) structure and looks like clusters which aggregate with each other particles. These results proposed that the catalyst was relatively stable.

(acidic) and NaOH (basic) and it is shown in Fig. 12.The rate of degradation increases in the pH range of up to 9 and then decreased. From the results, the more degradation efficiency of CIP (96.92 % at pH = 9) was observed under basic condition. In basic pH condition, higher degradation efficiency was observed, due to the surface of SWZG material have negative charges and CIP surface have a positive charge which induced the strong electrostatic repulsion between CIP solution and the catalyst [62]. Conversely, due to the electrostatic attraction, the SWZG nanocomposite contains a negative charge and the adsorption of CIP molecules tends to strong in low pH range [63]. Therefore, we performed the degradation reaction under the pH range of (pH = 9) for all other experiments.

3.7.8. CIP degradation pathway and the by-product formation In this study, the photocatalytic degradation intermediates of CIP in presence of SWZG suspension were detected by GC/MS spectroscopy obtained after 45 min. irradiation (Fig. S8). From the results, we got six predominant peaks with the retention times of 17.714, 18.534, 18.929, 19.280, 20.042 and 20.745 min. We could only one intermediate compound were identified namely, 7-amino-1-cyclopropyl-6-fluoro-2hydroxy-4-oxo-1,4-dihydro quinolone-3-carboxylic acid with the retention time of 18.929. The other retention time peaks do not match with any intermediates, so we could not be analyzed. Therefore the figures are not given. In the table-S2 shows the list of fragmentation peaks for those retention times. The degradation pathway for the formation of the intermediate product is delivered in Scheme 4. From the first step, the attack of (OH·)

3.7.7. Photocatalyst recyclability After the degradation reaction, the recovered photocatalyst was involved in recyclability test and it reveals the stability and reusability of the sample in after five consecutive runs. Fig. 13a &b contains the efficiency of the reusability test for CIP and MB. Increases in the number of reusable cycles and the efficiency of degradation are decreased. The degradation efficiency of CIP & MB by SWZG remains 86.397 % and 90.43 % in after 5th runs. After getting the results, we assumed that the catalyst contains good mechanical stability and reusability [64]. The efficiency of SWZG nanocomposite is higher than the related previous works (Table 2). The XRD diffraction pattern of before 11

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hydroxyl radical in C8 position in the quinolone moiety of CIP and to form intermediate I. Subsequently, hydroxylation and hydrolysis of C12 and C16 position in intermediate I to form primary quinolone derivative (D1) called 7-amino-1-cyclopropyl-6-fluoro-2-hydroxy-4-oxo-1,4dihydro quinolone-3-carboxylic acid (m/z 278.07) (Scheme 4). Furthermore, the repetitive attack of (·OH) hydroxyl radical, the primary intermediates are involved in the reaction of quinolone ring cleavage and to form oxygenated aliphatic compounds. Finally, these by-products (aliphatic compounds) would be mineralized to ecofriendly CO2 and H2O. This degradation process and the structure of the by-products were shown in Scheme. This degradation process could be verified from some other previous reports [70–72].

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4. Conclusion In this work, we delivered Sm2WO6/ZnO-GO by simple ultrasonicassisted hydrothermal method. Fig. 3 reveals the SEM image of the SWZG composite in presence hexagonal like ZnO and algae-like structure of Sm2WO6 in an elongated form on the graphene oxide sheet. The degradation efficiency of the as-prepared SWZG composite is 96.92 % of CIP in 90 min & 98.24 % of MB in 25 min, it is a promising catalyst and more efficient than other catalysts compared with previous reports. The CIP degradation process, the transformations of CIP products are analyzed by using GC–MS. For the results, CIP has been reduced to 7amino-1-cyclopropyl-6-fluoro-2-hydroxy-4-oxo-1,4-dihydro quinolone3-carboxylic acid (m/z 278.07), and its further oxidation by hydroxyl radical gets mineralised. In the ROS process, holes and superoxide radicals play a major role in the (CIP and MB) degradation reaction. Even after fifth runs, the catalyst SWZG having more degradation efficiency, it shows the excellent reusability of the photocatalyst. CRediT authorship contribution statement M. Arunpandian: Conceptualization, Investigation, Writing - original draft, Data curation. K. Selvakumar: Resources, Validation. A. Raja: Methodology, Formal analysis. P. Rajasekaran: Formal analysis. C. Ramalingan: Resources, Visualization. E.R. Nagarajan: Supervision, Data curation. A. Pandikumar: Supervision, Formal analysis. S. Arunachalam: Supervision, Writing - review & editing. Declaration of Competing Interest None. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfa.2020.124721. References [1] L.M. Pastrana-Martinez, J.L. Faria, J.M. Dona-Rodriguez, C. Fernandez-Rodriguez, A.M.T. Silva, Degradation of diphenhydramine pharmaceutical in aqueous solutions by using two highly active TiO2 photocatalysts: operating parameters and photocatalytic mechanism, Appl. Catal. B 113 (2012) 221–227. [2] M. Muruganandham, R.P.S. Suri, M. Sillanpaa, J.J. Wu, B. Ahmmad, S. Balachandran, M. Swaminathan, Recent developments in heterogeneous catalyzed based environmental remediation processes, J. Nano Nanotechnol. 14 (2014) 1898–1910. [3] R. Fagan, D.E. McCormack, D.D. Dionysiou, S.C. Pillai, A review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern, Mater. Sci. Semicon. Process. 42 (2016) 2–14. [4] Y.M. Hunge, A.A. Yadav, V.L. Mathe, Oxidative degradation of phthalic acid using TiO2 photocatalyst, J. Mater. Sci.: Mater. Electron. 29 (2018) 6183–6187. [5] T. Ali, Y.M. Hunge, A. Venkatraman, UV assisted photoelectrocatalytic degradation of reactive red 152 dye using spray deposited TiO2 thin films, J. Mater. Sci.: Mater. Electron. 29 (2018) 1209–1215. [6] Y.M. Hunge, A.A. Yadav, V.L. Mathe, Ultrasound assisted synthesis of WO3-ZnO nanocomposites for brilliant blue dye degradation, Ultrasonics Sonochem. 45 (2018) 116–122.

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