Green synthesis of SnO2-bentonite nanocomposites for the efficient photodegradation of methylene blue and eriochrome black-T

Journal Pre-proof Green synthesis of SnO2-bentonite nanocomposites for the efficient photodegradation of methylene blue and eriochrome black-T Moones Honarmand, Morteza Golmohammadi, Atena Naeimi PII:

S0254-0584(19)31231-3

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122416

Reference:

MAC 122416

To appear in:

Materials Chemistry and Physics

Received Date: 23 May 2019 Revised Date:

16 October 2019

Accepted Date: 6 November 2019

Please cite this article as: M. Honarmand, M. Golmohammadi, A. Naeimi, Green synthesis of SnO2bentonite nanocomposites for the efficient photodegradation of methylene blue and eriochrome black-T, Materials Chemistry and Physics (2019), doi: https://doi.org/10.1016/j.matchemphys.2019.122416. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Addition of extract

Methylene blue

SnO2-bentonite nanocomposite

Degradation

Eriochrome black-T

SnCl2.2H2O + Bentonite

Green synthesis of SnO2-bentonite nanocomposites for the efficient photodegradation of methylene blue and eriochrome black-T Moones Honarmanda*, Morteza Golmohammadia, Atena Naeimib a b *

Department of Chemical Engineering, Birjand University of Technology, Birjand, Iran

Department of Chemistry, Faculty of Science, University of Jiroft, Jiroft, 7867161167 Iran

Corresponding author, e-mail: [email protected], [email protected]; Tel: +98 56 32391306; Fax: +98 53 32391238

Abstract In this paper, for the first time, SnO2-bentonite nanocomposites were synthesized through immobilization of SnO2 nanoparticles on natural bentonite using aqueous extract of jujube fruit. In this simple and safe procedure, the jujube acted as a naturally-sourced reducing agent. The biosynthesized SnO2-bentonite nanocomposites were characterized by various techniques such as SEM (scanning electron microscopes), EDX (energy-dispersive X-ray spectroscopy), TEM (transmission electron microscope), XRD (X-ray diffraction), FT-IR (Fourier-transform infrared spectroscopy), and N2 adsorption-desorption to investigate the morphology, chemical elemental composition, crystalline size, functional groups and surface area of the biosynthesized nanocomposites. The supported SnO2 nanoparticles on bentonite were observed obviously in the SEM image of SnO2-bentonite nanocomposites. The EDX analysis of biosynthesized nanocomposites was well indicated the successful immobilization of SnO2 nanoparticles on bentonite. The average particle size of SnO2-bentonite nanocomposites was evaluated based on the TEM and found to be 18 nm. The crystalline nature of the SnO2-bentonite nanocomposites was confirmed via XRD analysis. The presented functional groups in biosynthesized SnO2-bentonite nanocomposites were identified by FTIR spectral analysis. Afterward, then, the photocatalytic performance of SnO2-bentonite nanocomposites was studied for the efficient degradation of organic dyes (methylene blue and eriochrome black-T) without using any toxic reducing agents under

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solar irradiation. The new SnO2-bentonite nanocomposites as a very stable catalyst could be tolerated the reaction conditions and reused at least three times without remarkable loss of activity. Keywords: Nanocomposites; Tin oxide; Bentonite; Organic dyes 1. Introduction The presence of organic dyes in the sewage of factories is currently a great problem because these dyes are toxic and their degradation process is naturally difficult. On the other, the use of organic dyes in various industrials such as printing, textile, food, cosmetic, leather, plastic, paper, and pharmaceutical industries is inevitable[1]. Due to the destructive effects of organic dyes and other pollutants on the environment and the ecosystem, their removal and degradation by the catalytic process is very necessary [2-19]. Over the past decades, many conventional techniques such as adsorption [20-26], flocculation [27], chemical oxidation [28], coagulation [29] and ion exchange [30] were applied for the removal of organic dyes. Despite being valuable of these methods, some these methods have limitations such as incomplete removal, slow process, high-energy requirement cost and production of secondary pollutants that require further disposal. These shortcomings create demands for efficient processes with economic viability, ecofriendliness, non-toxicity and no generation of secondary pollutants. The photocatalytic degradation is a suitable choice for the removal of organic dyes from wastewaters because of its efficiency and simplicity [4,31-33]. The photocatalytic degradation can completely destroy toxic organic pollutants under mild conditions. In this method, initially the dye molecules adsorbed on the surface of photocatalyst, after illumination of photocatalyst by UV irradiation, redox reactions take place on the interface of the photocatalyst/dye solution and active radicals are generated. The formed active radicals can attack to pollutant molecules and disintegrate them into non-toxic molecules. The efficiency of photocatalytic

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degradation of organic dyes is related to several parameters such as morphology, crystallinity, size, and shape of photocatalyst. Nanotechnology is an excellent option for the solution of the environmental challenges. In recent years, interesting findings have been reported on the degradation of harmful and toxic compounds using nanocatalysts [34]. Among of different nanocatalysts, photoactive materials have better performance because of their photocatalytic properties. SnO2 is a famous n-type semiconductor with an extensive band gap of 3.6 eV, which is used in many photocatalytic processes [35-38]. SnO2 in the nano scale, with high ratio of its surface to volume, leads to increasing its sensitivity and adsorption. There are some chemical and physical methods for the synthesis of SnO2 nanoparticles, which the most of them suffer from shortcomings such as harsh reaction conditions, high temperature and using organic solvents, hazardous capping agents or toxic stabilizers [39-42]. Thus, it is necessary to search about the green synthesis of SnO2 nanoparticles using plants as reducing agent under mild conditions [36, 38, 43-52]. Nowadays, the use of unique properties of plants attracted a lot of interest [53-63]. The Ziziphus jujuba (jujube) belongs to the Rhamnaceous family and jujube trees grow often in Europe, Australia, and Asia. The main components of jujube are flavonoids, polysaccharides, phenolics, and triterpenic acids [64]. From ancient times, jujube was used as the fruit and also for the treatment of some complications. There are limited reports for the green synthesis of nanoparticles using jujube extract that in most of them, leaves of jujube trees have been used [65-68]. In the case of chemical or green synthesis of nanoparticles, unfortunately, the agglomeration of nanoparticles happens due to their small size, and consequently their catalytic activity is decreased. To avoid the agglomeration of nanoparticles, they are commonly immobilized on/into solid supports [69-84]. Bentonite is natural clay, which is found in many countries

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and originated from the activities of volcanoes. Due to its specific properties such as low cost, high surface area, uniform pore volume, thermal stability, high exchange capacity and high safety could be a good candidate as catalyst support and adsorbent [85-89]. Since our goal in this study is the degradation of organic dyes using nanoparticles, it is better that nanoparticles are immobilized on the support which itself acts as an adsorbent. The bentonite with fantastic properties was the best choice for us. Therefore, we synthesized SnO2-bentonite nanocomposites using extract of jujube fruit for the first time. In the following, the catalytic activity of the biosynthesized SnO2-bentonite nanocomposites was studied for the degradation of two organic dyes (methylene blue and eriochrome black-T) under solar irradiation. Moreover, the recovery of SnO2-bentonite nanocomposites was investigated and the obtained results were specified which new catalyst could be tolerated the reaction conditions and reused at least three times without remarkable loss of activity. 2. Experimental section 2.1. Materials and Characterization The dyes of methylene blue, eriochrome black-T and also SnCl2.2H2O with chemical purity of 98% were purchased from Sigma-Aldrich Chemical Company. The double distilled water was utilized in all steps of experiment. X-ray diffraction (XRD) was conducted by a Philips X’pert diffractometry (PW 1800) with Cu-Kα radiation line to study the crystal structure and composition of the bentonite and SnO2-bentonite nanocomposites. Fourier-transform infrared (FTIR) spectra of SnO2-bentonite nanocomposites and bentonite were recorded using a Thermo Nicolet Smart Golden Gate MKII single reflection ATR spectrometer. The morphological features and the composition of bentonite, SnO2 nanoparticles and SnO2bentonite nanocomposites were studied by SEM and EDX (ZEISS, EVO18). TEM imagess were acquired on a Zeiss - EM10C (100 KV). The specific surface area of SnO2-bentonite

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nanocomposites was determined according to the Brunauer–Emmett–Teller (Quantachrome NovaWin2 BET). 2.2. Preparation of jujube extract 20 g of dried powdered fruit of jujube was added to 200 mL double distilled water and refluxed on a heating mantle at 80 ͦC for 30 min. After cooling of solution, the extract of jujube was centrifuged at 6000 rpm, filtered and kept in the refrigerator for further use. 2.3. The green synthesis of the SnO2 nanopartilces using the aqueous extract of the jujube fruit The aqueous extract of the jujube fruit (100 mL) was drop-wisely added to stirring solution of the SnCl2.2H2O (50 mL, 0.05 M) at room temperature for 30 min. During this time, the color of solution changes from white to light brown. After 30 min, the flask containing solution was transferred to bath oil and the stirring continued at 80 °C for 30 min. The light brown precipitations were centrifuged at 6000 rpm, filtered, washed with double distilled water and then dried at room temperature. In the end, the biosynthesized SnO2 nanoparticles were calcined at 500 °C for 1 h. 2.4. The green synthesis of the SnO2-bentonite nanocomposites using the aqueous extract of the jujube fruit The SnCl2.2H2O solution (50 mL, 0.05 M) and bentonite (2 g) was vigorously stirred for 30 min at room temperature. Then aqueous extract of the jujube fruit (100 mL) was added dropwise to the well-mixed solution of the SnCl2.2H2O and bentonite for 30 min. The rest of the steps are similar to the one described above. 2.5. Photocatalytic degradation of dyes To study the photocatalytic activity of SnO2-bentonite nanocomposites, several photocatalytic experiments under sunlight were conducted on August 25, 2018 in Birjand, Iran. In all of these experiments, 80 mg of nanocomposites were added to 15 mL of a 100

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ppm aqueous solution of the desired dye, and then stirred in the dark to accomplish the adsorption-desorption equilibrium. After 30 min, the mixture was transferred to the outdoor to expose direct sunlight for various intervals such as 0.5, 1, 2.5, and 5 hours (10 am to 3 pm, ambient temperature 28-32 °C). After the end of the experiment, the SnO2-bentonite nanocomposites were separated from the solution by centrifugation (5 min and 6000 rpm). The concentration of non-degraded dye in the solution was then estimated using a UV spectrophotometer and the process efficiency was calculated as follows: DE(% ) =

( C 0 − C f ) × 100 C0

Where, the DE was defined as decomposition efficiency of methylene blue or eriochrome black-T, and C0 and Cf denoted the initial and final concentration of these organic dyes, respectively. 3. Result and Discussion 3.1. Synthesis and characterization of SnO2-bentonite nanocomposites The SnO2-bentonite nanocomposites are synthesized via a simple and safe route involving the immobilization of SnO2 nanoparticles on the bentonite in the presence of the aqueous extract of jujube fruit without the using any toxic and hazardous surfactants and capping agents. In this green process, the jujube extract acts as a natural stabilizer and efficient reducing agent. The flavonoids and other presented compounds in extract of jujube fruit[68] contributed to the bioreduction of the Sn2+ ions to Sn0 and stabilization of SnO2 nanoparticles [52] (Scheme 1). For better understanding, both the synthesis of SnO2 nanoparticles and SnO2-bentonite nanocomposites are described separately in experimental section. Scheme 1 The morphology of the SnO2 nanoparticles, bentonite and SnO2-bentonite nanocomposites are exhibited by images of SEM in Fig. 1. As shown Fig. 1a and 1b, the morphology of

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SnO2 nanoparticles and natural bentonite is spherical and sheet, respectively. The spherical SnO2 nanoparticles on bentonite are observed obviously in the SEM image of SnO2bentonite nanocomposites (Fig. 1c). The elemental composition of the SnO2 nanoparticles, bentonite and SnO2-bentonite nanocomposites are determined by energy dispersive X-ray spectroscopy (EDX) analysis (Fig. 1d, 1e and 1f). The presence of tin and oxygen elements is specified in the EDX spectrum of the SnO2 nanoparticles (Fig. 1d). The EDX analysis of the bentonite is well indicated components of bentonite such as Si, Al, Mg, Na and O (Fig. 1e). In addition to peaks of bentonite, the related peaks to Sn element are seen in Fig. 1f, which confirms the presence of SnO2 nanoparticles on bentonite. Fig. 1. The XRD patterns of the synthesized SnO2 nanoparticles, bentonite and SnO2-bentonite nanocomposites are displayed in Fig. 2. In XRD pattern of SnO2 nanoparticles (Fig. 2a), the seen peaks at 2θ= 26.48, 33.52, 37.80, 51.64, 62.12 and 65.32 correspond to the (110), (101), (200), (211), (310) and (301) planes of SnO2 nanoparticles, respectively. The XRD pattern of the synthesized SnO2 nanoparticles is exactly accordance with the tetragonal rutile structure (JCPDS 77-0448) [90]. The crystallite size of nanoparticles is calculated using the Debye-Scherrer equation and is found to be 18 nm corresponding to (110) plane. The main constituents of the bentonite are montmorillonite, quartz and feldspar[91] which their diffraction patterns are completely shown in Fig 2b. The diffraction peaks are located at 2θ=7.23°, 19.83°, 35.92°, 61.79° correspond to (001), (100), (006) and (300) planes of montmorillonite, respectively. The XRD peaks are positioned at 2θ= 21.94°, 28.41° and 35.92° relevant to (101), (111) and (102) planes of cristobalite, respectively. The presence of quartz is also indicated by a peak at 2θ= 26.53° (011). As shown in Fig. 2c, after the immobilization of SnO2 nanoparticles on bentonite, the related peaks to SnO2 nanoparticles

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appear at 2θ = 22.64°, 34.16°, 52.08°, 55.40°, 62.32° and 66.48°. The obtained results are confirmed successful immobilization of SnO2 nanoparticles on bentonite. Fig. 2 The size, shape and surface morphology of SnO2-bentonite nanocomposites are characterized by using TEM. The TEM results are supported very well by SEM and XRD results of the SnO2-bentonite nanocomposites. The TEM images of the SnO2-bentonite nanocomposites at different magnifications are shown in Fig. 3. The attachment of the sphere-like SnO2 nanoparticles with average size of 18 nm on the surface of bentonite is well confirmed by the TEM images. Fig. 3 The surface area of SnO2-bentonite nanocomposites are determined by BET (BrunauerEmmett-Teller). The N2 adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) pore size distribution plot of SnO2-bentonite nanocomposites are exhibited in Fig. 4. The BET surface area, total pore volume and average pore diameter of SnO2-bentonite nanocomposites are 51.60 m2 g-1, 0.13 cm3 g- 1 and 9.88 nm, respectively. Fig. 4 Fig. 5 illustrates the FT-IR spectra of bentonite and SnO2-bentonite nanocomposites. In both of spectra, the absorption band at around 3650 cm−1 and 3488 cm−1 is related to free hydroxyl groups and the hydroxyl groups confirming the hydrogen bonding in these compounds, respectively [34]. The typical band at 1654 cm−1 is corresponding to O-H bending vibrations of water [92]. The absorption bands at around 1100, 530 and 480 cm−1 are due to Si–O–Si stretching vibrations and Si-O-Al and Si-O-Si bending vibrations, respectively [93]. The band of Sn–O stretching vibrations should be exhibited at around 500 cm-1 [94] but it overlaps with strong bands in this region and only causes the broadening of peaks.

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Fig. 5 3.2. Degradation of organic dyes in presence of SnO2-bentonite nanocomposites After the synthesis and characterization of SnO2-bentonite nanocomposites, their photocatalytic activity was studied for the degradation of two organic dyes, namely methylene blue and eriochrome black-T. The results demonstrated that the SnO2-bentonite nanocomposites can remove the over mentioned dyes from aqueous solution. Fig. 6 presents the UV spectra of dye solutions after different reaction times. It is obvious that the intensity of characteristic peak of methylene blue (maximum absorbance wavelength at ca. 670nm) and eriochrome black-T (at ca. 530 nm) reduced through passing the reaction time, so that the characteristic peak completely disappeared after 5 hours. These results, in turn, confirm the efficient degradation of organic dyes in presence of SnO2-bentonite nanocomposites under solar radiation. Fig. 6 Moreover, the changes of different normalized dyes concentration as well as degradation efficiency (DE) with reaction time are plotted in Fig. 7 and 8. As can be seen from these figures, the concentration of both dyes strongly decreases with time implying the phtocatalytic degradation of them by SnO2-bentonite nanocomposites. Fig. 7 Fig. 8 The suggested mechanism for the photocatalytic degradation of methylene blue and eriochrome black-T as organic dyes using photocatalyst under solar irradiation can be demonstrated as follows [35]: At first, due to high adsorbability of bentonite, organic dye is adsorbed on surface of SnO2-bentonite nanocomposites [95]. Then, the photocatalyst surface is illuminated by solar irradiation. The energy of sunlight is higher than band gap energy of photocatalyst. This phenomenon lead to the formation of a hole (h+) in the valence band and

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an electron (e-) in the conduction band of SnO2-bentonite nanocomposites. The h+ acts as an oxidizing agent and oxidizes directly the pollutant or water to form hydroxyl radicals. The ein the conduction band of photocatalyst acts as a reducing agent and leads to reducing of the adsorbed oxygens (O2) on the SnO2-bentonite nanocomposites and conversion them to oxygen radicals. These active radicals are excellent driving forces for an efficient photodegradation process. SnO2-bentonite nanocomposites play the significant role of an electron carrier. Such assisted photo processes are an attractive route to treatment of organic dyes using sunlight. As shown in Fig. 6, there is no characteristic peak in the UV spectra after the degradation process and it is suggested that the degradation products are H2O, CO2, NO2 and SO3 [96].

For study of morphology-property correlation, the degradation of methylene blue are examined in the presence of spherical SnO2, sheet bentonite and SnO2-bentonite nanocomposites. The catalytic performance of SnO2 nanoparticles, bentonite and SnO2bentonite nanocomposites for degradation of methylene blue are 90%, 15% and 100%, respectively, after 5 h. The presence of spherical SnO2 nanoparticles (Fig. 1a), an efficient photocatalyst on sheet bentonite (Fig. 1b), as a strong adsorbent increase the efficiency of

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the degradation process. For further investigation, the photocatalyst is not incorporated into the furnace after the synthesis and its SEM images are taken. As shown in the Fig. 9, due to the coating of the catalyst surface by the extract, the morphology of SnO2 nanoparticles and bentonite are not well observed. The catalytic activity of this catalyst are examined for degradation of methylene blue. The obtained result shows that yield of degradation is 88% after 5 h. It seems that because of the low availability of active sites on the catalyst surface, process efficiency is decreased. Fig. 9 Finally, in order to investigate the stability of biosynthesized SnO2-bentonite, after the degradation of methylene blue, the catalyst is separated by centrifugation, washed with doubly distilled water, dried and then used in the next cycle. This process continues for three successive cycles. The obtained result indicates which the reduction in the activity of catalyst is somewhat negligible after three times of reuse. The comparison of EDX and FTIR spectra, XRD pattern and BET isotherms of the recovered catalyst after three cycles (Fig. 10) with fresh catalyst (Figures 1f, 5, 2c and 4) is confirmed high structural stability and durability of biosynthesized SnO2-bentonite nanocomposites in during degradation process. The BET surface area, total pore volume and average pore diameter of recovered catalyst are 45.84 m2 g-1, 0.10 cm3 g-1 and 9.25 nm, respectively. Comparison of these results with fresh catalyst reveals that the surface area and porosity of the SnO2-bentonite nanocomposites remains almost unchanged after the degradation process. Fig. 10 Also, the morphology of the recycled SnO2-bentonite nanocomposites after three cycles of photodegradation of methylene blue is studied. Fig. 11 shows SEM and TEM images of the recovered SnO2-bentonite nanocomposites. It could be observed that the recovered photocatalyst kept morphology and initial particle size.

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Fig. 11 4. Conclusions The present study described a green and facile procedure for the green synthesis of SnO2bentonite nanocomposites in the presence of jujube extract acting as a natural reducing agent. In this green process, no expensive or toxic reagent or solvent was used and only employed a natural and environmentally-friendly material. After characterization of SnO2bentonite nanocomposites by various techniques, it was used as a new and effective photocatalyst for degradation of organic dyes without using any toxic reducing agents under solar radiation. The presence of bentonite as an inexpensive and natural support not only prevented from the aggregation of SnO2 nanoparticles and also improved catalytic activity of catalyst for efficient degradation of methylene blue and eriochrome black T. The obtained results from photocatalytic degradation experiments confirmed the excellent ability of biosynthesized nanocomposites in degradation of both organic dyes, so that after 5 h the degradation efficiencies approached to 100%.

The suggested mechanism for the

photocatalytic degradation of organic dyes was presented using SnO2-bentonite nanocomposites under solar irradiation. As UV spectrum of the affected solution by the degradation process, was suggested that the degradation products were safe compounds. Moreover, the SnO2-bentonite nanocomposites could be readily recycled several times without significant loss in its photocatalytic activity. The comparison of EDX, FT-IR, XRD, TEM, SEM, and BET results of the recovered catalyst after three cycles with fresh catalyst was confirmed high stability and durability of SnO2-bentonite nanocomposites in during degradation process. Acknowledgments We gratefully acknowledge the Birjand University of Technology and University of Jiroft for the support of this work.

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(a)

(b)

(c)

(d) O

Sn

(e) Si

O

Al

Mg Na

Sn

(f)

O Si Al Mg Na

Fig. 1. SEM images (a) SnO2 nanoparticles, (b) bentonite, (c) SnO2-bentonite nanocomposites, EDX spectra (d) SnO2 nanoparticles, (e) bentonite, (f) SnO2-bentonite nanocomposites.

(a) (110)

Intensity (a.u.)

(101) (211)

(200)

10

(b)

20

Intensity (a.u.)

(001)

30

40

(310) (301)

2θ

50

60

70

80

70

80

(101)

(011)

(100) (111) (102) (300)

0

10

20

30

40

2θ θ

50

60

(c)

Intensity (a.u.)

(011)

SnO2

(101) (102)

(001)

0

10

SnO2 SnO2

SnO2

(100)

20

SnO2

(300)

30

40

50

60

70

80

2θ θ

Fig. 2. XRD patterns of (a) SnO2, (b) bentonite and (c) SnO2-bentonite nanocomposites.

Fig. 3. TEM image of SnO2-bentonite nanocomposites.

90

(a)

Volume (cc/g)

75 60 45 30 15 0 0

0.2

0.4

0.6

0.8

1

Relative Pressure (P/P0) 0.018

(b)

0.016

dv/dr (cc/nm/g)

0.014 0.012 0.01 0.008 0.006 0.004 0.002 0 1

10

r (nm)

100

'

Fig. 4. The N2 adsorption–desorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot of SnO2-bentonite nanocomposites.

(a)

Transmittance (% )

(b)

4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wave number (cm-1) Fig. 5. The FT-IR spectra of: (a) bentonie and (b) SnO2-bentonite nanocomposites.

(a)

0.5 h 1h 2.5 h

Absorbance (a.u)

5h

200

300

400

500

600

700

Wavelength (nm) (b) 0.5 h 1h

Absorbance (a.u)

2.5 h 5h

200

300

400

500

600

700

Wavelength (nm)

Fig. 6. UV–vis absorption spectra of dye solutions after various reaction times: (a) methylene blue, (b) eriochrome black-T.

1

(a) 0.8

C/C0

0.6

0.4

0.2

0 0

2

0

2

Time (h)

4

6

4

6

1

(b) 0.8

C/C0

0.6

0.4

0.2

0

Time (h) Fig. 7. The plot of normalized dye concentration versus time: (a) methylene blue, (b)

eriochrome black-T.

(a)

100

80

DE (% )

60

40

20

0 0

1

2

3

4

5

4

5

Time (h)

(b) 100

80

DE (% )

60

40

20

0 0

1

2

3

Time (h) Fig. 8. The change of degradation efficiency with time: (a) methylene blue, (b) eriochrome

black-T.

Fig. 9. SEM image of the biosynthesized catalyst before being placed inside the furnace.

(a)

Transmittance (%)

(b)

4000 3600 3200 2800 2400 2000 1600 1200 Wave number (cm-1)

800

400

(c)

Intensity (a.u.)

(011)

SnO2

(101) (001) SnO2

(102)

(100)

SnO2

SnO2

SnO2

(300)

0

10

20

30

40

2θ

50

60

70

80

75

(d)

Volume (cc/g)

60

45

30

15

0 0

0.2

0.4

0.6

0.8

1

Relative Pressure (P/P0) 0.014

(e)

dv/dr (cc/nm/g)

0.012 0.01 0.008 0.006 0.004 0.002 0 1

10

100

r (nm) Fig. 10. (a) EDX and (b) FT-IR spectra, (c) XRD pattern and (d) N2 adsorption–desorption

isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot of recovered SnO2bentonite nanocomposites after degradation of methylene blue.

(a)

(b)

Fig. 11. (a) SEM and (b) TEM images of recovered SnO2-bentonite nanocomposites after

degradation of methylene blue.

Scheme 1. The proposed mechanism for the green synthesis of Sn nanoparticles.

Highlights •

SnO2-bentonite nanocomposites were synthesized via a green and simple route.

•

SnO2-bentonite nanocomposites exhibited superior photocatalytic ability for degradation of organic dyes.

•

The biosynthesized photocatalyst showed remarkable stability and reusability.

Statement The presence of organic dyes in the sewage of factories is currently a great problem because these dyes are toxic and their degradation process is naturally difficult. Due to the destructive effects of organic dyes and other pollutants on the environment and the ecosystem, their removal and degradation by the catalytic process is very necessary. Over the past decades, many conventional techniques were applied for the removal of organic dyes. Despite being valuable of these methods, some these methods have limitations. These shortcomings create demands for efficient processes with economic viability, eco-friendliness, non-toxicity and no generation of secondary pollutants. Nanotechnology is an excellent option for the solution of the environmental challenges. Among of different nanocatalysts, photoactive materials have better performance because of their photocatalytic properties. SnO2 is a famous n-type semiconductor with an extensive band gap of 3.6 eV, which is used in many photocatalytic processes. There are some chemical and physical methods for the synthesis of SnO2 nanoparticles, which the most of them suffer from shortcomings such as harsh reaction conditions, high temperature and using organic solvents, hazardous capping agents or toxic stabilizers. Thus, it is necessary to search about the green synthesis of SnO2 nanoparticles using plants as reducing agent under mild conditions. In this paper, for the first time, SnO2-bentonite nanocomposites were synthesized through immobilization of SnO2 nanoparticles on natural bentonite using aqueous extract of jujube fruit. In this simple and safe procedure, the jujube acted as a naturally-sourced reducing agent. The biosynthesized SnO2-bentonite nanocomposites were characterized by various techniques such as SEM, EDX, TEM, XRD, FT-IR, and N2 adsorption-desorption to investigate the morphology, chemical elemental composition, crystalline size, functional groups and surface

area of the biosynthesized nanocomposites. The supported SnO2 nanoparticles on bentonite were observed obviously in the SEM image of SnO2-bentonite nanocomposites. The EDX analysis of biosynthesized nanocomposites was well indicated the successful immobilization of SnO2 nanoparticles on bentonite. The average particle size of SnO2-bentonite nanocomposites was evaluated based on the TEM and found to be 18 nm. The crystalline nature of the SnO2bentonite nanocomposites was confirmed via XRD analysis. The presented functional groups in biosynthesized SnO2-bentonite nanocomposites were identified by FTIR spectral analysis. After characterization of SnO2-bentonite nanocomposites, it was used as a new and effective photocatalyst for degradation of organic dyes without using any toxic reducing agents under solar radiation. The presence of bentonite as an inexpensive and natural support not only prevented from the aggregation of SnO2 nanoparticles and also improved catalytic activity of catalyst for efficient degradation of methylene blue and eriochrome black T. The obtained results from photocatalytic degradation experiments confirmed the excellent ability of biosynthesized nanocomposites in degradation of both organic dyes, so that after 5 h the degradation efficiencies approached to 100%. The suggested mechanism for the photocatalytic degradation of organic dyes was presented using SnO2-bentonite nanocomposites under solar irradiation. As UV spectrum of the affected solution by the degradation process, was suggested that the degradation products were safe compounds. Moreover, the SnO2-bentonite nanocomposites could be readily recycled several times without significant loss in its photocatalytic activity. The comparison of EDX, FT-IR, XRD, TEM, SEM, and BET results of the recovered catalyst after three cycles with fresh catalyst was confirmed high stability and durability of SnO2-bentonite nanocomposites in during degradation process.