Tin(IV) Sulfide Biofoam for

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Three Dimensional Functionalized Carbon/Tin(IV) Sulfide Biofoam for Photocatalytical Purification of Chromium(VI)-Containing Wastewater Yunlei Zhong, Liu Han, Xunqing Yin, Haifeng Li, Dong Fang, and Guo Hong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01994 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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ACS Sustainable Chemistry & Engineering

Three Dimensional Functionalized Carbon/Tin(IV) Sulfide Biofoam for Photocatalytical Purification of Chromium(VI)Containing Wastewater

Yun-lei Zhong1a, Liu Han2a, Xunqing Yin1, Haifeng Li1, Dong Fang2* and Guo Hong1* 1. Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau SAR. 2. School of Chemistry and Environmental Engineering, Yancheng Teachers University, No. 2, South Hope Avenue, Tinghu District, Yancheng 224002, Jiangsu, China. a These authors contribute equally. *

Corresponding author: Guo Hong: [email protected], Dong Fang: [email protected].

Keywords: Loofah biofoam, visible light-driven, carbonized, carbon-based materials, microporous, flexible structures.

Abstract Carbon-based materials are widely used for environmental remediation because of their unique and excellent performances. Due to the huge daily consumption of such materials, the economic and environmental friendly derivations from natural biomass are highly desired. Herein, a new bio-carbon composite, carbonized loofah/Tin(IV) Sulfide (CLF@SnS2) foam, was successfully prepared using loofah biofoam through an efficient and scalable method. The hierarchical CLF@SnS2 foam has a high-porous structure, which can provides channels for light travelling through the whole material. It is confirmed that such three dimensional photocatalytic material can quickly purify Cr(VI)-containing waste water under mild visible light irradiation,

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with a efficiency of 99.7% Cr(VI) reduction within 120 mins. By contrast, CLF@SnS2 showed much better visible photocatalytic capacity than the uncarbonized counterpart (UCLF@SnS2), because the photoelectrons produced by the SnS2 nanosheets can be rapidly exported by the continuous channels provided by the carbon shell. Besides the high visible light-driven photocatalytic activity, CLF@SnS2 also exhibits excellent cycling stability. More importantly, this study demonstrated that CLF@SnS2 can be used for practical applications due to its flexibility and economic availability.

Introduction The effective treatment of toxic contaminants from municipal wastewaters is always a huge challenge for human society and becomes even urgent along with the increase of population and the intensification of industrial activities.1-3 The main pollutants in wastewater can be divided into two categories: organic and inorganic pollutants.4, 5 Phenol, organic dyes, antibiotics are organic pollutants, which can cause serious environmental problems.6, 7 The main inorganic pollutants are heavy metal elements, such as Pb, Cr, Hg, Co, Cd and As,8 which are extremely toxic to biosphere. Heavy metals contaminants are difficult to be biodegraded under natural conditions.9-11, which is a significant difference from organic pollutants. Hexavalent chromium is one of human carcinogens for its high toxicity and high biocompatibility.12 Cr(VI)-containing wastewater is mainly derived from the discharge of electroplating, leather tanning, chromate production and so on.13 In order to protect the freshwater resources and the public health, the Cr(VI) concentration in the industrial wastewater must be strictly controlled within the scope of the law before releasing.14-16 The safety limit of Cr(VI) concentration in drinking water is 0.05mg/L, according to the World Health Organization (WHO) recommendation.17 In order to


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solve these problems, various studies have been carried out, but no breakthrough has been made to date.18 Because of this, academics and industry have given considerable attentions on the efficient and economical treatment of Cr(VI)-containing wastewater. In order to effectively remove Cr(VI) from industrial wastewater, different methods have been tried, such as adsorption, ion exchange, electrochemical precipitation and so on.19 Unfortunately, these methods are inefficient and require a lot of reductants, which is uneconomical and harmful to the environment.20 Compared with these traditional means,21 semiconductor photocatalytic reduction of Cr (VI) aqueous solution abundant.22 Besides this, there is no undesired byproducts released in the process of aqueous Cr(VI) treatment.23 As a result, visible photocatalytic treatment of Chromium(VI)-Containing wastewater has been considered as the most promising method so far.24 TiO2 is the most studied semiconductor photocatalyst, due to its economy, biocompatibility and chemical stability.25 However, TiO2 is only excited with UV-light, because the bandgap of TiO2 is 3.2 eV.26 This means that only about 46% of the solar spectrum can be used during the treatment of Cr(VI) aqueous.27, 28 Based on the above reasons, it is necessary to develop a new semiconductor photocatalyst which can efficiently use the solar energy. Semiconducting metal sulfides can produce light responsiveness under both visible and short-wavelength near-infrared light due to their relatively small bandgap.29 Among them, Tin(IV) Disulfide, with a band gap about 2.08‒2.44 eV,30 is non-toxic and chemically stable under acidic or neutral conditions, which is much more efficient and quicker than biodegradation methods during the treating of Cr(VI) solution under visible light irradiation, according to the previous reports.31 However, the application of Tin Disulfide nanoparticles has some problems, such as strong agglomeration effect and light corrosion, which restrict its promotion.32-35 Carbon



materials have got great attentions in the fields of nano research due to its excellent chemical stability and conductivity.36, 37, 38 Therefore, they have been widely applied to energy storage and environmental governance.37 Since the carbon-based material has a strong sp2 hybrid structure, it can exhibit good stability under strong acid and alkali conditions.39, 40 So far, the carbon-based materials has yielded excellent results for dealing with contaminants from wastewater by physical adsorption.41-43 Combining the both advantages, SnS2 nanoparticles modified on the

surface of a 3D carbon-based material would show excellent photocatalytic efficiency and circulation performance during treating Cr(VI) solution, preventing SnS2 aggregation. Figure 1. Schematic illustration of the synthesis of the UCLF@SnS2 and CLF@SnS2 foam, and the process of reducing Cr(VI) to Cr(III). Thioacetamide (TAA) is used to the sulfur source of SnS2. In this work, we developed an efficient and scalable method for preparing CLF@SnS2 foam from natural loofah. CLF@SnS2 foam and the uncarbonized counterpart, UCLF@SnS2, are obtained by hydrothermal reaction, as shown in Figure 1. Both foams are new types of photocatalytic materials with a three dimensional macroporous structure. Such structure provides


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channels for lights travelling through the material which can be used to quickly treat Cr(VI)containing waste water under visible light exposure. The photocatalytic performance of CLF@SnS2 was judged by the speed of reduction of Cr(VI) solution under visible light irradiation. By contrast, CLF@SnS2 shows better visible photocatalytic capacity than UCLF@SnS2. Under visible light irradiation, the CLF@SnS2 can achieve 99.7% of Cr(VI) reduction efficiency within 120 min. Besides higher visible light-driven photocatalytic activity, it also exhibits much better cycling stability than that of UCLF@SnS2 and SnS2 powder. More importantly, this work shows that CLF@SnS2 composites have enormous potential applications due to its flexibility and economic availability.

Experimental section Reagents Loofah flesh, a common biofiber, was obtained from a supermarket in Shuyang County, Suqian City, JiangSu province, China. Tin tetrachloride (SnCl4·5H2O), thioacetamide (TAA) and 1,5-Diphenylcarbazide were purchased from Sinopharm Chemical Reagents Co., Ltd. All the experiments were conducted with ultra-pure water. All chemical reagents

no need further

purified before use. Preparation of 3D-biofoam UCLF@SnS2 and 3D-Carbon biofoam CLF@SnS2 The loofah flesh was washed thoroughly with ultrapure water to remove dust particles and dried in ambient conditions, defined as UCLF. Then, the prepared sample was slightly carbonized in a conventional oven under 110oC for 0.5 h after soaking in 1mol/L of tin tetrachloride solution, defined as CLF. Then, the SnS2 nanosheets were modified to the surface of the UCLF and the CLF by the chemical bath deposition (CBD). The resulting 3D-biofoam



composites were named UCLF@SnS2 and CLF@SnS2. Briefly, 290 mg SnCl4·5H2O and 120 mg thioacetamide were dissolved in ultrapure water (20mL) and ethyl alcohol (20 mL) under vigorous mechanical agitation. When the above solution is transparent, Both CLF and UCLF were immersed into the above solution. Then, the reactant was transferred into a stainless steel autoclave and heated to 120 °C with an oven for 6 h. For a control experiment, pure SnS2 particles were also prepared following the same procedure, without the addition of CLF or UCLF foam. Photocatalytic reduction of Cr(VI) Under natural conditions, hexavalent chromium mainly exists in the form of Cr2O72- in water bodies. Practically, specific concentration of K2Cr2O7 solution was selected to simulate hexavalent chromium-containing industrial wastewater. To study the catalytic effect of CLF@SnS2 and UCLF@SnS2, respectively, parallel experiments were carried out. Specifically, CLF@SnS2 foam (50 mg), UCLF@SnS2 foam (50 mg), CLF foam (50 mg), UCLF foam (50 mg), and SnS2 (10mg) were added to 50 ml of Cr(VI) solutions (50 mg/L) at pH 2.In all experiments, the SnS2 on the surface of CLF or UCLF is characterized to be less than 10 mg. In order to minimize the systematic error, we use 10 mg SnS2 as the control sample. All the Cr(VI) solutions were exposed to the visible light for 120min under magnetic stirring. Xenon lamp (300 W, λ>400 nm) is selected as the emitting source of visible light. Each sample was characterized by UV-vis spectroscopy at intervals of 10 minutes. In order to study the degradation process, the concentrations of hexavalent chromium in the solution at different time points were detected by UV-visible spectrophotometers. In order to study the cycle performance of CLF@SnS2 foam in photocatalytic degradation of hexavalent chromium solution, 50 mg CLF@SnS2 foam was added to the hexavalent


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chromium solution with a concentration of 50 mg/L and a volume of 50 ml. The mixture was exposed to visible light for 120min under stirring. After photocatalytic reaction, the CLF@SnS2 was recycled twice and then characterized by SEM and XRD. Cr ions adsorbed on CLF@SnS2 surface were studied by XPS. The concentration of the remaining Cr(VI) solutions were detected by UV-vis spectrophotometry, to evaluate the performance of CLF@SnS2. Characterization The morphology, size and components of the CLF@SnS2 and UCLF@SnS2 were studied by the Scanning electron microscopy (Hitachi S-4800). The structure of SnS2 on the surface of the CLF@SnS2 and UCLF@SnS2 were confirmed by XRD (X’ Pert-Pro MPD). XPS was carried out on Axis Ultra HAS. The concentrations of hexavalent chromium in the solution at different time points were detected by UV-visible spectrophotometers. The optical properties of composites were detected by Photoluminescence (PL) emission spectra (Horiba LABHRev).

Results and discussion Figure 2. Scanning electron microscope (SEM) images of Loofah at a) low and b, c) high magnifications; Carbonized Loofah at d) low and e, f) high magnifications.



The SEM images of loofah flesh foam (UCLF) and the carbonized loofah flesh foam (CLF) were shown in Figure 2. The fiber of UCLF foam had an average diameter about 300 um, and a rough surface under low magnification (Figure 2a). At high magnification of SEM images (Figure 2b and c), small debris can be clearly found on the surface of UCLF. However, the Figure 2 (d, e and f) showed that the surface of the carbonized loofah flesh foam (CLF) had much smoother surface.

Figure 3. Scanning electron microscope (SEM) images of UCLF@SnS2 at a) low and b, c) high magnifications; CLF@SnS2 at d) low and e, f) high magnifications. The SnS2 nanosheets crystalline were modified to the surface on the UCLF and CLF by the methods of Low-temperature chemical bath deposition. Hybrid foam prepared using thioacetamide and SnCl4·5H2O are denoted as UCLF@SnS2 and CLF@SnS2, respectively. The SEM images of the Figure 3 have shown the surface details of the products. The SnS2 nanoflakes with capreolary shapes, have successfully and perpendicularly loaded on the surface of UCLF (Figure 3a) and CLF (Figure 3d) foams. The high magnification of SEM images clearly showed that the surface of UCLF and CLF were completely covered with SnS2 nanosheets (Figure 3b and 3e). The nanosheets would also form quite a lot of nanopores between each other with three-


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dimensional structure on the UCLF and CLF foam (Figure 3c and 3f). According to the Figure S1of Supplementary, pure SnS2 nanoflakes alone would produce severe agglomeration without UCLF and CLF. Thus, the construction of hierarchical SnS2 nanostructures on UCLF and CLF foam can effectively prevent the reunion of SnS2 nanoflakes and provide more active sites for catalysis.

Figure 4. XRD patterns of the CLF, UCLF, SnS2, CLF@SnS2 and UCLF@SnS2. To analyze the crystal structure of the CLF, UCLF, SnS2, CLF@SnS2, and the UCLF@SnS2, the XRD measurement of the CLF, UCLF, SnS2, CLF@SnS2, and the UCLF@SnS2 were shown in Figure 4. The characteristic broad peak at 23oC can be attributed to the CLF and UCLF. All the other peaks of CLF@SnS2 and UCLF@SnS2 can be perfectly indentified as berndtite-2T SnS2 (JCPDS no. 1-1010).42-43 The results of the Figure 4 suggested the SnS2 nanoflakes have been successfully loaded on the surface of the CLF and UCLF.



Figure 5. SEM image of UCLF@SnS2 and CLF@SnS2, and the elemental maps of UCLF@SnS2 a) and CLF@SnS2 b). In order to further investigate the surface composition of UCLF@SnS2 and CLF@SnS2, the elemental mapping of UCLF@SnS2 and CLF@SnS2 analysis was shown in Figure 5. The results indicated the coexistence and the homogenous dispersion of C, S and Sn elements on the surface of UCLF and CLF (Figure 5a and b). Through the SEM of elemental analysis, the carbon content of CLF@SnS2 had a significantly increase compared with UCLF@SnS2. The electronderived rate would increase along with the carbon content, which can effectively accelerate the catalytic process under light exposure conditions. The SEM-EDS (Figure S2) indicated the result was consistent with Figure 5.


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Figure 6. The spectra of XPS. a) Full survey; b) Sn 3d spectra; and c) S 2p spectra of SnS2, CLF@SnS2, UCLF@SnS2, respectively. d) C 1s spectra of CLF@SnS2 and UCLF@SnS2. In order to further confirm the surface information of the SnS2, CLF@SnS2, UCLF@SnS2 (Figure 6), X-ray photoelectron spectroscopy was performed. According to the results of XPS full survey in Figure 6a, the surface of the CLF@SnS2 and UCLF@SnS2 contain Sn, S, and C elements. The XPS peaks at 486.6 and 495.1 eV were assigned to the Sn3d5/2 and Sn3d3/2 states, respectively. (Figure 6b). The peak at 162.6 eV are assigned to the S2p3/2 state of S22- moieties (Figure 6c). The peak at 284.8 eV are assigned to C1s state. In Fig. 6d, 284.8, 286 and 288.5 eV are assigned to the C-C, C-O-C and O-C=O components of CLF@SnS2 and UCLF@SnS2. The intensity of C-O-C of CLF@SnS2 was significantly weaker than that of UCLF@SnS2 while the intensity of C-C increased when the loofah was carbonized. This phenomenon can be caused by the decomposition of C-O-C on the surface of UCLF after thermal treatment. This result is consistent with the thermogravimetric analysis (TGA) (Figure S3, Surpporting information).



Figure 7. a) Photocatalystic reduction of 50 mg/L Cr(VI) with different catalysts. UV−vis absorption spectra of Cr(VI) residual after b) CLF@SnS2, c) UCLF@SnS2, d) SnS2, e) UCLF and f) CLF treatment. In order to investigate the catalytic activity, CLF@SnS2, UCLF@SnS2, SnS2, UCLF and CLF were used to reduce Cr(VI) solution under light radiation with stirring. To eliminate the influence of adsorption, light irradiation started after 30 mins. For the case of UCLF or CLF foam alone, the Cr(VI) concentration reached equilibrium after 75 mins, and only 4.66% of Cr(VI) was reduced from the wastewater after 120 mins (green and purple lines, respectively). When treated with the SnS2 nanoparticles, the corresponding removal efficiency of Cr(VI) is 42.3% after 120 min (blue line). Alternatively, the reduction of Cr(VI) by UCLF@SnS2 foams had an efficiency of 73.7% while it was 99.7% when treated with the CLF@SnS2 foam within 120 min. The ultraviolet absorption spectrum of the remaining hexavalent chromium in the solution with the CLF@SnS2, UCLF@SnS2, SnS2, UCLF, and CLF treated were shown in Figure 7b, c, d, e and f, respectively. It is obviously that the CLF@SnS2 foam (Figure 7b) reduce Cr(VI) more efficiently than UCLF@SnS2 (Figure 7c), SnS2 (Figure 7d), UCLF (Figure 7e), and CLF (Figure 7f). According to the results of Figure 7(b, c, d, e and f), the characteristic absorption peak of hexavalent chromium is clearly observed at 255 nm and 355 nm. And it can be clearly observed that the ultraviolet absorption peak will be rapidly weakened at 255nm and 355nm after the same concentration of hexavalent chromium solution with treated CLF@SnS2 foam (Figure 7b), UCLF@SnS2 (Figure 7c), SnS2 (Figure 7d), UCLF (Figure 7e), and CLF (Figure 7f). However, the ultraviolet absorption peak intensity of residual hexavalent chromium in the solution after treatment CLF@SnS2 foam decreases faster than that of UCLF@SnS2 (Figure 7c), SnS2 (Figure


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7d), UCLF (Figure 7e), and CLF (Figure 7f). In addition, it can be observed from Figure 7e and Figure 7f that the intensity of the characteristic absorption peak of hexavalent chromium does not change significantly after 120 min. And this result is consistent with the corresponding curve in Figure 7a. Therefore, Figure 7(e and f) showed that the concentration of hexavalent chromium in the solution after treatment with UCLF and CLF does not decrease significantly. In Figure 7b, the value of the absorption peak decreased fastest and reached equilibrium after 90 min. When the treatment time is extended to 120 min, no absorption peak has been observed in the Figure 7b. Besides, the characteristic absorption peak of hexavalent chromium could be clearly watched after 120min in Figure 7c. Based on the above results, it can be proved that CLF@SnS2 foam exhibits high catalytic activity for hexavalent chromium under visible light irradiation. The result of photocatalytic reduction of Cr(VI) and the results of PL (Figure S6, Supporting information) can be well matched.



Figure 8. Photocatalytic reduction of Cr(VI) solution (50 mL, 50 mg/L) with 50 mg CLF@SnS2 under the condition of pH 2 and recycle three times a). UV-vis absorption spectra of the remaining Cr(VI) in the solution after three cycles b, c and d). Photocatalytic stability of CLF@SnS2 foams were demonstrated through recycling. The cycling performance of CLF@SnS2 foams were shown in Figure 8 (b, c and d) by the UV absorption spectrum. As the Figure 8a shown, the removal efficiency of Cr(VI) solution was 88.6% after three cycles. And the results of UV-vis absorption spectra in the Figure 8 (b, c and d) also proved that the CLF@SnS2 foams have good photocatalytic stability and recyclability. At the same time, the SEM and XRD of the CLF@SnS2 foams after three cycling in the Figure S5 showed that SnS2 can be firmly present on the surface of CLF foams. Although the reduction of Cr(VI) is slightly lower than that of the first in the second and third use of the CLF@SnS2 foams. But, it is still better than the SnS2 nanoflowers under visible light exposure.

Figure 9. The flexibility of a) UCLF@SnS2 and b) CLF@SnS2. The flexibility of UCLF@SnS2 foam and CLF@SnS2 foam were shown in Figure 9. UCLF@SnS2 and CLF@SnS2 foams could easily restore to their original state after being


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compressed. The flexibility of UCLF@SnS2 foam was better than that of CLF@SnS2, which was due to the degree of carbonization of CLF@SnS2 than UCLF@SnS2 foam. However, the flexibility of CLF@SnS2 is good enough to benefit the handling and recycling of the industrial wastewater treatment.

Figure 10. Schematic of the mechanism of photocatalytic reduction of Cr(VI). Figure 10 depicts the process of photocatalytic reduction of Cr(VI) on the surface of CLF@SnS2 foam with continuous exposure to visible light.44-46 As shown in Equation 1, SnS2 was excited to generate photoelectrons and holes under visible light irradiation. However, the H2O undergo oxidation-reduction reactions to produce H+ (Equation 2). In the coexistence of photoelectrons and H+, CrO42- reacted with them and produced Cr3+ and H2O (Equation 3). And the XPS results of the CLF@SnS2 after three cycling (Figure S4, Supporting information) have proven the existence of Cr3+ during photocatalytic reduction. CLF@SnS2 + hν → e− + h+

(1)

H2O + 2h+ → 1/2O2 + 2H+

(2)

CrO42- + 8H+ + 3e− →Cr3+ + 4H2O

(3)



The electronic transmission rate could be effectively enhanced by the carbon of the CLF foam surface. The fibers of loofah flesh uncarbonated could effectively maintain mechanical strength and facilitate recycling of CLF@SnS2 foam.

Conclusion A new type of three dimensional photocatalytic material, CLF@SnS2 foam, was prepared for the treatment of Cr(VI) containing wastewater under visible light exposure. By contrast, CLF@SnS2 showed better visible photocatalytic capacity than the uncarbonized counterpart, UCLF@SnS2. Compared to SnS2 nanoparticles, the CLF@SnS2 foam has obvious advantages, such as 3-dimensional macroporous and flexible structures. By modifying the hierarchical nanostructure on the surface of UCLF and CLF foams, this method prevented aggregation of the SnS2 nanoparticles during the process of photocatalytic reduction of Cr(VI). These synergistic effects improved the visible light-driven ability of the CLF@SnS2 foams. Under visible light irradiation, CLF@SnS2 did not only show high catalytic activity in the visible light region, but also provided excellent cycling stability. The CLF@SnS2 can achieve 99.7% of Cr(VI) remove efficiency for 50 mg/L Cr(VI) solution within 120 min. More importantly, CLF@SnS2 composites are hopefully applied to the purification treatment of chromium-containing industrial wastewater due to its flexibility and economic availability.

Supporting Information Supporting information is available from the http://pubs.acs.org or from the author.

AUTHOR INFORMATION


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Corresponding Author *Guo Hong. E-mail: [email protected] *Dong Fang. E-mail: [email protected].

Acknowledgements Financial support for this project is provided by the Start-up Research Grant (SRG2016-00092IAPME), Multi-Year Research Grant (MYRG2018-00079-IAPME) of University of Macau and Science and Technology Development Fund (081/2017/A2), Macao S.A.R (FDCT).

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For Table of the Contents Use Only


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The CLF@SnS2 foam with 3D structure can quickly purify Cr(VI)-containing wastewater and exhibits excellent cycling stability under visible light irradiation.


Tin(IV) Sulfide Biofoam for

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