Reactive Yellow 161 Decolorization by TiO2 ... - ACS Publications

Dec 9, 2016 - Geelong, Victoria 3220, Australia. ‡. School of Textile Science and Engineering, Wuhan Textile University, 1 Sunshine Avenue, Wuhan, ...
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Reactive Yellow 161 Decolorization by TiO2/Porous Boron Nitride Nanosheet Composites in Cotton Dyeing Effluent Wanjie Xie,† Mingwen Zhang,† Dan Liu,*,† Weiwei Lei,† Lu Sun,*,†,‡ and Xungai Wang†,‡ †

Australia Future Fibers Research and Innovation Center, Institute for Frontier Materials, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia ‡ School of Textile Science and Engineering, Wuhan Textile University, 1 Sunshine Avenue, Wuhan, Jiangxia District, 430073, China S Supporting Information *

ABSTRACT: Titanium dioxide/porous boron nitride nanosheet composites (TiO2/P-BNNSs) were prepared in this study to photo decolorize Reactive Yellow 161 (RY161) in the simulated cotton dyeing effluent, where alkaline and inorganic salts were presented. The prepared TiO2/P-BNNSs were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). The photocatalytic performance of TiO2/P-BNNSs was also compared with that of synthesized TiO2 and commercial TiO2 (P25). Although the addition of alkaline and inorganic salts negatively influenced the RY161 decolorization, the RY161 decolorization efficiency remained the highest in the simulated systems (unhydrolyzed and hydrolyzed systems) with the application of TiO2/P-BNNSs. The excellent reusability of TiO2/P-BNNSs was also demonstrated after five successive cycles of use in the simulated cotton dyeing effluent, during which the RY161 decolorization percentage remained at 87%−90% in the unhydrolyzed system and at 100% in the hydrolyzed system. The excellent RY161 decolorization by TiO2/P-BNNSs and their exceptional reusability suggest TiO2/P-BNNSs are potential candidates for practical dye effluent treatment. KEYWORDS: TiO2 photocatalysis, Porous boron nitride nanosheets, Cotton dyeing effluent treatment, Inorganic salts, Alkaline, Sustainable recyclability

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INTRODUCTION Cotton textiles constitute one of the world’s mostly used textiles, and 50% of them are dyed with reactive dyes; during this process, the reactive dyes can lose up to 40% of its hydrolyzed or unfixed form in the dye bath depending on the dyeing efficiency.1−4 These dyes in the effluent appear strong color even in a very low concentration (1 mg L−1), generating byproducts through oxidation, hydrolysis, or other chemical reactions, posing a threat to both the environment and human health.5,6 In order to alleviate these negative effects, it is of paramount significance to develop an efficient technique for residual dye removal before discharging the cotton dyeing effluent into waterways. Titanium dioxide (TiO2) photocatalysis is such a green and promising technique to remove dyes due to its utilization of solar light and the stability, cost-effectiveness, and commercial availability of TiO 2. 7−12 Gonça lves et al. studied the degradation of Reactive Orange 4 either alone in water, or in its hydrolyzed and unhydrolyzed forms in a simulated dyebath by Degussa P25 TiO2 (P25) and Riedel-de-Häen (RH) and they found that P25 performed better than RH at RO4 decolorization when using sunlight as the light source.13 Arslan et al. applied even more kinds of TiO2, including Degussa P25 © 2016 American Chemical Society

(P25), Millenium PC 500 (PC 500), Sachtleben Mikroanatas IF9308/18 (Miroanatas), and Platinized P25 (Pt-P25) to decolorize a mixture of reactive dyes in two simulated cotton dyebaths. They systematically investigated the effects of pH and inorganic anions on decolorization degrees and the kinetics of the decolorization process with these TiO2 products in simulated cotton dyebaths.14 On the other hand, extended applications of this technique have been suffering from these problems: (1) High electron−hole recombination rate of TiO2 leading to inefficient dye removal.15−18 (2) Except for dyes, cotton dyeing effluent often contains auxiliaries such as inorganic salts, which might jeopardize the TiO2 for dyes removal. The most commonly accepted opinion is that inorganic ions might act as hydroxyl radicals (•OH) scavengers to reduce the oxidative species for organic dyes.19,20 In recent years, doping TiO2 with metal, nonmetal or coupling it with other semiconductors have been studied to improve its photocatalytic performance.21 Increasing the active (001) facets exposal of TiO2 is another popular method16 while Received: August 9, 2016 Revised: December 1, 2016 Published: December 9, 2016 1392

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into a 50 mL stain steel autoclave, which was put in the oven at 180 °C for 20 h. The obtained TiO2/P-BNNSs were then collected by centrifugation and washed with ethanol for 3 times, followed by being dried at room temperature overnight. The weight percentage of TiO2 on TiO2/P-BNNSs was determined as 38 wt % by weighing the final products. Synthesized TiO2 was prepared using the same method without the addition of porous BNNSs. Characterization. X-ray diffraction (XRD) patterns were obtained on a PANalytical’s X’Pert Powder X-ray diffraction (40 kV, 30 Ma) with Cu−Kα radiation from 20° to 80° at room temperature. Fourier transform infrared spectroscopy (FTIR) data were determined from a Bruker FTIR spectrophotometer in transmission mode, with accumulation of 32 scans at 4 cm−1 resolution. Transmission electron microscopy (TEM) was applied on a JEOL 2100F high resolution mode operating at 110 kV apparatus. Scanning electron microscopy was conducted on a Zeiss Supra 55 VP SEM. Nitrogen adsorption and desorption isotherms were obtained using a Tristar 3000 apparatus at 77 K. The optical absorption spectra of different concentrations of dye solution were obtained using a Varian Carry 3E UV−vis. spectrophotometer. Simulated sunlight test was carried out in the Altas Suntest CPS2 instrument (Ameteck, United States) equipped with 1500 W air cooled xenon arc lamp (light range 300−800 nm wavelengths). Simulated Cotton Dyeing Effluent. The RY161 concentration in the effluent was considered to be 50 mg L−1 while pH value was selected as 11. The concentrations of NaCl and Na2CO3 were selected as 80 and 25 g L−1, respectively. The dye bath was diluted in the rinsing stage and it was considered to be diluted by 40-fold. The concentrations of NaCl and Na2CO3 were then diluted to 2 and 0.625 g L−1, respectively. The compositions of the simulated dyeing effluent can be summarized in Table S1 while the molecular structure of RY161 is presented in Figure S1. Hydrolysis of RY161 was simulated by keeping the prepared simulated cotton dyeing effluent in the water bath at 80 °C for 2 h, followed by putting in the room temperature overnight to ensure the completed hydrolysis. Photocatalytic Activity Test. All prepared-solutions (50 mL) were added with TiO2/P-BNNSs, and the concentrations of all the compositions are shown in Table S1. The suspensions were stirred in the dark condition for 1 h to ensure the adsorption−desorption equilibrium of RY161 and were then put in the Altas Suntest CPS2 instrument (Ameteck, United States) equipped with 1500 W air cooled xenon arc lamp (light range 300−800 nm wavelengths, 350 W). A 2 mL suspension was extracted every 10 min and centrifuged to obtain the supernatant, the UV−vis absorbance spectra of which was obtained by Varian Cary 3E spectrophotometer, to track down the concentration changes of RY161. The RY161 decolorization can be calculated by eq 1:

special focus has been paid on combining TiO2 with carbonbased materials such as graphene, which is beneficial to reduce the recombination of photoinduced electrons and holes on TiO2.22,23 Boron nitride (BN) is an analogue to graphene and also called as “white graphene”. It possesses unique properties such as high chemical inertness and high adsorption ability and also has been applied for wastewater cleaning.24−27 To date, very few work has been done regarding combining BN with TiO2 though BN has been proven to be an excellent support for metal oxide particles.28 Tang et al. significantly improved the photocatalytic activity of TiO2 by functionalization of BN nanotubes and they summarized the improved dyes degradation was largely attributed to the strong underlying electrostatic potential of the high-purity nanotubes.29 Weng et al. designed the TiO2/BN porous sheets which performed well in removing organic compounds.30 We have also exploited the advantages of porous BNNSs such as their large surface area and high volume of pores to prepare TiO2/porous boron nitride nanosheets composites (TiO2/P-BNNSs, TiO2 weight percentage was 38 wt %). Exceptional photocatalytic performance of these composites for Rhodamine B (RhB) degradation in distilled water was well demonstrated and the mechanism for these phenomena was thoroughly discussed.31 Despite these progresses, the improved photocatalytic activity of TiO2/BN hybrids has been demonstrated only for excellent dyes removal in pure dye solutions. In order to extent their practical applications for textile dyeing wastewater, this study aimed to investigate the dyes decolorization by TiO2/PBNNSs in a simulated cotton dyeing effluent which contained reactive dyes, inorganic salts (NaCl and Na2CO3) and alkaline (NaOH, pH = 11). Reactive Yellow 161 (RY161) was selected because it is a commercial azo dye which is targeted at dyeing cotton fibers in the textile industry normally. Excellent RY161 decolorization by TiO2/P-BNNSs in the cotton dyeing effluent was demonstrated and the reasons for their better performance than both synthesized TiO2 and commercial TiO2 (P25) were specifically explained. The influences of the inorganic salts and alkaline on RY161 decolorization were also studied. The excellent reusability of TiO2/P-BNNSs implicates their potentials for practical textile dyeing wastewater treatment.

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EXPERIMENTAL SECTION

Materials. Boron trioxide (99%) and guanidine hydrochloride (98%) were purchased from Alfa Aesar and used to prepare porous BNNSs. Tetrabutyl titanate (TBT, Ti(OC4H9)4, 97%) and terephthtalic acid (98%) were obtained from Aldrich. Nitric acid (HNO3, 70%), ethanol (100%), and commercial TiO2 (P25, 80% anatase and 20% rutile TiO2, 21 nm particle size) were from Sigma-Aldrich (Australia). Reactive Yellow 161 (Reactive Yellow H-2G) was obtained from CIBA. Sodium chloride (NaCl) and sodium hydroxide (NaOH) were provided by Chem-Supply and sodium carbonate (Na2CO3) were from MERCK Pty Limited. All the chemicals were used as received without further purification. Synthesis of TiO2/P-BNNSs. The porous BNNSs were synthesized as described elsewhere.32 The synthesized TiO2 was prepared from TiO2 precursor which was prepared by mixing 1 mL TBT and 5 mL ethanol under ultrasonication for 5 min, followed by adding 2 mL HNO3 (70%) with vigorous stirring. The obtained solution was further diluted to 10 mL by adding ethanol to form the TiO2 precursor. The TiO2/P-BNNSs were synthesized as follows: porous BNNSs (40 mg) were first treated with HNO3 (8 M) by ultrasonic treatment for 2 h at room temperature, and then were dispersed in ethanol with ultrasonic treatment for 10 min. The obtained mixture was added with TiO2 precursor (2 mL) by vigorous stirring, after which the dispersion was diluted by ethanol to 25 mL. This dispersion was then transferred

decolorization (%) =

A0 − A × 100% A0

(1)

Where A0 is the absorbance of original RY161 solution at the maximum absorption wavelength (λmax), and A is the absorbance of RY161 solution at λmax after simulated sunlight irradiation. In order to figure out whether the photocatalytic activity is enhanced by using TiO2/P-BNNSs, synthesized TiO2 and commercial TiO2 (P25) were also used to carry out photocatalysis in the same solution compositions. (TiO2 quantity was maintained at 7.6 mg)

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RESUTLS AND DISCUSSION Characterization of TiO2/P-BNNSs. Figures 1a and b and S2 exhibit clearly that porous BNNSs is a wrinkled structure with pores. Uniform distribution of TiO2 particles on porous BNNSs is observed from Figure 1c, and the HRTEM image of TiO2/P-BNNSs (Figure 1d) explicitly reveals the well-defined crystallinity of the loaded TiO2 with a lattice spacing of 0.35 nm that assigns to the (101) plane of anatase TiO2 (JCPDS, no. 211272). It can also be seen that the fringe of porous BNNSs can be clearly identified from the TiO2/P-BNNSs edges. 1393

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BNNSs, where the hydroxyl groups (−OH) were introduced before synthesizing process or from the bonds (B−O−Ti) that were formed during the synthesizing process. These results suggest the successful loading of TiO2 on porous BNNSs. RY161 Decolorization in Distilled Water by P25, Synthesized TiO2 and TiO2/P-BNNSs. As shown in Figure 3, the RY161 decolorization degrees by using P25, synthesized TiO2 and TiO2/P-BNNSs were 71.2%, 85.8%, and 96.5%, respectively.

Figure 1. (a) TEM image of porous BNNSs. (b) Rectangular area in part a. (c) TEM image of TiO2/P-BNNSs. (d) High resolution TEM (HRTEM) image of TiO2/P-BNNSs.

The XRD pattern of TiO2/P-BNNSs is shown in Figure 2a. The diffraction patterns of synthesized TiO2 and porous BNNSs are also presented for comparison. For porous BNNSs, two diffraction peaks appeared in the (100) and (002) planes of porous BNNSs.32 All diffraction peaks of synthesized TiO2 are associated with the anatase phase.33 The diffraction peaks of synthesized TiO2 can also be found in the XRD pattern of TiO2/P-BNNSs while the diffraction peaks of porous BNNSs are overlapped by those of TiO2. This overlap is confirmed by the XRD patterns of TiO2/P-BNNSs with different TiO2 loading (Figure S3). FTIR spectra of porous BNNSs, synthesized TiO2, and TiO2/P-BNNSs are presented in Figure 2b. Two strong peaks of porous BNNSs located at 785 and 1349 cm−1 are assigned to B−N bending and B−N stretching, respectively.25 In terms of synthesized TiO2, the peak near 540 cm−1 corresponds to Ti− O stretching while the peak at 1633 cm−1 associated with the vibration of adsorbed water and the peak near 3300 cm−1 indicates the presence of hydroxyl group.34 For TiO2/PBNNSs, apart from the typical peaks aroused from porous BNNSs and synthesized TiO2, a characteristic peak near 1220 cm−1 was also found, which is ascribed to B−O bonds. These bonds may be originated from the pretreatment of porous

Figure 3. RY161 decolorization by P25, synthesized TiO2, and TiO2/ P-BNNSs in RY161 solution. Co and C are the concentration of original RY161 solution and concentration of RY161 solution at different irradiation times, respectively.

Compared with synthesized TiO2, TiO2/P-BNNSs showed higher RY161 decolorization percentage, suggesting that porous BNNSs play an important role in enhancing the photocatalytic performance of TiO2. This enhancement may be mainly attributed to the large adsorption capacity of porous BNNSs. In the adsorption experiments, 20% RY161 was adsorbed on porous BNNSs and TiO2/P-BNNSs, around 13% higher than those of synthesized TiO2 and P25 (Figure S4), suggesting the high adsorption capability of porous BNNSs and TiO2/PBNNSs. Porous BNNSs had a strong adsorption capability for

Figure 2. (a) XRD patterns and (b) FTIR spectra of porous BNNSs, synthesized TiO2, and TiO2/P-BNNSs. 1394

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ACS Sustainable Chemistry & Engineering RY161 because of the strong π−π interactions, which were induced by the similarity of benzene molecules and B−N rings on (002) plane of BN.35 In addition, the large surface area of porous BNNSs (∼1400 m2 g−1) enabled a large number of RY161 molecules to be adsorbed on their surface through “surface effect”. A high volume of pores and unsaturated atoms along the edges of nanosheets might also be beneficial to the strong adsorption.31 Therefore, the combination of porous BNNSs and TiO2 endowed TiO2/P-BNNSs with high surface area (226.9 m2 g−1, Figure S5) and high adsorption ability accordingly. Given that TiO2 particles were intimately anchored on porous BNNSs, the quick and high adsorption of RY161 on porous BNNSs increased the contact of RY161 and TiO2 surfaces, which could promote the RY161 decolorization rate as photoinduced reaction species were mainly located on TiO2 surfaces. Our results was also consistent with the published work from Gao et al, who synthesized TiO2 on the activated carbon (AC) to obtain TiO2/AC composites, which had higher photocatalytic efficiency for Acid Red 88 (AR88) than the unsupported TiO2 due to the high adsorption ability of activated carbon.36 Apart from the higher adsorption ability of TiO2/P-BNNSs, a greater number of hydroxyl radicals (•OH) produced from TiO2/P-BNNSs was confirmed in Figure S6. •OH are extremely powerful oxidants which are responsible for oxidizing organic species, thus a higher number of •OH leads to a higher RY161 decolorization percentage.37 Influence of pH and Inorganic Salts on RY161 Decolorization in the Cotton Dyeing Effluent. In order to understand how inorganic salts (NaCl and Na2CO3) and alkaline (NaOH, pH = 11) in this simulated cotton dyeing effluent influence the RY161 decolorization, different solution compositions were prepared: (1) RY161 solution, (2) RY161 (pH = 11) solution, (3) RY161 (NaCl + Na2CO3) solution. P25, synthesized TiO2, and TiO2/P-BNNSs were added in the as-prepared solutions, and the suspensions underwent a photocatalysis process being irradiated by the simulated sunlight for 70 min. The results are presented in Figure 4. Influence of Alkaline (pH = 11) for the RY161 Decolorization. The pH of effluent produced from practical cotton dyeing with reactive dyes is around 9−11.38 In order to understand how this pH influences the RY161 decolorization,

RY161 (pH = 11) solution was prepared with sodium hydroxide. In this condition, as shown in Figure 4, RY161 decolorization percentage was 30.8% in the presence of synthesized TiO2, 55% lower than that of RY161 in distilled water. This result is consistent with previous reports, which explained this reduction may be due to the surface property of TiO2.39−42 Specifically, the point of zero charge (pzc) of TiO2 is around 4.5−7,42 and its surface is negatively or positively charged when the solution pH is higher or lower than the pzc: pH < pzc : Ti − OH + H+ → TiOH2 + pH > pzc : Ti − OH + OH− → TiO− + H 2O

The pH of this RY161 solution was 11, higher than the pzc of TiO2, meaning that the surface of synthesized TiO2 was negatively charged. Given that RY161 is an anionic dye and is also negatively charged in solution, electrostatic repulsive forces occurred between synthesized TiO2 surface and RY161, decreasing the RY161 adsorption on synthesized TiO2. This reduced RY161 adsorption was confirmed by the adsorption− desorption equilibrium process, it is clearly seen from Figure S7, the RY161 adsorption was 6.8% in RY161 solution, but no RY161 adsorption was found in RY161 (pH = 11) solution. TiO2/P-BNNSs were also negatively influenced in this condition. As can be seen in Figure S7, the RY161 adsorption in RY161 (pH = 11) solution was 7% lower than that in the RY161 solution. This reduced adsorption ability of TiO2/PBNNSs can be ascribed to electrostastic repulsion between RY161 molecules and the synthesized TiO2 particles that have been loaded on porous BNNSs. Similar reduced adsorption and decolorization trend of synthesized TiO2 and TiO2/P-BNNSs toward the RY161 (pH = 11) solution and the achieved enhanced photocatalytic performance of TiO2/P-BNNSs suggested the successful loading of synthesized TiO2 on porous BNNSs. It is worth noting that unlike the synthesized TiO2, RY161 decolorization by P25 tended to increase when the pH of RY161 solution was adjusted to 11. It can be seen from Figure S7 that although no RY161 adsorbed on P25 surface, which is a similar adsorption trend to the synthesized TiO2, the RY161 decolorization in RY161 (pH = 11) solution was slightly higher than that in RY161 solution (Figure 4). Bouanimba et al. studied the effect of pH on the photocatalytic decolorization of Methyl Orange in the presence of two different kinds of TiO2 (P25 and PC500) and concluded that these two photocatalysts reacted differently with the variation of pH.43 Similarly, the synthesized TiO2 and P25 applied in this study also reacted differently when the pH of RY161 solution was 11. The synthesized TiO2 was negatively influenced while P25 was slightly positively affected by the increased alkalinity. Influence of Mixed Inorganic Salts (NaCl and Na2CO3) for the RY161 Decolorization. RY161 decolorization was decreased greatly with the addition of inorganic salts (NaCl and Na2CO3). It can be seen from Figure 4, for TiO2/P-BNNSs, RY161 decolorization decreased from 96.5% to 70%, while it reduced from 85.8% to 18.6% and from 71.2% to 41.8% for synthesized TiO2 and P25, respectively. A number of previous researches have claimed that inorganic salts may inhibit photodegradation of dyes. The most commonly seen explaination is that inorganic anions may act as scavengers for holes (h+) or hydroxyl radicals (•OH).44−46 For chloride ions (Cl−) and (CO32−), their scavenging behavior can be expressed as follows:

Figure 4. RY161 decolorization in RY161, RY161 (pH = 11) and RY161 (NaCl + Na2CO3) solutions by P25, synthesized TiO2, and TiO2/P-BNNSs under simulated sunlight for 70 min. 1395

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Figure 5. RY161 decolorization by P25, synthesized TiO2, and TiO2/P-BNNSs in the (a) unhydrolyzed and (b) hydrolyzed systems. (c) TiO2/PBNNSs under the simulated sunlight with five successive cycles in the hydrolyzed and unhydrolyzed system.

Cl− + •OH → HOCl•−

[DyeSO3Na]2 or n = dye aggregation unit which consists

Cl− + h+ → Cl•

of two or many single dye molecules

Electrostatic repulsion between dyes can be reduced if salts were added, leading to increased aggregation and decreased ionization of dyes. Less dyes in water negatively affect their reactions with •OH, resulting in reduced degradation.47 It should be mentioned that for the application of TiO2/PBNNSs, RY161 decolorization percentage with addition of mixed salts (70%) was lower than that with NaCl (97%) while higher than that with Na2CO3 (49.5%). This may be because the presence of NaCl was beneficial to RY161 adsorption on porous BNNS, thus alleviating the negative influence brought by the Na2CO3. (Figure S7) With the addition of mixed salts, TiO2/P-BNNSs decolorized 70% RY161, 51% higher than synthesized TiO2 and 29% higher than P25. TiO2/P-BNNSs gave the higheset RY161 decolorization percentage. RY161 Decolorization in the Simulated Cotton Dyeing Effluent. Two cotton dyeing effluent systems were simulated in this study. One was made up of unhydrolyzed RY161 with relevant auxiliaries (unhydrolyzed) as described in Table S1 while another one contained fully hydrolyzed RY161 with auxiliaries (hydrolyzed). In the later system, solution was put in a water bath at 80 °C for 2 h and then was kept at the room temperature overnight to ensure complete hydrolysis of monochlorotriazinyl group of RY161. These systems were then added with P25, synthesized TiO2 and TiO2/P-BNNSs respectively and were irradiated in the simulated sunlight. As shown in Figure 5a and b, during the 70 min irradiation, the RY161 decolorization values were 40%, 19%, and 70% in the unhydrolyzed system, while they were 35.7%, 27.4%, and 77.7% in the hydrolyzed system for P25, synthesized TiO2, and TiO2/ P-BNNSs, respectively. As shown in Figure S9a and b, the ln(Co/C) plots show a linear relationship to irradiation time, revealing that the RY161 decolorization in both systems went through the pseudo-first-order kinetic reaction regardless of P25, synthesized TiO2, or TiO2/P-BNNSs were applied. The equation of pesudo-first-order kinetic model can be expressed as eq 2:

CO32 − + •OH → HO− + CO3•−

In order to understand whether the decreased RY161 decolorization is originated from the scavenging behavior of Cl− and CO32−, RY161 (NaCl) and RY161 (Na2CO3) solutions, where NaCl and Na2CO3 were added into the RY161 solutions, respectively, were prepared. Decolorization results are shown in Figure S8. Specific influences of the added Cl− and CO32− are precisely discussed in the Supporting Information. Our results suggested that the added Cl− did not perform as •OH and h+ scavengers while the negative influence from the added CO32− for RY161 decolorization was due to their savangers for •OH. It also should be mentioned that by applying synthesized TiO2 and P25, the total RY161 decolorization in the presence of mixed salts (Figure 4) was much lower than that in the respective presence of NaCl or Na2CO3 only at the same concentration (Figure S8). Specifically, for P25, with presence of mixed salts, RY161 decolorization percetange was 41.8% while the degrees were 78.3% when NaCl was added and 68.5% with the addition of Na2CO3. For synthesized TiO2, RY161 percentage was 19% with the addition of mixed salts and 88.8% with mere presence of NaCl while 21.4% when there was only Na2CO3 added. These results are consistent with the results of Dong et al., who studied the decolorization of Reactive Blue MS with addition of mixed salts, and concluded that different proportioned mixed salts differently affect the decolorization of dyes.47 This could be explained by the enhanced “common ion effect” of the combined salts. Dissolving situations of dyes are determiend by the aggregation and ionization of dyes, which can be described as follows: Aggregation 2DyeSO3Na ↔ [DyeSO3Na]2 nDyeSO3Na ↔ [DyeSO3Na]n

⎛C ⎞ ln⎜ o ⎟ = kt ⎝C⎠

Ionization

(2)

Where C is the concentration of RY161 at different irradiation time; Co is the original concentration of RY161 after being stirred in the dark condition; k is the pseudo-first-order rate constant; t is the irradiation time.

DyeSO3Na ↔ DyeSO3− + Na + DyeSO3Na = single dye molecule 1396

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the synthesized TiO2, a higher number of •OH was produced on TiO2/P-BNNSs, and this is beneficial to the RY161 decolorization, especially when •OH is the predominant oxidation specie at high pH levels.48 The better performance of TiO2/P-BNNSs than either synthesized TiO2 or commercial TiO2 (P25) and the proven excellent reusability for RY161 decolorization in the sumulated cotton dyeing effluent demonstrate their potential applications in real cotton dyeing effluent.

The slopes of the fitting curves are k values, reflecting the photo reaction rate. All k (min−1) values are summarized in Table 1. Table 1. Pseudo-First-Order Rate Constants of Photo Reaction Using P25, Synthesized TiO2, and TiO2/P-BNNSs in the Unhydrolyzed and Hydrolyzed Systems k (min

−1

)

unhydrolyzed hydrolyzed

kP25

ksynthesizedTiO2

kTiO2/P‑BNNSs

0.00691 0.00537

0.00298 0.00444

0.01275 0.01749

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CONCLUSIONS Despite the fact that the RY161 decolorization was inhibited by the addition of either mixed inorganic salts or alkaline, RY161 decolorization percentage remained the highest with the application of TiO2/P-BNNSs. In the simulated cotton dyeing effluent, RY161 decolorization process by TiO2/P-BNNSs fit the pseudo-first-order kinetic reaction, and the RY161 decolorization rate with TiO2/P-BNNSs approximately four times the results by synthesized TiO2 in both unhydrolyzed and hydrolyzed systems while twice and three times the figure for P25 in the unhdrolyzed and hydrolyzed system, respectively. The good reusability of TiO2/P-BNNSs was proven after 5 successive cycles of use, during which the RY161 decolorization percentage remained at 87%−90% in the unhydrolyzed system and at 100% in the hydrolyzed system.

From Table 1, we can see that in both systems, RY161 decolorization rate by TiO2/P-BNNSs was higher than either P25 or synthesized TiO2. This rate by TiO2/P-BNNSs four times the figure for synthesized TiO2, suggesting loading TiO2 on porous BNNSs is of vital significance to improve the decolorization of reactive dyes in the cotton dyeing effluent. It can also be seen that compared with P25, the RY161 decolorizaiton rate by TiO2/P-BNNSs also twice the figure for P25 in the unhydrolyzed system and three times the rate by P25 in the hydrolyzed system. It should be mentioned RY161 decolorization rate by P25 in the hydrolyzed system was lower than that in the unhdyrolyzed system. On the contrary, the rates by synthesized TiO2 and TiO2/P-BNNSs in the hydrolyzed system were higher than that in the unhydrolyzed system. It may be because the hydrolysis of RY161 (monochlorotriazinyl group react with the hydroxyl groups of water molecule) slightly decreased the alkalinity of the solution. These results corresponds to the results obtained from RY161 (pH = 11) solution, which suggest the high alkalinity is beneficial to RY161 decolorization by P25 while has adverse effect on RY161 decolorization by synthesized TiO2 and TiO2/P-BNNSs. To check the reusable ability of TiO2/P-BNNSs, both systems were irradiated continuously, after another 90 min, RY161 decolorization by TiO2/P-BNNSs in the unhydrolyzed system reached 90% while reached 100% in the hydrolyzed system. TiO2/P-BNNSs were then collected by centrifugation and added into the newly prepared unhydrolyzed and hydrolyzed systems and were subsequently irradiated under the simulated sunlight until 160 min. As shown in Figure 5c, after five successive recycles, the RY161 decolorization by TiO2/P-BNNSs in the unhydrolyzed system remianed at 87− 90%; on the other hand, in the hydrolyzed system, RY161 decolorization remained at 100%. Decolorization Mechanism of RY161 by TiO2/P-BNNSs in the Cotton Dyeing Effluent. By concluding what have been demonstrated above, the mechanism of TiO2/P-BNNSs for RY161 decolorization in cotton dyeing effluent can be proposed. Specifically, once the TiO2/P-BNNSs are dispersed in the cotton dyeing effluent, RY161 molecules can quickly adsorb on the surface of TiO2/P-BNNSs. During the irradiation in simulated sunlight, RY161 molecules were then decolorized through reacting mainly with the holes (h+) and hydroxyl radicals (•OH) that were produced from the loaded TiO2 particles. Although the performance of the TiO2 is greatly hindered in such complex cotton dyeing effluent, after loading it on porous BNNSs, this adverse effect was greatly impaired. First of all, porous BNNSs ensure the high adsorption of RY161 on TiO2/P-BNNSs even though in a high alkaline (pH = 11) medium, where the RY161 adsorption on the synthesized TiO2 was reduced at a large extent. Additionally, compared with

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01896. Analysis procedures of hydroxyl radicals; additional figures and tables for RY161 molecular structure, RY161 adsorption percentage, RY161 decolorization kinetics data, SEM image, XRD spectra, N2 adsorption desorption equilibrium, and components of simulated cotton dyeing effluent (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +61 3 522 73247. E-mail: [email protected] (L.S.). *E-mail: [email protected] (D.L.). ORCID

Lu Sun: 0000-0003-2999-1250 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Australian Research Council Discovery Program (DE150101617 and DE140100716), the Australian Research Council Discovery Early Career Researcher Award scheme, the National Natural Science Foundation of China (NSFC 51302197), and Deakin University, Central Research Grant Scheme, are acknowledged.

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