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Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants
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JECE 648 1–7 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants
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Jinfeng Wang a , Haijin Zhu b , Christopher Hurren a,c , Jian Zhao a,d, Esfandiar Pakdel a , Zhenyu Li a , Xungai Wang a,c, * a
Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, Victoria 3220, Australia ARC Centre of Excellence for Electromaterials Science (ACES), Institute for Frontier Materials (IFM), Deakin University, Burwood, Victoria 3125, Australia School of Textile Science and Engineering, Wuhan Textile University, Wuhan 430073, China d Key Laboratory of Advanced Textile Composites, Ministry of Education of China, Tianjin Polytechnic University, Tianjin 300387, China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 December 2014 Accepted 12 May 2015
Treatment of coloured effluent treatment is a major issue for the textile industry. In this study, catalyst P 25–graphene was prepared and applied for degrading dye from an aqueous solution. Three types of dyes were selected to determine the feasibility of the catalyst for the dye degradation, including sulphonic, azoic, and fluorescent dyes. P25–graphene catalyst showed good ability to degrade all selected dyes. The influence of inorganic salts and surfactants on the photocatalytic degradation of rhodamine B using catalyst P25–graphene was also investigated. The degradation of rhodamine B was suppressed by the presence of NaCl, but the effect of Na2SO4 was negligible. The degradation of rhodamine B was significantly suppressed by all three types of surfactant, namely anionic, cationic and non-ionic surfactants. NMR technique was used to investigate the mechanisms associated with this suppression. ã 2015 Published by Elsevier Ltd.
Keywords: Graphene Photocatalyst Dye removal Surfactant Salt
9
Introduction
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Large quantities of highly coloured effluent are produced in the textile industry. It was reported that of the 700,000 tons of dyes produced annually worldwide, about 10–15% of the dyes are disposed off in effluent through incomplete exhaustion and washing process [1]. Their release causes serious damage to the aquatic environment due to high toxicity, low biodegradation and potential carcinogenicity [2]. Physical and traditional biological treatments are not sufficient to remove textile dyes with strong colour [3]. Semiconductor photocatalyst has emerged as one of the most promising materials for environmental remediation. It represents an easy way to utilize the energy of either natural sunlight or indoor illumination light. The mechanism of these functions is based on the in-situ generated highly reactive oxygen species (OH, O2) for mineralization of organic compounds. Photocatalysts, such as TiO2, ZnO, CdS, WO3, SnO2 and ZnS, have demonstrated their efficiency in degrading a wide range of
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* Corresponding author at: Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, Victoria 3220, Australia. Tel.: +61 3 52272894; fax: +61 3 52272539. E-mail address: [email protected] (X. Wang).
organics into readily biodegradable compounds, and eventually mineralizing them to carbon dioxide and water. Among the catalysts developed, TiO2 has been extensively studied. However, the application of TiO2 is limited by the fast recombination of the electron and photogenerated electron–hole pairs. The addition of graphene toTiO2 prevents the recombination of the electron–hole pairs in the TiO2@graphene composites. The TiO 2@graphene composite has a number of attributes, such as increased adsorptivity of pollutants, extended light absorption range, and facile charge transportation and separation, which were rarely reported for other composite carbon composites [4]. Enhanced photocatalytic activity of TiO2 dispersed on graphene has been reported by Zhang et al. [4]. This high performance photocatalysis is anticipated to open new possibilities in the application of TiO2@graphene in environment remediation (Table 1). The actual dye effluent contains not only dyes, but also other auxiliaries such as salts and surfactants. High concentrations of various inorganic salts such as NaCl and Na2SO4 are frequently used in the dyeing process [5]. Surfactants are also widely used as wetting, penetrating, dispersing, and levelling agents in dyeing processes. Hence, large amounts of inorganic salts and surfactants are discharged with dyes. However, a large number of the published papers on the treatment of textile effluent have focused on the principle colour removal, neglecting the influence of
http://dx.doi.org/10.1016/j.jece.2015.05.008 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Table 1 Reported work of applying TiO2@graphene composite for dye removal. Composites
Synthesis procedure
Pollutant
Light Sources
Ref.
P25@graphene P25@graphene oxide
Hydrothermal in water Mixing and sonication Hydrothermal in water and ethanol
UV light (365 nm) Mercury vapour lamp delivering near-UV/vis irradiation (>350 nm) Xe lamp with UV cutoff filter (>420 nm)
[6] [7]
P25/CoFe2O4/graphene
Methylene blue and reactive black Diphenhydramine pharmaceutical and methyl orange Methylene blue, methyl orange and neutral dark yellow Rhodamine B
UV light (365 nm) and LED lamp (>420 nm)
[9]
Methyl orange
High-pressure Xenon Lamp to simulate sunlight [10]
Methylene blue
UV light (220–280 nm) and visible light (400– 1050 nm) UV light (310–400 nm)
P25@reduced graphene oxide Hydrothermal and annealed at 400 under argon P25@graphene (loading of Hydrothermal and annealed at 500 YF3:Yb3+,Tm3+) under argon Reduced by hydrazine hydrate then P25@graphene annealed at 500 P25@reduced graphene oxide Hydrothermal in water at 500
Phenol
[8]
[11] [12]
Table 2 The characteristics of six organic dyes. Dye
Formula
Mw (g/Mol)
Rhodamine B (RhB)
Chemical structure
C28H31N2O3Cl
479
Methyl orange (MO)
C14H14N3O3SNa
327
Procion Red (PR) MX-5B
C19H10N6O7S2Cl2Na2
615
Terasil blue (TB) 3RL-02
C14H9BrN2O4
349
Lanasol yellow (LY) CE 4G
C19H12BrN5Na2O8S2
699
Acid red (AR) 88
C20H13NaN2O4S
400
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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surfactants and salts. In this study, three types of dye were selected to determine the feasibility of the catalyst TiO2@graphene for dye degradation. We further investigated the influence of inorganic salts and surfactant on dye degradation by a TiO2@graphene composite, with rhodamine B as a model molecule. To our best knowledge, this is the first report on the influence of the inorganic salts and surfactants on the photocatalytic performance of TiO 2@graphene nanocomposites. Experimental
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Materials
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Graphite powder (99.95% purity, 325 mesh) was purchased from Sigma–Aldrich. TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa. Rhodamine B and methyl orange were purchased from Sigma–Aldrich. Terasil blue, procion red and acid red were obtained from CIBA. The chemical structures, formula and molecular weights of organic dyes used in this study are listed in Table 2. Sodium chloride and sodium sulphate were provided by Chem-Supply Pty., Ltd., Australia. All the chemicals were used as received without further treatment. P25-reduced graphene oxide (P25@RGO) was fabricated using a hydrothermal reaction as reported [4]. Briefly, GO suspension was prepared through Hummer’s method [13] by reacting commercially obtained graphite powder in a mixture of H2SO4, NaNO3 and KMnO4. After the reaction, H2O2 was then added into the mixture. Subsequently, the mixture was filtered and washed with deionized water. The as-prepared black solid GO was then dried at 40 in the vacuum oven. 10 mg GO was then dispersed a mixture of water (40 mL) and ethanol (25 mL) by sonication for 30 min to obtain GO suspension. P25 colloid suspension (0.01 g/mL) in ethanol was prepared by ultra-sonication. Then 5 mL of P25 ethanol suspension was added into the GO suspension. After stirring for 2 h, the mixing solution was transferred into a 100 mL of Teflon-sealed autoclave and maintained at 120 C for 2 h. The resulting composite was recovered by centrifugation, rinsed with DI water for several times, and fully dried in vacuum at 60 C for 12 h.
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visible (UV–vis) spectrophotometer. Simulated sunlight tests were carried out using an Atlas Suntest CPS1 instrument equipped with 150 W Xenon lamp and a filter (coated quartz dish). The flux intensity was 300 W/m2. The PFG-NMR diffusion experiments were carried out on a Bruker Advance III 500 MHz wide bore spectrometer (with proton Larmor frequency of 500.07 MHz) equipped with a 5 mm Diff50 pulse-field gradient (PFG) probe. 1H NMR signal was used for the determination of diffusion coefficients of different spices. The concentration of RhB is 6 g/L in surfactant aqueous solution. The concentrations of surfactants were chosen to be far above their critical micelle concentrations (4 CMC). All the experiments were performed at room temperature (20 C). The pulse-field gradient stimulated echo (PFG-STE) pulse sequence was used to obtain diffusion coefficients. The maximum gradient strength of the amplifier is 29.454 T/m.
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Photocatalytic activity test
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Six different dyes, including sulphonic dyes (acid red, lanasol yellow and procion red), azoic dyes (methyl orange, terasil blue) and fluorescent dyes (rhodamine B), were selected to investigate the photocatalytic activity of P25@RGO composite. Their characteristics are listed in Table 2. The degradation process was monitored by the change of absorbance at the maximum absorbance wavelength. The experiments were carried out according to the following procedure: dried powder of P25@RGO was dispersed in 50 mL of dye aqueous solution (12 ppm) in a 100 mL beaker (for all photodegradation experiments, the net amount of P25 was maintained to be 1 mg/mL). The suspension was stirred in the dark for 1 h to ensure the establishment of adsorption and desorption equilibrium of dye molecules on the surface of the above catalysts. Subsequently the suspension liquor was irradiated with simulated sunlight using an Atlas Suntest CPS1 instrument. At given intervals, 3 mL of the suspension was extracted and then centrifuged at 4226 rcf for 10 min to separate the catalysts from the supernatant. UV–vis absorbance spectra of the supernatant were measured with a Varian Cary 3E spectro photometer. The degradation ratio was estimated by the following formula:
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removalð%Þ ¼ Characterization X-ray diffraction (XRD) data were obtained on a PANalytical’s X’Pert Power X-ray diffraction (40 kV, 30 mA) with Cu-Ka radiation at a scanning rate of 2.4 min1. The morphologies and structures of the samples were investigated by transmission electron microscopy (TEM) on a JEOL-2100 with an acceleration voltage of 200 kV. Thermal gravity analysis (TGA) data were collected on a thermogravimetric analyser (Netzsch Inc., Germany) in air with a temperature ramp rate of 10 min/ C. The optical absorption spectra of the samples were obtained using a Varian Cary 3E ultraviolet
3
97 98 99 100 101 102 103 104 105 106 107 108 109 110
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A0 A 100% A0
where A0 represents original absorbance of dye solution at the maximum absorption wavelength (lmax), A is residual absorbance of dye at lmax after UV light irradiation.
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Results and discussion
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Characterization of P25@RGO composite
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A typical morphological and structure of the prepared P25@RGO are shown in Fig. 1. The TEM results (Fig. 1A) show
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Fig. 1. (A) Typical TEM image of P25@RGO. (B) TGA curves of P25@RGO.
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Fig. 2. Time-dependent absorption spectra of RhB solution under simulated sunlight irradiation in the presence of (A) P25 and (B) P25@RGO. (C) Photocatalytic activity of P25 and P25@RGO, where C and C0 are the actual and original concentration of RhB solution.
Fig. 3. (A) Removal efficiency of different dyes as a function of the irradiation time. (B) Photocatalytic activity of P25@RGO as a function of the irradiation time, where C and C0 are the actual and initial concentration of dye solution.
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that obtained composite retained a two-dimensional sheet structure. P25 nanoparticles dispersed densely and evenly on the graphene carbon support. As shown in Fig. 1B, the weight loss of the P25@RGO was 2% at 120 C and 6.5% between 120 and 450 C. The remaining weight was assigned to P25. The weight loss at 120 C is assigned to the loss of absorbed water. Hence the weight ratio between GO and P25 in the sample was 6.5%:91.7%.
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Photocatalytic activity of P25@RGO composite
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Rhodamine B (RhB) molecule was selected as the model molecule to investigate photocatalytic activity of P25@RGO composite under simulated sunlight irradiation. The absorption peaks corresponding to the dye RhB disappeared under simulated sunlight irradiation, indicating degradation of the dye [1,14]. Fig. 2 shows the absorption spectra of RhB solution in the presence of catalyst P25 and P25@RGO under simulated sunlight irradiation. The degradation of RhB is negligible without the catalyst. With the presence of P25 (Fig. 2A), 76.5% degradation was observed after 45 min irradiation, which was compared with 98.3% degradation for P25@RGO (Fig. 2B). Fig. 2C shows the relative change of adsorption peak as a function of the irradiation time. The dye solution after being mixed with catalyst was stirred for 1 h in the dark to achieve the adsorption and desorption equilibration. Then the concentration of the dye was measured and used as the original concentration C0. Fig. 2C shows the relative change in concentration as a function of the irradiation time, where C0 and C are the
initial and actual concentration of RhB solution at different reaction times, respectively. The ln(C0/C) plots show a linear relationship with the irradiation time, which suggests that photodegradation of the dye solution went through a pseudofirst-order kinetic reaction [15]. To investigate the photocatalytic activity of the P25@RGO, six different dyes were selected in this study, including sulphonic dyes (AR, LY and PR), azoic dyes (MO, TB), fluorescent dyes (RhB) as shown in Table 2. The degradation process was monitored by the change of absorbance at the maximum absorbance wavelength. For the three sulphonic dyes (AR, LY and PR), the removal of AR in the presence of P25@RGO is the highest, which was 96% removal after irradiation for 30 min. This was compared with 89% for LY and 78% for PR. It is suggested that the chemical structure of dye molecules and the substitutes of aromatic nuclei of the dyes have significant influence on the reactivity of dyes, thereby resulting in difference in the removal efficiency [16]. Fig. 3 indicates that the P25@RGO had high photocatalytic activity on different dyes. As shown in FiG. 3B, the ln(C0/C) plots show a linear relationship with the irradiation time, which reveals that photodegradation of dye solution went through a pseudo-first-order kinetic reaction [15]. The removal rate was described as:
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C ¼ ekt C0 where C0 and C are the initial and actual concentration of RhB solution at different reaction times, respectively. k (min1) denotes
Table 3 The parameter of the photocatalytic activity of P25@RGO for degradation of organic dyes. Dye
Acid red 88
Lanasol yellow CE 4G
Procion red MX-5B
Rhodamine B
Terasil blue 3RL-02
Methyl orange
K (min1)
0.103
0.083
0.053
0.103
0.035
0.028
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Fig. 4. Removal efficiency of RhB at different concentration of inorganic salts: (A) NaCl (B) Na2SO4. 188
the pseudo-fist-order rate constant, t is the reaction time. The reaction constant rates for all the six dyes are listed in Table 3.
ions could lose an electron to P25 valence band holes or to a hydroxyl radical and become sulphate radicals as shown below:
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SO42 + h+ ! SO4
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Effect of salt on the photocatalytic activity of P25@RGO
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The photodegradation of dye RhB was carried out in the presence of two common used salts, NaCl and Na2SO4. The concentration of salt used for dyeing 100% cellulose is between 10 and 25 g/L. Therefore, the salt concentration range was selected from 0 to 20 g/L in this study, which is comparable to actual textile wastewater. As shown in Fig. 4, NaCl shows a detrimental effect on RhB degradation. The degradation rates decreased with increasing amounts of NaCl in the experimental range. For example, without NaCl, 88% of RhB was degraded after 20 min irradiation, compared with only 46% degradation at [NaCl] = 5 g/L, 39% degradation at [NaCl] = 10 g/L and 36% degradation at [NaCl] = 20 g/L. The inhabitation of NaCl on the photocatalytic activity of P25@RGO can be explained with the scavenging of OH radicals by ions. The reactions of chloride with OH are given below [17]:
SO42 + OH ! SO4 + OH
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However, the ability of the sulphate radicals to oxidize organics is limited to the inhibitory effect of SO42 [19].
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Effect of surfactant on the photocatalytic activity of P25@RGO
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In order to study the effect of surfactant on the RhB degradation, RhB degradation was carried out in the presence of three surfactants as listed in Table 4. These three surfactants are anionic surfactant SDS, cationic surfactant CTAB, and non-ionic surfactant Terjitol. For dyeing 100% cotton, the concentration of surfactant in actual textile waste water is 0.1–3 g/L (Ciba Specialty Chemicals Inc.). RhB degradation process was investigated at surfactant concentrations both below and above the critical micelle concentrations (CMC). Fig. 5A shows that anionic surfactant SDS had a significant suppressing effect on the removal of dye RhB. The higher the concentration of SDS was, the lower was the removal efficiency of RhB. For example, without the SDS, 99% of dye was removed after irradiation for 60 min, compared with only 31% removal in the presence of 1.24 g/L of SDS. When the concentration of SDS increased to 4.96 g/L, only 7% dye removal was reached. With the presence of non-ionic surfactant Tergitol, the dye removal efficiency decreased with the concentration increase of the surfactant. The removal efficiency declined from 99.2% with no surfactant Tergitol to 54.6% with 104 mg/L of surfactant Tergitol
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192 193 194 195 196 197 198 199 200 201 202 203 204 205
+
Cl + h ! Cl
206
208 207
The inhibition effect may also be caused by the competitive adsorption of Cl and the dye. The anions could be adsorbed on the surface of catalyst, thereby blocking the active sites [18]. The effects of sulphate in the same concentration range are shown in Fig. 4B. The addition of the sulphate had no significant impact on the photodegradation rate of RhB in the experimental range. According to Burns et al., report, surface-bound sulphate
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OH + Cl ! Cl + OH
Table 4 The characteristics of the three surfactants. Surfactant
Formula
CMC (g/L)
Sodium dodecyl sulphate (SDS)
Chemical structure
NaC12H25SO4
2.3
Cetyltrimethylammonium bromide (CTAB)
C19H42BrN
0.36 [20]
Tergitol np-9
C55H60O10
0.06a
Note: CMC represents critical micelle concentrations. a Technical data sheet from Dow Chemical Company.
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Fig. 5. Removal efficiency of RhB at different concentration of surfactant: (A) SDS and (B) Tergitol and (C) CTAB. 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271
after 60 min irradiation. Similar results were found in in the presence of CTAB. The higher the concentration of CTAB, the lower was the removal efficiency of dye RhB. When CTAB concentration reached 728 mg/L, the removal efficiency declined from 99.2% to 35.1% after irradiation for 60 min. The suppressing effect on dye RhB removal by surfactants may be attributed to the interactions between surfactants and RGO sheets. It has been investigated extensively that surfactants are adsorbed by RGO due to the parallel orientation of alkyl chains of surfactant to the basal plane of graphite, eventually forming a monolayer[21–23]. The adsorption of surfactant molecules on graphitic carbons hinders the interaction between dye molecules and the catalyst P25@RGO, thus suppressing the dye removal efficiency of P25@RGO. On the other hand, dye molecules can competitively interact with both surfactants and the catalyst. The formation of dye– surfactant aggregates could hinder dye RhB to react with catalyst, resulting in a reduction of removal efficiency of dye RhB. Herein, the interactions between dye RhB and surfactants were further investigated. The interactions between negatively charged surfactant SDS and positively charged dye RhB lead to the formation of a complex, which is confirmed by the shift of the UV–vis spectrum ( see Supporting information Fig. S1). With the ammonium groups of RhB interacting with the sulphonate in the head group of SDS, this complex gets incorporated into the micelles [24], in which RhB may be inert to react with P25@RGO, leading to a significant reduction of removal efficiency of dye RhB. The interactions between the dye RhB molecules with nonionic surfactant Tergitol, cationic surfactant CTAB and anionic surfactant SDS were investigated by the 1H NMR spectra, which are
shown in Fig. 6. The peaks in the region of 8.8–5.5 ppm are attributed to the aromatic protons of the dye RhB. The addition of surfactants led to significant chemical shifts of the aromatic resonance lines of dye RhB, which confirmed the molecular interactions between the dye RhB and surfactants Tergitol, CTAB and SDS. It can also be seen that the dye RhB shows different peak positions in the presence of Tergitol and CTAB, which may suggest that these two surfactants interact on different sites of dye RhB molecules. The percentage of dye RhB existing as free dye and dye– surfactant aggregates was further investigated by PFG NMR experiments. The peak 1 in Fig. 6 was used to estimate their diffusion coefficients. As shown in Table 5, the diffusion coefficients of dye RhB decreased in the presence of both surfactants Tergitol and CTAB due to the formation of dye– surfactant aggregates [25]. The diffusion coefficient of dye RhB was 2.61 1010 m2/s in D2O solution. After the addition of the nonionic surfactant Tergitol, the coefficient of dye RhB decreased to 0.25 1010 m2/s, while diffusion coefficient of dye molecules was 1.73 1010 m2/s after the addition of the cationic surfactant CTAB. Because the free dye and dye–surfactant aggregates are in dynamic equilibrium, if the exchange rate is fast, then only one diffusion coefficient is observed, and/or if the exchange rate is slow, two separate diffusion coefficients should be observed. Only one diffusion coefficient was observed in this experiment, which indicates that there is fast exchange between free dye RhB and dye –surfactant aggregates. Therefore, the mean values of the selfdiffusion coefficients of free dye and dye–surfactant aggregates can be expressed as the following equation [26]:
273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
Dobserve ¼ ð1 pagg ÞDmon þ pagg Dagg where Dobserve is the observed diffusion coefficient of the dye RhB molecules, Dmon and Dagg are the free dye and dye–surfactant aggregates diffusion coefficients, respectively. pagg indicates the dye–surfactant aggregates’ contribution to the observed diffusion coefficient. According to the above equation, pagg was calculated to be 98 mol% and 44 mol% for dye/Tergitol and dye/CTAB, respectively. This suggested that 98 mol% of the dye molecules were associated with the non-ionic surfactant Tergitol and 44 mol% for the cationic surfactant CTAB in the experimental conditions. With the presence Table 5 Diffusion coefficients (D 1010 m2/s) of D2O, surfactant and dye obtained from PFG NMR experiments at room temperature (20 C).
Fig. 6. 1H NMR of dye RhB and mixture of dye RhB and non-ionic surfactant Tergitol, cationic surfactant CTAB and anionic surfactant SDS in D2O.
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Dye + D2O Tergitol + D2O Dye + Tergitol + D2O CTAB + D2O Dye + CTAB + D2O Dye + SDS + H2O
D2O
Surfactant
15.6 14.6 16.0 15.0 15.7 16.1
0.19 0.20 0.61 0.61 0.59
Dye 2.61 0.25 1.73 0.51
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Dye + Tergitol Dye + CTAB
Dobserve
Dmon
Dagga
0.25 1.73
2.61 2.61
0.20 0.61
a
Dagg is the diffusion coefficient of dye–surfactant aggregates, which is assumed to be identical to the diffusion coefficient of the surfactant molecules. 312
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of anionic surfactant SDS, coefficient of dye changed from 2.61 1010 m2/s as monomer to be 0.51 1010 m2/s in the presence of surfactant SDS. The coefficient of dye–surfactant (dye/SDS) aggregates became very close to that of surfactant 0.59 1010 m2/s, which suggests that almost 100% of dye molecules form dye–surfactant aggregates. Compared to nonionic surfactant aqueous solution, there is a lower percentage of dye–surfactant aggregates formed in cationic surfactant CTAB aqueous solution. This is because the positively charged ammonia head of dye RhB gives rise to dye–CTAB repulsion [27]. The formation of the dye–surfactant (dye/CTAB) aggregates displays a balance between electrostatic and hydrophobic competitive force [28]. This result further indicates a large percentage of dye RhB formed dye–surfactant aggregates in the surfactant aqueous solution, which may hinder dye RhB to react with catalyst, resulting in a significant decrease of the reduction rate of dye (Table 6).
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Conclusion
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Inorganic ions and organic additives on the photocatalytic performance of P25@RGO composite were investigated. The results show that Cl has an inhibition effect on the degradation of RhB. All three types of surfactants exhibit significant suppressing effect on degradation of RhB by P25@RGO, including anionic surfactant SDS, cationic surfactant CTAB, and non-ionic surfactant Tergitol. The interactions between surfactants and dye RhB were confirmed by NMR, which may be the key factors for the inhibition effect of surfactants on the reduced dye removal efficiency of P25@RGO. This work reveals the photocatalytic performance of P 25–graphene in the presence of inorganic and organic additives, which will be valuable for future application of graphene-based photocatalysts in treatment of industrial dyehouse effluents.
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Acknowledgement Q7
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ARC is acknowledged for funding Deakin University’s Magnetic Resonance Facility through LIEF grant LE110100141.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jece.2015.05.008.
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Wang, A magnetically separable P25/ CoFe2O4/graphene catalyst with enhanced adsorption capacity and visiblelight-driven photocatalytic activity, RSC Adv. 3 (44) (2013) 22490–22497, doi: http://dx.doi.org/10.1039/c3ra43416h. [9] F. Wang, K. Zhang, Reduced graphene oxide–TiO2 nanocomposite with high photocatalystic activity for the degradation of rhodamine B, J. Mol. Catal. A Chem. 345 (1–2) (2011) 101–107, doi:http://dx.doi.org/10.1016/j.molcata.2011.05.026. [10] L. Ren, X. Qi, Y. Liu, Z. Huang, X. Wei, J. Li, L. Yang, J. Zhong, Upconversion-P25– graphene composite as an advanced sunlight driven photocatalytic hybrid material, J. Mater. Chem. 22 (23) (2012) 11765–11771, doi:http://dx.doi.org/ 10.1039/c2jm30457k. [11] S. Liu, H. Sun, S. Liu, S. Wang, Graphene facilitated visible light photodegradation of methylene blue over titanium dioxide photocatalysts, Chem. Eng. J. 214 (2013) 298–303, doi:http://dx.doi.org/10.1016/j.cej.2012.10.058. [12] P. Wang, J. Wang, X. Wang, H. Yu, J. Yu, M. Lei, Y. Wang, One-step synthesis of easy-recycling TiO2–rGO nanocomposite photocatalysts with enhanced photocatalytic activity, Appl. Catal. B: Environ. 132–133 (2013) 452–459, doi: http://dx.doi.org/10.1016/j.apcatb.2012.12.009. [13] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (6) (1958) 1339, doi:http://dx.doi.org/10.1021/ja01539a017. [14] G. Liu, J. Zhao, Photocatalytic degradation of dye sulforhodamine B: a comparative study of photocatalysis with photosensitization, New J. Chem. 24 (6) (2000) 411–417, doi:http://dx.doi.org/10.1039/b001573n. [15] Q. Wan, T.H. Wang, J.C. Zhao, Enhanced photocatalytic activity of ZnO nanotetrapods, Appl. Phys. Lett. 87 (8) (2005) 083105, doi:http://dx.doi.org/ 10.1063/1.2034092. [16] P. Niu, J. Hao, Efficient degradation of organic dyes by titanium dioxide– silicotungstic acid nanocomposite films: influence of inorganic salts and surfactants, Colloids Surf. A: Physicochem. Eng. 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Chiou, Water solubility enhancements of DDT and trichlorobenzene by some surfactants below and above the critical micelle concentration, Environ. Sci. Technol. 23 (7) (1989) 832–838, doi:http://dx.doi. org/10.1021/es00065a012. [21] M. Sammalkorpi, A.Z. Panagiotopoulos, M. Haataja, Surfactant and hydrocarbon aggregates on defective graphite surface: structure and dynamics, J. Phys. Chem. B 112 (41) (2008) 12954–12961, doi:http://dx.doi.org/10.1021/ jp8043835. 18811186. [22] Z. Király, G.H. Findenegg, Á. Mastalir, Chain-length anomaly in the two-dimensional ordering of the cationic surfactants CTAB at the graphite/water interface, revealed by advanced calorimetric methods, J. Phys. Chem. B 107 (45) (2003) 12492–12496, doi:http://dx.doi.org/10.1021/jp035466t. [23] Z. Király, G.H. Findenegg, E. Klumpp, H. Schlimper, I. 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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants
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Jinfeng Wang a , Haijin Zhu b , Christopher Hurren a,c , Jian Zhao a,d, Esfandiar Pakdel a , Zhenyu Li a , Xungai Wang a,c, * a
Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, Victoria 3220, Australia ARC Centre of Excellence for Electromaterials Science (ACES), Institute for Frontier Materials (IFM), Deakin University, Burwood, Victoria 3125, Australia School of Textile Science and Engineering, Wuhan Textile University, Wuhan 430073, China d Key Laboratory of Advanced Textile Composites, Ministry of Education of China, Tianjin Polytechnic University, Tianjin 300387, China b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 December 2014 Accepted 12 May 2015
Treatment of coloured effluent treatment is a major issue for the textile industry. In this study, catalyst P 25–graphene was prepared and applied for degrading dye from an aqueous solution. Three types of dyes were selected to determine the feasibility of the catalyst for the dye degradation, including sulphonic, azoic, and fluorescent dyes. P25–graphene catalyst showed good ability to degrade all selected dyes. The influence of inorganic salts and surfactants on the photocatalytic degradation of rhodamine B using catalyst P25–graphene was also investigated. The degradation of rhodamine B was suppressed by the presence of NaCl, but the effect of Na2SO4 was negligible. The degradation of rhodamine B was significantly suppressed by all three types of surfactant, namely anionic, cationic and non-ionic surfactants. NMR technique was used to investigate the mechanisms associated with this suppression. ã 2015 Published by Elsevier Ltd.
Keywords: Graphene Photocatalyst Dye removal Surfactant Salt
9
Introduction
10
Large quantities of highly coloured effluent are produced in the textile industry. It was reported that of the 700,000 tons of dyes produced annually worldwide, about 10–15% of the dyes are disposed off in effluent through incomplete exhaustion and washing process [1]. Their release causes serious damage to the aquatic environment due to high toxicity, low biodegradation and potential carcinogenicity [2]. Physical and traditional biological treatments are not sufficient to remove textile dyes with strong colour [3]. Semiconductor photocatalyst has emerged as one of the most promising materials for environmental remediation. It represents an easy way to utilize the energy of either natural sunlight or indoor illumination light. The mechanism of these functions is based on the in-situ generated highly reactive oxygen species (OH, O2) for mineralization of organic compounds. Photocatalysts, such as TiO2, ZnO, CdS, WO3, SnO2 and ZnS, have demonstrated their efficiency in degrading a wide range of
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Q3
* Corresponding author at: Australian Future Fibres Research and Innovation Centre, Institute for Frontier Materials, Deakin University, Victoria 3220, Australia. Tel.: +61 3 52272894; fax: +61 3 52272539. E-mail address: [email protected] (X. Wang).
organics into readily biodegradable compounds, and eventually mineralizing them to carbon dioxide and water. Among the catalysts developed, TiO2 has been extensively studied. However, the application of TiO2 is limited by the fast recombination of the electron and photogenerated electron–hole pairs. The addition of graphene toTiO2 prevents the recombination of the electron–hole pairs in the TiO2@graphene composites. The TiO 2@graphene composite has a number of attributes, such as increased adsorptivity of pollutants, extended light absorption range, and facile charge transportation and separation, which were rarely reported for other composite carbon composites [4]. Enhanced photocatalytic activity of TiO2 dispersed on graphene has been reported by Zhang et al. [4]. This high performance photocatalysis is anticipated to open new possibilities in the application of TiO2@graphene in environment remediation (Table 1). The actual dye effluent contains not only dyes, but also other auxiliaries such as salts and surfactants. High concentrations of various inorganic salts such as NaCl and Na2SO4 are frequently used in the dyeing process [5]. Surfactants are also widely used as wetting, penetrating, dispersing, and levelling agents in dyeing processes. Hence, large amounts of inorganic salts and surfactants are discharged with dyes. However, a large number of the published papers on the treatment of textile effluent have focused on the principle colour removal, neglecting the influence of
http://dx.doi.org/10.1016/j.jece.2015.05.008 2213-3437/ ã 2015 Published by Elsevier Ltd.
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Table 1 Reported work of applying TiO2@graphene composite for dye removal. Composites
Synthesis procedure
Pollutant
Light Sources
Ref.
P25@graphene P25@graphene oxide
Hydrothermal in water Mixing and sonication Hydrothermal in water and ethanol
UV light (365 nm) Mercury vapour lamp delivering near-UV/vis irradiation (>350 nm) Xe lamp with UV cutoff filter (>420 nm)
[6] [7]
P25/CoFe2O4/graphene
Methylene blue and reactive black Diphenhydramine pharmaceutical and methyl orange Methylene blue, methyl orange and neutral dark yellow Rhodamine B
UV light (365 nm) and LED lamp (>420 nm)
[9]
Methyl orange
High-pressure Xenon Lamp to simulate sunlight [10]
Methylene blue
UV light (220–280 nm) and visible light (400– 1050 nm) UV light (310–400 nm)
P25@reduced graphene oxide Hydrothermal and annealed at 400 under argon P25@graphene (loading of Hydrothermal and annealed at 500 YF3:Yb3+,Tm3+) under argon Reduced by hydrazine hydrate then P25@graphene annealed at 500 P25@reduced graphene oxide Hydrothermal in water at 500
Phenol
[8]
[11] [12]
Table 2 The characteristics of six organic dyes. Dye
Formula
Mw (g/Mol)
Rhodamine B (RhB)
Chemical structure
C28H31N2O3Cl
479
Methyl orange (MO)
C14H14N3O3SNa
327
Procion Red (PR) MX-5B
C19H10N6O7S2Cl2Na2
615
Terasil blue (TB) 3RL-02
C14H9BrN2O4
349
Lanasol yellow (LY) CE 4G
C19H12BrN5Na2O8S2
699
Acid red (AR) 88
C20H13NaN2O4S
400
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surfactants and salts. In this study, three types of dye were selected to determine the feasibility of the catalyst TiO2@graphene for dye degradation. We further investigated the influence of inorganic salts and surfactant on dye degradation by a TiO2@graphene composite, with rhodamine B as a model molecule. To our best knowledge, this is the first report on the influence of the inorganic salts and surfactants on the photocatalytic performance of TiO 2@graphene nanocomposites. Experimental
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Materials
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Graphite powder (99.95% purity, 325 mesh) was purchased from Sigma–Aldrich. TiO2 (P25, 20% rutile and 80% anatase) was purchased from Degussa. Rhodamine B and methyl orange were purchased from Sigma–Aldrich. Terasil blue, procion red and acid red were obtained from CIBA. The chemical structures, formula and molecular weights of organic dyes used in this study are listed in Table 2. Sodium chloride and sodium sulphate were provided by Chem-Supply Pty., Ltd., Australia. All the chemicals were used as received without further treatment. P25-reduced graphene oxide (P25@RGO) was fabricated using a hydrothermal reaction as reported [4]. Briefly, GO suspension was prepared through Hummer’s method [13] by reacting commercially obtained graphite powder in a mixture of H2SO4, NaNO3 and KMnO4. After the reaction, H2O2 was then added into the mixture. Subsequently, the mixture was filtered and washed with deionized water. The as-prepared black solid GO was then dried at 40 in the vacuum oven. 10 mg GO was then dispersed a mixture of water (40 mL) and ethanol (25 mL) by sonication for 30 min to obtain GO suspension. P25 colloid suspension (0.01 g/mL) in ethanol was prepared by ultra-sonication. Then 5 mL of P25 ethanol suspension was added into the GO suspension. After stirring for 2 h, the mixing solution was transferred into a 100 mL of Teflon-sealed autoclave and maintained at 120 C for 2 h. The resulting composite was recovered by centrifugation, rinsed with DI water for several times, and fully dried in vacuum at 60 C for 12 h.
61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
visible (UV–vis) spectrophotometer. Simulated sunlight tests were carried out using an Atlas Suntest CPS1 instrument equipped with 150 W Xenon lamp and a filter (coated quartz dish). The flux intensity was 300 W/m2. The PFG-NMR diffusion experiments were carried out on a Bruker Advance III 500 MHz wide bore spectrometer (with proton Larmor frequency of 500.07 MHz) equipped with a 5 mm Diff50 pulse-field gradient (PFG) probe. 1H NMR signal was used for the determination of diffusion coefficients of different spices. The concentration of RhB is 6 g/L in surfactant aqueous solution. The concentrations of surfactants were chosen to be far above their critical micelle concentrations (4 CMC). All the experiments were performed at room temperature (20 C). The pulse-field gradient stimulated echo (PFG-STE) pulse sequence was used to obtain diffusion coefficients. The maximum gradient strength of the amplifier is 29.454 T/m.
96
Photocatalytic activity test
111
Six different dyes, including sulphonic dyes (acid red, lanasol yellow and procion red), azoic dyes (methyl orange, terasil blue) and fluorescent dyes (rhodamine B), were selected to investigate the photocatalytic activity of P25@RGO composite. Their characteristics are listed in Table 2. The degradation process was monitored by the change of absorbance at the maximum absorbance wavelength. The experiments were carried out according to the following procedure: dried powder of P25@RGO was dispersed in 50 mL of dye aqueous solution (12 ppm) in a 100 mL beaker (for all photodegradation experiments, the net amount of P25 was maintained to be 1 mg/mL). The suspension was stirred in the dark for 1 h to ensure the establishment of adsorption and desorption equilibrium of dye molecules on the surface of the above catalysts. Subsequently the suspension liquor was irradiated with simulated sunlight using an Atlas Suntest CPS1 instrument. At given intervals, 3 mL of the suspension was extracted and then centrifuged at 4226 rcf for 10 min to separate the catalysts from the supernatant. UV–vis absorbance spectra of the supernatant were measured with a Varian Cary 3E spectro photometer. The degradation ratio was estimated by the following formula:
112
removalð%Þ ¼ Characterization X-ray diffraction (XRD) data were obtained on a PANalytical’s X’Pert Power X-ray diffraction (40 kV, 30 mA) with Cu-Ka radiation at a scanning rate of 2.4 min1. The morphologies and structures of the samples were investigated by transmission electron microscopy (TEM) on a JEOL-2100 with an acceleration voltage of 200 kV. Thermal gravity analysis (TGA) data were collected on a thermogravimetric analyser (Netzsch Inc., Germany) in air with a temperature ramp rate of 10 min/ C. The optical absorption spectra of the samples were obtained using a Varian Cary 3E ultraviolet
3
97 98 99 100 101 102 103 104 105 106 107 108 109 110
113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131
A0 A 100% A0
where A0 represents original absorbance of dye solution at the maximum absorption wavelength (lmax), A is residual absorbance of dye at lmax after UV light irradiation.
133 132 134
Results and discussion
136
Characterization of P25@RGO composite
137
A typical morphological and structure of the prepared P25@RGO are shown in Fig. 1. The TEM results (Fig. 1A) show
138
Fig. 1. (A) Typical TEM image of P25@RGO. (B) TGA curves of P25@RGO.
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Fig. 2. Time-dependent absorption spectra of RhB solution under simulated sunlight irradiation in the presence of (A) P25 and (B) P25@RGO. (C) Photocatalytic activity of P25 and P25@RGO, where C and C0 are the actual and original concentration of RhB solution.
Fig. 3. (A) Removal efficiency of different dyes as a function of the irradiation time. (B) Photocatalytic activity of P25@RGO as a function of the irradiation time, where C and C0 are the actual and initial concentration of dye solution.
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that obtained composite retained a two-dimensional sheet structure. P25 nanoparticles dispersed densely and evenly on the graphene carbon support. As shown in Fig. 1B, the weight loss of the P25@RGO was 2% at 120 C and 6.5% between 120 and 450 C. The remaining weight was assigned to P25. The weight loss at 120 C is assigned to the loss of absorbed water. Hence the weight ratio between GO and P25 in the sample was 6.5%:91.7%.
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Photocatalytic activity of P25@RGO composite
141 142 143 144 145
148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164
Rhodamine B (RhB) molecule was selected as the model molecule to investigate photocatalytic activity of P25@RGO composite under simulated sunlight irradiation. The absorption peaks corresponding to the dye RhB disappeared under simulated sunlight irradiation, indicating degradation of the dye [1,14]. Fig. 2 shows the absorption spectra of RhB solution in the presence of catalyst P25 and P25@RGO under simulated sunlight irradiation. The degradation of RhB is negligible without the catalyst. With the presence of P25 (Fig. 2A), 76.5% degradation was observed after 45 min irradiation, which was compared with 98.3% degradation for P25@RGO (Fig. 2B). Fig. 2C shows the relative change of adsorption peak as a function of the irradiation time. The dye solution after being mixed with catalyst was stirred for 1 h in the dark to achieve the adsorption and desorption equilibration. Then the concentration of the dye was measured and used as the original concentration C0. Fig. 2C shows the relative change in concentration as a function of the irradiation time, where C0 and C are the
initial and actual concentration of RhB solution at different reaction times, respectively. The ln(C0/C) plots show a linear relationship with the irradiation time, which suggests that photodegradation of the dye solution went through a pseudofirst-order kinetic reaction [15]. To investigate the photocatalytic activity of the P25@RGO, six different dyes were selected in this study, including sulphonic dyes (AR, LY and PR), azoic dyes (MO, TB), fluorescent dyes (RhB) as shown in Table 2. The degradation process was monitored by the change of absorbance at the maximum absorbance wavelength. For the three sulphonic dyes (AR, LY and PR), the removal of AR in the presence of P25@RGO is the highest, which was 96% removal after irradiation for 30 min. This was compared with 89% for LY and 78% for PR. It is suggested that the chemical structure of dye molecules and the substitutes of aromatic nuclei of the dyes have significant influence on the reactivity of dyes, thereby resulting in difference in the removal efficiency [16]. Fig. 3 indicates that the P25@RGO had high photocatalytic activity on different dyes. As shown in FiG. 3B, the ln(C0/C) plots show a linear relationship with the irradiation time, which reveals that photodegradation of dye solution went through a pseudo-first-order kinetic reaction [15]. The removal rate was described as:
165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186
C ¼ ekt C0 where C0 and C are the initial and actual concentration of RhB solution at different reaction times, respectively. k (min1) denotes
Table 3 The parameter of the photocatalytic activity of P25@RGO for degradation of organic dyes. Dye
Acid red 88
Lanasol yellow CE 4G
Procion red MX-5B
Rhodamine B
Terasil blue 3RL-02
Methyl orange
K (min1)
0.103
0.083
0.053
0.103
0.035
0.028
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Fig. 4. Removal efficiency of RhB at different concentration of inorganic salts: (A) NaCl (B) Na2SO4. 188
the pseudo-fist-order rate constant, t is the reaction time. The reaction constant rates for all the six dyes are listed in Table 3.
ions could lose an electron to P25 valence band holes or to a hydroxyl radical and become sulphate radicals as shown below:
215
189 190
SO42 + h+ ! SO4
217
Effect of salt on the photocatalytic activity of P25@RGO
191
The photodegradation of dye RhB was carried out in the presence of two common used salts, NaCl and Na2SO4. The concentration of salt used for dyeing 100% cellulose is between 10 and 25 g/L. Therefore, the salt concentration range was selected from 0 to 20 g/L in this study, which is comparable to actual textile wastewater. As shown in Fig. 4, NaCl shows a detrimental effect on RhB degradation. The degradation rates decreased with increasing amounts of NaCl in the experimental range. For example, without NaCl, 88% of RhB was degraded after 20 min irradiation, compared with only 46% degradation at [NaCl] = 5 g/L, 39% degradation at [NaCl] = 10 g/L and 36% degradation at [NaCl] = 20 g/L. The inhabitation of NaCl on the photocatalytic activity of P25@RGO can be explained with the scavenging of OH radicals by ions. The reactions of chloride with OH are given below [17]:
SO42 + OH ! SO4 + OH
218
However, the ability of the sulphate radicals to oxidize organics is limited to the inhibitory effect of SO42 [19].
220 219
Effect of surfactant on the photocatalytic activity of P25@RGO
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In order to study the effect of surfactant on the RhB degradation, RhB degradation was carried out in the presence of three surfactants as listed in Table 4. These three surfactants are anionic surfactant SDS, cationic surfactant CTAB, and non-ionic surfactant Terjitol. For dyeing 100% cotton, the concentration of surfactant in actual textile waste water is 0.1–3 g/L (Ciba Specialty Chemicals Inc.). RhB degradation process was investigated at surfactant concentrations both below and above the critical micelle concentrations (CMC). Fig. 5A shows that anionic surfactant SDS had a significant suppressing effect on the removal of dye RhB. The higher the concentration of SDS was, the lower was the removal efficiency of RhB. For example, without the SDS, 99% of dye was removed after irradiation for 60 min, compared with only 31% removal in the presence of 1.24 g/L of SDS. When the concentration of SDS increased to 4.96 g/L, only 7% dye removal was reached. With the presence of non-ionic surfactant Tergitol, the dye removal efficiency decreased with the concentration increase of the surfactant. The removal efficiency declined from 99.2% with no surfactant Tergitol to 54.6% with 104 mg/L of surfactant Tergitol
223
192 193 194 195 196 197 198 199 200 201 202 203 204 205
+
Cl + h ! Cl
206
208 207
The inhibition effect may also be caused by the competitive adsorption of Cl and the dye. The anions could be adsorbed on the surface of catalyst, thereby blocking the active sites [18]. The effects of sulphate in the same concentration range are shown in Fig. 4B. The addition of the sulphate had no significant impact on the photodegradation rate of RhB in the experimental range. According to Burns et al., report, surface-bound sulphate
209 210 211 212 213 214
OH + Cl ! Cl + OH
Table 4 The characteristics of the three surfactants. Surfactant
Formula
CMC (g/L)
Sodium dodecyl sulphate (SDS)
Chemical structure
NaC12H25SO4
2.3
Cetyltrimethylammonium bromide (CTAB)
C19H42BrN
0.36 [20]
Tergitol np-9
C55H60O10
0.06a
Note: CMC represents critical micelle concentrations. a Technical data sheet from Dow Chemical Company.
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Fig. 5. Removal efficiency of RhB at different concentration of surfactant: (A) SDS and (B) Tergitol and (C) CTAB. 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271
after 60 min irradiation. Similar results were found in in the presence of CTAB. The higher the concentration of CTAB, the lower was the removal efficiency of dye RhB. When CTAB concentration reached 728 mg/L, the removal efficiency declined from 99.2% to 35.1% after irradiation for 60 min. The suppressing effect on dye RhB removal by surfactants may be attributed to the interactions between surfactants and RGO sheets. It has been investigated extensively that surfactants are adsorbed by RGO due to the parallel orientation of alkyl chains of surfactant to the basal plane of graphite, eventually forming a monolayer[21–23]. The adsorption of surfactant molecules on graphitic carbons hinders the interaction between dye molecules and the catalyst P25@RGO, thus suppressing the dye removal efficiency of P25@RGO. On the other hand, dye molecules can competitively interact with both surfactants and the catalyst. The formation of dye– surfactant aggregates could hinder dye RhB to react with catalyst, resulting in a reduction of removal efficiency of dye RhB. Herein, the interactions between dye RhB and surfactants were further investigated. The interactions between negatively charged surfactant SDS and positively charged dye RhB lead to the formation of a complex, which is confirmed by the shift of the UV–vis spectrum ( see Supporting information Fig. S1). With the ammonium groups of RhB interacting with the sulphonate in the head group of SDS, this complex gets incorporated into the micelles [24], in which RhB may be inert to react with P25@RGO, leading to a significant reduction of removal efficiency of dye RhB. The interactions between the dye RhB molecules with nonionic surfactant Tergitol, cationic surfactant CTAB and anionic surfactant SDS were investigated by the 1H NMR spectra, which are
shown in Fig. 6. The peaks in the region of 8.8–5.5 ppm are attributed to the aromatic protons of the dye RhB. The addition of surfactants led to significant chemical shifts of the aromatic resonance lines of dye RhB, which confirmed the molecular interactions between the dye RhB and surfactants Tergitol, CTAB and SDS. It can also be seen that the dye RhB shows different peak positions in the presence of Tergitol and CTAB, which may suggest that these two surfactants interact on different sites of dye RhB molecules. The percentage of dye RhB existing as free dye and dye– surfactant aggregates was further investigated by PFG NMR experiments. The peak 1 in Fig. 6 was used to estimate their diffusion coefficients. As shown in Table 5, the diffusion coefficients of dye RhB decreased in the presence of both surfactants Tergitol and CTAB due to the formation of dye– surfactant aggregates [25]. The diffusion coefficient of dye RhB was 2.61 1010 m2/s in D2O solution. After the addition of the nonionic surfactant Tergitol, the coefficient of dye RhB decreased to 0.25 1010 m2/s, while diffusion coefficient of dye molecules was 1.73 1010 m2/s after the addition of the cationic surfactant CTAB. Because the free dye and dye–surfactant aggregates are in dynamic equilibrium, if the exchange rate is fast, then only one diffusion coefficient is observed, and/or if the exchange rate is slow, two separate diffusion coefficients should be observed. Only one diffusion coefficient was observed in this experiment, which indicates that there is fast exchange between free dye RhB and dye –surfactant aggregates. Therefore, the mean values of the selfdiffusion coefficients of free dye and dye–surfactant aggregates can be expressed as the following equation [26]:
273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300
Dobserve ¼ ð1 pagg ÞDmon þ pagg Dagg where Dobserve is the observed diffusion coefficient of the dye RhB molecules, Dmon and Dagg are the free dye and dye–surfactant aggregates diffusion coefficients, respectively. pagg indicates the dye–surfactant aggregates’ contribution to the observed diffusion coefficient. According to the above equation, pagg was calculated to be 98 mol% and 44 mol% for dye/Tergitol and dye/CTAB, respectively. This suggested that 98 mol% of the dye molecules were associated with the non-ionic surfactant Tergitol and 44 mol% for the cationic surfactant CTAB in the experimental conditions. With the presence Table 5 Diffusion coefficients (D 1010 m2/s) of D2O, surfactant and dye obtained from PFG NMR experiments at room temperature (20 C).
Fig. 6. 1H NMR of dye RhB and mixture of dye RhB and non-ionic surfactant Tergitol, cationic surfactant CTAB and anionic surfactant SDS in D2O.
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Dye + D2O Tergitol + D2O Dye + Tergitol + D2O CTAB + D2O Dye + CTAB + D2O Dye + SDS + H2O
D2O
Surfactant
15.6 14.6 16.0 15.0 15.7 16.1
0.19 0.20 0.61 0.61 0.59
Dye 2.61 0.25 1.73 0.51
Please cite this article in press as: J. Wang, et al., Degradation of organic dyes by P25-reduced graphene oxide: Influence of inorganic salts and surfactants, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.05.008
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Q5 308 309 310 311
G Model
JECE 648 1–7 J. Wang et al. / Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx Table 6 Diffusion coefficients (D 1010 m2/s) obtained from PFG NMR experiments at room temperature (20 C).
Dye + Tergitol Dye + CTAB
Dobserve
Dmon
Dagga
0.25 1.73
2.61 2.61
0.20 0.61
a
Dagg is the diffusion coefficient of dye–surfactant aggregates, which is assumed to be identical to the diffusion coefficient of the surfactant molecules. 312
328
of anionic surfactant SDS, coefficient of dye changed from 2.61 1010 m2/s as monomer to be 0.51 1010 m2/s in the presence of surfactant SDS. The coefficient of dye–surfactant (dye/SDS) aggregates became very close to that of surfactant 0.59 1010 m2/s, which suggests that almost 100% of dye molecules form dye–surfactant aggregates. Compared to nonionic surfactant aqueous solution, there is a lower percentage of dye–surfactant aggregates formed in cationic surfactant CTAB aqueous solution. This is because the positively charged ammonia head of dye RhB gives rise to dye–CTAB repulsion [27]. The formation of the dye–surfactant (dye/CTAB) aggregates displays a balance between electrostatic and hydrophobic competitive force [28]. This result further indicates a large percentage of dye RhB formed dye–surfactant aggregates in the surfactant aqueous solution, which may hinder dye RhB to react with catalyst, resulting in a significant decrease of the reduction rate of dye (Table 6).
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Conclusion
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Inorganic ions and organic additives on the photocatalytic performance of P25@RGO composite were investigated. The results show that Cl has an inhibition effect on the degradation of RhB. All three types of surfactants exhibit significant suppressing effect on degradation of RhB by P25@RGO, including anionic surfactant SDS, cationic surfactant CTAB, and non-ionic surfactant Tergitol. The interactions between surfactants and dye RhB were confirmed by NMR, which may be the key factors for the inhibition effect of surfactants on the reduced dye removal efficiency of P25@RGO. This work reveals the photocatalytic performance of P 25–graphene in the presence of inorganic and organic additives, which will be valuable for future application of graphene-based photocatalysts in treatment of industrial dyehouse effluents.
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Acknowledgement Q7
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ARC is acknowledged for funding Deakin University’s Magnetic Resonance Facility through LIEF grant LE110100141.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jece.2015.05.008.
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References
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