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Synergistic effect of Cu adsorption on the enhanced photocatalytic degradation of bisphenol A by TiO2 /titanate nanotubes composites Ruey-an Doong∗, Chia-wei Tsai Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Road., Hsinchu 30013, Taiwan

a r t i c l e

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Article history: Received 11 December 2014 Revised 14 April 2015 Accepted 9 May 2015 Available online xxx Keywords: One-dimensional titanate nanotubes (TNTs) Copper ions Bisphenol A (BPA) Photocatalytic activity Adsorption

a b s t r a c t The synergistic effect of Cu(II) adsorption on the enhanced photodegradation of bisphenol A (BPA) by TiO2 /titanate nanotube (TNTs) was investigated under UV light irradiation. TNTs were fabricated by hydrothermal method and calcined at 500 °C for 4 h (TNT-500) to adsorb Cu(II) as well as to increase the photocatalytic activity. The TNT-500 exhibited an excellent adsorption ability on copper ions and the adsorption isotherm of Cu(II) followed the Langmuir model with the maximum adsorption capacity of 54 mg/g at pH 5. In addition, the photocatalytic activity of TNT-500 toward BPA degradation was significantly enhanced by factors of 20–34 after the adsorption of 5–40 mg/g Cu(II). The pseudo-first-order rate constant (kobs ) for BPA photodegradation was highly dependent on Cu(II) concentration and an optimized value of 20 mg/L was obtained. The reaction rate of BPA by optimized Cu-loaded TNT-500 was 5.2 times higher than that of P25 TiO2 . In addition, the photocatalytic activity of Cu-loaded TNT-500 was significantly enhanced at pH > 4.8 and can be recycled for at least 5 times to completely photodegrade BPA under 365 nm UV light irradiation. Results obtained in this study clearly indicate that TNTs are promising nanomaterials for coupled removal of Cu(II) and BPA in aqueous solutions which can open an avenue to develop an effective strategy for removal of mixed contaminants in water and wastewater treatment. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Bisphenol A (BPA), one of the well-known emerging pollutants, is widely used as the industrial ingredient in polycarbonate plastic products [1,2]. Due to its biorefractory property and improper disposal, BPA can be released into the aquatic environments through the discharge of domestic sewages and industrial wastewater because the degradation efficiency of BPA in wastewater treatment is usually not high [3]. Therefore, the development of a cost-effective technology for removal of BPA is highly needed. Several treatment strategies including adsorption, chemical oxidation, and biological reaction have been developed for removal of emerging contaminants in the aquatic environments [4–8]. Photocatalytic degradation using TiO2 -based photocatalysts is one of the most effective method for elimination of BPA [9–11]. Since the discovery by Kasuga et al. [12], the one-dimensional (1-D) titanate nanotubes (TNTs) have recently been demonstrated as the promising nanostructured materials for removal of environmental contaminants because of their high specific surface area, good ion-exchange property and photocatalytic ability [13–16]. Several studies have demonstrated that the TNTs are excellent adsorbents for ∗

Corresponding author. Tel.: +886 3 5726785; fax: +886 3 5718649. E-mail address: [email protected] (R.-a. Doong).

removal of heavy metal ions, radioactive elements and rare earth elements such as Cs(I), Cd(II), Pb(II), Cu(II), Tl(II), and Eu(III) [17–19]. A previous study has reported that the adsorption capacity of metal ions onto hydrothermally as-prepared TNTs followed the sequence Pb(II)  Cd(II) > Cu(II) > Cr(III), and the adsorption capacity was up to 2.64 mmol/g for Pb(II) and 1.92 mmol/g for Cu(II) [20]. In addition, TNTs can be used for adsorptive removal of ammonia and organic compounds after surface modification [21,22]. Recently, the application of TNTs on photocatalysis of organic pollutants has also received much attention because of their suitable redox potential and bandgap. The as-prepared TNTs usually have a large band gap of 3.3–3.8 eV and show the inferior photodegradation efficiency on pollutant removal when compared to that of commercial TiO2 materials [23,24]. However, the photocatalytic activity of TNTs toward pollutant degradation can be enhanced by phase transformation or by doping with catalytic metal ions such as Cu(II) Ag(I), and Ce(III) [25–27]. Several studies have reported that the doping with 0.5–2 wt% Cu(II) could significantly enhance the removal efficiency and rate of environmental pollutant by TNTs and Cu species may serve as the active sites on Cu/TiO2 catalysts [26,28]. Although the doping or deposition of Cu ions can effectively enhance the photocatalytic activity of calcined TNTs toward organic removal, high temperature calcination is usually needed to deposit Cu ions onto the surface of TNTs, which is time- and energy-consuming.

http://dx.doi.org/10.1016/j.jtice.2015.05.013 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: R.-a. Doong, C.-w. Tsai, Synergistic effect of Cu adsorption on the enhanced photocatalytic degradation of bisphenol A by Tio2 /titanate nanotubes composites, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.05.013

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It is noteworthy that the metal ions and organic pollutants often coexist in wastewaters and concentration of Cu(II) is usually in the mg/L range [29]. This gives a great impetus to absorb Cu(II) ions from the wastewater by calcined TNTs first and then use the Cu-adsorbed TNT for enhanced photodegradation of organic pollutants without the calcination processes. This would be an ideal treatment strategy for simultaneous removal of Cu ions and organic compounds in a single operation unit. In spite of many studies on the enhanced photocatalysis of organic pollutants by metal-doped TNTs, the synergistic effect of adsorbed Cu(II) ions on the enhanced photodegradation of BPA by calcined TNT has received less attention. In this study, the synergistic effect of Cu(II) adsorption on the enhanced photocatalytic degradation of BPA by calcined TNTs was investigated under 365 nm UV light irradiation. TNTs were synthesized by alkaline hydrothermal method and then post-thermally treated at 500 °C for 4 h to form TiO2 /TNT nanocomposites (TNT-500) for adsorption of Cu(II). The Cu-adsorbed TNT-500 was then used to photodegrade BPA under UV light irradiation. Effect of environmental parameters including initial Cu(II) concentration, pH and initial BPA concentration on the photodegradation efficiency and rate of BPA was also evaluated. In addition, the change in total organic carbon (TOC) and photo-generated radicals of BPA solutions were also examined to elucidate the role of Cu(II) in the enhanced photodegradation of BPA by TNT-500. 2. Materials and methods 2.1. Reagents The ST-01 TiO2 nanoparticles were purchased from Ishihara Sangyo Ltd (Tokyo, Japan). Absolute ethanol (99.8%) and copper(II) nitrate pentahemihydrate (Cu(NO3 )2 •2.5H2 O, 98%) was obtained from Riedel-de Haën (Seelze, Germany). Hydrochloric acid (36.5–38.0%) was purchased from J. T. Baker (Phillipsburg, NJ). Bisphenol A (> 99%) was obtained from Sigma-Aldrich Co. (Milwaukee, WI). All chemicals were of analytical grade and were used as received without further purification. In addition, all aqueous solutions were prepared with bidistilled deionized water (bd H2 O, Millipore Co., 18.3 M cm) unless otherwise mentioned. 2.2. Fabrication of as-synthesized TNTs and TNT-500 The 1-D TNTs were synthesized using ST-01 TiO2 as the raw material. Briefly, 1.6 g of TiO2 powders were added into 20 mL of 10 M NaOH in the 30 mL Teflon-lined vessels. After ultrasonication for 1 h at room temperature, the mixtures were hydrothermally heated to 150 °C for 24 h and then cooled down to room temperature. The assynthesized TNTs were washed with 0.1 N HCl and bdH2 O repeatedly until the solution pH was near 7. Ethanol was added to the solution to replace water, and the TNTs were harvested from the solution by filtration and dried at 60 °C for 10 h in oven. The TNTs were then postthermally treated to form TiO2 /TNT nanocomposites at 500 °C for 4 h in air (TNT-500). 2.3. Adsorption of copper ions by TNT-based nanomaterials Batch adsorption experiments were carried out by adding 0.02 g of TNT-500 into 20 mL of bdH2 O and incubated in the shaker at 100 rpm and at 25 °C. The Cu(NO3 )2 solutions were added into the above solutions to get the final concentrations of 1–120 mg-Cu/L for adsorption. After the adsorption equilibrium for 1 h, the obtained Cu-adsorbed TNT-500 was harvested by filtration and the Cu concentrations in solutions were analyzed by Perkin Elmer model 100 atomic adsorption spectroscopy. 2.4. Photodegradation of BPA by Cu-adsorbed TNT-500 The photodegradation of BPA by Cu-adsorbed TNT-500 was carried out in a hollow cylindrical photoreactor equipped with a water

jacket to maintain the temperature at 25 °C. Four 8 W black light blue lamps (Winstar Lighting Co., Taipei, Taiwan) with 365 nm as the major peak wavelength were used as the light source. The 1 g/L Ti-based nanomaterials including Cu-adsorbed TNT-500 and commercial TiO2 were added into 20 mL solutions containing various concentrations of BPA ranging from 10 to 80 mg/L at pH 2.9–7.6. The mixtures were first stirred in the dark for 60 min to ensure the adsorption equilibrium of BPA onto the catalysts before the irradiation. After the adsorption equilibrium, UV light was turned on and solutions were sampled from the photoreactor periodically for analysis. After centrifugation to remove photocatalysts at 14,000 rpm for 5 min, the aqueous concentration of BPA in supernatants was determined by an Agilent high-performance liquid chromatograph equipped with C-18 column (LUNA 5 u 100 A, 4.6 mm × 250 mm, Phenomenex) and a diode array detector at 225 nm. The isocratic methanol/acetonitrile/water mixture (50:30:20, v/v) at a flow rate of 0.5 mL/min was used as the elute. 2.5. Characterization and measurement The surface morphology of as-synthesized TNTs and TNT-500 was characterized by JOEL scanning electron microscope (SEM, JEOL JSM6700F) at an accelerating voltage of 5 kV. A thin layer of Pt was coated on the samples to improve the resolution. In addition, the dimension and morphology of titanate-based materials were examined by transmission electron microscopy (TEM, JEOL JEM-2010) and high resolution TEM (JEOL, JEM-3000F) at accelerating voltages of 200 and 300 kV, respectively. The optical properties of the TNT-based nanomaterials were analyzed by UV–vis diffuse reflectance spectroscopy using a UV–vis spectrophotometer in the wavelength range of 250–500 nm. The X-ray photoelectron spectroscopy (XPS) measurements were performed by an ESCA PHI 1600 photoelectron spectrometer (Physical Electronics, Eden Prairie, MN) using Al Kα X-ray source (1486.6 ± 0.2 eV photon energy). During the data acquisition, the pressure in the sample chamber did not exceed 2.5 × 10−8 Torr. The binding energies of the photoelectrons were determined under the assumption that carbon has a binding energy of 284.8 eV. The specific surface areas and pore size distributions of TNT-based nanomaterials were determined by nitrogen adsorption–desorption isotherms at 77 K using N2 adsorption analyzer (Micromeritics, ASAP 2020). The specific surface areas were determined by the BrunauerEmmett-Teller method using adsorption data in a relative pressure (P/P0 ) range of 0.02–0.2. In addition, Barrett, Joyner and Halenda’s (BJH) mathematical models were used to calculate the pore size distributions and pore volumes of the TNT-based nanomaterials. The crystallinity of Ti-based nanomaterials was examined by using X-ray diffractometer (XRD, Bruker NEW D8 ADVANCE, Germany) with Ni˚ operating at 40 kV and 40 mA. filtered Cu Kα radiation (λ = 1.5406 A) The wide-angle XRD patterns were acquired from 5° to 80° 2θ with sampling step width of 0.05°. The electron paramagnetic resonance (EPR) spectrometer (Bruker, EMX-10, Germany) working at X-band frequency of 9.49–9.88 GHz with power of 8.02 mW was used to determine the photo-generated free radicals from the photodegradation of BPA by Cu-adsorbed TNT-500. Oxygen-saturated Cu-adsorbed TNT-500 containing 4.4 mM 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and BPA were irradiated with UV light produced from a 250 W Xe lamp at room temperature. The spectra of trapped charges in solutions were recorded at room temperature during the irradiation of 5–30 min. 3. Results and discussion 3.1. Characterization of as-synthesized and calcined TNTs The SEM and TEM were first used to understand the morphology and dimension of as-synthesized TNTs and TNT-500. As shown

Please cite this article as: R.-a. Doong, C.-w. Tsai, Synergistic effect of Cu adsorption on the enhanced photocatalytic degradation of bisphenol A by Tio2 /titanate nanotubes composites, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.05.013

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Fig. 1. SEM and TEM images of the as-synthesized and calcined TNTs. Fig. (a) and (c) are SEM and TEM images of as-synthesized TNTs, while Fig. (b) and (d) are SEM and TEM images of calcined TNTs at 500 °C, respectively. The insert of Fig. 1(c) is the tubular structures of TNT-500.

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in Fig. 1, the SEM and TEM images clearly showed the tubular structures of as-synthesized TNTs prepared at hydrothermal temperature of 150 °C for 24 h. The lengths of as-synthesized TNTs were of few μm with uniform diameter along with the length (Fig. 1(a)). After postthermal treatment at 500 °C for 4 h, the SEM image showed the coexistence of tubular structures and TiO2 nanoparticles (Fig. 1(b)) and the tubular structures of TNTs still remained after calcination (insert Fig. 1(c)). The well-crystallized TiO2 nanoparticles with mean particle size of 12 nm were clearly observed on the tube walls (Fig. 1(c)), indicating the phase transformation from nanotubes to crystalline TiO2 nanoparticles. In addition, the lattice fringes of TiO2 nanoparticles shown in Fig. 1(d) was about 0.35 nm, which corresponded to the (101) plane of anatase TiO2 [30,31]. This result confirms that the crystalline TiO2 nanoparticles were produced onto the surface of TNTs to form TiO2 /TNTs composites. The crystallinity and specific surface area of as-synthesized TNTs and TNT-500 were further examined. As shown in Fig. 2(a), several peaks centered at 10.3°, 24.3°, 28.4°, and 48.4° 2θ were clearly observed in XRD patterns of as-synthesized TNTs. The energy dispersive spectral result (Fig. S1, see supplementary data) showed that the Ti, O and Na contents were 64.1, 31.9 and 4.0 wt%, respectively, which suggested that the chemical structure of the TNTs was Nax H2-x Ti3 O7 [24,26]. No anatase peak of TiO2 appeared, indicating the complete conversion of TiO2 nanoparticles to tubular structures at 150 °C for 24 h. In addition, The XRD patterns of TNT-500 showed several new peaks at 25.33°, 37.78°, 48.07°, 53.92°, 55.11°, 62.72°, 68.59°, 70.35°, and 75.02° 2θ , which can be assigned as (101), (004), (200), (105), (211), (204), (116), (220) and (215) orientations of anatase TiO2 (JCPDS 21-1272). Several studies have depicted that the phase transformation of titanate–titania is a simple structural rearrangement and TNTs have a metastable nature and are intermediate species during the transformation of TiO2 [14,32]. Calcination is one of the most commonly used method to obtain the controlled and well-defined crystalline TiO2

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Please cite this article as: R.-a. Doong, C.-w. Tsai, Synergistic effect of Cu adsorption on the enhanced photocatalytic degradation of bisphenol A by Tio2 /titanate nanotubes composites, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.05.013

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phase [17,25]. The temperature for phase transformation is dependent on the morphology of TNTs, addition of additives and synthesis temperature and duration of TNTs [25,32]. Yu et al. [25] indicated that anatase TiO2 can be formed when TNTs were calcined at 300–600 °C. Our previous study [24] showed that the significant conversion of assynthesized TNTs to anatase TiO2 occurred at 400–500 °C, which is in good agreement with the result obtained in this study. The specific surface area as well as pore texture of TNT-based nanomaterials was also characterized. As shown in Fig. 2(b), a typical type IV isotherm with H3 hysteresis loop in the relative pressure (P/P0 ) range of 0.5–0.95 with the average pore diameter of 5.7 nm was observed for as-synthesized TNTs. The hysteresis loops of TNT-500 shifted to a high P/P0 region of 0.8–0.95 and the average pore diameter increased to 14.6 nm, indicating the mesoporous nature of TNT-based nanomaterials. The increase in pore size and the shift of hysteresis loop after calcination is mainly attributed to the change in morphology from tubular structures to the mixture of TNTs and TiO2 nanoparticles. In addition, the specific surface area of TNT-based material decreased from 398 to 95 m2 /g after calcination at 500 °C for 4 h, which is in good agreement with the TEM images. The optical property of as-synthesized TNTs and TNT-500 was analyzed using UV–vis diffuse reflectance spectroscopy. As shown in Fig. 3, the diffuse reflectance spectra of as-synthesized TNTs and TNT-500 showed absorption edges at around 340 and 370 nm, which corresponded to the bandgaps of 3.67 and 3.36 eV, respectively. This clearly indicates that calcination process reduced the bandgap of TNT-based nanomaterials and the wavelength of 365 nm is sufficient for TNT-500 to effectively photodegrade BPA. 3.2. Adsorption of Cu(II) by TNT-500 The adsorption performance of Cu(II) onto TNT-500 was examined at pH 5 and at 25 °C. Since Cu2+ easily precipitates at neutral pH, a weak acidic solution at pH 5 was used for adsorption to avoid the precipitation of Cu(OH)2 . In addition, the adsorption capacity of Cu(II) onto TNT-500 at pH 5 can be compared with the reported data using different adsorbents. Fig. S2 (see supplementary data) shows the adsorption isotherms of copper ions by ST-01 TiO2 and TNT-500 in solution. It is clear that ST-01 TiO2 nanoparticles had little effect on the adsorption of Cu(II) at pH 5, and the adsorbed amounts was lower than 5 mg/g. On the contrary, the adsorption of Cu(II) onto TNT-500 increased significantly. After adsorption equilibrium of 1 h, the adsorption efficiency of Cu(II) was higher than 90% at 1–10 mg/L, and then decreased to 67%, 50%, and 32% when the initial concentrations Cu(II) were 30, 60 and 120 mg/L, respectively. Several adsorption isotherm models including Langmuir and Freundlich can be employed to understand the adsorption behaviors of Cu(II) onto TNTs [20]. Both the adsorption of ST-01 TiO2 and TNT-500

Time (min) Fig. 4. The photodegradation of 10 mg/L BPA by as-synthesized TNTs and TNT-500 under the irradiation of 365 nm UV light at pH 7 and at 25 °C.

fitted well with the Langmuir isotherm and the calculated maximum adsorption capabilities (qm ) were 54 mg/g for TNT-500 and 3.8 mg/g for ST-01 TiO2 , showing that TNT-500 is a promising nanomaterial for Cu(II) adsorption. Several studies have used carbon nanotubes (CNTs) to adsorb metal ions and found that the adsorption of Cu(II) ions was 2.6–3.3 mg/g for pristine CNT [33] and 47–85 mg/g for activated CNTs [34,35]. Demirbas et al. [36] indicated that the adsorbed amount of Cu(II) could be up to 58.3 mg/g at 50 °C when hazelnut shell activated carbon was used to adsorb Cu(II) ions from aqueous solutions. In this study, the qm values of TNT-500 was superior or comparable to those of activated CNTs, presumably attributed to the negatively charged surface of TNT-500, which would undergo the electrostatic interaction between cationic metal ions and TNT-500 to increase the adsorption amounts of Cu(II).

3.3. Photodegradation of BPA by TNTs in the presence of Cu(II) Fig. 4 shows the photodegradation of BPA by as-synthesized TNTs and TNT-500 under UV light irradiation. Little BPA was adsorbed onto the surface of titanium-based nanomaterials after 60 min of reaction, depicting that adsorption of BPA can be neglected during photocatalytic process. No obvious photodegradation of BPA was observed in the absence of TNTs within 90 min at neutral pH, indicating that the direct photolysis of BPA can be neglected under UV light irradiation. In addition, the photocatalytic activity of as-synthesized TNTs toward BPA degradation was low and only 8% of BPA was photodegraded within 90 min. However, the photocatalytic activity of TNT-500 was enhanced and 42% of the original BPA was removed. The photodegradation of BPA followed the pseudo-first-order kinetics and the kobs for BPA photodegradation increased from 0.0012 min−1 by as-synthesized TNTs to 0.0074 min−1 by TNT-500, clearly depicting the enhanced photocatalytic activity of TNTs after thermal treatment. It is noteworthy that the commercial TiO2 nanoparticles showed good photocatalytic activity toward BPA degradation and the removal efficiency of BPA was 92% for Degussa P25 TiO2 and 70% for ST01 TiO2 after 90 min of irradiation. The kobs for BPA photodegradation was 0.049 min−1 for P25 TiO2 and 0.02 min−1 for ST01 TiO2 , which was 2.8–6.6 times higher than that of TNT-500. Several studies have depicted that addition of copper ions can enhance the photodegradation efficiency of titanium-based materials [26,37]. Fig. 5(a) shows the photodegradation of BPA by TNT-500 as a function of Cu(II) concentration. The photodegradation efficiency of BPA was 68.5% at 1 mg/L Cu(II) and then increased to > 99.9% when the concentration was higher than 5 mg/L. In addition, less than 6% of BPA were photodegraded within 90 min when solutions contained only 20 mg/L Cu(II) in the absence of TNT-500, which indicated that

Please cite this article as: R.-a. Doong, C.-w. Tsai, Synergistic effect of Cu adsorption on the enhanced photocatalytic degradation of bisphenol A by Tio2 /titanate nanotubes composites, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.05.013

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Binding energy (eV) the addition of low concentration of Cu(II) enhanced the photodegradation efficiency of BPA by TNT-500. Fig. 5(b) shows the kobs for BPA photodegradation by TNT-500 as a function of Cu(II) concentration. The kobs for BPA photodegradation increased from 0.0074 ± 0.0005 min−1 in the absence of Cu(II) to 0.173 ± 0.012 min−1 at 10 mg/L Cu(II) and then reached the maximum value of 0.253 ± 0.032 min−1 at 20 mg/L Cu(II). Further increase in Cu(II) concentration decreased the rate constant to 0.152 ± 0.026 min−1 . The kobs for BPA photodegradation by Cu-adsorbed TNT-500 increased by factors of 21–34 when compared with that of pure TNT-500. In addition, these rate constants were 1.6–5.5 and 4.1–12.4 times higher than those of Degussa P25 (0.049 min−1 ) and ST01 TiO2 (0.02 min−1 ), respectively, clearly showing that adsorption of Cu(II) significantly enhances the photodegradation efficiency and rate of BPA by TNT-500. The Cu(II) concentration is a vital factor influencing the photodegradation behavior of BPA by TNT-500 and low Cu(II) concentration increases the photodegradation efficiency of BPA. This enhancement is mainly attributed to the increase in reactive sites for radical production. However, adsorption of high concentration of Cu(II) would block the reactive sites, resulting in the decrease in photodegradation rate of BPA by TNT-500. A previous study has depicted that Cu(II) ions may serve as the electron trap center to reduce the hole-electron recombination rate [26]. At low initial Cu(II) concentration, most adsorbed Cu(II) can serve as the electron trap to enhance the photodegradation efficiency of BPA. However, the adsorbed Cu(II) may compete and block the photoreactive sites with BPA when the concentration was high, and subsequently results in the decrease in photodegradation efficiency and rate of BPA. Fig. S3 (see supplementary data) shows the kobs for BPA photodegradation as a function of adsorbed Cu(II) concentration. It is clear that the kobs for BPA photodegradation increased positively with the increase in adsorbed amount of Cu(II). However, the kobs for BPA photodegradation decreased when the adsorbed Cu(II) was high, reflecting the fact that

Fig. 6. The (a) Ti 2p and (b) Cu 2p XPS spectra of Cu-loaded TNT-500 before and after UV light irradiation.

the adsorbed Cu(II) plays an important role in the enhancement of photodegradation efficiency and rate of BPA by TNT-500. To further understand the role of Cu species in photodegradation, XPS was used to identify the change in chemical species of Ti and Cu elements in Cu-adsorbed TNT-500. As shown in Fig. 6, the Ti 2p spectra showed two peaks associated with Ti 2p3/2 at 458 eV and Ti 2p1/2 at 464.5 eV, which were the characteristic peaks of Ti4+ on TiO2 (Fig. 6(a)). No obvious peak shift of Ti 2p peak was observed after UV light irradiation. In addition, the Cu 2p spectra in Fig. 6(b) showed the peaks of Cu2+ phase with the 2p3/2 and 2p1/2 peaks at 932.6 and 952.4 eV, respectively. Similar to the Ti 2p spectra, no obvious peak shift of Cu 2p was observed after UV light irradiation, showing that the major species of Cu and Ti elements in Cu-loaded TNT-500 are stable before and after the photodegradation. Nian et al. [38] used X-ray absorbance spectroscopy to identify the structural feather of CuO species onto TNTs and found that the Cu species on all the TiO2 supports were in the +2 state and Cu2+ dissolution into the TiO2 lattices generated the reactive sites of Cux Ti1- x O2 species on the Cu/TiO2 catalysts. This also suggests that the adsorbed Cu species in this study may form reactive sites onto TNT-500. EPR was further used to identify the production of free radicals in the presence of various concentrations of copper ions. Fig. 7 shows the EPR spectra of free radicals produced from TNT-500 under UV light irradiation. No radical signal was found in solutions containing BPA and DMPO in the dark. After irradiation of UV light for 5 min, The six-line EPR spectra were obtained in the BPA-containing suspensions, depicting the production of oxygen-centered radical adducts such as •OH, •OOH, and ROO• [10,39]. Addition of Cu(II) enhanced the production of radical adducts, and the signal intensity increased with the increase in Cu(II) concentration from 0 to 20 mg/L. However,

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2.02

2.03

g factor Fig. 7. The EPR spectra of radicals obtained from the illumination of TNT-500 by UV light for 5 min in the presence of 10 mg/L BPA and various concentrations of Cu(II) ranging from 5 to 40 mg/L.

the EPR signals decreased slightly when further increased the Cu(II) concentration to 40 mg/L. These results are in good agreement with the photodegradation results shown in Fig. 5, clearly indicating that adsorption of copper ions onto TNTs can serve as the electron traps to effectively produce radicals during the UV irradiation processes and 20 mg/L Cu(II) is the optimal concentration for BPA photodegradation. Therefore, 20 mg/L Cu(II) was selected for further experiments. Several studies have depicted the feasibility of using TiO2 -based materials for effective photodegradation of BPA in aqueous solution [10,39]. Our previous study has identified several intermediates including 1,1-bis(4-hydroxyphenyl)ethane, 3-hydroxy-2,2-bis(4hydroxyphenyl)propane and bis(4-hydroxyphenyl)methanol when BPA was photodegraded by Cu–TiO2 nanorods [37]. However, the extent of mineralization has been reported less. Fig. S4 (see supplementary data) shows the change in TOC concentration of 10 mg/L BPA by Cu-adsorbed TNT-500 under UV light irradiation. The aqueous TOC concentrations decreased gradually with time and 39% of TOC were removed after 120 min of irradiation when 10 mg/L BPA was photodegraded by Cu-adsorbed TNT-500. It is noted that the mineralization of BPA by Degussa P25 TiO2 was 21%, which was lower than that of Cu-adsorbed TNT-500. These results clearly indicate that the photo-mineralization efficiency of BPA by TNT-500 can be effectively enhanced after the adsorption of Cu(II) under UV light irradiation. 3.4. Effect of pH value The pH value is one of the pronounced parameters influencing the photodegradation activity of TNTs. Fig. 8 shows the effect of pH on the photodegradation efficiency of BPA by Cu-adsorbed TNT-500 and the kobs for BPA photodegradation as a function of initial pH. Less than 40% of BPA was photodegraded within 60 min at pH < 4 when Cuadsorbed TNT-500 was irradiated with 365 nm UV light. The increase in pH to 4.8 significantly enhanced the photodegradation efficiency of BPA and 90% of the original BPA was removed within 60 min. In addition, the photodegradation efficiency of BPA by TNT-500 was higher than 99% at pH > 6.0. The high removal efficiency of BPA at pH > 4.8 is mainly due to the low isoelectric point (IEP) of TNT-500. As shown in Fig. S5 (see supplementary data), the pHIEP of TNT-500 was determined to be 4.2, which means that the surface of TNT-500 should be negatively charged at pH > 4.8. It is noted that pKa of BPA is 9.6, which means that BPA will remain in the molecular form in the pH range used in this study. Therefore, the adsorption of BPA onto negatively charged TNT is lower than that of Cu(II) and the adsorbed Cu(II) onto

0.35

(b)

0.30 0.25 0.20 0.15 0.10 0.05 0.00 3

4

5

6

7

8

pH value Fig. 8. (a) Effect of pH on the photodegradation of BPA by Cu-adsorbed TNT-500 and (b) the kobs for BPA photodegradation as a function of pH under irradiation of UV light at pH 7.0 and at 25 °C.

TNT-500 serves electron trap center to enhance photodegradation efficiency of BPA. Previous studies [40,41] have depicted that the photodegradation rate and efficiency of BPA by TiO2 particles increased upon increasing solution pH values, which is in good agreement with the results obtained in this study. In addition, the kobs for BPA photodegradation increased exponentially from 0.004 ± 0.001 min−1 at pH 2.9 to 0.311 ± 0.02 min−1 at pH 7.6, clearly indicating that the photodegradation activity of TNT-500 toward BPA degradation was significantly enhanced at high pH value. 3.5. Effect of initial BPA concentration The effect of initial BPA concentration on the photodegradation efficiency and rate of BPA by Cu-adsorbed TNT-500 was further examined. Fig. 9 shows the photodegradation efficiency of BPA at various initial BPA concentrations ranging from 10 to 80 mg/L. The removal efficiencies of various concentrations of BPA were all > 99% after 40 min of irradiation. However, the photodegradation rate of BPA decreased upon increasing initial concentration and the kobs for BPA photodegradation decreased from 0.342 ± 0.022 min−1 at 10 mg/L BPA to 0.099 ± 0.001 min−1 at 80 mg/L of BPA. The decrease in kobs for BPA photodegradation with the increase in initial concentration is mainly attributed to the limited reactive sites onto the TNT-500 surfaces. Previous studies [42,43] have depicted that the reaction kinetics for organic removal by catalysts followed the surface-mediated processes where the initial rate of reactants was positively related to the adsorption of reactant/reductant onto the metal surfaces and the reaction rate constant decreases upon increasing initial concentration. The Langmuir–Hinshelwood kinetic model can thus be used to describe the relationship between the initial concentrations and initial rate of BPA.

r0 =

dC KaC = kr dt 1 + KaC

(1)

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4. Conclusions

Remaining ratio (C/Co)

(a) 1.0

10 mg/L 20 mg/L 40 mg/L 60 mg/L 80 mg/L

0.8

In this study, we have demonstrated that the calcined TNTs can effectively adsorb Cu(II) and then the adsorbed Cu(II) increases the production rate of radicals on TNT-500 surface, resulting in the enhancement of photocatalytic degradation of BPA under UV light irradiation. The negatively charged TNTs can effectively adsorb Cu(II) and the maximum adsorption capability was 54 mg/g. After adsorption of 5–40 mg/L Cu(II), the photocatalytic activity of Cu-adsorbed TNT-500 toward BPA photodegradation was enhanced by factors of 20–34 compared with that of pure TNT-500, and the kobs values for BPA photodegradation ranged between 0.0074 ± 0.0005 and 0.253 ± 0.032 min−1 , which were 1.6–12.4 times higher than those by commercial TiO2 nanoparticles. The photodegradation of BPA by Cu-adsorbed TNT-500 is a surface-mediated process and can be welldescribed by Langmuir–Hinshelwood kinetics. In addition, the Cuadsorbed TNT-500 can be recycled for at least 5 times to photodegrade BPA. Results obtained in this study clearly indicate that TNTs is a promising nanomaterial for coupled removal of Cu(II) and BPA in aqueous solutions which can open an avenue to design the environmentally friendly wastewater treatment strategies for simultaneous removal of metal ions and organic pollutants in aqueous solutions.

0.6

0.4

0.2

0.0 0

5

10

15

20

25

30

35

40

Illumnation time (min)

(b)

8

Initial rate (mg/L-min)

7 r2

6

7

11.75 0.197 C 1 0.197 C 0.996

5 4

Acknowledgeents

3

The authors thank the Ministry of Science and Technology (MOST), Taiwan for financial support under Contract No. NSC 98–2221–E– 007–007–MY3 and NSC 99–2627–M–007–006.

2 1 0

0

10

20

30

40

50

60

70

80

90

BPA concentration (mg/L) Fig. 9. (a) The photodegradation of various initial concentrations of BPA and (b) the initial rate for BPA photodegradation by Cu-adsorbed TNT-500 as a function of initial BPA concentration.

where C is the aqueous concentration of BPA, kr is the intrinsic rate constant, and Ka is the Langmuir adsorption coefficient of BPA on the reactive sites. As shown in Fig. 7(b), the initial photodegradation rates increased positively from 2.07 ± 0.13 mg/L min at 10 mg/L BPA to 6.03 ± 0.22 mg/L min at 80 mg/L BPA. A good fit between the initial BPA concentration and the initial rate with kr and Ka of 11.8 mg/L min and 0.197 L/mg, respectively, was obtained (r2 = 0.996, n = 7), clearly indicating that the photodegradation of BPA by TNT-500 is a surfacemediated process. 3.6. Reusability of Cu-adsorbed TNTs for BPA photodegradation Fig. S6 (see supplementary data) shows the photocatalytic degradation efficiency of BPA and the reusability of P25 TiO2 and Cuadsorbed TNT-500. 46% of original BPA was photodegraded by P25 TiO2 after 60 min of UV irradiation. Re-injection of 10 mg/L BPA into the P25 TiO2 suspensions decreased the photodegradation efficiency of BPA and only 26% and 12% of BPA was removed within 60 min for the 2nd and 3rd injection, respectively. In contrast, Cu-adsorbed TNT-500 exhibited a good reusability on photodegradation of BPA. A nearly complete photodegradation of BPA by Cu-adsorbed TNT-500 was observed for the first 2 cycles and the removal efficiency only slightly decreased to 90% for the 3rd cycle. The efficiency of BPA decreased to 58% after 5 cycles, clearly showing that Cu-adsorbed TNT-500 is a superior photocatalyst to eliminate BPA in aqueous solutions. The decrease in photodegradation efficiency may be attributed to the accumulation of intermediates after photocatalytic degradation of BPA under UV light irradiation. These accumulated intermediates would compete with the adsorption and reactive sites on TNT-500 surface with BPA, resulting in the decrease in photodegradation efficiency and rate of BPA.

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Please cite this article as: R.-a. Doong, C.-w. Tsai, Synergistic effect of Cu adsorption on the enhanced photocatalytic degradation of bisphenol A by Tio2 /titanate nanotubes composites, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.05.013