Immobilized TiO2-Polyethylene Glycol: Effects of Aeration and pH of Methylene Blue Dye
Immobilized TiO2-Polyethylene Glycol: Effects of Aeration and pH of Methylene Blue Dye
Nawawi, Wan Izhan;Zaharudin, Raihan;Zuliahani, Ahmad;Shukri, Dyia Syaleyana;Azis, Tun Firdaus;Razali, Zainab
2017-05-12 00:00:00
applied sciences Article Immobilized TiO -Polyethylene Glycol: Effects of Aeration and pH of Methylene Blue Dye 1 , 1 , 2 1 1 Wan Izhan Nawawi *, Raihan Zaharudin , Ahmad Zuliahani , Dyia Syaleyana Shukri , 1 1 Tun Firdaus Azis and Zainab Razali Faculty of Applied Sciences, Universiti Teknologi MARA, Perlis, Arau 02600, Malaysia; nurraihanzaharudin@gmail.com (R.Z.); zuliahani@perlis.uitm.edu.my (A.Z.); dyia839@perlis.uitm.edu.my (D.S.S.); firdausbinazis@gmail.com (T.F.A.); zainab215@perlis.uitm.edu.my (Z.R.) Faculty of Applied Sciences, Universiti Teknologi MARA, Selangor, Shah Alam 40450, Malaysia * Correspondence: wi_nawawi@perlis.uitm.edu.my; Tel.: +60-49-882-305; Fax: +60-49-882-304 Academic Editor: Rajender S. Varma Received: 24 February 2017; Accepted: 4 May 2017; Published: 12 May 2017 Abstract: Immobilized TiO and immobilized TiO -polyethylene glycol (TiO /PEG) films have been 2 2 2 prepared via brush coating method. The formulation of immobilized TiO film was prepared by mixing distilled water with P25, while the formulation containing P25 combined with 8% PEG in distilled water was used in preparing immobilized TiO /PEG. A double sided adhesive tape (DSAT) was stacked onto a glass surface prior to coating with the formulations and annealing by a thermal treatment at 100 C for 15 min. The photocatalytic activity of immobilized photocatalysts was evaluated under photodegradation of methylene blue (MB). It was observed that immobilized TiO /PEG has showed a higher rate of photocatalytic activity compared to immobilize TiO . The X-ray 2 2 photoelectron spectroscopy and Fourier transform infrared (FT-IR) spectra of immobilized TiO /PEG sample proved that the existence of C=O led to enhanced photoactivity efficiency under normal light and visible light irradiations. The photocatalytic activity performance of immobilized TiO /PEG was the highest at 75 mLmin aeration rate and pH 11 of MB dye. The correlation between of all these parameters was investigated in this study. Keywords: immobilized TiO ; polyethylene glycol; double sided adhesive tape; methylene blue dye 1. Introduction Dye pollutants or effluents have attracted wide attention due to their toxicity and resistance for removal-feature. It has been a challenging task to decompose these effluents (e.g., methylene blue (MB) dye) [1,2]. Some of the major works conducted on MB dye were in aqueous solutions using photocatalysis method, and some of these photocatalytic studies were also employed in immobilization systems [3–6]. In aqueous systems such as slurry or suspension method, photocatalysts require an expensive photocatalyst recovery process which limits its application due to high scale-up cost and because it is not environmentally friendly [7,8]. Even though there are several commercial photocatalysts available, the TiO photocatalyst is always an option used in photocatalysis such as in wastewater remediation due to its high performance, low equipment cost, and low toxicity [9,10]. Alternatively, immobilization of TiO has been explored, and displayed better catalytic compared to TiO suspension [11]. Moreover, a comprehensive review of TiO films was conducted by Varshney et al. 2 2 (2016), in which various types of coating methods (e.g., doctor-blade, chemical vapour deposition, hydrothermal, electrophoretic deposition, sputter deposition, spray pyrolysis, and flame aerosol coating methods) are thoroughly explained [12]. In an earlier study by Sökmen et al. [13], immobilized TiO was used for the photodegradation of MB dye. This study revealed that more MB dye was Appl. Sci. 2017, 7, 508; doi:10.3390/app7050508 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 508 2 of 10 removed in a shorter amount of time through immobilized TiO –polymer. Studies have shown that the incorporation of polymer such as polyethylene glycol (PEG) in photocatalysis is highly beneficial for the enhancement of the catalytic activity of the immobilized sample. PEG is a polymeric material that can be employed to boost the functionalization of TiO surface which acted as a matrix agent or as a structure directing agent [14–16]. Curkovic ´ et al. [17] claimed that immobilized TiO with PEG exhibited higher catalytic activity and achieved higher surface density compared to other immobilized sample without the addition of PEG. The addition of PEG was proven to create a remarkable porosity in immobilized TiO sample [18]. It has been realized that the size of pores increased with increased molecular weight of PEG [19]. This factor indirectly promotes an excellent performance of immobilized photocatalyst sample. In another study by Liu et al. [20], the addition of high molecular weight PEG (i.e., PEG 6000) presented a major increase in specific surface area, well-defined particle size, and enhanced photo-generated charge separation rate between conduction band (CB) and valence band (VB) during oxidation process. Meanwhile, studies have shown that adhesive tapes (e.g., scotch tape, double sided adhesive tape (DSAT), and hatch tape) were previously used to measure the adhesiveness of TiO films onto substrate [21,22]. However, the functionalization of adhesive tapes (i.e., DSAT) as a support binder has not been studied extensively. Besides, improving the adherence of TiO film with flat supports such as glass plates is crucial to obtain an optimal photocatalytic effect. This is because film adhesion is one of the biggest challenges in polymer-immobilized photocatalyst [23]. Based on our previous works [24,25], DSAT was not only proven to strengthen the adhesion of TiO film to glass plate, but also improved the reusability of immobilized photocatalyst for up to 30 cycles. The immobilized TiO –DSAT successfully demonstrated an outstanding photocatalytic activity on the decolorization of Reactive Red 4 (RR4) and methylene blue dye. During the 30 cycles, the photocatalyst sample maintained a high photodegradation rate, indicating good stability of immobilized TiO –DSAT. Therefore, in this study, efforts have been made to immobilize TiO photocatalyst with pore matrix PEG 6000 and DSAT as a thin layer binder. The immobilization system utilized a paint brush-coating technique with low heat treatment method. The MB dye was used as a target pollutant in aqueous media to assess the photocatalytic activity. This study serves as an extension of our immobilization reports [26]; thus, the influence of experimental parameters such as aeration flow rate, initial dye concentration, and effect of pH were also investigated. 2. Experimental 2.1. Preparation of Immobilized TiO /PEG and TiO Films 2 2 Based on previous works [21], the immobilized TiO /PEG was prepared by dissolving 6.5 g of titanium dioxide (TiO ) Degussa P25 powder in 50 mL of distilled water added with 1 mL of 8% (w/v) of polyethylene glycol (PEG) solution (Merck, Darmstadt, Germany, molecular weight (MW) = 6000). The sample solution was thoroughly shook through a sonication process for one hour, producing a homogenized formulation. The sample films were prepared by using a brush-coating method applied onto glass substrates [24,25], which were cleaned and taped with double-sided adhesive tape (DSAT) prior to coating. The coated glass was then dried using a hot air blower at 100 C for 15 min. The thickness of the immobilized films was controlled by the optimal amount of photocatalyst loading and number of coatings [27]. The same procedure was repeated for immobilized TiO with the exception of the addition of PEG. 2.2. The Washing Process of Immobilized Samples and Photoactivity Test under Different Light Conditions The cleaning process was conducted to remove all unwanted contaminants by irradiating the immobilized samples using distilled water. The washing process was carried out for one hour under normal light (NL) irradiation to increase the permeability performance prior to photoactivity test. The photoactivities of the immobilized samples were tested by degradation of methylene blue (MB; Fluka Analytical, St. Louis, MO, USA). The experimental setup used was to apply immobilized Appl. Sci. 2017, 7, 508 3 of 10 Appl. Sci. 2017, 7, 508 3 of 10 sample with 20 mL of 12 mg/L concentration of MB dye into a glass cell (dimension: 50 mm length 10 mm width × 80 mm height). An NS 7200 aquarium pump was used as an aeration source along 10 mm width 80 mm height). An NS 7200 aquarium pump was used as an aeration source along the photodegradation process. Area of photocatalytic activity is equal to length × width of DSAT the photodegradation process. Area of photocatalytic activity is equal to length width of DSAT applied, which is 6.6 cm × 4.6 cm. The photoactivity tests of the immobilized samples were applied, which is 6.6 cm 4.6 cm. The photoactivity tests of the immobilized samples were conducted conducted under normal light and visible light (VL) irradiation. The NL intensity of the lamp used under normal light and visible light (VL) irradiation. The NL intensity of the lamp used was measured was measured at about 461 and 6.7 W/m for visible and UV lights, respectively, while an intensity of 2 2 at about 461 and 6.7 W/m for visible and UV lights, respectively, while an intensity of 430 W/m 430 W/m was used for VL source. Turning on the 55-W compact fluorescent lamp as the normal was used for VL source. Turning on the 55-W compact fluorescent lamp as the normal light source as light source as shown in Figure 1 initiated the photocatalytic reaction, and outlet absorbance was shown in Figure 1 initiated the photocatalytic reaction, and outlet absorbance was measured. A 4 mL measured. A 4 mL aliquot was extracted from the glass cell reactor at pre-specified intervals. aliquot was extracted from the glass cell reactor at pre-specified intervals. The absorbance of the aliquot The absorbance of the aliquot was measured by a UV spectrophotometer (Varian UV-Vis detector was measured by a UV spectrophotometer (Varian UV-Vis detector model HACH DR 1900, HACH, model HACH DR 1900, HACH, Loveland, CO, USA) set to 661 nm, which corresponds to the MB absorban Loveland, ce. CO, MB concentr USA) set to ation trends w 661 nm, which ere simulated corresponds usi to the ng a pseudo fi MB absorbance. rst-order ra MB concentration te constant pl trends ot of were simulated using a pseudo first-order rate constant plot of ln(C /C ) = kt, where Co represents the ln(Co/Ct) = kt, where Co represents the MB concentration at t = 0 min, Ct is the concentration at time o t −1 t MB , and concentration k is kinetic const at t = a 0nt min, or phot C is oact the concentration ivity efficiency at at time min t, and . The k isconcentration kinetic constant of M orB photoactivity at different efficiency at min . The concentration of MB at different experimental times was spectrometrically experimental times was spectrometrically evaluated [28]. The final MB concentration can be spectrometric evaluated [28a ].lly determ The final iMB ned with a calibration c concentration can beuspectr rve usin ometrically g the Beer–L determined ambert equation, wh with a calibration ich is curve using the Beer–Lambert equation, which is A = l"[MB], where l is the path length, [MB] is the A = lε[MB], where l is the path length, [MB] is the concentration of the dye, and ε is the MB molar absorptivity [27]. For VL irrad concentration of the dye, and " iis atthe ion, a glass MB molar ceabsorptivity ll that contai [ned 27]. For MB sol VL u irradiat tions wa ion, s exposed to the a glass cell that contained MB solutions was exposed to the visible light from a 55-W fluorescent lamp attached with visible light from a 55-W fluorescent lamp attached with a UV filter (>420 nm) at room temperature condit a UV filter ions. (>420 With t nm) he pse at u room do first temperatur -order plot e conditions. , absorbance w With as co the nverted to concentration, pseudo first-order plot, absorbance and rate of was converted to concentration, and rate of degradation (k) was calculated. degradation (k) was calculated. Figure 1. The photoactivity test of methylene blue (MB) dye for immobilized TiO2 and TiO2/PEG. Figure 1. The photoactivity test of methylene blue (MB) dye for immobilized TiO and TiO /PEG. 2 2 PEG: polyethylene glycol. PEG: polyethylene glycol. 2.3. Characterization of Immobilized Samples Using Fourier Transform Infrared Spectroscopy (FT-IR) and 2.3. Characterization of Immobilized Samples Using Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelecton Spectroscopy (XPS) X-ray Photoelecton Spectroscopy (XPS) FT-IR spectra of immobilized TiO2 and TiO2/PEG samples in powder form were analysed on FT-IR spectra of immobilized TiO and TiO /PEG samples in powder form were analysed on 2 2 Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device (Frontier, Perkin Elmer Spectrum Version equipped with an attenuated total reflectance device (Frontier, Waltham, Waltham, MA, USA) with a diamond crystal. Spectra were collected in a wavelength range of MA, USA) with a diamond crystal. Spectra were collected in a wavelength range of 600–4000 cm −1 −1 600–4000 cm with four scans and spectral resolution of 4 cm . The binding energy of immobilized with four scans and spectral resolution of 4 cm . The binding energy of immobilized TiO /PEG was TiO2/PEG was determined using X-ray photoelectron spectroscopy (XPS) with a Thermo ESCALAB determined using X-ray photoelectron spectroscopy (XPS) with a Thermo ESCALAB 250 spectrometer 250 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using a radiation source of (Thermo Fisher Scientific, Waltham, MA, USA) using a radiation source of monochromatic Al K with monochromatic Al Kα with the energy of 1486.6 eV. the energy of 1486.6 eV. 22.4. .4. Effects of Effects of Aerationand Aerationand Initial p Initial pH H Condition Condition The phot The photocatalytic ocatalytic st study udy of immobi of immobilized lized samp samples les in the degrad in the degradation ation of of MB MB dye as a mo dye as a model del pollut pollutant ant was studied was studied under under d differient fferent parameters parameters (i.e., dif (i.e., differen ferent aeration t aer rate, ation pH, rate, pH, an and initial condition). d initial condit The photocatalytic ion). The phot degradation ocatalytic degr studies adatifor on st allud pie arameters s for all p ar ae ram based eters on are the based on previous the pr Section evious 2.2. −1 Sect Four ion dif 2 fer .2ent . Foair ur di flow fferent rates air (25, flo 50, w ra 75,tes (25 and 100 , 50 mL , 75min , and )1wer 00 mL·m e used into)study were the useef d to fectstudy of aeration the effect rate. of aerat Under NL ion r irradiation, ate. Under theNL irradiation, the ef effect of pH was investigated fect of pH w by irradiating as invest anigimmobilized ated by irrad TiiO ating an /PEG immobilized TiO2/PEG sample using different pHs of MB dye solutions (pH 3, 6, 8, 10, 11, and 12). Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to change the pH of MB Appl. Sci. 2017, 7, 508 4 of 10 Appl. Sci. 2017, 7, 508 4 of 10 sample using different pHs of MB dye solutions (pH 3, 6, 8, 10, 11, and 12). Hydrochloric acid (HCl) solution. Four MB solutions with different concentrations were used (6, 12, 24, and 36 ppm) for and sodium hydroxide (NaOH) were used to change the pH of MB solution. Four MB solutions with initial concentration test. The solutions were subjected to an adsorption process to observe the initial different concentrations were used (6, 12, 24, and 36 ppm) for initial concentration test. The solutions degradation rate. The determination of the pH of the zero charge was conducted by adding a 0.01 g were subjected to an adsorption process to observe the initial degradation rate. The determination of unmodified TiO2 and modified TiO2/PEG powder into several 50 mL of pH-adjusted ultra-pure of the pH of the zero charge was conducted by adding a 0.01 g of unmodified TiO and modified water solutions ranging from pH 2 to 10 followed by the addition of 10 mL of 0.1 M KCl. TiO /PEG powder into several 50 mL of pH-adjusted ultra-pure water solutions ranging from pH 2 to The solutions were stirred, and the final pH values were recorded after 24 h of stirring. The discrepancies 10 followed by the addition of 10 mL of 0.1 M KCl. The solutions were stirred, and the final pH values of the pH were obtained by subtracting the initial pH with the final pH. A plot of pH discrepancies were recorded after 24 h of stirring. The discrepancies of the pH were obtained by subtracting the initial versus the initial pH was constructed, and the point of zero charge was obtained from the values pH with the final pH. A plot of pH discrepancies versus the initial pH was constructed, and the point of which cut the x-axis. The degradation of MB (concentration against time) was presented according to zero charge was obtained from the values which cut the x-axis. The degradation of MB (concentration pseudo first-order rate constant plot. against time) was presented according to pseudo first-order rate constant plot. 3. Result and Discussion 3. Result and Discussion 3.1. FT-IR and XPS Analysis of Immobilized TiO and TiO /PEG 3.1. FT-IR and XPS Analysis of Immobilized TiO2 2 and TiO2 2/PEG The FT-IR spectra of immobilized TiO and TiO /PEG were determined and are presented The FT-IR spectra of immobilized TiO22 and TiO2/PE 2 G were determined and are presented in −1 in Figure 2a. The spectra revealed a strong and sharp absorption peak at 1705.02 cm which is Figure 2a. The spectra revealed a strong and sharp absorption peak at 1705.02 cm which is attributable to the existence of C=O bond in immobilized TiO /PEG sample. At the lower wavenumber attributable to the existence of C=O bond in immobilized TiO 22/PEG sample. At the lower wavenumber −1 of 1160.06 cm , another bond was discovered corresponding to the stretching vibration of carbonyl of 1160.06 cm , another bond was discovered corresponding to the stretching vibration of carbonyl oxygen or the C–O bond. However, the C=O and C–O bonds were not found in the immobilized TiO oxygen or the C–O bond. However, the C=O and C–O bonds were not found in the immobilized 2 sample. These newly-discovered peaks observed in immobilized TiO /PEG were the result of the TiO2 sample. These newly-discovered peaks observed in immobilized TiO 2 2/PEG were the result of introduction of high molecular weight PEG 6000 into the immobilized sample. The oxidation of PEG the introduction of high molecular weight PEG 6000 into the immobilized sample. The oxidation of 6000 in washed film was detected owing to the strong absorption by the carbonyl group in the FT-IR PEG 6000 in washed film was detected owing to the strong absorption by the carbonyl group in the spectrum [29]. The strong absorption peaks of C=O and C–O bonds represent a successful interaction FT-IR spectrum [29]. The strong absorption peaks of C=O and C–O bonds represent a successful of TiO nanoparticles with PEG 6000 that favours good photocatalytic activity, as shown previously. interact 2 ion of TiO2 nanoparticles with PEG 6000 that favours good photocatalytic activity, as shown The surface analysis of immobilized TiO /PEG was also studied by X-ray photoelectron spectroscopy, previously. The surface analysis of imm 2obilized TiO2/PEG was also studied by X-ray photoelectron as displayed in Figure 2b. spectroscopy, as displayed in Figure 2b. x10 a) b) C1s spectra of R2 C1s C-C : 284.8 eV C-O : 286.7 eV Immobilized TiO 2 C-C C=O : 288.7 eV R1 Immobilized TiO /PEG R2 C-C 284.8 eV C-O C=O C-O 286.7 eV C=O C-O C=O 288.7 eV 1195 1200 1205 3000 2000 1000 292 288 284 280 -1 Wavelength (cm ) 4000 3000 2000 1000 292 288 284 280 276 Binding Energy (eV) -1 Wavelength (cm ) Binding energy (eV) Figure 2. The spectrum for: (a) the Fourier transform infrared spectroscopy (FT-IR) for immobilized Figure 2. The spectrum for: (a) the Fourier transform infrared spectroscopy (FT-IR) for immobilized TiO2 and TiO2/PEG samples; and (b) X-ray photoelectron spectroscopy (XPS) deconvolution peaks of TiO and TiO /PEG samples; and (b) X-ray photoelectron spectroscopy (XPS) deconvolution peaks of 2 2 C1s for immobilized TiO2/PEG. C1s for immobilized TiO /PEG. In this study, the C1s spectra of XPS analysis revealed the significant presence of a C=O bond In this study, the C1s spectra of XPS analysis revealed the significant presence of a C=O bond according to its characteristic binding energy at 288.7 eV. Other peaks detected were found to be according to its characteristic binding energy at 288.7 eV. Other peaks detected were found to be C–O C–O bond and C–C bond observed at 286.7 and 284.8 eV, respectively. This finding is very important bond and C–C bond observed at 286.7 and 284.8 eV, respectively. This finding is very important because because C=O and C–O bonds were the two main species responsible for the wider photoactivity C=O and C–O bonds were the two main species responsible for the wider photoactivity response response of TiO2 photocatalyst in the VL spectrum [30]. Due to the C=O and C–O bonds in immobilized TiO2/PEG, the immobilized sample is able to have a good photoactivity efficiency under both NL and VL irradiation. The bonds act as electron injectors that speed up the Tr ansmit tance (%) Transmittance % CPS CPS 20 30 40 50 60 70 Appl. Sci. 2017, 7, 508 5 of 10 photodegradation process when they are adsorbed on TiO2 surface and become excited by VL. From here, new electrons are formed and are injected to the conduction band of the TiO2 photocatalyst. The conduction band transferred the electrons from the bonds to electron acceptors on the immobilized TiO2/PEG surface. The excited electrons in the conduction band then performed a series of chain reactions that contribute to enhanced performance of immobilized TiO2/PEG under Appl. Sci. 2017, 7, 508 5 of 10 VL spectrum. 3.2. Photoactivity Tests of Immobilized TiO2 and TiO2/PEG under Different Light Irradiations of TiO photocatalyst in the VL spectrum [30]. Due to the C=O and C–O bonds in immobilized TiO /PEG, the immobilized sample is able to have a good photoactivity efficiency under both NL and The degradation of MB dye under NL and VL irradiations was used to study the photocatalytic VL irradiation. The bonds act as electron injectors that speed up the photodegradation process when activity of immobilized TiO2 and TiO2/PEG samples. Under identical conditions, it was found that they are adsorbed on TiO surface and become excited by VL. From here, new electrons are formed only 18.7% of MB dye solution was adsorbed on the immobilized TiO2 in the dark after 30 min, while and are injected to the conduction band of the TiO photocatalyst. The conduction band transferred the amount of MB removed by the TiO2/PEG was 47.4% [26]. This suggests that the greater the electrons from the bonds to electron acceptors on the immobilized TiO /PEG surface. The excited adsorption of MB on TiO2/PEG as compared to immobilized TiO2 is attributed to the higher surface electr area of TiO ons in the 2/Pconduction EG (Brunau band er–Em then meperformed tt–Teller (BET a series ) suof rface chain are ra eactions of the p that hot cont ocat ribute alyststo : T enhanced iO2/PEG, 2 2 performance of immobilized TiO /PEG under VL spectrum. 88 m /g; TiO2, 49 m /g). As can be seen in Figure 3a, the immobilized TiO2/PEG sample exhibited higher photocatalytic activities than immobilized TiO2 under NL irradiation, with the dye remaining 3.2. Photoactivity Tests of Immobilized TiO and TiO /PEG under Different Light Irradiations 2 2 for immobilized TiO2/PEG being 0.29 as compared to 0.58 for immobilized TiO2 in 15 min of irradiation time. At this stage, the photoactivity efficiency of immobilized TiO2 was proven to be two The degradation of MB dye under NL and VL irradiations was used to study the photocatalytic times better than immobilized TiO2. The high photoactivity efficiency rate of immobilized TiO2/PEG activity of immobilized TiO and TiO /PEG samples. Under identical conditions, it was found that 2 2 can first be attributed to the synergistic effect of adsorption and photocatalysis [27]. Results revealed only 18.7% of MB dye solution was adsorbed on the immobilized TiO in the dark after 30 min, that the enlarged surface area was dominant in the TiO2/PEG film and was found to be twice as large while the amount of MB removed by the TiO /PEG was 47.4% [26]. This suggests that the greater as the latter. The washings of immobilized TiO2/PEG film improved the photocatalytic ability in adsorption of MB on TiO /PEG as compared to immobilized TiO is attributed to the higher surface 2 2 degrading the MB under NL illumination. Through the photooxidation process in washing, the PEG area of TiO /PEG (Brunauer–Emmett–Teller (BET) surface area of the photocatalysts: TiO /PEG, 2 2 2 2 became oxidized and created more pores and active sites on the TiO2 surface. Thus, the photoactivity 88 m /g; TiO , 49 m /g). As can be seen in Figure 3a, the immobilized TiO /PEG sample exhibited 2 2 −1 −1 efficiency rate for immobilized TiO2/PEG was found to be 0.065 min , and 0.055 min for higher photocatalytic activities than immobilized TiO under NL irradiation, with the dye remaining immobilized TiO2. It is obvious that the k value for immobilized TiO2/PEG is 1.2 times higher than for immobilized TiO /PEG being 0.29 as compared to 0.58 for immobilized TiO in 15 min of irradiation 2 2 the immobilized TiO2 sample. time. At this stage, the photoactivity efficiency of immobilized TiO was proven to be two times better In addition, the decolorization for immobilized TiO2 under VL irradiation was nearly a straight than immobilized TiO . The high photoactivity efficiency rate of immobilized TiO /PEG can first be 2 2 line, as presented in Figure 3b. Through the pseudo first-order equation, the photoactivity efficiency attributed to the synergistic effect of adsorption and photocatalysis [27]. Results revealed that the −1 rate of immobilized TiO2/PEG was found to be 0.012 min . This result is in contrast to immobilized enlarged surface area was dominant in the TiO /PEG film and was found to be twice as large as the TiO2 photocatalyst, where it was inactive under VL irradiation due to its large band gap energy. latter. The washings of immobilized TiO /PEG film improved the photocatalytic ability in degrading From Figure 3b, the dye remaining for immobilized TiO2/PEG was colourless in only 180 min of the MB under NL illumination. Through the photooxidation process in washing, the PEG became irradiation. Compared to immobilized TiO2, the remaining MB was found to be 0.75 in the same oxidized and created more pores and active sites on the TiO surface. Thus, the photoactivity efficiency 1 1 conditions. Thus, the immobilized TiO2/PEG sample performed better photodegradation as rate for immobilized TiO /PEG was found to be 0.065 min , and 0.055 min for immobilized TiO . 2 2 compared to immobilized TiO2 under VL spectrum. This is mainly because of the presence of C=O It is obvious that the k value for immobilized TiO /PEG is 1.2 times higher than the immobilized and C–O bonds that contributed to the VL response ability in immobilized TiO2/PEG. TiO sample. a) b) 1 1 Immobilized TiO2 Immobilized TiO2/PEG Immobilized TiO2 Immobilized TiO2/PEG 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 0 1530 4560 050 100 150 200 Time (min) Time (min) Figure Figure 3. 3. Immobilized ImmobilizedT TiO iO 2and andT T iO iO2 /PEG /PEG sample for MB sample for MB photodegradation photodegradation under: ( under: (a a)) normal normal light light 2 2 irradiation irradiation and ( and (b b )) visible visiblelight light irradiat irradiation ion at at MB MBo = 12 = 12 ppm, ppm, pH pH 11, 28 °C. 11, 28 C. In addition, the decolorization for immobilized TiO under VL irradiation was nearly a straight line, as presented in Figure 3b. Through the pseudo first-order equation, the photoactivity efficiency rate of immobilized TiO /PEG was found to be 0.012 min . This result is in contrast to immobilized TiO photocatalyst, where it was inactive under VL irradiation due to its large band gap energy. From Figure 3b, the dye remaining for immobilized TiO /PEG was colourless in only 180 min of irradiation. Compared to immobilized TiO , the remaining MB was found to be 0.75 in the same Decolorizat ion (C/Co) Decolorization (C/Co) Appl. Sci. 2017, 7, 508 6 of 10 conditions. Thus, the immobilized TiO /PEG sample performed better photodegradation as compared to immobilized TiO under VL spectrum. This is mainly because of the presence of C=O and C–O bonds that contributed to the VL response ability in immobilized TiO /PEG. Appl. Sci. 2017, 7, 508 2 6 of 10 3.3. Effects of Aeration Rate 3.3. Effects of Aeration Rate Aeration flow rate affects the photoactivity efficiency of immobilized TiO /PEG because it acts as Aeration flow rate affects the photoactivity efficiency of immobilized TiO2/PEG because it acts an oxygen supply for the photocatalyst [31]. The effect of aeration flow rate was identified based on as an oxygen supply for the photocatalyst [31]. The effect of aeration flow rate was identified based first-order rate constant (k) value against flow rate and is shown in Table 1, and decolorization (C/C ) on first-order rate constant (k) value against flow rate and is shown in Table 1, and decolorization is shown in Figure 4, respectively. With PEG and DSAT in immobilized TiO /PEG, the concentration (C/Co) is shown in Figure 4, respectively. With PEG and DSAT in immobilized TiO2/PEG, the of MB dropped from 12 to 0 ppm in one hour with the highest removal efficiency rate at 75 mL/min. concentration of MB dropped from 12 to 0 ppm in one hour with the highest removal efficiency rate As can be seen in Figure 4, the dye remaining was up to 0.29 in just 15 min as compared to 0.53, 0.36, at 75 mL/min. As can be seen in Figure 4, the dye remaining was up to 0.29 in just 15 min as and 0.62 for 100, 50, and 25 mL/min aeration, respectively. The value of photodegradation efficiency (k) compared to 0.53, 0.36, and 0.62 for 100, 50, and 25 mL/min aeration, respectively. The value of at 0.0347, 0.058, 0.068, and 0.053 min for 25, 50, 75, and 100 mL/min−1flow rate accordingly indicates photodegradation efficiency (k) at 0.0347, 0.058, 0.068, and 0.053 min for 25, 50, 75, and 100 mL/min that the photocatalytic degradation of MB increased with increased aeration up to 75 mL/min; beyond flow rate accordingly indicates that the photocatalytic degradation of MB increased with increased that, however, the k value dropped by 0.22 to become 0.053 min at 100 mL/min. In other words,−1 aeration up to 75 mL/min; beyond that, however, the k value dropped by 0.22 to become 0.053 min at 75 mL/min, optimum flow rate for photodegradation of MB reached as high as 0.068 min of at 100 mL/min. In other words, at 75 mL/min, optimum flow rate for photodegradation of MB removal efficiency. High aeration −1 flow rate supply produced a high oxygen concentration and reacted reached as high as 0.068 min of removal efficiency. High aeration flow rate supply produced a high with excited electrons from photon flux, producing O anions. The production of O caused the 2 2 oxygen concentration and reacted with excited electrons from photon flux, producing O2 anions. photodegradation rate of MB to increase as the aeration flow rate increased [32]. However, the excessive The production of O2 caused the photodegradation rate of MB to increase as the aeration flow rate flow rate beyond 75 mL/min affected the penetration of light due to the production of a huge amount increased [32]. However, the excessive flow rate beyond 75 mL/min affected the penetration of light of bubbles during the photocatalysis irradiation. due to the production of a huge amount of bubbles during the photocatalysis irradiation. Table 1. The kinetic constants (k) of decolorizing MB at different aeration flow rates for optimum Table 1. The kinetic constants (k) of decolorizing MB at different aeration flow rates for optimum loading of 0.3 g of immobilized TiO /PEG. loading of 0.3 g of immobilized TiO2/PEG 1 2 −1 2 Aeration (mL/min) Aeration (mL/min) k,k Rate , Rate Consta Constantnt (min (min )) Correlation Correlati Coef on Co ficient effici(ent ( R ) R ) 25 0.035 0.9969 25 0.035 0.9969 50 0.058 0.9950 50 0.058 0.9950 75 0.068 75 0.068 0.99130.9913 100 0.053 0.9940 100 0.053 0.9940 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 1020 304050 6070 Time (min) Figure 4. Aeration flow rate (mL/min) of immobilized TiO /PEG photocatalyst film. Figure 4. Aeration flow rate (mL/min) of immobilized TiO 22/PEG photocatalyst film. 3.4. 3.4. Ef Effects fects of of pH pH Resear Researche chers rs have have p paid aid m much uch attention attention to studying the effect to studying the effect of of pH on the photocatalytic pH on the photocatalytic degradation degradation ofof d dyes yes bec because ause the the re real wastewater al wastewater effluences effluenc from es findustries rom indus may tries may not be have not be have a neutrala pH neutral pH (pH 7). More (pH 7). Moreover, the pH over, the pH of the reac of the reaction mixturetiinfluences on mixture theinfluence surface-char s the sur ged- f pr acoperties e-charged of- properties of the photocatalysts. The effect of pH is important to study, as it impacts the surface state of titanium and the ionization state of the dye molecules [33]. In this study, the effect of pH on MB degradation was evaluated at various pH conditions (i.e., 3, 6, 8, 10, 11, and 12). The significant effect of pH on the calculated pseudo first-order rate constant for the photocatalytic decolourization of MB is depicted in Figure 5a. It was observed that the photocatalytic degradation rate increased by increasing the pH value. pH 11 showed the highest photocatalytic activity as compared to other pH. Decolorization (C/Co) Appl. Sci. 2017, 7, 508 7 of 10 the photocatalysts. The effect of pH is important to study, as it impacts the surface state of titanium and the ionization state of the dye molecules [33]. In this study, the effect of pH on MB degradation was evaluated at various pH conditions (i.e., 3, 6, 8, 10, 11, and 12). The significant effect of pH on the Appl. Sci. 2017, 7, 508 7 of 10 calculated pseudo first-order rate constant for the photocatalytic decolourization of MB is depicted in Figure 5a. It was observed that the photocatalytic degradation rate increased by increasing the This is also due to the significant effect of high adsorption existing between positively-charged pH value. pH 11 showed the highest photocatalytic activity as compared to other pH. This is also cationic MB dye with the negative charge on the surface of immobilized TiO2/PEG. As can be seen in due to the significant effect of high adsorption existing between positively-charged cationic MB dye Figure 5, there is a difference in the values of MB photodegradation rate between pH 4 and pH 8. with the negative charge on the surface of immobilized TiO /PEG. As can be seen in Figure 5, there Figure 5b shows the plot graph of the pH value corresponding to the point of zero charge (pHpzc) of is a difference in the values of MB photodegradation rate between pH 4 and pH 8. Figure 5b shows immobilized TiO2/PEG surface where the value of pHpzc is 6.5. This means that the surface charge of the plot graph of the pH value corresponding to the point of zero charge (pH ) of immobilized pzc immobilized TiO2/PEG is positive in acidic pH (pH < 6.5) and negatively charged in alkaline pH TiO /PEG surface where the value of pH is 6.5. This means that the surface charge of immobilized 2 pzc (pH > 6.5). The pHpzc is a pH value where the net electrical charge of the photocatalyst is zero and is TiO /PEG is positive in acidic pH (pH < 6.5) and negatively charged in alkaline pH (pH > 6.5). used to qualitatively assess the TiO2 surface charge. Due to the negatively-charged TiO2/PEG surface The pH is a pH value where the net electrical charge of the photocatalyst is zero and is used to pzc in alkaline conditions, adsorption dominates between the negative TiO2 surface and the cationic MB qualitatively assess the TiO surface charge. Due to the negatively-charged TiO /PEG surface in 2 2 dye (pKa < 2) [34]. Hence, a pH value lower that 6.5 will cause a poor degradation of MB dye, and alkaline conditions, adsorption dominates between the negative TiO surface and the cationic MB dye this explains the gap in photocatalytic degradation rate obtained beyond pH 6.5 seen in Figure 5b. (pK < 2) [34]. Hence, a pH value lower that 6.5 will cause a poor degradation of MB dye, and this −1 However, the photoactivity efficiency decreased at pH 12 to become 0.062 min due to the rapid explains the gap in photocatalytic degradation rate obtained beyond pH 6.5 seen in Figure 5b. However, adsorption of dye which covered the photocatalyst surface, thus resulting in a slower photocatalytic the photoactivity efficiency decreased at pH 12 to become 0.062 min due to the rapid adsorption of activity [35]. Nonetheless, the removal rate of MB by TiO2/PEG film was largely attributed to dye which covered the photocatalyst surface, thus resulting in a slower photocatalytic activity [35]. adsorption, since the visible blue-stained surface indicated that the film was not fully Nonetheless, the removal rate of MB by TiO /PEG film was largely attributed to adsorption, since the photocatalytically degraded. This is due to the agglomeration of MB, which reduced the amount of visible blue-stained surface indicated that the film was not fully photocatalytically degraded. This is light needed for photocatalysis [36]. Thus, the optimal pH obtained in this study was discovered to due to the agglomeration of MB, which reduced the amount of light needed for photocatalysis [36]. be at pH 11. Thus, the optimal pH obtained in this study was discovered to be at pH 11. a) 0.1 b) 0.1 1 0.5 0.08 0.08 01 23 45 67 89 10 11 0.06 0.06 -0.5 0.04 -1 0.04 -1.5 pH pzc 0.02 0.02 -2 -2.5 24 68 10 12 02468 10 12 pH Initial pH pH (a) (b) Figure 5. Immobilized TiO2/PEG photocatalyst film for: (a) Initial pH effect and (b) plot of the Figure 5. Immobilized TiO /PEG photocatalyst film for: (a) Initial pH effect and (b) plot of the pH pH value corresponding to the point of zero charge. value corresponding to the point of zero charge. 4. Conclusions 4. Conclusions In our study, immobilized TiO2/PEG best demonstrated its properties in normal light In our study, immobilized TiO /PEG best demonstrated its properties in normal light applications −1 applications at 75 mL·min aeration and pH 11. Due to the C=O and C–O bonds found in at 75 mLmin aeration and pH 11. Due to the C=O and C–O bonds found in immobilized TiO /PEG, immobilized TiO2/PEG, the photoactivity tests were well performed under NL and VL irradiation. the photoactivity tests were well performed under NL and VL irradiation. The bonds accelerated The bonds accelerated the photoactivity efficiency by acting as an electron injector. It was also found the photoactivity efficiency by acting as an electron injector. It was also found that the presence of that the presence of PEG and DSAT assisted in the high removal efficiency of MB dye due to the ease PEG and DSAT assisted in the high removal efficiency of MB dye due to the ease of usage and good of usage and good photocatalytic activity, as reported in our previous study. The findings promised photocatalytic activity, as reported in our previous study. The findings promised a potent feasible a potent feasible application in photocatalysis regarding the simple deposition method as well as the application in photocatalysis regarding the simple deposition method as well as the incorporation of incorporation of a pore matrix polymer like PEG. For future recommendation, some works will be a pore matrix polymer like PEG. For future recommendation, some works will be developed to enhance developed to enhance the photocatalytic ability of immobilized TiO2/PEG sample. First, the the photocatalytic ability of immobilized TiO /PEG sample. First, the mechanism of PEG-DSAT will mechanism of PEG-DSAT will be studied as to why and how the C–O in PEG turned to a C=O bond. be studied as to why and how the C–O in PEG turned to a C=O bond. Then, the effect of sintering or Then, the effect of sintering or calcination under elevated temperature will also be conducted. It is calcination under elevated temperature will also be conducted. It is also necessary to further exploit the also necessary to further exploit the bulk properties modification of TiO2/PEG DSAT, and further study of its overall kinetic and adsorption isotherm would be beneficial. Next, the novel low heat-brush coating method will be re-adopted in order to create a smooth and uniform layer of immobilized TiO2/PEG before further implementation in an industrial reactor such as semi-batch photocatalytic reactor [37]. The increased specific surface area of PEG 6000 in immobilized TiO2/PEG should contribute to a closer and full contact of dye pollutant with the photocatalyst sample; hence, enhancing the efficiencies of photo-generated charge carriers should effectively improve the -1 K (min ) -1 K (min ) Discrepancy pH Discrep ancy p H Appl. Sci. 2017, 7, 508 8 of 10 bulk properties modification of TiO /PEG DSAT, and further study of its overall kinetic and adsorption isotherm would be beneficial. Next, the novel low heat-brush coating method will be re-adopted in order to create a smooth and uniform layer of immobilized TiO /PEG before further implementation in an industrial reactor such as semi-batch photocatalytic reactor [37]. The increased specific surface area of PEG 6000 in immobilized TiO /PEG should contribute to a closer and full contact of dye pollutant with the photocatalyst sample; hence, enhancing the efficiencies of photo-generated charge carriers should effectively improve the photocatalysis performance [38]. Therefore, the effect of different flat supports (e.g., graphite plate, stainless steel plate, and ceramic plate) on the photocatalysis performance of immobilized TiO /PEG will be studied. Under suitable conditions, the utilization of graphite plate could possibly trigger a photo-electro-catalytic process in immobilized TiO /PEG, which is more powerful than a conventional oxidation method [39]. Studies on the photodegradation of a complex molecule (i.e., phenol) as a general pollutant in real waste water is also essential to expand the application of immobilized TiO /PEG to a wider range of situations, instead of one specific readily-oxidized pollutant and its by-products [40]. It also becomes of great importance to explore the existence of other species in immobilized TiO /PEG (i.e., C=C) that may contribute to an expanded photo response from ultraviolet (UV) spectrum to VL spectrum of TiO photocatalyst [41]. Acknowledgments: We would like to thanks the Malaysian Ministry of Education (KPM) for providing generous financial support under REI grants: 600-IRMI/DANA 5/3/REI (1/2017) in conducting this study and Universiti Teknologi Mara (UiTM) for providing all the needed facilities. Author Contributions: The experimental work and drafting of the manuscript was carried out by Raihan Zaharudin and assisted by Dyia Syaleyana Shukri, Tun Firdaus Azis, Zainab Razali and Ahmad Zuliahani participated in the interpretation of the scientific results and the preparation of the manuscript. Wan Izhan Nawawi supported the work and cooperation between UiTM Perlis and UiTM Shah Alam, supervised the experimental work, commented and approved the manuscript. The manuscript was written through comments and contributions of all authors. All authors have given approval to the final version of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. References 1. Ali, R.; Siew, O.B. Photodegradation of new methylene blue N in aqueous solution using zinc oxide and titanium dioxide as catalyst. J. Teknol. 2007, 45, 31–41. [CrossRef] 2. Joshi, K.; Shrivastava, V. Removal of methylene blue dye aqueous solution using photocatalysis. Int. J. 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