Abstract: Titanium dioxide (TiO2) powder and TiO2 nanotubes (NTs) were used as the catalysts for sonocatalytic degradation of Rhodamine B in aqueous solution. The effect of parameters such as initial concentration of between 50-150 mg L-1, catalyst loading of between 500-3000 mg L-1, solution temperature of between 20-70°C, hydrogen peroxide (H2O2) concentration of between 4-16 mM and the presence of air flow at ultrasonic power of 50 W and 35 kHz had been investigated. Then, the optimum condition for sonocatalytic degradation Rhodamine B had been established. Higher sonocatalytic degradation efficiency of Rhodamine B can be achieved through TiO2 nanotubes as compared to TiO2 powder. The experimental results show that the best degradation efficiency of Rhodamine B can be obtained when the experimental conditions occurred at the initial concentration of 50 and 2000 mg L-1 of TiO2 NTs, 8 mM of H2O2, ultrasonic power of 50 W and frequency of 35 kHz with the presence of air flow which give the efficiency of above 85% in 120 min.
INTRODUCTION
Presently, there is over 7x105 tons annual worldwide production of dyes has been reported in the literature and it is estimated that 10-15% of the dyestuff is lost in the effluent during the dyeing processes (Abdullah and Ling, 2010). Methods such as coagulation, filtration and adsorption by activated carbon are only able to transfer the organic pollutants from one phase to another (Inoue et al., 2006). Besides, the presence of one or several benzene rings in the dye structure can always cause conventional treatment process to be ineffective in degrading and destroying these organic pollutants. Aromatic ring is a highly stable compound which is hardly degraded by using chemical and biological methods (Wang et al., 2007c). Therefore, a lot of studies have been dedicated to treat this wastewater by using advanced oxidation process which involves hydroxyl (•OH) radicals to mineralize the organic compounds (Abbasi and Asl, 2008; Harir et al., 2009; Shimizu et al., 2007; Wang et al., 2007b).
The method of ultrasonic irradiation is usually used to treat some dyestuff wastewaters owing to its advantages such as cleanness, high efficiency and non-secondary pollution generated (Wang et al., 2007a). Application of ultrasounds in water will cause the chemical effect of acoustic cavitation which represented by the formation, growth and implosive collapse of cavities. The temperature and pressure could rise up to 5000 K and 100 MPa, respectively inside the cavitation bubbles (Abdullah and Ling, 2010; Merouani et al., 2010). When water is sonicated, some reactive radicals such as •OH, •H and •OOH are produced, the former radical being a strong oxidizing agent which able to react actively with organic compounds.
In the sonocatalytic degradation of organic dye, the TiO2 was used as sonocatalyst because of its biological and chemical properties such as high catalytic activity, high chemical stability, inexpensive and low toxicity (Deng et al., 2009; Wang et al., 2007a). Many researchers have shown that the presence of TiO2 could increase the sonocatalytic degradation of organic pollutants (Abdullah and Ling, 2010; Wang et al., 2007a; Tuziuti et al., 2005). Recently, there are a few studies using modification of TiO2 such as mixing with other elements (rare metals, rare-earth metals) through doping or composite method in order to restrain the recombination of generated holes and electron pairs in addition to reduce the energy band gap of anatase TiO2 powder (Wang et al., 2008b, 2009, 2010). This could enhance the sonocatalytic degradation efficiency of organic dyes.
The aim of this study is to investigate the effect of sonocatalytic degradation using titanium dioxide nanotubes (TiO2 Nts) as compared to TiO2 powder. In this study, the TiO2 NTs were prepared by the combination of sol-gel process with hydrothermal treatment before treating the organic dye in aqueous solution under ultrasonic irradiation. Recently, extensive researches have been conducted on the synthesis of TiO2 NTs because of their novel properties compared to TiO2 powder such as high aspect ratio, high surface to volume ratio, high sedimentation rate and versatile chemistry with the conventional advantages and properties of TiO2 (Menzel et al., 2006; Poudel et al., 2005). The special feature of tubular structure with high aspect ratio provide more opportunities for separation of electron-hole pairs to enhance the electron transfer and improve the interpenetration of holes transport materials due to its delocalization of charge carriers in nanotubes (Menzel et al., 2006; Tachikawa et al., 2006). Thereby, this might help to increase the sonocatalytic degradation efficiency.
MATERIALS AND METHODS
Materials and apparatus: Rhodamine B was purchased from Sigma-Aldrich and was chosen as a model organic compound. Hydrochloric acid, HCl 37%, sodium hydroxide, NaOH (99% purity), tetra-n-butyl-orthotitanate, C16H36O4Ti or Ti(OBu)4 for synthesis (≥98% purity), ethanol, CH3CH2OH (96% extra purity), hydrogen peroxide solution, H2O2 (30% w/w) were obtained from Merck. Potassium hydroxide, KOH (≥98 % purity) was purchased from System Chemical Company. The controllable serial-ultrasonic bath apparatus (Elma Transsonic series TI-H5) was operated to irradiate the aqueous solution of Rhodamine B at a frequency of 35 kHz and effective power output up to 100 W through manual adjusting. A general schematic diagram of experimental scheme is illustrated in Fig. 1. The experimental conditions adopted throughout the whole experiments were initial dyes concentration of 50 and 2000 mg L-1 of TiO2 amount and neutral initial pH unless other conditions is noted. The concentrations of the samples were determined using AquamateTM Plus UV-vis spectrophotometer (Thermo Scientific Company) to monitor the degradation of Rhodamine B.
Procedures on catalyst preparation: TiO2 powder was prepared using the sol-gel and subjected to the heat-treatment. Ti(OBu)4 was dissolved in ethanol solution with a volume ratio of 4:1 for 1 h under constant stirring. Then, a known quantity of 0.1 M HCl was added to control the pH of solution to prevent the precipitation of TiO2. The mixture was hydrolyzed at room temperature under stirring for 2 h to obtain a transparent sol. Next, the sol was aged for 2 days at room temperature to obtain a dry gel. The resulting gel was washed with distilled water and dried overnight at 100°C in an oven. After that, the gel powder was calcined at 500°C for 3 h. TiO2 NTs were synthesized using hydrothermal method which was modified by Bavykin et al. (2008).
| Fig. 1: | Schematic illustration of sonocatalytic degradation experiment apparatus |
Three gram of synthesized TiO2 powder was mixed with 98 mL of 10 M NaOH and 2 mL of 10 M KOH solution in a Teflon vessel. The mixture was refluxed at 100°C under stirring for 48 h. The resulted white powder was thoroughly washed with distilled water to remove excess Na+ ions. Then, the powder was repeatedly treated with 0.1 M of HCl until the washing solution reached pH 3. Then, it was washed with water before drying it in an oven for several hours at 100°C. Finally, the dried samples were calcined at a temperature of 300°C for 2 h.
Sonocatalytic reaction and analysis of liquid samples: The prepared 200 mL of Rhodamine B in aqueous solution and TiO2 powder or NTs were put into the glass reactors. Next, the mixed solution was stirred to make a good dispersion of TiO2 for 10 min. After that, the mixed solution was put into the ultrasonic bath to ultrasonic treat the organic compound. Lastly, after certain time of ultrasonic irradiation, the liquid samples were centrifuged three times to separate the TiO2 nanotubes. After that, the liquid sample’s concentration was monitored by UV-vis spectrometer to estimate the degradation efficiency concentration of dye solution.
| (1) |
| Co | = | Initial dye concentration |
| Ct | = | Dye concentration at time t (min) |
RESULTS AND DISCUSSION
The UV-vis spectra of Rhodamine B in the presence of TiO2 NTs and powder: In order to exhibit the degradation of Rhodamine B, the original and degraded Rhodamine B at the interval time of 60 min within 180 min were monitored using UV-visible spectrophotometer in the wavelength of 400 to 800 nm. Figure 2a and b show the UV-visible spectra for the degradation efficiency of Rhodamine B in the presence of TiO2 powder and TiO2 NTs, respectively. The maximal absorbency of 40 mg L-1 Rhodamine B solution obeyed by Lambert-Beer’s law and the calibration curve of Rhodamine B solutions (figure not shown) were used to estimate the degradation efficiency of Rhodamine B at higher concentration of dye. It was observed that there was only a slightly decreased in the absorbance peak when ultrasonic irradiation using TiO2 powder was employed after 180 min (Fig. 2a). On the other hand, the absorbance peak of Rhodamine B solutions at 554 nm were decreased sharply along the ultrasonic irradiation time in the presence of TiO2 NTs. After 180 min of ultrasonic irradiation, the absorbance peak at 554 nm decrease significantly in the presence of TiO2 NTs as shown in Fig. 2b. The slightly shifted peak from 554 to 550 nm appeared in Fig. 2b is due to the intermediate products formed during ultrasonic irradiation (Wang et al., 2007b). However, the details on the intermediate products forms do not reveal through the absorbance in the UV-vis spectra.
Effect of ultrasonic irradiation time: The effect of ultrasonic irradiation time of the degradation of Rhodamine B was studied under several experimental conditions. The sonolysis of Rhodamine B in the absence of TiO2 catalyst was act as a control experiment. It was found that the presence of TiO2 play a significant role in the ultrasonic irradiation. Figure 3 shows that ultrasonic treatment with the presence of TiO2 had accelerated the degradation efficiency of organic dye. This explanation was supported by Priya and Madras (2006) which claimed that the existence of particles in liquid would provide a nucleation site for cavities formation due to its surface roughness. This may increase the dissociation of H2O molecules to form more •OH radicals which are highly reactive and therefore short-lived.
Meanwhile, it was observed that Rhodamine B degraded faster in the presence of TiO2 NTs (85%) than TiO2 powder (25%). This could be attributed to its higher specific surface area and porosity characteristic which provide more nucleation sites for cavitation bubble formation as compared to TiO2 powder. Besides, the inner and outer surfaces of TiO2 Nts could increase, which resulted in more surfaces available to restrain the recombination of electron-hole pairs. Tachikawa et al. (2006) reported that TiO2 in the form of NTs.
| Fig. 2: | UV-vis spectra of Rhodamine B solution in the presence of (a) TiO2 powder and (b) TiO2 Nts |
| Fig. 3: | Effect of irradiation time on the sonocatalytic degradation of Rhodamine B. (Initial dye concentration = 50 mg L-1, catalyst loading = 2.0 g L-1, solution temperature = 30°C) |
Could increase of delocalization of charge carriers in nanotubes where the separated electrons and holes were free to move throughout the length of the crystal in order to reduce the recombination probability.
| Fig. 4: | The reaction kinetics plot for the sonocatalytic degradation of Rhodamine B |
These holes not only can decompose the organic dyes adsorbed on the surface of TiO2 particles directly, but also could accelerate the generation of •OH radicals through oxidizing H2O molecules in the bulk solution (Wang et al., 2007b).
In addition, the degradation kinetics of Rhodamine B in aqueous solution by using ultrasonic irradiation with and without TiO2 NTs or TiO2 powder was investigated. As shown in Fig. 4, the results are presented in the form of a plot of ln(Co/Ct) as a function of irradiation time (t) indicated that the degradation of Rhodamine B satisfactory followed a pseudo-first-order kinetics. The pseudo-first-order rate constant, k values and corresponding regression coefficients (R2) under various operational conditions are listed in Table 1. It was observed that there was a double increased in k value when ultrasonic was applied in the presence of TiO2 as compared to the control experiment (without TiO2). Again, it was clearly shown that k value for the case of ultrasonic irradiation in the presence of TiO2 NTs was significant higher than in the presence of TiO2 powder. This implied that the degradation rate of Rhodamine B was higher and could achieve higher degradation efficiency in a shorter time. Similar degradation kinetic for reported for the photocatalytic degradation (Grzechulska and Morawski, 2002) and sonocatalytic degradation of organic dyes (Abdullah and Ling, 2010; Abbasi and Asl, 2008).
Effect of initial dye concentration: Initial concentrations of Rhodamine B solution ranging from 50 to 150 mg L-1 were used to review the effect of initial dye concentration on the degradation efficiency of Rhodamine B. Results from Fig. 5 shows that the degradation efficiency decreased with the increasing of initial dye concentration in the presence of either TiO2 powder or TiO2 NTs. In this respect, the sonocatalytic degradation process showed some similarities with those of a photocatalytic process (Grzechulska and Morawski, 2002; Harir et al., 2009).
| Fig. 5: | Effect of initial dye concentration on the sonocatalytic degradation of Rhodamine B. (Catalyst loading = 2 g L-1, solution temperature = 30°C, reaction time = 180 min) |
| Table 1: | Rate coefficients for the sonocatalytic degradation of Rhodamine B under various conditions |
This might be due to the excessive adsorption of the Rhodamine B molecules on the surface of TiO2 until it was fully covered the surface of TiO2. Besides, when increasing the initial dye concentration, the mutual screens between Rhodamine B molecules and TiO2 increased too. These two phenomena would prohibit TiO2 from absorbing heat and energy resulting from the acoustic cavitation and also disturbed the transmission of ultrasound. Thus, this would limit the •OH radicals that could attack certain dye molecules and leading to decrease in the degradation efficiency.
Effect of TiO2 loading: In order to optimize the addition amount of TiO2 catalyst to achieve highest sonocatalytic degradation efficiency, the addition amount starting from 500 to 3000 mg L-1 were adopted. Results from Fig. 6 show that the degradation efficiency of Rhodamine B rised sharply when the addition amount of TiO2 NTs increased from 500 to 2500 mg L-1. It began to decline after addition amount was increased to 3000 mg L-1 and further. Similar trend occurred when using TiO2 powder. This phenomenon can be explained by the fact that the excessive TiO2 particles caused TiO2 particles to come nearness and aggregation. This would result the decreasing number of surface active sites at TiO2. In addition, mutual screens among TiO2 increased that would make the Rhodamine B molecules received extra protection.
| Fig. 6: | Effect of catalyst loading on the sonocatalytic degradation of Rhodamine B. (Initial dye concentration = 50 mg L-1, solution temperature = 30°C, reaction time = 180 min) |
Then, it would prohibit TiO2 from absorbing heat and energy resulting from the acoustic cavitation (Abdullah and Ling, 2010). Consequently, it might result in a small chance that dye molecules encountered •OH radicals produced by ultrasound. Such a phenomenon was not observed in this study. The amount of TiO2 NTs required (2000 mg L-1) to achieve highest sonocatalytic degradation efficiency was found more than the amount of TiO2 powder (1500 mg L-1). The different optimal catalyst adding amount for TiO2 NTs and TiO2 powder was generally depended on the nature of the catalyst, organic pollutant and other many factors (Wang et al., 2006a).
Effect of the solution temperature: The effect of aqueous temperature ranges from 20 to 70°C on the sonochemical reaction rate were investigated at ultrasonic frequency of 35 kHz with an ultrasonic power of 50 W. Figure 7 shows that as the solution temperature increased from 20 to 70°C, degradation rate of Rhodamine B decreased sharply and slowly in the presence of TiO2 Nts and powder, respectively. Similar decreasing trend had been reported by Jiang et al. (2006) and Wang et al. (2006a) on the sonocatalytic degradation of 4-chlorophenol and methyl parathion, respectively. It was reported that an increase in temperature will lead to an increase in the possibility of coalescence among the cavitation bubbles, resulting in some of the bubbles losing their activity (Jiang et al., 2006).
Besides, the tiny cavitation bubbles would volatilize from the solution before growing and collapsing under ultrasonic irradiation when increasing the solution temperature (Wang et al., 2006a). Another reason was due the fact that high temperature would prohibit the absorbability of organic dyes on the surface of TiO2.
| Fig. 7: | Effect of solution temperature on the sonocatalytic degradation
of Rhodamine B. (Initial dye concentration = 50 mg L-1, TiO2
NTs = 2 g L-1 or TiO2 powder = 1.5 g L-1,
reaction time = 180 min) |
The proper quantities of organic dyes adsorbed on the surface of TiO2 catalyst can directly decomposed by the holes (Wang et al., 2006b). These entire phenomenons would decrease the degradation efficiency as the solution temperature was increased. Higher degradation efficiency was obtained at lowest solution temperature but it was tough to maintain its low temperature for longer irradiation time. Thus, the room solution temperature was chosen for our next study.
Effect of addition of H2O2: The sonocatalytic degradation of Rhodamine B in the presence of either TiO2 NTs or powder was studied for comparison of the different addition amount of H2O2 ranges between 4 to 16 mM. From Fig. 8, it was found that the degradation rate of Rhodamine B in the presence of either TiO2 powder or TiO2 NTs increased when H2O2 concentration was increased up to 8 mM but at higher levels, there was a drop in the degradation rate. Similar trend of results were reported for sonocatalytic degradation of Congo Red (Abdullah and Ling, 2010), Methylene Blue (Abbasi and Asl, 2008; Shimizu et al., 2007) and photocatalytic degradation of organic pollutant (Harir et al., 2009) in the presence of TiO2 and H2O2. The addition of H2O2 below 8 mM could increase the formation rate of •OH radicals in two ways. Firstly, the reduction of H2O2 at the conduction band (Eq. 2) and secondly was the self-decomposition of H2O2 to produce more •OH radicals (Eq. 3) (Abbasi and Asl, 2008).
| Fig. 8: | Effect of the added amount H2O2 on the
sonocatalytic degradation of Rhodamine B. (Initial dye concentration = 50
mg L-1, TiO2 NTs = 2 g L-1 or TiO2
powder = 1.5 g L-1, solution temperature = 30°C, reaction
time = 120 min) |
This could enhance the degradation rate of Rhodamine B in aqueous solution. Another reason might be due to relatively easier decomposition of into •OH radicals under ultrasonic irradiation as the dissociation energy for the O-O bond in H2O2 is 210 kJ mol-1 which is less than that of the O-H bond in H2O that is 4663 kJ mol-1 (Wang et al., 2008a).
| (2) |
| (3) |
It is a fact that H2O2 molecule could act as a hydroxyl source as well as hydroxyl scavenger under sonocatalytic condition (Abdullah and Ling, 2010). Thus, at higher concentration of H2O2 (above 8 mM), the surplus •OH radicals produced from the dissociation of H2O2 might undergo 3 pathways: react with H2O2 and produce less reactive species hydroperoxyl (•OOH) radicals, react with •OOH radicals to form H2O and self-recombination back to form H2O2 (Minero et al., 2005). This could inhibit the production of valence bond holes and •OH radicals that able to cause less organic dye to be degraded.
Effect of the presence of air flow: In order to investigate the effect of dissolved air, four experiments were performed in the absence and presence of air bubbling in the Rhodamine B solution. For all these experiments run, the degradation of Rhodamine B increases with the use of 1 L min-1 of air flow as shown in Fig. 9. This result indicated that the sonocatalytic degradation of Rhodamine B in the presence of H2O2 enhanced as a result of addition of air flow to the solution. The sonocatalytic degradation Rhodamine B using TiO2 NTs with the presence of air increased from 76 to 85%. Similar tend appeared on the degradation when using TiO2 powder as sonocatalyst which increased from 43 to 53%. The increasing degradation rate with the supply of air flow into the solution might be due to the production of •OH and other radicals such as •NO2 and •NO3. Some of the radicals formation that might take place during injection of air into Rhodamine B solutions have been reported by Inoue et al. (2006) and Kidak and Ince (2006). Goel et al. (2004) reported that 10% of these reactive radicals generated in the cavitation bubbles could diffuse into the bulk solution.
| Fig. 9: | Effect of the presence of air flow on the sonocatalytic degradation
of Rhodamine B. Filled and open symbols indicated TiO2 NTs and
TiO2 powder, respectively. (Initial dye concentration = 50 mg
L-1, TiO2 NTs = 2 g L-1 or TiO2
powder = 1.5 g L-1, solution temperature = 30°C, [H2O2]0
= 8 mM) |
Then, the degradation rate of Rhodamine B could increase as non-volatile organic (Rhodamine B) was mostly attacked by •OH radicals in the region of bulk solution and interface bubble-water (Goel et al., 2004; Kidak and Ince, 2006).
CONCLUSIONS
In the present study, it was found that TiO2 NTs had better degradation effect than TiO2 powder in sonocatalytic activity. The highest sonocatalytic degradation efficiency for Rhodamine B (85% within 120 min) occurred at 50 mg L-1 initial concentration of dye with concentration of TiO2 NTs at 2000 mg L-1, concentration of H2O2 at 8 mM, ultrasonic power of 50 W, ultrasonic frequency of 35 kHz, air flow rate of 1 L min-1 and neutral pH at room temperature. Hence, TiO2 NTs material was proven as a highly potential environmental catalyst and can become an alternative choice for large-scale applications for ultrasonic irradiation.
ACKNOWLEDGMENTS
Financial support provided by the Research University (RU) Grant and A Fellowship from Universiti Sains Malaysia are gratefully acknowledged.