Abstract
A new immobilized TiO2 technique was prepared by coating TiO2 solution onto double-sided adhesive tape (DSAT) as a thin layer binder without adding any organic additives. Glass plate was used as support material to immobilized TiO2/DSAT. Two different charges of dyes were applied, namely, anionic reactive red 4 (RR4) and cationic methylene blue (MB) dyes. Photocatalytic degradation of RR4 and MB dyes was observed under immobilized TiO2/DSAT with the degradation rate slightly lower and higher, respectively, compared with TiO2 in suspension mode. It was observed that DSAT is able to provide a very strong intact between glass and TiO2 layers thus making the reusability of immobilized TiO2/DSAT be up to 30 cycles. In fact, a better photodegradation activity was observed by number of cycles due to increasing formation of pores on TiO2 surface observed by SEM analysis.
1. Introduction
Advanced oxidation processes (AOPs) are one of the promising ways to eliminate dangerous pollutants into harmless treated products. Photocatalysis is one of the widely applied processes in advanced oxidation. Titanium dioxide (TiO2) is the most commonly used semiconductor in photocatalysis process due to its high activity, being inert to the biological and chemical environment, nontoxicity, and low cost. Conventional method in the photocatalytic studies is suspension of TiO2 in aqueous solution, which provides high surface to volume ratio [1]. However, the suspension of TiO2 powder caused treated wastewater in slurry form and required the filtration process. Due to its small particles, they stay suspended in water, clogging filter membranes, and penetrate through porous filter materials [2]. Realizing this issue, TiO2 immobilized techniques are implemented onto the support materials such as glass, silica gel, and metals. Numerous additives are also added in TiO2 as a binder to improve durability, temperature resistance, and strong absorbance affinity towards pollutants [3–6].
Recently, researchers used organic additive to promote proper adhesion in immobilized TiO2 substrate. Polyvinyl alcohol (PVA), polyethylene glycol (PEG), polyvinyl chloride (PVC), and polyvinylpyrrolidone (PVP) are examples of polymers that have been employed by previous researchers in their studies [7–10]. The immobilizations by using different coating techniques have been applied by some workers to produce smooth coating as well as thin layer like spinning, doctor blade, dipping, brushing, and spraying [11–15]. It is important to determine an ideal mixing ratio of TiO2 and its binder. Polymer added in TiO2 composite which acts as a binder induces a strong intact among the immobilized TiO2 matrix thus making it reusable over times. However, excessive amount of polymer makes TiO2 embedded in polymer matrix which causes reduction between TiO2 and pollutant surface contact and eventually reduces photocatalysis process. The lower the amount of polymer mixing ratio is, the better the performance of photocatalytic process will be observed, but it makes TiO2 leach out easily. To the best of our knowledge, no work used double-sided adhesive tape (DSAT) in replacing a polymer binder under TiO2 immobilization system. Since commercial DSAT is water proof, durable, and strong intact with any materials and is made from nonhazardous substances [16], it is highly potential to be used as a thin layer binder for TiO2 immobilized system.
In this work, TiO2 photocatalyst was immobilized on glass plate support material by using DSAT as a new binding technique. The photocatalytic activity of the immobilized photocatalyst was investigated by monitoring the degradation of RR4 and MB dyes as well as their durability and reusability.
2. Methods
2.1. Preparation of Immobilized TiO2
Typically, TiO2 solution was prepared by mixing 13 g of TiO2 (P25, Deggusa, 80 : 30 of anatase : rutile) with 100 mL of distilled water in 250 mL reagent bottle. The solution was undergoing shaking process for 30 minutes using orbital shaker model PSU-20i, Grant-bio, to make it homogenised. TiO2 solution was then coated (immobilized) by using brush technique and DSAT was stacked onto glass plate with dimensions of 13 × 4.8 cm () priorly coated with TiO2. The glass cell with immobilized TiO2 was then dried in the oven at 100°C for 20 minutes. A dried immobilized TiO2 sample was cleaned by using distilled water under irradiation of 55 W fluorescent lamp model Qusun E27, 6400 K in aerated condition for 1 hour prior to photocatalytic degradation.
2.2. Photocatalytic Degradation
25 mL of 120 mg L−1 of anionic RR4 dye was poured into a glass cell with dimensions of 150 mm × 10 mm × 80 mm (). Immobilized TiO2 was then entered into glass cell containing RR4 dye and irradiated with a 55-watt fluorescent lamp at specific time interval until it turns colourless. An aquarium pump model NS 7200 was used as an aeration source to supply oxygen. Photocatalytic degradation of MB was repeated by replacing RR4 with 12 mg L−1 of MB, following the same procedure. Percentage of colour remaining after photodegradation process was determined based on absorbance values measured by using HACH DR 1900 spectrophotometer at 517 and 661 nm for RR4 and MB, respectively. Two control tests were carried out for both RR4 and MB dyes by following photocatalytic degradation setup without any presence of TiO2. DSAT was included in the first test while another test was performed without both TiO2 and DSAT, namely, photolysis.
2.3. Reusability and Strength
The study of reusability was carried out following the photocatalytic degradation procedure by repeating the same procedure up to 30 times by using RR4 and MB dyes. Sample was alternately clean with ultrapure water under 55 W fluorescent lamp in aerated condition for 1 hour prior to repetitions process of photocatalytic degradation. The strength study was carried out by measuring the remaining weight in immobilized TiO2/DSAT sample after sonication process with ultrasonic bath Cress Ultrasonic, Model 4HT-1014-6.
2.4. Scanning Electron Microscopy (SEM)
The study on surface morphology and cross section of immobilized TiO2/DSAT was carried out by using SEM analysis model Leica Cambridge S360.
3. Result and Discussions
The photocatalytic degradation rate of anionic RR4 under immobilized TiO2/DSAT shows a significant improvement with increasing amount of loading catalyst as can be seen in Figure 1(a). The optimum catalyst loading of immobilized TiO2/DSAT was observed at 0.2 g and degradation rate of RR4 was ca. 0.040 min−1. The optimum catalyst loading under suspension mode was also observed at 0.2 g and the rate of RR4 removal was ca. 0.048 min−1 and it is just slightly higher than immobilized TiO2/DSAT. The plots percentage of RR4 and MB remaining at different catalysts loading is provided in Supplementary Figures and , respectively, in Supplementary Material available online at http://dx.doi.org/10.1155/2015/232741. A slightly lower photocatalytic activity of RR4 under immobilized TiO2/DSAT system compared to the suspension mode was considered as a good photoresponse under immobilized system since the photodegradation of immobilized TiO2 is always much lower than suspension mode. Based on reports by other researchers, photodegradation rate of immobilization system is more than two times slower than the suspension mode [17, 18]. However, different result will be obtained if another pretreatment process was applied prior to photocatalysis, namely, photoetching (cleaning process) that was successfully studied and reported by Nawi et al. [19]. Photoetching is responsible for removing the organic additive. They reported that the photodegradation of immobilization system was better than the suspension mode. Nevertheless, the system was found to be less effective for commercialization since this photoetching is a time-consuming process which took 8 hours to complete. Figure 1(b) shows the effect of catalyst loading towards photodegradation of cationic MB under suspension TiO2 and immobilized TiO2/DSAT. The optimum loading of immobilized TiO2/DSAT was recorded at 0.3 g where photodegradation rate of MB was ca. 0.069 min−1 and the rate is higher than optimum photodegradation rate for TiO2 suspension by 22% (0.054 min−1).
(a)
(b)
High photocatalytic degradation under immobilized TiO2/DSAT sample compared to suspension TiO2 is due to two main factors. Since TiO2 sample is negatively charged, positive charges in cationic MB make the sample have higher adsorption capacity with MB dye. Higher surface contact of photocatalyst with MB increases the adsorption capacity thus making the adsorption become dominant. Generally, good photodegradation of dye is a combination of adsorption and photocatalysis processes in equilibrium condition [20]. In this study, photocatalytic removal of MB in suspension mode has higher surface area where the adsorption is dominant in the entire process thus making TiO2 particle less able to perform photocatalysis process due to the scattering effect of adsorbed MB dye on the surface of TiO2 particle. In case of immobilized TiO2/DSAT sample, the surface area is less compared with suspension; moreover, the sample is more stable due to static condition, thus making the balance between adsorption and photocatalysis processes for enhanced photodegradation of MB. Second, immobilized TiO2/DSAT samples have undergone cleaning process prior to photodegradation of dyes. Cleaning process is proven to activate the photocatalyst. Nawi and Zain [21] reported that photocatalytic degradation of MB will become higher after washing of immobilized TiO2/PVC sample compared with sample without washing. Besides, an organic binder that is present in this TiO2 immobilization system which is DSAT makes this cleaning process become even more essential to oxidize the organic compound. In this study, sample in suspension mode was carried out without washing, thus making the photocatalysis in suspension mode become lower as compared with immobilized TiO2/DSAT which are under washing condition. As a comparison study, we have applied immobilized TiO2/DSAT to degrade MB without washing process. A photocatalytic degradation rate of MB under immobilized TiO2/DSAT without washing is drastically reduced to 0.463 min−1 and it is even lower than the rate under TiO2 in suspension mode (0.054 min−1). A control test has been conducted without the presence of TiO2 under both dyes to prove the importance of TiO2 photocatalyst in this study. As expected, there is no photocatalysis reaction occurring during the experiment. From Figure 2, the percentages of RR4 and MB remaining after photolysis are both beyond 90%. This happened due to the absence of TiO2 in the reaction since photocatalysis activity will only occur when the light from fluorescent lamp strikes the TiO2 surface thus degrading the organic pollutant. However, 45% of MB was removed for control test with presence of DSAT. This happened due to the adsorption dominant properties of MB which results in the MB itself being adsorbed onto the DSAT surface. This phenomenon explained the results in higher photodegradation of cationic MB dye compared to anionic RR4 dye under immobilization system.
Immobilized TiO2/DSAT sample has shown a very good photoactivity upon reusability. Figure 3 shows the photocatalytic degradation rate of RR4 and MB upon cycles. It was observed that the photocatalytic degradation rate of immobilized TiO2/DSAT is getting increased under RR4 as well as MB dye after 20 times of cycles. Beyond 20 until 30 cycles, the degradation rates remained constant for both RR4 and MB dyes. This might be due to the fact that, after the 20th cycle, the formations of porous structure on TiO2 surface have already exceeded its limit as proven by SEM image in Figure 4(b). Figure 4 shows the surface morphology and cross section images for immobilized TiO2/DSAT for 1st cycle and 30th cycle. It can be seen that the photodegradation improvement is due to the formation of porous TiO2 particles on surface of immobilized TiO2/DSAT after 30th cycle (Figure 4(b)) compared with 1st cycle of immobilized TiO2/DSAT (Figure 4(a)). The formation of pores on TiO2 surface after 30th cycle is due to oxidation of organic binder (DSAT) during cleaning process. From the cross section images of immobilized TiO2/DSAT in Figures 4(c) and 4(d), it is obviously shown that TiO2 sample is embedded in DSAT layer thus making the immobilized TiO2/DSAT become very strong intact and these cross section images (Figures 4(d) and 4(e)) explain the effect of sample reusability in Figure 3. The heating process after coating TiO2 on top of DSAT layer during sample preparation is the main cause that makes DSAT condition become semimelted and eventually allowed TiO2 layer to embed into DSAT. This explanation is supported by comparing of immobilized TiO2/DSAT without heating process. Figures 4(e) and 4(f) show the cross section images of immobilized TiO2/DSAT without heating process and the TiO2 layer is just attached on top of DSAT surface without strong intact. However, TiO2 layer in immobilized TiO2/DSAT only remains 50–60 wt.% after 30 minutes of sonication process at strength test as can be seen in Figure 5. It is well expected because no binder was applied on the entire immobilization system. But the immobilized TiO2 by using DSAT as a thin layer binder significantly improved the photocatalytic degradation rate of cationic and anionic dyes and it is almost the same as TiO2 suspension or even higher than TiO2 suspension in certain condition (Figure 1(b)).
4. Conclusion
An immobilized TiO2 sample was successfully carried out by using DSAT as a thin layer binder. The optimum catalyst loading under photodegradation of RR4 and MB was observed at 0.2 and 0.3 g, respectively. The photodegradation rate of RR4 removal was observed at slightly lower suspension TiO2 while higher rate of photodegradation under MB was observed. Higher removal rate under MB in immobilized TiO2/DSAT sample is due to the equilibrium conditions between adsorption and photocatalysis processes and the effect of washing process generated the surface of immobilized TiO2/DSAT into active photocatalyst. The reusability of immobilized TiO2/DSAT also is getting improved up to 30 cycles and this cannot be achieved by TiO2 in suspension mode. A good reusability of immobilized TiO2/DSAT was revealed whereby TiO2 was embedded into semimelted DSAT in heating process during sample preparation.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The authors would like to thank the Malaysian Ministry of Education (MOE) for providing generous financial support under RAGS Grants 600-RMI/RAGS 5/3 (35/2014) to conduct this study and Universiti Teknologi MARA for providing all the needed facilities.
Supplementary Materials
Supplementary Figure 1 shows the optimum loading (0.2 g) gives the best degradation rate for both immobilized and suspension system.
Supplementary Figure 2 shows the optimum loading (0.3 g) gives the best degradation rate for both immobilized and suspension system.