Photocatalytic Study of New Immobilized TiO2 Technique Towards Degradation of Reactive Red 4 Dye

The study on TiO2 for wastewater remediation has gained interest among researchers. However, the application of this photocatalyst is limited due to non-recyclability of conventional TiO2. Thus, immobilization technique has been developed to solve this issue. Hence, a comparison study between two types of immobilized photocatalysts namely titanium dioxide (TiO2) and TiO2 mixed with polyvinyl alcohol (PVA) has been conducted in this work to observe the significant effect of PVA polymer in photocatalysis reaction of reactive red 4 (RR4) dye. Double sided adhesive tape (DSAT) was used as thin layer binder in this immobilization system. The result shows that the photocatalytic performance of TiO2PVA/DSAT was higher than that of TiO2/DSAT under both normal UV and visible light irradiations due to the conjugated unsaturated polymer from PVA serve as electron donor for TiO2 thus increase the photocatalysis process. Besides, TiO2-PVA/DSAT was also found to possess much better adhesion strength to the support material compared to TiO2/DSAT. Based on the findings, this TiO2 immobilization system is expected to be beneficial in the industrial wastewater treatment. Thus, further study to improve the photocatalytic activity of this immobilized TiO2 will be in our future work.


Introduction
Advanced oxidation processes (AOPs) have seen to be growing interest recently. The practical application of AOPs is commercially known as photocatalysis. Titanium dioxide (TiO 2 ) is the best known phototcatalyst and has been employed in almost studies. Immobilization of TiO 2 has been introduced to replace the existing suspension system. This is due to the main drawback from the suspension system that is formation of sludge which requires further purification steps thus resulting in difficulties to recycle the photocatalyst. Since past decades, researchers have taken various approaches to modify the TiO 2 for a better performance of photocatalysis reaction. Thus, many different methods that have been developed to prepare the immobilized TiO 2 .
Adding polymer as matrices has been widely used by researchers in immobilized TiO 2 preparation to produce photocatalyst with better strength, adsorption capability and surface morphology. Some examples of the widely used polymers are polyvinyl chloride (PVC) [1], polyethylene glycol (PEG) [2] and polyvinyl alcohol (PVA) [3]. Under certain state, polymers with carboxyl or hydroxyl groups particularly, will bond chemically with the hydroxyl groups on the surface of TiO 2 . PVA which possess good film-forming ability is a hydrophilic polymer and potentially useful in immobilization of TiO 2 [4][5][6]. Its low market price is another factor that many researchers employed this polymer in their work. Yang et al. [7] prepare C-PVA/TiO 2 composites by calcinating the mixture of TiO 2 sol and C-PVA solution for photocatalytic degradation of rhodamine B. Meanwhile, H-PVA/ TiO 2 composite was synthesized by Song et al. [8] under sol-gel and thermal treatment process with tetrabutyl titanate as titanium source.
In this paper, a comparison study between two types of immobilized photocatalyst which were TiO 2 /DSAT and TiO 2 -PVA/DSAT has been carried out successfully. The photocatalytic activities of both photocatalysts were investigated by studying the degradation of reactive red 4 (RR4) dye under two different light sources namely normal and visible light. Besides, the adhesion strength of the photocatalysts was also compared through sonication procedure.

Materials
TiO 2 powder used (P25, 20% rutile and 80% anatase) was supplied by Evonic-Aeroxide. PVA was provided by R&M Chemicals. The model pollutant used was Reactive red 4 (RR4) dye which was purchased from Sigma-Aldrich. Deionized water was used to prepare all experimental solutions throughout this study.

Fabrication of immobilized TiO 2 /DSAT and TiO 2 -PVA/DSAT plates
The TiO 2 coating formulation was prepared by mixing TiO 2 with water while the TiO 2 -PVA coating formulation was prepared by adding a specific amount of PVA solution into the TiO 2 formulation. Clean glass plates (130 mm × 50 mm) were prepared as support material to immobilize both photocatalyst formulations by brush-coating method. DSAT was stuck onto one side of the plates to cover an area of 65 mm × 47 mm prior to coating. The plates were each coated with fixed amount of both immobilize photocatalysts.

Photocatalytic study
A custom made glass cell with dimension of 50 mm × 10 mm × 80 mm (L × B × H) was used as a container to fill in 20 mL of the 200 mg L−1 RR4 sample together with the coated plate. An aerator made from aquarium pump model NS 7200 was connected to the glass cell via PVC tubing and a glass Pasteur pipette to supply oxygen and bubbles constantly throughout the experiment. The plate was placed inside the cell. A 55 W Qusun E27, 6400K fluorescent lamp was used to irradiate the normal light source on to the plate. Hoya ultraviolet (UV) filter was used to the normal light to get visible light source. Samples of the irradiated solutions were taken at every 15 min interval to determine their concentration by using HACH DR1900 ultraviolet-visible (UV-vis) spectrophotometer.

Immobilized TiO 2 /PVA adhesion strength test
To test the adhesion strength of the immobilized TiO 2 /PVA, the plate was sonicated for 30 seconds with Cress Ultrasonic, Model 4HT-1014-6. The sonicated plate was then taken out after every 5 seconds interval and its weight was recorded. The differences of weight before and after every 5 seconds intervals for up to 30 seconds sonication of the catalyst were measured in order to evaluate the percentage of the catalyst particles adhered onto the surface of the glass plates. The mechanism is similar with the photosensitized mechanism of the dye sample towards TiO 2 under visible light. Visible light will cause the excitation of the sensitizer molecules adsorbed onto the surface of TiO 2 while consequently excite electrons to the conduction band (CB) of TiO 2 . Electrons will be transferred from the sensitizer to the electron acceptor substrate present on TiO 2 surface by the CB which acts as a media while the valence band (VB) will remains unaffected in a standard photosensitization process. The visible light activity of the immobilized polymer/TiO 2 sample happens due to series of chain reactions that will be formed by the excited electrons in CB of TiO 2 .

Adhesion strength test
This test identifies the adhesion strength of the immobilized TiO 2 -PVA/DSAT and TiO 2 /DSAT onto the support material. Figure 3 shows percentage of each photocatalyst weight remained after 30 seconds of sonication. Immobilized TiO 2 -PVA/DSAT shows much better adhesion strength towards the glass plate compared to the TiO 2 /DSAT. There is still as much as 87% of catalyst remaining for immobilized TiO 2 -PVA/DSAT while only 47% catalyst left on TiO 2 /DSAT after sonication. This was due to the significant effect of PVA that binds well chemically with TiO 2 matrices which results in stronger adhesion strength toward the support material. Since there is no binder present in immobilized TiO 2 /DSAT, a great amount of its photocatalyst was removed during sonication.