TiO₂ based membranes and nanostructures for wastewater treatment

Organic material is a critical pollutant in the water body due to its impact to the drinking water treatment process. For instance, the problem of color, taste and odor, increment of the doses of coagulant and disinfectant in conventional water treatment system, promoting biofilm growth in the water distribution system. More seriously chemical disinfection process by chlorine results in increases in the amount of complex heavy metals and undesired disinfection by-products (DBPs), which has been proven as one of the carcinogen. Photocatalytic oxidation process by TiO₂ nanoparticles, which are suspended in the solution, has been proven as an effective and efficient method to remove organic materials from the wastewater. The organic materials can be degraded and decomposed to CO₂ and H₂O, without generating secondary waste. However, the recovery of nanoparticles from the solution after the reaction is a challenging problem in the industry. Thus, TiO₂ with different morphologies such as spheres and rods were used to couple with alumina disc to perform as photocatalytic membrane. Under the irradiation of UV light, the photocatalytic membrane can perform the physical separation of organic material from the solution and decompose the organic layer remained on the membrane layer. Therefore, the membrane fouling can be reduced and effluent flux of the system can be preserved. TiO₂ spheres and branched rods photocatalytic membranes were successfully fabricated in this dissertation, which achieved 92% and 89% of humic acid removal, respectively. A layer of TiO₂ on top of the alumina disc has enabled the resulted membranes to preserve the effluent flux when the filtration process was taken place under the irradiation of UV light. Synthesizing the micro-sized TiO₂ particles can become another method to cease the problem to recovery TiO₂ from the solution, as larger particles of TiO₂ are easier to separate from the solution, such as sedimentation process. However, most of the micro-sized particles will have lower photocatalytic activity due to its lower surface area. To overcome this drawback, micro-sized TiO₂ particles, which consisted of nanostructured crystals agglomerated together, were synthesized. In addition, anatase TiO₂ particles that consisted of nanostructured crystals with exposed {001} facets have been produced and the photocatalytic activity is better than P25 due to its favorable mesoporous structure, surface area and superior light harvesting property. The first-order rate constant of the prepared TiO₂ with exposed {001} facets is 2.18 sˉ¹ which is higher than P25, 1.72 sˉ¹. Furthermore, the micro-sized crystals can be easily recovered from the solution by precipitation process. Flower-like and coral-like TiO₂ particles that composed by agglomeration of nanorods have been developed with enhanced adsorption capability. After deposited with Ni(OH)₂, the coral-like TiO₂ particles were active under visible light irradiation. Both products performed better in pollutant removal under visible light irradiation when compared to P25. The studies found that Ni(OH)₂-modified coral-like rutile TiO₂ has efficiently removed 76% of 100 mg/L of tetracycline solution whereas only 57% of the tetracycline removal achieved by P25. Moreover, the TiO₂ samples have shown great adsorption capability after depositing Ni(OH)₂.