Polymer-Titania Composites for Photocatalysis of Organics in Aqueous Environments
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Polymer-Titania Composites for Photocatalysis of Organics in Aqueous Environments Cecil A. Coutinho and Vinay K. Gupta Department of Chemical & Biomedical Engineering University of South Florida, Tampa, FL – 33620 ABSTRACT Microcomposites composed of titanium dioxide nanoparticles embedded within cross-linked, thermally responsive microgels of poly(N-isopropylacrylamide) were prepared. These microcomposites showed rapid sedimentation, which is useful for gravity separation of the titania nanoparticles in applications such as environmental remediation. To investigate the degradation kinetics using these microcomposites in aqueous suspensions, methyl orange was employed as a model contaminant. The decline in the methyl orange concentration was monitored using UV-Vis spectroscopy. Degradation of methyl orange was also measured using only nanoparticles TiO2 (DegussaTM P25) for comparison with the microcomposites. Experiments were performed at different pH conditions that spanned acidic, neutral, and basic conditions to gain insight into the interplay of TiO2 surface charge, ionization of the polyelctrolyte chains in the microcomposites, and ionization of the methyl orange. INTRODUCTION Titanium dioxide is a common and widely studied photocatalyst due to its appealing attributes such as non-toxicity, chemical inertness and high photocatalytic activity1-3. The large band gap of TiO2 (3.2eV) permits it to absorb photons in the UV region, which results in production of electron-hole pairs that participate in redox reactions known to degrade simple organic species4. In recent years, there has been increased interest in use of nanosized titania powders due to enhancements in photocatalytic activity5-7. Because separation of suspended titania nanoparticles from water has been a major obstacle, use of very fine particles of titania in applications such as waste-water treatment have been limited. In a recent report8, we demonstrated the synthesis of novel microcomposites in aqueous media comprising of polymer gels on micron length scales that were loaded with DegussaTM P25 TiO2 nanoparticles. Microcomposites with high mass fractions of titania (50-75wt%) were prepared that showed rapid sedimentation (~minutes), which is a useful characteristic for gravity separations. In this study we build on our previous body of work by investigating the photodegradation of a model organic dye, methyl orange, using the novel polymer-titania microcomposites. Kinetics of the photodegradation are evaluated by monitoring the changes in the methyl orange concentration using UV-Vis spectroscopy. The influence of pH of the solution, which influences the interactions between the poly(acrylic acid) (PAAc) in the polymer microgels, the titania surface and the methyl orange adsorbate is studied. Degradation of methyl orange using freely suspended titania is also conducted for comparison with the microcomposites. EXPERIMENTAL: PHOTOCATALYTIC STUDY Degradation studies were done using aqueous solutions containing 5 ppm of methyl orange (
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