Nanostructured TIO₂ based materials for the photocatalytic degradation of emerging organic pollutants from aqueous solution

Abstract : Water is regarded as the most vital of natural resources for the sustainability of life, yet freshwater systems are directly threatened by pollution. Among the many pollutants are organic emerging pollutants such as synthetic chemicals and pharmaceuticals. Bisphenol A (BPA) and ibuprofen are examples of such a synthetic organic compounds. Current wastewater treatment technologies such as membrane technology, chemical treatment and biodegradation are not always efficient in removing toxic emerging organic pollutants. This is because they were not specifically designed to remove these contaminants. Photocatalysis on the other hand, has shown great potential to remove toxic emerging pollutants from the environment. Over the years, TiO2 catalyst has been used widely for water remediation applications. Surface modification of TiO2 is necessary to extend its use in the visible region of the solar spectrum and to reduce the inherent fast recombination rate of charges. Polyaniline (PANI)-wrapped TiO2 nanorods (PANI-TiO2), obtained through the oxidative polymerization of aniline at the surface of hydrothermally pre-synthesized TiO2 nanorods, were evaluated as photocatalysts for the degradation of BPA. Fourier-transform infrared spectroscopy (FTIR) analysis revealed the successful incorporation of PANI into TiO2 by the appearance of peaks at 1577 cm-1 and 1502 cm-1 that are due to the C=C and C=N stretch of the benzoid and quinoid. Brunauer- Emmett-Teller (BET) analysis revealed the presence of mesoporous material in PANI-TiO2. Transmission electron microscopy (TEM) analysis showed that TiO2 nanorods with different diameters were synthesized. The TEM analysis showed that a thin layer of PANI wrapped the TiO2 nanorods. X-ray diffraction (XRD) and Raman spectroscopy revealed that anatase phase TiO2 was synthesized with typical Raman vii vibration peaks at 637 cm-1, 514 cm-1, 396 cm-1, and 195 cm-1. X-ray Photon Spectroscopy (XPS) survey scan of the PANI-TiO2 nanocomposite revealed the presence of C, O, Ti, and N. Photocatalytic activity evaluation under UV radiation through the effect of key parameters, including, pH, contact time, dosage and initial concentration of BPA was carried out in batch studies. Within 80 min, 99.7% of 5 ppm BPA was attained using 0.2 g/L PANI-TiO2 photocatalyst at pH 10. PANI-TiO2 showed a better performance than as-synthesized TiO2 with a rate constant of 4.46 x 10-2 min-1 compared to 2.18 x 10-2 min-1. Nitrate ions increased the rate of degradation of BPA whilst humic acid consistently inhibited the degradation of BPA. LC-MS analysis identified degradation products with m/z 213.1, 135.1 and 93.1. The PANI-TiO2 nanocomposite was reused up to five cycles with a removal of at least 80% in the fifth cycle. PANI capped WO3@TiO2 nanocomposite prepared through a three–stage synthetic route was evaluated for the degradation of ibuprofen under visible light. XRD analysis confirmed the anatase phase of TiO2 and monoclinic and orthorhombic WO3 crystalline structures were formed. The XRD analysis confirmed that the phases were not affected by wrapping in PANI.TEM analysis confirmed that TiO2 nanorods were synthesized with different diameters. TEM analysis showed that a WO3@TiO2 heterojunction was formed. A PANI layer was wrapping the heterojunction was observed. Photoluminescence analysis revealed that pairing TiO2 and WO3 resulted in improved charge separation. The charge separation was further improved by wrapping the heterojunction in a PANI matrix. DRS calculations showed that pairing TiO2 with WO3 extended the band edge to about 420 nm thus facilitating the use of the nanocomposite in the visible region of the solar spectrum. XPS analysis revealed viii the presence of W, C, O, Ti, N, and their corresponding photo electron peaks were found to be W4f, W4d, C1s, O 1s, N1s, and Ti 2p. The degradation of ibuprofen was influenced by pH with maximum degradation observed at pH 9. The degradation conformed to the Langmuir-Hinshelwood kinetic model. The rate constant, K for the degradation of ibuprofen by WO3@TiO2 and PANI/WO3@TiO2 was 2.59 x 10-2 and 3.5 x 10-2 respectively that were significantly higher than that of pristine TiO2, which was 1.92 x10-2...Ph.D. (Chemistry