Photocatalytic oxidation of emerging pollutants by nano TiO

Emerging contaminants pose health threats to flora and fauna even at trace level concentrations. Among these pollutants, estrogenic steroidal hormones such as estrone (E1), 17β-estradiol (E2), estriol (E3) and 17α-ethynylestradiol (EE2) are known to cause endocrine disruption, especially in aquatic systems. The successful treatment of these chemicals in water requires advanced oxidation processes (AOPs) in addition to the conventional treatment methods. Photocatalysis by TiO₂ that utilises free radicals for the photodegradation of organic pollutants is an AOP that has attracted recent research interest. TiO₂ photocatalysis face challenges such as its inability to degrade pollutants under visible light irradiation, the requirement for suitable immobilisation techniques for catalyst reuse and the need for appropriate methods to transmit light over long distances including under water. Each of the aforementioned shortcomings should be addressed for TiO₂ to be successfully applied. This study focusses on addressing the challenges to effectively degrade estrogenic steroidal hormones using TiO₂ photocatalysis. Commercial Aeroxide P25 TiO₂ was modified with gold nanoparticles to achieve visible light photocatalytic activity. Au-TiO₂ photocatalysts were synthesised using deposition-precipitation (DP) method and characterised using thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, UV-Vis spectroscopy, scanning electron microscopy (SEM), inductively coupled plasma mass spectrometer (ICP-MS), zeta potential and particle size analysis The performance of the new catalysts was compared to that of commercial P25 TiO₂ under different LED light sources: UVA, cool white and green. For the degradation of E1, E2, E3 and EE2, 0 – 8 wt.% gold loadings to P25 TiO₂ were studied, where 4 wt.% Au-TiO₂ was found to provide the fastest degradation rate of the pollutants. The catalysts’ performance decreased for the light sources in the order, UVA > cool white > green light. Photocatalysis of E1 (1 mg l⁻¹) was found to follow pseudo 1st order kinetics. E1 degradation was significantly more efficient by using 4 wt.% Au-TiO₂ than P25 TiO₂ under UVA (k = 0.28 ± 0.01 min⁻¹ vs. 0.01 min⁻¹) and cool white light (k = 2.44 ± 0.36 h⁻¹ vs. 0.06 ± 0.01 h⁻¹). The photocatalytic activity under visible light decreased in the order: 4 wt.% Au-TiO₂ > 8 wt.% Au-TiO₂ > 2 wt.% Au-TiO₂ > 1 wt.% Au-TiO₂ > P25 TiO₂. The enhanced activity of the Au-TiO₂ catalysts was attributed to the gold nanoparticles acting as electron sinks to minimise electron-hole recombination under UVA and due to increased absorption of light in the 500-600 nm wavelength as a result of localised surface plasmon resonance (LSPR). The stability of the catalysts for reuse is an important factor for consideration in photocatalysis. This was studied by reusing the catalysts over three cycles, for the photodegradation of E1. After three cycles of photocatalysis, the activity did not diminish by any significant amount (< 3%), showing the reusability of the…