This thesis work investigates the use of more energy efficient near-ultraviolet (UV)-to-visible light (365 nm) to activate titanium dioxide (TiO₂) and graphitic carbon nitride (g-C₃N₄) to produce primary reactive species, h_vb⁺ , e_cb⁻ and O₂^•−, which are then used to activate peroxymonosulfate (PMS) and chlorine to produce HO• and other radicals. These processes are termed Vis/g-C₃N₄/PMS, Vis/g-C₃N₄/chlorine, and UV/TiO₂/chlorine processes.

The Vis/g-C₃N₄/PMS process increased the pseudo-first order degradation rate constant of a model micropollutant, carbamazepine (CBZ), by 2 times compared with that in the absence of PMS. The enhanced CBZ degradation was attributed to the production of HO• and SO₄^•– from the PMS activation, but not the enhanced charge separation of g-C₃N₄ due to the p...[ Read more ]

This thesis work investigates the use of more energy efficient near-ultraviolet (UV)-to-visible light (>365 nm) to activate titanium dioxide (TiO₂) and graphitic carbon nitride (g-C₃N₄) to produce primary reactive species, h_vb⁺ , e_cb⁻ and O₂^•−, which are then used to activate peroxymonosulfate (PMS) and chlorine to produce HO• and other radicals. These processes are termed Vis/g-C₃N₄/PMS, Vis/g-C₃N₄/chlorine, and UV/TiO₂/chlorine processes.

The Vis/g-C₃N₄/PMS process increased the pseudo-first order degradation rate constant of a model micropollutant, carbamazepine (CBZ), by 2 times compared with that in the absence of PMS. The enhanced CBZ degradation was attributed to the production of HO• and SO₄^•– from the PMS activation, but not the enhanced charge separation of g-C₃N₄ due to the presence of PMS. The Vis/g-C₃N₄/chlorine process increased the degradation rate constant of CBZ by 16 and 7 times higher than those without g-C₃N₄ and HOCl/ClO^-, respectively. The large enhancement in the CBZ degradation is attributed to the production of HO• and ClO• from the chlorine activation by photo-induced h_vb⁺, e_cb⁻ and O2^•−. The UV/TiO₂/chlorine process degrades CBZ at an apparent first-order rate constant (k_CBZ) 34.2 and 3.9 times higher than those without TiO₂ and chlorine, respectively. In this process, chlorine serves as a catalyst to enhance the yield of hydroxyl radicals (HO•) without being consumed, in addition to its role as a radical precursor to produce HO• and reactive chlorine species. The near-UV-to-visible light photocatalytic oxidant activation process is practically relevant, feasible and easily implementable, and it expands the potential types of light sources (e.g., LEDs, solar light) to activate oxidants. To better simulate the diffusion-mediated micropollutant degradation in the photocatalytic chlorine activation processes, surface affinities of each micropollutant to g-C₃N₄ and TiO₂ were defined as the differential capacitance variation on the TiO₂ or g-C₃N₄ working electrodes measured in a three-electrode system when adding each micropollutant to the system. A diffusion-mediated degradation model was then established to successfully correlate the affinities of the 5 selected micropollutants to g-C₃N₄ and TiO₂ with their apparent first-order degradation rate constants by the Vis/g-C₃N₄/chlorine and UV/TiO₂/chlorine processes.

g-C₃N₄- and TiO₂-coated side-emitting plastic optical fibers (POFs) with high photocatalyst loading and porous polymeric outer layer were then fabricated by a novel dip-coating method. The high photocatalyst loading and porous coating layer structure were achieved by the use of acetone as a PVDF solvent and a morphology control agent during the fabrication. These features increased the exposed surface area of the fabricated POFs and minimized light irradiation loss due to light scattering or occlusion without sacrificing the mass transfer efficiency for adsorption and degradation of target pollutants. Of great significance is the methylene blue degradation rate constants achieved by the g-C₃N₄-POFs are all higher than those by g-C₃N₄ in suspension, especially at high g-C₃N₄ dosages. Such good performance maintained stable even after 20 cycles. The presence of chlorine increased the CBZ degradation rate constants of the fabricated POFs by 4-7 times and their photocatalytic activities remained unchanged even after 20 cycles in DDI water and simulated water. The good photocatalytic activities and stabilities of g-C₃N₄- and TiO₂-coated POFs show their tremendous potential in advanced drinking water purification, especially in the implementation of the near-UV-to-visible light photocatalytic oxidant activation process that takes the advantage of chlorine residuals available in drinking water.