Development of novel far-UVC-driven advanced oxidation processes for micropollutant abatement in water

Abstract

Micropollutants are of emerging concern in the context of achieving the United Nations’ Sustainable Development Goals 6 (Clean Water and Sanitation) and 14 (Life Below Water) because they pose significant threats to human health during water use and consumption. Ultraviolet (UV) light-driven advanced oxidation processes using a light source at a wavelength of 254 nm (UV₂₅₄-AOPs) are commonly proposed and added as an advanced treatment to remove the micropollutants. However, their real-world application remains limited because the processes demand high energy and chemical inputs resulting from low utilization of the light and the dosed chemicals. UV₂₅₄ light is not well absorbed by many micropollutants. Thus, photodegradation of the micropollutants is limited, and the degradation is mainly achieved by dosing chemicals to yield reactive species in the UV₂₅₄-AOPs. UV₂₅₄ light is also not well absorbed by the dosed chemicals (e.g., hydrogen peroxide), and the yields of reactive species are low. This thesis work proposed a novel strategy for improving micropollutant degradation by using Far-UVC light (UV₂₂₂) as an alternative to UV₂₅₄, where the direct photolysis of micropollutants and the production of reactive species from the photolysis of oxidant precursors (Far-UVC-AOPs) are more efficient. Direct photolysis of selected micropollutants (e.g., halogenated aromatic disinfection byproducts) was greatly enhanced under UV₂₂₂ radiation, which was mainly attributed to the higher molar absorption coefficients of selected micropollutants at 222 nm than 254 nm. Switching the UV wavelength from 254 nm to 222 nm significantly promotes the radical generation in the UV/hydrogen peroxide (UV/H₂O₂), UV/peroxydisulfate (UV/PDS), UV/chlorine, and UV/chlorinated cyanurates (UV/Cl- cyanurates) AOPs. The molar absorption coefficients and innate quantum yields of H₂O₂, PDS, chlorine, and Cl-cyanurates at 222 nm were determined and incorporated into kinetic models. The models enable accurate prediction of oxidant photodecay rates and radical generation in the Far-UVC-AOPs. The pseudo-first-order degradation rate constants of selected micropollutants by the Far-UVC-AOPs are higher than those by the UV₂₅₄-AOPs, attributing to the enhanced radical generation and stronger direct UV photolysis. Water matrix components, including bicarbonate, chloride, nitrate, and dissolved organic matter, show different impacts on the Far-UVC-AOPs and the UV₂₅₄-AOPs, due to their different photochemistry at 222 nm and 254 nm. For example, nitrate was found to increase the concentration of hydroxyl radicals (HO^•) in the Far-UVC-AOPs but not the UV₂₅₄-AOPs. This thesis advances the fundamental photochemistry of micropollutants and oxidant precursors at 222 nm and offers highly effective engineering tools for combating micropollutants in water.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Chii SHANG (Supervisor)