Abstract
Chlorite (ClO2-) is a significant by-product of oxidative water treatment using chlorine dioxide, and due to its ecotoxicological concerns, its concentration in drinking water is strictly regulated. Ultraviolet (UV) technology, a widespread disinfection method in water treatment, can degrade chlorite because chlorite absorbs UV light with a high quantum yield for its transformation into other compounds. Understanding the photolysis mechanism of chlorite is crucial to potentially integrate UV/chlorite processes into water treatment strategies.The combination of UV technology with chemical disinfectants like chlorine and ozone, results in advanced oxidation processes (AOPs), which boosts the disinfection efficacy while mitigating the formation of harmful disinfection by-products. UV-based AOPs produce a variety of reactive species, such as hydroxyl radicals (•OH) and reactive chlorine species (RCS). These species are effective against contaminants of emerging concerns (CECs), including pesticides, pharmaceuticals, industrial chemicals, and personal care products (PPCPs), which pose environmental and potential health hazards.
Despite its importance, there is little of knowledge on the UV/chlorite process. This PhD thesis investigates chlorite photolysis and evaluates the potential of UV/chlorite as an AOP. It also addresses the increasing interest in chlorite’s fate during UV-induced chlorine dioxide processes. A critical review on the photolysis of chlorine dioxide in water treatment was conducted to fill the gap in comprehensive literature on the subject. This review identifies several research needs, including identifying better the role of specific oxidants, better data on pathogen inactivation, and the parameters affecting UV-chlorine dioxide processes.
The thesis presents a mechanistic study of chlorite photolysis using experimental and kinetic modeling approaches. Experimental studies in buffered water with tert-butanol (TBA) as a radical scavenger across various pH levels were conducted and the main phototransformation products were quantified. Kinetic modelling elucidated the reaction pathways and predicted product formation. Furthermore, a study in UV/chlorite irradiation systems was undertaken to understand the fate of CECs during phototransformation of chlorite. Studied substances included caffeine, the insect repellent DEET, the preservative para-nitro benzoic acid (pNBA), and the •OH probe compound nitrobenzene. The study examined both direct and indirect phototransformation mechanisms in different scenarios, achieving successful degradation of these CECs and suggesting the potential of UV/chlorite as an AOP.
In conclusion, this thesis contributes insights into the phototransformation of chlorite, both in the context of chlorine dioxide-based water treatment and as an independent process. The research has resulted in three manuscripts in preparation for peer-reviewed journals. It provides the first detailed exploration of the primary radicals involved in micropollutant degradation during chlorite photolysis in a UV-based AOP. Additionally, this work includes the first thorough review of the UV-chlorine dioxide process. The results are impactful, offering vital information on chlorite removal and advancing water treatment efficiency and micropollutant elimination in drinking water treatment.
| Date of Award | 26 Jun 2024 |
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| Original language | English |
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| Supervisor | Jannis Wenk (Supervisor), John Chew (Supervisor) & Jan Hofman (Supervisor) |