THESIS
2019
xxv, 216 pages : illustrations ; 30 cm
Abstract
The effective control of emerging contaminants is becoming a priority in securing safe
potable water and it is particularly true in the countries and regions where water shortage
and quality deterioration necessitate the consideration of less pristine water as a crucial part
of the water supply formula. As many of these contaminants are recalcitrant to conventional
water treatment processes, advanced abatement processes need to be added. This thesis
work developed new advanced oxidation processes (AOPs) by integrating the UV light
emitting diodes (UV-LEDs) that are considered as the next-generation UV radiation sources
with free chlorine/inorganic chloramines/chlorine dioxide. This thesis work overcame
challenges in the design and operation of the UV-LED-based AOPs: understandin...[
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The effective control of emerging contaminants is becoming a priority in securing safe
potable water and it is particularly true in the countries and regions where water shortage
and quality deterioration necessitate the consideration of less pristine water as a crucial part
of the water supply formula. As many of these contaminants are recalcitrant to conventional
water treatment processes, advanced abatement processes need to be added. This thesis
work developed new advanced oxidation processes (AOPs) by integrating the UV light
emitting diodes (UV-LEDs) that are considered as the next-generation UV radiation sources
with free chlorine/inorganic chloramines/chlorine dioxide. This thesis work overcame
challenges in the design and operation of the UV-LED-based AOPs: understanding the
unique complexity of the photochemistry and radical chemistry of UV-LED photolysis of
free chlorine, inorganic chloramines, and chlorine dioxide under potable water treatment
conditions. The UV photolysis of free chlorine, inorganic chloramines, and chlorine dioxide
were all highly wavelength dependent. At neutral and slightly alkaline pHs, the photolysis
of chlorine and the consequential radical formation increased with increasing UV
wavelength from 255 to 300 nm, and the radical yields at 265, 285 and 300 nm were higher
than that at 254 nm (i.e., the emission wavelength of the conventional low-pressure UV
lamps). The photolysis of monochloramine and subsequent radical generation decreased
with increasing UV wavelength from 255 to 300 nm, while the photolysis of dichloramine
exhibited an opposite wavelength-dependence. The photolysis of chlorine dioxide and
subsequent radical production increased with increasing UV wavelength from 255 to 365
nm. The wavelength-dependences on the photolysis of free chlorine, inorganic chloramines
and chlorine dioxide were more strongly affected by the molar absorptivity than the
apparent/innate quantum yields of these oxidants. Selected micropollutants were destructed
by the UV-LED-based AOPs via several mechanistic pathways, e.g., chlorination alone,
direct UV photolysis, and radical oxidation, depending on their molecular structures. The
micropollutants were partially mineralized and transformed to undesired (by)products in
the UV-LED-based AOPs, and the remaining micropollutants and (by)products could be
removed by post-adsorption process. The UV-LED-based AOPs also produced higher
concentrations of the halogenated by-products from natural organic matter, compared to
the oxidation by the chlorine/inorganic chloramines/chlorine dioxide alone. Reactive
chlorine species (e.g., Cl
•) were responsible for the enhanced (by)product formation. The
newly established “wavelength-pH-photodecay-radical nexus” in this thesis work provides
references and implications for water engineers and participants in selecting sources of UV
radiation for chlorine/inorganic chloramines/chlorine dioxide photodecay and
micropollutant abatement by the UV-LED-based AOPs in different water settings. The
findings also offer a new direction in designing the UV-LED-based point-of-use or point-of-entry water purifiers in treating chlorine-/inorganic chloramine-/chlorine dioxide-containing
potable water.
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