Evaluating Peroxyacid and UV/Peroxyacid Disinfection: Pathogen Inactivation, Byproduct Formation, and Co-Removal Of Micropollutants
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Wang, Junyue
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Abstract
Recently, peroxyacids (POAs) are extensively investigated and applied as an alternative disinfectant to chlorine. Compared with chlorine, POAs could provide comparable bacterial inactivation and largely mitigate the formation of halogenated disinfection byproducts (DBPs). However, a dearth of information on the chemical properties of POA hinders the comprehensive understanding of their pathogen inactivation capabilities and byproduct mitigation mechanisms. Thus, this thesis work comprehensively evaluated the disinfection performance and delineated the underlying mechanisms of POAs, by assessing their reactivity with biomolecules, organic compounds, and inorganic constituents, and further investigating the potential of combing ultraviolet (UV) and POAs for enhanced removal of water contaminants.
First, this study employed multidimensional bioanalysis, including plate cultivation, (reverse transcriptional-) quantitative polymerase chain reaction ((RT-)qPCR), flow cytometry, and fluorescence microscopy, to unveil the bacterial and viral disinfection kinetics and mechanisms of POAs (i.e., performic acid (PFA), peracetic acid (PAA), and perpropionic acid (PPA)). Results showed that POAs exhibited satisfactory bacterial culturability inactivation, while the removal of enveloped and non-enveloped virus surrogates was mediocre. Furthermore, the bacterial inactivation was mainly attributed to intracellular accumulation and protein damage, rather than genome or cell integrity damage, resulting in a minimal bacterial inactivation count when the analytical methods were switched from cultivation to qPCR or flow cytometry. Finally, kinetic studies using simple biomolecules (i.e., amino acids and nucleotides) showed that POAs selectively reacted with S-containing compounds through sequential O-atom transfer reactions, suggesting that POA disinfection can be highly selective and dependent on the protein compositions of the microbes.
Second, the DBP formation potential of POAs was comprehensively studied at different background halide levels. Compared to chlorine, DBP formation by PAA and PFA was minimal in regular wastewater. However, during 24-h disinfection of saline wastewater, PAA surprisingly produced more brominated and iodinated DBPs than chlorine, while PFA kept all tested DBPs at bay effectively. To understand this phenomenon, a kinetic model was developed based on literature and additional kinetic investigation of POA decay and DBP [e.g., bromate, iodate, and iodophenol] generation in the POA/halide systems. Results showed that PFA oxidized halides 4-5 times faster than PAA to corresponding HOBr or HOI, but also efficiently oxidized HOI/IO- to IO3-, thereby mitigating iodinated DBP formation. Additionally, PFA’s rapid self-decay and slow release of H2O2 limited the HOBr level over long-term oxidation in bromide-containing water. For saline water, this study revealed the DBP formation concern of PAA and identified PFA as an alternative to minimize DBPs.
Moreover, as POAs themselves may not efficiently oxidize some microbes and organic contaminants due to their selective reactivity, research was conducted to investigate the potential of combing UV irradiation and POAs for co-removal of antibiotic resistance genes (ARGs) and organic micropollutants, with PAA as the representative POA. The photolysis of PAA under UV254 (254 nm) effectively generated hydroxyl radical (●OH) and multiple organic radicals (e.g., CH3C(O)OO●) and achieved synergistic degradation of ARGs and micropollutants. The ARG removal by UV/PAA was compared with other UV advanced oxidation processes (AOPs), i.e., UV/chlorine and UV/H2O2. The extracellular ARG (eARG) removal efficiency followed the order of UV/chlorine > UV/H2O2 > UV/PAA. ●OH and reactive chlorine species (RCS) largely contributed to eARG removal, while organic radicals had a minor contribution. For intracellular ARGs (iARGs), UV/H2O2 did not remove better than UV alone due to the scavenging of ●OH by cell components, whereas UV/PAA provided a modest synergism, likely due to diffusion of PAA into cells and intracellular ●OH generation. Comparatively, UV/chlorine achieved significant synergistic iARG removal, suggesting the critical ability of RCS to resist cellular scavenging and inactivate ARGs. Additionally, flow cytometry analysis demonstrated that cell membrane integrity was significantly damaged by chlorine oxidation, while the radicals, H2O2, and PAA exerted negligible impacts.
In light of the fact that ●OH could not account for all the micropollutant degradation in UV/PAA, the crucial role of organic radicals is implicated. In particular, acetylperoxyl radical (CH3C(O)OO●), a major organic radical in UV/PAA, is also known to play important roles in atmospheric chemistry, aqueous chemistry, and biochemistry. However, knowledge for the reactivity of CH3C(O)OO● with organic compounds has been lacking. Herein, three independent experimental approaches were exploited for kinetic data generation and cross-validation. First, laser flash photolysis of biacetyl was developed to provide a clean source of CH3C(O)OO●, and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was used as the probe for competition kinetics. The rate constants between CH3C(O)OO● with naproxen and trans-cinnamic acid were determined. Then, these compounds were utilized as kinetic competitors in a UV/biacetyl photoreactor to quantify the rate constants between CH3C(O)OO● and organic compounds of various structures. Further, pulse radiolysis of acetaldehyde solution was employed as an alternative system for CH3C(O)OO● generation to validate the rate constants. Using the same ABTS competition method, consistent kinetic data for representative compounds were reproduced. Overall, CH3C(O)OO● displays distinctive selectivity, reacting especially favorably with naphthyl and diene compounds but sluggishly with N- and S-containing groups. The quantified rate constants were integrated into the kinetic model for UV/PAA. Incorporating acetylperoxyl radical-oxidation reactions significantly enhanced the accuracy in modeling the degradation of micropollutants by UV/PAA.
Overall, this study demonstrated the promising potential of POAs for effective bacterial inactivation and DBP control, and delineated the underlying mechanisms. Meanwhile, the high chemical selectivity (e.g., toward S-containing moieties) of POAs restricts their efficiency in direct oxidation of microbial cell membranes, ARG and micropollutants. Combination of UV and POAs could produce reactive species (i.e., ●OH and CH3C(O)OO●) and achieve efficient degradation of ARGs and micropollutants. Reactivity of the organic radical CH3C(O)OO● toward various compounds is quantitatively characterized for the first time, and significantly improves the accuracy of UV/PAA kinetic modeling. The application and effectiveness of POAs and UV/POA for degrading a broader range of opportunistic pathogens and emerging contaminants in varying water matrices (e.g., bicarbonate, halides, organic matter) warrant further investigation. Results and methodologies developed by this study filled critical knowledge gaps for POAs and can be useful to facilitate future research.
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2024-04-30
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Dissertation