Degradation And Transformation Mechanisms Of PhACs During Chlorination And UV/Peracetic Acid Disinfection

As emerging micropollutants, pharmaceutically active compounds (PhACs) have been frequently detected all over the world, which bring a potential threat to human health and aquatic ecological security. Coventional wastewater treatment processes cannot effectively remove PhACs, which causes PhACs reach into the potable water system through the water cycle. As the last barrier of drinking water reaching into human life—disinfection, especially for the widely used chlorine disinfection, the removal effecay and mechanism of PhACs during this process is also an important issue, which will directly affect the drinking water security. UV/peracetic acid (PAA), as a new disinfection process, has gradually been the focus of attention by abroad scholars. Until now, the study on the degradation and mechanism of PhACs during UV/PAA is still seldom, and to investigate the removal mechanism of PhACs by this process has important theoretical and practical significance for evaluating the treatment efficiency and application prospect. Thus, based on the occurrence and removal of PhACs during the potable water system, this study chose typical PhACs as target compounds to investigate the removal efficiency, kinetics and transformation pathways of them during chlorination and UV/PAA process.First, a solid phase extraction (SPE)—liquid chromatography/mass spectrometry technology (LC/MS/MS) was employed to develop a method to simutaneously analyze 14 typical PhACs. Nine, ten and six PhACs were detected in tap water, the raw water and finished water of drinking water plant in certain city. Antipyrine, carbamazepine, isopropylantipyrine, aminopyrine and bezafibrate showed high detection frequencies in drinking water plant, and three of them were pyrazole ketones drugs, including antipyrine, isopropylantipyrine and aminopyrine. Conventional coagulation, clarification treatments can remove some PhACs with removal rates above 40%. Activated carbon filter, ozone oxidation and chlorine disinfection showed good removal efficiencies to certain PhACs. Until now, the concentrations of PhACs in the potable water system of this city were at low levels.Pyrazole ketones drugs, including antipyrine, isopropylantipyrine and aminopyrine, were chosen as target compounds to explore the removal mechanism of them during conventional chlorine disinfection process. The results showed that chlorine disinfection can rapidly degrade pyrazole ketones drugs. The degradation rate of antipyrine was lower than that of isopropylantipyrine, but higher than that of aminopyrine. The initial concentration of PhACs, chlorine dosage and solution pH can affect the removal efficacy of PhACs by chlorination. The reaction orders of the chlorination reaction rate of pyrazole ketones drugs with chlorine dosage were between 1～2, indicating HOC1, Cl2 and Cl2O contributed to the degradation of these drugs. HOCl, Cl2 and Cl2O showed different reaction rates and contributions to different PhACs. The rate constants of antipyrine with HOC1, Cl2 and Cl2O were 3.23×103 M-1s-1,2.86×107 M-1s-1 and 8.38x109 M-1s-1, respectively. The rate constants of isopropylantipyrine with HOC1, OC1-, Cl2 and Cl2O were 9.20x103 M-1s-1,4.68x102 M-1s-1,8.61×107 M-1s-1 and 1.39x102 M-1s-1, respectively. The rate constants of the protonated aminopyrine with HOC1, Cl2 were 5.73×102 M-1s-1 and 1.77x107～4.48*107, and the rate constants of unprotonated aminopyrine with HOC1, Cl2 and Cl2O were 9.68x102 M-1s-1,4.96x106-6.62x106 M-1s-1 and 4.33×109 M-1s-1, respectively. In contrast to the degradation rates of pyrazole ketones drugs by chlorine, the contribution of Cl2O to pyrazole ketones drugs degradation has a trend with aminopyrine (33%)>antipyrine (23%)>isopropylantipyrien (<0.01%) at neutral pH. The contribution of Cl2 was associated with chloride ion and hydrogen ion in solution. Conventional chlorine disinfection was observed to be difficult to completely mineralize pyrazole ketones drugs by TOC, UV, IR, LC/MS/MS and SPE-GC/MS analyses. The main transformation pathways included halogenated reaction, hydroxylation, dealkylation and dehydrogenation.Seven PhACs which were hard to be degraded by chlorination were chosen as target compounds to study the degradation and transformation of PhACs during UV/PAA process. The results showed that UV/PAA can effectively remove anti-inflammatory analgesic, sterols and antiepileptic drugs. By direct photosis experiments of PAA, the quantum yields (Φ) of undissociated and dissociated PAA at 254 nm were calculated and those were 1.20 mol/Einstein and 2.09 mol/Einstein, respectively. It indicated that the photolysis of PAA under UV irradiation can product·OH and initate the chain reactions of radicals. Through the competition kinetics experiments, the rate constants of undissociated and dissociated PAA with ·OH were determined, which were 9.33x108 M-1s-1 and 9.97×109 M-1s-1, respectively. It can be inferred that PAA has a scavenging effect on·OH. Different PhACs showed different degradation mechanisms during UV/PAA process.·OH was the main oxidant in the degradation of carbamazepine by UV/PAA. The degradation of ibuprofen during UV/PAA was attributed to both direct photolysis and ·OH oxidation. Direct photolysis,·OH and certain carbon-centred radicals contributed to naproxen degradation during UV/PAA. By investigating the degradation of naproxen substructure compounds and approximate structure compound during UV/PAA, the carbon-centred radicals were observed to be able to selectively degrade naphthl compounds. Combined with previous literatures with the free radical quenching experiments, these carbon-centred radicals should be CH3CO2· and CH3CO3·. In addition, the LC/MS results showed that the transformation byproducts of naproxen, ibuprofen and carbamazepine during UV/PAA were basically identical with that under UV/H2O2, indicating that there have similar degradation mechanisms in two processes.