Enhanced Degradation of Pharmaceuticals during Solar Photolysis of Free Available Chlorine
Date Issued
2014
Date
2014
Author(s)
Li, Chia-Pei
Abstract
The presence of emerging contaminants in aquatic system is a critical issue over the last decade because of the water scarcity resulted in the need of water recycling. Their existence in natural waters poses potential risks to ecosystem and human health. Because chlorination is widely used for disinfection and the photolysis of free available chlorine (FAC; in the form of HOCl and OCl-) generates various secondary oxidants (such as OH‧ and O3), the degradation of microcystin-LR (MC-LR), a well-known algal toxin, and seven chlorine-resistant pharmaceuticals (ketamine, metoprolol, atenolol, pentoxifylline, clofibric acid, gemfibrozil and ibuprofen) during FAC photolysis was investigated in this work. Objectives of this work were to study the natural photolysis of MC-LR in lake (Kinmen) and to investigate FAC solar photolysis of MC-LR and chlorine-resistant pharmaceuticals in chlorination process.
Indirect photolysis was the major natural attenuation process for MC-LR; the degradation rates depended on the concentrations of photosensitizers (NO3- and DOM) in the water. The photolysis half-lives of MC-LR were in the range of 11-21 hours. Similar to MC-LR, target pharmaceuticals also undergo relatively slower photolysis processes in the natural water systems. During chlorination process, solar photolysis of FAC effectively enhanced the degradation/transformation of MC-LR and chlorine-resistant pharmaceuticals. After three hours of reaction time with [FAC]0=0.5 ppm Cl2 at pH 7, target compounds were completely removed. The rate of chlorine-resistant pharmaceuticals during FAC photolysis was described using a pseudo-second-order equation (except for clofibric acid) when sufficient FAC was present in the system. FAC photolysis rate constants (k) were found to vary with initial concentrations of FAC; for example, the k of 50 ppb ketamine was between 0.0132 and 0.4518 M-1 min-1 when [FAC]0=0.05-2.00 ppm Cl2 at pH 7. Lower pH values (pH 6 (HOCl) compared to pH 9 (OCl-) resulted in higher FAC photolysis rates. During FAC photochemical reactions, OH‧ and O3 were not the only major secondary oxidants involved in enhanced MC-LR and pharmaceutical degradation. In addition, pharmaceuticals in the mixture competed for the FAC or for secondary oxidants, resulting in slower FAC photolysis rates.
Ketamine was used as a model compound for further mechanism study of FAC photolysis. Ketamine FAC photolysis was found to be a merely transformational process because TOC remained constant over the three-hour reaction period. Hydroxyl-ketamine (HK) was found to be one of the major oxidation byproducts of ketamine with FAC (HK: 2.4% yield). Similar to ketamine, HK was chlorine-resistant but could be degraded during the FAC photolysis. The FAC photolysis byproducts of ketamine differed from those of ketamine from sunlight photolysis alone. The toxicity of ketamine byproducts of this system was found to be much lower than that generated through natural sunlight photolysis.
Subjects
抗氯氧化藥物
自由餘氯
光轉化
加氯消毒
藻毒
Type
thesis
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