Degradation of triclosan by sulfate radicals generated by metal-mediated activation of oxidants.

摘要

Recent advances in environmental health and analytical chemistry make it possible to detect and identify many organic contaminants in water resources at trace levels. Many of these compounds are not completely degraded in conventional water and wastewater systems. The presence of persistent and toxic compounds, especially pharmaceuticals and personal care products (PPCPs) in water resources has generated considerable scientific, regulatory, and public interests, requiring the development of cost-effective technologies for the management of PPCPs-contaminated water resources. In particular, triclosan (TCS, 5-chloro-2-(2,4-dichlorophenoxy)phenol), an important antimicrobial agent widely used in many PPCPs, has attracted significant attention due to its potential endocrine disrupting capabilities.;In order to address these concerns, advanced oxidation processes (AOPs), involving the generation of strong oxidizing species including hydroxyl radicals (HRs) and sulfate radicals (SRs), have been studied and proposed to be effective to degrade a wide variety of organic compounds. However, most of research studies and application of the technologies have been focused exclusively on HRs-based AOPs. As a result, first I evaluated SRs-based AOPs as a new environmental risk management option for PPCPs-contaminated water. Triclosan, sulfamethoxazole, and acetaminophen, as model PPCPs, were effectively decomposed and mineralized by the attack of SRs generated by the activation of peroxymonosulfate (PMS) and persulfate (PS) with transition metals.;Special attention was given to understanding the effects of the oxidants and metals on TCS decomposition. PMS and PS were conjugated with transition metals (Fe2+, Co2+, Cu2+, and Ag+) in various ways. TCS was decomposed much faster with PMS than PS regardless of the metals conjugated. PMS/Co, PMS/Cu, and PS/Ag systems showed best reactivity with TCS while the other combinations exhibited negligible or much less TCS decomposition. More oxidants at a fixed oxidant:metal molar ratio resulted in faster decomposition of TCS, while excessive amounts of metals rather hindered the reaction due to undesired competition between the metal and TCS for SRs generated. Some metals, such as Co exhibiting catalytic behavior during the reaction, required less doses than their stoichiometric amounts to fully activate the oxidants, whereas other metals (e.g., Fe) required more doses. A series of the dose-effect results implied there should be optimum doses of oxidants and metals to maximize TCS decomposition.;Lastly, I investigated the detailed changes in metal speciation (solid vs. dissolved and Me2+ vs. Me3+) in Co/PMS and Fe/PMS systems over time and correlated it with TCS decomposition at different pH conditions. In spite of its significance as a key to understanding the efficiency of radical generation and catalytic/non-catalytic nature of the oxidation reaction, metal speciation has not been properly highlighted in previous studies probably due to associated analytical challenges. It was found that a rapid oxidation of Co2+ to Co3+ and Fe2+ to Fe3+ generally corresponded with the pseudo-steady state decomposition kinetics of TCS after its initial fast decomposition. I also found the presence of a potential threshold concentration of metals to effectively activate PMS. Fe required a higher threshold concentration than Co. A strong catalytic activity was observed for Co/PMS system in particular at pH 3 where most of Co added was present in the form of dissolved Co 2+. The pH impacts were different for Co/PMS and Fe/PMS, and TCS oxidation was fast at pH 5 for Co and pH 3 for Fe. As an alternative to established hydroxyl radicals, SRs exhibited high potential for the decomposition and mineralization of TCS. However, long term mineralization of TCS seemed less dependent on pH conditions.;In summary, SRs-based AOPs are effective for the degradation of TCS and other PPCPs and have the potential of providing options for the destruction of a wide range of organic contaminants in water. However, more studies are required, including identification of reaction intermediates and monitoring of toxicity in order to propose SRs-based AOPs for large scale practical applications.