Oxidative degradation of triclosan by potassium permanganate: Kinetics, degradation products, reaction mechanism, and toxicity evaluation
Graphical abstract
Introduction
Triclosan (5-chloro-2-(2,4-dichlorophenoxy)-phenol, TCS) is a broad-spectrum antimicrobial and preservative agent that is widely used in a range of consumer products such as toothpastes, antiseptic soaps, detergents, cosmetics, plastic kitchenware, socks, carpets and toys (Bedoux et al., 2012, Reiss et al., 2002, Singer et al., 2002). The global demand for TCS has continued to increase over the last thirty years, and about 350 tons are consumed every year in Europe (Halden and Paull, 2005, Pintado-Herrera et al., 2014, Young et al., 2008). Given the widespread use, this compound has been detected in surface water, wastewater, sediment, soil, organisms, and even in human milk (Fu et al., 2016, Toms et al., 2011). The high octanol water partitioning coefficient of TCS (log Kow = 4.8 at pH = 7) indicates that it can bioaccumulate in the biota and be biomagnified along the food chain. Triclosan is highly toxic to freshwater aquatic species like green algae, the water flea Daphnia magna, and fish (zebrafish, fathead minnows, bluegill sunfish) (Chen et al., 2014, Dann and Hontela, 2011). Previous researches have also shown that TCS may cause bacterial resistance, skin irritation, endocrine disruption, and even increase the formation of carcinogenic by-products (Brausch and Rand, 2011, Dayan, 2007, Novo et al., 2013).
Over the past few decades, various water treatment techniques, including ozonation, Fenton, photolysis, and permanganate treatment have been used to degrade TCS (Buth et al., 2009, Suarez et al., 2007, Sirés et al., 2007, Zhang and Huang, 2003). Compared with other oxidizing agents, permanganate possesses the attractive characteristics of high efficiency, relatively low cost, comparative stability, and ease of handling. Jiang et al. (2009) investigated the kinetics of triclosan degradation by permanganate, revealing the catalytic role of MnO2 formed in situ under slightly acidic conditions. Wu et al. (2012) found that degradation of TCS by permanganate followed pseudo-first order kinetics in a buffered DI water system at pH 7.0 and 8.6. In a drinking water system treated with permanganate, 2,4-dichlorophenol was identified as the major oxidation product of TCS. Unfortunately, to date, very little information is available on the transformation products, reaction mechanism and toxicity changes of TCS during oxidative degradation by KMnO4, though such knowledge is essential for practical implementation of the permanganate oxidation technique.
In this study, we systematically investigate the oxidation of TCS by permanganate. A series of operating parameters influencing the degradation processes, including the solution pH, temperature, and the presence of typical inorganic anions, humic, and fulvic acid, are evaluated to explore the optimum reaction conditions. The degradation products are identified by liquid chromatography-mass spectrometric analysis, and the reaction pathways are thus proposed. Frontier electron density (FED) calculations and point charges analysis are also employed to further confirm the identity of the intermediate products. Changes in the toxicity of triclosan during the oxidation process are evaluated using the luminescent bacteria Photobacterium phosphoreum and water flea Daphnia magna. In addition, the generation of disinfection-by-products from triclosan degradation products after implementing chlorination and chloramination in a full-scale treatment plant are discussed herein.
Section snippets
Chemicals
Triclosan (purity ≥ 99%) was purchased from Aladdin (Shanghai, China). The HPLC grade methanol and formic acid were supplied by Merck (Darmstadt, Germany). Other chemicals were of analytical grade and were used as received. Stock solutions of TCS, KMnO4 and NaNO2 were prepared with ultrapure water at concentrations of 500 mg/L, 5 g/L, and 30 g/L, respectively, and were all stored in brown bottles at 4 °C in a refrigerator.
Oxidation procedures
The oxidation reactions were initiated by adding 30 μL of potassium
Effect of initial solution pH
The pH of the aqueous solution may greatly influence the oxidative degradation of TCS by potassium permanganate, due to changes in the existing species of TCS (pKa = 8.1) (Kim et al., 2013) and the oxidation-reduction potential (E0) of KMnO4 (Yan and Schwartz, 1999). The following reactions proceed under the stated conditions, under acidic conditions: MnO4− + 8H+ + 5e– ⇌ Mn2+ + 4H2O, E0 = 1.51 V; under neutral or slightly alkaline conditions: MnO4− + 2H2O + 3e– ⇌ MnO2 + 4OH−, E0 = 0.58 V; under
Conclusions
Potassium permanganate treatment is demonstrated to be effective for the removal of TCS in water. Under the optimum reaction conditions of pH = 8.0, [TCS]0:[KMnO4]0 = 1:2.5 and T = 25 °C, TCS at a concentration of 20 mg/L can be completely degraded in 120 s. However, a high KMnO4 dosage is required for the efficient removal of TCS at μg/L levels in water and wastewater treatment.
Mass spectrometry analysis reveals the formation of 11 products, including phenol, derivatives of phenol,
Acknowledgements
This research was financially supported by the National Natural Science Foundation of China (No. 21577063, 21377051) and the Major Science and Technology Program for Water Pollution Control and Treatment of China (No. 2012ZX07506-001).
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