Formation of Chloroform and Chlorinated Organics by Free-Chlorine-Mediated Oxidation of Triclosan

Department of Civil and Environmental Engineering, 418 Durham Hall, Virginia Polytechnic and State University, Blacksburg, Virginia 24060-0361, USA.
Environmental Science and Technology (Impact Factor: 5.33). 06/2005; 39(9):3176-85. DOI: 10.1021/es048943+
Source: PubMed


The widely used antimicrobial agent triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) readily reacts with free chlorine under drinking water treatment conditions. Overall second-order kinetics were observed, first-order in free chlorine and first-order in triclosan. Over the pH range of 4-11.5, the kinetics were pH sensitive as a result of the pH dependent speciation of both triclosan and free chlorine. Using a Marquardt-Levenberg routine, it was determined that this pH effect indicates that the dominant reaction in this system is between the ionized phenolate form of triclosan and hypochlorous acid (HOCl). The overall second-order rate coefficient was determined to be kArO- = 5.40 (+/- 1.82) x 10(3) M(-1) s(-1). Three chlorophenoxyphenols and two chlorophenols were identified by gas chromatographic-mass spectroscopic analysis. The chlorophenoxyphenol compounds include two monochlorinated triclosan derivatives (5,6-dichloro-2-(2,4-dichlorophenoy)phenol and 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol) and one dichlorinated derivative (4,5,6-trichloro-(2,4-dichlorophenoxy)phenol); these species form via bimolecular electrophilic substitution of triclosan. 2,4-Dichlorophenol was detected under all reaction conditions and forms via ether cleavage of triclosan. In experiments with excess free chlorine, 2,4,6-trichlorophenol was formed via electrophilic substitution of 2,4-dichlorophenol. Chloroform formation was observed when an excess of free chlorine was present. A Hammett-type linear free-energy relationship (LFER) using Brown-Okamoto parameters (sigma+) was established to correlate the reactivity of HOCI and the phenolate forms of triclosan and other chlorophenols (log kArO- = -(10.7 +/- 2.2)Sigmasigma(+)o,m,p + 4.43). This LFER was used to obtain estimates of rate coefficients describing the reactivity of the intermediates 5,6-dichloro-2-(2,4-dichlorophenoy)phenol (kArO- approximately equal to 6 x 10(2)), 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol (kArO- approximately equal to 3 x 10(2)), and 4,5,6-trichloro-(2,4-dichlorophenoxy)phenol (kArO- approximately equal to 4 x 10(1)).

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    • "Previous studies have reported several degradation intermediates from chlorination or photolysis of triclosan. Rule et al. (2005) identified three chlorophenoxyphenols and two chlorophenols in the reaction of triclosan with free chlorine. Ferrer et al. (2004) proposed the replacement of chlorine atoms by hydroxyl groups and chlorine losses as the major photolytic degradation pathways of triclosan, and 2,4-dichlorophenol could also be formed from photoinduced hydrolysis of triclosan through cleavage of the CeO ether bond. "
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    ABSTRACT: The co-exposure to UV irradiation and free chlorine may occur in certain drinking water and wastewater treatment systems. This study investigated the effects of simultaneous low pressure ultraviolet (LPUV) irradiation and free chlorination on the formation of chloroform from triclosan which is a commonly used antibacterial agent. Different treatment systems (i.e., combined UV/chlorine, UV alone, and chlorine alone) were applied to examine the degradation of triclosan and formation of chloroform. The fate of representative intermediates, including chlorinated triclosan, dechlorinated triclosan intermediates and 2,4-dichlorophenol, were tracked to deduce the effect of combined UV/chlorine on the transformation of chloroform formation precursors. The relation between intermediates degradation and chloroform formation was investigated in depth by conducting stepwise experiments with UV and chlorine in different sequences. Results indicate that the combined UV/chlorine notably enhanced the chloroform formation from triclosan. From the reaction mechanism perspective the combined UV/chlorine, where the direct photolysis may play an important role, could accelerate the decay of intermediates and facilitate the generation of productive chloroform precursors. The radicals had modest influence on the degradation of triclosan and intermediates and partly hindered the formation of chloroform. These results emphasize the necessity of considering disinfection by-products formation in the application of combined UV/chlorine technology during water treatment.
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    • "It is well known that phenolic compounds, are generally more reactive upon deprotonation . As postulated by Rule et al. (2005), this effect occurs because phenolic AO À is better at activating the aromatic ring toward substitution reactions than AOH. Given that TCS and BPA have a pKa value around 8.1, their degradation at pH values around 10 is increased besides the lower reactivity of hypochlorite anion which results in the decrease of the degradation of other target compounds . "
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    • "fish and algae at ␮g/L level [16] [17]. Moreover, triclosan can react with free chlorine and produce toxic chlorinated phenoxyphenols during water disinfection [18]. Carcinogenic 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) can also be generated during the photolysis of triclosan [19–24]. "

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