Formation of chloroform and chlorinated organics by free-chlorine-mediated oxidation of triclosan.
by
Rule KL, Ebbett VR, Vikesland PJ.
Department of Civil and Environmental Engineering,
418 Durham Hall, Virginia Polytechnic and State University,
Blacksburg, Virginia 24060-0361, USA. Environ Sci Technol. 2005 May 1;39(9):3176-85
ABSTRACT
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)).