Triclosan (TCS), an antimicrobial agent, is an emerging and persistent environmental
pollutant that is often found as a contaminant in surface waters and sediments; hence,
knowledge of its degradability is important. In this study we investigated laccase-mediated
TCS transformation and detoxification, using laccase (from the fungus Ganoderma lucidum)
in the presence and absence of redox mediators. Transformation products were identified
using HPLC, ESI-MS and GC-MS, and transformation mechanisms were proposed. In the
absence of redox mediator, 56.5% TCS removal was observed within 24h, concomitant
with formation of new products with molecular weights greater than that of TCS. These
products were dimers and trimers of TCS, as confirmed by ESI-MS analysis. Among the
various mediators tested, 1-hydroxybenzotriazole (HBT) and syringaldehyde (SYD) significantly
enhanced TCS transformation ( approximately 90%). The presence of these mediators
resulted in products with lower molecular weights than TCS, including 2,4-dichlorophenol
(2,4-DCP; confirmed by GC-MS) and dechlorinated forms of 2,4-DCP. When SYD was used
as the mediator, dechlorination resulted in 2-chlorohydroquinone (2-CHQ). Bacterial
growth inhibition studies revealed that laccase-mediated transformation of TCS effectively
decreased its toxicity, with ultimate conversion to less toxic or nontoxic products.
Our results confirmed the involvement of two mechanisms of laccase-catalyzed TCS removal:
(i) oligomerization in the absence of redox mediators, and (ii) ether bond cleavage
followed by dechlorination in the presence of redox mediators. These results suggest
that laccase in combination with natural redox mediator systems may be a useful strategy
for the detoxification and elimination of TCS from aqueous systems.