Triclosan removal from surface water by ozonation - Kinetics and by-products formation

Removal of triclosan from surface water by ozonation was investigated. The results showed that complete elimination of triclosan from a surface water bearing 1-5 mg/L triclosan via continuous ozonation at 5 mg/L, require an ozonation time of 20-30 min depending on pH. Triclosan oxidation followed pseudo-first order kinetics with an apparent reaction rate constant varying from 0.214 min^-1 to 0.964 min^-1 depending on pH, initial triclosan concentration and water composition. Although the effect of pH was complex due to possible existence of different moieties, higher TCS removal efficiencies were obvious at weak-base conditions. Experiments performed to identify degradation by-products showed the formation of four by-products, namely, 2,4-dichlorophenol, 4-chlorocatechol and two unidentified compounds. Additionally, 2,4-dichloroanisole was detected when a methyl moieties exist in water. By-products were found to be eliminated upon further ozonation. The required exposure time varied from 20 to 30 min depending on pH of water. The ozone demand exerted for the complete oxidation of triclosan and its by-products was calculated as 13.04 mg ozone per mg of triclosan. A triclosan degradation pathway, which was found to be highly pH dependent, was proposed.