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THE AQUEOUS PHOTOLYSIS OF NICLOSAMIDE
Christina K. Remucal1, Megan McConville1, Terrance Hubert2
1University of Wisconsin-Madison, 660 N. Park St., Madison, WI 53706
2USGS Upper Midwest Environmental Services Center, 2630 Fanta Reed Road
La Crosse, WI 54603
The artificial sunlight photodegradation of 2’,5-dichloro-4’nitrosalicylanilide (niclosamide) was studied in aqueous buffer solutions at pH 5-9 using a xenon lamp and fixed-wavelength bulbs (maximum irradiance at 365 nm). The direct photodegradation of niclosamide was pH dependent, with faster degradation observed under acidic conditions. The photodegradation of niclosamide in the pH 8 buffer solution followed first-order kinetics with a quantum yield of 1.10 (± 0.02) x 10-6. Modeled solar irradiation was used to predict the half-life of niclosamide under natural sunlight. The predicted half-lives were 11.4 ± 0.2 days (continuous irradiation at the surface of the water column), 34.2 ± 0.7 days (actual calendar days at the surface of the water column), and 136.5 ± 2.9 days (continuous irradiation integrated over 55 cm of the water column). The photodegradation of niclosamide forms a series of aromatic intermediates and small acids, all of which are also susceptible to photolysis. Additionally, >20 unknown compounds with unique retention times were tentatively identified by liquid chromatography-tandem mass spectrometry. Many of these compounds are likely ring cleavage products of niclosamide or the aromatic intermediates. All organic compounds identified were present at less than 10% of the original parent compound abundance and/or concentration. The photolysis of niclosamide also produces chloride and nitrate/nitrite, corresponding to a mass balance of 97.8 ± 2.4% and 54.6 ± 3.4% with respect to the chloro- and nitro-groups, respectively. The generation of these inorganic species in solution during photolysis indicates that significant degradation of niclosamide and the resulting organic intermediate photoproducts has occurred. Kinetic modeling demonstrated that the slow first photolysis reaction of niclosamide is the rate-limiting step.