**ABSTRACT NOT FOR CITATION WITHOUT AUTHOR PERMISSION. The title,
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please contact the author via e-mail at remucal@wisc.edu or via telephone at
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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
June 2015
ABSTRACT:
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.