Photodegradation of the antihistamine cetirizine in natural waters

The photodegradation rate of the anti‐histamine cetirizine (Zyrtec®) was investigated in various water matrices. The average observed first‐order photodegradation rate coefficient (kₒbₛ), obtained by linear regression of the logarithmic‐transformed cetirizine concentrations versus irradiation time in simulated sunlight, was 0.024 h⁻¹ (n = 6; standard deviation ± 0.004) in deionized water corresponding to a half‐life of approximately 30 h. There was no statistical difference in the kₒbₛ of cetirizine photodegradation in coastal seawater compared with deionized water or deionized water amended with dissolved chromophoric organic matter. The quantum yield of cetirizine photodegradation decreased dramatically with increasing wavelength and decreasing energy of incoming radiation, with the average value ranging from 5.28 × 10⁻⁴ to 6.40 × 10⁻³ in the ultraviolet wavelength range (280–366 nm). The activation energy of cetirizine photodegradation was 10.3 kJ mol⁻¹ with an observed increase in cetirizine photodegradation as temperature increased. This is a significant environmental factor influencing half‐life and an important consideration, given that cetirizine has been detected in wastewater and receiving waters from different locations globally. Environ Toxicol Chem 2014;33:2240–2245. © 2014 SETAC