In this study photochemical fate of sulfathiazole was investigated using a sunlight simulator (ATLAS, USA). To mimick real water conditions, different natural organic matter (NOM) were used, including Suwannee River fulvic acids (SRFA, SUVA = 4.6), Beaufort River fulvic acids (BFFA, SUVA = 3.9), South Platte River fulvic acids (SPRFA, SUVA = 2.9), and a transphilic fraction of Suwannee River NOM (SRTPI, SUVA = 3.1). A high performance liquid chromatography (HPLC, Agilent) and a high resolution mass spectrometer (HRMS) (LTQ Orbitrap XL) were used for the quatification of STZ and the identification of photoproducts. Antimicrobial activity of irradiated STZ samples was evaluated by the minimum inhibition concentration against Escherichia coli. STZ undergoes rapid direct photolysis with an observed first-order rate constant (kobs) of 3.28 ± 0.06 hr-1 in buffered water (pH = 8.0); the photolytic quantum yield of STZ at pH = 8.0 was 0.079. Due to competitive light attenuation, natural organic matter isolates decreased the photolysis rate. We established a kinetic model that is capable of estimating the photochemical degradation rate of STZ in real waters using the absorbance profile of the water. This model successfully predicted the kobs of STZ in a real water collected from Denmark, Western Australia. Sulfanilic acid and 2-aminothiazole were identified as photoproducts using HRMS. An isomeric photoproduct of sulfathiazole, namely 2-imino-3-(p-aminobenzenesulfinyl-oxy)-thiazole, was also observed and its structure was postulated based on the MS/MS spectra. The photoreaction mechanism was proposed, the formation of isomer was postulated to be a result of intermediate radical addition to the endo-nitrogen of 2-AT that was formed. This was favoured by photochemical experiments spiking synthetic 2-aminothiazole. The formation of the isomer was treated by a near diffusion-controlled formation kinetics model. The three identified products showed significantly enhanced photo-stability. Nonetheless, antimicrobial assay of irradiated sulfathiazole solutions with Escherichia coli indicated little antimicrobial potency ascribed to photoproducts. This study suggests the efficacy of sunlight in rapidly degrading STZ at a predictable rate, leading to photoproducts of little antimicrobial potency. The mass spectrometry and mechanism work are both new insights into the photochemistry of sulfonamides.

Photodegradation of sulfathiazole: Kinetics, photo-induced structural rearrangement, and photoproducts antimicrobial activities

255th National Meeting (2018)

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