Fluoroquinolones (FLQs) are among the most commonly prescribed antibiotics, the occurrence in different waterways at μg/L level was frequently recorded worldwide. The dissipation of FLQs in waters is mainly governed by absorption and photolysis. Their photolysis in buffered water was found to be extremely high (e.g., &gt; 2.0 hr-1 at pH 8.0 for norfloxacin under simulated sunlight). However, in real waters the degradation rate is inhibited by the presence of natural organic matter (NOM). The phototransformation mechanism of FLQs has been widely studied in the past two decades. To the best of our knowledge, the photolytic degradation of FLQ, e.g., norfloxacin (NOR), is occurring from their triplet excited state (3NOR●). The role of NOM in the phototransformation of NOR could take place at two levels, i.e., competitive light screening thus inhibiting the photolytic rates (kNOR), and interacting with 3NOR●. The former was already described in several past studies; while little information is available for the interactions between 3NOR● and NOM. In the current work, 3NOR● interactions with different NOM isolates were investigated using a pump-probe laser spectroscopic instrument. In aerated buffer solutions, 3NOR● was observed with a lifetime (τ3NOR●) of 1.27 ± 0.015 msec. The four NOMs showed different effects on τ3NOR●. Suwannee River fulvic acid (SRFA, SUVA = 4.6), Gartempe River fulvic acid (GRFA, SUVA = 4.0), and Beaufort River fulvic acid (BFFA, SUVA = 3.8) exhibited a quenching effect. The quenching rate constants were determined to be 3.0×107 (SRFA), 1.4×107 (GRFA), and 2.8×107 (BFFA) L molC-1s-1, respectively. Surprisingly, SPRFA (SUVA = 2.9) significantly increased τ3NOR● during the laser experiments. Possible reason is the more efficient energy transfer from excited state of SPRFA (SPRFA●) to NOR. This observation has important implications for the photochemical fate of NOR in sunlit waters. NOR photodegradation experiments performed in a sunlight simulator showed that NOM has an impact on kNOR values. In agreement with above-discussed time-resolved result, SRFA, GRFA, and BFFA decreased kNOR, whereas a slight increase in kNOR was observed in the presence of SPRFA. The results presented herein are important new insights into the aquatic photochemistry of FLQ antibiotics and the role of NOM interacting with transients of micropollutant on nsec to msec timescale.

Excited triplet state of antibiotic norfloxacin interactions with dissolved natural organic matter: A laser spectroscopic study

255th National Meeting (2018)

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