(225j) UV-Based Treatment of Fluoroquinolone Antibiotics in Wastewater

Over the past few decades, pharmaceutical use has become significantly more prevalent due to advances in technology and medicine; however, this rise in pharmaceutical consumption has been correlated with increases in the concentrations of pharmaceuticals detected in municipal wastewater.  Several reports have expressed concerns over the presence of antibiotics in wastewater due to the potential for inhibition of select microorganisms and the development of antibiotic resistance.  Furthermore, low concentrations of novel biologically active compounds may pose threats to aquatic ecosystems.  Trace concentrations of pharmaceuticals persist through wastewater treatment plants and are discharged to surface water, compounding the issue.

This research examines transformation of pharmaceutical antibiotics using UV irradiation and the UV-H₂O₂ advanced oxidation process.  Five fluoroquinolone antibiotics, namely ciprofloxacin, enrofloxacin, levofloxacin, norfloxacin, and sarafloxacin, were investigated.  Transformation occurred through direct photolysis (UV), as well as through reaction with hydroxyl radicals (UV-H₂O₂).  The reaction kinetics were examined by quantifying the quantum yield and the fluence-based rate constants.  For each experiment, 10^-7 M of the antibiotics were dosed into 2 L of 10 mM phosphate buffer (for pH control).  In the case of UV-H₂O₂ testing, 10^‑5-10^-7 M hydrogen peroxide was added to the experimental solution.  The solution was then run through a batch-recycle reactor containing a low-pressure UV lamp that emits 2.55×10^-4 Einstein/min at 254nm.  Experiments generally lasted ten minutes for direct photolysis and five minutes for UV-H₂O₂; samples were withdrawn at appropriate time intervals.  Fluoroquinolone concentrations in experimental samples were analyzed using high performance liquid chromatography with fluorescence detection (HPLC-FLD), which allowed for measurement of all five fluoroquinolones in one sample run. 

To examine the effect of complex water matrices on transformation, combinations of multiple fluoroquinolones (total concentration of 10^-7 M) were simultaneously dosed into the reactor at varying molar ratios.  Data resulting from these experiments verified the quantum yield and fluence-based rate constants for all five fluoroquinolones.  In addition, the impact of real wastewater matrices was examined by spiking fluoroquinolones into influent and tertiary effluent from a regional wastewater treatment plant.  The resulting transformation efficiencies were used to quantify the impact of wastewater-derived organic matter on treatment of fluoroquinolone antibiotics.  The impact of dissolved organic matter on UV photolysis and UV-H₂O₂advanced oxidation was determined and provides important insight into transformation of pharmaceuticals by direct photolysis and hydroxyl radicals.

The results of this research include the following:  fluence-based rate constants for the five fluoroquinolones at wastewater treatment-relevant pH; the impact of complex wastewater matrices on transformation kinetics (i.e., multiple fluoroquinolones and wastewater-derived organic matter); and the improvement in transformation kinetics as a function of H₂O₂ concentration.  As more wastewater treatment plants utilize UV disinfection processes, the results of this research will prove useful in developing effective solutions to the emerging threat from pharmaceuticals.