Degradation of selected pharmaceuticals with pulsed corona discharges generated in water

摘要

Summary form only given. Highly industrialized societies are facing growing problems with pollution of drinking water by chemically and biologically inert contaminants. In particular pharmaceutically active compounds (PhACs) are withstanding destruction by conventional means and can be found in increasing concentrations [1]. Advanced oxidation processes (AOPs), including plasmas that are generated at atmospheric pressure, can provide a solution for water decontamination. In particular the generation of reactive species like hydroxyl radicals offers a possibility to break up even recalcitrant substances. We developed a plasma source with a coaxial geometry to create large volume corona discharges directly in water. For diagnostic reasons, The reaction chamber was made of a glass tube with the length of 150 mm and a diameter of 45 mm, hence holding a treatment volume of about 240 ml. Discharges were generated by applying positive high voltage pulses from a 6-stage Marx-bank generator. Streamers were forming around a thin tungsten wire (diameter of 50 μm) which was fixed in the center of the tube. Repetition rates of 20 Hz and peak voltages up to 80 kV were investigated. To evaluate the efficacy of the setup we decomposed 6 selected pharmaceuticals: carbamazepine (anti-epileptic), diatrizoate (x-ray contrast agent), diazepam (hypnotic, anticonvulsant), ethinylestradiol (hormone), diclofenac and ibuprofen (analgesics, anti-inflammatories), which are known to be ecologically problematic or in general may cause problems for water purification. Solutions of 0.5 mg/l for each substance were prepared and treated with up to 80,000 discharges. Concentrations after the treatment were determined with HPLC-MS. Depending on the recalcitrance of the selected pharmaceuticals, we achieved degradation rates of 45 % (diatrizoate) to 99 % (diclofenac). During the treatment pH-values stayed almost constant, but concentrations of dissolved oxygen doubled. Experiments in pure wate- showed that the amount of nitrate and nitrite that was created was insignificant and far below legal limits for drinking water. Experiments on the reaction pathways and kinetics that are responsible for the observed degradation are underway.