Degradation of diclofenac in aqueous solution by homogeneous and heterogeneous photolysis

Background: Pharmaceuticals have arisen as emerging environmental pollutants due to their potential impact on humans, animals and microorganisms even at low concentrations. Conventional wastewater treatment plants are known to be ineffective for removal of many pharmaceuticals present in wastewater. Advanced oxidation processes are one of the most promising treatment technologies for degradation of such persistent compounds. The current study was focused on the efficacies of various oxidation processes for degradation of diclofenac (DCF) as one of the most widespread pharmaceuticals. Methods: The efficacy of UV photolysis, hydrogen peroxide photolysis, homogeneous and heterogeneous Fenton/photoFenton treatment for removal of DCF from aqueous solution was examined. The impact of pH, hydrogen peroxide concentration, and catalyst type on DCF removal was assessed. The identification of DCF photo-degradation products with GC-MS (EI) technique was carried out. Results: According to results of the present study direct photolysis proved main contributing reaction pathway in all studied systems with UV irradiation. Fast decrease in pH value observed during all studied processes started at pH 5, 7, and 9 led to the system controlled by DCF precipitation-degradation-re-dissolution conditions. The enhanced efficacy of promoted photolysis proved dependent on the rate of pH decrease to value below pKa at the beginning of the process. After reaching acidic pH values of the surrounding solution the DCF degradation was controlled by the solubility independently of treatment method applied. The highest DCF degradation efficacy was attained by the heterogeneous photo-Fenton treatment at initial pH 11. Thus, DCF 90% conversion time decreased from 48 to 23.5 min for UV photolysis and heterogeneous photo-Fenton process (3.8 mM H2O2), respectively. Conclusions: DCF was quite resistant to all tested processes. The application of the Fenton-based treatment and UV/H2O2 did not show vital advantages compared to UV photolysis when the initial pH was 7 or 9. Direct photolysis proved main process contributing to DCF degradation in all studied systems combined with UV irradiation. The present study was the first to evaluate the efficacy of photo-Fenton catalysed by goethite for DCF degradation in aqueous solution. The latter proved the most efficient one among the Fenton-based processes.