Formation of Nitroaromatic Compounds in Advanced Oxidation Processes: Photolysis versus Photocatalysis

There is a growing demand for efficient treatment of organic polluted wastewaters by advanced oxidation processes (AOPs). Besides optimization of the processes, the detailed understanding of degradation mechanisms and interactions of organic pollutants with inorganic substrates is important for technical applications of AOPs. Therefore, the aim of the present study was to investigate the influence of nitrate ions on the photooxidation of phenol for various AOPs at different pH values. Three different oxidation processes were compared in these studies: direct photolysis, TiO2/UV, and H2O2/UV. Special emphasis has been laid on the analysis of byproducts, especially on the formation of nitroaromatic compounds. The formation of intermediates as well as the depletion of phenol were monitored by HPLC technique. The total organic carbon content, TOC, was measured to monitor the mineralization. Highest degradation rates and lowest concentrations of intermediates were observed with TiO2/UV being the AOP. Formation of highly toxic nitrophenols was only observed when homogeneous AOPs were employed. For the TiO2/UV process no formation of nitroaromatic byproducts occurred. At pH 5 formation of nitrophenols was observed employing direct photolysis in the presence of NO3-, while with H2O2/UV nitrophenols were detected only when the concentration of NO3- was higher than that of H2O2. At pH 11 no nitroaromatic intermediates were found for any AOPs compared in this study.