Catalytic wet air oxidation of a high strength p-nitrophenol wastewater over Ru and Pt catalysts: Influence of the reaction conditions on biodegradability enhancement

p-Nitrophenol (PNP) is widely used as a raw material in several industries, therefore it can be released to the environment, being mandatory the treatment of the PNP-contaminated industrial wastewaters. In this sense, the influence of temperature, oxygen partial pressure, type of catalyst, pH and ionic strength on the wet air oxidation (WAO) of a highly concentrated PNP wastewater was studied. Several 480 min batch tests have been performed and four Pt and Ru-based catalysts have been tested. The PNP elimination, total organic carbon (TOC) abatement and the intermediates distribution were monitored. Moreover, respirometric screening tests were completed after each experiment to assess the biodegradability enhancement of the catalytic WAO (CWAO) treated effluents. The results showed that PNP elimination was higher than 90% in most cases, being the temperature the most important operating parameter upon CWAO. Additionally, all the catalysts showed a similar behaviour in terms of PNP and TOC conversions. Besides, CWAO increased the biodegradability by more than 50% in most of the tested conditions, being the carboxylic acid fraction the key factor to be taken into account, as the best biodegradability enhancement was observed when this fraction was the highest. The partial pressure of oxygen had a negligible effect on the biodegradability enhancement. The ionic strength influence over the CWAO was studied and even though it did not affect the CWAO performances, the presence of NaCl in the solution resulted in a decrease of the effluent biodegradability. In terms of pH, the most suitable scenario was the one with no pH adjustment. Conclusively, this work demonstrated that an integrated CWAO and biological treatment would allow an easy removal of PNP and the intermediates formed during the first step of the treatment, being the best CWAO conditions for this pre-treatment to work at 180 °C under stoichiometric oxygen pressure (i.e. 7.6 bar of oxygen partial pressure) with a Ru/TiO2 catalyst.

Figure optionsDownload as PowerPoint slideHighlights▸ Conversion higher than 90% were systematically achieved, being temperature the most important parameter. ▸ Identification of oxidation intermediates allowed establishing a reaction pathway. ▸ In general, CWAO increased biodegradability by more than 50%. ▸ All the catalysts exhibited a similar behaviour. ▸ Carboxylic acid fraction was the key factor for biodegradability enhancement.