Effect of inorganic salt on the removal of typical pollutants in wastewater by RuO2/TiO2 via catalytic wet air oxidation

The treatment of high-salinity and high-organic wastewater is a tough task, with the removal of organic matter and the separation of salts often mutually restricting. Catalytic wet air oxidation (CWAO) coupled desalination technology (membrane distillation (MD), membrane bioreactor (MBR), ultrafiltration (UF), nanofiltration (NF), etc.) provides an effective method to simultaneously degrade the high-salinity (via desalination) and high-organic matters (via CWAO) in wastewater. In this work, five kinds of RuO2/TiO2 catalysts with different calcination temperatures were prepared for CWAO of maleic acid wastewater with a theoretical chemical oxygen demand (COD) value of 20,000 mg L−1. RuO2/TiO2 series catalysts demonstrated prominent salt resistance, with more than 80% TOC removal rates in the CWAO system containing 5 wt% Na2SO4; while RuO2/TiO2-350 showed the best degradation performance in both non-salinity and Na2SO4-containing conditions. Multiple characterization techniques, such as XRD, BET, XPS, NH3-TPD and TEM etc., verified the physicochemical structure of RuO2/TiO2 catalysts, and their influence on the degradation of pollutants. The calcination temperature was found to have a direct impact on the specific surface area, pore volume, oxygen vacancies and acid sites of catalysts, which in turn affected the ultimate catalytic activity. Furthermore, we also investigated the performance of the RuO2/TiO2-350 catalyst for the treatment of acids, alcohols and aromatic compounds with the addition of NaCl or Na2SO4, proving its good universality and excellent salt resistance in saline wastewater. Meanwhile, the relationship between the structure of three types of organic compounds and the degradation effect in the CWAO system was also explored.