Catalytic wet air oxidation: developing a continuous process

Past and present techniques to remove toxic organic pollutants from industrial wastewaters have involved biological, oxidative and thermal treatment, but long biological degradation lifetimes and harmful emissions released via incineration type processes poses an environmental problem. Much of the new and emerging technologies have steered away from chemical treatment and progressed towards more sustainable and environmentally friendly processes. Wet air oxidation (WAO), being one of them, uses air as the oxidant mixed with the wastewater solution to oxidise the pollutants. This technology has evolved over the years to include catalysis (CWAO) which offers a greener and more cost effective form of industrial wastewater treatment. The majority of CWAO studies involve batch treatment in autoclave reactors, but this project’s aim was to make the treatment process continuous, using an active, stable heterogeneous catalyst in a trickle-flow reactor. Phenol was chosen as the model pollutant and the goal was to reduce its concentration from 1000 ppm to below the EPA limit under the least energyintensive conditions possible. The initial stages were made up of commissioning a reactor, followed by catalyst screening and optimisation, which included correlating activity with catalyst structure and composition. HPLC followed by UV detection was used to quantify phenol conversion, while a range of surface and bulk characterisation techniques were used to determine catalyst structure. Of the catalysts screened platinum supported by silicon carbide provided the most successful results in terms of conversion. SiC’s hydrophobic nature limits the wetting experienced during a CWAO reaction; a process that hinders oxygen activation. Doping with ceria improved the catalyst’s performance, allowing the metal loading to be reduced while maintaining high conversion of phenol. However when ruthenium was the active component, the more hydrophilic alumina was the preferred support. The reaction with ruthenium relies more on catalyst wetting as it is already in the oxide form. When tests were subsequently carried out on Pt/alumina catalysts, they confirmed the need to increase the hydrophobicity in order to achieve high activity. It is proposed that when the active sites are metallic, the optimum support surface is highly hydrophobic; whereas when metal oxide provides the active sites, the optimum support surface is hydrophilic. These findings explain how some of the catalytic components contribute towards CWAO’s reaction mechanism, and activity controlled, in a way not yet shown by previous publications