Water purification by Catalytic Wet Air Oxidation (CWAO)

The generation of hazardous wastewaters from industrial and domestic processes, poses a serious environmental threat if left untreated and allowed to enter water sources. Current techniques for removing toxic organic pollutants from wastewaters include biological, thermal and chemical treatment, but they are often coupled with an inability to degrade high concentrations of pollutants, long biological degradation lifetimes or the release of harmful emission gases. As we steer towards more sustainable and environmentally friendly techniques, advanced oxidation processes have become increasingly popular. Wet Air Oxidation (WAO) and subsequently, Catalytic Wet Air Oxidation (CWAO), is proving to be one of the most economical and environmentally friendly processes for the removal of toxic organic compounds found in industrial wastewater streams. Using oxygen as a clean source of oxidant, CWAO oxidises the pollutants, over a catalyst, to CO2 and H2O. The addition of a catalyst offers increased reaction rates and less severe operating conditions, which in turn leads to lower capital costs. Phenol was selected as a model pollutant, for CWAO. Building on previous research, alternate catalyst compositions are investigated with particular attention paid to the wettability of catalyst support. Noble metals supported on silicon carbides were investigated for the CWAO of phenol and bisphenol-A (BPA) in a trickle bed reactor (TBR). Complete phenol conversion was observed for Pt/SiC at 7 bar(g) and 160 oC, with 99.9 % selectivity towards CO2. The partial substitution of platinum with ruthenium was investigated, as well as the effect of surface area of the silicon carbide support. An explanation is proposed for the increased reaction rates seen when using comparatively hydrophobic catalyst supports; the presence of gas bubbles within the catalyst pores on a hydrophobic support allows for a secondary mass transfer route of oxygen directly from the gas phase to the catalyst surface, prohibited by the more typical hydrophilic catalysts. iii The catalysts screened were characterised using a number of techniques including, powder x-ray diffraction (XRD), N2 adsorption BET specific surface area, thermogravimetric analysis (TGA), temperature programmed reduction (TPR), scanning election microscopy (SEM), energy dispersive x-rays (EDX), x-ray photoelectron spectroscopy (XPS) and helium pycnometry porosity measurements