CWAO of phenol over activated carbon catalysts with hydrogen peroxide as promoter

Catalytic wet air oxidation (CWAO) of phenol over activated carbon catalysts in presence of a typical promoter such as hydrogen peroxide has been studied. The experiments were carried out in a trickle-bed reactor at given operating conditions (Cinlet phenol= 1 g/L, Cinlet H2O2= 5g/L, T=127 oC, PT=8 atm and WCAT=2.5 g) in a wide range of space time values (W/QL=20-320 gCAT·h/L). The effect of the addition of hydrogen peroxide was analyzed in terms of initial reaction rate, distribution of intermediates and toxicity of the effluent. To this purpose, the experimental profiles of phenol and TOC conversions vs. space time were obtained, intermediates were properly identified and the phenol oxidation pathway proposed. Toxicity of the effluents was also measured. The results showed that due to the employ of hydrogen peroxide which leads to the production of hydroxyl and hydroperoxy radicals to initiate the reaction, faster initial reaction rates of phenol and TOC were achieved (rates were increased around 90 and 60%, respectively). Phenol was fully hydroxylated to hydroquinone which was instantaneously oxidized to pbenzoquinone. This quinone was oxidized to CO2 and low molecular weight acids such as maleic, oxalic, acetic and formic. Acetic acid was the only refractory specie at the operating conditions employed. Concentrations of the cyclic intermediates (hydroquinone and p-benzoquinone) were lower in presence of hydrogen peroxide; therefore, detoxification of the wastewater was significantly enhanced. Wet oxidation at space times above 160 gCAT·h/L should be enough for the disposal of the liquid effluent in a municipal wastewater treatment plant. These results present CWAO with activated carbon catalysts promoted by hydrogen peroxide as a promising technology to treat phenolic wastewaters. As far as toxicity of the effluent is concern, it is preferable to use a promoter than to work only with activated carbon as catalyst or support.