Comparative study of alcohol co-oxidation for enhanced phenol degradation in wastewater under supercritical water oxidation (SCWO) conditions

The treatment of phenol-containing wastewater is garnering significant interest due to phenol's toxicity and resistance to degradation. Supercritical Water Oxidation (SCWO) is noted for its effective breakdown of such waste. However, the process's high energy consumption is a barrier to industrial scaling. The introduction of auxiliary fuels to the SCWO process could enhance co-oxidation, thereby improving phenol decomposition rates and reducing energy demands. This study examines the effectiveness of methanol, ethanol, n-propanol, and isopropanol as auxiliary fuels in the SCWO treatment of phenol-containing wastewater, with hydrogen peroxide as the oxidant. Through simulations and experimental studies, the co-oxidation mechanisms of these alcohols in augmenting phenol oxidation were analyzed. Results indicated that H₂O₂ initially formed abundant OH· and HO₂· radicals. These radicals, further amplified by attacking the more reactive auxiliary fuels, accelerated reactions with phenol, significantly improving its degradation rate. The removal of phenol from phenol-containing wastewater increased from 77.15 % to 82.77 % (methanol), 99.43 % (ethanol), 99.12 % (n-propanol), and 98.04 % (isopropanol), respectively, within a reaction time of 15 s at a reaction temperature of 400 °C and 25 MPa. This indicates that the effectiveness of simple alcohols in removing phenol decreases in the order: ethanol > n-propanol > isopropanol > methanol. Furthermore, while auxiliary fuels enhanced degradation by increasing reactive intermediates, higher oxidation coefficients also improved phenol removal by boosting initial OH· and subsequent HO₂· formation from H₂O₂ decomposition. This investigation provides insights into the optimal application of alcohols as co-oxidants in SCWO, charting a course for more energy-efficient treatment of phenol-containing wastewater.