Catalytic wet oxidation of phenol under mild operating conditions: development of reaction pathway and sludge characterization

Several process industries discharge wastewater with enormous amount of toxic phenolic compounds. Wet oxidation (WO) is considered among the potential cleaner treatment methods for such waste streams. The present study demonstrates the effectiveness of two homogeneous copper salts (nitrate and sulfate) as catalysts during batch WO process conducted under mild temperature and pressures (i.e., 120 °C and oxygen pressure = 0.5 MPa). The catalytic oxidation showed around 90 % reduction of phenol, chemical oxygen demand and total organic carbon from the wastewater within 2 h. The oxidation reaction pathway at mild conditions is also proposed based on the presence of intermediates/by-products. Fourier transform infrared spectroscopy confirmed the formation of polymerized compounds containing alcoholic/phenolic species. The inductively coupled plasma-atomic emission spectroscopy analysis revealed the presence of ~40 % of the total copper in sludge. The copper recovery from the treated wastewater and sludge and its reuse in the oxidation process should be studied in future.