Efficient degradation of phenol wastewater using CuFe2O4 as highly active catalyst in the microwave-assisted catalytic wet peroxide oxidation process

Abstract

Six kinds of CuFe2O4, including capsule-shaped CuFe2O4-C, plate-shaped CuFe2O4-P, polyhedral-shaped CuFe2O4-O, and irregular shaped catalysts prepared by co-precipitation (CuFe2O4-CO), by microwave heating (CuFe2O4-MW), and by hydrothermal synthesis (CuFe2O4-S), were used in the microwave-assisted catalytic wet peroxide oxidation (MW-CWPO) process to remove phenol from wastewater. The physicochemical properties of the catalysts were dependent on preparation methods. The catalytic activities were in the order of CuFe2O4-CO (95.3 %) >> CuFe2O4-P (78.8 %) > CuFe2O4-C (72.7 %) > CuFe2O4-MW (67.1 %) > CuFe2O4-S (63.3 %) > CuFe2O4-O (60.0 %). Effects of CuFe2O4-CO dosage, H2O2, pH, phenol initial concentration, anions, and MW in the MW-CWPO process were then systematically studied. Under optimal conditions of pH = 3, H2O2 = 1 mL/L, and CuFe2O4-CO = 0.5 g/L, MW energy of 99 kJ (550 W, 3 min) and 132 kJ (550 W, 4 min) were found essential to achieve 99.6 % removal rate of phenol and 95.3 % removal rate of chemical oxygen demand (COD). Possible degradation pathways were suggested: phenol was firstly oxidized into hydroquinone and catechol, which further degraded into p-benzoquinone, 1,2-benzoquinone, respectively. Finally, these compounds transformed into oxalic acid, CO2, and H2O within 4 min. The proposed mechanism suggested that the catalyst CuFe2O4-CO generates more “hot spots” due to its irregular shape, which enhances MW scattering and absorbing abilities. Consequently, the chemical composition and crystal structure of the catalyst significantly promote the decomposition of H2O2 to generate free radicals. The CuFe2O4-CO has an opportunity of potential engineering application due to its high efficiency and stability.