Comparative Study on Oxidative Removal of Acid Orange 7 from Water by Iron-Modified Corn Straw Biochar Activated Urea-Hydrogen Peroxide and Its Mechanisms
Acid orange 7 (AO7), a recalcitrant azo dye, poses significant threats to aquatic ecosystems and human health. This study investigates the activation of urea-hydrogen peroxide (UHP) by iron-modified corn straw biochars (Fe-CSBs) for AO7 degradation. Fe-CSBs were synthesized via impregnation-pyrolysis at 300, 500, and 700 °C using ferrous sulfate as modifier. The oxidative removal efficiencies of AO7 by Fe-CSB-activated UHP were compared, and the effects of Fe-CSB dosage, UHP dosage, initial AO7 concentration, initial pH, and coexisting anions (CO3^2−, HCO3^−, Cl^−) were systematically examined. Quenching experiments identified reactive oxygen species, and LC-MS analysis determined degradation intermediates. Results showed that all Fe-CSBs effectively activated UHP, achieving degradation rates of 98.54%, 97.38%, and 98.54% for Fe-CSB300, Fe-CSB500, and Fe-CSB700, respectively, under optimal conditions (0.2 g·L−1 UHP, 0.2 g·L−1 Fe-CSB, 20 mg·L−1 AO7, pH 3, 60 min). CO3^2− and HCO3^− inhibited degradation, while Cl^− had negligible effect. The primary reactive species were hydroxyl radicals (·OH) and singlet oxygen (^1O2). Degradation proceeded via cleavage of the azo bond, forming benzene-containing intermediates, which underwent deamination, desulfurization, and oxidation to smaller organics, ultimately mineralizing to CO2 and H2O. This work provides insights into UHP-based advanced oxidation processes for dye wastewater treatment.