Key Takeaways & Executive Findings
- •• • Fe-CSB300, Fe-CSB500, and Fe-CSB700 achieved AO7 degradation rates of 98.54%, 97.38%, and 98.54%, respectively, within 60 min under optimal conditions (0.2 g·L−1 UHP, 0.2 g·L−1 Fe-CSB, 20 mg·L−1 AO7, pH 3), demonstrating high efficacy for dye removal. • • Coexisting CO3^2− and HCO3^− significantly inhibited AO7 degradation, while Cl^− had no effect, indicating the need for pre-treatment in carbonate-rich wastewaters. • • Quenching experiments identified ·OH and ^1O2 as the dominant reactive species, providing mechanistic insight for optimizing UHP activation. • • LC-MS analysis revealed that AO7 degradation proceeds via azo bond cleavage, leading to benzene-containing intermediates and eventual mineralization to CO2 and H2O, confirming complete detoxification potential.
Abstract
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.
1. Introduction
The widespread use of azo dyes, such as Acid Orange 7 (AO7), in textile and dye industries has led to severe water pollution, as these compounds resist natural degradation and exhibit toxic and carcinogenic properties. Conventional wastewater treatment methods, including adsorption and biological processes, often fail to achieve complete mineralization, leaving hazardous intermediates. Advanced oxidation processes (AOPs) have emerged as promising alternatives, generating reactive oxygen species (ROS) capable of non-selectively oxidizing organic pollutants. Among these, Fenton-like systems using solid peroxides like urea-hydrogen peroxide (UHP) offer advantages such as stability and ease of handling, yet their activation by heterogeneous catalysts remains underexplored.
Iron-modified biochars have gained attention as cost-effective and recyclable catalysts for activating peroxides, owing to their high surface area, iron content, and surface functional groups. However, systematic comparisons of biochars pyrolyzed at different temperatures for UHP activation are lacking, and the underlying mechanisms are not fully understood. This study addresses this gap by preparing Fe-CSBs at 300, 500, and 700 °C and evaluating their performance in UHP activation for AO7 degradation. The influence of operational parameters and coexisting anions is assessed, and the degradation pathway is elucidated, providing critical data for designing efficient and sustainable AOPs for dye wastewater remediation.
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YANG Ke, ZHAO Baowei, GUO Qi, LIU Hui, PAN Jianglong (2026). Comparative Study on Oxidative Removal of Acid Orange 7 from Water by Iron-Modified Corn Straw Biochar Activated Urea-Hydrogen Peroxide and Its Mechanisms. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2025012002
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Frequently Asked Questions
What is the optimal pyrolysis temperature for Fe-CSB to achieve highest AO7 degradation efficiency?
All three Fe-CSBs (300, 500, 700 °C) achieved high degradation rates (98.54%, 97.38%, 98.54%) under optimal conditions, indicating that pyrolysis temperature has minimal impact on catalytic activity. However, Fe-CSB300 and Fe-CSB700 showed slightly higher efficiency, suggesting that lower and higher temperatures may favor certain iron species or surface properties.
How do coexisting anions (CO3^2−, HCO3^−, Cl−) affect the degradation process?
CO3^2− and HCO3− significantly inhibited AO7 degradation, likely due to scavenging of ·OH and ^1O2, whereas Cl− had negligible effect. This implies that in carbonate-rich wastewaters, pre-treatment or pH adjustment may be necessary to maintain high degradation efficiency.
What are the dominant reactive oxygen species responsible for AO7 degradation?
Quenching experiments revealed that ·OH and ^1O2 are the primary reactive species contributing to AO7 degradation. This suggests that both radical and non-radical pathways are involved, which is crucial for understanding the activation mechanism and optimizing the system.
What is the degradation pathway of AO7 in this system?
LC-MS analysis indicated that AO7 degradation proceeds via cleavage of the azo bond (—N=N—), forming benzene-containing intermediates. Subsequent deamination, desulfurization, and oxidation reactions lead to smaller organic molecules, which are eventually mineralized to CO2 and H2O, confirming complete degradation.
What are the optimal operational conditions for maximum AO7 removal?
The optimal conditions were found to be 0.2 g·L−1 UHP, 0.2 g·L−1 Fe-CSB, 20 mg·L−1 AO7, and pH 3, achieving >97% degradation within 60 minutes. These parameters provide a baseline for scaling up the process.
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