Efficient Removal of Acid Orange 7 by Activating Persulfate Using Paper Sludge Biochar: Singlet Oxygen-Dominated Mechanism

Excess sludge in the paper industry is a hazardous solid waste that requires urgent and proper disposal for environmental protection and resource utilization. In this study, a novel magnetic biochar (Fe-SDBC) synthesized from paper sludge through one-step pyrolysis was employed to activate persulfate (PDS) for the efficient removal of acid orange 7 (AO7). The results indicated that Fe-SDBC could effectively activate PDS to remove 97.8% of AO7 within 90 min, with 89.4% removed within 5 min. Fe-SDBC had unique properties with abundant adsorption and active sites, including iron-containing compounds and oxygen-containing functional groups. The addition concentrations of Fe-SDBC (0.5 g/L) and PDS (10 mM) were optimized based on response surface methodology. Furthermore, Fe-SDBC presented good stability over a wide range of pH (3 ~ 11) and reusability in cyclic experiments. Coexisting ions, such as CO₃²⁻, HCO₃⁻, and PO₄³⁻, had an inhibitory effect on AO7 removal. Both radical and non-radical pathways were proved to be involved in the Fe-SDBC/PDS system for AO7 removal, with singlet oxygen (¹O₂) being the dominant species. Additionally, the degradation pathways were investigated and toxicity assessment was evaluated. This work will provide a potential approach for paper sludge recycling in the wastewater treatment.