N-doped graphitic biochars from C-phycocyanin extracted Spirulina residue for catalytic persulfate activation toward nonradical disinfection and organic oxidation

Biochars are low-cost and environmental-friendly materials, which are promising in wastewater treatment. In this study, biochars were manufactured from C-phycocyanin extracted (C-CP) Spirulina residue (SDBC) via thermal pyrolysis. Simultaneously, N-doping was also achieved from the protein in the algae for obtaining a high-performance carbocatalyst for peroxydisulfate (PDS) activation. The SDBC yielded large specific surface areas, nitrogen loading, and good conductivity, which demonstrated excellent oxidation efficiencies toward a wide array of aqueous microcontaminants. An in-depth mechanistic study was performed by integrating selective radical scavenging, solvent exchange (H2O to D2O), diverse organic probes, and electrochemical measurement, unveiling that SDBC/PDS did not rely on free radicals or singlet oxygen but a nonradical pathway. PDS intimately was bonded with a biochar (SDBC 900-acid, pyrolysis at 900 °C) to form a surface reactive complex that subsequently attacked an organic sulfamethoxazole (SMX) adsorbed on the biochar via an electron-transfer regime. During this process, the SDBC 900-acid played versatile roles in PDS activation, organic accumulation and mediating the electron shuttle from SMX to PDS. This nonradical system can maintain a superior oxidation efficiency in complicated water matrix and long-term stable operation. More importantly, the nonradical species in SDBC 900-acid/PDS system were capable of inactivating the bacteria (Escherichia coli) in wastewater. Therefore, the biochar based nonradical system can provide a mild and high-efficiency strategy for disinfection in waste and drinking water by green carbocatalysis. This study provides not only a value-added biochar catalyst for wastewater purification but also the first insight into the bacteria inactivation via nonradical oxidation.