Heterogeneous photo-Fenton degradation of acid orange 7 activated by red mud biochar under visible light irradiation

The heterogeneous photo-Fenton process is an effective technology for degrading organic contaminants in wastewater, and Fe-based catalysts are recently preferred due to their low biotoxicity and geological abundance. Herein, we synthesized a Fe-containing red mud biochar (RMBC) via one-step co-pyrolysis of red mud and shaddock peel as a photo-Fenton catalyst to activate H2O2 and degrade an azo dye (acid orange 7, AO7). RMBC showed excellent AO7 removal capability with a decolorization efficiency of nearly 100% and a mineralization efficiency of 87% in the heterogeneous photo-Fenton process with visible light irradiation, which were kept stable in five successive reuses. RMBC provided Fe2+ for H2O2 activation, and the light irradiation facilitated the redox cycle of Fe2+/Fe3+ in the system to produce more reactive oxygen species (ROS, i.e., •OH) for AO7 degradation. Further investigation revealed that •OH was the predominant ROS responsible for AO7 degradation in the light-free condition, while more ROS were produced in the system with light irradiation, and 1O2 was the primary ROS in the photo-Fenton process for AO7 removal, followed by •OH and O2•-. This study provides insight into the interfacial mechanisms of RMBC as a photo-Fenton catalyst for treating non-degradable organic contaminants in water through advanced oxidation processes under visible light irradiation.