Synergistic degradation of formaldehyde and chlorophenols in N-CQDs/BiOBr photocathode-coupled microbial fuel cells and the enhancement of power production efficiency

p-Chlorophenol (4-CP) is a recalcitrant pollutant with high toxicity that bioaccumulates and persists in the environment. In order to remove most of 4-CP in a short time, this study constructed an N-CQDs/BiOBr photocatalytic cathode microbial fuel cell (PMFCs) system, introduced formaldehyde wastewater into the biological anode, and enhanced the electronic interaction between the cathode and anode, thus realizing the high efficiency of the degradation of 4-CP and the production of electric energy. The results showed that the removal rate of 4-CP (30 mg/L) by PMFCs was 86.8%, which was higher than that of MFC (65%) and photocatalysis (11.73%) combined, showing a good synergistic effect. In addition, driven by the continuous negative bias provided by the anode, the electrons generated at the anode preferentially combine with the photogenerated holes at the cathode, which increases the power generation of the PMFCs, and achieves a maximum power density of 286.225 W/ m2 and a maximum open-circuit voltage of 0.518 V. The electrochemical properties of PMFCs were characterized by cyclic voltammetry and electrochemical impedance spectroscopy, and the results showed that the charge transfer resistance of PMFCs was only half that of MFC. Free radical trapping experiments have confirmed that ∙O2- is the main reactive oxidizing substance. Subsequently, a possible electron transport mechanism was proposed, and five degradation pathways of 4-CP were identified using liquid chromatography-mass spectrometry. In conclusion, this technology is a good alternative method to improve the treatment efficiency of refractory wastewater in practical applications. •Possible degradation pathways and electron transfer mechanisms of p-chlorophenol.•A coupled system of biological anode and photocathode was constructed.•Formaldehyde and p-chlorophenol have synergistic removal effects.