Heterogeneous electro-Fenton and photoelectro-Fenton processes: A critical review of fundamental principles and application for water/wastewater treatment

This exhaustive review focuses on the fundamental principles and applications of heterogeneous electrochemical wastewater treatment based on Fenton's chemistry reaction. The elementary equations involved in formation of hydroxyl radical in homogeneous electro-Fenton (EF) and photo electro-Fenton (PEF) processes was presented and the advantages of using insoluble solids as heterogeneous catalyst rather than soluble iron salts (heterogeneous electro-Fenton process) (Hetero-EF) was enumerated. Some of the required features of good heterogeneous catalysts were discussed, followed by the mechanisms of catalytic activation of H2O2 to reactive oxygen species (ROS) especially hydroxyl radical (center dot OH) by heterogeneous catalyst in Hetero-EF system. Extensive discussion on the two configuration of Hetero-EF system vis-a-vis added solid catalysts and functionalized cathodic materials were provided along with summaries of some relevant studies that are available in literature. The solid catalysts and the functionalized cathodic materials that have been utilized in Hetero-EF wastewater treatment were grouped into different classes and brief discussion on their synthesis route were given. Besides, the use of solid catalysts and iron-functionalized cathodic materials in bioelectrochemical system (BES) especially bioelectro-Fenton technology (BEF) using microbial fuel cells (MFCs) with concurrent electricity generation for Hetero-EF treatment of biorefractory organic pollutants was discussed. In the final part, emphasis was made on the challenges and future prospects of the Hetero-EF for wastewater treatment.