Trends of bio-Fenton, bio-photo-Fenton, and bio-electro-Fenton as emerging and sustainable applications to wastewater treatment

Bio-Fenton (BF), bio-photo-Fenton (BPF), and bio-electro-Fenton (BEF) can effectively remove organic pollutants from real wastewaters and synthetic solutions with dyes, drugs, and industrial chemicals. This review presents the fundamentals and applications of these emerging treatments for wastewater remediation. Homogeneous BF yielded oxidant ● OH from Fenton reaction between H 2 O 2 formed from a biocatalyst and added Fe 2+ , optimal in acidic medium. In heterogeneous BF, the biocatalyst was supported onto an iron material and ● OH was formed with its surface Fe 2+ , operating at neutral pH. The oxidation process was accelerated in BPF due the additional ● OH generation from photo-Fenton reaction by the light irradiated. The BEF process has been mainly developed with microbial fuel cells (MFCs) composed of separated chambers by a proton exchange membrane that removed organics via Fenton oxidation and co-generated bio-electricity. The anode was coated with a biofilm fed with a nutrient solution as the anolyte originating electrons that passed to the cathode immersed in the catholyte with the contaminated wastewater. In homogeneous BEF, the cathode electrogenerated H 2 O 2 from injected O 2 and ● OH was formed by reaction with soluble Fe 2+ , optimal under acidic conditions. Heterogeneous BEF involved the H 2 O 2 reaction with surface Fe 2+ of solid iron catalysts or functionalized iron cathodes to operate at neutral pH. For each process, the system used, the ability of H 2 O 2 generation, the degradation/mineralization performance, the role of generated oxidants, the stability of biocatalysts, catalysts, and/or electrodes, and the by-products formed are remarked. A final section details the challenges and prospects for future development.