Strategies based on electron donors to accelerate Fe(III)/Fe(II) cycle in Fenton or Fenton-like processes

Low Fe(II) generation is the key limitation for Fenton process, the solution is to provide electrons to accelerate Fe (III) reduction. Many technologies were currently developed to enhance Fenton reactions, yet some mechanisms of the enhanced processes were reported to be inconsistent and the most promising approach remains to be identified. This work systematically reviewed the current strategies to promote Fe(III)/Fe(II) cycle based on electron donors, which were classified into five categories according to electrons being provided by photo, electricity, reductants, oxidants and target contaminants, labelled as e-PF, e-EF, e-Re, e-Ox and e-TC, respec-tively. Principles of each enhanced Fenton process were clarified. Chelate agents were widely used in the e-Re, e-PF, e-Ox, e-EF, e-TC Fenton processes. Roles of chelate agents like carboxylates, natural antioxidants in all the enhanced Fenton processes were reviewed. Chelate agents with reducing property like tannic acid, tea poly -phenols and catechin were found to increase the most degradation rate of target pollutants. Carbon-based ma-terials are another widely used catalysts in Fenton process. The same carbon-based catalysts can play different roles via distinct mechanisms during electron donation in the e-Re, e-Ox, e-TC, and e-PF processes, which were all clearly elucidated in this work. The stability of the catalysts and the pH adaptability of each approach fol-lowed the trend of e-PF > e-Re > e-Ox > e-TC. In addition, Fenton process enhanced by the e-Re exhibited the best performance on pollutant degradation. Through comprehensive consideration of pollutant degradation, catalyst stability and pH adaptability, e-Re might be the most promising enhanced Fenton process. Development of reductants that can supply electrons to heterogeneous iron would be necessary for future practical application.

Automated summary

Low Fe(II) generation is the main limitation for Fenton process, and providing electrons is the solution to promote Fe(III) reduction. This study systematically reviewed the strategies for enhancing Fe(III)/Fe(II) cycle based on electron donors, which can be classified into five categories: e-PF, e-EF, e-Re, e-Ox, and e-TC. The principles and mechanisms of each enhanced Fenton process were clarified, and the role of chelate agents and carbon-based materials were discussed. Among them, e-Re showed the best performance in pollutant degradation, making it the most promising approach. The development of reductants for future practical application is necessary.