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

• Fe(III)/Fe(II) cycle enhanced by light, electricity, reductants, oxidants and contaminants were clarified. • Roles and mechanisms of chelate agents and carbon materials in electron donation were summarized. • Electrons donated by reductants showed the best pollutant degradation and pH adaptability. • Enhanced Fe(III)/Fe(II) cycle by reductants shows the similar degradation rate in homogeneous and heterogeneous systems. 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, respectively. 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 polyphenols and catechin were found to increase the most degradation rate of target pollutants. Carbon-based materials 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 followed 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.