Critical review of reductant-enhanced peroxide activation processes: Trade-off between accelerated Fe3+/Fe2+ cycle and quenching reactions

In this review, we systematically summarized the origins, development, basic mechanism and limits of reductant-enhanced iron-based peroxide activation processes (including H2O2, peroxymonosulfate and peroxydisulfate). Notably, the use of reductants would simultaneously accelerate the reduction of Fe3+ and quench the production of reactive species. This trade-off between Fe3+ reduction and reactive species consumption is mainly determined by two aspects, i.e. (i) intrinsic chemical properties like electron transfer pathways and reaction rates with reactive species, and (ii) experimental conditions like reagent dosage ratio, dissolved O-2 and pH. Therefore, we first reviewed the characteristics of different iron/peroxide/reductant systems in terms of the above two aspects. Then, we evaluated the practical application potential of different reductants based on their toxicity, costs and self-transformation products. Finally, newly-emerging enhancement strategies like multiple dosing and regulation of phase and crystalline degree were summarized, and future research needs were proposed.