A Mini-Review on the Use of Chelating or Reducing Agents to Improve Fe(II)-Fe(III) Cycles in Persulfate/Fe(II) Systems

Persulfates (PSs) have recently gained recognition as strong oxidants notably for soil and groundwater remediation. Ferrous iron (Fe(II)) has been extensively employed in activating PS. However, the PS/Fe(II) system still suffers from some inherent drawbacks, including the fast accumulation of ferric ion (Fe(III)) or precipitation of Fe(III), sluggish conversion from Fe(III) to Fe(II), and the unwanted scavenging of SO4•− by excess Fe(II). To overcome these issues, the following two types of reagents are usually used: chelating agents and reducing agents. There are 11 chelating agents mentioned in this paper (EDTA, CA, etc.) and 3 reducing agents (HA, thiosulfate and A-Boron). This review extracted data from studies to give a data-based overview on how the added agents can improve the PS/Fe(II) system under varied conditions. The mechanisms of improvement by chelating agents could be classified into the following three categories: to control excess Fe(II), reduce Fe(III) to Fe(II), and inhibit Fe precipitation. The data analysis suggested an optimal working agent/Fe(II) ratio of 0–2 with acidic and neutral pH. The mechanism of using a reducing agent for PS/Fe(II) system improvement is simply reducing Fe(III) to Fe(II), using hydroxylamine, thiosulfate, etc. The improvement was significant at a hydroxylamine/Fe (II) molar ratio between 0 and 10, yet overdosed hydroxylamine can scavenge the reactive radicals, resulting in negative effects. The future research trend was also proposed in this review.