Iron-mediated activation of persulfate and peroxymonosulfate in both homogeneous and heterogeneous ways: A review

Various organic contaminants accumulated in the environment pose great threat to ecosystems and human health. Sulfate radical-based advanced oxidation processes (SR-AOPs) have attracted increasing attention for the removal of these contaminants in recent years. Iron species, including ferrous and ferric iron, zero-valent iron, iron oxides and oxyhydroxides, iron sulfides and various supported iron catalysts, are known to be effective in activating persulfate (PS) or peroxymonosulfate (PMS) to generate sulfate radicals. This review is dedicated to summarize the up-to-date research progresses of iron-mediated activation of PS and PMS mediated by these iron-based species in both homogeneous and heterogeneous ways. The activators are categorized based on their chemistry, and the up-to-date knowledge regarding the activation mechanisms are summarized and discussed. Then, a summary of frequently-used synthesis methods of heterogeneous iron catalysts is presented. In addition, the effects of anions, solution pH, dissolved oxygen, and external energy on the activation processes are discussed. Finally, future research perspectives on the iron-based PS/PMS activation method are proposed and how to further improve such a technology for practical application are also discussed.