Review of iron-free Fenton-like systems for activating H2O2 in advanced oxidation processes

•Elements with multiple redox states efficiently decompose H2O2 at neutral pH.•Activation of H2O2 is entirely governed by solution pH and catalyst composition.•Metal leaching and toxicity is an important factor for practical applications.•Iron-free Fenton systems work only in specialized reaction conditions. Iron-catalyzed hydrogen peroxide decomposition for in situ generation of hydroxyl radicals (HO•) has been extensively developed as advanced oxidation processes (AOPs) for environmental applications. A variety of catalytic iron species constituting metal salts (in Fe2+ or Fe3+ form), metal oxides (e.g., Fe2O3, Fe3O4), and zero-valent metal (Fe0) have been exploited for chemical (classical Fenton), photochemical (photo-Fenton) and electrochemical (electro-Fenton) degradation pathways. However, the requirement of strict acidic conditions to prevent iron precipitation still remains the bottleneck for iron-based AOPs. In this article, we present a thorough review of alternative non-iron Fenton catalysts and their reactivity towards hydrogen peroxide activation. Elements with multiple redox states (like chromium, cerium, copper, cobalt, manganese and ruthenium) all directly decompose H2O2 into HO• through conventional Fenton-like pathways. The in situ formation of H2O2 and decomposition into HO• can be also achieved using electron transfer mechanism in zero-valent aluminum/O2 system. Although these Fenton systems (except aluminum) work efficiently even at neutral pH, the H2O2 activation mechanism is very specific to the nature of the catalyst and critically depends on its composition. This review describes in detail the complex mechanisms and emphasizes on practical limitations influencing their environmental applications.