A critical review of the application of chelating agents to enable Fenton and Fenton-like reactions at high pH values

To overcome the drawback of low pH requirement of the classical Fenton reaction, researchers have applied chelating agents to form complexes with Fe and enable Fenton reaction at high pHs, which is reviewed in this article. The chelating agents reviewed include humic substances, polycarboxylates, aminopolycarboxylic acids, and polyoxometalates. Ligands affect the reactivity of Fe-complexes by changing their redox potentials, promoting their reaction with H2O2, and competing with target contaminants for the oxidative species. Fe(III)-complexes are reduced to Fe(II)-complexes by O2− not H2O2, as indicated by their redox potentials. The stability constants of Fe-complexes increase with increasing pKa values of the corresponding ligands and also with increasing charge density of the metal ions. A higher stability constant of Fe(III)-complex indicates higher reaction rate of corresponding Fe(II)-complex with H2O2 and lower reduction rate of Fe(III)-complex to Fe(II)-complex. OH, O2−, and ferryl species were reported to be the reactive species on the contaminant removal in the chelate-modified Fenton process. The generation of these species depends on the chelating agents and reaction conditions. The process is very efficient in degrading contaminants, indicating a potential treatment approach for the pollution remediation at natural pH.