Fenton, photo-Fenton, electro-Fenton, and their combined treatments for the removal of insecticides from waters and soils. A review

Insecticides have been widely used in industry, medicine, agriculture, and by-consumers since the World War II to control, destroy, or prevent any insect pests. The application of these compounds to farmlands has ensured an increasing agricultural activity for larger food production. Due to its widespread use, these toxic products have been detected in waters and soils where they represent a great health risk for living beings. Since the high stability and biorecalcitrance of insecticides at mild conditions prevents their effective removal in conventional wastewater treatment plants, powerful advanced oxidation processes have been developed for their effective destruction. This review presents a critical, exhaustive, and detailed analysis on the application of Fenton and Fenton-based processes to remediate synthetic and actual waters and soils contaminated with common insecticides, covering up to July 2021. Homogeneous and heterogeneous Fenton, homogeneous and heterogeneous photo-Fenton with UV light, homogeneous solar photo-Fenton, and electro-Fenton and related processes, as well as combined ones involving hybrid and sequential treatments, have been examined. The fundamentals and characteristics of each technology are briefly detailed, and the main results obtained for the removal of the most used insecticide families are carefully exposed and discussed. The role of generated oxidizing agents is explained, and the by-products formed, and reaction sequences proposed are described. Finally, the oxidation ability of homogeneous Fenton and homogeneous photo-Fenton treatments over insecticides is compared with that of other available advanced oxidation processes like photocatalysis with TiO2/UV, UV/O3, and UV/H2O2, as well as O3/H2O2 and persulfate.