# Catalytic Degradation of 4-Ethylpyridine in Water by Heterogeneous Photo-Fenton Process
## 1 Introduction  
Pyridine derivatives are synthetic compounds that are commonly used in the production of commercial products such as food aromas, paints, dyes, rubber products, waterproof fabrics, coatings, pesticides, fungicides, and antioxidants [1; 2; 3]. Pyridine derivatives include mainly alkyl pyridines, halogenated pyridines, and amino-pyridines, which have been largely used as the basic raw materials for the manufacturing of industrial solvents, agriculture herbicides, and pesticides. 4-ethylpyridine (4 EP) is among the pyridine derivatives utilized in coking, chemical synthesis, pharmaceutical, and pesticide industries [4; 5; 6; 7; 8]. It is characterized by its high toxicity and risk of carcinogenicity, and can pose human health and environment concerns [5; 6; 7; 8; 9]. In addition, 4 EP is soluble in water, very stableand difficult to biodegradable [10; 11; 12; 13; 14]. It is largely spread across the environment due to its extensive use and can contaminate groundwater and surface waters [8; 9; 10; 11; 12; 13; 14]. The conventional biological remediation process treatment methods are not able to completely remove 4 EP from contaminated water [15; 16; 17]. The search for a treatment method to eliminate it from water, transform it to a biocompatible form, or lessen its toxicity is urgently desired. Particularly, advanced oxidation processes (AOPs) showed high potential to degrade various bioresistant and hazardous organic pollutants in water [18; 19; 20]. These processes are capable of mineralizing several types of organic pollutants, including phenols, polyphenols, pesticides, pharmaceuticals, etc. [19; 21]. The great efficacy of AOPs is mainly due to the generation of powerful oxidizing radicals, mainly hydroxyl radicals (HO\({}^{\bullet}\)) [22; 23]. These unstable radical species with a very short lifetime and a high standard potential (E\({}^{\circ}\) = 2.80 V/SHE) react immediately and non-selectively with organic pollutant molecules leading to total mineralization into CO\({}_{2}\), H\({}_{2}\)O, and other inorganic ions, or partial transformation to substances easy to biodegradable [22; 23; 24]. In particular, photo-Fenton oxidation (UV/H\({}_{2}\)O\({}_{2}\)/Fe\({}^{2+}\)) is a good option to reduce and eliminate many pollutants during wastewater treatment. It is considered as the most effective H\({}_{2}\)O\({}_{2}\)-based process in the treatment of various effluents containing highly toxic and bio-resistant organic compounds [20; 25; 26]. In this process, HO\({}^{\bullet}\) radicals generated from H\({}_{2}\)O\({}_{2}\) decomposition by UV photolysis and by ferrous iron catalyst (Equations (1) and (2)):  
\[\mathrm{Fe}_{\mathrm{aq}}^{2+}+\mathrm{H}_{2}\mathrm{O}_{\mathrm{2 aq}} \rightarrow\mathrm{Fe}_{\mathrm{aq}}^{3+}+\mathrm{HO}^{\bullet}+\mathrm{H}^{+ }_{\mathrm{aq}} \tag{1}\]  
\[\mathrm{H}_{2}\mathrm{O}_{\mathrm{2 aq}}+\mathrm{hv}\rightarrow\mathrm{HO}^{ \bullet}+\mathrm{HO}^{\bullet} \tag{2}\]  
However, high amounts of Fe\({}^{2+}\) are needed to attain complete removal of organic pollutants, which results in another environmental problem related to the discharge of treated water or sludge contaminated with iron, and it also increases the operational costs of the treatment process. Using heterogeneous iron catalysts can solve these economic and environmental problems since the catalyst can be assembled at the end the treatment and recycled many times [27; 28; 29; 30; 31; 32; 33; 34]. Natural and synthetic magnetite, Fe\({}_{3}\)O\({}_{4}\), containing both Fe\({}^{2+}\) and Fe\({}^{3+}\) catalytic sites, was successfully used to degrade organic pollutants [35; 36; 37; 38; 39; 34].