## 1 Introduction  
Environmental pollution generated by industrial wastewater is one of the greatest problems the world is facing today and dyes are widely applied in many industries, such as leather, textile, plastics, paper, and other entities emanating from pharmaceutical and food industries [1; 2; 3; 4; 5]. Dyes effect the photosynthetic activities of aquatic lives and their ensuing byproducts are not biodegradable but are stable, carcinogenic, and hazardous to human health [6; 7; 8]. Dyes are usually classified on the basis of their chemical structure, such as anionic, cationic, and non-ionic [9; 10]; reactive dyes are stable and anionic in character, and display resistance towards light [11; 12]. Generally, the methods for the removal of dyes can be classified into the following categories, namely biological methods (anaerobicand aerobic processes) [13; 14; 15], physical methods (sedimentation) [16], membrane processes [16], and adsorption [17; 18], and chemical methods (coagulation) [19], flocculation [20], ozonation [21], and electrochemical [21; 22]. Advanced Oxidation Processes (AOPs) involve the generation of reactive oxygen species using a combination of strong oxidants such as H\({}_{2}\)O\({}_{2}\), O\({}_{3}\), and catalysts by completely mineralizing the biodegradable compounds and the degradation of non-biodegradable organic pollutants [23; 24]. The AOPs are an attractive option due to the usage of environmentally friendly chemicals and their high efficiency for the removal of persistent and synthetic organic compounds with short treatment times [25]. Some of the most popular AOPs are Fenton's reaction, Photo-Fenton, Sono-Fenton, Electro-Fenton, electrochemical oxidation, TiO\({}_{2}\) photocatalysis, advanced ozonolysis, cavitation, and Fenton-like reaction [26], among others. In the Fenton reaction, the formation of hydroxyl radicals is the main event accomplished by different processes: (1) homogeneous Fenton process, involving iron salts and an oxidant under an acid medium; (2) heterogeneous Fenton process, involving a solid catalysis and oxidant ('Fenton-like catalyst'); (3) photo reduction of Fe(III) to Fe(II) by ultrasound and UV ('photo-Fenton and sono-Fenton processes', respectively) [27]. Heterogeneous Fenton-like processes by solid catalysts (iron or other transition metals) have been applied over a wide pH range; reduce hydroxide precipitation and possibly facilitate solid/liquid phase separation [28]. Recently, an innovative treatment method based on heterogeneous Fenton-like catalysis oxidant activation has been researched as an alternative to conventional methods of the AOPs [29]. Persulfate (PS), as a strong oxidizing agent, for removal of a wide variety of inorganic and organic components, has received much attention owing to some of its great advantages [30; 31]. The PS salts usually form S\({}_{2}\)O\({}_{8}\)\({}^{2-}\) anions, which efficiently remove most of the pollutants, being a powerful oxidant with high oxidation potential of SO\({}_{4}\)\({}^{-\bullet}\) at natural pH [32; 33]. The PS can be activated using ultrasound, UV (Equation (1)) and heat that causes the hemolysis of the peroxide bond in the persulfate molecule. The persulfate is an oxidant (E\({}^{0}\) = 2.01 V) and the powerful oxidizing species of sulfate free radicals (E\({}^{0}\) = 2.6 V) can be formed through the cleavage of the O\(-\)O bond via activation or/and catalysis process [34]. \[\text{S}_{2}\text{O}_{8}\text{${}^{2-}$+ hv}\to 2\text{SO}_{4}\text{${}^{\bullet}$}. \tag{1}\]  
Spinel ferrite (XFe\({}_{2}\)O\({}_{4}\)) is a stable, low-cost magnetic material, and widely used for the removal of various pollutants. Cobalt (Co) can be used to replace the cation X\({}^{2+}\) in XFe\({}_{2}\)O\({}_{4}\) to enhance the catalytic activity of Fe spinel and in view of the separation of Fe\({}^{2+}\) from the catalyst structure the high activity for activation of the PS has been demonstrated (Equation (2)) [32].