## 5 Novel Magnetic Ferrite Nanoparticle Applications in Wastewater Treatment  
### Magnetic Ferrite Nanoparticles for Photocatalytic Degradation of Wastewater Pollutants  
The fundamental principle of photocatalysis is generating electron-hole pairs upon the interaction of photoactive material with visible light photons [236]. These resulting charge carriers will induce redox reactions and eventually generate reactive species that further degrade the target pollutant into harmless products [237]. The ideal photocatalyst should possess three key parameters: (i) it should absorb a broad spectrum of light, (ii) it should support the separation of charge carriers and less recombination, and (iii) it should support a redox reaction via the contribution of charges at active sites. These key parameters depend on bandgap, surface area, surface functionality, crystallite size, porosity, etc. [238]. The smaller bandgap will make it feasible to absorb light from large spectra of light [239]. Small magnetic ferrite nanoparticles, specifically in the quantum domain, create discrete energy levels that lead to high light absorption and unique material responses due to the quantum confinement effect [240]. This process can be carried out by directly or indirectly irradiating photocatalysts or reactions via light. Figure 11 demonstrates the fundamental mechanism of photocatalysis; the central photoactive material will absorb photons as per its bandgap and excite ground state valence band (VB) electrons to excited conduction band (CB) electrons. This transition of electrons eventually creates a positive hole in the VB. These generated e-h pairs can directly react with pollutant molecules and react with it, and e-h pairs can convert pollutants indirectly. The presence of a hole in the VB causes the oxidation of water molecules at the active site and forms an OH\({}^{\bullet}\) radical. Similarly, negative electrons in the VB will create superoxide radicals (O\({}_{2}\)\({}^{\bullet}\)). These reactive oxygen species (OH\({}^{\bullet}\), O\({}_{2}\)\({}^{\bullet}\)) eventually convert pollutants into harmless products, e.g., H\({}_{2}\)O and CO\({}_{2}\)[241]. Recently, ferrites have received great attention in the scientific community for the photocatalytic degradation of various water-soluble organic and inorganic pollutants (Figure 12) [69]. In this regard, nanoparticles have been synthesized using solvothermal, precipitation, combustion, sonochemical, thermal decomposition, green synthesis, sol-gel, and microwave-assisted methodologies [55]. The morphology and porosity of photo-catalysts are highly dependent on the synthesis technique, while elemental composition will influence the optoelectronic properties of nanoparticles [242]. Most magnetic ferrite  
Figure 11: Photocatalytic degradation mechanisms of pollutants in water. Reproduced with permission from Ref. [241]. nanoparticles show ferromagnetic, ferrimagnetic, or superparamagnetic properties and can be separated from the dispersed medium in the presence of a magnetic field [243]. In the past years, CuFe\({}_{2}\)O\({}_{4}\) nanoparticles and their composites have been applied as photocatalysts for the degradation of different pollutants in wastewater [244]. As compared to other magnetic ferrite nanoparticles, such as NiFe\({}_{2}\)O\({}_{4}\) and CoFe\({}_{2}\)O\({}_{4}\) nanoparticles showing a bandgap of 2.2 eV and 2.7 eV, respectively [245], Cu-based magnetic ferrite nanoparticles show a relatively small bandgap (1.7-1.9), which offers better bandgap tuning and visible light harvesting, which is the most freely available energy in the form of sunlight. In addition, Cu-based magnetic ferrite nanoparticles show high photostability, a low cost, and toxicity of the catalyst [246]. Due to its small bandgap, CuFe\({}_{2}\)O\({}_{4}\) can decompose pollutants in visible sunlight. However, hydrogen peroxide must be added for high degradation capacities or when ultraviolet (UV) energy is applied as an irradiation source.