## 4 Chemical Methods for Synthesis of Magnetic Ferrite Nanoparticles
### Hydrothermal Synthesis  
Hydrothermal synthesis is a widely utilized method for producing magnetic ferrite nanoparticles, which are increasingly applied in water treatment. This technique involves chemical reactions under high pressure and temperature in a sealed environment, resulting in nanoparticles with controlled size, shape, and crystallinity [156; 157; 158; 159; 160; 161; 162]. As shown in Figure 6, the process begins with preparing precursor materials, typically involving iron salts such as iron (III) chloride or iron (II) sulfate, often combined with other transition metal salts to achieve desired properties. These precursors are dissolved in a solvent, commonly water or a water-alcohol mixture, and placed in a sealed autoclave capable of  
Figure 5: Schematic representation of magnetite synthesis by coprecipitation method. Reproduced with permission from Ref. [145]. withstanding high pressure and temperature. Hydrothermal synthesis involves specific conditions to produce magnetic ferrite nanoparticles effectively. The temperature during the process is maintained between 150 \({}^{\circ}\)C and 250 \({}^{\circ}\)C. The duration of the synthesis can span several hours, depending on the desired properties of the nanoparticles. The water content in the reaction mixture plays a crucial role, as higher water content typically leads to larger nanoparticles. The reaction mixture generally includes iron salts, such as iron (III) chloride and iron (II) sulfate, along with other transition metal salts. The solvent used is commonly water or a water-alcohol mixture. These conditions are carefully controlled to achieve the nanoparticles' desired size, shape, and crystallinity [157; 158; 160]. For instance, [PERSON] et al. demonstrated that cobalt ferrite nanoparticles synthesized via hydrothermal treatment at varying temperatures (ranging from 100 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, from 2 to 120 h) exhibited an increase in size from 11.9 to 18.7 nm, enhanced dispersity, and improved crystallinity [163]. After the reaction, the system is cooled, and washing solvents remove impurities or excess reagents from the nanoparticles. The washed nanoparticles are then collected using centrifugation or filtration. [PERSON] et al., in this way, synthesized CuO- and WO\({}_{3}\)-modified Fe\({}_{2}\)O\({}_{3}\) nanoparticles for biomedical applications [164]. The treatment of nanoparticles, whether during or after synthesis, generally involves several essential steps to enhance the final product's quality and functionality. Magnetic ferrite nanoparticle surface modifications, such as polymer or silica coatings, play a crucial role in improving the stability and functionality of magnetic ferrite nanoparticles synthesized through the hydrothermal method [158; 165; 166]. In this case, [PERSON] et al. reported that by varying the Co\({}_{2}\)Fe\({}_{3}\)O\({}_{4}\) to tetraethoxysilane ratio such as 1:1, 2:1, and 3:1 during the synthesis of silica-coated magnetic ferrite nanoparticles, they achieved remarkable stability in aqueous solutions and uniformity of size [167]. The ability to modify and functionalize the surface of magnetic ferrite nanoparticles and their easy separation through magnetic fields renders them highly efficient in removing water contaminants. Applying magnetic ferrite nanoparticles synthesized through hydrothermal methods can further expand sustainable water treatment by leveraging their superior properties in various treatment processes. Magnetic ferrite nanoparticles have been proven effective in multiple environmental applications, such as photocatalysis, adsorption, coagulation, flocculation, and filtration, for removing pollutants like dyes, heavy metals, pesticides, and organic contaminants. These applications depend on specific conditions to maximize efficiency and  
Figure 6: A schematic of the synthesis of iron oxide magnetic nanoparticles using the hydrothermal method. Reproduced with permission from Ref. [30]. functionality.