## 1 Introduction
In recent years, to remove or mineralize the organic pollutants, the sorption [22; 23] and heterogeneous photocatalysis methods have been widely adopted, deriving from modern advanced oxidation processes (AOPs) [24; 25; 26]. Recent overwhelming research interest in using titanium dioxide (TiO\({}_{2}\)) as a photocatalyst is attributed to the excellent capabilities of this catalyst to degrade a wide array of organic compounds in CO\({}_{2}\) and H\({}_{2}\)O. Several studies reported TiO\({}_{2}\) used in suspension form in wastewater treatment, resulting in the slurry formation in photoreceptors due to its fine powdery nature; this is one major limitation in TiO\({}_{2}\) based photocatalysis [27] and its moderate bandgap of 2.7 eV makes the catalyst active only in the UV region [28]. The photochemical side reactions and decomposition of catalyst causes the fast termination of photocatalytic cycles and limit the efficiency of the process, and also leads to fastback electron transfer processes and recombination reactions that are the reasons for limiting the photocatalysis performance [29]. Much attention has been paid by several research groups on the modification of TiO\({}_{2}\) surface area, mainly by the atomic mixing with SiO\({}_{2}\) to enhance the activity of TiO\({}_{2}\) catalyst [30]. Nevertheless, as SiO\({}_{2}\) is inserted in the TiO\({}_{2}\) lattice, the improvement of the specific surface area of the resulting catalyst is also accompanied by the broadening of its bandgap energy. Consequently, TiO\({}_{2}\)-SiO\({}_{2}\) mixed oxide inherently absorbs much more UV light than the visible counterpart, compared to bare TiO\({}_{2}\). Although nitrogen-doped TiO\({}_{2}\) (N-TiO\({}_{2}\)) catalyst has been intensively studied for the absorption of visible light, nitrogen-doped TiO\({}_{2}\)-SiO\({}_{2}\) (N-TiO\({}_{2}\)-SiO\({}_{2}\)) catalyst has not been much explored. In this present research, the N-TiO\({}_{2}\)-SiO\({}_{2}\) mixed oxide is synthesized using the sol-gel method. For simultaneous improvement of the specific surface area and crystallinity of N-TiO\({}_{2}\)-SiO\({}_{2}\) catalyst, hydrothermal treatment is also employed immediately after the sol-gel process [31]. Many techniques are used for the deposition of TiO\({}_{2}\) onto silica-coated iron oxide nanoparticles (NPs). Impregnation, precipitation, and sol-gel methods are used. Magnetic visible-activephotocatalysts are promising materials for overcoming the two fundamental drawbacks of titanium dioxide: NPs that hinder TiO\({}_{2}\) application and development in the fields of environmental pollutants and energy sources. Recently, many techniques have been proposed to induce visible light activity and magnetic separability to TiO\({}_{2}\) NPs [32]. Very recently, large-scale synthesis of discrete and uniformly sized Fe-based nanoparticles and superparamagnetic Fe\({}_{3}\)O\({}_{4}\)/SiO\({}_{2}\), based on the use of spinning disk reactor (SDR), has been reported [15]. Fe-based metallic glass technique was reported for remediation of industrially contaminated water with arsenic and nitrate by mass production [33]. Among all the mixing devices used for precipitation, SDR has many advantages when compared to other mixing devices, such as the rotational speed grater. Good results on controlling size and shape of the nanoparticles and NPs production of narrow particle distribution size (PDSs) at a specific target size and also through continuous operation, compatible to industrial practice, can be performed [34]. The proposed work is also economically effective; the catalyst is reusable and reduces operating cost. This approach has been considered in order to achieve economic feasibility of the process since immobilized systems may not be used due to the high capacity of the wastewater. However, in this process suspended photocatalytic particles allow for the operation of the system without any interruption, and the catalyst could be reused for several cycles [32; 35].