# Application of Spinel and Hexagonal Ferrites in Heterogeneous Photocatalysis
## 1 Introduction  
Nowadays, increasing global pollution and energy issues constitute the main threats to the natural environment. Therefore, new strategies and approaches have been developed to solve those issues. Among them, materials science might be considered as one of the crucial fields for environmental protection [1]. For example, oxide systems, defined as a material consisting of at least two components (oxides) aimed at improving the application properties of the final product, are becoming more and more popular. The growth of the application potential of oxide systems is due to the synergistic effects between components (e.g., heterojunction, \(Z\)-scheme mechanism, etc. [2]) or the formation of mixed crystallographic structures (e.g., ZnTiO\({}_{3}\), SrTiO\({}_{3}\)[3]). It should be emphasized, however, that apart from the chemical composition, the control of crystallite/particle sizes, in particular at the nano scale, is fundamental for further applications of oxide materials in photoactive processes (photocatalytic, photovoltaic, etc.) [4]. For this reason, the classic approach to materials' synthesis, where the size of the particles is not the main concern for the process efficiency, is currently one of the crucial issues in modern materials science. Since the famous publication by [PERSON] and [PERSON] in 1972 [5], photocatalysis study using semiconductor materials has received increasing attention [6; 7; 8; 9]. Each year, scientists from around the world are developing new photoactive materials, as well as modifying the known ones, in order to overcome their limitations with mass transfer, charge carrierrecombination, visible-light activity, separation, generation of reactive oxygen species, and effective irradiation [10; 11; 12; 13; 14; 15]. Comparing the number of recently published manuscripts in the field of photocatalysis (Figure 1), only about 7% consider the material separation after the reaction, which is considerably less than those focusing on activity increase (84%) and visible-light response (80%). This clearly shows that the recycling and reuse of photocatalysts is still undervalued. Furthermore, it should be mentioned that the study of magnetic separation began relatively late (around beginning of the 21 st century) in comparison to the other aspects of heterogeneous photocatalysis. Of course, other types of separation/recycling have been proposed and investigated, such as sedimentation, coagulation, (ultra)filtration, immobilization of photocatalysts on elements of photoreceptors (walls [16], quartz protecting tubes of lamps and LEDs [17; 18], and optical fibers [19]) or additional supporters (glass beads [16], microspheres [20], balls [21], mesh [22], cloth [22], rings [23], membranes [24; 25; 26], and plates/films/foils [27; 28; 29; 30]) [31]. The main limitation of simple separation by gravitational sedimentation is caused by the nanometric size of photocatalysts. [PERSON] et al. [32] as well as [PERSON] et al. [33] proposed photocatalyst separation by coagulation. The addition of inorganic (FeCl\({}_{3}\)[33], [Al\({}_{2}\)(OH)\({}_{n}\)Cl\({}_{6-n}\)]\({}_{m}\)[32] and poly-ferric sulfate (PFS) [34]) or organic coagulants (e.g., cationic polyacrylamide (CPAM) [35]) might effectively accelerate the aggregation of photocatalyst nanoparticles (NPs) by modifying their double layer potential. Another method of suspended photocatalyst recovery is ultrafiltration. For example, [PERSON] et al. [36] separated fine titania NPs by both cross-flow ultrafiltration (CFU) and coagulation. It has been found that  
Figure 1: The number of scientific articles published in 2020 (according to the Web of Science database) for various keywords concerning photocatalysis. titania loses activity during coagulation, in contrast to CFU, which does not cause activity loss. However, the application of ultrafiltration is associated with significantly higher operation costs. In this respect, the immobilization of photocatalysts seems to be a better option.