## 3 Environmental Applications of Graphene-Based Photocatalysts
### Magnetically Separable Graphene-Based Composite Photocatalysts
Various types of magnetic materials have been proposed for graphene-based heterojunction systems and mostly spinel ferrites with general chemical formula of \(\text{MFe}_{2}\text{O}_{4}\), where M is Co, Ni, Mn, Fe, Zn, Bi, etc. [158]. Common use of those materials is mainly caused by simple sol-gel or hydrothermal/solvothermal synthesis methods [159; 160; 161]. The structure of spinel ferrite is based on a closed cubic oxygen lattice, in which tetrahedra and octahedra are occupied by metal cations (Figure 16). Spinel that contains only divalent ions at tetrahedral sites is called normal, whereas that with divalent ions at octahedral sites is called inverse [162]. The unpaired electrons exist in most of spinel ferrites structures, which in an external magnetic field, create the partial magnetic moments, due to their ordered orbits movement around individual atoms. The created magnetic moments are equaled to the macroscopic magnetic properties of magnetic material. Moreover, the existence of the magnetic domains, i.e., areas in the material structure where partial magnetic moments are directed in one direction, is characteristic for ferromagnets [163]. In photocatalysis, magnetic separation is efficient for high saturation magnetization (Ms) values. Ms represents the force of interaction between the magnetic particle and the current magnetic field. It has been suggested that Ms value of 1 Am\({}^{2}\)/kg is enough for efficient separation [164], but there are no many reports to support this, and thus there is disagreement between researches on this aspect. An overview of the most widespread ferromagnets-graphene heterojunction systems, together with their photocatalytic properties for degradation of organic pollutants, is summarized in \[\text{ ferrite NP}\left(\text{e}^{-}\right)+\text{G}/\text{GO}/r\text{GO} \rightarrow\text{ ferrite NP}+\text{G}/\text{GO}/\text{rGO}\left(\text{e}^{-}\right), \tag{2}\]0  
Another important issue of graphene-based magnetic photocatalysts is their recovery after photocatalytic process and reuse in the next experiments. The aforementioned Ms parameter, determined by means of a magnetic hysteresis loop (Figure 17), is crucial for the evaluation of the possibility of material for external magnetic field separation. \[\text{ ferrite NP}\left(\text{e}^{-}\right)+\text{G}/\text{GO}/r\text{GO} \rightarrow\text{ ferrite NP}+\text{G}/\text{GO}/\text{rGO}\left(\text{e}^{-}\right), \tag{2}\]1 Spinel ferrite structure on \(\text{Fe}_{3}\text{O}_{4}\) NP example (octahedra not marked for clearer structure visibility). For unmodified spinel ferrites NPs, the saturation magnetization value fluctuates around 70-90 Am\({}^{2}\)/kg, depending on M cation type [165; 166; 167]. The Ms values decrease after increasing the percentage of non-magnetic compound in the nanocomposite. In the recent literature reports, even up to 9-10 reuse cycles with almost no loss in photocatalytic activity have been observed for G-based magnetic nanocomposites [168; 169; 146]. Recycled photocatalysts, before reusing in the next photodegradation process, are often recovered by simple washing with demineralized water and drying [170] up to alkaline solution treatment [171]. In summary, it is clear that coupling of graphene and its composite with magnetic particles might results in efficient, stable and easily separable photocatalyst for various environmental application.