## 3 Environmental Applications of Graphene-Based Photocatalysts
### Magnetically Separable Graphene-Based Composite Photocatalysts  
In recent years, the composites of graphene with magnetic particles have been intensively studied for various applications, including biomedical purposes (controlled-release drug delivery [136], nano-supports [137], protein and peptides extraction [138], biosensing [139], etc.) and environmental purification (heavy metal removal [140], inorganic and organic contaminants sorption [141; 142]). Graphene, due to good electron conductivity, high adsorption capacity and extremely high specific surface area [143], is also widely used as photocatalytic material. The coupling of graphene with ferromagnets allows the formation of effective material for organic recalcitrant photodegradation. Magnetic NPs in photocatalysis could perform a dual function, i.e., as (i) a semiconductor in heterojunction, and (ii) a material applied for post-process photocatalyst separation. In most cases, nanoferrites are used as photocatalytically active materials since efficient separation of photogenerated charge carriers is related to synergistic effect between two semiconductors [144]. Ferrites, as semiconductors with narrow bandgap, e.g., CuFe\({}_{2}\)O\({}_{4}\)--1.42 eV [145], AgFeO\({}_{2}\)--1.9 eV [146], ZnFe\({}_{2}\)O\({}_{4}\)--1.68 eV [147], Fe\({}_{3}\)O\({}_{4}\)--0.1 eV [148], could be easier photoexcited with sunlight than graphene. Consequently, photogenerated electrons are transferred to graphene sheets, where they take part in formation of ROS, and thus causing the degradation of organic pollutants. General chemical reactions (Equations (1)-(5)) as well as schematic mechanism of organic compounds' degradation on G/BiFeO\({}_{3}\) (Figure 15) are shown below [149]. \[\text{ ferrite NP}+\text{hv}\rightarrow\text{ ferrite NP}\left(\text{h}^{+}+ \text{e}^{-}\right), \tag{1}\]  
\[\text{ ferrite NP}\left(\text{e}^{-}\right)+\text{G}/\text{GO}/r\text{GO} \rightarrow\text{ ferrite NP}+\text{G}/\text{GO}/\text{rGO}\left(\text{e}^{-}\right), \tag{2}\]  
\[\text{G}/\text{GO}/\text{rGO}\left(\text{e}^{-}\right)+\text{O}_{2} \rightarrow\text{G}/\text{GO}/\text{rGO}+\text{O}_{2}{}^{\bullet-}, \tag{3}\]  
\[\text{ ferrite NP}\left(\text{h}^{+}\right)+\text{OH}^{-}\rightarrow\text{ ferrite NP}+{}^{\bullet}\text{OH}, \tag{4}\]  
\[\text{organic pollutant}+\text{O}_{2}{}^{\bullet-}+{}^{\bullet}\text{OH} \rightarrow\text{degradation products}, \tag{5}\]  
Nonetheless, especially in acidic conditions, direct ferromagnet-semiconductor structure could not be stable due to photobleaching and oxidation-reduction of iron ions in ferromagnet NPs, caused by charge carriers side reactions (Equations (6)-(9)) [150]. As a  
Figure 15: Schematic mechanism of photodegradation of organic pollutants on BiFeO\({}_{3}\)-graphene heterojunction. Reprinted with permission from [149]. Copyright (2013) Creative Commons Attribution. result, iron dissolution could cause poisoning of photocatalyst and, as a consequence, breakdown of the heterojunction structure [151]. \[\text{Fe}^{3+}\left(\text{s}\right)+\text{e}^{-}\rightarrow\text{Fe}^{2+}\left( \text{s}\right), \tag{6}\]  
\[\text{Fe}_{2}\text{O}_{3}+6\text{H}^{+}+2\text{e}^{-}\to 2\text{Fe}^{2+}+3 \text{H}_{2}\text{O}, \tag{7}\]  
\[\text{Fe}_{3}\text{O}_{4}+8\text{H}^{+}+3\text{e}^{-}\to 3\text{Fe}^{2+}+4 \text{H}_{2}\text{O}, \tag{8}\]  
\[\text{Fe}^{2+}\left(\text{aq}\right)+\text{h}^{+}\rightarrow\text{Fe}^{3+} \left(\text{aq}\right), \tag{9}\]  
Therefore, the junction of ferrite NPs and graphene sheets is a conceivable solution for creating universal material with both photocatalytic activity and magnetic separability. As photogenerated electrons migrate only from ferrite to graphene, the leaching of ions is inhibited, and the adsorption and photocatalytic properties are increased [152; 153]. In practice, ferrite-graphene hybrid photocatalysts are successfully used for degradation of organic recalcitrant chemicals, mostly dyes, e.g., methylene blue (MB) or methyl orange (MO), but also pesticides and antibiotics [154; 155; 156; 157].