Advances in Magnetically Separable Photocatalysts: Smart, Recyclable Materials for Water Pollution Mitigation

Organic and inorganic compounds utilised at different stages of various industrial processes are lost into effluent water and eventually find their way into fresh water sources where they cause devastating effects on the ecosystem due to their stability, toxicity, and non-biodegradable nature. Semiconductor photocatalysis has been highlighted as a promising technology for the treatment of water laden with organic, inorganic, and microbial pollutants. However, these semiconductor photocatalysts are applied in powdered form, which makes separation and recycling after treatment extremely difficult. This not only leads to loss of the photocatalyst but also to secondary pollution by the photocatalyst particles. The introduction of various magnetic nanoparticles such as magnetite, maghemite, ferrites, etc. into the photocatalyst matrix has recently become an area of intense research because it allows for the easy separation of the photocatalyst from the treated water using an external magnetic field. Herein, we discuss the recent developments in terms of synthesis and photocatalytic properties of magnetically separable nanocomposites towards water treatment. The influence of the magnetic nanoparticles in the optical properties, charge transfer mechanism, and overall photocatalytic activity is deliberated based on selected results. We conclude the review by providing summary remarks on the successes of magnetic photocatalysts and present some of the future challenges regarding the exploitation of these materials in water treatment.


Introduction
Rapid development in terms of industrialisation and population growth over the years has put pressure on sustainable clean water supply. This is partly due to the ever increasing demand and the significant contribution towards compromised fresh water quality that comes with population growth and industrialisation. Similarly, climate change (drought conditions and elevated temperatures) has also impacted negatively on the amount of fresh water available. Consequently, the development of effective water treatment technologies remains an important area of research. This would ensure adequate utilisation of the available fresh water sources, which are constantly polluted through various domestic and industrial activities. Moreover, it would allow efficient treatment of industrial effluent water and enable it to be recycled which would lessen the burden on constant fresh water supply. This is particularly key to the smooth operations of the industries in water scarce localities where the amount of fresh water allocated per industry per day is highly restricted [1].
Over the years, a number of water treatment technologies such as membrane processes [1,2], biological methods [3,4], advanced oxidation processes (AOPs) [5,6], adsorption [7,8], electrochemical lowering its activity. Moreover, after the treatment has been completed, the photocatalyst needs to be separated and recovered from the water, and this presents an expensive and time-consuming exercise, especially when it comes to nanometre-sized particles. This often leads to inefficient recovery of the photocatalyst, which not only affects its recyclability but also causes secondary pollution and escalates the treatment costs.
Engineering of materials with high visible light photocatalytic activity that are easily separable and recoverable from the treated water remains the ultimate goal of ongoing research. With the increased usage of nanophotocatalysts, efficient separation and recovery of these materials from water is becoming increasingly difficult. Immobilising the photocatalyst particles on solid supports such as glass, ceramics, polymers, zeolites, sand, etc. has been widely investigated and has successfully enabled easy separation of the photocatalyst [66][67][68][69][70][71]. However, in the process of attaining easy separation, the photocatalytic activity of the immobilised semiconductors decreased remarkably. This was due to a decrease in the surface area, poor interaction with the reaction medium, and limited exposure of the photocatalyst to light [72,73]. Since neither the photocatalytic activity nor the easy separation can be sacrificed, researchers turned their interests to magnetic nanoparticles as integral ingredients for developing photocatalysts, which were easily separated from water with an external magnetic field [74][75][76]. Magnetic nanoparticles such as haematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ), magnetite (Fe 3 O 4 ), ferrites (MFe 2 O 4 , M = Mg, Ni, Zn, Cu, Co, Cd, etc.), etc. have been successfully coupled with various photocatalysts to induce magnetic behaviour in the composite material and allow easy magnetic separation [39,[77][78][79]. Since the photocatalyst remains powdery upon incorporation of the magnetic nanoparticles, this does not compromise its surface area and the distribution of the photocatalyst in the aquatic medium. In addition, some of these magnetic nanoparticles have good visible light absorption and photocatalytic properties, which further enhance the overall pollutant degradation kinetics over the magnetic nanocomposite [39,40,64,[80][81][82].
Considering the future prospects of magnetic photocatalysts, the design and preparation of magnetic visible-light-active nanocomposites has emerged as a hot topic in photocatalytic environmental pollution control. Therefore, it is necessary to bring into perspective the current developments in the tailoring and application of such materials in water treatment. A few reviews are available mainly on the synthesis, structure, and various applications of magnetic materials [83][84][85][86][87][88]. This review exclusively discusses the most recent developments in terms of the synthesis and exploitation of various magnetic nanocomposite photocatalysts in water pollution mitigation. Details on the synthesis routes and structure of the various magnetic nanoparticles have been well-reviewed and have been excluded in this review. Furthermore, this review only focusses on iron oxide magnetic nanoparticles resulting in four categories of magnetic nanocomposite photocatalysts: haematite, maghemite, magnetite, and ferrite-based magnetic photocatalysts. Under each category, we discuss the influence of the magnetic nanoparticle on optical properties, charge transfer mechanism, and the overall photocatalytic properties of the nanocomposites. We conclude the review by providing some summary remarks and highlight the future challenges towards tailoring and utilising magnetic photocatalysts in water treatment.

Haematite (α-Fe 2 O 3 )-Based Magnetic Photocatalysts
As a pristine material, haematite is a narrow band gap semiconductor (2-2.2 eV) with good visible light response, good chemical stability, low-cost, ferromagnetic behaviour, abundantly available, high resistance to corrosion, and environmentally friendly [89][90][91]. On its own, haematite shows poor photocatalytic activity due to the high recombination rate of the charge carriers and poor conductivity [90,91]. Despite its poor photocatalytic activity, the narrow band gap of haematite makes it an ideal sensitiser for improving the visible light response of various semiconductors. Zhao et al. developed a nanostructured α-Fe 2 O 3 -AgBr nonwoven cloth (Figure 1a,b) via a simple electrospinning-calcination method and studied its visible light photocatalytic behaviour towards rhodamine B (RhB) and para-chlorophenol (4-CP) decomposition. For both pollutants, the α-Fe 2 O 3 -AgBr nonwoven cloth showed superior activity over α-Fe 2 O 3 and AgBr clothes individually, attaining 91.8% 60 min and 74.2% in 120 min for RhB and 4-CP decomposition, respectively [81]. Coupling AgBr with α-Fe 2 O 3 resulted in a significant red-shift and resulted in the formation of heterojunctions, which facilitated charge separation, thereby improving the overall photocatalytic activity of the α-Fe 2 O 3 -AgBr nonwoven cloth. In addition, the cloth was easy to handle and recycle during photocatalysis by simple dipping and removal or by using an external magnet. It was found that the composite cloth was stable over four cycles with only a slight decrease in photocatalytic performance [81]. maghemite, magnetite, and ferrite-based magnetic photocatalysts. Under each category, we discuss the influence of the magnetic nanoparticle on optical properties, charge transfer mechanism, and the overall photocatalytic properties of the nanocomposites. We conclude the review by providing some summary remarks and highlight the future challenges towards tailoring and utilising magnetic photocatalysts in water treatment.

Haematite (α-Fe2O3)-Based Magnetic Photocatalysts
As a pristine material, haematite is a narrow band gap semiconductor (2-2.2 eV) with good visible light response, good chemical stability, low-cost, ferromagnetic behaviour, abundantly available, high resistance to corrosion, and environmentally friendly [89][90][91]. On its own, haematite shows poor photocatalytic activity due to the high recombination rate of the charge carriers and poor conductivity [90,91]. Despite its poor photocatalytic activity, the narrow band gap of haematite makes it an ideal sensitiser for improving the visible light response of various semiconductors. Zhao et al. developed a nanostructured α-Fe2O3-AgBr nonwoven cloth (Figures 1a,b) via a simple electrospinning-calcination method and studied its visible light photocatalytic behaviour towards rhodamine B (RhB) and para-chlorophenol (4-CP) decomposition. For both pollutants, the α-Fe2O3-AgBr nonwoven cloth showed superior activity over α-Fe2O3 and AgBr clothes individually, attaining 91.8% 60 min and 74.2% in 120 min for RhB and 4-CP decomposition, respectively [81]. Coupling AgBr with α-Fe2O3 resulted in a significant red-shift and resulted in the formation of heterojunctions, which facilitated charge separation, thereby improving the overall photocatalytic activity of the α-Fe2O3-AgBr nonwoven cloth. In addition, the cloth was easy to handle and recycle during photocatalysis by simple dipping and removal or by using an external magnet. It was found that the composite cloth was stable over four cycles with only a slight decrease in photocatalytic performance [81].  Xue and coworkers fabricated magnetically separable α-Fe 2 O 3 /graphitic carbon nitride (g-C 3 N 4 ) nanocomposite photocatalysts with varying α-Fe 2 O 3 loadings (5%, 10%, 30%, 50%) and investigated their performance for degradation of RhB. The highest degradation efficiency (96.7% in 4 h) was recorded using the sample containing 10% α-Fe 2 O 3 , which suggests optimum composition [92]. The introduction of α-Fe 2 O 3 nanoparticles into g-C 3 N 4 matrix enhanced its visible light utilisation and charge separation efficiency. Moreover, the composite photocatalyst displayed good stability and could be easily separable and recycled using an external magnet [92]. Similarly, core-shell magnetically separable Ag/AgCl/Fe 2 O 3 showed remarkable photocatalytic activity towards bisphenol A (BPA) and E. coli under visible light exposure. At a 5% α-Fe 2 O 3 loading, the composite photocatalyst reached the highest activity, which was 13 times that of Ag/AgCl for BPA degradation, and complete destruction of E. coli was observed after just 30 min of irradiation [39]. The nanocomposite displayed good stability and sufficient magnetic response to an external magnet.
In terms of charge transfer mechanism, the path followed by the electrons and holes depends on the semiconductor coupled with haematite and how their band structures match. In any case, under visible light illumination, haematite is excited and electrons accumulate in the conduction band while leaving positive holes in the valence band. In the case of α-Fe 2 O 3 -AgBr, both semiconductors are excited by visible light, and the electrons from AgBr transfer to the conduction band of α-Fe 2 O 3 , while the holes transfer in the opposite direction ( Figure 1e). The holes will directly attack the organic pollutants to form the degradation products, while the electrons may react with adsorbed oxygen leading to the eventual formation of hydroxyl radicals, which are strong oxidising agents [81]. In the Ag/AgCl/Fe 2 O 3 ternary nanostructure, the charge transfer mechanism (Figure 1f) is a bit complex. Only Ag and α-Fe 2 O 3 are excited upon visible light absorption, and the plasmon-generated electrons in Ag will be injected into the conduction band of AgCl and further transfer to the conduction band of α-Fe 2 O 3 . Meanwhile, the holes in α-Fe 2 O 3 can directly decompose the pollutants, while the holes left in Ag react with Cl´ions to form Cl 0 , which then attacks the pollutants [39].

Maghemite (γ-Fe 2 O 3 )-based Magnetic Photocatalysts
Maghemite is another important form of iron oxide but, unlike haematite, it shows good magnetic properties with saturation magnetisation values of up to 76 emu/g [103]. Consequently, the incorporation of maghemite into various photocatalyst matrices has been explored to fabricate magnetically separable composite photocatalysts for easy recovery and recycling [104,105]. Interestingly, despite the sensitivity to an external magnetic field, when the magnetic field is removed, the γ-Fe 2 O 3 -incorporating nanocomposites do not retain any significant residual magnetism that would otherwise cause problems by creating large clusters of magnetised particles during water treatment [106]. Maghemite shows good visible light response owing to its narrow band gap (2.2 eV) and possesses good adsorption properties. However, its photocatalytic activity is poor as a result of the high recombination rate of the photogenerated charge carriers and photodissolution [74,107]. Two approaches are usually adopted when coupling maghemite with other semiconductors. In the first approach, maghemite is directly coupled with other semiconductors to form heterojunctions, and it contributes towards visible light response of the nanocomposite photocatalyst as well as the overall charge transfer process [74,108,109]. According to the second approach, maghemite and the other semiconductor do not form heterojunctions (not in direct contact), but they are separated by an inert layer or shell usually silica (SiO 2 ). This is aimed at preventing photodissolution of the iron oxide and charge transfer between the two materials, which could otherwise result in the recombination on the maghemite surface. In this case, maghemite nanoparticles do not play any part in the photocatalysis process but function solely in the magnetic separation of the photocatalyst from the reaction medium [104,106,110].
A magnetically separable quaternary γ-Fe 2 O 3 /N and Fe-codoped TiO 2 nanocomposite was prepared using a sol-gel method (as illustrated in Figure 2a) and employed for the decomposition of reactive blue 4 (RB 4) under visible light illumination. Significantly higher photocatalytic activity was observed over the quaternary nanocomposite photocatalyst compared to N,Fe-TiO 2 , N-TiO 2 , and TiO 2 , reaching 100% degradation in 180 min (Figure 2b) [111]. The remarkable improvement in activity of the nanocomposite could be explained in terms of the combined contribution of codoping TiO 2 and coupling with γ-Fe 2 O 3 , which resulted in significant band gap narrowing, efficient visible light response, and efficient charge separation and transfer. Furthermore, the multicomponent photocatalyst demonstrated good stability and recyclability over four cycles without any significant loss in activity or magnetic properties, which allowed for easy magnetic separation [111]. Yu and coworkers probed the adsorption and UV photocatalytic oxidation of As(III) over TiO 2 decorated on magnetic (γ-Fe 2 O 3 ) mesoporous SBA-15 nanocomposite (γ-Fe 2 O 3 /SBA-15/TiO 2 ) derived from simple inner-pore hydrolysis combined with solvent evaporation route [112]. The nanocomposite photocatalyst displayed a dual function of adsorption and photocatalytic degradation. Firstly, As(III) ions were adsorbed onto the photocatalyst and then upon UV irradiation, undergo photocatalytic oxidation to As(V), which is less toxic and also removable by the photocatalyst. In addition, the photocatalyst showed good stability and could be regenerated for five cycles using NaOH (0.01 M) and still maintained over 90% As(III) removal and good magnetic properties for easy separation [112]. The incorporation of the SBA-15 ensured good dispersibility of the nanoparticles and improved the surface area of the nanocomposite. All three components of the nanocomposite were equally important in the overall removal of As(III).
Catalysts 2016, 6, 79 6 of 35 gap (2.2 eV) and possesses good adsorption properties. However, its photocatalytic activity is poor as a result of the high recombination rate of the photogenerated charge carriers and photodissolution [74,107]. Two approaches are usually adopted when coupling maghemite with other semiconductors. In the first approach, maghemite is directly coupled with other semiconductors to form heterojunctions, and it contributes towards visible light response of the nanocomposite photocatalyst as well as the overall charge transfer process [74,108,109]. According to the second approach, maghemite and the other semiconductor do not form heterojunctions (not in direct contact), but they are separated by an inert layer or shell usually silica (SiO2). This is aimed at preventing photodissolution of the iron oxide and charge transfer between the two materials, which could otherwise result in the recombination on the maghemite surface. In this case, maghemite nanoparticles do not play any part in the photocatalysis process but function solely in the magnetic separation of the photocatalyst from the reaction medium [104,106,110]. A magnetically separable quaternary γ-Fe2O3/N and Fe-codoped TiO2 nanocomposite was prepared using a sol-gel method (as illustrated in Figure 2a) and employed for the decomposition of reactive blue 4 (RB 4) under visible light illumination. Significantly higher photocatalytic activity was observed over the quaternary nanocomposite photocatalyst compared to N,Fe-TiO2, N-TiO2, and TiO2, reaching 100% degradation in 180 min (Figure 2b) [111]. The remarkable improvement in activity of the nanocomposite could be explained in terms of the combined contribution of codoping TiO2 and coupling with γ-Fe2O3, which resulted in significant band gap narrowing, efficient visible light response, and efficient charge separation and transfer. Furthermore, the multicomponent photocatalyst demonstrated good stability and recyclability over four cycles without any significant loss in activity or magnetic properties, which allowed for easy magnetic separation [111]. Yu and coworkers probed the adsorption and UV photocatalytic oxidation of As(III) over TiO2 decorated on magnetic (γ-Fe2O3) mesoporous SBA-15 nanocomposite (γ-Fe2O3/SBA-15/TiO2) derived from simple inner-pore hydrolysis combined with solvent evaporation route [112]. The nanocomposite photocatalyst displayed a dual function of adsorption and photocatalytic degradation. Firstly, As(III) ions were adsorbed onto the photocatalyst and then upon UV irradiation, undergo photocatalytic oxidation to As(V), which is less toxic and also removable by the photocatalyst. In addition, the photocatalyst showed good stability and could be regenerated for five cycles using NaOH (0.01 M) and still maintained over 90% As(III) removal and good magnetic properties for easy separation [112]. The incorporation of the SBA-15 ensured good dispersibility of the nanoparticles and improved the surface area of the nanocomposite. All three components of the nanocomposite were equally important in the overall removal of As(III).  Improved optical and photocatalytic properties have also been reported for other magnetic photocatalysts such as ZnO/γ-Fe 2 O 3 [113], g-C 3 N 4 /γ-Fe 2 O 3 [109], and γ-Fe 2 O 3 /TiO 2 [74] towards the degradation of various pollutants including RhB, methylene blue (MB), and methyl orange (MO). Moreover, these nanostructures showed good stability and could be magnetically separated and recycled without a significant loss in activity. However, the maghemite nanoparticle loading needs to be carefully controlled in order to strike a good balance between sufficient magnetism to facilitate separation and the detrimental effect of recombination of the charge carriers on the maghemite nanoparticles. This means that, while enjoying the luxury of easy separation, the detrimental effect of recombination must not be ignored in order to positively benefit from the incorporation of the maghemite nanoparticles.
According to the proposed charge transfer route in γ-Fe 2 O 3 /N,Fe-TiO 2 (Figure 3a), both maghemite and codoped titania absorb visible light, and electrons are excited from the valence band to the conduction band in γ-Fe 2 O 3 , leaving holes in the valence band. Similarly, in the codoped titania, electrons are excited from the valence band of TiO 2 to the impurity states formed by Fe doping and also from N sub-band gap states to the conduction band of TiO 2 . Subsequently, electrons transfer from the conduction band of maghemite to the Fe impurity states in TiO 2 , while the holes migrate to the N states in TiO 2 . The electrons and holes will be captured by molecular oxygen and water to form the superoxide and hydroxyl radicals, respectively [111]. These radical species are important oxidising agents for pollutant decomposition. Moreover, these nanostructures showed good stability and could be magnetically separated and recycled without a significant loss in activity. However, the maghemite nanoparticle loading needs to be carefully controlled in order to strike a good balance between sufficient magnetism to facilitate separation and the detrimental effect of recombination of the charge carriers on the maghemite nanoparticles. This means that, while enjoying the luxury of easy separation, the detrimental effect of recombination must not be ignored in order to positively benefit from the incorporation of the maghemite nanoparticles. According to the proposed charge transfer route in γ-Fe2O3/N,Fe-TiO2 (Figure 3a), both maghemite and codoped titania absorb visible light, and electrons are excited from the valence band to the conduction band in γ-Fe2O3, leaving holes in the valence band. Similarly, in the codoped titania, electrons are excited from the valence band of TiO2 to the impurity states formed by Fe doping and also from N sub-band gap states to the conduction band of TiO2. Subsequently, electrons transfer from the conduction band of maghemite to the Fe impurity states in TiO2, while the holes migrate to the N states in TiO2. The electrons and holes will be captured by molecular oxygen and water to form the superoxide and hydroxyl radicals, respectively [111]. These radical species are important oxidising agents for pollutant decomposition. The strategy of preventing the formation of heterojunctions between maghemite and the semiconductor coupled with it has been widely explored using silica as the barrier/shell surrounding the magnetic core. For example, Szeto et al. used a layer-by-layer assembly route to fabricate urchinlike γ-Fe2O3@SiO2@TiO2 composite microparticles (Figures 3b-e) and observed enhanced phenol decomposition under UV irradiation [114]. The tailored composite photocatalyst was shown to possess a large surface area, a prolonged life of the charge carriers, remarkable permeability, and significantly higher photocatalytic activity compared to commercial P25. In addition, incorporation of the maghemite nanoparticles ensured easy and efficient separation of the photocatalyst using an external magnetic field [114]. Similar results have been reported for other magnetic heterostructures such as γ-Fe2O3@SiO2@Ce-TiO2 [110], γ-Fe2O3@SiO2@AgBr:Ag [104], and γ-Fe2O3@SiO2@TiO2 [106], whereby a silica coating separates the maghemite nanoparticles from direct contact with the other components of the heterostructure. In these nanocomposites, the maghemite nanoparticles were only incorporated to facilitate the magnetic separation and play no part in the charge transfer process and overall photocatalytic decomposition of the pollutants. Although this approach is logical and has yielded some positive results, a critical comparison between the heterojunction type of photocatalyst and the core-shell structure of the same nanocomposite is rarely documented. This could map a way The strategy of preventing the formation of heterojunctions between maghemite and the semiconductor coupled with it has been widely explored using silica as the barrier/shell surrounding the magnetic core. For example, Szeto et al. used a layer-by-layer assembly route to fabricate urchin-like γ-Fe 2 O 3 @SiO 2 @TiO 2 composite microparticles (Figure 3b-e) and observed enhanced phenol decomposition under UV irradiation [114]. The tailored composite photocatalyst was shown to possess a large surface area, a prolonged life of the charge carriers, remarkable permeability, and significantly higher photocatalytic activity compared to commercial P25. In addition, incorporation of the maghemite nanoparticles ensured easy and efficient separation of the photocatalyst using an external magnetic field [114]. Similar results have been reported for other magnetic heterostructures such as γ-Fe 2 O 3 @SiO 2 @Ce-TiO 2 [110], γ-Fe 2 O 3 @SiO 2 @AgBr:Ag [104], and γ-Fe 2 O 3 @SiO 2 @TiO 2 [106], whereby a silica coating separates the maghemite nanoparticles from direct contact with the other components of the heterostructure. In these nanocomposites, the maghemite nanoparticles were only incorporated to facilitate the magnetic separation and play no part in the charge transfer process and overall photocatalytic decomposition of the pollutants. Although this approach is logical and has yielded some positive results, a critical comparison between the heterojunction type of photocatalyst and the core-shell structure of the same nanocomposite is rarely documented. This could map a way forward in terms of the most efficient approach towards engineering the maghemite-based magnetic photocatalyst and benefitting from the properties of this material.

Magnetite (Fe 3 O 4 )-based Magnetic Photocatalysts
Magnetite is by far one of the most widely exploited iron oxide magnetic nanoparticle in the fabrication of magnetic photocatalysts due to its low toxicity, good magnetic properties, biocompatibility, and remarkable adsorption properties [115,116]. The magnetisation saturation for magnetite can reach values of up to 92 emu/g, making it the ideal candidate for synthesis of magnetically separable photocatalysts [103]. However, unlike maghemite, magnetite shows no photocatalytic properties, and, like maghemite, the formation of heterojunctions with other semiconductors in nanocomposite photocatalysts is undesirable because magnetite may suppress the overall photocatalytic efficiency by acting as a recombination centre for the charge carriers [117,118]. Silica is commonly utilised to enclose the magnetite core, which is wrapped with the other semiconductor nanoparticles to form the magnetic composite photocatalysts. The silica barrier prevents electron transfer between magnetite and the other semiconductor, additionally preventing photodissolution of the iron oxide [119,120].
Despite the possible negative effect of magnetite in heterojunction nanocomposites, several reports on successful tailoring and improvement in photocatalytic activity over heterojunction nanocomposites have emerged, and careful control of the Fe 3 O 4 loading in the nanocomposite is an important point of consideration. For example, Zhao et al. observed enhanced photocatalytic activity towards ampicillin (AMP) over Fe 3 O 4 /TiO 2 /Ag with a sea urchin-like morphology, both under visible and UV light irradiation. The composite photocatalyst reached 98.7% and 91.5% AMP degradation after 360 min of UV and visible light illumination, respectively [116]. Furthermore, the nanocomposite possessed good magnetic behaviour with a magnetisation saturation of 26.5 emu¨g´1 (Figure 4), which was high enough to facilitate magnetic separation (Figure 4 insert). In addition to the degradation of AMP, the nanocomposite photocatalyst showed remarkable photocatalytic properties towards S. aureus (G + ), E. coli (G´), and A. niger [116]. In another example, improved MO decomposition was realised over Fe 3 O 4 /N-TiO 2 /Ag hollow nanospheres, reaching 99.5% in 80 min of visible light exposure when the Ag loading was 1.0% [121]. Ma and coworkers employed a low-temperature crystallisation method to wrap Fe 3 O 4 spheres with TiO 2 nanoparticles and subsequently immobilised Au nanoparticles on the core-shell Fe 3 O 4 /TiO 2 to yield a Fe 3 O 4 /TiO 2 /Au plasmonic magnetic photocatalyst. RhB was used as a model pollutant to evaluate the photocatalytic properties of the nanocomposite, and it showed superior performance over Fe 3 O 4 and Fe 3 O 4 /TiO 2 , with good magnetic properties (magnetisation saturation (Ms), 44.6 emu¨g´1), and recyclability [122]. The heterojunctions formed between Fe 3 O 4 and titania allow electrons to transfer between the two materials, which improves charge separation. However, the Fe 3 O 4 loading needs to be kept in check as excess Fe 3 O 4 will cause detrimental effects. Therefore, Fe 3 O 4 nanoparticles have a dual function in the heterojunction nanostructures-electron trapping and enabling magnetic separation.
Li et al. fabricated Fe 3 O 4 magnetic core coated with SiO 2 prior to the deposition of TiO 2 and surface modification with lysine, and the quaternary nanocomposite photocatalyst (Fe 3 O 4 @SiO 2 @TiO 2 -lysine) showed multifunctional properties as it was employed as an adsorbent, photocatalyst, and sensor for dissolved organic and inorganic phosphorus in seawater [123]. The photocatalyst displayed remarkable stability and could be magnetically separated and recycled for 10 cycles without any significant loss in its adsorption and photocatalytic properties [123]. Other core-shell Fe 3 O 4 @SiO 2 nanostructures decorated with TiO 2 have been fabricated and evaluated for the decomposition of various pollutants such as RhB, 2-chlorophenol (2-CP), phenol, MB, and MO. These materials showed improved photocatalytic properties, good stability, and sufficiently high magnetic response that enabled separation using an external magnetic field [124][125][126][127]. Table 1 provides a summary of the synthesis and photocatalytic properties of various other magnetite-incorporating titania-based photocatalysts towards water treatment applications. In the magnetic core-shell nanostructures, the Fe 3 O 4 nanoparticles encapsulated with the silica shell do not partake in the charge transfer process but only in the magnetic separation of the photocatalyst.  Chidambaram and coworkers prepared a novel plasmonic, magnetic Fe3O4/ZnO/Ag nanocomposite and employed it for the photocatalytic degradation of MB under visible light illumination. Complete MB removal was observed after 6 h of visible light exposure on Fe3O4/ZnO/Ag, while only 20% MB removal was realised over Fe3O4/ZnO during the same irradiation time. The improved activity could be explained in terms of improved charge separation and the surface plasmon resonance (SPR) effect of Ag nanoparticles, which is responsible for the visible light activity of the nanocomposite photocatalyst [139]. Fe3O4 nanoparticles played a dual role of facilitating charge separation and enabling magnetic separation. In addition, the photocatalyst showed great stability over 10 cycles without much loss in activity or magnetic properties. Upon visible light irradiation, the SPR-generated electrons are injected into the conduction band of ZnO and further transfer to the Fe3O4 nanoparticles. Subsequently, the electrons reduce Fe 3+ to Fe 2+ , which in turn is oxidised back to Fe 3+ upon reaction with adsorbed oxygen forming the superoxide radicals. Meanwhile, the SPR-generated holes in Ag could react with water to form the hydroxyl radicals and together with the superoxide radicals are responsible for pollutant degradation [139]. Yan et al. observed higher visible light photocatalytic performance over urchin-like Fe3O4@SiO2@ZnO/CdS core-shell microspheres (Figures 5a,b) towards RhB compared to Fe3O4@SiO2@ZnO microspheres. Coating Fe3O4 with SiO2 prevented the formation of heterojunctions with ZnO and CdS, and the nanocomposite showed good magnetic properties with magnetisation saturation value of 18.97 emu/g [140]. On visible light exposure, only CdS nanoparticles were excited and played the role of a sensitiser for ZnO, with electrons transferred to the conduction band of ZnO while the holes remained in the valence band of CdS (Figure 5c), resulting in efficient charge separation and the formation of the oxidising species [140].
Habibi-Yangjeh's group has recently prepared and investigated the visible light photocatalytic properties of various magnetic nanocomposites-based on Fe3O4 and ZnO such as ZnO/AgI/Fe3O4 [141], ZnO/AgBr/Fe3O4/Ag3VO4 [142], ZnO/Ag3VO4/Fe3O4 [143], Fe3O4/ZnO/AgCl [144], and Fe3O4/AgBr-ZnO [145] towards model pollutants such as RhB, phenol, MB, and MO. All of the nanocomposite photocatalysts showed good photocatalytic properties, which can be credited to efficient charge separation and improved visible light absorption as a result of the heterojunctions formed between ZnO and the co-catalysts. Moreover, it was shown that despite the formation of the composite photocatalysts, the presence of Fe3O4 nanoparticles ensured that the photocatalysts still  The improved activity could be explained in terms of improved charge separation and the surface plasmon resonance (SPR) effect of Ag nanoparticles, which is responsible for the visible light activity of the nanocomposite photocatalyst [139]. Fe 3 O 4 nanoparticles played a dual role of facilitating charge separation and enabling magnetic separation. In addition, the photocatalyst showed great stability over 10 cycles without much loss in activity or magnetic properties. Upon visible light irradiation, the SPR-generated electrons are injected into the conduction band of ZnO and further transfer to the Fe 3 O 4 nanoparticles. Subsequently, the electrons reduce Fe 3+ to Fe 2+ , which in turn is oxidised back to Fe 3+ upon reaction with adsorbed oxygen forming the superoxide radicals. Meanwhile, the SPR-generated holes in Ag could react with water to form the hydroxyl radicals and together with the superoxide radicals are responsible for pollutant degradation [139]. Yan et al.  Owing to its meta-free nature, polymeric structure, non-toxicity, good visible light response, good photocatalytic properties, and high thermal and chemical stability, tailoring graphitic carbon nitride (g-C3N4)-based magnetic photocatalysts is an important exploit towards potential practical application of these materials in water treatment. Using a combination of calcination and coprecipitation, Yang and coworkers prepared Fe3O4/g-C3N4 nanocomposite photocatalyst and probed its photocatalytic behaviour towards 2,4,6-trichlorophenol (2,4,6-TCP) under visible light illumination [77]. In a similar work, Jia et al. fabricated Fe3O4/g-C3N4 via a hydrothermal route and studied its visible light photocatalytic performance towards RhB decomposition [146]. In both studies, higher photocatalytic performance was observed over the nanocomposites compared to Fe3O4 and g-C3N4 individually, reaching 95.5% in 60 min [146] and 96.5% in 100 min [77] for RhB and 2,4,6-TCP, respectively. Coupling g-C3N4 with Fe3O4 improved charge separation since only g-C3N4 is excited by visible light, and the electrons in the conduction band quickly transfer to Fe3O4, where they are captured by oxygen to form the radical species. Meanwhile, the holes remain in the valence band of g-C3N4 and directly oxidise the pollutants to form the degradation products. Therefore, the holes and electrons occupy different locations in the nanocomposite. Moreover, magnetic separation was made possible by the presence of the Fe3O4 nanoparticles [77,146].
Habibi-Yangjeh's group investigated the visible light photocatalytic performance of novel magnetic nanocomposites: g-C3N4/Fe3O4/BiOI [147], g-C3N4/Fe3O4/AgCl [148], and g-C3N4/AgBr/Fe3O4 [149] towards the decomposition of RhB, MO, and MB. Significantly higher   [145] towards model pollutants such as RhB, phenol, MB, and MO. All of the nanocomposite photocatalysts showed good photocatalytic properties, which can be credited to efficient charge separation and improved visible light absorption as a result of the heterojunctions formed between ZnO and the co-catalysts. Moreover, it was shown that despite the formation of the composite photocatalysts, the presence of Fe 3 O 4 nanoparticles ensured that the photocatalysts still retained sufficient magnetic response to allow for efficient separation using an external magnetic field. Obviously, the magnetisation saturation of Fe 3 O 4 decreased sharply upon the formation of the nanocomposites due to their interaction and coverage by the non-magnetic components of the photocatalysts [141][142][143][144][145]. Under visible light irradiation, ZnO remains inactive and only the co-catalyst with a narrow band gap can be excited. For example, in ZnO/AgI/Fe 3 O 4 and ZnO/AgBr/Fe 3 O 4 /Ag 3 VO 4 , only AgI, AgBr, and Ag 3 VO 4 , respectively, are excited, and electrons are promoted to the conduction band, while the holes remain in the valence band. Subsequently, the electrons transfer to the conduction band of ZnO where they react with adsorbed oxygen to form the radical species (superoxide and hydroxyl radicals). Accordingly, the holes are directly involved in the degradation of the pollutants since they are not positive enough to oxidise water for the hydroxyl radicals [141,142]. The electrons and holes occupied different sites in the nanocomposite photocatalysts, which ensured efficient charge separation and the formation of the reactive species for pollutant degradation.
Owing to its meta-free nature, polymeric structure, non-toxicity, good visible light response, good photocatalytic properties, and high thermal and chemical stability, tailoring graphitic carbon nitride (g-C 3 N 4 )-based magnetic photocatalysts is an important exploit towards potential practical application of these materials in water treatment. Using a combination of calcination and co-precipitation, Yang and coworkers prepared Fe 3 O 4 /g-C 3 N 4 nanocomposite photocatalyst and probed its photocatalytic behaviour towards 2,4,6-trichlorophenol (2,4,6-TCP) under visible light illumination [77]. In a similar work, Jia et al. fabricated Fe 3 O 4 /g-C 3 N 4 via a hydrothermal route and studied its visible light photocatalytic performance towards RhB decomposition [146]. In both studies, higher photocatalytic performance was observed over the nanocomposites compared to Fe 3 O 4 and g-C 3 N 4 individually, reaching 95.5% in 60 min [146] and 96.5% in 100 min [77] for RhB and 2,4,6-TCP, respectively. Coupling g-C 3 N 4 with Fe 3 O 4 improved charge separation since only g-C 3 N 4 is excited by visible light, and the electrons in the conduction band quickly transfer to Fe 3 O 4 , where they are captured by oxygen to form the radical species. Meanwhile, the holes remain in the valence band of g-C 3 N 4 and directly oxidise the pollutants to form the degradation products. Therefore, the holes and electrons occupy different locations in the nanocomposite. Moreover, magnetic separation was made possible by the presence of the Fe 3 O 4 nanoparticles [77,146].
Habibi-Yangjeh's group investigated the visible light photocatalytic performance of novel magnetic nanocomposites: g-C 3 N 4 /Fe 3 O 4 /BiOI [147], g-C 3 N 4 /Fe 3 O 4 /AgCl [148], and g-C 3 N 4 /AgBr/Fe 3 O 4 [149] towards the decomposition of RhB, MO, and MB. Significantly higher photocatalytic activity was observed over the ternary heterostructures compared to single and binary nanocomposites, and this could be ascribed to the synergistic effect of the various components of the ternary nanocomposites, resulting in improved visible light response and charge separation efficiency. Moreover, the presence of Fe 3 O 4 in the heterostructures endowed them with magnetic response, which allowed separation using an external magnetic field [147][148][149]. In terms of charge transfer, only g-C 3 N 4 is excited by visible light in the case of g-C 3 N 4 /Fe 3 O 4 /AgCl, while both g-C 3 N 4 and BiOI are excited in g-C 3 N 4 /Fe 3 O 4 /BiOI, and the electrons are promoted to their conduction bands, leaving positive holes in the valence bands. This is followed by the transfer of electrons from the conduction band of g-C 3 N 4 , which is more negative, to the conduction bands of AgCl and BiOI, which are less negative, whereby photoreduction reactions involving adsorbed oxygen take place and result in the formation of the radical species [147,148]. Accordingly, in both materials, the holes accumulate in the valence band of g-C 3 N 4 where they directly oxidise the pollutants to form the degradation products.
Generally, the incorporation of Fe 3 O 4 nanoparticles in the photocatalysts can serve as a means to prolong the life of the photogenerated charge carriers by acting as electron traps and induce magnetic response in the nanocomposites, which enables easy separation using an external magnetic field. In both magnetic core-shell and heterojunction nanostructures, the magnetic influence of the  Table 2.

Ferrites (MFe 2 O 4 )-based Magnetic Photocatalysts
The spinel ferrites with the chemical formula MFe 2 O 4 , where M is a cation such as Mg 2+ , Cu 2+ , Zn 2+ , Ni 2+ , Co 2+ , etc., are popular materials due to their narrow band gaps, which endow them with remarkable visible light response. Moreover, ferrites show good chemical, thermal, and ferromagnetic properties, numerous oxygen vacancies, and a high density of surface hydroxyl groups [166,167]. In terms of the crystal structure of ferrites, the oxygen atoms form a face-centred cubic (fcc) close packing, while the M 2+ and Fe 2+ cations occupy either octahedral or tetrahedral interstitial sites [168]. Generally, ferrites have attracted tremendous interests in various applications such as fabrication of memory devices, biotechnology, catalysts for organic reactions, water splitting, pollution remediation, lithium ion batteries, etc. [169][170][171][172]. Among the ferrites, zinc ferrite (ZnFe 2 O 4 ) is one of the most attractive ferrite due to its photochemical and thermal stability, low toxicity, narrow band gap (1.9 eV), natural abundance, good magnetic properties, and easy preparation [64,173,174]. However, as a pristine photocatalyst, ZnFe 2 O 4 show low photocatalytic activity (low quantum efficiency) due to the high recombination rate of the photogenerated charge carriers, unconvincing photoelectric conversion, and low valence band potential [64].
In an attempt to improve the photocatalytic performance of ZnFe 2 O 4 , laser ablation obtained ZnO x (OH) x and FeO x colloids were used as precursors to uniformly deposit ZnFe 2 O 4 nanoparticles on RGO sheets to form a magnetic RGO/ZnFe 2 O 4 nanocomposite. The RGO/ZnFe 2 O 4 nanocomposite exhibited improved charge separation and photocatalytic activity towards MB degradation under visible light irradiation [175]. In a similar work by Yang et al., ZnFe 2 O 4 nanocrystals were confined within an interconnected graphene network, which served as dispersing agents, as transport channels for electrons, and as electron scavengers and reservoirs to minimise recombination. Enhanced visible light photocatalytic degradation of MB was observed over the nanocomposite photocatalyst [176]. In the RGO/ZnFe 2 O 4 nanocomposite, electrons are excited by visible light from the valence band to the conduction band of ZnFe 2 O 4 , and these electrons quickly transfer to the RGO skeleton where they are trapped by adsorbed oxygen to form superoxide radicals (Figure 6a). The holes are confined to the valence band of ZnFe 2 O 4 where they directly oxidise the pollutants since they are not positive enough to react with water to form the hydroxyl radicals [175,176]. More interestingly, the RGO/ZnFe 2 O 4 nanocomposites showed good stability and recyclability and were easily separable using an external magnetic field owing to the good magnetic response (Figure 6b). The presence of RGO ensured that the electrons and holes are located at different parts of the nanocomposite, thereby ensuring efficient charge separation.
Enhanced visible light photocatalytic degradation of 17α-ethinylestradiol (EE 2) was observed over magnetic ZnFe 2 O 4 -Ag/RGO nanocomposite prepared via a hydrothermal method. In terms of degradation kinetics, the nanocomposite photocatalyst showed the highest activity, which was 14.6 and 5.6 times that of ZnFe 2 O 4 and ZnFe 2 O 4 /RGO, respectively. This could be credited to the combined contribution of RGO, Ag, and ZnFe 2 O 4 in the nanocomposite, resulting in an improved surface area, efficient charge separation and transportation, minimal aggregation, and improved visible light utilization [177]. Chen and coworkers used a combination of solvothermal and in situ precipitation routes to tailor a magnetic RGO/ZnFe 2 O 4 /Ag 3 PO 4 nanocomposite and examined its photocatalytic behaviour towards 2,4-DCP under visible light exposure. The ternary nanocomposite exhibited significantly superior activity over pure Ag 3 PO 4 [178]. In the ZnFe 2 O 4 -Ag/RGO heterostructure, electrons excited by visible light from the valence band of ZnFe 2 O 4 to its conduction band transfer to the Ag nanoparticles and, further, to the RGO skeleton where they are captured by adsorbed oxygen to form the radical species (Figure 6c). The holes directly attack the pollutant to form the degradation products [177]. Similarly, in the RGO/ZnFe 2 O 4 /Ag 3 PO 4 ternary heterostructure, a heterojunction type of charge transfer mechanism was the most plausible route (Figure 6d Several other interesting ZnFe2O4-based magnetic nanocomposite photocatalysts such as ZnFe2O4/ZnO/Ag3PO4 [64], ZnFe2O4/ZnO [179], N-TiO2/ZnFe2O4 [180], ZnFe2O4/Ag3PO4 [38], and Ag/ZnO/ZnFe2O4 [181] have recently been fabricated, and their photocatalytic properties were examined for the degradation of various pollutants in water. Notably, coupling the various semiconductors with ZnFe2O4 not only improved the overall visible response and photocatalytic properties of the photocatalysts but also introduced magnetic behaviour, which then allowed for easy separation using an external magnetic field. Nickel ferrite (NiFe2O4) is another commonly exploited ferrite with a narrow band gap (1.7 eV), good magnetic properties, and high chemical and thermal stability. Structurally, NiFe2O4 has a typical inverse spinel structure and is ferromagnetic in nature. Its magnetism stems from the magnetic moment of antiparallel spins between Fe 3+ cations occupying tetrahedral sites and Ni 2+ cations sitting at octahedral sites [172,182,183]. Pure NiFe2O4 exhibits very low photocatalytic activity due to a high recombination rate of the charge carriers and the aggregation of the nanoparticles. However, NiFe2O4 is an attractive semiconductor to couple with other semiconductors or carbon nanomaterials and   [181] have recently been fabricated, and their photocatalytic properties were examined for the degradation of various pollutants in water. Notably, coupling the various semiconductors with ZnFe 2 O 4 not only improved the overall visible response and photocatalytic properties of the photocatalysts but also introduced magnetic behaviour, which then allowed for easy separation using an external magnetic field.
Nickel ferrite (NiFe 2 O 4 ) is another commonly exploited ferrite with a narrow band gap (1.7 eV), good magnetic properties, and high chemical and thermal stability. Structurally, NiFe 2 O 4 has a typical inverse spinel structure and is ferromagnetic in nature. Its magnetism stems from the magnetic moment of antiparallel spins between Fe 3+ cations occupying tetrahedral sites and Ni 2+ cations sitting at octahedral sites [172,182,183]. Pure NiFe 2 O 4 exhibits very low photocatalytic activity due to a high recombination rate of the charge carriers and the aggregation of the nanoparticles. However, NiFe 2 O 4 is an attractive semiconductor to couple with other semiconductors or carbon nanomaterials and form nanocomposite photocatalysts with improved activity and, most importantly, could easily be separated using a magnet [167,184,185]. For example, magnetically separable AgBr/NiFe 2 O 4 nanocomposite photocatalyst prepared via a combination of hydrothermal and an ultrasound-assisted precipitation method showed improved photocatalytic performance towards RhB under visible light illumination [186]. Ji and coworkers observed a significant improvement in the photocatalytic activity of magnetic g-C 3 N 4 /NiFe 2 O 4 towards MB degradation, reaching 87% in 4 h of visible light irradiation [172]. In another example, magnetic Ag 3 PO 4 /NiFe 2 O 4 nanocomposite tailored using an in situ precipitation method exhibited better visible light-driven photocatalytic activity towards MB compared to Ag 3 PO 4 and NiFe 3 O 4 , individually [184]. NiFe 2 O 4 -based magnetic nanocomposites incorporating carbon nanomaterials such as Pd-NiFe 2 O 4 /RGO [27] and NiFe 2 O 4 /MWCNTs [187] have also shown remarkable enhancement in photocatalytic activity owing to the synergy between the carbon nanomaterials and NiFe 2 O 4 , which resulted in improved optical properties, charge separation, and transportation and specific surface area [27,187].
In  [184,186]. Meanwhile, in g-C 3 N 4 /NiFe 2 O 4 (both semiconductors are visible active), the electrons transfer from the conduction band of g-C 3 N 4 to the conduction band of NiFe 2 O 4 , and the holes transfer in the opposite direction to the valence band of g-C 3 N 4 (Figure 7a) [172]. Charge separation efficiency is greatly improved upon coupling NiFe 2 O 4 with the other semiconductors since the holes and electrons are located at different sites in the nanocomposites. Moreover, the presence of NiFe 2 O 4 nanoparticles in the nanocomposites endowed them with good magnetic response, which allowed for easy magnetic separation. form nanocomposite photocatalysts with improved activity and, most importantly, could easily be separated using a magnet [167,184,185]. For example, magnetically separable AgBr/NiFe2O4 nanocomposite photocatalyst prepared via a combination of hydrothermal and an ultrasoundassisted precipitation method showed improved photocatalytic performance towards RhB under visible light illumination [186]. Ji and coworkers observed a significant improvement in the photocatalytic activity of magnetic g-C3N4/NiFe2O4 towards MB degradation, reaching 87% in 4 h of visible light irradiation [172]. In another example, magnetic Ag3PO4/NiFe2O4 nanocomposite tailored using an in situ precipitation method exhibited better visible light-driven photocatalytic activity towards MB compared to Ag3PO4 and NiFe3O4, individually [184]. NiFe2O4-based magnetic nanocomposites incorporating carbon nanomaterials such as Pd-NiFe2O4/RGO [27] and NiFe2O4/MWCNTs [187] have also shown remarkable enhancement in photocatalytic activity owing to the synergy between the carbon nanomaterials and NiFe2O4, which resulted in improved optical properties, charge separation, and transportation and specific surface area [27,187].
In all the NiFe2O4-incorporating photocatalysts, NiFe2O4 is excited upon the absorption of visible light. The charge transfer process depends on the other semiconductors involved in the nanocomposites. For example in AgBr/NiFe2O4 and Ag3PO4/NiFe2O4, in addition to NiFe2O4, AgBr and Ag3PO4 are also excited by visible light, and electrons are promoted to their conduction bands, leaving holes in the valence bands. The electrons transfer from the conduction band of NiFe2O4, since it is more negative, to the conduction bands of AgBr and Ag3PO4. Accordingly, the holes transfer from the valence bands of AgBr and Ag3PO4 to the valence band of NiFe2O4 where they are involved in the degradation of the pollutants [184,186]. Meanwhile, in g-C3N4/NiFe2O4 (both semiconductors are visible active), the electrons transfer from the conduction band of g-C3N4 to the conduction band of NiFe2O4, and the holes transfer in the opposite direction to the valence band of g-C3N4 (Figure 7a) [172]. Charge separation efficiency is greatly improved upon coupling NiFe2O4 with the other semiconductors since the holes and electrons are located at different sites in the nanocomposites. Moreover, the presence of NiFe2O4 nanoparticles in the nanocomposites endowed them with good magnetic response, which allowed for easy magnetic separation. Cobalt ferrite (CoFe2O4) is another important magnetic material with several attractive properties such as a crystallisation temperature similar to that of TiO2, remarkable electromagnetic behaviour, high cubic magnetocrystalline anisotropy, good chemical stability, and high mechanical hardness [188][189][190][191]. Coupling CoFe2O4 with other semiconductors, carbon nanomaterials and Cobalt ferrite (CoFe 2 O 4 ) is another important magnetic material with several attractive properties such as a crystallisation temperature similar to that of TiO 2 , remarkable electromagnetic behaviour, high cubic magnetocrystalline anisotropy, good chemical stability, and high mechanical hardness [188][189][190][191].
Coupling CoFe 2 O 4 with other semiconductors, carbon nanomaterials and conducting polymers are some of the strategies explored in order to benefit from its optical and magnetic properties. Pristine CoFe 2 O 4 shows poor photocatalytic efficiency due to the high recombination rate of the charge carriers [61,192,193]. Kim et al. prepared hollow cobalt ferrite-polyaniline (CoFe 2 O 4 -PANI) nanofibres using the electrospinning method, calcination, and in situ chemical oxidative polymerisation, in succession. It was observed that the nanocomposite was magnetically separable and also showed significantly higher visible light photocatalytic degradation of MO, which was 80 times the activity of neat CoFe 2 O 4 [61]. Hydrothermally prepared magnetic CoFe 2 O 4 /graphene (GR) nanocomposites with varying GR loadings exhibited improved visible light photocatalytic properties towards MB degradation compared to pristine CoFe 2 O 4 . At an optimum GR loading of 10%, the nanocomposite reached 100% MB degradation in 60 min, while the neat CoFe 2 O 4 could only manage a low 34% degradation in twice the irradiation time [192]. The enhanced activity could be ascribed to the formation of heterojunctions between CoFe 2 O 4 and GR, which allowed for the efficient separation of the charge carriers. Upon excitation by visible light, the electrons in the conduction band of CoFe 2 O 4 transfer to GR, where they are trapped by molecular oxygen adsorbed on the surface [192]. A similar scenario was highlighted as responsible for the improved activity of CoFe 2 O 4 -PANI, whereby the electrons are transferred to the conduction band of CoFe 2 O 4 from PANI while the holes transfer in the opposite direction to PANI [61]. The electrons and holes are kept apart, which ensures their availability to form the active species responsible for pollutant degradation.
Enhanced visible light photocatalytic decomposition of MB was reported over CoFe 2 O 4 decorated CdS nanorods (CoFe 2 O 4 /CdS, prepared via a soft chemical method. The composite nanorods exhibited better activity than CdS and CoFe 2 O 4 , individually, and were magnetically separable [193]. Three-dimensional (3D) urchin-like TiO 2 , prepared hydrothermally, was decorated with CoFe 2 O 4 using a co-precipitation strategy to yield the nanocomposite photocatalyst CoFe 2 O 4 /TiO 2 with improved photocatalytic properties compared to the individual materials. Upon UV light illumination, the nanocomposite reached 98.9% MB degradation, while 79.9% MB decomposition was realised when the urchin-like TiO 2 was used as the photocatalyst [194]. Moreover, the nanocomposite photocatalyst showed good stability and recyclability, reaching 93.8% MB degradation in the fifth cycles while still maintaining good magnetic properties [194]. Impressive photocatalytic activity and versatility was observed for CoFe 2 O 4 /BiOX (X = Cl, Br and I) nanocomposites towards the UV and visible light photocatalytic decomposition of MO, RhB, MB, and their mixture (MO + MB + RhB) [195]. Similarly, Xu and coworkers examined the photocatalytic properties and versatile nature of the Ag/AgCl/CoFe 2 O 4 nanocomposite by employing the photocatalyst for the decomposition of MO, bisphenol A (BPA), ciprofloxacin (CIP), and the deactivation of E. coli. The nanocomposite photocatalyst displayed outstanding photocatalytic performance towards both coloured (MO) and colourless (BPA and CIP) pollutants as well as microbial pollutants, reaching 93.4% in 150 min for MO and 100% in 90 min for CIP, and almost all of the E. coli were deactivated in just 30 min of visible light illumination [196]. In all the CoFe 2 O 4 -incorporating nanocomposites, the enhancement in activity could be ascribed to the formation of a heterojunction between CoFe 2 O 4 and the other semiconductor, which allowed efficient charge separation and formation of the reactive species responsible for pollutant degradation. In addition, the presence of CoFe 2 O 4 endowed the nanocomposite photocatalysts with sufficient magnetic sensitivity for easy separation using an external magnetic field [193][194][195][196]. Similar results have been reported for other CoFe 2 O 4 -based magnetic photocatalysts such as CoFe 2 O 4 /MCM-41/TiO 2 [197], Fe,N-TiO 2 /CoFe 2 O 4 [198], and BiOBr/CoFe 2 O 4 [65].
A host of other ferrite-based magnetic nanocomposite photocatalysts (Table 3) have been designed and tailored for the removal of various pollutants in water, for which their photocatalytic properties were also examined. Enhanced photocatalytic performance has been observed upon the incorporation of the ferrites. Moreover, the presence of these ferrites induced magnetic response in the photocatalyst, which aids the separation process for easy recovery and recycling of the photocatalysts. Despite the decrease in magnetisation saturation of the nanocomposites compared to the neat ferrites, the photocatalyst retained enough magnetic response to enable separation using an external magnet.

Summary Remarks and Future Outlook
The development of efficient visible light photocatalytic degradation of pollutants and easy recovery and recyclability of the photocatalyst is an exciting prospect in water treatment. The incorporation of magnetic nanoparticles such as haematite, maghemite, magnetite, and the ferrites into various photocatalyst matrices provides an attractive strategy to enhance the optical properties, charge separation efficiency, and the overall photocatalytic activity. Most interestingly, the presence of the magnetic nanoparticles endow the nanocomposite photocatalyst with sufficient magnetic response to enable separation of the photocatalyst using an external magnetic field. However, careful control of the magnetic nanoparticle, especially loading maghemite and magnetite, is important in ensuring a balance between good magnetic response and the detrimental effect of recombination that occurs on the surface of the nanoparticles. Alternatively, silica is used as a barrier between the magnetic core and the other semiconductor coupled with maghemite or magnetite. This prevents charge transfer between the coupled materials that could otherwise result in photodissolution and recombination. The incorporation of the ferrites in various photocatalysts provides more beneficial properties than just good magnetic response; they contribute towards visible light absorption, charge separation, and the photocatalytic performance of the nanocomposite photocatalyst. Therefore, the ferrites are the most attractive nanoparticles compared to haematite, maghemite, and magnetite, in terms of their photocatalytic performance and multifaceted contribution in the magnetic nanocomposites. However, a systematic, comparative study of the photocatalytic performance of the ferrites, haematite, magnetite, and maghemite is seldom reported, and this makes it difficult to map a way forward in terms of exploiting the best materials for photocatalytic applications.
Despite the promising results regarding visible-light-active magnetic photocatalysts, there are still some challenges that possibly hinder the practical exploitation of these materials. The most obvious problem relates to the reaction kinetics; most of the degradation experiments are slow (may take several hours) despite the improvements in visible light absorption of the photocatalyst, and this presents a massive challenge for practical applications. Secondly, work still needs to be done to develop synthesis routes that will ensure uniform shape and size of the magnetic particles as well as their uniform distribution within the nanocomposite matrix in order to ensure good magnetic response and efficient recovery. Strong contact between the magnetic nanoparticles and the other semiconductor particles is crucial in ensuring minimal leaching out and loss of the photocatalytic activity and magnetic response. In the magnetic core-shell structures, careful control of the thickness of the silica shell is important, as this will affect the magnetic response of the composite material. Extensive theoretical studies are needed in order to understand the interactions between the magnetic nanoparticles and the other semiconductor nanoparticles in the nanocomposites; this can help design and tailor materials with good heterojunctions that promote efficient charge separation. Moreover, theoretical studies can provide a good idea of the optimum loadings of the magnetic nanoparticles in the photocatalyst to ensure good magnetic response and minimal recombination. In most cases, the magnetisation saturation of the magnetic nanoparticles decrease significantly upon coupling with the non-magnetic components of the nanocomposite photocatalyst, which has a negative bearing on the separation efficiency and recovery of the photocatalyst. Work still needs to be done to ensure that nanocomposites retain sufficiently high magnetic response in order to enable efficient magnetic separation.