Ferrite Nanoparticles for Water Decontamination Applications

Abstract

“No water, no life. No blue, no green.”—Sylvia Earle.

Despite 75% of aqueous flora fauna, humans are still struggling with potable water. In this regard, research groups have discovered various methodologies to recycle the used water. Among these approaches, advanced oxidation process (AOP) gained significant attention of researchers. More specifically, photocatalytic oxidation of organic pollutants (dyes, pigments, pesticides, etc.) seems simple and quick with non-toxic by-products. Here, semiconducting metal nanoparticles, metal oxides, and other chemical compounds have been exploited as photocatalysts due to inclined structural and optical properties. While the effect of magnetic features of such catalysts has not been studied much, here, magnetic field-directed electron spin may also affect the electron transfer in multi-component catalyst system. Moreover, the effect of external magnetic field and catalyst’s magnetic domains might also alter the electronic pathway for interfacial charge transfer phenomenon during illumination. Further, different morphological features can also be introduced to optimize the objective of development of an efficient and reusable photocatalyst with easier separation from solution.

1 Need of Wastewater Treatment

“If we move forward and don’t clean up the messes of the past, they’ll just get swept under the rug.”—Erin Brokovich.

Water is a source of survival for all forms of life on earth. One cannot die without love and surely can without water. Globally, the supply of safe, secure, and inexpensive water has been one of the biggest difficulties faced in recent years. Fresh water availability per capita has been decreased as a result of the alarming rate of industrialization and population growth [1, 2]. Water covers over 71% of the surface of the globe. Of the total available water, around 97% is found in oceans and 3% is fresh water. Only 0.3% of the freshwater on earth's surface is located in fresh lakes, rivers, and streams, while 68.7% of the remaining fresh water is frozen in icecaps and glaciers. The remaining freshwater is found beneath the ground [2]. The water distribution over the earth's surface is depicted in a pie chart in Fig. 1.

Fig. 1

Water distribution over the earth’s surface

Along with the very low availability of fresh water on the earth, industrialization and urbanization are also causing the deterioration of water quality. As a result, water crises are becoming unimaginably severe over the globe. Even though the planet’s freshwater supply has mostly remained steady over time through constant recycling via the atmosphere and back. The rise in population implies that the rivalry for a plentiful supply of clean water for drinking, cooking, bathing, and supporting life grows more intense every year. According to a recent report by UNICEF, nearly 2/3rd of the world’s population, i.e., 4.0 billion people endure extreme water scarcity for at least one month each year. Due to the acute water shortage, 700 million people may be displaced worldwide by 2030 [3]. These key factors of water scarcity in the future are enough to be alarmed and step up for water conservation. Due to the dumping of polluted water effluents from industries to rivers and other freshwater streams, water-borne diseases are rising. According to recent data from a WHO assessment, at least 2 billion people drink polluted (feces) water. Microbial contamination brought on by feces contamination poses the biggest threat to the potable water. Drinking water contaminated by microorganisms increases the risk of contracting diseases such as diarrhea, cholera, typhoid, polio and contributes to the 4,85,000 annual deaths from diarrhea. The most dangerous chemicals in drinking water include arsenic, fluoride, and nitrate, but novel pollutants like pesticides, drugs, per- and polyfluoroalkyl substances (PFASs), and microplastics are now a source of public concern [4]. These pollutants generally emerge from wastewater from industries and households. Recycling wastewater effluents is therefore a top issue in every community worldwide in terms of environmental conservation and sustainable development.

2 Background of Photocatalytic Wastewater Treatment

The major water pollutants are of three types, viz. organic, inorganic, and microbial contaminants. The organic pollutants include pesticides, phthalates, phenols, dyes, and other VOCs from industries. These pollutants are usually carcinogenic and mutagenic in nature even present in very small fractions and can lower dissolved oxygen causing harm to humans and aquatic life. Metals, salts, and other substances free of carbon are examples of inorganic pollutants. From an ecotoxicological point of view, metal ions like Hg²⁺, Pb²⁺, Cr³⁺, Cr⁴⁺, Ni²⁺, Co²⁺, Cu²⁺, Cd²⁺, Ag⁺, As⁵⁺, and As³⁺ are toxic. In addition, the contamination caused by radioactive elements is a serious problem given their potentially dangerous long-term effects. Microbes/pathogens including bacteria, viruses, and parasites may only be present in very small concentrations in drinking water, but they are a major risk factor for the safety of water since they spread many infectious diseases. The pathogens such as bacteria, viruses, protozoa, and helminths are responsible for disease such as diarrhea, cholera, gastrointestinal illness [5,6,7,8,9,10,11,12,13].

Primary, secondary, and tertiary treatments are some of the processes in the treatment of wastewater as shown in Fig. 2. In the preliminary treatment procedure, heavy objects like sand, stones, etc. are separated from water using a range of processes, such as screening followed by grit removal, then pre-aeration, flow metering, and finally sampling. After initial treatment, primary treatment using sedimentation and floatation process removes the settleable organic solids. Biological techniques are employed in secondary treatment to further decompose organic materials, remove dissolved and colloidal particles (aquatic microorganisms). The most important and final stage entails a thorough tertiary treatment to clean the secondary-treated water and make it fit for consumption [14, 15]. There are tertiary treatments available to extract or degrade waste from wastewater using physical, chemical, and biological techniques (Fig. 2). These methods include electrochemical techniques, ozonation, adsorption, reverse osmosis, coagulation and flocculation, precipitation, and fungal decolonization [16,17,18,19,20,21,22]. For tertiary treatment, typical purification systems are either inefficient or excessively expensive. When using chemical techniques, ozonation is the least effective because of its short life and high expense, while the limitation of chlorination is that it may produce trihalomethane, a harmful disinfection by-product. Reverse osmosis is an expensive and ineffective physical approach for treating wastewater, as are flocculation and filtering [23]. When using biological methods, system handling becomes extremely important. Therefore, there is still a need for an effective, affordable, and manageable wastewater treatment method.

Fig. 2

Wastewater treatment stages

Among various wastewater treatment techniques, advanced oxidation process (AOP) compared to other techniques is rapid, effective, and inexpensive. It can overcome obstacles including hazardous by-products, high costs, and partial removal of pollutants. AOPs are employed for the complete mineralization of complex bio-refractory pollutants from aqueous solution [1]. Different AOPs include Fenton-like reactions, ozonation, and photocatalysis which require costly catalysts, equipment and may lead to incomplete degradation to form toxic secondary products which adds to environmental stress. One form of promising AOP technology is sunlight-driven photocatalysis. Solar irradiation has the ability to activate photocatalysts, resulting in the production of highly reactive photo-induced charge carriers that react with contaminants. In AOP photocatalytic process, the photocatalyst is generally mixed with the aqueous sample in which pollutants are present. The mechanism of photocatalysis is shown in Fig. 3. The basic principle on which AOP photocatalysis works involves the generation of exciton pair upon irradiation exposure of photocatalyst with electromagnetic radiation of energy, equal or greater than the optical band gap of the catalyst. The transition of electron (e⁻) from the valence band to the conduction band occurs, and holes (h⁺) are left in the valence band. Therefore, a photocatalyst with a smaller band gap is more likely to absorb more photons from visible light. The generated e⁻ and h⁺ further react with water and dissolved oxygen in the water to form superoxide anion radicals and hydroxyl radicals. These radicals further react on the adsorbed pollutant on the surface of the catalyst to decompose it to form minerals and CO₂ [24]. The following equations represent the mechanism of photocatalysis:

$${\text{catalyst}} + h\nu \to {\text{catalst}} + {\text{h}}^{ + } + {\text{e}}^{ - } ,$$

(1)

$${\text{catalyst}} \left( {{\text{e}}^{ - } } \right) + {\text{O}}_{2} \to {\text{O}}_{2}^{ - } ,$$

(2)

$${\text{catalyst}} \left( {{\text{h}}^{ + } } \right) + {\text{H}}_{2} {\text{O}} \to {\text{OH}}^{ \cdot } ,$$

(3)

$${\text{O}}_{2}^{ - } + {\text{OH}}^{ \cdot } + {\text{pollutant}} \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}}{.}$$

(4)

Fig. 3

Mechanism of photocatalytic wastewater treatment

3 Magnetic Materials as Photocatalyst

There are variety of materials which have been used so far as photocatalyst for wastewater treatment including metal oxide (ZnO, TiO₂, BiTiO₃, SrTiO₃, Fe₂O₃, BiOBr, CaFe₂O₄, ZnFe₂O₄, etc.), metal organic frameworks, carbon nanostructures (carbon nanosphere, carbon nanotube, carbon nitride, etc.), metal sulfide (ZnS, CdS, CuInS₂, etc.), piezoelectric materials, and noble metals (Pt, Ag, and Au). Due to its great chemical inertness, non-toxicity, abundance (0.44% of the earth’s crust), and cost, titanium dioxide (TiO₂) is currently the most used photocatalyst. Possessing 3.2 eV as optical band gap, TiO₂ can absorb UV light (λ < 387 nm), which only makes up 5–8% of the whole solar spectrum. As a result, efforts have made on developing novel composite of TiO₂ with various components, such as carbon nanotubes, activated carbons, metal ions (Co, Ag, Au, etc.), and non-metals (nitrogen, carbon, sulfur, etc.). A good photocatalyst needs to have a low bandgap energy, with CB and VB at the proper positions to increase visible light activity. Other than that, a common problem with all the photocatalysts is their recovery from solution after use and thereafter their reuse, which restricts the scope of applications for photocatalysis [25,26,27].

Recently, magnetic materials have proven to be more useful for application of photocatalysis. The advantages of magnetic materials as photocatalysts over non-magnetic photocatalysts are their high efficiency, sustainability, and ease of preparation. When the particle size decreases (to nano-level), solid/liquid separation becomes increasingly challenging. On the other hand, magnetic filtering can be used for separation of catalyst from solution when using magnetic sorbents based on metal oxides. Additionally, using magnetic fields to remove particles from solutions is more efficient and effective (and frequently much faster) than centrifugation or filtering. The benefits of employing magnetic nanoparticles as an adsorbent in water treatment procedures include their high reusability, biocompatibility, selective solid/liquid separation using magnetic filtration, which is speedier than centrifugation and filtration procedures. However, the most important advantage of using magnetic materials is that catalysts are simply removed using a magnetic field from outside to separate the reaction liquid [28, 29]. The entire processes can be done with low cost in less time and improved reusability of the catalyst (Fig. 4).

Fig. 4

Illustration of photocatalysis and removal of magnetic nanoparticles

Numerous types of magnetic materials have been fabricated, developed, and employed successfully in a variety of applications, including biomedicine, MRI, catalysis, spintronics, robotics, engineering, environmental remediation, etc. [2]. For a magnetic material being able to show best photocatalytic behavior under visible light, the following conditions must be satisfied:

• Their band gap should lie in the region of semiconductor (2–3 eV).

• Should be applicable at ambient temperature and pressures.

• Good selective adsorption.

• Lower rate of recombination of electrons and holes.

• Complete mineralization with no harmful by-products.

There are several methods to synthesize magnetic materials, including co-precipitation, solvothermal, hydrothermal, microemulsion, sonochemical, etc., which affect their magnetic properties, particle size, distribution, shape, and other characteristics. There are variety of metals which show magnetic properties, but the strongest magnetic materials consists of iron, cobalt, and nickel or their alloys [2]. All these magnetic photocatalysts are typically the composites made of magnetic material and non-magnetic photocatalytic material. In general, iron is used as one of the major components for the synthesis of magnetic photocatalysts. The most common and potentially useful magnetic minerals for water treatment are α-Fe₂O₃ (hematite), β-Fe₂O₃ (maghemite), Fe₃O₄ (magnetite), and FeO (wustite) out of the eight known phases of iron oxides [30]. Initially, single-phase magnetic material such as Fe₂O₃ is used in environmental applications [31]. Rarely, reports are there on magnetic photocatalysts with a single phase that had high catalytic performance [32]. Most single-component, single-phase photocatalysts have a weak capacity for visible adsorption, which results in a low catalytic efficiency. Development of binary and ternary photoactive ferrites has been used to address this difficulty. The general formula for a material containing Fe³⁺ is MFe₂O₄, where M is a transition metal (Cu, Zn, Co, Mn, Ba, or Ni). Size- and shape-dependent magnetic response (saturation magnetization) of ferrites enables their utility as photocatalyst under external magnetic field. In terms of a photocatalytic application, the narrow band gap caused by the additional metal (M) enables ferrite as a very potent catalyst under visible radiation. Strong photocatalytic activity is demonstrated by ferrites, particularly when paired with other binary or photoactive materials such as BiFeO₃, ZnFe₂O₄, CoFe₂O₄, NiFe₂O₄ [33]. The examples of other binary ferrite photocatalysts along with synthesis method, pollutant, and efficiency of degradation are given in Table 1.

Table 1 Magnetic metal oxide-based photocatalysts with synthesis method, pollutant, and efficiency of degradation

The interaction of a magnetic oxide with other oxide phase has also been extensively investigated as a way to produce highly photoactive materials. The catalytic activity can be significantly increased and charge recombination can be reduced when two different oxides are combined. Other than that, to overcome the difficulty of separation of photocatalyst along with the achievement of superior efficiency, combination of already available photocatalysts with the magnetic materials is came into existence. As TiO₂ is the most widely used catalyst, synthesis of magnetic photocatalyst in combination with TiO₂ has been studied broadly. For the very first time in 1994, Japanese patented the synthesis of Fe₃O₄/TiO₂ nanocomposites via titanium oxide deposition onto a magnetic core via hydrolysis of TiOSO₄, which led to its precipitation. Towata et al. in 1997 patented the dispersion of ferromagnetic particles in a TiO₂ suspension led to the creation of another magnetic photocatalyst  [34]. Afterward, various researchers reported the deposition of photocatalyst onto magnetic core particles [35,36,37,38]. According to Beydoun et al. [35], charge carrier recombination of Fe₃O₄ could be caused by the contacts between the Fe oxide (core) and TiO₂ (shell) during illumination. Iron is less likely to be transported into the lower conduction band of the Fe₃O₄ core due to photogenerated electrons which are elevated to the conduction band of TiO₂. The iron oxide core’s conduction band is preferred by the photogenerated holes in the valence band of TiO₂, which causes iron to oxidize and then leach iron ions into the solution. Therefore, it was concluded that interaction between magnetic Fe oxide and photoactive TiO₂ caused reduced photocatalytic activity when compared to solo TiO₂ against organic pollutant degradation. It was recommended to sandwich an inert layer between the magnetic core and TiO₂ shell to fend off photo-dissolution. The three-component photocatalyst is composed of three layers: (i) magnetic core, (ii) interlayer that prevents photo-dissolution and charge carrier recombination, and (iii) photocatalytic outer layer for the degradation of target molecule. Gad-Allah et al. [39] synthesized TiO₂/SiO₂/Fe₃O₄ composite as a magnetic photocatalyst in which silica was used as inert layer between TiO₂ and Fe₃O₄ to protect the magnetic and recombination of electron holes. In a similar manner, other reports also showed the use of silica or carbon as inert layer material between TiO₂ and magnetic core to form composite which was completely magnetically separable with high photocatalytic efficiencies [40,41,42]. Other material that has been extensively used as photocatalyst is zinc oxide (ZnO), which has a bandgap energy that is similar to that of TiO₂ (3.37 eV). Other benefits that have prompted researchers to extensively study its photoactivity include its electron mobility and inexpensive production costs. Researchers reported the synthesis of Fe₃O₄-embedded ZnO magnetic materials as semiconductor photocatalysts for wastewater treatment. Some other examples of important magnetic metal oxide-based photocatalysts with synthesis method, pollutant, and efficiency of degradation are given in Table 1.

Successful immobilization or covalent bonding of organic moieties and non-metals, such as S, N, Cl, and P, onto magnetic photocatalysts has also been used to aid in the photodegradation of organic contaminants. It has been demonstrated that adding a well-selected organic modifier can produce a material with a greater photoactivity than its pure inorganic counterpart. When a material is doped with a suitable dopant, such as N doping in TiO₂ photocatalyst leads to increased photoactivity. The substitution of N-atoms for oxygen in the lattice sites has been connected to an increase in photoactivity, and this will result to an occupied N_2p level above valence band of catalyst (O_2p). The semiconductor’s new N_2p level is situated halfway between the valence and conduction bands enabling easier transportation of N_2p electrons to conduction band.

The common examples of organic/inorganic/magnetic photocatalysts involve the combination of ferrite with carbon, silica, poly(propylene oxide), polydopamine, poly(N-isopropylacrylamide), multiwalled carbon nanotubes, carbon spheres, graphene, etc., for usage in wastewater treatment via photocatalysis. Along with their appropriate magnetic properties—namely, being ferrimagnetic, ferromagnetic, and super paramagnetic (nanoparticle size < 10 nm)—their synthesis process is easy and inexpensive. According to their intended usage, these magnetic materials' size, shape, and magnetism may be easily changed, making it simple to disperse them in liquid media and maintain their stability for various applications. Additionally, these materials are biocompatible, chemically inert, thermally stable, and non-toxic or less hazardous [2].

In addition, numerous researches have been conducted in which the various magnetic properties, such as electron spin, external magnetic field, or size/morphology of the catalysts, were identified and might potentially have an impact on the photocatalytic efficiency of the magnetic material.

4 Properties of Magnetic Photocatalysts

The saturation magnetization, coercivity, magnetoresistance effect, etc. of magnetic materials’ photocatalysts typically affect their effectiveness. These properties directly affect the rate of charge separation and recombination and hence the photocatalytic efficiency of the material. These properties are directly affected by externally applied magnetic field, tuning of electron spin, and size of the photocatalyst sample. In contrast to expectations, the photocatalytic process takes a period of 10^–5–10^–3 s for the charge separation and transportation of excitons, which is significantly higher than the time of recombination (10^–12–10⁻³ s) in a typical semiconductor [61]. Here, the preparation of composite materials would solve the splitting and transfer of photoexcited electron–hole pairs-related issues. When a metal and semiconductor are combined, the semiconductor's energy band forms an interfacial electronic antiblocking layer, delaying the recombination of excitons. This results from the disparity between the Fermi level of the semiconductor and the work function of the metal. Another method to improve the separation and transmission of photogenerated electron–hole pairs is to apply an external magnetic field while the photocatalytic process is underway.

External magnetic field and Electron spin orientation

An acceptable, effective, non-contact way for increasing photocatalytic efficiency is to apply an external magnetic field. The magnetic field can alter the electrical configurations of the photocatalyst in its ground state, specifically the direction of the electron spin, in addition to producing the Lorentz force to split the photogenerated electrons and holes. Additionally, the chance that reaction intermediates will come into contact with catalyst-active sites is increased by an external magnetic field.

In case of non-ferromagnetic materials, external magnetic field does not impact electron spin orientation, and both show separate effect on the photocatalytic efficiency of the materials. When an external magnetic field is applied, a moving charged species in a magnetic field encounters a Lorentz force for non-ferromagnetic materials. Lorentz force is determined by the equation F = q.v.B, where q is the charge, v is the velocity, and B is the strength of the magnetic field. Light activates the electron–hole pairs during the photocatalytic reaction. They will experience opposing forces and deviate in the opposite direction when traveling in an external magnetic field as a result of their opposite charges. More carriers can interact with the catalyst surface and take part in the catalytic activity as a result of the longer lifetimes of electrons and holes.

Since charge and spin are two characteristics that electrons inherently possess. Spin orientations can be categorized as upward and downward, and inverse spin orientations will produce magnetic fields in the opposite direction. The electron spin states within a catalyst determine the electronic configurations of the catalyst’s ground state, which in a notable way influence a catalyst’s ability to absorb light, photogenerated carriers’ splitting and recombination, and the intermedia species reaction barrier. Enhancing the effectiveness of electron transfer is possible through the dissipation-less spin transporting generated by spin polarization. Spin orientation can be tuned by adjusting the vacancies available in the catalyst via doping. In case of ferromagnetic sample, the spin orientation can be simply tuned by applying external magnetic field [62].

Electron spin states and outside magnetic fields have an impact on photocatalysis performance when ferromagnetic catalysts are exposed to a magnetic field. Electrons with spin preference can be produced by ferromagnetic catalysts, and these electrons will align with the external magnetic field. The interface of composite catalysts could easily permit the passage of electrons with precise spin alignments. A common trait is a negative magnetoresistance (MR) effect. The formula for MR can be given in Eq. (5) below:

$${\text{MR}} = \left[ {\left( {R_{H} - R_{0} } \right)/R_{0} } \right] \times 100\% ,$$

(5)

where the symbols R_H and R₀, respectively, stand for the resistances with and without a magnetic field. A substance exhibiting the MR effect can change its resistance when the magnetic field is present. Therefore, the resistance will drop in a magnetic field if a photocatalyst material displays a negative MR effect. A negative MR photocatalytic material will have its spin moments align parallel to one another in the presence of a magnetic field, which allows for more photogenerated carriers to participate in the surface photocatalytic processes. As a result, photogenerated carriers will be used more frequently, and photocatalytic performance will be improved with high sunlight conversion efficiency. In the meanwhile, a magnet can easily recycle photocatalysts having ferromagnetic properties [61, 63]. The schematic of charge separation with induced Lorentz force by applying external magnetic field and thus leading to enhanced photocatalytic activity is shown in Fig. 5.

Fig. 5

Schematic of improved charge separation and enhanced photocatalysis via application of external magnetic field

Shi et al. [61] reported that hydrothermal and photoreduction methods used to create ZnFe₂O₄/Ag were used to deposit it on a 3D-cs substrate (to create 3D-cSZA) for increased surface area. In a 3D-cSZA photodegradation experiment, MO degraded 18% within 90 min and the photodegradation performance is further enhanced by the addition of a magnetic field, and at the same time, 24% of MO was degraded. Due to the photocatalyst's negative magnetic resonance (MR), when a magnetic field is provided, excessive electrons and holes can move to the surface to degrade pollutants, which are advantageous for increasing photocatalytic efficiency. Li et al. [64] reported the photocatalytic removal of Rhodamine B dye at equivalent rates of 59%, 74%, 83%, and 84% (magnetic fields 0, 3, 6, and 8 kOe, respectively) in the presence of α-Fe₂O₃/rGO nanocomposites. The applied magnetic field led to a notable increase in photocatalytic efficiency. In contrast, the photocatalytic efficiency of pure α-Fe₂O₃ was barely observed at zero magnetic field. When the α-Fe₂O₃/rGO nanocomposites were subjected to various magnetic fields, an apparent MR effect can be seen. While α-Fe₂O₃ nanoparticles did not exhibit MR impact due to alignment of magnetic moment (α-Fe₂O₃ + rGO) and applied magnetic field lines, electronic spin orientation could easily pass through the resulted interface, which resulted in lowered resistance of α-Fe₂O₃/rGO nanocomposites when a magnetic field was applied. In a similar work, Wang et al.  [65] used the composite photocatalyst of α-Fe₂O₃ and rGO (mass ratio, 10:1) in 1000 Oe magnetic field. The visible light-induced degradation efficiency of Congo red (CR) was improved from 60 to 87%. In comparison to pure α-Fe₂O₃, the composite photocatalyst had a greater negative MR and more vigorous magnetization intensity, according to magnetic behavior measurement. By applying a magnetic field, it was possible for the electrons with the precise spin alignments to flow over the contact with ease. He et al.  [63] prepared ZnFe₂O₄/AgBr magnetic photocatalyst. The % degradation of Congo red using the synthesized photocatalyst without the external magnetic field was 40% in 90 min. The photodegradation efficiency of ZnFe₂O₄/AgBr was significantly increased by applying 1000 Oe of magnetic field; 53% of condo red was photodegraded in just 90 min. It was observed that only AgBr catalyst showed no changes in their photocatalytic efficiency with variation in magnetic field. Also, pure ZnFe₂O₄ showed significant enhancement in the photoactivity with external applied magnetic field. Therefore, it was concluded that due to its greater negative MR, ZnFe₂O₄/AgBr has the maximum photocatalytic activity when exposed to a magnetic field.

Hence, adding the magnetic field to photocatalytic processes is a useful and effective way to increase the efficiency of solar energy conversion while also improving adsorption on surfaces and charge separation.

Size and Morphology

The particle size induced magnetic features, e.g., coercivity and saturation magnetization, has already been well-documented. Depending on the material, saturation magnetization and coercivity for small nanoparticles up to a threshold size have been observed to increase as particle size has risen. The following equation gives the separating magnetic force (F_m) operating on the particles from solution with the help of external magnetic field:

$$F_{{\text{m}}} = \, \mu_{0} .V_{{\text{p}}} .(M_{{\text{p}}} .\ N )H_{{\text{a}}} ,$$

(6)

where V_p and M_p are the particle’s volume and magnetism, respectively, and H_a is applied magnetic field strength. Therefore, the force needed to separate the magnetic particles from the solution increases with respect to increased particle size. The surface-to-volume ratio is decreased by increasing particle diameter, which lowers the photocatalytic material’s effectiveness. So, in order to obtain the desired outcomes in these methods: improved photochemical kinetics and magnetic recovery, these parameters must be regulated [66].

The factors such as size, coercivity, saturation magnetization, which directly affect their performance, are influenced by the morphology of the nanoparticles in addition to the size and nature of the material. Exploring various nanomaterials morphologies is therefore vital to fully comprehend the potential benefits and drawbacks that each one of them may have in relation to applications. The foundation for the development of advanced multicomponent magnetic systems will be the structure–properties relationship as well as the selection of synthesis techniques in terms of resilience, particle size uniformity, scalability, and chemical characteristics. The saturation magnetization directly affects the magnetic moment which in turns consequences the photocatalytic activity of the magnetic materials. An increase in photocatalytic degradation is related to the maximum net magnetic moment [67]. The cube-shaped particles exhibit the highest saturation magnetization, while rod-like particles exhibit the lowest, notwithstanding the difficulty in accurately controlling the morphology, particle size, and crystallinity of the nanosized particles [68]. Baghshahi et al. [68] reported that after 120 min of exposure to light, the photocatalytic MB removal efficiency of Fe₃O₄-TiO₂ NPs was greater than 90, 73, and 64% (spherical, cube, and rods, respectively). On the contrary, in another report by Baghshahi et al. [69], magnetite nanoparticles of different morphologies were developed by hydrothermal method with magnetic saturation of 60.74, 66.36, and 35.11 emu/g for the sphere, cube, and rod-like morphologies. The results were interpreted in terms of particle size, and the greater critical size of rod-shaped nanoparticles can be used to explain the magnetic drop in saturation. In contrast to spherical nanoparticles, the magnetic saturation of cubic nanoparticles rises with size.

Only a very few reports are available in which the effect of morphology in correlation with particle size of magnetic materials on their photocatalytic activity is explained. The variation in morphology contributes in the saturation magnetization of the magnetic material which in turn affects its photocatalytic activity, but the results are dependent on particle size too. In conclusion, both particle size and morphology play altogether to put an impact on the saturation magnetization and thus photocatalytic activity of the magnetic material.

5 Conclusion

This chapter explains how magnetic materials’ structure–property connection affects their photocatalytic efficiency for removing organic contaminants from aqueous solutions. The availability of drinkable water is still a problem for mankind, aquatic vegetation, and animals. In this context, research teams have found a number of techniques for recycling spent water. Among them, the oxidation of organic pollutants (such as dyes, pigments, pesticides) by photocatalysis appears simple and quick, and it produces non-toxic by-products. The role of semiconducting magnetic nanoparticle in the photocatalysis has been explained in detail. Different types of magnetic materials, viz ferrites, their organic, inorganic, metallic composites, and their role in photocatalysis have been discussed thoroughly. However, a very few researchers studied the influence of these catalysts' magnetic properties on photocatalytic activity. In case of non-ferromagnetic materials, external magnetic field does not impact electron spin orientation, and both show separate effect on the photocatalytic efficiency of the materials. Electron spin states and outside magnetic fields have an impact on the efficiency of photocatalysis when ferromagnetic catalysts are exposed to a magnetic field. Spin-preferential electrons can be produced by ferromagnetic catalysts, and these electrons will align with the surrounding magnetic field. The common trait is negative MRs. More photogenerated carriers can take part in the surface photocatalytic processes because a negative MR photocatalytic material will have its spin moments align parallel to one another in the presence of a magnetic field. As a result, photogenerated carriers will be used more frequently, and high sunlight conversion efficiency will enhance photocatalytic activity. Other than external magnetic field, effect of particle size and morphology on their photocatalytic activity was also discussed. It was concluded that the variation in morphology influences the saturation magnetization of the magnetic material, which in turn impacts its photocatalytic activity, but the outcomes also depend on particle size. To achieve a suitable magnetic performance for such photocatalytic applications, magnetic particles must be of the appropriate sizes and have a morphology that exhibits high saturation magnetization.