Bismuth-based magnetic Z-scheme heterojunctions as emerging photocatalysts for the environmental remediation of pharmaceuticals

1. Introduction

Water is a crucial element for the sustainability of life; it is not only essential for the sustenance of aquatic life but also vital for irrigation, food production, human survival, and the stability of the whole ecosystem. However, escalated industrialization and globalization produce the majority of the liquid and solid waste that pollutes water sources worldwide [1]. The primary cause of illness and disease worldwide is poor water quality. According to reports, the frequent release of 2 million tons of garbage into waterways lead to some major health issues. Additionally, there are an estimated 14,000 deaths per day, attributed to contaminated drinking water [2]. Water pollution is caused by various factors such as excessive energy usage, radioactive waste, urbanization, improper management of sewage and wastewater, industrial waste, mining activities, pesticides, and chemical fertilizers [3]. The rapid industrial expansion has accompanied the world’s rapid population growth, producing more industrial and domestic wastewater that contains harmful organic contaminants that withstand most current treatments. Due to their high persistence, low biodegradability, promotion of disease resistance, carcinogenicity, and mutagenicity, new pollutants like antibiotics, even at low doses, have attracted a lot of attention. Pharmaceuticals, such as antibiotics (ciprofloxacin (CIP), ofloxacin-OFL, norfloxacin-NOR), non-steroidal anti-inflammatory drugs (NSAIDs) (diclofenac-DCF, ibuprofen), and psychiatric drugs (carbamazepine) have been continuously introduced into aquatic environments through the waste streams of factories, medical centers, and wastewater treatment plants [4]. Antibiotics such as CIP, OFL, NOR, and tetracycline (TC) are most found in wastewater effluents and surface waters. Their extensive use, ineffective elimination through conventional therapeutic methods, and incomplete metabolism in both people and animals are the primary causes of their existence in wastewater. Because of their extreme toxicity and carcinogenicity, the European Union (EU) registered several commonly used pharmaceuticals and personal hygiene items in 2007 as recently discovered pollutants [5].

Pharmaceuticals can persist in the environment and are typically non-biodegradable. However, natural attenuation processes like direct/indirect photolysis can lower the concentration of these pollutants in water bodies. Photocatalysis is considered a very effective methodology for pharmaceutical degradation because of its strong oxidation capability, economic viability, and environmental friendliness [6]. According to the technique’s basic idea, photocatalytic reactions are fueled by external energy after light is absorbed and various reactive oxygen species (ROSs), such as non-selective hydroxyl radicals, are produced on the catalyst’s surface. The first stage in semiconductor photocatalysis is the absorption of light energy that is equal to or higher than the semiconductor’s bandgap. Electrons and holes surface recombination (route A), electrons and holes bulk recombination (route B), surface oxidation reaction initiated by photoexcited holes (route C), and surface reduction initiated by photoexcited electrons (route D) are the four distinct pathways that the photoexcited electrons and holes may follow [7]. Although single-component photocatalysis has made strides in pollutant degradation, it still faces numerous challenges, including low photoconversion efficiency, poor light absorption capacity, low redox capability, and facile electron-hole pair recombination, which has restricted its practical applications. Metallic doping has been praised as an efficient and successful method of overcoming some of the restrictions when compared to other approaches. Metal oxide photocatalytic activity can be increased by doping pristine materials with metals and non-metals, surface engineering, morphology regulation, and facet engineering. Recombination of charge carriers can also be decreased, which increases the quantum efficiency when semiconductor heterojunctions with properly aligned conduction and valence bands (VBs) are formed [8].

The creation of heterojunctions has gradually developed into a successful method for resolving the conflict between the photocatalysts’ redox capacity and narrow bandgap in recent years. Two semiconductor photocatalyst heterojunction complexes have garnered considerable attention from researchers. Physical contact established between two semiconductors when they are in close contact with each other is called a heterojunction. The advantageous spatial potential variation on each side of the junction facilitates the division of charge carriers triggered by light, hence increasing the photocatalytic activity [9]. A lot of interest has recently been shown in the Z-scheme photocatalyst because it has a higher redox capacity than the typical type-II heterojunction photocatalyst [10]. Furthermore, Z-scheme heterojunctions can enhance photocatalytic activity by reducing charge recombination because of the effective electron-hole pair dynamics across the interface. In a Z-scheme photocatalyst, with the aid of an electron-transfer mediator, the energized electrons of photosystem II (PSII) are transferred to photosystem I (PSI) (Figure 1) and recombined with the holes on it, simulating the natural process of photosynthesis [11].

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Figure 1.

Schematic representation of electron transfer in oxygenic photosynthesis using the Z-scheme. Reprinted from [13]. This is an open-access article licensed under Creative Commons Attribution (CC BY-NC 3.0) https://creativecommons.org/licenses/by-nc/3.0/.

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The envisioned increased photodegradation activity of Z-scheme heterojunction required the selection of semiconductors with suitable band edge potentials for the cooperative production of holes, hydroxyl radicals, and superoxide anions [12]. Due to their special characteristics, compounds containing bismuth have recently been examined as potential replacements for commercial photocatalysts. They can convert light into heat, have excellent antibacterial properties, are non-toxic, and have highly effective photocatalytic performance. They can also strongly absorb at near-infrared wavelengths [14]. The ability to absorb light, their tunable band structures, their distinctive morphologies, and the possibility of in situ reduction of Bi³⁺ to Bi⁰, which enhances light absorption, make bismuth-based semiconductors intriguing. Its surface plasmon resonance (SPR) property has made the creation of metallic bismuth (Bi⁰) during the synthesis of Bi-based photocatalysts, which was previously thought to be a limitation, extremely advantageous [15]. Magnetic SPR-active Bi-based catalysts, which combine the photocatalytic benefits of SPR with the usefulness of magnetic recoverability, have also shown promise in recent developments [16]. The rhombohedral crystal structure of metallic bismuth has a melting point of 271°C and a density of 9.7 g cm⁻³ [17]. Bismuth-based materials, in particular, have an unusual electronic structure, and their VB is created by the hybridization of the Bi-6s and O-2p orbitals, giving it a sharp absorption edge in the visible light spectrum [18]. The formation and migration of holes are more favored by the reverse bond between the anions and cations, which facilitates the photocatalytic reaction. One of the major problems associated with heterogeneous photocatalysis is the reusability of the photocatalysts due to their inefficient recovery from the reaction mixture. One of the most effective techniques to preserve the samples’ catalytic activity and make it easier for them to be conveniently extracted from the reaction medium is to incorporate magnetic materials to generate magnetic photocatalytic materials. In photocatalytic systems, magnetic materials can actively enhance charge carrier dynamics through mechanisms such as the Lorentz force, which alters electron trajectories and facilitates effective charge separation [19]. Furthermore, the magnetic field-induced electron spin polarization assists in preventing photogenerated electron–hole pair recombination, enhancing overall photocatalytic efficiency [20].

A variety of semiconductor photocatalysts have been efficiently combined with magnetic nanoparticles, such as γ-Fe₂O₃, Fe₃O₄, MFe₂O₄ (M = Mg, Ni, Zn, Cu, Co, etc.), creating composite materials that exhibit induced magnetic behavior along with high photocatalytic activity [21]. A lot of research studies have been published summarizing the achievements in the field of Z-scheme photocatalysis [15,22,23], but to the best of the authors’ knowledge, there is not a single review featuring the bismuth-based magnetic Z-scheme heterojunction for the photodegradation of pharmaceuticals. Considering the benefits of the synergistic effects of bismuth-based materials, Z-scheme, and magnetic components incorporated in heterojunctions, the advancements in this field should be summarized to further accelerate the work and achieve high-efficiency materials for different photocatalytic purposes. This review highlights the progress made in bismuth-based magnetic Z-scheme heterojunctions for the mitigation of pharmaceuticals. Here, we provide a brief overview of the history, sources of emissions, and effects of pharmaceuticals on water pollution and the environment. This review’s main section discusses the latest advancements in customized magnetic composites as environmentally friendly photocatalysts to prevent pharmaceutical deterioration. Different kinds of bismuth-based magnetic composites have been explored in terms of their magnetic and photocatalytic characteristics. This summary of existing data with limitations and a future research outlook may be helpful to the scientific readers on this topic.

2. Environmental impacts of pharmaceuticals

Modern healthcare sectors have been utilizing pharmaceutical compounds for numerous beneficial purposes; however, many pharmaceutical industries are also releasing extremely harmful pollutants into the environment, either directly or through chemical modifications. Figure 2 illustrates the various pathways through which pharmaceuticals and their metabolites can enter the environment.

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Figure 2.

Potential sources of pharmaceutical contaminants in the environment. Reprinted from [24] with permission from Elsevier. License Number: 5798900481732.

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Antibiotics, chemotherapy products, hormones, analgesics, antipyretics, and antidepressants are the primary ingredients of pharmaceutical waste. The most significant concentrations (365.5 μg L⁻¹) of antibiotics have generally been discovered in the surface water of various emerging Asian countries. CIP a common fluoroquinolone (FQ), has been found in a variety of water sources at different concentrations, including drinking water (3.2 × 10⁻⁵ mg L⁻¹), hospital effluents (1.1 × 10⁻² to 9.9 × 10⁻² mg L⁻¹), and sewage treatment plant influents (1.4 × 10⁻⁴ mg L⁻¹ [25]. Drugs that are frequently found in water bodies in Africa include antibiotics and NSAIDs. The range of 53.8–56.6 μg L⁻¹ for sulfamethoxazole and 0.087–272.2 μg L⁻¹ for amoxicillin was found to have the highest concentrations among different African countries. Ibuprofen has been observed to be present in the highest concentrations among the different NSAIDs, with levels as high as 67.9 μg L⁻¹ [26]. The likely causes of pharmaceutical pollution in Africa and Asia are careless usage and disposal, inadequate sanitation, and a lack of adequate wastewater treatment resources [27].

Lower concentrations of complex pharmaceutical mixtures, when exposed to stream biota over an extended period, may cause acute and chronic harm, behavioral changes, tissue accumulation, reproductive damage, and inhibition of cell proliferation in living organisms. Pharmaceutical effluents in drinking water can be dangerous for elderly people, infants, and those with liver or kidney failure. The presence of estrogens in drinking water has also been known to reduce male fertility. It may also raise the risk of testicular and breast cancer as well [22]. Long-term exposure to low concentrations of pharmaceuticals causes high morbidity rates and behavioral changes in aquatic organisms.

3. Recent trends in heterojunction development

Following Honda and Fujishima’s groundbreaking work on photocatalysis in 1972, semiconductor-based photocatalysts have been widely applied in various functions to address global issues due to their exceptional electrical and physical properties. As an alternative to produce energy repeatedly, photocatalytic water splitting can degrade a variety of organic contaminants to produce value-added carbon compounds by reducing CO₂ and cleansing our environment in ambient conditions [28].

Many semiconductors, including TiO₂ [29], CdS [30], ZnO [31], BiVO₄ [32], and C₃N₄ [33] have been thoroughly investigated and suggested as potential photocatalysts. However, these semiconductors have certain shortcomings that make it difficult to use them in practical applications, which inspires the creation of heterojunction-based photocatalysts. A heterojunction is often formed by two semiconductor photocatalysts that are coupled together due to their complementary features, relative energy band locations, and variances in Fermi levels (EFs), which allow for the creation of carrier routes between them [34]. This technique enhances the redox ability, increases the absorbable spectral range, and encourages the separation of electron-hole pairs, thus compensating for the deficiencies of the individual semiconductors. Different types of heterojunctions, including traditional heterojunction, Schottky heterojunction, p-n heterojunction, S-schemes, and Z-schemes, have been widely discussed in the previous studies [35].

Traditional heterojunctions are classified based on relationships between energy and band position, such as type-I, type-II, and type-III. In a traditional type I heterojunction photocatalyst, both photogenerated electrons and holes could move from one semiconductor, which had a lower VB and a higher conduction band (CB), to another semiconductor. The photocatalytic performance of type I heterojunctions cannot be effectively enhanced because of the fast recombination of accumulated charge carriers within the semiconductor. The extreme band levels of type III heterojunction prevent the band gaps of the two semiconductors from overlapping at the interface [36]. Consequently, type-III heterojunctions have very scarcely been reported, and their energy band structure is staggered, which does not offer favorable pathways for the separation of charge carriers [37]. Meanwhile, in type-II scheme heterojunctions, the electrons move towards a semiconductor which has a lower CB and holes move to a semiconductor that has a higher VB level, lowering electron-hole pair contact and recombination, as shown in Figure 3. This transfer of charge carriers reduces the recombination efficiency of charge carriers but overall decreases the photocatalytic performance of the catalysts.

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Figure 3.

Three types of heterojunction photocatalysts. Reprinted from [39]. This is an open-access article licensed under Creative Commons Attribution (CC-BY). (http://creativecommons.org/licenses/by/4.0/).

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To overcome the problem associated with type-II scheme heterojunctions, the concept of advanced Z-scheme heterojunctions was introduced. The Z-scheme heterojunction differs in its electron transport pathway from the type-II heterojunction despite sharing the same energy band arrangement [38]. The Z-scheme heterojunction overcomes the limitations of type-II heterojunction while maintaining a maximal redox capacity because of its unique migration pattern [40]. Magnetic composites are ideally suited to construct Z-scheme heterojunctions rather than traditional type II because of their ability to capture charge dynamics induced by magnetic fields. High redox potential is maintained by the complementary band alignments in Z-scheme complexes, which promote effective electron transport and are necessary for catalytic activity. Unlike type-II heterojunctions, which often lower redox activity by isolating electrons and holes at opposite ends, Z-schemes maintain strong oxidative and reductive characteristics. These advantages make magnetic Z-schemes ideal for processes like photocatalysis and pollutant degradation [41]. A photocatalyst using the Z-scheme is typically made up of both n-type and p-type semiconductors. Due to their superior photocatalytic activity over type-I, type-II, and type-III heterojunctions, Z-scheme heterojunctions are widely being developed for different photocatalytic purposes, including photocatalytic mitigation of pharmaceuticals from wastewater [42].

The Z-scheme heterojunction was initially utilized in liquid-phase based photocatalytic reactions, as these reactions rely on the redox electron mediator’s charge transfer. All liquid-phase Z-scheme photocatalysts have greater redox capacity compared to conventional heterojunctions. But the most common drawback is a significant amount of reverse reaction [43]. In 2006, Tada et al. originally presented the concept of an all-solid-state Z-scheme heterostructure [44]. The primary aim of all-solid-state Z-scheme photocatalysts was to increase the range of applications and do away with the need for redox pairs by substituting them with nanoconductors. Electronic media come in a variety of forms, including oxides, carbon quantum dots, and noble metals. Similarly, some major shortcomings, such as noble metals’ high price, limit their widespread application. Economically, this type of Z scheme is not useful for long-term usage [45].

In 2013, Yu et al. first proposed the idea of a direct Z-scheme heterojunction, based on an all-solid-state Z-scheme heterojunction [23]. It lowers the Z-scheme photocatalytic system’s construction costs. It can counteract the effect of light-shielding, which is caused by the loading of metal-based mediators. There are conflicting and ambiguous theories regarding the transfer of electrons and holes. Its working is still not completely studied [46]. Liquid media are necessary for conventional photocatalysts, while electronic media with light-shielding properties are necessary for all-solid-state photocatalysts. Consequently, direct Z-scheme photocatalysts can maintain the benefits of the previously listed Z-scheme photocatalysts while also optimizing redox capacity, efficiently utilizing sunlight, and enhancing the separation capability of electron-hole pairs to achieve the desired spatial separation of electrons and holes. The removal of electronic media also prevents light-shielding effects, simplifies preparation, and significantly lowers costs. A recently published Z-scheme heterostructure made of few-layer MoS₂/Bi₂MoO₆ demonstrated exceptional visible-light photocatalytic activity toward the inactivation of bacteria in water and the photooxidation of Rhodamine B (RhB) [47]. It has also been reported that direct Z-scheme Bi₂MoO₆/ZnIn₂S₄ composite semiconductors can be formed, and this can lead to an increase in their photocatalytic activity when exposed to visible light [48]. The incorporation of a magnetic component to the Z-scheme increases the redox potential for pollutant degradation because it effectively separates a hole-electron pair [49]. Therefore, magnetic Z-scheme heterojunctions are preferred for effective degradation of organic and inorganic pollutants.

4. Bismuth-based magnetic Z-scheme photocatalysts

Bismuth-based photocatalysts are known to have suitable band edge potentials for photocatalytic action, and their band structures can be easily tuned by modification strategies. The Z-scheme heterojunctions of bismuth-based photocatalysts with other materials not only impart a suitable mechanism for charge transfer but also help to attain stable physicochemical characteristics. For instance, Bi₂MoO₆, BiOBr, Bi₂WO₆, BiOI, and other bismuth-based materials are typical layer-structured semiconductors (Figure 4) [50-52]. These layered structures, when integrated into a Z-scheme, offer numerous exposed facets that favor redox reactions at distinct locations when coupled with other semiconductors. Moreover, the layered configuration also promotes the mobility of photoinduced charge carriers, which in turn helps to reduce recombination at the Z-scheme interface. Therefore, due to their enlarged surface area, improved stability, and higher electron mobility, bismuth-based Z-schemes have drawn much interest as photocatalysts for water purification. The oxygen to metal charge transfers originating from the orbital configurations of Bi and O also aid the visible light responsiveness of these materials [53]. Therefore, a material containing bismuth (Bi) is a good choice for building the Z-scheme-containing photocatalytic system by merging it with another type of semiconductor [54]. The recovery efficiency of Bi-based Z-scheme heterojunctions can be achieved by incorporating magnetic elements into the photocatalyst structure. This not only facilitates the easy removal of the catalyst from the reaction mixture using an external magnetic field but also enhances the charge separation efficiency of the catalyst [55]. Magnetic Z-scheme photocatalysts surpass conventional Z-schemes in pollutant degradation due to their unique charge dynamics brought about by magnetic fields, which enhance electron-hole separation and reduce recombination. The magnetic properties in the Z scheme promote efficient charge transfer and the generation of reactive species, including superoxide and hydroxyl radicals, for the degradation of pollutants. Strong redox potentials and easy recovery and reusability by magnetic separation make magnetic Z-schemes more effective and sustainable for environmental applications than conventional Z-scheme photocatalysts [56,57]. For example, the appropriate band gap (2.4 eV), nontoxicity, and chemical inertness of BiVO₄ have led to many recent research studies employing it as a catalyst. Nevertheless, single BiVO₄ had certain shortcomings that made it difficult to meet practical application requirements. These included a maximum photo-induced carrier recombination efficiency, an insufficient quantum yield, and a poor visible-light response. Besides being a promising option for treating water, large-scale application of this catalyst is also hampered by the difficulty in recovering and reusing nanoscale catalyst material from aquatic environments [58]. Typically, the photocatalytic performance of BiVO₄ can be improved by the formation of a Z-scheme heterojunction, and a magnetic component can be incorporated to address the catalyst recovery issue [59].

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Figure 4.

Schematic diagram of the layered structure of (a) BiWO₆ reprinted from [60] with permission from Elsevier. License No. 6024350937186 and (b) BiOX reprinted from [61]. This is an open-access article licensed under Creative Commons Attribution (CC BY-NC 3.0) https://creativecommons.org/licenses/by-nc/3.0/

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5. Antibiotic degradation using Bismuth-based magnetic Z-scheme photocatalysts

The advent of antibiotics has increased the likelihood of human survival by reducing bacterial infections. TC, a pharmaceutical antibiotic, is one of the commonly used antibiotics that is highly soluble in water [62]. Koutavarapu [63] and companions observed the 98% TC removal using the NiFe₂O₄/BiPO₄ nanocomposite in 100 min. Nickel ferrite (NiFe₂O₄) is well known for being an appropriate and suitable visible material for the degradation of organic pollutants because of its proper bandgap (1.73 eV), non-toxicity, and moderate magnetic separation. Nevertheless, due to the quick reunion of photo-induced charge carriers, pure NiFe₂O₄ exhibits relatively poor catalytic activity. Because of the unique characteristics brought about by its band position, bismuth phosphate (BiPO₄) stands out among other wide bandgap semiconductor photocatalysts as one of the best photocatalysts, which degrades organic dyes when exposed to solar light. Due to the quick recombination of photo-induced charge carriers of bismuth phosphate, its catalytic activity is limited to the ultraviolet spectrum. This issue has been solved with the formation of a heterojunction between two semiconductors. In BiPO₄ and NiFe₂O₄, electron-hole pairs are produced by exposure to solar light. Consequently, electrons move from the CB of NiFe₂O₄ towards the CB of BiPO₄. Similarly, holes move from the VB of BiPO₄ towards the VB of NiFe₂O₄ (Figure 5a). The oxidation mechanism of NiFe₂O₄ produces ^•O₂⁻ radical species at its CB, while photogenerated holes at BiPO₄’s VB react with water molecules to produce ^•OH radicals. These generated active species, such as ^•O₂⁻ and ^•OH, are advantageous for the elimination of antibiotic TC [63]. Thus, the formation of Z-scheme heterojunction between BiPO₄ and NiFe₂O₄ provided a suitable pathway for electron transfer, which actively generated reactive species necessary for the photocatalytic action. Recently, Dang et al. proposed a similar strategy for the degradation of TC using Fe₃O₄/BiOCl/BiOI heterojunction with 89% removal efficiency [64]. The increased photocatalytic performance was primarily associated with the Z-scheme heterojunction, which prevented electrons and holes from recombining and preserved the valence and CBs with greater redox potential. ^•O₂⁻ and h⁺ were the predominant active species in the composite photocatalysis system, and their synergistic effect allowed for the effective degradation of TC. Moreover, the highest TOC removal rate of Fe₃O₄/BiOCl/BiOI (78%) compared to that of BiOCl (30%), BiOI (59%), and BiOCl/BiOI (CI3) (71%) displayed the good mineralization of TC over the Fe₃O₄/BiOCl/BiOI nanocomposite (Figure 5b).

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Figure 5.

(a) Schematic representation of the degradation of TC. Reprinted from [63] with permission from Elsevier. License Number: 5798141166711, (b) The TOC removal rates over the BiOCl, BiOI, CI3, and Fe₃O₄/BiOCl/BiOI nanocomposite in the photodegradation of TC. Reprinted from [64] with permission from Elsevier. License Number: 6023770130806.

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Among the most widely prescribed FQ antibiotics, OFL, has been found in a variety of aquatic environments, including surface water, sewage plant effluents, and medical wastewater [65]. Due to its chemical stability and biological toxicity, it can lead to drug resistance and other health risks [66]. Thus, there is a lot of interest in creating new OFL degradation techniques. Zhao et al. synthesized a dual Z-scheme BiVO₄/g-C₃N₄/NiFe₂O₄ that degrades OFL with 93.8% efficiency in 60 min. Single BiVO₄ was limited in its ability to meet practical applications due to a few disadvantages, including maximum photo-induced carrier recombination efficiency, an inadequate quantum yield, and a poor visible-light response. While many advantages are present in the typical semiconductor g-C₃N₄, including a large effective surface area, an appropriate electronic band structure, and excellent stability towards chemicals. The process of recovery and reutilization of nano-scale catalyst material from aquatic environments is a confrontation that prevents the extensive use of BiVO₄ and g-C₃N₄-based heterojunction catalysts, despite their potential as water treatment agents [59]. To address this issue, magnetic NiFe₂O₄ coupled with a catalyst was typically used. The direct dual Z-scheme layout was formed by the combination of excited electrons in the CB of BiVO₄ and photo-induced holes in the VB of g-C₃N₄ and NiFe₂O₄. It could have happened because electrons migrate more quickly than holes do. Because of this, under the supposition of a direct Z-scheme structure, the electrons residing in the CB of g-C₃N₄ and NiFe₂O₄ can be significantly separated from the holes present in the VB of BiVO₄ instead of a single semiconductor. BiVO₄’s holes possessed enough energy to produce ^•OH, whereas the electrons found in g-C₃N₄ and NiFe₂O₄ could reduce O₂ to ^•O₂⁻. Thus, OFL is degraded by the generation of these active species. The mineralization efficiency was recorded around 40.5% by BiVO₄/g-C₃N₄/NiFe₂O₄ which was higher than those of pure BiVO₄. Further, the transformation products of OFL were identified by high performance liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry (HPLC-ESI-Q-TOF-MS), as shown in Figure 6.

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Figure 6.

Proposed transformation pathways for the degradation of OFL employing BiVO₄/g-C₃N₄/NiFe₂O₄ photocatalytic system. Reprinted from [59] with permission from Elsevier. License Number: 6023780813911.

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Another Z-scheme heterojunction, Bi₂O₃@NiFe₂O₄, was synthesized by Jing et al. using the two-step hydrothermal technique for photocatalytic activation of peroxymonosulfate (PMS) to break down OFL in wastewater simulation. Through electron paramagnetic resonance studies and quenching experiments,·O₂^- was found to be the primary reactive species in the breakdown of OFX. The composite exhibited outstanding cyclic stability and magnetic recovery properties [67].

Animal and human bacterial infections are frequently treated with FQ antibiotics due to their strong and broad-spectrum antibacterial properties [68]. The most common antibiotic in the FQ category is ciprofloxacin, which is used to treat typhoid, infectious diarrhea, gonorrhea, and skin infections. Wang et al. studied a novel photocatalyst based on biochar (BC) that was assembled using flower-shaped microspheres made of Bi₂WO₆ and Fe₃O₄ nanoparticles which showed excellent photocatalytic activity. Following a 60-min exposure to visible light emitting diode (LED) light, 91.5% of the CIP was removed. Bismuth tungstate Bi₂WO₆, a significant member of the Aurivillius family, consists of two layers: [WO₄]²⁻ and [Bi₂O₂]²⁺. Bismuth tungstate photocatalyst activity was increased using BC, a common and affordable carbon source. The condensed aromatic ring structure and surface functional groups of BC can speed up electron transfer during the photocatalytic activity. Subsequently, ferroferric oxide (Fe₃O₄) was incorporated into Bi₂WO₆ to create a heterostructure, which improved its performance even more and facilitated material recycling for practical uses. Compared to single Bi₂WO₆ and Bi₂WO₆ /Fe₃O₄, the surface area of Bi₂WO₆/Fe₃O₄/BC was increased to 35.89 m²/g after a very small amount of reed straw BC, which had a wide surface area of 306.63 m²/g, was added. Bi₂WO₆ and Fe₃O₄ combined to produce a heterojunction. Visible light was responsible for the generation of the e^– and h⁺ of Bi₂WO₆ by the process of photoinduction. The holes in the VB of Bi₂WO₆ recombine with the photogenerated electrons in the CB of Fe₃O₄. This recombination mechanism effectively reduces O₂ to •O₂⁻ by preserving the electrons in the CB of Bi₂WO₆ (−0.85 eV), which are more negative than the reduction potential of O₂/•O₂⁻ (−0.33 eV). In the meantime, the holes that are still present in the VB of Fe₃O₄ have enough oxidative potential to either take part in direct oxidation or start the production of hydroxyl radicals (•OH) through surface-bound processes or H₂O₂ intermediates. BC’s oxygen-containing functional groups can also encourage electron transfer to O₂, which would increase the formation of •O₂⁻. Simply expressed, h⁺, ^•O₂⁻, and ^•OH broke down CIP [69]. Li et al. recently created a new magnetic g-C₃N₄/α-Fe₂O₃/Bi₃TaO₇ (CN/FO/BTO) heterojunction with a dual Z-scheme system using an ultrasound-assisted calcination procedure. Within 120 min of visible light exposure, the results demonstrated that the optimized CN/FO/BTO heterojunction could remove 95.6% of the CIP, which was 6.1, 15.9, 5.2, 3.7, and 2.3 times more than that of single CN, FO, BTO, binary CN/FO, and CN/BTO, respectively [70].

NOR is an antibiotic that shows severe side effects on humans, including headache, nausea, and dyspepsia. Kumar et al observed that in visible, ultraviolet, near-infrared, and natural solar light, Ag@BiPO₄/BiOBr/BiFeO₃ exhibited superior photocatalytic activity for the deterioration of norfloxacin (NOR). NOR is degraded by catalyst APBF-3 (0.3 wt.% Ag@BiPO₄/BiOBr/BiFeO₃) in 90 min under visible light and by 99.1% in less than 45 min under UV exposure. Although there is expanding curiosity in the treatment of wastewater, the use of bismuth phosphate (BiPO₄) is constrained to the visible region because of its large band gap. BiOBr is another well-liked photocatalyst that has uses in disinfection and detoxification due to its high stability, low toxicity, and visible active properties. Bismuth ferrite (BiFeO₃) is used in photocatalytic and sensing applications. A drawback shared by BiFeO₃ and BiOBr is a greater rate of photogenerated electron-hole surface recombination. Absorption in the near-infrared range is increased by adding Ag to BiPO₄, BiOBr, and BiFeO₃. Due to the Schottky barriers at interfaces, photogenerated electrons from the VB of BiPO₄ and BiFeO₃ migrate to Ag, which is in metallic form. Ag recombines these electrons and holes, maintaining the electron accumulation at the high potential CB of BiOBr. Likewise, Ag may once more receive the holes from the CB of BiPO₄ and BiFeO₃. As a result, BiPO₄ and BiFeO₃ have more electrons in their VB, which can combine with H₂O and OH^– to form ^•OH radicals. Conversely, electrons accumulate on the CB of BiOBr and BiPO₄, which hold a greater capacity to produce ^•O₂⁻ (Figure 7a) [71]. This mechanism demonstrates how exceptionally high potential electron and hole energies in CB and VB produce remarkable photocatalytic activity. The magnetisation-hysteresis curve for Ag@BiPO₄/BiOBr/BiFeO₃ showed a ferromagnetic character with magnetization of 6.8 emu/g, which is enough for magnetic separation and to avoid secondary pollution (Figures 7b and c). Another Z-scheme heterojunction of a magnetic oxygen vacancy-BiOBr / CoFe₂O₄ (OV-BiOBr / CoFe₂O₄) nanocomposite was fabricated by the Zhu group, which can degrade NOR at a rate of up to 94.61%. More active sites and improved absorbability could be provided by an OV-BiOBr / CoFe₂O₄ composite with a larger specific surface area than BiOBr, which was advantageous for increasing the catalyst’s photocatalytic activity [72].

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Figure 7.

(a) The schematic diagram for the charge separation and movement under visible light. Reprinted from [71] with permission from Elsevier. License Number: 5798820705935, (b) M-H curve for APBF-3, and (c) Separation efficiency of APBF-3 under magnetic field for photocatalytic degradation of NOR over APBF-3 under visible light irradiation. Reprinted from [71] with permission from Elsevier. License Number: 6023921360807.

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6. NSAIDs degradation using Bismuth-based magnetic Z-scheme photocatalysts

A common non-steroidal anti-inflammatory medication used to treat painful rheumatoid and non-rheumatoid diseases is called DCF. DCF is a persistent pollutant that negatively impacts tissue and embryonic development as well as the reproductive system because of its non-volatility and poor biodegradability [73]. Kumar et al. have studied a novel catalyst Bi₄O₅I₂/Fe₃O₄@SrTiO₃, which showed 98.4% removal of DCF in 90 min. SrTiO₃, a perovskite compound, has been employed in photocatalysis because of its high CB edge energy level. However, there are some drawbacks as well, like its high band gap energies, which prevent it from being used frequently in the visible region. Bi₄O₅I₂ is a better candidate for a photocatalyst because of its significantly elevated VB edge, which, when combined with strategically positioned CB, can enhance oxidative capabilities. The photogenerated charge has a fast transit channel provided by Fe₃O₄, which has led to an increased charge carrier lifetime. Thus, it makes sense to form a solar-active Z-scheme heterojunction of SrTiO₃ and Bi₄O₅I₂. Based on the Z-scheme charge transfer, the photogenerated electrons move from the CB of SrTiO₃ to the VB of Bi₄O₅I₂ and merge with holes. Electrons accumulate as a result of the high potential CB of Bi₄O₅I₂ and the VB of SrTiO₃ (Figure 8). Thus, the generation of ^•OH and ^•O₂⁻ was highly feasible thermodynamically and these generated active species were responsible for degradation of DCF [74]. Further, magnetic properties of the Bi₄O₅I₂/Fe₃O₄@SrTiO₃ heterojunction showed that the catalyst exhibits ferromagnetic behavior with very low coercivity. The material possessed 48.4 emu g⁻¹ saturation magnetization which is very high for magnetic separation from the aqueous reaction medium.

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Figure 8.

Photocatalytic degradation mechanism of DCF using (a) conventional charge transfer, (b) dual Z-scheme pathway. Reprinted from [74] with permission from Elsevier. License Number: 5946031128184, and (c) Magnetism-Hysteresis (M-H) plot. Reprinted from [74] with permission from Elsevier. License Number: 6023930656045.

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The typical central pain reliever, carbamazepine (CBZ), has been found to have negative environmental impacts in addition to being helpful in treating trigeminal neuralgia, epilepsy, and other mental diseases. CBZ is persistent and bioaccumulative because of its hard biodegradation property, which is caused by the symmetrical aromatic heterocycle’s special chemical structure [75]. According to Xi et al, the high photocatalytic response for the removal of carbamazepine is exhibited by Fe₃O₄/BiOBr/CQDs due to their improved visible light absorption as well as better efficiency of photogenerated charge transfer, which allows for the elimination of 99.52% of CBZ in 120 min of light intensity. Due to their exceptional electron-transfer capability and favorable biocompatibility, carbon quantum dots (CQDs), unique “zero-dimensional” carbon-based nanomaterials, have been regarded as superior catalysts. By building a magnetic bismuth-based heterojunction photocatalyst, it is possible to address the issue of photogenerated charge recombination while simultaneously enhancing the material’s capacity for regeneration and separation through the use of an external magnetic field. Because of its inherent advantages of strong magnetism, good stability, and low cost, Fe₃O₄ is a characteristic iron oxide that is frequently employed to create bismuth-based and magnetic heterojunction photocatalysts. The electrons e^- on the VB of Fe₃O₄ and BiOBr were liberated and transferred to the CB upon exposure to visible LED light, resulting in the formation of photogenerated holes h⁺ on the VB. Then, as an electron donor and receptor, biomass CQDs helped the electron transfer from the CB of BiOBr to the VB of Fe₃O₄ (Figure 9). Thus, strong oxidation reducibility could be maintained by the e^- on the CB of Fe₃O₄ and the h⁺ on the VB of BiOBr, enhancing the photocatalyst’s catalytic activity [76]. Lan et al. developed La-doped bismuth ferrite (BiFeO₃) containing multiferroics (La-BFOs), another effective photocatalyst, which exhibited 94.5% removal efficiency in 30 min of solar light irradiation for the effective degradation of carbamazepine. The piezo-potential from the piezoelectric effect functioned within an electric field to alter the structure of band energy and speed up the dissociation of photo-induced carriers when La-BFOs nanocomposites were subjected to simulated solar light. This produced the most reactive species, which improved the elimination of CBZ from wastewater [77]. Also, the incorporation of La improved the magnetization value, reaching 6.6 emu/g for La-BFO, resulting in a magnetic material suitable for the recycling process of the catalyst. Table 1 highlights some recently developed bismuth-containing magnetic Z-scheme heterojunctions for pharmaceutical degradation.

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Figure 9.

Photocatalytic mechanism of Fe₃O₄/BiOBr/CQDs on CBZ photodegradation. Reprinted from [76] with permission from Elsevier. License Number: 5798170221433.

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7. Current challenges and future recommendations

Bismuth-based magnetic composites have demonstrated remarkable efficiency in treating wastewater containing pollutants such as pharmaceuticals. Nevertheless, there are still certain obstacles related to their development that require further investigation in the future.

• 1.

  According to the databases that were scanned, the focus was solely on the withdrawal of common antibiotics, but it is impossible to overlook the risks connected to other medications, including anticancer drugs, beta-blockers, steroids, and hormones. It is advised to conduct additional research on the photocatalytic functions of magnetic composites against pharmaceuticals in order to close this gap.

• 2.

  Further, future studies must examine the toxicological evidence and secondary pollution linked to the use of these magnetic composites.

• 3.

  As actual wastewater is a mixture of different pollutants, the photocatalytic ability of bismuth-based magnetic composites must also be directed toward the elimination of multiple pollutants at once. According to this, it has been observed that most research on the use of magnetic nanocomposites for treating wastewater is currently being conducted with simulated wastewater, while on the other hand, actual wastewater is required to provide experimental support for the materials’ commercial or practical application.

• 4.

  The research mentioned above has mostly been done on a lab scale, which is quite doable; nevertheless, consideration must be given to how economical these materials are when used on a large scale.

• 5.

  To increase the efficiency of Z-scheme photocatalysts, future research should focus on constructing well-integrated interfaces to facilitate fast electron transport. It would also help to reduce interfacial resistance, promote directed electron flow, and significantly improve the spatial separation of charge carriers.

8. Conclusions

Photocatalysis has emerged as an impressive technique to remove these micro-persistent contaminants from water. Bismuth-based materials, due to their unique properties, are suitable candidates for making Z-scheme heterojunctions for improved efficiency of photocatalysts. Bi-based magnetic Z-scheme heterojunctions’ synergistic structural and electrical characteristics enable them to demonstrate exceptional photocatalytic performance. Moreover, recovering the photocatalyst from the retraction media is one of the main issues with heterogeneous photocatalysis. In addition to making catalyst recovery easier with an external magnetic field, the use of magnetic components like Fe₃O₄ improves electron transport and interfacial charge separation. Recently, many composites of bismuth-based materials with magnetic components have been developed. This review summarizes the advancements in the field of magnetically separable bismuth-based Z-scheme heterojunctions for the deterioration of pharmaceuticals. We have discussed the potential sources of pharmaceutical contaminants and their potential environmental threats. Then we have discussed the various kinds of heterojunctions, including type-I, type-II, type-III, and Z-scheme heterojunctions, and the latter type further includes liquid phase Z-scheme, all solid mediator Z-scheme, and direct Z-scheme heterojunctions. Lastly, we have also highlighted some current challenges associated with Z-scheme photocatalysts and future perspectives to further explore the dimensions of bismuth-based magnetic Z-scheme heterojunctions to enhance the photocatalytic performance of the materials.