Surface-Modified Magnetic Nanoparticles for Photocatalytic Degradation of Antibiotics in Wastewater: A Review

Abstract

Recent advancements in nanotechnology and materials science have enabled the development of magnetic photocatalysts with improved efficiency, stability, and reusability, offering a promising approach for wastewater treatment. The integration of magnetic nanoparticles (MNPs) into photocatalytic processes has gained significant attention as a sustainable method for addressing emerging pollutants—such as antibiotics and pharmaceutical compounds—which pose environmental and public health risks, including the proliferation of antibiotic resistance. Surface modification techniques, specifically applied to MNPs, are employed to enhance their photocatalytic performance by improving surface reactivity, reducing nanoparticle agglomeration, and increasing photocatalytic activity under both visible and ultraviolet (UV) light irradiation. These modifications also facilitate the selective adsorption and degradation of target contaminants. Importantly, the modified nanoparticles retain their magnetic properties, allowing for facile separation and reuse in multiple treatment cycles via external magnetic fields. This review provides a comprehensive overview of recent developments in surface-modified MNPs for wastewater treatment, with a focus on their physicochemical properties, surface modification strategies, and effectiveness in the removal of antibiotics from aqueous environments. Furthermore, the review discusses advantages over conventional treatment methods, current limitations, and future research directions, emphasizing the potential of this technology for sustainable and efficient water purification.

1. Introduction

Water treatment is an essential process to ensure the availability of clean and safe water for human consumption, industrial applications, and environmental sustainability. The increasing demand for clean water is driven by rapid industrialization, climate change, and urbanization, which collectively lead to heightened pressure on freshwater resources [1,2]. According to the United Nations (UN) World Water Development Report, industrial activities consume substantial volumes of water and significantly contribute to water pollution, with estimated water-related risks amounting to approximately $126 billion [3]. The UN emphasizes the strong interconnection between water resources and climate change, noting that extreme weather events exacerbate water scarcity and pollution [4].

Wastewater pollution poses a critical environmental and public health concern, as it can degrade ecosystems, compromise food safety, and pose direct and indirect risks to human health. Among the various pollutants of concern, a class of substances known as contaminants of emerging concern (CECs)—including pharmaceuticals and antibiotics—has become a significant environmental issue due to their widespread use and improper disposal [5,6]. These compounds, frequently detected in aquatic environments even at trace concentrations, are of particular concern because they exhibit cumulative toxicity and can promote the development of antimicrobial-resistant bacteria when persistently present [7]. Antibiotics, in particular, enter water bodies through multiple pathways, including effluent discharges from pharmaceutical manufacturing facilities, improper disposal of unused medications, and excretion from humans and livestock (Scheme 1) [8,9,10]. Given their persistence and biological activity, antibiotics and related pharmaceuticals constitute a critical area of investigation within environmental science and public health.

Increased antibiotic usage and production contribute directly to environmental contamination, as elevated consumption rates lead to higher concentrations of these compounds in aquatic systems. Global antibiotic consumption increased by 46% between 2000 and 2008, based on data from 76 countries [11]. According to NAVADHI Market Research Private Limited (an Indian private company), the global pharmaceutical market is projected to reach approximately $1.57 trillion, reflecting the scale of pharmaceutical manufacturing [12]. Antibiotic residues have been detected in diverse aquatic environments, including rivers, lakes, and wastewater treatment plants worldwide [13,14,15,16]. Wilkinson et al. reported the presence of 34 active pharmaceutical ingredients, including antibiotics, in rivers across all continents, with a detection frequency of 62% across sampled sites [17]. Bhagat et al. documented concentrations of six antibiotics in untreated wastewater ranging from 3.24 ng L⁻¹ to 31,000 µg L⁻¹, with the highest levels observed in India, followed by China and the United States [18].

Contaminated water adversely affects both aquatic and terrestrial organisms by disrupting habitats, altering reproductive cycles, and increasing mortality rates [19,20]. In agricultural settings, the use of contaminated water for irrigation introduces antibiotic residues into the food chain, thereby compromising food safety and consumer health [6]. The widespread production, distribution, and consumption of antibiotics contribute to the release of active pharmaceutical compounds into the environment, facilitating the development of antimicrobial resistance (AMR) [21,22]. Environmental antibiotic contamination creates conditions that favor the selection and proliferation of resistant microbial strains. The Centers for Disease Control and Prevention (CDC) classifies AMR as a “One Health” issue, emphasizing its interconnected impact on human, animal, and environmental systems [23]. AMR disseminates through water sources, soil, and broader ecosystems, driven not only by pharmaceutical manufacturing but also by improper disposal practices, agricultural runoff, and food production pathways. These factors collectively contribute to the persistence and spread of resistant microbial communities.

Antibiotics further impact aquatic environments because most compounds are not fully metabolized by humans or animals and are subsequently excreted into sewage systems, from which they can enter natural water bodies [9]. Once present, pharmaceuticals can exert toxic effects on aquatic organisms, disrupting physiological functions, reproductive processes, and behavior. Exposure to certain antibiotics also disturbs microbial communities within aquatic ecosystems, leading to alterations in nutrient cycling and degradation of water quality [10,24].

Continuous exposure of pathogens to subtherapeutic concentrations of antibiotics promotes the emergence of AMR, which poses substantial public health challenges [22]. In 2019, AMR was estimated to be directly responsible for 1.27 million deaths globally, according to The Lancet [25]. The World Health Organization (WHO) has identified AMR as one of the top ten global public health threats, noting its significance in both 2019 and 2021 assessments [26,27]. The presence of antibiotics in aquatic systems represents a multifaceted challenge that necessitates effective strategies for monitoring, regulation, and protection of the water quality to safeguard environmental and human health. The persistence of antibiotics and other CECs in aquatic systems, along with their contribution to ecological disruption and AMR development, underscores the urgent need for efficient and reliable removal technologies. Therefore, a variety of advanced water treatment approaches have been explored to mitigate these pollutants.

Conventional treatment technologies for removing pharmaceutical contaminants include membrane filtration, adsorption, precipitation, reverse osmosis, coagulation, and flocculation [28,29,30]. However, these approaches exhibit several limitations, including the requirement for tightly controlled operational conditions and incomplete mineralization of pharmaceutical compounds [31,32]. Furthermore, many wastewater treatment facilities are not adequately equipped to fully eliminate antibiotics, thereby exacerbating environmental contamination [33]. These technologies may also involve complex downstream processes, leading to extended treatment times and increased operational costs [34,35]. Collectively, these limitations highlight the need for more efficient, cost-effective, and sustainable strategies to address antibiotic contamination in aquatic environments.

The application of nanotechnology in wastewater treatment systems represents a promising approach to address the increasing complexity and diversity of contaminants in aquatic environments. Nanoparticles (NPs) offer considerable potential to enhance existing treatment technologies by improving the removal efficiency of various pollutants, thereby contributing to improved water quality [36,37]. Owing to their high specific surface-area-to-volume ratio, reactivity, and tunable selectivity, NPs can be engineered to target specific classes of contaminants, including heavy metals, microorganisms, and organic compounds [38]. Their incorporation not only enhances treatment performance but also supports the development of sustainable, energy-efficient, and cost-effective water purification platforms. Additionally, NPs can facilitate degradation processes by promoting the breakdown of hazardous substances into simpler and less harmful products [39].

Photocatalysis offers a versatile and environmentally compatible approach to water treatment, relying on the light-induced activation of photocatalysts to initiate redox reactions. The efficiency of photocatalytic processes is governed by several catalyst-specific factors, including band gap energy (E_g), surface area, and the recombination rate of photogenerated electron–hole pairs [40,41]. Despite its potential, photocatalysis faces several practical constraints such as limited light absorption efficiency, challenges associated with catalyst recovery and reuse, and scalability limitations for industrial implementation [33,42]. The integration of photocatalysts with magnetic nanoparticles (MNPs) has emerged as a promising strategy to address these limitations by enabling magnetic separation, enhancing degradation performance, and improving overall process sustainability.

Typically composed of iron oxide nanoparticles (IONPs), MNPs exhibit magnetic properties that allow for straightforward recovery after treatment, thereby minimizing secondary pollution [43]. In addition, surface-functionalized MNPs combine high specific surface area, magnetic separability, and tunable selectivity, making them particularly suitable for water purification applications [44,45]. These characteristics also promote catalyst reusability, contributing to more cost-effective wastewater treatment processes.

This review provides an overview of recent developments in photocatalysis using surface-modified MNPs for wastewater treatment. It highlights advances in catalyst design, elucidates mechanisms of pollutant degradation, and discusses strategies to enhance photocatalytic performance. In addition, it examines the physicochemical characteristics influencing the photocatalytic performance of MNPs, including their composition and surface functionalization. Understanding these parameters is essential for the rational design of NPs tailored to specific wastewater treatment applications. Challenges and limitations associated with this approach are also addressed.

2. Magnetic Nanoparticle-Assisted Photocatalysis

NPs have been widely recognized for their potential to enhance the removal of antibiotics in water treatment, particularly through advanced oxidation processes (AOPs). NPs exhibit strong effectiveness in treating water contaminated with persistent emerging contaminants and other difficult-to-degrade organic compounds that resist conventional treatment methods [46]. AOPs operate by generating hydroxyl (•OH) radicals, which oxidize a broad range of organic pollutants, including antibiotic residues, into less harmful products [47]. Common AOPs include ozonation, Fenton oxidation, sonolysis, photocatalysis, and ultraviolet (UV)/hydrogen peroxide (H₂O₂) photolysis [46]. Among these approaches, photocatalysis has demonstrated high efficacy in achieving complete mineralization of contaminants, particularly antibiotics [40]. Moreover, photocatalysis does not require external chemical additives, as it relies on light energy—either natural sunlight or low-energy UV irradiation—making it a sustainable and cost-effective water treatment strategy.

Photocatalysis typically employs semiconducting materials, such as metal oxides, that generate reactive oxygen species (ROS) upon light irradiation. When a semiconductor with a specific E_g absorbs photons with energy equal to or greater than its E_g, electrons are excited from the valence band (VB) to the conduction band (CB), generating electron–hole pairs. These charge carriers participate in surface redox reactions, leading to the formation of ROS—including •OH radicals and superoxide (•O₂⁻) anions—that degrade contaminants and ultimately mineralize them (Scheme 2) [48,49,50]. The E_g is a key parameter governing light absorption and photocatalytic activity [51]. Semiconductors with narrow band gaps (1.8–3.1 eV) are activated under visible light (400–700 nm), whereas those with wider band gaps (>3.1 eV) typically require UV irradiation (<400 nm) [42]. This tunable nature allows semiconducting photocatalysts to operate across a broad portion of the solar spectrum, supporting their application in diverse environmental remediation settings.

In addition to radical-mediated pathways involving •OH and •O₂⁻ species, recent studies suggest that non-radical ROS such as singlet oxygen (¹O₂) may also contribute to the photocatalytic degradation of antibiotics in certain systems [52,53]. ¹O₂ is an electronically excited form of molecular oxygen that has been reported in certain AOPs [54]. Compared to radical ROS, ¹O₂ exhibits greater resistance to scavenging by inorganic ions commonly present in real wastewater and higher selectivity toward electron-rich organic pollutants [52,55]. Although ¹O₂ generation has been suggested in selected photocatalytic systems, its contribution has been reported in only a limited number of antibiotic degradation studies involving surface-modified MNPs. Nonetheless, acknowledging this pathway provides a more complete mechanistic perspective on photocatalytic antibiotic degradation beyond conventional radical-driven processes.

Based on these principles, MNPs have been extensively explored for their potential to enhance photocatalytic processes. The incorporation of MNPs enables facile catalyst recovery via an external magnetic field, thereby improving reusability while reducing toxicity and operational costs [45,56]. Among MNPs, IONPs—including magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), and other spinel ferrites (XFe₂O₄, X = Zn, Cu, Mn, etc.)—are the most widely applied due to their biocompatibility, favorable physicochemical and magnetic properties, and ease of surface functionalization [57,58]. These materials can be incorporated into photocatalytic systems to enhance the degradation of antibiotic contaminants. Surface modification of MNPs can further improve photocatalytic performance by facilitating light absorption and promoting charge separation, making them particularly attractive for environmental remediation [43].

Despite their promising properties, bare MNPs exhibit certain limitations such as weak visible-light activity and rapid electron–hole recombination rate, both of which hinder photocatalytic efficiency [59]. To overcome these challenges, various surface modification strategies have been developed to tailor the optical and electronic properties of MNPs, thereby improving charge separation, light harvesting, and overall catalytic performance. Consequently, the development of surface-modified MNPs represents a crucial approach for enhancing the photocatalytic degradation of persistent pollutants, particularly antibiotics.

3. Surface-Modified Magnetic Nanoparticles: A Potential Candidate for Photocatalysis

MNPs have gained considerable attention in environmental remediation due to their unique physicochemical properties, low cost, natural abundance, and environmentally compatible synthesis routes. Their nanoscale dimensions and high surface-area-to-volume ratio provide abundant active sites for molecular interactions, boosting adsorption capacity and catalytic activity [60,61,62]. One of the major advantages of MNPs in water treatment is their ability to be rapidly and selectively separated from solution using an external magnet, eliminating the need for energy-intensive filtration or centrifugation and thereby reducing operational time and cost.

The magnetic behavior of IONPs is strongly influenced by their structure, crystallinity, morphology, particle size, and surface characteristics. Depending on these parameters, IONPs may exhibit either superparamagnetic or ferrimagnetic behavior [60,63]. At sufficiently small dimensions, IONPs display superparamagnetism, becoming magnetized only in the presence of an external magnetic field and losing magnetization upon its removal. This property prevents NP aggregation and is advantageous in applications requiring controlled magnetic responses [44,45,56]. In contrast, larger IONPs typically exhibit ferrimagnetism, retaining magnetization even without an applied magnetic field, a characteristic beneficial in processes requiring stable magnetic behavior [44,60]. Magnetic strength is commonly evaluated by saturation magnetization (M_S), defined as the maximum magnetization achieved under an applied magnetic field [64]. Ferrimagnetic NPs generally exhibit higher M_S values than superparamagnetic ones, reflecting their distinct functionalities in photocatalyst recovery [60].

Despite their promising attributes, bare MNPs present several limitations that restrict their performance. A primary challenge is their tendency to aggregate due to magnetic dipole–dipole interactions, which reduces their active surface area and limits the number of available active sites for contaminant interactions. In addition, IONPs are susceptible to oxidation or structural degradation when exposed to oxygen, moisture, or acidic conditions, leading to diminished magnetic strength and reduced catalytic efficiency [43,65]. These limitations underscore the need for strategies that improve MNP stability, reactivity, and durability in aqueous environments.

Surface modification represents an effective approach to overcome the limitations of bare MNPs. In surface-modified MNPs, the magnetic core—typically Fe₃O₄, γ-Fe₂O₃, or XFe₂O₄—is coated with functional layers such as metal oxides, carbon-based materials, silica shells, or polymer coatings. These surface layers significantly improve dispersion stability, selectivity, and catalytic activity while preserving the inherent physicochemical and magnetic properties of the core [44]. Such modifications also enhance key photocatalytic features, including light absorption range, charge separation efficiency, suppression of electron–hole recombination, and resistance to environmental degradation [66,67,68]. In addition, they mitigate aggregation, strengthen chemical durability, and enable repeated use without substantial loss of magnetic performance [43]. Based on these considerations, a general framework for the rational design of surface-modified MNPs for photocatalytic anti-biotic degradation is summarized in Scheme 3.

A typical application of surface-modified MNPs in wastewater treatment is illustrated in Scheme 4. The NPs are dispersed into contaminated water, where they adsorb and photocatalytically degrade pollutants through the generation of ROS under light irradiation. After treatment, the NPs are magnetically recovered, washed, and reused, enabling repeated operation with minimal material loss. This integrated adsorption–photodegradation–magnetic separation process offers an efficient, sustainable, and cost-effective approach for water purification.

Various photocatalytic materials have been incorporated as functional shells around MNP cores for water purification. Metal oxides such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten trioxide (WO₃) are commonly used due to their intrinsic photocatalytic activity. TiO₂ and ZnO, with E_g of approximately 3.2 and 3.3 eV, respectively, generate electron–hole pairs under UV irradiation. Surface modification strategies, such as metal or non-metal doping and the integration of heterogeneous co-catalysts, extend their optical absorption into the visible range and create charge-trapping sites that suppress electron–hole recombination [48,69,70,71]. WO₃, with a narrower E_g of 2.6–2.8 eV, can be activated by visible light, and its photocatalytic performance has been shown to depend on morphology control, doping, and coupling with co-catalysts [72,73]. Carbon-based materials, including graphene and its derivatives, also serve as effective surface modifiers due to their tunable electronic structures, high conductivity, and modifiable surface chemistry, which facilitate efficient charge transfer and separation [74].

Optimizing the optical and electronic properties of these systems—including band-gap engineering, heterojunction formation, and charge-carrier regulation—is essential for designing effective MNP-based photocatalysts. Approaches such as metal or non-metal doping, coupling with complementary semiconductors, and engineering surface defects improve light harvesting, suppress electron–hole recombination, and extend photocatalytic activity into the visible-light region (Scheme 5). Collectively, these strategies enhance the degradation efficiency of persistent contaminants under a broader light spectrum [75,76,77].

Given the limitations of bare MNPs, a variety of modification strategies have been developed to improve their stability, light absorption, and charge-carrier dynamics. These strategies include surface coating, elemental doping, plasmonic enhancement, surface functionalization, and the construction of heterojunction structures. Each approach offers distinct advantages in expanding visible-light absorption, promoting charge separation, and suppressing electron–hole recombination.

From an application-oriented standpoint, heterojunction structures and surface coating strategies offer substantial improvements in photocatalytic efficiency and have been widely investigated due to their ability to regulate charge-carrier dynamics through engineered band alignment, thereby reducing recombination losses and improving light-driven reaction efficiency [71,78]. Core–shell materials and semiconductor heterojunctions are therefore frequently reported to exhibit high activity under both UV and visible-light irradiation. Despite these advantages, their functional performance is strongly governed by interfacial quality, crystallographic compatibility, and shell thickness. In particular, structural imperfections at interfaces or excessive shell growth may impede electron transfer and, in magnetically recoverable systems, compromise separation efficiency [79,80,81]. Furthermore, the reliance on multistep synthesis procedures and precise control over morphology and crystallinity presents challenges for cost-effective and large-scale production [82].

Metal and non-metal doping provide an alternative route for tuning electronic properties and broadening optical absorption while maintaining a relatively simple synthesis procedure. Although controlled dopant levels can enhance photocatalytic performance, excessive incorporation tends to introduce deep trap states that facilitate charge recombination and accelerate material degradation under prolonged irradiation [42,83]. Noble-metal-based plasmonic modifications further enable visible-light activation via localized surface plasmon resonance (LSPR) effects; however, their practical deployment is constrained by economic considerations and stability issues, including metal aggregation and leaching [84,85]. Surface functionalization provides an effective means to enhance interfacial charge transport and reactant adsorption while maintaining relatively low processing energy requirements [86].

Overall, these modification strategies provide effective and versatile pathways for enhancing photocatalytic efficiency, stability, and light utilization. The following subsections discuss these modification techniques and their specific roles in enhancing the performance of MNP-based photocatalysts.

3.1. Surface Coating

Surface coating, particularly with metal oxides, modifies the optical properties of magnetic photocatalysts by introducing photocatalytic activity and reducing the recombination rate of photogenerated charges [71]. This is often achieved through the design of core–shell structures in which a magnetic core is encapsulated by a shell composed of a photocatalytically active material (Figure 1a) [87]. Such configurations enable spatial separation of charge carriers and enhanced interfacial interactions, thereby improving overall photocatalytic efficiency. For example, a magnetic molecularly imprinted polymer (MIP)-based Fe₃O₄/SiO₂/TiO₂ system exhibited two distinct optical absorption edges at approximately 1.7 and 3.0 eV, attributed to the Fe₃O₄ core and the TiO₂ shell, respectively. The visible-light activity (below ~3 eV) was linked to interfacial charge transfer between Fe₃O₄ and TiO₂, which facilitated improved charge separation and consequently enhanced photocatalytic performance (Figure 1b) [88]. In another study, a core–shell ZnFe₂O₄/SiO₂/TiO₂ composite synthesized via the sol–gel method exhibited a slight increase in E_g from 3.13 eV to 3.25 eV due to the outer TiO₂ coating [89]. These examples demonstrate that surface coatings may induce only minor changes in E_g while substantially improving photocatalytic efficiency by promoting effective charge separation between photogenerated electrons and holes. In addition, surface coatings can reduce defects and passivate active sites within the shell, while improving interfacial charge transfer between the core and the shell. These effects collectively suppress electron–hole recombination and sustain photocatalytic activity under prolonged operational conditions.

3.2. Elemental Doping

Doping, achieved by incorporating foreign metal or non-metal atoms into the photocatalyst lattice, introduces new energy levels and modifies the material’s band gap [91]. This modification enhances charge separation and extends light absorption into the visible region, addressing a common limitation of unmodified photocatalysts. For example, a TiO₂@ZnFe₂O₄/Pd nanocomposite demonstrated improved photocatalytic activity for diclofenac (DCF) degradation. The enhancement was attributed to Pd doping, which reduced electron–hole recombination and prolonged charge-carrier lifetimes [92]. Similarly, embedding Fe₃O₄/SiO₂ nanocomposites into a nitrogen-doped carbon xerogel–titania (N-CXTi) matrix improved photocatalytic performance. The N-CXTi matrix exhibited an E_g of 3.22 eV, lower than that of the commercial TiO₂ P25 photocatalyst (E_g = 3.5 eV) [93]. As shown in Figure 1c, photoluminescence (PL) spectra of Nb-doped and undoped Fe₃O₄@SiO₂@TiO₂ NPs indicate that Nb doping significantly reduces PL intensity, reflecting effective suppression of electron–hole recombination [90]. By introducing dopants, trap energy levels are generated that prolong charge-carrier lifetimes, suppress recombination, and thereby improve photocatalytic activity under visible-light irradiation.

3.3. Plasmonic Enhancement

Plasmonic enhancement, particularly through noble metal NPs (gold, silver, and platinum), based on localized surface plasmon resonance (LSPR), significantly increases light absorption, especially within the visible range [84]. The superior performance of plasmonic nanocomposites is largely attributed to their tailored structural architecture, which promotes efficient charge-carrier separation and transfer. In one study, CuFe₂O₄@WO₃/Ag NPs showed improved UV and partial visible-light absorption for the photodegradation of the drug pollutants Gemfibrozil and Tamoxifen when compared to CuFe₂O₄@WO₃ NPs. This enhancement was accompanied by a reduction in E_g from 2.21 eV (CuFe₂O₄@WO₃) to 2.13 eV (CuFe₂O₄@WO₃/Ag), as shown in Figure 2a [94]. Similarly, in the plasmonic photocatalyst Ag/AgBr/ZnFe₂O₄, incorporation of metallic Ag NPs reduced the E_g to 1.7 eV, improving the degradation of carbamazepine (CBZ) [95].

The plasmonic resonance effect promotes the generation of hot electrons, which are subsequently transferred into the semiconductor conduction band. This process suppresses electron–hole recombination and thereby enhances photocatalytic activity under visible-light irradiation. As illustrated in Figure 2b, in a Fe₃O₄@SiO₂/d-TiO₂/Pt photocatalyst, the inclusion of a Pt cocatalyst facilitates the extraction of photogenerated electrons from the TiO₂ shell, while the SiO₂ interlayer inhibits hole migration toward the Fe₃O₄ core. This spatial separation prevents the formation of Ti–Fe secondary phases and promotes hole trapping at Ti⁴⁺ vacancies, resulting in more efficient charge-carrier dynamics under oxidative conditions [96].

3.4. Surface Functionalization

Functionalizing MNPs with polymers, complex molecules containing targeted binding sites, or other functional groups improves light absorption, reduces aggregation, and enhances pollutant interactions while preserving magnetic properties [43,97]. γ-Fe₂O₃@SrCO₃ NPs functionalized with PW₁₂O₄₀³⁻ polyoxometalate (POM) anions demonstrated high efficiency in the photocatalytic degradation of ibuprofen (IBP) in aqueous solutions under solar irradiation. This performance is associated with a direct E_g of 1.8 eV, which allows absorption in the visible region [98]. Similarly, magnetic conjugated polymers incorporating hydrophilic anions (MCP[Br]) exhibit visible-light-responsive photocatalytic behavior, reflected in a reduced E_g of 2.0 eV. The UV–visible diffuse reflectance spectra (DRS) (Figure 3a) show the enhanced light absorption for MCP[Br] relative to Fe₃O₄ NPs, highlighting the contribution of the polymeric component to visible-light activity [99]. In a related approach, poly(1,4,8,11-cyclotetradecane [2,2-bipyridine]-5,5-dicarboxamine)-coated Fe₂O₃ (Fe₂O₃@PNH) NPs operate via a visible-light photo-Fenton mechanism in which the polymer promotes interfacial charge separation and facilitates Fe(III)/Fe(II) cycling to generate •OH radicals from H₂O₂. Figure 3b summarizes the band structure, with X-ray photoelectron spectroscopy (XPS) valence-band onsets of 1.87 eV (Fe₂O₃@PNH) and 2.03 eV (Fe₂O₃), yielding estimated CB positions of approximately 0.11 and 0.19 eV, respectively. Tauc-derived E_g values of 3.15 eV (PNH), 1.92 eV (Fe₂O₃), and 1.68 eV (Fe₂O₃@PNH) support a downhill electron-transfer pathway that accelerates H₂O₂ activation [100].

Overall, surface functionalization enhances surface reactivity and introduces specific binding sites, thereby improving charge-separation efficiency and suppressing electron–hole recombination. In addition, it increases nanoparticle selectivity toward target contaminants and elevates overall photocatalytic performance under visible-light irradiation.

3.5. Construction of Heterojunction Structures

An effective strategy to enhance photocatalytic efficiency is the construction of heterojunctions by integrating semiconductors with complementary band gaps and band alignments to facilitate charge separation. For example, magnetic γ-Fe₂O₃ ultrathin nanosheets paired with mesoporous black TiO₂ hollow sphere form γ-Fe₂O₃/b-TiO₂ heterojunctions capable of degrading tetracycline (TC) under visible light, with an optimized E_g of ~2.41 eV. Individually, black TiO₂ and γ-Fe₂O₃ exhibit E_g of approximately 3.07 and 1.94 eV, respectively [103]. The nanosheet and hollow morphologies promote separation and transport of photogenerated charge carriers, thereby enhancing photocatalytic activity. Similarly, Fe₃O₄/activated carbon/cetyltrimethylammonium bromide–bismuth oxychloride (Fe₃O₄/AC/CTAB–BiOCl) exhibits an E_g of 3.18 eV, compared to 3.46 eV (BiOCl), 3.20 eV (CTAB–BiOCl), and 3.27 eV (AC/CTAB–BiOCl), as derived from Tauc plots (Figure 3c), consistent with improved visible-light absorption in the heterojunction [101].

In another study, γ-Fe₂O₃ and ZnO show band gaps of 2.05 and 3.20 eV, respectively, while a γ-Fe₂O₃@ZnO core–shell composite exhibits a 206 nm redshift in its absorption edge relative to ZnO, thereby improving its visible-light harvesting capability [104]. Furthermore, a 2D/2D Fe₃O₄/bismuth tungstate (Bi₂WO₆) heterostructure (Figure 3d) operates via a direct Z-scheme mechanism. The measured conducted-band potential of Fe₃O₄ (≈−0.42 eV) and valence-band potential of Bi₂WO₆ (≈+3.0 eV) enable the generation of O₂•⁻ radicals by electrons retained on Fe₃O₄ and •OH radicals by holes on Bi₂WO₆ under visible light, preserving strong redox potentials while enhancing charge separation and overall photocatalytic performance [102]. Collectively, heterojunctions effectively suppress electron–hole recombination, thereby significantly increasing photocatalytic efficiency.

4. Applications of Surface-Modified Magnetic Nanoparticles for Antibiotic Degradation

4.1. Most Common Antibiotics and Their Photodegradation by Magnetic Nanoparticle-Based Catalysts

The presence of antibiotics and pharmaceuticals in wastewater constitutes a major environmental concern due to their persistence and potential adverse effects on ecosystems and human health. Table 1 summarizes numerous studies reporting the occurrence of commonly detected antibiotics in wastewater, including tetracycline (TC), sulfamethoxazole (SMX), ciprofloxacin (CIP), trimethoprim (TMP), metronidazole (MTZ), and norfloxacin (NOR) [10,13,14,18,105]. These compounds are also listed in the 23rd WHO Model List of Essential Medicines, highlighting their widespread use and global importance [106]. In addition to antibiotics, other pharmaceutical groups, such as analgesics, anti-inflammatory drugs, and antiepileptics, specifically acetaminophen (ACP), ibuprofen (IBP), carbamazepine (CBZ), and diclofenac (DCF), are frequently detected in wastewater [16,105,107,108,109].

Surface-modified MNPs are widely applied in the removal of pharmaceutical contaminants from wastewater, with their performance strongly dependent on the nature of their interactions with target compounds. Selectivity is a key factor governing this process, as removal efficiency depends on the specificity of interactions between the material surface and the contaminant. This selective behavior is controlled by the compatibility between surface chemistry and the physicochemical characteristics of the target molecules, including polarity, molecular structure, and functional groups [97,110]. Consequently, certain materials exhibit high removal efficiency toward specific pharmaceutical classes while showing limited effectiveness toward others. From a treatment perspective, selectivity primarily reflects preferential adsorption or transformation of target antibiotics rather than complete mineralization. Although selective interactions can result in high or complete degradation, the extent of mineralization depends on compound-specific degradation pathways. Several studies report that selective degradation may proceed through partial oxidation, leading to the formation of transformation by-products whose persistence depends on the material properties and operating conditions.

Tetracycline (TC). TC is a broad-spectrum antibiotic used to treat a wide range of bacterial infections, including respiratory infections, sexually transmitted diseases (STDs), acne, and ocular infections [106,111]. It inhibits bacterial protein synthesis and is also extensively used in veterinary medicine for livestock disease control [112]. Due to its persistence in aquatic environments, numerous photocatalytic systems have been developed for TC degradation [103,113]. Various photocatalysts have demonstrated effective TC removal in aqueous media. The visible-light-assisted MCP[Br] photocatalyst achieved 99% TC removal at an initial concentration of 10 mg L⁻¹ within 2 h, while 20% removal in the dark was attributed to adsorption [99]. A TiO₂/CoFe₂O₄ composite outperformed the individual components, achieving 75.3% TC degradation (10 mg L⁻¹) under UV light and 50.4% under visible light within 180 min [114]. In addition, a microcapsulated phase-change material (MEPCM) with an n-docosane core and a SiO₂/Fe₃O₄ composite shell coated with bismuth oxyiodide (BiOI) nanosheets (BiOI–MEPCM) exhibited 85.3% TC degradation under visible light after 120 min (Figure 4a), demonstrating its potential for solar-driven wastewater treatment [115]. Beyond removal efficiency, mechanistic insight has been reported for selected systems; for example, defect-engineered γ-Fe₂O₃/black TiO₂ heterojunctions enabled identification of stepwise tetracycline degradation pathways involving successive bond cleavage, ring-opening reactions, and intermediate formation [103]. More comprehensive mineralization was demonstrated using a TiO₂–ferroferric oxide–activated carbon composite (TFOC) showed rapid photocatalytic activity, achieving 96% degradation of TC (60 mg L⁻¹) under UV light within 60 min through oxidation, demethylation, dehydroxylation, and subsequent mineralization to CO₂ and H₂O via active species such as h⁺, •OH, and O₂•⁻ [116]. The multilayer core–shell MnFe₂O₄@Bi₂₄O₃₁Br₁₀/Bi₅O₇I (M@B/B) structure achieved 95% TC degradation (50 mg L⁻¹) within 100 min, outperforming MnFe₂O₄ NPs and MnFe₂O₄@Bi₂₄O₃₁Br₁₀ (M@B) [117]. Furthermore, a Fe₂O₃@PNH catalyst exhibited enhanced visible-light-driven Fenton-like degradation of TC, achieving approximately 90% removal at an initial concentration of 50 mg L⁻¹ within 120 min. This performance was 2–5 times higher than that of pristine Fe₂O₃, while PNH alone showed negligible activity (Figure 4b). Under identical conditions, complete degradation of methylene blue was observed, whereas TC derivatives such as oxytetracycline and doxycycline exhibited removal efficiencies of up to 90% [100]. These results emphasize the role of selectivity, whereby differences in molecular structure influence interactions with the catalyst surface and lead to variable degradation efficiencies.

Sulfamethoxazole (SMX). SMX is a widely used sulfonamide antibiotic in both human and veterinary medicine, primarily for the treatment of bacterial infections such as urinary tract infections (UTIs), respiratory infections, diarrhea, and prostatitis [21,106]. Despite its therapeutic importance, SMX is frequently detected in wastewater, surface waters, and wastewater treatment plant (WWTP) effluents [10,107,118]. Recent advances in photocatalytic technologies have demonstrated significant potential for SMX removal. For example, Fe₃O₄/ZnO nanocomposites achieved complete degradation of 100 μg L⁻¹ SMX after 240 min of irradiation [21]. A magnetic carbon nanotube–TiO₂ (MCNT–TiO₂) composite achieved approximately 90% SMX removal (150 μg L⁻¹) under solar irradiation within 30 min [119]. Beyond removal efficiency, mechanistic investigations reported in the literature indicate that SMX degradation typically proceeds via initial cleavage of the sulfonamide bond, followed by hydroxylation and ring-opening reactions, with multiple intermediate species identified depending on the photocatalyst and reaction conditions [118]. An illustrative example of selective pharmaceutical degradation was observed using a magnetic ZnO/γ-Fe₂O₃/bentonite nanocomposite. Under solar irradiation, the material achieved rapid degradation of SMX and CIP, with removal efficiencies of approximately 97% and 98%, respectively, within 30 min at 5 mg L⁻¹. In contrast, CBZ exhibited strong resistance to degradation, with only 3% removal in a ternary mixture under the same conditions (Figure 4c) [120]. This pronounced difference highlights the selective nature of the photocatalytic process and is attributed to variations in molecular structure and chemical reactivity. Specifically, the chemical stability of CBZ and the absence of readily degradable functional groups reduce its susceptibility to photocatalytic attack compared with the more reactive structures of SMX and CIP.

Figure 4. (a) Photocatalytic degradation of tetracycline using BiOI–MEPCM as the photocatalyst under light irradiation. Reproduced with permission from ref. [115]. Copyright 2023, MDPI. (b) Photocatalytic degradation efficiencies of tetracycline, oxytetracycline, doxycycline, and methylene blue using Fe₂O₃@PNH, Fe₂O₃, and PNH under visible-light irradiation. Reproduced with permission from ref. [100]. Copyright 2020, ACS. (c) Photodegradation of ciprofloxacin (CIP; 5 mg L⁻¹), sulfamethoxazole (SMX; 5 mg L⁻¹), and carbamazepine (CBZ; 5 mg L⁻¹) using a ZnO/γ-Fe₂O₃/bentonite nanocomposite. Reproduced with permission from ref. [120]. Copyright 2022, MDPI. (d) Photodegradation of ciprofloxacin by Fe₃O₄/Bi₂WO₆ photocatalysts with varying iron content, with individual Fe₃O₄ and Bi₂WO₆ components shown for comparison. Reproduced with permission from ref. [102]. Copyright 2021, ACS.

Figure 4. (a) Photocatalytic degradation of tetracycline using BiOI–MEPCM as the photocatalyst under light irradiation. Reproduced with permission from ref. [115]. Copyright 2023, MDPI. (b) Photocatalytic degradation efficiencies of tetracycline, oxytetracycline, doxycycline, and methylene blue using Fe₂O₃@PNH, Fe₂O₃, and PNH under visible-light irradiation. Reproduced with permission from ref. [100]. Copyright 2020, ACS. (c) Photodegradation of ciprofloxacin (CIP; 5 mg L⁻¹), sulfamethoxazole (SMX; 5 mg L⁻¹), and carbamazepine (CBZ; 5 mg L⁻¹) using a ZnO/γ-Fe₂O₃/bentonite nanocomposite. Reproduced with permission from ref. [120]. Copyright 2022, MDPI. (d) Photodegradation of ciprofloxacin by Fe₃O₄/Bi₂WO₆ photocatalysts with varying iron content, with individual Fe₃O₄ and Bi₂WO₆ components shown for comparison. Reproduced with permission from ref. [102]. Copyright 2021, ACS.

Ciprofloxacin (CIP). CIP is one of the most widely prescribed fluoroquinolone antibiotics used to treat bacterial infections, including UTIs, respiratory infections, and gastrointestinal infections. It acts by inhibiting bacterial DNA replication [121] and is therefore effective against invasive bacterial diarrhea, enteric fever, and mild pyelonephritis or prostatitis [106]. CIP is frequently detected in wastewater due to its poor removal by conventional wastewater treatment processes [14,18]. Recent advances in photocatalysis have demonstrated significant potential for enhancing CIP removal from aquatic systems. Notably, Fe-modified Fe₃O₄/Bi₂WO₆ photocatalysts exhibit strong activity dependence on Fe loading, with maximal degradation efficiency (99.7%) observed at 4% Fe content. Figure 4d shows that, under identical conditions, pure Bi₂WO₆ and Fe₃O₄ achieved only 51.8% and 5.0% degradation, respectively, whereas Fe-modified photocatalysts containing 0.5%, 2%, and 8% Fe reached 93.5%, 97.4%, and 96.5% CIP degradation, respectively [102]. A metal–organic framework (MOF) layer supporting the growth of an MIP enables enhanced selective removal in aqueous systems. Performance evaluation in real wastewater spiked with CIP (5, 10, and 20 mg L⁻¹) demonstrated that MOF-based Fe₃O₄@MIL-100(Fe)@MIP achieved over 95% removal of 10 mg L⁻¹ CIP within 24 h under visible light [122]. Within 60 min of irradiation, the γ-Fe₂O₃@ZnO core–shell photocatalyst achieved 92.5% degradation of CIP at an initial concentration of 10 mg L⁻¹, significantly outperforming pure γ-Fe₂O₃ (11.7%) and ZnO (52.4%). Mechanistic investigations for this system indicated that CIP degradation proceeded through hydroxyl radical–mediated oxidation and piperazine ring cleavage, accompanied by the formation of intermediate species [104]. In addition, the Fe₃O₄/SiO₂/TiO₂ composite exhibited high photocatalytic activity under UV light, achieving 95% degradation of 5 mg L⁻¹ CIP within 90 min [123]. These findings demonstrate the effectiveness of such materials in minimizing CIP contamination in wastewater, therefore mitigating its environmental impact.

Trimethoprim (TMP). TMP is an antibiotic that inhibits a key bacterial enzyme involved in DNA synthesis [124]. It is frequently co-administered with SMX for the treatment of UTIs, bronchitis, and certain types of pneumonia [21,106]. Recent studies have demonstrated the feasibility of photocatalytic degradation of TMP in water treatment processes. For instance, a magnetic multifunctional photocatalyst, g-C₃N₄@TiO₂@Fe₃O₄, achieved nearly complete degradation of TMP (200 mg L⁻¹) within 120 min under UV irradiation (Figure 5a) [125]. Similarly, TiO₂ NPs and carbon nanofiber-modified Fe₃O₄ (TiO₂@Fe₃O₄@C-NFs) catalysts achieved a high degree of mineralization of TMP (5 mg L⁻¹) within 125 min under UV irradiation [126]. In contrast, a Fe₃O₄/ZnO nanocomposite removed only 36% of TMP after 240 min of UV irradiation, although the same material completely eliminated SMX under identical conditions [21]. This observation highlights the selective nature of the photocatalytic activity of the Fe₃O₄/ZnO nanocomposite, which arises from differences in chemical reactivity and molecular structure among the target compounds.

Acetaminophen (ACP). Paracetamol (PA), also known as ACP, is one of the most widely used pharmaceuticals worldwide for pain relief, including headaches, muscle pain, joint pain, and dental pain, as well as for fever reduction [93]. Due to its extensive use and disposal, particularly from healthcare facilities, ACP is frequently detected in aquatic environments. Recent photocatalytic technologies show strong potential for mitigating this form of water contamination. For instance, a ZnO/Fe₃O₄–graphene oxide/MOF (ZnO/Fe₃O₄–GO/ZIF) nanocomposite achieved 99.1% degradation of ACP (10 mg L⁻¹) under simulated solar irradiation within 45 min [127]. Likewise, a flower-like TiO₂/Fe₂O₃ photocatalyst achieved complete mineralization of PA (50 mg L⁻¹) within 90 min under UV light [128]. A Fe₃O₄/SiO₂/N-CXTi system also exhibited excellent photocatalytic performance, achieving 99.2% ACP removal under UV irradiation within 180 min [93]. Furthermore, cobalt ferrite functionalized with niobium pentoxide (CoFe₂O₄@Nb₂O₅, CFNb) NPs achieved 97.5% degradation of PA (20 mg L⁻¹) within 60 min, surpassing the performance of CoFe₂O₄ extract (CFext, 96.5%) and pure Nb₂O₅ (Nb600, 90.2%), as shown in Figure 5b [129]. These advanced photocatalysts represent highly effective approaches for reducing the ecological impact of ACP/PA in wastewater treatment systems.

Figure 5. (a) Degradation rates of trimethoprim and isoniazid using a g-C₃N₄@TiO₂@Fe₃O₄ multifunctional nanohybrid material. Reproduced with permission from ref. [125]. Copyright 2022, ACS. (b) Comparison of the photocatalytic degradation efficiency of paracetamol using CoFe₂O₄@Nb₂O₅ (CFNb), CoFe₂O₄ extract (CFext), and pure Nb₂O₅ (Nb600). Reproduced with permission from ref. [129]. Copyright 2023, MDPI. (c) Comparison of the photocatalytic performance of Fe₃O₄/SiO₂/TiO₂/MIP and Fe₃O₄/SiO₂/TiO₂ nanocomposites for diclofenac degradation. Reproduced with permission from ref. [88]. Copyright 2025, MDPI.

Figure 5. (a) Degradation rates of trimethoprim and isoniazid using a g-C₃N₄@TiO₂@Fe₃O₄ multifunctional nanohybrid material. Reproduced with permission from ref. [125]. Copyright 2022, ACS. (b) Comparison of the photocatalytic degradation efficiency of paracetamol using CoFe₂O₄@Nb₂O₅ (CFNb), CoFe₂O₄ extract (CFext), and pure Nb₂O₅ (Nb600). Reproduced with permission from ref. [129]. Copyright 2023, MDPI. (c) Comparison of the photocatalytic performance of Fe₃O₄/SiO₂/TiO₂/MIP and Fe₃O₄/SiO₂/TiO₂ nanocomposites for diclofenac degradation. Reproduced with permission from ref. [88]. Copyright 2025, MDPI.

Ibuprofen (IBP). IBP is a nonsteroidal anti-inflammatory drug (NSAID) commonly administered for the relief of pain, inflammation, fever, arthritis, and headaches. Although conventional water treatment processes are generally effective in removing IBP, residual concentrations are still detected in aquatic environments, including rivers and lakes [108,130]. Recent studies have demonstrated efficient photocatalytic degradation of IBP in water systems. For example, POM–γ-Fe₂O₃/SrCO₃ nanocomposite showed significant degradation of IBP at an initial concentration of 10 mg L⁻¹ under solar irradiation within 2 h [98]. A visible-light-driven N-TiO₂@SiO₂@Fe₃O₄ photocatalyst achieved 94% degradation of IBP (2 mg L⁻¹) within 5 h, while showing lower degradation efficiencies for benzophenone-3 and carbamazepine, highlighting its selective activity toward IBP [131]. However, Fe₃O₄/SiO₂/TiO₂ particles exhibited only 60% degradation of IBP (15 mg L⁻¹) after 90 min under UV light [123]. In contrast, a mesoporous BiOBr/Fe₃O₄@SiO₂ photocatalyst achieved nearly complete mineralization of IBP (2 mg L⁻¹) after 6 h of visible-light irradiation [132]. These findings demonstrate that photocatalytic performance toward IBP is strongly dependent on catalyst composition and structure, and that magnetically separable systems can achieve both efficient degradation and, in selected cases, mineralization.

Carbamazepine (CBZ). CBZ is an anticonvulsant drug commonly prescribed for the treatment of epilepsy, bipolar disorder, and neuropathic pain. Its widespread occurrence in surface waters has drawn significant attention [10,96,109]. Numerous photocatalytic materials have been investigated to enhance CBZ removal from water. MCNT–TiO₂ composites achieved 60% removal of CBZ (10 mg L⁻¹) after 120 min under light irradiation [119]. Similarly, a Fe₃O₄@SiO₂/d-TiO₂/Pt photocatalyst achieved 96% CBZ degradation under UV light within 120 min, compared to 57% removal under visible light over the same period [96]. In contrast, an Ag/AgBr/ZnFe₂O₄ photocatalyst exhibited limited visible-light responsiveness, degrading only 22.7% of CBZ (10 mg L⁻¹) within 240 min [95]. These results emphasize the necessity of optimizing photocatalytic systems to improve CBZ removal efficiency in water treatment applications.

Diclofenac (DCF). DCF is another widely used NSAID prescribed for the treatment of arthritis, inflammation, and musculoskeletal pain. Its extensive use and partial metabolic degradation in the human body have been associated with toxic effects on aquatic organisms [133,134]. Photocatalytic technologies have emerged as promising approaches [10,96,109] for DCF removal from water. TiO₂@Zn Fe₂O₄/Pd nanocomposites achieved 84.8% degradation of DFC (10 mg L⁻¹) under direct solar irradiation within 120 min, outperforming TiO₂@ZnFe₂O₄ (73.3%) and ZnFe₂O₄ (49.9%) [92]. A Fe₃O₄/SiO₂/TiO₂/MIP photocatalyst exhibited enhanced selectivity and performance, removing approximately 80% of DCF (10 mg L⁻¹) after 30 min of adsorption followed by 120 min of photocatalytic treatment under both UV and simulated solar light. As shown in Figure 5c, this material outperformed its non-imprinted counterpart (Fe₃O₄/SiO₂/TiO₂), confirming the effectiveness of molecular imprinting in improving photocatalytic efficiency [88]. Rose Bengal-functionalized Fe₃O₄@SiO₂ NPs (Fe₃O₄@SiO₂-RB) achieved complete DCF removal under visible light within 3 h [135]. In addition, a Fe₃O₄/Bi₂S₃/BiOBr (FeSBr) magnetic heterojunction photocatalyst demonstrated superior visible-light-driven DCF degradation, achieving 93.8% removal within 40 min with a rate constant of 0.0527 min⁻¹, outperforming BiOBr, Bi₂S₃, and Bi₂S₃/BiOBr [134]. These systems demonstrate strong potential for mitigating DCF contamination in aquatic environments.

Overall, numerous studies have demonstrated the effective removal of antibiotics and pharmaceuticals from aquatic systems using various catalytic processes with high photocatalytic efficiency. In addition to the compounds discussed above, several other pharmaceuticals—including metronidazole (MNZ) [127,136], norfloxacin (NFX) [123,127], naproxen (NPX) [137], levofloxacin (LVX) [117,138], ketamine [139], ofloxacin (OFL) [101], penicillin (PCN) [140], erythromycin (ERY), roxithromycin (ROX) [21], tamoxifen (TAM), gemfibrozil (GEM) [94], triclosan (TCS) [117] and sulfamethazine (SMZ) [127]—have also been successfully treated using photocatalysis involving surface-modified MNPs.

Table 1. Overview of antibiotics targeted for water treatment using surface-modified magnetic photocatalysts.

Antibiotic	Application	Antibiotic Amount	Magnetic Photocatalyst	Degradation Efficiency	Time (min)	Ref.
Tetracycline (TC)	Broad-spectrum antibiotic for bacterial infections	10 mg L−1	MCP[Br]	99%	120	[99]
		10 mg L−1	TiO2/CoFe2O4	75.3%	180	[114]
		50 mg L−1	BiOI–MEPCM	85.3%	120	[115]
		60 mg L−1	TFOC	96%	60	[116]
		50 mg L−1	M@B/B	95%	100	[117]
		50 mg L−1	Fe2O3@PNH	90%	120	[100]
Sulfamethoxazole (SMX)	Antibacterial for UTIs and respiratory infections	5 mg L−1	ZnO/γ-Fe2O3/Bentonite	97%	30	[120]
		100 μg L−1	Fe3O4/ZnO	100%	240	[21]
		150 μg L−1	MCNT–TiO2	90%	30	[119]
Ciprofloxacin (CIP)	Fluoroquinolone for UTIs, gastrointestinal, and respiratory infections	10 mg L−1	Fe3O4/Bi2WO6	99.7%	25	[102]
		5 mg L−1	ZnO/γ-Fe2O3/bentonite	93%	30	[120]
		10 mg L−1	γ-Fe2O3@ZnO	92.5%	60	[104]
		10 mg L−1	Fe3O4@MIL-100(Fe)@MIP	95%	1440	[122]
		5 mg L−1	Fe3O4/SiO2/TiO2	95%	90	[123]
Trimethoprim (TMP)	Co-administered with SMX for UTIs, bronchitis, and pneumonia	200 mg L−1	g-C3N4@TiO2@Fe3O4	100%	120	[125]
		5 mg L−1	TiO2@Fe3O4@C-NFs	100%	150	[126]
		100 mg L−1	Fe3O4/ZnO	36%	240	[21]
Acetaminophen (ACP)/Paracetamol (PA)	Analgesic and antipyretic for mild to moderate pain and fever	10 ppm	ZnO/Fe3O4–GO/ZIF	99.1%	45	[127]
		50 mg L−1	TiO2/Fe2O3	100%	90	[128]
		25 mg L−1	Fe3O4@SiO2/N-CXTi	99.20%	180	[93]
		20 mg L−1	CoFe2O4@Nb2O5	97.5%	60	[129]
Ibuprofen (IBP)	NSAID for pain, inflammation, fever, arthritis, and headaches	10 mg L−1	POM–γ-Fe2O3/SrCO3	Not reported	120	[98]
		2 mg L−1	N-TiO2@SiO2@Fe3O4	94%	300	[131]
		15mg L−1	Fe3O4@SiO2@TiO2	60%	90	[123]
		2 mg L−1	BiOBr/Fe3O4@SiO2	100%	360	[132]
Carbamazepine (CBZ)	Anticonvulsant for epilepsy, neuropathic pain, and bipolar disorder	10 mg L−1	MCNT–TiO2	60%	120	[119]
		14 mg L−1	Fe3O4@SiO2/d-TiO2/Pt	96%	120	[96]
		10 mg L−1	Ag/AgBr/ZnFe2O4	22.7%	240	[95]
Diclofenac (DCF)	NSAID for pain and inflammation, used for arthritis, migraines, and muscular pain	10 mg L−1	TiO2@ZnFe2O4/Pd	84.8%	120	[92]
		10 mg L−1	Fe3O4/SiO2/TiO2/MIP	80%	120	[88]
		5 × 105 M	Fe3O4@SiO2-RB	100%	180	[135]
		10 mg L−1	Fe3O4/Bi2S3/BiOBr	93.80%	40	[134]

Table 1. Overview of antibiotics targeted for water treatment using surface-modified magnetic photocatalysts.

Antibiotic	Application	Antibiotic Amount	Magnetic Photocatalyst	Degradation Efficiency	Time (min)	Ref.
Tetracycline (TC)	Broad-spectrum antibiotic for bacterial infections	10 mg L−1	MCP[Br]	99%	120	[99]
		10 mg L−1	TiO2/CoFe2O4	75.3%	180	[114]
		50 mg L−1	BiOI–MEPCM	85.3%	120	[115]
		60 mg L−1	TFOC	96%	60	[116]
		50 mg L−1	M@B/B	95%	100	[117]
		50 mg L−1	Fe2O3@PNH	90%	120	[100]
Sulfamethoxazole (SMX)	Antibacterial for UTIs and respiratory infections	5 mg L−1	ZnO/γ-Fe2O3/Bentonite	97%	30	[120]
		100 μg L−1	Fe3O4/ZnO	100%	240	[21]
		150 μg L−1	MCNT–TiO2	90%	30	[119]
Ciprofloxacin (CIP)	Fluoroquinolone for UTIs, gastrointestinal, and respiratory infections	10 mg L−1	Fe3O4/Bi2WO6	99.7%	25	[102]
		5 mg L−1	ZnO/γ-Fe2O3/bentonite	93%	30	[120]
		10 mg L−1	γ-Fe2O3@ZnO	92.5%	60	[104]
		10 mg L−1	Fe3O4@MIL-100(Fe)@MIP	95%	1440	[122]
		5 mg L−1	Fe3O4/SiO2/TiO2	95%	90	[123]
Trimethoprim (TMP)	Co-administered with SMX for UTIs, bronchitis, and pneumonia	200 mg L−1	g-C3N4@TiO2@Fe3O4	100%	120	[125]
		5 mg L−1	TiO2@Fe3O4@C-NFs	100%	150	[126]
		100 mg L−1	Fe3O4/ZnO	36%	240	[21]
Acetaminophen (ACP)/Paracetamol (PA)	Analgesic and antipyretic for mild to moderate pain and fever	10 ppm	ZnO/Fe3O4–GO/ZIF	99.1%	45	[127]
		50 mg L−1	TiO2/Fe2O3	100%	90	[128]
		25 mg L−1	Fe3O4@SiO2/N-CXTi	99.20%	180	[93]
		20 mg L−1	CoFe2O4@Nb2O5	97.5%	60	[129]
Ibuprofen (IBP)	NSAID for pain, inflammation, fever, arthritis, and headaches	10 mg L−1	POM–γ-Fe2O3/SrCO3	Not reported	120	[98]
		2 mg L−1	N-TiO2@SiO2@Fe3O4	94%	300	[131]
		15mg L−1	Fe3O4@SiO2@TiO2	60%	90	[123]
		2 mg L−1	BiOBr/Fe3O4@SiO2	100%	360	[132]
Carbamazepine (CBZ)	Anticonvulsant for epilepsy, neuropathic pain, and bipolar disorder	10 mg L−1	MCNT–TiO2	60%	120	[119]
		14 mg L−1	Fe3O4@SiO2/d-TiO2/Pt	96%	120	[96]
		10 mg L−1	Ag/AgBr/ZnFe2O4	22.7%	240	[95]
Diclofenac (DCF)	NSAID for pain and inflammation, used for arthritis, migraines, and muscular pain	10 mg L−1	TiO2@ZnFe2O4/Pd	84.8%	120	[92]
		10 mg L−1	Fe3O4/SiO2/TiO2/MIP	80%	120	[88]
		5 × 105 M	Fe3O4@SiO2-RB	100%	180	[135]
		10 mg L−1	Fe3O4/Bi2S3/BiOBr	93.80%	40	[134]

4.2. Recovery, Reusability, and Long-Term Performance of Magnetic Nanoparticles

The magnetic properties of NPs play a critical role in facilitating their recovery and enabling repeated reuse over multiple photocatalytic cycles, which is essential for the large-scale application of environmental remediation technologies. Magnetic responsiveness allows NPs to be separated directly from treated water using an external magnetic field, thereby overcoming the limitations of conventional, energy-intensive, and time-consuming separation techniques such as filtration and centrifugation. This rapid and efficient magnetic separation not only enhances operational efficiency but also reduces the generation of secondary waste associated with non-recyclable catalysts [141]. Most importantly, magnetic separation significantly contributes to the sustainability of photocatalytic treatment systems, as NPs can be readily recovered and reused with minimal loss of catalytic activity [142]. Such reusability is crucial for cost reduction, as it lowers both the frequency and quantity of photocatalyst required, thereby improving the long-term economic feasibility of nanotechnology-based water treatment.

Numerous studies have demonstrated that magnetic core NPs—such as Fe₃O₄, γ-Fe₂O₃, and XFe₂O₄—when coupled with metal oxides, exhibit high structural stability and consistent photocatalytic performance over multiple cycles. Their durability during repeated photocatalytic reactions and recovery processes makes them well-suited for water treatment applications. These NPs are typically washed and, in some cases, dried prior to reuse, ensuring sustained functionality across cycles. Their ability to maintain consistent activity over repeated use highlights their robust performance and sustainability for long-term operation in wastewater treatment systems [59,142]. A summary of recent advances in degradation efficiency and recyclability of surface-modified magnetic photocatalysts for antibiotic removal is presented in Table 2.

Fe₃O₄-Based Systems. Magnetite (Fe₃O₄) is the most widely used magnetic core due to its high saturation magnetization and excellent chemical stability. When integrated with metal oxides, Fe₃O₄ provides efficient magnetic recovery while enhancing photocatalytic durability through improved charge transfer and structural protection [143]. The mesoporous BiOBr/Fe₃O₄@SiO₂ composite retained approximately 80% of its initial photocatalytic activity after five consecutive 60-min cycles and achieved nearly 99% magnetic separation efficiency within 5 min. Despite a reduced saturation magnetization (M_S ≈ 40 emu g⁻¹) compared with bare Fe₃O₄ (~65 emu g⁻¹), the material remained suitable for rapid magnetic recovery [132]. A similar level of stability was observed for the Fe₃O₄/AC–CTAB–BiOCl photocatalyst, which maintained high OFL degradation efficiency over five cycles (Figure 6a). Fe₃O₄ and Fe₃O₄/AC–CTAB–BiOCl exhibited M_S values of 91.7 and 2.2 emu g⁻¹, respectively, and their narrow hysteresis loops confirmed superparamagnetic behavior and excellent recyclability [101]. The TiO₂/SiO₂/Fe₃O₄ composite also showed high reusability, with less than a 10% decrease in the apparent rate constant (k_app) over four cycles for ACP removal, compared with TiO₂/Fe₃O₄. Adequate magnetic separation was maintained, with M_S values of 38.9 emu g⁻¹ for TiO₂/SiO₂/Fe₃O₄ and 45.0 emu g⁻¹ for TiO₂/Fe₃O₄, despite partial magnetization loss due to the SiO₂ coating [144].

As shown in Figure 6b, Fe₃O₄@SiO₂@Nb-TiO₂ NPs retained a strong magnetic response while preserving ferrimagnetic behavior. The M_S decreased from 73 emu g⁻¹ for Fe₃O₄ to 41, 22, and 19 emu g⁻¹ for Fe₃O₄@SiO₂, Fe₃O₄@SiO₂@TiO₂, and Fe₃O₄@SiO₂@Nb-TiO₂, respectively, due to the presence of non-magnetic shells. Nevertheless, rapid magnetic separability was retained, and only a moderate decline in CIP degradation efficiency was observed, decreasing from 94% to 77% after five cycles [90].

The Fe₃O₄/SiO₂/TiO₂@reduced graphene oxide (rGO) nanocomposite also demonstrated good recyclability, with MNZ removal decreasing from 94.2% to 63.5% over four cycles. Although the M_S value declined to 14.8 emu g⁻¹ due to the TiO₂ and rGO layers, rapid magnetic separation within 30 s was still achieved [136]. Similarly, the Fe₃O₄@SiO₂/d-TiO₂/Pt photocatalyst demonstrated comparable robustness, maintaining CBZ degradation efficiency over eight successive cycles without detectable performance loss, confirming its strong magnetic stability and structural integrity (Figure 6c) [96].

Among the reported systems, Fe₃O₄/ZnO demonstrated the highest durability, maintaining stable photocatalytic efficiency over eight cycles without the need for additional catalysts. Although its M_S value was relatively low (4.3 emu g⁻¹) due to minimal Fe₃O₄ loading, it remained sufficient to sustain superparamagnetic behavior and prolonged operational stability [21]. Comparatively, Fe₃O₄/ZnO and Fe₃O₄@SiO₂/d-TiO₂/Pt exhibited superior durability, whereas BiOBr/Fe₃O₄@SiO₂ and Fe₃O₄/AC–CTAB–BiOCl provided the highest separation efficiency. TiO₂/SiO₂/Fe₃O₄ and Fe₃O₄@SiO₂@Nb-TiO₂ offered balanced recyclability and magnetic responsiveness, while Fe₃O₄/SiO₂/TiO₂@rGO, despite its lower magnetization, exhibited the fastest magnetic response. Collectively, these systems demonstrate the complementary advantages and trade-offs among Fe₃O₄-based photocatalysts in terms of durability, magnetization, and recyclability.

γ-Fe₂O₃-Based Systems. Hematite (γ-Fe₂O₃) exhibits high chemical stability and visible-light photoactivity, making it suitable for photocatalytic applications due to its favorable optical properties (E_g ≈ 2.2 eV). Although its saturation magnetization is lower than that of Fe₃O₄, coupling γ-Fe₂O₃ with metal oxides or conductive phases significantly enhances charge separation and recyclability [145]. For example, POM–γ-Fe₂O₃/SrCO₃ nanohybrids efficiently degraded IBP over four cycles with minimal loss of activity, while their superparamagnetic behavior (M_S = 11 emu g⁻¹) enabled rapid magnetic separation [98]. The Fe₂O₃@PNH system also demonstrated high structural stability and reproducibility, retaining nearly 90% catalytic efficiency over five consecutive cycles with negligible performance loss (Figure 7a) [100]. Similarly, γ-Fe₂O₃@ZnO core–shell NPs maintained a CIP degradation efficiency of 90.3% over six cycles, with only a 2.8% decline attributed primarily to material loss. Despite a reduction in M_S from 55.2 to 39.1 emu g⁻¹, the composite preserved excellent ferromagnetic properties, with a remanent magnetization of 5.2 emu g⁻¹ and a coercivity of 87.7 Oe [104]. The incorporation of nonmagnetic components such as ZnO and TiO₂ further enhanced photocatalytic efficiency but introduced trade-offs in magnetic performance. This effect was evident in ZnO/Fe₂O₃–GO/ZIF composites, which achieved more than 95% SMZ degradation over ten cycles while reducing M_S from 54.9 to 21.2 emu g⁻¹ [127].

Similarly, flower-like TiO₂/Fe₂O₃ core–shell NPs maintained high PA degradation efficiency over four cycles; however, the efficiency decreased to 58% in the fifth cycle, coinciding with a decline in M_S from 44.1 to 20.8 emu g⁻¹ [128]. Figure 7b shows a similar trend for branched polyethyleneimine (bPEI)-functionalized γ-Fe₂O₃–TiO₂ nanocomposites, where both bare γ-Fe₂O₃ and γ-Fe₂O₃–TiO₂ exhibited superparamagnetic behavior, with M_S decreasing from 67 to 49 emu g⁻¹ following TiO₂ coating and bPEI coupling. Despite this reduction, the γ-Fe₂O₃–TiO₂ nanocomposite maintained efficient magnetic recovery and stable CIP degradation for two cycles, followed by only a marginal decline after the fourth cycle [146]. Overall, these studies highlight the critical balance between maintaining high photocatalytic activity, preserving structural stability, and optimizing magnetic properties to ensure effective separation and long-term reusability in γ-Fe₂O₃-based photocatalyst systems.

XFe₂O₄-Based Systems. Spinel ferrites (XFe₂O₄) have shown significant potential in photocatalytic degradation processes due to their unique narrow band gaps (E_g ≈ 1.8–2.2 eV), which enable visible-light activity. These materials combine stable magnetic behavior with tunable electronic properties and, when integrated with semiconductor oxides, allow efficient catalyst separation and recovery using external magnetic fields [142]. For instance, the M@B/B composite exhibited strong magnetic separation capability with high material recovery rates (80–87%) and effective removal of LVX, TC, and TCS over five cycles. Although the saturation magnetization decreased due to the incorporation of nonmagnetic components, both the M@B/B and M@B composites retained sufficient magnetic responsiveness to ensure recyclable photocatalytic performance [117].

Similarly, the CuFe₂O₄@WO₃/Ag system exhibited negligible catalytic loss, with TAM and GEM degradation decreasing only slightly from 83.15% to 72.64% and from 81.47% to 68.25%, respectively, over five cycles. During this process, M_S value declined from 62.57 to 29.49 emu g⁻¹ after coating [94]. The TiO₂@ZnFe₂O₄/Pd and ZnFe₂O₄@TiO₂/Cu composites also demonstrated strong recyclability over five cycles. For TiO₂@ZnFe₂O₄/Pd, the M_S of ZnFe₂O₄ decreased from 42.57 to 27.28 emu g⁻¹ after TiO₂ coating and subsequent Pd doping. Likewise, in the ZnFe₂O₄@TiO₂/Cu composite, the M_S decreased from 40.57 to 37.32 emu g⁻¹ after TiO₂ coating and further declined to 26.54 emu g⁻¹ following Cu incorporation. Under these conditions, the degradation efficiencies of DCF and NPX decreased only slightly, from 86.1% to 71.38% and from 80.73% to 72.31%, respectively, indicating good structural stability and reusability [92,137]. The TiO₂/CoFe₂O₄ composites also exhibited a pronounced reduction in M_S compared with pristine CoFe₂O₄ (87.6 emu g⁻¹), with the TiO₂/CoFe₂O₄ (TC10) composite showing an M_S of 8.4 emu g⁻¹. Nevertheless, this reduction did not compromise magnetic separability, and the composite maintained effective recyclability, as evidenced by stable TC degradation under UV irradiation, which decreased only from approximately 75% in the first cycle to 65% after five cycles (Figure 7c) [114]. Lastly, the CoFe₂O₄@CuS catalyst retained good photocatalytic activity, showing only a 12.1% reduction in PCN degradation after five cycles, with its M_S decreased slightly from 7.76 to 7.34 emu g⁻¹ [140].

Overall, surface modification of MNPs inevitably leads to a reduction in saturation magnetization; however, these materials consistently exhibit outstanding photocatalytic activity and operational stability, making them well-suited for sustainable water treatment applications. Notably, photocatalytic efficiency generally decreases only marginally after multiple reuse cycles, indicating excellent structural robustness and recyclability. Although the incorporation of nonmagnetic components often enhances photocatalytic performance, optimization of the trade-off between magnetic separability and catalytic efficiency remains essential for ensuring long-term system sustainability. Collectively, the integration of optical activity and magnetic recoverability contributes to cost reduction and improved resource efficiency in water and wastewater treatment, thereby offering a potential pathway toward environmentally friendly and economically sustainable remediation technologies.

Table 2. Degradation and reusability performance of surface-modified magnetic photocatalysts for antibiotic removal from water.

MNP Core	Photocatalyst	Contaminant	Time (min)	Removal Efficiency (%)	Reusability Cycles	MS (emu g−1)	Ref.
Fe3O4	BiOBr/Fe3O4@SiO2	IBP	60	80–60	5	∼40	[132]
	Fe3O4/AC–CTAB–BiOCl	OFL	60	100	5	2.2	[101]
	TiO2/SiO2/Fe3O4	ACP	300	-	4	38.9	[144]
	Fe3O4@SiO2@Nb-TiO2	CIP		94–77	5	22	[90]
	Fe3O4@SiO2@TiO2/rGO	MNZ	60	94–63	4	14.8	[136]
	Fe3O4@SiO2/d-TiO2/Pt	CBZ	120	96	8	-	[96]
	Fe3O4/ZnO	SMX ROX ERY TMP	240	100	8	4.3	[21]
				94
				66
				36
Fe2O3	POM–γ-Fe2O3/SrCO3	IBP	120	-	4	11	[98]
	Fe2O3@PNH	TC	60	90	5	-	[100]
	γ-Fe2O3@ZnO	CIP	60	92.5	6	5.2	[104]
	ZnO/Fe2O3–GO/ZIF	SMZ	60	97.7	10	21.2	[127]
	TiO2/Fe2O3	PA	180	98–57.5	5	20.8	[128]
	γ-Fe2O3–TiO2	CIP	150	70	4	49	[146]
XFe2O4	MnFe2O4@Bi24O31Br10/BiO7I	LVX	100	93.2–70.2	5	32.40	[117]
		TC		95–71.7
		TCS		87.8–68.1
	CuFe2O4@WO3/Ag	TAM GEM	150	83.15–72.64 81.47–68.25	5	29.49	[94]
	TiO2@ZnFe2O4/Pd	DCF	120	86.1–71.4	5	27.28	[92]
	ZnFe2O4@TiO2/Cu	NPX	120	80.7–72.3	8	26.45	[137]
	TiO2/CoFe2O4	TC	180	75–65	5	8.4	[114]
	CoFe2O4@CuS	PCN	120	70.7–58.6	5	7.76	[140]

Table 2. Degradation and reusability performance of surface-modified magnetic photocatalysts for antibiotic removal from water.

MNP Core	Photocatalyst	Contaminant	Time (min)	Removal Efficiency (%)	Reusability Cycles	MS (emu g−1)	Ref.
Fe3O4	BiOBr/Fe3O4@SiO2	IBP	60	80–60	5	∼40	[132]
	Fe3O4/AC–CTAB–BiOCl	OFL	60	100	5	2.2	[101]
	TiO2/SiO2/Fe3O4	ACP	300	-	4	38.9	[144]
	Fe3O4@SiO2@Nb-TiO2	CIP		94–77	5	22	[90]
	Fe3O4@SiO2@TiO2/rGO	MNZ	60	94–63	4	14.8	[136]
	Fe3O4@SiO2/d-TiO2/Pt	CBZ	120	96	8	-	[96]
	Fe3O4/ZnO	SMX ROX ERY TMP	240	100	8	4.3	[21]
				94
				66
				36
Fe2O3	POM–γ-Fe2O3/SrCO3	IBP	120	-	4	11	[98]
	Fe2O3@PNH	TC	60	90	5	-	[100]
	γ-Fe2O3@ZnO	CIP	60	92.5	6	5.2	[104]
	ZnO/Fe2O3–GO/ZIF	SMZ	60	97.7	10	21.2	[127]
	TiO2/Fe2O3	PA	180	98–57.5	5	20.8	[128]
	γ-Fe2O3–TiO2	CIP	150	70	4	49	[146]
XFe2O4	MnFe2O4@Bi24O31Br10/BiO7I	LVX	100	93.2–70.2	5	32.40	[117]
		TC		95–71.7
		TCS		87.8–68.1
	CuFe2O4@WO3/Ag	TAM GEM	150	83.15–72.64 81.47–68.25	5	29.49	[94]
	TiO2@ZnFe2O4/Pd	DCF	120	86.1–71.4	5	27.28	[92]
	ZnFe2O4@TiO2/Cu	NPX	120	80.7–72.3	8	26.45	[137]
	TiO2/CoFe2O4	TC	180	75–65	5	8.4	[114]
	CoFe2O4@CuS	PCN	120	70.7–58.6	5	7.76	[140]

4.3. Influencing Factors in Photocatalytic Reactions

Photocatalytic reactions are governed by a range of physicochemical parameters that collectively influence reaction kinetics, charge-carrier dynamics, and overall catalytic efficiency. Key factors include the characteristics and intensity of the light source, catalyst morphology and particle size, surface area and porosity, and the presence of electron donors or acceptors. In addition, variables such as catalyst dosage, dispersibility in the reaction medium, and the physicochemical properties of the target pollutants can significantly affect degradation performance [67,78,147].

Among these factors, three critical variables—(1) the pH of the reaction medium, (2) the H₂O₂ concentration, and (3) the calcination temperature of the catalyst—play a decisive role in modulating photocatalytic efficiency. The pH of the solution influences the surface charge distribution of the catalyst and reactants, as well as the ionization state of pollutants [92]. The addition of H₂O₂ enhances the generation of ROS, which are essential for pollutant degradation; however, its concentration must be carefully optimized to avoid radical quenching [21]. The calcination temperature affects photocatalytic performance by altering crystallinity, phase composition, surface area, and defect density, all of which significantly influence overall process efficiency [123,148]. Consequently, optimization of these parameters is essential for achieving high photocatalytic activity, particularly for the degradation of antibiotic contaminants in aqueous systems.

Effect of Reaction Solution pH. The pH of the reaction solution has a substantial impact on photocatalytic antibiotic degradation by modifying the surface charge of the catalyst, the ionization state of pollutants, and the generation of ROS. In addition, pollutant solubility and stability are influenced by pH, making it a critical parameter for effective photocatalysis in environmental remediation and water treatment. A comparative analysis of recent studies highlights the role of pH across different catalytic systems and antibiotic pollutants. For example, the photocatalytic degradation of DFC using a TiO₂@ZnFe₂O₄/Pd nanocomposite exhibited maximum efficiency at pH 4, achieving 81.11% removal under 120 min of solar irradiation. This efficiency progressively declined with increasing pH, with degradation efficiencies of 73.48%, 65.98%, and 46.21% observed at pH 5, 7, and 9, respectively. The superior performance at pH 4 was attributed to the increased production of •OH radicals, which are highly effective in breaking down antibiotic contaminants. Additionally, at this pH, the catalyst surface carries a positive charge, facilitating electrostatic attraction with negatively charged DFC molecules [92]. In contrast, the photocatalytic degradation of CIP using MIL-100(Fe)-based MIPs was optimal at approximately neutral pH (~7), achieving over 95% removal efficiency. When the pH decreased below 5 or increased above 9, the CIP removal efficiency declined by 10–30%. At pH < 4, the catalyst surface is predominantly positively charged, while it becomes increasingly negative at higher pH. Under extreme conditions (pH < 3 or pH > 11), the system exhibited poor performance, suggesting that maintaining near-neutral conditions is crucial for effective CIP adsorption and degradation [122]. Figure 8a illustrates the effect of pH on the photocatalytic degradation of cefalexin using an α-Fe₂O₃/g-C₃N₄ nanocomposite, which showed maximum efficiency (97.8%) at pH 7. This value was significantly higher than those obtained at pH 3 (67.1%), pH 5 (72.9%), and pH 9 (44.2%) [149]. Similarly, the CoFe₂O₄@CuS nanocomposite exhibited a maximum removal efficiency of 75% for PC at pH 5, while only approximately 20% removal was observed at pH 3. Under extreme acidic conditions, H⁺ ions compete with •OH radicals, thereby reducing their availability for degradation [140]. These studies collectively demonstrate that pH is a critical determinant of antibiotic degradation efficiency in photocatalysis. Adjusting the pH in accordance with the physicochemical properties of both the pollutant and the catalyst is therefore essential for optimizing photocatalytic performance.

Effect of H₂O₂ Concentration. The addition of H₂O₂ plays a crucial role in the photocatalytic removal of antibiotics and other pollutants by acting as an electron acceptor, capturing conduction band electrons, and thus suppressing charge recombination. This promotes the formation of •OH radicals through both reduction and photolytic pathways. Dissolved O₂ from air also contributes by generating O₂•⁻ through electron capture. Therefore, both H₂O₂ and O₂ promote ROS generation through complementary pathways [150]. In Fe₂O₃@PNH nanocomposites, an optimal H₂O₂ concentration of 10 mM under visible-light irradiation yielded the highest TC degradation efficiency (Figure 8b). Higher H₂O₂ concentrations reduced oxidation activity due to •OH scavenging and excessive Fe³⁺ formation [100]. For Fe₃O₄/ZnO nanocomposites under UV irradiation, the addition of 100 mg L⁻¹ H₂O₂ led to the complete removal of SMX and near-complete degradation of TMP (94%), ERY (95%), and ROX (98%) within 4 h. In contrast, without H₂O₂, the degradation efficiencies were significantly lower, with removal rates of approximately 79% for SMX, 67% for TMP, 71% for ERY, and 85% for ROX. These results highlight the synergistic interaction between H₂O₂ and the photocatalyst in promoting oxidative degradation [21].

Similarly, the POM–γ-Fe₂O₃/SrCO₃ catalyst achieved nearly complete mineralization of IBP under solar irradiation with 80 μL of H₂O₂. However, increasing the H₂O₂ dose to 160 μL led to competitive scavenging of OH• radicals by excess H₂O₂, thereby reducing degradation efficiency [98]. In contrast, the Ag/AgBr/ZnFe₂O₄ catalyst exhibited a negative response to H₂O₂ addition during CBZ degradation, with efficiency decreasing from 22.7% (without H₂O₂) to 18.3%. Alternatively, the use of potassium peroxodisulfate enhanced degradation to 28.4% via sulfate radical (SO₄•⁻) generation, indicating that this catalyst more effectively activates persulfate than H₂O₂. Excess H₂O₂ likely hindered the degradation process due to unfavorable catalyst–oxidant interactions [95]. In another study, Figure 8c shows that the Fe₃O₄@SiO₂/d-TiO₂/Pt nanocomposite achieved 92% CBZ degradation in the presence of H₂O₂ under neutral pH. Under mildly acidic conditions (pH 5), the efficiency further increased to 96%, which was attributed to enhanced H₂O₂ activation and increased ROS production [96]. These findings underscore the importance of optimizing both H₂O₂ concentration and solution pH to maximize photocatalytic efficiency while minimizing the adverse effects of excessive oxidant dosage.

Effect of Calcination Temperature. The calcination temperature of photocatalysts plays an important role in determining their photocatalytic activity for water pollutant degradation by influencing crystallinity, phase composition, and surface area. These structural characteristics directly affect key parameters such as adsorption capacity, charge-carrier mobility, and the availability of active sites. For Fe₃O₄/SiO₂/TiO₂ composites, the sample calcined at 500 °C exhibited degradation efficiencies of 78% for CIP and 82% for NOR. In contrast, the sample calcined at 600 °C achieved significantly higher efficiencies of approximately 95% for both CIP and NOR under identical conditions. This improvement is attributed to the formation of highly crystalline anatase TiO₂, which provides an optimal balance between crystallinity and surface area, thereby maximizing photocatalytic activity [123]. Similarly, calcination of N-TiO₂@SiO₂@Fe₃O₄ at 500 °C resulted in the optimal IBP degradation, achieving 90% efficiency compared with 88% at 400 °C. The superior performance at 500 °C was attributed to greater nitrogen incorporation, which enhances light absorption and improves charge separation. However, calcination temperatures above 500 °C induced excessive crystal growth and reduced surface area, ultimately leading to lower photocatalytic performance [131]. These findings underscore the importance of optimizing the calcination temperature with respect to catalyst composition, as achieving an appropriate balance between crystallinity and surface area is essential for maximizing photocatalytic activity.

5. Challenges and Future Perspectives

Despite the significant potential of metal-based NPs for addressing environmental and energy-related challenges through photodegradation processes, their practical application remains constrained by several technical, economic, and environmental limitations. Overcoming these barriers is essential for the efficient large-scale deployment of MNP-based technologies (Scheme 6).

One major challenge is the long-term stability of surface modifications, which directly governs the durability and photocatalytic performance of these NPs. External factors such as pH fluctuations, oxidative environments, and prolonged light exposure can degrade surface coatings, leading to NP aggregation, reduced effective surface area, and diminished catalytic efficiency. In addition, strong magnetic interactions among MNPs can further promote aggregation, thereby reducing dispersibility and complicating recovery during both synthesis and application. These effects increase operational costs and elevate the risk of secondary pollution. Addressing these limitations requires the development of robust surface-engineering strategies that effectively suppress aggregation while preserving or enhancing the photocatalytic functionality of surface-modified MNPs under practical operating conditions.

Additionally, the synthesis of surface-modified MNPs poses a further challenge, as it often requires multistep procedures that increase production costs and limit scalability for industrial implementation. Transitioning from laboratory-scale research to industrial deployment involves both economic and technical constraints, including the need to maintain uniform NP properties, ensure long-term performance stability, and control production costs during scale-up. Scalability is further complicated by the difficulty of preserving consistent NP characteristics during large-scale manufacturing [82]. Advances in material science—particularly surface modifications using hydrophilic polymers or MOFs—offer promising strategies to improve dispersibility, selectivity, and stability in complex aqueous systems [99,122]. However, achieving high selectivity toward specific contaminants in heterogeneous wastewater matrices remains a major challenge. This limitation can be addressed only through precise surface-engineering strategies that promote selective interactions with target molecules while minimizing nonspecific binding.

Environmental considerations represent another critical aspect. The potential toxicity of surface-modified MNPs, including risks associated with NP leaching into aquatic ecosystems, highlights the need for comprehensive lifecycle and toxicity assessments. Secondary pollutants generated during photocatalytic reactions must also be considered, as incomplete mineralization of contaminants can produce by-products that pose additional ecological risks [151]. Furthermore, many laboratory studies rely on synthetic water systems that differ markedly from real wastewater, emphasizing the need for more realistic testing conditions to establish practical applicability [152]. These challenges become even more pronounced in complex environmental matrices, where multiple contaminants compete for active sites and reactive species, thereby reducing the overall photocatalytic efficiency of surface-modified MNP systems. A thorough evaluation of environmental and health impacts is therefore essential to ensure the safe and sustainable deployment of these technologies on a large scale.

Beyond environmental risk considerations, the long-term structural stability of surface-modified MNPs under repeated photocatalytic operation remains insufficiently understood. While recyclability tests often indicate stable catalytic activity over several cycles, they do not necessarily capture gradual structural degradation or chemical transformations occurring at the nanoparticle surface [153]. Prolonged exposure to reactive oxygen species, fluctuating pH conditions, and continuous light irradiation can induce surface reconstruction, coating degradation, or lattice defects, which may ultimately affect photocatalytic efficiency and material integrity [82,154]. Such changes can also promote slow metal or dopant release, even in the absence of detectable activity loss, underscoring the need for systematic post-cycle characterization. Comprehensive evaluation using techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), XPS, and inductively coupled plasma (ICP)-based analyses is therefore required to correlate performance stability with structural evolution under realistic operating conditions [142,155]. Addressing these durability-related challenges will be critical for translating laboratory-scale photocatalytic systems into long-term, environmentally compatible water treatment technologies.

Future research should focus on developing cost-effective and scalable synthesis strategies that preserve the physicochemical properties of surface-modified MNPs, including high surface area, crystallinity, and uniform particle size distribution. Incorporating these NPs into photocatalytic reactors—particularly continuous-flow systems—may help maintain stable performance during wastewater treatment. Such reactor designs must ensure uniform light distribution and efficient interaction between contaminants and the photocatalyst to overcome current technological limitations [156]. Expanding their application into energy storage and conversion systems may also reveal multifunctional behavior arising from the combined catalytic and magnetic properties of these materials. Green synthesis approaches—utilizing plant extracts, biomass, or other environmentally benign precursors—offer additional advantages by reducing energy consumption, minimizing the formation of toxic by-products, and lowering the environmental footprint of the NP fabrication process [157].

From a process-engineering perspective, the magnetic properties of surface-modified MNPs provide a practical advantage for large-scale and continuous wastewater treatment, as external magnetic fields can be employed for in situ catalyst retention, separation, and recycling within flow-through photoreactors. Such configurations can minimize catalyst loss, reduce downstream solid–liquid separation requirements, and support stable long-term operation under continuous hydraulic conditions [142,156,158]. Coupling magnetic photocatalysts with energy-efficient light sources and an optimized reactor, therefore, represents a realistic pathway toward pilot-scale implementation. Nevertheless, validation under real wastewater conditions and extended operational times remains essential to assess durability, fouling resistance, and overall process efficiency.

Optimization of operational conditions, including calcination temperature and solution pH, also provides pathways to enhance photocatalyst performance and degradation efficiency. Another important parameter is the choice of energy source; sustainable systems based on light-emitting diodes (LEDs) offer improved energy efficiency, environmentally benign operation, and extended service life, thereby reducing the overall energy footprint of photocatalytic processes [159]. Furthermore, integrating photocatalysis with complementary technologies such as membrane filtration, flocculation, precipitation, and adsorption presents opportunities for developing large-scale, multifunctional water purification systems [160]. To support sustainability goals, extensive toxicity evaluations and environmental impact assessments are required. Considering these future directions, advancements in synthesis, scale-up, and process integration will enable surface-modified MNPs to function as transformative materials for energy generation, environmental remediation, and water purification, contributing to the development of sustainable technological solutions.

6. Conclusions

This review summarizes the recent progress in the development of surface-modified MNPs as advanced photocatalysts for the mitigation of antibiotic pollution in wastewater. Through tailored surface engineering, the photocatalytic performance of MNPs has been substantially enhanced, with improvements in adsorption selectivity, structural stability, light-responsive behavior, and magnetic recoverability. These attributes enable the efficient degradation of a wide range of pharmaceutical contaminants under both UV and visible-light irradiation, while also allowing material reuse over multiple treatment cycles. The versatility of these nanomaterials, together with their adaptability to diverse environmental conditions and their potential integration into hybrid treatment systems, highlights their promise for scalable and sustainable water purification technologies.

Despite these advances, significant challenges related to large-scale implementation, economic feasibility, environmental safety, and long-term operational stability remain. Overcoming these barriers requires continued efforts in green synthesis strategies, advanced reactor design, and the development of multifunctional hybrid systems. Such efforts are critical to enable the successful translation of laboratory-scale research into industrial applications. Looking ahead, surface-modified MNPs hold potential not only for advancing wastewater treatment technologies but also for broader applications in environmental remediation and energy conversion, thereby contributing to the sustainable management of global water resources.