Advanced ZnO Nanorods and Metal–Organic Frameworks for Sustainable Photocatalytic Microplastic Degradation

Abstract

The increasing presence of microplastics in the aquatic and terrestrial food chains calls for the need to come up with innovative and effective remediation approaches. Such innovations as zinc oxide (ZnO) structures and metal–organic frameworks (MOFs) are examined as the second generation of photocatalysts for degrading microplastics under sunlight. We will focus on the latest advances and discuss the structure of photocatalytic processes, their functioning under various light conditions, and their environmental impacts, especially environmental safety and ecotoxicity. ZnO structures are even better photocatalysts because they form reactive oxygen species (ROS) as good as other metal oxides. However, their possible cytotoxicity and the ability to generate oxidative stress require serious evaluation. MOFs, on the contrary, offer physicochemical properties, environmental safety, ecotoxicity, and environmentally friendly synthesis pathways, making them a worthy substitute. The review underscores the urgency of incorporating environmental safety and ecotoxicity into the design of photocatalysts, thereby unlocking their full potential while avoiding environmental or human health risks. Moving forward in the field of sustainable nanotechnology to remove microplastics will provide the way to come up with green innovations and hence guarantee the effectiveness of combating plastic pollution in long-term stability.

1. Introduction

Microplastics are the smallest plastic particles, which are less than 5 mm; they are one of the pollutants that are of primary and secondary sources [1,2]. Primary microplastics are purposely produced at sub-millimetric dimensions and used in varied purposes, such as the use of microbeads in cosmetics and personal-care products, as well as abrasives integrated in industrial use. In comparison, the existence of secondary microplastics is a by-product of bigger plastic waste. The major sources of microplastic pollution include degraded plastic wrappings, discarded fishing equipment, synthetic clothes, wear particles (tires), and plastic pellets used in production [3,4]. Microplastics can be divided into groups based on their polymer structure and morphology. The most frequently used polymers are polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), which are widely used in plastic baggage, bottles, and packaging materials (Figure 1). Their morphological manifestations differ and include fibers, fragments, beads, and films. For example, microfibers are commonly released during the laundering of synthetic textiles, and films form mainly from the breakdown of plastic bags. Microplastics are widely distributed across aquatic, terrestrial, and atmospheric ecosystems, posing significant ecological risks. Microplastics have also been found in isolated places like deep-sea sediments and the poles, and they have penetrated the food web [5,6,7,8]. Their endurance in the environment, due to their resistance to degradation, enables them to last for a few decades. Their small size makes them easy for a wide range of organisms, from plankton to sea mammals, to ingest, resulting in physical damage, chemical toxicity, and bioaccumulation throughout the trophic web. Ingestion may result in obstructive lesions in digestive tracts, loss in energy levels, and reproductive performance [2,6,9]. In addition, microplastics serve as carriers of toxic chemicals, such as persistent organic pollutants (POPs) and heavy metals, which adsorb to their surfaces. Such pollutants increase toxicity and risks to organisms, and they exacerbate ecological and human health issues. With further disintegration of micro-scale plastics into nanoplastics, their ability to penetrate biological barriers increases, increasing the possibility of cellular-level toxicity and long-term environmental consequences [10,11].

In terrestrial systems, microplastics undermine the soil structure, disrupt the dynamics of the microbial communities, and block the growth of vegetation. Their build-up in the soils of agricultural land, often due to the utilization of plastic mulches or irrigation with treated wastewater, has raised concerns about the prospective long-term consequences on edaphic health and crop production. By cutting-edge, anthropocentrically friendly curative mechanisms, the sustained residual effects of microplastics, with the concomitant adverse impacts, underscore the urgent need to implement novel and sustainable mitigation frameworks to curb this pervasive type of anthropogenic pollution [12,13,14,15,16]. The ubiquity of microplastics in various ecological compartments, along with the nature of the harm they cause physically, chemically, and biologically, captures the irreplaceable need to have specific interventions aimed at dampening their ecological footprints [17,18]. The development of sustainable abatement modalities, such as state-of-the-art photocatalytic degradation methods, requires a comprehensive understanding of microplastic provenance and typology.

1.1. Challenges in Microplastic Degradation and Existing Solutions

The synthetic pollutant known as microplastic pollution, which is mostly formed through the degraded materials of plastic products of microscopic size, has become a major concern in the world. These particles are generally smaller than 5 mm in diameter, and they have filled the seas, freshwater, and the atmosphere, posing serious problems to the biodiversity, aquatic life, and human health [19,20]. The resistance and inherent chemical stability of microplastics to biodegradation are its sources of persistence, which complicates the mitigation efforts to reduce the accumulation of this group in the environment.

Among the main difficulties connected with the issue of microplastic degradation, there are their polymeric heterogeneous structure, heterogeneous size, and widespread dispersion. Common polymers used in everyday applications, such as PE, PP, PS, and polyvinyl chloride (PVC), exhibit varying resistance to degradation due to differences in their molecular structures and additives. Breaking down of bigger plastic materials also produces more microplastics of irregular shapes, complicating the degradation process. These materials are also hydrophobic and thus favor the sorption of toxic pollutants, making the environmental and health hazards worse [21]. The traditional procedures used to treat the pollution, that is, physical collection, incineration, and landfill, have significant drawbacks, like being inefficient, too expensive, and releasing other types of pollution (resulting in greenhouse gases or leaches) [21,22,23,24]. Even though biodegradation has a theoretical future, its use is slowed down by low microbial activity and moot conditions in the environment. Similarly, the chemical degradation processes such as photodegradation and thermal oxidation face the same problem of the partial breakdown of the material, and the by-products that form could be dangerous [25,26,27].

1.2. Emerging Solutions: Photocatalytic Degradation

The use of solar irradiation as a novel method in photocatalytic degradation has become the solution to a sustainable and energy-saving approach in reducing the effects of microplastic pollution. In particular, the use of zinc oxide (ZnO) nanorods has significant potential due to their unique optical and structural properties [28,29]. ZnO is also a wide-bandgap semiconductor (3.37 eV) that works better at photocatalytic wavelengths between ultraviolet and near-visible wavelengths. Large surface areas and good charge separation properties of ZnO nanorods result in maximum light absorption and provision of more active sites to photocatalytic reactions, which makes them more effective than bulk ZnO [30]. The strong electron mobility of the ZnO nanorods overcomes the electron–hole recombination, which is one of the most common limitations in photocatalysis, and encourages the generation of reactive oxygen species (ROS), including hydroxyl radicals (HO) and superoxide anions (O₂⁻) responsible for polymer oxidation. These ROS play a major role in degrading microplastics into smaller, less harmful molecular structures. In addition, the quasi-one-dimensional structure of ZnO nanorods enhances photon collection and the transfer of large numbers of photons, thereby promoting strong interaction with microplastic particles [31,32]. Despite these benefits, ZnO-based photocatalysts face real-world challenges. Fast recombination of generated photo charges can limit its overall effectiveness, and the long-term stability can be reduced by long-term irradiation, causing photocorrosion. To overcome these difficulties, the incorporation of techniques including nitrogen-doped ZnO with transition metals, or the inclusion of it with other semiconductors, e.g., TiO₂ or graphene, has been researched to enhance charge separation and widen the photocatalytic to visible frequency range [33]. The surface functionalization and protective coating are also being sought after in order to increase the durability of ZnO nanorods in photocatalytic assemblies [34]. Current studies have focused on attaching ZnO nanorods to metal–organic frameworks (MOFs) to further enhance photocatalytic activity [33,35,36]. MOFs will also enhance surface areas and act as auxiliary light absorbers, thus complementing ZnO nanorods. Early research has shown that this hybrid design would significantly increase the rate of decomposition of microplastics under natural light [37,38,39,40]. In addition, the role of the MOF is structure-dependent, where photoactive MOFs act as sensitizers, while non-photoactive ones (e.g., ZIF-8) mainly improve adsorption and dispersion of microplastics. However, the direct experimental evidence for this mechanism in ZnO nanorods or MOF-based materials degradation systems remains limited in microplastics, and the proposed pathway is present as a plausible interpretation. Though there is laudable progress in treating the degradation of microplastics, there is an obvious need for more effective, sustainable, and scalable approaches to microplastic degradation. The synthesis of ZnO nanorod–photocatalytic systems offers an interesting platform for the exploitation of solar power and non-toxic catalysts to overcome the weaknesses of the traditional methods. The design, synthesis, and stability of these systems will be optimized and will form a central point in the practical implementation of the systems in the environmental remediation processes. This review will provide an all-encompassing evaluation of the prospects of ZnO nanorods and MOFs as next generation photocatalysts for the sustainable degradation of microplastics under sunlight. We examine their photocatalytic characteristics, mechanisms of degradation, environmental safety, and ecotoxicity, with special reference to the applicability in real-world settings. One of these is the synergistic improvement in the photocatalytic efficiency of ZnO nanorods and MOFs when they are coupled, thus making the difference between this work and the preceding studies that analyzed them separately. In addition, the comparative analysis with other extant photocatalysts is also provided to determine the best and most sustainable solutions for degrading microplastics. This review, unlike other traditional works that use artificial light sources, emphasizes the effectiveness of ZnO nanorods and MOFs under natural sunlight, thereby supporting the applicability for ecologically friendly operations. Issues related to the difficulties, future visions, and plans for large-scale environmental remediation are also discussed. By applying the principles of environmental safety, ecotoxicity, and sustainability, this review has assisted in the further development of scalable, efficient photocatalytic devices the address microplastic pollution.

2. Photocatalysis as a Promising Solution for Microplastic Degradation

2.1. Basics of Photocatalysis and Its Relevance in Environmental Remediation

Photocatalysis, a process driven by light and mediated by semiconductor materials, has emerged as a powerful method for environmental cleanup. Its proven effectiveness in breaking down persistent organic pollutants, dyes, toxic chemicals, and, more recently, microplastics, highlights its versatility. This technique leverages solar energy, a sustainable and renewable resource, making it an eco-friendly solution for addressing various pollution challenges. Among the semiconductor materials explored, titanium dioxide (TiO₂) and ZnO stand out for their favorable electronic properties, particularly their ability to absorb light and generate reactive intermediates that can initiate complex chemical reactions [41,42].

In this process, photons must have energy equal to or greater than the semiconductor’s bandgap. These photons excite electrons from the valence band to the conduction band. This leaves positive “holes” in the valence band. The excitation creates electron–hole pairs (e⁻/h⁺) that move to the photocatalyst surface. There, they take part in redox reactions with adsorbed molecules, such as oxygen and water. These reactions generate ROS like hydroxyl radicals (•OH), superoxide ions (O₂⁻•), and hydrogen peroxide (H₂O₂). These ROS are highly reactive and help break down complex organic compounds, including the polymers found in microplastics.

Photocatalysis offers several significant advantages for environmental applications. Utilizing solar energy reduces reliance on external power sources, thereby enhancing sustainability. Furthermore, photocatalysts like TiO₂ and ZnO are known for their chemical stability, non-toxicity, and reusability, making them cost-effective for prolonged use [42,43]. The degradation products, typically carbon dioxide (CO₂) and water (H₂O), are non-toxic, offering a cleaner alternative to traditional methods like incineration or chemical treatments, which often produce harmful by-products. This makes photocatalysis an attractive option for addressing microplastic pollution, offering both environmental and economic benefits.

2.2. Photocatalytic Mechanisms for Polymer Breakdown

ZnO nanorods exhibit limited visible light absorption due to their wide bandgap (3.37 eV). To address this, coupling ZnO with MOFs improves the utilization of visible light and enhances overall photocatalytic efficiency [44,45]. The degradation of microplastic polymers via photocatalysis involves oxidative pathways initiated by light-excited semiconductors. Upon absorbing photon energy, the semiconductor generates electron–hole pairs, which drive a series of chemical reactions that result in the breakdown of polymeric microplastics (Figure 2). The critical steps in this process include:

2.2.1. Photon Absorption and Charge Generation

Integrating MOFs with ZnO nanorods extends light absorption into the visible range. Under sunlight, MOFs generate excited electrons and holes that transfer to ZnO nanorods, facilitating charge separation:

* ZnO/MOF + hν → e⁻_CB + h⁺_VB

(1)

This effective charge transfer minimizes photoexcited carrier recombination and enhances overall photocatalytic performance under sunlight. Consequently, the photocatalytic behavior of ZnO/MOF systems is highly dependent on the specific MOF and its role, whether as a photosensitizer, adsorbent, charge-transfer mediator, or additional redox-active center within the polymer chain of the microplastic. Teng et al. [46] proposed a Type II charge-transfer mechanism for the NU66/ZnO composites. Photogenerated electrons (e⁻) travel from UN66 conduction band (CB) to ZnO CB, and photogenerated holes (h⁺) transfer from ZnO valence band (VB) to UN66 h⁺ under visible light. A Type II heterojunction was the result. After that, O₂ in the environment interacted with e⁻ to create O₂⁻ in the conduction band of NU66. After the active species O₂⁻ attacks MB/MG pollutants, they produce CO₂ and H₂O, thereby accelerating photodegradation. Similarly, the photocatalytic activity of ZnCDs/ZnO@ZIF-8 is due to the heterojunction and ZIF-8, which enhance electron transport and electron–hole pair separation under UV irradiation, exciting electrons from the excited state to ZnCDs/ZnO, converting O₂ to O₂⁻ and •O₂⁻ (ROS), and allowing holes at VB to contribute to the composites [47]. The ZMFe-3 catalyst generates electron–hole pairs under visible light irradiation. Due to the heterojunction structure, the electrons (e⁻) in the conduction band (CB) of ZnO recombine with the holes (h⁺) in the valence band (VB) of NH2-MIL-88B, resulting in efficient charge separation during the degradation process [48]. Similarly, the photocatalytic behavior of ZnO/MOF systems involves CBs and VBs interactions that promote photosensitizers, redox reactions, and charge-transfer during the degradation process [49,50,51].

The photocatalytic degradation pathways of microplastics differ significantly depending on polymer structure. For polyolefins such as PE and PP, degradation typically initiates with C–H bond activation at the polymer backbone. Reactive oxygen species, particularly •OH, abstract hydrogen atoms to generate alkyl radicals, which subsequently react with oxygen to form peroxy radicals and hydroperoxides. These intermediates undergo chain scission, producing smaller oxygenated compounds such as alcohols, aldehydes, and carboxylic acids. In other words, aromatic polymers such as PS and PET exhibit different degradation behavior due to the presence of aromatic rings and functional groups. In PS, oxidation can occur at the benzylic position, facilitating chain scission, while in PET, photocatalytic degradation often involves cleavage of ester bonds in addition to oxidative processes. The presence of conjugated structures may also influence light absorption and electron transfer, thereby altering degradation pathways compared to those of polyolefins. Consequently, C–H bond activation plays a key role in initiating degradation in polyolefins, whereas aromatic polymers undergo more complex pathways involving both backbone oxidation and functional group cleavage. These differences highlight the importance of polymer-specific considerations when evaluating photocatalytic degradation mechanisms.

2.2.2. Generation of ROS

The photoexcited electrons in the conduction band of ZnO reduce adsorbed oxygen molecules, forming superoxide radicals (O₂⁻•), while the holes in the valence band simultaneously oxidize water molecules to produce hydroxyl radicals (•OH):

$$* O_2+e_{CB}^{-}\to O_2^{•}$$

(2)

* H₂O + h⁺_(VB) → •OH + H⁺

(3)

These reactive oxygen species, especially hydroxyl radicals, play a crucial role in initiating the oxidative degradation of microplastic polymers.

2.2.3. Oxidative Attack on Microplastics

ROS, mainly •OH, attack microplastic carbon backbones, breaking bonds and reducing molecular weight to smaller fragments and intermediates. Continued ROS activity further breaks them down.

2.2.4. Photoinduced Chain Scission

Beyond ROS-mediated oxidation, direct light absorption by the MOF-ZnO system induces polymer chain scission. This mechanism disintegrates microplastics, such as PE and PS, into smaller fragments, thereby exposing additional surface area for further ROS-driven degradation.

2.2.5. Complete Mineralization

Sustained photocatalytic activity under sunlight can eventually lead to the full mineralization of microplastics. This involves breaking down residual polymer fragments into non-toxic products, such as CO₂, H₂O, and inorganic salts, effectively mitigating environmental pollution.

2.3. Key Factors Influencing Photocatalytic Efficiency

The effectiveness of photocatalytic degradation depends on a number of things, such as the kind of catalyst, the wavelength and intensity of the light source, its shape, and how it interacts with microplastics. ZnO nanorods work very well because they have a large surface area, can separate charges, and generate numerous ROS when exposed to UV light. But their large bandgap makes them less effective in visible light. To solve this problem, hybrid solutions like ZnO-MOF composites are being developed. MOFs process tunable structure and a large surface area, which enhance their light absorption ability, charge separation efficiency, and charges transfer properties [52]. These characteristics make ZnO/MOF composites very effective photocatalysts that help break down microplastics that do not break down naturally [53].

2.4. Expanding Photocatalytic Materials

In addition to ZnO, researchers are also examining other photocatalysts, such as TiO₂ and graphitic carbon nitride (g-C₃N₄), to determine whether they can break down microplastics when exposed to sunlight [54,55,56,57,58]. To enable photocatalytic activity in the visible spectrum, scientists are exploring strategies such as altering the surface and combining different photocatalysts, including ZnO-MOF composites [59,60]. These improvements are very important for scaling up photocatalytic devices and for their use in large-scale environmental cleanup projects.

2.5. Determination and Analysis of Microplastics in Water

The accurate evaluation of photocatalytic degradation of microplastics requires reliable methods for their identification and quantification. Spectroscopic techniques such as Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy are widely used to identify polymer types and monitor chemical changes during degradation. FTIR, particularly in ATR or micro-FTIR modes, enables rapid identification of common polymers such as PE, PP, PS, and PET, while Raman spectroscopy provides higher spatial resolution and is suitable for smaller particles (<10 µm) [61,62,63].

These spectroscopic methods are often combined with microscopy techniques, including scanning electron microscopy (SEM), to evaluate morphological changes such as surface cracking, fragmentation, and particle size reduction during photocatalytic treatment. In ZnO-based systems, degradation is frequently assessed using complementary indicators such as the carbonyl index (CI), weight loss measurements, and total organic carbon (TOC) analysis, which provide insight into oxidative degradation and partial mineralization processes [64,65]. Furthermore, chromatographic techniques such as gas chromatography–mass spectrometry (GC–MS) or liquid chromatography–mass spectrometry (LC–MS) are employed to identify intermediate degradation products, including aldehydes, ketones, and carboxylic acids. These analyses are essential for understanding degradation pathways and confirming that microplastic breakdown proceeds through stepwise oxidation rather than immediate mineralization. Consequently, integrating spectroscopic identification, morphological characterization, and chemical analysis is essential for accurately evaluating photocatalytic degradation efficiency and mechanisms in microplastic systems.

3. ZnO Nanorods in Photocatalysis

3.1. Properties and Synthesis Methods of ZnO Nanorods

ZnO nanorods are well-known one-dimensional nanostructures with unique properties that make them highly useful in photocatalytic applications. Their long shape helps charge carriers move quickly along the rod axis, reducing the recombination of photoexcited electrons and holes. This is an important feature for improving photocatalytic activity [66]. Because ZnO has a wide bandgap (3.37 eV), it is an UV-active semiconductor. Its high exciton binding energy (60 meV) also allows excitons to form even at ambient temperature, making it even more efficient as a photocatalyst. The large surface area-to-volume ratio of ZnO nanorods also provides many active sites for surface reactions, making them well suited for photocatalytic oxidation and reduction processes [33]. Additionally, several ways to make ZnO nanorods (see Figure 3) have been developed to manage their shape, size, and surface properties, making them better for certain photocatalytic uses. Some important ways to make these are:

Hydrothermal Synthesis: An economical, low-temperature technique for fabricating ZnO nanorods, allowing precise regulation of parameters such as reaction temperature, precursor concentration, and growth duration. This method is particularly suited for large-scale production, providing versatility in tailoring nanorod properties for targeted applications [67,68,69].

• Chemical Vapor Deposition (CVD): This very precise method makes ZnO nanorods with the same size and better crystallinity by depositing vapor-phase precursors at high temperatures. CVD is effective for producing high-quality nanorods with superior structural characteristics, even though it requires substantial resources [70,71,72,73].
• Electrochemical Deposition (ED): This energy-efficient approach is good for making ZnO nanorods on conductive substrates. It allows the control of factors such as voltage and electrolyte composition, leading to nanostructures with large surface areas and enhanced catalytic efficiency [74,75,76,77].

These methods can fine-tune the features of ZnO nanorods, such as their aspect ratio, crystallinity, and defect density, to make them work better for things like cleaning up the environment and turning energy into electricity.

3.2. Applications of ZnO Nanorods in Environmental Photocatalysis

ZnO nanorods show considerable promise in environmental photocatalysis, especially for degrading organic contaminants in water and air. Their large surface area lets pollutants adhere easily. When exposed to UV radiation, they generate ROS such as hydroxyl radicals (•OH) and superoxide anions (O₂⁻•). These ROS help break down toxic chemicals, dyes, and other harmful compounds into less hazardous or fully mineralized products. Some important uses are:

• Water Treatment: ZnO nanorods break down POPs, including medicines, insecticides, and dyes, that are often present in wastewater, turning them into safe by-products [53,78].
• Air Purification: ZnO nanorods are used in photocatalytic systems to remove VOCs and other air pollutants, thereby improving air quality [79].

Figure 4a shows the length-to-diameter ratio of the ZnO nanorods as 3.2 µm and 94 nm, which is an advantage in improving photocatalytic activity. Photocatalytic degradation of low-density PP microplastics (particle size 300 nm) was measured by means of a 100 mL batch photoreactor (27 particles, which was given an approximation of 26 mg) suspended in natural solar irradiation during a total time of 196 h [80]. The degradation of fragmented microplastics, especially low-density polyethylene (LDPE) films in water using visible light-induced plasmonic photocatalysts comprising platinum nanoparticles deposited on zinc oxide nanorods (ZnO–Pt), is shown in Figure 4b [81]. In Figure 4c, the LDPE microplastic residues are degraded through heterogeneous photocatalysis by visible light with the use of ZnO nanorods. The increased plasmonic interfaces led to greater interfacial exciton separation and hydroxyl radical production, as well as a 78% increase in visible light absorption. The LDPE films were photocatalyzed using various catalysts, and the indices of carbonyl (CI) and vinyl (VI) were demonstrated. The larger the catalyst, the greater the photocatalytic performance, as evidenced by a 30% increase in both CI and VI for the larger ZnO nanorods [82]. Figure 4d demonstrates the visible light photocatalytic degradation of the microplastics of PP by providing a flow-through photocatalytic reactor with ZnO nanorods coated on glass fibers. In total, 2 weeks of constant photocatalytic processing at visible wavelengths diminished the average volume of particles of the PP microplastics by an estimated 65%. Various health organizations (HSDB, IARC, and NIH) assert that the by-products detected in water samples following the photodegradation of PP microplastics in this research pose low toxicity to humans and aquatic life [83]. Moreover, T. S. Tofa [84] reported the design of visible light-responsive photocatalysts based on zinc oxide nanorods (ZnO NPs) and platinum nanoparticles loaded onto the modified surface of ZnO nanorods (Pt-ZnO NPs) for photocatalytic applications. The optimal deposition time for Pt nanoparticles was determined, resulting in photodegradation efficiencies of about 38% under UV irradiation and 16.5% under visible light irradiation.

3.3. Photocatalytic Degradation of Organic Pollutants

Photocatalysis enables ZnO nanorods to degrade a broad range of organic contaminants. When exposed to UV light, ZnO nanorods absorb photons with energy greater than their bandgap, promoting electrons to the conduction band and creating electron–hole pairs [85,86]. These charge carriers make redox reactions happen on the surface of the nanorods:

• Electrons (e⁻): These change molecular oxygen into superoxide anions (O₂⁻•).
• Holes (h⁺): These oxidize water molecules or hydroxyl ions (OH⁻, negatively charged oxygen–hydrogen ions) to make hydroxyl radicals (•OH, highly reactive forms of oxygen), which are strong oxidants that start the destruction of pollutants. The photocatalytic process includes the following.
• Adsorption of Pollutants: Organic pollutants stick to the surfaces of ZnO nanorods. They interact with the ROS that are generated during photocatalysis.
• Oxidative Degradation: ROS break chemical bonds (such as C–C and C–H), turning contaminants into smaller, less harmful intermediates.
• Mineralization: When ROS are present for a long time, they turn pollutants into harmless end products. These include CO₂ and H₂O.
• Dyes: ZnO nanorods efficiently degrade methylene blue, rhodamine B, and methyl orange into simpler molecules [87,88].
• Pesticides and Herbicides: ZnO nanorods degrade agrochemicals such as atrazine and 2,4-D under UV irradiation [89,90].
• Medicines: ZnO nanorods are proven to work in breaking down pollutants, including antibiotics and anti-inflammatory medications [91].

Recent developments in hybrid photocatalytic systems, including the integration of ZnO with graphene oxide or MOFs, have markedly enhanced photocatalytic efficacy, particularly in the presence of visible light [92]. These new ideas make ZnO nanostructures more useful for cleaning up the environment on a broad scale.

4. MOFs in Photocatalysis

4.1. Structure, Properties, and Tunability of MOFs

MOFs are a special class of porous materials with highly organized three-dimensional structures. They comprise metal ions or clusters bound to organic ligands. This complex structure endows MOFs with remarkable features, including very large surface areas, tunable porosity, and a wide range of chemical functionalities. Because of these qualities, MOFs can be used in many different ways, but they are particularly useful in photocatalysis [93,94,95]. MOFs can be tuned by choosing alternative metal centers and organic ligands. This allows frameworks to be created with specific characteristics for specific photocatalytic applications [37]. Researchers may alter key parameters such as bandgap energy, surface area, and structural stability under different environmental conditions by employing different metal ions, including zinc (Zn), copper (Cu), or zirconium (Zr), and varying the organic linkers [96]. Post-synthetic alterations enhance these qualities and allows MOFs to be tuned for specific photocatalytic processes, making them more useful for cleaning the environment and converting energy into usable forms.

4.2. Applications of MOFs in Photocatalysis

MOFs are well known for their potential in photocatalytic processes because they have a large surface area and a porous structure that facilitates the adsorption of reactants and improves light absorption. These materials have been used in diverse photocatalytic processes, encompassing water splitting, CO₂ reduction, and the destruction of organic contaminants [97,98]. MOFs have shown significant potential for degrading pollutants in air and water as part of environmental cleanup [99]. Their porous nature makes it easy for contaminants to stick to them, and the metal centers in the frameworks function as catalytic sites for redox processes. When exposed to light, MOFs produce ROS, which are necessary for converting harmful organic pollutants into safer forms. When mixing MOFs with other photocatalysts, such as ZnO or TiO₂, their photocatalytic performance can be improved by increasing their light absorption and enhancing their performance in real-world environmental applications [100,101].

In Figure 5a, the optimized NH2-MIL-53(Fe/Ti) (1.5:1) showed a better degradation with respect to norfloxacin (NOR) under the irradiation of visible light, which is attributed to the electron exchange between Fe³⁺ and Ti⁴⁺ metal centers. Effortless NOR degradation efficiency of 84.6% was realized at the point after 120 min of the irradiation [102]. Figure 5b shows that, as the bias was applied, the catalytic efficiency of the MOF-based porous Fe₂O₃ reached a maximum degradation rate of 0.438 at 1.2 V under sunlight irradiation with an intensity of 120 mW cm⁻¹. It is worth noting that the catalytic performance was not significantly reduced after four cycles [103]. Figure 5c shows that in situ deposition of MoS₂ onto NH₂-MIL-88B(Fe) yields a high-performance heterostructured photocatalyst. The composite successfully degraded microplastic debris and HDPE films in water, as well as tetracycline (TC) hydrochloride, achieving 96.44% degradation and 72.36% salinity reduction. The heterojunction increased the absorption of light, charge separation, carrier lifetime, and surface area. Photocatalysis was also shown to cause a significant reduction in TC toxicity, as confirmed by liquid chromatography–mass spectrometry (LC-MS) to identify degradation intermediates and pathways. The NH₂-MIL-88 B(Fe)/MoS₂ has tremendous remediation potential in the environment due to its increased ability to form reactive species [104]. In Figure 5d, Ag₂O nanoparticles of an average size of around 6 nm incorporated in Fe-MOF had an extensive solar light collection and a high quantity of active sites, as. Ag₂O/Fe-MOF, Ag₂O, and Fe-MOF catalysts were used to photocatalytically convert PE and PET microplastics. The findings established that the PE and PET weight loss values achieved with Ag₂O/Fe-MOF (78.0 mg of PE and 24.7 mg of PET). Than the values achieved using pure Ag₂O, and were 1.7× and 2.4× higher than the values achieved using Fe-MOF. Ag₂O/Fe-MOF yielded higher H₂ evolution rates of 1.7 mmol g⁻¹ h⁻¹ and 1.9 mmol g⁻¹ h⁻¹ in PE and PET systems, respectively, relative to pristine Fe-MOF, and produced low levels of H₂ [105].

Additionally, Jiang et al. [106] synthesized the soluble BiOCl-OH semiconductor-organic framework (BOCH-SOF) of xylitol, and reported the influence of PS microplastics on the photocatalytic degradation of TC. This was the result of enhanced light absorption in the BOCH-SOF/PS system, with the synergistic effect of PS and TC interaction, and subsequent TC degradation and PS aging occurring simultaneously. Rincón et al. [107] presented an enzyme-based CrL-MOF, consisting of the enzyme Candida rugosa lipase (CrL), as a new approach to degrading plastics in water while retaining the catalyst recyclability. The most prevalent degradation product of PET, bis-(hydroxyethyl) terephthalate (BHET), was chosen as a model contaminant. In aqueous conditions, CrL-MOFs eliminated 37% of BHET in 24 h through two complementary stages: enzymatic degradation of CrL and adsorption of the degradation products onto the MOF framework. To obtain high surface area concentration, Haris et al. [108] have reported that MOF nanosheets effectively inhibited aggregation. In addition, the C@FeO nanopillars were found to be magnetic, thus allowing the stamping of the adsorbent once the water was used. It was a highly efficient conversion of 71.7% for the microplastic concentration up to 1000 mg L⁻¹ in 60 min.

4.3. MOFs for Microplastic Degradation: Current Research

Research into the potential of MOFs to degrade microplastics has increased as concerns about microplastic pollution have grown. Research has shown that MOFs can effectively adsorb and degrade microplastics under various types of light. Their distinctive porosity structures and reactive sites make it easier for ROS to form. ROS are important for breaking down the long polymer chains that make up microplastics [38,107,109,110,111,112]. Recent attempts are being made to optimize the synthesis of MOFs so that they can better break down microplastics [38,113]. Methods such as adding metal nanoparticles or combining MOFs with other materials have shown promise, especially for improving performance under visible light. Research has also focused on understanding how MOFs degrade microplastics, including identifying intermediate degradation products and the pathways leading to mineralization [114,115,116]. The sequential reaction process of microplastic degradation enabled by MOFs under sunshine exposure is as follows.

Step 1—Exposing the MOF to sunlight: The MOF’s surface is first hit by sunlight, and the photons that are absorbed provide the photocatalytic process energy. The MOF is a photosensitizer that separates charges. It is made up of metal nodes and organic linkers.

Step 2—Producing photoinduced charge carriers: In photoactive MOFs, light absorption can induce ligand-to-metal or metal-to-ligand charge-transfer processes, generating excited electrons and holes; however, not all MOFs exhibit semiconductor-like behavior, and many primarily function as adsorptive or structural components in photocatalytic systems. This generates reactive charge carriers, including positively charged holes in the valence band and excited electrons in the conduction band. These are very important for photocatalytic degradation.

Step 3—Adsorption and reaction with microplastics: Adsorbed microplastic particles interact with the charge carriers generated by light. Reactive species, such as superoxide anions (O₂•⁻), formed when oxygen is reduced by electrons, and hydroxyl radicals (•OH), generated during oxidation events, target the chemical bonds in microplastic polymers, initiating oxidative deterioration.

Step 4—Breaking down into smaller molecules: In the last step, the large microplastic macromolecules break down into smaller, less hazardous substances, such as low-molecular-weight organic molecules or mineralized products like CO₂ and H₂O. The arrows indicate the progression of the photocatalytic process, in which MOF catalyzing the complete degradation of microplastics via a series of redox reactions.

This process shows how the MOF may use solar energy to speed up photocatalytic reactions that break down long-lasting microplastics into safe compounds for the environment.

5. Synergistic Effects of ZnO Nanorods and MOFs

The ZnO nanorods are combined with MOFs in composite photocatalysts; the two materials interact synergistically, leading to enhanced photocatalytic efficiency [35,117]. The high specific surface area and tunable pore architectures of MOFs complement the effective electron–hole separation and photocatalytic reactivity of ZnO nanorods. This facilitates the adsorption of pollutant molecules and increases the generation of ROS. This integration not only improves photocatalytic degradation processes but also broadens the photoresponse range, making the material more suitable for environmental remediation. ZnO nanorod–MOF composites also offer high adsorption capacity for organic pollutants and improved solar light utilization due to their large surface area and adjustable porosity. Under UV irradiation, ZnO nanorods generate reactive oxygen species, which contribute to their photocatalytic function. MOFs also provide additional catalytic sites for redox reactions and enhance the structural stability of the composite. These combined properties facilitate charge carrier separation, inhibit electron–hole recombination, and extend photon absorption into the visible region [118,119]. Furthermore, the morphology further governs surface area, light absorption, and charge transport pathways. One-dimensional structures such as ZnO nanorods facilitate directional electron transport and reduce recombination, while porous or hierarchical structures increase surface area and improve contact with microplastic particles. In ZnO/MOF composites, the porous framework of MOFs enhances adsorption of hydrophobic microplastics, increasing local reactant concentration and improving interfacial charge transfer. Moreover, the formation of heterojunctions between ZnO and photoactive components can significantly enhance photocatalytic efficiency by promoting charge separation. However, the overall performance is determined by the interplay between structural properties, morphology, and reaction conditions, highlighting the need for rational design of photocatalysts tailored to microplastic degradation.

Research indicates that these hybrid materials exhibit superior photocatalytic activity compared to their separate constituents. Adding ZnO nanorods to MOFs increases the production of hydroxyl radicals and superoxide anions, which are important for oxidizing microplastics. Moreover, the customized production of ZnO nanorod–MOF composites enhances their structural and electrical characteristics, hence improving photocatalytic activity.

Mechanistic Insights—How ZnO Nanorods and MOFs Improve Microplastic Degradation

The synergistic effects in ZnO nanorod–MOF composites arise from essential molecular connections. The large surface area and porous structure of MOFs make it easier for microplastics to adhere to them. This raises the concentration of pollutants on the surface of the composite and speeds up the breakdown process. When light hits ZnO nanorods, they create photoexcited electrons and holes that cause ROS to form on the surface of the hybrid material.

The metal centers of MOFs also serve as additional catalytic sites, accelerating redox reactions and generating more ROS. The ROS initiate gradual oxidation at the polymer surface, particularly at defect sites or pre-existing functional groups, leading to chain scission and the formation of oxygenated intermediates; this process progressively enhances the susceptibility of microplastics to further degradation rather than causing immediate breakdown. Also, the structural stability of MOFs maintains the ZnO nanorods’ photocatalytic activity, prolonging the composite’s lifespan and improving its performance. Combining ZnO nanorods with MOFs is a promising approach to enhance photocatalytic activity, particularly for degrading microplastics. To find effective ways to address microplastic contamination, we need to understand how these hybrid materials work and improve the production. Current research in this domain has the potential to provide substantial progress in photocatalysis and sustainable environmental cleanup initiatives [120,121].

Figure 6 shows the step-by-step reaction process that leads to the faster breakdown of microplastics when ZnO nanorods and MOFs operate together under sunlight:

Step 1—Shine sunlight on ZnO nanorods and MOFs: The process starts when ZnO nanorods and MOFs, which are photocatalysts, absorb sunlight. The photons absorbed provide these materials with the energy they need to excite electrons, which initiates the photocatalytic process.

Step 2—Producing photoinduced charge carriers: When photons are absorbed, electrons in both ZnO and MOFs move from the valence band to the conduction band, producing electron–hole pairs (e⁻ and h⁺).

Step 3—Charge transfer between ZnO and MOFs: The photoexcited electrons and holes transfer between ZnO and MOFs because their electronic properties are well matched. This charge transfer at the interface makes it easier for electrons and holes to separate, reducing recombination and boosting photocatalytic performance.

Step 4—Redox reactions with microplastics adsorbed: Microplastic particles adsorbed to the surfaces of ZnO nanorods and MOFs interact with the charge carriers generated by light. Electrons and holes drive redox reactions that produce highly reactive species such as hydroxyl radicals (•OH) and superoxide anions (O₂•⁻). These molecules break down the chemical bonds in the microplastic polymers.

Step 5—Breaking down into smaller, less dangerous molecules: The microplastics undergo progressive photocatalytic oxidation, which leads to the formation of intermediate products such as oligomers, alcohols, aldehydes, and carboxylic acids, with complete mineralization to CO₂ and H₂O, representing an ideal but not always achieved redox process or organic molecules with low molecular weight. This phase turns microplastic trash that does not break down into materials that are good for the environment.

When ZnO nanorods and MOFs are exposed to sunlight, they work together to improve photocatalytic degradation by making charge separation more efficient and by making it easier for redox reactions to happen one after the other. This breaks down complicated microplastic polymers.

6. Photocatalytic Mechanisms Under Sunlight

6.1. Sunlight-Driven Photocatalytic Degradation Pathways

Photocatalysis driven by sunlight has become a viable approach for environmental cleanup, leveraging abundant solar energy. ZnO nanorods are primarily active under UV irradiation due to their wide bandgap (~3.2–3.37 eV); therefore, their performance under sunlight is limited unless modified (e.g., MOFs via coupling with photoactive materials or defect engineering) to enhance visible light utilization and charge separation. Photocatalysts such as ZnO nanorods and MOFs can break down organic contaminants, including microplastics, using both ultraviolet (UV) and visible light. The photocatalytic process that uses sunlight operates in two main ways: it produces ROS, and it directly transfers photoinduced electron–hole pairs to pollutant molecules. Photons from sunshine move electrons from the valence band to the conduction band of the photocatalyst. This leaves behind positively charged holes in the valence band. These pairs of electrons and holes then undergo redox reactions, producing ROS such as hydroxyl radicals (•OH) and superoxide anions (•O₂⁻). These disrupt the chemical bonds in pollutant molecules.

6.2. Role of UV/Visible Light in Activating ZnO Nanorods and MOFs

ZnO nanorods work well under UV light because photons with energies higher than their bandgap excite electron–hole pairs [122,123]. Because of its large bandgap, ZnO’s photocatalytic activity is poor under visible light. To solve this problem, ZnO nanorods can be added to MOFs to enable them the absorb visible light. MOFs are good at collecting visible light and transferring excited electrons to ZnO nanorods. Overall, this makes them better photocatalysts [33,35,124,125,126]. This hybrid system not only increases the range of light it can absorb but also improves charge separation by reducing the rate of electron–hole recombination. This improves the photocatalytic performance in sunlight.

6.3. Kinetics of Degradation and Intermediate Products

The pace of photocatalytic degradation is affected by things like the production of ROS, the adsorption of pollutants onto the surface of the photocatalyst, and the prevention of electron–hole recombination [127,128,129,130,131]. Due to their large surface area and improved charge separation, ZnO nanorod–MOF composites degrade more rapidly. Usually, degradation follows pseudo-first-order kinetics, meaning the amount of pollutant decreases exponentially over time [132]. It takes many stages for complex pollutants, such as microplastics, to break down completely into non-toxic end products, such as CO₂ and H₂O [133,134]. The first step is oxidative breakage of polymer chains, which produces smaller pieces and intermediates such as alcohols, carboxylic acids, and aldehydes. Long-term exposure to ROS causes these intermediates to undergo further oxidation, ultimately leading to mineralization. It is important to understand these degradation pathways and identify intermediate products to improve photocatalytic systems that efficiently break down materials without producing dangerous by-products. Also, photocatalytic processes that use ZnO nanorods and MOFs powered by sunlight are a good way to break down long-lasting contaminants in the environment. The ZnO-MOF hybrid structure works much better as a photocatalyst when both UV and visible light are used to activate it. Understanding the kinetics and paths of degradation further makes it clear that these systems may be used to create improved, long-lasting photocatalytic technologies for use in the environment.

Reaction Kinetics of Photocatalytic Microplastic Degradation: The kinetics of photocatalytic microplastic degradation are commonly described using pseudo-first-order and Langmuir–Hinshelwood (L–H) models, which are widely applied to heterogeneous photocatalytic systems. In many ZnO-based studies, the degradation process follows pseudo-first-order kinetics, as seen in Equation (4):

$$In \left({\displaystyle \frac{C_0}{C}}\right)=kt$$

(4)

where C₀ and C represent the initial and residual concentrations (or relative mass/oxidation indices) of microplastics; k is the apparent rate constant; and t is the irradiation time. This simplified model is frequently used to describe ROS-driven oxidation processes in microplastic degradation systems under UV or visible light irradiation.

For systems involving adsorption–reaction interactions, such as ZnO/MOF composites, the Langmuir–Hinshelwood model provides a more realistic description with Equation (5):

$$r={\displaystyle \frac{kKC}{1+KC}}$$

(5)

where r is the reaction rate, k is the intrinsic reaction constant, and K is the adsorption equilibrium constant. This model highlights the critical role of surface adsorption, particularly for hydrophobic microplastics, where MOFs can enhance local concentration and facilitate interfacial charge transfer.

Recent studies on photocatalytic microplastic degradation have confirmed that kinetic behavior is strongly influenced by catalyst properties, light intensity, and polymer type, with apparent rate constants often reflecting combined effects of adsorption, surface oxidation, and fragmentation rather than true bulk mineralization [39,48,135]. In practice, kinetic evaluation in microplastic systems is often based on indirect parameters such as carbonyl index (CI), weight loss, or total organic carbon (TOC) removal. Therefore, the derived rate constants should be interpreted as apparent values that capture the overall degradation behavior, including intermediate formation and partial oxidation.

7. Comparative Analysis of ZnO Nanorods, MOFs, and Other Photocatalysts

Photocatalysts, such as ZnO nanorods, MOFs, and other semiconductor materials such as TiO₂, cadmium sulfide (CdS), and graphitic carbon nitride (g-C₃N₄), play a significant role in pollutant degradation [55,136,137]. The photocatalytic performance of these materials is determined by their structural features, charge-transport behavior, and optical characteristics.

• ZnO nanorods: ZnO nanorods are commonly utilized photocatalysts owing to their large surface area, efficient charge transfer, and high oxidative potential. ZnO, a wide bandgap semiconductor (~3.2–3.37 eV), reacts to UV radiation by creating electron–hole pairs, which contribute to the creation of ROS. The one-dimensional (1D) nanorod shape promotes directed charge transport while minimizing recombination losses. However, its large bandgap reduces visible light absorption, limiting its total solar-driven efficiency [137,138].
• MOFs: MOFs are porous materials made up of metal nodes and organic linkers, with a large surface area and adaptable topologies. Although not all MOFs are inherently photoactive, certain systems demonstrate photocatalytic activity via ligand-to-metal charge-transfer pathways. MOFs may also operate as effective adsorbents and charge-transfer mediators when combined with semiconductor materials, improving interfacial interactions and pollutant degradation efficiency. Their structural tunability enables the creation of materials that can absorb both UV and visible light. However, MOFs may have low structural stability when exposed to severe circumstances such as aqueous environments, high pH, or extended irradiation [139,140].
• Other semiconductor photocatalysts: Several semiconductor photocatalysts, including TiO₂, CdS, and g-C₃N₄, have been intensively studied. TiO₂ is a well-studied photocatalyst because to its chemical stability, cheap cost, and high oxidative activity. However, like ZnO, it is most active under UV light due to its broad bandgap [141]. CdS has improved visible light absorption but suffers from photocorrosion and low stability. Although g-C₃N₄ has received attention as a visible light-responsive photocatalyst, its high electron–hole recombination rate restricts its efficiency, requiring future modification techniques to increase performance [142,143,144,145,146,147,148,149,150,151].

Advantages and Limitations of ZnO Nanorods and MOFs in Microplastic Degradation

ZnO nanorods and MOFs have several advantages for breaking down microplastics using light, but they also have certain problems.

Table 1. Comparison of ZnO nanorods, MOFs, and other semiconductor photocatalysts (TiO₂, CdS, g-C₃N₄) for the solar-driven photocatalytic breakdown of pollutants, listing their pros and cons, along with key references.

Photocatalyst	Advantages	Limitations	Citations
ZnO Nanorods	High UV photocatalytic activity due to wide bandgap (~3.37 eV). 1D structure enhances charge carrier mobility and reduces recombination. Cost-effective synthesis, suitable for large-scale applications.	Limited absorption in the visible spectrum. Prone to photocorrosion under prolonged UV exposure.	[152,153]
MOFs	Tunable bandgap for absorption of both UV and visible light. High surface area and porosity for efficient pollutant adsorption. Flexibility in design, allowing integration with other catalysts.	Stability issues under moisture and high temperatures. Complex and time-consuming synthesis.	[154,155,156,157,158]
TiO2	Excellent chemical stability. High UV activity. Low cost and readily available.	Limited visible light absorption due to large bandgap (~3.2 eV).	[159,160,161]
CdS	Visible light absorption. Higher photocatalytic activity under visible light compared to TiO2.	Photocorrosion issues. Toxicity concerns.	[162,163]
g-C3N4	Visible light absorption. Metal-free, non-toxic.	High electron–hole recombination rate. Requires modifications for enhanced efficiency.	[164,165,166,167]

summarizes the most important performance variables for each material. This makes it easier to compare key parameters relevant to photocatalytic degradation, especially when treating microplastics and other pollutants. ZnO nanorods and MOFs each have their own strengths. ZnO works well in UV-driven processes, while MOFs are more flexible and can be tailored to operate across a wider range of solar spectra. Combining these materials may address their respective problems, such as ZnO’s low activity under visible light and MOFs’ stability issues. This can lead to better photocatalytic efficiency for more complex applications, such as breaking down microplastics. To develop sophisticated photocatalysts for long-term environmental cleanup, it is important to understand the pros and cons of each material [121,122,123,124,125,126,127,128,129,143,145,146,147,148].

Table 2. Comparison of photocatalysts for breaking down organic compounds and microplastics under different types of light, along with their photocatalytic efficiency.

Photocatalyst	Target Organic Compounds/ Microplastics	Light Source	Photocatalytic Efficiency (High/Moderate/Low)	Main Findings	Citations
ZnO Nanorods	PE Microplastics	UV Light (365 nm)	Moderate	High degradation efficiency due to strong ROS generation but limited by UV absorption only. Photocorrosion after extended use.	[81,82,168]
TiO2 (P25)	PS Microplastics	UV Light (365 nm)	Moderate	Efficient under UV light with high stability, but suffers from a large bandgap, limiting visible light activity.	[169]
ZnO/Ag Nanocomposite	Dyes (Methylene Blue)	Visible Light (450 nm)	High	Enhanced photocatalytic activity due to silver nanoparticle plasmonic effects, improving visible light absorption and reducing electron recombination.	[170,171]
g-C3N4	PP Microplastics	Sunlight (Natural)	Low	Visible light absorption with low recombination rates, but lower overall efficiency compared to UV-driven processes. Requires surface modification.	[172]
CdS/TiO2 Heterojunction	PET	Visible Light (450 nm)	Moderate	Improved charge separation due to heterojunction, enabling visible light activity, but suffers from photocorrosion and toxicity of CdS.	[173,174,175]
ZnO Nanorods/Graphene Oxide	PE Microplastics	Solar Light (Full Spectrum)	High	Synergistic effect of graphene oxide improves charge separation and expands light absorption into visible range, enhancing overall performance.	[92,176,177,178,179]
MOF (MIL-53)	PVC Microplastics	Visible Light (420 nm)	Low	Tunable light absorption due to metal–organic framework structure, with high surface area and adsorption properties. Stability is a concern.	[124,180,181]
BiVO4	Pharmaceuticals (Antibiotics)	Visible Light (450 nm)	Moderate	Good visible light absorption and stability, with relatively low recombination rates. Efficiency improves with co-catalysts.	[182,183,184,185,186]

compares several photocatalysts, such as ZnO nanorods, TiO₂, g-C₃N₄, CdS, and MOFs, for breaking down organic pollutants and microplastics when exposed to different types of light, such as UV, visible light, and sunshine. How well any material works as a photocatalyst relies on how well it absorbs light, how well it makes ROS, and how well it separates charges. ZnO nanorods and TiO₂ are more effective under UV light because they have broad bandgaps. On the other hand, g-C₃N₄ and CdS are more effective under visible light because they have narrow bandgaps. MOFs can absorb a lot of light because their porosity and flexibility can be tuned, though they need to be further optimized to improve stability and long-term performance. This study shows how important it is to match the properties of photocatalysts to light sources and target contaminants to break them down effectively. This information is useful for scaling these materials for use in environmental cleanup.

8. Environmental Implications and Safety Considerations

Despite the considerable potential of ZnO nanorods and MOFs in the degradation of environmental contaminants, it is important to evaluate the fate of degradation products to determine ecological concerns [144,187]. During photocatalytic degradation, smaller intermediates, such as short-chain hydrocarbons, alcohols, aldehydes, and carboxylic acids, are produced before total mineralization into CO₂ and H₂O. However, incomplete breakdown might result in deleterious by-products, such as oligomers or low-molecular-weight molecules, which may persist in the environment or provide toxicity hazards [188,189]. To track intermediates and assess the environmental impacts, advanced analytical methods are needed, such as GC-MS and high-performance liquid chromatography (HPLC). There are also concerns about the potential release of photocatalysts, such as ZnO nanorods and MOFs, into ecosystems. Nanoparticles may harm living things in water and on land because they are reactive, mobile, and can accumulate in living organisms. To reduce environmental footprints, it is important to regulate catalyst stability, recyclability, and reuse [190,191]. Life cycle analysis (LCA) provides a comprehensive framework for assessing the sustainability of ZnO nanorod- and MOF-based photocatalysis by accounting for energy consumption, emissions, and environmental impacts from manufacturing to disposal [192,193]. The environmental effects of ZnO nanorods depend on how they are made, such as hydrothermal or sol–gel processes, which use a lot of energy and produce waste. To make them more sustainable, it is important to find ways that use less energy, avoid toxic materials, and work more effectively [194]. MOFs are difficult to work with because they involve organic solvents and metal precursors, which can be harmful to the environment. But new developments in green chemistry, such as solvent- and water-free syntheses, have made them more sustainable. Post-synthetic alterations and hybrid catalyst designs may extend MOF lifespans, reducing the need for replacement and their environmental impact [195]. Photocatalysis powered by sunlight is a low-energy alternative to conventional ways of treating pollutants. However, it is crucial to consider other environmental issues, such as the source of the materials, the efficiency of the synthesis, and what happens at the end of the process, to ensure that the benefits outweigh the risks. ZnO nanorod- and MOF-based photocatalysts hold significant promise for environmental remediation; however, understanding their environmental impacts and sustainability is essential. To develop environmentally friendly photocatalytic technologies that effectively reduce pollution while avoiding ecological hazards, it is important to optimize synthesis, improve catalyst stability, and conduct life cycle evaluations.

9. Environmental Safety and Ecotoxicity of ZnO Nanorods and MOFs for Photocatalytic Degradation

It is important to include environmental safety and ecotoxicity in the study of ZnO and MOFs for photocatalytic microplastic degradation, especially when it comes to safety for the environment, people, and ecosystems. ZnO and MOFs have considerable potential for pollutant degradation; however, it is crucial to ensure that these photocatalysts do not introduce novel risks to ecosystems or organisms [53,196]. Environmental safety and ecotoxicity is the capacity of a substance to interact with living things without hurting them. When using these materials for photocatalytic purposes, particularly when cleaning up the environment on a broad scale, it is very important to find out whether they pose a threat to people, fish, or soil microbes. ZnO nanoparticles are recognized for their strong photocatalytic activity, but we need to carefully assess their potential cytotoxicity, capacity to induce oxidative stress, and environmental persistence to prevent unforeseen ecological effects [197,198]. MOFs are often appreciated for their adaptability and large surface area; releasing metal parts or organic linkers into the environment might be dangerous [160,199]. Researchers have examined the antibacterial and UV-blocking properties of ZnO nanorods, which make them well suited for environmental safety and ecotoxicity applications, including sunscreens and medical devices [200,201,202]. However, the oxidative stress processes that degrade pollutants in ZnO may also damage living cells [203]. It is important to study how Zn²⁺ ions and ROS interact with biological tissues and aquatic creatures during photocatalysis. To ensure that ZnO nanostructures are safe for the environment and living organisms, it is important to optimize them for improved photocatalytic performance while reducing their cytotoxicity. MOFs have a wide range of structures and properties that can be tailored, making them well-suited for environmental safety and ecotoxicity applications, especially when made with non-toxic metals and environmental safety and ecotoxicity organic linkers [204]. For instance, MOFs constructed from metals that are not harmful, such as iron or zinc, may be less hazardous when designed with green chemistry principles. Research is still exploring post-synthetic modifications and hybrid designs that improve photocatalytic efficiency without compromising environmental safety and ecotoxicity. It is essential to verify that MOFs do not leach deleterious metal ions or decompose into hazardous by-products, thereby ensuring their safe use in environmental remediation [205,206]. Integrating environmental safety and ecotoxicity into photocatalyst design corresponds with the objective of creating sustainable, eco-friendly technology. This emphasis on safety and sustainability also enhances the societal and regulatory acceptability of innovative photocatalytic technologies, thereby promoting their practical use. Environmental safety and ecotoxicity is an important factor to think about when using ZnO and MOFs in real life to break down microplastics in sunshine. To make sure that these materials break down pollutants safely for the environment and for living things, they must be safe for the environment and for living things [39,207,208,209].

Table 3. Comparison of environmental safety and ecotoxicity of several photocatalysts, such as ZnO nanorods, TiO₂, g-C₃N₄, CdS, and MOFs, looking at how these substances may be hazardous and how they affect the environment when used to break down other substances.

Photocatalyst	Environmental Safety	Toxicity Mechanisms	Potential Environmental Impact	Ref.
ZnO Nanorods	Moderate biocompatibility; widely used in biomedical applications, including sunscreen and antibacterial coatings.	Potential cytotoxicity due to ROS generation and Zn2+ ion release, inducing oxidative stress in cells and aquatic organisms.	Zn2+ ions and ROS may lead to cellular damage; potentially harmful to aquatic organisms and ecosystems.	[212,213]
TiO2 Nanoparticles	High biocompatibility; widely used in food, cosmetics, and biomedical devices.	Low cytotoxicity under visible light; potential phototoxicity under UV exposure.	Generally considered safe, but UV-induced ROS generation could affect aquatic life in high concentrations.	[214,215]
g-C3N4	Generally biocompatible; lower toxicity compared to metal-based photocatalysts.	Low cytotoxicity, but potential accumulation in living tissues due to hydrophobic nature.	Minimal adverse effects; non-metallic structure leads to lower toxicity, making it suitable for environmental applications.	[216,217]
CdS Nanoparticles	Low biocompatibility; high toxicity due to Cd2+ ion release.	Cd2+ ions cause severe cytotoxicity and environmental toxicity, affecting aquatic organisms, inducing oxidative stress, and damaging biological systems.	Cadmium is a heavy metal with known toxicity; CdS photocatalysts pose significant environmental risks due to leaching of cadmium ions.	[218,219]
MOFs	Variable biocompatibility depending on metal centers and organic linkers; Fe-based and Zn-based MOFs show good biocompatibility.	Potential toxicity from metal ion leaching (depending on metal center); organic linker degradation could release harmful compounds.	Careful selection of biocompatible metals and green synthesis methods can reduce risks; Fe-MOFs are less toxic compared to MOFs using heavy metals like Cu or Cd.	[220,221]

shows the comparison of environmental safety and ecotoxicity of different photocatalysts, like ZnO nanorods, TiO₂, g-C₃N₄, CdS, and MOFs. It highlights key factors, including toxicity, environmental impacts, and potential for photocatalytic degradation. Every material has a different environmental safety and ecotoxicity profile depending on how it interacts with biological systems and the environment. ZnO nanorods, for example, are known to kill bacteria, but they may also be harmful to cells at higher concentrations. On the other hand, TiO₂ is usually thought to be environmentally safe and not very poisonous regarding ecotoxicity, which makes it good for use in the environment [210,211]. g-C₃N₄ is not very toxic, but CdS is a concern because it may leak and harm aquatic life. MOFs can be engineered to be more environmentally safe and less ecotoxic by changing their characteristics, however, further research is needed to find their long-term on the environment impacts. To get the most out of photocatalysts for sustainable environmental cleanup, you need to understand the environmental safety and ecotoxicity criteria.

10. Challenges and Future Perspectives

Current Limitations in Scaling Up Photocatalytic Microplastic Degradation: Although photocatalytic materials such as ZnO nanorods and MOFs have shown promise for breaking down microplastics in the lab, scaling these methods for industry remains difficult. One big problem is that certain materials, like ZnO, only absorb ultraviolet (UV) light, which makes up approximately 5% of the solar spectrum. This means that they do not work very well in natural sunshine. This makes them less useful for outdoor use. Also, generating large numbers of active photocatalysts often requires substantial energy, hazardous chemicals, and expensive materials, making photocatalytic technology hard to afford [211,222]. Another important problem is how to build photocatalytic reactors capable of handling large volumes of polluted air or water. This is because it is hard to get light to spread evenly over the catalyst surface on a large scale, which makes the photocatalytic activity go down. Long-term operational stability is particularly difficult due to catalyst deactivation, surface fouling, and the need for regular regeneration. Additionally, the breakdown of microplastics produces many metabolites, some of which may remain harmful or persist for a long time in the environment. To keep the environment safe, it is important to fully understand these intermediates and ensure they are completely mineralized into non-toxic end products.

New strategies for making photocatalysts work better and last longer: Researchers have developed several strategies to make photocatalysts more stable and effective at breaking down microplastics [223]. One important way to use more sunlight is to add dopants or co-catalysts to ZnO nanorods and MOFs. This makes the materials absorb light in the visible range. For instance, combining ZnO nanorods with plasmonic nanoparticles (e.g., gold or silver) or mixing MOFs with graphene oxide can enhance photocatalytic activity by improving light absorption, promoting charge separation, and reducing electron–hole recombination [224,225,226,227]. Another good idea is to develop composite materials that combine the best features of different photocatalysts. Combining ZnO nanorods with MOFs can yield hybrid materials that can hold more adsorbent, transmit charge more effectively, and have greater surface area for photocatalytic processes. These composites may also exhibit better long-term stability, meaning the catalyst will last longer and perform better [60,228]. To address environmental concerns associated with photocatalyst production, researchers are also exploring green synthesis techniques, such as solvent-free methods or the use of bio-based ligands to make MOFs.

Future Research Directions for Photocatalytic Microplastic Remediation: Future research on photocatalytic microplastic degradation is anticipated to focus on several critical areas. One goal is to develop new photocatalysts that absorb more light, perform better, and last longer [16,229]. This might include discovering new MOFs or semiconductors with unique structural and electrical properties, as well as improving composite materials that combine distinct photocatalytic processes to enhance performance. It will also be important to understand how microplastics break down, particularly how big polymer chains break down into smaller intermediates and, finally, into harmless mineralized products. Advanced characterization technologies and computational modeling will be crucial in enhancing reaction conditions and facilitating photocatalyst-microplastic interactions. Another key goal will be to develop photocatalytic reactors that perform better and can operate continuously on a large scale. New reactor designs that enhance the photocatalyst’s exposure to light, such as immobilizing catalysts on support materials or creating floating photocatalytic films, might greatly improve their performance in practice. In addition, it will be important to address environmental and safety issues that arise with photocatalytic devices. This means assessing the long-term stability, potential toxicity, and the environmental fate of photocatalyst materials, as well as ensuring that degradation by-products do not pose additional ecological hazards. Life cycle assessments (LCAs) will be crucial in directing the development of sustainable photocatalytic methods for microplastic cleanup. There has been significant improvement, but there are still issues with scalability, efficiency, and environmental safety. To fully realize the promise of photocatalytic technologies in reducing environmental pollution, it will be important to address these challenges with new strategies and research focused on reactor design and safety evaluations.

11. Summary and Conclusions

This study shows that ZnO nanorods and MOFs might be the next generation of environmental safety and ecotoxicity photocatalysts for breaking down microplastics in sunlight in a way that is good for the environment. These materials show a lot of potential for eco-friendly and effective microplastic cleanup because they combine the unique qualities of ZnO nanorods and MOFs, namely a large surface area, adjustable porosity, and the ability to produce ROS. The synergistic combination of ZnO nanorods with MOFs improves photocatalytic effectiveness by separating charges, lowering electron–hole recombination, and increasing light absorption. This review also focuses on environmental safety and ecotoxicity, which is different from most other photocatalysis research. Comparative comparison with other photocatalysts shows that ZnO-MOF systems are flexible, scalable, and can be used in the real world for more than just breaking down microplastics. They can also be used to clean water, fix air pollution, and get rid of organic pollutants. However, there are still problems with long-term stability, large-scale use, and figuring out how dangerous things are. This means that further study is needed on sustainable synthesis processes, sophisticated reactor designs, and thorough life cycle evaluations. For long-term, sustainable solutions to plastic pollution that protect the health of people and the environment, it will be important to develop new and environmentally safe photocatalyst technologies. ZnO nanorods and MOFs might change green photocatalysis for the better with additional study. This could lead to a cleaner, more sustainable future.