Photocatalytic and Enzymatic Degradation of Microplastics: Current Status, Comparison, and Combination

Abstract

Microplastics (MPs), as emerging environmental pollutants, pose a significant global environmental challenge due to their persistence, widespread distribution, and ecological health risks. In this review, we summarize recent advances in the photocatalytic and enzymatic degradation of MPs. The mechanism, treatment efficiency, advantages, and disadvantages of degradation techniques are compared and analyzed, together with their scope of application. Photocatalytic degradation exhibits high efficiency but may generate secondary pollution; enzymatic degradation operates under mild conditions with strong specificity but at a slower rate. Both methods possess distinct advantages and disadvantages in terms of mechanism and applicability. The combined methods exhibit a superior performance compared to standalone techniques by overcoming the inherent limitations of each approach. Prospects for future development trends and challenges in MP treatment technologies are also discussed, together with proposed directions and recommendations for further research.

1. Introduction

Plastics, valued for their durability, versatility, stability, and low cost, are indispensable in modern life and industry [1]. However, their ubiquity has led to severe environmental pollution [2]. As persistent pollutants, they can remain in the natural environment for centuries or longer [3]. During physical, chemical, and biological aging processes, plastics fragment and ultimately break down into microplastics [4].

Microplastics (MPs) are defined as plastic fragments smaller than 5 mm in any dimension, possessing no specific lower size limit [5]. Based on their origin, microplastics are further classified into primary and secondary microplastics. Primary MPs are manufactured particles generated directly during industrial processes, such as milling or extrusion [6]. Examples include plastic microbeads incorporated into personal care products (e.g., facial cleansers and toothpaste), tire wear particles, and microfibers shed from synthetic textiles (e.g., nylon) during the laundering process [7]. Secondary MPs form from the fragmentation of larger plastics in the environment due to UV radiation, heat, mechanical forces, and weathering. Examples include plastic packaging breaking down, agricultural mulch degrading in soil, and aged fishing gear fragmenting in oceans [8]. The predominant types of microplastics identified in environmental matrices and human samples comprise polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA), and polystyrene (PS). These MPs undergo progressive degradation, generating particles < 100 nm in size, which are classified as nanoplastics (NPs) [9].

MPs, as an emerging class of environmental contaminants, exhibit the characteristics of small size, low density, and pervasive distribution, which enable their atmospheric transport and facilitate prolonged suspension in the air [10,11,12]. MPs are now widely distributed in aquatic systems, terrestrial soils, and within living organisms, posing significant potential threats to ecosystems and constituting a major pollution risk [13,14]. Substantial quantities of MPs pose a severe threat to the marine ecological balance [15,16]. Upon ingestion by marine biota—including fish, shellfish, and plankton—MPs can induce multiple physiological and ecological effects [17]. Primarily, their ingestion can cause digestive tract obstruction, reducing feeding efficiency, suppressing growth and reproduction, and potentially causing mortality. Secondly, due to their resistance to degradation within organisms, MPs undergo bioaccumulation in tissues [18]. In terrestrial environments, MPs accumulate within the soil matrix, altering soil structure and thereby impacting plant growth and development [19]. The presence of associated chemical additives, such as plasticizers, further increases the risk of MP accumulation in soil [20,21,22]. These chemicals, particularly those characterized by hydrophobicity, persistent covalent bonds, and functional groups resistant to degradation, enhance their adsorption capacity onto MPs and inhibit the biodegradation processes of said MPs [23,24,25]. Furthermore, atmospheric MPs can interfere with cloud formation processes, influencing Earth’s radiation balance and climate systems [26]. MPs also pose significant threats to animal and human health [27]. They are pervasive in water and food, entering organisms through dietary intake. They can be absorbed via the lungs or digestive tract, enter the circulatory system, and accumulate within cells across various tissues [28,29]. Scientists have now detected MP contamination in human blood and lung tissue, with subsequent identification in diverse human tissue samples, including those from patients with liver cirrhosis and the great saphenous vein [9,30,31]. Given the severity and complexity of MP pollution, the development of efficient technologies for the degradation of MPs has become a major focus of research.

In recent years, extensive research has been dedicated to developing methods for degrading MPs from the environment. These approaches can be broadly categorized into physical, chemical, biological, and combined treatment techniques [32], exemplified by methods such as mechanical degradation, adsorption [33], thermal degradation [34], photocatalytic degradation [35], and biodegradation [36]. In Figure 1, the classification of MPs and some degradation methods is presented. While current physical treatment technologies serve as high-efficiency pretreatment methods to concentrate MPs for subsequent degradation, their application is limited and requires a combination with chemical or biological processes [37]. Research hotspots in chemical degradation focus on developing highly efficient catalysts, optimizing reaction conditions (such as low-temperature hydrolysis), and pursuing complete degradation [35]. Biological degradation research prioritizes discovering highly efficient microorganisms and elucidating their enzymatic mechanisms, while simultaneously exploring synergistic strategies [36]. The aim of all three approaches—physical, chemical, and biological—is to achieve high efficiency, low energy consumption, and environmental friendliness. They emphasize the systematic assessment of degradation product risks and promote the development of synergistic strategies integrating multiple technologies [32]. Each of these methods has its advantages and disadvantages and, when taken together, there is no single perfect degradation method [38].

Over the past five years, numerous review articles have been published, with the authors summarizing and discussing methods for the degradation of MPs and their underlying mechanisms and evaluating their degradation performance. In Table 1, research reviews on several MP degradation techniques are summarized. The authors of these studies have focused on diverse aspects, including degradation mechanisms and advanced oxidation processes (AOPs).

The ideal degradation technology aims to completely mineralize MPs into CO₂ and H₂O in the shortest time and at the lowest cost. However, due to the complex composition of MPs, degradation is often incomplete. In light of this fact, current research also focuses on innovative technologies for MP degradation, with photocatalytic and enzymatic degradation being particular research hotspots. Photocatalytic degradation, which is based on light-activated semiconductor materials, leads to the in situ generation of reactive oxygen species and causes the degradation of MPs [35]. This technology offers fast degradation rates, utilization of solar energy, and effectiveness against various types of plastics [39]. However, under incomplete mineralization, this process generates a series of complex intermediates, including chemicals such as aldehydes, ketones, and carboxylic acids, as well as a large number of NPs [40,41]. These products may more easily penetrate biological barriers, accumulate in organs, and induce oxidative stress and inflammatory responses, posing potential health risks that cannot be overlooked [42]. Even when mineralization is nearly complete, trace concentrations of transformation products can still interact synergistically with other pollutants in the environment, thereby exacerbating their environmental impact [43,44]. Enzymatic degradation breaks down polymers into smaller components (such as oligomers and monomers) for metabolic processes. Its advantages include high reaction specificity, good biocompatibility, and being generally environmentally friendly and low risk [45]. However, its use is challenging, because each enzyme is typically effective only for specific types of plastics, and the degradation rate may be slow. Moreover, if the enzyme efficiency is low, it may generate undegraded MPs and some toxic intermediates that take a long time to degrade. These undegraded components can leach into water or soil, and after being absorbed by organisms, may cause direct chemical toxicity or disrupt the normal metabolic cycles of ecosystems [46,47]. Figure 2 presents the pathways and potential risks of photocatalytic and enzymatic degradation of MPs.

Strategies for the optimal selection of methods and enhancing removal efficiency remain key areas for further investigation. In this review, we specifically synthesize research on the photocatalytic and enzymatic degradation of MPs. We critically examine their characteristics, advantages, and limitations and further evaluate their respective application scopes and prospects. The overarching aim is to provide a foundation for exploring innovative, interdisciplinary, and integrated strategies designed to overcome current technological bottlenecks and achieve efficient and environmentally friendly MP degradation.

Table 1. Reported reviews focused on MP degradation.

Title	Main Topic	References
From macro to micro: The key parameters influencing the degradation mechanism and the toxicity of microplastics in the environment	The mechanisms (photodegradation, thermal, mechanochemical, photocatalytic, and biodegradation) influencing MP degradation and its ecotoxicological impacts on terrestrial and aquatic ecosystems	[48]
Light-driven degradation of microplastics: Mechanisms, technologies, and future directions	Photocatalytic degradation mechanisms, catalyst advancements (e.g., TiO2 and ZnO), and value-added upcycling of microplastics into hydrogen/chemicals	[49]
Microplastics: a global threat to life and living	Mechanisms of action, degradation pathways (biological/chemical/photodegradation), and analytical techniques for MPs	[50]
Advances in chemical removal and degradation technologies for microplastics in the aquatic environment: A review	Chemical removal and degradation technologies for MPs in aquatic environments (such as coagulation, advanced oxidation, and photocatalysis), analyzing their efficiency, mechanisms, and influencing factors	[51]
Review of Soil Microplastic Degradation Pathways and Remediation Techniques	Degradation pathways (e.g., pyrolysis, hydrolysis, and biodegradation) and photocatalytic remediation techniques for MPs in soil environments, with emphasis on agricultural mulch film	[52]
From bulk to bits: understanding the degradation dynamics from plastics to microplastics, geographical influences and analytical approaches	Degradation dynamics from plastics to microplastics with emphasis on geographical influences and analytical characterization techniques	[53]
Occurrence, Degradation Pathways, and Potential Synergistic Degradation Mechanism of Microplastics in Surface Water: A Review	Occurrence, accumulation, and synergistic degradation mechanisms (physical–chemical–biological coupling and microbial community interactions) of MPs, specifically in surface waters (rivers, lakes, and reservoirs)	[54]
On the degradation of (micro)plastics: Degradation methods, influencing factors, environmental impacts	Degradation methods, influencing factors (intrinsic properties and external environment), and environmental impacts of degradation products for (micro)plastics.	[55]
Microbial strategies for effective microplastics biodegradation: Insights and innovations in environmental remediation	Microbial consortia and enzymatic pathways for enhancing MP biodegradation, with emphasis on pretreatment strategies and environmental remediation applications	[56]
Engineered technologies for the separation and degradation of microplastics in water: A review	State-of-the-art engineered technologies for both the separation and degradation of MPs in freshwater, identifying knowledge gaps and future research directions for real-scale application	[57]

Table 1. Reported reviews focused on MP degradation.

Title	Main Topic	References
From macro to micro: The key parameters influencing the degradation mechanism and the toxicity of microplastics in the environment	The mechanisms (photodegradation, thermal, mechanochemical, photocatalytic, and biodegradation) influencing MP degradation and its ecotoxicological impacts on terrestrial and aquatic ecosystems	[48]
Light-driven degradation of microplastics: Mechanisms, technologies, and future directions	Photocatalytic degradation mechanisms, catalyst advancements (e.g., TiO2 and ZnO), and value-added upcycling of microplastics into hydrogen/chemicals	[49]
Microplastics: a global threat to life and living	Mechanisms of action, degradation pathways (biological/chemical/photodegradation), and analytical techniques for MPs	[50]
Advances in chemical removal and degradation technologies for microplastics in the aquatic environment: A review	Chemical removal and degradation technologies for MPs in aquatic environments (such as coagulation, advanced oxidation, and photocatalysis), analyzing their efficiency, mechanisms, and influencing factors	[51]
Review of Soil Microplastic Degradation Pathways and Remediation Techniques	Degradation pathways (e.g., pyrolysis, hydrolysis, and biodegradation) and photocatalytic remediation techniques for MPs in soil environments, with emphasis on agricultural mulch film	[52]
From bulk to bits: understanding the degradation dynamics from plastics to microplastics, geographical influences and analytical approaches	Degradation dynamics from plastics to microplastics with emphasis on geographical influences and analytical characterization techniques	[53]
Occurrence, Degradation Pathways, and Potential Synergistic Degradation Mechanism of Microplastics in Surface Water: A Review	Occurrence, accumulation, and synergistic degradation mechanisms (physical–chemical–biological coupling and microbial community interactions) of MPs, specifically in surface waters (rivers, lakes, and reservoirs)	[54]
On the degradation of (micro)plastics: Degradation methods, influencing factors, environmental impacts	Degradation methods, influencing factors (intrinsic properties and external environment), and environmental impacts of degradation products for (micro)plastics.	[55]
Microbial strategies for effective microplastics biodegradation: Insights and innovations in environmental remediation	Microbial consortia and enzymatic pathways for enhancing MP biodegradation, with emphasis on pretreatment strategies and environmental remediation applications	[56]
Engineered technologies for the separation and degradation of microplastics in water: A review	State-of-the-art engineered technologies for both the separation and degradation of MPs in freshwater, identifying knowledge gaps and future research directions for real-scale application	[57]

2. Photocatalytic Degradation

Photocatalysis refers to the process in which a catalyst absorbs light energy and converts it into chemical energy under illumination, thereby accelerating chemical reactions [35]. This technology primarily involves homogeneous and heterogeneous catalytic systems. Although homogeneous photocatalytic systems offer advantages such as high reaction rates and short treatment times, high costs and difficulties in product separation limit their practical application. In contrast, heterogeneous photocatalysis, owing to its high catalyst stability and ease of recovery and reuse, is more suitable for implementing process engineering strategies. Consequently, in the field of photocatalytic degradation, particularly for MP degradation, heterogeneous photocatalysis demonstrates significant advantages. It enables the complete mineralization of MPs into harmless small molecules, such as CO₂ and H₂O, under efficient and mild reaction conditions [39,58]. This approach overcomes the limitations of traditional physical and chemical methods, including low efficiency, high energy consumption, secondary pollution, and incomplete mineralization, making it a promising alternative treatment strategy [59]. The core mechanism of this technology relies on the generation of photogenerated electrons (e⁻) and holes (h⁺) from a photocatalyst upon excitation by light of specific wavelengths (ultraviolet or visible). These charge carriers subsequently react with environmental species (e.g., H₂O and O₂) to generate highly reactive oxygen species (ROS, such as ·OH and $O_2^{.}$⁻) [60]. ROS attack chemical bonds (e.g., C-C and C-H) within the polymer chains of MPs through strong oxidation, initiating chain scission and oxidation, ultimately leading to fragmentation into small molecules or complete mineralization [61,62]. This process encompasses three key steps (Equations (1)–(6)): light absorption and excitation; charge carrier migration and ROS generation; and the oxidative degradation and mineralization of MPs [39]. The mechanism of photocatalytic degradation is shown in Figure 3.

$$photocatalyst+hv\to e^{-}+h^{+}$$

(1)

$$h^{+}+H_2\mathrm{O}\to ·OH+H^{+}$$

(2)

$$e^{-}+O_2\to O_2^{·-}$$

(3)

$$MPs+·OH\to Radicalintermediates$$

(4)

$$Radicalintermediates+·OH/O_2^{·-}\to Smallmoleculealdehydes,acids,carboxylicacids$$

(5)

$$smallmoleculeor ganicsubstance+·OH\to {CO}_2+H_{2}O$$

(6)

2.1. Ultraviolet-Induced Photocatalysis

Ultraviolet (UV) photocatalysis forms the foundation of early photocatalysis research, utilizing high-energy UV light to excite wide-bandgap semiconductor catalysts. TiO₂ possesses a bandgap of 3.2 eV, limiting its photocatalytic activity to the UV range. Owing to its high stability, low toxicity, cost-effectiveness, and excellent UV photocatalytic activity, it is the most widely used photocatalyst [63,64]. Nabi et al. [65] demonstrated in their study that TiO₂ nanoparticle films prepared using Triton X-100 as a solvent exhibit remarkable photocatalytic degradation performance under UV irradiation, achieving complete mineralization of 400 nm PS MPs within 12 h. Furthermore, ZnO, a highly promising photocatalytic material, has garnered extensive attention due to its excellent optical properties, high redox potential, favorable carrier mobility, and environmental friendliness [66]. Its synthesis is straightforward, and its morphology and size can be controlled via low-temperature hydrothermal methods to obtain nanostructures with specific surface characteristics [67]. Studies indicate that ZnO photocatalysis achieves a 97% degradation efficiency for polyacrylamide MPs within less than 48 h, reducing the COD by 95% and effectively eliminating any toxic intermediates.

Although UV light catalysts (such as TiO₂ and ZnO) have the advantages of high activity and non-toxicity, they can only use UV light, accounting for roughly 3-5% of solar energy, and recombination of electron–hole pairs is also limited, which reduces the availability of ROS required for effective degradation of MPs and greatly limits their large-scale application in the actual environment [68].

2.2. Visible Light-Induced Photocatalysis

To achieve more efficient removal of MPs, it is essential to expand research on the utilization of solar energy in the visible light region. Visible light-induced photocatalysis not only enables effective pollutant degradation but is also environmentally friendly. Significant progress has been made in the visible light photocatalytic degradation of MPs through modifications of conventional catalysts or the development of novel photocatalytic materials.

Researchers have developed various modification strategies in this field, including noble metal deposition (e.g., Ag), elemental doping (e.g., N), and heterojunction construction. Examples of photocatalysts used for MP degradation are listed in Table 2. For instance, Maria et al. [69] demonstrated significantly enhanced degradation efficiency for PE MPs in aqueous media using green-synthesized nitrogen-doped TiO₂ (N-TiO₂), achieving a 6.40% mass loss within 20 h and a first-order rate constant (38.2 × 10⁻⁴ h⁻¹) far exceeding its performance in solid media (1.85%, 12.2 × 10⁻⁴ h⁻¹). Further research by Llorente-García et al. [70] effectively degraded high-density polyethylene (HDPE) and low-density polyethylene (LDPE) under visible light using mesoporous N-TiO₂ coatings, revealing a pronounced size effect: smaller HDPE MPs (382 ± 154 μm) exhibited significantly higher mass loss (4.65 ± 0.35%) after 50 h compared to larger particles (814 ± 91 μm, 0.22 ± 0.02%). The rise in carbonyl index from 0.07 to 0.45 further corroborated the higher degree of oxidative degradation.

Significant progress has also been made in other high-performance photocatalyst systems beyond modified TiO₂. Heterojunction catalysts effectively suppress e⁻-h⁺ recombination and enhance ROS yield by establishing an internal electric field. Research has shown that In₂O₃-reduced graphene oxide (rGO) nanocomposites can reduce the size of 500 nm PS MPs to 280 nm within 12 h under visible light, achieving a degradation efficiency of 56%, with FTIR analysis confirming carbonyl group formation [71]. Zhang et al. [72] constructed an S-scheme Cs₃Bi₂Br₉/BiOCl heterojunction, leveraging its strong internal electric field to promote charge separation, achieving a 42.3 ± 3.89% mass loss for PS MPs under visible light irradiation. MOFs have emerged as efficient materials due to their highly tunable pore sizes, large specific surface areas, and abundant surface functional groups. Their unique hybrid metal–organic structures confer performance advantages that surpass those of other porous materials. For example, in one study, NH₂-UiO-66 exhibited an exceptionally high removal rate (98% within 30 min) for PS [73]. The BiOI/MIL-101 composite (40 wt% BiOI) significantly accelerated charge separation through an optimized heterojunction structure and increased specific surface area. After 6 h of reaction, the increase in carbonyl index (0.127) for the treated PE MPs was 5.3 times higher than that achieved with pristine BiOI [73]. Plasmonic catalysts exploit the localized surface plasmon resonance effect to enhance light absorption and charge carrier dynamics. Tajik et al. [74] developed plasmonic Pt/ZnO nanorod catalysts, whereby the LSPR effect boosted visible light absorption by 78% and promoted ·OH generation, effectively degrading low-density polyethylene (LDPE) fragments in water. SEM observations of surface physical damage and FTIR-detected changes in carbonyl/vinyl indices collectively confirmed the degradation. Sima et al. [75] devised a two-step plasma–catalytic system combining dielectric barrier discharge (DBD) plasma (which generates ROS for PS MP degradation) with a plasma-activated Hopcalite catalyst (selectively oxidizing intermediates to CO₂). This system achieved 98.4% mineralization efficiency for PS MPs within 60 min and maintained 93.2% CO_x conversion and 99.5% CO₂ selectivity even after 10 cycles. The excellent stability is attributed to plasma-enhanced surface-adsorbed oxygen species and micro-discharge oxidation reactions within the pores [76].

The efficiency of photocatalytic MP degradation is influenced by multiple factors, including the inherent properties of MPs (polymer type, size, shape, and crystallinity), photocatalyst characteristics (band structure, specific surface area, porosity, and visible light responsiveness), environmental conditions (light wavelength and intensity, humidity, and pH), and the interfacial contact efficiency between the catalyst and MPs. Research indicates that the visible light responsiveness of the catalyst, the design of porous structures, and effective contact with MPs are crucial for enhancing degradation efficiency [77]. Compared to visible light, UV irradiation exhibits higher degradation efficiency for MPs [78]. Due to its shorter wavelength, UV light possesses higher photon energy. Within a certain range, an increase in light intensity enhances the photocatalytic degradation efficiency of MPs. However, as the light intensity continues to increase, the MP degradation rate slows down and eventually stabilizes. This finding can be explained by the fact that, with the other conditions remaining constant, the number of active sites on the catalyst surface is finite. Consequently, light intensity beyond a certain threshold does not further increase the degradation rate [35]. Table 3 provides a comparison of UV-induced photocatalysis with visible light-induced photocatalysis.

While the photocatalytic degradation of MPs demonstrates significant advantages under laboratory conditions, with moderate catalyst costs and recyclability offering long-term economic viability and sustainability, this approach still faces limitations [59]. For instance, in natural water bodies, the quenching of ROS by dissolved organic matter (DOM) and competitive adsorption from coexisting contaminants can reduce catalytic efficiency by 40–65% [79]. Furthermore, the potential generation of NPs during degradation and catalyst component leaching may cause secondary pollution. Additionally, the activity of immobilized catalysts suffers a 30–50% loss compared to their suspended counterparts [61,77], further hindering engineering-scale implementation. Against this backdrop, China is rapidly deploying resource recovery applications leveraging cost-effective materials [80], and all countries are innovating and developing to achieve the United Nations Sustainable Development Goals [81]. Future development necessitates creating broadband-responsive photocatalysts to mitigate DOM interference, establishing real-time NP monitoring technologies to control secondary risks, designing coupled systems to enhance immobilized catalyst efficiency, and conducting comprehensive full life-cycle assessments of long-term ecological safety and engineering feasibility [35].

Table 2. Results of photocatalysts used for MP degradation.

Type	Photocatalyst	MPs	Light Source Type	Degradation Efficiency (%)	Time	References
TiO2	TiO2	PP	Visible light	Mass loss, 50.5 ± 0.5%	50 h	[82]
	Pac-Man TiO2	PS	UV light	28%	70 h	[83]
Doped TiO2	N-TiO2	PE	Visible light	Mass loss, 6.4%	20 h	[69]
	N-TiO2	HDPE, LDPE	Visible light	Mass loss, 4.65 ± 0.35%	50 h	[70]
	MJMPs	PMMA, PE, PS	Visible light	34.6%, 48.9%, and 11.3%	36 h	[84]
	Nb2O5	PE, PP, PET	Visible light	100%	55–64 h	[85]
Heterojunctions	In2O3-rGO	PS	Visible light	56%	12 h	[71]
	Cs3Bi3Br9/BiOCl S	PS	Visible light	Mass loss, 42.3 ± 3.89%	50 h	[72]
	CuO/TiO2	Nylon	UV-Vis light	-	5 h	[86]
	Fe1−xS/FeMoO4/MoS2	PS	Visible light	58.46%	30 h	[87]
	CdS-16% CuInSe2	PET	Visible light	-	-	[88]
	C,N-TiO2/SiO2	PET	Visible light	9.35–16.22%	-	[89]
	g-C3N4/TiO2/WCT-AC	PE	Visible light	67.58%	60 h	[90]
	0D Co3O4@2D Co (OH)2	PE	Visible light	40%	9 h	[91]
	FeB/TiO2	PS	UV light	92.3%	12 h	[76]
	TiOX/ZnO	PET, PES	UV light	100%	480 h	[92]
	NH2-UiO-66	PS	Visible light	98%	30 min	[93]
	CuO/Bi2O3/g-C3N4	PET	Visible light	Mass loss, 41.6%	240 h	[94]
	g-C3N4/CQD/FeNi-BTC	PS, PET	Visible light	82.16%	72 h	[95]
MOFs	BiOI/MIL101	PE	Visible light	CI increased by 0.127	6 h	[73]
	Ag2O/Fe-MOF	PEG	Visible light	Mass loss, 5.5%	3 h	[96]
Plasmonic Photocatalyst	Ionophore-assisted Hopcalite	PS	Visible light	98.4%	1 h	[75]
	Plasmonic Platinum/ZnO	LDPE	Visible light	78%	-	[74]

“-” represents unknown.

Table 2. Results of photocatalysts used for MP degradation.

Type	Photocatalyst	MPs	Light Source Type	Degradation Efficiency (%)	Time	References
TiO2	TiO2	PP	Visible light	Mass loss, 50.5 ± 0.5%	50 h	[82]
	Pac-Man TiO2	PS	UV light	28%	70 h	[83]
Doped TiO2	N-TiO2	PE	Visible light	Mass loss, 6.4%	20 h	[69]
	N-TiO2	HDPE, LDPE	Visible light	Mass loss, 4.65 ± 0.35%	50 h	[70]
	MJMPs	PMMA, PE, PS	Visible light	34.6%, 48.9%, and 11.3%	36 h	[84]
	Nb2O5	PE, PP, PET	Visible light	100%	55–64 h	[85]
Heterojunctions	In2O3-rGO	PS	Visible light	56%	12 h	[71]
	Cs3Bi3Br9/BiOCl S	PS	Visible light	Mass loss, 42.3 ± 3.89%	50 h	[72]
	CuO/TiO2	Nylon	UV-Vis light	-	5 h	[86]
	Fe1−xS/FeMoO4/MoS2	PS	Visible light	58.46%	30 h	[87]
	CdS-16% CuInSe2	PET	Visible light	-	-	[88]
	C,N-TiO2/SiO2	PET	Visible light	9.35–16.22%	-	[89]
	g-C3N4/TiO2/WCT-AC	PE	Visible light	67.58%	60 h	[90]
	0D Co3O4@2D Co (OH)2	PE	Visible light	40%	9 h	[91]
	FeB/TiO2	PS	UV light	92.3%	12 h	[76]
	TiOX/ZnO	PET, PES	UV light	100%	480 h	[92]
	NH2-UiO-66	PS	Visible light	98%	30 min	[93]
	CuO/Bi2O3/g-C3N4	PET	Visible light	Mass loss, 41.6%	240 h	[94]
	g-C3N4/CQD/FeNi-BTC	PS, PET	Visible light	82.16%	72 h	[95]
MOFs	BiOI/MIL101	PE	Visible light	CI increased by 0.127	6 h	[73]
	Ag2O/Fe-MOF	PEG	Visible light	Mass loss, 5.5%	3 h	[96]
Plasmonic Photocatalyst	Ionophore-assisted Hopcalite	PS	Visible light	98.4%	1 h	[75]
	Plasmonic Platinum/ZnO	LDPE	Visible light	78%	-	[74]

“-” represents unknown.

Table 3. Comparison of UV-induced photocatalysis and visible light-induced photocatalysis.

Characteristics	UV-Induced Photocatalysis	Visible Light-Induced Photocatalysis
Light source	UV light	Visible light
Photocatalysts	Wide Bandgap Semiconductors TiO2 ZnO	Narrow-bandgap or modified semiconductors g-C3N4 Modified TiO2 CdS Composite materials
Advantages	Catalysts (such as TiO2) are stable, non-toxic, and low-cost and have strong redox ability and high reactivity.	It can efficiently use solar energy and has greater application potential, which is the current research hotspot.
Disadvantages	The utilization rate of solar light is extremely low (UV light only accounts for 3–5%), and additional UV light sources are usually required, resulting in high energy consumption and cost.	Many catalysts (such as CdS) have poor stability and are prone to photocorrosion. The design and preparation of catalysts are more complicated, and the cost may be higher. The oxidation ability is sometimes weaker than that of the ultraviolet catalytic system.

Table 3. Comparison of UV-induced photocatalysis and visible light-induced photocatalysis.

Characteristics	UV-Induced Photocatalysis	Visible Light-Induced Photocatalysis
Light source	UV light	Visible light
Photocatalysts	Wide Bandgap Semiconductors TiO2 ZnO	Narrow-bandgap or modified semiconductors g-C3N4 Modified TiO2 CdS Composite materials
Advantages	Catalysts (such as TiO2) are stable, non-toxic, and low-cost and have strong redox ability and high reactivity.	It can efficiently use solar energy and has greater application potential, which is the current research hotspot.
Disadvantages	The utilization rate of solar light is extremely low (UV light only accounts for 3–5%), and additional UV light sources are usually required, resulting in high energy consumption and cost.	Many catalysts (such as CdS) have poor stability and are prone to photocorrosion. The design and preparation of catalysts are more complicated, and the cost may be higher. The oxidation ability is sometimes weaker than that of the ultraviolet catalytic system.

3. Enzymatic Degradation

Different physical and chemical treatments explored over the years have been effective in degrading MPs. Issues such as high energy consumption, chemical use, or the need for complex infrastructure limit their large-scale applicability as environmentally sustainable solutions [97]. To address these challenges, biological treatments—which utilize the metabolic capacity of microorganisms to degrade environmental pollutants—have become an emerging research focus [98,99,100]. This biodegradation technique is widely used for the degradation of MPs in aqueous environments, soils, and waste, with little risk to the environment and human health [101]. Compared with other degradation methods, enzymatic degradation (enzymes secreted by microorganisms play a key role in the transformation of MPs) has received extensive attention due to its specificity, reduced impact on the environment, and the ability to convert MPs into less harmful products. Microorganisms can further use these low-molecular-weight degradation products for metabolism and ultimately generate gases such as CO₂, thereby reducing environmental pollution. Figure 4 shows the enzymatic degradation mechanism for MPs.

Each enzyme exhibits a distinct mechanism of action in degrading MPs. In general, the core mechanism of enzymatic degradation involves the recognition and binding of specific chemical bonds (such as ester and amide bonds) on the MP surface by specific hydrolases (e.g., esterases, lipases, and cutinases) or oxidoreductases (e.g., laccases and peroxidases) via their active sites [102]. Acting as biocatalysts, they efficiently cleave these bonds using hydrolytic or oxidative reactions, progressively breaking down recalcitrant polymers into oligomers or monomers [103]. In Table 4, the different types of MP-degrading enzymes are summarized.

3.1. Main Enzymes Involved in the Degradation of MPs

3.1.1. Hydrolase

Hydrolases serve as the primary agents for MP degradation, particularly excelling in the breakdown of MPs containing ester or amide bonds, such as PET, polyurethane (PUR), and PA. These enzymes cleave specific chemical bonds within the polymer backbone through hydrolysis [102]. Enzyme molecules first adsorb onto the hydrophobic MP surface via substrate-binding sites, after which the catalytic residues in the active center activate water molecules to attack the target chemical bonds, resulting in their cleavage into smaller oligomers or monomers [104]. A representative hydrolase is PETase, derived from Ideonella sakaiensis, which directly and efficiently degrades low-crystallinity PET and is regarded as a milestone discovery in the field [105]. Additionally, cutinase exhibits broad-spectrum degradation activity toward various polyesters, including PET and polycaprolactone (PCL), while esterases and lipases are mainly employed to degrade aliphatic polyesters. The advantages of hydrolases lie in their typically non-toxic reaction products, which can be further utilized by microorganisms, and the fact that they require no additional cofactors. However, their limitations include being more effective on MPs containing hydrolyzable bonds and relatively low degradation efficiency for highly crystalline materials [106].

3.1.2. Oxidoreductase

Compared with hydrolytic enzymes, oxidoreductases mainly act on non-hydrolytic MPs, such as inert polymers, namely, PE, PP, and PS, which are only composed of C-C main chains [102]. Laccase is one of the representatives in this regard. It often requires mediator molecules as electron shuttles to diffuse to the MP surface through mediator-free radicals to cause degradation [107]. Karthikeyan et al. [108] found that laccase can break the polymer chain through a redox reaction and can bind to mediator molecules (such as ABTS and HBT) to enhance degradation efficiency and degrade PET, PE, PS, PVC, etc., particularly for aromatic polymers (such as PS). Peroxidases, such as lignin peroxidase (LiP) and manganese peroxidase (MnP), use hydrogen peroxide to produce high-valence enzyme–oxygen complexes that directly degrade MPs. The advantage of oxidoreductases is that they can degrade inert polyolefins, with a wide range of substrates; however, their process is random, the product is complex, and it usually relies on cofactors or mediators, with high cost and relatively low efficiency [108,109].

3.2. Strategies to Improve the Efficiency of the Enzymatic Degradation of MPs

The efficiency of the enzymatic degradation of MPs is affected by numerous factors, such as operating parameters, the chemical structure of MPs, and enzyme immobilization strategies. Operating conditions such as temperature, pH, and water availability significantly affect enzyme activity. High temperatures can accelerate the reaction but may cause enzyme inactivation. Deviation from the optimum pH will reduce the catalytic efficiency. The crystallinity, hydrophobicity, and molecular structure of MPs are the key internal factors affecting the accessibility of enzymes [110]. High crystallinity and hydrophobicity hinder the binding of enzymes to substrates. Additionally, enzyme immobilization improves the degradation efficiency by enhancing its stability and reusability; however, during carrier selection, it is necessary to take into account substrate accessibility and enzyme activity retention. Pretreatment (such as thermal, chemical, or oxidative treatment) can reduce crystallinity and increase surface hydrophilicity, thereby significantly improving the enzymatic hydrolysis effect. The synergistic optimization of these factors is the core of achieving efficient and sustainable enzymatic degradation of MPs [111,112].

At present, researchers primarily focus on techniques such as molecular modification and immobilization, aiming to optimize degradation efficiency through enhancements in enzyme structure and presentation formats. Protein engineering strategies (including rational design, directed evolution, and machine learning-assisted approaches) have significantly improved the thermostability and catalytic efficiency of native polyester-degrading enzymes, particularly PET hydrolases, achieving high degradation rates (up to 90%) and monomer recovery for PET [113]. Synthetic biology approaches, such as constructing bacterial curli fiber-based whole-cell catalysts displaying PETase (BIND-PETase), facilitate stable PET degradation in wastewater environments, achieving 9.1% degradation efficiency for post-consumer PET waste with high crystallinity (>20%), combined with good storage stability and reusability [103]. Furthermore, techniques such as enzyme immobilization and the enhancement of carrier systems have been demonstrated to significantly improve enzyme activity and stability, thereby boosting the degradation efficiency of MPs [108]. In addition, studies on immobilized enzymes for MP degradation are mainly centered on hydrolases; immobilized oxidases, despite their considerable potential for the degradation of MPs with C-C backbones, remain at an early stage of research due to their reliance on radical pathways and mediator systems [114]. Schwaminger et al. [115] fixed the PETase enzyme on magnetic iron oxide nanoparticles through His-tag for the degradation of PET and found that the immobilized enzyme has high affinity and load capacity, can be recycled multiple times, and can effectively degrade PET, resulting in an increase in crystallinity. Zurier et al. [116] connected a silicon-binding peptide to the C-terminus of PETase to achieve directional self-immobilization of the enzyme on mesoporous silica. The activity of the immobilized enzyme (PETase-SBP-Si) was 6.2 times higher than that of the free enzyme under simulated influent conditions, and the overall catalytic activity was increased by 2.1 times. In light of the above findings, the authors of future studies should prioritize enzyme engineering and elucidate the primary degradation mechanisms to advance more efficient technological solutions for MP remediation.

Table 4. Different types of MP degradation enzymes.

Enzyme Type	Enzymes	MPs	Possible Sources	References
Hydrolases	Esterases	PU, PE, PET, PVC	Pseudomonas spp.	[117]
	Lipases	PET, PCL	Thermomyces Lanuginosus	[118]
	PETase	PET	Ideonella sakaiensis	[105]
	MHETase	PET	Ideonella sakaiensis	[105]
	Cutinases	PET, PCL, PU	Resulting from phytopathogenic fungus infection	[106]
Oxidoreductases	Laccases	PET, PE, PS, PVC	Ascomycetes, Basidiomycetes, and Deuteromycetes fungi	[119]
	LiP	PVC, PET, PE, PP, PS	White-rot fungus	[108]
	MnP	PVC, PE, PP	Phanerochaete chrysosporium	[109]

Table 4. Different types of MP degradation enzymes.

Enzyme Type	Enzymes	MPs	Possible Sources	References
Hydrolases	Esterases	PU, PE, PET, PVC	Pseudomonas spp.	[117]
	Lipases	PET, PCL	Thermomyces Lanuginosus	[118]
	PETase	PET	Ideonella sakaiensis	[105]
	MHETase	PET	Ideonella sakaiensis	[105]
	Cutinases	PET, PCL, PU	Resulting from phytopathogenic fungus infection	[106]
Oxidoreductases	Laccases	PET, PE, PS, PVC	Ascomycetes, Basidiomycetes, and Deuteromycetes fungi	[119]
	LiP	PVC, PET, PE, PP, PS	White-rot fungus	[108]
	MnP	PVC, PE, PP	Phanerochaete chrysosporium	[109]

Enzymatic degradation leverages natural processes, offering the core advantages of environmentally friendly and minimal infrastructure requirements [120]. It can be implemented in diverse environments, such as wastewater treatment plants and natural water bodies, avoiding the need for high energy consumption and complex equipment [101]. Its environmental friendliness is particularly noteworthy, as it theoretically converts target MPs entirely into harmless end products such as water, carbon dioxide, and biomass, without generating chemically harmful byproducts [121]. However, the large-scale application of this technology faces significant bottlenecks. In one instance, the degradation efficiency for recalcitrant MPs is limited, resulting in protracted degradation cycles [122]. This efficiency barrier to mass production persists despite concerted efforts; MPs are not strictly managed in parts of the world such as Africa, and green recycling and simple biodegradation are still pursued, with China focusing on synthetic biology for resource utilization [123,124]. Conversely, complex environmental conditions significantly impact degradation. Furthermore, enzymatic degradation could generate intermediates such as oligomers and monomers, some of which may be toxic or persistent if not further degraded or mineralized [46]. To realize its dual original intent of low cost and environmental benefit, thereby transitioning the technology from theoretical advantages into practical application, it is necessary to shorten the degradation cycle through high-efficiency strain breeding (such as using directional evolution to enhance enzymatic activity) and develop intelligent monitoring devices to optimize the reaction microenvironment in real time [124,125].

4. Comparison and Combination Between Photocatalytic and Enzymatic Degradation

Photocatalytic and enzymatic degradation each possess distinct advantages and disadvantages. In

Table 5. Comparison of photocatalytic and enzymatic degradation for MPs.

Method	Photocatalytic Degradation	Enzymatic Degradation
Reaction mechanism	Radical oxidation reaction.	Enzymatic reaction.
Reaction conditions	Requires light (ultraviolet or visible light) and catalysts; affected by temperature, pH, light intensity, etc.	Suitable temperature and pH are required.
Degradation efficiency	It is faster in the early stage; however, it may slow down with the accumulation of products. It is effective for a variety of MPs; however, the mineralization rate (completely converted to CO2 and H2O) may not be high.	The efficiency is slow, and the specificity is strong.
Advantages	Strong oxidation capability and deep degradation potential, relatively straightforward operation, and readily available catalysts [39].	Minimal by-products, mild processing conditions, and low energy consumption. Environmentally friendly, with no secondary pollution.
Disadvantages	It may not be completely degraded, resulting in secondary pollution, high energy consumption of the light source, and difficult large-scale application.	Enzymes exhibit poor stability, are readily inactivated, degrade slowly, demonstrate limited efficacy on composite MPs, and carry relatively high costs, presenting challenges for large-scale application.

, the comparison of photocatalytic and enzymatic degradation methods is summarized.

Photocatalytic technology utilizes semiconductor materials to generate highly oxidative free radicals under light irradiation, which can attack polymer chains, achieving fragmentation and mineralization of MPs [126]. However, its widespread application is limited by issues such as high recombination rates of photogenerated carriers and low utilization efficiency of visible light [127,128]. Coupling with other technologies can effectively overcome these limitations. The integration of photocatalysis and Fenton reactions establishes a highly efficient synergistic system, in which electrons generated during photocatalysis continuously reduce Fe³⁺ to Fe²⁺ in the Fenton process, significantly accelerating the cyclic generation rate of hydroxyl radicals and markedly enhancing the degradation efficiency and mineralization degree of MPs such as PE and PP [73].

Conversely, the combination of photocatalysis and membrane filtration technology can realize the interception and in situ degradation of MPs. After the MPs are retained by the membrane, the photocatalytic material loaded on the membrane surface degrades under light, which not only alleviates the issue of membrane fouling and prolongs the service life of the membrane material but also alleviates the secondary disposal problem of pollutants [129]. Additionally, the combination of photocatalysis and adsorption technology also shows excellent performance. The g-C₃N₄/CQD/FeNi-BTC composite material constructed by combining g-C₃N₄ with carbon quantum dots (CQDs) and FeNi-BTC MOFs can achieve nearly 100% adsorption of 1200 mg/L MPs in 45 min [95]. Oliva et al. [130] combined photocatalysis, electrostatic adsorption, and magnetic separation technology, using magnetic bismuth ferrite (BiFO) particles to achieve 100% removal of PS MPs in 90 min, and reached 195.5 mg/g adsorption capacity and a 95.5% mineralization rate.

Enzymatic degradation relies on the action of enzymes (such as esterases, cutinases, laccases, etc.), produced by specific microorganisms, on the chemical bonds of polymers. It offers notable advantages, including mild reaction conditions and environmental friendliness [131]. However, its efficacy is often constrained by various factors. To improve degradation efficiency, physical or chemical pretreatments such as UV irradiation, thermal treatment, or mechanical abrasion are frequently employed. These processes oxidize the MP surface and generate microcracks, thereby exposing more enzymatic active sites. Such efforts pave the way for subsequent enzymatic reactions and enhance monomer recovery yield.

Furthermore, enzymatic degradation synergizing with entire biological systems is currently a research hotspot. A synergistic consortium comprising Pseudomonas aeruginosa and Xanthomonas campestris has been successfully employed in the biodegradation of MPs. The key mechanism lies in the effective hydrolysis of MPs’ molecular chains by enzymes (such as PHB depolymerase) produced by the microorganisms, confirming the feasibility of biodegradation [132]. In one study, the degradation efficiency of LDPE MPs by fungi (Sarocladium strictum) and bacteria (Bacillus velezensis) reached 26.3% within 60 days through enzyme synergy, and the degradation rate was 2–4 times that of single culture [133].

In recent years, the development of photocatalytic-enzymatic degradation synergistic systems has emerged as a highly attractive frontier research direction. The aim of this approach is to achieve more efficient degradation of mineralized phosphates by simultaneously harnessing the oxidative capacity of photocatalysis and the specificity of enzymatic activity. Such synergy enables immediate enzymatic of intermediates formed during photocatalytic degradation, thereby enhancing mineralization efficiency and reducing costs. At present, research on photocatalytic–enzymatic degradation synergy remains scarce; however, preliminary studies demonstrate promising potential. For instance, Zhang et al. [134] developed an MCN/HRP photocatalyst/enzyme heterojunction (PEH) by immobilizing horseradish peroxidase (HRP) onto mesoporous carbon nitride (MCN). PEH demonstrated enhanced degradation of bisphenol A (BPA), a common MP component, achieving an 85.7% degradation rate. This rate significantly surpassed the 2.7% achieved by the enzyme alone or the 48.2% achieved by the photocatalyst alone. This superiority stems not only from enhanced visible light absorption and charge separation efficiency but also from the crucial role of photoenzymatic synergistic catalysis. Additionally, as a novel autonomous material, magnetic micromotors/microrobots (MNMs) can integrate physical motion to enhance mass transfer, chemical catalysis to produce reactive oxygen species and biological enzyme immobilization potential, and simultaneously realize adsorption and degradation, and magnetic components can be rapidly recovered to avoid secondary pollution [135]. This multi-method joint mode breaks through the limitations of a single technology, improves the removal efficiency of MPs through functional integration, and provides an efficient and sustainable technical path for environmental pollution control [136]. The results presented in

Table 6. Examples of specific use of synergetic methods.

Combination		MPs	Degradation Efficiency (%)	Time	References
Photocatalytic degradation	Combined with electrostatic adsorption and magnetic separation technology	PS	100%	90 min	[130]
	Combined with the Fenton reaction	PE	CI increased 0.127	6 h	[73]
	Fixing Ag-TiO2 onto an Al2O3 Ceramic Membrane	PMPF	23.3%	28 h	[129]
	ZIF-67/g-C3N4 synergistically with PMS under visible light	PET	60.63%	6 h	[137]
Enzymatic degradation	Combined with high-temperature composting	-	43.7%	45 d	[138]
	Combined with UV pretreatment	PE	-	6 d	[38]
Photocatalysis-enzyme	Horseradish peroxidase (HRP) immobilized on mesoporous carbon nitride (MCN)	BPA	85.7%	-	[134]

“-” represents unknown.

demonstrate the examples of specific use of synergetic methods.

5. Conclusions and Prospects

5.1. Conclusion

MPs represent a pervasive environmental threat due to their persistence, mobility, and capacity to adsorb toxic pollutants. In this review, we have comprehensively analyzed the photocatalytic and enzymatic degradation of MPs. Photocatalytic degradation exhibits relatively high efficiency and, utilizing light as the primary energy source, predominantly generates non-toxic by-products, rendering it environmentally favorable. However, its efficiency remains subject to improvement, whilst the preparation and modification of high-performance catalysts incur substantial costs, thus maintaining a distance from large-scale application. Although enzymatic degradation provides eco-friendly pathways, the associated degradation rates present an ongoing threat to the environment. In general, each degradation technology has its own advantages and disadvantages. In practical use, appropriate degradation methods should be selected after comprehensive consideration. At present, combined treatment technology is used more frequently and is a research hotspot, because it can harness advantages in terms of both efficiency and cost.

The current degradation technology still has some limitations. Most of the existing technologies are in the experimental stage at the laboratory scale; large-scale implementation is challenging, and the actual degradation effect may be unstable. In light of the current landscape in MP degradation research, the authors of future studies should focus on the formation, degradation, and toxicity of MPs. It is still necessary to develop new degradation schemes, combine technologies to improve efficiency, reduce pollution and cost, and enhance the practical application scale of technology. Additionally, to assess pollution levels and degradation success rates, it will be necessary to build real-time monitoring systems for MPs in various environments. Researchers should also focus on the production, disposal, and recycling of sustainable MPs and aim to solve the problem at the source by formulating laws and strict management strategies. Furthermore, the safe degradation of MPs can aid in purifying community water sources, soil, and air; reducing health risks; saving governance costs; enhancing the environment and cohesion; and providing more sustainable homes for the future.

5.2. Challenges and Prospects

Microplastic pollution has emerged as a global environmental crisis. Different degradation technologies represent critical countermeasures, yet their development entails both significant opportunities and formidable challenges [48]. The core difficulty resides in the uncertainties associated with technological implementation. The diverse sources and complex distribution of MPs, combined with their intrinsic structural stability, constitute the primary barrier to degradation [28,48]. Furthermore, the coexistence of multiple contaminants and dynamic physicochemical conditions within actual environmental matrices (aquatic systems, soils, and sediments) substantially compromises the degradation efficiency [2]. The photocatalytic and enzymatic degradation methods highlighted in this review frequently incur prohibitive costs when scaled for engineering applications, raising concerns about economic viability [139]. The degradation processes themselves risk generating toxic intermediates or end-products, presenting secondary contamination hazards [35]. Additionally, standardized, efficient, and real-time monitoring techniques for nanoscale degradation products and their ecotoxicity within complex environmental media remain notably underdeveloped [4,140].

Despite these challenges, microplastic degradation presents distinct opportunities. The most promising approach involves the strategic integration of diverse technological strategies, proven to markedly enhance the degradation efficiency and kinetics [141]. Certain pretreatment methods, such as ultraviolet radiation and mechanical fracturing, can be employed to disrupt the surface of MPs, thereby increasing their surface roughness and enhancing degradation efficiency. Furthermore, emerging technologies like MNMs can better integrate photocatalysis and enzymatic degradation with other techniques, thereby improving the MP degradation efficacy. Concurrently, the development of novel catalytic materials, the ongoing optimization of enzyme engineering, and synergistic technological breakthroughs offer potential pathways to overcome existing limitations [142]. Through these advancements, the aim is not only end-of-pipe remediation but also to establish foundations for source-directed solutions (e.g., designing degradable materials).