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Review

Photocatalytic Technologies for Transformation and Degradation of Microplastics in the Environment: Current Achievements and Future Prospects

by
Anyou Xie
1,
Meiqing Jin
1,*,
Jiangwei Zhu
2,
Qingwei Zhou
1,
Li Fu
1,* and
Weihong Wu
1
1
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China
2
Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing 210037, China
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(5), 846; https://doi.org/10.3390/catal13050846
Submission received: 27 March 2023 / Revised: 2 May 2023 / Accepted: 4 May 2023 / Published: 6 May 2023
(This article belongs to the Special Issue Chemical Catalysis for Waste Plastics Recycling and Upcycling)
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Abstract

:
Microplastic (MP) pollution has emerged as a significant environmental concern, with exposure to it linked to numerous negative consequences for both ecosystems and humans. To tackle this complex issue, innovative technologies that are capable of effectively eliminating MPs from the environment are necessary. In this review, we examined a variety of bare and composite photocatalysts employed in the degradation process. An in-depth assessment of the benefits and drawbacks of each catalyst was presented. Additionally, we explored the photocatalytic mechanisms and factors influencing degradation. The review concludes by addressing the current challenges and outlining future research priorities, which will help guide efforts to mitigate MP contamination.

1. Introduction

In recent years, the rapid growth of industrialization and consumerism has given rise to a burgeoning environmental concern: MPs. The definition, sources, prevalence, and potential ecological risks of MPs have become critical topics in the field of environmental science [1]. MPs are small plastic particles that are less than 5 mm in diameter, originating from a variety of sources [2]. They can be classified into two groups: primary and secondary MPs. Primary MPs are intentionally manufactured at small sizes, such as microbeads used in personal care products, industrial abrasives, and pre-production plastic pellets, also known as “nurdles”. Secondary MPs are derived from the fragmentation and degradation of larger plastic items, such as plastic bags, bottles, and fishing gear, due to physical, chemical, and biological processes in the environment [3].
The prevalence of MPs in the environment has been increasingly documented in various ecosystems, including marine, freshwater, terrestrial, and even atmospheric environments [4,5]. MPs have been detected in remote locations, such as the Arctic and deep-sea sediments, indicating their extensive distribution and long-range transport potential. Research has shown that the primary sources of MPs in the environment are urban runoff, wastewater treatment plants, and direct littering, among others [6]. The accumulation of MPs in the environment has become a global issue, posing threats to ecosystems and human health [7].
MP pollution has rapidly emerged as a pressing environmental issue, posing significant threats to ecosystems, biodiversity, and human health [8]. The ubiquitous presence of MPs in various environments, coupled with their potential to cause a myriad of adverse effects, underscores the importance and urgency of addressing this global problem [9]. Ecologically, MPs have been shown to cause harm to a wide range of organisms across various trophic levels [10]. Ingestion, entanglement, and habitat alteration are some of the key mechanisms through which MPs can detrimentally affect the health and survival of aquatic and terrestrial species [11]. Moreover, MPs can act as vectors for the adsorption and transport of toxic contaminants, which can accumulate in organisms and biomagnify through food webs [12]. This exacerbates the risk of adverse effects on ecosystems, leading to potential disruptions in ecosystem functioning, food web dynamics, and species interactions [13].
From a human health perspective, the presence of MPs in the environment and their potential for bioaccumulation raises concerns about their potential impacts on human health [14]. MPs have been detected in various food sources, including seafood, bottled water, and even table salt, posing potential risks of ingestion by humans [15]. Although the precise health implications of MP ingestion are not yet fully understood, the current evidence suggests that MPs may cause inflammation, oxidative stress, and genotoxicity, among other effects [16]. Furthermore, the potential transfer of toxic contaminants adsorbed onto MPs to humans through food consumption may exacerbate the health risks that are associated with MP pollution [17]. As such, the need to address MP pollution is not only an ecological imperative but also a matter of public health urgency.
Given the importance and urgency of addressing this issue, the development and application of innovative and effective remediation strategies are crucial. Among the various emerging technologies, photocatalytic degradation has demonstrated promising potential in addressing MP pollution [18,19,20,21,22,23]. Photocatalytic degradation is a process that employs photocatalysts, typically semiconductor materials such as titanium dioxide (TiO2) or zinc oxide (ZnO), to promote the degradation of organic pollutants, including MPs, under the influence of light. Upon irradiation with ultraviolet (UV) or visible light, photocatalysts generate electron–hole pairs, which can initiate oxidation and reduction reactions with surrounding molecules, such as water and dissolved oxygen [24,25,26,27]. These reactions produce highly reactive species, which can effectively break down the chemical bonds in MPs, ultimately converting them into less harmful by-products, such as carbon dioxide and water. One of the main advantages of photocatalytic technologies is their potential to degrade a wide range of MP types, including polyethylene (PE) [28], polypropylene (PP) [29], and polystyrene (PS) [30], which are commonly found in the environment. Moreover, photocatalytic degradation can be applied to both primary and secondary MPs, offering a versatile solution to MP pollution from various sources. Additionally, photocatalytic degradation can potentially lead to the complete mineralization of MPs. However, it is important to note that some hazardous by-products may be produced during the process.
Despite the promising potential of photocatalytic technologies, several challenges need to be addressed to improve their efficiency and applicability in real-world scenarios. First, the efficiency of photocatalytic degradation is highly dependent on factors such as the photocatalyst material, light source, and environmental conditions, which may vary significantly between different settings. Furthermore, the scalability of photocatalytic processes remains a challenge, as most studies have been conducted at the laboratory scale, and the transition to larger-scale applications may require substantial technical advancements and optimization. Additionally, the potential ecological impacts of photocatalysts, particularly in aquatic environments, should be carefully assessed, as some photocatalyst materials may have toxic effects on aquatic organisms or accumulate in the environment. The role of photocatalytic technologies in addressing MP pollution is an important and emerging area of research that holds significant potential for mitigating this global environmental issue. Although several challenges remain in the development and implementation of photocatalytic processes, ongoing research and advancements in materials science, process optimization, and understanding of the underlying mechanisms are expected to pave the way for the development of efficient, scalable, and environmentally friendly solutions for MP pollution. As we continue to explore the potential of photocatalytic technologies in mitigating MP pollution, it is imperative that we also consider the broader context of plastic waste management and adopt a holistic approach to reduce plastic consumption, promote recycling and the circular economy, and ultimately, protect our ecosystems and human health. In this review, we investigated various bare and composite photocatalysts utilized in the degradation of MPs. A thorough evaluation of the advantages and disadvantages associated with each catalyst was provided. Furthermore, we delved into the photocatalytic mechanisms and elements that impact degradation. This review culminates by discussing the present challenges and delineating future research directions, aiding in guiding endeavors to tackle MP pollution.

2. Types of Photocatalysts Used for MP Degradation

2.1. Zinc Oxide (ZnO)

Among various metal oxide photocatalysts, ZnO stands out due to its properties such as its exceptional optical characteristics, excellent electron mobility, and non-toxic nature. Additionally, ZnO is easy to produce and can be formed into diverse shapes and sizes. Nanoscale materials are particularly valuable due to their increased surface-to-volume ratio, since photocatalysis relies on surface interactions. Tofa et al. [31] reported the degradation of MPs, specifically low-density polyethylene film (LDPE), in water using visible light-activated heterogeneous ZnO photocatalysts. The oxidation of LDPE led to the creation of low molecular weight compounds, causing brittleness and surface changes in the LDPE. Furthermore, the catalyst’s surface area proved critical for enhancing LDPE degradation. These findings present new insights into eco-friendly technology for solving MP pollution while minimizing the by-products.
Uheida et al. [32] conducted a study on the visible light photocatalytic degradation of PP MP using ZnO nanorods (NRs) coated on glass fibers. The FTIR spectroscopy results confirmed an efficient photodegradation of PP MPs, as evidenced by carbonyl group formation and a high carbonyl index. The degradation of PP MPs occurred through chain scissions, leading to a smaller chain reorganization. Photodegradation for two weeks under visible light (equivalent to four weeks of half-day sunlight) resulted in a 65% reduction in the average particle volume compared to the initial PP MPs. These promising outcomes suggest that photocatalytic reactors could be successfully employed for sustainable MP removal.
Currently, the application of ZnO as a photocatalyst for MP degradation encounters several challenges that restrict its practical use. ZnO possesses a wide bandgap (3.37 eV) that enables absorption primarily in the UV region, which comprises a minor portion (approximately 5%) of the solar spectrum. This narrow absorption range results in poor utilization of solar energy and diminished photocatalytic performance. Secondly, the rapid recombination of photogenerated electron–hole pairs in ZnO curtails the generation of reactive species, such as hydroxyl radicals, which are crucial for MP degradation. The short lifetime of these charge carriers translates to reduced photocatalytic efficiency. ZnO NPs often exhibit a limited surface area and adsorption capacity, which can impede effective contact between the photocatalyst and MPs in aquatic environments. This decreased interaction further lowers the photocatalytic degradation efficiency. Additionally, the photocorrosion of ZnO under UV irradiation can lead to the formation of zinc hydroxide and the eventual dissolution of Zn2+ ions, which may raise concerns about their environmental and ecological impacts. The practical application of ZnO photocatalysts in the field necessitates efficient recovery and reuse, as well as a proper assessment of their ecological consequences. The potential release of ZnO NPs into the environment could pose risks to aquatic organisms and ecosystems, thus warranting further investigation. To address these challenges, researchers are exploring various strategies, including bandgap engineering, doping, and the development of composite photocatalysts to enhance the performance of ZnO-based photocatalytic systems for efficient MP degradation.
To further improve the photocatalytic capabilities of ZnO, various materials can be incorporated to adjust its optical attributes and promote electron–hole pair separation. Studies have shown that incorporating a plasmonic metal can boost photocatalytic efficiency. These factors are linked to the surface plasmon resonance phenomenon, where a specific wavelength of the electromagnetic spectrum interacts with platinum nanostructures, causing resonance of interfacial conduction electrons and resulting in heightened light absorption in the nanocomposite [33,34,35]. The amplified visible light absorption driven by plasmon absorption in Pt NPs, and the transfer of photogenerated electrons from ZnO nanorod interfaces to Pt diminish electron–hole recombination (Figure 1) [36]. Consequently, a 15% increase in photocatalytic performance is achieved under visible light exposure. The ZnO-Pt effectively degraded MP pollutants, such as residual LDPE films in water. The creation of oxygenated groups led to surface alterations. The modified ZnO-Pt catalysts displayed a roughly 13% greater capacity for oxidizing LDPE films compared to unmodified ZnO nanorods. As a result, enhancing ZnO with a plasmonic metal could be a promising strategy for accelerating the oxidation of MPs.

2.2. Titanium Oxides (TiO2)

TiO2 is commonly employed as a model photocatalyst due to its remarkable ability to oxidize organic pollutants [37]. In various research studies, TiO2 has demonstrated exceptional performance in removing organic pollutants under UV light. The potential of TiO2 for MP degradation merits further investigation, although some studies have utilized TiO2/plastic composite materials to develop photodegradable plastics, such as PP [29], polystyrene (PS) [38], and high- and low-density polyethylene (LDPE) films [39,40,41]. An enhanced removal of LDPE was observed using TiO2 nanotubes under visible light, while the activity of TiO2 was improved with copper phthalocyanine, polypyrrole, and multiwalled carbon nanotubes for PE removal [42]. Iron phthalocyanine and ferric stearate were employed for photodegradable PS [43]. Photocatalytic degradation as a means of addressing MPs has not been explored since 2020.
Nabi et al. [44] proposed an effective method for degrading and fully mineralizing MPs using a TiO2 NPs under UV exposure. Among the prepared films, Triton X-100 (TXT) demonstrated superior performance in ability compared to ethanol and water. TXT showed remarkable photocatalytic degradation and mineralization of polystyrene particles of various sizes as well as polyethylene. The enhanced activity of the TXT film can be ascribed to its surface hydrophilicity and structure. The degradation mechanism was explored using in situ DRIFTS and mass spectrometry, revealing the emergence of hydroxyl, carbonyl, and carbon–hydrogen groups. This study presents an eco-friendly and sustainable approach for addressing MP waste in the environment.
Similar to ZnO, TiO2 as a photocatalyst for MP degradation also faces several challenges that limit its practical application. First, TiO2 has a wide bandgap (3.2 eV for anatase) that allows absorption only in the UV region. This results in low utilization of solar energy and reduced photocatalytic efficiency. Second, the rapid recombination of photogenerated electron–hole pairs in TiO2 hinders the generation of reactive species, such as hydroxyl radicals, which is necessary for MP degradation. The limited surface area and adsorption capability of TiO2 may impede effective contact between the photocatalyst and MPs in aquatic environments. To overcome these challenges, researchers are exploring strategies such as bandgap engineering, doping, and the development of composite photocatalysts to improve TiO2-based photocatalytic systems for efficient MP degradation.
In a similar study, Ariza-Tarazona et al. [45] investigated the photocatalytic degradation of MPs using nitrogen-doped titanium dioxide (N-TiO2). The environmentally friendly N-TiO2 displayed a remarkable ability to cause mass loss in high-density polyethylene (HDPE) MPs in both solid and aqueous environments. Another photocatalyst created using a conventional, less sustainable sol-gel method also showed a strong capacity to promote mass loss in the extracted MPs within a water-based environment. The results indicated that the environmental conditions, interactions between pollutants and N-TiO2, and the surface area should be carefully adjusted or designed to avoid photocatalysis disruption. Zhou and colleagues [46] further improved the degradation of polyethylene terephthalate (PET)-fiber-based microplastics (FMPs) by incorporating platinum (Pt) into N-TiO2. Their findings showed that a hydrothermal pretreatment is crucial for creating an initial rough surface and reducing the molecular weight. This study not only achieves enhanced degradation of PET-FMPs but also provides valuable insights for developing reduction strategies in the field of environmental remediation in the future.
Silver-doped TiO2 demonstrated enhanced performance in degrading MPs in water samples from bottles [47]. The findings reveal that the catalyst can break down MP compounds in water under UV radiation, with a 3% Ag/TiO2 catalyst displaying the most effective degradation capability after 4 h of exposure. After the degradation process, only 9.5 mg of MPs remained, representing an 81% mass degradation percentage. In contrast, analyses without a catalyst showed no change in the MP’s mass. This highlights that the MP’s degradation process under UV light is reliant on the presence of photocatalysts and will not occur without them. Allé et al. [48] demonstrated the potential to mineralize polymethylmethacrylate and polystyrene nanobeads using TiO2 irradiation in an aqueous solution. They employed TiO2–P25/β-SiC foams in a flow-through setup, achieving a 50% TOC conversion after 7 h of treatment for removing polymethylmethacrylate MPs. The study also revealed that the photocatalytic process is effective for polymers with diverse molecular structures, such as polystyrene and polymethylmethacrylate, as well as various average nanobead sizes.

2.3. Heterojunction Photocatalyst

The formation of heterojunctions by combining two distinct semiconductors has proven to be one of the most effective techniques for achieving the spatial separation of photo-induced electron–hole pairs [49,50]. When the two semiconductors possess differing Fermi energy levels (EF) or work functions (W), a built-in electric field (ED) is expected to develop at the heterojunction interface due to spontaneous electron diffusion from the higher EF semiconductor to the lower EF one. Upon light exposure, photo-induced electrons and holes may be driven to move between the semiconductors through the built-in electric field, thereby reducing carrier recombination. It is crucial to consider that the formation of a built-in electric field at the heterojunction interface and photo-induced carrier transfer behavior rely on various factors, such as semiconductivity, work function, and conduction band/valence band potentials of semiconductors. Zhou et al. [51] reported a Z-scheme Bi2O3@N-TiO2 heterojunction. Bi2O3@N-TiO2 degrades approximately 10.23 wt% of PET-FMPs at pH 9, nearly triple the rate at pH 7. The experimental findings suggest that PET-FMP hydrolysis under alkaline conditions primarily contributes to the enhanced performance. The polyester modified with a g-C3N4/TiO2 composite was created using centrifugal electrospinning, which endowed it with photodegradable properties under sunlight [52]. The results demonstrated that the polyester altered with a 5% g-C3N4/TiO2 mixture exhibited exceptional self-degradation capabilities in aquatic environments when exposed to sunlight. After 400 h of illumination, the polyester matrix was nearly entirely degraded. The breakdown of the polyester macromolecule resulted in the formation of formic acid, sodium sulfate, and short-chain compounds, which subsequently decomposed into CO2 and H2O. The macromolecular chain degradation progressed from larger to smaller fragments until none remained.
Qin et al. [53] introduced a photochemical approach for the in situ creation of semiconductors within the porous framework of metal-organic frameworks (MOFs). The resulting heterojunction photocatalysts showcased potential for MP conversion applications. This innovative photochemical strategy facilitated the generation of evenly distributed Ag2O particles inside MOF pores, revealing numerous active sites. Enhanced charge transfer rates and a wide light-harvesting spectrum were achieved using Ag2O/Fe-MOF heterojunction structures. Exhibiting high photocatalytic performance, the Ag2O/Fe-MOF photocatalyst was effective in converting PEG, PE, and PET MPs. Furthermore, the Ag2O/Fe-MOF photocatalyst allowed for the highly selective conversion of MPs into valuable products.
Acuña-Bedoya et al. [54] explored photocatalytic degradation to tackle pollution caused by PS. They employed immobilized Cu2O produced through anodizing for the first time to degrade PS particles of around 350 nm. The anodization process utilized two growth media, resulting in Cu2O/CuO semiconductors featuring distinct morphologies and a bandgap. The degradation of MPs revealed intermediate compounds containing carbonyl groups, verifying PS degradation. The findings demonstrated that visible light photocatalysis using Cu2O/CuO promotes polymer chain breakage and decreases PS concentrations by up to 23%, which is a six-fold improvement over photolysis. Moreover, up to 15% mineralization was achieved.

2.4. Other Photocatalysts

ZnxCd1−xS has attracted considerable interest due to its exceptional optical properties, adjustable and suitable band gap, and high efficiency in utilizing visible light [55,56]. Simultaneously, binary alloy sulfides demonstrate enhanced capacity, chemical stability, and electrochemical activity compared to single metal sulfides [57,58]. Moreover, MXene has been found to have a favorable Fermi level position and remarkable electrical conductivity [59,60], enabling it to serve as an efficient catalyst promoter that quickens charge–hole separation, thus improving the performance [61,62]. Cao et al. [63] designed MXene/ZxC1−xS for achieving high-efficiency H2 evolution alongside MP degradation. The experimental results reveal that the H2 evolution rate of M−2/Z0.6C0.4S is the highest at 14.17 mM/h/g. Concurrently, the PET bottle undergoes oxidation, ultimately breaking down into smaller organic compounds such as glycolate, acetate, and methanol. This study introduces an outstanding method that enables simultaneous H2 production and plastic pollution degradation through photocatalyst band structure engineering.
Concurrently, research has been conducted on the photocatalytic degradation of MPs using BiOCl-based catalysts. Jiang et al. [64] developed a novel hydroxy-rich ultrathin BiOCl for degrading HDPE MPs. After five h of the reaction, the mass loss of polyethylene MPs reached 5.38%, a 24-fold increase compared to BiOCl. This could be attributed to the higher number of surface hydroxyl groups in BiOCl, which leads to a greater production of ·OH radicals under light exposure [65]. Similar mechanisms to those reported in traditional photocatalysts were observed, with ·OH radicals playing a significant role in the photocatalytic degradation. The proposed degradation pathway suggests that the initial event in this reaction is triggered by the hydroxyl radicals generated from BiOCl-1 attacking the C–H bond of the polyethylene molecule. This causes the polymeric chains to continuously fracture and ultimately degrade into CO2 and H2O.

3. Principles and Mechanisms of MP Degradation

Tofa et al. [31] proposed the breakdown of LDPE film using ZnO as a photocatalyst. The hydroxyl and superoxide radicals generated by the catalyst initiate degradation at susceptible sites, followed by chain breaking. Oxygen incorporation then leads to the formation of peroxy radicals, and hydrogen atoms are removed from the polymeric chains to create hydroperoxide groups. These hydroperoxide groups act as the primary oxygenated products controlling the degradation rate, where their dissociation into alkoxy radicals undergoes successive reactions to produce carbonyl and vinyl group-containing species, which ultimately cause chain cleavage. However, further oxidation can result in complete mineralization, yielding carbon dioxide and water [38,66,67]. Photocatalysis involves the production of reactive species such as holes, electrons, hydroxyl, and superoxide ion radicals. Comprehending the role of these species in the degradation process is crucial for designing an efficient photocatalytic system. Vital-Grappin et al. [68] explored the contributions of these reactive species in the degradation of HDPE MPs using C,N-TiO2. Tert-butanol, isopropyl alcohol, Tiron, and Cu(NO3)2 were identified as appropriate scavengers for OH•, h+, O2•−, and e−, respectively. The findings demonstrated that the generation of free OH• through pathways involving photogenerated e− plays a vital role in MP degradation. Additionally, when h+ and O2•− were removed from the reaction system, the observed degradation patterns indicated that these species could also initiate the degradation process. The generation of these reactive species is influenced by various factors, including the pH, temperature, and microplastic concentration. For instance, at lower pH levels, the generation of OH• may be favored, while at higher pH levels, s O2•− could be more prevalent. In addition, higher temperatures can potentially accelerate the generation of a reactive species and the overall degradation process due to the increased reaction rate at elevated temperatures. Furthermore, higher microplastic concentrations might lead to increased competition for active sites on the photocatalyst surface, influencing the overall degradation efficiency.
The photodegradation process of PS in a solid state commences with holes and reactive oxidative species such as ·OH and O2·− [44]. Upon UV light exposure, TiO2 becomes excited, generating an electron in the conduction band (e−CB) and a positive hole in the valence band (h+VB). Two potential pathways for initiating the reaction exist. In the first route, the oxidation reaction takes place at the valence band. The h+VB, a potent oxidizing agent, can oxidize organic compounds, leading to CO2 and H2O formation, and generate hydroxyl radicals through oxidation. Electrophilic ·OH radicals can non-selectively oxidize electron-rich organic compounds, ultimately resulting in their mineralization. In the second pathway, the e−CB interacts with oxygen, forming a superoxide radical. This superoxide radical reacts with water, producing ·OOH, which then generates H2O2 [69]. Finally, hydrogen peroxide transforms into ·OH, which reacts with MPs, causing their degradation, as photocatalysis involves generating reactive species such as holes, electrons, hydroxyl, and superoxide ion radicals.
The mechanism of photocatalytic degradation of PE MPs using C,N-TiO2 photocatalyst is thoroughly examined [70]. As depicted in Figure 2, the degradation process begins with hydroxyl radicals attacking fragile polymer chains, producing polyethylene alkyl radicals. Subsequently, the alkyl radical reacts with oxygen atoms, propagating the degradation process by forming a peroxy radical. This new peroxy radical cleaves a hydrogen bond from another PE chain, generating a hydroperoxide species. Hydroperoxide then decomposes into two free radicals, oxy and hydroxyl radicals, by breaking the O-O bonds. The generated radicals proceed to detach unstable hydrogen atoms from other polymer chains. An equilibrium is demonstrated, showing the ratio of peroxy radicals to hydroperoxide. In a separate study, Jiang et al. [64] developed BiOCl for PE MP degradation. The electron paramagnetic resonance, capture experiments, and BiOCl’s surface hydroxyl groups contribute to increased hydroxyl radical production and efficient photocatalytic degradation. Moreover, the abundance of hydroxyls in BiOCl-X may aid in dispersing it in aqueous solutions and enhancing the active surface sites and charge transfer.

4. Morphological Changes of MPs during Photocatalytic Degradation

The morphology of photocatalysts plays a crucial role in the photocatalytic degradation process. Different morphologies can lead to variations in surface area, porosity, and active site accessibility, ultimately influencing the overall photocatalytic performance. The interplay between the impact factors, photocatalytic mechanisms, and the morphology of the catalysts is essential for understanding the process and optimizing the photocatalytic performance. For instance, impact factors such as the pH, temperature, and microplastic concentration can affect the catalyst morphology, which in turn, can influence the generation of reactive species and the photocatalytic mechanism. By optimizing the morphology of the catalysts, it is possible to enhance the performance of the photocatalytic process by promoting better adsorption of microplastics, increased generation of reactive species, and efficient utilization of active sites on the catalyst surface.
Figure 3 displays the morphological changes in PS MPs on TiO2 NPs during the photocatalytic degradation. PS MPs were uniformly distributed across all films. Figure 3A reveals the exact dimensions and morphology of PS on FTO (without a photocatalyst) at 0 h. A barely noticeable change in PS size occurred throughout the irradiation process (Figure 3B). However, PS MPs exhibited changes in their morphology and size during the first 3 h of irradiation. Photodegradation-induced coalescence of PS occurs, suggesting that degradation transpires at the PS-air interface. More significant alterations in PS morphology and size were observed after an additional 3 and 6 h of irradiation (Figure 3D,F,H). The TXT film demonstrated a more pronounced change in PS morphology (Figure 3H) and size compared to ethanol and water films. Additionally, the behavior of different sized PS MPs on the TXT film was investigated, which exhibited substantial changes in morphology and a decrease in size. Based on these observations, TXT is highly effective in the photocatalytic degradation of PS. AFM-IR imaging was performed on selected PS MPs subjected to photo-aging for various durations [72]. Figure 3 displays topographical images and AFM-IR images of the carbonyl group. The AFM-IR signal intensity is influenced by factors such as surface morphology, roughness, thickness, and other MPs’ material properties, including individual compounds’ thermal expansion coefficient [73,74]. The bright purple particles in the topographical images (Figure 4a–c) were identified as TiO2 NPs that were embedded in the MPs’ protruding areas. The images in Figure 4d–f represent the C=O bond distribution, with brightness indicating this functional group’s signal intensity. As seen in Figure 4b,e, the bright purple circular shape in the image corresponds to the TiO2 NPs, matching the dark brown regions in the AFM-IR image [70]. A strong purple signal surrounding the brown area indicates significant surface oxidation around the TiO2 NPs during the photocatalytic aging process.
The morphological changes of PP MPs during the photodegradation were further examined using SEM [32]. Notable alterations in the surface microstructure of the MP particles can be attributed to a combination of factors, such as the elimination of photodegradation by-products, reorganization of the surface amorphous content, and the rise in crystalline fractions, which ultimately lead to the surface layer’s contraction and the development of cracks and cavities. The emergence of surface cracks and cavities can intensify the degradation process by allowing oxygen to permeate more deeply into the sample, thereby promoting photooxidation. As photocatalytic treatment duration increases, the size and density of the cavities consistently expand. The formation of cavities may also result from the release of volatile degradation products from the polymer particle surfaces.

5. Factors Affecting Photocatalytic Degradation Efficiency

The impact of operating parameters on the photocatalytic degradation of MPs should be taken into account [70]. The photocatalytic degradation efficiency of MPs is impacted by various factors, such as the solution’s pH, the termination rate, and the ease of generating free radicals that are essential for extracting hydrogen atoms from polymer chains. Understanding the impact of these factors on the photocatalytic mechanism and morphology of the photocatalysts is crucial for optimizing the process and enhancing its performance. For example, the pH of the solution influences the generation of reactive species (e.g., hydroxyl radicals and superoxide radicals) that participate in the photocatalytic mechanism, as well as the adsorption of MPs onto the photocatalyst surface. The temperature affects the reaction rate and the generation of reactive species, ultimately influencing the overall degradation efficiency. Lastly, the MP concentration can impact the available active sites on the photocatalyst surface, consequently affecting the morphology and performance of the photocatalysts. By examining these factors and their effects on both the mechanism and morphology, a more holistic understanding of the photocatalytic degradation process can be achieved, ultimately leading to optimized processes with enhanced performance.
Figure 5 displays the degradation graphs of photocatalytic tests conducted under various temperatures and pH conditions. The enhanced degradation efficiency at pH 3 could be explained through Le Chatelier’s principle, which suggests that hydroperoxide synthesis accelerates with an increased H+ ion concentration. Furthermore, the photocatalyst’s surface charge and inter-particle electrostatic attraction can also be influenced by the pH level [75]. MPs in alkaline solutions are more prone to degradation in acidic conditions due to Coulomb repulsion, which inhibits degradation by shielding the MPs [70].
For instance, photocatalysis under low pH conditions is more conducive to the reduction in HDPE MP concentration levels [70]. At pH 3 and 0 °C, the average mass loss after 50 h of irradiation reached 71.77%. This result can be credited to the combined effects of the pH and temperature on MP degradation. Continuing with the experiments at pH 3, the average mass loss after 50 h of irradiation at 40 °C was 12.42%, nearly one-sixth of the value obtained at the same pH and 0 °C (71.77%). As no plastic fragmentation occurs at 40 °C, the limited degradation was primarily attributed to the interaction between MPs and the C,N-TiO2. The test carried out at 20 °C and pH 7 produced the lowest mass loss. If the pH of the reaction system is further increased, degradation is not favored under alkaline conditions. At pH 11, degradation in terms of mass loss was 1.55% and 0.78% for temperatures of 0 °C and 40 °C, respectively. The morphology of the MPs after photocatalysis under these conditions is similar to that of the as-extracted MPs. The SEM micrograph further confirms that the MPs exposed to photocatalysis under alkaline conditions display a surface that closely resembles that of the as-extracted MPs.
The adsorption of MPs on a photocatalyst’s surface can be influenced by their size. While MPs with diameters of a few nanometers are likely to adsorb on the semiconductor, the larger sizes of most common MPs hinder their adsorption. For instance, MPs from a commercially available facial scrub have an average particle size of 850 µm [45,70], which is 17,000 times larger than Degussa P25 TiO2 particles [76]. Consequently, a significant challenge in MP photocatalysis is addressing their limited or non-existent adsorption on the semiconductor due to their substantial particle size. Furthermore, the shape of MPs may impact their dispersion behavior and contact with the semiconductor, warranting investigation when developing photocatalytic systems that aim for complete MP mineralization to CO2 and H2O. Llorente-García et al. [77] explored visible light photocatalysis of HDPE and LDPE MPs using a mesoporous N-TiO2 coating as a potential solution. The findings indicate that the size and shape of the MPs affected their photocatalytic degradation using the N-TiO2 coating. Smaller MPs resulted in greater degradation, while film-shaped MPs led to reduced degradation.

6. Micromotors Related to Photocatalytic Degradation of MPs

Micromotors are specifically designed to transform local chemical energy, external field energy (such as magnetic or electric), or light into a self-directed motion, allowing them to carry out specialized functions in aquatic settings [78,79]. These micromotors come in a variety of forms [80]. Both chemical- and light-driven micromotors have been widely studied for numerous water purification applications [81]. In comparison to stationary alternatives, micromotors generate a micromixing effect through self-propulsion, which increases mass transfer in their vicinity and improves the elimination of target pollutants in the solution [82]. Orozco et al. [83] showcased the positive influence of H2O2-driven micromotors on the decontamination of organophosphate nerve agents. Apart from their microstirring capabilities, catalytic micromotors can also act as environmental microcleaners, as H2O2 is recognized as an eco-friendly oxidant for wastewater treatment [84]. Biotemplated tubular Fe3O4–MnO2 micromotors have been demonstrated to decompose H2O2, generating both oxygen bubbles for self-propulsion and hydroxyl radicals for efficient removal of organic dyes [85].
The collection and elimination of minuscule MPs particles with neutral surface properties pose a significant challenge, leading to the failure of current systems in filtering them out of tap water. Consequently, devising innovative methods to capture and degrade these particles is crucial. Beladi-Mousavi et al. [86] have developed visible light-driven microrobots that are capable of capturing and transporting substantial quantities of MPs in complex environments in response to sunlight, as well as effectively breaking them down into smaller oligomers (Figure 6). This energy-efficient approach, powered solely by wireless sources, eliminates the need for pretreatment processes or bulky mechanical stirrers that are typically found in traditional systems. It offers a combined solution for MP removal and in situ degradation. Considering the ongoing research into sunlight-driven photocatalysts for MP degradation, the innovative strategy presented in this study, which incorporates autonomous motion, enables an efficient capture and degradation of MPs. This could lay the foundation for a new, intelligent system that could be applicable in hard-to-access environments.
In a similar vein, Wang et al. [87] have designed an innovative passive particle elimination system composed of photocatalytic Au@Ni@TiO2 micromotors. They examined the motion of individual particles and assembled chains in different fuels, specifically H2O and H2O2. They proposed and implemented two strategies for MPs and suspended matter removal, which were applied to both the suspended matter and MPs found in personal care products and collected from open waters. The latter system functions efficiently regardless of the fuel type, in dilute peroxide solutions and water, providing a practical advantage for MP removal in real-world settings. They also demonstrated, for the first time, that material and shape limitations do not affect the removal of matter by light-driven micromotors. The most significant challenge moving forward is to enhance selectivity, allowing the micromotors to recognize MPs and, in turn, improve MP removal efficiency.
Aurivillius-based compounds, particularly Bi2WO6, have been widely investigated as promising visible light-responsive photocatalysts for environmental restoration [88]. The creation of perovskite-like Bi2WO6 micromotors involves the self-assembly of NPs into 3D microspheres [89]. These micromotors demonstrate self-propulsion at a speed of 1.9 μm/s under visible light in water-based environments. Moreover, individual micromotors cluster and move collectively due to the chemical gradients. The micromotors were evaluated for their capability to break down commercial baby wipes, which consist of PP, modified cellulose, and other additives. Wet wipes and sanitary towels are personal care textile items that contribute to waste MP fiber pollution in aquatic systems when disposed of in toilets. After 50 h of continuous light exposure, the micromotors actively adhered to the textile fibers and degraded them, as evidenced by the emergence of cracks and the development of larger holes [90].

7. Challenges, Future Prospects, and Directions for Research

Despite the promising potential of photocatalytic degradation for the removal of MPs, several challenges need to be addressed to ensure the practical applicability and efficiency of this approach:
(1)
Stability and reusability: The long-term stability and reusability of photocatalysts are critical factors for their sustainable application. Developing robust catalysts that can maintain their performance over multiple cycles will be essential for large-scale deployment.
(2)
Toxicity of degradation products: As discussed earlier, the complete degradation of MPs may generate potentially toxic intermediates. It is crucial to assess the toxicity of these intermediates and develop photocatalysts that minimize the formation of harmful by-products.
(3)
Selectivity: The presence of multiple pollutants in water bodies may affect the selectivity of photocatalysts toward MPs. Developing selective photocatalysts that can specifically target MPs without being affected by other pollutants will improve the overall efficiency of the degradation process.
(4)
Scalability: Most studies on photocatalytic degradation of MPs are conducted at the laboratory scale. Scaling up these processes for real-world applications while maintaining their efficiency is a challenge that needs to be addressed.
(5)
Economic feasibility: The cost-effectiveness of photocatalytic degradation methods is a critical aspect for their large-scale implementation. Research should focus on reducing the costs that are associated with the photocatalysts, reactor design, and energy consumption.
Photocatalysis has been widely investigated for a variety of emerging contaminants; however, research on MP removal has only recently started gaining traction, leaving numerous research gaps to be addressed. The majority of existing studies concentrate on individual photocatalytic systems, which may be constrained by factors such as rapid electron–hole recombination, unsuitable band values and positions, and sluggish surface reaction kinetics [91,92]. These limitations lead to less-than-optimal photocatalytic degradation performance, marked by low efficiency and extended irradiation periods, thus impeding further applications. As a result, there is a demand for optimizing these photocatalysts, such as by developing composite photocatalysts that exhibit an improved performance compared to single systems.
In addition to metal-based catalysts, metal-free catalysts, regarded as environmentally friendly options, have demonstrated the effective removal of numerous emerging contaminants, including endocrine disruptors and antibiotics [60]. However, research on MP removal using metal-free catalysts is lacking, necessitating further investigation. Photocatalysis, although promising for complete MP removal in water, lacks the adequate analysis of treatment costs. Additionally, catalyst reusability and stability during the MP removal process are crucial for assessing the sustainable applicability of the catalyst, but the relevant research is scarce.
Research has been conducted to assess the impact of MPs on living organisms, demonstrating increased toxic effects on non-selective filter feeders that accidentally consume MPs rather than nutritious substances. MPs may cause a range of internal damage, including obstructions in the digestive tract, reduced food intake, or infiltration into the circulatory system [93,94]. Chronic effects from MP ingestion have been observed, such as negative impacts on the tissues and intestinal tracts of Mytilus edulis caused by HDPE MPs. Exposure to 20 μm MP beads led to reduced fecundity in Calanus helgolandicus (pelagic copepods) [95]. A positive correlation was found between MPs concentrations in sediment and both plastic particle uptake and weight loss in A. marina. Lungworms exposed to micro-PS and PCBs experienced a decreased feeding ability and weight loss [96]. Chemicals absorbed by plastics were found to cause liver toxicity in Japanese medaka [97]. Luís et al. [98] studied the potential influence of MPs on the short-term toxicity of chromium to Pomatoschistus microps juveniles. They found a significant reduction in predatory performance (up to 67%) and the inhibition of acetylcholinesterase activity. Additionally, MPs have a large surface area, allowing them to act as vectors for various pollutants, such as POPs and heavy metals, during environmental transport [99]. At present, achieving complete photocatalytic degradation of MPs is challenging, leading to the generation of potentially toxic degradation intermediates [32,64]. Nevertheless, most studies have not evaluated the toxicity of these intermediates. Such research gaps result in an incomplete assessment of the feasibility of photocatalysts.
Another direction for improving photocatalytic degradation performance is through band gap engineering and studying band edge potentials. By manipulating the band gap and band edge potentials, it is possible to generate specific radical species that are more effective in degrading MPs. Tailoring the band gap can enhance the photocatalytic activity by increasing the absorption of light in the visible spectrum and facilitating the generation of electron–hole pairs [100]. Adjusting the band edge potentials ensures that the photogenerated electrons and holes are effectively utilized in redox reactions to generate reactive radical species, ultimately improving the degradation of MPs [54]. Future research should focus on developing photocatalysts with optimal band gap and band edge potentials to maximize the efficiency of photocatalytic degradation of MPs in water.
Global plastic production reaches approximately 300 million metric tons (367 m mt) annually. Given this immense plastic production, the development and commercialization of plastic degradation and removal methods should be a primary focus for researchers. One crucial aspect to consider in terms of commercialization is the Technology Readiness Level (TRL) [101]. Evaluating the TRL of proposed and utilized degradation and removal techniques is a critical subject. However, photocatalytic degradation methods currently sit at the lower end of the TRL scale and require significant development before they can be commercially applied to plastic or MP degradation. In addition to the TRL scale, the Electrical Energy per Order (EEO) is another important value to examine for photocatalytic degradation methods [102]. These two factors can help determine the progress of photocatalytic degradation from laboratory experiments to commercial use. Pang et al. [103] identified several key requirements for elevating the TRL of photocatalytic degradation methods, including enhancing photocatalytic activity, advancing photocatalytic reactor and system technology, and exploring the scalability of photocatalytic technology.

8. Conclusions

MP pollution in urban water bodies is a pressing concern due to its potential risks to aquatic life and human health. This review highlights the characteristics of MPs, including their sources, environmental transport, and ecological consequences. MPs have been demonstrated to cause physical harm, interfere with nutrient intake, and serve as vectors for other pollutants in aquatic ecosystems.
Photocatalytic degradation has emerged as a promising technique for the removal of MPs in water. The effectiveness of photocatalytic degradation largely depends on factors such as catalyst type, light source, and operational conditions. However, current photocatalytic degradation methods struggle to break down MPs fully and efficiently, often requiring extensive time to achieve satisfactory results.
Composite photocatalysts have shown potential for an improved performance compared to single systems, and metal-free catalysts represent environmentally friendly alternatives that warrant further research. Despite the promising potential of photocatalytic degradation, several challenges remain, such as the stability and reusability of photocatalysts, the toxicity of degradation intermediates, and the scalability and economic feasibility of the technology.
In conclusion, while photocatalytic degradation offers a potential solution for the mitigation of MP pollution in urban water bodies, significant advancements are required to address the challenges and limitations associated with this technique. By doing so, researchers can contribute to the development of effective and sustainable strategies for MP removal, ultimately safeguarding aquatic ecosystems and human health.

Author Contributions

Conceptualization, W.W. and L.F.; software, A.X.; validation, A.X. and M.J.; formal analysis, A.X., Q.Z. and J.Z.; investigation, A.X., M.J. and J.Z.; resources, L.F., Q.Z. and W.W.; data curation, M.J., J.Z. and Q.Z.; writing—original draft preparation, A.X., M.J., J.Z. and Q.Z.; writing—review and editing, L.F. and W.W.; visualization, L.F.; supervision, L.F. and W.W.; project administration, W.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by the Ningbo Science and Technique Plan Project (2022S110).

Data Availability Statement

No applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Photodegradation of LDPE films with plasmonic ZnO−Pt photocatalysts. Reprinted/adapted with permission from Ref. [36]. 2023, MDPI.
Figure 1. Photodegradation of LDPE films with plasmonic ZnO−Pt photocatalysts. Reprinted/adapted with permission from Ref. [36]. 2023, MDPI.
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Figure 2. Mechanism of PE degradation via photocatalytic C,N−TiO2. Reprinted/adapted with permission from Ref. [71]. 2023, Elsevier.
Figure 2. Mechanism of PE degradation via photocatalytic C,N−TiO2. Reprinted/adapted with permission from Ref. [71]. 2023, Elsevier.
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Figure 3. FE-SEM images of PS spheres after different irradiation time under 365 nm UV light. (A) PS spheres on FTO (without catalyst) before irradiation. (B) Without catalyst after 12 h of irradiation. (C) Water-based TiO2 (WT) after 6 h of irradiation. (D) WT after 12 h of irradiation. (E) Ethanol-based TiO2 (ET) after 6 h of irradiation. (F) ET after 12 h of irradiation. (G) Triton X-100-based TiO2 (TXT) after 6 h of irradiation. (H) TXT after 12 h of irradiation. Reprinted/adapted with permission from Ref. [44]. 2023, Elsevier.
Figure 3. FE-SEM images of PS spheres after different irradiation time under 365 nm UV light. (A) PS spheres on FTO (without catalyst) before irradiation. (B) Without catalyst after 12 h of irradiation. (C) Water-based TiO2 (WT) after 6 h of irradiation. (D) WT after 12 h of irradiation. (E) Ethanol-based TiO2 (ET) after 6 h of irradiation. (F) ET after 12 h of irradiation. (G) Triton X-100-based TiO2 (TXT) after 6 h of irradiation. (H) TXT after 12 h of irradiation. Reprinted/adapted with permission from Ref. [44]. 2023, Elsevier.
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Figure 4. AFM topographical images of nano−TiO2−coated PS MPs after photocatalytic aging for (a) 0 h, (b) 2 h, and (c) 4 h; AFM-IR images of MPs collected at the carbonyl peak position after photo-aging for (d) 0 h, (e) 2 h, and (f) 4 h. Reprinted/adapted with permission from Ref. [72]. 2023, Elsevier.
Figure 4. AFM topographical images of nano−TiO2−coated PS MPs after photocatalytic aging for (a) 0 h, (b) 2 h, and (c) 4 h; AFM-IR images of MPs collected at the carbonyl peak position after photo-aging for (d) 0 h, (e) 2 h, and (f) 4 h. Reprinted/adapted with permission from Ref. [72]. 2023, Elsevier.
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Figure 5. (a) HDPE MP concentration and (b) mass loss of the MPs after the photocatalytic experiments at different experimental conditions. Reprinted/adapted with permission from Ref. [70]. 2023, Elsevier.
Figure 5. (a) HDPE MP concentration and (b) mass loss of the MPs after the photocatalytic experiments at different experimental conditions. Reprinted/adapted with permission from Ref. [70]. 2023, Elsevier.
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Figure 6. Schematic illustration of photocatalytic degradation of plastics using conventional systems and light−driven microrobots. Reprinted/adapted with permission from Ref. [86]. 2023, ACS.
Figure 6. Schematic illustration of photocatalytic degradation of plastics using conventional systems and light−driven microrobots. Reprinted/adapted with permission from Ref. [86]. 2023, ACS.
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Xie, A.; Jin, M.; Zhu, J.; Zhou, Q.; Fu, L.; Wu, W. Photocatalytic Technologies for Transformation and Degradation of Microplastics in the Environment: Current Achievements and Future Prospects. Catalysts 2023, 13, 846. https://doi.org/10.3390/catal13050846

AMA Style

Xie A, Jin M, Zhu J, Zhou Q, Fu L, Wu W. Photocatalytic Technologies for Transformation and Degradation of Microplastics in the Environment: Current Achievements and Future Prospects. Catalysts. 2023; 13(5):846. https://doi.org/10.3390/catal13050846

Chicago/Turabian Style

Xie, Anyou, Meiqing Jin, Jiangwei Zhu, Qingwei Zhou, Li Fu, and Weihong Wu. 2023. "Photocatalytic Technologies for Transformation and Degradation of Microplastics in the Environment: Current Achievements and Future Prospects" Catalysts 13, no. 5: 846. https://doi.org/10.3390/catal13050846

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