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Review

Systematic Review of Degradation Processes for Microplastics: Progress and Prospects

by
Peng Xiang
,
Ting Zhang
,
Qian Wu
and
Qiang Li
*
Key Laboratory of Coarse Cereal Processing, Ministry of Agriculture and Rural Affairs, Sichuan Engineering & Technology Research Center of Coarse Cereal Industrialization, School of Food and Biological Engineering, Chengdu University, Chengdu 610106, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(17), 12698; https://doi.org/10.3390/su151712698
Submission received: 24 June 2023 / Revised: 11 August 2023 / Accepted: 20 August 2023 / Published: 22 August 2023
(This article belongs to the Special Issue Urban Environment and Human Health)
Browse Figures
Review Reports Versions Notes

Abstract

:
Microplastics (MPs) have been shown to be more hazardous than large plastics. In recent years, many studies have confirmed the hazards of MPs to organisms and summarized various MP degradation techniques, but there is a lack of discussion on the prospects of the application of these degradation techniques and their degradation efficiency. Therefore, this paper reviewed the degradation techniques of MPs, such as adsorption, direct photodegradation, photocatalytic oxidation, electrochemical oxidation, and biological methods, and their application prospects. By focusing on the biodegradation mechanism and degradation efficiency, the potential for efficient and sustainable development of biodegradation processes and the prospect of large-scale application are highlighted, enabling readers to better understand the current status of research on MP biodegradation. This review provides direction for research on MP degradation, suggestions for governmental environmental governance and policy development, and references for the sustainability and large-scale application of MP biodegradation.

Graphical Abstract

1. Introduction

Due to their low price, extreme durability, light weight, and good ductility, plastics are now widely used in the construction, healthcare, electronic components, automotive, agriculture, and food packaging industries [1,2,3,4]. And because of the durability of plastics, they also pose a huge environmental hazard. Studies have shown that global plastic waste is expected to reach 270 million tons by 2060 [5]. Waste plastics undergo physical, chemical, biological, and other forms of wear, consumption, and decomposition, resulting in particles less than 5 mm in diameter being defined as microplastics (MPs) and particles less than 100 nm being defined as nanoparticles (NPs) [6,7,8,9]. Figure 1 shows the source of microplastics. Plastic particles, cosmetics, laundry wastewater, sewage sludge, and atmospheric deposition generated during the friction between motor tires and road surfaces are also important sources of MPs [10,11,12].
MPs can be absorbed by aquatic plants and animals and adversely affect their growth, development, and reproduction [13,14,15,16,17,18,19,20]. On the other hand, MPs can absorb various pollutants in the ocean, such as antibiotics [21,22], polycyclic aromatic hydrocarbons [23,24], heavy metals [25,26,27], organic compounds [28,29], and pathogenic microorganisms [30,31,32], and are capable of serving as carriers of pollutants that are fed on by aquatic organisms and thus entering the organisms. In addition, MPs can directly enter agricultural soils through sewage sludge, irrigation water, domestic water, and atmospheric deposition, or indirectly enter agricultural soils through the degradation of plastic residues (such as mulch films) in agricultural activities. Figure 2 shows the pathway of MPs into plants. When absorbed by terrestrial plants, MPs will inhibit the growth and development of the plants and remain in the plant bodies. The accumulation of MPs in the plant body will eventually reach the human body along with the enrichment of the food chain, causing harm to the human body.
At present, most of the waste plastics in the world are disposed of in landfills [33], which require a large amount of land. Due to the strong stability of plastic, it is not easy to decompose in the soil, which seriously affects the sustainable use of the soil. Moreover, a large number of microorganisms breed in the soil, producing harmful gases that adversely affect the surrounding air and environment [34]. Not only that, but the leachate from plastic waste will also enter the river through groundwater, causing harm to the environment and ecology. With the deepening of research, there are an increasing number of treatment methods for waste plastics. The MPs contained in the sewage treated by the sewage treatment plant are reduced, but there are still many small particles that are difficult to remove. Adsorption, advanced oxidation processes (AOPs), and biodegradation can accelerate the degradation of MPs through a series of physical and chemical reactions, thus increasing their degradation rate [35,36]. Among them, AOPs include direct photodegradation, photocatalytic oxidation, and electrochemical oxidation. However, there are shortcomings to these methods. The process of photodegradation is uncontrollable. Even under laboratory conditions, the degree of photoaging and the types of intermediates in the photochemical system cannot be completely determined. In addition, photodegradation consumes more energy. Prolonged exposure to sunlight may also cause light pollution [37]. For photocatalytic oxidation, although it uses free solar energy, most plastics can only be partially degraded under ultraviolet radiation, and the degree of degradation is not up to the requirements. The catalyst added in the reaction is also difficult to recover and can easily result in secondary pollution [35]. Electrochemical oxidation has broad application prospects in the treatment of degradable plastics due to its strong controllability, simple operation, and low secondary pollution. However, for these processes, the intermediate products obtained by their degradation are uncertain as to whether they are harmful or not, and it is difficult to control the reaction process. Therefore, much research is being conducted on the biodegradation process. MPs can serve as substrates for microbial biofilm growth and provide energy for microbial growth and reproduction. And the selection of biodegradation conditions is a key factor in improving the efficiency of degradation. pH is a critical factor for the survival of microorganisms, as it has a key influence on their life activities and substance metabolism [38]. An increase or decrease in pH during biodegradation may be due to the production and accumulation of alkaline aromatic compounds or other metabolites during degradation. As the biofilm grows, the plastic structure breaks, and during the assimilation process, it is taken up by the microorganisms (bacteria, fungi) in the biofilm and finally decomposed into smaller molecules (CO, N2, H2, H2O, H2S) [39]. These molecules are further used by microorganisms as a usable energy source and eventually returned to the atmosphere, completing the conversion from small molecules to usable products [35]. Not only is it better than other processes in terms of energy savings, environmental pollution, and degradation efficiency, but the biodegradation of its intermediate products and final products will not cause secondary pollution, which is a more efficient and ideal degradation process. Therefore, bioremediation is also considered to be the most ideal method for removing MP contamination.
In general, a problem that human beings must face is that MPs will only be produced by humans, and after a series of migrations, they will eventually return to humans. With the passage of time and the accumulation of MPs, it is time to think about and solve these problems. As of May 2022, a total of 113 articles on microplastic degradation were found through a literature analysis performed using the Stork software (https://www.storkapp.me, accessed on 23 June 2023) and screening of article titles and abstracts. By analyzing the above articles, this study reviews various degradation techniques for MPs and discusses for the first time the efficiency, sustainability, and prospects for large-scale application of biodegradation methods. This review provides direction for research on the degradation of MPs, suggestions for governmental environmental governance and policy development, and references for the sustainability and large-scale application of MP biodegradation.

2. Physical and Chemical Processes

MPs are considered more serious persistent pollutants than plastics [40]. In the past ten years, China and European countries have taken corresponding measures to limit the use of plastics by issuing laws and regulations to reduce the pollution of MPs from the source [35]. However, the situation of plastic pollution around the world is still serious. In recent years, many studies have reported the degradation processes of MPs. These include physical and chemical methods.

2.1. Physical Law

Physical methods mainly include sol–gel, coagulation filtration, and adsorption. Structural composite silica gel is obtained by the sol–gel process to polymerize and interact with MPs [41,42], and then separation technology is used to eliminate these agglomerates to eliminate the MPs. Coagulation filtration causes MPs to form larger agglomerates through coagulation to achieve the separation effect. The sol–gel process exhibits a pH-induced reaction and is more suitable for application in liquids [43]. Excessive use of coagulants will cause secondary pollution and harm organisms. Leppänen et al. [44] captured microplastics in the water column using a hygroscopic nanocellulose network. In addition, for a long time, biochar has been regarded as the most promising adsorbent due to its porous structure and easy fabrication, and the adsorption of pollutants has been widely studied [45,46]. MPs can be adsorbed by different adsorbent materials through mechanisms including electrostatic interactions, hydrogen bonding interactions, and π-π interactions. Wang et al. [47] studied a highly efficient Mg/Zn-modified magnetic biochar adsorbent for the removal of MPs from aqueous solutions with a maximum efficiency of 99.46%. Tiwari et al. [48] studied the interaction between a Zn–Al layered double hydroxide (LDH) and MPs, indicating that the Zn–Al–LDH can adsorb MPs in water and that its efficiency can reach 164.49 mg/g. Sun et al. [49] found that a sponge made of chitin and graphene oxide (ChGO) as raw materials can effectively adsorb different types of MPs. Even after multiple adsorption cycles, its efficiency can still reach 89.8%. Both magnetic and composite adsorbents have ideal removal efficiencies, but their material synthesis is complex and the cost is high; additionally, more research is needed to explore the adsorption mechanism of MPs; therefore, the development of MPs adsorbents will still be the focus of attention [50].

2.2. Advanced Oxidation Processes (AOPs) Degradation

Some recent studies have shown that AOPs are an efficient chemical elimination technology that can lead to the formation of various reactive oxygen species (ROS), chemical chain scission, or cross-linking and exhibit excellent performance in degrading MPs [51,52]. AOPs include three main methods: Direct photodegradation, photocatalytic oxidation, and electrochemical oxidation. The oxidation process, degradation mechanism, advantages and disadvantages, and future application prospects of the three processes will be discussed in detail below.

2.2.1. Direct Photodegradation

Direct photodegradation is the main transformation pathway of atmospheric organic matter and is considered an important process in the decomposition of hydrocarbons and polymers [53,54]. At present, the degradation of MPs has also been studied. Different types of light are important factors influencing the photodegradation of MPs, with UV light having the greatest effect [55]. Under strong UV irradiation, MPs can cause UV absorption of unsaturated surface groups, formation of polymer radicals, oxidation, hydrogen extraction, and chain scission or crosslinking [56,57]. Ainali et al. [58] analyzed pyrolysis-gas chromatography–mass spectrometry (Py-GC/MS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and other methods to observe and analyze the degradation process of MPs. FTIR revealed that MPs formed new functional groups during UV irradiation, including carbonyl, vinyl, and hydroxyl/hydroperoxides, and XRD and DSC measurements enhanced the apparent effect of UV irradiation on their crystalline and thermal properties [58]. SEM found significant morphological changes on the surface of MPs, demonstrating the degradation of plastic properties and their progressive fragility due to UV irradiation [58]. In addition, the plastic degradation mechanisms of different types of MPs before and after UV irradiation were investigated by Py-GC/MS, demonstrating that the respective quantitative ratios of low molecular weight compounds and relatively high molecular weight hydrocarbons varied with UV irradiation [58]. Zhu et al. [37] studied PS-MPs after simulating sunlight for 150 days and observed obvious signs of aging: the surface roughness increased, and the particle size decreased. Ding et al. [59] found that soils with different properties had different degradation rates during the same photodegradation process. The soil containing clay, iron oxides, and MnO2 enhanced the degradation rate, while the soil containing organic carbon inhibited the degradation rate. Electrostatic interactions may be the dominant factor influencing the rate of photodegradation of MPs in soils with different properties. Wang et al. [60] found that natural organic acids in the aqueous environment can promote the aging of PVC microplastics, which may be related to the hydroxyl radicals produced by the photolysis of these organic acids. Although modern technology can realize the degradation of MPs by artificial light sources, according to current research, its degradation efficiency cannot reach 100%. Coupled with its aging degree and the uncertainty of the type of intermediate products in this photochemical system, it may be harmful to the environment, causing secondary damage or even more harmful effects. It has been reported that the residues of photodegraded MPs can cause serious harm to organisms [61,62,63]. In recent years, some studies have shown that the photocatalytic reaction based on Mie resonance can lead to the coupling reaction of carbon–carbon (C–C) in an aromatic polymer that can achieve a degradation effect [64,65]. In addition, Kwon et al. [66] synthesized different Cu2−xS nanoparticles (CuS and Cu1.8S NPs) with localized surface plasmon resonance (LSPR) absorbance in the near-infrared (NIR) region and photodegradation properties leading to polydimethylsiloxane (PDMS) polymer. In addition, photodegradation causes a slight amount of pollution in the environment. The emergence of new technologies offers new ideas for the degradation of polymers. Therefore, future research should focus on the toxicological analysis of intermediates in the photodegradation process and the development and application of new technologies.

2.2.2. Photocatalytic Oxidation

Photocatalytic oxidative degradation is a redox process that uses solar energy as an energy source and utilizes the free radicals generated by semiconductors to react with MPs to break the polymer chain, thereby initiating the degradation process of the MPs in order to achieve the effect of removal [67]. At present, numerous studies have shown that nano-TiO2 can be used in the photocatalytic degradation of MPs. The process is accompanied by the generation of hydroxyl, carbonyl, and hydrocarbon groups, resulting in a harder surface [68,69]. Nabi et al. [68] showed in their 2020 paper that the TiO2 nanoparticle film had a high degradation efficiency of 98.40% for 400 nm PS within 12 h and the highest degradation efficiency for PE after 36 h. In addition, it was mentioned in a paper in 2021 that N–TiO2 materials synthesized by two routes showed a good degradation effect on MPs [70]. Not only on nano-TiO2, Tofa et al. [70] found in 2019 that zinc oxide (ZnO) nanorods (ZnO-Pt) also have a good effect on the degradation of MPs in water. The use of solar energy as an energy source is the greatest advantage of photocatalytic oxidation. Cao et al. [71] successfully prepared a series of MXene/ZnxCd1-xS photocatalysts in 2022, which can degrade MPs and utilize light energy to catalyze hydrogen evolution. The optimal photocatalytic hydrogen evolution rate can reach 14.17 mmol/g/h, which solves the pollution and energy problems of MPs in one fell swoop. Zhou et al. [72] concluded that Cu2O is an excellent visible-light photocatalyst. In 2021, Zhu et al. [73] successfully prepared a material (rGO@Fe3O4/Cu2O@ZnO) with strong hydrophobicity, high photocatalytic performance, and recyclability, and its photocatalytic degradation efficiency of acrylamide (AM) reached 97.3%. In addition to this, a large number of studies have reported the study of photocatalysts such as Cu2O [74,75], α-Fe2O3 [76,77], Au [78], Ag [79], and Cu [80] for polymer degradation. Photocatalytic oxidation can utilize solar energy and save energy. It is economically feasible to apply on a large scale, but the photocatalytic oxidation process may release volatile organic compounds (VOCs), which will inevitably have an impact on the environment. In addition, the catalyst is not easy to recycle, and the residue in the water will cause secondary pollution. Therefore, the future application of photocatalytic oxidative degradation technology needs further research.

2.2.3. Electrochemical Oxidation

At present, there are few studies on the effective degradation of MPs by electrochemical oxidation treatment. Miao et al. [81] proposed a similar electro-Fenton degradation method for MPs based on a TiO2/graphite (TiO2/C) cathode, which also generates free radicals through electrode redox to react with the MPs and achieve degradation. This method will not result in secondary pollution, but its electrolysis intermediates are uncontrollable. Therefore, future research should focus on controlling the environmental impact of intermediates and final decomposition products.

3. Biodegradation Processes

Biodegradation technology has the characteristics of high efficiency, being green, low cost, and sustainable development, and is considered to be the most likely technology to be applied to solve the pollution of MPs in the future. Although MPs can persist in the environment and have a certain resistance to degradation due to their chemical stability, studies have shown that some bacteria and fungi can participate in the degradation of MPs [82]. In addition, animals also have a certain degradation effect on MPs.

3.1. Animals

Animals can degrade MPs through phagocytosis or enzymes secreted by microorganisms in vivo. Figure 3 summarizes the process of MP removal in animals. Some studies have reported that yellow mealworms can survive by eating PS, which leads to a decrease in the quality of PS [83]. Baeza et al. [84] found that MPs were present in Lumbricus terrestris living in soil contaminated with MPs, and the number of particles in the hindgut was higher. Songet et al. [85] studied the invertebrate snail (Achatina fulica) in soil and found that it also had a certain ability to degrade PS. Only a few animals achieve degradation through ingestion, and most animals achieve degradation through enzymes secreted by their microbiota. Billen et al. [86] showed that both animal mealworms (Tenebrio molitor) and larvae of the greater wax moth (Galleria mellonella) accelerated the biodegradation efficiency of PE. To explore its specific degradation pathway, Zhang et al. [87] isolated a PE-degrading fungus from the gut of the wax moth Galleria mellonella and found that it had a significant effect on degrading PE-MPs. In addition, Luo et al. [88] found that larvae of Zophobas atratus (Coleoptera: Tenebrionidae) have certain degradation effects on three types of polystyrene (PS), PE, and polyurethane (PU), and their degradation is related to changes in the intestinal microbial community and digestive enzyme activity. Most of the degradation ability of animals is determined by the microorganisms in their bodies, so microbial degradation is worthy of our attention.

3.2. Bacteria

Bacteria are efficient degrading microorganisms capable of degrading MPs [89]. The degradation of MPs is closely related to enzymes produced by microorganisms [90]. Bacteria decompose organic polymers into simple CO2, H2O, and inorganic substances by secreting enzymes; this is the product obtained by combining enzymes with polymers and catalyzing their hydrolysis [91]. In the process of degradation, MPs are regarded as one of the essential carbon sources necessary for bacterial survival. Bacteria attach to the surface of MPs and form a biofilm, which leads to the corrosion and cracking of the MPs. As the bacteria survive and reproduce on the biofilm, they continue to soften the MP structure and finally absorb it to achieve the purpose of removal (see Figure 4 for details). Diverse types of bacteria secrete different types of enzymes, which have different degradation effects and degradation products on various types of MPs. According to reports, bacteria may release toxic and harmful substances during the degradation process to inhibit their growth [36]. In addition, the enzymes secreted by bacteria may not work on different types of plastic substrates, and it is difficult to degrade MPs. Therefore, different kinds of efficient degrading bacteria have been found continuously, which improve the degradation efficiency of MPs [92]. The bacteria currently capable of degrading MPs are summarized in Table 1.
The microbial degradation of PE-MPs, PS-MPs, PP-MPs, PET-MPs, PVC-MPs, etc., has attracted extensive attention. The intestinal tract of animals is the main gathering place for microorganisms, and some studies have found that bacteria isolated from the intestines can effectively degrade MPs. Yin et al. [93] isolated Acinetobacter sp. strain NyZ450 and Bacillus sp. strain NyZ451 from Tenebrio molitor larvae. After the two were co-cultured for 30 days, the quality of the PE decreased by approximately 18%. Yang et al. [94] also isolated a PS-degrading strain, Exiguobacterium sp. Strain YT2, from the Tenebrio molitor larvae. It could form biofilms on PS membranes during a 28-day incubation period. Through experiments, it was found that the suspension culture of its strains could reduce the weight of PS by 7.4% within 60 days. Lwanga et al. [95] also isolated bacteria from the gut of earthworms that had a degradation effect on LDPE-MPs. Microorganisms can use MPs as a source of energy to survive. Therefore, Vimala et al. [96] used PE as the sole carbon source to study the degradation efficiency of Bacillus subtilis, and the results showed that its mass was reduced by 9.26% after 30 days.
The soil is rich in nutrients and is the main gathering place for bacteria; furthermore, it is the main area of pollution. Therefore, a large number of researchers have screened for an efficient MP-degrading bacterium in the soil. Park et al. [97] added a mixed flora to the sediments of the landfill site as the basic culture medium (without a carbon source) and screened the viable strains by adding PE MPs. Finally, the content of Bacillus and Bacteroides isolates was higher, which reduced the weight of PE MPs by 14.7%. Auta et al. [98] isolated Bacillus cereus and Bacillus gottheili from the sediments of mangroves in Malaysia. After 40 days of culture, Bacillus cereus caused a massive reduction in PE, PET, and PS to 1.6%. B. gottellii caused PE, PET, PP, and PS weight loss percentages of 6.6%, 7.4%, 3.6%, and 5.8%, respectively [98]. Auta et al. [99] isolated mixed colonies from mangroves in different environments, and after adding PS and PET for 90 days, the weight loss reached 18%. To screen for more efficient degrading bacteria of PP-MPs, Auta et al. [100] isolated Bacillus sp. strain 27 and Rhodococcus sp. strain 36 from the Matang Mangrove area in Perak and the Cherating Mangrove area in Pahang in Peninsular Malaysia. The study showed that both bacteria could use PP-MPs for growth and reproduction, and after 40 days of culture, their weight loss rates were 4.0% and 6.4%, respectively [100].
Different bacteria have suitable conditions for their growth, and the influence of temperature on them cannot be ignored [101,102]. With an increase or decrease in temperature, the performance of bacteria’s in vivo adaptation to the environment is distinct, so the degradation effect of MPs may be different. Sun et al. [103] studied the effect of microorganisms on the degradation of PE, PVC, and PHA-MPs before and after composting. The results showed that the abundance of PE, PVC, and PHA-MPs decreased after composting, and the MPs were oxidized, with their oxygen content increasing by 3–30%. The surface morphology was rougher than that of the initial MPs, and obvious cracks and grooves were observed in all of them. This shows that the composting technology can degrade MPs, resulting in a weight reduction of 13%, 3%, and 29%, respectively [104]. Chen et al. [104] used high-temperature composting (HTC) technology to accelerate the microbial degradation of MPs in sewage sludge. Thermus, Bacillus, and Geobacillus were the dominant strains for efficient degradation during HTC [105]. After 45 days of culture, the degradation efficiency reached 43.7%, and after co-culturing PS-MPs with bacteria at 70 °C for 56 days under laboratory conditions, the degradation rate was 7.3% [104]. Novel thermophilic bacteria have become the focus of research. Skariyachan et al. [106] screened eight strains of plastic-degrading bacteria after culturing 36 strains of plastic-degrading bacteria collected from sewage treatment plants, landfills, and other areas at 50 ℃ for 140 days. Then, a mixed bacterial colony was formed through various combinations. Finally, it was found that the degradation efficiency of four combinations, Aneurinibacillus aneurinilyticus btDSCE01, Brevibacillus agri btDSCE02, Brevibacillus sp. btDSCE03, and Brevibacillus brevis btDSCE04, was higher than that of the other groups and caused the weight losses of LDPE, HDPE, and PP to reach 58.21%, 46.6%, and 56.3%, respectively [106]. In contrast, Habib et al. [107] isolated two bacteria, Pseudomonas sp. ADL15 and Rhodococcus sp. ADL36, from the Antarctic soil. Infrared spectroscopy analysis showed that the functional groups of PP-MPs changed significantly after they were cultured with the Antarctic strain for 40 days, and the two bacteria reduced the weight of PP-MPs by 17.3% and 7.3%, respectively.
In addition to bacteria isolated from soil, a large number of microorganisms that can degrade MPs are also enriched in sewages, rivers, and oceans. Grgic et al. [108] isolated a mixed strain of Bacillus licheniformis, Lysinibacillus mas-siliensis, Delftia acidovorans, and Bacillus sp. from sludge and sediment in sewage treatment plants and studied their effect on LDPE-MPs and the PS degradation efficiency of MPs. After 22 days of culture, the results showed that mixed bacterial cultures degraded LDPE-MPs and PS-MPs better than pure bacterial cultures, and the biodegradation efficiency of LDPE-MPs was higher than that of PS-MPs [108]. Devi et al. [109] isolated four bacteria from the Vaigai River in Madurai, India, including Bacillus sp. (BS-1), Bacillus cereus (BC), Bacillus sp. (BS-2), and Bacillus paramycoides (BP). The results showed that a single colony had high degradation of PE and PP and that Bacillus paramycoides (BP) and Bacillus cereus (BC) reduced the weight of PP and PE by 78.99% and 63%, respectively [109]. Li et al. [110] isolated strain M. hydropicus ire-31 from a lignin-rich marine pulp mill. After culturing for 30 days, the changes in surface morphology and functional groups of PE-MPs were analyzed by SEM and FTIR, and cracks were observed on the surface. Using FTIR, it was found that additional hydroxyl and carbonyl functional groups were formed on the surface of the polymer, indicating that it had a degradation effect on PE-MPs through oxidation. Giacomucci et al. [111] studied the biodegradation of PVC by anaerobic microorganisms in the ocean. After 7 months of cultivation, a dense biofilm was displayed on the polymer surface with a weight loss of 11.7%, and the thermal stability and average molecular weight were significantly reduced, indicating the potential degradation effect of anaerobic microorganisms on PVC-MPs.
Harshvardhan et al. [112] isolated three marine bacteria from the Arabian Sea in India, named Kocuria palustris M16, Bacillus pumilus M27, and Bacillus subtilis H1584. After 30 days of growth in a medium containing PE as the sole carbon source, it was found by FTIR that the ketone carbonyl bond index, ester carbonyl bond index, and vinyl bond index on the surface of PE had increased, indicating the degradation effect of PE, which resulted in a weight loss of 1%, 1.5%, and 1.75%, respectively [112]. Raghul et al. [113] isolated V. parahaemolyticus (BTTV4 and BTTN18) and V. alginolyticus (BTTC10 and BTTC27) from marine sediments and studied their degradation efficiency of a polyvinyl alcohol-low linear density polyethylene (PVA-LLDPE) blend plastic film as a combination. After 15 weeks of incubation at 120 rpm and 37 °C in shake flasks, SEM showed that visible cracks and grooves appeared on the surface of the PVA-LLDPE blend film, indicating that the combination could lead to the degradation of the PVA-LLDPE plastic blend [113]. The study also pointed out that the percentage of tensile strength loss of PVA-LLDPE plastic films containing 25% and 30% PVA was greater, and the tensile strength decreased with increasing PVA content, indicating that the PVA component can be used as a carbon source to promote the degradation of PVA-LLDPE by bacteria [113]. Gao et al. [114] screened plastic debris-contaminated marine sediment samples collected from a bay in China and found a marine bacterial community, CAS6, that efficiently colonized and degraded PET and PE. Biofilms were found on the surface of PET and PE films by SEM, and significant changes in the surface morphology of PET and PE were observed, especially severe cracks and deep pores [114]. Soil and oceans are the main habitats of bacteria, which bring together a large number of MP-degrading bacteria. MPs are used by bacteria as the energy needed for growth to form biofilms on their surfaces. With the growth of biofilms, the surface of MPs will be cracked and damaged, and the changes in functional groups show the process of oxidation. Bacterial degradation has the advantages of being green, environmentally friendly, and sustainable, and numerous studies have proven the efficacy of efficient degradation of polysulfonic acid mucopolysaccharides by bacteria [115,116]. In future research, we should pay attention to the study of degradation efficiency and degradation mechanisms of bacterial degradation methods so that they have a better prospect for environmental governance and sustainable development.
Table 1. Research status of bacterial degradation of MPs.
Table 1. Research status of bacterial degradation of MPs.
The Bacteria TypesTypes
of MPs		Duration of
Degradation		Weight LossReferences
Acinetobacter sp. NyZ450/Bacillus sp. NyZ451PE30 d18%[93]
Exiguobacterium sp. YT2PS60 d7.4%[94]
Bacillus simplex, and Bacillus sp.LDPE21 d [95]
Bacillus subtilisPE30 d9.26%[96]
Bacillus sp. and Paenibacillus sp.PE60 d14.7%[97]
Bacillus cereusPE/PET/PS40 d1.6%/6.6%/7.4%[98]
Bacillus gottheiliiPE/PET/PP/PS40 d6.2%/3.0%/3.6%/5.8%[98]
B. cereus, S. globispora and B. flexus and B. gottheiliiPS and PET90 d18%[99]
Bacillus sp. 27PP40 d4.0%[100]
Rhodococcus sp. 36PP40 d6.4%[100]
Firmicutes, Bacteroidetes, Proteobacteria, Gemmatimonadetes, Deinococcus-ThermusPE/PVC/PHA60 d13%/3%/29%[103]
Thermus, Bacillus, and GeobacillusPS56 d7.3%[104]
Aneurinibacillus aneurinilyticus btDSCE01, Brevibacillus agri btDSCE02, Brevibacillus sp. btDSCE03, and Brevibacillus brevis btDSCE04LDPE/HDPE/PP140 d58.21%/46.6%/56.3%[106]
Pseudomonas sp. ADL15PP40 d17.3%[107]
Rhodococcus sp.PP40 d7.3%[107]
Bacillus licheniformis/Lysinibacillus mas-siliensis, Bacillus sp. and Delftia acidovaransLDPE and PS22 d [108]
Bacillus paramycoides (BP)PP21 d78.99%[109]
Bacillus cereus (BC)PE21 d63%[109]
Bacillus paramycoides (BP) and Bacillus cereus (BC)PP/PE21 d78.62%/72.50%[109]
M. hydrolyticus IRE-31LDPE30 d [110]
Anaerobic bacteriaPVC210 d11.7%[111]
Kocuria palustris M16PE30 d1%[112]
Bacillus pumilus M27PE30 d1.5%[112]
Bacillus subtilis H1584PE30 d1.75%[112]
V. parahaemolyticus (BTTV4 and BTTN18) and V.alginolyticus (BTTC10 and BTTC27)PVA-LLDPE105 d [113]
Exiguobacterium sp., Halomonas sp., and Ochrobactrum sp.PET/PE28 d2.7%/19.6%[114]

3.3. Fungi

In addition to bacteria, fungi have the potential to adhere to and utilize MPs [117]. Few studies have been reported on the degradation of MPs by fungi, which suggests there are some difficulties in screening fungal strains with MP-degrading properties [92]. However, in recent years, many researchers have screened out fungi that can degrade MPs from different places, mainly from soil and the ocean. The ability of fungi to degrade polymers is due to several extracellular enzymes secreted by their enzymatic systems, including a manganese peroxidase (MnP), a lignin peroxidase (LiP), a multifunctional peroxidase, and a laccase (Lac). The polymer is decomposed by these enzymes to produce monomeric substances, and these monomers are then absorbed by the fungus and assimilated or mineralized by its intracellular enzyme system to achieve degradation [118,119,120,121,122]. In addition, fungi can produce hydrophobins that adhere hyphae to plastic surfaces, and they can also penetrate the surface of the polymer material and move deep into it, which can degrade the matrix to the greatest extent possible [123]. Daly et al. [120] summarized the methods of fungal degradation of lignocellulose and proposed the feasibility of fungal degradation of plastics. This fungal degradation mechanism also exists in the degradation process of MPs. Table 2 summarizes the fungi currently capable of degrading MPs.
The animal body is also the habitat of fungi and a potential site for screening MP-degrading fungi. Zhang et al. [87] isolated the PE-degrading fungus Aspergillus flavus-PEDX3 from the intestinal contents of Galleria mellonella. After 28 days of mixed culture with HDPE, through Fourier transform infrared spectroscopy analysis, it was found that carbonyl and ether groups had appeared on the surface of HDPE, indicating that pedx3 could degrade MPs. In addition, two laccase-like multicopper oxidase (LMCO) genes, AFLA_006190 and AFLA_053930, were found to be upregulated during degradation by reverse transcription-polymerase chain reaction (RT-PCR). It was confirmed that fungi can degrade HDPE by secreting enzymes [87].
There are many types of microorganisms in the soil [124]. Landfills are the places where most waste plastics accumulate, resulting in many microorganisms that can degrade MPs, of which fungi are an important part. At present, many researchers have isolated fungi that can degrade MPs from landfills and have further studied their degradation effects on these MPs. Gajendiran et al. [125] studied the degradation of LDPE by Aspergillus clavatus-JASK1 isolated from a landfill soil, and after 90 days of culture, its weight loss reached 35%. Verma et al. [126] isolated two fungi, A. flavus and A. terreus, from the landfill in Agra, and after culturing them in soil and LDPE for 9 months, their weights decreased by 30.6% and 11.4%, respectively. After 4 months of mixed culture with LDPE in the liquid medium, their weights dropped by 14.3% and 13.1%, respectively. Balasubramanian et al. [127] isolated the fungus Aspergillus terreus MF12 from a plastic waste dump. Through weight loss and Fourier transform infrared spectroscopy analysis, it was found that the strain could effectively degrade HDPE, and after 30 days of culture, the HDPE lost 9.4% of its weight [127]. In addition, the effect of different environmental factors on the degradation process of HDPE was also studied in this experiment. Under optimal conditions, the degradation rate could reach up to 20.8% [127]. Kunlere et al. [128] reported that two fungi, Aspergillus flavus MCP5 and Aspergillus flavus MMP10, were able to grow using LDPE as carbon and nitrogen sources, and Fourier transform infrared spectroscopy showed changes in the functional groups of their samples. Compared with the control, the peak intensities of the other spectra were increased or decreased, indicating that the two fungi were able to participate in the degradation process of LDPE. Ameen et al. [129] isolated six fungi from the Saudi Arabian mangrove sediments. After co-cultivation with LDPE, examination under light and SEM revealed that a large number of fungi were attached to the surface, more lignin-decomposing enzymes were produced, and more CO2 was released. These observations suggest that the screened fungi can decompose and consume LDPE. In addition to screening landfills for MP-degrading fungi, fungal species capable of degrading MPs also exist in other soils. Russell et al. [130] isolated the fungus Pestalotiopsis microspora from Amazonian plant samples in Ecuador. Under aerobic and anaerobic conditions, the isolate was able to grow with a unique polyester polyurethane (PUR) as the sole carbon source, which could secrete serine hydrolases to degrade PUR.
Fungi that can utilize and decompose MPs also exist in the ocean. Paco et al. [131] studied the response of the marine fungus Zalerion maritimum to PE particles at different incubation times. The results showed that the fungus could utilize PE and reduce the size and quality of PE particles, and the removal rate of PE was as high as 43% on the 14th day of culture [131,132]. Devi et al. [133] isolated two fungal strains, Aspergillus tubingensis VRKPT1 and Aspergillus flavus VRKPT2, from PE waste near the coast, and the fungi could survive using native PE as a carbon source. After 30 days of culture, the weight of HDPE was reduced by 6.02% and 8.51%, respectively. Although there are few reports on the degradation of MPs by fungi, according to the analysis of the reported literature, fungi degrade MPs very efficiently, so it is imperative to screen more kinds of fungi in future research to reduce the impact of MPs on the environment.
Table 2. Research status of fungal degradation of MPs.
Table 2. Research status of fungal degradation of MPs.
Fungi TypeTypes
of MPs		Duration of
Degradation		Weight LossReferences
Aspergillus flavus PEDX3HDPE28 d3.9%[87]
Aspergillus clavatu JASK1LDPE90 d35%[125]
A. flavusLDPE120 d14.3%[126]
A. terreusLDPE120 d13.1%[126]
A. flavusLDPE270 d30.6%[126]
A. terreusLDPE270 d11.4%[126]
Aspergillus terreus MF12.HDPE30 d9.4%[127]
Aspergillus flavus MCP5LDPE14 d1.67%[128]
Alternaria alternata, Aspergillus caespitosus, Aspergillus terreus, Eupenicillium hirayamae, Paecilomyces variotii, and Phialophora albaLDPE28 d [129]
Pestalotiopsis microsporaPUR14 d [130]
Zalerion maritimumPE14 d43%[131]
Aspergillus tubingensis VRKPT1HDPE30 d6.02%[132]
Aspergillus flavus VRKPT2HDPE30 d8.51%[132]

3.4. Combined Degradation of MPs by Bacteria and Fungi

In terms of MP biodegradation, most previous research has focused on the isolation of a single class of microorganisms with degrading ability; however, studies have shown that the combination of bacteria and fungi has a more efficient degradation efficiency. Esmaeili et al. [134] isolated two strains with a significant ability to degrade LDPE from a landfill soil in Tehran: the bacterium Lysinibacillus xylanilyticus and the fungus Aspergillus niger F1. FTIR, XRD, and SEM analysis proved the degradation potential of the mixed bacteria using PE as a carbon source. After 126 days of treatment, the biodegradation of UV-irradiated and non-UV-irradiated films reached 29.5% and 15.8%, respectively [134].

4. Summary and Outlook

In the future, the production of plastic products will continue to exhibit an upward trend. Plastic pollution has become an ecological and environmental problem that cannot be ignored in the Earth’s biosphere. Plastics undergo physical, chemical, biological, and other forms of wear, consumption, and decomposition to produce MPs with smaller particle sizes, which have been proven to be ubiquitous in the environment. With the increasing seriousness of MP pollution, it is imperative to explore effective MP degradation methods. In this paper, for the first time, the current degradation technologies of MPs and their advantages and disadvantages are systematically reviewed. The degradation mechanism and efficiency of biodegradation processes are mainly introduced, and the high efficiency and sustainable development potential of biodegradation are emphasized, as are the prospects of large-scale biological applications in the future. This review described the degradation efficiency of MPs by different microorganisms, including 45 bacteria and 18 fungi. Some of these microorganisms used MPs as the only source of nutrient elements. Through this paper, it is found that the microbial removal process has the advantages of high efficiency, being environmentally friendly, low cost, and sustainability; it is the most effective, energy-saving, and applicable degradation method for future large-scale applications.
Based on the above summary, in order to reduce the harm of MPs and understand the feasibility of the biodegradation process, we provide several directions for future research. First, we can screen plants that can effectively absorb MPs in the soil to improve the soil environment. Second, screen for microorganisms, such as fungi and mixed flora, that can efficiently degrade MPs. Third, remediation of MPs-contaminated soil by inoculation with degrading bacteria. In conclusion, this paper provides a solution for the management of MPs, a reference for the sustainable development and large-scale application of microplastic degradation in the future, and a suggestion for environmental management and policymaking by governments around the world.

Author Contributions

Literature retrieval and information collection, P.X. and Q.L.; analyzed the data, T.Z. and Q.W.; wrote and reviewed the paper, Q.L. and P.X.; project management, Q.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by Sichuan Natural Science Foundation Project (2023NSFSC1229) and Open Foundation of Hebei Key Laboratory of Wetland Ecology and Conservation (No. hklk202203).

Institutional Review Board Statement

This article does not contain any studies with human participants or animals performed by any of the authors.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data analyzed during this study are included in this article.

Conflicts of Interest

The authors declare no competing interests.

Abbreviations

MPs: microplastics; NPs: nanoparticles; PS: polystyrene; PP: polypropylene; PE: polyethylene; PU: polyurethane; PC: polycarbonate; PHA: polyhydroxyalkanoates; PVC: polyvinyl chloride; PVA: polyvinyl alcohol; PET: polyethylene terephthalate; LDPE: low-density polyethylene; HDPE: high-density polyethylene; PVA-LLDPE: linear low-density polyethylene; AOPs: advanced oxidation process; HTC: high-temperature composting

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Figure 1. Source and migration characteristics of soil microplastics (MPs).
Figure 1. Source and migration characteristics of soil microplastics (MPs).
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Figure 2. Absorption and transport of microplastics (MPs) in terrestrial plants. Arrows indicate microplastics absorbed by plants.
Figure 2. Absorption and transport of microplastics (MPs) in terrestrial plants. Arrows indicate microplastics absorbed by plants.
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Figure 3. Pathways for animal decomposition of microplastics (MPs).
Figure 3. Pathways for animal decomposition of microplastics (MPs).
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Figure 4. Pathways of bacteria- and fungi-degrading microplastics (MPs).
Figure 4. Pathways of bacteria- and fungi-degrading microplastics (MPs).
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Xiang, P.; Zhang, T.; Wu, Q.; Li, Q. Systematic Review of Degradation Processes for Microplastics: Progress and Prospects. Sustainability 2023, 15, 12698. https://doi.org/10.3390/su151712698

AMA Style

Xiang P, Zhang T, Wu Q, Li Q. Systematic Review of Degradation Processes for Microplastics: Progress and Prospects. Sustainability. 2023; 15(17):12698. https://doi.org/10.3390/su151712698

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Xiang, Peng, Ting Zhang, Qian Wu, and Qiang Li. 2023. "Systematic Review of Degradation Processes for Microplastics: Progress and Prospects" Sustainability 15, no. 17: 12698. https://doi.org/10.3390/su151712698

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