An Overview of Micro(Nano)Plastics in the Environment: Sampling, Identification, Risk Assessment and Control

Abstract

Advances in urban infrastructure, a flourishing polymer sector, and more traffic have all contributed to a rise in micro(nano)plastics in the environment. Researchers are exploring the production, fate, toxicity threshold, and severity of micro(nano)plastic exposure. Albeit, understanding sampling protocols, preservation of samples, and characterization of micro(nano)plastics obtained from the different mediums (e.g., soil, water, air, and living bodies) is still challenging. Particularly identification of micro(nano)plastics, on the other hand, is restricted and limited to the typical generic definition of contaminating sources. In addition, before micro(nano)plastics degrade naturally, many challenges must be overcome, enhancing the need for research on assisted degradation. Thus, a systematic review is presented, which begins by discussing micro(nano)plastic identification, sampling, and handling; then showcases the environmental and health consequences and how to control them; finally, it discusses environmental micro(nano)plastics management options. According to studies, biological and chemical methods to break down micro(nano)plastics have risen in popularity. However, these methods often only cover one type of plastic. Furthermore, these solutions can transform polymers into micro(nano)plastics and may also produce byproducts, increasing environmental contamination risk. Therefore, control, prevention, and management strategies are all investigated to generate more realistic and long-term solutions. The literature suggests a combination of different microorganisms (e.g., different bacterial species) and different approaches (e.g., filtration with degradation) could be more effective in the treatment of micro(nano)plastics. Furthermore, according to the literature, relevant health risks associated with micro(nano)plastics to humans from various exposure routes are currently unclear. Likewise, standardization of methods supported with sophisticated state-of-the-art apparatus for detecting micro(nano)plastics is required. Overall, precision in micro(nano)plastic identification and treatment strategy selection is critical, and their usage should be regulated if their environmental behavior is not properly addressed.

1. Introduction

Plastics have been massively produced since the 1940s, when society started to consume large quantities of plastics in every segment of our lives, including industry, agriculture, healthcare, and others [1,2]. The production of plastics has been expanded in the last decades because of their favored characteristics, such as lightness, durability, flexibility, versatility, and cost-effectiveness [3]. However, the extensive consumption of plastics has resulted in severe environmental consequences. It is of particular relevance that once plastic litters enter the environment, they can persist for a long time and will be broken down into small debris (alias mesoplastics), microplastics (MPs, <5 mm), and even nanoplastics (NPs, <100 nm) via physical, chemical and biological processes before reaching complete decomposition [4]. The MPs and NPs have attracted public concern due to their small size, worldwide distribution, and potential ecotoxicological effects [5]. As a result, the studies have been increasing in the past few years (Figure 1, retrieval time from 01.2016 to 01.2022). The figure shows that the studies of MPs and NPs have expanded in the past few years.

Figure 1. Visual analysis diagram of research studies relating to MPs and NPs since 2016. To date, many previous studies have investigated the global distribution of MPs and recently focused on the NPs [6]. For example, as compared with the studies of MPs, the analysis methodologies of NPs are pretty limited, and only 405 publications (including 71 review papers and 334 research papers) can be retrieved according to the literature retrieval of ‘nanoplastic(s)’with other keywords such as ‘analysis’, ‘extraction’, ‘methodology’, ‘separation’, ‘characterization’, ‘identification’, or ‘quantification’ (Figure 2) (Date up to 1 May 2022). The figure shows that most NPs studies focused on the fate and toxicity in the environment, while studies regarding the measuring methodology are still scarce.

Figure 1. Visual analysis diagram of research studies relating to MPs and NPs since 2016. To date, many previous studies have investigated the global distribution of MPs and recently focused on the NPs [6]. For example, as compared with the studies of MPs, the analysis methodologies of NPs are pretty limited, and only 405 publications (including 71 review papers and 334 research papers) can be retrieved according to the literature retrieval of ‘nanoplastic(s)’with other keywords such as ‘analysis’, ‘extraction’, ‘methodology’, ‘separation’, ‘characterization’, ‘identification’, or ‘quantification’ (Figure 2) (Date up to 1 May 2022). The figure shows that most NPs studies focused on the fate and toxicity in the environment, while studies regarding the measuring methodology are still scarce.

Figure 2. Visual analysis diagram of MPs and NPs measurement since 2016. Note: The retrieval was accomplished with the keywords such as “nanoplastic”, “analysis”, “extraction”, “methodology”, “separation”, “characterization”, “identification”, and “quantification”, via database, including Web of Science, Science Direct.

Figure 2. Visual analysis diagram of MPs and NPs measurement since 2016. Note: The retrieval was accomplished with the keywords such as “nanoplastic”, “analysis”, “extraction”, “methodology”, “separation”, “characterization”, “identification”, and “quantification”, via database, including Web of Science, Science Direct.

Recently, Cai et al. [7] summarized the progress and challenges of environmental NPs analysis, in which 95 studies have been reviewed while only 12 of them focused on the field studies (Date up to 1 May 2022). For example, Fang et al. [8] introduced the identification and visualization of MPs and NPs by a newly developed Raman mapping image method; Peiponen et al. [9] also demonstrated the optical identification of MPs and NPs in aquatic environments. However, they all pointed out that improving analytical reliability and accuracy is still challenging, especially for NPs. For instance, how to enhance the weak Raman signal from NPs, and the identification requires a combination of optical detection, etc. In other words, it is a common challenge that the analytic techniques remain to be standardized and explored, especially for improving the detection efficiency and accuracy.

Various physical, biological/microbial, and chemical techniques have been implemented to combat the threat posed by micro(nano)plastic. Currently, some research is being carried out on (micro)plastic degradation. The degradation of (micro)plastics is split into biological and chemical degradation [10]. Sudhakar et al. [11] conducted in situ experiments using polyethylene (PE) and polypropylene (PP) sheets in marine water. They studied Pseudomonas spp., Clostridium spp., unidentified anaerobic, heterotrophic, and iron-reducing bacteria and fungi and found that degradation dependent on study site, plastic type, and season. Another study addressed an in vitro study of Nylon pellets degradation by Bacillus cereus, B. sphericus, Vibrio furnisii, and Brevundimonas vesicularis and found that degradation varied across microbial taxa (highest for B. cereus) [12]. On the other hand, in vitro degradation of PE sheets with B. cereus subgroup A and B. sphericus GC subgroup IV showed degradation rates varied with PE [13]. Bao and coworkers recently studied microplastics released from biodegradable blended plastic and demonstrated bacterial communities in contrasting environments such as water and air, showing variable trends [14]. For instance, plastics having hydrolyzable covalent links (such as polyethylene terephthalate (PET)) can be hydrophilic, which accelerates the hydrolytic degradation of the polymer [14]. Microplastics’ C–C and C–H bonds can be destroyed by ultraviolet (UV) light, causing them to deteriorate [15,16]. However, more research is needed on the chemical breakdown of microplastics, which is currently being carried out via advanced oxidation processes (AOPs) [17].

This work intends to provide detailed insight into recent advancements in micro and nano plastics characteristics, including sampling, identification, and handling. Furthermore, different environmental segments, such as seawater, soil, and sediments, were assessed for micro and nano plastics. Studies comprised plastics detection in birds, fish, and other organisms, and their characteristics inside living bodies were also discussed. The emphasis will be on risk assessment and management of micro and nano plastic pollution leading to their control and degradation strategies.

2. Overall Introduction on Techniques for MPs and NPs Measurement

The studies of MPs and NPs pollution are gaining more and more attention around the globe; however, it should be cautioned that their protocols remain to be standardized profoundly in terms of sampling, preservation, digestion, separation, and identification in particular to some comprehensive, fast and efficient approaches [18]. At present, most of the studies adopted the approaches recommended by NOAA. Figure 3 summarizes the most common methods for the determination of MPs in water [19,20], soil (or sediment, sand) [21,22], organism [23,24], and air [25] in the previous studies

2.1. Separation Methods for MPs and NPs in Different Matrices

The analytical steps for MPs and NPs in different environmental matrices are generally similar, including sample collection, processing (e.g., centrifugation, digestion, staining), flotation, filtration, and then a series of physical and chemical methods for characterization (Figure 3). In general, trawl or Neuston nets are usually used for MPs sampling from the aqueous environment such as freshwater and seawater. At the same time, stainless steel products (e.g., shovels and spoons) are commonly applied in sampling soil, sediment, and sludge. By comparison, pumps and atmospheric wet and dry deposition (e.g., active and passive sampling) are frequently applied to sampling MPs in the air; and planktonic nets are used to collect aquatic organisms [26,27,28]. After sampling, flotation is essential in separating MPs from the collected samples. At this stage, the large materials will be removed using saturated sodium chloride and sodium iodide [29,30]. Sodium chloride applies to the flotation of low-density plastics, while sodium iodide is for the flotation of high-density plastics. After flotation, the supernatant will be filtered using filter membranes made of cellulose nitrate, nylon, PTFE, sartorius, polycarbonate, and glass membranes [31,32,33]. However, digestion is essential to the flotation of MPs in complex samples (e.g., sludge, wastewater), and a 30% hydrogen peroxide solution is commonly used for digestion [34,35]. In addition, the digestion solution varies with the types of samples and the characteristics of MPs, aiming to achieve maximum recovery efficiency [36]. For example, potassium hydroxide solution is generally used for the digestion of biological samples [37], while proteinase K is for tissues [38] and Fenton’s reagent for sediments [39].

2.2. Identification of MPs and NPs

Various methods, including visual, spectroscopic, and scanning electron microscopy (SEM), are recommended to identify the MPs in environmental samples. Among them, the microscope’s visual method is aimed at observing MPs’ surface morphology (e.g., shape, length, color, etc.) [40], and high-resolution images of surface structure MPs can be achieved using SEM. SEM pictures can be used to compare MPs before and after aging or to see MP biofilm [41]. Further, the spectroscopic method, mainly through Micro Fourier Transform Interferometer (μ-FTIR) or μ-Raman can identify the composition of polymers by analyzing the functional groups on MPs [42], in which the μ-FTIR is applicable to the identification of MPs in size of ca. 20 μm, while μ-Raman to those in size of ca. 1 μm and even smaller [43]. However, since both of these procedures are costly and time-consuming, it is useful to quickly establish techniques for detecting MPs. Next, some new approaches were suggested, such as a support vector machine (SVM) approach is recommended to determine MPs based on hyperspectral imaging and achieving steady detection of various kinds of plastics [18]. Some studies investigated a portable optical sensor to detect transparent and translucent MPs in freshwater. For understanding a variety of MPs and plastic additives, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) are recommended in addition to visual and spectral verification. For instance, LC-MS/GC-MS can be used to examine the degradation products and additives in natural and manufactured fibers [44]. In addition, other integrated methods such as thermal desorption-proton transfer reaction-mass spectrometry (TS-PTR-MS), thermochemolysis coupled to GC-MS, pyrolysis-gas chromatography/mass spectrometry (py-GC-MS) and surface-enhanced Raman spectroscopy (SERS) have been shown to be efficient in the identification of NPs in real-world studies [45]. However, exploring novel methods for rapid quantitative and qualitative identification of MPs in various media, particularly for tiny plastic particles such as NPs, remains challenging [46].

3. Techniques for MPs and NPs Measurement in Various Matrices

3.1. Techniques for MPs Studies in Seawater

As shown in Figure 3 and

Table 1. Common methods used in the sampling and identification of MPs in seawater.

Procedures	Common Methods	Refs
Sampling Tool	Plankton net; Manta trawl; Neuston net; Continue intake; Teflon pump	[47,48,49,50,51]
Reservation	Formalin solution (4%, 5%); Refrigerated; Ethanol (70%, 90%)	[52,53,54]
Digestion	Hydrogen peroxide (30%); Sodium hydroxide (5M); Nitric acid (65%); Fe (II) solution (0.05 M)	[47,55,56]
Separation	Sieving; Density separation, filtration	[57]
Pore Size (μm)	0.45; 0.70; 0.75	[58]
Temperature for drying	50 ℃; 55 ℃; 60 °C; Air-dried	[59]
Identification	Stereomicroscope; SEM; μ-FTIR or Raman	[60]

, the main steps for MPs studies in seawater include sampling, preservation, digestion, filtration, and identification.

3.1.1. Sampling

Seawater samples can be collected by plankton net [61], manta trawl [47], catamaran, neuston net [48] (Figure 4), teflon pump [49] and intake system of the vessel [51], with a common range of mesh size from 120~335 μm [47,54]. Manta trawl and Neuston net with a mesh size of 330 μm or 333 μm [62,63] is the most frequently used in seawater sampling. In general, the vessel is dragged at a speed of 1~5 knots [50,54] for 5~30 min [61,64]. Next, a flow meter is installed at the entrance of the sampling device to measure the water flow [52]. Finally, transfer the sampled surface water to a glass bottle for further analysis.

3.1.2. Preservation and Digestion

In general, the aforementioned seawater samples are directly used for detecting MPs [19,33], except for some specific purpose of investigating MPs distribution in the environment [56]. Under such circumstances, it is suggested to preserve fresh seawater with reagents (e.g., 4% or 5% formalin solution) [52], 70 or 95% ethanol [53], or directly refrigerated at 4 °C [65], since that some MPs may transfer from seawaters to phytoplankton, zooplankton and other organisms in higher trophic levels. Subsequently, seawater is digested to degrade the organic matter inside by adding reagents such as 5 M NaOH [66], 65% HNO₃ [56], 30–35% H₂O₂ [20], or wet peroxide oxidation (WPO, a combination of 30% H₂O₂ and 0.05 M Fe(II) solution (i.e., 7.5 g of FeSO₄·7H₂O to 500 mL of water and 3 mL of concentrated sulfuric acid)) [53] and others, as discussed later in Section 3.2. Next, the digested solution of the samples on the net will be transferred to a density separator for flotation, which separates the light component MPs (with a density of 0.8–1.4 g/mL) from heavy ones and collects the floating solids using 0.3-mm custom sieve [57]. In contrast, the samples in the volume-reduced water can be directly separated from the mixtures through sieving [53] and filtration with a set of vacuum suction filter devices with membrane pore sizes of 0.45~0.75 μm [54]. Next, the filter paper is subsequently transferred into a petri dish with a cover for drying at 25~60 °C [53].

3.1.3. Observation, Enumeration, and Identification

The items similar to MPs (based on morphology and color) will be selected from the above-dried samples for further analysis. In general, MPs can be analyzed by qualitative analysis and quantitative analysis, in which the qualitative analysis applies to the analysis of the physical morphologies and the identification of the chemical components of MPs [67], while the quantitative analysis is mainly to calculate the quantity and weight of MPs by inspection [68]. First, the MPs’ shape, color, and size were observed by a stereoscopic microscope [55] and characterized by SEM [69]. Next, the suspected MPs particles or fibers can be selected using tweezers. The quantity of MPs is counted by manual counting (prone to the miscarriage of justice) [68]. At the same time, the qualitative analysis is achieved by μ-FTIR [61], μ-Raman [60] are commonly used in previous studies, other approaches such as SEM-EDS [53], ESEM-EDS [67], Pyr-GG-MS [70], or other methods such as tagging method [71].

3.2. Techniques for MPs Studies in Sediment and Soil

Among the types of sediments, such as sea (core) sand, beach sand, and municipal soil [34,38], beach sand is the most studied type. Beach sands are collected by stainless steel spoon, shovel, or frame with a quadrat from the surface to a depth of 1~5 cm [59,72] (As shown in

Table 2. Typical sampling and identification of MPs and NPs in sediment or soil.

Parameters	Range/Chemicals/Instruments	Refs
Sampling depth (cm)	1; 2; 3; 5; 10	[59,72]
Mesh size (mm)	1; 2; 5	[59,75]
Dry temperature	Freeze-dried; Air-dried; 40 ℃; 50 ℃; 60 ℃	[74,75]
Separation solution	NaCl; NaI; KI; ZnCl2 (1.37~1.7 g/mL); ZnBr2; Canola oil	[77,78]
Filter paper mesh size (μm)	0.7; 0.8; 1.0; 1.2	[50,79]
Identification	Stereomicroscope; SEM; μ-FTIR or μ-Raman; Py-GC-MS	[60,61,80]

). Sands are usually preserved in aluminum foil or stainless containers and then dried through freeze-dried [50], air-dried [73], or oven at 40–60 °C [74,75] [76]. Once dried, the samples can be sieved with a mesh size of 1~5 mm [75,77].

Next, the dried and sieved samples will be separated by density flotation [79], which separates the light component MPs (with a density of 0.8–1.4 g/mL) from the reconstituted sediment (with a density of 2.65 g/mL) [68]. In detail, a certain volume of sediment samples can be mixed evenly by stirring and shaking, letting it settle down until reaching solid-liquid separation, and collecting the MPs suspended or floating on the supernatant using filter paper.

As summarized in

Table 3. Advantages and disadvantages of density separation solutions.

Solution (Density)	Advantages	Disadvantages	Refs
NaCl (1.2 g/mL)	Low cost, environmental sound	High-density plastic particles are not applicable	[75]
NaI (1.57 g/mL)	Cheaper, less environmental dangerous	Oxidizes filter paper, makes filter black, and makes it difficult to distinguish MPs	[81]
KI (1.254 g/mL)	Applicable to high-density polymer	Expensive, oxidize filter paper	[83]
ZnCl2 (1.6 g/mL)	The high recovery rate of plastic particles (even high-density ones)	High cost, hazardous to the environment	[82]
ZnBr22 (1.71 g/mL)	Good recoveries for larger MPs	Expensive, severely hazardous to the environment	[81]
Canola oil	High recovery rate	Interfere the chemical analysis of MPs	[85]
NaBr	Low cost, Environmental friendly	Not applicable to some types of plastics	[84]

, the common saturated salt solution includes 1.2 g/mL NaCl [75], 1.71 g/mL ZnBr₂ [81], 1.6 g/mL ZnCl₂ [82], and 1.57 g/mL NaI [77], and others. NaCl is the most widely used because of its low cost and no harm to the environment [79]. However, some high-density plastic particles cannot float in the solution. Flotation reagents such as NaI, KI [83], ZnCl₂, NaBr [84], and ZnBr₂ [81] are also used to float denser polymers in some studies, but they are costly and may cause chemical hazards to filter paper and therefore disturb the identification of samples, and be a hazard to the environment.

After floatation, the supernatant is filtered with a mesh size of 0.47~2 μm through vacuum suction filter equipment [72,86]. More MPs would be collected by the use of a 0.7 μm or smaller filter [50] as compared with larger ones (e.g., 2 μm or larger mesh size) [76]. At last, the suspected items similar to MPs can be identified according to the same procedures as the seawater sample [87].

3.3. Techniques for MPs Studies in Organisms

The existence of MPs in marine organisms have been studied in birds [88], fish [55], mussel [89], terrestrial invertebrate [90], earthworms [91], bivalves [92], and invertebrates [93], in terms of the whole animal [93], digestive tract [88], all soft tissue [92], and gastrointestinal tract [94]. Two sampling devices are commonly used to collect organism samples in water [95]. One is the direct use of plankton nets such as Bongo nets and shrimp nets, the residual organisms on the net can be collected after being washed with 10% formalin solution. The other is that collect a certain volume of the water column with a clean container and then extract the organism from the water column for further analysis.

Preservation methods used in biological organisms are stricter than seawater or sediment samples. Immediately, frozen the organism samples at −20 °C [96] or preserved them in formalin solution [97] or ethanol [98] (

Table 4. Common methods used in the sampling and identification of MPs in organisms.

Items	Parameters	Refs
Biological species	Birds; Fish; Mussel; Bivalves; Invertebrates	[55,89,93]
Tissue type	Digestive tract; All soft tissue; Whole animal; Gastrointestinal tract	[93,94]
Preservation	Frozen at −20 °C; Ethanol; Formalin	[60,89]
Digestion	Acid (HNO3); Enzymatic; H2O2; NaClO; Alkaline	[56,66]
Digestion temperature	25 ℃; 65 °C; 100 ℃	[76,104,105]
Separation	Flotation; Filtration; Sieving	[71,104]
Identification	Stereomicroscope; SEM; μ-FTIR or μ-Raman	[52,60,61]

). The pretreatment of organism samples is a combination of digestion and filtration. According to the size of the biological sample, digestion can be divided into two types. One is the complete digestion (i.e., digestion of the whole biological sample [93], which applies to small organisms such as mussels [89] and oysters [99]. The other is partial digestion dissolves part of the soft tissue or digestive organs obtained by dissection and tissue section [100], which applies to larger biological samples such as fish [101], waterfowl [102], and others (e.g., turtles) [103].

Digestion is classified as acid, alkaline, and enzyme digestion [67] (

Table 5. Advantages and disadvantages of different digestion methods in MPs study.

Digestion Method	Digestion Solution	Advantage	Disadvantage	Refs
Acid digestion	HCl (5% or 37%)	Fully digested fish tissues	May degrade some plastic polymers	[66]
	HNO3 (65% or 69%)	Full digestion and inexpensive	Corrosive acid, may degrade plastics	[92]
	HClO4 (65% or 68%)	Common reagent in labs, easy to obtain	Corrosive acid, may degrade plastics	[105]
	HF (40%)	Full digestion	may impact the structural or chemical integrity of plastics	[56]
	H2O2 (30% or 35%)	No effect on any plastic particles	Incomplete digestion	[96]
Alkaline digestion	KOH (10%)	The most suitable solution to digest fish tissue	May hinder FTIR if not cleaned	[66]
	NaOH (5M)	Low chemical harm and low cost	Unknown effects on polymer types	[55]
Enzymatic digestion	Enzyme (proteinase and collagenase)	High digestion efficiency	Incomplete digestion and may be worse for nylon fibres	[60]

). Based on the previous studies, acid digestion solutions mainly include H₂O₂ (30% or 35%) [47], HNO₃ (65% or 69%) [56], NaClO [66] and other combined acids (i.e., 5% or 37% HCl, 65% or 69% HNO₃, 40% HF) [66,104] at room temperature (usually 25 °C) [104], 65 °C [106], and 100 °C [76], in which 65 °C is the most frequently used. They show different advantages and disadvantages. For example, H₂O₂ has no effects on plastic particles, but it cannot remove all organic materials; NaClO can completely eliminate organisms, but it fails to reach an optimum digestion efficiency in a short time [66], and the concentrated acid, especially corrosive acid may adversely impair the structural and chemical integrity of MPs and may result in strong chemical hazards to the environment as well [56,66,76]. By comparison, enzymatic digestion has high efficiency, but it may destroy nylon fiber plastics and cannot completely digest biogenic materials [92,96]. Once the biological tissues are completely digested, a vacuum suction filter device is further used to filter the digested mixtures, and the identification of MPs follows the procedures of seawater samples.

3.4. Research Methods of MPs in Air

Air is a mixture of gases and suspended particles [107]. Recent literature suggests the presence of MPs in the air [28]. There are quite limited studies on the occurrence of MPs in the air [108,109,110]. In general, atmospheric fallout with dry and wet deposition can be collected through a stainless-steel funnel, which connects with a 20 L glass bottle at the bottom [111,112]. Once the fallout was collected, rinse the funnel with reverse osmosed water to recover all particles (including MPs) adhering to the funnel. After being rinsed, all samples in a 20 L glass bottle were immediately filtered on quartz fiber GF/A (with a size of 1.6 μm). At last, observe the MPs on the filter using a stereomicroscope and measure the length of the fibers during the enumeration (e.g., the software ‘Histolab (Microvision instruments—Evry—France)’ coupled with the stereomicroscope) [111,112]. The other method for indoor air sampling is as follows, a pump (Stand-alone sampling pump GH300, Deltanova, France) at 8 L/min allowed to sample indoor air on quartz fiber GF/A (1.6 μm, 47 mm), and the height and time of samplings depends on the location selected. Next, 50 mL zinc chloride solution (1.6 g/cm³) is used as a flotation solution to separate the 5.5 mg sample in a separation funnel. Next, a subsample of 1 mL was taken from the homogenized floating fraction and filtered on quartz fiber GF/A (1.6 μm, 47 mm). At last, all samples were observed by stereomicroscope and qualitatively analyzed by using μ-FTIR or μ-Raman [111].

3.5. Measurement of MPs in Other System

In addition to the natural environmental media, there are some techniques for detecting the MPs in other specific media, such as wastewater [65], drinking water or tap water [113,114], and sludge [115]. Among them, grabbing samples (i.e., single samples collected at one time) is the most typical method for collecting wastewater samples from wastewater treatment plants [43]. In detail, a telescopic sampling rod (Telescoop, Waterra Ltd., Solihull, UK) was used to collect wastewater samples, which can be further transferred into a 50 mL centrifuge tube for centrifugation at 2038× g for 2 min, and then the resulting pellet was treated with 30 mL of Fenton’s reagent [61]. By comparison, the sampling and pretreatment of tap water and drinking water is much easier because their components are simpler than wastewater. The samples are mainly taken from bottled water and water plants [116,117]. Next, the samples will be processed in terms of staining [118], filtration [119], or digestion [120] for subsequent observation. Since the sludge in wastewater treatment plants commonly contains complex components, they are usually to be air-dried, sieved, and kept as soil samples [34] and then analyzed using the same methods as that of MPs analysis in soil.

4. Accumulation and Translocation of MPs in the Human Body

MPs can transport and deposit in tissues and organs when breathed or inhaled [28,121]. The non-absorbed component of MPs is excreted out [122]. Few MPs have been known to infiltrate the respiratory tract. The different densities and configurations determine MPs’ aerodynamics, which is used to estimate MPs settling velocities [123]. Small aerodynamic equivalent diameter MPs are more ready to attain the deeper respiratory track. Fibers were found in the lungs compared to other shapes, suggesting their high risk [124]. Mehmood and Peng [28] proposed that the persistence, transportation, and health risks posed to MPs are directly related to their types, sizes, shapes, and environment, such as indoor and outdoor locations. After 180 days in synthetic lung fluid, PP and PE fibers showed no disintegration or alterations, indicating a high potential for MPs persistence in the respiratory system [125]. Some nanoscale particles have been demonstrated to pass through the various protecting barriers, cell membranes, blood, brain, and placenta barriers [126,127]. Nevertheless, there is no direct proof that MPs are distributed and accumulated in human organs. According to the sole mouse-model-based study, MPs can be collected in different organs, including the kidney, gut, and liver [128].

Little information is available on the possibility of MPs intake and the accompanying health effects in humans. However, the harmful effects of MPs have been observed in other living organisms, including animal studies, but their findings cannot be extrapolated to human toxicity. Ingestion of MPs showed inflammation in the digestion system in Mytilus [129]. Fish’s immune systems are similarly vulnerable to micro(nano)plastic attacks. For example, Fathead minnow showed a substantial increase in primary granule degranulation and innate immune system release neutrophil extracellular when exposed to nano-plastics [130]. Polystyrene microspheres were administered intramuscularly into rats to increase blood coagulability or to cause vascular occlusions, resulting in pulmonary hypertension, inflammation, and chemotactic activity [131]. Intracellular structure damage was observed in vivo polystyrene exposure in macrophages, erythrocytes, and rat alveolar epithelial cells [132]. Microbes, pathogens, metals, persistent organic contaminants, and chemicals are among the pollutants in soil, water, and air [133]. When MPs coexist with such contaminants, MPs can absorb these pollutants, which can then be released into the human body, exacerbating MPs toxicity [134,135]. The consequences of MPs on human health are still being debated [121]. Since plants can also uptake several types of soil pollutants, which ultimately enter the food chain [136], MPs has also potential to accumulate in plant biomass [137]. Some researchers underlined the concerns of food chain transfer, while others argued that MPs or MPs additions have no negative effects [134]. Furthermore, the deposition of suspended particles in the air on plants, whether dry or wet, can disrupt photosynthesis and be intake by humans and other animals through the consumption of such polluted plants. The majority of the disagreements arise from the ambiguity in the estimation intake of MPs, so further research and modeling are required for a more comprehensive demonstration of intake routes, fate, and persistence.

Despite the fact that MPs toxicity is still in its early stages, occupational illnesses have been related to inhaling MPs particles. Workers exposed to PP have respiratory symptoms that are 3.6 times more severe [124]. Long-term inhalation and excessive exposure to fine particles is more dangerous to health and also result in gene mutation [138]. Exposure to PP and other synthetic plastics over 10 to 20 years enhanced the number of cancer patients among textile workers. Workers who worked with polyvinyl chloride had a higher risk of lung cancer as they got older, worked more years, and spent more time in the factories [123]. More research is needed to estimate MPs concentrations in the atmosphere and tissues and understand their mechanical toxicity. In additions, the toxicity associated with micro(nano)plastics should further studies based on different age groups and genders. Previous studies suggest children are more susceptible to MPs exposure compared to adults due to their activities such as playing on artificial turf in grounds and parks [28]. Likewise, MPs may pose a threat to fetus development thus their toxicity to women during pregnancy should also explore in detail [139,140].

5. Management and Treatment of Plastic Pollution

5.1. Eco-Friendly Safety Procedures against the MPs

Due to higher sales and market share in the international market and a wide range of use in daily life, plastic appears as an unavoidable commodity [141,142,143]. A very small share of the total produced plastic is collected for recycling, so plastic waste is continuously gathered in the environment due to its high stability and persistence [142]. Although the global community has aimed directly at a range of regulatory initiatives in recent years to regulate plastic pollution and its environmental damage, there are presently restrictions on the manufacturing and using plastics.

Some nations worldwide have implemented regulations limiting the manufacturing of plastics (e.g., beverage bottles and carrier bags). For example, in the Canadian province of Ontario, a law prohibiting the manufacture of microbeads was passed [144]. As a result, legislation has been passed in the United States prohibiting the use of small plastics in order to reduce pollution [145]. These approaches, taken together, aid in lowering the micro(nano)plastic existence on the ground, in the steam, and the air.

The manufacturing industry also performs an integral role in decreasing micro(nano)plastic in the overall supply chain. IKEA (Ingvar Kamprad Elmtaryd and Agunnaryd), for instance, has included the EPR (Extended Manufacturer) strategy in its marketing strategy by encouraging the reusing of polymeric products across its production network [146]. The growth of a circular financial system, such as recycling, reuse, and waste management process, may also assist in controlling plastic utilization, with significant zigzag social and ecological consequences. Among the most effective and sustainable solutions to avoid pollution has been recommended: regulating micro(nano)plastic in a well. For instance, outlawing plastic bags can successfully prevent excess plastic use, and pursuing that goal can minimize plastic contamination with MP [147]. Other producers must also label the plastics commodities with proper guidance and information regarding corresponding recycling and environmental feasibility. Furthermore, campaigns raise awareness of MPs challenges among colleges, schools, organizations, and networks, as well as teach individual responsibility for decreasing MPs by deciding to cast-off, limit, recycle, and reuse. Children’s educational initiatives (e.g., sea waste education) will increase their understanding, visions of outcomes, and willingness to report them [148].

5.2. Governmental Policies and Nongovernmental Campaigns to Curb MPs Pollution

In 1970s, the distribution of plastic wastes in the Pacific Ocean firstly attracted the attention of researchers [149], and it was more than two decades later when Thompson coined the term “microplastic” in 2004 [4]. Micro(nano)plastic pollution has attracted public concerns in recent years. For example, MP was detected in some brands of table salts in China [150,151] and Turkey [152] and even in 83% of all tap water samples collected from cities and towns from five continents [153]. Under such circumstances, some policies and campaigns relating to plastic litter have been enacted or launched, aiming to control the quantities of plastic litter from the human side. Some selected policies or campaigns curbing MP pollution are shown with the time axis in Figure 5.

In the past few years, more and more countries made all kinds of efforts to slow down the release of plastic wastes (including MPs and NPs) into the natural environment, in terms of banning the supplement of pellets in daily skincare products, limiting the use of disposable plastic products, or strengthening the monitoring and remediation of plastics pollution. For example, some provinces in China banned the production, supplement, sale, and use of pellets, disposable non-degradable plastic products in the past years [27], and the European Commission held a side event as a response to Resolution UNEP/EA.4/R.9 on “Addressing Single-use Plastic Products Pollution” in 2021. It is expected that the distribution and transfer of (micro-/nano-)plastics pollution can be controlled with the efforts of all parties.

5.3. Biological Control

Researchers have found biological management viruses positively impact micro(nano)plastic degradation. Biodegradation is one of the most important strategies to control micro(nano)plastic. Plastics are decomposed by microbes, fungi, and meal worms. It has already been discovered as a dependable and environmentally beneficial solution for dealing with plastic contamination by decomposing it without causing any negative consequences. Bombelli et al. [154] found that ethylene glycol produced by wax moth larvae Galleria mellonella (at a rate of 0.23 mg cm⁻² per hour) can rapidly bio-degrade PE.

Similarly, microorganisms of the earth’s crust (Lumbricus Terrestris) degrade low-density PE (LDPE) MP particles [155]. MPs’ hazardous has also been found to be declined by the Bacillus gottheilii bacteria. Bacillus gottheilii and Bacillus cereus were utilized by Auta et al. [156] to eliminate MPs from PE, PP, PET, and polystyrene. For morphological and structural alterations and weight loss in MPs, the findings were examined using (FTIR).

B. cereus and B. gottheilii presented a higher reduction of polystyrene and polyethylene and showed removal rates constant of 0.0019 and 0.0016 per day, respectively, while the corroding half-life was 363.16 and 431.25 days, indicating that B. gottheilii has the potential to degrade micro(nano)plastic. These plastic predators have demonstrated a novel approach to reducing plastic trash and MPs [156]. In other studies, Bacillus siamensis demonstrated 8.46% degradation of LDPE in 90 days [157], 5.5% of HDPE in100 days by Aspergillus flavus [158], 14.7% of PE in 60 days by Bacillus sp. In addition, Paenibacillus sp. [159]. Similarly, Aspergillus nomius RH06 degraded 6.63, while Trichoderma viride RH03 showed 5.13% of LDPE in 45 days [160]. Comparatively, Bacillus cereus strain A5 and Aspergillus oryzae strain A5 showed 35.72 and 36.4% degradation of LDPE in 112 days [161]. Previously, Thomas et al. [162] reported 18.0% degradation of PE by Pseudomonas fluorescens in 270 days and Klebsiella pneumoniae CH001 showed 18.4% degradation of HDPE in 60 days [143]. At the same time, the highest plastic degradation efficiency was found by Bacillus vallismortis bt-dsce01, which degraded 75% of LDPE in 120 days [163].

As a result, governments must invest more in research to uncover species capable of adequately breaking down plastic. Destruction of MPs and their associated species is a promising way to taint micro(nano)plastic’s reputation as a green method.

6. Methods and Mechanisms of Degradation of (Micro)Plastics

The following section discusses the most popular micro(nano)plastic degradation approaches, i.e., (1) biodegradation; (2) chemical degradation. Biodegradation includes bacterial-assisted biodegradation, fungal-assisted biodegradation, algal-assisted biodegradation, Integrated enzymatic biodegradation, and combined biodegradation [133].

6.1. Biodegradation

Biodegradation is the disintegration of pollutants (e.g., plastics) by microbes such as fungi, bacteria, and viruses accompanied by microbial catabolism [133]. The development of biofilms on micro(nano)plastic and plastic surfaces leads to the breakdown of their fundamental structure frame and cross-links by particular enzymes, resulting in monomers, oligomers, dimers, and other depolymerization byproducts (e.g., CO₂ and H₂O) [164]. Inorganic and organic metabolites or conversion constituents result from incomplete mineralization [165]. In an alkaline environment, biodegraded polymers produce H₂O and CO₂ [166], while in anaerobic conditions, they are CH₄ and CO₂ [167]. Plastic biodegradation has gained widespread acceptability due to its environmental neutrality and convenience. Aside from ecological considerations, the variety of microbes, the nature of plastics, the shape of polymers, and the enzyme selection are all major candidates influencing the biodegradation of plastic. Biodegrading by bacteria, enzymes, fungi, and collective biodegrading is being researched. Biodegradation occurs in various ways, each with its own mode of action.

6.1.1. Bacterial Assisted Degradation

Many scientists are studying the bacterial degradation of plastics and the formation of micro(nano)plastics. Based on Web of Science and Scopus online sources, bacteria that decompose plastics originated from landfills belong to the phyla Proteobacteria, Actinobacteria, and Firmicutes in the majority of biodegradable plastic-related studies [168]. The gut of a super worm contained Pseudomonas aeruginosa DSM 50071 (bacteria), which exhibited polystyrene degradation in lab experiment [169]. Likewise, Bacillus strains isolated from Peninsular Malaysian mangrove environments showed successful PP degradation in 40 days [170].

In association with bacteria in the body, certain bacteria performed biodegradation indirectly. In yellow mealworm larvae, polyvinylchloride can disintegrate and be converted into monomers, but gentamicin slows down this process by inhibiting intestinal microorganisms [171]. Gut microorganisms are claimed to be involved in polystyrene breakdown. For instance, polystyrene consumption substantially increased the gut microbial population in Achatina fulica (snails) and enhanced flora growth [172]. Likewise, Tribolium castaneum larvae have Acinetobacter bacterium, and Endosymbiotic bacteria in citrus mealybugs demonstrated successful polystyrene and polyethylene degradation, respectively [173,174]. Noticeably, a mixture of different microorganisms showed more efficient micro(nano)plastic degradation than a single bacterium whose degradation performance in a mixture of varying micro(nano)plastic is poor. Micro(nano)plastic is more complex and degraded by the environment, making its makeup more challenging to interpret. As a result, bacterial degradation is limited.

6.1.2. Fungal-Assisted Degradation

A laboratory study demonstrated that Zalerion maritimum (marine fungus) utilized polyethylene as a growth substrate and significantly reduced the proportion of polyethylene particles in 28 days [175]. Polyester yarn mineral media showed filamentous fungal tip proliferation in the presence of F. oxysporum and F. solani [166]. Fungi can degrade micro(nano)plastic using polymers for carbon and energy use [176]. Fungi can use enzymes that are not substrate-specific to detoxify contaminants and invade substrates. Mycelium may bind to hydrophobic substances and even infiltrate three-dimensional surfaces thanks to fungi’s hydrophobic proteins. For this purpose, the wax borer’s intestinal contents were used to isolate the Aspergillus flavus PEDX3 polyethylene-degrading fungus strain and successfully employed in HDPE conversion into lower molecular weight MPs during a four-week experiment [37]. In contrast with bacteria, fungi can degrade a wide range of plastics, which can only degrade specific plastics based on their enzymes. Thus, fungi can function on non-specific substrates, transcending the restrictions of bacterial degradation alone. Fungi have a lot of potential for breaking down plastics and micro(nano)plastic.

6.1.3. Algae-Assisted Biodegradation

Algae have also been found to produce chemicals to break down polymers utilizing these plastics as a carbon source. Future study into algal degradation holds a promising prospect [177]. Conventional bio-based biodegradable plastics have low water tolerance and mechanical qualities, whereas algal synthesized MPs are light, hydrophobic, and strong [178]. Furthermore, algal is simple to produce and harvest, making them time and space-efficient [179]. Although algal-based polymer synthesis is still limited to libraries scale, these polymers have enormous scope for application. Only a few algae species have been able to degrade plastic objects. The surface of plastics provides feasible sites for algal growth [177], where the colonies of algae degrade plastics by lignin and extracellular polysaccharides [179,180]. Plastic degradation has also been witnessed on plastic surfaces with algae (blue-green algae, green algae, diatom, Navicula pupula, and Scenedesmus dimorphus [181]. Furthermore, Khoironi et al. [181] discovered PET plastics degrade faster than PE plastics, implying plastic-type as a determinant factor in algal-based degradation. Furthermore, several bacteria and fungi which has the potential to degrade plastic are not well-tailored to the marine ecology, wherein a majority of plastic garbage build-up [182]. Still, such limitation endures by some harmless and non-toxic algae by establishing algal biofilms in varied polluted watercourses successfully impair plastic-oriented waste [182]. Algae can be utilized as microbial processing plants to create tailored PETase and directly degrade the waste of plastics [183]. Synthetic biology is a viable method for developing a green solution for lessening biological PET utilizing microalgae [177]. Establishing of Diatom, Navicula pupula, blue-green alga, Dimorphus, and Anabaena spiroides on the plastic surface showed breakdown [181]. Furthermore, Khoironi et al. [181,184] reported PET plastics break down faster than PE plastics, demonstrating that algae-based MPs biodegradation depends on the types of MPs. However, further research and analysis into the process and algae efficacy as micro(nano)plastic degraders is required.

6.1.4. Integrated Enzymatic Biodegradation (IEB)

The integrated enzymatic breakdown is among the main characteristics of biodegradable polymers. About all-polymer breakdown in the environment requires IEB [185]. Plastic-degrading microbes have been discovered in the digestive tracts of insects, on land, and in water, but they are quite particular and have very variable disintegrated [186,187]. IEB is a convenient tool to degrade plastics and micro(nano)plastic, despite relying on the enzyme of the precise organism, a distinct catalytic process, and a long incubation period. This is especially true nowadays, as a study on engineered enzymes raises the dominance of this biological breakdown process [185]. Only naturally occurring enzymes are insufficient to reply to breakdown MPs because their potency is low, and their range is not as diverse as we would prefer. As a result, research is being conducted to manufacture enzymes to maximize the effectiveness of enzyme breakdown.

6.1.5. Combined Biodegradation

Many studies have shown that one bacterium’s polymer biodegradation is poor, resulting in reduced microbial growth due to hazardous end products produced by polymer breakdown [188]. Some other investigated using many bacteria to build resilient microbial colonies to biodegrade MPs to overcome this limitation [186]: (1) products of microbial degradation by one bacterial strain often work as a substrate for the proliferation of a second bacterial strain [165]; (2) an ample and durable microbial group can confront sole microorganism precision and disintegrate in various MPs; (3) A healthy microbial community can efficiently degrade micro(nano)plastic in a synergistic symbiotic way. Many scientists have succeeded in developing a biodegradation method based on a range of microorganisms [14]. Taniguchi and colleagues [166] successfully demonstrated PET degradation by Three routes that can degrade PET: Ideonella sakaiensis 201-F6; Microbial Consortium No. 46; and MHETase and PETase. Park and Kim [159] speed up the decomposition of polyethylene MPs using hybrid bacterial culture primarily comprised of Paenibacillus sp. and Bacillus sp. obtained from landfills. To a significant extent, cumulative biological degradation surpasses the limits of sole bacterial degradation [159]. Nevertheless, due to the direct contact of numerous microorganisms and diverse enzymes, the breakdown and consumption of plastics and micro(nano)plastic by bacterial communities is a rather complex procedure, and the degradation mechanism and kinetics are still not precise. Moreover, the co-existence of other pollutants may alter the microbial community structure and their response to target pollutants [189,190]. Resultantly interaction of microorganisms with MPs and the efficiency of MPs degradation may change. Therefore, exploration of the main determinant elements and mechanisms is required.

6.2. Chemical Degradation

Miao and colleagues proposed a TiO₂/graphite (TiO₂/C) cathode, and six hours of electrolyzed Ag/AgCl at 0.7 V showed 75% dichlorination, oxidized the hydroxyl radicals, and degraded PVC simultaneously [191]. In another study, a nanocomposite membrane made of PLA/PE and TiO₂ nanoparticles was subjected to simulated sunshine, which accelerates PLA and PE breakdown and modifies polymer structure [192]. By combining LDPE with a photothermosensitizer containing cobalt stearate or nanoparticles of silver, eco-friendly materials can be produced with promising PE degradation potential [193].

The process of hydrolysis chemically degrades plastics. The potential of plastic to disintegrate in water is determined by the presence of hydrolyzable covalent bonds such as ester, ether, anhydride, amide, carbamide, or ester amide groups, etc. The effectiveness of hydrolysis is influenced by water properties, time, pH, and temperature [194]. Plastics having hydrolyzable covalent linkages (for example, PET) can be hydrophilic, accelerating the polymer’s hydrolytic breakdown [14]. Polyester can cause hydrolysis when hydrogen ions target the ester bonds under alkaline or acidic conditions. Hydrolytic breakdown in alkaline conditions causes surface corrosion of polyesters in parallel to chain breaking [195]. Likewise, Lin et al. discovered that elevated UV radiation (3600 mJ cm²) could cause changes (such as cracks, wrinkles, and protrusions) in PET, PVC, and PS when compared to a regular dose of UV [196]. This phenomenon was likened to intense UV, causing the chemical bond to crack microplastics. UV radiation degrades micro(nano)plastics by cracking their C–H and C–C linkages [15].

Another efficient form of chemical degradation for MPs degradation is oxidative degradation. When oxygen is introduced to polymers, functional groups such as hydroxyl and carbon monoxide are generated, which assist in biological degradation [10]. Light or heat can stimulate the degradation activity, and nonhydrolyzable materials are especially susceptible to light and heath initiative OD [18]. Oxygen is found to be the most potent variable which causes material breakdown; ambient oxygen can strike covalent bonds in polymers, exacerbating the formation of free radicals; it has an oxidative degradation significantly affected by the structure of polymers, and OD produces peroxy radicals which break bonds and initiate side reactions [10]. Furthermore, the aging of plastics is substantially enhanced in the presence of ozone, causing them to degrade faster [194]. Due to its low economic efficiency, complete sulfate breakdown with a greater level of mineralization, and capacity to break down randomly tough plastics by bypassing the selectivity of microbial enzymatic biodegradation, advanced persulfate-based oxidation technologies have been developed and received a lot of interest [197]. Advanced oxidation processes (AOPs) are a chemical degradation type preceded by high oxidative reactive organic species (ROS). These species perform the degradation of organic pollutants in soil and water. AOPs include electrochemical oxidation, photocatalytic oxidation, and photochemical oxidation, which successfully apply MPs degradation [17]. The cosmetics sources’ MPs were degraded by the sulfate-based AOPs (SR-AOPs) method via catalytic activation of peroxynitrite by magnetic nanocomposites. Under a hydrothermal environment, activation of peroxynitrite yields reactive radicals with a high potential for MPs degradation [198]. ROS, an AOPs product, directly caused MP degradation, resulting in chain breakdown, product generation, and even total conversion of MPs into subunits [17].

Further studies are required to combine chemical treatment with other strategies. For instance, Mustafa et al. [199] recently concluded that combining degradation with filtration significantly enhanced the removal of emerging organic pollutants from water. Such strategies would be effective for the efficient removal of MPs.

7. Conclusions and Future Prospects

Systematically and explicitly, this review summarized the sampling, pretreatment, identification, health risk, and control of MPs pollution around the globe. In addition, both governmental policies and nongovernmental campaigns are listed to show the efforts to curb MPs worldwide. Undoubtedly, public concerns about MPs pollution, but more collaborative efforts are needed in the future to facilitate solution-searching efforts. For instance, the investigation protocols remain standardized, the fate of MPs in the environment requires more effort, either laboratory studies or modeling of transfer and transportation, and the ecotoxicological effects on living organisms and even humans based on the background concentrations of MPs in the environment. Furthermore, government organizations and public and private sectors will develop more innovative, long-term solutions to deal with plastic litters and MPs production.

Some countermeasures to curb plastic pollution may be achieved by changing industry behavior and influencing regulation, limiting the consumption of disposable plastic products, forbidding the supplement of plastic pellets into products (e.g., skin washing products, shower products, and others) at a much larger scale or worldwide, modifying plastic recycling actions and inventing feasible biodegradable plastics, and others. This article explores further the degradation mechanisms, influencing factors, and environmental interactions of degradable plastics. Public awareness regarding the wise use of plastic items and their reduction and recycling should also be extended to underdeveloped countries and small cities.

Additionally, a link has been demonstrated between plastic degradation and micro/nanoplastics. With additional research, it is possible to transform those non-biodegradable plastics into biodegradable plastics. Simultaneously, studies may conduct into the environmental impact and health risks of microplastic substances, laying the theoretical foundation for the future scientific selection of environmentally benign microplastic degradation procedures. Based on chemical and microbiological-assisted degradation information presented in this research, there are still some knowledge gaps, including (i) studies should focus more on designing methodologies to hinder the formation of hazardous intermediates by degradation, (ii) meanwhile parallel approaches that preferentially break chemical bonds in polymer MPs to produce usable organics at a high rate (for example, CH₂O₂, C₂H₄O₄, CH₄O, and C₂H₆O) and extract these molecules safely, (iii) following assisted degradation processes, the remaining MPs are detrimental to the environment, and MP biodegradation products have not been collected or recognized in a timely manner in most of the available literature, and (iv) also determine if interactions between various microbial species and distinct enzymes influence the degradation and utilization of MPs by microbial consortiums.