Microplastics in the Environment: A Review Linking Pathways to Sustainable Separation Techniques

Abstract

Since the mid-20th century, the quantity of microplastics (MPs) has increased significantly, becoming a persistent environmental pollutant widely distributed in global water bodies, soils, and the atmosphere. While plastic materials have brought significant convenience to daily life, the MPs resulting from their degradation pose increasing threats to ecosystems and human health. This comprehensive review examines the sources, migration pathways, and ecological impacts of MPs, and critically evaluates the current separation techniques from physical, chemical, and biological perspectives. In particular, numerical simulations of the hydrocyclone separation technique reveal its unique flow characteristics, including turbulent velocity gradients and axial pressure differences, with a separation efficiency of up to 93%. This technique offers advantages such as high efficiency, low energy consumption, and environmental friendliness. In response to the growing microplastic pollution issue, this review emphasizes that the development of future microplastic separation techniques should prioritize separation efficiency, sustainability, and environmental compatibility. Continued research in this field will provide theoretical support for optimizing microplastic pollution control technologies and contribute to achieving environmental protection and sustainable development goals.

1. Introduction

In recent decades, society’s heavy reliance on plastic products has exacerbated global plastic pollution, presenting a significant challenge in managing microplastics (MPs). Plastics are primarily used in packaging, accounting for 39.9% of total plastic usage in Europe in 2016. Plastics are extensively employed in construction (20.1%), electronics (5.7%), automotive (8.6%), and agriculture (3.4%) [1,2]. This widespread application is due to plastic’s favorable properties, including geometric stability and corrosion resistance, as well as its historical affordability. However, the extensive use of plastics has been accompanied by significant waste generation, with only a minimal fraction being recycled, and the majority ending up in landfills. The outbreak of the COVID-19 pandemic, in 2019, significantly increased the global consumption of polypropylene (PP) and polyethylene (PE)-based face masks, exacerbating the already critical issue of plastic pollution. By 2050, the plastics industry is expected to account for 20% of global oil consumption, unless current trends are reversed [3]. Plastics often take centuries or longer to decompose, posing significant environmental challenges [4]. The historical trajectory of plastic production and its unintended consequences are illustrated in Figure 1, based on the relevant information. The inception of modern plastics dates to 1907, when Leo Baekeland synthesized the first commercially viable synthetic plastic (phenolic resin) from phenol and formaldehyde [5]. Since then, global mass production has made plastics indispensable in daily life. However, the mismanagement of plastic waste has led to its accumulation in ecosystems, where it fragments into MPs—solid plastic particles typically smaller than 5 mm [6,7]. These particles, made of a mixture of polymers and functional additives, can unintentionally form when larger plastic items, such as car tires or synthetic textiles, undergo wear and tear [7]. It is also important to note that synthetic textiles are products made from synthetic fibers. The widespread use of synthetic fibers leads to the continuous release of microfibers into the environment through processes such as production and washing. These microfibers are extremely small in size (with lengths reaching up to 100 μm and diameters as small as 2 μm), presenting unique challenges for pollution management [8,9,10]. MPs are transported through various pathways, such as the atmosphere, rivers, and soil, exhibiting distinct physicochemical properties in different environments. For example, microplastics in soil can form complexes with organic matter and mineral particles, complicating their separation. This environmental behavior is further influenced by their adsorption capacity—MPs efficiently adsorb a wide range of toxins due to their minute size and high dispersion [11,12,13], which provide a large surface area and numerous adsorption sites. Additionally, the hydrophobic nature of the polymeric components in MPs further enhances this process [14]. With advancing knowledge of microplastic migration and transformation, researchers can develop targeted separation techniques based on pathway-specific characteristics, which is critical for assessing their risks to ecosystems and human health.

Figure 2 presents a visual analysis of over 7200 research articles related to MPs based on VOSviewer (Version 1.6.20). The articles were retrieved from the Web of Science database (2019–2024) using the keywords “microplastics” and “pollution”. The low node density and weak interconnections observed in Cluster 1 suggest that separation techniques remain an area requiring further development in MPs research. Significantly, the advancement of efficient separation techniques is crucial for achieving high-efficiency recovery and targeted treatment of MPs [19]. So far, various technical approaches have been developed for the separation of MPs. Among the physical employed approaches, physical adsorption and filtration techniques stand out [20], which involve the capture or adsorption of microplastic particles using membranes or agents with varying pore sizes. Although their efficiency in capturing smaller particles remains limited. While offering advantages such as mature technique and straightforward operation, it may exhibit lower efficiency in separating minute particles [21]. Density separation, on the other hand, relies primarily on the density disparity between microplastic particles and the surrounding medium (e.g., water or salt solution), utilizing centrifugal force or buoyancy for segregation [22]. Although it boasts higher separation efficiency, the equipment requirements are contingent upon the types of MPs and their sizes, necessitating stringent standards for separation devices [23,24,25]. Chemical treatment methods, such as dissolution, extraction, and chemical reactions, alter the interaction between MPs and other substances to achieve separation [26]. While being effective for specific microplastic types or environmental samples, this approach necessitates complex processing steps and chemical reagents, potentially causing secondary pollution [27]. Microbial treatment methods offer environmentally friendly and sustainable solutions for microplastic treatment. The microbial metabolism of plastics and the dissolved organic matter leached from them can directly or indirectly influence critical biogeochemical cycles [28]. However, their degradation rate, efficiency, and environmental requirements warrant further research [29,30]. According to relevant research, MPs can serve as reservoirs and new vectors for pathogenic bacteria in aquatic environments, thereby posing risks to ecosystems [28,31,32].

This review systematically elucidates the extensive migration characteristics and transport pathways of MPs in the environment, with a focused analysis on current primary microplastic separation techniques. Traditional methods are limited by their reliance on chemical reagents, potential secondary pollution, and low separation efficiency, highlighting the urgent need to develop more efficient, targeted, and environmentally friendly separation techniques that align with the principles of green and sustainable development. Notably, the hydrocyclone separation technique has emerged as a highly promising approach for microplastic removal due to its high separation efficiency, low energy consumption, and environmental friendliness. Moreover, future research should concentrate on integrated MP separation methods that combine multiple techniques. By leveraging the complementary advantages of physical, chemical, and biological approaches, the performance limitations of single techniques can be overcome, thereby enhancing the removal efficiency of MPs in environmental matrices (e.g., water). This not only provides a scalable solution for the removal and pollution control of MPs, but holds significant strategic importance for addressing the increasingly severe global challenge of MPs pollution.

2. Environmental Pathways and Hazards of MPs

Widely acknowledged to be a pervasive category of anthropogenic pollutants, MPs are extensively distributed across various environmental compartments, including the atmosphere, aquatic systems, and soils. Their spatial distribution patterns are profoundly influenced by human activities. Urbanization and industrialization contribute to the release of MPs through multiple pathways, such as the laundering of synthetic textiles, abrasion of vehicle tires, and degradation of agricultural plastics, leading to their persistent accumulation in the environment. Notably, atmospheric transport facilitates the long-range dispersion of MPs to remote regions. They can undergo trophic transfer through both aquatic and terrestrial food chains, posing significant risks of human exposure via ingestion and inhalation. The cross-media mobility and persistent bioaccumulation potential of MPs represent critical threats to ecosystem integrity.

2.1. MPs in Aquatic Environments

In recent years, MPs have become widespread in various aquatic ecosystems, posing a serious threat to aquatic organisms and their habitats. MPs in the aquatic environments are typically categorized into two types: primary MPs and secondary MPs [33]. Primary MPs are directly released into the environment in the form of MPs (smaller than 5 mm), and primarily originate from personal care products containing microplastic ingredients, such as facial cleansers, cosmetics, and toothpaste, which enter environmental systems through household waste discharge [34,35,36]. Studies in the European Union have suggested that individuals in Norway, and Switzerland may generate approximately 17.5 ± 10 mg of MP beads per person per day solely from using soap [37]. Moreover, daily facial cleansing by British women can contribute to the release of 4594–94,500 microplastic particles into sewage systems [38]. Secondary MPs, also defined as plastic particles with a diameter of less than 5 mm, differ from primary MPs in that they are not directly released into the environment in the form of MPs. Instead, they result from the prolonged physical and chemical degradation of plastics [33]. For example, fishing gear, such as nets, ropes, and buoys, used in aquaculture undergo abrasion and damage over time due to exposure to external forces, like ultraviolet radiation [4], making aquaculture a significant source of MPs in water. In a survey, ropes, fishing lines, and buoys and floats, respectively, accounted for 5.5%, 3.4%, and 1.5% of the total fragments collected during the investigation [39]. Globally, discarded fishing gear comprises about 10% of total marine debris, although significant variations may exist among different regions [40,41].

Since the mid-20th century, plastic production has surged dramatically. By the end of 2014, the world’s oceans have harbored at least 15–51 trillion microplastic fragments, weighing between 93 and 236 million tons [17]. Plastic remains a pervasive and enduring pollutant in marine ecosystems worldwide, predominantly concentrated along the coasts of densely populated or industrialized areas [42]. Taking the United States as an example, plastic waste underwent a nearly ninefold increase from 1970 to 2003, making it one of the fastest-growing urban waste types. Water samples from two California rivers were analyzed from 2004 to 2005 in a study by Moore et al., revealing that these rivers discharged over 2 billion plastic particles into the ocean in just three days [43]. In China, it is estimated that approximately 2995.7 tons of MPs are discharged annually through wastewater, with urban areas accounting for 74.9%, and rural areas contributing 25.1% [44]. Although wastewater treatment plants intercept some larger polymer plastics and small plastic fragments in oxidation tanks or sewage sludge, some MPs manage to evade the treatment process and are discharged into rivers and lakes [33,45]. The concentration of MPs in rivers is significantly influenced by seasonal and watercourse variations. During the rainy season, high precipitation and increased water flow enhance the transport of MPs from the source to downstream, leading to a more uniform distribution in rivers and tributaries. By contrast, reduced rainfall and slower flow in the dry season cause MPs to accumulate near the source, resulting in higher concentrations in tributaries than in the main river [46]. These differences are closely related to flow patterns as well as pollution sources from urban, industrial, and agricultural activities. According to the statistics, the Yangtze River, which flows through the key economic city of Shanghai in China, discharges approximately 1.5 million tons of plastic waste into the Yellow Sea [3]. Additionally, MPs have been detected in bottled water, tap water, and drinking water treatment plants (DWTPs) across Europe, Asia, and the Americas [47]. Detailed analyses reveal that the most common microplastic particles in drinking water are primarily small-sized (<10 μm) fibers and fragments composed of polyethylene terephthalate (PET), PE, PP, and polystyrene (PS) [47]. It is estimated that 57% of plastic particles found in food primarily originate from aquatic sources [48]. The smaller the size of MPs, the larger their surface-area-to-volume ratio, enabling them to absorb more pollutants and thereby increasing their toxic effects on organisms [49].

Moreover, a substantial quantity of ultrafine fibers from the textile industry finds its way into the oceans, primarily originating from indoor and outdoor washing, domestic drainage systems, or the direct disposal of discarded garments into rivers [50]. These rivers serve not only as temporary sinks for MPs but as critical pathways for their transport into marine environments [51]. Synthetic fibers are widely used in the textile industry due to their excellent wear resistance, wrinkle resistance, and elasticity. They serve as a complement to natural fibers, such as cotton, wool, and linen, in textile products. Statistics indicate that nearly 7 × 10⁵ fibers are shed from every 6 kg of clothing [52]. Water discharged from washing machines contains approximately 100–300 fibers per liter [53], which results in the release of around 64,000 pounds of microfibers into the oceans daily, totaling approximately 2 million tons annually [18]. By the end of 2019, there were at least 1.5 quadrillion microfibers in the oceans. Consequently, the oceans accumulate a substantial amount of MPs and microfibers originating from human activities. Over the past few decades, the annual consumption of synthetic fibers in clothing has experienced staggering growth, from 16 tons to 42 million tons [18]. Therefore, ultrafine fibers also significantly contribute to the microfiber load in water within the textile industry. Studies have shown that microplastic exposure significantly inhibits the growth of microalgae in both freshwater and marine environments [54], leading to cyst formation and cell membrane damage [55]. Furthermore, research has shown that PP MPs can cause severe damage to the intestines and liver of freshwater organisms, such as zebrafish, resulting in a reduction of cellulose in the gut and an increase in rhizobia [56].

2.2. MPs in Soil Environments

Agricultural plastic film is a widely employed product in agricultural practices, serving as a traditional method to boost productivity. Globally, over 128,652 square kilometers of farmland are covered by agricultural plastic film [57], aiding in soil moisture retention and weed suppression [58,59]. However, recent research has shown that residual microplastic film creates a plastic ring, which has a significantly higher abundance of genes related to denitrification and sulfate reduction activity than soil, increasing the potential risk of nitrogen and sulfur loss [60]. MPs in the soil mainly manifest as fragments and fibers, typically ranging from 1 to 3 mm in size [61]. Global annual consumption of agricultural plastic film has stabilized at approximately 700,000 metric tons since the mid-20th century, with China dominating global usage, accounting for nearly 80% of the total. Approximately 20 million hectares of farmland were covered by agricultural plastic film [62]. In China, agricultural film with a thickness of 0.005 mm is primarily used, significantly thinner than the 0.02 mm film standard in developed countries [61]. Due to its susceptibility to rupture during cultivation, coupled with farmers’ limited environmental awareness, this film is often discarded in the soil, leading to a low recycling rate and substantial accumulation of film residues in farmland. Over time, these residual plastic mulches degrade, fragmenting into MPs, which pose further environmental challenges. Furthermore, residual plastic film in fields gradually deteriorates, breaking down into smaller plastic remnants, and ultimately transforms into MPs [61]. Since agricultural coverings primarily consist of PE, residual MPs are non-biodegradable and often challenging to extract from the soil, posing potential environmental risks with prolonged use [63]. Therefore, agricultural plastic film emerges as a significant contributor to soil MPs. Additionally, other agricultural products, such as linen, wrapping, containers, and nets, also have the potential to disperse in the environment [64]. Exposure to sunlight and high temperatures may cause fragmentation, making it difficult to separate and treat from the soil. Consequently, these MPs may persist in the soil and could be transported to deeper layers through processes such as bioturbation by soil organisms, leaching, or water infiltration.

Soil amendments (such as nutrient-rich municipal compost and sewage sludge) are widely applied in agricultural production due to their ability to improve soil structure and enhance nutrient content [65,66,67,68]. However, the hidden risks of microplastic contamination cannot be overlooked. Compost, a widely used agricultural fertilizer, has been shown to significantly increase the abundance of MPs in soil, particularly small-sized MPs (<1 mm) [69]. Colombini et al. [70] demonstrated that 21 years of continuous compost application led to the significant accumulation of coarse MPs (2–5 mm in size) in soil. Moreover, MPs derived from compost exhibit a higher propensity to adsorb pollutants and facilitate their transport, thereby exacerbating environmental and ecological risks [69]. Yet, removing the majority of MPs remains a persistent challenge in nearly all composting techniques and sludge treatment facilities [68]. Based on the estimated application rates, compost alone could introduce approximately 0.016 to 1.2 kg per hectare per year (annual application rate: 7 tons per hectare) and 0.08 to 6.3 kg per hectare per year of plastic input (annual application rate: 35 tons per hectare) [62]. Despite partial plastic removal achieved through screening before and after composting, significant amounts of microplastic residues remain in the soil [62]. Weithmann et al. [2] conducted a study on soil treated with different substrates in 2018, revealing varying degrees of microplastic pollution, with particle abundances exceeding 895 particles per kilogram for particles larger than 1 mm in diameter. On the other hand, it can be concluded from the efficiency of wastewater treatment plants that up to 90% of MPs are removed from water and retained in sludge [71,72,73]. Lares et al. [74] found that fiber content in Finnish sludge samples was possibly as high as 80%, with microplastic concentrations detected in sludge ranging from 1500 to 24,000 kg [73,75]. As a result, using sludge as fertilizer leads to microplastic accumulation in the soil. Related data indicate that, in North America, approximately 44,000 to 300,000 tons [76], and in Australia, the total amount of MPs generated annually from sludge application can reach 2.8 × 10³ to 1.9 × 10⁴ tons [5]. Moreover, microfibers retain their original characteristics for up to 15 years after sludge application [68], and the accumulation of fibers in sludge is likely a risk factor, as ultrafine plastic fibers could spread via wastewater, polluting not only the land but water sources as well, thus affecting various organisms [77]. Further research suggests that owing to excessive use of soil amendments, the microplastic burden in terrestrial ecosystems may have surpassed the total amount of MPs in the global marine ecosystem [61].

MPs can also originate from the extensive accumulation of plastic waste [62,78]. Landfills are one of the most widely used methods for solid waste disposal globally [79]. Plastics in landfills undergo physical compression, chemical oxidation, and biological degradation, breaking down into MPs. As a result, landfill leachate contains significant amounts of MPs, which can further migrate into surrounding environments, posing ecological risks. For example, soil near the Coimbra landfill in Portugal detected microplastic abundances of up to 150,000 items/kg [79]. Similarly, the leachate lagoon zone in Hamedan landfill, Iran, exhibits the highest abundance of MPs (76,513 particles/kg dry soil), with older landfills exceeding active ones and MPs mainly ranging from 0.45 μm–25 μm (59.32%) to 25 μm–5 mm (40.68%) [80]. During the rainy season, MPs near landfills are transported by water flow into deeper soil layers, potentially reaching the groundwater. Landfill proximity further influences MPs distribution, with closer landfills leading to more pronounced MP characteristics in aquatic systems [79,81]. This highlights the critical role of landfill location in the dynamics of microplastic pollution. In addition, a significant portion of MPs originates from e-waste. The annual MP load of electronic waste (e-waste) recycling plants is estimated to be 4.01 tons [82]. As a result of the improper handling of e-waste, plastic parts fragment and pyrolyze, releasing a large number of microplastic particles [83]. Although e-waste recycling mainly focuses on metals and rare earth elements, their contribution to microplastic contamination is often underestimated [84]. Zhan et al. [82] found that metals and polymers in e-waste may release pollutants, such as heavy metals and polybrominated diphenyl ethers (PBDEs), during the disassembly and fragmentation process. These untreated MPs are dumped haphazardly into the soil, leading to soil degradation [85]. Once plastics accumulate in the soil, they become part of a complex mixture of organic and mineral substitutes. Through interactions between organic minerals, soil organic matter may become highly stable and can persist for hundreds of years [62].

MPs not only reduce soil microbial biomass, microbial activity, and functional diversity, but they affect the cycling of plant nutrients in the soil [1,16]. Additionally, MPs can co-induce phytotoxicity mechanisms with other pollutants. For example, MPs may induce toxicity to plants by promoting the accumulation of polycyclic aromatic hydrocarbons (PAHs) and dibutyl phthalate (DBP) [86]. The co-contamination of MPs (such as PS or PE) with cadmium (Cd) can exacerbate the growth inhibition and toxicity effects in crops like sorghum [87]. This toxicity manifests as damage to photosynthesis and antioxidant defense systems, leading to reduced chlorophyll content and Rubisco activity, mitochondrial disruption, and impaired water and nutrient uptake [86,88]. Due to the small size of MPs, small soil invertebrates, such as earthworms, can ingest and accumulate them in their bodies, which not only disrupts their intestinal functions but transfers MPs from the soil surface to deeper layers [57,89,90]. Furthermore, alterations in mitochondrial functions and the activation of compensation mechanisms in earthworms vary depending on the type of MPs they are exposed to [91].

2.3. MPs in Atmosphere

Fibers from textiles, automobile tires, and their by-products are the primary sources of MPs in the atmosphere. Research indicates that 34.8% of microfibers originate from washing synthetic fibers, while 28.3% result from tire abrasion [18]. Studies conducted in the Greater Paris area have identified fibers as the predominant type of atmospheric MPs [92]. Annually, 3 to 10 tons of ultrafine fibers are deposited through atmospheric deposition, with 29% comprising petrochemical fibers [93]. It is estimated that approximately 1900 fibers from clothing are released into the environment during washing processes [53,94], with clothing fibers generating ultrafine particles through photodegradation. These lightweight MPs can then become airborne [94,95]. There is concern that they can potentially enter the human body via respiration, thereby triggering adverse health effects and illnesses. Consequently, textiles are recognized as significant contributors to airborne MPs [96,97].

Additionally, automobile tires are also one of the sources of MPs. In addition, it is also a significant source of MPs in soil and aquatic environments, especially in urban areas [98]. They are complex polymers, which are composed of various synthetic compounds and natural rubber [99,100]. During tire operation, substantial amounts of nanoscale to micrometer-sized debris are generated [39], including fine dust particles, such as PM0.1 (0.001–0.1 μm), PM2.5 (0.1–2.5 μm), and PM10 (2.5–10 μm). Due to their lightweight nature, some microplastic particles are suspended directly in the air, forming what is commonly known as “urban dust” [101]. When these particles accumulate on road surfaces, they are either transported by wind or flow into the stormwater drainage system, eventually depositing into surface water, groundwater, and the soil [98]. Annual tire dust emissions in Sweden are estimated to be approximately 10,000 tons, while in Germany, they can reach as high as 110,000 tons [62]. Global evaluations based on scientific reports and commercial data show that per capita microplastic generation from tires varies, ranging from 0.23 kg·y⁻¹ (India) to 1.9 kg·y⁻² (Japan), and even as high as 4.9 kg·y⁻³ in the United States [102]. In 2018, tire abrasion on Swiss roads released approximately 10,600 ± 3800 tons of rubber, while artificial turf released 357 ± 30 tons of rubber particles [39]. Therefore, particles from tires and various rubber products remain prominent sources of airborne MPs and warrant attention [103]. Other notable sources of microfibers include curtains, carpets, old indoor coatings, and mattresses, which can directly release microfibers into the air, contributing to microfiber pollution [104,105]. Due to the lightweight nature of MPs, air currents and wind can facilitate particle transport, leading to dispersion from their original sources [106]. Sandstorms, in particular, facilitate both long-distance and short-term MP transport, carrying large quantities of plastic particles from degraded lands to sensitive areas, such as densely populated regions, thereby causing transboundary pollution [107].

Airborne MPs can enter various organisms through pathways such as sedimentation, inhalation, or food chain transfer. Moreover, the number of MPs and nanoplastics entering the body through respiration may exceed those ingested via food [92,108,109]. It is estimated that an adult inhales 1.9 × 10³–1.0 × 10⁵ microplastic particles annually from indoor air and 0–3.0 × 10⁷ particles from outdoor air [109]. A report from the August 2020 American Chemical Society (ACS) meeting highlighted the detection of MPs in human lung, liver, spleen, and kidney tissues [110]. Inhalation of microfibers can cause damage to the respiratory system and, although the lungs are exposed to several to dozens of microplastic particles daily, the respiratory tract cannot fully absorb or clear them, potentially allowing MPs to enter the digestive or circulatory systems [111,112]. Recent studies have even detected MPs in the placenta, with a 100% detection rate and concentrations ranging from 6.5 to 790 μg per gram of tissue [113,114]. Professor Matthew Campen further suggested that rising microplastic concentrations in human tissues could be linked to increased rates of inflammatory bowel disease, colon cancer, and declining sperm counts in individuals under 50 years old [115]. These MPs could induce respiratory system injury, and people’s lungs would be exposed to several or even dozens of MPs every day, but people’s respiratory tract could not completely absorb or remove these MPs, so as to further promote the development of MPs to enter the digestive system or circulatory system.

3. Separation Methods

MPs exhibit high mobility and pose potential ecological risks, making the development of efficient and environmentally friendly separation techniques essential. Currently, MP separation methods are primarily categorized into physical, chemical, and biological approaches, each leveraging different physicochemical properties of MPs. However, these methods vary in separation efficiency, environmental adaptability, and practical feasibility. Therefore, the rational selection and optimization of separation techniques are crucial for mitigating MP pollution. This section examines the mechanisms of different separation techniques and their applicability.

3.1. Physical Methods

Density separation is a widely used technique for isolating MPs from environmental samples, particularly in sediments and soils. It is characterized by simplicity, significant efficacy, low cost, and a short separation duration [116,117]. The density of plastic particles varies depending on the polymer type, typically ranging from 0.8 to 1.4 g/cm³. For example, common plastics, like PE, have densities of 0.85–0.94 g/cm³, while PS ranges from 0.96 to 1.04 g/cm³. However, these values are based on raw resin and may not account for additives used during manufacturing. By contrast, sand and sediment have a much higher density of approximately 2.65 g/cm³, making it possible to separate MPs from these heavier materials using density-based methods, as illustrated in Figure 3a. The process involves mixing the sample with a saturated solution and oscillating it for a specified duration. During this process, lighter plastic particles remain suspended or float on the surface, while heavier sediment particles settle to the bottom. The supernatant, containing the plastic particles, is then extracted for further analysis [118]. Saturated sodium chloride (NaCl) solution, with a density of 1.2 kg/L, is commonly used for density separation due to its affordability and non-toxic properties [33]. However, its relatively low density limits its effectiveness for extracting high-density polymers like polyvinylchloride (PVC) and PET. Additionally, soil organic matter (SOM), which typically has a density of 1.0–1.4 g/cm³, cannot be effectively separated from MPs using NaCl [62]. To address these limitations, higher-density solutions, such as saturated zinc chloride (ZnCl₂, 1.4–1.6 g/cm³) and sodium iodide (NaI, 1.6–1.8 g/cm³), are often employed [119]. These solutions significantly improve the extraction efficiency of high-density MPs and are widely used for the separation and flotation of MPs in environmental samples [94].

In density separation processes, pretreatment aims to remove the interfering substances while minimizing damage to the target materials. A common method involves using a 30% H₂O₂ solution to remove natural organic matter (NOM) from the surface of MPs, which is cost-effective and increases extraction efficiency [120,121]. Loöder et al. [122] proposed an enzymatic digestion pretreatment method (BEPP), which utilizes stepwise enzyme digestion (protease at pH 9/50 °C and cellulase at pH 5/50 °C). By optimizing buffer solutions and 10% SDS concentration, this method effectively removes organic matter (e.g., proteins and cellulose) from environmental samples, while preserving the morphological integrity of MPs and enhancing the efficiency of density separation. These pretreatment methods, when combined with optimized density separation solutions, enhance the accuracy and representativeness of microplastic quantification in environmental samples.

Wang et al. [123] developed an efficient method based on optimized density separation and nonionic surfactants (e.g., Tween 20) to purify single microplastic species from mixed systems. By adjusting the density of the flotation solution, PET with a purity of 97 ± 2% was successfully extracted from a PS)/PET mixture. The nonionic surfactant enhanced the separation purity of microplastic to 76–96% and demonstrated high efficiency in simulated environmental water samples. This cost-effective method significantly improves the quality of plastic recycling. However, this separation method has certain limitations when applied to real environmental samples, primarily due to the competitive adsorption of complex matrices (such as minerals and humic acids), which inhibits the efficiency of surfactants. Additionally, their experimental design was limited to investigating mixed systems composed of binary combinations of the four microplastics (PS, PVC, PET, and thermoplastic polyurethane [TPU]). Therefore, further development of pre-treatment processes for environmental MPs is necessary to meet practical separation needs.

Zhang et al. [124] developed a flotation-based method for extracting MPs from soil samples, as illustrated in Figure 3b. The method is mainly based on flotation and ultrasound-assisted separation. Specifically, 10 g of soil was mixed with 50 mL of distilled water in an aluminum cup and manually stirred to form a homogeneous suspension. After rinsing the glass rod with distilled water, the mixture is left overnight to allow soil particles to settle. Floating MPs and surface impurities are then decanted and filtered through 3 μm pore-size filter paper. This step is repeated at least four times to ensure complete extraction. The soil solution is further subjected to ultrasonic vibration for 2 h to enhance microplastic detachment from soil particles, followed by additional filtration. The filter, along with the collected material, is oven-dried at 60 °C to a constant weight for quantification. However, MPs strongly adhere to soil particles, making extraction via flotation challenging [124].

Shen et al. [125] demonstrated that cationic surfactant-modified aluminosilicate filters achieved a microplastic removal efficiency of 96%, significantly outperforming rapid sand filters (60–80%). This improvement stems from three retention mechanisms: capture, entrapment, and entanglement (Figure 4), enhanced by increased hydrophobicity and altered surface charge properties due to surfactant modification. This low-cost, energy-efficient approach shows promise for enhancing tertiary treatment in wastewater treatment plants (WWTPs). The only drawback is that the experiment used clean water solutions, which may not reflect the characteristics of real WWTPs.

Recently, Yu et al. [126] proposed an innovative study utilizing a high-efficiency interfacial solar evaporation platform (ISEP) to remove MPs from water. This platform not only produces freshwater but effectively removes MPs. The researchers developed a carbon felt–polyethyleneimine (CF-PEI) composite by grafting polyethyleneimine (PEI) onto the surface of carbon felt. Due to the abundant amino groups in PEI, which exhibit excellent adsorption capabilities, the CF-PEI composite significantly enhanced the efficiency of microplastic removal from water.

3.2. Chemical Methods

The chemical methods for microplastic separation can be broadly classified into two categories: one involves techniques that separate MPs through chemical action, such as adsorption and coagulation; the other involves processes that degrade MPs into smaller molecules, such as photocatalysis and advanced oxidation. This chapter primarily introduces the applications and limitations of adsorption and photocatalysis methods in the removal of MPs from aqueous environments, such as wastewater and natural water bodies.

Adsorption-based chemical methods for treating MPs offer a simple and environmentally friendly approach, with high adsorption efficiency, low cost, reusability, and no secondary pollution. However, the adsorption capacity diminishes over time, leading to a decline in the microplastic adsorption capability. The adsorption separation process results from the combined effects of physical adsorption (such as van der Waals forces, electrostatic interactions, hydrogen bonding, and hydrophobic interactions) and chemical adsorption [127]. Various factors, including choice of adsorbent, pH level, particle size, temperature, and other variables, affect the separation performance in microplastic separation experiments [128]. Sajid et al. [129] demonstrated that neutral media achieve optimal separation of adsorbents for MPs. Compared to alkaline conditions, a higher adsorption efficiency was observed in acidic environments [130]. However, the availability of effective adsorbents for microplastic removal remains limited. To effectively remove MPs, Sun et al. [128] utilized chitosan and graphene oxide to create a highly compressible sponge, effectively adsorbing different types of MPs within pH 6–8. Misra et al. [131] prepared a magnetic nanoparticle composite material by adsorbing polyacrylic anion ionic liquid (PIL) onto Fe₂O₃/SiO₂ particles. The adhesive coating of PIL on the magnetic nanoparticles enhanced the binding of MPs to pollutants in wastewater. In experiments, PS beads (size = 1 μm; concentration = 1 g/L) were completely removed at an adsorption concentration of 10 g/L, demonstrating the excellent effectiveness of this adsorbent. Additionally, the use of free radical species–functionalized efficient catalysts for deep oxidation of pollutants has attracted widespread attention [132]. Kang et al. [133] prepared magnetic nitrogen-doped carbon nanohelixes for the sulfate oxidation of MPs in cosmetics under hydrothermal conditions. Firstly, carbonized manganese nanoparticles were encapsulated in spiral nitrogen-doped carbon nanotubes via a one-pot pyrolysis process, then functionalized with asymmetrical peroxomonosulfate (POMS). It was reported that, after 8 h of pyrolysis, there was approximately a 54% weight loss of MPs. Reineccius et al. [134] compared different microplastic separation methods and found that adsorption-based separation achieved 98% average microplastic recovery rate and 96.3% matrix removal with the lowest cost and minimal effort. Moreover, this technique offers the advantages of high precision, ease of operation, and low cost. Liu et al. [135] obtained highly crystalline layered double hydroxides (LDHs) via the hydrothermal method, which were then mixed with PDMS, epoxy resin, and a polyurethane sponge to form a PLT@PU sponge, demonstrating a good adsorption of micro/nanoplastics in water. Zhang et al. [136] found that, during the aging process, the carbon chains in PE and PLA MPs break and combine with oxygen, forming oxygen-containing functional groups. As a result of this aging process, the adsorption capacity of PE and PLA MPs for Cd(II) and Cr(VI) significantly increased. Specifically, the adsorption capacity of Cd(II) on PE and PLA MPs increased by 40.61% and 25.49%, respectively, while the adsorption capacity of Cr(VI) increased by 37.50% and 69.29%, respectively.

Photocatalysis, as shown in Figure 5, is a process that accelerates reactions via active sites generated on the catalyst surface by light energy, without the need for direct contact with the catalyst [137]. Photocatalytic degradation involves redox processes, such as superoxide radicals (O₂·) and hydroxyl radicals (·OH), which react with organic polymers, leading to their decomposition, polymer chain breakage, and potential mineralization [138,139]. While photocatalytic degradation offers the advantage of not requiring additional chemical substances besides stable photocatalysts, its widespread application is hindered by limitations such as inadequate selectivity for MPs, difficulty in catalyst regeneration, and varying degradation efficiency [140]. In another study, organic solvents were used to mix polymers with TiO₂ to form composite films, and TiO₂ was embedded into PE, PS, and PP plastics for photocatalytic treatment [141,142]. Tofa et al. [143] employed ZnO nanorods and ZnO nanorods decorated with Pt nanoparticles for photocatalytic degradation of low-density polyethylene (LDPE) in water. After photocatalysis, surface wrinkling, cracking, and cavities were observed on the polymer surface due to the formation of oxygen-containing groups and volatile organic compounds, validating the chemical conversion of the polymer. Chen et al. [144] induced aging and degradation of MPs using electron beam technology, resulting in carbon chain breakage due to hydroxyl radicals generated by electron beam radiation. This process gradually degraded the MPs into small molecule esters and alcohols. It was found that the oxygen-to-carbon ratio of aged MPs reached 0.071, with a mass loss of 48%, and a carbonyl index value of 0.69. Chokejaroenrat et al. [145] deposited CuO onto BiVO₄ nanocomposites and found improved photocatalytic activity, increasing the generation of ROS from e-h pairs. After incineration with H₂O, ROS generation further escalated, causing significant surface wear and an elevation in carbon-based and vinyl indices in MPs. Jiang et al. [146] found the outstanding degradation performance of MPs using a novel quaternary layered double hydroxide composite photocatalyst, CuMgAlTi-R400. Following 300 h of illumination, an average particle size reduction of 54.2% was achieved, with higher degradation efficiency correlating with smaller particle sizes. Xue et al. [147] studied a visible light-driven catalytic and photothermal Fenton-like reaction method, revealing that, without external H₂O₂ dosage, the degradation rate of 310 nm PS spheres approached 100% after 4 h at 75 °C under visible light irradiation.

3.3. Biological Methods

Biological methods for microplastic separation primarily rely on natural biological processes to remove or transform MPs. This section focuses on the mechanisms and limitations of microorganism-mediated (both in vivo and in vitro) and plant-based approaches for microplastic removal. Biodegradation refers to the microbial-mediated breakdown of polymers, offering a cost-effective and environmentally friendly approach to address plastic waste issues [148]. The biodegradation of MPs occurs in four stages: formation of a microbial biofilm, biodegradation, biofragmentation, and mineralization [149]. Microorganisms, such as bacteria and fungi, play key roles in this process, with hydrolytic enzymes, such as lipases, esterases, and keratinases, catalyzing the degradation of polymers [150]. Microorganisms initially adhere to the surface of MPs, forming a biofilm and initiating biodegradation [151]. Subsequently, microorganisms degrade the MP polymer chains by secreting hydrolytic enzymes (e.g., esterases and lipases) [152,153,154]. The degradation products are further broken down into micron-sized or nano-sized particles via biofragmentation, with a portion of these fragments internalized by the microorganisms and ultimately metabolized into CO₂, H₂O, and biomass [150]. Microorganisms, such as Bacillus and Rhodococcus, have the capability to break down organic polymers into simpler components, including water, carbon dioxide, and methane [140,152,155,156]. However, this method is highly susceptible to environmental conditions due to microbial adaptation variances, leading to microbial death or reduced degradation efficiency with changes in temperature, sunlight, and humidity, among others [157]. Since extracellular enzymes are only accessible on the surface of MPs, the biodegradation process proceeds slowly, often taking days to weeks [158]. Nevertheless, microorganisms still contribute to reducing the molecular weight of polymers and inducing physical and chemical changes [159]. Additionally, microbial community competition may hinder microplastic removal efficiency, thus integrating biodegradation with complementary techniques during pretreatment could enhance overall degradation effectiveness [148].

Paço, et al. [160] found that marine fungi can degrade PE particles, confirming biodegradation by reducing the size and mass of microplastic particles. Over 14 days, the microplastic content slowly decreased, ultimately dropping by 56.7% [156]. Park and Kim [159] studied the degradation of PE by Penicillium-mixed bacterial cultures and reduced the polymer’s molecular weight by 14.7% after 60 days of cultivation. Zhang et al. [161] investigated microplastic biodegradation by Aspergillus isolated from wax moths, observing a 3.9% polymer weight loss after 28 days of cultivation. Tiwari et al. [162] found that Brevibacillus brevis can effectively degrade PE MPs, with a 19.8% decrease in microplastic weight observed after 35 days of treatment. Jeyavani et al. [163] discovered that certain marine bacterial biota can degrade MPs, leading to a gradual weight loss of PP MPs over 28 days of biodegradation, accompanied by the appearance of cracks and voids on the surface. Zhang et al. [164] examined the degradation of polylactic acid (PLA), polyhydroxyalkanoates (PHAs), and LDPE at 4, 15, and 22 °C. They found that PHA films exhibited more significant degradation at each temperature, and the toxicity of the degradation products was highest at 22 °C. Due to the wide variety of methods used to quantify biodegradation, studies on plastic biodegradation lack a common baseline for comparison. Additionally, the enzymatic biodegradation of MPs will become a research hotspot in the future [149].

Currently, scientists are screening environmental microorganisms (in vitro) and insect gut microorganisms (in vivo) to evaluate their ability to degrade plastics. Utilizing the natural capabilities of insects, such as Tenebrio molitor, Zophobas morio, silkworms, and waxworms, to decompose and recycle plastics can significantly reduce the long-term environmental impact of plastic waste [165]. Insect digestive enzymes and gut microorganisms play a crucial role in this process. Therefore, insects and their microbial communities are considered a promising and sustainable solution to the plastic waste problem.

In addition to biodegradation, plant-based strategies are also showing potential in mitigating plastic pollution. For example, Long et al. [166] reported that plant species and density affect MPs removal efficiency in rural wastewater treatment plants. Recent studies have shown that bioretention systems can effectively remove MPs, with aquatic plants playing a key role. For example, the combined action of common reed roots and substrates significantly influences MPs retention through physical filtration, substrate ratio, and root biofilm interactions [167]. However, the accumulation of MPs on roots or root hairs can impact plant health, highlighting the complexity of the retention process.

3.4. Exploration of Hydrocyclone Separator for MPs Separation

The hydrocyclone, based on the principle of centrifugal sedimentation, utilizes the tangential velocity gradient of the rotating flow to overcome gravity, triggering rotational motion in particles. This causes particles to migrate toward the outer wall vortex region for effective separation. This technique is suitable for the separation of liquids and suspensions.

The discrete phase model (DPM) is employed to simulate particle motion within the hydrocyclone, treating particles as discrete entities and tracking their trajectories to evaluate separation efficiency. The data presented in Figure 6 were obtained from computational fluid dynamics (CFD) simulations using the discrete phase model (DPM) in ANSYS Fluent 2022 R1, performed for a DN18 (which indicates that the internal diameter at the interface between the conical and cylindrical sections of the hydrocyclone is 18 mm) hydrocyclone. The particles have a size of 10 μm and a density of 1.5 g/cm³, with a fluid flow velocity of 5.67 m/s, and were injected perpendicular to the inlet surface. Figure 6b shows the pressure distribution, with the longitudinal cross-sectional pressure increasing progressively from the center toward the outer wall. Figure 6c illustrates the velocity distribution, showing a distinct pattern where flow velocity increases from the center, peaking at about one-third of the hydrocyclone’s radius. Figure 6a reveals a pressure drop of 0.82 MPa from the inlet to the overflow outlet. In the inner vortex region, flow velocity increases linearly from the core to the periphery, resembling a “quasi-forced vortex” and then decays nonlinearly in the outer vortex, exhibiting “quasi-free vortex” behavior. Near the hydrocyclone wall, the flow velocity sharply drops to 1 m/s. These results highlight the pronounced internal pressure field, which enhances separation efficiency, reduces energy consumption, and optimizes equipment size. Under specific conditions, the hydrocyclone achieves a separation efficiency of 93%, largely due to the minimal density difference between microplastic particles and water. It is noteworthy that while increasing velocity contributes to higher separation efficiency, the corresponding increase in energy consumption requires careful consideration. Therefore, a comprehensive evaluation of energy consumption and system operating conditions is essential.

There have been studies on the separation of MPs used in hydrocyclones. For example, Fu et al. [168] investigated the separation performance of a light-medium separation technique using a hydrocyclone, leveraging the density difference between high-density polyethylene (HDPE) and PP through a combination of experimental studies and CFD simulations. Similarly, Bu et al. [169] investigated the separation efficiency of MPs and flow dynamics in a 10 mm mini-hydrocyclone through experiments and CFD simulations. Inoue et al. [170] developed a hydrocyclone-based technique capable of efficiently extracting low-density MPs (<1.14 g cm⁻³) from 10 kg of fluidized sediment within 30 s. Thiemsakul et al. [171] optimized hydrocyclone geometry via CFD and ANOVA, proposing a design with an underflow-to-cylinder diameter ratio of 0.264, an overflow-to-cylinder diameter ratio of 0.3, a cone-to-cylinder height ratio of 13.22:70.78, and a tangential inlet, achieving 76% microplastic recovery efficiency.

The hydrocyclone offers efficient separation capabilities, capable of processing large volumes of liquid or suspension. Its simple structure and low cost make it an attractive option [169]. Future development of hydrocyclones for microplastic separation will focus on enhancing the separation efficiency for fine particles, and those with minimal density differences, while reducing energy consumption [172]. This can be achieved by optimizing the hydrocyclone structure, adjusting operational parameters, such as flow rate and pressure, and integrating intelligent control techniques to balance high efficiency and energy savings. Furthermore, the incorporation of advanced materials and multi-stage separation processes is expected to further improve performance, meeting the practical demands of MPs separation.

4. Conclusions

This review examines the generation, transport pathways, and hazards of MPs, highlighting the urgency of increasing public awareness regarding microplastic pollution. It critically evaluates the widely used microplastic separation techniques. The current microplastic separation methods can be broadly classified into physical, chemical, and biological techniques, each with its limitations. Physical methods, such as density separation and screening, leverage the differences in size, density, and shape between MPs and other materials, offering simplicity, cost-effectiveness, and broad applicability. However, these methods are less effective in separating small and low-density MPs. Chemical methods, including adsorption and photocatalysis, are relatively simple and cost-effective but require optimization of adsorbents or catalysts based on the characteristics of the MPs. Biological methods, such as biodegradation, offer high selectivity and specificity, and compared to chemical methods, do not rely on chemical reagents, making them more environmentally friendly and avoiding secondary pollution. However, challenges remain in controlling operational conditions and ensuring the stability of these methods. To overcome these limitations, future microplastic separation techniques should focus on enhancing efficiency, adaptability to various microplastic forms, and environmental conditions, while maintaining eco-friendliness. The hydrocyclone separation technique, with its high efficiency, low energy consumption, and environmentally friendly features, shows significant potential. As technological innovations continue, the efficiency and accuracy of microplastic separation are expected to improve, providing more effective solutions to the increasingly severe microplastic pollution problem.