Occurrence and Control of Microplastics and Emerging Technological Solutions for Their Removal in Freshwaters: A Comprehensive Review

Abstract

Plastic remains a cheap material for numerous uses in households, industries, and engineering; however, it disintegrates in aquatic ecosystems to form smaller particles termed microplastics. Microplastics (MPs) have become a cause for concern due to their persistence and potential effects on freshwater ecosystems. Moreover, the toxicity of microplastics can be achieved through different mechanisms, including physical blockage and additive leaching, or they can function as vectors for other chemical pollutants. Microplastics were found to provide a growing surface for microbial communities, forming a biofilm termed the plastisphere. Microplastic pollution seems to need urgent attention globally; however, the comparability of results becomes a challenge due to the different techniques employed by different researchers. Moreover, the complete removal of MPs has proven to be an impossible task. This review explored MP occurrence in freshwater ecosystems, the role of microbial communities in the dynamics of microplastics, removal techniques, strategies for reduction in the environment, and their effect on freshwater ecosystems. Moreover, techniques to reduce microplastic release, such as recycling, plastic–fuel conversion, and biodegradable plastics, are explored. The review provides recommendations for reducing microplastic release and removal in freshwater ecosystems. This review stresses existing gaps to explore going forward in addressing microplastic pollution and possible removal techniques.

1. Introduction

Our current lifestyles revolve around convenience, and plastics have become a major commodity in our society. Plastics are considered affordable, extremely durable, and lightweight compared to other materials [1]. Due to the increasing and widespread use of plastic materials, microplastic pollution has emerged as a significant environmental concern. It is estimated that around 5 to 13 million metric tonnes of plastic debris are introduced into aquatic environments annually [2]. These plastic debris disintegrate through mechanical degradation, which can be enhanced by chemical and biological weathering to form microplastics (MPs) [3]. Other anthropogenic activities driving microplastics’ enrichment in freshwater environments include wastewater effluents, urban and agricultural runoff, manufacturing industries, and consumer products [4,5,6]. Microplastics are fragments smaller than 5 mm that are pervasive across freshwater environments. Their overwhelming numbers in aquatic ecosystems range from 15 trillion to as many as 51 trillion particles, with a combined mass estimated to be between 93,000 and 236,000 metric tonnes [7].

Due to their small size, large surface area-to-volume ratio, and hydrophobic nature, microplastics can adsorb a wide range of chemical pollutants (e.g., heavy metals, pesticides, and persistent organic pollutants) from the surrounding environment [8]. Therefore, microplastic toxicity can be achieved through the physical blockage of permeable membranes, leaching of additives, and conveyance of other chemical contaminants [9,10]. Moreover, chemicals linked to microplastics can disrupt the endocrine system, potentially affecting human reproduction and development [11]. Water acts as a carrier for microplastics, allowing them to spread along the longitudinal gradient of rivers. Moreover, microplastics accumulate in organisms, disrupt enzymatic activities in the gastrointestinal tract and other endocrine systems, and threaten the overall ecological integrity of the system [12].

Moreover, microplastics provide a surface for microbial growth, creating a new ecological niche called the plastisphere. A plastisphere is formed when microbial cells adhere to a surface, forming a community embedded in an extracellular matrix (EPS) [13,14]. The plastisphere facilitates the co-transport of pollutants and microorganisms, enhancing the risk of spatial and trophic transfer, bioaccumulation, and microbial community disruption in aquatic food webs [15]. Moreover, the plastisphere gives rise to serious concerns by serving as a vector, transporting pathogenic bacteria, viruses, invasive species, and antibiotic resistance genes across ecosystems [16]. Numerous studies have reported high bacterial diversity and antibiotic-resistant genes on microplastics in rivers receiving domestic sewage [17,18,19]. Galafassi et al. [20] emphasised that microplastics contribute to the spread of pathogenic bacteria in treated wastewaters. Moreover, Chen et al. [21] found that microplastics shape the microbial communities in a lentic water body. This dual role as both pollutant carriers and microbial incubators makes microplastics an urgent ecological and public health concern.

Given these growing challenges, there is an urgent need to understand the dynamics of microplastics and their associated ecological risks, as well as the innovative technologies to help reduce microplastic abundance in freshwaters. This review takes a closer look at microplastics’ behaviour and their association with biota, as well as highlights cutting-edge technologies for monitoring, controlling, and reducing their influx into freshwater environments. More relevant research gaps are also explicitly defined.

2. Distribution of Microplastics in Freshwater

Microplastics have recently become a hot topic due to their ubiquitous nature. However, extensive studies have been conducted in marine environments [22,23], with freshwaters having only received attention within the past few decades. Freshwater ecosystems are the first recipients of microplastic-forming plastic garbage from industries, urbanisation, agricultural activities, and wastewater effluents (Figure 1) before marine environments. Microplastics occurring on the Earth’s surface are washed into freshwater ecosystems through runoff, with urban catchments being among the significant contributors [24,25]. Microplastics are persistent in aquatic environments, and they were found to accumulate in different environmental matrices and the food web [4,5,6]. Upon reaching freshwater ecosystems, microplastics will either stay suspended in the surface water, sink to the bottom sediment, or be taken up by organisms depending on their size, structure, and density. Studies have reported microplastics in surface waters, sediment, and biota [26,27].

Numerous freshwater ecosystems receiving water due to different types of land use have been explored to determine the potential occurrence of microplastics in the water, sediment, and biota. A high abundance of microplastics was reported in the surface waters in urban rivers [24,25], in caves [28,29], and protected and remote rivers [27,30]. Moreover, Townsend et al. [31] reported microplastics in the sediment in urban wetlands. In addition, Ramaremisa et al. [32] reported high microplastic abundance in the surface water and sediment in the Vaal River, whereas Graham et al. [33] reported microplastics in the surface water, sediment, and fish species in the Orange-Senqu River system. Microplastics were also found in dams, with a higher abundance at the shoreline [34,35]. Moreover, Pojar et al. [36] reported microplastic abundance in the surface water and sediment in a dam in the Eastern Carpathians, Romania. Microplastics were also reported in freshwater macroinvertebrates such as dragonfly nymphs [37], shrimp [38] and crabs [39], amphibians [40], and large fish [41,42]. However, their potential to biomagnify up the food webs and influence higher trophic levels remains poorly understood (Figure 2). There is a pressing need for detailed ecological studies that explore trophic transfer, bioaccumulation, and the biomagnification of microplastics, particularly in freshwater vertebrates such as fish and amphibians. The implications of such a transfer on ecosystem stability are substantial and warrant urgent, holistic investigation.

3. Techniques Used for Analysing Microplastics in Environmental Samples

3.1. Instruments Used for Microplastic Analysis and Polymer Identification

Different techniques have been employed to analyse microplastics in environmental samples. Stereomicroscopes are commonly used for morphological identification (size and shape) and counting, whereas scanning electron microscopy (SEM) is used to further evaluate the surface morphological features (texture) at the micrometre level to produce high-resolution images [31,43]. Zhao et al. [44] employed a microfluidic–microwave system (MMS) and showed that the limit of detection is much higher than the typical levels observed in most freshwater systems. In contrast, Wen et al. [45] reported the efficiency of MMS in detecting low concentrations of microplastics. Similarly, Barrancos et al. [46] reported a low sensitivity for MMS with 0.1 dB for microplastics as small as 600 µm.

Nevertheless, other sensitive techniques such as Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and laser direct infrared imaging (LDIR) are used [32,33]. The sensitivity of FTIR was observed in environmental samples with low microplastic concentrations, such as in caves [28] and groundwater [47]. Raman spectroscopy has been extensively used in microplastic analyses of surface waters, sediment, and biota [42,48,49]; however, it has its shortcomings, which include interference from pigments and additives [50]. Laser direct infrared imaging has shown to be reliable for different types of samples. For instance, Liu et al. [51] and Sun et al. [52] used LDIR to detect microplastics in human placenta, whereas Nizamali et al. [53] used it in bottled water. Despite the reliability of FTIR, Raman spectroscopy, and LDIR in microplastic analysis, these instruments are expensive, and not all researchers have access to them. There is a need for the development of cost-effective techniques if we are to easily document microplastic occurrence in the environment across the globe. Moreover, to enhance the comparability of results, the techniques used should be the same, which is unlikely to happen if there are no affordable instruments. Microplastic studies currently lack harmonised methodologies for sampling, analysis, and quantification, making it difficult to compare data across time, space, and research groups [28,47,54]. Nevertheless, efforts have been made to standardise a methodology to enhance the comparability of results [55,56], although polymer characterisation instruments are still not affordable for researchers in economically disadvantaged countries. Establishing global or regional guidelines, similar to those for chemical water quality testing, would support data reproducibility and cross-border policy development, particularly for transboundary river systems.

3.2. Machine Learning (ML) and Internet of Things (IoT)

Despite the collection of real-time data, existing data may be used to model microplastic pollution in freshwater ecosystems. The ML would need real-time data collected by technologies such as FTIR, Raman spectroscopy, and LDIR [57]. However, the integration of ML and the IoT sensor networks can enhance real-time data collection over a large area. The IoT sensor uses technologies such as electrochemical and optical methods to detect and quantify microplastics in aquatic environments [58,59]. This IoT technique is known to produce comprehensive data that can yield predictive results for understanding the behaviour of microplastics in aquatic environments [60]. Tran et al. [57] reported the reliability of machine learning for the prediction of microplastics in peatlands. Moreover, a machine learning technique was successfully used to predict the transportation of microplastics between different sites and environmental matrices in an open channel with hydrologic dynamics [61]. Sajan et al. [62] successfully employed artificial neural networks and hidden Markov models (ANN-HMM) for predicting the trend of microplastic pollution in a freshwater ecosystem. Despite the potential of machine learning in addressing microplastic pollution, the technique may not be user-friendly for beginners, and special skills are required before its employment.

3.3. Geographic Information Systems and Remote Sensing

Geographic information systems (GISs) and remote sensing have been used for mapping, visualising, and exploring the spatial distribution of microplastic hotspots and potential sources. Sharma et al. [63] successfully employed fuzzy logic-based GIS tools to map potential sources of microplastics. Moreover, Tajwar et al. [64] mapped the spatial distribution of microplastics using a GIS, whereas El-Alfy et al. [65] employed a GIS to develop a predictive model of the vulnerability of freshwater ecosystems to non-point sources of microplastics. Rauf et al. [66] emphasised that the integration of GISs and remote sensing technologies can provide a comprehensive solution to addressing microplastic pollution, particularly when it comes to the exploration of spatial distribution. Nevertheless, the integration of this technology is still limited in microplastic pollution studies.

4. Toxicity Mechanisms and Effects

During plastic manufacturing processes, plasticiser chemicals are utilised to enhance their flexibility and strength [67]. Commonly used plasticiser chemicals include bisphenol A (BPA), heavy metals, phthalates, and flame retardants [68,69]. These plasticisers eventually leach from microplastics to surface waters and could become detrimental to aquatic life in high concentrations [70]. The leaching rate may be increased by the enhanced solubility of the aqueous boundary layer, which lowers the mass transfer resistance, whereas biofilm may reduce the rate of additive leaching from microplastics [71,72]. Dissolved plasticisers may become biologically available and be taken up by aquatic biota, eventually causing toxic effects [73]. On the other hand, microplastic particles were found to block the secretion of digestive enzymes and hormones in Salmo truta [74], whereas Zink and Wood [75] emphasised that microplastics may result in respiratory blockages in fish gills. Despite plasticiser leaching and blockages, microplastic particles may act as vectors for chemical contaminants and enhance bioaccumulation, hence, toxicity [8,9]. This may result in a complex combined effect of the adsorbed chemicals and microplastic particles. This complex interactive effect could exacerbate physiological stress in aquatic organisms, yet mechanistic toxicology studies remain limited in scope. More integrative ecotoxicological assays are needed to assess these toxicity mechanisms under environmentally relevant conditions.

The effect of microplastics on toxicity seems to occur through both direct and indirect routes. Nevertheless, the endpoints are usually the same. Mbugani et al. [76] reported histo-morphological alterations in the small intestine of Oreochromis urolepis exposed to PE-MPs, whereas Ruthsatz et al. [77] reported effects on the growth, development, and metabolism of the African clawed frog (Xenopus laevis). Additionally, Gupta et al. [78] reported a disruption of hormonal homeostasis in female zebrafish exposed to polystyrene microplastics (PS-MPs). In another study, exposure to polyethylene MPs ruined the reproductive organs of adult loach (Paramisgurnus dabryanus), triggered germ cell apoptosis, and reduced the quality of gametes. Moreover, exposure to microplastics has resulted in physical damage to tissues, oxidative stress, and reproductive and developmental issues in aquatic biota [79,80]. The effects of microplastics in freshwater ecosystems have also been observed at the community level, with Wang et al. [81] reporting a shift in macrobenthos community in association with microplastic abundance. Moreover, change in macrobenthos community structure was observed in an artificial river after exposure to PE-MPs [82].

In aquatic plants, Ceschin et al. [83] reported reduced growth and chlorophyll content, as well as elongated roots due to microplastic exposure. Physical growth, physical damage, and reduced leaf growth were observed on Salvinia cucullata after exposure to microplastics [84]. Exposure to microplastics resulted in the inhibition of photosynthesis in freshwater algae [85]. It is evident that the exposure to microplastics by freshwater organisms has been associated with a wide range of effects, including biochemical, physiological, and ecological disturbances. However, the underlying mechanisms remain underexplored, and this poses serious threats and uncertainties regarding global environmental health issues.

5. Microplastic Interaction with Microbial Communities

There is a complex interaction between microplastics and microbial communities, which results in a broader ecological impact. Due to their extensive surface areas, microplastics constitute an ideal environment for the colonization of diverse microbial communities in freshwater systems, forming a biofilm termed the plastisphere [14]. The plastisphere provides physical support, nutrient access, and a shield against harsh environments [13]. It produces a distinctive ecological niche on plastic debris, involving complex interactions between microorganisms and their environment. Microbial communities colonising the plastisphere comprise bacteria, archaea, fungi, protozoa, and algae [15,16]. The formation of the plastisphere commences with microbial adhesion to plastic surfaces, which is mediated by biological fouling [86]. The pioneer microbial species promptly adhere to plastic surfaces using surface appendages such as flagella, fimbriae, and pili [87]. The microbes in the plastisphere participate in the adsorption, degradation, and transformation of environmental pollutants. The community of microorganisms within the plastisphere secretes extracellular polymeric substances (EPSs) that aid in stable attachment, surface conditioning, and biofilm development [13].

Over time, microorganisms begin to firmly anchor to the surface via stronger interactions, including specific ligand–receptor binding and the production of adhesion molecules like fimbriae and extracellular polymeric substances (EPSs) [88]. Notable phenomena such as cell-to-cell signalling enhance quorum sensing, which in turn regulates biofilm thickness, structural integrity, and nutrient sharing [89,90]. This is an irreversible stage in which the synthesis of secondary metabolites such as bacteriocins and antimicrobial peptides helps maintain microbial dominance and excludes competitors [91,92]. Microcolonies are formed within the EPS while the biofilm matures. The growth will lead to thicker EPSs, which become complex and chemically diverse, containing polysaccharides, proteins, lipids, and extracellular DNA (eDNA). In this mature stage, increased resistance to antimicrobials, environmental protection, and host immune responses (in pathogenic contexts) is developed [93]. The final stage involves some cells exiting the biofilm and reverting to a planktonic state, enabling the colonization of new surfaces. Dispersion is a survival strategy, allowing microbes to escape unfavourable conditions and spread to new niches. Understanding this process is crucial as microplastic debris serves as a persistent and stable surface that facilitates microbial colonization and enables the transfer of microorganisms. It also acts as a vector for the dissemination of antibiotic resistance genes, promoting the formation of microbial biofilms that can support the proliferation of opportunistic pathogens and harmful algal species [94]. As the EPS accumulates, it increases the effective density of MPs by trapping inorganic particles, organic detritus, and mineral sediments within its hydrated matrix [90,93]. This progressive fouling can shift buoyant plastics toward neutral or negative buoyancy, promoting their sedimentation into benthic zones. The downward transport of biofilm-coated MPs has ecological implications, as it introduces persistent synthetic particles into sedimentary habitats, where they may be ingested by deposit-feeding invertebrates or disrupt benthic microbial processes [92,94]. In this way, EPS-mediated changes in MP density serve as both a driver of vertical particle transport and a pathway for the transfer of associated contaminants and microbial communities to deeper aquatic ecosystems.

The integrity of the plastisphere may be influenced by environmental factors such as temperature, salinity, pH, UV exposure, and nutrient availability. For instance, variations in temperature and nutrient concentration can influence microbial metabolism and succession, while salinity differences lead to selective colonization by halotolerant or halophilic species (

Table 1. Environmental parameters and their impact on plastisphere microbial communities in freshwater ecosystems [87,95].

Environmental Parameter	Impact on Microbial Communities	Implications for Plastisphere Ecology
Temperature	Modifies microbial metabolism, enzyme activity, and community succession.	Influences biofilm growth rate and biodiversity; may drive ecosystem-specific profiles.
Salinity	Selects for halotolerant and halophilic species; inhibits sensitive organisms.	Shapes community composition based on habitat (marine vs. freshwater).
pH	Affects microbial viability, gene expression, and EPS production; extreme pH limits diversity.	Controls biofilm stability and species compatibility in various environments.
UV Exposure	Causes oxidative stress, DNA damage, and microbial mortality; favours UV-resistant species.	Alters surface colonization and may reduce primary producers in the plastisphere.
Nutrient Availability	Stimulates colonization, metabolic activity, and biofilm thickness; scarcity triggers stress response.	Dictates biofilm development, succession, and potential for pollutant degradation.

) [87,95]. These dynamic factors contribute to the heterogeneity observed in plastisphere communities across different ecosystems.

6. Control and Management of Microplastics in Freshwaters

6.1. Bioremediation of Microplastic Pollution

Traditional remediation techniques such as filtration or chemical treatment often prove inefficient or ecologically disruptive [96]. As such, bioremediation, the use of biological agents to degrade or detoxify pollutants, has emerged as a promising and eco-friendly alternative. This innovation has emerged as a compelling strategy for mitigating microplastic pollution in freshwater systems by harnessing the metabolic capabilities of microorganisms [96]. Naturally occurring microorganisms capable of degrading synthetic polymers offer a biologically sustainable pathway for mitigating microplastic contamination [97]. Several bacterial and fungal species have demonstrated the ability to colonise and enzymatically degrade various plastic types, including polyethylene terephthalate (PET), polyethylene (PE), and polystyrene (PS) [96,97].

Various bacterial families, including Rhodobacteraceae and Comamonadaceae, and fungal groups such as Ascomycota and Basidiomycota, have been identified as key players in breaking down microplastic particles into nanoplastics, which are particles smaller than 100 nm [98]. These microbes employ enzymatic processes to fragment and degrade polymers, and their activity is highly influenced by environmental conditions such as pH, temperature, and nutrient availability [96,99]. The efficiency of this biological approach is not uniform across all aquatic systems; for instance, microplastic degradation occurs at higher rates in humic lakes, reaching up to 45% per year compared to clearer water bodies [100].

From a sustainability perspective, bioremediation is recognised for its ecological compatibility and minimal environmental footprint, especially in contrast to physicochemical treatments that may generate secondary pollutants [96]. However, this method is not without limitations. One critical concern is the partial degradation of plastics, which can result in smaller, potentially more bioavailable nanoplastics that still pose ecological threats [101]. El-Kurdi et al. [102] emphasised that nanoplastics fragmenting from microplastics are known to cause serious environmental problems due to their smaller size, which results in high mobility and increased toxicity. Nevertheless, nanoplastics may enter cells and further degrade into fragments with the help of enzymes. The fragments could be used as a carbon source for the growth and development of microbes, which may further enhance bioremediation [103,104]. Therefore, optimizing the microbial consortia and environmental parameters for enhanced degradation remains a significant research frontier. Continued investigation is necessary to identify robust microbial strains and develop standardised protocols that can be scaled to field applications [96]. Overall, while bioremediation holds substantial promise, addressing these challenges is crucial for its integration into comprehensive microplastic management strategies.

Notably, Ideonella sakaiensis has garnered significant attention due to its capacity to hydrolyse PET using two key enzymes: PETase and MHETase, which break PET down into its monomeric components, terephthalic acid (TPA) and ethylene glycol (EG) [105]. A body of evidence has shown that other genera, such as Pseudomonas, Bacillus, and Rhodococcus, have also shown the potential to degrade plastics, often through oxidative or hydrolytic enzyme activity, including laccases, esterases, and oxygenases [106,107,108]. Fungi such as Aspergillus and Penicillium have exhibited similar capacities, particularly for polyethylene and polyurethane degradation [109]. To overcome the inherent limitations of native microbial degradation, recent advances in synthetic biology have facilitated the development of genetically engineered microorganisms (GEMs) optimised for enhanced microplastic degradation. By overexpressing or engineering plastic-degrading enzymes such as PETase, MHETase, cutinase, and lipase [110], researchers aim to significantly accelerate the breakdown of persistent plastic polymers under environmentally relevant conditions.

For instance, the directed evolution of PETase variants has yielded enzymes with increased thermostability and activity, capable of functioning at ambient temperatures prevalent in freshwater bodies [111]. Additionally, the coupling of PETase with MHETase in co-expression systems has improved the conversion efficiency of PET into monomers [112]. E. coli and Bacillus subtilis are commonly used as chassis organisms for expressing these recombinant enzymes due to their genetic tractability and environmental compatibility [113]. It is worth noting that microorganisms, including GEMs, provide an innovative solution to microplastic pollution in freshwater environments. This emerging trend deserves attention in future research on environmental sustainability.

6.2. Nanotechnology-Based Solutions for Microplastic Removal

Nanotechnology is a promising alternative for increasing the efficiency of microplastic removal in surface waters [114]. Nanomaterials (NMs) (size: 1–100 nm) have unique physical and chemical properties, such as a large surface area, photocatalytic activity, and excellent adsorptive properties, which are ideal for the degradation of contaminants in surface waters [114,115]. NMs can be characterised by multiple parameters, including their dimensions, surface chemistry, and crystal structures [116]. Based on their geometry and dimensions, NMs are classified into four groups: zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) [117]. The functions and roles of NMs are closely related to their chemical composition, dimensions, and geometry [114]. The chemical composition and corresponding surface chemistry, such as hydrophilicity and charge, are vital aspects that affect their physico-chemical properties, including their antibacterial and antifouling properties [118]. Furthermore, the dimensions and geometry of nanostructures have a major effect on their distribution, permeability, and transport behaviour, as well as their interactions with the surrounding matrix or types of impurities [119].

Nanomaterial-based smart membrane filtration techniques have been recommended as a solution for microplastic removal in surface waters [120]. Membrane filtration techniques, such as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, forward osmosis, dynamic membrane systems, and membrane bioreactors (MBRs), are promising strategies for minimizing microplastics in surface waters [116,121]. The advantages of using these filtration techniques include their ease, low energy requirements, low cost, safe and flexible up-scaling, high selective permeability for component transport, stable effluent quality, and environmental compatibility [116,122]. According to the European Commission [123], the membrane method can be regarded as a green technology based on the 17 sustainable development goals, guidelines, and process intensification strategy. The incorporation of NMs such as MXene [124], zeolites [125], carbon NMs [126], metals, and metal oxides [127] into filtration membranes has shown significant potential in improving the removal of MPs from surface waters. These advanced membranes leverage the unique properties of NMs to enhance selectivity, permeability, and mechanical stability, hence addressing the restrictions of traditional filtration techniques [128].

Nevertheless, there are some limitations associated with the filtration technique (

Table 2. The pros and cons of filtration techniques employed for microplastics in surface waters [130,131,132,134].

Filtration	Pros	Cons
Microfiltration	Pore sizes require low pressure (0.1–10 µm) Good pre-treatment	Efficiency may be low compared to UF, NF, and RO Abrasion can easily occur
Ultrafiltration	Removes smaller particles (1–100 nm)	Allows for membrane scaling Repulsion between ultrafiltration manufacturing materials and microplastics affects the removal efficiency
Nanofiltration	High efficiency, particles smaller than 10 nm (1–10 nm)	Allows for membrane scaling High pressure requirements
Reverse osmosis	High efficiency, smaller pore sizes (<1 nm) Low water flux	High energy requirements Fibres may pass through
Forward osmosis	Fouling resistance Water flux enhanced	Expensive
Dynamic membrane filtration	Effective for fibre removal	Low efficiency for fragments
Membrane bioreactor	May function efficiently in the absence of a secondary settling tank	High operational cost Membrane fouling

). Microfiltration allows notable-sized microplastics to pass through, and the sharp-cornered ones may cause membrane abrasion [129]. With UF and NF, regular cleaning may be required as they clog quickly, and they have poor thermal and chemical resistance. Moreover, the hydrophobicity of microplastics may result in repulsion or attraction due to the charges of the membrane surfaces [130]. Reverse osmosis was shown to require high amounts of energy, and fibres and fragments were found in the permeate that went through the RO [129,131,132]. Nevertheless, FO showed some resistance in fouling and enhanced water flux; however, its limitations included the cost of improving its hydrophilicity to allow for the rejection of hydrophilic and negatively charged MPs [133]. Dynamic membrane filtration was found to be effective in removing fibres, with low efficiency in fragment removal [134], whereas the membrane bioreactor was found to incur high operational costs, with high amounts of membrane fouling. Despite the filtration technique’s efficiency in reducing microplastics, the filtration membranes may be clogged and require energy. The integration of multiple membranes needs further exploration to provide insight into their efficiency and strategies to reduce membrane degradation.

6.2.1. MXene

MXene, a 2D NM, has also gained substantial attention in a variety of applications due to its exceptional chemical and thermal stability [124]. MXene is prepared by etching A-element layers, which produces MXene with a large surface area and several folds [135]. These 2D NMs are chemically stable, functional on the surface, hydrophilic in nature, extremely conductive, and environmentally friendly [124]. This has led to their use in different fields, including water purification [135]. A study by Yang et al. [136] showed that the h-Ti₃C₂T_x MXene membrane had a very high removal performance (up to 99.3%) for non-degradable MPs (fluorescent PS (FP) microspheres). Yang et al. [136] further stated that this membrane showed an excellent water flux (196.7 L m⁻² h⁻¹ kPa⁻¹), which is greater than that of the untreated membranes formed from 2D NMs. The h-Ti₃C₂T_x membrane’s high water flux and maximum MP removal efficiency indicate its potential for practical applications in the separation of MPs and other suspended solids from water.

6.2.2. Zeolites

Zeolites, which are microporous minerals based on silicon, have a distinctive crystalline structure, high surface area, ion-exchange capability, and network of uniform pores, making them effective for adsorption and filtration [125]. They are highly effective at trapping suspended solids in water and have gathered increasing interest due to their low cost and high potential for local manufacturing [137]. Additionally, their periodic and distinct structure, abundant adsorption sites, and variable pore topologies facilitate exceptional selectivity in the process of adsorption, which enables the effective removal of different pollutants, including MPs [125]. A study by Babalar et al. [138] showed that a magnetically activated biochar–zeolite composite had a higher adsorption capacity of 736 mg/g on 2 μm and 769 mg/g on 15 μm PS MPs. Sponza and Öztekin [139] showed that a zeolitic imidazolate/Fe₃O₄ nanocomposite attained the adsorption of 99% of PS MPs within 30 min of contact time. Although zeolite has not been used specifically for removing MPs in membrane form, these studies demonstrated that it has a high capacity to adsorb MPs from water due to its porous structure. Furthermore, zeolites’ design allows them to selectively adsorb different MPs based on their charge, size, and chemical properties [138]. Zeolites are also environmentally friendly and economically feasible for long-term use as they occur naturally and can be easily obtained [139]. On the other hand, the cost and availability of zeolites pose challenges for large-scale water treatment.

6.2.3. Carbon Nanomaterials

The primary benefit of carbon nanomaterials (CNMs) is their porosity [117]. The surface of CNMs can be functionalised with several chemically active groups (hydroxyl and fluorinated alkyl groups, carboxyl, aldehyde, and amino groups) to affect certain pollutants [126]. Some CNMs, including graphene and CNTs, can be used as fillers for conventional membranes for the removal of pollutants [117]. Graphene is made up of a single layer of sp2-hybridised carbon atoms arranged in a honeycomb structure [140], making it a potential alternative for pollution remediation across a wide pH range [126]. Sun et al. [141] reported that chitin and GO-made sponges had effective reusability and that the efficiency for the removal of PE MPs was 89.8%. Another study by Dey et al. [142] demonstrated that a GO–polyvinyl alcohol composite membrane removed 95% of HDPE MPs in 15 s and exhibited excellent permeability (179 Lm⁻² h⁻¹ kPa⁻¹) with a transmembrane pressure of 3.5 bar.

Graphene sheets undergo a process of coiling to create tubular structures known as CNTs [143]. CNTs have great potential as adsorbent materials for water treatment due to their hollow architectures, their many active sites both on the surface and inside, and their strong attraction to specific pollutants [144]. Membranes made of CNTs have the benefits of being both resistant and flexible, similar to ceramic membranes [145]. A study by Sajid et al. [146] showed that CNTs had adsorption capabilities of 1600 mg/g for PE, 1400 mg/g for PET, and 1100 mg/g for polyamide. Sajid et al. [146] further stated that a reusability trial showed that even after undergoing four cycles of reuse, the efficacy of removing MPs remained consistent at 80%. This could be attributed to the CNTs’ higher surface areas and unique structures, which allow them to effectively remove MPs [143]. The remarkable mechanical strength and tailored properties of CNTs promise durability under harsh treatment conditions and provide regeneration potential, therefore increasing their sustainability and economy [144]. Nonetheless, CNTs are expensive to develop, which may restrict their implementation in large-scale water treatment plants.

6.2.4. Metals and Metal Oxides

Nanomaterials based on metals and metal oxides consist of one, two, or three metals and/or their oxides, and their incorporation into membranes stands out as an innovative solution [127]. Despite the wide variety of these materials, only silver (Ag), zinc oxide (ZnO), iron oxide (Fe₂O₃), and titanium oxides (TiO₂) are commonly incorporated into the membrane method due to their unparalleled reactivity and catalytic properties [117,127]. These membranes are easy to design and possess antibacterial properties [128]. Moreover, their capacity to participate in photocatalytic reactions under UV light allows for effective MP degradation [147]. Metal and metal oxide membranes offer substantial adsorption capacities due to their electrostatic interactions, enabling effective MP removal [148]. Their rapid kinetic adsorption makes them ideal for an expedited water treatment process [147]. Additionally, their surfaces can be modified to enhance their efficiency and selectivity, and they are widely available for various applications. However, these membranes may release toxic nanoparticles and ions, posing health and environmental risks [127]. Also, their production costs hinder their application in large-scale water treatment plants.

7. Innovative Technologies to Reduce Microplastics’ Release into the Environment

7.1. Plastic-to-Fuel Technologies

Plastic-to-fuel technologies are emerging as a promising solution to the growing plastic waste crisis, offering a way to turn non-biodegradable plastics into valuable energy sources. These approaches not only help reduce the environmental burden of plastic waste but also support the principles of a circular economy by transforming waste into usable fuels [149]. Several thermochemical processes underpin these technologies, each with unique methods and outputs. Pyrolysis, for example, involves heating plastics without oxygen to produce liquid fuels, gases, and solid char. Innovations such as microwave-assisted pyrolysis have shown potential for generating diesel-like oils that can power internal combustion engines [150]. Another method, gasification, converts plastics into syngas, a mixture of hydrogen and carbon monoxide, through high-temperature reactions. This process can produce hydrogen and other valuable chemicals, making it ideal for integrated biorefineries [150,151]. Liquefaction, on the other hand, breaks down plastics in a solvent under high temperature and pressure, yielding liquid fuels that can be refined further [152].

Economically and environmentally, plastic-to-fuel technologies offer multiple advantages. They can significantly reduce the volume of plastics going to landfills and decrease environmental pollution, contributing to sustainable development goals [153]. However, many of these processes are still in early development stages, and their economic feasibility requires further research and refinement to improve efficiency and scalability [150]. Despite these challenges, integrating renewable energy sources into plastic-to-fuel systems could enhance their sustainability and overall impact. As research progresses, these technologies hold strong potential to support cleaner energy transitions and protect ecosystems from plastic pollution.

7.2. Use of Biodegradable Plastics

The use of biodegradable plastics has the potential to reduce plastic waste, ultimately decreasing microplastics in the environment. Biodegradable plastics (BPs) have emerged as an alternative solution to conventional plastic (CP) pollution due to their capability to degrade and reintegrate into biogeochemical cycles [154]. Therefore, there has been an increasing global interest in BPs in recent decades. It is estimated that global BP production capacity will reach over 3.5 million tonnes by 2027 [155]. BPs are commonly used for food packaging, garbage sacks, shopping bags, and agricultural applications [154,156]. The main biodegradable plastics are starch-based blends (SB), polylactic acid (PLA), polybutylene adipate-co-terephthalate (PBAT), and polybutylene succinate (PBS) [157,158,159]. However, the increasing use of BPs by numerous sectors has raised concerns about their actual biodegradability. The degradation of BPs predominantly depends on their properties, including composition, crystallinity, and surface area [158,160]. Furthermore, environmental factors such as the abundance and activity of microorganisms, pH, and temperature greatly impact BP biodegradation [160,161]. The lower microbial diversity in freshwater ecosystems, due to variations in nutrient availability and water quality, results in slower BP biodegradation rates compared to soil environments [158,160,161]. In comparison to non-biodegradable plastics, BPs can be completely degraded in a controllable environment, such as an industrial composting environment, due to its high temperature, humidity, and microbial richness, within a month [162]. On the other hand, once BPs enter an open environment with uncontrollable conditions such as natural water, their degradation may be challenging to assess [163,164].

Biodegradation of Microplastics in Freshwater Environments

A study by Nabeoka et al. [165] showed that the biodegradation of PLA in freshwater at 25 °C can take up to 11.4 months to commence (Table 2). Eronen-Rasimus et al. [164] reported that poly (3-hydroxybutyrate/3-hydroxy valerate) (PHB/HV), plasticised starch (PR), and cellulose acetate (CA) almost completely degraded in one year in the brackish Baltic Sea, while PLA showed no signs of degradation. Massardier-Nageotte et al. [166] and Bagheri et al. [167] reported that the maximum degradation exhibited by PLA in freshwater was 5% (Table 2). In addition, Bagheri et al. [167] reported that the incubation of PLA for 52 weeks in freshwater in a thermostatic chamber at 25 °C and under fluorescent light (as part of a 16 h light and 8 h dark cycle) resulted in <2% degradation. In contrast, Massardier-Nageotte et al. [166] reported that a shorter incubation period of 4 weeks in freshwater resulted in a slightly higher degradation rate of 3.7% for PLA (Table 2).

The BPs in freshwater involve microbial-driven degradation, in which specific microbes, particularly from the Burkholderiaceae family, utilise enzymes such as PLA depolymerase and polyesterase to break down biodegradable plastics, promoting weight loss and biofilm formation in aquatic environments [167,168]. Alkaline conditions may have promoted the degradation of PLA, releasing lactic acid as a byproduct [166]. Also, the released lactic acid may have been rapidly utilised by bacteria via their metabolism, resulting in the low degradation of PLA [166,167,168]. The slow degradation process of PLA in an open environment raises concerns about plastic pollution and its effectiveness as a potential sustainable material.

Polyhydroxybutyrate showed a 43.5% weight loss (degraded by Cytophaga-Flavobacterium-Bacteroides, γ-Proteobacteria, and β-Proteobacteria) in freshwater at 20 °C after 6 weeks [169] (Table 2). Also, Narancic et al. [170] and Sridewi et al. [171] stated that 8 weeks of PHB incubation in freshwater at 21 and 32 °C showed degradability rates of 90 and 70%, respectively. These variations in degradation could be due to factors including microbial activity and environmental conditions such as temperature and pH [166,167,168]. Also, PHB, being more crystalline and having a higher melting temperature, is less accessible to degrading microorganisms and enzymes compared to the less crystalline PHB/PHV [169]. A study by Syahirah et al. [172] showed that Cupriavidus sp., Acidovorax sp., Variovora sp., Streptomyces sp., and Ideonella sp. degraded P(3HB-co-3HV) by 21% in lake water (Table 2). The degradation of alginate is pH-dependent; thus, the lake’s pH may promote the high degradation rate of alginate P(3HB-co-3HV) blend films [172]. Although biodegradable plastics (BPs) are often positioned as sustainable alternatives to conventional plastics, real-world studies on their degradation in freshwater ecosystems are scarce. Most assessments are conducted under laboratory or industrial composting conditions, which do not reflect the variability of natural freshwater environments such as rivers, wetlands, and lakes [164]. Long-term field trials are essential to assess degradation rates, byproduct toxicity, and ecological compatibility across diverse climatic zones, including semi-arid regions in Southern Africa. In addition, even biodegradable plastics can undergo fragmentation into secondary microplastics during their partial degradation, potentially posing significant ecological risks [173].

7.3. Use of Recyclable Products

Innovations in recyclable product design, mainly through eco-design strategies, can effectively decrease MP generation from products such as textiles, packaging, and tyres [174]. Monomaterial designs and/or biodegradable polymers are the main innovations aimed to create products that are recyclable and environmentally friendly [175]. Mono-material product designs, including packaging, textiles, and various consumer goods, are based on circular and biodegradable polymers made of a single monomer [176], delivering tailorable properties through molecular or macromolecular engineering without changing their chemical makeup or composition [177]. Mono-material designs offer a sustainable alternative to traditional plastic products, which not only improves recyclability but also reduces the fragmentation of plastics into microplastics during degradation processes [178]. In addition, mono-material designs simplify recycling by using a single polymer type, which avoids the challenges associated with separating and processing multi-material products [179]. This leads to a more efficient recycling process and reduces the likelihood of plastics breaking down into MPs during recycling [178]. Also, mono-material products made from biodegradable polymers such as polylactic acid (PLA) and polyhydroxyalkanoates (PHAs) can degrade more completely in natural environments, reducing the accumulation of MPs [180,181]. As mono-materials can also be tailored through molecular and macromolecular engineering to exhibit a wide range of properties, they can replace multi-material products without compromising functionality [178]. This versatility reduces the need for additives and other materials that contribute to MP pollution. The effective mitigation of microplastic pollution cannot rely solely on technological innovation [182]. A notable gap exists in addressing socioeconomic drivers, policy frameworks, and public behaviour, especially in low- and middle-income countries (LMICs). Limited recycling infrastructure, lack of economic incentives, and weak policy enforcement contribute to the ongoing release of microplastics. Integrating behavioural science and environmental economics into future research could drive more inclusive, scalable solutions. Figure 3 summarises key similarities and differences between conventional and emerging technologies aimed at minimising microplastic discharge into the environment.

8. Conclusions and Way Forward

Microplastics have been shown to be ubiquitous, and their effects on freshwater ecosystems are becoming non-negligible. The most reported effects on aquatic biota are the blockage of their respiratory systems and gastrointestinal tracts, leading to the secretion of enzymes and hormones, and oxidative stress. Nevertheless, the mechanism underlying their toxicity effects in a wide variety of organisms, particularly those occupying different trophic levels, still needs further investigation. Moreover, the biomagnification capacity of microplastics is still poorly understood. Microplastics as a conveyor for other chemical contaminants have been adequately documented; however, the combined effect of adsorbed chemicals and microplastic additives has never been studied. Insignificant progress has been made regarding techniques for microplastic pollution remediation and control in freshwater ecosystems. Nevertheless, despite filtration techniques showing some advantages, limitations such as their inefficiency, fouling, vulnerability to sharp-edged particles, and high energy requirements are yet to be addressed. Therefore, more studies on the development of innovative technologies to control and reduce microplastic pollution are recommended. Challenges for addressing microplastic pollution include the following:

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The lack of globally standardised techniques, which can enhance the comparability of results between different regions;

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The lack of guidelines for microplastic load in different environmental compartments;

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Poor enforcement of legislation or policies regarding microplastic pollution, especially in low- and middle-income countries.

Other research gaps worthy of further exploration include the following:

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The dynamics governing the leaching of plasticisers and their effect on freshwater ecosystems;

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The main and combined effect of plasticiser and metals, as well as physical variables influencing toxicity;

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The integration of AI into field-based microplastic assessments;

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The potential for microplastics to biomagnify along the food web;

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Spatio-temporal dynamics in rivers, wetlands, and pans to allow for modelling and prediction through different exposure routes;

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The use of mesocosm experimental systems for cause-and-effect studies to enhance the comparability of findings to those of a natural/field environment;

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Microplastic partitioning between different matrices in aquatic environments;

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Technological advancements to enhance the resilience of filtration systems.