The invention of plastic was a terrific innovation; now, it has penetrated everywhere, and the estimated plastic production reached 367 million tonnes in 2020 globally. ¹ Plastic is a synthetic polymer made from crude oil and natural gas. The dependence of the public on plastic merchandise has inevitably led to a prompt rise in plastic manufacturing volume. Plastic-related research is increasingly in focus in recent times and more volume of studies are being pursued. ^{2 , 3} Due to the presence of different forms of plastics, the waterbodies (ocean, river, lake, wetland, etc), the atmosphere, the terrestrial environment, and even the groundwater are being polluted by microplastics (<5 mm) and nanoplastics (<100 nm) ( Figure 1). ⁴ The large plastics products are fragmented into smaller pieces and get converted into MNPs through various processes. ^{5 , 6} At the same time, the hydrocarbon chains in plastic compounds make them difficult for plastic wastes to decompose in the environment. MNPs are reaching either directly into the environment or after the degradation of plastic debris. Although the MNPs have different sizes (1 nm to 5 mm), various shapes (fibre, film, foam, etc.), and unique chemical structures (polyethene, polypropylene, polyethene terephthalate, etc.), they are generally invisible in nature. MNPs not only pose direct threats to marine life through ingestion but also serve as carriers for toxic chemicals, further amplifying their ecological impact. Understanding their distribution, behavior, and long-term effects is crucial for developing effective strategies to mitigate their pervasive presence in the environment. At the same time, MNPs interact with the neighbouring compounds and microorganisms, which is crucial for animal, human, and plant health; i.e., these invisible compounds comfortably react/interact with the surrounding environment, including soil, organic and inorganic compounds, microorganisms, heavy metals, and plants. ⁷ As a result, MNPs-related research has been getting significant attention in recent years, both in societies and in scientific communities, due to their potential impacts on ecological systems and human health. ^{8 , 9} Groundwater contamination by plastic microfibers is a pressing concern in environmental science ( Figure 2). ¹⁰ These tiny, nearly invisible plastic fibres shed from clothing, textiles, and other sources can infiltrate the soil and eventually reach groundwater reserves. Once there, they pose a risk of contamination, potentially affecting both aquatic ecosystems and human drinking water sources. ¹⁰ Understanding and mitigating this hidden threat is essential for safeguarding our precious groundwater resources and the overall health of our environment. However, the current research on interactions of these invisible MNPs is in its early stages and data are limited in the important connected sectors. The objective of this article is to highlight the perspectives of current characterization methodologies, health impact, important source pathways, and available remediation options for the invisible MNPs present in the environment.

The quantitation and analysis of MNPs are essential for accurately assessing their existence and intensity in different environmental samples. ¹¹ A number of techniques are employed for quantification depending on the type of MNPs sample and the size range of MNPs being investigated such as density separation and filtration, ¹² microscopic and spectroscopic techniques, ¹¹ chromatographic approach, ¹³ nanoparticle tracking analysis (NTA), ¹⁴ quantitative polymerase-chain-reaction (qPCR) approach ¹⁵ ( Figure 3).

Density separation and filtration are common tools to isolate and concentrate MNPs from soil or water samples. It is employed to segregate MNPs from the collected soil or water samples for further characterization. Optical microscopy is a traditional method of visualization of MNPs. In this method of direct visualization, the segregated MNP samples are collected and prepared on glass slides, and they are manually counted and quantified. Although this method can provide valuable information about the presence of MNPs, it may not be suitable for quantifying relatively small particles or dealing with large sample sizes. However, infrared or Raman spectroscopy is being used for identifying and quantifying MNPs based on unique molecular vibrations of plastic compounds to better understand the polymeric types of these invisible plastic compounds present in the environmental sample. ¹¹

More recently, a combination of microscopy and infrared spectroscopy has been employed to identify and quantify MNPs in individual samples. ¹¹ They provide spatial distribution and allow for a more detailed characterization of MNPs in complex soil or water samples. The pyrolysis-gas chromatography-mass spectrometry is also used to identify and quantify MNPs based on the polymer’s chemical breakdown (pyrolysis process). ¹³ It conveniently provides information about the types and concentrations of plastics in the soil or water sample.

In addition, nanoparticle tracking analysis (NTA) has also been considered an optical technique for tracking individual MNP particles based on their Brownian motion. ¹⁴ It provides information on particle size distribution and concentration as well. More recently, quantitative polymerase chain reaction (qPCR), a technique within molecular biology based on MNP ingested by microorganisms or organisms, has also been practised. ¹⁵ All of these techniques for quantifying and analysing MNPs have benefits and drawbacks, but the choice of technique mostly depends on the kind of sample, the goals of the research, and the resources at hand. In order to do more thorough analyses of these invisible plastic compounds in environmental samples, a combination of various approaches is frequently used. To guarantee the accuracy and comparability of MNP data across the globe, these methodologies must be standardised.

The potential impact of MNPs on the environment and human health is a noble area of research. ¹⁶ The extent of the health impacts of MNPs is not yet fully understood, several potential pathways of exposure and adverse health outcomes have been recognised. It is important to note that the current scientific understanding of MNP toxicity on human health is still in its early stages, ¹⁶ and much more research is needed to fully understand the extent of the risk. The presence of these invisible plastic compounds in the human body and their potential health effects have prompted calls for more comprehensive studies, improved waste management, and the development of sustainable alternatives to reduce plastic pollution and potential human exposure.

Although the current literature had properly covered the source, pathways, quantification, and even interaction of these invisible MNPs. The MNPs originate from a multitude of sources, including the fragmentation of larger plastic debris, the shedding of microfibers from synthetic textiles during laundering, the abrasion of tires on roads, the release of microbeads from personal care products, and the breakdown of industrial plastic pellets (nurdles). These MNPs can enter the environment through various pathways, including direct deposition, atmospheric deposition, surface runoff, and wastewater discharge. So it is crucial to understand the sources and pathways of MNPs of the contaminated sites to develop effective mitigation strategies and prevent further pollution. ³⁰ For example, the release of fibre from textiles, the presence of microbeads in cosmetic products, and the degradation of plastic debris could be potential sources of MNPs. ³¹ Most of this research emphasizes the amount and degradation of plastics, recognition and quantification of MNPs in the environment and in the microorganisms, toxicological effects on single species, and combined effects with others pollutantss. The presence of MNPs in the environment poses numerous ecological and human health risks. MNPs can be ingested by a wide range of organisms, leading to physical harm, internal blockages, and potential transfer through the food chain. MNPs can adsorb and transport toxic chemicals, including persistent organic pollutants and heavy metals, thereby magnifying their toxicity and bioavailability. Additionally, MNPs have been linked to adverse effects on reproductive health, immune function, and developmental processes in aquatic organisms. However, there are still some critical issues that urgently need to be addressed. Such as the impact of these invisible plastic compounds on the structural stability of ecosystems or their remediation, especially from the soil and groundwater, is limited. ³² The land-use pattern and soil types play a critical role in the infiltration of MNPs into subsurface systems and groundwater. ³³

The leaching of MNPs can happen predominantly from agricultural soils or from urban infrastructure wastes. The improper disposal of plastic waste, such as plastic bags, bottles, and packaging materials, is a significant source of MNPs in the environment. Plastics that end up as litter on land can be transported by wind, rain, and water runoff into aquatic bodies. Generally, larger plastic items, such as bottles and bags, break down into MNPs through processes like photodegradation, mechanical weathering, and chemical degradation. They are called secondary MNPs. However, the primary MNPs have resulted from synthetic textiles, like polyester and nylon, shed microfibers during laundering, wear down of vehicle tires over time, industrial wastes containing plastic pellets (known as nurdles), as well as microbeads from personal care products. The biosolids treated from sewage sludge applied on agricultural fields are also a critical source of these invisible plastic compounds in soil and groundwater. ³⁴

Although a large volume of literature has focused on the presence of invisible MNPs in marine and freshwater systems, some of the recent studies have started focusing on soil, groundwater, and remote locations. ^{32 , 33} Surface water bodies such as rivers, lakes, and oceans are often the first recipients of MNPs from various sources, including urban runoff, industrial discharges, and plastic debris from land. All of these investigations have shown that MNP pollution of surface waterways is widespread, with significant concentrations occurring around urban areas or plastic manufacturing facilities.

The studies on MNPs in soil are relatively new and not as abundant as those of surface water, soil contamination by MNPs also occurs through various pathways, including the direct application of plastic mulches, sewage sludge, and atmospheric deposition. A common methodology of MNPs separation and characterization is being adopted for soil as well. While MNP concentrations in soil may not be as high as in surface waters, they can still pose a risk to terrestrial ecosystems and potentially enter the food chain through plant uptake. ³² Further, groundwater contamination can occur through the percolation of MNPs from surface soils, landfills, and septic systems which raises concern about their potential impacts on drinking water quality and groundwater-dependent ecosystems. ^{35 , 36} Due to the challenges of accessing and sampling groundwater, research in this area has been relatively challenging. ^{32 , 37} To investigate MNP content in groundwater, researchers have used well water sampling and then proceeded with common filtration, separation, and microscopic techniques. ³³

In short, studies on MNPs in surface water are more extensive and established compared to those in soil and groundwater but the potential source and the presence of MNPs in all three compartments have been confirmed, and each environment poses unique challenges for research and monitoring. To create efficient mitigation methods and protect both human health and the environment, it is crucial to keep researching the source pathways, fate, and potential effects of MNPs on these environmental compartments.

Plastics are polymers based on chains of carbon atoms and carbon as the main component. Micro (nano) plastic releases in the dissolved form as organic carbon or as a greenhouse gas (such as CO ₂) while degrading in the ecosystem. So, MNPs are also closely associated with the current climate crisis because they are directly or indirectly involved in greenhouse gas emissions from their production to disposal stages. ³⁸

Micro (nano) plastics have a significant effect on ecosystem functions, leading to ecological disruptions and alterations in natural processes. ³⁹ These effects can impact both aquatic and terrestrial ecosystems. They alter the nutrient cycling of the ecosystem when ingested by organisms. Micro (nano) plastics have an impact on biodiversity, community structure, and the distribution of species within ecosystems. Other species that rely on them for food or ecological interactions are affected in a cascade manner by this. The accumulation of MNPs in sediments, soils, and water bodies modifies the habitat, bioaccumulates the hazardous substances around it, changes how different animals feed and reproduce, and lowers the ecosystem's ability to withstand environmental stressors. ³⁹ All these have long-term consequences for population dynamics and ecosystem stability. Organisms exposed to MNPs may be more vulnerable to other ecological pressures, such as changes in temperature, salinity, or pollution, leading to greater susceptibility to diseases and overall ecosystem instability. Micro (nano) plastics also affect the delivery of ecosystem services to human populations and decrease agricultural productivity and fisheries yield, and affect the carbon sequestration capacity of ecosystems. ⁴⁰ Micro (nano) plastics influence the carbon burial rates in marine sediments and ultimately affect the carbon sequestration capacity of ecosystems and alter the global carbon cycle. The intentional and unintentional input of these invisible plastic compounds into the environment may have a latent impact on global carbon stocks. So, a strong commitment is required to find a replacement for plastic products.

The remediation of MNPs is a challenging task due to their widespread distribution, small size, persistence in terrestrial and aquatic ecosystems and their consequent impacts on natural systems. ⁴¹ The removal of MNPs from the contaminated ecosystem is necessary for a benign environment. Two strategies have been used in practice to systematically manage or minimise the pollution caused by MNPs: the first is to block the natural ecosystem's access points, and the second is to remove MNPs in situ from the already contaminated bodies. However, researchers are further exploring various other strategies to address the issue. For example, the upgradation of existing wastewater treatment technologies using granular activated carbon filtration, membrane filtration, and ozonation has a significant impact on MNPs removal. At the same time, stormwater management and beach clean-up initiatives have a promising outcome for preventing the generation of secondary MNPs. Some microorganisms and specialized filter-feeding organisms have also shown the ability to consume and break down MNPs. ⁴¹ The development and use of biodegradable and environmentally friendly plastics can help reduce the accumulation of MNPs in the environment. Increasing public awareness about MNP pollution and educating people about the sources and impacts of MNPs can help reduce their release into the environment.

The reduction of MNPs using biochar is another emerging area of research and holds promise as a potential solution to tackle MNP pollution. ⁴² According to Wang et al., ⁴³ biochar is an effective and affordable investment in the removal of MNPs. They observed that sand filters mediated with biochar enhanced the removal efficiency of MNPs. The porous nature of biochar offers a lot of binding sites and a lot of surface area, making it a good adsorbent for MNPs. ⁴² Biochar attracts and traps MNP after being added to contaminated settings, such as soil or water, by limiting its mobility and availability to living things. To lessen the movement of these invisible plastic compounds from sediments to water columns during erosion events or tidal action, biochar can be used as a sediment amendment in coastal locations. It was observed that biochar can help MNPs degrade more quickly by enhancing the vital microbial communities. ⁴²

In the ongoing battle to mitigate the detrimental effects of MNP on our environment, innovative solutions and concerted efforts have emerged as critical strategies. The preventive measures include reducing plastic consumption, improving waste management practices, implementing bans on microplastics in consumer products, and promoting the use of biodegradable alternatives. The remediation strategies encompass techniques such as filtration, sediment dredging, and the development of innovative nanomaterial-based sorbents for capturing MNPs from water and soil environments. Furthermore, enhanced monitoring and surveillance programs are essential for assessing the distribution, abundance, and ecological impacts of MNPs, thereby guiding targeted interventions and policy decisions. The cutting-edge wastewater treatment technologies are being developed to capture and filter out these minuscule plastic particles before they reach our water bodies, significantly reducing their input. ³⁵ Biodegradable plastics are also under development to curtail the persistence of MNPs in the environment, though challenges remain in their widespread adoption. Nanotechnology offers promise with materials like magnetic nanoparticles that can attract and remove microplastics from water sources. Furthermore, regulatory bodies are increasingly implementing bans and restrictions on single-use plastics, while citizen science initiatives and educational campaigns are raising awareness and contributing to data collection efforts. Innovations in cleanup technologies for oceans and waterways, as well as a shift towards a circular economy, underscore the diverse range of approaches being pursued to combat the MNP crisis. These collective endeavours aim to safeguard ecosystems and human health from the pervasive impact of these invisible plastic compounds.

As plastic-related research is an epicentre from the past decade, this paper emphasizes the technological advancements in MNP research and development with their health implications, types, sources, and environmental implications. •

Although the presence and spread of MNPs are well-known in the terrestrial and aquatic environment, the focus on the fate of MNP in groundwater and their remediation process is scarce. More studies are required to understand the properties, concentrations, and types of MNPs in soil and groundwater in order to assess the public health risks. Therefore, it is urgently required to delineate MNP pollution by the policymaker and researcher for their monitoring, assessment, and handling to safeguard the environment from these invisible compounds.

Prabhakar Sharma and Prateek Sharma conceived and planned the study, wrote the main manuscript text, and reviewed the manuscript. Prabhakar Sharma prepared the figure.

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