Out of all the places of contamination, WWTPs can be undoubtedly considered as the storehouse for MPs. Domestic waste, pharmaceutical and personal care products, and industrial wastes contribute to MPs colonization in wastewater. According to a study conducted on a Turkish WWTP, a miserable concentration of 2936×10⁶ particles/ day was released in the effluent into the Sea of Marmara [41]. Ngo et al. [42] provided a litany of factors responsible for MPs proliferation in WWTPs. The textile industry is another major contributor of MPs in WWTPs [43]. The author further highlights that the fibers and polymers released from these industries in effluent go to WWTPs. In the same study, the author informed that a WWTP in China had contamination of 4.9 to 12 microplastic fibers per liter. Microbeads, used in tooth-pastes, face, and body wash products, and other cosmetic products, reach WWTP’s influent cycle via domestic discharge [44]. Usually, eroded plastic debris from the construction processes, like admixtures used in concrete and bitumen remnants, abrasion of tires, and packaging of construction machinery and tools, remain suspended in the atmosphere before finally falling and scattering onto the ground. During the time of precipitation, these scattered particles are washed away with the run-off and eventually end up in sewer pipes, taking them straight to WWTPs [45].

Microplastics have also been extensively found in water bodies. A study by Ayyamperumal et al. [46] on the Noyyal River in India reported high concentrations of microplastics in both the freshwater and sediments, ranging from 500 to 6,500 items per cubic meter. The cause of such pollution was attributed to the nearby industries and urbanization. Fishing activities are often the major polluter of MPs in the river system. Wang et al. [47] documented that MPs were found in abundance in the water of the Yangtze River, which was a result of the fishing practices of the local fishermen. Usually, nylon nets, fishing lines, and fishing tools degrade and become the source of MPs in the river system. Kiss et al. [48] also stressed that high population density, the spatial distribution of residential and industrial areas, and improper waste management are the root causes of MPs abundance in river water and are ostensible from the study on the Tisza River in Central Europe. Sometimes, flood becomes a major contributor to MPs proliferation in the rivers and other surface water bodies. Previously deposited MPs in any catchment area of an urbanized locality are washed away with flood water and redistributed into the nearby water body [49]. These rivers further become the vectors for MP pollution in seawater. The grim situation of the Black Sea is one primary example of it. 2.6 tons of MPs was discharged to the Black Sea in 2010 by three major transboundary rivers: the Danube, Dnieper, and Don River [50]. The study further added that human activities and mismanagement of the sewage effluent were responsible for the pollution. An unfortunate figure highlighted that 1.15–2.4 million tons of plastic enter oceans globally from the land and out of this, 0.27 million tons of plastic remains in the sea current [51–52]. It can be safer to imagine that the remaining concentration gets either submerged and settled on the ocean floor, washed away and left on coastal sediments, or consumed by marine animals.

Talking about another water body upon which not enough studies have been conducted and can be a future potential hotspot for MPs is the groundwater. Nizzetto et al. [19] list various parameters responsible for the MP contamination of the groundwater. The lists include precipitation run-offs, discharge of WWTPs into rivers and lakes, soil erosion, leaching of leachates, infiltration of wastewater from WWTPs, overflowing of sceptic tanks, and repetitive use of reusable irrigation water coming from municipal WWTPs. Generally, soil characteristics, roots, and branches of the plants forming crevices in rocks and sinkholes provide a pathway for MPs to migrate into groundwater [53]. Groundwater monitoring for MPs’ contamination should be of paramount importance because it is one of the major sources of water for human beings. Any contamination in this water can pose direct and indirect effects on human health. A study conducted in India reported that concentrations up to 80 items/L of MPs were found in the groundwater samples of Chennai city [54]. The author also highlighted that landfill dumping sites were close to the sampling sites. This can positively correlate between landfill sites and groundwater contamination with MPs. In Germany, MPs of the size 1 μm, 2 μm, and 5 μm were found to be transported in an alluvial aquifer in Upper Rhine Valley. The worrying fact about this study was that the respective aquifer is a trans-boundary aquifer running between the boundaries of Alsace (France) and Baden-Wuerttemberg (Germany) and caters up to a population of 3 million people [55]. Since there is a dearth of studies encompassing the migration of MPs to the groundwater, scientific communities should pay attention and work on the topic catering to the transportation behavior of MPs in the ground and groundwater. However, it can be stated that the porosity of the soil media must play an important role in the migration of such particles. The greater the porosity and the pore size, the easier the transportation would be. Along with that, soil texture can also affect the migration ability of MPs reaching up to groundwater. Coarser the texture, smoother will be its transition from ground to groundwater level. Finer soil grains will retain the MPs onto its surface, thus retarding its downward progression. Hallaq [56] also supports that the percolation of MPs into the soil media is a function of soil texture. Apart from the leaching around landfill sites and WWTPs, irrigation can significantly increase the concentration of MPs in the groundwater. In a study, an unconfined aquifer was contaminated with MPs, with its maximum concentration reaching up to 55 particles/L [57]. Interestingly, an irrigable horticulture site was in close proximity to the sampling site. This further proves that the irrigation method can work as a carrier for MPs for groundwater.

An abundance of MPs has also been identified in the coastal and estuarian sediments. Li et al. [58] specifically mentioned that the factors involved in the proliferation of MPs in coastal and estuarian sediments are anthropogenic activities like tourism and recreational activities, coastal urbanization, ocean currents, and meteorological conditions. Xu [59] describe warm ocean currents as one of the key factors influencing MPs’ presence in coastal and estuarian sediments. The respective author discussed the MP’s pollution in the Changjiang estuary. The author attributed the Taiwan warm currents for circulating these MPs from ocean towards the terrestrial zone. A similar study supported the argument that strong tidal current served as a carrier of these pollutants over coastal sediments. Strong tidal currents were found to be responsible for polluting Hangzhou Bay in Eastern China. The sampling sites on the bay were accumulated with MPs, which were probably the by-products of the degradation of fishing gears, ropes, scuba gear, plastic bottles, and marine plastic littering by cruise and commercial ships. This plastic debris degrades and floats on seawater, which is brought to the shores by ocean currents. Socio-economic development plays a crucial role in the regulation of MPs in coastal sediments and is evident from the study of Li et al. [58]. The study found a maximum concentration of 2250 particles/kg of MPs in Mangroves of Southern coastal areas of China. Sometimes, geography of an area also becomes conducive for trapping MPs loads over its soil. The semi-enclosed area of Bohai Sea was found to be a sinking ground of MPs [60]. The waves enter and then breaks due to semi-enclosed coastal geography and lose its strength. They eventually dump the MPs load on coastal sediments.

The rapid proliferation of plastic has raised concern not only for the environment but also for human health. Although there is a lack of trophic transfer of MPs into the human digestive system, the studies on this issue presented some unsettling results. It is well known that human beings are consumers of aquatic animals. Studies have reported that enough MPs fragments have been found in the digestive tracts, circulatory systems, and tissues of marine animals [61–62]. It has been forecasted that the European population will have an intake of 11000 particles of MPs per year due to the consumption of shellfish [63]. MPs have also been found to catalyze reactive oxygen species in human epithelial and cerebral cells [64]. Exposure to MPs has also affected the reproductive health of pregnant women. Pregnant women with traces of polycyclic aromatic hydrocarbon (PAH) in their system were found to be giving birth to under-weight with restricted growth babies [65]. Another study recorded the short-term effects of exposure to PAH on human health, in which it was found that frequent exposures to MPs can affect the functioning of the lungs. In some cases, it was even found that exposure to PAH increased the risk of asthma and thrombosis [66]. Goudarzi et al. [67] also covered some short-term effects of PAH’s exposure to humans’ digestive tract. From nausea to vomiting, eye, and skin irritation were common symptoms. The MPs in human intestines show sufficient cytotoxicity in intestinal cell lines and increased in oxidative stress activities [68]. John et al. [69] described certain long-term effects on human health, where it was found that repetitive, prolonged exposure in small doses to PAH could alter the DNA structure. Huang et al. [70] also documented the dire effects of PAH on reproductive health. Abnormal birth weight, side effects in the fetus, lesser testosterone level, the irregular circumference of the head, etc., were some of the comprehensions of the respective study. Can it be posited that it may be a beginning of mutation in humans? Several reports have also discussed the oxidative stress induced due to the exposure of MPs in humans. Sun et al. [65] purported that PAH induces oxidative stress and causes chromosomal damage in women. Although sufficient data is yet to be researched before concluding the impact of MPs on the liver, heart, and kidneys, authors have not ruled out the potential damage that these contaminants can cause [71–73]. However, further research is needed to quantify microplastics and correlate their presence with human health impacts. A detailed Fig. 3 diagrammatically represents the effects of MPs on various human body organs.

The toxicological effects of MPs on aquatic and terrestrial animals have been extensively studied and reported and several studies. The MPs can either float on the surface, suspended in water columns at varying depths or settle down and accumulate on bed sediments of any water source. Various authors have verified the presence and adverse effects of MPs in crustaceans, birds, mammals, turtles, fish, bivalves, and so on [62, 74–78]. Aquatic animals, especially fishes, feed upon these MPs due to the appearance and size of plankton [79]. Once the MPs are ingested into aquatic fauna’s digestive system, they are not easily emoved by their body as time ensues, and the MPs accrue. The most severe effect of this accumulation is the blocking of the digestive tracts of these animals. Wang et al. [80] argue that once these MPs are retained for a longer duration, they can be transferred to higher trophic predators. In an experimental study by Deng et al. [81] discovered a disturbance in energy and lipid metabolism in mice. In fishes, Danio rerio suffered intestinal damage when introduced to PVC MPs with a size of roughly 70 μm size. The MP’s presence has not only affected the biological metabolism of animals but also interfered with the dynamics of animals’ habitats. MPs accumulation in shore sediments led to an increase in thermal diffusivity, increased heat capacity, and increased permeability of the sand grains, which has affected the hatching cycle of eggs of sea turtles [82]. A positive correlation was also found between the amount of MPs in the fishes of the North Atlantic Sea, chiefly Dicentrarchus labrax, Trachurus trachurus, Scomber colias, and the bisphenol A detected in their cells and tissues [83]. There is no wonder that these toxic fishes can easily find their way into the food chain of humans. Another study highlighted the adverse effect of MPs affecting the reproduction ability and energy balance among black-lip pearl oyster, Pinctada margaritifera, was highlighted by Gardon et al. [84].

In avian species, polybrominated diphenyl ethers were found in the stomach and fatty tissues of birds like Puffinus tenuirostris and was reported at a concentration of 0.3–186 ng/g-lipid weight [75]. This suggests that these birds must have preyed on the fishes which had already accumulated the respective MPs in their digestive system. This can be inferred as a prime example of the transfer of toxic elements climbing up in the food chain. Likewise, other fish species, like Ardea Cinerea, Cygnus olor, and Anas platyrhynchos, were also found to be infested with the fragments of MPs with a concentration of 4.3 ± 2.6 particles per bird in Switzerland [85].

Mammals have also been found to be affected by MP contamination. In the Netherlands, a study conducted on harbor seals provided sufficient evidence for the MPs present in the stomachs and intestines of their corpses [78]. Although it was not clear the actual reason for MPs in the body of seals, it can be hypothesized that either the harbor area is very much polluted with plastic or a lot of anthropogenic activities and recreational activities take place in that area, such as fishing, boating, and cruising. The biggest mammal on this planet, whales, have been found to be highly infested with MPs. Study on Baleen whales revealed the presence of MPs with a size range of 1mm to 17cm of several polymers (polyethylene, polypropylene, polyethylene terephthalate, nylon, etc.) have been recovered from their intestine [86]. Even though, in most cases, the intake of MPs happens accidentally by these animals, but the matter of the fact is their existence and our imprudent attitude in disposing of them. In an experimental study on mice, a worrying result surfaced where it was found that polycyclic aromatic hydrocarbons in mice caused a rapid growth of tumours in the nasal cavity and respiratory tract [65]. MPs in mammals is also a growing cause of concern because it has various associated indirect dangers. The MPs in a mammal biological system can trigger or alter the immune system, oxidative stress, inflammation, reproductive outputs, energy reserves, feeding capacity, etc. [87–91].

The wide application of plastics and their derivatives has led to a huge pollution crisis in our natural environment. Furthermore, the degradation of plastics into MPs has worsened the situation. The presence of MPs in the soil environment has the potential to alter the physical and chemical properties of soil, nutrient content, soil flora and fauna, etc [92]. However, certain soil microorganisms, such as bacteria and fungi, break down MPs using enzymes, improving soil health and promoting crop growth by converting MPs into harmless byproducts like CO2, H2O, and CH4 [93]. Often, continuous use of sewage sludge on the farm fields is considered to be the common source of MPs driving into the soil [94]. Currently, the global production of sewage sludge is estimated to be 45 million Megagrams, out of which the EU’s share is about 13 million Megagrams in 2020 [95]. Due to the colossal generation of sludge, many developed and developing nations are struggling to find ways in order to dispose of this waste, and that is why sewage sludge is not regarded as waste anymore, and focus has been shifted to its utilization, such as composting, landfilling, and agricultural use [96–97]. Although, sewage sludge is primarily used as a fertilizer and source of irrigation water in soil because of its high moisture content, richness in organic matter content which is up to 70%, and biogenic matters such as carbon (4%), nitrogen (4%) and phosphorous (2.5%) [95]. But being a carrier of MPs, itself cannot be overlooked easily. Apart from the application of sewage sludge on soil, there are other sources as well responsible for introduction of MPs in soil and sediments environment, such as plastic mulching for water retention and heat preservation, use of plastic pipes, plastic storage tanks, polypropylene woven bags and ropes, refuse from dumping sites, public places like parks and industrial areas, etc [98–100]. The introduction of MPs in soil has affected not only the crops but also the animals and organisms living there. The earthworms residing in soil with MPs’ presence weigh relatively much less than those who were living in soil without any MPs’ presence [101]. A similar study on earthworms highlighted that these invertebrates showed recalcitrance in their growth after 60 days of exposure to polyethylene MPs [102]. A similar study also reported a reduction in the germination rate of Lepidium sativum (garden cress), as MPs might be blocking the nutrients and water from being sucked by the pores of seed capsules [101]. A field study conducted on rice crops highlighted the adverse effects of nano polystyrene. In the study, it was found that the given MPs induced oxidative stress, as confirmed by the increased number of antioxidant enzymes in rice roots. This phenomenon caused a reduction in the root length of rice and an increased number of lateral branches of the root [103]. The author also reported several other aftereffects of MPs in the soil system. Some of them are reduced rate of seed germination, genotoxicity in the crops, and bio-accumulation of MPs in the plants transported through vascular tissues of the plants. This reduced the rate of seed germination of the respective plant species. Another invertebrate responsible for sustaining the quality of soil, E. crypticus, was found to be displaying a reduction in its reproduction ability and growth rate when it was exposed to soil with MPs in it [104].

The ocean is a major carbon sink, absorbing approximately 25% of human-made carbon emissions from the 1960s to 2010 [105]. Not just that, oceans also sequester atmospheric carbon, with studies showing the capacities of sequestration up to 20,000 Gt [106]. The presence of plastic in the ocean causes it to absorb more oxygen, reducing the amount available for other marine life. Additionally, the accumulation of organic matter on the ocean’s surface hinders the transfer of oxygen from the ocean to the atmosphere [107]. The study further states that MPs influence marine microorganisms’ ability to absorb carbon dioxide and release oxygen. Additionally, MPs alter the biogenic composition of the sea-surface microlayer, leading to a reduction in dissolved CO₂ concentration in the water, thus disrupting the marine carbon cycle [108]. They also hinder algae’s uptake of essential elements like copper, reducing carbon sequestration and affecting the ocean’s role in converting atmospheric carbon into oxygen [109]. Marine microplastics have the potential to impact phytoplankton photosynthesis and growth, induce toxicity in zooplankton, affect their development and reproduction, disrupt the marine biological pump, and alter the ocean’s carbon stock [110]. However, there is still a wide scope and need for further research to understand the impact and interaction of MPs and OCS in a better way.

Synthetic organic chemicals have emerged as an inseparable part of modern human lives and society in the last few decades, and PPCPs play one such crucial role in it. Polyaromatic hydrocarbons (PAHs), polycarbonate biphenyls (PCBs), perfluorinated alkyl substances (PFAs), etc, are some of those synthesized organic chemicals.

The widespread use of these substances (PPCPs) has made them ubiquitous in every sphere of our surroundings, and their persistence and long residence time are making the scenario worse [111–112]. A study revealed that there are more than 160 pharmaceutical chemicals have been found in surface and wastewater [113].

The adsorption of MPs and PPCPs is mainly governed by the adsorption capacity of the MPs, specific surface area, surface texture, shape and size, polymeric properties of MPs, and secondary parameters such as salinity, pH, ionic strength and dissolved organic contents [114].

In several cases, the high ionic strength of the surroundings was found to be a positive catalyst for the interaction between MPs and PPCPs. As reported by Ma et al. [115], the sorption of triclosan upon MPs enhanced by 44% and 74%, with an increase of 35% in the salinity of NaCl. Surface polarity also plays an important role in the interaction of MPs with other entities. MPs that are highly polar and hydrophobic displayed a greater propensity for the hydrophilic PPCPs [114]. Wang et al. [116] state the effect of pH on the interaction where it was reported that when pharmaceutical chemicals, namely estrone, 17β-estradiol, and 17α-ethinylestradiol showed hesitation in their sorption over MPs as the pH of the water raised from 2 up to 12 while perfluorooctanesulfonate (PFOS) and perfluorooctanesulfonamide (PFOSA) improved its sorption over MPs upon the reduction of pH from 7 to 3. Electrostatic interaction between MPs and PPCPs, such as hydrogen bond and van der Waals force, restrict the bond, whereas ionic bond promotes [117]. π-π interaction between aromatic structured compounds and plastic polymer keeps them intact. The strong cohesion between polystyrene and PPCPs is attributed to the strength of the conjugated π cloud of the aromatic structure of PPCPs and polymer [118]. Elizalde-Velázquez et al. [119] state that π-π interaction promotes the interaction between non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, diclofenac, and naproxen. Interestingly, DOM plays two roles. It can either work for the sorption of PPCPs over MPs, or it may restrict [114]. Tetracycline displayed reticence in interacting with polystyrene, polypropylene, and polyethylene, and the interaction rate dropped by 93%, 95%, and 97%, respectively, upon an increase of fulvic acid by 20 mg/L [120]. Physical characteristics also play a deciding role in aggregating PPCPs over MPs. As the size of polystyrene decreased from micron to submicron to nano-sized particles, phenanthrene, and nitrobenzene were found to be aggregating efficiently on the MPs’ surface [114].

ARGs are the genes that have developed resistance to antibiotics and other similar drugs used to treat human infections and diseases [121]. ARGs have been found in almost every zone of our environment, be it water bodies, aquaculture systems, drinking water, STPs, sediments, soil, and air [122–128]. Recently, this has been a hot topic of discussion among scientific peers where the interaction of MPs and ARGs has been detected to cause mutation among the microbial communities, thereby providing them resistance against antibiotic medicines [129]. Biofilm around the MPs facilitates the interaction between the polymers and antimicrobialresistant (AMR)bacteria and helps their transmission [130]. MPs hydrophobic surface provides a suitable spot for the aggregation of bacteria and promotes the formation of biofilm [131]. Stenger et al. [132] studied this phenomenon and reported the accumulation of AMR bacteria such as Flavobacteriaceae, Pseudomonas, Desulfovibrio, Morganella morganii, and Acinetobacter beijerinckii on the biofilms of MPs in the Norwegian sea. The interaction between the two contaminants poses serious effects. The retention time of ARGs has increased significantly in the soil ecosystem, with 0.1% polystyrene [133].

According to Shrivastava [134], heavy metals are widely used in industries such as medicine, toys, food processing, electronics, chemicals, and household applications.. Vehicle exhaust releases heavy metals like lead (Pb), cadmium (Cd), nickel (Ni), zinc (Zn), and iron (Fe) into the air [135]. These heavy metals, which are persistent pollutants in our environment, have shown greater affinity toward MPs [136].. Heavy metal-laden dust on the ground can also enter surface water bodies through precipitation and runoff, carrying these contaminants into nearby rivers or lakes, the author further added.

There are several factors supporting the adsorption of heavy metals onto MPs surface. The higher the specific surface area, polarity, the polymer’s age, and the surface’s polarity influence the adsorption process [136]. Excessive weathering or aging of these polymers due to adverse environmental conditions like abrasion with other material or UV disintegration increases the surface roughness, thereby increasing its surface area. This makes the surface of MPs as a site for anionic activities and attracts cationic metallic pollutants onto its surface [137]. pH and salinity also affect the interaction process, but there is a certain non-uniformity as the mechanism depends from metal to metal. An experiment conducted by Holmes et al. [138] demonstrated the variation in affinity among the heavy metals for the MPs. The experiment included a mixture of river water and seawater. Upon increasing the salinity gradient, chromium adsorption increased, and that of cadmium, nickel, and cobalt decreased. Wang et al. [139] also manifested a similar kind of experiment to check the effect of pH on the adsorption efficiency. The result showed that higher pH generally leads to more metallic ion adsorption on the polymer surface. A higher specific surface area provides more adsorbing ground for metallic ions and thus promotes the interaction [140]. Zeta potential, or the surface charge on MPs determines the attraction and repulsion trend and is a determining factor for adsorbing the metal ion pollutants over it [141]. Lower zeta potential can trigger the adsorption mechanism of metallic ions due to electrostatic attraction [140]. It was reported that the elevated potential of an aqueous solution of arsenic increased the adsorption affinity over polystyrene and polytetrafluoroethylene [142–143].

There are some dire effects of this interaction. Adsorption of heavy metals on MPs is lethal for the growth of the crops. Their combination affects the development of cells and tissues of humans and causes a disruption in enzyme activities in aquatic animals, like zebrafish [144–145].

Microscopy is a widely used physical analytical method in which the small size of MPs is magnified. This method is useful for the identification of texture and structural information of MPs. However, this method has its own limitations. MPs with sizes of less than 100 microns, which have amorphous shapes, unlike the other typical shapes of MPs (fiber, film, fragment, etc.), are difficult to identify using microscopy alone. But, another mode of microscopy i.e., scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) is able to present a highly clear and high-resolution image of MPs. However, the SEM-EDS method is expensive and requires enough amount of time and skilled workers to perform the experiment [146].

Spectroscopy is another technology used to identify MPs, which works on the principle of emission and radiation absorption by any matter. One of the several methods of spectroscopy is Raman spectroscopy. This method involves the knowledge of the vibration of a substance and the scattering of light upon being incident by a light ray [147]. The author further describes the benefits of Raman spectroscopy, which include coverage of a wider spectral range, high-resolution image, higher sensitivity to a non-polar functional group, measurement of particles as small as 1-micron, lower water interference, etc. Furthermore, the sample required for the analysis is also very small. But there are also a few downvotes for this method. For example, the procedure often heats up the sample, which may alter its chemical or physical properties due to using a laser as its light source. The procedure can be affected by fluorescence and has a low signal-to-noise ratio [147]. Although Raman spectroscopy has been extensively used for the identification of MPs, it still has to match the popularity of Fourier-transformation infrared spectroscopy (FT-IR). This state-of-the-art technology is able to analyze the chemical bonds, bond composition, and differentiation between organic and inorganic molecules of MPs and other similar minute particles by generating their unique spectra [148]. Working on the same principles as Raman spectroscopy, FT-IR has its own in-built library, which facilitates the researcher in identifying not only the size of the polymer but also its type [146]. To analyze MPs along with its chemical composition in the environment micro attenuated total reflection FTIR i.e., μ-ATR-FTIR is used [149]. However, this method is not suitable when the size of the MP particle is less than 50 μm [150]. Along with that, sometimes, the pressure produced by the probe tip of ATR may alter and damage the fragile MPs structure [146].

Thermal analysis is another technique that is beneficial in understanding the variation of physical and chemical characteristics of MPs depending upon their thermal stability [146]. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are two such approaches in this category for the identification of MPs. DSC provides the result of the physical characteristics of a polymer, evaluating its variation in transition, crystallization, dissolution, and its respective entropy and enthalpy [151]. TGA studies the variation in the mass of polymer when it is heated and provides qualitative and quantitative insight into MPs by calculating their dependence on temperature and time [152]. Even though both are modern technologies but they have their own sets of limitations. In a study, it was discovered that a combination of DSC and TGA identified the polyethylene and polypropylene but were unable to detect polyvinyl chloride, polyamide, polyester, and polyurethane due to the overlapping phase transition signal [153]. Pyro gas chromatography mass spectroscopy (PY-GC-MS) is another thermal analysis technique used for the identification of MPs by analyzing the thermally decomposed gas from the polymer. Thermal analysis can be used as an alternative to spectroscopy, but these are destructive methods and entail careful observation and required experience [146]. This method is capable of identifying polyethylene, polypropylene, polyvinyl chloride, polyurethane, polystyrene, polyamide, and polyethylene [154–156].

Another novel technology, atomic force microscopy (AFM) coupled with infrared, is a promising analyzing method for identifying MPs. This method uses thermal expansion by absorbing infrared, thereby producing oscillation in the sample. Later, amplitude and frequency are studied based on the oscillation produced [157].

The take-away information from here can be that the identification of MPs depends on various parameters such as their shapes and sizes, their type, composition, polarity, etc. Each technique has its own pros and cons. FT-IR provides detailed chemical composition and polymer type but struggles for particles less than 50 μm. Meanwhile, Raman Spectroscopy gives a detailed resolution for particles as small as 1 μm, but during the identification process, the sample heating can alter the polymer’s properties. On the other hand, PY-GC-MS is excellent with polymer types and their quantification but requires expert intervention to interpret the result. Thus, saying that any single method is superior and the best among all would be incorrect. Rather, to ensure comparability, sampling methods should adhere to standardized protocols, which still need more research, including subsampling and extrapolation techniques, and future work should focus on refining these techniques and standardizing methodologies across studies to enhance the comparability and reliability of MP pollution data globally.

When it comes to the quantification of these contaminants, there are a few methods for that, such as the gravimetry method, mass quantification method, sub-sample method [158]. During visual/ microscopy and FTIR analysis, sub-sampling and extrapolation techniques are commonly used. Given the potentially large number of microplastics on the filters, analyzing or counting every particle is labor-intensive and time-consuming. To streamline this, filters are divided into sections, and subsamples are randomly taken from one or more parts. The microplastics counted in these selected areas are then extrapolated to estimate the total microplastic count on the entire filter [159–160].

The second method is particle mass quantification, which measures the mass of microplastics using various techniques such as thermos-extraction/desorption-gas chromatography-mass spectrometry (TED-GC/MS), differential scanning calorimetry (DSC), FTIR imaging combined with weighing on an analytical balance, and plane array FTIR imaging [161–164].

The third method is gravimetry, which also uses the principle of microplastic mass. However, this method is rarely used since it has been reviewed by only one research study by Sheriff et al. [158]. In this technique, filters are weighed before and after sample filtration and drying. The difference in mass (in 100 mL) is then used to estimate the quantity of microplastics [165].

Another challenge that comes up in front of the research fraternity is the identification of a suitable quantifying unit for MPs. The lack of standardized units for measuring microplastics has been a significant barrier to the comparability of studies. The three commonly used units—items/m³, particles/kg, and mg/kg—each have distinct advantages and limitations. For instance, items/m³ provide particle counts suitable for water-based systems but lack information on mass, which is critical for assessing environmental load. Conversely, mg/kg captures the total mass of microplastics but overlooks the number and morphology of particles, essential for understanding ecological impacts [166]. A few things can be suggested as a first step towards standardizing the measurable unit for MPs. For instance, adopting a dual reporting system by combining mg/kg for mass-based pollution load and particles/m³ or particles/kg for ecological impact assessment. Also, efforts should be made to develop global protocols for sampling and analysis, for example, establishing standardized mesh sizes for particle collection (e.g., 300 μm for initial sieving) and consistent size thresholds for microplastics (e.g., 5 mm upper limit).

Biochar and activated carbon are two extensively used methods to efficiently treat stormwater containing MPs and nanoplastics using their adsorbent surface and porous texture [168–171]. Siipola et al. [170] discovered in an experiment that the removal efficiency of biochar-activated carbon effectively impedes the further flow of PE MPs.

Ultrafiltration, microfiltration, reverse osmosis, etc, are some membrane processes that have been widely used in the last five years [172]. The process involves the combination of a porous membrane and a biological process that can improve the removal efficiency of MPs from the effluent by 99.99% [173]. Out of all the membrane processes, ultrafiltration is considered to be the most capable of removing macro molecules and colloidal particles weighing up to a few thousand Dalton suspended in the system and allows the process to perform under a low pressure of 1–10 bar [172]. The author further reports that the ultrafiltration combined with a membrane bioreactor having a pore size of not more than 0.2 μm can remove 100% of MPs from the sample. Meanwhile, reverse osmosis is a promising budding process used to remove MPs from the wastewater working on high pressure to improve efficiency; however, it creates nanoparticles of plastics by fragmenting them during the process [174–175]. Dynamic membrane technology is another physical process for the removal of poor-settling and non-biodegradable particles from the effluent by quickly forming a second membrane of microparticles. Less resistance offered by filtrate and easy cleaning make it a suitable option to remove MPs from synthetic wastewater under gravity-driven operation [172, 176]. Traditional WWTPs adjoined with granular activated carbon filtration methods have also proven to be reliable for the removal of such micro contaminants. Yet it is still inefficient relative to the other processes discussed above. Wang et al. [13] reported a removal efficiency of 73.7 to 98.5% in removing MPs with a size range from 1–5 μm. Another process, magnetic separation, suitable only for sediment and water samples and not for the effluent from WWTPs, was found to be efficient for the recovery of these contaminants having a size index of 200μm to 1 mm [171, 177].

Other traditional methods under physical treatment are primary sedimentation tanks with grit chambers. The maximum efficiency for such unit was found to be 79% recently by Ziajahromi et al. [178]. For large-size MPs, this process can remove up to 98% of the contaminants from the WWTPs. Filtration using granular media also offers an option to remove MPs from WWTPs where the porous media of the size range 0.45–1 μm is used [172]. However, using such small-sized porous media frequently clogs the media, which interrupts the flow. Therefore, ferrous salt is used in response to the clogging in which the solid particles form flocculate at one place, which is later skimmed off [179].

The activated sludge process utilizes the reintroduction of microbes in a treatment unit of a WWTP. The activated sludge process followed by oxidation ditch or anaerobic tanks has been found to remove MPs from the wastewater with an efficiency of 43%, where the reintroduced microbes degrade the MPs by feeding upon them for the source of their nourishment [180–182]. Biofilm is another process in which a film is enveloped around the surface of MPs, and then the microbes are adsorbed on it and degrade the particles [172]. Degradation of MPs by fungal and bacterial actions is one of the most eco-friendly methods currently being used for MPs’ removal. Different fungi feed upon MPs since these particles serve as a source of their nutrients, finally degrading them permanently [183]. A study by Anand et al. [184] informs the growing trend of utilizing green algae for the treatment of the MPs (especially LDPE and BPA) infested water. The major advantage of this technique is that this process does not require a rich carbon source to grow and can adapt to diverse types of habitats.

The clarification process and biological treatment after the primary treatment of wastewater in a WWTP constitutes biological treatment. Not very efficient in removing MPs from wastewater, and in fact, many researchers have different opinions on the caliber of this process. Sun et al. [185] report the range of removal efficiency of up to 98% if the preliminary and primary treatment has been provided. Another report suggested a minimal removal efficiency range of 2–55% of wastewater [186]. These variations in the removal can be attributed to changes in physical characteristics (size, shape, surface texture), changes in concentration of microbes, and abiotic components such as pH, temperature, etc. [172]. Constructed wetlands is also a natural treatment unit of wastewater infested with MPs. Macro invertebrates such as snails, bristle worms, beetles, etc, present in these wetlands play a conducive role in stopping the accumulation of sludge [187]. Wang et al. [188] highlighted the role of these invertebrates in removing these contaminants where these wetlands were efficient as high as 90%. Treatment by corals, sea clams, ingestion by marine organisms, and zooplanktons are considered to be highly effective and novel biological approaches for the removal of MPs pollution from the marine environment [189].

Removal of MPs by oxidising agents like ozone, hydrogen peroxide, photo-catalytic oxidation, electro-Fenton process, etc., comes under oxidation, where the polymers are mineralized into CO₂ and water [172]. Although these processes are very efficient, they are uneconomical. Treatment using ozone is one of the oldest oxidant-procedure where degradation of polymers has been found up to 90% at a temperature of 35–45°C [190]. The author further suggests the process to be more effective against polyethylene, polypropylene and polyethylene terephthalate. In this process, the hydrogen peroxide and iron (Fe²⁺) react and generate free hydroxyl radicles. These hydroxyl radicles consist of heterogeneous catalyst that targets the organic impurities, such as polymers present in the water environment. After the reaction, the organic impurities are converted into either CO₂, water, or mineralized [172]. However, very low removal efficiency has been reported by this method in which roughly 26% of MPs were recovered from the sample, after 24 hours at different doses of hydrogen peroxide and Fe²⁺ [191]. The electro-Fenton process is a modification of its traditional process, which is still a novice technology where the hydrogen peroxide is generated electrochemically for the removal of MPs, especially polyvinyl chloride. However, not enough scientific reports have been published discussing its efficacy for the treatment of this emerging contaminant [172, 192].

Photolysis and photocatalytic are other advanced oxidation processes used to degrade polymers. The photolysis process utilizes ultraviolet rays on samples infested with MPs for a certain period of time, after which the organic impurities eventually convert into water and CO₂ [193]. However, Brandon et al. [194] questioned its efficacy where the author found a low degradation rate of MPs (polyethylene and polypropylene) when compared with FT-IR. Coagulation is one of the most widely used processes in which aluminium and ferrous salts are used as flocculating agents, and Murphy et al. [195] found a positive correlation between this salt and the removal of MPs via the flocs formed by these flocculating agents. Ma et al. [196] experimented with a new flocculating agent, polyacrylamide, where the removal efficiency reached up to 90% by varying the dose of the flocculating agent between 3–15 mg/L.

Most of the methods mentioned above are for WWTPs because the wastewater acts as a hub for MPs in them. Although, with new progressing technologies coming forth day by day, we still have a long way to go to develop cost-efficient and bio-inspired materials that should be synthesized to cater to this problem. Moreover, investigation and studies should be conducted on the development of a uniform method for the effective removal of MPs from the environment.

As per the report by BGR [234], groundwater caters to the two-thirds population of Germany as their vital and essential resource. As the population grows, the demand will increase, implying that more groundwater will be used for drinking. According to Umwelt Bundesamt [235], groundwater contributes up to 70% of drinking water in Germany. It is astonishing to see that only a dearth of studies has been conducted on German groundwater to detect and analyze MPs. Mintenig et al. [236] confirmed the presence of MPs in groundwater, which was used for drinking purposes. The detected concentration was very low (7 particles/m³), and the dominant shape was fibre. Another study conducted by Weber et al. [237] on German groundwater could not detect any MPs impurities. Although the concentration of MPs in the two studies discussed (details shown in Table 1) did not pose a threat, more research is needed on groundwater for the presence of MPs.

Germany has a vast network of rivers and streams across its land. Danube, Rhine, Ems, Weser, Elbe and Oder are the chief rivers of the country. However, the rivers and their sediments have been gravely infested with MPs, as seen in Table 1.

Roscher et al. [238] report the heavy concentration of MPs in the Weser River water. The contamination was recorded up to 9700 particles/m³. The possible reason was attributed to the estuarian currents that were distributing the MPs at the confluence of the River Weser and the North Sea. Eibes and Gabel [239], in a similar kind of study, reported the presence of MPs in the water of River Ems, where 1.54 ± 1.54 particles/m³ were detected. The weirs present in the downstream impeded the flow of river water which could cause the sedimentation of MPs in the region; the study reported. Another study conducted on the Rhine River by Mani et al. [240] reported the massive MP contamination of 11050 particles/L. The study samples were collected in the vicinity of urban centres and industrial zones having plastic production facilities and air blasting processes along the stretch of the river Rhine. The massive pollution of MPs can be attributed to these locations. Schmidt et al. [241] conducted the analysis of canal water in another megacity of Germany, Berlin, where the author reported maximum contamination of 96 particles/L. The migration of MPs into the canal water could be correlated to the presence of a WWTPs outfall nearby. This is, once again, a strong indication that WWTPs act as the carrier of MPs. In addition to that, the author also stressed the possibility of precipitation to be a chief factor for carrying MPs into the canal water. It can be hypothesized that the stormwater washes off all the MPs of the catchment area and pour it into the canal. 233 ± 36 particles/m³ were also reported in the freshwater river Warnow in north-eastern Germany by Siegel et al. [242]. The sampling site was chosen near a drinking water treatment plant. The author stressed that the flocculants used in the treatment plant were the reason for MPs’ proliferation in the river water. A surprising figure of 30 × 10³ particles/kg dry weight was reported in the hyporheic zone of the river [243]. The reason to choose riverbed sediment was because the sedimentation process can lead to the accumulation of MPs at the bottom. The author of the respective study also stressed the fact that river water can easily infiltrate groundwater or a local aquifer, which can be further used as a source of drinking water for the inhabitants. Various authors have already pressed upon the issue to consider the interface between groundwater and river, as this site can be a potential doorway for MPs transportation towards either direction [244–246]. Rocío et al. [247] explain that benthic zone organisms can transport MPs or can migrate through the hyporheic zone into the groundwater. Depending upon the situation, water from either the entire river or a specific section can flow into shallow groundwater or vice-versa. This flow of MPs from one zone to another depends on various properties such as the shape of the streambed, material heterogeneities, turbulence in the stream, the flow of porewater, etc. A study in Switzerland revealed that around 33% of microplastics infiltrate groundwater from river water, with concentrations increasing from 8 ± 7 MPs/m³ in groundwater to 36 ± 11 MPs/m³ in nano-filtered water in the three sampling campaigns [248]. Thus, it is important to analyse the riverbed as well, which can also act as a sink and gateway for MPs. The author, once again, focused upon the involvement of WWTPs as a potential carrier of MPs as they are not designed to remove particles of such small size. Dierkes et al. [249] also reported a minuscule trace of 0.028 ± 0.006 mg/g of MPs in Rhine water. A freshwater stream in Varel was investigated by Stolte et al. [250]. The samples were taken and filtered from the site which was close to the vicinity of a paper recycling plant. The author discovered a moderate concentration of 19 fibres/L, which was still higher than the other sampled sites, beach sediments, and seawater. The effluent from the paper recycling plant contributed to the MP’s fiber and flake pollution to the freshwater stream the study comprehended.

Mani et al. [251] studied the 20 km stretch of Rhine between Cologne to Duisburg, where the author discovered a maximum concentration of 9.2 particles/m³. The presence of MPs in the river water was attributed to direct industrial discharge and the three communal WWTPs located along the stretch of the study area.

So far, it can be posited that rivers carry a large fraction of MP pollution with them, acting as a source and sink for these contaminants.

Fig. 6 provides a visual representation of the geographical distribution of microplastic studies conducted across Germany. The map highlights a significant disparity in research coverage, with a concentration of studies in the western regions and a notable scarcity in the central and southern parts of the country. While some research has been carried out in the eastern regions, particularly in major cities like Berlin and Rostock, the western region remains the primary focus, largely due to the presence of significant water bodies such as the Rhine River.

Despite the Elbe River traversing the eastern part of the country and major urban centers being situated along these rivers, the limited number of studies in these areas is concerning. The central region of Germany presents an even starker gap, with minimal research identified, and the southern region, home to major cities like Munich, similarly lacks substantial studies. This distribution underscores the urgent need for expanded research efforts to address microplastic pollution across these underrepresented regions.

The studies depicted in Fig. 6 span various aquatic environments, including groundwater, household tap water, wastewater treatment plants (WWTPs), river sediments, and surface water.

Germany has an approximately 3700 km long coastline, with the North and Baltic Seas. Moreover, Europe’s two major rivers, Rhine and Danube, flow through Germany. Rivers have always been considered as a vector for MPs, and European rivers have been detected with MPs traces as small as 0.03 particles/m³ up to 187000 particles/m³ [251–252]. Talking about the German coastline, the north and north-west states of Germany contain 1000 companies of plastic alone and, let alone the other industries, several municipal and industrial WWTPs [251]. North Sea and Baltic Sea have also been found to be polluted with MPs. Adding more to it, Germany is the biggest contributor of polyethylene to ocean debris, in terms of absolute mass, in the whole of Europe, and the amount ranged up to 24,461 tons [211]. Mani et al. [251] reported that the Rhine River delivers almost 10 tons of MPs toward the North Sea with it. Dibke et al. [253] corroborate this claim in the study where it was reported that the concentration of MPs in the North Sea, as mentioned in Table 1, reached up to 1396 mg/m³. The author listed several potential reasons that could cause such heavy contamination in the region. Firstly, the area witnesses so many offshore and recreational activities. They can be cruising, fishing, scuba diving, etc. Fishing gears contribute majorly to the MPs’ pollution in marine water. The next reason stated by the author was that the sampled sites were among the busiest shipping routes of the world. The cargo can be a potential proliferator of microcontaminants. The German Bight and the surrounding areas have a number of ports and harbours. The area is also visited by millions of visitors. Thus, anthropogenic activity can lead to spreading of MPs as well. Roscher et al. [238] reported a concentration of 23 particles/m³ in a study conducted at the margin of the North Sea and the estuarian region. However, the same study reported a relatively higher concentration of MPs in the estuarian zones, and the variation decreased towards the sea.

Baltic sea is one of the most studied marine region and many authors have confirmed the presence of MPs [254–256]. In a similar way, Stolte et al. [250] confirmed the presence of MPs with a concentration of 3.3 fibres/L which is relatively lower than the concentration obtained from freshwater in that same study. The author corelated this contamination chiefly to municipal WWTPs, industrial discharges, off-shore recreational activities and anthropogenic activities.

It is not a hidden fact that the marine waters are the hot-spot of MPs, yet very few studies have been conducted on German coasts for the identification of MPs. More studies would certainly benefit the researchers and policy maker to assess the situation and respond in an appropriate sense.

It is not unknown that WWTPs receive MPs’ load every single day from domestic and industrial outlets abridged the path between domestic and industrial wastes and rivers. So many authors have already reported that the WWTPs act as the biggest source and sink for the MPs [261]. Currently, in Germany, there are roughly 9100 WWTPs, and every year, these WWTPs emanate close to 7.3 × 10¹² particles/year, on average, into the river [241]. Table 2 will provide a list of studies conducted on German WWTPs to detect the concentration of MPs emanating from them. Likewise, Dierkes et al. [249] conducted a study in two WWTPs of the Bavaria region catering to a population of 70000 and 170000, respectively. The study revealed that a massive concentration of MPs (3.3 ± 0.3 mg/g) were recovered from the sludge of WWTP catering to a population of 170,000. Clearly, it can be judged that population affects the influent and effluent load of MPs in WWTPs. Mintenig et al. [261] comprehensively examined twelve WWTPs and discovered a large concentration of 9000 particles/ m³ being discharged by a WWTP at Holdorf. The study also revealed that the detected MPs were predominantly composed of polyester and purported that the WWTPs can act as a source and sink for these micro contaminants. An interesting takeaway of this study was about a tertiary WWTP out of all the studied, which was installed with a post-filtration unit. This unit helped to reduce the concentration of MPs’ discharge by 97%. A systematic comparison was observed by Wolff et al. [262], where the average concentration of MPs was compared between dry and wet weather days of a WWTP having an effluent rate of 10,000 m³/d serving a population of 98,500. The concentration of MPs discharged by the WWTP in wet weather days was almost two folds than in dry weather days. While the wet weather days discharge concentration for MPs stood up to 5900 particles/m³, on the other hand, only 3000 particles/m³ were recovered during dry weather days. Residential waste entering into the WWTPs could be the major reason for the recovery of MPs in the effluent. Altmann et al. [263] conducted a study on an urban WWTP, which showed an enormous concentration of 2.2 μg/mg dry weight. Bitter et al. [264] conducted study on four German municipal WWTPs which showed high concentrations of MPs in their effluent. 19.6 μg/L of MPs were recovered from the WWTP, serving a population of 70,000 having an outflow discharge rate of 10,300 m³/d. Out of all the studied WWTPs, WWTP installed with a post-treatment pile cloth media filter unit (PCMF) showed the best removal efficiency of MPs from its system. The removal rate of >94% of MPs was observed at the WWTP. The WWTP, which had a removal unit of continuous up-flow granular activated carbon filter was found to be least efficient for the removal of MPs as its removal rate was only 2%, roughly. This study also mentioned the use of DSC for the analysis of MPs in the sample and thoroughly mentioned its swiftness and user-friendly functions for analysing the samples.

Although Germany has made strides in addressing plastic waste, critical research gaps remain in understanding microplastic pollution in its aquatic environments. For instance, studies on rivers like the Rhine have reported alarming concentrations of up to 11,050 particles/L, predominantly fibers and polymers like polyethylene and polypropylene. However, limited investigations into groundwater contamination, such as a study showing up to 7 particles/m³ in aquifers, highlight the need for broader spatial and temporal coverage. Wagner et al. [267] has already mentioned the wide presence of MPs in the European freshwater system. At the same time, Kye et al. [268] has cited Annex XV restriction report for intentionally added microplastics by the European Chemicals Agency that the granule type of MPs was heavily found in industrial wastewater other than fibers. Meanwhile, limited studies on MPs in the German aquatic system and the associated gaps restrict our ability to fully comprehend the pathways and accumulation mechanisms of microplastics, particularly their transition from surface water to groundwater, which serves as a major drinking water source. Without detailed knowledge of these interactions, devising effective mitigation strategies becomes challenging, leaving ecosystems and human health at continued risk.