Raman spectroscopy for microplastic detection in water sources: a systematic review

The increasing demand and use of plastics in our daily lives have caused an increase in microplastics (MPs) concentration in water bodies. Increasing MP in water affects aquatic life and is associated with several health issues. All sources of water whether fresh, marine, or sewage have reported the presence of various MPs. It is clear from relevant literature that the presence of MP with a particular chemical composition could be indicative of its source and could contribute to its removal. Increasing population density, plastic litters, fishing activities, and industrial wastes are major contributors of MP in water. This review is systematically undertaken where Raman spectroscopy (RS) is used as an indispensable tool to identify the chemical composition of the MP in various water sources (fresh/ground/drinking; ocean/sea; waste/sewage) between 2015 and 2021. Based on the Raman spectra, polystyrene (PS), polyethylene terephthalate (PET), polyethylene (PE), and polypropylene (PP) are some of the common MP identified in the water sources.


Introduction
Plastics are semi-synthetic or synthetic polymeric materials that are light in weight and can be molded into any form or dimension. It has been referred to as a boon to humanity since its invention in the early 1900s and has since replaced metal in a wide range of applications. Because of its low cost and resistance to harsh environmental conditions, it has become the preferred material for the manufacture of bottles, bags, buckets, pipes, furniture, nets, and even clothes which have become commonplace in daily life (Thompson et al. 2009). Plastics are mostly non-biodegradable and take hundreds of years to naturally degrade. Overproduction of plastics and their indiscriminate usage have resulted in the Editorial responsibility: Samareh Mirkia.
1 3 production of millions of tonnes of waste each year, which often end up in the environment including air, landfills, water bodies, and seas. Furthermore, appropriate human actions such as improper disposal behavior, inadequate recycling, and dumping waste into the ocean have been a matter of great concern (Eriksen et al. 2014). Owing to the impact of weathering, these plastics that persist in soil or water bodies often degrade resulting in the creation of tiny plastic particles known as microplastic (MP). MPs are particles with a size less than 5 mm and are found to be present in both marine and terrestrial environments (Scheurer and Bigalke 2018). MP has recently been discovered inside the human placenta, making it a global problem of the twentyfirst century (Ragusa et al. 2021). MPs in water bodies, in particular, are a major source of concern because they frequently enter the food chain, eventually making their way through our foods and beverages. Figure 1 depicts the typical mechanism for the generation of MP in water bodies and their return back to humans. MP has since been found to mess with human physiology, affecting health conditions (Smith et al. 2018). MP with different chemical compositions from various materials enters water sources as a result of a variety of human actions. Personal grooming products, plastic litter such as bottles, and shopping bags are among the most common of these items. Laundry and fishing are two other main activities that lead to the increase in MP emissions in water as small, microscopic fibers detach from products during such process (Cesa et al. 2020). Other MPs, such as the tiny plastic beads used in exfoliating cleaners in detergents and cosmetics, are purposefully produced. As a result, it is important to determine the existence of MP on a large scale by looking at their abundance, length distribution, and chemical composition. There are several methods used for identifying the chemical composition of MPs including density gradient separation with subsequent elemental analysis, pyrolysis-gas chromatography in combination with mass spectrometry, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy (RS), etc. Many of these approaches have shown that they can be used to identify MP in a variety of situations. And most of these techniques, however, have limitations when it comes to the size of MP that can be detected (Koelmans et al. 2019). Alternatively, RS has demonstrated the ability to detect MP of sub-micron dimensions and has emerged as a key Fig. 1 The diagram depicts the activities that contribute to the generation of MP in water bodies and their movement across food chains 1 3 technology in this field. RS detects molecular vibrations which are represented in the Raman spectrum as the fingerprints of chemical structure facilitating the identification of the sample with high specificity. Thus, such identification can provide hints about the origin of plastic debris. Some of the benefits of RS include non-destructiveness, molecular characterization with sub-micron resolution, low sample requirement leading to analysis with small sample volumes, rapid (information obtained in the order of seconds), and practically no interference from water during sample characterization (Ribeiro-Claro et al. 2017). RS is a certified and potent means for figuring out the chemical composition of MP. With the increasing number of research papers involving RS for MP detection in water bodies, it's critical that recent advances in the field are organized in a systematic way to highlight the field. This systematic review serves as a summary of the use of RS in the detection of MP in various water sources namely marine, fresh, and wastewater spread around the world in different countries.

Search strategy
Searches were performed in the two databases Scopus and Web of Science (WOS) in April 2021. Three search terms were used during the search: "Raman" and "Microplastic" and "Water" in the field of title, keywords, and abstract. The options for "Advanced search" were utilized to exclude reviews, book chapters, letters, reports, conference proceedings, and foreign language articles. The search was restricted to studies dated from the year 2015-2021.

Inclusion criteria
Original research articles dealing with detection and qualitative characterization of MP in water sources including, ocean/sea, fresh/ground/surface, bottled, and wastewater using RS are taken into consideration.

Exclusion criteria
Reviews, book chapters, letters, reports, conference proceedings, and foreign language articles were excluded using the "Advanced search" filters in both databases, Scopus and Web of Science (WOS). The title, keywords, and abstracts, and full texts were screened again to exclude studies that did not meet the inclusion requirements. Further, those studies that did not use detailed RS for qualitative MP analysis or mention a country, type of water source (fresh/ground/drinking; ocean/sea; waste/sewage), or the chemical composition of MP identified by RS were excluded. Recently, Dąbrowska et al. 2021 described Raman analyses as the efficient and standard method for the study of microplastics characterization in marine environments (Dąbrowska, 2021). However, the authors did not mention any source of water. Hence, such works were not included in our study.

Study selection
The authors reviewed the title and abstract of the studies followed by a full-text screening to identify the studies based on the inclusion criteria. Initially, 319 studies (213 studies from Scopus and 106 studies from WOB) were identified by searching the above-mentioned databases based on the search keywords. These obtained articles did not include any reviews, book chapters, letters, reports, conference proceedings, and foreign language articles which were excluded using "Advanced search" filters. A total of 79 duplicates were removed following which 240 studies were shortlisted. Further, from the 240 studies, 186 studies were excluded based on the exclusion criteria (Sect. 2.3), and 54 studies were shortlisted by screening and analyzing the title, keyword, and abstracts of the articles. Finally, 40 studies that met the inclusion criteria (Sect. 2.2) with the required data were selected and systematically reviewed.

Data extraction
From the studies meeting the inclusion criteria, data were recovered including the country, type of water source (fresh/ ground/drinking; ocean/sea; waste/sewage), and the chemical composition of MP identified by RS.

Results and discussion
In the end, a total of 40 studies were chosen for data extraction and analysis in this review. The workflow for selecting eligible articles is depicted in Fig. 2.

Detection of MP in water using Raman spectroscopy
At scales beneath 100 microns, visual characteristics alone cannot tell whether a particle is composed of plastic, rubber, stone, glass, or organic matter. Therefore, one must use a characterization technique that yields chemical information about the investigated samples. RS has been recognized as a useful tool to identify the chemical composition of such microparticles, MP, and characterize them. RS is a vibrational spectroscopy method based on inelastic light scattering that provides information about detailed molecular structures. Briefly, laser light with a specific wavelength is used as an excitation source. Typically, inelastically scattered photons with longer wavelengths (called stokes Raman scattering) from the sample are collected. Depending on the molecules under study, collected Raman photons contain a mixture of various wavelengths which is then dispersed using a grating/prism and detected using a sensitive camera to produce a Raman spectrum. Raman spectrum gives characteristic vibrational frequencies such as bond stretching and bending which is unique to each molecule, thereby generating a molecular fingerprint spectrum. These quantized modes within the molecule have different energies, leading to peaks in the Raman spectrum that can span anywhere from 0 to 4000 cm −1 (Adamson 1991). Sampling options for Raman microscopy are generally trivial and do not require extensive pre-processing. The key considerations for Raman systems are around the choice of laser, which affects signal strength and sample fluorescence (Wahadoszamen et al. 2015). RS utilizes sub-micron wavelength lasers as it can resolve particles smaller than 1 micron. Several improvements to RS have been made as the technique has progressed, to collect more chemically applicable data. Micro-RS (μRS) is one such technique, which uses a combination of a Raman spectrometer and an optical microscope to measure spatially resolved Raman spectra and generate chemical images of samples. Such images allow visualization of molecular heterogeneity at sub-micron scales in analyzed samples. Further, as the low-energy vibrational domains are often missed in RS, surface-enhanced Raman scattering (SERS) spectroscopy is used in this regard to detect more efficient Raman signals. This technique involves adsorbing the sample to a plasmonic metallic surface (gold or silver) or nano-structured substrates. The plasmonic oscillations of the surface electrons to boosts the inelastic Raman scattering signal of the sample (Zhou et al. 2021a, b).
RS has been used for the detection of MPs in various water samples from around the world (Fig. 4). The Raman spectra of commonly occurring MPs in water sources are shown in Fig. 3. Additionally, the Raman fingerprints along with their assignment of MP are mentioned in Table 1. These Raman fingerprints have been used as a library to compare and identify the chemical composition of MP sampled from various water sources. Likewise, there are also other databases available like KnowItAll® Raman Library by Wiley for rapid spectral analysis and MP identification.
MPs are known to have adverse effects on human health. Most MP consumed via drinking water is likely to pass by the digestive tract without being absorbed. Therefore, gut and oral tissues are more likely to show effects like irritation and inflammation due to exposure to MPs. PE induces ROS (reactive oxygen species) generation and has proven to have a cytotoxic effect in both T98G and HeLa cells (Schirinzi et al. 2017). A recent study also exhibited that PS acts as an immune stimulant that induces cytokine and chemokine production. It is shown to be toxic to cells, causes oxidative stress, and affects membrane integrity and fluidity. It also disrupts the mitochondrial membrane and inhibits the plasma membrane ATP-binding cassette (ABC) transporter activity. Recent studies show that PP of the smaller size (20 µm particles) shows signs of cytotoxicity and ROS induction at high dosages (Hwang et al. 2019). Therefore, the following sections deal with the chemical compositions

MP detection in ocean/seawater
Oceans and seas are vast bodies of water that occupy about 71% of the planet's surface [35]. They are home to a diverse range of animals, plants, and other species and assist in the maintenance of people's livelihoods by providing them with food, water, and other necessities. Plastics have been liberally littered around in oceans and seas since its discovery, making it the primary cause of pollutants in these bodies of water (Haward 2018). Since these plastics are non-biodegradable in nature, they take a long time to degrade and, because of the continuous impact of weathering in oceans, MPs are generated. MPs have been shown to infiltrate the food chains of aquatic animals posing a threat to their biological functions (EFSA Panel 2016). As a result, it is critical to keep a watch on MP in the oceans and seas in order to limit their effect. This section focuses on using RS to accurately detect MP in sea and ocean water.
The most visible MP in coastal surface waters surrounding the subtropical island of Okinawa, Japan, were of PE, PP, PVC, PA, and PS in origin with sizes varying from 1.38 to 47.8 μm. PE was the most common form (representing around 75% of the overall MP found), and the sources of these MPs were found to be from anthropogenic activities along the sea, polymer bags, damaged fishing nets, and household materials (Ripken et al. 2021). Another study demonstrated that the sea surface water along the Guangdong coastal areas of the South China Sea had an MP abundance of 850-3500 items/L. MP of the fiber type was more prevalent in this area than fragments and pellets. Furthermore, various colors of MP objects were detected with translucent forms being the most noticeable (62.5%). Rayon was the most common polymer form found at all the sampling sites according to RS. Polyethylene terephthalate, ethylene/vinyl acetate copolymer, polyacrylamide, and cotton particles were also found which are commonly used in packaging, electrical equipment, and construction materials (Li et al. 2021). The majority of the MP particles found along the Mediterranean Sea's Lebanese coast were red and blue and were made of polymers such as PP, PE, PS, PE, PET, and PUR, as well as PVC and polylactic acid. MP concentration in surface water was 4.3 items/m 3 , mostly particles, fibers, and microbeads that are assumed to enter water bodies from household refuse, water drains, and poorly maintained landfills. Furthermore, the study showed that the high abundance of MP in European anchovy (Engraulis encrasicolus) and spiny oyster (Spondylus spinosus) populations is a significant concern for the food chain (Kazour et al. 2019). The study in Admiralty Bay, Antarctica showed an abundance of 2.40 (± 4.57) microfibers/100m 3 of size range 10-22 μm. The microfibers were found to be mostly made up of polyethylene glycols, polyurethanes, polyethylene terephthalates, and polyamides, which are thought to have arrived at the study site through water current from nearby high-population areas, as Antarctica is devoid of any factories or human population. As a result, there is concern that these MPs can move through the water reaching the most inhabited places (Absher et al. 2019). Studies have reported the presence of MP particles within various products of human consumption including fish and shellfish as well as in the environment including oceans, soils, and the atmosphere worldwide. SERS successfully detected PS in aqueous solutions based on key peaks of interest accredited to ring vibrations (i.e., v(C-C) near 1002 cm −1 and β(C-H) near 1032 cm −1 ) of the benzenes within the polymer backbone. Similarly, vital peaks for PET in the Raman spectra are present at 1615-1620 cm −1 and 1730 cm −1 due to the ring breathing and carbonyl stretching, respectively. For all PS particles, the primary peak of interest is present at 1002 and 1032 cm −1 (Caldwell et al. 2021). Alternatively, the use of machine learning techniques for spectroscopic data analysis has allowed for real-time object analysis. For automated detection of MP in a marine water sample, a portable Raman device with extra tree algorithm was utilized. The machine was able to distinguish and classify a variety of MP including PS, PMMA, PET, PE, PA, PVC, and PP  (2019) Polycarbonate ( Vibration of Amide I band Lee et al. (2015) with a selectivity of 98.82 percent (Yang et al. 2020). Even though Raman spectroscopy showed versatility in detection of MP, sensitivity and fluorescence interference in the method still remains a limitation in the detection of MP.
As the Raman signals from these plastics are sometimes weak, conjugation into active Raman surfaces has shown to enhance detection ability. SERS was used for the detection of MP in aquatic environments. Silver colloid was used as the liquid SERS substrate to enhance Raman signals along with sodium chloride as the aggregating agent. Based on the Raman intensity at 1026 cm −1 , the method showed detection of polystyrene particles of ~ 100 nm size down to the limit of 40 μg/mL in seawater samples (Lv et al. 2020). In another study, Klarite was used as the SERS substrate for the detection of MP. Klarite contains a dense grid of inverted pyramidal cavities of gold which promoted strong focusing of the laser beam onto the MP and enabled detection of sub-micron-sized particles. This method rapidly detected the presence of polystyrene and PMMA in water sample based on the SERS intensity at 1003 and 1453 cm −1 , respectively . Nanopore AuNSs@Ag@AAO SERS substrate embedded with nanostar dimers was used for the identification of MP in water samples. The method was able to detect PS particles of 0.4 μm at 50 ppm (Lê et al. 2021). It is understood that Raman detection methods enable only chemical characterization of the MP and not visualization. Therefore, alternative methods for quickly visualization and characterization of MP would be advantageous and rapid for MP studies. There are recent studies where a combination of holography and RS was used for the detection of PS and PMMA. A laser beam of 785 nm was used for holographic image and Raman spectrum of the MP in a large volume of the water sample, thereby providing structural and chemical quantification simultaneously (Takahashi et al. 2020). Further, staining MP with Nile Red, aided in quick mapping and chemical identification of MP. The 442 nm laser of μRS produces Nile Red luminescence, which can be used to identify different suspected MPs as well as to selectively characterize them (Kang et al. 2020).

MP detection in fresh/ground/tap/packaged drinking water
Fresh/ground/tap/packaged waters are the most popular sources of drinking water. The majority of these waters which come from natural sources are treated before consumption. Plastic pollution is becoming more prevalent, and its presence has been detected in most water bodies. MP made from these plastics has been shown to contaminate almost all water sources (Koelmans et al. 2019;Pivokonský et al. 2020). As a result, it is important to detect its presence in drinking water supplies to protect human health. As discussed further in this section, RS was found to have an advantage in the rapid detection of MP from water sources. Two studies indicated that PP and PE are quite common sources of MP in freshwater sources in China. The high degree of MP pollutants in lakes typically takes place in the vicinity of severe human activity. PP and PE had been broadly utilized in enterprise and everyday life. Laundry wastewater can be a huge supply of nylon particles, because nylons are broadly utilized in textiles, including garments and ropes. The substantial fisheries activities might also additionally result in big quantities of nylon particles, as they are the principal element of fishing tools (Yuan et al. 2019;Pan et al. 2020). In the Ciwalengke River, Indonesia, polyester was the most plentiful MP, derived from the shredding of clothing from textile industries and the laundry activities of citizens. The rivers in Tibet Plateau have fewer MP than more populated and advanced areas, and many common sources of MPs are not present here. PET and PS are common in apparel and water bottles, colored fibers, meal packaging, furniture, prescription drugs, and private care products. However, studies have demonstrated that standard RS is unable to identify PS particles of size 1 µm or less. Hence, SERS was used to detect PS nanoparticles (smaller than 1 µm) based on the characteristic bands at 616, 793, 1028, 1152, 1455, and 1600 cm − 1 , consistent with the ν6b radial ring-stretching mode, ν1 symmetric ring-stretching mode, ν18a tangential C-H bending mode, ν15 mode, ν19b or δ(CH2), and ν12 C-C stretching mode, respectively (Zhou et al. 2021a, b). A small variety of MP debris was recognized as PP or PA, which might be used within the manufacture of diverse fabrics and bags (Jiang et al. 2019). PMMA was detected in streaming tap water in Germany. PMMA is a synthetic resin, used as a substitute for glass in various products (Kniggendorf et al. 2019). PE and PP are two of the most highly demanded polymers in Europe for activities like packaging, construction, and agriculture. All these activities are responsible for the presence of MPs located in Lake Tollense, in Germany (Tamminga and Fischer 2020). A study has proven the presence of MP in drinking water in Germany that occasionally enters from the supply network (Weber et al. 2021). However, next to Germany, water bodies in Poland also show a high level of MP in its water bodies like the Vistula River. Studies show that MP abundance is also affected by atmospheric conditions (precipitation and wind), increased surface runoff, and water level. Further, population density and proximity to city and business areas also affect the MP abundance in the rivers. (Sekudewicz et al. 2021). µRS was used to detect the presence of MP in packaged mineral water in glass bottles, plastic bottles, and beverage cartons. Since micro-Fourier transform infrared spectroscopy (m-FT-IR) was unable to detect particles smaller than 20 µm, micro-Raman was found to be the most suitable in detecting the chemical composition of MP in the low micrometer range. Most common MPs in water from plastic bottles were categorized as PES: (PET; 84%) and polypropylene (PP; 7%) indicating the chemical composition of the bottles and the caps used for packaging. In the water from breverage cartons and glass bottles, polyethylene or polyolefins were found indicating that the cartons are coated with them. Additionally, it was also observed that recycled bottles contained more amount of MP than those which were single used (Schymanski et al. 2018). The prevalence of MP in tap water samples was studied in different cities of China using micro-RS. The amount of MP in the tap was 440 ± 275 particles/L with most of the particles below 50 μm in size and of various shapes such as fibers, fragments, and spheres. RS identified 14 different types of MP, the most common being polyethylene and polypropylene (Tong et al. 2020). In another study, an in-line system was developed complemented with RS for detection of MP in tap water. The system contained perfluorohexane filter which effectively captured the flowing MP with a recovery of 95.9% and a wide-area illumination laser source was used for illuminating the particles. Based on the intensity ratio of PE and perfluorohexane peaks, the amount of PE MP in water was measured in real-time (Cho et al. 2021).

MP detection in waste/sewage water
Human activities such as the use of toilets, washing clothes and utensils, as well as various industrial activities, all produce wastewater. These waters contain a broad range of pollutants including bacteria, heavy metals, and MP. Since MPs are mostly micron to sub-micron in size, they are often left unremoved or only partially removed using current treatment methods. As a result, it's critical to detect and characterize MP in wastewater and at various stages of water treatment to effectively remove them and avoid secondary pollution (Prata et al. 2018). The use of RS to monitor MP in wastewaters has proven to be efficient, and its applications are discussed in this section.
In Israel, untreated wastewater was found to have ~ 65 items/L among which the most dominant form of MP observed was in fiber form and black color. Micro-RS revealed that the particles were mainly PE, PVC, PP, PC, PTFE, PS, PUR, PA, and polyolefin elastomer. Apart from these, some cellulose-based particles were also observed. Another important finding from the study was that secondary treatment (use of microorganisms to biologically remove contaminants from wastewater) significantly reduces the amount of MP by 88.2%. However, tertiary treatment did not offer any significant reduction in the amount of MP. Additionally, it was found that fibers smaller than than 20 μm diameter are less prone to removal during secondary treatment (Ben-David et al. 2021). In China, plastic polymers such as polyesters, polyamide, polyethylene terephthalate, and polyethylene made up about 89.5% of the MP detected in wastewaters in Harbin region. The influent water contained about 126.0 ± 14.0 items/L in the sizes of 0.02-0.1 and 0.1-0.5 mm. The MPs were mainly observed to be in fiber and fragment forms which may have been derived from personal care and household products. Moreover, it was also observed that a significant amount of these MPs during the waste treatment process is left behind in the sludge, which when later used as fertilizer leads to secondary pollution of MP (Jiang et al. 2019). Fiber, fragment, and microbead forms were the most common types in wastewaters in the Wuhan region of China with abundances of 23-80 items/L. PVC was the most popular type of MP whereas the microbeads found were mainly made of polyethylene PE, PP, EVA. It was attributed that the prevalence of microbeads in toiletries, as well as household wastewater, were among the sources of MP (Tang et al. 2020). In another study at Changzhou, China, the abundance of MP in domestic, industrial, agricultural and aquacultural wastewater was found to be 18-890, 8-23, 8-40, and 13-27 items/L, respectively. Nearly 83% of the overall MPs were made up of polyethylene PE, PP, and PS. The most common shapes were fragments and film, and the majority of MPs were observed to be smaller than 500 μm . Further, in South Korea, the most evident form of MP in wastewater was polyethylene (PE) and polypropylene (PP) in the size range of ~ 20 to ~ 1000 μm (Nguyen et al. 2021). Whereas in Virginia, USA, about 95% of the MPs in wastewater were made of polyethylene, others include glycerol monostearate, styrene-ethylene-butylene copolymer, etc. as a result of industrial discharge (Fortin et al. 2019). Additionally, it was also observed that Raman microspectroscopy has a detection limit of 1 μm. Many MPs, on the order of millions per m 3 were found in a wastewater receiving stream in Argentina. MP with size of fewer than 500 μm accounted for 56% of the total, while fragments with sizes in the ~ 10 μm account for 44%. This study highlighted that freshwater sources are getting contaminated with MP from wastewater (Montecinos et al. 2021). Further, the presence of MP was detected in advanced wastewater treatment systems such as activated carbon filtration and reverse osmosis nanofiltration systems using Raman microspectroscopy. The system was able to detect MP as small as 1 μm and also provided removal of background signals from organic matter present along with MP samples (Fortin et al. 2019). Apart from studying samples from the wastewater treatment plant, Raman imaging has been used to investigate wastewater released from various commercial activities. This method helped in both structural and chemical quantification of the MP. The wastewater collected from the car service center showed that the car body and engine polishing waste contained billions to trillions of MP in the size of 7 μm down to ∼200 nm mostly dominated by PS and polyacrylic particles (characterized based on peaks at 1000 and 1 3 2850 cm −1 , respectively) (Sobhani et al. 2020). Similarly, wastewater generated from eyeglass polishing was found to have a large amount of MP. One liter of such wastewater was found to contain 1380-62,539 mg MPs in the size range of 1.2-600 μm. RS found that MP was mainly polystyrene and PMMA origin based on their specific Raman peaks at 814 and 1001 cm −1 , respectively (Lee et al. 2021). In an alternative approach to monitoring contamination of other water sources by wastewater effluent, Domogalla-Urbansky et al. studied freshwater bivalves. It was observed that bivalves exposed to contaminated wastewater for six months had 2597 particles < 50 μm and 454 particles > 50 μm as detected by Raman microscopy. Polypropene and PVC were found to be the most abundant type of MP in the bivalents with their size reaching up to 1 μm (Domogalla-Urbansky et al. 2019). The commonly used methods for isolation and detection of MP from wastewater are often challenging and tend to contaminate the isolated MP with organic matter. An alternative approach proposed by Sujathan et al., involved a heat and bleach method followed by imaging using confocal Raman microscopy. This method shows a 78 ± 8% efficiency in the detection of MP and detected items as small as 20 μm size with increased efficiency (Sujathan et al. 2017) ( Table 2).
Identification of chemical composition helps in understanding the prevalence of MP in a particular region and strategizing its removal and preventing its ill effects. Figure 3 depicts the common MPs identified in countries included in the study between 2015 and 2021.

Prevention and removal of MPs from water bodies
Although the identification of MPs can be confirmed using Raman spectroscopy methods. The main challenge is attempting to prevent and removing MPs from bodies of water. The government is handling the hassle of MPs and in the subsequent years more significant measures for pollution prevention, like constrained use of plastic bags, plastic bottles, and different plastic substances must be implemented (Picó et al. 2019). Several common methods such as carbon block filters, reverse osmosis, and distillation filters known to remove MPs from water. Carbon block filters are known  Ben-David et al. (2021) to be the most efficient ones that can remove the major content of bioplastics from water. Because reverse osmosis can remove particles down to 0.001 micron, it can remove all known microplastics, however, it is more expensive and requires maintenance. Distillation filters are also known to provide pure water and also filter microplastics. Further, water treatment plants can also efficiently remove MPs from water. The initial coagulation and sedimentation at a drinking water treatment plant can remove 1.8 to 54.5% of MPs, and advanced treatment eliminates approximately 88.6% of MPs (Tang et al. 2021). The common methods of MP removal from water are exhibited in Fig. 5.
Recently, ferrofluid, a nontoxic magnetic liquid consisting of oil and magnetite (iron oxide powder) was used to separate MPs from water. The ferrofluid attracts the microplastics because of the nonpolar properties of both.
However, the removal of MPs may become more efficient if the removal methods are specific to the type of MPs found in a particular water body or source. Several studies have been conducted that study the removal of specific MPs from water. A study recently reported that polyaluminum chloride (PACl) isn't always able to successfully eliminate virgin polypropylene MPs from water, thereby exposing humans to eco-toxicological impacts of PPMPs through tap water. Another study exhibited the efficiency of PAC and FeCl3 coagulation in removal of polyethylene (PE) MPs from water. It was also reported that PAC exhibited better coagulation properties than FeCl3 (Zhou et al., 2021a, b). Further, the efficiency of anionic polyacrylamide (PAM), sodium alginate (SA), and activated silicic acid (ASA) when using polyaluminum chloride (PAC) to remove polyethylene terephthalate (PET) MPs was studied. The experimental results showed that ASA had the highest removal efficiency (54.70%) under conventional dosage, while PAM achieved the best removal effect (91.45%) at high dosage . The effects of water composition and temperature on the agglomeration-fixation reaction of microplastics using organosilanes were examined. The expulsion of MPs with various chemical compositions such as polyethylene, polyamide, polypropylene, and polyvinylchloride, polyester from biologically treated municipal wastewater, seawater, and demineralized water was investigated. It was observed that the polarity of microplastics and organosilanes had a significant impact on MP removal efficiency . Organosilanes' organic groups can be chemically adapted to various types of MPs (Sturm et al., 2021).

Conclusion
Disposal of plastics into the immediate environment has led to high environmental risk putting the life forms at stake. The presence of MP has been observed to reach all the corners of the earth including the pristine regions like the Arctic and Antarctica (Absher et al. 2019;Lusher et al. 2015). Thus, it is important to study MP and how they spread from one region to another over time, to deeply help in mitigating this problem. Comprehensive studies can help enhance and expand current waste disposal and treatment regulations for coping with and eliminating MP. There are many challenges related to the investigation of MP pollution. These particles being in the micrometer order and transparent often skip the visual detection method using light microscopes. Thus, alternate methods such as spectroscopy are looked into as they can determine the chemical composition of contaminants regardless of their size. This article discusses the applications of RS in the detection and identification of MP in three different sources of water such as sea/ocean water, freshwater, and wastewater. RS has demonstrated great ability in the identification of wide varieties of MP of different shapes, sizes, and colors, irrespective of the location of the study and water sources. Seawater showed the source of these MPs depended on the nearby industrial, shipping, fishing, and commercial human activities on the seaside. The presence of MP was observed in almost all the sources of freshwater such as rivers, lakes, ponds, groundwater sources, and even packaged drinking water bottles. RS revealed that the most common MP sources in rivers, lakes, and ponds were PET, PE, PP, and PS, mainly caused by fishing, and household activities. The main source of MP detected in packaged drinking water was the plastic bottle itself, where single used plastic bottles were observed to have lesser MP contamination compared to recycled ones. In wastewater, the amount of MP was observed to reduce significantly after primary treatment whereas a limited decrease in their amount was observed after secondary treatment. Studying MP contamination in wastewater further helped in determining the reasons for cross-pollution of other water sources as well as soil. The main advantage of Raman is that it can also detect particles less than 10 μm which is often missed by FTIR spectroscopy and even FTIR imaging systems. Further incorporation of Raman systems with in-line water supply for real, on-time monitoring of the contaminants has also been demonstrated. Miniaturization of systems has led to the development of portable handheld Raman spectroscopes which can be used for characterizing MP in the field setting to get a greater understanding of its effect. Advanced instruments such as the PLastic Explorer (PLEX) are being developed by physicists from the Northern Water Problems Institute at the Russian Academy of Sciences' Karelian Research Centre, in collaboration with the Atlantic Department of the Russian Academy of Sciences' Shirshov Institute of Oceanology, for MP investigation in the Baltic Sea (Zobkov et al. 2019). These devices helped to discover the adversaries of plastics in the environment in a manner that was not possible before. RS combined with other techniques such as holographic imaging or visualizing stained MP has shown to map the particles as well as chemically characterize them at the same time. Even though RS showed high specificity, the sensitivity of the method sometimes seems to be a drawback. To overcome this, Stimulated Emission Raman spectroscopic techniques are used in which various Raman active substances aid in increasing the signal intensity from the MP. SERS have been shown to further increase the efficiency of RS in determining sub-micron MP particles and also reduce the fluorescence contamination often observed in traditional Raman spectra. With the rapid development of science and technology, combining machine learning with RS can serve as a potential method of identification. This is also expected to help in automating the whole process thereby reducing errors in detection rates. Therefore, it is clear that RS is a powerful tool that offers versatility in the detection and characterization of MP in water. And with the help of a systematic review, global trends and the application of specialized techniques are being highlighted to the wider audience. Further, such systematic reviews help to collect recent information from peerreviewed literature to develop a reference library of Raman spectra of MP detected in water sources which will help researchers and environmentalists across the globe in rapid identification of the particles. This will provide an efficient framework for effective monitoring of this global issue.