Plastic is one of the leading materials that play a vital role in the destruction of quite a lot of things. It is made up of a combination of polymers which play a role in the harmful effects on that certain thing. According to a lot of researches and studies, it has been noticed that our waters are now being constantly filled up with nano plastics that are not seen by the naked eye and that is why their adversity too, is more than that of the normal plastics to be found.
These have a drastic impact on the ecology and as well as the human life itself, all of which are briefly explained in this article along with the classifications and working of nano plastics and their categories.
Since the inception of plastic as phenol-formaldehyde resin in 1907, Plastic is now one of the most abundant materials. Due to the production of plastic in a wide range, the throw-away culture has started taking place as most of the plastics are used only once. When plastic was introduced, it was considered a dream material because of its extreme versatility and imperviousness to water. Other than making plastic a very beneficial material, plastic’s attributes also have an impact on the Earth. In 1950, 1.5 million tons of plastic was globally produced and in 2017, the quantity was 348 million tons. Due to this lack of plastic degradation, low recycling rate, and increased production rate, a huge problem has arisen which is the accumulation of plastics in the environment at alarming rates.
The research on microplastic is majorly conducted in the marine environments but the actual amount of plastic discharger in the marine environment is more than what the marine sampling records show, therefore helping in emphasizing that there is still a need for research on the terrestrial and freshwater micro-and nano-plastic. Not only the marine environment but also the land and freshwater environment is infiltrated by microplastics. As compared to oceans, the terrestrial and freshwater sink of microplastics is larger, with the plastic’s amount being from 4 to 23×.
For developing an integrative mass-balance model of micro-and nano-plastics in the global environment, the investigation of exploring the terrestrial and freshwater sink for micro-and nano-plastics is very important. This review is on evaluating the concepts of micro-and nano-plastics currently in the terrestrial and freshwater environments and recognizing the knowledge gaps in it and find the important areas to put the focus on for future studies.
Plastics and their Additives
For an average consumer, the name ‘plastic’ may be misleading because many materials fall in this category and all of them have their different properties. Plastic is defined by the International Union of Pure and Applied Chemistry (IUPAC) as a polymeric material, which contains other constituents and substances for lessening its cost and enhancing its performance. Although they missed the word ‘synthetic’ while defining plastic.
Polymers are of many types. They are commonly classified into synthetic and natural. Silk, DNA, proteins, cellulose, starch, and wool are water-based and natural polymers. Natural polymers are constituents of the majority of living tissue structures. Synthetic polymers are polyurethane (PUR), polystyrene (PS), polypropylene (PP), low-density polyethylene (LDPE), polyvinyl chloride (PVC), high-density polyethylene (HDPE), and polyethylene terephthalate (PET). Synthetic polymers are produced by using raw materials like natural gas, oil, and coal and then are classified as plastic.
Constituents of world plastic production
PUR, PS, PP, LDPE, PVC, HDPE, and PET comes in 90% of the plastic that is produced in the whole world. Bioplastic and biodegradable plastic are the plastic’s two other types that do not fall in either the synthetic or natural category too. Biomass-based resources originate bioplastic whereas both biomass-based and petroleum-based resources originate the biodegradable plastic. As compared to synthetic plastic, both these types biodegrade more readily, that’s why they are alternatives for synthetic plastic. Although, there is no proof that in the natural environment, as compared to synthetic plastics, the bioplastics or biodegradable plastics will degrade any better.
Classifications of plastics
Resin codes are the seven categories into which plastics are classified.
Resin code number 1
Resin code number 1 is PET, which is utilized typically in the polyester coating, carbonated beverages containers, juices, and water bottles. For storing food, it is considered a safe plastic as it can be recycled easily, although, if exposed to high temperatures, the resin code number 1 leaches the chemical antimony. In many organic forms, that chemical antimony can be a potential human carcinogen.
Resin code number 2
Resin code number 2 is HDPE, which is utilized for liquids, containers, and beverages like a beauty product, cleaning supplies, milk, juice, and water. When exposed to sunlight, boiling water, and high temperatures, HDPE leaches endocrine-disrupting agents (estrogenic-behavioral chemicals or estrogenic chemicals). Compounds made by humans, which mimic estrogen and affect the reproduction and health of exposed aquatic organisms severely are known as estrogenic chemicals.
Resin code number 3
Resin code number 3 is PVC, which is utilized commonly for plumbing in pipes, it is also utilized for jackets, bath toys, and food wraps. When endocrine-disrupting agents (bisphenol A (BPA) and phthalates) comes in contact with the water, they are dangerous. Endocrine-disrupting agents are leached by PVC.
Resin code number 4
Resin code number 4 is LDPE, which is assumed as the plastic of low hazard which is utilized for cling wrap, dry cleaning, newspapers, coffee cups, milk cartons, and bread packaging.
Resin code number 5
Resin code number 5 is PP, which is a safe plastic as it is utilized for different numerous items like paper currency, lab equipment, automotive part, carpets, cold weather clothing, take-out, and yogurt containers.
Resin code number 6
Resin code number 6 is PS. It is utilized typically in the fishing and building/construction industry, for taking out food as a packaging material. Styrene is an “anticipated human carcinogen” and when in contact with hot liquid, it will leach from PS.
Resin code number 7
Resin code number 7 is a combination of plastic resins or an amalgamation of the plastic resin’s all other types. Typically, these plastics are utilized for packaging and larger water containers.
Working of BPA
In such plastics (Polycarbonate (PC)), the common additive, BPA, can cause obesity, asthma, reproductive problems, and hormonal changes in humans. In many countries, BPA has been phased out of plastic products. Alternative chemicals like Bisphenol F (BPF) and Bisphenol S (BPS) are just as hormonally active as the BPA, therefore continuing the same health problems.
When plastic combines with other additives, it doesn’t remain just an environmental contaminant, but its properties can be even more dangerous. By adding other organic compounds, many plastic compounds can be enhanced or altered, making up 50 percent of the product at times. Brominated flame retardants, nonylphenol (NP), phthalate acid esters, BPA, and perfluoroalkyl substances (PFASs).
Nanoplastics: Size, Shape, and Density
Like microplastics, there is no standard size definition for nano plastics too. A naked eye can’t see the nano plastics as they are very small. The plastic particles which are smaller than 0.1 µm are usually nano plastics. When the micro-and macro-plastics breaks down, the nano- and micro-meter sized particles are formed, also resulting in the production of nano plastics, which are the smallest plastic particles.
Nanoplastics are not researched much because of their size and that’s why they are difficult to be characterized. Although, some nano plastic’s commercial sources include hygiene and beauty products, particularly those having exfoliating characteristics, air-blasting mechanisms, abrasive cleaning supplies, and plastic powders that are utilized for making larger plastics.
Quantification and Characterization of Microplastics
Visual identification methods like microscopic or naked eye analyses are used in most characterization studies. Despite being cheap and simplistic, below 500 µm, accurate identification is not allowed by visual identification methods. Also, there is a threat in visual identification, of not properly identifying natural particles, like castor oil and stearic acid, calcium carbonate or quartz, aluminum silicate (from coal ash). Often the microscopic fibers and fragments are misidentified as rayon, cotton, or some other organic materials. Plastic materials have many attributes, some of them include visual cues, like luster or shininess, and unnatural or bright colors. Such visual cues start to diminish more and more when the plastic’s size and the time duration of plastic’s exposure to the natural environment decreases.
Infrared Spectroscopy (IR)
Near-infrared (NIR) (4000-12,500 cm-1) and mid-infrared (MIR) are the two different systems of measurement for infrared (IR) spectroscopy. Combinations of vibrational bands are detected typically by NIR spectroscopy, that’s why in that manner, polymers cannot be identified, whereas deformation of polymeric bonds and their different vibrational stretching is detected by MIR spectroscopy, therefore it is easy to differentiate between the polymer’s different types in a sample. In NIR spectroscopy, the differentiation between polymers is done based on the vibrational band’s shapes and not the bands.
Both MIR and NIR spectroscopy can be used to recognize different polymers within a sample, but, the NIR spectroscopy is not used much to identify microplastics as commonly as the MIR spectroscopy. Handheld FT-MIR, FT-MIR microscopy, and conventional stationary FT-MIR spectroscopy are the three types of MIR spectroscopy, which is also called Fourier transform mid-infrared (FT-MIR) spectroscopy.
Like IR spectroscopy, Raman spectroscopy uses vibrational bands for detecting and characterizing the polymeric substances. Although, the difference between these two spectroscopies is that how are those vibrational bands interpreted. In Raman Spectroscopy, the photon scattering is utilized along with the subsequent shifts between the elastic and inelastic scattering. Raman Shift corresponds to the difference of energy that the IR spectroscopy identifies and then detects the different particular polymers in a sample.
Fate and Behaviour of Micro- and Nano plastics and Their Ecological Implications
In sediment environments and soil, the study of the behavior and fate of micro-and nano plastics is rare. Only some studies are published, and in them, the focus is on the sedimentary organism’s role in the movement of nano- and micro-plastics in soils. A significant role is played by the sedimentary organisms in the transportation and movement of microplastics to deeper soil depths.
The movement of LDPE microplastics (<150 µm, 250-150 µm, 1mm-250 µm) in sandy soils (2.40 percent clay, 3.20 percent silt, and 94.40% sand) was studied by Yu et al. because of earthworm (Lumbricus Terrestris) burrowing. A 7 percent w/w microplastic concentration resulted when LDPE microbeads were added to the soil’s surface with dry litter (Populusnigra). It was observed that in 14 days, a major movement was made by LDPE particles from the sample’s surface to the sample’s bottom (50 cm vertical) because of the burrowing and movement of earthworms.
Contribution of earthworms
The microplastic’s concentration was greatly increased at depth because of the earthworms. Although, a tendency was shown by the earthworms in moving the smaller particles than 250 µm, which led to a gradient of microplastics with depth.
Rillig et al. collected soils from Berlin’s fields to perform a test. He analyzed the earthworm in those soils (7.6 percent clay, 18.8 percent slit, 73.6 percent sand) and had the same results when they added the dry litter (Populusnigra) and LDPE microbeads (2360-2800 µm, 1700-2000 µm, 1180-1400 µm, and 710–850 µm) to the sample’s surface leading to a 15.5 percent w/w microplastic concentration. It was a positive effect from the earthworms on LDPE microbead’s movement from the surface to the sample’s bottom, with the smaller LDPE microbeads slowly moving to the lowest depths.
Rillig et al. interestingly noticed that the microplastics were not only transported from subsequent egestion and ingestion, but they were also transported by adhering themselves to the earthworm’s exterior. Maaß et al. studied the collembolan species (Proisotoma Minuta and Folsomia candida), but he also found that there is a major impact of soil-dwelling organisms on the microplastic’s (smaller than 200 µm) movement.
Effects of rainfall
For transporting micro-and nano plastics to the groundwater sources through the vadose zone, events of rainfall may be functioning in tandem with the sedimentary organisms. Such results may apply and extrapolate to the sediments of beaches that are continuously facing exposure to the cycles of wet-dry because of the subsequent change of tides and water levels and their proximity to the waterbodies.
One of the issues in the micro-and nano-plastics presence in the environment is that they are capable of altering the sediment’s physio-chemical characteristics. Maximal temperature is decreased by microplastics, along with an increase in the permeability of the sediment. Microplastics also decrease the sediment’s heat absorbency when they come in contact with the microplastics. Biota can be extremely affected by this decrease in heat absorbency. The temperature of the soil determines the gender of eggs of many animals (alligators and turtles), and because of the excess permeability, many sediment-dwelling biotas may dry out. Changes of such types can cause a drastic effect on the food webs and may also cause major legacy effects.
All the organisms that are found in the microplasts are usually in the form of either fibres or fragments which play a part in a lot of environmental and health implications. If in any case, a certain biota is consuming the microplastics then as a result they will suffer from gastrointestinal tract issues and maybe issues related to obstruction too. These issues can lead towards some really life threatening issues such as false satiety or starvation and ultimately even death. Anyhow, these factors are only considering the physical implications whereas the chemical effects are not taken into account.
According to the experiments and researches, it has been found out that it is quite easy for the plasticides to move away from the plastics which can cause harmful effects to the biota. The additives which are present in this are basically lipophilic and are able to penetrate the cell membranes. Many POPs and these additives are toxic to the biota, therefore, it can lead to abnormalities, resulting in the organism’s potential death.
As compared to microplastics, the biota faces a more major threat by the nano plastics because of their increasingly small size. Both Nanoplastics and plasticides can penetrate the cell membranes. They can enter the cell and accumulate in any of the organism’s parts.
Latex particles of smaller than 50 nm are capable of accumulating in Japanese rice fish (brain, blood, liver, and testes), they can also be the reason for reduced rates of survival to their embryos. In Japanese rice fish, there was a breaching of the blood-brain barrier, such kinds of breaching causes extreme health risks to all humans and animals. In fishes, nano plastics can influence metabolic, physiological, and behavioral shifts in crucian carp (Carassius carassius), reduced reproduction rates and fertility in Tigriopus japonicas and crustaceans Daphnia Magna, growth inhibition in green algae (Scenedesmus sp. and Chlorella sp.), and significant developmental defects in the embryos of sea urchin (Paracentrotus lividus). According to all these studies on marine biota, it is predicted that when exposed to nano plastics, all the terrestrial living forms that are close to or living in the water sources will face issues as the marine organisms do.
Dependence on the biota
Any ecosystem’s or environment’s health is extremely dependent on the biota that lives within it, so the health effects associated with nano- and microplastics, influence the ecosystems to suffer along with the organisms. The suffering of the critical food networks and chains may occur, and the ecosystem’s deterioration will be induced by the deterioration of the organisms. Unlike microplastics which are not capable of penetrating the cell walls of plants, nano plastics can penetrate the cell walls. Microplastics have minimal effects on flora, unlike nano plastics.
In comparison with fauna, the difference with flora is that the uptake of each plant varies and depends on some factors; pH of vacuoles and cytoplasm, tonoplast potential, plasmalemma (bio-membrane) potential, lipid and water fractions and sorption potential, rate of growth, transpiration, sap pH and surface area, xylem volume, surface area, density, and root volume.
Due to so many major differences between the plants, it is tough to identify the effects that are caused by nano plastics. Although in some studies, engineered carbonaceous nanoparticles, similar in shape, size, and surface functional groups to microplastics, are used and have been documented in whole plants (soybean (Glycine max), maize (Zea mays), thale cress (Arabidopsis thaliana), and rice (Oryza sativa)), and enters via one of the following pathways: soil carbon or root exudate mediation, aquaporins, carrier proteins, ion transport channels, and endocytosis through the plasmodesmata. Although, on plant’s response, stress, and toxicity towards nano plastics, only some studies have been conducted.
Alarming effect of plastics
Plastic pervades all the terrestrial and aquatic environments, therefore becoming one of the most deleterious and prevalent environmental contaminants. It’s difficult to quantify and evaluate because of its varying sources, sinks, pathways, additives, and many forms. On plastic pollution, there has been limited research but to date, the primary focus is the marine environment and the gap between the knowledge regarding transport, fate, quantity, and impact of nano- and microplastics in terrestrial and freshwater ecosystems needs to be filled.
In the natural environment, the increasingly smaller size of plastics causes many complications for accurate analysis and sampling, limiting their capability of mitigating dangerous effects that are associated with their presence. In future research, an emphasis on small nano- and microplastics is vital for developing precise techniques of sampling and quantification. To make a particular database of information and to understand the health implications for both humans and ecosystems, the standardization of nano- and microplastic analysis and sampling, including units of measurement, is significant.
Plastic, as explained above, is comprised of a wide variety of polymers and hence hold the characteristics and properties which can be harmful in certain ways. Nanoplastics which can nowadays be found easily in the marine life and water around us affect the ecosystem and human life at a very large scale. However, if we combine and look for their degeneration then surely their adversity can be minimized at quite a large scale which can be proved beneficial for both the ecology and human life.