General introduction on plastic pollution

Plastic pollution is nowadays a global and ubiquitous problem, including marine environment surface waters, soils, sludges, sediments, biota, food and air. To have an idea of how widespread this problem is, over 350 million tonnes are produced every year [1, 2]. River plastic pollution was estimated to be between 0.8 and 2.7 million tons per year at the global scale. A recent collaborative study performed for almost two years on 42 rivers and 11 European countries measured riverine macro-litter floating to the sea. An estimated 307 to 925 million litter items are released from Europe to the ocean, which represents up to 5000 tons/year [3]. Recently, on May 20, 2021, a fire started on the cargo ship X-Press Pearl off the coast of Colombo, Sri Lanka. The ship transported 1680 tonnes of plastic pellets, about 70 billion of them, each about 5 mm wide. These pellets are also called nurdles, which are melted to make many plastic products afterwards. It has been reported as the biggest marine plastic spill in history [4, 5].

Globally, the amount of microplastics (MPs) in soils is estimated to be between 1.5 and 6.6 million tons, of which China is number one on the list, accounting for 660 kilotons of MPs in soils. The abundance and characteristics of MPs in soils vary by agricultural soil type, cultivation, year of cultivation and sampling depth. The main reason is attributed to the irrigation of agricultural fields with wastewater. MPs in Spanish and Italian soils have reached up to 38 and 28 kilotons, respectively. This has been attributed to increasing water reuse for agriculture and it is a direct consequence of water scarcity in the Mediterranean. In this sense, it is estimated that the total amount of MPs present in soils around the globe is probably higher than the amount of MPs present on the surface of the oceans. Consequently, many studies were carried out in the last few years on the occurrence, fate and ecological risks of MPs in solid organic wastes. Composting and anaerobic digestion processes were also investigated multiple sources of MPs rather than plastic mulching film [2, 6]. A recent study collected 477 soils from China from 109 locations and MPs varied from 2462 to 3767 items/kg, but could be up up to 40,000 items/kg [7]. A more recent paper [8] reported data on MPs at the global level. Pollution levels of MPs in agriculture, roadside, urban and landfill sites were reported too. Higher values were also reported in China with similar levels to that in a previous global study [7] of up to 4000 and 5215 items/kg in agricultural soils of Shandong and Jilin provinces, respectively. Surprisingly, at the global scale, very high values were reported for Austria, up to 1.2 × 107 items/kg, but unfortunately not enough information was provided on the sampling sites [8].

Having said that, this Feature Article discusses different aspects of MP analysis and analytical chemistry methods, including green analytical chemistry (GAC) in the first place. Secondly, MPs are an excellent sorption material for a variety of chemical compounds with an obvious risk to biota and lately discrepancies in the impact on human health were reported too. Recommendations to better understand and mitigate MP contamination are summarized.

Analytical chemistry methods

MP analysis is not yet solved. Although we do have in place several analytical methods, there is no agreement yet on the protocols to be used. In the meantime, until we reach an agreement on the methods, a useful recommendation for MP analysis would be to add a detailed protocol of sampling and sample preparation in publications and theses related to MP analysis. It can also be part of the supplemental material. In this sense, a few questions need to be answered, like have positive and negative controls been included and were the LOD/LOQ of the analytical method reported? How does a given analytical method compare to those used by other scientists? One of the ongoing problems in MP analysis is the comparison of methods and data obtained between laboratories.

Obviously, more recommendations for the different analytical steps of the protocols for analyzing MP are needed. First, for sample preparation protocols, there is the need to agree on specific details like mesh size for the sampling nets in continental waters, on particle size for MPs in soils and other parameters. In this sense, the most common particle size for sampling water is 300 µm, although there are a few papers that use nets of 50–80 µm, and for soils the particle size can vary between 20 and 5000 µm. Regarding detection, there is a lack of universally accepted detection methods. In this sense, destructive mass-based MP detection methods ignore smaller MPs and nondestructive particle-based MP detection methods fail to provide mass-related information. There is also a lack of differentiation of smaller and larger particles and additional instrumentation will be needed for this purpose like the use of field-flow fractionation (FFF). For polymer identification purposes, we do have a list of techniques available like pyrolysis-GC–MS, FTIR, or Raman. The use of only microscopic techniques or visual inspection is not enough for MP analysis, although most of the literature being published on MP detection and occurrence is based on this technique. Why is this happening? This is easy to answer: almost any laboratory can afford a microscope.

In short, analytical chemistry procedures require appropriate controls, blanks and validated protocols to determine MPs in aquatic and terrestrial environments. In this context, certified reference materials would be of great help for MP analysis. At present, there is an obvious need for interlaboratory comparison tests, the first steps towards standardization and harmonization. This aspect was recently reported in an interlaboratory study carried out by > 20 expert laboratories in Europe using only standard polymer samples and qualitative data to identify the different polymers. The results were a disaster and called for the improved and continuous training of laboratories involved in the analysis of MPs [9].

Regarding the results obtained, e.g., items/liter (water) or items/kg (soil) or items/m3 of air, what size are you reporting on, and how does this compare to the literature and indeed the probable natural size range? It is highly recommended that any given author publishing data on MPs compares their data with published information at the international level. If the discrepancy between these values is higher than an order of magnitude, an explanation is needed, e.g., use of sampling nets of different mesh sizes (water) or not representative sampling (soils, biota). Most of these points cited above were already indicated in a recent paper [10], where it was also suggested to include a ‘Limitations’ section in scientific papers. We believe this will be of added value for future submitted papers and a positive contribution to better understanding MP/nanoplastic (NP) pollution in the environment.

Additionally, several review papers on sampling and analytical methods for MPs were recently published [11,12,13]. Advantages and disadvantages of a comprehensive list of different analytical methods for MPs were listed, including impedance spectroscopy, naked-eye visual methods, optical sensing, polarized light scattering, microfluidics, FTIR spectroscopy, Raman spectroscopy, fluorescence spectroscopy with Nile red dye, and pyrolysis GC–MS. Another review paper on the characterization, identification, and quantification of MPs/NPs obtained from diverse samples in the soil environment was reported [11]. Summarizing the capabilities of each analytical technique, it can be pointed out that FTIR is probably the most used technique nowadays, and it can analyze particle size distributions down to 20 μm, whereas Raman spectroscopy is suitable for analyzing MPs down to 1 µm, and can be applied to the physicochemical characterization of NPs (< 1 μm) in combination with SEM. Most of these techniques will provide information to distinguish MPs from soil particles. Lastly, a high-throughout application of ASTM D8332, standard practice for the collection of water samples with high, medium and low levels of suspended solids, for the identification and quantification of MP particles and fibers was recently published [13]. This standardized method is capable of pumping 1500 L of water in 45 min to collect particles as small as 45 µm.

GAC protocols for the analysis of MPs in water are getting more attention nowadays [14, 15]. Within the last years, aspects such as green, ecofriendly and sustainability are making their way into analytical chemistry. GAC protocols in a broader sense include ecotoxicological methods [15]. The use of invertebrates in ecotoxicology as an alternative to vertebrates was highlighted, as well as increased acceptance, better protocols, and a growing recognition of the ecological relevance of invertebrate studies. The integration of in vitro, in silico, and non-vertebrate methods into personalized toxicology approaches is expected to be increasingly used. Collaborative efforts between researchers, industries, and regulatory bodies are anticipated to lead to standardized protocols and validation frameworks. Streamlining the regulatory approval process for in vitro and in silico green methods, along with invertebrate models, will facilitate their integration into standard toxicological assessments.

Sorption materials

It is well known that MPs act as sorption materials for persistent organic compounds, and recently, papers on the sorption of pharmaceuticals and other polar chemicals were reported [16, 17]. The properties of pharmaceuticals related to the hydrophobicity of the compounds, such as the octanol–water partition coefficient (log Kow) and pKa, also have a crucial role in the sorption affinity of pharmaceuticals on MPs. MPs behaving as vectors for such chemicals have helped in the transportation of pharmaceutical compounds like antibiotics, in turn, contributing to the grave danger of antibiotic resistance genes (ARGs). Many examples were reported in the last few years for antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), psychiatric drugs or cardiovascular drugs that could sorb to the MP surface.

A recent comprehensive work examined the interaction mechanisms of MPs and antibiotics, and the effect of MPs on antibiotic bioavailability and ARG abundance in aquatic environments [17]. In this respect, mechanisms for antibiotic adsorption on MPs are summarized, mainly including hydrophobic, hydrogen-bonding, and electrostatic interactions. Additionally, environmental factors (such as pH, ionic strength, dissolved organic matter, minerals, and aging conditions) affecting antibiotic adsorption by MPs were discussed. This study concluded that research on MP − antibiotic interactions and their related implications were still in the early stage. However, a more recent work [18] evaluated the influence of MPs on the availability of antibiotics in soil using diffusive gradients in thin films (DGT). Three types of MPs, polyethylene (PE), PVC and PP, were used and it was concluded that, in general, it decreased the availability of MPs in different types of soils. The last example of MPs as sorption materials for contaminants was reported in a recent paper that examined the sorption capacity, among other properties, of MPs derived from mulch films towards pesticides [19]. This work also concluded that MPs derived from mulching films could sorb and desorb pesticides, affecting the degradation of pesticides as well. Considering that mulching is a global activity in agriculture, it is worth mentioning this example here and encouraging researchers at the global scale to carry out further studies in the future to examine the sorption capacity of MPs for pesticides under different climatic conditions and soil types. Thus, the adsorption mechanism and ecological impacts of MPs and antibiotics, pesticides, and many other chemicals still need further investigation. Most importantly, and similarly to other environmental experiments, microcosm and mesocosm studies and field investigations are recommended to investigate the adsorption and accumulation of antibiotics and pesticides on MPs, always using environmentally relevant concentrations and different climatic conditions of higher/lower temperatures, drought/flooding events, and diverse soil types.

Risks

In a similar way to many emerging contaminants and nanomaterials, MPs will affect communities, biological diversity, and ecosystem processes. MPs increase the abundance of some taxa, while decreasing the abundance of other taxa, indicating trade-offs among taxa and altered microbial community composition in both the natural environment and animals’ guts. Potential threats to biodiversity were discussed [18]. In regard to the toxicity of MPs, size and shape in particular are relevant, as smaller MP and NP particles are more toxic to organisms. Higher toxicity in terms of size and shape is also attributed to MPs due to their longer residence time in organisms [20].

The ubiquity of MPs can have adverse effects on the environment and biota. MPs are capable of absorbing and accumulating pollutants from the air, soil, and seawater as well as microorganisms. The formation of biofilms inevitably changes the properties of MPs, where MPs are more toxic for aquatic organisms. Biofilms are believed to have good adsorption properties and accumulate heavy metals, antibiotics, and other toxic substances that can easily spread through the aquatic food web. Many aquatic organisms are used as promising tools for biomonitoring programs and assessing environmental quality. Recently, a new “mussel-watch” program, using M. galloprovincialis, has been proposed by the International Council for the Exploration of the Sea to monitor MP pollution in the marine environment [20]. The presence of MPs in lakes has been reported in high mountain lakes [21] and the presence of MPs was assessed in abiotic (water and sediment) and biotic (zooplankton, tadpoles, fish) compartments of two high-mountain lakes (Upper Lake Balma and Lower Lake Balma) in the Cottian Alps (northwest Italy). No MPs were found in water and zooplankton (Arctodiaptomus spp.) samples. Finally, further studies are desirable to better understand the source and dynamics of MPs in high-mountain lakes. Another recent study on the possible effects of MPs on biofilms was reported as well [22].

MPs can impact many different aspects of the terrestrial ecosystem, affecting the toxicity of organisms. In this respect, soil properties can be changed and MPs can alter the diversity and abundance of the microbiome and different MPs have different effects on bacteria and fungi [2]. For soil animals, MPs can affect their mobility, growth rate and reproductive capacity [8]. Effects of MPs and NPs on soil animals and plants were extensively reported in a recent review [11]. The diverse sizes of MPs/NPs yield distinct impacts on various plant growth parameters and have the potential to modify both plant growth and photosynthetic parameters via modulation of microbial metabolism and interrelationships among microorganisms. MPs did not solely have detrimental impacts on plants, but induced changes to the soil properties, which were the dominant factors influencing the toxicity of the MPs. Plant growth can be influenced by the MP polymer type, size, and shape, along with the time of exposure and concentrations. Consequently, the impact of MPs on soil-based plant growth is limited, necessitating the need for comprehensive investigations.

Regarding the impacts of MPs on soil organisms, ingestion, efflux, and bioturbation contribute to the detrimental impact of MPs on soil fauna by facilitating the incorporation and transport of MPs through soil organisms. Earthworms, commonly used as indicator organisms, have been extensively studied to assess the physiological toxicity of MPs on soil organisms. Compared to MPs, NPs exhibit greater toxicity towards earthworms. It has been reported that exposure to NPs at concentrations exceeding 1 g kg−1 (0.1% w/w) may lead to reduced earthworm growth and survival.

Another recent example selected here describes the stress response to oxytetracycline (OTC) and MP-based PE in wheat during seed germination [23] The phytotoxicity of combined contaminants in plants, such as antibiotics and MPs, is of concern and rarely studied. The effects of individual and combined exposure of wheat (Triticum aestivum L) to OTC and PE MPs using physiological and metabolic profiles were examined. The effect of PE MPs depended on the OTC level in the combined exposure groups. Dominant metabolites included carboxylic acids, alcohols, and amines in the control group and all treatment groups.

In short, it is worth mentioning that there are much fewer studies on the toxicological evaluation of MPs in terrestrial organisms compared to aquatic organisms, in agreement with a recent paper [19]. Additionally, most of the citations reported in this work indicate that the addition of MPs increased the toxicity risk of pesticides to terrestrial organisms. Importantly, in the case of mulching activities, MPs act as carriers and will absorb and desorb pesticides, and the bioaccumulation of both MPs and pesticides in organisms will occur. Lastly, there is a lack of toxicity studies of MPs and pesticides in terrestrial soil–plant agroecosystems. This applied research needs further attention in the coming years.

The possible risk of MPs to human health was also reported in a recent review paper [24], although there is not yet enough evidence on the risks. Recently, the possibility of hemodialysis patients being contaminated with MPs and NPs during dialysis treatments was reported [25]. In this context, hemodialysis patients may be at greater risk due to potential increased exposure during dialysis treatments and their inability to excrete these potential toxins in the urine.

The risk of MPs in drinking water, where the source is close to dumping or waste sites like landfills, has been reported recently. One of these studies was performed in Chennai, India [26]. The incidence and spatial distribution pattern of MPs in surface water and ground water near landfill sites was reported. In the North Chennai region, not far from the Kodungaiyur landfill, 23 items/liter were reported and could reach 93 items/liter. Polymers were reported in decreasing order of PE > PVC > PS > PP > PET. According to the projected daily consumption (PDC), PE was the polymer at higher risk, especially for children, and it was higher too for lake water than tap water. The presence of MPs in lake water that can be used for drinking water without enough treatment is obviously a risk, especially in countries like India where water treatment is performed in different ways within the country. Interestingly, another recent paper from China [27] on the presence of MPs in waste disposal treatment sites and landfills reported values up to 123 items/liter in groundwater near dump sites, with an average value of 27 items/liter, with similar values to the previous Indian study. These values were 3–7 times higher than the average values in the groundwater upstream of the dumping site [26].

Summarizing, the risk of MPs in drinking water was basically reported in cases where there was not enough treatment of the water and where the source, in the case of Indian lake water [26], was close to a landfill site, as the worst-case scenario. We do not believe that this is the case for most drinking water systems, since a lot of treatment is required before it is distributed through the sanitation system of a certain city or metropolitan area. In this sense, a critical paper from the Department of Marine Sciences at the University of Connecticut, USA [28], reported that there was no clear evidence that MPs were demonstrated to be a threat to human health. They added that the rather common practice of including vague statements and references indicating potential impacts of MPs on human health to publicize studies and attract the attention of the media should be avoided. Indeed, we do agree with this and although MPs have been detected in many human body fluids, there is still no clear evidence of a major threat, as compared to other emerging chemicals present in our bodies like perfluoroalkyl substances (PFAS) or PAHs and the increasing threat of climate change. A global vision of the impacts on human health is needed, in line with the exposome and other studies that are being carried out where multiple chemicals are being investigated in the context of nontarget screening methodologies generally using advanced mass spectrometric and spectroscopic techniques in combination with chemometric tools. The situation is completely different in those countries where sanitation is not efficient, as in the case of North Chennai, India [26]. Unfortunately, predictions for the years to come are even worse since the increase in population will occur mainly in poor countries that still need a lot of investment in water treatment facilities.

Sustainable solutions

Recommendations regarding sustainable solutions were recently reported in a well-documented critical review on MPs, including the need for comparison exercise studies at the global scale, as well as the establishment of an international database to collect data on MP distributions [28]. The same authors insisted on stopping the publication of qualitative studies in different regions of the planet, which deliver very poor information. This usually happens because the identification of MPs, either visually or with a microscope, is easy and cheap, so everybody can do it. We recall a few years ago when we started to look at the literature on MP determination, over 50% of the papers used different types of microscopes, with no chemical information on the polymer composition. Such research is possible worldwide, and this is also the way so-called “citizen science” has been very active in this field for many years, Australia is a good example [29]. Summarizing the analytical chemistry section of MP papers, we would say that attention should be given to 1) clearly describing the method used, including sample preparation; 2) the validity of the conclusions, i.e., that enough representative samples were collected to write strong conclusions; 3) referencing prior publications to indicate differences and similarities with a link to the method used; and 4) presenting the significance of the results with regard to the literature on MPs with a global critical view, pointing out the most relevant findings in comparison with others.

Recently, the OECD recommended Global Ambition will require new challenges and strong international cooperation [30]. In this sense, sub-Saharan Africa, the Middle East and North Africa are projected to represent an increasing share of globally mismanaged waste over time due to their fast growth of plastic use combined with weak waste management systems. The major points cited in this report were [27]: 1) curb production and demand, by implementing reuse systems; 2) enhance waste collection and treatment, especially in developing countries; 3) encourage improvements in recycling; 4) enhance municipal litter management; 5) encourage research targeting microplastic leakage; 6) ensure strong international cooperation and support; 7) ensure adequate financing of waste treatment; 8) align legally binding financial flows to cope with plastic pollution, and 9) develop bioplastics and new materials.

Considering such strong recommendations and being a bit more precise for point 9, the development of bioplastics and new materials involves quite a lot of research, investment and technology. Goods news was reported by researchers at the University of Chicago Pritzker School of Molecular Engineering (PME), who have developed a new material, which they call a “pluripotent plastic” [31]. Like pluripotent stem cells, which can give rise to any type of adult cell in the human body, their plastic can take on many final forms. The pluripotent material is based on polymers containing “dynamic covalent bonds”, which can break and re-form reversibly. Heating the material to low temperatures (around 60 °C) results in the formation of more bonds and a material that is stiff and of high strength at room temperature. It can then be used for plastic utensils and other purposes. Heating to higher temperatures (around 110 °C) yields the formation of fewer bonds and, therefore, a softer, extensible material that can be used, for instance, as a pressure-sensitive adhesive. The researchers of this study believe that it paves the way toward a different way of thinking about material design and will help to recycle plastics for multiple uses. This new material is certainly an excellent development and it would help, if produced commercially in a few years, to reduce the amount of plastic waste and MP contamination worldwide.

Under point 9, there is also the development of bioplastics, which is very relevant for our future as a society and has become very popular in the last few years. Bioplastics will grow in the coming years and are expected to hit almost 3 million tons in 2024. Certification will indicate if a given bioplastic is compostable or not and under which limitations. In this sense, very common industrially compostable plastics based on poly(lactic) acid (PLA) are certified only for an industrial composting environment, whereas polyhydroxyalkanoates (PHAs), cellulose, or starch are certified for a broader range of environments. Overall, the commercial spectrum of compostable plastics is dominant and it is expected to hit almost 3 million tons this year for PLA and PHAs, as well as polybutylene succinate adipate (PBA), polybutylene adipate terephthalate (PBAT) and carbohydrates like cellulose and starch blends. Europe is the largest market for biodegradable plastics, over 52%. Additionally, a recent review on biodegradable plastics [32] summarizes the main drivers for the better implementation of biodegradable plastics: financial, regulatory, consumer and technology driven.