Plastic waste can be divided into macroplastics (> 25 mm), mesoplastics (5–25 mm), microplastics (< 5mm) and nanoplastics (< 100 nm) depending on their size. Accordingly, plastic particles smaller than 5 mm are microplastics, without specifying the lower size limit. Publications [36] address the fact that the removability of microplastic particles depends primarily on their size and shape; that is why a lower limit (e.g. 20 or 50 µm) has been specified. The hole size of the nets used for sampling can be considered as a lower limit. In the literature, e.g. [39] the size of nanoparticles between 1-100 nm is now accepted, which is the lower part of the colloid size range (1–1000 nm) defined decades ago. In an older article [40], but also in a critical review published in 2021 [41], particles in the range of 1–1000 nm are considered to be nanoplastics, or plastic nanoparticles (PNP). In several publications [36], [42], [43] this colloidal size range is excluded from the studies. The literature is not uniform on the issue of size, there are overlaps in the intervals. The point is not the exact specification of the sizes, but the appearance of the colloidal properties below the micrometer range. Due to their small size, colloidal particles cannot be seen with an optical microscope, move with thermal motion, do not settle or pass through an ordinary filter, and can be filtered only with membrane. Thus, on the one hand, they have great mobility in the environment, and on the other hand, they have a high surface area to volume ratio, which means they are present with a huge active surface area, although their mass is small. Therefore, particular attention should be paid to colloidal (nano) particles, as they are more difficult to separate, identify and remove from the water body, and also pose a much greater health risk than micron-sized particles. We only mention them here as an example, in connection with their large specific surface area, which enables them to dissolve monomers, plasticizers, etc. much more effectively, as well as their ability to adsorb and transport many hazardous substances to living organisms, where due to their small size, they are more easily absorbed in the digestive system, penetrate the wall of blood vessels, enter the cells, etc.

Based on the main morphological characteristic of the particles, they can be isometric (same extent in all three spatial directions, e.g. spherical, cubic) and anisometric (different sizes in one or two spatial directions, e.g. cigar-shaped, rod-shaped, disk-shaped, lamellar or significantly different [e.g. films, fibrous materials]) [38], [39]. Aggregations of various morphologies (e.g. spherical aggregates due to random adhesion of lamellae) can be formed by the adhesion of the particles. In terms of their shape, microplastic spheres, pellets, beads, granules, threads, fibers, films, and fragments with irregular shape have also been detected [41]. The most commonly observed shapes are fragments and fibers. Plastic nanoparticles are often tiny spheres [41]. Surface roughness is also a morphological characteristic. The microplastic particles produced by fragmentation have a rough surface.

Plastic nanoparticles dispersed in water show a similar (mostly negative) non-zero surface charge over a wide pH range, so they repel each other electrostatically, their dispersion is in a stable colloidal state [41]. Surface charge can come from the production (polymers with polar groups, polymerization aids for primary particles), it can be formed by the adsorption (eco-corona formation) of surface-active substances (e.g. soaps) or polyelectrolytes (e.g. humic substances, dissolved proteins), and during the aging process due to environmental effects (e.g. photooxidation) as well.

The hydrophobic/hydrophilic nature of the surface is also important. The surface of polymers is hydrophobic and cannot be wetted by water; the wettability of plastics can be modified by additives. The hydrophobicity of the surface is beneficial for protein adsorption and interaction with biological entities in the formation of the bio-nano interface.

Several studies have shown that the size, shape and surface physicochemical properties of microplastic particles (e.g. their positive or negative charge) are fundamental in terms of their biological effects [11]. Depending on their chemical structure (Figure 2), microplastics can bind the ions and molecules of pollutants through hydrophobic interaction, van der Waals forces and via their active groups through π-π and electrostatic interactions and H-bonds [44].

 

Some of the plastic micro/nanoparticles that enter the environmental systems are in aqueous suspensions. In these, the surface of the particles hydrates and forms an aqueous interface that usually contains an electrical double layer. The structure of the latter changes with the composition of the aqueous phase due to the adsorption of dissolved ions and molecules, and particle-particle interactions generally depend on it [45]. In the case of PNPs (polymer nanoparticles) in natural waters, the following dissolved and other dispersed components need to be examined.

Dissolved salts, pH: Charge screening becomes more effective with increasing salt concentration. Therefore, PNPs are generally less stable and aggregate with increasing ionic strength. Multivalent cations (e.g. Ca2+, Mg2+) are more effective than monovalent ones (e.g. Na+). In natural waters, e.g. the high salinity (ionic strength) of seawater dramatically increases PNP aggregation compared to that in fresh water (rivers, lakes, groundwater) [46]. The change of the charge state of the particles can be followed by measuring the electrophoretic mobility; their calculated ζ-potential approaches zero, which indicates the disappearance of the repulsive effect between the particles. The stability of PNP also depends on the pH of the system, but only indirectly through the pH-dependent speciation of the hydrolyzing metal ions. Under normal environmental conditions, the effect of pH on PNP aggregation is minimal [41].

Dissolved and dispersed organic matter: Natural waters contain dissolved organic matter (DOM), their amount and components (mainly small- and large-molecule organic acids and proteins) vary greatly. Their effect on the mobility of PNP is significant and various, depending on the properties of PNP and DOM as well as on the salts dissolved in the water and the pH. The acidic components of DOM (e.g. humic acids, alginate) are in a dissociated state at the most frequently occurring pH values, so they are negatively charged. At increasing DOM concentrations, due to the adsorption of polyanions the absolute ζ-potential of the opposite, i.e., positive net surface charged PNPs decreases, it becomes zero at the isoelectric point, thus reducing the electrostatic repulsion between particles, which leads to particle aggregation. The average hydrodynamic diameter of the particles may increase significantly. A large excess of organic matter can completely cover the PNPs, their charge and ζ-potential will have the opposite sign, they will be sterically stabilized, and their aggregation will decrease [47]. In addition to electrostatic effects, DOM, especially protein (e.g., albumin) components can bind to the surface of PNPs through hydrophobic interactions, which can cause aggregation. On the other hand, the adsorption layer of bound organic matter prevents particles from sticking together, steric repulsion reduces aggregation. In simple cases, DOM does not have much effect on negatively charged PNPs, the electrostatic repulsion between them restricts their interaction. In such systems, the absolute ζ-potential of the PNP may increase slightly with increasing DOM content, and the stability of the suspension is unchanged or even slightly improved. However, in the presence of bridging metal cations (e.g. Ca2+, even heavy metal ions), the increasing concentration of DOM leads to the aggregation of PNP [46]. The interaction of PNPs with particulate organic matter (POM) and microbial cells through hydrophobic interactions or through metal cation bridges can increase or decrease the heteroaggregation of PNP depending on the environmental conditions. The surface chemistry of PNPs in nature differs essentially from the original characteristics of the plastic, ultimately determined by the interaction with organic matter and other aqueous components. Studying the eco-corona and its formation requires thorough research in the future.

Mineral particles, inorganic colloids: Dispersed mineral microparticles and colloidal particles (e.g. clay minerals, metal oxides, carbonates) are always present in natural waters, especially in fresh waters. PNPs tend to bind to the surface of inorganic colloids, forming relatively unstable heteroaggregates [46]. This interaction depends greatly on the ζ-potential of the particles under the given conditions. With the same charge sign, the mineral surfaces can repel PNPs, thus promoting their stabilization in the medium (e.g. the interaction of negatively charged clay lamellae with PNPs covered with humic acid) [47]. However, with increasing ionic strength, this electrostatic repulsion between particles of the same charge can be minimized, and multivalent cations can form cation bridges between particles, promoting heteroaggregation. Oppositely charged particles attract each other (e.g. negatively charged iron[III]oxide minerals stick together with positively charged PNPs, this heteroaggregation occurs even in pure water). Some minerals can have a permanent negative charge but also localized positive charge regions and vice versa [46]. For example, at certain pH and ionic strength, clay mineral lamellae bind negatively charged PNPs at their edges, which may be positively charged even though the net surface charge of the clay lamellae is negative [46]. Suspended minerals mostly lower the stability of PNPs in water, depending on the environmental conditions and their colloidal properties; it appears that the presence of inorganic colloids most often reduces the mobility of PNPs [41]. Figure 3 schematically shows the colloidal interactions between suspended plastic nanoparticles and other dissolved or suspended components in natural waters. In part (a) of Figure 3, the electrostatic repulsion between PNPs with the same charge sign prevents aggregation of the particles, thus preserving the individuality of the particles and the colloidal stability of the suspension. Part (b) shows that metal cations can increase PNP aggregation in two ways: i) the charge-screening effect of cations, electrolytes in general, reduces the repulsion between the particles so that the van der Waals attraction can prevail between them, ii) the adsorption of at least divalent cations forms cation bridges between the particles. In part (c), dissolved organic matter (DOM) is adsorbed on both the positively charged PNPs and the negatively charged PNPs through cationic bridges, which leads to heteroaggregation. In part (d) we can see the adsorption of dispersed organic matter (POM - particulate organic matter) on the PNPs, which sterically stabilizes the PNPs, thus minimizing their aggregation. However, under certain conditions, POM can be incorporated into larger heteroaggregates. Part (e) shows that both positively and negatively charged PNPs can be adsorbed on inorganic colloids, which can lead to heteroaggregation of many other suspended and dissolved components. Part (f) illustrates the settling of large aggregates, which reduces the PNP concentration in the aqueous phase.

Microplastic particles have complex physicochemical properties that alter their mobility, bioavailability and toxicity to organisms, as well as their interactions with surrounding pollutants [48]. Similar to nanoparticles, accurate and reliable detection and measurement of microplastics and/or nanoplastics as well as their characteristics are important to understand their impact on the environment and ecology. The authors review various analytical tools/techniques, discuss the strengths and weaknesses of instrumental methods, compare them, and analyze their applicability in the separation, morphological and physical classification, chemical characterization and quantification of microplastics. They emphasize the need for standardized experimental procedures and characterizations due to the complex transformation, cross-contamination, and heterogeneous properties of microplastics in terms of particle size and chemical composition. It is already known that microplastic (e.g. PS, PE, PVC, PP) particles bind toxins, industrial additives [49] and well-known toxic substances, inorganic and organic micropollutants (e.g. Ni, Hg, Cu, Ag, polychlorinated biphenyls [PCBs], drug residues, hydrophobic organic pollutants [HOC], triclosan, polycyclic aromatic compounds [PACs], polycyclic aromatic hydrocarbons [PAHs] benzopyrene) on their surface and transport them in enriched amounts to living organisms, increasing their toxicity more or less depending on the adsorption capacity [11], [44].