Several studies deal with the adsorption/desorption equilibrium of organic pollutants, heavy metals, and pharmaceuticals bound to microplastic particles under various conditions [15], [105], [113], [139], but we know little about the extent of desorption in a physiological environment [46], [103]. In an in vitro study, a thirty-fold higher desorption rate of the studied organic pollutants from PVC and PE particles was observed compared to seawater when intestinal juice was also present in the system [140]. The adsorption-desorption balance depends on the type, size, color, shape, physicochemical properties of the polymer (elasticity, crystal structure, porosity, degree of degradation), additives, concentration and chemical properties of the pollutant (hydrophobicity, presence of H-bonding functional groups), other pollutants present, and environmental conditions (pH, ion concentration, temperature) [105], [138], [141]. In the study of adsorption of antibiotics to polymers, it was found that for some antibiotics the degree of adsorption was higher in fresh water than in salt water [139]. The adsorption of non-steroidal anti-inflammatory drug molecules on microplastics is significantly affected by the pH of the solution, at lower pH the adsorption is greater for these compounds [141]. An amorphous polymer has a larger free surface area for binding than a crystalline polymer. Glass-like polymers, such as PVC, PS, tend to adsorb, while rubber-like polymers, such as PE, PP, tend to absorb more. The smaller the particles, the more they can bind to their surface, but below a certain size, the tendency of nanoparticles to form aggregates increases, which prevents further binding [14]. The formation of biofilms on the surface of the microplastic can also affect the sorption capacity [106]. The area available for adsorption can be further increased by the degradation of microplastics. Microplastics can acquire polar functional groups (-C=O, -C-O-, -OH, -COOH) during their aging, which can serve as new adsorption sites and even form stronger H-bonds with pollutants of the corresponding molecular structure [44], [139]. In the case of PS, the formation of a carbonyl group (ketone) is characteristic, and in the case of PE, the formation of a carboxyl group is common. The sorption of hydrophobic materials generally decreases with aging as the proportion of oxygen-containing functional groups increases, making the polymer more hydrophilic. In contrast, sorption of metals and some antibiotics increases with aging, due to increased polarity and surface charge. However, environmental factors also affect sorption capacity (organic matter, e.g. humic acid content, salinity, charged mineral particles) [106]. Binding of metals can occur by direct adsorption of cations onto the charged surface of the particle or by complexation. In an aqueous medium, an increase in pH significantly increases the adsorption capacity of metals. At a higher pH, the functional groups of the microplastic are deprotonated, so that the electronegativity becomes higher and more binding sites for metal ions are formed on the surface of the microplastic [15]. The degree of heavy metal binding also depends on the size of the microplastic and the presence of bound natural organic matter on its surface. Natural organic matter may contain functional groups that readily bind heavy metals and affect the aggregation of nanoplastics. Aged/eroded particles are more susceptible to heavy metal binding [102], [113].

Several studies have investigated the synergistic or additive toxic effects of microplastics and other pollutants and additives that are present along with it. The presence of microplastics enhances the toxic effects of nickel, some pharmaceuticals, perfluorinated compounds, or dimethylmercury [81], [106]. The binding of antibiotics to the surface of microplastics may exacerbate the negative effects of both pollutants. When microplastics and antibiotics were administered simultaneously to clam (Tegillarca granosa), downregulation of some genes responsible for detoxification was observed, so that the antibiotic concentration in the blood of the mollusks was higher than in the antibiotic-only treated groups [142]. The amount of antibiotics remaining in the mollusks does not pose a direct toxic threat to humans who consume them, but this amount may already be sufficient for the development of antibiotic resistance in the intestinal tract.

Not only drugs, heavy metals and organic pollutants can adhere to the surface of microplastic particles, but also microorganisms. In aqueous media, the formation of a biofilm on the surface of microplastics is typical. Among the microorganisms adhering to the surface, there may be pathogenic strains that may pose a threat to the living organism [7], [16], [99]. Among others, Vibrio parahaemolyticus, which causes gastrointestinal infection with acute diarrhea and vomiting in humans, has already been detected on PE, PP and PS particles [98]. Figure 6 shows the microorganisms that have been detected on the surface of microplastic particles [143]. Microplastic particles covered with biofilm altered the normal intestinal flora in animal studies, leading to metabolic changes, obesity and immunological deficits [144].