## 4 Interactions of MPs and Other Pollutants
The hydrophobic or lipophilic properties of pollutant molecules and the specific surface area or the surface-to-volume ratio of MP particles turned out to be the main contributors to the capacity of MPs to concentrate pollutants [47; 48]. Smaller particles have a higher adsorption capacity due to a larger specific surface area. MPs are exposed to mechanical, thermal, chemical and photo-oxidative stress and biodegradation in the environment, which cause surface structure changes and molecular weight decrease, thereby increasing the adsorption capacity. Thus, MPs that are present longer in the environment have a greater adsorption capacity for pollutants [47; 58; 60]. A greater diversity and concentration of pollutants was observed in black-colored MPs, which indicates complex physical and chemical interactions [47; 60]. The color is often conditioned by the age or degree of decay of the MP. As a result of the degradation process, the surface of the MP changes and the polarity increases, which increases the porosity and charge so particles become more reactive [48; 61]. In general, higher concentrations of pollutants were found in discolored and yellowed MPs [48]. Interactions of chemicals and MPs also depend on environmental conditions, such as weather conditions, humidity, solar radiation, pH, salinity, water temperature, dissolved organic substances and microbiological activity [41; 47; 61; 62]. Higher salinity favors changes in the size and surface area of particles and an increase in adsorption capacity; therefore, the transport of pollutants via MPs is more pronounced in the sea environment [61]. Lower pH and higher temperature contribute to a faster release of bound pollutants [47]. The presence of organic matter in the surrounding water significantly reduces the binding of organic pollutants to MPs because of direct competition, i.e., part of the pollutants bind to the organic matter itself [61]. Biofilm formed by environmental microorganisms on the surface of MP particles, can improve the adsorption of pollutants, especially metals, by increasing the surface area of the particles and changing the polarity [47; 48]. Most studies on pollutant binding to biofilms have been conducted on metals, so further research on organic chemicals is needed [48]. Figure 3 highlights factors that determine interactions of MPs with pollutants in the environment. Due to the lack of standard procedures for the preparation and analysis of MP samples, the results of studies examining the affinity of different types of MPs for different groups of pollutants are often incomparable [58]. There is an urgent need for a coordinated strategy to develop mechanical models and analytical tools as well as to standardize sampling methods and laboratory analyses, which would strengthen the overall understanding of the environmental fate, transport and exposure of MPs and provide reliable data on MPs in air, water, soil, sediment and biota [56]. Also, more detailed research is needed to reveal the source of toxic chemicals found in MPs to determine whether they originate from additives added in production procedure, from the environment to which MPs are exposed or as a consequence of the joint action of both mentioned [48].