Interaction with Other Compounds
The exchange of additives or contaminants between microplastic particles and the surrounding water depends on
- • Concentration gradient,
- • Matrix in which the microplastics are present,
- • Physical and chemical properties of the plastic polymer and
- • Degradation processes acting upon microplastic particles [111,112].
The sorption of hydrophobic pollutants to the noncrystalline regions of plastic polymers occurs, and the smaller additives tend to move out of plastics fast. Apart from the compounds used in plastic manufacturing, there is also potential for exposure of aquatic biota to other contaminants that adsorb to microplastic particles. Microplastics are typically hydrophobic and have large surface areas, allowing them to accumulate organic pollutants such as polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenylethers (PBDEs), polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane. Ashton et al.  reported concentrations of metals in composite plastic pellet samples retrieved from the high-tide line along a stretch of coastline in southwest England. In the freshwater environment, MPs are likely to co-occur with other contaminants such as pharmaceuticals, personal care products, flame retardants and other industrial chemicals which enter the environment as parts of complex solid and liquid waste streams .
The sorption processes takes place through physical and chemical adsorption as well as pore-filling processes. Physical adsorption is the reversible sorption to surfaces of the polymer matrix, that is, it does not involve the formation of covalent bonds. Chemical adsorption involves chemical reactions between the polymer surface and the sorbate which generates new chemical bonds at the polymer surface depending on how aged the polymer surface is. The changes in pH, temperature and ionic strength of the localized environment influence these processes . In pore-filling processes, the hydrophobic pollutants with lower molecular weights will more easily move through a polymer matrix with larger pores.
Factors Influencing Adsorption Process
- • Particle size and texture-. Decreased particle size increases the potential for surface chemical interactions and, thus, binding with hydrophobic chemicals. Also, the physically weathered particles have more surface area and may be exposed to high levels of UV radiation and wind.
- • Structure of the polymer: Polymers that have structures with short and repeating units, a high symmetry and strong interchain hydrogen bonding will have a lower sorption potential.
- • Polymer density and crystallinity: The sorption and diffusion of hydrophobic contaminants are most likely to take place in the amorphous area of a plastic material, because the crystalline region consists of more ordered and tightly structured polymer chains.
- • Pollutants present: Pollutants with lower molecular weight have high sorption potential.
- • Surrounding environment .