**5.4 Plastic biodegradation**

*Bacterial Biofilms*

**5.2 Chemicals**

volatile compounds.

**5.3 Buoyancy and aggregation**

turbulence leads to vertical mixing.

the marine ecosystems [106].

environmental solids is by physisorption.

where molecules are confined at the interface between fluid and solid phases as an adherent physical form [93]. Sorption is directly related to properties of the solid, a chemical, and the surface-to-volume ratio of the solid which for microplastic particles is quite large [94]. Apart from surface area, plastics exhibit a range of properties and dimensions, implying the relevance of absorption and adsorption to understanding the importance to the understanding of microplastics' fate and effect. Physisorption or physical sorption occurs from noncovalent intermolecular interactions such as van der Waals interactions. The interaction forces of solids and chemicals though the noncovalent interactions and their combinations and physisorption are usually reversible. Generally, the sorption of materials and chemicals to

Microplastics can sorb and accumulate both organic and inorganic contaminants

Sorption evaluations can identify the chemicals with higher affinity to microplastics under a variety of environmental conditions. Bench scale sorption studies permit the evaluation of the mass balance for a specific chemical or chemical mixtures. The distribution of chemicals in an environment contaminated with microplastics can be estimated from experimentally determined sorption capacities. Toxicity parallels sorption data, but greater sorption to microplastics does not necessarily lead to higher toxicity or bioaccumulation of a pollutant chemical.

Biofilm formation at the surface of microplastics may lead to density changes of particles that alter the specific gravity for the mass of microplastic debris [104]. Mineral detritus when incorporated in microplastic debris will increase the density which leads to sinking. Biofilm distribution and bioavailability are expected to be adjusted in response to the buoyancy of microplastics [105]. Biofouling causes changes in the buoyancy of microplastics and, with increasing specific gravity, leads to descension in the water column to a depth of comparable density. Microplastic sampling in the water column can lead to an underestimation of quantities since

Aggregate debris formation can be enhanced by biofilm formation on microplastic surfaces commonly expected in situations where diverse bacterial communities colonize the microplastic surfaces. Aggregation has been confirmed by experiment as a factor leading to the apparent removal of microplastics from the surface layer of

detrimental to humans and ecosystem life when released to organisms that may ingest them [95]. Sorption is a major determinant for bioavailability and contributes to the effects of combined exposure to chemicals and microplastics related to the toxicity and bioaccumulation in humans and ecosystem flora. Neutral charged areas of the microplastic surface offer attractive settings for deposition of chemicals due to attractive hydrophobic forces. This is in contrast with hydrophilic or charged compounds that are attracted to the negative-charged areas on the microplastic surface through electrostatic interactions and aquatic media characteristics [94, 96]. Organic chemicals associated with microplastic debris are typically in the semi-volatile or non-volatile categories such as polychlorinated biphenyls and some organic pesticides [97, 98]. Inorganic chemical species are generally ionic. Fuel chemicals and other higher-boiling constituents can be found in the microplastic debris [88, 99–103]. Weathering can be significantly changed the composition containing

**312**

Significant abiotic and biotic conditions exist to show that plastics are vulnerable to these forces found in the environment. Plastic weathering contributes to structural defects and size reduction but incomplete decay. Chemical and physical degradation processes contribute to the overall weathering process. Plastics are composed of a wide variety of chemical structure features that degrade in a spectrum of kinetics under biotic and abiotic conditions. Biodegradation of plastics under aerobic conditions forms new products during the degradation path leading potentially to mineralization forming process end-products such as CO2, H2O, or CH4 depending on the terminal electron acceptor [108]. Oxygen is the terminal electron acceptor for the aerobic degradation process. Aerobic conditions lead to the formation of CO2 and H2O in addition to the cellular biomass of microorganisms during the degradation of the plastic forms. When sulfidogenic conditions are encountered, plastic biodegradation can lead to the formation of CO2 and H2O. Polymer degradation accomplished under anaerobic conditions produces organic acids, H2O, CO2, and CH4. The aerobic process has been found to be more efficient than anaerobic conditions. The anaerobic process produces less energy due to the absence of O2, serving the electron acceptor, which is more efficient in comparison to CO2 and SO4 −2 [109]. The exposed surface of plastics is where the initial effects of biodegradation are encountered. The biodegradation rate is directly related to the composition of the plastic. The increase of microbial-colonized surface area leads to faster biodegradation rates assuming all other environmental conditions to be equal [110]. Microorganisms can break organic chemicals into simpler chemical forms through biochemical transformation. Plastic biodegradation is a process in which any change in the polymer structure occurs through the structure altering action of microbial enzymes leading to plastic property changes in the form of molecular weight reduction, mechanical strength changes, and surface properties. A more complete understanding of plastic daughter products of environmental degradation is required to more thoroughly understand the effectiveness of environmental plastic degradation.
