Interaction of Chemical Pollutants with Microplastics
The industrial, agricultural and other anthropological activities on land lead to significant pollution of the aquatic environment with hazardous chemicals [1,57]. Approximately
625,000 barrels of oils are discharged into the coastal waters every year as a consequence of runoff from land. Plastic materials have been estimated to release between 35 and 917 tonnes of chemical additives into the marine environment every year, with a majority being released from plasticized PVC [1,58].
Plastic fragments which get deposited in the shallow or deep waters of the marine environment not only pose physical pollution problems but are also a potential source of significant chemical hazard. Although plastics are considered to be biochemically inert material which do not interact with the human endocrine system owing to their large molecular size, the plastic debris existing in the marine environment may well carry chemicals of smaller molecular size which can enter the cell, chemically interact with important biological entities or molecules and disrupt the endocrine system . Such chemicals can be categorized as [1,48]:
- • Chemical pollutants, which can get absorbed into the bulk of plastic material by diffusion
- • Hydrophobic chemicals, which can be adsorbed from the surrounding seawater owing to their affinity for the hydrophobic surface of plastics
- • Additives, monomers and oligomers of the component molecules of the plastics
Many of these chemical pollutants are considered to be persistent because they are highly resistant to environmental degradation and consequently remain in the marine environment for considerable periods of time [1,61]. In a study conducted for contaminants of growing concern, the time period in which a contaminant decreases from being of highest concern to a baseline level of the lowest concern was typically 14.5 ± 4.5 years . Many such pollutants can bioaccumulate in organisms, including humans, and possess the ability to traverse the food chain as well as being passed from mothers to their offspring . The persistent hazardous chemicals of this category are termed as persistent organic pollutants (POPs). Some classes of pollutants have been demonstrated to interact with microplastics and are designated as POPs. The Stockholm Convention on Persistent Organic Pollutants was created in 2001 by the United Nations Environment Programme (UNEP) with an objective to protect human health and environment from POPs. The convention lists POPs with considerable health effects and undertakes measures to reduce, restrict or eliminate the manufacture of POPs . The chemicals that are included in the list follow specific criteria, such as possessing a half-life of at least two months in the aquatic environment, showing evidence of widespread dispersal from its source, remaining stable and demonstrating bioaccumulative and toxicological effects .
The list initially consisted of 12 chemical pollutants, namely aldrin, chlordane, dichlorodiphenyltrichloroethane (DDT), dieldrin, endrin, hexachlorobenzene, heptachlor, mirex, polychlorinated biphenyls, polychlorinated dibenzofurans (PCDF), polychlorinated dibenzo-p-dioxins (PCDD) and toxaphene that were known as the “dirty dozen.” It has now been extended to include up to 26 chemical substances of potential concern. Some other classes of pollutants that pose considerable hazards to biological organisms include perfluoroalkylates, phthalates, polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs). The plastic monomer bisphenol A (BPA) and alkvlphenol additives have been known to exert estrogenic effects while some phthalate plasticizers are associated with reduced testosterone production . PCBs and PBDEs are persistent, bioaccumulative and toxic (PBT) chemicals and pose serious concern to human health which has resulted in their inclusion in the List of chemicals for Priority Action adopted by OSPAR (Oslo and Paris Conventions) Commission [61,62]. PCBs are highly resilient in nature and have been estimated to remain as the most widespread contaminant in the aquatic environment and organisms until at least 2050 .
The interaction of microplastics with POPs involves three distinct phenomena :
- • Absorption: Waterborne chemical pollutants diffuse into the bulk of the plastic material by absorption. The PAH phenanthrene diffuses into the bulk of polypropylene matrix by surface diffusion.
- • Adsorption: During the adsorption of POPs to the microplastics, the POP acts as the adsorbate and microplastic as the adsorbent/substrate. Some microplastics, such as polyethylene, polystyrene and polypropylene are nonpolar in nature, while some others, such as polyamide and polycarbonate, are polar. The more nonpolar a plastic material is, the greater is its affinity for hydrophobic POPs.
- • Desorption: The pollutants which contaminate microplastics may be desorbed after ingestion of the microplastic by aquatic organisms. A study has demonstrated that the microplastics composed of polyethylene that are contaminated with the PAH phenanthrene exhibited the highest potential for transfer from microplastics to organisms .
Microplastics in the marine environment can be capable of concentrating waterborne POPs by up to 1 million times higher than their background concentration in the seawater . The ingestion of contaminated microplastics by aquatic organisms represents a unique pathway for introduction and subsequent traversing of toxic chemicals into the food web. However, there is wide scope for research in the ways by which microplastics can act as vectors for the introduction of chemicals in the marine food web.