Chemicals (POPs and EDCs) in Marine Litter Plastics: Fate in the Marine Environment
Besides the adverse physiological effects to marine organisms that arise from ingestion of pieces of plastic, plastics in the marine environment may also pose an additional chemical hazard, especially those containing known or suspected endocrine disrupting chemicals as additives or contaminants. Although plastics will not be the only route by which marine species are exposed to hazardous chemicals, existing evidence supports mounting concern in the scientific community that plastics may nonetheless make a significant contribution to exposures to complex mixtures of chemical contaminants [14,18,20,26,29,32-39,69]. The chemicals found in plastic marine litter can be classified in the following four categories of origin:
- • Chemicals intentionally added during the production process (additives such as flame retardants, plasticizers, antioxidants, UV stabilizers and pigments);
- • Unintentional chemicals coming from the production processes, including monomers (e.g., vinyl chloride, BPA, etc.)'—which may also originate from UV radiation onto the plastic waste—and catalysts, normally present in traces (ppm);
- • Chemicals coming from the recycling of plastic waste1-; and finally,
- • Hydrophobic chemicals adsorbed from environmental pollution onto the surface of the plastics.
Whatever their origin, such substances may be directly released from plastics when they reach the guts of marine species, and may otherwise leach to the marine environment when the plastic weathers, at a rate depending on factors such as the nature and strength of the bond between additive and polymer (reactively bonded compounds requiring more energy), pore diameter, molecular weight of the additive, temperature, pressure and biofouling.
Chemicals with endocrine disrupting properties are a major concern for the marine environment. A compilation of lists of chemicals recognized as endocrine disrupting chemicals (EDCs) or suggested as potential EDCs has been developed by the International Panel on Chemical Pollution (IPCP) . The SINList,' developed by ChemSec, compiles those chemicals with most urgent action needed.
In more general terms, experimental research on animals shows that low-level, non-linear exposures to endocrine disruptor chemicals (EDCs) lead to both transient and permanent changes to endocrine systems, as EDCs can mimic, compete with, or disrupt the synthesis of endogenous hormones [20,43,44]. This results in impaired reproduction and consequent low birth rates and potential loss of biodiversity, thyroid function and metabolism, and increased incidence and progression of hormone-sensitive cancers . The research suggests that embryo and developmental periods are critical-sensitive periods to EDCs.5 EDCs may cause effects in cellular and/or animal models at extremely low concentrations .
Some of those intentional chemical additives in plastics with toxic and endocrine- disrupting properties might be present at levels of 1000-500,000 mg/kg (ppm). This is the case of polybrominated diphenyl ethers (PBDEs) used as flame retardants in plastics, polyurethane foams and textiles; tetrabromobisphenol A (TBBPA) , used as flame retardant in epoxy, vinyl esters and polycarbonate resins; or hexabromocyclododecane (HBCDD), used in polystyrene foam (EPS/XPS) or di-2-ethylhexyl phthalate (DEHP) in PVC. It is also recognized that such chemicals can be found as particularly prominent contaminants in marine species collected from areas in which flame retardant-treated plastics are in use. For example, elevated HBCDD levels were found in oysters from aquaculture farms at which EPS/XPS buoys containing HBCDD were present . The observation that high levels of the y-HBCDD isomer, which dominates commercial mixtures of this flame retardant [47,48], can be detected in fish in some European waters
 indicates that direct exposure to technical HBCDD present in the polymer matrix can also be a relevant exposure pathway for fish, as well as the wider environmental exposure to the more stable o-HBCDD.
Further evidence that some POP chemicals are transferred to animal tissues directly from ingested plastic rather than from polluted prey, for example, arises from a study by Tanaka et al.  on short-tailed shearwaters that frequently ingest plastics that they mistake for food. These researchers focused on the presence of specific congeners of PBDEs present in the plastic but not commonly found in their prey (pelagic fish), confirming the presence of those congeners in both the fatty tissues of the birds and in the plastics found in their stomachs.
Other plastic additives of concern in the marine environment include chlorinated paraffins5
 added as flame retardants; polychlorinated biphenyls (PCBs) and polychlorinated naphthalenes (PCNs) included in PVC coatings/paints, and sometimes released as fine particles from abrasive blasting from (e.g., bridges into waters in tonnes scale1) [51,52] and per- and polyfluorinated compounds (PFCs)i [53,54]. Fluorinated polymers containing perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) precursors used in some textile fibers and in paper and paperboard articles (i.e., fast-food packaging and paper plates, cups, etc.) to provide grease and water resistance , can become microplastics/ fibers in the aquatic environment and release PFOS when degrading or ingested  .
Other chemicals of concern include plastic additives with known or suspected endocrine disrupting properties, including alkylphenols (octylphenol and nonylphenol) used mainly as antioxidants, bisphenol A (BPA) present in polycarbonate plastics as trace monomer, phthalate esters—e.g., di(2-ethylhexyl) phthalate (DEFIP), diisodecyl phthalate (DIDP), diisononyl phthalate (DINP) and butyl benzyl phthalate (BPP)—widely used as plasticizers in proportions up to 60% of the weight of a plastic to increase properties such as flexibility, transparency or longevity, and organotin compounds (based on methyl, butyl or octyl groups, such as tributyltin5) used as stabilizing additives in some PVC polymers. For example, Takada et al.  and Flirai et al.  analyzed a wide range of chemicals in marine plastics collected from urban and remote beaches and open oceans, including theoretically “non-persistent” additives such as alkylphenols (i.e., nonylphenol, octylphenol and BPA) that were detected in concentrations ranging from ng/g to pg/g in polyethylene and polypropylene debris.4 Moreover, a significant correlation has been demonstrated [18,60] among seven different phthalate esters (phthalates or PAEs) present in samples taken in the same area of microplastics, plankton and bubbler samples of different cetacean species.'”
Some of these chemicals with endocrine disruptor properties may not qualify as “persistent” under the strict criteria of the Stockholm Convention, which requires in the screening criteria of its Annex D, evidence of its half-life in water, soil, sediments and air. Nevertheless, when present in a polymeric matrix in marine conditions, they may be potentially as harmful as officially recognized POPs in terms of behavior and consequences in the marine environment, as their presence is “topped-up” by the continuous flow of “fresh” plastic waste in river discharges, urban runoff and wastewater and associated sediments [41,61]. Their adsorption to microplastics, combined with the harsher environmental conditions of low temperature and salinity, combined also with low light and low oxygen content in subsurface waters and sediments, may also enhance their persistence in marine systems and their mobility and fluxes through all the compartments of the marine environment . Sorption of contaminants in nanopores of plastics may further inhibit contaminant biodegradation . Taking into account also that is very difficult or even impossible to establish a threshold of toxicity for many EDCs, as low dose effects and non-monotonic dose responses (NMDR) are common , the overall result would be that those substances in plastics in the marine environment may, through their widespread and pervasive distribution, present equivalent levels of concerns to those of recognized POPs. In this regard, such characteristics and evidence would allow equating EDCs in marine plastic waste with the defining properties of a POP under the Stockholm Convention. This is further discussed in “Contribution from the Stockholm Convention on Persistent Organic Pollutants,” on potential measures for the consideration of the Stockholm Convention.
It should be noted that recycled plastic/polymers can also carry a high content of toxic chemicals carried over from their source plastics, and may also therefore contribute to chemical exposure of the marine environment' when they reach the ocean. The fact that much of the plastic waste collected for recycling is exported to countries with low legal requirements or technical capabilities on the control of the different types and concentrations of hazardous substances contained in the plastics is an added source of concern, as the concentration of those toxic chemicals may increase in the recycled products.
With regard to the pollutants present in sea water and adsorbed onto the plastic surface, it has been estimated that fluxes of PCBs, PBDEs and PFOA to the Arctic caused by plastic debris was in the order of four to six times smaller than fluxes caused by atmospheric or seawater currents . It is important to keep in mind, however, that the significance of pollutant transport routes does not only depend on the absolute amount of pollutants, but also on their impact from direct plastic ingestion and bioaccumulation in food chains . In this regard, a qualitative distinction has to be made between microplastics and nanoplastics.
In microplastics, the adsorption of pollutants has been experimentally demonstrated from virgin plastic pellets in seawater, which implies that plastics constitute both a transport medium and a potential source of toxic chemicals in the marine environment [22,58,59]. The mechanisms of concentration of these chemicals is a complex issue depending of multiple variables including hydrophobicity of the pollutant, type of polymer, age of the plastic, water, temperature, pressure, presence of biofouling on the plastic surface and salinity. It is without doubt that other media present in the oceans, including natural sediments and the sorbent organic matter (SOM)—composed of suspended organic particulates, black carbon and natural diet and planktonic species—also have the capacity to adsorb hydrophobic organic chemicals (HOCs), such that ingestion of plastics will not be the only source of exposure to such chemical agents. Indeed, on average, the fraction of HOCs adsorbed to marine plastics appears to be statistically smaller when compared to that adsorbed fraction in other media in the ocean, such that chemical exposure of marine biota might be dominated by those other matrices . Nonetheless, for certain chemical groups and/or specific local conditions with high concentrations of plastic matter, the importance of contaminant transfer from plastics may well be of quantitative significance.
In nanoplastics, the high surface area may present exceptionally strong sorption affinities for pollutants, thus changing the exposure and risk to these chemicals  and further increasing their significance as contributors to overall chemical exposure. In this regard, Koelmans et al.  affirm that: “because of the surface effect, it may be possible that nanoplastics retain organic toxic chemicals or heavy metals at higher concentrations than microplastics, thus leading to a fugacity gradient to organism tissue once ingested. If nanoplastics are capable of permeating membranes, passing cell walls, translocate and/or reside in epithelial tissues for prolonged times, the combination of particle and chemical toxicity may yield unforeseen risks.” Velzeboer et al.  affirm that: “Nano- plastics have been shown to pass through the chorion of fish eggs and have been shown to move directly from the digestive tract of mussels into their circulatory system. This implies that occurrence of HOC contaminated nanoplastics in the environment may potentially enhance uptake.” Unfortunately, there are currently no sufficiently developed analytical methods adequate to detect and quantify nanoplastics in the environment or food chain , let alone to analyze their chemical signature in detail.
-  TBBPA degrades to bisphenol A and to TBBPA dimethyl ether. Bisphenol A and phthalates are rapidly metabolizedonce ingested but their concentration within the tissues varies between species for the same exposure. s Short-chained chlorinated paraffins are listed in Annex A (elimination) of the Stockholm Convention sinceMay 2017.
-  PCBs and PCNs have been used to some extent as flame retardants in cables and other polymers includingPVC coatings for corrosion protection. Such coatings are sometimes removed from bridges and dams byabrasive blasting and end up in rivers and the sea. f http://greensciencepolicy.org/highly-fluorinated-chemicals/. * PFCs in the environment can last for millions of years. s Marine paint containing tributyltin was forbidden by the International Convention on the Control ofHarmful Anti-fouling Systems in Ships (entered into force in 200S), signed by most of the countries. Teuten et al.  tested the sorption uptake and desorption kinetics of HOCs in different polymers inlaboratory conditions, showing that glassy polymers such as PVC exhibit larger sorption capacities andslower HOC release rates than rubbery polymers such as high-density polyethylene. Mato et al.  showedthat polyethylene has higher affinity than polypropylene for HOCs.
-  This finding suggests a new non-invasive method, which is to use the PAEs found in plankton as tracers ofthe exposure/ingestion in cetaceans or other endangered species.
-  Articles with any substance listed under the Stockholm Convention, such as HBCDD used mainly in EPS/XPS polymers, are not allowed to undergo recycling processes, except articles (plastics) with hexa-, hepta-,tetra- or pentabromodiphenyl ethers that would allow some countries to recycle them until 2030 under anexemption of the Convention. f For example, 50% of the plastic waste collected for recycling in the EU is exported to third countries withno sound environmental waste management guarantees (source: Plastic Recyclers Europe).