Plastics can contain a “cocktail of chemicals”
The harmful chemicals associated with plastics can be divided into three categories: ingredients of the plastic material, byproducts of manufacturing and chemicals adsorbed from the environment. The possible toxicological responses caused by plastic can thus be a combination of all of these chemicals. Some of these chemicals are defined as priority pollutants, which are regulated by governmental agencies because of their toxicity or persistence in organisms and food webs. These chemicals include heavy metals, pesticides, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), which can disrupt important physiological processes of animals causing for example diseases and problems in reproduction. It has been found that at least 78 % of priority pollutants listed by EPA (US Environmental Protection Agency) and 61 % listed by EU are associated with plastic litter either deriving from the manufacturing or from the environment.
All plastic, from macro- to nanoscale are concerned to leach and adsorb hazardous substances. The loss of additives by leaching can have an effect on the polymers’ fragmentation behavior in the marine environment on a longer time scale, and this fragmentation might further facilitate the leaching or adsorption of hazardous substances from the environment. The size and surface area are important factors influencing the leaching and adsorption behavior: the smaller the particle the larger the surface-volume ratio is, and therefore also the capacity to release or bind compounds is higher for smaller particles compared to larger ones. It can sometimes be hard to determine whether a compound has been in the plastic since manufacturing or adsorbed from the environment. For example polycyclic aromatic hydrocarbons (PAHs) can be formed during polystyrene production or be adsorbed to the plastic from the environment.
Since different chemicals are present in the marine environment, such as in the sediments, water column, plastics and biota, in different concentrations, their interactions and possible synergic effects have to be taken into account when assessing the impacts to marine life. Up to date most of the studies have assessed the fate and impacts of plastics and their leachates or adsorbed contaminants as a whole without being able to separate the effects caused by individual substances, or on the contrary, examined only the influences of one specific substance without taking into account the chemical cocktail present in the material.
Due to their large molecular size, polymers are usually considered to be biochemically inert and not posing threat to environment. However, unreacted residual monomers or small oligomers can be found in the plastic material since polymerization reactions are seldom complete. Their amounts in the products may vary between few parts per million to several percent depending on the polymer type and manufacturing process. Some of the monomers used are considered harmful: BPA disrupts endocrine function, whereas styrene and vinyl chloride monomers have both shown to be carcinogenic and mutagenic. In contrast, some plastic monomers, such as ethylene and propylene, are not considered hazardous.
In a study assessing the harmfulness of plastic polymers based on their chemical composition, 29 % of studied 55 polymer types were partly or completely made of monomers that are classified either as carcinogenic, mutagenic or toxic for reproduction. Polyurethanes (PUR), polyacrylonitriles (PAN) and polyvinyl chlorides (PVC) were considered as the most hazardous polymer types according to their monomer composition, whereas polypropylene (PP), ethylene-vinyl acetate (EVA), polyvinyl acetate (PVAc) and polyethylenes (PE) were evaluated to be the least hazardous. This ranking does, however, take into account only the monomers and does not pay attention to the numerous additives used in plastic production. Therefore, the overall harmfulness of some of the plastics can probably be different than proposed in this study.
Despite their harmfulness, in many cases the most hazardous substances in plastics are not the monomers but other compounds, such as solvents, initiators, catalysts and other polymerization additives. For example some solvents may be toxic and flammable, and they can sometimes be hard to completely evaporate or precipitate from the polymer when manufacturing. Solvents toxic to aquatic life include for example methanol, cyclohexane and heptane. The most hazardous initiators include potassium persulfate and benzoyl peroxide, which cause respiratory problems and skin irritation. Catalysts are usually based on different metals, and they include for example tributyltin, zinc oxide and copper chloride, which are considered very toxic to aquatic life.
Several thousand different additives are used in the plastic production. Brominated flame retardants, phthalates and lead compounds used as heat stabilizers are considered the most hazardous additive types. Some of the brominated flame retardants, such as PBDEs, structurally resemble polychlorinated bisphenyls (PCBs) which are well-known environmental contaminants accumulating to the fat tissues of aquatic animals. They are for example able to cause neurotoxic effects and alter the function of thyroid hormone. Phthalates are estrogenic compounds that can disrupt endocrine function and reproductive systems of animals, and studies have shown that especially lower molecular weight phthalate plasticizers are acutely and chronically toxic to a variety of aquatic micro-organisms, algae, invertebrates and fish.
Their impacts to the marine life are later discussed in section Impacts of hazardous substances in more detail.
Hazardous substances, such as some plastic monomers, solvents, additives and byproducts, or their degradation products can be released during all phases of plastic life cycle. This is an unwanted situation for the manufacturer, since the leaching of additives shortens the polymer lifetime, but is certainly harmful also to the environment.
Because additives are not usually chemically (covalently) bound to the plastic structure, they are able to leach out from the polymer matrix. Moreover, their leaching is even facilitated by the low molecular weight of the additive. Since additives can form a large proportion of the plastic mass, their leaching may form a substantial amount of all chemical substances leaching out of plastic material. For example PVC may contain more than 40 % by weight of plasticizers, mostly phthalates.
The amounts of chemical emissions depend on various factors. Firstly, the content of these chemicals in the plastic determines how much and what can be leached. The properties of the polymer, such as permeability of the polymer structure, have also a key role. The size of the gaps in the polymer depends largely on the physical state, which can be either glassy, rubbery or crystalline. For example rubbery polymers have larger gaps and therefore higher diffusion rates compared to other types of polymers.
Also the properties of the chemical, such as the size of the chemical compared to the polymer gaps, affect leaching. The molecular weight (size), linearity (shape) and polarity of additives are important factors; for example the volatility and diffusion is higher for additives with small molecular weight. Branched additives are not so easily migrated into liquid compared to linear ones, but instead they volatilize easier to air.
Besides the polymer properties, also surrounding media and its properties have an impact on leaching. When considering the leaching of additives into liquids one must consider the molecular size of the extractant liquid. If it is small enough to penetrate into the polymer structure, it can dissolve additives inside the polymer matrix. Then both the extractant and additive diffuses out of the polymer to the surrounding environment. In addition to the factors described above, also the environmental factors, such as temperature and micro-organisms, are able affect to the leaching behavior.
Leaching of additives and residual monomers from plastic pellets has been shown in a laboratory experiment where virgin plastic pellets made of polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and high-density polyethylene (HDPE) were kept in artificial seawater for 24 hours. Leachable components were found in all of the plastics except polypropylene. The leached compounds from PVC were not identified, but from the other plastic types monomers, such as styrene, and oligomers were recognized.
Similar results have also been obtained in a study which investigated leaching of additives from common plastics items made of low-density polyethylene (LDPE), polyethylene terephthalate (PET), polystyrene (PS) and polyvinyl chloride (PVC). None of the studied plastics were chemically inert and leaching of additives or oligomers were observed in all cases. Leaching rates were high in the beginning of the experiment and decreased with time. However, the leached additives were cumulating giving a highest concentration at the end of the experiment after 57 or 78 days. The detected leached additives and oligomers included for example BPA, phthalates, citrates, styrene oligomers. Interestingly, 98% of leached additives, mainly citrates, from a studied plastic bag made of polyethylene were released from the print on its side.
The study also showed differences in the leaching potential between different plastic types. From the studied plastic types, printed PE and PVC had the highest leaching ratios whereas PET had the lowest leaching potential. Turbulence of the water was shown to increase leaching of additives in all tested plastics. For example PBA leaching from PVC was 11 % higher and in turbulent conditions and even 15–70 fold increase was observed in leaching of phthalates. Largest increase in leaching under turbulent conditions was observed for PVC and PE, which might be due to their flexible structure. In contrast, from the other studied physical parameters, salinity and UV radiation did not affect to the leaching behavior. It was hypothesized that the continuous exposure to UV radiation during the experiment resulted in immediate photodegradation of released additives, since many of them are UV sensitive.
This is also supported by results obtained in a separate study, which exposed the plastic to UV radiation only prior to the experiment and showed that weathering of plastics can increase the overall leaching of hazardous substances. The leaching of BPA from plastic litter was experimentally studied by placing unaged and UV treated 0.8–2 mm granulated polycarbonate plastics (PC) in water collected from the Baltic Sea for 60 days. The leaching of BPA was higher from unaged plastic during the first days of the experiment, but at the end of the experiment leaching was considerably higher from aged plastic which had been treated with UV radiation prior to the experiment. At the end of the experiment, the aged plastic (72 mg /100 ml) had leached BPA so that the concentration in seawater was higher than 600 ng/L, whereas unaged plastic resulted in a concentration less 20 ng/L.