Impacts of hazardous substances

IMPACTS OF HAZARDOUS SUBSTANCES

1. Pathways of exposure in marine organisms

The concentrations of various monomers and additives, such as BPA, PBDEs and phthalates are reported to be high in marine plastics. Therefore it is assumed that plastic litter may serve as a pathway for hazardous chemicals to biota. In general, hazardous chemicals can enter marine organisms and food webs via the surrounding environment and diet. A process where chemicals are taken up and stored from the aqueous media to animal tissues through respiratory surfaces or skin is called bioconcentration. It is also possible to obtain chemicals when feeding: the uptake and retention of chemicals is called bioaccumulation, when also diet is taken into account in addition to the surrounding environment.

Sometimes the concentration of a particular substance may be low in the water, but reach harmful levels when biomagnified in the trophic chain. Biomagnification is the process where chemicals are transferred only from food to an organism resulting in higher concentration compared with the source. This is is most frequently detected in animals on higher trophic levels, such as in predatory seabirds and marine mammals.

The major route of chemical transport depends on the organism and the physic-chemical properties of the contaminant and its surroundings. It has been suggested that the chemical components in the digestive tract, such as surfactants, may enhance the leaching of hazardous substances from plastic.The ingestion of plastic and adhered chemicals by various marine organisms is discussed in section Plastic ingestion.

In lower trophic levels, the major route of plastic-derived compound intake is estimated to occur passively via body surface or respiratory organs by diffusion, which is the process of bioconcentration. This can happen for example in small invertebrates, such as plankton, polychaete worms, bivalves and molluscs, but also to fish. These contaminants are accumulated as long as the animal reaches equilibrium with its surrounding environment. Ingested plastic particles may also contribute to accumulation in lower extent.

In higher trophic levels, the animals can get plastic-derived substances directly by ingesting plastic particles or indirectly via food web. Many additives have relatively short half-lives and are easy to metabolize, which makes the biomagnification through food web unlikely. It is therefore estimated that the most important pathway for additives to higher trophic level species is direct ingestion of plastic. It is also estimated that leaching of additives or residual monomers from ingested plastics might be more relevant for large species, which live long, can be chronically exposed and have a long gut retention time.

In theory, the transfer of hazardous substances from plastic upon ingestion depends on the concentration gradient between the hazardous substances in the plastic particle and the animal. If the concentration for the substance is the same and no gradient exists, the plastic is supposed to go through digestive tract without any chemical transfer. If the concentration in plastic is higher than in the surrounding tissues, the hazardous substances are likely to transfer to the animal. Consequently, a negative gradient implies that the ingested plastic might actually reduce the animals’ body burden of hazardous chemicals. However, all the adhered substances in plastic it must be taken into account when assessing the hazards they may pose to an organism. The same plastic particle could theoretically leach additives inside an animal but at the same time clean the organism from some persistent organic pollutants (POPs). The capability of plastic to act both as a source and a sink to hazardous substances in marine environment makes it hard to assess the risks associated to plastics.

The processes described above apply also to other harmful substances present in the ambient seawater and adsorbed from the environment. One of the greatest differences between additives derived from plastics and pollutants adsorbed from the environment lays on their persistence in the marine environment and in the food webs. Whereas many additives, such as phenols, are relatively short-lived and metabolized easily, POPs may persist long times in the aquatic environment and be biomagnified in food webs.

However, the extent of chemical uptake of hazardous substances through ingestion of plastics compared to other exposure pathways is not yet fully understood. It has been estimated, that even though plastics contain and are able to adsorb high concentrations of hazardous substances, the total fraction of POPs sorbed to plastic litter would be < 1 % compared to all other media globally (i.e. water, sediments and biota). It is therefore discussed, whether the plastics are relatively important vector for persistent, bioaccumulative, toxic substances (PBTs) compared to other exposure pathways, such as bioconcentration occurring through skin and gills and consumption of natural prey.

2. effects of plastic leachates

Acute toxicity of leachates derived from different plastics have been studied with a cladoceran (Daphnia magna), which is widely used in toxicology tests due to its sensitivity and suitability for laboratory testing. Tested plastics including polypropylene (PP), high-density polyethylene (HDPE), polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene (ABS) and epoxy were incubated in +50 °C deionized water for 3 days in a concentration of 250 g plastic in one liter of water. Individuals of newborn D. magna were then exposed to leachates for 48 hours. The leachate was considered toxic when it reached the limit of EC50, which is the concentration which causes effect in 50% of the test organisms. After 48 hours 42% of the 26 plastic leachates were acutely toxic in concentrations between 2 and 235 g of plastic per liter. All tested PVC and epoxy products leached acutely toxic chemicals, whereas PP and ABS products did not cause any adverse effects to D. magna. One of the five tested HDPE products caused toxic effects, which indicates that also plastics made from non-toxic monomers (e.g. ethylene) can leach enough additives to harm organisms. In another similar study also polyurethane (PU) and polycarbonate (PC) were found toxic to D. magna in concentrations ranging between 5-80 g/L.

Besides the above-mentioned cladoceran, the impacts of plastic leachates have also been studied in Sweden using copepod Nitocra spinipes. Polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), polyurethane (PU) and three bioplastics were grinded to powders prior to the experiment. 10 grams of each plastic were mixed with 100 ml of brackish water obtained from the Baltic Sea and incubated 72 hours. Adult copepods were exposed to the leachates for 96 hours at +20 °C in darkness and as a result, 38 % of the 21 studied plastic products caused acute toxicity. It was shown that soft and elastic plastics, such as PVC, were most commonly causing toxic effects compared to rigid materials, which is probably related to their chemical structure. Additionally the study examined the effect of artificial sunlight to leaching, but did not find a constant pattern.

The effect of different plastic types, including polylactic acid, high-density polyethylene and polyvinylchloride, and their concentrations to the lugworm (Arenicola marina) have been experimentally studied. After one month of exposure, the lugworms showed elevated metabolic rate in the highest concentration (2 % from the sediment weight) compared to lower concentrations (0.02 % and 0.2 % of sediment weight), which might be a stress response induced by microplastics. Also feeding activity decreased when the microplastics were present in the sediment. The effects were most prominent for PVC. It was not clear, whether the effects of the plastics were physical or chemical; the stronger effects were suggested to be due to chemical leaching of toxic residual vinyl chloride monomers.

3. Harmful substances adsorbed from environment

Many harmful substances, such as PCBs and DDTs, are present in seawater and surface sediments. These persistent, bioaccumulative and toxic substances (PBTs) have low solubility to water and are therefore hydrophobic; thus they have a tendency to sorb onto materials that have similar hydrophobic properties. Traditionally hydrophobic organic compounds have been adsorbed to so-called sorbent organic material, which includes the organic fraction of soils and sediments. However, recently it has been shown that many hydrophobic organic compounds have higher affinity to many types of plastics compared to natural sediments. In addition to organic pollutants, also inorganic heavy metals, such as aluminum (Al), lead (Pb) and zinc (Zn), have been shown to accumulate to plastic pellets.

Many anthropogenic contaminants, including POPs, are known to accumulate on sea surface microlayer, which is a thin (40–100 µm) boundary layer between the atmosphere and the ocean. Given the fact that majority of plastic litter floats on the sea surface, they can easily sorb contaminants from this enriched microlayer. For example field experiments have revealed the potential of polypropylene (PP) virgin pellets to adsorb PCBs and DDE from the surrounding seawater in high concentrations: in a study conducted in Japanese coast the concentration factors for PCBs was reported being as high as 10⁶.

The types of chemicals sorbed to plastics reflect the types and concentrations of these substances present in the marine environment. Spatial variation in the distribution and concentration of these substances has an impact on what compounds and how much is adsorbed to the plastic in different areas. For example, in South Africa and Vietnam higher concentrations of DDT in plastic resin pellets and plastic fragments have been detected, which is suggested to be due to their use as pesticides in these areas. The spatial differences underline the possibility of different risks on different areas. On the other hand, floating plastic litter can travel long distances and end up far from the original discharge site. The pollutant content of plastic item may therefore reflect not only local pollution in the collection site but also pollution along its route to collection site.

4. ADSorption properties of plastics

The concentration and rate of accumulation of different substances on plastic varies depending on the polymer type and substance. The structure of the polymer matrix determines whether the chemical can be adsorbed into the polymer matrix or only to the plastic surface. Adsorbing to the matrix provides greater surface area and therefore greater possible concentration. Sorption will tend towards equilibrium between plastic and seawater and depend on the adsorption kinetics and the relative concentrations of substance.

It has indeed been observed that sorption rates and concentrations of PCBs and PAHs in the environment vary greatly depending on the plastic type. For example polyethylene terephthalate (PET) and polyvinyl chloride (PVC) have been noted to reach the equilibrium with the concentrations of PAHs and PCBs in the environment faster than high-density and low-density polyethylene (HDPE and LDPE) and polypropylene (PP), and their concentrations were also lower. Faster saturation for PVC and PET were suggested to depend on their glassy structure which allows sorption only to the surface, whereas in the case of PE the substances can diffuse slowly into the polymer matrix and end up accumulating more contaminants. This tendency of PE to accumulate more contaminants, such as PCBs, has also been verified when comparing PE and PP plastic pellets and fragments found from the environment.

UV radiation alters the surface properties and surface area of plastics by causing microscopic cracks and therefore accelerating and increasing the sorption capacity. This weathering is retarded in the water due to lower temperatures and lower oxygen content of the water as well as possible foulants covering the plastic surface. It is therefore assumed, that plastic that has weathered long time on land before ending up in the marine environment might have higher capacity to adsorb contaminants from the seawater.

A study comparing the concentrations of PCBs in beached resin pellets found large differences in the pellets collected from the same beach on the coast of Japan. It was observed that the concentrations were higher in discolored particles that had turned yellowish in the environment, which was suggested to be due to their longer residence times in the ocean. Fouled pellets had higher concentration of PCBs, which can also be explained by longer residence time. In another study the discolored particles were observed to have a higher degree of cracking, fissuring and chalking than new pellets.

Finally, also plastic size has an impact on the sorption. Smaller particles have larger surface-volume ratio and have therefore higher size-related capacity to adsorb contaminants from the surrounding environment. When this is combined with the greater possibility of smaller plastic particles to be ingested by various organisms, it is easy to see why there is a rising concern related to microplastics in the marine environment. The ingestion of plastic and hazardous substances by various marine organisms is discussed in section Plastic ingestion.