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Déchets marins et microplastiques

4. Can micro-plastics transport contaminants into the marine environment?

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    3.4.2 The relevance of plastic particles as a contaminant transport route

    The workshop considered the importance of plastics as a possible transport route for PBTs relative to the atmosphere or in dissolved or adsorbed form in seawater. It has been demonstrated that marine microplastics contain a wide-range of organic contaminants including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons, organochlorine pesticides (DDTs, HCHs), polybrominated diphenylethers (PBDEs), alkylphenols and bisphenol A (BPA), at concentrations from sub ng/g to μg/g (Mato et al., 2000; Rios et al., 2007, Teuten et al., 2009). Concentrations of PBTs adsorbed on plastics showed distinct spatial variations reflecting global pollution patterns (Ogata et al., 2009). Together with the spatial pattern, non-uniform distribution (i.e., piece-to-piece variation) in the concentrations of PBTs in the microplastics was observed (Endo et al., 2005; Ogata et al., 2009).

    The workshop discussed three basic scenarios, with which the fate of transported chemicals in microplastics might be examined. It should be stressed that what follows here are hypotheses and that the workshop did not reach conclusions on the specifics of this issue:

    Hypothesis 1; the sorption of PBTs to micro-plastics is reversible.

    Micro-plastics will act as reversible passive samplers of pollutants to and from the water column (and atmosphere). This could mean that micro-plastics take up (absorb) PBTs in regions where PBT concentrations are high, and could release (desorb) PBTs in cleaner, remote regions. Depending on the type of micro-plastic, sorption could be slow due to internal diffusion (e.g., LDPE), resulting in the core of the micro-plastic not being in equilibrium with the outer surface of the particle.

    Hypothesis 2; for most PBTs, atmospheric transport dominates.

    Micro-plastics may matter as a source of PBT’s only where long-range atmospheric transport (LRAT) is low. In view of the low concentrations of micro-plastics reported in the Ocean, it seems likely that long-range atmospheric transport will dominate along wind trajectories (i.e., within hemispheric transport cells, and into the Arctic; cross-equatorial transport in the troposphere is slow - a year or more - but transport to remote ocean regions within a hemisphere is rapid).

    Hypothesis 3;

    micro-plastics are stable in the surface water Micro-plastics will serve as a stable phase in addition to organic matter in the water column and biota, so stabilizing PBTs in the water column, thereby reducing their sinks. PBTs then partition between air, water, sediment and biota, preferentially into the organic carbon and lipid phase of the latter. The presence of micro-plastics will provide an additional, mostly attractive phase for PBTs to diffuse into. As micro- plastics are not expected to be degraded in an organism’s gut, micro-plastics could stabilize PBTs in the environment and reduce other sinks, such as sedimentation with organic carbon.

    Zarfl and Matthies (2010) estimated mass fluxes of polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), and perfluorooctanoic acid (PFOA) sorbed on plastics to the Arctic via the main ocean currents and compared this route to the dissolved state and via the atmosphere. Substance fluxes of these chemicals in which atmospheric transport or sea water currents account for several tons per year are predicted, whereas those mediated by plastics are four to six orders of magnitude smaller. However, these authors also considered that the significance of various pollutant transport routes does not depend only on absolute mass fluxes but also on bioaccumulation in marine food chains.

    There is a strong theoretical basis and also plenty of empirical data to show that PE and other (micro) plastics emitted to the environment can absorb chemicals of concern, adsorption capacity is increased by deterioration and depends on the type of polymer, e.g. Endo et al. (2005), Ogata et al. (2009), Teuten et al. (2009) and Frias et al. (2010). Plastic pellets (nurdles) are even utilized as passive samplers, e.g. Ogata et al. (2009), Lohmann and Muir (2010) and Smedes et al (2009).

    Based on the fugacity modelling approach a “fugacity-capacity” can be estimated to assess the tendency of chemicals to partition between air, water, plastics and organic carbon present in sediments as a result of their relative volumes. Previous work has established that in general, plastics favours the accumulation of organic chemicals with high octanol- water partitioning coefficient (log KOW), thus acting similar to lipids in organisms and organic carbon in sediments (See Box below for an example). A limitation to modelling approaches, which are based on equilibrium partitioning, is that they fail to consider the dynamics of the system, for instance the kinetics of partitioning between environmental media and the plastic or the influence of accumulating plastic with time. Additionally, this model assumes an unrealistically uniform distribution of both pellets pellets and PBTs.

    3.4.3 Contaminant uptake and release

    It is suspected that plastics may transfer PBTs which do not undergo long-range atmospheric transport from coasts to the interior of Oceans (See Zarfl and Matthies, 2010 in relation to transport routes to the Arctic and the possible role of plastic particles). Time-scales of sorption and desorption are a function of the type of plastic (Teuten et al. 2009), its size, the compound of interest and diffusion across the water-plastic interface. Karapangioti and Klontza (2008) studied the absorption kinetics of phenanthrene in plastic pellets and concluded that the material from which the pellet is made, the size of the plastic particle and its state of ageing or weathering can influence kinetic processes of uptake and the diffusion rate within the polymer. For LDPE, times to reach equilibrium are ca. 50 – 100 days for particles the size of plastic pellets, but far shorter, e.g. a couple of days for PE films that are 50 μm thick.

    Modelling approaches

    An example was presented of a modelling approach to assess the potential behaviour of chemicals absorbed in PE beads that have been released to the environment as a consequence of their use as an exfoliator in personal care products such as facial cleansers (Fendall and Sewell, 2009). It is estimated that ca. 260 tonnes is currently formulated per year in the USA (with an estimated per capita consumption of 0.88g/person/year). A typical concentration of PE beads in formulations is 0.5 – 5%, and the particles are from 4 μm to 1mm with a median of 250 μm. Based on a conservative estimate, for modelling purposes, it is assumed that only 25% of PE beads discharged to municipal waste water treatment plants (WWTPs), as a result of there use in facial cleansers, is retained and that therefore 75% can escape to surface waters. It is further estimated that ca. 43m3/y might reach the sea along the west coast off the USA. Modelling was then applied using and area of 1000 km x 100 km to represent the California coastline) with the help of chemical space diagrams. However, with only 43m3 of PE micro-plastic, in this area, it was demonstrated that chemicals will partition predominately between air and water. Adding a sediment compartment results in an increase of partitioning of substances with Log KPE-W > 5 to accumulate in sediment, i.e. introduces competition with the plastic. Therefore due to the volume ratios in the scenario, a significantly large amount of PE micro-plastic would be needed to be present in the aquatic environment from the above source to result in significant partitioning; otherwise biological exposure to chemicals in the water and air will be of much greater concern.

    Among the microplastic studies by Endo et al., (2005) and Ogata et al. (2009), pellets with sporadic high concentrations of PCBs were observed. Large (up to 3 orders of magnitude) piece-to-piece variation was observed among the plastic resin pellets collected from a single beach, indicating slow sorption/desorption. These microplastics with sporadically high concentrations of PCBs could expose significant amount of PCBs to biota which ingest the plastics (Endo et al., 2005). For instance, if we recognize that it takes 7 to 180 days for substances with a high log Kow such as PCB’s and PBDE’s to reach equilibrium in plastic particles (200μm thick, then it is reasonable to expect that it may take a comparable amount of time for contaminants to desorb once ingested by an organism, if environmental conditions within the gastrointestinal gut of an organism are such that desorption would be favoured.

    Teuten et al. (2007) carried out adsorption/desorption experiments in-vitro with combinations of clean media and phenanthrene equilibrated sediments and PE particles and predicted that the presence of phenanthrene contaminated plastic particles was likely “to give a significant increase in phenanthrene accumulation in the lugworm Arenicola marina, a sedimentary deposit feeder known to ingest plastic particles. Citing Voparil & Mayer (2000), who demonstrated experimentally that the presence of digestive surfactants in polychaete worms increases the ‘bioaccessibility’ of sediment-bound contaminants Teuten et al. (2007) considered that gut-surfactant mediated desorption may play an important role in the transfer of contaminants from plastic particle to benthic deposit feeders. In this context, Voparil and Mayer (2000) noted that gut fluid concentrations of high molecular weight PAHs are greater than those predicted from equilibrium partitioning theory, indicating the importance of the digestive pathway for hydrophobic organic contaminant exposure and bioaccumulation. The workshop considered that the quantification of the size ranges and identification of the type of plastic particles present in the environment needs to be given priority; this will allow a better understanding of the kinetics of plastic absorbed contaminants as well as potential chemical and physical effects related to particle size. Furthermore, uptake and distribution patterns of micro-plastic particles along food-chains needs to be analysed in different geographic areas.

    Source & ©: ,  Proceedings of the GESAMP International Workshop
    on micro- plastic particles as a vector in transporting persistent, bio- accumulating and toxic substances in the oceans. 28-30th June 2010, UNESCO-IOC,
    Paris. 3.4 Transport, distribution and fate including deterioration and degradation routes. P.21-24.

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