<\/p>\n
<\/p>\n
<\/p>\n
Plastic litter in marine environments was first observed in the 1970s1<\/a>,2<\/a><\/sup>. Since then, plastic litter in the environment has become an issue of global concern. Global plastic production has been increasing steadily, and so has the amount of plastic found in the environment3<\/a>,4<\/a><\/sup>. For example, between the 1960s and 1990s, fibre concentration in the northeast Atlantic increased from 0.01 to 0.038 fibres per cubic metre4<\/a><\/sup>. Only in the past decade have the occurrence and distribution of plastic particles in the size range 1 \u03bcm to 5 mm, defined as microplastics by Thompson in 20044<\/a><\/sup>, been studied intensively in the marine environment (ref.\u00a05<\/a><\/sup>\u00a0and citations therein). Freshwater and terrestrial environments came into the focus of research as sources and emission pathways5<\/a>,6<\/a>,7<\/a><\/sup>, and a similar degree of plastic contamination was found in both8<\/a>,9<\/a>,10<\/a><\/sup>. Still, environmental microplastic concentrations mostly remain well below the concentrations of natural particles such as colloids, detritus and black carbon11<\/a><\/sup>, and below threshold concentrations that lead to adverse effects in organisms12<\/a>,13<\/a><\/sup>.<\/p>\n<\/div>\n As recently as 2015, it was speculated that plastic particles smaller than 1 \u03bcm, referred to as nanoplastics, might occur in the environment14<\/a>,15<\/a><\/sup>: indeed, this seems to be common sense even in the absence of reliable data. Laboratory experiments have shown that these plastic nanoparticles may be generated during fragmentation of larger plastic specimens15<\/a>,16<\/a><\/sup>\u00a0or be released directly from products that contain nanoplastic particles17<\/a>,18<\/a><\/sup>. However, no comprehensive field data on exposure exist, and there is limited information about the potential impacts of nanoplastics in the environment. Eventually, assessing the risks posed by nanoplastic will depend on addressing four main questions: namely, how much nanoplastic is in fact released in the environment, where it comes from, what happens to it after its release, and what its impact is.<\/p>\n <\/p>\n<\/div>\n Assessing exposure<\/strong><\/p>\n To answer the question of how much nanoplastic is in the environment, sensitive and selective analytical techniques are required to detect different types of nanoplastic as single particles or in bulk. The analytical methods must be sensitive towards particle sizes in the nanometre range and concentrations down to nanograms per litre. Further, the methods must be selective towards various types of polymers in environmental matrices. Therefore, it was suggested that methods that had already proved useful for nanoparticle detection in laboratory experiments might be applicable to detect nanoplastic and investigate its fate and effects19<\/a><\/sup>. But are these methods also suitable for environmental samples? For instance, nanoparticle tracking analysis, scanning and transmission electron microscopy, or dynamic light-scattering techniques are not selective to plastic particles but require additional sample preparation. Fourier transform infrared spectroscopy, molecular imaging such as time-of-flight secondary ion mass spectrometry and pyrolysis gas chromatography promise to be selective and may therefore allow the detection of nanoplastic directly from the particulate matrix. Some of these methods, although not yet mature, have been applied to detect nanoplastic and microplastic in the lower micrometre size range, but mostly in laboratory experiments14<\/a>,20<\/a>,21<\/a><\/sup>. A further limitation is that in the laboratory, these methods use nanoscale polymer beads as reference material. Such materials typically have a well-defined particle size, concentration and structure, unlike nanoplastic fragments in the environment with their wide variety of structure, polymer type, shape and particle size22<\/a><\/sup>. Hence, reference materials mimicking the properties of nanoplastics from the environment are required.<\/p>\n To assess the environmental impact of nanoplastic, we need data on exposure in marine, freshwater and terrestrial settings. These data are currently very limited20<\/a><\/sup>, primarily because of the lack of analytical methods, so environmental concentrations are only estimates. They show an increasing trend over time23<\/a><\/sup>\u00a0because of the potential release by fragmentation and degradation of macro- and microplastic24<\/a><\/sup>, an increased application in products18<\/a><\/sup>, and its generation as by-product during manufacturing25<\/a><\/sup>. Number concentrations of nanoplastic in the environment may be simply estimated based on fragmentation of microplastic in the environment: the hypothetical complete fragmentation of one spherical microplastic particle of 5 mm in diameter would result in 1014<\/sup>\u00a0spherical fragments of 100 nm in diameter. However, such fragmentation could require timescales of several hundreds of years17<\/a>,23<\/a><\/sup>. This example shows that environmental exposure assessment for nanoplastics is largely speculative. The contribution of nanoplastics to the total colloidal matter concentration, including ubiquitous natural colloids, will vary strongly and is expected to be minor in most cases.<\/p>\n <\/p>\n<\/div>\n<\/div>\n<\/section>\n Origin of nanoplastic<\/strong><\/p>\n Emissions of nanoplastic can be direct, or they can be released from microplastic and macroplastic because of fragmentation and degradation17<\/a><\/sup>. Direct emission from products and applications includes waterborne paints, adhesives, coatings, biomedical products (drug delivery, medical diagnostics), electronics, magnetics and optoelectronics17<\/a><\/sup>. A recently acknowledged source is 3D printing, in which polymer particles in the nanometre size range are generated25<\/a><\/sup>.<\/p>\n Fragmentation of microplastics may be considered as a source in the environment26<\/a><\/sup>. Researchers have started to investigate fragmentation processes of macro- and microplastics14<\/a>,15<\/a>,27<\/a>,28<\/a><\/sup>\u00a0in simplified laboratory experiments, but we still lack comprehensive data that allow us to predict the fragmentation process in nature and its contribution to the overall mass balance of plastic.<\/p>\n <\/p>\n<\/div>\n<\/div>\n<\/section>\n Environmental fate<\/strong><\/p>\n Because its sources vary, nanoplastic is highly polydisperse in physical properties and heterogeneous in composition14<\/a>,15<\/a>,20<\/a><\/sup>. Natural particles also exhibit a wide variety of physical properties and composition. Hence, the expected fate processes of nanoplastics will be highly variable, and homo- and hetero-aggregation, advective flow transport, sedimentation, re-suspension, photo- and biodegradation, and sediment entrapment may occur. For instance, at particle sizes below a few micrometres, Brownian motion becomes increasingly relevant29<\/a><\/sup>, whereas for larger particles sedimentation controls the fate30<\/a><\/sup>. Hence, unattached nanoplastic particles are likely to remain in the water column, and sedimentation may not occur, leading to potentially long transport distances in surface waters. However, interactions with other particulate matter are likely to occur, forming larger hetero-aggregates which then can be subjected to sedimentation30<\/a>,31<\/a><\/sup>. Modelling studies showed that particles in the size range of 5 \u03bcm had the highest mobility, whereas larger plastic particles and nanoplastics were retained preferentially32<\/a><\/sup>.<\/p>\n Besides these particle\u2013particle interactions, release of chemicals (for instance anti-oxidants and flame-retardants) from the nanoplastics, or sorption of chemicals to the nanoplastics, may take place, modulating the availability of potential hazardous chemicals in the aquatic environment. This assumption must be handled with care because of low nanoplastic concentration compared with natural particle concentration11<\/a><\/sup>. Given that particles of all size ranges are found in the environment and that these particles interact with each other as well as with solutes, research on nanoplastic may focus on properties that differ significantly from those of natural particles.<\/p>\n <\/p>\n<\/div>\n<\/div>\n<\/section>\n Effects in the ecosystem<\/strong><\/p>\n Like the ubiquitous natural colloids, nanoplastics may affect individual organisms, habitats and ecosystems. In particular, plastic particles in the nanosize range may cause adverse effects owing to their higher potential for uptake into cells and tissues, and higher surface-area-to-volume ratios that make them prone to sorb and release chemicals13<\/a><\/sup>. Although the number concentrations of nanoplastics are expected to be much higher than for microplastics, estimated environmental concentrations have not yet exceeded threshold values for effect concentrations23<\/a><\/sup>\u00a0\u2014 but, owing to continuous emissions, they may do so in future.<\/p>\n Given the higher natural particulate concentrations, interactions between nanoplastic and natural colloids and other particulate matter may modulate the exposure and, thus, the risk associated with nanoplastics. It is recommended to compare the toxicity of natural particles, nanoplastic and heteroaggregates to identify hazards posed specifically by nanoplastic to aquatic and terrestrial organisms. The nanoplastic-specific risk may be relevant only if the hazard due to nanoplastics is higher than for natural particles or if the exposure to nanoplastics is above natural particle concentrations.<\/p>\n<\/div>\n<\/div>\n<\/section>\n <\/p>\n <\/p>\n <\/p>\n (\uc6d0\ubb38: \uc5ec\uae30<\/a>\ub97c \ud074\ub9ad\ud558\uc138\uc694~)<\/p>\n <\/p>\n <\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":"