Microplastics as vectors for environmental contaminants: Exploring sorption, desorption, and transfer to biota.
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Microplastics as Vectors for Environmental Contaminants : Exploring Sorption, Desorption, and Transfer to Biota
- Microplastics as Vectors for Environmental Contaminants : Exploring Sorption, Desorption, and Transfer to Biota.
- Page 1 of 17 Microplastics as vectors for environmental contaminants: Exploring sorption, desorption, and transfer to biota.
Keywords
- Microplastics, contaminants, hydrophobic organic chemicals (HOCs), exposure, transfer, ecotoxicity Page 3 of 17 I. INTRODUCTION Interactions between microplastics (MPs) and organic contaminants take place before, during, and after their release to natural environments.
- The role of MPs as contaminant vectors has been the topic of experimental studies and review papers, both supporting and challenging this hypothesis.
- Some argue that plastic debris and HOCs form complex cocktails that increase the overall bioavailability of HOCs to aquatic organisms and ultimately to humans (Vethaak and Leslie 2016).
- The authors discuss the impact of different uptake routes on MP-facilitated HOC transfer into biota, including direct MP contact exposure (internal and external).
- Finally, the authors point to knowledge gaps and research required for a more comprehensive understanding and modelling of these processes.
EXTRINSIC PROPERTIES
- Due to their hydrophobicity and lipophilicity, HOCs sorb to non-polar phases in natural aquatic environments, including sediment particles, suspended organic matter, and MPs.
- Recent tests with seven different HOCs and four polymers showed that the sorption process depends strongly on the polymer type (Hüffer and Hofmann 2016).
- Polymers often contain both crystalline and amorphous (i.e., non-crystalline) regions.
- Page 8 of 17 Desorption of adsorbed and absorbed molecules will depend on many factors and will generally decrease with increasing partition ratios and increasing binding strength.
- Even if seemingly counter-intuitive, these mechanisms suggest that desorption and leaching of HOCs from MPs often will occur to a greater extent and faster when they are absorbed rather than adsorbed.
III. TRANSFER OF HYDROPHOBIC ORGANIC CHEMICALS BETWEEN
- A conceptual framework for mechanisms involved in the role of MPs as vectors for HOCs and their transfer into aquatic organisms has been proposed (Koelmans et al. 2016).
- What is not implicitly included in these model frameworks, however, is direct contact exposure.
- HOCs sorbed to MPs are thus expected to be Page 10 of 17 transferred more rapidly to biota through digestive fluid than through water, and direct contact of MPs with exterior or interior parts of the organism might be an important and so far overlooked uptake route.
- In oceans, the mass of water is estimated to be a factor of 1013 higher than the mass of plastic, and the volume of organic carbon in marine coastal environments was modelled to be more than 107 times greater (Koelmans et al. 2016; Gouin et al. 2011).
IV. LINKING LABORATORY AND ENVIRONMENTAL CONDITIONS
- Environmental sampling experiments provide a relatively unbiased view of the sorption of HOCs to MPs under given environmental conditions.
- Directly linking cause and effects, for both chemical processes and biological effects, is challenged by the vast number of parameters that will have varied at a given location over time.
- Laboratory experiments, on the contrary, apply Page 12 of 17 conditions that may mimic, but not replicate, environmental conditions.
- Field sampling and laboratory testing are thus complementary.
V. KNOWLEDGE GAPS AND FUTURE RESEARCH DIRECTIONS
- The mechanisms and kinetics of degradation of MPs, and the consequences for MPs as HOC vectors, are not well understood (Eerkes-Medrano et al. 2015; Rocha-Santos and Duarte 2015) and should thus be further investigated.
- The processes of adsorption versus absorption of HOCs to MPs need to be better understood, especially with regards to how sorption and desorption kinetics may differ between MPs and other naturally occurring particulate matter, and how this may influence transfer into biota.
- For micro- and especially nano-sized particles, whose sizes are minute compared to the matrix they are in, this poses a range of challenges inherent to the related scientific field of nanotoxicology.
- To more precisely assess the relative importance of MPs as vectors for HOCs, compared to natural pathways, there is a clear need for more appropriate sampling methods to avoid underestimations of actual environmental MP concentrations.
CONCLUSION
- While the relative role of MPs as vectors for HOCs to organisms is generally considered minor in comparison to that of natural exposure pathways (such as water, food, and natural particulate matter) under present conditions, it is important to emphasize that: MP concentrations and environmental conditions change over time; and, spatiotemporal MP hotspots do (and will) occur.
- To better evaluate the role of MPs as pathways for HOC transfer into biota under such temporally and spatially varying conditions, an improved understanding of the governing processes is needed.
- Sorption/desorption processes differ between different polymers as well as between MPs and various natural particulate matter.
- Additionally, weathering will change HOC sorption and desorption.
- How these factors influence the role of MPs as HOC vectors is a topic of further research.
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"Microplastics as vectors for enviro..." refers background in this paper
...Furthermore, glassy polymers have internal pores (“nanovoids”), creating strong adsorption sites and contributing to slow HOC release rates (Teuten et al. 2009)....
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...The amorphous regions, where sorption of HOCs generally occurs (Teuten et al. 2009), can be either “glassy” or “rubbery,” depending on the glass transition temperature of the polymer....
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...Microplastic shape and size define the surface-to-volume ratio and diffusional length scales, which in turn determine the time to reach equilibrium and the rate of absorption and desorption (Teuten et al. 2009)....
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...In contrast, reactions with O2 can increase the surface polarity, which decreases the affinity for HOCs (Endo et al. 2005; Teuten et al. 2009)....
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1,688 citations
"Microplastics as vectors for enviro..." refers background in this paper
...First, it is very likely that the actual occurrence of MPs in the environment is currently underestimated due to sampling method issues (Eerkes-Medrano et al. 2015; Kooi et al. 2016) and a lack of consensus regarding samplingmethods (RochaSantos and Duarte 2015)....
[...]
...The mechanisms and kinetics of degradation of MPs, and the consequences for MPs as HOC vectors, are not well understood (Eerkes-Medrano et al. 2015; Rocha-Santos and Duarte 2015) and should thus be further investigated....
[...]
...First, it is very likely that the actual occurrence of microplastics in the environment is currently underestimated due to sampling method issues (Kooi et al. 2016; Eerkes-Medrano et al. 2015) and a lack of consensus regarding sampling methods (Rocha-Santos and Duarte 2015)....
[...]
1,481 citations
"Microplastics as vectors for enviro..." refers background in this paper
...Surface polarity plays a role as HOCs interact more strongly with nonpolar surfaces (Mato et al. 2001)....
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...Increasing crystallinity results in decreasing capacity and rate of absorption of a contaminant into the polymer matrix (Mato et al. 2001; Karapanagioti and Klontza 2008)....
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...Weathering can also lead to an increase in crystallinity of the polymer (Mato et al. 2001; Karapanagioti and Klontza 2008), thereby reducing sorption of HOCs....
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1,339 citations
1,033 citations
"Microplastics as vectors for enviro..." refers background in this paper
...However, at higher concentrations of the chemical, absorption often takes over as thedominant retention process due to themuch larger volume to accommodate the molecules (Luthy et al. 1997; Cornelissen et al. 2005)....
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Q2. What are the future works mentioned in the paper "Microplastics as vectors for environmental contaminants: exploring sorption, desorption, and transfer to biota" ?
How these factors influence the role of MPs as HOC vectors is a topic of further research.