186
temperatures of some plastics are around 50–80 °C or even
below for PE (Osswald et al. 2006; Qiu et al. 2016).
Therefore, it is generally recommended to use temperatures
of less than 50 °C.
For H 2 O 2 , negative effects on synthetic polymers have
been shown by Nuelle et al. (2014), but just after a weeklong treatment. The needed incubation time and effectiveness can be further improved by a new approach from Tagg
et al. (2017) who used Fenton’s reagent, a mixture of iron
sulphate (FeSO 4 ) and H 2 O 2 . The digestion with enzymes is
regarded to be non-destructive to MPs, targeting specifically
proteins, polysaccharides and lipids. Cole et al. (2014) presented an approach with Proteinase-K and an up to 97.7%
effective removal of biogenic matter. Courtene-Jones et al.
(2017) digested mussel tissue with trypsin with an efficiency
of 88%. The biggest disadvantage of these treatments is the
high cost of these specific enzymes. The succession of several technical enzymes in combination with sodium dodecyl
sulphate (SDS) and an oxidative agent (i.e., H 2 O 2 ) seems to
be an effective, inexpensive, and non-hazardous alternative
(Löder and Gerdts 2015; Löder et al. 2015; Fischer and
Scholz-Böttcher 2017; Mintenig et al. 2017; Primpke et al.
2017b).
When choosing the most suitable digestion method several factors have to be considered: time, cost, destructiveness, and effectiveness.
Purification can take several minutes (Tagg et al. 2017),
several hours (Cole et al. 2014; Dehaut et al. 2016) or several
days (Foekema et al. 2013; Löder and Gerdts 2015; Karami
et al. 2017). Generally, longer incubation times improve the
effectiveness but might also negatively impact MPs. For
example, Nuelle et al. (2014) showed a negative effect of a
week-long treatment with H 2 O 2 while no significant effect
has been shown for shorter application periods (Nuelle et al.
2014; Tagg et al. 2017). Application time should be reduced
to the maximum time before causing negative effects and to
the minimum time necessary to cause the highest possible
effectiveness.
Specific enzymes like Proteinase-K and trypsin are very
expensive. Technical enzymes, on the other hand, can be
used as an inexpensive alternative (Löder and Gerdts 2015;
Löder et al. 2017; Mintenig et al. 2017).
It is noticeable that methods using acids are more destructive, especially at higher temperatures, than other methods.
Only at low concentrations and low temperatures (5%,
25 °C) HCl and HNO 3 are less destructive than non-acid
based methods, although they are also less effective at low
temperatures and concentrations. For the alkaline treatments,
KOH is more effective than NaOH with the same level of
destructiveness. When comparing two oxidative treatments
most frequently used, H 2 O 2 is more effective than NaClO
and less destructive.
Next to the potential destructiveness, the effectiveness of
the treatment has to be taken into account when considering
the most suitable digesting agent (Fig. 2). For most treatments, an increase in temperature provokes an increase in
effectiveness but often also an increase in destructiveness.
Some treatments might be very effective but also relatively
destructive to MPs like HNO 3 (69%) and HCl (37%) and
other treatments are less destructive but also less effective
like NaOH and NaClO (Karami et al. 2017). Enzymatic
treatments represent the best choice in terms of being nondestructive to MPs. Several working groups have shown the
high effectiveness of enzymatic digestion with different
enzymes (Cole et al. 2014; Courtene-Jones et al. 2017;
Karlsson et al. 2017; Löder et al. 2017; Mintenig et al.
2017).
Microplastics Identification
Once the environmental samples have been purified and concentrated by removing the biogenic and inorganic matter the
MPs within the samples have to be identified. This identification is most easily performed by visual inspection either with
the naked eye or with the use of a (stereo) microscope (Shim
et al. 2017). The sorting is based on several criteria defined
in a pilot-study by Norén (2007), which include having no
visible cell-structure, homogenous coloration, and equal
thickness for fibers (Enders et al. 2015). Nonetheless,
Hidalgo-Ruz et al. (2012) stated that up to 70% of particles
that potentially resembled MPs based on merely visual
inspection could not be confirmed to be of synthetic origin.
These limits of visual identification, even by experienced
operators, have been shown by several studies (Eriksen et al.
2013; Dekiff et al. 2014; Lenz et al. 2015; Löder and Gerdts
2015; Song et al. 2015). Despite this high proneness to
errors, many studies still rely on the visual identification of
MPs. An overestimation can be avoided when a chemical
characterization is subsequently performed to confirm plastics. If the chemical characterization is based on a prior
visual sorting of potential MPs, an underestimation, especially of very small particles is still very likely (Song et al.
2015). Stains can be used to facilitate visual analysis, like
Nile Red (Desforges et al. 2014; Shim et al. 2016; ErniCassola et al. 2017; Maes et al. 2017a) or rose bengal (Ivleva
et al. 2016). Maes et al. (2017a) presented an approach using
Nile Red that enabled for a reliable identification of MPs
(96.6% recovery for MPs of a 100–500 μm size range).
Nevertheless, this approach does not allow for a differentiation of distinct polymer types (Maes et al. 2017a), and may
only be suitable for identification of MPs used in organism
studies, where the specific polymer type is known. For environmental samples, chemical characterization is needed and
can be achieved by spectroscopic analyses like Fourier transform infrared (FTIR), Raman and energy dispersive X-ray
(EDX) spectroscopy or thermal analysis (Ivleva et al. 2016;
Shim et al. 2017).
T. Hamm et al.
temperatures of some plastics are around 50–80 °C or even
below for PE (Osswald et al. 2006; Qiu et al. 2016).
Therefore, it is generally recommended to use temperatures
of less than 50 °C.
For H 2 O 2 , negative effects on synthetic polymers have
been shown by Nuelle et al. (2014), but just after a weeklong treatment. The needed incubation time and effectiveness can be further improved by a new approach from Tagg
et al. (2017) who used Fenton’s reagent, a mixture of iron
sulphate (FeSO 4 ) and H 2 O 2 . The digestion with enzymes is
regarded to be non-destructive to MPs, targeting specifically
proteins, polysaccharides and lipids. Cole et al. (2014) presented an approach with Proteinase-K and an up to 97.7%
effective removal of biogenic matter. Courtene-Jones et al.
(2017) digested mussel tissue with trypsin with an efficiency
of 88%. The biggest disadvantage of these treatments is the
high cost of these specific enzymes. The succession of several technical enzymes in combination with sodium dodecyl
sulphate (SDS) and an oxidative agent (i.e., H 2 O 2 ) seems to
be an effective, inexpensive, and non-hazardous alternative
(Löder and Gerdts 2015; Löder et al. 2015; Fischer and
Scholz-Böttcher 2017; Mintenig et al. 2017; Primpke et al.
2017b).
When choosing the most suitable digestion method several factors have to be considered: time, cost, destructiveness, and effectiveness.
Purification can take several minutes (Tagg et al. 2017),
several hours (Cole et al. 2014; Dehaut et al. 2016) or several
days (Foekema et al. 2013; Löder and Gerdts 2015; Karami
et al. 2017). Generally, longer incubation times improve the
effectiveness but might also negatively impact MPs. For
example, Nuelle et al. (2014) showed a negative effect of a
week-long treatment with H 2 O 2 while no significant effect
has been shown for shorter application periods (Nuelle et al.
2014; Tagg et al. 2017). Application time should be reduced
to the maximum time before causing negative effects and to
the minimum time necessary to cause the highest possible
effectiveness.
Specific enzymes like Proteinase-K and trypsin are very
expensive. Technical enzymes, on the other hand, can be
used as an inexpensive alternative (Löder and Gerdts 2015;
Löder et al. 2017; Mintenig et al. 2017).
It is noticeable that methods using acids are more destructive, especially at higher temperatures, than other methods.
Only at low concentrations and low temperatures (5%,
25 °C) HCl and HNO 3 are less destructive than non-acid
based methods, although they are also less effective at low
temperatures and concentrations. For the alkaline treatments,
KOH is more effective than NaOH with the same level of
destructiveness. When comparing two oxidative treatments
most frequently used, H 2 O 2 is more effective than NaClO
and less destructive.
Next to the potential destructiveness, the effectiveness of
the treatment has to be taken into account when considering
the most suitable digesting agent (Fig. 2). For most treatments, an increase in temperature provokes an increase in
effectiveness but often also an increase in destructiveness.
Some treatments might be very effective but also relatively
destructive to MPs like HNO 3 (69%) and HCl (37%) and
other treatments are less destructive but also less effective
like NaOH and NaClO (Karami et al. 2017). Enzymatic
treatments represent the best choice in terms of being nondestructive to MPs. Several working groups have shown the
high effectiveness of enzymatic digestion with different
enzymes (Cole et al. 2014; Courtene-Jones et al. 2017;
Karlsson et al. 2017; Löder et al. 2017; Mintenig et al.
2017).
Microplastics Identification
Once the environmental samples have been purified and concentrated by removing the biogenic and inorganic matter the
MPs within the samples have to be identified. This identification is most easily performed by visual inspection either with
the naked eye or with the use of a (stereo) microscope (Shim
et al. 2017). The sorting is based on several criteria defined
in a pilot-study by Norén (2007), which include having no
visible cell-structure, homogenous coloration, and equal
thickness for fibers (Enders et al. 2015). Nonetheless,
Hidalgo-Ruz et al. (2012) stated that up to 70% of particles
that potentially resembled MPs based on merely visual
inspection could not be confirmed to be of synthetic origin.
These limits of visual identification, even by experienced
operators, have been shown by several studies (Eriksen et al.
2013; Dekiff et al. 2014; Lenz et al. 2015; Löder and Gerdts
2015; Song et al. 2015). Despite this high proneness to
errors, many studies still rely on the visual identification of
MPs. An overestimation can be avoided when a chemical
characterization is subsequently performed to confirm plastics. If the chemical characterization is based on a prior
visual sorting of potential MPs, an underestimation, especially of very small particles is still very likely (Song et al.
2015). Stains can be used to facilitate visual analysis, like
Nile Red (Desforges et al. 2014; Shim et al. 2016; ErniCassola et al. 2017; Maes et al. 2017a) or rose bengal (Ivleva
et al. 2016). Maes et al. (2017a) presented an approach using
Nile Red that enabled for a reliable identification of MPs
(96.6% recovery for MPs of a 100–500 μm size range).
Nevertheless, this approach does not allow for a differentiation of distinct polymer types (Maes et al. 2017a), and may
only be suitable for identification of MPs used in organism
studies, where the specific polymer type is known. For environmental samples, chemical characterization is needed and
can be achieved by spectroscopic analyses like Fourier transform infrared (FTIR), Raman and energy dispersive X-ray
(EDX) spectroscopy or thermal analysis (Ivleva et al. 2016;
Shim et al. 2017).
T. Hamm et al.