185
beaker or flask where the supernatant is decanted or removed
with a pipette or in a separatory funnel, where the inorganic
material is removed via the bottom valve (Maes et al. 2017b;
Mintenig et al. 2017; Zobkov and Esiukova 2017).
Constructed devices like the Munich/MicroPlastic Sediment
Separator (MPSS) by Imhof et al. (2012), designed for the
extraction of MPs from large quantities of sediment (up to
6 kg), and the small-scale Sediment-Microplastic Isolation
(SMI) unit by Coppock et al. (2017) usually achieve very
good recovery rates (96%) even for small MPs (< 1 mm;
(Imhof et al. 2012), when applied with ZnCl 2 . According to
Kedzierski et al. (2017), it is possible to extract 54% of the
plastics produced in Europe with NaCl of 1.18 g cm
−3
density while with a 1.8 g cm
−3
solution (achievable with, e.g.,
NaI, polytungstate, ZnCl 2 ) the extraction of 93–98% is feasible. Therefore, achieved recovery rates are not only dependent on the device but mainly on the separation liquid used.
Another density based technique to separate MPs from
sediment matrix is elutriation/fluidization, where water or air
is pumped through the fluid column containing the sample
and water or a salt solution (Claessens et al. 2013; Nuelle
et al. 2014; Zhu 2015; Kedzierski et al. 2016). Recently, a
non-density based extraction approach with canola oil has
been developed by Crichton et al. (2017). The approach
makes use of the oleophilic properties of MPs. So far it has
only been tested with MPs larger than 500 μm, but showed
high recovery rates of 96% (Crichton et al. 2017). When
choosing one of the available methods, factors like sample
volume or mass, time needed, costs, safety, toxicity, and
extraction efficiency have to be considered.
For small amounts of sediment, approaches in flasks or
funnels can be used or the novel developed SMI unit
(Coppock et al. 2017; Maes et al. 2017b). If larger sediment
volumes (1–6 L) are processed, elutriation systems or the
MPSS would be a better choice (Imhof et al. 2012; Nuelle
et al. 2014).
The time necessary for shaking should be adjusted to the
sediment amount. The more sediment, the longer the mixing
interval should be to assure that all MP particles are separated from the sediment particles. For settling, the span
depends on the density gradient between MPs and liquid as
well as the length of the fluidization column. Furthermore,
the settling times have to be adjusted to the solutions used
since particles rise and settle more slowly in more viscous
solutions like CaCl 2 or ZnCl 2 (Crichton et al. 2017).
The most inexpensive approaches are simple setups with
flasks and NaCl or oil. Zinc chloride is more expensive in
relation to NaCl, especially when adjusted to higher densities but by far less expensive than NaI and polytungstate
(Coppock et al. 2017). At best, an effective and cost efficient
setup is used with a high density solution that can be refurbished and that allows for a proper mixing of the sediment as
well as a proper settling time.
Concentrated NaCl solutions as well as canola oil do not
pose any hazard to the environment. Other salt solutions are
more hazardous to health and the environment in ascending
order: NaI, CaCl 2 , polytungstate, ZnCl 2 . These solutions
should therefore be recycled as far as possible due to financial and environmental reasons (Löder and Gerdts 2015).
Kedzierski et al. (2017) showed that NaI can effectively be
recycled without major density loss. Zinc chloride can be
refurbished in large quantities quite easily via pressure filtration (Löder and Gerdts 2015). Miller et al. (2017) did an
extensive comparison of different separation techniques on
the basis of current literature and listed advantages and disadvantages. Based on this list, the authors recommended the
use of ZnBr 2 (Miller et al. 2017). Nevertheless, ZnBr 2 has to
date just been used by one study (Quinn et al. 2017) and
ZnCl 2 is not included in the list although it is suitable for the
same density range, less expensive (ZnBr 2 : 165 € kg
−1
,
ZnCl 2 : 92.50 € kg
−1
, Merck Millipore, December 2017) and
more widely used. Therefore, other authors have recommended the use of ZnCl 2 as well (Löder and Gerdts 2015;
Ivleva et al. 2016; Primpke et al. 2017a).
Independent of the extraction method chosen the next
step is to filter the residual fluid or the supernatant of the
(density) separation containing MPs to remove the respective salt solution and to concentrate the sample to certain size
fractions.
Sample Purification
Before the samples can be analyzed the biogenic matter has
to be removed. Sediment samples after density separation
contain usually a relatively low amount of biogenic matter
(benthic diatoms, copepods, polychaetes, bivalves, etc.). In
contrast, samples from the sea surface, mostly taken with
plankton nets, are normally very rich in biogenic matter
(phyto- and zooplankton) as well as biota samples. The main
digesting agents used for the removal of biogenic matter are
acids like hydrochloric acid (HCl), nitric acid (HNO 3 ) and
sulphuric acid (H 2 SO 4 ) (Claessens et al. 2013; De Witte et al.
2014; Klein et al. 2015), bases like sodium hydroxide
(NaOH) and potassium hydroxide (KOH) (Foekema et al.
2013; Dehaut et al. 2016; Karami et al. 2017; Wagner et al.
2017), oxidative agents like sodium hypochlorite (NaClO)
and hydrogen peroxide (H 2 O 2 ) (Nuelle et al. 2014; Avio et al.
2015; Collard et al. 2015; Tagg et al. 2017) and enzymes
(Cole et al. 2014; Löder and Gerdts 2015; Courtene-Jones
et al. 2017; Fischer and Scholz-Böttcher 2017; Mintenig
et al. 2017). Several studies showed the destructive effects,
i.e., discoloration, embrittlement or a loss in surface area, of
acids (e.g., HNO 3 ) and bases (e.g., NaOH) on MPs especially
at high temperatures (Cole et al. 2014; Nuelle et al. 2014;
Bürkle GmbH 2015; Karami et al. 2017). Heat deflection
Microplastics in Aquatic Systems – Monitoring Methods and Biological Consequences
beaker or flask where the supernatant is decanted or removed
with a pipette or in a separatory funnel, where the inorganic
material is removed via the bottom valve (Maes et al. 2017b;
Mintenig et al. 2017; Zobkov and Esiukova 2017).
Constructed devices like the Munich/MicroPlastic Sediment
Separator (MPSS) by Imhof et al. (2012), designed for the
extraction of MPs from large quantities of sediment (up to
6 kg), and the small-scale Sediment-Microplastic Isolation
(SMI) unit by Coppock et al. (2017) usually achieve very
good recovery rates (96%) even for small MPs (< 1 mm;
(Imhof et al. 2012), when applied with ZnCl 2 . According to
Kedzierski et al. (2017), it is possible to extract 54% of the
plastics produced in Europe with NaCl of 1.18 g cm
−3
density while with a 1.8 g cm
−3
solution (achievable with, e.g.,
NaI, polytungstate, ZnCl 2 ) the extraction of 93–98% is feasible. Therefore, achieved recovery rates are not only dependent on the device but mainly on the separation liquid used.
Another density based technique to separate MPs from
sediment matrix is elutriation/fluidization, where water or air
is pumped through the fluid column containing the sample
and water or a salt solution (Claessens et al. 2013; Nuelle
et al. 2014; Zhu 2015; Kedzierski et al. 2016). Recently, a
non-density based extraction approach with canola oil has
been developed by Crichton et al. (2017). The approach
makes use of the oleophilic properties of MPs. So far it has
only been tested with MPs larger than 500 μm, but showed
high recovery rates of 96% (Crichton et al. 2017). When
choosing one of the available methods, factors like sample
volume or mass, time needed, costs, safety, toxicity, and
extraction efficiency have to be considered.
For small amounts of sediment, approaches in flasks or
funnels can be used or the novel developed SMI unit
(Coppock et al. 2017; Maes et al. 2017b). If larger sediment
volumes (1–6 L) are processed, elutriation systems or the
MPSS would be a better choice (Imhof et al. 2012; Nuelle
et al. 2014).
The time necessary for shaking should be adjusted to the
sediment amount. The more sediment, the longer the mixing
interval should be to assure that all MP particles are separated from the sediment particles. For settling, the span
depends on the density gradient between MPs and liquid as
well as the length of the fluidization column. Furthermore,
the settling times have to be adjusted to the solutions used
since particles rise and settle more slowly in more viscous
solutions like CaCl 2 or ZnCl 2 (Crichton et al. 2017).
The most inexpensive approaches are simple setups with
flasks and NaCl or oil. Zinc chloride is more expensive in
relation to NaCl, especially when adjusted to higher densities but by far less expensive than NaI and polytungstate
(Coppock et al. 2017). At best, an effective and cost efficient
setup is used with a high density solution that can be refurbished and that allows for a proper mixing of the sediment as
well as a proper settling time.
Concentrated NaCl solutions as well as canola oil do not
pose any hazard to the environment. Other salt solutions are
more hazardous to health and the environment in ascending
order: NaI, CaCl 2 , polytungstate, ZnCl 2 . These solutions
should therefore be recycled as far as possible due to financial and environmental reasons (Löder and Gerdts 2015).
Kedzierski et al. (2017) showed that NaI can effectively be
recycled without major density loss. Zinc chloride can be
refurbished in large quantities quite easily via pressure filtration (Löder and Gerdts 2015). Miller et al. (2017) did an
extensive comparison of different separation techniques on
the basis of current literature and listed advantages and disadvantages. Based on this list, the authors recommended the
use of ZnBr 2 (Miller et al. 2017). Nevertheless, ZnBr 2 has to
date just been used by one study (Quinn et al. 2017) and
ZnCl 2 is not included in the list although it is suitable for the
same density range, less expensive (ZnBr 2 : 165 € kg
−1
,
ZnCl 2 : 92.50 € kg
−1
, Merck Millipore, December 2017) and
more widely used. Therefore, other authors have recommended the use of ZnCl 2 as well (Löder and Gerdts 2015;
Ivleva et al. 2016; Primpke et al. 2017a).
Independent of the extraction method chosen the next
step is to filter the residual fluid or the supernatant of the
(density) separation containing MPs to remove the respective salt solution and to concentrate the sample to certain size
fractions.
Sample Purification
Before the samples can be analyzed the biogenic matter has
to be removed. Sediment samples after density separation
contain usually a relatively low amount of biogenic matter
(benthic diatoms, copepods, polychaetes, bivalves, etc.). In
contrast, samples from the sea surface, mostly taken with
plankton nets, are normally very rich in biogenic matter
(phyto- and zooplankton) as well as biota samples. The main
digesting agents used for the removal of biogenic matter are
acids like hydrochloric acid (HCl), nitric acid (HNO 3 ) and
sulphuric acid (H 2 SO 4 ) (Claessens et al. 2013; De Witte et al.
2014; Klein et al. 2015), bases like sodium hydroxide
(NaOH) and potassium hydroxide (KOH) (Foekema et al.
2013; Dehaut et al. 2016; Karami et al. 2017; Wagner et al.
2017), oxidative agents like sodium hypochlorite (NaClO)
and hydrogen peroxide (H 2 O 2 ) (Nuelle et al. 2014; Avio et al.
2015; Collard et al. 2015; Tagg et al. 2017) and enzymes
(Cole et al. 2014; Löder and Gerdts 2015; Courtene-Jones
et al. 2017; Fischer and Scholz-Böttcher 2017; Mintenig
et al. 2017). Several studies showed the destructive effects,
i.e., discoloration, embrittlement or a loss in surface area, of
acids (e.g., HNO 3 ) and bases (e.g., NaOH) on MPs especially
at high temperatures (Cole et al. 2014; Nuelle et al. 2014;
Bürkle GmbH 2015; Karami et al. 2017). Heat deflection
Microplastics in Aquatic Systems – Monitoring Methods and Biological Consequences