|Narrative summary of dataset||Contents and composition of microplastic particles have been analysed in ten
sediment samples from the Danish part of the open North Sea in 2015. The contents
of microplastic particles in the size range 20-5000 µm were determined to be in the
range of 192-675 particles per kg dry weight sediment, dominated by blue and black
coloured particles, mainly as fibres, and particle sizes < 300 µm were dominating. |
|Summary of processing methodology||Sediment samples consisting of ~50 ml (~100 g wet weight (ww)) were first
digested by two hours of thoroughly shaking with 100 ml of a 30 % VIP1 solution (industrial agent from NOVODAN aps, Denmark) consisting of ~3 %
potassium hydroxide, ~1 % potassium tripolyphosphate, ~1 % potassium silicate and ~7 % sodium hypochlorite, so dominant fractions of natural organic
matter were decomposed. Similar treatment of different types of common
plastic materials showed no larger external changes in the particle structure
Figure 2.1. North Sea sampling
stations for ten sediments analysed for microplastics in 2015.
and colour (Strand et al. 2013). The chemical digestion step was followed by
five minutes of ultrasonic treatment. The remaining particles were then allowed to settle in the glass for five minutes. Consequently, the supernatant
was removed into a new glass. The remaining sediment sample was then
treated two times with saturated saline solution, to separate particles according to density, by adding 100 ml of saline solution, followed by ten minutes
of shaking and consequently five minutes of ultrasonic treatment. The sample
was allowed to sediment for five minutes after each of the two steps. Pooled
supernatant was divided into six size fractions, i.e. 20-38, 38-100, 100-300,
300-1000 and 1000-5000 µm, by being flushed with filtered freshwater
through the sediment test sieves (Retsch) placed on top of each according to
decreasing mesh size. The remaining sediment sample was also flushed
through the test sieves of sizes 300-1000 and 1000-5000 µm.
For each size fraction, particles retained on the test sieves and regarded as of
synthetic origin were visually identified based on their relatively homogenous texture and structure using a stereo microscope (20-50x magnification).
The identified microplastic particles were isolated, counted and characterized with respect to their colour and type/shape, i.e. as elongated filaments
(i.e. “fibres”), plastic film and plastic fragments, granules or round spherules
according to the descriptions adopted from EU guidance on monitoring of
marine litter (EU 2013) (figure 2.2). Plastic film and plastic fragments were
counted as one category, as they were relatively difficult to distinguish from
each other, especially at sizes less than 300 µm. This group also only represented a minor group compared to other identified types of microparticles.
An additional group of particles was characterised as particles of “uncertain
origin”, as during the visual microscope analyses it was not possible to
properly assess whether they consisted of remaining natural matter or were
made of synthetic polymer materials. This “uncertain” group consisted
mainly of transparent fibres.
From the fractions of 20-38 and 38-100 µm, only fibres longer than 100 µm
were isolated and counted.
The following fractions were stored for potential, later analysis by Fourier
transform infrared (FT-IR) spectroscopy:
1) all isolated particles of plastic origin including particles categorised as
2) all remains of samples from size fractions 20-38 and 38-100 µm.
For quality assurance of the analyses, all glassware was cleaned and baked
in the oven at 450 °C prior to the start of the microplastic analysis, and all the
reagents were filtered using 20 µm mesh sizes. Additionally, the risk for airborne particulate contamination was controlled every day of sample analysis, by leaving two open Petri dishes with filtered water that were put out
every day prior to the start of the sample preparation and analysis. On a daily
basis, 2-8 particles on average were recorded in the Petri dishes that all days
were dominated by transparent fibre particles (70-100 %). Data for all analysed samples were therefore assessed to have adequate quality, because the
potential, airborne contamination did not exceed 10 % of the total amount of
the collected microparticles from the samples analysed the same day. Two
replicate samples were analysed for sample DMU1073 only, and average data are reported in the result section whereas data for both replicates are
shown in Appendix 2.
The characterisation of colours of microplastic particles was performed according to the colour codes listed in the EU guidance document (see table in
figure 2.2), and were as following: transparent, crystalline, white, red, orange,
blue, black, grey, brown, green, pink and yellow. |