Since the beginning of the production of plastics in the 40s of the last century, microplastics that are no longer visible to the eye has been a growing problem of the global pollution of the marine environment . Therefore the Marine Strategy Framework Directive (MSRL) has the amendment that the “composition of micro-particles (in particular microplastics) has to be characterised in marine litter in the marine and coastal environment” .
Beside the microplastic by itself, the contamination of microplastics with hydrophobic organic chemicals (HOCs) is also ecologically relevant and of vital interest . It may become bioavailabe for benthic organisms and may cause toxic effects.
In the present study, ex-situ solid phase micro extraction (SPME) was used to measure the pollution of microplastics with PAHs and PCBs in North and Wadden Sea sediments. Silicone (polydimethylsiloane, PDMS) was applied as representative for microplastic. PDMS was also suggested as a reference partitioning phase and used for polymer-polymer partitioning as a necessary link for accurate polymer-based partitioning calculations. The capacity of polyethylene (PE) for HOCs was calculated fom PDMS via polymer-polymer partition, because PE has been – beside polypropylene – the most common plastic material in marine environment [1,3].
Sediment samples were collected at nine stations of the German Bight (North Sea) and 30 stations in the Wadden Sea during campaigns in 2013 and 2014. For equilibrium sampling SPME technique was applied . SPME were conducted with commercially available glass fibers with a nominal PDMS coating of 12.7 µm (Fiberguide Industries, Stirling, NJ, USA). The fibers were cut into 10 cm pieces and cleaned by treating twice with methanol and ultrapure water in an ultrasonic bath. Approximately 10 g (wet weight) homogenized sediment was placed in a cleaned 9 mL glass vial and closed with polytetrafluoroethylene (PTFE) lined caps. The septum was pierced with a syringe needle for placing 10 cm pieces of the clean fiber into the vial.
Three fibers were inserted into each vial. The vials were shaken on an end-over-end shaker for 240 h until equilibrium was reached . The equilibrated fibers were cut off with a scalpel, rinsed with ultraclean water and gently wiped with lint free tissue. The exposed as well as cleaned, unexposed fibers (blanks) were wrapped in cleaned aluminum foil and stored at -18 °C until analysis via gas chromatography-mass spectrometry (GC-MS).
Approximately 9 cm of equilibrated and blank PDMS fibers were transferred and stored contamination-free in separate GC liners (ALEX liner tray, Gerstel) for analysis. Prior to loading, the liners were plugged with deactivated glass wool, rinsed twice with both acetone and hexane and pre-heated at 250 °C for 19 min under helium flow in the GC injector to remove organic residues. The fiber loaded liners were automatically placed in the injector (MultiPurpose Sampler, MPS 2XL, Gerstel) and thermally desorbed in a Cooled Injection System (CIS). The injector temperature was increased from 50 °C to 250 °C at 12 °C s-1 and hold for 15 min for assuring thermal desorption from the fiber. Then the desorbed sample was transferred splitless to the column and the injector purge flow to the split vent was set at 50 mL min-1. The GC (7890A, Agilent Technologies) contained a DB-5 fused silica gel column (325 °C: 30 m x 250 µm x 0.25 µm, J&W Scientific) with helium as the carrier phase. A quadrupole MSD (mass selective detector, 5975C inert XL MSD with triple-axis detector, Agilent Technologies) with a transfer line temperature set at 310 °C was used for the quantification of analytes by mass spectrometry. In total the run of one fiber analysis took 47 min. For quantification purposes with mass spectrometry a five-point external calibration curve with standard solutions was generated (PAH-Mix 9 and PCB-Mix 3; Dr. Ehrenstorfer GmbH, Germany). For each compound of interest the masses of two ions (target mass and qualifier) were monitored. The target mass was used for quantification, while the mass of the second ion and the retention time were used for qualitative purposes. The MSD operated in the SIM (selected ion mode) acquisition mode to assure highest sensitivity. The dwell time of the MSD, was set to collect a minimum of 15-20 data points across each chromatographic peak.
Calculation of CPE
The concentrations of the HOCs in the PDMS fiber coating (CPDMS) was calculated as ratio of analyte mass of each analyte sorbed to the fiber (mPDMS) and the PDMS volume (VPDMS) of the fiber. A PDMS volume of 0.0877 µL cm-1 was calculated based on the measured polymer thickness from the confocal microscopy .
LDPE concentrations (CPE) of HOCs were calculated from the concentration in the silicone fiber coating (CPDMS) and the compound specific polymer to polymer partitioning coefficient between the used PDMS and PE (KPDMS-PE) derived by Gilbert :
Results and Discussion
The results of this study showed, that PAHs and PCBs were identified in all silicone samples in the ng/g range. The concentrations range for PCB between 26 and 360 ng/g (sum of 5 PCBs) and for PAHs between 220 and 950 ng/g (sum of 16 US-EPA PAHs). The results of this study show that PCB-153 and PCB-138 were the most abundent PCBs detected, pyrene and fluoranthene were the most common PAH compounds in the samples. The results showed as well, that silicone contamination depends on the contamination level of sediment porewater. So highest concentrations were observed in organic rich highly contaminated muddy sediments whereas sandy sediments show low silicone contamination levels. PDMS concentrations were similar or higher than those found in the sediments (enrichment factors: PCB-153: 32-1377, fluoranthene: 0.2-16). The factors depend on the chemical-physical properties (e.g. water solubility, lipophilicity, molecular weight) of the compounds. The composition of PAHs and PCBs within the North Sea and Wadden Sea sampling stations differed marginally. Elevated PDMS concentrations (CPDMS) of PAHs were observed near river estuaries (Elbe/Weser) and at at a former dumping ground for sewage sludge (fig. 1). Compositions between North Sea and Wadden Sea are substantially different due to a higher amounts of PCBs in Wadden Sea sediments. For example, PCB 153 showed rather low concentrations in North Sea samples. In contrast, PDMS concentrations were more than three times higher at the Weser estuary (Wadden Sea). These results indicate a still high contribution to pollution load of plastics by contaminated river sediments. Sorption of HOCs on the used PE was twice as high compared to the used silicone (fig. 1), probably due to larger molecular cavities in this polymer and a higher area to volume ratio of the example polymer.
This study provides the first comprehensive dataset on microplastic contamination (silicone and PE) with PAHs and PCBs in North Sea sediments including the Wadden Sea as well as Elbe and Weser estuary. The analytical strategy combines sampling and automated analysis and minimizes those artifacts and errors related to sample storage, transport and treatment. Both sampling and analysis procedures are unsophisticated, robust and inexpensive. From the SPME experiments the spatial HOC contamination of two polymers were examined and compared with the sediment pollution of North and Wadden Sea. The study shows that plastics can be an important point source carrying HOCs. Beside the initial organism that ingests the plastics and may be affected by the HOCs, also the organisms within its food web can be influenced by the plastic contamination. In future studies, other common plastic materials will be included, whereas the capacity for HOC contamination will be calculated via polymer-polymer partition coefficients.
We would like to thank Berit Brockmeyer (BSH) for her support during the sampling campaigns. The support from the Federal Ministry of Education and Research (BMBF) project contract no. 03F0669D is gratefully acknowledged.
Nora Claire Niehus, Moritz Kielmann, Gesine Witt,
University of Applied Sciences Hamburg, Ulmenliet 20, 21033 Hamburg, Germany
Prof. Gesine Witt
Life Sciences / Environmental Engineering