Extraction of microplastic from marine sediments: A comparison between pressurized solvent extraction and density separation
Introduction
It is estimated that a trillion of plastic pieces is currently floating in the sea, moving throughout the oceans by the prevailing winds and surface currents (Eriksen et al., 2014). Researchers have also shown as a fraction of this plastic degrades into microplastics (Browne et al., 2007; Cole et al., 2011; Corcoran et al., 2009) and escapes the sea surface by settling. Several are the factors involved in the process: the intrinsic polymer density, the degradation and fragmentation extent, the ingestion by marine organisms (Choy et al., 2019), the increase of density caused by biofilm formation (Lobelle and Cunliffe, 2011), the entrainment into aggregates (de Haan et al., 2019). Noteworthy, even light density polymer fractions were repeatedly found in the seafloor as result of this “fallout” (Sanchez-Vidal et al., 2018; Egger et al., 2020). In this perspective, several authors pointed to the seafloor as the possible final sink for the plastic pollution (Eriksen et al., 2014; Woodall et al., 2014; Sanchez-Vidal et al., 2018). UNEP in 2005 estimated that approximately 70% of the marine litter ends up in the seabed. However, the numbers are repeatedly object of review, as the most up-to-date surveys and estimations are released (Lebreton et al., 2019). Overall, the close correlation observed between plastic production and its deposition in the sedimentary record suggests that the amount of plastic in the deep sea is constantly growing (Brandon et al., 2019). This is a matter of great concerns because the plastics and microplastics trapped in the seafloor sediments may be further decomposed and/or remobilized (Liu et al., 2014) causing further impacts to marine organism throughout the food web (Eriksen et al., 2014; Panio et al., 2020). For this reason, the monitoring of the plastic abundance in the deep sea is considered very important (Browne et al., 2010).
Despite the effort of regulatory bodies to provide guidelines (OSPAR, 2009), and the effort of the scientific community to develop and test new analytical approaches, the definition of an established and harmonized approach for measuring the concentration of microplastics in the marine sediments is still lacking (Imhof et al., 2012). At the present time, the choice of the sample preparation method is strongly influenced both by the size of the investigated plastic particles and sediments, as like as the researcher preferences.
Larger microplastics are in general separated from fine sediments (mud or silt) by visual sorting, sieving or filtration. These procedures are considered relatively easy and provide reliable determination, especially when the plastic identity is confirmed by spectroscopy (Saliu et al., 2018).
Smaller microplastics, due to the possible colloidal interaction with the finer sediment fractions, are often submitted to physical separation methods i.e. elutriation, density separation and froth flotation. In particular, density separation has become the method of choice for the analysis of marine sediments (Hidalgo-Ruz et al., 2012). Here, microplastics are isolated due to their densities, that are in general lower than the densities of the sediment particles: separation occurs because the microplastics are floating whereas sediments particles are sinking (Quinn et al., 2017). Particles may be finely selected by changing the density of the liquid, and this is commonly obtained by using a specific brine solution (Thompson et al., 2004). The method may display drawbacks that end up in a difficult application for large scale monitoring campaigns: an extensively handling of the sample with few possibilities of automatization, the use of large equipment, the use of dense salts such as NaI and ZnCl2 that might be expensive and hazardous, (Corcoran et al., 2009). Froth flotation is a technique that is mostly applied in the recycling industry, whereas few are the examples of an application for analytical microplastic separation (Ihmof et al., 2007). It relies on the wettability of the plastic (due to a combination of bulk density, particle size, shape, surface energy, and surface roughness). The segregation of the microparticles is obtained by altering the surface tension of the medium and/or by chemical conditioning (Imhof et al., 2012) influencing the strength of the hydrophobic interactions, whereby hydrophobic particles adhere to the surface of air bubbles and are carried out to the air–liquid interface. A certain unpredictability of the bubbles motion may determine particles losses, and this is the main reason of the poor use as analytical separation method.
In all these approaches, since the morphology of the plastic is retained, the detection is mostly carried out by microscopy or micro-spectroscopy. Data are therefore collected as particles counts. It is also possible to convert the particle numbers into total particle mass by calculation that consider the shape and density of the particles, which require approximations (Mai et al., 2018). On the other hand, the literature provides also examples of direct gravimetric determinations obtained by measuring the mass recovered onto pre-weighted filters (Okoffo et al., 2020).
Solvent extraction is an emerging alternative approach that may overcome some of the limitation observed when the physical separation methods are applied. In this case, since the extraction of plastic particles is achieved through either partially emulsification or solubilization, the recoveries are considerably less affected by the particle size, and even the extraction of submicron particles is theoretically possible. On the other hand, the different solubility of different polymers in the different solvents, and the pronounced changes induced by photo-oxidative degradation should be carefully taken in account to avoid underestimation (Saliu et al., 2021). For instance, Castelvetro et al. (Castelvetro et al., 2021) recently evaluated the use of hot organic solvent to extract microplastic from sand samples collected from sediments. To perform the authors used a Kumagawa-type apparatus consisting of a stainless steel cylinder loaded with 160 g of sample. Dichloromethane (DCM) was used as first solvent for the extraction of PS, low molecular and low-to-medium molecular weight oxidized polyolefins (that are formed during the photo-oxidative plastic degradation); whereas a further extraction was performed with boiling xylene, aiming to recover the high molecular weight polyolefins (i.e. PP and PE), which are insoluble in boiling DCM. Extracts were then analyzed by multi-analytical approach comprising infrared spectroscopy, mass spectrometry and nuclear magnetic resonance spectroscopy. The extraction procedure displayed consistent recoveries (evaluated on weight basis) and highlighted interesting features regarding the chemical modification of the weathered plastic particles, highlighting the presence of a “hidden microplastic” fraction that is often poor considered in the literature.
Fuller and Gautam (Fuller and Gautam, 2016) pioneered the application of pressurized fluid for the extraction of microplastic by extracting microplastics from municipal wastes and soil collected from an industrial area. The authors observed how the use of dichloromethane at high temperature (180–190 °C) and pressure (104 bar) enables to quantitatively recover of polar polymers such as PET and PS, and polyolefin such PP and PE, spiked into glass beads (several milligrams). Successive literature works displayed how the hyphenation of this extraction technique with thermal degradation-mass spectrometry enables the quantification of microplastics even in trace amounts (Fischer and Scholz-Bottcher, 2019). Specifically, Okoffo et al. (Okoffo et al., 2020) reported a limit of detection of 0,003 mg/g for PE microplastics in biosolids by applying PSE and double-shot pyrolysis gas chromatography–mass spectrometry (Pyr-GC/MS). Similar detection limits were reported by La Nasa et al. for the extraction of PS from sea sand by applying microwave solvent extraction and Pyr-GC/MS (La Nasa et al., 2020). Dierkes et al. (Dierkes et al., 2019) observed quantification limit of 0.007 mg/g for PE and PP in sewage sludge by using THF in PSE at 185 °C and 100 bar and by analyzing the extracts with pyr-GC/MS.
Due to the lack of harmonization, the use of different detection instruments and different metrics, a thorough comparison of the performance reported in the literature for the physical and chemical extraction of microplastics is not possible. Moreover, it must be considered that the results reported in terms of limit of quantification might be severely affected by representativeness of the subsample used and by the possible background contamination related to the procedures employed and the lab/equipment conditions (Saliu et al., 2020a). In the attempt to provide a comparison that is less affected by these factors, we performed, in the same lab condition, the extraction of light density microplastics (PS, PE and PP) from marine sediments, by employing both pressurized solvent extraction and density separation. Results were determined by measuring the mass of recovered material and the pros and cons of both the procedures discussed.
Section snippets
Samples
Extra pure sea sand was purchased from Sigma Aldrich (Supelco 17701). As indicated by the producer, the material was constituted by SiO2 grains in the 0.1–0.315 mm size range, with a density of 2.65 g/cm3 and a melting point of 1713 °C. Seafloor sediments were collected with a multicorer system at 358 m water depth, 2,7 miles off the city of Barcelona (41°20′46.6″N 2°32′30.1″E) during an oceanographic cruise onboard RV Garcia del Cid in February 2019. Once onboard, the undisturbed top centimeter
Characterization of the plastic particles
As already indicated in the experimental section of this paper, a fraction of the plastic microparticles used to run the extraction experiments were firstly characterized by means of infrared micro spectroscopy. This was principally done to determine the extent of the photo-oxidative degradation on the selected polyolefin particles (since obtained from municipal waste recycled plastics) and to ensure the substantial homogeneity of the subfraction used to perform the spikes into the sediment
Conclusion
PSE and density separation were compared as methods to isolate microplastics from marine sediments. PSE showed the highest absolute recoveries, obtained from the smallest particle size fractions, whereas density separation displayed a better repeatability with largest size fraction. Both the approaches ensured the extraction of the microparticles from large sample volumes with a good control of background contamination.
CRediT authorship contribution statement
Nicolo' Stile: experiment execution.
Clarissa Raguso: experiment execution.
Alice Pedruzzi: PSE technical assistance.
Emir Cetojevic: PSE technical assistance.
Marina Lasagni: supervision and funding.
Anna Sanchez-Vidal: sediment samples and characterization.
Francesco Saliu: Conceptualization, Methodology, Original draft preparation, statistical analysis.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: the instruments used to perform the pressurized solvent extraction method and the glass apparatus used for the filtration were provided by Buchi Laborthenik. Two authors of the paper work for the company and provided in the setup of the extraction method. However, test was run in the university by university employees and the aim of the paper is comparison of
Acknowledgments
This work was supported by Univesity of Milano Bicocca, grant FAR2019.
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