Analysis techniques for quantifying nano-and microplastic particles and their degradation in the marine environment

Here we provide a summary of the main activities and achievements by SINTEF Ocean within the JIP Oceans project ANDROMEDA. The project aimed to develop an instrument platform for advanced characterization of nanoplastic (NP; <1 µm) and small MP (sMP; 1-10 µm). SINTEF Ocean’s activities primarily focused on the development and optimisation of advanced techniques to measure and quantify sMP and other challenging types of MP particles (e.g. microfibres, tyre wear particles and paint flakes). This included characterisation of the partially degraded sMP/NP generated in studies investigating the degradation and fragmentation mechanisms of plastic into MP and NP. We also studied the release of plastic-associated chemicals during fragmentation and degradation processes. SINTEF Ocean also had a strong focus on developing and implementing methods for producing environmentally relevant sMP and NP reference materials (RMs). A range of 'pristine' bulk polymer materials (including polyethylene, polypropylene, polystyrene and polyethylene terephthalate) with varying degrees of additive chemicals were sourced and cryomilled by CARAT GmbH, who supplied a fraction comprising particles <100 µm. Characterisation work indicated the cryomilling process is unable to produce significant quantities of particles <10 µm. SINTEF Ocean used these materials as a start point for developing alternative methods to produce RMs <10 µm through a combination of UV degradation (UVC ozonation) and probe sonication, as well as using a partial solubilisation approach. Results indicate a significant increase of the sMP and NP fractions and the processes are ready for further optimisation. To isolate the particle size fractions of interest for the different studies planned in ANDROMEDA (MP, 10-100 µm; sMP, 1-10 µm; NP <1 µm), we used commercially available plastic particles to develop a fractionation procedure based on a combination of stirred cell filtration, using a series of different pore-sized filters and centrifugal filtration (NPs; <1 µm). The methodology has been validated and was used in the project. Within ANDROMEDA, SINTEF Ocean helped to establish a hyphenated identification and mass-based sMP and NP quantification method for individual polymer types using field flow fractionation (FFF) and pyrolysis GC-MS. Specific focus was placed on developing a solvent extraction technique for isolating sMP and NP from aqueous samples and preparing them for pyGC-MS analysis. This work was completed for polyethylene, polyester, polystyrene and a number of other polymer types. These analysis approaches, along with other characterisation techniques for particle size and abundance (Morphology G3 particle size analyser, nanotracking analysis) were used to support the development of the RMs and to evaluate their suitability. SINTEF has also undertaken a collaborative study with the instrument producer Agilent to investigate improved analysis and quantification of microplastic fibres, which are a challenging form of MP as conventional techniques such as µFTIR can identify a single fibre as multiple particles. We have purchased and modified CaF2 slides and tested whether these can be used in µFTIR analysis to ensure all parts of the fibres remain in a single focal plane. The slides offered limited improvement, but testing with an LDIR instrument indicated a significant improvement over the conventional µFTIR approach and appears a good alternative. The slides proved successful in keeping fibres in the optical plane in the µFTIR, but reduced the sensitivity of the instrument to some degree, indicating a compromise is needed for fibres. ANDROMEDA has developed and published a method for controlled accelerated hydrolytic degradation of selected polymers. The degree of degradation can be controlled easily with full degradation achieved within 3 hours using the conditions employed. Andromeda has used accelerated simulated sunlight to facilitate rapid UV degradation and fragmentation of the bulk materials. Comprehensive degradation studies have been conducted to study the mechanism of UV degradation in detail, as well as to investigate the influence of parameters such as temperature and pH, where attention was given to additive chemical and degradation product leaching. Partners specialised in dissemination, communication and data management ensured a strong stakeholder involvement and efficient outreach of the project results. A project webpage was created (https://www.andromedaproject.net/) and regular posts were made on social media. ANDROMEDA engaged with the EU MSFD technical group on marine litter (TGML) and contributed directly to the development of a 'White Paper' on microplastic and a guidance document on plastic litter monitoring. It is also involved indirectly in the Global Plastic Treaty negotiations via the Scientists' Coalition for an Effective Plastics Treaty (SCEPT) and the MICRO Conference series.

Optimised sampling methods were developed, enabling validated sampling that can be compared with conventional MP techniques in water and atmosphere. An optimised MP sampling device for water, which can be attached to a pumping system or ferrybox, was developed, tested, compared, and used in different European seas. Similar work was carried out for two MP sampling devices. An Andromeda smartphone allowing in-situ MP monitoring on beaches was developed and launched. Cost-effective analysis techniques based on automated image analysis using hyperspectral imaging or fluorescence microscopy were developed for polymer identification. Andromeda demonstrated the importance of using reference materials for calibration and degradation experiments. Project outcomes include the use of auto-coding networks to reconstruct FTIR and Raman spectra particularly when targeting specific particles. It was also demonstrated that the analytical combination of FFF and pyGC-MS could be an interesting approach for the analysis of polymers in heterogeneous samples. Our efforts have led to the development of a deep learning method for signal enhancement of FTIR and Raman spectra in the analysis of environmental samples. Similarly, certain techniques (LC-HRMS, pyGC-MS, SEM) have proved highly effective for the analysis of TWPs in the marine environment. The use of platinized membranes proved particularly effective for the analysis of MPs (1 to 10 µm) by Raman microscopy, while SEM-Raman proved suitable for the identification of NPs. Research into plastic (bio)degradation showed that degradation processes vary greatly depending on the material properties and the environmental conditions. Material were degraded in situ and in the laboratory under different light and pressure conditions. Results showed bacteria produce dissolved compounds from various polymers such as TWPs, PET and PVC. Sediments were shown to be a significant source of toxic organic additives for the entire water column. The degradation of plastic polymers induces the production of plastic fragments and dissolved compounds that are toxic to organisms in the marine environment. Communication and dissemination activities amplified scientific achievements and reached intended target audiences. Interaction with key stakeholders was achieved by applying classic communication tools (website and social media). To increase visibility, the project developed information-rich documents for the general public on microplastic pollution in the marine environment. Stakeholder workshops ensured dialogue and mutual learning between researchers and key stakeholders around the cost effectiveness of microplastic analysis methods for seawater samples, which specifically focused on engaging policy and decision makers. Relevant communication material and documents such as the project brochure, ANDROMEDA factsheets and the stakeholder workshop reports are all available for download from the project website.

Current methods for microplastic (MP) analysis can be divided into low-cost versus more advanced methods. ANDROMEDA recognizes that further development and validation is needed for both approaches. Low-cost methods are needed that can identify a broad range of MP polymers with acceptable accuracy. Advanced methods need further development in order to push the limit of detectability for smaller sizes of MP and nanoplastics (NP) and improve their ability to analyze MP types that are currently difficult to analyze by microspectroscopy. Moreover, to study plastic degradation mechanisms over a reasonable time frame, lab-based accelerated degradation approaches are required that mimic natural fragmentation and additive chemical leaching. Within ANDROMEDA, in situ MP detection, efficient sampling and cost-effective laboratory methods will be developed and optimized to analyze MP. Approaches will be based on hyperspectral imaging, chemical markers and fluorometric detection techniques. Advanced analysis techniques making use of µFTIR, Raman imaging and SEM-EDX (amongst others) will be applied to quantify and characterize MP and NP down to 1 µm, 0.2 µm or lower. Specific tasks will focus on challenging types of MP such as microfibers, tire wear particles (TWPs) and paint flakes. UV, hydrolytic and thermo-oxidative methods to study accelerated plastic degradation at the lab-scale will be developed and used to prepare partially degraded reference materials. Comprehensive degradation studies will be conducted to study in detail the mechanisms of UV and microbial degradation, as well as to investigate the influence of parameters such as temperature, pH and hyperbaric pressure, where attention will be paid to additive chemical leaching. Quality assurance will be a central theme in all aspects of the project. Partners specialized in dissemination, communication and data management will ensure strong stakeholder involvement and efficient outreach of the project results.