title: Occurrence and fate of micro- and nanoplastic in the terrestrial environment
Creators: name: Foetisch, Alexandra
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 550 Earth sciences &amp; geology
subject: 910 Geography &amp; travel

description: The worldwide production of plastic has grown exponentially since the 1950’s and revolutionized our daily life. Simultaneously, plastic pollution in the environment has become a global issue and micro- (MP) and nanoplastics (NP) have now been detected even in the most remote ecosystems. There is currently a data gap due to a lack of analytical methods on the occurrence and characterisation of two highly relevant categories of plastic in the soil environment: tire wear particles (TWP), which concentration in the environment is expected to be high and carry toxic additives, and NP, which toxicity has been demonstrated on soil organisms and is characterized by its ability to cross cell membranes. The effects of micro- and nanoplastic (MNP) on their surrounding environment are determined by their size, morphology, surface characteristics and chemical composition, which can be affected by soil residence time. As the soil is often considered as sink for MNP, it is crucial to investigate and understand the different weathering factors which might affect the MNP properties. To address these knowledge gaps, three main objectives were identified in the scope of this study: develop an extraction and single particle identification method for the quantification and characterisation of (i) TWP and (ii) NP in soil samples and (iii) characterise the physico-chemical properties at the surface of plastic debris occurring in the soil environment, as well as assess the effect of soil and UV weathering as single ageing factors.
In order to realise the first objective, a method of extraction and identification of TWP in soil samples based on their black colour was developed using optical microscopy. Cryo-grinded TWP down to a size of 35 μm could be detected with a >85% but the tests conducted with environmental TWP showed that the density used in this study was not efficient to separate the whole range of TWP occurring in different densities. Yet, TWP concentration in highway adjacent soil samples ranged between 8084 ±1059 and 2562 ± 1160 TWP kg-1 dry soil and showed similar trends and magnitude order than previously reported concentrations. Thus, the developed protocol was estimated sufficiently accurate for TWP monitoring in soil samples.
Regarding the second objective, an extraction and identification method for NP in soil samples was developed using X-ray spectro-microscopy (STX-NEXAFS). The results demonstrated the suitability of the technique for the imaging and chemical characterisation of individual NP with a minimum dimension of ≈100 nm and its application to the analysis of pure NP and for NP present in environmental and food matrices. However, it was not possible to obtain quantitative data on the NP present in the samples, as the method was too time consuming to allow the measurement of a high number of particles.
For the last objective, STXM-NEXAFS was applied to the characterisation of the surface alterations of natural-soil weathered, soil-incubated and UV exposed polymers. A surface alteration on a depth varying between 150 and 1000 nm on could be observed and the analysis of the replicate’s measurement acquired on the same plastic debris highlighted the heterogeneity of the processes affecting polymers surface. The comparison of UV weathered and natural-soil weathered samples showed that the two treatments led to different surface alterations and the absence of surface alteration after one-year soil incubation indicated slow aging of polymers in this medium. Moreover, the very first step of surface fragmentation was observed on a PS fragment, providing an insight on the factors and processes leading to the release of MP and NP in soils.
Overall, the present research contributed significantly to the development of innovative methods to characterise MNP in the soil environment. The results obtained helped to provide ground information on the characteristic of environmental MP and NP, which is of high importance to design ecotoxicological test using environmentally relevant material as well as validate predictive models to better understand the potential risk that MP and NP represent for the ecosystems.