Investigation of microplastics’ and potentially toxic elements’ contents in marine table salts from Europe

This thesis is dedicated to the identification and quantification of microplastics (MPs) in different samples of marine table salt. MPS recently became a hot topic for the scientific community and their connection with the marine environment is due to wastewater discharge in surface water bodies and to the weathering of marine litter. Specifically, the main sources of MPs in wastewater are textile fibres, tyres’ erosion and atmospheric particulate matter. The presence of MPs in marine table salt was already proven, however there are still some relevant issues to explore. In details, a reference analytical procedure is still missing; the available literature data are mostly related to non-European salt samples; the correlation between MPs presence and marine salt origin has not yet been explored; the toxicological aspects related to MPs are still unknown, while the ongoing research is mostly focused on MPs identification and quantification. This thesis has three main objectives. Firstly, to compare two analytical methodologies (i.e. microFTIR and optical microscopy); secondly, to establish whether MPS’ presence could affect the concentration of potential toxic elements (PTEs) in marine table salt, considering that the toxicological effects of PTEs are well-known; thirdly, to evaluate the health risk related to PTEs exposure deriving from marine salt ingestion. 23 samples of different brands of marine table salt, produced in European countries (Greece, Italy, France, Portugal, Slovenia), were studied. Micro-FTIR is considered by literature the most reliable technology for MPs’ characterisation, therefore it is used to verify the results of the visual identification, usually through optical microscopy, which is subjected to human errors. The MPs were firstly identified through a LEICA MZ7.5 stereomicroscope, coupled with ImageJ processing software, improving MPs visual identification though Nile red stain. Later the specific polymer types were analysed through a Nicolet iN10 Thermo Scientific micro-FTIR. Finally, the salt samples underwent a chemical characterization through a NEX DE VS Rigaku XRF spectrometer to detect the eventual presence of PTEs. The particle-size distribution and the polymeric composition of MPs were estimated for each brand of marine table salt. A statistical analysis was conducted to establish any connection with the sample’s characteristics (i.e. origin, processing treatments and packaging). From the statistical analysis, the MPs’ pollution resulted significantly correlated with the sea of origin of the salt but not with the processing treatments. It wasn’t possible to find a causal correlation with the packaging, since Italian salts, more polluted by MPs, were packed in cardboard while Greek salts, less polluted, were all packed in plastic bags. Finally, the amount of MPs ingested every year in different countries was estimated considering the average use of table salt in the countries of origin of the analysed samples. Even though literature described possible effects on sea biota exposed to high concentration of microplastics, the consequences of ingestion of microplastics by humans are still unknown. On the grounds of the amounts of PTEs detected in the considered samples, the correlations between the PTEs concentrations and number of MPs were investigated. This thesis was partly developed during an Erasmus+ mobility at Technical University of Crete between February and December 2020.