Age-related changes in uptake and release of trace elements in marine microplastics

Abstract

Due to the growing demand and use of plastics, the disposal and waste of plastic products is getting more important. A greater amount of the discarded plastic ends up in the oceans. There it undergoes degradation processes, resulting in the formation of microplastics (MP). The particles get overgrown by microorganisms, ingested by marine life, and further enter the human food chain. MPs are under suspicion to release contained additives and adsorb trace elements from the ocean. In this thesis four different commonly used polymers were investigated. Polystyrene (PS), Polyamide (PA), Polyethylentherepthalate (PET) and Polyvinylidenchloride (PVDC). These samples were artificially aged by UV irradiation and oxidizing reagents. The changes caused by degradation were investigated with common methods, such as Raman and FTIR spectroscopy. Further the native and degraded polymers were treated with artificial seawater spiked with selected heavy metals such as Pb and Cd. To obtain quantitative bulk information of the age-related changes in uptake and release of inorganic species, microwave assisted digestion followed by liquid inductively coupled plasma mass spectrometry (ICP-MS) analysis was carried out. Laser ablation ICP-MS (LA-ICP-MS) and laser induced breakdown spectroscopy (LIBS) were used for qualitative spatially resolved analysis in solid MPs. Investigated were both the distribution of trace elements, and O as a marker for oxidation/degradation within individual particles. The aged PS samples clearly show an increased uptake of heavy metals such as Pb and increased release of additives containing Zn. PA showed leaching of additives with no noticeable change in molecular structure with ageing and did not show increased uptake for any of the elements examined. Especially in the depth profiles, PET showed the washing out of Sb in the edge areas of the particles. Further it showed an increased absorption of some added trace elements like Nd and Th. PVDC showed major changes due to artificial ageing, which was also reflected in an increased release of trace elements, which was particularly evident in the depth profiles whereas the increased adsorption of some elements seemed to be independent from ageing. The uptake and leaching behaviour is different for each trace element and each additive respectively. The measured depth profiles and images of the single MP particles showed various results depending on the polymer type and elements. Biggest changes were observed in surface near regions, whereas for others also the bulk composition was affected. Degradation was noticed only in the first 200 nm from surface by the change of oxygen content. It turned out that the combination of the results of bulk analysis and spatially resolved information is particularly valuable to be able to better assess the situation.