Degradation of bioplastics (polylactic acid-PLA and thermoplastic starch-TPS) in the marine environment

This master thesis was conducted in collaboration with the Biochemical Engineering & Environmental Biotechnology lab of the Technical University of Crete within an Erasmus+ mobility (February-September 2022, most of experimental activity) and the Circular Economy lab of the Politecnico of Torino (data analysis and some experimental activity). Bioplastics are biodegradable and/or biobased polymers that are attracting increasing interest in the marketplace and in the literature as sustainable alternatives to conventional fossil-based plastics. Bioplastics undergo the same degradation mechanisms as conventional plastics, and their degradation depends on their physico-chemical properties as well as on the exposure environment. Biopolymers are often biodegradable only under specific and controlled conditions, therefore their behavior in the marine environment is still unclear and raises concerns in the scientific community. The aim of this work is the study of the degradation of two common and, so-called “biodegradable”, bioplastics, namely polylactic acid (PLA) and thermoplastic starch (TPS), in the coastal and pelagic zones of the marine environment. Accelerated weathering (indoor conditions) and natural light (outdoor conditions) were simulated in the BEEB lab of the Technical University of Crete. Pellets degradation over a 5-months period was studied. Weight and size variations of pellets were monitored; Scanning Electron Microscope (SEM) for surface topography changes, and spectroscopy, such as X-ray fluorescence (XRF) and Fourier Transform Infrared (ATR-FTIR) for elemental composition and chemical bonds variations, were used. Microplastics (MPs) formation was described and quantified using fluorescence microscope and Nile Red, Dynamic Light Scattering (DLS), and Nanoparticle Tracking Analysis (NTA). Degradation (visible as surface cracks, topography changes, weight and size differences, surface bonds and elemental variations) was much more evident in pelagic pellets (in seawater) than in coastal ones (on sand). Weight reduction for PLA in indoor conditions was 7.6% in the coastal zone and 33.2% in the pelagic one. Weight reduction for TPS in indoor conditions was negligible in the coastal zone and 16.8% in the pelagic one. An increase in the concentration of Si, S, and Cl was observed on the pellets surface, especially for the pelagic ones. Due to the TPS porous structure, these concentrations were up to two orders of magnitude higher than PLA (from 102 to 104 mg/kg). Biofilm formation for TPS in the outdoor pelagic environment likely enhanced elements uptake. The Carbonyl Index (CI) increased from 0.30 to 0.85 and 0.47 for PLA indoor and outdoor, respectively; while decreased from 5.60 to 3.40 and 4.90 for TPS indoor and outdoor, respectively. MPs formation (mainly in the form of fibers) was similar for the pelagic zone under indoor and outdoor conditions (from 104 MPs/ml of the pure seawater to 106) and its increase over the time was two orders of magnitude higher than in the coastal zone. To conclude, degradation effects and microplastics generation were more visible for pelagic zone with respect to coastal zone, likely due to hydrolysis effect. Photo-oxidation, along with its degradation effects, was more powerful for indoor conditions (concentrated UV light) compared to outdoor conditions.