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Advancing Marine Plastic Degradation Studies using Novel, Open-Source, Action Spectra-Guided Weathering Methods
Summary
Scientists built new, low-cost lab tools to test how sunlight, seawater, and even ocean microbes break down common plastics (like polypropylene and nylon) in ways that better mimic the real ocean, instead of relying on unrealistic super-intense UV lab tests. They found that plain plastic breaks down fastest in dry, sunny conditions, but a coating of microbes (biofilm) actually slows this breakdown by shielding plastic from UV light — meaning previous estimates of how quickly ocean plastic degrades into microplastics may not reflect what's really happening in the sea. This matters because it helps resear
Plastics are now pervasive across marine and terrestrial environments, yet their long-term degradation under realistic environmental conditions remains poorly constrained. Many accelerated weathering studies use intence ultraviolet radiation sources, high temperatures, and unrealistic exposure regimes, that do not accurately represent marine settings, making it difficult to relate laboratory degradation rates to environmental persistence. This thesis develops and validates a suite of open-source, environmentally relevant methodologies for quantifying polymer weathering under controlled but more realistic exposure conditions.Polypropylene (PP), polyamide (PA), and polylactic acid (PLA) were selected to represent conventional, industrial, and biodegradable polymer types relevant to marine plastic pollution. First, a custom LED-based action-spectra apparatus was developed using narrowband UV LEDs between 285 and 395 nm, 3D-printed components, controlled sample positioning, and spatial irradiance mapping. This enabled wavelength-specific photo-oxidative responses to be quantified using FTIR-derived Carbonyl Index measurements. The results showed that UVB wavelengths produced the strongest chemical changes, particularly in PP and PA, while PLA showed limited chemical response under the tested conditions. Polymer-specific action spectra were successfully generated for PP and PA, demonstrating that LED-based systems can be used to quantify wavelength-dependent polymer degradation.A second simulated environmental weathering (SEW) system was then developed to examine longer-term degradation under dry, abiotic seawater, and biotic seawater conditions, with controlled UVR, temperature, salinity, humidity, and biological exposure. Long-term weathering showed that virgin PP was the most susceptible polymer, undergoing pronounced oxidation and physical surface damage under UV-exposed dry and abiotic conditions. PA (nylon) developed surface cracking but showed more limited chemical oxidation, while PLA remained largely stable both chemically and physically. Biotic treatments consistently reduced Carbonyl Index development, indicating that microbial growth and biofilm formation can shield polymer surfaces from UVR and suppress photo-oxidative degradation.The findings demonstrate that polymer degradation in marine-relevant environments is controlled not only by UV dose, but also by wavelength distribution, sample exposure history, oxygen availability, the presence of water, and biological surface colonisation. The work also shows that action spectra can provide useful first-order rankings of wavelength importance, but that spectra derived under narrowband, UVB-enhanced conditions cannot directly predict degradation under complex broadband environmental exposures. Overall, this thesis provides a novel open-source methodological framework for more environmentally relevant polymer weathering studies and shows that realistic marine conditions can substantially alter both the rate and pathway of plastic degradation compaired to standard industrial accelerated weathering.