Materials: High-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polycarbonate (PC) plastic sheets, as the primary plastic without weathering previously, were obtained from McMaster-Carr Supply Company (USA). These polymers were chosen because of their large-scale global production and their widespread presence in the marine environment. The plastic sheets (122 cm × 61 cm) had a thickness of 1.6 mm. The exact formula and additive information were not provided by the vendor. However, plastic additives in various concentrations were confirmed in different plastics through Pyr-GC/MS. Plastic sheets were cut into small tabs (1.3 cm × 4.4 cm) and then cleaned with DI water and dried in a laminar-flow hood before further use.

Weathering Experiment: To simulate long-term weathering conditions in the marine environment, plastic tabs underwent a controlled experimental setup. Four plastic tabs of each polymer type were placed within a 10 cm Pyrex crystallizing dish filled with 200 mL of filtered natural seawater (1 μm spiral wound cartridge filter, Pall, USA, salinity 32 psu) and maintained at 55 °C with three replicates. Additionally, approximately 10,000 glass beads (1 mm diameter) were introduced to the dish, covering 80% of the dish’s bottom surface area, with continuous agitation provided by a rotary table (stroke length 10 cm, speed 60 rpm) to simulate the effects of physical contact with sediment particles in coastal water. UVA lighting (315–400 nm) was applied using three overhead LED UV lamps (Isuerfy, 120 W, F120W-UV-US), positioned 3 cm above the dishes, with an intensity of 230 W/m2 each, verified by a UV light meter (UV513AB, General Tools). UVA was selected due to its prevalence and deeper penetration depth over the other UV bands. On average, the simulated UVA irradiance was approximately 50 times that of natural UVA strength, estimated at 4.5 W/m2 reaching the earth on a global average, i.e., 3 months of continuous light exposure in this experiment equals approximately 25 years of natural diel UVA exposure in the ocean. To maintain constant salinity and water levels, distilled water was replenished every other day.

The plastic tabs were subsampled at four different time points: T0 (before weathering), T1 (3 months ≈ 25 years), T2 (6 months ≈ 50 years), and T3 (9 months ≈ 75 years). These time points were chosen to capture the progressive changes in the plastic tabs over the course of the weathering process. The experimental design, involving the coincubation of plastic tabs of the same polymer with agitation, posed challenges for tracking the weight change of a specific tab at a given time point. Additionally, due to incubation in seawater, effectively removing all sea salt adhered to the plastic surface through water rinsing was difficult. Thus, we did not measure the weight loss of the tabs with the exposure time.