Dataset: Thermalgravimetric analysis

Name: Thermalgravimetric analysis
Published: 2024-12-26
Keywords: macroplastic, photodegradation, pyrolysis-gc/ms, oceans

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N/A. Laboratory incubation experiment.

Temporal Extent: 2020-10-01 - 2022-09-01

Project:

EAGER: Collaborative Research: NSF2026: Is Plastic Degradation Occurring in the Deep Ocean Water Column? (Deep Ocean Plastic Degradation)

Principal Investigator:

Rut Pedrosa Pàmies (Marine Biological Laboratory, MBL)

Co-Principal Investigator:

Scott Gallager (Coastal Ocean Vision)

Zhanfei Liu (University of Texas at Austin, UT Austin)

Emil Ruff (Marine Biological Laboratory, MBL)

Student:

Xiangtao Jiang (University of Texas at Austin, UT Austin)

Contact:

Xiangtao Jiang (University of Texas at Austin, UT Austin)

BCO-DMO Data Manager:

Karen Soenen (Woods Hole Oceanographic Institution, WHOI BCO-DMO)

Version:

Version Date:

2024-12-26

Restricted:

Validated:

Current State:

Data not available

Abstract:

The photodegradation of macroplastics in the marine environment remains poorly understood. Here, we investigated the weathering of commercially available plastics (tabs 1.3 × 4.4 × 0.16 cm), including high-density polyethylene, low-density polyethylene, polypropylene, polystyrene, and polycarbonate, in seawater under laboratory-simulated ultraviolet A radiation for 3–9 months, equivalent to 25–75 years of natural sunlight exposure without considering other confounding factors. After the exposure, the physical integrity and thermal stability of the tabs remained relatively intact, suggesting that the bulk polymer chains were not severely altered despite strong irradiation, likely due to their low specific surface area. In contrast, the surface layer (∼1 μm) of the tabs was highly oxidized and eroded after 9 months of accelerated weathering. Several antioxidant additives were identified in the plastics through low temperature pyrolysis coupled with gas chromatography/mass spectrometry (Pyr-GC/MS) analysis. The Pyr-GC/MS results also revealed many new oxygen-containing compounds formed during photodegradation, and these compounds indicated the dominance of chain scission reactions during weathering. These findings highlight the strong resistance of industrial macroplastics to weathering, emphasizing the need for a broader range of plastics with varying properties and sizes to accurately estimate plastic degradation in the marine environment.

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Methods & Sampling

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.

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.

Data Processing Description

TGA was performed using a thermogravimetric analyzer (Shimadzu TGA-50) under a nitrogen atmosphere (50 mL/min flow rate) with three replicates for each polymer type. Briefly, the plastic tabs were cut into small pieces, and a sample weighing 5–10 mg was analyzed. The sample in a platinum pan was heated from 30 to 650 °C at a rate of 5 °C/min. TG and derivative TG (DTG) curves were recorded to assess the thermal behavior of the sample. The TG onset temperatures (Tonset), defined as a 3% (w/w) loss, and the DTG maximum temperatures (Tmax) were calculated from the curve to determine the thermal stability of the plastic, following the method described by Yamada et al.

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Funding Source	Award Number
NSF Division of Ocean Sciences	OCE-2033828

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Dataset: Thermalgravimetric analysis