An FTIR and EA-IRMS Application to the Degradation Study of Compostable Plastic Bags in the Natural Marine Environment

The present study aims to explore the degradation process of compostable, starch-based (i.e., Mater-Bi®) shopping bags in the marine environment using isotope ratio mass spectrometry and Fourier-transform infrared spectroscopy (FTIR). The mixing model applied to the isotopic data suggested that the compostable shopping bags had a mixed composition with a higher percentage of polyesters (61% to 72%). Changes in the isotopic composition over a 73-day period of marine water immersion showed a decrease in the corn starch constituent (of 14% to 13%), with a similar rate in both types of bags in accordance with the evidence derived from the infrared spectra. The time required for complete degradation of the starch fraction was estimated by an isotopic approach from 124 to 180 days, following zero-order kinetics. The coupled application of these two analytical methodologies promises to (i) show complementary evidence regarding the time-course degradability of different polymers via FTIR changes, and (ii) infer potential degradation mechanisms via carbon isotope analyzes. We encourage the use of this information to support advancements in the development of more sustainable-by-design plastic materials.