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Environmental aging behavior and organic matter release of biodegradable microplastics PLA, PBAT and PHBV under different coastal environmental conditions
Summary
"Biodegradable" plastics used in coastal areas (like PLA, PBAT, and PHBV) don't just harmlessly disappear—sunlight and seawater conditions break them down into smaller particles and release dissolved substances at different rates depending on the environment. This matters because these breakdown products, including tiny nano-sized particles, could persist in coastal waters longer than expected and potentially enter the food chain, meaning "biodegradable" plastics may still pose environmental and health risks similar to conventional microplastics under certain conditions.
To better understand the environmental fate of biodegradable microplastics (MPs) in coastal environments, this study investigated the aging behavior and organic matter release of PLA, PBAT, and PHBV MPs with typical environmental factors (UV irradiation, water, salinity, and DOM). After 30-days incubation, all materials aged rapidly under UV, with significant changes in particle size distribution and number-average molecular weight (M) decreased to 20-50% of initial value. The increases in carbonyl index (CI) and -COOH/-COO- ratio of the carbonyl bond indicated significant photooxidation, Norrish reactions and ester bond hydrolysis. While in darkness, the decrease of M was substantially lower and -COOH/-COO- ratio increased but no change in CI were detected, indicating hydrolytic cleavage only. Among tested polymers, PHBV exhibited the largest changes regardless of irradiation. The influence from environmental matrix was also pronounce: aging proceeded faster in air than in DI water; dissolved salts in seawater inhibited chain scission, whereas the present of DOM markedly accelerated chain scission but reduced the relative abundance of chain-scission product. Moreover, with the analysis of TOC, particle abundance and chemical component of leached OC, we found that UV irradiation promoted the conversion of shed particulates into dissolved molecules and accelerated humification simultaneously in the leachate, yielding more humic-like DOM. Conversely, dark aging leached higher relative abundances of submicron/nanoscale particles and protein-like DOM into surrounding water. After all, by altering aging behaviors and the plastic derived organic matter release, these environmental factors may further change the environmental persistency and possible risks of biodegradable plastics in coastal ecosystems.