Aerobic microbial degradation of PLA, PHB, and PBAT microplastics in freshwater: Different structural changes and byproduct formation

초록

Biodegradable plastics (BPs) are advocated as sustainable alternatives, however their fate in freshwater remains poorly understood. We evaluated the aerobic biodegradation of poly(3-hydroxybutyrate) (PHB), polylactic acid (PLA), and poly(butylene adipate-co-terephthalate) (PBAT) microplastics (450-550 mu m) in sludge-inoculated mineral medium and in natural lake water. Biodegradation was considerably higher in sludge, with PHB exhibiting the highest biodegradation, PLA moderate, and PBAT negligible. Surface and molecular characterizations (FE-SEM, XRD, XPS, and LC-MS) revealed polymer-specific pathways: PHB showed surface erosion, reduced crystallinity, and modest XPS shifts without detectable extracellular 3-hydroxybutyrate; PLA showed slow hydrolysis, increased crystallinity, and C-C enrichment with lactic acid intermediates; PBAT remained largely recalcitrant, releasing terephthalic and adipic acids while retaining aromatic backbone integrity. Principal component analysis of XPS and LC-MS data resolved distinct degradation trajectories among polymers. The persistence, bioaccumulation, and toxicity (PBT) characteristics of the LC-MS-identified degradation products were evaluated using a screening-level approach (REACH Annex XIII Section 3.1). For PBT screening, ready biodegradability data, octanol-water partition coefficients, and acute aquatic toxicity values were collected from literature. All three acids showed no screening-level indication of P, B, or T concern, although terephthalic acid exhibited comparatively higher toxicity than adipic and lactic acids. Overall, biodegradable microplastics degrade slowly via divergent pathways in freshwater and the screening-level hazard potential of their degradation products remains low.

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