From degradation to fragmentation: physicochemical properties influence microplastic formation and persistence of P(3HB-co-4HB) in soil

The environmental fate of biodegradable polyhydroxyalkanoates (PHA) remains insufficiently understood, particularly regarding how soil biodegradation generates transient microplastic intermediates and controls their persistence. Here, the degradation-fragmentation behavior of P(3HB- co -4HB) copolymers containing 10mol%4HB and 34mol%4HB was investigated by combining microscopic and topographical observations, individual-particle persistence monitoring, and quantitative microplastic analysis. Degradation was spatially heterogeneous, with localized erosion features spatially associated with filamentous structures consistent with fungal hyphae. In the 34mol%4HB copolymer, selected for detailed microscale observations because surface alteration was more clearly resolved, topographical analyses revealed channel-like eroded regions with micrometer-scale depth variations and residual domains. The two copolymers exhibited distinct degradation and fragmentation behaviors. The 34mol%4HB copolymer showed pore formation, heterogeneous erosion and morphological features consistent with secondary fragmentation, whereas the 10mol%4HB material better preserved particle integrity and underwent more gradual surface erosion. Persistence analysis showed that PHA microplastics remained detectable in soil for days to weeks, with smaller particles and those from the 34mol%4HB copolymer becoming non-detectable earlier. At the population scale, the two grades generated markedly different particle populations in terms of abundance, size distribution and morphology. The 34mol%4HB copolymer generated more numerous and smaller particles, whereas the 10mol%4HB material produced fewer, larger and more irregular residual fragments. Circularity analysis further showed that the higher overall circularity of the 34mol%4HB population was partly associated with its enrichment in smaller particles. Overall, this study shows that, for biodegradable PHAs, microplastic formation can represent a transient intermediate stage within the degradation process rather than a terminal fragmentation state, as the generated particles can continue to degrade and progressively become non-detectable over time.