Lifetimes and mechanisms of biodegradation of polyhydroxyalkanoate (PHA) in estuarine and marine field environments

Coastal cities face significant challenges from plastic pollution, with most plastics being resistant to biodegradation. Biodegradable plastics are increasingly used to address this issue, particularly for items prone to entering, and then accumulating, in waterways, through littering or leakage. Among biodegradable plastics, polyhydroxyalkanoates (PHAs) are notable as bioderived, bacterially synthesised aliphatic polyesters that are readily biodegradable in varied environments. This study focuses on the lifetimes and biodegradation behaviour of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) sheets submerged in five different aquatic environments (under both surface and benthic conditions) within a single coastal zone over 51 weeks. The biodegradation was characterised through mass and thickness loss, and changes in surface morphology, thermal and mechanical properties, and molecular weight. The findings revealed that the lifetimes of PHBV sheets varied between benthic and surface sites, with all benthic sites exhibiting faster biodegradation rates (0.068 ± 0.019 mg.d^-1.cm^-2 to 0.163 ± 0.048 mg.d^-1.cm^-2) compared to the surface (0.032 ± 0.015 mg.d^-1.cm^-2). Lag times to initiation of biodegradation in the Marina benthic and River benthic sites were similar (9-25 days) with the two other benthic sites (Sea and Mesocosm) comparable with the Marina surface ranging from 41 to 110 days), indicating that the local environment has a stronger influence on lag time as opposed to the specific rate of mass loss following biodegradation onset. UV exposure did not impact the crystallinity of the surface sheets, which remained stable throughout the exposure period. Overall, if thin- walled, (∼150 μm) products made from PHA do leak into the aquatic environment and remain buoyant, then their lifetimes are forecast to be within 1-2 years; if they settle in benthic environments, their lifetimes are likely to be between 4 and 9 months.