No disintegration was observed for HD-PE. Additionally, polyhydroxybutyrate (PHB, MIREL P5001) was used as a positive control and high-density polyethylene (HD-PE) as a negative control. In a follow-up experiment, the <500 µm fraction of sediment from four different beaches around Isola d'Elba, Italy, was used to test plastic disintegration as above. We assume that the higher surface-to-volume ratio in fine sediment compared to coarse sediment led to a higher bacterial abundance and thus to faster disintegration rates. Disintegration half-life was shortest on mud (139 days) and increased with the grain size of the beach sediment fractions (63-250 µm: 296 days 250-500 µm: 310 days 500-1,000 µm: 438 days >1,000 µm: 428 days). First, to test the effect of sediment grain size, film of Mater-Bi HF03V, a blend of thermoplastic starch and biodegradable polyesters, was placed on the surface of mud as well as on four different grain size fractions of beach sand.
To gain more insight into the effects of different sediments on the degradation rate of biodegradable plastic we performed two iterative seawater tank experiments. Biodegradable polymers are available on the market as a substitute for conventional plastic and could potentially end up in the same environment. The seafloor is considered the major sink for plastic debris in the world's oceans.
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