Biodegradability of plastics in soil
According to ISO 17556, the determination of ultimate aerobic biodegradability in soil can be performed by measuring the oxygen consumption or depletion caused by aerobic respiration of microorganisms using Gas Endeavour® as a fully automated closed respirometer. The plastic materials, which is the sole source of carbon and energy, is mixed with the sandy-loam soil and incubated at temperature 22 ±1 oC. 1.5 g plastic per 100 g soil was used for all plastic samples. During biodegradation of plastic materials over 200 days, the consumed oxygen is measured continuously by Gas Endeavour® and the biodegradability is calculated as the ratio of the consumed oxygen to the theoretical value.
The extremely low relative standard deviations can be observed for the cumulative oxygen consumption of blank, cellulose positive control, PHB powder samples (±1.6%, ±2.5% and ±6.0%, respectively). This supports a high level of confidence of the results obtained with Gas Endeavour®. Accordingly, the validity criteria of ISO 17556:2019 are fulfilled (i.e., the positive control reached 75.1% which is more than 60% at the end of the test) biodegradation and relative standard deviation of BOD values of all blanks are only ±1.6% which is far below ±20%). PHB powder material reaches about 100±10.2% of biodegradation in 100 days, but only 9.5±2.0% degradation for PHB granule over 200 days. PBS powder has 26.5±4.6% degradation in 200 days. No significant degradation can be observed for PLA granule and PLLA film over the full duration of incubation.