Assessing Marine Biodegradability of Plastics

Regarding assessment criteria, 60% of ThOD has been frequently invoked as a threshold for ready biodegradability [23 ,27 ,28 (link),29 ], and 20% of ThOD is used by [30 ] as a pre-screening criterion to consider a material as potentially biodegradable in the marine environment. However, during microbial biodegradation, a relevant proportion of the polymer carbon is not mineralized to CO₂ but assimilated by the heterotrophic microbial consortium and converted into biomass, setting an actual maximum of BOD between 30 and 50% below ThOD [31 (link)]. On the other hand, current biodegradable materials are frequently heteropolymers and complex mixtures whose exact atomic composition is unknown, which prevents calculation of the ThOD. Because of these limitations, we have recently proposed replacing the percentage of ThOD by the percentage of the BOD recorded in the positive control (C+), using as C+ the truly marine-biodegradable polymer PHB [11 (link)].
In addition, since the current method is based on short-term (28 days) incubations, it is advisable to include a third benchmark intended to differentiate fully non-biodegradable materials from slightly biodegradable materials, which can be arbitrarily set at 5% C+. Therefore, a provisional scheme for the assessment of marine biodegradability of plastic materials can be based on these benchmarks, resulting in the following classes (Table 1): (i) non-biodegradable (<5% C+ in 28 days), (ii) slightly biodegradable (between 5 and 20%), (iii) moderately biodegradable (between 20 and 60%), and (iv) readily biodegradable (>60%) (see Table 1).