shows the biodegradation results for the 28 days of incubation obtained for each material. According to UNE-EN ISO 14851:2019 guidelines [38], a substance is considered biodegradable if the BOD is higher than 60% ThOD. PHB was used as positive control due to its well-known biodegradability in seawater, as shown by Tachibana et al. who reported 80% biodegradation in 25 days [28]. Similar percentages of biodegradation were found in the present study, where the PHB control achieved 70% degradation after 28 days. Some standards propose an extended incubation period of 60 days, and up to 180 days (ISO 19679, ISO 23977-2, ASTM D7991-15) [44,45]. This extension considerably increases the technical complexity of the experimental setup and increases the chances of failures during the execution of the tests. Moreover, even though we may need to consider a longer period for certain materials, it was observed that the degradation became stable after 28 days for both positive control and tested materials. Materials PVA.029 and PVA.030, including 15% glycerol in their composition, showed a final biodegradation percentage of 5.3 and 8.4% ThOD, respectively, and are thus classified as slightly biodegradable. For the PVA.031 sample, composed by PVA only, we observed that the biodegradation was negligible. However, studies about degradability of PVA on freshwater inoculated with municipal sewage sludge reported percentages of biodegradation of 13% during 21 days [12]. The marine medium used in the present studies (pH 8.3) may lack microbial strains present in wastewater. Additionally, seawater shows a pH > 8, which may have retarded degradation, since biodegradation of PVA was reported to be higher in acidic aqueous solutions than in alkaline ones [46].
Expressed as a percentage of the positive control, the biodegradation rate of the sample PVA.029 was 7.8 and 12.4% for the 030.PVA one, values that still represent a low degree of biodegradation. As expected, the material with the higher degree of hydrolysis (030.PVA) results as the most biodegradable, as reduced molecular weight is a precondition for microbial attack, and the hydrolytic mechanisms enhance the biodegradation processes [47]. Moreover, PVA materials can reach up to 60% degradation in 32 days depending on the degree of solubility of the polymer [48].
The detected increase of 6.85% on average in the biodegradation rate for glycerol-containing PVA may be due to the changes in the hydrophilic characteristics of the glycerol that reduce internal hydrogen bonds in the polymer chain and decrease the residual mass as described by Abdullah and Dong (2019) [49]. They observed an increase of 23.33% in biodegradation rates when adding glycerol. Moreover, raw glycerol has biodegradation on natural water between 68 and 78% [50].
It should be noted that due to the lack of research about biodegradability on microparticles from bioplastics, this study can be only compared to studies using larger fragments (films, pellets, etc.). There is a gap of information in this area that needs to be covered, given that the final faith of microplastics and their degradation products is not well known [51].
Some labels have been developed by industry to distinguish plastics that can biodegrade in the environment. For instance, the label created by Vinçotte OK Biodegradable WATER applies the standard BS EN ISO 14851:2019 [38,52]. Nevertheless, as Harrison et al. pointed out, there is no agreement on which standards to use for plastic biodegradation [53], and a more realistic point of view unifying all the criteria required for these specific materials is needed. It is also important to bear in mind that biodegradability is an intrinsic property of a material, but performed biodegradation will necessarily depend on environmental conditions, and thus no single standard will be able to be representative of the multiple end-of-life scenarios present in the aquatic environments.