Biodegradation of plastics and ecotoxicity testing: when should it be done

Most biodegradable plastics are waterinsoluble solid materials under normal environmental conditions (Bohlmann, 2005). The biodegradation of plastics is a surface erosion process which happens at the solid/liquid interface, where extracellular enzymes, in the liquid phase, start depolymerisation of the solid phase (Tosin et al., 1996). The depolymerisation releases monomers that are assimilated by the surrounding microorganisms (the “central dogma” for biodegradation of polymers; Kaplan et al., 1993). The depolymerisation is the limiting factor, while the subsequent assimilation of monomers by the microbes is expected to be immediate (Saponaro et al., 2008). For example, a cube shaped plastic item made with an isotropic and homogeneous biodegradable polymer (a “Polyester X”) with a volume of 1 cm3 and a density of 1 g/cm3 is subjected to biodegradation. When biodegradation starts, the cube will shrink as a consequence of the erosion of the six surfaces. Therefore, the first phenomenon of biodegradation is surface erosion that can be measured as a decrease in thickness per unit of time per unit area (cm day−1 cm−2). In this simple model, suppose a constant erosion rate of 0.0095 cm day−1 cm−2. With this erosion rate it takes about 100 days to have a complete biodegradation of the plastic cube. After 16 days, each side of the cube will be reduced to 0.85 cm (Figure 1), the residual volume will be 0.61 cm3 (equal to 0.61 g) and 0.39 g of plastic will have being assimilated (i.e., the percentage of biodegradation will be 39%). Assume that the process is stopped at this point. The cube is still a plastic item, intact, apart from the surface, a thin, partly degraded layer where the degradation processes is “frozen” in an intermediate phase. A reduction in molecular weight and a loss of material properties has happened (especially in the amorphous regions, more sensitive to degradation) but without weight loss (Eldsäter et al., 2000) The missing mass has been converted into CO2 and H2O (for simplicity, we consider that all the plastic is totally mineralised, with no conversion into biomass and no recalcitrant bio-products being formed). Is a percentage of 39% a substantial biodegradation level? If referred to the Polyester X 39% sounds incomplete. But if we consider more carefully the matter, it is the