## 4 Discussion
On the other hand, the less negative \(\delta^{13}\)C values were evaluated for PLA and PHB, suggesting that the food bags (ULB and LSB) analyzed in this study (\(-21.14\)\(\pm\)\(0.13\%\) and \(-22.87\)\(\pm\)\(0.09\%\), for ULB and LSB, respectively), are not constituted of these polymers, but rather of polyesters such as PCL (repeatedly mentioned in the literature as a Mater-Bi\({}^{\circleddelta}\) component) or PBS and corn starch (the only component declared by the producer). Our \(\delta^{13}\)C results suggested a mixed composition of blends used in the shopping bags, as indicated by the results of the mixing model. This showed a higher percentage of polyesters in LSB with respect to ULB (72% and 61%, respectively), suggesting different purposes of the shopping bags' use. In fact, the LSB sample represents a bag type which requires higher tensile strength and flexibility [47]. However, the more negative \(\delta^{13}\)C values reported for PE bags could suggest that bio-bags employed in the experiment do not contain PE, or at least not in large detectable quantities. It must, however, be taken into account that, according to the data obtained, vegetable oils of such common crops like sunflower and rapeseed would give co-polyesters an isotopic footprint between that of PCL (as well as PBS) and PE. In summary, both ULB and LSB seem to be composed of polyester and starch, as also confirmed by FTIR analysis. In fact, the diverse relative proportion of the two components in ULB and LSB explains the spectral differences found among them, confirming the higher percentage of starch in ULB than that in LSB. Regarding the changes in composition of the shopping bags (ULB and LSB) over the 73-day period of immersion in marine water, data showed a depletion of \(\delta^{13}\)C values for ULB and LSB (\(\Delta\)Bi\({}^{3}\)C of \(-2.16\) and \(-2.84\%\), respectively), which was not observed for the PE control bags. The isotopic shift observed could be reasonably related to the degradative actions that occur in the marine environment (e.g., chemical and/or biological processes) such as photodegradation, thermo-oxidation, hydrolysis and biodegradation. These processes indeed could be responsible for the depolymerization and the subsequent assimilation and metabolism of the monomers by microorganisms ([15] and literature therein). The lack of carbon depletion in the PE control bags suggested a very low PE degradation (if any) with respect to Mater-Bi\({}^{\circleddelta}\) bags, as already observed by some authors [48; 49]. In the work by [PERSON] et al. [48], PE bags exposed for 20 months at a depth of 2 m in the Baltic Sea showed no biodegradation and low changes in tensile elongation at break. Moreover, in the present study, the isotopic data of Mater-Bi\({}^{\circleddelta}\) bags showed a faster loss of corn starch which has been suggested to be a more bioavailable and hydrophilic component compared to polyesters. Similar results were reported elsewhere [50]. Results obtained by FTIR spectroscopy showed that both ULB and LSB seem to be composed of polyester and starch confirming the outputs of the isotopic analysis approach. In particular, the absence of aromatic overtone signals in the region between 2000 and 1650 cm\({}^{-1}\) could suggest the absence of co-polyesters containing aromatic monomers like terephthalic acid, in favor of the presence of PCL. In fact, during the shopping bags degradation experiment, FTIR spectra showed an increase in hydroxyl groups (ratio I300/I730), ascribed to the hydrolytic degradation of the constituting polymers. The decrease in molecular weight with an increase in the number of terminal groups, which contributes to the strengthening of the OH signals, is a well-known phenomenon. Hydrolysable ester bonds in a polymer backbone make polymers more susceptible to microbial degradation [16; 45; 51].