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Assessment of bioplastics degradation in the environment

Morfogianni Dimitra

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Summary

The accumulation of plastic waste is one of the various challenges our planet has faced in recent years. Some plastic waste accumulates in landfills, while others end up in aquatic and terrestrial ecosystems, where they remain for a very long time, due to the fact that petrochemical-based plastics are mainly non-degradable. One way of overcoming, at least in part, the problems associated with the production of plastic goods and their end of life is to develop and produce biodegradable plastics of biological origin, also known as bioplastics. Bioplastics are produced from renewable biomass sources and most of them can be broken down by micro-organisms in the environment. The use of bioplastics is gaining ground, but their study is still at an early stage. In this thesis, the fate and degradation potential of the bioplastics PLA and PBAT in ECOFLEX form, separately and in combination with Coffee Silverskin (CS) and MMT Clays additives, were studied under the influence of environmental conditions in seawater and soil environment. The experimental procedure was divided into three time-intervals of 45 days and in total lasted about 4 months. The bioplastic samples were kept in seawater in the laboratory and in soil containers outdoors. The rate of biodegradation was monitored by studying the weight of the bioplastics, the behaviour of the bacterial community in seawater and soil, and the growth of microorganisms on the surface of the bioplastics, also known as biofilm. During the experiments the bioplastics were the only source of food/energy for the microorganisms. In addition, the protein concentration in the biofilm of the samples was tested in seawater and soil samples. Regarding the results related to weight change, PLA and ECOFLEX samples did not show significant weight reduction up to the end of 135 days in either water or soil. However, discoloration and greater flexibility of the samples were observed after 90 and 135d. The samples that were a mixture of CS and MMT Clays showed a greater weight reduction. Specifically, PLA4043D + ECOFLEX(10-15%) + 30%CS + 8%MMTClays lost 17.9% of their weight after 90d in water and 51.6% after 45d in soil. PLA4043D+30%CS and ECOFLEX+30%CS samples had lost about 10% of their weight by the end of the experiment in seawater, and even more in soil. Regarding the biofilm study, in both environments, more bacterial colonies were measured on the surface of the combined materials. The bacterial density was higher by two orders of magnitude on combined bioplastics than PLA and ECOFLEX. These results are consistent with the above results concerning weight loss in terms of degradation rate. Finally, regarding the study of proteins in the biofilm, in the seawater samples, a maximum concentration of 3.75 μg/mL was observed in the PLA4043D + ECOFLEX(10-15%) + 30%CS + 8%MMT Clays sample of 90d. This sample was the one that showed the highest degradation rate. As for the soil samples, a maximum concentration of proteins was observed in the samples of the first 45d, indicatively 4.33 μg/mL for PLA4043D + 30%CS. This may be due to external environmental factors such as temperature. In conclusion, PLA and ECOFLEX(PBAT) bioplastics are difficult to be degraded and require a longer time by microorganisms in aquatic and terrestrial environments. However, in combination with CS and MMT Clays, the degradation of these materials starts earlier, as due to their biological origin it is easier for microorganisms to degrade them. Consequently, the remaining polymer becomes vulnerable and the degradation rate increases.

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