Study of the biodegradation in soil of new generation plactics

The intense use of plastic contributes to increase the amount of municipal waste that are generally disposed in landfill. For some applications and sectors, an important alternative to the conventional plastic materials can be found in the use of the new generation materials: the biodegradable polymers. Their use can be an alternative to landfill disposal and can thus reduce the cost of waste management and the accumulation in the environment. The biodegradable polymers, in fact, are used by microorganisms as source of carbon and are converted by into carbon dioxide (or methane), water and mineral salts of any other element present (mineralization) plus new biomass. Generally, laboratory test methods developed for determining the biodegradability of polymeric material are based on the evaluation of the carbon dioxide production (respirometric test), but in order to completely describe the biodegradation process and to avoid an underestimation of the biodegradation percentage, it is very important to quantify and identify possible by-products and biomass production. The experimental work presents in this thesis concerned the study of the different aspects of the biodegradation of new generation plastics in soil. Particular attention was focused on the fate of the possible by-products of biodegradable polymers (the monomers) and on the determination of the biomass generated during the process. To summarize: 1) The effects of the soil pH on the mineralization of ten monomers, chosen between the most widely used for the synthesis of plastic materials, was evaluated by respirometric tests and the experimental data were used to validate a numerical model that can estimate the amount of carbon used by microorganism for biochemical synthesis. 2) The mineralization of a model polyester was investigated by respirometric tests in different soil mixtures in order to evaluate the effects of the initial soil pH and of the addition of organic matter. 3) The combustion of soil samples at 550 °C was used for determining the amount of organic matter and biomass in soil samples. The sensitivity of the method was evaluated by adding different low amounts of organic matter to a natural and a synthetic soil. 4) In order to describe the carbon balance during the biodegradation of the model polyester, biomass production and polyester residues in soil were estimated. Biomass and organic matter deriving by polyester biodegradation were studied by combustion of soil samples. Polyester residues were estimated by extractions in soxhlet of soil samples (with chloroform) and the extracts were characterized by 1H-NMR and 31P-NMR acquisitions and GPC.