Valorisation of plastic waste: degradation based on enzymatic depolymerisation

• Autor(a) principal: Aguiar, Mariana Isabel dos Santos
• Data de Publicação: 2022
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositórios Científicos de Acesso Aberto de Portugal (RCAAP)
• Texto Completo: http://hdl.handle.net/10773/36848
• topic: Plastic waste; Polyethylene; Additives; Laccase; Biocatalysis; Depolymerisation; Plastic recycling

Plastics are ubiquitous synthetic polymers that have become an indispensable part of daily life due to their many uses, ranging from packaging to electronic applications and biomedicine. It is therefore not surprising that their annual production exceeds 350 million tons. However, only a small fraction of the polymers produced each year are recycled, in part because the physical properties of the recycled material decrease significantly during the process. This work aimed to achieve a recycling process based on the concept of circular economy by developing an enzymatic depolymerisation process for packaging waste commonly produced of high-density polyethylene (HDPE). For this purpose, the enzyme laccase from Trametes versicolor was used. In addition, two methodologies were evaluated, i.e. the enzyme was applied to solid HDPE or HDPE solubilized in solvent - liquid HDPE. The enzymatic reaction occurred for 14 days at mild temperature (37 ⁰C) under constant agitation. For the solid HDPE, three different approaches were evaluted in order to determine the best conditions to enzymatic reaction: (i) biocatalytic media (citrate-phosphate buffer pH 4.5 or aqueous mixture of deep eutectic solvent (DES - cholinium dihydrogen citrate : xylitol at 50 (w/w)%), (ii) mediators (ABTS or HBT) and (iii) polymers (HDPE with and without additive - orange pigment, PO64). The extent of the reactions was evaluated by weight loss, FTIR, TGA, DSC and XRD, and the best system was the one composed of HDPE without additive and treated with buffer, laccase and ABTS. This system showed a weight loss of 32.90%, new bands in the FTIR spectra (e.g., carbonyl peak), a higher diffraction angle in XRD, and similar results in TGA and DSC compared to orange HDPE. The results indicate that enzymatic oxidation has occurred, preferentially attacking the amorphous domain. Regarding the liquid HDPE, different L-menthol-based DES and their constituents were used to dissolve HDPE to understand whether it would be easier for the enzyme to attack the HDPE in the liquid state. This approach was also evaluated in the presence or absence of a mediator (ABTS) and using HDPE with and without additive. The DES L-menthol : decanoic acid was the most suitable, since the enzyme presents activity after 24 h. The extent of the reactions was evaluated by FTIR, DSC, and FT-Raman. The FTIR and DSC results indicate that some structural changes occurred in most systems, as new bands (i.e., carbonyl peak) appeared, and the melting temperature and crystallinity values showed some differences, especially in the systems in which HDPE was dissolved with DES of L-menthol : decanoic acid and treated with buffer, laccase and ABTS. In addition, it was found that, the system of HDPE dissolved with heptanoic acid and treated with buffer, laccase and ABTS allowed the removal of 100% of the additive (PO64) from the HDPE. Promising results were obtained for the enzymatic depolymerisation of HDPE using laccase, especially the results obtained for the solid HDPE approach, i.e., the system of HDPE without additive treated with buffer, laccase, and ABTS. However, further studies are needed to understand the polymer changes and optimise the process to achieve the desired recycling process.