The bacterial synthesis and characterisation of polyhydroxyalkanoates using waste plastics as carbon sources for novel applications

Abstract

Polyhydroxyalkanoates (PHAs) are a group of biocompatible, environmentally neutral, biodegradable plastics that can be produced by bacteria. The structure of PHAs can be adapted for a wide range of applications, including biomaterials, 3D printing filaments, packaging and general household items. Some of the factors limiting the mass usage of PHAs are the high costs of the carbon sources required for bacterial metabolism and the chemical processing required to extract stable, contaminant-free PHA structures from biomass, that are comparable to the wide range of petrochemical plastics currently in use. This study has considered the novel use of oxidised and non-oxidised wax substrates derived from waste polyethylene (PE) as a carbon source for wild-type bacteria. In addition, industrial waste polypropylene (PP) and polystyrene (PS) were used as potential carbon sources. The bacterial strain Cupriavidus necator H16 was used to generate polyhydroxybutyrate (PHB) and other PHAs with differing polyester compositions, depending on the type of waste carbon source used in 48 hour fermentations in nutrient rich media. These products were then chemically analysed to assess their structures. The methods for the chemical analysis included nuclear magnetic resonance (NMR) and electrospray ionisation tandem mass spectrometry (ESI-MS/MS). It can be concluded, that oxidised PE (O-PEW), non-oxidised PE (N-PEW), thermally treated PP and PS could be promising carbon sources for PHA production in the future. In terms of PHA applications, it has been demonstrated that human cells can be grown on 3D-printed PHA-blend scaffolds and through collaborative studies PHA-blend biofilms containing herbicides that can biodegrade efficiently to distribute their herbicides to successfully prevent weed growth. Further research in this field could focus on further modification of the recombinant microbial strain, more accurate methods of using bioprobes or optical instruments to monitor bacterial growth during fermentation, mixed cultures of bacterial species and the use of other recyclable ionic liquids to produce PHAs of higher quality and purity. Moreover, other waste plastics could be attempted as carbon sources and further research into emulsion formation to improve viability could lead to greater PHA yield. If biodegradable alternatives such as PHAs can become the bioplastic of choice, we could look forward to a greener and more economically stable future.