Biosynthesis and characterisation of polyhydroxyalkanoate biopolymers and their oligomers for circular economy

Abstract

Polyhydroxyalkanoates (PHAs) are biodegradable bioplastics that can potentially replace non-biodegradable petroleum-based plastics. However, the high production cost of PHAs which is associated with the high cost of starting substrate extraction solvents limits its integration into large scale biotechnology process. To overcome this limitation, this research examined the upcycling and bioconversion of plastic wastes to PHA. Novel plastic wastes investigated in this study were oxidised low-density polyethylene (LDPE), LDPE separated from Tetra Pak® waste (PE-T) and [text redacted]. These plastic wastes were supplied directly to Cupriavidus necator for use as potential carbon sources for PHA accumulation in a 48-hour shake flask cultivation study, in either tryptone soy broth or basal salt medium. LDPE and PE-T produced the most PHA yield with high purity (29% CDW and 40% CDW respectively). While cultures with [text redacted] had the highest yield (32-68% CDW), characterisation results showed this was due to high contamination from the [text redacted] starting material. Electrospray ionisation mass spectrometry (ESI-MS) confirmed the monomer composition of the polymer produced with LDPE and PE-T to be 3-hydroxybutyrate, 3-hydroxyvalerate and 3-hydroxyhexanoate and that with [text redacted] to be 3-hydroxybutyrate and 3-hydroxyvalerate. Chloroform solvent extraction and soap digestion were also compared to determine the most cost-effective, characterized by high PHA yield and purity. Chloroform extraction technique resulted in higher PHA yields (40% CDW) than soap digestion technique (14% CDW). In PHA oligomer production studies, thermal degradation of PHBV to PHA oligomers proved to be a better method for obtaining PHA oligomers than from yeast biomass in brewery waste. The outcome from this study provides preliminary evidence for further developmental work on the cost-effective microbial recycling of LDPE and Tetra Pak® plastic wastes for PHA production.