Conversion of bioplastics into poly(3-hydroxybutyrate) via thermochemical depolymerisation and fermentation: A model for post-consumer waste recycling

This study investigates the thermochemical depolymerisation of commercial bioplastics - including pure poly(3-hydroxybutyrate) powder (PHB_P), PHB-based compounded pellets (PHB_CP) and polylactic acid (PLA) - followed by bioconversion of the resulting monomers into PHB by Paraburkholderia sacchari and Cupriavidus necator. Hydrothermal hydrolysates of PHB_P and PHB_CP at 200 °C and 5-6 h treatment resulted in 72.9-74.0 % 3-hydroxybutyric acid (3HB) and 21.9-27.1 % crotonic acid (CA) content, whereas PLA fully degraded into lactic acid at 140 °C. Alkaline treatment of PHB_P and PHB_CP resulted in higher CA:3HB ratio (0.43-0.550) than hydrothermal treatment (0.20-0.39). Shake-flask fermentations using PHB_P (2.2 g_PHB/L by P. sacchari) or PLA (3.5 g_PHB/L by C. necator) hydrothermal hydrolysates showed higher polymer production than PHB_CP hydrothermal and PHB_P alkaline hydrolysates. Hydrothermal co-hydrolysates of PLA and PHB_P reduced polymer production (ca. 1.4 g/L) by both strains. Benchmark shake flask fermentations using succinic acid and 1,4-butanediol - representative monomers of poly(butylene succinate) - showed that P. sacchari efficiently converted succinic acid to PHB (2.1 g/L), whereas 1,4-butanediol led to low bacterial growth and polymer formation. Bioreactor fed-batch fermentations with P. sacchari showed that a pH-stat feeding strategy with a concentrated hydrothermal PHB_P hydrolysate led to 8.2 g_PHB/L (54.9 % w/w intracellular content), 0.29 g/(L·h) productivity, 0.54 g/g yield and 356 kDa weight average molecular weight. This work highlights the importance of depolymerization strategy on hydrolysate composition and microbial conversion efficiency, identifies 3HB and LA as key recyclable monomers for PHB biosynthesis, and presents a framework for thermochemical recycling of post-consumer bioplastic wastes.