Integrated chemo-biotechnological process for upcycling polyesters into new PHB

This study presents a two-step upcycling approach for biodegradable polyesters — poly(lactic acid) (PLA), poly(caprolactone) (PCL), and polyhydroxybutyrate (PHB) — via chemical depolymerization and microbial biosynthesis of PHB. Alkaline hydrolysis in methanolic KOH at 50 °C was used to generate potassium salts of monomers and oligomers. Three KOH concentrations (deficient, stoichiometric, and excess) were tested. Depolymerization efficiency reached up to 100% for PCL and 97% for the in-house produced PHB under stoichiometric conditions. FTIR analysis confirmed ester bond cleavage and carboxylate formation in all cases. Among six screened bacterial strains, Cupriavidus necator B4383 was the only one capable of significant PHB production. Fermentations on depolymerized feedstocks demonstrated the highest biomass yields with depolymerized commercial PHB D(PHB-com) under KOH deficiency (1.43 g/L), and the highest PHB content with depolymerized PHB synthesized in our laboratory D(in-house-PHB) under KOH excess (86% CDW). Maximum PHB titers reached 0.93 g/L. Feedstocks from PLA and PCL were less efficient. Upcycled PHB was structurally confirmed via FTIR and showed thermal stability in the 291–296 °C range across all feedstocks, consistent with PHB polymers mentioned in available literature. These results demonstrate that PHB, PLA, and PCL can be chemically degraded and re-converted into PHB using a microbial system, offering a viable strategy for polyester valorization in circular bioprocesses.