Integrated chemical-biological upcycling of polylactic acid waste into polyhydroxyalkanoates via lactate and volatile fatty acid platforms

Polylactic acid (PLA) waste is an increasing environmental challenge that demands valorization routes aligned with circular bioeconomy principles. This study demonstrates an integrated chemical-biological strategy to upcycle PLA into polyhydroxyalkanoates (PHAs) through two pathways: (i) alkaline depolymerization to lactate-rich hydrolysates, acidogenic fermentation to volatile fatty acids (VFAs), and PHA accumulation, and (ii) direct PHA production from lactate-rich chemical hydrolysates. PLA was depolymerized using NaOH pretreatment (0.5–4 M) at 37 and 55 °C, achieving >90% carbon solubilization with kinetics governed by alkali strength and particle size. Mixed-culture fermentation converted lactate mainly into propionate and acetate, with hydrolysates produced at 2 M NaOH giving the highest VFA titers and activity. Cupriavidus necator assays revealed stronger interaction between substrate type and nutrient regime for PHA accumulation. VFA-rich streams under nutrient-sufficient condition supported the highest biomass and PHA production at 2 M, whereas, lactate-rich hydrolysates under nutrient-deficient (nitrogen-limited) condition yielded maximum PHB content of 84.3% ( w /w) at 0.5 M. Comparatively, the direct lactate-based route showed greater practical potential as it enabled higher polymer accumulation while avoiding the intermediate fermentation step required in the VFA-mediated pathway. Overall, the integration of alkaline depolymerization with microbial conversion highlights potential for developing simplified and scalable bioconversion strategies for plastic waste valorization, while supporting sustainable materials recovery within a circular bioeconomy framework.