Mild Hydrolytic Depolymerisation of Poly(lactic acid) Waste to Added-Value Chemicals Promoted by Alkali and Alkaline-Earth Metal carbonates

The rapid growth of poly(lactic acid) (PLA) usage in packaging, food and additive manufacturing applications has led to a rising accumulation of PLA-containing waste streams. Efficient chemical recycling strategies capable of returning PLA to its monomer, lactic acid (LA), are crucial to closing the material loop and enabling a circular PLA economy. However, existing hydrolytic depolymerisation methods are still underdeveloped, typically requiring strongly basic conditions or soluble zinc catalysts that hampers sustainability and cost-effectiveness of the process. Herein, we demonstrate that readily available potassium carbonate and calcium carbonate function as mild, homogeneous and heterogeneous promoters, respectively, for the selective hydrolysis reaction of PLA in subcritical water. 100% LA yield and full PLA conversion were achieved at 130 °C reaction temperature, over a reaction time of 24 h for K2CO3 and 48 h for CaCO3, with the cogeneration of reusable by-products (KCl or calcium lactate). Reaction time and temperature, carbonate loading, and promoter / PLA ratios were screened to elucidate the optimal operational window for depolymerisation. The strategy was successfully extended to post-consumer and industrial PLA wastes, including disposable beverage cups and 3D-printing residues, underscoring its practical relevance. This carbonate-mediated approach offers a sustainable route to hydrolytic depolymerisation, contributing to more circular polymer management.