A rapid, robust, and reliable method to distinguish plastics and compostable bioplastics residues in compost

Compostable bioplastics can be assimilated to biowaste and represent a valid component of the organic fraction of the municipal solid waste, whereas conventional plastics are considered undesirable contaminants, especially in composting plants. In this scenario, the lack of analytical methods able to differentiate plastics from compostable bioplastics is becoming a major challenge. First, distinguishing plastics and bioplastics residues in compost is important to understand whether the composting process has correctly completed the degradation of the compostable bioplastic introduced. Furthermore, the difficulty of distinguishing compostable bioplastics from plastics hinders the accurate assessment of the organic recycling rate of compostable bioplastics. In this context, this study aimed to develop a rapid, robust, and reliable analytical method to differentiate plastic and compostable bioplastic residues in compost, based on their differential solubility in an alkaline medium. Optimal conditions for selective compostable bioplastics dissolution were first established. The method was then tested and validated on five compost samples from composting plants treating the organic fraction of municipal waste. Selective dissolution under thermal alkaline conditions (80 °C, 5% w/v NaOH, 2 h) effectively dissolved compostable bioplastics (i.e. starch-based and polylactic acid-based compostable bioplastics) without quantitatively and qualitatively affecting plastics. Following the isolation of impurities via hydrogen peroxide bleaching, the method successfully distinguished plastics from compostable bioplastics residues, with results fully consistent with FT-IR spectroscopy. Recovery tests confirmed the method’s robustness across various concentrations of plastics and compostable bioplastics (ranging from 0 to 1% w/w), with a recovery efficiency of 98%. Apart from serve for composting performance and compost quality assessment, this rapid and reliable approach may also represent the foundation for updating official methods referenced in national and European fertilizer regulations.