The Analysis of Polylactic Acid Oligomers and Their Fate in Laboratory and Agricultural Soil

Polylactic acid (PLA) is the most widely consumed biodegradable plastic worldwide. Though PLA products have the advantages of easy degradation and a small carbon footprint, poly(lactic acid) oligomers (OLAs) released from PLA have been found to exhibit toxicity. Accurate quantification of the presence of the OLAs in soil is crucial for understanding their occurrence and environmental fate. In this study, we synthesized sequence-defined OLA standards and deuterated OLAs (d-OLAs) and developed a broad-spectrum extraction method with water-saturated ethyl acetate and 0.1% formic acid, achieving sensitive and precise quantification of the OLAs across various soil environments. The simulated experiments showed that PLA-based products released amounts of OLAs, reaching their peak within 24 h, followed by degradation, with the OLAs remaining detectable in the soil for up to 168 h. Meanwhile, the OLA with a degree of polymerization (DP) of 2 exhibited the highest concentration, while higher-DP OLAs demonstrated greater stability in soil compared to others. In the field samples, OLAs were detected in soil over the long term beneath PLA-based biodegradable mulch films with residual concentrations reaching up to 16.10 ± 0.37 ng/g. This study bridges foundational analytical methods and addresses data gaps for further investigations into the environmental fates of the OLAs and PLA.