Microplastic extraction method optimisation in solid marine matrices, to preserve integrity and polymer recovery

The development of effective methodologies for extracting microplastics from marine matrices is key for designing appropriate measures to reduce and mitigate their effects. The diversity of protocols developed shows problems and limitations that hinder the comparability and reproducibility of the results obtained in different studies. The aim of the present study is to optimise an efficient method for the extraction and isolation of microplastics from solid marine matrices without causing polymer degradation. Three solutions were evaluated for the digestion of organic matrices (10% KOH, 15% H₂O₂, 35% HNO₃) and three salts were tested for the density separation of microplastics from sediments (NaCl, ZnCl₂, NaI). The potential chemical degradation associated with both steps was monitored using Raman spectroscopy. The results showed that digestion of organic matrices with 10% KOH at 60 °C for 24 h, combined with density separation using a 30% ZnCl₂ solution in water, was optimal and did not compromise polymer integrity. The optimised method was subsequently applied to sediment samples and holothurian digestive tracts collected from a coastal area in Calpe (Alicante, Spain). Holothurians accumulated microplastics, with the highest abundances in the 1-500 μm and 500-1000 μm size ranges. Sediment samples exhibited greater quantities in the 500-1000 μm and 1000-1500 μm fractions. Overall, 95% of the recovered microplastics were identified using a Raman spectral database created during the optimization protocol. The new optimised protocol offers a robust and polymer-preserving approach for the extraction and characterisation of microplastics from solid marine matrices, facilitating more reliable assessments of microplastic pollution in coastal environments.