Comprehensive assessment of various digestion protocols for extraction microplastics from organic-rich environmental matrices

Digestion plays a vital role in extracting microplastics (MPs) from organic-rich environmental matrices (OEMs). The diversity of MPs and OEMs complicates the application of a unified digestion reagent to various environmental media. In this study, we comprehensively evaluated the efficiency of removing organic matter (OM) from four typical OEMs (animal tissue, plants, soils, and sludges) using 27 digestion protocols through a preliminary screening experiment followed by optimization of digestion conditions (such as concentration, time, and temperature). Moreover, the impact of the digestion protocols on 26 types of MPs was examined to ensure minimal alteration in their morphology, weight, contact angle, surface functional groups, and backbone structures. The optimized digestion protocol for each OEM is as follows: 2 % KOH at 40 °C for animal tissue; Fenton's reagent at 60 °C for plants; 0.2 M K₂S₂O₈ at 60 °C for soil; and a three-step protocol for sewage sludges (0.2 M K₂S₂O₈ at 65 °C, 15 % H₂O₂ at 40 °C, and 2 % KOH at 40 °C). These protocols achieve the highest removal rate of OM while minimizing the impact on the 26 MPs. Finally, the methods were validated by adding 75-95 μm standard polyethylene (PE) spherical particles into the tested natural OEMs. A custom-made counting device integrated with a computer-assisted counting method was employed to ensure complete retrieval of these added particles. The proposed digestion protocols resulted in recovery rates of 98.9 ± 1.2 % (animal tissue), 85.5 ± 0.9 % (plants), 74.9 ± 3.5 % (soil), and 82.7 ± 2.3 % (sludge). Our results offer an essential reference for the establishment of standard MPs extraction methods from complex environmental matrices.