Oxidative purification of microplastics in riverine suspended matter samples — Solving the challenge of plant debris removal for microplastic analysis

Riverine suspended matter (river-SPM) contains large amounts of natural particles consisting of cellulose and lignin, posing a challenge for microplastic (MPs) analysis. Additionally, organic matter composition under seasonal and discharge-related dynamics varies for each river. Therefore, this study attempted to identify a universally applicable clean-up procedure to remove matrix particles with high organic matter content, mainly plant debris, from the river-SPM samples. This study tested six digestion procedures adapted from existing (ligno)cellulosic digestion/oxidation methods with a river-SPM sample followed by density separation using sodium polytungstate. From these, NaOCl treatment (CL) showed the highest efficiency of organic matter removal, eliminating 96-100 % of the matrix weight. Exposure of tested MPs (in size range of 100-500 μm) in the CL protocol showed no adverse effect on polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyethylene terephthalate (PET). Similarly, no detrimental matrix effects were found on 100 μm spherical PS standard particles spiked in the river-SPM. This procedure achieved high recovery rates of tested plastics (92-100 %). In terms of method applicability, the procedure was successfully applied to samples from different seasons containing various matrix concentrations and compositions. Although samples with high amounts of plant debris needed to undergo this procedure twice, only minor alteration of the particle surface and IR spectrum of PS presented and no adverse effect on PP. To further tackle the high and varied concentration of plant-derived matrix in river-SPM samples, a novel sequential oxidation protocol (2DOCL) combining cellulose dissolution, Fenton's oxidation, and NaOCl oxidation was developed, resulting in a more (time) effective and predictable process, demonstrating no severely destructive effect on tested plastics. The sequential digestion protocol can be optimized for certain matrices as applying all steps will not be necessary.