To further purify the microplastic sample and reduce debris before microscopic analysis, wet peroxidation digestion using Fenton's reagent was carried out (Masura et al. 2015). The dried filtered material, recovered after density separation was transferred to a large beaker (1-2 L depending on the volume of the sample, and to prevent overflow since the reaction is volatile) and an initial 20 mL of 30% H2O2, followed by 20 mL of 0.05 mol L -1 acidic Iron Fe(II) solution (prepared by adding 7.5 g of FeSO4.⋅7H2O to 500 mL of ultrapure water (Advantec® RFU685DA) and 3 mL of concentrated sulfuric acid). This mixture was allowed to sit uncovered for at least 5 minutes for the initial reaction to begin. After the initial reaction, ultrapure water was used to wash down the sides of the beaker. A magnetic stir bar was added to the beaker and then covered with a watch glass. The mixture was then heated to 75℃ (using a hot plate with a stirrer) until gas bubbles were observed. To prevent overflow ultrapure water was added periodically to wash down the sides of the beaker. The temperature was kept constant at 75℃ and after every 30 minutes (or until no visible reaction) another 20 mL of 30% H2O2 was added to the beaker. This was repeated until no organic matter was visible within a four-hour limit. After the organic matter was dissolved, 6 g of NaCl (FUJIFILM Wako Pure Chemical Co., Ltd.) per 20 mL of the digested mixture was added to the beaker and stirred until completely dissolved. After cooling, the mixture (including undissolved matter) was transferred to a large funnel (~3 L) with a stopcock attached at the end (using rubber hose and hose clip). This mixture was then allowed to settle for approximately 24-72 hours depending on the amount of solids, which was drained periodically until no large solids were visible. The remaining solution containing microplastics was vacuum filtered onto a 90 mm Whatman paper filter. The filter was then allowed to dry overnight in a loosely covered glass petri dish in preparation for microscopic analysis.