Assessment of microplastic digestion methods in source and treated drinking waters

Analysis of microplastic particles in source and treated drinking waters requires filtration, isolation of microplastics from non-plastic organic and inorganic particles, and subsequent quantification and characterization using spectroscopic techniques. Microplastic isolation has been achieved in previous studies using a range of acids, bases, oxidants, and enzymes, however no study has systematically assessed multiple methods to identify one that is optimal for source and treated drinking waters in terms of reduction of non-plastic particles. In this study, seven oxidation, digestion, and acidification methods which have been applied individually in previous studies were directly evaluated to compare their relative performance for the isolation of microplastics >2 μm in size from non-plastic particles in drinking water. Among all seven methods evaluated, oxidation using Fenton's reagent followed by enzymatic digestion using cellulase and trypsin resulted in the greatest improvement in the amount of clean filter area as well as reduction in particle counts. Subsequent trials were conducted to improve the method by applying acidification at varying timesteps, as well as reducing oxidation and digestion reaction times. These modifications served to minimize the formation of iron precipitates as well as reduce overall sample processing time. The improved method was then evaluated using three different surface waters to confirm its applicability and reproducibility. The modified digestion method may be applied as a standard procedure across a range of source and treated waters, prior to microplastic characterization using spectroscopic techniques.