We are trying to quantify microplastics in effluent and are following ASTM D8333-20 to remove interfering organics in wastewater having medium to high suspended solids. The method suggests using reference spheres for QA/QC and adding them prior to the wet oxidation step. We added fluorescent polyethylene spheres (250-350 um) which tend to float on the surface. After adding the 30% hydrogen peroxide, placing the sample in the tube rocker for 60 minutes, and centrifuging for 3 minutes, the micropheres (250-300 um) were on the surface or mid-way in the supernatant, making it impossible not to remove some of them in the process of pipetting off the overlying liquid. Should we be adding a surfactant and , if so, how much? Are there suggestions for surrogates to mimic wastewater. We tried microcrystalline cellulose and soy bean meal.
Joan Ikoma
Surfactants are not recommended. They will help wet the polyethylene balls and lower them to the bottom of the container. Try PTFE balls. They are worse oxidized by hydrogen peroxide than polyethylene balls.
density of polyethylene is mostly smaller than water, that is why your spheres are floating, not because of entrapped air.
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