Controlled modification of the hydrophilicity of polyethylene terephthalate micro- and nanoplastic particles and their analysis in bottled drinking water.

We have previously developed methods to artificially age cryogenically milled polyethylene terephthalate (PET) and polyethylene (PE) particles using alkaline and acidic treatments. This approach reliably resulted in large quantities of micro- and nanoplastic particles with significantly increased hydrophilicity. To investigate the relationship between treatment time and hydrophobicity, we studied the hydrophobicity index (Hy) of PET particles treated with 0.25 M KOH for different treatment times in an ultrasonic bath (1.5-6 h). We obtained a specific size fraction of interest (0.2-1.2 µm), by centrifugation of the initial suspension of artificially aged PET (aPET). For this, a stepwise centrifugation protocol was used, and we characterized the specific size fraction using an array of different techniques. The characterization techniques included dark field (DF) microscopy for determining the hydrophobicity index (Hy), particle counting by single particle extinction and scattering (SPES), dynamic light scattering (DLS), Raman microscopy, and scanning electron microscopy (SEM). The results showed clear instability and hydrophobic behavior for shorter treatment times. Stable suspensions were obtained after 6 h of treatment with KOH yielding increased hydrophilicity and a sufficiently high zeta potential. The decrease in hydrophobicity index values over time should make it possible to design future reference materials where the artificially aged micro- and nanoplastic particles become qualitative reference materials (RMs) with an ordinal property. A known amount of the most hydrophilic fraction (6 h) was added to bottled drinking water, and the particle mass and particle number concentration recoveries were assessed. The resulting mass recovery was 68 ± 13% (n = 3). The approach allows the preparation of microplastic and nanoplastic particles with different hydrophobicity indices that can be easily controlled (or tuned) to result in particles with higher or lower hydrophobicity suitable for further studies.