## 3 Results and Discussion  
### Determination of Point of Zero Charge  
Point of zero charge (pH\({}_{\text{PZC}}\)) indicates the pH value at which the surface of the material is neutrally charged. The point of zero charge is an important parameter for explaining the adsorption behavior of the sorbent, in this case, the MPs. In order to assay the possible influence of the constituents of the water matrix on the pH\({}_{\text{PZC}}\), the experiments were conducted in both distilled water and a synthetic water matrix. The obtained results for the determination of the pH\({}_{\text{PZC}}\) for selected MPs in distilled water and the synthetic water matrix are shown in Figure S3. The point of zero charge obtained for the selected MPs in distilled water ranged from pH = 3.83 to pH = 8.17 in the following order: PP < PLA < PET < PEp < PEg < PE_PCPs_2 < PE_PCPs_1. Furthermore, the point of zero charge obtained for the MPs in the synthetic water matrix was in the range from pH = 4.04 to pH = 8.71, with the sequence PP < PLA < PET < PEg < PE_PCPs_2 < PE_PCPs_1. Based on the obtained results shown in Figure S3, the constituents of the water matrix had no effect on pH\({}_{\text{PZC}}\) for the selected MPs, except in the case of PEp, where the pH\({}_{\text{PZC}}\) in the distilled water was pH\({}_{\text{PZC}}\) = 4.70, while in the synthetic matrix it was pH\({}_{\text{PZC}}\) = 8.21. [PERSON] et al. (2018) also determined the point of zero charge of polyethylene, where its obtained value in distilled water was pH\({}_{\text{PZC}}\) = 4.30. The results obtained by [PERSON] et al. (2018) also indicated that the constituents of the water matrix significantly affect the charge of MPs. They also implied that it is important to use same water matrix during experiments in order to prevent the uneven diffusion in solution, and ensure the accuracy of the determination [42]. Additionally, research by [PERSON] and [PERSON] (2012) indicated that the water matrix has a significant effect on the point of zero charge of the material [43]. On the other hand, based on the obtained results of the point of zero charge for the powdered polyethylene isolated from the cosmetic products (PE_PCPs_1 and PE_PCPs_2), it could be observed that the change in the water matrix had a weaker effect on these materials in comparison with PEp. Furthermore, the pH values at which PE_PCPs_1 and PE_PCPs_2 were neutrally charged in distilled water were lower at pH\({}_{\text{PZC}}\) = 8.11 and pH\({}_{\text{PZC}}\) = 6.14, respectively, compared to the values observed with the synthetic water matrix, which were pH\({}_{\text{PZC}}\) = 8.71 and pH\({}_{\text{PZC}}\) = 7.29, respectively. This behavior of isolated MPs in relation to pure PEp can be explained by the change in the structure of polyethylene during the production process of cosmetic products, which possibly stabilizes the surface of the polymer and significantly reduces the influence of the water matrix on the point of zero charge of PE_PCPs_1 and PE_PCPs_2 [44]. Such an effect occurring in the production of personal care products may also be the reason for a significant difference in surface charge between isolated microplastic particles (pH\({}_{\text{PZC}}\) = 8.11 and 6.14 for PE_PCPs_1 and PE_PCPs_2, respectively) and Pep (pH\({}_{\text{PZC}}\) = 4.70) in distilled water. The pH\({}_{\text{PZC}}\) values obtained for Peg, PET and PP in distilled water were 5.06, 4.66, and 3.83, respectively, which is in accordance with the results of other authors [22; 45]. By comparing the obtained results for the point of zero charge of the granulated MPs in distilled water and synthetic matrix to the results for powdered MPs, it can be noted that in the case of granular MPs (PEg, PET, PP), the influence of the water matrix on pH\({}_{\text{PZC}}\) was significantly smaller. This behavior can be attributed to the greater granulation and smooth surface of MPs, which means that the presence of salt in the synthetic matrix did not affect the pH\({}_{\text{PZC}}\)[43; 46; 47; 48; 49].