## 3 Results and Discussion
### Long-Term Material Exposure  
Given the results of the first part of the study and the fact that other authors already proved PVC, PTFE and PVDF for estrogen sorption, albeit they used much higher concentrations, we chose EPDM material for a more detailed evaluation. EPDM is a non-crystalline thermoplastic with similar polarity to perfluorinated polymers. Its most notable advantages are UV and ozone resistance, elasticity with 600% elongation and tensile strength (see Table S2), making it a suitable material for constructing, for example, AOP units. In this phase of the study, sorption at higher estrogen concentrations (300 and 500 ng/L) in two different matrices (deionized water and tap water) and a prolonged exposure period (1-12 h) was tested. The results of the sorption experiments are graphically illustrated in Figure 3.  
As can be seen in Figure 3a,b, the only significant sorption was observed for EE2 after 12 h at both concentration levels and water matrices. The sorption of E1 and E2 reached up to 20% of the initial concentration, and the sorption of E3 was negligible in all experiments. The presence of ions in solution can also affect the sorption behavior of plastics by affecting the solubility of organic compounds in water. In general, increasing ionic strength reduces the solubility of non-polar and weakly polar organic contaminants in water, known as the salting-out effect [38]. Thus, the higher salt level increases the availability of certain hydrophobic contaminants for adsorption onto plastics. This effect was observed at a higher tested concentration (500 ng/L), whereas in tap water (\(\kappa\) = 417 \(\pm\) 2.0 \(\upmu\)S/cm), the adsorbed amount of EE2 was two times higher than in deionized water.  
Subsequently, after 12 h sorption experiments, desorption into the deionized water was evaluated. Estrogen cumulative concentrations desorbed from the EPDM surface are shown in Figure 3c. Although the sorbed amount of EE2 was at least 20%, the desorbed amount of all estrogens did not exceed 10% of the initial concentration at both concentration levels (300 and 500 ng/L) and both water matrices. EE2 was not leached into the deionized water, most probably due to the hydrophobic interactions. Although the highest log K\({}_{\text{OW}}\) value has estrone, higher EE2 adsorption was found in both matrices on EPDM. The dissociation constants (\(pK_{a}\)) of selected estrogens have values around 10.3 (Table S1), indicating that in both matrices (pH of 5.8 and 7.1 for deionized water and tap water, respectively), they stay uncharged. This way, the charge interaction was minimal and not responsible for significant sorption. Another property that may affect interaction with polymers is proton donor and acceptor characteristics, in particular, the ability to form hydrogen bonds, which has been attributed to play a predominant factor in the transport of estrogens in biological systems [27; 39].  
The EPDM material was evaluated as the most proper, mainly regarding chemical properties. In addition to the fact that this material showed low sorption of estrogens, its mechanical properties are equally important, as these materials can be bent with a smaller radius than PTFE or PVDF, which have similar polarity. The flexibility of EPDM material can significantly save costs and reduce material consumption while eliminating the adverse effects of possible, albeit minimal, undesirable estrogen sorption.