The zeta potential became positive (see Figure S3 in SI) when a cation was added, indicating that cations were adsorbed onto the PAF-45 surface. We have previously found that bridging can occur between Mg2+ ions and the sulfonate functional groups in PFOS molecules and we have quantified the calcium-bridging mechanism using Density functional theory calculations23. The presence of Fe3+ was found to decrease the UPLC-MS/MS response to PFOS, which indicates that PFOS may have formed a complex with the Fe3+ 48. The UPLC-MS/MS total ion current of a sample with a known PFOS concentration was significantly lower when one of the test cations was present than when the test cations were absent and new peaks were formed. The UPLC-MS/MS results shown in Fig. 5 show that PFOS could be adsorbed while coordinated with Na+ (see Fig. 5b), Mg2+ (see Fig. 5c), or Fe3+ (see Fig. 5d). This indicates that the cations may have formed complexes with the PFOS, decreasing the concentration of non-complexed PFOS (measured by UPLC-MS/MS). The geometries of the structures formed when PFOS interacts with the different cations, from Density functional theory calculations, are shown in Fig. 6 and the thermodynamic parameters and the dipole moment parameters are listed in Table 2.
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