A Porous Aromatic Framework Constructed from Benzene Rings Has a High Adsorption Capacity for Perfluorooctane Sulfonate

A low-cost and easily constructed porous aromatic framework (PAF-45) was successfully prepared using the Scholl reaction. PAF-45 was, for the first time, used to remove perfluorooctane sulfonate (PFOS) from aqueous solution. Systematic experiments were performed to determine the adsorption capacity of PAF-45 for PFOS and to characterize the kinetics of the adsorption process. The adsorption of PFOS onto PAF-45 reached equilibrium in 30 min, and the adsorption capacity of PAF-45 for PFOS was excellent (5847 mg g−1 at pH 3). The amount of PFOS adsorbed by PAF-45 increased significantly as the cation (Na+, Mg2+, or Fe3+) concentration increased, which probably occurred because the cations enhanced the interactions between the negatively charged PFOS molecules and the positively charged PAF-45 surface. The cations Na+, Mg2+, and Fe3+ were found to form complexes with PFOS anions in solution. Density functional theory was used to identify the interactions between PFOS and Na+, Mg2+, and Fe3+. We expect that materials of the same type as PAF-45 could be useful adsorbents for removing organic pollutants from industrial wastewater and contaminated surface water.

The adsorption capacity of PAF-45 for PFOS and the adsorption kinetics. Many adsorbents have been used to remove PFOS from water, and the adsorption capacities that have been found for a range of adsorbents are summarized in Table 1. In the study presented in this paper, we demonstrated for the first time that PAF-45 has a remarkable capacity for adsorbing PFOS from water. As shown in Fig. 2, the adsorption of PFOS onto PAF-45 almost reached equilibrium within 30 min. The pseudo-second-order rate equation constant, k 2 , for the adsorption of PFOS onto PAF-45, which was determined using a nonlinear model, is shown in Table 1. The pseudo-second-order model gave an adsorption capacity, q e , of 5096.17 mg g −1 , an initial adsorption rate, ν 0 , of 2842.75 mg g −1 min −1 , and a final pH of 3.08, and the coefficient of determination, r 2 , was 0.995. The data were clearly described better by the Freundlich model (which gave a K value of 2000 mg (1−1/n) L 1/n g −1 , an n −1 value of 0.2, and an r 2 value of 0.953) than by the Langmuir model (which gave a q m value of 5847.39 mg g −1 , a b value of 0.32 L mg −1 , and an r 2 value of 0.942) over all the PFOS concentration range that was tested and especially at high concentrations. The adsorption capacity of PAF-45 for PFOS is, to the best of our knowledge, much higher than the adsorption capacity of any adsorbent that had previously been tested. We also tested the adsorption capacity of PAF-45 for PFOS in neutral condition, the results indicated that the high adsorption capacity of PAF-45 was obtained at pH 7 (see Figure S6 in SI). The high adsorption capacity of PAF-45 can be ascribed to (i) its hydrophobic aromatic rings (see Figure S2 in SI), which match the hydrophobic C-F chains in PFOS and (ii) its positive charge at pH 3 (see Figure S3 in SI). These characteristics mean that PAF-45 can interact with the SO 3 − group in PFOS through electrostatic attraction 24,25 . It is possible for hemi-micelles and even micelles to form within the pores of PAF-45 once many PFOS molecules have been adsorbed because the PFOS concentration is likely to be  Reusability of PAF-45. PAF-45 was used repeatedly to adsorb PFOS from a solution; it was regenerated between adsorption tests by treating it with a mixture of NaOH and acetone and then, rinsing it with deionized water. The rates at which PFOS was removed from solution by PAF-45 in six consecutive adsorption-regeneration cycles are shown in Fig. 3. We found that PAF-45 could be used in at least six cycles without losing any of its capacity to adsorb PFOS, which indicates that the adsorption properties of PAF-45 were completely refreshed by the regeneration process.

Effects of pH and the ionic strength of the solution on PFOS adsorption by PAF-45.
The adsorption of PFOS onto PAF-45 was strongly affected by both the pH and the ionic strength of the solution [26][27][28][29][30][31][32][33][34][35][36][37] . As shown in Fig. 4a, the adsorption capacity of the PAF-45 increased as the pH decreased, changing from 4814 mg g −1 at pH 9 to 5721 mg g −1 at pH 3. The PAF-45 was found to be positively charged at acidic pH values, and its point of zero charge was pH 8.8 ( Figure S3 in SI). The increase in the capacity of PAF-45 to adsorb PFOS as the pH decreased was probably related to the increase in the strength of the electrostatic attraction as the pH decreased 10,[38][39][40][41][42] . Non-electrostatic interactions (mainly hydrophobic interactions) tend to contribute more than electrostatic interactions to the amount of a species adsorbed, but adsorption through non-electrostatic interactions tends to be little affected by changes in the pH of a solution.
The ionic strength of the solution and the type of cation present within it had critical effects on the adsorption of PFOS onto PAF-45. The effects of the presence of Na + , Mg 2+ , or Fe 3+ at different concentrations on the adsorption of PFOS by PAF-45 were determined at pH 3, and the results are shown in Fig. 4b. More PFOS was adsorbed and less time was taken reach equilibrium when any of the cations tested were present than when  none of the cations tested were present. Surprisingly, the adsorption capacity of PAF-45 for PFOS was higher (reaching 6439 mg g −1 ) in the presence of Fe 3+ than in the presence of Na + or Mg 2+ . Such a high adsorption capacity for PFOS had not previously been reported for any other adsorbent. Ultrahigh performance liquid chromatography-tandem quadrupole mass spectrometry (UPLC-MS/MS) chromatograms of PAF-45 with PFOS adsorbed to it in the presence of Na + , Mg 2+ , and Fe 3+ are shown in Fig. 5, and the Fourier transform infrared spectra and X-ray diffraction patterns of PAF-45 under the same conditions are shown Figures S4 and S5 in SI, respectively. As we can see from Figures S7 and S8 in SI, between PFOS and cations (Na + , Mg 2+ , or Fe 3+ ), PFOS is the priority to interact PAF-45 whether stimulated PFOS or real PFOS contaminants 43 .
Several factors contribute significantly to the effects the presence of the cations tested have on the adsorption capacity of PAF-45. First, there are electrostatic interactions between the cations, the PFOS on the surface, and the  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 Mg 2+ ions and the sulfonate functional groups in PFOS molecules, and we have quantified the calcium-bridging mechanism using Density functional theory calculations 23 . The presence of Fe 3+ was found to decrease the UPLC-MS/MS response to PFOS, which indicates that PFOS may have formed a complex with the Fe 3+ 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), Mg 2+ (see Fig. 5c), or Fe 3+ (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.
Thermodynamics of the adsorption processes. The structure of H 2 O is shown in Fig. 6a, and   , and (c) Fe(H 2 O) 6 3+ and (d) the HOMO orbitals of perfluorooctane sulfonate (PFOS), (e) the geometry of an Na + ion bound to one PFOS molecule, (f) the geometry of a K + ion bound to one PFOS molecule, (g) the geometry of an Mg 2+ ion bound to one PFOS molecule, (h) the geometry of an Mg 2+ ion bound to two PFOS molecules, (i) the geometry of an Fe 3+ ion bound to one PFOS molecule, and (k) the geometry of an Fe 3+ ion bound to two PFOS molecules. The isosurface plots of the HOMO orbitals were generated using an isodensity value of 0.02 a.u. are shown in Fig. 6b,c, respectively. Na + and K + are strongly ionized in water; therefore, they are present as free ions. A frontier orbital analysis suggested that the HOMO of the anionic PFOS surfactant is primarily localized on the sulfonate head group (Fig. 6d), which indicates that the -SO 3 group is the active site that binds to the positively charged Na + , Mg 2+ , and Fe 3+ ions. Mg 2+ and Fe 3+ ions may bind to one or two PFOS anions to create bridged structures. The reactions between PFOS and K + , Na + , Mg 2+ , and Fe 3+ are shown, with the changes in their Gibbs free energies (Δ G θ ), in reactions 1-6. The Gibbs free energy changes for all the reactions are negative, which shows that all these reactions occur spontaneously. The commercially available form of PFOS is PFOS − K + . The Gibbs free energy change is lower for reaction 1 than for reaction 2, which indicates that the form CF 3 (CF 2 ) 7 SO 3 Na is energetically more favorable than the form CF 3 (CF 2 ) 7 SO 3 K (Fig. 6e). Density functional theory calculations showed that the configurations CF 3 (CF 2 ) 7 SO 3 Mg(H 2 O) 4 (Fig. 6g) and CF 3 (CF 2 ) 7 SO 3 MgO 3 S(CF 2 ) 7 CF 3 (H 2 O) 2 (Fig. 6h) form when Mg 2+ binds with one or two PFOS molecules, respectively. We found that PFOS bound to Mg 2+ is more polar than unbound PFOS (μ = 22.9 for PFOS, but μ = 31.2 for CF 3 (CF 2 ) 7 SO 3 Mg(H 2 O) 4 ), which means that interactions between Mg 2+ -bound PFOS and the PAF-45 surface are more favorable than interactions between unbound PFOS and the PAF-45 surface. This is in agreement with the higher adsorption efficiency that was found when Mg 2+ was present. The Gibbs free energy changes for reactions 3 and 4 were found to be − 656.80 and − 1001.55 kJ mol −1 , respectively (also see Table 2), which suggests that the formation of CF 3 (CF 2 ) 7 SO 3 MgO 3 S(CF 2 ) 7 CF 3 (H 2 O) 2 is favorable. The configurations of Fe 3+ bound to one and two PFOS molecules are shown in Fig. 6I,k, respectively. Three PFOS molecules cannot bind to Fe 3+ due to steric hindrance. The Gibbs free energy changes for reactions 5 and 6 were found to be − 1219.26 and − 2078.66 kJ mol −1 , respectively, which suggests that an Fe 3+ ion easily   forms a bridge between two PFOS anions by generating an attractive force that is effective at the length of an Fe-O(S) bond. This attraction is sufficiently strong to cause the PFOS molecules to be effectively linked together through a bridge-like PFOS-Fe-PFOS configuration. Our calculations show that Fe 3+ is more able than Na + or Mg 2+ to form a bridge between PFOS molecules. This is consistent with our finding that the PFOS adsorption equilibrium was reached more quickly in the presence of Fe 3+ than in the presence of Na + or Mg 2+ .

Conclusions
The PAF-45 produced was found to be very stable in moist aerobic conditions and to contain uniform pores. We found that PFOS could diffuse into and adsorb onto PAF-45. The adsorption of PFOS onto PAF-45 reached equilibrium within 30 min, and the adsorption capacity of PAF-45 for PFOS at pH 3.0 was 5847 mg g −1 . We attributed the high adsorption capacity of PAF-45 for PFOS to PAF-45's positively charged surface and extremely hydrophobicity. The pH had a significant effect on the amount of PFOS adsorbed, and the composition of the solution affected the amount of PFOS adsorbed and the kinetics of the adsorption process. We found that certain cations can form bridges between PFOS molecules. Our results show that PAFs have a remarkable amount of potential for removing pollutants from contaminated water.

Methods
Chemicals and general information. Potassium PFOS (99% pure) was purchased from AccuStandard.
HPLC-grade methanol and acetonitrile were purchased from Fisher Scientific. Sodium chloride, magnesium chloride, iron chloride, sodium hydroxide, and hydrochloric acid were purchased from Sinopharm Chemical Reagent Company. Biphenyl (99% pure) was provided by Aladdin. Other materials were purchased from commercial suppliers and used without further purification unless otherwise noted. The ultrapure water used was produced using a Milli-Q integral water purification system. The morphology of the PAF-45 was examined using a Hitachi S-3000N field emission scanning electron microscope and a JEOL JEM-2100F transmission electron microscope. The crystallinity and regularity of the PAF-45 were determined by X-ray diffractometry using an AXS instrument. The surface area of the PAF-45 was determined using the Brunauer, Emmett, and Teller method. The functional groups were identified using Fourier transform infrared spectroscopy. The zeta potential was determined at different pH values. The pH was measured using a pH meter. The contact angle of the PAF-45 was determined using a DataPhysics DCAT21 dynamic contact angle measuring instrument.
Anhydrous aluminum chloride was added to a round-bottomed flask, and then, the flask was attached to a vacuum to remove the gases that were present. The flask was then filled with N 2 , and the gas was evacuated three times. Dry CHCl 3 (60 mL) was then injected using a syringe, and the mixture was kept at 60 °C for 3 h. Biphenyl in CHCl 3 (40 mL) was added to the flask, and the mixture was kept at 60 °C for 24 h while being stirred. The contents were cooled to room temperature and filtered to separate the crude product, which was washed with hydrochloric acid, then methanol, and then acetone to remove unreacted monomers and catalyst residues. The product was further purified using Soxhlet extraction with ethanol, tetrahydrofuran, and trichloromethane, successively, for 48 h each.

Sorption experiments.
The sorption experiments were conducted in polypropylene centrifuge tubes (50 mL), each of which contained PAF-45 (0.5 ± 0.02 mg) and a PFOS solution (40 mL) of the required concentration. The tubes were shaken at 180 rpm and kept at 25 °C for 48 h. The pH of the solution in each tube was adjusted using 0.1 M HCl and 0.1 M NaOH, and the ionic strength was controlled by adding 1 M NaCl (aq) , MgCl 2(aq) , or FeCl 3(aq) . The initial PFOS concentration used in the adsorption kinetics experiments was 100 mg L −1 . The adsorption isotherm experiments were conducted using PFOS concentrations of 50-200 mg L −1 , and the solutions were kept at pH 3 by making regular adjustments during the sorption experiments. All the tests were performed in triplicate.