Unique fluorophilic pores engineering within porous aromatic frameworks for trace perfluorooctanoic acid removal

ABSTRACT Perfluorooctanoic acid (PFOA), a representative of per/polyfluorinated alkyl substances, has become a persistent water pollutant of widespread concern due to its biological toxicity and refractory property. In this work, we design and synthesize two porous aromatic frameworks (PAF) of PAF-CF3 and PAF-C2F5 using fluorine-containing alkyl based monomers in tetrahedral geometry. Both PAFs exhibit nanosized pores (∼1.0 nm) of high surface areas (over 800 m2 g−1) and good fluorophilicity. Remarkable adsorption capacity (˃740 mg g−1) and superior efficiency (˃24 g mg−1 h−1) are achieved toward the removal of PFOA with 1 μg L−1 concentration owing to unique C–F···F–C interactions. In particular, PAF-CF3 and PAF-C2F5 are able to reduce the PFOA concentration in water to 37.9 ng L−1 and 43.3 ng L−1, below EPA regulations (70 ng L−1). The reusability and high efficiency give both PAFs a great potential for sewage treatment.

PFOA Contents Measurements.PFOA contents of the water samples were determined by a high-resolution liquid chromatography coupled with a quadrupole/orbitrap mass spectrometer (Q EXACTIVE FOCUS, Thermo Scientific).The mobile phase was 100% acetonitrile with a flow rate of 0.2 mL min -1 .The setting parameters for mass spectrometry with electrospray ionization (ESI) operated in negative ion mode and selected ion monitoring mode were listed in the following: sheath gas flow rate 30, aus gas flow rate 15, sweep gas flow rate 0, spray voltage 4.2 keV, capillary temperature of 320 C and aus gas heater temperature of 300 C.All subsequent adsorption experiments were repeated in triplicate for minimizing experimental errors.
Calculation Method.The molecular interaction energy was calculated using the program Gaussian16 [1].The geometry optimizations were performed through the combination of B3LYP functional [2] and D3 version of Grimme's dispersion correction (GD3) [3].The vibrational frequencies of the optimized configurations were analyzed to validate that the configurations correspond to a local minimum.Two basis sets, 6-31G(d) and 6-31++G(d,p), were used in the geometry optimization and single-point energy calculation, respectively.The interactions between adsorbent and adsorbate were directly obtained from the counterpoise correction calculation.

Synthesis of 1-trifluoromethyl-3-tritylbenzene
1-Bromo-3-tritylbenzene (800 mg), CF 3 COONa (1088 mg) and CuI (762 mg) were suspended in NMP (30 mL).The mixture was heated at 160 C under nitrogen atmosphere for 24 h.After cooled to room temperature, the reaction mixture was diluted with ethyl acetate and washed with H 2 O, and then the solvent of the organic layer was removed under reduced pressure.Pure product was separated and purified by column chromatography using hexane as an eluent.Yield: 96.0%.

Synthesis of (4-bromo-3-trifluoromethylphenyl)tris(4-bromophenyl)methane
Bromine (4.0 mL) was dripped slowly to 1-trifluoromethyl-3-tritylbenzene (2.25 g) while stirring at room temperature.The reaction solution was stirred overnight and then poured into EtOH which was precooled at -78 C.The crude product was obtained by filtration and then washed with saturated aqueous NaHSO 3 solution.Pure product was obtained by column chromatography purification using hexane as an eluent.Yield: 87.1%.
Langmuir adsorption model: q e (mg g -1 ) is the amount of PFOA adsorbed by adsorbent at each concentration in equilibrated state.
q max,e (mg g -1 ) is the maximum adsorption capacity of adsorbent at equilibrium.K L (L mmol -1 ) is the Langmuir equilibrium constant.
C e (mmol L -1 ) is each concentration of PFOA in equilibrated state.
Freundlich adsorption model: q e (mg g -1 ) is the amount of PFOA adsorbed by adsorbent at each concentration in equilibrated state.
C e (mg L -1 ) is each concentration of PFOA in equilibrated state.
n is an indicator of adsorption strength.The PFOA removal degree (R, %) was determined by the following equation:

Adsorption Kinetics
) and C t (μg L -1 ) are the initial and residual concentrations of PFOA solution.
The amount of PFOA adsorbed at the adsorbent was determined using the following equation: q t (mg g -1 ) is the amount of PFOA adsorbed on per gram of adsorbent at time t (h).
C 0 (μg L -1 ) is the initial concentration of PFOA solution.
C t (μg L -1 ) is the residual concentration of PFOA solution at time t (h).
C A (mg L -1 ) is the concentration of adsorbent.
Data were fitted by Ho and McKay's pseudo-second-order adsorption model and intraparticle diffusion model for analysis.
Ho and McKay's pseudo-second-order adsorption model: q e (mg g -1 ) is the amount of PFOA adsorbed by adsorbent at equilibrium.
Intraparticle diffusion model: ) is the intraparticle diffusion rate constant.
C (mg g -1 ) is an indicator of the boundary layer effect.

Equilibrium Adsorption at Low Concentration
The batch adsorption experiments of PFOA (1 μg L -1 ) by GAC, PAC, PAF-1, PAF-CF 3 and PAF-C 2 F 5 (10 mg L -1 ) were performed by consulting the same procedures as above. 1 mL of the sample was collected at 9 h and filtered with 0.22 μm PES inorganic syringe filter.The control experiments were carried out under the same condition without addition of adsorbents, giving the losses of PFOA as references.And the blank sample was also taken at 9 h.

Regeneration Studies
Adsorption Experiments: 50 mg of PAF-CF 3 were added to a solution of PFOA (100 mL, 200 μg L -1 ) and kept stirring for 24 h. 1 mL of the suspension was taken at 24 h and then filtered with a 0.22 μm PES inorganic syringe filter, and the concentration of resulting solution was determined by LC-MS.PFOA-loaded PAF was recycled by filtration through a filter membrane.

Ⅳ. Supporting Figures and Tables
10 g (4-bromo-3-trifluoromethylphenyl)tris(4bromo-3-pentafluoroethylphenyl)tris(4bromophenyl)methane which was separately dissolved in 60 mL dehydrated DMF.The mixture was stirred at this temperature for 3 d.After cooling to room temperature, the reaction system was added by conc.HCl and the resultant precipitate was filtered off and washed with H 2 O, CHCl 3 and THF by Soxhlet extraction in turn.Pale yellow solid product of PAF-CF 3 or PAF-C 2 F 5 was obtained after drying at 80 C for 12 h in vacuum.Ⅲ.Batch Adsorption Studies of PFOA and Regeneration1.Adsorption IsothermsPAFs (4 mg) were added into the PFOA solutions with concentrations of 10, 20, 30, 50, 100, 200, 400, 600, 800 and 1000 mg L -1 respectively and stirred for 48 h.The suspensions were filtered with 0.22 μm PES inorganic syringe filters and the concentrations of filtrates were analyzed by LC-MS.Taking account of the losses of PFOA, control experiments were performed under the same condition with no adsorbent addition.
Concentration (200 μg L -1 ): Prior to the adsorption kinetics experiment, 50 mg of PAFs were rehydrated by fully dispersing in 95 mL of deionized H 2 O through vigorous sonication and agitation.5 mL of PFOA solution (4 mg L -1 ) was added to form an initial PFOA solution of 200 μg L -1 . 1 mL of samples were taken at predetermined time (1, 2, 5, 30, 60, 120, 240, 360 and 480 min) and filtered with 0.22 μm PES inorganic syringe filters.Control experiments were carried out under the same condition without adsorbent, giving the losses of PFOA as references.All the actual concentrations of initial PFOA solution and samples were measured by LC-MS.Low Concentration (1 μg L -1 ): 1 mg of PAF was added to 100 mL of deionized H 2 O followed by vigorous sonication and agitation for rehydration before the adsorption kinetics experiment. 1 mL of PFOA solution (0.1 mg L -1 ) was added to form an initial PFOA solution of 1 μg L -1 .Next steps were the same as that for the adsorption of PFOA solution with high concentration (200 μg L -1 ), except the sampling time of 1, 5, 30, 60, 180, 300, 420 and 540 min.
Figure S2.Liquid 1 H, 19 F NMR spectra (a, b) and mass spectrum (c) of Monomer 2 in specific

Figure S11 .
Figure S11.(a) The variations of PFOA adsorbed amounts at equilibrium in function of initial

Table S1 .
Element contents of carbon and fluorine in PAF-CF 3 and PAF-C 2 F 5 determined by EDX.

Table S2 .
Langmuir and Freundlich parameters for the adsorption of PFOA on different adsorbents.

Table S3 .
Comparison of adsorption kinetics parameters by different adsorbents reported in the literature.