A Deep Insight into Perfluorooctanoic Acid Photodegradation Using Metal Ion-Exchanged Zeolites

Treating perfluorooctanoic acid (PFOA) in an aqueous environment is problematic due to its low concentration and its high resistance to biological and chemical degradation. To tackle this challenge, combinations of pre-enrichment and photodegradation processes are promising solutions. In this work, we investigated metal ion-exchanged zeolites as adsorbents and photocatalysts for PFOA treatment. Among various transition metal ion-exchanged BEA zeolites, Fe-exchanged BEA (Fe-BEA) zeolites showed significant activity for the photodegradation of PFOA. The isolated iron species in Fe-BEA zeolite are responsible for PFOA photodegradation, whereas other iron species present from excess iron loading in the zeolite will lower its photocatalytic activity. Furthermore, it was proved via size exclusion tests using branched PFOA isomers that the photodegradation of PFOA took place on the internal surface rather than the external surface of Fe-BEA zeolite. Photodegradation of PFOA was also tested to be effective with Fe-exchanged BEA-type zeolites having various SiO2/Al2O3 ratios, but ineffective with FAU-type zeolites. The optimal Fe-BEA zeolite showed a sorption coefficient Kd of 6.0 × 105 L kg–1 at an aqueous phase PFOA concentration of 0.7 μg L–1 and a PFOA half-life of 1.8 h under UV-A irradiation. The presented study offers a deeper understanding of the use of metal ion-exchanged zeolites for photodegradation of PFOA.


Contents
. Selected parameters of transition metal ion-exchanged BEA35 zeolites.Several metal ion solutions with 1 mM concentration were used for ion-exchange into zeolites during the preparation (for details see section 2.  S4.Adsorption and kinetic data on PFOA degradation using iron-exchanged FAU zeolite and ironexchanged BEA zeolites with various SiO2/Al2O3 ratios.Reaction conditions: 1 g L -1 zeolite, C0,PFOA = 48 μM, pH0 = 5, 1 day pre-adsorption before irradiation. .

Catalyst preparation.
In this study, commercially available BEA type and FAU type zeolites were used for preparing metal cationexchanged zeolites.The procedure is based on our previous study and described as follows 1 : (a) 10 g of target zeolite is suspended in 100 mL DI water and treated in an ultrasonic bath for 15 min; (b) after stirring with nitrogen purging for 30 min, a certain amount of target metal salts is added under nitrogen atmosphere; (c) the pH of the zeolite suspension is adjusted to 3.0 with H2SO4 or NaOH under nitrogen atmosphere; (d) the vial is then closed and heated at 90 °C with stirring overnight; (e) the zeolite sample is separated from the liquid phase by centrifugation and washed with DI water three times; (f) the zeolite sample is then re-suspended in 100 mL 10 wt.% H2O2 solution and stirred overnight; (g) the zeolite sample is separated from the liquid phase by centrifugation, washed with DI water three times, and finally dried at 105 °C overnight.

Photochemical degradation.
The reaction setup has been described in our previous work. 2Where not otherwise stated, 30 mL aqueous PFOA solution (48 M) was mixed with 0.03 g of metal-cation-exchanged zeolite in a 40 mL quartz reactor followed by 24 h shaking in order to approach adsorption equilibrium.The reactor was shaken on a rotating shaker with 240 rpm constantly during the photochemical process in order to guarantee a good dispersion of the zeolite in the suspension.The scheme of the photochemical setup is shown in Figure S2.The UV-A mercury lamp (6 W, central wavelength 365 nm, Herolab GmbH Laborgeräte, Germany) was placed beneath the quartz reactor.The distance between the bottom of the quartz reactor and the UV lamp window was 20 mm.The spectral curve of the UV-A lamp is shown in Figure S9.The photon flux was measured by ferrioxalate actinometry to be 4.47×10 -6 mol s -1 .

Analysis.
In order to determine the total concentrations (freely dissolved and adsorbed fraction) of PFOA and shorterchain acids (C4 -C7) in the zeolite suspension, 0.1 mL aliquots of the suspension were transferred into a 4 mL vial and the pH was adjusted to below 2. Afterwards, 2 mL acetonitrile was added and shaken for 24 h to extract the adsorbed fractions into the liquid phase.The suspension was separated by centrifugation and the clear supernatant was transferred to LC/MS analysis.In the current study, the PFOA recovery after this extraction process was 94 ± 3 %.In order to determine the concentrations of fluoride and shorter-chain acids (C2 and C3) in the suspension, 1 mL aliquots were taken and the pH was adjusted to ≥ 11.The suspension was shaken for 2 h, separated by centrifugation and the clear supernatant was used for IC analysis.
The concentration of PFOA and shorter-chain acids (C4-C7) were determined by LC/MS (LCMS-2020; SHIMADZU Corp.), and the concentration of fluoride and shorter-chain acids (C2-C3) were determined by ion chromatography (IC, Dionex) as described. 3,4 he defluorination ratio ( F − ) is calculated as follows: where  F − is the fluoride concentration (µM) and  0 is the initial concentration of PFOA (µM).XPS spectra were recorded on an Axis Ultra photoelectron spectrometer (Kratos, Manchester, UK) using monochromatized Al Kα radiation (hν=1486.6eV).Pass energies of 160 eV and 40 eV were set for measuring the survey and the high-resolution spectra, respectively.The main component of the C 1s signal was set as reference at 284.8 eV for binding energy determination.
X-ray powder diffraction patterns were recorded at room temperature on a Bruker D8-Advance diffractometer, equipped with a one-dimensional silicon strip detector (LynxEye) using Cu-K radiation and a counting time of 1 s per data point.
So far, Fe-loaded BEA zeolites with moderate SiO2/Al2O3 in the range of about 30 are recognized as suitable photodegradation catalysts.With a half-life ( ½) in the range of 1.6 to 1.8 h, PFOA photodegradation is, however, relatively slow.Thus, the reaction cannot realistically be applied to treat large volumes of PFOAcontaminated water directly.Nevertheless, the Fe-BEA zeolites differ from other photocatalysts in their sorption performance for PFOA, which allows a concentrate-and-degrade strategy to be applied.This means that the PFOA is firstly separated from the contaminated water by fast and safe adsorption to the Fe-BEA zeolite, and afterwards degraded in the adsorbed state using UV-irradiation, which in turn regenerates the adsorbent.In order to characterize in more detail the sorption properties of BEA35 which showed the best compromise between adsorption and catalytic activity, adsorption isotherms were obtained in standard soft water 5 for PFOA.
As Figure S10(a) shows, PFOA adsorption can be well fitted by the Freundlich isotherm in the range of aqueous phase PFOA concentrations of 0.7 to 700 µg L -1 , with a Freundlich coefficient of KF = 10 4.5 mg 1-n kg -1 L -n and n = 0.63.Beyond this range, sorption is approaching maximum loading, which was determined by fitting the data in the higher concentration range to the linearized Langmuir isotherm with qmax = 7.7 wt. % (Figure S11).
Compared to an all-silica BEA zeolite, the maximum loading of the BEA zeolite with moderate Al content (and thus uptake capacity for Fe) is significantly lower (7.8 vs. 35 wt.%).This maximum loading is relevant for PFOA removal from high-concentrated solutions in the tens of mg L -1 range.However, PFOA contamination in the environment is frequently detected in the ng to tens of µg L -1 concentration range.Due to the non-linear isotherm shape, Kd values increase with decreasing PFOA aqueous phase concentration (Figure S10(b)), which is beneficial for adsorptive enrichment.The highest Kd determined at Cfree = 0.7 µg L -1 in this study for the BEA35 zeolite is 6.0×10 5 L kg -1 , while at Cfree = 10 µg L -1 it is still 1.5×10 5 L kg. 6 This ranks the BEA35 zeolite among the very good adsorbents for PFOA.While some organic and/or carbon-based adsorbents (ion exchange resins, cyclodextrin polymers or high-performance activated carbons as summarized by Aumeier et al. 7 ) show even higher Kd values up to 10 6 L kg -1 at Cfree in the µg L -1 range, the Fe-loaded zeolite has the advantage of regeneration simply by sunlight.

Text S3: Photodegradation of PFOA under UV irradiation in the presence of ferric ions and
Fe-zeolites.
In the first step, Fe-BEA will adsorb PFOA in water, and some fraction of PFOA present as complexed PFOA ([C 7 F 15 COO-Fe] 2+ -zeolite, eq.S1).The PFOA complex can be excited under UV-A irradiation to produce Fe 2+ -zeolite and C7F15COO• via carboxyl-to-metal electron transfer (eq.S2).
The C7F15 will preferentially react with dissolved oxygen to form C7F15OO (eq.S4).
The subsequent radical and hydrolysis reactions have been proposed in the literature (eqs.S5 -S9).
2C The produced C6F13COOH with one CF2 unit less than PFOA can complex again with ferric ions and be decomposed further until complete mineralization.Additionally, the produced Fe 2+ -zeolite can be re-oxidized by molecular oxygen into Fe 3+ -zeolite, and the photocatalytic cycle is formed (eq.S10).

Text S4: Stability and reusability test
The surface chemical states of introduced Fe in the Fe-BEA35 before and after reaction was analyzed by X-ray photoelectron spectroscopy (XPS).The broad Fe 2p spectra were deconvoluted into several peaks and fitted according to the literature (Figure S12) 15,16 .The peak at 710.4 eV was assigned to Fe(II) species.
The peaks at 712.0 and 714.1 eV were assigned to Fe(III) species, which may be ascribed to the ferric ions in octahedral and tetrahedral coordination, respectively.These XPS results fit well with our expectations that Fe-BEA35 consists mainly Fe(III) species, which contain active sites for PFOA photodegradation.The fraction of Fe(II) species slightly increased after reaction (from 19% to 23%), yet the major fraction of Fe(III) species was not affected (from 81% to 77%).To evaluated the stabilities of crystallographic structure of Fe-BEA35 before and after photodegradation of PFOA, the X-ray powder diffraction (XRD) measurement was performed (Figure S13).As seen in Figure x, almost identical XRD patterns were obtained for the Fe-BEA35 before and after reaction, which indicated the relatively stable crystallographic structure of Fe-BEA35.
In view of practical applications, experiments on the reusability of 1.26 Fe-BEA35 for photodegradation of PFOA were performed (Figure S12).After each 8 h irradiation period, the supernatant was separated via centrifugation and replaced by fresh aqueous PFOA solution with the same volume.The zeolite suspension was prepared for the next reaction run after 1 day-shaking for adsorption equilibration.As seen in Figure S12, the 1.26 Fe-BEA35 continues to exhibit a relatively high activity for PFOA photodegradation after four consecutive runs, as only a slight decrease in PFOA degradation performance was observed.The possible reasons for this decrease might be: (i) the remaining PFOA and its byproducts (PFCAs) remain on the zeolite between the runs; (ii) some zeolite separation losses during reusability tests; (iii) the slightly decreased fraction of Fe(III) species relative to Fe(II) after the catalytic reaction compared to the fresh material.As the inherent catalyst activity is not affected during the reusability test, it shows good stability and is thus suitable potential for practical application.

Langmuir isotherm fit for the adsorption of PFOA on BEA35 zeolite
The linearized form of the Langmuir isotherm (eq.S14) was applied to fit the adsorption equilibrium data.
In contrast to the Freundlich isotherm, the fit was in this case better in the high loading range.In this way, the maximum loading (qm = 7.7 wt.%) and Langmuir coefficient (KL = 1100 L g -1 ) were determined.Adsorption kinetics is biphasic with a fast initial part where >90% of PFOA are adsorbed and a slower part most likely due to slow diffusion of PFOA in the zeolite pore system with approach to equilibrium within 24 h.

Figures S1 to S15, referred to in the main text and supportingFigure S1 .
Figures S1 to S15, referred to in the main text and supporting information.

Figure S2 .
Figure S2.Scheme of the experimental setup for photochemical experiments.

Figure S5 .Figure S6 .
Figure S5.Schematic diagram of PFOA configurations on Fe-BEA35 with and without specific

Figure S7 .Figure S8 .
Figure S7.Schematic diagram of four isolated Fe species (as examples) in Fe-containing zeolites

Figure S9 .
Figure S9.Spectral beam intensity of UV-A lamp.The intensity varies along with power of the

Figure S10 .
Figure S10.Adsorption isotherms (a) and single-point sorption coefficients as function of

Figure S11 .Figure S12 .
Figure S11.Adsorption isotherm for adsorption of PFOA on BEA35 zeolite in EPA standard soft water, pH = 7 fitted by the Langmuir equation (for data points with qm ≥ 5 g kg -1 or 0.5 wt.%)

Figure S13 .Figure S14 .
Figure S13.The X-ray powder diffraction pattern of 1.26 Fe-BEA35 before and after

Table S3 .
Selected parameters of transition metal ion-exchanged BEA35 zeolites.Several metal ion solutions with 1 mM concentration were used for ion-exchange into zeolites during the preparation (for details see section 2.2).

Table S5 .
List of tested zeolites with additional information.The BET surface area was provided by supplier or determined by N2 adsorption/desorption experiment.
b The SiO2/Al2O3 molar ratio was provided by the supplier.c