High-Valent Iron-Oxo Complexes as Dominant Species to Eliminate Pharmaceuticals and Chloride-Containing Intermediates by the Activation of Peroxymonosulfate Under Visible Irradiation

Generally, the sulfate (SO4·−) and hydroxyl (HO·) radicals are the dominant active species in most catalytic oxidation processes with peroxymonosulfate (PMS). However, the existence of various natural organic and inorganic matters in aquatic environments might influence the oxidation efficiency of these radicals, and/or form more toxic and refractory intermediates than the parent, especially in chlorine-ion-containing conditions. Here, we constructed a novel visible-light catalytic system with PMS based on iron hexadecachlorophthalocyanine-poly (4-vinylpyridine)/polyacrylonitrile nanofibers through pyridine ligands to generate high-valent iron-oxo (Fe(IV)=O) species as the main active species. The coordination structure was characterized by UV–Vis diffuse reflection, X-ray photoelectron spectroscopy, etc. The high-valent iron-oxo generation from peroxysulfate O–O bond heterolytic cleavage was proved by high-definition electrospray ionization mass spectrometer. Ultra-performance liquid chromatography coupled with high-definition mass spectrometry showed that the photocatalytic system was efficient for the degradation of carbamazepine and the chlorinated intermediates by iron-oxo active species in chlorine-ion-containing conditions.


Introduction
In recent decades, increasing environmental pollution that has resulted from recalcitrant organics, such as pharmaceuticals and their metabolites, personal-care products and endocrine disruptors has obtained increasing attention because of their high persistence and adverse impacts on the ecosystem [1][2][3]. Advanced-oxidation processes for the destruction of recalcitrant organic contaminants have been applied extensively [4,5]. Among the various advanced-oxidation processes, peroxymonosulfate (PMS, 2KHSO 5 ·KHSO 4 ·K 2 SO 4 , oxone) as the effective oxidant has been developed gradually since it can generate hydroxyl (HO · ) and sulfate (SO 4 ·− ) radicals that improve the flexibility over a broad pH range [6][7][8].
To generate such radicals, numerous activators have been investigated, including ultraviolet (UV) [9], transition metals [10][11][12], ultrasound [13], electrochemical activation [14] and carbon catalysts [15]. Although HO · and SO 4 ·− have a strong oxidizing capability because of their high redox potential [16,17], ubiquitous natural organic/inorganic matters in aquatic ecosystems will influence its oxidizing capacity and form byproducts that may be more toxic and refractory than the parent [14,18]. In particular, the chlorine ion, which can be oxidized by sulfate radicals to form active chlorine species HOCl/Cl 2 or chlorine radicals that may cause the occurrence of chlorinated intermediates, as a usual inorganic ion, exist in a variety of aquatic environments [19][20][21][22]. Thus, it is necessary not to overlook the transformation products and pollutant pathways.
Metalloporphyrins (MPs) as the key catalytic structures in biological enzymes activate O 2 or other peroxide to react with substrates under mild conditions [23][24][25].
Metallophthalocyanines (MPcs) are attractive as catalysts for analogous structures with MP complexes and because of their cost-effective and simple preparation [26]. MPcs exhibit a better spectral response in the visible-light region compared with MPs that can be applied widely in the photocatalytic field [27,28]. Iron phthalocyanine (FePc) and its numerous derivatives with various substituent groups as environmentally friendly catalysts have been applied extensively in catalytic reactions [29][30][31]. Iron hexadecachlorophthalocyanine (FePcCl 16 ) is one of the stable derivatives because of the macrocyclic structure was protected by chlorinated substituents at the peripheries due to their strong electron-withdrawing. However, the limitation of FePcCl 16 is applied in aqueous solution because of its catalytically inactive dimers [32], which could be overcome by the supported catalytic systems [26]. In previous studies, several general approaches (e.g., physical adsorption, physical mixing, covalent anchoring, electrostatic interaction, coordination bonding) have been used to immobilize MPcs on various supports to improve stability and cyclic performance, such as polymers [33,34], carbon nanotubes [35][36][37], carbon fiber [38], graphitic carbon nitride [39][40][41], cellulose [42][43][44] and mesoporous carbon [45]. In various MPc-supported catalysts, HO · /SO4 ·− as the main species from homolytic cleavage of the peroxide O-O bond to remove recalcitrant organic contaminants. High-valence metal-oxo species are generated from the heterolytic cleavage of the peroxide O-O bond and compete with the homolytic cleavage process, which have been recognized as active species in enzymatic processes [46].
We used poly (4-vinylpyridine) (P4VP)/polyacrylonitrile (PAN) nanofibers (NF) (P4VP/PAN NFs), which were prepared in our previous work [33], to immobilize FePcCl 16 by the nitrogen atom of the pyridinyl moieties of P4VP to synthesize FePcCl 16 -P4VP/PAN NFs. Carbamazepine (CBZ) as the most frequently detected recalcitrant organic in aquatic environments to evaluate the catalytic ability at neutral pH. The intermediates and degradation pathway of CBZ was proposed by ultraperformance liquid chromatography (UPLC) coupled with high-definition mass spectrometry (HDMS). When NaCl was introduced, FePcCl 16 -P4VP/PAN NFs/PMS/ visible-light and PMS/visible-light systems showed an excellent catalytic activity for CBZ decomposition. The FePcCl 16 -P4VP/PAN NFs/PMS/NaCl/visible-light system could degrade CBZ and its transformation products rapidly and completely, including the chlorinated intermediates. However, in the PMS/NaCl/visible-light system, residual chlorinated intermediates were still observed. The reason for this phenomenon is that, in the FePcCl 16 -P4VP/PAN NFs/PMS/NaCl/visible-light system, visible-light-induced FePcCl 16 and the electron donors of pyridine were identified as major factors in the formation of Fe(IV)=O species, which could efficiently degrade chlorinated intermediates from CBZ.

Materials and Reagents
FePcCl 16 was prepared by previously reported method [47]. All other chemicals used have been described in a previous study [33].

Characterization
ATR-FTIR spectroscopy, XPS, UV-Vis analysis and UV-Vis diffuse reflection spectra were used to detect the structure of FePcCl 16 powders, P4VP/PAN NFs and FePcCl 16 -P4VP/PAN NFs. Detailed parameters have been described in the literature [36]. The morphology of catalytic fibers were observed by VHX-2000 digital microscope (Keyence, Japan).

Photocatalytic Experiments
The detailed photocatalytic experiments were referenced our previous work [39]. All experiments were conducted in a 40-mL glass vessels at room temperature. The vessels contained 0.3 g/L catalytic NFs with an initial CBZ concentration of 0.025 mM and PMS of 2 mM. The sample preparation and the measurements is provided in the Supplementary Material. Figure 1a shows the UV-Vis diffuse reflectance spectra of the P4VP/PAN NFs, FePcCl 16 /PAN NFs and FePcCl 16 -P4VP/PAN NFs. In the visible region, a strong absorption band of FePcCl16/PAN NFs at 721 nm was observed. The absorption band of FePcCl 16 -P4VP/PAN NFs was blue-shifted by ~ 38 nm, which may because of a ligandto-metal charge-transfer transformation from the pyridine of P4VP toward the metal atom of FePcCl 16 [48]. The digital microscope images of FePcCl 16 -P4VP/PAN NFs (Fig. S2A) and P4VP/PAN NFs (Fig. S2B) show that FePcCl 16 powders loaded on P4VP/PAN NFs. Figure 1b showed the UV-Vis absorption spectra of P4VP, PAN and FePcCl 16 in DMF solution. FePcCl 16 showed a Q-band at 684 nm. In contrast, the maximum absorption of the FePcCl 16 /P4VP and FePcCl 16 /P4VP/ PAN were blue-shifted by ~ 5 nm and ~ 4 nm, respectively, whereas FePcCl 16 /PAN was shifted by ~ 1 nm only, which indicates the coordination between the FePcCl 16 and P4VP.

Characterization
XPS was used to investigate the chemical state and surface composition between FePcCl 16 and P4VP/PAN NFs. As shown in Fig. 2d, the survey spectra of a wide scan showed that new bands of iron and chlorine were observed when FePcCl 16 was supported on P4VP/PAN NFs. The N1s core-level spectrum of the FePcCl 16 has two distinct peaks centered at 398.44 eV and 399.07 eV, which are assigned to the C-N=C and C-N-C bonds of FePcCl 16 (Fig. 2a). 399.28 eV and 399.88 eV are attributed to the imine moiety (-N=) of the pyridine rings [49] and the C≡N of PAN [50], respectively (Fig. 2b). After the interaction of FePcCl 16 and P4VP/PAN NFs, the peak for the C-N=C and C-N-C bonds of FePcCl 16 moved to 398.14 eV and 398.80 eV, respectively, and the imine moiety (-N=) of the pyridine rings moved to 399.39 eV, whereas the C≡N of PAN was unchanged (Fig. 2c). It is because that the pyridine rings in the P4VP as the electron donors rather than the C≡N groups to coordinate with FePcCl 16 [48]. According to the TEM images of P4VP/PAN NFs (Fig. S3A, B) and FePcCl 16 -P4VP/PAN NFs (Fig. S3C, D), after FePcCl 16 loaded on P4VP/PAN NFs, the textural properties are maintained and FePcCl 16 are well dispersed on P4VP/PAN NFs.

Photocatalytic Activity
CBZ was selected as the model pollutant to detect the catalytic properties of FePcCl 16 -P4VP/PAN NFs. The influence of PMS dosage is shown in Fig. S5. The FePcCl 16 -P4VP/ PAN NFs could not degrade CBZ without PMS under visible-light irradiation. With an increase in PMS concentration, the removal rate of CBZ increased. When the concentration of PMS was 2 mM, FePcCl 16 -P4VP/PAN NFs exhibited the highest photocatalytic activity. Therefore, in this study, 2 mM PMS was selected to explore the photocatalytic activity of FePcCl 16 -P4VP/PAN NFs. As show in Fig. 3 CBZ did not decrease in the presence of FePcCl 16 -P4VP/PAN NFs with PMS in the dark, which indicates that CBZ cannot The pH is a non-negligible factor in the degradation of organic contaminants in aqueous solution. Here, we researched the photocatalytic oxidation of CBZ with PMS, FePcCl 16 /PMS and FePcCl 16 -P4VP/PAN NFs/PMS under visible-light irradiation at pH 3, 7 and 10. Figure 4 shows a good photocatalytic activity in the FePcCl 16 /PMS/visiblelight system at pH 3 and 7, whereas it possesses a relatively weak photocatalytic capacity at pH 10. This system is a Fenton-like catalytic system, which produces SO 4 ·− and HO · radicals as the dominant species. And high pH could inhibit the reaction ability of SO 4 ·− and HO · radicals [51]. In the FePcCl 16 -P4VP/PAN NFs/PMS/visible-light system, CBZ was removed almost completely after 120 min at pH 3, 7 and 10. The results indicate that FePcCl 16 -P4VP/ PAN NFs exhibited an excellent photocatalytic ability over a broader pH range than pure FePcCl 16 . The results show that FePcCl 16 anchored on P4VP/PAN NFs may lead to a unique photocatalytic mechanism that differs from FePcCl 16 .
Inorganic salts usually exist in practical wastewater and may influence the pollutant-oxidation performance [52]. Hence, the impact of inorganic salts is necessary to evaluate the FePcCl 16 -P4VP/PAN NFs/PMS/visible-light system. There was no obvious influence on the degradation of CBZ in the FePcCl 16 -P4VP/PAN NFs/PMS/visible-light system in the presence of inorganic salts (Na 2 SO 4 , CaSO 4 and MgSO 4 ) (Fig. 5a). The influence of NaHCO 3 and Na 3 PO 4 was in Fig. S6. The removal rate of CBZ was slightly inhibited by Na 3 PO 4 and inhibited by NaHCO 3 severely. Except for the aforementioned ions, chloride ion (Cl − ) is another major inorganic anion in water resources and almost all natural waters. When NaCl is introduced in the FePcCl 16 -P4VP/ PAN NFs/PMS/visible-light system, as shown in Fig. 5b, the removal rate increased significantly and CBZ almost degraded completely in ~ 20 min. Without visible light, NaCl could not promote adsorption and the catalytic ability of FePcCl 16 -P4VP/PAN NFs. Only PMS with NaCl under visible light could accelerate the degradation of CBZ. Based on the aforementioned phenomenon, the addition of NaCl would promote the photocatalytic degradation of CBZ with PMS, and FePcCl 16 -P4VP/PAN NFs led to a greater promotion effect. According to the literature [19,20], PMS and sulfate radicals can oxidize chloride ions to active chlorine species HOCl/Cl 2 or chlorine radicals under visible light, which can degrade CBZ rapidly. In addition to the aforementioned radicals, other catalytic mechanisms may exist in the FePcCl 16 -P4VP/PAN NFs/PMS/Cl − /visible-light system that promote the removal rate further.

Mechanism Analysis
PMS could be activated by visible light to produce SO 4 ·and HO · radicals [18,53]. TBA and ethanol are as the quencher of SO 4 ·and HO · [39,54]. The HO · can be quenched by ethanol and TBA, whereas the SO 4 ·can be quenched only by ethanol. Ethanol could quenched both SO 4 ·and HO · , whereas TBA quenched SO 4 ·only. Thus, the difference in CBZ degradation rates in the FePcCl 16 -P4VP/PAN NFs/ PMS system under visible light in existence of ethanol and TBA to identify the contribution of the two radicals. The influence of two quenchers of CBZ degradation efficiency was showed in Fig. 6a. Removal rates of CBZ without quencher was ~ 98%, whereas the corresponding values in existence of TBA decreased by ~ 15% and ethanol decreased by ~ 30%, respectively. These results suggest that only 30% removal of CBZ was contributed by HO · and SO 4 ·-, and other oxidation processes existed in the FePcCl 16 -P4VP/ PAN NFs/PMS/visible-light system as the dominant agent of 70% CBZ degradation. No obvious EPR signals existed for the FePcCl 16 -P4VP/PAN NFs/PMS system in the dark (Fig. 6b). Under visible light after 300 s, DMPO-OH signals were observed. However, we could not detect the DMPO-SO 4 signals. SO 4 ·radicals may not be stable in aqueous solution, but these react with H 2 O to form HO · radicals [55].
GC-MS and HDMS were used to investigate other potent active species. Sulfoxides can be oxidized by the two-electron transfer step from Fe(IV)=O species to produce the corresponding sulfones (Eq. 1), which differ from the HO · products (Eq. 2), and the SO 4 ·products are similar to the HO · products [39,56].
ESI-MS with a soft MS ionization technique was used to observe high-valency iron-oxo species (Fe(IV)=O) [57,58]. Figure 8a   The DFT reaction process was calculated by The B3LYP/6-311G method. The length of the Fe-N (4VP) bond (2.1985 Å) with PMS was longer than without PMS (2.1608 Å) (Fig. S8B and D). The results indicate that the promotion of O-O bond cleavage through pyridine ligands to form iron-oxo center. The detailed DFT calculations (Tables S2-S5) show the high electron spin densities at the iron-oxo center ( Fig. S8A and C), which achieves electrophilic addition on CBZ and its intermediates.
According to the aforementioned results, a possible photocatalytic mechanism to activate PMS by FePcCl 16 -P4VP/ PAN NFs is shown in Fig. 9. FePcCl 16 could be motivated to the excited state (*FePcCl 16 ) under visible-light irradiation, which coordinates with the oxygen atoms of PMS to form an O-O bond [39,59]. The electron-donating effect of pyridine in P4VP promotes the heterolytic cleavage of the O-O bond and generates the high-valent iron-oxo (Fe(IV)=O) species, which is the major active species in this reaction system [60]. Under visible-light irradiation, PMS also could be activated to produce SO 4 ·− , which could react with OH − to form HO · [61]. SO 4 ·− and HO · are the minor active species which would be captured by Cl − to produce Cl · [62].

CBZ Transformation Products and Pathway by FePcCl 16 -P4VP/PAN NFs/PMS System Under Visible Light
During CBZ degradation by the FePcCl 16 -P4VP/PAN NFs/ PMS system under visible light, a typical intermediate totalion chromatogram (TIC) was obtained according to eleven intermediates (A-K), which were detected in positive mode (Fig. 10c). Detailed information on the intermediates is shown in Table S6. We provide a comprehensive view of CBZ degradation in Fig. 11. Two isobaric species, B and C with empirical formula C 15 H 13 N 2 O 2 , and three isobaric species, A, I and K, with empirical formula C 15 H 11 N 2 O 3 were formed. Intermediate B contains one more oxygen atom than the CBZ molecule, whereas the oxidizing species easily attack the reactive 10,11-double bond of the CBZ molecule [63,64]. Compound A formed from 10-11-dihydroxyl-CBZ by dehydrogenation, which was not observed in this catalytic system [65]. Intermediate B underwent two ring-contraction mechanisms to form intermediates C and D. Detailed mechanisms have been described in previous studies [66,67]. Intermediate H was produced with carboxyl moieties from compound B by opening the heterocyclic ring [68]. A detailed mechanism has been provided in previous studies, in which compound H underwent an intramolecular reaction to compound I [69]. I was oxidized further to J. Intermediate K originated from H via electrophilic aromatic substitution with intramolecular cyclization. Compound E formed from compound C by the loss of a -CONH 2 group, and it was oxidized to form compound F. Intermediate G could result from D and F through the removal of a carbonyl group or it could be oxidized, respectively. As the reaction proceeds, the intermediate area increased and then decreased (Fig. 10a, b).
After 120 min of treatment, all intermediates were degraded to small acids or were mineralized completely.

Comparison of the Removal Efficiency of Chlorine Intermediates With or Without FePcCl 16 -P4VP/PAN NFs Under Visible Light
In the chapter on photocatalytic activity, when NaCl was introduced in this photocatalytic system, the degradation of CBZ was enhanced significantly. The catalytic efficiency of the FePcCl 16 -P4VP/PAN NFs/PMS/NaCl photocatalytic system was more rapid than that of the PMS/NaCl photocatalytic system. Although Cl − could enhance the catalytic efficiency, according to other literature [21,22], chlorine radicals and active chlorine species that formed via the oneelectron oxidation of Cl − caused the occurrence of chlorinated intermediates, which may be more toxic and degrade with more difficulty than the parent. Hence, an investigation of the CBZ transformation products and their evolution processes are very important. Figures 12c and 13d show that the total-ion chromatogram of CBZ degraded in the presence of PMS and NaCl with or without FePcCl 16 -P4VP/PAN NFs. Detailed information on the detected intermediates is listed in Tables S7 and S8. The Three isomorphisms of organic chloro-derivatives were detected in the two photocatalytic systems. Unfortunately, the specific structure of the three isomorphisms is uncertain. The fate of the chloro-derivatives was not the same in the two photocatalytic systems. In the FePcCl 16 -P4VP/PAN NFs/PMS/NaCl photocatalytic system, all transformations, including three chloro-derivatives were degraded completely after 40 min reaction (Fig. 12a, b). No other chlorine-containing organic compounds could be detected. In the PMS/ NaCl photocatalytic system, intermediates that do not contain chlorine were oxidized easily by the radicals (e.g., HO · , SO 4 ·− , Cl · ) (Fig. 13a-c). However, chlorine-containing derivatives were also detected after 60 min reaction (Fig. 13c). According to the aforementioned phenomenon, high-valency iron-oxo species (O=Fe(IV)) could rapidly destroy chlorinecontaining derivatives, whereas other radicals were unable to achieve this effect.

Conclusions
A novel heterogeneous photocatalyst FePcCl 16 -P4VP/PAN NFs was synthesized to activate PMS for refractory contaminant oxidation in high constituent backgrounds, especially for those that contain chlorine ions. The removal of CBZ was enhanced when NaCl was added to the FePcCl 16 -P4VP/ PAN NFs/PMS and the PMS photocatalytic systems. The PMS/NaCl photocatalytic system was inefficient to degrade the chlorine-containing intermediates, which are harmful to the aquatic environment. However, the FePcCl 16 -P4VP/PAN NFs/PMS/NaCl photocatalytic system could remove the parent and all transformation products including the chlorinecontaining intermediates promptly and completely, since Fe(IV)=O was a critical and effective species in the system. Moreover, FePcCl 16 molecules anchored by the pyridyl-containing P4VP, could be excited under visible irradiation to promote the generation of Fe(IV)=O active species through a heterolytic cleavage of the O-O bond in the presence of PMS. This work provides a new strategy and simple route to treat chlorine-containing wastewater efficiently.