High-Valent Iron-Oxo Complexes as Dominant Species to Eliminate Pharmaceuticals and Chloride-Containing Intermediates by the Activation of Peroxymonosulfate Under Visible Irradiation
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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.
KeywordsVisible light Peroxymonosulfate High-valency iron-oxo species Chlorinated intermediates Degradation pathway
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, 2KHSO5·KHSO4·K2SO4, oxone) as the effective oxidant has been developed gradually since it can generate hydroxyl (HO·) and sulfate (SO4·−) 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) , transition metals [10, 11, 12], ultrasound , electrochemical activation  and carbon catalysts . Although HO· and SO4·− 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/Cl2 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 O2 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 . 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 (FePcCl16) 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 FePcCl16 is applied in aqueous solution because of its catalytically inactive dimers , which could be overcome by the supported catalytic systems . 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 , graphitic carbon nitride [39, 40, 41], cellulose [42, 43, 44] and mesoporous carbon . 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 .
We used poly (4-vinylpyridine) (P4VP)/polyacrylonitrile (PAN) nanofibers (NF) (P4VP/PAN NFs), which were prepared in our previous work , to immobilize FePcCl16 by the nitrogen atom of the pyridinyl moieties of P4VP to synthesize FePcCl16-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 ultra-performance liquid chromatography (UPLC) coupled with high-definition mass spectrometry (HDMS). When NaCl was introduced, FePcCl16-P4VP/PAN NFs/PMS/visible-light and PMS/visible-light systems showed an excellent catalytic activity for CBZ decomposition. The FePcCl16-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 FePcCl16-P4VP/PAN NFs/PMS/NaCl/visible-light system, visible-light-induced FePcCl16 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.
2 Materials and Methods
2.1 Materials and Reagents
2.2 Preparation of P4VP/PAN Nanofibers (P4VP/PAN NFs)
A detailed method has been described in a previous study .
2.3 Preparation of FePcCl16-P4VP/PAN Nanofibers (FePcCl16-P4VP/PAN NFs)
FePcCl16 (15 wt% FePcCl16 vs. P4VP/PAN NFs) was dispersed in anhydrous tetrahydrofuran (THF) according to the detailed method as described previously . Figure S1 shows the preparation method of the FePcCl16-P4VP/PAN NFs. The content of FePcCl16 in FePcCl16-P4VP/PAN NFs based on inductively-coupled plasma (ICP) results (Optima 2100DV), which was 1.029 × 10−4 mol/g.
ATR-FTIR spectroscopy, XPS, UV–Vis analysis and UV–Vis diffuse reflection spectra were used to detect the structure of FePcCl16 powders, P4VP/PAN NFs and FePcCl16-P4VP/PAN NFs. Detailed parameters have been described in the literature . The morphology of catalytic fibers were observed by VHX-2000 digital microscope (Keyence, Japan).
2.5 Photocatalytic Experiments
The detailed photocatalytic experiments were referenced our previous work . 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.
3 Results and Discussion
Figure 1b showed the UV–Vis absorption spectra of P4VP, PAN and FePcCl16 in DMF solution. FePcCl16 showed a Q-band at 684 nm. In contrast, the maximum absorption of the FePcCl16/P4VP and FePcCl16/P4VP/PAN were blue-shifted by ~ 5 nm and ~ 4 nm, respectively, whereas FePcCl16/PAN was shifted by ~ 1 nm only, which indicates the coordination between the FePcCl16 and P4VP.
3.2 Photocatalytic Activity
Recycling experiments were conducted with the FePcCl16-P4VP/PAN NFs/PMS system under visible-light irradiation (Fig. S7). After six reaction cycles, the photocatalytic ability of FePcCl16-P4VP/PAN NFs had not weakened. The degradation of other organic contaminants is listed in Table S1.
3.3 Mechanism Analysis
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 SO4·‒ products are similar to the HO· products [39, 56].
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.
3.4 CBZ Transformation Products and Pathway by FePcCl16-P4VP/PAN NFs/PMS System Under Visible Light
3.5 Comparison of the Removal Efficiency of Chlorine Intermediates With or Without FePcCl16-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 FePcCl16-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 one-electron 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.
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 FePcCl16-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·, SO4·−, 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 chlorine-containing derivatives, whereas other radicals were unable to achieve this effect.
A novel heterogeneous photocatalyst FePcCl16-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 FePcCl16-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 FePcCl16-P4VP/PAN NFs/PMS/NaCl photocatalytic system could remove the parent and all transformation products including the chlorine-containing intermediates promptly and completely, since Fe(IV)=O was a critical and effective species in the system. Moreover, FePcCl16 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.
This work was supported by the National Natural Science Foundation of China (No. 51703201), and Zhejiang Provincial Natural Science Foundation of China (No. Q19E030051), and the Public Welfare Technology Application Research Project of Zhejiang Province (No. GF18E030003).
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