Catalytic Wet Peroxide Oxidation of Anionic Pollutants over Fluorinated Fe 3 O 4 Microspheres at Circumneutral pH Values

: Fluorinated Fe 3 O 4 microspheres with 7.1 ± 1.4 wt% of ﬂuoride (F-Fe 3 O 4 -1) were prepared via glycothermal synthesis. Fluorination signiﬁcantly enhanced the activity of F-Fe 3 O 4 -1 in catalytic wet peroxide oxidation of anionic dyes (including orange G (OG) and congo red) at pH ~7. However, the promotional effect of ﬂuorination became less obvious for amphoteric rhodamine B and was not observed for cationic methylene blue. After reacting with H 2 O 2 (40 mM) for 2 h at pH 6.5 and 40 ◦ C, the decolorization rates of OG (0.1 mM) and the pseudo-ﬁrst-order rate constant were 96.8% and 0.0284 min − 1 over F-Fe 3 O 4 -1 versus 17.6% and 0.0011 min − 1 over unmodiﬁed Fe 3 O 4 . The effects of reaction parameters (initial H 2 O 2 concentration and pH value and reaction temperature) on OG decolorization with H 2 O 2 over F-Fe 3 O 4 -1 were investigated. The reusability of F-Fe 3 O 4 -1 was demonstrated by OG decolorization in eight consecutive runs. Fluorination increased the isoelectric point of F-Fe 3 O 4 -1 to 8.7 and facilitated the adsorption and degradation of anionic dyes on the surface of F-Fe 3 O 4 -1 at pH ~7. Scavenging tests and EPR spectra supported that hydroxyl radicals were the main reactive species for the OG decolorization over F-Fe 3 O 4 -1.


Introduction
Organic compounds constitute one of the most common pollutants found in wastewater that pose potential hazards to the ecosystem and the environment. Fenton oxidation is effective for organic wastewater remediation since it can generate strongly oxidative hydroxyl radicals (•OH, E 0 = 2.80 V) that are capable of degrading most recalcitrant organic pollutants. However, the Fenton process requires acidic condition (pH~3) and generates undesired iron sludge. Catalytic wet peroxide oxidation (CWPO) may overcome these problems since it can work at circumneutral pH values and avoid the formation of iron sludge [1][2][3][4]. However, most heterogeneous Fenton-like catalysts exhibit lower activity than the conventional homogeneous Fenton process [2,[5][6][7][8] since the kinetics of a heterogeneous catalytic reaction are also affected by the mass transfer process inside the reaction system (e.g., adsorption and diffusion) besides the intrinsic activity of a catalyst. In this regard, the development of highly active and stable heterogeneous Fenton-like catalysts with favorable mass transfer is critical for a successful CWPO process.
Magnetite (Fe 3 O 4 ) is a promising heterogeneous Fenton-like catalyst owing to its intrinsic peroxidase-like activity and efficient magnetic separation [2,[9][10][11][12]. However, it exhibits very low catalytic activity for the degradation of anionic organic contaminants under circumneutral pH conditions [10,13]. One reason for this is its low isoelectric point (IEP, about 6-7) [14][15][16]. As such, Fe 3 O 4 particles are negatively charged at pH~7 and cannot effectively adsorb anionic contaminants due to electrostatic repulsion [14]. In addition, the activity of Fe 3 O 4 particles in activating H 2 O 2 under mild conditions (e.g., pH 7 and room temperature) is still too weak and has to be further improved for practical applications [2].
Doping is a simple and useful methodology to enhance the activity of magnetite, wherein metallic dopants (e.g., Ti 4+ , Cu 2+ , and Cr 3+ ) are the most commonly used [2,6,14,17,18]. In sharp contrast, nonmetal-doped Fe 3 [19][20][21][22][23][24]). In fluorinated Fe 3 O 4 (F-Fe 3 O 4 ) microspheres [25], fluoride may replace lattice oxygen or coordinate with surface lattice iron via ligand exchange with hydroxide groups. The substitution of lattice O 2− with F − results in a framework with extra positive charge due to a charge imbalance. A number of possibilities exist for charge compensation, including by attracting anions (e.g., OH − , F − , and Cl − ) from the solution, by reducing the average valence of the iron cations (e.g., increasing the content of low-valence ferrous (Fe 2+ ) ions), or by introducing some cationic vacancies. Therefore, fluorination presents largely unexplored opportunities for adjusting the physicochemical properties of F-Fe 3 O 4 for specific applications, e.g., it is expected to increase the surface zeta potential of the pristine Fe 3 O 4 for the enhanced adsorption of anionic compounds via electrostatic attraction and to improve the H 2 O 2 -activation capability via the Fenton-like reaction (Equation (1)).
≡Fe 2+ + H 2 O 2 → ≡Fe 3+ + ·OH + OH − (≡ represents the particle surface) (1) In this work, we found that in comparison with unmodified Fe 3 O 4 , fluorination significantly enhanced the catalytic activity of F-Fe 3 O 4 microspheres for the degradation of anionic pollutants (e.g., orange G (OG) and congo red (CR) in Scheme 1). However, this promotional effect became less obvious for an amphoteric dye rhodamine B (RhB), and no promotion was observed for a cationic dye methylene blue (MB)). The different activities of F-Fe 3 O 4 in the degradation of cationic and anionic dyes were mainly attributed to the adsorption-enhanced catalytic oxidation of anionic dyes on its surface via a free-radical reaction pathway.

Catalytic Performances of F-Fe 3 O 4 -r Microspheres
As shown in Figure 1a, fluorination significantly enhanced the decolorization rate (DR) of OG (0.1 mM) with H 2 O 2 (40 mM) after reacting for 2 h at pH 6.5 and 40 • C over F-Fe 3 O 4 -r (e.g., the 2 h DR increased from 17.6% for Fe 3 O 4 -blank to 62.2% for F-Fe 3 O 4 -0.1). The best performance (96.8% of DR after 2 h) was obtained over F-Fe 3 O 4 -1. A further increase in the fluoride content resulted in a gradual decrease in the 2 h DRs of OG (0.1 mM) (68.4% for F-Fe 3 O 4 -2 and 47.5% for F-Fe 3 O 4 -3), which may be related to the increase in the Na 3 FeF 6 impurity in F-Fe 3 O 4 -r (r = 2, 3) [25]. By assuming that the instantaneous concentration of •OH is constant since the amount of H 2 O 2 is in larger excess with regard to OG (40 mM vs. 0.1 mM), the decolorization process of OG in water was well fitted with the pseudo-first-order kinetic model (R 2 : 0.990-0.997 in Figure 1b). The rate constants, k s1 , were obtained from the slopes of the straight lines by linear regression of ln(C 0 /C t )-time plots. The change of k s1 with fluorination is similar to that of the DR (Table 1) 1 Reaction reactions were the same as Figure 1; 2 Tert-butanol (10 mM) added. Figure 2a  The OG decolorization at different pH values is presented in Figure 2c,d. Both the 2 h DR and k s1 value increased when the initial pH value reduced from 9.0 to 3.0. It is noteworthy that the high decolorization efficiencies of the OG (0.1 mM) solution could be achieved (2 h DR: 94.8-96.8% and k s1 : 0.0252-0.0284 min −1 in Table S2) at the natural pH value of the OG (0.1 mM) solution (6.4-6.8). This has technological and operational advantages since it is not required to pre-adjust its pH value.

Shown in
The temperature effect on the OG decolorization is displayed in Figure 2e,f. It is obvious that the 2 h DE and k 1 value increased as the reaction temperature increased from 25 • C to 50 • C (e.g., they were 32.6% and 0.0028 min −1 at 25 • C, and increased to 99.5% and 0.0445 min −1 at 55 • C in Table S3). The kinetic parameters, including the activation energy (E a ) and collision frequency (A, also called pre-exponential factor), of the OG decolorization, were estimated from the linear Arrhenius equation of lnk s1 = lnA − E a /RT, where R is the universal gas constant (8.314 J/(mol·K)). The E a value was estimated to be 75.7 kJ/mol from the slope of the fitted linear equation of the plot of lnk s1 − 1000/T ( Figure S1), which is significantly lower than the dissociation energy of H 2 O 2 to ·OH (213.8 kJ/mol) due to the catalytic effect of F-Fe 3 O 4 -1 [28] but higher than those obtained for the RhB degradation on Fe 3 O 4 MNPs (45.6 kJ/mol [26], 46.6 kJ/mol [29], 47.6 kJ/mol [10] or 51 kJ/mol [30]), which may be attributed to the smaller surface area of the F-Fe 3 O 4 -1 microspheres. However, it is interesting to note that the pre-exponential factor A in the reaction system of OG/F-Fe 3 O 4 -1 microspheres is five orders of magnitude larger than that obtained in RhB/Fe 3 O 4 MNPs (6.88 × 10 10 min −1 vs. 8.26 × 10 5 min −1 [10]), which indicates that collision frequency between reacting species are greatly enhanced on F-Fe 3 O 4 -1 microspheres in agreement with the proposed adsorption-enhanced catalytic degradation of anionic molecules (vide infra).
To check the scope of the applicability of F-Fe 3 O 4 -1 in organic wastewater treatment, the CWPO degradations of other organic dyes (CR, MB, and RhB) were investigated and displayed in Figure 3 and Table 2. As shown in Figure 3a [32,33], or 6.41 [27]. Therefore, the carboxylic acid group of RhB is deprotonated and becomes negatively charged at the tested pH (~6.6), which may partially contribute to the enhanced adsorption and degradation of RhB on F-Fe 3 O 4 -1.  (Figure 4 and Table 3). The 2 h DR, k s1, and chemical oxygen demand (COD) reduction rate were 84.9%, 0.0151 min −1, and 65.2%, respectively, in the eighth run of F-Fe 3 O 4 -1. The low-to-moderate COD reduction rate (65.2-83.2%) indicated the formation of some organic intermediates during the degradation of OG with H 2 O 2 . These intermediates may cover some active sites on F-Fe 3 O 4 -1, resulting in a gradual reduction in the activity of reused F-Fe 3 O 4 -1.

Characterization of F-Fe 3 O 4 -r Microspheres
The F-Fe 3 O 4 -r microspheres used in this work have been characterized in our previous work [25]. The XRD patterns shown in Figure 1 of Reference 25 indicate that F-Fe 3 O 4 -r (r ≤ 1) consist of the single magnetite (PDF #19-0629) phase while F-Fe 3 O 4 -r (r = 2, 3) contain the Na 3 FeF 6 (PDF #22-1381) impurity. Unless otherwise specified, the fluorinated Fe 3 O 4 in this work referred to F-Fe 3 O 4 -1 microspheres, which comprise pure magnetite with the highest fluoride content (7.1 ± 1.4 wt% determined by EDX in Table S4) and the best catalytic performance (Figure 1). In comparison with Fe 3 O 4 -blank, F-Fe 3 O 4 -1 microspheres have a reduced average particle size (360 ± 80 nm vs. 220 ± 34 nm) with slightly more aggregation (Figure 2 in Reference [25]), which resulted in their smaller S BET value (28.0 and 17.6 m 2 /g) (Figure 3 in Reference [25]). However, there is no obvious change in the magnetic properties of Fe 3 O 4 upon fluorination (Figure 4 in Reference [25]), which is beneficial for the magnetic separation and recycling of the F-Fe 3 O 4 -1 microspheres.

Mechanistic Investigation
The concentrations of dissolved F − and iron ions after OG decolorization were, respectively, 0.65 mg/L and 0.15 mg/L, which meet the criteria set in the fourth edition of the World Health Organization's Guidelines for drinking-water quality (0.5-1 mg/L fluoride and <0.3 mg/L iron). In addition, the decolorization of OG (0.1 mM) was negligible (~5%) after replacing F-Fe 3 O 4 -1 with 0.65 mg/L of F − or 0.15 mg/L of Fe 2+ (or Fe 3+ ) under otherwise identical conditions ( Figure 5), which supported the notion that the decolorization of OG with H 2 O 2 was catalyzed by heterogeneous F-Fe 3 O 4 -1 solid catalyst, not by soluble species.
The addition of tert-butanol (10 mM) as a scavenger of hydroxyl radicals significantly inhibited the degradation of OG with H 2 O 2 at pH 6.5 over F-Fe 3 O 4 -1. As shown in Figure 6 and Entry 6 in Table 1, the 2 h DR and k s1, respectively, decreased from 96.8% and 0.0284 min −1 to 35.2% and 0.0026 min −1 , implying that hydroxyl radicals are the main reactive oxygen species contributing to the OG degradation. Since the decolorization of OG was not completely suppressed by tert-butanol (10 mM), other reactive oxygen species such as superoxide radical anions may also make a minor contribution to the decolorization of OG.  The EPR spectra with the DMPO spin-trapping technique were used to identify the generation of ·OH radicals. As shown in Figure 7, a typical 1:2:2:1 quadruplet pattern corresponding to the DMPO-·OH adduct was observed for both F-Fe 3 O 4 -1 and Fe 3 O 4blank, supporting the formation of ·OH radicals in the H 2 O 2 /F-Fe 3 O 4 -1 (or Fe 3 O 4 -blank) reaction system. However, the signal intensities for F-Fe 3 O 4 -1 were obviously stronger than those for Fe 3 O 4 -blank, indicating the higher activity of the former to activate H 2 O 2 due to the larger number of Fe 2+ ions generated after fluorination of F-Fe 3 O 4 -1 [25]. The improved catalytic activity of F-Fe 3 O 4 -1 for the degradation of anionic dyes (e.g., OG and CR) at pH~7 was related to their enhanced adsorption. According to the measurement of zeta potentials at different pH values (Figure 8), the IEPs of F-Fe 3 O 4 -1 and Fe 3 O 4 -blank were determined to be 8.7 and 4.9, respectively. The higher IEP (8.7) of F-Fe 3 O 4 -1 was attributed to the partial substitution of F − for lattice O 2− in Fe 3 O 4 . As a result, the surface of F-Fe 3 O 4 -1 was positively charged, while Fe 3 O 4 -blank was negatively charged at pH~7. Therefore, the adsorption of anionic dye molecules on F-Fe 3 O 4 -1 was facilitated via electrostatic attraction but unfavorable on Fe 3 O 4 -blank at pH~7 due to electrostatic repulsion (e.g., the 15-min adsorption of OG was 5.9% on Fe 3 O 4 -blank vs. 12.3% on F-Fe 3 O 4 -1). It is generally accepted that surface-generated hydroxyl radicals are highly reactive and short-lived [34] and are mostly consumed on the surface before diffusing to the solution [35]. Therefore, the adsorbed anionic molecules (OG or CR), not those in the solution, were more likely to be oxidized by surface-generated hydroxyl radicals. This strengthens the critical role of the adsorption of the solid surface in determining the oxidation rate of pollutants in iron oxide/H 2 O 2 systems [36].

Synthesis of Fluoride-Modified Fe 3 O 4 Microspheres
The fluoride-modified Fe 3 O 4 microspheres were made via glycothermal synthesis according to the reported method [25] and named as F-Fe 3 O 4 -r, where r indicates the nominal F/Fe molar ratio (r: 0-3). The product made at r = 0 was unmodified (denoted Fe 3 O 4 -blank). In brief, NaOAc (22 mmol) and NaF (0-21 mmol) were sequentially added at an interval of 10 min to 4.5 mL of FeCl 3 ·6H 2 O (1.5 M) solution in ethylene glycol and then heated in a 60-mL Teflon-lined stainless steel autoclave at 198 • C for 14 h to obtain F-Fe 3 O 4 -r black particles.

CWPO of Organic Dyes
A general procedure of CWPO was as follows: A mixture of 0.025 g of F-Fe 3 O 4 -r microspheres and 50 mL of OG (C s = 0.1 mM) stock solution was first stirred at 750 rpm and 40 ± 2 • C for 15 min to reach the adsorption equilibrium. The concentration of OG was then measured and taken as the concentration at time zero (C 0 ). The degradation of OG was initiated by rapidly adding 0.230 g of H 2 O 2 (30%) into the mixture. At selected intervals, about 3 mL of the samples was taken by a syringe and passed through a filter membrane, and the residual concentration of OG in solution, Ct, was analyzed. Multiple measurements of the catalytic activity were performed with the relative standard deviation usually below 5%. In recyclability tests, the used F-Fe 3 O 4 -1 was magnetically recovered and directly reused for the next run according to the same procedure. This procedure was also applied to degrade CR, MB, and RhB. The concentrations, C t , of OG, CR, MB, and RhB in aqueous solutions were analyzed by measuring the absorbance at 478, 495, 664, and 554 nm, respectively, using a UV-vis spectrophotometer (UV-1800PC, Shanghai Mapada Instrument, Shanghai, China). Before measurement, the pH value of the CR solution in the quartz cuvette was adjusted by adding one drop of 2 wt% KOH.

Physicochemical Characterization
Powder X-ray diffraction (XRD) patterns were collected on Bruker D8 Advance Diffractometer with Cu Kα radiation at 30 kV and 20 mA. Scanning electron microscope (SEM) images were taken on JEOL JSM-5510LV SEM (Tokyo, Japan) at 20 kV. N 2 adsorptiondesorption isotherms were measured on Micromeritics ASAP 2020 (Georgia, United States) at 77 K. The samples were degassed under vacuum at 100 • C for 2 h before measurements. The BET specific surface area S BET was calculated from the adsorbed quantities in the relative pressure (P/P 0 ) range of 0.05-0.30. M-H curves were measured on a JDAW-2000D (Jilin, China) vibrating sample magnetometer (VSM) at room temperature. Zeta potentials were determined at different pH values on Malvern Zetasizer Zen3690 (Malvern, United Kingdom). Electron-spin resonance (EPR) spectra were collected on Bruker EMXplus (Berlin, Germany) at room temperature. The concentrations of fluoride and iron ions in the solution were analyzed using ionic chromatography (ICS-1500) and atomic absorption spectroscopy (Agilent 240 AA, California, United States), respectively.

Conclusions
Fluorination increased the isoelectric point from 4.9 for undoped Fe 3 O 4 -blank microspheres to 8.7 for F-Fe 3 O 4 -1 with 7.1 ± 1.4 wt% of fluoride. In comparison with Fe 3 O 4 -blank, F-Fe 3 O 4 -1 could efficiently catalyze the decolorization of anionic dye solutions at pH~7. This was mainly attributed to the enhanced adsorption of anionic dyes on the surface of F-Fe 3 O 4 -1 via electrostatic attraction, followed by oxidative degradation via a free-radical reaction pathway. This work strengthens the critical role of the adsorption of the solid surface in determining the oxidation rate of pollutants in iron oxide/H 2 O 2 systems.
Author Contributions: Conceptualization, F.C.; methodology, investigation, data curation, and formal analysis, H.L., W.C. and F.C.; resources, F.C. and R.C.; and writing-original draft preparation, review, and editing, F.C. All authors have read and agreed to the published version of the manuscript.