Catalytic Wet Peroxide Oxidation of Anionic Pollutants over Fluorinated Fe₃O₄ Microspheres at Circumneutral pH Values

Abstract

Fluorinated Fe₃O₄ microspheres with 7.1 ± 1.4 wt% of fluoride (F-Fe₃O₄-1) were prepared via glycothermal synthesis. Fluorination significantly enhanced the activity of F-Fe₃O₄-1 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 H₂O₂ (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-Fe₃O₄-1 versus 17.6% and 0.0011 min⁻¹ over unmodified Fe₃O₄. The effects of reaction parameters (initial H₂O₂ concentration and pH value and reaction temperature) on OG decolorization with H₂O₂ over F-Fe₃O₄-1 were investigated. The reusability of F-Fe₃O₄-1 was demonstrated by OG decolorization in eight consecutive runs. Fluorination increased the isoelectric point of F-Fe₃O₄-1 to 8.7 and facilitated the adsorption and degradation of anionic dyes on the surface of F-Fe₃O₄-1 at pH ~7. Scavenging tests and EPR spectra supported that hydroxyl radicals were the main reactive species for the OG decolorization over F-Fe₃O₄-1.

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, E⁰ = 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 (Fe₃O₄) 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, Fe₃O₄ particles are negatively charged at pH ~7 and cannot effectively adsorb anionic contaminants due to electrostatic repulsion [14]. In addition, the activity of Fe₃O₄ particles in activating H₂O₂ 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., Ti⁴⁺, Cu²⁺, and Cr³⁺) are the most commonly used [2,6,14,17,18]. In sharp contrast, nonmetal-doped Fe₃O₄ have been seldom investigated. Fluoride doping is known for modulating semiconductor band structures with improved photocatalytic performances (e.g., F-doped TiO₂, SnO₂, Co₃O₄, γ-Fe₂O₃, Cu₂O, and Bi₂WO₆ [19,20,21,22,23,24]). In fluorinated Fe₃O₄ (F-Fe₃O₄) microspheres [25], fluoride may replace lattice oxygen or coordinate with surface lattice iron via ligand exchange with hydroxide groups. The substitution of lattice O²⁻ 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 (Fe²⁺) ions), or by introducing some cationic vacancies. Therefore, fluorination presents largely unexplored opportunities for adjusting the physicochemical properties of F-Fe₃O₄ for specific applications, e.g., it is expected to increase the surface zeta potential of the pristine Fe₃O₄ for the enhanced adsorption of anionic compounds via electrostatic attraction and to improve the H₂O₂-activation capability via the Fenton-like reaction (Equation (1)).

≡Fe²⁺ + H₂O₂ → ≡Fe³⁺ + ·OH + OH⁻ (≡ represents the particle surface)

(1)

In this work, we found that in comparison with unmodified Fe₃O₄, fluorination significantly enhanced the catalytic activity of F-Fe₃O₄ 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-Fe₃O₄ 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.

2. Results and Discussion

2.1. Catalytic Performances of F-Fe₃O₄-r Microspheres

As shown in Figure 1a, fluorination significantly enhanced the decolorization rate (DR) of OG (0.1 mM) with H₂O₂ (40 mM) after reacting for 2 h at pH 6.5 and 40 °C over F-Fe₃O₄-r (e.g., the 2 h DR increased from 17.6% for Fe₃O₄-blank to 62.2% for F-Fe₃O₄-0.1). The best performance (96.8% of DR after 2 h) was obtained over F-Fe₃O₄-1. 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-Fe₃O₄-2 and 47.5% for F-Fe₃O₄-3), which may be related to the increase in the Na₃FeF₆ impurity in F-Fe₃O₄-r (r = 2, 3) [25]. By assuming that the instantaneous concentration of •OH is constant since the amount of H₂O₂ 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 (R²: 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(C₀/C_t)-time plots. The change of k_s1 with fluorination is similar to that of the DR (

Table 1. Effect of fluorination on catalytic performances of F-Fe₃O₄-r microspheres for the OG decolorization ¹.

Entry	r	15-min Adsorption (%)	2-h DR (%)	ks1/min−1 (R2)
1	0	5.9	17.6	0.0011 (0.997)
2	0.1	10.8	62.2	0.0070 (0.990)
3	1	12.3	96.8	0.0284 (0.991)
4	2	9.8	68.4	0.0092 (0.994)
5	3	7.8	47.5	0.0045 (0.991)
6 2	1	8.3	35.2	0.0026 (0.996)

¹ Reaction reactions were the same as Figure 1; ² Tert-butanol (10 mM) added.

). A 2.5-fold increment in k_s1 was achieved from Fe₃O₄-blank to F-Fe₃O₄-1 (0.0011 vs. 0.0284 min⁻¹) at 40 °C.

Shown in Figure 2a,b is the effect of the initial H₂O₂ concentration, [H₂O₂]₀, on the decolorization of OG at 40 °C. The decolorization efficiency is lower at lower [H₂O₂]₀—the 2 h DR and k_s1 value were only 65.9% and 0.0082 min⁻¹, respectively, at 20 mM of [H₂O₂]₀ in Table S1. On the other hand, higher [H₂O₂]₀ was not beneficial either—the 2 h DE and k_s1 value were 90.3% and 0.0182 min⁻¹, respectively, at 80 mM of [H₂O₂]₀. The optimal [H₂O₂]₀ was about 40–60 mM for efficient decolorization of OG (0.1 mM, pH 6.5) on F-Fe₃O₄-1 at 40 °C (the 2 h DE and k_s1 value were ~97% and 0.030 min⁻¹, respectively). The existence of the optimal H₂O₂ concentration is due to the dual roles of H₂O₂. Firstly, it is the source of ⋅OH radicals for the OG degradation. Secondly, excess H₂O₂ scavenges ⋅OH radicals (Equation (2)) [26,27], thus lowering the concentration of ⋅OH radicals in the solution.

H₂O₂ + ⋅OH → HOO⋅ + H₂O

(2)

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.

The temperature effect on the OG decolorization is displayed in Figure 2e,f. It is obvious that the 2 h DE and k₁ 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 H₂O₂ to ⋅OH (213.8 kJ/mol) due to the catalytic effect of F-Fe₃O₄-1 [28] but higher than those obtained for the RhB degradation on Fe₃O₄ 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-Fe₃O₄-1 microspheres. However, it is interesting to note that the pre-exponential factor A in the reaction system of OG/F-Fe₃O₄-1 microspheres is five orders of magnitude larger than that obtained in RhB/Fe₃O₄ MNPs (6.88 × 10¹⁰ min⁻¹ vs. 8.26 × 10⁵ min⁻¹ [10]), which indicates that collision frequency between reacting species are greatly enhanced on F-Fe₃O₄-1 microspheres in agreement with the proposed adsorption-enhanced catalytic degradation of anionic molecules (vide infra).

To check the scope of the applicability of F-Fe₃O₄-1 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. CWPO of organic dyes over F-Fe₃O₄-r microspheres.

Organic Dyes	r	15-min Adsorption (%)	Degradation Rate (%)	ks1/min−1 (R2)
CR (100 mg/L, pH 6.8)	0	5.2	19.2	0.0012 (0.975)
	1	34.3	91.7	0.024 (0.976)
MB (0.01 mM, pH 6.5)	0	11.6	44.2	0.0036 (0.989)
	1	7.3	42.4	0.0040 (0.990)
RhB (0.01 mM, pH 6.6)	0	17.4	49.3	0.0039 (0.984)
	1	16.6	67.1	0.0080 (0.993)

. As shown in Figure 3a,b, the degradation of another anion dye CR was also significantly promoted on F-Fe₃O₄-1 than on Fe₃O₄-blank (the 2 h DR and k_s1 increased from 19.2% and 0.0012 min⁻¹ for Fe₃O₄-blank to 91.7% and 0.024 min⁻¹ for F-Fe₃O₄-1). However, the promotional effect of fluorination was not observed for cationic dye MB (2 h DR and k_s1: 44.2% and 0.0036 min⁻¹ on Fe₃O₄-blank versus 42.4% and 0.0040 min⁻¹ on F-Fe₃O₄-1) and became less obvious for the RhB dye (2 h DR and k_s1: 16.4% and 0.0039 min⁻¹ on Fe₃O₄-blank versus 67.5% and 0.0080 min⁻¹ on F-Fe₃O₄-1). The pKa value of RhB was reported to be 3.5 [31], 4.1 ± 0.1 [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-Fe₃O₄-1.

F-Fe₃O₄-1 maintained its high catalytic activity after continuously running for 16 h in recyclability tests (Figure 4 and

Table 3. Recyclability of F-Fe₃O₄-1 for catalytic degradation of OG with H₂O₂ ¹.

Run	2-h DR (%)	ks1/min−1 (R2)	COD Reduction Rate (%)
1	96.8	0.0284 (0.988)	83.2
2	92.7	0.0191 (0.978)	80.1
3	89.6	0.0178 (0.990)	75.5
4	89.1	0.0170 (0.988)	70.4
5	87.1	0.0163 (0.985)	68.6
6	86.4	0.0160 (0.990)	66.4
7	85.9	0.0157 (0.987)	66.1
8	84.9	0.0151 (0.987)	65.2

¹ Reaction conditions were the same as Figure 4.

). 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-Fe₃O₄-1. The low-to-moderate COD reduction rate (65.2–83.2%) indicated the formation of some organic intermediates during the degradation of OG with H₂O₂. These intermediates may cover some active sites on F-Fe₃O₄-1, resulting in a gradual reduction in the activity of reused F-Fe₃O₄-1.

2.2. Characterization of F-Fe₃O₄-r Microspheres

The F-Fe₃O₄-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-Fe₃O₄-r (r ≤ 1) consist of the single magnetite (PDF #19-0629) phase while F-Fe₃O₄-r (r = 2, 3) contain the Na₃FeF₆ (PDF #22-1381) impurity. Unless otherwise specified, the fluorinated Fe₃O₄ in this work referred to F-Fe₃O₄-1 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 Fe₃O₄-blank, F-Fe₃O₄-1 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 m²/g) (Figure 3 in Reference [25]). However, there is no obvious change in the magnetic properties of Fe₃O₄ upon fluorination (Figure 4 in Reference [25]), which is beneficial for the magnetic separation and recycling of the F-Fe₃O₄-1 microspheres.

2.3. 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-Fe₃O₄-1 with 0.65 mg/L of F⁻ or 0.15 mg/L of Fe²⁺ (or Fe³⁺) under otherwise identical conditions (Figure 5), which supported the notion that the decolorization of OG with H₂O₂ was catalyzed by heterogeneous F-Fe₃O₄-1 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 H₂O₂ at pH 6.5 over F-Fe₃O₄-1. 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-Fe₃O₄-1 and Fe₃O₄-blank, supporting the formation of ·OH radicals in the H₂O₂/F-Fe₃O₄-1 (or Fe₃O₄-blank) reaction system. However, the signal intensities for F-Fe₃O₄-1 were obviously stronger than those for Fe₃O₄-blank, indicating the higher activity of the former to activate H₂O₂ due to the larger number of Fe²⁺ ions generated after fluorination of F-Fe₃O₄-1 [25].

The improved catalytic activity of F-Fe₃O₄-1 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-Fe₃O₄-1 and Fe₃O₄-blank were determined to be 8.7 and 4.9, respectively. The higher IEP (8.7) of F-Fe₃O₄-1 was attributed to the partial substitution of F⁻ for lattice O²⁻ in Fe₃O₄. As a result, the surface of F-Fe₃O₄-1 was positively charged, while Fe₃O₄-blank was negatively charged at pH ~7. Therefore, the adsorption of anionic dye molecules on F-Fe₃O₄-1 was facilitated via electrostatic attraction but unfavorable on Fe₃O₄-blank at pH ~7 due to electrostatic repulsion (e.g., the 15-min adsorption of OG was 5.9% on Fe₃O₄-blank vs. 12.3% on F-Fe₃O₄-1). 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/H₂O₂ systems [36].

3. Materials and Methods

3.1. Chemicals and Regents

All chemicals, including FeCl₃·6H₂O, NaOAc, NaF, ethylene glycol, orange G (OG), congo Red (CR), methylene blue (MB), rhodamine B (RhB), and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), were of analytic grade or above and used as received. All aqueous solutions were prepared with deionized (DI) water (<20 μS/cm) produced from Heal Force NW 15VF.

3.2. Synthesis of Fluoride-Modified Fe₃O₄ Microspheres

The fluoride-modified Fe₃O₄ microspheres were made via glycothermal synthesis according to the reported method [25] and named as F-Fe₃O₄-r, where r indicates the nominal F/Fe molar ratio (r: 0–3). The product made at r = 0 was unmodified (denoted Fe₃O₄-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 FeCl₃·6H₂O (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-Fe₃O₄-r black particles.

3.3. CWPO of Organic Dyes

A general procedure of CWPO was as follows: A mixture of 0.025 g of F-Fe₃O₄-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 (C₀). The degradation of OG was initiated by rapidly adding 0.230 g of H₂O₂ (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-Fe₃O₄-1 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.

3.4. 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. N₂ adsorption–desorption 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/P₀) 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.

4. Conclusions

Fluorination increased the isoelectric point from 4.9 for undoped Fe₃O₄-blank microspheres to 8.7 for F-Fe₃O₄-1 with 7.1 ± 1.4 wt% of fluoride. In comparison with Fe₃O₄-blank, F-Fe₃O₄-1 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-Fe₃O₄-1 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/H₂O₂ systems.