Facile Synthesis of n-Fe₃O₄/ACF Functional Cathode for Efficient Dye Degradation through Heterogeneous E-Fenton Process

Abstract

In order to put forward an efficient and eco-friendly approach to degrade dye-containing industrial effluents, an n-Fe₃O₄/ACF nanocomposite was synthesized using the facile precipitation method and applied as a functional cathode for a heterogeneous electro-Fenton (E-Fenton) process. In particular, optimal initial pH value, current density, pole plate spacing, and electrode area were confirmed through systematical experiments as 5.73, 30 mA/cm², 3 cm, and 2 × 2 cm², respectively. Under such optimal reaction conditions, 98% of the methylene blue (MB) was degraded by n-Fe₃O₄/ACF after 2 h of E-Fenton treatment. In addition, n-Fe₃O₄/ACF could still decolor about 90% of the methylene blue (MB) for five rounds with some reductions in efficiency. Furthermore, results of electrochemical impedance spectroscopy and heterogeneous E-Fenton performance tests indicated that the loading of metal material Fe₃O₄ could enhance the overall electron transport capacity, which could accelerate the whole degradation processes. Moreover, the rich pores and large specific surface area of n-Fe₃O₄/ACF provided many active sites, which could greatly improve the efficiency of O₂ reduction, promote the generation of H₂O₂, and shorten the reaction length between •OH and the pollutant groups.

1. Introduction

Recently, dye-containing industrial effluent degradation has attracted more and more attention because the effluents cannot be easily decomposed by conventional methods [1] such as coagulation, precipitation, flotation, adsorption, membrane processes, and reverse osmosis. Therefore, researchers are committed to developing efficient, multipurpose, and environmentally friendly wastewater treatment approaches [2,3]. The heterogeneous electro-Fenton (E-Fenton) method is an advanced oxidation process (AOP) that has been successfully applied to degrade many organic pollutant groups [4,5,6,7]. The highly oxidative and non-selective hydroxyl radicals (•OH) generated during the heterogeneous E-Fenton process are the key, as they can react with organic pollutant groups to decompose these pollutants into CO₂, water, and inorganic ions [8,9,10]. During the heterogeneous E-Fenton process, H₂O₂ is in situ-generated by the cathodic reduction of oxygen (Equation (1)) [11], which then reacts with the Fe²⁺ (Equation (2)) to form highly reactive •OH [12]. In addition, the Fe³⁺ ions formed as a result of the Fenton reaction are constantly reduced to Fe²⁺ on the cathode surface (Equation (3)); thus, they contribute further to hydroxyl radical production [13,14,15].

$${{\displaystyle O}}_2+{{\displaystyle 2H}}^{+}+{{\displaystyle 2e}}^{-}\to {{\displaystyle H}}_2{{\displaystyle O}}_2$$

(1)

$${{\displaystyle F\mathrm{e}}}^{2+}+{{\displaystyle H}}_2{{\displaystyle O}}_2\to {{\displaystyle Fe}}^{3+}+•{{\displaystyle OH}}^{}+{{\displaystyle OH}}^{-}$$

(2)

$${{\displaystyle F\mathrm{e}}}^{3+}+{{\displaystyle e}}^{-}\to {{\displaystyle Fe}}^{2+}$$

(3)

In recent years, scientists have devoted themselves to developing Fe-containing standalone carbonaceous cathodes for heterogeneous E-Fenton processes. Fe-containing carbonaceous cathodes are comprised of two main parts: (1) a carbonaceous substrate used to conduct electricity and generate H₂O₂; and (2) Fe-containing catalysts immobilized on the carbonaceous substrate aimed at the formation of •OH [16]. On the one hand, carbonaceous substrates such as activated carbon [17], graphene [18], carbon nanotubes (CNTs) [19], and carbon aerogels (CAs) [20] are promising candidates owing to their superior chemical and physical stability, high over-potential for hydrogen evolution reaction, and inertness toward electro-generated H₂O₂. Among them, activated carbon fiber (ACF) has attracted great interests because of its excellent conductivity, high adsorption capacity, and multi-functional catalytic activity [21]. Furthermore, the superior mechanical integrity of ACF allows it to serve as a stable electrode to generate hydrogen peroxide by the reduction of O₂ on its surface [13]. On the other hand, a great deal of attention has been paid to the magnetic iron oxides, such as Fe₃O₄ [22], Fe₂O₃ [23], FeOOH [22], and NiFe₂O₄ [24]. These magnetic iron oxides are distinguished from other catalyst due to their strong recycling performance, large surface area, non-toxicity, eco-friendliness, and stability in a wide range of pH values and temperatures [25]. Among them, magnetite (Fe₃O₄) is considered to be an environmentally friendly and inexpensive catalyst in heterogeneous E-Fenton systems [26,27] because ferrous and ferric ions in Fe₃O₄ can activate H₂O₂ and generate •OH, according to Equation (2) [28]. Furthermore, Fe₃O₄ can be applied at basic and near neutral pH values, which makes the process easy to operate and avoids sludge generation and the need to recycle the catalysts [29,30].

Herein, we precipitated Fe₃O₄ nanoparticles on activated carbon fiber (ACF) as a nanocomposite (n-Fe₃O₄/ACF) through the facile precipitation method and applied it as a functional cathode for the heterogeneous E-Fenton process. In addition, the morphology, microstructure, and element composition features of n-Fe₃O₄/ACF were systematically characterized by SEM, TEM, XRD, and XPS. In particular, the optimal reaction conditions, such as initial pH value, current density, pole plate spacing, and electrode area, were confirmed by comparing the COD removal rate of degrading methylene blue (MB) at different conditions. Furthermore, cycling tests of n-Fe₃O₄/ACF were implemented to evaluate its degradation efficiency decay after five rounds of E-Fenton treatment. Furthermore, electrochemical impedance spectroscopy (EIS) was performed, and a schematic degradation mechanism diagram was proposed to reveal the methylene blue (MB) removal mechanism of n-Fe₃O₄/ACF.

In order to put forward an efficient and eco-friendly approach to degrade dye-containing industrial effluents, an n-Fe₃O₄/ACF nanocomposite was synthesized through the facile precipitation method and applied as a functional cathode for the heterogeneous electro-Fenton (E-Fenton) process. Notably, the optimal initial pH value, current density, pole plate spacing, and electrode area were confirmed through systematically experiments as 5.73, 30 mA/cm², 3 cm, and 2 × 2 cm², respectively. Moreover, n-Fe₃O₄/ACF could still decolor about 90% of MB for five rounds with few efficiency decays. Furthermore, results of electrochemical impedance spectroscopy and heterogeneous E-Fenton performance tests indicate that compositing Fe₃O₄ nanoparticles with ACF as functional cathodes could dramatically increase the COD removal of methylene blue with the help of rich pores and the large specific surface area of n-Fe₃O₄/ACF to promote the generation of H₂O₂ and shorten the reaction length between •OH and pollutant organics.

2. Results and Discussion

SEM and EDS were firstly implemented to investigate the surface topography and microstructure of n-Fe₃O₄/ACF. Different magnification SEM images (Figure 1a–c) showed that n-Fe₃O₄/ACF consists of activated carbon fiber filaments roughly 10 μm in diameter and Fe₃O₄ nanoparticles or loose clusters immobilized on the filaments. The composition of the nanocomposites was confirmed as C, Fe, and O according to the results of EDS (Figure 1d). In order to evaluate the specific surface area and pore structure of n-Fe₃O₄/ACF, N₂ adsorption–desorption and pore size analyses were carried out carefully. As shown in Figure 1e, the surface area (SBET) of n-Fe₃O₄/ACF was around 756.86 m²/g, and the pore size distribution was mainly microporous and mesoporous, which was very similar to activated carbon fiber. This shows that the Fe₃O₄ nanoparticles did not significantly change the structural characteristics of activated carbon fiber. In addition, according to IUPAC classification, the n-Fe₃O₄/ACF shows the characteristics of the type II adsorption isotherm H3 hysteresis loop, which reflects that the pore structure of the sample is generally formed by the loose accumulation of flake particles. The magnetic properties of n-Fe₃O₄/ACF were characterized by VSM (Figure 1f). When the applied magnetic field was removed, the magnetization of the sample was also changed to zero with no hysteresis, and its magnetization curve showed that its hysteresis loop is reversible, which showed that n-Fe₃O₄/ACF exhibited superparamagnetic properties at room temperature. The saturation magnetization of the n-Fe₃O₄/ACF composite was about Ms = 0.0035 emu/g when the magnetic field strength was 30,000 Oe. The low saturation magnetization level might be caused by the large amount of non-magnetic carbon in the n-Fe₃O₄/ACF materials.

According to the TEM images of the n-Fe₃O₄/ACF in Figure 2a, it is known that the surface of the sample consists of Fe₃O₄ with a diameter about 10 nm, and it is clear to see that there are multiple orientations of Fe₃O₄ crystals on the surface of the sample. Additionally, the electron diffraction pattern (Figure 2b) also proved that the sample is polycrystalline and composed of regular single crystals in multiple various directions. Owing to the tendency of Fe₃O₄ to grow linked together, the interaction between the dipoles of Fe₃O₄ nanoparticles is dominant, and the Fe₃O₄ particles tend to agglomerate and form chain-like or ring-like structures when Fe₃O₄ particles are in a certain small size, which is also consistent with that typically observed by SEM. Moreover, the interplanar spacing of the lattice fringes was around 0.2436 nm, which corresponds to the (111) facet of Fe₃O₄ [31]. As shown in Figure 2c, three elements including C, O, and Fe in the mapping layered image of n-Fe₃O₄/ACF prove that the element distribution is very uniform, and no element agglomeration occurs. The crystal phase of n-Fe₃O₄/ACF was further confirmed by X-ray diffraction: six broad and low peaks at 24.9° and 43.3°, and 30.1°, 35.2°, 55.9°, and 62.7°, corresponding to (002) and (100) and (220), (311), (511), and (400) of carbon and Fe₃O₄, respectively [32], which also proves that the n-Fe₃O₄/ACF is polycrystalline and composed of single crystals in multiple directions.

X-ray photoelectron spectroscopy (XPS) was used to characterize the composition and valence state of n-Fe₃O₄/ACF. As presented in Figure 3a, XPS survey spectra revealed the coexistence of Fe, C, and O in n-Fe₃O₄/ACF. As for the high-resolution spectrum of C 1s (Figure 3b), three characteristic peaks could be assigned to carbon atom sp² orbital hybridization (284.5 eV), C–O–C (286 eV), and O=C–O (288.5 eV), respectively. The peaks of Fe 2p 3/2 and Fe 2p1/2 are shown in Figure 3c, and two peaks were located at 711.2 eV and 724.9 eV, respectively, which is close to the standard XPS data of Fe₃O₄. In addition, Fe²⁺ and Fe³⁺ were determined from the peaks at 710.80 eV, 712.9 eV, 724.3 eV, and 726.27 eV, and the satellite peak simultaneously located near 718.7 eV was a γ-characteristic peak of Fe³⁺ in Fe₂O₃, indicating that Fe₃O₄ nanoparticles were partially oxidized. Moreover, Figure 3d shows the high-resolution spectrum of O 1s, and the three characteristic peaks at 532.9 eV, 531.4 eV, and 529.8 eV were assigned to C=O, C–O, and metal oxides, respectively [33]. The above results verified that the as-prepared sample was modified ACF with Fe₃O₄ nanoparticles.

In order to investigate the influences of the reaction conditions (initial pH of the reaction system, current density, plate spacing and electrode area) on the COD removal in the process of the heterogeneous E-Fenton system treating dye methylene blue (MB) with n-Fe₃O₄/ACF functionalized cathode catalysis, a single factor method was designed to obtain the best reaction conditions for a heterogeneous electro-Fenton system treating organic methylene blue dye. Firstly, the results of the effects of initial pH value on COD removal are shown in Figure 4a. For the COD removal, the maximum difference of the sample was about 10%, in the range of pH = 3–12. There was no dependence of the heterogeneous E-Fenton process on the acidic environment, as reported in the previous literature, indicating that n-Fe₃O₄/ACF showed high catalytic activity in a very wide pH range, thus overcoming the defect that the traditional homogeneous E-Fenton process is only applicable in a narrow pH range, and the most suitable initial pH value for future experiments was set at 5.73 (original pH value of methylene blue) for operational convenience and low cost of pre-treatment. Secondly, the influence of current density played an important role in COD removal (Figure 4b); different current densities (from 5 to 50 mA/cm²) were applied to study the effects on the COD removal. Apparently, the most suitable current density was 30 mA/cm², because too low a current density induced an insufficient generation rate of H₂O₂ on the cathode surface, and an excessive current density might trigger side electrolytic water reactions.

Thirdly, the results of the influence of different plate spacing is shown in Figure 4c. The COD removal of electrode plate spacing with 3 cm was 98%, which was higher than those of other electrode plate spacing values. When the plate spacing is too short, it will cause concentration polarization, which will reduce the amount of freely organic molecules and affect the mass transfer efficiency, and treatment efficiency of the heterogeneous E-Fenton will decrease. When the plate spacing is too long, the rates of electro transport mass transfer and diffusion mass transfer will decrease, which could also decrease the treatment efficiency of the heterogeneous E-Fenton process. Finally, when the modified activated carbon fiber is applied to the actual wastewater treatment, its degradation effect may be different due to the amplification effect. The effects of different electrode areas are shown in Figure 4d. The experimental group with the highest COD removal was 2 × 2 cm². In this experiment, the single factor method was used to analyze the influence of the electrode area, namely, 3 × 3 cm² and 4 × 4 cm², due to relative area of 2 × 2 cm² being larger, and its current density lower. Furthermore, the larger area may lead to an unbalanced catalyst load on its surface.

To verify the stable catalytic capacity of the n-Fe₃O₄/ACF cathode, a cycle test of methylene blue (MB) removal was performed under the above optimal conditions. Figure 5a shows that n-Fe₃O₄/ACF could still decolor about 90% of methylene blue (MB) for five rounds with few efficiency decays. The above results showed that n-Fe₃O₄/ACF as a functional cathode showed good stability in heterogeneous E-Fenton treatment of methylene blue (MB). In addition, electrochemical impedance spectroscopy (EIS) was performed to reveal the methylene blue (MB) removal mechanism of n-Fe₃O₄/ACF. As shown in Figure 5b, in the medium frequency region, the arc span-modified n-Fe₃O₄/ACF was smaller than that of pure ACF, indicating that the transmission and diffusion resistance of electrolyte ions are smaller in n-Fe₃O₄/ACF. In the high-frequency region, the intersection of the EIS curve and the z’ axis of n-Fe₃O₄/ACF was smaller than that of pure ACF, which also showed that the modified ACF had smaller equivalent resistance and stronger conductivity [34]. It could be seen that the loading of Fe₃O₄ metal material could enhance the overall electron transport capacity of the sample. The mechanism of dye removal by n-Fe₃O₄/ACF was discussed based on a literature review (Figure 5c) [13,17,32]. The rich pores and large specific surface area of n-Fe₃O₄/ACF provided many active sites, which could greatly improve the efficiency of O₂ reduction and promote the generation of H₂O₂ based on Equation (1). Furthermore, n-Fe₃O₄/ACF could maintain a stable concentration of Fe²⁺ through Equation (3) to aid in constant •OH formation based on Equation (2). Furthermore, the unique 3D nanostructure of n-Fe₃O₄/ACF could facilitate the adsorption capacity of methylene blue (MB) and intermediate products. Hence, active •OH free radicals could reach the pollutant organics efficiently on the surface of n-Fe₃O₄/ACF.

3. Materials and Methods

3.1. Raw Materials and Reagents

Activated carbon fiber (purity 99.9%) was purchased from Sinopharm Company (Beijing, China). The analytical pure reagents such as methylene blue (MB), hydrochloric acid (HCl), sodium sulfate (Na₂SO₄), ferric chloride hexahydrate (FeCl₃·6H₂O), ferrous sulfate heptahydrate (FeSO₄·7H₂O), sodium hydroxide (NaOH), and ammonium hydroxide (NH₃·H₂O) were purchased from China National Pharmaceutical Group Co., Ltd. (Beijing, China), and used without purification in the whole experiments. Deionized water was used in the whole experimental process.

3.2. Preparation of Activated Carbon Fibers

Activated carbon fiber (2 cm × 2 cm) was placed in a 250 mL beaker, and deionized water was added for ultrasonic washing 2–3 times to remove the residual impurities and ash on the surface, followed by the addition of 100 mL of 0.1 mol/L HCl solution to soak for 6–8 h, and then filtration, washing by deionized water to remove the residual acid, and oven drying at 80 °C to obtain the activated carbon fibers without further treatment.

3.3. Deposition of n-Fe₃O₄ on ACF

In total, 13.14 g FeCl₃ 6H₂O and 7.2 g FeSO₄ 7H₂O were dissolved in 450 mL ethanol aqueous solution (ethanol to water volume ratio was 3:7) at 70 °C with magnetic stirring for 30 min. Subsequently, the pretreated activated carbon was mixed with the solution. After further stirring for 30 min and heating to 75 °C with a water bath, 5 mol/L NaOH was added dropwise until the solution pH reached 10. After reaction for 60 min, the beaker was allowed to stand and age, and then it was cooled to room temperature, washed with deionized water, and dried in a preheated vacuum drying oven at 80 °C. Subsequently, the modified activated carbon fiber could be obtained.

3.4. Characterization

The morphology and microstructure of the samples were systematically investigated by scanning electron microscopy (SEM, Hitachi SU4800, Tokyo, Japan) and transmission electron microscopy (TEM, JEOL JEM 2100F, Tokyo, Japan). Furthermore, X-ray powder diffraction (XRD, Ultimo IV, Tokyo, Japan) was carried out using Cu Kα radiation over the measurement range of 5–90°. X-ray photoelectron spectra (XPS) were recorded by a Kratos AXIS SUPRA XPS system (Manchester, England).

3.5. Heterogeneous E-Fenton Performance Test

The E-Fenton performance of n-Fe₃O₄/ACF was evaluated by degrading the simulated dyeing wastewater (methylene blue (MB)). COD removal is calculated by a similar formula:

$$\mathrm{COD} \mathrm{removal} (\%)=\frac{{}_{{{\displaystyle COD}}_0{{\displaystyle -COD}}_t}}{{{\displaystyle COD}}_0}\times 100\%$$

where COD₀ is the initial chemical oxygen demand (mg/L), and COD_t is the chemical oxygen demand at t min in the reaction process (mg/L). The n-Fe₃O₄/ACF was used as a catalyst for the heterogeneous electro-Fenton oxidation system to treat simulated dyed wastewater. The experiment was carried out in a beaker with an effective volume of 250 mL. A Pt sheet electrode was used as an anode, the n-Fe₃O₄/ACF material was used as a functional cathode (both sizes were 2 cm × 2 cm), and Na₂SO₄ was used as the supporting electrolyte. After the addition of 200 mL methylene blue (MB) solution (100 mg/L) into the beaker, air was pumped into the solution at a flow rate of 200 mL/min to form a heterogeneous E-Fenton oxidation system. During the whole reaction period, the solution was stirred and homogenized with a magnetic stirrer, and the current was provided by an adjustable constant voltage DC power supply.

4. Conclusions

In conclusion, an n-Fe₃O₄/ACF nanocomposite was synthesized through the facile precipitation method, and its morphology, microstructure, and element composition features were systematically characterized by SEM, TEM, XRD, VSM, and XPS. More importantly, n-Fe₃O₄/ACF was applied as a functional cathode for the heterogeneous E-Fenton process. Notably, optimal initial pH value, current density, pole plate spacing, and electrode area were confirmed as 5.73, 30 mA/cm², 3 cm, and 2 × 2 cm², respectively, through systematically experiments. Moreover, 98% of methylene blue (MB) could be degraded by n-Fe₃O₄/ACF after 2 h of E-Fenton treatment, and there was less than 10% efficiency decay after the functional cathode was used for five rounds. Compositing Fe₃O₄ nanoparticles with ACF as functional cathodes could dramatically increase COD removal through two mechanisms. Firstly, loading of Fe₃O₄ metal material onto ACF can enhance the overall electron transport capacity, which accelerates the whole degradation processes. Secondly, active sites are furtherly provided by n-Fe₃O₄/ACF owing to their rich pores and large specific surface areas, which can greatly improve the efficiency of O₂ reduction, promote the generation of H₂O₂, and shorten the reaction length between •OH and pollutant groups. We hope our work will provide a new approach to degrading dye-containing industrial effluents efficiently and in an eco-friendly manner.