Magnetically Recyclable Carbon-Nitride-Wrapped Nano-Fe⁰ as Active Catalyst for Acid Red G Dye Decoloration

Abstract

Heterogeneous catalytic degradation of organic dyes can effectively achieve the goals of reducing the chromaticity of aqueous solutions and completely removing pollutants. We here present a carbon-nitride-wrapped zero-valent Fe catalyst (CNFe), which can directly degrade Acid Red G (ARG) dye without additional oxidants. CNFe exhibited a nanotube-like morphology, wherein the zero-valent Fe (Fe⁰) was wrapped by a carbon layer to effectively enhance its dispersibility and prevent its oxidative deactivation. Meanwhile, the large specific surface area (169.19 m²/g), along with abundant active sites such as Fe and O, endowed CNFe with excellent activity. Under strongly acidic conditions, even in the presence of various anions, CNFe can still remove approximately 91.6% of ARG within 30 min. In a 10 h continuous flow column experiment, the removal efficiency of ARG consistently exceeded 67.6%, indicating that CNFe had great potential for treating actual dyeing wastewater. Catalytic mechanism studies showed that, under neutral conditions, CNFe mainly removed ARG through adsorption, whereas, under acidic conditions, the Fe⁰ in CNFe can not only activate molecular oxygen to generate HO· for the oxidative degradation of ARG but also remove ARG via reduction. Furthermore, CNFe can adsorb ARG through hydrogen bonding of surface hydroxyl groups. The developmental toxicity of the generated intermediates was effectively reduced, demonstrating lower environmental risks. Therefore, this study provided a simple, high-efficiency, and economical method for removing dyes from water, which can offer guidance for the treatment of practical dye wastewater.

1. Introduction

Dye wastewater is characterized by substantial organic content, high chroma, and difficulty in degradation [1]. When discharged into water bodies, its high chroma blocks light, inhibiting algal photosynthesis [2]. The high organic matter concentration will consume the oxygen in water, causing hypoxia and death of aquatic organisms. Toxic dyes pose a serious threat not only to living organisms but also to human health [3]. Typical approaches for treating dye-containing wastewater encompass adsorption [4,5], biological degradation [6,7], and chemical oxidation [8,9]. Among these, adsorption fails to completely degrade pollutants. Biological methods are sensitive to toxic dyes and struggle with azo/anthraquinone dyes [10]. Conventional chemical oxidation (e.g., Fenton) has strong oxidizing power to degrade organic dyestuff but consumes large reagent and easily generates secondary pollution (e.g., iron sludge) [6]. In contrast, heterogeneous catalytic degradation can not only efficiently degrade organic dyestuff but also avoid sludge generation [11,12]. Therefore, heterogeneous catalytic degradation of organic pollutants has become a research hotspot.

Recently, a growing number of studies have highlighted the application of catalysts to trigger the activation of oxidants (e.g., hydrogen peroxide, persulfates, or ozone) for degrading organic dyes in water [13,14,15]. However, this process required the additional addition of large amounts of oxidants, which increased the treatment cost. If catalysts can be used to directly degrade organic dyes without adding oxidants, it would be more cost-effective. Zero-valent iron (Fe⁰) can not only degrade pollutants by activating oxidants but also directly act on organic pollutants to cause their degradation [16]. Yu et al. [17] have demonstrated through column experiments that using Fe⁰ to remove nitroguanidine from water was a feasible strategy. The removal rates of lignin, cellulose and hemicellulose by Fe⁰ can reach 20.77%, 30.35% and 44.7%, respectively [18]. Starch-modified nanoscale Fe⁰ can remove tetracycline from water through flocculation, adsorption, and degradation [19].

Because of magnetic forces and high surface energy, however, Fe⁰ particles were prone to adhering to each other, forming large-sized aggregates that led to decreased reactivity [20,21]. The addition of supports such as biopolymers, bentonite, biochar, graphene, etc., can enhance the stability of Fe⁰ nanoparticles [22]. Chen et al. [23] modified Fe⁰ with polyvinylpyrrolidone to prepare highly dispersed Fe⁰ nanoparticles. The obtained catalysts exhibited excellent removal ability toward TC compared to the parent Fe⁰. Additionally, the surface of Fe⁰ was susceptible to being oxidized, which also caused a reduction in activity [24,25]. Due to the surface oxidation of Fe⁰ during repeated use, the ciprofloxacin removal rate significantly declined to about 30% [26]. To address this, coating Fe⁰ with a zero-valent copper (Cu⁰) layer could suppress Fe⁰ oxidation [27]. Notably, recycling tests revealed that the resulting Fe⁰-Cu⁰ catalyst retained both superior activity and outstanding durability. Coating the exterior of Fe⁰ with a carbon layer could prevent Fe⁰ particle agglomeration and hinder Fe⁰ oxidation, thereby enhancing its activity and stability [28]. Studies have demonstrated that nitrogen-doped carbon materials were capable of tuning the coordination environment of doped metals, subsequently adjusting the charge transfer between the carbon support and metals [29]. Notably, the N atoms in carbon nitride exhibit a strong coordination with Fe, which can effectively inhibit the oxidation and dissolution of Fe⁰ [30]. Drawing inspiration from this, the catalyst obtained by coating carbon nitride on the surface of Fe⁰ may exhibit excellent activity in organic pollutants degradation. The outer carbon nitride coating on Fe⁰ might not only inhibit particle aggregation but also suppress Fe⁰ oxidation.

In this study, a carbon-nitride-coated Fe⁰ catalyst (CNFe) was constructed. Comprehensive characterization of its physicochemical properties was performed using XRD, SEM, TEM, BET, and XPS. Subsequently, a comprehensive investigation was performed using Acid Red G (ARG) as the target contaminant to examine the influences of solution pH, catalyst amount, pollutant concentration, and coexisting ions on ARG removal efficiency. Furthermore, the practical applicability of CNFe for removing pollutants was explored via continuous-flow column experiments. Finally, the mechanism of ARG removal by CNFe, as well as the degradation pathways and toxicity of intermediate products, were studied.

2. Results and Discussion

2.1. Characterization of CNFe

Figure 1a illustrates the XRD patterns of CNFe. A wide diffraction peak centered at 26.23° attributed to the graphitic carbon (002) plane [31]. Moreover, a series of peaks at 44.65°, 64.99° and 82.33° corresponded to (110), (200) and (211) planes of Fe⁰, respectively [32]. It indicated that the prepared catalyst was a composite of carbon nitride and Fe⁰. As depicted in Figure 1b, the magnetic hysteresis curve of CNFe revealed a saturation magnetization value of about 74.8 emu/g. The presence of a clear hysteresis loop demonstrated that CNFe exhibited magnetic behavior [33], allowing for easy separation under an external magnetic field. This characteristic was beneficial for catalyst recovery in practical applications. Figure 1c shows the N₂ adsorption–desorption isotherms of CNFe. According to the classification of IUPAC, the isotherms of CNFe belonged to type-IV pattern with a distinct hysteresis loop, confirming abundant mesopores in CNFe [34]. This aligned with the pore diameter distribution curves (Figure 1d). The specific surface area (S_BET) of CNFe was 169.19 m²/g, and the external surface area (S_exter) was 165.36 m²/g, which were 5.06 and 1.92 times that of CN, respectively (

Table 1. Texture properties of CNFe before and after reaction.

Catalyst	SBET m2/g	Sexter m2/g	Vtotal cm3/g	Vmicro cm3/g	Vmeso cm3/g	D nm
CNFe	169.19	165.36	0.34	0.00067	0.34	7.99
CNFe Used	161.83	156.90	0.38	0.00035	0.38	9.34

and Table S1). Moreover, the average pore diameter (D) was 7.99 nm. Even after reaction for ARG removal, the specific surface area of CNFe only slightly decreased to 161.83 m²/g. It was noteworthy that the total volume and average pore diameter increased after the reaction. Due to the blockage of the micropores in catalyst by adsorbed pollutants during the reaction, the micropore volume of the catalyst decreased. The reduction in micropores led to an increase in the average pore diameter. Regarding the increase in total pore volume, it is likely due to the formation of new mesopores between the adsorbed pollutants on the catalyst. However, the total pore volume only increased slightly from 0.34 cm³/g to 0.38 cm³/g, and it is speculated that this was most likely caused by instrumental testing errors. Overall, the large S_BET, S_exter, and D increased the accessibility of active sites and facilitated the transformation of reactants inside the material [35].

Figure 2 displays the SEM images of CNFe. CNFe was composed of many nanotubes (Figure 2a). For CN, it exhibits a very smooth surface (Figure S2). The constituent elements of CNFe, including C, N, and O, were homogeneously dispersed (Figure 2c–e), while, for Fe, it was present as nanoparticles (Figure 2f). From the TEM image in Figure 2g, it can also be seen that many nanotubes existed in CNFe. A lattice fringe spacing of 0.202 nm was measured, in exact agreement with the (111) plane of Fe⁰ (Figure 2h) [36]. In addition, the nano-Fe⁰ crystals were encapsulated within the carbon layers (Figure 2i), which could restrain the aggregation of nano-Fe⁰.

Figure 3 illustrates the high-resolution XPS spectra for Fe 2p and O 1s. For the Fe 2p spectrum (Figure 3a), two main peaks were observed: 2p^1/2 and 2p^3/2. Both peaks were composed of components such as Fe⁰ (707.47 eV and 719.44 eV), Fe₃C (709.75 eV and 721.28 eV), Fe²⁺ (711.46 eV and 724.1 eV), Fe³⁺ (713.12 eV and 725.96 eV), and satellite peaks (715.38 eV and 728.81 eV). Abundant Fe⁰, Fe²⁺, and Fe³⁺ endowed CNFe with good magnetic properties, which aligned with previous XRD and magnetic test results. Additionally, the O 1s spectrum of CNFe in Figure 3b indicated that the surface was mainly composed of –OH and C–O groups. Abundant O-containing functional groups might contribute to pollutant removal [37].

The elemental composition of the catalyst is shown in Table S2. It is widely accepted that X-ray Photoelectron Spectroscopy (XPS) exclusively probes the surface elemental composition of a sample, whereas Inductively Coupled Plasma (ICP) spectrometry quantifies its bulk elemental content. Notably, the Fe content on the catalyst surface was significantly lower than the total Fe content, indicating that Fe was predominantly located within the catalyst interior. This further confirms that Fe was encapsulated by carbon nitride.

2.2. Removal of ARG

Figure 4a presents the influence of solution pH on CNFe-mediated ARG removal. At pH = 2, ARG removal was effective, with an efficiency of 91.6% attained within 30 min. The performance, however, declined remarkedly when the solution pH rose above 4. The removal rate in CNFe/ARG system at pH = 2 reached 0.21 min⁻¹ (Figure S3), which was 5.18 times higher than that at pH = 12. It was speculated that, under highly acidic conditions, the catalyst surface acquired positive charges, promoting the removal of the negatively charged ARG [38]. In contrast, the removal efficiencies of ARG by CN at pH = 2 and 6 were only 11.0% and 3.03% after 30 min, respectively (Figure S4). It is speculated that Fe⁰ in CNFe played a major role in ARG removal, rather than carbon nitride. In addition, it may due to the corrosion of Fe⁰ and generation of hydroxyl radicals (HO·) under strong acid conditions [39]. The generated HO· could effectively degrade ARG.

Figure 4b,c display the effect of catalyst dosage and ARG concentration on ARG removal, respectively. A higher catalyst dosage provided more active sites, thereby increasing removal efficiencies (Figure 4b). In contrast, when the concentration of ARG increased and the catalyst dosage remained constant, the active sites were relatively insufficient, resulting in a gradual decrease in removal efficiency (Figure 4c).

The presence of co-existing ions in the solution may inhibit ARG removal by competing for active sites, leading to a lower removal efficiency. Figure 4d demonstrates the impact of the co-existing ions on ARG removal. Despite the presence of coexisting ions (NO₃⁻, SO₄²⁻, Cl⁻, H₂PO₄⁻ and HCO₃⁻), the removal efficiencies of ARG were comparable to the control, indicating that the coexisting ions have no effect on the removal of ARG by CNFe.

To investigate the potential of CNFe in practical applications, a column experiment was conducted for continuous ARG wastewater treatment, with results presented in Figure 5. After treatment, the red solution turned colorless, and the removal efficiency of ARG reached approximately 95%, indicating that the catalyst can effectively treat wastewater and achieve decolorization. Moreover, after 10 h of continuous operation, the removal efficiency of ARG still exceeded 67.6%, further demonstrating that using CNFe for ARG wastewater treatment is a feasible strategy.

2.3. Pathways of ARG Degradation and Toxicity of the Intermediates

At pH = 6, the solution color faded gradually but stayed red throughout the experiment (Figure 6a). While at pH = 2, the color first changed from red to yellow, then to colorless (Figure 6b). The change in solution color indicated that ARG may have been converted into other substances. To reveal whether CNFe removed ARG through adsorption or degradation, variations in the UV–Vis absorption spectra of ARG during the reaction process at pH = 2 and pH = 6 were monitored. At pH = 6, as the reaction proceeded, the UV–Vis absorption spectrum of ARG gradually decreased while the spectral shape remained unchanged (Figure 6c). Moreover, the trend of change in ARG and total organic carbon (TOC) concentration was highly consistent (Figure 6d), indicating that ARG was primarily removed via adsorption under this condition. However, when the reaction solution pH was 2, the UV–Vis absorption spectrum of ARG rapidly decreased as the reaction proceeded and, notably, the spectral shape changed significantly (Figure 6e). Especially, the relative intensities of the two shoulder peaks in the 500–550 nm range undergo significant variation. This indicated that the molecular structure of ARG may change under this condition. Nevertheless, the TOC removal efficiency was substantially lower than ARG removal efficiency (Figure 6f). It suggested that, at pH = 2, the ARG removal process may include degradation.

Therefore, the components of the post-reaction solution at pH = 2 were analyzed using a HPLC-MS instrument. A sum of 13 intermediates was identified (Table S3). Figure 7 illustrates the proposed degradation pathway of ARG. The degradation of ARG primarily proceeded via three pathways, with the first step in all cases being the breakage of the azo bond (–N = N–). It was noteworthy that the chromaticity (color-forming properties) of azo dyes mainly originates from their core functional group (azo group), as well as the extended conjugated π system formed between the azo group and the aromatic rings [40]. The cleavage of the azo bond caused the ARG solution to change from red to colorless. Subsequently, the bonds including C–N, C–O, and C–S in the intermediate products were vulnerable to cleavage by reactive species [28]. Benzene rings underwent ring-opening reactions upon attack by reactive species. Therefore, ARG was progressively broken down into small molecules and ultimately completely mineralized. After reaction for 30 min, about 45.8% of ARG was completely removed (Figure 6f).

The developmental toxicity of organic compounds is often used to assess their toxicity [41]. Thus, a comparative study was conducted on the developmental toxicity of intermediates and ARG. Figure 8 shows that, except for intermediate P4, all remaining intermediates exhibited lower developmental toxicity than ARG, indicating that the treatment by CNFe could not only reduce the chromaticity of the solution but also significantly reduce the solution toxicity.

2.4. Reaction Mechanism of CNFe

To identify the dominant reactive species during CNFe-mediated ARG removal, reactive species quenching experiments were conducted. Tert-butanol is a common quencher of hydroxyl radicals (HO·). Upon addition of tert-butanol to the reaction solution, the ARG removal rate declined markedly, with the removal efficiency dropping from 91.8% to 86.8% after reaction for 30 min (Figure 9). This indicated that HO· played a certain role in ARG removal. To reveal the source of reactive radicals, the XPS spectra of CNFe pre- and post-reaction were comparatively studied to identify the main active sites in CNFe. After reaction, the content of Fe⁰ in the catalyst decreased from 23.52% to 6.93%, whereas the contents of Fe²⁺ and Fe³⁺ increased from 23.48% to 24.74% and 28.09% to 35.82%, respectively (Table S4). Moreover, the diffraction peak intensity of Fe⁰ in the XRD patterns significantly decreased (Figure 1a). It indicated that Fe⁰ was the main active site in CNFe. It was reported that a lower solution pH accelerated Fe⁰ corrosion, leading to the release of additional electrons. Meanwhile, dissolved O₂ can capture these electrons and generate hydrogen peroxide (H₂O₂) (Equation (1)) [42,43]. Furthermore, the Fe²⁺ produced from the corrosion of Fe⁰ further activated H₂O₂ to produce HO· (Equation (2)), which can effectively degrade ARG in water [44]. As shown in Figure 3b, the O 1s spectrum revealed the formation of a new Fe–O bond post-reaction, which further confirmed the corrosion of Fe⁰ during the reaction. To further verify this hypothesis, citric acid was used to complex iron ions, inhibiting their activation of the generated H₂O₂. After adding citric acid, the removal efficiency of ARG decreased to 74.4%. Thus, this result confirmed the process of Fe⁰ corrosion and HO·-mediated degradation of ARG under acidic conditions. In addition, the corrosion of Fe⁰ released abundant electrons, which may promote the reductive removal of ARG. A strong oxidizing agent, potassium dichromate, was added to the solution, which can capture electrons generated from the corrosion of Fe⁰. After adding potassium dichromate, the removal of ARG was significantly inhibited, demonstrating that CNFe can also remove ARG through reduction. According to the detected intermediate products, it was also evident that aromatic rings in many products were transformed into aliphatic rings, further confirming the mechanism of reductive action of CNFe on ARG. In addition, after the reaction, the binding energy of –OH peak in CNFe shifted from 531.67 to 532.11 eV, owing to the hydrogen bonding interaction between –OH and ARG [45]. Therefore, CNFe can also remove ARG from water through adsorption.

Fe⁰ + O₂ +2H⁺ → H₂O₂ + Fe²⁺

(1)

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + HO

(2)

3. Experimental

3.1. Chemicals

Melamine, glycine, ferric chloride hexahydrate, sodium hydroxide, hydrochloric acid, methanol, tert butyl alcohol, citric acid, potassium dichromate, sodium dihydrogen phosphate, sodium sulfate, sodium chloride, and sodium bicarbonate were of analytical purity and sourced from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Synthesis of Carbon-Nitride-Wrapped Nano-Fe⁰ (CNFe)

The carbon-nitride-wrapped nano-Fe⁰ (CNFe) was synthesized according to our previous study [46]. Firstly, melamine (4.00 g), glycine (1.00 g) and ferric chloride hexahydrate (1.00 g) were mixed and grinded adequately to obtain a uniformly mixed catalyst precursor. Subsequently, catalyst precursor underwent pyrolysis at 700 °C for 2 h in a N₂ atmosphere to obtain CNFe. For comparison, carbon nitride (CN) was prepared using the same method as above but without adding an Fe source.

3.3. Characterization

An XRD instrument (Smartlab9, Rigaku Corporation, Tokyo, Japan) was used to determine the crystal structure of CNFe, employing a scan rate of 10°/min and a scanning range of 10–90°. The microstructure of CNFe was detected using SEM (S 4800, Hitachi, Tokyo, Japan) and TEM (Tecnai G2 F 20, FEI, Hillsboro, OR, USA). Characterization of the texture properties was performed on a physical adsorption apparatus (ASAP 2460, Micromeritics, Norcross, GA, USA). The magnetism was characterized via a magnetometer (735 VSM Controller, LakeShore, Westerville, OH, USA) under a magnetic field ranging from −20,000 to 20,000 Oe. Additionally, XPS (K-Alpha⁺, ThermoFisher, Waltham, MA, USA) was employed to characterize the chemical bonds and surface functional groups of the catalyst. The binding energies of each element were revised according to the C 1s peak (284.8 eV).

3.4. ARG Removal Experiment

Typically, CNFe (20 mg) was introduced into ARG solution (100 mL, 50 mg/L) and stirred at 500 rpm. At predetermined intervals, samples (3 mL) were withdrawn, filtered through a membrane (0.22 µm), and their absorbance was analyzed at 504 nm using a UV–visible spectrophotometer (UV752, Yoke Instrument, Shanghai, China). The ARG removal efficiency was represented by C/C₀. Additionally, the removal rate was obtained by fitting the removal efficiency data using a pseudo-first-order kinetic model (Equation (3)).

$$\ln {\displaystyle \frac{C}{C_0}}=-kt$$

(3)

where C is the concentration of ARG at t min, mg/L; C₀ is the initial concentration of ARG, mg/L; and k is the rate constant of the reaction, min⁻¹.

In the pH effect experiment, the pH of the ARG solution was tuned to 2, 4, 6, 8, 10, and 12 with NaOH (0.1 M) or HCl (0.1 M), after which CNFe was added to study the change in ARG concentration over time. In the catalyst dosage effect experiment, the dosage of CNFe was adjusted to 0.05, 0.10, and 0.20 g/L, respectively. In the pollutant concentration effect experiment, the ARG concentration was fixed at 10, 20, and 50 mg/L, respectively. In the coexisting ion effect experiment, 20 mM sodium salts containing Cl⁻, SO₄²⁻, NO₃⁻, HCO₃⁻, or H₂PO₄⁻ were added to the ARG solution to study the pollutant concentration change over time. Except for the pollutant concentration effect experiment, the ARG concentration in all other experiments was 50 mg/L. In addition, the performance of catalyst towards ARG removal was also evaluated by the dynamic column experiment. Specifically, the experiment was conducted with 30 mg of catalyst, 10 mg/L ARG concentration, and a solution flow rate of 0.55 mL/min. The mineralization rate of ARG was determined using a total organic carbon analyzer (TOC-L CSN, Shimadzu, Kyoto, Japan). Reaction-generated intermediates were examined via a HPLC-MS (Xevo G2-XS QTof, Waters, Milford, CT, USA). To assess toxicity evolution, the developmental toxicity of both ARG and the detected intermediate products was comparably assessed using the Toxicity Estimation Software Tool (TEST).

4. Conclusions

To sum up, an Fe⁰ catalyst wrapped with carbon nitride (CNFe) was fabricated by one-step pyrolysis. The nanotube-like structure endowed it with a large S_BET (169.19 m²/g), S_exter (165.36 m²/g) and D (7.99 nm). Abundant Fe- and O-containing active sites endowed it with excellent activity. A lower solution pH corresponded to higher catalyst activity. At pH = 2, the ARG removal efficiency achieved approximately 91.6%. Even in the presence of coexisting ions, ARG could still be effectively removed. In the 10 h column experiment, the removal efficiency of ARG remained above 67.6% throughout. Under strongly acidic conditions, CNFe can activate molecular oxygen to generate HO·, thereby oxidizing and removing ARG from water. Additionally, ARG could also be removed by CNFe through reduction and adsorption. Thirteen intermediate products were generated during the reaction, of which only one exhibited higher developmental toxicity than the original ARG, indicating a significant reduction in solution toxicity post-treatment. Therefore, this study provided a method for efficiently removing dyes from water without adding oxidants, which was expected to serve as a reference for treating practical dye wastewater.