Effects of Sn Addition and Fe Content Adjustment on the Decolorization Performance of Fe-Si-B Amorphous Alloys for Azo Dyes

Abstract

This study compares the effects of adding Mo, Cu, and Sn elements on the decolorization performance of Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn) amorphous alloys. After the addition of Cu and Sn elements, the Fe-Si-B amorphous alloys generate three-dimensional (3D) petal-like nanostructured corrosion products during the decolorization process. These petal-like nanostructures possess a high specific surface area and excellent adsorption capacity, thereby effectively promoting the decolorization of dyes. Furthermore, the influence of Fe content variation on the decolorization performance of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) and Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) alloys was investigated. The glass-forming ability of Fe_77+xSi_13−xB₉Cu₁ alloys decreases with increasing Fe content, leading to the precipitation of α-Fe crystalline phases starting from Fe₇₉Si₁₁B₉Cu₁. As the crystallinity increases, the decolorization performance of the alloys gradually deteriorates. In contrast, the Fe_77+xSi_13−xB₉Sn₁ alloys maintain their amorphous structure even with increasing Fe content, and their decolorization performance for Orange II improves accordingly. The high decolorization efficiency of FeSiBSn amorphous alloys for Orange II can be attributed to their unique self-refreshing properties.

1. Introduction

Azo dyes, as predominant pollutants in textile, printing, and leather industries, are characterized by high chromaticity, toxicity, and recalcitrance, posing severe threats to ecosystems and human health upon wastewater discharge [1,2,3]. Traditional treatment methods include biological, physical adsorption, and chemical oxidation approaches. The biological method primarily utilizes microbial enzymes to break down the unsaturated bonds and chromophore groups within dye molecules, converting large molecules into smaller ones, ultimately achieving complete degradation of azo dyes [4,5]. However, the biological method exhibits poor adaptability to high-concentration dye wastewater and may generate intermediate products that are biologically toxic. The physical method mainly relies on differences in the physical properties of various substances to separate or collect them. Common physical methods include adsorption, membrane separation, magnetic separation, and extraction [6,7]. While physical methods are generally straightforward to operate, they are associated with high costs. The chemical method primarily involves chemical reactions with chromophore groups to cleave these groups, transforming large organic molecules into smaller ones, which are further degraded, ultimately leading to the mineralization of azo dyes. This approach includes methods such as electrochemical oxidation, photocatalytic oxidation, ozone oxidation, Fenton oxidation, and zero-valent iron reduction [8,9]. Oxidation methods typically require external inputs of electricity, light, or chemical reagents, which results in higher operational costs and substantial sludge production. Consequently, the development of efficient, stable, and environmentally benign treatment technologies has emerged as a research priority.

Zero-valent iron (ZVI, Fe⁰) reduction is one of the commonly used chemical methods for the degradation of azo dyes. The main reasons are its high production yield, low cost, and non-toxic, harmless nature to biological organisms. Fe-based amorphous alloys contain a large amount of metastable Fe⁰, which imparts high reactivity and energy. At the same time, the corrosion resistance of Fe-based amorphous alloys is superior to that of conventional crystalline ZVI, addressing the issue of easy passivation in crystalline ZVI [10,11,12]. Studies have shown that their efficiency in degrading azo dyes can be tens to hundreds of times greater than that of commercial iron powders [13]. This superior performance originates from abundant active sites and anti-passivation capabilities on the amorphous alloy surface, where the metastable structure enables continuous release of active Fe atoms, facilitating reduction reactions and free radical generation [14,15,16].

It is worth noting that, under normal circumstances, researchers aim to alter certain properties of the original alloy by introducing trace amounts of other metals or metalloid elements [17,18,19,20]. This approach is intended to enhance the degradation performance of azo dyes. When coupled with less-active metals, Fe-based amorphous alloys exhibit accelerated Fe⁰ oxidation rates via galvanic cell effects, thereby amplifying reactivity [21,22,23,24]. Yao et al. developed Fe-AR-NCI, a Fe-based amorphous ribbon with nanoscale compositional heterogeneity, by incorporating Cu into FeSiB alloy [25]. This modification achieved exceptional degradation performance for Orange II, demonstrating a half-life (t_1/2) of 5 min—80-fold faster than ZVI-P300 (t_1/2: 160 min) under identical conditions.

The decolorization mechanism involves two primary reactions (Equations (1) and (2)): Fe⁰ atoms and adsorbed Fe²⁺ donate electrons to reduce the azo bond (−N=N−), cleaving the conjugated chromophore system. This underscores the necessity to investigate how enhanced Fe content influences decolorization performance [26,27].

$$2{\mathrm{Fe}}^0+\mathrm{R}-\mathrm{N}=\mathrm{N}-\mathrm{R}’+4{\mathrm{H}}^{+}\to 2{\mathrm{Fe}}^{2+}+\mathrm{R}-{\mathrm{NH}}_2+\mathrm{R}’-{\mathrm{NH}}_2$$

(1)

$$4{\mathrm{Fe}}^{2+}\left(\mathrm{adsorbed}\right)+\mathrm{R}-\mathrm{N}=\mathrm{N}-\mathrm{R}’+4{\mathrm{H}}^{+}\to 4{\mathrm{Fe}}^{3+}+\mathrm{R}-{\mathrm{NH}}_2+\mathrm{R}’-{\mathrm{NH}}_2$$

(2)

The standard electrode potential of Fe²⁺/Fe⁰ is −0.447 V. Therefore, this study intends to introduce small amounts of elements with electrode potentials higher than that of Fe⁰, forming a galvanic cell structure to accelerate the degradation of azo dyes. In addition, Mo elements can stimulate the reaction activity of Fe⁰ by coordinating with surface electronic transfer [28], while Sn elements can enhance the amorphous formation ability [29]. The effects of adding Mo (−0.2 V, Mo³⁺/Mo⁰), Cu (+0.345 V, Cu²⁺/Cu⁰), and Sn (−0.138 V, Sn²⁺/Sn⁰) elements on the decolorization performance of Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn) amorphous alloys were investigated. Finally, the influence of Fe content variation on the decolorization performance of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) and Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) alloys was explored, and the mechanism of efficient decolorization by FeSiBSn amorphous alloys was clarified.

2. Experimental

All raw materials used in this study were of industrial-grade purity: iron (Fe, 99.99 wt.%), silicon (Si, 99.99 wt.%), copper (Cu, 99.99 wt.%), tin (Sn, 99.99 wt.%), molybdenum (Mo, 99.99 wt.%), and boron (B, 99.99 wt.%). The alloys were prepared with nominal compositions of Fe₇₈Si₁₃B₉, Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn), Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4), and Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4). Ingot melting was performed in a vacuum arc melting furnace under an argon atmosphere. The alloy ingot was inductively melted under an argon atmosphere until it reached a molten state. After maintaining the molten state for 3 s, the melt was ejected by gas flow onto a rapidly rotating copper wheel, with a wheel speed of 55 m/s, resulting in the formation of a ribbon-shaped amorphous alloy. The resulting ribbons exhibited a thickness of approximately 10 μm and were cut into 10~15 mm ribbons for subsequent experiments. Orange II (C₁₆H₁₁N₂Na₄S), employed as the target pollutant, was purchased from Sigma-Aldrich (Bayswater, Australia).

This experiment utilizes X-ray diffraction (XRD, X′Pert Powder, PANalytical, Netherlands) to identify the phase structure of the surface of the ribbon, employing Cu target Kα radiation with a wavelength of λ = 1.54056 Å. The surface morphology of the samples is observed and analyzed using a thermal field emission scanning electron microscope (SEM, NOVA NANOSEM 430, FEI, Peabody, MA, USA). Additionally, an Energy Dispersive Spectrometer (EDS) attached to the SEM is used to perform compositional analysis of the elements on the surface of the sample.

The dye decolorization experiments were conducted as follows: First, a 250 mL solution of Orange II azo dye (40 mg/L) was prepared in a 500 mL beaker. The beaker was placed in a thermostatic water bath maintained at a set temperature. The pH of the solution was adjusted to the desired value using 1 mol/L hydrochloric acid (HCl) and 1 mol/L sodium hydroxide (NaOH). Pre-weighed ribbons were added to the beaker, and mechanical stirring was initiated at a rate of 350 rpm. During the degradation process, 5 mL aliquots of the solution were periodically sampled using a disposable syringe and filtered through a 0.45-μm membrane to remove impurities. The filtered solution was immediately analyzed for absorbance using a UV-Vis spectrophotometer (UV–vis, TU-1900, Persee, Beijing, China).

3. Results and Discussion

Figure 1 presents the XRD patterns of the as-prepared Fe₇₈Si₁₃B₉ and Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn) amorphous alloys. As observed, all alloy ribbons exhibit a broad diffuse scattering peak centered at approximately 45°, with no sharp crystalline peaks corresponding to long-range atomic order. This characteristic is attributed to the short-range ordered but long-range disordered atomic arrangement typical of amorphous structures, resulting in diffuse scattering patterns [30]. These findings confirm the fully amorphous nature of the prepared samples. Figure 1b–e show the SEM images of the Fe₇₈Si₁₃B₉ and Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn) amorphous alloys. The images reveal smooth surfaces of the alloy ribbons without noticeable defects.

Figure 2 illustrate the time-dependent color changes of Orange II solutions degraded using Fe₇₈Si₁₃B₉ and Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn) amorphous ribbons under the following reaction conditions: temperature T = 35 °C, initial pH₀ = 7, ribbon dosage = 2 g/L, and initial AO II concentration C_{Orange II} = 40 mg/L. As shown in Figure 2, the AO II solutions degraded by Fe₇₈Si₁₃B₉, Fe₇₇Si₁₃B₉Cu₁, and Fe₇₇Si₁₃B₉Sn₁ ribbons became nearly colorless after 40 min, indicating almost complete decolorization. In contrast, Figure 2b reveals that the Orange II solution treated with Fe₇₇Si₁₃B₉Mo₁ ribbons still exhibited noticeable coloration even after 60 min, suggesting incomplete decolorization at this time point. Notably, during the initial 10 min of the decolorization process, the Fe₇₇Si₁₃B₉Cu₁ and Fe₇₇Si₁₃B₉Sn₁ ribbons demonstrated superior decolorization efficiency compared to the Fe₇₈Si₁₃B₉ ribbon.

According to the Lambert–Beer law, the ratio of Orange II concentrations at different reaction times is directly proportional to the ratio of the intensities at the maximum absorption peak (λ_max) [31]. Therefore, the degradation efficiency of Orange II at various reaction times, treated with different alloy ribbons, can be calculated using Equation (3), as shown in Figure 3a. Under identical reaction conditions, the decolorization process initiated more rapidly with Fe₇₇Si₁₃B₉Cu₁ and Fe₇₇Si₁₃B₉Sn₁ ribbons, achieving decolorization efficiencies of 58.5% and 59.4%, respectively, at 10 min. In contrast, the Fe₇₈Si₁₃B₉ ribbon exhibited a decolorization efficiency of only approximately 50% at the same time point. Over a 60-min period, the Fe₇₈Si₁₃B₉, Fe₇₇Si₁₃B₉Cu₁, and Fe₇₇Si₁₃B₉Sn₁ ribbons all achieved decolorization efficiencies of around 96%, outperforming the Fe₇₇Si₁₃B₉Mo₁ ribbon, which reached only 81.5% efficiency at 60 min.

D = [(C₀ − C_t)/C₀] × 100

(3)

Here, D represents the decolorization efficiency, C_t is the concentration of the Orange II solution at time t, and C₀ is the initial concentration of the Orange II solution.

To investigate the degradation rates of the three materials, the experimental data obtained from the degradation process were fitted using a pseudo-first-order kinetic model. The pseudo-first-order kinetic equation has been widely applied in the degradation of azo dyes by zero-valent metals and is expressed as follows [32]:

$$C_t/C_0=\exp (-k_{\mathrm{obs}}t)$$

(4)

where k_obs is the apparent rate constant (min⁻¹), and t is the reaction time.

Figure 3b shows the (C_t/C₀) vs. reaction time curves for the four amorphous alloy compositions. The fitted k_obs for Fe₇₈Si₁₃B₉, Fe₇₇Si₁₃B₉Mo₁, Fe₇₇Si₁₃B₉Cu₁, and Fe₇₇Si₁₃B₉Sn₁ ribbons were determined to be 0.066 min⁻¹, 0.034 min⁻¹, 0.074 min⁻¹, and 0.074 min⁻¹, respectively. Compared to the k_oks of the Fe₇₈Si₁₃B₉ ribbon, the Fe₇₇Si₁₃B₉Cu₁ and Fe₇₇Si₁₃B₉Sn₁ ribbons exhibited an approximately 12% increase in k_oks, indicating that the addition of Cu and Sn elements effectively enhanced the decolorization performance of FeSiB amorphous alloys toward Orange II. In contrast, the incorporation of Mo hindered the decolorization process of FeSiB amorphous alloys for Orange II.

Since the decolorization process of Orange II azo dye by Fe-based amorphous ribbons involves a series of heterogeneous reactions on their surfaces, the surface morphology of the ribbons after reaction was analyzed. As clearly shown in Figure 4, the surfaces of all Fe-based amorphous ribbons exhibit typical characteristics of selective corrosion. Specifically, the Fe₇₈Si₁₃B₉ and Fe₇₇Si₁₃B₉Mo₁ ribbons show a significant amount of granular corrosion products on their surfaces after the reaction. However, the granular corrosion products on the Fe₇₈Si₁₃B₉ ribbon are more widely distributed, with three-dimensional (3D) petal-like structures surrounding the selective corrosion regions. These structures possess a high specific surface area, enabling more effective adsorption of Orange II molecules [10,25]. In contrast, the granular corrosion products on the Fe₇₇Si₁₃B₉Mo₁ ribbon are primarily concentrated in the selective corrosion regions. Compared to other amorphous alloys with different compositions, the surface outside the selective corrosion zone of Fe₇₇Si₁₃B₉Mo₁ is smoother. This may be attributed to the addition of Mo, which enhances the corrosion resistance of the amorphous alloy [33]. On the other hand, the Fe₇₇Si₁₃B₉Cu₁ and Fe₇₇Si₁₃B₉Sn₁ ribbons exhibit a large number of cotton-like deposits on their surfaces. These deposits are loose and porous, providing channels for mass and electron transfer during the reaction, which facilitates the diffusion and adsorption of Orange II dye molecules [34]. The surfaces surrounding the deposits feature 3D petal-like nanostructured corrosion products. Unlike the Fe₇₈Si₁₃B₉ ribbon, the Fe₇₇Si₁₃B₉Cu₁ and Fe₇₇Si₁₃B₉Sn₁ ribbons display a higher quantity of these petal-like nanostructures, predominantly distributed in non-selective corrosion regions. Consequently, the Fe₇₇Si₁₃B₉Cu₁ and Fe₇₇Si₁₃B₉Sn₁ ribbons demonstrate superior Orange II dye removal performance, while the Fe₇₇Si₁₃B₉Mo₁ ribbon, lacking such structures, exhibits the poorest dye removal performance among the four types of ribbons. Additionally, EDS analysis of the deposits in Figure 4h reveals the presence of sulfur (S) elements. Since the ribbons themselves do not contain S, its presence indicates that azo dye molecules or their degradation intermediates/products are adsorbed on the ribbon surfaces [35]. Figure 4i presents the EDS elemental mapping corresponding to the location depicted in Figure 4g. It is evident that the selectively corroded regions exhibit higher concentrations of Fe, Si, and Sn, whereas the surrounding areas display an elevated oxygen (O) content, indicating that iron hydroxides are predominantly distributed outside the selectively corroded regions.

Figure 5a presents the XRD patterns of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4). As Fe substitutes for Si, the Fe₇₉Si₁₁B₉Cu₁ alloy ribbon begins to exhibit a distinct diffraction peak near 65°, corresponding to the α-Fe phase. With further increases in Fe content, the intensity of the diffraction peak significantly increases, indicating enhanced crystallinity of the alloy. These results demonstrate that the substitution of Fe for Si reduces the glass-forming ability of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) alloys, leading to the formation of α-Fe crystalline phases starting from the Fe₇₉Si₁₁B₉Cu₁ alloy. Figure 5b shows the XRD patterns of Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4). It is evident that all alloys exhibit a typical broad diffuse scattering peak at 2θ = 45° without any crystalline diffraction peaks, confirming the fully amorphous structure of the prepared samples. These results indicate that, within the FeSiB alloy system, the addition of Sn enhances the glass-forming ability more effectively than the addition of Cu.

Table 1. Elemental composition (at.%) of the Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) and Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) ribbons measured by EDS.

Sample	Fe	Si	Cu	Sn
Fe77Si13B9Cu1	86.8	11.6	1.6	-
Fe79Si11B9Cu1	89.0	9.7	1.3	-
Fe81Si9B9Cu1	90.8	7.9	1.3	-
Fe77Si13B9Sn1	87.3	11.5	-	1.2
Fe79Si11B9Sn1	89.3	9.4	-	1.3
Fe81Si9B9Sn1	91.5	7.3	-	1.2

summarizes the elemental composition of the Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) and Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) ribbons measured by EDS, which generally aligns with the nominal compositional trends.

Figure 6a shows the time-dependent decolorization efficiency of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) alloys for Orange II. Under identical conditions, the decolorization efficiency of Fe_77+xSi_13−xB₉Cu₁ alloys gradually decreases with increasing Fe content. At 60 min of reaction, the decolorization efficiencies of Fe₇₉Si₁₁B₉Cu₁ and Fe₈₁Si₉B₉Cu₁ alloys for Orange II are 94.1% and 91.2%, respectively, both lower than that of Fe₇₇Si₁₃B₉Cu₁ (95.6%). Figure 6b presents the k_oks and R² for the degradation reactions of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) alloys. It can be observed that the degradation rate constants of Fe₇₉Si₁₁B₉Cu₁ and Fe₈₁Si₉B₉Cu₁ alloys for Orange II azo dye are 0.062 min⁻¹ and 0.054 min⁻¹, respectively, representing reductions of 16.2% and 27.0% compared to that of Fe₇₇Si₁₃B₉Cu₁ (0.074 min⁻¹). Previous studies have shown that the reactivity of amorphous Fe⁰ in Fe-based alloys of the same composition is higher than that of crystalline Fe⁰. Therefore, the amorphous Fe₇₇Si₁₃B₉Cu₁ exhibits superior decolorization performance for Orange II compared to the partially crystallized Fe₇₉Si₁₁B₉Cu₁ and Fe₈₁Si₉B₉Cu₁ alloys, and the decolorization performance deteriorates progressively with increasing crystallinity.

Figure 7a–d present the surface morphology of the Fe₇₉Si₁₁B₉Cu₁ and Fe₈₁Si₉B₉Cu₁ alloy ribbons after the reaction. Compared to the amorphous Fe₇₇Si₁₃B₉Cu₁, the Fe₇₉Si₁₁B₉Cu₁ and Fe₈₁Si₉B₉Cu₁ alloys exhibit numerous nanoscale pores within the selective corrosion regions, which are likely formed due to the detachment of α-Fe during the decolorization of Orange II [36]. In addition, the Fe₇₇Si₁₃B₉Cu₁ amorphous ribbon exhibited significantly more cotton-like deposits on its surface after the reaction compared to the Fe₇₉Si₁₁B₉Cu₁ and Fe₈₁Si₉B₉Cu₁ alloy ribbons, indicating that more Fe⁰ participated in the reaction during the decolorization process for Fe₇₇Si₁₃B₉Cu₁. The EDS data in Figure 7e reveal that the Fe content in the pore-free regions of the selective corrosion zones is 76%, significantly higher than the 64% observed in the nanoporous regions.

Figure 8a shows the time-dependent decolorization efficiency of Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) alloys for Orange II. Under identical conditions, the decolorization efficiency of Fe_77+xSi_13−xB₉Sn₁ alloys gradually increases with higher Fe content. At 60 min of reaction, the decolorization efficiencies of Fe₇₉Si₁₁B₉Sn₁ and Fe₈₁Si₉B₉Sn₁ amorphous ribbons for Orange II both reach 97.0%, surpassing that of Fe₇₇Si₁₃B₉Sn₁ (96.1%). Notably, the Fe₈₁Si₉B₉Sn₁ ribbon achieves a decolorization efficiency of 71.3% within just 10 min. Figure 8b presents the k_oks and the R² for the degradation reactions of Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) alloys. It can be observed that the degradation rate constants of Fe₇₉Si₁₁B₉Sn₁ and Fe₈₁Si₉B₉Sn₁ alloys for Orange II azo dye are 0.084 min⁻¹ and 0.094 min⁻¹, respectively, representing improvements of 13.5% and 27.0% compared to that of Fe₇₇Si₁₃B₉Sn₁ (0.074 min⁻¹). These results demonstrate that, while maintaining the amorphous structure of the ribbons, increasing the Fe content effectively enhances the decolorization performance of FeSiBSn amorphous alloys for azo dyes.

Table 2. Part of research on azo dye using Fe-base amorphous alloys.

Composition	Dye	C0 (mg/L)	pH	Mass Dosage (g/L)	Temp. (°C)	Kobs (min−1)	Ref.
Fe78(Si,B)22	Orange II	100	6	9-12	Room	0.125	[37]
(Fe0.99Mo0.01)78Si9B13	Direct blue 2B	200	7	13.3	60	0.136	[38]
Fe78Si13B9	Orange II	40	7	2	35	0.071	[11]
Fe80B13C7	acid orange 7	20	3	2	25	0.08	[10]
Fe85P11C2B2	acid orange 7	20	3	2	35	0.0118	[12]
Fe78Si11B9P2	Orange II	40	7	2	35	0.082	[27]
Fe81Si9B9Sn1	Orange II	40	7	2	35	0.094	This work

shows the removal performance of certain Fe-based amorphous alloys for azo dyes. A comparison reveals that the Fe₈₁Si₉B₉Sn₁ amorphous ribbon exhibits exceptional removal performance under neutral conditions, which can even rival the removal performance of some compositions at pH = 3.

Figure 9a–d present the surface morphology of Fe₇₉Si₁₁B₉Sn₁ and Fe₈₁Si₉B₉Sn₁ amorphous ribbons after the reaction. With increasing Fe content, the Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) alloys exhibit numerous cracks on their surfaces after the reaction, with fresh ribbon surfaces exposed within the cracks. As the degradation reaction progresses, these cracks gradually peel off, continuously exposing fresh internal surfaces and providing a sustained supply of Fe⁰ for the degradation process. The EDS data in Figure 9e reveal that the Fe content in the regions where cracks have peeled off is 91.5%, nearly identical to the Fe content on the ribbon surface before the reaction (as shown in Table 1). Additionally, the absence of O indicates that these regions remain unoxidized. This self-renewing behavior endows the FeSiBSn amorphous ribbons with exceptional decolorization performance for azo dyes.

The UV-Vis absorption spectra of Orange II solutions at different reaction times for Fe₇₇Si₁₃B₉Sn₁, Fe₇₉Si₁₁B₉Sn₁, and Fe₈₁Si₉B₉Sn₁ amorphous ribbons are shown in Figure 10. From the 0-min curve, it can be observed that Orange II solutions exhibit three characteristic absorption peaks at 484 nm, 310 nm, and 228 nm. Among them, the λ_max at 484 nm originates from the n-π* transition of the azo structure (-N=N-), while the peaks at 228 nm and 310 nm are attributed to the π-π* transitions of the conjugated systems of the benzene and naphthalene rings, respectively [39]. The concentration of Orange II solution is proportional to the intensity at λ_max. Therefore, the degradation efficiency of the amorphous alloys for Orange II dye can be represented by the intensity changes of the absorption peak at 484 nm. The results indicate that the intensity of the absorption peak at 484 nm gradually decreases with increasing reaction time, nearly disappearing after 60 min, suggesting that the chromophore (-N=N-) in the dye molecules is almost completely broken. Additionally, the characteristic absorption peaks at 228 nm and 310 nm disappear after 30 min of reaction, indicating that the benzene and naphthalene ring structures in the dye molecules are also degraded [27,40]. At this stage, a new characteristic absorption peak appears at 248 nm, corresponding to the amino structure (-NH₂), which is identified as a product of the cleavage of the azo bond. In the initial stages of the decolorization process, the decolorization of azo dyes by FeSiBSn is dominated by surface adsorption, and thus no new characteristic peaks are observed. By comparing the data in Figure 10, it is evident that the intensity of the amino characteristic peak increases with higher Fe content, indicating that the degradation efficiency for Orange II follows the order: Fe₈₁Si₉B₉Sn₁ > Fe₇₉Si₁₁B₉Sn₁ > Fe₇₇Si₁₃B₉Sn₁.

4. Conclusions

This study compares the decolorization performance of Fe₇₈Si₁₃B₉ and Fe₇₇Si₁₃B₉M₁ (M = Mo, Cu, or Sn) amorphous alloys for azo dyes, as well as the influence of Fe content variation on the decolorization performance of Fe_77+xSi_13−xB₉Cu₁ (x = 0, 2, or 4) and Fe_77+xSi_13−xB₉Sn₁ (x = 0, 2, or 4) alloys. For the Fe₇₇Si₁₃B₉M₁ amorphous alloys, the addition of Cu and Sn elements enhances their decolorization performance for azo dyes, while the incorporation of Mo has a detrimental effect. The underlying mechanisms can be primarily attributed to the following aspects: The incorporation of Mo enhances the corrosion resistance of amorphous alloys, consequently delaying the surface reaction of Fe⁰ and reducing the reactive activity at the alloy surface. In contrast, the addition of Cu and Sn induces the formation of abundant 3D petal-like nanostructures corrosion products during surface reactions. These unique nanostructures significantly enhance the adsorption capacity of amorphous alloys toward azo dye molecules, thereby facilitating the decolorization process through improved interfacial interactions. In the Fe_77+xSi_13−xB₉Cu₁ alloys, increasing Fe content reduces the glass-forming ability, leading to the precipitation of α-Fe crystalline phases, thereby weakening the decolorization performance of FeSiBCu alloys for azo dyes. In contrast, the Fe_77+xSi_13−xB₉Sn₁ alloys exhibit excellent glass-forming ability, maintaining their amorphous structure even with increasing Fe content, and their decolorization performance for azo dyes improves accordingly. The outstanding decolorization performance of Fe_77+xSi_13−xB₉Sn₁ amorphous alloys for Orange II are attributed to the formation of numerous cracks during the reaction, which peel off to expose fresh surfaces, enabling a self-renewing behavior. This self-renewing behavior results in a degradation rate of 0.094 min⁻¹ for Fe₈₁Si₉B₉Sn₁, significantly higher than that of the conventional Fe₇₈Si₁₃B₉ amorphous alloy (0.066 min⁻¹). This study provides a new perspective for designing FeSiB amorphous alloys with high-efficiency decolorization performance for azo dyes.