Efficient photodecomposition of rhodamine B by an Fe-based metallic glass in an oxalic acid solution

Abstract

Fe-based metallic glass (amorphous Fe–Si–B ribbons) was successfully fabricated and shown to be an efficient catalyst of the photo-decomposition of rhodamine B (RhB). The effects of various factors on the photodecomposition were investigated. The catalyst dosage and the wavelength and intensity of the light source were all found to strongly influence the reaction rate. A nearly complete degradation of RhB (20 mg L⁻¹) was achieved within 15 min by 50 mg L⁻¹ Fe–Si–B catalyst and 1 mM oxalic acid when illuminated with 125 W UV light. The Fe–Si–B catalyst was also found to exhibit good structural stability and no loss of performance for three cycles. Based on the results of these experiments, we conclude that the amorphous Fe–Si–B ribbons and oxalic acid constitute a promising heterogeneous Fenton-like system for industrial wastewater treatment.

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1. Introduction

During the production of textile dyes, more than 15% of it is lost in wastewater streams, which causes environmental problems.¹ It is very important to treat dye waste effluents to lessen its destruction of the environment. Conventional methods to treat dyes include physical adsorption,² flocculation³ and biological degradation.⁴ These methods usually do not work efficiently as they are non-destructive and merely involve the transfer of pollutants from water to sludge, and hence result in the production of secondary waste. Thus, there is a need for developing more effective treatment technologies to remove dyes from wastewaters. Recently, advanced oxidation processes (AOPs) such as the Fenton reaction have been found to be very effective at removing organic pollutants from wastewater.^5,6 The Fenton system can generate hydroxyl radicals (˙OH), which can in turn degrade most organic pollutants to water and carbon dioxide due to their high oxidation potential (E⁰ = 2.8 V).⁵ More recently, ozone, light and electricity have been involved in the Fenton AOP system to increase the efficiency of ˙OH generation. Thus, photo-Fenton and photo-Fenton-like reactions have been extensively used to degrade dyes.^7–11 The major drawbacks of the Fenton process are that it operates optimally at a pH of 3 and uses excess amounts of ferrous ions.^12,13 To solve these problems, better Fenton-like catalysts need to be developed. On the other hand, H₂O₂ is added as the direct source of ˙OH in these reactions.¹⁴ However, H₂O₂ is a very reactive compound and does not survive in nature for long. These defects have limited the usefulness of Fenton systems in a natural environment.

It is well known that a combination of iron oxides and polycarboxylic acids can form a photochemical system that gives a photo-Fenton-like reaction without the addition of H₂O₂.^15,16 Since there are abundant polycarboxylic acids in the natural environment, this photochemical oxidation process can directly utilize natural materials to decompose organic pollutants economically. It is important to investigate the photodecomposition of dyes in the Fenton systems with polycarboxylic acids and gain a better understanding of how the dyes are transformed. Oxalic acid is one of the most active polycarboxylic acids.¹⁷ So in this paper, oxalic acid was selected as both a new ˙OH source and the pH buffer, which provided a proper reaction environment.

Metallic glasses, unlike crystalline metals, are formed by processes involving conditions far from equilibrium, and the constituent atoms do not reside at the thermodynamic equilibrium positions.^18,19 This metastable nature imparts many excellent properties onto amorphous alloys that are unachievable for crystalline alloys, such as good catalytic and chemical properties.^20,21 Meanwhile, Fe-based metallic glasses have been widely studied due to their low cost, good resistance to corrosion, and excellent soft magnetic properties.²⁰ Therefore, there is great potential for using Fe-based metallic glasses as a Fenton-like catalyst to degrade dyes.

In this work, amorphous Fe₇₈Si₉B₁₃ ribbons were prepared by applying the melt-spinning technique and were used as catalysts. The catalytic performance of the Fe–Si–B amorphous ribbons was investigated for the degradation of rhodamine B (RhB), which was used as a model pollutant. RhB, one of the important xanthene dyes, has become a common organic pollutant and exhibits quite high resistance to photodegradation and oxidative degradation.²² The major objectives of this work were (i) to develop a new Fenton system combining amorphous Fe–Si–B alloys and oxalic acid; (ii) to evaluate the degradation performance of the new Fenton system for RhB; and (iii) to investigate the influences of various reaction parameters on the rate of degradation. We also discuss the RhB degradation mechanism and the stability of amorphous Fe–Si–B ribbons.

2. Materials and methods

2.1 Materials

RhB (>99%) was purchased from Tianjin Chemical Reagent Research Institute, China. Oxalic acid was purchased from Sinopharm Chemical Reagent Co., Ltd, China. All of the chemicals were of analytical grade and were used as received without further purification. Deionized water was used for preparing the RhB solutions in the experiments.

2.2 Preparation and characterization of amorphous Fe–Si–B ribbons

The master ingot of Fe₇₈Si₉B₁₃ was produced by arc melting a mixture of Fe, Si, and B (each with a purity greater than 99%) under a Ti-gettered argon atmosphere. The ingot was then remelted in a quartz tube by carrying out induction melting, followed by single roller spinning to obtain the amorphous ribbons. The ribbons were 2 mm in width and 20 μm in thickness.

The crystal structures of the catalysts were determined by performing X-ray diffraction (XRD). The XRD data were collected on a D8 Bruker diffractometer with Cu Kα radiation.

2.3 Procedure and approach

Unless indicated otherwise, the reaction mixture was formed by adding various amounts of an Fe-based amorphous alloy and oxalic acid to 100 mL samples of an aqueous solution containing 20 mg L⁻¹ of RhB. Before the photoreaction, the mixture was magnetically stirred in the dark for 0.5 h to establish an adsorption–desorption equilibrium. The various mixtures were then illuminated with ultraviolet (UV) or visible radiation while being magnetically stirred. The UV reactions were conducted with a 300 W mercury lamp and a 125 W mercury lamp, respectively, to study the effect of UV intensity on the reactions. The reaction with visible light was achieved by using a 350 W xenon lamp. At given time intervals, 3 mL aliquots were collected and then centrifuged, and the resulting supernatant was analyzed by using UV-Vis spectroscopy. The UV-Vis absorption spectra of the aqueous solutions were recorded from 200 to 800 nm using a UV-Vis spectrophotometer (TU1800-PC, Beijing). The RhB concentration was determined based on the constructed calibration curve at the maximum absorption wavelength (λ_max) of 554 nm. Moreover, (Fe³⁺, Fe²⁺) concentrations in the solution after the reaction were determined by carrying out inductively coupled plasma emission spectrometry (ICP) using a SPECTROBLUE ICP-OES (Spectro, Inc.).

3. Results and discussion

3.1 Characterization of the Fe–Si–B catalyst

The XRD pattern of the Fe₇₈Si₉B₁₃ ribbons obtained by the melt-spinning technique is shown in Fig. 1. The XRD pattern of Fe is also included in Fig. 1 for comparison. In addition, the XRD pattern of the Fe₇₈Si₉B₁₃ ribbon after being irradiated with UV light for 3 cycles in the presence of 1 mM oxalic acid is also shown in Fig. 1. The amorphous nature of the as-received ribbon was indicated by the broadness of the peak in the XRD pattern. The results demonstrated that the ribbon displayed only short-range order, and the zero-valent iron (ZVI) in the ribbon was successfully frozen into an amorphous state. Meanwhile, the amorphous nature of the Fe–Si–B ribbon was maintained after being used for the degradation of RhB.

Fig. 1 XRD patterns of as-received and reused Fe–Si–B amorphous ribbons.

3.2 Photodecomposition of RhB

3.2.1 Effect of combinations of Fe–Si–B, oxalic acid and UV. The photodecomposition of RhB under various reaction conditions is shown in Fig. 2. The UV source was a 125 W mercury lamp. UV illumination of the Fe–Si–B ribbon decomposed only 5% of the RhB (curve a). The combination of Fe–Si–B ribbon and 2 mM oxalic acid without UV illumination (in the dark) degraded only 6% of the RhB after 1 h of reaction (curve b). For these two reactions, the degradation of RhB that did occur may have been due to adsorption rather than photodecomposition. In the presence of an acidic environment and with UV illumination, however, the Fe–Si–B ribbon degraded 51% of the Rhb after 1 h (curve d). When combining the effects of UV irradiation and an acidic environment, the ZVI in the Fe–Si–B ribbon may have reacted with the dissolved oxygen in the solution to produce H₂O₂,²³ and hence yield a considerably increased rate of degradation. UV illumination of an oxalate solution without the ribbons resulted in the degradation of 36% of the RhB after 1 h. UV irradiation with the oxalate solution alone was therefore able to slowly degrade the dye. By contrast, UV illumination with the combination of the Fe–Si–B ribbon and 2 mM oxalic acid showed very fast RhB photodecomposition (nearly 100% at 20 min, curve e). These results suggest the new Fenton system to have a great ability to decompose dyes, with the Fe–Si–B ribbons catalyzing the decomposition.

Fig. 2 Photodecomposition of 20 mg L⁻¹ RhB in various conditions: (a) 100 mg L⁻¹ Fe–Si–B ribbon with UV, (b) 100 mg L⁻¹ Fe–Si–B ribbon in 2 mM oxalic acid without UV, (c) 2 mM oxalic acid with UV, pH = 2.7, (d) 100 mg L⁻¹ Fe–Si–B ribbon in acetic acid with UV, pH = 2.7, (e) 100 mg L⁻¹ Fe–Si–B ribbon in 2 mM oxalic acid with UV, pH = 2.7, (f) 100 mg L⁻¹ Fe–Si–B ribbon in 2 mM oxalic acid and 10 mM isopropanol with UV, pH = 2.7.

3.2.2 Degradation analysis. In order to determine whether hydroxyl radicals (˙OH) take part in the degradation of RhB, isopropanol was used as a trap. The result is shown as Fig. 1 curve (f). After using the hydroxyl radical trap, the degradation rate decreased rapidly. Only 8% of the RhB was degraded after 1 h. This result indicated that the hydroxyl radicals generated in the reaction were used to effect the degradation.

There have been few studies about the combination of ZVI and oxalic acid. On the other hand, the photochemistry of Fe(III)–oxalate complexes has been studied by many workers.^24–26 So the degradation mechanism of Fe(III)–oxalate complexes could be a reference for the new photo-Fenton system. According to this extrapolated mechanism, during the photochemical reaction of ZVI and oxalic acid under UV illumination, superoxides (˙O₂⁻) and hydroperoxyl radicals (˙O₂H) (eqn (2)–(5)) would have formed as the key intermediates. H₂O₂, formed from ˙O₂⁻ and ˙O₂H, (eqn (6) and (7)) would have then taken part in a Fenton reaction to produce ˙OH (eqn (8)):

ZVI + nH₂C₂O₄ + hν → [Fe(III)(C₂O₄)_n]^(2n−3)− + H₂ (1)

[Fe(III)(C₂O₄)_n]^(2n−3)− + hν → [Fe(II)(C₂O₄)_(n−1)]⁽⁴⁻²ⁿ⁾ + ˙(C₂O₄)⁻ (2)

˙(C₂O₄)⁻ → CO₂ + ˙(CO₂)⁻ (3)

˙(CO₂)⁻ + O₂ → CO₂ + ˙O₂⁻ (4)

˙O₂⁻ + H⁺ → ˙O₂H (5)

˙O₂⁻ + Fe³⁺ → Fe²⁺ + O₂ (6)

˙O₂⁻/˙O₂H⁻ + nH⁺ + Fe²⁺ → Fe³⁺ + H₂O₂ (7)

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + ˙OH (8)

Oxalic acid would have first adsorbed on the surface of the ribbons in the presence of UV radiation to form [Fe(III)(C₂O₄)_n]^(2n−3)− (eqn (1)). This complex could have been excited under illumination to form [Fe(II)(C₂O₄)_(n−1)]⁽⁴⁻²ⁿ⁾ and the oxalate radical ˙(C₂O₄)⁻ (eqn (2)). The oxalate radical would have then transformed to the carbon-centered radical ˙(CO₂)⁻ (eqn (3)),²⁷ and the excited electrons would have transferred to the adsorbed oxygen to form superoxide ions ˙(O₂)⁻ (eqn (4)). Fe(III) would have reacted with ˙(O₂)⁻ to form O₂ and Fe(II), and Fe(II) would have reacted with ˙O₂⁻ and ˙O₂H to form H₂O₂ in acidic solution with Fe(III) (eqn (6) and (7)).²⁸ After the formation of H₂O₂, hydroxyl radicals could have been generated by the reaction of Fe(II) and H₂O₂ as described in eqn (8). Meanwhile, there was no significant leaching of Fe ions. The concentration of leached Fe (Fe²⁺, Fe³⁺) in the solution was 0.1 mg L⁻¹, which was comparable to the results obtained by Zheng²⁹ (2 mg L⁻¹) and Zhang³⁰ (0.18 mg L⁻¹). This result indicated that all of the reactions occurred on the ribbon surface.

3.2.3 Effect of Fe–Si–B concentration. Since the photoreaction of RhB was carried out with the help of Fe–Si–B ribbons, the ribbon content was considered as an important experimental parameter. Fig. 3 shows the effect of Fe–Si–B concentration on the degradation of RhB in a 2 mM oxalate solution with UV. The RhB photodecomposition was found to depend strongly on the concentration of Fe–Si–B. The degradation rate of RhB increased as the Fe–Si–B concentration was decreased from 200 mg L⁻¹ to 50 mg L⁻¹, and nearly 100% of the RhB was degraded within 15 min at an Fe–Si–B concentration of 50 mg L⁻¹. However, when the Fe–Si–B concentration was further decreased to 25 mg L⁻¹, the degradation rate of RhB decreased.

Fig. 3 Effect of Fe–Si–B ribbon concentration on the degradation of RhB with 2 mM oxalic acid and UV (125 W mercury lamp).

We attributed this phenomenon to the amount of Fe ions taking part in the reaction. Since this type of photoreaction proceeds by the generation of Fe(III)–oxalate complexes, the presence of Fe ions was essential. The concentration of Fe ions of course increased with increasing Fe–Si–B concentration. Thus, the formation of [Fe(III)(C₂O₄)_n]^(3−2n)−, which has been shown to have high photoactivity under UV illumination,³¹ appears to have occurred in sufficient quantity at an appropriate Fe ion concentration. Hence more ˙OH radicals were apparently produced during the photochemical reaction in the present Fe–Si–B ribbon-oxalate system. A lower concentration of Fe ions (25 mg L⁻¹) led to less photodecomposition of RhB due to the formation of less [Fe(III)(C₂O₄)_n]^(3−2n)−. On the other hand, too high an Fe ion concentration (100 mg L⁻¹ to 1000 mg L⁻¹) also led to lower photodecomposition of RhB. Such a high concentration of Fe ions would have promoted scavenging reactions (eqn. (9) and (10)),³² and hence decrease the concentration of hydroxyl radicals involved in degradation.

Fe²⁺ + ˙O₂H → Fe³⁺ + HO₂⁻ (9)

Fe²⁺ + ˙OH → Fe³⁺ + HO⁻ (10)

3.2.4 Effect of oxalic acid concentration. A series of experiments were performed to examine the effect of the initial oxalic acid concentration under the following conditions: initial RhB concentration = 20 mg L⁻¹, Fe–Si–B concentration = 50 mg L⁻¹ and UV light illumination (125 W mercury lamp). The RhB concentrations at various oxalic acid dosages are shown in Fig. 4 as a function of the UV illumination time. As seen in these curves, the oxalate solution clearly enhanced the photodecomposition of RhB. The optimal oxalate concentration was found to be 1 mM under these experimental conditions. Excess oxalic acid is likely to occupy the adsorption sites on the surface of Fe–Si–B ribbon, and the excess oxalic acid also competes with RhB in reaction with the generated ˙OH radicals competitively with RhB.²⁵ Thus, under these conditions, fewer of the generated hydroxyl radicals would have been available for helping to decompose the RhB.

Fig. 4 Effect of oxalic acid concentration on the degradation of RhB with 50 mg L⁻¹ Fe–Si–B ribbons and UV (125 W mercury lamp).

3.2.5 Effect of the light source. The photodecomposition of RhB was examined under visible light (λ = 420 nm, 350 W xenon lamp) and compared with the results with UV light (λ = 365 nm, 125 W mercury lamp). The results are shown in Fig. 5. The degradation rate under visible light was much lower than that under UV with the same Fe–Si–B dosage. The UV light was clearly better at driving the degradation reaction. In this regard, note that visible light, being of lower energy than UV light, likely generated fewer Fe ions, explaining why the best dosage of Fe–Si–B ribbons under visible light was increased to 200 mg L⁻¹. Although the rate of decomposition was relatively low under visible light, 95% of the RhB was still degraded in 1 h in the presence of 200 mg L⁻¹ Fe–Si–B ribbons. Thus, the new Fenton system has the potential to work in solar light.

Fig. 5 Degradation of RhB at various concentrations of Fe–Si–B under visible light. (UV data under its optimal Fe–Si–B concentration shown as stars for comparison.) Initial RhB concentration, 20 mg L⁻¹; oxalic acid dosage, 1 mM.

The effect of the UV illumination intensity was also examined. Fig. 6 shows the degradation of RhB with a 300 W mercury lamp light source. For a given concentration of the amorphous Fe–Si–B ribbons, increasing the illumination intensity was expected to yield more Fe ions. For the 125 W mercury lamp, 50 mg L⁻¹ of Fe–Si–B was determined to be optimum. However, at this Fe–Si–B concentration, increasing the power of the mercury lamp to 300 W would have produced more Fe ions, and hence exceeded the optimum concentration of Fe ions. And indeed, the degradation rate was observed to decrease. Moreover, as the power of the mercury lamp was increased to 300 W, the optimum Fe–Si–B dosage decreased to 25 mg L⁻¹, which is consistent with the interpretation above. The best degradation rate for the 300 W mercury lamp was lower than that for the 125 W mercury lamp due to the lower Fe–Si–B concentration, which led to a smaller reaction area.

Fig. 6 Degradation of RhB under different intensities of UV light (initial RhB concentration, 20 mg L⁻¹; oxalic acid dosage, 1 mM).

3.2.6 Stability and reusability of the Fe–Si–B catalyst. The stability and reusability of the amorphous Fe–Si–B ribbons were evaluated by monitoring the degradation of RhB in the presence of 1 mM oxalic acid under UV illumination (using the 125 W mercury lamp). At the end of each cycle, the Fe–Si–B catalyst ribbons were separated from the reaction solution by carrying out a simple filtering procedure. Then they were washed with deionized water and dried in a glovebox under N₂. As shown in Fig. 7, RhB was completely degraded within 15 minutes in the second cycle and in the third cycle. The amorphous Fe–Si–B ribbon could therefore be reused for at least three cycles with no significant loss of activity. The amorphous nature of the Fe–Si–B catalyst was also maintained after three cycles of use, as indicated by the XRD measurements shown in Fig. 1. Therefore, these results indicated that the Fe–Si–B catalyst has great long-term stability and excellent potential application value due to the ease with which it can be easily separated from the reaction solution.

Fig. 7 Stability of the Fe–Si–B catalyst for the degradation of RhB (initial RhB concentration, 20 mg L⁻¹; Fe–Si–B dosage, 50 mg L⁻¹; oxalic acid dosage, 1 mM).

4. Conclusions

The photodecomposition behavior of RhB was investigated using amorphous Fe–Si–B ribbons in an oxalate solution, and illuminated with UV and visible radiation. The following results were obtained.

(1) A new Fenton system including amorphous Fe-based ribbons and oxalic acid was developed. The combination of amorphous Fe–Si–B ribbons, oxalic acid and light illumination was essential for the photodecomposition of RhB.

(2) The new Fenton system showed a considerable ability to effect degradation. A nearly complete degradation of RhB (20 mg L⁻¹) was achieved within 15 min by the use of 50 mg L⁻¹ Fe–Si–B catalyst and 1 mM oxalic acid when illuminated by 125 W UV light.

(3) The Fe–Si–B concentration was found to be an important parameter. An optimum Fe-based ribbon concentration was found to be associated with each experimental condition. In particular, when the intensity of the light source was increased, the optimum dosage of the Fe–Si–B decreased.

(4) The amorphous Fe–Si–B ribbon functioned as a catalyst in the Fenton system. It has great long-term stability and excellent potential application value due to the ease with which it can be easily separated from the reaction solution.

Acknowledgements

The authors acknowledge financial support from the National Natural Science Foundation of China (Grant no. 51671056) and the Jiangsu key laboratory for advanced metallic materials (BM2007204).

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