Rapid malachite green degradation using Fe73.5Si13

Materials and Design 119 (2017) 244–253

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Rapid malachite green degradation using Fe73.5Si13.5B9Cu1Nb3 metallic glass for activation of persulfate under UV–Vis light S.X. Liang a, Z. Jia a, W.C. Zhang b, W.M. Wang c, L.C. Zhang a,⁎ a b c

School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, Perth, WA 6027, Australia Environmental Protection Administration of Ji'an City, Ji'an, Jiangxi Province 343000, China School of Materials Science and Engineering, Shandong University, Jinan, Shandong 250061, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Influence parameters of malachite green dye degradation using Fe73.5 Si13.5B9Cu1Nb3 metallic glass are investigated. • First-order kinetic model fits well with malachite green degradation process. • Inclusion of Nb and Si enhances the surface stability of Fe73.5Si13.5B9Cu1Nb3. • High reusability with acceptable dye degradation efficiency is observed.

a r t i c l e

i n f o

Article history: Received 14 October 2016 Received in revised form 19 December 2016 Accepted 12 January 2017 Available online 13 January 2017 Keywords: Metallic glass Reusability Surface stability Persulfate Malachite green Degradation

a b s t r a c t In this work, it is the first time to report that Fe73.5Si13.5B9Cu1Nb3 metallic glass having unique atomic structure was employed for activation of persulfate under UV–Vis light. The investigation evaluated the importance of influencing parameters, including dye concentration, persulfate concentration, ribbon dosage and light intensity, on malachite green (MG) dye degradation. The results reveal that 100% dye color removal with a reaction rate of k = 0.0849 min−1 could be achieved within 30 min under specific parameters control. In addition, surface decay behavior of the catalyst also plays a significant effect on the reusability and sustainability. The inclusion of Nb atom in Fe73.5Si13.5B9Cu1Nb3 promotes enrichment of Si atom on the ribbon surface, causing the formation of Si and Nb oxides to further improve the surface stability on both of free and roller-contacted surfaces. The precipitations on reused ribbon surface are confirmed as α-Fe, iron oxide and Si, Nb oxides, revealing a high potential of catalytic reusability for wastewater treatment. The present work will open a new gate for further realizing the high performance of industrial water treatment using metallic glass. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction

⁎ Corresponding author. E-mail addresses: [email protected], [email protected] (L.C. Zhang).

http://dx.doi.org/10.1016/j.matdes.2017.01.039 0264-1275/© 2017 Elsevier Ltd. All rights reserved.

Amorphous alloys, also known as metallic glasses, with a shortrange ordered and long-range disordered atomic structure have recently attracted increasing attention for researchers as advanced materials. For example, Al-based amorphous alloys with higher wear resistance

S.X. Liang et al. / Materials and Design 119 (2017) 244–253

and corrosion resistance are employed as coating by cold gas spray [1]. Fe-based amorphous alloys with high mechanical properties, soft ferromagnetic and high glass forming ability (GFA), exhibit super potential in industrial applications [2]. In addition to the easy alteration of chemical compositions, the amorphous alloys present a high surface stability and corrosion resistance in various applications [3], such as the high anticorrosion ability in strong acids or alkalis for Zr-based amorphous alloy [4]. Recently, due to the ultrafast water treatment efficiency and the superior chemical property [5], amorphous alloys are also treated as superior candidates for environmental wastewater remediation. It is found that the Fe\\Si\\B\\Mo and Fe\\Si\\B metallic glasses present a very high potential for the acid orange II dye degradation [6,7]. Using Fe\\B amorphous alloy as a catalyst exhibits 89 and 1.8 times faster than the commercial iron powder and Fe\\B crystalline alloy, respectively, when treating the direct blue 6 dye [8]. The reusability of the amorphous alloys is also a very attractive catalytic performance in wastewater purification [5,9-11]. The reusability of the Fe\\Si\\B\\Y powder in amorphous state is achieved to 13 cycles [11]. Fe\\Si\\B metallic glass exhibits 30 times of reusability while activating persulfate for methylene blue (MB) degradation [12]. The dye adsorption behavior of Fe\\Si\\B and Fe\\Si\\B\\Cu\\Nb amorphous alloys for brilliant red 3B-A has also been studied [13]. The Fe\\B\\Y metallic glass with weak atomic bonds of amorphous atoms presents a high reactivity in degrading Orange G dye [14]. Extensive endeavors due to different compositions of metallic glasses have been made for investigating the mechanism for water treatment, such as Mg-based [15–17], Co-based [18], and Fe-based alloys such as Fe\\B [8], Fe\\Si\\B [19], Fe\\Mo\\S\\B [6,20] and Fe\\Si\\B\\Cu\\Nb [21]. Therefore, the study of metallic glass as a catalyst for water remediation is an active and challenging topic. The water treatment technology is a significant issue in industrial settings. In recent years, various water treatment processes have been established, such as biological methods (microorganisms, biosorption, etc.) [22,23], physical methods (adsorption on solid phases, ion exchange, etc.) [24,25] and chemical methods (ozonation, chemical precipitation, etc.) [26,27]. Among these methods, advanced oxidation processes (AOPs) have been extensively studied as a promising technique due to their superior degradation and mineralization efficiency on pollutants in wastewater [12,13,28–31]. Compared to other methods, most of the non-biodegradable compounds are easy to be mineralized to CO2, small molecules or other harmless products [32, 33]. For example, Jiang et al. [34] reported that bisphenol A removal rate in aqueous solution increased up to 97% by Fe2 +/persulfate and 100% by Fe0/persulfate system; Jia et al. [29] employed nano-sized ZnO as a photocatalyst to degrade cibacron brilliant yellow 3G-P dye; Idel-aouad et al. [35] reported that 99.3 ± 0.2% decolorization and 84 ± 5% mineralization were achieved by using heterogeneous Fenton reaction for C.I. acid red 14 under rational conditions. It is recently reported that sulfate radicals (SO4•−) with a high redox potential of 2.60 V are easy to be produced by UV [36], ultrasound [37], transition metal ions (such as Fe2+) [38], or thermal activation [39] by persulfate. The produced sulfate radicals are relatively stable in water at neutral pH and can be homogeneously distributed in the aqueous solution [40]. On the other hand, the persulfate (E0 = 2.01 V) with the benefits of convenient storage, pH-independence and low cost also can be solely used for degrading various organic pollutants [41]. Due to the superior properties of non-toxicity, effectiveness and low price [38,42], zero valent iron (ZVI) has been promising attracted by many researchers as a persulfate activation catalyst [34]. The ZVI can convert to Fe2+ through corrosion in persulfate or aqueous solutions under both aerobic (Eq. (2)) and anaerobic (Eq. (3)) conditions by the reactions Eqs. (1)–(4) [43]:

245

Fe0 þ 2H2 O→Fe2þ þ H2 þ 2OH−

ð3Þ

Fe0 þ S2 O8 2− →Fe2þ þ 2SO4 2−

ð4Þ

Fe0 þ 2S2 O8 2− →Fe2þ þ 2SO4 2− þ 2SO4 •

−

ð5Þ

Thus, the ZVI can act as a slow releasing source of Fe2+ and also provide a novel way to generate sulfate radicals without involving iron ions (Eq. (5)) [44]. However, the high ferrous ions leaching rate will cause secondary pollution, which requires additional pressure to eliminate redundant ferrous ions [44]. Therefore, an appropriate Fe-based catalyst that features sustainable development is highly in demand for wastewater remediation. Malachite green (MG), i.e. basic green 4 (C23H25ClN2, 4-[(4dimethylaminophenyl)-phenyl-methyl]-N,N-dimethyl-aniline), a cationic triphenylmethane dye, is one of the largest group of hazardous dyes. It is commonly utilized in the aquaculture industry as an effective fungicide [45], textile industry as a coloring agent [46] and industries of leather, paper and pharmaceutical owing to its low price, readily availability and high efficiency [47]. However, due to its genotoxic and carcinogenic nature, extensive studies report that MG has serious drawbacks when existing in the water, such as the potential influences on human reproductive and immune systems [48]. Therefore, it is significant to explore an effective technique of dye removal for remediating toxic components in MG solution. In this work, the MG dye degradation and mineralization were investigated by using Fe73.5Si13.5B9Cu1Nb3 metallic glass ribbons as a catalyst, an alternative Fe2 + releasing source, for activating persulfate under UV–Vis light. The influenced parameters on MG degradation and pseudo-first-order kinetic model were studied in detail. Moreover, recycling experiments were carried out to fully discuss the stability and feasibility of the Fe73.5Si13.5B9Cu1Nb3 ribbons. 2. Experimental methods 2.1. Materials and chemicals Malachite green dye was supplied by Ji'an Haomai Fine Chemical Industry Co., Ltd., China (Product Number: 150120). Fig. 1 shows the MG dye nature ability of self-conversion between MG molecules and MG leucocarbinol. The physical characteristics of MG are summarized in Table 1. The sodium persulfate (Na2S2O8) was supplied by BDH Chemicals Ltd., Poole, England. Amorphous alloy Fe73.5Si13.5B9Cu1Nb3 was manufactured by vacuum melt-spinning [49–51]. All the alloy ribbons having the thickness of 30–40 μm were cut into approximately 5 × 20 mm (brittleness). The 18.2 MΩ·cm Milli-Q water was employed throughout experiments for all aqueous solutions. All chemicals used in the experiments are in analytical grade and there is no need to further purification. 2.2. Catalyst characterizations X-ray diffraction (XRD) was employed to analyze the structure and phase contents of the ribbons by using a PANalytical Empyrean diffractometer with Co-Kα radiation at ambient temperature. The scanning electron microscope (SEM) equipped with EDS detector (JEOL 6000, Japan) was used to investigate the surface topography and atomic composition. The surface characterization was also recorded by ultravioletvisible diffuse reflectance spectrum (UV–Vis DRS) by a Lambda 35 UV–Vis Spectrometer using BaSO4 as the reference (Shelton, CT, USA).

Fe0 →Fe2þ þ 2e−

ð1Þ

2.3. Analytical methods

2Fe0 þ O2 þ 2H2 O→2Fe2þ þ 4OH−

ð2Þ

All the initial dye solutions were freshly prepared before each experiment in order to minimize variance in concentration due to hydrolysis

246


Fig. 1. Conversion between MG (left) and MG leucocarbinol (right).

and direct photolysis of MG [52]. Specific MG dye aqueous solution of 1000 ppm was extracted by a macropipette (SocorexIsba S.A., Switzerland) and diluted to a specific dye concentration. The absorbance peak of MG was located at λmax = 618 nm. All dye decolorization experiments were conducted in a 250 mL glass beaker by a VortexGenie 2 mixer (Scientific Industries, Inc. USA) under ambient temperature. The light source was supplied by a 300 W simulated solar light lamp (Perfectlight Scientific Pty Ltd., Beijing, China). At given time intervals (i.e. 1, 2, 5, 10, 15, 30, 45 and 60 min), approximately 4 mL of the dye aqueous sample was extracted from the reacting solution following with the characterization in turn by UV–Vis spectrometer (Shelton, CT, USA) and the total organic carbon (TOC) removal of the specimen was also analyzed by a TOC-5000 CE analyzer (Shimadzu, Japan) for the mineralization analysis. For Fe, Si leaching experiment, the sample solutions were diluted 10 times with 2% w/w nitric acid (HNO3) and then filtered by a 0.45 μm filter before the ICP-OES test (Optima 8300 ICP-OES Spectrometer, Perkin-Elmer). The pH value (~2.8) after adding persulfate was treated as initial pH value of MG solution throughout all experiments in this work. The MG removal rate was obtained using Eq. (6):

3. Results and discussions 3.1. Characterizations The XRD patterns of as-received and 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons are shown in Fig. 2(a). Identical recycled ribbons (0.5 g·L−1) are obtained after dye treatment under the following conditions: MG concentration of 20 ppm, persulfate concentration of 1.00 mmol·L−1 and irradiation intensity of 7.7 μW·cm−2. Both XRD patterns of as-received and recycled ribbons present a broad diffraction peak at 2θ = 40–60°, indicating that all the ribbons are mainly in the amorphous state [53–56]. However, it is noticed that the maximum diffraction peak at 2θmax = 51.49° of 5th run recycled ribbons exhibits a higher diffraction intensity, indicating α-Fe is gradually crystallized on the surface of recycled Fe73.5Si13.5B9Cu1Nb3 ribbons during dye degradation process under UV–Vis irradiation [57,58]. The atomic d-space in Fe73.5Si13.5B9Cu1Nb3 alloy is calculated as 0.253 nm by applying Eqs. (9) and (10). d ¼ 7:7=k

Color Removal ð%Þ ¼ ½ðC 0 −C Þ=C 0 100%

ð6Þ

where C0 and C are the initial dye concentration and dye concentration at time t (min), respectively. The MG decomposition was fitted by the pseudo-first-order kinetic model as shown in Eq. (7): C ¼ C 0 expð−kobs t Þ

ð7Þ

where C0 is the initial dye concentration; C is the dye concentration at time t and kobs is the calculated reaction rate. The Eq. (7) can be rewritten as Eq. (8): ln ðC 0 =C Þ ¼ kobs t

ð8Þ

For analysis of catalytic stability and reusability, identical Fe73.5Si13.5B9Cu1Nb3 ribbons were reused for dye degradation throughout 5 times. After each time, all ribbons were taken out from the beaker and then washed by Milli-Q water followed by ultrasonic cleaning for 3 min. Table 1 Structure and characteristics of malachite green (chloride compound). Structure

Empirical formula Molecular weight (g·mol−1) λmax (nm)

C23H25ClN2 364.911 618

k ¼ ð4π sinθÞ=λ

ð9Þ ð10Þ

where d is the adjacent atoms distance (d-space), λ is the wavelength of X-ray source used (Co-Kα: 0.179 nm), and k is the modulus of the scattering vector. The relatively larger distance between atoms results in higher free energy in ribbons compared with d = 0.248 nm of Fe78Si9B13 ribbons [59]. This result indicates that the Fe atoms in Fe73.5Si13.5B9Cu1Nb3 ribbons are easier to be excited to Fe2 + compared to the more compact atomic arrangement in Fe78Si9B13 alloy [60]. However, Nb atoms in Fe73.5Si13.5B9Cu1Nb3 alloy tend to form oxide films covering on the ribbon surface leading to lower reaction activity but a more stable protection of buried Fe atoms during the dye degradation [3,61]. Therefore, the Fe73.5Si13.5B9Cu1Nb3 ribbon might lead to a high surface stability when performing the MG dye degradation. Fig. 2(b) presents the UV–Vis DRS pattern of the as-received and 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons. The peaks at 236 nm and 300 nm of the as-received Fe73.5Si13.5B9Cu1Nb3 ribbons indicate isolated iron species in the tetrahedrally or octahedrally coordinated framework position and a small degree of oligomeric iron species, respectively [62,63]. No obvious band is observed from 300 to 450 nm and after 450 nm, revealing that no large oxide is formed and the iron species on the as-received ribbon surface distributes homogeneously, respectively [62]. After recycled for 5 times, a new intense peak at the band of 272 nm is observed, indicating that the framework position of isolated iron species is different from the as-received ribbons. In addition, the iron oxides (excepting α-Fe) are gradually formed on the surface of ribbons due to the band at 590 nm. Fig. 3 shows the SEM images of the as-received and 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons. As seen from Fig. 3(a), the free surface of the as-received ribbons is practically smooth without any apparent defects, whereas Fig. 3(c) presents the roller-contact surface being relatively rough. The inclusion of Nb enhances deposition rate of Si atom


247

Fig. 3. SEM micrographs of free surfaces of (a) as-received and (b) 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons and roller-contact surfaces of (c) as-received and (d) 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons.

the maximum absorbance peak at 618 nm rapidly decreases to invisible within 30 min, indicating that the chromophore in MG molecules has been completely removed. It is also interesting to notice that there is a slight blue-shift of maximum absorbance peak from 618 nm to 606 nm, which is attributed to N-demethylation reactions in conjunction with the reaction of an adduct of sulfate radical [46]. Notably, there is a new absorbance peak appeared at λ = 295 nm after 60 min, which is possibly caused by benzene ring opening and cleavage of the central carbon [65]. Fig. 5 shows the comparable results of MG degradation by solely adding persulfate and Fe73.5Si13.5B9Cu1Nb3/persulfate under UV–Vis irradiation. Persulfate (E0 = 2.01 V) having strong redox potential is easy to be activated to sulfate radicals (SO4•−, E0 = 2.60 V) under the light or heat. [36,39]. It is commonly treated as the dominated reagent for organic compounds degradation [42]. The MG degradation by sole persulfate activation under UV–Vis irradiation is presented in Fig. 5. The corresponding reactions are shown as Eqs. (11) and (12): Fig. 2. (a) XRD patterns and (b) UV–Vis DRS of as-received and 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons (MG concentration: 20 ppm, ribbons dosage: 0.5 g·L−1, persulfate concentration: 1.00 mmol·L−1 and irradiation intensity: 7.7 μW·cm−2).

S2 O8 2− þ hv→2SO4 •

−

ð11Þ

−

SO4 • þ MG→products and growth of a thick SiO2 film on Fe73.5Si13.5B9Cu1Nb3 ribbons surface, leading to a stable protection of buried Fe from corrosion. After 5th recycled, the morphology of free surface in Fig. 3(b) only presents a slight corrosion, whereas in Fig. 3(d) on the roller-contact surface, some newly generated products are formed near the “grooves”. Table 2 shows the EDS analysis of both surfaces of the as-received and the 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons. Clearly, the value of the Fe atomic percentage on the as-received free/roller-contacted surface is very close to the nominal value in Fe73.5Si13.5B9Cu1Nb3 alloy (73.5%). The sharp decrease of Fe atoms on the marked corrosion area for the 5th run free surface reveals that the new precipitations are generated on the surface, and they are mainly in the form of Si/Nb oxides. The generated SiO2 films are easy to be removed by stirring, however, the Nb oxides are more stable on the ribbons surface and hard to be removed [64].

In comparison, the MG degradation efficiency sharply increases after adding Fe73.5Si13.5B9Cu1Nb3 ribbons. As shown in Fig. 5, 100% degradation of MG is achieved within 30 min after adding Fe73.5Si13.5B9Cu1Nb3, whereas only 77% of MG degradation is achieved by solely persulfate at the same condition. It demonstrates that the Fe atom in the Table 2 EDS result of atomic ratio on the as-received and 5th run recycled Fe73.5Si13.5B9Cu1Nb3 surface. Fe73.5Si13.5B9Cu1Nb3

Element

Energy (keV)

Mass%

Atom%

As-received free surface

Si Fe Cu Nb Si Fe Cu Nb Si Fe Cu Nb Si Fe Cu Nb

1.739 6.398 8.040 2.166 1.739 6.398 8.040 2.166 1.739 6.398 8.040 2.166 1.739 6.398 8.040 2.166

10.88 74.65 0.58 13.90 23.79 47.08 0.38 28.74 10.34 75.55 0.76 13.34 14.62 59.95 1.00 24.42

20.58 71.00 0.48 7.94 42.24 42.03 0.30 15.43 19.62 72.09 0.64 7.65 27.79 57.33 0.84 14.04

5th run free surface

3.2. Dye degradation 3.2.1. UV–Vis spectra The UV–Vis spectrum of MG degradation is shown in Fig. 4. It is observed that the absorbance peaks of MG are located at λ = 315 nm, λ = 425 nm and λ = 618 nm, respectively. The peaks at λ = 315 nm and λ = 425 nm show an apparent decrease after 30 min under UV–Vis irradiation, indicating that the breakage of whole conjugated aromatic structure in MG [46]. Simultaneously,

ð12Þ

As-received roller-contact surface

5th run roller-contact surface

248


demethylation, reduction, benzene ring-removal and oxidation to the N,N-dimethylaniline [66] and further mineralized to final products 2− H2O, CO2, NO− thereby requiring a longer performance for 3 , and SO4 complete mineralization [67]. In addition, as seen from Fig. 6, there is no significant leaching of Fe (Fe2 + or Fe3 +) or Si ions for Fe73.5Si13.5B9Cu1Nb3 catalyst during the degradation of MG dye. Such performance indicates the advanced surface stability of the catalyst and further evidences the superiority of the catalyst in wastewater treatment. In order to investigate the mechanism of MG degradation, various parameters (i.e. dye concentration, ribbon dosage, persulfate concentration and light intensity) are studied in the following sections.

Fig. 4. UV–Vis spectra of MG degradation by Fe73.5Si13.5B9Cu1Nb3/UV–Vis/persulfate system at different time intervals (MG concentration: 20 ppm, ribbons dosage: 0.5 g·L−1, persulfate concentration: 1.00 mmol·L−1, UV–Vis irradiation intensity: 7.7 μW·cm−2).

Fe73.5Si13.5B9Cu1Nb3 alloy plays an important role (donating electrons) for activating persulfate (Eqs. (3)–(5) and (13)) [39,43,44]. The valence electrons receive a relatively weak constraint due to the absence of periodicity in the atomic arrangement of amorphous alloy [7]. The Fe atoms in the amorphous alloy having weak atomic bonds are easy to donate electrons, thereby accelerating the conversion from Fe0 to Fe2+ to further enhance the dye degradation efficiency. −

Fe2þ þ S2 O8 2− →Fe3þ þ SO4 • þ SO4 2−

ð13Þ

3.2.2. Effect of MG concentration Fig. 7(a) shows the effect of MG concentration on color removal from 5 ppm to 100 ppm under UV–Vis irradiation. It is observed that the color removal rate increases with decreasing MG concentration. 100% of decolorization can be achieved within 15 min at 5 ppm concentration of MG. In contrast, only 38% decolorization is observed at 100 ppm at the same time. The first-order kinetic model with a range of dye concentrations is shown in Fig. 7(b) and the corresponding rate constants kobs are presented in Table 3. Notably, the highest kobs value (0.1991 min−1) is observed at the 5 ppm dye concentration and it sharply decreases with increasing dye concentration to 100 ppm. This result indicates that lower dye concentration is more favorable to the MG decomposition [68]. The likely reason is that the UV–Vis light is easy to transmit through dye molecules for persulfate activation at low concentration of MG dye. The sufficient activated SO4•− ensures the high degradation efficiency for MG molecules, which can be explained by Eqs. (4), (11)–(13). Increasing the dye concentration would increase the numbers of MG molecules and thus block the light activation for persulfate, resulting in slower dye degradation efficiency [29,30].

To further investigate the mineralization efficiency of MG, the TOC removals during the dye degradation are shown in Fig. 6. During the dye degradation and mineralization, chromophore bond in the dye molecules is firstly attacked by the produced SO4•− to cause a fast dye color removal. Afterward, six by-products are proposed to be generated, including leucomalachite green (LMG), desmethyl malachite green, (4Dimethylamino-phenyl)-phenyl-methanone, 3-Dimethylamino-phenol, N-methylated diaminotriphenylmethane and N,N-dimethylaniline [66]. A series of dye mineralized progress can be involved as N-

3.2.3. Effect of catalyst dosage Fig. 8(a) presents the effect of ribbon dosages in the range of 0.1 g·L−1–2.0 g·L−1 on MG degradation (20 ppm) in the presence of 1.00 mmol·L−1 persulfate under UV–Vis irradiation of 7.7 μW·cm−2. All the dye solutions reach complete degradation after 60 min at various ribbon dosages. Only persulfate without adding ribbons still can fully degrade MG solution after the same irradiation time, but presenting a much slower reaction rate (Table 3). The organic matters

Fig. 5. Comparable UV–Vis spectra of MG degradation by using and without using Fe73.5Si13.5B9Cu1Nb3 ribbons within 30 min under UV–Vis/persulfate system, (inset) comparable color removals of MG in percentage vs. time (MG concentration: 20 ppm, ribbons dosage: 0.5 g·L−1, persulfate concentration: 1.00 mmol·L−1, UV–Vis irradiation intensity: 7.7 μW·cm−2).

Fig. 6. TOC removals of MG dye and Fe, Si leaching concentrations (catalyst dosage: 0.5 g·L−1, irradiation intensity: 7.7 μW·cm−2, dye concentration: 20 ppm and persulfate concentration: 1.0 mmol·L−1).


249

contacting area between the persulfate anions and Fe73.5Si13.5B9Cu1Nb3 ribbons surface, supplying faster production rate of SO4•−. The first-order kinetic constants kobs in Fig. 8(b) and Table 3 also show that the reaction rates increase with increasing ribbon dosages. The 2.0 g·L− 1 ribbon dosage for degrading 20 ppm MG shows the highest reaction rate of 0.1939 min−1, whereas the lowest reaction rate of 0.0455 min− 1 occurs at 0.1 g·L−1 ribbon dosage. It is known that excess Fe2+ would scavenge sulfate radicals which would cause a reduction of degradation efficiency (Eq. (14)) [70,71]. However, increasing the ribbon dosage to 2.0 g·L−1 would not lead to the scavenging of SO4•− due to the relatively slow Fe leaching from the ribbons. This is one of the most important advantages by using metallic glass as the catalyst. Further increasing the ribbon dosage from 1.0 g·L− 1 to 2.0 g·L−1 has slight effect on MG degradation, only from 89% to 99% within 15 min. As a consequence of the economic view, 1.0 g·L− 1 is the desired ribbons dosage in the following experiments. −

Fe2þ þ SO4 • →Fe3þ þ SO4 2−

ð14Þ

3.2.4. Effect of persulfate concentration In order to further study the importance of persulfate on the MG degradation, persulfate concentrations from 0 to 2.00 mmol·L−1 were

Fig. 7. (a) Effect of dye concentration on color removals in percentage vs. time; (b) variation of ln (C0/C) vs. time at different dye concentrations (ribbons dosage: 0.5 g·L−1, persulfate concentration: 1.00 mmol·L−1 and irradiation intensity: 7.7 μW·cm−2).

decomposition by iron metals normally takes place with direct surface contact [69]. Increasing the catalyst dosage would cause more

Table 3 The pseudo-first-order kinetics of MG degradation at different reaction conditions. Ribbons MG concentration dosage (g·L−1) (ppm)

Persulfate concentration (mmol·L−1)

Irradiation intensity (μW·cm−2)

5 10 20 50 100 20 20 20 20 20 20 20 20 20 20 20

1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 0.10 0.25 0.50 2.00 1.00 1.00 1.00

7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 0 11.1 14.8

0.5 0.5 0.5 0.5 0.5 0.1 0.3 1.0 2.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Rate constant k (min−1) k

R2

0.1991 0.1073 0.0849 0.0473 0.0414 0.0455 0.0706 0.1179 0.1939 0.0244 0.0458 0.0558 0.2246 0.0446 0.1450 0.1624

0.9966 0.9966 0.9923 0.9938 0.9963 0.9969 0.9929 0.9899 0.9898 0.9880 0.9907 0.9983 0.9974 0.9904 0.9868 0.9945

Fig. 8. (a) Effect of ribbon dosages on color removals in percentage vs. time; (b) variation of ln (C0/C) vs. time at different ribbon dosages (MG concentration: 20 ppm, persulfate concentration: 1.00 mmol·L−1 and irradiation intensity: 7.7 μW·cm−2).

250


3.2.5. Effect of irradiation intensity Fig. 10(a) shows the effect of irradiation intensity from 0 to 14.8 μW·cm−2 on degrading MG. Notably, only 93% MG degradation is achieved without irradiation in 60 min, indicating that the Fe73.5Si13.5B9Cu1Nb3 alloy still presents a strong ability for activating persulfate in darkness. After introducing 7.7 μW·cm−2 of irradiation intensity, a significant increase of MG degradation occurs as a consequence of completely degraded within 30 min. Increasing intensity

from 7.7 to 11.1 μW·cm− 2 could enhance the dye degradation from 75% to 95% within 15 min, however, further increasing the irradiation intensity from 11.1 to 14.8 μW·cm−2 only slightly enhances the MG degradation rate. It is known that the UV–Vis irradiation could promote the production rate of SO4•−, leading to faster reaction rate for dye degradation than that without light irradiation. In addition, the light energy also could much enhance the conversion from Fe0 to Fe2+, the electrons on Fe atoms are in more active state owing to external energy from irradiation [7]. The reaction rates in Fig. 10(b) also show a close reaction rate of 0.1450 min−1 at 11.1 μW·cm−2 and 0.1624 min−1 at 14.8 μW·cm−2 (Table 3), indicating that the irradiation intensity of 11.1 μW·cm−2 is the threshold in the present condition. Basically, sulfate radicals are generated by the destruction of peroxide bond in persulfate. The estimated peroxide bond length and energy in persulfate molecules are 1.497 Å and 33.5 kcal·mol−1, respectively [72]. However, it cannot predict the behavior of persulfate under UV–Vis irradiation only based on the need of bond energy because the energy absorption ability of molecules is also important [73]. For further study the superior catalytic performance of Fe73.5Si13.5B9Cu1Nb3 alloy, Table 4 summarizes the rate constants of different types of ZVI including micro-sized ZVI (micro-ZVI), nano-sized ZVI (NZVI) and amorphous ZVI in this work. According to Nam and Tratnyek [74], the rate constant of 0.380 min− 1 is obtained from

Fig. 9. (a) Effect of persulfate concentration on dye color removals in percentage vs. time; (b) variation of ln (C0/C) vs. time at different persulfate concentration (MG concentration: 20 ppm, ribbon dosage: 0.5 g·L−1 and irradiation intensity: 7.7 μW·cm−2).

Fig. 10. (a) Effect of irradiation intensity on persulfate concentration in percentage vs. time; (b) variation of ln (C0/C) vs. time at different irradiation intensity (MG concentration: 20 ppm, ribbon dosage: 0.5 g·L−1, persulfate concentration: 1.00 mmol·L−1).

used for investigation in this work. As shown in Fig. 9(a), only less than 10% of dye decolorization could be achieved by solely using Fe73.5Si13.5B9Cu1Nb3 ribbons within 60 min. It is a consequence of the MG self-photosensitization under UV–Vis irradiation [52] in addition to slight dye molecules adsorption on the catalyst surface. At the concentrations of 0.10 mmol·L− 1 and 0.25 mmol·L−1, the dye color removals increase to 75% and 100% within 60 min, respectively, indicating the quantity of sulfate radicals plays a significant role in the dye degradation. The first-order kinetic model is shown in Fig. 9(b) and the corresponding data are summarized in Table 3. Clearly, the rate constant kobs at 2.00 mmol·L−1 persulfate (0.2246 min−1) is almost triple times higher than that at persulfate concentration of 1.00 mmol·L−1 (0.0849 min−1), indicating sufficient sulfate radicals are generated for dye degradation when persulfate concentration is at 2.00 mmol·L−1.


251

degrading orange II dye using 200 g·L−1 micro-ZVI. A much lower dosage of 0.165 g·L−1 NZVI particles is used to degrade acid black 24 dye but achieving the same order of magnitude of the rate constant as micro-ZVI [75], which is attributed to the significant specific surface area of NZVI. In this work, the identical magnitude of the rate constant can be observed by only using 1.0 g·L−1 amorphous ZVI ribbons. According to Zhang et al. [7], the surface area normalized rate constants (kSA) can be described as the materials ability for dye degradation due to linear dependence between rate constants and iron surface area concentration, as shown in Eqs. (15) and (16) [7,8]: ρa ¼ S=V

ð15Þ

where S is the total surface of ZVI and V is the volume of the solution; kSA ¼ kobs =ρa

ð16Þ

where kobs is the calculated reaction rate constant and ρa is the area dosage. As seen from Table 4, the value of kSA in this work is far greater than those using micro-ZVI and NZVI, indicating that amorphous ZVI would receive much larger degradation potential when using the same catalyst dosage. 3.2.6. Stability and reusability Surface stability and catalytic reusability are two of the most important capabilities using amorphous alloy as the catalyst. All the recycling experiments in this work are operated under the same conditions: 20 ppm dye concentration, 0.5 g·L−1 ribbons dosage, 1.00 mmol·L−1 persulfate concentration and 7.7 μW·cm−2 light intensity. Fig. 11 shows catalytic reusability using Fe73.5Si13.5B9Cu1Nb3 ribbons from 1st run to 5th run for degrading MG dye. Clearly, 100% MG degradation is reached within 30 min for the fresh use and only slight decrease is observed from 2nd to 5th run. After 30 min reaction from 2nd to 5th run, the dye solution still shows at least 90% of degradation rate, suggesting that the catalyst can be reused at least 5 times with acceptable MG dye degradation. Fig. 11(b) shows the corresponding reaction rates from the fresh used to the 5th used Fe73.5Si13.5B9Cu1Nb3. Notably, the reaction rates of the fresh used ribbon and 2nd used ribbon are 0.0852 min−1 and 0.0599 min−1, respectively. It is interesting to notice that the reaction rate of the 5th used ribbon (k = 0.0549 min−1) is very close to 2nd used. The possible reason of the slight decay after the 2nd used is that SO2 layers are gradually formed on the ribbon surface during the MG degradation process. Such a layer is easy to be removed [64] by the vortex-stirrer to continuously supply ferrous ions for activating persulfate [43]. Such performance provides a significant enhancement for the surface stability, thereby further improving the catalytic reusability [64,76], as shown in the aforementioned SEM images (Fig. 3(d)). Therefore, it is believed that Fe73.5Si13.5B9Cu1Nb3 ribbons presenting an outstanding long-term stability and recycle feasibility in this work will open a new opportunity for the wastewater remediation. 4. Conclusion The investigation of MG dye degradation indicates that persulfate could be effectively activated by Fe73.5Si13.5B9Cu1Nb3 metallic glass. According to the characterizations by XRD, UV–Vis DRS and SEM, the 5th run recycled Fe73.5Si13.5B9Cu1Nb3 ribbons present a slight decay. The

Fig. 11. (a) MG degradation efficiency by Fe73.5Si13.5B9Cu1Nb3/UV–Vis/persulfate system with fresh and reused Fe73.5Si13.5B9Cu1Nb3 ribbons within 30 min; (b) variation of ln (C0/C) vs. time of fresh and reused Fe73.5Si13.5B9Cu1Nb3 ribbons (MG concentration: 20 ppm, ribbons dosage: 0.5 g·L−1, persulfate concentration: 1.00 mmol·L−1 and irradiation intensity: 7.7 μW·cm−2).

precipitated substances on the 5th run ribbons surface are confirmed as α-Fe, iron oxide and Si, Nb oxides. The production of Nb oxides promotes the protection of Fe73.5Si13.5B9Cu1Nb3 ribbon surface. It is found that the MG with concentration of 20 ppm could be completely degraded within 30 min under the conditions of persulfate concentration of 1.00 mmol·L−1, ribbons dosage of 0.5 g·L−1 and light irradiation intensity at 7.7 μW·cm−2. The increase of persulfate concentration, ribbons dosage and light irradiation apparently improves reaction rates. All the dye decomposition processes fit well with the pseudo-first-order kinetic model. The recycled experiments show that Fe73.5Si13.5B9Cu1Nb3 metallic glass can be reused at least 5 times with an acceptable dye degradation rate, presenting a superior surface stability and reusability. The Fe73.5Si13.5B9Cu1Nb3 metallic glass in this work receives a high potential

Table 4 Comparable dye degradation results of rate constants (k) by using various Fe-based catalyst. ZVI type Amorphous ZVI Micro-ZVI NZVI a

Mass dosage (g·L−1) 1.0 200 0.165

Area dosagea (m2·L−1) 6.7 × 10 1.4 23.2

−3

Dye

kobs (min−1)

kSA (L·m−2·min−1)

Ref.

Malachite green Orange II Acid black 24

0.118 0.380 0.199

17.612 0.268 0.009

This work [74] [75]

The area is calculated by assuming each Fe73.5Si13.5B9Cu1Nb3 ribbon as approximately 5 × 20 mm.

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for activating persulfate in dye degradation, providing a fundamental basis for developing alternative catalyst in wastewater treatment.

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