Abstract
The commercial Fe-Si-B metallic glass (Fe-Si-BMG) powder and the widely used powder of zero valent iron (ZVI), namely Fe0, were used to eliminate the Ni(II) from aqueous solution. Kinetic analysis results indicate that the removal efficiency of Fe-Si-BMG powder in removing Ni(II) is about 38 times faster than that of the Fe0 powder. Morphology observation shows that the product layers on the surface of Fe-Si-BMG powder are composed of homogeneous and loose whiskers, which peel off more easily from the surface during the agitation process, compared with those of Fe0 powder. Chemical composition analysis about the surface of Fe-Si-BMG powders before and after reaction shows that Fe-Si-BMG powder can eliminate the Ni(II) through surface adsorption, reduction, and coprecipitation mechanisms; while Fe0 powder removes the Ni(II) in solution mainly through surface adsorption and coprecipitation mechanisms.
Science Press
Enormous industry wastewater polluted by heavy metal has been discharged every year with the development of manufacturing industry[1]. Heavy metal pollution has already become a worldwide problem due to its toxicity to the plants and animals[2]. For example, the intake of Ni(II) of high concentration may cause headache, dizziness, nausea, vomiting, chest pain, tightness of the chest, dry cough, tachypnea, cyanosis, extreme weakness, and even cancer of lungs, nose and bone[3]. Therefore, great effort has been made to deal with the industrial wastewater containing heavy metal. The powder of zero valent iron (ZVI), namely Fe0, is widely used to treat the industrial wastewater polluted by various heavy metals due to its advantages of environmental friendliness, low cost, and relatively low standard redox potential (E0=Fe2+/Fe0=-0.44 V). For example, Ni(II) with rela-tively positive standard reduction potential of -0.24 V can theoretically be reduced and adsorbed by Fe0 powder. But the fast corrosion of Fe0 powder leads to the rapid decrease in absorption efficiency[4]. Therefore, it is of great significance to develop the effective materials for wastewater treatment.
Recent reports showed that the Fe-based metallic glass with special atomic arrangement and metastable structure can degrade azo dyes more efficiently than Fe0 does. It is reported that Fe-B metallic glass can degrade the tetrasodium salt (Direct Blue 6) 89 times faster than Fe0 powder does[5]; Fe-Si-B metallic glass, namely Fe-Si-BMG, can degrade tetrasodium salt and Sudanorange (Orange II) 60 and 1300 times faster than Fe0 powder does, respectively[6-10]. Hence, the Fe-based metallic glass with special structures may eliminate the heavy metal in wastewater more efficiently.
In this research, the removal efficiency of Fe-Si-BMG and Fe0 powders in eliminating Ni(II) from aqueous solution was investigated. Additionally, the kinetic analyses and removal mechanism of the reaction process were also studied.
The Fe-Si-BMG powder with a nominal composition of Fe78Si8B14 in atomic percentage was purchased from Haoxi Nano Technology Co., Ltd (Shanghai, China). The Fe0 powder was provided with purity>99.5%. The Ni(II) solution was prepared by dissolving NiCl2·6H2O salt in ultra-pure water (18 MΩ·cm). Other reagents of analytical grade were used in this research.
A series of experiments were conducted by agitating 500 mg/L Ni(II) solution with a specific amount of Fe-Si-BMG powder. The mixed solution was kept at a fixed temperature by water-bath method and stirred at 200 r/min. About 5 mL solution was taken out by a syringe and filtered with membrane filter of 0.2 μm at set intervals. The morphology and structure of the powders before and after reaction were analyzed by scanning electron microscopy (SEM, JSM-6700) and X-ray diffraction (XRD, Rigaku D/max-2400) with Cu Kα radiation. The concentration of Ni(II) in filtrate was analyzed via atomic absorption spectroscopy (AAS, M31-AA2600). The Brunauer-Emmett-Teller (BET) surface area analyses of the Fe-Si-BMG powder and Fe0 powder were performed using the nitrogen method with a surface analyzer model (NOVA4000). X-ray photoelectron spectroscopy (XPS) analysis was performed on the ESCALAB250 instrument with Al Kα radiation. The potential dynamic polarization curves were obtained in 100 mg/L NiCl2 solution at room temperature using the CHI660E electrochemical workstation.
2.1 Characterization of Fe-Si-BMG and Fe0 powders
Fig.1 shows XRD patterns of the Fe-Si-BMG and Fe0 powders. It can be seen that the XRD pattern of Fe-Si-BMG powder exhibits a diffraction peak at 2θ=45° corresponding to the amorphous state, and the α-Fe phase can be observed in the XRD pattern of Fe0 powder.
Fig.1 XRD patterns of original Fe-Si-BMG and Fe0 powders
Fig.2 shows SEM morphologies of the Fe-Si-BMG and Fe0 powders. It can be seen that all the particles are well dispersed without aggregation. Compared with the round and smooth surface of Fe-Si-BMG powder (Fig.2a), the surface of Fe0 powder is rather irregular (Fig.2b). Therefore, although the average diameter of Fe-Si-BMG and Fe0 powders is about 10 and 50 μm, respectively, the specific surface area (as) of Fe-Si-BMG and Fe0 powders measured by BET analysis is almost the same, which is 0.329 and 0.341 m2·g-1, respectively.
Fig.2 SEM morphologies of original Fe-Si-BMG (a) and Fe0 (b) powders
The removal efficiency of Fe-Si-BMG and Fe0 powders in removing Ni(II) from aqueous solution was investigated. These two powders with the dosage of 3 g/L were separately put into the Ni(II) solution of 100 mg/L at 298 K. Fig.3 shows the variation of Ni(II) content after addition of Fe-Si-BMG and Fe0 powders for different durations. The removal processes of Ni(II) by Fe-Si-BMG and Fe0 powders fit well with the pseudo-first-order reaction, as expressed by Eq.(1):
Fig.3 Relationship between Ct/C0 and reaction time for Fe-Si-BMG and Fe0 powders
where Ct and C0 are the current and initial Ni(II) content, respectively; t is the reaction time; k is the reaction rate constant; C1 and C2 are constants. The reaction rate constant k for Fe-Si-BMG and Fe0 powders is estimated to be 0.006 and 0.022 min-1, respectively. Considering that the reaction rate is related to the specific surface area, the surface area normalized rate constant kSA=k/ρa is used to describe the interior reaction rate of the materials. ρa is the surface area concentration of the specimen (ρa=asρm, where ρm is the mass concentration of the material). The calculated kSA is 0.0352 and 1.337 L·m-2·h-1 for Fe-Si-BMG and Fe0 powders, respectively. The reaction rate of Fe-Si-BMG powder in removing Ni(II) from aqueous solution is about 38 times faster than that of the Fe0 powders.
The reaction active energy is also a very effective parame-ter[11] to compare the removal efficiency of Ni(II) by Fe-Si-BMG and Fe0 powders. The removal efficiency of Fe-Si-BMG and Fe0 powders in removing Ni(II) from aqueous solution highly depends on temperature. Fig.4a shows the variation of Ct /C0 at different temperatures of Fe-Si-BMG powder, indicating that high temperature is beneficial to the reaction process between Fe-Si-BMG powder and Ni(II) due to the high diffusion rate. The rate constants at different temperatures were calculated by nonlinear regression. Based on these reaction rate constants, the reaction active energy ΔE can be derived using Arrhenius-type equation, as expressed by Eq.(2):
Fig.4 Relationship between Ct/C0 and reaction time at different temperatures (a); Arrhenius plots of -lnk-1000/RT of removing Ni(II)
by Fe-Si-BMG and Fe0 powders (b)
where R is the gas constant, T is the temperature, and A is a constant[12]. The plots of -lnk against 1000/RT of the two powders are shown in Fig.4b. The calculated reaction active energy for Fe0 powder in removing Ni(II) from aqueous solution is 42.0 kJ/mol, indicating a surface-controlled chemical reaction (>29 kJ/mol). Nevertheless, ΔE for the Fe-Si-BMG powder is 21.5 kJ/mol, indicating a diffusion-controlled reaction (8~21 kJ/mol)[13]. Since the Fe-Si-BMG powder with metastable structure has a higher free energy than the thermally stable crystal solid, Fe-Si-BMG powder has much lower reaction active energy than Fe0 powder does in removing Ni(II) from aqueous solution. This result is consistent with the calculated reaction rate constants.
The morphologies of the Fe-Si-BMG and Fe0 powders after reaction for 100 min are shown in Fig.5a and 5b, respectively. It can be seen that the Fe-Si-BMG powder after reaction still keeps the spherical shape and is well dispersed without aggregation. But the Fe0 powder becomes aggregated. It can be noticed that the reaction product peels off from Fe-Si-BMG powder layer by layer during the reaction process due to agitation. Therefore, the used particles remain their spherical shape. Fig.5c shows that some small pieces of product peel off from the powder, indicating that the next layer of product is formed and starts to peel off from the powder. At the same time, there are also some pieces of product representing the last layer which can be observed on the surface of the other powder. Furthermore, Fig.5d shows that the product layers have homogeneous and loose porous structure consisting of numerous whiskers, which may be related to the uniform amorphous structure of metallic glass and the uniform drop style of the product layers. Interestingly, the similar morphology of product layers can also be observed in the removal process of azo dyes and Cu2+ from aqueous solution by amorphous alloys[14-17]. However, for the Fe0 powder, the reaction product layers show an inhomogeneous distribution and seldomly peel off from the powders, which may be responsible for the rapid decay of its efficiency in wastewater treatment. Tang et al[5,6] studied the reaction rate of Fe100-xBx (x=16at%~20at%) amorphous ribbon, FeSiB amorphous ribbon, and their crystalline counterparts in degrading azo dye, and found that the reaction rates of amorphous ribbons are much higher than that of their crystalline counterparts. Elements B and Si contribute to the formation of an incompact product layer and can improve the degradation rate. The results are similar to the ones in this research. Therefore, it can be concluded that the formation of loose product layer on the surface of Fe-Si-BMG powder is related to the Si and B elements in the powder.
Fig.5 Surface morphologies of Fe-Si-BMG (a) and Fe0 (b) powders after reaction for 100 min; magnified morphologies of Fe-Si-BMG powder (c, d)
Fig.6 shows XRD patterns of Fe-Si-BMG and Fe0 powders after reaction. The Fe-Si-BMG powder still retains its amor-phous structure with the product layer covering the surface which is undetectable due to the low resolution of XRD analyzer. However, some diffraction peaks appear in the XRD pattern of Fe0 powder, indicating that the product layer on the surface of Fe0 powder is much thicker than that of Fe-Si-BMG powder. This phenomenon may be related to the fact that the product layer of Fe-Si-BMG powder peels off more easily from the surface than that of Fe0 powder does.
Fig.6 XRD patterns of Fe-Si-BMG and Fe0 powders after reaction
Fig.7 shows the potential dynamic polarization curves of pure iron ribbon, Fe-Si-B amorphous ribbon, and Fe-Si-B crystalline ribbon with the similar size. It can be seen that the Fe-Si-B amorphous ribbon has lower pitting potential and larger current density, compared with the Fe-Si-B crystalline ribbon, indicating that the Fe-Si-B amorphous ribbon is prone to corrosion in the Ni2Cl solution[18]. According to above analysis, the high reactivity of Fe-Si-B amorphous ribbon is due to the metastable amorphous structure and low reaction active energy. Furthermore, Fe-Si-B crystalline ribbon is prone to reacting with the Ni2Cl solution, compared with the pure Fe ribbon. Therefore, this phenomenon should be related to the formation of loose product layer of Fe-Si-B crystalline ribbon due to the addition of Si and B elements. In contrast, besides the high reaction active energy of pure Fe ribbon, the compact product layer also makes it hard to react in Ni2Cl solution, compared with the Fe-Si-B materials.
Fig.7 Polarization curves of pure iron ribbon, Fe-Si-B amorphous ribbon, and Fe-Si-B crystalline ribbon in Ni(II) solution
In order to clarify the removal mechanism of Fe-Si-BMG powder, the composition of Fe-Si-BMG powder before and after reaction for 100 min was investigated by XPS depth analysis. Fig.8 shows XPS spectra of the Fe-Si-BMG powders before and after reaction. The element composition is summarized in Table 1. It can be seen that the oxygen content in the outermost surface layer of the as-received Fe-Si-BMG powder is up to 78.50at%. After the Fe-Si-BMG powder was sputtered by Ar+ for 20 s at the sputtering rate of 0.4 nm/s, the oxygen con-tent is decreased to 3.10at%, indicating that the as-received Fe-Si-BMG powder is surrounded by oxidation layer with the thickness of about 8 nm. Furthermore, the relative atomic ratio of Fe:Si:B=38.9:35.4:25.7 for the outermost surface layer of as-received Fe78Si8B14 amorphous particles, indicating that Si and B are significantly enriched in the surface layer. According to Ref.[19-25], the possible components for the outmost layer of the as-received powder are: FeOx (Fe2+, 710 eV), FeOOH (712, 725.1 eV), Fe3O4 (713.87, 727.2 eV), Si0 (99.2 eV), SiO (Si2+, 102.28 eV), and BxOy (192 eV). After reaction, the surface components of the Fe-Si-BMG powder change slightly into FeOx (710.8 eV), FeOOH (712.5, 724.7 eV), Fe3O4 (715.2, 727.2 eV), Si0 (99.28 eV), SiO (102.13, 101.3 eV), BxOy (192.2 eV), Ni0 (852.2 eV), and Ni2+ (855.7 eV). But it is obvious that all the peak areas of the powder after reaction are larger than those of the as-received one, indicating that Fe, Si, and B elements are more prone to losing electrons to form oxides in NiCl2 solution than in the atmosphere. These oxides peel off from the powder due to agitation and are precipitated in the solution during the reaction process.
Fig.8 XPS spectra of Fe-Si-BMG powders before and after reaction: (a) overall spectra, (b) Fe 2p, (c) Si 2p, and (d) B 1s
Table 1 XPS results of surface element composition of Fe-Si-BMG powders before and after reaction (at%)
| Specimen | Fe | Si | B | O |
|---|
|
Before reaction |
8.37 |
7.61 |
5.52 |
78.50 |
|
Sputtered for 20 s |
77.60 |
6.10 |
13.20 |
3.10 |
|
After reaction |
12.59 |
8.74 |
8.13 |
70.54 |
To further explore the reaction mechanism of Fe-Si-BMG powder in the solution, Ni element on the surface of the Fe-Si-BMG and Fe0 powders was detected by an advanced XPS analyzer. Fig.9 shows XPS spectra of Ni 2p3/2 of the two specimens. Ni2+ (855.8 eV) can be detected on the surface of both Fe-Si-BMG and Fe0 powders, indicating that Ni ions can combine with oxides on the surface and be removed from the solution through adsorption and coprecipitation. Furthermore, the reduced Ni0 (852.4 eV) can be found on the surface of Fe-Si-BMG powder, indicating that the initially dissolved Ni ions may gain electrons during the reaction process and the Ni0 is precipitated on the surface of the Fe-Si-BMG powder. The redox reaction can be described as follows[26]:
Fig.9 XPS spectra of Ni 2p3/2 on the surface of Fe-Si-BMG and Fe0 powders
Therefore, it can be concluded that the dissolved Ni ions in the solution is removed by Fe0 powder mainly through the surface adsorption and coprecipitation, while Fe-Si-BMG powder eliminates the Ni ions through surface adsorption, chemical reduction, and coprecipitation. Thus, Ni ions can be removed by Fe-Si-BMG powder more efficiently.
1) Both the removal processes of Ni(II) by Fe-Si-B metallic glass (Fe-Si-BMG) and powder with zero valent iron (Fe0) fit well with the pseudo-first-order reaction equation. The calculated reaction constant of Fe-Si-BMG powder is 38 times larger than that of Fe0 powder; the calculated reaction active energy of Fe-Si-BMG powder is smaller than that of Fe0 powder, indicating that Fe-Si-BMG powder is more efficient in removing Ni(II) ions from solution.
2) Microstructure observations show that the removal mechanism of Ni(II) in solution by Fe0 powder is mainly surface adsorption and coprecipitation, while that by
Fe-Si-BMG powder is the surface adsorption, chemical reduction, and coprecipitation.
3) The product layers of Fe-Si-BMG powder in Ni(II) solution can peel off easily from the particles by agitation, which also contributes to the better reaction efficiency between Fe-Si-BMG powder and Ni(II) in solution. Therefore, this work provides a new effective material for heavy metal treatment in wastewater.
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