Effect of the addition of Mg, Ti, Ni on the decoloration performance of AlCrFeMn high entropy alloy

1. Introduction

Recently, high entropy alloys (HEAs) are becoming new research frontier in the metallic materials community [1]. Different from conventional metallic alloys based on one or two principal elements, HEAs generally contain at least four principal elements [2,3]. Recent researches have reported many attractive properties of HEAs, such as mechanical properties [[4], [5], [6]], good temper-softening resistance [4,5], good thermal stability [7,8], excellent corrision resistance [9,10], etc. The promising properties of high entropy alloys offer the potential to be used in many applications. In order to further broaden the application of HEAs, we plan to apply HEAs in the research of wastewater treatment. The reason why we want to apply HEAs in sewage treatment is that metals or alloys have become the important research objects in wastewater treatment due to their fast decolorization, environmental protection, low cost and low efficiency [[11], [12], [13]]. As a kind of special alloys, HEAs own many properties superior to those of traditional metals or alloys. What's more, HEAs are in metastable state which prenents high active and high energy, so we suppose that HEAs will have good capacity of wastewater treatment. Moreover, Lv et al. [14] have reported that AlCoCrTiZn HEA show remarkable properties in decolorization of azo dyes. Azo dyes are the largest synthetic dyes used in the textile industry and are considered to be potential genotoxic and carcinogenic [12,15]. Most of azo dyes have complex aromatic structure, strong resistance to microorganisms, and strong resistance to traditional wastewater treatment processes [[16], [17], [18]]. So, we decided to design a new HEA system with good decolorization performance of azo dyes.

It is well known that the composition and particle size of powder alloys have a large effect on their catalytic and decolorization performance [19]. Therefore, in order to obtain HEAs powder with good decolorization ability, we select high activity elements to synthesize HEAs by ball milling. HEAs usually consist of a simple solid solution phase, such as single FCC phase or single BCC phase [1,20]. Since BCC phase is hard and brittle, we predict that HEAs containing single BCC phase is more easily milled into relatively uniform fine powder particles during ball milling process. In addition, HEAs composed of higher active elements also exhibit higher activity. Therefore, we intend to design a high entropy alloy system composed of active elements containing single BCC phase. At present, Al-based metallic glass alloys [21], Mn-Al binary alloys [22] and zero-valent iron [16,23] have been reported, and they have significant decolorization properties of azo dyes. It also indicates that elements such as Al, Cr, Fe, Mn, Ti, Zn and Co all have high activity, among which Fe, Cr and Mn are of all BCC crystal structures. Although Al is of FCC crystal structure, but Al contributes to the formation of BCC phase when it acts as a component of a high entropy alloy, and Al has very high activity [24]. Then we choose AlCrFeMn as the main research object, and hope that it will show good decolorization performance. In order to further improve the decoloration efficiency of AlCrFeMn, we consider adding the fifth elements (Mg or Ti or Ni) to AlCrFeMn. Here, equiatomic AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) HEAs synthesized by mechanical alloying for the decoloration of azo dye DB6 is reported in this work.

2. Experimental

Raw materials of aluminum, chromium, iron, manganese, magnesium, titanium, and nickel having a particle size ranging from 1 to 10 μm and a purity of more than 99.9 wt% were used for mechanical alloying to synthesize AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni). The zirconia jars and the balls were used as a grinding medium, and stearic acid was a process control agent. The ball milling process was carried out in a high energy planetary mill with a rotational speed of 350 rpm and a ball to powder ratio of 25:1. In order to prevent the powder from overheating, self-polymerizing or adhering to the inner wall of the jars and the grinding balls, a grinding procedure was established that the ball grinding mill rotated for 50 min and then stood for 10 min. High purity argon was added to each zirconia tank prior to milling to prevent oxidation of the feedstock during the synthesis. After processing for 50 h, samples were taken from zirconia jars for further characterization. DB6 (C₃₂H₂₀N₆S₄O₁₄Na₄) was provided by Tianjin Hai Lan Chemical Pigment Co., Ltd. A certain amount of BM HEAs powders were put into 200 ml DB6 azo dye solution each time, and reacted in a beaker with distilled water as solvent. The beaker was placed in a constant temperature water bath, and the dye solution rotated at a fixed speed during the reaction. The pH values of azo dye solutions were adjusted by adding 0.1 M HCl solution or 0.1 M NaOH solution. Deionized water was used throughout this work.

The decolorization process was recorded by UV-vis absorption spectrophotometry (Shimadzu Corporation, Japan) in the range of 400-800 nm, and the decolorization activity of HEAs was evaluated. Approximately 2.5 ml of dye solution was taken out for test each time. The crystal structure of the samples was investigated by X-ray diffraction (XRD, D8 progress, CuKα) and a standard F20 transmission electron microscope (TEM, Tecnai). The surface area was analyzed by means of a nitrogen adsorption method under a Brunauer Emmett Teller (BET, 3h-2000beta). The statistical distribution of the particle size was obtained under a laser particle size analyzer (FJUL1076). Inductively coupled plasma (ICP, PE ICP 8000) was used to detect the residual metal ion concentration in the dye solution, and the pH values of the DB6 solution were measured using a digital pH meter (FE20, Mettler Toledo). An electromechanical stirrer (JB90-D) rotated at a certain speed to evenly distribute the HEAs powders in the dye solution.

3. Results and discussion

3.1. Characterization of samples before decoloration

Fig. 1(a) shows that the XRD patterns of AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) HEAs. It can be found that all the four alloys are composed of BCC phase. AlCrFeMn, AlCrFeMnMg, AlCrFeMnNi are only composed of BCC phases and the corresponding crystal planes of the peaks are (110), (200) and (211), respectively. But for AlCrFeMnTi, expect BCC phase, it also contains FCC (111) phase [12]. The TEM micrograph and the selected area electron diffraction (SAED) pattern of AlCrFeMnMg are presented in Fig. 1(b). It can be seen from the bright field image that some of the powders obtained during the ball milling process are close to nanometer in size. Some defects occur during the milling process, which greatly increase the active sites and improve the decolorization ability of HEAs powders. The top right corner of Fig. 1(b) shows selected area electron diffraction (SAED) pattern of AlCrFeMnMg with three crystal planes (110), (200) and (211), which are corresponding to three peaks in the XRD pattern of AlCrFeMnMg.

It is obvious that the activity of metals is closely related to their grain size, specific surface area, morphology and structure, and it can be improved by various processes. At present, the cheapest and simplest method to synthesize HEAs powders is high energy ball milling method. Fig. 2 depicts the SEM images of AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) powders synthesized by mechanical alloying. As presented in Fig. S1, the average paticle sizes of AlCrFeMn, AlCrFeMnMg, AlCrFeMnTi and AlCrFeMnNi are 4.2 μm, 9.8 μm, 7.2 μm and 6.6 μm, respectively, and their particle sizes are close. During ball milling process, powder particles are trapped between the balls or between the balls and the wall.

The obtained HEAs powders are in a metastable state, which is a property that many conventional alloys cannot achieve. Many micro-nanostructured materials have good chemical and catalytic properties, which is a well-known example. The BM HEAs powders shown in Fig. 2 have rough surface and many corrugations and micropores, indicating that the powder have a large specific surface area and a large number of active sites.

3.1.1. Effect of elements addition on decoloration efficiency

The kinetics of DB6 decolorized by AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) is studied by UV-vis spectroscopy. The evolution of the decolorization efficiency (%) during the process was calculated using Eq. (1) [22]:

(C₀ - C_t)/C₀ × 100 (1)

where C_t is the dye concentration at time t and C₀ is the initial concentration of DB6.

As illustrated in Fig. 3, the changes in the absorbance reflect the evolution of the DB6 chromophores, the peaks at maximum value become weaker, indicating cleavage of the azo bonds. Combining with the results of UV-vis spectrum and other similar research, it can deduced that the decolorization products of DB6 are likely to be a small amount of anilines, benzenes and their homologues, as well as some small molecules.

The relationship between the dye concentration and the decoloration time is not a positive proportion. Through the nonlinear curve fitting, it can be found that the decolorization process of DB6 by AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) is in good agreement with the pseudo first order exponential decay kinetics using Eq. (2) [14,25]:

I= I₀ + I₁ exp(-t/t₀) (2)

where I is the normalized intensity of concentration, I₀ and I₁ are fitting constants, t is the reaction time and t₀ is the time when the intensity decrease to e^-1 of the initial intensity. For comparison purpose, all parameters of the decoloration time in this work are represented by t₀.

As shown in Fig. 4, Fig. 4(a) shows the degradation fitting curves, and Fig. 4(b) presents the decolorization time obtained after fitting. The decolorization time values of AlCrFeMn, AlCrFeMnMg, AlFeCrMnTi and AlCrFeMnNi are 12.6 min, 6 min, 10.4 min and 25.6 min, respectively (Fig. 4(b)). Both of Mg and Ti can improve the decoloration performance of AlCrFeMn, but Ni plays a negative role. This may be due to the high activity of Mg and Ti, which increases the overall activity of the alloys, while nickel, due to its lower activity, reduces the activity of the alloy after addition. The activity rank of the three elements is Mg > Ti > Ni. The decoloration rating of the three HEAs (AlCrFeMnM (M = Mg, Ti, Ni)) powders is consistent with the activity rank of the three elements (Mg, Ti, Ni). The reaction activity of AlCrFeMnMg and AlCrFeMnTi is about 2 and 1.2 times higher than that of AlCrFeMn. The specific surface areas of AlCrFeMn, AlCrFeMnMg, AlCrFeMnTi and AlCrFeMnNi are 2.68 m²/g, 2.07 m²/g, 6.08 m²/g, 7.43 m²/g, respectively. Although the specific surface area rank of AlCrFeMnMg, AlCrFeMnTi and AlCrFeMnNi is AlCrFeMnMg < AlCrFeMnTi < AlCrFeMnNi, the activity of AlCrFeMnMg is higher than that of AlCrFeMnTi, and the activity of AlCrFeMnTi is higher than that of AlCrFeMnNi. This indicates that the activity of the fifth elements (Mg, Ti, Ni) has greater influence on the activity of AlCrFeMn than that of the specific surface area.

3.1.2. Comparision with other alloys materials

As presented in Fig. 5, in order to compare with other materials as reported in other papers, the concentration of AlCrFeMn and AlCrFeMnMg is increased to 6 g/L. BM AlCrFeMn and AlCrFeMnMg show excellent performance in the decoloration of azo dye DB6, and their reaction activity is 3 times and 7 times faster than that of BM MgZn-based metallic glass, which is the best one reported in the metallic glasses so far, respectively [14]. The BM MgZn-based metallic glass is about 20 times faster than the BM Fe-based glassy powders, and the decoloration efficiency of the BM Fe-based glassy powders about 200 times faster than that of the widely used commercial ZVI powders [25,26].

3.2. Effect of different experimental conditions on decolorization efficiency of AlCrFeMnMg

3.2.1. Effect of different dye concentration

Fig. 6 presents the decoloration efficiency of 50 mg/L, 100 mg/L, 200 mg /L DB6 solution by 0.5 g/L AlCrFeMnMg during the decoloration process at 25 °C, initial pH = 7. The 200 mg /L DB6 solution is completely decolorized by AlCrFeMnMg within 6 min. The 50 mg/L and 100 mg/L DB6 solution completely decolorized by AlCrFeMnMg only need 1.2 min and 3.5 min, respectively. Obviously, the higher the dye concentration is, the harder it is to be decolorized.

3.2.2. Effect of initial pH

Fig. 7 illustrates the decoloration efficiency of 200 mg/L DB6 by 0.5 g/L AlCrFeMnMg at different pH values. The decoloration reaction of DB6 by AlCrFeMnMg particles is studied by changing the initial pH from 3 to 10, and the reaction times change significantly. The decoloration time of 200 mg/L DB6 by 0.5 g/L AlCrFeMnMg at pH = 3, 7 and 10 is 0.16 min, 0.36 min and 6 min, respectively. The reaction activity of AlCrFeMnMg in alkaline and acidic azo dye solution is about 37.5 and 16.6 times faster than that of in neutral solutions, respectively.

Under acidic conditions, there are three reasons for the decolorization of DB6, the first reason is the reduction of the zero valence metal; the second reason is the reduction of [H] produced by the acid reacting with metals, which plays the most important role in the decoloration of DB6; the third reason is the electrochemical reduction, that is, the different elements can form nano-galvanic cells in AlCrFeMnMg, lively metals act as anode, inert metals act as cathode, dye solution act as electrolyte, the dye molecules are reduced by electron and hydrogen ions at the cathodes. Under neutral conditions, electrochemical reduction plays the most important role. But under alkaline conditions, except electrochemical reduction and zero valence metal reduction, hydroxides are maybe produced in the dye aqueous solution such as Mg(OH)₂, Al(OH)₃, which have strong adsorption capacity and can adsorb a large number of dye molecules, flocculating and precipitating dye molecules.

3.2.3. Effect of temperature

Based on the obtained kinetic rate constants at different temperatures, the activation energy (△E, kJ/mol) of the decoloration process of DB6 by AlCrFeMnMg can be obtained according to the following Eq. (3) [14,27]:

t₀ = $\tau_{0}exp(\Delta{E/RT})$ (3)

where $\tau_{0}$ is a time pre-factor, R is the gas constant, △E is the activation energy and t₀ is the time when the intensity decrease to e^-1 of the initial intensity.

The normalized concentration as a function of treatment time at diff;erent temperatures ranging from 25 to 55 °C of the decoloration of DB6 by AlCrFeMnMg is decipted in Fig. 8a. Plot of (lnt₀) vs. (1000/RT) for estimation of the activation energy of decoloration of DB6 by AlCrFeMnMg is shown in Fig. 8b (initial pH = 7, 0.5 g/L AlCrFeMnMg). For ordinary thermal reactions, the activation energy is usually between 60 and 250 kJ/mol. The activation energy of AlCrFeMnMg is 44.5 KJ/mol for decolorizing azo dye DB6. From Fig. S2, we can find that the activation energy of AlCrFeMn is 54.6 KJ/mol for the reaction of decolorizing azo dye DB6 [28]. It is obvious that the activation of AlCrFeMnMg is 10 KJ/mol lower than that of AlCrFeMn, which is also one reason that the decoloration capacity of AlCrFeMnMg is improved. The decoloration time of 200 mg/L DB6 by 0.5 g/L AlCrFeMnMg at 25 °C, 35 °C, 45 °C and 55 °C is 6 min, 2.18 min, 1.1 min and 0.6 min, respectively. The result shows that the higher the temperature is, the shorter the decoloration time is, so the reaction temperatures also have great influence on the decolorization efficiency of AlCrFeMnMg. The activity of AlCrFeMnMg is greatly improved as the reaction temperature increases.

3.3. Characterization of samples after decoloration

3.3.1. XRD, SEM and TEM analysis of AlCrFeMnMg after reaction

AlCrFeMnMg presents the best decolorization performance in this work, so it is taken as an example for characterization. As seen from the XRD diffraction pattern in Fig. 9a, there are some changes in the composition of AlCrFeMnMg after decolorizing DB6. In addition to the BCC phases, there appears Mg₂Al(OH)₇ phase in AlCrFeMnMg after reaction. In the process of decolorization of DB6 by AlCrFeMnMg, Mg²⁺ and Al³⁺ are combined with hydroxyl ions in water to produce hydroxides Mg₂Al(OH)₇, which has strong ability to adsorb dye molecules. The surface micrograph of AlCrFeMnMg after decoloration are provided in Fig. 9b, and it demonstrates that nanobristles uniformly and loosely distribute on all the surface of the powders, indicating the homogenous corrosion on the surface. The TEM image of AlCrFeMnMg after decoloration shown in Fig. 9c that the nanowires structure is attached to the surface of the powders.

3.3.2. ICP analysis

From Table.S1, one can find that the concentration of each ion left in the solution after treatment is very low and close to the minimum detection values of the instrument. The ICP result indicates that the HEAs will not cause the secondary pollution during the decoloration process.

3.4. Decolorization reaction mechanism analysis

On the basis of the results discussed above, the decolorization mechanism of DB6 using AlCrFeMn, AlCrFeMnMg, AlFeCrMnTi and AlCrFeMnNi can be proposed as follows: When the HEAs powders are introduced to the dye solution, the DB6 molecules are first adsorbed on the surface of the HEAs particles, then the metal atoms in the four HEAs perform as catalyst and react with the –N=N– bonds and break them to single phenyl ring compounds with amine groups as possible products, thereby decolorizing the DB6. However, HEAs exhibit better decolorization properties in this work than most previously reported alloys for the following reasons:

(1) Compared with ZVI, due to special alloy component, the surface of the BM HEAs powders appear electrochemical inhomogeneity and form a large number of tiny galvanic cells between elements such as Mg-Fe, Al-Fe, Al-Cr. These tiny galvanic cells act as the electron donors during the decoloration process that lose electrons and H₂O provides H⁺, while one –N=N– bond of the azo dye receive four H⁺ and four e^- and form –NH₂.

(2) The second reason is the reduction of [H] produced by the reaction between dye solution and metals in HEAs. The appearance of bubbles inside the solution suggests the formation of hydrogen gas as a direct reaction product of the redox process. Quick transfer of electrons between the metals could produce a large number of [H] around the active sites, the electrons derived from the metals are captured by protons to generate nascent hydrogen [H].

(3) The third reason is the combination of unique crystal structure with severe lattice distortion, residual stress stored plastic deformation energy, which is responsible for the excellent capacity of HEAs in decolorizing azo dyes. The unique solid solution structure of BM HEAs virtually is in a non-equilibrium state where atoms possess high potential energy and much more active sites than those in conventional alloys.

(4) The fourth reason is the high reducibility and high activity of the zero-valent metal elements in the BM HEAs. Zero-valent Al, Cr, Fe, Mn, Mg, Ti and Ni all have strong reducibility and high activity.

4. Conclusions

The decolorization process of DB6 by AlCrFeMn and AlCrFeMnM (M = Mg, Ti, Ni) is in good agreement with the pseudo first order exponential decay kinetics. AlCrFeMn shows excellent performance in the decoloration of DB6, 3 times faster than BM MgZn-based glassy powders. The reaction activity of AlCrFeMnMg and AlCrFeMnTi is about 2 and 1.2 times higher than that of AlCrFeMn. BM AlCrFeMn and AlCrFeMnMg show excellent performance in the decoloration of azo dye DB6, and their reaction activity are 3 times and 7 times faster than that of BM MgZn-based metallic glass, respectively. The higher the dye concentration is, the harder it is to be decolorized. The reaction activity of AlCrFeMnMg in alkaline and acidic azo dye solution is about 37.5 and 16.6 times faster than that in neutral solutions, respectively. The ICP result indicates that the HEAs do not cause the secondary pollution during the decoloration process. In conclusion, HEAs have good research value in the application of azo dye decoloration and degradation.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No.51671056) and Jiangsu key laboratory for advanced metallic materials (Grant No. BM2007204).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.03.025.

The authors have declared that no competing interests exist.

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