Electrochemical oxidation of methylene blue dye in wastewater using mechanically alloyed high entropy alloy modified carbon paste electrode using cyclic voltammetry

Mechanical alloying is one of the popular, simple, and easy powder metallurgy methods to prepare nanostructured high entropy alloys (HEA). HEAs are modern-day alloys that exhibit significantly improved properties and are used in many unique applications. One such application is using HEA powders for determining the methylene blue dye in wastewater using cyclic voltammetry. We have successfully synthesized the HEA powder of composition 25Fe-19Cr-19Ni-18Ti-19Mn by planetary ball mill and studied their phases, surface morphology, and particle sizes by x-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) respectively. We have fabricated the HEA-modified carbon paste electrode (HEA-MCPE) to study the electrochemical oxidation of methylene blue (MB) dye present in the wastewater. MB is a cationic dye that is toxic, and carcinogenic in high doses; generally used in textile, paper, and leather industries for coloring purposes and discharged into the water sources and thus creating a threat to aquatic animals and humans. Therefore, we must determine the MB dye in waste water regularly. Our fabricated electrode can detect MB dye in wastewater over a pH range of 6 to 7.6 with a significant current response. We have found that, the 4 mg HEA-MCPE and pH 6 are the optimal experimental conditions for achieving a higher rate of electro-oxidation of MB dye. The calculated active surface area for bare and HEA-MCPE is found to be 0.180 and 0.918 cm2 respectively. We have found out that, increase in the concentration of MB from 1 mM to 5 mM increases the anodic peak current linearly due to the increased molecular interaction and the mobility of electrons between the analyte and the electrode surface.


Introduction
Over the past few decades, the research in metallurgy is accelerating with greater innovation in a wide variety of alloys, manufacturing techniques, and their applications. Researchers are developing incredible alloys with various combinations of metals and methods. One such alloy is high entropy alloys (HEA). These are the combination of 5 or more metals of almost equal atomic composition [1]. All the multi-elements in the alloy impart their parental properties to the alloy and result in extraordinary and unique properties like high resistance to corrosion, thermal stability, electrochemical properties, oxidation resistance, excellent electrical conductivity, and fracture toughness even at cryogenic conditions. Apart from all the improved and unique properties of HEAs, we still need to find a better answer for the atomic arrangement of multicomponent lattice structure and the atomic transport, phase diagrams, and their stability at high entropies [2]. Generally, they are Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. composed of multi-elements and exhibit high mixing entropy than the entropy of mixing pure metals. The main feature of HEAs is the increase in the enthalpy with an increase in the entropy of mixing alloys, this permits the addition of more elements to the alloy resulting in excellent properties. Therefore, HEAs are one of the superalloys used in turbines, coatings, soft-magnetic materials [3][4][5], electrochemical sensors [6], and photothermal and nuclear applications [7].
HEAs are prepared from casting, arc melting, deposition, and by various powder metallurgical methods like mechanical alloying, atomization, additive manufacturing methods, etc But, mechanical alloying is proved to be one of the best, simple, most economic, and easy methods to prepare HEAs. This is a solid-state, nonequilibrium method used to produce various alloys at atomic scale through solid solution [8][9][10][11][12][13]. Table 1 shows the list of reported literature on fabrication of HEA by mechanical alloying.
The refined structure (nano range) also imparts unique properties to the mechanically alloyed HEAs. This improves the surface area of the HEA powders and results in excellent surface energy. Due to the excellent surface area, the prepared HEAs could also be used as an electrochemical sensor to detect various neurotransmitters, surfactants, toxic dyes, pharmaceutical drugs, pesticides, heavy metal ions, etc As we know, the transition elements exhibit excellent electrochemical properties and our fabricated HEA is composed of 5 d-block elements namely Fe, Mn, Ti, Ni, and Cr. All these elements impart their combined electrochemical properties to the HEA. Therefore, in near future, HEA could be one of the potential electrochemical sensors. This opens up a new window for metallurgists and electrochemists to use them for various electrocatalytic applications. In the present paper, we have used 15 h ball-milled 25Fe-19Cr-19Ni-18Ti-19Mn nanopowders to fabricate HEA-MCPE to detect MB in wastewater.
Methylene Blue is a thiazine dye mainly used in the treatment of methemoglobinemia, a blood disorder that produces an abnormal amount of methemoglobin in blood [21]. Sometimes, it is also used to treat cyanide poisoning and urinary tract infections [21]. Excessive usage of MB leads to the breakdown of red blood cells, allergy, high blood pressure, vomiting, shortness of breath, headache, and confusion. MB is also used as a coloring material in textile, leather, and paper industries. These companies will discharge a huge amount of MB dye into the surface and groundwater and this causes many diseases. Therefore, we must determine the toxic MB dye in wastewater by a suitable and simple method. There are a handful number of analytical, chemical, chromatographic, and photochemical methods available to determine MB in wastewater, but these methods are tedious, expensive, require large space for big instruments, need multiple processing steps, and lack sensitivity and selectivity. Therefore, there is a great requirement to develop economical, stable, robust, and miniaturized devices to detect the MB dye and other synthetic dyes in less time in an efficient way. Electrochemical methods like cyclic voltammetry are one of the best and well-suited methods which meets all the requirement mentioned above.
In the present paper, we have fabricated the 25Fe-19Cr-19Ni-18Ti-19Mn nanopowders modified carbon paste electrode and determined the electrochemical oxidation of MB dye in wastewater using cyclic voltammetry. Many researchers have used metal oxide, metal nanoparticles, polymers, and composite materials as modifiers to detect various dyes, but no one reported the use of alloys to electrochemically determine the MB

Synthesis of 25Fe-19Cr-19Ni-18Ti-19Mn nanopowders by planetary ball mill
The HEA nanopowders were synthesized by Retsch Planetary Ball Mill PM 100. The Fe, Cr, Ti, Ni, and Mn elemental powders were used as starting metals of high purity and milled for 15 h at 300 rpm speed and a 6:1 ball-to-powder weight ratio under a toluene atmosphere. The milling media is composed of one 500 ml chrome steel jar with 10 mm diameter chrome steel balls.

Preparation of BCPE and MG-MCPE
Zive SP1 galvanostat/potentiostat was used to carry out the electrochemical studies of HEA nanopowders. For this study, we have used 3 electrodes and the details of the electrode used and the preparation of BCPE is reported by the authors in their previous publication [6]. The MCPE was prepared by grinding 2, 4, 6, 8, 10, and 12 mg of HEA nanopowders separately with BCPE composition. The fabricated paste was inserted into a polymer electrode having a 3 mm diameter cavity and which is connected by a copper wire for electrical conductivity and to measure the current response for BCPE and all the different concentrations of HEA-MCPE.
In the present study, 4 mg HEA-MCPE has shown excellent electrocatalytic properties than others. We have also studied the effect of scan rate, pH, and the concentration of the modifier on the electro-oxidation of 1 mM MB dye in wastewater. The MB dye was purchased from Sigma-Aldrich and a buffer electrolyte was prepared from sodium dihydrogen phosphate and disodium hydrogen phosphate purchased from Merck. All the solutions were prepared using Milli-Q water. Figure 1 shows the graphical representation of the experimental studies carried out in the present paper including the electrochemical studies of MB by HEA-MCPE.

Phase analysis by XRD
The phase analysis of 15 h ball-milled HEA nanopowders was carried out using XRD as shown in figure 2. We have noticed that 15 h ball milling is optimum to get the highest reduction in the particle size of HEA nanopowders. Beyond the 15 h of milling could result in agglomeration or re-welding of HEA powders due to the increased surface area and surface energy. Therefore, the XRD spectra reveal the amorphous nature of the powder with peak broadening as shown. As we start the milling, nickel and titanium metals diffuse into the interstitial sites of the Iron and form a solid solution.
As we know from our previous research work in the case of stainless steel powders, before milling one case see the sharp crystalline elemental peaks, but with the ball milling broadening of peaks takes place with decreased peak intensity followed by the diminishing of a few peaks due to the dissolution of metals in the solid solution of the soft metal in the alloy. Here, in the case of our HEA composition, all the metals such as Ni, Ti, Cr, and Mn go into the Fe lattice and result in the homogeneous solution of refined crystallites. The peaks are very much coincideds with the JCPDF reference numbers 01-081-8772, 01-075-9974, and 01-076-8673 respectively. The rate of solid solution formation is also triggered by a high entropy caused during the milling of HEA powders and also by their differences in atomic sizes. The formation of broad peaks in figure 2 is mainly due to the piling up of  residual stress due to the interaction of ball-jar-powders with great impact, and also due to the refinement of particle size and increased lattice strain. This increased defect storage sites, and enhanced surface area is an added advantage for the electro catalytic applications. Therefore, these alloys are potential candidates for electrochemical sensor applications.

Morphological investigation
We have successfully investigated the morphology of the HEA powders, BCPE and HEA-MCPE respectively by SEM. Figure 3 represents the SEM microstructure of 15 h ball milled 25Fe-19Cr-19Ni-18Ti-19Mn nanopowders. As we mentioned earlier, we have optimized the milling time to 15 h, any milling above 15 h could lead to agglomeration and thus reduces the surface area and energy; a negative effect on electrochemical sensor applications. Therefore, 15 h ball-milled HEA powder gives excellent refinement and maximum surface area.
From the figure, it is confirmed that the HEA powders are irregular in shape and size. We can also control the HEA powder morphology by varying the milling parameters like the ball-to-powder ratio, mill speed, size of the milling media, etc The optimized milling conditions used in the present paper to fabricate HEA powder bestowed us with the maximum refinement and highest surface area. For electrochemical sensor applications, the surface area of the modifier plays an important role. Figures 4(a) and (b) represent the SEM morphology of BCPE and HEA-MCPE respectively. Both the electrodes have shown different surface morphology as we see from the figure. The surface of BCPE shows less surface area, does not tightly adhere to neighboring particles, less reactive sites, and exhibits limited porosity. On the other hand, HEA-MCPE depicts excellent bonding, holds firm, maximum surface roughness, reactive sites, and surface area. The increased surface area and the reactive sites in MCPE are due to the extra surface imparted by the HEA particles at the surface of the electrode as shown in figure 4(b) marked by circles. The irregular and uneven surface of HEA-MCPE could also be responsible to increase the active sites, where an electrochemical reaction takes place. Therefore, HEA-MCPE showed maximum anodic peak current during the determination of MB.
We have also performed HR-TEM analysis to study the exact particle size, shape, and lattice spacing of ballmilled HEA powders. Figure 5(a) shows the TEM image of the 15 h ball-milled HEA nanopowders and it depicts the slightly spherical and more irregularly shaped particles of average size 15 nm. The TEM image shows accurate particle morphology compared to SEM; this is due to the differences in the sample preparation. For TEM analysis, HEA powders were dispersed in ethanol and sonicated for complete dispersion to obtain a deagglomerated sample. Whereas SEM does not involve dispersion, this resulted in the agglomeration of HEA powders more rapidly due to their refined structure. Therefore, TEM analysis is more accurate in providing correct morphology compared to SEM. To understand the lattice spacing of HEA powders, we have performed HR-TEM analysis as shown in figure 5(b). The distance between the two marked lines in figure 5(b) is the d-spacing of HEA powder and it was found to be 0.203 nm. Therefore, the calculated lattice parameter for the 15 h ball-milled HEA nanopowders was found to be 0.351 nm. We have observed from the XRD that, up to 10 h ball milling the lattice parameter values were ≈ 0.287 nm; but after 15 h milling, there is a significant increase in the lattice parameter. This is due to the complete refinement in the crystallite at 15 h milling time and also due to the solid solution formation of different metals (Ni, Ti, Mn, Cr, and Fe) having different atomic radii [28].

Electro-oxidation of MB in wastewater 3.3.1. Optimizing the concentration of HEA nanopowders
Fixing the optimum concentration of the modifier decides the efficiency of the electrode in determining any analytes. Therefore, in the first step, we optimized the concentration of the HEA nanopowders to detect MB in the wastewater. To do so, we fabricated the HEA-MCPE of concentrations 0 (BCPE), 2, 4, 6, 8, 10, and 12 mg and studied the anodic peak current response during the oxidation of MB in the wastewater. We have compared the anodic peak current obtained for different concentrations of modifier and the results are plotted in figure 6(a). The plot confirms that the 4 mg HEA-MCPE depicted a maximum anodic peak current of 508.4 μA compared to other HEA-MCPEs. This increase in the current sensitivity is due to the increased electrode surface area, roughness, reactive sites, etc Another reason for the improved current sensitivity of the HEA-MCPE compared to BCPE is the composition of the HEA nanopowders, all the elements used to fabricate HEA powders belongs to  d-block and these transition elements showcase excellent electrochemical sensitivity than most of the other elements. From figure 6(a) it is confirmed that the anodic peak current increased up to 4 mg HEA-MCPE and then decreased gradually to 12 mg HEA-MCPE. Most of the time, an increase in the concentration of the modifier decreases the current sensitivity, therefore selecting the optimum concentration of the modifier is of prime importance to fabricate greater electrodes. Therefore, at the beginning of the electrochemical studies, we must select the proper concentration of the modifier which gives the highest sensitivity. Hence, we have selected 4 mg HEA-MCPE as our standard working electrode for the determination of MB and studied the further electrochemical properties.
Our 4 mg HEA-MCPE not only increased the anodic peak current (Ipa) but also decreased the overpotential. The cyclic voltammogram of BCPE and 4 mg HEA-MCPE is represented in figure 6(b). The Ipa at 4 mg HEA-MCPE was reported to be 508.4 μA and the same anodic peak current at BCPE was recorded to be 99.74 μA only. This huge anodic peak current difference between 4 mg HEA-MCPE and BCPE has depicted the importance of the modifier to increase the sensitivity, robustness, and selectivity of the electrode sensor. This confirms the excellent surface area and porosity of HEA powders formed because of plastic deformation and mechanical refinement during milling. This in turn increased the reactive sites and the mobility of electrons during the electrochemical studies at a 4 mg modifier concentration. We have also calculated the active surface area of the 4 mg HEA-MCPE and BCPE using the Randles-Sevcik equation [29][30][31][32][33][34] as below and the calculated diffusion coefficient of methylene blue for two-electron transfer redox reaction is found to be 5.30 × 10−6 cm 2 s -1 as per the literature [35]. The calculated active surface area for BCPE and HEA-MCPE is found to be 0.180 and 0.918 cm 2 respectively.

Effect of pH
Optimizing the pH plays an important role to obtain good current sensitivity and also the stability of the analytes [36,37]. Some of the analytes are more stable in acid and some are in basic media therefore we must carefully choose the pH of the electrolyte. Choosing the optimum pH can improve the sensitivity, and selectivity and give a clear-cut demarcation of electron mobility and the number of protons involved in the electrochemical reaction. Figure 7(a) depicts the cyclic voltammogram of HEA-modified electrodes in 1mM MB solution at PBS of different pH respectively at a scan rate of 0.1 Vs −1 . The voltammogram shows the irregular variation in the anodic peak current with an increase in pH and this confirms that the MB analyte is unstable at higher pH. But, on the other hand, the electrode potential (Epa) shifts towards the lower potentials (negative potential) as shown in figure 7(b). This negative shift of Epa confirms the participation of protons in the electro-oxidation of MB.
The plot of anodic peak potential with pH is linear and follows the equation, Ep (V) = 0.5235−(0.0651) pH (V/pH) (R 2 = 0.9938), with an excellent linear regression coefficient (R 2 ). This confirms that, the transfer of electrons during the redox reaction of MB mainly depends upon protonation. Further, we have also calculated the number of protons (m) involved in redox reaction based on a graph of pH versus Ep by the below equation, Where R is the gas constant, T is the temperature in kelvin, n is the number of electrons and F is the Faraday constant. The number of protons (m) involved in the electrode reaction is found to be 2.201 and the value is nearly equal to 2. Therefore, the electrochemical redox reaction of MB is taking place with the involvements of 2 electrons and 2 protons. The correlation coefficient for the linear decrease in peak potential with an increase in the pH from pH 6.0 to 7.6 was found to be 0.9938. The MB can be easily oxidized to cationic reactive radical by applying a suitable voltage [38]. MB dye in its cationic form can act as a strong electron/proton acceptor on the surface of the HEA-MCPE [39]. Therefore, oxidation of MB on the electrode can increase the number of protons [22]. Ju et al reported the participation of only 1 H + in the electrode reaction when the pH of the electrolyte is more than 6, as a result of the least effective proton, the rate constant (k0) value increases with the pH [22].
Few authors reported only one oxidation and reduction peak similar to our studies in the present paper and mentioned that reduction of MB takes place through a very fast 2 electron transfer step [24,34,40]. But, some papers reported the existence of 2 oxidation peaks for the methylene blue product leucomethylene due to the adsorption of methylene blue products on the surface of the electrode [41][42][43]. Leventis and Chen reported the importance of pH for the oxidation of MB and they reported that the possibility of 2 oxidation peaks at pH 5.5 to 7.4 is more but at higher scan rates [44]. The second oxidation peak is due to the electro-oxidation of the MB products or intermediates which are very difficult to oxidize [44]. This also depends upon the type of electrode under consideration. Our fabricated electrode HEA-MCPE does not show a second oxidation peak at a scan rate of 0.1 Vs −1 .

Effect of scan rate
The current response is a very important phenomenon that will decide the efficiency, stability, selectivity, sensitivity, and robustness of the electrode [45][46][47][48][49]. Scan rate also reflects the type of electrochemical reactions involved; whether adsorption or diffusion controlled [50][51][52]. Therefore, we have decided to study the stability and sensitivity of the HEA-MCPE by varying the scan rate from 0.1 to 0.8 Vs −1 . We have recorded the anodic peak current of electro-oxidation of MB at the respective scan rate in a PBS of pH 6 as shown in figure 8(a).
The plot shows the increase in the anodic peak current with an increase in the scan rate from 0.1 to 0.8 Vs −1 respectively. This linear increase in Ipa is because of the fast and the direct electron transfer between the MB and the electrode surface (HEA-MCPE) respectively. This linear electrochemical response is also evident that the MB binds strongly with an exceptional electronic coupling on the HEA-MCPE [23]. To understand the type of electrochemical reaction in detail, we have also plotted the anodic peak current versus square root of scan rate as shown in figure 8(b). The correlation coefficient for figures 8(a) and (b) was calculated to be 0.9962 and 0.9836 respectively, and their values are almost equal to 1 therefore, the mass transfer process is diffusion-controlled respectively.

Effect of MB concentration variation
As we know, peak current mainly depends upon the concentration of the analyte, therefore to understand the selectivity and the robustness of the fabricated electrode, we have studied the effect of analyte concentration on the oxidation peak current [53][54][55]. Figure 9(a) depicts the cyclic voltammogram of the different concentrations of the MB analyte using 4 mg HEA-MCPE at 0.1M PBS at pH 6. From the figure, it is confirmed that the increase in the concentration of MB from 1mM to 5mM increases the anodic peak current linearly from 581 to 928 μA respectively. This is due to the increased molecules of MB at higher concentrations, which in turn increases the molecular interaction and the mobility of electrons between the analyte and the electrode surface. Therefore, we can observe the maximum anodic peak current at 5 mM MB concentration in the wastewater. Figure 9(b) shows the plot of different concentrations of MB and the anodic peak current. We can observe the gradual increment in the Ipa with an increase in MB concentration from 1 to 5 mM with a correlation coefficient of 0.9974. The increased current response is also due to the successive increase in the number of MB molecules at the interface of prepared HEA-MCPE.

Conclusions
Successfully fabricated the novel 25Fe-19Cr-19Ni-18Ti-19Mn alloy powder-modified carbon paste electrode to determine MB in the wastewater. No researcher has reported the use of 25Fe-19Cr-19Ni-18Ti-19Mn alloy powder as a modifier in carbon paste electrodes to determine MB or any other analyte. Our fabricated HEA-MCPE can determine the MB in wastewater with excellent sensitivity, selectivity, robustness, and in a simple method. Among all the different concentrations of the modifiers (2, 4, 6, 8, 10, and 12 mg), the 4 mg HEA-MCPE showed an excellent anodic peak current of 508.4 μA and BCPE showed only 99.74 μA. The increased current response is due to the increased porosity, surface area, and surface roughness of the modified carbon paste electrode as confirmed by SEM. We have also investigated the effect of scan rate, pH, and the analyte concentration variation on MB electro-oxidation. Scan rate studies confirmed that the mass transfer process is diffusion-controlled. MB was easily oxidized to cationic radical, which acts as a strong electron/proton acceptor  on the surface of the HEA-MCPE. This increased the number of protons and electrons to two with single oxidation and reduction peaks, unlike the other reported articles. We have also observed the shifting of anodic peak potential to negative values confirming the participation of protons in the electro-oxidation of MB. Therefore, the electrode reaction is very fast in the present study. The pH studies revealed the irregular variation in the anodic peak current with an increase in pH confirming the instability of MB at higher pH. An increase in the concentration of MB from 1mM to 5mM increases the anodic peak current linearly from 581 to 928 μA respectively due to the increased molecular interaction and the mobility of electrons between the MB and the electrode surface. Our fabricated novel HEA-MCPE exhibited excellent sensitivity in electro oxidizing the MB and could be the potential electrode for determining various dyes, pharmaceutical drugs, toxic heavy metals, pesticides, etc in the future. This will create a new research domain for metallurgists and mechanical engineers to use different alloy powders for various electrocatalytic applications.