264 © Published by College of Education for Pure Science (Ibn Al-Haitham), University of Baghdad. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License Synthesis, Characterization of Nickel Cobaltite Nanoparticles and Its Use in Removal Methyl Green Dye from Aqueous Solution Maryam Abdulsatar Abduljabar1* and Sundus Hadi Merza 2 1,2Department of Chemistry, College of Education for Pure Science (Ibn Al-Haitham), University of Baghdad, Baghdad, Iraq. *Corresponding Author. Received: 11 April 2023 Accepted: 23 May 2023 Published: 20 July 2024 doi.org/10.30526/37.3.3398 Abstract In this study, nickel cobaltite (NC) nanoparticles were created using the sol-gel process and used as an adsorbent to adsorb methyl green dye (MG) from aqueous solutions. The adequate preparation of nickel cobaltite nanoparticles was verified using FT-IR, SEM, and X- ray diffraction (XRD) studies. The crystalline particle size of NC nanoparticles was 10.53 nm. The effects of a number of experimental variables, such as temperature, adsorbent dosage, and contact time, were examined. The optimal contact time and adsorbent dosage were 120 minutes and 4.5 mg/L, respectively. Four kinetic models—an intraparticle diffusion, a pseudo-first- order equation, a pseudo-second-order equation, and the Boyd equation—were employed to monitor the adsorption process. Modeling of the experimental data showed that the pseudo- second-order model accurately captured the adsorption kinetics due to the high value of the correlation coefficients (R2). MG dye is gradually adsorbed to the NC nanoparticles through boundary layer diffusion and intraparticle diffusion. The results of the thermodynamic analysis showed that the MG dye adsorption was endothermic and a nonspontaneous phyisorption process. Keywords: Weber and Morris, boyd model, EDX analysis, percentage removal. 1. Introduction The presence of dyes in the water stream has a significant impact on daily life. Because dyes are produced in millions of tons globally and utilized in both small- and large-scale industries, such as the leather industry, food industry, cosmetic industry, textile industry, and pharmaceutical industry [1]. Dyes are primarily responsible for the higher mortality rates of kidney, liver, and bladder malignancies. Dyes have complicated structures with aromatic rings linked to various functional groups. Due to their high thermal and chemical stability, many dyes are resistant to degradation by light, heat, and natural oxidants, making dye removal from wastewater extremely important [2,3]. A number of technologies have been employed to remove dyes, such as advanced oxidation [4], biological treatment [5], using natural materials https://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0002-2698-8112 mailto:mariam.abd2105@ihcoedu.uobaghdad.edu.iq?subject=mariam.abd2105@ihcoedu.uobaghdad.edu.iq https://orcid.org/0000-0003-4206-3133 mailto:sundus.h.m@ihcoedu.uobaghdad.edu.iq IHJPAS. 2024, 37( 3 ) 265 for biosorption, adsorption on activated carbon, coagulation/flocculation [6], ultrafiltration [7], and reverse osmosis [8]. Most of these technologies are very expensive. In particular, nanotechnology presents a promising method for the adsorption of dyes from aqueous solutions due to the distinctive physicochemical features of the nanoparticles, such as high chemical reactivity, conductivity, magnetic and optical properties, as well as catalytic potential. Nanomaterials have been investigated for the adsorption of various contaminants, metal ions [9, 10], dyes [11–13], and antibiotics [14]. Multiple types of nano adsorbents have been created and are being used to treat wastewater, such as nanoparticles, nanofibers, nano clays, silica nanoparticles, and carbonaceous nanomaterials. In recent years, low-cost and nontoxic [15] nanoparticles, particularly metallic and bimetallic oxides, have been employed to eliminate contaminants. Nickel cobaltite (NC) in a spinel structure, AB2O4, is one of the essential bimetallic oxides. In comparison to a single metal element, bimetallic oxides demonstrated better adsorption performance [16]. In this study, NC was prepared and characterized using FT-IR, XRD, SEM, and EDX analysis. NC is used as an adsorbent to remove methyl green (MG) dye from an aqueous solution. MG is a cationic triphenylmethane that is frequently used in biology and medicine to modify the color of solutions as well as as a photochromophore to ignite coagulated films [17]. The MG molecular structure is depicted in Figure 1. The kinetic and thermodynamic characteristics, as well as factors like temperature and adsorbent dosage that affect adsorption, were taken into consideration when carrying out the study. Figure 1. Methyl green dye structure 2. Materials and Methods 2.1 Chemicals and Materials Methyl green (MG) dye with a maximum wavelength (λ max =618 nm), chemical formula C26H33Cl2N3 and molecular weight of 458.47 mole/L were provided by WINLAB LIMITED- UNITED KINGDOM and utilized without further purification. BDH supplied Co(NO3)2•6H2O and Ni(NO3)2•6 H2O. The NaOH was purchased from the GCC Company. 2.2 Characterization methods The average size of the NC was studied using an X-ray powder diffractometer (XRD), 6000 Shimadzu (Japan), using Cu (1.54060), voltage: 40.0 kV, and current: 30.0 mA. Shimadzu 8400s (Japan) spectrophotometers were used to study the characteristics of functional groups. IHJPAS. 2024, 37( 3 ) 266 Moreover, scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques (MIRA3 TESCAN, Czech) were used to examine the morphology and chemical components of NC adsorbent. 2.3 Preparation of nickel cobaltite Nickel cobaltite (NC) was synthesized following the procedure outlined by Imranullah et al. with some modifications [18]. About 0.1 M Co (NO3)2.6H2O and 0.1 M Ni (NO3)2.6H2O were mixed using a magnetic stirring hotplate to form a pink solution. When the temperature reached 50–60 °C, a solution of NaOH (50 mL) was added gradually using a separating funnel (pH around 9–10). The color of the solution gradually changed from dark red to blue and finally to green. The gel form was obtained after the addition of NaOH was completed. The gel was washed several times with water and ethanol using a centrifuge and finally dried at 100 °C for 2 hours. The NC nanoparticles were obtained after calcining the dried sample at 300 °C for 2 hours.. 2.4 Adsorption Experimental Prior to the adsorption procedure, standard solutions in the range of 1–15 mg/L were prepared daily by serial dilution from the MG stock solution, which was prepared by dissolving 0.1 g in 1000 ml of distilled water. The maximum absorption was assessed using a UV-visible spectrophotometer (Shimadzu 1800, Japan). The calibration curve was created using the Beer- Lambert law by plotting the absorption results against the standard dye solution concentration. The determined slope served as a guide for determining concentration in the remaining experiments. Figure 2 displays the calibration curve. 0 2 4 6 8 10 12 14 16 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 y= 0.0351+0.1253x R2= 0.9958 A b s o rb a n c e C (mg/L) Figure 2. Calibration curve of MG dye solution. Equation 1 was used to calculate the MG dye adsorption capacity at time t (mg/g) [19]. 𝑞𝑡 = (𝑐° − 𝑐𝑡)𝑉 𝑚 (1) Where c° (mg/L) and ct(mg/L) are the initial MG concentrations and the concentration of the MG dye solution at t time, respectively. The volume of the MG solution is V (L), and the weight of the NG adsorbent is m (g). The removal efficiency (R%) of MG was estimated using Eq 2. R = (𝐶°−𝐶𝑡) 𝐜° × 100 (2) IHJPAS. 2024, 37( 3 ) 267 3. Results and Discussion 3.1 Characterization of NC 3.1.1 The XRD analysis The X-ray pattern of NC nanoparticles is presented in Figure 3. It has been found that NC has a crystalline structure. The XRD diffractogram of NiCo2O4 exhibits Bragg reflection peaks at 2θ=19.145°, 31.261°, 36.849°, 44.792°, 55.066°, 59.231°, and 65.022°. All Bragg peaks are in good agreement with the Joint Committee on Powder Diffraction Standards (JCPDS) (card no. 073-1702)[20]. Table 1 lists the interplanar spacing (d) and diffraction peak positions (2θ) of the prepared NiCo2O4 and the JCPDS. The matching in (d) and the small displacement change in (2θ) support the production of NC. Using Debye-Scherer Equation 3[21], the average crystallite size (d) of the NiCo2O4 particles was computed. 𝑑 = (0.94 𝜆)/(𝛽𝐷 cos 𝜃) (3) Where βD is the diffraction peak's full width at half maximum (FWHM) in radians, θ is the Bragg diffraction angle peak, and λ is the X-ray wavelength of Cu-K =0.1542 nm. The calculated average crystallite size is 10.53 nm. The broad peaks and low intensity support the crystalline grain's modest size. 10 20 30 40 50 60 70 80 0 20 40 60 80 100 120 140 In te n s it y Two theta NC (4 0 0 ) (5 1 1 ) (4 4 0 ) (4 2 2 ) (1 1 1 ) (2 2 0 ) (3 1 1 ) Figure 3. The XRD diffractogram of NC nanoparticles. The lattice constant (unit cell dimension) of the spinel structure NC crystal was calculated using the cubic lattice formula. 𝑎 = √ℎ2 + 𝑘2 + 𝑙2 (4) Where the Miller indices are h, k, and l. The computed value of (a) is tabulated in Table 1. The values of a are quite similar to the NiCo2O4 provided in the JCPDS 20-0781 file [22]. IHJPAS. 2024, 37( 3 ) 268 Table 1. Compares NiCo2O4 diffraction peak positions and interplanar spacing (d) with JCPDS (card no. 073- 1702 and 00-020-0781). (2θ) degree (d) spacing Mille index NiCo2O4 prepared JCPDS ( 073-1702) NiCo2O4 prepared JCPDS ( 073-1702) Lattice constant (111)a 19.145° 18.928° 4.63 - 8.019 (220) 31.261° 31.152° 2.85 2.87 8.06 (311) 36.849° 36.705° 2.43 2.45 8.059 (400) 44.792° 44.635° 2.02 2.03 8.08 (422)b 55.066° 44.635° 1.65 - 8.083 (511) 59.231° 59.115° 1.55 1.56 7.79 (440) 65.022° 64.963° 1.43 1.43 8.089 a (4.69) b (1.65) JCPDS (card no. ) 00-020- 0781[23] 3.1.2 The FT-IR analysis The NiCo2O4 exhibits a wide band in its spectra between 2500 and 3500 cm-1, which is caused by O-H stretching [24]. The H-O-H bending vibration mode is indicated by the bands between 1300 cm-1 and 1500 cm-1 . The bands between 520 cm-1 and 650 cm-1 were attributed to the stretching vibration of the Ni–O and Co–O [21]. 4000 3500 3000 2500 2000 1500 1000 500 T ra n sm it ta n ce (Wavenumber cm-1) NC HOH bending Co- O Ni- O OH streshing Figure 4. The FT-IR spectrum of NC nanoparticles. 3.1.3 SEM analysis The main controlling parameters to obtain various NC structures are solvents, reaction time, and temperature [25]. Figure 5 depicts the SEM image of NC prepared using distilled water and calcined at 300 °C for 2 hours at 135 kx and 5.00 kx magnification. The NC-prepared images showed the growth of nanorods as well as aggregated nanoparticles as plates. The EDX analysis of NC nanoparticles confirms that there are no foreign elements in the sample. IHJPAS. 2024, 37( 3 ) 269 Figure 5. The SEM image and EDX analysis of NC nanoparticles. 3.2 Effects of temperature, contact time and dosage on MG removal The impact of contact time and temperature on the percentage removal of MG using NC adsorbent is clearly shown in Figure 6. The adsorption achieved equilibrium within 100 minutes and continued after this point. This can be explained by the fact that at the beginning of the adsorption process, there were a lot of empty active adsorption sites on the surface of the NC. As the contact time increased, the active adsorption sites were gradually occupied and diminished. The optimal contact time for MG is 120 minutes. The effect of temperature on the adsorption percentage of MG dye from solution on NC adsorbent was determined at different temperatures of 288 K, 298 K, and 308 K. As seen in Figure 6, the adsorption percentage increased significantly from 27.59% at 288 K to 56.37% at 308 K. The enhanced interactions between the empty spaces on the adsorbent surface and the MG dye molecules, as well as the increased molecular mobility of the dye molecules, are likely the causes of the preferred removal of MG dye with temperature increases. It is also evident that as the temperature rises, the removal percentage rises as well [26]. The outcomes of the experiment show that the adsorption of MG onto the NC adsorbent is an endothermic process. IHJPAS. 2024, 37( 3 ) 270 0 20 40 60 80 100 120 140 160 180 200 0 10 20 30 40 50 60 R % Time (min) 288 k 298 K 308 K Figure 6. Effect of temperature and contact time on MG removal. With a set starting concentration of 15 mg/L and a constant temperature of 298 K, the adsorption of MG on the NC was tested with different dosages of adsorbent ranging from 0.001 g to 0.005 g. The initial dye removal percentage increased as the adsorbent dosage increased, as shown in Figure 7. This may be attributed to the increased surface area and the unoccupied dye-binding sites on the adsorbent surface. The saturation of the free active sites by MG dye caused a modest increase in the removal percentage. The optimal adsorbent dosage for future adsorption investigations was found to be 0.0045 g. 0.001 0.002 0.003 0.004 0.005 25 30 35 40 45 50 R e m o v al ( % ) Dosage (g) Figure 7. Effect of adsorbent dosage on percentage removal of MG dye. 3.3 Adsorption kinetics study To characterize the kinetics of pollutants (MG dye), the movement of the adsorbate within and onto the surface sites of the adsorbent, and possible rate-limiting steps, a variety of adsorption kinetic models such as pseudo-first-order (PFO), pseudo-second-order (PSO), Weber's intraparticle diffusion, and Boyd have been used [27–29]. Equations 5 and 6 define the linearized forms of the PFO and PSO models, respectively. ln(qe − qt) = lnqe − k1t (5) IHJPAS. 2024, 37( 3 ) 271 t qt = 1 k2qe 2 + t qe (6) The slope and intercept of the ln (qe-qt) vs. time plot can be used to calculate the rate constant k1 (1/min) and the amount of MG absorbed at equilibrium (qe) (mg/g). The weight of the adsorption capacity of MG absorbed at the time (t) in minutes is represented by the qt (mg/g). The rate constant of the second order, k2(g/mg. min), was determined from the intercept of the plot of t/qt vs. t. Table 2 displays the PFO and PSO kinetic parameters. Table 2. Modeling coefficients for the kinetic adsorption of PFO and PSO (MG = 15 mg/L, NC = 3 mg/L). Table 2 clearly shows that the correlation coefficient R2 of PFO kinetics is lower than that of PSO and that the computed qe (mg/g) values do not closely match the experimental qe (mg/g) values. Also, the value of the sum of squares error (SSE) is higher than the PSO. Therefore, it can be inferred that the PFO kinetic model Figure 8 is inappropriate for predicting the adsorption kinetics of MG dye onto NC adsorbent. The PSO model accurately predicts the adsorption process with a higher R2 value, as shown in Figure 8. This implies that the chemical process was in control of the adsorption process. Similar behavior has been seen in the adsorption of MG by multi-walled carbon nanotubes decorated with nickel nano ferrite adsorbent and graphene sheet adsorbent [30, 31]. Temperature Pseudo-first order model 288 298 308 qe, exp (mg/g) 9.19 16.52 18.79 qe, cal (mg/g) 19.90 12.11 9.01 K1(1/min) -0.0286 -0.0252 -0.0229 SSE 1.183 0.1173 1.565 R2 0.9576 0.9854 0.7019 Pseudo-second order model qe, cal (mg/g) 11.54 18.08 19.84 K2( g/mg. min) 0.00196 0.00317 0.00477 SSE 1.070 0.692 0.271 R2 0.9627 0.9939 0.9971 IHJPAS. 2024, 37( 3 ) 272 0 20 40 60 80 100 120 140 160 -3 -2 -1 0 1 2 3 ln ( q e -q t) Time (min) 288 K 298 K 308 K PFO 0 20 40 60 80 100 120 140 160 180 200 0 5 10 15 20 288 K 298 K 308 K PSO t/ q t time (min) Figure 8. The PFO and PSO models for MG dye on NC adsorbent at different temperatures. In order to forecast the rate-controlling diffusion mechanism within the adsorption system, the Weber and Morris intraparticle diffusion and Boyd models were applied. The linearized form of the Weber and Morris model is shown in Eq 7. 𝑞𝑡 = 𝑘𝑑𝑡0.5 + 𝑐 (7) Where the slope and intercept of the qt against the t0.5 plot can be used to get the diffusion rate constant, kd (mg/g.min-0.5), and the boundary layer thickness constant, c (mg/g). The plots of intra-particle diffusion are depicted in Figure 9. The plot of the entire process consists of three linear plots rather than a single line. The kinetic variables with the regression coefficient R2 for each step are presented in Table 3. The first step is assigned to external diffusion. In this step, the adsorbate penetrates the liquid film surrounding the adsorbent. The concentration difference between the bulk solution and the adsorbent's surface acts as the driving force of external IHJPAS. 2024, 37( 3 ) 273 diffusion. The low driving force may be the cause of the low k value [32]. The second stage is focused on adsorbate diffusion and adsorption within the pores and active sites of the adsorbent. The high value of kd demonstrates the high adsorption rate and verifies the existence of many active sites. The third step shows the equilibrium in the adsorption process. The kd values have minimum values due to the low MG dye concentration remaining in the solution and the saturation of the pores and active sites of the adsorbent. On the other hand, boundary diffusion (C) values increased as temperature rose, indicating that surface adsorption became more prominent [33]. According to the adsorption data, the process of MG dye removal from an aqueous solution is complicated and involves both boundary layer diffusion and intra-particle diffusion. 2 4 6 8 10 12 14 2 4 6 8 10 12 14 16 18 20 288 K 298 K 308 K q t t 0.5 Figure 9. Weber and Morris intraparticle diffusion. Table 3. Parameters for intra-particle diffusion based on three steps. T/ K Step 1 Step 2 Step 3 Kd (mg.g-1. min-0.5) C (mg/g) R2 Kd (mg.g-1. min-0.5) C (mg/g) R2 Kd (mg.g-1. min-0.5) C (mg/g) R2 288 0.6615 0.4685 0.9162 1.2415 -3.6134 0.9848 0.9321 7.9356 0.9741 298 0.7095 5.0737 0.9818 0.790 8.4322 0.9695 0.2230 13.5222 0.9977 308 0.5043 9.6947 0.9127 0.8106 9.6693 0.9203 0.0724 17.8309 0.9613 The Boyd equation [34]was used to forecast the regulating mechanism for MG adsorption on NC adsorbent. It is stated as following equation −ln(1 − 𝐹) = 𝑘𝑏𝑡 (8) F is determined as qt/qe, and the Boyd constant is kb (min-1). According to the Boyd equation (linear behavior, zero intercept value), the diffusion of adsorbate in a bounded liquid layer around the adsorbent is the slowest process; otherwise, film diffusion and pore diffusion regulate the adsorption mechanism. As can be observed from Figure 10, linear behavior with a zero value of intercept is not reported, highlighting the significance of both film diffusion and pore diffusion as the governing processes for the mechanism of MG adsorption onto NC. Thus, IHJPAS. 2024, 37( 3 ) 274 it was found that there were multiple steps involved in the adsorption mechanism of MG onto NC. 0 20 40 60 80 100 120 140 160 0 1 2 3 4 5 6 288 K y=-0.1700+0.0286x (R2=0.9576) 298 K y=0.3141+0.0251x (R2=0.9918) 308 K y=0.3445+0.0335x (R2=0.9421) - ln (1 -F ) time (min) Figure 10. Boyd model for MG adsorption on NC nanoparticles 3.4 Activation energy and thermodynamic parameters The Arrhenius relationship is used to express the pseudo-second-order rate constant k2 of dye adsorption as a function of temperature. [35]: ln 𝑘2 = ln 𝐴 − 𝐸𝑎 𝑅𝑇 (9) Where R and T are the gas constant (8.314 J/mol K) and T is the absolute temperature, Ea is the Arrhenius activation energy of adsorption, and A is the Arrhenius factor. A straight line with a slope of -Ea/R is produced when ln k2 is plotted against 1/T, as shown in Figure 11. A value of 32.79 (kJ/mol) was determined as the activation energy. This value indicats that the adsorption has a low potential barrier and relates to a phyisorption process [31, 36]. 0.00325 0.00330 0.00335 0.00340 0.00345 0.00350 -6.4 -6.2 -6.0 -5.8 -5.6 -5.4 -5.2 y= 7.4713-3944.9x R2=0.9993 ln k 1/T Figure 11. Plot of the activation energy for the MG adsorption on NC nanoparticles. IHJPAS. 2024, 37( 3 ) 275 The Eyring equation was used to calculate the thermodynamic activation parameters of the process, such as enthalpy (H*), entropy (S*), and free energy (G*), to demonstrate the impact of solution temperature on the transport/kinetic process of MG dye adsorption [35, 37]. 𝑙𝑛 𝑘2 𝑇 = 𝑙𝑛 𝑘𝐵 ℎ + ∆𝑆∗ 𝑅 − ∆𝐻∗ 𝑅𝑇 (10) Where kB and h are the Boltzmann constants 1.3807×10−23 (J/K) and the Planck constant 6.6261×10−34 (J s), respectively. k2 is the PSO rate constant. When plotting ln(k2/T) against 1/T, a straight line should be drawn. The slope (-∆H*/R) and intercept (ln kB/h + ∆S*/R) of the line were used to determine the ∆H* and ∆S* Figure 12. The activation enthalpy change value (30.32 kJ/mol) indicates that the adsorption is endothermic in nature. A negative value of ∆S* (191.09 J/mole) indicates the presence of an associative mechanism in the adsorption process. The free energy of activation (∆G*) for the adsorption of MG onto NC was calculated using Equation. 11. ∆𝐺∗ = ∆𝐻∗ − 𝑇∆𝑆∗ (11) The free energies were calculated to be 85.35, 87.27, and 89.18 at 288, 298, and 308 K, respectively. The fact that the positive values of Gibbs free energy of the MG adsorption at all temperatures suggest a non-spontaneous process. Similar behaviors have already been reported in other research [35, 38]. 0.00325 0.00330 0.00335 0.00340 0.00345 0.00350 -12.0 -11.8 -11.6 -11.4 -11.2 -11.0 y=0.7748-3647.2x R2=0.99909 (l n k 2 /T ) 1/T Figure 12. Plot of lnK2/T versus 1/T. 4. Conclusion The SEM-EDX, XDR, and FT-IR analyses confirmed the formation of NC. The adsorption of MG dye onto NC nanoparticles was investigated under optimum conditions. The fitness of the experimental data was assessed using PFO and PSO kinetic models. The PSO reaction model is a good fit for the MG dye's adsorption kinetics. The computed qe values from the PSO model were in good agreement with the experimental findings. The results of the adsorption of IHJPAS. 2024, 37( 3 ) 276 MG onto NC showed that adsorbent dosage and temperature influenced the adsorption and removal of the adsorbent. The percentage removal of MG dye increased with an increase in the dosage of the adsorbent. At 308 K, the highest percentage removal of 56.37% was observed. According to the kinetic studies, equilibrium in the MG adsorption on NC was reached after 120 minutes. The results indicated that both film diffusion and intraparticle diffusion are involved in the mechanism of adsorption, in accordance with the Weber and Morris equation and Boyd plot. The adsorption of MG is a phyisorption process that is non-spontaneous and endothermic. Acknowledgment The authors the Department of Chemistry, College of Education for Pure Science (Ibn Al- Haitham), University of Baghdad, for facilitating the work of the practice in this article. Conflict of Interest The authors declare that they have no conflicts of interest Funding There is no financial support. References 1. Hassaan, M.; Nemr, A. El; Hassaan, M.A. Health and Environmental Impacts of Dyes: Mini Review. American Journal of Environmental Science and Engineering. 2017, 1(3), 64–67. https://doi.org/10.11648/j.ajese.20170103.11. 2. Katheresan, V.; Kansedo, J.; Lau, S. Y. Efficiency of various recent wastewater dye removal methods: A review. 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