Facile Synthesis of CuFe₂O₄ Nanoparticles for Efficient Removal of Acid Blue 113 and Malachite Green Dyes from Aqueous Media

Abstract

This work studies the synthesis, characterization, and application of CuFe₂O₄ nanoparticles for the removal of acid blue 113 and malachite green dyes from aqueous media. Utilizing the combustion procedure, CuFe₂O₄ nanoparticles were synthesized using two different fuels: L-alanine (CFA) and L-valine (CFV). Besides, the synthesized CuFe₂O₄ nanoparticles were characterized through some tools, including Fourier transform infrared (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and field emission scanning electron microscope (FE-SEM). XRD analysis verified the creation of a CuFe₂O₄ cubic spinel structure without any contaminants, revealing average crystallite sizes of 26.37 and 17.65 nm for the CFA and CFV samples, respectively. The FTIR spectra exhibited peaks indicative of metal-oxygen bond stretching, verifying the presence of a spinel formation. Elemental analysis via EDX confirmed the stoichiometric composition typical of copper ferrite. In addition, FE-SEM displayed that the CFA and CFV samples are composed of particles with spherical and irregular shapes, measuring average diameters of 188.35 and 132.78 nm, respectively. The maximum adsorption capabilities of the CFA and CFV samples towards acid blue 113 dyes are 281.69 and 297.62 mg/g, respectively. Also, the maximum adsorption capabilities of the CFA and CFV products towards malachite green dye are 280.11 and 294.99 mg/g, respectively. Kinetic and equilibrium studies revealed that the adsorption process of acid blue 113 and malachite green dyes onto the CFA and CFV samples followed the pseudo-second-order model and Langmuir isotherm. Thermodynamic analysis indicated that the adsorption process was physical, spontaneous, and exothermic.

1. Introduction

Water contamination with organic dyes primarily stems from industrial activities, particularly those associated with textile manufacturing, dye production, and other related industries. These industries often discharge colored wastewater into nearby water bodies without adequate treatment, leading to significant environmental pollution [1,2,3]. The use of synthetic dyes, which are complex organic molecules that are not readily biodegradable, contributes to persistent contamination that is difficult to remediate. Factors such as improper disposal practices, a lack of stringent regulatory frameworks, and inefficient wastewater treatment facilities exacerbate the issue, allowing dyes to enter and persist in the aquatic environment [4,5]. Organic dyes pose serious threats to environmental and human health due to their toxic, mutagenic, and carcinogenic properties [6,7]. Dyes can accumulate in ecosystems, affecting both aquatic and terrestrial life. For humans, exposure to contaminated water can lead to various health issues, including skin irritation, allergies, respiratory problems, and, in severe cases, cancer. The persistence of these dyes in water bodies also affects photosynthesis by reducing light penetration, thus disturbing aquatic ecosystems and reducing biodiversity [8,9]. Specific dyes such as acid blue 113 and malachite green are particularly notorious for their adverse effects on human health. Malachite green, for example, has been banned or restricted in various jurisdictions due to its confirmed carcinogenic and mutagenic effects [10]. Acid blue 113 is also under scrutiny for its potential toxic effects [11]. These dyes can enter the human body through various routes, including direct contact, ingestion of contaminated water, or consumption of contaminated seafood, leading to significant health risks. There are several methods for removing organic dyes from wastewater, including physical, chemical, and biological processes [12,13]. Some of the most common techniques include adsorption [14], electrodialysis [15], advanced oxidation processes [16], and biodegradation [17]. Adsorption is often favored for dye removal due to its simplicity, efficiency, and cost-effectiveness. It does not require the use of expensive chemicals and can be achieved using various natural and synthetic adsorbents. Unlike other methods, adsorption is capable of removing a wide range of dye concentrations, and it is relatively easy to design and operate. Furthermore, adsorbents can sometimes be regenerated and reused, reducing the overall cost of the treatment process [18,19]. Nano-metal oxides, due to their high surface area and unique chemical properties, have shown excellent potential in the adsorption of organic dyes. These nanomaterials provide a high number of working sites for dye particles, leading to superior adsorption capacities compared to conventional materials. Their nanoscale size also facilitates faster kinetics, making the adsorption process more efficient [20,21]. The combustion method is an effective technique for synthesizing nano-metal oxides due to its simplicity, scalability, and cost-effectiveness [22]. This method allows the rapid synthesis of nanomaterials with controlled sizes and morphologies, which are crucial for optimizing their adsorption properties and overall performance in dye removal. Several studies have reported the synthesis of CuFe₂O₄ nanoparticles. For instance, George et al. [23] and Tajik et al. [24] have both highlighted the synthesis of CuFe₂O₄ nanoparticles using different methods, such as sol–gel and hydrothermal, respectively. These studies primarily focused on the physical properties, such as microwave dielectric parameters and the electrochemical behavior of CuFe₂O₄ nanoparticles. In this work, the combustion method was selected for synthesizing CuFe₂O₄ nanoparticles due to its simplicity, cost-effectiveness, and ability to produce nanoparticles with controlled characteristics. L-alanine and L-valine were chosen as fuel precursors in this method due to their role in facilitating the combustion synthesis method. Both amino acids have desirable properties, such as high nitrogen content, which ensures the release of significant heat during the combustion process. This heat promotes the formation of nanostructured CuFe₂O₄ with a high surface area and uniform morphology. Hence, these properties promote efficient adsorption of acid blue 113 and malachite green dyes from aqueous media. Besides, CuFe₂O₄ nanoparticles were chosen as adsorbents due to their magnetic properties, which enable efficient separation and recovery using external magnetic fields, thus facilitating their reuse in multiple treatment cycles. This unique property gives CuFe₂O₄ a significant advantage over other oxide-based adsorbents like ZnO and zeolites.

2. Results and Discussion

2.1. Synthesis and Characterization of CuFe₂O₄ Nanoparticles

Copper ferrite (CuFe₂O₄) nanoparticles were synthesized using L-valine and L-alanine as fuels in a combustion method, as illustrated in Scheme 1. The method involved a complex reaction between fuel and iron and copper nitrates, yielding nanoparticles alongside gaseous by-products. The synthesis exhibited a high exothermic nature, indicated by the release of nitrogen, carbon dioxide, and water vapor. The difference in fuel led to a distinct stoichiometric ratio and potentially different properties. This synthesis strategy demonstrates a simple, quick, and energy-efficient pathway to produce tailored nanoparticles.

The crystalline properties of copper ferrite products, CFV and CFA, were analyzed by XRD, as depicted in Figure 1A,B, respectively. According to JCPDS No. 77-0010 (Figure 1C), both patterns exhibit distinct diffraction peaks, indicating a cubic spinel crystalline structure of copper ferrite (CuFe₂O₄). It was observed that the diffraction peaks at 2θ = 24.28°, 33.19°, 35.66°, 38.79°, 40.89°, 43.37°, 53.93°, 57.33°, and 62.38° correlate to (111), (220), (311), (222), (400), (311), (511), (422), and (440) Miller indices of CuFe₂O₄, respectively. In addition, the average crystal size, which was determined by the Scherrer equation [25,26], of the CFV and CFA products is 17.65 and 26.37 nm, respectively.

The EDX analysis of copper ferrite products, CFV and CFA, was clarified in Figure 2A,B, respectively. The fundamental elements, copper (Cu), iron (Fe), and oxygen (O), are present in both samples of CuFe₂O₄. The CFV sample composition in atomic percentages is Cu: 13.98%, Fe: 28.42%, and O: 57.60%, while the CFA sample composition is Cu: 13.53%, Fe: 28.36%, and O: 58.11%. The results show that the elements in both samples are similar to the chemical structure of copper ferrite (CuFe₂O₄), which is usually shown by the ratio Cu:Fe:O (1:2:4).

The FTIR spectra of CuFe₂O₄ nanoparticles, identified as CFV and CFA, reveal their unique functional groups, as shown in Figure 3A,B, respectively. The observation of broad absorption bands at 3439 cm⁻¹ for CFV and 3442 cm⁻¹ for CFA is indicative of O–H stretching vibrations, implying the existence of hydroxyl groups on the surface of the nanoparticles or the presence of adsorbed water molecules. Additionally, the absorption bands observed at 1629 cm⁻¹ for CFV and 1632 cm⁻¹ for CFA can be attributed to the bending vibrations of O-H. The CFV and CFA samples exhibit two significant bands at approximately 544 and 444 cm⁻¹ for CFV and 545 and 446 cm⁻¹ for CFA. These frequencies correspond to the vibrational modes of metal–oxygen bonds situated in the octahedral (Fe–O) and tetrahedral (Cu–O) environments, a hallmark of the CuFe₂O₄ spinel configuration [27].

Figure 4A,B represents the FE-SEM images of the CFV and CFA samples, respectively. The CFV and CFA samples consist of irregular and spherical shapes with average diameters of 132.78 and 188.35 nm, respectively. Variations in size assessments from SEM relative to XRD could stem from the inherent divergences in the operational principles and measurements of these two techniques. SEM assesses the physical dimensions, which may encompass aggregates or groupings of primary particles. On the other hand, XRD provides an estimate of the crystalline domain sizes, which typically refer to single or smaller conglomerations of crystals. As a result, the tendency of SEM to report larger sizes could be due to the presence of particle aggregations that are not within XRD’s detection capabilities.

In Figure 5A,B, the particle size distributions for the CFV and CFA samples are illustrated by histograms, respectively. For the CFV sample, a narrower particle size distribution is observed, predominantly concentrated around 80 to 160 nm. The modal size is identified at around 120 nm, with a tail extending towards larger sizes, suggesting a majority of smaller particles accompanied by a lesser quantity of larger particles. In addition, the CFA sample exhibits a broader particle size distribution, ranging widely from 50 to 350 nm. The most frequent particle size is found to be approximately 200 nm, as evidenced by the peak of the histogram. This wider distribution indicates a greater variation in particle sizes within the sample.

The nitrogen adsorption/desorption isotherms of the CFA and CFV samples (Figure 6A) demonstrated an increase in volume as the relative pressure (P/P_o) increased, suggesting mesoporosity in both samples. The pore size distribution (Figure 6B) indicated that both samples primarily consist of pores smaller than 10 nm. The CFV sample exhibited a more significant distribution in the smaller range, while the CFA sample showed a slightly broader pore size range. The surface textures of the CFV and CFA samples were tabulated in

Table 1. Surface textures of the CFV and CFA samples.

Sample	BET Surface Area (m2/g)	Total Pore Volume (cc/g)	Average Pore Size (nm)
CFV	69.38	0.1232	3.34
CFA	39.78	0.068	2.89

. The CFV sample was found to have a higher BET surface area (69.38 m²/g) compared to the CFA sample (39.78 m²/g). The total pore volume of the CFV sample (0.1232 cc/g) exceeded that of the CFA sample (0.068 cc/g). Additionally, the average pore size of the CFV sample was determined to be 3.34 nm, whereas that of the CFA was 2.89 nm.

The thermal stability of the CFV sample (as an illustrative example) can be evaluated based on the thermogravimetric analysis (TGA), as shown in Figure 7. A pronounced weight loss (3.70%) is observed between 25 °C and 400 °C, suggesting that decomposition of a particular component occurs, possibly attributed to the removal of adsorbed moisture. Beyond 400 °C, weight stabilization is achieved, and the mass remains consistent up to 800 °C, reflecting the high thermal stability of CuFe₂O₄.

2.2. Removal of Acid Blue 113 and Malachite Green Dyes from Aqueous Media

Equation (1) represents the amount of dye adsorbed per unit mass of the adsorbent (O, mg/g) [21].

$$\mathrm{O}=\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)\times \frac{\mathrm{V}}{\mathrm{W}}$$

(1)

In this Equation, C_o is the initial concentration of the dye in the solution before adsorption takes place (mg/L), C_e is the concentration of the dye at equilibrium after the adsorption has occurred (mg/L), W is the mass of the adsorbent (g), and V is the volume of the dye solution (L).

Also, Equation (2) calculates the percentage removal of the dye (% A) [21].

$$\%\mathrm{A}=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}\times 100$$

(2)

2.2.1. Influence of pH

Figure 8A presents the adsorption percentage of acid blue 113 and malachite green dyes on the CFA and CFV samples across a range of pH values. The adsorption behavior is notably influenced by the pH of the solution, which can be attributed to the point of zero charge (pH_PZC) of the adsorbents. The point of zero charge of the CFA and CFV adsorbents is 4.64 and 5.18, respectively, as shown in Figure 8B. In the case of anionic acid blue 113 dye, it is observed that the adsorption percentage decreases sharply as the pH increases from 2 to 10. This trend suggests that at a pH below the pH_PZC of adsorbents, the surface is more positively charged, facilitating greater adsorption of the anionic acid blue 113 dye through electrostatic attraction, as shown in Scheme 2. Also, as the pH exceeds the pH_PZC, the surface charge of adsorbents becomes more negative, reducing the adsorption efficiency of anionic acid blue 113 dye due to electrostatic repulsion. The adsorption percentages of the acid blue dye on the CFA and CFV adsorbents at pH 2 are 89.25 and 95.28%, respectively. In the case of the cationic malachite green dye, the adsorbents show an inverse relationship between pH and adsorption percentage when compared to the anionic acid blue 113 dye. This could be due to the differing chemical nature of the malachite green dye, which has cationic characteristics, leading to decreased adsorption at a pH below the pH_PZC of the adsorbents owing to electrostatic repulsion between the cationic malachite green dye molecules and the positively charged surface of the adsorbents. Also, as the pH exceeds the pH_PZC, the surface charge of adsorbents becomes more negative, enhancing the adsorption efficiency owing to the electrostatic attraction between the cationic malachite green dye molecules and the negatively charged surface of the adsorbents, as shown in Scheme 2. The adsorption percentages of the malachite green dye on the CFA and CFV adsorbents at pH 10 are 85.88 and 93.29%, respectively.

2.2.2. Influence of Contact Time

Figure 9 depicts the effect of contact time on the adsorption percentage of acid blue 113 and malachite green dyes on the CFV and CFA copper ferrite nanoparticles. It is observed that the adsorption percentage increases rapidly from 10 to 80 min for both dyes, which suggests a strong affinity between the dyes and the synthesized nanoparticles. Following the initial rapid adsorption, an equilibrium is noticed after 80 min due to the saturation of active sites. The removal percentages of acid blue 113 dye on the CFA and CFV samples after 80 min are 88.86 and 95.12%, respectively. Also, the removal percentages of malachite green dye on the CFA and CFV samples after 80 min are 85.53 and 93.12%, respectively.

Furthermore, in this study, pseudo-first-order and pseudo-second-order kinetic models were used to evaluate the removal kinetics of acid blue 113 and malachite green dyes on the CFA and CFV samples [28,29].

$$\mathrm{Pseudo-first-order}:\log \left({\mathrm{O}}_{\mathrm{e}}-{\mathrm{O}}_{\mathrm{t}}\right)={\mathrm{logO}}_{\mathrm{e}}-\frac{{\mathrm{L}}_1}{2.303}\mathrm{t}$$

(3)

$$\mathrm{Pseudo-second-order}:\frac{\mathrm{t}}{{\mathrm{O}}_{\mathrm{t}}}=\frac{1}{{\mathrm{L}}_2{\mathrm{O}}_{\mathrm{e}}^2}+\frac{1}{{\mathrm{O}}_{\mathrm{e}}}\mathrm{t}$$

(4)

where O_t (mg/g) represents the amount of acid blue 113 or malachite green dye removed at contact time t whereas O_e (mg/g) represents the equilibrium adsorption capacity. In addition, the rate constants L₁ and L₂ indicate the rate of the pseudo-first-order and pseudo-second-order models, with the units of 1/min as well as g/mg·min, respectively.

Figure 10, along with

Table 2. Kinetic parameters for the removal of acid blue 113 dye using the CFA and CFV samples.

Samples	OExp (mg/g)	Pseudo-First-Order			Pseudo-Second-Order
		L1 (1/min)	Oe (mg/g)	R2	L2 (g/mg·min)	Oe (mg/g)	R2
CFA	266.58	0.0254	99.06	0.9589	0.000695	266.67	0.9998
CFV	285.37	0.0235	97.79	0.9244	0.000740	281.69	0.9999

and

Table 3. Kinetic parameters for the removal of malachite green dye using the CFA and CFV samples.

Samples	OExp (mg/g)	Pseudo-First-Order			Pseudo-Second-Order
		L1 (1/min)	Oe (mg/g)	R2	L2 (g/mg.min)	Oe (mg/g)	R2
CFA	256.58	0.0219	141.49	0.9602	0.00037	255.75	0.9999
CFV	279.36	0.0249	119.24	0.9493	0.00053	280.11	0.9999

, collectively provide a comprehensive assessment of the kinetics involved in the removal of acid blue 113 and malachite green dyes using the CFA and CFV samples. The experimental capacity (O_Exp) and the correlation coefficients (R²) displayed in Table 2 and Table 3 suggest that the pseudo-second-order kinetic model aligns more closely with the actual observations than the pseudo-first-order model. For both types of samples, CFA and CFV, the R² values derived from the pseudo-second-order model are closer to unity, indicating a superior fit with the experimental data. Moreover, the equilibrium capacities (O_e) predicted by the pseudo-second-order equation closely match the experimental values (O_Exp), suggesting that the adsorption processes are more accurately depicted by the pseudo-second-order kinetics.

2.2.3. Influence of Temperature

Figure 11A demonstrates how the CFA and CFV samples adsorb acid blue 113 and malachite green dyes at different temperatures. The reduction in adsorption percentages for acid blue 113 and malachite green dyes as temperatures increase indicates that the dye adsorption by the CFA or CFV samples is an exothermic process.

Equations (5)–(7) were used to determine the Gibbs free energy (ΔG^o), entropy change (ΔS^o), and enthalpy change (ΔH^o) associated with the removal of acid blue 113 and malachite green dyes using the CFA and CFV samples [21].

$${\mathrm{L}}_{\mathrm{d}}=\frac{{\mathrm{O}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{e}}}$$

(5)

$${\mathrm{lnL}}_{\mathrm{d}}=\frac{△{\mathrm{S}}^{\mathrm{o}}}{\mathrm{R}}-\frac{△{\mathrm{H}}^{\mathrm{o}}}{\mathrm{RT}}$$

(6)

$$△{\mathrm{G}}^{\mathrm{o}}=△{\mathrm{H}}^{\mathrm{o}}-\mathrm{T}△{\mathrm{S}}^{\mathrm{o}}$$

(7)

R is defined as the gas constant (KJ/molK), T is defined as the temperature (K), and L_d is defined as the distribution coefficient (L/g).

Figure 11B, along with

Table 4. Thermodynamic constants for acid blue 113 dye removal by the CFA and CFV samples.

Samples	△So (KJ/molK)	△Ho (KJ/mol)	△Go (KJ/mol)
			298	308	318	328
CFA	0.0535	−21.19	−37.14	−37.67	−38.21	−38.74
CFV	0.0759	−30.01	−52.63	−53.39	−54.15	−54.91

and

Table 5. Thermodynamic constants for malachite green dye removal by the CFA and CFV samples.

Samples	△So (KJ/molK)	△Ho (KJ/mol)	△Go (KJ/mol)
			298	308	318	328
CFA	0.0287	−13.02	−21.58	−21.87	−22.16	−22.44
CFV	0.0607	−24.63	−42.73	−43.34	−43.94	−44.55

, presents a detailed analysis of the thermodynamic properties associated with the removal of acid blue 113 and malachite green dyes by the CFA and CFV samples. An increase in entropy (ΔS°) in both samples suggests greater disorder at the interface between the solid and solution phases during adsorption. Moreover, the negative Gibbs free energy (ΔG°) values across various temperatures confirm the spontaneous nature of the adsorption. In addition, the values of enthalpy (ΔH°) indicate that the adsorption process is exothermic and primarily physical.

2.2.4. Influence of Concentration

Figure 12 shows the effect of varying initial concentrations of acid blue 113 and malachite green dyes on the adsorption performance of the CFA and CFV samples. For both samples, there is a decrease in the removal percentage of dyes adsorbed as the initial concentration increases. At lower concentrations of dyes, the availability of active sites on the samples exceeds the number of dye molecules, facilitating higher adsorption percentages. However, as the concentration of dyes increases, these sites become increasingly occupied until they reach capacity. Once saturation is achieved, no further dye molecules can be effectively adsorbed, resulting in reduced adsorption efficiency.

Additionally, the following Equations depict the linear forms of the Langmuir and Freundlich equilibrium isotherms that were used to analyze the adsorption of acid blue 113 and malachite green dyes on the CFA and CFV samples [28,29].

$$\mathrm{Freundlich}\mathrm{isotherm}:{\mathrm{lnO}}_{\mathrm{e}}={\mathrm{lnL}}_3+\frac{1}{\mathrm{Z}}{\mathrm{lnC}}_{\mathrm{e}}$$

(8)

$$\mathrm{Langmuir}\mathrm{isotherm}:\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{O}}_{\mathrm{e}}}=\frac{1}{{\mathrm{L}}_4{\mathrm{O}}_{\max }}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{O}}_{\max }}$$

(9)

1/Z is defined as the adsorption intensity. The rate constants L₄ and L₃ indicate the rate of the Langmuir as well as Freundlich isotherms, with the units of (L/mg) and ((mg/g)(L/mg)^1/n), respectively. Moreover, O_max is defined as the maximum uptake capability (mg/g). Equation (10) was employed to estimate O_max by applying the Freundlich equilibrium isotherm [21].

$${\mathrm{O}}_{\max }={\mathrm{L}}_3\left({\mathrm{C}}_{\mathrm{o}}^{1/\mathrm{Z}}\right)$$

(10)

Figure 13, along with

Table 6. Equilibrium constants for acid blue 113 dye removal by the CFA and CFV samples.

Samples	Langmuir			Freundlich
	Omax (mg/g)	L4 (L/mg)	R2	Omax (mg/g)	L3 (mg/g)(L/mg)1/n	1/Z	R2
CFA	281.69	0.2141	0.9990	475.15	67.60	0.3419	0.8112
CFV	297.62	0.3114	0.9986	565.15	81.50	0.3395	0.7037

and

Table 7. Equilibrium constants for malachite green dye removal by the CFA and CFV samples.

Samples	Langmuir			Freundlich
	Omax (mg/g)	L4 (L/mg)	R2	Omax (mg/g)	L3 (mg/g)(L/mg)1/n	1/Z	R2
CFA	280.11	0.1082	0.9973	463.37	47.23	0.4004	0.8719
CFV	294.99	0.2038	0.9976	556.00	66.01	0.3736	0.7611

, provides a comprehensive analysis of the equilibrium characteristics of acid blue 113 and malachite green dyes adsorbed by the CFA and CFV samples. The correlation coefficients (R²) from Table 6 and Table 7 indicate that the Langmuir plot demonstrates a more accurate fit compared to the Freundlich plot. For both the CFA and CFV samples, the R² values from the Langmuir isotherm are closer to 1, suggesting a more precise correspondence with the experimental data observed.

The data comparison of various adsorbents (

Table 8. Comparison of maximum adsorption capacities towards acid blue 113 dye for various adsorbents, including the new CFA and CFV samples.

Adsorbent	Omax (mg/g)	Ref
Activated carbon	188.89	[30]
Chitosan	143.00	[31]
Calcium hydroxide	153.53	[32]
Magnetic graphene oxide/biomass-activated carbon	32.20	[33]
Hydroxyapatite	120.48	[34]
Magnetite	128.00	[35]
CFA	281.69	This study
CFV	297.62	This study

and

Table 9. Comparison of maximum adsorption capacities towards malachite green dye for various adsorbents, including the new CFA and CFV samples.

Adsorbent	Omax (mg/g)	Ref
Carboxylate functionalized multi-walled carbon nanotubes	11.70	[36]
Chitosan-functionalized graphene oxide	71.90	[37]
Cellulose nanofibril aerogel	212.70	[38]
Graphene/Fe3O4/polyaniline nanocomposite	150.27	[39]
Zinc-tungstate based materials	251.758	[40]
Reduced graphene oxide	279.85	[41]
CFA	280.11	This study
CFV	294.99	This study

) reveals that the CFA and CFV samples demonstrate superior adsorption capacities of 281.69 and 297.62 mg/g for acid blue 113 dye and 280.11 and 294.99 mg/g for malachite green dye, respectively. Hence, the synthesized samples outperformed other adsorbents for the removal of acid blue 113 and malachite green dyes. The following factors contribute to this superior performance: average crystallite size and surface morphology. The average crystallite sizes of CFA (26.37 nm) and CFV (17.65 nm) indicate that the CFV sample has smaller crystallites, leading to a higher surface area and improved adsorption capacity. Also, SEM analysis revealed that the CFA and CFV samples exhibit spherical and irregular shapes, with average diameters of 188.35 and 132.78 nm, respectively. The smaller particle size distribution in the CFV sample results in a higher surface area, facilitating greater dye interaction and high adsorption efficiency.

2.2.5. Effect of Regeneration and Reusability

Figure 14 explores the impact of five repeated adsorption/desorption cycles on the removal efficiency of acid blue 113 and malachite green dyes using the CFA and CFV samples. The results show a marginal decline in the dye adsorption percentage as the cycle count increases. Consequently, it can be deduced that the CFA and CFV adsorbents exhibit excellent reusability for adsorbing acid blue 113 and malachite green dyes.

XRD analysis was performed on the CFV and CFA samples after regeneration (Figures omitted for brevity). It was found that there was no difference between the regenerated samples and those before adsorption, and this confirms the stability of the adsorbed materials.

3. Experimental

3.1. Materials

Copper(II) nitrate trihydrate (Cu(NO₃)₂·3H₂O), hydrochloric acid (HCl), L-valine (C₅H₁₁NO₂), iron(III) nitrate nonahydrate (Fe(NO₃)₃·9H₂O), L-alanine (C₃H₇NO₂), potassium chloride (KCl), malachite green dye (C₂₃H₂₅N₂Cl), sodium hydroxide (NaOH), and acid blue 113 dye (C₃₂H₂₁N₅Na₂O₆S₂) were purchased from Sigma-Aldrich and used without further purification.

3.2. Synthesis of Copper Ferrite (CuFe₂O₄) Nanoparticles

Scheme 3 represents the practical steps for the synthesis of CuFe₂O₄ nanoparticles by the combustion method using L-valine and L-alanine fuels. In this regard, 5.50 g of Fe(NO₃)₃·9H₂O was dissolved in 60 mL of distilled water to obtain an iron(III) solution. Subsequently, 1.64 g of Cu(NO₃)₂·3H₂O was also dissolved in 60 mL of distilled water to obtain a Cu(II) solution. In parallel, 1.18 g of L-valine was dissolved in 60 mL of distilled water to obtain a L-valine fuel solution. The Fe(III), Cu(II), and L-valine fuel solutions were then mixed using a magnetic stirrer at 120 °C until dryness. The resultant dry sample was subjected to calcination at 650 °C for 3 h to obtain CuFe₂O₄ nanoparticles, which were designated as CFV. In a similar manner, 1.62 g of L-alanine was dissolved in 60 mL of distilled water in a separate process to obtain L-alanine fuel solution. The Fe(III), Cu(II), and L-alanine fuel solutions were then mixed using a magnetic stirrer at 120 °C until complete evaporation. Afterward, the resultant dry sample was subjected to calcination at 650 °C for 3 h to obtain CuFe₂O₄ nanoparticles, which were designated as CFA.

3.3. Instrumentation

The chemical crystalline structures of the CFV and CFA products were analyzed using an X-ray powder diffractometer (XRD, D8 Discover, Bruker, Billerica, MA, USA) equipped with a Cu Kα radiation source (wavelength of 1.5406 Å) and operated at a generator voltage and current of 40 kV and 40 mA, respectively. Infrared spectra of the CFV and CFA products were obtained using the Fourier transform infrared spectrometer (FTIR, Nicolet IS10, Thermo Fisher Scientific, Waltham, MA, USA). The surface morphology and elemental constitution of the CFV and CFA samples were obtained by scanning electron microscope/energy-dispersive X-ray spectrophotometer (SEM/EDX, JSM-6510LV, JEOL Ltd., Tokyo, Japan). The surface textures of the CFV and CFA products were obtained using an N₂ gas analyzer (NOVA2000 series, Quantachrome, Boynton Beach, FL, USA).

3.4. Removal of Acid Blue 113 and Malachite Green Dyes from Aqueous Media

The effects of pH on the adsorption process were assessed over a range from 2 to 10, with a constant dye concentration of 300 mg/L, a dye volume of 50 mL, and an adsorbent amount of 0.05 g at a temperature of 298 K for a contact time of 180 min. The influence of contact time was examined between 10 and 120 min under identical conditions of dye concentration, volume, and adsorbent amount, with pH values fixed at 2 for acid blue 113 dye and 10 for malachite green dye. Furthermore, the effect of temperature was evaluated within the range of 298 to 328 K, keeping the dye concentration, volume, and adsorbent amount constant, whereas a contact time of 80 min was maintained; pH levels were set at 2 for acid blue 113 dye and 10 for malachite green dye. Finally, the impact of varying dye concentrations, ranging from 50 to 400 mg/L, was also investigated, maintaining the volume of dye and amount of adsorbent constant at a temperature of 298 K and a contact time of 80 min, with the pH set as before for both dyes. The adsorbent is removed with a magnet after studying each effect. Scheme 4 represents the experimental conditions followed to remove acid blue 113 and malachite green dyes from aqueous media using the synthesized CuFe₂O₄ nanoparticles.

In the experimental study depicted in Scheme 5, the regeneration and reusability of a dye-laden adsorbent were explored. The adsorbent, after being saturated with dye, underwent a regeneration process through calcination at a temperature of 550 °C for a duration of 2 h to remove the dye. Subsequently, the efficacy of the regenerated adsorbent was tested under optimal adsorption conditions over the course of five cycles to evaluate its reusability. The optimal conditions maintained during each cycle of adsorption included a dye concentration of 300 mg/L and a volume of dye solution of 50 mL, with 0.05 g of the adsorbent. The pH was kept at 2 for the adsorption of acid blue 113 dye and at 10 for malachite green dye. Each adsorption cycle was conducted for a contact time of 80 min at a constant temperature of 298 K. These parameters were carefully controlled to ensure the reliability of the results concerning the adsorbent efficiency and its potential for repeated use in dye removal from aqueous solutions.

3.5. Point of Zero Charge (pH_PZC) of the CFV and CFA Samples

A series of 50 mL solutions with varying preliminary pH values (ranging from 2.50 to 11.50) were prepared using a 0.1 M KCl solution. The initial pH (pH_i) of each solution was adjusted using either 0.1 M HCl or 0.1 M NaOH. After that, 0.25 g of the adsorbent was added to each solution, and then the mixtures were agitated continuously for 12 h. The mixtures were filtered, and the final pH of the supernatant (pH_f) was measured. The change in pH (ΔpH) was calculated by subtracting the final pH from the initial pH, and a graph was plotted of ΔpH against the initial pH based on the collected data. The point at which the plot of ΔpH versus initial pH intersected the x-axis was identified as the point of zero charge (pH_PZC) of the adsorbent [21].

4. Conclusions

This research is dedicated to the synthesis, investigation, and application of CuFe₂O₄ nanoparticles for the effective removal of acid blue 113 and malachite green dyes from aqueous solutions. Utilizing the combustion method, these nanoparticles were synthesized with L-alanine (CFA) and L-valine (CFV) serving as fuels. Comprehensive analyses of the CuFe₂O₄ nanoparticles were conducted using some techniques such as XRD, EDX, FTIR, and FE-SEM. XRD results verified the nanoparticles’ cubic spinel formation, devoid of any contaminants, displaying average crystallite sizes of 26.37 nm for CFA and 17.65 nm for CFV. The maximum adsorption capabilities of the CFA and CFV samples towards malachite green dye are 280.11 and 294.99 mg/g, respectively. Besides, the maximum adsorption capabilities of the CFA and CFV samples towards acid blue 113 dye are 281.69 and 297.62 mg/g, respectively. The removal processes of acid blue 113 and malachite green dyes by the CFA and CFV samples adhered to a pseudo-second-order kinetic model and aligned with the Langmuir equilibrium isotherm. This study provides crucial insights into the development of highly efficient CuFe₂O₄ nanoparticles for water treatment. The innovative synthesis via the combustion method with L-alanine and L-valine as fuels demonstrates a scalable and environmentally friendly approach to producing adsorbents with superior performance in dye removal. These findings can pave the way for the practical implementation of CuFe₂O₄ nanoparticles in industrial wastewater treatment systems, reducing environmental contamination by effectively eliminating harmful dyes like acid blue 113 and malachite green.