Process Optimization and Equilibrium, Thermodynamic, and Kinetic Modeling of Toxic Congo Red Dye Adsorption from Aqueous Solutions Using a Copper Ferrite Nanocomposite Adsorbent

Abstract

In the present investigation of copper ferrite, a CuFe₂O₄ nanocomposite adsorbent was synthesized using the sol-gel method, and its relevance in the adsorptive elimination of the toxic Congo red (CR) aqueous phase was examined. A variety of structural methods were used to analyze the CuFe₂O₄ nanocomposite; the as-synthesized nanocomposite had agglomerated clusters with a porous, irregular, rough surface that could be seen using FE-SEM, and it also contained carbon (23.47%), oxygen (44.31%), copper (10.21%), and iron (22.01%) in its elemental composition by weight. Experiments were designed to achieve the most optimized system through the utilization of a central composite design (CCD). The highest uptake of CR dye at equilibrium occurred when the initial pH value was 5.5, the adsorbate concentration was 125 mg/L, and the adsorbent dosage was 3.5 g/L. Kinetic studies were conducted, and they showed that the adsorption process followed a pseudo-second-order (PSO) model (regression coefficient, R² = 0.9998), suggesting a chemisorption mechanism, and the overall reaction rate was governed by both the film and pore diffusion of adsorbate molecules. The process through which dye molecules were taken up onto the particle surface revealed interactions involving electrostatic forces, hydrogen bonding, and pore filling. According to isotherm studies, the equilibrium data exhibited strong agreement with the Langmuir model (R² = 0.9989), demonstrating a maximum monolayer adsorption capacity (q_max) of 64.72 mg/g at pH 6 and 302 K. Considering the obtained negative ΔG and positive ΔH_ads and ΔS_ads values across all tested temperatures in the thermodynamic investigations, it was confirmed that the adsorption process was characterized as endothermic, spontaneous, and feasible, with an increased level of randomness. The CuFe₂O₄ adsorbent developed in this study is anticipated to find extensive application in effluent treatment, owing to its excellent reusability and remarkable capability to effectively remove CR in comparison to other adsorbents.

Keywords: Congo red dye adsorption; adsorption kinetics; copper ferrite nanocomposite; equilibrium isotherms; response surface methodology; reusability; sol–gel synthesis; thermodynamics.

Figure 1

Characterization of the CuFe₂O₄ nanocomposite adsorbent: (A) Fourier transform infrared spectroscopy (FT-IR) spectra; (B) thermogravimetric analysis (TGA) profile; (C) zero-point charge (pH_zpc) graph; (D) magnetic hysteresis profile; (E) X-ray diffraction (XRD) analysis; and (F) X-ray photoelectron spectroscopy (XPS) spectra.

Figure 2

Energy-dispersive X-ray spectroscopy (EDS)/field-emission scanning electron microscopy (FE-SEM) images of (A–C) copper ferrite (CuFe₂O₄) nanocomposite adsorbent; and (D–F) bare Fe₃O₄ NPs.

Figure 3

Graphical representations of residuals in CR dye uptake onto CuFe₂O₄ adsorbent. (A) Standard probability plot of standardized residuals, Red dot indicates that the residuals in the plot follow a straight line (B) fitted values plotted against standardized residuals, (C) frequency of observation versus standardized residuals, Blue line indicates that the normal distribution of the standardized residuals and (D) standardized residuals plotted against data order, Red dot indicate the standardized residuals in the order of corresponding observations. Blue line indicates that the residuals in the plot fluctuate in a random pattern.

Figure 4

Contour plots for the interactive effect of (A) initial dye concentration and pH, and (B) CuFe₂O₄ adsorbent dosage and initial adsorbate concentration, on the equilibrium dye uptake of CR.

Figure 5

Response surface plots for the interactive effect of (A) initial pH and CuFe₂O₄ adsorbent dosage, and (B) initial adsorbate concentration and dosage of CuFe₂O₄ adsorbent, on the equilibrium uptake of CR dye.

Figure 6

CR dye uptake onto CuFe₂O₄ nanocomposite. (A) Langmuir isotherm model; (B) Freundlich isotherm plot; (C) Temkin isotherm model; (D) Dubinin–Radushkevich isotherm plot; and (E) equilibrium adsorption capacity against various isotherm models. (Initial pH: 6; initial adsorbate concentration: 40–240 mg/L; dosage of CuFe₂O₄ adsorbent: 3.5 g/L; nanocomposite particle size: 742.5 nm; stirring speed: 150 rpm: duration of contact 24 h; operating temperature: 302 K).

Figure 7

A suggested mechanism for CR dye uptake onto CuFe₂O₄ nanocomposite adsorbent.

Figure 8

CR dye uptake onto CuFe₂O₄ nanocomposite. (A) Ho’s pseudo-second-order (PSO) kinetic plot; (B) Lagergren pseudo-first-order (PFO) kinetic plot; (C) Elovich kinetic plot; (D) intraparticle diffusion kinetic model, Blue color (First region) indicates that the external boundary layer diffusion of the adsorbate molecules and the process is rapid, Pink color (Second region) is attributed to the progressive adsorption stage, where pore diffusion is rate-controlling. It indicates the diffusion of the adsorbate molecules through the pores of the adsorbent, Green color (Third region) refers to the final saturation stage and the pore diffusion starts to slow down due to the low adsorbate concentration in the aqueous solution; (E) Boyd kinetic plot; and (F) Bangham kinetic plot. (Initial pH: 6; initial adsorbate concentration: 40–240 mg/L; dosage of CuFe₂O₄ adsorbent: 3.5 g/L; nanocomposite particle size: 742.5 nm; stirring speed: 150 rpm: duration of contact 24 h; operating temperature: 302 K).

Figure 9

(A) Influence of temperature on equilibrium CR dye uptake onto CuFe₂O₄ nanocomposite; (B) Van’t Hoff plot; and (C) Arrhenius plot. (Initial pH: 6; initial dye concentration: 40–240 mg/L; dosage of CuFe₂O₄ adsorbent: 3.5 g/L; nanocomposite particle size: 742.5 nm; stirring speed: 150 rpm: duration of contact 24 h; operating temperature: 302–330 K).

Figure 10

(A) Desorption efficacy of CR dye in various runs. (Desorbing reagent volume: 0.1 L; stirring speed: 150 rpm: duration of contact 24 h; operating temperature: 302 K). (B) Reusability of CuFe₂O₄ adsorbent for the uptake of CR dye in various runs. (Initial pH: 6; initial adsorbate concentration: 175 mg/L; volume of adsorbate solution: 100 mL; nanocomposite particle size: 742.5 nm; stirring speed: 150 rpm: duration of contact 24 h; operating temperature: 302 K).