Synergistic effect of Co3O4/CdAl2O4 nanocomposite for photocatalytic decontamination of organic dyes under visible light irradiation

Abstract

Water supply and the removal of contaminants are critical challenges for modern societies. Various techniques have been developed to treat industrial wastewater. This study synthesized a Co3O4/CdAl2O4 nanocomposite using a hydrothermal method to remove Direct Blue and Basic Yellow organic dyes from aqueous media. The nanocomposite was characterized through FTIR, XRD, FESEM, EDX, TEM, PL, and UV-DRS spectroscopy to assess its physical, chemical, and optical properties, including morphology, particle size, crystalline structure, chemical composition, and band gap. DRS analysis revealed that the band gaps of Co₃O₄, CdAl₂O₄, and the synthesized nanocomposite were 1.5 eV, 3.66 eV, and 2.4 eV respectively. The formation of the heterogeneous Co₃O₄/CdAl₂O₄ structure reduced the recombination rate compared to pure Co₃O₄ and CdAl₂O₄ samples. The study examined how operational parameters such as pH, contact time, initial dye concentration, and catalyst dosage affected dye removal efficiency. Optimal removal efficiency for Direct Blue was achieved at pH 2 and for Basic Yellow at pH 12 with a dye concentration of 10 ppm in a 20 mL solution using a catalyst dosage of 0.01 g. Maximum removal efficiencies were 91% for Direct Blue and 86% for Basic Yellow. Kinetic studies showed that the dye removal process followed pseudo-first-order kinetics indicative of a surface-controlled reaction. The experimental data fit well with this model, allowing calculation of the reaction rate constant and activation energy determination. The regression coefficients were 0.992 for Direct Blue and 0.991 for Basic Yellow with rate constants of 0.00379 s^{- 1} and 0.00125 s^{- 1} respectively. The photocatalytic regeneration results indicate that the synthesized nanocomposite demonstrates excellent reusability for up to four cycles, achieving final degradation rates of 80% and 74% for Direct Blue and Basic Yellow, respectively. Mechanistic studies reveal that superoxide radicals are the primary reactive species, serving as powerful oxidants that break down the chemical bonds of pollutant molecules. This process results in bond cleavage and ultimately mineralizes pollutants into simpler compounds like water and carbon dioxide. Consequently, the cobalt oxide/cadmium alumina nanocomposite, with its superior photocatalytic properties and high stability, shows significant potential for treating industrial wastewater containing organic dyes.

Keywords: Basic yellow 28; Cadmium alumina; Cobalt oxide; Direct blue 199; Nanocomposite.

Fig. 1

FTIR spectra of (a) Co₃O₄, (b) CdAl₂O₄, (c) Co₃O₄/CdAl₂O₄ nanocomposite.

Fig. 2

XRD patterns of the synthesized (a) Co₃O₄ (b) CdAl₂O₄ (c) Co₃O₄/CdAl₂O₄ nanocomposite.

Fig. 3

FESEM images of (a) Co₃O₄ (500 nm), (b) CdAl₂O₄ (500 nm), (c) Co₃O₄/CdAl₂O₄ nanocomposite and (d) TEM image of the synthesized Co₃O₄/CdAl₂O₄ nanocomposite.

Fig. 4

EDX spectrum and elemental quantitative results of the Co₃O₄ (a), CdAl₂O₄ (b), and Co₃O₄/CdAl₂O₄ nanocomposite (c).

Fig. 5

Thermogravimetric analysis of Co₃O₄, CdAl₂O₄ and the synthesized nanocomposite.

Fig. 6

The adsorption-desorption isotherms of Co₃O₄, CdAl₂O₄ and the synthesized nanocomposite.

Fig. 7

Raman spectra of Co₃O₄ (a), CdAl₂O₄ (b), (c) the synthesized nanocomposite and the room temperature photoluminescence emission spectrum at 390 nm excitation wavelength for prepared samples (d).

Fig. 8

(a) The UV-Vis DRS spectrums (b) and Tauc plots for the synthesized Co₃O₄ , CdAl₂O₄ , and the prepared nanocomposite.

Fig. 9

The pseudo first order plot for the photodegradation.

Fig. 10

Effect of pH on the synthesized nanocomposite for basic yellow amd direct blue (a) and pH_PZC determination test for the synthesized nanocomposite (b) .

Fig. 11

Effect of photocatalyst dosage on the photocatalytic reduction (a), initial concentration (b), and contact time on the photocatalytic degradation.

Fig. 12

Photocatalytic stability of four repeated cycles for degradation of (a) basic yellow (b) direct blue.

Fig. 13

XRD pattern and FESEM of the reused photocatalyst.

Fig. 14

Influence of several reactive species on the degradation of DB and BY dyes using nanocomposite under visible light.

Fig. 15

Schematic representation of photocatalytic mechanism of nanocomposite.