Kinetic and thermodynamic study of the removal of acid black 234 and acid brown 113 dyes from aqueous solution using a low-cost synthesized activated carbon supported iron oxide (AC/Fe₂O₃) nanocomposites

In the present study nanocomposites (AC/Fe₂O₃) were synthesized using chemical co-precipitation method. The formation of AC/Fe₂O₃ was confirmed through FTIR, XRD, SEM, and TGA techniques. The XRD data showed highly crystalline nature of AC/Fe₂O₃ with the average size 17.3 nm, SEM analysis indicated that the texture of AC is tile like and highly porous while spongy aggregates of Fe₂O₃. The sharp peak at 521 cm⁻¹ in FTIR shows Fe–O stretching. TGA results revealed that the synthesized AC/Fe₂O₃ are stable thermally. The characterized AC/Fe₂O₃ was evaluated as adsorbent for the adsorption of two hazardous textile dyes (Acid black 234 and acid brown 113). The effect of different parameters like pH, shaking time, solution concentration, dosage of adsorbent, and temperature was also considered. The process of adsorption of both the dyes was close-fitting in pseudo-second-order kinetics. For intra-particle diffusion Webber-Morris and Reichenberg equation was also used. Further adsorption isotherm such as Freundlich, Dubinin-Radushkevich, and Langmuir was also applied on the adsorption study. The adsorption mechanism for the acid black 234 is best described by the Freundlich while that of acid brown 113 by Langmuir isotherm model. Thermodynamics study revealed that calculated Gibbs free energy was −1.911 kJ/mol for acid black 234 and for acid brown 113 was −2.657 kJ/mol, enthalpy (ΔH) was −28.127 kJ/mol for acid black 234 while for Acid brown 113 was −7.468 kJ/mol, for acid black 234 entropy (ΔS) was −92.851 J/K.mol and for acid brown 113 was −16.357 J/K.mol, respectively confirming the exothermic and spontaneous nature of adsorption.