Adsorptive Removal of Methyl Orange Using Calcined Mg–Al LDHs: A Comprehensive Kinetic, Isotherm, and Thermodynamic Study

Abstract: This work investigates the adsorption efficiency of mixed oxides derived from calcined Mg–Al–CO₃ layered double hydroxides (MA-LDH), synthesized via the co-precipitation technique, for the removal of methyl orange (MO) from aqueous solutions, aiming to reduce associated health hazards. A series of batch experiments were conducted to assess the influence of key operational parameters such as pH, contact duration, adsorbent dosage, initial dye concentration, and thermodynamic variables. Equilibrium was achieved swiftly within 15 minutes at ambient temperature (25 °C), and the kinetic behavior adhered closely to the pseudo-second-order model. Adsorption isotherm analysis indicated monolayer adsorption, consistent with the Langmuir model, with a maximum adsorption capacity of 500 mg/g. Thermodynamic evaluation across 25, 35, and 45 °C confirmed that the process is both endothermic and spontaneous, as shown by the positive enthalpy change (ΔH°) and negative free energy values (ΔG°). The increase in ΔG° with temperature also reflects an enhancement of adsorption at elevated temperatures. Characterization techniques such as XRD, SEM–EDX, and TGA provided evidence that the MO dye uptake occurs through the rehydration of the calcined material and the subsequent reformation of the layered double hydroxide structure via intercalation of dye molecules into the interlayer space. Theoretical calculations using DFT and SWISSADME were used. Keywords: Layered double hydroxides; mixed oxide; methyl orange dye; adsorption; reconstruction; equilibrium,