Adsorption of light green dye using Mg–Al-layered double hydroxides (LDHs) with carbonate and nitrate anions

Mg–Al LDHs intercalated with carbonate (CO₃²⁻) and nitrate (NO₃⁻) anions were synthesized using the coprecipitation technique. Both materials were utilized to uptake light green dye from aquatic systems through batch adsorption experiments. The fabricated LDHs were characterized using FTIR, XRD, surface area, and SEM. Several factors influencing the uptake process were systematically evaluated, including contact time, pH, dosage, and initial dye content at various temperatures (293, 303, and 313 K). The finding showed that the adsorption efficiency of Mg–Al LDH samples peaked at pH 5, with Mg–Al–NO₃ LDH (76.4%) outperforming Mg–Al–CO₃ LDH (62.5%) due to differences in surface charge and anion mobility. Adsorption equilibrium was reached within 30 min, with increased adsorbent dosage improving removal efficiency up to a threshold. The dye uptake performance was more remarkable for Mg–Al–NO₃ LDH (19.33 mg. g⁻¹) than Mg–Al–CO₃ LDH (15.65 mg. g⁻¹) at pH 5 and 293 K. Isotherm analysis identified Henry’s model for the initial adsorption stage and Temkin’s model for higher concentrations, confirming an endothermic process with ΔS > 0. The pseudo-second-order model (R² = 0.9999) ascribed adsorption kinetics, while the Weber-Morris model indicated surface and intraparticle diffusion contributions. Thermodynamic analysis confirmed that the process was predominantly physisorption, driven by hydrogen bonding interactions between dye molecules and the –OH groups on the LDH surface. The superior removal capacity of Mg–Al–NO₃ LDH was attributed to stronger hydrogen bonding, facilitated by the presence of nitrate anions, which enhanced the electrostatic-interactions between the adsorbent and the dye molecules.