Kinetics, isotherm, thermodynamics, and DFT studies for the simultaneous adsorption of two anionic dyes using MgAl layered double hydroxide

Efficient removal of synthetic dyes from wastewater is crucial to mitigate their environmental impact. In this study, MgAl-layered double hydroxides (MgAl LDHs) with Mg²⁺:Al³⁺ ratios of 2:1, 3:1, and 4:1 and two interlayer anions (Cl⁻ and CO₃²⁻) were synthesized by co-precipitation and characterized by XRD, FTIR, BET, SEM, and TEM. Batch adsorption experiments optimized key variables, with best performance at pH 5, 120 min contact time, and 0.02 g adsorbent dosage. The Mg₃Al–Cl sample showed the highest capacities–even when both dyes were present and interfering with each other–reaching 166.62 mg·g⁻¹ for methyl orange (MO) and 123.35 mg·g⁻¹ for congo red (CR). Kinetic data were best described by the pseudo-second-order model, and isotherms fitted the Langmuir equation. Thermodynamic parameters indicated a feasible, spontaneous, and endothermic process. The interlayer anion critically affected performance: chloride-intercalated LDHs outperformed carbonate-intercalated ones under identical conditions, consistent with differences in anion exchange and interaction strength with the galleries. The material retained >80% capacity after five regeneration cycles. Density functional theory (DFT) supported the experiments, showing adsorption governed by electrostatic attraction, hydrogen bonding, surface complexation, and n–π stacking, and clarifying how both the Mg²⁺:Al³⁺ ratio and the interlayer anion identity tune adsorption efficiency. The originality of this work lies in a combined experimental–DFT framework that co-optimizes cation ratio and interlayer anion to predict and enhance simultaneous removal of multiple anionic dyes.