Role of Interlayer Expansion and Surface Chelation in Ternary NiCuAl Layered Double Hydroxides for Ultrahigh Capacity Dye Adsorption.

Developing high-performance adsorbents for anionic dye removal is essential for sustainable water remediation. However, conventional methods struggle with the persistent, non-biodegradable nature of synthetic azo dyes. In this study, we report robust hydrothermal synthesis of ternary NiCuAl layered double hydroxide (LDH), exploiting its tunable interlayer chemistry and high surface reactivity for efficient anionic dye capture. Characterization via XRD, XPS, and BET confirmed a mesoporous framework (D pore = 3.78 nm) with an expanded interlayer spacing of 8.58 Å, facilitating superior molecular accessibility. Langmuir isotherm fitting indicated the dyes underwent monolayer adsorption with theoretical adsorption capacities (q max) of 6628 mg g-1 for Eriochrome Black T (EBT) and 567 mg g-1 for Allura Red AC (AR). Kinetic analysis followed the pseudo-second-order model, confirming chemisorption as the primary mechanism. The Weber-Morris model revealed a multi-stage uptake governed by film diffusion and pore transport. The remarkable 13-fold difference in capacity is attributed to the chelation effect of EBT, which acts as a tridentate ligand forming stable complexes with Ni2+ and Cu2+ cations. DFT analysis indicates that EBT exhibits stronger electrostatic and geometric anisotropy, promoting more directional interactions. Overall, metal coordination and an expanded framework make NiCuAl LDH an exceptional material for high-capacity wastewater treatment.