Combined experimental and DFT study on the adsorption of congo red dye using self-assembled hierarchical microspheres of lanthanum hydroxide

Abstract

Self-assembled hierarchical microspheres of lanthanum hydroxide (La(OH)3) were used to investigate the adsorption of the azo-dye Congo Red (C.R.) in an aqueous medium. The versatile coordination chemistry of lanthanum ions makes them very efficient in adsorption processes involving organic ligands. The microspheres were synthesized using the reaction-diffusion framework (RDF) at ambient temperature, which consists of diffusing a concentrated ammonia solution onto an agar hydrogel matrix containing the dissolved lanthanum salt. The microspheres, which emerged with distinct Liesegang bands ranging in size from ∼300 nm to ∼70 µm, demonstrated rapid adsorption, reaching equilibrium within 15 min, with optimal adsorption of 395 mg g–1 at pH 6 and 30 °C. The kinetic and thermodynamic data were analyzed and fitted to a pseudo-second-order equation and the Sips model. The adsorption mechanism, driven by the dye's structure, the adsorbent's surface chemistry, and the medium, involves complex interplays of electrostatic attractions, physisorption, and chemisorption, with water playing a pivotal role. The positive enthalpy of adsorption extracted from a Sips isotherm, diverging from Langmuir behavior, and FTIR spectral comparisons pre- and post-adsorption provide empirical insights. Density functional theory (DFT) calculations, complemented by implicit solvation models, reveal significant solvation effects on the adsorption enthalpy of the sulfonate group of C.R. on the La(OH)3 surface, highlighting the solvent's vital role. The adsorption enthalpy of +62.63 kJ mol–1 closely aligns with the primary termination of the lanthanum hydroxide surface by water molecules. Remarkably, the bond length between the surface lanthanum and water's oxygen undergoes a significant shift when bonded to the sulfonate group. This study offers a deeper understanding of C.R. dye adsorption dynamics, emphasizing both experimental findings and theoretical insights, underscoring the complex interplay of forces and the central role of water in the process.