Interfacial dynamics of Al3+ ions and water on hydrophilic adsorbents: A molecular dynamics study

The residues generated from the anodizing industry contain metallic ions like Al³⁺, and the presence of these pollutants in water bodies cause serious concern because of the ecological and human health risks. The adsorption of heavy metal ions is a widely used option due to it is an easy to implement, eco-friendly, and cost-effective process. This work reports the effect of different surfaces on the adsorption process of Al³⁺ ions to clarify the adsorption mechanism of the ions using molecular dynamics simulations. The models of three adsorbents: hydroxyapatite (HAP), an activated carbon (AC), and clinoptilolite (CLI) were computationally investigated to identify the most suitable adsorbent. The calculated adsorption isotherms allowed the evaluation of three different adsorbent materials for the removal of Al³⁺ ions from an aqueous solution. The findings were compared with experimental data of Al³⁺ adsorption onto bone char and a good qualitative similarity with experimental data was obtained for the HAP model. The results obtained by simulation as well as the experimental data fit satisfactorily to the Freundlich model, the fitting parameters show differences less than 3 % between the simulated and experimental data. For the three adsorbents, the simulated data fit better to the Freundlich isotherm adsorption model, suggesting heterogeneous adsorption on surfaces with variable adsorption capacity in each case. To study the adsorption mechanism, density profiles, RDFs, number of H-bonds, molecular minimum distances and intermolecular interactions were calculated. The efficiency of Al³⁺ ion adsorption is strongly influenced by the hydrophilic or amphiphilic nature of the adsorbent surface, as well as by the solvation structure of the ions in the solution. In each system, the solvation phenomenon occurs, although only the AC adsorbent exhibits it to a lesser extent compared to CLI and HAP surfaces. The CLI model had the highest adsorption capacity, due to its intense hydrophilic behavior, which leads to greater affinity and stability in the water molecules adsorbed on its surface and, therefore, allows a greater adsorption of Al³⁺ ions.