Adsorption of Sulfamethoxazole on Layered Double Hydroxides:\nMolecular Dynamics Modeling

Sulfamethoxazole\nhas been extensively detected in aquatic environments\nworldwide, posing a great risk to human health. Although clay minerals\nare effective adsorbents for contaminants, investigations of the adsorption\nof sulfamethoxazole on clay minerals remain limited. In this paper,\nthe adsorption of anionic sulfamethoxazole (SMX<sup>–</sup>) from bulk to layered double hydroxides (LDH) surface was simulated\nby molecular dynamics. The electrostatic interaction between LDH and\nSMX<sup>–</sup> drove the adsorption. Four adsorption configurations\nof SMX<sup>–</sup> were identified: two with NO<sub>3</sub><sup>–</sup> ions inserted between SMX<sup>–</sup> and\nthe LDH surface, one via water bridging and one directly forming hydrogen\nbonding with the hydroxyl hydrogen atoms of LDH. The adsorbed SMX<sup>–</sup> exhibited significantly lower diffusion coefficient\nthan that in the bulk, indicating that LDH has an advantage over common\nclay adsorbents (e.g., montmorillonite) in removing SMX<sup>–</sup> from water. The adsorption process was demonstrated to be enthalpy-driven\nby thermodynamic analysis. The free energy well in the weakly ordered\nwater layer was deeper than that in the strongly ordered water layer,\nwhich explained why SMX<sup>–</sup> preferred to stay in the\nformer. To enter the free energy barrier in the latter, SMX<sup>–</sup> was required to overcome the barrier by exchanging with adsorbed\nNO<sub>3</sub><sup>–</sup> ions or breaking the ordered water\nlayer.