A combined molecular dynamics simulation, DFT calculations, and experimental study of the adsorption of Rhodamine B dye on kaolinite and hydroxyapatite in aqueous solutions

Abstract

The adsorption efficiency of Rhodamine B (RhB) onto the natural kaolinite and hydroxyapatite (HAP) was investigated. Both adsorbents were characterized using XRD, FTIR, SEM, XRF, and BET surface area analysis. The experimental adsorption tests were optimized by the Box-Behnken design (BBD) which led to conclude the maximal RhB removal obtained by kaolinite (95%) and HAP (84%) using pH=6.5, [RhB]=60 mg/L, and adsorbent mass=1.5 g/L. Molecular dynamics (MD) simulations were finally applied to investigate and predict the mechanisms involved in the adsorption of RhB molecules onto the surface of the kaolinite and HAP. This indicated that the adsorption phenomenon was spontaneous and favorable in the acidic medium with the dominance of Van der Waals interactions. The kinetics investigation showed that the adsorption is well described by the pseudo-second order kinetic (R2 =0.99). The thermodynamics and isotherms studies exhibit that the adsorption phenomenon for both adsorbents is endothermic and significantly fitted with the Freundlich model. The significant RhB adsorption efficiency suggests the potential application of the natural HAP and kaolinite for the removal of the dyes. Moreover, the theoretical study would be of interesting contribution to the advances in the proposed mechanisms of the adsorption process.