Phenol adsorption mechanism on the zinc oxide surface: Experimental, cluster DFT calculations, and molecular dynamics simulations

Abstract

Pollution by phenols is considered a major environmental issue. Therefore, their removal from wastewaters is of great practical significance and is directly related to human health. Herein, a porous zinc oxide (ZnO) was synthesized by a simple precipitation method and used as an adsorbent for the removal of phenol at different operating conditions. Besides experimental investigations, a comprehensive understanding of the mechanism of phenol adsorption on ZnO surface at the molecular level was achieved by using cluster Density Functional Theory (DFT) and molecular dynamics (MD) simulations. The prepared adsorbent was characterized using a variety of techniques, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), as well as the Brunauer Emmett Teller (BET) method. The effects of different variables, such as pH, pH(pzc) (pH at the point of zero charge), initial concentration, and temperature, were investigated to optimize and understand the adsorption process. Possible controlling mechanism and the potential rate-limiting steps were analyzed using Lagergren's pseudo-first-order and pseudo-second-order models, and the data were found to follow the pseudo-second-order equation. The maximum removal of the phenol molecule was observed at pH=4 with adsorption capacities of 3.61 and 4.51mg/g at 30 degrees and 50 degrees, respectively. Furthermore, Cluster DFT calculations and molecular dynamics simulations were used to get deeper mechanistic insights on the adsorption behavior of the phenol molecule onto zinc oxide. DFT results confirmed the predominance of chemisorption while those from MD simulations indicated an increased interaction of ZnO with phenol in the presence of solvent due to water bridged H-bonds.