Equilibrium, kinetics and thermodynamics study on the adsorption of Cr(VI) and as(III) by diatomite-modified MnO2

Abstract

MnO2 was selected to modify the diatomite (Mn-D) to improve the adsorption capacity as a new adsorbent, which was employed to explore its adsorption capacity for Cr(VI) and As(III), respectively. Mn-D was characterized via SEM, BET, FTIR, XRD, TG and pHzpc to investigate its morphology and structural properties. The structure and surface chemical properties of diatomite changed after coating with MnO2, which improved the adsorption capacity for the metal ions. The various influencing factors, such as the pH, initial concentration, temperature and adsorbent dosage, were systematically studied. Based on these results, both the adsorption processes of Cr(VI) and As(III) were pH dependent, and the optimal pH for Cr(VI) and As(III) adsorption were 2 and 6, respectively. To further elucidate the mono/competitive adsorption process, different kinds of isotherm and kinetics models were applied to simulate the experimental data. In the single system, the Dubinin-Radushkevich isotherm model was more suitable than the other models to fit the adsorption process of Cr(VI) and As(III), indicating that the adsorption of Cr(VI) was dominated by a chemical process, while the adsorption of As(III) was physically dependent. The extended Langmuir and Freundlich multicomponents were employed to fit adsorption in the binary system. For Cr(VI), the pseudo-first-order model was better than the other models at describing the adsorption process, both in the single system and binary system. However, the kinetics data of As(III) well followed the pseudo-second-order model. In addition, the analysis of the thermodynamic study revealed that both the adsorption processes of Cr(VI) and As(III) were endothermic and spontaneous.