Adsorption kinetics of Eu(III) and Am(III) onto bentonite: analysis and application of the liquid membrane tidal diffusion model

Abstract

A new liquid membrane tidal diffusion model (LMTD) was applied based on three assumptions to analyze the kinetics of Eu(III) adsorption on Ca-bentonite and Am(III) adsorption on Na-bentonite. The changes in adsorption quantities as a function of contact time presented the phenomenon of tidal fluctuation according to the experiments. The tidal fluctuation showed a periodic change, and the change period was slightly affected by temperature. An adsorption event includes at least one fluctuation and each fluctuation can be divided into three steps: liquid membrane diffusion, surface diffusion and internal diffusion. Each step consists of one to three processes: physical adsorption, physical desorption and chemical adsorption. The pseudo-first-order or pseudo-second-order kinetic model did not fit the experimental curves well at t = 6 h. The first-order reaction and the second-order reaction were introduced to establish the LMTD and to explain the adsorption steps and processes. This new kinetic model is helpful to better explain the mechanism of adsorption reactions.