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The crossover from normal Fickian diffusion, where mean-squared displacement goes astto single-file diffusion, where t 0.5is studied as function of particle size confined in zeolite zsm-22 using molecular dynamics simulations. The simulation results indicate that the crossover is smooth as the particle size increases and the diffusion reaches single file through a series of subdiffusion processes. A small number of mutual passage of particles can destroy the correlated single-file diffusion. The density dependence on single-file mobility obtained from molecular dynamics simulations are in very good agreement with theoretical predictions. [ABSTRACT FROM AUTHOR]

An investigation has been undertaken to determine the removal of heavy metals (Cd2+, Cu2+, Ni2+ and Zn2+) of high environmental priority due to their toxicity from dilute aqueous solutions by biosorption using inexpensive biomaterials like activated sludge. Activated sludge is used widely in water treatment plants and is easily available. Each experiment was performed over a period of time to determine the biosorption of heavy metals from the aqueous phase to the solid phase. The maximum sorption uptake of the studied metal ions by activated sludge showed the following order: Cd2+ > Cu2+ > Ni2+ > Zn2+. The pseudo first- and second-order kinetic models were used to describe the kinetic data. The experimental data fit with the second-order kinetic model very well. The type of mechanism involved is analyzed in terms of the intraparticle diffusion model. Other models are also reviewed. Quantification of metal-biomass interactions, nature of adsorption, kinetics, ion-exchange as well as models used to characterize activated sludge biosorption are reviewed. All solutions are analyzed using inductively coupled plasma (ICP). [ABSTRACT FROM AUTHOR]

In the transport processes in many physical systems, the temperature dependence of diffusivity is assumed to be exhibit Arrhenius behavior, i.e., a linear relationship l o g (D) ∝ 1 ∕ T. When extended to lower temperatures, deviations are observed in many physical systems. The phenomenological description of these deviations identifies them as sub-Arrhenius and super-Arrhenius regimes based on the concave or convex nature of l o g (D) ∝ 1 ∕ T curve and the activation energy becomes temperature dependent. In general, super-Arrhenius behavior is observed in classical systems such as supercooled liquids where correlated motions play an important role in the dynamical behavior of the system, whereas sub-Arrhenius behavior is proposed to be intimately related to quantum tunneling effect of penetration of an energy barrier along the reaction path as in the case of chemical reactions. In this article, we show that a purely classical system, such as binary colloidal mixture under an external repulsive potential, can undergo sub-Arrhenius diffusion. The depletion interactions between the repulsive potential barrier and larger particles in the presence of the smaller particles contribute to the temperature dependence of activation energy which in turn lead to the sub-Arrhenius diffusion of larger particles in the mixture. [ABSTRACT FROM AUTHOR]

An analytic expression is derived for the diffusion coefficient of a sorbate in a crystalline porous solid with bottlenecks. The diffusion coefficients obtained from the analytic expression is found to agree well with the molecular dynamics results. It is also found to reproduce the temperature dependence of the levitation effect for zeolites Y and A. The present calculations provide a strong theoretical support for the levitation effect obtained so far purely from molecular dynamics calculations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]