(499e) Unsteady-State Facilitated Diffusive Membrane Systems

Separation processes are vital to the energy and chemicals industries as a means of transforming reaction mixtures into purified products, and increasingly as a tool to mitigate environmental contamination. Membrane separations are often proposed over other separation techniques, such as distillation, because they do not require energy-intensive phase changes to accomplish their goal. In a typical membrane separation, the chemical potential gradient of the solute itself is the force that drives the target solute through a membrane. In the case of dilute solutions, which characterize many environmental contaminants, the chemical potential gradient is low resulting in low fluxes and poor separations.

Membranes that exhibit facilitated transport mechanisms whereby chemically selective reactive â??carriersâ? that promote solute permeation are incorporated within the membrane are often proposed as a means of addressing the low fluxes. Despite its potential, facilitated transport has proven difficult to realize in practical membrane systems. Supported liquid membranes, which are one method for realizing facilitated transport, allow for highly selective processes to be developed; however, because these membranes comprise a thin organic layer suspended between two aqueous solutions, they are prohibitively unstable.In our design, the carriers, which exhibit pH-dependent solute binding, are fixed to the membrane matrix and are used to enhance flux by forcing the membrane into an unsteady process. This system resembles a barrier membrane wherein reactive sites capture target solutes as they attempt to cross the membrane; however by using an external stimulus, such as pH, we manipulate the behavior of the reactive sites over time. The pH dependent reactive sites are periodically forced into a loading state, where a target solute is captured by the membraneâ??s active sites, followed by an unloading state, where the solute is released from the membraneâ??s structure, by varying the pH of the membraneâ??s surroundings. As a result the concentration gradient, and hence the soluteâ??s flux across the membrane, are constantly varying. A theoretical model that describes the performance of the system, e.g., the net flow of material across the membrane, is developed. This model describes the flux-enhancement ability of this membrane system as a function of membrane properties, such as thickness, density of reactive sites, and soluteâ??s diffusivity, and as a function of operating parameters such as the solute feed concentration, the loading time, and the unloading time.

An experimental membrane system was assembled to test the validity of theoretical predictions. The experimental system consists of chelation resins as the reactive carrier suspended in a poly(vinyl alcohol) hydrogel. Iminodiacetic acid groups within the resins bind the target solute, divalent metallic ions, reversibly as a function of solution pH. Experiments conducted under a variety of operating conditions demonstrate strong agreement between the model predictions and experimental results. The theoretical model, corroborated by the bench-scale experiments, allows for predictions for the performance of unsteady facilitated diffusion membrane systems as a function of membrane and system properties.