Carbon dioxide transport by proteic and facilitated transport membranes.

Membrane separation of gases is governed by the permeability of each species across the membrane. The ratio of permeabilities yields the selectivity. Use of certain organic carriers in facilitated transport membranes and the CO2 converting enzyme carbonic anhydrase (CA) in proteic and facilitated transport membranes allows a dramatic increase in CO2 selectivity over other gases. CA has a low Km (9 mM), which we predicted would allow it to scavenge CO2 to very low partial pressures. Our goal was to determine if CA could remove CO2 from an environment at levels of 0.1% or less. Prior measurements of CO2 transport across thin supported liquid membranes showed that addition of CA enhanced CO2 flux by 3- to 100-fold. Proteic films use bifunctional reagents (e.g., glutaraldehyde) to cross-link the enzyme forming a gel. Bovine serum albumin (BSA) is often added for structural stability. Using such a preparation we examined the ability of proteic films to improve CO2 selectivity and to scavenge CO2 from a mixed gas stream. Proof-of-concept results, measured by mass spectrometry, showed a fivefold improvement in CO2 capture rate with maximal improvement at CO2 values of 1% partial pressure difference in the presence of 0 atm absolute difference. At 0.1% CO2 the membrane exhibited a 76% improvement over controls. At 0.3% CO2 the improvement is about threefold. CA proteic membranes exhibit selectivity for CO2 over oxygen and nitrogen in excess of three orders of magnitude. A CA-based proteic or facilitated transport membrane should readily achieve CO2 partial pressures of 0.05% under CELSS conditions. In addition to proteic membranes we are exploring direct immobilization of engineered CA to ultra-high-permeability teflon membranes. Site-directed mutagenesis was used to add functional groups while retaining full enzymatic activity. These results provide a basis for development of far more efficient CO2 capture proteic and facilitated transport membranes with increased selectivity to values closer to 100-fold at 1% CO2. The result will be CO2 selectivity at 0.1% on the order of 400-fold. These results exceed those obtained with other technologies.