Mechanisms of Transport through Reverse Osmosis Membranes

Reverse osmosis membrane transport mechanisms are briefly reviewed. There are several approaches which basically involve pore flow and/or diffusive flow. At one extreme, as in the solution-diffusion model, there is no clear-cut route for the transport, while others, like the preferential sorption-capillary flow mechanism, invoke definite pores and capillaries. The fundamental concept of interfacial adsorption combined with mass transport through the capillaries, which led to the development of asymmetric porous membranes, are the prominent features of the latter mechanism. This has been developed further to enable specification and prediction of performance of a membrane in terms of average pore size and its distribution. A quantitative description of penetrant solution and diffusion in microheterogeneous media as described by the dual sorption theory and a mechanism based on irreversible thermodynamics are also discussed. The usefulness and limitations of various approaches are pointed out.