Colloquium

Reverse Osmosis (RO)/Nanofiltration (NF) are pressure driven membrane processes, which are widely used to separate dissolved solids from water. RO and NF are non-selective processes i.e., the semi-permeable membrane is not capable of removing targeted solutes selectively. It has however been observed that membranes reject different electrolytes to a different extent. Also rejection of a particular electrolyte is affected by accompanying electrolytes in the feed water, all other conditions remaining identical. In the area of water and wastewater treatment and water reuse, the degrees of rejection (or permeation) of environmentally regulated species are important. Progresses have been made in developing models, which integrate Nernst Planck equation with Donnan effect to predict the relative permeation of ions. This approach is unable to differentiate the permeation between ions of identical valence i.e. homovalent ions. In this work, a novel approach has been developed to determine the relative permeation rates of different electrolytes through semi-permeable membranes; of particular importance is the ability of this new approach to predict differential permeation among ions of the same valance. The central theme of the model rests on the premise that ions diffuse through membranes as electrically neutral solutes and their permeation rates are functions of their interdiffusion coefficients. Stokes-Einstein equation relates diffusivity of ions with their individual hydrated ionic radii (rH) and relative rH is related to their ion exchange selectivities towards polymeric ion exchangers. The theoretical premise is validated by extensive experiments performed in the laboratory as well as with data from open literature and from operating RO plants.