Interactions between feed solutes and inorganic electrolytic draw solutes in forward osmosis

Abstract

A comprehensive transport model for Forward Osmosis (FO) is presented, based on Maxwell-Stefan theory. In FO, the oppositely directed fluxes give rise to frictional interactions, while the salinity gradient also causes to thermodynamic non-ideal behaviour of organic feed solutes, in the form of salting out. When using electrolytic draw solutes, unequal ion permeance of the draw solute creates an electrostatic potential difference across the membrane, which is an additional driving force for transport of ionic feed solutes. A sensitivity analysis is presented, assessing the effect of frictional interactions, partitioning of feed and draw solutes and salting out on feed solute rejection. It is shown that feed solute rejection is determined primarily by friction with the membrane polymer and partitioning, and secondary by salting out. Frictional interaction between feed and draw solutes is not significant for active layer transport, for a wide range of parameter variation. It can however be significant for transport in the support layer, once feed solutes have permeated through the active layer. Electromigration can be as important as diffusively-driven transport, provided that the length over which the electrostatic potential is established is limited to about the thickness of the active layer. Finally, additional interactions between membranes, organic and inorganic solutes are discussed.