(18c) Ion Exchange in Forward Osmosis: A New Membrane and Draw Solution Design Problem

For the past decade, most of the high profile research on osmotic membrane processes has focused on making new membranes with low structural parameter support layers.  As such, the active layer has been by comparison ignored in its design.  Conventional wisdom in the field has suggested that a typical reverse osmosis-type selective layer (aromatic polyamide) would suffice as a selective layer for FO as well, so long as the supporting structure were designed appropriately.  Unfortunately, this view may have been misguided.  Polyamide layers contain carboxylic acid groups which deprotenate and become negatively charged in most aqueous environments.  Many draw solutions contain cations which can exchange with cations in the feed solution across this negatively charged membrane.  Such transport will negatively impact permeate water quality in FO.  This study examines this phenomenon both from the practical perspective of desalination performance as well as a more systematic look at the transport.  We believe that understanding this problem will help redirect some attention of osmotic membrane design back toward the selective layer properties where adjustments can be made to mitigate cation transport and improve solute rejection in FO.