pH Sensitivity of Ion Exchange through a Thin Film Composite Membrane in Forward Osmosis

The uneven permeation of cations and anions through forward osmosis membranes offers a new technical challenge in the development of forward osmosis processes. Cation exchange in polyamide thin film composite membranes is caused by carboxylic acid functional groups within the structure of these membranes' selective layers. These functional groups will gain or lose a proton depending on the external solution pH. The deprotonation of a polyamide at alkaline pHs results in a net negative charge, allowing for the exchange of cations between feed and draw solutions having monovalent cations. In this study, the importance of solution pH in influencing cation transport across a commercial thin film composite forward osmosis membrane was examined. It was found that cation transport across this membrane varies significantly with changes in pH and occurred fastest at alkaline pH.