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.
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