The effect of feed pH on the performance of a reverse osmosis membrane

Membranes are predominantly characterized using single salts at one pH. In this paper, a more comprehensive characterization of a reverse osmosis membrane is provided. The zeta potential, flux and rejection behaviour of a brackish water membrane is examined over a wide pH range. Performance in both single salt solutions (sodium chloride, calcium chloride and sodium sulfate) and a binary solution of calcium and sodium chloride are considered. Results are consistent with the Donnan exclusion theory. That is, larger, more highly charged ions are rejected more strongly than smaller or less strongly charged species. In particular, this behaviour can lead to hydronium ion rejections of up to −300% as this ion permeates preferentially over other cations. Membrane flux is relatively constant with respect to pH although there is some evidence of an increase in flux in doubly charged salt solutions at high pH.