pH shift of permeate during RO operation

When reverse osmosis process is operated, the cases where the pH of feed solution is different from the pH of permeate is frequently observed. In general, the pH of the permeate is higher than the feed pH at low pH and the pH of permeate is lower than the pH of feed at high pH. It has long been known to be due to the electrostatic interaction between proton in feed and membrane surface charge which is positively charged at low pH and negatively charged at high pH. In this experiment, the effect of surface charge on pH shift during the RO operation was investigated using two membranes having different surface charge but similar performance. Then, the critical pH???s of the membranes were observed in the absence or presence of carbonate species in order to see the influence of carbon dioxide in atmosphere. The systematic investigation in this study shows that the gross salt rejection of RO membranes is one of dominant factor determining the degree of pH shift and the critical pH is determined by the feed water characteristics