Effect of pH on rejection of hexavalent chromium by nanofiltration

Removal of hexavalent chromium (Cr+6) was carried out with different nanofiltration (NF) membranes for varying concentration and pH of the membrane feed solution. Two membranes were used for this investigation: one high rejection membrane (NFI) and the other a low rejection membrane (NFII). The percent rejection of chromium was found to increase with the increase of feed solution pH. It has been observed that the effect of feed concentration on the percent rejection was quite low, but the nature of effect varies with the pH of the solution with a transition happening at above pH 7.0. The rejection was observed to increase with increasing in concentration for feed pH below 7.0, whereas at higher pH a reverse trend occurs with lowering of rejection for higher feed concentration. The permeate flux was found to be lower at higher feed concentration, but varied only marginally with change in pH value. UV-spectrograph of the permeate solution at different pH value was carried out to determine the nature of the ionic species of chromium present in the system. Results show that the variation of rejection at different pH is due to the nature of ionic component present at the specific pH. The role of feed solution pH for membrane separation of chromium has been explained with the help of this spectrograph.