Effects of chlorine exposure on nanofiltration performance of polyamide membranes

Composite polyamide nanofiltration membranes comprising of an inner sublayer of polyethylenimine (PEI)/trimesoyl chloride (TMC) crosslinks and an outer sublayer of piperazine (PIP)/TMC crosslinks were fabricated via layer-by-layer sequential interfacial polymerization, and the chlorine resistance of the membranes was shown to be improved by the outer sublayer. The effects of chlorine exposure on the nanofiltration performance of the positively-charged polyamide membrane were investigated at different chlorination conditions (pH, chlorine concentration, exposure duration). In general, membrane chlorination resulted in an increase in membrane permeability, whereas the solute rejection could increase or decrease, depending on the charge properties of the solutes. Controlled chlorine exposure at low concentrations could enhance the water flux of the membrane without significant reductions in solute rejections for MgCl2 and MgSO4 (and, to a lesser extent, NaCl); the membrane retention of Na2SO4 was actually enhanced by the chlorine treatment. At a given chlorine concentration, the effect of membrane chlorination was intensified at both alkaline or acidic pHs as compared to membrane chlorination at pH 7. It was also shown that the customarily used chlorination intensity (ppm h), which is a composite parameter based on the product of chlorine concentration and chlorination time, was not adequate for use as a standalone parameter to characterize the chlorination conditions.