The role of nanofiltration membrane surface charge on the scale-prone ions concentration polarization for low or medium saline water softening

Based on the fact that when inlet bulk solution with low or medium concentration was involved, scaling was apt to appear on nanofiltration (NF) membrane surface in retentate side. Therefore, study on its surface charge performance under threshold concentration condition is necessary. Two polyamide composite NF membranes (DL and DK) were characterized with streaming potential (Es), zeta (ξ) potential and other measurements encountering with a wide ionic strength range (1.5-108 mol·m−3). Then, the interactional relations between membrane surface charge and concentration polarization (CP) degree, along with the relevant constituting factors were investigated. The results showed that even when the sparingly soluble salt concentration for CaSO4 exceeded its threshold point, both NF membranes at pressure drop of 180 kPa still exhibited low and medium-intensity surface electrokinetic phenomena (−5.94 ≤ Es ≤ −3.41 mV; −38.82 ≤ ξ-potential ≤ −36.34 mV). It turned out that solvent flux decreased almost linearly, but measured concentrations of the scalant ions in NF retentate streams increased slightly at first and then rapidly with the enlargement of ξ-potential, respectively. Meanwhile, mass transfer coefficient and boundary layer thickness had a negligible change in the scope of testing ξ-potential. Therefore, CPSO42− and CPCa2+ decreased apparently with ξ-potential becoming less negative for the adopted NF membranes.