The negative rejection of H+ in NF of carbonate solution and its influences on membrane performance

Nanofiltration (NF) experiments were conducted with simulated solution containing bicarbonate hardness and with three membranes: NF90, NF270 and NF-. The operating pressure was kept at 1.0 MPa, the water sample temperature was kept at 25 °C, the velocity on membrane surface was approximately 1 m s−1 and the feeding pH values were 6.0, 6.5, 7.0, 7.5 and 8.0. Except for sampling, both the concentrate and the permeate were cycled to the feeding tank in order to keep the concentration constant. Experimental results demonstrated that the flux of NF90 membrane was slightly affected by the feeding pH, while the flux of NF270 and NF membranes was augmented when the feeding was acidified. The permeate pH of each membrane was always lower than the concentrate pH, indicating that the H+ was negative rejected. This phenomenon was observed not only in the filtration of carbonate solution, but also in the filtration of the NaCl solution and the tap water. It is assumed that the small positive H+ can easily permeate the NF membranes having negative charges. The more charges the membrane has, the more H+ is negative rejected. High permeability of H+ can speed decomposition of HCO3- into CO32- and then accelerate CaCO3 crystallization on membrane surface, which is unfavorable for membrane performances. In NF processes, it is better to considerate the membrane’s selection and the feeding’s pretreatment simultaneously, and it is beneficial for preventing scaling when selecting the membranes with less negative charges if only the water qualify can meet the demands.