The transmembrane electrical potential (TMEP) across two commercial nanofiltration membranes (ESNA1-K and Filmtec NF) was investigated in KCl and MgCl_2 solutions. TMEP was measured in a wide range of salt concentrations (1-60 mol · m~(-3)) and pH values (3-10) at the feed side, with pressure differences in the range of 0.1-0.6 MPa. A two-3 layer model based on the Nernst-Planck equation was proposed to describe the relation between TMEP and permeation flux. From the pattern of these curves, the information of membrane structure could be deduced. In the concentration range investigated, TMEP in KCl solutions was always positive and decreased as the salt concentration increased. The contribution of the membrane potential to the TMEP decreased. TMEP was greatly affected by the feed pH. When the feed pH increased, the mobility of cations increased, which indicated that the charges of NF membranes were more negative. The zero point of TMEP and the minimum of rejection in KCl solution were consistent and occurred at the isoelectric point of NF membranes, while in MgCl_2 solution the zero point of TMEP located at a higher pH value. The TMEP in MgCl_2 solutions changed its sign at a given concentration, and by calculating the transport number the location of the minimum rejection could be determined.
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