Characterization of the membrane charge of Al2O3 membranes

Study on transmembrane electrical potential of nanofiltration membranes in KCl and MgCl2 solutions

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-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.

Control of alpha-alumina surface charge with carboxylic acids

In this work, we studied the surface charge of alpha-alumina treated with carboxylic acids with different carbon chain length. The results show the possibility of controlling surface charges of alumina by using different concentrations of carboxylic acids or changing the size of the carbon chain of the acids. We also report that part of the acid found on the surface is strongly bound, therefore making it possible to obtain pH-resistant samples of alpha-alumina with an isoelectric point (IEP) of 5.5. It is found, that IEP values obtained for modified samples have a linear correlation with the number of carbon atoms of dicarboxylic acids for up to five carbon atoms. From a practical perspective, the method presented in this work has many advantages. First, it maintains the same hydrophilicity of the alumina surface. Second, the modification of the surface is stable in a long-range of pH. Finally, the presented method is easy-to-use and cheap, as the modification consists of only two simple steps carried out at low temperatures with inexpensive and nontoxic reagents.

Influence of steric, electric, and dielectric effects on membrane potential

The membrane potential arising through nanofiltration membranes separating two aqueous solutions of the same electrolyte at identical hydrostatic pressures but different concentrations is investigated within the scope of the steric, electric, and dielectric exclusion model. The influence of the ion size and the so-called dielectric exclusion on the membrane potential arising through both neutral and electrically charged membranes is investigated. Dielectric phenomena have no influence on the membrane potential through neutral membranes, unlike ion size effects which increase the membrane potential value. For charged membranes, both steric and dielectric effects increase the membrane potential at a given concentration but the diffusion potential (that is the high-concentration limit of the membrane potential) is affected only by steric effects. It is therefore proposed that membrane potential measurements carried out at high salt concentrations could be used to determine the mean pore size of nanofiltration membranes. In practical cases, the membrane volume charge density and the dielectric constant inside pores depend on the physicochemical properties of both the membrane and the surrounding solutions (pH, concentration, and chemical nature of ions). It is shown that the Donnan and dielectric exclusions affect the membrane potential of charged membranes similarly; namely, a higher salt concentration is needed to screen the membrane fixed charge. The membrane volume charge density and the pore dielectric constant cannot then be determined unambiguously by means of membrane potential experiments, and additional independent measurements are in need. It is suggested to carry out rejection rate measurements (together with membrane potential measurements).