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Streaming Potential Measurements As a Characterization Method for Nanofiltration Membranes

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Streaming Potential Measurements As a Characterization Method for Nanofiltration Membranes Colloids and Surfaces A: Physicochemical and Engineering Aspects 150 (1999) 247–259 Streaming potential measurements as a characterization method for nanofiltration membranes J.M.M. Peeters, M.H.V. Mulder *, H. Strathmann Uni6ersity of Twente, Faculty of Chemical Technology, P.O. Box 217, 7500 AE, Enschede, The Netherlands Received 17 June 1998; accepted 19 October 1998 Abstract The streaming potentials of two different nanofiltration membranes were studied with several electrolyte solutions to investigate the influence of salt type and concentration on the zeta potential and kinetic surface charge density of the membranes. The zeta potentials decreased with increasing salt concentration, whereas the kinetic surface charge densities increased. The kinetic surface charge densities could be described by Freundlich isotherms, except in one case, indicating that the membranes had a negligible surface charge. The kinetic surface charge density observed was caused by adsorbed anions. Salt retention measurements showed different mechanisms for salt separation for the two investigated membranes. One membrane showed a salt retention that could be explained by a Donnan exclusion type of separation mechanism, whereas for the other membrane the salt rejection seemed to be a combination of size and Donnan excluion. Comparing the results obtained by the streaming potential measurements with those of the retention measurements, it could be concluded that the membrane with the highest kinetic surface charge density showed the Donnan exclusion type of separation, whereas the membrane with the lower surface charge density showed a separation mechanism that was not totally determined by Donnan exclusion, size effects seemed to play a role as well. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanofiltration; Salt retention; Streaming potential 1. Introduction responsible for the separation, whereas concern- ing the separation of charged species, e.g. ions, The separation mechanism of nanofiltration both size and charge effects may play a role [3–5]. membranes is assumed to be based on a combina- The charge effects that are partly causing the tion of several processes, like size exclusion, rejection of ions are resulting from interactions charge exclusion and, as sometimes referred to as, between the charged particles and the charged dielectric exclusion [1,2]. In the case of uncharged membrane. These electrochemical interactions be- molecules, mainly size exclusion is thought to be tween membrane and ions can be investigated in different ways. Separation experiments have been * Corresponding author. Tel.: +31-53-4892950; fax: +31- performed, varying parameters like salt concen- 53-4894611; e-mail: [email protected]. tration and membrane flux, resulting in a value 0927-7757/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0927-7757(98)00828-0 248 J.M.M. Peeters et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 150 (1999) 247–259 Fig. 1. Schematic drawing of potential decrease as a function of the distance from the surface in an electrolyte solution. for the membrane charge [6–8]. Membrane poten- ions near the membrane will be higher than that tial measurements have been carried out as well to in the bulk of the solution. The charges at and determine the membrane charge density [6,9,10]. adjacent to the surface will cause a potential Furthermore, electrical impedance measurements difference between the region near the surface and have been carried out giving an indication for the the bulk of the solution. The potential decreases resistance of electrolyte transport through the within the solution as a function of the distance membrane [11]. from the charged surface, as shown in Fig. 1. In this paper we will use electrokinetic measure- Three different potentials are shown in this figure, ments to investigate the membrane charge of two the surface potential, c 0, the potential at the nanofiltration membranes. Streaming potential Stern plane, c d, and the electrokinetic or zeta measurements have been carried out to determine potential, z. All these potentials are defined with the zeta potential and the kinetic surface charge respect to the potential at infinite distance from density of the membranes. To investigate the infl- the surface. Although the surface potential is an uence of the type of the electrolyte and of its important parameter, the potential at the Stern concentration, different types of salts at different plane is practically of more importance. This concentrations were used for the streaming poten- Stern plane is the interface between the fixed part, tial measurements. Finally, the results of the i.e. a layer of immobile ions near the charged streaming potential measurements will be com- surface, and the diffusive (mobile) part of the pared to the results of retention measurements electrical double layer. The potential at the Stern carried out with the same electrolyte solutions as plane is the actual potential influencing the behav- used in the retention measurements. ior of the charged species. However, as this poten- tial cannot be measured directly, the electrokinetic potential is often considered as an adequate sub- 2. Theory stitute. The plane at which the zeta potential is located should be outside the Stern layer, and 2.1. Streaming potential represents the potential at the surface of shear between surface and solution where there is rela- Ions in an electrolyte solution that is brought in tive motion between them. The position of the contact with a charged surface will not be dis- surface of shear is close to, and from practical tributed randomly. The concentration of counter- reasons assumed to be identical to, the Stern J.M.M. Peeters et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 150 (1999) 247–259 249 ozDP R surface [12]. The zeta potential is shown in Fig. 1 DE = exp (2) str hl R as well. The zeta potential can be determined by 0 th electrokinetic measurements, like streaming poten- In the latter case the streaming potential is not tial and electro-osmosis measurements. dependent on the geometry of the capillary. A streaming potential is the potential difference In the case of a low potential, the relation at zero current caused by the convective flow of between the surface charge density at the hydrody- charge due to a pressure gradient through a namic shear plane and the zeta potential is given charged capillary, plug, diaphragm or membrane. in the following equation: A streaming potential is generated by exerting a oz force on the double layer that has been built up in s d = (3) the solution near the charged surface. Since an k−1 excess of counter charges is present, movement of where k-1 is the Debije length. The Debije length those counter charges causes a current. This cur- can be calculated by Eq. (4): rent which is streaming through the double layer is called the streaming current. The accumulation of ' okT k−1 = 2 (4) counter charges downstream generates a streaming 4e NAI potential across the capillary which on its turn 2 o causes a conduction current through the capillary with I=0.5 z i ci: where is the permittivity of in the reverse direction. In steady state, the stream- the medium, k the Boltzmann constant, T the ing current equals the conduction current. temperature, e the elementary charge, NA the Measurement of a streaming potential can Avogadro number, I the ionic strength, zi the provide both the zeta potential, z, and the net valency and ci the concentration of species i. charge density, s d, at the hydrodynamic shear Although Eq. (4) was derived for symmetrical plane. The streaming potential is defined to be electrolytes, like 1-1 and 2-2 electrolytes, we will positive if the higher potential is at the high use it for 1-2 and 2-1 electrolytes as well. pressure side. Concerning the membrane charge density, a The relation between the zeta potential, z, the distinction can be made between the actual charge D of the membrane, which originates from mem- streaming potential, Estr, and the pressure differ- ence applied, DP, is given by [12,13]: brane fixed charges, like carboxylic or sulphonic groups, and adsorbed charges. ozDP DE (1) Benavente et al. [14] introduced a method to str = l 2 s discriminate between these two types of charges. h l + 0 r Therefore, it was assumed that three components where o is the permittivity of the medium, h the added to the total charge of the system. First, the S0 viscosity, l the bulk conductivity, l the surface fixed charges at the membrane surface, , second, 0 s Ss conductivity, and r the radius of the pore or the charges of the Stern-layer, , and finally, the capillary or half the width of a slit. charges within the diffusive part of the electric Sd It is difficult to measure directly the surface double layer, . The charge of the Stern-layer is conductivity and therefore, at low electrolyte con- caused by the adsorption of ions. As electroneu- centrations, where the surface conductivity is not trality is assumed, the total charge equals zero. be negligible, Eq. (1) cannot easily be applied. This problem can be circumvented by measuring the %0 +%s +%d =0 (5) actual resistance of the electrolyte solution across the slit or pore, Rexp, and comparing this value with Assuming that the diameter of the pore or the the resistance that can be calculated from experi- width of the slit through which the solution is ments at high concentrations, where the surface streamed is much larger than the Stern-layer thick- s conductivity can be neglected, Rth [13]. Then, Eq. ness, the relation between the charge densities, , (1) can be written as: of each of the layer can be written as: 250 J.M.M.
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