Transport Properties and Electrokinetic Characterization of an Amphoteric Nanofilter

Abstract

Transport properties of a tubular nanofilter with amphoteric properties have been investigated by means of the SEDE (steric, electric, and dielectric exclusion) homogeneous model. Within the scope of this 1D model, the separation of solutes results from transport effects (described by means of extended Nernst-Planck equations) and interfacial phenomena including steric hindrance, the Donnan effect, and dielectric exclusion (expressed in terms of (i) the Born dielectric effect, which is connected to the lowering of the dielectric constant of a solution inside nanodimensional pores, and (ii) the interaction between ions and the polarization charges induced at the dielectric boundary between the pore walls and the pore-filling solution). The effective volume charge density of the membrane has been determined from tangential streaming potential experiments coupled with conductance experiments in a potassium chloride solution at various pH values ranging from 2 to 11. The inferred values have been used in the SEDE model to compute the ion rejection rates with the dielectric constant of the solution inside the pores as a single adjustable parameter. The model provides a relatively good description of experimental data even at extreme pH values for which a ternary system has been considered (K+, Cl-, and H+ or OH- depending on the pH). The fit to experimental data at the membrane isoelectric point indicates that the confinement effect decreases the dielectric constant inside the pores only slightly (with respect to its bulk value). However, the (pH-dependent) ionization of surface sites has been found to lead to a substantial lowering of the dielectric constant inside the pores.

Evaluation of the steric, electric, and dielectric exclusion model on the basis of salt rejection rate and membrane potential measurements

“…This is done numerically from Eqs. (13)-(19) [5,24]. For all calculations, the diffusion coefficients of calcium and chloride ions were set at 0.792 and 2.031 × 10 −9 m 2 s −1 , respectively.…”

“…An advantage of NF is that it permits operation at low pressure while keeping a relatively high permeate flux compared with that for conventional reverse osmosis. Another feature of NF membranes is that most of them are electrically charged in aqueous media due to ionization of surface functional groups [3][4][5] and/or * Corresponding author.…”

“…It was shown that dielectric exclusion could not be neglected in the analysis of the rejection properties of the membrane under consideration and that the widely adopted steric/electric exclusion theory was unable to describe experimental data on the rejection rate. This new approach was also applied to study the rejection properties of an amphoteric ceramic membrane in the case of KCl solutions [5]. Again, the model has proven to provide a relatively good description of experimental data even at extreme pH values for which a ternary system had been considered.…”

“…However, the charge density deduced from these measurements may provide only a rough estimate of the charge density within the active layer pores [5]. As to the dielectric constant, the measurement of this parameter is also extremely difficult due to the multilayer (thick support layers) and composite (membrane material and confined solution) structure of NF membranes.…”

Ex-Situ vs. In-Situ Defluoridation of the Contaminated Aquifers: A State-of-the-Art Review

“…Nano filtration membranes work particularly well due to its diffusional behaviour for fluoride rejection, its high hydraulic permeability and high selectivity for rejection of fluoride (Pontié et al, 2006). A particular advantage of NF membranes is that in Reverse Osmosis virtually everything is removed from water whereas NF membranes are selective and can function in a way that it doesn't remove the essential salts from water (Bason et al, 2006;Cervera et al, 2003;Diawara, 2008;Hu & Dickson, 2006;Szymczyk & Fievet, 2005;Szymczyk et al, 2006;Yadav et al, 1999) . But the membrane process incurs huge water losses as brine, have high capital cost, are very energy intensive and generally requires remineralisation as a post treatment step (Hindin et al, 1968;; U.S. Environmental Protection Agency(USEPA), 2002).…”