Sébastien Déon

Concentration polarization phenomenon during the nanofiltration of multi-ionic solutions: influence of the filtrated solution and operating conditions.

One of the major difficulties for the prediction of separation performances in the case of multi-ionic mixtures nanofiltration lies in the description of the concentration polarization phenomenon. Usual models available in literature do not take account of the polarization phenomenon or only describe it cursorily. Very few studies dedicated to the understanding and the specific description of the concentration polarization phenomenon are available in literature and a 2-D multi-ionic model describing the layer heterogeneity along the membrane length has never been proposed yet. The model used in the present work, called Pore and Polarization Transport Model (PPTM), allows an accurate description of the concentration polarization layer occurring during the filtration of multi-ionic solutions by taking account of the radial electromigrative transport in the layer, the turbulence, as well as the axial heterogeneity. In this context, the present paper aims at proposing a numerical investigation of the influence of operating conditions on the behavior of the polarization layer occurring at the membrane vicinity. The input parameters governing the transport through the membrane have been assessed in a previous study in the same experimental conditions so that only the polarization layer is investigated here. The proposed model which was previously validated on experimental observed rejection curves is then used to understand how operating conditions, such as applied pressure, feed flow-rate, or divalent ion proportion, govern the polarization phenomenon. For this purpose, concentration and thickness axial profiles along the membrane length and radial profiles within the polarization layer are investigated for various conditions. Finally, the impact of the type of divalent ion and the number of ions is also studied on various mixtures.

Determining the Dielectric Constant inside Pores of Nanofiltration Membranes from Membrane Potential Measurements

The membrane potential technique was applied to a nanofiltration polyamide membrane to determine its mean pore radius and the dielectric constant of electrolyte solutions inside pores. To our knowledge, this is the first attempt to assess these features from membrane potential measurements. Membrane potential data were analyzed by means of the SEDE (steric electric and dielectric exclusion) transport model. Experiments were conducted with single-salt solutions of NaCl and CaCl(2) and mixed-salt solutions of NaCl and CaCl(2) at various concentrations. It was shown that the pore-size values deduced from the high-concentration limit of the membrane potential measured with the two single-salt solutions are in good agreement. With this parameter being known, the membrane potential measured at high salt concentration with electrolyte mixtures was further used to compute the dielectric constant inside pores. The latter was found to be smaller than its bulk value and to decrease when sodium ions were replaced by calcium ions.

Novel modified poly vinyl chloride blend membranes for removal of heavy metals from mixed ion feed sample

Herein, an attempt has been made to prepare a novel membrane with good efficiency for removal of heavy metal ions namely lead (Pb), cadmium (Cd) and chromium (Cr). 4-amino benzoic acid (ABA) was covalently grafted onto the poly vinyl chloride (PVC) backbone by CN bond to enhance the hydrophilicity. 1H NMR and ATR-IR spectroscopy analysis confirmed the chemical modification of PVC. Further the modified polymer was blended in different compositions with polysulfone (PSf) for optimization. Morphological changes that occurred in blend membranes, due to the incorporation of modified PVC was studied by AFM and SEM techniques. The effect on hydrophilicity and performance of blends owing to incorporation of modified PVC was evaluated by water uptake, contact angle and flux studies. The density of functional groups in blends was analyzed by its ion-exchange capacity. Batch wise filtration of metal ions was carried out and the effect of pressure, feed pH and interference of ions was thoroughly investigated. Essentially, 100% rejection was obtained for all the metal ions in acidic pH with a productivity of 2.56l/m2h. The results were correlated with the results of commercially available NF 270 membrane under the same operating conditions.

Numerical Ways to Characterize the Deterioration of Nanofiltration Membranes

In this study, a transport model is used to characterize structural and physico-chemical changes in a nanofiltration membrane during the filtration of ionic mixtures. The membrane state is analyzed by a set of four model parameters identified from glucose and salts filtration: the membrane water permeability (Lp), the mean pore radius (rp), the membrane charge density (Xd), and the dielectric constant of the solution inside pores ( p). The study of these structural and physico-chemical properties allows us to determine if deterioration or fouling occurred during filtration. Two distinct identification procedures from filtration of synthetic solutions are investigated in this paper. One is based on the filtration of single salt solutions, whereas the other lies in parameters identification from mixtures containing at least three ions. These methods are applied here to characterize influence of fouling deposit formation and membrane cleaning.

Theoretical Understanding of How Solution Properties Govern Nanofiltration Performances

Mechanisms governing transfer of ions through nanofiltration membranes are complex and it is primordial to understand how rejection and selectivity performances depend on the properties of the solution. For this purpose, a knowledge model based on a coupling between equilibrium partitioning induced by steric, electric and dielectric exclusions and transport inside pores by diffusion, convection and electro-migration is proposed to theoretically discuss the influence of solution properties on performances. After detailing the physical description of this model, the influence of ion size on rejection is firstly discussed from simulations obtained in several appropriate cases. Since electrostatic interactions are known to play a role on ion rejection, the influence of ion valence and concentration is then studied and different behaviors are brought to light depending on ions considered. Finally, the influence of confinement within nanopores on water dielectric properties and its consequences for ion separation are also addressed.