Philippe Sistat

Intensive current transfer in membrane systems: Modelling, mechanisms and application in electrodialysis

Usually in electrochemical systems, the direct current densities not exceeding the limiting current density are applied. However, the recent practice of electrodialysis evidences the interest of other current modes where either the imposed direct current is over the limiting one or a non-constant asymmetrical (such as pulsed) current is used. The paper is devoted to make the mechanisms of mass transfer under these current regimes more clear. The theoretical background for mathematical modelling of mass transfer at overlimiting currents is described. Four effects providing overlimiting current conductance are examined. Two of them are related to water splitting: the appearance of additional charge carriers (H(+) and OH(-) ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. These effects result from formation of concentration gradients (known as concentration polarization) caused by the current flowing under conditions where ionic transport numbers are different in the membrane and solution. Similar effects take place not only in electrodialysis membrane systems, but in electrode ones, in electrophoresis and electrokinetic micro- and nanofluidic devices such as micropumps. The relation of these effects to the properties of the membrane surface (the chemical nature of the fixed groups, the degree of heterogeneity and hydrophobicity, and the geometrical shape of the surface) is analyzed. The interaction between the coupled effects is studied, and the conditions under which one or another effect becomes dominant are discussed. The application of intensive current modes in electrodialysis, the state-of-the-art and perspectives, are considered. It is shown that the intensive current modes are compatible with new trends in water treatment oriented towards Zero Liquid Discharge (ZLD) technologies. The main idea of these hybrid schemes including pressure- and electro-driven processes as well as conventional methods is to provide the precipitation of hardness salts before the membrane modules and that of well dissolved salts after.

Separation of H+/Cu2+ cations by electrodialysis using modified proton conducting membranes

The separation properties of Nafion® and sulfonated polyimide (SP) membranes have been compared for the recovery of acids from effluents containing metallic salts. To improve the selectivity, we have deposited a polyethylene imine (PEI) anion-exchange layer on the membrane surface by using two different techniques, a conventional electrodeposition and the plasma polymerization technique involving an ethylene/ammonia monomers mixture. We have pointed out the efficiency of the latter technique since the transport number of multivalent cations has been decreased by 74 and 54% for the Nafion® and the SP membranes, respectively. Plasma modified sulfonated polyimides are therefore good candidates for electrodialysis processes and constitute an alternative to the Nafion® membrane.

Diffusion layer thickness in a membrane system as determined from voltammetric and chronopotentiometric data

The thickness of the diffusion boundary layer (DBL) in solution near the surface of an ion-exchange membrane is compared at current densities (δ0) much less than the limiting value and at the limiting current density (δlim). The initial linear part of the current-voltage curve (CVC) of the membrane and the initial part of its chronopotentiogram are used to calculate δ0. Values of δ0 are found in a flow-through cell with an active membrane area of 2 × 2 cm2 and 0.02 M NaCl solution for three membranes: AMX, Nafion-117, and MK-40. It is shown that δ0 is more than 20% less for Nafion-117 than for AMX and MK-40. Values of δ0 are close together for the latter two membranes and do not differ greatly from the value calculated from convective diffusion theory (the Leveque equation). In all cases, δ0 is significantly greater than δlim found from the value of the limiting current density by the method of intersecting tangents, which are drawn to the initial segment of the CVC and to the sloping plateau. The effects that determine the dependence of DBL thickness on not only hydrodynamic conditions, but also the state of the membrane surface, are discussed. The principal phenomenon responsible for the decrease in DBL thickness with increasing current is termed coupled convection, more likely, electroconvection. Among the significant properties of the surface are singled out its electrical heterogeneity and degree of hydrophobicity. The different rate of electroconvection near cation- and anion-exchange membranes is related to the Stokes radius of the counterions. The latter explains the well-known observation in the literature that the limiting current density in dilute NaCl solutions is approximately the same for cation- and anion-exchange membranes in spite of the fact that the mobility of Cl− ions is approximately 1.5 times higher than that of Na+ ions.

The crossed interdiffusion of sodium nitrate and sulfate through an anion exchange membrane, as studied by Raman spectroscopy

Raman spectroscopy is applied to visualize concentration profiles in solution adjacent to the top of DSV anion exchange membranes in horizontal positions. The concentration profiles of sulfates, nitrates and their fluxes through the membrane are recorded as a function of time during 13–15 h experiments. A three layer mathematical model based on extended Nernst–Planck equations with a convective term is developed. The membrane’s kinetic and static parameters, such as ionic diffusion and ion exchange coefficients, are determined by separate experiments. Only one parameter, the diffusion boundary layer thickness, δ, is fitted. It is found that the model described fluxes well, especially when the variation of δ is taken into account. At the same time, the model gives nearly linear concentration profiles, whereas the measured profiles are much more complicated. This complicated shape is explained by natural convection and the emergence of the Benard vortexes in the system.

Confocal Raman micro-spectroscopy and electrochemical investigation of anion transport through ion-exchange membranes

Abstract The aim of this study is the determination of the ionic distribution versus the spatial coordinates within the membrane polymers. The confocal Raman spectroscopy is a useful tool for the study because it allows investigation of the species nature within materials. The studied membrane is a AW from Solvay (with a poly-4-vinylpyridine graft and an ethylene tetrafluoroethylene matrix). The first results are the pK value (2.25), verification of the grafts homogeneity in the whole membrane and an initial study about ionic transport within membrane and spectra analysis with factor analysis (principal components analysis and evolving factor analysis with multivariate curve resolution).

Chronopotentiometry and impedancemetry of homogeneous and heterogeneous ion-exchange membranes

AbstractIt is known that the behavior of a membrane with homogeneous and heterogeneous surface under the same current density is different. In order to find out this difference, a two-dimensional non-stationary model of mass transfer through electrically inhomogeneous surface of ion-exchange membrane under direct current is proposed. In this paper, the general formulation of the boundary conditions at the conductive sections has the form of the integral function of the local current density. The problem was solved by applying an electric current stream function, which is quite similar to the stream function in hydrodynamics. For the first time, it is proved that the distribution of current lines over the conductive sections of the membrane is not uniform. It is shown experimentally and theoretically that the shape of chronopotentiograms and electrochemical impedance spectra changes with changing conductive portion of the ion-exchange membrane surface.

Brackish Water Desalination by Electrodialysis: CaCO3 Scaling Monitoring During Batch Recirculation Operation

Abstract The composition of feed water used in electrodialysis (ED) desalination causes the risk of CaCO3 scale crystallization in ED equipments, which leads to the decrease of process efficiency. To control scaling in water systems, several scaling indices and tests, permitting to predict the scaling occurrence, have been devised. In this study, an accelerated scaling test allowing to follow CaCO3 formation in water desalination plant using ED process was proposed. Tests were performed using a pilot unit as a conventional ED in batch recirculation mode. By simple in situ measurements of pH and flow rate of the concentrate, the crystallization onset and growth of CaCO3 were followed during the pilot operation. This leads to precise determination of the number of concentrate recirculation times and therefore to the highest recovery rate without scaling risk as a function of the inlet water quality and the antiscale pretreatment.

Effect of coating and plasma treatments on the induced coupled plasma-reactive ionic etching of boron-doped diamond for microelectromechanical systems (MEMS) applications

Induced coupled plasma etching was performed on poly-crystalline boron-doped diamond (BDD) films synthesized by microwave assisted chemical vapor deposition (MWCVD). Effects of the input power and of the gas composition and pressure on etching performances have been investigated. A basic aluminium layer and a new hybrid material have been studied as protecting film for the patterning. Best conditions to get high etch rates were a gas ratio Ar/O2 of 27% (vol.) under 10xa0mTorr at 200xa0W (input power). Aluminium (Al) masking induced un-intentional whiskers formation which was removable using CHF3 as a reactive gas with the drawback to lower the process selectivity. In order to get both a high etching rate and a high selectivity, a hybrid material based on an organic/mineral resin was then used as the masking material. This performing photoresist film allowed etching a 3.6-μm-thick BDD film at 200xa0nmxa0min−1.