Study the transport properties of anion and cation exchange membranes toward various ions using chronopotentiometry
The transport properties of various anion and cation exchange membranes were studied in different electrolyte solutions using chronopotentiometry technique to get insight about the influence of the counter ion on the transport properties of the membranes. The investigated samples include heterogeneous ion exchange membranes varying in the functionality of fixed charged groups, physical properties of the polymer and the preparation procedure. Chloride and nitrate were investigated as counter-ion in their sodium salts for anion exchange membranes while sodium, calcium, potassium, and magnesium were selected as counter-ion in their chloride salts to study cation exchange membranes transport properties. First, the current – voltage curve of an ion exchange membrane in a given solution was recorded to obtain the limiting current, then a constant current above the limiting current was used for chronopotentiometry measurement. In all the studied membranes the potential across the membrane remained constant for a time interval called the transition time , when a constant current was applied. Then the potential increased rapidly to reach the steady state potential. The permselectivty and the transport number of a membrane were calculated using the corresponding transition time.
Morphology and microtopology of cation-exchange polymers and the origin of the overlimiting current.
In electrodialysis desalination processes, the operating current density is limited by concentration polarization. In contrast to other membrane processes such as ultrafiltration, in electrodialysis, current transport above the limiting current is possible. In this work, the origin of the overlimiting current at cation-exchange polymers is investigated. We show that, under certain experimental conditions, electroconvection is the origin of the overlimiting conductance. The theory concerning electroconvection predicts a shortening of the plateau length of membranes with increased conductive or geometrical heterogeneity. We investigate the influence of these two parameters and show that the creation of line undulations on the membrane surface normal to the flow direction, having distances in the range of approximately 50-200% of the boundary-layer thickness, lead to an earlier onset of the overlimiting current. The plateau length of the undulated membranes is reduced by up to 60% compared to that of a flat membrane. These results verify the existence of electroconvection as a mechanism destabilizing the laminar boundary layer at the liquid-membrane interface and causing ionic transport above the limiting current density.
Nanofiltration of highly concentrated salt solutions up to seawater salinity
Abstract Recently, Nanofiltration (NF) membranes have been employed in pre-treatment unit operations in both thermal and membrane seawater desalination processes. This has resulted in reduction of chemicals used in pre-treatment processes as well as lowering the energy consumption and water production cost and, therefore, has led to a more environmentally friendly processes. In order to predict NF membrane performance, a systematic study on the filtration performance of selected commercial NF membranes against brackish water and seawater is required. In this study, three commercial nanofiltration membranes (NF90, NF270, N30F) have been used to treat highly concentrated (NaCl) salt solutions up to 25,000 ppm, a salinity level similar to that of seawater. The membranes were firstly characterized using the Atomic Force Microscopy (AFM) technique. Pore size and pore size distribution obtained from AFM measurements were used to analyse both experimental data of pure water permeation and data obtained from salt rejection. The main parameters studied in this paper are feed pressure and salt concentration. The experimental data of this work was correlated and analysed using the Spigler-Kedem model. In particular, the reflection coefficient ( σ ) of all studied membranes and the solute permeability of the salt ( P s ) have been determined for all membranes and at all salinity levels studied. For a salinity of 5000 ppm and a pressure of 9 bars, the experimental results showed that NF90 could achieve a salt rejection up to 95%, whereas its rejection dropped to 41% at a salinity of 25,000 ppm and the same pressure. Rejection levels achieved by NF270 have been in range of 11–29% while N30F gave the lowest rejection in the range of 3–6%.