Water desalination by neutralization dialysis with ion-exchange membranes: Flow rate and acid/alkali concentration effects

Kinetics of surface water desalination by neutralization dialysis (ND) was studied in a three-compartment stack with ion-exchange membranes, and the process feasibility was evaluated. The pH and the ionic conductivity of a model saline solution (0.02 M NaCl) were measured during different ND operations in batch mode. The influence of the flow rate, the solution concentration in the alkali and acid compartments was investigated. It was found that, at the beginning of the operation, the ion-exchange kinetics was mainly controlled by the membrane diffusion. The maximal fluxes of exchanging ions (Jmax) (Na+ and H+ across CMX membrane, and Cl− and OH− across AMX membrane) were theoretically evaluated. The calculated and the experimentally measured fluxes at the beginning of batch operation were close. However, after some minutes of solution desalination, the salt concentration decreased, which involved diffusion boundary layer controlled kinetics. Hence, with time, the fluxes decreased and were considerably smaller than Jmax. The increase/decrease in the acid and alkali solution concentration resulted in corresponding increase/decrease in the ion-exchange rate. Recommendations have been provided to reduce the desalination duration. © 2015 Elsevier B.V. All rights reserved.

In this study, we established that DBLs as well as the acidic and alkaline solution concentrations play an important role in the ND kinetics. At the start of the operation, when the NaCl concentration is relatively high (0.02 M in all cases), the diffusion is mainly controlled by the membrane. However, while NaCl concentration decreases, the diffusion control progressively passes to the DBLs even if their thickness is quite low (evaluated at about 80 μm). This change in the kinetics mechanism is accompanied by a fast decrease in the rate of ion-exchange expressed in lowering values of the ion fluxes across corresponding ion-exchange membranes: Na+ and H+ across the cation-exchange CMX membrane, and Cl− and OH− ions across the anion-exchange AMX membrane. The maximum possible (limiting) for the given membrane ion-exchange fluxes (Jmax) was evaluated theoretically by knowing the electric conductivity and the exchange capacity of IEMs. These values were found in a good agreement with the experimental fluxes at the beginning of batch operation. As expected, an increase in the acid solution concentration resulted in increasing ion-exchange rate across the CEM and in reducing pH of the saline solution, while increasing alkaline solution concentration led to lower exchange rate across the AEM and higher pH values in the saline solution. The ion-exchange rate strongly depended on the saline solution pH value. Indeed, low pH reduced the ion-exchange rate across the CEM, while high pH reduced that across the AEM. This correlation resulted in saline solution pH oscillations. Taking into account high dependence of the ion-exchange rate on the flow rate, it is recommended to use thin and effective spacers and to apply high flow rates in each compartment. As well, relatively high concentrations of the alkaline and acidic solutions are preferential, as they allow low molar fraction of salt ions to be maintained in the corresponding compartments. However, certain constraints should be taken into account concerning the safety of operation and the membrane life time in aggressive conditions. Finally, special attention should be paid to maintain the saline solution pH close to 7 in order to maximize the fluxes. Even if the ND process costs are relatively high compared to the ED and RO costs (1.38 $/m3 compared to 0.1–0.4 $/m3 or 0.17–1 $/m3, respectively), we can state that it is a promising technique due to its relatively easy implementation. In practice, the ND costs can be essentially reduced if using local chemical products, e.g. lime water instead of caustic soda. List of symbols A acid compartment AEM anion-exchange membrane B alkali compartment D desalination compartment CA acid concentration (mol·L−1) CB alkali concentration (mol·L−1) CD concentration in desalination compartment (mol·L−1) CEM cation-exchange membrane DBL diffusion boundary layer IEM ion-exchange membrane ND neutralization dialysis Q flow rate (mL·min−1) S Membrane active area (m2) VA Solution volume in the A compartment (L) VB Solution volume in the B compartment (L) VD Solution volume in the D compartment (L) i = 1, 2, 3, 4 H+, OH−, Na+ and Cl− ions ci ion concentration in the solution (mol·L−1) ci ion concentration in the membrane (mol·L−1) C A ci , ci ion concentration in the CEM and AEM (mol·L−1) Di, Di diffusion coefficient in the solution and in the membrane (m2·s−1) Dij, Dij mutual diffusion coefficient in the solution and in the membrane (m2·s−1) D membrane thickness (μm) Ji flow density (mol·m−2·s−1) C A J ,J ion fluxes in the CEM and AEM (mol·m−2·s−1) C A JDBL, JDBL maximal fluxes in the CEM and AEM (mol·m−2·s−1) R membrane relative resistance C A X ; X concentrations of ions in the CEM and AEM (mol·L−1) zi charge number σ solution conductivity (mS·cm−1) δ DBL thickness (μm) Acknowledgments The study was realized within French-Russian International Associated Laboratory “Ion-exchange membranes and related processes”. We are grateful to CNRS, France, and to RFBR Russia (projects Nbs 1308-01168 and 13-08-96525), as well as to FP7 Marie Curie Action “CoTraPhen” project PIRSES-GA-2010-269135.