%0 Journal Article
%T Enhancing Ion Transfer in Overlimiting Electrodialysis of Dilute Solutions by Modifying the Surface of Heterogeneous Ion-Exchange Membranes
%A Natalia Pismenskaya
%A Nadezhda Melnik
%A Ekaterina Nevakshenova
%A Kseniya Nebavskaya
%A Victor Nikonenko
%J International Journal of Chemical Engineering
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/528290
%X The desalination of dilute NaCl solutions with heterogeneous Russian commercial and modified ion-exchange membranes was studied in a laboratory cell imitating desalination channels of large-scale electrodialysers. The modification was made by casting a thin film of a Nafion-type material on the surface of cation-exchange membrane, and by processing with a strong polyelectrolyte the surface of anion-exchange membrane. It was shown that the modifications resulted in an increase of mass transfer coefficient and in a decrease in water splitting rate, both by up to 2 times. The effect of mass transfer growth is explained by higher surface hydrophobicity of the modified membrane that enhances electroconvection. The decrease in water splitting rate in the case of cation-exchange membrane is due to homogenization of its surface layer. In the case of anion-exchange membrane the effect is due to grafting of quaternary ammonium bases onto the original membrane surface layer. The suppression of water splitting favors development of electroconvection. In turn, intensive electroconvection contributes to deliver salt ions to membrane surface and thus reduces water splitting. 1. Introduction The applications of electrodialysis (ED) for water recovery in hybrid systems with RO [1] and in near zero liquid discharge (ZLD) systems [2], for ultrapure water production [3, 4], and for salt production from sea water [5] are some examples of successful use of this process, characterized by high economic and ecological effectiveness [6]. Desalination/deionization of dilute solutions is one of the largest ED applications [6, 7]. However, the process rate in this case is limited by the delivery of electrolyte from bulk solution to the membrane interface. This delivery occurs mainly as electrolyte diffusion while the contribution of forced convection is vanishing when approaching the interface [8¨C10]. The usage of intensive current modes might be of practical interest, since it can significantly raise the ED process rate [6, 10]. The latest researches [10¨C12] show that one of the most promising ways of reducing diffusion limitations and enhancing the ED rate is the stimulation of current-induced convection, namely electroconvection. Electroconvection occurs as volume transport under the effect of an electric field imposed through the charged solution, in particular, through the electrical double layer (EDL). In the case where the space charge region (SCR) remains quasiequilibrium, electroconvection occurs as classical electroosmotic flow, named electroosmosis of the first kind
%U http://www.hindawi.com/journals/ijce/2012/528290/