The effect of increasing ion capacity in the porous phase of a shock electrodialysis cell

MEDINA, SABRINA X; PADILLA, FLORENCIA; ÁVILA, ADOLFO M.; MADRID, ROSSANA E.

Buenos Aires

Workshop; II Brazil-Argentine Microfluidics Congress and V Congreso de Microfluídica Argentina; 2022

Microfluídica Argentina

Shock electrodialysis (SED) is an emerging electrokinetic purification method which is ableto deionize solutions. This process only requires power for moving fluids through porous media,which avoid the need for high temperature (distillation) or high pressure (reverse osmosis, RO)systems. SED is essentially based on ion concentration polarization phenomenon. Concentrationpolarization is an interphase phenomenon which typically occurs when there is a mass transportlimitation for the faster diffusing species in the fluid phase compared to the transport in theselective layer. Consequently, there is a reduction of the faster diffusing component in theboundary layer next to the selective layer and thus creating a depleted zone near the interphase.If a porous material is next to the interphase, now concentration polarization becomes higherdue to the added diffusion limitation provided by the porous material and thus reducing evenmore the concentration of the faster diffusing species in the interphase. In our previous work,it was shown that it is possible to extract a less concentrated electrolyte solution by continuousflow experiments in a miniaturized cell1. For this, a biochar obtained through the pyrolysisof compressed residues from the sugarcane harvest was used as a porous phase to improve thepolarization of the ion concentration. Through this system, the conductivity of a 0.1 mM KClfeed solution can be lowered by 25 percent over a period of 4 h working at a flow rate of 0.7 ccmand 5.0 V of applied voltage. This process was reversible soon after the voltage was shut off.Different concentration ratios between feed and dilute phase can be obtained by changing the flowrate. This is an important advantage in terms of microfluidic device design where operating flowrates are typically a few microliters. On the other hand, the separation performance between thefeed and the more dilute outlet stream was improved by using a less permeable biochar material.In this work, the effect of increasing the ion capacity of the porous phase was studied. For thispurpose, a zeolite material layer was included between two biochar disks. Figure 1 shows theconductivity response when a pulse of low concentration solution ( 2 µS/cm) was injected intothe feed. In the first injection, voltage was not applied. Voltage was switched on during thesecond injection. In this latter case, a significant depleted concentration stream at the cell outletwas observed. This is associated with the improved ion capacity of the porous phase due thepresence of the zeolitic material.