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Membranes 2020, 10, 347 8 of 15 outlet streams (mixed together), were characterized by measuring volumes, conductivity using WTW Cond 3310 together with WTW TetraCon 25 cell, and pH using WTW pH 3310 with a SenTix®41 cell, all purchased from WTW (Prague, Czech Republic). Deionization performance D (hereinafter referred to as ion-removal) was estimated based on conductivity measurements as D= 1−κ ×100% (3) κ0 where κ is the conductivity of the diluted stream and κ0 is the conductivity of the feed. This method had been chosen due to the lack of equipment in our laboratory. More precise estimation could be obtained by measuring concentrations using spectroscopic methods because conductivity might be influenced by the H+ and OH−, products of present water splitting [19]. However, as we compared the ratio of product conductivity to feed conductivity, the obtained deionization factor was considered illustrative enough to count with. 3. Results 3.1. Desalination Performance As expected, an increase in dimensionless current led to increased desalination, as displayed in Figure 6a,b. For both NaCl and Na2SO4 with Siltep 11, the desalination reached 99% around a dimensionless current of three. For foamed fireclay, the desalination did not even reach 90% for a dimensionless current of three (and more for Na2SO4). We compared these data using a model reported by [20], who observed quite a precise collapse of all the measured desalination data on a single master curve approximately following exponential equation logcc =αI (4) 0 where parameter α = −0.619. For our case, this parameter, α = −0.472 ± 0.031, for Siltep 11 and α = −0.220 ± 0.020 for foamed fireclay. Finally, plotting the master curve for each porous material and Schlumpberger’s data respectively provided a good comparison of the desalination performance with dimensionless current as the single variable parameter (Figure 7). We could clearly see both of our cases fell behind the unit with glass frit, with foamed fireclay reaching the desired desalination at a very high dimensionless current of approximately ten and more. In practice, reaching such a large dimensionless current would require applying a current of 0.48 A for Na2SO4 and 0.24 A for NaCl (with 10mM solution and flow rate 1.5 mL/min), which would drive the voltage well above 200 V with our unit. There are a few possible reasons for the worse performance of foamed fireclay. At first, it could be that the pore size was too large to sustain uninterrupted deionization shocks along the channels (as explained above in the Section 3.4: Porous Media Choice and Shape). Second, it might have been the surface charge and pH changes, respectively. The fireclay used is a material composed mostly of alumina and silica, with alumina being prevalent (2.4:1 Al2O3:SiO2). The average pH of the diluted stream (anode side) for experiments with Na2SO4 was 9.9 ± 0.4, indicating a negative charge of both silica and alumina (IEP = 3.9 and 8.8 respectively [25], and 6.8 ± 2.4 for NaCl, indicating more-less positive charge for alumina (and negative for silica), forming the overall rather positive charge, although probably reduced by the negative silica. With Na2SO4 and anion-exchange membranes (AEMs) used, the negative charge would not (in theory) promote deionization shocks in the desired way because the anions would be driven along the charged walls in the opposite direction. That said, zeta potential as a function of pH of the material should be measured to verify this hypothesis. The observed performance suggests only limited deionization shock propagation and functionality. For Siltep 11, the pH varied significantly between 3 and 11 for Na2SO4 and 6.5–9 for NaCl without any correlation with the applied current or desalination performance. The cause of these significant, rather random changes in pH, is unknown.PDF Image | Desalination Performance Assessment Anion-Exchange Membranes
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