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filtration of micron-scale particles and aggregates of nanoscale particles present in the feedwater, and we hypothesize that this was due to steric hindrance by our microporous frit. We further demonstrate that ap- proximately 99% of Escherichia coli bacteria placed in the feedwater were killed or removed upon flow through our shock ED prototype, il- lustrating the potential for in-situ and additive-free disinfection in shock ED. In addition, we show that our prototype can continuously separate electrochemically inactive ions by charge, consistent with the- ory [26] but contradicting claims that ICP acts as a “virtual barrier” to all charged species [24]. 2. Materials and methods 2.1. Shock ED device A schematic and a photograph of our shock ED device are shown in Fig. 2a and b. The setup consists of a cylindrical silica glass frit (Adams & Chittenden Scientific Glass), which is 1 mm thick and has a 5 mm radius. The frit is placed against a Nafion membrane, and the membrane is in direct contact with the copper disk cathode. The frit is separated from the copper disk anode by a reservoir of copper sulfate (CuSO4) electro- lyte (3 mm thick reservoir). The frit has a random microstructure with pores roughly 500–700 nm in diameter (Fig. 2c, d), an internal surface area (measured via BET) of am = 1.75 m2/g, porosity of 0.4, and a den- sity of ρm = 1.02 g/cm3. The pore surfaces are negatively charged, and the magnitude of the charge depends on copper sulfate concentration [11]. The charged surfaces promote the (faster-than-diffusion) trans- port of positive copper ions to the cathode via electroosmotic flows, leading to overlimiting currents [11]. 2.2. Sample preparation A 1 M CuSO4 stock solution was prepared by dissolving 2.5 g of CuSO4 (Science Company) into 10 mL of deionized water. This stock so- lution was further diluted 10 times to obtain 0.1 M CuSO4 solution, and again diluted to obtain 1 mM CuSO4 solution. To demonstrate size- based filtration, two suspensions were prepared by adding 50 μm diameter green fluorescent polymer microspheres (Thermo Scientific) and 50 nm diameter red fluorescent nanoparticles (Thermo Scientific) into 1 mM CuSO4 solution. The concentration of these suspensions was 20 mg/mL and 2 mg/mL, respectively. To demonstrate charged- based separation, positively and negatived charged fluorescent dye so- lutions were also prepared. For positively charged dye, 2 × 10− 5 g/mL of Rhodamine B fluorescent dye was mixed into 1 mM CuSO4. The pH of this solution was measured to be 4.2, far enough below Rhodamine's isoelectric point of 6 to ensure that the dye is positively charged [27–29]. For negatively charged dye solution, 1 mg/mL of fluorescein dye (Sigma-Aldrich) was mixed into 1 mM CuSO4 [30], and 50% volume of isopropyl alcohol was added to ensure solubility. In order to evaluate the disinfection capabilities of the device, we prepared suspensions of E. coli K12 (ATCC). The bacteria were cultured in LB broth at 37° with shaking, and when they reached log phase, were resuspended in 1.5 M NaCl solution. This concentration was selected to minimize osmotic shock upon transfer from the LB broth. After exper- iments were completed, the bacteria were stained with a BacLight live/ dead staining kit as per the manufacturer's instructions (Invitrogen) and thus the live cells could be observed with a microscope. Control samples of E. coli were left suspended in the NaCl solution while testing was con- ducted, and little or no degradation was observed to their viability upon completion of the experiment. 2.3. Device operation An electrochemical analyzer (Uniscan instruments PG581) was used to apply a voltage to the device. The analyzer's reference and counter electrode leads were connected to the anode, and the working electrode lead was connected to the cathode. After about 10 min, the current reached steady state, and collection of fluid from the outlet of the device began. As indicated in Fig. 2a, the fluid flow was directed towards the cathode side of the device (to force flow through the depletion zone). Flow rate was precisely controlled by a syringe pump (Harvard Appara- tus), and the fluid extraction time varied from several minutes to several tens of minutes in order to collect roughly 1 mL of fluid from the outlet for accurate post-experiment analysis. ab D. Deng et al. / Desalination 357 (2015) 77–83 79 anode (Cu) reservoir glass frit Nafion membrane cathode (Cu) c outlet 1 cm d counter/reference electrode working electrode 1 μm Fig. 2. Description of the prototype shock ED device used in this work. (a) A sketch of the frit/membrane/electrode sandwich structure (not to scale), where arrows indicate the fluid flow direction; (b) a photograph of shock ED device; (c) a SEM micrograph of glass frit showing its pore structure, and (d) enlarged micrographs indicating pore size around 500–700 nm. 0.1 1 3 mm outletPDF Image | Water Purification by Shock Electrodialysis
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