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82 D. Deng et al. / Desalination 357 (2015) 77–83 [37]. Jeon et al. further developed a device which separated negatively charged particles via the deflection of their path through the depletion zone (where the extent of deflection correlated to the particles' electro- phoretic mobility) [37]. In this work, we show that ICP can in fact accelerate the passage of positively charged species, thus contradicting the virtual barrier claim and demonstrating, apparently for the first time, charge-based separa- tion by the ion depletion zone. Naturally this nonlinear electrophoretic effect only applies to particles small enough to pass through the pores and avoid filtration. In order to demonstrate the effect of the depletion zone on positively charged species, we injected the positively charged Rhodamine dye solution into the device reservoir, and applied a con- stant voltage of 1.5 V. Note that Rhodamine is considered a non- electrochemically active species, as it does not participate in the elec- trode reactions (electrode reactions include the positively charged cop- per ions). The solvated molecule has an effective size of only a few nanometers, so no size-based filtration takes place in our device. Based on the applied electric field, the positively charged dye was ex- pected to move via electrophoretic forces towards the cathode where they accumulate, forming an enrichment region near the outlet. The outlet and reservoir concentrations of Rhodamine were calculated by injecting samples into a hemocytometer chip, and measuring their inte- grated fluorescent intensities (I) using an optical microscope. As expected, the dye concentration at the outlet was observed to be greater than at that of the reservoir. Quantitatively, we defined the en- hancement ratio of fluorescent intensity as Iinlet/Ioutlet, where Iinlet is the integrated fluorescent intensity of solution in the inlet or reservoir, and Ioutlet is the integrated fluorescent intensity of solution extracted from the outlet. This enhancement ratio is shown in Fig. 5a as a function of extraction flow rate. As the flow rate decreased, the effluent became more enriched in Rhodamine as the integrated fluorescent intensity of solution increased. When no voltage was applied to the shock ED device, as expected, no concentration enhancement was observed in the efflu- ent solution. The experiment was then repeated with the negatively charged fluo- rescein dye solution. In contrast to the positively charged dye, the fluo- rescein was expected to migrate electrophoretically towards the anode, thus being “repelled” from the depletion zone by the large electric field there. When the dye concentration was measured quantitatively at the inlet and outlet (at the cathode-side), we observed a depletion of fluo- rescein at the outlet as expected. The removal ratio, defined as Iinlet/ Ioutlet, is shown as a function of flow rate in Fig. 5b, and this ratio in- creased with increasing flow rate. a Fig. 5. Results demonstrating the charge-based separation of non-electrochemically active ions by our shock ED device. (a) The observed enhancement ratio of a fluorescent, positively charged dye versus flow rate of feedwater through the device. Enhancement ratio is the ratio of dye concentration (fluorescent intensity) of the outlet sample to the feedwater sample. Enhancement ratio is always greater than unity, indicating that positively charged dye accumulated in the depletion zone of the device. Enhancement ratio decreases as flow rate is in- creased. (a) The observed removal ratio of a fluorescent, negatively charged dye versus flow rate. Removal ratio is also defined as the ratio of dye concentration (fluorescent intensity) of the outlet sample to the feedwater sample, but in this case, this ratio is always below unity indicating that negatively charged dye was repelled from the depletion zone. Removal ratio increases as flow rate is increased. b 4. Discussion and conclusion In summary, we have demonstrated that shock ED devices can per- form many functions in addition to (and simultaneously along with) water desalination, including filtration, disinfection and separations by charge and by size. As shock ED employs a microporous frit within the flow channel between the membranes, we were able to demonstrate steric filtration of microscale particles and aggregates of nanoscale par- ticles. Further, we were able to kill or remove approximately 99% of vi- able E. coli bacteria present in the feedwater upon flowing through the shock ED device with applied voltage. We hypothesize that further re- ductions in bacteria viability are achievable via the presumably strong electric fields present within the shock region, and future work will focus on building a prototype capable of testing this hypothesis. Lastly, we demonstrated that our shock ED device can continuously separate positively charged species from negatively charged ones that are small enough to pass into the porous frit. These demonstrated functionalities can be applied to water purifica- tion systems (filtration, disinfection), as well as particulate, molecular or biological separation systems, and demonstrate the potential of shock ED as a versatile new technique for chemical engineering. As in our initial experimental publication [11], here we used a simple first prototype that sustains direct current by electrodeposition/dissolution reactions at copper electrodes and does not separate the brine produced near the anode, as required for continuous operation. We are currently building and testing a proposed scalable prototype capable of con- tinuous desalination and water purification of arbitrary feed water (see Fig. 6 of Ref. [11]). The complete shock ED system consists of a stack of negatively charged porous media separated by cation exchange membranes with electrode streams sustaining the current by water electrolysis. An important potential application could be to treat produced water from hydraulic fracturing of unconventional oil and gas reservoirs [46]. It has recently been argued that classical ED is an economically viable technology to address this grand challenge in water treatment, if mem- brane fouling and pre/post-processing were not overly costly issues [47]. Our results suggest that shock ED could be even more attractive, as it retains most of the benefits of classical ED, while incorporating filtration and charge-based colloidal separation in a single compact system. Moreover, the demonstrated electrical disinfection capabili- ty should dramatically reduce fouling of the cation exchange mem- branes, thus eliminating a major lifetime and cost concern. Unlike classical ED, any size-filtered or electrophoretically separated particles μμPDF Image | Water Purification by Shock Electrodialysis
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