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between the inlet and outlet of the device. It is difficult to assess the im- pact of the shock electrodialysis process for disinfection in the original experiments because copper sulfate solution is not a good medium for cells, so a different electrolyte, 1.5 M sodium chloride (NaCl), was used for these experiments to ensure bacteria viability (see the Materials and methods section.) A sample of the inlet E. coli solution is shown in Fig. 4a. The bacteria were stained with a live-dead kit prior to microscopy, and so live bacteria appear green, and dead as red. Using our microscopy setup, we performed a cell counting analysis which showed that the inlet sample contains bacteria which were 99 ± 0.4% viable. Outlet samples were taken under two conditions: one with a flow rate of 0.2 μL/min through the device under the applied voltage at 1.5 V, shown in Fig. 4c, and the other with the glass frit re- moved and without any applied voltage, as shown in Fig. 4b. When Fig. 4b is compared with Fig. 4a, there is an observable drop in concen- tration, possibly due to adhesion at points in the system, but the key point is that the bacteria are still largely viable with only very few dead cells. The data demonstrate significant disinfection that depends on forced convection through the porous medium, as well as the action of the ap- plied electric field. When Fig. 4c is compared with Fig. 4a, using the sys- tem with the frit results in yet lower concentrations, and larger numbers of dead cells. More quantitatively, with the frit removed, cell viability was measured to be to 96.4 ± 0.9%. With the frit in place, we measured a much reduced viability of the bacteria in the outlet sample, which was 28 ± 8%. We also observed (with the frit in place and under the applied voltage) a strong reduction in the concentration of cells in the outlet sample, as shown in Fig. 4c. Here, in the outlet sample, 96.5 ± 1.8% of bacteria were absent relative to the reservoir sample. Combined with the reduced viability of the outlet sample, roughly 1% of initially viable bacteria remained present and viable in the outlet sample when the frit was in place. We hypothesize that the latter results are due to both steric exclusion of a majority of the bacteria from the frit pore space (E. coli have typically micron-scale dimensions [36]), and an inhospitable environ- ment to the bacteria which are able enter the glass frit. a We further hypothesize that the strong electric fields arising in the ion depletion zone can also reduce further the viability of the bacteria passing through the device [37]. Previous studies have shown that an electric field of roughly 0.8–2 V/μm can promote cell death [38,39]. However, we were unable to reliably test this hypothesis in this work, because the use of sodium chloride rather than copper sulfate likely inhibited the formation of a concentration shock (and thus a strong electric field) in our prototype. With sodium chloride (and copper elec- trodes), our device relied on water electrolysis electrode reactions rath- er than copper redox reactions to pass a current through the device, which can cause zones of perturbed pH to enter the system [40]. Future work will develop prototypes capable of generating shocks in sodium chloride and other general electrolytic solutions, allowing us to test the effect of depletion zone electric fields on bacteria survival. 3.3. Separation based on charge In microfluidic devices leveraging ICP to perform molecular sample stacking [41] and water desalination [24], the ion depletion zone was reported to act as a “virtual barrier” for all charged particles (both positively and negatively charged). A possible mechanism for such charge-independent attraction is diffusio-phoresis [42–44], in which charged particles climb a salt concentration gradient in the absence of an applied electric field. When current is being passed, however, this effect competes with classical electrophoresis in the joint phenomenon of electro-diffusiophoresis studied by Malkin and A. Dukhin [45]. Rica and Bazant have shown that electrophoresis generally dominates diffusiophoresis in large concentration gradients due to the enhanced electric field of the depleted solution [26], thus enabling separation by charge in particle flows through regions of ICP. Recently, Jeon et al. an- alyzed the forces acting on charged particles flowing through the deple- tion region in microfluidic ICP with negatively charged channel walls and reached consistent conclusions, that negatively charged (counter- ionic) species are repelled from the depletion zone via strong electro- phoretic forces, while positively charged species cannot be repelled D. Deng et al. / Desalination 357 (2015) 77–83 81 bc Fig. 4. Results demonstrating the disinfection of a feedwater containing E. coli as a model bacteria. (a) Microscopic image of the feedwater, (b) image of a sample from the outlet stream of the device without the porous frit and at zero voltage, showing some cell adsorption but little disinfection (cell death), and (c) an outlet stream image from the device with porous frit included at an applied voltage (1.5 V) and outlet flow rate (0.2 μL/min), showing strong disinfection and filtration. The live bacteria are fluorescent green, and the dead bacteria are red. Scale bar is 200, 500, and 500 μm, for (a)–(c) respectively.PDF Image | Desalination 357
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