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80 D. Deng et al. / Desalination 357 (2015) 77–83 2.4. Microscopy A Nikon Ti–U inverted fluorescence microscope, a 4× or 10× objec- tive, and a Photometrics Coolsnap HQ2 CCD camera were used to cap- ture optical micrographs of samples of fresh solutions or solutions from the outlet of the device 4× and 10× objectives were used in con- junction with a hemocytometer to count the bacteria. To count live E. coli cells or particles, samples were loaded into a hemocytometer (INCYTO). The grid pattern on the hemocytometer is explicitly designed for use with lower-powered objectives. In our case, the lower magnifi- cation was desirable in order to achieve decent counting statistics on low concentration samples. Illumination was provided by a mercury lamp (Intensilight) for the fluorescence images, and a standard lamp for the bright field images. For E. coli, the live/dead cell counting was done using a Nikon C-FL B-2E/C FITC Filter for the live bacteria (dyed with Syto BC, thus appearing green) and a Nikon C-FL Texas Red HYQ for the dead bacteria (dyed with Propidium Iodide, thus appearing red). Image analysis was performed using ImageJ software, either to count the number of particles, count live and dead cells, or to measure the integrated fluorescent intensity of the fluorescent dye solution. In experiments involving fluorescent dyes, any photobleaching effects were not significant as we confirmed that there was no decrease in the integrated fluorescence of a control sample kept next to the device over the course of the experiment. 3. Results and discussion 3.1. Filtration based on size In contrast to electrodialysis [31], shock electrodialysis utilizes a charged porous glass frit placed between ion exchange membranes to enable overlimiting current [14] and propagate a deionization shock wave [12,13,15,16]. The frit in a shock ED system can also be used as a filter to sterically exclude undesirable particles from the flow (for exam- ple, particulates in feedwater in a water purification process). The glass frit in our prototype has pore diameters between roughly 500 and 700 nm, so micron-scale particles can potentially be largely excluded from the fluid at the outlet. In the first experiment, the reservoir was filled with the suspension of 50 μm particles in copper sulfate solution. We then flowed this sus- pension through the shock ED device (from anode to cathode), without applying electric field, and captured the effluent. The microscopy image taken of the inlet solution is shown in Fig. 3a, and here we can observe the particles as dark dots. As shown in Fig. 3b, the absence of particles was observed in the outflow solution, demonstrating that our shock ED prototype successfully filtered out these particles. In a similar experiment, we investigated the behavior of particles with diameter less than the frit pore size using the suspension of 50 nm-diameter particles in aqueous copper sulfate solution. From the image of the inlet solution in Fig. 3c, the nanoparticles appeared to floc- culate into aggregates due to surface interactions [32,33]. Due to the spatial resolution limitations of optical microscopy, we could not ob- serve directly any unaggregated 50 nm-diameter particles potentially present in solution. However, we observed that the aggregates were large enough to be filtered by porous medium, as no aggregates were observed in the effluent samples (Fig. 3d). 3.2. Disinfection Removing waterborne pathogens is a critical part of many water pu- rification processes [1,34]. State-of-the-art reverse osmosis (RO) desali- nation plants utilize dedicated post-treatment procedures for water disinfection, typically the addition of chlorine or chlorine by-products, to eliminate most harmful microorganisms [35]. The shock ED device in- vestigated here utilizes a glass frit with submicron-radius pores that can potentially serve as a filter to remove many biological species during de- ionization, eliminating the need for any post-treatments. In addition, we postulate that the high electric fields present in the deionization region may further reduce the survival rate of any microorganisms that make it through the glass frit. To demonstrate the disinfection functionality in our device, we mea- sured the change in concentration and viability of a suspension of E. coli ab cd Fig. 3. Results demonstrating the size-based filtration of our shock ED device. Bright field images for the feedwater (a) and (c), and images on a sample from the corresponding outflow (b) and (d). The particles with 50 μm diameter were completely removed from inlet (a) to outlet (b). Aggregates of 50-nm-diameter nanoparticles were also filtered from inlet (c) to outlet (d). Scale bars are 200 μm.PDF Image | Water Purification by Shock Electrodialysis
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