Environ. Sci. Technol. 2008, 42, 6160–6166
New Method of Water Purification Based on the Particle-Exclusion Phenomenon IVAN KLYUZHIN, ANDREW SYMONDS,† JEFFREY MAGULA, AND GERALD H. POLLACK* Department of Bioengineering, Box 355061, University of Washington, Seattle, Washington 98195
Received December 17, 2007. Revised manuscript received April 29, 2008. Accepted May 7, 2008.
Colloidal particles in suspension are excluded from the vicinity of various hydrophilic surfaces. On the basis of this phenomenon, a novel method of water purification is proposed and tested. Proof-of-concept is demonstrated using a custommade extractor that collected clean water from the annular “exclusion zone” within a Nafion tube. Up to 99.6% of particles could be removed from the suspension. The experimental results suggest that particle exclusion may provide a new framework for water purification from both organic and inorganic matter, as well as from harmful pathogens.
Introduction Recent studies have shown unexpected particle behavior next to various hydrophilic surfaces (1–3). Particles, such as microspheres, migrate away from these surfaces, leaving unexpectedly large regions of particle-free water (Figure 1). At equilibrium, these regions may be as large as several hundred micrometers, and their physical properties differ from those of bulk water. These interfacial areas have been termed “exclusion zones” because of their distinctive ability to exclude particles and solutes. Exclusion zones had in fact been noted as early as 1970, when microspheres were shown to be excluded from the vicinity of certain biological tissues (4). More recently, extensive studies have shown that the exclusion phenomenon is quite general, both in biological and non-biological systems (1, 2). The phenomenon of exclusion provides a new basis for water purification. With a large enough exclusion zone next to the exclusion-generating surface, clean water can be easily collected, leaving behind particle-containing and solutecontaining water. We chose Nafion as the excluding surface. Nafion is a perfluorosulfonic ionomer that consists of a polytetrafluoroethylene backbone and regularly spaced side chains terminated by sulfonate ionic groups (5). Previous studies have demonstrated that Nafion has negative charge in deionized water, with a surface electrical potential of -160 mV (2). The advantages of using Nafion as the excluding surface are its relatively large exclusion zone and its commercially availability in convenient tubular shape. We developed a system in which water could be drawn continuously along the tube’s annular exclusion zone and extracted * Corresponding author phone: (206) 685-1880; fax: (206) 6853300; e-mail:
[email protected]. † Current address: MRC Centre for Developmental Neurobiology, King’s College London, Strand, London WC2R 2LS, England, U.K. 6160
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 16, 2008
at the end of the tube, with promising levels of separation of both artificial and natural water-borne particles.
Materials and Methods The purification process employed a Nafion tube. With a pressure gradient imposed along the tube, suspensions could be forced to flow inside. An annular exclusion zone developed with distance along the tube (Figure 2). By drawing water from the annular zone using an extractor at the end, virtually particle-free water could be obtained. Flow Chamber. To observe whether various particle types were excluded from the zones next to the Nafion surface, both in static conditions and during flow, the experimental chamber illustrated schematically in Figure 2 was used. A 5-mm-thick transparent plastic sheet with window cut out was set on a glass plate. A 3-mm hole was drilled on either end of the sheet, allowing the Nafion tube (O.D. 2.5 mm, TT-110, Perma Pure LLC, NJ) to traverse the chamber. The chamber was filled with distilled, deionized water and sealed with a second glass plate. The tube was connected via Tygon tubing to a container on the input end and to a syringe pump (YA-12, Yale Apparatus) on the output end. The pump worked in the withdraw mode, diminishing the pressure and forcing the fluid suspension to flow through the tube. To observe particle behavior near the Nafion surface in static situations, the syringe pump was engaged until the tube was primed; then, after 5 to 10 min, the equilibration time required to ensure that the exclusion zone was fully formed, the near-surface regions were examined. To observe particle behavior during flow, the syringe pump was set to operate at a constant rate. Once the flow was established, the interfacial region within the Nafion tube was examined in an inverted microscope (Diaphot, Nikon) equipped with a 5× objective lens. Images were acquired and analyzed using a digital CMOS camera (DFC 290, Leica). Images were also taken occasionally using a digital camera (EasyShare P880, Kodak) through an eyepiece of the microscope. Observations of bacterial and viral suspensions were made in a biosafe laboratory, where high magnification objectives (20× or higher) were the only ones available for use. The short focal distances prompted the use of thinner Nafion tubes (TT-030, ID 0.64 mm, Perma Pure) that were fixed between a microscope slide and a coverslip. The Nafion tube was superfused with the bacterial suspension and observed under an upright microscope (Axioskop, Zeiss). Bacteria were imaged close to the Nafion tube under both bright field and fluorescent illumination. Adenoviruses were also studied and imaged via a laser scanning confocal microscope (Leica SP1, Keck Imaging Center at University of Washington). Because they were small (cylindrical, 80 nm long) and could not therefore be resolved with light microscopy, the viruses were fluorescently labeled so that the exclusion zone would appear as a fluorescence “void” next to the surface of the Nafion tube. Differential Extractor and Separation. The experimental setup used for the separation experiments is shown in Figure 3. A custom-made “differential extractor” was employed to separate exclusion zone water and bulk solution. The extractor was made of two concentric stainless steel tubes of different diameter held together by low temperature silver solder. The Nafion tube clasped the extractor’s outer tube. The extractor’s inner tube extended out 0.5 mm into the Nafion tube, which made it possible to visualize the purification process under the microscope. The inner and outer channels of the extractor led to separate outputs, each connected to a syringe pumps via Tygon tubing. 10.1021/es703159q CCC: $40.75
2008 American Chemical Society
Published on Web 07/11/2008
FIGURE 1. Example of development of exclusion zone (EZ) near the edge of Nafion film. An aqueous microsphere suspension was injected between two microscope slides with a flat Nafion film squeezed between. Following injection, the microspheres migrated away from the edges of the Nafion film, leaving a region of particle-free water. Carboxylate-coated 3-µm microspheres were used. Images were acquired with an inverted microscope.
FIGURE 2. Experimental chamber used for exclusion-zone observations, not to scale. A motor pulled on the syringe plunger at a constant, controllable rate. Particles condensed in the tube’s core, as shown schematically. For exclusion-zone observations in static situations, the syringe motor could be stopped. Observation point marked with dashed circle.
FIGURE 3. Experimental setup used for water purification experiments, not to scale. Syringe pumps (not shown) generated flow through Nafion tube and channels of differential extractor. Arrows denote direction of flow. The pumps generated reduced pressures inside the extractor’s inner and outer channels, forcing the suspension to flow. When the suspension reached the extractor’s interface, the contents of the tube’s core was sucked into the inner channel, while the annular water was sucked into the outer channel (Figure 3). Thus, the solution near the Nafion surface was separated from the core solution. Suspension Preparation. To test the method’s effectiveness, particle suspensions of varied composition were prepared. They included topsoil, clay soil, silt, bacteria, viruses, and microspheres. Turbidity, expressed in NTUs (nephelometric turbidity units) was used as a measure of particle concentration. This approach is commonly used for
that purpose (6–9). Turbidity was measured using a turbidimeter (2020i, LaMotte, MD). The instrument was calibrated using 0.5, 10, and 100 NTU standards (Amco Clear, GFS Chemicals, OH). Although turbidity of river and lake waters can reach hundreds of NTUs during rains and floods, typical values lie within 1-20 NTU (7, 8), well within the range of calibration. The suspensions of topsoil, clay soil, and silt were prepared by mixing the soil samples (Ward’s Natural Science, Rochester NY) with deionized water in a commercial blender (51BL32, Waring, New Hartford, CT). After they were mixed, the suspensions were filtered through a paper filter with 8- to 12-µm pore size (Sharkskin General-Purpose filter paper, Whatman). This procedure removed the larger particles that would ordinarily settle out inside the Nafion tube. The turbidity of the filtered solutions lay within a 100-300 NTU range, and this required further dilution for experiments. Dilution yielded input suspensions with turbidities 5, 20, and 100 NTU. To supplement the studies of natural particles, carboxylate-coated 3-µm latex microspheres (Polybead, Polysciences Inc.; coefficient of variance, 5%) were used as synthetic test particles. Concentrated stock suspensions (1.68 × 1012 particles/L) were diluted with distilled, deionized water. For the experiments that employed flow, the stock solution was diluted to yield 5, 20, and 100 NTU suspensions. Water for the experiments (Type 1, HPLC grade, 18.2 ΜΩ) was obtained from a standard water purification system (Diamond TII, Barnstead). Because of their hazardous nature, bacterial and viral suspensions were observed under static conditions only, near the surface of the Nafion tube. No flow experiments were carried out. Two gram-negative bacterial species were studied, Escherichia coli and Nitrousomonus europea. E. coli were cultured overnight at 37 °C in liquid broth media. N. europea were grown in special media at room temperature until fully grown. Cells were spun down and washed three times in 10% glycerol solution to remove salt, which has been shown to reduce the size of exclusion zones (3). The washed bacteria were then diluted 1:1 with sterile deionized water, giving a final concentration of approximately 5 × 108 particles/mL. Some bacterial preparations were incubated VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6161
FIGURE 4. Exclusion zones within static colloidal suspensions. Photographs were taken with a 5× objective 5 min after priming. with a fluorescent cell-viability marker SYTO-9 (Molecular Probes) for 15 min. Viral particles were obtained from the University of Washington Molecular Genetics Laboratory. The preparation used was an aerosol adenovirus of rare serotype, usually seen in patients with compromised immune systems. Viral particles were labeled with Cy3 dye by dialysis and then frozen in 0.5 mL aliquots at -80 °C for storage. Once the virus preparation was thawed on ice, it was dialyzed against 500 mL sterile deionized water for 3.5 h to remove the salt from the virus particle buffer (initially 150 mM NaCl). For the preparation used in these experiments, the stock concentration was 1.4 × 1012 viral particles/mL. This was diluted 3:2 with deionized water, giving a working concentration of 8.4 × 1011 viral particles/mL.
Results Solute Exclusion. To determine if topsoil particles, clay soil, and silt are excluded, the Nafion tube was primed with the respective suspension at 100 NTU and observed as described in Figure 2. Exclusion zones were seen with all suspensions (Figure 4). Their size at equilibrium depended on suspension type. For clay particles, the size (∼170 µm) was generally larger than that of topsoil and silt particles (∼120 µm). By comparison, the exclusion zone of 3-µm carboxylate microspheres under the same conditions was greater than 300 µm. Soil-exclusion zones were also more ambiguously defined than microsphere-exclusion zones, that is, their termination boundaries were less sharp. Exclusion zones were also seen with bacterial particles. Both bacterial species (N. europea and E. coli) were excluded. An example is shown in Figure 5. The exclusion zone was revealed by a fluorescence gap. Viral suspensions revealed a different kind of behavior. After the viral suspension was superfused around the Nafion tube, “clusters” were observed which were much larger than the individual particles (Figure 6). These fluorescent clusters, 2-5 µm in diameter, were absent from a region approximately 150-µm wide. They were also adsorbed onto the Nafion tube’s outer wall, possibly as a result of immediate binding to the Nafion upon infusion. Whether or not individual viral particles might be present within the cluster-exclusion zone remains uncertain. Flow Observations. To study particle exclusion during flow, the experimental chamber illustrated in Figure 2 was 6162
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 16, 2008
FIGURE 5. Exclusion of E. coli inside and outside the Nafion tube.
FIGURE 6. Medium of viral particles near Nafion surface. Observed fluorescent clusters were absent in the zone next to Nafion (EZ). Some clusters attached to the Nafion surface. modified so that Nafion tubing of different i.d.’s (2.2, 1.6 and 1.2 mm) could be employed. The exclusion zone inside the tube was measured at midlength along the tube and studied at different flow rates. Starting from 5 mL/h, the flow rate was increased in 5 mL/h increments to 50 mL/h. At each
FIGURE 7. Development of exclusion zone during flow at a fixed observation point. Flow rate of 15 mL/h was used. A steady state was reached after approximately 60 s. Right: microsphere exclusion in static situation.
FIGURE 8. Effect of flow rate on exclusion-zone size. flow rate, approximately five minutes were allowed for the exclusion zone to come to steady state, for measurement. An example showing the development of the steady state is presented in Figure 7. Along the length of the Nafion tube, the exclusion zone was not uniform. At the tube’s entry, particles were evenly distributed over the full cross-section. As particles moved through the tube during steady flow, they were progressively excluded from the vicinity of the wall, both with distance and time. At a flow rate of 15 mL/hr, for example, the exclusion zone reached its steady-state size at a distance of 1-2 mm. At that position and flow rate, the exclusion-zone size was smaller than in the static condition (see Figure 7). When the flow rate was increased, the exclusion-zone size diminished. This dependence, for each of several tube diameters, is shown in Figure 8. Note that the tube with the largest i.d. showed the largest exclusion zones. When solutions of topsoil, clay soil, or silt were pumped through the Nafion tube, sediment formed. The sedimented particles accumulated progressively on the bottom of the tube in a thin layer. In general, sediment was heavier toward the tube’s distal end, although the amounts were not quantified. Sediment formation did not obscure the flow through the tube. It is apparent from Figure 8 that lower flow rates yielded larger exclusion zones. However, lower flow rates also brought lower overall water-collection rates. As a compromise, an intermediate flow rate of 15 mL/h was selected for the separation experiments, below. At this flow rate, the linear velocity at the centerline was 1.91 ( 0.06 mm/s. Pure-Water Extraction. Extraction experiments were carried out according to the setup shown in Figure 3. When particles reached the differential extractor, the exclusion zone was fully formed (Figure 9). The differential extractor separated the suspension into core and annular fractions (c.f., Figure 3). The latter fraction contained relatively purified water.
We define R as the ratio of turbidity in the collected purified suspension to that in the input suspension. Lower R values mean better purification. It was more convenient to express results in terms of 1 - R, which we define as purification efficacy. Higher values imply higher efficacy. Purified-water yield, on the other hand, depends on how fast water within the exclusion zone is delivered to the differential extractor. Both yield and efficacy are increased with larger exclusion zones. To determine efficacy and yield at various levels of contamination, the turbidities of the input suspensions were varied. Suspensions of topsoil, clay soil, silt, and microspheres were used. The inner channel intake rate was kept at 13 mL/ h, and the outer channel intake rate was 2 mL/h. Together, this created a 15 mL/h flow through the Nafion tube. Thus, for every 15 mL of input suspension, 2 mL of purified annular fraction were extracted. Sample collection continued for two hours, yielding 4 mL of exclusion-zone water. Because the minimum amount of solution required for turbidity measurement was 10 mL, the collected annular fractions were diluted with 6 mL of deionized water. After the turbidity was measured, the result was multiplied by 2.5 to take into account the dilution. The results are presented in Table 1. The purification efficacy depended on suspension type. The highest was achieved with the microspheres. With 100 NTU input suspension, 99.6% efficacy was achieved. For the input suspensions of lower turbidity (20 and 5 NTU), efficacy was 98%. Results obtained with the soil suspensions showed the opposite tendency: higher initial particle concentrations resulted in lower efficacy. Approximately 70-90% of particles were removed. Overall, the microsphere suspensions were purified with greater efficacy than the soil suspensions. To determine how efficacy varied with annular flow rate, the ratio of outer/inner intake rate ratio was varied, without changing either input turbidity (20 NTU) or overall flow rate (15 mL/h). The differential extractor separated an annular layer from the solution in the core. The width of the annular layer collected depended on the outer-channel intake rate: higher intake rates tended to suck water closer to the core, which was “dirtier”. Thus, with lower outer-channel intake rate, cleaner water should be collectible. To test this premise, the outer/inner channel flow ratio was changed in five steps, from 14/1 to 7/8 mL/hr, and the turbidities were measured. Changing the ratio did not affect the size of exclusion zone because the flow rate was maintained constant. The results are presented in Table 2. With all suspensions, purification efficacy decreased as the intake rate through the outer channel was increased. Values of purification efficacy were always highest for the microsphere suspensions. However, with the 1 mL/h intake rate, topsoil and clay results were similar to that of microspheres. In general, reduction VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6163
FIGURE 9. Photographs of the zones near the extractor. Left: Microsphere suspension. Right: Clay suspension. Intake rate of the outer channel: 2 mL/h. Inner channel: 13 mL/h.
TABLE 1. Purification Efficacy for Various Suspensions As a Function of Particle Concentrationa purification efficacy (%) input turbidity (NTU)
microspheres
topsoil
silt
clay soil
98 98 99.6
93 73 76
84 93 86
90 84 79
5 20 100
a Solution intake rates: inner channel, 13 mL/h; outer channel, 2 mL/h. Samples collected for 2 h.
TABLE 2. Purification Efficacy Achieved with Various Outer Channel Intake Ratesa purification efficacy (%) intake rate through outer channel (mL/h) suspension type microsphere clay soil topsoil
1
2
4
6
7
98 98 96
91 71 80
80 47 59
70 48 50
56 46 39
a Inner channel intake rate was changed to maintain a 15 mL/h flow rate through the tube. Samples were collected for two hours and then diluted to yield 10 mL.
of the intake rate through the outer channel resulted in cleaner water. This was observed for all particle types. Results varied somewhat among experiments. For example, the conditions in the second data column of Table 2 (2 mL/h) and the second data row of Table 1 (20 NTU) are similar: both represent collection rates of 2 mL/h at 20 NTU. Yet the results varied somewhat. Variation was typically on the order of 10-20%. Tables 1 and 2 show the best results among those achieved under nearly similar conditions, typically among five trials. The deviations were mainly the result of technical deficits, such as slightly noncircular tubular cross-section or off-center collector position, and because the goal was to test proof-of-principle, the data presented in the tables report the best results achieved, rather than the means. In the absence of the noted technical deficits, it is estimated that the best results would have been achieved with more consistency.
Discussion Solutes are excluded from an unexpectedly large zone next to many hydrophilic surfaces (1–4). On the basis of this phenomenon, we constructed a device that separates various 6164
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 16, 2008
particulates from water and collects the purified fraction. Representative particulates commonly found in water were effectively removed, without the need for a fine physical filter. The scientific basis of this phenomenon is under intense study. Exclusion of particles and solutes from zones spanning hundreds of micrometers from surfaces is not expected from conventional theory, although some aspects such as doublelayer and osmotic phenomena may play a role. The bulk of evidence suggests that a major player is a long-range reorganization of water molecules in the vicinity of hydrophilic surfaces, including Nafion, the reorganized, or structured “phase” of water excluding particles and solutes (2). A presentation of the evidence and implications is available at http://uwtv.org/programs/displayevent.aspx?rID)22222, the link to the 2007/2008 University of Washington Annual Lectureship Award, where this phenomenon is featured. Exclusion of Water-Borne Particles. The experiments incorporated various soil suspensions and biological particles. The fact that they were excluded from the region next to Nafion adds to the growing list of particles and solutes confirmed to be excluded from the zone next to hydrophilic surfaces (1–4). Soil suspensions had smaller exclusion zones than microspheres (Figure 4). This difference may result from two factors. One is particle-size distribution. The exclusion-zone size is larger for larger particles (1). Soil suspensions, after having passed through a 12-µm filter, contained an array of smaller particle sizes, including those smaller than the 3-µm microsphere size used here. The presence of these smaller particles may explain the relatively smaller exclusion zones. Another reason for the relatively smaller zones may be the presence of dissolved substances. Soils usually contain salts and other molecular compounds such as proteins, peptides, and polysaccharides (10), which are known to cause diminution of exclusion-zone size (3). We found also that the boundaries of the exclusion zone were less sharp for soil than for microspheres (Figure 4). This fuzziness also may arise from the soil’s particle-size diversity. For the 3-µm carboxylate microspheres, by contrast, the standard deviation of size was only 0.112 µm, generating a very distinct border (Figure 4). Exclusion zones observed for bacterial particles and viral clusters imply that a disinfection device may be possible. Currently, bacterial particles are filtered out by membranes or filters, and the residual organisms are deactivated by various disinfectants such as chlorine, ozone, or UV (11, 12). If bacterial particles are separable in a same manner as soil particles, then a new disinfection method could be on the horizon. Exclusion-Based Purification. Built on the observation that water-borne particles are excluded from near-surface
regions of hydrophilic substances, the prototype filtration system performed well. Various soils could be separated, resulting in relatively pure water. Purification of soil suspensions depended on concentration (Table 1). Lower-concentration suspensions generally showed higher purification efficacy than those with higher concentrations. This may be a result of the lower salt concentration associated with lower soil concentration. On the other hand, the efficacy of microsphere-suspension purification did not depend significantly on concentration. This result agrees with the previous observation that microsphere concentration does not affect the exclusionzone size (1). The observed relation between exclusion-zone size and flow rate (Figure 8) sheds light on the conditions under which pure-water yield may be maximized. Pure-water yield depends mainly on two features: exclusion zone size and flow rate. Larger exclusion zones allow extraction of more clean water per unit volume of original suspension. Higher flow rate should allow faster collection. On the other hand, higher flow rate also reduces the size of exclusion zone (Figure 8). Hence, to maintain high purification efficacy a lower outer-channel intake rate must be used, which in turn decreases the pure water collection rate. However, the curves on the graph in Figure 8 suggest a plateau at flow rate of approximately 50 mL/h; thus, at flow rates higher than 50 mL/h, the exclusion-zone size should not change significantly. Potentially, this may allow pure water collection at much faster rates. At an expedient outer-channel flow rate of 2 mL/h, soil turbidity was reduced by 90%, whereas the reduction for microspheres was up to 99.6% (Table 1). This difference stems from the observation that microsphere-exclusion zones were larger than soil-particle exclusion zones; the flow collected by the differential extractor’s outer channel was therefore essentially all exclusion-zone water. If the exclusion zone is smaller than 200 µm, as in the case of soil suspensions, then the particles from the core are sucked into the outer channel, contaminating the annular fraction. This problem was solved by lowering the outer channel’s intake rate. Although this procedure increased the purification efficacy (Table 2), it also reduced pure water yield, the tradeoff between pure water yield and purification efficacy. Methodological Limitations. Execution of the experiments revealed several shortcomings of the approach. They arise both from the exclusion phenomenon itself and the specific implementation of the principle. Geometrical Limitations. Geometric uniformity was an important consideration. To prevent particles from flowing into the extractor’s outer channel, the gap between the extractor’s two concentric tubes had to be uniform over the circumference; otherwise, some core solution could flow into the outer channel. This contaminates the annular fraction and reduces purification efficacy. To minimize this problem, the setup had to be carefully aligned each time prior to the experiment and not always perfectly. If this concentric approach were adapted for larger scale purification, careful engineering would need to address this limitation. Similarly, the Nafion-tube shape is also critical. If the cross-section is noncircular, then the flow profile will be nonuniform, and particles in some core regions may enter the annulus. Indeed, some samples coming from the manufacturer had a distinctly elliptical cross-section. Such tubes could be forced into an almost circular cross-section by feeding them through a hole made in the chamber wall. When the tube was dry, it fit through the hole easily. Upon swelling in water, the tube sealed the hole and acquired a more circular cross-section. Noncircularity is a lingering
problem that could be circumvented by using a system design based on flat surfaces. Sediment Formation. The observed sediment formation is probably caused by the rapid settling of the heavier particles or by some chemical change inside the Nafion tube that created heavier particles. If sedimentation is caused by the weight of the larger particles, then the problem could probably be solved through finer prefiltering, or the issue could be circumvented altogether through use of a vertically oriented Nafion tube. Although the amount of sediment was minute relative to the total number of particles within the suspension, its existence must have affected the results presented in Table 1, presumably elevating the apparent purification efficacy. Indeed, one could argue that the turbidity reduction in the annular fraction originated entirely from sediment formation, the clear water arising entirely because of settling. However, sediment fallout was not observed at all with microspheres, which showed the greatest turbidity reduction (Table 1). Hence, the effect seems to be relatively minor, although further consideration is warranted. Low Purification Rate. The relatively small size of the exclusion zone places a limit on the water-purification rate. Flow rates were considerably smaller than those involved in standard purification techniques. To increase throughput, multiple units could be arranged in parallel, or relatively larger exclusion zones could be generated by using surfaces that might be better exclusion-zone nucleators than Nafion. On the other hand, the tubular design employed here was adopted solely for obtaining proof of principle, with no consideration given to generating high throughput. For commercial applications, other designs may be more effective. If these improved designs cannot yield purification efficacies beyond the 80-90% obtainable here with common particles, then cascading of units in series might eventually be necessary. The current design is preliminary; it reflects only the modest level of engineering required for obtaining proof-of-principle. Advantages. The principal advantage of this approach is its inherent simplicity. Removal of particles occurs naturally; in principle, it is merely a matter of collecting the purified water. Modern purification techniques, by contrast require the use of physical filters or membranes that eventually become clogged and must be cleaned (backwashed) or replaced (11, 13–15). Here, there are no physical filters. On the other hand, scaling up this new approach for implementation in realistic settings will evidently require appreciable engineering. This is for the future. Another advantage of this approach is its inherent capacity to remove pathogens. Bacterial particles were excluded, and preliminary data indicate that viruses might be excluded as well (Figures 7 and 8). This feature would make the method especially useful in locales where drinking water may contain biological contaminants. In summary, the exclusion-zone phenomenon offers a new, previously unforeseen route toward water purification. Proof-of-principle has been demonstrated, with good separation ratios in a simple design. The prototype could easily be improved for both faster and cleaner collection, making it a practical approach with considerable promise because of its inherent simplicity.
Acknowledgments This research was supported by ONR Grant N00014-05-10773 and NIH Grant 1 R21 AT-002362. We thank Chenyang Wang, Vincent Thuc Wu, and Iris Pang for help with preliminary measurements that aided in the initial design.
Literature Cited (1) Zheng, J.-M.; Pollack, G. H. Long-range forces extending from polymer-gel surfaces. Phys. Rev. E 2003, 68, 031408. VOL. 42, NO. 16, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6165
(2) Zheng, J.-M.; Chin, W.-C.; Khijniak, E.; Khijniak, E., jr.; Pollack, G. H. Surfaces and interfacial water: Evidence that hydrophilic surfaces have long-range impact. Adv. Colloid Interface Sci. 2006, 127, 19–27. (3) Zheng, J.-M.; Pollack, G. H. Solute exclusion and potential distribution near hydrophilic surfaces. In Water and the Cell; Pollack, G. H., Cameron, I. L., Wheatley, D. N., Ed.; Springer: Dordrecht, The Netherlands, 2006; pp165-174. (4) Green, K.; Otori, T. Direct measurements of membrane unstirred layers. J. Physiol. 1970, 207, 93–102. (5) Heitner-Wirguin, C. Recent advances in perfluorinated ionomer membranes: structure, properties and applications. J. Membr. Sci. 1996, 120, 1–33. (6) Ebie, K. New measurement principle and basic performance of high-sensitivity turbidimeter with two optical systems in series. Water Res. 2006, 40, 683–691. (7) U.S. Environmental Protection Agency. Guidance Manual for Compliance with the Interim Enhanced Surface Water Treatment Rule, Turbidity Provisions; EPA MDBP Technical Guidances; U.S. EPA: Washington, DC, 1999. (8) LeChevallier, M. W. Effect of turbidity on chlorination efficiency and bacterial persistence in drinking water. Appl. Environ.
6166
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 16, 2008
Microbiol. 1981, 42, 159–167. (9) McGuire, M. J. Eight revolutions in the history of U.S. drinking water disinfection. J. AWWA 2006, 98 (3), 123–149. (10) Oliveira, J. L.; Boroski, M.; Azevedo, J. C.; Nozaki, J. Spectroscopic investigation of humic substances in a tropical lake during a complete hydrological cycle. Acta Hydrochim. Hydrobiol. 2006, 34, 608–617. (11) Droste, R. L. Theory and Practice of Water and Wastewater Treatment; John Wiley: New York, 1997. (12) Gehr, R.; Wagner, M.; Veerasubramanian, P.; Payment, P. Disinfction efficiency of peracetic acid, UV and ozone after enhanced primary treatment of municipal wastewater. Water Res. 2003, 37, 4573–4586. (13) Logsdon, G. S. Filtration processessA distinguished history and a promising future. J. AWWA 2006, 98 (3), 150–162. (14) Ahmad, R.; Amirtharajah, A. Detachment of particles during biofilter backwashing. J. AWWA 1998, 90 (12), 74–85. (15) Chellam, S.; Wiesner, M. R. Particle transport in clean membrane filters in laminar flows. Environ. Sci. Technol. 1992, 26 (8), 1611–1621.
ES703159Q
