Method of building massively-parallel ion concentration polarization separation device

Abstract

Ion Concentration Polarization (ICP) purification devices and methods for building massively-parallel implementations of the same, said devices being suitable for separation of salts, heavy metals and biological contaminants from source water.

Claims

1. A water separation device comprising: a chamber comprising an inlet port, a bottom support and a volume to receive water comprising ions to be separated; wherein said bottom support comprises a porous ion concentration polarization matrix comprising at least one layer of a mesh with holes partially or entirely coated with an ion selective material, said matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for a concentrated water stream comprising said ions; at least one anode; at least one cathode in electrical contact with said ion concentration polarization matrix; wherein the volume to receive the water comprising ions is located above the bottom support such that gravity directs the water comprising ions through the chamber and water depleted in said ions through the ion concentration polarization matrix.

2. The device of claim 1 wherein the anode is comprised of a conductive metal mesh and is positioned proximate to the inlet port.

3. The device of claim 1 wherein the porous ion concentration polarization matrix comprises a two dimensional matrix of holes, wherein each hole is coated with an ion selective material.

4. The device of claim 3 wherein each hole has a diameter of less than about 500 microns.

5. The device of claim 1 wherein the mesh is electrically conductive.

6. The device of claim 1 wherein the mesh is electrically non-conductive.

7. A water separation device comprising: a chamber comprising an inlet port, a bottom support and a volume to receive water comprising ions to be separated; wherein said bottom support comprises a porous ion concentration polarization matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for a concentrated water stream comprising said ions; at least one anode; at least one cathode in electrical contact with said ion concentration polarization matrix; wherein the matrix comprises a plurality of layers of mesh with holes partially or entirely coated with an ion selective material; wherein the plurality of layers of mesh are characterized by at least two different hole sizes.

8. The device of claim 1 wherein the porous ion concentration polarization matrix comprises a plurality of microchannels of less than 100 nm in diameter.

9. The device of claim 1 wherein the ion concentration polarization matrix has a surface area of at least about 3 cm.sup.2.

10. A water separation device comprising: a chamber comprising an inlet port, a bottom support and a volume to receive water comprising ions to be separated; wherein said bottom support comprises a porous ion concentration polarization matrix comprising at least one layer of a mesh with holes partially or entirely coated with an ion selective material, said matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for a concentrated water stream comprising said ions; at least one anode; at least one cathode in electrical contact with said ion concentration polarization matrix; wherein the porous ion concentration polarization matrix comprises a two dimensional matrix of holes wherein each hole is coated with an ion selective material; wherein the at least one outlet for the concentrated water stream comprises one or more holes in the bottom support none of which are coated with an ionic polymer in electrical contact with a power supply.

11. A water separation device comprising: a chamber comprising an inlet port, a bottom support and a volume to receive water comprising ions to be separated; wherein said bottom support comprises a porous ion concentration polarization matrix comprising at least one layer of a mesh with holes partially or entirely coated with an ion selective material, said matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for a concentrated water stream comprising said ions; at least one anode; at least one cathode in electrical contact with said ion concentration polarization matrix; wherein the at least one outlet for the concentrated water stream is located at the periphery of the bottom support.

12. A water separation device comprising: a chamber comprising an inlet port, a bottom support and a volume to receive water comprising ions to be separated; wherein said bottom support comprises a porous ion concentration polarization matrix comprising at least one layer of a mesh with holes partially or entirely coated with an ion selective material, said matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for a concentrated water stream comprising said ions; at least one anode; at least one cathode in electrical contact with said ion concentration polarization matrix; wherein the at least one outlet for the concentrated water stream is located in the wall of the chamber, proximate to the bottom support.

13. The device of claim 1 wherein the at least one cathode is in electrical contact with the ion concentration polarization matrix via an electrolyte solution disposed within the chamber.

14. The device of claim 1 wherein the water comprises cells, particulates, emulsions, biomolecules or combinations thereof.

15. The device of claim 4 wherein each hole has a diameter of less than about 300 microns.

16. The device of claim 1 wherein the matrix comprises a mesh characterized by holes having a diameter of about 100 microns.

17. The device of claim 1 wherein the ion concentration polarization matrix has a surface area of at least about 5 cm.sup.2.

18. The device of claim 1 wherein the ion concentration polarization matrix has a surface area of at least about 200 cm.sup.2.

19. The device of claim 1 wherein the volume and matrix are configured to be capable of accommodating a water flow rate of about 1 mL per minute or more.

20. The device of claim 1 wherein the volume and matrix are configured to be capable of accommodating a water flow rate of about 5 mL per minute or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

(2) FIG. 1 depicts schematic diagrams of (a) metal buffered and (b) self-buffered multiplexed desalination system.

(3) FIG. 2 depicts Nafion coated meshes: (a) Nylon woven mesh of 125 μm hole, (b) stainless steel mesh of 75 μm opening and (c) PCB of 500 μm drilling hole.

(4) FIG. 3 depicts (a) schematic diagram of PCB based metal buffer system and (b) layout of bottom PCB plate.

(5) FIG. 4 depicts fabrication of metal buffer system employing commercial metal mesh: (a) top cap part, (b) bottom filter part, and (c) conductivities vs. electrical voltage plot to show the desalting efficiency.

(6) FIG. 5 depicts (a) schematic diagram of ¼ inch outlet prototype (not to scale) and its building block, (i)˜(iv) the fabrication steps for each building block.

(7) FIG. 6 depicts (a) ICP desalination demonstration and the analysis of desalted water by (b) optical comparison, and (c) electrical conductivity measurement.

(8) FIG. 7 depicts schematic diagram of Nafion sheet sandwiched self-buffered system and its fabrication steps.

(9) FIG. 8 depicts (a) ICP desalination demonstration and the analysis of desalted water by (b) optical comparison, and (c) electrical conductivity measurement.

(10) FIG. 9 depicts (a) conductivity and concentration of each heavy metal species from filtered samples as a function of applied electrical voltage.

DETAILED DESCRIPTION OF THE INVENTION

(11) Thus, the invention provides for a water separation device, such as a microporous device, comprising:

(12) a chamber characterized by an inlet port, a bottom support and a volume to receive water comprising ions to be separated, such as a brine;

(13) wherein said bottom support comprises a porous ion concentration polarization matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for concentrated water stream comprising said ions;

(14) at least one anode and at least one cathode functionally connected to said ion concentration polarization matrix.

(15) Referring to FIG. 3, the chamber 4 is preferably located above the bottom support 5 to permit gravity to cause the water to flow through the device. The chamber further defines a volume 6 to receive and hold water to be purified. The volume of the chamber is not critical to its utility. In designing devices for personal use, the volume can be less than 1 liter, such as less than 0.5 liters. The material used to manufacture the chamber is also not critical. In general, the chamber is made of non-conductive material and/or lined with non-conductive material, such as polyethyleneterephthalate (PET).

(16) The chamber is characterized by an inlet port 7 to receive the water to be purified. The chamber can be designed to be a closed system (for example, where the inlet port receives a conduit optionally controlled by a valve) or an open system (for example, where the inlet port is the open end of a graduated cylinder where the water is poured into the volume).

(17) The chamber volume is further defined by a bottom support 5. The bottom support comprises a porous ion concentration polarization matrix configured to permit water depleted in said ions to exit the chamber and at least one outlet for concentrated water stream comprising said ions. The matrix is preferably a mesh, such as a conductive metal mesh or a mesh coated with a conductive material or polymer. Alternatively, the matrix can be a plate configured with a matrix of holes where the holes or plate are coated with an ion selective material. Alternatively, the matrix can be a microporous material with randomly distributed holes and inherent conductivity through the material. The matrix can be a separate material and fitted into or on the support. Alternatively, the matrix can be an integral part of the support and molded as a single piece.

(18) Preferably, the diameters of the holes in the porous ion concentration polarization matrix 2 comprises are less than about 500 microns, preferably less than 300 microns. Mesh is preferred as the open surface area permits a greater flow rate. Preferably, the matrix comprises at least one layer of a mesh with microscopic holes partially or entirely coated with an ion selective material. The mesh material itself can be conductive or non-conductive. In one embodiment, the matrix can comprise a plurality of layers of mesh characterized by the same or at least two different hole sizes. Alternatively, the porous ion concentration polarization matrix comprises a plurality of microchannels (less than 500 microns), or nanofilters, e.g., of less than 100 nm in diameter.

(19) The size of the matrix will generally depend on the volume of the chamber and the desired rate of flow. For an individual-sized device, the ion concentration polarization matrix will preferably have a surface area of at least about 3 cm.sup.2, preferably at least about 5 cm.sup.2, such as at least about 200 cm.sup.2.

(20) In one embodiment, the anode is comprised of a conductive metal mesh and is positioned proximate to the inlet port. Referring to FIG. 3, the anode 1 is located on the top face of the bottom support 5 and assists in defining the ICP, or ion depletion, zone. See also FIG. 1.

(21) The device also contains an outlet for collecting a purified water stream and at least one outlet for collecting ions and impurities in a concentrated water stream. The outlet for the concentrated water stream can comprise one or more holes in the bottom support none of which are coated with an ionic polymer functionally connected to a power supply.

(22) It can be convenient to locate the outlet(s) for the concentrated water stream at the periphery of the bottom support. In this embodiment, the centrally located matrix creates an ion depletion zone and forces the ions and impurities to the periphery of the device. Alternatively, the outlet for the concentrated water stream can be located in the wall of the chamber, proximate to the bottom support.

(23) The cathode of the device can be functionally connected to the ion concentration polarization matrix via an electrolyte solution disposed within the chamber.

(24) The invention provides for a parallelized microporous separation device comprising: at least one separation chamber 8 in fluid communication with a brine source 9; and at least one separation core 5 comprising an anode 1, an insulation layer 10, and a cathode 12, and an ion concentration polarization matrix with ion selective material 2 in between; and an electrical power supply functionally connected to anode and cathode; and at least one fluid outlet 13 that is blocked or affected by an ion concentration polarization zone created by the said ion concentration polarization matrix (e.g coated with an ion-selective material); and at least one fluid outlet 14 that is not blocked nor affected by ion concentration polarization zone created by the said ion concentration polarization matrix; wherein separation of ions, molecules, and/or particles are achieved by the differential interaction between the said ions, molecules, and/or particles with ion concentration polarization zone, with different ions, molecules and/or particles exiting different fluid outlets.

(25) The invention also provides for methods of purifying water, separating ions from water or concentrating ions in water. The methods comprise the steps: a. Adding water comprising ions to a device described herein through the inlet port; b. Supplying a voltage, such as at least 20 volts, thereby creating an ion depletion zone in the chamber above the porous ion concentration polarization matrix; whereby desalted water flows through the ion depletion zone and porous ion concentration polarization matrix and is collected from the device and concentrated salt water is collected from the at least one outlet therefor.

(26) The flow rate of water through the device can be about 1 mL per minute or more, preferably about 5 mL/min or more.

(27) The water comprising ions that can be subjected to the method is selected from seawater, brackish water, or a biological sample, such as blood, protein, virus, pathogen, DNA, and/or heavy metals such as arsenic, cadmium and lead.

(28) One embodiment of the purification/separation device utilizes direct vertical feeding of source water and lets only pure water flow down to a collection chamber by creating an ICP zone around bottom outlet holes as shown in FIG. 1. A metal buffer system such as that shown in FIG. 1(a) enables a simpler design than the first generation device since ICP can initiate directly from a nanojunction right on the deposited metal (or conducting mesh). A metal buffer system can work with low concentration samples (less than 100 mM) but becomes unsuitable at higher concentrations due to serious bubble formation on the cathode side. Compared to the metal buffer system, the self-buffering system as shown in FIG. 1(b) provides a bubble-free environment inside the separation chamber. Since conductivity of nanoporous junction (Nafion in this device) is much higher than sample electrolyte solution (20 times greater than seawater (500 mM) sample), most of the electrical current can pass through nanojunction, not directly through the solution so that ICP can still be initiated around the outlet holes. Bubbles due to electrolysis can be generated around cathodes without substantially impacting the ICP zone and separation since the electrodes are located outside the separation chamber. In this design, a fine meshwork (˜100 μm mesh holes) of Nafion-coated cathode interacts with a more coarse mesh (˜300 μm mesh holes) serving as the anode to create a dense array of ion-depletion zones within few inch circular structure such as wafer. Essentially, this improved design provides for dramatic parallelization of the ICP phenomena and is closer to the vertical feeding system such as those found in household water purifiers. For example, around 2×10.sup.6 holes can serve individual ICP separator so that the flow rate coming out from 8 inch wafer stack could be 1˜10 L/min.

(29) These vertical approaches have several advantages in terms of multiplexing and manufacturability. First of all, since it utilizes vertical inlet/outlet, multiplication of these holes can be implemented in mesh type structures. In terms of integration, mesh structures are already the most efficient due to minimization of loss of unused (or useless) surface area. The brine stream travels above the ICP zone and slips toward the edge of the device so that the fluid channeling for separation of brine/fresh water is relatively easy. For manufacturability, fabrication processes of fine structures involves only non-lithographical methods such as utilizing commercially available conducting/non-conducting mesh and printed circuit board (PCB), etc. One-inch diameter stacks are massively parallelized but can also be massively parallelized themselves into variably sized arrays, to provide water flows for any type of demand from facility scale (e.g. a laboratory) to municipal scale (e.g. aircraft carrier). One hundred stacks can operate from the power required of a standard incandescent light bulb (<3.5 Wh/L product water capacity), yet provide >1 gallon/min of product water from seawater. In addition, such an embodiment doesn't have nano-scale shrinkage and bonding issues due to its macro-scale and mechanical sealing.

(30) Commercially available metal/plastic meshes or PCB can be used for core substrates for the multiplexed desalination device. Meshes provide the best integration efficiency while one can design PCB for customizing flow properties. FIG. 2 shows various Nafion coated meshes: (a) Nylon woven mesh (Industrial Netting) with 125 um hole, (b) stainless steel mesh (Dual Manufacturing inc.) with 75 μm hole, and (c) PCB (Advanced Circuits) with 500 μm hole. The coating processes are as follows: Dipping into 20 wt % Nafion resin solution (Sigma Aldrich), followed by scrubbing and removing excess resin. Nitrogen gas is then blown so that individual holes do not remain clogged with resin. Drying at room temperature (1 hour) and baking at 75° C. on a hotplate were needed for complete curing of the resin.

(31) FIG. 3(a) shows the metal buffer system employing printed circuit boards, or PCB. It comprises two PCB plates (1) and (2), film spacer (3) and inlet chamber (4). The top and bottom PCB served as anode and cathode, respectively. The bottom PCB was coated with 20 wt % ionic polymer, such as Nafion or other sulfonated tetrafluoroethylene based fluoropolymer-copolymer, by aforementioned manner so that ICP zones were created at the individual desalted outlet holes with applying voltage. FIG. 3(b) shows a bottom PCB with dimensions of 2 inch (width)×2.5 inch (length)× 1/31 inch (thickness), although other dimensions can be used. This board had 456 desalted holes (diameter=300 μm) and 20 brine holes (diameter=500 μm) so that the two streams could equally split. One can arbitrarily design the number of each group of holes, e.g., for 1:1 division. Several stacks of commercial Mylar (3M Over Head Projector film) was used for the spacer (400 μm thick spacer was used in this device.) However, the non-transparency of the PCB didn't allow one to observe inside, and flow controlling and splitting were more difficult than if the materials had been more transparent. More importantly, there were large areas of waste due to PCB fabrication/cost limit (only 456 holes in 1 inch diameter) leading to a limited flow rate. In some sense, this could be an advantage of PCB if one completed the investigation of control parameter because one can design the hole spacing and shape, etc. to suit specific needs. Meshes (or nets), as shown in FIGS. 2(a) and 2(b), could be a better choice of filter to maximize the flow rate and economic cost.

(32) With the metal buffer system scheme, the metal mesh system was fabricated as shown in FIG. 4. FIG. 4(a) shows the anodic compartment which consisted of base metal mesh (1 inch diameter and 500 μm hole diameter), an acrylic plate ( 1/32 inch thick) and plastic chamber. The mesh was sandwiched between the chamber and the plate. For the cathodic compartment, a thin OHP film-Nafion coated metal mesh (anode)-collecting chamber was assembled as shown in FIG. 4(b). The diameter of anodic metal mesh was ½ inch (smaller than anodic mesh) so that brine outlets were placed along the ¾ inch perimeter. Since the film in this example was too flexible, another rigid plastic plate could be used to support the film for the assembly. FIG. 4(c) shows the demonstration of desalting 10 mM sodium phosphate buffer solution at high flow rate. Though the flow feeding of 5 mL/min and 2.5 mL/min were too high to be desalted, we can successfully (around 70%) desalt the solution at 1 mL/min. However, the metal buffer system was not suitable to desalt at highly concentrated source water since bubble generations were too severe due to electrolysis.

(33) To the contrary, the self-buffered system provides a bubble-free environment inside the ICP chamber since the bubbles should form at the cathodic side located outside the chamber. FIG. 5 depicts the fabrication of an embodiment employing a self-buffered system. It consists of four parts: (1) Nafion coated core meshes, (2) Cathode, (3) Nafion coated bottom plastic substrate, and (4) Cap part that had bare metal mesh (anode). Details were as follows: (i) Nafion solution was applied for coating the meshes by aforementioned method. Fine mesh (125 μm holes and diameter=½ inch) was sandwiched between two coarse meshes (381 μm holes and diameter=½ inch) in order to create microfluidic environment near the meshes. The stabilization of ICP phenomenon is enhanced by a bottleneck structure 10, thus a coarse-fine-coarse sandwich will serve for preventing an instability. Super glue (Home Depot) was applied for bonding each mesh. (ii) A piece of FlexiGlass (Home Depot) was round cut (diameter=2 inch) and outlet hole (¼ inch) was drilled at the center for fresh water drain. Aluminum (or copper) tape (Home depot) as a cathode was attached around the rim of the bottom plastic piece. Another Nafion coating was done by brushing with proper mask so that 1/32 inch gap between Nafion coating and cathode was maintained. (iii) Commercially available over-head projector (OHP) film was round cut for insulating layer and then, the mesh stack, OHP film and the bottom plastic piece were bonded by Super glue. Additional Nafion solution may be applied for guaranteeing electrical connections. (iv) Clear Extruded Acrylic Round Tube (McMaster, OD=1 inch, ID=¾ inch and length=¼ inch) was used for the chamber. At the bottom of the tube, we sawed 8 notches for brine outlet. Due to the transparency, the direct observation of depletion zone was possible through a side view. Coarse metal mesh as anode was sandwiched between the chamber and 20 mL plastic syringe head.

(34) FIG. 6 depicts a prototype of the just-described embodiment being used for desalination. For initial performance check, 10 mM phosphate buffer solution with red dye was used as source water. Electrical voltage was applied by Keithley 236 source measure unit in the range of 1˜110 V and a syringe pump (Harvard apparatus, PHD 2000) was connected to the prototype head. The flow rate was set to 0.25 mL/min. As shown in FIG. 6(a), outlet water turned to be clear, while inlet water was red. Dissolved red dyes were eliminated from the source water as applied voltage increased and it was almost clear above 100 V as shown in FIG. 6(b). To confirm the salt removal, the conductivities of desalted water were measured using a conductivity meter (VWR, symphony pH/conductivity meter) since the electrical conductivity is linearly proportional to an electrolyte concentration. It showed that the conductivity of desalted water was dropped to 95% at 100 V which was equal to 95% salt removal efficiency.

(35) In yet another embodiment, another self-buffered system designed to minimize manual fabrication, and with enhanced precision and reusability was developed as shown in FIG. 7. It consists of the following parts: (1) Nafion coated core meshes, (2) Upper plastic plate ( 1/32 inch thickness) with copper (or aluminum) tape as cathode, (3) Nafion sheet (Ion power, 10 mil), (4) bottom plastic plate ( 1/32 inch thickness) and (5) Cap part that had bare metal mesh (anode). Details of fabrications were as following. In order to obtain uniform thickness of Nafion coating, commercially available Nafion sheet was sandwiched between two plastic plates instead of brushing Nafion solution. The center hole of bottom plate was slightly ( 1/32 inch) smaller than top plate so that the stepped structure can hold Nafion coated plastic mesh stack. Copper (or aluminum) film was taped around the rim. Nafion sheet and the copper foil could contact each other through liquid filled in the radially positioned group of holes. Three holes around the center hole were placed for tightening with cap part. By placing several washers, we can control the amount of liquid leaking, i.e. brine flow. Final assembled prototype with top cap was shown in. FIG. 8(a) shows the desalting operation demonstration with 10 mM sodium phosphate buffer solution and yellow food dye. The color becomes gradually clear as a function of applied voltage (FIG. 8(b)). Corresponding electrical conductivities were also decreased as shown in FIG. 8(c) and 95% of salts were finally removed at 90 V of applied voltage.

(36) As shown in FIG. 9(a), conductivity increments (blue dots) from brine stream were also plotted with calculated values (black dotted line) from the conductivity difference between source water and filtered stream. Since the collection of brine stream retained from 0V, the measured increments of conductivity (blue dots) were lower than estimated values (dotted line). FIG. 9(b) shows conductivity changes of modeled seawater (yellow dye was intentionally mixed) as a function of applied electrical voltage. In both cases, the contents of the filtered stream dropped below the safe requirement, once the electric field value reached the threshold. This also coincided with the colorimetric observation. All experiments were repeated more than 5 times during more than 10 hours of operation by a single prototype to get reliable data points.

(37) We tested direct heavy metal removal using the prototype device with the filtered-stream outlet size of ⅜ inch diameter. External power supply (Keithley 236 source measure unit) and syringe pump (set to 1 mL/min; Harvard PHD 2000) were connected to the prototype. The system was horizontally set up over the collection bottle. Direct way to monitor the operation is the continuous measurement of electric current through the system. Since the generation of ICP zone rapidly increases the electrical resistance, one can typically observe a precipitous current drop (about two orders of magnitude) within a few seconds for normal operation, followed by a slower decrease thereafter. Both initial and steady-state current values depend on the salinity of the source water, and applied voltage. The common failure modes of the system include; i) no ICP generation, caused by electrical breakdown or shortage, in which case there will not be any current drop, ii) unbalancing between the input flow and the electric field, resulting in salt leakage through the mesh. ICP zone can be disrupted or destroyed if subjected under excessive flow speed, either locally (at some holes of the mesh) or globally. In such cases, one can observe a higher steady-state current or unsteady current profile long after the beginning of the operation.

(38) We prepared a water sample, mimicking the ionic components of metal contaminated Bangladeshi groundwater for testing our system. The resulting source water sample contained As3+ and As5+ (initially 500 ppb for each Arsenic ion) and Cd2+ and Pb2+ (initially 200 ppb for each ion). To quantify the concentration of each ion species in the purified water sample, an Agilent 7700x Inductively Coupled Plasma-Mass Spectrometer (Agilent Technologies) was employed. Also Pb and Cd detection kit (Invitrogen, Measure-iT™ Assay kit) and Arsenic detection kit (HACH co., EZ High Range Test Kit) were used to detect the amount of remaining substances. FIG. 9(a) showed the conductivity drop and concentration changes as a function of applied voltage. Majority ion concentrations (such as Na+ and Cl−) were monitored by conductivity measurement, which showed a precipitous drop once the electric voltage was applied. On the other hand, the conductivity increased at the brine stream, confirming that actual separation of salts from the desalted stream into the brine stream has occurred. At 80V, the concentrations of As5+, As3+, Cd2+ and Pb2+ were down to 5.76 ppb, 4.90 ppb, 4.26 ppb and 3.97 ppb which are below WHO's suggested safe level (As5+: 10 ppb, As3+: 10 ppb, Cd2+: 5 ppb and Pb2+: 15 ppb).sup.13 showing >99% overall removal efficiency. It was shown that As (M.W. 74.92160) were removed more efficiently than Pb (M.W. 207.2) or Cd (M.W. 112.411) potentially due to the inherent mobility differences. The power requirement was measured to be 0.5˜0.8 Wh/L for this groundwater sample. After a single purification process, the concentration of heavy metal and salts can be doubled with 50% recovery. If dirty water at the initial salt concentration of 10 mM (heavy metal contaminated groundwater) is filtered, we would get approximately 20 mM of concentrated brine with half of initial volume. Then we can put this concentrated water into the system again to get 40 mM of more concentrated brine yielding 75% recovery in total. In the same manner (repeating the process with brines at each step), we can increase the recovery and minimize the volume of brine as needed. However, this cascade concept is not tested in this work due to serious safety issues with highly concentrated heavy metals in an academic level laboratory. In addition to the multi-stage ICP purification scheme described above, we also can achieve high water recovery by controlling flow rate ratio of desalted/concentrated water. While we do not demonstrate high water recovery with large-scale device, in our previous experiments.sup.21, we have shown this water recovery can be achieved up to 99.5% (i.e. 200-fold). Since the processing volume of contaminated water in this manuscript and future portable applications is not too huge (less than 30 L/day) compared to plant scale reverse osmosis (usually produces fresh water at greater than 105 tons/day), we believe that concentrated brine from our device merely has critical impact to the environment. Also the nature of ICP does not require the cleaning process necessary for reverse-osmosis membranes since it utilizes a “pushing” force from the membrane, i.e. fouling-free process. If the core membrane is worn out, we can easily replace the membrane since the price is low. Based on the aforementioned differences, ICP purification process has completely different application areas compared to existing RO process.

(39) In addition to heavy metal removal, seawater desalination in this prototype device was also demonstrated. Sodium chloride solution of 500 mM (30,000 mg/L) was prepared as a model for seawater and introduced into the same prototype device with the input flow rate of 1 mL/min. Due to the high salt concentration there was a small fluctuation of steady-state current signal during the desalting operation. These variations could be minimized by reducing electrical resistance of electrode and proper choice of high-end performance nanoporous material. Still, as shown in FIG. 9(b), the conductivity of initial sample (˜50 mS/cm) was reduced down to less than 1 mS/cm (corresponding to 10 mM or 600 mg/L TDS, below the drinkable water criterion.sup.13), demonstrating over 98% salt removal efficiency (recovery rate ˜50%). The small current fluctuation we observed in this experiment may signify a localized leakage (compromised ICP zone) of salt caused by high salt concentration of the source water. This means that a local failure in the mesh does not lead to a systemic operational failure, but simply lowers the removal efficiency in a graded fashion. Such feature would be critically important for reliability when one consider a massive parallelization of the unit device (our prototype) into much higher flow systems. Required power was measured to be around 5 Wh/L, which is comparable to previous single microfluidic system's efficiency.sup.6 and current state-of-art reverse osmosis plant system.sup.1.

REFERENCES

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(41) While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.