WO2016057430A2

WO2016057430A2 - Devices and methods for removing dissolved ions from water using a voltage-driven charge pulse

Info

Publication number: WO2016057430A2
Application number: PCT/US2015/054091
Authority: WO
Inventors: Adam J. RAUSCH; Ashok J. GADGIL; Robert Kostecki
Original assignee: The Regents Of The University Of California
Priority date: 2014-10-03
Filing date: 2015-10-05
Publication date: 2016-04-14
Also published as: WO2016057430A3

Abstract

The present invention provides for a device useful or removing dissolved ions from water configured to comprise delivering a high voltage pulse to water containing said dissolved ions.

Description

Devices and methods for removing dissolved ions from water using a voltage-driven charge pulse

RELATED PATENT APPLICATIONS

[0001] The application claims priority to U.S. Provisional Patent Application Ser. No.

62/059,790, filed October 3, 2014; which is incorporated herein by reference.

STATEMENT OF GOVERNMENTAL SUPPORT

[0002] This invention was made with government support under Contract No. DE-AC02- 05CH11231 awarded by the United States Department of Energy. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] This invention relates generally to removing dissolved ions from water.

BACKGROUND OF THE INVENTION

[0004] In many areas of the world, surface water is unavailable for much of the year, and groundwater has high concentrations of dissolved ions. WHO recommends total dissolved solids (TDS) which consist of dissolved salts and silica in drinking water to be not more than 500 ppm, however in many parts of the world, the population has no recourse but to drink groundwater with TDS concentrations as high as 800 ppm, 1000 ppm, and even in some cases higher than 1400 ppm.

[0005] Methods to reduce ionic concentrations to more palatable levels include distillation, solar distillation, reverse osmosis, and ion exchange. In many cases these are not considered affordable, practical, or effective (e.g., owing to lack of adequate capital, land surface, solar insolation, or energy access). For years, membranes and ion exchange have been used to lower TDS from water and wastewater.

[0006] The basic concept for separating compounds that are dissolved in water using electrical means is quite old. The technology began to be refined starting in the 1990s.

Capacitive Deionization (CDI) is a process that applies a direct current electrical bias across a pair of electrodes in contact with the aqueous electrolyte to separate positively charged cations and negatively charged anions via migration and physical adsorption at the electrodes. For instance, in saline waters the positively charge sodium migrates to the negative electrodes and the negatively charged chloride ion will migrate to the positive electrode.

[0007] The Electronic Water Purifier (EWP) is a new technology developed in the last 10 years that has low operating costs, low rejection wastewater volume, low capital expenditure, no chemical requirements, a small footprint and is now available in sizes ranging from under- the-sink water purifiers to large commercial units. They use a CDI process (called membrane CDI) to remove dissolved ions from water by using a semi-permeable membrane that coats the electrodes. The device consists of multiple layers including coated electrodes that contain a conductive surface sandwiched between layers of activated carbon. A non-conductive spacer material separates the plates from one another. These electrodes are alternately connected to the two sides of a DC power supply. The device works on the principle of capacitive deionization to purify water, with the application of a low voltage DC potential to attract and discharge ions to the electrode surface. The high-surface-area carbon electrode layers attract and hold ions on their surface removing them from the water stream flowing through the device. Eventually, all the charged sites are filled, and the device must then be regenerated by discharging the ions from the electrode surfaces. This is accomplished by shorting the electrodes and reversing the polarity of the applied DC potential. Once a substantial number of the newly displaced ions are flushed in the waste stream, after a length of time, the unit begins to charge again by attracting ions from the feed solution under the influence of the reverse potential.

[0008] However, this method is still limited by a relative slow mass transfer across the electrolyte and inadequate ion storage capacity. Currently, there are no comparably inexpensive methods to reduce ionic salts appearing as total dissolved solids (TDS) in drinking water. Those who can afford it, purchase bottled water, or water treated with Reverse Osmosis or other methods. However, much of the population exposed to high levels of TDS is poor. Individuals from these populations do not have access to effective means to reduce TDS of potable water.

SUMMARY OF THE INVENTION

[0009] The present invention provides for a device useful for removing dissolved ions from water configured to comprise delivering a high voltage pulse to water containing said dissolved ions. The present invention also provides for a method for removing dissolved ions from water comprising providing said device, and using it thereof.

[0010] The present invention provides for a system for removing dissolved ions from water, comprising: (a) a first pair of electrodes, (b) optionally a second pair of electrodes, (c) a first voltage source capable of producing a low DC voltage, (d) a second voltage source capable of producing a plurality of short-duration, voltage-driven charge pulses, (e) a chamber defined by the first pair of electrode, and optionally by the second pair of electrodes, (f) an inlet, and (g) an outlet, wherein (i) the first pair of electrodes and the first voltage source are in electrical communication, and the first pair of electrodes and the second voltage source are in electrical communication or (ii) the first pair of electrodes and the first voltage source are in electrical communication, and the second pair of electrodes and the second voltage source are in electrical communication, and the chamber is in fluid communication with the inlet and the outlet, wherein a fluid flows in a direction from the inlet through the chamber and out of the outlet.

[0011] In some embodiments, the chamber has a linear dimension of about 1 mm from a first electrode to a second electrode of the first pair of electrodes, and/or a linear dimension of about 1 mm from a first electrode to a second electrode of the second pair of electrodes. In some embodiments, each electrode has a high surface area, wherein each electrode has an internal surface area of equal to or more than about 2000 m for each projected area of 0.0175 m . In some embodiments, the system comprises a circuit as shown in Figure 4. In some embodiments, each electrode comprises a material suitable for adsorbing an ion, such as activated carbon, carbon nanofoam, aerogel, ion exchange resin (IER), or the like. In some embodiments, each electrode comprises an IER particle as taught in U.S. Provisional Patent Application Ser. No. 62/059,777, filed October 3, 2014; which is incorporated herein by reference.

[0012] In some embodiments, the first pair of electrodes and/or second pair of electrodes are configured such that a direction of delivery of the low DC voltage, or a voltage gradient generated by the low DC voltage, and/or the charge pulses are perpendicular to the direction from which the fluid flows through the chamber. The low DC voltage is high enough to induce ion movement, but low enough to limit redox reactions at the any of the electrodes, such as producing 0₂ and/or H₂. In some embodiments, the low DC voltage is about 1.0 to about 1.4 V. In some embodiments, the charge pulses have a voltage of about 5 V to about 25 V, or 10 kV to about 25 kV. The duration of each pulse is equal to or less than about 1 μ8. In some embodiments, the duration of each pulse is equal to or more than about 50 ns to equal to or less than about 1 μ8. In some embodiments, the duration between pulses is or has an average of equal to or more than about 1 μ8. In some embodiments, the duration between pulses is or has an average of equal to or less than about 25 μ8. In some embodiments, the duration between pulses is or has an average of equal to or more than about 1 μ8 to equal to or less than 25 μ8.

[0013] In some embodiments, the system for removing dissolved ions from water, comprises: (a) a first pair of electrodes, (b) a first voltage source capable of producing a low DC voltage, (c) a second voltage source capable of producing a plurality of short-duration, voltage-driven charge pulses, (d) a chamber defined by the first pair of electrodes, (e) an inlet, and (f) an outlet, wherein the first pair of electrodes and the first voltage source are in electrical communication, the first pair of electrodes and the second voltage source are in electrical communication, and the chamber is in fluid communication with the inlet and the outlet, wherein a fluid flows in a direction from the inlet through the chamber and out of the outlet.

[0014] In some embodiments, the system for removing dissolved ions from water, comprises: (a) a first pair of electrodes, (b) a second pair of electrodes, (c) a first voltage source capable of producing a low DC voltage, (d) a second voltage source capable of producing a plurality of short-duration, voltage-driven charge pulses, (e) a chamber defined by the first pair of electrode, and optionally by the second pair of electrodes, (f) an inlet, and (g) an outlet, wherein the first pair of electrodes and the first voltage source are in electrical

communication, the second pair of electrodes and the second voltage source are in electrical communication, and the chamber is in fluid communication with the inlet and the outlet, wherein a fluid flows in a direction from the inlet through the chamber and out of the outlet.

[0015] The present invention provides for a device or apparatus comprising the system of the present invention.

[0016] The present invention provides for a method for removing dissolved ions from ion- containing water comprising: (a) providing the system or apparatus for removing dissolved ions from ion-containing water of the present invention; (b) flowing ion-containing water into the chamber via the inlet, (c) applying a low DC voltage with the first voltage source through the water via the first pair of electrodes, and (d) applying a plurality of short-duration, voltage-driven charge-pulses with the first voltage source, or the second voltage source, through the water via the first pair of electrodes, or the second pair of electrodes, such that the water flowing out of the outlet has fewer ions than the ion-containing water flowing into the inlet.

[0017] In some embodiments, the ion removed is any cation having an atomic number equal to or larger than 3. In some embodiments, the cation is an element of Group 1 or 2, or any cation with a valence of 1+ or 2+. In some embodiments, the cation is Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, or Ca²⁺. In some embodiments, the ion removed is any anion having an atomic number equal to or larger than 5. In some embodiments, the anion is an element of Group 16 or 17, or any anion with a valence of 1- or 2-. In some embodiments, the anion is F^", CI^", Br -^", O2-^", S^2", or Se^2".

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.

[0019] Figure 1. Schematic of CDI apparatus. Brackish water flows between two electrodes held at a constant voltage difference. The ions in the influx are drawn to and held at oppositely charged electrode surfaces. The layer of charge inside the electrode and the layer of oppositely charged ions outside the electrode constitute the "electric double layer." Once the electrode surface area is saturated with ions, the flow is stopped and voltage shunted or reversed, so the ions re-enter the solution. This highly concentrated solution is passed to a waste stream, and the process repeats.

[0020] Figure 2. Energy consumption for CDI as a function of influent NaCl concentration, compared with RO desalination. Adapted from Oren (cited in Example 1).

[0021] Figure 3. Proposed method of electrode preparation starting from commercial IER beads. In some embodiments, the final electrode contains pyrolyzed IER along with carbon black (black) and polymer binder (fibers) on a metal contact.

[0022] Figure 4. Representative circuit for characterizing transient response to voltage pulses in CDI. Charges at b and f represent electric charges in the electrodes matched by ionic charges c and d in the corresponding electric double layer. Charges at a and e represent electric charges at the electrodes that are unmatched by ionic charge in the corresponding double layer.

DETAILED DESCRIPTION

[0023] Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

[0024] Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.

[0025] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.

[0026] It must be noted that as used herein and in the appended claims, the singular forms "a", "and", and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "an ion" includes a plurality of such ions, and so forth. [0027] The term "about" refers to a value including 10% more than the stated value and 10% less than the stated value.

[0028] These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below.

[0029] The basic technology used in this invention is based on increased ion-mobility in aqueous electrolytes under a strong electric field (Wien effect), such as when the voltage is from 10 kV to 25 kV, and when the applied voltage has a very high frequency. One of the main dominant effect is the reduction of the effective shielding of the double layers by ramping up the voltage (leading edge of the pulse) faster than the electrical double layers can form in response. Positively and negatively charged ions in water are hydrated, i.e., multiple water dipolar molecules cluster around them. When the voltage is from 10 kV to 25 kV, one expects to see the following: The hydration shell causes the ions to move in water under the potential gradient. However, it is possible to get the ions to shed the hydration cluster by delivering an intense short pulse of high voltage. A very short intense voltage pulse causes the ions to move rapidly outside their hydration shell. The hydration shells again build builds up in the order of milliseconds. So, rapid short intense voltage pulses spaced about a millisecond apart will keep the ion mobility high. In some embodiments, low-voltage drift electrodes cause the ions to drift towards the electrodes where they are adsorbed on activated carbon, or caused to enter the interstitial spaces within a hydrogel immersed in water. In some embodiments, the ion-electrosorption medium has a very low diffusivity for ions and keeps the ions away from the bulk volume of free water, until the time comes to discharge ions by reversing, or shunting, the drift electrodes polarity.

[0030] In one aspect, the high voltage pulse is delivered in a direction perpendicular to the direction of the solution flow, from a pair of polarizable electrodes. The electrodes are polarized with a low voltage DC bias and the charge pulses are superimposed on the constant voltage polarization. The pulse duration must be short (microseconds) to avoid redox reactions, such as generation of 0₂ and H₂, at the electrodes.

[0031] In another aspect, the high-voltage electrodes are separate from the low- voltage electrodes that provide for steady ion mobility, but both sides of electrodes are aligned perpendicular to the same axis. The high voltage electrodes increase the ion mobility and the low-voltage electrodes provide the steady electric field to promote ions migration toward the ion-electrosorption medium (activated carbon, carbon nanofoam, carbon aerogel, EIR, or similar material).

[0032] In another aspect, only a pair of electrodes is used, and employed for both the high voltage millisecond-apart pulses, and the low voltage gradient that causes ion drift towards the electrodes.

[0033] This technology may be potentially attractive for creating new affordable water sources by desalination of low salinity sources such as brackish and surface waters but not for seawater and for water saving (e.g., municipal and industrial wastewater reclamation) by removing trace contaminants from polluted streams. Another possible application of this invention is for removing toxic naturally occurring ionic contaminants (e.g., fluoride or nitrate from underground water supplies).

[0034] The invention having been described, the following examples are offered to illustrate the subject invention by way of illustration, not by way of limitation.

EXAMPLE 1

Treating Brackish Drinking Water

[0035] Summary

[0036] We aim to develop a novel technology to affordably make brackish water potable for the rural developing communities that are increasingly dependent on brackish sources.

[0037] The Brackish Water Challenge

[0038] One in three people lived in water— stressed regions in 2006, and that number is expected to double by 2015.1 Traditionally reliable surface sources, fed by precipitation or glacial melt, face threats from climate change, while fresh groundwater is often chronically oversubscribed. Indeed, 60% of India's groundwater resources are expected to be depleted within 20 years.2 While such water stress contributes to a host of problems, shortages in potable drinking water are among the most acute.

[0039] Seawater intrusion, agricultural practices, and contamination from industry are making fresh sources increasingly saline.3 Freshwater shortages, combined with these salinization effects, mean communities are increasingly dependent on brackish sources for drinking water. Already, saline groundwater is the sole source in much of Namibia, and peninsular India.3 Today, an estimated 1.1 billion people live in places with saline ground water.3

[0040] Without treatment, this water is so salty that drinking it comes with great suffering. For this reason, the US EPA limits drinking water to less than 500 ppm total dissolved solids (TDS), as a secondary regulation.4 Currently, the dominant method for treating such waters is reverse osmosis (RO), but high energy requirements make it costly.5 Not surprisingly, areas with rising dependence on saline sources have observed a corresponding rise in the cost of potable drinking water.3 As such, the burdens of this issue are felt disproportionately by the poorest in such regions.

[0041] The brackish water challenge is then to make affordable (<$l/tonne) potable water (TDS < 500 ppm) from the abundantly available brackish (TDS 1,000—3,500 ppm) sources. Such a treatment method could provide new sources of drinking water for those who cannot afford the current alternatives.

[0042] The CDI Approach

[0043] To address this challenge, we will explore two novel and potentially game-changing approaches to capacitive deionization (CDI) technology, in the expectation of lowering initial costs, decreasing energy requirements, and improving efficiency, thereby developing a viable brackish water treatment method for rural developing world communities. CDI is a low- energy, electrochemical method for removing ions from water by drawing them to charged electrodes and trapping them in electric double layers that form at the electrode surfaces. In a typical CDI configuration (Figure 1A), water flows between two parallel— plate electrodes, which are charged by applying an external voltage. As the brackish water enters, the charged ions - which constitute most of TDS - drift to the oppositely charged electrodes, where they are held at the surface. The applied cell voltage is kept low enough (1.0—1.4 V) that trapped ions do not transfer their charge, and chemical bonds do not form. Eventually, the surface area of the electrodes becomes saturated with ions, and the electrodes must be regenerated. At this point, the flow is stopped, the voltage is turned off (or reversed), and the ions re— enter solution where they can be routed to a waste stream. Following regeneration, the process can begin again. [0044] The energy requirements for CDI are a strong function of the influent concentration. More energy is required to treat higher— concentration influents. As a result, the energy required to treat seawater is higher than that with RO, but CDI has the potential to treat brackish waters for significantly less energy than is required for RO (see Figure 2).6

[0045] The electrode material properties are also important. To perform well, the material must have high electrical conductivity, chemical stability over a wide variety of possible influents, and high surface area available for adsorption of ions.6 Carbon aerogels, used frequently for such applications, have specific surface areas of 400—600 m2/g.7

[0046] Currently, much of CDI research is aimed at increasing the capacity of electrodes, usually by exploring exotic and expensive materials.8, 9 As such, these efforts are unlikely to provide affordable solutions for developing communities' needs. Correspondingly, technology development has focused largely on industrial— world applications. 10,11

[0047] Despite the current focus of CDI research, the technology offers unique advantages for developing— world markets. The low energy requirement means smaller operating costs and lower infrastructure requirements. Operation requires no external chemistry or consumables, so it does not require supply chains for rare materials. Finally, the ability to operate efficiently at small scales means CDI can be affordable even in remote, rural settings.

[0048] By focusing on brackish water and the unique needs of rural developing communities, we were able to identify CDI as a technology with particular promise for this specific problem. To realize that promise, however, we must still tailor CDI to address the brackish water challenge. To that end, we propose to explore two novel approaches, for which invention disclosures have been filed. The first has the potential to reduce electrode cost, thereby lowering first costs of a CDI cell. The second has the potential to dramatically improve efficiency, thereby lowering operational energy costs. We see such efforts as the necessary first steps to achieving a breakthrough in such treatment systems.

[0049] Affordable Electrodes

[0050] Currently, the high cost of electrode materials represents the largest single cost for a CDI cell, and likely the largest barrier to CDI as a solution in the developing world. Our first approach focuses on fabricating an effective, affordable electrode which incorporates both double layer trapping and chemical bonds. [0051] Our affordable electrode incorporates ion exchange resins (IERs). IERs capture ions via chemical bonds, releasing other, harmless ions back into solution. Indeed, IERs have already been used in water purification,12 but existing IER— based water purifiers require regeneration using corrosive chemicals. The resulting risks and chemical supply chains make them non— ideal for developing world applications. However, an IER— based CDI electrode may allow us the benefits of ion exchange, but with an electrical regeneration mechanism, thus eliminating the need for these supply chains.

[0052] IERs are affordable and commercially available in a variety of bead sizes (0.3—1.25 mm). They are offered with different functional groups, which dictate the polarity of the ions exchanged and the ions released into solution. Off— the— shelf IER beads, however, are not electrically conductive and have small specific surface areas (-45 m2/g),13 two issues that will have to be addressed in our fabrication process (Figure 3).

[0053] Surface area can be increased by decreasing the average particle size, which can be achieved by common techniques such as ball milling (or cryomilling for soft polymer— based materials). Such methods have been shown to increase the total surface area by as much as 250 times.14 To further improve the surface area and simultaneously increase electrical conductivity, the surface of the milled IER can be pyrolyzed by heating to 700— 1000°C for a very short time. Wilson, et al. have demonstrated this using heated argon gas, 13 though we may also explore alternative methods (plasma torch, fluidized bed heating, or gunpowder reaction) to achieve the required properties (pyrolysis depth, carbon structure, transition thickness). One pyrolyzed, the conducting high— surface— area IER can be mixed with carbon black and polymer binders, and painted as a thin film over metal to form a functional electrode. These binding and adherence methods can be adapted from those already optimized for battery electrodes.15

[0054] We expect this process to produce an electrically conductive, high surface area electrode capable of ion exchange. Here, the electric field will drive ions to the electrode edge. Migration through the electrode to the IER surface will be driven by diffusion. Finally, the contaminant will be retained due to chemical attraction at specific functional groups in the resin. As with traditional CDI, regeneration will be affected electrically, rather than chemically.

[0055] This method represents an entirely new approach to CDI with the potential for affordable, high— capacity electrodes. Here, the central challenge is developing and optimizing the fabrication methods (milling, pyrolysis, binding, adherence) in a way that produces optimal material properties. Success here will not only constitute the first electrically regeneratable ion exchange resin, but also lower first costs for CDI.

[0056] Voltage Pulsing

[0057] Our second approach to make CDI more affordable aims to decrease the amount of electrode material required. Here, we propose a novel voltage pulsing design to increase the drift rate of influent ions. This will make CDI cells smaller, electrodes shorter, and perhaps improve overall capacity.

[0058] A number of molecular arrangements can slow electrically induced motion of ions. In a strong electrolyte, an ion will be surrounded by a cloud of oppositely charged ions, with will be pulled in the opposite direction by a field. In a weak electrolyte, many ions remain paired with oppositely charged counterparts, also pulled in the opposite direction. Finally, ions attract a bulky sphere of water molecules, which must be dragged through the solution. All such molecular arrangements, however, can be disrupted by sufficiently strong electric fields. These disruptions are collectively known as the Wien effects.16

[0059] In normal configurations, the voltages required to produce these fields would be sufficient to induce undesirable reduction and oxidization (redox) reactions at the electrode surfaces, rendering the two designs incompatible. However, the relative time— scales offer an exciting possibility. The time scales for breakup of molecular arrangements via the Wien effects are shorter than those for instigating redox reactions. Therefore, short (likely μ8), high voltage pulses may allow us to break up the detrimental molecular arrangements without inducing redox reactions.

[0060] With increased ion mobility, the ions would reach the electrodes sooner, decreasing the required residence time for water being treated. This, in turn, would mean the same treatment rate could be achieved with considerably less electrode material, which would have a substantial impact on overall cost. Alternatively, increased ion mobility would allow for operation at lower DC voltages, improving overall efficiency. Finally, the approach may increase the electrode's total ion capacity, as experimental evidence suggests double layer capacity is dominated by the ion's hydrated radius, which includes the sphere of water molecules.7 [0061] If successful, this effort would represent the first use of Wien effects in CDI. Further, achieving Wien effects with pulsed fields has seen little exploration. Most importantly, the combined efforts would create a substantially more affordable CDI cell, opening the door to markets that could otherwise not afford treatment.

[0062] Measuring Outcomes

[0063] For the IER approach, we intend to develop a method for fabricating the electrode material and confirm its physical properties. This confirmation will require advanced characterization methods, as well as the expertise available at Lawrence Berkeley National Lab's Batteries Group. Specifically, we intend to measure IER particle size and carbon thickness via electron microscopy and characterize carbon surface formations via Raman spectroscopy. Other methods will likely be introduced as they prove necessary. This will set the stage for electrode creation, device testing, and iterative tuning of our methods.

[0064] For the voltage pulsing approach, we aim to observe pulse— based Wien effects, and understand what this arrangement can provide in terms of improved efficiency, improved surface capacity, and reduced electrode size. In this grant period, we intend to design, build, and test a prototype apparatus capable of performing CDI both with and without such pulsing. Using this system, we can perform side— by— side measurements to identify improvements in ion mobility, electrode capacity, and required residence time. Specifically, we will measure changes in ion mobility and residence time by comparing effluent concentrations over varying flow rates. We can observe electrode capacity by running the system until breakthrough, marked by a rapid rise in the effluent's ion concentration. This occurs when all usable spaces in the electric double layers are filled and ions can no longer be removed from solution.

[0065] Finally, we will produce an economic estimate of the cost to bring brackish water to potable levels under real conditions, based on the current state of our efforts.

[0066] References cited:

1 The United Nations World Water Development Report 2. UNESCO: Paris (2006).

2 Deep Wells and Prudence: Towards Pragmatic Action for Addressing Groundwater Overexploitation in India. World Bank: Washington DC (2010). 3 van Weert, F., et al. Global Overview of Saline Groundwater Occurrence and Genesis. IGRAC Report: Utrect (2009).

4 Drinking Water Standards and Health Advisories. US Environmental Protection Agency (EPA 822-S-12-001): Washington, DC (2012).

5 Subramani, A., et al. Water Research 47 (2007) 1907.

6 Oren, Y. Desalination 228 (2008) 10.

7 Gabelich, C. J., et al. Environmental Science & Technology 36 (2002) 3010.

8 Li, H. B., et al. J Materials Chemistry 19 (2009) 6773.

9 Gao , Y., et al. Thin Solid Films 517 (2009) 1616.

10 Conway, B. E., et al. Electrochimica Acta 47 (2005) 705.

11 Welgemoed, T. J., and C. F. Schutte. Desalination 183 (2005) 327.

12 Hubicki, Z., et al. In: Adsorption and Its Applications in Industry and Environmental Protection, Vol II: Applications in Environmental Protection 120— A (1999) 497.

13 Wilson, M. S., et al. Journal of Materials Chemistry 21 (2011) 7418.

14 Yim, S. P., et al. Waste Management Symposia 2 (1991) 295.

15 Marks, T., et al. Journal of the Electrochemical Society 158 (2011) A51.

16 Luitjen, E. Nature Physics 9 (2013) 606.

17 Novogratz, J. Innovations 2 (2007) 19.

EXAMPLE 2

Pulse-charged eapadtive deionizatio : increasing effective ion mobility with transient

[0067] 2.5 billion people rely solely on groundwater for basic needs,¹ but an estimated 20% of aquifers are being tapped unsustainably. In addition, remaining freshwater sources face salinization threats from seawater intrusion, agricultural practices, and industrial contamination. The result is an estimated 1.1 billion people living in places with brackish ground water, - and communities increasingly dependent on these brackish sources for drinking water. Drinking this water causes great suffering.

[0068] As with many such problems, this dependence disproportionately affects the poorest among us. For example, 60% of India's fresh groundwater resources are expected to be depleted within 20 years.⁴ And while mature technologies for removing these salts exist, costs and infrastructure requirements often make these solutions impractical in rural developing settings.

[0069] Capacitive deionization (CDI) offers particular promise for low-cost, off-grid treatment of brackish groundwater, due to low energy requirements at typical influent concentrations (1000-3500 ppm).⁵ It is also well-matched for intermittent, off-grid power sources. Unfortunately, traditional CDI can also be slow, which presents a challenge for application at scale, especially in developing settings. The traditional configuration relies on electrically-induced ion flow across a (typically mm-scale) channel, which is initially rapid, but slows as electrical double-layers form at the electrodes. To address this, one approach pumps the water through thick electrodes built with μιη-scale pathways.⁶ Another method adds carbon beads that become charged within the channel, adsorb ions, and can later be separated mechanically. Both methods reduce the distance necessary for ions to travel prior to adsorption, but increase required pressure, and hence energy requirements.

[0070] It appears possible, however, to operate CDI in an altogether different way. In our arrangement, the electrodes are not held at a constant voltage with the resultant slow, steady transfer of charge. Instead, the cell is charged in discrete steps, transferred to the electrodes by periodic, short-duration, high-voltage pulses. Each pulse creates a transient voltage spike. The rise is induced by the high-voltage pulser, but the fall is due to the response of the double layers within the cell. Instead of seeing one brief high-magnitude field, ions in the channel see periodic high-magnitude fields and react to each pulse. Redox reactions are still avoided, so long as each spike is shorter than the timescales over which redox reactions are induced.

[0071] Though the high-magnitude fields are present for only a fraction of the duty cycle, they are many orders of magnitude higher than those in the bulk after double-layers have been established. Thus, this arrangement induces higher effective ion mobility, a nonlinear current response to applied voltage, and hence an opportunity to improve performance at only modest energy cost.

[0072] Our presentation will discuss early theoretical formulations, as well as experimental results from side-by- side comparisons of pulse-charged and traditional CDI.

[0073] Background

[0074] Capacitive deionization (CDI) is a water-treatment technology which can reduce TDS by removing ions from solution. In a typical CDI configuration (Figure 1), influent water flows between two parallel electrodes held at a constant voltage difference. Dissolved ions are drawn to these electrodes and held in the electrical double layers that form at the electrode surface. When the double layers become saturated, flow is stopped, voltage is shunted (or reversed), and the ions are released into water that is then routed to a waste stream. Following this regeneration step, the process can begin again. Cell voltages are typically 1-1.4 V, high enough to induce ion movement, but low enough to limit undesirable redox reactions at the electrodes.

[0075] CDI offers particular promise for low-cost, off-grid treatment of brackish

groundwater, due to low energy requirements at typical influent concentrations (1000-3500 ppm).⁴ It is also well-matched for intermittent, off-grid power sources. Unfortunately, traditional CDI can also be slow, which presents a challenge for application at scale, especially in developing settings. The traditional configuration relies on electrically- induced ion flow across a (typically mm-scale) channel, which is initially rapid, but slows as electrical double-layers form at the electrodes. To address this issue, one recent approach pumps the water through thick electrodes built with μιη-scale pathways.⁶ Another method adds carbon beads that become charged within the channel, adsorb ions, and can later be separated mechanically. Both methods reduce the distance necessary for ions to travel prior to adsorpsion, but increase required pressure, and hence energy requirements.

[0076] Theory

[0077] It appears possible, however, to operate CDI in an altogether different way. In our arrangement, the electrodes are not held at a constant voltage with the resultant slow, steady transfer of charge. Instead, the cell is charged in discrete steps, transferred to the electrodes by periodic, short-duration, higher-voltage pulses. Each pulse creates a transient voltage spike. The rise is induced by a voltage pulser, but the fall is due to the response of the double layers within the cell. Instead of seeing one brief high-magnitude field, ions in the channel see periodic high-magnitude fields and react to each pulse. Redox reactions are still avoided, so long as each spike is shorter than the timescales over which redox reactions are induced.

[0078] Though the high-magnitude fields are present for only a fraction of the duty cycle, they are many orders of magnitude higher than those in the bulk after double-layers have been established. Thus, this arrangement induces higher effective ion mobility, a nonlinear current response to applied voltage, and hence an opportunity to improve performance at only modest energy cost. Ion-mobility may be further increased if applied voltage spikes are sufficiently large to induce the Wien effect within the bulk. Once past this threshold, the electric fields within the bulk are sufficient that ions are no longer affected by counter-ions, and their mobility approaches the diffuse limit.

[0079] Methods

[0080] The Pulse-charged CDI arrangement requires attaching the electrodes of a CDI device to a voltage supply capable of short-duration, voltage-controlled, high-current pulses. As this supply will drive only the rising edge, a diode is required in series. This supply may operate independently or in parallel with a traditional CDI power source (low voltage, DC, high current capacity) similarly protected with a series diode. In practical application, this would likely be supplemented with pump(s), valve(s), electronics to control regeneration steps, and conductivity testing to monitor total dissolved solids at output and trigger regeneration.

[0081] Practical application of pulse-charged capacitive deionization requires that pulse voltage, pulse duration, and period between pulses be well matched to the physical characteristics of the CDI cell. To determine these, a cell can be characterized by observing response to voltage steps, as represented by the circuit shown in Figure 4. In this

configuration, the formation of the double layer, in response to a voltage step, represented by if(t), can be approximated by di(£)

i_f i t) ¾ ¾ ¾—- + i{t)

at for C∑ » Ci, where i(t) and Ri can be measured directly, and Ci can be found by monitoring current response to with varying salt concentrations and extrapolating to a ion-less electrolyte (¾ = 0Ω), at which point the process is analogous to charging a simple R-C circuit. The time evolution of charge at e (and a), can be found as

[0082] With q_e and Q known, the electric field magnitude applied to the bulk is just

for x « A, and x » D, respectively, where x is the distance between electrodes and A is the cross-sectional area of the electrodes, and XD is the Debye length. The time-evolution of this field informs the operational parameters for this device. Specifically, the timescale for the decay of this field informs choice of period between driven pulses, as pulse pileup could increase voltage and likely induce undesirable redox reactions. Ensuring the decay of V_e-_a reaches sub-redox levels (<1.2-1.4 V) in less than redox timescales provides a maximum pulse- voltage for redox-free operation. Pulse duration should be sufficient to reach peak field strength. Adjustment of either pulse period or pulse duration (beyond minimums noted above) can be used to achieve a specific mean current , which translates to mean ion removal rate.

[0083] Preliminary Results

[0084] As constructed, our prototype CDI cell has channel distance x ~ 1mm and projected

2 2

area ^4 ~ 0.0175 m , with tortuous internal surface area -2000 m . Although experimental characterization is still ongoing, early experimental data and modelling of the prototype cell have provided some insight. Ci is estimated at 10-100 nF; C∑ estimated at 10-50 F; Ri can be kept under 1Ω so charge transfer is relatively fast. We therefore expect our prototype CDI cell should operate well under the following parameters. Optimal pulse voltage is expected to be between 5 and 25 V. Pulse duration is expected between 50 ns and 1 μ8. Period between pulses is expected between 1 and 20 μ8.

[0085] As our prototype cell represents relatively standard CDI parameters (on a per-cell basis), a pulse-charged CDI device capable of operating in the Wien-effect regime (pulse voltages ~kV) would likely require a substantially different design focused on operation at higher voltages, or in systems treating water of substantially higher concentrations. [0086] References cited:

1. UN World Water Development Report 2015; UNESCO: Paris, 2015.

2. Gleeson, T.; Wada, Y.; Bierkens, M. F. P.; van Beek, L. P. H. Water balance of global aquifers revealed by groundwater footprint. Nature 2012, 488: 197-200.

3. van Weert, F.; van der Gun, J.; Reckman, J.; Global Overview of Saline Groundwater Occurrence and Genesis; IGRAC: Utrect, 2009.

4. Deep Wells and Prudence: Towards Pragmatic Action for Addressing Groundwater Overexploitation in India; World Bank: Washington, D.C., 2010.

5. Oren, Y. Capacitive deionization (CD I) for desalination and water treatment - past, present and future (a review). Desalination 2008, 228 10-29.

6. Suss, M. E.; Baumann, T. F.; Bourcier, W. L.; Spadaccini, C. M.: Rose, K. A.;

Santiago, J. G.; Stadermann, M. Capacitive desalination with flow-through electrodes. Energy and Environmental Science 2012, 5, 9511-9519.

7. Hatzell, K. B.; Owama, E.; Ferris, A.; Daffos, B.; Urita, K.; Tzedakis, T.; Chauvet, F.;

Taberna, P.; Gogotsi, Y.; Simon, P. Capacitive deionizaiton concept based on suspension electrodes without ion exchange membranes. Electrochemistry

Communications 2014, 43, 18-21.

[0087] While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.

Claims

What we claim is:

1. A system for removing dissolved ions from water, comprising: (a) a first pair of

electrodes, (b) optionally a second pair of electrodes, (c) a first voltage source capable of producing a low DC voltage, (d) a second voltage source capable of producing a plurality of short-duration, voltage-driven charge-pulses, (e) a chamber defined by the first pair of electrode, and optionally by the second pair of electrodes, (f) an inlet, and (g) an outlet, wherein (i) the first pair of electrodes and the first voltage source are in electrical communication, and the first pair of electrodes and the second voltage source are in electrical communication or (ii) the first pair of electrodes and the first voltage source are in electrical communication, and the second pair of electrodes and the second voltage source are in electrical communication, and the chamber is in fluid communication with the inlet and the outlet, wherein a fluid flows in a direction from the inlet through the chamber and out of the outlet.

2. The system of claim 1, wherein the chamber has a linear dimension of about 1 mm from a first electrode to a second electrode of the first pair of electrodes, and/or a linear dimension of about 1 mm from a first electrode to a second electrode of the second pair of electrodes.

3. The system of claim 1, wherein each electrode has a high surface area, wherein each electrode has an internal surface area of equal to or more than about 2000 m for each projected area of 0.0175 m .

4. The system of claim 1, wherein each electrode comprises activated carbon, nanofoam, aerogel, or ion exchange resin (IER).

5. The system of claim 1, wherein the first pair of electrodes and/or second pair of

electrodes are configured such that a direction of delivery of the low DC voltage, or a voltage gradient generated by the low DC voltage, and/or the charge-pulses are perpendicular to the direction from which the fluid flows through the chamber.

6. The system of claim 1, wherein the low DC voltage is about 1.0 to about 1.4 V.

7. The system of claim 1, wherein the voltage-driven charge-pulses have a voltage of about 5 V to about 25 V or about 10 kV to about 25 kV, the duration of each pulse is equal to or more than about 50 ns to equal to or less than about 1 μ8, and the duration between pulses is or has an average of equal to or more than about 1 μ8 to equal to or less than 25 μ8.

8. The system of claim 1, wherein the system comprises: (a) a first pair of electrodes, (b) a first voltage source capable of producing a low DC voltage, (c) a second voltage source capable of producing a plurality of short-duration, voltage-driven charge- pulses, (d) a chamber defined by the first pair of electrodes, (e) an inlet, and (f) an outlet, wherein the first pair of electrodes and the first voltage source are in electrical communication, the first pair of electrodes and the second voltage source are in electrical communication, and the chamber is in fluid communication with the inlet and the outlet, wherein a fluid flows in a direction from the inlet through the chamber and out of the outlet.

9. The system of claim 1, wherein the system comprises: (a) a first pair of electrodes, (b) a second pair of electrodes, (c) a first voltage source capable of producing a low DC voltage, (d) a second voltage source capable of producing a plurality of short- duration, voltage-driven-charge pulses, (e) a chamber defined by the first pair of electrode, and optionally by the second pair of electrodes, (f) an inlet, and (g) an outlet, wherein the first pair of electrodes and the first voltage source are in electrical communication, the second pair of electrodes and the second voltage source are in electrical communication, and the chamber is in fluid communication with the inlet and the outlet, wherein a fluid flows in a direction from the inlet through the chamber and out of the outlet.

10. A method for removing dissolved ions from ion-containing water comprising: (a) providing the system of claim 1; (b) flowing ion-containing water into the chamber via the inlet, (c) applying a low DC voltage with the first voltage source through the water via the first pair of electrodes, and (d) applying a plurality of short-duration, voltage-driven charge-pulses with the first voltage source, or the second voltage source, through the water via the first pair of electrodes, or the second pair of electrodes, such that the water flowing out of the outlet has fewer ions than the ion- containing water flowing into the inlet.

11. The method of claim 10, wherein the ion removed is an elemental or molecular cation.

12. The method of claim 10, wherein the ion removed is an elemental or molecular anion.