: Deionization, ltration,

ouadBaza

etts Institusetts Insute of Tec

Bacteria in the feedwater are either l-

ores near to an ion selective element. While shock ED has been demonstrated as

r B.V. All rights reserved.

Desalination 357 (2015) 7783

Contents lists available at ScienceDirect

Desalin

j ourna l homepage: www.e l1. Introduction

Notes: The authors declare no competing nancial interest. Corresponding author.

E-mail address: bazant@mit.edu (M.Z. Bazant).1Water treatment 2014 ElsevieKeywords:Shock electrodialysisOver-limiting currentDisinfectionFiltrationSeparationsDeionization

an effective water desalination tool, we here present evidence of other simultaneous functionalities. We showthat shock ED can thoroughly lter micron-scale particles and aggregates of nanoparticles present in thefeedwater.We also demonstrate that shock ED can enable disinfection of feedwaters, as approximately 99% of vi-able bacteria (here Escherichia coli) in the inow were killed or removed by our prototype. Shock ED also sepa-rates positive from negative particles, contrary to claims that ICP acts as a virtual barrier for all chargedparticles. By combining these functionalities (ltration, separation and disinfection) with deionization, shockED has the potential to enable highly compact and efcient water purication systems.shock waves in microscale pAccepted 13 November 2014Available online xxxx

lination, and disinfection of a feedstream. Shock electrodialysis (shock ED) is a newly developed technique forwater desalination, leveraging the formation of ion concentration polarization (ICP) zones and deionizationa r t i c l e i n f o

Article history:Received 9 September 2014Received in revised form 10 November 2014Present address: St Edmund's College, University of C2 Present address: School of Engineering and Applied

Cambridge, MA, USA.3 Present address: Giner Inc., Newton, MA, USA.4 Present address: Faculty of Mechanical Engineering

Technology, Haifa, Israel.

http://dx.doi.org/10.1016/j.desal.2014.11.0110011-9164/ 2014 Elsevier B.V. All rights reserved.a b s t r a c t

The development of energy and infrastructure efcient water purication systems is among the most critical en-gineering challenges facing our society.Water purication is often amulti-step process involving ltration, desa-tered or killed by large electric elds. Besides deionization and ltration, nano-particles can be separated by charge.Daosheng Deng a,2, Wassim AMatthew E. Suss a,4, Martin Z.a Department of Chemical Engineering, Massachusb Department of Mechanical Engineering, Massachc Department of Mathematics, Massachusetts Instit

H I G H L I G H T S

Experiments demonstrate the multi-functionality of shock electrodialysis.

a,1, William A. Braff b,3, Sven Schlumpberger a,nt a,c,ute of Technology, Cambridge, MA, USAtitute of Technology, Cambridge, MA, USAhnology, Cambridge, MA, USA

G R A P H I C A L A B S T R A C Tseparation, and disinfection

Water purication by shock electrodialysisambridge, Cambridge, UK.Sciences, Harvard University,

, Technion - Israel Institute ofation

sev ie r .com/ locate /desa lThe purication of sea or brackishwater is an increasingly importantprocess in areas suffering from water stress or scarcity [1]. State of theart water purication is performed primarily by reverse osmosis (RO)plants and in some cases by electrodialysis (ED) plants [1,2]. In RO and

EDplants, the completewater purication process can be roughly divid-ed into three sequential steps: i) upstream feedwater processing, ii) saltremoval (desalination), and iii) downstream processing of the productwater [35]. In RO plants, to perform desalination, the feedwater ispressurized to above its osmotic pressure, and then ows through anRO membrane which inhibits the transport of salts. In ED, feedwater isows through an open channel between an anion and cation exchangemembrane, and an appropriately directed ionic current is applied to thesystem, removing anions and cations (dissolved salts) fromwater [6,7].Many upstream steps are required in both RO and ED purication sys-tem to prevent membrane fouling, including ltration to remove silt(membrane foulants), and pH adjustments of the feedwater [8]. Down-stream processes include disinfection of the desalted water through theuse of chemical additives such as chlorine [8].Modern ROplants typical-ly require roughly 4 kWh/m3 of energy to purify sea water to potablewater [3,5], but nearly one third of this total energy is devoted to up-stream and downstream processes rather than the desalination itself[4,5]. In some cases, there can also be economic benets of combiningED and RO in a hybrid desalination process [9].

Shock electrodialysis (shock ED) is a new technique for water desa-lination that differs from classical ED in several key aspects [10,11]. Thetheory behind shock ED is a subject of active research [1117], so herewe briey summarize the basic concepts needed to understand our ex-periments. In its current realization, a shock ED cell consists of two ionselective elements (ion exchange membranes or electrodes) betweenwhich feedwater ows through a charged porous medium with thindouble layers that acts as a leaky membrane [11,15,16] (Fig. 1). LikeED, when current is passed through the shock ED cell, an ion depletedzone is formed along an ion selective element (the cathode in Fig. 1).

in submicron pores, and surface convection by electro-osmotic owvortices in the depleted region [14,1719], which dominates inmicron-scale or larger pores. Experiments in microchannels or poresof different sizes have recently demonstrated and visualized the surfaceconduction [20] and electro-osmotic ow [11] mechanisms, as well asthe transition between them [21]. As a result of this over-limiting cur-rent, the depletion zone can be propagated through the pores as ashock wave (i.e. with a sharp boundary between the depleted andundepleted zones) [12,13,15,16,22,23]. Water owing through the de-pletion zone is separated and emerges from the cell as desalinatedwater [11]. In shock ED [10,11], an ion enrichment or brine zone isformed at the opposite ion selective element, and the formation ofenriched and depleted zones at opposite ends leads to strong ion con-centration polarization (ICP) [6,7].

Previously, we developed a shock ED prototype using a porous silicaglass frit with micron-scale pores as the porous medium, a copper elec-trode as the anode-side ion selective element, and a Naon ion ex-change membrane as the cathode-side ion selective element [11].With this device, we demonstrated the deionization of a copper sulfatesolution by reducing its concentration by roughly 4 orders ofmagnitudein two passes (to 10 M). Further, our measurements of overlimitingconductance suggested that the overlimiting current mechanism inour prototype devicewas electroosmotic ow rather than surface trans-port [11]. Compared to recently-developed microuidic approachesleveraging ICP for water desalination [24,25], shock electrodialysis is amore scalable technology, as its use of porous media can enable highthroughput without requiring the fabrication of many parallel micro-uidic systems [11]. Another unique feature of shock ED is the abilityto propagate the depletion zone controllably through micron-scale frit

cel, a sa

78 D. Deng et al. / Desalination 357 (2015) 7783As the applied voltage is increased, ion concentration near this elementapproaches zero, and the system can reach the classical diffusion-limited current [6]. However, unlike ED, in shock ED the presence of asurface charge along the porous media's internal surfaces can enabletransport of ions faster than diffusion. There are two theoretically pre-dicted mechanisms [14]: surface conduction by electromigrationthrough the electric double layers of the pores [14,18], which dominates

+

+

+

+

+

+++

+

anode

cathode

rese

rvoi

rpo

rous

mat

eria

ls

inlet

Fig. 1. Schematic demonstrating water purication with our shock ED device. Our shock EDwhich is placed aporousmedia. Bypassing an ionic current between the ion selective elements

produce desalinated water, the device demonstrates other unique functionalities, including ltratpores, enabling a tunable ion depletion zone which can extend to milli-meters or larger in length to further increase throughput.

In this work, we demonstrate that our shock ED cell can perform anumber of functions in addition to (and simultaneously with)water de-salination, including ltration, disinfection, and ion separations (seeFig. 1). Both ltration and disinfection are important processes in mod-ern water purication plants [3,8]. With our cell, we demonstrate the

+

micro-particle

+

+

positive dye

negative dye

salinity

outlet (purified water)

bacterium

l consists of two ion selective elements (electrodes or ion exchange membranes) betweenlt depletion zone is formednear to the cathode. In addition to leveraging thedepletion zone to

ion of particulates, spatial separation of species by valence sign, and disinfection.

ltration of micron-scale particles and aggregates of nanoscale particlespresent in the feedwater, andwe hypothesize that this was due to sterichindrance by our microporous frit. We further demonstrate that ap-proximately 99% of Escherichia coli bacteria placed in the feedwaterwere killed or removed upon ow through our shock ED prototype, il-lustrating the potential for in-situ and additive-free disinfection inshock ED. In addition, we show that our prototype can continuouslyseparate electrochemically inactive ions by charge, consistent with the-ory [26] but contradicting claims that ICP acts as a virtual barrier to allcharged species [24].

2. Materials and methods

2.1. Shock ED device

A schematic and a photograph of our shock ED device are shown inFig. 2a and b. The setup consists of a cylindrical silica glass frit (Adams &Chittenden Scientic Glass),which is 1mmthick and has a 5mmradius.The frit is placed against a Naon membrane, and the membrane is indirect contact with the copper disk cathode. The frit is separated fromthe copper disk anode by a reservoir of copper sulfate (CuSO4) electro-lyte (3 mm thick reservoir). The frit has a random microstructure withpores roughly 500700 nm in diameter (Fig. 2c, d), an internal surfacearea (measured via BET) of am = 1.75 m2/g, porosity of 0.4, and a den-sity of m = 1.02 g/cm3. The pore surfaces are negatively charged, andthe magnitude of the charge depends on copper sulfate concentration[11]. The charged surfaces promote the (faster-than-diffusion) trans-

diameter green uorescent polymer microspheres (Thermo Scientic)and 50 nm diameter red uorescent nanoparticles (Thermo Scientic)into 1 mM CuSO4 solution. The concentration of these suspensionswas 20 mg/mL and 2 mg/mL, respectively. To demonstrate charged-based separation, positively and negatived charged uorescent dye so-lutions were also prepared. For positively charged dye, 2 105 g/mLof Rhodamine B uorescent dye was mixed into 1 mM CuSO4. The pHof this solution was measured to be 4.2, far enough below Rhodamine'sisoelectric point of 6 to ensure that the dye is positively charged[2729]. For negatively charged dye solution, 1 mg/mL of uoresceindye (Sigma-Aldrich) wasmixed into 1mMCuSO4 [30], and 50% volumeof isopropyl alcohol was added to ensure solubility.

In order to evaluate the disinfection capabilities of the device, weprepared suspensions of E. coli K12 (ATCC). The bacteria were culturedin LB broth at 37 with shaking, and when they reached log phase,were resuspended in 1.5MNaCl solution. This concentrationwas selectedto minimize osmotic shock upon transfer from the LB broth. After exper-iments were completed, the bacteria were stained with a BacLight live/dead staining kit as per the manufacturer's instructions (Invitrogen) andthus the live cells could be observed with a microscope. Control samplesof E. coliwere left suspended in the NaCl solution while testing was con-ducted, and little or no degradation was observed to their viability uponcompletion of the experiment.

2.3. Device operation

An electrochemical analyzer (Uniscan instruments PG581)was used

b

outle

t

it/m

79D. Deng et al. / Desalination 357 (2015) 7783port of positive copper ions to the cathode via electroosmotic ows,leading to overlimiting currents [11].

2.2. Sample preparation

A 1 M CuSO4 stock solution was prepared by dissolving 2.5 g ofCuSO4 (Science Company) into 10mL of deionized water. This stock so-lution was further diluted 10 times to obtain 0.1 M CuSO4 solution, andagain diluted to obtain 1 mM CuSO4 solution. To demonstrate size-based ltration, two suspensions were prepared by adding 50 m

anode (Cu)

cathode (Cu) Nafion membrane

a

c

reservoir

glass frit

3 m

m1

0.1

Fig. 2.Description of the prototype shock ED device used in this work. (a) A sketch of the fr

direction; (b) a photograph of shock ED device; (c) a SEM micrograph of glass frit showing itsto apply a voltage to the device. The analyzer's reference and counterelectrode leadswere connected to the anode, and theworking electrodelead was connected to the cathode. After about 10 min, the currentreached steady state, and collection of uid from the outlet of the devicebegan. As indicated in Fig. 2a, the uid ow was directed towards thecathode side of the device (to force ow through the depletion zone).Flow rate was precisely controlled by a syringe pump (Harvard Appara-tus), and the uid extraction time varied from several minutes to severaltens ofminutes in order to collect roughly 1mL ofuid from the outlet foraccurate post-experiment analysis.

1 m

d

working electrode

counter/reference electrode

outlet

1 cm

embrane/electrode sandwich structure (not to scale), where arrows indicate the uid ow

pore structure, and (d) enlarged micrographs indicating pore size around 500700 nm.

2.4. Microscopy

A Nikon TiU inverted uorescence microscope, a 4 or 10 objec-tive, and a Photometrics Coolsnap HQ2 CCD camera were used to cap-ture optical micrographs of samples of fresh solutions or solutionsfrom the outlet of the device 4 and 10 objectives were used in con-junction with a hemocytometer to count the bacteria. To count liveE. coli cells or particles, samples were loaded into a hemocytometer(INCYTO). The grid pattern on the hemocytometer is explicitly designedfor use with lower-powered objectives. In our case, the lower magni-cation was desirable in order to achieve decent counting statistics onlow concentration samples. Illumination was provided by a mercurylamp (Intensilight) for the uorescence images, and a standard lampfor the bright eld images. For E. coli, the live/dead cell counting wasdone using a Nikon C-FL B-2E/C FITC Filter for the live bacteria (dyedwith Syto BC, thus appearing green) and a Nikon C-FL Texas Red HYQfor the dead bacteria (dyed with Propidium Iodide, thus appearingred). Image analysis was performed using ImageJ software, either tocount the number of particles, count live and dead cells, or to measurethe integrated uorescent intensity of the uorescent dye solution. Inexperiments involving uorescent dyes, any photobleaching effectswere not signicant as we conrmed that there was no decrease inthe integrated uorescence of a control sample kept next to the deviceover the course of the experiment.

3. Results and discussion

3.1. Filtration based on size

700 nm, so micron-scale particles can potentially be largely excludedfrom the uid at the outlet.

In the rst experiment, the reservoir was lled with the suspensionof 50 m particles in copper sulfate solution. We then owed this sus-pension through the shock ED device (from anode to cathode), withoutapplying electric eld, and captured the efuent. Themicroscopy imagetaken of the inlet solution is shown in Fig. 3a, and here we can observethe particles as dark dots. As shown in Fig. 3b, the absence of particleswas observed in the outow solution, demonstrating that our shockED prototype successfully ltered out these particles.

In a similar experiment, we investigated the behavior of particleswith diameter less than the frit pore size using the suspension of50 nm-diameter particles in aqueous copper sulfate solution. From theimage of the inlet solution in Fig. 3c, the nanoparticles appeared to oc-culate into aggregates due to surface interactions [32,33]. Due to thespatial resolution limitations of optical microscopy, we could not ob-serve directly any unaggregated 50 nm-diameter particles potentiallypresent in solution. However, we observed that the aggregates werelarge enough to be ltered by porous medium, as no aggregates wereobserved in the efuent samples (Fig. 3d).

3.2. Disinfection

Removing waterborne pathogens is a critical part of manywater pu-rication processes [1,34]. State-of-the-art reverse osmosis (RO) desali-nation plants utilize dedicated post-treatment procedures for waterdisinfection, typically the addition of chlorine or chlorine by-products,to eliminatemost harmfulmicroorganisms [35]. The shock EDdevice in-

agutlet

80 D. Deng et al. / Desalination 357 (2015) 7783In contrast to electrodialysis [31], shock electrodialysis utilizes acharged porous glass frit placed between ion exchange membranes toenable overlimiting current [14] and propagate a deionization shockwave [12,13,15,16]. The frit in a shock ED system can also be used as alter to sterically exclude undesirable particles from theow (for exam-ple, particulates in feedwater in a water purication process). The glassfrit in our prototype has pore diameters between roughly 500 and

a

c

Fig. 3. Results demonstrating the size-based ltration of our shock ED device. Bright eld im(b) and (d). The particleswith 50 mdiameterwere completely removed from inlet (a) to o

(d). Scale bars are 200 m.vestigated here utilizes a glass frit with submicron-radius pores that canpotentially serve as alter to removemanybiological species duringde-ionization, eliminating theneed for any post-treatments. In addition,wepostulate that the high electric elds present in the deionization regionmay further reduce the survival rate of anymicroorganisms thatmake itthrough the glass frit.

To demonstrate the disinfection functionality in our device, wemea-sured the change in concentration and viability of a suspension of E. coli

d

b

es for the feedwater (a) and (c), and images on a sample from the corresponding outow(b). Aggregates of 50-nm-diameter nanoparticleswere alsoltered from inlet (c) to outlet

between the inlet and outlet of the device. It is difcult to assess the im-pact of the shock electrodialysis process for disinfection in the originalexperiments because copper sulfate solution is not a good medium forcells, so a different electrolyte, 1.5 M sodium chloride (NaCl), wasused for these experiments to ensure bacteria viability (see theMaterials and methods section.) A sample of the inlet E. coli solution isshown in Fig. 4a. The bacteria were stained with a live-dead kit priorto microscopy, and so live bacteria appear green, and dead as red.Using our microscopy setup, we performed a cell counting analysiswhich showed that the inlet sample contains bacteria which were99 0.4% viable. Outlet samples were taken under two conditions:onewith a ow rate of 0.2 L/min through the device under the appliedvoltage at 1.5 V, shown in Fig. 4c, and the other with the glass frit re-moved and without any applied voltage, as shown in Fig. 4b. WhenFig. 4b is compared with Fig. 4a, there is an observable drop in concen-tration, possibly due to adhesion at points in the system, but the keypoint is that the bacteria are still largely viable with only very fewdead cells.

The data demonstrate signicant disinfection that depends on forcedconvection through the porous medium, as well as the action of the ap-plied electric eld. When Fig. 4c is compared with Fig. 4a, using the sys-temwith the frit results in yet lower concentrations, and larger numbersof dead cells. More quantitatively, with the frit removed, cell viabilitywasmeasured to be to 96.4 0.9%. With the frit in place, we measuredamuch reduced viability of the bacteria in the outlet sample, whichwas28 8%.We also observed (with the frit in place and under the appliedvoltage) a strong reduction in the concentration of cells in the outletsample, as shown in Fig. 4c. Here, in the outlet sample, 96.5 1.8% ofbacteria were absent relative to the reservoir sample. Combined withthe reduced viability of the outlet sample, roughly 1% of initially viable

We further hypothesize that the strong electric elds arising in theion depletion zone can also reduce further the viability of the bacteriapassing through the device [37]. Previous studies have shown that anelectric eld of roughly 0.82 V/m can promote cell death [38,39].However, we were unable to reliably test this hypothesis in this work,because the use of sodium chloride rather than copper sulfate likelyinhibited the formation of a concentration shock (and thus a strongelectric eld) in our prototype. With sodium chloride (and copper elec-trodes), our device relied on water electrolysis electrode reactions rath-er than copper redox reactions to pass a current through the device,which can cause zones of perturbed pH to enter the system [40]. Futurework will develop prototypes capable of generating shocks in sodiumchloride and other general electrolytic solutions, allowing us to testthe effect of depletion zone electric elds on bacteria survival.

3.3. Separation based on charge

In microuidic devices leveraging ICP to perform molecular samplestacking [41] and water desalination [24], the ion depletion zone wasreported to act as a virtual barrier for all charged particles (bothpositively and negatively charged). A possible mechanism for suchcharge-independent attraction is diffusio-phoresis [4244], in whichcharged particles climb a salt concentration gradient in the absence ofan applied electric eld. When current is being passed, however, thiseffect competes with classical electrophoresis in the joint phenomenonof electro-diffusiophoresis studied by Malkin and A. Dukhin [45].Rica and Bazant have shown that electrophoresis generally dominatesdiffusiophoresis in large concentration gradients due to the enhancedelectric eld of the depleted solution [26], thus enabling separation bycharge in particle ows through regions of ICP. Recently, Jeon et al. an-

c

cterittledisi

81D. Deng et al. / Desalination 357 (2015) 7783bacteria remained present and viable in the outlet sample when the fritwas in place. We hypothesize that the latter results are due to both stericexclusion of amajority of the bacteria from the frit pore space (E. coli havetypically micron-scale dimensions [36]), and an inhospitable environ-ment to the bacteria which are able enter the glass frit.

a

b

Fig. 4. Results demonstrating the disinfection of a feedwater containing E. coli as amodel bathe device without the porous frit and at zero voltage, showing some cell adsorption but lincluded at an applied voltage (1.5 V) and outlet ow rate (0.2 L/min), showing strong

red. Scale bar is 200, 500, and 500 m, for (a)(c) respectively.alyzed the forces acting on charged particles owing through the deple-tion region in microuidic ICP with negatively charged channel wallsand reached consistent conclusions, that negatively charged (counter-ionic) species are repelled from the depletion zone via strong electro-phoretic forces, while positively charged species cannot be repelled

ia. (a)Microscopic image of the feedwater, (b) image of a sample from the outlet stream ofdisinfection (cell death), and (c) an outlet stream image from the device with porous fritnfection and ltration. The live bacteria are uorescent green, and the dead bacteria are

[37]. Jeon et al. further developed a device which separated negativelycharged particles via the deection of their path through the depletionzone (where the extent of deection correlated to the particles' electro-phoretic mobility) [37].

In this work, we show that ICP can in fact accelerate the passage ofpositively charged species, thus contradicting the virtual barrier claimand demonstrating, apparently for the rst time, charge-based separa-tion by the ion depletion zone. Naturally this nonlinear electrophoreticeffect only applies to particles small enough to pass through the poresand avoid ltration. In order to demonstrate the effect of the depletionzone on positively charged species, we injected the positively chargedRhodamine dye solution into the device reservoir, and applied a con-stant voltage of 1.5 V. Note that Rhodamine is considered a non-electrochemically active species, as it does not participate in the elec-trode reactions (electrode reactions include the positively charged cop-per ions). The solvated molecule has an effective size of only a fewnanometers, so no size-based ltration takes place in our device.Based on the applied electric eld, the positively charged dye was ex-pected to move via electrophoretic forces towards the cathode wherethey accumulate, forming an enrichment region near the outlet. Theoutlet and reservoir concentrations of Rhodamine were calculated byinjecting samples into a hemocytometer chip, andmeasuring their inte-

4. Discussion and conclusion

In summary, we have demonstrated that shock ED devices can per-form many functions in addition to (and simultaneously along with)water desalination, including ltration, disinfection and separations bycharge and by size. As shock ED employs a microporous frit within theow channel between the membranes, we were able to demonstratesteric ltration of microscale particles and aggregates of nanoscale par-ticles. Further, we were able to kill or remove approximately 99% of vi-able E. coli bacteria present in the feedwater upon owing through theshock ED device with applied voltage. We hypothesize that further re-ductions in bacteria viability are achievable via the presumably strongelectric elds present within the shock region, and future work willfocus on building a prototype capable of testing this hypothesis. Lastly,we demonstrated that our shock ED device can continuously separatepositively charged species from negatively charged ones that are smallenough to pass into the porous frit.

These demonstrated functionalities can be applied towater purica-tion systems (ltration, disinfection), as well as particulate, molecularor biological separation systems, and demonstrate the potential ofshock ED as a versatile new technique for chemical engineering. As inour initial experimental publication [11], here we used a simple rst

b

iontio oumuw rnit

82 D. Deng et al. / Desalination 357 (2015) 7783grated uorescent intensities (I) using an optical microscope.As expected, the dye concentration at the outlet was observed to be

greater than at that of the reservoir. Quantitatively, we dened the en-hancement ratio of uorescent intensity as Iinlet/Ioutlet, where Iinlet is theintegrated uorescent intensity of solution in the inlet or reservoir,and Ioutlet is the integrated uorescent intensity of solution extractedfrom the outlet. This enhancement ratio is shown in Fig. 5a as a functionof extraction ow rate. As the ow rate decreased, the efuent becamemore enriched in Rhodamine as the integrated uorescent intensity ofsolution increased.Whennovoltagewas applied to the shock EDdevice,as expected, no concentration enhancement was observed in the efu-ent solution.

The experimentwas then repeatedwith thenegatively charged uo-rescein dye solution. In contrast to the positively charged dye, the uo-rescein was expected tomigrate electrophoretically towards the anode,thus being repelled from the depletion zone by the large electric eldthere. When the dye concentration was measured quantitatively at theinlet and outlet (at the cathode-side), we observed a depletion of uo-rescein at the outlet as expected. The removal ratio, dened as Iinlet/Ioutlet, is shown as a function of ow rate in Fig. 5b, and this ratio in-creased with increasing ow rate.

a

Fig. 5. Results demonstrating the charge-based separation of non-electrochemically activecharged dye versus ow rate of feedwater through the device. Enhancement ratio is the raEnhancement ratio is always greater than unity, indicating that positively charged dye acccreased. (a) The observed removal ratio of a uorescent, negatively charged dye versus oof the outlet sample to the feedwater sample, but in this case, this ratio is always below u

ratio increases as ow rate is increased.prototype that sustains direct current by electrodeposition/dissolutionreactions at copper electrodes and does not separate the brine producednear the anode, as required for continuous operation. We are currentlybuilding and testing a proposed scalable prototype capable of con-tinuous desalination and water purication of arbitrary feed water(see Fig. 6 of Ref. [11]). The complete shock ED system consists of astack of negatively charged porousmedia separated by cation exchangemembranes with electrode streams sustaining the current by waterelectrolysis.

An important potential application could be to treat produced waterfrom hydraulic fracturing of unconventional oil and gas reservoirs [46].It has recently been argued that classical ED is an economically viabletechnology to address this grand challenge in water treatment, if mem-brane fouling and pre/post-processingwere not overly costly issues [47].Our results suggest that shock ED could be even more attractive, asit retains most of the benets of classical ED, while incorporatingltration and charge-based colloidal separation in a single compactsystem. Moreover, the demonstrated electrical disinfection capabili-ty should dramatically reduce fouling of the cation exchange mem-branes, thus eliminating a major lifetime and cost concern. Unlikeclassical ED, any size-ltered or electrophoretically separated particles

s by our shock ED device. (a) The observed enhancement ratio of a uorescent, positivelyf dye concentration (uorescent intensity) of the outlet sample to the feedwater sample.lated in the depletion zone of the device. Enhancement ratio decreases as ow rate is in-ate. Removal ratio is also dened as the ratio of dye concentration (uorescent intensity)y indicating that negatively charged dye was repelled from the depletion zone. Removal

are prevented from reaching the membranes, thereby further reducingclogging and fouling of the most expensive component of the system.In contrast, the frit or other porous medium is cheap and could beperiodically replaced.

[20] J.-H. Han, E. Khoo, P. Bai, M.Z. Bazant, Over-limiting current and control of dendriticgrowth by surface conduction in nanopores, Sci. Rep. 14 (2014) 7056.

[21] S. Nam, I. Cho, J. Heo, G. Lim, M.Z. Bazant, G. Sung, S.J. Kim, Experimental Veri-cation of Overlimiting Current by Surface Conduction and Electro-osmoticFlow in Microchannels, 2014. (arXiv:1409.2956 [physics.u-dyn]).

[22] T.A. Zangle, A. Mani, J.G. Santiago, On the propagation of concentration polarizationfrom microchannel/nanochannel interfaces. Part II: numerical and experimental

83D. Deng et al. / Desalination 357 (2015) 7783ple, in order to avoid the need for Faradaic reactions, such aswater split-ting, to sustain the current, shock ED could also be performed withblocking porous electrodes to drive salt depletion, as in (membrane[48]) capacitive deionization [49,50]. The use ofmetal electrodepositionto sustain the current at the cathode, even without a membrane, couldalso enable selective metal separation to be added as yet another simul-taneous functionality. The dissolved molecules or colloidal particleswould separate via shock-ED fractionation in the leaky membranein cross ow, while dissolved metal ions could be selectively removedby shock electrodeposition at the cathode [20], all in one continuousunit process.

Acknowledgments

This work was supported by grants from the Weatherford Interna-tional and the MIT Energy Initiative. W. A. would like to thank theDepartment of Chemical Engineering of Ecole Polytechnique deMontrealfor the internship support at MIT.

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Water purification by shock electrodialysis: Deionization, filtration, separation, and disinfection1. Introduction2. Materials and methods2.1. Shock ED device2.2. Sample preparation2.3. Device operation2.4. Microscopy

3. Results and discussion3.1. Filtration based on size3.2. Disinfection3.3. Separation based on charge

4. Discussion and conclusionAcknowledgmentsReferences