Desalination 343 (2014) 8–16

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Desalination

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Solute and water transport in forward osmosis using polydopaminemodified thin film composite membranes

Jason T. Arena a, Seetha S Manickam a, Kevin K. Reimund a, Benny D. Freeman b, Jeffrey R. McCutcheon a,⁎a University of Connecticut, Department of Chemical and Biomolecular Engineering, Center for Environmental Sciences and Engineering, Storrs, CT 06269, USAb The University of Texas at Austin, Department of Chemical Engineering, Austin, TX 78712, USA

H I G H L I G H T S

• Reverse osmosis membrane support layers were modified with polydopamine.• The modified membranes were tested under forward osmosis conditions.• An ammonia–carbon dioxide draw solution was used for the testing.• Sodium ion rejection was found to be low, while chloride ion rejection was high.• Differing ion rejection is an evidence of ion exchange between the feed and draw.

⁎ Corresponding author. Tel.: +1 860 486 4601; fax: +E-mail address: jeff@engr.uconn.edu (J.R. McCutcheon

0011-9164/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.desal.2014.01.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 August 2013Received in revised form 4 December 2013Accepted 2 January 2014Available online 2 April 2014

Keywords:Forward osmosisPressure retarded osmosisThin film composite membranePolydopamineMembrane modification

Forward osmosis is a rapidly emerging technology that has potential to enable low cost water treatment anddesalination. Previous investigations have found that reverse osmosis (RO) membranes were unsuitable forforward osmosis in part due to their hydrophobic support layers, which inhibit wetting. Poor wetting hin-ders water and solute transport in the support layer, dramatically increasing the severity of internal concen-tration polarization. In this study, ROmembrane support layers were modified with polydopamine (PDA) toincrease their hydrophilicity and promote wetting. The results indicate that the modified RO membranesexhibited a four to six fold increase in forward osmosis (FO) water flux under test conditions relative tounmodified membranes. Additional tests were performed under model desalination conditions using anammonia–carbon dioxide draw solution with a sodium chloride feed. The sodium and chloride rejectionswere measured independently and in some instances substantial differences were observed. Additionally sodi-um and chloride rejections were lower than anticipated with a peak rejection of 90%. The substantial differencebetween sodium and chloride rejections was attributed to a cationic exchange effect between the draw and feedsolutions.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Forward osmosis (FO) is an emerging process being consideredfor the desalination, purification, and treatment of water [1–6]. Afunctional FO process requires an easily recoverable draw solutioncapable of generating high osmotic pressures as well as a highly pro-ductive and selective membrane [1,4,7]. Various draw solutes exist,but only the ammonia–carbon dioxide (NH3–CO2) draw solutionhas been demonstrated as both an effective and recyclable solutethat may enable osmotically driven desalination [1,4,7–10]. Amongstthe most commonly studied membrane for forward osmosis is theasymmetric cellulose acetate (CA) manufactured by Hydration Tech-nology Innovations™ (HTI) [1,4,9–12,16]. This membrane's morpholo-gy has been optimized for use in osmotically driven membrane

1 860 486 2959.).

ghts reserved.

processes [12]. The CA membrane while offering acceptablepermselectivity and desirable hydrophilicity has inherent chemicalcompatibility drawbacks, notably hydrolysis in alkaline conditions [13–15]. Hydrolysis reduces salt rejection, which in FO translates to higherdraw solute cross-over and a lower osmotic pressure difference acrossthe membrane [14,15]. The NH3–CO2 draw solution will hydrolyze CAas this draw solution can be expected to have pHs above 7.7 [16,17].

This leads to the consideration of alternative membrane chemistriesfor use with the NH3–CO2 draw solution. The commercial alternativeto the CA membranes is the thin film composite (TFC) membraneplatform. These membranes, typically used in reverse osmosis, com-prise an ultra-thin aromatic polyamide layer supported by a polysulfone(PSu) or polyethersulfone (PES) layer that has been cast onto a polyes-ter (PET) nonwoven [18]. Each of these layers is capable ofwithstandinga broad range of pH and temperature conditions making them suitablefor usewith theNH3–CO2 drawsolution.Despite these desirable charac-teristics for use FO processes early studies which attempted to use TFC

9J.T. Arena et al. / Desalination 343 (2014) 8–16

membranes in FO found the performance of TFC ROmembranes to beinferior to that of HTI's CA FO membrane [1,2]. In later work, the lackof TFC support layer wetting was demonstrated as a hindrance to os-motic flux due to a reduced effective porosity and enhanced internalconcentration polarization (ICP) [19,22]. To address this problem theuse of TFC membranes with an intrinsically hydrophilic supportwould be desirable. This would require a retuning of the delicate in-terfacial polymerization process, which can be impacted by the sup-port layer properties [21,22]. Furthermore, hydrophilic supportsmay plasticize in the presence of water and cause damage to thefragile selective layer. Ideally, one could start with a TFC membranemade from a non-swelling hydrophobic support that also exhibitsgood permselectivity; then modify that membrane's support layerto increase its hydrophilicity. Recently commercial TFC FO mem-branes have just begun to enter the market with limited availability,with only HTI providing theirs for sale at the time of writing[6,11,23,24].

A recently developed technique to impart a hydrophilic characteronto microfiltration, ultrafiltration, and reverse osmosis membrane se-lective layers for enhanced fouling resistance to oil/water emulsionsand protein mixtures was reported by McCloskey and co-workersusing polydopamine (PDA) [25–28]. PDA is a polymer with a chemistrysimilar to the adhesive secretions of mussels [29–31]. It is formed fromthe spontaneous polymerization of dopamine in an alkaline aqueous so-lution. A subsequent study byArena examined thefirst use of PDAmod-ified membranes for osmotically driven membrane process. This wasdone through the application of PDA to TFCmembrane support layer(s).Significant improvements in the water flux of PDA modified TFC ROmembranes were observed in the pressure retarded osmosis (PRO) ori-entation [32]. Others, such as Han, adopted this technique prior to syn-thesis of the membrane [33].

With the improved performance of these membranes in the PROmode, we hypothesized that a similar improvement would be possiblein the FO mode as well. The excellent selectivity of these membranesas well as tolerance to the often used ammonia–carbon dioxide drawsolution make such a platform appealing. Membranes were tested fordesalination performance using this draw solution in hopes of demon-strating the promise of these modified membranes; however, it wasfound that rejection, especially for cations, was far lower than anticipat-ed. This was attributed to an ion exchange phenomenon occurringthrough the polyamide selective layer.

2. Materials and methods

2.1. Selected membranes and chemicals

The membranes selected for this investigation are the DowWater &Process Solutions™ BW30 and SW30-XLE. Both membranes' supportlayers are made of PSu supported by a PET nonwoven [34]. Thesemem-braneswere chosen for their high permselectivity, use in earlier studies,and reported properties [46]. Sodium chloride, tris–hydrochloride, sodi-um hydroxide, ammonium bicarbonate, and ammonium hydroxidewere purchased from Fisher Scientific (Pittsburgh, PA). Dopamine-hydrochloride was purchased from Sigma-Aldrich (St. Louis, MO).Isopropanol, sodium tetraphenyl boron, potassium chromate, calciumnitrate, and silver nitrate were purchased from Acros Organics (Geel,Belgium).Water used in this studywas ultrapureMilli-Qwater produceby aMillipore Integral 10water system (Millipore Corporation, Billerica,MA).

2.2. PDA modification of TFC membranes

The PDA modification followed the procedure set forth in previouswork [32]. Since the PDA formation only occurs in the aqueous phase,it was necessary to prewet the support in isopropanol (IPA) prior toPDAmodification. The supportwas soaked IPA for 1 hr and thenwashed

in a series of three deionizedwater baths for 45min each. Following theIPA wetting and DI water rinsing, the membranes were stored in DIwater at 4 °C before being modified with PDA. The dopamine polymer-ization took place within a custom built coating container where themembrane separates two reservoirs. This container ensures that nearlyall of the PDA polymerizes within the PSu layer and not the selectivelayer (which would negatively impact permeability [26–28]). Bothsides of the membrane were placed in contact with a pH 8.8 Tris buffersolution. Dopamine-HCl was added to the solution in contact withmembranes' PSu support layers to bring the support layer coating solu-tion to a concentration of 2 g·L−1 dopamine. Polymerization occurs atroom temperature with non-agitated solutions exposed to the air. ThePDA polymerization can be observed upon the addition of dopaminewhere the formation of PDA is indicated by the change in color of thepolymerizing dopamine solution from clear to orange and finally tobrown.

2.3. Mercury intrusion porosimetry

Amercury intrusion porosimeter (MIP) (AutoPoreIV, Micrometrics)was used to characterize the membranes for pore diameter and totalpore volume. TheWashburn equationwas used to calculate the pore di-ameters from the intrusion pressure.

d ¼ −4γ � cos θð ÞP

: ð1Þ

In Eq. (1), P is the intrusion pressure (MPa), d is the pore diameter(μm), γ is the surface tension of mercury (485 dyn·cm−1) and θ is thecontact angle of mercury (a value of 130° was assumed) with the sam-ple. The sample was tested in the pressure range of 1–720 bar. It is to benoted that Eq. (1) assumes that measured pore diameters are cylindri-cal. While this assumption is idealized for themembrane supports test-ed in this study, the resulting values for d calculated in Eq. (1) representthe equivalent cylindrical pore diameters of the support. It is also to benoted that the intrusion technique can detect both through and blindpores but not closed pores [38].

2.4. Fourier transform infrared spectroscopy

The modified TFC RO membranes were tested in Fourier transforminfrared (FTIR) spectroscopy to examine the surface functionalgroups of the membranes' selective layers. Membranes were tested,after drying, in a Thermo Scientific (Waltham, MA) Nicolet iS10 FTIRspectrophotometer with Smart iTR attachment was used to performthese measurements on a dried membrane. Measurements were takenon the selective layer using 64 scans with a resolution of 4 cm−1.

2.5. Osmotic flux testing of modified membranes

2.5.1. Sodium chloride as the draw soluteBoth neat andmodified TFC ROmembranes were tested under os-

motic flux conditions with the membrane oriented in the FO mode(with the support layer facing the draw solution) [35]. Bothmembraneswere tested in each of the four following varieties neat (used as-received), no PET (where the PET backing has been removed from themembrane), one hour PDA modified, and forty-two hour PDAmodifiedmembranes (both modified using the method reported above). Mem-branes not modified with PDA were tested following storage in DIwater. Prior to testing no wetting technique was implemented. Themembrane area exposed to the feed and draw solutions were ap-proximately 19 cm2 (3 in.2). Sodium chloride (NaCl) was used asthe draw solute at concentrations of 0.05, 0.1, 0.5, 1.0, and 1.5 M.The osmotic flux testing procedure has been described previously[11,22,32,37,39]. Temperature was maintained at 23 ± 1 °C. Fluxwas measured gravimetrically using a balance (Denver Instruments

Fig. 1.Porosity data fromMIP of BW30 (solid) and SW30-XLE (cross-hatched)membranes(neat, no PET and PDA modified).

10 J.T. Arena et al. / Desalination 343 (2014) 8–16

PI-4002, Denver Instruments Bohemia, NY) connected to a computermeasuring the mass of the draw solution tank once per minute. Theosmotic pressures produced by these draw solutions (as presentedin the figures) were calculated using the van't Hoff equation. Testswere run in triplicate using fresh membrane samples. Reverse soluteflux was monitored by measuring of the feed solution conductivity.

2.5.2. Determination of the effective structural parameterThe structural parameter is a measure of the effective diffusive dis-

tance of a solute through a porous media [5,11,35–37]. Solutes andwater are assumed to be only capable of diffusing only through awettedpore, thus a lack of wetting can have a large impact on the effectivestructural parameter of a porous material [19,21,32]. The importanceof the structural parameter is shown in the governing equation forwater flux in the FO orientation, which including feed solution external

Fig. 2. Pore diameter histograms from MIP for of BW30 and SW3

mass transfer limitations can be represented by the following equa-tion [37].

Jw ¼ Aπd;b exp − Jw � S

D

� �−π f ;b exp − Jw

k

� �

1þ BJw

expJwk

� �− exp − Jw � S

D

� �� �8>><>>:

9>>=>>;

ð2Þ

In Eq. (2), Jw is the water flux (m·s−1), A is the water permeance(m·s−1·bar−1), πd,b is the osmotic pressure of the draw solution(bar), S is the structural parameter (m), D is the solute diffusivity inwater (m2·s−1), t is the thickness of the membrane support layer(s)(m), πf,b is the osmotic pressure of the feed solution (bar), and B is thesolute permeability (m·s−1). Osmotic pressures can be calculatedusing the van't Hoff equation [38]. The water permeance (A) and solutepermeability (B) are commonly determined using reverse osmosis [32].The structural parameter can be determined from a numerical solutionto Eq. (2) using experimental water flux data. The structural parameteris often defined as a function of support layer thickness (t), porosity (ε),and tortuosity (τ) (S= tτ / ε), and is representative of the effective dif-fusion distance through the support; however, rather than measuringeach of these values individually (which can be difficult to do accurate-ly),fitting S to themodel above using experimental data provides an “ef-fective structural parameter” since the approach accounts for poorwetting in the support since these regions not available for solute trans-port [19,21,32].

2.5.3. Ammonia–carbon dioxide draw solutionPDA modified TFC membranes were tested for NaCl rejection in

forward osmosis using an NH3–CO2 based draw solution. Thesetests were performed in a laboratory scale osmosis test systemsusing a 2.0 M draw solution with an ammonia to carbon dioxideratio of 1.2:1 on a molar basis and a feed solution of 0.25 M sodiumchloride. These solutions were run counter-current with a crossflowvelocity of 0.25m·s−1 at 23±1 °C,matching the testing conditionsusing the NaCl draw solution. The membrane support layer was in con-tact with the NH3–CO2 draw solution (FO mode). Experiments were

0-XLE membranes (with PET removed and PDA modified).

Fig. 3. FTIR spectra of the selective layer for PDAmodified commercial TFCmembranes atwave numbers from1800 to 600 cm−1. Thepeaks are consistentwith those typically found for thefully aromatic TFC [45].

11J.T. Arena et al. / Desalination 343 (2014) 8–16

also run for a short timewith the draw solution against a DIwater feed tomeasure the pure water flux for the NH3–CO2 draw solution.

Loss of draw solutes via permeation through the membrane neg-atively impacts the overall cost and efficiency of FO processes be-cause draw solutes that are lost must be replaced after drawsolution recovery [6,40,41]. Unrecovered draw solutes may also con-taminate the brine complicating its disposal. The flux of ammoniaspecies (both as ammonia and ammonium) from the draw to the feedsolution was measured gravimetrically using sodium tetraphenylboron as a precipitating agent [42,43]. A small sample of feed solutionwas removed from the feed tank and analyzed. When added to a solu-tion containing ammonia species, ammonium tetraphenyl borate isformed and precipitates out of solution. This precipitate was capturedusing fine porosity filter paper, washed with 1 °C DI water, dried, andmassed on an analytical balance (Denver Instruments PI-114, DenverInstruments Bohemia, NY). Following filtration of the ammoniumtetraphenyl borate mixture a small amount of sodium tetraphenylboronwas added to the filtered solution to ensure that all of the ammo-nia species in solution were precipitated.

Sodium flux was determined from a mass balance based on thefinal concentration of sodium in the draw solution, analyzed via atomicabsorption spectroscopy in a Perkin-Elmer 3100 AA (Perkin-Elmer,

Fig. 4. FTIR spectra of the selective layer for both PDAmodified and unmodified commer-cial TFC membranes at wave numbers from 3700 to 2700 cm−1. The peak from 3000 to2800 is likely attributed to solid state hydrogen bonded hydroxyl stretch in the polyamidelayer's carboxylic acid moieties [49].

Waltham, MA) equipped with a sodium cathode lamp (Perkin-ElmerIntensitron Part# 303-6065, Perkin-Elmer, Waltham, MA). Solutionswere analyzed using an air-acetylene flame with the detector setat 589 nm. Standard solutions were made with sodium chloride indiluted ammonium bicarbonate solution at concentrations rangingfrom 2 to 12 ppm. The instrument was blanked against an ammoniumbicarbonate draw solution with the same dilution factor as the sodiumchloride-containing draw solution. Ammonium bicarbonate draw sam-ples were diluted to give an absorbance in the range of the standardsolutions.

Chloride flux was determined from a mass balance based on thefinal concentration of chloride in the draw solution, which was de-termined using theMohr titration [42]. In theMohr titration chlorideis titrated with silver nitrate in the presence of a potassium chromateindicator. At the end point of the titration excess silver ions formsilver chromate producing a reddish brown color within the solution.Due to the presence of bicarbonate in the draw solution being ana-lyzed the solution was boiled to dryness prior to the titration to vol-atilize all of the ammonia and carbon dioxide within the solution.Following drying, the residual solutes were rehydrated and theresulting solution was titrated. A complete validation of this tech-nique is presented in the Supplementary material.

3. Results and discussion

3.1. Porosimetry characterization

Mercury intrusion porosimetry (MIP) was performed on bothmodified and unmodified membranes to examine the effect of thePDA modification on membrane support layer pore diameters andporosity. As shown in Fig. 1, the porosity for both membrane types(i.e., BW30 and SW30-XLE) decreased as a result of polydopaminedeposition, and the samples exposed to the dopamine coating solutionfor a longer time (i.e. 42 hr) had a lower porosity than those treatedfor only 1 hr. This decrease in porosity directly competes with the in-creased wettability as measured by contact angle goniometry as report-ed by Arena [32].

Fig. 2 presents the effective pore size distribution for themembranesconsidered in this study. Therewereminimal changes in the pore diam-eter distribution formembraneswith higher coating times. Thesemem-branes exhibited a slight shift toward smaller pores, but given thethinness of PDA layers [26,44] the pore diameter distributions do notchange dramatically. Care should be taken when scrutinizing MIP datatoo closely as the high pressures employed by cause irreversible samplecompression and skew results; however, for comparative purposes the

Fig. 5.Osmoticflux performance of BW30 and SW30-XLEmembranes (neat, PET removed and PDAmodified) at 23± 1 °C, 0.25m/s feed and draw cross-flow velocity, and no transmem-brane hydrostatic pressure.

12 J.T. Arena et al. / Desalination 343 (2014) 8–16

unmodified and modified membranes would deform similarly and sothis technique is reasonable for comparing porosity changes.

3.2. FTIR spectra

The FTIR spectra for these membranes, shown in Figs. 3 and 4, arecharacteristic for those membranes based upon a fully aromaticpolyamide [45]. The strong similarities in the FTIR spectra for theBW30 and SW30-XLE membranes are to be expected given theircommon lineage stemming from the FT30 membrane originally de-veloped by Cadotte [46,47]. Also, based upon the FTIR spectra PDAcannot be detected. This is unsurprising many of the functionalgroups characteristic of PDA are already present in an aromatic poly-amide, which based upon the structure proposed by Dreyer consistsof an indole or indoline like structure (containing a N–H), carbonyland hydroxyl functional groups [48]. Overall the application of PDAto the membrane support layers does not appear to significantlyalter the surface functional groups of themembrane's selective layer.

An interesting peak of the spectra (found in Fig. 4) is the 3000–2800 cm−1 peak. This peak can be only attributed to a hydrogenbonded hydroxyl stretch of a solid state carboxylic acid [49]. Thispeak implies incomplete cross-linking between the trimesoyl chlorideand m-phenylene diamine monomers of the polyamide, producing afunctional group that can be expected to de-protonate at elevatedpHs. This deprotonation of the polyamide selective layer would giverise to negative surface charges of the membranes as detailed in theliterature [50,51]. Additionally, deprotonation of carboxylic acid groupsof a polyamide can also be attributed to improved rejections of thesemembranes at slightly basic pHs [51,52]. As will be discussed below,

Fig. 6. Salt (sodium chloride) flux of BW30 and SW30-XLEmembranes (neat, PET removed andbrane hydrostatic pressure.

these charged groups may play a significant role in other transport pro-cesses during FO.

3.3. Osmotic flux performance

3.3.1. Water flux for a sodium chloride draw solutionFig. 5 shows that osmotic water flux was increased significantly

following modification of the BW30 and SW30-XLE membraneswith PDA. PDA modification caused water flux to increase by up toa factor of 4 for the BW30 and up to a factor of 6 for the SW30-XLEmembrane. This observation is similar to those reported previously,where the PDAmodified BW30 and SW30-XLEmembranes exhibitedan 8 and 12 fold increase in flux, respectively [32]. The 42 hour PDAmodified membranes showed slightly decreased (but not statistical-ly significant) water flux when compared to the 1 hour PDAmodifiedmembrane. This can be explained to be a result of decrease porositywithin the membrane support layers as shown in Fig. 1. The increasewater flux for the PDAmodifiedmembranes may be attributed to theincreased wettability of membrane support layer increasing the rateof draw solute transport through the membrane support layer. Thiswill increase the concentration, and osmotic pressure, of the drawsolution at the membrane interface.

3.3.2. Reverse solute flux for a sodium chloride draw solutionThe salt flux increased (Fig. 6) after PDAmodification for both the

BW30 and SW30-XLE membranes as a result of the improved wetta-bility of the membranes' support layer. As support layer wettingimproves, solutes can more easily diffuse through a membrane's

PDAmodified) at 23±1 °C, 0.25m/s feed and draw cross-flowvelocity, and no transmem-

Fig. 7. Structural parameters of BW30 (solid bars) and SW30-XLE (cross-hatched bars)membranes (neat, no PET and PDA modified), calculated from RO data presented inArena et al. [32].

13J.T. Arena et al. / Desalination 343 (2014) 8–16

support layer. This increases the concentration of those solutes at theselective layer interface and results in increased solute flux.

3.3.3. Membrane structural parametersEffective structural parameters for the membranes considered in

this study were calculated using Eq. (2). Water permeance and sodiumchloride permeability values reported in Arena [32] were used for thisanalysis. As shown in Fig. 7, removal of the PET backing layer resultedin a 70% reduction in the effective structural parameters for both theBW30 and SW30-XLE. Following removal of the PET layers these mem-branes still exhibit structural parameter orders of magnitude higherthan their structure would suggest is possible based upon their thick-ness and porosity [53].

This finding suggests that the poor wetting of the PSu layer is theprimary cause of the high effective structural parameters for both theBW30 and SW30-XLE; however, poor wetting of the PSu layer seemsto be more severe for the SW30-XLE as shown by this membrane'shigher effective structural parameters. Modification of thesemembrane's PSu layer with PDA resulted in a near order of magni-tude decrease in the effective structural parameter for both mem-branes. This result is particularly interesting given that membraneporosity is reduced by the PDA coating process, as shown in Fig. 1.That is, the resistance to mass transfer has been reduced due to

Fig. 8. Osmotic flux data for pure water (solid bars) and 0.25M sodium chloride (cross-hatchedcross-flow velocity, and no transmembrane hydrostatic pressure.

PDA coating despite the fact that the porosity of the support layeris decreased as a result of PDA coating.

3.4. Desalination performance of PDA modified TFC membranes

3.4.1. Water flux in forward osmosis desalinationBy comparing the pure water fluxes for the NH3–CO2 draw solution

in Fig. 8 to water fluxes for a NaCl draw solution presented in Fig. 5 itbecomes apparent that the NH3–CO2 draw solution produces similarwater fluxes the 1 M sodium chloride draw solution under these testconditions. Upon addition of sodium chloride water, fluxes de-creased by more than 50%. This is likely due to external concentra-tion polarization effects, increasing the osmotic pressure of thefeed solution at the membrane selective layer interface.

3.4.2. Solute flux in forward osmosis desalinationReverse solute flux was measured for the ammonia species per-

meating through the membrane in both the molecular and ionicforms (as ammonia and ammonium respectively) from the draw so-lution into the feed solution. The ammonia species crossover wasmeasured between 0.75 and 0.9 mol·m−2·h−1. Ammonia being polarmolecule like water and of similar size to water with a more mobilehydration shell than ammonium [54] prevents themembrane fromeas-ily discriminating between water and ammonia molecules [15].

Sodium and chloride ion fluxes are given in Fig. 9. As would be ex-pected, the SW30-XLE exhibited significantly lower forward sodiumflux (cross-hatched bars) than the BW30 due to its higher selectivity.On the other hand, chloride flux is dramatically lower for both mem-branes. This was an unanticipated finding since, in early studies on FOdesalination using HTI's CA membrane and this draw solute foundhigh NaCl rejections [1,20].

The unequal sodium and chloride ion fluxesmustmean that a cationfrom the draw solution is moving to the feed solution from the drawsolution, since electroneutrality must be maintained. The only cationavailable in the draw solute is ammonium. It is interesting to notethat in all instances the ammonia flux was greater than or equivalentto the sodium flux. This supports evidence of ion exchange since itwould close the mass balance for both ammonia and sodiummovingbetween the two solutions.

The ion flux data is converted to rejection values in Fig. 10 (doneby multiplying forward solute flux by water flux to determine theconcentration of water passing through the membrane then dividingthis by the concentration of the feed water). The SW30-XLE hadbetter sodium and chloride rejection under these process conditionswith around 65% rejection of sodium and 85–90% rejection of chlo-ride for both the 1 hour and 42 hour PDAmodification. The BW30 ex-hibited a large disparity in sodium and chloride rejections. The

bars) feed solutions against a 2 M NH3/CO2 solution at 23± 1 °C, 0.25m/s draw and feed

Fig. 9. Solute fluxes for osmotically driven sodium chloride rejection. The lined bar represents ammonia species reverse solute fluxes, the solid bar represents sodium ion forward solutefluxes, and the cross-hatched bard represents chloride ion forward solute fluxes at 23 ± 1 °C, 0.25 m/s draw and feed cross-flow velocity, and no transmembrane hydrostatic pressure.

14 J.T. Arena et al. / Desalination 343 (2014) 8–16

rejections of the sodium ion were 15–25% while the chloride ionrejections were 80–85%. The cation exchange occurring betweenthe 0.25 M NaCl feed and the 2.0 M NH3–CO2 draw solutions presentphenomena never directly reported. These data could also explainlow sodium chloride rejections [55] or uneven anion and cationrejections of various electrolytes [24] reported by others using TFCmembranes.

3.4.3. Ion exchange mechanismsThere are two possible mechanisms for the ion exchange behav-

ior exhibited between the NaCl feed and NH3–CO2 draw solutions.The first is reliant upon the equilibria amongst ammonia specieswithin the draw solution [17]. Three nitrogen containing speciesare present within the draw solution: ammonia, ammonium, andcarbamate. These species are in equilibrium, but ammonia is un-charged, has chemical interactions similar to water, and a less rigidhydration shell (in relation cation and anion species) [54]. As suchammonia can easily diffuse through the membrane selective layerwithout affecting electroneutrality between the two solutions.Ammonia present within feed solution can now speciate into ammo-nium, causing an imbalance of charge. This charge imbalance drives asodium ion (the only cation available on the feed side) to diffuse intothe draw solution thus producing the unequal feed solution ionfluxes. The second mechanism for ion exchange is the selectivelayer functioning as a cation exchanger where negatively charged

Fig. 10. Observed rejection for a 2 M NH3/CO2 draw solution versus 0.25M sodium chloride feerejection at 23 ± 1 °C, 0.25 m/s draw and feed cross-flow velocity, and no transmembrane hy

functional groups of a membrane's selective layer allow for preferen-tial transport of cations.

Similar ion exchange behavior to those illustrated in Fig. 9 (thisbeing unequal anion to cation transport for electrolytes) was report-ed in a recent publication by Coday observed unequal feed solute iontransport using non-volatile solutes with commercial TFC FO mem-branes [24]. As these solutes do not exist in equilibrium between acharged and uncharged species this would imply that the membranechemistry is the dominating factor in ion transport behavior. This isfurther reinforced by observations also by Coday where HTI's CA FOmembrane was also tested displaying lower cation fluxes than TFCFO membranes [24]. Additionally the high rejections of sodium chlo-ride in studies using HTI's CA membrane with the NH3–CO2 draw so-lution further demonstrate the importance of membrane chemistry[1,20].

A classical cation exchange resin should be a cross-linked waterinsoluble structure with acidic functional groups (i.e. sulfonic,carboxylic, and phenolic). These acid functional groups whendeprotonated would have a negative charge allowing for ionic inter-actions with cations, specifically cations residing within the polymerstructure and exchanging cations in solution. Cation exchangerswith carboxylic acid functionality are pH sensitive only functioningas such at pHs above 7 [42]. The FTIR spectra for these membranesindicate that carboxylic functional groups are part of the polyamideselective layers of these membranes (see Fig. 4) [49]. So the mostlikely reason for the ion exchange behavior is the deprotonation of

d. The solid bars represent sodium rejection and the cross-hatched bars represent chloridedrostatic pressure.

15J.T. Arena et al. / Desalination 343 (2014) 8–16

carboxylic acid functional groups of polyamide making availablecation exchange site within the polyamide layer [42,51,52]. This al-lows for the movement of cations between the feed and draw solu-tions; therefore, in order to mitigate this behavior in polyamidebased TFC membranes the pH would need to be below 7 (not possi-ble with all draw solutions). Alternatively other selectively layerchemistries can be synthesized or revisited.

4. Conclusions

This study explored the impact on FO properties resulting fromthe application of a thin film of PDA on the PSu support structure ofa commercial TFC RO membrane. A four and six fold enhancementin the FOmode osmotic flux of the BW30 and SW30-XLEmembranes,respectively, was observed after modification with PDA. Overall,these membranes were shown to have modest flux under desalina-tion conditions with a 2.0 MNH3–CO2 draw solution and a 0.25M so-dium chloride feed; however, low sodium rejections were observeddue to cation exchange between the draw and feed solutions.Evidence for this ion exchange is provided by the unequal fluxesbetween sodium and chloride. A tuning of process conditions ormembrane chemistry may enable higher rejections for both ionswithin the feed solution.

Acknowledgments

The authors acknowledge funding from the NSF CBET Chemicaland Biological Separations Program #1160098 and #1160069, andUSEPA (Project No. R834872). The authors also acknowledge theNWRI-AMTA Fellowship forMembrane Technology and National Sci-ence Foundation GK-12 Program, which provided support for JasonT. Arena. The authors alsowish to thankDowWater & Process Solutionsfor providingmembranes for this study. Additionally the authorswouldlike to thank Dr. James V. Arena at Central Connecticut State Universityand Dr. Abhay Vaze at the University of Connecticut for providingtechnical assistance and feedback for some of the analytical tech-niques used.

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