Treatment of PerfluorinatedChemicals by Electro-MicrofiltrationY U - T I N G T S A I , A N G E L A Y U - C H E N L I N , *Y U - H S I A N G W E N G , A N D K U N G - C H E H L I

Graduate Institute of Environmental Engineering,National Taiwan University, 71, Chou-Shan Rd.,Taipei 106, Taiwan (R.O.C.)

Received October 28, 2009. Revised manuscript receivedAugust 22, 2010. Accepted September 15, 2010.

Perfluorinated compounds (PFCs) are negatively chargedand have low pKa values in water; therefore, a laboratory-scale electro-microfiltration (EMF) unit that applies a direct-current electrical field across its membrane can greatly enhancetheir removal from aqueous systems. We examined theeffects of an aqueous inorganic matrix (pH: 4, 7, or 10; ionicstrength: 0.4-4.8 mM; ionic composition: Na2SO4, NaCl, NH4Clor CaCl2) and an organic matrix such as dissolved organicmatter (DOM) on the ability of EMF to remove perfluorooctanoicacid (PFOA) and perfluorooctane sulfonate (PFOS). Decreasedremoval of PFOX (X ) A or S) was observed when theproton concentration and the ionic strength increased. Whenthe applied electrical field strength was less than the criticalelectrical field strength (Ecritical, HA), PFOX removal was lowerin the presence of DOM. We hypothesize that these matricesaffect PFOX rejection by altering membrane zeta potentialduring filtration in the presence of an electrical field. In addition,EMF was found to remove three other PFCs effectively(perfluorodecanoic acid, perfluorohexane sulfonate, andperfluorohexanoic acid), and was also able to remove 70%PFOX and 80% DOC from real industrial wastewaters.

1. IntroductionSince the 1950s, perfluorinated compounds (PFCs) have beenused extensively in industrial and commercial products suchas repellents, surfactants, fire-fighting foams, and cosmeticsfor their distinctive surface activity and chemical and thermalstability. Nevertheless, PFCs are considered to be a neworganic pollutant due to their persistence, toxicity, andbioaccumulation (1, 2). Perfluorooctanoic acid (PFOA) andperfluorooctane sulfonate (PFOS) are the PFCs most oftendetected in aqueous environments (3, 4). It is difficult toremove PFOX (X ) A or S) from water due to their lowconcentrations (ng/L-µg/L) in aqueous environments. There-fore, it is essential to understand the fate of these trace organiccontaminants and to develop effective treatment methods.

Membrane technology has recently received extensiveattention because it provides an alternative method formeeting stricter water treatment regulations. Microfiltration(MF) with large pore sizes has been used to efficiently removefine particulates and pathogens from water (5). Ceramic MFmembranes in particular allow higher flux and betterchemical compatibility compared to organic membranes andare therefore suitable for treating wastewaters from variouswaste streams. In theory, PFCs are not excluded by MF

because they are smaller than the average membrane poresize. However, PFCs are negatively charged in water, sug-gesting that the performance of MF process may be improvedby applying an electrical force that attracts these chargedcompounds. This combination technology is called electro-microfiltration (EMF).

EMF is a pressure-driven membrane process in which anadditional electrical current is applied simultaneously duringmechanical filtration. Since the applied electrical fieldgradient is parallel to the flow, charged substances would beretained by the electrical force, thus increasing the perfor-mance of the membrane. Electrophoresis and electroosmosis,two important electrokinetic phenomena, participate con-currently in EMF. Electrophoresis is the movement of acharged species under the influence of an electrical field,whereas electroosmosis uses counterions under the influenceof an electrical field to draw a liquid through a membrane.In addition, electrochemical reactions may occur if targetcompounds are exposed to the electrodes (6). Many studieshave reported the application of EMF to water and wastewatertreatments on laboratory and pilot scales (6-9). Althoughelectrofiltration requires more energy than traditional mem-brane filtration does, the incremental flux that results fromthe applied electrical field strength may compensate for theincreased energy consumption when there is significant fluxenhancement. Furthermore, rejection of the pollutant isincreased after voltage is applied, indicating that additionalcostly treatments may not be necessary.

Water inorganic matrices (pH, ionic strength and com-position) and organic matrices such as dissolved organicmatter (DOM) may influence filtration flux and soluterejection by altering the membrane zeta potential or thechemical speciation of solutes; this might affect many solute-membrane or solute-solute interactions (10-15).

When an electrical force is applied to the filtration systemto counter the hydrodynamic force, the membrane zetapotential may significantly affect solute rejection. To the bestof our knowledge, no reports have discussed the efficacy ofPFC removal by EMF. In this work, we have used a laboratory-scale EMF unit to study PFC removal from water undervarious conditions. The effect of membrane zeta potentialon EMF performance was examined under the followingconditions (1): in acidic, neutral or basic feeding solutions(pH 4, 7, or 10) (2), in electrolyte solutions of various ionicstrengths, and (3) in the presence of DOM. Finally, a morerealistic trial was conducted by applying EMF to realwastewater.

2. Materials and Methods2.1. Filtration Module. We used a tubular ceramic MFmembrane with nominal pore size 0.1 µm. Membranecharacteristics are listed in Supporting Information (SI) TableS1; Figure 1 shows a schematic layout of the experimentalsetup. The main filtration module consisted of an externalhousing (a hollow plastic tube), an internal cathodic layerencircling the MF membrane and a concentric anodic rod.The internal diameter of the filter and the diameter of theconcentric anodic rod were 6 and 1.3 mm, respectively. Theanode was made of platinum, and the cathode was made oftitanium. This tubular ceramic membrane was assembled inthe external housing to form a cross-flow cell with a filtrationsurface area of 20.7 cm2.

2.2. Feed Solution. The PFCs used in this study are listedin SI Table S2. In addition, the pKa and effective diametersare also reported (16-22). We purchased PFHxA, PFOA, andPFDA from Sigma-Aldrich (St. Louis, MO), and PFHxS and

* Corresponding author phone: (886)-2-33664386; fax: (886)-2-33669828; e-mail: yuchenlin@ntu.edu.tw.

Environ. Sci. Technol. 2010, 44, 7914–7920

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PFOS from Fluka (Buchs, Switzerland). All PFC standardswere of purity >97%. The initial concentration of each PFCwas 100 µg/L in all experiments.

Humic acid (HA) was used to represent the DOM insynthetic water. The stock solution was prepared by mixing1 g of powdered sodium salt of HA (Sigma-Aldrich, St. Louis,MO, USA) with 1 L of Milli-Q water. The solution was filteredthrough a 0.45 µm filter to remove particulates and storedat 4 °C until usage. The concentration of dissolved organiccarbon (DOC) in the feed solution was approximately 5 mg/L. For EMF experiments with varying pH or DOM, NaCl wasused to adjust the conductivity to 100 µS/cm (0.8 mM). ThepH was adjusted separately.

2.3. Filtration Experiment. Five liters of PFC-containingsolution with or without HA were prepared for each experi-ment. A direct-current (DC) power supply (Chroma, model6210K-600/1000 W) provided electrical voltage. A peristalticpump was used to introduce the feed solution into thefiltration cell, and pressure was maintained at 49 kPa witha pressure gauge. The cross-flow velocity was 0.18 m/s.Permeate was collected in a polypropylene container,measured with an electrical balance, and recycled back intothe system to maintain a constant feed concentration.Samples were taken for determination of PFC and DOMconcentrations during the course of filtration.

The membrane was cleaned by sequential flushing with0.1 N NaOH, 50% aqueous methanol and Milli-Q water aftereach experiment. After each 10 min washing cycle, themembrane was soaked in a 0.1 N NaOH solution for 12 h andin a 50% aqueous methanol solution for another 12 h.Afterward, the membrane was immersed in Milli-Q water toremove residual methanol and NaOH. Lastly, the clean waterflux was measured after Milli-Q water was filtered throughfor 1 h. After the cleaning process, the clean water fluxrecovery was 98-100%.

The membrane zeta potential (�) at different solutionconditions can be determined from the electroosmotic flux-versus-electrical current curve described by the Helmholtz-Smoluchowski equation (23):

where Q is the electroosmotic flux (m3/s), I is the appliedelectrical current (A), D is the constant ) ε0εr (C/Vm), ε0 andεr are the vacuum permittivity and relative dielectric constantof the medium, and η and κ are the solution viscosity (Ns/m2) and the conductivity of the electrolytes in the bulksolution (S/m). The experimental setup for measurement ofthe electroosmotic flux was the same as for the EMFexperiments. The typical measurement time was 5 min.

2.4. LC-ESI-MS/MS and Other Chemical Analysis. Theconcentrations of PFOX and three other PFCs were quantifiedusing liquid chromatography tandem mass spectrometry (LC-MS/MS). Chromatographic separation was performed usingan Agilent 1200 HPLC (Agilent, Palo Alto, CA) equipped witha ZORBAX Eclipse XDB-C18 column (150 × 4.6 mm, 5 µm).Mass spectrometric measurements were carried out on aSciex API 4000 (Applied Biosystems, Foster City, CA) equippedwith an electrospray ionization interface in negative mode.Data acquisition was performed in multiple reaction moni-toring mode with a dwell time of 30 ms and unit massresolution on both mass analyzers. The other LC and MS/MS parameters used in this study have been described indetail elsewhere (4). Quantification was based on a 7-pointcalibration curve with a linear range from 1 to 125 µg/L.

DOC was analyzed with an organic carbon analyzer (modelIL-550 TOC-TN, Lachat Instruments, Germany). The pH andconductivity were measured by an EC-212 pH meter andEC220 conductivity meter, respectively.

FIGURE 1. Schematic diagram of a laboratory scale electro-microfiltration (EMF) system.

QI) -D�

ηκ(1)

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3. Results and Discussion3.1. Effect of Electrical Field Strength on PFOX Remov-als. The removal efficiencies and rejections (η) of PFOX andthe other PFCs were determined by the following equation:

where C0 is the initial PFOX concentration in the bulk solution,and Cp is the PFOX concentration in the permeate stream.PFOX rejection remained consistent over time, indicatingthat these compounds were separated immediately fromwater when an electrical field was applied (SI Figure S1).Figure 2 depicts the removal efficiencies of PFOX as a functionof solution pH at three different electrical field strengths (0,29, and 58 V/cm). In the absence of an electrical field, PFOXremoval efficiencies were low, indicating that the MFmembrane alone was insufficient to remove these pollutants.The effective diameters of PFOA and PFOS are 0.91 and 0.99nm, respectively (SI Table S2). Therefore an MF membranewith a nominal size of 100 nm should not be able to removeeither PFOX. These results also suggest insignificant PFOXadsorption onto the MF membrane. However, the applicationof a DC electrical field through the membrane greatlyenhanced PFOX removal. At pH 4, as electrical field strengthincreased to 29 and 58 V/cm, the average removal efficienciesof PFOA and PFOS increased to 42.4% (PFOA) and 45.6%(PFOS) at 29 V/cm and to 61.3% (PFOA) and 65.6% (PFOS)at 58 V/cm. In addition, PFOA and PFOS removals at solutionpH 7 increased to 56.3% and 57.6% in a 29 V/cm electricalfield, and further yet to 72.3% and 72.2% in a 58 V/cm

electrical field. Moreover, PFOA and PFOS rejections in-creased to >84% at pH 10 in a 58 V/cm electrical field. Theseresults clearly demonstrate that EMF increased PFOX removalefficiencies and are consistent with findings from earlierstudies that used EMF to treat water containing arsenic andoxide-chemical mechanical polishing (oxide-CMP) waste-water (7, 24). Weng et al. (24) reported that rejection ofarsenate (V) increased from 30% to >90% after applying anelectrical field, whereas Yang et al. (7) found that removalsfrom oxide-CMP wastewater were high for turbidity and totalorganic carbon.

To determine the mechanism of PFOX removal by EMF,we monitored PFOX concentrations in the tank, reasoningthat if electrochemical degradation were important for PFOXremoval, their concentrations in the feed tank would decreasesignificantly. From mass balance analysis, we found thatrecoveries of PFOA and PFOS were 101% and 99.5%,respectively, indicating insignificant electrochemical loss inthe EMF system. Electrochemical degradation of PFOX byEMF has been previously excluded as a major removalmechanism when conductivity is low (100 µS/cm), filtrationcell retention time is short (few seconds), or inert material(Pt) is used for the anode electrode (6, 25). Other studieshave pointed out that PFOX are resistant to decompositionby advanced oxidation processes such as ozone, ozone/UV,ozone/H2O2 and the Fenton reagent (26, 27). Consequently,physical separation due to electrophoretic attraction duringEMF may be the dominant mechanism in PFOX removal.

3.2. Effect of pH and Ions on PFOX Removals andTheir Removal Mechanism. The effects of different pH levels(4, 7, and 10) on the speciation of PFOX were expected to beof minimal significance, since these compounds have fairlylow pKas and are always in their negative forms. However,PFOX removal efficiency was strongly and positively cor-related with solution pH (Figure 2). Solution pH may affectsolute speciation and membrane zeta potential; these in turnmay influence the electrostatic repulsion between themembrane and solute.

First, the membrane zeta potential (�) was determined byusing the electroosmotic flux method described in Section2.3. The titanium dioxide/zirconium dioxide (TiO2/ZrO2)incorporated into the membrane carries a negative zetapotential at solution pH 4, 7, and 10. Results from themeasurements were appeared in SI Figure S2 and Table S3.The � values at different pH environments were -23.1 mV(pH 10), -16.7 mV (pH 7), and -12.2 mV (pH 4) (Table 1),indicating that our membrane’s zeta potentials were negativeat all pH levels studied. In addition, the higher the pH value,the more negative the � observed. This finding is consistentwith an earlier study (10) that also used a TiO2/ZrO2 tubularceramic membrane.

Figure 3 depicts the three forces proposed to affect PFOXfiltration: the electrostatic repulsion force that originates frommembrane-solute interaction, the electrophoretic force aris-ing from the presence of an electrical field, and thehydrodynamic force stemming from advective transport ofPFOX, which in turn derives from the pressure gradient and

FIGURE 2. Removal efficiencies of (a) PFOA (b) PFOS at variouselectrical field strengths at pH 4, 7, and 10.

η )C0 - Cp

C0× 100% (2)

TABLE 1. Zeta Potential Measurements for the MicrofiltrationMembrane at Different pH Values and Electrolyte Solutions

electro-osmotic flux method(ionic strength of 0.8 mM)

pH 4 7 10

zeta potential, NaCl (mV) -12.2 -16.7 (-13.5)a -23.1zeta potential, NH4Cl (mV) -17.2zeta potential, CaCl2 (mV) -11.3zeta potential, Na2SO 4 (mV) -19.1

a Zeta potential of membrane after HA adsorption.

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advective flow. In the absence of applied electrical field, thehydrodynamic force dominated PFOX transport through themembrane, as evidenced by the small change in PFOXremoval efficiency when the membrane zeta potentialchanged (Figure 2). The minimal removal observed at pH 10(Figure 2) was attributed to the membrane zeta potential.When an electrical field was applied, the resultant electro-phoretic attraction force and the electrostatic repulsion forcefrom the membrane zeta potential offset the hydrodynamicforce. When an electrical voltage was applied at the samemagnitude, a significant difference in PFOX rejection atvarious pH values was observed. The increased removalefficiency of PFOX with increasing pH value was attributedto high membrane zeta potential when solution pHincreased from 4 to 10.

Because acids, bases, and salts are used extensively tofabricate electronics, wastewaters from electronic industrycan contain substantial amounts of electrolytes. High-ionic-strength wastewater may decrease the performance of EMFbecause of the high electricity consumption required. Tounderstand the effect of ionic strength on PFOX rejection,EMF experiments were conducted in various solutionenvironments, and results were compared with experimentsconducted in the absence of background electrolytes. Figure4 shows the effect of ionic strength on PFOX removal by EMFin a 58 V/cm electrical field. The electrolytes used were

Na2SO4, NaCl, NH4Cl or CaCl2, and concentrations rangedfrom 0.4 to 4.8 mM. Decreases in PFOX removal wereobserved as ionic strength increased, suggesting decreasedmembrane zeta potential due to compression of the electricdouble layer, in turn reducing the electrostatic repulsion forcebetween the membrane and PFOX. In addition, PFOXremovals in the following electrolyte solutions decreased(given in descending order): Na2SO4 > NaCl ∼ NH4Cl > CaCl2.This is consistent with the membrane zeta potentialspreviously measured for each solution (Table 1).

3.3. Effect of DOM on PFOX Removals and FiltrationFlux. DOM is ubiquitous in wastewater. To explore the effectof water organic matrix on filtration performance, HA wasused as background DOM and spiked into a solutioncontaining 100 µg/L PFOX. Because DOM is a fouling agentthat can cause flux decline during filtration, it is essential tomonitor the flux during the filtration process. Figure 5a showsthe normalized flux as a function of filtration time in thepresence of HA at pH 7. A blank test with Milli-Q waterdemonstrated constant flux throughout filtration. In theabsence of HA, no flux decline was observed during the MFof PFOX, with results similar to those from the blank test(data not shown). However, the flux decreased to 73% of itsinitial value during the MF of PFOX in the presence of 5mg/L HA. Although the size of HA is smaller than themembrane’s pore size, its aggregation makes it one of the

FIGURE 3. Proposed mechanism for removal of PFOA and PFOS anions during (a) microfiltration (b) electro-microfiltration.

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most common fouling agents found during MF filtration (28);alternatively, a highly concentrated layer of HA may form onthe membrane surface during filtration. We found that theflux increased substantially and proportionally with anapplied electrical field and plateaued within 25 min (20, 37,and 47% flux increases were observed after 120 min atelectrical fields of 29, 43.5, and 58 V/cm, respectively). Theincrease in flux after electrical field application was attributedto electrophoresis, which pulled HA away from the membranesurface, thus reducing filtration resistance. In addition, theelectroosmosis flux induced simultaneously contributed tothe increase in the filtration flux. The flux during EMF ofPFOX in the absence of HA at 58 V/cm was higher than theMilli-Q water flux by 25%. In the presence of an electricalfield, the incremental flux below the Milli-Q water flux wasmainly attributed to electrophoresis, while the flux exceedingthe Milli-Q water flux was mainly ascribed to electroosmosis.To quantify the effect of electrical field strength on flux, wecalculated the critical field strength (Ecritical, V/cm), that is,the electrical field strength that counterbalances the convec-tive migration of charged species toward a membrane (29).

In eq 3, J denotes the permeate flux (cm/s), and µp (cm2/(Vs)) is the electrophoretic mobility of the charged species.

From eq 3, the Ecritical, HA was 41 V/cm, assuming anelectrophoretic mobility of Aldrich HA’s µp value of 3.3 ×10-4 cm2/ (Vs) (23) and an averaged filtration flux of 1.35 ×10-2 cm/s. When the electrical field strength was less thanthe critical value, HA tended to move toward the membranesurface, resulting in a flux lower than with Milli-Q water (J/J0

) 90%). Applying an electrical field close to Ecritical,HA enabledretention of charged compounds on the concentrate side,yielding a flux equal to that of Milli-Q water. Previousinvestigators have also noted that electroosmosis should beconsidered when the applied field strength exceeds the criticalvalue (8).

Figure 5b shows the PFOX and DOC removal efficienciesin the presence or absence of HA at pH 7. Without an electricalfield, PFOX rejection was low (<3%), and addition of HA didnot change its removal efficiency significantly, demonstratingthe insignificance of HA-PFOX interactions. This is consistentwith findings from Dong et al. (12, 13), who reported thatremoval of bisphenol A by ultrafiltration and hollow fibermicrofiltration remained unchanged in the presence of HA.Note that the membrane pore size (0.1 µm) used in our studywas relatively larger than the size of PFOX (see SI Table S2);therefore, interactions between PFOX and the membranewere not expected when MF was used.

Figure 5b also reveals two distinct phenomena after anelectrical field was applied. The removal efficiencies of PFOXin the presence of HA were lower than those in the absenceof HA at 29 V/cm. However, when field strength was

FIGURE 4. The ionic strength effect of Na2SO4, NaCl, NH4Cl orCaCl2 (0.4-4.8 mM) on the (a) PFOA (b) PFOS removal byelectro-microfiltration (E ) 58 V/cm; pH 7).

Ecritical )J

µp(3)

FIGURE 5. (a) Normalized flux in the presence of HA at variouselectrical field strengths at pH 7 (PFOS displays identicalbehavior). (b) Removal efficiencies of PFOX and DOC in thepresence or absence of HA at pH 7.

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maintained at 58 V/cm, similar removal efficiencies wereobserved regardless of the presence or absence of HA. It isclear that the electrical force dominates all other effects whenthe applied electrical field strength is greater than the criticalvalue, and therefore no significant differences in PFOXremoval efficiencies were seen with or without HA. In orderto account for the decreases in PFOX removal efficienciesobserved when the applied electrical field was less than thecritical value, we hypothesize that membrane zeta potentialplayed an important role in modulating PFOX rejection. Withregards to the forces acting on PFOX molecules (Figure 3),the hydrodynamic force would tend to promote their passagethrough the membrane, whereas the electrical force wouldbe expected to keep them on the concentrate side. In addition,membrane zeta potential may influence the solute transportthrough a membrane. Without an electrical force, thehydrodynamic force dominates. The sieving mechanismdetermines the removal efficiency of the membrane. On theother hand, when the electrical force is able to contribute,that is, is greater than the hydrodynamic force, the electricalforce dominates. Note that when the electrical force is smallerthan the hydrodynamic force, the membrane zeta potentialshould influence the removal efficiency of pollutants. At 29V/cm, the electrical field was less than the Ecritical, HA, conditionsunder which HA could be transported toward the membrane;thus, its adsorption would decrease the membrane zetapotential (10, 11). Such a decrease in membrane zeta potentialfrom -16.7 to -13.5 mV (absolute value) after fouling wasconfirmed (Table 1). Consequently, HA adsorption woulddecrease the PFOX rejection efficiency during EMF with anelectrical field strength lower than its critical value.

DOC rejections increased dramatically from 5% at 0 V/cm,to 65% at 29 V/cm, and finally to 80% at 58 V/cm. The DOCcould not be completely removed by EMF, even though theapplied electrical field exceeded the Ecritical, HA. HA is knownto be heterogeneous, with a typical molecular structureconsisting of multiple functional groups, including carboxyl,amine, hydroxyl and carbonyl groups (30). Since the calcu-lated Ecritical, HA was based on averaged electrophoreticmobility, it is likely that the uncharged and positively chargedfractions could not be altered even at a high electrical fieldstrength. Weng et al. (6) have reported that uncharged HAdid pass through membranes despite an applied electricalfield strength near the critical value.

3.4. Removal of Coexisting PFCs by EMF. In addition toPFOX, three other PFCs were also detected at high frequenciesor concentrations. Perfluorodecanoic acid (PFDA), a precur-sor of PFOA, is one of the most commonly detected PFCs inwastewater effluents and their receiving river waters (4).Additionally, Sinclair and Kannan (31) reported that per-fluorohexane sulfonate (PFHxS) concentrations were similarto PFOS levels in wastewater treatment plant (WWTP)effluents. Moreover, perfluoroalkyl carboxylates or perfluo-roalkyl sulfonates (intermediates during photochemical orsonochemical degradation) are toxic to the biota even attrace levels (32).Thus, a model solution was prepared tosimulate an aquatic system concurrently polluted by PFHxA,PFOA, PFDA, PFHxS and PFOS. SI Figure S3 shows theremoval of five PFCs by EMF at 58 V/cm. Their removalefficiencies were 70-76% and 81-86% at pH 7 and pH 10,respectively. These results confirm that EMF is of potentiallysignificant value for treating water containing negativelycharged PFCs with low pKa values.

3.5. Evaluation of Energy Consumption. We roughlycalculated the energy consumption and compared it withresults from a previous review paper (33) based on the appliedelectricity (kJ), treated concentration (µM) and treated volume(L) given. This review found energy consumptions for PFOAand PFOS treatment to be 1-8200 and 11-4500 kJ/µmole,respectively. In our study, the energy needed to separate five

coexisting PFCs by EMF was ∼180 kJ/µmole, a level com-parable to other treatment technologies. In general, theapplication of electrical field strength increases MF perfor-mance but also incurs additional operational and capitalcosts. Based on our results, the additional operating costafter applying electricity was 2.61-23.6 kWh/m3 (this broadrange is due to different ionic strength). Given an averageindustrial electricity cost of USD 6.54 cent/kWh (34), theincurred cost would be USD 0.171-1.54/m3, an amountcomparable to the cost of wastewater treatment for irrigation(USD 0.3-0.6/m3) (35). Note that the higher the conductivityof water, the more electricity is consumed. EMF is more suitedfor treatment of wastewater with low conductivity. Becauseof increases in water prices stemming from water shortagesand pollution, MF has been extensively used as a polishingstep for water reuse. Furthermore, an upgrade from MF toEMF should not only increase a separation system’s per-formance but also eliminate membrane fouling.

3.6. Treatment of Real Wastewater by EMF. The waste-water used in this study was sampled from an electronics/optoelectronics fabrication plant in Taiwan (4). As summa-rized in SI Table S4, this wastewater had a DOC concentrationof 8.2 mg/L and contained primary cations and anions suchas Na+, NH4

+, Ca2+, Cl-, and SO42- at mg/L levels. The

concentrations of PFOA and PFOS in the sampled wastewaterwere 21 and 33 µg/L, respectively. In order to understand thewastewater’s matrix effects on PFOX removal, additionalPFOX was spiked into the wastewater to increase theconcentration to approximately 100 µg/L (36, 37). Figure 6ashows PFOX and DOC removals during EMF. The pH values

FIGURE 6. (a) PFOX and DOC removals in wastewater byelectro-microfiltration. (b) The normalized flux as a function offiltration time in the wastewater.

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of the wastewater and Milli-Q water were 8.6. Applying anelectrical field strength of 58 V/cm increased PFOX and DOCremovals from 0 to 70% and from 10 to 80%, respectively.However, the removal efficiencies of PFOX in wastewaterwere about 10% lower than that in Milli-Q water. This canbe explained by a reduced membrane zeta potential due tothe presence of DOM and electrolytes.

Figure 6b shows the normalized flux as a function offiltration time. In the absence of an electrical field, the fluxdecreased to 40% of its initial value, while the flux was 83%of its initial value at electrical field stength of 58 V/cm. Notethat the flux at 58 V/cm was lower than the Milli-Q waterflux, indicating the differences in the compositions of thereal vs synthetic wastewater used in our study. This couldalso be explained that the applied electrical field was lessthan the Ecritical in this wastewater. In this situation, DOMtended to be adsorbed onto the membrane surface andresulted in a decrease in the zeta potential of the membrane.This explained why the PFOX removal was lower in thewastewater than that in Milli-Q water. Overall, EMF was foundto remove PFOX from real wastewater effectively.

Supporting Information AvailableFigures S1-S2 and Tables S1-S4. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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