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An Effective Design of Electrically Conducting Thin-Film Composite (TFC) Membranes for Bio and Organic Fouling Control in Forward Osmosis (FO) Qing Liu, Guanglei Qiu, Zhengzhong Zhou, Jingguo Li, Gary Lee Amy, Jianping Xie, and Jim Yang Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03402 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 9, 2016

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Environmental Science & Technology

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An Effective Design of Electrically Conducting Thin-Film Composite

2

(TFC) Membranes for Bio and Organic Fouling Control in Forward

3

Osmosis (FO)

4

Qing Liu a, Guanglei Qiu a, Zhengzhong Zhou a, Jingguo Li a, Gary Lee Amy a, b, Jianping Xie a*, Jim

5

Yang Lee a*

6

a

7

Ridge Crescent, Singapore, 119260.

8

b

Department of Chemical & Biomolecular Engineering, National University of Singapore, 10 Kent

College of Engineering and Science, Clemson University, Clemson SC 29634 USA

9 10

*

Corresponding author:

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E-mail: [email protected] & [email protected].

12

Phone: (65) 65162899

13

Fax: (65) 6779193

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ABSTRACT: The organic foulants and bacteria in secondary wastewater treatment can seriously

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impair the membrane performance in a water treatment plant. The embedded electrode approach using

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an externally applied potential to repel organic foulants and inhibit bacterial adhesion can effectively

17

reduce the frequency of membrane replacement. Electrode embedment in membranes is often carried

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out by dispensing a conductor (e.g. carbon nanotubes, or CNTs) in the membrane substrate, which gives

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rise to two problems: the leaching-out of the conductor and a percolation-limited membrane

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conductivity that results in an added energy cost. This study presents a facile method for the embedment

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of a continuous electrode in thin-film composite (TFC) forward osmosis (FO) membranes. Specifically,

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a conducting porous carbon paper is used as the understructure for the formation of a membrane

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substrate by the classical phase inversion process. The carbon paper and the membrane substrate

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polymer form an interpenetrating structure with good stability and low electrical resistance (only about

25

1Ω/ ). The membrane-electrode assembly was deployed as the cathode of an electrochemical cell, and

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showed good resistance to organic and microbial fouling with the imposition of a 2.0V DC voltage. The

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carbon paper-based FO TFC membranes also possess good mechanical stability for practical use.

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INTRODUCTION

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Forward osmosis (FO) used in conjunction with other membrane processes can significantly

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increase the overall energy efficiency in membrane water desalination and wastewater reuse1-5.

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In the FO process, water is drawn from the feed solution (low osmotic pressure) to the draw

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solution (high osmotic pressure) by the osmotic pressure gradient across a semipermeable

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membrane. Due to the asymmetry in the FO membrane structure, there are two operating modes

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in a FO process: the AL-FS mode (where the active layer is facing the feed solution) or the AL-

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DS mode (where the active layer is facing the draw solution). Generally, water flux is higher in

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the AL-DS mode due to a smaller internal concentration polarization6-14. The AL-DS mode can

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also be used for osmotic pressure generation (the pressure retarded osmosis (PRO) process). The

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AL-DS mode however has a higher fouling tendency since the foulants in the feed solution can

40

easily enter the porous membrane substrate.

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Biopolymers, proteins, and microbes are the predominant foulants in the FO treatment of

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secondary wastewater15. The organic compounds can adsorb on the membrane exterior surface

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and on the porous membrane interior to form a gel layer, which not only plugs the pores to lead

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to a water flux reduction, but also causes bacterial growth16.

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polymeric substances (EPS) during the bacterial growth could culminate in biofilm formation.

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The biofilm under the protection of EPS is stubbornly resistant to physical and chemical

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cleaning17, 20-23. It is still a challenge to design FO membranes which are resistant to both organic

48

and bacterial fouling.

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The enhancement of membrane hydrophilicity is commonly used to improve the membrane

50

fouling resistance. It is reported that antifouling properties are derived from the strong hydration

The release of extracellular



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layer on a hydrophilic surface, which inhibits the adsorption of foulants23-26. Since the majority

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of organic foulants are negatively charged27, the other commonly used method is to impart

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negative charge to the membrane surface to electrostatically repel the adhesion of negatively

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charged organic foulants and bacteria28. The fabrication of hydrophilic or negatively charged

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membranes is often complex and has so far been targeted mostly at organic foulants29-34.

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Biofouling resistance, on the other hand, is often addressed by adding antimicrobial

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nanomaterials to a membrane formulation35-37. The stability and durability of the modified

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membranes is often an issue due to the leachability of the biocides, which becomes a source of

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secondary contamination.

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An alternative method to alter and introduce charges to a membrane surface is the application of

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an external potential, which has shown good antifouling properties38, 39. The membrane needs to

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be modified with a conductor to allow the application of an electric potential for microbial

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control but without the loss of the desired membrane properties40-50. Among the different

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additives that have been used to produce composite conducting membranes, the highly

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conducting CNTs are the most common. In the study by Vecitis et al., a PTFE filter coated with

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CNTs was used as the anode. The application of a 3 V DC voltage was found to be effective for

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annihilating bacteria and viruses43. Direct anodic oxidation and indirect oxidation of bacteria by

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the aqueous oxidants formed in anodic oxidation were proposed as the bacteria inactivation

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mechanisms. Lannoy et al. reported a thin film composite (TFC) membrane with cross-linked

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CNTs in the polyamide (PA) selective layer. Improved biofouling resistance was shown with the

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application of 1.5V AC49. The authors theorized that an alternating potential causes instabilities

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in the local pH and the electrical double layer, thus creating a non-ideal environment for bacterial

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growth. In addition, bacteria are repelled by an applied negative potential and oxidized by a 4 ACS Paragon Plus Environment

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positive applied potential. This bacterial inhibition mechanism was also mentioned in the study

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of Ronen et al. 41 where the bacterial growth on CNTs-PVA composite membranes was reduced

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by a -1.5V applied potential. The authors suggested that bacterial inhibition was due to the H2O2

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formed in the cathodic reaction of oxygen reduction.

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The anti-organic fouling properties of CNT-incorporated composite conducting membranes have

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also been examined with the application of a DC voltage. In the work of Zhang et al on CNT-

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containing ultrafiltration (UF) membranes45, the authors attributed the fouling reduction with the

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application of a negative potential to the increased energy barrier and the decreased collision

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efficiency of negatively charged organic matters with the membrane surface. The composite

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CNT-UF membranes were also used in the Dudchenko study40. These conducting membranes

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when used as the cathode were able to reduce fouling by polyacrylic acid (PAA) at a moderate

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applied potential (3-5V). However, different from bacterial inactivation, a positive applied

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potential aggravated the organic fouling. These observations corroborated the electrostatic

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repulsion mechanism proposed by Zhang et al. in their study45.

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The above studies showed that membranes which are rendered conducting by CNTs can reduce

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organic fouling by applying a small negative potential, while bacterial attachment can be

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inhibited by applying a small positive or negative potential. These conducting membranes are

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often produced by the “blending” or “coating” methods. The conducting additives often present

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themselves as a dispersed phase, which gives rise to two potential issues: the possibility of CNT

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leaching (unless covalently bonded); and a percolation-limited conductivity improvement. Thus

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it is hypothesized that a contiguous phase of conducting materials should provide a better long-

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term stability and greater conductivity enhancements. In addition, since organic fouling and 5 ACS Paragon Plus Environment


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microbial fouling are performance limiting factors in the water treatment process, this study not

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only investigated the membrane resistance to microbial deposition and growth, but also the

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membrane resistance to organic foulants; in the presence of a low and negative applied potential.

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Carbon paper with a porous architecture and good conductivity is often used as the diffusion

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layer in polymer electrolyte membrane fuel cells. It has also been used as the electrode for

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electrochemical disinfection51. Herein, we have designed polyethersulfone (PES) FO TFC

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membranes with carbon paper as the continuous conducting phase. Specifically, a conducting

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porous carbon paper is used as the understructure for the formation of the membrane substrate by

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the classical phase inversion process. The carbon paper and the substrate polymer form an

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interpenetrating structure with good stability and low electrical resistance (at only 1Ω/ ). The

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conductivity of the composite membrane can be as high as 5500S/m. This membrane-electrode

107

assembly was deployed as the cathode of an electrochemical cell, and good organic and

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microbial fouling resistance was shown with the imposition of a 2.0V DC voltage. Furthermore,

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these carbon paper-based polymer membranes also have significantly improved mechanical

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properties, which are important for their practical use.

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EXPERIMENTAL

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Materials

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Radel

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polyethylene glycol 400 (PEG, Mn=400g/mol) from Merck; m-phenylenediamine (MPD, ＞

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99%), 1,3,5-benzenetricarbonyl trichloride (TMC, 98%), n-hexanes (＞99.9%), sodium alginate,

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sodium chloride and fluorescein isothiocynateconjugated bovine serum albumin (BSA-FITC)

®

A PES from Solvay Advanced Polymers; N-methyl-2-pyrrolodinone (NMP, ＞99.5%),


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from Sigma-Aldrich, Carbon paper (NOS1005) from CeTech, were used as received. De-ionized

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(DI) water was produced by a Milli-Q ultrapure water system (Millipore, USA).

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Membrane preparation

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The pristine PES membrane substrate was formed by the Loeb-Sourirajan phase inversion

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method. In brief, the PES polymer after drying in vacuum overnight (for moisture removal) was

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mixed with PEG-400, NMP and water in the ratio of PES: PEG: NMP: water = 20:37.9:37.9:4.2

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(wt.%) to form a casting dope. The dope after overnight degassing was cast onto a glass plate

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with a casting knife. The as-cast membrane was quickly immersed in a room temperature water

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coagulation bath and kept in DI water for 24 h to ensure complete precipitation.

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For the preparation of carbon paper-containing PES membranes, a carbon paper was placed on

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the glass plate and the dope was cast onto the carbon paper instead (Figure S1). All other steps

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were the same as in the preparation of pristine PES membranes. The carbon paper-containing

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PES membrane was designated as Car-PES.

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A PA selective layer was formed on the membrane top face by the following procedure: a

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membrane substrate was immersed in a 2wt% MPD aqueous solution for 1min. The excess MPD

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solution was carefully removed from the membrane surface with a filter paper. This was

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followed by contacting the same membrane surface with a 0.1wt% TMC solution in n-hexane for

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30s. The TFC membrane fabricated as such was air-dried for 5min and then stored in DI water.

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Membrane characterization



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Membrane morphology was examined by field-emission scanning electron microscopy

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(FESEM). The membrane sample for microscopy examination was freeze-dried, cryo-fractured

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in liquid nitrogen, and then surface coated with Pt. A JSM 6700F (JEOL, Japan) microscope

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operating at 5kV accelerating voltage was used for the FESEM. Water contact angle

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measurements were carried at room temperature, using a Contact Angle Goniometer (Rame

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Hart) and Milli-Q deionized probe liquid to evaluate the hydrophilicity of the membrane

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substrate. The mechanical properties of the membrane substrate were measured by an Instron

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5542 tensile test equipment. A flat sheet membrane was cut into 5 mm wide stripes and clamped

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at the both ends. The initial gauge length of 30 mm was stretched at the rate of 10 mm/min. At

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least five stripes were tested for each casting condition to obtain the mean values of membrane

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tensile stress, Young’s modulus and elongation at break. A four-point conductivity probe

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(Signatone S-302-4) was used for the measurements of electrical sheet resistance.

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A dead end stainless steel RO filtration cell was used to measure the substrate water permeability

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(A). The effective membrane area in the cell was 9.62 cm2. Water permeability A was calculated

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from the pure water permeation flux under a transmembrane pressure of 3 bar by equation 1

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where Jv is the volumetric water flux and ∆P is the applied hydraulic pressure.

A=

Jv ∆P

(1)

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Membrane substrate porosity ε, defined as the total pore volume divided by the membrane

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volume, was determined by a gravimetric method and calculated as follows: (w2-w1) ρ1 ε=

(2) 8

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where w1 is the dry weight of PES substrate membrane, w2 is total weight of substrate saturated

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with water. Vm is the total volume of the membrane.

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The mean (µp) and the standard deviation (σp) of the effective pore size of each membrane

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substrate were measured by the solute rejection method with a dead-end stainless steel RO

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filtration set-up. 1 bar of pressure was applied to a 200 ppm solution of a neutral solute (PEG or

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polyethyleneoxide (PEO)) to permeate the solute through the membrane. The solute

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concentrations in the feed and in the permeate were measured by a total organic carbon analyzer

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(TOCASI-5000A, Shimadzu, Japan). A more detailed description of the measurement

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procedures can be found in the open literature 52.

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The FO performance of membrane was measured by the water flux (Jv in L/m2h, or LMH) and

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reverse salt flux (Js in g/m2h, or gMH) calculated from the following equations. Jv=

∆V 1 (5) ∆t Am

Js=

(CtVt)-(C0V0) 1 ∆t Am

(6)

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where ∆V(L) is the volume of water permeated through the membrane in a given period of time

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∆t (h), Am is the effective membrane surface area (m2). Ct and Vt are the salt concentration (g/L)

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and the volume of the feed solution (L) at the end of the FO test respectively. Their

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corresponding values at the start of the experiment are C0 and V0. Salt concentrations were again

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measured conductometrically. The dilution of the draw solution in the experiments was



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negligible since the amount of water permeated to the draw solution was less than 1% of the

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latter.

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Microbial fouling on the FO TFC membranes

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Gram-negative E-coli and Gram-positive S. aureus strain were used to evaluate the membrane

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bacterial resistance. E-coli and S.aureus were cultured in a Lysogeny broth (LB) and incubated

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in an Isotemp incubator (Fisher Scientific, Inc., Pittsburgh, PA) overnight at 37℃ (shaking

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speed: 150rpm). The culture was centrifuged at 3000rpm for 10min, and the precipitated pellet

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was re-suspended in a LB solution. The cell suspension prepared as such was used as the

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bacterial stock solution, and was diluted to 107colony-forming units (CFU)/ml before use. Each

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PES and Car-PES membrane was immersed in the bacterial suspension for 4 hours. Meanwhile,

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a negative potential (-2V) was also applied to a Car-PES membrane in the bacterial suspension

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under the same condition. The value of -2V was determined after a limited scoping experiment

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and provided the “best” membrane performance without any gas liberation. The bacteria adhered

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to the membrane back surface was examined by SEM. This involved fixing the bacteria in the

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membrane with a 3vol.% glutaraldehyde phosphate-buffered saline (PBS) solution at 4°C,

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followed by rinsing with PBS solution several times to remove the excess glutaraldehyde. The

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“fixed” samples were dehydrated with ethanol solutions (25, 50, 75 and 100v/v), air-dried, and

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coated with platinum (20mA, 30s) for SEM examination. The quantifications of bacterial

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adhesion and viability on the membrane substrate were carried out by the spread plate method. A

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membrane sample was put in 5ml of PBS, followed by ultrasonication for 7min to remove the

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loose bacterial deposit on the sample surface. The bacterial solution was sequentially diluted, and

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spread on the agar plate. The colonies were counted after 24h of growth. The inactivation rates 10 ACS Paragon Plus Environment

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were calculated by comparing the cell density of the Car-PES membranes with that of the PES

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membrane.

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Two additional tests were also used to evaluate the effect of microbial fouling on membrane

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performance under simulated operating conditions. In the stationary test, a membrane sample

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after the FO test was immersed in the E-coli and S.aureus suspension for 3 days. The bacterial

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suspension was refreshed every 24 h in order to maintain a constant desired bacterial

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concentration. The membrane FO performance was measured again after a 3-day period and

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washing with PBS. In the continuous microbial fouling test, the AL-DS orientation was chosen

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because of its high water flux and a greater fouling propensity. The set-up shown in Figure S2

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was used to apply the potential to the Car-PES membrane. The same set-up without the applied

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potential was used for the control PES and Car-PES membranes. Prior to fouling the water flux

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through the membrane was measured with a DI water feed and a 2 M NaCl draw solution for 1h.

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The DI water was then replaced with a LB solution containing 107 CFU/ml of E-coli. After the

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fouling test, the membrane was cleaned by flowing DI water at 0.1 L/min on both sides of the

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membrane. The water flux through the membrane was then re-measured with DI water (feed

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solution) and 2 M NaCl solution (draw solution). Baseline experiments were conducted under the

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same conditions using a LB solution without the bacteria. All processes were operated at room

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temperature (20 ± 1) °C.

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Protein adsorption on membranes

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BSA-FITC solution was used to measure the protein adsorption behavior of FO TFC membranes.

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A membrane was rinsed initially with a PBS solution and then soaked in the BSA-FITC solution

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(0.5 mg/mL PBS solution) at room temperature for 1 h. The adsorption was imaged by a Leica 11 ACS Paragon Plus Environment


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DMLM fluorescence microscope (Leica Microsystems, Wetzlar, Germany) equipped with an

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excitation filter of 495 nm and an emission filter of 525 nm.

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Organic fouling of FO TFC membranes

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The organic fouling of the membranes was evaluated on the same laboratory-scale FO setup at

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the AL-DS orientation. Since natural surface water contains not only organic foulants but also

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dissolved salts, the alginate solutions were also prepared to contain 4 mM NaCl. The NaCl

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concentration was chosen to emulate the conductivity of typical natural surface water (150-500

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µs cm-1, corresponding to 1-4mM NaCl). In this continuous fouling test, the feed was DI water or

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the foulant solution (400ppm alginate + 4mM NaCl) and the draw solution was 2M NaCl

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solution. The feed and the draw solution flew counter-currently to each other at room

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temperature (20 ± 1) °C at a flow rate of 0.1 L/min. Prior to the test, the FO membrane was

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stabilized and tested with DI water for 1h. To initiate the fouling test, the DI water feed was

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replaced with the foulant solution. Control experiments were carried out under the same

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conditions in the absence of foulants. 12 hours later, the membrane was cleaned for 1h by

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running a FO process using DI water as the feed and the draw solution at 0.1L/min. The FO

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performance of the cleaned membrane was retested again with a DI water feed and a 2M NaCl

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draw solution.

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RESULT & DISCUSSION

232

Substrate characterizations

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The preparation of a FO-TFC membrane on carbon paper was a facile process, similar to the

234

preparation of a TFC membrane. In this design the carbon paper provided the continuum for 12 ACS Paragon Plus Environment

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electron conduction. This can be seen from the SEM image of a typical carbon paper in Figure

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S3 where the carbon fibers interwove into a connected network. The pore size of the carbon

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paper was hundreds of µm large, and hence it is reasonable to expect the PES polymer could

238

easily infiltrate the pores of the carbon paper during casting. Figure 1 shows the morphologies of

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a pristine PES membrane versus a Car-PES membrane as examined by FESEM. The pristine

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PES membrane was cast to almost the same thickness as the carbon paper (~ 200 µm). The PES

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membrane showed the characteristic finger-like cavities with a denser top layer. The Car-PES

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membrane was morphologically different showing a distinct two layer structure: a thin (~10 µm)

243

dense PES layer in the membrane top surface; and PES-infused carbon fibers in the membrane

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bottom layer (~ 190 µm). The morphology and the cross-section of the top layer were the same

245

in both membranes, which is to be expected since there were no carbon fibers in this layer. The

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dense top layer structure was formed by the rapid solvent outflow to the surrounding water when

247

a nascent membrane was immersed in the coagulation bath. Consequently the PA layers formed

248

by IP on the membrane top surface were also similar (Figure S4), suggesting that the presence of

249

a carbon paper in the composite membrane would not affect the membrane selectivity. The

250

presence of the carbon paper, however, created a very different bottom layer. The large pores in

251

the PES back surface ~100 µm in size were caused by the incoming water flux being faster than

252

the outgoing solvent (NMP) flux in phase inversion. For the Car-PES membrane, the PES

253

polymer and the carbon fibers formed an interlocked structure. However, the presence of the

254

carbon paper inhibited the water influx in phase inversion and disrupted the formation of the

255

finger-like porous structure. On the other hand, the pores in the carbon paper were not

256

completely occupied by PES due to filling from the top. Thus some pores in the Car-PES

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membrane back surface were larger than those in the PES membrane. The interlocking carbon13 ACS Paragon Plus Environment


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PES structure is expected to improve the membrane stability in water treatment since the

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mechanically more robust carbon paper could adhere strongly to the membrane.

260 261

Figure 1: Morphologies of the cross-section, top and bottom surfaces of PES (a, c, e) and Car-

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PES membrane substrate (b, d, f)

263

Membrane durability is determined by its mechanical strength, which can be undermined by the

264

defects formed in a membrane fabrication process. The measurements of the mechanical

265

strengths of the PES and Car-PES membranes (Table 1); however, allayed such a concern.

266

Instead the Car-PES substrate was mechanically strong; showing significantly enhanced

267

mechanical properties to surpass the PES membrane, the carbon paper, and all of the previously

268

reported polymer membranes. The interlocking structure is the most likely reason for the synergy

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in integrating PES with carbon paper.

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Material

Young’s modulus/MPa

Tensile strength/ MPa

Elongation at break /%

PES

152.0±22.4

4.2±0.6

17.1±4.3

Car-PES

5520.0±420.5

23.6±1.3

0.5±0.2

Carbon paper

2610.2±205.6

8.3±0.7

0.6±0.3

273

Table 1: Mechanical properties of PES and Car-PES membranes and carbon paper

274

The water permeability (A) of PES and Car-PES membranes was evaluated in a RO filtration

275

set-up. It can be seen that water permeability was slightly higher for the Car-PES membrane. It is

276

well known that in a pressure driven process like RO, water permeability is mainly determined

277

by the membrane porosity and pore size4. While Table S1 shows comparable porosities for the

278

two membranes; the average effective pore size was slightly larger for the Car-PES membrane.

279

Its larger standard deviation also suggests a broader pore size distribution. This is consistent with

280

the SEM images in Figure 1 where some large pores can be easily found on the Car-PES

281

substrate back surface.

282

The inclusion of a carbon paper in the membrane might bring about two structural effects that

283

could impact the membrane water flux. One was the suppression of the membrane finger-like

284

structure leading to lower porosity and reduced water flux, while the other was the creation of

285

enlarged pores which increased porosity, reduced tortuosity and increased water flux. The two

286

effects were mutually compensating to result in an incidental increase in the water flux.

287

Contact angle measurements were carried out next to evaluate the membrane hydrophilicity.

288

Table S1 shows that the contact angles on the membrane top surface were quite close; which is 15 ACS Paragon Plus Environment


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to be expected due to the likeness of the PES and Car-PES and PES membrane top layers. More

290

interestingly, despite the hydrophobic nature of the carbon fibers, the contact angle on the back

291

surface of Car-PES was only slightly higher than that of PES. This is indication of the

292

encapsulation of the carbon fibers by the infiltrated PES polymer. Previous research has shown

293

that the water flux in FO water is constrained by internal concentration polarization (ICP), which

294

depends on the water permeability and substrate hydrophilicity52-55. A more hydrophilic substrate

295

not only facilitates water and salt transport but also suppresses air bubble formation in the

296

substrate to increase the effective porosity and reduce the ICP. The comparable porosity and

297

contact angle of PES and Car-PES is a good sign since it implies that the inclusion of the

298

conducting carbon paper would not bring about an adverse effect to the FO water flux.

299

The electrical properties of the composite conducting membrane are important to the use of

300

external potential for fouling control. The sheet resistance of the Car-PES membrane as

301

measured by the four point probe method was a low 1.0 Ω/ . This compares very favorably to

302

previous work using CNTs where the sheet resistance was a high 6000 Ω/

303

connectivity of dispersed CNTs in an insulating polymer matrix. The conductivity of the Car-

304

PES membrane calculated from the measured sheet resistance and a membrane thickness of 180

305

µm was 5500S/m.

306

The FO performance of PES and Car-PES TFC membranes

307

The water flux and the reverse salt flux of the PES and Car-PES TFC membranes in FO were

308

measured in a conventional FO set-up using a DI water feed solution and a 2M NaCl draw

309

solution. The membrane performance was evaluated in two typical membrane orientations (AL-

310

FS and AL-DS) and the results are summarized in Figure 2. In comparison with the PES

48

due to the poor


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membrane, the FO water flux and the reverse salt flux of the Car-PES membrane showed a slight

312

decrease. As mentioned earlier, the carbon paper inclusion could bring about two effects: an

313

increase in water permeability (A) in RO and a decrease in hydrophilicity (stronger ICP effect).

314

The measured FO data were the result of a balance of these two competing factors.

315 316

Figure 2: FO performance of PES and Car-PES TFC membranes: (a) water flux; (b) reverse salt

317

flux

318

Microbial resistance of PES, Car-PES and electrically charged Car-PES TFC membranes

319

The pristine PES, Car-PES and electrically charged Car-PES TFC membranes were fouled with

320

two model bacteria: S.aureus which is gram-positive and E-coli which is gram-negative. The

321

SEM images in Figure 3 were taken after the membranes were exposed to the bacterial

322

suspensions for 4h. The deposition and growth of the E-coli and S.aureus bacteria was extensive

323

on the PES and Car-PES membranes. These bacteria also tended to form colonies on the

324

membrane surface resulting in pore plugging. By comparison, when a -2V DC voltage was

325

applied to the Car-PES membrane, a very clean membrane surface was obtained. Only a few

326

bacteria were found on the surface without colony formation or pore plugging. Hence an 17 ACS Paragon Plus Environment


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327

electrically negatively charged membrane could effectively repel bacteria to inhibit their

328

deposition and growth.

329 330

Figure 3: SEM images of bacterial deposition and growth on the PES, Car-PES and electrically

331

charged Car-PES TFC membranes.

332

The antimicrobial properties of the membranes were evaluated by a quantitative antifouling

333

assay using the spread plate method. Figure 4 shows that the amount of viable bacteria was

334

somewhat higher on the Car-PES membrane. This could be due to the slightly more hydrophobic

335

surface of the Car-PES membrane found earlier in the contact angle measurements. A

336

hydrophobic surface is known to promote the interaction between the bacteria and the membrane

337

surface. The electrically charged Car-PES membrane, on the other hand, showed a substantial

338

decrease of bacterial adhesion, reducing the adherent fraction to only about 10%. The

339

mechanism of bacterial inhibition in a low potential is still not well understood, and different

340

theories about the application of positive or negative surface charge have been proposed. In this

341

study where the applied potential was negative, the major mechanism of bacterial inhibition 18 ACS Paragon Plus Environment

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342

should be the like-charge electrostatic repulsion between bacteria and a negatively charged

343

membrane surface, which has been proven to effectively repel bacterial approach and prevent

344

biofilm formation

345

of H2O2 by the electrochemical reduction of oxygen. Ronen et al. have shown that the

346

electrochemically produced H2O2 could decrease microbial cell viability and increase cell

347

membrane permeability. A microscopic amount of H2O2 near the membrane surface could be

348

sufficiently lethal to the bacteria to inhibit biofilm formation in a membrane filtration

349

operation41.

49, 56

. However we could not rule out the formation of a microscopic amount

350 351

Figure 4: The static bacteria adhesion behavior of S.aureus and E.coli on PES, Car-PES and

352

electrically charged Car-PES TFC membranes

353

In order to examine the effects of bacterial attachment on the performance of these FO

354

membranes in water treatment, a comparison of the water flux (Figure 5) and the reverse salt flux

355

(Figure S5) before and after bacterial attachment was carried out after a brief physical wash. For

356

the PES and Car-PES membranes, the water flux decrease of over 50% was caused by serious 19 ACS Paragon Plus Environment


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357

fouling by bacterial colonies and biofilm formation. On the contrary, water flux recovery was

358

almost 100% for the negatively charged Car-PES membrane, consistent with the SEM

359

observation of very limited bacterial deposition and growth. While no current flow and no gas

360

liberation were detected, we still could not rule out the possible formation of a microscopic

361

amount of H2O2.

362 363

Figure 5: Effects of microbial fouling on the water flux of PES, Car-PES and electrically charged

364

Car-PES TFC membranes

365

Prior to the continuous microbial fouling test, baseline measurements were carried out to

366

quantify the water flux reduction due to the adsorption of compounds from the LB solution

367

and the decrease in osmotic pressure gradient caused by draw solution dilution and feed

368

solution concentration. The baseline curves for the PES and Car-PES membranes trended

369

similarly, showing about 10% water flux reduction over 24h of operation (Figure S6). The

370

biofouling resistance of these membranes was measured in continuous FO tests using a LB

371

suspension of E-coli as the feed to simulate biological wastewater, and compared with the 20 ACS Paragon Plus Environment

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372

membrane performance in DI water. The results showed ~80% of water flux decrease for both

373

PES and Car-PES membranes in 24h (Figure 6). This substantial reduction should again be

374

attributed to bacterial growth and biofilm formation. In the AL-DS orientation, bacterial

375

deposition occurred in the membrane substrate, and was difficult to remove by physical

376

washing due to the lack of shear forces in the membrane structure. Consequently, the

377

water flux of these two membranes could only restore to ~30% of its initial value after

378

cleaning (Figure S7). The electrically charged Car-PES membrane fared much better by

379

comparison, showing about 30% of water flux decrease in 24h (Figure 6); and 84%

380

recovery of the initial water flux after cleaning (Figure S7). These measurements are

381

consistent with the result from SEM observations, verifying that bacteria could barely

382

deposit and grow on the electrically charged membrane.

383 384

Figure 6: Comparison of membrane resistance to microbial fouling with bacteria LB suspension

385

as feed solution. The draw solution was 2M NaCl. The flow rates on both sides of the membrane

386

were 0.1L/min. 21 ACS Paragon Plus Environment


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387

Adsorption of organic foulants on PES, Car-PES and electrically charged Car-PES TFC

388

membranes

389

Protein adsorption resistance is a common test for evaluating the anti-organic fouling

390

performance of water treatment membranes. Hence the PES, Car-PES and electrically charged

391

Car-PES TFC membranes after exposure to a fluorescent BSA-FITC protein solution (0.5

392

mg/ml) for 1 hour were examined with a fluorescence microscope. Figure 7 showed that such

393

treatment conditions caused the complete coverage of the pristine PES membrane with BSA

394

molecules, which gave rise to a strong fluorescence. This is consistent with previous studies that

395

the hydrophobic-hydrophobic interactions between the benzene rings of PES and protein

396

molecules would promote BSA adsorption34. Since carbon paper did not show any affinity for

397

BSA adsorption, the strong fluorescence of the Car-PES membrane is another indication of the

398

encapsulation of the carbon fibers by the PES polymer; and the permeation of the BSA

399

molecules through the pore system of the carbon paper. However, in the presence of an

400

externally applied negative potential, the adsorption of the BSA was greatly suppressed and

401

fluorescence was barely visible. The applied potential was therefore successful in repelling the

402

BSA molecules from the membrane surface.

403 404

Figure 7: Protein adsorption on PES (a), Car-PES (b) and electrically charged Car-PES (c)

405

membranes 22 ACS Paragon Plus Environment

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406

Effects of organic fouling on the performance of PES, Car-PES and electrically charged

407

Car-PES TFC membranes

408

Prior to the fouling test, a baseline experiment was also carried out using 4mM NaCl solution as

409

the feed solution and 2M NaCl as the draw solution, in order to correct for the effects of draw

410

solution dilution and feed solution concentration. The water flux during the 12 hour test was

411

approximately constant, and the final water flux through both membranes was about 94% of the

412

initial water flux (Figure S8). The organic fouling in FO was tested using a general model

413

organic foulant, alginic acid. With a pKa of 3.21, alginate acid could easily ionize in water to

414

provide an abundance of alginate anions. Prior to the addition of alginate, the membrane was

415

conditioned by running FO with a DI water feed for 1 h. After alginate fouling, the membrane

416

was washed with DI water for 1h, and the draw solution was restored to the initial concentration.

417

As shown in Figure 8, the water flux through the PES and Car-PES membranes decreased

418

significantly and immediately after the addition of alginate to the feed solution. The permeate

419

drag carried the alginate to the selective PA layer through the porous substrate; causing alginate

420

accumulation in the porous membrane substrate. The water flux through the PES and Car-PES

421

membranes declined gradually with the accumulation of alginate, and was about 42% and 48%

422

of the initial water flux respectively at the end of the test. The more severe fouling of the Car-

423

PES membrane could be associated with its slightly more hydrophobic character. By comparison

424

the Car-PES membrane with the application of a negative potential showed a much smaller water

425

flux decline (17 %) over the same period of time due to the like-charge repulsion between the

426

alginate anions and the externally applied potential. It should be noted that some positive ions

427

might adsorb on the membrane surface and neutralized part of the membrane surface charge45.

428

However, the experimental results showed that within the scope of this study, the application of a 23 ACS Paragon Plus Environment


Page 24 of 35

429

2V potential was sufficient to reject the anionic organic foulants even with the presence of

430

positive ions.

431 432

Figure 8: Comparison of membrane resistance to organic fouling in a feed solution with 400 ppm

433

alginate. The draw solution was 2M NaCl. The flow rates on both sides of the membrane were

434

0.1L/min.

435

The fouled membranes was washed with DI water for 1h, and then retested for their FO

436

performance using a DI water feed. Figure S9 shows the results of water recovery. The water

437

flux could return to 64% of its initial performance for the PES membrane. For the Car-PES

438

membrane, the value was about 60%. While the alginate on the membrane surface could be

439

removed by a washing regiment, the alginate within the porous membrane substrate could not.

440

On the contrary, the water flux for the electrically charged Car-PES membrane could be restored

441

to about 92% of its initial value, similar to the results of the baseline experiment (Figure S9).

442

This is indication that under an externally applied potential, alginate could only be loosely held 24 ACS Paragon Plus Environment

Page 25 of 35


443

by the membrane surface and as such was easily removable by physical washing. In addition, the

444

test for membrane selectivity showed that all three membranes could retain a good selectivity

445

(Figure S9). Therefore positive ions (Na+) that might have been accumulated on the membrane

446

surface under the applied negative potential could also be removed by a simple physical wash.

447

One of the concerns in the application of an external negative potential is Ca2+ adsorption which

448

could bridge between the organic foulant (alginate) to increase the severity of alginate gelation21,

449

57, 58

450

of a control and electrically-charged PVDF membranes with embedded CNTs. Even though Ca2+

451

could in principle accelerate the fouling of the CNTs-PVDF membranes, the authors found that

452

in practice, the electrically-induced surface charge was not effective for the Ca2+ bridging of

453

organic foulants; and hence there was no increase in the fouling of the electrically charged

454

membrane in the presence of Ca2+ 45.

455

There are reports in the literature where a positive potential was used for the oxidative killing of

456

bacteria and the inhibition of biofilm formation. In actual wastewater treatment, however, the

457

application of positive charge could worsen organic fouling due to the attractive forces between

458

the membrane and the foulants40, 50. In this study, a facile method was developed to prepare

459

carbon paper-PES membranes with high electrical conductivity and enhanced mechanical

460

properties and membrane integrity (stability). When a relatively small and negative potential

461

(2V) was applied to these membranes, the deposition and growth of both gram-positive and

462

gram-negative bacteria were inhibited, which could be attributed to the repulsive forces between

463

like charges and the possible presence of a microscopic amount of H2O2 formed by

464

electrochemical oxygen reduction reaction. This electrically charged composite membrane also

. Zhang et al. added Ca2+ to the test solution to examine their effects on the fouling behavior



Page 26 of 35

465

showed an increased resistance towards organic fouling as tested by the model foulant alginate.

466

The water flux decrease due to organic fouling was substantially reduced; and a very high water

467

flux recovery rate was possible after a simple physical wash. The experimental results indicated

468

that this membrane design approach (of forming TFC on an existing porous conductor rather

469

than trying to connect dispersed conductors in a polymer matrix) is effective for producing

470

energy efficient antifouling composite membranes which inhibit both microbial and organic

471

fouling with the application of a negative external potential.

472

ASSOCIATED CONTENT

473

Supporting Information: The Supporting Information is available free of charge via the Internet

474

at http://pubs.acs.org.

475

Details on the preparation process of PES and Car-PES substrates (Figure S1), the flow cell for

476

FO operations with an externally applied electric potential (Figure S2), the surface morphology

477

of carbon paper used in this study(Figure S3), surface morphology of the PA layer on PES and

478

Car-PES TFC membranes (Figure S4), effect of microbial fouling on the reverse salt flux of

479

PES, Car-PES and electrically charged Car-PES TFC membranes (Figure S5), base line

480

experiments using the LB solution and a 2M NaCl draw solution(Figure S6), water flux and

481

reverse salt flux of PES, Car-PES and electrically charged Car-PES membranes after continuous

482

microbial fouling and a brief physical wash(Figure S7), base line experiments using a 4mM

483

NaCl feed and a 2M NaCl draw solution (Figure S8), water flux and reverse salt flux of

484

organically fouled PES, Car-PES and electrically charged Car-PES membranes after a brief

485

physical wash(Figure S9), and properties of PES and Car-PES substrates (Table S1).

486 26 ACS Paragon Plus Environment

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487

AUTHOR INFORMATION

488

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4

489

Engineering Drive 4, 117585, Singapore Author to whom correspondence should be addressed:

490

Email: [email protected] & [email protected]

491

Notes

492

The authors declare no competing financial interest.

493

ACKNOWLEDGMENT

494

The authors would like to thank the Singapore National Research Foundation for funding this

495

study through the project “Advanced FO membranes and membrane systems for wastewater

496

treatment, water reuse and seawater desalination” (R-279-000-336-281). Thanks are also

497

extended to Professor Neal TS Chung for providing the experimental equipment.

498

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