Mitigation of Thin-Film Composite Membrane Biofouling via

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Mitigation of Thin Film Composite Membrane Biofouling via Immobilizing Nano-Sized Biocidal Reservoirs in the Membrane Active Layer Alireza Zirehpour, Ahmad Rahimpour, Ahmad Arabi Shamsabadi, Mohammad Sharifian Gh., and Masoud Soroush Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00782 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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Environmental Science & Technology

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Mitigation of Thin Film Composite Membrane Biofouling

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via Immobilizing Nano-Sized Biocidal Reservoirs in the

3

Membrane Active Layer

4 5 6

Alireza Zirehpour1, Ahmad Rahimpour1*, Ahmad Arabi Shamsabadi2,

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Mohammad Sharifian Gh.3, Masoud Soroush2*

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Key Words: Thin-film composite membrane, metal-organic framework, biofouling

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mitigation, forward osmosis, biocidal reservoir

April 10, 2017

THIRD REVISED VERSION

Submitted for Publication in ACS Environmental Science and Technology

25 26 27 28 29 30 31 32 33 34 35

1

Department of Chemical Engineering, Babol Noushirvani University of Technology, Shariati Ave., Babol, Iran 2 Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104, USA 3 Department of Chemistry, Temple University, Philadelphia, PA 19122, USA *Corresponding authors: Rahimpour: [email protected], [email protected], 98-11-32334204 (tel/fax) Soroush: [email protected], 1-215-895-1710 (tel), 1-215-895-5837 (fax)

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ABSTRACT

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This work investigates the use of a silver-based metal-organic framework (MOF) for

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mitigating biofouling in forward-osmosis thin-film composite (TFC) membranes. This is the

39

first study of the use of MOFs for biofouling control in membranes. MOF nanocrystals were

40

immobilized in the active layer of the membranes via dispersing them in the organic solution

41

used for interfacial polymerization. Field emission scanning electron microscopy (FE-SEM)

42

and X-ray photoelectron spectroscope (XPS) characterization results showed the presence of

43

the MOF nanocrystals in the active layer of the membranes. The immobilization improved

44

the membrane active layer in terms of hydrophilicity and transport properties, without

45

adversely affecting the selectivity. It imparted antibacterial activity to the membranes; the

46

number of live bacteria attached to the membrane surface was over 90% less than that of

47

control membranes. Additionally, the MOF nanocrystals provided biocidal activity that lasted

48

for 6 months. The immobilization improved biofouling resistance in the membranes, whose

49

flux had a decline of 8% after 24 hours of operation in biofouling experiments, while that of

50

the control membranes had a greater decline of ~21%. Investigating of the membranes using

51

showed that the improvement in the biofouling resistance is due to simultaneous

52

improvement of anti-adhesive and antimicrobial properties of the membranes. Fluorescence

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microscopy and FE-SEM indicated simultaneous improvement in anti-adhesive and

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antimicrobial properties of the TFN membranes, resulting in a limited biofilm formation.

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INTRODUCTION

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The demand for water sources has been increasing with the world population. The decreasing

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water supply in arid and semiarid areas has motivated more use of unconventional water

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sources (for example, brackish and seawater desalination, and wastewater treatment) 1.

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Membrane-based technologies can play a major role in addressing the increasing water

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demand. In these technologies, membrane fouling is a major issue, as it lowers membrane

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performance, increases operational costs, and shortens membrane life 1-3.

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Forward osmosis (FO) is a high water-recovery and low-cost membrane-based

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technology that has potential applications in desalination 4 and wastewater treatment

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advantage of FO (beneficial for fouling control) over pressure-driven membrane technologies

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is that it does not need hydraulic pressure to operate 6-12. Nevertheless, biofouling has limited

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the use of FO, in particular, for feeds containing microorganisms 9, 12-13.

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2, 5

. An

There are four main types of membrane fouling: organic, inorganic, colloidal, and 14-17

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microbial (biofouling)

. Compared to organic and inorganic fouling, biofouling is much

75

more complex, because in biofouling, the foulants are microorganisms in the feed solution.

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The microorganisms attach to the membrane surface, propagate, and produce sticky

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extracellular polymeric substances (EPSs), leading to the formation of aggregated biofilms 18-

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20

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filtration process consumes more energy

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entire membrane surfaces. Therefore, a small amount of them in the feed solution can result

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in severe biofouling 18. Furthermore, EPS matrices enhance the adhesion of microorganisms

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on the membrane surface and prevent the biofilms from being treated by oxidizing agents,

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biocides, and antibiotics 23-24.

. Once the biofilms are stabilized on membrane surfaces, water flux decreases and the 21-22

. Microorganisms are capable of colonizing

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Thin film composite (TFC) membranes based on polyamide materials have been used

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predominantly as osmotic membranes in water and energy applications 25-29. Biofouling poses



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a unique challenge to these membranes, because the membranes cannot tolerate oxidants such

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as chlorine that is inexpensive and widely used

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mitigation strategies are needed.

30-31

.

Therefore, effective biofouling

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Surface modification of TFC membranes by directly incorporating or anchoring

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biocide agents is a promising approach to improving membrane properties (such as

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biofouling resistance). The appeal of this approach is mainly due to the ability of biocidal

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agents on membrane surfaces to deactivate bacteria upon a contact. Metal/metal oxide

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nanoparticle systems are commonly used to prepare antimicrobial membranes. For instance,

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different types of biocidal nanomaterials including silver (Ag) nanoparticles and silver-based

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compounds 32-35, copper (Cu) nanoparticles 35-36, titanium dioxides (TiO2) 37, and zinc oxides

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(ZnO) 38 have been used to develop antibacterial membranes. Applying silver-based materials

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has been recognized as an effective approach to decreasing membrane biofouling

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these materials have strong and broad biocidal activity against bacteria, fungi, and viruses 41.

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Nevertheless, the direct blending method has some disadvantages. First, the biofouling

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mitigation does not last long, as the particles have weak resistance to washing (they are easily

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released), causing the membranes to quickly lose their antimicrobial function 36, 42-44. Second,

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membrane performance (membrane selectivity) deteriorates as a result of low compatibility

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between the inorganic nanomaterials and organic membrane

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32-33, 39-40

;

44-45

.

Recent studies have generally focused on forming nanomaterial agents on membrane

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surface to endow membranes with direct, effective and long-lasting biofouling mitigation

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39-40, 46-48

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(NaBH4), which is not environmentally friendly, has been recommended

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permeability loss during membrane modification is still a major problem 18.

36,

. Among these studies, the use of toxic reductants such as sodium borohydride 39, 48

. Moreover,

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In recent years, metal-organic frameworks (MOFs), which are compounds consisting

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of metal ions or clusters coordinated to organic ligands, have been studied for their great


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49-52

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potential in bactericidal applications

. MOFs are promising antibacterial materials,

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because their inorganic and organic components can provide platforms to generate strong

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bactericidal activity and biocompatibility 52-56. A major advantage of MOFs is their ability to

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act as a reservoir of metal ions that are inherent parts of their molecular structure. The metal

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ions are stabilized by the formation of chemical bonds to the organic linker that are

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sufficiently strong to make the MOF structure adequately robust without blocking their

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antibacterial activity. Another advantage of MOFs is their uniform distribution of metal

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active sites. Thus, MOFs provide a long-lasting antibacterial effect and prevent metal

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agglomeration and oxidation 50.

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In this work, we investigate the use of silver-based MOF nanocrystals to mitigate the

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biofouling in FO membranes. The MOF nanocrystals have a good compatibility with the

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polyamide layer because of their organic ligand. Consequently, the membrane active layer is

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improved without detrimental effects on the layer selectivity. Results presented in this paper

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highlight the great potential of MOF compounds in improving the biofouling resistance of

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membranes. To best of our knowledge, this work is the first study of MOFs for biofouling

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control in membranes.

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MATERIALS AND METHODS

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MOF nanocrystal synthesis and characterization. The synthesis protocol was adapted

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from typical conditions reported in the literature 57. The MOF nanocrystals were synthesized

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under ultrasonic irradiation at a frequency of 24 KHz (Heilscher UP400s, Germany) for a

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reaction time of 60 min. The output and pulse of ultrasonic waves were kept at 80W and 0.6,

133

respectively.

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dissolved in 40 ml dimethylformamide (DMF) as solvent, and then mixed with 40 ml solution

135

of silver nitrate [AgNO3, (1 g)] in DMF. Afterwards, the product was centrifuged, and the

In a typical synthesis, 1,3,5-benzenetricarboxylic acid [BTC, (1 g)] was



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precipitate was washed with a mixture of water and ethanol (1:1), and then dried at 60°C for

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24 h. The morphology of MOF nanocrystals was observed using transmission electron

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microscopy (TEM, Zeiss EM900), operated at 20 kV. Chemical and functional groups of the

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MOF nanocrystals were determined using a Bruker-IFS 48 FTIR spectrometer (Ettlingen,

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Germany) with a horizontal ATR device. X-ray powder diffraction (XRD) patterns for the

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MOF nanomaterial were recorded at 298 K using a XPERT-PRO X-ray diffractometer. The

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chemical characterization of MOF block was investigated via energy-dispersive X-ray (EDX)

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spectroscopy. Size distributions of MOF nanocrystals were measured in n-hexane organic

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solution via dynamic light scattering (DLS, Nano ZS ZEN 3600).

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Immobilization of MOF nanocrystals in the active layer of FO membranes. The

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selective layer of FO membranes was prepared via interfacial polymerization (IP) on

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polyethersulfone (PES) substrates. Further information on the PES substrate synthesis and the

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membranes characterization are given in the Supporting Information (SI). Thin film

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nanocomposite (TFN) membranes were prepared by immobilizing the silver-based MOF

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nanocrystals within their selective layer during IP process. The PES substrate was immersed

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in a 2.0 wt.% 1,3-phenylendiamine (MPD) solution for 2 min. The excess MPD solution was

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carefully removed from the membrane surface by an air-knife. The MPD-soaked membrane

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was then dipped in a 0.1 wt./v% trimesoylchloride (TMC) in a n-hexane solution for 30 sec.

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The MOF nanocrystals were loaded in a TMC-n-hexane solution (0.02 wt./v%) and dispersed

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by ultrasonicating for 30 min. The reaction of MPD and TMC at the interface resulted in the

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formation of an ultrathin polyamide rejection layer on the PES substrates. Afterward, the

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membranes were cured in an oven at 80 °C for 5 min. The prepared membranes were stored

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in 20 °C deionized water before being tested.

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Statistical Analysis. To confirm any observed difference is due to the MOF

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modification but not an experimental error, significant differences (α = 0.05) were


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determined via Student’s t-test with two-tailed distribution and reported as p-values.

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Microsoft Excel® software was used for the calculations. P-values less than 0.05 suggest that

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the differences are statistically significant.

164 165

REDULTS AND DISCUSSIONS

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Characterization of the MOF nanocrystals. The silver-based MOF was characterized using

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TEM, IR and EDX. The TEM image in Fig. 1a shows the morphology of the MOF

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nanocrystals prepared by the ultrasonic method. The chemical composition of the MOF was

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characterized via energy-dispersive X-ray (EDX) analysis, as presented in Fig. 1b. The EDX

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analysis indicates that oxygen-to-silver atom ratio in the MOF building block is 2.52. The IR

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spectrum of the MOF nanocrystals (Fig. 1c) shows the characteristic peaks for C–H (690-760

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cm-1), C–O (1090 cm-1), and C=C bonds (1620 cm-1), and also oxygenated C=O (1735 cm-1)

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and O–H (3300-3600 cm-1) groups. The absorption bands of the carboxylate group and the

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aromatic benzene ring are consistent with the nature of the organic ligand

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measurement indicated that the size distribution of nanocrystals has a sharp and narrow peak

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with an approximately 33 nm mean size (Fig. 1d), which is in the range of the MOF size

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measured from the TEM image. The organic ligands and the small size of the MOFs lead to

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no defects in the membrane active layer when MOFs are present in the active layer. The

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XRD pattern of the MOF nanocrystals presented in the SI (Fig. S1) had three diffraction

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peaks for silver: Ag (111), Ag (200), and Ag (220)

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and intense peaks at 6.6°, 9.8°, 11.2° and 13.1°, confirming the crystalline structure in the

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synthesized nanomaterial.

58

. The DLS

35, 59

. In addition, there were other sharp

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Characterization of the TFN membranes. Loading MOF nanocrystals alter

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polyamide-layer specifications of the TFN membrane. Fig. 2 shows the surface micrographs

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of the TFC and TFN membranes obtained using different detectors of field emission scanning



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electron microscopy (FE-SEM). The left-side images were taken using a common secondary

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detector, while the right-side images using a backscattered imaging detector. The bright areas

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in each of the modes have a different meaning. In the secondary imaging mode, the bright

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areas represent a projection or a hill on the membrane surface. Instead, the bright points in

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the backscattered imaging mode indicate the existence of a high-atomic-number element on

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the sample surface. Heavy elements (such as silver in this case) backscatter electrons more

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strongly than light elements (such as carbon, nitrogen and oxygen in the polyamide layer),

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and thus they appear brighter in the image

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changes in the overall morphology of the membrane surface (“ridge and valley”). The

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backscattered images showed bright spots on the surface of the TFN membranes, which can

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be attributed to the silver, as characteristic element of MOF nanocrystals used (Fig. 2b, right-

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side). This observation clearly identified the MOF nanocrystals in the active layer of TFN

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membrane.

60-61

. Loading MOF nanocrystals did not cause

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A higher magnification of the FE-SEM images of the TFN membrane surfaces is

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shown in Fig. 2c. The images indicate that MOF nanocrystals exist within the thin selective

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layer of the TFN membranes and on their surfaces as well. It seems that the MOF

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nanocrystals have been completely surrounded by the thin-film polyamide matrix (orange

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arrows). This agrees well with the good compatibility of the MOF nanocrystals and

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polyamide network, causing the two phases to match well (without any gaps between the two

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phases). In addition, the EDX spectrum was obtained for different bright points on the TFN

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membrane surface, and the results again revealed that these points were associated with the

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MOF nanocrystals (Fig. 2c).

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Fig. 3 illustrates cross-section FE-SEM micrographs of the TFC and TFN membranes.

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As can be seen, both membranes have identical active-layer thicknesses (Figs. 3a and 3b).

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The atomic force microscopy (AFM) results shown in Fig. 3d also indicate no considerable


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alteration in surface roughness of the membranes after immobilizing MOF nanocrystals. The

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results presented in Figs. 2 and 3 suggest that MOF nanocrystals do not significantly affect

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overall morphology of TFN membrane surface, as its thin-film polyamide features were

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similar to those of the TFC membranes.

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Fig. 3f compares the water contact-angle of the TFN membrane to that of the control

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TFC membrane. A statistical analysis of the contact angle results indicate that the presented

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results are statistically significant (P-value < 0.05). The contact angle results demonstrate that

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the surface of TFN membrane become more hydrophilic by immobilizing the MOF

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nanocrystals. The enhanced hydrophilicity of the TFN membrane is due to the hydrophilic

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property of the MOF nanocrystals coming from the functional groups of the nanocrystals

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(Fig. 1c).

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The polyamide active layers of the FO membranes were analyzed for the elemental

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composition, chemical bonding, and cross-linking degree through X-ray photoelectron

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spectroscope (XPS). The XPS surveys spectra of TFC and TFN membranes presented in Fig.

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4 confirm the presence of oxygen (O 1s), nitrogen (N 1s), and carbon (C 1s) elements at the

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membranes surface (~ 10 nm depth). Signals at around 368 eV and 374 eV, attributed to Ag

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3d orbitals, appeared in the spectrum of the TFN membrane, prove the existence of the MOF

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nanocrystals in the top 10 nm depth of TFN membrane surface.

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The element ratios of O/N, important properties of the polyamide layer which reflect

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the layer cross-linking degree, were calculated from the XPS spectra. These results are

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summarized in Table 1. Theoretically, O/N ratio varies between 1.0 and 2.0. A value of 1.0

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indicates that the polyamide layer is fully cross-linked, while a value of 2.0 corresponds to a

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fully linear structure

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MOF nanocrystals into the polyamide layer increases O/N ratio of the TFN membranes. This

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behavior can be due to the formation of a less-cross-linked structure in the polyamide

62-63

. The elemental-composition analysis shows that immobilizing the



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network or the existence additional oxygen sources of the MOF organic block in the

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membrane. Atomic concentrations determined from the MOF EDX analysis (Fig. 1b) and

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XPS analysis (Table 1) were used to estimate the oxygen percent added from the MOF. The

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concentration values allow one to determine if an increase in the O/N ratio is due to a smaller

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cross-linking degree or the existence of additional oxygen sources in the MOF. Table 1

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shows two kinds of O/N ratios (X and Y). The O/N ratio denoted by X was determined

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directly from the XPS results, while the ratio denoted by Y was calculated after eliminating

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the oxygen content added by the MOF. As shown in Table 1, the O/N ratio of the TFC

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membrane was 1.02, indicating 97% cross-linking in its selective layer. Loading the MOF

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nanocrystals to the TMC organic solution increased the O/N ratios of the TFN membrane to

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1.04 (ratio denoted by Y), indicating 93% cross-linking. The change in cross-linking is due to

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the interrupting polymer chains in the polyamide network (caused by the presence of MOF

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nanocrystals).

249

High-resolution XPS spectra of carbon (1s) are presented in Figs. 4b and 4c for the

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TFC and TFN membranes, respectively. Peak fitting was carried out to provide information

251

of the chemical bonding. The carbon (1s) spectrum of TFC membrane shows two peaks (Fig.

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4b): a major peak at 284.8 eV corresponds to carbons without adjacent electron withdrawing

253

groups (such as carbons in aliphatic and aromatic C–C or C–H bonds)

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peak at 287.9 eV. The peak with the 3.1 eV shift corresponds to carbons in a strong electron-

255

withdrawing environment (most likely those in carboxylic O=C–O and amide O=C–N

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groups)

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peak at 285.9 with an intermediate binding energy shift (Fig. 4c), which is likely associated

258

with carbons in a weak electron withdrawing environment (most likely carbons in C–O

259

bonds). This exclusive peak is likely to a sign of the organic building block of the MOF

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nanocrystals in the active layer of TFN membrane. The results indicate that loading MOF

62, 64-65

, and a minor

62-63, 66

. Compared to the TFC membranes, the TFN membranes have an additional


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nanocrystals alters the physiochemical properties of the polyamide layer of the TFN

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membrane. These alterations are expected to modify the transport properties of the resultant

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membrane.

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Effects of MOF nanocrystals on transport through the active layer of TFN

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membranes. The effects of immobilizing MOF nanocrystals on the transport properties of

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the membranes were investigated, and the results are summarized in Table 2. The intrinsic

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water and solute permeability coefficients (denoted by A and B, respectively) of the

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membranes were calculated based on the mean of each three independent FO experimental

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data and using the four-step FO characterization protocol described in Ref. 67. As can be seen

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in Table 2, immobilizing the MOF nanocrystals enhanced the water permeability by about

271

55%. Likewise, the solute permeability coefficient (B) of the TFN membrane is slightly

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higher than that of the TFC membrane. Immobilizing the MOF nanocrystals reduces the

273

solute permeability/water permeability ratio (B/A) in the TFN membrane, pointing to an

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improved permselectivity of the membrane (Table 2). Generally, a low B/A ratio is desired to

275

enhance selectivity and decrease fouling tendency, thus improving the FO process stability 68-

276

70

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membrane. This verifies that there is no gap between the incorporated nanomaterials and the

278

polyamide network. The results suggest the presence of a lower transport resistance in the

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selective layer of TFN membrane, which can be attributed to the following factors. First,

280

loading the MOF nanocrystals in the TMC organic solution may affect the IP process, which

281

increases the fractional free-volume in the polyamide matrix due to the disrupted polymer

282

chain packing

283

have a lower cross linking degree of polyamide layer compared to the TFC membranes

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(Table 1). Second, the more hydrophilic surface of the TFN membrane can attract water

285

molecules, initiating a faster overall flow of water molecules through the layer. In summary,

. Moreover, the TFN membrane provided a slightly higher salt rejection than the TFC

71

. This is supported by the XPS results, showing that the TFN membranes



286

these results indicate that immobilizing the MOF nanocrystals facilitate the transport across

287

the selective layer of TFN membrane, while preserving the membrane selectivity.

288

Effects of MOF nanocrystals on the biocidal activity of the TFN membranes. The

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bactericidal activity of the membranes was assessed for two model bacteria: E. coli and S.

290

aureus, as detailed in the SI. Fig. 5 shows the results of antibacterial assays of the TFN

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membrane, normalized by the number of attached live bacteria on the TFC membrane. As

292

can be seen, the presence of the MOF nanocrystals significantly reduced the number of viable

293

bacteria attached to the TFN membrane, demonstrating antimicrobial activity of the TFN

294

membrane surface. One-hour incubation tests of the TFN membranes achieved bacterial

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inactivation rates of over 96% and 90% for E. coli and S. aureus, respectively, relative to

296

those of the TFC membranes. A statistical analysis indicated that the presented results of the

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membranes' biocidal activity are statistically significant, with the P-value less than 0.05

298

(shown by the stars above the bars). This bactericidal activity was achieved with a very low

299

loading (0.02%) of the MOF nanocrystals. This loading level is much less than the loading

300

levels of silver compounds used in previous studies35,

301

antibacterial activity. This can be attributed to the minimal aggregation of MOF nanocrystals,

302

leading to an effective distribution of biocidal agents over the membrane surface. This

303

finding is in agreement with previous studied49, 73 that silver-based MOFs have much better

304

antibacterial activities than many commonly-used silver-based compounds at low

305

concentrations, and that they have long-term efficiency for biocidal capability. In summary,

306

the MOF nanocrystals are still active when immobilized into the membrane selective layer

307

and impart biocidal properties to the surface of TFN membranes.

72

to provide membranes with

308

To evaluate the stability of the biocidal activity of the TFN membranes, the

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membranes were stored in a water container for two different durations: 24 days and six

310

months. The stored membranes were then evaluated for their antibacterial properties again. 11 ACS Paragon Plus Environment

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After 24 days of membranes being in water, there was a slight increase in the number of

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attached live bacteria to the membranes, indicating no significant change in their bactericidal

313

activity (Fig. 5). Likewise, there was no considerable change in biocidal activity of the TFN

314

membranes after six months of being in water. These results point to another very appealing

315

feature of MOF nanocrystals; that is, imparting a stable antibacterial activity to the TFN

316

membranes for long-term applications.

317

The MOF nanocrystals activity against bacteria comes from one or more biocidal

318

agent in their structure. The silver sites in the framework can be active against bacteria.

319

Furthermore, the organic ligand used in the framework may have antimicrobial action owing

320

to its functional groups. Leaching of Ag+ from the MOF nanocrystals may be the cause of

321

bacterial cell inactivation. The released Ag+ may interact with the thiol groups of proteins and

322

disrupt integrity of bacterial membrane 74-75. Also, functional groups of organic ligand in the

323

framework can bond with cations in cell, causing modification and fragmentation of DNA 76.

324

Ultimately, these leads to cytoplasm outflow and death of the bacteria.

325

As the dynamics of Ag+ release from the MOF nanocrystals immobilized in TFN

326

membrane active layer control the duration of its biocidal activity, we examined the silver-

327

ion release from the TFN membrane in batch experiments. As Fig. 6 shows, the silver-ion

328

release from the TFN membrane decreased sharply from the initial value of about 0.07

329

µg.cm-2.day-1 during the first 2 days. The release rate then decreased very slowly afterwards.

330

This Ag+-release rate demonstrates the long-term durability of biocidal activity exhibited by

331

the TFN membranes. This implies that the MOF can work as an Ag+ reservoir immobilized in

332

the membrane active layer, providing a controlled sustained Ag+ release

333

indicate that the MOF imparts a long-lasting antibacterial activity to the membrane surface to

334

mitigate biofouling in a prolonged period of the membrane operation. This attractive

335

performance is much better than those reported in previous studies where other silver


50, 77

.

These results


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compounds, such as silver nanoparticles, were used and the resulting membranes had

337

antibacterial activity only over a short period of time 39-40, 48.

338

Effects of MOF nanocrystals on the biofouling resistance of the TFN

339

membranes. To determine how the TFN membrane surface inhibits the biofilm formation,

340

the membranes were examined in biofouling experiments according to the protocol described

341

in ref.14,

342

biofouling experiments were performed twice independently. Fig. 7 shows the corresponding

343

results. During the experiments, a gradual decline in the water flux was observed. The decline

344

is due to the adhesion of bacteria and hence biofouling on the membrane surface. The

345

presence of E. coli in the feed solution affected the flux of the TFC membrane considerably.

346

Over the course of 24 h, the FO water flux through the TFC membrane dropped about 20-

347

24%, indicating that the membrane easily fouled under the conditions. In contrast, the TFN

348

membrane showed only a 6-10% decline in water flux, indicating much better biofouling

349

resistance. Cross-flow cleaning yielded more than 90% recovery in FO water flux for both

350

membranes (higher level of recovery for the TFN membrane) (Fig. 7c).

78

and detailed in the SI. To show that the observed trends are reproducible, the

351

To understand the role of MOF nanocrystals in biofouling mitigation, attached

352

bacteria on the biofouled membranes were further investigated. Fig. 8 shows representative

353

images of the TFN and TFC membranes using FESEM and fluorescence microscopy. As can

354

be seen, less bacteria were found on the surface of the TFN membranes than on the surface of

355

the TFC membranes (Figs. 8a and 8b). The quantitative analysis presented in Fig. 8e

356

supported this issue so that considerably fewer numbers of bacteria were attached to TFN

357

membrane surface (~ 48 %) than to the TFC membrane surface. This is in agreement with the

358

anti-adhesion properties of the TFN membrane surface against the bacteria

359

live/dead staining experiment revealed that a large portion of the bacteria on the TFC

360

membranes appeared in green (seemed alive) (Fig. 8c). In contrast, the cells on the TFN


18, 79-80

. The

Page 15 of 30


361

membranes appeared in red, indicating the strong capability of these membranes to kill the

362

bacteria (Fig. 8d). This observation is in good agreement with the biocidal activity results

363

shown in Fig. 5.

364

This study showed that biocidal activity imparted by MOF nanocrystals contribute to

365

the mitigation of biofouling on TFN membranes. It demonstrated that it is possible to

366

improve both the biocidal activity and the hydrophilicity of the membrane active layer,

367

without any adverse effects on the membrane selectivity. MOFs are very attractive materials

368

for developing high-performance FO membranes. They have great potential in improving the

369

biofouling resistance of FO membranes that require both antibacterial and anti-adhesion

370

features.

371

Supporting Information

372

Additional Materials and Methods, FO performance results (Table S1), and XRD pattern of

373

the MOF nanocrystals (Fig. S1) are available free of charge via the Internet at

374

http://pubs.acs.org.

375

Acknowledgment

376

The authors would like to express their thanks to Prof. Joel B. Sheffield, Department of

377

Biology, Temple University, for permitting them to use his fluorescence microscope for this

378

work.

379 380 381



382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429

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(39) Ben-Sasson, M.; Lu, X.; Bar-Zeev, E.; Zodrow, K. R.; Nejati, S.; Qi, G.; Giannelis, E. P.; Elimelech, M., In situ formation of silver nanoparticles on thin-film composite reverse osmosis membranes for biofouling mitigation. Water Res. 2014, 62, 260-270. (40) Yin, J.; Yang, Y.; Hu, Z.; Deng, B., Attachment of silver nanoparticles (AgNPs) onto thin-film composite (TFC) membranes through covalent bonding to reduce membrane biofouling. J. Membr. Sci. 2013, 441, 73-82. (41) Rai, M.; Yadav, A.; Gade, A., Silver nanoparticles as a new generation of antimicrobials. Biotechnol. Adv. 2009, 27 (1), 76-83. (42) Miller, D. J.; Araújo, P. A.; Correia, P. B.; Ramsey, M. M.; Kruithof, J. C.; van Loosdrecht, M. C. M.; Freeman, B. D.; Paul, D. R.; Whiteley, M.; Vrouwenvelder, J. S., Short-term adhesion and long-term biofouling testing of polydopamine and poly (ethylene glycol) surface modifications of membranes and feed spacers for biofouling control. Water Res. 2012, 46 (12), 3737-3753. (43) Yu, D. G.; Teng, M. Y.; Chou, W. L.; Yang, M. C., Characterization and inhibitory effect of antibacterial PAN-based hollow fiber loaded with silver nitrate. J. Membr. Sci. 2003, 225 (1), 115-123. (44) Ong, C. S.; Goh, P. S.; Lau, W. J.; Misdan, N.; Ismail, A. F., Nanomaterials for biofouling and scaling mitigation of thin film composite membrane: A review. Desalination 2016, 393, 2-15. (45) Daer, S.; Kharraz, J.; Giwa, A.; Hasan, S. W., Recent applications of nanomaterials in water desalination: a critical review and future opportunities. Desalination 2015, 367, 37-48. (46) Zhang, S.; Qiu, G.; Ting, Y. P.; Chung, T.-S., Silver–PEGylated dendrimer nanocomposite coating for anti-fouling thin film composite membranes for water treatment. Colloids Surf., A 2013, 436, 207-214. (47) Rahaman, M. S.; Thérien-Aubin, H.; Ben-Sasson, M.; Ober, C. K.; Nielsen, M.; Elimelech, M., Control of biofouling on reverse osmosis polyamide membranes modified with biocidal nanoparticles and antifouling polymer brushes. J. Mater. Chem. B 2014, 2 (12), 1724-1732. (48) Soroush, A.; Ma, W.; Silvino, Y.; Rahaman, M. S., Surface modification of thin film composite forward osmosis membrane by silver-decorated graphene-oxide nanosheets. Environmental Science: Nano 2015, 2 (4), 395-405. (49) Lu, X.; Ye, J.; Zhang, D.; Xie, R.; Bogale, R. F.; Sun, Y.; Zhao, L.; Zhao, Q.; Ning, G., Silver carboxylate metal–organic frameworks with highly antibacterial activity and biocompatibility. J. Inorg. Biochem. 2014, 138, 114-121. (50) Wyszogrodzka, G.; Marszałek, B.; Gil, B.; Dorożyński, P., Metal-organic frameworks: mechanisms of antibacterial action and potential applications. Drug Discov. Today 2016, 21 (6), 1009-1018. (51) Rodríguez, H. S.; Hinestroza, J. P.; Ochoa‐Puentes, C.; Sierra, C. A.; Soto, C. Y., Antibacterial activity against Escherichia coli of Cu‐BTC (MOF‐199) metal‐organic framework immobilized onto cellulosic fibers. J. Appl. Polym. Sci. 2014, 131 (19), 4081540819. (52) Zhuang, W.; Yuan, D.; Li, J. R.; Luo, Z.; Zhou, H. C.; Bashir, S.; Liu, J., Highly Potent Bactericidal Activity of Porous Metal‐Organic Frameworks. Adv. Healthc. Mater. 2012, 1 (2), 225-238. (53) Cavicchioli, M.; Massabni, A. C.; Heinrich, T. A.; Costa-Neto, C. M.; Abrão, E. P.; Fonseca, B. A. L.; Castellano, E. E.; Corbi, P. P.; Lustri, W. R.; Leite, C. Q. F., Pt (II) and Ag (I) complexes with acesulfame: Crystal structure and a study of their antitumoral, antimicrobial and antiviral activities. J. Inorg. Biochem. 2010, 104 (5), 533-540.


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(54) Wang, K.; Ma, X.; Shao, D.; Geng, Z.; Zhang, Z.; Wang, Z., Coordination-induced assembly of coordination polymer submicrospheres: promising antibacterial and in vitro anticancer activities. Cryst. Growth Des. 2012, 12 (7), 3786-3791. (55) Ng, N. S.; Leverett, P.; Hibbs, D. E.; Yang, Q.; Bulanadi, J. C.; Wu, M. J.; AldrichWright, J. R., The antimicrobial properties of some copper (II) and platinum (II) 1, 10phenanthroline complexes. Dalton Trans. 2013, 42 (9), 3196-3209. (56) Horcajada, P.; Chalati, T.; Serre, C.; Gillet, B.; Sebrie, C.; Baati, T.; Eubank, J. F.; Heurtaux, D.; Clayette, P.; Kreuz, C., Porous metal-organic-framework nanoscale carriers as a potential platform for drug delivery and imaging. Nat. Mater. 2010, 9 (2), 172-178. (57) Israr, F.; Chun, D.; Kim, Y.; Kim, D. K., High yield synthesis of Ni-BTC metal–organic framework with ultrasonic irradiation: Role of polar aprotic DMF solvent. Ultrason. Sonochem. 2016, 31, 93-101. (58) Autie-Castro, G.; Autie, M. A.; Rodríguez-Castellón, E.; Aguirre, C.; Reguera, E., CuBTC and Fe-BTC metal-organic frameworks: Role of the materials structural features on their performance for volatile hydrocarbons separation. Colloids Surf., A 2015, 481, 351-357. (59) Akhavan, O., Lasting antibacterial activities of Ag–TiO 2/Ag/a-TiO 2 nanocomposite thin film photocatalysts under solar light irradiation. J. Colloid Interface Sci. 2009, 336 (1), 117-124. (60) Echlin, P.; Fiori, C. E.; Goldstein, J.; Joy, D. C.; Newbury, D. E., Advanced scanning electron microscopy and X-ray microanalysis. Springer Science & Business Media: 2013. (61) Goldstein, J.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Romig Jr, A. D.; Lyman, C. E.; Fiori, C.; Lifshin, E., Scanning electron microscopy and X-ray microanalysis. Springer Science & Business Media: 2012. (62) Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O., Probing the nano-and micro-scales of reverse osmosis membranes—a comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements. J. Membr. Sci. 2007, 287 (1), 146-156. (63) Tang, C. Y.; Kwon, Y.-N.; Leckie, J. O., Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry. Desalination 2009, 242 (1), 149-167. (64) Boussu, K.; De Baerdemaeker, J.; Dauwe, C.; Weber, M.; Lynn, K. G.; Depla, D.; Aldea, S.; Vankelecom, I. F. J.; Vandecasteele, C.; Van der Bruggen, B., Physico‐Chemical Characterization of Nanofiltration Membranes. ChemPhysChem 2007, 8 (3), 370-379. (65) Benavente, J.; Vázquez, M. I., Effect of age and chemical treatments on characteristic parameters for active and porous sublayers of polymeric composite membranes. J. Colloid Interface Sci. 2004, 273 (2), 547-555. (66) Wagner, C. D.; Muilenberg, G. E., Handbook of X-ray photoelectron spectroscopy. Perkin-Elmer: 1979. (67) Tiraferri, A.; Yip, N. Y.; Straub, A. P.; Castrillon, S. R.-V.; Elimelech, M., A method for the simultaneous determination of transport and structural parameters of forward osmosis membranes. J. Membr. Sci. 2013, 444, 523-538. (68) Wei, J.; Qiu, C.; Tang, C. Y.; Wang, R.; Fane, A. G., Synthesis and characterization of flat-sheet thin film composite forward osmosis membranes. J. Membr. Sci. 2011, 372 (1), 292-302. (69) Phillip, W. A.; Yong, J. S.; Elimelech, M., Reverse draw solute permeation in forward osmosis: modeling and experiments. Environ. Sci. Technol. 2010, 44 (13), 5170-5176. (70) Zirehpour, A.; Rahimpour, A.; Seyedpour, F.; Jahanshahi, M., Developing new CTA/CA-based membrane containing hydrophilic nanoparticles to enhance the forward osmosis desalination. Desalination 2015, 371, 46-57. 18 ACS Paragon Plus Environment


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(71) Moaddeb, M.; Koros, W. J., Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles. J. Membr. Sci. 1997, 125 (1), 143-163. (72) Lee, S. Y.; Kim, H. J.; Patel, R.; Im, S. J.; Kim, J. H.; Min, B. R., Silver nanoparticles immobilized on thin film composite polyamide membrane: characterization, nanofiltration, antifouling properties. Polym. Adv. Technol. 2007, 18 (7), 562-568. (73) Chamakura, K.; Perez-Ballestero, R.; Luo, Z.; Bashir, S.; Liu, J., Comparison of bactericidal activities of silver nanoparticles with common chemical disinfectants. Colloids Surf., B 2011, 84 (1), 88-96. (74) Matsumura, Y.; Yoshikata, K.; Kunisaki, S.-i.; Tsuchido, T., Mode of bactericidal action of silver zeolite and its comparison with that of silver nitrate. Appl. Environ. Microbiol. 2003, 69 (7), 4278-4281. (75) Feng, Q. L.; Wu, J.; Chen, G. Q.; Cui, F. Z.; Kim, T. N.; Kim, J. O., A mechanistic study of the antibacterial effect of silver ions on Escherichia coli and Staphylococcus aureus. Journal of biomedical materials research 2000, 52 (4), 662-668. (76) Pulido, M. D.; Parrish, A. R., Metal-induced apoptosis: mechanisms. Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis 2003, 533 (1), 227-241. (77) Berchel, M.; Le Gall, T.; Denis, C.; Le Hir, S.; Quentel, F.; Elléouet, C.; Montier, T.; Rueff, J.-M.; Salaün, J.-Y.; Haelters, J.-P., A silver-based metal–organic framework material as a ‘reservoir’of bactericidal metal ions. New J. Chem. 2011, 35 (5), 1000-1003. (78) Perreault, F.; Jaramillo, H.; Xie, M.; Ude, M.; Elimelech, M., Biofouling Mitigation in Forward Osmosis using Graphene Oxide Functionalized Thin-Film Composite Membranes. Environ. Sci. Technol. 2016, 50 (11), 5840-5848. (79) Yang, H.-L.; Chun-Te Lin, J.; Huang, C., Application of nanosilver surface modification to RO membrane and spacer for mitigating biofouling in seawater desalination. Water Res. 2009, 43 (15), 3777-3786. (80) Zhu, X.; Bai, R.; Wee, K.-H.; Liu, C.; Tang, S.-L., Membrane surfaces immobilized with ionic or reduced silver and their anti-biofouling performances. J. Membr. Sci. 2010, 363 (1), 278-286.

605 606 607 608 609 610 611 612 613 614


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615 616 617 618 619

Table 1. Elemental compositions, O/N ratio, and the degree of cross-linking of the polyamide selective layer. O/N Ratio

Atomic concentration (%)

Cross-linking degreec (%)

Membrane

620 621 622 623

a

b

C (1s)

O (1s)

N (1s)

Ag (3d)

X

Y

TFC

74.18

13.02

12.81

0

1.021

1.021

97

TFN

75.44

12.55

11.89

0.12

1.054

1.042

93

a

The ratio was calculated directly from atomic concentrations determined using XPS results. The ratio was estimated after eliminating the oxygen content added by the organic block of the MOF. c The degree was calculated from the Y O/N ratios.

b

624 625 626 627

Table 2. Transport parameters of the FO membranes (calculated using the protocol described in Ref. 67). They

628

were estimated from the flux means and standard deviations reported in Table S1. The solute rejection results (R

629

values) are from experiments with a 2000 ppm NaCl feed and at 2.5 bar in the RO mode. A = water

630

permeability coefficient, B = solute permeability coefficient, = water flux coefficient of determination,

631

= solute flux coefficient of determination, and R = solute rejection.

A

B

B/A

(L/(m2.h.bar))

(L/(m2.h))

(1/bar)

TFC

2.10±0.14

0.27±0.02

TFN

3.25±0.18

0.36±0.02

R (%)

0.129±0.018

96.1

0.975

0.992

0.111±0.012

96.8

0.988

0.991

Membrane

632 633 634 635 20 ACS Paragon Plus Environment


636 637 638 639 640

641 642

Fig. 1. Characterization of MOF nanocrystals. (a) TEM micrograph, (b) Element concentration determined via

643

EDX analysis, (c) IR spectrum, identifying the functional groups of MOF, (d) DLS measurement, representing

644

size distribution of the nanocrystals in n-hexane organic solution.

645 646


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647 648

Fig. 2. FE-SEM images of the membranes using different imaging detectors. (a) TFC membrane, (b) TFN

649

membrane, (c) A higher-magnification image of the TFN membrane surface. The left-hand images were taken

650

using a secondary detector, while the right-hand image were taken using a backscattered imaging detector. The

651

backscattered image of TFN membrane shows numerous bright spots on the surface, attributed to the silver

652

element as a MOF nanocrystal characteristic. The orange arrows in the right-side image show the positions of

653

some of the bright spots in the backscattered image. EDX results of three different spectra, revealing the

654

presence of silver as the characteristic element of the MOF nanocrystals used.



655 656

Fig. 3. Thin film polyamide layer characterization of TFC and TFN membranes, (a) Cross-section FE-SEM

657

image of a TFC membrane, (b) Cross-section FE-SEM image of a TFN membrane, (c) AFM image of a TFC

658

membrane, (d) AFM image of a TFN membrane, (e) Surface roughness parameters of TFC and TFN membranes

659

[the mean roughness (Ra), the root mean square of the Z data (Rq), and the mean difference between the five

660

highest peaks and lowest valleys (Rm)], (f) water contact angles of TFC and TFN membrane surfaces (each bar

661

represents the standard deviation of the trials. Asterisks above the bars represent the statistical significance (P-

662

value < 0.05), determined by a student’s t-test).

663 23 ACS Paragon Plus Environment

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664 665

666 667

Fig. 4. XPS analysis of the elemental composition and chemical bonding. (a) Survey spectra of TFC and TFN

668

membranes, (b) high-resolution XPS spectra of carbon (1s) for a TFC membrane, and (c) high-resolution XPS

669

spectra of carbon (1s) for a TFN membrane.

670 671 24 ACS Paragon Plus Environment


672 673

674 675

Fig. 5. Biocidal activity of TFC and TFN membranes, the normalized number of attached live bacteria on the

676

TFC and TFN membranes for E. coli and S. aureus bacteria. The values were normalized by the number of

677

attached live bacteria on the TFC. The orange and violet bars illustrate the antibacterial activity of the TFN

678

membranes after 24 days and 6 months of storing the TFN membranes in water, respectively. Each bar indicates

679

the standard deviation of three independent test, each with a fresh membrane. Asterisks above the bars represent

680

the statistical significance (P-value < 0.05), determined by a student’s t-test.

681 682 683 684 685 686 687 688 25 ACS Paragon Plus Environment

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689 690

691 692

Fig. 6. Release rate of Ag+ from the TFN membrane.

693 694 695 696 697 698 699 700 701 702



703 704

Fig. 7. Normalized water fluxes of TFC and TFN membranes vs. time in biofouling experiments using E. coli

705

(experimental conditions: initial water flux = 27 L/m2h, velocity = 8.5 cm/s, T = 25 °C). (a) First run, (b)

706

Repeated run (Each one represents the biofouling experiment obtained from independent runs with a fresh

707

membrane), and (c) FO water flux recovery ratio after cross-flow cleaning.


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708

709 710

Fig. 8. Bacterial inactivation properties of the membrane surface after biofouling experiment with E. coli. FE-

711

SEM image of (a) TFC membrane, and (b) TFN membrane. Fluorescence microscopy image of (c) TFC

712

membrane surface, and (d) TFN membrane surface, (e) quantitative analysis of the number of attached bacteria

713

to the membranes surface, determined via observing different areas of the membranes. The images c and d were

714

taken using the live/dead staining experiment.



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Mitigation of Thin-Film Composite Membrane Biofouling via

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