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Relating Organic Fouling in Membrane Distillation to Intermolecular Adhesion Forces and Interfacial Surface Energies Chanhee Boo, Seungkwan Hong, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05768 • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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Environmental Science & Technology

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Relating Organic Fouling in Membrane Distillation to Intermolecular Adhesion Forces and Interfacial Surface Energies

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Submitted to Environmental Science & Technology Revised: November 19, 2018 10 11 12 13 14 15 16 17 18 19 20

Chanhee Boo,1, 3 Seungkwan Hong,2* and Menachem Elimelech3*

1Department

21 22

2School

23 24

3Department

25 26 27 28 29 30 31 32

2*E-mail: 3*E-mail:

of Earth and Environmental Engineering, Columbia University, New York, New York 10027-6623, United States

of Civil, Environmental and Architectural Engineering, Korea University, 145 Anam-ro, Seongbuk-Gu, Seoul 02841, Republic of Korea of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States

[email protected]; phone: +82-2-928-7656. email: [email protected]; phone: +1 (203) 432-2789.

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ABSTRACT

34

This study investigates the fouling mechanisms in membrane distillation, focusing on the impact

35

of foulant type and membrane surface chemistry. Interaction forces between a surface-

36

functionalized particle probe simulating a range of organic foulants and model surfaces,

37

modified with different surface energy materials, were measured by atomic force microscopy

38

(AFM). The measured interaction forces were compared to those calculated based on the

39

experimentally determined surface energy components of the particle probe, model surface, and

40

medium (i.e., water). Surfaces with low interfacial energy exhibited high attractive interaction

41

forces with organic foulants, implying a higher fouling potential. In contrast, hydrophilic

42

surfaces (i.e., surfaces with high interfacial energy) showed the lowest attractive forces with all

43

types of foulants. We further performed fouling experiments with alginate, humic acid, and

44

mineral oil in direct contact membrane distillation using polyvinylidene fluoride membranes

45

modified with various materials to control membrane surface energy. The observed fouling

46

behavior was compared to the interaction force data to better understand the underlying fouling

47

mechanisms. A remarkable correlation was obtained between the evaluated interaction force data

48

and the fouling behavior of the membranes with different surface energy. Membranes with low

49

surface energy were fouled by hydrophobic, low surface tension foulants via “attractive” and

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subsequent “adsorptive” interaction mechanisms. Furthermore, such membranes have a higher

51

fouling potential than membranes with high or ultralow surface energy.

52 53

TOC Art

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INTRODUCTION

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Membrane distillation (MD) is an emerging thermal separation process with potential

58

applications for desalination of hypersaline industrial wastewaters, such as those from the oil and

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gas industry and thermoelectric power-generating facilities.1-4 Challenges in treating hypersaline

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industrial wastewaters with membrane technologies arise not only from the high level of total

61

dissolved solids, but also from the high fouling potential of such wastewaters.5,

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wastewaters contain a wide range of organic and inorganic substances that induce severe

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membrane fouling.7, 8 Membrane fouling critically limits MD system efficiency, especially when

64

treatment of wastewaters at high water recovery is desired.9, 10

6

Industrial

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In MD, water vapor transports across the hydrophobic pores of microporous membrane

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following a phase transition at the liquid-pore interface.11 Membrane surface properties are

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expected to govern the fouling behavior in MD, because foulants in the feed stream are in direct

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contact with the membrane surface.12 Surface charge and wettability represent properties that

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strongly influence the fouling of MD membranes like other osmosis and pressure-driven

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membranes.13,

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operation for high-salinity applications where the effect of electrostatic interactions between

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foulants and the membrane surface is suppressed by charge screening.15-17 Thus, elucidating the

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impact of surface wettability on fouling of MD membranes is important for understanding the

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underlying mechanisms and for the development of fouling-resistant membranes.

14

Charge properties, however, may not play a significant role during MD

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Recent innovations in membrane materials and surface modification techniques enable

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fabrication of MD membranes with tailored surface wettability.18-21 Such developments include

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MD membranes with superhydrophobic, omniphobic, and hydrophilic surfaces produced by

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engineering

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Superhydrophobic membranes have been explored most widely to obtain enhanced MD

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performance over conventional hydrophobic membranes, including long-term wetting

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resistance22, and anti-fouling23 and anti-scaling24 properties. Roughening the substrate surface by

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attaching inorganic nanoparticles, such as silica and titanium oxide, followed by coating with

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low surface energy materials, is a typical strategy to fabricate superhydrophobic MD

84

membranes.25, 26

the

surface

chemistry

and

nanostructure


of

a

hydrophobic

substrate.


85

Omniphobic membranes that resist wetting from both water and oil have recently been

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suggested as a means to expand MD application for desalination of challenging wastewaters that

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contain low surface tension contaminants.27,

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omniphobicity can be realized by constructing a nanoscale structure with an increased air-to-

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solid ratio, also known as a re-entrant structure, which provides a local kinetic barrier to

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transition from the meta stable Cassie-Baxter state to the fully wetted Wenzel state for low

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surface tension substances.29 Materials having cylindrical or spherical morphology, such as

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electrospun nanofibers and silica nanoparticles, serve as an ideal platform for omniphobic

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membrane fabrication.30, 31 Other studies have developed composite MD membranes comprising

94

a thin hydrophilic layer on top of a hydrophobic substrate to enhance resistance to oil fouling.32,

95

33

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or inorganic nanoparticles (e.g., silica36 and titanium oxide34) have been employed to obtain a

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hydrophilic coating layer.

98 99

28

In addition to ultralow surface energy,

In such membranes, polymeric materials (e.g., polyethylene glycol34 and polyethyleneimine35)

Measurements of foulant–membrane interaction forces using atomic force microscopy (AFM) have been employed to evaluate the fouling potential of osmosis and pressure-driven

100

membranes.37,

38

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target surface and then retracted after it has been in contact with the surface in a fluid cell filled

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with the test solution. Interaction forces measured when the particle probe approaches the surface

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and when the particle probe is withdrawn after being in contact with the surface allow the

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quantification of electrostatic double layer and adhesion forces, respectively.38 The strengths of

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AFM force measurement in studying membrane fouling mechanisms include: (i) providing

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information on molecular level membrane–foulant interactions that describe the initial fouling

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mechanism, (ii) enabling simulation of the surface chemistry of the target foulant using a

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chemically modified particle probe, and (iii) allowing interaction force measurements under a

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solution chemistry similar to that of the foulant solution. Despite these benefits, to date, there are

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no systematic studies investigating the fouling mechanisms in MD using interfacial forces

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measured by AFM.

During AFM measurement, a functionalized particle probe is brought to the

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In this paper, we investigate organic fouling mechanisms in MD, focusing on the impact of

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foulant type and membrane surface chemistry. Interaction forces between a surface

114

functionalized particle probe and model surfaces modified with different surface energy

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materials were evaluated by atomic force microscopy and theoretical calculations using the 4 ACS Paragon Plus Environment

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measured interfacial energy components. Fouling experiments were performed in direct contact

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membrane distillation with organic foulants having different surface chemistry. By comparing

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molecular level interaction force data obtained from AFM and results from MD organic fouling

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experiments, we provide new insights into the fouling mechanisms of MD membranes with

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different surface wettability and suggest guidelines for the design of antifouling MD membranes.

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MATERIALS AND METHODS

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Materials and Chemicals. A flat sheet polyvinylidene fluoride (PVDF) membrane with a

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nominal pore size of 0.1 m and an average thickness of 125 m was supplied from Millipore

125

(VVHP, Billerica, MA). A mechanical grade silicon wafer was purchased from University Wafer

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(Single side polished 1196, South Boston, MA). Hydrogen peroxide (ACS reagent, 30 wt %) and

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sulfuric acid (ACS reagent, 95.0–98.0%) from Sigma Aldrich (St. Louis, MO) were used to

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prepare a piranha cleaning solution. Sodium hydroxide (ACS reagent, ≥97%), polyvinyl alcohol

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(MW: 13,000-23,000 Da, 98% hydrolyzed), and glutaraldehyde solution (50 wt % in H2O) from

130

Sigma Aldrich (St. Louis, MO) were used for hydrophilic surface modification of the PVDF

131

substrate. (Nonafluorohexyl)triethoxysilane (C12H19F9O3Si, hereafter denoted as 9FAS),

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(heptadecafluorotetrahydrodecyl)triethoxysilane (C16H19F17O3Si, hereafter denoted as 17FAS),

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and n-Decyltriethoxysilane (C16H36O3Si, hereafter denoted as C10) from Gelest (Morrisville, PA)

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and hexane from J.T. Baker (ACS reagent, 98.5%) were used to obtain surfaces with low

135

interfacial energy. Deionized (DI) water was supplied by Millipore System (Millipore Co.,

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Billerica, MA).

137

Surface Modification and Characterization. Model surfaces with different interfacial

138

energy were prepared using a silicon wafer with perfectly smooth surface topology. The silicon

139

wafer was first cleaned using a piranha solution (a mixture of sulfuric acid and hydrogen

140

peroxide at 3:1 volume ratio) for 2 h to maximize silanization efficiency by removing organic

141

contaminants. After thorough washing with DI water, followed by drying on a hot plate (~120 °C)

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for 1 h, the silicon wafer surface was functionalized with 9–FAS, 17–FAS, or C10 via well-

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established silane chemistry.39 The piranha cleaned silicon wafer was immersed in 1% v/v silane

144

solutions in hexane for 24 h, followed by thorough rising with hexane and heat treatment at



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100 °C for 1 h. The prepared model surfaces were used for the evaluation of interfacial energy

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components and AFM interaction force measurements.

147

Two different surface modifications were employed to render the PVDF membrane surface

148

hydrophilic. First, the PVDF membrane was coated with polyvinyl alcohol (PVA), a water-

149

soluble synthetic polymer widely used for surface hydrophilization.40,

150

adsorbs on a hydrophobic surface via physisorption and subsequently polymerizes through

151

crystallization.42 To apply PVA coating exclusively on the top side, the PVDF membrane coupon

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was sandwiched between a clean glass plate and a rubber gasket with a 2.54 cm  2.54 cm

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central hole cut out. The sandwiched membrane was secured by steel clamps, thus creating a

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sealed well. The coating solution was prepared by dissolving 0.5 % w/v PVA in DI water (0.5 g

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PVA in 100 mL DI water) at 90 °C for 12 h under stirring, followed by pH adjustment to 2–3

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using hydrochloric acid (1.0 M). Glutaraldehyde (0.5 mL) was added as a cross-linking agent

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immediately before applying the PVA coating solution to the PVDF membrane. Surface coating

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was carried out for 1 h on an orbital shaker that was placed inside the incubator at a temperature

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of 60 °C, followed by a thorough rinse with DI water to remove any residues. Alkaline treatment

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was used as another method to modify the surface of the PVDF membrane to be hydrophilic.43

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The PVDF membrane was immersed in a 7.5 M NaOH solution at ~70 °C for 3 h to generate

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hydroxyl functional groups on PVDF polymer chain, thereby obtaining a hydrophilic surface. To

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ensure complete soaking of the PVDF membrane in the NaOH solution, the membrane was

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prewetted by ethanol. The PVA coated and alkaline treated PVDF membranes were used for MD

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fouling experiments and oil adsorption tests, respectively.

41

PVA spontaneously

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The PVDF membranes were coated with silane molecules to tailor the surface chemistry with

167

low interfacial energy. The silane coating solutions were prepared by dissolving each 9–FAS or

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17–FAS in hexane at a concentration of 1 % v/v. The PVDF membrane coupon was soaked in

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the silane coating solution for 12 h, followed by a thorough rinse with hexane, and then heat

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treatment at 100 °C for 1 h. The silane molecules bind to the membrane via physisorption and

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attractive hydrophobic-hydrophobic interaction, lowering the surface energy of the PVDF

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membranes.

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Surface morphology of the PVDF membranes coated with PVA, 9–FAS, and 17–FAS was

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investigated by scanning electron microscopy (SEM, Hitachi SU-70). Before imaging, 6 ACS Paragon Plus Environment

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membrane samples were sputter-coated with a chromium layer (BTT-IV, Denton Vacuum, LLC,

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Moorestown, NJ). Acceleration voltage of 5.0 kV was applied to image all membrane samples.

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The elemental composition of the surface-modified PVDF membranes was analyzed by X-ray

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photoelectron spectroscopy (XPS, PHI VersaProbe II, Physical Electrons Inc., MN). The XPS

179

spectra were collected using monochromatic 1486.7 eV Al Kα X-ray source with a 0.47 eV

180

system resolution. The energy scale was calibrated using Cu 2p3/2 (932.67 eV) and Au 4f7/2

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(84.00 eV) peaks on a clean copper plate and a clean gold foil.

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Water and mineral oil contact angles in air and the mineral oil contact angle underwater of

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the PVDF membranes modified with PVA, 9–FAS, and 17–FAS were measured by a contact

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angle goniometer (OneAttension, Biolin scientific instrument). The in-air contact angles were

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measured using the sessile drop method by placing a 2-L liquid on the membrane surface. The

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underwater oil contact angle was measured in a custom-built liquid cell filled with water in

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which mineral oil was injected beneath the membrane surface using a U-shaped needle. The

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liquid droplet placed on the membrane sample was photographed using a digital camera for 10 s.

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The left and right contact angles were analyzed from the digital images by a post-processing

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software (OneAttension software). The measurements were conducted on a minimum of two

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random locations with three different membrane samples, and the data were averaged.

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AFM Measurements. Atomic force microscopy (AFM) was employed to measure the

193

interaction forces between a surface-functionalized particle probe and the model silicon wafer

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surfaces modified with different surface energy materials, adapting the procedure described by

195

Li and Elimelech.38 The particle probe was prepared by attaching a 4-m diameter, silicon

196

dioxide-based particle (Sigma-Aldrich, St. Louis, MO), to the tip of a silicon nitride AFM

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cantilever (Bruker, NP-O10, spring constant of 0.06 Nm-1), using optical adhesive (Optical

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Adhesive 68, Norland Products, Inc., Cranbury, NJ), cured by exposure to UV light for 20 min.

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The cantilever with the attached particle probe was immersed in a 1 % v/v C10 solution in hexane

200

for 12 h to modify the silica particle surface to be hydrophobic, followed by a three-time

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thorough wash with hexane. After surface functionalization, the cantilever was re-examined by

202

microscope to ensure the presence of the particle probe (Figure S1). The bare and C10–

203

functionalized silica particle probes were used as surrogates for hydrophilic and hydrophobic

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organic foulants, respectively. 7 ACS Paragon Plus Environment


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AFM adhesion force measurements were performed in a fluid cell filled with a 0.6 M NaCl

206

solution, which is the same salinity employed for the following MD fouling experiments.

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Cantilever deflection versus separation distance measurements were collected in AFM contact

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mode for the probe particle approach and retraction from the model silicon wafer surfaces. Data

209

were collected at three random locations on each model surface under ambient conditions and 50

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force measurements were taken at each location to minimize inherent variability in the force data.

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Such variability is mainly attributed to the heterogeneity of surface modification. Only the

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retracting (pull-off) force data were analyzed using Nanoscope Analysis Software (Version 1.40,

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Bruker Corporation) to obtain maximum adhesion forces between the particle probe and the

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surface.

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Static and Dynamic Oil Adsorption Tests. Mineral oil-in-water emulsion at a

216

concentration of 500 ppm was used for static and dynamic oil adsorption tests. To prepare the

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emulsion, mineral oil was added to DI water, followed by 60 min of probe sonication (Misonix

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3000, Misonix Inc., Farmingdale, NY) and an additional 10-min sonication in an ultrasonic bath

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(FS60 Ultrasonic Cleaner, Fisher Scientific Co., Pittsburgh, PA). The prepared emulsion was

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highly turbid and stable with no significant phase separation during the oil adsorption tests. The

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hydrodynamic diameter of the oil droplets was measured by dynamic light scattering (DLS)

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(ZetaPals, Brookhaven Instrument, Holtsville, NT). The measured size (diameter) of the oil

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droplets was 1.1 ± 0.2 m before the membrane adsorption tests and increased slightly to 1.3 ±

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0.3 m after the adsorption tests. We note that the size of the oil droplets is much larger

225

compared to the size of the membrane pores (0.1 m).

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The surface-modified PVDF membranes were cut into small pieces (i.e., 0.95 cm in diameter)

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and the edge was covered with water-resistant tape. The edge-covered membrane coupons were

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placed in a 20 mL scintillation vial filled with the mineral-oil-in-water emulsion (500 ppm).

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Static oil adsorption tests were performed by promoting contact between oil and membranes

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using a tube rotator (Elmeco Arma-Al, Rockville, MD) at a rotating speed of 50 rpm for 3 h.

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Dynamic oil adsorption was evaluated using a crossflow membrane filtration system in the

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absence of applied hydraulic pressure, in order to exclude the effect of permeate drag force on oil

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adsorption to the membrane. The surface-modified PVDF membranes were mounted on a

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membrane cell and exposed to the crossflow of mineral-oil-in-water emulsion at a flow rate of 8 ACS Paragon Plus Environment

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0.4 L/min (crossflow velocity of 8.5 cm/s) for 3 h. After static and dynamic oil adsorption tests,

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the membrane coupons were dried in a desiccator for 24 h and punched into small pieces of

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identical size (i.e., 0.64 cm in diameter) for the following thermogravimetric analysis.

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The PVDF samples were analyzed using thermogravimetric analysis (TGA) (TA Instruments

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Q50, New Castle, DE) to quantify the amount of mineral oil adhered to the membrane during

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static and dynamic oil adsorption tests. The weight change of the membrane samples was

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measured with increasing temperature up to 600 °C at a heating rate of 20 °C/min in an air

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atmosphere. The total mass fraction as a function of temperature was analyzed to trace thermal

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decomposition of the mineral oil from the different membrane samples.

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Membrane Distillation Fouling Tests. Fouling behavior of the PVDF membranes

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coated with different surface energy materials was evaluated using a laboratory-scale direct

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contact membrane distillation (DCMD) unit. Three representative organic foulants — alginate,

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Suwannee River humic acid (SRHA or humic acid), and mineral oil — were employed to

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investigate the impact of foulant type on MD membrane fouling. Stock solutions (10 g/L) of

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alginate (Sigma-Aldrich, St. Louis, MO) and humic acid (International Humic Substances

250

Society, St. Paul, MN) were prepared by dissolving the organic foulant in DI water, followed by

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filtration with a 0.45 m filter (Millipore, Bellerica, CA). Mineral-oil-in-water emulsion feed

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was prepared by the procedure described earlier. Feed solutions were prepared with a foulant

253

concentration of 500 ppm in 0.6 M NaCl for all MD fouling experiments.

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Membranes were inserted into a custom-built transparent acrylic cell with an effective

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membrane area of 2.54 cm  2.54 cm. Feed solution temperature was set at 65 °C and permeate

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solution temperature was adjusted to achieve an identical initial water flux of ~10 L m-2 h-1 for

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all membranes. A higher cross-flow rate for the feed stream, 0.4 L/min (cross-flow velocity of

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8.5 cm/s), than for the permeate stream, 0.35 L/min (cross-flow velocity of 7.5 cm/s), was used

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to promote fouling by applying a slightly higher hydraulic pressure on the feed side. The water

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vapor flux, Jw, across the membrane was determined by measuring the increase in permeate

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weight. Electric conductivity of the permeate solution was monitored to assess the salt rejection

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during fouling experiments.

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RESULTS AND DISCUSSION

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Interaction Energy Calculated from van Oss theory. Foulant–membrane interactions

267

govern the initial fouling mechanisms in MD, because the membrane surface is in direct contact

268

with the feed stream while the pores serve as a distillate (permeate) pathway. Hence, quantitative

269

analysis of foulant–membrane interactions is essential to better understand MD membrane

270

fouling behavior.

271

We calculated the interaction energy between a particle probe and the model surfaces with

272

varying interfacial energies using the van Oss theory to investigate the impact of foulant type and

273

membrane surface chemistry on the organic fouling mechanism in MD.44 The total interaction

274

energy between the particle (denoted with subscript 1) and the surface (subscript 2) immersed in

275

water (subscript 3) is obtained by summing the energies from Lifshitz–van der Waals (LW) and

276

Lewis acid-base (AB) interactions45:

277

Total LW AB G132  G132  G132

(1)

278

Following the Depré equation, the interaction energy between two condensed materials

279

immersed in a medium (i.e., water) can also be expressed in terms of interfacial LW and AB

280

forces45:

281

LW G132   12LW   13LW   23LW

(2)

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AB G132   12AB   13AB   23AB

(3)

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Here the interfacial LW force is derived from a balance between the attractive force toward the

284

bulk material i and surface tension of material j as proposed by Girifalco and Good46, 47

285

 ijLW 

286

Note that interfacial LW force,  ijLW , can only be positive or zero, which implies attractive LW

287

interaction between two condensed materials in most cases.48 In contrast, interfacial AB force is

288

determined by the interplay between electron acceptor,   , and electron donor,   , components,

289

which are essentially asymmetric and can be negative (i.e., attractive) or positive (i.e., repulsive):

290

 ijAB  2



 iLW   LW j



 i   j





2

(4)

 i   j



(5)


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Table 1 summarizes our calculated surface energy components of the particle probe (i.e.,

292

OH– and C10–functionalized silica particles) and model surfaces with different interfacial energy

293

(i.e., OH, 9–FAS, and 17–FAS functionalized silicon wafer) by the extended Young-Dupré

294

equation using the measured intrinsic contact angles of probe liquids (Table S1 and S2). Details

295

on the calculation of surface energy components are described in the Supporting Information. TABLE 1

296 297

Once the surface energy components (i.e., LW, +, and +) of all involved materials in a given

298

system have been measured, the interaction energy between the particle and the surface

299

immersed in water, G132 , can be determined by

TOT 300 G 132  2



 3LW   1LW



  

 2LW   3LW  2

 3



 1   2   3   3

301





 1   2   3   1 2   1 2

(6)

302

Figure 1 shows the calculated interaction energy between (A) hydrophilic or (B) hydrophobic

303

particle and surfaces having different interfacial energy. The interfacial force (or interfacial

304

tension) presented on the left axis describes the miscibility of two different materials. Generally,

305

a positive interfacial force indicates a repulsive tendency between two materials while a negative

306

value implies their affinity. For instance, the interfacial force of hydrophilic particle–water is

307

calculated to be negative 5.7 mN/m (Figure 1A, orange bar with horizontal pattern), while that of

308

hydrophobic particle–water is positive 28.4 mN/m (Figure 1B, orange bar with horizontal

309

pattern), indicating their affinity and immiscibility with water, respectively. The interaction

310

energy (presented as red bar with checked pattern) on the right axis describes an interplay that

311

involves three components: a repulsive (positive value) or an attractive (negative value) force

312

between the foulant (denoted with subscript “1”) and the surface (subscript “2”) immersed in

313

water (subscript “3”) as described in eq 6.

314

FIGURE 1

315

In most cases, interaction energy values were negative (red bar with checked pattern),

316

indicating attractive interaction when the particle (or foulant) approaches the surface (or

317

membrane) in water. A positive interaction energy, which indicates a repulsive force, was

318

obtained only for a contact between the hydrophilic particle and the hydrophilic surface (first red

319

bar with checked pattern in Figure 1A). This result implies that deposition or adsorption of

320

hydrophilic foulants on membranes with high interfacial energy (or hydrophilic surface) is not 11 ACS Paragon Plus Environment




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favorable.49 Interaction energies between the hydrophilic particle and the surfaces functionalized

322

with 9–FAS and 17–FAS were negative, –2.4 and –7.5 mJ/m2, respectively; however, these

323

values were relatively small, indicating weak attractive forces. In contrast, very strong attractive

324

forces were identified between the hydrophobic particle and the surfaces modified with low

325

surface energy materials (–51.2 and –61.5 mJ/m2 for the 9–FAS and 17–FAS functionalized

326

surfaces, respectively, Figure 1B), which is about an order of magnitude higher than the cases

327

with the hydrophilic particle (Figure 1A). This observation suggests that deposition of

328

hydrophobic foulants on membranes with low surface energy is more favorable compared to that

329

on membranes with a hydrophilic surface.

330

Adhesion Forces Measured by AFM. Adhesion forces measured using AFM provide a

331

quantitative assessment of the interactions between the foulant and the membrane surface.38

332

Results from AFM measurements are presented in the form of adhesion force normalized by the

333

radius of the particle probe, Fad/R, to allow meaningful comparison of particle–surface

334

interactions per unit contact area (Figure 2). These Fad/R values are proportional to the energy

335

per unit area required to separate a particle from a flat surface to an infinite distance, W(∞), as

336

described by Fad  2 RW () .50 Therefore, Fad/R values are also viewed as an indicator of the

337

membrane fouling potential, similar to the interaction energy calculated based on the van Oss

338

theory shown in the previous section.38 FIGURE 2

339 340

We conducted AFM force measurements in a fluid cell filled with a 0.6 M NaCl solution to

341

suppress the effect of electrostatic interactions on the overall interaction between the particle and

342

the surface, in accordance with the assumption made for the interaction energy calculation.

343

Negligible contribution of electrostatic interaction to the overall foulant–membrane interaction at

344

solution salinities above ~0.15 M is corroborated by findings from previous studies.51,

345

identical solution salinity (0.6 M NaCl) was employed for the MD fouling experiments,

346

described later in this paper.

52

An

347

As shown in Figure 2A, very small adhesion force (–0.08 mN/m) was measured for the

348

interaction between the hydrophilic particle probe and the hydrophilic model surface (control Si

349

wafer, blue bar with checked pattern). Adhesion forces between the hydrophilic particle probe

350

and the surfaces with low interfacial energy were also relatively small: –0.8 and –1.3 mN/m for 12 ACS Paragon Plus Environment

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351

the 9–FAS and 17–FAS functionalized surfaces, respectively. These results correlate well with

352

the previous theoretical interaction energy calculations where the attractive forces between the

353

hydrophilic particle and the two FAS–modified surfaces were small (Figure 1A). Attractive

354

interactions between hydrophobic foulants and the hydrophilic surface are also expected to be

355

low, as indicated by a relatively small adhesion force between the hydrophobic particle probe

356

and the control silicon wafer (–1.1 mN/m, presented in the blue bar with checked pattern in

357

Figure 2B). In contrast, much stronger AFM adhesion forces of –7.2 and –9.4 mN/m were

358

measured between the hydrophobic particle probe and the 9–FAS and 17–FAS functionalized

359

surfaces, respectively (orange and red bars with diagonal pattern in Figure 2B), which are close

360

to an order of magnitude higher than that measured with the hydrophilic particle probe.

361

The attractive interactions between the particle probe and chemically modified model

362

surfaces measured by AFM are generally in good agreement with the results calculated based on

363

the van Oss theory. Both approaches reveal that the hydrophilic surface (i.e., high interfacial

364

energy) provides relatively small attractive forces with both the hydrophilic and hydrophobic

365

particles, while the surfaces with low interfacial energy exhibit stronger attractive forces to the

366

particles, especially to those that are hydrophobic. In the following subsections, we relate the

367

interaction forces evaluated above to the fouling behavior of MD membranes to depict the

368

underlying organic fouling mechanisms.

369

Organic Fouling Behavior of Membranes with Different Surface Wettability. To

370

investigate the impact of surface chemistry on the fouling behavior of MD membranes, PVDF

371

membranes modified with different surface energy materials were employed for the fouling

372

experiments. Specifically, we coated the PVDF membranes with PVA, 9–FAS, and 17–FAS to

373

have MD membranes with high, moderately low, and ultralow surface energies, respectively.

374

SEM analysis shows that membrane surface morphologies after coating with 9–FAS and 17–

375

FAS did not change much compared to the pristine PVDF substrate (Figures 3A-2 and 3A-3).

376

Although a portion of pore cavity is filled with PVA coating, the intrinsic surface roughness of

377

the PVDF substrate, which can influence the membrane fouling behavior, remained intact for the

378

PVA coated membrane (Figure 3A-1). XPS analysis reveals the elemental composition of

379

oxygen for the PVA coated PVDF membrane resulting from alcohol groups of PVA (Figure 3B-

380

1). The ratio of elemental fractions of fluorine to carbon was shown to increase in the order of

381

the PVA, 9–FAS, and 17–FAS coated PVDF membranes (Figures 3B-1 to 3B-3). The obtained 13 ACS Paragon Plus Environment


382

XPS results substantiate successful surface coating of the PVDF substrate to produce MD

383

membranes with different surface chemistry and, correspondingly, different surface energies. FIGURE 3

384 385

Three model foulants — alginate, humic acid, and mineral oil — were employed to assess

386

the impact of foulant type on the organic fouling mechanism of MD membranes. Permeate water

387

flux decline was monitored for a short period of time (i.e., 3 h) using a feed solution with a

388

relatively high organic foulant concentration (i.e., 500 mg/L) to focus on the initial stage of

389

foulant deposition on the membrane surface. Flux decline curves for the PVA, 9–FAS, and 17–

390

FAS coated PVDF membranes obtained from fouling with alginate, humic acid, and mineral oil

391

are presented in Figure 4. Duplicate fouling runs are provided in Figure S2 of the Supporting

392

Information. FIGURE 4

393 394

For all fouling experiments described below, salt (NaCl) rejection was monitored to be

395

>99.8%, suggesting that the change in MD membrane performance is solely attributed to

396

deposition of foulants on the membrane surface or pore blocking. Relatively stable water fluxes

397

were monitored for the PVA-coated PVDF membrane (i.e., MD membrane with high interfacial

398

energy or hydrophilic surface) during MD experiments with all types of foulants, indicating low

399

organic fouling propensity of the hydrophilic membrane. The observed low fouling of the

400

hydrophilic surface MD membrane compared to conventional hydrophobic MD membranes has

401

been demonstrated in previous studies targeting the development of an MD membrane resistant

402

to oil fouling.32-35 These studies attributed the enhanced oil fouling resistance to a hydration layer

403

formed on the hydrophilic surface coating that effectively repels non-polar oil foulants. Our

404

results further prove that such a hydration layer is also effective in preventing deposition of

405

diverse organic foulants on the membrane surface, including alginate and humic acid, as shown

406

in Figures 4A and 4B.

407

Polysaccharides are hydrophilic macromolecules ubiquitous in natural waters and wastewater

408

effluents.53,

54

409

membrane fouling studies.54, 55 As Figure 4A shows, water flux declines by alginate fouling were

410

negligible for all surface-modified PVDF membranes. This observation agrees with the

411

interaction forces evaluated by AFM measurements, which indicated negligible attraction

Alginate is often selected as a reference polysaccharide for a wide range of


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412

between the hydrophilic particle and model surfaces with different interfacial energies. Such a

413

weak attractive force results in reduced deposition and accumulation of alginate on the

414

membrane surface and, hence, marginal influence on water flux. This result implies that

415

hydrophilic organic foulants may not cause significant fouling during MD operation for

416

wastewater treatment.55, 56

417

Humic acid is relatively more hydrophobic than alginate due to its aromatic constituents.55, 57

418

We observed only a slight water flux decline for the 9–FAS and 17–FAS modified PVDF

419

membranes during fouling with humic acid, with the PVA coated membrane exhibiting stable

420

water flux during the 3 h MD experiments (Figure 4B). As demonstrated earlier by AFM

421

measurements, a hydrophobic particle probe (i.e., C10–functionalized silica particle) exhibited a

422

higher adhesion force with model surfaces than a hydrophilic particle. Further, such attractive

423

forces between the hydrophobic particle and model surfaces increased with decreasing surface

424

interfacial energy. The observed greater fouling potential of the membranes modified with 9–

425

FAS and 17–FAS than the PVA-coated PVDF membrane is ascribed to the higher attractive

426

hydrophobic-hydrophobic interaction between humic molecules and the membrane surface

427

(Figure 4B). Our results suggest that organic and biological foulants that are hydrophobic in

428

nature have a stronger tendency to deposit on a hydrophobic surface, causing more severe MD

429

membrane fouling compared to the hydrophilic foulants.49, 55, 58

430

The PVA coated PVDF membrane was not fouled much by mineral oil, owing to the

431

hydration layer on the hydrophilic PVA coating (Figure 4C).32, 35 In contrast, the 9–FAS and 17–

432

FAS coated PVDF membranes experienced significant flux decline by mineral oil fouling

433

(Figure 4C). We attribute the observed significant oil fouling potential of the 9–FAS and 17–

434

FAS modified membranes to the strong attractive interactions between the low interfacial energy

435

surfaces and mineral oil, which comprises a long hydrocarbon chain. It is interesting to note that

436

flux decline of the 9–FAS modified PVDF membrane was much more significant compared to

437

that of the 17–FAS modified membrane. This observation is contradicted by the adhesion force

438

measured by AFM where a higher attractive interaction between the hydrophobic particle (C10-

439

functionalized silica particle) and the surface modified with 17–FAS was estimated compared to

440

one modified with 9–FAS. Further discussion on this conflicting result from particle–surface

441

interaction force evaluation and MD fouling experiments is provided in the following subsection.



442

Membrane Fouling Mechanisms. Fouling behavior of the MD membranes with

443

different surface wettability is well corroborated by the results obtained from interaction force

444

measurements, except in the case of mineral oil fouling. To better understand the MD membrane

445

fouling mechanism caused by mineral oil, we measured water and mineral oil contact angles in

446

air, and mineral oil contact angle under water on the membrane surface. The PVA coated PVDF

447

membrane was in-air hydrophilic and oleophilic as evidenced by low contact angles with water

448

and mineral oil (Figure 5A). The underwater mineral oil contact angle of the PVA coated

449

membrane was measured to be very high (~150°). The measured high mineral oil contact angle is

450

attributed to the hydration layer formed by PVA coating that offers an effective barrier for

451

contact between mineral oil and the membrane surface in water. This observation also explains

452

the excellent fouling resistance of the PVA coated PVDF membrane to mineral oil shown in

453

previous MD fouling experiments (Figure 4C).

454

FIGURE 5

455

Both the 9–FAS and 17–FAS modified PVDF membranes exhibited a high water contact

456

angle in air, suggesting their hydrophobic surface wettability. Interestingly, the 9–FAS modified

457

PVDF membrane was oleophilic in air, and thus, easily wetted by mineral oil (Figure 5B, red bar

458

with diagonal pattern), while the 17–FAS modified PVDF membrane showed high wetting

459

resistance to mineral oil (Figure 5C, red bar with diagonal pattern). Similar trends were obtained

460

from underwater oil contact angle measurements; the 17–FAS modified membrane exhibited a

461

mineral contact angle of 91° under water, but a contact angle of only 52° was measured for the

462

9–FAS modified membrane. The small underwater oil contact angle measured for the 9–FAS

463

modified PVDF membrane was predictable, given the relatively high surface hydrophobicity of

464

this membrane. During underwater oil contact angle measurements, water does not remain on a

465

hydrophobic surface, thereby allowing an effective contact between mineral oil and the 9–FAS

466

modified surface via attractive hydrophobic-hydrophobic interaction. Similar or even higher

467

attractive force between mineral oil and the membrane surface coated with 17–FAS is expected,

468

compared to that modified with 9–FAS, as we have measured a stronger AFM adhesion force

469

between the hydrophobic particle and model surface functionalized with 17–FAS (Figure 2B).

470

Results from AFM force measurement describe foulant deposition on the surface of MD

471

membrane, but those results give only a limited explanation of adsorptive behavior of foulants 16 ACS Paragon Plus Environment

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472

into the pores. Different from other organic foulants, such as alginate and humic acid, mineral oil

473

can readily adsorb and penetrate into the hydrophobic membrane pores due to its low surface

474

tension (~30 mN/m). Such an adsorptive fouling mechanism is distinguished from the initial

475

foulant deposition on the membrane surface driven by foulant–surface interaction. Our previous

476

studies have demonstrated that resistance of an MD membrane to adsorptive fouling by low

477

surface tension contaminants is enhanced by creating a surface with ultralow interfacial energy

478

and a nanoscale structure with an increased air-to-solid ratio, referred to as a re-entrant

479

structure.29, 30 Such surfaces enable a metastable Cassie-Baxter state for the liquid-solid-vapor

480

interface without substrate wetting even for low surface tension substances. As presented in

481

Figure 5C, the 17–FAS modified PVDF membrane did not allow mineral oil to fully penetrate

482

the pores in air as well as under water. Although the microporous PVDF substrate does not

483

feature a distinct re-entrant structure, a substantially rough texture created by high surface

484

porosity and the relatively large pores (i.e., 0.1 m in diameter) are likely to develop a local

485

kinetic barrier to adsorptive oil intrusion into the pores when the surface chemistry satisfies

486

ultralow interfacial energy; this result is similar to the high wetting resistance observed in

487

omniphobic surfaces.59, 60

488

To further verify the adsorptive oil fouling mechanism, we quantified the amount of mineral

489

oil that penetrated into the membrane pores using thermogravimetric analysis (TGA) after static

490

and dynamic oil adsorption tests. Instead of a PVA coated membrane, an alkaline-treated PVDF

491

membrane was used as a representative MD membrane with a hydrophilic surface for oil

492

adsorption tests to minimize interference of PVA coating with TGA. The compositional change

493

of the surface-modified PVDF membranes after the oil adsorption tests is provided in Figure S3

494

of the Supporting Information. While both the 17–FAS modified and alkaline-treated PVDF

495

membranes showed a single thermal decomposition at ~500 °C, the 9–FAS modified PVDF

496

membrane exhibited additional decomposition at ~250–300 °C, which is close to the boiling

497

point of mineral oil.61 Mineral oil adhered to the 9–FAS modified PVDF membranes accounts

498

for ~30% and ~22% of total weight of the membrane sample after static and dynamic oil

499

adsorption tests, respectively. This observation clearly demonstrates that a fairly large amount of

500

mineral oil adsorbed into the pores of the 9–FAS modified PVDF membrane, while the other two

501

membranes effectively prevent such oil penetration.



502

Implications. We demonstrated the significant role of foulant chemistry on fouling

503

behavior of MD membranes with different surface wettability. Contrary to the water “repelling”

504

nature of surfaces with low and ultralow interfacial energies (i.e., hydrophobic,

505

superhydrophobic, and omniphobic surfaces), a relatively high adhesion tendency of foulants to

506

these surfaces was shown due to the attractive hydrophobic-hydrophobic interaction. Such

507

attractive interactions become stronger between the hydrophobic foulants and surfaces with

508

lower interfacial energies. We also showed that conventional hydrophobic MD membranes were

509

fouled by hydrophobic, low surface tension foulants via “attractive” and subsequent “adsorptive”

510

interaction mechanisms. Membranes having surfaces with ultralow interfacial energy (i.e.,

511

omniphobic membranes), however, did not allow adsorptive penetration of low surface tension

512

foulants into the pores, thus exhibiting a lower fouling potential than conventional hydrophobic

513

membranes.

514

MD membranes are considered to suffer less from fouling than other types of desalination

515

membranes, such as reverse osmosis and nanofiltration membranes, due to their larger pore size

516

and operation without hydraulic pressure. However, fouling is still of particular concern for

517

treatment of hypersaline industrial wastewaters where MD is envisioned to have niche

518

desalination applications. MD membranes with tailored surface wettability will play a key role in

519

overcoming practical limitations of the MD process, including wetting, scaling, and fouling.

520

Understanding the impact of membrane surface chemistry on such practical limitations will

521

provide the scientific base for MD membrane fabrication.

522

ASSOCIATED CONTENT

523

The Supporting Information is available free of charge on the ACS Publications website at DOI:

524

Surface energy components of probe liquids (Table S1); Summary of contact angle and

525

interfacial energy data of surface functionalized model substrate (Table S2); Procedure for AFM

526

tip functionalization (Figure S1); Duplicate water flux decline curves for surface-modified PVDF

527

membranes during MD fouling experiments (Figure S2); Amount of mineral oil adsorbed during

528

oil adsorption tests measured by TGA (Figure S3) (PDF)

529

AUTHOR INFORMATION

530

Corresponding Author 18 ACS Paragon Plus Environment

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Page 19 of 29


531

2*E-mail:

[email protected]; phone: +82-2-928-7656.

532

3*E-mail:

email: [email protected]; phone: +1 (203) 432-2789.

533

Notes

534

The authors declare no competing financial interest.

535

ACKNOWLEGMENTS

536

We acknowledge the support received from the National Science Foundation through the

537

Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC-1449500).

538

The characterization facilities were supported by the Yale Institute of Nanoscale and Quantum

539

Engineering (YINQE) and Yale West Campus Materials Characterization Core (MCC).

540 541



543

References

544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587

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48. Fowkes, F. M., Attractive Forces at Interfaces. Ind Eng Chem 1964, 56, (12), 40-&. 49. Qin, W.; Zhang, J.; Xie, Z.; Ng, D.; Ye, Y.; Gray, S. R.; Xie, M., Synergistic effect of combined colloidal and organic fouling in membrane distillation: Measurements and mechanisms. Environmental Science: Water Research & Technology 2017, 3, (1), 119-127. 50. Israelachvili, J. N., Intermolecular and surface forces. 3rd ed.; Academic Press: Burlington, MA, 2011; p xxx, 674 p. 51. Chen, Y.; Tian, M.; Li, X.; Wang, Y.; An, A. K.; Fang, J.; He, T., Anti-wetting behavior of negatively charged superhydrophobic PVDF membranes in direct contact membrane distillation of emulsified wastewaters. J Membrane Sci 2017, 535, 230238. 52. Coday, B. D.; Luxbacher, T.; Childress, A. E.; Almaraz, N.; Xu, P.; Cath, T. Y., Indirect determination of zeta potential at high ionic strength: Specific application to semipermeable polymeric membranes. J Membrane Sci 2015, 478, 58-64. 53. Ang, W. S.; Tiraferri, A.; Chen, K. L.; Elimelech, M., Fouling and cleaning of RO membranes fouled by mixtures of organic foulants simulating wastewater effluent. J Membrane Sci 2011, 376, (1-2), 196-206. 54. Boo, C.; Elimelech, M.; Hong, S., Fouling control in a forward osmosis process integrating seawater desalination and wastewater reclamation. J Membrane Sci 2013, 444, 148-156. 55. Naidu, G.; Jeong, S.; Kim, S. J.; Kim, I. S.; Vigneswaran, S., Organic fouling behavior in direct contact membrane distillation. Desalination 2014, 347, 230-239. 56. Khayet, M.; Mengual, J. I., Effect of salt concentration during the treatment of humic acid solutions by membrane distillation. Desalination 2004, 168, 373-381. 57. Lee, S.; Elimelech, M., Salt cleaning of organic-fouled reverse osmosis membranes. Water Res 2007, 41, (5), 1134-1142. 58. Van Oss, C. J.; Good, R. J.; Chaudhury, M. K., The role of van der Waals forces and hydrogen bonds in “hydrophobic interactions” between biopolymers and low energy surfaces. J Colloid Interf Sci 1986, 111, (2), 378-390. 59. Choi, W.; Tuteja, A.; Chhatre, S.; Mabry, J. M.; Cohen, R. E.; McKinley, G. H., Fabrics with Tunable Oleophobicity. Adv Mater 2009, 21, (21), 2190-+. 60. Tuteja, A.; Choi, W.; Mabry, J. M.; McKinley, G. H.; Cohen, R. E., Robust omniphobic surfaces. P Natl Acad Sci USA 2008, 105, (47), 18200-18205. 61. Droz, C.; Grob, K., Determination of food contamination by mineral oil material from printed cardboard using on-line coupled LC-GC-FID. Z Lebensm Unters F A 1997, 205, (3), 239-241.



717 718 719 720 721

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Table 1. Interfacial energy components of particle “1” and surface “2” in medium “3” estimated by the Owens-Wendt method. Total interfacial force, TOTAL, is the sum of Lifshitz–van der Waals (LW) and Lewis acid-base (AB) components, with AB derived from electron acceptor (+) and electron donor (-) components using the relation AB = 2 (+-)1/2. Data for the medium (i.e., water) were taken from the literature.45 All values were expressed in units of mN/m. LW

+

-

AB

TOTAL

OH

37.7

3.7

39.8

24.3

62.0

C10

30.2

0.002

5.2

0.2

30.4

OH

37.7

3.7

39.8

24.3

62.0

9–FAS

18.5

0.1

6.5

1.1

19.6

17–FAS

11.1

0.1

1.7

0.8

11.9

Water

21.8

25.5

25.5

51

72.8

Component Particle (1)

Surface (2) Medium (3)


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723 724 725 726 727 728 729 730 731

Figure 1. Interaction energy G132 (red bar with checked pattern) between (A) hydrophilic particle (i.e., bare silica particle) or (B) hydrophobic particle (i.e., C10–functionalized silica particle) and surface modified silicon wafer in an aqueous medium. Control, 9–FAS modified, and 17–FAS modified silicon wafers were used as model surfaces having different surface energies. Interaction energy between a particle and a surface in a medium, G132 , was calculated from interfacial forces between particle-surface (12), particle-medium (13), and surface-medium (23) using the relation G132   12   13   23 .

732 733



735 736 737 738 739 740 741 742

Figure 2. Adhesion forces (normalized by particle radius, Fm/Rp) between (A) bare silica particle and (B) n-Decyltriethoxysilane (C10) functionalized silica particle and control, 9–FAS, and 17– FAS modified silicon wafers measured by AFM in contact mode. Bare silica particle and C10– functionalized silica particle represent model hydrophilic and hydrophobic foulants, respectively. The measurements were conducted in a fluid cell using a 0.6 M NaCl solution at ambient conditions. Force measurements were conducted at three different locations, and at least 50 measurements were taken at each location to minimize inherent variability in the force data.


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(A-2) 9–FAS Coated PVDF

(A-1) PVA Coated PVDF

(A-3) 17–FAS Coated PVDF

5 m

5 m

5 m

F/C=0.2

60 40 20 0

745 746 747 748 749 750 751

C

F Element

O

80 (B-2) 9–FAS Coated PVDF F/C=1.2

60 40 20 0

C

F Element

Area Fraction (%)

80 (B-1) PVA Coated PVDF

Area Fraction (%)

Area Fraction (%)

744 80 (B-3) 17–FAS Coated PVDF F/C=1.6

60 40 20 0

C

F Element

Figure 3. SEM images depicting the surface of the PVDF membranes coated with (A-1) PVA, (A-2) 9–FAS, and (A-3) 17–FAS. XPS analysis of the surface of the PVDF membranes coated with (B-1) PVA, (B-2) 9–FAS, and (B-3) 17–FAS. The elemental fraction was calculated using the CasaXPS software package, using Shirley-type background from the XPS survey scan. The fluorine to carbon (F/C) elemental fraction ratio, an indicator of surface wettability or surface hydrophobicity, is also presented.

752 753


755 756 757 758 759 760 761

10 8 6 4

PVA Coated PVDF 9FAS Coated PVDF 17FAS Coated PVDF

2 0 0

60

120

Time (min)

180

12 (B) Humic Acid (500 mg/L) in 0.6 M NaCl

Water Flux (L m-2 h-1)

12 (A) Alginate (500 mg/L) in 0.6 M NaCl




10 8 6 4


2 0 0

60

120

Time (min)

180

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12 (C) Mineral Oil (500 mg/L) in 0.6 M NaCl 10 8 6 4


2 0 0

60

120

Time (min)

180

Figure 4. Water flux decline curves for PVA, 9–FAS, and 17–FAS coated PVDF membranes obtained from (A) alginate, (B) humic acid, and (C) mineral oil fouling experiments in DCMD. Feed solution was prepared by adding 500 mg/L foulant to 0.6 M NaCl. Feed solution temperature was set at 65 °C and permeate solution temperature was adjusted to achieve an identical initial water flux of ~10 L m-2 h-1 for all membranes. Crossflow velocities of 8.5 cm/s and 7.5 cm/s were employed for feed and permeate streams, respectively.

762


Page 29 of 29


Water CA in Air Oil CA in Air Oil CA Underwater

Contact Angle (

180 (A)

(C)

150 120 90 60 30 0

764 765 766 767 768 769 770 771

(B)

PVA Coated PVDF

9FAS Coated PVDF

17FAS Coated PVDF

Figure 5. Water and mineral oil contact angles in air and mineral oil contact angle underwater on the PVDF membranes coated with PVA, 9–FAS, and 17–FAS. The in-air contact angles were measured using the sessile drop method by placing 2 L liquid on the membrane surface. The underwater oil contact angle was measured in a custom-built liquid cell in which the mineral oil was injected beneath a membrane surface in water using a U-shaped needle. Contact angles 1 min after placing the test liquid were determined to be the steady-state value. Error bars represent standard deviations of two contact angles from three different membrane samples.

772 773 774


Relating Organic Fouling in Membrane Distillation ... - ACS Publications

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