Membrane Characerizations Net 4102

7/27/2019 Membrane Characerizations Net 4102

1/179

Subject Area: Water Quality

Web Report #4102

Evaluation of Membrane

Characterization Methods


2/179



2012 Water Research Foundation. ALL RIGHTS RESERVED.


3/179

About the Water Research FoundationThe Water Research Foundation is a member-supported, international, 501(c)3 nonprot organization that

sponsors research that enables water utilities, public health agencies, and other professionals to provide

safe and affordable drinking water to consumers.

The Foundations mission is to advance the science of water to improve the quality of life. To achieve this

mission, the Foundation sponsors studies on all aspects of drinking water, including resources, treatment,

and distribution. Nearly 1,000 water utilities, consulting rms, and manufacturers in North America and

abroad contribute subscription payments to support the Foundations work. Additional funding comes

from collaborative partnerships with other national and international organizations and the U.S. federal

government, allowing for resources to be leveraged, expertise to be shared, and broad-based knowledge

to be developed and disseminated.

From its headquarters in Denver, Colorado, the Foundations staff directs and supports the efforts ofmore than 800 volunteers who serve on the Board of Trustees and various committees. These volunteers

represent many facets of the water industry, and contribute their expertise to select and monitor research

studies that benet the entire drinking water community.

Research results are disseminated through a number of channels, including reports, the Website,

Webcasts, workshops, and periodicals.

The Foundation serves as a cooperative program providing subscribers the opportunity to pool their

resources and build upon each others expertise. By applying Foundation research ndings, subscribers

can save substantial costs and stay on the leading edge of drinking water science and technology. Since

its inception, the Foundation has supplied the water community with more than $460 million in appliedresearch value.

More information about the Foundation and how to become a subscriber is available at www.WaterRF.org.



4/179

Jointly sponsored by:

Water Research Foundation6666 West Quincy Avenue, Denver, CO 80235

and

U.S. Environmental Protection AgencyWashington DC 20460

Published by:

Prepared by:

Amy E. ChildressDepartment of Civil and Environmental Engineering, University of Nevada, Reno, NV 89557

Jonathan A. BrantDepartment of Civil and Architectural Engineering, University of Wyoming, Laramie, WY 82071

Pawel RempalaDepartment of Civil and Environmental Engineering, University of Nevada, Reno, NV 89557

Donald W. Phipps Jr.

Orange County Water District/Research and Development Department, 10500 Ellis Avenue,

Fountain Valley, CA 92708-8300

and

Pierre KwanHDR Engineering, Inc., 500 108th Avenue NE, Suite 1200, Bellevue, WA 98004-5549





5/179

DISCLAIMER

This study was funded by the Water Research Foundation (Foundation) and the U.S. Environmental

Protection Agency (USEPA) under Cooperative Agreement No. X-83294801-1. The Foundation

and USEPA assume no responsibility for the content of the research study reported in this

publication or for the opinions or statements of fact expressed in the report. The mention of

trade names for commercial products does not represent or imply the approval or endorsementof the Foundation or USEPA. This report is presented solely for informational purposes.

Copyright 2012by Water Research Foundation

ALL RIGHTS RESERVED.

No part of this publication may be copied, reproduced

or otherwise utilized without permission.



6/179

v

CONTENTS

TABLES ....................................................................................................................................... viiLIST OF FIGURES ....................................................................................................................... ixFOREWORD ................................................................................................................................ xvACKNOWLEDGMENTS .......................................................................................................... xviiEXECUTIVE SUMMARY ......................................................................................................... xixCHAPTER 1: INTRODUCTION AND BACKGROUND ............................................................ 1

Contact Angle Measurements ............................................................................................. 1Zeta Potential Measurements .............................................................................................. 6Surface Roughness Atomic Force Microscopy (AFM) ................................................... 8Transmission Electron Microscopy (TEM) ........................................................................ 9Scanning Electron Microscopy (SEM) ............................................................................. 10Chemical Force Microscopy (CFM) ................................................................................. 12X-Ray Photoelectron Spectroscopy (XPS) ....................................................................... 14Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) ..... 15Objectives ......................................................................................................................... 16

CHAPTER 2: MATERIALS AND METHODS .......................................................................... 18Membrane Samples ........................................................................................................... 18Statistical Analysis ............................................................................................................ 19Membrane Community Survey On Membrane Characterization Techniques .................. 19Development Of Standard Techniques For Characterizing Membrane Surfaces ............. 19

AFM Surface Roughness Measurements .................................................................... 20Contact Angle Measurements ..................................................................................... 21Streaming Potential Measurements............................................................................. 22

Assessment of Membrane Performance and Fouling ....................................................... 22

Temperature Correction Factors for Normalizing Membrane Permeate Flux ............ 25Membrane Performance Testing ................................................................................. 28Overview of Membrane Performance Test Method ................................................... 29Normalizing Membrane Performance Response ........................................................ 29

CHAPTER 3: RESULTS AND DISCUSSION ............................................................................ 31Method Development For AFM Surface Roughness........................................................ 31

Imaging and Roughness Analysis of Flat Membrane Surfaces .................................. 31Imaging and Roughness Analysis of Curved (Hollow Fiber) Membrane Surfaces .... 36Standard Method for AFM Surface Roughness Measurements on Flat-Sheet and

Hollow Fiber Membranes ........................................................................................... 38Method Development For Contact Angle Measurements ................................................ 38

Contact Angle Analysis Of The Flat-Sheet Teflon Standard Surface ........................ 38Contact Angle Analysis Of Flat Sheet Membrane Surfaces ....................................... 41Statistical Analysis Of Contact Angle Results For Flat Surfaces ............................... 44Contact Angle Analysis Of Curved Surfaces .............................................................. 47Standard Method For Contact Angle Measurements On Flat-Sheet And Hollow FiberMembranes .................................................................................................................. 48



7/179

vi

Method Development For Streaming Potential (Zeta Potential) Measurements .............. 48PMMA Standard Surface ............................................................................................ 48Streaming Potential (Zeta Potential) Analysis Of Flat Sheet Membrane Surfaces .... 49Streaming Potential (Zeta Potential) Analysis Of Hollow Fiber Membrane Surfaces 52Standard Method For Streaming Potential Measurements On Flat-Sheet And Hollow

Fiber Membranes ........................................................................................................ 53

Method Feedback From Partner Utilities .......................................................................... 53Guidance For Integrity Test Calculations ......................................................................... 61Evaluating Correlations Between Membrane Characteristics And Performance ............. 65

Multiple Linear Regression (MLR) Methods ............................................................. 70Assessing the Predictive Ability of MLR Model Inputs by Iteratively Withholding a

Test Exemplar ............................................................................................................. 73Assessing the Influence of MLR Model Input Parameters ......................................... 75Modeling Conclusions ................................................................................................ 87

CHAPTER 4: CONCLUSIONS ................................................................................................... 89Development Of Standard Techniques For Characterizing Membrane Surfaces ............. 89

Correlation Between Membrane Properties And Membrane Fouling ........................ 90

APPENDIX ALIST OF UTILITIES USING MEMBRANES AS A TREATMENTTECHNOLOGY (AS OF 2010) ........................................................................................92

APPENDIX B SURVEY / QUESTIONNAIRE ON MEMBRANE CHARACTERIZATIONTECHNIQUES AND PROCEDURES ............................................................................117

APPENDIX C INSTRUCTIONS TO PARTICIPATING LABORATORIES FORCONDUCTING AFM, CONTACT ANGLE, AND STREAMING POTENTIAL

MEASUREMENTS .........................................................................................................126APPENDIX D STANDARD PROTOCOLS FOR CHARACTERIZING MEMBRANE

SURFACES: CONTACT ANGLE, ZETA POTENTIAL, SURFACE ROUGHNESS .133REFERENCES ........................................................................................................................... 152ABBREVIATIONS .................................................................................................................... 154



8/179

vii

TABLES

Table 1.1. Distribution of adhesion across the HL and SG membrane surfaces as a function of

surface area coverage measured with methyl, carboxyl and hydroxyl functionalized probes (I =

0.01 M NaCl; pH 6; T = 20 C). Taken from Brant et al. (2006). ................................................ 12Table 1.2 Measured atomic concentration percentages (%) of C1s, O1s, N1s and S2p obtained by X-ray photoelectron spectroscopy (XPS) for commercially available and experimental membranes.

Taken from Boussu et al. (2007). ................................................................................................. 15Table 2.1 Names and select properties of membranes used in this investigation (MF-

microfiltration; UF- ultrafiltration; NF- nanofiltration; RO- reverse osmosis, PP- polypropylene,PES- polyethersulfone, PA- polyamide, TFCthin film composite). ......................................... 18Table 2.2 Proposed standard surfaces for atomic force microscopy surface roughness

measurements. ............................................................................................................................... 21Table 2.3 Proposed standard surfaces for contact angle measurements. ...................................... 21Table 2.4 Summary of primary system components and accessories for the standard membrane

test unit. ......................................................................................................................................... 24

Table 2.5 Chemistry and composition of the feed solution used for membrane foulingexperiments25

Table 2.6 Summarized temperature correction factors (TCFs) for the NF-270, ESNA1-LF,

ESPA-2, SW30HR, and MUNIRO-400 membranes, which were used in the membrane fouling

experiments. .................................................................................................................................. 27Table 3.1 Surface roughness statistics generated from atomic force microscope (AFM) imagingof the ESNA1-LF membrane surface by the OCWD and DU laboratories (resolution = 256

256, measurement mode = tapping, medium = water). Numerals (1, 2, ..) correspond to a samplesite on the membrane. ................................................................................................................... 32Table 3.2 Surface roughness statistics generated from atomic force microscope (AFM)measurements of the SW30HR membrane surface from the OCWD and DU laboratories

(resolution = 256 256, measurement mode = tapping, medium = water). Numerals (1, 2, ..)

correspond to a sample site on the membrane. ............................................................................. 33Table 3.3 Precision statistics for the surface roughness data that was collected for the ESNA1-LFand SW30HR membranes (celltest results from one laboratory on one material, repeatability

standard deviation (sr), reproducibility standard deviation (sR). ................................................... 34Table 3.4 Surface roughness statistics as measured using an atomic force microscope (AFM) for

the NF-270, ESNA1-LF, ESPA-2, and SW30HR membranes. All measurements were performed

on flat sheet membrane samples by the Orange County Water District (OCWD) laboratory. Allmeasurements were performed on wet membrane samples in contact mode and using a liquid

cell. ................................................................................................................................................ 34Table 3.5 Surface roughness statistics for Teflon tubing, which was being considered as astandard surface for analyzing hollow fiber membranes using atomic force microscopy

(resolution = 256 256, medium = water, OCWDcontact mode, DUtapping mode). ......... 39



9/179

viii

Table 3.6 Contact angle data for the Teflon standard surface from the three participating

laboratories (Colorado School of Mines - CSM, Duke UniversityDU, and the University of

Nevada, RenoUNR). All contact angles were measured using the captive bubble method anddoubly deionized water as the contact angle probe liquid (T = 20C). ........................................ 39Table 3.7 Contact angle data for the ESNA1-LF membrane from the three participating

laboratories (Colorado School of Mines - CSM, Duke UniversityDU, and the University ofNevada, RenoUNR). All contact angles were measured using the captive bubble method anddoubly deionized water as the contact angle probe liquid (T = 20C). ........................................ 41Table 3.8 Contact angle data for the SW30HR membrane from the three participating

laboratories (Colorado School of Mines - CSM, Duke UniversityDU, and the University of

Nevada, RenoUNR). All contact angles were measured using the captive bubble method anddoubly deionized water as the contact angle probe liquid (T = 20C). ........................................ 42Table 3.9 Average contact angle values for the PTFE standard surface, ESNA1-LF, andSW30HR membrane. The associated statistical measures (repeatability and reproducibility) from

the inter-laboratory study are also reported for each test surface. ................................................ 45Table 3.10 Reported and measured contact angle values for the 2D reference tool. All resultswere acquired by the laboratory at the University of Nevada, Reno (UNR). ............................... 47Table 3.11 Contact angle results for hollow fiber membranes. All results were obtained using theexperimental apparatus that was developed as part of this project. .............................................. 48Table 3.12 Specific comments on the usefulness and technique for carrying out contact angle

measurements in membrane applications. .................................................................................... 56Table 3.13 Specific comments on the usefulness and technique for carrying out streamingpotential measurements to calculate the zeta potential of membranes in water and wastewater

treatment applications. .................................................................................................................. 58Table 3.14 Specific comments on the usefulness and technique for carrying out surfaceroughness measurements using an atomic force microscope for membranes in water and

wastewater treatment applications. ............................................................................................... 61Table 3.15 Multiple linear regression (MLR) model input parameters and values for describing

bentonite clay fouling. .................................................................................................................. 71Table 3.16 Constants, coefficients and statistics for the best MLR model: J/Jo (at 5g m

-2Load) =

0.67233+ (2.2910-3 Jinitial, L m-2hr-1)(2.1210-3 Rq, nm) + (3.0010-3 , degrees)........................................................................................................................................................ 72

Table 3.17 Values ofKandJ/Jo, Plateauobtained by fitting normalized permeate flux (J/Jo) data

and clay loading (g m-2

) data for each membrane to equation Eq. 3.3. R-squared values and 95%

asymptotic confidence intervals are reported for each nonlinear regression membrane model80

Table 3.18 Membrane properties selected as independent variables for MLR analyses ofKand

J/Jo, Plateau . The zeta potential values were determined at pH=5.3, 2mM KCl saturated withambient CO2. Surface roughness statistics were collected by the OCWD, while zeta potential and

water contact angle values were collected by UNR. Note: Rq was slightly cross-correlated with

Jo (r=0.5206) and with (r=0.5084)..81



10/179

ix

LIST OF FIGURES

Figure 1.1 (a) Representative digital image of a liquid droplet on a dry surface in which the

contact angle () is measured according to the sessile drop method. (b) Representative digital

image of an air bubble on a wetted membrane surface (the membrane is immersed in a liquid)and the contact angle () is measured according to the captive bubble technique. ........................ 2Figure 1.2 Techniques commonly used for carrying out contact angle measurements: (a) static

sessile dropadvancing and receding; (b) static captive bubbleadvancing and receding; and

(c) dynamic captive bubble. Figure adapted from Drelich (1997). For those illustrations for thesessile drop technique the term advancing refers to the advancement of the liquid (increase in the

volume of the liquid droplet) over a dry surface, while the term receding refers to the recession

of the liquid droplet (decrease the in the volume of the liquid droplet) over a previously wet

surface. A similar relationship exists for the captive bubble measurements, where the water inwhich the surface is immersed either advances or recedes across the surface through a decrease

or increase in the volume of the air bubble. .................................................................................... 3Figure 1.3 The effect of drop (bubble) size on advancing (Adv.) and receding (Rec.) contact

angles for the air /water/polyethylene film system as obtained with the static sessile-drop (SD),static captive-bubble (CB), and dynamic captive-bubble (DCB) techniques. Taken from Drelich

et al. (1996). .................................................................................................................................... 4Figure 1.4 Illustration of the relative locations of the Stern layer and Shear plane from a charged

surface in water and the associated change in surface potential with distance. Taken fromChapman-Wilbert et al. (1999). ...................................................................................................... 7Figure 1.5 Schematic illustration of the operating principles associated with the atomic force

microscope (AFM). Taken from Wyart et al. (2008). .................................................................... 8Figure 1.6 Transmission electron microscope (TEM) image of a graft copolymer film. Takenfrom Kim et al. (2008). ................................................................................................................... 9Figure 1.7 Ordered and disordered co-polymer blend morphologies visualized using TEMTmethod. Taken from Jinnai et al. (2006)....................................................................................... 10Figure 1.8 Scanning electron microscope (SEM) images of cross-sections of polymer blend

membranes. Taken form Li et al. (2008). ..................................................................................... 11Figure 1.9 Topographical and corresponding CFM images for the HL membrane acquired with a

CH3-, COOH-, and OH-functionalized tip (I = 0.01 M NaCl; pH = 6.1, and T = 20 C). Takenfrom Brant et al. (2006). ............................................................................................................... 13Figure 1.10 Changes in adhesion force of carboxyl-modified microspheres (a) and hydroxyl-

modified microspheres (b) to PE membranes that were sampled during the pilot-filtration test.

An SEM image of a functionalized polystyrene bead glued to a cantilever tip used in this study isshown on the right. Taken from Yamammura et al. (2008). ........................................................ 14Figure 1.11 ATR-FTIR spectra of several commercial and one lab-made membrane, containing

sulfone groups. Ar-SO2-Ar typical frequencies are (S=O asym) = 1325 cm-1

, (S=O symm) =

1140 cm-1

. Differences in IR spectra stem from C(CH3)2 bridges present between aryl rings inNTR7450 and Desal 5DL. Taken from Boussu et al. (2007). ...................................................... 16



11/179


12/179

xi

Figure 3.11. Representative digital images of air bubbles on the SW30HR membrane immersed

in water at the CSM, DU, and UNR laboratories. Each of these air bubbles are representative of

those from which contact angle measurements were performed. ................................................. 44Figure 3.12. Picture of the 2D certified contact angle fixed-drop calibration reference tool(Ram-Hart, Netcong, NJ). ........................................................................................................... 46Figure 3.13. (left) Picture of a 3D contact angle reference tool, which includes the sample surface

stage, flat stainless steel plate, and stainless steel sphere. (right) Schematic illustration and

equations associated with using the 3D contact angle reference tool (= contact angle measuredthrough the stainless steel sphere,D = diameter of the stainless steel sphere,H= distance from

the top of the sphere to the top elevation of the stainless steel cross member, andP= distance

from the top elevation of the sample stage to the top elevation of the stainless steel crossmember. ........................................................................................................................................ 46Figure 3.14. Photographs of the experimental apparatus that was designed and used to measurecontact angles on hollow fiber membranes. (left and middle) Image of the PTFE tubing

sandwiched between two glass slides and secured in the sample stage. (right) Image of an air

bubble that has been placed on the PTFE tubing immersed in water. .......................................... 48Figure 3.15. Zeta potential data collected for the PMMA control surface from UNRUniversityof Nevada, Reno (I = 2 mM KCl) and Anton Paar (I = 1 mM KCl). Polynomial fits (cubic) to

both data sets are also provided. . ................................................................................................. 49Figure 3.16. Zeta potential as a function of solution pH for the ESNA1-LF membrane (I = 2 mM

KCl). Streaming potential measurements were by the UCRUniversity of California Riverside,UNRUniversity of Nevada, Reno, the FKKT - University of Maribor, and Anton Paar. Those

data points from UCR, FKKT, and Anton Paar represent the average of three separate tests on

three different membarne coupons................................................................................................ 50Figure 3.17. Zeta potential as a function of solution pH for the SW30HR membrane (I = 2 mM

KCl). Streaming potential measurements were by the UCRUniversity of California Riverside,UNRUniversity of Nevada, Reno, the FKKT - University of Maribor, and Anton Paar. Those

data points from UCR, FKKT, and Anton Paar represent the average of three separate tests on

three different membrane coupons................................................................................................ 51Figure 3.18. Zeta potential as a function of solution pH for the inner surface of the Norit X-Flow

hollow fiber membrane (I = 2 mM KCl, n = 3). ........................................................................... 53Figure 3.19 Distribution of survey responses amongst each of the four respondent categories

(water/wastewater utilities, membrane manufacturers, characterization equipment vendors, andacademia). The total number of survey responses that were received was eleven. ...................... 54Figure 3.20. Distribution of responses to the question on the usefulness of contact angle

measurements in membrane applications. .................................................................................... 55Figure 3.21. Distribution of responses to the question on the usefulness of zeta potential values,

acquired through streaming potential measurements, in membrane applications. ....................... 57Figure 3.22. Distribution of responses to the question on the usefulness of surface roughnessmeasurements, performed using an atomic force microscope, in membrane applications. .......... 60



13/179

xii

Figure 3.23 Difference in the calculated values for the minimum direct integrity test pressure

(Ptest) for obtaining a resolution of 3 m assuming a standard error in the contact angle value of

5. The values for Ptest were calculated using Eq. 3.1 ( = 1, = 74.9 dynes/cm, BPmax = 3 psi)........................................................................................................................................................ 64Figure 3.24. Results of simple regression analysis for the NF-270 membrane. 100 ml of permeate

corresponds to 1.94 g loaded m-2 membrane. Correlation Coefficient = -0.996635; R-squared =99.3281 percent; R-squared (adjusted for d.f.) = 99.1042 percent; Standard Error of Est. =

0.02384; Mean absolute error = 0.015953; Durbin-Watson statistic = 2.77534 (P = 0.6576); Lag1 residual autocorrelation = -0.434792. The central line represents the regression models, the

inner boundaries represent the 95% confidence limit and the outer boundaries represent the

model prediction limit. .................................................................................................................. 65Figure 3.25. Results of simple regression analysis for the ESNA1-LF membrane:. 100 ml ofpermeate corresponds to 1.94 g loaded m

-2membranemembrane. Correlation Coefficient = -

0.999234; R-squared = 99.8469 percent; R-squared (adjusted for d.f.) = 99.7959 percent;

Standard Error of Est. = 0.00762926; Mean absolute error = 0.00468329; Durbin-Watson

statistic = 2.83777 (P=0.7339); Lag 1 residual autocorrelation = -0.62428. The central linerepresents the regression models, the inner boundaries represent the 95% confidence limit and

the outer boundaries represent the model prediction limit. ........................................................... 66Figure 3.26. Results of simple regression analysis for the ESPA2 membrane. 100 ml of permeate

corresponds to 1.94 g loaded m-2

membrane. Correlation Coefficient = -0.998841; R-squared =99.7683 percent; R-squared (adjusted for d.f.) = 99.691 percent; Standard Error of Est. =

0.0134216; Mean absolute error = 0.00948347; Durbin-Watson statistic = 1.50241 (P=0.0550);

Lag 1 residual autocorrelation = 0.0623037. The central line represents the regression models,the inner boundaries represent the 95% confidence limit and the outer boundaries represent the

model prediction limit. .................................................................................................................. 67Figure 3.27. Results of simple regression analysis for the SW30HR membrane. 100 ml of

permeate corresponds to 1.94 g loaded m-2 membrane. Correlation Coefficient = -0.996067; R-squared = 99.215 percent; R-squared (adjusted for d.f.) = 98.9534 percent; Standard Error of Est.= 0.00316966; Mean absolute error = 0.0020736; Durbin-Watson statistic = 2.6779 (P=0.5877);

Lag 1 residual autocorrelation = -0.561779. The central line represents the regression models, the

inner boundaries represent the 95% confidence limit and the outer boundaries represent themodel prediction limit. .................................................................................................................. 68Figure 3.28. Results of simple regression analysis for the MUNIRO-400 membrane. 100 ml of

permeate corresponds to 1.94 g loaded m-2

membrane. Correlation Coefficient = -0.994937; R-

squared = 98.9899 percent; R-squared (adjusted for d.f.) = 98.8215 percent; Standard Error ofEst. = 0.00624946; Mean absolute error = 0.00416953; Durbin-Watson statistic = 2.42804

(P=0.5686); Lag 1 residual autocorrelation = -0.352731. The central line represents theregression models, the inner boundaries represent the 95% confidence limit and the outer

boundaries represent the model prediction limit. .......................................................................... 69Figure 3.29. Normalized membrane flux (J/Jo) predicted by the simple regression models as afunction of bentonite clay loading. From this, membrane performance of all five membranes at at

a given clay load can be determined. A 5g m-2

bentonite load was chosen with which to generate

performance data for MLR analysis because this falls within a nearly linear portion of all five



14/179

xiii

relationships and membrane performance could be determined in all cases at this point by

interpolation of experimental data. ............................................................................................... 70Figure 3.30. Results: Best MLR model prediction and UNR membrane data determined from

laboratory measurements. Statistics: R-Squared = 99.2736%; Adj. R-Squared = 97.0943%; Std.Error of Est. = .00718711; M.A.E = .00251155; P-Value = 0.1069. Horizontal bars = 95%

confidence interval for J/Jo @ 5 g m-2 clay load estimated from laboratory data; vertical bars =MLR model standard error of the estimate. .................................................................................. 73Figure 3.31. Comparison of the prediction of membrane behavior by the MLR model constructedfrom all the exemplars (filled circles) with prediction of five MLR models that were constructed

using data missing each of the individual membranes indicated (open circles). Horizontal bars =

95% confidence interval for J/Jo @ 5 g m-2

clay load estimated from laboratory data; verticalbars = MLR model standard error of the estimate. ....................................................................... 75Figure 3.32. Illustration of the decline in normalized permeate flux rate (J/Jo) with increasingclay load. The plateau value (J/Jo, Plateau) is taken as a measure of the severity of the impact of

bentonite clay on the permeate flux rate for a membrane and is determined as the point where

J/Jo becomes zero order with respect to clay loaded. .................................................................... 78Figure 3.31. Intrinsic water flux (J/Jo) as a function of the clay loaded (g m

2) for the a) NF-270

and b) MUNIRO-400 membranes. Lines show Eq. 3.3 fitted to the data by nonlinear regression.

....................................................................................................................................................... 79Figure 3.33. Linearly regressed relationship betweenJ/Jo and K. The intervals are the asymptotic

95% confidence intervals from the individual nonlinear models. ................................................ 80Figure 3.34. Plot of the observed and predictedKvalues for each of the different membranesfouled by bentonite clay. R-squared = 98.82%, adjusted R-squared = 95.27%, p-value = 0.1363.

The value ofKdetermined in the nonlinear regression models for the individual membranes

loaded with bentonite clay could be predicted well from roughness, initial water flux and contact

angle data. Zeta potential data were not required for the prediction (probably due to smallvariation amongst the test membranes)......................................................................................... 82Figure 3.35. Plot of the observed and predictedJ/JoPlateau values for each of the different

membranes fouled by bentonite clay. R-squared = 97.83%, Adjusted R-squared = 91.33%, p-value = 0.1843. The MLR model was able to describe the majority of the variance inJ/JoPlateau

using roughness, contact angle and initial water flux. As withK, zeta potential data were not

required to explain the variance observed inJ/JoPlateau. ............................................................... 86Figure 3.36. Mechanistic explanation of fouling by bentonite clay. ............................................ 87



15/179



16/179

xv

FOREWORD

The Water Research Foundation (Foundation) is a nonprofit corporation dedicated to the

development and implementation of scientifically sound research designed to help drinking

water utilities respond to regulatory requirements and address high-priority concerns. TheFoundations research agenda is developed through a process of consultation with Foundation

subscribers and other drinking water professionals. The Foundations Board of Trustees and

other professional volunteers help prioritize and select research projects for funding based uponcurrent and future industry needs, applicability, and past work. The Foundation sponsors

research projects through the Focus Area, Emerging Opportunities, and Tailored Collaboration

programs, as well as various joint research efforts with organizations such as the U.S.

Environmental Protection Agency and the U.S. Bureau of Reclamation.This publication is a result of a research project fully funded or funded in part by

Foundation subscribers. The Foundations subscription program provides a cost-effective and

collaborative method for funding research in the public interest. The research investment that

underpins this report will intrinsically increase in value as the findings are applied incommunities throughout the world. Foundation research projects are managed closely from their

inception to the final report by the staff and a large cadre of volunteers who willingly contribute

their time and expertise. The Foundation provides planning, management, and technicaloversight and awards contracts to other institutions such as water utilities, universities, and

engineering firms to conduct the research.

A broad spectrum of water supply issues is addressed by the Foundation's researchagenda, including resources, treatment and operations, distribution and storage, water quality and

analysis, toxicology, economics, and management. The ultimate purpose of the coordinated

effort is to assist water suppliers to provide a reliable supply of safe and affordable drinking

water to consumers. The true benefits of the Foundations research are realized when the results

are implemented at the utility level. The Foundation's staff and Board of Trustees are pleased tooffer this publication as a contribution toward that end.

Roy L. Wolfe, Ph.D. Robert C. Renner, P.E.

Chair, Board of Trustees Executive Director

Water Research Foundation Water Research Foundation



17/179



18/179

xvii

ACKNOWLEDGMENTS

The PIs and Key Research Team Members would like to acknowledge the participation

of Dr. Ryan Heck of UNR in confidentiality negotiations with Pall Microza and GE Water. We

would also like to thank OCWD participants, Jana Safarik, and Richard Bold for contact angleand AFM imaging measurements. We would like to express our gratitude to the Duke

University teamProfessor Mark Wiesner, Zachary Hendren, and Dr. Soryong Chae for contact

angle and AFM measurements; the Colorado School of Mines team Professor Tzahi Cath andMatt Bolt for zeta potential and contact angle measurements); the University of California,

Riverside teamProfessor Sharon Walker, Olgun Zorlu for zeta potential measurements; and the

University of Maribor teamDr. Irena Petrini for zeta potential measurements. Consultation of

Dr. K.C. Khulbe (University of Ottawa) with regard to AFM measurements is alsoacknowledged. We greatly appreciate the assistance of Dr. Thomas Luxbacher of Anton Parr in

evaluating the zeta potential results and initiating an additional round robin of measurements.

Assistance from additional UNR personnel, Viktoriya Weirauch and Serife Ozger, is also

acknowledged. Lastly, we would like to acknowledge the WaterRF Project Manager, JonathanCuppett and our Project Advisory Committee members, Michelle Chapman, Jess Garcia-

Aleman, Carl Spangenberg, and Shahram Tabe.



19/179



20/179

xix

EXECUTIVE SUMMARY

INTRODUCTION AND BACKGROUND

A variety of techniques and approaches are routinely used for characterizing the physical

and chemical properties of membrane surfaces. Measuring the contact angle that is formed whena droplet of water is placed on the membrane surface is used to quantify the

hydrophobicity/hydrophilicity of the membrane surface. Atomic force microscopy is used to both

quantify and visualize the morphology or roughness of membrane surfaces. Streaming potentialmeasurements are used to calculate a zeta potential, an indicator of surface charge, for membrane

surfaces under variable solution chemistries. While numerous other characteristics may be

measured for membrane surfaces these three aforementioned ones have perhaps received the

greatest interest and use from scientists, engineers, and membrane manufacturers. This stronginterest is attributed to the relative importance of each of these properties in determining

permeate flux rates, solute rejection, and fouling characteristics for microporous (microfiltration,

ultrafiltration) and nonporous (nanofiltration, reverse osmosis) membranes. For example, it is

now widely accepted that the specific flux rate (m3

m-2

day-1

Pa-1

) increases with increasinghydrophilicity of the material that makes up the membrane. Similarly, hydrophilic and smooth

membrane surfaces tend to be less prone to severe fouling events. Contact angle with water, and

other liquids, in addition to zeta potential values are instrumental for describing the chemicalinteractions that occur between colloids, microorganisms, and other materials and the membrane

surface. Such descriptions are generally made within the context of the Derjaguin-Landau-

Verwey-Overbeek (DLVO) model and its various extensions. Using this approach more specificsurface energy parameters, for example the Lewis acid-base values, for membranes may be

calculated using contact angle measurements. Despite the wealth of information that may be

derived from the aforementioned surface characterization techniques a standard method for

applying these techniques to membrane surfaces is lacking.

Surface characterization techniques rely on rather exhaustive sample preparationprocedures and a thorough understanding of the properties of the sample material. In the absence

of either of these requirements it is difficult at best to generate reproducible results. The situation

for polymeric membranes is further complicated by the lack of specific information about

membrane material composition/chemistry and the use of preservatives for long-term storage.Not surprisingly these complicating factors have resulted in numerous disagreements between

any two labs studying the same membrane under similar operating conditions. Furthermore, in

light of the often-conflicting results it is nearly impossible to derive usable information fromthese laboratory studies for utilities and other end users. It is therefore necessary that a standard

method be developed for conducting and interpreting the results from contact angle, streaming

potential and atomic force microscope measurements.

OBJECTIVES

The overall objective of this project was to develop standard methods for both conducting

and interpreting the results from contact angle, streaming potential, and atomic force microscope

measurements for characterizing membrane surfaces. The motivation for this overall objective is

the improvement of our ability to evaluate the role(s) of membrane properties in determining



21/179

xx

membrane performance. The specific research goals that were pursued to achieve the overall

objective of this research project were as follows:

1.Identify three techniques for characterizing the physical and chemical properties ofmembrane surfaces,

2.Quantify the variability between data generated using the selected characterization

techniques in order to develop the associated standard methods for their use by themembrane community,

3.Provide drinking water utilities with information on the value of membranecharacterization methods, a means by which to compare membrane properties on an

equal basis, and guidance for accurate contact angle measurements for integrity test

calculations,

4.Evaluate the ability of those surface characteristics determined using the proposed

standard methods to accurately predict membrane performance and fouling.

APPROACH

The following research tasks were conducted in order to achieve the overall objective of

this project:

Task A: Identification and review of characterization methods

Task B: Identification and selection of membranes for analysis

Task C: Development of standard methods for characterizing membrane surfaces

Task D: Obtain feedback from partner utilities on the proposed standard methods

Task E: Characterize select membrane properties using the proposed standard methods

Task F: Conduct membrane fouling studies

Task G: Evaluate relationships between membrane characteristics and membraneperformance

Task H: Develop guidance for utilities doing integrity test calculations

Following a comprehensive review of the available literature on membrane surface

characteristics and performance the following three characterization techniques were selected for

inclusion in this project: contact angle (surface energy), atomic force microscopy (AFM)(surface morphology), and streaming potential (zeta potential) measurements. Membrane

samples that included microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse

osmosis (RO) membranes were selected and acquired based on their use in the water treatmentindustry. Standard (control) surfaces were also identified and acquired for each of

aforementioned characterization techniques that were studied as part of this project. Surface

samples (membranes and standard surfaces) were distributed, along with guidance on how to

conduct each type of characterization technique, to three separate laboratories. The participatinglaboratories took measurements, assessed precision, and provided input on the proposed standard

techniques. Next, membrane fouling (or more specifically, membrane resistance to fouling) was

evaluated under laboratory conditions using a suspension of bentonite (clay) as the foulingsolution. Membrane properties were then correlated with membrane performance metrics.



22/179

Executive Summary| xxi

xxi

RESULTS AND CONCLUSIONS

The following conclusions were reached regarding the development and application ofstandard techniques for characterizing membrane surfaces, and specifically for executing surface

characterization using contact angle, surface roughness, and streaming potential measurements:

Operating an AFM in contact mode produces more consistent roughness statistics whenimaging the outside of tubular structures. Therefore, when imaging hollow fiber

membrane surfaces, whether the concave or convex surface, the AFM should be operated

in contact rather than tapping mode.

A linear relationship exists between the root mean square and average roughness statistics.Therefore, both parameters may be equally applied for drawing correlations between

membrane surface morphology and membrane fouling.

The inherent physical heterogeneity of membrane surfaces requires that greater than threesites be imaged (i.e., their roughness measured) in order to develop an accurate assessment

of membrane surface roughness.

Surface roughness measurements using the AFM technique is a precise measure ofmembrane surface morphology based on the reproducibility and repeatability of surface

roughness statistics between two laboratories. The precision of the measurement is,

however, based on the two laboratories using the standard technique.

Using the standard technique, the captive bubble method produces precise contact angleresults for membrane surfaces with a repeatability and reproducibility between any two

laboratories of approximately 5. This level of precision allows for equal comparison of

relative assessments of membrane surfaces with regards to their hydrophobicity (i.e., is themembrane hydrophobic or hydrophilic). However, the current level of precision does not

lend itself well to the comparison of surface energy parameters calculated between two

laboratories using contact angles measured at each laboratory. Further study is needed to

determine how variations in contact angle results affect the magnitude and sign of surfaceenergy parameters determined through contact angle analysis to establish the broader

significance of these values to the membrane community (i.e., are these surface energy

parameters useful in membrane operations?).

The proposed standard method for determining the zeta potential of membrane surfacesusing streaming potential measurements produces relatively reproducible resultsindependent of the type of electrokinetic analyzer used. However, minor deviations from

the proposed standard method (e.g., differences in sample pretreatment) can result in

substantial errors.

The availability of characterization equipment and operator skill level requirements are thegreatest impediment to membrane characterization tests (contact angle, streaming

potential, surface roughness) being carried out by utilities.

From the perspective of utilities, characterization results, such as contact angle, surfaceroughness, and streaming potential, are only useful during membrane selection and lose



23/179

xxii

their usefulness once membranes are put into service. A key challenge that was identified

in this regard is how to determine membrane characteristics without damaging the

membrane modules or elements (i.e., how to determine these characteristics without

conducting a destructive autopsy).

The following conclusions were reached regarding the correlation between membranesurface properties determined using the proposed standard techniques and membrane fouling (or

more specifically, membrane resistance to fouling):

The permeate flux loss that was observed upon fouling of the five membrane samples witha bentonite clay suspension at a constant feed pressure could be modeled statistically to

express normalized permeate flux as a function of the clay loading rate (5 g m-2

). Based onthese observations a multiple linear regression (MLR) model was developed and found to

be capable of describing >97% of the observed variance in normalized product flux.

Those membrane surface properties that were determined to be necessary for describingthe observed membrane performance included root mean square roughness, contact angle

and initial permeate flux but not zeta potential. This implies that surface roughness andhydrophobicity have a greater influence on bentonite clay cake density/water permeabilitythan do surface charge interactions. Overall, better membrane performance (higher

observed normalized flux after 5 g m-2

bentonite loading) was associated with greater

RMS roughness, higher initial water flux and greater contact angle (greater surface

hydrophobicity).

Membrane surface properties (roughness, hydrophobicity and charge) could be used to

predict membrane performance in the presence of foulant materials using numerical

modeling approaches.

APPLICATIONS AND RECOMMENDATIONS

The membrane and standard surface characterization results demonstrate the reproducible

and accurate AFM generated surface roughness, contact angle, and zeta potential results can be

generated independent of the type of characterization equipment used. The generation of such

data is however dependent on the use of standard methods and experimental techniques, whichare presented in this report. The absence of such standard methods and techniques is likely to

result in variability in the aforementioned measures of membrane surface characteristics.

Process Design and Operational Impacts

Improving the accuracy and reliability of membrane characterization results, in particular

contact angle with water (hydrophobicity) and zeta potential (charge) is expected to furtherimprove the process by which membranes are selected for different applications. While further

research is needed to fully realize the implications of improved characterization results forpredicting membrane performance and predicting membrane fouling, the preliminary results

given in this report suggest that it could substantially improve the accuracy of predictive fouling

models. The development, and ultimate application, of more accurate predictive membrane

performance models is likely to facilitate better membrane selection and operation. For example,



24/179

Executive Summary| xxiii

xxiii

membranes may be selected whose surface chemistry is suited for minimizing fouling under a

given set of feed water quality conditions.

Data Interpretation

Use of the standard methods developed as part of this project can facilitate the generationof reproducible contact angle, streaming potential (zeta potential), and AFM produced surface

roughness statistics for membrane surfaces. These findings appear to be independent of the make

and model of the respective characterization equipment used. However, data that is produced inthe absence of these standard methods are open to substantial variability and error. Therefore,

membrane characterization data, and any extensions (interfacial modeling), must be analyzed

and interpreted with great caution. This is particularly important in the absence of anyaccompanying detailed information on the methods and techniques used. Based on these findings

it is highly recommended that laboratories carrying out membrane characterization studies

strictly adhere to the standard methods given in this report. Furthermore, consideration should be

given to developing a certification program for those laboratories seeking to carry out membranecharacterization work. Implementing these, and perhaps additional, standardization measures

will greatly improve the reproducibility and accuracy of membrane characterization results

(contact angle with water, zeta potential, and surface roughness statistics).

Regulatory Impacts

Based on the data collected as part of this effort the standard error in the measured

contact angle value for a given surface between any two labs is approximately 5. This

variability in contact angle results has implications for the calculated minimum direct integritytest pressure for obtaining a resolution of 3 m (Ptest), which is required for direct integrity

testing of hollow fiber membranes. The difference in calculatedPtestvalues, assuming an error in

the measured contact angle of 5, is dependent on the magnitude of the measured contact angle.

Over a range on contact angle values of 0 to 90 and an error in the contact angle value of 5, theerror in the calculatedPtestvalue may range from 0.1 to 1.3 psi.



25/179

xxiv

PARTICIPANTS

A list of subcontractors, participating utilities, and other individuals who contributed tothis project are listed below. We gratefully acknowledge the valuable contributions to this project

from all of the participants.

Subcontractors, Participating

Utilities and Other Participants

Pierre Kwan, P.E.

HDR Engineering, Inc. (HDR)

Prof. Jonathan A. Brant

University of Wyoming

Don Phipps

The Orange County Water District (OCWD)

Greg Turman

City of Clearwater

Annika M. Bankston, P.E.Minneapolis Water Works

Uzi DanielWest Basin Municipal Water District

Prof. Mark Wiesner

Duke University

Prof. Tzahi CathColorado School of Mines

Prof. Erik Hoek

University of California, Los Angeles

Carl Clegg

Ram-Hart Instruments Company

Thomas Luxbacher, Ph.D.

Anton Paar GmbH



26/179

1

CHAPTER 1INTRODUCTION AND BACKGROUND

Membrane-based pressure driven processes constitute essential and mature technologywith numerous applications in water treatment and drinking water production. Despite advancesin membrane science, the application of membrane processes continues to be hampered by thepersistent problem of membrane fouling. Membrane fouling results from the deposition,adsorption, and/or accumulation of rejected species on, or within the membrane, resulting in thedeterioration of water flux and quality.

Membrane processes are playing an ever-increasing role as environmental engineerscontinue to seek sustainable methods for water treatment, wastewater reclamation, groundwaterrestoration, and pollution prevention. Mitigation of membrane fouling and maximization ofwater productivity are essential for optimizing membrane processes. Membrane fouling results

from the attachment, accumulation, or adsorption of substances onto the membrane surfaceand/or within the membrane pores; it hinders membrane performance and shortens membranelife. Physical and chemical interactions between solutes or particles and the membraneinterface substantially affect water productivity and the rate at which membrane foulingoccurs. Previous research has unambiguously linked membrane characteristics to the rate andextent of membrane fouling, and therefore, to the water productivity and overall performance ofmembranes. Contact angle, surface roughness, and streaming potential are three surfacecharacterization methods that appear to be relatively simple; the measurements appear to beroutine and the physics behind the equations describing these properties appears to be straight-forward. However, literature on these surface characterization methods has shown that they arequite complex and far from being completely understood, especially zeta potential via streaming

potential measurements (Yaroshchuk and Ribitsch, 2002).

CONTACT ANGLE MEASUREMENTS

The contact angle () that is formed at the three-phase interface between solid, liquid, andgas/vapor phases (Figure 1.1) may be used to elicit information regarding membrane surfaceenergy properties. There are two accepted techniques for measuring contact angle, sessile dropand captive bubble, as shown in Figure 1a and 1b, respectively. The information that is gatheredfrom contact angle analysis may be used to calculate specific surface energy properties (van derWaals, Lewis acid-base) for detailed interfacial analyses, as well as for qualitatively assessingthe wettability, or hydrophobicity/hydrophilicity, of a membrane surface. This latter application

is perhaps the most common use for contact angle data by utilities and other membrane users.Perhaps the greatest challenge with goniometric contact angle measurements is contact anglehysteresis, which is the difference in the measured contact angle depending on whether it is anadvancing or receding measurement. Advancing measurements involve the spreading of a liquiddrop over a dry surface. Conversely, receding measurements involve the liquid droplet shrinkingand thus the contact angle is measured on an already wet surface. Receding measurements aremost commonly associated with the captive bubble technique in which an air bubble is placed ona surface that is immersed in water. In this way, the water recedes from that volume that is



27/179

2

occupied by the air bubble (Figure 1.1b). The effect can be as large as a 20 difference inadvancing and receding contact angle values, but can also be negligible (Adamson and Gast,1997; Garbassi et al., 1998). For a brief, recent account of the origin of contact angle hysteresissee Gao and McCarthy (2006). Although theories of contact angle and contact angle hysteresisare still controversial, the measurement does have numerous practical applications (e.g.,

calculation of surface energy components and pore-liquid entry pressures).

Figure 1.1 (a) Representative digital image of a liquid droplet on a dry surface in which thecontact angle () is measured according to the sessile drop method. (b) Representativedigital image of an air bubble on a wetted membrane surface (the membrane is immersedin a liquid) and the contact angle () is measured according to the captive bubbletechnique.

Perhaps the greatest advantages of contact angle measurements are the relatively simpleprinciples of the measurement technique and that the required equipment (goniometer) isrelatively inexpensive. Note that these relative comparisons are all being made to therequirements for other characterization measurements, such as streaming potential and electron

microscopy techniques. That said, details associated with the algorithms that are used for tangentline determination in digital images by vendor supplied software can be complicated and are notusually revealed by the vendors.

From a practical point of view, the captive bubble technique, in which the gaseous phaseis introduced as a bubble under the membrane surface, is more viable than the sessile droptechnique, which requires rigorous drying of the membrane surface. However, with the captivebubble technique, it is sometimes confusing as to which angle should be reported. As contactangle is defined as the angle measured through the denser fluid phase, the contact angle throughthe liquid phase should always be reported. This means that if the goniometer (because of objectconfiguration software requirements) measures the angle through the gas phase (*) then thesupplementary angle to the * angle (180- *) should be reported. Interestingly, two referenceson this subject cited here (Adamson and Gast, 1997; Garbassi et al., 1998) have figuressuggesting otherwise; also an example in recent literature (Roudman and DiGiano, 2000) isfurther indication of confusion on this issue. Another aspect of the contact angle measurementthat can be confusing is the distinction between advancing and receding angles. The use of theseterms is apparent for sessile drop measurements, but is sometimes confused for captive bubblemeasurements. As can be seen, in Figure 1.2 attention should not be placed on whether the drop

a) b)



28/179

3

or bubble is growing or shrinking, but instead on whether the liquid phase is advancing orreceding, because it is the solid-liquid contact angle that is of interest (Drelich, 1997).

Figure 1.2 Techniques commonly used for carrying out contact angle measurements: (a)static sessile drop advancing and receding; (b) static captive bubble advancing andreceding; and (c) dynamic captive bubble. Figure adapted from Drelich (1997). For thoseillustrations for the sessile drop technique the term advancing refers to the advancement ofthe liquid (increase in the volume of the liquid droplet) over a dry surface, while the termreceding refers to the recession of the liquid droplet (decrease the in the volume of theliquid droplet) over a previously wet surface. A similar relationship exists for the captivebubble measurements, where the water in which the surface is immersed either advancesor recedes across the surface through a decrease or increase in the volume of the air

bubble.

Another important issue for contact angle measurement reproducibility is the effect ofdrop and bubble size on contact angle. Drelich et al. (1996; 1997) performed a study on a varietyof surfaces (polymers, monolayers on gold, minerals) and made several conclusions. First, forsmooth, homogenous, nearly-ideal surfaces, contact angle does not depend on drop size (Figure1.3), although hysteresis can remain detectable. Second, dynamic captive bubble resultsapproach results for receding sessile drop or bubble. Third, receding sessile drop results are most

Gas

Liquid

c)

Receding or intermediate contact angle

Pressure air

Buoyancy force

Gas

Liquid

b)

Advancingcontact angle

Recedingcontact angle

Gas

Liquid

a)

Advancingcontact angle

Recedingcontact angle

Liquid

Gas



29/179

4

prone to change with drop size on non-ideal surfaces. The advancing angle should be morereproducible, as it depends on bubble/drop size to a lesser degree, on both smooth and imperfectsurfaces. However, information on surface quality that can be gained from receding angle/bubblesize relationship will be lost if only advancing angle is measured. Drelich et al. (1996; 1997)offered explanations for some of these facts based on the concept of line tension. Line tension is

the energy associated with the line where the three phases meet and has dimensions ofenergy/length, as opposed to energy/surface units for surface tension. It was concluded thatfurther discussion and research is necessary to better understand observed phenomena.

Tadmor (2004) used the line tension (energy) concept to express the experimentallyelusive equilibrium contact angle as a function of the advancing and receding contact angles.However, calculations of the equilibrium value require precise measurement of both theadvancing and receding angles, something that is not practical in non-laboratory applications.

Figure 1.3 The effect of drop (bubble) size on advancing (Adv.) and receding (Rec.) contactangles for the air /water/polyethylene film system as obtained with the static sessile-drop(SD), static captive-bubble (CB), and dynamic captive-bubble (DCB) techniques. Source:Reprinted fromJournal of Colloid and Interface Science, 179 (1), Drelich, J., Miller, J.D.,Good, R.J., The Effect of Drop (Bubble) Size on Advancing and Receding Contact Anglesfor Heterogeneous and Rough Solid Surfaces as Observed with Sessile-Drop and Captive-Bubble Techniques,37-50, 1996, with permission from Elsevier.

Another interesting factor is the exposure time during which contact angle is measured.For special cases, when the solvent alters the surface being characterized (e.g., when swelling

occurs) significant time drift of contact angles (both advancing and receding) can be observed(Adamson and Gast, 1997). However, use of the captive bubble technique and a conditionedmembrane can eliminate the time drift problem. Measurement temperature is another factor thatmay potentially affect the measured contact angle value. For example, Schonhorn (1966)concluded based on theoretical considerations, that for polar liquids and polypropylene, thedependence of contact angle on temperature should be minimal ( = 2 for T 100C). Hefurther concluded that contact angle should decrease as the temperature of the probe liquidincreases. Such a dependence of contact angle on temperature is likely attributed to the



30/179

5

dependence of liquid viscosity on temperature. Surface titration can provide information on thedependence of contact angle on the chemical interactions of a solution with a solid surface(Adamson and Gast, 1997). Mnttri et al. (2006)report contact angle changes as function of pHfor several commercial membranes. In the same paper, zeta potentials as functions of pH are alsoshown. It is interesting to note that these plots of contact angle as a function of pH correlated

with zeta potentials plotted as absolute values. See also Brant et al. (2006) for further examplesof contact angle titration. Even though performing surface titration bears prohibitive labor costfor routine analysis (multiple samples and solutions to assure statistically significant data), itunderscores the need for control of pH of the water used in the study.

Boussu et al. (2008) used multiple linear regression (MLR) to study the influence ofsurface hydrophobicity, surface roughness, surface charge, molecular weight cut-off,permeability, and porosity of the top layer on nanofiltration membrane performance. Contactangle, volume fraction of small pores, and membrane charge were the most significant variablesin predicting relative flux and adsorbed amount as well as retention of dissolved organiccompounds by five commercial NF membranes. Also, contact angle measurements are useful inmembrane integrity testing (if one wishes to use a realistic estimate, rather than a worst casescenario value of 1 for cos ) as outlined by EPA in Membrane Filtration Guidance Manual(Alklaibi and Lior, 2005). The need for determination of the pressure necessary for integritytesting stems from requirements of EPAsLong Term 2 Enhanced Surface Water Treatment Rule(LT2ESWTR) (Allcock et al., 2006). For example, Cryptosporidium oocysts constitute a publichealth risk as they cause gastro-intestinal illness and resist chlorine-based disinfectants. The sizeof Cryptosporidium oocysts (spores) is approximately 3 m. The net pressure applied duringdirect integrity tests must ensure that any breach large enough to pass Cryptosporidium oocysts(3 m) would also pass air during the test (EPA, 2005). The minimum direct integrity testpressure, otherwise known as the pore liquid entry pressure, is a function of both the liquidcharacteristics (surface tension) and the membrane characteristics (hydrophobicity and porestructure):

maxcos193.0 BPPtest += (1.1)

where Ptest is the minimum direct integrity test pressure (psi), is the pore shape correctionfactor (dimensionless), is the surface tension at the air-liquid interface (dynes/cm), is theliquid-membrane contact angle (degrees), BPmax is the maximum backpressure on the systemduring the test (psi), and 0.193 is the constant that includes the defect diameter (i.e., 3-mresolution requirement) and unit conversion factors (EPA, 2005). This equation appears asEquation 4.1 in theMembrane Filtration Guidance Manual (EPA, 2005):

The LT2ESWTR does not establish the minimum test pressure to be used during a

pressure-based direct integrity test, but rather only requires that the test achieve a 3-

m resolution. If a membrane manufacturer has information to support the use of

values other than = 1 and= 0, and these less conservative values are approved

by the State, then Equation 4.1 can be used to calculate the minimum required test

pressure. It is essential that the use of values other than = 1 and = 0 be

scientifically defensible, since the use of inappropriate values could result in the use

of a test pressure that does not meet the resolution criterion established by the rule.



31/179

6

One approach for determining membrane-specific values for and is through

direct experimental evaluation. Because these parameters can have a significant

effect on the required direct integrity test pressure, it is strongly recommended that

States require sufficient justification from a membrane manufacturer prior to

approving the use of values other than = 1 and= 0, such as independent third

party testing results using a method accepted by the scientific community anddemonstrating statistically significant data.

It is clear that in order to reduce the pressure difference necessary for integrity testing andto achieve the resolution requirement, reliable estimation of membrane contact angle is required.The linear dependence of pressure drop on cos (Eq. 1.1) implies that for= 30, a pressuredrop 13% less than the worst case scenario estimate (= 0) can be used. An uncertainty of 1 for= 30 (i.e., = 30 1) will translate into 1% error bars for the required pressure drop. Thesame absolute error bar forwill translate into greater absolute and relative cos uncertainty formore hydrophobic membranes (increasing in degrees: |d cos | = (/180)|sin||d |).

ZETA POTENTIAL MEASUREMENTS

Membrane surface charge is not directly experimentally accessible. Instead, zeta potentialis used as an approximation for both the magnitude and sign (+ or ) of membrane surfacecharge. Zeta potential () is the potential difference between the bulk of solution and the sheer(slipping) plane of the interfacial double layer[Figure 1.4] (Delgado et al., 2005). Zeta potentialis a function of surface and solution chemistry (pH, ionic composition, and ionic strength) at thesolid-liquid interface and is an important membrane characteristic for assessing membranefouling potential and developing chemical cleaning protocols. Membrane zeta potential istypically determined from streaming potential measurements. A streaming potential is generatedwhen an electrolyte solution flows through a thin channel or porous media (e.g., a sand column)and is related to zeta potential by the Helmholtz-Smoluchowski equation:

0E

p

=

(1.2)

whereE is the streaming potential due to electrolyte flow through a capillary channel, p is theapplied pressure driving the flow, is the zeta potential, is the electrolyte conductivity, is theviscosity of the electrolyte solution, is the relative permittivity of the solution (dimensionless),and0 is the vacuum permittivity (fundamental constant). Values ofE,p, and are measured bythe streaming potential analyzer; while and are calculated based on temperature measurement(empirical fit functions for pure water data are used).

In experimental investigations of membrane charge, streaming potential measurementshave typically been used to calculate zeta potential. Streaming potential is the potential inducedwhen an electrolyte solution is pumped across a stationary, charged surface. Streaming potentialcan be used to calculate zeta potential using the Helmholtz-Smoluchowski equation, whichrelates the pressure dependence of streaming potential to the properties of the solution (i.e.,conductivity and viscosity). Unlike earlier streaming potential investigations, the majority ofmore recent investigations (e.g., (Nystrm et al., 1995; Childress and Elimelech, 1996;



32/179

7

Hagmeyer and Gimbel, 1999; Peeters et al., 1999; Schfer, 1999; Childress and Elimelech, 2000;Ernst et al., 2000; Tay et al., 2002)) measure streaming potential over a range of solution pH.Investigating surface charge as a function of pH is crucial for understanding acid-base propertiesof membrane surface functional groups.

Although streaming potential measurements are the most frequently used method forevaluating charge properties, they have also been criticized. Results from prior studies revealuncertainty in individual measurements as well as data scatter (Chapman-Wilbert et al., 1999).Further, the question of overall reproducibility has not been addressed. Other concerns includethe effect of membrane roughness on the measurement, vagueness of the relationship betweenthe measured zeta potential and the double layer structure, the inherent assumptions of theHelmholtz-Smoluchowski equation (i.e., laminar flow), and the lack of a calibration standard.

Figure 1.4 Illustration of the relative locations of the Stern layer and Shear plane from acharged surface in water and the associated change in surface potential with distance.Source: Reprinted fromJournal of Membrane Science, 161 (1-2), Chapman-Wilbert, M.,Delagah, S., Pellegrino, J., Variance of streaming potential measurements, 247-261, 1999,

with permission from Elsevier.

Although the operating principles and requirements for measuring streaming potential(e.g., voltage, pressure) are relatively straightforward, commercial instruments are much morecommon than lab-made instruments. However, due to the small market for these instruments,they are relatively expensive. This is an additional disadvantage of the technique. However, zetapotential from streaming potential measurement is one of few techniques capable of describingthe charge properties of membranes.



33/179

8

SURFACE ROUGHNESS ATOMIC FORCE MICROSCOPY (AFM)

Surface roughness may be measured with atomic-level resolution using atomic forcemicroscopy (AFM). AFM is a non-optical surface imaging technique which approaches atomicresolution. In the most common implementation, a sharp tip on a cantilever is scanned at

constant force across a surface to provide a surface profile. The force between the tip and thesurface always has a van der Waals (dispersion) force component, but cantilevers carryingcharge are also available and can be used to image electric potential. A laser beam is reflected offthe back of a cantilever and its position determined using a photo-diode. A surface may bescanned using any number of operating modes (e.g., contact, tapping, non-contact). Tappingmode is most commonly used to characterize membrane surfaces as contact mode may result indamage to the membrane surface. Tapping mode uses a rapidly oscillating cantilever in thevicinity of the surface, and amplitude damping is used for imaging. Only short, intermittentcontact of the AFM tip with the sample (tapping) occurs, which is especially suitable formembrane surfaces. Measuring the position and movement of the cantilever as it is scanned overa membrane surface allows for the direct measurement of surface features.

The membrane surface can be scanned in vacuum, air, or water and no samplepreparation is necessary (Mulder, 1996). An example illustration of an AFM experimental setupis given in Figure 1.5. Based on the sample profile, h(x, y), it is possible to calculate mean andRMS roughness (Ra andRrms, respectively) and 10-point mean roughness (Rz) parameters (Chunget al., 2002). Chung et al. (2002) determined the relationship betweenRa and the water flux andseparation performance of hollow-fiber UF membranes. As Ra decreased, flux decreased andrejection increased. This example illustrates the relevance of AFM characterization to membraneperformance.

Figure 1.5Schematic illustration of the operating principles associated with the atomicforce microscope (AFM). Taken from Wyart et al. (2008). Source: Reprinted fromJournalof Membrane Science, 315 (1-2), Wyart, Y., Georges, G., Demie, C., Amra, C., Moulin, P.,Membrane characterization by microscopic methods: Multiscale structure, 82-92, 2008,with permission from Elsevier.



34/179

9

TRANSMISSION ELECTRON MICROSCOPY (TEM)

In transmission electron microscopy (TEM) an image is produced by passing electronsunder high vacuum through a sample. Near-atomic resolution is attainable using TEM; howeverthis technique is limited to thin samples that are at most several tens of nanometers in thickness.

Therefore, TEM is restricted to imaging thin-film samples rather than fully constructedmembranes. An example TEM image of a graft copolymer is shown in Figure 1.6. Kim et al.(2008) concluded that the differences in electron densities between the two components of thisamphiphilic graft copolymer were sufficient to be observed as image contrast. Dark regionsrepresent the hydrophobic crystalline regions of the main chains, whereas lighter regionsrepresent the hydrophilic side chains. In another example, use of sophisticated cutting tools (e.g.,a diamond knife ultramicrotome) enabled Blom et al. (2003) to study cross-sectional samples ofa proton exchange membrane.

Figure 1.6 Transmission electron microscope (TEM) image of a graft copolymer film.Taken from Kim et al. (2008). Source: Reprinted fromJournal of Membrane Science,325 (1), Kim, Y., Park, J.T., Koh, J.H., Roh, D.K., Kim, J.H., Anhydrous protonconducting membranes based on crosslinked graft copolymer electrolytes, 319-325,2008, with permission from Elsevier.

More recently, tomography capability has been added to the TEM technique (TEMT).The majority of publications using TEMT pertain to polymers. In TEMT, TEM images arecollected from single specimens at different tilt angles and are processed to generate 3Drepresentations of nano-structures. In a paper by Jinnai et al. (2006), 200-nm thick co-polymersamples were studied. Images clearly show the porous structure of the material (Figure 1.7). Noreports have been found pertaining to TEMT for polymeric membranes for water treatment. Interms of sample preparation, the vacuum environment requires dry samples. Also, coating may



35/179

10

be necessary to prevent sample destruction by the electron beam. These requirements constitutedisadvantages of TEM as they can lead to artifacts.

Figure 1.7 Ordered and disordered co-polymer blend morphologies visualized using TEMTmethod. Taken from Jinnai et al. (2006).

SCANNING ELECTRON MICROSCOPY (SEM)

For scanning electron microscopy (SEM) measurements a sample surface is exposed to anarrow beam of electrons in vacuum. Secondary electrons are liberated form the surface anddetected. These conditions require sample preparation drying and coating with conductivesurface (in most cases). Sample preparation can lead to artifacts in observed images (Mulder,

1996). Resolution of 10 nm or better can be achieved, but varies with equipment available. Poresize, surface porosity, and pore geometry can be visualized (Mulder, 1996) and porosity and poresize distribution can be estimated. Asymmetry in composite membranes can also be readilyobserved. Li et al. (2008) used SEM to study polysulfone/sulfonated PEEK membranes. Changesin pore structure resulting from different casting conditions and composition are shown in Figure1.8.



36/179

11

Figure 1.8 Scanning electron microscope (SEM) images of cross-sections of polymerblend membranes. Taken form Li et al. (2008).Source: Reprinted fromJournal ofMembrane Science, 324, Li, X., De Feyter, S., Vankelecom, I.F.J.,Poly(sulfone)/sulfonated poly(ether ether ketone) blend membranes: Morphologystudy and application in the filtration of alcohol based feeds, 67-75, 2008, withpermission from Elsevier.

Obtaining cross-section samples usually involves razor blades and can result incompression and tearing. The direct freeze fracture method, where a membrane is made brittle in

liquid nitrogen and is then broken, can also be used. Co-principal investigator Phipps wasinvolved in developing a new cryo-snap technique, where the membrane is embedded in icebefore a fracture is made. Perhaps the resolving power of SEM measurements is the greatestadvantage of this technique not reaching atomic level, but well beyond optical microscoperesolution, it is perfectly suited for the study of membrane pores. Very rough samples that cancause a large amount of tip artifacts in AFM (Mller and Mehnert, 1997) are not a problem inSEM. Also, the preparation of a thin film, as is required in TEM, is not required for SEM. The



37/179

12

only disadvantage is that membrane pore structure can be affected by drying and coating, soSEM-observed porosity may not exactly correspond to in situ membrane porosity.

CHEMICAL FORCE MICROSCOPY (CFM)

Chemical force microscopy (CFM) is an innovative extension of AFM. CFM uses achemically modified AFM tip to investigate specific interactions with, and ultimately thechemistry of, a membrane surface. Often a microsphere, or a colloidal particle, with specificfunctional groups is placed on the tip to create a functionalized probe. For example, a tip coveredwith acidic carboxylic groups will exhibit enhanced attraction to polar, rather than hydrophobic,sites on the surface. CFM experiments on membranes are frequently performed in aqueoussolutions in order to best simulate in situ interactions. There is no need for surface preparation asin SEM. The distribution of adhesion values that were measured across the HL and SGmembrane surfaces using various (hydrophobic and hydrophilic) probes are reported in Table1.1. This data may be used to characterize the relative chemical heterogeneity across a membranesurface (i.e., the distribution of hydrophilic [low adhesion] and hydrophobic [high adhesion]areas). This data may be further supported using CFM resolved images of the membrane surface(Figure 1.9), which are similar to the 3D rendered images of membrane surfaces that areproduced using conventional AFM. For example, Brant et al. (2006)used CFM images of twomembrane surfaces in order to understand their associated fouling tendencies.

Table 1.1Distribution of adhesion across the HL and SG membrane surfaces as a function of surface

area coverage measured with methyl, carboxyl and hydroxyl functionalized probes (I =0.01 M NaCl; pH 6; T = 20 C).

Probe High Adhesion Moderate Adhesion Low Adhesion

Surface area coverage (%)

HL Membrane

Methyl 5.2 88.0 6.8

Carboxyl 97.2 1.6 1.2

Hydroxyl 7.8 88.9 3.3

SG Membrane

Methyl 5.1 22.2 72.7

Carboxyl 93.0 6.7 0.4

Hydroxyl 0.3 21.7 78.0Source: Reprinted from Colloids and Surfaces A:- Physicochemical and Engineering Aspects, 280 (1-3),Brant, J.A., Johnson, K.M., Childress, A.E., Characterizing NF and RO membrane surface heterogeneityusing chemical force microscopy,45-57, 2006, with permission from Elsevier.

http://www.sciencedirect.com/science/article/pii/S0927775706000689?_rdoc=8&_fmt=high&_origin=browse&_srch=hubEid(1-s2.0-S0927775706X03011)&_docanchor=&_ct=37&_refLink=Y&_zone=rslt_list_item&md5=48f56cdc0c5011fc5389a77e9fd16325http://www.sciencedirect.com/science/article/pii/S092777570600

Membrane Characerizations Net 4102

Documents

Transcript of Membrane Characerizations Net 4102