Journal of Membrane Science 209 (2002) 93–106
a
b
, ∗
a
Department of Chemical Engineering, Colorado School of Mines, Golden, CO 80401, USA b
Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, USA
Received 12 December 2001; received in revised form 29 April 2002; accepted 21 May 2002
Abstract
The initial fouling behavior of a clean membrane surface was studied using flow field-flow fractionation (flow FFF), an analytical technique typically used to separate and characterize macromolecules and particulates. This work represents the first time flow FFF has been used to quantitatively evaluate membrane performance. Flow FFF is an ideal tool for expeditiously studying sample–membrane interactions for the following reasons: membranes can be quickly installed into the flow FFF channel, each analysis requires only microgram amounts of sample, and sample–membrane interactions can be rapidly quantitated for different flowrates and solution compositions.
Suwannee River humic acids were used as a probe to investigate the initial fouling of an XLE reverse osmosis membrane and an NF-200 nanofiltration membrane. Flow FFF was successfully used to quantitate the fouling of each membrane and to demonstrate that the majority of sample loss was due to irreversible adsorption. The fouling on both membranes was enhanced by increasing the flowrate perpendicular to the membrane surface and by adding calcium ions to the solution. The
NF-200 membrane was more resistant than the XLE membrane to fouling in the presence of calcium ions, whereas, the fouling resistance of both membranes improved to similar levels with the addition of EDTA to a solution containing calcium ions.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: Flow field-flow fractionation; Humic acids; Fouling; Solute–surface interactions; Reverse osmosis; Water treatment
1. Introduction
Humic substances comprise a large group of organic compounds that are the natural decomposition products of plant and animal debris. Since they are present in aquatic and terrestrial systems, humic materials pose a wide variety of problems, especially for municipal and commercial water treatment plants and systems
[1–3] . For example, humic substances
have the ability to form carcinogenic trihalomethanes
∗
Corresponding author. Tel.:
+
1-303-273-3245; fax:
+
1-303-273-3629.
E-mail address: krwillia@mines.edu (S.K.R. Williams).
during chlorine disinfection
[4–10] . Some water treat-
ment plants use membranes to remove humics from potable water, but substantial membrane fouling occurs due to the wide molecular weight distribution and high surface activity of humic materials. Humic fouling directly impacts membrane performance and thus the efficiency of water treatment plants. The expenses associated with membrane cleaning, replacement, and plant down time are incentives to minimize fouling.
Up to this point, stirred cell set-ups and a variety of membrane modules have been used to study various aspects of membrane fouling
[11–14] . The main
parameters of interest are flux, pressure, and/or permeate/retentate concentrations since these are direct
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 2 8 0 - 6

94
Nomenclature
A
A
A
A
BR
DT
RA
SR
R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 mean peak area of the blank runs mean peak area of the dilution tube runs mean peak area obtained when the field was removed at the end of each sample recovery run mean peak area of the sample recovery runs
D
R diffusion coefficient retention ratio
%SIA percent sample irreversibly adsorbed
%SR percent sample recovery t
%SRA percent sample reversibly adsorbed void time t r w
V
0
V
˙ c sample retention time channel thickness channel volume or void volume volumetric cross flowrate indicators of fouling. These experiments are time consuming and require large quantities of sample and solution. Flow field-flow fractionation (flow FFF), on the other hand, is an ideal tool for investigating membrane fouling because of the small quantities of membrane
(6 cm
×
32 cm), sample (1–2
␮ g), and solution (
∼
0.2 l) required and the short analysis times.
FFF is a chromatography-like technique traditionally used for separating and characterizing macromolecules and particulates
[15–21] . A sample plug
injected into the flow of carrier solution is transported along the length of the FFF channel. The channel flowrate is such that laminar flow conditions are present, and a parabolic fluid velocity profile is es-
tablished across the breadth of the channel ( Fig. 1a ).
The maximum flow velocity occurs at the center of the channel and approaches zero at the channel walls.
The driving force responsible for separating the sample into its components is induced by a field that is applied perpendicular to the channel flow (
Fig. 1a ).
This external “field” can be electrical, magnetic, crossflow, thermal, sedimentation, etc. depending on the properties of the sample being separated
[20,22–26]
In this work, the “field” is a crossflow of fluid that is identical in composition to that of the carrier solution. The crossflow passes through a porous ceramic
.
frit, enters the FFF channel, and flows across the channel thickness while driving the sample to the surface of a semi-permeable membrane located on the opposite wall. The membrane retains sample in the channel while allowing the carrier liquid to leave the channel and exit the system via a second porous ceramic frit
(
Fig. 1b ). It is important to note that in standard FFF
terminology the term “crossflow” refers to the fluid flow across the membrane thickness as opposed to over the membrane surface.
The crossflow causes a sample concentration build-up on the membrane surface. A steady-state condition is established when the field-induced sample migration towards the membrane equals the diffusive transport of sample back towards the center of the channel. Different sample species form zones of different thickness extending out from the membrane surface. In this normal mode separation, smaller
“particles” form thicker zones due to their high diffusion rates and occupy the faster velocity streamlines of the laminar flow profile. These particles elute more rapidly than larger particles which are in the slower velocity streamlines near the membrane wall (
Fig. 1b ).
When the particle size exceeds approximately 1 ␮ m, the steric/hyperlayer mode prevails, and the elution order is reversed in that larger particles elute before smaller particles
[27–29] . The humic acids used in
this work are smaller than 1
␮ m and thus elute by the normal mode only.
The separated sample components elute from the channel within minutes and flow through a UV detector that detects and records their passage. For normal mode separations, the measured retention time (i.e.
amount of time for a sample component to elute from the channel) can be used to calculate sample hydrodynamic diameters and diffusion coefficients. In addition, the retention times and peak areas can provide information about sample–membrane interactions and sample recoveries (amount of sample eluting from the channel). This quantitative capability of flow FFF offers a unique analytical perspective in the study of membrane fouling.
FFF is typically used to measure molecular weight and particle size distributions. This work is a novel use of flow FFF to quantitate the amount of sample adsorbed to a clean membrane surface and to promptly assess the performance of a membrane. Suwannee
River humic acids (SRHA) were used to investigate

R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 95
Fig. 1. (a) Flow FFF channel with laminar flow profile; (b) exploded view of channel.
the initial fouling of an XLE reverse osmosis membrane and an NF-200 nanofiltration membrane. The objectives were (1) to demonstrate that flow FFF is a reliable analytical technique capable of quantitating membrane fouling, (2) to measure and compare the fouling of an XLE membrane and an NF-200 membrane, (3) to differentiate between reversible and irreversible sample adsorption, and (4) to quantitate the effect of cross flowrate, calcium ions, and EDTA on membrane fouling.
2. Materials and methods
2.1. Membranes
2.1.1. Samples
Rolls of an XLE reverse osmosis membrane and an NF-200 nanofiltration membrane were supplied by FilmTec Corporation (Edina, MN). The XLE membrane is made from 1,3,5-benzene tri-carbonyl tri-chloride (TMC) and 1,3-phenylene diamine
(MPD), whereas, the NF-200 membrane is made from
TMC and piperazine (
Fig. 2 ). Each membrane con-
sists of a non-woven backing covered with a porous layer of polysulfone. The polyamide layer formed on the surface has a thickness of several thousand angstroms
[30] .
2.1.2. Storage
The membrane rolls were each sealed in a plastic bag and stored at 2
◦
C. Exposure of the XLE membrane to light was kept to a minimum because air oxidation of the residual MPD group was catalyzed by light
[30] .
2.1.3. Preparation
A membrane strip (6 cm
×
32 cm), cut to fit the flow FFF channel, was soaked overnight in distilled deionized water. The membrane was then placed in a
25% isopropyl alcohol–water solution for 20 min and immersed in freshly distilled deionized water for 1 h.
Care was taken to avoid touching the membrane surface that would later be exposed to sample in the channel. The XLE and NF-200 membranes were prepared in the same manner with the exception that the XLE membrane sample was minimally exposed to light.
2.2. Solution chemistry
2.2.1. Carrier solutions
The American Society for Testing Methods
(ASTM) procedure for preparing reconstituted fresh water
[31,32] was modified to produce a carrier liquid
that was generally representative of rivers
[30,33–36] .
The ASTM standard solution for soft waters, composed of NaHCO
3
, CaSO
4
·
2H
2
O, MgSO
4
, and KCl,

R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 was selected because it had a pH range of 7.3–7.5
which is similar to the pH found in many rivers. Experimental variables were minimized by excluding
MgSO
4 and KCl from all carrier solutions. These two salts were selected because Mg smaller amounts than Ca
2
+
2
+ is present in in river waters and KCl is an indifferent electrolyte.
Three variations of the ASTM soft water standard solution were used as flow FFF carrier solutions. The first solution, composed of 2.27 mM NaHCO
3
, was used to establish a baseline for comparing the extent of fouling with other solutions. A second solution of 1.57 mM NaHCO
3 and 0.17 mM CaSO
4
·
2H
2
O was employed to study the effect of calcium ions on membrane fouling. The third solution, consisting of 1.57 mM NaHCO
3
, 0.17 mM CaSO
4
·
2H
2
O, and
0.17 mM EDTA, was applied to investigate the impact
EDTA had on membrane fouling in the presence of calcium ions. The ionic strength affected the conformation of humic substances and was kept constant by adjusting the amount of NaHCO
3 in each solution to match the ionic strength (2.27 mM) of the modified
ASTM soft water standard solution. The first two solutions had a pH of ∼ 7.5, whereas, the pH of the third solution was
∼
6.5.
2.2.2. Chemical reagents
Reagent grade calcium sulfate dihydrate (CaSO
4
·
2H
2
O) and ethylenediamine-tetraacetic acid disodium salt dihydrate (C
10
H
14
N
2
Na
2
O
8
·
2H
2
O) were acquired from J.T. Baker (Phillipsburg, NJ). ACS certified sodium bicarbonate (NaHCO
3
) was purchased from Fisher Scientific (Pittsburgh, PA).
2.2.3. Suwannee River humic acids
A freeze-dried sample of Suwannee River humic acid (SRHA) reference was obtained from the International Humic Substances Society (Department of
Soils, University of Minnesota, St. Paul, MN). The
SRHA was dissolved in a 2.27 mM NaHCO
(1
␮ g/50
␮ l) and stored at 2
◦
C.
2.2.4. Distilled deionized water
3 solution
Distilled water was processed through a Module
Type 1 Pretreat Feed Barnstead (Dubuque, IA) water purification system. The distilled deionized water was used to make all carrier solutions and had a conductivity of
∼
12 M cm.
2.3. Flow FFF
2.3.1. Equipment
97
The flow FFF system is composed of several
pieces of equipment ( Fig. 3 ). An HPLC model 420
pump (ESA Inc., Bedford, MA) and an HPLC pump
Series II (Lab Alliance, State College, PA) generated the crossflow and channel flows, respectively.
Samples were injected into the channel via a 50
␮ l
Rheodyne loop injector (Model 7125, Rheodyne Inc.,
Cotati, CA). The flow FFF channel (Model F-1000,
Fig. 3. Flow FFF system set-up.

98 R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106
FFFractionation LLC, Salt Lake City, UT) consisted of two Plexiglass blocks that were each inlaid with a porous ceramic frit. A 2 cm wide channel with triangular tips 29.5 cm apart was cut out of a 0.02 cm thick
Teflon spacer. The UV detectors (Model 757, Applied
Biosystems, Ramsey, NJ) were set at a wavelength of
254 nm. Data acquisition and analysis were accomplished with a PC compatible computer. The computer was also connected to two balances (Model TS400S,
Ohaus, Florham Park, NJ) that measured flowrate by monitoring changes in the mass of carrier liquid eluting from the FFF channel as a function of time.
The flow FFF system was modified to withstand the high pressures that accompanied the use of nanofiltration and reverse osmosis membranes. These modifications included replacing the system’s three-way valves with high-pressure three-way valves and the use of Gortex sealant around the channel. The pressure in the system varied between 50 and 350 psi depending on the membrane and flowrate conditions employed.
2.3.2. Preparation of flow FFF system
Installation of the membrane into the FFF channel occurred immediately after the wetting procedure.
This entailed placing Gortex sealant, the membrane, and the Teflon spacer between the two Plexiglass blocks of the flow FFF channel and tightening the bolts to 55 in. lb. Air bubbles were removed, and the channel was connected to the rest of the flow FFF system. The membrane was then equilibrated with the carrier solution by flushing the system overnight.
2.3.3. Experimental method
A goal of this work was to study the initial fouling of clean membrane surfaces and thus experiments were performed the first 4 days after a new membrane was installed. Each experiment consisted of three steps: a sample recovery run to determine the amount of sample eluting (30–60 min), a blank run to correct for system fluctuations (30–60 min), and a dilution tube run to establish 100% sample recovery (15–25 min). At the start of every run, the channel and cross flowrates were adjusted to the desired flowrates using a back
pressure regulator ( Fig. 3 ) to equalize each flowrate
entering the channel with the respective flowrate exiting the channel.
For each sample recovery run, approximately 1 ␮ g of SRHA was injected into the flow FFF channel.
Once the SRHA entered the channel, valves 1 and 2
(
Fig. 3 ) were rotated to divert the channel flow di-
rectly to the UV
1 detector (flow path represented by dashed line), allowing the crossflow to drive sample to the membrane surface. After a steady-state sample concentration profile had developed across the channel thickness, valves 1 and 2 were returned to their original position, and the flow was rerouted back through the channel. The sample was then separated, eluted, and recorded by the UV
1 detector as a peak.
The recorded peak area was used to calculate the sample recovery (i.e. the amount of eluted sample compared to the amount of sample injected).
After the sample had completely eluted from the channel, the crossflow was diverted around the channel (dashed line) using valves 3 and 4 (
Fig. 3 ). Any
sample reversibly adsorbed to the membrane as a result of the field was released, eluted, and recorded by the UV
1 detector. The UV
2 detector connected to the crossflow outlet measured sample loss due to permeation through the membrane. The amount of sample reversibly adsorbed and the amount of sample lost through the membrane were calculated from their respective peak areas.
Diverting the channel and cross flowrates caused pressure fluctuations in the flow FFF system, which in turn, led to baseline shifts in the UV detector traces.
Blank runs with 50
␮ l injections of distilled deionized water were performed to ascertain the extent of these baseline shifts and to obtain any necessary correction factors for the sample recovery measurements.
The dilution tube runs consisted of injecting 1
␮ g samples of SRHA into a 1-ml polystyrene pipette connected to the channel flow pump and to the UV
1 detector. The resulting UV signal peak area corresponded to 100% sample recovery. The tube diluted the sample to a concentration comparable to that experienced in the FFF channel and yielded an on-scale detector peak. Sample interactions with the dilution tube wall were assumed to be negligible.
2.4. Method of calculation
The peak areas for each sample recovery run, blank run, and dilution tube run were calculated using software from FFFractionation, LLC (Salt Lake
City, UT). The percent sample recovery (%SR) was determined by comparing the mean peak area of the

R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 99
Fig. 4. A
DT is the SRHA peak area associated with 100% sample recovery and is obtained by injection through a dilution tube. A
SR the peak area of an SRHA sample eluted from the flow FFF channel and represents the sample recovery. A
RA is is the resulting peak area after the crossflow is turned off and corresponds to sample that has been reversibly adsorbed to the membrane.
sample recovery runs (A SR ) to the mean peak area of the dilution tube runs (A
DT
) ( Fig. 4 )
%SR
=
A
SR
A
DT
×
100 (1)
The sample reversibly adsorbed (%SRA) to the membrane as a result of the field was calculated using the mean peak area obtained when the field was removed at the end of each sample recovery run (A
RA
) and the
mean peak area of the dilution tube runs ( Fig. 4 )
%SRA
=
(A
RA
−
A
DT
A
BR
)
×
100 (2)
The mean reversibly adsorbed peak area was corrected for pressure fluctuations by subtracting the mean peak area of the blank runs (A
BR
). There was no need to correct the peak areas of the sample recovery runs since the baseline fluctuations occurred after these peaks were recorded.
The amount of sample irreversibly adsorbed (%SIA) to the membrane was determined via a mass balance.
The UV
2 detector monitoring the crossflow outlet indicated that no sample was passing through the membrane (note: sample passage through the membrane was observed at 280
␮ g/50
␮ l). Thus, the mass balance reduced to
%SIA
=
100
−
%SR
−
%SRA (3)
This %SIA is measured using typical flow FFF channel flowrates and is inclusive of both surface and
“pore” fouling.
2.5. Surface analysis
2.5.1. Scanning electron microscopy (SEM)
Samples cut from the membrane rolls were prepared according to the wetting procedure and air dried. The membrane samples were mounted on stubs using conductive double-sided tape and coated with gold (Hummer VI sputter coater, Anatech, Springfield, VA). The SEM images were obtained using a Model JXA-840 microprobe (JEOL USA Inc.,
Peabody, MA). The acceleration voltage was 15 kV, and the probe current was 3
×
10
−
10
A.
2.5.2. Atomic force microscopy (AFM)
Membrane samples were prepared following the wetting procedure previously described and stored in distilled deionized water. The samples were mounted on AFM stubs using double-sided tape. AFM imaging
(Nanoscope IIIa module, Digital Instruments, Santa
Barbara, CA) was performed in air at room temperature using the contact/retrace mode with a scan rate of 1 Hz and 256 samples/line.
3. Results and discussion
3.1. Optimization of flowrates
Humic acids have large diffusion coefficients due to their small size and hence require high cross flowrates

100 R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 to drive them into equilibrium layers near the membrane wall. Experiments were conducted to determine the flowrates that would ensure sample retention and thus separation of the SRHA according to molecular weights. Several flowrate combinations were tested by varying the channel flowrate while keeping the cross flowrate constant and vice versa. The UV signal was recorded to determine how each flowrate combination affected the SRHA retention time, the amount of
SRHA eluting from the channel, and the system pressure. A channel flowrate of 0.5 ml/min and a cross flowrate of 3.5–5.0 ml/min resulted in a well-retained sample peak with minimal channel leakage and were selected as the optimum flowrates.
3.2. Effect of flowrate on sample recovery
The recoveries of 1
␮ g of SRHA from flow FFF channels equipped with either an NF-200 or an XLE membrane were measured and compared. In this first set of experiments, the carrier solution was 2.27 mM
NaHCO
3
. A channel flowrate of 0.5 ml/min and a cross flowrate of 3.5 ml/min resulted in a 75–87% sample recovery for the XLE membrane, whereas, the sample recovery decreased to 37–61% when the cross flowrate was increased to 5.0 ml/min (
Fig. 5 ). The sample re-
covery was approximately constant over 4 days of membrane usage.
The NF-200 membrane showed similar trends to that of the XLE membrane. The sample recovery was
75–91% at a channel flowrate of 0.5 ml/min and a cross flowrate of 3.5 ml/min and decreased to 51–75% at a cross flowrate of 5.0 ml/min. The sample recovery was also relatively constant over a 4-day period (not shown).
The average distance between the sample and the membrane surface can be controlled by adjusting the cross flowrate. Higher cross flowrates drive sample into slower velocity streamlines near the membrane surface thereby increasing the probability of sample–membrane interactions. An increase in sample retention time (
Table 1 ) and a decrease
in sample recovery (
Table 2 ) with increasing cross
flowrate were observed for both the XLE and NF-200 membranes.
The magnitudes of the changes in the experimental retention times of SRHA with increasing cross
flowrate ( Table 1 ) demonstrated some discrepancies
with predictions from FFF theory for both membranes.
The retention ratio (R) is defined as the time for an unretained sample to elute from the channel, t
0
, divided by the sample retention time, t r
. This ratio can also be written in terms of experimental variables
R = t 0 t r
6
V w 2
0
V
˙
D c
(4)
Fig. 5. Effect of flowrate on SRHA recovery for the XLE membrane. The carrier solution was 2.27 mM NaHCO
3 was 0.5 ml/min for all experiments. The S.D. was from
±
1% to
±
11%.
, and the channel flowrate

R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 101
Table 1
Mean retention times (t r
) (min) of SRHA at two different cross flowrates (
V ˙ c
) (ml/min)
XLE NF-200
3.5 ml/min 5.0 ml/min 5.0 ml/min
(expected)
a
3.5 ml/min 5.0 ml/min 5.0 ml/min
(expected)
a
NaHCO
3
NaHCO
3
NaHCO
3
+
CaSO
4
+
CaSO
4
·
2H
2
·
2H
2
O
O
+
EDTA
6.7
4.0
–
12.9
6.6
10.9
9.6
5.7
–
5.6
6.0
–
6.9
7.0
4.1
8.0
8.6
–
Channel flowrate was 0.5 ml/min.
a
Calculated by multiplying t r at 3.5 ml/min by the cross flowrate ratio of 5.0/3.5. The S.D. varied from
±
0.1 min to
±
0.8 min for the first two solutions listed and from
±
0.1 min to
±
3.2 min for the EDTA containing solution.
where V
0 is the channel void volume, D the sample diffusion coefficient, w the channel thickness, and
V ˙ c is the cross flowrate.
Eq. (4)
demonstrates that the sample retention time is directly proportional to the cross flowrate. Thus, an increase in cross flowrate from
3.5 to 5.0 ml/min should yield a 43% increase in t r
.
The earlier than expected elution of SRHA for the
NF-200 membrane is indicative of a sample zone that, on average, occupies faster velocity streamlines.
This positioning of the SRHA further from the membrane surface than theoretically predicted suggests that the SRHA experiences repulsive interactions with the membrane. The longer retention times observed for the XLE membrane is suggestive of a sample zone that resides closer to the membrane surface than predicted by theory and infers possible attractive interactions. These observed changes in retention time relative to theoretical values can be useful in monitoring sample–membrane interactions. Physical measurements were made of both membranes using calipers to ensure that the observed differences in t r were not due to variations in channel void volume.
3.3. Effect of calcium on sample recovery
Experiments were done to quantitate the effect of calcium ions on the SRHA recovery for the NF-200 and XLE membranes. Sample recoveries in a 1.57 mM
NaHCO
3 and 0.17 mM CaSO
4
·
2H
2
O carrier solution were 8–11 and 49–63% for the XLE and NF-200 membrane, respectively, at a channel flowrate of 0.5 ml/min
and a cross flowrate of 3.5 ml/min ( Fig. 6 ). These were
significant decreases relative to the previously measured sample recoveries in a solution containing only
NaHCO
3
.
The presence of calcium ions in solution has been reported to greatly enhance the fouling of membranes by humic substances
[7,12,13,37–43] . Upon the ad-
dition of calcium, the ionic strength of the solution increases, and the electrical double layers around the humic acids and membrane surface decrease. The resulting decrease in shielding allows the humics to more closely approach each other and the membrane.
The repulsion between the negatively charged groups of the humic material also decreases causing the
Table 2
Comparison of percent sample recovery (%SR), percent sample reversibly adsorbed (%SRA), and percent sample irreversibly adsorbed
(%SIA) of SRHA for each membrane for various cross flowrates and carrier solutions
XLE NF-200
3.5 ml/min 5.0 ml/min 3.5 ml/min 5.0 ml/min
%SR %SRA %SIA %SR %SRA %SIA %SR %SRA %SIA %SR %SRA %SIA
NaHCO
3
NaHCO
3
NaHCO
3
81
+
CaSO
4
+
CaSO
4
·
2H
2
·
2H
2
O 10
O
+
EDTA –
3
1
–
16
89
–
52
4
42
5
4
5
44
92
53
85
56
–
2
1
–
12
43
–
65
31
43
3
2
2
32
68
54
Channel flowrate was 0.5 ml/min; values are the average of 4 days of experiments and will not necessarily add up to 100%. The S.D.
varied from
±
2% to
±
11% for the %SR, from
±
1% to
±
4% for the %SRA, and from
±
2% to
±
10% for the %SIA.

102 R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106
Fig. 6. Effect of calcium ions on SRHA recovery for the XLE and NF-200 membranes. Data presented is for the third day of a 4-day series of experiments. The channel flowrate was 0.5 ml/min for all experiments while the cross flowrate was varied: (A) 2.27 mM NaHCO
3
V ˙ c
=
3
.
5 ml/min; (B) 1.57 mM NaHCO
NaHCO
3
+
0
.
17 mM CaSO
4
·
2H
2
O,
V ˙ c
3
+
=
0
5
.
.
17 mM CaSO
4
·
2H
2
O,
V
0 ml/min. The S.D. was
˙ c
=
±
3
.
5 ml/min; (C) 2.27 mM NaHCO
1% to
±
8%.
3
,
V ˙ c
=
5
.
0 ml/min; (D) 1.57 mM
, humics to become more coiled and compact. This smaller configuration allows the humics to form a more tightly packed, dense adsorption layer on the membrane surface. Calcium ions can also act as bridges to connect negatively charged humic substances to each other and to the negatively charged membrane surface.
As in the NaHCO
3 case, increasing the cross flowrate to 5.0 ml/min decreased the sample recovery (
Fig. 6 ) and increased the sample retention time
(
Table 1 ) for both membranes. The XLE and NF-200
membrane had a sample recovery of 2–8, and 22–34%, respectively. It is apparent that in the presence of calcium ions, the NF-200 membrane is more resistant to fouling by SRHA than the XLE membrane.
The substantial differences in the sample recoveries of the two membranes (
Table 2 ) can be partially
explained by their relative surface roughness
[44,45] .
Atomic force microscopy and scanning electron microscopy images revealed that the XLE membrane had a rougher surface than the NF-200 membrane
(
Figs. 7 and 8 ). Consequently, the XLE membrane has
a larger surface area available for interactions with the humic sample. These qualitative data supported the flow FFF measurements which showed a greater loss of the SRHA sample for the XLE membrane. AFM imaging of the membranes in the carrier solutions would be interesting to perform if our instrument had this capability.
Other factors that may influence SRHA interactions with the membranes are of chemical and electrostatic origin. The XLE and NF-200 membranes have different types of amine groups and degrees of aromaticity (
Fig. 2 ). X-ray photoelectron spectroscopy
(Katz Analytical Services, Chanhassen, MN) showed similar results for the XLE and NF-200 membranes.
In the NaHCO
3 and CaSO
4
·
2H
2
O carrier solution, streaming potential measurements (BI-EKA, Anton
Paar, Austria) yielded isoelectric points of 3.8 and 3.3
for the XLE and NF-200 membranes, respectively.
Hence, both membranes were negatively charged in the pH 7.5 carrier solution. More detailed zeta potential studies may yield additional explanations for the observed differences in membrane performance.
3.4. Effect of EDTA on sample recovery
In order to quantitate the effect of EDTA on membrane fouling, experiments were conducted in a carrier solution containing 1.57 mM NaHCO
3
, 0.17 mM
CaSO
4
·
2H
2
O, and 0.17 mM EDTA. This third carrier solution had a similar ionic strength (2.27 mM) but

R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 103
Fig. 7. AFM (300 nm section) and SEM (2
␮ m section at 10,000
× magnification) of XLE membrane.
Fig. 8. AFM (250 nm section) and SEM (2
␮ m section at 10,000
× magnification) of NF-200 membrane.

104 R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106
Fig. 9. Effect of EDTA on SRHA sample recovery for the XLE and NF-200 membranes. Data presented is for the third day of a 4-day series of experiments. The channel flowrate was 0.5 ml/min and the cross flowrate was 5.0 ml/min for all experiments: (A) 2.27 mM
NaHCO
3
; (B) 1.57 mM NaHCO was from
±
1% to
±
8%.
3
+
0
.
17 mM CaSO
4
·
2H
2
O; (C) 1.57 mM NaHCO
3
+
0
.
17 mM CaSO
4
·
2H
2
O
+
0
.
17 mM EDTA. The S.D.
slightly different pH (
∼
6.5) to that of the two previous carrier solutions (pH
∼
7.5). A channel flowrate of 0.5 ml/min and a cross flowrate of 5.0 ml/min were used since the most dramatic decreases in sample recovery in the presence of calcium ions were observed at these flowrates (
Table 2 ). The sample recoveries in
this EDTA-containing solution were
∼
45% for both the XLE and NF-200 membranes (
Fig. 9 ). Exper-
iments repeated at pH
∼
7.5 and ionic strength of
10.7 mM showed no significant differences in sample recoveries to those at pH
∼
6.5.
The observed experimental results can be explained on the basis of EDTA–calcium complexation. EDTA reduces the amount of free calcium ions in solution and prevents the calcium ions from facilitating interactions between the membrane surface and the humic acids as well as among the humic acids themselves
[39,41] .
A comparison of the effect of calcium ions with and without EDTA on sample recovery demonstrated that the addition of EDTA significantly improved the sample recovery for both the XLE and NF-200 membranes
(
Table 2 ).
Since calcium and EDTA bind in a 1:1 ratio, one would expect to observe sample recoveries similar to those seen in the 2.27 mM NaHCO
3 solution.
However, the sample recoveries for the NaHCO
3
,
CaSO
4
·
2H
2
O, and EDTA solution were less than those for the NaHCO
3
solution ( Table 2 ). A solu-
tion of 1.57 mM NaHCO
3 and 0.17 mM EDTA at a channel flowrate of 0.5 ml/min and a cross flowrate of 5.0 ml/min gave sample recoveries of
∼
40 and
∼
30% for the XLE and NF-200 membranes, respectively (data not shown). The lower sample recoveries observed with carrier solutions containing EDTA implies that EDTA is also involved with interactions between the membrane and the humic acids.
3.5. Reversible and irreversible sample adsorption
Calculations based on the flow FFF results ( showed that a significant amount of SRHA adsorbed irreversibly to the membrane surface. Depending on the carrier solution, the membrane, and the cross flowrate used, the percent sample irreversibly adsorbed (%SIA) ranged from 16 to 92%. An approximately three-fold increase in %SIA was observed for the NaHCO
3
Table 2 )
solution when the cross flowrate was changed from 3.5 to 5.0 ml/min. The same increase in cross flowrate for experiments using the calcium containing carrier solution did not produce a significant change in the %SIA for the XLE membrane.
The wide molecular weight distribution, high surface activity, and diversity in humic acids chemistry make it difficult to elucidate the mechanisms

R.L. Hartmann, S.K.R. Williams / Journal of Membrane Science 209 (2002) 93–106 responsible for the sample loss that was observed.
Flow FFF is not capable of differentiating the specific functional groups that interact with the surface and whether the adsorption is to the membrane surface and/or to the pore. These results do, however, show that flow FFF is a reliable technique that can be used as a tool to predict, quantitate, and compare membrane performance.
References
105 invaluable discussion. The opinions expressed herein are that of the authors and not necessarily that of NSF.
4. Conclusions
Suwannee River humic acids were used to investigate the initial fouling of a clean membrane surface.
This is the first time flow FFF has been used in a novel manner to evaluate membrane surfaces and performance instead of determining molecular weight or particle size. Flow field-flow fractionation was shown to be a reliable analytical technique capable of rapidly quantitating membrane performance. In addition, the results indicated that the majority of the sample was irreversibly adsorbed to the membrane surface. The fouling of an XLE reverse osmosis membrane and an
NF-200 nanofiltration membrane was enhanced by increasing the flowrate perpendicular to the membrane surface and by adding calcium ions to the solution.
The NF-200 membrane was more resistant than the
XLE membrane to fouling in the presence of calcium, whereas, the fouling resistance of both membranes improved to similar levels with the addition of EDTA to a solution containing calcium ions.
Acknowledgements
This project was funded by the NSF I/U CRC for
Membrane Applied Science and Technology. Partial support for this work was provided by the National
Science Foundation’s Computer Science, Engineering, and Mathematics Scholarships Program under grant
DUE-9987037. W. Mickols of FilmTec Corporation
(Edina, MN) supplied the membranes and served as the industrial mentor. A. Latal of Anton Paar (Austria) made the streaming potential measurements. From the
Colorado School of Mines, the Suwannee River humic acids samples were a gift from Dr. Patrick MacCarthy.
R. McGrew and J. Oakey performed SEM and AFM imaging, and G. Kassalainen and H. Lee provided
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