Journal of Membrane Science 164 (2000) 89–110
Membrane filtration of natural organic matter: factors and mechanisms
affecting rejection and flux decline with charged ultrafiltration (UF)
membrane
Jaeweon Cho a,b , Gary Amy b , John Pellegrino c,∗
a
c
KJIST, Department of Environmental Science and Engineering, 1 Oryong-dong, Puk-gu, Kwangju 500-712, South Korea
b Civil, Architectural, and Environmental Engineering, University of Colorado at Boulder, Boulder, CO 80309, USA
National Institute of Standards and Technology, Physical and Chemical Properties Division, MS 838.01, Boulder, CO 80303, USA
Received 14 September 1998; received in revised form 14 May 1999; accepted 18 May 1999
Abstract
We studied natural organic matter (NOM) rejection and the membrane’s flux decline during natural water filtration using
a charged ultrafiltration membrane based on thin-film-composite technology. NOM rejection mechanisms such as steric
exclusion and aromatic/hydrophobic and charge interactions were considered. Water composition factors affecting NOM
rejection and flux decline were investigated, including ionic strength, pH, and calcium ion concentration. The membrane’s
effective relative molecular mass cutoff for the NOM in our study was between 1500 and 2300 (significantly lower than
the manufacturer’s nominal value of 8000) and depended on the NOM characteristics in the source water. In particular the
ratio of UV absorbance at 254 nm to dissolved organic carbon (related to the humic content) correlated with the rejection.
Comparison of relative molecular mass distributions between fractionated NOM and recovered membrane foulants indicates
that the foulants are the larger-sized neutral and/or basic NOM components, and not the humic substances that were efficiently
rejected by this membrane. ©2000 Elsevier Science B.V. All rights reserved.
Keywords: Flux decline; Fouling; Natural organic matter; NOM; Ultrafiltration: water treatment
1. Introduction
This paper focuses on the filtration of natural organic matter (NOM) from natural waters
(and NOM isolates from those waters) using a
thin-film-composite (TFC) ultrafiltration (UF) membrane. The natural waters were prefiltered using a microfilter (with nominal 0.45 ␮m pore size) to minimize
contributions to flux decline due to biofilm formation.
∗ Corresponding author. Tel.: +1-303-497-3416;
fax: +1-303-497-5259.
E-mail address: [email protected] (J. Pellegrino).
The emphasis is on describing the electrostatic and
molecular-size interactions between NOM and the
membrane that provide much of the basis for solute
rejection and adsorptive flux decline. The measurements and analytical protocols we are studying may
also be useful for the further development of predictive correlations for the filtration figures-of-merit. In
addition, the NOM size-exclusion characteristics and
transport properties for this particular membrane are
more completely measured in the context of drinking
water treatment.
Flux decline and NOM rejection are two important
issues in membrane filtration. Both are influenced by
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 1 7 6 - 3
90
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
water composition factors (bulk NOM concentration,
humic/non-humic NOM fraction, molecular mass distribution, pH, Ca++ concentration and ionic strength);
membrane physical and chemical properties (pore size,
water permeability, charge, hydrophobicity/philicity);
and filtration process conditions (the NOM concentration at the membrane interface, controlled by flux rate
and mass transfer in the fluid boundary layer). NOM
adsorption and gel-layer formation not only affects
flux decline but will also affect the observed rejection
characteristics.
A rigorous characterization of NOM in raw
and membrane-treated waters can provide guidance as to the applicability of various membrane
processes. NOM characterization includes several
analytical methods such as NOM fractionation by
humic/nonhumic character, molecular size analysis by high-pressure size exclusion chromatography
(HPSEC), aromaticity by UV absorbance (an index
of the NOM structure), and acidic functional group
content by potentiometric titration of NOM isolates.
Membrane characterization includes contact angle as
an index of relative hydrophobicity, zeta potential for
indirect measurement of surface charge, and nominal relative molecular mass cutoff (MWCO) using
polyethylene glycols (PEGs).
2. Background
NOM properties, including structure (aromatic versus aliphatic, or hydrophobic versus hydrophilic), size
(average relative molecular mass (RMM), and RMM
distribution), and charge density, are important factors
in the formation of disinfection by-products (DBPs)
[1–3] and can also be influential factors in membrane
processes due to hydrophobic and charge interactions
[4–6].
UV absorbance at 254 nm (UVA254 ) has been used
to monitor not only NOM concentration, but also the
humic content or aromaticity of NOM based on specific UVA254 (SUVA = UVA254 /dissolved organic carbon (DOC)) [2,7]. NOM fractionation by XAD-8 (a
nonionic adsorbent from acrylic ester polymer with
nominal pore size 23.5 nm) and XAD-4 (a nonionic
adsorbent from polystyrene with nominal pore size
5 nm) resins has been used to obtain hydrophobic DOC
(XAD-8 isolate), transphilic DOC (XAD-4 isolate),
and hydrophilic DOC (effluent from XAD-8 followed
by XAD-4) [8–10]. For example, XAD-8 was used to
obtain hydrophobic NOM and hydrophilic NOM water (the effluent of XAD-8) for hollow-fiber membrane
filtration by Nilson and DiGiano [6]. These groups
of organic matter have different characteristics of hydrophobicity, size, shape, and charge density.
The molecular size of NOM must always be an important factor influencing its rejection by membrane
filtration. The relative molecular mass or size of NOM
has been estimated by both UF membrane gel filtration and HPSEC methods. Ultrafiltration was performed with several different regenerated cellulose
membranes (coded as YM series) and a stirred-cell
dead-end unit to obtain RMM fractions between two
nominal MWCOs [5,6,11]. In this case, using UF for
RMM fractionation is not significantly influenced by
pH due to the uncharged nature of YM membranes
[11]. However, when NOM concentration is relatively
high, the concentration polarization near the membrane surface is high and may influence the rejection
(as measured by either DOC or UVA). When concentration polarization exists, a permeation coefficient
model was suggested to calculate the percent rejection
[12].
Chin et al. [13] used the HPSEC technique
to analyze RMM distribution and to calculate
weight-averaged RMM (Mw ), number-average RMM
(Mn ), and polydispersivity (Mw /Mn ) (an index of
NOM homogeneity). RMMs determined by HPSEC
were smaller than those measured by the UF method,
according to Chin et al. [13] and a comparison of
results for fulvic acids from Jucker and Clark [5].
Charge density may also be an influential factor
on NOM rejection and flux decline during filtration,
because membranes are generally negatively charged
[14,15]. The NOM charge density of bulk natural waters is not easy to measure by titration with base because of interference from inorganic solutes. Instead,
just the hydrophobic acids (XAD-8 isolate) have been
used in potentiometric titrations (pH between 3 and 8)
to measure carboxylic acidity [16]. For the phenolic
group content of NOM, twice the amount of NaOH required to titrate from pH 8–10 was used as an estimate
[7]. It is desirable to measure the charge density of
transphilic (XAD-4 isolate) and hydrophobic (XAD-8
isolate) acids in order to more completely elucidate
electrostatic effects on rejection.
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Membrane properties, including hydrophobicity,
pore size and surface charge, are also necessary for
evaluation of NOM rejection, and associated fouling,
during membrane filtration. Membrane hydrophobicity can be inferred by the material itself and by contact
angle measurements. Contact angle is an index of hydrophobicity of the membrane surface, which can be
measured by captive bubble and sessile drop methods.
Zhang and co-workers [17,18], and Gekas et al. [19]
used cellulose acetate (CA) and polysulfone (PSf)
membranes to compare contact angles of relatively
hydrophilic and hydrophobic membranes. There was
no significant difference in contact angles between CA
and PSf membranes in the measurements of Zhang
and Hallstrom [18] and Gekas et al. [19]; however, CA
exhibited higher contact angles than PSf in the results
of Zhang et al. [17]. PSf membranes from different
manufacturers showed very different contact angles
[17,18]. Polyethersulfone (PES) membranes (coded
as PM series) and regenerated cellulose membranes
(the YM series) exhibited a large difference in contact angles based on the measurements of Jucker and
Clark [5]. The contact angle of membrane surfaces is
not easy to accurately measure because of the effects
of surface texture, porosity and water wicking, but
it is still regarded as a convenient semi-quantitative
index of membrane hydrophobicity/hydrophilicity.
Comparisons of contact angle measurements between
material surfaces are more meaningful if the entire
protocol (including elapsed times) is kept consistent
for each sample.
Membrane MWCO is the basic tool for predicting
NOM rejection and other solute separations by membrane filtration because size exclusion is such an important mechanism. Manufacturers provide membrane
MWCO information from the results of solute rejection tests using macromolecules such as proteins, dextrans and PEGs — often uncharged. As an alternative,
recently, Ohya et al. [20] determined the MWCO of
an asymmetric aromatic polyimide membrane using
toluene solutions of aliphatic hydrocarbons with different RMMs.
Direct measurement of the membrane surface
charge is very difficult. Instead, the zeta potential
(which is the potential at the shear plane in the diffuse
layer) is used. It can be estimated using the streaming
potential method. Causserand et al. [21] and Jucker
and Clark [5] measured the zeta potentials of UF
91
membrane pores, while Elimelech and co-workers
[14,15] measured the zeta potentials of the membrane
surface (active layer). The Helmholtz–Smoluchowski
equation was used to calculate the zeta potential from
the streaming potential by all of these researchers. The
zeta potentials of clean membranes were measured
at different pH’s and ionic strengths, and membranes
with either a protein [21] or humic acids [5] adsorbed
were also measured. Also a Suwannee River humic
acid solution itself was used as a background electrolyte for the zeta potential measurement of Childress
and Elimelech [15].
The zeta potentials of membranes adsorbed with a
protein or a humic acid were less than those of clean
membranes before adsorption [5,21]. Zeta potentials
became more negative as pH increased for PES and
sulfonated PSf membranes (sulfonated PSf had the
higher negative charge) [21], and for PA TFC membranes [15]. The influence of ionic strength on PEG
rejection by a sulfonated PSf membrane was shown
to provide higher PEG rejection with higher ionic
strength, thus indicating that the pore radii of the membranes are decreased by higher ionic strength [22].
This is an indirect evidence of the surface electrostatic
forces of the membrane material.
NOM removal by nanofiltration (NF) and UF membranes is generally based on polyamide TFC, polysulfone, polyethersulfone, and sulfonated-polyethersulfone polymers. Beyond the membrane’s ability to
reject the target solutes, flux decline and fouling of
the membranes are important factors for economic
reasons. Ideally, the membrane surface should have a
negative charge to repel NOM, which also has a negative charge, thus inhibiting its adsorption and helping
to maintain a high flux. The charge repulsion between
NOM and the membrane surface, and NOM adsorption, will also be influenced by water-quality factors
such as pH and ionic strength.
Flux declines were measured for PSf and sulfonated
PSf hollow-fiber membrane filtration of Suwannee
River organic matter at different pH’s by Braghetta
et al. [22], while Nilson and DiGiano [6] filtered a bulk
NOM solution, a hydrophilic NOM solution (XAD-8
effluent), and a hydrophobic NOM solution (XAD-8
isolate). They observed that flux declines were relatively significant at pH 4 and 7, but were negligible
at pH 10, probably due to the fact that the acidic
components of NOM have a higher negative charge
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J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
density at higher pH (from carboxylic and phenolic
functional groups) thus inhibiting NOM adsorption
and the resulting flux decline.
Hydrophobic interactions are influential factors on
flux decline, as indicated by results that a hydrophobic
NOM solution (XAD-8 isolate) exhibited more flux
decline than a hydrophilic NOM solution (effluent of
XAD-8) [6]. A hydrophobic PSf membrane with a hydrophobic nonionic surfactant was also used to demonstrate more flux decline than a relatively hydrophilic
membrane with the same surfactant [23]. This result
substantiates the significance of hydrophobic interactions in the foulant-adsorption process. Further results along this vein were obtained with a stirred-cell,
dead-end filtration apparatus used to measure flux declines with PES and RC UF membranes [4]. The PES
membrane (PM series) exhibited more flux decline
than the RC membrane (YM series). According to
these studies, it seems likely that PSf and sulfonated
PSf membranes can be fouled more easily by hydrophobic components of NOM due to the hydrophobicity of the membrane surface.
Several NF membranes based on PA TFC (NF90
and TFCS) and sulfonated-PES (NTR7450) were used
to measure flux declines with an NOM-containing natural (Ohio River) water [24]. The PA TFC membranes
exhibited very little or almost no flux decline for 4–13
days even with a recycle line (which increased the
bulk solute concentrations), while the sulfonated PES
membrane showed greater flux decline over 4–7 days
and required frequent membrane cleaning to maintain
the specified flux.
This review of prior work provides the basis for the
measurements we have applied. The results in this paper will focus on filtration with a relatively ‘tight’ UF
TFC that is negatively charged. The results provide
both an improved qualitative understanding of the interaction of the heterogeneous NOM mixture and a
negatively charged membrane, and further experimental measurements with which to advance our predictive capability.
ments. According to the manufacturer, the surface of
this membrane is made of a combination of aromatic
and aliphatic multifunctional monomers, and has ionizable functional groups such as carboxylic acids.
A regenerated cellulose membrane (coded as YM3)
which is highly hydrophilic but does not have ionizable functional groups was also used only for NOM
rejection experiments. Thus, these filtration measurements provided a comparison of the solute rejection
between charged and uncharged membranes.
The surface zeta potentials for both membranes
were estimated from streaming potential measurements using a KCl electrolyte solution (30 mS/m) and
a commercial electrokinetic analyzer (EKA) measurement apparatus.
The contact angle was measured between a water
droplet, the membrane surface, and air, using a goniometer (sessile drop method). The membrane is first
rinsed by floating it, skin side down, in a container
of DI water (a deionized water prepared by filtration
with two proprietary cation-exchange mixed beds, an
anion-exchange bed, and a 0.2 ␮m filter) for 1 day,
and changing the water three times. This procedure
extracts water-soluble coating materials (for example,
glycerine). The rinsed membranes are dried in a closed
desiccator for a day and stored in a closed petri dish
before measurements. Membrane samples are cut into
small pieces and mounted on a support. An approximately 2.0 ␮l droplet of DI water is placed on the
membrane specimen and the contact angle is measured with the goniometer immediately after the drop
placement.
Table 1 presents a listing of the characteristics of the
membranes, all of which were provided by the manufacturers except for the contact angles, zeta potentials, and average clean-water permeance. The YM3
and GM membranes may be considered relatively hydrophilic and hydrophobic, respectively, based on our
contact angle measurements.
3.2. Source waters
3. Methods and analyses
3.1. Membrane
A membrane of the TFC (coded as GM) type was
used for the NOM filtration and flux decline measure-
Two drinking water sources, Silver Lake surface
water (SL-SW) and Orange County groundwater
(OC-GW), representing relatively hydrophilic versus hydrophobic sources of NOM, respectively, were
used to perform bench-scale membrane-filtration
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
93
Table 1
Summary of membranes tested
Code
Material
Contact
angle
Manufacturer’s
nominal MWCO
Zeta potential
at pH 7 (mV)
Clean water permeance
(l per day/m2 /kPa)
YM3
GM
Regenerated Cellulose
Polyamide TFC
13.3◦
54.7◦
3000
8000
−8.6
−17.0
4.51
8.00
Table 2
Characteristics of source watersa
Source
DOC
(mg/l)
UVA
(/cm)
SUVA
(/cm/mg l)
Baseflow
SL-SW
Runoff
SL-SW
OC-GW
2.00
(±0.10)
3.88
(±0.19)
9.80
(±0.49)
47.80
(±2.39)
0.048
0.024
0.172
Twitchell
Conductivity
(␮S/cm)
pH
Alkalinity
(ppm CaCO3 )
Humic content
(% DOC)
21.4
6.2
0.045
29.9
6.4
11.7
(±0.1)
–
0.480
0.049
477.0
8.8
1.770
0.037
1066.0
7.1
43.3
(±3.1)
56.9
(±3.4)
80.0
(±5.7)
60.6
(±3.9)
222.0
(±18.0)
79.0
(±6.4)
Ca
(mg/l)
8.1
–
4.7
34.3
a
Note: In general, three replicate measurements were made and the coefficient of variation was less than 5%. Insignificant variations
were observed in measurements of UVA, conductivity, pH, and Ca++ . The numbers in parenthesis are one standard deviation.
tests. SL-SW samples were collected during baseflow (normal) and runoff (spring snowmelt) periods
to include seasonal variations in NOM. Unless otherwise indicated, all membrane filtration tests were
performed on 0.45 ␮m pre-filtered water, corresponding to the definition of dissolved organic carbon
(DOC). A third water source was also used: Twitchell
water, representing an agricultural drainage feeding
into the California State Water Project. Each source
water was analyzed for DOC, UV absorbance at
254 nm (UVA254 ), specific UV absorbance at 254 nm
(SUVA = UVA254 /DOC), conductivity, pH, alkalinity,
Ca++ concentration, and humic (XAD-8 adsorbable)
content of the NOM. The humic fractions of NOM
source waters were determined by performing a DOC
mass balance across an XAD-8 resin column, with the
column effluent representing the non-humic fraction.
These results are tabulated in Table 2. A typical natural water contains 40–60% humic matter as defined
by isolation/adsorption onto XAD-8 resin. A water
with a 40/60 split between humic and non-humic
would be considered a ‘more non-humic water’ while
a 60/40 split would infer the opposite. Our waters
ranged from about 40% humic (fairly representative)
to 80% (an ‘outlier’).
All the source waters used in these filtration measurements were stored under refrigeration prior to their
use. The bulk water analysis was always repeated at
the same time as the filtration measurements. We observed no significant change in composition over the
period of this study and since the feed waters were
prefiltered by nominal 0.45 ␮m microfilters, biological organisms were not considered to be a significant
part of the membrane challenge.
HPSEC was used to determine the RMM distribution of NOM. We used a modified silica column (separation range of 138–35 000 mass units) and a commercial UV spectrophotometric detector. Eluent for
the HPSEC was composed of DI water buffered with
phosphate (pH 6.8) and NaCl to provide ionic strength
of 0.1 M [13]. Standard solutions for the RMM calibration curve were made with sodium polystyrene
sulfonates (PSS) (1800, 4600, 8000, and 35000 mass
units), and salicylic acid (138 mass units) was used to
confirm the lower range of the calibration curve. The
RMM distributions of baseflow and runoff SL-SW,
OC-GW, Twitchell water, and several XAD-isolate solutions were determined. These RMM distributions
and average RMMs are shown in Figs. 1 and 2and
Table 3, respectively.
94
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 1. Relative molecular mass distribution (by SEC) of the NOM contained in the natural water sources: (䊐) baseflow Silver Lake surface
water (SL-SW), (䊏) runoff Silver Lake surface water (RSL-SW ), (䊊) Orange County ground water (OC-GW), and (∇) Twitchell drainage.
Fig. 2. Relative molecular mass distribution (by SEC) of the NOM fractions contained in the (䊏) runoff Silver Lake surface water: (heavy
solid line) hydrophobic DOC, (thin solid line) hydrophilic DOC, and (dashed line) transphilic DOC.
XAD-8 and XAD-4 resins were used to isolate
the hydrophobic DOC (primarily polycyclic aromatic acids, the XAD-8 isolate), transphilic DOC
(primarily the relatively hydrophilic aliphatic acids,
the XAD-4 isolate), and hydrophilic DOC (the ef-
fluent from XAD-4 column whose feed had passed
through an XAD-8 column) from the runoff SL-SW.
The RMM distributions of the hydrophobic DOC,
transphilic DOC, and hydrophilic DOC derived from
the runoff SL-SW are presented in Fig. 2. We also ar-
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
95
Table 3
Relative molecular mass determinationsa
Source
Mn (number-averaged RMM)
Mw (weight-averaged RMM)
polydispersivity (Mw /Mn )
Runoff SL-SW
1270
(±15)
1330
785
804
984
965
1160
(±96)
1350
(±58)
1490
(±95)
1750
953
941
1200
1120
1590
(±41)
1700
(±45)
1.17
Hydrophobic DOC of runoff SL-SW
Transphilic DOC of runoff SL-SW
Hydrophilic DOC of runoff SL-SW
Baseflow SL-SW
Hydrophilic DOC of baseflow SL-SW
OC-GW
Twitchell water
1.31
1.21
1.17
1.22
1.16
1.37
1.26
a Note: three replicate measurements for the RMM distribution were used to calculate the M and M of the runoff SL-SW, OC-GW,
n
w
and Twitchell feed waters. The calculated standard deviation for those results are in the parenthesis.
tificially increased the DOC concentration of baseflow
SL-SW from 2.0 to 9.2 mg/l by using the retentate
from a nanofiltration step with a membrane having
MWCO of 300 (NF70). From this ‘DOC-augmented’
baseflow SL-SW we produced an additional water sample: the effluent from an XAD-8 column
providing a hydrophilic, nonhumic NOM solution
(DOC = 5.2 mg/l).
3.3. Continuous crossflow, flat sheet membrane unit
A commercial bench scale crossflow membrane cell
was used to evaluate the filtration properties of the flat
sheet specimens. The system is comprised of the membrane unit, the feed, permeate and retentate streams.
The retentate line is also divided into recycle and
waste lines (see Fig. 3). The system accommodates
60 cm2 flat sheet specimens under tangential feed flow
with a channel height of 0.04 cm. Feed flowrates are
variable between 100 and 450 ml/min. The crossflow
velocity can be adjusted by varying the bulk feed
flow, and this crossflow velocity was kept approximately constant for all measurements in this report at
∼8.6 cm/s by setting up a constant feed flowrate of
200 ml/min. The temperature was maintained at 298 K
and the transmembrane pressure was kept constant at
approximately 345 kPa (50 psi). At these conditions
the Reynolds number is laminar, nominally 36. The
system recovery R, which is defined as ratio of permeate to fresh feed mass flow, can be controlled by
changing the amount of recycle from the retenate back
to the feed line (see Fig. 3). Higher system recovery
ratios (for a given fresh feed concentration) will result in a higher NOM concentration at the membrane
surface.
For each filtration test, a new membrane specimen
was soaked in DI water for 1 day to rinse away water soluble surface coatings. Clean water was filtered
through the membrane until approximately constant
flux was obtained, then the NOM solution (equilibrated to room temperature) was filtered. The flow
rate, UVA, and DOC of the permeate were measured
over time. The SUVA of the permeate was compared
with that of the feed sample to provide a measure of
the preferential rejection of the aromatic (hydrophobic) fraction of the NOM.
System recovery R, as defined above, is the fraction
of fresh feed flow that is recovered as permeate. In this
case, retentate recycle is used to maintain the crossflow
velocity at a consistent level. The purpose of this mode
of operation is to obtain higher bulk concentrations of
NOM. When retentate recycle was used to increase
the system recovery, bulk rejection Rj (bulk) and feed
rejection Rj (feed) was defined by
R (bulk) =
R(feed) =
Cb − Cp
Cb
CR (1 − R)
.
Cf
(1)
(2)
Cb is the bulk NOM concentration in the module, Cp
is the NOM concentration in the permeate, CR is the
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J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 3. Schematic of crossflow membrane filtration apparatus.
NOM concentration of the retentate, and Cf is NOM
concentration of fresh feed.
3.5. Flux decline and adsorption tests using
crossflow filtration unit
3.4. Polyethylene glycol MWCO determinations of
the membrane
Extending on the approaches of many prior investigations that used three-parameter models [25–28],
a five-parameter membrane resistance-in-series model
was used to quantify the influences on flux decline:
A range of PEGs (RMM = 200–10 000) were used
to determine the membrane’s MWCO under the same
crossflow filtration operating conditions as used in the
NOM filtration measurements. A 20 mg/l concentration as DOC was used for each of the three PEG rejection measurements: in DI water; in DI water containing 10 mM NaCl; and in DI water containing 4 mM
Ca++ . NaCl and Ca++ were added to determine the
effects of increased ionic strength and calcium binding
on the membrane’s apparent MWCO. The purpose of
these measurements were not to ‘define’ the MWCO
of the membrane but to provide a standard benchmark
with a neutral, linear macromolecule with which to
compare NOM rejection.
Jv =
1P
µ(rm + rc + rg + ra1 + ra2 )
(3)
where Jv is flux through the membrane (cm/s), 1P is
transmembrane pressure (Pa), µ is dynamic viscosity
(Pa s or g s cm−1 ), rm is membrane hydraulic resistance, rc is concentration polarization resistance, rg is
gel layer resistance, ra1 is weak adsorption resistance,
ra2 is strong adsorption resistance (all resistances are
in cm−1 ). For our study, the concentration polarization, gel layer, and weak adsorption resistances may
be considered to be reversible by clean water and
NaOH extraction, while strong adsorption resistance
is not. Note that in this model the osmotic pressure
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
97
Fig. 4. Schematic representation of the filtration protocol used to determine resistances in series.
term is subsumed into the concentration polarization
resistance.
DI water and NOM water were crossflow filtered
using the following protocol (illustrated schematically
in Fig. 4) to obtain all resistances. Clean water was
first filtered through the membrane until a constant flux
was obtained (Step 1); then NOM-containing water
was introduced and the permeate rate was monitored
over time (Step 2). After the permeate rate reached a
constant value (that is, the permeate rate of the fouled
membrane), DI water replaced the NOM-containing
water, and the applied pressure was released to remove
concentration polarization (Step 3). The fouled membrane was then rinsed (the flow rate was increased
from 8.6 to 19.3 cm/s) with DI water so that the gel
layer (highly concentrated NOM layer) was removed
from the membrane surface and DI water filtration
was again performed (Step 4). The membrane was
then soaked in a 0.1 M NaOH solution for 1 day so
that weakly adsorbed NOM on the membrane surface
could be desorbed, then DI water was again filtered
through it (Step 5).
Using the flux values from steps 1–5, we could calculate rm , rc , rg , ra1 , and ra2 . The difference between
concentration polarization and gel layer resistances is
that the former is a thermodynamic modification of the
pressure driving force, and the latter is the viscous resistance for flow through highly concentrated (precipitated or gelled) solutes. The weak adsorption can be
defined as the NOM adsorption that can be removed by
chemical cleaning with 0.1 M NaOH. Strong adsorp-
tion is attributed to the NOM that cannot be desorbed
even with this chemical cleaning. The RMM distribution was determined (using HPSEC) for the NOM
desorbed by 0.1 M NaOH extraction of a fouled membrane. The foulants were compared with the RMM distributions of hydrophobic, transphilic and hydrophilic
DOC fractions from the runoff SL-SW feed water.
3.6. General discussion of measurement uncertainties
Based on systematic uncertainties from resolution
of the mass balance, volumetric standards, and timing
devices, we estimate the uncertainties in the permeate flux measurements to be 0.2–1% of the reported
values. The lower uncertainties are for the initial periods when the permeation rates are higher. Carrying
this uncertainty into the calculation of the tabulated
resistances-in-series, we estimate an uncertainty between 1% and 10% of the reported value, again depending on its magnitude. The expanded uncertainties
(coverage factor of 2) due to random and systematic
effects (based on 3 replicate analyses) in the reported
NOM rejections are ±2–5% when based on DOC, and
±0.5–2% when based on UVA254 . The expanded coverage on the specific RMM values are estimated to be
±60 mass units based on the variance of peak times
observed for replicate measurements with the RMM
standards. 2–3 replicate measurements for the peak retention times on the mass standards were made such
that standard deviation was within 0.01 min (the coefficient of variation was 0.1–0.2% over the range of
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J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 5. Mass % rejection of PEG by YM3 (solid line) and GM (dashed line) versus the PEG relative molecular mass under crossflow
filtration and different solution conditions.
retention times). The same strategy was applied for
the determination of RMM distribution of the water
samples.
4. Results and discussion
4.1. PEG MWCO of membrane
The rejection of PEG by the GM and YM3 membranes are presented against the average RMMs of
the PEG fractions in Fig. 5. The MWCO of a membrane is typically defined as the RMM that is 90% rejected when the macromolecules used are non-charged
model compounds [22,29]. As shown in Fig. 5, the
GM membrane apparently has a MWCO value greater
than 8000 for neutral linear macromolecules. As the
RMM of PEG increases, its rejection by the GM increases almost linearly. Thus, the GM membrane exhibits a diffuse type of cut-off which implies a wide
pore size distribution. With added NaCl or Ca++ in
the DI water, PEG rejections (at a given RMM) increased. The YM3 membrane’s nominal MWCO exhibited no effect from the added ions and matched its
manufacturer’s specification.
We attribute our results to the supposition that the
pore size of the GM membrane is affected by the
charge interactions between its functional groups, such
as the electrostatic repulsion between nearby carboxylates. Therefore, an increase in ionic strength and/or
ion binding (such as with Ca++ ) would decrease the
double layer and allow functional groups to approach
closer to each other. Whether such a phenomenon
would increase or decrease the apparent mean of the
membrane’s pore size distribution depends on the specific morphology present. In the case of the GM membrane, it appears that the effective pore size distribution has a lower mean value when charge screening
occurs. Our results and interpretations are consistent
with the general observations of Braghetta et al. [22]
for their measurements using sulfonated polysulfone
nanofiltration membranes to filter solutions of varying
pH and ionic strength.
4.2. NOM rejection mechanisms: size exclusion,
hydrophobic interactions, and charge interactions
When other factors are excluded, membrane filtration is a physical barrier process, rejecting macro-
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
99
Fig. 6. Relative molecular mass distribution (by SEC) of the NOM contained in the (䊏) runoff Silver Lake surface water (RSL-SW ) feed,
(thin solid line) permeate at 5% recovery, (medium dashed line) retentate at 5% recovery, and (heavy dashed line) retentate at 95% recovery.
Table 4
Size exclusion by GM membrane based on weight-average RMM
Baseflow SL-SW OC-GW Runoff SL-SW Twitchell
Mw
Feed
1200
Permeate 827
NOM rejection (%)
DOC
37.6
UVA
63.2
1590
925
84.2
94.0
1490
958
60.2
84.8
1700
1210
59.9
65.2
molecules larger than the membrane pore size. As UF
membranes may have a wide pore size distribution
(diffuse type), and NOM comprises a range of macromolecule structures and sizes, it is not straightforward
to illustrate (or predict) macromolecule rejection by
size exclusion effects. The Mw of the different feed
waters and GM permeates are compared in Table 4.
The Mw of the permeates are 20–24% less than those
of feed waters for the various NOM sources.
The RMM distributions of the GM permeate (recovery ratio = 5%) and GM retentate (recovery ratios = 5%
and 95%), collected at steady state flux conditions, are
compared to the RMM distribution of the feed (runoff
SL-SW) in Fig. 6. (Note: The retentate from a significantly more concentrated stream was required in
order to discern any significant shift in the RMM distribution between fresh feed and retentate). A shift to
smaller and larger RMMs, for the permeate and reject stream occurred, respectively. The vertical axis of
Fig. 6 represents a normalized fraction percentage obtained by dividing each incremental height of the chromatogram with a sum of the heights when the chromatogram is divided into incremental mass intervals.
As shown in Fig. 6, the 200–300 RMM fraction for the
GM permeate is greater than that of the feed; meanwhile, the RMM fraction larger than 2000 is almost
completely rejected. The RMM distribution shape of
the GM retentate stream is generally similar to that
of feed (runoff SL-SW), however, the distribution is
shifted to a larger RMM by about 1000.
Several source waters with different RMMs, SUVAs and humic contents (% DOC), were used for
NOM filtration through the GM membrane. In general, when the source waters have a larger Mw , NOM
based on DOC and UVA is more easily rejected by the
GM membrane than source waters having a smaller
Mw (see Fig. 7). But if we were to consider only av-
100
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 7. NOM% rejection based on (clear bars) DOC and (shaded bars) UVA during GM membrane filtration of the various feed waters.
Crosshatched band between 10% and 20% represents expected DOC rejection based on size exclusion from the PEG measurements.
Tabulated below each feed water are several NOM metrics.
erage molecular mass, then based on the PEG measurements all rejections should have been in the range
of 10–20%. There is a generally monotonic relationship between NOM rejection and increasing SUVA
content of the feed stream. In fact, even though the
OC-GW NOM has a smaller Mw than Twitchell water, since its SUVA is higher, so is the rejection for
both DOC and UVA. On the other hand, hydrophilic
NOM (effluent of XAD-8 derived from the baseflow
SL-SW) with both very low RMM and hydrophobicity (lowest SUVA), has virtually no measurable rejection of either DOC or UVA. Indeed, there is also a
positive correlation between NOM rejection and humic content (based upon DOC). These results suggest
that NOM aromaticity/hydrophobicity may become a
quantitative predictor of NOM rejection.
It appears that all the humic fraction of the NOM
can be rejected by the GM membrane. This assertion is
illustrated by the GM retentate stream from the runoff
SL-SW having almost the same RMM distribution as
the hydrophobic DOC (XAD-8 isolate derived from
the runoff SL-SW) (see Fig. 8). In fact, for all the
waters (except the hydrophilic NOM solution) the hydrophobic fraction of NOM is preferentially excluded
from the membrane, as indicated by the fact that the
UVA rejection is higher than the DOC rejection.
NOM rejections by the YM3 membrane are compared with those by the GM membrane (see Fig. 9)
to evaluate charge interactions using several natural
waters. The YM3 is presumably non-charged (actually less negatively charged based on zeta potential
measurements) than the GM membrane (Table 1) and
has a MWCO of 3000 (manufacturer’s rating and our
measurements, Fig. 5). Based on a comparison of their
nominal MWCOs, we anticipated that the YM3 membrane would exhibit higher NOM rejection than the
GM membrane. However, the opposite was true. The
GM membrane had higher NOM rejections than the
YM3 membrane except for the case of the hydrophilic
NOM-containing water. The higher NOM rejections
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
101
Fig. 8. Relative molecular mass distribution (by SEC) of the NOM contained in the (䊐) hydrophobic fraction from runoff Silver Lake
surface water and (heavy dashed line) retentate at 95% recovery from the same feed water.
Fig. 9. DOC% rejection during membrane filtration of the various feed waters using (clear bars) YM3 and (shaded bars) GM membranes
filtration. Crosshatched band between 10% and 20% and 30% and 40% represents expected DOC rejection for GM and YM3, respectively,
based on size exclusion from the PEG measurements and the average relative molecular mass of the DOC in the feed water.
102
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 10. pH versus acidity (milliequivalents per gram of C from DOC measurements) of the NOM isolates from the (䊉) XAD-8 and (䊊)
XAD-4 fractionation of runoff Silver Lake surface water.
by the GM membrane are believed to be due to charge
repulsions between the negative surface charge of the
GM membrane and the negative charge density of
NOM acidic fractions.
The charge density of NOM fractions from runoff
SL-SW were measured by potentiometric titrations
(see Fig. 10). The charge density (per mass of DOC)
of XAD-4 isolate (hydrophilic acids with aliphatic
structure) is higher than that of the XAD-8 isolate
(hydrophobic acids with aromatic structure), but both
have significant acidity. The number of charged groups
in the two types of isolates on an average molar basis would seem to be relatively equivalent when the
average RMM is considered (see Table 3).
The average solute molecular size (based on number
average) of the OC-GW < runoff SL-SW < Twitchell
(see Table 3) but they are all between 1150 and
1350. The YM3’s NOM rejection follows the order,
OC-GW < Twitchell < runoff SL-SW, but the variation in measured rejection between the waters is
small (between 47% and 55% based on DOC). Therefore, one can judge that the primary mechanism of
NOM rejection for the YM3 membrane is a steric
hindrance.
On the other hand, the GM’s NOM rejection follows OC-GW > Twitchell ≈ runoff SL-SW, which is
the same order as the humic fraction. But it is instructive to note that it is only with the OC-GW that the
NOM rejection of the GM significantly exceeds that
of the YM3. The combination of the higher humic
content and negative charge density of the OC-GW resulted in a solute that was significantly more excluded
from the GM membrane than the YM3. This leads
us to speculate that there is a strong synergy between
the aromaticity (and therefore, shape instead of simply
RMM) and charge density of the solutes that controls
the partitioning of NOM constituents into membrane
pores. This synergy may find its strongest expression
when the membrane material has significant intrinsic
charge density.
4.3. Solution conditions affecting NOM rejection
The effects on NOM rejection by the GM membrane from changes in ionic strength, Ca++ concentration, and pH were evaluated with runoff SL-SW
and Twitchell water (see Figs. 11 and 12). Only
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
103
Fig. 11. NOM % rejection based on (clear bars) DOC and (shaded bars) UVA during GM membrane filtration of runoff Silver Lake
surface water under various feed water modifications. Ambient means the unmodified R SL-SW.
Fig. 12. NOM% rejection based on (clear bars) DOC and (shaded bars) UVA during GM membrane filtration of Twitchell drainage water
under various feed water modifications. Ambient means the unmodified Twitchell.
[Ca++ ] significantly influenced NOM rejection with
either water. This is likely attributable to calcium
binding between NOM molecules [30] and possibly
also the negatively-charged membrane surface. Either
case would result in reduced surface charge repulsions
between the NOM and the membrane.
Hong and Elimelech [31] reported a decrease in water permeation through an aromatic-polyamide TFC
104
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
nanofiltration (NF) membrane when increasing the
Ca++ concentration. A thicker, more dense ‘fouling’
layer due to the rejected, coiled NOM was their reasonable explanation for those results. But they also
reported almost no difference in the NOM rejection
(based on total organic carbon — TOC) for their model
humic substances with the increase in Ca++ concentration. The NF membrane they used had NOM rejection (under baseline conditions) typically over 90%,
while the GM UF membrane has only 55–60% DOC
rejection with the waters in these measurements.
We consider that the intramolecular Ca++ binding
decreases the electrostatic interaction, in one case between a molecule and a relatively inpenetrable surface, and in the other case, between the molecule and
a pore mouth that it can actually ‘fit’ in! Diminishing
the strength of the electrostatic repulsion in the former case allows the rejected molecules to more tightly
aggregate as a layer on the surface, but in the latter
case allows the formerly-rejected molecules to pass
into and through the pore structure. One can also suppose that the intramolecular Ca++ binding decreases
the NOM molar volume (as Hong and Elimelech [31]
suggested by the descriptor ‘coiled’), then that fraction of the NOM size distribution in our measurements
that was close to passing through the membrane (in
the absence of Ca++ ) would then be small enough to
permeate and lower the observed rejection.
Slightly lowering the pH only has a minor effect
on NOM rejection because a significant portion of the
NOM acidity for both hydrophobic and hydrophilic
acids is within a pH range of 3–4 (see Fig. 10). In the
pH range between 6 and 8, there is significantly less
acidity for those components. Clearly, our justification
for a minor pH effect on rejection also requires no
major change in the membrane surface charge characteristics in the same range of pH values. We did
in fact do the streaming potential measurements with
varying pH and ionic strength. The zeta potential varied smoothly from −15 to −17.7 mV as the pH was
changed from 5.5 to 8.5.
The increase in ionic strength also appeared to insignificantly influence NOM rejection in our measurements. It was anticipated that since increased ionic
strength can cause a compacted double layer of the
NOM macromolecules a significant reduction in rejection (due to the reduced negative charge density)
would occur. However, our other measurements (see
Fig. 5) indicated an apparent reduction in GM membrane pore size was caused by a similar increase in
ionic strength. Therefore, we infer that the effects on
macromolecule charge and size and the membrane
pore geometry were offsetting.
When a recycle line was used to increase the recovery ratio up to 95%, NOM rejections can be calculated
by two methods: based on bulk rejection by Eq. (1)
and feed rejection by Eq. (2). The NOM bulk rejections at the 95% recovery was more than 90% based
on both DOC and UVA with the increased bulk concentration (near the membrane surface) caused by the
recycle stream (see Fig. 11). When NOM feed-based
rejections were calculated, DOC and UVA rejections
were 45.8% and 56.1%, respectively.
4.4. Solution conditions affecting flux decline and
resistances in series
Flux declines were monitored over time with runoff
SL-SW as the bulk water and with that bulk water
spiked with 10 mM NaCl; spiked with 4 mM Ca; pH
adjusted to 4.32; and filtered at 95% recovery (see Fig.
13). The permeance, or water mass transfer coefficient
(MTCw , l m−2 per day kPa−1 ), calculated at various
times in the filtration process, is used as the general
index of flux. When normalized by the initial flux, the
decline (versus unmodified bulk water) was similar for
all cases except that of the increased recovery ratio,
that is, the increased bulk NOM concentration lead to
a larger flux decline, as expected.
The flux decline was also measured (Fig. 14) using each of the different source waters (runoff SL-SW,
OC-GW, and Twitchell). Filtration of OC-GW has
slightly less flux decline than runoff SL-SW, even
though the OC-GW has a higher DOC concentration
and hydrophobicity. More intuitively, though, the very
high DOC concentration of the Twitchell water causes
the largest flux decline of all three waters. These results indicate that the DOC fractions that are most
effectively excluded (rejected) by the GM membrane
are not the same fractions that contribute to increased
flux decline.
To evaluate the flux decline more quantitatively,
the resistances in series defined in Eq. (3) were calculated. These are presented in Table 5, including
for runoff SL-SW at the modified solution conditions
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
105
Fig. 13. Permeance (l/m2 per day/kPa) versus time in crossflow filtration of runoff Silver Lake surface water under various feed water
modifications. The unmodified feed water filtered at 5% recovery is the heavy solid line which connects the actual data points, whose
symbols were omitted for visual clarity. (䊉) unmodified at 95% recovery, (䊊) with 10 mM NaCl at 5% recovery, (䊐) with 4 mM Ca2+
at 5% recovery, and () pH 4.32 at 5% recovery. The increase in permeance at end is due to flux recovery conditions.
(ionic strength, pH, Ca, recovery ratio). There are no
significant differences except for the increased recovery ratio (95%) case and the Twitchell water. The concentration polarization and gel layer resistances are
very small compared with weak adsorption resistance
ra1 , suggesting that NOM-fouled membranes need to
be chemically cleaned. Membrane flux recoveries (ratio of clean water flux after chemical cleaning to initial clean water flux) from fouled membranes are more
than 90% for all cases, indicating effective cleaning
for NOM-fouled membranes using a caustic solution.
4.5. Effective MWCO determination
As previously mentioned, the charged GM membrane rejected NOM mass fractions are smaller than
expected based on the membrane’s nominal MWCO
(8000). It can be envisioned that there is a certain apparent MWCO for the charged membrane with the
charged NOM. This was determined using the NOM
fractional rejection by Eq. (4) with the RMM distribution of the membrane permeate [29].
RMi =
WMi (feed) − WMi (perm)(1 − Roverall )
WMi (feed)
(4)
Here, RMi is the fractional rejection for a certain
RMM, WMi is the mass fraction of that RMM, and
Roverall is overall NOM rejection based on DOC.
The NOM fractional rejections by the GM membrane with runoff SL-SW are shown in Fig. 15, along
with the RMM distributions of runoff SL-SW, and the
corresponding permeate after filtration at 5% recovery. The effective MWCO (based on a RMi value of
90%) was determined using Eq. (4) and are listed with
SUVA values and Mw in Table 6 for the three source
waters. The much smaller effective MWCOs for the
GM membrane for the NOM contained in our test waters, compared to either the manufacturer’s nominal
value or PEG-based measurements we made, are attributed to charge and hydrophobic interactions (note
106
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 14. Permeance (l/m2 per day/kPa) versus time in crossflow filtration of: (solid line) runoff Silver Lake surface water, (short dashes
line) Orange County ground water, and (long dashes line) Twitchell drainage water. The lines connect the actual data points whose symbols
have been eliminated to increase visual clarity. All filtrations were at 5% recovery with GM membrane. Increase in permeance at end is
due to flux recovery conditions.
Table 5
Resistances in series
Feed waters
rm
(cm−1 )
rc
(cm−1 )
rg
(cm−1 )
ra1
(cm−1 )
ra2
(cm−1 )
Flux recovered (%)
runoff SL-SW
runoff SL-SW and NaCl (10 mM)
runoff SL-SW and Ca++ (4 mM)
runoff SL-SW @ pH = 4.3
runoff SL-SW @ R = 95%
OC-GW
Twitchell water
213.4
187.66
209.2
180.8
213.4
210.6
209.4
0.0
0.88
3.2
0.44
0.0
0.50
9.06
6.26
11.16
6.18
4.68
26.98
4.98
45.82
61.68
26.94
62.78
46.44
245.16
34.02
135.22
0.0
6.16
1.42
0.0
0.0
1.08
5.08
100
96.8
99.3
100
92.5
99.4
97.6
the inverse correlation between SUVA and MWCO)
between the membrane surface and NOM. Thus, we
see once again that the effective or observed MWCO
is not an absolute membrane quantity, but is dependent
on the specific solute chemistry that is presented to it.
4.6. Analysis of GM membrane foulants
The results presented in Figs. 13 and 14 suggest that
hydrophobic fractions of NOM did not cause significant flux decline. Even after pH was decreased and
ionic strength was increased to screen the NOM’s negative charge and enhance the hydrophobic interactions,
flux decline was not significantly increased (see Fig.
13). This fact suggests that other NOM fractions such
as transphilic DOC (XAD-4 isolate) and hydrophilic
DOC (effluent of XAD-8 and 4, the neutrals and bases)
may be foulants of the GM membrane. RMM distribution of NOM desorbed from the fouled GM membrane by a NaOH solution was compared to that of
the hydrophilic DOC derived from runoff SL-SW (see
Fig. 16). These SEC chromatograms reveal that the
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
107
Fig. 15. Relative molecular mass distribution of NOM in runoff Silver Lake surface water (䊏) feed and (solid line) permeate (filtration
at 5% recovery with GM membrane). Also plotted is the (dashed line) calculated % rejection of specific relative molecular masses. The
90% rejection point is ∼1580.
Table 6
Effective MWCOs of the GM membrane (manufacturer’s nominal MWCO = 8000)
NOM source
SUVA (/cm/mg l)
Mw of NOM
Effective MWCO
Twitchell water
Runoff SL-SW
OC-GW
0.037
0.045
0.049
1670
1490
1590
2220
1580
1500
hydrophilic DOC has two main peaks in the RMM
distribution, and the larger RMM peak (between 700
and 2000) is very similar to the larger peak of desorbed NOM from the fouled GM membrane. The Mw
and polydispersivity of the larger RMM peak of hydrophilic DOC are 937 and 1.04, respectively. These
are very close to those of the desorbed NOM from the
fouled GM membrane, 1,150 and 1.04, respectively. If
we compare this larger RMM peak of desorbed NOM
with the four different RMM distributions of XAD
isolates from runoff SL-SW, there are no matching
peaks except for the larger RMM peak of hydrophilic
DOC (see Figs. 2 and 16). This supports the assertion that the fraction described as hydrophilic DOC
is one of the membrane foulants responsible for flux
decline.
The smaller RMM region of the hydrophilic DOC
seems to correlate with a similar region in the GM per-
meate when comparing the RMM distributions of hydrophilic DOC and GM permeate (see Fig. 17). There
is also good agreement between RMM distributions of
the GM permeate and transphilic DOC associated with
runoff SL-SW. This suggests that the GM permeate is
composed of a combination of transphilic DOC (hydrophilic acids) and the smaller RMM of hydrophilic
DOC (neutrals or bases). The hydrophobic DOC (hydrophobic acids) is rejected by the GM membrane due
to its negative charge (electrostatic repulsion) and high
RMM.
Thus, in the case of the GM membrane, we hypothesize that the larger hydrophilic fractions of the NOM
(without significant ionizable functionality) are major
components of the adsorbed foulants that lead to significant, long-term flux decline. The specific mechanisms may include pore mouth adsorption and subsequent narrowing of the pores, since these species are
108
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
Fig. 16. Relative molecular mass distribution of (䊊) hydrophilic DOC fraction and (heavy solid line) NaOH desorbed DOC from filtration
(at 5% recovery with GM membrane). Both DOCs are from runoff Silver Lake surface water.
Fig. 17. Relative molecular mass distribution of (䊊) hydrophilic DOC fraction, (䊏) transphilic DOC fraction, and (heavy solid line)
permeate from filtration (at 5% recovery with GM membrane). All DOCs are from runoff Silver Lake surface water.
small enough not to be excluded from the membrane
on strictly steric considerations. Initial adsorption by
these species would screen electrostatic repulsions,
thus allowing further mass to accumulate including
the larger aromatic NOM components.
5. Conclusions
NOM rejection by a (negatively) charged UF membrane is higher than expected based solely on steric
considerations, because of the combination of the size,
J. Cho et al. / Journal of Membrane Science 164 (2000) 89–110
shape, aromaticity and the negative charge density
of this NOM. The NOM’s aromaticity (with larger
RMM) and charge are contained in the hydrophobic
acids (i.e., XAD-8 resin column isolate) which have
a greater negative charge due to carboxylic and phenolic moieties than neutrals and bases (effluent of
XAD-8/4). The hydrophilic acids, even with a high
negative charge density, could not be rejected completely by this negatively charged UF membrane. The
lower RMM fractions permeate the membrane presumably due to the combinations of overall size and
shape. NOM rejection by the charged UF membrane
is decreased when calcium ions are in solution. This
may be due to charge neutralization of both the NOM
and the membrane surface. Small changes in ionic
strength and pH had minor effects on NOM rejection by this charged UF membrane. There is preferential rejection of the aromatic/hydrophobic NOM fraction, which is indicated by the higher rejection based
on UVA versus DOC. This fraction also has a larger
RMM and negative charge/molecule than the other
fractions.
With the same source water, the recovery ratio
(based on the fresh feed rate) can influence flux decline by increasing the bulk NOM concentration.
Again, small changes in ionic strength and pH are minor factors affecting flux decline by the charged UF
membrane, but, in this case, added Ca++ concentration is not a significant factor. This latter observation
is presumably due to the small role of humic substances in causing flux decline for this membrane
and natural waters. This presumption was supported
by further measurements with different source waters
where variations in NOM aromaticity/hydrophobicity
apparently did not influence the flux decline of this
charged UF membrane. We think the neutral and base
fraction of the NOM to be a factor in this membrane’s
fouling based on RMM distributions determined using HPSEC. Our flux recovery measurements indicate
that the fouled membrane can be cleaned effectively
only by chemical cleaning. In our case, we used 0.1 N
NaOH but other cleaning formulations may be even
more effective.
Effective relative molecular mass cutoff (MWCO)
was confirmed to be dependent on the specific source
waters, and SUVA was a consistent indicator of the
trend. Increasing SUVA content correlated with lower
apparent effective MWCO.
109
The emergent physical picture is that the molecular shape and molar charge density of the NOM
determines whether it will be excluded from the
membrane’s pore structure. RMM (by SEC) is one
indirect indicator of these characteristics of the NOM.
Other chromatographic measures like XAD-8 and
XAD-4 resin fractionation are further indicators.
NOM molecules that are not completely excluded
from the membrane’s pores, but that are large enough
and can adsorb near the pore openings do so, and
are the primary species leading to flux decline. These
species are removed with NaOH cleaning, an observation that supports the hypothesis that they are adsorbing near the pore mouth. NOM molecules whose
combination of size and charge are low enough not to
be influenced by the membrane’s pore structure are
found in the permeate.
Further measurements to determine chromatographic indicators of molecular size, shape, functionality, and charge will be helpful in providing improved
quantitative correlations for the membrane filtration
behavior of complex mixtures.
Acknowledgements
This work was supported by the American Water
Works Association Research Foundation - Traci Case,
project manager.
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