j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 119 (1996) 253-268
Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes Amy E. Childress, Menachem Elimelech * Department of Civil and Environmental Engineering, School of Engineering and Applied Science, University of California, Los Angeles, CA 90095-1593, USA
Received 13 February 1996; revised 17 April 1996; accepted 22 April 1996
Abstract A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaC1), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCI2, Na2SO 4, and MgSO 4) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge. Keywords: Zeta potential; Streaming potential; Membrane charge; Reverse osmosis membranes; Nanofiltration membranes
1. Introduction During the past decade a variety of " n e w generation" water treatment membranes have been developed [1]. These polymeric films may generally be divided into two categories. The first consists of dense polymers with high salt rejection for reverse osmosis (RO) processing of sea water and brackish water. The second category of films displays a wide range of porosity which is appropriate for nanofiltra-
* Corresponding author. Tel.: (310) 825-1774; Fax: (310) 2062222; E-mail:
[email protected].
tion (NF) and ultrafiltration (UF) systems. These membranes are used in such diverse applications as wastewater reclamation, water softening, and selective removal of certain molecular and ionic species [21. In both categories, the membranes have been vastly improved in the areas of water flux, salt rejection, and especially in their ability to maintain high performance levels at substantially lower operating pressures than their predecessors. Despite these improvements, a decline in membrane performance over time results from membrane fouling [3-5]. Since the interaction of organic and inorganic colloidal substances with membrane surfaces in aqueous me-
0376-7388/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0376-7388(96)00127-5
254
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
dia is dependent on the charge of the membrane surface [6-8], determination of the zeta potential of the membrane surface is critical to membrane fouling research. This property has generally been overlooked in efforts to select optimum operating conditions for various membrane separation processes. A polymeric membrane acquires surface charge when brought into contact with an aqueous medium. This charge influences the distribution of ions at the membrane-solution interface; co-ions are repelled from the membrane surface while counterions are attracted to it. Consequently, a so-called electrical double layer forms at the membrane surface. The distribution of ions in this layer can be described by several models which are discussed elsewhere [9-12]. Fundamental to all models of the electrical double layer is the concept of a plane of shear. This plane separates the fixed part of the electrical double layer from the mobile part. The electrical potential at the shear plane is called the zeta (or electrokinetic) potential. This potential is of significant importance to colloid and surface science since the surface potential itself cannot be determined experimentally [9-12]. Zeta potential can be determined from one of the following electrokinetic measurements: streaming potential, sedimentation potential, electro-osmosis, or electrophoresis [9,10,12]. Of these techniques, streaming potential is the most suitable for a flat membrane surface [9,13]. Streaming potential is the potential induced when an electrolyte solution flows across a stationary, charged surface. Streaming potential quantifies an electrokinetic effect reflecting the properties of the surface, the flow characteristics, and the chemistry and thermodynamics of the electrolyte solution [9]. Several works on streaming potential measurements of UF membranes have appeared in the literature [14-16]. For UF membranes, however, these measurements reflect the streaming potential of the pores rather than the membrane surface. Pore streaming potential is induced when the electrolyte solution flows through the membrane whereas surface streaming potential (for RO and NF) is the induced potential when the electrolyte solution flows tangential to the membrane. A limited number of studies involving streaming potential measurements of RO or NF membranes are available in the literature [13,17-22]. The majority of these studies were limited in scope
and involved rather simple electrolytes. Moreover, only two investigations [13,22] involved streaming potential measurements over a range of solution pH. Investigating surface charge as a function of pH is crucial for understanding acid-base properties of membrane surface functional groups. Elimelech et al. [13] performed preliminary experiments on the feasibility of using a streaming potential analyzer to determine the zeta potential of RO membranes. This work not only demonstrated the use of a streaming potential analyzer, but also discussed the origin of charge of polymeric surfaces in aqueous solutions and the basic principles of streaming potential measurements. More recently, Nystr~m et al. [22] performed streaming potential measurements on NF membranes in a study on the fouling and retention of these membranes. Zeta potential was calculated as a function of pH and ionic strength. In these studies, however, only a limited range of solution chemistries and membrane types were evaluated. More complex solution chemistries, relevant to RO and NF processing of natural waters and wastewaters, have not been considered. The main objective of this paper is to investigate the effect of solution chemical composition on the surface charge properties of various modem commercial RO and NF membranes. Solution chemistries selected for this investigation directly relate to major chemical species found in a variety of natural waters and wastewaters. Included are the effects of solution pH, indifferent monovalent ions (Na +, C1-), divalent hardness cations (Ca2+, Mg2+), divalent anions (SO4z ), a variety of humic substances representing the ubiquitous dissolved organic matter of natural waters, and anionic and cationic surfactants. The mechanisms of surface charge acquisition by the RO and NF membranes in the presence of these solution chemistries are delineated and discussed.
2. Experimental
2.1. Streaming potential analyzer A commercial streaming potential analyzer (BIEKA, Brookhaven Instruments Corp., Holtsville, NY) was used in this research. This instrument includes an analyzer, a measuring cell, electrodes, and a data
A.E. ChiMress, M. Elimelech/ Journal of Membrane Science 119 (1996) 253-268
10
0/
UpperFormer
fO f~?
0/ if/
UpperParafilm UpperMembrane
/C
.7/
Spacers
/ ~/o
/ /
LowerMembrane LowerParafilm
f
/
LowerFormer
Fig. 1. Schematicdescriptionof the seven layers of the measuring cell.
control system. The analyzer consists of a mechanical drive unit to produce and measure the pressure that drives the electrolyte solution from a reservoir into and through the measuring cell. Sensors for measuring the temperature and conductivity are located internally and pH is measured externally. The measuring cell is composed of seven layers (Fig. 1). The outermost layers are called formers. Next to the formers is Parafilm which serves to minimize leakage from the cell. Next to the Parafilm are the membrane samples. Two pieces of membranes are used for each measurement. One piece, with its active layer facing down, is attached to the upper part of the cell and the other piece, with its active layer facing up, is attached to the lower part of the cell. A channel with dimensions of 86 x 10 mm is created between the membranes using three polytetrafluoroethylene (PTFE) spacers (each 0.254 mm thick). The Ag/AgC1 electrodes that measure the induced streaming potential are mounted at each end of the channel. To prevent polarization of the electrodes, the direction of flow through the cell alternates for each run. Additionally, the electrodes are stored in 0.01 M KC1 solution overnight to prevent build-up of charge. 2.2. Representative membranes
Two RO and two NF membranes were used in the investigation. A thin-film composite polyamide RO
255
membrane (TFCL) and a thin-film composite NF membrane (TFCS) were obtained from Fluid Systems Corporation (San Diego, CA). A thin-film composite NF membrane (NF-70) was obtained from FilmTec (Minneapolis, MN), and an asymmetric cellulose acetate RO membrane (CE) was obtained from Desalination Systems (Escondido, CA). 2.3. Membrane storage
The TFCL, TFCS, and NF-70 membranes were originally stored in a 0.75% sodium meta-bisulfite (Na28205) solution and kept in a refrigerator at approximately 5°C. This storage procedure was proposed by Chapman-Wilbert [23] and also agreed on by the manufacturers. However, it was eventually determined that this storage procedure altered the membrane surface charge. Thus, the experiments were repeated with the membranes stored in deionized water at approximately 5°C. All results shown (except for the results comparing membrane storage) are for experiments using membranes stored in deionized water. The cellulose acetate membrane was supplied as a dry, rolled sheet and was stored as received at room temperature. Based on the manufacturers suggestion, this membrane was dipped in 65% methanol prior to use to remove preservatives. Following that, the membrane was rinsed in deionized water and left overnight (approximately 16 h) in deionized water. 2.4. Solution chemistry 2.4.1. Inorganic electrolytes
Certified ACS grade sodium chloride (NaC1), calcium chloride (CaCI2), sodium sulfate (NazSO4), and magnesium sulfate (MgSO 4) salts (Fisher Scientific, Pittsburgh, PA) were used. Two concentrations of CaC12, NazSO 4, and MgSO 4 were selected (10 4 and 10 -3 M). The effects of these salts were evaluated with 0.01 M NaC1 as a background electrolyte. 2.4.2. Humic substances
Three humic acids from different sources, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used in this investigation. Some of the relevant characteristics of these humic acids are summarized in Table 1.
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A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
Table 1 Molecular weight and acidity of humic acids Humic acid
Suwannee River humic acid (SRHA) Peat humic acid (PHA) Aldrich humic acid (AHA)
Molecular
Acidity (meq/g)
weight (g/mol)
Carboxylic
Phenolic
3000-5000 [24] 10 000-30 000 [26] > 50 000 [27]
4.1 [25] 3.7 [27] 3.3 [28]
2.1 [25]
Suwannee River humic acid reference (SRHA) was received in a freeze-dried form from the International Humic Substances Society (Golden, CO). The molecular weight of this humic acid is between 3000 and 5000 g / m o l [24] and the major functional groups are carboxylic (4.1 m e q / g ) and phenolic (2.1 m e q / g ) [25]. The humic acid powder was dissolved in deionized water and the pH was adjusted to 7.5. Peat humic acid (PHA) was also received in a freeze-dried form from the International Humic Substances Society (Golden, CO). Amirbahman and Olson [26] determined the molecular weight distribution of PHA using an ultrafiltration technique and found that approximately 30% of the PHA macromolecules are smaller than 10000 g / m o l and approximately 30% are larger than 30 000 g/mol. Preliminary investigations regarding the acidity of PHA have determined the carboxylic content to be approximately 3.7 m e q / g [27]. The humic powder was dissolved in deionized water and the pH was adjusted to 7.2. Commercial humic acid (AHA) was received in a powder form from Aldrich Chemicals (Milwaukee, WI). The humic acid was purified using a method given by Hering and Morel [29] with the addition of a dialysis step. In the dialysis step, the humic acid was placed in a regenerated cellulose membrane dialysis bag with a 50000 molecular weight cut-off (Fisher Scientific, Pittsburgh, PA) and was dialyzed against deionized water until the conductivity of the solution was less than l0 /xS/cm (more than 7 days). Following dialysis, the Aldrich humic acid was freeze-dried. The stock solution was made by dissolving the freeze-dried humic acid in deionized water and raising the pH to about 8. The resulting solution had a molecular weight greater than 50 000 g / m o l . Kim et al. [28] purified Aldrich humic acid to obtain a solution with 94% of the macromolecules
2.5 [28]
having a molecular weight greater than 50000. The Aldrich humic acid purified by Kim et al. [28] was found to have 3.3 m e q / g of carboxyl groups and 2.5 m e q / g of phenol groups.
2.4.3. Surfactants Certified grade sodium dodecyl sulfate (SDS) (Fisher Scientific, Pittsburgh, PA) and dodecyltrimethylammonium bromide (DTAB) (Aldrich Chemicals, Milwaukee, WI) were used as model anionic and cationic surfactants, respectively. The surfactant concentrations used in the streaming potential measurements were 10 -5 and 10 - 4 M. These concentrations are much below the corresponding critical micelle concentrations of approximately 10 -2.5 and 10 -2"1 M (in the presence of 0.01 M NaC1) [30] for SDS and DTAB, respectively.
2.5. Reproducibility and equilibration experiments A set of experiments was run to determine the variability in streaming potential results when two different samples of the same membrane were used. Eight of the nine measurements for the two different samples varied by less than 1 mV ( ~ 10%) and the other measurement varied by less than 2 mV ( ~ 20%). Additionally, it was found that new membrane samples should be used each day; results varied by more than 2 mV when a membrane sample was reused the next day after being stored overnight in the measuring cell. Experiments were also performed to determine the time needed for equilibration with the inorganic salt solutions. Based on these experiments, it was determined that a 30-min equilibration period was necessary. At 30 min and beyond, the streaming potential values varied minimally. Similar experiments were performed for the humic acid and surfac-
A.E. Childress, M. Elimelech/ Journal of Membrane Science 119 (1996) 253-268 tant test solutions and results indicated that 20 min of equilibration is satisfactory for both. Additionally, 10 min equilibration time was found to be adequate for pH adjustments. 2.6. Streaming potential measurement procedure 2.6.1. Electrode plating The Ag/AgC1 electrodes were replated intermittently throughout the experiments. Replating was necessary if "spots" of Ag were visible on the electrode surface. Such spots indicated that the AgC1 coating on the electrode surface was uneven. Additionally, replating was performed when the asymmetry potentials were high or when the standard deviation in the measured streaming potentials was consistently high, Before the electrodes were replated, the old AgC1 coating was removed. In this procedure, the electrodes were dipped in a vial of concentrated ammonium hydroxide in an ultrasonic bath and then polished with extra fine sand paper. Next, the electrodes were dipped again in the ammonium hydroxide and thoroughly rinsed with deionized water. In the last step, the electrodes were dipped in concentrated nitric acid to remove any trace organics on the electrode surface. The electrodes were replated by an electrolytic deposition process where the Ag electrode was used as the anode and a gold (Au) electrode was used as the cathode. Both electrodes were attached to electrolysis connectors on the EKA and placed in a beaker of 0.1 M HC1. The HC1 solution was stirred throughout the entire deposition process, which lasted approximately 3 - 5 rain. After the first Ag electrode was replated, the second Ag electrode was replated for exactly the same length of time. Both electrodes were then rinsed with deionized water and placed in 0.01 M KC1 for 24 h before being used. This minimized asymmetry potential resulting from the plating process. 2.6.2. Preparation of membranes Two 127 × 50 mm membrane samples were cut from the membrane sheet (which was stored as previously mentioned). Using a template, the necessary holes and channels were cut into the samples. The membranes were then rinsed with deionized
257
water and put into a beaker containing the first electrolyte solution to be tested. After the membranes were in the solution for approximately 20 min, the membranes (along with the Parafilm, formers, and spacers) were assembled in the measuring cell. 2.6.3. Preparation of cell The cell was first flushed with deionized water for approximately 3 min and then with the test solution for long enough to displace all the deionized water in the system. The test solution was later recirculated through the cell for 30 min (20 min for humic acid and surfactant runs). The pH of the test solution was adjusted with hydrochloric acid (HC1) (Fisher Scientific, Pittsburgh, PA) and the solution was recirculated for 10 min before the first measurement (at pH 2 or 3) was made. Subsequently, the pH was adjusted with sodium hydroxide (NaOH) (Fisher Scientific, Pittsburgh, PA) and the same procedure was followed until the last measurement (at pH 9) was made. In all cases, the pH did not exceed 10 to avoid possible stripping of the Ag/AgC1 electrodes. At the end of the run, the system was flushed again for approximately 3 min with deionized water. 2.6.4. Measurements All streaming potential measurements were performed at room temperature (approximately 23°C). Eight measurements were taken (four in each direction) at each pH. The first two measurements were automatically dropped and the remaining six (three in each direction) were averaged to calculate the zeta potential at each pH. If there was one stray value within the six measurements, that value was dropped and the zeta potential was calculated from the average of the remaining measurements. 2.7. Calculation of zeta potential Zeta potential was calculated from the streaming potential using the Helmholtz-Smoluchowski equation [31]: A~ ~= --
/x L 1
AP Eeo A R
(1)
Here, ~ is the zeta potential, Us is the streaming potential, P is the applied pressure, ( A U J A P is the
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
258
slope of the streaming potential versus applied pressure curve), /x is the dynamic viscosity of the solution, e is the permittivity of the test solution, E0 is the permittivity of free space, L is the channel length, A is the channel cross-sectional area, and R is the channel resistance. The ratio L / A was determined using the Fairbrother and Mastin approach [32]. For solutions with low surface conductivity (i.e. electrolyte concentration greater than approximately 10 -3 M) this ratio is given by L - - = KR
(2)
A
10
|
•
|
•
|
•
|
ZlUs - -
--,,
A P ee o
|
•
|
•
|
•
|
•
.
, 8
.
, 9
.
5
> E v
E
0
•- ' - a - " - C . \ ," '=. ~ . , 2 3~.
-5
~) o_ - 1 0
N
-15
-20
--1-- TFGL membrane --0-- TFCS membrane --A-- NF-70 membrane CE membrane
where s: is the solution conductivity. With this expression, Eq. (1) reduces to its usable form
•
~ A A
-25
Fig. 2. Comparison of zeta potentials of four different membranes over the pH range 2-9. Experiments were carried out with a background electrolyte of 0.01 M NaC1.
(3)
3. R e s u l t s a n d d i s c u s s i o n
3.1. Inorganic salt experiments Results of zeta potential measurements with inorganic salt solutions (NaC1, CaC12, NazSO 4, and MgSO 4) are presented in this section. The effects of CaC12, NazSO 4, and MgSO 4 on the membrane surface charge were evaluated with a background electrolyte of 0.01 M NaC1. The concentrations of divalent ions used in the experiments are typical of those found in natural waters [33]. The effect of membrane storage in sodium meta-bisulfite compared to storage in deionized water is also presented. 3.1.1. Effect of solution pH Zeta potential versus pH data for experiments performed in the presence of 0.01 M NaC1 are shown in Fig. 2. From these results, a few generalizations can be made regarding the thin-film composite membranes. The two NF membranes (TFCS and NF-70) are more negatively charged than the RO membrane (TFCL). More specifically, the TFCS and NF-70 membranes have lower isoelectric points (pH 3 and pH 4, respectively) and they acquire more negative charge at the highest pH tested ( - 16 and - 2 3 mV,
respectively). The TFCL membrane is most positively charged as it has the highest positive zeta potential (almost 9 mV) and the highest isoelectric point (pH 5.2). For all the membranes, the surface charge is positive in the lower pH range, passes through an isoelectric point (between pH 3 and 5), and then becomes negative in the higher pH range. The shape of the zeta potential curves is indicative of amphoteric surfaces. Since the thin-film composite membranes are made by the interracial polymerization reaction of a monomeric polyamine with a polyfunctional acyl halide, ionizable carboxyl and amine functional groups may be expected on the membrane surface [13]. Close approach of co-ions (anions, in our case) may also be responsible for a portion of the negative surface charge. In aqueous solutions, since anions are less hydrated than cations, they can more closely approach the membrane surface. The surface will then acquire a more negative zeta potential due to the presence of anions (C1- and O H - ) beyond the plane of shear [34]. Preferential adsorption (or close approach) of anions has been used to explain surface charge behavior of several non-ionogenic surfaces (i.e. surfaces with no ionizable functional groups). For example, preferential adsorption has been suggested in the study of hydrocarbon droplets [31,35,36]
259
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
and in the study of hydrophobic polystyrene latex colloids [34,37,38]. Based on a study of the relationship between maximum zeta potential and contact angle for several polymers in KC1 solution, Jacobasch and Schurz [6] concluded that adsorption of co-ions is most applicable to hydrophobic surfaces. Hence, co-ion adsorption cannot solely explain the surface charge development of hydrophilic surfaces, such as the cellulose acetate (CE) membrane used in this investigation. As with the composite membranes, the zeta potential for the CE membrane is positive at low pH values and negative at high pH values. This behavior cannot be explained by dissociation of the acetyl and hydroxyl groups of the polymeric structure of the cellulose acetate membrane since they do not dissociate at the given chemical conditions. Instead, the acid-base behavior of the membrane surface may be attributed to chemical post-treatment with an additive containing acidic functional groups. This post-
treatment would explain why the zeta potential curve of the CE membrane has a shape characteristic of a surface containing weakly acidic functional groups. The membrane may also undergo a post-treatment which would result in a positive charge on the membrane surface at low pH. Due to the proprietary nature of membrane fabrication, no additional information is available from the manufacturer on such post-treatments. The observed zeta potential behavior of the CE membrane may also be attributed to dissociation of carboxyl groups of the cellulose acetate. Minning and Spiegler [20] reported that cellulose acetate membranes possess a low fixed charge and thus behave as cation exchange membranes at low electrolyte concentration. Later, Demisch and Pusch [39,40] attributed the fixed charge of cellulose acetate to dissociation of membrane carboxyl groups and developed a method to determine the amount of carboxyl groups. None of these studies, however, • • • i - . • • - • - o - • - i -
10
10
(a) TFCL
\
5 ~ -"~'~O 0 *5
13.
"6
N
>
pH
.......
o
; ;
2
3~i'~6
7
8
9
-5
-5
O.. {1~ - 1 0
-10
N
-15 -20
(b) TFCS
5
- - m - - no divalent ions •- - O - - 0.001 M CaCI 2 IAI
--~:
;%;'lalMentaiC;s
A-- ~ m A
--~--
0,001 M Na2SO 4
-20
0.001 M Na2SO 4
-25
-25
1 0
5I
"q.%m_.
-15
~z~.
.
~
[
-
i
-
•
•
•
-
i
-
i
-
0
-
•
-
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-
(d) CE
pH
v
2
-sE
o ,
~[\ "$
N
-20
-25
--I---O--
no divalent o s 0.001 M CaCI 2
I~l
0.001 M Na2SO 4
1
~
O
--I--20
-25
o
O
~AI.~ zx.... k l ~
-15
~b.a ~l
0
o
no divalent ions
- - ' O - - 0.001 M CaCI 2 _ / x . _ 0.001 M Na2SO 4
Fig. 3. Zeta potential of (a) TFCL, (b) TFCS, (c) NF-70, and (d) CE membranes as a function of pH measured in the presence of divalent cations Ca 2+ (by adding CaCI 2) and divalent anions SO2- (by adding Na2SO4). All experiments were carried out in the presence of 0.01 M NaC1.
260
A.E. Childre ss, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
provided an explanation for the origin of the carboxyl groups.
3.1.2. Effect of divalent ions Three different salts were used to evaluate the effect of divalent ions on the membrane surface charge. Fig. 3 presents zeta potential versus pH curves for 0.001 M CaC12 and 0.001 M N a z S O 4. Results for 0.001 M MgSO 4 are shown in Fig. 4. All experiments were carried out with 0.01 M NaC1 as a background electrolyte. For the TFCL membrane (Figs. 3a and 4a), the divalent ions generally have no effect on the zeta potential of the membrane. A possible explanation for this observation in experiments with CaC12 is that the close approach of co-ions (C1-) may counteract the effect of the adsorbed divalent cations. For the TFCS membrane (Fig. 3b), CaC12 and NazSO 4 have a noticeable effect on the surface charge. In the presence of divalent cations (Ca 2+), the TFCS membrane is more positively charged over the entire pH range and the isoelectric point shifts from pH 3.0 to 3.6. On the other hand, when divalent anions (SO 2- ) are present, the membrane is more negatively charged over the entire pH range. When both divalent cations (Mg 2+) and anions (SO42-) are present in solution (Fig. 4b), the zeta potential of the TFCS membrane above the isoelectric point does not change substantially, most likely because of the counteracting ef-
10
.
.
.
.
.
.
.
.
.
.
.
.
i C]'~--O~o~,I 5
+:
I[:--ram-- no divalent ions .15t--0-- O.O01MMgSO4
.
.
.
.
.
fects of the adsorbed Mg 2+ and SO 2- ions. Below the isoelectric point, the sulfate anions adsorb to the surface of the TFCS membrane and eliminate its positive surface charge. Since the membranes investigated are negatively charged above the isoelectric point, complex formation of the Ca 2+ ion with the membrane surface would be electrostatically favorable. In surface complex formation, the ion may form an inner-sphere complex (chemical bond), an outer-sphere complex (ion pair), or be in the diffuse swarm of the electric double layer [11]. If electrostatic attraction is the only bonding mechanism, the Ca 2+ would form an outer-sphere complex. However, if covalent bonding or some combination of covalent and ionic bonding occurs, inner-sphere complexes are formed. In both cases, adsorption of Ca 2+ to the membrane surface reduces the negative charge of the membrane. The effect of the divalent anion, SO 2-, is opposite to the effect of the divalent cation and would be most obvious below the isoelectric point where the membrane is positively charged. The effect of the divalent ions is less obvious for the NF-70 membrane (Fig. 3c). Below the isoelectric point, there appears to be no effect of the sulfate anions, and above the isoelectric point, the effect of the calcium cations is only noticeable above pH 8. In the case of the CE membrane (Fig. 3d), the effect of the divalent cations is again more substantial than
10 . ,
.
.
.
.
.
.
.
.
.
.
(a) TFCL
.
.
.
.
.
(b) TFCS 5
0H
F - n - no divalent ions
,
,N
.15f--0-- O.O01MMgSO4 \ m ~ " O
Fig. 4. Zeta potential of (a) TFCL and (b) TFCS membranes as a function of pH measured in the presence of both divalent cations (Mg 2+ ) and divalent anions (SO4z- ). The divalent cations and anions originated from the addition of MgSO4 salt. All experiments were carried out in the presence of 0.01 M NaC1.
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
the effect of the divalent anions. Overall, the divalent anions (SO 2 - ) do not appear to adsorb as readily to the membrane surface as the divalent cations (Ca 2+ ). This is because the membrane has a positive charge over only a small portion of the pH range investigated.
10
..... 2
3.1.3. Effect of membrane storage Results of an experiment showing the effect of membrane storage on the zeta potential of the TFCL membrane are shown in Fig. 5. Since the membrane stored in sodium meta-bisulfite (Na2S205) is more negatively charged and has a lower isoelectric point, it is likely that meta-bisulfite anions ($2 O2-) have adsorbed to the membrane surface. Strong interaction of the $2052- anions is expected at low pH where the membrane is positively charged. On the other hand, at higher pH values where the membrane is negatively charged, the effect of the meta-bisulfite anions is not as significant. 3.2. Humic acid experiments The results of zeta potential experiments with Suwannee River humic acid (SRHA), peat humic acid (PHA), and Aldrich humic acid (AHA) are discussed in this section. In all experiments, 0.01 M NaC1 was used as a background electrolyte. Over the majority of the pH range, the presence of the humics increases the negative zeta potential (i.e. the membrane becomes more negatively charged). These results indicate that humic macromolecules readily adsorb to the membrane surface and that the negatively-charged functional groups of the humics dominate the membrane surface charge. It is widely accepted that humic substances account for the observed surface charge properties of various types of naturally occurring particles and minerals [24,41-46]. O'Melia [47] and Tipping [48] provide reviews on the adsorption of dissolved organic matter to natural surfaces. Only a few studies, however, have been reported on the surface charge properties of membranes in the presence of humic substances [49,50]. The studies of humic adsorption onto oxide surfaces, although not directly applicable, may provide some insight into adsorption mechanisms of humics onto membrane surfaces. For example, in a study on the adsorption of humics onto aquatic surfaces, Ochs
~.
-5
I~1-10 -15
TFCL
i~I~ o\
g
261
,, ,....p. . ....
3
4 ~0~6
o,,. 7
8
,
9
\=
--II-- stored in deionized water - - 0 - - stored in Na2S205
0
Fig. 5. Effect of membrane storage on zeta potential of TFCL membrane as a function of pH. All experiments were carried out with a background electrolyte of 0.01 M NaC1.
[51] concluded that humic substances possess a very high affinity for both hydrophilic and hydrophobic surfaces (hence, they are "amphipathic") and that their adsorption is very fast. Numerous studies have also shown that the adsorption of humic substances onto negatively-charged mineral surfaces is enhanced in the presence of divalent cations (Ca 2+, Mg 2+ ) [24,42-46,52]. 3.2.1. Suwannee River humic acid In Fig. 6, the dramatic effect of the higher concentration of Suwannee River humic acid (SRHA) on the membrane surface charge is most apparent at low pH. As the pH increases, the effect of SRHA on the membrane surface charge diminishes, and eventually, in the high pH range, the effect is very small. This pH-dependent behavior can be explained by changes in humic-membrane interactions. Below the isoelectric point of the membranes, where the membranes and SRHA are oppositely charged, adsorption of SRHA is favorable because of both electrostatic and hydrophobic interactions. Just above the isoelectric point, where the SRHA and the membrane are similarly charged, adsorption is likely to be dominated by hydrophobic interactions. At higher pH values, only minimal adsorption occurs because of the electrostatic repulsion between the SRHA and the membrane surface and the increased hydrophilicity of the SRHA.
262
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
In their studies on the adsorption of aquatic humic substances onto hydrophobic ultrafiltration membranes, Jucker and Clark [49] found that in the low pH range there is increased adsorption of humics onto membranes. Results from potentiometry titrations and octanol-water partitioning showed that, at low pH (pH 4), SRHA is essentially hydrophobic. At pH 6 and above, however, deprotonation of the carboxylic and phenolic functional groups leads to increased water solubility and greater hydrophilicity [49]. In NF experiments, Braghetta [50] found that permeate flux and solute rejection decreased significandy at low pH. This was partially attributed to the accumulation of humic macromolecules at the membrane surface in a layer of high packing density. 3.2.2. Suwannee Riuer humic acid in the presence of diualent cations The effect of SRHA on the surface charge of the TFCL membrane in the presence of divalent cations is shown in Fig. 7. The addition of divalent cations .
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increases the negative charge of the membrane surface. This is attributed to increased humic adsorption on the membrane surface. The same mechanisms responsible for increased adsorption of humics onto oxide surfaces in the presence of divalent cations may also explain the increased adsorption onto membrane surfaces. The effect of divalent cations on humic adsorption is attributed to either simple electrostatic charge shielding or complexation reactions [24]. Numerous investigations have shown that divalent cations form complexes with the functional groups of humic macromolecules, and thus reduce the interchain repulsion of the humics [42-46,52]. Calcium may also adsorb to the membrane surface and thereby reduce the negative charge of the membrane (especially in the case of the TFCS and CE membranes), making interaction between the membrane and humics more favorable. Jucker and Clark [49] suggested that calcium acts as a bridge between the membrane surface and the negatively-charged humic molecules and also between the negatively. ,
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A.E. Childress, M. Elimelech/ Journal of Membrane Science 119 (1996) 253-268 •
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263
different humic acids: Suwannee River humic acid (SRHA), peat humic acid (PHA), and Aldrich humic acid (AHA). All humic acids cause the membrane to be more negatively charged, with A H A having the most pronounced effect and SRHA having the least. The difference in the effect of these humics on the membrane surface charge is attributed to the difference in their adsorption behavior. Although the charges (or acidity) of the humics tested do not differ substantially, their molecular weights do (Table 1). AHA, the humic with the largest molecular weight, adsorbs to the greatest extent, whereas SRHA, the humic with the smallest molecular weight, adsorbs to the least extent. Theoretical arguments and experimental evidence show that with increasing molecular weight, and hence, increasing hydrophobicity, the adsorption of polyelectrolytes increases [11,53,54]. Jucker and Clark [49] demonstrated that more adsorption onto ultrafiltration membranes occurs with Suwannee River humic acid (molecular weight between 3000 to 5000 g / m o l as shown in Table 1) than with Suwannee River fulvic acid which has a molecular weight of 1000-1700 g / m o l [24]. 3.3. Surfactant experiments The effect of anionic surfactant (SDS) and cationic surfactant (DTAB) on the zeta potential of membrane surfaces was investigated in the presence of 0.01 M NaC1. The adsorbed anionic surfactant has a somewhat similar effect as the adsorbed humic substances: the negatively-charged sulfate functional groups of the surfactant molecule cause the membrane to become more negatively charged. The effect of the adsorbed cationic surfactant is opposite to that of the adsorbed anionic surfactant in that the membrane becomes more positively charged. The adsorption characteristics of surfactants are governed by the molecular structure of the surfactant molecules (e.g. type of polar head, structure and length of hydrocarbon chain) and the characteristics of the membrane surface (e.g. charge, hydrophobicity) [55-57] as discussed below. 3.3.1. Anionic surfactant (SDS) For the three membranes shown in Fig. 9, the addition of 10 -5 M SDS causes the membrane to be more negatively charged over the majority of the pH
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
264
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range. At this concentration, the surfactant ions adsorb as individual molecules, and at low pH, where the membrane and surfactant are oppositely charged, adsorption results primarily from attractive electrostatic forces. As the pH increases, the membrane becomes less positively charged, and adsorption remains constant. Around the isoelectric point of the membrane, the importance of electrostatic forces diminishes and adsorption results more from hydrophobic interactions. As the pH increases further, surfactant adsorption due to hydrophobic interactions remains constant, but the zeta potential continues to become more negative because of the ongoing dissociation of membrane functional groups. The surfac-
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A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
point at which there is a sharp increase in adsorption which reflects the transition from individual surfactant ion adsorption to surfactant association at the surface through lateral interaction of the hydrocarbon chains [59]. Hunter [55] points out that the concept of hemimicelle formation is not universally accepted. For example, Koopal [60] proposes a model which suggests that flat adsorption will prevail at the HMC. Koopal and Keltjens [61] argue that although the models of Fuerstenau and Healy [62], Healy [63], and Ottewill and Rastogi [64] have attracted the most attention in the literature, they do not consider cooperative lateral hydrophobic attraction; more specifically, the models do not account for hydrocarbon chain length and flexibility of the surfactant molecule. Chandar et al. [65], using fluorescence probing techniques, suggest that below the HMC, adsorption of SDS onto the positively-charged alumina surface is due to electrostatic attraction of individual surfactant ions, and no significant aggregation takes place. On the other hand, above the HMC, adsorption of SDS onto the alumina surface occurs through the formation of surfactant aggregates of limited size; the increase in adsorption is attributed to the increasing number of surfactant aggregates. Chandar et al.
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[65] further suggest that, at higher surfactant concentrations, adsorption occurs through growth of the existing surfactant aggregates. The HMC for the SDS-alumina system was found to be approximately 10 -4.5 M. Accordingly, it is likely that in the present study, hemimicelle formation occurs in the lower pH range of the 10 - 4 M SDS experiments. Hemimicelle formation in the low pH range of the experiments with 10 - 4 M SDS is schematically illustrated in Fig. 10b. As the pH increases, the membrane becomes less positively charged and SDS adsorption decreases proportionately. Above the isoelectric point of the membrane, adsorption decreases dramatically as the adsorption mechanism shifts from being primarily electrostatic interaction to primarily hydrophobic interaction. In the latter case, hemimicelles cannot form because the hydrophobic hydrocarbon chain of the surfactant molecule is attached to the membrane surface (Fig. 10b). At the higher end of the pH range, in the case of the TFCS (Fig. 9b) and CE (Fig. 9c) membranes, electrostatic repulsion appears to outweigh the hydrophobic interactions resulting in no significant SDS adsorption.
-15 TFCL
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-20 Fig. 11. Effect of an anionic surfactant, sodium dodecyl sulfate (SDS), versus the effect of a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), on the zeta potential of the TFCL membrane over the pH range 2-9. Experiments were carried out in the presence of 0.01 M NaCI.
3.3.2. Cationic surfactant (DTAB) For the experiments with 10 - 4 M cationic surfactant (DTAB) shown in Fig. 11, hydrophobic interactions dominate in the lower pH range because the membrane and surfactant molecules are similarly charged. This is illustrated schematically in Fig. 12.
266
A.E. Childress, M. Elimelech / Journal of Membrane Science 119 (1996) 253-268
Just above the isoelectric point, electrostatic interactions become more important. Eventually in the higher pH range, where adsorption is most substantial, hemimicelle formation may occur. As shown in Fig. 12, the behavior with the cationic surfactant is opposite to that with the anionic surfactant (Fig. 10b) at similar molar concentration. Nishimura et al. [66] performed atomic force microscopy studies to provide direct evidence for the formation of surfactant aggregates (hemimicelles) at the m i c a - w a t e r interface. In their study, the HMC for DTAB adsorption onto mica (in the presence of 0.001 M KBr or KC1 and at pH 5.3-5.8) was found to be 10 3.6 M. Hence, it is quite reasonable to assume that hemimicelles form at the m e m b r a n e - s o l u t i o n interface in the higher pH range of the 10 -4 M DTAB experiments.
4. Conclusions Streaming potential measurements can be effectively used to determine the zeta potential of a membrane surface and to investigate the effects of solution chemical composition on membrane surface charge. Solution chemistry has a marked effect on the surface charge characteristics of polymeric membranes. In the presence of an indifferent (1:1) electrolyte, all membranes investigated were positively charged in the lower pH range, had an isoelectric point between 3 and 5, and were negatively charged in the higher pH range. This behavior is typical of amphoteric surfaces. Humic substances readily adsorb to membrane surfaces and markedly influence the membrane surface charge. The adsorption of humic acid to the membrane surface is enhanced with increasing molecular weight of the humic macromolecules and with the addition of divalent cations to the solution. Anionic and cationic surfactants also readily adsorb to the membrane surface. A dramatic effect on membrane surface charge is observed at relatively low surfactant concentrations. The striking change in membrane surface charge in the presence of surfactants is attributed to the formation of hemimicelles at the m e m b r a n e - s o l u t i o n interface.
Acknowledgements We acknowledge the support of the U.S. Bureau of Reclamation, Water Treatment Engineering and Research Group, and of Fluid Systems Corporation, a member of Anglian Water Group. The findings reported in this paper do not necessarily reflect the views of these agencies and no official endorsement should be inferred.
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