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Sodium chloride rejection by a UF ceramic membrane in relation to its surface electrical properties
Separation and Purification Technology 49 (2006) 122–129
Sodium chloride rejection by a UF ceramic membrane in relation to its surface electrical properties P. Narong, A.E. James ∗ School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK Received 11 June 2005; received in revised form 9 September 2005; accepted 15 September 2005
Abstract The rejection of salt (NaCl) by a ceramic (titanium dioxide–alumina layered) ultrafiltration membrane having a nominal pore size of 5 nm and being operated in the cross flow mode is investigated. Measurements were undertaken using different concentrations of salt over range of pH at relatively low transmembrane pressure (TMP). The rejection was assessed by determining the concentrations of sodium and chloride ions in the permeate and retentate and comparing them with the feed concentration. Rejections in the range 15–40% were found and at the same pH lower rejections are found at higher salt concentrations. Increased concentration reduces the rejection by up to 15%. Overall the rejection is seen to be a mainly function of pH which causes the rejection to vary by up to 50%. The observed rejection characteristics were compared with both filtration potential and ζ-potential data over the same range of pH and salt concentration. The filtration potential data was obtained in situ whilst the ζ-potentials were determined from electrophoretic measurements ground membrane material. The membranes are positively charged at low pH and negatively charged at high pH, the isoelectric points being pH 3.1 and 3.8 from electrophoretic and filtration potential measurements, respectively, the difference being attributed to differences in surface characteristics. The minimum rejection occurs at around pH 4 which corresponds to the isoelectric point (i.e.p.) found using filtration potentials showing that the electrostatic interactions between ions and the membrane surface are an important factor in salt rejection. © 2005 Elsevier B.V. All rights reserved. Keywords: Salt rejection; Ultrafiltration; ζ-Potential; Filtration potential
1. Introduction Clean water is very essential for all aspects of human life and in some regions can be difficult to obtain from wells or rivers in these situations brackish water and seawater can provide useful sources for purification. Pressure driven membrane processes are an important category of techniques for desalination of such waters. In particular, reverse osmosis (RO) is a well known method for producing desalinated water, but nanofiltration (NF) and even ultrafiltration (UF) can be used in the desalination process if the membrane pore diameters are less than about 10 nm [1]. Extensive research has been published about the rejection of ions and mechanisms for solute rejection by membranes [1–4]. In case of neutral RO membranes, the observed rejection of
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solute is generally very high and the membrane is considered to be permeable only to the solvent provided the transmembrane pressure is higher than the osmotic pressure. In case of mesoporous membranes, typically those employed in ultrafiltration, the pore diameter is larger than ion diameter and it has been found that salt rejection can be high even though neutral solutes are not rejected. These phenomena can be explained taking into account the charges which remain on the surface of the membrane pores thus the electrostatic interactions between ions and the membrane surface is important [1,2,5,6]. For seawater which is a mixture of salt and large charged organic molecules, it is found that the nominal monovalent salt rejection is usually negative. This has been exploited to remove salts from solution while concentrating dyestuffs, pharmaceuticals and whey sugars and protein. In a previous study involving reverse osmosis [3], the phenomenon of apparent negative rejection at the membrane has been reported and shown to arise from the effect of the Donnan distribution of the salt between the solution and the membrane. Applying Spiegler–Keden analysis to solute transport in the
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membrane and introducing a Donnan distribution factor, it is possible to calculate theoretical curves that qualitatively demonstrate the observed phenomenon of apparent negative rejection [4,5]. While there has been much interest in the rejection phenomenon in RO and also nanofiltration less attention has been paid to UF. Typically, [1] describe an investigation for the filtration of different saline solution using several ceramic UF membranes such as ␥ alumina, CoAl2 O4 and TiO2 /ZnAl2 O4 on the surface support in ␣ alumina. They remark that there is a correlation of the membrane selectivity in terms of rejection with the surface charge developed on the membrane material. The ability to predict the effect of the processes involved in the UF membrane separation would be very useful for planning and optimisation of system. Such predictions would ideally utilise available physical property data of a process stream and a membrane. For UF membranes, this involves the considering the structure of the membrane in terms of its pore size, materials of construction and electrokinetic properties such as the zeta potential (hereafter called ζ-potential) and the streaming potential. Electrokinetic phenomena allow the charge distribution at the interface between membrane surface and the fluid to be characterised. It is not possible to measure the ζ-potential directly and some model has to be invoked to link it to the measured electrokinetic data. The ζ-potential can be related to any of the four main electrokinetic phenomena, i.e. electrophoresis, streaming potential, electroosmosis and sedimentation potential. The first two of these are the basis for the most popular methods and the last is rarely used. Investigations of the electrophoretic mobility of particles in suspension characterise the particle’s net charge by measuring their velocity in an applied external electrical field. In contrast, the streaming potential results from forcing fluid through a porous medium and its value is that it can provide a useful in situ characterisation for solid charged surfaces such as those found in UF and microfiltration (MF) membranes. However, some authors acknowledge that, for some types of membrane, streaming potential measurements may be difficult to obtain and also that the results seem to be characteristic of the outer region of the porous surface [6]. There authors suggest that calculation of the ζ-potential may be impossible for composite membranes because often the internal pore size distribution and pore geometry are known inexactly and the influence of each part of the membrane contributes to the final result. To this end, they advocate the adoption a quantity which may be conveniently termed the flow through or filtration potential which is dimensionally the same as the streaming potential but reflects the overall relationship between the measured transmembrane electrical potential and pressure. It appears that this filtration potential characterises the overall influence of the surface potentials in a composite membrane and the electrolyte environment in which these different surfaces are immersed. Using these electrokinetic techniques, the variations in the net charge of a membrane surface and its isoelectric point (i.e.p.) can be measured as a function of the physical–chemical environment [7]. The performances of ceramic membranes in desalination depend on the concentration of ions and on the complexity of
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the solution to be filtered, but also on the type of membrane material. Typically, ceramic membranes used are generally made of metal oxides, which have an amphoteric behaviour, so that the electric surface charge depends on the pH of the feed solution [1,8–10]. Therefore, repulsion or attraction occurs between the ionic species present in the solution and the charged membrane. Beside the electrostatic effect, the size of the filtered species is also another parameter which should be considered, particularly if the solution contains complexed cations with mineral or organic materials. Asymmetric or composite membranes represent an important and popular type of membrane which are formed from two or more sublayers having different transport properties. Since these composite membranes may be constructed from different material the overall performance may be related to contributions from each of the different layers. For instance, if one sublayer shows high salt retention (as usually happens with active layers of commercial membranes like reverse osmosis and nanofiltration membranes), the permeate solution has much lower concentration than the feed solution. Generally, the higher or lower contribution of indirect effects in the parameters measured for a composite membrane is related to the different structures and transport properties of the sublayers forming the composite membrane [7,11]. In these cases, the separation efficiency of the UF membrane in the treatment of ionic solution can be explained by the combination of size and charge effect. The membrane charge depends on the pH of the solution and the i.e.p. of the membrane [1,12–14]. The aim of the present work is to investigate the relationships between salt rejection and electrokinetic potentials (ζ-potential and filtration potential) for a commercial UF ceramic membrane at different pH and electrolyte concentrations, since correlation of the membrane selectivity in terms of salt rejection with respect to electrokinetic potentials represents a direct way of relating the electrostatic properties of the membrane surface with its rejection characteristics.
2. Materials and methods 2.1. Membranes Tubular ceramic membranes (Schumasiv® single channel TI 01070, produced by USF Schumacher, Germany) were used. These are constructed so that the inorganic oxide particles comprising the inner surface layer of the circular tube determine the pore size of the membrane. This surface layer is supported by a more porous ceramic tube which can be made from the same or alternative metal oxide. Membranes having 0.005 m nominal pore sizes were selected for the present study and these are classified as UF membranes. The active inner surface is made of a layer of titanium dioxide which is deposited on a porous alumina support having a nominal pore diameter of 1.2 m. The overall dimensions for the membranes are: 6.67 mm i.d., 10 mm o.d. and 131 mm in length and the permeate area for each tubular membrane tube is approximately 2.75 × 10−3 m2 .
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Fig. 1. A schematic diagram of ultrafiltration rig for testing flux rejection properties.
2.2. Filtration experiments Sodium chloride solutions were prepared using a solid dried form of analytical grade sodium chloride (Sigma® –Aldrich, U.K.) with ultra pure water (ELGASTAT® Spectrum B unit Cartridge type: SC2 , USF Memcor, U.K.) with a resistivity of 18 M cm−1 and an organic contamination of less than 0.03 mg l−1 . Sodium chloride in the feed solution was prepared at different concentrations (10−1 , 10−2 and 10−3 M) which were run in the cross flow UF system schematically shown in Fig. 1. The main components were glass containers (6 dm3 ), a magnetic stirrer (RW20, IKAMAG® , U.K.), a variable speed peristaltic pump (Type 603S, Watson-Marlow Ltd., England), 9.6 mm flexible piping (Marprene, Watson Marlow, U.K.), reinforced PVC flexible piping and various fittings, cylindrical uPVC pressure dampening vessels and a rubber pressure dampening T-piece, a tubular UF membranes module, a uPVC valve (Georg Fischer, Switzerland), uPVC pieces to fit transducer connectors, a pressure relief valve pre-set at 5 bar (NABIC, U.K.). The permeate flow rate was determined using a measuring cylinder placed on top an analytical balance and a stop clock. Various pieces of piping, connectors and valves, a variable area flow meter (Gem¨u Gebr. M¨uller, Germany) complete the apparatus. In addition, the concentrations of sodium and chloride ions in the permeate were monitored. In this work, each filtration experiment was repeated a minimum of three times to ensure good reproducibility. The filtration rig was operated at using a cross flow velocity of 0.5 m s−1 at a transmembrane pressure (TMP) of 2.5 bar. The concentration of chloride anions in the permeate were determined using an argentometric titra-
tion method [15] while concentration of sodium cations were measured using inductive coupled plasma atomic emission spectrometry (ICP-AES) (Model: VISTA-MPX CCD Simultaneous ICP-OES, Australia). 2.3. Membrane characterisation A membrane sample was cut to a length of 2–3 mm to allow inspection of the membrane surface using a scanning electron microscope (FEI QUANTA 200, Purge, Czech Republic) operated at an accelerating voltage of 30 kV. In addition, an elemental analysis was carried using an energy dispersive X-ray spectrometer (EDXS). The ζ-potential of the membrane was measured as well as its filtration potential. Samples of membrane were cut and finely ground. The electrophoretic mobility of the ground particles was determined (Zetasizer 3000HS Advance, Malvern Instrument GmbH, U.K.) at three different ionic strengths of NaCl (10−1 , 10−2 , 10−3 M). In order to hold the ionic strength constant, the pH of the oxide suspensions was adjusted in the range pH 3–10 using 0.1 M H2 SO4 or NaOH where appropriate and the ζ-potentials of the particles were determined using the Helmholtz–Smoluchowski equation: ς=
µU εε0
(1)
where µ is the viscosity of the liquid, ε the dielectric constant, ε0 the permittivity of free space, U the electrokinetic mobility of the particle =u/E and u is the velocity of the particle in an electrical field of strength E. Although under certain limiting circumstances the ζ-potential may be derived from the filtra-
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tion potential by the application of appropriate theory but if one is interested only in detecting the i.e.p. of the system it is not necessary to attempt this conversion. In terms of membrane characterisation the filtration potential represents a non destructive in situ method in contrast electrophoresis measurements which require particles of the membrane material which can often only be obtained by destroying the membrane. This often involves destroying the membrane, although this would have to be done if a microscopic examination of the membrane surface, for instance to examine the effects of fouling, is required. In this study, measurement of the filtration potential were made on the membrane over a range of pH 3–7, because higher and lower pH compress the electrokinetic double layer, and moreover, high pH cause damaging of the electrodes, using several different concentration of NaCl (10−1 , 10−2 , 10−3 M). The electric potential difference over a range of pressure driving forces (0 bar ≤ TMP ≤ 2.0 bar) was measured using a pair of platinum electrodes [8] (Alfa Aesar, Johnson Matthey Chemical, U.K.) attached to a high impedance milli-voltmeter (Wavetek Meterman, RS Component, U.K.). One was placed along the central line of the tubular membrane and the other was wrapped around the outer wall of the membrane. The electrodes were connected to the voltmeter which recorded the potential difference generated by the electrolyte flow. The filtration potentials used in this study are defined as the instantaneous potential difference per unit difference of applied pressure (Φ/TMP). This is because the observed potential changes quite rapidly due to polarisation of the electrodes if it is measured continually and in the absence of fully reversible electrodes the use of the instantaneous potential provides a useful means of estimating both streaming and filtration potentials and good reproducibility is achieved using this methodology [7]. 3. Results and discussion 3.1. Effect of salt concentration The rejection factor, α, for different concentrations of sodium chloride at pH 7 and 3 are shown as a functions of time in Figs. 2 and 3. It is seen that the salt rejection increases with time for all concentrations of electrolyte used. At any time, the rejection is reduced as the feed concentration of salt is increased. A similar pattern has been reported by other workers who have compared modelled single salt rejection with experimental results as a function of pH for nanofiltration systems [16]. At pH 7, after 3000 s, the rejection factors rise to 0.31, 0.35 and 0.44 for feed concentrations 10−1 , 10−2 and 10−3 M sodium chloride, respectively, while at pH 3, the rejection factors rise to 0.35, 0.36 and 0.45 for feed concentrations 10−1 , 10−2 and 10−3 M sodium chloride, respectively. In general, all fluxes decay with time and the best flux is seen for 10−3 M sodium chloride and the worst for 10−1 M sodium chloride. As has been noted elsewhere [14], the permeate flux decays with time but in this study, it is beneficial to the separation process as this corresponds to an increase in the salt rejection. These observations broadly agree with those of other workers [12] who used a Carbosep M2 tubular ZrO2 membrane (UF membrane with a cut-off
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Fig. 2. Filtration of salt solution at a 2.5 bar pressure through the titania membrane: salt rejection factor, α, as a function of time in different concentrations of salt solution at pH 7 (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
value 15 kDa) in studies of phosphate rejection. Other workers [17] state that the permeate flux of all polymeric NF membranes, used to study rejection characteristics of organic and inorganic pollutants of surface water, decreased significantly during the time of operation. This is attributed to the accumulation of a fouling layer on membrane surface during the period of operation. 3.2. SEM and EDXS of membranes The SEM cross-section image of membrane surface is shown in Fig. 4. The membrane used in this study is a composite membrane having active side surface which is made from a thin layer of titanium dioxide and alumina that can be seen clearly. A typical membrane pore is indicated and three different membrane support layers the outer of which is made of sintered alumina are seen. Fig. 5 shows the specific spectrums of the active surface of the membrane obtained using EDXS. It
Fig. 3. Filtration of salt solution at a 2.5 bar pressure through the titania membrane: salt rejection factor, α, as a function of time in different concentrations of salt solution at pH 3 (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
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Fig. 4. SEM cross-section image of titanium dioxide UF membrane (magnification ×500).
is seen that the UF membrane used in this study contains five elements: oxygen, O (39.90 wt%); titanium, Ti (33.88 wt%); aluminium, Al (23.55 wt%); zirconium, Zr (1.84 wt%) and calcium; Ca (0.82 wt%) confirming the composition of the membrane. The Zr and Ca are low level contaminants originating from either the manufacturing process or from when the membrane section was cut. 3.3. Effect of pH and electrolyte concentration on the electrokinetic potentials of membrane The effect of pH on the ζ-potential of the titanium membrane particles as a function of increasing electrolyte pH and concentration of sodium chloride is shown in Fig. 6 for three different concentrations (10−3 , 10−2 and 10−1 M sodium chloride). The
Fig. 5. EDXS spectrum of titanium dioxide UF membrane.
Fig. 6. The ζ-potential of titanium dioxide membrane as a function of pH dispersed in different background electrolyte of sodium chloride (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
results show that the ζ-potentials of the membrane used in this study is positive at pH 3 being 2.7, 2.5 and 0.4 mV for 10−3 , 10−2 and 10−1 M sodium chloride, respectively. The i.e.p. is found between pH 3.1 and 3.3. Some workers [18,19] have attributed shifts in the i.e.p. to the adsorption of anions and cations on the membrane surface but here the changes in the i.e.p. are small in comparison to the large change in electrolyte concentration and may arise from experimental error. As the pH is increased the ζ-potentials become more negative, with the maximum magnitudes being found at pH 7 (−25.3, −26.0, −29.4 mV for 10−1 , 10−2 and 10−3 M sodium chloride, respectively). From these results, it is seen that at constant sodium chloride concentration the sign of the ζ-potential can be significantly altered by varying pH, while at constant pH the changes in salt concentration do not have such a great effect. This observation is in agreement with other researchers [20] who studied the ζ-potential of composite membranes (aluminium oxide, Al2 O3 , titanium oxide, TiO2 and silica) and explained this type of behaviour in terms of the proton equilibrium that occurs at the surface of the membrane. The average magnitude of the ζ-potential decreases as the electrolyte concentration increases, a result which may be explained by the decrease in the effective thickness of the diffuse layer as the ionic strength increases so that in this system sodium chloride acts as an indifferent electrolyte. The filtration potential is an in situ measurement which is useful for indicating the sign of the membranes electrokinetic potential and in particular the i.e.p. As noted above, in principle the ζ-potential can be determined from the filtration potential but often this is not necessary because both ζ-potential and filtration potential will be zero at the i.e.p. which consequently can readily be detected from filtration potential data. In Fig. 7 shows the variation of the filtration potential with pH for three concentrations of sodium chloride (10−3 , 10−2 and 10−1 M). The i.e.p. is found between pH 3.5 and 3.6 which is higher than that found from the ζ-potential of ground membrane. This discrepancy may
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3.4. Correlation between salt rejection and the electrokinetic potentials
Fig. 7. The streaming potential (SP) of titanium dioxide membrane as a function of pH dispersed in different background electrolyte of sodium chloride (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
arise because of changes in the surface properties of the material caused in the communition process [1,7,17]. The grain size could also affect the electrokinetic characteristics and Moritz et al. [18] in an investigation using streaming potentials into the materials of construction of asymmetric ceramic membranes, having similar materials and construction to those used in the present study, show that the i.e.p. changes with the grain size of titanium dioxide found in the different layers of the membrane. Typically, the i.e.p. of the alumina support is found at pH 4, while the i.e.p. for the coarse titanium dioxide and fine titanium dioxide layers are found at around pH 3.5 and 3.8, respectively. Finally, they report that the i.e.p. for the complete membrane depends on the ionic strength of the electrolyte and varies slightly between pH 3.7 and 4.0 for 10−2 and 10−4 M KCl, respectively. In the present work, the low i.e.p. found using ζ-potentials could indicate that the membrane particles used in the mobility study could be fairly coarse as increase in size seems to lower the i.e.p. Apart from the dependence on salt concentration which is smaller than that reported by Moritz et al. [18], there is reasonable agreement however in the i.e.p. of the membranes found using streaming and filtration potentials. As noted previously the lowering of the i.e.p. with increasing salt concentration is explained simply in terms of the specific adsorption of weakly hydrated chlorine ions [18,19].
The surface charge of the material, which depends on the pH of the solution, is an important parameter realising the efficiency of a membrane separation process, especially when removing ionic species. It should be kept in mind that the Cl− anion has a smaller ionic radius (0.098 nm) than the Na+ cation (0.181 nm) and that both of them are smaller than the pore size membrane used (5.0 nm). Therefore, since the radius of membrane pore is large compared to the ionic radii, the rejection of electrolyte is not governed by the effect of size and the main mechanism responsible for the rejection of the salt is the electrostatic interactions between ions and the membrane surface. The electrokinetic potentials which are linked to its surface charge can be a useful indication of a membrane’s propensity for salt rejection. The rejection of sodium chloride and the ζ-potential of the membrane as a function of sodium concentration at the i.e.p. (∼pH 3) and also at pH 7 (neutral pH) is shown in Table 1. It is seen that, for the composite TiO2 /Al2 O3 membranes used in the present work, the surface has positive charge at low pH (at pH 3), so that the adsorption of cations, at least the H+ ions from water must be also taken into account and this may help to increase the rejection rate and in addition the rejection rate of the neutral salts associating a monovalent anion and a cation could be expected to decrease when the pH increases [13]. However, one of the main factors governing ion rejection by the membrane is the electrostatic repulsion between ions and membrane and thus changes in the ζ-potential and the filtration potential should be reflected by changes in salt rejection. In this study, the ζ-potential and the related filtration potential are shown to be functions of pH and salt concentration. Increased salt concentration lowers the ζ-potential by compressing the electrical double layer whilst change in pH alters the electrokinetic potential of amphoteric ceramic (metal oxide) membranes through reactions of the type [7,19,21]: MOH + H+ ↔ MOH2 + ↔ M+ + H2 O
(2)
MOH + OH− ↔ M(OH)2 − ↔ MO− + H2 O
(3)
The first and second reactions produce positively and negatively charged surfaces, respectively, thus the i.e.p. corresponds to the point where there is no net charge on the surface. In addition to and separate from the dissociation of the amphoteric oxide surface hydroxyl and hydronium groups may also be physically adsorbed thus altering the surface charge. As
Table 1 Rejection of sodium chloride and ζ-potential of the membrane as a function of sodium concentration at pH 3 and 7 NaCl concentration (M)
10−3 10−2 10−1
pH 3
pH 7
Rejection (%)
ζ-Potential (mV)
Rejection (%)
ζ-Potential (mV)
45.0 36.4 34.6
2.7 0.4 2.6
43.7 34.6 31.3
−29.4 −26.0 −25.3
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chloride due to the excess adsorption of chloride ions. Viewed as a sequence, the salt rejection rate decreases continually with increasing sodium chloride concentrations at every pH, a minimum in sodium chloride rejection is found at pH ∼ 4.0 for the composite TiO2 membrane, which is close to the pH of the i.e.p. of the membrane. For a simple electrolyte such as sodium chloride, the salt rejection rate depends mainly on the electrostatic interactions, which are controlled by the surface charge developed on the membrane. References
Fig. 8. Percentage of salt rejection as a function of pH in different concentrations of sodium chloride (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
there is no electrostatic repulsion between the membrane surface and ions when there is no surface charge a membrane will be most inefficient at its i.e.p. and this will be observed as a minimum in the salt rejection. In the present study the i.e.p. is around pH 3.2 (electrophoresis) and pH 3.6 (filtration potential), thus a minimum in the salt rejection is expected between pH 3 and 4. Fig. 8 shows the rejection of sodium chloride as a function of pH at three different concentrations of sodium chloride; this represents the sequence of the electrokinetic interactions developed between the ions and the membrane charge. The minimum in the salt rejection is found at about pH 4 which is broadly in agreement with the i.e.p. determined using filtration potentials [18]. It seems that the i.e.p. found in situ from filtration potentials is a better predictor of the pH of minimum rejection in a ceramic membrane. Since the minimum rejection is still about 15% close to the i.e.p., it is possible that electrostatic repulsion is not the only mechanism involved. 4. Conclusions The results of analysis using SEM images and an EDXS spectrum analyser used to determine the structure and composition of the membrane confirm that the membrane is of composite construction and is comprises an active surface made of TiO2 supported by three layers of Al2 O3 each of different thickness. In the presence of low concentrations of an indifferent electrolyte (NaCl) used to maintain a constant ionic strength, the ζ-potential varies with pH and is seen to be positive at low pH (
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Sodium chloride rejection by a UF ceramic membrane in relation to its surface electrical properties P. Narong, A.E. James ∗ School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK Received 11 June 2005; received in revised form 9 September 2005; accepted 15 September 2005
Abstract The rejection of salt (NaCl) by a ceramic (titanium dioxide–alumina layered) ultrafiltration membrane having a nominal pore size of 5 nm and being operated in the cross flow mode is investigated. Measurements were undertaken using different concentrations of salt over range of pH at relatively low transmembrane pressure (TMP). The rejection was assessed by determining the concentrations of sodium and chloride ions in the permeate and retentate and comparing them with the feed concentration. Rejections in the range 15–40% were found and at the same pH lower rejections are found at higher salt concentrations. Increased concentration reduces the rejection by up to 15%. Overall the rejection is seen to be a mainly function of pH which causes the rejection to vary by up to 50%. The observed rejection characteristics were compared with both filtration potential and ζ-potential data over the same range of pH and salt concentration. The filtration potential data was obtained in situ whilst the ζ-potentials were determined from electrophoretic measurements ground membrane material. The membranes are positively charged at low pH and negatively charged at high pH, the isoelectric points being pH 3.1 and 3.8 from electrophoretic and filtration potential measurements, respectively, the difference being attributed to differences in surface characteristics. The minimum rejection occurs at around pH 4 which corresponds to the isoelectric point (i.e.p.) found using filtration potentials showing that the electrostatic interactions between ions and the membrane surface are an important factor in salt rejection. © 2005 Elsevier B.V. All rights reserved. Keywords: Salt rejection; Ultrafiltration; ζ-Potential; Filtration potential
1. Introduction Clean water is very essential for all aspects of human life and in some regions can be difficult to obtain from wells or rivers in these situations brackish water and seawater can provide useful sources for purification. Pressure driven membrane processes are an important category of techniques for desalination of such waters. In particular, reverse osmosis (RO) is a well known method for producing desalinated water, but nanofiltration (NF) and even ultrafiltration (UF) can be used in the desalination process if the membrane pore diameters are less than about 10 nm [1]. Extensive research has been published about the rejection of ions and mechanisms for solute rejection by membranes [1–4]. In case of neutral RO membranes, the observed rejection of
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solute is generally very high and the membrane is considered to be permeable only to the solvent provided the transmembrane pressure is higher than the osmotic pressure. In case of mesoporous membranes, typically those employed in ultrafiltration, the pore diameter is larger than ion diameter and it has been found that salt rejection can be high even though neutral solutes are not rejected. These phenomena can be explained taking into account the charges which remain on the surface of the membrane pores thus the electrostatic interactions between ions and the membrane surface is important [1,2,5,6]. For seawater which is a mixture of salt and large charged organic molecules, it is found that the nominal monovalent salt rejection is usually negative. This has been exploited to remove salts from solution while concentrating dyestuffs, pharmaceuticals and whey sugars and protein. In a previous study involving reverse osmosis [3], the phenomenon of apparent negative rejection at the membrane has been reported and shown to arise from the effect of the Donnan distribution of the salt between the solution and the membrane. Applying Spiegler–Keden analysis to solute transport in the
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membrane and introducing a Donnan distribution factor, it is possible to calculate theoretical curves that qualitatively demonstrate the observed phenomenon of apparent negative rejection [4,5]. While there has been much interest in the rejection phenomenon in RO and also nanofiltration less attention has been paid to UF. Typically, [1] describe an investigation for the filtration of different saline solution using several ceramic UF membranes such as ␥ alumina, CoAl2 O4 and TiO2 /ZnAl2 O4 on the surface support in ␣ alumina. They remark that there is a correlation of the membrane selectivity in terms of rejection with the surface charge developed on the membrane material. The ability to predict the effect of the processes involved in the UF membrane separation would be very useful for planning and optimisation of system. Such predictions would ideally utilise available physical property data of a process stream and a membrane. For UF membranes, this involves the considering the structure of the membrane in terms of its pore size, materials of construction and electrokinetic properties such as the zeta potential (hereafter called ζ-potential) and the streaming potential. Electrokinetic phenomena allow the charge distribution at the interface between membrane surface and the fluid to be characterised. It is not possible to measure the ζ-potential directly and some model has to be invoked to link it to the measured electrokinetic data. The ζ-potential can be related to any of the four main electrokinetic phenomena, i.e. electrophoresis, streaming potential, electroosmosis and sedimentation potential. The first two of these are the basis for the most popular methods and the last is rarely used. Investigations of the electrophoretic mobility of particles in suspension characterise the particle’s net charge by measuring their velocity in an applied external electrical field. In contrast, the streaming potential results from forcing fluid through a porous medium and its value is that it can provide a useful in situ characterisation for solid charged surfaces such as those found in UF and microfiltration (MF) membranes. However, some authors acknowledge that, for some types of membrane, streaming potential measurements may be difficult to obtain and also that the results seem to be characteristic of the outer region of the porous surface [6]. There authors suggest that calculation of the ζ-potential may be impossible for composite membranes because often the internal pore size distribution and pore geometry are known inexactly and the influence of each part of the membrane contributes to the final result. To this end, they advocate the adoption a quantity which may be conveniently termed the flow through or filtration potential which is dimensionally the same as the streaming potential but reflects the overall relationship between the measured transmembrane electrical potential and pressure. It appears that this filtration potential characterises the overall influence of the surface potentials in a composite membrane and the electrolyte environment in which these different surfaces are immersed. Using these electrokinetic techniques, the variations in the net charge of a membrane surface and its isoelectric point (i.e.p.) can be measured as a function of the physical–chemical environment [7]. The performances of ceramic membranes in desalination depend on the concentration of ions and on the complexity of
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the solution to be filtered, but also on the type of membrane material. Typically, ceramic membranes used are generally made of metal oxides, which have an amphoteric behaviour, so that the electric surface charge depends on the pH of the feed solution [1,8–10]. Therefore, repulsion or attraction occurs between the ionic species present in the solution and the charged membrane. Beside the electrostatic effect, the size of the filtered species is also another parameter which should be considered, particularly if the solution contains complexed cations with mineral or organic materials. Asymmetric or composite membranes represent an important and popular type of membrane which are formed from two or more sublayers having different transport properties. Since these composite membranes may be constructed from different material the overall performance may be related to contributions from each of the different layers. For instance, if one sublayer shows high salt retention (as usually happens with active layers of commercial membranes like reverse osmosis and nanofiltration membranes), the permeate solution has much lower concentration than the feed solution. Generally, the higher or lower contribution of indirect effects in the parameters measured for a composite membrane is related to the different structures and transport properties of the sublayers forming the composite membrane [7,11]. In these cases, the separation efficiency of the UF membrane in the treatment of ionic solution can be explained by the combination of size and charge effect. The membrane charge depends on the pH of the solution and the i.e.p. of the membrane [1,12–14]. The aim of the present work is to investigate the relationships between salt rejection and electrokinetic potentials (ζ-potential and filtration potential) for a commercial UF ceramic membrane at different pH and electrolyte concentrations, since correlation of the membrane selectivity in terms of salt rejection with respect to electrokinetic potentials represents a direct way of relating the electrostatic properties of the membrane surface with its rejection characteristics.
2. Materials and methods 2.1. Membranes Tubular ceramic membranes (Schumasiv® single channel TI 01070, produced by USF Schumacher, Germany) were used. These are constructed so that the inorganic oxide particles comprising the inner surface layer of the circular tube determine the pore size of the membrane. This surface layer is supported by a more porous ceramic tube which can be made from the same or alternative metal oxide. Membranes having 0.005 m nominal pore sizes were selected for the present study and these are classified as UF membranes. The active inner surface is made of a layer of titanium dioxide which is deposited on a porous alumina support having a nominal pore diameter of 1.2 m. The overall dimensions for the membranes are: 6.67 mm i.d., 10 mm o.d. and 131 mm in length and the permeate area for each tubular membrane tube is approximately 2.75 × 10−3 m2 .
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Fig. 1. A schematic diagram of ultrafiltration rig for testing flux rejection properties.
2.2. Filtration experiments Sodium chloride solutions were prepared using a solid dried form of analytical grade sodium chloride (Sigma® –Aldrich, U.K.) with ultra pure water (ELGASTAT® Spectrum B unit Cartridge type: SC2 , USF Memcor, U.K.) with a resistivity of 18 M cm−1 and an organic contamination of less than 0.03 mg l−1 . Sodium chloride in the feed solution was prepared at different concentrations (10−1 , 10−2 and 10−3 M) which were run in the cross flow UF system schematically shown in Fig. 1. The main components were glass containers (6 dm3 ), a magnetic stirrer (RW20, IKAMAG® , U.K.), a variable speed peristaltic pump (Type 603S, Watson-Marlow Ltd., England), 9.6 mm flexible piping (Marprene, Watson Marlow, U.K.), reinforced PVC flexible piping and various fittings, cylindrical uPVC pressure dampening vessels and a rubber pressure dampening T-piece, a tubular UF membranes module, a uPVC valve (Georg Fischer, Switzerland), uPVC pieces to fit transducer connectors, a pressure relief valve pre-set at 5 bar (NABIC, U.K.). The permeate flow rate was determined using a measuring cylinder placed on top an analytical balance and a stop clock. Various pieces of piping, connectors and valves, a variable area flow meter (Gem¨u Gebr. M¨uller, Germany) complete the apparatus. In addition, the concentrations of sodium and chloride ions in the permeate were monitored. In this work, each filtration experiment was repeated a minimum of three times to ensure good reproducibility. The filtration rig was operated at using a cross flow velocity of 0.5 m s−1 at a transmembrane pressure (TMP) of 2.5 bar. The concentration of chloride anions in the permeate were determined using an argentometric titra-
tion method [15] while concentration of sodium cations were measured using inductive coupled plasma atomic emission spectrometry (ICP-AES) (Model: VISTA-MPX CCD Simultaneous ICP-OES, Australia). 2.3. Membrane characterisation A membrane sample was cut to a length of 2–3 mm to allow inspection of the membrane surface using a scanning electron microscope (FEI QUANTA 200, Purge, Czech Republic) operated at an accelerating voltage of 30 kV. In addition, an elemental analysis was carried using an energy dispersive X-ray spectrometer (EDXS). The ζ-potential of the membrane was measured as well as its filtration potential. Samples of membrane were cut and finely ground. The electrophoretic mobility of the ground particles was determined (Zetasizer 3000HS Advance, Malvern Instrument GmbH, U.K.) at three different ionic strengths of NaCl (10−1 , 10−2 , 10−3 M). In order to hold the ionic strength constant, the pH of the oxide suspensions was adjusted in the range pH 3–10 using 0.1 M H2 SO4 or NaOH where appropriate and the ζ-potentials of the particles were determined using the Helmholtz–Smoluchowski equation: ς=
µU εε0
(1)
where µ is the viscosity of the liquid, ε the dielectric constant, ε0 the permittivity of free space, U the electrokinetic mobility of the particle =u/E and u is the velocity of the particle in an electrical field of strength E. Although under certain limiting circumstances the ζ-potential may be derived from the filtra-
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tion potential by the application of appropriate theory but if one is interested only in detecting the i.e.p. of the system it is not necessary to attempt this conversion. In terms of membrane characterisation the filtration potential represents a non destructive in situ method in contrast electrophoresis measurements which require particles of the membrane material which can often only be obtained by destroying the membrane. This often involves destroying the membrane, although this would have to be done if a microscopic examination of the membrane surface, for instance to examine the effects of fouling, is required. In this study, measurement of the filtration potential were made on the membrane over a range of pH 3–7, because higher and lower pH compress the electrokinetic double layer, and moreover, high pH cause damaging of the electrodes, using several different concentration of NaCl (10−1 , 10−2 , 10−3 M). The electric potential difference over a range of pressure driving forces (0 bar ≤ TMP ≤ 2.0 bar) was measured using a pair of platinum electrodes [8] (Alfa Aesar, Johnson Matthey Chemical, U.K.) attached to a high impedance milli-voltmeter (Wavetek Meterman, RS Component, U.K.). One was placed along the central line of the tubular membrane and the other was wrapped around the outer wall of the membrane. The electrodes were connected to the voltmeter which recorded the potential difference generated by the electrolyte flow. The filtration potentials used in this study are defined as the instantaneous potential difference per unit difference of applied pressure (Φ/TMP). This is because the observed potential changes quite rapidly due to polarisation of the electrodes if it is measured continually and in the absence of fully reversible electrodes the use of the instantaneous potential provides a useful means of estimating both streaming and filtration potentials and good reproducibility is achieved using this methodology [7]. 3. Results and discussion 3.1. Effect of salt concentration The rejection factor, α, for different concentrations of sodium chloride at pH 7 and 3 are shown as a functions of time in Figs. 2 and 3. It is seen that the salt rejection increases with time for all concentrations of electrolyte used. At any time, the rejection is reduced as the feed concentration of salt is increased. A similar pattern has been reported by other workers who have compared modelled single salt rejection with experimental results as a function of pH for nanofiltration systems [16]. At pH 7, after 3000 s, the rejection factors rise to 0.31, 0.35 and 0.44 for feed concentrations 10−1 , 10−2 and 10−3 M sodium chloride, respectively, while at pH 3, the rejection factors rise to 0.35, 0.36 and 0.45 for feed concentrations 10−1 , 10−2 and 10−3 M sodium chloride, respectively. In general, all fluxes decay with time and the best flux is seen for 10−3 M sodium chloride and the worst for 10−1 M sodium chloride. As has been noted elsewhere [14], the permeate flux decays with time but in this study, it is beneficial to the separation process as this corresponds to an increase in the salt rejection. These observations broadly agree with those of other workers [12] who used a Carbosep M2 tubular ZrO2 membrane (UF membrane with a cut-off
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Fig. 2. Filtration of salt solution at a 2.5 bar pressure through the titania membrane: salt rejection factor, α, as a function of time in different concentrations of salt solution at pH 7 (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
value 15 kDa) in studies of phosphate rejection. Other workers [17] state that the permeate flux of all polymeric NF membranes, used to study rejection characteristics of organic and inorganic pollutants of surface water, decreased significantly during the time of operation. This is attributed to the accumulation of a fouling layer on membrane surface during the period of operation. 3.2. SEM and EDXS of membranes The SEM cross-section image of membrane surface is shown in Fig. 4. The membrane used in this study is a composite membrane having active side surface which is made from a thin layer of titanium dioxide and alumina that can be seen clearly. A typical membrane pore is indicated and three different membrane support layers the outer of which is made of sintered alumina are seen. Fig. 5 shows the specific spectrums of the active surface of the membrane obtained using EDXS. It
Fig. 3. Filtration of salt solution at a 2.5 bar pressure through the titania membrane: salt rejection factor, α, as a function of time in different concentrations of salt solution at pH 3 (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
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Fig. 4. SEM cross-section image of titanium dioxide UF membrane (magnification ×500).
is seen that the UF membrane used in this study contains five elements: oxygen, O (39.90 wt%); titanium, Ti (33.88 wt%); aluminium, Al (23.55 wt%); zirconium, Zr (1.84 wt%) and calcium; Ca (0.82 wt%) confirming the composition of the membrane. The Zr and Ca are low level contaminants originating from either the manufacturing process or from when the membrane section was cut. 3.3. Effect of pH and electrolyte concentration on the electrokinetic potentials of membrane The effect of pH on the ζ-potential of the titanium membrane particles as a function of increasing electrolyte pH and concentration of sodium chloride is shown in Fig. 6 for three different concentrations (10−3 , 10−2 and 10−1 M sodium chloride). The
Fig. 5. EDXS spectrum of titanium dioxide UF membrane.
Fig. 6. The ζ-potential of titanium dioxide membrane as a function of pH dispersed in different background electrolyte of sodium chloride (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
results show that the ζ-potentials of the membrane used in this study is positive at pH 3 being 2.7, 2.5 and 0.4 mV for 10−3 , 10−2 and 10−1 M sodium chloride, respectively. The i.e.p. is found between pH 3.1 and 3.3. Some workers [18,19] have attributed shifts in the i.e.p. to the adsorption of anions and cations on the membrane surface but here the changes in the i.e.p. are small in comparison to the large change in electrolyte concentration and may arise from experimental error. As the pH is increased the ζ-potentials become more negative, with the maximum magnitudes being found at pH 7 (−25.3, −26.0, −29.4 mV for 10−1 , 10−2 and 10−3 M sodium chloride, respectively). From these results, it is seen that at constant sodium chloride concentration the sign of the ζ-potential can be significantly altered by varying pH, while at constant pH the changes in salt concentration do not have such a great effect. This observation is in agreement with other researchers [20] who studied the ζ-potential of composite membranes (aluminium oxide, Al2 O3 , titanium oxide, TiO2 and silica) and explained this type of behaviour in terms of the proton equilibrium that occurs at the surface of the membrane. The average magnitude of the ζ-potential decreases as the electrolyte concentration increases, a result which may be explained by the decrease in the effective thickness of the diffuse layer as the ionic strength increases so that in this system sodium chloride acts as an indifferent electrolyte. The filtration potential is an in situ measurement which is useful for indicating the sign of the membranes electrokinetic potential and in particular the i.e.p. As noted above, in principle the ζ-potential can be determined from the filtration potential but often this is not necessary because both ζ-potential and filtration potential will be zero at the i.e.p. which consequently can readily be detected from filtration potential data. In Fig. 7 shows the variation of the filtration potential with pH for three concentrations of sodium chloride (10−3 , 10−2 and 10−1 M). The i.e.p. is found between pH 3.5 and 3.6 which is higher than that found from the ζ-potential of ground membrane. This discrepancy may
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3.4. Correlation between salt rejection and the electrokinetic potentials
Fig. 7. The streaming potential (SP) of titanium dioxide membrane as a function of pH dispersed in different background electrolyte of sodium chloride (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
arise because of changes in the surface properties of the material caused in the communition process [1,7,17]. The grain size could also affect the electrokinetic characteristics and Moritz et al. [18] in an investigation using streaming potentials into the materials of construction of asymmetric ceramic membranes, having similar materials and construction to those used in the present study, show that the i.e.p. changes with the grain size of titanium dioxide found in the different layers of the membrane. Typically, the i.e.p. of the alumina support is found at pH 4, while the i.e.p. for the coarse titanium dioxide and fine titanium dioxide layers are found at around pH 3.5 and 3.8, respectively. Finally, they report that the i.e.p. for the complete membrane depends on the ionic strength of the electrolyte and varies slightly between pH 3.7 and 4.0 for 10−2 and 10−4 M KCl, respectively. In the present work, the low i.e.p. found using ζ-potentials could indicate that the membrane particles used in the mobility study could be fairly coarse as increase in size seems to lower the i.e.p. Apart from the dependence on salt concentration which is smaller than that reported by Moritz et al. [18], there is reasonable agreement however in the i.e.p. of the membranes found using streaming and filtration potentials. As noted previously the lowering of the i.e.p. with increasing salt concentration is explained simply in terms of the specific adsorption of weakly hydrated chlorine ions [18,19].
The surface charge of the material, which depends on the pH of the solution, is an important parameter realising the efficiency of a membrane separation process, especially when removing ionic species. It should be kept in mind that the Cl− anion has a smaller ionic radius (0.098 nm) than the Na+ cation (0.181 nm) and that both of them are smaller than the pore size membrane used (5.0 nm). Therefore, since the radius of membrane pore is large compared to the ionic radii, the rejection of electrolyte is not governed by the effect of size and the main mechanism responsible for the rejection of the salt is the electrostatic interactions between ions and the membrane surface. The electrokinetic potentials which are linked to its surface charge can be a useful indication of a membrane’s propensity for salt rejection. The rejection of sodium chloride and the ζ-potential of the membrane as a function of sodium concentration at the i.e.p. (∼pH 3) and also at pH 7 (neutral pH) is shown in Table 1. It is seen that, for the composite TiO2 /Al2 O3 membranes used in the present work, the surface has positive charge at low pH (at pH 3), so that the adsorption of cations, at least the H+ ions from water must be also taken into account and this may help to increase the rejection rate and in addition the rejection rate of the neutral salts associating a monovalent anion and a cation could be expected to decrease when the pH increases [13]. However, one of the main factors governing ion rejection by the membrane is the electrostatic repulsion between ions and membrane and thus changes in the ζ-potential and the filtration potential should be reflected by changes in salt rejection. In this study, the ζ-potential and the related filtration potential are shown to be functions of pH and salt concentration. Increased salt concentration lowers the ζ-potential by compressing the electrical double layer whilst change in pH alters the electrokinetic potential of amphoteric ceramic (metal oxide) membranes through reactions of the type [7,19,21]: MOH + H+ ↔ MOH2 + ↔ M+ + H2 O
(2)
MOH + OH− ↔ M(OH)2 − ↔ MO− + H2 O
(3)
The first and second reactions produce positively and negatively charged surfaces, respectively, thus the i.e.p. corresponds to the point where there is no net charge on the surface. In addition to and separate from the dissociation of the amphoteric oxide surface hydroxyl and hydronium groups may also be physically adsorbed thus altering the surface charge. As
Table 1 Rejection of sodium chloride and ζ-potential of the membrane as a function of sodium concentration at pH 3 and 7 NaCl concentration (M)
10−3 10−2 10−1
pH 3
pH 7
Rejection (%)
ζ-Potential (mV)
Rejection (%)
ζ-Potential (mV)
45.0 36.4 34.6
2.7 0.4 2.6
43.7 34.6 31.3
−29.4 −26.0 −25.3
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chloride due to the excess adsorption of chloride ions. Viewed as a sequence, the salt rejection rate decreases continually with increasing sodium chloride concentrations at every pH, a minimum in sodium chloride rejection is found at pH ∼ 4.0 for the composite TiO2 membrane, which is close to the pH of the i.e.p. of the membrane. For a simple electrolyte such as sodium chloride, the salt rejection rate depends mainly on the electrostatic interactions, which are controlled by the surface charge developed on the membrane. References
Fig. 8. Percentage of salt rejection as a function of pH in different concentrations of sodium chloride (, 10−1 M NaCl; , 10−2 M NaCl; , 10−3 M NaCl).
there is no electrostatic repulsion between the membrane surface and ions when there is no surface charge a membrane will be most inefficient at its i.e.p. and this will be observed as a minimum in the salt rejection. In the present study the i.e.p. is around pH 3.2 (electrophoresis) and pH 3.6 (filtration potential), thus a minimum in the salt rejection is expected between pH 3 and 4. Fig. 8 shows the rejection of sodium chloride as a function of pH at three different concentrations of sodium chloride; this represents the sequence of the electrokinetic interactions developed between the ions and the membrane charge. The minimum in the salt rejection is found at about pH 4 which is broadly in agreement with the i.e.p. determined using filtration potentials [18]. It seems that the i.e.p. found in situ from filtration potentials is a better predictor of the pH of minimum rejection in a ceramic membrane. Since the minimum rejection is still about 15% close to the i.e.p., it is possible that electrostatic repulsion is not the only mechanism involved. 4. Conclusions The results of analysis using SEM images and an EDXS spectrum analyser used to determine the structure and composition of the membrane confirm that the membrane is of composite construction and is comprises an active surface made of TiO2 supported by three layers of Al2 O3 each of different thickness. In the presence of low concentrations of an indifferent electrolyte (NaCl) used to maintain a constant ionic strength, the ζ-potential varies with pH and is seen to be positive at low pH (
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