pH and ionic strength effects on the performance of ceramic membranes in water filtration

ELSEVIER JoumalofMembraneScience93 (1994) 91-103

joumal of

MEE2E

pH and ionic strength effects on the performance of ceramic membranes in water filtration

Faten F. Nazzal, Mark R. Wiesnef Department ofEnvironmental Science and Engineering, Rice University, P.O. Box 1892, Houston, TX 77251, USA

Received 30 December 1993; accepted in revised form 22 March 1994

Abstract

The effects of pH and ionic strength on the performance of ceramic microtiltration membranes are evaluated. An a-alumina membrane, characterized by a 0.8 pm nominal pore diameter, was operated in a crossflow configu- ration with three types of feed waters: particle-free electrolyte solutions, dispersions of mineral colloids and solutions of humic acid. The permeate flux was affected significantly by the pH of the feed water. The experimental results are interpreted in the context of electroviscous, visco-electric, and solvation effects. These results suggest that counterions exert an influence on the permeate flux at distances substantially greater than a Debye length from the pore walls.

Keywords: Ceramic membranes; Microtiltration; Electroviscous effects; Membrane Potentials; Electrochemistry

1. Illtroduction

Membrane filtration technologies are being applied with increasing frequency to treat water for potable use and to treat industrial and do- mestic wastewaters. The pH and ionic strength of the feed waters to membrane units are often altered in conjunction with pretreatment prac- tices aimed at protecting the membrane or enhancing membrane performance. Feed waters may be acidified to avoid precipitation of scale, iron or aluminum salts may be added to precip- itate otherwise soluble species such as phospho- rus and arsenic as well as to coagulate colloidal materials and improve permeate flux. In the latter case, research and practice have shown that

vorrespondingauthor. Tel.: (7 13)527-495 1; Fax: (713) 285- 5203; E mail: ENVI @ rice.edu.

coagulation pretreatment is essential for main- taining higher permeate flux using ceramic membranes [ l-41.

In this work we consider the effects of pH and ionic strength on the performance of a ceramic a-alumina membrane in treating suspensions of mineral colloids and humic materials.

2. Background

Ceramic membranes made from metal oxides, such as, Alu203, ZrO*, TiOz, and other materials are commercially available. Ceramic membranes typically exhibit excellent tolerance to pH extremes and oxidants as well as high mechani- cal strength. Moreover, because ceramic membranes are made of metal oxides, their composition is very well characterized, making them

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92 F.F. Nazzal, A4.R. Wiesner / Journal ofMembrane Science 93 (1994) 91-103

excellent research tools for the study of surface phenomena related to membrane permeability, fouling, and rejection. The oxide surfaces of these membranes are amphoteric and therefore their surface charge density and sign vary as a function of pH [ 5 1. For example, A&O3 is reported to have a zero point of charge near pH 9 [ 61. Hence, alumina membranes, in the absence of specifically adsorbed ions, are expected to have a net positive charge below this pH and should be negatively charged at higher values of pH. Surface complexation of ionic species may further modify the membrane charge. Surface charge and the related distribution of ionic species near pore surfaces within the matrix of the membrane can have a significant effect on the permeability of the membrane and adsorptive fouling.

The pH and ionic strength may also change the stability and conformation of colloidal species in the feed water with important implications for membrane-foulant interactions. The tendency for natural organic matter to sorb, the permeability of cake or gel layers deposited on membranes, and the potential for aggregation of colloids are all expected to be affected by pH and ionic strength.

Numerous observations of the effect of pH and ionic strength on permeate flux have been reported in the literature. Moosemiller et al. [ 71 reported that the permeation of particle-free electrolyte solutions through Al2O3 and TiOz membranes was greatest near the isoelectric points of these membranes. Hoogland et al. [ 8 ] reported that the permeate flux was at a minimum near pH 9 when filtering suspensions of SiOs on an A1203 membrane. The highest permeation rates occurred at pH 2 and pH greater than 10. McDonogh and coworkers [ 9, lo] reported on the effect of the zeta potential of silica particles on the specific resistance of cakes of silica colloids deposited on charged polymeric membranes. Cake resistance was observed to decrease with increasing zeta potential of the silica particles. They attributed reductions in the specific resistance of the cakes to tertiary electroviscous effects as manifested in a more porous spacing of particles within the cake due to increased diffuse layer repulsion.

Taken together, these studies suggest that numerous phenomena affected by pH and ionic strength may be responsible for the observed changes in permeation rates. In the following sections we briefly discuss possible mechanisms by which pH and ionic strength may affect the permeation rate of membranes used to treat raw waters containing natural organic materials and mineral colloids. It should also be noted that for small pore radii (reverse osmosis and nanoliltra- tion membranes), these mechanisms also have important implications for the rejection of ionic and macromolecular species.

2.1. Membrane permeability

Permeate flux, J, through a microporous membrane of thickness 6 with effective pore radius rpore can be modeled as Poisseuille flow through a bundle of capillaries:

(1)

where fis the fraction of open pore area on the membrane surface, and r is the pore tortuosity factor. pH and ionic strength may exert an effect on membrane permeability that can be interpreted as a change in the net pressure applied (AP), the pore (capillary) radius, or the viscosity of the water, p. Most approaches to describ- ing flow through porous media have described deviations from ideality in terms of changes in the viscosity of the permeating fluid near the capillary walls. Mechanistically, these changes can be ascribed to electrokinetic and solvation phenomena near the membrane surface.

Primary electroviscous effects on flow through porous media, including membranes, have been recognized for some time. These effects may be important, even for relatively porous membranes that do not reject ions. The flow of permeate through pores displaces ions that have ac- cumulated in the diffuse layer near the charged walls of membrane pores. As a result, an electri- cal potential is established that produces an ad-

F. F. Nazzal. M. R. Wiesner /Journal of Membrane Science 93 (I 994) 91- 103 93

ditional resistance to flow as back-migrating ions exert a drag force on the water moving through the membrane capillaries. The resultant permeate flux is lower than that predicted by Eq. ( 1). This reduced permeability can be interpreted as an increase in the apparent viscosity of the permeating water. The apparent viscosity, A, of the water flowing through charged capillaries can be related to the zeta potential, [, of the pore wall and the double layer thickness, 1 /K, by [ 111:

Pa

[

1 88”C’(l-G)F -l -= - P Krpore 1 (24

where fl is a dimensionless parameter that de- pends solely on electrolyte properties, given by:

P= ~*k*l%* 16w2&e,2 (2b)

T is the absolute temperature; k is the universal Boltzmann constant; E is the dielectric constant; e, is the proton charge; 2 is the conductivity of the electrolyte; K is the Debye-Hiickel parameter; 4 is a dimensionless zeta potential, r=e,,[/ kT; G is the ratio of the mean electrostatic potential across the pore to the zeta potential; F is a factor used to correct Smoluchowski’s equation for effects of high zeta potential and fine capillaries; and r,,, is the radius of the pores. Ionic strength affects apparent viscosity through its effects on both the Debye parameter, K, and the surface potential.

As pH changes and functional groups on membrane surfaces become more or less protonated, the surface charge density and surface potential on the membrane may change. When the magnitude of the surface potential increases, Eq. (2 ) predicts that the effective viscosity of the fluid moving through the membrane pores should increase, thereby reducing the permeate flux [ Eq. ( 1) 1. Eqs. ( 1) and (2) suggest that if primary electroviscous effects dominate, the membrane permeability should pass through a maximum near the zero point of charge of the membrane where the apparent viscosity is at a minimum. An increase in ionic strength reduces the Debye length and is therefore predicted to reduce the apparent viscosity. Thus Eqs. ( 1) and (2) also

predict that the permeate flux should be greater when filtering solutions of higher ionic strength. Finally, at a given ionic strength, counterions with smaller hydrated radii and therefore smaller drag coefftcients would be predicted to allow for greater membrane permeability as compared with ions of larger hydrated radii [ 12 1.

The presence of structured water or so-called solvation effects in membrane pores may also reduce permeate flux. Ordered water may be 2-3 molecules thick at a water-solid interface [ 13,141. Hydrogen bonding with functional groups on the membrane may extend this layer of structured water to several hundred molecular monolayers [ 141. The adsorption of hydropho- bic materials on surfaces greatly reduces the structured water film thickness. Structured water effects are predicted to increase with increasing surface charge density and have been observed to be somewhat insensitive to ionic strengths up to - lo-* M.

2.2. pH and ionic strength eflects on colloidal foulants

Deposition of colloidal materials on the membrane surface may block membrane pores and/ or create an additional layer of resistance to permeation. An extension of Darcy’s law is typically used to describe permeate flux as being proportional to the transmembrane pressure drop and inversely proportional to the sum of resistances presented by the membrane and deposited materials,

J= AP

/dRm +Rc +Rc,) (3)

where the membrane resistance, R,, the resistance of the cake or gel layer, R,, and the resistance of the concentration polarization layer, KP, are functions of time. Typically, R,, << R, and R, may be neglected. By comparison of Eq. (3) with Eq. (l),

Rm If=o=fz (4)

adsorption or deposition of materials over time

94 F.F. Nazzal, M.R. Wiesner / JournalofMembraneScience 93 (1994) 91-103

60

-40 - b

-60 1 I I

4 6

P6H

10 12

Fig. 1. The effect ofpH on zeta potential of alumina (ionic strength (IS): A, 10m3 M; A, 10m2 M) and silica (IS: 0, lo-’ M; 0, lo-‘M).

in membrane pores leads to a decrease in E or rpore and results in an increase in R, which is often difftcult to reverse. Irreversible losses in permeate flux are operationally referred to as fouling. Humic materials have been implicated as important factors in membrane fouling [ 1,15 1. The hydrophobicity and conformation of polye- lectrolytes, such as macromolecular humic material in water, are significantly affected by pH and ionic strength. Repulsion between deprotonated functional groups at higher pH values increases the hydrodynamic radius and resistance of an adsorbed humic molecule. Reduc- tions in charge at lower pH and/or charge screening at high ionic strength may reduce the size of these molecules while enhancing their adsorption on membrane surfaces. However, these effects may be mitigated by pH and ionic strength effects on the membrane surface.

As in the membrane, electroviscous and solvation effects may be factors in determining the resistance R, of a porous cake of deposited particles. This suggests a modification to Eq. ( 3 ) of the form (neglecting R,, ) :

J= AP

p ++&R, ( P >

where the effective viscosity in the deposited cake and membrane (p_ and pa,,, , respectively) are applied to each of the resistance terms.

The cake resistance will also be affected by the sizes of the materials making up the cake. By the Kozeny equation, a cake of a given thickness composed of larger particles will produce less resistance than a cake of the same thickness composed of smaller particles. Thus, changes in pH and/or ionic strength that lead to aggregation (high ionic strength, zero point of charge) may reduce the cake resistance and increase the permeation rate. On the other hand, tertiary electroviscous effects, diffuse layer interactions that alter the geometry of the system, may affect the cake permeability by increasing the cake poros- ity, as suggested by McDonogh and coworkers [ 9 1. The implication under this mechanism is that low ionic strengths and high particle surface

F.F. Nazzal, h4.R. Wiesner /Journal ofMembrane Science 93 (1994) 91-103

2000

IS-le-3M IS-le-2M

< ./ /. 3

20 40 60 80 100 120

95

Time (min)

Fig. 2 The effect of pH on permeate flux (electrolyte solution ) .

charge (pH values far from the zero point of charge of the suspended particles) should tend to lower the cake resistance and therefore increase the permeate flux.

A series of membrane filtration experiments was conducted in which the pH and ionic strength of synthetic feed waters was varied and the resultant effect on the permeate flux was evaluated.

3. Experimental

3.1. Filtration unit

The experiments were done on a bench scale filtration unit utilizing a single tubular cr-alumina ceramic membrane (U.S. Filter, Rockford, IL); 0.8 fl nominal pore size, 0.65 cm i.d. and 33.1 cm long. Flow to the bench-scale apparatus was provided by a 0.5 horsepower centrifugal pump. The flow rate through the tubular mem-

brane element was fixed at 30 cm3/s; at this rate, the flow is turbulent (Re= 6,000).

3.2. Feed water

Three types of dispersions adjusted for pH and ionic strength were introduced to the membrane: ( 1) particle-free electrolyte solutions, (2) silica suspensions, and ( 3 ) humic acid solutions.

Feed suspensions of silicon dioxide (Min-U- Sil 5, Mill Creek, OK, marketed by U.S. Silica Co.) particles were prepared by mixing 0.6 g of silica with ultrapure water deionized, low dissolved organic carbon water, filtered on a 0.22 pm filter (Milli-Q-System, Millipore ) 1. The silica particles had a number-averaged particle diameter of 1 +- 0.195 (urn.

A stock solution of humic acid (Aldrich) was prepared by mixing 0.45 g of humic acid with ultrapure water to yield a concentration of 300 mg/ 1. This stock solution was left to mix for 24 h at

F.F. Nazzal, M.R. Wiesner /Journal ofMembrane Science 93 (1994) 91-103

k . .

l IS-le-3 _> ‘; -I IS-1 e-2 c

Time(min)

Fig. 3. Effect of pH and ionic strength on permeate flux (silica suspension, C, = 100 mg/l )

pH 12 and was filtered on a 0.45 pm filter to insure that all the humic acid in the resultant stock solution was dissolved. The feed solutions in the experiments were prepared by mixing 200 ml of the stock solution with ultrapure water to obtain a solution of 10 mg/l humic materials measured as total organic carbon (TOC; Shimadzu TOC 500).

The pH of the suspension was adjusted by adding either HNO3 or NaOH. The ionic strength of the suspension was adjusted using NaN03. The experiments were performed at low ( - 10V3 M) and high ( - 10m2 M) ionic strengths.

3.3. Analytical methods

The electrophoretic mobility of silica particles and membrane fragments were measured mic- roscopically (Zeta-Meter Microscope Module, Zeta-Meter Inc., Long Island city, NY). The concentrations of humic acid in the rejectate and

permeate flows were measured as TOC. The particle size distributions in the feed, rejectate and permeate of the experiments on colloidal silica were characterized using an electronic particle counter (Coulter Multisizer, Coulter Electronics Inc., Hialeah, FL) which operates on the electri- cal sensing zone principle. All particle counts were performed using a 19 pm diameter orifice.

3.4. Experimental procedure

Prior to each experiment, the membrane was cleaned using 0.0 1 M NaOH solution ( pH N 12 ) . The caustic solution was circulated through the membrane for 30 min, then drained. The system was rinsed with ultrapure water, and then a solution of 0.01 M HCl (pH - 2) was circulated through the system for another 30 min. The system was rinsed again with ultrapure water until the pH of the rinse water reached a value of - 6. The permeate flux of the membrane was then

F.F. Nazzal, M.R. Wiesner /Journal ofMembraneScience 93 (1994) 91-103 91

0 2 4 6 8 10 12

PH

Fig. 4. Effect of pH and ionic strength on % deposition (silica suspension, C,= 100 mg/l; IS: q , lo-’ M, A, lo-’ M).

evaluated using ultrapure water. These clean water flux tests (CWFI) were performed before each experiment as a check on the membrane cleaning procedure and to insure that the initial membrane permeability was nearly identical for each experiment.

The experiments were carried out by introduc- ing the feed to the clean membrane at a feed pressure of 2 10 kPa. The flow to the system was adjusted using a calibrated rotameter. Permeate flow was effected at a constant transmembrane pressure of -200 kPa. The experiments were done at a temperature of 20 + 2 o C, and the measured permeation rates were adjusted to 20°C in the analysis. The unit was operated at the same setting of permeate flow, with pressure and permeate flux data recorded at 2-3 min intervals until the permeation rate reached a steady state condition. Rejectate and permeate samples were taken at the beginning and when conditions of stable permeate flux prevailed. After obtaining a

series of these measurements at one pH, the permeate valve was closed, and the feed water pH was adjusted to the next value. The feed water was then recycled through the system and the above procedure was repeated. This sequence of adjusting the pH and allowing the system to stabilize was repeated to obtain data on permeate flux at three pH values, each one lower than the previous. The three pH value sequence was performed once at an initial ionic strength of 10m3 M and once at an ionic strength of lo-* M. The experiments were carried out in duplicate to es- tablish reproducibility. In similar experiments, not reported here, pH and ionic strength were varied in no particular order, and the temperature was allowed to increase in the course of each experiment. The results from these experiments support the conclusions drawn from the constant temperature experiments reported here.

A single “fast flush” was done at the conclusion of each sequence of experiments. The fast

98 F. F. Nazzal, M. R. Wiesner /Journal of Membrane Science 93 (1994) 91- 103

2.5

2.0

I-

/ -

0.5 2

0

0

D

n Cm # n

4 6 8 10 12

PH

Fig. 5. Effect of pH and ionic strength on silica average particle diameter at 10m3 M ionic strength: 0, number average particle diameter (dp, ); n , volume average particle diameter (dp, ) ; and at 1 Ow2 M ionic strength: 0, dp,; 0, dp,.

flush procedure introduces a burst of high veloc- ity flow across the membrane that results in a pulse of materials removed from the surface of the membrane. These materials were collected for analysis.

4. Results and discussion

The zeta potential of membrane fragments and silica particles at the ionic strengths under study as a function of pH are shown in Fig. 1. These results are similar to those commonly reported for A1203 and silica with several exceptions [ 6 1. The isoelectric point (i.e.p. ) for the Alu203 particles was observed to be N 8.3. The literature values for the i.e.p. of suspensions of clean A1203 are near pH 9. Also in departure from observations with pure A1203, the ionic strength had lit-

tle effect on the zeta potential. The i.e.p. for silica is reported to be near pH 2. The calculated zeta potential for silica was relatively flat ( - 30 to -40 mV) over the range of 4 to 11; a steep increase at the lower pH values as has been reported for pure colloidal silica was not observed. Deviations from the ideal electrokinetic behavior of suspensions of clean alumina or silica are attributed to impurities or adsorbed materials on the silica and alumina surfaces studied in this work. The insensitivity of zeta potential to ionic strength suggests that a reduced sensitivity of phenomena such as electroviscosity to changes in ionic strength can be anticipated.

4.1. Particle-free electrolyte solutions

The permeate flux of particle-free electrolyte solutions filtered at a constant transmembrane

F.F. Nazzal, M.R. Wiesner /Journal ofMembraneScience 93 (1994) 91-103 99

2000 ,

t IS-le-3 I IS-1 e-2

1000 -

500 - b

0 0 20

9 9: I,

I

40

b#i

N d -i

I,

60 60

Time (min)

m

2 e P

100 120

Fig. 6. Effect of pH and ionic strength on permeate flux (humic solution, C,= 10 mg/l as TOC).

pressure through the cY-alumina membrane increased monotonically with decreasing pH at both ionic strengths (Fig. 2). For an ionic strength of N 10M3 M, the permeate flux at pH 4.3 was N 20% greater than that measured at pH 10. Although a membrane with a 0.8 pm nominal pore diameter will not reject ions to any significant degree, accumulation of ions near the pore walls within the membrane will produce electrokinetic effects that can potentially affect the membrane permeation. Electroviscous con- siderations [ Eq. (2 ) ] imply a maximum in permeate flux at the i.e.p. of the membrane. The effect of pH on permeate flux at an ionic strength of N 1 O-* M was similar to that observed at 1 0m3 M, but was less pronounced. This latter obser- vation is not inconsistent with an electroviscous effect. However, in contrast, the absolute value of permeate flux was higher at the lower ionic strength. Moreover, electroviscous effects are calculated to reduce the permeate flux by N 5%

or less under the conditions in these experiments. Higher ionic strengths are predicted to increase rather than reduce the permeate flux. Thus, these observations do not appear to con- form with the behavior of permeate flux as ex- plained by electroviscous effects alone.

The observed permeation behavior appears to be related to the relative composition of counterions within the membrane pores. The permeating water may experience a drag produced by hydrated counter- and co-ions near the pore walls. At the lower pH, when the membrane is posi- tively charged, NO, ions are the predominant counterions. These ions have a smaller hydrated radius than the Na+ ions that dominate the diffuse layer at higher pH values. At the i.e.p., the concentrations of Na+ and NO, ions in the diffuse layer are approximately equal. At a low ionic strength, hydroxide and hydronium ions play a more important role as counter- and co-ions in comparison with the higher ionic strengths where

100 F. F, Nazzal, M. R. Wiesner /Journal of Membrane Science 93 (I 994) 91- IO3

50

10

4 6 8 10 12

L % Rem

100

80

60 3

i

d

bp

40

20

0

PH

Fig. 7. Effect of pH on humic materials deposition and removal (ionic strength - 1 O-’ M).

the hydrated Na+ and NO, ions increasingly predominate in the diffuse layer. By this explanation, higher ionic strengths dampen the pH effect through diffuse layer compression, while decreasing the overall permeate flux due to co- and counter-ion substitution. However, to account for the magnitude of the observed changes in membrane permeability, this explanation implies an influence of counter-ions at distances substantially greater than a Debye length from the pore wall.

4.2. Silica suspensions

The permeation behavior of the membranes in filtering silica suspensions in electrolyte solutions was similar to that observed for the particle-free electrolyte solutions. In Fig. 3 the permeate flux observed in replicate experiments carried out with silica suspensions with a constant transmembrane pressure, at three pH val-

ues, and at two ionic strengths is plotted as a function of time. The permeate flux was observed to stabilize within several minutes after decreasing the pH. Each decrease in pH produced a higher stable permeate flux than that observed at the previous higher pH. Overall, a 20% increase in permeate flux was observed when the pH value was reduced from pH 10 to 4 at an ionic strength of 10e3 M. As with the particle-free electrolyte solutions, higher permeation rates were measured at the lower ionic strength. The trend of increasing permeation rate with decreasing ionic strength is further evidenced by comparing the initial permeate flux obtained using particle-free deionized water (pH - 7) with the permeate fluxes obtained in the experiments using either particle-free or silica-suspension electrolyte solutions.

Silica deposition on the membrane decreased the permeate flux to - 75% of the permeation rate obtained with the particle-free electrolyte solutions. Silica deposition ranged from - 5 to

F. F. Nazzal, M. R. Wiesner /Journal of Membrane Science 93 (1994) 91-103 101

% Rem.

4 6 8 10 1

/

100

60

Z

60 z

f

:

40

20

1

PH

Fig. 8. Effect of pH on humic materials deposition and removal (ionic strength - 1 O-’ M).

30% of the silica mass initially introduced in the feed suspension (Fig. 4). More silica was deposited and subsequently removed by fast flushing at the lower ionic strength. A lower permeation rate was observed at the higher ionic strength, despite a decrease in silica deposition on the membrane. When normalized for electrolyte effects alone on the membrane, the stable flux through the silica cake was nearly constant with respect to pH and ionic strength. Particle size did not change greatly during these experiments (Fig. 5). Evidence of some aggregation was observed at an ionic strength of I 0m2 M; the volume average diameter increased from - 2.1 to 2.2 ,um as the pH was decreased from 10 to 4 over time. These observations suggest that cake resistance was not a factor in the overall effect of pH and ionic strength on the permeation rate and that the silica particles reduced the permeate flux primarily by pore blockage.

4.3. Humic acid solution

The solutions of humic acid produced significant reductions in the permeate flux. The permeate flux was reduced to levels between 50 and 60% of the initial deionized water permeation rate (Fig, 6). In contrast with the silica suspensions and particle-free electrolyte solutions, the permeation rate in the presence of humic material was relatively insensitive to changes in pH and ionic strength. The data may suggest a minimum stable permeate flux near the i.e.p. of the membrane. The humic acid masks and modifies the effects of the pH and ionic strength on the membrane permeability. The relatively large reductions in the permeate flux produced by the humic acid obscures the changes in the membrane permeability due to the ionic strength and pH. Moreover, adsorption of humic acid on membrane pores modifies the surface chemistry of the membrane. The small but reproducible effects of the pH and ionic strength on the permea-

102 F.F. Nazzal, M.R. Wiesner / JournalofMembraneScience 93 (I 994) 91-103

tion rates may be due in part to the effects of these variables on the size and hydrophobicity of the humic molecules. Protonation of the functional groups at low pH decreases the hydrodynamic radii of humic materials, while increasing their hydrophobicity and tendency to adsorb. Larger amounts of humic material may adsorb at lower pH values. However, the adsorbed layer may present less resistance to flow per molecule of humic material due to its compact conformation. Near the i.e.p. of the membrane, adsorption of humic materials on the membrane may be enhanced. In addition, deprotonated functional groups on the humic acid may, due to the repulsion between groups, extend molecules adsorbed in the pores into the path of the permeating water. These factors, in combination with the solvation/electrokinetic effects on the membrane, produce a weak minimum in the permeation rate with respect to the pH at the i.e.p. of the membrane. In contrast, the pH effects on the membrane alone appear to partially offset the reductions in the permeate flux due to the adsorption of humic acid despite the enhanced adsorption of humic acid at the lower pH values (Figs. 7 and 8).

The influence of pH and ionic strength on the adsorption of humic acid on the membrane and on its removal across the membrane was evaluated by monitoring the TOC of the concentrate and permeate flows over time. The mass of humic acid adsorbed or deposited within the membrane system was calculated from the difference in the initial TOC of the feed solution and the TOC in the concentrate. The concentrate TOC was observed to decrease over time correspond- ing to the deposition or adsorption of humic acid on the surfaces in the membrane system. The percents of humic acid adsorbed reported in Figs. 7 and 8 are calculated from the measurements taken at the beginning and at the end of each pe- riod of stabilization following a readjustment of pH or ionic strength. Both the deposition and removal of humic materials decreased with increasing pH. This trend was observed for both ionic strengths. However, the effect of pH on deposition at the higher ionic strength was not as pronounced.

5. Summary and conclusions

The permeate flux of clean a-alumina membranes is affected significantly by the pH of the feed stream. The 0.8 pm membrane used in this work operated at a significantly higher permeation rate at a pH well below the i.e.p. of the membrane. This conclusion is at some variance with a previous study of ceramic membranes in which the membrane permeability was found to be at a maximum at the i.e.p. of the membrane. Both the permeation rate and its sensitivity to pH are reduced at higher ionic strengths.

These observations of membrane permeability cannot be accounted for in terms of electroviscosity alone. In the absence of other dissolved species and surface active materials, it appears that changes in the permeability of alumina membranes are largely due to changes in the composition of counterions that accumulate near the pore walls within the membrane. At a pH above the i.e.p. of the membrane, the counterions are cations with hydrated radii which are large and produce greater drag on the permeate flow than the anions that predominate in the diffuse layer below the i.e.p.. The magnitude of the observed effects suggests that the counterions influence the permeate flux at distances from the pore walls substantially greater than a Debye length.

The electrokinetic properties of silica deposits on the membranes played a negligible role in determining the membrane permeability. Al- though the presence of silica reduced the permeate flux, we attribute this primarily to the blockage of the membrane pores by silica particles. Had the permeate flux been reduced by the resistance of a deposited cake, an increase in the silica deposited on the membrane should have yielded a reduction in permeate flux.

The presence of humic materials in the feed water had a significant effect on the membrane permeation. By adsorbing to membrane surfaces, humic materials both impede the flow of the permeate and modify the surface chemistry of the membrane. In the presence of humic materials, the performance of ceramic membranes should be less sensitive to pH.

F. F. Nazzal, MR. Wiesner /Journal of Membrane Science 93 (1994) 91-103 103

Acknowledgments

This material is based in part upon work supported by the Energy Research in Program (ERAP- 1990) under 003604019.

Applications Grant No.

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pH and ionic strength effects on the performance of ceramic membranes in water filtration

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