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Humic acid fouling during microfiltration
Journal of Membrane Science 157 (1999) 1±12
Humic acid fouling during micro®ltration Wei Yuan, Andrew L. Zydney* Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA Received 24 July 1998; received in revised form 11 October 1998; accepted 16 October 1998
Abstract A major factor limiting the use of micro®ltration for surface water treatment is membrane fouling by natural organic matter. The extent and mechanisms of humic acid fouling during micro®ltration have been examined using stirred cell ®ltration experiments and scanning electron microscopy. The extent of fouling was strongly dependent on both the source and preparation of the humic acid solutions. The large ¯ux decline observed during constant pressure micro®ltration was caused by the formation of a humic acid deposit located on the upper surface of the membrane. Pre®ltration of the humic acid solutions dramatically reduced the rate of fouling through the removal of large humic acid aggregates. The initial fouling in this system was determined almost entirely by the convective deposition of these large particles/aggregates on the membrane surface. This initial deposit accelerated the subsequent rate of humic acid fouling, possibly serving as a nucleation site for deposition of macromolecular humic acids. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Fouling; Micro®ltration; Water treatment; Humic acid; Natural organic matter
1. Introduction Micro®ltration (MF) has been widely applied in drinking water treatment for the removal of particles, turbidity, and microorganisms from surface water and groundwater as an alternative to conventional water treatment processes (coagulation, sedimentation and sand ®ltration) [1,2]. Micro®ltration offers several advantages including superior water quality, easier control of operation, lower maintenance, and reduced sludge production. The greater removal of particles and microorganisms is of particular interest in meeting the more stringent requirements of the surface water
*Corresponding author. Tel.: +1-302-831-2399; fax: +1-302831-1048; e-mail: [email protected]
treatment rule (SWTR) and disinfectants/disinfection by-products regulations. One of the critical factors limiting the use of membrane ®ltration for drinking water treatment is membrane fouling, the irreversible loss of system productivity over time caused by interactions between the membrane and the various components in the process stream. Naturally occurring organic matter has been identi®ed as a major foulant in both surface and ground waters. For example, Mallevialle et al. [3] showed that the structure of the fouling layer formed during ultra®ltration or micro®ltration of natural river waters was determined largely by the organic matrix which served as a ``glue'' for inorganic constituents (e.g., iron, aluminum, silicon and calcium) in the fouling layer. Similar results were reported by Bersillon et al. [4] in their analysis of the deposited cake
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00329-9
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formed on the membrane surface after ®ltration of natural waters. Lahoussine-Turcaud et al. [5] studied the ultra®ltration of several different organic and inorganic macromolecules and concluded that organic substances like humic and tannic acids had a much greater effect on the ¯ux decline than did the inorganic colloids. The ¯ux decline observed during ultra®ltration of Seine River water was primarily caused by the deposition of naturally occurring organic macromolecules, and in particular humic materials, on the membrane surface [5]. Kaiya et al. [6] analyzed the composition of the deposited layer formed on a hollow ®ber micro®ltration membrane during ®ltration of water obtained from a eutrophic lake. Again, the dominant factor causing the ¯ux decline was the deposition of natural organic matter, in this case in combination with manganese, on the membrane surface. A major fraction of the natural organic matter (NOM) present in surface or ground waters is composed of humic substances (HS), which are complex macromolecular products of the chemical and biological degradation of plant and animal residues, including lignin, carbohydrates, and proteins [7]. Humic substances are conveniently divided into three categories: humic acids, fulvic acids, and humin according to their solubility in acidic solutions. Humic acids refer to the fraction of humic substances that is not soluble in water under acidic conditions (pH<2) but is soluble at higher pH. They are a heterogeneous mixture having both aromatic and aliphatic components and containing three main functional groups: carboxylic acids (COOH), phenolic alcohols (OH), and methoxy carbonyls (C=O) [8]. Previous studies of humic acid fouling have focused on their interactions with either fully retentive nano®ltration membranes or partially retentive ultra®ltration membranes. For example, Jucker and Clark [9] showed that humic acid adsorption on hydrophobic ultra®ltration membranes was greater at low pH, which was attributed to the reduction in net charge and the increase in hydrophobicity of humic acids caused by the neutralization of the acid side groups at low pH. Humic acid adsorption was also enhanced in the presence of Ca2, possibly due to electrostatic shielding by the divalent cation. Similar results were reported by Hong and Elimelech [10] during nano®ltration, with the rate of ¯ux decline controlled by the
magnitude of the electrostatic interactions in combination with the hydrodynamic drag due to the ®ltrate ¯ow. Experimental studies of humic acid fouling on micro®ltration membranes are more limited. Although it might be expected that humic acid fouling would be minimal on large pore size (>0.1 mm) micro®ltration membranes, NystroÈm et al. [7] found a rapid decline in ¯ux during humic acid ®ltration through 1.9 mm pore diameter inorganic capillary membranes. For example, the ¯ux for a 10 ppm humic acid solution decreased by 50% after only 5 min of ®ltration at 2 bar, while the ¯ux for a 100 ppm humic acid solution decreased by more than 90% within 5 min. NystroÈm et al. attributed this dramatic ¯ux decline to electrostatic interactions between the negatively charged humic acid and the positively charged inorganic membrane, resulting in the nearly complete blockage of the membrane pores. The detailed mechanisms governing the blockage of such large pores by the humic macromolecules were not determined. The overall goal of this study was to develop a more fundamental understanding of the mechanisms governing humic acid fouling during micro®ltration. Experiments were thus designed to: (1) quantify the rate and extent of ¯ux decline through different micro®ltration membranes by commercially available humic acids under well-de®ned ®ltration conditions; (2) characterize the molecular weight distribution of the different humic acid solutions and its in¯uence on fouling; (3) use scanning electron microscopy to characterize the nature of the humic acid deposit formed on the membrane surface; and (4) use these results to obtain insights into the underlying physical phenomena governing humic acid fouling during micro®ltration. 2. Experimental The bulk of the experimental studies were performed using a humic acid obtained from Aldrich (Milwaukee, WI). Limited data were also obtained with a Suwannee River humic acid obtained from the International Humic Substances Society (St. Paul, MN). Humic acid solutions were prepared by dissolving pre-weighed quantities of humic acid powder in 2 l of ®ltered deionized (DI) water (pre®ltered through
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a 0.2 mm ®lter) obtained from a NANOPure water puri®cation system (Barnstead, Dubuque, IA) with resistivity greater than 18 M cm. The solution pH was adjusted to 7.00.1 by addition of small quantities of 0.1 M NaOH or HCl as needed. pH was measured using an Acumet 915 pH meter (Fischer Scienti®c). Humic acid concentrations were evaluated spectrophotometrically using a Perkin-Elmer Lambda 4B spectrophotometer (Perkin Elmer, Norwark, CT) with the absorbance measured at 254 nm. The overall humic acid concentration was determined by comparison of the absorbance data with an appropriate calibration curve. The apparent molecular weight (MW) distribution of the humic acid samples was determined using the ultra®ltration fractionation method originally developed by Aiken [11]. Humic acid samples were fractionated in a 25 mm diameter stirred cell (Model 8010, Amicon, Beverly, MA) using a series of OMEGA polyethersulfone ultra®ltration membranes (Filtron Technology, Northborough, MA) with nominal molecular weight cut-offs of 3, 10, 30, 50, 100, 300 kD, and 1 M. The ®ltration was performed at a constant pressure of 207 kPa (30 psi), with the fractional amount of humic acid within each size range calculated from the difference in humic acid concentration between adjacent ®ltrate samples (e.g., the difference in concentration between the ®ltrates from the 30 and 10 kD membranes). In order to obtain additional insights into the fouling behavior, a series of experiments were performed with humic acid solutions that had ®rst been pre®ltered through a 100 kD, 300 kD, 1 M, or 0.16 mm polyethersulfone membrane (Filtron Technology, Northborough, MA) to remove the larger molecular weight components. This pre®ltration was done using 60 mg/l humic acid solutions at 69 kPa (10 psi) in a 64 mm diameter stirred cell (Model 8200, Amicon, Beverly, MA) at a stirring speed of 350 rpm. Each membrane was used for about 500 ml of ®ltration. The collected ®ltrate was then diluted with ®ltered DI water to obtain the desired concentration. Most ®ltration experiments employed 0.16 mm pore size OMEGA polyethersulfone (PES) micro®ltration membranes provided by Filtron Technology (Northborough, MA). These PES membranes have an asymmetric structure with an upper skin approximately 0.5 mm in thickness providing the membrane its
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permselectivity. Limited data were also obtained with 0.22 mm Durapore polyvinylidene ¯uoride membranes (Millipore, Bedford, MA) and 0.2 mm tracketched polycarbonate membranes (Poretics, Livermore, CA). The membranes were placed in a 25 mm diameter stirred cell (Model 8010, Amicon, Beverly, MA) connected to an air-pressurized 2 l solution reservoir. The stirred cell and reservoir were initially ®lled with ®ltered DI water. Each membrane was ¯ushed with at least 50 ml of ®ltered DI water prior to use to remove any wetting agents. The water ¯ux was then measured as a function of time at a constant pressure until a quasi-steady ¯ux was attained (usually within 10 min). The stirred cell and reservoir were then emptied and re®lled with a humic acid solution. The system was repressurized to the initial pressure, and the stirring speed was set to 600 rpm using a Strobotac type 1531-AB strobe light (General Radio, Concord, MA). The ®ltrate ¯ow rate was measured by timed collection, with ®ltrate samples collected periodically for subsequent analysis. At the end of the experiment, the stirred cell and membrane were gently rinsed with ®ltered DI water, and the water ¯ux through the fouled membrane was reevaluated at the same pressure used during the humic acid ®ltration. All experiments were conducted at room temperature (2228C). Selected membranes were examined by scanning electron microscopy (SEM). The membranes were ®rst ¯ushed with at least 200 ml of ®ltered deionized water to remove any labile humic acid. The membranes were dehydrated by repeated washing in ethanol/water solutions of increasing ethanol concentration, with the ®nal washing performed using pure ethanol. The membranes were then mounted on specimen studs using carbon paste, sputtered with gold under an argon environment, and imaged with a JEOL JXA-840 SEM at 5.0 kV. 3. Results Fig. 1 shows experimental data for the ®ltrate ¯ux as a function of time for the ®ltration of 2 mg/l solutions of the Aldrich and Suwannee River humic acids. Two runs are shown for the Aldrich humic acid performed under identical conditions but using two
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Fig. 1. Normalized filtrate flux during filtration of 2 mg/l solutions of the Aldrich and Suwannee River humic acids through 0.16 mm polyethersulfone microfiltration membranes at 69 kPa (10 psi).
Fig. 2. Apparent molecular weight distribution for solutions of the Aldrich and Suwannee River humic acids at pH 7.0.
separate membranes. All of the data were obtained at a constant pressure of 69 kPa (10 psi) and a stirring speed of 600 rpm. In each case, the ®ltrate ¯ux has been normalized by J0, the steady state water ¯ux evaluated just before switching to the humic acid solution. The J0 values for the three membranes varied between 1.4210ÿ3 and 1.5810ÿ3 m/s, with this small difference re¯ecting the inherent variability in membrane properties. The two runs with the Aldrich humic acid were essentially identical, with reproducibility better than 10%. The ®ltrate ¯ux declined much more rapidly for the Aldrich humic acid, with the ¯ux decreasing to 10% of its initial value within the ®rst 20 min of ®ltration while J/J00.23 after 120 min of ®ltration for the Suwannee River humic acid. This large difference in fouling behavior was likely due to the different physical and chemical characteristics of the humic acids. The Suwannee River humic acid has a greater concentration of functional groups (i.e., carboxylic acids), which makes it more negatively charged and more hydrophilic than the Aldrich humic acid that is obtained from soil [12,13]. In addition, the Aldrich humic acid had a greater percentage of larger molecular weight components as determined by the ultra®ltration fractionation (Fig. 2). The error bars in the ®gure represent the standard deviation as determined from repeat measurements on the same humic acid sample (evaluated with a separate clean set of ultra®ltration membranes). For example, more than 60% of the Aldrich humic acid had a molecular weight greater than 50 000, while only 20% of the Suwannee River humic acid had that
large a molecular weight. As discussed subsequently, these large molecular weight components, and in particular the fraction with molecular weight greater than 1 M, play a critical role in humic acid fouling during micro®ltration. Although the Suwannee River humic acid is more representative of most aqueous systems, we decided to examine the fouling behavior of the Aldrich humic acid in more detail in order to determine the origin of the dramatic ¯ux decline seen in Fig. 1. Fig. 3 shows experimental data for the ®ltrate ¯ux (top panel) and observed rejection coef®cient (lower panel) as a function of time for the ®ltration of 2 mg/l solutions of the Aldrich humic acid through several different micro®ltration membranes: a 0.16 mm polyethersulfone (PES) membrane, a 0.22 mm polyvinylidene ¯uoride (PVDF) membrane, and a 0.2 mm track-etched polycarbonate (PCTE) membrane. The J0 values for the three membranes were similar: 1.310ÿ3 m/s for the PES membrane, 1.010ÿ3 m/s for the PVDF membrane and 0.9510ÿ3 m/s for the PCTE membrane. The ®ltrate ¯ux declined rapidly for all three membranes, with J/J0 decreasing to less than 10% of its initial value within the ®rst 20 min. The most rapid ¯ux decline was seen with the PCTE membrane, which was likely due to the smaller porosity (<10%) for this track-etched membrane compared to the other polymer membranes (porosity70%). The ¯ux at long times appeared to reach a quasisteady value, ranging from Jss4.310ÿ5 m/s for the PCTE membrane to 5.610ÿ5 m/s for the PES membrane. All three micro®ltration membranes were
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Fig. 4. Water flux as a function of transmembrane pressure through a single 0.16 mm polyethersulfone microfiltration membrane after different stages in the filtration of a 15 mg/l humic acid solution. Detailed conditions are described in text.
Fig. 3. Normalized filtrate flux and observed rejection coefficient during filtration of a 2 mg/l solution of the Aldrich humic acid through polyethersulfone (PES); polyvinylidene fluoride (PVDF); and polycarbonate (PCTE) microfiltration membranes at 69 kPa (10 psi), with J01.310ÿ3 m/s (PES); 1.010ÿ3 m/s (PVDF); 0.9510ÿ3 m/s (PCTE).
essentially non-retentive to the humic acids at the start of the ®ltration, with the humic acid concentration in the ®ltrate being essentially identical to that in the bulk solution. However, the apparent rejection coef®cient, de®ned as one minus the ratio of the ®ltrate to bulk humic acid concentration at each time, increased rapidly during the micro®ltration approaching a value of 87% for the PCTE membrane and as much as 95% for the PES membrane. In order to understand the origin of the large ¯ux decline seen in Fig. 3, a series of experiments were performed to measure the hydraulic permeability of the PES membrane after different fouling steps. Fig. 4 shows data for the ®ltrate ¯ux of DI water through a single PES membrane: (1) clean; (2) after static exposure to a 15 mg/l humic acid solution at 48C for 24 h; (3) after ®ltration of a 15 mg/l humic acid solution at 69 kPa (10 psi) for 2 h; (4) after physical
cleaning; (5) after chemical cleaning using 0.1 N NaOH for 30 min. The data for the clean membrane (after ¯ushing with DI water) were highly linear (r20.997) indicating that the membrane was incompressible over this pressure range. The membrane hydraulic permeability Lp, Lp
J ; P
(1)
where is the water viscosity at room temperature; J the ®ltrate ¯ux; and P is the transmembrane pressure, was thus a constant with a value Lp1.9 10ÿ9 m. The ®ltrate ¯ux data after static adsorption in a 15 mg/l humic acid solution were also linear (r20.994), with the permeability of the pre-adsorbed membrane approximately 10% smaller than that for the clean membrane. A similar reduction in ¯ux was seen after humic acid adsorption on the PVDF and PCTE membranes, even at humic acid concentrations as high as 50 mg/l. This reduction in membrane hydraulic permeability would correspond to a pore Ê as evaluated using constriction of approximately 80 A the Hagen±Poiseuille equation for ¯ow through a model PES membrane with uniform cylindrical pores of 0.16 mm diameter. This type of pore constriction is reasonably consistent with a monolayer of humic Ê as deteracids with effective size from 30 to 100 A mined previously by electron microscopy [8]. The preadsorbed membrane was then used to ®lter a 15 mg/l humic acid solution at a constant pressure of 69 kPa
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(10 psi) for 120 min. The membrane was then gently rinsed with ®ltered deionized water to remove any labile humic acid before being returned to the stirred cell. This ``fouled'' membrane had a de®nite yellowish color caused by the presence of a humic acid deposit on the membrane surface (discussed in more detail subsequently). The hydraulic permeability of the fouled membrane was 1.510ÿ10 m, more than an order of magnitude smaller than that for the clean membrane. In addition, the water ¯ux at each pressure showed a small (approximately 20%) transient decline over about 20 min; the data in Fig. 4 are the steadystate values at each pressure. Similar behavior has been reported by Opong and Zydney [14] for protein deposits formed during micro®ltration and is likely due to the compressibility of the deposit. Also note that the water ¯ux at 69 kPa was about 30% higher than the ®ltrate ¯ux obtained during the humic acid ®ltration at this pressure. This small increase in ¯ux was seen after all of the humic acid ®ltrations and is likely due to the absence of bulk concentration polarization and/or the removal of weakly bound humic acids in switching from the humic acid solution to water. After measuring the water ¯ux, the surface of the fouled membrane was carefully wiped with several paper towels. This physical cleaning removed essentially all of the yellowish deposit, and it returned the hydraulic permeability to nearly 80% of its initial (clean) value. In addition, the water ¯ux was again linear, indicating a constant permeability. This dramatic increase in ¯ux indicated that the bulk of the fouling occurred on the upper surface of the membrane and was not located throughout the membrane pore structure (where it would have been inaccessible to the physical cleaning). The membrane was then removed from the stirred cell, soaked in a 0.1 N NaOH solution for 30 min, rinsed in deionized water and returned to the stirred cell. The hydraulic permeability of this chemically-cleaned membrane was Lp1.7 10ÿ9 m, which was only about 8% smaller than that determined initially for the clean membrane. Thus, the NaOH cleaning was able to remove most of the small amount of residual (internal) humic acid from the membrane pores. In order to further quantify the nature of the fouling deposit, scanning electron micrographs were obtained of the top (feed side) surface of 0.16 mm PES membranes after different ®ltration times. Each
SEM was obtained with a separate membrane used to ®lter a 2 mg/l humic acid solution at a constant pressure of 69 kPa (10 psi). Fig. 5 shows the SEMs for the clean membrane and for the membranes after t1 min (J/J00.87); 5 min (J/J00.5); 20 min (J/J00.1); and 120 min (J/J00.04) of ®ltration. Even after only 1 min of ®ltration, the membrane surface was partially covered by several large particles or aggregates, although most of the pores remained unblocked as expected from the relatively small (13%) decline in ®ltrate ¯ux. The surface after 5 min of ®ltration was covered by a greater number of aggregates along with a more amorphous layer (at least at the resolution of the SEMs). By the end of the 120 min of ®ltration, the membrane surface was almost completely covered by the humic acid deposit, consistent with the large (96%) reduction in ®ltrate ¯ux. The effects of bulk concentration and applied transmembrane pressure on humic acid fouling of the 0.16 mm PES micro®ltration membranes are shown in Fig. 6. The top panel shows data with an initial feed concentration from 2 to 30 mg/l all obtained at P20 kPa (3 psi), while the bottom panel shows data at P from 20 to 103 kPa (3±15 psi) with Cb2 mg/l. Each experiment was performed using a separate (fresh) membrane with similar initial water permeability. The rate of ¯ux decline clearly increased with increasing humic acid concentration; however, the quasi-steady ¯ux attained at long ®ltration times was essentially independent of the concentration, varying from 8.010ÿ5 to 9.210ÿ5 m/s over the 15-fold range of humic acid concentration. The initial ®ltrate ¯ux increased essentially linearly with increasing pressure, but the rate of ¯ux decline also increased sharply with increasing P. The net result was that the run at 15 psi had the lowest ®ltrate ¯ux for t10 to 20 min. The quasi-steady ¯ux was a weak function of the applied pressure, varying from 6.510ÿ5 to 9.210ÿ5 m/s with slightly lower values obtained at higher pressures. Similar behavior has been reported by Palecek and Zydney [15] for protein micro®ltration, with the quasi-steady ¯ux attributed to the absence of fouling when the hydrodynamic drag force towards the membrane was balanced by the repulsive (electrostatic) interactions between the bulk protein and the protein deposit on the membrane surface.
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Fig. 5. Scanning electron micrographs of the upper surface of polyethersulfone microfiltration membranes used to filter 2 mg/l humic acid solutions at 69 kPa for different periods of time (a) clear membrane, (b) t1 min, J/J00.87, (c) t5 min, J/J00.5, (d) t20 min, J/J00.1, and (e) t120 min, J/J00.04.
The data in Fig. 6 were used to evaluate the instantaneous rate of ¯ux decline Kÿ
1 dJ J dt
(2)
with the derivative dJ/dt evaluated numerically using a ®nite difference representation accurate to order (t)2. The results for the initial rate of ¯ux decline are shown in Fig. 7 as a function of the initial rate of solute
transport to the membrane (J0Cb). The data for the experiments at different Cb and P collapse to a single line when plotted in this manner, with r20.97 and a slope of 17 m2/g. These results clearly indicate that the initial rate of ¯ux decline is due to the convective deposition of the humic acids, or more likely the humic acid aggregates or particles, on the membrane surface as opposed to any adsorption or kinetically-limited fouling process (which would be unlikely to vary linearly with J0Cb).
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Fig. 8. Effect of prefiltration (through 100 kD, 300 kD, 1 M or 0.16 mm polyethersulfone membranes) on the flux decline during filtration of a 2 mg/l humic acid solution through a 0.16 mm polyethersulfone microfiltration membrane at 69 kPa (10 psi). J0 ranged from 1.310ÿ3 to 1.510ÿ3 m/s.
Fig. 6. Effect of bulk concentration (top panel) and transmembrane pressure drop (bottom panel) on the flux during humic acid microfiltration. For runs at different Cb, J03.810ÿ4 (2 mg/l); 4.410ÿ4 m/s (5 mg/l); 3.910ÿ4 m/s (15 mg/l); 4.510ÿ4 m/s (30 mg/l). For runs at different P, J03.810ÿ4 m/s (3 psi); 5.910 ÿ4 m/s (5 psi); 8.310 ÿ4 m/s (8 psi); 1.310 ÿ3 m/s (10 psi); 1.810ÿ3 m/s (15 psi).
Fig. 7. Initial rate of flux decline as a function of the initial convective mass flux towards the membrane for filtration experiments shown in Fig. 6.
In order to obtain additional insights into the fouling mechanisms, a series of experiments were performed using humic acid solutions that had ®rst been pre®ltered through different pore size membranes prior to use in the micro®ltration experiment. In each case, a 60 mg/l humic acid solution was used for the pre®ltration, with the concentration in the collected ®ltrate adjusted to 2 mg/l by dilution with ®ltered deionized water. These pre®ltered solutions were stored at 48C until use (within 4 h of the initial pre®ltration). The micro®ltration experiments were performed at 69 kPa (10 psi) using 0.16 mm PES membranes having essentially identical water permeability, with the results for the normalized ¯ux shown in Fig. 8. The pre®ltration signi®cantly reduced the rate and extent of fouling compared to that for the un®ltered (standard) humic acid solution, suggesting that the components responsible for the bulk of the rapid initial fouling were removed by the pre®ltration step. The magnitude of the improvement in ¯ux increased as the pre®ltration was conducted through smaller pore size (i.e., smaller nominal molecular weight cut-off) membranes, with the ¯ux for the 100 kD pre®ltered solution dropping by less than 50% after 120 min of ®ltration (8500 l/m2 of membrane area). Scanning electron micrographs of the membranes used with the pre-®ltered solutions showed a complete absence of large aggregates, although an amorphous layer was seen over certain regions of the membrane (Fig. 9). The pre®ltration
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Fig. 10. Effect of storage conditions on the flux decline through 0.16 mm polyethersulfone microfiltration membranes at 69 kPa (10 psi). J0 ranged from 1.310ÿ3 to 1.510ÿ3 m/s.
Fig. 11. Apparent molecular weight distribution for 100 kD prefiltered solutions at pH 7.0 after different storage conditions. Fig. 9. Scanning electron micrographs of the upper surface of the polyethersulfone microfiltration membranes used to filter 2 mg/l solutions at 69 kPa for 2 h of (a) 100 kD prefiltered humic acid and (b) 300 kD prefiltered humic acid.
also altered the apparent molecular weight distribution of the humic acid solutions; the percentage of humic acids with MW greater than 300 kD decreased from 36% for the fresh (un®ltered solution) to 22% for the 0.16 mm pre®ltered solution and less than 1% for the 100 kD pre®ltered solution. Fig. 10 shows data for the normalized ¯ux for 2 mg/ l solutions of the 100 kD pre®ltered humic acid solution after storage for 24 h at either 508C or 48C in deionized water or at 48C in the presence of 1 mM Ca2. Each solution was stored at a humic acid concentration of 10 mg/l and was diluted and returned
to room temperature (2228C) immediately prior to the micro®ltration. Thus, the solution stored with 1 mM Ca2 actually had a 0.2 mM CaCl2 concentration during the micro®ltration. The rate of ¯ux decline for the solutions stored at 508C and at 48C in the presence of Ca2 was quite dramatic, with the ¯ux decreasing by more than 90% within the ®rst 60 min of ®ltration. This large increase in fouling was due to the re-aggregation of the humic acids as shown by the change in the apparent molecular distributions for these stored solutions (Fig. 11). For example, the percentage of humic acids with MW greater than 300 kD increased from only 1% for the fresh 100 kD pre®ltered solution to 11% for the solution stored at 508C and about 9% for the solution stored at 48C in the presence of 1 mM Ca2. Storage of the
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Fig. 12. Normalized filtrate flux during sequential filtration experiments using a 2 mg/l standard humic acid solution at 69 kPa (10 psi) followed by either DI water, a 2 mg/l 100 kD prefiltered humic acid solution, or a 2 mg/l standard humic acid solution.
Fig. 13. Normalized filtrate flux during the filtration of 2 mg/l mixtures of the standard and 100 kD prefiltered humic acid solutions through 0.16 mm polyethersulfone microfiltration membranes at 69 kPa (10 psi). J 0 ranged from 1.310 ÿ3 to 1.510ÿ3 m/s.
100 kD pre®ltered solution at 48C without Ca2 caused almost no increase in fouling, consistent with the absence of any signi®cant reaggregation of this humic acid solution (Fig. 11). These results indicate that both Ca2 and high temperature increase the rate of humic acid aggregation, which in turn increases the fouling seen during humic acid micro®ltration. Fig. 12 shows the results of a series of ``sequential'' ®ltration experiments. In each case, a standard (un®ltered) 2 mg/l humic acid solution was ®rst ®ltered through a 0.16 mm membrane at 69 kPa (10 psi) for about 10 min (until J/J00.25). The stirred cell was then emptied, the membrane rinsed with ®ltered DI water, and the system re®lled with either DI water, a 2 mg/l solution of a 100 kD pre®ltered humic acid, or a 2 mg/l solution of fresh (un®ltered) humic acid. The pressure and stirring speed were reset at their original values, and the ®ltration continued for an additional 110 min. The ¯ux at the start of this second ®ltration was slightly larger than that obtained at the end of the ®rst ®ltration, even when the second ®ltration was performed with the standard humic acid solution. This slight increase in ¯ux was likely due to some decompression of the fouling deposit and/or the removal of labile humic acid from the deposit during rinsing. The ¯ux for the deionized water remained essentially constant throughout the second ®ltration step as expected. However, the ¯ux for the 100 kD pre®ltered
humic acid solution declined by more than a factor of 4 over 110 min of ®ltration, attaining a quasi-steady ¯ux that was nearly identical to that for the standard (un®ltered) solution. This is in contrast to the relatively small decline in ¯ux (less than two-fold) seen during ®ltration of the 100 kD pre®ltered humic acid solution under the same conditions but without the initial ®ltration of the standard humic acid solution (Fig. 8). These data clearly indicate that the 100 kD pre®ltered humic acid solution was able to foul the membrane much more extensively after the membrane was ®rst fouled by a standard (un®ltered) solution, suggesting that the deposition of large particles or aggregates on the membrane surface is able to accelerate, or catalyze, the addition of smaller humic acid macromolecules to the growing deposit. Fig. 13 shows the fouling behavior for mixtures of the standard and 100 kD pre®ltered solutions, each with a total humic acid concentration of 2 mg/l. Although the rate of ¯ux decline does decrease as one reduces the fraction of the standard solution in the mixture, the behavior is highly non-linear with the ¯ux decline for the mixtures with >50% (volume) standard solution being almost identical, even though the 100 kD pre®ltered solution alone is largely nonfouling. Even the solution with only 20% standard solution fouls quite substantially, with the quasisteady ¯ux being similar to that for the 100% standard solution. These data are very consistent with the
W, Yuan, A.L. Zydney / Journal of Membrane Science 157 (1999) 1±12
results in the sequential ®ltration experiments. In this case, the presence of small amounts of the un®ltered solution, and most likely the large aggregates/ particles present in this solution, signi®cantly accelerates the overall rate of fouling caused by the humic acids. 4. Discussion The data obtained in this study clearly demonstrate that humic acid fouling during micro®ltration is governed by the convective deposition of a large fouling component on the upper surface of the membrane. Although the exact nature of this fouling species has not been quantitatively determined, it is most likely an aggregated form of humic acid (and not some other particulate matter present in the humic acid solutions) since even the 100 kD pre®ltered humic acids regained their fouling tendencies after storage under appropriate conditions. The large difference in fouling behavior seen for the Aldrich and Suwannee River humic acids is most likely due to the different amounts of aggregated humic materials present in these samples. Previous studies [12,13] have demonstrated that the Suwannee River humic acid has a higher concentration of carboxylic acid groups, giving it a greater negative charge and increased hydrophilicity. This suggests that humic acid aggregation may occur by some type of intermolecular hydrophobic interaction involving the aromatic and/or aliphatic regions of the humic acids. A similar hypothesis was presented by Wershaw et al. [16] who proposed that the aggregates were held together by a combination of hydrophobic interactions, bonding between aromatic groups, and hydrogen bonding between polar groups. The addition of calcium may accelerate the aggregation by reducing electrostatic repulsion between the negatively charged macromolecules and/or enhancing the strength of the hydrophobic interactions [17,18]. The increased aggregation at high temperature could be a kinetic effect caused by an increase in the rate of aggregation or it could be due to an increase in the strength of the hydrophobic interactions. The very rapid initial ¯ux decline is dominated by the physical deposition of large humic acid aggregates, with these aggregates then accelerating the subsequent deposition of non-aggregated humic acid
11
macromolecules. This behavior is similar to the twostep fouling process described for protein micro®ltration [19,20], in which case both the protein aggregation and chemical attachment to the growing deposit appear to occur through the formation of speci®c intermolecular disul®de linkages. Note that it may also be possible for large clay±humic±metal complexes or inorganic particles present in natural water to accelerate humic acid aggregation and deposition. The resulting fouling layer would then be composed of inorganic particles within a humic acid matrix, which is exactly the type of structure observed by Mallevialle et al. [3] and Bersillon et al. [4] in their studies on natural water ®ltration. Additional experiments are clearly needed to identify the molecular basis for humic acid aggregation and for the attachment of humic acid macromolecules to the humic acid deposit during micro®ltration. 5. Conclusions This study provides the ®rst quantitative analysis of the mechanisms governing humic acid fouling during micro®ltration. Humic acid adsorption caused only a 10% reduction in ®ltrate ¯ux (or membrane hydraulic permeability), which is consistent with the adsorption of approximately a monolayer of macromolecular humic acids within the membrane pores. The ¯ux decline during actual humic acid ®ltration was much more dramatic, with the ¯ux declining by well over an order of magnitude within 30 min of ®ltration for a 2 mg/l solution of Aldrich humic acid at 69 kPa (10 psi). This ¯ux decline is due to the formation of a humic acid deposit located on the upper surface of the membrane, with relatively little internal fouling within the membrane pore structure. The initial step in humic acid fouling is the convective deposition of large humic acid aggregates/particles on the membrane surface, with this initial deposit accelerating the subsequent deposition of macromolecular humic acids. Pre®ltration of the humic acid solution signi®cantly reduced the rate and extent of fouling by removing the large humic acid aggregates/particles from the solution. However, the humic acids in these pre®ltered solutions were able to reaggregate when stored at high temperature or in the presence of small amounts of calcium.
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Acknowledgements The authors would like to acknowledge Filtron Technology Corporation for donation of polyethersulfone membranes used in this work and the assistance of Robert Wieland in the preparation of scanning electron micrographs. References [1] J.G. Jacangelo, C.A. Buckley, Microfiltration, in: J. Mallevialle, P.E. Odendaal, M.R. Wiesner (Eds.), Water Treatment Membrane Processes, Chapter 11, McGraw-Hill, New York, 1996, pp. 11.1±11.39. [2] AWWA Membrane Technology Research Committee, Committee Report: Membrane processes in potable water treatment, J. AWWA 84(1) (1992) 59±67. [3] J. Mallevialle, C. Anselme, O.Marsigny, 1989. Effects of humic substances on membrane processes, in: I.H. Suffet, P. MacCarthy (Eds.), Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants, ACS, Washington, DC, pp. 749±767. [4] J.L. Bersillon, Fouling analysis and control, in: L. Cecille, J.C. Toussaint (Eds.), Future Industrial Prospects of Membrane Processes, Elsevier, Oxford, 1988, pp. 234±247. [5] V. Lahoussine-Turcaud, M.R. Wiesner, J.Y. Bottero, Fouling in tangential-flow ultrafiltration: effect of colloid size and coagulation pretreatment, J. Membr. Sci. 52 (1990) 173±190. [6] Y. Kaiya, Y. Itoh, K. Fujita, S. Takizawa, Study on fouling materials in the membrane treatment process for potable water, Desalination 106 (1996) 71±77. [7] M. NystroÈm, K. Ruohomaki, L. Kaipia, Humic acid as a fouling agent in filtration, Desalination 106 (1996) 79±87. [8] F.J. Stevenson, Humus Chemistry, Wiley, New York, 1982. [9] C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes, J. Membr. Sci. 97 (1994) 37±52.
[10] S. Hong, M. Elimelech, Chemical and physical aspects of natural organic matter (NOM) fouling of nanofiltration membranes, J. Membr. Sci. 132 (1997) 159±181. [11] G.R. Aiken, Evaluation of ultrafiltration for determining molecular weight of fulvic acid, Environ. Sci. Technol. 18 (1984) 978±981. [12] P. MacCarthy, R.L. Malcolm, The nature of commercial humic acids, in: I.H. Suffet, P. MacCarthy (Eds.), Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants, ACS, Washington, DC, 1989, pp. 55±64. [13] C.T. Chiou, D.E. Kile, T.I. Brinton, R.L. Malcolm, J.A. Leenheer, A comparison of water solubility enhancement of organic solutes by aquatic humic materials and commercial humic acids, Environ. Sci. Technol. 21 (1987) 1231±1234. [14] W.S. Opong, A.L. Zydney, Hydraulic permeability of protein layers deposited during ultrafiltration, J. Colloid Interface Sci. 142 (1991) 41±60. [15] S.P. Palecek, A.L. Zydney, Hydraulic permeability of protein deposits formed during microfiltration: effects of solution pH and ionic strength, J. Membr. Sci. 95 (1994) 71±81. [16] R.L. Wershaw, K.A. Thorn, D.J. Pinckney, P. MacCarthy, J.A. Rice, H.F. Hemong, Application of a membrane model to the secondary structure of humic materials in peat, in: C.H. Fuchsman (Ed.), Peat and Water, Elsevier, New York, 1986, pp. 133±157. [17] S.E. Manahan, Interactions of hazardous-waste chemicals with humic substances, in: I.H. Suffet, P. MacCarthy (Eds.) Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants, ACS, Washington, DC, 1989, pp. 83±92. [18] R.R. Engerbretson, R.V. Wandruszka, Kinetic aspects of cation-enhanced aggregation in aqueous humic acids, Environ. Sci. Technol 32 (1998) 488±493. [19] S.T. Kelly, W.S. Opong, A.L. Zydney, The influence of protein aggregates on the fouling of microfiltration membranes during stirred cell filtration, J. Membr. Sci. 80 (1993) 175±187. [20] S.T. Kelly, A.L. Zydney, Effects of intermolecular thioldisulfide interchange reactions on BSA fouling during microfiltration, Biotech. Bioeng. 44 (1994) 972±982.
Humic acid fouling during micro®ltration Wei Yuan, Andrew L. Zydney* Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA Received 24 July 1998; received in revised form 11 October 1998; accepted 16 October 1998
Abstract A major factor limiting the use of micro®ltration for surface water treatment is membrane fouling by natural organic matter. The extent and mechanisms of humic acid fouling during micro®ltration have been examined using stirred cell ®ltration experiments and scanning electron microscopy. The extent of fouling was strongly dependent on both the source and preparation of the humic acid solutions. The large ¯ux decline observed during constant pressure micro®ltration was caused by the formation of a humic acid deposit located on the upper surface of the membrane. Pre®ltration of the humic acid solutions dramatically reduced the rate of fouling through the removal of large humic acid aggregates. The initial fouling in this system was determined almost entirely by the convective deposition of these large particles/aggregates on the membrane surface. This initial deposit accelerated the subsequent rate of humic acid fouling, possibly serving as a nucleation site for deposition of macromolecular humic acids. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Fouling; Micro®ltration; Water treatment; Humic acid; Natural organic matter
1. Introduction Micro®ltration (MF) has been widely applied in drinking water treatment for the removal of particles, turbidity, and microorganisms from surface water and groundwater as an alternative to conventional water treatment processes (coagulation, sedimentation and sand ®ltration) [1,2]. Micro®ltration offers several advantages including superior water quality, easier control of operation, lower maintenance, and reduced sludge production. The greater removal of particles and microorganisms is of particular interest in meeting the more stringent requirements of the surface water
*Corresponding author. Tel.: +1-302-831-2399; fax: +1-302831-1048; e-mail: [email protected]
treatment rule (SWTR) and disinfectants/disinfection by-products regulations. One of the critical factors limiting the use of membrane ®ltration for drinking water treatment is membrane fouling, the irreversible loss of system productivity over time caused by interactions between the membrane and the various components in the process stream. Naturally occurring organic matter has been identi®ed as a major foulant in both surface and ground waters. For example, Mallevialle et al. [3] showed that the structure of the fouling layer formed during ultra®ltration or micro®ltration of natural river waters was determined largely by the organic matrix which served as a ``glue'' for inorganic constituents (e.g., iron, aluminum, silicon and calcium) in the fouling layer. Similar results were reported by Bersillon et al. [4] in their analysis of the deposited cake
0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00329-9
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formed on the membrane surface after ®ltration of natural waters. Lahoussine-Turcaud et al. [5] studied the ultra®ltration of several different organic and inorganic macromolecules and concluded that organic substances like humic and tannic acids had a much greater effect on the ¯ux decline than did the inorganic colloids. The ¯ux decline observed during ultra®ltration of Seine River water was primarily caused by the deposition of naturally occurring organic macromolecules, and in particular humic materials, on the membrane surface [5]. Kaiya et al. [6] analyzed the composition of the deposited layer formed on a hollow ®ber micro®ltration membrane during ®ltration of water obtained from a eutrophic lake. Again, the dominant factor causing the ¯ux decline was the deposition of natural organic matter, in this case in combination with manganese, on the membrane surface. A major fraction of the natural organic matter (NOM) present in surface or ground waters is composed of humic substances (HS), which are complex macromolecular products of the chemical and biological degradation of plant and animal residues, including lignin, carbohydrates, and proteins [7]. Humic substances are conveniently divided into three categories: humic acids, fulvic acids, and humin according to their solubility in acidic solutions. Humic acids refer to the fraction of humic substances that is not soluble in water under acidic conditions (pH<2) but is soluble at higher pH. They are a heterogeneous mixture having both aromatic and aliphatic components and containing three main functional groups: carboxylic acids (COOH), phenolic alcohols (OH), and methoxy carbonyls (C=O) [8]. Previous studies of humic acid fouling have focused on their interactions with either fully retentive nano®ltration membranes or partially retentive ultra®ltration membranes. For example, Jucker and Clark [9] showed that humic acid adsorption on hydrophobic ultra®ltration membranes was greater at low pH, which was attributed to the reduction in net charge and the increase in hydrophobicity of humic acids caused by the neutralization of the acid side groups at low pH. Humic acid adsorption was also enhanced in the presence of Ca2, possibly due to electrostatic shielding by the divalent cation. Similar results were reported by Hong and Elimelech [10] during nano®ltration, with the rate of ¯ux decline controlled by the
magnitude of the electrostatic interactions in combination with the hydrodynamic drag due to the ®ltrate ¯ow. Experimental studies of humic acid fouling on micro®ltration membranes are more limited. Although it might be expected that humic acid fouling would be minimal on large pore size (>0.1 mm) micro®ltration membranes, NystroÈm et al. [7] found a rapid decline in ¯ux during humic acid ®ltration through 1.9 mm pore diameter inorganic capillary membranes. For example, the ¯ux for a 10 ppm humic acid solution decreased by 50% after only 5 min of ®ltration at 2 bar, while the ¯ux for a 100 ppm humic acid solution decreased by more than 90% within 5 min. NystroÈm et al. attributed this dramatic ¯ux decline to electrostatic interactions between the negatively charged humic acid and the positively charged inorganic membrane, resulting in the nearly complete blockage of the membrane pores. The detailed mechanisms governing the blockage of such large pores by the humic macromolecules were not determined. The overall goal of this study was to develop a more fundamental understanding of the mechanisms governing humic acid fouling during micro®ltration. Experiments were thus designed to: (1) quantify the rate and extent of ¯ux decline through different micro®ltration membranes by commercially available humic acids under well-de®ned ®ltration conditions; (2) characterize the molecular weight distribution of the different humic acid solutions and its in¯uence on fouling; (3) use scanning electron microscopy to characterize the nature of the humic acid deposit formed on the membrane surface; and (4) use these results to obtain insights into the underlying physical phenomena governing humic acid fouling during micro®ltration. 2. Experimental The bulk of the experimental studies were performed using a humic acid obtained from Aldrich (Milwaukee, WI). Limited data were also obtained with a Suwannee River humic acid obtained from the International Humic Substances Society (St. Paul, MN). Humic acid solutions were prepared by dissolving pre-weighed quantities of humic acid powder in 2 l of ®ltered deionized (DI) water (pre®ltered through
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a 0.2 mm ®lter) obtained from a NANOPure water puri®cation system (Barnstead, Dubuque, IA) with resistivity greater than 18 M cm. The solution pH was adjusted to 7.00.1 by addition of small quantities of 0.1 M NaOH or HCl as needed. pH was measured using an Acumet 915 pH meter (Fischer Scienti®c). Humic acid concentrations were evaluated spectrophotometrically using a Perkin-Elmer Lambda 4B spectrophotometer (Perkin Elmer, Norwark, CT) with the absorbance measured at 254 nm. The overall humic acid concentration was determined by comparison of the absorbance data with an appropriate calibration curve. The apparent molecular weight (MW) distribution of the humic acid samples was determined using the ultra®ltration fractionation method originally developed by Aiken [11]. Humic acid samples were fractionated in a 25 mm diameter stirred cell (Model 8010, Amicon, Beverly, MA) using a series of OMEGA polyethersulfone ultra®ltration membranes (Filtron Technology, Northborough, MA) with nominal molecular weight cut-offs of 3, 10, 30, 50, 100, 300 kD, and 1 M. The ®ltration was performed at a constant pressure of 207 kPa (30 psi), with the fractional amount of humic acid within each size range calculated from the difference in humic acid concentration between adjacent ®ltrate samples (e.g., the difference in concentration between the ®ltrates from the 30 and 10 kD membranes). In order to obtain additional insights into the fouling behavior, a series of experiments were performed with humic acid solutions that had ®rst been pre®ltered through a 100 kD, 300 kD, 1 M, or 0.16 mm polyethersulfone membrane (Filtron Technology, Northborough, MA) to remove the larger molecular weight components. This pre®ltration was done using 60 mg/l humic acid solutions at 69 kPa (10 psi) in a 64 mm diameter stirred cell (Model 8200, Amicon, Beverly, MA) at a stirring speed of 350 rpm. Each membrane was used for about 500 ml of ®ltration. The collected ®ltrate was then diluted with ®ltered DI water to obtain the desired concentration. Most ®ltration experiments employed 0.16 mm pore size OMEGA polyethersulfone (PES) micro®ltration membranes provided by Filtron Technology (Northborough, MA). These PES membranes have an asymmetric structure with an upper skin approximately 0.5 mm in thickness providing the membrane its
3
permselectivity. Limited data were also obtained with 0.22 mm Durapore polyvinylidene ¯uoride membranes (Millipore, Bedford, MA) and 0.2 mm tracketched polycarbonate membranes (Poretics, Livermore, CA). The membranes were placed in a 25 mm diameter stirred cell (Model 8010, Amicon, Beverly, MA) connected to an air-pressurized 2 l solution reservoir. The stirred cell and reservoir were initially ®lled with ®ltered DI water. Each membrane was ¯ushed with at least 50 ml of ®ltered DI water prior to use to remove any wetting agents. The water ¯ux was then measured as a function of time at a constant pressure until a quasi-steady ¯ux was attained (usually within 10 min). The stirred cell and reservoir were then emptied and re®lled with a humic acid solution. The system was repressurized to the initial pressure, and the stirring speed was set to 600 rpm using a Strobotac type 1531-AB strobe light (General Radio, Concord, MA). The ®ltrate ¯ow rate was measured by timed collection, with ®ltrate samples collected periodically for subsequent analysis. At the end of the experiment, the stirred cell and membrane were gently rinsed with ®ltered DI water, and the water ¯ux through the fouled membrane was reevaluated at the same pressure used during the humic acid ®ltration. All experiments were conducted at room temperature (2228C). Selected membranes were examined by scanning electron microscopy (SEM). The membranes were ®rst ¯ushed with at least 200 ml of ®ltered deionized water to remove any labile humic acid. The membranes were dehydrated by repeated washing in ethanol/water solutions of increasing ethanol concentration, with the ®nal washing performed using pure ethanol. The membranes were then mounted on specimen studs using carbon paste, sputtered with gold under an argon environment, and imaged with a JEOL JXA-840 SEM at 5.0 kV. 3. Results Fig. 1 shows experimental data for the ®ltrate ¯ux as a function of time for the ®ltration of 2 mg/l solutions of the Aldrich and Suwannee River humic acids. Two runs are shown for the Aldrich humic acid performed under identical conditions but using two
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Fig. 1. Normalized filtrate flux during filtration of 2 mg/l solutions of the Aldrich and Suwannee River humic acids through 0.16 mm polyethersulfone microfiltration membranes at 69 kPa (10 psi).
Fig. 2. Apparent molecular weight distribution for solutions of the Aldrich and Suwannee River humic acids at pH 7.0.
separate membranes. All of the data were obtained at a constant pressure of 69 kPa (10 psi) and a stirring speed of 600 rpm. In each case, the ®ltrate ¯ux has been normalized by J0, the steady state water ¯ux evaluated just before switching to the humic acid solution. The J0 values for the three membranes varied between 1.4210ÿ3 and 1.5810ÿ3 m/s, with this small difference re¯ecting the inherent variability in membrane properties. The two runs with the Aldrich humic acid were essentially identical, with reproducibility better than 10%. The ®ltrate ¯ux declined much more rapidly for the Aldrich humic acid, with the ¯ux decreasing to 10% of its initial value within the ®rst 20 min of ®ltration while J/J00.23 after 120 min of ®ltration for the Suwannee River humic acid. This large difference in fouling behavior was likely due to the different physical and chemical characteristics of the humic acids. The Suwannee River humic acid has a greater concentration of functional groups (i.e., carboxylic acids), which makes it more negatively charged and more hydrophilic than the Aldrich humic acid that is obtained from soil [12,13]. In addition, the Aldrich humic acid had a greater percentage of larger molecular weight components as determined by the ultra®ltration fractionation (Fig. 2). The error bars in the ®gure represent the standard deviation as determined from repeat measurements on the same humic acid sample (evaluated with a separate clean set of ultra®ltration membranes). For example, more than 60% of the Aldrich humic acid had a molecular weight greater than 50 000, while only 20% of the Suwannee River humic acid had that
large a molecular weight. As discussed subsequently, these large molecular weight components, and in particular the fraction with molecular weight greater than 1 M, play a critical role in humic acid fouling during micro®ltration. Although the Suwannee River humic acid is more representative of most aqueous systems, we decided to examine the fouling behavior of the Aldrich humic acid in more detail in order to determine the origin of the dramatic ¯ux decline seen in Fig. 1. Fig. 3 shows experimental data for the ®ltrate ¯ux (top panel) and observed rejection coef®cient (lower panel) as a function of time for the ®ltration of 2 mg/l solutions of the Aldrich humic acid through several different micro®ltration membranes: a 0.16 mm polyethersulfone (PES) membrane, a 0.22 mm polyvinylidene ¯uoride (PVDF) membrane, and a 0.2 mm track-etched polycarbonate (PCTE) membrane. The J0 values for the three membranes were similar: 1.310ÿ3 m/s for the PES membrane, 1.010ÿ3 m/s for the PVDF membrane and 0.9510ÿ3 m/s for the PCTE membrane. The ®ltrate ¯ux declined rapidly for all three membranes, with J/J0 decreasing to less than 10% of its initial value within the ®rst 20 min. The most rapid ¯ux decline was seen with the PCTE membrane, which was likely due to the smaller porosity (<10%) for this track-etched membrane compared to the other polymer membranes (porosity70%). The ¯ux at long times appeared to reach a quasisteady value, ranging from Jss4.310ÿ5 m/s for the PCTE membrane to 5.610ÿ5 m/s for the PES membrane. All three micro®ltration membranes were
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5
Fig. 4. Water flux as a function of transmembrane pressure through a single 0.16 mm polyethersulfone microfiltration membrane after different stages in the filtration of a 15 mg/l humic acid solution. Detailed conditions are described in text.
Fig. 3. Normalized filtrate flux and observed rejection coefficient during filtration of a 2 mg/l solution of the Aldrich humic acid through polyethersulfone (PES); polyvinylidene fluoride (PVDF); and polycarbonate (PCTE) microfiltration membranes at 69 kPa (10 psi), with J01.310ÿ3 m/s (PES); 1.010ÿ3 m/s (PVDF); 0.9510ÿ3 m/s (PCTE).
essentially non-retentive to the humic acids at the start of the ®ltration, with the humic acid concentration in the ®ltrate being essentially identical to that in the bulk solution. However, the apparent rejection coef®cient, de®ned as one minus the ratio of the ®ltrate to bulk humic acid concentration at each time, increased rapidly during the micro®ltration approaching a value of 87% for the PCTE membrane and as much as 95% for the PES membrane. In order to understand the origin of the large ¯ux decline seen in Fig. 3, a series of experiments were performed to measure the hydraulic permeability of the PES membrane after different fouling steps. Fig. 4 shows data for the ®ltrate ¯ux of DI water through a single PES membrane: (1) clean; (2) after static exposure to a 15 mg/l humic acid solution at 48C for 24 h; (3) after ®ltration of a 15 mg/l humic acid solution at 69 kPa (10 psi) for 2 h; (4) after physical
cleaning; (5) after chemical cleaning using 0.1 N NaOH for 30 min. The data for the clean membrane (after ¯ushing with DI water) were highly linear (r20.997) indicating that the membrane was incompressible over this pressure range. The membrane hydraulic permeability Lp, Lp
J ; P
(1)
where is the water viscosity at room temperature; J the ®ltrate ¯ux; and P is the transmembrane pressure, was thus a constant with a value Lp1.9 10ÿ9 m. The ®ltrate ¯ux data after static adsorption in a 15 mg/l humic acid solution were also linear (r20.994), with the permeability of the pre-adsorbed membrane approximately 10% smaller than that for the clean membrane. A similar reduction in ¯ux was seen after humic acid adsorption on the PVDF and PCTE membranes, even at humic acid concentrations as high as 50 mg/l. This reduction in membrane hydraulic permeability would correspond to a pore Ê as evaluated using constriction of approximately 80 A the Hagen±Poiseuille equation for ¯ow through a model PES membrane with uniform cylindrical pores of 0.16 mm diameter. This type of pore constriction is reasonably consistent with a monolayer of humic Ê as deteracids with effective size from 30 to 100 A mined previously by electron microscopy [8]. The preadsorbed membrane was then used to ®lter a 15 mg/l humic acid solution at a constant pressure of 69 kPa
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(10 psi) for 120 min. The membrane was then gently rinsed with ®ltered deionized water to remove any labile humic acid before being returned to the stirred cell. This ``fouled'' membrane had a de®nite yellowish color caused by the presence of a humic acid deposit on the membrane surface (discussed in more detail subsequently). The hydraulic permeability of the fouled membrane was 1.510ÿ10 m, more than an order of magnitude smaller than that for the clean membrane. In addition, the water ¯ux at each pressure showed a small (approximately 20%) transient decline over about 20 min; the data in Fig. 4 are the steadystate values at each pressure. Similar behavior has been reported by Opong and Zydney [14] for protein deposits formed during micro®ltration and is likely due to the compressibility of the deposit. Also note that the water ¯ux at 69 kPa was about 30% higher than the ®ltrate ¯ux obtained during the humic acid ®ltration at this pressure. This small increase in ¯ux was seen after all of the humic acid ®ltrations and is likely due to the absence of bulk concentration polarization and/or the removal of weakly bound humic acids in switching from the humic acid solution to water. After measuring the water ¯ux, the surface of the fouled membrane was carefully wiped with several paper towels. This physical cleaning removed essentially all of the yellowish deposit, and it returned the hydraulic permeability to nearly 80% of its initial (clean) value. In addition, the water ¯ux was again linear, indicating a constant permeability. This dramatic increase in ¯ux indicated that the bulk of the fouling occurred on the upper surface of the membrane and was not located throughout the membrane pore structure (where it would have been inaccessible to the physical cleaning). The membrane was then removed from the stirred cell, soaked in a 0.1 N NaOH solution for 30 min, rinsed in deionized water and returned to the stirred cell. The hydraulic permeability of this chemically-cleaned membrane was Lp1.7 10ÿ9 m, which was only about 8% smaller than that determined initially for the clean membrane. Thus, the NaOH cleaning was able to remove most of the small amount of residual (internal) humic acid from the membrane pores. In order to further quantify the nature of the fouling deposit, scanning electron micrographs were obtained of the top (feed side) surface of 0.16 mm PES membranes after different ®ltration times. Each
SEM was obtained with a separate membrane used to ®lter a 2 mg/l humic acid solution at a constant pressure of 69 kPa (10 psi). Fig. 5 shows the SEMs for the clean membrane and for the membranes after t1 min (J/J00.87); 5 min (J/J00.5); 20 min (J/J00.1); and 120 min (J/J00.04) of ®ltration. Even after only 1 min of ®ltration, the membrane surface was partially covered by several large particles or aggregates, although most of the pores remained unblocked as expected from the relatively small (13%) decline in ®ltrate ¯ux. The surface after 5 min of ®ltration was covered by a greater number of aggregates along with a more amorphous layer (at least at the resolution of the SEMs). By the end of the 120 min of ®ltration, the membrane surface was almost completely covered by the humic acid deposit, consistent with the large (96%) reduction in ®ltrate ¯ux. The effects of bulk concentration and applied transmembrane pressure on humic acid fouling of the 0.16 mm PES micro®ltration membranes are shown in Fig. 6. The top panel shows data with an initial feed concentration from 2 to 30 mg/l all obtained at P20 kPa (3 psi), while the bottom panel shows data at P from 20 to 103 kPa (3±15 psi) with Cb2 mg/l. Each experiment was performed using a separate (fresh) membrane with similar initial water permeability. The rate of ¯ux decline clearly increased with increasing humic acid concentration; however, the quasi-steady ¯ux attained at long ®ltration times was essentially independent of the concentration, varying from 8.010ÿ5 to 9.210ÿ5 m/s over the 15-fold range of humic acid concentration. The initial ®ltrate ¯ux increased essentially linearly with increasing pressure, but the rate of ¯ux decline also increased sharply with increasing P. The net result was that the run at 15 psi had the lowest ®ltrate ¯ux for t10 to 20 min. The quasi-steady ¯ux was a weak function of the applied pressure, varying from 6.510ÿ5 to 9.210ÿ5 m/s with slightly lower values obtained at higher pressures. Similar behavior has been reported by Palecek and Zydney [15] for protein micro®ltration, with the quasi-steady ¯ux attributed to the absence of fouling when the hydrodynamic drag force towards the membrane was balanced by the repulsive (electrostatic) interactions between the bulk protein and the protein deposit on the membrane surface.
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Fig. 5. Scanning electron micrographs of the upper surface of polyethersulfone microfiltration membranes used to filter 2 mg/l humic acid solutions at 69 kPa for different periods of time (a) clear membrane, (b) t1 min, J/J00.87, (c) t5 min, J/J00.5, (d) t20 min, J/J00.1, and (e) t120 min, J/J00.04.
The data in Fig. 6 were used to evaluate the instantaneous rate of ¯ux decline Kÿ
1 dJ J dt
(2)
with the derivative dJ/dt evaluated numerically using a ®nite difference representation accurate to order (t)2. The results for the initial rate of ¯ux decline are shown in Fig. 7 as a function of the initial rate of solute
transport to the membrane (J0Cb). The data for the experiments at different Cb and P collapse to a single line when plotted in this manner, with r20.97 and a slope of 17 m2/g. These results clearly indicate that the initial rate of ¯ux decline is due to the convective deposition of the humic acids, or more likely the humic acid aggregates or particles, on the membrane surface as opposed to any adsorption or kinetically-limited fouling process (which would be unlikely to vary linearly with J0Cb).
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Fig. 8. Effect of prefiltration (through 100 kD, 300 kD, 1 M or 0.16 mm polyethersulfone membranes) on the flux decline during filtration of a 2 mg/l humic acid solution through a 0.16 mm polyethersulfone microfiltration membrane at 69 kPa (10 psi). J0 ranged from 1.310ÿ3 to 1.510ÿ3 m/s.
Fig. 6. Effect of bulk concentration (top panel) and transmembrane pressure drop (bottom panel) on the flux during humic acid microfiltration. For runs at different Cb, J03.810ÿ4 (2 mg/l); 4.410ÿ4 m/s (5 mg/l); 3.910ÿ4 m/s (15 mg/l); 4.510ÿ4 m/s (30 mg/l). For runs at different P, J03.810ÿ4 m/s (3 psi); 5.910 ÿ4 m/s (5 psi); 8.310 ÿ4 m/s (8 psi); 1.310 ÿ3 m/s (10 psi); 1.810ÿ3 m/s (15 psi).
Fig. 7. Initial rate of flux decline as a function of the initial convective mass flux towards the membrane for filtration experiments shown in Fig. 6.
In order to obtain additional insights into the fouling mechanisms, a series of experiments were performed using humic acid solutions that had ®rst been pre®ltered through different pore size membranes prior to use in the micro®ltration experiment. In each case, a 60 mg/l humic acid solution was used for the pre®ltration, with the concentration in the collected ®ltrate adjusted to 2 mg/l by dilution with ®ltered deionized water. These pre®ltered solutions were stored at 48C until use (within 4 h of the initial pre®ltration). The micro®ltration experiments were performed at 69 kPa (10 psi) using 0.16 mm PES membranes having essentially identical water permeability, with the results for the normalized ¯ux shown in Fig. 8. The pre®ltration signi®cantly reduced the rate and extent of fouling compared to that for the un®ltered (standard) humic acid solution, suggesting that the components responsible for the bulk of the rapid initial fouling were removed by the pre®ltration step. The magnitude of the improvement in ¯ux increased as the pre®ltration was conducted through smaller pore size (i.e., smaller nominal molecular weight cut-off) membranes, with the ¯ux for the 100 kD pre®ltered solution dropping by less than 50% after 120 min of ®ltration (8500 l/m2 of membrane area). Scanning electron micrographs of the membranes used with the pre-®ltered solutions showed a complete absence of large aggregates, although an amorphous layer was seen over certain regions of the membrane (Fig. 9). The pre®ltration
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9
Fig. 10. Effect of storage conditions on the flux decline through 0.16 mm polyethersulfone microfiltration membranes at 69 kPa (10 psi). J0 ranged from 1.310ÿ3 to 1.510ÿ3 m/s.
Fig. 11. Apparent molecular weight distribution for 100 kD prefiltered solutions at pH 7.0 after different storage conditions. Fig. 9. Scanning electron micrographs of the upper surface of the polyethersulfone microfiltration membranes used to filter 2 mg/l solutions at 69 kPa for 2 h of (a) 100 kD prefiltered humic acid and (b) 300 kD prefiltered humic acid.
also altered the apparent molecular weight distribution of the humic acid solutions; the percentage of humic acids with MW greater than 300 kD decreased from 36% for the fresh (un®ltered solution) to 22% for the 0.16 mm pre®ltered solution and less than 1% for the 100 kD pre®ltered solution. Fig. 10 shows data for the normalized ¯ux for 2 mg/ l solutions of the 100 kD pre®ltered humic acid solution after storage for 24 h at either 508C or 48C in deionized water or at 48C in the presence of 1 mM Ca2. Each solution was stored at a humic acid concentration of 10 mg/l and was diluted and returned
to room temperature (2228C) immediately prior to the micro®ltration. Thus, the solution stored with 1 mM Ca2 actually had a 0.2 mM CaCl2 concentration during the micro®ltration. The rate of ¯ux decline for the solutions stored at 508C and at 48C in the presence of Ca2 was quite dramatic, with the ¯ux decreasing by more than 90% within the ®rst 60 min of ®ltration. This large increase in fouling was due to the re-aggregation of the humic acids as shown by the change in the apparent molecular distributions for these stored solutions (Fig. 11). For example, the percentage of humic acids with MW greater than 300 kD increased from only 1% for the fresh 100 kD pre®ltered solution to 11% for the solution stored at 508C and about 9% for the solution stored at 48C in the presence of 1 mM Ca2. Storage of the
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Fig. 12. Normalized filtrate flux during sequential filtration experiments using a 2 mg/l standard humic acid solution at 69 kPa (10 psi) followed by either DI water, a 2 mg/l 100 kD prefiltered humic acid solution, or a 2 mg/l standard humic acid solution.
Fig. 13. Normalized filtrate flux during the filtration of 2 mg/l mixtures of the standard and 100 kD prefiltered humic acid solutions through 0.16 mm polyethersulfone microfiltration membranes at 69 kPa (10 psi). J 0 ranged from 1.310 ÿ3 to 1.510ÿ3 m/s.
100 kD pre®ltered solution at 48C without Ca2 caused almost no increase in fouling, consistent with the absence of any signi®cant reaggregation of this humic acid solution (Fig. 11). These results indicate that both Ca2 and high temperature increase the rate of humic acid aggregation, which in turn increases the fouling seen during humic acid micro®ltration. Fig. 12 shows the results of a series of ``sequential'' ®ltration experiments. In each case, a standard (un®ltered) 2 mg/l humic acid solution was ®rst ®ltered through a 0.16 mm membrane at 69 kPa (10 psi) for about 10 min (until J/J00.25). The stirred cell was then emptied, the membrane rinsed with ®ltered DI water, and the system re®lled with either DI water, a 2 mg/l solution of a 100 kD pre®ltered humic acid, or a 2 mg/l solution of fresh (un®ltered) humic acid. The pressure and stirring speed were reset at their original values, and the ®ltration continued for an additional 110 min. The ¯ux at the start of this second ®ltration was slightly larger than that obtained at the end of the ®rst ®ltration, even when the second ®ltration was performed with the standard humic acid solution. This slight increase in ¯ux was likely due to some decompression of the fouling deposit and/or the removal of labile humic acid from the deposit during rinsing. The ¯ux for the deionized water remained essentially constant throughout the second ®ltration step as expected. However, the ¯ux for the 100 kD pre®ltered
humic acid solution declined by more than a factor of 4 over 110 min of ®ltration, attaining a quasi-steady ¯ux that was nearly identical to that for the standard (un®ltered) solution. This is in contrast to the relatively small decline in ¯ux (less than two-fold) seen during ®ltration of the 100 kD pre®ltered humic acid solution under the same conditions but without the initial ®ltration of the standard humic acid solution (Fig. 8). These data clearly indicate that the 100 kD pre®ltered humic acid solution was able to foul the membrane much more extensively after the membrane was ®rst fouled by a standard (un®ltered) solution, suggesting that the deposition of large particles or aggregates on the membrane surface is able to accelerate, or catalyze, the addition of smaller humic acid macromolecules to the growing deposit. Fig. 13 shows the fouling behavior for mixtures of the standard and 100 kD pre®ltered solutions, each with a total humic acid concentration of 2 mg/l. Although the rate of ¯ux decline does decrease as one reduces the fraction of the standard solution in the mixture, the behavior is highly non-linear with the ¯ux decline for the mixtures with >50% (volume) standard solution being almost identical, even though the 100 kD pre®ltered solution alone is largely nonfouling. Even the solution with only 20% standard solution fouls quite substantially, with the quasisteady ¯ux being similar to that for the 100% standard solution. These data are very consistent with the
W, Yuan, A.L. Zydney / Journal of Membrane Science 157 (1999) 1±12
results in the sequential ®ltration experiments. In this case, the presence of small amounts of the un®ltered solution, and most likely the large aggregates/ particles present in this solution, signi®cantly accelerates the overall rate of fouling caused by the humic acids. 4. Discussion The data obtained in this study clearly demonstrate that humic acid fouling during micro®ltration is governed by the convective deposition of a large fouling component on the upper surface of the membrane. Although the exact nature of this fouling species has not been quantitatively determined, it is most likely an aggregated form of humic acid (and not some other particulate matter present in the humic acid solutions) since even the 100 kD pre®ltered humic acids regained their fouling tendencies after storage under appropriate conditions. The large difference in fouling behavior seen for the Aldrich and Suwannee River humic acids is most likely due to the different amounts of aggregated humic materials present in these samples. Previous studies [12,13] have demonstrated that the Suwannee River humic acid has a higher concentration of carboxylic acid groups, giving it a greater negative charge and increased hydrophilicity. This suggests that humic acid aggregation may occur by some type of intermolecular hydrophobic interaction involving the aromatic and/or aliphatic regions of the humic acids. A similar hypothesis was presented by Wershaw et al. [16] who proposed that the aggregates were held together by a combination of hydrophobic interactions, bonding between aromatic groups, and hydrogen bonding between polar groups. The addition of calcium may accelerate the aggregation by reducing electrostatic repulsion between the negatively charged macromolecules and/or enhancing the strength of the hydrophobic interactions [17,18]. The increased aggregation at high temperature could be a kinetic effect caused by an increase in the rate of aggregation or it could be due to an increase in the strength of the hydrophobic interactions. The very rapid initial ¯ux decline is dominated by the physical deposition of large humic acid aggregates, with these aggregates then accelerating the subsequent deposition of non-aggregated humic acid
11
macromolecules. This behavior is similar to the twostep fouling process described for protein micro®ltration [19,20], in which case both the protein aggregation and chemical attachment to the growing deposit appear to occur through the formation of speci®c intermolecular disul®de linkages. Note that it may also be possible for large clay±humic±metal complexes or inorganic particles present in natural water to accelerate humic acid aggregation and deposition. The resulting fouling layer would then be composed of inorganic particles within a humic acid matrix, which is exactly the type of structure observed by Mallevialle et al. [3] and Bersillon et al. [4] in their studies on natural water ®ltration. Additional experiments are clearly needed to identify the molecular basis for humic acid aggregation and for the attachment of humic acid macromolecules to the humic acid deposit during micro®ltration. 5. Conclusions This study provides the ®rst quantitative analysis of the mechanisms governing humic acid fouling during micro®ltration. Humic acid adsorption caused only a 10% reduction in ®ltrate ¯ux (or membrane hydraulic permeability), which is consistent with the adsorption of approximately a monolayer of macromolecular humic acids within the membrane pores. The ¯ux decline during actual humic acid ®ltration was much more dramatic, with the ¯ux declining by well over an order of magnitude within 30 min of ®ltration for a 2 mg/l solution of Aldrich humic acid at 69 kPa (10 psi). This ¯ux decline is due to the formation of a humic acid deposit located on the upper surface of the membrane, with relatively little internal fouling within the membrane pore structure. The initial step in humic acid fouling is the convective deposition of large humic acid aggregates/particles on the membrane surface, with this initial deposit accelerating the subsequent deposition of macromolecular humic acids. Pre®ltration of the humic acid solution signi®cantly reduced the rate and extent of fouling by removing the large humic acid aggregates/particles from the solution. However, the humic acids in these pre®ltered solutions were able to reaggregate when stored at high temperature or in the presence of small amounts of calcium.
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Acknowledgements The authors would like to acknowledge Filtron Technology Corporation for donation of polyethersulfone membranes used in this work and the assistance of Robert Wieland in the preparation of scanning electron micrographs. References [1] J.G. Jacangelo, C.A. Buckley, Microfiltration, in: J. Mallevialle, P.E. Odendaal, M.R. Wiesner (Eds.), Water Treatment Membrane Processes, Chapter 11, McGraw-Hill, New York, 1996, pp. 11.1±11.39. [2] AWWA Membrane Technology Research Committee, Committee Report: Membrane processes in potable water treatment, J. AWWA 84(1) (1992) 59±67. [3] J. Mallevialle, C. Anselme, O.Marsigny, 1989. Effects of humic substances on membrane processes, in: I.H. Suffet, P. MacCarthy (Eds.), Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants, ACS, Washington, DC, pp. 749±767. [4] J.L. Bersillon, Fouling analysis and control, in: L. Cecille, J.C. Toussaint (Eds.), Future Industrial Prospects of Membrane Processes, Elsevier, Oxford, 1988, pp. 234±247. [5] V. Lahoussine-Turcaud, M.R. Wiesner, J.Y. Bottero, Fouling in tangential-flow ultrafiltration: effect of colloid size and coagulation pretreatment, J. Membr. Sci. 52 (1990) 173±190. [6] Y. Kaiya, Y. Itoh, K. Fujita, S. Takizawa, Study on fouling materials in the membrane treatment process for potable water, Desalination 106 (1996) 71±77. [7] M. NystroÈm, K. Ruohomaki, L. Kaipia, Humic acid as a fouling agent in filtration, Desalination 106 (1996) 79±87. [8] F.J. Stevenson, Humus Chemistry, Wiley, New York, 1982. [9] C. Jucker, M.M. Clark, Adsorption of aquatic humic substances on hydrophobic ultrafiltration membranes, J. Membr. Sci. 97 (1994) 37±52.
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