Effects of Humic Substances on Membrane Processes - Advances in

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch041

Effects of Humic Substances on Membrane Processes J. Mallevialle, C. Anselme, and O. Marsigny Centre de Recherche Lyonnaise des Eaux-Degrémont, 38 rue du Président Wilson, 78230 Le Pecq, France

Analytical methods are being developed to help characterize the mechanism of membrane fouling. For example, pyrolysis-gas chromatography-mass spectrometry provides global information on the nature of macromolecules present in the water and in the cake that forms at the membrane surface. Scanning electron microscopy, transmission electron microscopy, elemental analysis, X-ray microanalysis, secondary ion mass spectroscopy, and Fourier transform IR spectroscopy also can be used to define the encrustation.

SUSPENDED MATTER AND MACROMOLECULES are removed from water pri marily by coagulation and sand filtration. Many water-treatment companies are involved in research into ways to optimize this clarification step, but so far the complexity of the problem has led to very involved solutions that create difficulties with regard to any automation. One alternative is to develop new liquid-solid separation processes such as membrane filtration, which seems to be promising from the viewpoints of both economics and auto mation. Some limitations inherent in this process must be resolved; per manent fouling, adsorption, and concentration polarization all lead to a quick flux decline (75-90%) during the first period of use (I). These phenomena must be understood if we are to overcome their disadvantages and develop an economically viable process. Figure 1 illustrates our approach to the goal of optimization of filteredwater production by cleaning cycles (backflushing, pulse flux, etc.). The main problem in achieving this goal is permanent fouling, which appears to 0065-2393/89/0219-0749$06.00/0 © 1989 American Chemical Society

In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

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AQUATIC H U M I C SUBSTANCES


. . . . Evolution of permeability with regular hydraulic cleaning (backflush) — Evolution of permeability without regular hydraulic cleaning (backflush)

Time (arbitrary scale) Figure 1. Permeability of a membrane as a function of time and reversibility offouling. be tied to the aqueous organic matrix. Inorganic compounds such as clay do not induce any definitive fouling, but filtration of water containing complex mixtures of organic and inorganic matter can lead to irreversible fouling (2). Three types of studies were conducted to characterize this fouling. The first study included measurement of general water-quality parameters (e.g., turbidity, total organic carbon (TOC), and elemental analysis). In the second study, a separation was performed by gel permeation chromatography (GPC) to determine the apparent molecular weight of the organic content. These samples were studied by pyrolysis-gas chromatography-mass spectrometry ( P y - G C - M S ) (3, 4). This process reveals the distribution of the different families of organic macromolecules in the feedwater, filtrate, and retentate and enables us to determine the organic mass balance of the effluents. This P y - G C - M S method can also be applied directly to any matter deposited in the membrane. Finally, methods such as electron microscopy coupled with analytical detectors (X-ray microanalyzer) and secondary ion mass spectrom etry (SIMS) were applied to obtain a structural view of the membrane and the deposit on it. The goals of this work were to characterize the composition and structure of filtration "cakes" and to point out the importance of the adsorption mech anism of some macromolecular compounds in the phenomenon of permanent fouling. From this complete characterization, further research works could be implemented in the area of fouling reduction. Such results could help to


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improve the fouling-reduction methods that are used in membrane-filtration processes: • pretreatment of raw water (coagulation or addition of powdered activated carbon before membrane filtration) • modification of the membrane surface to avoid adsorption of specific foulant macromolecules


• dynamic cleaning cycles (backflush, pulse flux, reverse circu lation) • chemical cleaning of membrane (oxidants, surfactants)

Experimental Materials and Methods Water Samples. Four different water samples were used in this work. One was a ground water from Normandy (France) at Bernay with low TOC and high mineral content. This water was rapidly recharged by rain, and that led to an occasional sharp increase in turbidity. A second water sample, from the Allier River at Bellerive, had a mountain origin. Turbidity in this case was normally between 1.5 and 10 nephelometric turbidity units (NTU), with some peaks surging to 70 NTU. This water had a low mineral content and medium TOC. The third sample was reservoir waterfromNebias, France, with strong turbidity and high TOC. The last sample was river water from Dunkirk, in the North of France, with high TOC, medium turbidity and very high algae content. These water samples will be referred to in this chapter as Bernay, Bellerive, Nebias, and Dunkirk. Membrane Samples. Various microfiltration and ultrafiltration hollow fibers were used, with porosities rangingfrom0.2 u,m to 20 À. The constitutive polymers were polypropylene, polyacrylonitrile, polysulfone, poly(ether sulfone), and a cellulosic polymer. Water Concentration Methods. Water samples werefirstconcentrated by rotary evaporation to obtain a TOC between 100 and 200 mg/L. These concentrated samples (in 10-mL aliquots) were injected into a gel permeation chromatograph packed with afinegel (Sephadex G25, Sephadex Pharmacia, Uppsala, Sweden). The conditions were as follows: column size, 0.25 x 90 cm; eluant, ultrapure water; flow rate, 150 mL/h. Elution was followed by a continuous UV absorbance detector (254 nm) and TOC measurement on a TOC meter (Dohrman DC80, Envirotech Corp., Santa Clara, CA). The samples were then dried for pyrolysis. Flash pyrolysis was performed with a temperature-control system (Pyroprobe 100, Chemical Data System, Oxford, PA). Each sample (300-500^g aliquots in a quartz tube) was pyrolyzed at 200-750 °C, with a temperature program of 20 °C/ms and afinalhold for 20 s. After pyrolysis, thefragmentswere separated on a 30-m fused-silica column (DBWAX) that was temperature-programmed from 30 to 220 °C at 3 °C/min, with afinalperiod of 10 min at 220 °C. This separation was followed by mass spectrometry (R 10-10 C, Nermag, Rueil Malmaison, France) operating in electron impact at 70 eV and scanningfrom20 to 400 mlz. The chromatograms were analyzed with a computer (PDP 11/73, DEC), using a 70,000-spectra library (WHILEY) on a hard disk.


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Pyrolysis IR is a step-by-step temperature-programmed pyrolysis technique that transforms organic carbon into C 0 , which is subsequently measured by IR spec troscopy. The various forms of carbon (aliphatic, aromatic, and inorganic) are detected at discrete temperatures; this method allows for the determination of the various sources of carbon in the sample from different fractions. 2


Deposit. The deposits were obtained from the fouled membranes by submit ting them to sonication. The solution was centrifuged and the fouling-matter fraction was freeze-dried. The dry residue was then pyrolyzed with flash P y - G C - M S , py rolysis IR analysis, or other analytical techniques. Overall Parameters. The parameters studied are shown in Table I. UV absorbance at 254 nm (Perkin Elmer, Lambda 3) and TOC (Dohrman, D C 80) were measured for each water sample. Some determinations were also made by standard methods for parameters such as N0 ~, Fe, alkalinity, and number of algae cells (5). 3

Microscopic Techniques. This study was performed with transmission (TEM) and scanning electron microscopy (SEM) (Figure 2). The scanning electron micro scope 0 E O L JSM2) was coupled with an energy-dispersing system (6) that provided information on inorganic elements contained in the samples, for Ζ > 20 amu. The samples for SEM were frozen with liquid nitrogen. Cryofracture of the hollow fiber was performed to allow observation within the fiber, and all samples were then lyophilized. Some samples were compared with air-dried membranes. The preparation of samples for T E M (Philips 300) was more difficult than for SEM because we used resin inclusion and ultramicrotom sampling equipment. The SIMS apparatus (CAM EC A SMI 300) gives an exact spatial distribution (7) for selected elements in cross sections of samples (Figure 3). Some fouled-membrane data were also obtained by means of Fourier transform IR spectroscopy with an attenuated total reflection technique. The general scheme of analysis given in Fig ure 4 represents all of the analytical tools used in this study.

Results and Discussion O r g a n i c M a t r i x o f the W a t e r . Table I shows the data for the overall parameters for each water, with few differences among the waters. Bernay water has a very low T O C , which indicates a low organic content, and this agrees with the elemental analysis (Table II). The U V measurement showed that a few aromatic structures are present in the organic matrix. For Bellerive water, the T O C is low-to-medium (3 ppm); U V absorbance at 254 nm is medium; turbidity is also medium ( ej Ι β Ο

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90 Bernay deposit 6.4 Bernay raw water 7.3 25 >95 Nebias deposit 2.5 3

The filtration "cakes" are generally more than 80% nonorganic matter; iron, aluminium, silicon, and calcium are the major elements. The A l / S i ratio is characteristic of kaolinite clays. Iron, particularly concentrated at the internal "skin" of the membrane, could come from precipitation of hydrox ides, complexation with organic compounds, or the natural residual iron contained by clays. In contrast, few inorganic elements (other than calcium, sodium, and potassium) seem to enter the structure of the membrane and foul its pores. Organic matter from raw waters could be included in such a fouling mechanism. Calcium carbonate is not heavily involved in the fouling of ultrafiltration membranes, even in highly alkaline waters (200-300 m g / L of alkalinity as C a C 0 ) . The carbon content in the deposit is generally between 10 and 20%, and more than 90% of this carbon has an organic origin. Organic carbon is concentrated on the internal skin of the membrane during filtration. The nature of these organic macromolecules does not depend on the types of waters treated. The organic composition of these "cakes" is about 50% carbohydrates ( M W >5000 daltons) and 25% proteins and polyhydroxyaro matic substances. The average organic composition of the waters suggested a great enrichment of proteins and polyphenols in the cake, probably caused by adsorption phenomena in the internal skin of the membrane and in the cake. Carbohydrates generally have low adsorptive affinity with the mem brane polymer. However, their high concentration in raw water leads to an important contribution to the permanent fouling of the membrane. 3

MORPHOLOGICAL STUDY. This study, conducted with S E M and T E M , enabled us to determine the structure of the unused membrane and its initial



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porosity. The fouled samples, suitably prepared, were observed with S E M at magnifications between X 100 and X 30,000. The membranes, except for the microfiltration membranes, had an internal skin whose porosity seemed to be the first limiting factor of filtration. O n this internal skin, a deposit (between 2 and 30 μιτι thick) showed holes corresponding to the placement of water before the preparation (Figure 8). Structures such as diatoms and other microorganisms were also present, but in very small quantities. We observed some types of small plates with typical inorganic structures. Fig ure 9 shows kaolinite (clay) plates encased in an organic cement. With appropriate preparation conditions for T E M , the deposit also showed this plate structure on its external part (Figure 10); near the internal skin, we found some globular structures that could correspond to organic matter or macromolecules. X-ray microanalysis, which uses an energy-dispersing system with a window before the diode, detected elements by their a- and β-rays (6). It provided a general spectrum specific to each sample. For the Bellerive river water, this analysis showed significant quantities of Fe, A l , and Si. It con firmed the presence of clay in the membrane deposit and the absence of carbonates. However, this instrument was not able to give any further quan titative information on organic compounds. Secondary ion mass spectrometry was applied to fouled membranes. The first example was filtration of the Bellerive water with a poly(ether sulfone) membrane. The plate showing C gives the structure of the mem-

Figure 8. SEM micrograph of a freeze-dried fouled cellulosic membrane.



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Figure 9. SEM micrograph of a fouled polysulfone membrane. brane and of the main organic part of the deposit, in conjunction with the Ν picture (Figure 11). The inorganic part of the deposit is shown in this case by the distribution of A l and Si (Figure 12), indicating clays. A difference of distribution is evident between two aspects of this deposit. The organic substances seem to be packed under the inorganic layer, an observation that could confirm the hypothesis of organic gel formation. The second example reports on a membrane washed with H N 0 (0.3%). The plates were taken for C , Fe, C a (Figure 13), and K . The Fe remains in the deposit, close to the membrane skin. This fact may be explained by precipitation of Fe or complexation with the organic matter. The localization of C a and K, deep in the thickness of the membrane, indicates that chemical cleaning with H N 0 can make these ions soluble, and therefore change the physical and chemical properties of the membrane. This solubilization could explain why, in this case, H N 0 does not lead to any flux recovery. Finally, the measurements on different plates enable us to estimate the thickness of the deposit between 30 and 50 μ ι η , and to confirm the S E M results on lyophilized samples. The SIMS technique appears to be very efficient in observing and analyzing fouled membrane samples, even in the case of fouling elements in the thickness of the membrane. Fourier transform IR-attenuated total reflection was conducted with a Bruker IFS 88 and applied on virgin membrane samples as well as on fouled 3

3

3



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Figure 10. TEM micrograph showing detail of clay in the deposit. membrane. Characteristic alkyl-group absorption may be observed for virgin membrane (Figure 14). In the case of 10-day fouled membrane, these peaks are attenuated and characteristic peaks of clays appear in the 1000-cm" area. The inorganic part of the deposit shows more clearly than the organic part. 1

Conclusion The first purpose of this study was to set up an analytical scheme to char acterize the foulant matter. We also attempted to determine which polymer is most appropriate to use for filtration with each water we study. Such




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Figure 11. SIMS repartition of carbon and nitrogen in a poly(ether sulfone) membrane and its foulant deposit.


M A L L E V I A L L E E T AL.

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4L

Figure 12. SIMS repartition of aluminum and silicon in a poly(ether sulfone) membrane and its foulant deposit.



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Figure 13. SIMS repartition of calcium in a poly(ether sulfone) membrane washed with HN0 (0.3%). 3

— Virgin PP membrane - Internal skin (CH3,CH2,CH,C-C) — Fouled (18 hours) membrane ^ . _ „. . Fouled (to days) membrane 0.16 ( S i

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2000

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Wavenumbers (cm" ) Figure 14. Fourier transform IR spectra of various membranes with total attenuated reflection.



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information could suggest combinations of types of water and membrane polymers that would help us to optimize the process. Our results show that fouling could be linked to the organic matrix, and especially to carbohydrates, proteins, and polyhydroxyaromatic compounds. This link has been shown by the analytical procedure used in this study, which effectively couples the balances of organic matter for different effluants to results obtained by ob servation and microanalysis. A future goal is to study the fouling mechanism by using the same testing setup and analytical procedure with synthetic solutions. We hope to confirm the contribution to the fouling of each family of each organic com pound. The final step of this study would be to implement pretreatment methods for feedwater and dynamic cleaning cycles in order to limit the influence of the fouling in the ultrafiltration process. This work could prob ably help to minimize the influence of fouling in membrane-filtration proc esses, or help to select the right chemical cleaning agent to remove the organic substances responsible for permanent fouling.

References 1. Cheryan, M. Handbook of Ultrafiltration; Technomic Publishing: Lancaster, Basel, Switzerland, 1986; pp 171-174. 2. Matthiason, E. J. MembraneSci.1983, 16, 23-36. 3. Bruchet, A. Thése de 3éme Cycle, Poitiers University, France, 1985. 4. Bruchet, Α.; Anselme, C.; Marsigny, O.; Mallevialle, J. THM Formation Po tential and Organic Content: A New Analytical Approach; Presented at the Grutte Association conference on humic substances, Rennes, France, October 1986. 5. Recueil des Normes Francaises. Eaux méthodes d'essais, 2nd ed.; AFNOR: Paris, France, 1985; ISBN 2-12-179021-7. 6. Seminaire Microscopie Analytique. Laboratoire de spectrochimie infrarouge et Raman, Centre National de Recherche Scientifique: Thiais, France, 1986. 7. Truchet, M. J. Microscopie 1975, 24(1), 1-22. 8. Schnitzer, M.; Khan, S. Humic Substances in the Environment; Marcel Dekker, New York, 1972. 9. Thurman, E. M. Organic Geochemistry of Natural Waters; Martinus Nijhoff/ Dr. W. Junk: Dorchester, Boston, Lancaster, 1985. 10. Malcolm et al. Reconnaissance Samplings and Characterization of Aquatic Humic Substances at the Yuma Desalting Test Facility, Arizona; U.S. Geological Survey: Denver, 1981; p 42; Water Resources Investigations. RECEIVED

for review October 15, 1987.

ACCEPTED

for publication May 18, 1988.


Effects of Humic Substances on Membrane Processes - Advances in

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