Fouling of inorganic membranes during whey ultrafiltration: Analytical methodology

of Membrane Science, 51 (1990) 293-307 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

Journal

293

FOULING OF INORGANIC MEMBRANES DURING WHEY ULTRAFILTRATION: ANALYTICAL METHODOLOGY

J.-P. LABBE Ecole Nationale Superieure de Chimie de Paris, 11, rue P. et M. Curie, 75231 Paris Cedex 05 (France)

A. QUEMERAIS Universite de Rennes 1, Avenue 35042 Rennes CCdex (France)

du General Leclerc,

Campus

de Beaulieu,

F. MICHEL and G. DAUFIN Institut National de la Recherche Laitiere, 65, rue de Saint Brieuc,

Agronomique, 35042 Rennes

Laboratoire de Recherches Cedex (France)

de Technologie

(Received September 12,1989; accepted in revised form February 15,199O)

Summary Ultrafiltration of various types of whey was carried out through an inorganic membrane (M, Carbosep, 20000 Da cut-off). Fouling was evaluated as a hydraulic resistance (R,) and analysed with infrared (IR) and X-ray photoelectron (XPS) spectroscopies, giving complementary results. Spectroscopic data are in a very good agreement with UF flux variations and Rrvalues: the higher the transmembrane pressure or the whey protein content, the higher the fouling protein content. Proteins are found in the bulk of the membrane as well as on its surface, their concentration being higher in the latter case, whereas the phosphate/protein ratio lies often in the range 25-45% whatever the whey type or the operating conditions. Qualitatively, phosphate organization involves at least adsorbed hydrogen-phosphates and apatite structures resulting from several interactions between phosphate, protein, calcium and membrane. Only when the pH is increased up to 6.9 does PO, organization typically reach the apatite lattice. Its level is highest in the bulk of the membrane (4.1% relative to ZrO,), representing nearly 85% of the protein content.

Introduction During ultrafiltration of food liquids, permeate fluxes represent but a fraction (down to a few percents) of their value for pure water. Mass transfer (solvent and small solutes) is carried out through a hydraulic resistance lo50 times greater than that of the membrane itself [ 11.The concentration polarization layer and membrane fouling are the main factors responsible for such resistance limiting ultrafiltration performance in the food industry. Much research has been devoted to the technological and industrial application of ultrafiltration of whey and its derivatives, as it is the main food fluid enhanced

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294

in value by this technique [ 2,3]. The mass transfer of whey and model solutions through an inorganic membrane has been studied. Flux variations in the course of time [ 4,5] have been understood using models based on osmotic pressure, protein adsorption and deposition by convection suggested elsewhere

L&71.

The study of fouling layers is likely to result in a better understanding of phenomena limiting mass transfer, thus allowing better process control (membrane permeability and selectivity, duration, cleaning efficiency). Physical or physico-chemical analysis methods require deposit recovery by applying spongy cylinders [8], ultrasonics [9] or surfactants [ 10-121. Analyses include proteins and amino acids [ 111, proteins, phosphorus, calcium, lactose with an electrophoretic identification of proteins [ 10,131. Proteins can also be measured spectrophotometrically at 750 nm [ 121 or by 14Clabelling [ 14,151. Qualitative analysis is therefore rather easy, whereas quantitative measurements lack accuracy owing to the small amounts of material recovered [ 161. Among physical methods allowing a direct study, SEM observation is often used [8,13,17-201 with the possibility of elemental analysis with an X-ray probe [ 21,221. Lately, more sophisticated methods, such as XPS and IR allowed the identification of the nature of organic membranes [ 11.Both techniques and secondary ions mass spectroscopy (SIMS) were used to analyze antifoaming agents adsorbed on polysulfone membranes [ 231. Having an experience in IR and XPS methods [ 24-261, we were interested in adapting these techniques to get valuable qualitative, quantitative and structural information on fouling layers obtained after tangential ultrafiltration of whey on ZrO, inorganic membranes. Materials and methods Ultrafiltration

tests

Tangential ultrafiltration was performed on laboratory equipment using an inorganic M, Carbosep membrane (38 cm2 area, 20 000 Da cut-off), from tubes kindly provided by TECH-SEP (Miribel, France). The experiments were performed at 50°C during 4 hr. Two transmembrane pressures (TP= 2 and 4 bars) and two fluid tangential flow-rates (1.8 and 4 m-set-l) were used; these conditions were chosen as to obtain, in certain cases, little fouling. The flow regime was always turbulent. Initial membrane cleaning

With filtered (5 ,um) tap water, two reagents were used successively (30 min, 50°C 2 m-see-l, 2 bars), each of them being followed by a water rinsing procedure: 0.2 M NaOH solution of hypochlorite (1000 mg-1-l chlorine) and 0.04 M HNOB. The water flux of the clean membrane, Jw, was measured by ultrafiltering during 15 min (2 bars, 2 m-set-‘, 50’ C). The effect of various clean-

295

ing procedures on fouled membranes has been studied and will be published soon. Fouling layer preservation The fouled membrane was usually characterized by a water flux measurement (& ) after emptying and rinsing all the equipment with tap water (50’ C ) ; therefore, the layers analyzed corresponded to the irreversible fouling [ 51. An attempt to analyse a fraction of the polarization layer by difference was made by excluding the rinsing procedure and simply maintaining the transmembrane pressure (0 or 15 set after flow stopping). A third type of procedure consisted in back-flushing (4 bars, 50” C, 1 hr ) after conventional rinsing. An additional water flux (J& ) measured the alteration of the membrane permeability linked to the residual fouling. After each type of experiment, the Carbosep tubes were cut in four parts (5 cm long). Spectroscopic analyses were performed on the middle samples, far from the ends of the tubes, to ensure a well established flow rate. Fluids used Five fluids were ultrafiltered: - Sweet cheese whey, first skimmed and cooled (4°C ) , fresh (L, ) and after freezing ( LI ). - Ls, sweet cheese whey, clarified according to the treatment described by Fauquant et al. [ 271: thermal treatment (pH 6-6.6; Ca approx. 1.2 g-l-‘) at 79’ C for 20 set followed by 2 hr decantation at 2 ‘C and filtration. - LB, sweet whey protein concentrated by ultrafiltration up to a volumetric concentration ratio (VCR) of 3 (50’ C; hollow fiber PM50 membrane ) . - Lq, clarified sweet whey protein concentrated by UF up to a VCR of about 2. The sweet cheese whey was clarified by tangential microfiltration (MI, Carbosep membrane). Ultrafiltration was performed either at the initial pH 5.6, or at pH 6.9 after sodium hydroxide addition. - Phosphate buffer solution (pH 6.3) containing a mixture of sodium hydrogenphosphates: 10.7 g-1-l NaH,P04, H,O+3.2 g-l-’ Na,HPO,. The composition of the whey samples appears in Table 1. Characterization of mass transfer The membrane permeability was expressed as permeation flux J (2% accuracy). The fluxes measured during ultrafiltration and water rinsing were converted into 3 types of hydraulic resistances: membrane, R,; boundary layer, Rbl; and irreversible fouling, Rf, after treatment of the data from the models, based on the hypothesis that fouling layers behave as apparent serial resistances opposing mass transfer [ 5,6] according to the basic Darcy’s law:

296 TABLE

1

Composition

of fluids (g-l-‘)

Dry matter Protein (NX6.38) /I-Lactoglobulin a-Lactalbumin

L/G

L,

L3

L4

61.5

59.6 6.6

76.0 21.4 11.3 2.4

66.0

8.7 3.2 0.85

Ash P

4.8 0.35

Ca

0.3 6.35

PH

3.6 0.8 6.8 0.32 1.03 5.95

4.9 0.29

12.0 7.0 1.6 4.5

0.36

0.23

6.2

5.616.9

ER=F (R(m-l) =pa_sec_m~~_2_sec_l) Fouling layer analysis

X-Ray photoelectron spectroscopy (XPS) Samples for analysis were cut with a jeweller saw ( 10 x 20 mm), then fixed on a transfer bar in the introduction/preparation bell ( 1O-7 Torr pressure). Subsequently they were transferred, on a sample holder, to the measuring part and placed in the correct position to be studied (5 x 10WIOTorr). The surface under study was exposed to non-monochromatized X-rays from a twin Mg-Al source (1253.6 eV Mg-Ka and/or 1486.6 eV Al-Kcr ). In the case of insulating substances, shifts which could be due to charged surfaces had to be taken into account. The energy calibration was then made from the C1, peak fixed at 285.1 eV (Fig. 1) . Measurements were taken through a HA 100 VSW hemispheric analyser and pulses treated by a computer controlled acquisition system. Results were usually obtained with a 50 eV scanning energy for general spectra, and 25 eV for profiles giving a 1.4 or 1.1 eV resolution. The angle between the source and the analyser lens axis was about 55” and the ejection angle of the electrons from the substrate was about 15” relative to the normal. In these particular conditions, layer thickness analysed is supposed to be not more than 9-10 nm. When interpreting spectra, the layers under study were supposed to be homogeneous and the possible variations resulting from the different attenuation lengths were not corrected. Raw signal values corresponding to Ols, Cls, N1,, Zr 3p312,Ca 2~“‘~ structures were divided by sensitivity factors, taking into account ionization cross-sections and apparatus factors. These sensitivity factors are those given by Briggs and Seah [28] on an experimental basis. XPS could give quantitative esti-

297

ev

-366

-1000

-800

- 600

-zoo

A00 BINDING

WI

- 336

u

ENERGY

1. XPS spectrum: M4 ZrOz membrane - UF of Lz clarified whey (4 bars, 4 m-set-‘, 5O”C, pH 6.9).

Fig.

4 hr,

mations, mainly relative values with calibration standards. In fact, it was difficult to measure less than 0.1 atom%. Moreover, some elements like phosphorus (with a small photoionization cross section) appeared only as traces in whey fouling layers, unless the phosphate precipitation was important. A blank membrane spectrum shows on the one hand good stoicheometric ratios Zr/O + Y/O, on the other hand a thin layer of carbon polluting matter as it always occurs after the unavoidable contact with air. Infrared spectroscopy (IR) Clean or fouled membrane surface was scraped with a microspatula under a binocular magnifying-glass, either lightly to obtain surface substances or more deeply to get both surface and bulk membrane substances, taking great care not to scrape the carbon support of the ZrO, membrane. Samples were made each with 3 mg of membrane from 25 random points. Grinding for at least 10 min. was a major factor for constant absorbance values and good reproducibility of statistical particle size distribution. It was performed in a monocrystalline cu-Al,O, mortar in the presence of CsBr as a dispersing medium; 3 mm diameter micropellets were obtained from O-200 pg membrane and spectra registered (absorbance mode) on Perkin-Elmer spectrometers, dispersive (783) and Fourier transform (1710) models. The spectra obtained from a fouled membrane (Fig. 2-5) revealed separate bands for ZrOz (500 cm-’ ) and the various constituents of the fouling layer: lactose and phosphates in the 1000-1150 cm-l range, proteins (1650 and 1540 cm-‘: amide I and II bands), lipids (1740 cm-’ ). A blank membrane spectrum shows but the 500 cm-’ band. The calculation programme was used in particular to subtract

a possible effect of lactose at 3400 cm-’ (eon) and the contribution of the water deformation effect at 1650 cm-‘. For fouling substances which represented but a few per cents of the membrane, the precision was limited: 8-20% for very low levels ( < 1 wt.% for phosphates and lipids) and small substrate masses ( < 40 pug).Precision was better with a larger amount of membrane (150-200 pug)and fouling substance (proteins: 448%; N= 10 samples). Results could be expressed in mass or atom % units. Results

The fouling of M4 membranes showed a great range of variations for hydraulic resistances ( 1012m-l units), starting from the value of the membrane itself (6.7-8.7) up to 20 times that value. The variation of ultrafiltration fluxes in the three groups of experiments performed reflected the effect of pH, transmembrane pressure and fluid nature. Surface and bulk composition - application to the effect of whey pH The study with L, fluid by XPS, directly on the fouled surface gave information on the outer layers, while CsBr pellets were also examined after scraping samples representative of the whole membrane thickness. Both types of samples were also analyzed by IR (light scraping to obtain surface substances ) (Fig. 2). At pH 5.6, nitrogen and carbon elements were more concentrated in the outer layers than in the bulk (C/Zr = 10 as compared to 0.8; N/Zr= 2.5 as

3000

Fig. 2. IR (1) 50 pg membrane membrane

2000

1500

1000

cm-’

spectra of M, membrane after L, whey ultrafiltration (absorbance vs. wavenumber): crystallized /Clactoglobulin standard; (2) pH 5.6, bulk; (3) pH 5.6, scraped from the surface (x3: scale magnification); (4) pH 6.9, bulk; (5) pH 6.9, scraped from the surface.

299

TABLE 2 Effect of whey pH on fouling layers for M, Carbosep ultrafiltration membrane pH 5.6

Surface Bulk

pH 6.9

Protein

PO,

Ca

Protein

PO,

Ca/Zr

1.3 3.8

1.8 1.0

trace -

6.3 4.8

3.5 4.1

0.001 0.35

Fluid: Ll, 4 bar, 4 m-set-‘,

5O”C, 4 hr (protein and PO, in % ZrO,; Ca mass % ).

compared to 0.17). Low Ca amounts could be detected only on the surface. With IR quantitative measurements (Table 2)) phosphates in addition to proteins also showed a greater content at the surface, with a rather constant PO,/ protein ratio (about 25%). Moreover, the band profile in the PO, range was totally original: two bands at 1015 and 1130 cm-’ did not correspond to any calcium phosphate. At pH 6.9, the layer thickness was so important that zirconium peaks could no longer be detected by XPS on the surface. Likewise, organic matter, particularly proteins, was preferably situated in the outer layers, as opposed to Ca and P. The higher protein content of the surface was confirmed by IR spectroscopy, whereas large amounts of phosphates were detected in the bulk. Moreover, an apatite structure was readily identified. The irreversible fouling was much higher with Rf= 166 as compared to 30 (10” m-l) at pH 5.6. These values are clearly correlated to the corresponding figures for fouling of the bulk of the membrane by proteins and phosphates, determined by IR spectroscopy: 8.9 and 4.8% respectively (Table 2). Cheryan and Merin [ 221 also attributed a greater adsorption when the pH of a cottage cheese whey was raised from 3 to 7 to an “apatite” or “hydroxyapatite”. Nevertheless, both techniques confirmed a greater amount of protein at both pH values. Such a conclusion seemed correct in view of the respective electric charges of proteins (negative) and ZrO, (positive or nearly zero). But phosphates also were present, with at least two formation mechanisms. Problem of layer preservation An attempt to analyse the layers remaining in the absence of a rinsing procedure resulted in a major additional complexity factor, as lactose was present in large amounts, absorbing in the same IR range as phosphates (1000-1150 cm-l ). Nevertheless information could still be obtained by measuring phosphates after a thermal treatment (400’ C) which destroyed the lactose. Unfortunately, no further information could be obtained in this way about the polarization layer: the amount of lactose was too high (30-50% of total fouling), as computed from the difference in spectra at 20 ’ C and 400 oC, especially when the transmembrane pressure was lower (the Zr signal is no longer detected in

300

XPS measurements: Table 3). A higher pressure resulted in a more efficient elimination of the lactose surplus, at the same time retaining a greater quantity of the sum proteins + phosphates (II, and II, compared to I, and I2 ) . Such a phenomenon reflected the reliability of the analytical methods, the results of which were in perfect agreement with Rbl and Rf values (Fig. 3). Lastly, when water back flushing was attempted followed by an additional water rinsing (II, 4)) the improvement observed on hydraulic characteristics TABLE 3 Layer preservation; residual fouling after various procedures Transmembrane pressure (TP )

Procedure

Atomic absorption

XPS (surface )

IR (mass % of ZrO,)

Ca (mass %)

N/C

Zr/C

Lactose

Proteins

PO,

I (2 bars)

1 Mere stop 2 TP maintained 3 Rinsing

0.1 0.1 0.02

0.15 0.18 0.32

0 0 0.02

6.6 3.5 0

3.8 4.4 3.3

1.2 2.0 0.8

II (4 bars)

1 Mere stop 2 TP maintained 3 Rinsing 4 Backflushing*

0.06 0.05 0.10 -

0.16 0.13 0.25 0.27

0 0 0.01 0.1

3.1 2.1 0 0

4.2 5.4 3.6 1.5

1.5 1.2 2.8 0.6

M4 Carbosep membrane; L; whey ultrafiltration 1.8 m-set-‘; *0.3% lipids.

01 0

100

200

50°C; pH 6.5.

D

TimeCmlnl

Fig. 3. Ultrafiltration flux J(t) before fouling layer analysis (L; whey, 1.8 m-set-‘,

50°C).

301

(Fig. 3) was obvious from the analytical point of view. This test was characterized by the lowest ZrjC XPS ratio. Infrared values confirmed the lowest fouling level, also for proteins, phosphates or their sum (overall fouling). A variation of the phosphate band profile was observed, intermediary between apatite and sodium hydrogen phosphate, as will be seen later. Nature of fouling fluid The first fluid (L,) with rather high fouling characteristics (&=23x 101’ m-l ) could serve as a reference. Several points should be stressed: - first of all, the phosphate content was low (approximately 1% ). The calcium value was even lower, which meant the phosphates did play a part in the process, but not as calcium phosphates; this was confirmed by the band profile. Insoluble particles presumably calcium phosphates contained in L’, after thawing were not available for precipitation within the membrane during the UF run. Higher phosphate and calcium contents after L, UF (Table 4) are likely to account for higher fouling resistances (Fig. 3 ), owing to their precipitation within the membrane during L, UF. - for L, (clarified whey), the lower fouling level (R,= 10 x 1012m-l ) was confirmed by several figures: protein content (IR), high value of Zr/C (XPS), which denoted a small surface thickness, and much greater flux value (Fig. 4). The lipid content was lower, but not zero. The presence of lipids on and within a whey microfiltration membrane was emphasized [ 29,301. In this work, such compounds were not totally eliminated by the thermocalcic treatment and still could be concentrated locally at the membrane surface. In retrospect such an observation justifies the efforts put into improving the clarifying process using microfiltration [ 311. TABLE 4 Fouling of a M4 Carbosep ultrafiltration membrane by different fluids”

XPS - Surface (thickness < 7.5 nm)

IR - Bulk (mass % ZrOz)

Zr/C C/C N/C Ca/C Lipids Proteins Phosphates Ca% (atomic absorption)

L

L

0.06 0.42 0.34 0.01 1.57 3.07 1.25 0.018

0.15 0.27 0.26 0 1.34 2.87 1.2 0.02

0

0.42 0.27 0.02 1.11 3.89 1.2 0.015

“1.8 m-set-‘, 5O”C, 4 hr. L,: sweet whey, Lz: clarified sweet whey, L,: clarified sweet whey protein concentrate (VCR= 3 ) .

302

Rbl

h

Rm

Ll

53

23

6.9

L3

85

13

7.6

_....._.________---___

0

100

T~maCm~nl

200

Fig. 4. Ultrafiltration flux as a function of time for various fluids (M4 Carbosep membrane, 2 bars, 1.8 m-set-‘, 50°C): L,, sweet whey; Lz, clarified sweet whey; L,, clarified sweet whey protein concentrate (VCR=3).

for LB,the limited hydraulic resistance (R,= 13x 1012m-‘) for a Rblvalue of 85 x 1012m-l did not appear to represent a good image of the thick film observed by XPS measurement (no zirconium peak) at the surface. IR (bulk) values showed a significantly higher value for proteins, as was expected from their greater concentration in the fluid, but the overall fouling (1 +p + PO,) was only 6.2 as compared to 5.9 for L’, . The idea of a thick film with low hydraulic resisting properties (Fig. 4) was supported by the presence of solid material suspended in such a fluid [ 41. -

Discussion Quantitative

IR determinationxproteins

With constant conditions (grinding, pellet diameter, etc. ) the absorbance A should be proportional to the mass (m) of absorbing substance: A=cm

For zirconium dioxide ZrO,, AzrOz was defined as the difference of the absorbances at 500 cm -i (maximum) and 850 cm-l (minimum). The linear regression coefficient as measured with 18 assays was 0.999 and eZrOy =0.016 pggl (valid for mc65 pup). The amount of ZrO, could thus be obtained from the 500 cm-’ absorbance, either directly or after dilution of a more concentrated pellet. Large amounts of substance (loo-250 pug) could also be determined easily from their mass.

303

The maximum absorbance was obtained for proteins from amide I band at 1650 cm-’ (Fig. 2: 1) with: A, =c,m, Their relative proportion referred to ZrO, was: --mr, -- Ezroz A p mZrOz

E,

Azroz

The main problem to be solved consisted in correcting the contribution of water (1630 cm - ’ ) . This H-O-H bending mode could be computed from the (more important) O-H stretching mode at 3400 cm-‘. As will be seen from Fig. 2 (1) , a crystallized /3-lactoglobulin standard showed a limited OH band at 3400 cm-l, as compared to samples obtained from UF experiments (Fig. 2.2-2.5). Sixteen /3-lactoglobulin standards mixed with wet ZrO, allowed the correction to be obtained: the amide II band (1540 cm-l, less sensitive to water) was used to compute A,, by referring to the same amount of water as in the crystallized standards. The linear regression coefficient was optimized at the p = 0.9976 level with: A, =A1650- 0.46 (&,0

- 0.63 Aw,)

The corresponding value of cZrOz/cp= 0.62 was identical to that obtained in the absence of water in the standards. The results (mass content) were expressed in percentage values with cZrOz/cp=62. Alc5,, was obtained from the spectrum using a tangent (1200 - 1800 cm-l ). For very low /3-lactoglobulin values (m, < 0.5 pg), the organic pollution had to be taken into account, and an overall correction was preferred; with 3 mm diameter pellets, A, =AIGsO -0.362 AzaOO Finally, the resolution was approximately 0.05 pg of protein. Additional phosphate structure From data published on phosphate and calcium stability in a solution, calcium phosphates were expected to precipitate. One interesting papers dealt with the 6-7.5 pH range from 25-37 oC [ 321. More acidic media (down to pH 3.5) and higher temperatures (up to 60” C ) were also studied [ 33-351. Crystallized phases are usually examined; first hydrogenphosphates are formed [ 351; they are converted to less acidic phases in the vicinity of pH 7: CaHP04*2 HaO-+Ca,H,(P0,),.5

H,O

This latter compound crystallizes easily, changing to the hydroxyapatite form in more alkaline media [ 361. The form in which Ca precipitates was claimed

304

not to be affected by milk proteins [ 351, which otherwise are known to inhibit the precipitation. Our results [ 24-261 showed that the thermodynamic view did not suffice to describe actual phenomena on heat exchange surfaces. Crystallized phosphates such as Ca,H,(PO,),*5 Hz0 did exist in such experiments, but only in the absence of other compounds in the solution, and a triple interaction existed _ at the solid surface - between phosphates, calcium and organic molecules of whey or milk. Amorphous compounds and cryptocrystalline apatites were favoured in these conditions, as is the case here with L, at pH 6.9 (Fig. 2: 4 and 5). As previously stated [ 371, fouling phenomena are still more complex in membrane processes. This was true for many experiments related in this paper since a peculiar double band profile (Fig. 2: 2 and 3) could not be assigned to any calcium phosphate. The problem was solved by noticing that the membrane was richer in sodium than in calcium. Ultrafiltering a sodium hydrogen phosphate solution at pH 6.3 gave a quasi zero level of fouling (&=0.8x 10” m-l). The IR spectrum of such a sample was identical to the unknown. The phosphate content was 0.73%, slightly lower than 1% found for L, (Table 2, pH 5.6, bulk) or 1.2% for L,, L2, L, (Table 3), pointing out a protein-ZrO, adsorption adding to the phosphate-ZrOz adsorption (same structure ). It is worth noting that when measuring phosphate at 1040 cm-’ the value of ezronlcp deP en d s on the phosphate structure: 0.32 here, as compared to 0.20 for a crystallized apatite. Area measurements were not significant. Comparison of IR and XPS methods These techniques were complementary: both could be used to elucidate phenomena occurring on the membrane surface or in the bulk. Routine XPS measurements described the surface, giving qualitative atomic information on very thin fouling layers (less than 10 nm thick). Using IR spectra the bulk of the membrane was incorporated, giving quantitative results. Elements and their binding type were recognized by the first technique, structures (including amorphous) by the second method. IR was very sensitive to phosphate but could not detect calcium, the latter being readily evidenced by XPS, contrary to phosphorus. When necessary, other techniques were employed. For example, calcium measurements were sometimes difficult in the vicinity of the Zr 3 p signal (Fig. 1). The presence of calcium in the phosphates was then checked by atomic absorption. From Table 2 there is no doubt that PO, and Ca contents were quite different, confirming the IR spectra (Fig. 2): the main mechanism was not calcium phosphate precipitation. Sodium hydrogen-phosphates in fact did adsorb together with proteins. The latter offered an additional possibility to fix phosphates to the membrane, as seen above. Such mechanisms explained why the PO,/P ratio had but a limited range of variation (25-45% ). A further calcium phosphate precipitation was a third - not independent - phenomenon to be taken into account, particularly with high pH.

305

The only analysis of Ca and P, found on and in the membrane allowed P/Ca ratios ranging from 1.3 to over 2.5 to be calculated [ 11.It clearly appears from this work that the structural information obtained by IR spectrometry substantially enhanced the possibility to explain the phenomena involved in the fouling process. The sensitivity of both spectroscopic methods was sufficiently high to allow the study of the weak fouling layers left after cleaning [ 381. Conclusion The present paper is a contribution to the development of a methodology allowing data to be gathered for a better understanding of the fouling mechanisms during whey ultrafiltration and therefore of the phenomena which control membrane permeability and selectivity. XPS and IR spectroscopy can be used to obtain a quantitative analysis of the main fouling constituents - organic and inorganic - as atoms, characteristic groups and structures (crystallized or amorphous). A good agreement is observed with hydraulic resistances of fouling (R,). The distribution of protein, phosphate and lipid relative to ZrOa is best evidenced after lactose elimination (by rinsing). The methodology suggested is sufficiently reliable and sensitive to be used for various fluids and operating conditions, to characterize the effect of backflushing and the basic influence of whey pH on the actual nature of the fouling layers: when the pH is raised from 5.6 to 6.9 a calcium phosphate precipitation phenomenon is observed supplementary to a sodium hydrogen phosphate adsorption at lower pH values. Proteins are the major constituent in all cases, mainly at the membrane surface, and the phosphate/ protein ratio is remarkably constant in the deposits observed after ultrafiltering three types of wheys. Membrane fouling therefore can be understood as a protein-ZrOz interaction (based on their relative electrical charges), but also includes the overall complex ZrOz-phosphates (Na-Ca)-proteins. Lastly, the importance of the solvent (water) should not be neglected: it is clearly evident in these fouling layers and plays a most important part in the ZrO,-PO,-protein interaction. This study suggests several improvements of the methods used: precision and accuracy of course, but also finer analysis of proteins and their interaction with the membrane (e.g. by exploiting the possibilities of amide II band and other inner ratios), study of fouling heterogeneity by local sampling. Acknowledgement We wish to thank J-L. Maubois for his comments on the manuscript.

306

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adsorption

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