Coupling of SEM-EDX and FTIR-ATR to (quantitatively ... · SEM-EDX and FTIR-ATR as complementary tools to analyse ... where flat PVDF MF membranes are immerged in fluid to be treated

Coupling of SEM-EDX and FTIR-ATR to (quantitatively) investigate organic fouling on porous organic composite membranes

M Rabiller-Baudry*,1 , F. Gouttefangeas 2, J. Le Lannic 2 and P. Rabiller3 1 Université Rennes 1, UMR 6226 « Institut des Sciences Chimiques de Rennes » CNRS, 263 avenue du Général Leclerc,

CS 74205, case 1011, 35042 Rennes cedex, France * corresponding author : [email protected] 2 Université Rennes 1, CMEBA, 263 avenue du Général Leclerc, CS 74205, batiment 10A, 35042 Rennes cedex, France 3 Université Rennes 1, UMR 6251 « Institut de Physique de Rennes» CNRS, 263 avenue du Général Leclerc, CS 74205,

35042 Rennes cedex, France

Membrane processes are widely used at industrial scale for separation/concentration purpose. Among filtered fluids are tape water and food fluids, both containing biological components. For this later case the most used membranes are composite organic materials made of three layers that are consecutively crossed by the filtered fluid. The main drawback of membrane processes is the built-up , in the course of the filtration, of a more or less thick fouling layer that can be located both in/on the membrane. As both fouling components and membrane are organic compounds, a specific approach is proposed in this chapter to be able to distinguish carbon, nitrogen and oxygen issued from the membrane and the fouling layer, respectively, by using a post-treatment of raw SEM-EDX results. The methodology is detailed in the case of ultrafiltration membrane made of PES fouled with skim milk , a complex media leading to membrane fouling mainly made of proteins. A complementary analysis is achieved using Fourier Transformed infrared in the total attenuated reflection mode (FTIR-ATR) and the use of a similar kind of raw signal post-treatment than that of SEM-EDX one is also explained.

Keywords SEM-EDX, FTIR-ATR, industrial organic membrane, organic fouling

1. About membrane processes

Membrane based separation techniques which are efficient at molecular level are widely used at industrial scale for concentration or selective extraction of target component(s) from a liquid medium. The main applications concern the drinking water production and effluent treatment as well as treatment of food fluid, and particularly dairy fluids. The separation of the fluid components is achieved trough a porous medium, called membrane, thanks to a driving force obtained by a pressure gradient applied across the membrane material. The initial feed is then separated in two fractions: a retained one, called retentate, and a transport fraction through the porous medium, called permeate (Figure 1a). Several complex mechanisms are required to explain the solute transfer toward the permeate side; among them are molecular sieving and electrostatic interactions between the membrane and the charged solute to be filtered. According to the rejected solute size different techniques are commonly distinguished: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) [1, 2]. MF retains suspension of the order of a micron as well as bacteria. UF is applied for retention of viruses and soluble polymers as proteins and polysaccharides. NF is able to retain small organic solutes within the range of molecular weight from 200 g.mol-1 to 300 g.mol-1. RO is a well-known desalination technique able to retain small inorganic ions as Na+, Cl- and so on. The most commonly used membranes are composite organic materials generally made of at least three layers which are consecutively crossed during filtration by the filtered fluid transmitted toward the permeate side (Figure 1). A sub-micron layer is directly in contact with the feed fluid to be filtered and is commonly called active or selective layer. It can be made of several polymer types (Figure 2) such as polyethersulfone (PES) with or without polyvinyl pyrolidone (PVP) added in order to increase the hydrophilic character, polysulfone (PSU), polyvinylidenedifluoride (PVDF), aromatic polyamides (PA), cellulose based polymers and so on. The intermediate layer is often made of polysulfone (PSU) or sometimes PES. Finally a macro-porous support often made of unwoven polyester gives its mechanical resistance to the whole membrane. Because of the inherent hydrophobic character of the membrane polymers, it easily induces attractive interactions with all types of organic molecules contained in the filtered fluid, the first consequence of it being logically the build up of an organic adsorbed layer on/in the membrane. Thanks to the applied pressure, the formation of the adsorbed layer is generally quickly followed by the build up of a thicker fouling layer, the cohesion of which is due to interactions developed between solutes. Moreover, inorganic components having perhaps less affinity toward the membrane material can be entrapped in the organic layer and thus participate to the global fouling. Systematic fouling is observed when filtering biological and food fluids with polymeric membranes. This is the main drawback of membrane processes and it is well admitted that understanding of membrane fouling by proteins remains a challenge [3].

Current Microscopy Contributions to Advances in Science and Technology (A. Méndez-Vilas, Ed.)

© 2012 FORMATEX 1066

For instance when filtering skim milk, that is a made of about 32 g.L-1 of proteins (molecular weight equal or higher than 14,000 g.mol-1), 45 g.L-1 of lactose and 8 g.L-1 of various organic and inorganic salts, an ultrafiltration PES membrane able to retain about 90 % of 5,000 -10,000 g.mol-1 molecules loose about 75 % of its permeate flux in only few minutes. Of course the choice of process parameters as pressure, cross-flow velocity, temperature, concentration factor have an impact on the fouling level, but it is not possible to totally avoid this phenomenon [4]. In food industry, to ensure hygienic safety of membrane equipment as well as of the that of the produced retentate and permeate fractions, an ‘in place’ cleaning step is regularly required. In dairy industry, cascades of several formulated detergents are used for the recovery of selectivity and flux of organic spiral wound membranes. The twice daily cleaning represents about 30 % of the filtration time. Moreover, the whole cleaning procedure limits the service lifetime of the membranes that must be replaced every 18 to 24 months, depending on the filtered fluid (skim milk, sweet whey, acid whey...). This situation is partly due to the difficulty to found simple and efficient cleaning procedures. It must be underlined that up to now only empirical approaches are used generally based on the determination of the recovery of the membrane permeate flux to water. Clearly such approach is not sufficient to appreciate the cleaning efficiency of a given detergent or a given cascade in an accurate manner. Better procedure would contribute to avoid production failure, the cost of which is always considered to be too high by industrial, and thus to propose more sustainable production processes. In order to be able to propose accurate cleaning procedures, it is necessary to both identify and quantify the components of the fouling layer composition at different state, namely before (few µm thickness) and after (less than 1 µm thickness) the cleaning step. In some cases or after an efficient cleaning, the thickness of the fouling layer is too small to allow elementary analysis of the fouling without large superimposition of the membrane signal. This is why we deliberately develop a dual methodology allowing the simultaneous analysis of the two phases by the use of scanning electron microscopy (SEM) coupled to Energy-Dispersive X-ray spectroscopy (EDX) micro-analysis and Infra-Red spectroscopy in the Attenuated Total Reflection mode (ATR)(fouling layer and membrane) As both fouling components and membrane are organic compounds, a specific treatment of raw data is proposed in this chapter enabling to make distinction of carbon, nitrogen and oxygen issued respectively from the membrane and from the fouling layer. For sake of clarity, the demonstration is made with a PES/PVP ultrafiltration membrane fouled by proteins issued from skim milk, the main membrane application of the nowadays worldwide dairy industry.

Fig. 1: PES composite membrane used in ultrafiltration- (a) Scheme - (b) FESEM pictures (magnification: x 200) highlighting the 3 layers – (c) FESEM pictures (magnification: x 2,000) evidencing more dense active top layer and the porous intermediate layer – (d) FESEM pictures (magnification: x 30,000) showing the continuity between the more dense active layer (pore size of about 6 nm diameter are to small to be evidenced by SEM) and the intermediate layer. See appendix 1 for FESEM experimental details.

Feed / Retentate side

active layer

intermediate layer

macroprous support

Permeate side

(a)

(b)

(c)

(d)

Feed / Retentate side

active layer

intermediate layer

macroprous support

Permeate side

(a)

(b)

(c)

(d)



PES PSU PVP

Fig. 2: Some polymer used in the active layer composition of organic membranes.

2. SEM-EDX and FTIR-ATR as complementary tools to analyse organic fouling on polymeric membranes

In the past, SEM and SEM-EDX has been regularly used to characterize the fouling of RO polyamide membranes of sea water desalination. The particularity of such case is that membranes are not regularly cleaned. Thus both mineral fouling issued from inorganic salts contained in the sea and organic fouling are present in the spiral membranes. The organic part of the fouling is generally a very thick biofilm (micro-organisms installed in the exo-polysaccharides and naturally produced proteins ) that can be observed with eyes without any specific tools. So the fouling layer can be easily scrapped and then analysed evidencing that RO membranes fouling are made of about 50 - 60% biofilm and the complement mainly corresponds to inorganic elements as Fe, Al, Ca, Mg, Si [5]. Such kind of characterisation is quite common now, but out of the scope of this chapter as the membrane is not present in the analysed samples. SEM pictures are often used only to visualise the morphology of fouling on polymeric MF membranes. Much attention is paid to the microbiological part of membrane fouling in membrane bioreactor but without any EDX complementary analysis [6]. SEM-EDX is generally used to show the occurrence of mineral fouling on organic membranes: a quite easy task considering that the inorganic elements which are looked for usually are not present into the membrane . For instance accumulation of metal species can be shown on PSU hollow fibre membranes used in UF of waste water [7]. More interesting for us are the attempts to show organic fouling on organic membranes. The general approach used consists in the EDX evidencing of various elements (for instance C, O, N, Cl, P, Na, Mg, Fe, Al, K, Ca) but no distinction is proposed concerning the C element that can be issued from the organic membrane itself. A typical

example is given in [8] where flat PVDF MF membranes are immerged in fluid to be treated in a membrane bioreactor. The identified fouling layer is, in that case, mainly made of an organic gel deposit and some inorganic species Sometimes, but rarely, a more accurate comment is proposed comparing the signal intensity of a given element only belonging to the membrane (as F for PVDF membrane) and lowered by the presence of fouling on the membrane, as in the case of hollow fibre MF membrane used in submerged membrane bioreactor and for which it is shown that the membrane fouling is mainly due to organic pollutants, whereas inorganic substance’s contribution to membrane fouling is minor [9]. The methodology presented here, illustrated through some examples, is an original and quite unique work which aims at quantifying or having a semi-quantification of organic substances on an organic membrane [1-14].

2.1. Raw signal acquisition by both techniques has similitude

First, in order to avoid any problem with water adsorbed in/on membranes, the samples are carefully dried in a dessiccator under dynamic vacuum during several days. Then dried sample are directly analysed.

2.1.2 SEM-EDX acquisition

Membrane samples are cut and stuck with carbon tape on aluminium or brass holders. A conducting layer is deposited by sputtering onto the insulating membrane to avoid electrostatic charges. The chosen alloy is made of gold (Au) and palladium (Pd). Of course carbon is avoided when dealing with evidencing of carbon on the sample. With the aim of developing a quantitative method, the penetration depth of the electron beam must be controlled as much as possible. This depth depends on different parameters.

- First is the membrane itself, in other words: polymer chemical nature and the different pore sizes of the membrane successive layers. Of course nothing can be done about that.

- Second is the accelerating voltage that is always chosen to be the same. In this case 20 kV is chosen, but it can be underlined that some tests have been made with 10 kV without any specific advantages.

- Others parameters concern the deposit layers on the membrane, either the Au, Pd metal layer and the fouling one.



We decided to control the Au, Pd thickness layer by carefully controlling the sputtering conditions; voltage drop is 1.2 kV and current is 7.5 mA. Because of the high insulating character of the membranes, 3 min is selected after several tests with lower times whereas the tension and current are those commonly used in the SEM centre of University Rennes 1 (CMEBA: www.cmeba.univ-rennes1.fr), regardless of the analysed samples (see appendix 2 for experimental details). Membranes and fouling often are heterogeneous. In order to have an average value that can be considered as representative and not a particular position that could be non representative, the EDX analysis is performed during 50 s on a wide surface of the sample and not on a specific point. For this purpose a 400 magnification of the sample is arbitrarily chosen corresponding to a 198 µm x 198 µm area and the whole picture is analysed. Finally, thanks to the sample preparation and the accelerating voltage of the electron beam during EDX analysis, the main remaining uncontrolled parameter of the penetration depth of electrons in the membrane is the thickness and porosity of the fouling layer (Figure 3). Proteins, polysaccharides and polyphenols are the main organic components encountered in the fouling of membranes used in food industry. In all of them at least C and O elements are present. Moreover proteins show nitrogen (N) and, in a more negligible amount, sulphur (S) due to the occurrence of some amino acids containing thiol groups (S-H) or disulfide bounds (S-S). Regardless of the active layer of the membranes, most UF and NF membranes in food industry are composite ones with a PES or PSU intermediate layer; both containing S element. Thus neglecting S amount coming out of proteins, which is a reasonable assumption according to the amino acid composition, S can be considered as a specific element of the membrane, regardless of the active layer polymer. Table 1 shows the raw EDX analysis of a virgin PES UF (containing a little amount of PVP) and of the same membrane after fouling at lab scale by proteins of skim milk. The fouling layer is about 1-2 µm thickness as shown on FESEM pictures (Figure 4). Clearly the S signal is significantly lower for the fouled membrane than for the virgin one. Conclusion can thus be drawn that the analysed membrane volume is lower in the case of the fouled membrane than in the case of the virgin one, and as a consequence the direct comparison of C amount does not reveal at first sight the part due to the membrane and that due to the fouling layer.

(a) (b)

Fig. 3: Scheme of electron beam penetration in the membrane – (a) fouled membrane - (b) pristine membrane

Table 1 Representative SEM-EDX analysis of a virgin PES membrane (containing few PVP) and of the same membrane fouled at lab scale by proteins issued from skim milk (see Figure 4, quantitative FTIR-ATR leads to a protein surface density of 125 µg.cm-2 for the fouling)

element atomic % atomic % fouled

membrane virgin

membrane C (K) 73.2 70.8 N (K) 2.6 3.0 O (K) 20.3 20.1 S (K) 3.9 6.1

total 100.0 100.0

incident incidentelectron beam electron beam

Au + Pdfouling layer

PES active layer

PES intermediate layer

polyester support

sample support sample support



(a) (b)

Fig. 4: FESEM pictures of a PES membrane fouled at lab scale by protein issued from skim milk- (a) magnification : x 1,000 – (b) magnification : x 30,000 (fouling is the white cloudy layer about 1-2 µm thick on the membrane surface. Such fouling layer corresponds to protein surface density of 125 µg.cm-2 as derived from FTIR-ATR quantitative analysis). See appendix 1 for FESEM experimental details.

2.1.2 FTIR-ATR acquisition

FTIR achieved in transmission mode is a current technique mainly used to evidence chemical bonds between atoms (functional groups) in organic compounds (only few vibrational modes concern inorganic compounds and are out of the scope of this chapter). Stretching or bending vibrational modes of the bonds of functional groups can couple to an IR excitation yielding selective resonant absorption. The characteristic absorption wavelengths

o are specific of the atoms belonging to the selected bound (CH, CO, CN...) o depend on the bound nature (simple, double, triple bound) between two atoms, for instance C-O and C= O are

different cases o depend on the environment of the bound in the whole compound (for instance for the vibration of C=O located

in the 1700-1600 cm-1 region, sub-regions exist according to the functional group, namely carboxylic acid, cetone, aldehyde, amide....

The principle of the ATR mode is slightly different than that of transmission mode but for a given compound similar spectra are obtained by the two acquisition techniques. (see appendix 3 for experimental details). In ATR mode, the IR beam is almost totally reflected in a 2.5 cm by 5.0 cm ZnSe crystal in close contact with the sample. An evanescent waves extend into the sample which can be attenuated by selective absorption in the sample. The IR intensity exiting from the crystal then carries this information which can be recorded. To get reproducible spectra, the contact between the ZnSe crystal and the membrane must be optimal; this is achieved with a press put over the sample to apply a controlled pressure. The IR evanescent waves penetration depth depends on:

o the ZnSe crystal refraction index o the angle of the IR incident beam with the sample surface (controlled by the ZnSe crystal cut configuration) o the membrane material (chemical nature and porosity) o and at least the thickness of the fouling layer, if any.

To achieve quantitative analysis, all parameters used for spectra recording are carefully controlled (coverage of the ZnSe crystal by the membrane sample, IR beam incident angle (45° here for the ZnSe crystal used, allowing 12 reflexions of the signal before the entrance in the DTGS detector). Each spectrum is the average of several scans obtained with a 2 cm-1 resolution. After preliminary experiments ranging from 5 to 100 scans, 20 scans are chosen as it gives an optimal signal to noise ratio for a quite short acquisition time of a few minutes.



(a) (b)

Fig. 5: Scheme of IR beam penetration in the membrane – (a) pristine membrane – (b) fouled membrane Before quantitative analysis of the fouling can be done directly on the membrane, several preliminary experiments have been performed. It was first checked that quantitative analysis of proteins, lactose and sucrose (two disaccharides of food interest) solubilised in water can be extracted from ATR spectra after an appropriate calibration. As said above, proteins, polysaccharides and polyphenols are the main organic components encountered in the fouling of membranes used in food industry. All of them have at least C-O, C-C, and C-H bonds. C-O-C ether bonds are frequent in polysaccharides and polyphenols. Amide functional group (-N-C=O) is typical of proteins and peptides (also known as peptide bond) As already said, regardless of the active layer of the membranes, most UF and NF membranes in food industry are based on either a PES or a PSU intermediate layer. It was checked that ATR-FTIR spectra of such a composite virgin membrane is the sum of those of the active layer and that of the intermediate layer. In the following, for sake of simplification, we have chosen a membrane with both an active layer and an intermediate layer mainly made of PES (but the global approach described here remains valid with polyamide active layer on PSU for instance). Figure 6 shows the raw ATR spectra of a virgin PES UF (containing a little amount of PVP) and of the same membrane after fouling by proteins of skim milk, the fouling layer is about 1-2 µm thickness as observed from FESEM pictures (Figure 4, Table 1). The increase of the absorption of the IR beam due to the occurrence of the fouling layer is obvious, regardless of the wave-number (proportional to the reciprocal value of the wavelength). By comparison between ATR spectra of pure proteins, pure PES and pure saccharides (lactose, sucrose) it can be drawn that the 1240 cm-1 band corresponding to a C-C-O vibration of the PES matrix (or of the PSU matrix) does not overlap with specific vibrations of the potential foulant of the membrane. This band can thus be considered as a valid signature of the PES/PSU membrane. As a consequence, at first sight, the more important its attenuation is, the thicker is the fouling layer. Nevertheless attempts for quantification of the fouling layer only based on this attenuation failed and more sophisticated treatment must be proposed to go ahead.

2.2. Post treatment of raw signals acquired by both techniques is similar

According to explanations given above, with both techniques, by controlling the sample preparation and the recording conditions it is possible to obtain a final signal (EDX element or ATR spectrum) corresponding to the superimposition of the fouling layer and of the membrane. In both cases the membrane signal can easily be to identified thanks to the active layer containing PES/PSU. The sulfur content can be used as the EDX marker of the membrane whereas the 1240 cm-1 absorption band plays this role with the FTIR-ATR. For both techniques raw data have to be processed as described in the following in order to highlight in a quantitative way the presence of the fouling species on the membrane. It mainly consists in the normalisation of the whole signal by that considered as membrane specific.

polyester support


PES active layer

ZnSe crystalincident spectrumIR beam

polyester support


PES active layerfouling layer


polyester support


PES active layer


polyester support


PES active layerfouling layer




Fig.6 Raw FTIR-ATR spectra of virgin PES membrane (blue) and of PES membrane fouled at lab scale with various amount of proteins (black: 125 µg.cm-2 corresponding to pictures in Figure 4, red: 5 µg.cm-2)

2.2.1 SEM-EDX

The treatment first consists in the normalisation of the raw signal of each detected element (further called X) by the S content, leading to the X/S atomic ratio. For organic compounds a particular focus is done on carbon, nitrogen and oxygen. With this kind of membrane and fouling, the identification nitrogen and quantification of its content which is very low, is often tricky and may appear a little bit confusing (Figure 7).

Fig.7 Raw SEM-EDX spectra of virgin PES membrane and of PES membrane fouled at lab scale with 125 µg.cm-2 proteins (corresponding to pictures in Figure 4 and analyses of Table 1) In most cases the N + C sum is constant although respective C and N contents may be very different for different data sets of collected on the same membrane large sheet (plausible explanation can be the heterogeneity of the membrane material or the problem of deconvoluting N and C signals as C is quite high and N quite low). In the following, two groups of elements are considered to highlight the presence of organic compounds, namely (C+N) on one hand and O on the other hand. So the X/S ratio is first systematically determined. As example, Table 2 shows calculations for the virgin and fouled membrane of Table 1. The increase in (C+N) and O is clearly highlighted with the fouled membrane compared to the virgin one. Finally, excess of a given element or group of elements is defined according to equation 1. An example of results is given Table 2.

4000.0 3 600 3200 2 80 0 2400 200 0 1800 1600 140 0 1200 1 00 0 800 600.0

0.01

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.19

cm-1

A

Abso

rban

ce (a

.u.)

1240

Wavenumber (cm-1)4000.0 3 600 3200 2 80 0 2400 200 0 1800 1600 140 0 1200 1 00 0 800 600.0

0.01

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.19

cm-1

A

Abso

rban

ce (a

.u.)

12401240

Wavenumber (cm-1)

Virgin membrane

Full scale 923 counts / cursor: 0.0014 (2985 counts) k eV

Virgin membrane


Fouled membrane


Fouled membrane




X/S excess = (X/S) fouled membrane – (X/S) virgin membrane (1)

Table 2. Post-treatment results of SEM-EDX raw analyse of Table 1

atomic ratio fouled

membrane virgin

membrane

(C+N)/S 19 12

O/S 5 3

S/S 1 1

(C+N)/S excess 7

O/S excess 2

2.2.2 FTIR-ATR

Each type of fouling species is identified thanks to a specific absorption band. In the following for sake of clarification, only the case of fouling by proteins is chosen. Proteins can be evidenced from their amide bands (bond length variation). The amide I band corresponds to the C=O vibration and is located in the range 1630-1650 cm-1 in close to a OH vibration mode of water (harmonic of the large band OH band in the 3600-3000 cm-1 region). The amide II band corresponds to the superimposition of CN and N-H vibrations and is located close to 1530-1550 cm-1. In order to overcome possible troubles due to the presence of remaining water traces, the amide II band is chosen for quantification of proteins on the membrane. As explained above the PES membrane is represented by the 1240 cm-1 band. In order to measure the height of H bands at 1539 cm-1 (H1539, amide II) and at 1240 cm-1 (H1240, PES), a baseline must be chosen. We used the line measured between 2240 cm-1 and 2060 cm-1 as reference because this is a range where non specific, nevertheless non zero absorbance of the membrane occurs (Figure 6) for the PES membrane used here. The treatment first consists in the normalisation of the H height by H1240 leading to the H1539/H1240 ratio (similar approach as that explained with S for EDX). Even if H1240 decreases strongly in presence of protein fouling when compared with the virgin membrane (Figure 6), the H1539/H1240 ratio significantly increases (Table 3). This ratio is then used for the establishment of a calibration curve using 19 membrane samples fouled with known proteins surface densities in the range 0.5 - 350 µg. cm-2.. Finally a linear relationship can be obtained (R2 = 0.997, equation 2) that is further used for direct quantification of proteins on the PES membrane. The precision on the protein amount is ± 1 µg.cm-2.

H 1539 / H1240 = 0.0034 [P] + 0.0165 (2) With: [P]: the protein concentration in µg cm-2

Table 3 Representative FTIR-ATR analysis of a virgin PES membrane (containing few PVP) and of the same membrane fouled by proteins issued from skim milk (see Figure 4 for fouling corresponding to 125 µg. cm-2 proteins surface density as derived from FTIR-ATR quantification)

H1539 H1240 H1539/H1240 virgin PES membrane 0.0349 2.1864 0.01596 PES membrane fouled by 5 µg.cm-2 proteins 0.0102 0.3212 0.03176 PES membrane fouled by 125 µg.cm-2 proteins 0.0271 0.0612 0.44281

2.3. Validation of quantitative or semi-quantitative SEM-EDX versus FTIR-ATR analyse

We have applied our methodology on the 23 m2 industrial spiral module made of 8 assembled membranes. Three membranes have been randomly taken in the cleaned membrane at end of its service life (see appendix 4 for details) from which sample patches are collected at regularly increasing distances from the winding axis of the membranes, the unwounded length ranging from 0 to 120 cm. The protein deposit are very low in this cleaned membrane ranging from 5 to 15 µg.cm-2. Results of both SEM-EDX and FTIR-ATR analyses are summarized in Figures 8 and 9 showing a quite good coherence in fouling distribution over the membrane despite an apparent lower sensitivity for EDX. Attempt of correlation between SEM EDX excess and protein amount is shown Figure 10 a suggesting that a EDX could be quantitative for protein deposit that are lower than 15 µg.cm-2, the exact range acceptable for quantification is not clearly identified here.



0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0 20 40 60 80 100 120 140unwounded membrane length (cm)

H15

39 /H

1240

by

FTIR

-ATR

membrane 1membrane 4membrane 5

Fig. 8 FTIR-ATR raw results for the 3 membranes sampled in the spiral PES membrane fouled and cleaned at industrial scale (see appendix 4 for experimental details)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100 120 140

unwounded membrane length (cm)

(C+N

)/S e

xces

s


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 20 40 60 80 100 120 140

unwounded membrane length (cm)

O/S

exc

ess


Fig. 9 SEM-EDX results for the 3 membranes sampled in the spiral PES membrane fouled and cleaned at industrial scale (see appendix 4 for experimental details) - Results are the average of five measurements on different zones of the same sample.

Similar attempt is also done with more fouled membranes, obtained from the lab scale fouled spiral membrane of 6.5 m2 (Figure 10 b) evidencing that no quantitative result can be expected for protein deposit higher than 15 µg.cm-2 as the SEM-EDX seems to saturate. Increase in the fouling density together with the fouling amount could be one explanation but a systematic study of this assumption is needed before giving a definitive answer.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 2 4 6 8 10 12 14 16 18

proteins (µg.cm-2)

exce

ss b

y ED

X

(C+N)/S excessO/S excess

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140Proteins on PES membrane (µg.cm-2)

Exce

ss in

ED

X

(C+N)/S excessO/S excess

(a) (b)

Fig. 10 Attempt of correlation between SEM-EDX and protein amount obtained from FTIR-ATR - (a) for the 3 membranes sampled in the spiral PES membrane fouled and cleaned at industrial scale – (b) for the different samples issued from a PES spiral membrane fouled (but not cleaned) at lab scale.



4. Conclusion

This chapter deals with an original post-treatment of SEM-EDX and FTIR-ATR raw data obtained when analysing an relatively thin organic fouling made of proteins directly on a polymer composite membrane made at least of a PES or PSU intermediate layer. The overall approach consists in normalisation of the raw signal by that of a specific one only due to the membrane. The specific signal in SEM-EDX in this case is the S atom owing to PES/PSU layer whereas it is a band located at 1240 cm-1 for FTIR-ATR. Then from SEM-EDX an excess of C+N and O elements can be calculated by comparison to the pristine membrane allowing evidencing presence of organic species in the fouling layer. Whereas quantification of proteins of the fouling layer can be achieved by FTIR-ATR in the range 1 to 350 µg.cm-2, some limitations in quantification appear with SEM-EDX, likely due to the evolution of the fouling layer density and structure. Nevertheless basis of quantitative analysis of organic fouling of polymer membranes are given here, and would probably be used in several cases, especially when dealing with polymer modification by organic functional groups. In the membrane field, we apply this methodology now for several years on industrial membranes to understand and overcome breaks in production and to study on a fundamental point of view the better way to efficiently clean membrane, that is a remaining bottleneck of membrane processes for a more sustainable production.

Acknowledgement. The author acknowledge Dorian Lebordais, trainee from IUT Rennes at ISCR membrane group for its technical assistance in performing UF of skim milk and the whole recording in FTIR-ATR analysis of the 336 samples issued from the 6.5 m2 spiral membrane out of them are chosen illustrations for this chapter.

Appendices

Appendix 1: Experimental details for FESEM pictures

Surface microscopy was determined with a field emission scanning electron microscope (FESEM, JSM JEOL 6301 F, Japan) working at a low accelerating voltage of 7 kV. Moreover, the dried membrane samples are put in liquid nitrogen and break in order to reveal the different layers of both the pristine membrane and the fouled ones.

Appendix 2: Experimental details for SEM-EDX

A conventional scanning electron microscope (SEM, JEOL JSM 6400, Japan) is used. It is equipped with ultra-thin window energy dispersive X-ray spectrometer (EDX) and a micro analysis system (LINK INCA Oxford Instruments) allowing quantitative determination of elements. The emission of the characteristic X-rays occurs in a "pear" of size of the order of the micrometer with two consequences: the spatial resolution of the analysis is of the order of the µm and the analyzed depth is also of the micron order, that can raise particular problems for the analysis of small particles or thin films. Experimental conditions have been optimized to have an average overview of the membrane fouling, thus surface elemental analysis and mapping are performed with the following experimental conditions: 20 kV accelerating voltage , 15 mm working distance , 400 magnification and spectrum acquisition during 50 s.

Elements of interest are mainly carbon, oxygen, nitrogen and sulphur for the pristine membrane and for the membrane fouled at lab scale by skim milk UF (see appendix 4). Additional elements are detected for the membrane fouled at industrial scale, namely potassium, sodium, phosphorous, calcium, chlorine, silicium, iron. The total amount of C + N+ O+S is close to 99.5 atom % , the sum of all others elements corresponding to 0.5 atom %..

It is difficult to evidence and quantify nitrogen in these samples because this element is always in very few amount (but its presence has been checked by SIMS analysis), so the deconvolution of C + N large peak is always bad and only a global response corresponding to the sum of the two elements gives reproducible results over many analyse performed through 10 years.

Appendix 3 Experimental details for FTIR-ATR

Infra-red (FTIR) analysis are performed using the Attenuated Total Reflection mode (ATR) directly on the composite membrane. The FTIR-ATR spectra is recorded between 4000 and 600 cm-1 with a spectrometer provided by Perkin-Elmer (Paragon 1000, spectrum for windows software) equipped with a ZnSe crystal with an incidence angle of 45° allowing 12 reflections. The background spectrum is recorded in the air. The acquisition conditions are as follows: 20 scans, 2 cm-1 resolution.

Appendix 4 Experimental details concerning the PES membrane and skim milk ultrafiltration

A PES ultrafiltration membrane (5-10 kg mol-1, HFK-131, Koch, USA) in spiral configuration (6.5 m2) is used for skim milk UF. This membrane is the worldwide main used membrane at industrial scale in dairy industry (about 70 % of cases).

The skim milk used is a commercial one (UHT, Lait de Montagne, Carrefour, France) containing an average of 31.5 g.L−1 proteins and 48 g.L−1 carbohydrates (mainly lactose) and only tracks of lipids (< 0.5 %). Water used either for



solution preparation is deionised and 1 µm filtered. Its conductivity is lower than 1 μS.cm-1. Fouling of the spiral PES membrane is achieved at lab scale by UF of 24 L of skim milk during 2 h at 50°C in batch mode (total recycling of both retentate and permeate in the feed tank). The average cross flow velocity (v) is close to 0.3 m.s-1 . The average transmembrane pressure is TMP= 2.6 bar, but the pressure drop along the spiral element is quite significant (3.7 bar at inlet and 1.5 bar at outlet) allowing a final heterogeneous distribution of the fouling in the membrane partly depending on the TMP gradient. Then membrane is carefully rinsed with deionised water and demounted and cut in 127 cm2 samples that are dried before being analyzed by SEM-EDX and FTIR-ATR. These membrane coupons are then cut again in 9 pieces exactly covering the ZnSe crystal of FTIR-ATR; the fouling ranges from 10 to 130 µg of proteins per square centimetre of membrane (geometric area is considered here and not specific area).

A second PES membrane of the same reference but fouled at industrial scale by ultrafiltration of skim milk is also used (23 m2). When analyzed this membrane was at the end of its service life (8,000 h of UF). A classical and quite efficient cleaning sequence has been performed at industrial scale before analysis. FTIR-ATR evidences protein fouling gradient ranging from 15 to few µg of proteins per square centimetre of membrane. The full autopsy of this membrane is published in [11].

References

[1] Schafer A.I., Fane A.G., Waite T.D. (Eds), Nanofiltration principles and applications, Elsevier, Oxford, UK, 2005. [2] Gorlay S. V (Ed) Handbook of membrane research, properties, performance and applications,), Nova Science Publishers, inc.,

New York, 2, USA, 2010. [3] Chan R., Chen V., Characterization of protein fouling on membranes: opportunities and challenges, J. Membr. Sci.., 2004; 242:

169-188. [4] Rabiller-Baudry M., Paugam L., Delaunay D., Membrane cleaning: a key for sustainable production in dairy industry, in S. V

Gorlay (Ed) Handbook of membrane research, properties, performance and applications, , New York, , USA, Nova Science Publishers, inc;, Chapter 6, 2010.

[5] van Hoof.S.C.J.M., Minnery J.G. and Mack B., Performing a membrane autopsy, Desalination and Water Reuse. 2002. 11/4: 40-46.

[6] Meng F., Liao B., Liang S., Yang F., Zhang H., Song L., Morphological visualization, componential characterization and microbiological identification of membrane fouling in membrane bioreactors (MBRs) Review Article, J. Membr. Sci.. 2010. 361: 1-14.

[7] Zhang G., Liu Z., Membrane fouling and cleaning in ultrafiltration of wastewater from banknote printing works, J. Membr. Sci.. , 2003. 211: 235-249.

[8] Wang Z., Wu Z., Yin X., Tian L., Membrane fouling in a submerged membrane bioreactor (MBR) under sub-critical flux operation: Membrane foulant and gel layer characterization, J. Membr. Sci.. 2008; 325: 238-244.

[9] Bin Z., Baosheng S., Min J., Taishi G., Zhenghong G., Extraction and analysis of extracellular polymeric substances in membrane fouling in submerged MBR, Desalination. 2008; 227: 286-294.

[10] Rabiller-Baudry M. Le Maux M., Chaufer B., Begoin L., Characterisation of cleaned and fouled membrane by ATR-FTIR and EDS analysis coupled with SEM: application to UF of skimmed milk with a PES membrane , Desalination, 2002; 146: 123-128.

[11] Bégoin L., Rabiller-Baudry M., Chaufer B., Faille C., Blanpain-Avet P., Bénézech T., Doneva T., Methodology of analysis of a spiral-wound module. Application to PES membrane for ultrafiltration of skimmed milk. Desalination, 2006; 192: 40-53.

[12] Bégoin L, Rabiller-Baudry M., Chaufer B., Hautbois M. C., Doneva T., Ageing of PES industrial spiral-wound membranes in acid whey ultrafiltration . Desalination, 2006; 192: 25-39.

[13] .Delaunay D., Rabiller-Baudry M., Gozalvez-Zafrilla J.M.,Balannec B., Frappart M., Paugam L., Mapping of protein fouling by FTIR-ATR as experimental tool to study membrane fouling and fluid velocity profile in various geometries and validation by CFD simulation Chem. Eng. Process.., 2008. 47: 1106-1117.

[14] Rabiller-Baudry M., Bégoin L., Delaunay D., Paugam L., Chaufer B., A dual approach of membrane cleaning based on physico-chemistry and hydrodynamics. Application to PES membrane of dairy industry Chem. Eng. Process.., 2008; 47: 267-275.



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