In-situ characterization of concentration polarization layers during
cross-flow membrane separation process of anisotropic colloidal
dispersions by small-angle X-ray scattering
F. Pignon1 M. Abyan1 C. David1 A. Magnin1 M. Sztucki2
1Laboratoire
de Rhéologie, UMR 5520 (UJF - Grenoble I, G-INP, CNRS), BP 53, F38041 Grenoble Cedex 9, France
2European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex 9
France
(E-mail : pignon@ujf-grenoble.fr)
Abstract
The development of new “SAXS cross-flow filtration cells” at the "Laboratoire de Rhéologie" combined with
in-situ time resolved small angle x-ray scattering (SAXS) performed at the European Synchrotron Radiation
Facility (ID02 beamline) has allowed to characterize the colloids structural organization inside the
concentration polarization layer during cross-flow separation process. The filtered suspensions are aqueous
Laponite clay dispersions composed of platelike particles, 30 nm in diameter and 1 nm in thickness. Different
rheological behaviors (shear thinning, thixotropic behavior) have been explored by controlling the mutual
interactions between the particles thanks to addition of peptizer. Concentration profiles in the dynamic
polarization concentration layer have been deduced from the analysis of SAXS patterns obtained along
filtration time as a function of the distance to the membrane (from 100 micrometer of the surface membrane)
with a 50 micrometers vertically collimated x-ray beam. The reversibility of the concentration polarization
layer has been demonstrated being in agreement with permeation flux measurements.
Keywords. Cross-flow Separation, Polarization Layers, Structure, SAXS, Colloid, Laponite.
INTRODUCTION
The performance of the separation process is mainly governed by the accumulation of the deposited colloidal
particles on the filtration membrane. Depending on the transmembrane pressure and cross-flow velocity
conditions, a transition from stable to unstable filtration performance has been underlined and a criteria of
limit of filtration stability has been proposed [1-2]. Some experimental investigation have probed the
structural organization of deposits [3-6] and some others have allowed to follow by small-angle x-ray or
neutron scattering [7-10] the structural organization of colloids near the membrane during frontal
ultrafiltration. But until now, only few attempts [11] have been done to obtain the evolution of the
concentration profile within the concentration polarization layer. The focus of this work is to characterize, insitu, at nanometer length scales, the induced structures in the vicinity of the ultrafiltration membranes, when
colloidal dispersions are simultaneously subjected to a transmembrane pressure and cross-flow over the
membrane.
METHODS
Aqueous Laponite clay dispersions have been studied. This clay is made of plate-like particles 30 nm in
diameter and 1 nm in thickness. The filtered dispersions have been prepared at initial volume fraction v =
0.01 in a solution of distilled water and NaCl (10-3 M) with different content in peptizer (tspp) tetrasodium
diphosphate Na4P2O7. The “SAXS cross-flow filtration cell” used for these measurements is made of
polycarbonate and contains a flat polysulfone ultrafiltration membrane (100kD). In the retentate channel, the
Q cross-flow and P transmembrane pressure are imposed and measured simultaneously and constantly.
Permeate flux J is recovered in a recipient and its mass is registered. The beam (50 m vertically x 250 m
horizontally) crosses the deposited particles allowing to probe multi-level structures and interactions as a
function of the height z above the membrane [3]. Additionally static SAXS measurements were performed in
a temperature controlled flow-through capillary cell and permitted to establish a calibration curve to correlate
the level of absolute scattering intensity to volume fraction v. Consequently from the SAXS patterns
measured during time in the filtration SAXS cell, the particle concentration or volume fraction can be
deduced reliably.
RESULTS
The reversibility of the concentration polarization layer in agreement with permeation flux measurements has
been demonstrated in the results presented in figure 1. In step (1), no change of the concentration is
detected above 100 micrometers from the membrane whatever the change in p for a sufficient cross-flow Q
of 0.45 L/min. When the cross-flow is reduced to 0.2 L/min (step (2)) the concentration in the polarization
layer starts to increase and stabilizes after a few minutes of ultrafiltration. At z = 100 micrometers, the
volume fraction is equal to 4 which correspond to the gel state of these dispersions in the phase diagram.
Furthermore, 2D SAXS patterns exhibit an important increase in orientation of the plate-like particles. A
further increase of the cross-flow to 0.45 L/min (step (3)) induces the disruption of this high concentrated
polarization layer. The corresponding permeation flux reaches then the same level as the one at the end of
step (1).
Q (L/min)
J Permeation Flux
5
P x10 (Pa)
a)
v
Volume fraction  (%)
2
(L/m /h)
6
1.4
30
b)
5
(1)
25
4
1.2
(3)
1
20
0.8
3
15
2
(2)
(4)
10
1
0.4
5
100
300
500
z(m)
5
time (min)Px10 (Pa) Q (L/min)
17
165
194
252
308
0.22
1.2
1.2
1.2
1.2
0.45
0.45
0.2
0.45
0.1
(1)
(2)
(3)
(4)
0.6
0.2
0
0
50
0
100 150 200 250 300 350
Time (min)
Figure 1. Cross-flow separation process of
Laponite dispersions. a) concentration profile
versus distance z deduced from in-situ SAXS and
corresponding 2D SAXS pattern. b) Q Cross-flow,
P transmembrane pressure and corresponding J permeation flux measured simultaneously with scattering
measurements during time. I = 10-3 M, pH = 10, tspp = 0%, T = 29 ± 1°C.
CONCLUSION
The structure and concentration properties of the deposited layers during cross-flow ultrafiltration have been
characterized at pertinent length scales thanks to filtration cells allowing in-situ time resolved SAXS. The
high level of concentration reached in accumulated layers and the highly anisotropic structure have been
identified as one of the main mechanisms controlling the filtration flux decrease. The change in mutual
particle interactions and associate changes in the thixotropic behavior of the dispersions have been linked to
the temporal reversibility of the concentration polarization layers. These presented experimental data
demonstrate the potential of in-situ SAXS methods to obtain pertinent structural information in the vicinity of
membrane surfaces in order to understand the mechanisms controlling the cross-flow separation processes.
REFERENCES
[1] Field R. W. Wu. D. Howell J. A. and Gupta B. B., J. Membr. Sci. 100, 259-272 (1995).
[2] Bacchin P. Si-Hassen D. Starov V. Clifton M. J. and Aimar P., Chem. Eng. Sci. 57, 77-91 (2002).
[3] Su T.J. Lu J.R. Cui Z.F. Thomas R.K. and Heenan R.K., Langmuir 14, 5517 (1998).
[4] Antelmi D., Cabane B., Meireles M., and Aimar P., Langmuir 17, 7137 (2001).
[5] Chen J.C. Li Q. and Elimelech M., Adv. Colloid Interface Sci. 107, 83 (2004).
[6] Madeline J. B. Meireles M. Bourgerette C. Botet R. Schweins R. Cabane B., Langmuir 23, 1645-1658 (2007)
[7] Pignon F. Magnin A. Piau J.-M. Cabane B. Aimar P. Meireles M. and Lindner P., J. Membr. Sci. 174, 189 (2000).
[8] Pignon F. Alemdar A. Magnin A. and Narayanan T., Langmuir, 19, 8638 (2003).
[9] Pignon F. Belina G. Narayanan T. Paubel X. Magnin A. and Gésan-Guiziou G., J. Chem. Phys. 121(16), 8138-8146
(2004).
[10] David C. Pignon F. Narayanan T. Sztucki M. Gésan-Guiziou G. and Magnin A., Langmuir, 24, 4523-4529 (2008).
[11] Pignon F. David C. Magnin A. and Sztucki M., XVth International Congress on Rheology, The Society of Rheology,
80th annual Meeting, Book Series: AIP Conference Proceedings 1027, 120-122 (2008).