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Structural characterisation of deposits formed during frontal filtration
Journal of Membrane Science 174 (2000) 189–204
Structural characterisation of deposits formed during frontal filtration Frédéric Pignon a , Albert Magnin a , Jean-Michel Piau a , Bernard Cabane b , Pierre Aimar c , Martine Meireles c,∗ , Peter Lindner d a
Laboratoire de Rhéologie, Institut National Polytechnique de Grenoble, Université Joseph Fourier Grenoble I, UMR 5520, B.P. 53, 38041 Grenoble Cedex 9, France b PMMH, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France c Laboratoire de Génie Chimique, 118, route de Narbonne, 31062 Toulouse Cedex, France d Institut Laue-Langevin, 6 rue Jules Horowitz, B.P. 156, 38042 Grenoble Cedex 9, France Received 20 August 1999; accepted 7 March 2000
Abstract Understanding the mechanisms that control the filtration of a complex medium is a major challenge for the development of membrane-based processes in bio-industries, agro-industries or sludge treatment, where the complexity of the fluids is seen in terms of composition (liquid–liquid or liquid–solid mixtures), or physico-chemical characteristics (rheology, stability, etc.). This complexity is likely to induce different material organisations within the fluid depending on concentration and hydrodynamics fields. One of the aims of this study is to determine a relation between structural mechanisms and macroscopic properties of cakes formed on filtration of a colloidal suspension. Our investigations were carried out on clay suspensions. Filtration characteristics of those suspensions were investigated through simple dead end filtration tests and structural characteristics of the deposits formed during filtration were determined by using small-angle neutron scattering (SANS), static light scattering (SLS) and local birefringence techniques, associated with rheometric studies. These led to the conclusion that in the cakes formed from Laponite suspensions (volumic fraction=0.48%, transmembrane pressure=0.5 bar), the particles are packed in an anisotropic arrangement parallel to the membrane. Moreover, we show that upon filtration, an aggregation mechanism is promoted, leading to the formation of a porous fractal structure. This porous structure maintains some permeability in the cake. Adding a peptizer to the suspension changes the characteristics of the cake to a more regular ordering of the particle, with concentration fluctuations that have a lower fractal dimension. This ordered arrangement leads to a lower cake permeability. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Cake filtration; Fractal structures; Neutron scattering; Colloidal fouling
1. Introduction One of the main limiting factors for the development of membrane separation process lies in the formation ∗ Corresponding author. Tel.: +33-561-55-83-04; fax: +33-561-55-61-39. E-mail address: [email protected] (M. Meireles)
of a deposit near the separating membrane. This situation becomes critical when the substances to be separated are macromolecules or colloidal particles, which can combine in concentrated solutions until they form a deposit or physical gel near the membrane. The physico-chemical and structural characteristics of this deposit and its porosity indeed control the efficiency of filtration [1]. Understanding the mechanisms that
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 3 9 4 - X
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control the filtration of a complex medium is a major challenge for the development of membrane-based processes in bio-industries, agro-industries or sludge treatment, where the complexity of the fluids is seen in terms of composition (liquid–liquid or liquid–solid mixtures), or physico-chemical characteristics (rheology, stability, etc.). This complexity is likely to induce different material organisation within the fluid depending on concentration and hydrodynamics fields. Theoretical work associated with numerical predictions and permeation flux measurements [2–4] enabled a description to be given for model particles or macromolecules. It has been shown that the conditions required for colloidal particles to attain their maximum density are strong inter-particle repulsion forces and an optimum size distribution. In the case of more complex media, little work to date has been performed with regard to the influence of morphology, deformability or variability of the initial suspended particle and their aggregates on the characteristics of the deposit. Most of the work undertaken assumes that the structure of the cakes formed on filtration is well described by an hexagonal or cubic packing. However, the structure of these media may be considerably oriented owing to physico-chemical and/or hydrodynamic interactions [5]. Consequently, it is of vital interest to understand the mechanisms whereby the deposit is formed and the influence of concentration and hydrodynamic fields on its structural arrangement in order to predict its occurrence and be in a position to reduce or control its effects. An important step is, therefore, to establish a relation between the microstructure of the cakes formed on filtration and their macroscopic properties, by using experimental and theoretical approaches. Recent studies on alumina and ceramic membranes demonstrate that small-angle neutron scattering is a sensitive technique for characterising pore size distribution [6] and for monitoring the in situ development of the foulant layer inside the membrane pores. During protein filtration, the change in scattering pattern allowed to follow the gradual build-up of an adsorbed layer in the internal porous matrix [7]. More than the assessment of a foulant layer thickness, the challenge here is to use physical techniques such as small-angle neutron scattering to characterise in terms of fractal or random organisation the microst ructure of a cake deposited onto the membrane
as a function of cake thickness and initial states of a suspension. This article presents the first results of a study performed to characterise the microstructure of deposits formed during frontal ultrafiltration of suspensions of anisotropic nano-particles, by means of in situ measurements, combining physical investigation techniques, namely small-angle neutron scattering, static light scattering and birefringence measurements, and rheometric measurements. The filtration characteristics are evaluated as a function of controlled physico-chemical conditions defining different initial states (sol or gel) of the suspension to be filtered. The material used here is an aqueous suspension of synthetic clay, Laponite, consisting of nanometer-sized disc-shaped particles. This suspension was chosen due to its transparency and its great purity, making it a model for natural clays like the bentonites found in the ultrafiltration of water [8], and also in other applications in the petroleum, cosmetics and pharmaceuticals industries. Laponite was also chosen because the strength of the attraction between particles can be modified by adding a peptiser making it possible to define two initial states of the suspension for a given volume fraction. When no peptiser was added to the suspension, the initial state of the suspension is a gel which exhibits a yield stress that has to be exceeded for the suspension to flow. On addition of peptiser, the suspension to be filtered is a liquid suspension that flows under its own weight and exhibits Newtonian-type behaviour law. Rheometric measurements, were used to characterise the mechanical behaviour of the initial state of the clay suspensions to be filtered. Physical techniques such as small-angle neutron scattering or static light scattering are used to identify the relevant length scales responsible for structuring the deposits. These techniques, allied with local birefringence methods, provide information on the structural orientations resulting from filtration in the deposit of anisotropic clay particles. 2. Materials and methods 2.1. Materials The clay used herein, Laponite XLG is manufact ured by Laporte Industrie. Its chemical composition is:
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66.2% SiO2 , 30.2% MgO, 2.9% Na2 O and 0.7% Li2 O, which corresponds to the following chemical formula: Si8 [Mg5.5 Li0.4 H4.0 O24.0 ]0.7− Na0.7 0.7+ The clay suspension consists of polydisperse disc-shaped particles with a diameter of about 30 nm and thickness of 1 nm [9]. The particles have a density of 2.53 g cm−3 [10]. The disc-shaped particles have marked anisotropy and exhibit positive surface charges on their edges and negative surface charges on their sides. Consequently, the arrangement of the particles within the aqueous medium will be strongly influenced by the particle volume fraction, by the pH and by the ionic content of the surrounding aqueous medium. Depending on these three parameters the suspensions can have different states: a stable colloidal solution, an elastic gel, a plastic paste or even separate solid and liquid phases. The gel state is reached above a volume fraction φv∗ , for a given ionic strength, pH and gelation time tp . The phase diagram of these suspensions has been studied recently [11–13]. Three different domains have been defined, an optically isotropic liquid, an optically isotropic gel (above φv∗ ), and an optically birefringent gel at a higher concentration. The sol–gel transition and the structure formation of the gel phase has been studied by means of static scattering measurements [9,11,14–18] and also by dynamic light scattering measurements [19]. The suspensions used in this study were prepared as follows. When a peptiser was not used, the clay powder was mixed in a solution of 10−3 M NaCl bi-distilled water at 20◦ C. The mixture was then stirred with a deflocculating vane at a speed of 800 rpm for 20 min. The peptiser used was tetrasodium-pyrophostate (abbreviated to tspp in the rest of the text): Na4 P2 O7 , 10H2 O, with a molar mass of 446 g mol−1 . This peptiser was added to the suspension at a concentration, denoted Cp , of 6% and calculated as a percentage of the mass of dry clay. This concentration was found to be the optimum concentration to fill the positive sites at the edges of the platelets. The tspp powder was mixed in a solution of 10−3 M NaCl distilled water. At 20◦ C, the mixture was stirred with a deflocculating vane at a speed of 800 rpm for 5 min. The clay was then added and mixed using the vane, at a speed of 800 rpm
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for 20 min. This preparation method ensures that the suspensions were completely homogeneous. The pH of all the preparations used in this study was adjusted to 9.5 in order to avoid any dissolution of the material. Thomson and Butterworth [20] showed that there is significant dissolution of magnesium silicate at pH values of less than 7 and the disappearance of such dissolution at pH values above 9. Two types of suspension were prepared with a clay volume fraction φ v =0.48%, a low ionic strength ([NaCl]=10−3 M) and a pH of 9.5. The first suspension (A1) does not contain any peptiser: φ v =0.48%, Cp =0%. This suspension which is a gel showing an initial connected structure with a fractal dimension D=1, exhibits a yield stress and a minimum stress must be applied for it to lose its connectivity. The second suspension (A2), contains a peptiser: φ v =0.48%, Cp =6%, and is a sol consisting of particles or clusters of disconnected particles with no fractal character and does not show a yield stress. Changes in the rheological properties and structural characteristics of the gels could be observed over time. This is partly due to the osmotic swelling produced by repulsion between the double layers, and partly due to the particles becoming gradually organised in fractal aggregates over increasingly large length scales [17]. The long-term gelation of these Laponite suspensions has also been studied by Mourchid and Levitz [21]. They have shown that under ambient atmosphere, Mg2+ is released from Laponite, suggesting that carbon dioxide from the atmosphere promotes acidification of the suspensions, resulting in a progressive Laponite dissolution and a slow increase of the ionic strength. The release of divalent cations Mg2+ can promote aggregative processes leading to the observation of fractal structures above some micrometers. In order to take into account these structural and mechanical evolutions of the suspensions, the time tp that has elapsed between the end of preparation and the test will always be indicated in the remainder of this text. Furthermore, the initial states (structural and mechanical) of the two suspensions A1 and A2, have been properly characterised at time tp by means of rheometric and scattering measurements. This time tp corresponds to the beginning of filtration.
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reservoir tube determined by a cathetometer with an accuracy of about 0.05 ml. It should be noted that internal fouling was not expected to occur, as the particle size (30 nm×1 nm) is larger than the mean diameter of the membrane pores (2–3 nm). In a typical experiment, where a pressure of 0.5 bar had been applied for 48 h, the permeate volume was about 1 cm3 , and the retentate about 0.4 cm3 . In the retentate, the lower part (up to 5 mm above the membrane) was a solid or gel like deposit, and the upper part (up to 20 mm above the membrane) was a liquid suspension or a gel. 2.3. Scattering techniques
Fig. 1. Diagrammetric frontal view of the ultrafiltration unit developed for the investigation of the deposit by small-angle neutron scattering.
2.2. Filtration apparatus A diagrammetric frontal view of the filtration cell is given in Fig. 1. It was designed to enable in situ scattering and birefringence measurements to be performed. It consists of two bottomless Suprasil rectangular quartz cells (outer dimensions 40 mm×13 mm×2.5 mm, inner dimensions 38 mm×10 mm×1 mm) which are located above and below the membrane. The flow section measures 10 mm×1 mm. The flat asymmetrical organic ultrafiltration membrane is made of sulphonated polysulphone (Diaflo ultrafilters PM 10, Amicon). A seal is inserted between the bottom cell and the membrane. To ensure complete sealing, the cells are tied by a system of rubber rings, then placed in a holder which ensures that the two cells are parallel. The holder is attached to a metal plate, which enables the two-cell and membrane assembly to be moved vertically for micrometer displacements and the control of the impact of the incident beam to a given distance d from the filter, with an accuracy of 0.1 mm. Transmembrane pressure is applied at the top of the column of fluid by purified compressed air. Solvent flux through the membrane and the deposit is measured through time variations of the level of permeate fluid in a
The small-angle neutron scattering (SANS) and static light scattering (SLS) techniques, are well established tools for characterising disordered condensed matter systems like colloidal dispersions. These systems have no rigorously periodic arrangement of atoms and the scattering originates from inhomogeneities in the electron density distribution in the medium. The aim of the scattering techniques is to define accurately the correlation of these structural inhomogeneities and then provide information on size, shape and arrangement of the constituent objects in these dispersions [22]. For SANS measurements, the nuclei of the atoms scatter the neutrons. For SLS measurements, the scattering is due to the fluctuations of dielectric constant of the medium. When coherent light is scattered from a disordered system it gives rise to a random diffraction pattern [23]. From these patterns, a limited structure factor can be interpreted as instantaneous, diffraction-limited structure factor measuring the exact spatial arrangement of the disorder [24]. Angular radiation measurements were carried out over the widest possible domain of scattering wave vectors, in order to obtain structural information ranging from a few nanometres, i.e. the scale of the particle size, to a few micrometres, in order to identify the scale of the largest structures. The wave vector Q =Q·qq of modulus Q is defined by Q = (4π n/λ) sin(θ/2), where n is the refractive index of the suspending medium (in the case of light scattering measurements), λ the wavelength of the radiation and θ the scattering angle.
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The light scattering measurements cover a wave vector domain ranging from 3×10−5 to 4×10−4 Å−1 . The small-angle neutron scattering measurements cover an additional wave vector domain ranging from 10−2 to 2×10−1 Å−1 . From the scattering patterns the angle dependence of the scattered intensity I can be deduced. The radial average of the scattering intenQ) is performed in order to get better statistic sity I(Q correlations. Furthermore, if the scattering intensity Q)∼Q Q−D ) in a parfollows a power law decay (I(Q ticular Q range domain, this can be interpreted as a fractal behaviour [25,26].
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between the glass slides, it might undergo mechanical stress, causing partial loss of structure. The sample was, therefore, left at rest for 10 min before pressure was applied and before any scattering measurements were carried out. Tests were performed to make sure that the rest time was sufficient for the scattering intensity as a function of wave vector not to be affected by any loss of structure. In spite of the slight structural breakdown caused by handling, it was possible to demonstrate a different structure in the initial suspension sample and the sample taken from the deposit. 2.4. Local birefringence
2.3.1. Small-angle neutron scattering (SANS) The measurements were carried out at the Institut Laue-Langevin in Grenoble, using the D11 multi-detector [27], at a wavelength of 6 Å. The distance between the detector and the sample was fixed at 2.5 m, with a beam collimation of 5.5 m. The ultrafiltration cell containing the deposit, with a scattering thickness of 1 mm, was positioned in front of the neutron beam. A diaphragm forming a 1 mm high, 4 mm wide slot was positioned to form a beam of the same size as the slot. The detector consists of 64×64 elements, each measuring 1 cm×1 cm. The isotropic mean of the total scattering intensity was then calculated. For the anisotropic spectra, the scattering intensity was integrated in a 30◦ angular sector in both y and z directions. 2.3.2. Static light scattering (SLS) The laser test facility used for these experiments was developed and built at the Rheology Laboratory [28,29]. It consists of a 2 mW laser beam (He–Ne) with a wavelength of 6328 Å, and a Fresnel lens (focal distance 122 mm and diameter 127 mm) acting as a scattering screen. A beam-stop cuts off beam transmission. The detector was a video camera with a 752×582 pixel CCD sensor. A shutter was used to vary the acquisition time from 1/50 to 1/10000 s. The results were analysed by means of image processing and a program for carrying out classical integration operations. The scattering spectra were obtained by taking samples from the filter cell and placing them between two optical glass slides. The sample has a fixed thickness of 1 mm and the transmission measurements, therefore, gave satisfactory results (I (transmitted)/I (incident)=0.95). When the sample was placed
The experiments were carried out to check the orientations of anisotropic particle deposits using a local birefringence measuring system with a photo-elastic modulator [30,31]. The optical equipment used for the experiments is shown in Fig. 2. It consists of a He–Ne laser beam with a wavelength of λ=6328 Å, polarised by a straight polariser set at 45◦ to the y-axis of the filter cell. The beam then crosses the following: a photo-elastic modulator (PEM) set at 45◦ to the polariser, consisting of a birefringent blade and piezoelectric plate, excited at a frequency of 50 kHz, a quarter-wave plate, set at 0◦ to the polariser, a converging lens with a focal length f=100 mm, giving a measurement that is as local as possible; as it crosses the sample, the beam has a diameter of 0.1 mm, a circular polariser, consisting of a quarter-wave plate and straight polariser set at 45◦ to each other, and a photodiode that detects the intensity of the light beam that has crossed all the previous optical elements. The laser
Fig. 2. Diagrammetric view of the local birefringence measuring apparatus.
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beam was directed perpendicular to the sides of the filter cell. The thickness of the sample e0 was fixed at 1 mm. The transmitted intensity was recorded on the photodiode and was sent to two lock-in amplifiers which allowed to access to the optical anisotropy δ. The birefringence 1n was then deduced by the relationship 1n = (λ/2πe0 )δ. Measurements of sin δ were recorded starting from the lower edge of the cell (near the membrane) and moving up through the retentate, with data being acquired every 0.1 mm. The fact of recording the sin δ of the optical anisotropy and not the optical anisotropy itself gives an uncertainty δ over the domain, where δ is close to 1. However, the values of δ on either side of this domain may be extended by continuity [31]. The birefringence measurements presented in this article were carried out on the deposit formed after a filtration test and after stopping the filtration pressure. The plane of observation (y, z) is the same as that in the scattering measurements. Thus, if the platelets are oriented along either the y or z-axis, the birefringence measurements can be used to compare the results of the different scattering patterns. 2.5. Rheometric techniques The mechanical behaviour of the suspensions under shear was studied with a Weissenberg–Carrimed rotating imposed shear rheometer. The tests were performed at a temperature of 22±1◦ C. Changes in stress were determined for steady conditions as a function of the applied shear rate. Two cone-plate configurations were used, of radius r1 =24.5 mm and r2 =96 mm and angle α 1 =0.076 rad and α 2 =0.07 rad. Torsion bar torque sensors were used. The atmosphere around the sample was saturated with water in order to prevent evaporation during the measurement [32]. An in-depth study of the fluids used for the experiments [33,34] enabled the main rheological characteristics of the suspensions to be determined. 3. Structure and rheological properties of fluids 3.1. Structure of initial suspensions 3.1.1. Suspensions without peptiser The structure of these clay suspensions without any peptiser was studied in a previous work [16,17]
Fig. 3. Schematic representation of a Laponite suspension structure at rest. The network consists of sub-units of oriented particles, micron-sized aggregates formed from a dense stack of sub-units and a fractal mass consisting of a loose mass of micron-sized aggregates.
by combining scattering measurements obtained with various types of radiation (X-rays, neutrons, light). Two characteristic length scales were revealed. It was shown that the structure of the suspensions at rest consists of sub-units measuring a few tens of nanometres, which become organised under the effect of attraction and repulsion forces acting between the particles. They then form regions of varying particle density up to the characteristic scale of a micron. Beyond a micron, the denser regions are aggregated to form a fractal structure of dimension D, giving the gel its three-dimensional structure that is responsible for the occurrence of a yield stress (Fig. 3). This fractal dimension D, which increases with the particle volume fraction, ionic force and gelation time, is correlated with a hardening of the mechanical properties of the gels at rest. Gels studies herein, have a fractal behaviour of dimension D=1, induced by an alignment of the micron-sized aggregates that leads to the formation of a mechanically fragile fibrous structure. 3.1.2. Suspensions with peptiser For the same volume fraction, the suspension changes from a gel to a sol when tetrasodium pyro-
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phosphate is added. The phosphate anion P2 O7 2− stabilises the Laponite suspension [35]. The effect of tspp is to fill in the positive sites at the edges of the platelets, thus tending to make the particles more uniformly repulsive. This effect is reinforced by the diffuse double layer of Na+ ions. Consequently, it may be expected that there will be further suspension of the objects present and a reduction in aggregate formation, possibly leading to a non-connected sol state. However, in addition to an excessive concentration of peptiser, an excess of Na+ counter ions causes the suspension to flocculate [36]. A peptiser concentration equal to 6% of the mass of clay is the optimum, with most of the electropositive sites of the Laponite discs being occupied by an anion, while avoiding any flocculation in the suspension. 3.2. Rheological properties of initial suspensions The rheological properties of colloidal suspensions are highly dependent on the interactions that may occur at microscopic scale between the particles dispersed in the aqueous medium. Within the suspensions, the particles are subjected to various attraction and repulsion forces which, above a certain particle volume fraction, may lead to the formation of a continuous network from one end of the sample to the other [37]. The structure that is formed in this way can only break up under shear once a certain yield stress has been reached. Consequently, these suspensions often have a thixotropic behaviour. Thixotropy can be defined as the tendency of matter to exhibit a time-dependent change in its reference properties (reference viscosity, yield value, time scale, elasticity modulus, loss modulus, etc. or conductivity) when stresses are applied, and to recover these reference properties progressively when stress is relieved [38]. Adding a peptiser reduces this thixotropic effect. Transient conditions are attained more quickly and the viscoelastic properties of the suspensions are less dependent on the sequence of stresses applied. Fig. 4 shows the variation of stress with applied shear rate for Laponite suspensions with and without a peptiser. In the case of a suspension with φ v =0.48% and without any peptiser, there is a stress plateau at low shear rate, while the stresses increase at the highest shear rates. This type of rheograph suggests the existence of a yield stress and shear-thinning behaviour
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Fig. 4. Rheometric shear behaviour of initial Laponite suspensions prior to filtration. [NaCl]=10−3 M, pH=9.5, tp =100 days.
at the highest shear rates. With these suspensions, simply shaking the bottle of gelified solution is enough to liquefy the suspension; it then recovers its initial consistency in a few hours when it is left to rest. In the case of the suspension containing 6% of tspp at a volume fraction φ v =0.48%, the flow curve in Fig. 4 follows a Newtonian-type behaviour, and has no stress plateau at low shear rates. This suspension does not possess a yield stress and is not thixotropic.
4. Characterisation of the ultrafiltration deposit 4.1. Filtration of Laponite suspensions The two types of suspension, A1 and A2, were filtered at a pressure of 0.5 bar for a period of 48 h. The variations of the permeate volume versus elapsed time are shown in Fig. 5. At the beginning of filtration, the variations are comparable for both types of suspension, accounting for experimental uncertainties. As time increases, results show that a higher filtration rate is obtained for the suspension A1 (no peptiser) than for the suspension A2 (with peptiser): the filtration rate measured at steady-state for suspension A1 is two-fold higher than those measured for suspension A2. 4.2. Small-scale structuring of deposits (SANS) Figs. 6 and 7 represent the spectra obtained by SANS in the cake at different distances d from the
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Fig. 5. Filtration characteristics of Laponite suspensions subjected to filtration at a pressure of 0.5 bar for 48 h. Influence of the initial structure of the suspensions on filtration kinetics; suspensions A1: φ v =0.48%, Cp =0%, suspension A2: φ v =0.48%, Cp =6%; [NaCl]=10−3 M, pH=9.5, tp =90 days.
membrane, for suspensions A1 and A2, respectively. The z-axis of each pattern corresponds to the horizontal axis of the detector plane, parallel to the membrane, and the y-axis to the vertical axis, along the flow direction (Fig. 1). An important result is that the spectra are increasingly anisotropic as the impact of the beam is moved from the top of the cell to the vicinity of the membrane. It is also clear from Figs. 6 and 7 that the patterns are not the same for the two suspensions. In order to measure the anisotropy, the intensities of the pattern were integrated in a 30◦ angular section along the z direction, and in a similar sector along the y direction (Fig. 8). This was done for the patterns obtained at two distances from the membrane, d=5 mm, i.e. far from the membrane, corresponding to the most isotropic spectrum, and d=0.5 mm, i.e. in the filtration deposit, corresponding to the most anisotropic spectrum. In the retentate of suspension A1, far from the membrane (d=5 mm, Fig. 8), the scattered intensities integrated along the z and y directions are identical and they both decay according to a Q −2 law over most of the range of Q vectors (2×10−2 to 2×10−1 Å−1 ). These Q values correspond to interferences between pairs of scatterers that are separated by distances of 30–300 Å, i.e. intraparticle distances or distances between neighbouring particles. The Q −2 law is the law expected for disks that are randomly oriented and ran-
domly spaced in the suspension. Accordingly, the retentate can be described, at short length scales, as a suspension of particles that have only weak correlations with their neighbours. In the deposit of suspension A1, close to the membrane (d=0.5 mm, Figs. 6 and 8) the intensity integrated along the y direction decays much less than along the z direction. This anisotropy demonstrates that the particles have their short axes along the y direction. For randomly spaced particles, the decay would follow the Guinier law: (I Q/I (Q → 0)) = 1 − (Q2 Ry2 /3) [39,40], where Ry is the half dimension of the particles in the y direction. The fit of Guinier law on experimental data yields Ry =1.21 nm, which is consistent with the known thickness of Laponite platelets (about 1 nm). In the deposit of suspension A2, (with a peptizer), the scattering pattern has two peaks located on either side of the beam, in the y direction, and a depression between these peaks (Figs. 7 and 10). The depression at low Q reflects repulsive correlations between neighbouring particles. As the beam is placed closer to the membrane, the depression becomes more pronounced, and the peaks more intense, indicating stronger correlations (Fig. 11). The peak locations correspond to an average interparticle distance along the y direcQmax . It is clear tion which is calculated as dy =2π /Q that dy decreases considerably upon approaching the membrane. At the shortest distance (d=1.2 mm) it is dy =78 Å, which would correspond to a lamellar array of volume fraction φ v =10%, i.e. 20 times more concentrated than the original suspension. 4.3. Large-scale structuring of deposits (SLS) Q) determined Figs. 12 and 13 show the intensity I(Q by static light scattering in suspensions A1 and A2 before filtration and in deposits formed on filtration at a pressure of 0.5 bar for 15 h. In Fig. 12, obtained with suspension A1 without any peptiser, for wave vectors between Q =4×10−5 and 2×10−4 Å−1 , the fractal dimension increases from 1 when the suspension is at rest, to 2 for the deposit. This demonstrates that the gel is more densely structured in the deposit in comparison with the suspension at rest. With regard to the higher wave vectors, it can be seen that the scattering intensity as a function of the wave vector follows a Q −3 power law that is characteristic
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Fig. 6. Small-angle neutron scattering patterns for the deposit formed in frontal filtration of a Laponite suspension under 0.5 bar pressure for 48 h filtration time. d=Observation distance to the membrane; suspension A1: φ v =0.48%, Cp =0% tspp, [NaCl]=10−3 M, pH=9.5, tp =38 days.
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Fig. 7. Small-angle neutron scattering patterns for the deposit formed in frontal filtration of a Laponite suspension under 0.5 bar pressure for 48 h filtration time. d=Observation distance to the membrane; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5 tp =121 days.
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Fig. 8. Variations of scattering intensity with wave number from small-angle neutron scattering of a deposit after 48 h ultrafiltration at a transmembrane pressure of 0.5 bar; suspension A1: φ v =0.48%, Cp =0% tspp, [NaCl]=10−3 M, pH=9.5, tp =38 days.
of the presence of micron-sized aggregates. It may be Q) curves that the deposit is strucdeduced from the I(Q tured in a different way from the initial suspension. It is suggested that at these length scales (0.5–2.5 mm),
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the deposit still consists of dense micron-sized aggregates, that are identical to those at rest but are more numerous and more connected. Thus upon filtration, the structure of the suspension has changed from a loose fibrous network to a denser and more connected fractal gel (Fig. 9a). The static light scattering experiments, obtained from suspension A2 with a peptiser, Fig. 13, also displays a structural modification upon filtration: while the initial suspension has no fractal structure, we could observe that within the deposit, for wave vectors ranging from Q =4×10−5 to 2×10−4 Å−1 , the scattering intensity is proportional to Q −1.3 . It is suggested that at these length scales, filtration has promoted a gathering of the particles into micron-sized aggregates and enabled the formation of a fractal structure. These results underline the great influence of the initial structure of the suspension on the subsequent structural characteristics of the deposit obtained with filtration time, pressure conditions and initial particle volume fractions being the same. Filtering structured A1 suspensions produces deposits with a larger frac-
Fig. 9. (a) Schematic representation of the microstructure of a deposit formed by frontal filtration at a transmembrane pressure of 0.5 bar; Laponite suspension A1: φ v =0.48%, Cp =0% tspp and (b) schematic representation of the microstructure of a deposit formed by frontal filtration at a transmembrane pressure of 0.5 bar; Laponite suspension A2: φ v =0.48%, Cp =6% tspp.
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Fig. 10. Variations of scattering intensity with wave number from small-angle neutron scattering of a deposit after 48 h ultrafiltration at a transmembrane pressure of 0.5 bar; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5, tp =121 days.
tal dimension (D=2) than the deposits obtained from initially unstructured A2 suspensions. It would be interesting to check whether longer filtration times with the A2 suspensions would produce more compact deposits, with a larger fractal dimension than D=1.3. 4.4. Orientation of particles near the deposit Fig. 14 shows the birefringence curve 1n as a function of the distance d to the membrane for the two
Fig. 11. Evaluation of the inter-particle distance from maximum scattering intensity along y as a function of distance d to the membrane. Small-angle neutron scattering of a deposit after 48 h ultrafiltration at a transmembrane pressure of 0.5 bar; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5, tp =121 days.
Fig. 12. Comparison of static light scattering of a Laponite suspension at rest with static light scattering of a deposit formed from a Laponite suspension after 15 h frontal filtration at a transmembrane pressure of 0.5 bar; suspension A1: φ v =0.48%, Cp =0% tspp, [NaCl]=10−3 M, pH=9.5, tp =22 days.
types of suspension A1 and A2 after 48 h filtration at 0.5 bar. With both initial types of suspension, there is considerable birefringence near the filter, which then decreases with distance to become zero beyond a certain distance dg . This indicates that the structure is highly birefringent near the filter, becoming less and less so at the top of the cell. It may be noted that for suspension A1 this distance dg =6 mm is larger than the distance dg =4 mm obtained with suspension A2. The distance dg must be related to the thickness of the
Fig. 13. Comparison of static light scattering of a Laponite suspension at rest with static light scattering of a deposit formed from a Laponite suspension after 15 h frontal filtration at a transmembrane pressure of 0.5 bar; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5, tp =24 days.
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Fig. 14. In situ local birefringence measurements performed on a deposit formed from a Laponite suspension after a 48 h ultrafiltration at a transmembrane pressure of 0.5 bar. Effect of initial state of suspension on state of orientation in the deposit φv =0.48%, [NaCl]=10−3 M, pH=9.5, tp =100 days.
filtration deposit as it is a characteristic signature of the orientations induced by the presence of the membrane. This increase in birefringence near the filter corresponds to orientation of the particles forming the filtration deposit. It should be noted that, for the case of the initial suspension with no peptiser, the birefringence fluctuates on approaching the filter, which is not the case with suspension A2 containing peptiser. Thus, for the deposits obtained from type A2 suspensions containing peptiser, it may be concluded that: (i) the orientation of objects parallel to the membrane is more homogeneous, and (ii) structuring takes place over smaller distances than with the type A1 suspensions with no peptiser. 5. Analysis of structuring mechanisms By examining the rheometric and structural characteristics of filtered deposits obtained from clay suspensions with and without peptisers, we can get some insights on the mechanisms whereby these deposits are formed. Small-angle neutron scattering tests revealed an anisotropic increase in scattering intensity in the deposits at decreasing observation distances from the membrane. Moreover, depending on the initial state of the suspension, considerable differences were observed in this anisotropy.
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In the case of a non-connected initial suspension, an increase in intensity is observed with the shape of two intensity peaks revealing a constant inter-particle distance. At the shortest distance from the membrane (d=1.2 mm) this constant inter-particle distance was estimated of the order of 78 Å, which corresponds to a lamellar array of volumic fraction φ v =10%. In the case of an initial suspension consisting of aggregated and interconnected particles, the increase in intensity is consistent with increased fluctuations in concentrations of the particles. For both suspensions, the anisotropy of the scattering spectra near the membrane indicates greater orientation of the particles, over inter-particle distances of the order of magnitude of the size of the elementary particles, i.e. a few nanometres. It is interesting to note that such orientations at these length scales have never before been observed on structured Laponite suspensions in the same concentration domains. Light scattering measurements show large-scale organisation of the particles, which become connected into micron-sized aggregates to form fractal clusters. The fractal dimension of this large-scale structure also depends on the initial conditions of the suspension before filtration. Filtration of a connected suspension characterised by a fractal dimension of 1, induces a densification of the structure, the fractal dimension of which reaches a value of 2. Filtration of a suspension of unconnected particles, also induces a structuration, but the resulting fractal dimension is less than for the previous case. Birefringence measurements clearly show the orientation of the particles and enable a gelification distance dg to be evaluated. With both types of initial conditions, the orientation increases on approaching the membrane, but the distance beyond which this orientation disappears is more important in the case where the initial structure is connected. Moreover, in this case, fluctuations in orientation are observed, which should be compared with the fluctuations in concentration observed with small-angle neutron scattering. These in situ physical characterisations enable a structural model to be proposed for the formation of a deposit from a Laponite suspension during filtration, depending on the initial state of the suspension.
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With a connected suspension, without any peptiser, the deposit is composed of clay particles which are oriented in a direction parallel to the membrane. Among these are dense micron-sized aggregates promoted by the concentration subsequent to the filtration. These dense micron-sized aggregates have a fractal dimension which is above that of the suspension at rest. The juxtaposition of domains consisting of highly oriented particles and micron-sized aggregates may explain the fluctuations in particle concentration detected by neutron scattering. At increasing distances from the membrane, the particles are oriented in an increasingly random manner, and it may be assumed that the micron-sized aggregates are arranged with a smaller fractal dimension than that of the cake. The filtration process will then result in a concentration of the initial areas of low particle density in such a way that particles are forced to adopt an orientation mainly parallel to the membrane. Areas of initially higher density, i.e. the micron-sized aggregates, seem to be hardly modified by filtration but their presence makes the deposit more heterogeneous (Fig. 9a). Such arrangement which forms a relatively porous three-dimensional structure presumably accounts for the observed filtration characteristics. With an unconnected initial suspension, the deposit consists mainly of isolated particles oriented parallel to the membrane. Further away from the membrane, the platelets are oriented in an increasingly random manner, until they are scattered isotropically in the upper part of the cell. Within this ordered arrangement of particles, certain more concentrated areas are formed. Indeed, a large-scale fractal structure appears, which seems to indicate that the isolated particles, subjected to filtration conditions, tend to form dense clusters connected in a fractal structure. However, in contrast to the case without any peptiser, these dense clusters have an internal orientation leading to a constant inter-particle distance that can be detected by the local birefringence techniques described above. Near the membrane, the particles in the deposit are subjected mainly to excluded volume effects and are arranged face to face, thus reducing the porosity of the medium and hence the filtration performance (Fig. 9b). In conclusion, initially connected suspensions tend to form more heterogeneous deposits, consisting of objects that are more widely spaced and less oriented
than the deposits formed from initially unconnected suspensions. These differences that have been evidenced at a microscales are consistent with the differences observed in filtration characteristics of both types of suspensions estimated by macroscopic filtration tests.
6. Conclusions This study clearly shows the major contribution made by using in situ physical investigation techniques to determine the structure of ultrafiltration deposits. The aim is to understand the mechanisms whereby these deposits are structured, in order to improve the separation processes used to filter complex media. By using angular radiation scattering and birefringence techniques associated with rheometric analyses, it was possible to obtain information on the structuring of anisotropic colloidal particles in an aqueous medium. It appears that the filtration fluxes are limited by an anisotropic arrangement of the particles parallel to the membrane, irrespective of the initial physico-chemical and structural conditions. In addition, it was shown that the initial structure of the suspensions at rest has a decisive effect on the efficiency of separation. Indeed, an initially connected gel increases filtration flux by adopting a more porous structure than a suspension initially in a liquid state. Lastly, these techniques hold out interesting prospects, as they can be used to observe the formation of deposits in real time and, thus, to monitor transient mechanical and structural changes occurring in the deposits. This work is indeed continuing and being extended to deposits formed during tangential ultrafiltration.
7. Nomenclature Cp D d Q n
concentration of peptiser in the suspension (%) fractal dimension distance from the membrane (m) wave vector (m−1 ) refractive index
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Greek letters φ v volumic fraction of Laponite in suspension (%) φv∗ critical gel volumic fraction (%) λ wavelength of radiation (m) References [1] P. Bacchin, P. Aimar, V. Sanchez, Model for colloidal fouling of membranes, AIChE J. 41 (2) (1995) 368. [2] A.A. Kozinski, E.N. Lightfoot, Protein ultrafiltration: a general example of boundary layer filtration, AIChE J. 18 (5) (1972) 1030–1040. [3] R.M. Mc Donough, C.D.J. Fell, A.G. Fane, Surface charge and permeability in the ultrafiltration of non-flocculating colloids, J. Membr. Sci. 21 (1984) 285. [4] L. Song, M. Elimelech, Theory of concentration polarization in cross flow filtration, J. Chem. Soc., Faraday Trans. 91 (1995) 3389. [5] T.D. Waite, A.I. Schäfer, A.G. Fane, A. Heuer, Colloidal fouling of ultrafiltration membranes: impact of aggregate structure and size, J. Colloid Interface Sci. 212 (1999) 264– 274. [6] K.L. Stefanopolous, G.E. Romanos, A.Ch. Mitropolous, N.K. Kanellopoulos, R.K. Heenan, Characterisation of porous alumina membrane by adsorption in conjunction with SANS, J. Membr. Sci. 153 (1999) 1–7. [7] T.J. Su, J.R. Lu, Z.F. Cui, R. K Thomas, R.K. Heenan, Application of small angle neutron scattering to in situ study of protein fouling on ceramic membranes, Langmuir 14 (1998) 5517–5520. [8] P. Bacchin, P. Aimar, V. Sanchez, Influence of surface interaction on transfer during colloid ultrafiltration, J. Membr. Sci. 115 (1996) 49–63. [9] J.D.F. Ramsay, S.W. Swanton, J. Bunce, Swelling, dispersion of smectite clay colloids: determination of structure by neutron and small-angle neutron scattering, J. Chem. Soc., Faraday Trans. 86 (1990) 3919. [10] L. Rosta, H.R. Von Gunten, Light scattering characterization of Laponite sols, J. Colloid Interface Sci. 134 (2) (1990) 397–406. [11] A. Mourchid, A. Delville, J. Lambard, E. Lécolier, P. Levitz, Phase, diagram of colloidal dispersions of anisotropic charged particles: equilibrium properties, structure, and rheology of Laponite suspensions, Langmuir 11 (1995) 1942–1950. [12] J.C. Gabriel, C. Sanchez, P. Davidson, Observation of nematic liquid–crystal textures in aqueous gels of smectite clays, J. Phys. Chem. 100 (1996) 11139–11143. [13] A. Mourchid, E. Lécolier, E.H. Vandamme, P. Levitz, On viscoelastic, birefringent, and swelling properties of Laponite clay suspensions: revisited phase diagram, Langmuir 14 (17) (1998) 4718–4723. [14] J.D.F. Ramsay, P. Lindner, Small-angle neutron scattering investigations of the structure of thixotropic dispersions of smectite clay colloids, J. Chem. Soc., Faraday Trans. 89 (23) (1993) 4207–4214.
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[15] M. Morvan, D. Espinat, J. Lambard, Th. Zemb, Ultrasmalland small-angle X-ray scattering of smectite clay suspensions, Colloid Surf. A: Physicochem. Eng. Aspects 82 (1994) 193– 203. [16] F. Pignon, J.M. Piau, A. Magnin, Structure and pertinent length scale of a discotic clay gel, Phys. Rev. Lett. 76 (1996) 4857–4860. [17] F. Pignon, A. Magnin, J.M. Piau, B. Cabane, P. Lindner, O. Diat, A yield Stress thixotropic clay suspension: investigations of structure by light, neutron and X-ray scattering, Phys. Rev. E 56 (1997) 3281–3289. [18] M. Kroon, W.L. Vos, G.H. Wegdam, Structure and formation of a gel of collo¨ıdal disks, Phys. Rev. E 57 (1998) 1962–1970. [19] M. Kroon, G.H. Wegdam, R. Sprik, Dynamic light scattering studies on the sol–gel transition of a suspension of anisotropic colloidal particles, Phys. Rev. E 54 (1996) 6541–6550. [20] D.W. Thomson, J.T. Butterworth, The nature of Laponite and its aqueous dispersions, J. Colloid Interface Sci. 151 (1992) 236. [21] A. Mourchid, P. Levitz, Long-term gelation of Laponite aqueous dispersions, Phys. Rev. E 57 (1998) R4887–R4890. [22] P. Lindner, Th. Zemb (Eds.), Neutron, X-ray and Light Scattering, Elsevier, Amsterdam, 1991. [23] B. Chu, Laser Light Scattering, Basic Principles and Practice, Academic Press, New York, 1991. [24] O. Diat, T. Nayayanan, D.L. Abernathy, G. Grübet, Small angle X-ray scattering from dynamic processes, Curr. Opinion Colloid Interface Sci. 3 (1998) 305–311. [25] G. Dietler, C. Aubert, D.S. Cannell, P. Wiltzius, Gelation of colloidal silica, Phys. Rev. Lett. 57 (14) (1986) 3117–3120. [26] M.Y. Lin, R. Klein, H.M. Linsday, D.A. Weitz, R.C. Ball, P. Meakin, The structure of fractal aggregates of finite extent, J. Colloid Interface Sci. 137 (1) (1990) 263–279. [27] P. Lindner, R. C Oberthür, Apparatus for the investigation of liquid systems in a shear gradient by small angle neutron scattering (SANS), Revue Phys. Appl. 19 (1984) 759. [28] M. Dorget, Propriétés rhéologiques des composés silice–silicone, thèse de l’Institut National Polytechnique de Grenoble, 1995. [29] J.M. Piau, M. Dorget, J.F. Palierne, A. Pouchelon, Shear elasticity and yield stress of silica–silicone physical gels: fractal approach, J. Rheol. 43 (2) (1998) 305–314. [30] G.G. Fuller, Optical Rheometry, Annu. Rev. Fluid Mech. 22 (1990) 387–417. [31] G.G. Fuller, Optical Rheometry of Complex Fluids, Oxford University Press, Oxford, 1995. [32] A. Magnin, J.M. Piau, Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel, J. Non-Newtonian Fluid Mech. 36 (1990) 85–108. [33] F. Pignon, A. Magnin, J.M. Piau, Thixotropic colloidal suspension and flow curves with minimum: identification of flow regimes and rheometrical consequences, J. Rheol. 40 (1996) 573–587. [34] F. Pignon, A. Magnin, J.M. Piau, Thixotropic behavior of clay dispersions: combinations of scattering and rheometric techniques, J. Rheol. 42 (1998) 1349–1373.
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[35] B.S. Neumann, K.G. Sansom, The study of gel formation and flocculation in aqueous clay dispersions by optical and rheological methods, Israel J. Chem. 8 (1970) 315–322. [36] H. Tateyama, P.J. Scales, M. Ooi, S. Nishimura, K. Rees, T.W. Healy, X-ray diffraction and rheology study of highly ordered clay platelet alignment in aqueous solutions of sodium tripolyphosphate, Langmuir 13 (1997) 2440–2446. [37] W.B. Russel, Review of the role of colloidal forces in the rheology of suspensions, J. Rheol. 24 (1980) 287–317.
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Structural characterisation of deposits formed during frontal filtration Frédéric Pignon a , Albert Magnin a , Jean-Michel Piau a , Bernard Cabane b , Pierre Aimar c , Martine Meireles c,∗ , Peter Lindner d a
Laboratoire de Rhéologie, Institut National Polytechnique de Grenoble, Université Joseph Fourier Grenoble I, UMR 5520, B.P. 53, 38041 Grenoble Cedex 9, France b PMMH, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France c Laboratoire de Génie Chimique, 118, route de Narbonne, 31062 Toulouse Cedex, France d Institut Laue-Langevin, 6 rue Jules Horowitz, B.P. 156, 38042 Grenoble Cedex 9, France Received 20 August 1999; accepted 7 March 2000
Abstract Understanding the mechanisms that control the filtration of a complex medium is a major challenge for the development of membrane-based processes in bio-industries, agro-industries or sludge treatment, where the complexity of the fluids is seen in terms of composition (liquid–liquid or liquid–solid mixtures), or physico-chemical characteristics (rheology, stability, etc.). This complexity is likely to induce different material organisations within the fluid depending on concentration and hydrodynamics fields. One of the aims of this study is to determine a relation between structural mechanisms and macroscopic properties of cakes formed on filtration of a colloidal suspension. Our investigations were carried out on clay suspensions. Filtration characteristics of those suspensions were investigated through simple dead end filtration tests and structural characteristics of the deposits formed during filtration were determined by using small-angle neutron scattering (SANS), static light scattering (SLS) and local birefringence techniques, associated with rheometric studies. These led to the conclusion that in the cakes formed from Laponite suspensions (volumic fraction=0.48%, transmembrane pressure=0.5 bar), the particles are packed in an anisotropic arrangement parallel to the membrane. Moreover, we show that upon filtration, an aggregation mechanism is promoted, leading to the formation of a porous fractal structure. This porous structure maintains some permeability in the cake. Adding a peptizer to the suspension changes the characteristics of the cake to a more regular ordering of the particle, with concentration fluctuations that have a lower fractal dimension. This ordered arrangement leads to a lower cake permeability. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Cake filtration; Fractal structures; Neutron scattering; Colloidal fouling
1. Introduction One of the main limiting factors for the development of membrane separation process lies in the formation ∗ Corresponding author. Tel.: +33-561-55-83-04; fax: +33-561-55-61-39. E-mail address: [email protected] (M. Meireles)
of a deposit near the separating membrane. This situation becomes critical when the substances to be separated are macromolecules or colloidal particles, which can combine in concentrated solutions until they form a deposit or physical gel near the membrane. The physico-chemical and structural characteristics of this deposit and its porosity indeed control the efficiency of filtration [1]. Understanding the mechanisms that
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 3 9 4 - X
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control the filtration of a complex medium is a major challenge for the development of membrane-based processes in bio-industries, agro-industries or sludge treatment, where the complexity of the fluids is seen in terms of composition (liquid–liquid or liquid–solid mixtures), or physico-chemical characteristics (rheology, stability, etc.). This complexity is likely to induce different material organisation within the fluid depending on concentration and hydrodynamics fields. Theoretical work associated with numerical predictions and permeation flux measurements [2–4] enabled a description to be given for model particles or macromolecules. It has been shown that the conditions required for colloidal particles to attain their maximum density are strong inter-particle repulsion forces and an optimum size distribution. In the case of more complex media, little work to date has been performed with regard to the influence of morphology, deformability or variability of the initial suspended particle and their aggregates on the characteristics of the deposit. Most of the work undertaken assumes that the structure of the cakes formed on filtration is well described by an hexagonal or cubic packing. However, the structure of these media may be considerably oriented owing to physico-chemical and/or hydrodynamic interactions [5]. Consequently, it is of vital interest to understand the mechanisms whereby the deposit is formed and the influence of concentration and hydrodynamic fields on its structural arrangement in order to predict its occurrence and be in a position to reduce or control its effects. An important step is, therefore, to establish a relation between the microstructure of the cakes formed on filtration and their macroscopic properties, by using experimental and theoretical approaches. Recent studies on alumina and ceramic membranes demonstrate that small-angle neutron scattering is a sensitive technique for characterising pore size distribution [6] and for monitoring the in situ development of the foulant layer inside the membrane pores. During protein filtration, the change in scattering pattern allowed to follow the gradual build-up of an adsorbed layer in the internal porous matrix [7]. More than the assessment of a foulant layer thickness, the challenge here is to use physical techniques such as small-angle neutron scattering to characterise in terms of fractal or random organisation the microst ructure of a cake deposited onto the membrane
as a function of cake thickness and initial states of a suspension. This article presents the first results of a study performed to characterise the microstructure of deposits formed during frontal ultrafiltration of suspensions of anisotropic nano-particles, by means of in situ measurements, combining physical investigation techniques, namely small-angle neutron scattering, static light scattering and birefringence measurements, and rheometric measurements. The filtration characteristics are evaluated as a function of controlled physico-chemical conditions defining different initial states (sol or gel) of the suspension to be filtered. The material used here is an aqueous suspension of synthetic clay, Laponite, consisting of nanometer-sized disc-shaped particles. This suspension was chosen due to its transparency and its great purity, making it a model for natural clays like the bentonites found in the ultrafiltration of water [8], and also in other applications in the petroleum, cosmetics and pharmaceuticals industries. Laponite was also chosen because the strength of the attraction between particles can be modified by adding a peptiser making it possible to define two initial states of the suspension for a given volume fraction. When no peptiser was added to the suspension, the initial state of the suspension is a gel which exhibits a yield stress that has to be exceeded for the suspension to flow. On addition of peptiser, the suspension to be filtered is a liquid suspension that flows under its own weight and exhibits Newtonian-type behaviour law. Rheometric measurements, were used to characterise the mechanical behaviour of the initial state of the clay suspensions to be filtered. Physical techniques such as small-angle neutron scattering or static light scattering are used to identify the relevant length scales responsible for structuring the deposits. These techniques, allied with local birefringence methods, provide information on the structural orientations resulting from filtration in the deposit of anisotropic clay particles. 2. Materials and methods 2.1. Materials The clay used herein, Laponite XLG is manufact ured by Laporte Industrie. Its chemical composition is:
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66.2% SiO2 , 30.2% MgO, 2.9% Na2 O and 0.7% Li2 O, which corresponds to the following chemical formula: Si8 [Mg5.5 Li0.4 H4.0 O24.0 ]0.7− Na0.7 0.7+ The clay suspension consists of polydisperse disc-shaped particles with a diameter of about 30 nm and thickness of 1 nm [9]. The particles have a density of 2.53 g cm−3 [10]. The disc-shaped particles have marked anisotropy and exhibit positive surface charges on their edges and negative surface charges on their sides. Consequently, the arrangement of the particles within the aqueous medium will be strongly influenced by the particle volume fraction, by the pH and by the ionic content of the surrounding aqueous medium. Depending on these three parameters the suspensions can have different states: a stable colloidal solution, an elastic gel, a plastic paste or even separate solid and liquid phases. The gel state is reached above a volume fraction φv∗ , for a given ionic strength, pH and gelation time tp . The phase diagram of these suspensions has been studied recently [11–13]. Three different domains have been defined, an optically isotropic liquid, an optically isotropic gel (above φv∗ ), and an optically birefringent gel at a higher concentration. The sol–gel transition and the structure formation of the gel phase has been studied by means of static scattering measurements [9,11,14–18] and also by dynamic light scattering measurements [19]. The suspensions used in this study were prepared as follows. When a peptiser was not used, the clay powder was mixed in a solution of 10−3 M NaCl bi-distilled water at 20◦ C. The mixture was then stirred with a deflocculating vane at a speed of 800 rpm for 20 min. The peptiser used was tetrasodium-pyrophostate (abbreviated to tspp in the rest of the text): Na4 P2 O7 , 10H2 O, with a molar mass of 446 g mol−1 . This peptiser was added to the suspension at a concentration, denoted Cp , of 6% and calculated as a percentage of the mass of dry clay. This concentration was found to be the optimum concentration to fill the positive sites at the edges of the platelets. The tspp powder was mixed in a solution of 10−3 M NaCl distilled water. At 20◦ C, the mixture was stirred with a deflocculating vane at a speed of 800 rpm for 5 min. The clay was then added and mixed using the vane, at a speed of 800 rpm
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for 20 min. This preparation method ensures that the suspensions were completely homogeneous. The pH of all the preparations used in this study was adjusted to 9.5 in order to avoid any dissolution of the material. Thomson and Butterworth [20] showed that there is significant dissolution of magnesium silicate at pH values of less than 7 and the disappearance of such dissolution at pH values above 9. Two types of suspension were prepared with a clay volume fraction φ v =0.48%, a low ionic strength ([NaCl]=10−3 M) and a pH of 9.5. The first suspension (A1) does not contain any peptiser: φ v =0.48%, Cp =0%. This suspension which is a gel showing an initial connected structure with a fractal dimension D=1, exhibits a yield stress and a minimum stress must be applied for it to lose its connectivity. The second suspension (A2), contains a peptiser: φ v =0.48%, Cp =6%, and is a sol consisting of particles or clusters of disconnected particles with no fractal character and does not show a yield stress. Changes in the rheological properties and structural characteristics of the gels could be observed over time. This is partly due to the osmotic swelling produced by repulsion between the double layers, and partly due to the particles becoming gradually organised in fractal aggregates over increasingly large length scales [17]. The long-term gelation of these Laponite suspensions has also been studied by Mourchid and Levitz [21]. They have shown that under ambient atmosphere, Mg2+ is released from Laponite, suggesting that carbon dioxide from the atmosphere promotes acidification of the suspensions, resulting in a progressive Laponite dissolution and a slow increase of the ionic strength. The release of divalent cations Mg2+ can promote aggregative processes leading to the observation of fractal structures above some micrometers. In order to take into account these structural and mechanical evolutions of the suspensions, the time tp that has elapsed between the end of preparation and the test will always be indicated in the remainder of this text. Furthermore, the initial states (structural and mechanical) of the two suspensions A1 and A2, have been properly characterised at time tp by means of rheometric and scattering measurements. This time tp corresponds to the beginning of filtration.
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reservoir tube determined by a cathetometer with an accuracy of about 0.05 ml. It should be noted that internal fouling was not expected to occur, as the particle size (30 nm×1 nm) is larger than the mean diameter of the membrane pores (2–3 nm). In a typical experiment, where a pressure of 0.5 bar had been applied for 48 h, the permeate volume was about 1 cm3 , and the retentate about 0.4 cm3 . In the retentate, the lower part (up to 5 mm above the membrane) was a solid or gel like deposit, and the upper part (up to 20 mm above the membrane) was a liquid suspension or a gel. 2.3. Scattering techniques
Fig. 1. Diagrammetric frontal view of the ultrafiltration unit developed for the investigation of the deposit by small-angle neutron scattering.
2.2. Filtration apparatus A diagrammetric frontal view of the filtration cell is given in Fig. 1. It was designed to enable in situ scattering and birefringence measurements to be performed. It consists of two bottomless Suprasil rectangular quartz cells (outer dimensions 40 mm×13 mm×2.5 mm, inner dimensions 38 mm×10 mm×1 mm) which are located above and below the membrane. The flow section measures 10 mm×1 mm. The flat asymmetrical organic ultrafiltration membrane is made of sulphonated polysulphone (Diaflo ultrafilters PM 10, Amicon). A seal is inserted between the bottom cell and the membrane. To ensure complete sealing, the cells are tied by a system of rubber rings, then placed in a holder which ensures that the two cells are parallel. The holder is attached to a metal plate, which enables the two-cell and membrane assembly to be moved vertically for micrometer displacements and the control of the impact of the incident beam to a given distance d from the filter, with an accuracy of 0.1 mm. Transmembrane pressure is applied at the top of the column of fluid by purified compressed air. Solvent flux through the membrane and the deposit is measured through time variations of the level of permeate fluid in a
The small-angle neutron scattering (SANS) and static light scattering (SLS) techniques, are well established tools for characterising disordered condensed matter systems like colloidal dispersions. These systems have no rigorously periodic arrangement of atoms and the scattering originates from inhomogeneities in the electron density distribution in the medium. The aim of the scattering techniques is to define accurately the correlation of these structural inhomogeneities and then provide information on size, shape and arrangement of the constituent objects in these dispersions [22]. For SANS measurements, the nuclei of the atoms scatter the neutrons. For SLS measurements, the scattering is due to the fluctuations of dielectric constant of the medium. When coherent light is scattered from a disordered system it gives rise to a random diffraction pattern [23]. From these patterns, a limited structure factor can be interpreted as instantaneous, diffraction-limited structure factor measuring the exact spatial arrangement of the disorder [24]. Angular radiation measurements were carried out over the widest possible domain of scattering wave vectors, in order to obtain structural information ranging from a few nanometres, i.e. the scale of the particle size, to a few micrometres, in order to identify the scale of the largest structures. The wave vector Q =Q·qq of modulus Q is defined by Q = (4π n/λ) sin(θ/2), where n is the refractive index of the suspending medium (in the case of light scattering measurements), λ the wavelength of the radiation and θ the scattering angle.
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The light scattering measurements cover a wave vector domain ranging from 3×10−5 to 4×10−4 Å−1 . The small-angle neutron scattering measurements cover an additional wave vector domain ranging from 10−2 to 2×10−1 Å−1 . From the scattering patterns the angle dependence of the scattered intensity I can be deduced. The radial average of the scattering intenQ) is performed in order to get better statistic sity I(Q correlations. Furthermore, if the scattering intensity Q)∼Q Q−D ) in a parfollows a power law decay (I(Q ticular Q range domain, this can be interpreted as a fractal behaviour [25,26].
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between the glass slides, it might undergo mechanical stress, causing partial loss of structure. The sample was, therefore, left at rest for 10 min before pressure was applied and before any scattering measurements were carried out. Tests were performed to make sure that the rest time was sufficient for the scattering intensity as a function of wave vector not to be affected by any loss of structure. In spite of the slight structural breakdown caused by handling, it was possible to demonstrate a different structure in the initial suspension sample and the sample taken from the deposit. 2.4. Local birefringence
2.3.1. Small-angle neutron scattering (SANS) The measurements were carried out at the Institut Laue-Langevin in Grenoble, using the D11 multi-detector [27], at a wavelength of 6 Å. The distance between the detector and the sample was fixed at 2.5 m, with a beam collimation of 5.5 m. The ultrafiltration cell containing the deposit, with a scattering thickness of 1 mm, was positioned in front of the neutron beam. A diaphragm forming a 1 mm high, 4 mm wide slot was positioned to form a beam of the same size as the slot. The detector consists of 64×64 elements, each measuring 1 cm×1 cm. The isotropic mean of the total scattering intensity was then calculated. For the anisotropic spectra, the scattering intensity was integrated in a 30◦ angular sector in both y and z directions. 2.3.2. Static light scattering (SLS) The laser test facility used for these experiments was developed and built at the Rheology Laboratory [28,29]. It consists of a 2 mW laser beam (He–Ne) with a wavelength of 6328 Å, and a Fresnel lens (focal distance 122 mm and diameter 127 mm) acting as a scattering screen. A beam-stop cuts off beam transmission. The detector was a video camera with a 752×582 pixel CCD sensor. A shutter was used to vary the acquisition time from 1/50 to 1/10000 s. The results were analysed by means of image processing and a program for carrying out classical integration operations. The scattering spectra were obtained by taking samples from the filter cell and placing them between two optical glass slides. The sample has a fixed thickness of 1 mm and the transmission measurements, therefore, gave satisfactory results (I (transmitted)/I (incident)=0.95). When the sample was placed
The experiments were carried out to check the orientations of anisotropic particle deposits using a local birefringence measuring system with a photo-elastic modulator [30,31]. The optical equipment used for the experiments is shown in Fig. 2. It consists of a He–Ne laser beam with a wavelength of λ=6328 Å, polarised by a straight polariser set at 45◦ to the y-axis of the filter cell. The beam then crosses the following: a photo-elastic modulator (PEM) set at 45◦ to the polariser, consisting of a birefringent blade and piezoelectric plate, excited at a frequency of 50 kHz, a quarter-wave plate, set at 0◦ to the polariser, a converging lens with a focal length f=100 mm, giving a measurement that is as local as possible; as it crosses the sample, the beam has a diameter of 0.1 mm, a circular polariser, consisting of a quarter-wave plate and straight polariser set at 45◦ to each other, and a photodiode that detects the intensity of the light beam that has crossed all the previous optical elements. The laser
Fig. 2. Diagrammetric view of the local birefringence measuring apparatus.
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beam was directed perpendicular to the sides of the filter cell. The thickness of the sample e0 was fixed at 1 mm. The transmitted intensity was recorded on the photodiode and was sent to two lock-in amplifiers which allowed to access to the optical anisotropy δ. The birefringence 1n was then deduced by the relationship 1n = (λ/2πe0 )δ. Measurements of sin δ were recorded starting from the lower edge of the cell (near the membrane) and moving up through the retentate, with data being acquired every 0.1 mm. The fact of recording the sin δ of the optical anisotropy and not the optical anisotropy itself gives an uncertainty δ over the domain, where δ is close to 1. However, the values of δ on either side of this domain may be extended by continuity [31]. The birefringence measurements presented in this article were carried out on the deposit formed after a filtration test and after stopping the filtration pressure. The plane of observation (y, z) is the same as that in the scattering measurements. Thus, if the platelets are oriented along either the y or z-axis, the birefringence measurements can be used to compare the results of the different scattering patterns. 2.5. Rheometric techniques The mechanical behaviour of the suspensions under shear was studied with a Weissenberg–Carrimed rotating imposed shear rheometer. The tests were performed at a temperature of 22±1◦ C. Changes in stress were determined for steady conditions as a function of the applied shear rate. Two cone-plate configurations were used, of radius r1 =24.5 mm and r2 =96 mm and angle α 1 =0.076 rad and α 2 =0.07 rad. Torsion bar torque sensors were used. The atmosphere around the sample was saturated with water in order to prevent evaporation during the measurement [32]. An in-depth study of the fluids used for the experiments [33,34] enabled the main rheological characteristics of the suspensions to be determined. 3. Structure and rheological properties of fluids 3.1. Structure of initial suspensions 3.1.1. Suspensions without peptiser The structure of these clay suspensions without any peptiser was studied in a previous work [16,17]
Fig. 3. Schematic representation of a Laponite suspension structure at rest. The network consists of sub-units of oriented particles, micron-sized aggregates formed from a dense stack of sub-units and a fractal mass consisting of a loose mass of micron-sized aggregates.
by combining scattering measurements obtained with various types of radiation (X-rays, neutrons, light). Two characteristic length scales were revealed. It was shown that the structure of the suspensions at rest consists of sub-units measuring a few tens of nanometres, which become organised under the effect of attraction and repulsion forces acting between the particles. They then form regions of varying particle density up to the characteristic scale of a micron. Beyond a micron, the denser regions are aggregated to form a fractal structure of dimension D, giving the gel its three-dimensional structure that is responsible for the occurrence of a yield stress (Fig. 3). This fractal dimension D, which increases with the particle volume fraction, ionic force and gelation time, is correlated with a hardening of the mechanical properties of the gels at rest. Gels studies herein, have a fractal behaviour of dimension D=1, induced by an alignment of the micron-sized aggregates that leads to the formation of a mechanically fragile fibrous structure. 3.1.2. Suspensions with peptiser For the same volume fraction, the suspension changes from a gel to a sol when tetrasodium pyro-
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phosphate is added. The phosphate anion P2 O7 2− stabilises the Laponite suspension [35]. The effect of tspp is to fill in the positive sites at the edges of the platelets, thus tending to make the particles more uniformly repulsive. This effect is reinforced by the diffuse double layer of Na+ ions. Consequently, it may be expected that there will be further suspension of the objects present and a reduction in aggregate formation, possibly leading to a non-connected sol state. However, in addition to an excessive concentration of peptiser, an excess of Na+ counter ions causes the suspension to flocculate [36]. A peptiser concentration equal to 6% of the mass of clay is the optimum, with most of the electropositive sites of the Laponite discs being occupied by an anion, while avoiding any flocculation in the suspension. 3.2. Rheological properties of initial suspensions The rheological properties of colloidal suspensions are highly dependent on the interactions that may occur at microscopic scale between the particles dispersed in the aqueous medium. Within the suspensions, the particles are subjected to various attraction and repulsion forces which, above a certain particle volume fraction, may lead to the formation of a continuous network from one end of the sample to the other [37]. The structure that is formed in this way can only break up under shear once a certain yield stress has been reached. Consequently, these suspensions often have a thixotropic behaviour. Thixotropy can be defined as the tendency of matter to exhibit a time-dependent change in its reference properties (reference viscosity, yield value, time scale, elasticity modulus, loss modulus, etc. or conductivity) when stresses are applied, and to recover these reference properties progressively when stress is relieved [38]. Adding a peptiser reduces this thixotropic effect. Transient conditions are attained more quickly and the viscoelastic properties of the suspensions are less dependent on the sequence of stresses applied. Fig. 4 shows the variation of stress with applied shear rate for Laponite suspensions with and without a peptiser. In the case of a suspension with φ v =0.48% and without any peptiser, there is a stress plateau at low shear rate, while the stresses increase at the highest shear rates. This type of rheograph suggests the existence of a yield stress and shear-thinning behaviour
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Fig. 4. Rheometric shear behaviour of initial Laponite suspensions prior to filtration. [NaCl]=10−3 M, pH=9.5, tp =100 days.
at the highest shear rates. With these suspensions, simply shaking the bottle of gelified solution is enough to liquefy the suspension; it then recovers its initial consistency in a few hours when it is left to rest. In the case of the suspension containing 6% of tspp at a volume fraction φ v =0.48%, the flow curve in Fig. 4 follows a Newtonian-type behaviour, and has no stress plateau at low shear rates. This suspension does not possess a yield stress and is not thixotropic.
4. Characterisation of the ultrafiltration deposit 4.1. Filtration of Laponite suspensions The two types of suspension, A1 and A2, were filtered at a pressure of 0.5 bar for a period of 48 h. The variations of the permeate volume versus elapsed time are shown in Fig. 5. At the beginning of filtration, the variations are comparable for both types of suspension, accounting for experimental uncertainties. As time increases, results show that a higher filtration rate is obtained for the suspension A1 (no peptiser) than for the suspension A2 (with peptiser): the filtration rate measured at steady-state for suspension A1 is two-fold higher than those measured for suspension A2. 4.2. Small-scale structuring of deposits (SANS) Figs. 6 and 7 represent the spectra obtained by SANS in the cake at different distances d from the
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Fig. 5. Filtration characteristics of Laponite suspensions subjected to filtration at a pressure of 0.5 bar for 48 h. Influence of the initial structure of the suspensions on filtration kinetics; suspensions A1: φ v =0.48%, Cp =0%, suspension A2: φ v =0.48%, Cp =6%; [NaCl]=10−3 M, pH=9.5, tp =90 days.
membrane, for suspensions A1 and A2, respectively. The z-axis of each pattern corresponds to the horizontal axis of the detector plane, parallel to the membrane, and the y-axis to the vertical axis, along the flow direction (Fig. 1). An important result is that the spectra are increasingly anisotropic as the impact of the beam is moved from the top of the cell to the vicinity of the membrane. It is also clear from Figs. 6 and 7 that the patterns are not the same for the two suspensions. In order to measure the anisotropy, the intensities of the pattern were integrated in a 30◦ angular section along the z direction, and in a similar sector along the y direction (Fig. 8). This was done for the patterns obtained at two distances from the membrane, d=5 mm, i.e. far from the membrane, corresponding to the most isotropic spectrum, and d=0.5 mm, i.e. in the filtration deposit, corresponding to the most anisotropic spectrum. In the retentate of suspension A1, far from the membrane (d=5 mm, Fig. 8), the scattered intensities integrated along the z and y directions are identical and they both decay according to a Q −2 law over most of the range of Q vectors (2×10−2 to 2×10−1 Å−1 ). These Q values correspond to interferences between pairs of scatterers that are separated by distances of 30–300 Å, i.e. intraparticle distances or distances between neighbouring particles. The Q −2 law is the law expected for disks that are randomly oriented and ran-
domly spaced in the suspension. Accordingly, the retentate can be described, at short length scales, as a suspension of particles that have only weak correlations with their neighbours. In the deposit of suspension A1, close to the membrane (d=0.5 mm, Figs. 6 and 8) the intensity integrated along the y direction decays much less than along the z direction. This anisotropy demonstrates that the particles have their short axes along the y direction. For randomly spaced particles, the decay would follow the Guinier law: (I Q/I (Q → 0)) = 1 − (Q2 Ry2 /3) [39,40], where Ry is the half dimension of the particles in the y direction. The fit of Guinier law on experimental data yields Ry =1.21 nm, which is consistent with the known thickness of Laponite platelets (about 1 nm). In the deposit of suspension A2, (with a peptizer), the scattering pattern has two peaks located on either side of the beam, in the y direction, and a depression between these peaks (Figs. 7 and 10). The depression at low Q reflects repulsive correlations between neighbouring particles. As the beam is placed closer to the membrane, the depression becomes more pronounced, and the peaks more intense, indicating stronger correlations (Fig. 11). The peak locations correspond to an average interparticle distance along the y direcQmax . It is clear tion which is calculated as dy =2π /Q that dy decreases considerably upon approaching the membrane. At the shortest distance (d=1.2 mm) it is dy =78 Å, which would correspond to a lamellar array of volume fraction φ v =10%, i.e. 20 times more concentrated than the original suspension. 4.3. Large-scale structuring of deposits (SLS) Q) determined Figs. 12 and 13 show the intensity I(Q by static light scattering in suspensions A1 and A2 before filtration and in deposits formed on filtration at a pressure of 0.5 bar for 15 h. In Fig. 12, obtained with suspension A1 without any peptiser, for wave vectors between Q =4×10−5 and 2×10−4 Å−1 , the fractal dimension increases from 1 when the suspension is at rest, to 2 for the deposit. This demonstrates that the gel is more densely structured in the deposit in comparison with the suspension at rest. With regard to the higher wave vectors, it can be seen that the scattering intensity as a function of the wave vector follows a Q −3 power law that is characteristic
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Fig. 6. Small-angle neutron scattering patterns for the deposit formed in frontal filtration of a Laponite suspension under 0.5 bar pressure for 48 h filtration time. d=Observation distance to the membrane; suspension A1: φ v =0.48%, Cp =0% tspp, [NaCl]=10−3 M, pH=9.5, tp =38 days.
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Fig. 7. Small-angle neutron scattering patterns for the deposit formed in frontal filtration of a Laponite suspension under 0.5 bar pressure for 48 h filtration time. d=Observation distance to the membrane; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5 tp =121 days.
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Fig. 8. Variations of scattering intensity with wave number from small-angle neutron scattering of a deposit after 48 h ultrafiltration at a transmembrane pressure of 0.5 bar; suspension A1: φ v =0.48%, Cp =0% tspp, [NaCl]=10−3 M, pH=9.5, tp =38 days.
of the presence of micron-sized aggregates. It may be Q) curves that the deposit is strucdeduced from the I(Q tured in a different way from the initial suspension. It is suggested that at these length scales (0.5–2.5 mm),
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the deposit still consists of dense micron-sized aggregates, that are identical to those at rest but are more numerous and more connected. Thus upon filtration, the structure of the suspension has changed from a loose fibrous network to a denser and more connected fractal gel (Fig. 9a). The static light scattering experiments, obtained from suspension A2 with a peptiser, Fig. 13, also displays a structural modification upon filtration: while the initial suspension has no fractal structure, we could observe that within the deposit, for wave vectors ranging from Q =4×10−5 to 2×10−4 Å−1 , the scattering intensity is proportional to Q −1.3 . It is suggested that at these length scales, filtration has promoted a gathering of the particles into micron-sized aggregates and enabled the formation of a fractal structure. These results underline the great influence of the initial structure of the suspension on the subsequent structural characteristics of the deposit obtained with filtration time, pressure conditions and initial particle volume fractions being the same. Filtering structured A1 suspensions produces deposits with a larger frac-
Fig. 9. (a) Schematic representation of the microstructure of a deposit formed by frontal filtration at a transmembrane pressure of 0.5 bar; Laponite suspension A1: φ v =0.48%, Cp =0% tspp and (b) schematic representation of the microstructure of a deposit formed by frontal filtration at a transmembrane pressure of 0.5 bar; Laponite suspension A2: φ v =0.48%, Cp =6% tspp.
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Fig. 10. Variations of scattering intensity with wave number from small-angle neutron scattering of a deposit after 48 h ultrafiltration at a transmembrane pressure of 0.5 bar; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5, tp =121 days.
tal dimension (D=2) than the deposits obtained from initially unstructured A2 suspensions. It would be interesting to check whether longer filtration times with the A2 suspensions would produce more compact deposits, with a larger fractal dimension than D=1.3. 4.4. Orientation of particles near the deposit Fig. 14 shows the birefringence curve 1n as a function of the distance d to the membrane for the two
Fig. 11. Evaluation of the inter-particle distance from maximum scattering intensity along y as a function of distance d to the membrane. Small-angle neutron scattering of a deposit after 48 h ultrafiltration at a transmembrane pressure of 0.5 bar; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5, tp =121 days.
Fig. 12. Comparison of static light scattering of a Laponite suspension at rest with static light scattering of a deposit formed from a Laponite suspension after 15 h frontal filtration at a transmembrane pressure of 0.5 bar; suspension A1: φ v =0.48%, Cp =0% tspp, [NaCl]=10−3 M, pH=9.5, tp =22 days.
types of suspension A1 and A2 after 48 h filtration at 0.5 bar. With both initial types of suspension, there is considerable birefringence near the filter, which then decreases with distance to become zero beyond a certain distance dg . This indicates that the structure is highly birefringent near the filter, becoming less and less so at the top of the cell. It may be noted that for suspension A1 this distance dg =6 mm is larger than the distance dg =4 mm obtained with suspension A2. The distance dg must be related to the thickness of the
Fig. 13. Comparison of static light scattering of a Laponite suspension at rest with static light scattering of a deposit formed from a Laponite suspension after 15 h frontal filtration at a transmembrane pressure of 0.5 bar; suspension A2: φ v =0.48%, Cp =6% tspp, [NaCl]=10−3 M, pH=9.5, tp =24 days.
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Fig. 14. In situ local birefringence measurements performed on a deposit formed from a Laponite suspension after a 48 h ultrafiltration at a transmembrane pressure of 0.5 bar. Effect of initial state of suspension on state of orientation in the deposit φv =0.48%, [NaCl]=10−3 M, pH=9.5, tp =100 days.
filtration deposit as it is a characteristic signature of the orientations induced by the presence of the membrane. This increase in birefringence near the filter corresponds to orientation of the particles forming the filtration deposit. It should be noted that, for the case of the initial suspension with no peptiser, the birefringence fluctuates on approaching the filter, which is not the case with suspension A2 containing peptiser. Thus, for the deposits obtained from type A2 suspensions containing peptiser, it may be concluded that: (i) the orientation of objects parallel to the membrane is more homogeneous, and (ii) structuring takes place over smaller distances than with the type A1 suspensions with no peptiser. 5. Analysis of structuring mechanisms By examining the rheometric and structural characteristics of filtered deposits obtained from clay suspensions with and without peptisers, we can get some insights on the mechanisms whereby these deposits are formed. Small-angle neutron scattering tests revealed an anisotropic increase in scattering intensity in the deposits at decreasing observation distances from the membrane. Moreover, depending on the initial state of the suspension, considerable differences were observed in this anisotropy.
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In the case of a non-connected initial suspension, an increase in intensity is observed with the shape of two intensity peaks revealing a constant inter-particle distance. At the shortest distance from the membrane (d=1.2 mm) this constant inter-particle distance was estimated of the order of 78 Å, which corresponds to a lamellar array of volumic fraction φ v =10%. In the case of an initial suspension consisting of aggregated and interconnected particles, the increase in intensity is consistent with increased fluctuations in concentrations of the particles. For both suspensions, the anisotropy of the scattering spectra near the membrane indicates greater orientation of the particles, over inter-particle distances of the order of magnitude of the size of the elementary particles, i.e. a few nanometres. It is interesting to note that such orientations at these length scales have never before been observed on structured Laponite suspensions in the same concentration domains. Light scattering measurements show large-scale organisation of the particles, which become connected into micron-sized aggregates to form fractal clusters. The fractal dimension of this large-scale structure also depends on the initial conditions of the suspension before filtration. Filtration of a connected suspension characterised by a fractal dimension of 1, induces a densification of the structure, the fractal dimension of which reaches a value of 2. Filtration of a suspension of unconnected particles, also induces a structuration, but the resulting fractal dimension is less than for the previous case. Birefringence measurements clearly show the orientation of the particles and enable a gelification distance dg to be evaluated. With both types of initial conditions, the orientation increases on approaching the membrane, but the distance beyond which this orientation disappears is more important in the case where the initial structure is connected. Moreover, in this case, fluctuations in orientation are observed, which should be compared with the fluctuations in concentration observed with small-angle neutron scattering. These in situ physical characterisations enable a structural model to be proposed for the formation of a deposit from a Laponite suspension during filtration, depending on the initial state of the suspension.
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With a connected suspension, without any peptiser, the deposit is composed of clay particles which are oriented in a direction parallel to the membrane. Among these are dense micron-sized aggregates promoted by the concentration subsequent to the filtration. These dense micron-sized aggregates have a fractal dimension which is above that of the suspension at rest. The juxtaposition of domains consisting of highly oriented particles and micron-sized aggregates may explain the fluctuations in particle concentration detected by neutron scattering. At increasing distances from the membrane, the particles are oriented in an increasingly random manner, and it may be assumed that the micron-sized aggregates are arranged with a smaller fractal dimension than that of the cake. The filtration process will then result in a concentration of the initial areas of low particle density in such a way that particles are forced to adopt an orientation mainly parallel to the membrane. Areas of initially higher density, i.e. the micron-sized aggregates, seem to be hardly modified by filtration but their presence makes the deposit more heterogeneous (Fig. 9a). Such arrangement which forms a relatively porous three-dimensional structure presumably accounts for the observed filtration characteristics. With an unconnected initial suspension, the deposit consists mainly of isolated particles oriented parallel to the membrane. Further away from the membrane, the platelets are oriented in an increasingly random manner, until they are scattered isotropically in the upper part of the cell. Within this ordered arrangement of particles, certain more concentrated areas are formed. Indeed, a large-scale fractal structure appears, which seems to indicate that the isolated particles, subjected to filtration conditions, tend to form dense clusters connected in a fractal structure. However, in contrast to the case without any peptiser, these dense clusters have an internal orientation leading to a constant inter-particle distance that can be detected by the local birefringence techniques described above. Near the membrane, the particles in the deposit are subjected mainly to excluded volume effects and are arranged face to face, thus reducing the porosity of the medium and hence the filtration performance (Fig. 9b). In conclusion, initially connected suspensions tend to form more heterogeneous deposits, consisting of objects that are more widely spaced and less oriented
than the deposits formed from initially unconnected suspensions. These differences that have been evidenced at a microscales are consistent with the differences observed in filtration characteristics of both types of suspensions estimated by macroscopic filtration tests.
6. Conclusions This study clearly shows the major contribution made by using in situ physical investigation techniques to determine the structure of ultrafiltration deposits. The aim is to understand the mechanisms whereby these deposits are structured, in order to improve the separation processes used to filter complex media. By using angular radiation scattering and birefringence techniques associated with rheometric analyses, it was possible to obtain information on the structuring of anisotropic colloidal particles in an aqueous medium. It appears that the filtration fluxes are limited by an anisotropic arrangement of the particles parallel to the membrane, irrespective of the initial physico-chemical and structural conditions. In addition, it was shown that the initial structure of the suspensions at rest has a decisive effect on the efficiency of separation. Indeed, an initially connected gel increases filtration flux by adopting a more porous structure than a suspension initially in a liquid state. Lastly, these techniques hold out interesting prospects, as they can be used to observe the formation of deposits in real time and, thus, to monitor transient mechanical and structural changes occurring in the deposits. This work is indeed continuing and being extended to deposits formed during tangential ultrafiltration.
7. Nomenclature Cp D d Q n
concentration of peptiser in the suspension (%) fractal dimension distance from the membrane (m) wave vector (m−1 ) refractive index
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Greek letters φ v volumic fraction of Laponite in suspension (%) φv∗ critical gel volumic fraction (%) λ wavelength of radiation (m) References [1] P. Bacchin, P. Aimar, V. Sanchez, Model for colloidal fouling of membranes, AIChE J. 41 (2) (1995) 368. [2] A.A. Kozinski, E.N. Lightfoot, Protein ultrafiltration: a general example of boundary layer filtration, AIChE J. 18 (5) (1972) 1030–1040. [3] R.M. Mc Donough, C.D.J. Fell, A.G. Fane, Surface charge and permeability in the ultrafiltration of non-flocculating colloids, J. Membr. Sci. 21 (1984) 285. [4] L. Song, M. Elimelech, Theory of concentration polarization in cross flow filtration, J. Chem. Soc., Faraday Trans. 91 (1995) 3389. [5] T.D. Waite, A.I. Schäfer, A.G. Fane, A. Heuer, Colloidal fouling of ultrafiltration membranes: impact of aggregate structure and size, J. Colloid Interface Sci. 212 (1999) 264– 274. [6] K.L. Stefanopolous, G.E. Romanos, A.Ch. Mitropolous, N.K. Kanellopoulos, R.K. Heenan, Characterisation of porous alumina membrane by adsorption in conjunction with SANS, J. Membr. Sci. 153 (1999) 1–7. [7] T.J. Su, J.R. Lu, Z.F. Cui, R. K Thomas, R.K. Heenan, Application of small angle neutron scattering to in situ study of protein fouling on ceramic membranes, Langmuir 14 (1998) 5517–5520. [8] P. Bacchin, P. Aimar, V. Sanchez, Influence of surface interaction on transfer during colloid ultrafiltration, J. Membr. Sci. 115 (1996) 49–63. [9] J.D.F. Ramsay, S.W. Swanton, J. Bunce, Swelling, dispersion of smectite clay colloids: determination of structure by neutron and small-angle neutron scattering, J. Chem. Soc., Faraday Trans. 86 (1990) 3919. [10] L. Rosta, H.R. Von Gunten, Light scattering characterization of Laponite sols, J. Colloid Interface Sci. 134 (2) (1990) 397–406. [11] A. Mourchid, A. Delville, J. Lambard, E. Lécolier, P. Levitz, Phase, diagram of colloidal dispersions of anisotropic charged particles: equilibrium properties, structure, and rheology of Laponite suspensions, Langmuir 11 (1995) 1942–1950. [12] J.C. Gabriel, C. Sanchez, P. Davidson, Observation of nematic liquid–crystal textures in aqueous gels of smectite clays, J. Phys. Chem. 100 (1996) 11139–11143. [13] A. Mourchid, E. Lécolier, E.H. Vandamme, P. Levitz, On viscoelastic, birefringent, and swelling properties of Laponite clay suspensions: revisited phase diagram, Langmuir 14 (17) (1998) 4718–4723. [14] J.D.F. Ramsay, P. Lindner, Small-angle neutron scattering investigations of the structure of thixotropic dispersions of smectite clay colloids, J. Chem. Soc., Faraday Trans. 89 (23) (1993) 4207–4214.
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