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Influence of pH and Al2(SO4)3 on the stability of whey suspensions
Separation and Purification Technology 67 (2009) 364–368
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Influence of pH and Al2 (SO4 )3 on the stability of whey suspensions P. Kaewkannetra a , F.J. Garcia-Garcia b , A.E. James c , T.Y. Chiu d,e,∗ a
FerVVAP Research Centre, Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand Corrosion and Protection Centre, School of Materials, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK c School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, M60 1QD, UK d Earth Tech Engineering Ltd, Wentworth Business Park, Maple Road, Tankersley, Barnsley S75 3DL, UK e Centre of Water Science, Cranfield University, Bedfordshire MK43 0AL, UK b
a r t i c l e
i n f o
Article history: Received 22 January 2009 Received in revised form 3 April 2009 Accepted 13 April 2009 Keywords: Stability Whey suspensions Electrokinetics -Potential Coagulation
a b s t r a c t The effects of pH and aluminium sulphate concentration on the stability of whey suspension have been investigated. The particle size and particle size distribution are examined by scanning electron microscopy and granulometric analysis, respectively. The zeta potential and residual turbidity of 0.01 wt% whey suspensions are determined over the pH range of 3–8 with aluminium sulphate as background electrolyte. The results show that the zeta potential of whey suspensions depends on pH and, to a lesser degree, aluminium sulphate concentration. The average particle size of flocs in suspension increases with decreasing zeta potentials which is accompanied by low turbidity. The degree of flocculation of whey suspension is linked to the zeta potential. The DLVO calculations are in good agreement with the observed trends indicating that the classical DLVO theory can be used to predict colloidal stability of the whey protein suspensions in the presence of higher strength electrolyte and varying pHs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the past few decades, the processing of whey, a liquid produced when milk is possessed into cheese or casein, has developed from dairy waste into a variety of dairy products with increasing added values [1]. Much research has been undertaken concerning various aspects of whey especially in relation to its use in food products and various investigators have studied either its behaviour in terms of aggregation and adsorption or treatment of whey effluents from dairy industries using membrane filtration [1–4] but to our knowledge few studies attempted to characterise the stability of whey suspensions in terms of their electrokinetic properties. Suspensions of whey are highly turbid and form biological suspensions containing settleable and colloidal materials; most colloid particles existing in natural waters and in many industrial effluent streams have negative surface charges, depending on the solution pH and the stability of these particles is often investigated in terms of the zeta-potential (-potential). The -potential of colloid suspensions is controlled by the surface chemistry of the colloidal particles as well as the chemistry of the solution [5]. Knowledge of the interaction forces between whey particles
∗ Earth Tech Engineering Ltd, Wentworth Business Park, Maple Road, Tankersley, Barnsley S75 3DL, UK. Tel.: +44 122 622 4176; fax: +44 122 622 4488. E-mail address: [email protected] (T.Y. Chiu). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.04.013
in suspensions is essential for interpreting the stability of these suspensions and also the adhesive behaviour of the particles. Of the interaction forces it is the electrical double layer forces that are characterised in terms of the -potential and these usually are repulsive as particles composed of the same material will have similar charges [6]. The sedimentation and filtration of colloidal particles can be linked with their stability and thus, knowledge of the -potential is fundamental to the understanding of these processes. In treating industrial wastewater, coagulation and flocculation are considered to be essential requirements for efficient separation using either filtration or sedimentation. The purpose of flocculation is to promote the interaction of particles so as to form aggregates that can be efficiently removed in subsequent filtration processes or to form larger faster settling particles in sedimentation. In order to achieve efficient flocculation, the suspensions must be destabilized by the addition of coagulants, such as aluminium sulphate, ferric salts and polyelectrolytes [7]. The rate of coagulation depends mainly on the stability of the colloid particles and the salt concentration and pH are two important parameters affecting colloid stability [5]. This is especially so for protein-stabilized emulsions such as whey suspensions where these two parameters are considered to be the most important characteristics in determining their stability [8–10]. Another way of assessing the changes in the colloidal stability of suspensions is to consider changes in their turbidity. It should be remembered that although turbidity measurement is an indirect technique, it provides an easy method of
P. Kaewkannetra et al. / Separation and Purification Technology 67 (2009) 364–368
appraising the performance of a flocculation process. The turbidity of a suspension can reflect the amount of colloidal material present and thus, the effectiveness of a flocculation process can be readily established in terms of the residual turbidity of the suspensions following physicochemical treatment and subsequent setting or coagulation [11]. In most applications the floc size can be controlled by the selection of an appropriate flocculating agent and subsequently through the correct implementation of the mixing and flocculation conditions [12]. If inorganic coagulants are used then the changes in the electrokinetic properties of the suspensions can sometimes be related to the residual turbidity. Electrokinetic phenomena occur when one electrically charged phase moves with respect to an adjoining phase. The interactions between the mechanical and electrical forces can be used to acquire some knowledge of the electrical potential in the interfacial region [12] which can be important in particle interactions. One of the more readily inferable electrokinetic quantities from electrokinetic measurements is the -potential. The -potential is related to the electrokinetic mobility through the Helmholtz-Smoluchowski equation [13]: =
ue ve = εr ε0 εr ε0 E
(1)
where ue , ve , E, , εr , and ε0 are the electrophoretic mobility of the aggregates, the velocity of the particles, the electric field, the viscosity of the solution, the dielectric constant of the liquid and the permittivity of free space, respectively. An aim of the present work is to use the -potential to investigate the colloidal stability of whey suspensions. Quantitatively colloidal stability is usually assessed in terms of DLVO theory [13]; where the total interaction energy, V(h), as a function of the distance between the surfaces of two particles along the center to center line, (h), is expressed as V (h) = VVDW (h) + VE (h)
(2)
where VVDW (h) and VE (h) are the van der Waals attractive energy and the electrical double layer, respectively. The van der Waals interaction is given by [14] A VVDW (h) = − 6
2R2 2R2 × ln + h2 +4Rh h2 +4Rh+4R2
h2 +4Rh h2 +4Rh+4R2
(3) And the electrostatic repulsion energy is [15] VE (h) = 2εr ε0 R 2 ln(1 + e−h )
(4)
where R, A, , and are the particle radius, the Hamaker function, zeta potential and the reciprocal Debye–Huckel length, respectively. The total interaction energy is usually divided by the product of Boltzman’s constant (k) and temperature (T), i.e. the average free energy involved in an encounter between two droplets by Brownian motion. The influence of mineral ions on the stability of proteinstabilized emulsions is complex and depends on the precise nature of the ions, the electrical characteristics of the droplet surface and the prevailing environmental conditions [16]. A number of studies have been conducted to provide insight into the effect of monovalent and divalent ions [10,17,18] on the physicochemical properties and stability of emulsions stabilized by whey proteins. However, many chemicals used in wastewater coagulation are trivalent such as aluminium ions in the form of aluminium sulphate. These trivalent minerals may alter the stability of protein-stabilized emulsions in a way different from mono- and di-valent minerals and to the authors knowledge, no studies have been carried out investigating the effects of aluminium sulphate on the stability of whey suspensions.
365
In this study, changes in the turbidity of a typical whey are attributed to various modes of colloidal stability and are considered in terms their size distributions and -potentials as functions of pH and electrolyte concentration. Aluminium sulphate was used as electrolyte and it seems that the results are adequately explained in terms of the classical DLVO theory. 2. Materials and methods 2.1. Materials All suspensions were prepared using spray dried powder from bovine milk (Sigma® , USA). The manufacturer’s analysis of the product was 65% lactose, 13% protein, 8% ash, 2% lactic acid and the rest of these are 12% total solids that can be separated to be fatty and non-fatty solids. Sodium hydroxide (0.1 M) and nitric acid (0.1 M) were used to adjust the pH of solutions. Aluminium sulphate was used as the electrolyte. All chemicals were analytical grade (Sigma® –Aldrich, UK), and all solutions were prepared with ultra pure water (ELGASTAT® Spectrum B unit Cartridge type: SC2 , USF Memcor, England) with a resistivity of 18 M cm and an organic contamination of less than 0.03 mg l−1 . 2.2. Preparation of flocculated whey suspensions Suspensions of the whey particles (0.1%, w/v) were prepared using ultra pure water in a flocculation rig fitted with an impellor (Flocculator SW1, Stuart Scientific, UK); various quantities of aluminium sulphate were added so that the concentration in the suspensions varied between 3 and 8 mg l−1 . The pH of suspensions was varied from 3 to 10 using dilute acid and alkali where appropriate. The suspensions were mixed for 1 min using an impeller speed of 200–222 rpm followed by 15 min of slow mixing at 20–22 rpm to promote floc growth. After flocculation the motor was switched off and the flocculated whey particles were allowed to settle for 15 min. 2.3. Determination of size and particle size distribution Granular dried whey was examined to determine its size and shape with a scanning electron microscope, SEM (FEI QUANTA 200, Purge, Czech Republic) operated at an accelerating voltage of 20 kV. The suspensions of whey from the flocculation rig were analysed to determine the particle size distribution using a laser scattering instrument (Mastersizer 2000, Malvern Instrument Ltd, UK) which is suitable for particle sizes ranging from 0.02 to 2000 m. The measurements made in this work are reported as the full particle size distribution. Each sample was analysed and the average of five replications is reported here. It should be remembered that the particles are considered to be spherical in these results. 2.4. Determination of turbidity and -potential The turbidity measured in nephelometric turbidity units (NTU) of the supernatant liquid obtained after the period of settling following flocculation was monitored using a laboratory turbidity meter (Model: TU1110, Cambridge, UK). Electrophoretic mobility measurements were obtained using a laser scattering instrument (Zetasizer 3000HS Advance, Malvern Instrument Ltd, UK). This equipment measures distribution of electrophoretic mobility and -potential of particles in liquid suspensions using Laser Doppler Velocimetry (LDV). Samples are injected into the measuring cell and the output from the instrument includes the average -potential and its standard deviation. The -potentials reported here were
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Fig. 1. SEM micrograph of (a) dried whey granular; (b) dried whey granule showing the loose aggregation of the primary whey particles; (c) particle size distribution of the unflocculated whey suspension.
calculated from the average of at least five separate injections per sample. Measurements were obtained over the desired pH range and at different aluminium sulphate concentrations. All experiments were carried out at the ambient temperature which was 25 ◦ C. 3. Results and discussion 3.1. Scanning electronic microscopy and particle size distribution The SEM micrographs show that dried whey granules, prior to being dispersed in water, are approximately spherical in shape (Fig. 1a). Each granule is composed of large numbers of smaller whey particles as shown in Fig. 1b. The surface texture is loosely agglomerate and the granules are readily disaggregated and form turbid suspension when dispersed in water. The resultant suspensions consist of disaggregated primary particles. The average size for the unflocculated whey dispersed in deionised water is 7.0 m (Fig. 1c) and it lies within the reported range of whey sizes of 0.3–10 m [19–21]. The presence of wide spread of particle sizes depends on the relative proportions of the main constituents in the whey as well as its source. The size distribution of prepared whey suspensions under various pH and Al2 (SO4 )3 electrolyte concentrations are shown in Fig. 2a and b, respectively. It is clear that the mean particle size decreases with increasing pH with the largest floc of 18.5 m observed at pH 3. Under increasing Al2 (SO4 )3 concentrations, the average size for the flocculated whey increases and reaches a maximum size of 23.5 m before the flocs break down to approximately 15.3 m at 5 and 8 mg l−1 , respectively.
Fig. 2. Mean particle size of whey suspensions as a function of (a) electrolyte concentrations at pH 3.0 and (b) pH at 3 mg l−1 Al2 (SO4 )3 .
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Table 1 Turbidity of whey suspensions as a function of pH and electrolyte concentration. pH
Al2 (SO4 )3 (mg l−1 )
Turbidity (NTU)
3.0 4.0 5.0 6.0 7.0 8.0 3.0 3.0 3.0 3.0 3.0
3 3 3 3 3 3 4 5 6 7 8
122 128 132 132 132 147 126 102 132 138 142
3.2. Effect of pH and electrolyte concentration on the residual turbidity Changes in the residual turbidity of the supernatant as functions of pH and Al2 (SO4 )3 concentrations following settlement of flocculated whey suspensions are shown in Table 1. The lowest turbidity of 122 NTU was obtained at pH 3.0 and the maximum turbidity occurring at pH 8 (147 NTU). There is generally an increase in turbidity with pH. It seems that at low pH there is a preponderance of larger flocs that will settle relatively quickly, while a higher pH the whey particles seem more dispersed with considerably smaller particles present. Elsewhere, the reported maximum turbidity was in agreement with the current study, however, the minimum turbidity was found to occur at pH 6 [22]. This discrepancy may arise from the difference in type of whey used which is reflected in the observed isoelectric point of whey in the current study and is discussed in later sections. In the presence of Al2 (SO4 )3 the turbidity is significantly reduced and increased Al2 (SO4 )3 concentration produces a small increase in turbidity. The turbidity minima of 102 NTU is found at 5 mg l−1 Al2 (SO4 )3 . When the applied dosage was higher than the optimum amount, the turbidity increased. This shows restabilization of the coagulated whey particles. At high doses of coagulant, a sufficient degree of over-saturation occurs to produce a rapid precipitation of large quantity of coagulant. This is a common phenomenon due to overdosing and is also observed in a number of works where hydrolyzable metal salts have been utilised [23–25]. It seems that the both pH and Al2 (SO4 )3 concentrations affect the interactions between whey particles in solution. This in turn affects the residual turbidity of the suspensions. 3.3. Effect of pH and electrolyte concentration on the -potential The effect of pH on the -potential of the supernatant whey particles in the presence of Al2 (SO4 )3 is shown in Fig. 3 at three different concentrations, together with a sample where there was no background electrolyte. The results show that the -potential of the whey used in this study is positive at pH 3 in the presence and absence of Al2 (SO4 )3 electrolyte. As the pH is increased the -potentials in all cases are negative, with the maximum magnitude being found at pH 8. From these results it is seen that the -potential seems to be affected by both changes in the pH and by the Al2 (SO4 )3 concentration. At constant Al2 (SO4 )3 concentration, the sign of the -potential is changed by varying pH and at constant pH it is impossible to change the sign of the -potential by changing the Al2 (SO4 )3 concentration. Generally, at any pH the magnitude of the -potentials is reduced as Al2 (SO4 )3 concentration is increased. The behaviour of the -potential changes and it is seen to be negative from the i.e.p. (the point where the -potentials become zero ( = 0 mV) is called the isoelectric point, i.e.p.) at pH between 3.0 and 4.0. As whey includes various proteins, the i.e.p. of proteins in
Fig. 3. Zeta potentials of whey suspensions in the presence of aluminium sulphate over the range of pH (() 0 mg l−1 Al2 (SO4 )3 ; () 3 mg l−1 Al2 (SO4 )3 (×) 5 mg l−1 Al2 (SO4 )3 , () 8 mg l−1 Al2 (SO4 )3 ).
the whey suspension can vary [26]. Since the i.e.p. does not change with increasing Al2 (SO4 )3 concentration it is clear that as far as the whey particles are concerned it is an indifferent electrolyte, that is increasing the concentration only serves to compress the double layer. Previous studies [27,28] have attributed shifts in the i.e.p. to the adsorption of whey suspension but here the changes in the i.e.p. are small in comparison to the large change in electrolyte concentration and may arise from experimental error. The trends in these results are broadly similar to those reported by others [10,17,18] although these authors use a whey protein isolate powder and different concentrations of copper chloride (0–100 M), calcium chloride (0–20 M) and potassium chloride (0–100 M). 3.4. Correlation between -potential and residual turbidity The effect of electrolyte concentration on the calculated interaction potential is shown in Fig. 4a. These potentials were calculated using the known electrolyte concentrations and their corresponding measured zeta potential. The total interactive energy curve of DLVO theory predicts primary maximum under various electrolyte concentrations. This implies that the electrostatic repulsive energy is sufficiently dominant in the present system for the suspension to resist flocculation and thereby results in forming a relatively stable colloidal system. Without flocculation, the suspension will remain turbid. This explains the high turbidities observed under the various electrolyte concentrations (Table 1). The height of the energy barrier to flocculation decreases as the electrolyte concentration increases. This lowered primary maximum energy barrier implies that particles can collide with sufficient energy to overcome the repulsive energy barrier and are not longer prevented from coming close together to flocculate which accounts for the lowered turbidity observed when increasing the concentrations of Al2 (SO4 )3 from 3 to 5 mg l−1 . Contrary to the DLVO predictions, further decline in turbidities associated with lowered primary energy barrier at Al2 (SO4 )3 concentrations >5 mg l−1 is not observed. The turbidity increase is attributed to the restabilization of the coagulated whey particles. At high doses of coagulant, a sufficient degree of over-saturation occurs to produce a rapid precipitation of large quantity of coagulant. The results are in agreement with other workers [29] who suggested that particle surface charge, reflected as zeta potential, shows the status of the particle stability in the system, but may not necessarily be proportional to the amount of coagulant needed for particle destabilization. Fig. 4b shows the calculated overall interaction potential as a function of the separation distance under various pHs assuming that Al2 (SO4 )3 concentration remains constant. The energy barrier to flocculation decreases as pH approaches the i.e.p. of the whey proteins. At approximately pH 3.3, the energy barrier disap-
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turbidity is low and when the -potential is increased the average floc size decreases. The numerical calculations of suspension stability to aggregation using the DLVO theory show that this theory can be applied to predict colloidal interaction and stability of the whey suspensions under varying pH conditions. References
Fig. 4. Interaction potential of whey particles in suspension as a function of separation distances: (a) variable electrolyte concentrations at pH 3.0 and (b) variable pH at 3 mg l−1 Al2 (SO4 )3 .
pears indicating that no electrostatic repulsion exists between the interacting particles because zeta potential at this point is 0 mV. The absence of the energy barrier at the i.e.p. means that attractive forces are a dominating factor and induces aggregation and is reflected in the lowest observed turbidity. It is seen that the height of the repulsive energy barrier is sufficiently greater than the thermal energy (kT) when the magnitude of the -potential is greater than 10 mV. This is where the suspensions tend to be stable to aggregation and is reflected in the increasing residual turbidity observed with an increasing pH (pH > 4). The results of the DLVO calculations are in good agreement with the observed trends under various pH conditions indicating that the DLVO theory can be used to predict colloidal stability of the whey protein suspensions. 4. Conclusions This study shows that the aggregation and particle stability of whey particles in the suspension is strongly dependent on pH and aluminium sulphate concentrations. The whey suspension is highly unstable to aggregation near the isoelectric point at pH between 3.2 and 3.4 because of the relatively low surface charge of the particles, which is demonstrated by the zeta potential measurements and supported by the turbidity results. The residual turbidity of a suspension following the addition of aluminium sulphate is a good indication of the effectiveness of the flocculation process. The minimum and maximum turbidity in aluminium sulphate occurs at the isoelectric point and pH 8, respectively. It is noted that when the -potentials of the supernatant whey are relatively high (at pH 8) then the residual turbidity is also high suggesting some degree of stabilization and also when the -potentials are low the residual
[1] J.H. Hanemaaijer, Microfiltration of whey processing, Desalination 53 (1985) 143–155. ´ S.D. Milanovic, ´ D.M. Krstic, ´ M.N. Tekic, ´ Fouling of inorganic mem[2] M.D. Caric, branes by adsorption of whey proteins, J. Membr. Sci. 165 (2000) 83–88. [3] H.S. Alkhatim, M.I. Alcaina, E. Soriano, M.I. Lora, J. Arnal, Treatment of whey effluent from dairy industries by nanofiltration membranes, Desalination 119 (1998) 177–184. [4] D. Barba, F. Beolchini, D. Cifoni, F. Veglio, Whey ultrafiltration in a tubular membrane: effect of selected operating parameters, Sep. Sci. Technol. 37 (2002) 1171–1788. [5] J. Lyklema, Fundamentals of Interface and Colloid Science, vol. 2, Academic Press, London/San Diego/New York, 1995. [6] I.Y. Chang, K.P. Chang, The role of hydration force on the stability of the suspension of Saccharomyces cerevisiae—application of the extended DLVO theory, Colloid Surf. A: Physicochem. Eng. Aspects Sci. 211 (2002) 66–77. [7] B. Keskinler, G. Akay, Y.K. Bayhan, E. Erhan, Effect of ionic environment on the cross flow microfiltration behaviour of yeast suspensions, J. Membr. Sci. 206 (2002) 351–360. [8] S.O. Agboola, D.G. Dalgleish, Effects of pH and ethanol on the kinetics of destabilization of oil-in-water emulsions containing milk proteins, J. Sci. Food Agric. 72 (1996) 448–454. [9] J.A. Hunt, D.J. Dalgleish, Effect of pH on the stability and surface composition of emulsions made with protein isolate, J. Agric. Food Chem. 42 (1994) 2131–2135. [10] A. Kulmyrzaev, H. Schubert, Influence of KCl on the physicochemical properties of whey protein stabilized emulsions, Food Hydrocoll. 18 (2004) 13–19. [11] B. Ersoy, Effect of pH and polymer change density on settling rate and turbidity of natural stone suspensions, Int. J. Miner. Process 3–4 (2005) 207–216. [12] D.M. Afonso, G. Hagmeyer, R. Gimbel, Streaming potential measurements to assess the variation of nanofiltration membranes surface charge with the concentration of salt solution, Sep. Sci. Technol. 22–23 (2001) 529–541. [13] J.R. Hunter, Zeta Potential in Colloid Science: Principle and Application, second ed., Academic Press Inc., London, 1988. [14] P.C. Hiemenz, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1986. [15] P. Walstra, Dispersed systems: basic considerations, in: O.R. Fennema (Ed.), Food Chemistry, Marcel Dekker, New York, 1996. [16] D.J. McClements, Food Emulsions—Principles, Practice and Techniques, CRC Press, Boca Raton, 1999. [17] A.A. Kulmyrzaev, H. Schubert, Influence of pH and CaCl2 on the stability of dilute whey protein stabilized emulsions, Food Res. Int. 33 (2000) 15–20. [18] M.P.C. Silvestre, E.A. Decker, D.J. McClements, Influence of copper on the stability of whey protein stabilized emulsions, Food Hydrocoll. 13 (1999) 419–424. [19] O. Al Akoum, Y.M. Jaffrin, L. Din, P. Paullier, C. Vanhoutte, An hydrodynamic investigation on electrokinetic properties of microfiltration and ultrafiltration in a vibrating membrane module, J. Membr. Sci. 197 (2002) 37–52. [20] H.M. Hudson, C.R. Daubert, Electrokinetic and electroviscous properties in derivatized whey protein isolate colloidal suspensions of defined size, in: IFT Annual Meeting, New Orleans, LA, USA, 2001. [21] L. Ricq, P. Andre, J.C. Reggiani, S. Zaragoza-Piqueras, J. Pagetti, G. Daufin, Effect of proteins on electrokinetic properties of inorganic membranes during ultraand micro-filtration, J. Membr. Sci. 114 (1996) 27–38. [22] W.A. Bough, D.R. Landes, Recovery and nutritional evaluation of proteinaceous solids separated from whey by coagulation with chitosan, J. Dairy Sci. 59 (1976) 1874–1880. [23] A. Adin, C. Klein-Banay, Pretreatment of seawater by flocculation and settling for particulates removal, Desalination 58 (1986) 227–241. [24] A.L. Ahmad, S. Sumathi, B.H. Hameed, Coagulation of residue oil and suspended solid in palm oil mill effluent by chitosan, alum and PAC, Chem. Eng. J. 118 (2006) 99–105. [25] N.D. Tzoupanos, A.I. Zouboulis, Y.C. Zhao, The application of novel coagulant reagent (polyaluminium silicate chloride) for the post-treatment of landfill leachates, Chemosphere 73 (2008) 729–736. [26] A. Higuchi, S. Mishima, T. Nagakawa, Separation of proteins by surface modified polysulfone membranes, J. Membr. Sci. 57 (1991) 175–185. [27] T. Moritz, S. Benfer, P. Arki, G. Tomandl, Influence of the surface charge on the permeate flux in the dead end filtration with ceramic membrane, Sep. Sci. Technol. 25 (2001) 501–508. [28] H.J. Jacobasch, F. Simon, C. Werner, C. Bellmann, Technishches Messen 63 (1996) 439–446. [29] E. Pefferkorn, Clay and oxide destabilization induced by mixed alum/ macromolecular flocculation aids, Adv. Colloid Interface Sci. 120 (2006) 33–45.
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Influence of pH and Al2 (SO4 )3 on the stability of whey suspensions P. Kaewkannetra a , F.J. Garcia-Garcia b , A.E. James c , T.Y. Chiu d,e,∗ a
FerVVAP Research Centre, Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand Corrosion and Protection Centre, School of Materials, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK c School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, M60 1QD, UK d Earth Tech Engineering Ltd, Wentworth Business Park, Maple Road, Tankersley, Barnsley S75 3DL, UK e Centre of Water Science, Cranfield University, Bedfordshire MK43 0AL, UK b
a r t i c l e
i n f o
Article history: Received 22 January 2009 Received in revised form 3 April 2009 Accepted 13 April 2009 Keywords: Stability Whey suspensions Electrokinetics -Potential Coagulation
a b s t r a c t The effects of pH and aluminium sulphate concentration on the stability of whey suspension have been investigated. The particle size and particle size distribution are examined by scanning electron microscopy and granulometric analysis, respectively. The zeta potential and residual turbidity of 0.01 wt% whey suspensions are determined over the pH range of 3–8 with aluminium sulphate as background electrolyte. The results show that the zeta potential of whey suspensions depends on pH and, to a lesser degree, aluminium sulphate concentration. The average particle size of flocs in suspension increases with decreasing zeta potentials which is accompanied by low turbidity. The degree of flocculation of whey suspension is linked to the zeta potential. The DLVO calculations are in good agreement with the observed trends indicating that the classical DLVO theory can be used to predict colloidal stability of the whey protein suspensions in the presence of higher strength electrolyte and varying pHs. © 2009 Elsevier B.V. All rights reserved.
1. Introduction During the past few decades, the processing of whey, a liquid produced when milk is possessed into cheese or casein, has developed from dairy waste into a variety of dairy products with increasing added values [1]. Much research has been undertaken concerning various aspects of whey especially in relation to its use in food products and various investigators have studied either its behaviour in terms of aggregation and adsorption or treatment of whey effluents from dairy industries using membrane filtration [1–4] but to our knowledge few studies attempted to characterise the stability of whey suspensions in terms of their electrokinetic properties. Suspensions of whey are highly turbid and form biological suspensions containing settleable and colloidal materials; most colloid particles existing in natural waters and in many industrial effluent streams have negative surface charges, depending on the solution pH and the stability of these particles is often investigated in terms of the zeta-potential (-potential). The -potential of colloid suspensions is controlled by the surface chemistry of the colloidal particles as well as the chemistry of the solution [5]. Knowledge of the interaction forces between whey particles
∗ Earth Tech Engineering Ltd, Wentworth Business Park, Maple Road, Tankersley, Barnsley S75 3DL, UK. Tel.: +44 122 622 4176; fax: +44 122 622 4488. E-mail address: [email protected] (T.Y. Chiu). 1383-5866/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2009.04.013
in suspensions is essential for interpreting the stability of these suspensions and also the adhesive behaviour of the particles. Of the interaction forces it is the electrical double layer forces that are characterised in terms of the -potential and these usually are repulsive as particles composed of the same material will have similar charges [6]. The sedimentation and filtration of colloidal particles can be linked with their stability and thus, knowledge of the -potential is fundamental to the understanding of these processes. In treating industrial wastewater, coagulation and flocculation are considered to be essential requirements for efficient separation using either filtration or sedimentation. The purpose of flocculation is to promote the interaction of particles so as to form aggregates that can be efficiently removed in subsequent filtration processes or to form larger faster settling particles in sedimentation. In order to achieve efficient flocculation, the suspensions must be destabilized by the addition of coagulants, such as aluminium sulphate, ferric salts and polyelectrolytes [7]. The rate of coagulation depends mainly on the stability of the colloid particles and the salt concentration and pH are two important parameters affecting colloid stability [5]. This is especially so for protein-stabilized emulsions such as whey suspensions where these two parameters are considered to be the most important characteristics in determining their stability [8–10]. Another way of assessing the changes in the colloidal stability of suspensions is to consider changes in their turbidity. It should be remembered that although turbidity measurement is an indirect technique, it provides an easy method of
P. Kaewkannetra et al. / Separation and Purification Technology 67 (2009) 364–368
appraising the performance of a flocculation process. The turbidity of a suspension can reflect the amount of colloidal material present and thus, the effectiveness of a flocculation process can be readily established in terms of the residual turbidity of the suspensions following physicochemical treatment and subsequent setting or coagulation [11]. In most applications the floc size can be controlled by the selection of an appropriate flocculating agent and subsequently through the correct implementation of the mixing and flocculation conditions [12]. If inorganic coagulants are used then the changes in the electrokinetic properties of the suspensions can sometimes be related to the residual turbidity. Electrokinetic phenomena occur when one electrically charged phase moves with respect to an adjoining phase. The interactions between the mechanical and electrical forces can be used to acquire some knowledge of the electrical potential in the interfacial region [12] which can be important in particle interactions. One of the more readily inferable electrokinetic quantities from electrokinetic measurements is the -potential. The -potential is related to the electrokinetic mobility through the Helmholtz-Smoluchowski equation [13]: =
ue ve = εr ε0 εr ε0 E
(1)
where ue , ve , E, , εr , and ε0 are the electrophoretic mobility of the aggregates, the velocity of the particles, the electric field, the viscosity of the solution, the dielectric constant of the liquid and the permittivity of free space, respectively. An aim of the present work is to use the -potential to investigate the colloidal stability of whey suspensions. Quantitatively colloidal stability is usually assessed in terms of DLVO theory [13]; where the total interaction energy, V(h), as a function of the distance between the surfaces of two particles along the center to center line, (h), is expressed as V (h) = VVDW (h) + VE (h)
(2)
where VVDW (h) and VE (h) are the van der Waals attractive energy and the electrical double layer, respectively. The van der Waals interaction is given by [14] A VVDW (h) = − 6
2R2 2R2 × ln + h2 +4Rh h2 +4Rh+4R2
h2 +4Rh h2 +4Rh+4R2
(3) And the electrostatic repulsion energy is [15] VE (h) = 2εr ε0 R 2 ln(1 + e−h )
(4)
where R, A, , and are the particle radius, the Hamaker function, zeta potential and the reciprocal Debye–Huckel length, respectively. The total interaction energy is usually divided by the product of Boltzman’s constant (k) and temperature (T), i.e. the average free energy involved in an encounter between two droplets by Brownian motion. The influence of mineral ions on the stability of proteinstabilized emulsions is complex and depends on the precise nature of the ions, the electrical characteristics of the droplet surface and the prevailing environmental conditions [16]. A number of studies have been conducted to provide insight into the effect of monovalent and divalent ions [10,17,18] on the physicochemical properties and stability of emulsions stabilized by whey proteins. However, many chemicals used in wastewater coagulation are trivalent such as aluminium ions in the form of aluminium sulphate. These trivalent minerals may alter the stability of protein-stabilized emulsions in a way different from mono- and di-valent minerals and to the authors knowledge, no studies have been carried out investigating the effects of aluminium sulphate on the stability of whey suspensions.
365
In this study, changes in the turbidity of a typical whey are attributed to various modes of colloidal stability and are considered in terms their size distributions and -potentials as functions of pH and electrolyte concentration. Aluminium sulphate was used as electrolyte and it seems that the results are adequately explained in terms of the classical DLVO theory. 2. Materials and methods 2.1. Materials All suspensions were prepared using spray dried powder from bovine milk (Sigma® , USA). The manufacturer’s analysis of the product was 65% lactose, 13% protein, 8% ash, 2% lactic acid and the rest of these are 12% total solids that can be separated to be fatty and non-fatty solids. Sodium hydroxide (0.1 M) and nitric acid (0.1 M) were used to adjust the pH of solutions. Aluminium sulphate was used as the electrolyte. All chemicals were analytical grade (Sigma® –Aldrich, UK), and all solutions were prepared with ultra pure water (ELGASTAT® Spectrum B unit Cartridge type: SC2 , USF Memcor, England) with a resistivity of 18 M cm and an organic contamination of less than 0.03 mg l−1 . 2.2. Preparation of flocculated whey suspensions Suspensions of the whey particles (0.1%, w/v) were prepared using ultra pure water in a flocculation rig fitted with an impellor (Flocculator SW1, Stuart Scientific, UK); various quantities of aluminium sulphate were added so that the concentration in the suspensions varied between 3 and 8 mg l−1 . The pH of suspensions was varied from 3 to 10 using dilute acid and alkali where appropriate. The suspensions were mixed for 1 min using an impeller speed of 200–222 rpm followed by 15 min of slow mixing at 20–22 rpm to promote floc growth. After flocculation the motor was switched off and the flocculated whey particles were allowed to settle for 15 min. 2.3. Determination of size and particle size distribution Granular dried whey was examined to determine its size and shape with a scanning electron microscope, SEM (FEI QUANTA 200, Purge, Czech Republic) operated at an accelerating voltage of 20 kV. The suspensions of whey from the flocculation rig were analysed to determine the particle size distribution using a laser scattering instrument (Mastersizer 2000, Malvern Instrument Ltd, UK) which is suitable for particle sizes ranging from 0.02 to 2000 m. The measurements made in this work are reported as the full particle size distribution. Each sample was analysed and the average of five replications is reported here. It should be remembered that the particles are considered to be spherical in these results. 2.4. Determination of turbidity and -potential The turbidity measured in nephelometric turbidity units (NTU) of the supernatant liquid obtained after the period of settling following flocculation was monitored using a laboratory turbidity meter (Model: TU1110, Cambridge, UK). Electrophoretic mobility measurements were obtained using a laser scattering instrument (Zetasizer 3000HS Advance, Malvern Instrument Ltd, UK). This equipment measures distribution of electrophoretic mobility and -potential of particles in liquid suspensions using Laser Doppler Velocimetry (LDV). Samples are injected into the measuring cell and the output from the instrument includes the average -potential and its standard deviation. The -potentials reported here were
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Fig. 1. SEM micrograph of (a) dried whey granular; (b) dried whey granule showing the loose aggregation of the primary whey particles; (c) particle size distribution of the unflocculated whey suspension.
calculated from the average of at least five separate injections per sample. Measurements were obtained over the desired pH range and at different aluminium sulphate concentrations. All experiments were carried out at the ambient temperature which was 25 ◦ C. 3. Results and discussion 3.1. Scanning electronic microscopy and particle size distribution The SEM micrographs show that dried whey granules, prior to being dispersed in water, are approximately spherical in shape (Fig. 1a). Each granule is composed of large numbers of smaller whey particles as shown in Fig. 1b. The surface texture is loosely agglomerate and the granules are readily disaggregated and form turbid suspension when dispersed in water. The resultant suspensions consist of disaggregated primary particles. The average size for the unflocculated whey dispersed in deionised water is 7.0 m (Fig. 1c) and it lies within the reported range of whey sizes of 0.3–10 m [19–21]. The presence of wide spread of particle sizes depends on the relative proportions of the main constituents in the whey as well as its source. The size distribution of prepared whey suspensions under various pH and Al2 (SO4 )3 electrolyte concentrations are shown in Fig. 2a and b, respectively. It is clear that the mean particle size decreases with increasing pH with the largest floc of 18.5 m observed at pH 3. Under increasing Al2 (SO4 )3 concentrations, the average size for the flocculated whey increases and reaches a maximum size of 23.5 m before the flocs break down to approximately 15.3 m at 5 and 8 mg l−1 , respectively.
Fig. 2. Mean particle size of whey suspensions as a function of (a) electrolyte concentrations at pH 3.0 and (b) pH at 3 mg l−1 Al2 (SO4 )3 .
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Table 1 Turbidity of whey suspensions as a function of pH and electrolyte concentration. pH
Al2 (SO4 )3 (mg l−1 )
Turbidity (NTU)
3.0 4.0 5.0 6.0 7.0 8.0 3.0 3.0 3.0 3.0 3.0
3 3 3 3 3 3 4 5 6 7 8
122 128 132 132 132 147 126 102 132 138 142
3.2. Effect of pH and electrolyte concentration on the residual turbidity Changes in the residual turbidity of the supernatant as functions of pH and Al2 (SO4 )3 concentrations following settlement of flocculated whey suspensions are shown in Table 1. The lowest turbidity of 122 NTU was obtained at pH 3.0 and the maximum turbidity occurring at pH 8 (147 NTU). There is generally an increase in turbidity with pH. It seems that at low pH there is a preponderance of larger flocs that will settle relatively quickly, while a higher pH the whey particles seem more dispersed with considerably smaller particles present. Elsewhere, the reported maximum turbidity was in agreement with the current study, however, the minimum turbidity was found to occur at pH 6 [22]. This discrepancy may arise from the difference in type of whey used which is reflected in the observed isoelectric point of whey in the current study and is discussed in later sections. In the presence of Al2 (SO4 )3 the turbidity is significantly reduced and increased Al2 (SO4 )3 concentration produces a small increase in turbidity. The turbidity minima of 102 NTU is found at 5 mg l−1 Al2 (SO4 )3 . When the applied dosage was higher than the optimum amount, the turbidity increased. This shows restabilization of the coagulated whey particles. At high doses of coagulant, a sufficient degree of over-saturation occurs to produce a rapid precipitation of large quantity of coagulant. This is a common phenomenon due to overdosing and is also observed in a number of works where hydrolyzable metal salts have been utilised [23–25]. It seems that the both pH and Al2 (SO4 )3 concentrations affect the interactions between whey particles in solution. This in turn affects the residual turbidity of the suspensions. 3.3. Effect of pH and electrolyte concentration on the -potential The effect of pH on the -potential of the supernatant whey particles in the presence of Al2 (SO4 )3 is shown in Fig. 3 at three different concentrations, together with a sample where there was no background electrolyte. The results show that the -potential of the whey used in this study is positive at pH 3 in the presence and absence of Al2 (SO4 )3 electrolyte. As the pH is increased the -potentials in all cases are negative, with the maximum magnitude being found at pH 8. From these results it is seen that the -potential seems to be affected by both changes in the pH and by the Al2 (SO4 )3 concentration. At constant Al2 (SO4 )3 concentration, the sign of the -potential is changed by varying pH and at constant pH it is impossible to change the sign of the -potential by changing the Al2 (SO4 )3 concentration. Generally, at any pH the magnitude of the -potentials is reduced as Al2 (SO4 )3 concentration is increased. The behaviour of the -potential changes and it is seen to be negative from the i.e.p. (the point where the -potentials become zero ( = 0 mV) is called the isoelectric point, i.e.p.) at pH between 3.0 and 4.0. As whey includes various proteins, the i.e.p. of proteins in
Fig. 3. Zeta potentials of whey suspensions in the presence of aluminium sulphate over the range of pH (() 0 mg l−1 Al2 (SO4 )3 ; () 3 mg l−1 Al2 (SO4 )3 (×) 5 mg l−1 Al2 (SO4 )3 , () 8 mg l−1 Al2 (SO4 )3 ).
the whey suspension can vary [26]. Since the i.e.p. does not change with increasing Al2 (SO4 )3 concentration it is clear that as far as the whey particles are concerned it is an indifferent electrolyte, that is increasing the concentration only serves to compress the double layer. Previous studies [27,28] have attributed shifts in the i.e.p. to the adsorption of whey suspension but here the changes in the i.e.p. are small in comparison to the large change in electrolyte concentration and may arise from experimental error. The trends in these results are broadly similar to those reported by others [10,17,18] although these authors use a whey protein isolate powder and different concentrations of copper chloride (0–100 M), calcium chloride (0–20 M) and potassium chloride (0–100 M). 3.4. Correlation between -potential and residual turbidity The effect of electrolyte concentration on the calculated interaction potential is shown in Fig. 4a. These potentials were calculated using the known electrolyte concentrations and their corresponding measured zeta potential. The total interactive energy curve of DLVO theory predicts primary maximum under various electrolyte concentrations. This implies that the electrostatic repulsive energy is sufficiently dominant in the present system for the suspension to resist flocculation and thereby results in forming a relatively stable colloidal system. Without flocculation, the suspension will remain turbid. This explains the high turbidities observed under the various electrolyte concentrations (Table 1). The height of the energy barrier to flocculation decreases as the electrolyte concentration increases. This lowered primary maximum energy barrier implies that particles can collide with sufficient energy to overcome the repulsive energy barrier and are not longer prevented from coming close together to flocculate which accounts for the lowered turbidity observed when increasing the concentrations of Al2 (SO4 )3 from 3 to 5 mg l−1 . Contrary to the DLVO predictions, further decline in turbidities associated with lowered primary energy barrier at Al2 (SO4 )3 concentrations >5 mg l−1 is not observed. The turbidity increase is attributed to the restabilization of the coagulated whey particles. At high doses of coagulant, a sufficient degree of over-saturation occurs to produce a rapid precipitation of large quantity of coagulant. The results are in agreement with other workers [29] who suggested that particle surface charge, reflected as zeta potential, shows the status of the particle stability in the system, but may not necessarily be proportional to the amount of coagulant needed for particle destabilization. Fig. 4b shows the calculated overall interaction potential as a function of the separation distance under various pHs assuming that Al2 (SO4 )3 concentration remains constant. The energy barrier to flocculation decreases as pH approaches the i.e.p. of the whey proteins. At approximately pH 3.3, the energy barrier disap-
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turbidity is low and when the -potential is increased the average floc size decreases. The numerical calculations of suspension stability to aggregation using the DLVO theory show that this theory can be applied to predict colloidal interaction and stability of the whey suspensions under varying pH conditions. References
Fig. 4. Interaction potential of whey particles in suspension as a function of separation distances: (a) variable electrolyte concentrations at pH 3.0 and (b) variable pH at 3 mg l−1 Al2 (SO4 )3 .
pears indicating that no electrostatic repulsion exists between the interacting particles because zeta potential at this point is 0 mV. The absence of the energy barrier at the i.e.p. means that attractive forces are a dominating factor and induces aggregation and is reflected in the lowest observed turbidity. It is seen that the height of the repulsive energy barrier is sufficiently greater than the thermal energy (kT) when the magnitude of the -potential is greater than 10 mV. This is where the suspensions tend to be stable to aggregation and is reflected in the increasing residual turbidity observed with an increasing pH (pH > 4). The results of the DLVO calculations are in good agreement with the observed trends under various pH conditions indicating that the DLVO theory can be used to predict colloidal stability of the whey protein suspensions. 4. Conclusions This study shows that the aggregation and particle stability of whey particles in the suspension is strongly dependent on pH and aluminium sulphate concentrations. The whey suspension is highly unstable to aggregation near the isoelectric point at pH between 3.2 and 3.4 because of the relatively low surface charge of the particles, which is demonstrated by the zeta potential measurements and supported by the turbidity results. The residual turbidity of a suspension following the addition of aluminium sulphate is a good indication of the effectiveness of the flocculation process. The minimum and maximum turbidity in aluminium sulphate occurs at the isoelectric point and pH 8, respectively. It is noted that when the -potentials of the supernatant whey are relatively high (at pH 8) then the residual turbidity is also high suggesting some degree of stabilization and also when the -potentials are low the residual
[1] J.H. Hanemaaijer, Microfiltration of whey processing, Desalination 53 (1985) 143–155. ´ S.D. Milanovic, ´ D.M. Krstic, ´ M.N. Tekic, ´ Fouling of inorganic mem[2] M.D. Caric, branes by adsorption of whey proteins, J. Membr. Sci. 165 (2000) 83–88. [3] H.S. Alkhatim, M.I. Alcaina, E. Soriano, M.I. Lora, J. Arnal, Treatment of whey effluent from dairy industries by nanofiltration membranes, Desalination 119 (1998) 177–184. [4] D. Barba, F. Beolchini, D. Cifoni, F. Veglio, Whey ultrafiltration in a tubular membrane: effect of selected operating parameters, Sep. Sci. Technol. 37 (2002) 1171–1788. [5] J. Lyklema, Fundamentals of Interface and Colloid Science, vol. 2, Academic Press, London/San Diego/New York, 1995. [6] I.Y. Chang, K.P. Chang, The role of hydration force on the stability of the suspension of Saccharomyces cerevisiae—application of the extended DLVO theory, Colloid Surf. A: Physicochem. Eng. Aspects Sci. 211 (2002) 66–77. [7] B. Keskinler, G. Akay, Y.K. Bayhan, E. Erhan, Effect of ionic environment on the cross flow microfiltration behaviour of yeast suspensions, J. Membr. Sci. 206 (2002) 351–360. [8] S.O. Agboola, D.G. Dalgleish, Effects of pH and ethanol on the kinetics of destabilization of oil-in-water emulsions containing milk proteins, J. Sci. Food Agric. 72 (1996) 448–454. [9] J.A. Hunt, D.J. Dalgleish, Effect of pH on the stability and surface composition of emulsions made with protein isolate, J. Agric. Food Chem. 42 (1994) 2131–2135. [10] A. Kulmyrzaev, H. Schubert, Influence of KCl on the physicochemical properties of whey protein stabilized emulsions, Food Hydrocoll. 18 (2004) 13–19. [11] B. Ersoy, Effect of pH and polymer change density on settling rate and turbidity of natural stone suspensions, Int. J. Miner. Process 3–4 (2005) 207–216. [12] D.M. Afonso, G. Hagmeyer, R. Gimbel, Streaming potential measurements to assess the variation of nanofiltration membranes surface charge with the concentration of salt solution, Sep. Sci. Technol. 22–23 (2001) 529–541. [13] J.R. Hunter, Zeta Potential in Colloid Science: Principle and Application, second ed., Academic Press Inc., London, 1988. [14] P.C. Hiemenz, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1986. [15] P. Walstra, Dispersed systems: basic considerations, in: O.R. Fennema (Ed.), Food Chemistry, Marcel Dekker, New York, 1996. [16] D.J. McClements, Food Emulsions—Principles, Practice and Techniques, CRC Press, Boca Raton, 1999. [17] A.A. Kulmyrzaev, H. Schubert, Influence of pH and CaCl2 on the stability of dilute whey protein stabilized emulsions, Food Res. Int. 33 (2000) 15–20. [18] M.P.C. Silvestre, E.A. Decker, D.J. McClements, Influence of copper on the stability of whey protein stabilized emulsions, Food Hydrocoll. 13 (1999) 419–424. [19] O. Al Akoum, Y.M. Jaffrin, L. Din, P. Paullier, C. Vanhoutte, An hydrodynamic investigation on electrokinetic properties of microfiltration and ultrafiltration in a vibrating membrane module, J. Membr. Sci. 197 (2002) 37–52. [20] H.M. Hudson, C.R. Daubert, Electrokinetic and electroviscous properties in derivatized whey protein isolate colloidal suspensions of defined size, in: IFT Annual Meeting, New Orleans, LA, USA, 2001. [21] L. Ricq, P. Andre, J.C. Reggiani, S. Zaragoza-Piqueras, J. Pagetti, G. Daufin, Effect of proteins on electrokinetic properties of inorganic membranes during ultraand micro-filtration, J. Membr. Sci. 114 (1996) 27–38. [22] W.A. Bough, D.R. Landes, Recovery and nutritional evaluation of proteinaceous solids separated from whey by coagulation with chitosan, J. Dairy Sci. 59 (1976) 1874–1880. [23] A. Adin, C. Klein-Banay, Pretreatment of seawater by flocculation and settling for particulates removal, Desalination 58 (1986) 227–241. [24] A.L. Ahmad, S. Sumathi, B.H. Hameed, Coagulation of residue oil and suspended solid in palm oil mill effluent by chitosan, alum and PAC, Chem. Eng. J. 118 (2006) 99–105. [25] N.D. Tzoupanos, A.I. Zouboulis, Y.C. Zhao, The application of novel coagulant reagent (polyaluminium silicate chloride) for the post-treatment of landfill leachates, Chemosphere 73 (2008) 729–736. [26] A. Higuchi, S. Mishima, T. Nagakawa, Separation of proteins by surface modified polysulfone membranes, J. Membr. Sci. 57 (1991) 175–185. [27] T. Moritz, S. Benfer, P. Arki, G. Tomandl, Influence of the surface charge on the permeate flux in the dead end filtration with ceramic membrane, Sep. Sci. Technol. 25 (2001) 501–508. [28] H.J. Jacobasch, F. Simon, C. Werner, C. Bellmann, Technishches Messen 63 (1996) 439–446. [29] E. Pefferkorn, Clay and oxide destabilization induced by mixed alum/ macromolecular flocculation aids, Adv. Colloid Interface Sci. 120 (2006) 33–45.