flocc. mechan. by cat. pol. investigated_franks_2006

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Flocculation Mechanism Induced by Cationic Polymers Investigated by

Light Scattering

Ying Zhou and George V. Franks*,†

Chemical Engineering and the Centre for Multiphase Process, The UniVersity of Newcastle,Callaghan, NSW 2308 Australia

ReceiVed January 29, 2006. In Final Form: April 23, 2006

Three cationic polymers with molecular weights and charge densities of 3.0× 105 g/mol and 10%, 1.1× 105 g/moland 40%, and 1.2× 105 g/mol and100% were chosen as flocculants to aggregate silica particles (90 nm), under variousconditions, including change in polymer dosage, particle concentration, background electrolyte concentration, andshear rate. The size and structure of flocs produced were determined using the static light scattering technique. Onthe basis of measurements of polymer adsorption and its effect on the zeta potential and floc properties, it has beenfound that the polymer charge density plays an important role in determining the flocculation mechanism. Polymerswith a 10% charge density facilitate bridging, 40% charged polymers bring about either a combination of chargeneutralization and bridging or bridging, depending on the polymer dosage, and polymers with the charge density of 100% induce electrostatic patch flocculation mechanism at the optimum polymer dosage and below but bring aboutbridging mechanismat the polymer dosage approaching the adsorption plateau value. Bridging aggregation can readilybe affected by the particle concentration, and an increase in particle concentration results in the formation of largerbut looser aggregates, whereas electrostatic patch aggregation is independent of particle concentration. The addition

of a background electrolyte aids in bridging aggregation while it is detrimental to electrostatic patch aggregation. Ithas also been found that the effect of shear rate on the mass fractal dimension depends on polymer charge density.

1. Introduction

Polyelectrolyte flocculants can significantly enhance solid-liquid separation processes and are increasingly being used ina wide range of industries, such as minerals recovery, industrialtailings dewatering, paper manufacturing, and water and waste-water treatment. When polyelectrolytes are added to oppositelycharged particles, electrostatic attraction is believed to be themaindriving force for adsorption and the postulated mechanismsby which polyelectrolytes can bring about flocculation arebridging, charge neutralization, or electrostatic patch models.1

Bridging flocculation occurs whensegments of the same polymermolecule are attached to more than one particle, thereby linkingthe particles together. This type of flocculation mechanism hasbeen found to be very efficient. Charge neutralization is causedby the reduction in the electric double layer repulsion betweenparticlesdue to adsorption of highly charged polyelectrolytes onoppositely charged particles. It is generally believed that lowmolecular weight polymers tend to adsorb and neutralize theopposite charges on the particles. The electrostatic patchflocculation is thought to be operative for polymers of very highcharge density interacting with oppositely charged particles of lowcharge density. The net residual charge of the polymer patchon oneparticlesurfacecan attach to thebare part of an oppositelycharged particle.

Several studies on polymer flocculation have been publishedin the literature,2-6 the choice of polymer in any particular case

still hasto be made on a largelyempiricalbasis,whichis primarilydueto tworeasons. Oneis that, flocculation is a very complicatedprocess involving the following stages:7 (a) particle-polymermixing,(b) attachment of the polymer molecules ontothe particlesurface, (c) reconformation of the polymer molecules on theparticle surface, (d) particle flocculation, and (e) floc breakupdueto shear mixing. These processescan take place concurrentlyandareoftencompeting. Therefore, to understandtheflocculationmechanisms, it is crucial to gain fundamental knowledge of thekey parameters which can significantly influence the time scaleof a particular stage. Polymer molecular weight,6,8,9 charge

density,6,8,10 dimensionsin solution,11 polymer concentration,8,12,13

background electrolyte concentration,8,14 and particle concen-tration13-16 areamong some of these parameters. Theother reasonmight be that usually indirect methods, such as turbidity,sedimentation rate, or visual observation, are applied to monitorthe flocculation progress. In this way, only some overallparameters are measured and no information is obtained on flocproperties, especially the floc structure. This wasdespite the factthatknowledge of flocproperties can leadto better understandingand control of many industrial separation processes. It is well-known that different industrial applications require aggregates(or flocs) of different properties.17,18 For example, in filtration

* Corresponding author. E-mail: [email protected].† Current address: Department of Chemicaland BiomolecularEngineering,

The University of Melbourne, VIC 3010 Australia.(1) Gregory, J., Flocculation by Polymers and Polyelectrolyte. In Solid/liquid

dispersions; Tadros, T. F., Ed.; Academic Press: London, 1987.(2) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448-56.(3) Yan, Y. D.; Glover, S. M.; Jameson, G. J.; Biggs, S. Int. J. Miner. Process.

2004, 73, 161-75.(4) Thomas, D. N.; Judd, S. J.; Fawcett, N. Water Res. 1999, 33, 1579-92.(5) Gregory, J. J. Colloid Interface Sci. 1976, 55, 35-44.(6) Mabire, F.; Audebert, R.; Quivoron, C. J. Colloid Interface Sci. 1984, 97 ,

120-36.

(7) Elimelech, M. Particle Deposition and Aggregation: Measurement,

Modelling, and Simulation; Butterworth-Heinemann: Oxford, 1995.(8) Graham, N. J. D. Colloids Surf. 1981, 3, 61-77.(9) Gill, R. I. S.; Herrington, T. M. Colloids Surf. 1987, 25, 297-310.(10) Gill, R. I. S.; Herrington, T. M. Colloids Surf. 1987, 28, 41-52.(11) Mpofu, P.; Addai-Mensah, J.; Ralston, J. J. Colloid Interface Sci. 2004,

271, 145-156.(12) Bratskaya, S.; Schwarz, S.; Liebert, T.; Heinze, T. Colloids Surf. A 2005,

254, 75-80.(13) Pelssers, E. G. M.; Stuart, M. A. C.; Fleer, G. J. Colloids Surf. 1989, 38,

15-25.(14) Wagberg, L.; Inger, A. Colloids Surf. A 1995, 104, 169-84.(15) Pelssers, E. G. M.; Cohen Stuart, M. A.; Fleer, G. J. J. Chem. Soc.,

Faraday Trans. 1990, 86 , 1355-61.(16) Gregory, J. Colloids Surf. 1988, 31, 231-53.(17) Moudgil, B. M.; Behl, S. Flotation Sci. Eng. 1995, 415-39.(18) Gregory, J. Filtr. Sep. 1998, 35, 367-71.

6775 Langmuir 2006, 22, 6775-6786

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processes, strong but porous flocs are often preferred. Theaggregate size and its size distribution are clearly important,because it will determine whether the flocs are suitable for aparticular separation process such as filtration or sedimentation.

The advent of fractal mathematics in the mid 1970s led tosignificant advancement in our understanding of the complexstructure of particle aggregates.19,20 It is now known thataggregates formed from random aggregation processes, in dilutesuspensions, possess mass fractal characteristics.7,19-23 This

means that thestructure of theaggregates is self-similarat differentlength scales or scale invariance. For a mass fractal aggregate,its mass, M ( R), is proportional to its radius, R, raised to power

Df

where Df stands for the mass fractal dimension and is not limitedto integer values. Df can further be used to characterize the massdensity of the aggregate, F( R), i.e.

Therefore, the mass fractal dimension is a powerful measure of the structural compactness or the space filling capacity of an

aggregate. In three-dimensional Euclidean spaces, Df rangesfrom1 to 3. An entirely compacted object such as a solid sphere hasa Df of 3, whereas aggregates with an open configuration of particles are characterized by smaller fractal dimensions.

Static light scattering has been extensively used in themeasurement of the size of particles and aggregates as well asin the study of aggregating and aggregatedcolloidalsystems.24,25

However, it is surprising that this technique has not, to ourknowledge, yet been widely applied to flocs induced bypolyelectrolytes. In a static light scattering experiment, a beamof light is directed onto a sample and the scattered light intensityis measured as a function of the magnitude of the scatteringvector, Q. Fora mass fractalaggregate consistingof monodisperseprimary particles and satisfying the criteria of the Rayleigh-

Gans-Debye regime, there is the following relationshipbetweenthescattered intensity fromthe aggregate, I (Q), and the scatteringwave vector, Q.24,25

where

and n is the refractive index of the fluid, λ, the wavelength invacuo of the laser light used, and θ, the scattering angle. Since1/ Q is the length scale probed in a scattering experiment, the low

andhigh regionsof Q reveal theoverallstructureof theaggregatesand the structure of the primary particles, respectively. The Q- Df

dependence with the scattered intensity is valid in a range of

length scales much larger than the primary particles and muchsmaller than aggregates.22

where r 0 is the radius of the primary particle and R stands forthe aggregate radius. The measure of fractal dimension can beestimated from the absolute slope of log I (Q) versus log Q byfitting a straight line through the fractal regime section of thescattering plot.

In this study, we used three cationic polymers with variouscharge densitiesas flocculants to aggregate modelsilica particles(90 nm) and investigated the effect of aggregation conditions,including polymer dosage, particle concentration, backgroundelectrolyte concentration, and shear rate, on the flocculationmechanism. Silica particles of 90 nm were chosen because theytend to form relatively small flocs whose size and structure aresuitableforcharacterizationby the static lightscattering technique.The aim of the current research is to gain an improved

understanding of therelationship between polymer characteristics,aggregation conditions, and flocculation mechanism as well asthe aggregate characteristics such as size and structure.

2. Experimental Section

2.1. Materials. Monodisperse spherical silica particles wereobtained from Nissan Chemical America Corporation, with a BETsurface area of 30 m2 g-1, a mean particle diameter of 90 nm, andadensityof2.2gcm-3. Thesilica wassupplied as aqueoussuspensionwith a concentration of 40.5% w/w and a pH of 10. Prior to use,pH was adjusted to 5.5 at which the silica is negatively charged.Two cationic copolymers of acrylamide/diallyldimethylammonium,chloride (D6010 and D6040) and one cationic homopolymer of diallyldimethylammonium, chloride (D6099) were received as giftfrom SNF Floerger. The molecular weights of these polymers in

0.01 M NaCl solution were determined by static light scattering atpH 5.5 and 25 °C on Zetasizer Nano ZS (Malvern Instruments,U.K.) and are listed in Table 1. The hydrodynamic diameters of these polymers at different NaCl concentrations were measured bydynamic light scattering at pH 5.5 and 25 °C on Zetasizer Nano ZSandare presentedin Table2. Thesuppliedliquid solutionsof polymerswere diluted using gentle stirring for 1 h to produce solutions of 0.1% w/w polymer concentration which were then adjusted to pH5.5, the same as that of silica suspensions. Reagents used to adjustthepH of thesuspensions andsolutions were analytic grade HClandNaOH. The background electrolyte used throughout this work wasNaCl. Distilled water was used in all experiments.

2.2. Minithickener and Aggregation Conditions. Figure 1 is adrawing of the minithickener fabricated for the production of aggregates. A 35 L PVC column was flange connected with a large

(19) Mandelbrot, B. B. The Fractal Geometry of Nature; W. H. Freeman:New York, 1983.

(20) Stanley, H. E.; Ostrowsky, N. On Growth and Form: Fractal and Non-Fractal Patterns in Physics; M. Nijhoff: Dordrecht, The Netherlands, 1986.

(21) Weitz, D. A.; Huang, J. S.; Lin, M. Y.; Sung, J. Phys. ReV. Lett. 1985,54, 1416-19.

(22) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin,P. Nature 1989, 339, 360-2.

(23) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin,P. Phys. ReV. A 1990, 41, 2005-20.

(24) Schaefer, D. W.;Martin,J. E.;Wiltzius, P.;Cannell,D. S. Phys. ReV. Lett.1984, 52, 2371-4.

(25) Bushell, G. C.; Yan, Y. D.; Woodfield, D.; Raper, J.; Amal, R. Ad V.Colloid Interface Sci. 2002, 95, 1-50.

M ( R) ∝ R Df (1)

F( R) ∝ R Df -3

(2)

I (Q) ∝ Q- Df (3)

Q )π n sin(θ /2)

λ(4)

Table 1. Charge Densities and Molecular Weights of ThreeCationic Polymers Used in This Study

polymer charge density % molecular weight g/mol

D6010 10 3.0 × 105

D6040 40 1.1 × 105

D6099 100 1.2 × 105

Table 2. Hydrodynamic Diameters of Three Cationic Polymersat Different Concentrations of NaCl

hydrodynamic diameter, nm

polymer0.01 M

NaCl0.03 M

NaCl0.06 M

NaCl0.1 MNaCl

D6010 66 63 59 55D6040 55 53 50 n.a.a

D6099 68 65 62 n.a.

a n.a. ) not available.

1

R, Q ,

1

r 0(5)

6776 Langmuir, Vol. 22, No. 16, 2006 Zhou and Franks



Perspex funnel with another flange attached to the bottom. Aremovable, clearPerspex cupwas fastened with screwsand a gasketonto the bottomof this flange. Four steel baffles were inserted alongthe inner diameter of the cylindrical section. The baffles extended

3 cm into thevessel. Mixingwithin themini-thickenerwas achievedwith a Shelton mixer having a 15 cm diameter 6 bladed Rushtonimpeller in the cylindrical section, and a 7.5 cm diameter 6 bladedRushton impeller in the conical section on the same shaft. Allflocculation experiments were conducted at 25°C. A volume of 25L suspension of silica particles was flocculated at pH 5.5 in allflocculation experiments. The silica particles were allowed toflocculate for 90 s after addition of polymer and were stirredcontinuously at the same shear rate during this time. Unless statedotherwise, thesilica particle concentration used was0.16% w/wandthemixing speed waskept at 142 rpm for allflocculation experimentsundertaken.

2.3. Characterization of Aggregate Size and Structure. Smallangle static light scattering was employed to study the size andstructure of the silica particle aggregates. The instrument used was

a Mastersizer S (Malvern Instruments, U.K.), which has a 633 nmHe-Ne laser as the light source. The Mastersizer simultaneouslymeasures scattered intensities at a range of scattering angles from0 to 46ο. It also provides a direct measure of the average sizedistributionof thematerial in the scattering cell using fullMie theory.It should be noted that most aggregates induced by polymersthroughout this work can grow very rapidly and form networks in3-5 min. To minimizethe effectof these aggregatenetworks, sampleswere taken fromthe bulksuspensionsin a 250mL beakerimmediatelyafter mixingwas completed. It took about 3 minto carry thesamplesto the room with the light scattering instrument where they weresubsequentlydiluted 20 to 1 in a 2 L beakerwith dispersion mediumat the same pH and salt concentration as in the minithickener toproduce optimum obscuration. The samples were then gravity fedinto the light scattering cell from the connected 2 L beaker and

flowed through the cell in the course of sizing measurement. Thissetup can prevent aggregates from producing networks and at thesame time bring about negligible shear and disruption to their sizeand structure.

2.4. Zeta Potential Measurement. The zeta potential measure-ments were performed with a Colloidal Dynamics, Acoustosizer(Warwick, RI). A concentration of 2 vol % solids were chosen forthis investigation as it is within the range of the volume fractionswhich (1) arehigh enough to produce a signalfrom theparticles thatis large relativeto thesignal from thepolyelectrolyte and electrolyte

ions and (2) are low enough so that the particle-particle interactiondoes not significantly affect the signal. The measurements werecarried out on agitated suspensions at 22 ( 0.1 °C and pH 5.5. Theconductivity, temperature, and pH of the slurry were continuouslymonitored in situ using probes attached to the instrument.

2.5. Polymer AdsorptionExperiments. A100mLaliquotofthesupernatant above the settled flocs was removed from the mini-thickener 1 h after flocculation and then centrifuged at 3500 rpmfor 25 min to remove any residual particles. The residual polymerconcentration was determined by total organic carbon analysis forthe supernatant obtained by centrifugation. The TOC analysis wascarried out using TOC-5000 (Total organic carbon analyzer,Shimadzu). It should be pointed out that adsorption experimentswere just performed over the polymer dosage at which flocculationoccurs in this study.

3. Results and Discussion

3.1. Effect of Polymer Dosage on the Aggregate Size and

Structure. Preliminary experiments suggestedthat 10% chargedpolymers are not able to flocculate 90 nm silica particles evenat polymer doses as high as 50 mg/g silica. This is presumablydue to the fact that the segments of the low charged polymerlayerthat protrudefromthe particlesurface, evenbefore flattening,cannot overcome the distance of closest approach between twosilica particles. This distance is, at an estimated 1-1 electrolyteconcentration, about 10-5 M, well above 200 nm, since underthis condition the reciprocal double layer thickness is about 100nm. The effective thickness of the electric double layer reduces

to 1.8 nm at 0.03 M NaCl. The polymer hydrodynamic diameterlisted in Table 2 is 63 nm, much larger than ∼ 2κ-1 (3.6 nm).Therefore, 0.03M NaClwas added to decreasethe electric doublelayer distance when 10% charged polymers were used asflocculants. The optimum polymer dosages in terms of the bestsupernatant clarity for 10% at the background salt concentrationof 0.03 M NaCl, 40% and 100% charged polymers were visuallydetermined to be 12, 12 and 2 mg/g silica, respectively. Figure2 presents the volume average aggregate diameter (D [4, 3]) andsize distribution with polymer dosage for 10%, 40%, and 100%charged polymers used as flocculants, respectively. It can beseen from Figure 2a,b that the average aggregate size (diameter,D [4, 3]) increases with the increase in polymer dosage for the10% and 40% charged polymers. At the optimum dosages, the

average aggregate size is 49 and 36 microns for 10% and 40%charged polymers, respectively. Figure 2c indicates that, in thecase of 100% charged polymers, the typical aggregate diameter(D [4,3]) attains the maximum value,20 microns, at itsoptimumdosage, below and above which the average aggregate sizedecreases. It is worth noting thatthere should be some uncertaintyin the absolute aggregate size information obtained from theMastersizer due to the fact that the commercial data analysisprogram used treats any scattering object as a solid sphere ratherthan a porous object such as an aggregate. However, we areconfident that our observed trend in the average aggregate sizeas a function of the polymer dosage is reliable.

Figure 3 shows log I (Q) versus log Q with polymer dosagefor 10%, 40% and 100% charged polymers used as flocculants,

Figure 1. Drawing of the mini-thickenerfabricated for the productionof aggregates (not to scale).

Flocculation Mechanism Induced by Cationic Polymers Langmuir, Vol. 22, No. 16, 2006 6777

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respectively. For the purpose of clarity, the scattering curves of all scattering figures in this paper are shifted vertically withrespect to one another. As shown in Figure3a, forthe aggregates

induced by 10% charged polymers, the mass fractal dimensionis 2.4 at the polymer dosages of 4 and 12 mg/g silica while 2.5at the dosages of 6 and 8 mg/g silica. In the case of aggregatesinduced by 40% charged polymers, as shown in Figure 3b, theslopes reveal a mass fractal dimension of 2.6 at all polymerdosages applied. When aggregates areinduced by 100% chargedpolymers (see Figure 3c), the mass fractal dimension is 2.6 atthe highest polymer dosage, 12 mg/g silica, and 2.7 at the otherthree polymer dosages.

The most noticeable distinction observed from Figures 2 and3 is that, at the optimum dosage, aggregates produced by 10%charged polymer possess large average sizes and relatively lowmass fractal dimensions, whereas aggregates induced by 100%charged polymer attain smallaverage sizes and high mass fractal

dimensions. This seems to indicate that the two polymers induceflocculation by different mechanisms and the difference inflocculation mechanism is responsible for the difference in thesize and mass fractal dimension of flocs observed. If the sameflocculationmechanism resultedfor each polymer at its optimumdosage, the average size and mass fractal dimension of flocs

should be very similar.To better understand the flocculation mechanism operating

for each charged polymer, it isessential to studythe zeta potentialand polymer adsorption of the flocculated particles as a functionof polymer dosage. The zeta potential of silica particles withpolymer dosages is shown in Figure 4. The magnitude of thenegative zeta potential slightly decreases with increasing thedosage of 10% charged polymers (D 6010) at the backgroundsalt concentration of 0.03 M NaCl over lower polymer dosages,whereas over high polymer dosages, the negative zeta potentialnearly levels off with changes in the polymer dosage. The zetapotential does not reach the isoelectric point even at the polymerdosage much higher than the optimum dosage of 12 mg/g silica.The charge of 10% charged polymer under 0.03 M NaCl, to

Figure 2. Typical floc size distribution and volume average (D [4,3]) floc sizes of silica flocculated with (a) 10% charged polymersat [NaCl] ) 0.03 M, (b) 40% charged polymers, and (c) 100%charge polymers, respectively, at various polymer dosages.

Figure 3. Typical scattering patterns of silica flocculated with (a)10% charged polymers at [NaCl] ) 0.03 M, (b) 40% chargedpolymers, and (c) 100% charge polymers, respectively, at variouspolymer dosages.


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some extent, should be screened by salt ions. So the reductionin zeta potential by the 10% charged polymers is not simply dueto charge neutralization but also the adsorption of a layer of polymer chains at the silica particle surface causes the shear

plane to shift farther from the surface of the particle, reducingthe measured zeta potential.26 In fact, Mpofu et al. 27 found thatthe adsorption of nonionic poly(ethylene oxide) (PEO) polymergives rise to the reduction in the magnitude of the zeta potentialwhen they investigated the zeta potential of kaolinitedispersionsas a function of PEO dosage. The zeta potential varied fromnegative to positive values with increasing dosages of 40%chargedpolymers (D6040)and 100%charged polymers (D6099).The zeta potential value is more positive for silica particlesflocculated by 100% charged polymers than by 40% chargedpolymers at the same polymer dosage. The isoelectric point isattained at 4.5 mg/g silica for 100% charged polymers and at 8.2mg/g silica for40% charged polymers. This is in agreement withthe larger cationic charge density of 100% charged polymers.

The adsorbed amounts of 10% charged polymers (D6010) atthe background salt concentration of 0.03 M NaCl, 40% chargedpolymers (D6040) and 100% charged polymers (D6099) ontothe silica particles are shown in Figure 5. The adsorbed amountincreases with increasing equilibriumconcentrationsof polymersfor three charged polymers. It can be seen that for 100% chargedpolymers the adsorbed amount nearly reaches the maximumplateau value at the equilibrium concentration of 6.7 mg dm-3,which corresponds to the polymer dosage of 12 mg/g silica,whereas for 40% and 10% charged polymers, the adsorbedamounts are far from approaching the maximum plateau valuesat equilibrium concentrations of 4.9 and 4.0 mg dm-3, corre-sponding to the optimum polymer dosages of 12 and 12 mg/gsilica. The Langmuir adsorption isotherm model 28 is applied to

analyze the adsorption of the three charged polymers onto silicaand is given by

where C eq (mg dm-3) is theequilibrium concentration of polymer, x is the amount of polymer adsorbed (mg dm-3), m is the total

solid particle surface area per unit volume (m2 dm-3), K is thelangmuir adsorption constant (dm3 mg-1), and ( x / m)max is themaximum amount of polymer adsorbed per unit solid surface

area. The model assumes that the solute (polymers) and solvent(water) have equal molecular cross-sectional surface areas andthat there is no net solute-solvent interaction. Although thisassumption may not be totally valid in the case of polymeradsorption, the use of the model in the context of this studyallows a direct and consistentcomparison between threechargedpolymers. A plot of C eq /( x / m) against C eq should yield a linearrelationship if the Langmiur model is followed. The value of ( x / m)max can be determined from the slope of such a plot. A plotof C eq /( x / m) as a function of C eq for three charged polymers isshown in Figure 6, which indicates that the isotherms conformto a langmuir type of adsorption as depicted by the linearity of the plots. Values of ( x / m)max are given in Table 3. It can be seenthat( x / m)max (themaximum valueof adsorbed amount) increases

(26) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: London, 1981.

(27) Mpofu, P.; Addai-Mensah, J.; Ralston, J. Int. J. Miner. Process. 2003,71, 247-68.

(28) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford,1990.

Figure 4. Electrokinetic zetapotential of silica particlesas functionof the polymer dosages.

C eq

( x / m))

1

K (( x / m))max

+C eq

(( x / m))max (6)

Figure 5. Adsorbed amounts of three cationic polymers on silicaparticles as a fuction of the polymer dosage.

Figure6. Langmiur plotof theadsorption of three cationic polymers.

Table 3. Maximum Value of Adsorbed Amount of ThreeCationic Polymers

maximum value of theadsorbed amount (mg m-2)

polymer absolute DADMAC

D6010a 1.49 0.149D6040 0.314 0.125D6099 0.126 0.126

a At the background salt concentration of 0.03 M NaCl.







with decreasing the charge densities of polymers. However, themaximum values of adsorbed amounts expressed in terms of charged DADMAC segments are almost the same for 40% and100% charged polymers (shown in Table 3). This is consistentwith the finding of Bauer et al.,29 who observed that the largeelectrostatic attraction between the copolymers of diallyl-dimethylammonium-chloride and N -methyl- N -vinyl-acetamidewith different charge densities and the silica surface leads to thesame numberof adsorbed chargesof thecopolymers andthereforeto a corresponding increase of the adsorbed amounts withdecreasing charge density of the copolymers at pH 5.8.Interestingly,the maximum valuesof adsorbed amounts expressedin terms of charged DADMAC segments for 10% charged

polymersis higherthanthosefor40% and100% charged polymers(Table 3). This is likely attributed to screening of the charge of 10% charged polymers by 0.03 M NaCl and formation of moreloops and tails. It can be concluded that the lower the chargedensity of the polymers, the more loops and tails the polymershave, and the adsorption is dominated by the strong electrostaticattraction between the positive segment charges and the negativesilica particle surface charges.

The number of adsorbed polymer moleculesper silica particleis also calculated and given in Table 4. To better understand theconformations of polymer molecules on the silica particles, it isnecessaryto calculatethe number of polymer moleculesrequiredif a silica particle is fully covered by three charged polymerswith a flat conformation (polymer patch), respectively. The size

or conformation of the three charged polymer molecules insolution can give an indication of the patch size formed by threecharged polymerswhen theyare adsorbedon the particlesurface.Here we can establish the relationship between polymerhydrodynamic diameter and patch area as a first approximation.Assuming that the patch consists of only one adsorbed polymermolecule and is circular with a polymer hydrodynamic diameterin solution then the patch area is 3115 nm2 for 10% chargedpolymers at the background salt concentration of 0.03 M NaCl,2374 nm2 for 40% charged polymers, and 3630 nm2 for 100%charged polymers. The surface area of a sphere of 90 nm is25434 nm2. So the number of polymer molecules required tofully cover a silica particle with flat patch conformations is 8 for10% charged polymers under 0.03 M NaCl, 10 for 40% chargedpolymers, and 7 for 100% charged polymers, respectively. Incomparison with these values, it is clear that 10% chargedpolymers at the polymer dosages of 6, 8, and 12 mg/g silica,100% charged polymers at the polymer dosage of 8 and 12 mg/gsilica, and 40% charged polymers at all four polymer dosagesapplied in flocculation cannot adopt flat conformations on thesilica particle surface, since the number of adsorbed polymermolecules per silica particleis equalto or larger thanthat requiredto fully cover one silica particle with flat patch conformations.This implies that in these cases, polymer molecules can onlyadopt extended conformations on the particle surface with loops

and tails protruding away from the particle surface. However,for100% chargedpolymers,it isvery likelythat adsorbed polymermolecules adopt flat configurations at the polymer dosages of 1 and 2 mg/g silica, since the silica particle surface is far frombeing fully covered.

We suggest that the flocculation of silica particles with 10%charged polymers at the background salt concentration of 0.03M NaCl wasmost likely attributedto bridging, since therelativelylarge aggregates were formed at the optimum dosage. At low

polymer dosage, the polymer molecules adopt a relatively flatconformation on the particle surface, so few at most polymerchains can extend beyond the range of the electric double layerrepulsion. At high dosage, a more extended configuration of thepolymer chains result 30 so that bridging flocculation can resultas the polymer chains extend beyond the range of the doublelayer. Therefore, at the optimum polymer dosage of 12 mg/g,flocs with large size are formed due to the increased polymerloops and tails protrudingfrom theparticlesurfacesand extendingbeyond theinfluence of the electric double repulsion. The numberof adsorbed polymer moleculesper silicaparticleand theadsorbedpolymer amount do confirm the formation of polymer loops andtails. The zeta potential value in Figure 4 does not support thecharge neutralization mechanism at this polymer dosage. These

provide extra evidence thatflocculation is a bridgingmechanism.In comparison, below the optimum polymer dosages, small flocsare produced. The relatively smaller aggregates obtained at thepolymer dosage of 6 and 8 mg/g silica should be attributed tothe ineffective flocculation, since not many loops and tails of adsorbed polymerswere ableto extend beyond theelectric doublelayer to invoke the efficient bridging. This is supported by thesmaller number of the adsorbed polymer molecules per silicaparticle (Table 4) and the relatively lower adsorbed polymeramount (Figure 5). We suggest that the lower mass fractaldimension of aggregates at the polymer dosage of 4 mg/g silicamainly results from the strong particles’ electric double layerrepulsion due to the low particle surface coverage. The zetapotential value in Figure 4 confirms that the silica is still highly

charged. Moreover, theadsorbed polymeramount andthenumberof adsorbed polymer molecules per particle both indicate thatthe polymer surface coverage is low.

Flocculationinduced by the100% charged polymers is causedby the electrostatic patch mechanism1 at the polymer dosages of 1 and 2 mg/g silica. It can be seen from Figure 4 that the zetapotential value is -48.3 mV at the polymer dosage of 1 mg/gsilica and -36.75 mV at 2 mg/g silica. This suggests chargeneutralization is not the operating flocculation mechanism atthese two polymer dosages. The polymer adsorbed amount of 0.0233 mg m-2 at the dosage of 1 mg/g silica and 0.0273 mgm-2 at 2 mg/g silica is below 50% of the maximum value of adsorbed amount of 0.126 mg m-2 in Table 3. Moreover, asdiscussed previously, the number of adsorbedpolymermolecules

per particle at these two polymer dosages indicates that thishighly charged polymer can readily be adsorbed with a very flatconformation and form the positive polymer patch on the silicaparticlesurface, since this processis energetically most favorable.As shown previously (Figures 2c and 3c), flocculation inducedby 100% charged polymers forms aggregates with high massfractal dimension and the size of the aggregates produced at theoptimum dosage is small. The formation of flocs with high massfractal dimension by 100% charged polymers is in agreementwith the finding by Wong et al.,31 that is, the more charges that

(29) Bauer,D.; Buchhammer, H.;Fuchs, A.;Jaeger,W.; Killmann,E.; Lunkwitz,K.; Rehmet, R.; Schwarz, S. Colloids Surf. A 1999, 156 , 291-305.

(30) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf. A 1998, 139,199-211.

(31) Wong, K.; Cabane, B.; Duplessix, R. J. Colloid Interface Sci. 1988, 123,466-81.

Table 4. Number of Adsorbed Polymer Molecules Per Particleat Different Polymer Dosages for Three Cationic Polymers

number of adsorbed polymer molecules per particle

polymer 1 mg/g 2 mg/g 4 mg/g 6 mg/g 8 mg/g 12 mg/g

D6010a n.ab n.a 6 8 11 16D6040 n.a n.a 10 15 18 25D6099 2 3 n.a. n.a. 12 13

a At the background salt concentration of 0.03 M NaCl. b n.a.) notavailable.




the macromolecules carry, the shorter the distance of separationbetweenneighboring sphereswithin a floc. At thepolymerdosageof 8 and12 mg/g silica, thenumber of adsorbed polymer moleculespersilica particle (Table 4) indicatesthat polymer moleculescanonlyadoptextended conformations withloopsand tailsprotrudingaway from the particle surface. The large positive zeta potentialvaluesat thepolymer dosage of 8 and 12 mg/g silica also provideadditional supporting evidence that flocculation at these twopolymer dosages is by the bridging mechanism. The lower mass

fractal dimension and the minimum size of flocs at the polymerdosageof 12 mg/g shouldbe attributed to the high particle surfacecoverage, since the adsorbed polymer amount of 0.107 mg m-2

shown in Figure 5 nearly reaches the maximum value of theadsorbed amounts of 0.126 mg m-2.

Flocculation mechanisms induced by 40% charged polymerscould be either bridgingor a combination of charge neutralizationand bridging, depending on the polymer dosage. At higherpolymer dose such as the optimum, the adsorbed polymermolecules tendto adopt more extendedconfiguration,26 and morepolymer loops and tail can extend beyond the influence of theelectric double layer repulsion, invoking quite efficient bridgingaggregation. Thisis consistent with the experimental observationthat relatively large flocs are produced at the optimum dosage,

as shown in Figure 2b. The number of adsorbed polymermolecules per silica particle confirms the bridging mechanism,since the polymer molecules can only adopt extended conforma-tions with loops andtailsprotruding away from theparticlesurfaceat theoptimum polymer dosage. Thepositive zeta potentialvalueof 9.9 mV at the polymer dosage of 12 mg/g silica also providessupporting evidence that the flocculation mechanismis bridging.At a polymer dosage of 8 mg/g silica, charge neutralization issupposed to be the dominant flocculation mechanism, since thezeta value in Figure 4 is close to 0. However, it is worth notingthatthe number of adsorbedpolymer moleculesper silica particle(Table 4) provides evidence that the adsorbed polymers adoptextended conformations with the formation of the loops andtails. This implies that bridging should still be operating

mechanism concurrently. The number of adsorbed polymermolecules per silica particle (Table 4) at polymer dosage of 4or 6 mg/g silica indicates the adsorbed polymers should adoptextended rather than flat conformations on the particle surface.Although the particles are negatively charged (zeta potentialvalue of - 23.3 and - 9.4 mV respectively) at the polymerdosageof 4 and6 mg/g silica,it islikely that charge neutralizationwould still be the operating mechanism since this polymer ismoderately charged. So a combination of charge neutralizationand bridging should be the operating flocculation mechanism atthe polymer dosage of 4, 6, and 8 mg/g silica.

Since 40% and 100% charged polymers have approximatelythe same molecular weights (of 40% (1.1 × 105 and 1.2 × 105

g/mol, respectively), we suggest that the difference in charge

densityof these polymers should be responsible for thedifferencein the properties of flocs formed and flocculation mechanismsinduced. Compared to 40% and 100% charged polymers, 10%charged polymers possess lower charge density and highermolecular weight (3.0× 105 g/mol), both of which can facilitatebridging flocculation. However, we believe that the low chargedensityof 10% polymers playsa moreimportant role. Otherwise,we would expect that 40% charged polymers should bring aboutelectrostatic patch flocculation rather than typical bridging at itsoptimum dosage. Our suggestion is also supported by Durand-Piana et al.,32 who believed that, of all the parameters that

determine flocculation mechanisms, the most important one isthe number of charges brought to the oppositely charged particlesurface by the cationic polymers.

3.2. Effect of Particle Concentration on the Aggregate Size

and Structure. Figure 7 shows the volume average aggregatediameter (D [4, 3]) and size distribution at two different particle

concentrations for aggregates induced by 10% polymer at thebackground salt concentration of 0.03 M NaCl, 40% and 100%charged polymers under their own optimum dosage. As shownin Figure 7a,b, increasing concentrations of particles results ina dramatic increase in the average aggregate size for 10% and40% charged polymers, whereas it has little effect on aggregatesize for 100% charged polymer. Figure 8 shows log I (Q) versuslog Q with the particle concentration for aggregates induced bythree polymers under the optimum dosage conditions. It can beseen from Figure 8 that an increase in particle concentrationleadsto a decrease inmass fractal dimensionof aggregates inducedby 10% and 40% charged polymers. For aggregates induced by100% charged polymers, the mass fractal dimension does notchange with particle concentration.

(32) Durand-Piana, G.; Lafuma, F.; Audebert, R. J. Colloid Interface Sci.1987, 119, 474-80.

Figure 7. Effect of particle concentration on typical floc sizedistributions and volume average (D [4, 3]) floc sizes of silicaflocculated with (a) 10% charged polymers under the optimumpolymer dosage of 12 mg/g silica and at [NaCl] ) 0.03 M, (b) 40%charged polymers under the optimum polymer dosage of 12 mg/gsilica, and (c) 100% charge polymers under the optimum polymerdosage of 2 mg/g silica, respectively.





According to conventional aggregation theory, a higherconcentration of particles will yield larger aggregates,33-35 whichhas been confirmed by numerous experimental studies and fieldinvestigations. However, this is not always true since manyresearchers have reported that an increase in the particleconcentration could result in a lower aggregate size in somecases36-38 or have little or no impact.39 In our case, the particleconcentrations (<1% w/w) were relatively dilute, and the

flocculation kinetics, based on the Smoluchowski model,4,40

should be adopted for analysis of our results. According to theSmoluchowski model, the flocculation process is second orderwith respect to particle concentration and breakage is first order.

The model predicts that the aggregate size should be larger athigher initial particle concentration as we find for the 10% and40% charged polymers. On the other hand, we find that when100% charged polymers are used the size of flocs is constantregardless of the solid concentration.

In an aggregation process involving adsorbingpolymers,thereare two important time scales: the polymer adsorption time (t A)and the particle collision time (t C). As a first approximation, onecan treat both the polymer adsorption and subsequent particle

flocculation processes as simple bimolecular reactions. Underorthokinetic conditions as used in this study, the rate of collision, J ij, between i (silica) particles and j (polyacid) species is givenby7

where k ijcan be calculatedfrom(ignoring hydrodynamic effects)

in the above equations, ni and n j are the respective numberconcentrationsof particleand polymer in solution.Theirrespectiveradii are ai and a j. G is the mean shear rate of mixing, which can

be calculated from the power input per unit mass of fluid. It isabout 1000 s-1 in this experiment. Note that, by setting j equalto i, eqs 7 and 8 can also be used to describe the collision processbetween thesilica particles. Ashas been pointed outby Gregory,1

it is usually necessary for a substantial fraction of the addedpolymer to be adsorbed before particles are sufficiently de-stabilized for flocculation to occur. It has been shown that thetime required to adsorb a fraction f of the initial concentrationcan be calculated from7

In our work, f is estimated from the polymer adsorption

experiments, which is 0.8 for the 10% charged polymer, 0.5 forthe 40% charged polymers, and 0.8 for the 100% chargedpolymers at their respective optimum polymer dosage. As forthe particle flocculation step,the characteristiccollisiontime, t C,can be estimated using the following expression:7

where k ii is the particle flocculation rate constant. We must saythat the above-discussed approach is a very simplistic model.What actually takes place must be a lot more complex processthan this. Nevertheless, this simplified picture does seem ableto offer an explanation to our experimental results. Since the

polymer reconformation rate and the corresponding time scalet r may be expected to be roughly constant for each chargedpolymer, whereas it can be seen from eqs 7 and 8 that bothpolymer adsorption rate and particle flocculation rate increasewithincreasing the initial number concentrationsof the particles,conversely, both t C and t A decrease. The polymer adsorptiontime (t A) and the particle collision time (t C) with solidsconcentrations based on eqs 8-10 are calculated and listed inTable 5. The decreased t C and t A will in turn decrease the timeavailable for a polymer molecule to adopt a flatter conformationon the silica surface before encountering another silica particleand hence causing aggregation. A more extended polymerconformation on the particle surface can greatly facilitateflocculation if the flocculation is via the bridging mechanism,

(33) Oles, V. J. Colloid Interface Sci. 1992, 154, 351-8.(34) Eisma, D.; Li, A. Neth. J. Sea Res. 1993, 31, 107-17.(35) Berhane, I.; Sternberg, R. W.; Kineke, G. C.; Milligan, T. G.; Kranck,

K. Cont. Shelf Res. 1997, 17 , 267-85.(36) Tsai, C. H.; Iacobellis, S.; Lick, W. J. Great Lakes Res. 1987, 13, 135-

46.(37) Lick, W.; Lick, J. J. Great Lakes Res. 1988, 14, 514-23.(38) Williams, R. A. Colloid and Surface Engineering: Applications in the

Process Industries; Butterworth-Heinemann: Oxford, 1992.(39) Milligan, T. G.; Hill, P. S. J. Sea Res. 1998, 39, 227-41.(40) Vande Ven,T. G.M. Colloidal Hydrodynamics; Academic Press: London,

1989.

Figure 8. Effect of particle concentration on typical scatteringpatterns of silica flocculated with (a) 10% charged polymers undertheoptimum polymer dosage of 12 mg/g silicaand at [NaCl]) 0.03M, (b) 40% charged polymers under the optimum polymer dosageof 12 mg/g silica, and (c) 100% charge polymers under theoptimumpolymer dosage of 2 mg/g silica, respectively.

J ij ) k ijnin j (7)

k ij )4G

3(ai + a j)

3(8)

t A ) -ln(1 - f )

k ijni

(9)

t C )1

k iini

(10)





but should have a negative influence on flocculation if particlesare destabilized by electrostatic patch flocculation due to failureto form a flat polymer patch. If the above two factors areconsidered together, it is then not difficult to comprehend whythe sizes and structures of aggregates induced by 10% and 40%charged polymer become larger with an increase in the initialparticle concentration;whereas the sizeand structureof aggregatesinducedby 100%charged polymer arenotaffected by theincreasein initial particle concentration.

Ourresults canfurtherbe explainedin another way. Thehighercharged polymers more readily relax to a flat conformation onthe silica particle surface than the lower charged polymer, sincethis is energetically most favorable. So 100% charged polymers

are initially adsorbed with a very flat conformation as suggestedby Wagberg,14 and, regardless of particle concentration, thispolymer thus always induces electrostatic patchflocculation thatis not influenced by particle concentration. In contrast, in thecase of 10% and 40% charged polymers, the polymer recon-formation rate is relatively low; conversely, the correspondingtime scale t r is longer since their charge density is relatively low.However, an increase in particle concentration will decrease thepolymer adsorption time (t A) and the particle collision time (t C),as discussed previously. If both t r > t A and t r > t C, a polymermolecule will adopt a more extended conformation on the silicaparticle owing to the shorter time between particle collisions,resultingin veryefficientbridging flocculation.13,16 Such a processis not second order (as commonly acknowledged), but strongly

biased toward larger aggregates.13

3.3. Effect of Background Electrolyte Concentration on

theAggregate Size andStructure. Strong electrolytesare alwayspresent in industrial suspension. Thusit is importantto understandthe influence of background salt on flocculation induced bypolyelectrolytes. It is well-known thatthe additionof backgroundelectrolyte can influence aggregation induced by polymers. Thisis because it can (1) screen the particles’ surface charge bycounterion binding; (2)reducethe doublelayer range;(3) decreasethemeanextension of the polymer;(4) affect polymer adsorption;(5) decrease the polymer effective charge. For example, theeffective thickness of the electric double layer is about 1000 nmin salt free suspension, and it reduces to 100 nm at an estimated1-1 electrolyte concentration about 10-5 M. The effective

thicknesses of the electric double layer are only 1.8, 1.2, and 1nm at 0.03, 0.06, and 0.1 M NaCl, respectively. The variationof polymer hydrodynamic diameter with background NaCl isshown in Table 2. However, it is not clear if the addition of backgroundsalt can influence aggregation induced by a differentmechanism to the same extent.

Figure 9 shows the volume average aggregate diameter (D[4,3]) and size distribution as a function of the backgroundelectrolyte concentration for flocs induced by 10%, 40%, and100% charged polymers under the optimum dosage conditions.As can be seen from Figure 9a,b, the increase in NaClconcentration initially results in a dramatic increase in floc size,followed by the slight increase in floc size, for 10% and 40%charged polymers, whereas Figure 9c show the increase in NaCl

results in a steady reduction in aggregate size for 100% chargedpolymer. The former is consistent with the findings of Graham,who found that modest increase in ionic strength can enhanceflocculation ratesduring bridging flocculationof amorphous silicaparticles by cationic polyelectrolytes,8 whereas the latter is inagreementwiththefindingsof Gregory whoobserveda systematicreduction in the peak-flocculation rate with increasing saltconcentration for the electrostatic patch mechanism.2

Figure 10 shows log I (Q) versus log Q with the background

electrolyte concentration for aggregates induced by 10%, 40%,and 100% charged polymers under the optimum dosage condi-tions. For flocs induced by 10% and 40% charged polymers, asshown in Figure 10a,b, the mass fractal dimension decreaseswith an initial increase in NaCl concentration, and remainsrelatively constant with further increase in NaCl concentration,whereas for flocs induced by 100% charged polymers, the massfractal dimension remains constant with the increase in NaClconcentration (Figure 10c).

The dramatic increase in aggregate size and the reduction inmass fractal dimension of aggregates for 10% and 40% chargedpolymers at the optimum dosage with the initial increase inbackground NaCl concentration should be attributed to theenhanced bridgingaggregation by the compression of the electric

Table 5. Polymer Adsorption Time and Particle Collision Timeat Different Solids Concentration for Three Cationic Polymers

t A (s) t C (s)

polymer 0.16 wt % 0.8 wt % 0.16 wt % 0.8 wt %

D6010a 3.0 0.59 2.3 0.45D6040 1.5 0.30 2.3 0.45D6099 2.7 0.54 2.3 0.45

a At the background salt concentration of 0.03 M NaCl.

Figure 9. Effect of background electrolyte concentration on typicalfloc size distributions and volume average (d [4, 3]) floc sizes of silica flocculated with (a) 10% charged polymers under the optimumpolymer dosage of 12 mg/g silica and at [NaCl] ) 0.03 M, (b) 40%charged polymers under the optimum polymer dosage of 12 mg/gsilica, and (c) 100% charge polymers under the optimum polymerdosage of 2 mg/g silica, respectively.





double layer and the screening of both the particles’ surfacecharge and polymer charge. The latter can effectively reduce theelectrostatic attraction between silica and polymer, and thus thepolymers can adopt a more extended configuration with moreloops and tails protruding away from the particle surface. Morepolymer loops and tails passing through the relatively narrower

extent of the double layer can predominantly facilitate theflocculation induced by the bridging mechanism. The slightincrease in aggregate size andtherelativelyconstant floc structureof aggregates with further increase in NaCl concentration for10% and 40% charged polymers at the optimum dosage implythat the positive effect of the background salt on the bridgingflocculation is only limited to a certain NaCl concentration.However, the formation of aggregates with large size and lowmass fractal dimension with the increased background salt in thecase of 10 and 40% charged polymers suggest that the reducedpolymer molecule extension (shownin Table 2) upontheadditionof NaCl does not play an important role in the enhanced bridgingaggregation. The slight decrease in aggregate size for 100%charged polymers at the optimum dosage upon the addition of

background NaCl should mainly result from the reduction inboth themean extension(shown in Table 2) of polymer moleculeand the charges on the patch. Such reduction can lead to thepolymer patch with decreased size and charge, thereby diminish-ing the attraction between the patch and the bare area of the

oppositely charged particles whose charges are also screened bythecounterionsof thebackgroundsalt. Therefore, it is not difficultto comprehend why the addition of NaCl causes the slightreduction in aggregate size. However, the effect of the additionof NaCl on the mass fractal dimension of aggregates induced by100% charged polymers is inappreciable, because the slightchange of the mass fractal dimension with NaCl concentrationliesin the uncertainty arising fromfitting the straight linethroughthe fractal regime section of the scattering plot and thus cannotbe determined more accurately than about 0.1.

3.4. Effect of Shear Rate on the Aggregate Size and

Structure. Almost all applications of polymeric flocculants areoperated under conditions where a suspension is subjected toshear. Whenflocculation occurs by bridging or electrostatic patch

Figure10. Effect of background electrolyte concentration on typicalscatteringpatternsof silica flocculated with(a) 10% charged polymersunder theoptimum polymer dosageof 12 mg/g andat [NaCl]) 0.03M, (b) 40% charged polymers under the optimum polymer dosageof 12 mg/g silica, and (c) 100% charge polymers under theoptimumpolymer dosage of 2 mg/g silica, respectively.

Figure 11. Effectof shear rate on typical floc size distributions andvolume average (D [4, 3]) floc sizes of silica flocculated with (a)10% charged polymers under the optimum polymer dosage of 12mg/g silica and at [NaCl] ) 0.03 M, (b) 40% charged polymersunder the optimum polymer dosage of 12 mg/g silica, and (c) 100%charge polymers under the optimum polymer dosage of 2 mg/gsilica, respectively.






mechanisms, the conformation of polymer chains may beconsiderably affected by the applied shear. Figure 11 shows thevolume average aggregate diameter (D [4,3]) and size distri-bution of aggregates induced by 10% at the background saltconcentration of 0.03 M NaCl, 40% and 100% charged polymer

at the optimum dosage conditions. As illustrated in this figure,the increased shear rate results in a decrease in aggregate sizefor all three charged polymers. A higher shear rate can increasethe aggregation rate by increasing mixing and particle collision,but on the other hand, it also increases aggregate breakage to amuch greater extent.16 As a result, aggregate breakage at higherrates prevails, resulting in a drop in the aggregate size in thiswork.

Figure 12 presents log I (Q) versus log Q with the shear ratefor flocs induced by 10% charged polymers at a background saltconcentration of 0.03 M NaCl, 40%and 100% charged polymersunder the optimum dosage conditions. As shown in Figure 12a,the mass fractal dimension of flocs from 10% charged polymersinvariably increases with shear rate, whereas the mass fractal

dimensions of flocs from 40%and 100% charged polymers remainconstant, being 2.6 and 2.7, respectively.

It is expected that polymer reconformation rate at its ownoptimum dosage follows the trend: 100%> 40%> 10%, sincethe electrostatic attraction is the main driving force for polymeradsorption. From eqs 8-10, the polymer adsorption time (t A)and particle collision time (t C) can be calculated, as listed inTable 6. It should be pointed out that the shear rates of 82 and318 rpm are estimated as 430 and 3333 s-1, respectively, in hisstudy. We can see that lowering the shear rate will lead to anincrease in thepolymer adsorption time (t A) and particlecollisiontime (t C). This will in turn allow more time for a polyelectrolytemolecule toadopt a much flatter conformation on thesilica surfacebefore encountering another silica particle and hence causingaggregation. A flatter polymer conformation on the particlesurface favors aggregation induced by the charge neutralizationand electrostatic patch mechanisms. In contrast, the increase intheshearrate canaid a polymer molecule to adopt a more extendedconformationon the particlesurface before encountering anothersilicaparticle, which cangreatlyimprove theflocculation inducedby the bridging mechanism. However, it should be pointed outthat more polymer tails and loops can be permanently brokenif flocculation is via a bridging mechanism.16

In all, our results suggest that flocculation induced by 10%charged polymers at its optimum dosage should be via bridging.The slight increase in aggregate mass fractal dimension withshear rate is possibly attributable to the floc breakup and partialreflocculation after the shear force is removed. Under high shearconditions, the flocs are broken either by disruption of theattachmentpointon a particlesurface or by thescissionof covalentbonds with the bridging polymer chains, followed by recon-formation of the polymer to form a flatter configuration on theparticle surface. As a result, the denser flocs were produced athigher shear rate. Flocculationinducedby 40% charged polymersis likely to exhibit a changover from a combination of chargeneutralization and bridging mechanism at low shear rate tobridging mechanism at higher shear rate. Flocculation inducedby 100% charged polymers occur via an electrostatic patchmechanism. It is likely that 100% charged polymers are initiallyadsorbed with a very flat conformation and induce electrostatic

patch flocculation regardless of shear rate. Although the flocswhich are initially formed via the electrostatic patch mechanismcan be broken by shear, the floc structure is not influenced dueto the flat conformation of this highly charged polymer on theparticle surface.

4. Conclusions

The flocculation of 90 nm diameter silica particles by theaddition of differently charged cationic polymers results in theformation of flocs with distinctive characteristics determined bythe flocculationconditions, includingpolymer dosage, solid con-centration, background electrolyte concentration, and shear rate.

Lower charged polymers are prone to produce flocs withsmaller mass fractal dimension than the higher charged one in

Figure 12. Effect of shear rate on typical scattering patterns of silica flocculated with(a) 10% charged polymers underthe optimumpolymer dosageof 12 mg/g andat [NaCl]) 0.03 M, (b)40% chargedpolymers under the optimum polymer dosage of 12 mg/g silica, and(c) 100% charge polymers under the optimum polymer dosage of 2 mg/g silica, respectively.

Table 6. Polymer Adsorption Time and Particle Collision Timeat Different Shear Rate for Three Cationic Polymers

t A (s) t C (s)

polymer82

rpm142rpm

318rpm

82rpm

142rpm

318rpm

D6010a 6.9 3.0 0.89 5.3 2.3 0.68D6040 3.5 1.5 0.45 5.3 2.3 0.68D6099 6.2 2.7 0.81 5.3 2.3 0.68

a At background salt concentration of 0.03 M NaCl.





the range of polymer dosages used. Furthermore, lower chargedpolymers favor the production of larger flocs at the optimumdosage than the higher charged ones. This is possibly due to thefact that 10% and 100% charged polymers induce bridging andelectrostatic patch flocculation, respectively, whereas 40%charged polymers induce either a combination of chargeneutralizationand bridging or bridging, depending on thepolymerdosage and shear rate.

Bridging aggregation induced by 10% and 40% cationic

polymers at their optimal dosages can readily be affected by theparticle concentration. Increasing particle concentration resultsin the formation of larger aggregates. Electrostatic patchaggregation induced by 100%chargedpolymers is not influencedby the solids concentration.

The addition of background electrolyte aids in bridgingaggregation induced by 10% and 40% charged cationic polymersat their optimal dosages, whereas it is detrimental to electrostaticpatch aggregation induced by 100% charged polymers. The

addition of the background electrolyte leads to the productionof large aggregates for10% and40% charged polymers,whereasthe increase in background electrolyte results in the formationof smaller aggregates for 100% charged polymers.

The increase in shear rate results in a reduction of floc sizefor all three charged polymers. However, the effect of shear rateon mass fractaldimensionis greatly dependent on polymerchargedensity.

Acknowledgment. Y.Z. thanks the Australian governmentfor the award of an IPRS and the University of Newcastle,Australia, for the award of UNRS Central. Thanks are also dueto SNF for the provision of cationic polymers. The authorsacknowledge financial support from the Australian ResearchCouncil (inparticular,Discovery Grant 0209669 and the SpecialResearch Centre for Multiphase Processes).

LA060281+


flocc. mechan. by cat. pol. investigated_franks_2006

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