Reprinted from

COLLOIDSANDSURFACESAN INTERNATow. .oJRNAl

A: PHYSICOCHEMICAL ANDENGINEERING ASPECTS

Colloids and SurfacesA: Physicochemical and Engineering Aspects 162 (2000) 141-148

Aixing Fan, Nicholas J. Turro, P. Somasundaran *

Langmuir Center for Colloids and Interfaces, Co/unlbia University, NeK' York, NY 10025. USA

ELSEVIER

COLLOIDSANDSURFAC~ A

Colloids and SurfacesA: Physicochemical and Engineering Aspects 162 (2<XX» 141-148ELSEVIER

www.elsevier.nl/locate/colsurfa

Aixing Fan, Nicholas J. Turro, P. Somasundaran *

Langmuir Center for Colloids and Interfaces, Columbia University, New York, NY 10025, USA

Received 12 November 1998; accepted 1 June 1999

Abstract

Flocculation of alumina fines with combinations of the anionic fonn of a polyacrylic acid (PAA) and a highmolecular weight cationic copolymer of acrylamide and quaternary acrylate salt (Percol) was investigated. Effects ofPAA and Percol molecular weights as well as Percol charge density on the rate of flocculation were also investigated.In order to determine the role of the polymer confonnation and changes in it due to the presence of another polymer,the fluorescence label method was employed to monitor PAA conformation at the alumina I water interface underdifferent polymer addition modes, such as the sequential addition of the two polymers vs. co-addition. Importantly,polymer size distribution was found to have a marked effect depending on the polymer type: in the case of PAA, abroader distribution yielded better flocculation; no such effect was observed with Percol. Based on the results, anoverall mechanistic picture of dual polymer flocculation is proposed. ((;) 2000 Elsevier Science B. V. All rights reserved.

Keywords: Polymer floculation; Alumina fines; Polyacrylic acid

I. Introduction

Many industrial processes such as paperrnak-ing, mineral processing, water treatment, andsludge dewatering involve solid-liquid separationusing polymeric flocculants. In many of theseapplications, use of combinations of oppositelycharged polyelectrolytes under suitable conditionsenhances the flocculation [1,2). Although suchcombinations are increasingly used as flocculantsin the industry, a limited amount of mechanisticor molecular level research has been done in thepast and as a result, fundamentals of this complex

process are far from being well understood, forexample, the influence of polymer molecularweight and polymer conformation on the floccula-tion process.

It has been known for a while that a pair ofoppositely charged polymers can produce syner-gism in the flocculation of paper pulp [3,4]. Al-though most studies have been empirical, somefundamental investigations have been performedin the recent years. For example, Yu et al. [1,2]have investigated dual polymer flocculation ofalumina particles by studying not only polymeradsorption, solution viscosity and particle zetapotential, but also an important parameter, poly-mer coiling index, as determined by the fluores-cence label technique. Petzold et al. [5,6] studiedthe effect of charge ratios of the two oppositely

. Corresponding author. Tel.: + 1-212-8542926; fax: + 1-212-8548362.

E-mail address: ps24@columbia.edu (p. Somasundaran)

(1)27-7757/00{$ - see front matter (:) 2000 Elsevier Science B.V. AD rights reserved.

PII: SO927-7757(99)OO252-6

142 A. FDIf et al. / CoJ/oids and Surfaces A: Physicochem. Eng. Aspects 162 (2fXXJ) 141-148

Table IProperties of the polymers used

Pen:ol 722Perw1728~7S7

soso80

5.07.15.2

}.7291.1

a Given by the manufacturer.b Measured by a viscometer in a 300C water bath.C Calculated using I,,] = kMo with k = 3.7 X 10-2 and a = 0.66 [15).

2. Experimental

2.1. Materials

The alumina powder (Praxair SurfaceTechnologies) used was of an average size of 0.3~ and a BET surface area of approximately15 m2 g-). Polyacrylic acids of molecular weights10, 90 and lOOOk were all obtained fromPolysciences. Pyrene labeled P AA (molecularweight detennined by gel permeability chromatog-raphy to be 76k) was synthesized accordingto a procedure reported earlier [7]. Cationic co-polymers of various molecular weights andcationic charge densities (Allied Colloids) werecopolymers of acrylamide and dimethylaminoethyl acrylate, the latter being fully quater-nized with methyl chloride and therefore posi-tively charged over a wide pH range.Characteristics of the cationic polyelectrolytesare listed in Table 1. Ionic strength was keptconstant in all the experiments using 0.1 M NaCIsolution.

charged polymers and the influence of polymermolecular weight on flocculation. In all of theprevious studies, two effects have been combined:(I) the strong adsorption and anchoring effect ofa highly oppositely charged polyelectrolyte of lowmolecular weight and (2) the bridging effect of ahigh molecular weight polymer of similar chargeto achieve enhanced flocculation.

In this study, the effect of combinations of apolyacylic acid (PAA) and a high molecularweight cationic copolymer of acrylamide and qua-ternary acrylate salt (Percol) was investigated. Tounderstand the mechanism behind this dual poly-mer flocculation, effects of the molecular weightsof P AA and Percol as well as charge density effectof Percol were studied. PAA conformation at thealumina I water interface was investigated at themolecular level, by the fluorescence label methods,under different polymer addition modes, i.e. se-quential addition of the two polymers versus co-addition. Polymer size distribution was alsoinvestigated. Based on the results, a flocculationmechanism is proposed for this system.

-+rn2-rn-+ - -+rn2-I mc=oI

Na

aI-t;I,=0Y

I0f2

I+N CI-

Ob013- -

&3

Percol PAA

A. F,., ~, 01. / Colloids and Surf~.r A: Physicoclltm. Eng. A.IJ¥CI.r 162 (2(XK) 141-148 143

2.2. Flocculation tests weight are preferred for polymer bridging,whereas polymers of high charge density are pre-ferred for charge neutralization. In addition tothese two mechanisms, a patch model [11] waslater developed for systems containing hjghlycharged polymers where the distance betweencharged groups on the polymer molecule is con-siderably less than that between charged groupson the partjcle surface. A mosaic of posjtivepatches on the surface is formed and results ininitial patchmse adsorption followed by bridging.In the case of patch neutralization, both polymermolecular weight and charge densjty are of impor-tance. Compared mth single polymer flocculation,flocculation involving dual polymers has receivedvery little attention.

For each experiment, a 1 g sample of aJuminawas put in 40 mJ of 0.1 M NaC1 in a 50 mJ beakerand sonicated for 30 s at a power setting of 30Watts (Lab-Line Ultratip Labsonic System, Lab-Line Instruments). The beaker was fitted with athree-bladed 2.5 crn diameter propeIJer, 45° pitch,rotating c1ockwise with pulp flow axiaIJydownwards. The desired amount of polymer solu-tion was then added to the suspension with thepropeIJer rotating at 300 rev mm - J. One minute

after the first polymer was added, the secondone was added and stirred for an additional twominutes at the same rpm, and then after thestirring was stopped for one minute, 16 mJ of thesupernatant from the top was transferred to atubidimeter (HF Scientific) for turbidity measure-ments.

3. J. Enhanced flocculation with dual polymers

The flocculation of alumjna suspensions wjthpolyacryljc acid alone is illustrated in Fig. 1 interms of supernatant turbidity vs. time. Floccula-tion was enhanced when the concentration of IOkP AA was increased from I to 5 ppm, andthen decreased upon further addition of PAA.The flocculation is not strong because neithercharge neutralization nor bridging is significantwjth IOk PAA at pH 8. The addjtion of Percol757 at 5 ppm dosage causes some flocculation butless than that by the same amount of PAA (Fig.2), due to negligible interactions betweenPercol and alumna which are both positivelycharged. However, when both polymers wereadded to the suspension, flocculation was foundto improve markedly. In addition, it can be seenthat sequential addition of the two polymers pro-duces better flocculation than co-addition. Thsphenomenon will be further discussed later in thispaper. For the purpose of discussions, we termthe flocs induced by PAA alone 'primary flocs',and those formed wjth two polymers 'binaryflocs'.

2.3. Fluorescence measurements

Fluorescence spectra were recorded on a Pho-ton International PTI-LS 100 spectrophotometer.For solution and solid samples, the fluorescenceexperiments were conducted in a I cm square and2 rom flat quartz cell respectjvely. The relativeintensities of pyrene emission peaks at 373 om(monomer) and 475 om (excimer) obtained withexcitation at 335 om were recorded. The ratio ofintensities of excimer to monomer peaks, IJlm,was calculated and termed the coiling index,which reflects the extent of coiling of polymerchains [8,9J.

3. Results and discussion

Single polymer flocculation has been well stud-ied in the past. Polymer bridging and chargeneutralization are accepted as the two majormechanisms [lOJ. Bridging flocculation occurswhen segments of a polymer chain adsorb ondifferent particles for inducing flocculation andlink the solid particles together. In charge neutral-ization, oppositely charged polymer adsorbs onthe particle, reduces the particle charge and desta-bilizes the suspension. Polymers of high molecular

3.2. Effect of P AA molecular weight and sizedistribution

In a set of experiments, PAA was added firstand was proposed to serve as the 'anchor' for the

A. Fan et aI./ Colloids and Surfaces A: Physicochem. Eng. Aspects 162 (2fXXJ) 141-148

- 1ppm PAA

-+- 5 ppm PM

1 Oppm P AA

-+- 20ppm PM

-Q- w/o PM

0 2 ~ 4 5

Time,min.

1 8 96

Fig. Diagram illustrating flocculation of alumina fines with polyacrylic acid (PAA) (IOk PAA, pH 8, 0.1 M NaG)

in the tested molecular weight range. Possibly, awider distribution of PAA M.W. results in awider size distribution of primary flocs. The bi-nary floc formation, which probably fo))ows thepatch model, i.e. patch adsorption and bridging,is possibly a non-equilibrium process. In singlepolymer flocculation, such non-equilibrium floc-culation has been found to cause entrapment[12,13); this could also be expected to occur in thebinary floc formation.

It has to be noted that while a wider sizedistribution is preferred for the first polymeradded, no effect on flocculation was observedwhen mixtures with the second polymers of differ-ent molecular weights were tested. This effect ofpolymer size distribution may possess some indus-trial significance.

adsorption of the second polymer. The additionof P AA alone produces primary flocs, which growinto binary flocs when the second polymer isadded. It can be seen from Fig. 3 that there is aclear effect of the molecular weight of the PAAeffect: the higher the P AA molecular weight, theless Percol 757 needed for attaining the maximumflocculation. This indicates that larger P AAmolecules produce bigger primary flocs, whichrequires a smaller amount of the tethering secondpolymer to form binary flocs. It is concluded thatpolymer bridging exists in the formation of pri-mary flocs for this system.

Most commercial polymers have wide molecu-lar weight distributions. In order to study theeffect of polymer weight distribution, 10 and 90kM. W. P AA molecules were premixed at a 1: 1ratio to obtain a bimodal and broad molecularweight distribution. The flocculation responsesobtained are shown in Fig. 4. It can be seen thatwith the 1:] mixture of ]Ok and 9Ok PAA, thePerrol dosage range for good flocculation iswider. Therefore, it is proposed that a wider sizedistribution of P AA is preferred for flocculation

3.3. Effect of Percol molecular weight and chargedensity

As discussed earlier, the patch model fits singlepolymer flocculation systems containing highlycharged polymers. Similarly, the patch model can

A. Fan et 01. / Colloidr and Sllr/~" A: Physicocltem. En,. A.fpecI" 162 (2tXXJ) 141-148

be used to explain the formation of binary flocswhere a highly charged large polymer is used [14].For this system, both the dangling chains of PAAand the highly charged Perro! ensure the patchadsorption of Percol.

The effect of Percol size and charge density onthe flocculation process is shown in Fig. 5. It canbe seen that a higher charge density can reducethe polymer dosage required to achieve optimumflocculation. This reduction is approximately pro-portional to the charge density increase, whichimplies charge neutralization. A larger moleculecan produce bigger flocs and thus better floccula-tion, which suggests a bridging mechanism forflocculation. The above observations illustrate theimportance of both the polymer size and thecharge density in forming the secondary ftocs,which are in agreement with predictions madeusing the patch model.

Fig. 3. Effect of molecular weight of polyacrylic acid (PM)on the flocculation of aJumjna

3.4. Polymer conformation

Yu et al. [2) employed fluorescence to investi-gate the conformational change of anionic highmolecular weight PAA as a function of the con-centration of a lower molecular weight cationicpolymer, polydiallyl dimethyl ammonium chloride(PDADMAC). In that case, PAA served as thebridging agent and PDADMAC as the anchoringpolymer. The coiling index of the larger polymer,PAA, was found to decrease continuously with an

llXXJ

900

*XI

700

~~~500

J400300

200

100

0 2 4 S

Ti~.min9

Fig. 2. Enhanced flocculation with dual polymeR: flocculationof alwnina particles with IOk polyacrylic acid (P AA) alone,Pen:ol 757 alone, and combination of the two polymers undertwo modes of addition.

Fig. 4. Effect of polyacrylic acid (PAA) molecular weightdistribution on the flocculation of alumina

A. Fan et al.jCo/Joids and Surfaces A: Physicochem. Eng. Aspects 162 (2lXXJ) 141-148146

~~:§~f-

Fig. 6. Polyacrylk acid (PAA) confonnational change andflocculation response as a function of Percol concentration

100 effect of Percol, since the number of anchors isdecreased and the bridging chain is coiled beforeadsorption on the particles. Since polymer confor-mation does playa critical role in determining theflocculation response, additional polymer confor-mation study was conducted by comparing thecoiling indexes of P AA at the alumina I waterinterface for two different addition modes (Fig.8). The conformational change of PAA due tocomplexation with Percol 757 in the solutionphase was also measured and is shown on thesame figure for reference.

10

:oneenIraIioo of ~. ppn

Fig. 5. Effect of Percol molecular weight and charge densityon the flocculation of alumina

~i:-'6i!-

increase in the concentration of the smaller poly-mer. In other words, PAA is more stretched whenthere is more preadsorbed PDADMAC. In thepresent study, P AA serves as the anchoring poly-mer and Percol as the bridging polymer. Interest-ingly, it was found in this study that PAAgets stretched more with an increase in concentra-tion of the second polymer (Fig. 6). In both cases,the conformation of PAA is dramaticallychanged. It can therefore be concluded that com-plexation of oppositely charged polymers doestake place at the solid I water interface. As theconcentration of one polymer increases, the otherpolymer, no matter whether it plays the role ofanchoring or bridging, will become morestretched.

It was seen from Fig. 2 that different floccula-tion resulted when the polymer addition modediffered. The influence of addition mode is furtherillustrated in Fig. 7. It can be seen that the wholecurve shifted to the left with sequential addition incomparison to the co-addition mode, whichmeans less dosage is required in the former mode.Premixing of P AA and Percol will weaken boththe anchoring effect of P AA and the bridging

Fig. 7. Flocculation response under different polymer additionmodes, sequential addition VS. co-addition.

A. Fan et al./Colloids and Surfaces A: Physicochem. Eng. Aspects 162 (2000) 141-148 147

which results in increased stretching of P AA. Inthe co-addition mode, PAA is first complexed withPercol and the surface adsorption sites have onlythe residual sites on PAA to complex with. Withgreater amounts of Percol in the complex, theparticle surface has less influence so that theco-addition curve is more similar to that of com-plexation in solution and deviates markedly fromthe sequential addition curve. It is clear that theconformation and hence flocculation is affecteddrastica)]y by the environment (free in bulk orbound on the surface) in which the two polymersencounter each other.

0.5

0.4

~ 0.3><OJ

-g

'Oil:§ 0.2'0() 4. Conclusions

o.The results of the investigations reported above

may be summarized as follows.0

1000 1 10Concentration of Perrol 757. ppm

Fig. 8. Polyacrylic acid (PAA) conformational changes due toPercol addition in solution and at the alumina I water inter-face.

.. ... .\. . ..... .. ..': f . '" ... ...~.. . . .. .~lMted altmirm particles

In solution, P AA chains become coiled and thenstretched until a constant coiling extent is reached.Coiling with Percol at low dosage is attributed tocharge neutralization. With further addition of thecationic polymer, the polymer complex may be-come positively charged with resultant restretch-ing. It is interesting to note that the stretchingreaches a constant value when the Percol concen-tration is above 20 ppm. This constant value issomewhat lower than that for P AA alone, and canbe attributed to the higher charge density of Percol757 relative to PAA. As expected, the polymerconformation curve for co-addition lies betweenthose for the solution phase and the sequentialaddition mode at the solid I liquid interface.

With both addition modes, there should becompetition between the particle and Percol forP AA due to their electrostatic attraction withPAA. In the case of sequential addition, PAAadsorption on the particle surface takes place first.Percol competes with the surface adsorption sitesand becomes dominant at higher concentrations,

add PM (a~mr)

~ .~ It":+ . --- ""

,..

Primary IkJcs

-I

add Pert:Oi (tehler)

BiMryflocs

Fig. 9. Schematic representation of the dual polymer floccula.tion process.

148 A. Fan et a/.jColloids and Surfaces A: Physicochem. Eng. Aspects /62 (2fXXJ) 141-148

Acknowledgements

The authors thank Environmental ProtectionAgency (EPA-R82330l-Ol-O) and the NationalScience Foundation (Center grant # NSFjEEC-984618) for their financial support.

References

(1) Flocculation can be markedly en-hanced by choosing an appropriate pair of oppo-sitely charged polymers (Fig. 9). The polymerthat is added first adsorbs on the aluminaparticles and produces primary flocs. It alsoserves as anchors for the adsorption of thesecond polymer. In addition to charge neu-tralization, bridging contributes to theprimary floc formation for the system understudy.

(2) Larger P AA is preferred since biggerprimary flocs can be formed which will decreasethe optimum dosage of the second polymerrequired. A wider molecular distribution ofPAA M.W. can also widen the optimum dosagerange of the second polymer and is hence pre-ferred.

(3) With the second polymer, bigger flocsare formed with the larger polymer, and an in-crease in charge density reduces the optimumpolymer dosage. This is explained using a patchmodel.

(4) Increase in the concentration of one of thetwo polymers is found to make the other polymermore stretched.

(5) Sequential addition is a better mode thanco-addition, i.e. polymer complexation should oc-cur after the adsorption of the first polymer, sincethe polymer adsorption is essentially irreversible.In this mode, a maximum amount of the secondpolymer is available for complexation coupledwith bridging.

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