Colloids and Surfaces .4.: Physicochemical and Engineering Aspects. 81 (1993) 17-230927-7757/93/$06.00 @ 1993 - Elsevier Science Publishers B. V. All rights reserved.

Xiang Yu, P. Somasundaran*Langmuir Center for Colloids and InterfaceNY 10027, USA

Henry Krumb School of Mines. Columbia University. New York,

(Received 22 December 1992; accepted 14 June 1993)

Abstract

Flocculation of alumina was investigated in this study with polystyrene sulfonate and cationic polyacrylamide aspolymers, alone and in combinations with each other. When used alone, polystyrene sulfonate showed some limitedflocculation, while cationic polyacrylamide did not produce any flocculation of the alumina powder. Electrokineticsand adsorption results suggest this to be due to charge neutralization in the former case and electrostatic repulsion inthe latter case. When these two polymers were added together, flocculation, measured in terms of settling rate, wasenhanced markedly. This is attributed to the bridging by cationic polyacrylamide of alumina particles with preadsorbedpolystyrene sulfonate. Viscosity results for the polymer combinations support the governing role of the polymer-polymercomplexation. It was also found that flocculation obtained with the two polymers premixed was lower than thatobtained with the polymers introduced individually. This suggests that polymer complex formation in bulk duringmixing is detrimental to the flocculation process since the interparticle bridging capacity of the polymer is thus reduced.

Key words: Double ftocculants; Enhanced ftocculation

Introduction

Polymers are used increasingly in solid-liquidseparation processes, water purification, mineralprocessing, oil recovery and paper making [1-3].For many years, conventional flocculation pro-cesses have depended on the use of polymericflocculants. Polymers can be used alone or inassociation with other flocculant aids, such asinorganic salts, surfactants or even a second poly-mer [4-5]. Britt et al. [6] and Moore [7] found acombination of a low-molecular-weight cationicpolymer and a high-molecular-weight anionic poly-mer to produce synergism in the flocculation ofpaper pulp. For fine and ultrafine solid suspen-sions, use of double flocculant systems seems tooffer a promising route for enhanced solid-liquidseparation. Most studies on double flocculants,however, have been empirical ones. In order to

utilize more fully the effect of multipolymerschemes, it will be helpful to develop an under-standing or the mechanisms by which they act.

In this investigation, flocculation or aluminafines with combinations of polystyrene sulfonateand cationic polyacrylamide, and with variousmodes or polymer additions, was investigated. Thestability of alumina suspensions, expressed in termsof settling rate, was measured along with polymeradsorption, solution viscosity and particle zetapotential. Based on the results, the mechanismscontrolling the flocculation process are discussed.

Experimental

Materials

Linde A alumina powder used in this study waspurchased from Union Carbide Corp. and has anaverage size of 0.3 J1ffi and a BET surface area of.Corresponding author.

X. Yu and P. Somasundaran{Col/oids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 17-1J

approximately 14m2 g-l. A 10mol%, 3-5 millionaverage molecular weight cationic polyacrylamidewas obtained from American Cyanamid.Polystyrene sulfonates of molecular weights 4600and 1.2 million were purchased from PolysciencesInc. ACS certified grade hydrochloric acid (HC1)and sodium hydroxide (NaOH) solutions fromFisher Scientific Inc. were used as pH modifiers.The ionic strength was maintained constant byusing 0.03 M NaO solution in all the experiments.

The samples were 'prepared in a fashion similar tothat used in the flocculation tests and supematant~then separated from sediments by centrifugationat 3000 rev min - 1 for 15 min. Polystyrene sulfonate

concentration was determined by a Beckman DU-8 spectrophotometer at approximately 225 nm.Cationic polyacrylamide and total polymer con-centrations were measured using a Dohrmann totalorganic carbon analyzer. Blank tests with polymeror polymer mixture solutions at the same concen-trations but without alumina were carried out toensure that adsorption results were not affected by

any precipitation.

Methods

Viscosity measurementsViscosity measurements were carried out in a

Ubbelohde viscometer with a flow time of98:t 0.1 s for water at 30°C.

Results and discussion

Flocculation with single polymer

The flocculation of alumina suspensions at aboutpH 4.5, expressed in terms of settling rate, is plottedin Fig. I as a function of polymer concentration.It can be seen that the cationic polyacrylamide,when used alone, does not produce any effect onthe stability of alumina suspensions. However,polystyrene sulfonate causes measurable floccula-tion which increases with the polymer concen-tration reaching a maximum around 20 ppm withthe polystyrene sulfonate of molecular weight 4600and 50 ppm with the polystyrene sulfonate ofmolecular weight 1.2 million. Further increase inpolystyrene sulfonate concentration causes adecrease in the settling rate. Adsorption testsshowed the positively charged cationic polyacryl-amide not to adsorb on alumina particles whichare also positively charged at this pH condition,while the negatively charged polystyrene sulfonatewas shown to adsorb to different extents dependenton the molecular weights. The zeta potential resultsobtained with alumina at this pH are shown in

Flocculation testsFor each test, a 5 g sample of alumina was

stirred in 190 ml of 0.03 M NaCI in a 250 ml beakerwith a magnetic stirrer for 30 min. The suspensionwas then adjusted to pH 4.5 and further condi-tioned for 30 min. After conditioning, the beakerwas fitted with four 0.25 in wide baftle plates forbetter mixing and a 1 in diameter propeller to stirthe sample after removing the magnetic stirrer. Thedesired amount of polymer stock solution was thenadded to the suspension over 1 min, using a syringepump (Sage Instruments Model 301) while thepropeller was rotating at 600 rev min - 1. In the

case of double flocculation, the second polymerwas added in the same manner, 1 min after theaddition of the first polymer. The weight ratio ofthe two polymers was kept at 1:1 for all experi-ments. The sample was further stirred for 4 mill at300 rev min - 1 before transferring it into a 250 ml

graduated cylinder for the settling ratemeasurement.

Zeta potential measurementsThe zeta potential was measured using a Laser

Zee Meter (Pen Kern Model 501). After the floccu-lation test was completed, flocs were ultrasonicatedfor 30 s to break them into small particles anddiluted with supernatant in order to obtain asuitable dilute sample.

Polymer adsorption testsThe depletion method was used to determine

the amount of polymer adsorbed onto alumina.

X. Yu and P. SomasundaranfColloids Surfaces A: Physicochem. Eng. Aspects 81 (1991) 17.;.»

Fig. 2 as a function of the polymer concentrationIt is seen that the addition of polystyrene sulfonatecauses a sharp decrease in the zeta potential ofalumina while cationic polyacrylamide does nolhave any effect. It is clear that, in single polymersystems, the observed flocculation behavior can beattributed to the adsorption of the oppositely

charged polystyrene sulfonate, resulting in thecharge neutralization of alumina particles. Thedecrease in flocculation at higher concentrationsof the polystyrene sulfonate is the result of reversalof surface charge and surface saturation of thealumina particles by the adsorbed polymer. It isalso seen that polystyrene sulfonates of twodifferent molecular weights have very similar effectson the zeta potential of alumina. The observeddifference between flocculation with polystyrenesulfonate of molecular weights 4600 and 1.2 millionis attributed to enhanced bridging of the particlesby the longer polymer leading to better flocculation

response.

[Toto. Polymer], ppm

Fig. I. Effect of single polymer concentration on flocculationof alumina; '\7, 1.2 million mol. wt. polystyrene sulfonate;O. 4600 mol. wt. polystyrene sulfonate; 0, cationicpolyacrylamide.

Flocculation with double flocculants

The effect of two polymers as flocculants foralumina is compared in Fig. 3 with that of thesingle polymers. It can be seen that the maximumsettling rate increases from 1.8 mm s -1 with20 ppm of polystyrene sulfonate (mol. wt. 4600)alone to 8 mm s - 1 if the polystyrene sulfonate is

followed by 50 ppm cationic polyacrylamide. Thecationic polyacrylamide did not produce any floc-culation by itself. Zeta potential results (Fig. 4)show that, even though the addition of the twopolymers caused a decrease in the zeta potentialof alumina, the magnitude of the zeta potentialdecrease is less than that caused by polystyrenesulfonate alone. It is to be noted that excellentflocculation occurs even when alumina particlescarry significant positive charge (zeta potential> + 30 m V). This suggests that the increased floc-culation obtained with the above two flocculantsis not controlled by electrostatic forces. Theadsorption isotherms of polystyrene sulfonate andcationic polyacrylamide as well as their mixture

>E

::§

~.,

"0C-

o

0;N

Fig. 2. Effect of single polymer concentration on zeta potentialof alumina; 'i7. 1.2 million mol. wt. polystyrene sulfonate;O. 4600 mol. wt. polystyrene sulfonate; 0, cationicpolyacrylamide.

20 X. ¥u and P. Somasundaran{Col/oids Surfaces A: Physicochem. Eng. Aspects 81 (1993) 17-11

on alumina are shown in Fig. 5. It can be seen thatcationic polyacrylamide, when used alone, doesnot adsorb on the alumina surface due to electro-static repulsion. When the two polymers were usedtogether, irrespective of the mode of polymer addi-tion, there was no residual polymer left in solutionat any of the polymer dosages tested. Thus bothpolystyrene sulfonate and cationic polyacrylamideadsorb completely on alumina when they are usedin combination with each other. The coadsorptionof polystyrene sulfonate and cationic polyacryl-amide is attributed to the formation of complexesbetween the two polymers at the solid-liquid inter-face. As discussed in the previous section, theanionic polystyrene sulfonate can be expected toadsorb on the alumina surface due to electrostaticattraction. The anionic polymer molecules thusadsorbed can be expected to act as anionic"anchors" for the adsorption of the long chaincationic polymer and to cause good interparticlebridging and thus superior flocculation.

Fig. 3. Comparison of ftocculation response of alumina withdouble ftocculanls to that with single ftocculants; l;,. polystyrenesulfonate followed by cationic polya('Tylamide; 0, cationicpolyacrylamide followed by polystyrene sulfonate; 'J, premix-ture of polystyrene sulfonate and cationic polyacrylamide;O. 4600 mol. wt. polystyrene sulfonate alone; O. cationicpolyacrylamide alone.

10.40

;JO>E

"0.,c.,'0a.00;N

"e"-0-~

j;.°0;Cto

ac0~'0..~

;to

1~

0

-10

~-~

-.100 ~ 40 60

[Toto! polymer]. ppm

80 ..{) 50: 100 1'50 200 250 .

Fig. 4. Effect of individual polymers and their combination onzeta potential of alumina; 6, polystyrene sulfonate followed bycationic polyacrylamide; 0, cationic polyacrylamide followedby polystyrene sulfonate; 'i7 , premixture of polystyrene sulfonateand cationic polyacrylamide; O. 4600 mol. wL polystyrenesulfonate alone; O. cationic polyacrylamide alone.

Residual PSS Canc ppm

Fig. 5. Adsorption isotherms of individual polymers and theircombination on the alumina surface; D, premixture of polysty-rene sulfonate and cationic polyacrylamide; \7. 1.2 million mol.wt. polystyrene sulfonate; <>. 4600 mol. wt. polystyrene sulfo-nate; O. cationic polyacrylamide.

21X. Yu and P. Somasundaran/Colloids Surf«D A: PllysIcoc~ E,.,. ASp«:tS 81 (1993) 17-23

8Effect of order of polymer addition " .,. ",:.~~, i:c!r' ~.' ...~~.. ..

Since the fonnation of polymer complex is pro-posed to be the mechanism for the flocculation.further studies were conducted to investigate theeffect of the order of polymer addition on theflocculation behavior. Polymer addition sequencestested were polystyrene sulfonate followed by cat-ionic polyacrylamide, cationic polyacrylamide fol-lowed by polystyrene sulfonate and both addedsimultaneously. The results obtained are shown inFig. 3. It can be seen that when the two polymersare added in premixed fonn, the flocculation is notas good as that obtained when the polymers wereadded one after another, even though it is stillsignificantly higher than that obtained with onlyone polymer. This indicates that the fonnation ofpolymer complexes in bulk solution is detrimentalto the flocculation process since the number ofpolystyrene sulfonate molecules available for fonn-ing "anchors" on the alumina surface is apparentlyreduced by interactions with cationic polyacryl-amide in the bulk. Any such reduction will indeedlead to a reduction in interparticle bridging.

It is interesting to note that, when the cationicpolyacrylamide is introduced prior to the polysty-rene sulfonate, the flocculation behavior obtainedis similar to that observed with the polystyrenesulfonate-i;ationic polyacrylamide sequence. Whencationic polyacrylamide is added first, since it doesnot adsorb on alumina by itself, the flocculationbehavior could be expected to be closely similar tothat obtained with the polymers in the premixedmode. Evidently polystyrene sulfonate can diffuseand then adsorb on the alumina surface morerapidly, due to its low molecular weight, at thehigh solid concentrations used. Thus polystyrenesulfonate might not be able to interact significantlywith cationic polyacrylamide in bulk solution.

e ~-,,5 A

:1i0 4~0-CE 3

'"

~

010 20 40 60 80 100

[Totol Polymer]. ppm

Fig. 6. Eft'ect of polystyrene sulfonate molecular weight on theOoa:ulation of alumina with combinations or polystyrene sulfo-nate and cationic polyacrylamide; O. 1.2 million moL wt.polystyrene sulfonate and cationic polyacrylamide; t.. 4&X>moL wt. polystyrene sulfonate and cationic polyacrylamide;yo. 1.2 million mol. wt. polystyrene sulfonate; 0, 4&X> moL wt.polystyrene sulfonate; O. cationic polyacrylamide.

combination with cationic polyacrylamide, theflocculation behavior is the same as that obtainedwith polystyrene sulfonate of molecular weight4600 in combination with the cationic polyacryl-amide, even though there is a marked difference in8occulation obtained with the two polystyrenesulfonates when used alone. It is suggested that insingle polymer systems, the molecular weight hasan effect since the 8occulation is the result ofbridging via the polystyrene sulfonate chain, inaddition to charge neutralization. When polysty-rene sulfonate is used along with cationic polyacryl-amide, however, the bridging of polystyrenesulfonate coated particles takes place via the cat-ionic polyacrylamide with a stretched chain lengthof ~ 17 000 om which is much longer than that ofpolystyrene sulfonate (~7 nm for polystyrene sulfo-nate of molecular weight 4600 and ~ 2000 nm forpolystyrene sulfonate of molecular weight 1.2million).

Effect of polystyrene sulfonate molecular weight

As shown in Fig. 6, when the polystyrene sulfo-nate of molecular weight 1.2 million was used in

22 x. yu and P. SDmaSWldaranlCol1oids SurfDCes A: PhysicocllelJl. £"8. ~ 81 (1993) 17-.lJ

Table 1Specific viICosities of polymer mixtures

Polymerconcn.(ppm)

4600 mol. wtpolystyrenesulfonate

Cationic

polyacrylamide

so100

0.05310.0735

0.02360.0455

0.11360.1974

Polymer-polymer interaction in solution sulfonatecombinations.

and polyacrylamidecationic

Polymer-polymer complexation, proposed asthe mechanism for the flocculation of alumina withdouble ftocculants, was tested by measuring viscos-ity changes of the polymer solutions. Resultsobtained for specific viscosities of the polymersalone and their mixtures are shown in Table I. Itcan be seen that the specific viscosity of the100 ppm polystyrene sulfonate and cationic poly-acrylamide mixture (0.21) is higher than that ofeither the 4600 mol. wt. (0.046) polystyrene sulfo-nate or the cationic polyacrylamide (0.074), indicat-ing formation of a complex of higher molecularweight. However, the addition of cationic poly-acrylamide decreases the viscosity of the 1.2 millionmol. wt. polystyrene sulfonate from 0.287 to 0.2,suggesting the formation of a more compact poly-styrene sulfonate-cationic polyacrylamide com-plex. While the polystyrene sulfonate molecule isstretched due to electrostatic repulsion betweenthe negatively charged sulfonate groups, the poly-styrene sulfonate-cationic polyacrylamide complexcan be expected to be coiled on account of theneutralization of negative sites on polystyrene sul-fonate by the positive sites of the cationic polyacryl-amide. The viscosity results show that the natureof the polymer complexes is similar in spite of thedifferences in polystyrene sulfonate molecularweights, since the dimension of the complex willbe determined essentially by the length of thelonger cationic polyacrylamide molecules to eachof which several polystyrene sulfonate moleculesmay possibly be attached. It is to be noted thatthe above viscosity results correlate well with theflocculation results obtained with the polystyrene

Summary

Use or two polymers simultaneously is shownto enhan~ the flocculation or alumina suspensionssignificantly. While in single polymer systems, theflocculation is mainly due to charge neutralizationor the positively charged alumina surface by nega-tively charged polymer, enhanced flocculation is

obtained with double tlocculants due to poly-mer-polymer interaction which results in excellentinterparticle bridging. The mode or polymer addi-tion also has a measurable effect on the ftoccula-tion. Viscosity results support the role of theproposed polymer-polymer complexation in tloc-culation. The results obtained also suggest that thebest flocculation is achieved when these two poly-mers are allowed to interact first at the solid-liquidinterface then in the bulk.

Acknowledgments

Support from the National Science Foundation,Nalco Chemical Company and EngelhardCorporation is gratefully acknowledged.

ReferelK:e5

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X. Yu and P. SomasundaranlColloids Surfaces A: Physicochem. Eng. Aspects ai (1993) 17-11 n

3 J. Montgomery, Precipitation, coagulation and flocculation,in Water Treatment Principles and Design, John Wiley andSons, New York., 1985, pp. 116-134.

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