Foaming-Antifoaming in Boiling Suspensions†

Darsh Wasan,* Alex Nikolov, and Anal Shah

Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 10 West 33rd Street,Chicago, Illinois 60616

Particle-stabilized aqueous foams are encountered in radioactive waste treatment and im-mobilization processes and in food, chemical, and agricultural products. The cause of foaminessin the presence of finely divided solids during boiling and in the absence of any surface-activeagents is not well understood. Our research has identified at least two kinds of particles insuch foaming systems, hydrophilic (i.e., water wet) colloidal particles dispersed in the aqueousphase and biphilic particles (partially wetted by water). The biphilic particles are attached tothe air-water surface. In this study we used an advanced optical technique to characterize andmonitor the number of nonattached (i.e., hydrophilic) and attached (i.e., biphilic) particles atthe gas-liquid surface. The results clearly show that foaming increases with an increase ineach of the two types of particles but to a different degree. The presence of biphilic particlescauses a significantly higher degree of foaminess than the hydrophilic colloidal particles.

Introduction

Particle-stabilized aqueous foams are encountered inthe processing of solid waste (e.g., during boiling), food,chemical, and agricultural products, froth flotation, andradioactive waste treatment and immobilization pro-cesses.1 Uncontrollable foaming can severely impact theproduction rate and ultimately the cost-effectiveness ofa chemical process. The solid particles in boiling sus-pensions, in the absence of any surfactants, stabilize thefoam lamella and enhance the foaminess. Previously,1we identified at least two types of particles in suchthree-phase foaming systems: hydrophilic colloidalparticles dispersed in the aqueous phase and biphilicparticles (i.e., with some area of the particle wetted bywater and the other part is not). These biphilic (oramphiphilic) particles attach to the surfaces of the foamlamella, provide a steric barrier against the coalescenceof bubbles, and thereby enhance foam lamella stabilityand foaminess.

A foam lamella is formed during the generation andinteraction of bubbles during the boiling of aqueoussuspensions. Hydrophilic colloidal particles get trappedinside the lamella. Subsequently, due to the confinedboundaries of the film (lamella), these particles form alayered (i.e., stratified) structure inside the foam lamel-la.2,3 Monte Carlo simulations of the film containingparticles show that the concentration of the colloidalparticles is higher in the film/lamella than that in thebulk.4 Furthermore, our theoretical calculations showthat, at a higher particle concentration, a better particlein-layer structure develops that increases the energystabilization barrier, inhibiting particle diffusion fromthe film to the bulk meniscus.3,5,6 The repulsive struc-

ture barrier (i.e., the structural disjoining pressure)arising due to the colloid particle in-layer structureformation offers a novel stabilization mechanism formacrodispersions such as foams and emulsions. In fact,we have produced aqueous foams in surfactant-freeparticle suspensions using nanosized silica particles.2,7

Our thin film experiments have clearly shown thatthere exists a critical lamella size below which at leastone layer of particles always stays in the film. Thiscritical lamella size is dependent upon particle size andconcentration. The critical lamella size seems to increasealmost exponentially with particle concentration.5 Ourobservations show the phenomenon of lamella stratifi-cation (i.e., layering) is very much dependent on lamella(or bubble) size.5,8 The classical concept of foam lamellastability is based on the disjoining pressure isotherm(a thermodynamic quantity which is independent oflamella size). Therefore, the disjoining pressure iso-therm is unable to predict the stability of the foamlamella stabilized by colloidal particles, the stability ofwhich depends on both particle concentration and itssize.

An important factor that affects the colloidal particlestructuring and layering phenomena in confined films,and thereby the film stability, is the polydispersity inparticle size. Polydispersity has a significant effect onthe structural disjoining pressure. Studies have shownthat a 30% polydispersity in particle size can decreasethe energy structural barrier by a factor of 3, while theeffect on the depletion well is smaller.3,4,9 This suggeststhat a simple way to destabilize a stable foam is toincrease the polydispersity of the suspension or simplyadd a small amount (e.g., 1 vol %) of large particles. Thelarge particles trapped inside the foam lamella weakenthe structure of colloidal particles, decrease the struc-tural barrier, and destabilize the foam lamella, reducingthe foaminess and foam stability.3,9

The first part of our paper presents results of foamingtests during the boiling of simulated nonradioactivewaste containing both hydrophilic and biphilic particles.We used an advanced optical technique to monitor theseparticles.

The second part of our paper describes using thisknowledge to develop a new antifoam to suppress severe

† During the past 8 years, our research group at IIT hascollaborated with the U.S. Department of Energy SavannahRiver Technology Center to obtain a fundamental understand-ing of the physicochemical cause of foaming and have usedthis knowledge to develop novel antifoaming agents that areeffective in the harsh environment of high-level radioactivewaste processing. The results of this effort are summarizedin this paper.

* To whom correspondence should be addressed. Tel.: (312)567-3001. Fax: (312) 567-3003. E-mail: wasan@iit.edu.

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10.1021/ie0306776 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 03/30/2004

foaming in the radioactive waste vitrification processfor immobilization of nuclear wastes at the DefenseWaste Processing Facility at the Savannah River Sitein Aiken, SC.10

Foam Generation

The foam was generated by boiling the simulatedwaste and collecting the foam using the experimentalapparatus shown in Figure 1. The original setup asdescribed in ref 1 was modified to collect particlescarried by the foam lamellae. A syringe with an innerdiameter of 5 mm attached to a vacuum chamber wasused to imbibe the foam lamellae containing particles.The foaminess was quantified by measuring the differ-ence in volume between the aqueous particle suspensionduring boiling (i.e., when it is foaming) and the suspen-sion volume without boiling (when it is not foaming);the foam collapsed when boiling ceased at the sameparticle concentration.

We used an optical technique for quantitativelycharacterizing the number of attached (biphilic) andnonattached (hydrophilic) particles at the gas-liquidsurface (Figure 2). A modified glass syringe (3.0 mminner diameter) with a threaded screw piston was usedto hold the suspension and to control the curvature ofthe meniscus at the air-aqueous surface. The syringe

was mounted vertically on the horizontal plate of theinverted microscope. The microscope was tuned to workin both reflected and transmitted light modes. Withtime, the particles in the suspension settled to thebottom part of the syringe.

The bottom of the syringe is open to the air and endswith an aqueous meniscus. The curvature of the me-niscus is controlled by the position of the threaded screwpiston and is adjusted to make the meniscus flat. Whenthe particles settle, they slowly approach the air-aqueous surface at the bottom part of the syringe. Thebiphilic particles approach the air-aqueous surface andinteract with the surface via the aqueous film (whichruptures), and the particles attach themselves to thesurface, forming a three-phase contact angle.

The number of particles attached (i.e., biphilic) to thegas-liquid surface is counted using reflected light. Withthe passage of time, the hydrophilic particles arrive andsettle at the air-liquid surface. Due to the hydrophilicnature of these particles, they do not attach to the air-layer surface, and they are not visible in reflected lightusing a low-aperture objective. However, in the trans-mitted light they are visible, as are the attachedparticles. Therefore, with this optical arrangement,numbers of both the biphilic and hydrophilic particlesare counted.

This unique method permits us to calculate thebiphilic and the hydrophilic particles. Mingins andScheludko11 used a similar optical arrangement to studythe attachment of small particles to a pendant drop andto estimate the contact angle and ratio of attached tounattached particles.

Our previous publication1 dealt with studying foam-ing during the boiling of the simulated nonradioactiveaqueous waste suspension. The simulant (i.e., the modelsuspension which mimicked the actual radioactivewaste) was prepared by the Savannah River TechnologyCenter. The waste suspension contained 7-20 wt %metal oxides and hydroxides of aluminum, iron, man-ganese, and mercury at a pH of 5-6 (see Table 1 in ref

Figure 1. Experimental setup to separate the biphilic particlesfrom the sludge under foaming conditions.

Figure 2. Experimental setup to characterize the nature of particles.

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1). The insoluble particles are irregular in shape andare polydisperse with particle sizes varying from 5 nmto 10 µm as estimated by the thin film interferometryand microscopic studies. Furthermore, these particleshave nonuniform surface energy. That is to say that apart of the particle surface is hydrophilic and the otherpart is hydrophobic. The particles with such nonuniformsurface energy are biphilic in behavior. These particlesare metal oxides and hydroxides, and these biphilicparticles attach to the air-aqueous surface. We usedour capillary force balance method to characterize therole of particle-particle interactions in foam lamellastability.1 These biphilic particles form a networkstructure due to the attractive capillary force andprovide a steric-type stabilization for the foam lamella.The suspension also contained colloidal particles, whichare hydrophilic with a radius of above 400 nm andeffective volume of about 20 vol %. These particlesstabilize foam lamellae as large as 200 µm, possibly dueto the formation of a layered structure inside the foamlamella.

We conducted boiling experiments using the samenonradioactive waste simulant containing both types ofparticles. To evaluate the presence of any surface activeagents in the suspension, the classic Barcht tests wereconducted at room temperature, and these tests showedno foaminess and foam stability. The initial solidconcentration in the sludge was 8.8 wt %. During theboiling of the sludge, the solid concentration increases(due to the evaporation of water), leading to an increasein foaminess (Figure 3). The particles carried by thefoam lamellae were removed using a capillary connectedto a chamber with partial vacuum and collected at themaximum foaminess of 475 vol % when the total particleconcentration in the suspension was 18.8 wt %. Thecollected particles in the suspension were then charac-terized by counting the number of attached (i.e., biphilic)particles and the total number of particles using theoptical method described above. The maximum foami-ness of the foam lamellae contained 16 wt % of biphilicparticles and 2.65 wt % of hydrophilic colloidal particles.The suspension was diluted back to the initial solidsconcentration of 8.8 wt % by adding distilled water atthe end of run 1, and run 2 was performed. Foaminesswas monitored during the boiling of the suspension.Results are shown in Figure 3. A reduction in foaminessfrom 475 to 375 vol % was observed at a total solidsconcentration of 18.65 wt %. This reduction in foaminessis attributed to the partial removal of biphilic particles

in run 2 since the biphilic particle concentration wasdetermined to be only 13.3 wt % at the end of this run.

This procedure was repeated, and the results of runs3-5 are also shown in Figure 3. The stepwise decreasein foaminess from 475 vol % (run 1) to 375 vol % (run2) to 320 vol % (run 3) to 260 vol % (run 4) was observed,which corresponds to the successive removal of particlesfrom the foam lamella for each run, respectively. Thedata for run 5 show that foaminess remained virtuallyunchanged even though it contained fewer biphilicparticles than run 4. These experimental results revealthat foaminess is caused by two types of particles,amphiphilic (biphilic) and hydrophilic, both of which arepresent in our simulated waste. Run 5 has the largestnumber of nonattached (i.e., hydrophilic) particles,which produces about 260 vol % foaminess.

Table 1 shows the number and concentration of bi-philic (i.e., attached) and hydrophilic (i.e., nonattached)particles and the degree of foaminess. It is evident thatthe foaminess decreases from 475 to 260 vol % as theratio of biphilic to hydrophilic particles decreases from3.3 to 0.06 (corresponding to runs 1 and 4, respectively).It should be noted that not all of the removed particleswere biphilic. Table 1 lists the number and concentra-tion of biphilic particles that were carried by the foamlamellae.

Figure 4 shows a three-dimensional surface plotdepicting the dependence of foaminess on the concentra-tion of both biphilic and hydrophilic particles. Curve Adepicts results of the experiment when the simulatedsludge contained biphilic particles only, and curve Bdepicts the foaming results when only hydrophilicparticles were present. The results of runs 1-4 are alsomarked on this figure. The figure shows that foaminessincreases with an increase in the concentration of thetwo types of particles. However, hydrophilic particlesproduce a maximum of about 260 vol % of foaminess(i.e., the amount of gas incorporated into the system),whereas biphilic particles resulted in a significantlyhigher degree of foaminess (i.e., 900 vol %) over thesame range of particle concentrations. The photographsof the attached (or biphilic) particles for different runsare also shown in this figure.

The major finding from our experimental observationsis that the particle hydrophilicity/biphilicity and particlesize determine the degree of foaminess in aqueous foamscontaining fine particles. Results show that foaminessincreases with an increase in concentration of each ofthe two types of particles. However, particles of inter-mediate hydrophobicity caused a greater degree offoaminess over the same range of particle concentration.The challenge is how to reduce the foaminess.

Development and Testing of AntifoamingAgents

Our research addresses two different but interrelatedstudies. The first deals with the fundamental under-

Figure 3. Effect of particle ratio biphilic/hydrophilic on thefoaminess: run 1, original sludge, particle ratio 3.3; run 2, particleratio 2.4; run 3, particle ratio 0.9; run 4, particle ratio 0.06; run 5,particle ratio 0.06.

Table 1. Number and Concentration of Attached andNonattached Particles at Maximum Foaminess

biphilic particles hydrophilic particles

runno. no.

concn(wt %) no.

concn(wt %)

vol % maxfoaminess

1 712 14.5 216 4.3 4752 663 13.3 276 5.5 3753 453 8.96 503 9.84 3204 63 1.05 1065 17.73 260

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standing of the causes of foaming and stability of thefoam as highlighted in the preceding section. The secondis the practical need to find an effective antifoam ordefoaming agent to minimize the production of foamduring processing. Antifoam and defoaming agents havebeen developed to help industries such as nuclear waste,paper, pulp, coatings, ink, and printing. The commercialantifoams were developed to eliminate foaminess in thepresence of surfactants, and these antifoams are notexpected to be effective when foam is stabilized byparticles in the absence of any surfactant. Mechanismsof stabilization of foams in the presence of particles andby surfactants are quite different. Moreover, the com-mercial antifoams are unstable in the harsh environ-ment in radioactive waste processing that includesstrong caustic solutions or strong acid solutions, hightemperature, radiation, reducing and oxidizing agents,etc.

There are a number of processing operations thatcould operate more efficiently and reliably if moreefficient antifoams could be found or developed. Threeareas of concern at the DOE’s Savannah River Site(SRS) are the development of a better antifoam for thehigh-level waste evaporators, development of an effec-tive antifoam in the tetraphenylborate processing, andthe development of an antifoam for the Defense WasteProcessing Facility. The Hanford River Protection projectalso needs an efficient antifoam for the high-level andlow-activity waste evaporators.

In our recent study on foams stabilized by thehydrophilic particles,2 we observed that when a smallquantity (2 wt %) of larger hydrophilic particles (100nm) was added to a suspension containing 8 vol % of 8nm microdispersed hydrophilic particle suspension, thefoam lamella ruptured and the foam was unstable. The

presence of a few large particles inside the foam leadsto the local destabilization of the ordered particlestructure, resulting in lamella rupture even at a highthickness. We explored this concept of using polydis-persity in particle size to reduce foam lamella stabilityto develop a new type of proprietary antifoam. Resultsof the experiments are shown in Figure 5. The foaminesswas reduced from 425% to 275% (a 35% reduction).

The foam stability in the presence of particles isgoverned by two mechanisms: colloidal particle layerformation (i.e., structural barrier); steric stabilizationby biphilic particles. We added 100 ppm of a proprietarywetting agent to modify the biphilic particle surface tobecome more hydrophilic. We observed a dramaticreduction in foaminess (Figure 5).

Figure 4. Effect of biphilic and colloidal particles on foaminess.

Figure 5. Effect of wetting agent and polydispersity on foaminess.

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On the basis of the mechanistic understanding offoam generation and stability, we developed an im-proved antifoam agent (IIT 747), since the commercialantifoam agent (Dow Corning 544) was found to beineffective in the aggressive physical and chemicalenvironment in the Defense Waste Processing Facility(DWPF) sludge receipt and adjustment process asindicated by the foaminess data presented in Figure 6.The addition of the IIT antifoam was found to be moreeffective in minimizing foam and was more effective overtime than Dow Corning 544. The improved antifoamagent, IIT 747, was subsequently tested in a pilot plantat the Savannah River Site and with real waste in theirshielded cells. The antifoam developed by us is nowbeing used in the Defense Waste Processing Facility.

Concluding Remarks

The research conducted by us on both the foaminessand foam stability and antifoaming action for theradioactive waste separation process in boiling suspen-sions has clearly shown that particles alone (withoutany surfactants) can cause severe foaming. Our researchfindings have major implications for treating actualsludges such as radioactive wastes containing crystal-line materials that have a nonuniform surface energy.These particles are neither completely hydrophilic norhydrophobic but are biphilic particles.

Earlier, we identified two major mechanisms offoaminess and stabilization of foams containing solidparticles, namely (i) steric stabilization due to theattachment of biphilic particles to the gas/liquid surfaceand (ii) structural stabilization due to the hydrophiliccolloidal particles forming a layered structure inside thefoam lamella. Other mechanisms for the formation andstability of foams have been discussed in the litera-ture.12-14 However, a recent review article by Binks15

points out the scarcity of pertinent literature on theability of fine particles to cause foaminess or foamstability, especially in the absence of any other surface-active material.

Our continuing study is aimed at systematicallyinvestigating the influence of the wettability of solidparticles by increasing the hydrophobicity/biphilicity as

determined using differential interferometry on foami-ness and foam stability. The effects of particle size andshape, as well as the pH of the aqueous solution andelectrolyte concentration, are being investigated. Re-search to understand the basic mechanisms controllingboth the foaminess, foam stability, and antifoamingaction in gas-liquid-solids systems continues to be ahigh priority need for the U.S. Department of Energy’sEnvironmental Management Science Program.

Acknowledgment

This research was supported by the EnvironmentalManagement Science Program of the Office of Environ-mental Management, U.S. Department of Energy, GrantDE-FG07-01ER14828.

Literature Cited

(1) Bindal, S. K.; Nikolov, A. D.; Wasan, D. T.; Lambert, D. P.;Koopman, D. C. Foaming in Simulated Radioactive Waste. Envi-ron. Sci. Technol. 2001, 35, 3941.

(2) Bindal, S. K.; Sethumadhavan, G.; Nikolov, A. D.; Wasan,D. T. Foaming Mechanisms in Surfactant Free Particle Suspen-sions. Am. Inst. Chem. Eng. 1987, 48 (10), 2307.

(3) Nikolov, A. D.; Wasan, D. T. Dispersion Stability Due toStructural Contributions to the Particle Interaction as Probed byThin Liquid Film Dynamics. Langmuir 1992, 8, 2985.

(4) Wasan, D. T.; Nikolov, A. D.; Trokhymchuk, A.; Henderson,D. Confinement-Induced Structural Forces in Colloidal Systems.In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed.;Marcel Dekker: New York, 2002.

(5) Sethumadhavan, G.; Nikolov, A. D.; Wasan, D. T. Stabilityof Liquid Films Containing Monodispersed Colloidal Particles. J.Colloid Interface Sci. 2001, 240, 105.

(6) Sethumadhavan, G.; Nikolov, A. D.; Wasan, D. T. FilmStratification in the Presence of Colloidal Particles. Langmuir2002, 17, 2059.

(7) Wasan, D. T.; Nikolov, A. D.; Henderson, D. New Vistas inDispersion Science and Engineering. Am. Inst. Chem. Eng. 2003,49 (3), 550.

(8) Nikolov A. D.; Wasan. D. T. Effects of Film Size and MicellarPolydispersity on Film Stratification. Colloid Surf., A 1997, 128,243.

(9) Sethumadhavan, G.; Bindal, S.; Nikolov, A.; Wasan, D. T.Stability of Thin Liquid Films Containing Polydisperse Particles.Colloid Surf. 2002, 204, 51.

(10) Koopman, D. C. Comparison of Dow Corning 544 Antifoamto IIT 747 Antifoam in the 1/240th SRAT. Report WSRC-TR-99-00377; Westinghouse Savannah River Co.: Feb 23, 2000.

(11) Mingens, J.; Scheludko, A. J. Attachment of SphericalParticles to the Surface of a Pendant Drop and the Tension of theWetting Perimeter. J. Chem. Soc., Faraday Trans. 1 1979, 1.

(12) Wasan, D. T.; Koczo, K.; Nikolov, A. D. Mechanisms ofAqueous Foam Stability and Antifoaming Action With and With-out Oil: A Thin Film Approach. In Foams, Fundamentals andApplications in the Petroleum Industry; Schramm, L. L., Ed.;Advances in Chemistry Series 242; American Chemical Society:Washington, DC, 1994.

(13) Wasan, D. T.; Christiano, S. P. Foams and Antifoams: AThin Film Approach. In Handbook of Surface and Colloid Chem-istry; Birdi, K. S., Ed.; CRC Press: Boca Raton, FL, 1997.

(14) Garret, P. R. The Effect of Poly(tetrafluoroethylene) Par-ticles on the Foamability of Aqueous Surfactant Solutions. J.Colloid Interface Sci. 1979, 69, 107.

(15) Binks, B. P. Particles as surfactants-similarities anddifferences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.

Received for review August 18, 2003Revised manuscript received January 27, 2004

Accepted January 29, 2004

IE0306776

Figure 6. Effect of antifoams on foaminess with time.

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