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Ind. Eng. Chem. Res. 2004, 43, 3812-3816
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 immobilization processes and in food, chemical, and agricultural products. The cause of foaminess in the presence of finely divided solids during boiling and in the absence of any surface-active agents is not well understood. Our research has identified at least two kinds of particles in such foaming systems, hydrophilic (i.e., water wet) colloidal particles dispersed in the aqueous phase and biphilic particles (partially wetted by water). The biphilic particles are attached to the air-water surface. In this study we used an advanced optical technique to characterize and monitor the number of nonattached (i.e., hydrophilic) and attached (i.e., biphilic) particles at the gas-liquid surface. The results clearly show that foaming increases with an increase in each of the two types of particles but to a different degree. The presence of biphilic particles causes a significantly higher degree of foaminess than the hydrophilic colloidal particles. Introduction Particle-stabilized aqueous foams are encountered in the processing of solid waste (e.g., during boiling), food, chemical, and agricultural products, froth flotation, and radioactive waste treatment and immobilization processes.1 Uncontrollable foaming can severely impact the production rate and ultimately the cost-effectiveness of a chemical process. The solid particles in boiling suspensions, in the absence of any surfactants, stabilize the foam lamella and enhance the foaminess. Previously,1 we identified at least two types of particles in such three-phase foaming systems: hydrophilic colloidal particles dispersed in the aqueous phase and biphilic particles (i.e., with some area of the particle wetted by water and the other part is not). These biphilic (or amphiphilic) particles attach to the surfaces of the foam lamella, provide a steric barrier against the coalescence of bubbles, and thereby enhance foam lamella stability and foaminess. A foam lamella is formed during the generation and interaction of bubbles during the boiling of aqueous suspensions. Hydrophilic colloidal particles get trapped inside the lamella. Subsequently, due to the confined boundaries of the film (lamella), these particles form a layered (i.e., stratified) structure inside the foam lamella.2,3 Monte Carlo simulations of the film containing particles show that the concentration of the colloidal particles is higher in the film/lamella than that in the bulk.4 Furthermore, our theoretical calculations show that, at a higher particle concentration, a better particle in-layer structure develops that increases the energy stabilization barrier, inhibiting particle diffusion from the film to the bulk meniscus.3,5,6 The repulsive struc† During the past 8 years, our research group at IIT has collaborated with the U.S. Department of Energy Savannah River Technology Center to obtain a fundamental understanding of the physicochemical cause of foaming and have used this knowledge to develop novel antifoaming agents that are effective in the harsh environment of high-level radioactive waste processing. The results of this effort are summarized in this paper. * To whom correspondence should be addressed. Tel.: (312) 567-3001. Fax: (312) 567-3003. E-mail:
[email protected].
ture barrier (i.e., the structural disjoining pressure) arising due to the colloid particle in-layer structure formation offers a novel stabilization mechanism for macrodispersions such as foams and emulsions. In fact, we have produced aqueous foams in surfactant-free particle suspensions using nanosized silica particles.2,7 Our thin film experiments have clearly shown that there exists a critical lamella size below which at least one layer of particles always stays in the film. This critical lamella size is dependent upon particle size and concentration. The critical lamella size seems to increase almost exponentially with particle concentration.5 Our observations show the phenomenon of lamella stratification (i.e., layering) is very much dependent on lamella (or bubble) size.5,8 The classical concept of foam lamella stability is based on the disjoining pressure isotherm (a thermodynamic quantity which is independent of lamella size). Therefore, the disjoining pressure isotherm is unable to predict the stability of the foam lamella stabilized by colloidal particles, the stability of which depends on both particle concentration and its size. An important factor that affects the colloidal particle structuring and layering phenomena in confined films, and thereby the film stability, is the polydispersity in particle size. Polydispersity has a significant effect on the structural disjoining pressure. Studies have shown that a 30% polydispersity in particle size can decrease the energy structural barrier by a factor of 3, while the effect on the depletion well is smaller.3,4,9 This suggests that a simple way to destabilize a stable foam is to increase the polydispersity of the suspension or simply add a small amount (e.g., 1 vol %) of large particles. The large particles trapped inside the foam lamella weaken the structure of colloidal particles, decrease the structural barrier, and destabilize the foam lamella, reducing the foaminess and foam stability.3,9 The first part of our paper presents results of foaming tests during the boiling of simulated nonradioactive waste containing both hydrophilic and biphilic particles. We used an advanced optical technique to monitor these particles. The second part of our paper describes using this knowledge to develop a new antifoam to suppress severe
10.1021/ie0306776 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/30/2004
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Figure 1. Experimental setup to separate the biphilic particles from the sludge under foaming conditions.
foaming in the radioactive waste vitrification process for immobilization of nuclear wastes at the Defense Waste Processing Facility at the Savannah River Site in Aiken, SC.10 Foam Generation The foam was generated by boiling the simulated waste and collecting the foam using the experimental apparatus shown in Figure 1. The original setup as described in ref 1 was modified to collect particles carried by the foam lamellae. A syringe with an inner diameter of 5 mm attached to a vacuum chamber was used to imbibe the foam lamellae containing particles. The foaminess was quantified by measuring the difference in volume between the aqueous particle suspension during boiling (i.e., when it is foaming) and the suspension volume without boiling (when it is not foaming); the foam collapsed when boiling ceased at the same particle concentration. We used an optical technique for quantitatively characterizing the number of attached (biphilic) and nonattached (hydrophilic) particles at the gas-liquid surface (Figure 2). A modified glass syringe (3.0 mm inner diameter) with a threaded screw piston was used to hold the suspension and to control the curvature of the meniscus at the air-aqueous surface. The syringe
Figure 2. Experimental setup to characterize the nature of particles.
was mounted vertically on the horizontal plate of the inverted microscope. The microscope was tuned to work in both reflected and transmitted light modes. With time, the particles in the suspension settled to the bottom part of the syringe. The bottom of the syringe is open to the air and ends with an aqueous meniscus. The curvature of the meniscus is controlled by the position of the threaded screw piston and is adjusted to make the meniscus flat. When the particles settle, they slowly approach the airaqueous surface at the bottom part of the syringe. The biphilic particles approach the air-aqueous surface and interact with the surface via the aqueous film (which ruptures), and the particles attach themselves to the surface, forming a three-phase contact angle. The number of particles attached (i.e., biphilic) to the gas-liquid surface is counted using reflected light. With the passage of time, the hydrophilic particles arrive and settle at the air-liquid surface. Due to the hydrophilic nature of these particles, they do not attach to the airlayer surface, and they are not visible in reflected light using a low-aperture objective. However, in the transmitted light they are visible, as are the attached particles. Therefore, with this optical arrangement, numbers of both the biphilic and hydrophilic particles are counted. This unique method permits us to calculate the biphilic and the hydrophilic particles. Mingins and Scheludko11 used a similar optical arrangement to study the attachment of small particles to a pendant drop and to estimate the contact angle and ratio of attached to unattached particles. Our previous publication1 dealt with studying foaming during the boiling of the simulated nonradioactive aqueous waste suspension. The simulant (i.e., the model suspension which mimicked the actual radioactive waste) was prepared by the Savannah River Technology Center. The waste suspension contained 7-20 wt % metal oxides and hydroxides of aluminum, iron, manganese, and mercury at a pH of 5-6 (see Table 1 in ref
3814 Ind. Eng. Chem. Res., Vol. 43, No. 14, 2004 Table 1. Number and Concentration of Attached and Nonattached Particles at Maximum Foaminess biphilic particles
Figure 3. Effect of particle ratio biphilic/hydrophilic on the foaminess: run 1, original sludge, particle ratio 3.3; run 2, particle ratio 2.4; run 3, particle ratio 0.9; run 4, particle ratio 0.06; run 5, particle ratio 0.06.
1). The insoluble particles are irregular in shape and are polydisperse with particle sizes varying from 5 nm to 10 µm as estimated by the thin film interferometry and microscopic studies. Furthermore, these particles have nonuniform surface energy. That is to say that a part of the particle surface is hydrophilic and the other part is hydrophobic. The particles with such nonuniform surface energy are biphilic in behavior. These particles are metal oxides and hydroxides, and these biphilic particles attach to the air-aqueous surface. We used our capillary force balance method to characterize the role of particle-particle interactions in foam lamella stability.1 These biphilic particles form a network structure due to the attractive capillary force and provide a steric-type stabilization for the foam lamella. The suspension also contained colloidal particles, which are hydrophilic with a radius of above 400 nm and effective volume of about 20 vol %. These particles stabilize foam lamellae as large as 200 µm, possibly due to the formation of a layered structure inside the foam lamella. We conducted boiling experiments using the same nonradioactive waste simulant containing both types of particles. To evaluate the presence of any surface active agents in the suspension, the classic Barcht tests were conducted at room temperature, and these tests showed no foaminess and foam stability. The initial solid concentration in the sludge was 8.8 wt %. During the boiling of the sludge, the solid concentration increases (due to the evaporation of water), leading to an increase in foaminess (Figure 3). The particles carried by the foam lamellae were removed using a capillary connected to a chamber with partial vacuum and collected at the maximum foaminess of 475 vol % when the total particle concentration in the suspension was 18.8 wt %. The collected particles in the suspension were then characterized by counting the number of attached (i.e., biphilic) particles and the total number of particles using the optical method described above. The maximum foaminess of the foam lamellae contained 16 wt % of biphilic particles and 2.65 wt % of hydrophilic colloidal particles. The suspension was diluted back to the initial solids concentration of 8.8 wt % by adding distilled water at the end of run 1, and run 2 was performed. Foaminess was monitored during the boiling of the suspension. Results are shown in Figure 3. A reduction in foaminess from 475 to 375 vol % was observed at a total solids concentration of 18.65 wt %. This reduction in foaminess is attributed to the partial removal of biphilic particles
run no. 1 2 3 4
hydrophilic particles
no.
concn (wt %)
no.
concn (wt %)
vol % max foaminess
712 663 453 63
14.5 13.3 8.96 1.05
216 276 503 1065
4.3 5.5 9.84 17.73
475 375 320 260
in run 2 since the biphilic particle concentration was determined to be only 13.3 wt % at the end of this run. This procedure was repeated, and the results of runs 3-5 are also shown in Figure 3. The stepwise decrease in foaminess from 475 vol % (run 1) to 375 vol % (run 2) to 320 vol % (run 3) to 260 vol % (run 4) was observed, which corresponds to the successive removal of particles from the foam lamella for each run, respectively. The data for run 5 show that foaminess remained virtually unchanged even though it contained fewer biphilic particles than run 4. These experimental results reveal that foaminess is caused by two types of particles, amphiphilic (biphilic) and hydrophilic, both of which are present in our simulated waste. Run 5 has the largest number of nonattached (i.e., hydrophilic) particles, which produces about 260 vol % foaminess. Table 1 shows the number and concentration of biphilic (i.e., attached) and hydrophilic (i.e., nonattached) particles and the degree of foaminess. It is evident that the foaminess decreases from 475 to 260 vol % as the ratio of biphilic to hydrophilic particles decreases from 3.3 to 0.06 (corresponding to runs 1 and 4, respectively). It should be noted that not all of the removed particles were biphilic. Table 1 lists the number and concentration of biphilic particles that were carried by the foam lamellae. Figure 4 shows a three-dimensional surface plot depicting the dependence of foaminess on the concentration of both biphilic and hydrophilic particles. Curve A depicts results of the experiment when the simulated sludge contained biphilic particles only, and curve B depicts the foaming results when only hydrophilic particles were present. The results of runs 1-4 are also marked on this figure. The figure shows that foaminess increases with an increase in the concentration of the two types of particles. However, hydrophilic particles produce a maximum of about 260 vol % of foaminess (i.e., the amount of gas incorporated into the system), whereas biphilic particles resulted in a significantly higher degree of foaminess (i.e., 900 vol %) over the same range of particle concentrations. The photographs of the attached (or biphilic) particles for different runs are also shown in this figure. The major finding from our experimental observations is that the particle hydrophilicity/biphilicity and particle size determine the degree of foaminess in aqueous foams containing fine particles. Results show that foaminess increases with an increase in concentration of each of the two types of particles. However, particles of intermediate hydrophobicity caused a greater degree of foaminess over the same range of particle concentration. The challenge is how to reduce the foaminess. Development and Testing of Antifoaming Agents Our research addresses two different but interrelated studies. The first deals with the fundamental under-
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Figure 4. Effect of biphilic and colloidal particles on foaminess.
standing of the causes of foaming and stability of the foam as highlighted in the preceding section. The second is the practical need to find an effective antifoam or defoaming agent to minimize the production of foam during processing. Antifoam and defoaming agents have been developed to help industries such as nuclear waste, paper, pulp, coatings, ink, and printing. The commercial antifoams were developed to eliminate foaminess in the presence of surfactants, and these antifoams are not expected to be effective when foam is stabilized by particles in the absence of any surfactant. Mechanisms of stabilization of foams in the presence of particles and by surfactants are quite different. Moreover, the commercial antifoams are unstable in the harsh environment in radioactive waste processing that includes strong caustic solutions or strong acid solutions, high temperature, radiation, reducing and oxidizing agents, etc. There are a number of processing operations that could operate more efficiently and reliably if more efficient antifoams could be found or developed. Three areas of concern at the DOE’s Savannah River Site (SRS) are the development of a better antifoam for the high-level waste evaporators, development of an effective antifoam in the tetraphenylborate processing, and the development of an antifoam for the Defense Waste Processing Facility. The Hanford River Protection project also needs an efficient antifoam for the high-level and low-activity waste evaporators. In our recent study on foams stabilized by the hydrophilic particles,2 we observed that when a small quantity (2 wt %) of larger hydrophilic particles (100 nm) was added to a suspension containing 8 vol % of 8 nm microdispersed hydrophilic particle suspension, the foam lamella ruptured and the foam was unstable. The
Figure 5. Effect of wetting agent and polydispersity on foaminess.
presence of a few large particles inside the foam leads to the local destabilization of the ordered particle structure, resulting in lamella rupture even at a high thickness. We explored this concept of using polydispersity in particle size to reduce foam lamella stability to develop a new type of proprietary antifoam. Results of the experiments are shown in Figure 5. The foaminess was reduced from 425% to 275% (a 35% reduction). The foam stability in the presence of particles is governed by two mechanisms: colloidal particle layer formation (i.e., structural barrier); steric stabilization by biphilic particles. We added 100 ppm of a proprietary wetting agent to modify the biphilic particle surface to become more hydrophilic. We observed a dramatic reduction in foaminess (Figure 5).
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determined using differential interferometry on foaminess and foam stability. The effects of particle size and shape, as well as the pH of the aqueous solution and electrolyte concentration, are being investigated. Research to understand the basic mechanisms controlling both the foaminess, foam stability, and antifoaming action in gas-liquid-solids systems continues to be a high priority need for the U.S. Department of Energy’s Environmental Management Science Program. Acknowledgment This research was supported by the Environmental Management Science Program of the Office of Environmental Management, U.S. Department of Energy, Grant DE-FG07-01ER14828. Literature Cited Figure 6. Effect of antifoams on foaminess with time.
On the basis of the mechanistic understanding of foam generation and stability, we developed an improved antifoam agent (IIT 747), since the commercial antifoam agent (Dow Corning 544) was found to be ineffective in the aggressive physical and chemical environment in the Defense Waste Processing Facility (DWPF) sludge receipt and adjustment process as indicated by the foaminess data presented in Figure 6. The addition of the IIT antifoam was found to be more effective in minimizing foam and was more effective over time than Dow Corning 544. The improved antifoam agent, IIT 747, was subsequently tested in a pilot plant at the Savannah River Site and with real waste in their shielded cells. The antifoam developed by us is now being used in the Defense Waste Processing Facility. Concluding Remarks The research conducted by us on both the foaminess and foam stability and antifoaming action for the radioactive waste separation process in boiling suspensions has clearly shown that particles alone (without any surfactants) can cause severe foaming. Our research findings have major implications for treating actual sludges such as radioactive wastes containing crystalline materials that have a nonuniform surface energy. These particles are neither completely hydrophilic nor hydrophobic but are biphilic particles. Earlier, we identified two major mechanisms of foaminess and stabilization of foams containing solid particles, namely (i) steric stabilization due to the attachment of biphilic particles to the gas/liquid surface and (ii) structural stabilization due to the hydrophilic colloidal particles forming a layered structure inside the foam lamella. Other mechanisms for the formation and stability of foams have been discussed in the literature.12-14 However, a recent review article by Binks15 points out the scarcity of pertinent literature on the ability of fine particles to cause foaminess or foam stability, especially in the absence of any other surfaceactive material. Our continuing study is aimed at systematically investigating the influence of the wettability of solid particles by increasing the hydrophobicity/biphilicity as
(1) Bindal, S. K.; Nikolov, A. D.; Wasan, D. T.; Lambert, D. P.; Koopman, D. C. Foaming in Simulated Radioactive Waste. Environ. Sci. Technol. 2001, 35, 3941. (2) Bindal, S. K.; Sethumadhavan, G.; Nikolov, A. D.; Wasan, D. T. Foaming Mechanisms in Surfactant Free Particle Suspensions. Am. Inst. Chem. Eng. 1987, 48 (10), 2307. (3) Nikolov, A. D.; Wasan, D. T. Dispersion Stability Due to Structural Contributions to the Particle Interaction as Probed by Thin 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. Stability of Liquid Films Containing Monodispersed Colloidal Particles. J. Colloid Interface Sci. 2001, 240, 105. (6) Sethumadhavan, G.; Nikolov, A. D.; Wasan, D. T. Film Stratification in the Presence of Colloidal Particles. Langmuir 2002, 17, 2059. (7) Wasan, D. T.; Nikolov, A. D.; Henderson, D. New Vistas in Dispersion Science and Engineering. Am. Inst. Chem. Eng. 2003, 49 (3), 550. (8) Nikolov A. D.; Wasan. D. T. Effects of Film Size and Micellar Polydispersity 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 Antifoam to IIT 747 Antifoam in the 1/240th SRAT. Report WSRC-TR-9900377; Westinghouse Savannah River Co.: Feb 23, 2000. (11) Mingens, J.; Scheludko, A. J. Attachment of Spherical Particles to the Surface of a Pendant Drop and the Tension of the Wetting Perimeter. J. Chem. Soc., Faraday Trans. 1 1979, 1. (12) Wasan, D. T.; Koczo, K.; Nikolov, A. D. Mechanisms of Aqueous Foam Stability and Antifoaming Action With and Without Oil: A Thin Film Approach. In Foams, Fundamentals and Applications 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: A Thin Film Approach. In Handbook of Surface and Colloid Chemistry; Birdi, K. S., Ed.; CRC Press: Boca Raton, FL, 1997. (14) Garret, P. R. The Effect of Poly(tetrafluoroethylene) Particles on the Foamability of Aqueous Surfactant Solutions. J. Colloid Interface Sci. 1979, 69, 107. (15) Binks, B. P. Particles as surfactants-similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21.
Received for review August 18, 2003 Revised manuscript received January 27, 2004 Accepted January 29, 2004 IE0306776
