Environ. Sci. Technol. 2001, 35, 3941-3947
Foaming in Simulated Radioactive Waste SUNIL K BINDAL, ALEX D. NIKOLOV, AND DARSH T. WASAN* Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616 DANIEL P. LAMBERT AND DAVID C. KOOPMAN Savannah River Technology Center, Aiken, South Carolina 29808
Radioactive waste treatment process usually involves concentration of radionuclides before waste can be immobilized by storing it in stable solid form. Foaming is observed at various stages of waste processing like SRAT (sludge receipt and adjustment tank) and melter operations. This kind of foaming greatly limits the process efficiency. The foam encountered can be characterized as a threephase foam that incorporates finely divided solids (colloidal particles). The solid particles stabilize foaminess in two ways: by adsorption of biphilic particles at the surfaces of foam lamella and by layering of particles trapped inside the foam lamella. During bubble generation and rise, solid particles organize themselves into a layered structure due to confinement inside the foam lamella, and this structure provides a barrier against the coalescence of the bubbles, thereby causing foaming. Our novel capillary force balance apparatus was used to examine the particleparticle interactions, which affect particle layer formation in the foam lamella. Moreover, foaminess shows a maximum with increasing solid particle concentration. To explain the maximum in foaminess, a study was carried out on the simulated sludge, a non-radioactive simulant of the radioactive waste sludge at SRS, to identify the parameters that affect the foaming in a system characterized by the absence of surface-active agents. This threephase foam does not show any foam stability unlike surfactantstabilized foam. The parameters investigated were solid particle concentration, heating flux, and electrolyte concentration. The maximum in foaminess was found to be a net result of two countereffects that arise due to particleparticle interactions: structural stabilization and depletion destabilization. It was found that higher electrolyte concentration causes a reduction in foaminess and leads to a smaller bubble size. Higher heating fluxes lead to greater foaminess due to an increased rate of foam lamella generation in the sludge system.
Introduction Processing of nuclear material produces radioactive waste as a byproduct. This radioactive waste is stored in large * Corresponding author e-mail:
[email protected]; fax: (312)5673003; phone: (312)567-3001. 10.1021/es0106319 CCC: $20.00 Published on Web 08/30/2001
2001 American Chemical Society
underground tanks. For the past few decades, storing the waste in large underground tanks proved to be a safe and efficient means of controlling the waste. However, over several years, several of the waste storage tanks have developed leaks due to corrosion. In the early 1980s, it was recognized that there would be significant cost and safety advantages associated with immobilizing the radioactive waste in stable solid form. The immobilization operations separate the waste into three parts: highly radioactive insoluble sludge, highly radioactive precipitate slurry, and less radioactive watersoluble salts. Much of the Savannah River Site (SRS) highlevel radioactive waste is in the form of alkaline suspensions of insoluble, inorganic (“sludge”) particulates. The chemical processing of the sludge to produce melter feed suitable for a glass melter involves the boiling of the sludge (1, 2). The Environmental Protection Agency (EPA) has designated vitrification as one of the accepted available technologies for immobilization of high level nuclear wastes. These wastes are immobilized by incorporating them into a borosilicate glass matrix to reduce the mobility of radionuclides (3, 4). During this processing, various unit operations are employed to concentrate radionuclides, but such engineered mass transfer operations are often inhibited by the problem of foaming. The WSRC (Westinghouse Savannah River Company) site has implemented vitrification technology to immobilize high level waste, and they observed significant foaming during SRAT (sludge receipt and adjustment tank) operation, SME (slurry mix evaporator) tank, and melter operations. Foaming in melters is attributed to the release of oxygen and water vapor from the melt (5). Foaming of this nature is highly detrimental to process efficiency and can lead to the plugging of filters and instrumentation lines (6). Boiling of sludge creates an environment often ignored in fundamental studies of foam formation, a three-phase gas/liquid/solid system. Therefore, such three-phase foams, which incorporate finely divided solids (colloidal particles), are quite different from foams usually encountered. Blair and Lukacs (7) and Plodinec (8) have investigated the foaming problem and found that oxides of the multivalent ions of manganese and iron are the source of foaming. Schreiber and Hockman (9) also studied effects of melt composition on foaming, but there is a lack of basic understanding of three-phase foam formation, which has led to the implementation of empirical approaches to address the foaming and its impact on the treatment process. The solid particles in the sludge are of different sizes and have different surface characteristics such as hydrophilicity and biphilicity. The solid particles can cause foaminess by two mechanisms: by adsorption of biphilic particles on the surface of foam lamella and by layering of particles trapped inside the foam lamella. Biphilic particles in the sludge are adsorbed at the gas-liquid surface, and the concentration of the particle increases during the boiling process, leading to steric interactions between adsorbed particles. The steric interactions between the adsorbed particles lead to stabilization of the foam lamella, resulting in an increase in foaminess. A similar kind of stability mechanism was observed by Menon and Wasan (10) in a crude oil emulsion due to the adsorption of biphilic asphaltene particles on the oil-water interface. Another mechanism that works in parallel to the adsorption mechanism is layering of solid particles inside the foam lamella. Layering of submicronsized particles and micelles has been extensively used to explain the stability of foam and emulsion films. Forces induced by layering are non-DLVO (Derjaguin, Landau, VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3941
TABLE 1. Composition of Sludge Prepared from Purex Simultant component
wt % on dry basis
Al Ca Fe Hg Mn Na Ni Ag, Cu, Mg, Pd, Ru, Rh, Zn, etc.
3.8 2.4 25.5 3.5 5.0 4.6 2.6 trace (i.e.,
