Through the Environmental Management Science Program (EMSP), DOE's Office of Environmental Management (EM) and Office of Science (SC) collaborate to fund basic research to solve intractable problems that threaten the successful closure of DOE sites. As one of the offices within the Office of Science and Technology, EMSP ensures that OST's projects cover the full spectrum of R&D.

The millions of gallons of radioactive waste stored in underground tanks is a top-priority remediation problem for the U.S. Department of Energy. From a waste processing standpoint, the tank waste components of greatest concern are insoluble sludges consisting of suspensions of colloids, particles between a nanometer and micrometer in diameter.

Under common circumstances, the colloidal particles can form agglomerate networks that impede several aspects of waste processing. They can trap flammable gases, making retrieval and transport more dangerous. They can clog transfer lines and extraction systems like filters and ion exchangers. And they resist efficient sedimentation, frustrating efforts to reduce the volume of waste to be treated and disposed of.

Given the range of chemistries present in DOE tanks, it's impractical to measure the properties of all tank wastes under all potential conditions to design rational treatment procedures. Instead, a sound scientific framework for predicting property trends needs to be established. That's the goal of an Environmental Management Science Program project titled “Colloidal Agglomeration in Tank Sludge: Impact on Waste Processing.” A team combining researchers from Pacific Northwest, Oak Ridge, and Sandia National Laboratories and the University of Washington is in the last year of this three-year project. The resulting enhanced understanding of agglomeration phenomena and the properties of complex colloidal suspensions will aid in the development of new methods and techniques for processing hazardous tank wastes.

Researchers are working to

• understand the factors controlling the nature and extent of colloidal agglomeration under expected waste processing conditions;
• determine how agglomeration phenomena influence physical properties relevant to waste processing including rheology, sedimentation, and filtration; and
• develop strategies for optimizing processing conditions by controlling agglomeration phenomena.

Significant findings

Transmission electron microscopy of actual wastes shows that most sludges consist of agglomerates of submicrometer-sized primary particles of hydrated oxides and insoluble salts. To aid experimentation, researchers identified model colloid suspensions that duplicate the compositions and particle morphologies of actual waste. Static light scattering measurements on both model suspensions and actual wastes showed that the primary particles in the basic salt solutions found in most tank wastes undergo extensive aggregation to form fractal agglomerates. This structure has an enormous impact on slurry properties because fractal objects occupy much more space than dense objects at the same solids loading.

Depending on the extent of agglomeration and solids loading, slurry viscosities can vary by over five orders of magnitude, and the most viscous suspensions contain colloidal agglomerates. To achieve desired viscosities for retrieval and transport, these sludges require 20-fold dilution, producing enormous quantities of wastes.

The researchers have also found that primary particle and agglomerate size can influence sedimentation rates by over three orders of magnitude. Laboratory tests on actual tank wastes revealed that final sediment densities ranging from only 1 to 8 volume percent are common. This means that particulate layers will occupy large volumes during steps such as sludge washing and leaching and that the volume occupied by sediment layers is predominantly interstitial water that cannot be removed during solid-liquid separations.

Experiments have further indicated that the solids loading in sediment layers is highly dependent on the degree of agglomeration and the interparticle interaction potential. Packing of individual particles can be highly efficient (up to 50 volume percent). For agglomerated systems, sediment densities are much lower, but can be increased up to threefold by weakening interparticle interactions. This increases the compressibility of the sediment, which compacts under its own weight. Experiments have shown that in the high-salt regime of most tank wastes, sediment densities first decrease, then increase with salt content because of the interplay of electrical double layer and hydration forces. The presence of calcium and other divalent cations is particularly effective in promoting sediment compression. Manipulation of such forces could be used to improve tank utilization and the efficiency of solid-liquid separations during steps such as sludge washing and leaching.

“We can now predict sludge properties if we know the distributions of primary particle and agglomerate sizes present in a given tank,” says initial principal investigator Bruce Bunker. “If tank sludges are properly characterized, we can predict processing conditions that are likely to be encountered. For example, we can predict solids loadings above which the contents of a given tank are likely to be transformed from pumpable liquids into viscous gels that will clog transfer lines.” Unfortunately, it may not be practical to measure agglomerate distributions in certain radioactive wastes. “Our long-term goal is to be able to predict agglomerate distributions based on interparticle interaction potentials, which are controlled by particle types and solution chemistry,” says Bunker.

Closing in on practical applications

While work to date suggests that short-range forces are important in controlling the properties of tank sludge, quantitative models do not yet exist to predict sludge properties such as sediment densities or slurry viscosities based solely on interaction potentials. Work in fiscal year 1999 is focused on measuring short-range forces, determining how different salts influence the fractal dimension and size of colloidal agglomerates, and modeling agglomerate properties. Once the framework relating interparticle forces, agglomeration, and sludge properties is established, the project will devise modifications of sludge suspensions to optimize properties for retrieval, transport, and sedimentation processes.

For further information, contact Joel Tingey, current principal investigator, PNNL, (509) 376-2580.

Readers interested in this project should take care not to miss the article on slurry monitoring.