PNNL-13501 - Pacific Northwest National Laboratory

Development of Aqueous Based Sol-Gel Systems for Producing Durable Salt Waste Forms
Harry D. Smith, Liang Liang, Renee L. Russell, Gary L. Smith, Troy N. Terry, Brian J.J. Zelinski
Study Control Number: PN00031/1438
This project explores new materials for encapsulating and storing a wide range of wastes including salt wastes, soot,
ashes, toxic metals, and other wastes containing low levels of radionuclides. One new material for this process is a solgel
organic-inorganic hybrid, which has good mechanical integrity, chemical durability, low processing temperature, and
is easy to fabricate.
Project Description
In the past, chemical approaches for the sol-gel synthesis
of ceramics and polymer-ceramic (polyceram) hybrids
have used organic solvents as the reaction medium with
little consideration of their high volatility and
flammability. However, alternative sol-gel processes in
aqueous media may offer acceptable results without the
need for hazardous solvents, precursors, or byproducts.
To accomplish this, emulsions consisting of polymer and
ceramic precursors suspended as micelles in water are
substituted for the organic-based systems previously
identified as being effective matrices for storing salt
waste. Cross-linking and curing of the emulsions at lowtemperatures
produces mechanically robust waste forms
able to accommodate high salt loadings. The leach
resistance of these waste forms is strongly influenced by
processing; where the use of faster acting cross-linking
agents produce a more leach resistant material.
Optimization of the cured emulsions should make them
attractive alternatives to vitrification for low-level wastes,
which would see broad use in many tank-related
applications.
Background
Polycerams have been demonstrated to be viable
candidates for use as waste forms for encapsulating salt
wastes (Smith and Zelinski 1999). The major drawback
to their use is the organic-based fabrication route with its
associated flammability hazards. The development of
water-based polymer-polyceram processing routes would
facilitate the conversion of complex aqueous mixtures and
low-level waste streams to stable waste forms using a
simple, low temperature flowsheet. This capability
represents a timely alternative to vitrification strategies
which are currently experiencing unforeseen obstacles to
their implementation. The process flowsheet for
polyceram waste forms is projected to be much simpler
and more cost-effective than vitrification and grouting
approaches and so presents the DOE complex with a near-
term, attractive alternative for quick retrieval and closure
of the many sites containing large concentrations of salts,
without generating secondary waste streams.
Polycerams based on the use of rubber (butadiene) as the
organic component and silica as the ceramic component
were first developed as candidate waste forms capable of
stabilizing greater than 10 wt% salt waste through a joint
collaboration between the Arizona Materials Laboratory
of the University of Arizona and Pacific Northwest
National Laboratory. These materials greatly exceeded
the strength requirements set by the Mixed Waste Focus
Area and exhibited leaching behavior sufficient to meet
the Resource Conservation and Recovery Act Land
Disposal Requirement standards. The current project
builds on this experience by exploiting the chemical
processing expertise at both participating institutions to
develop waterborne emulsion systems for butadiene and
other polymeric materials, in anticipation of eventual
modifications to incorporate the ceramic component. The
developed emulsions resemble latexes and consist of
small, colloidal liquid droplets, or miscelles, suspended in
water by surface charges introduced using a surfactant.
With the addition of the appropriate cross-linking agent,
the miscelles act like mini-reaction vessels in which the
polymer precursors combine to form a tough, insoluble,
chemically resistant material upon removal of water and
cure.
Results and Accomplishments
Commercially available aqueous emulsion systems were
screened for further development by assessing the
strength, chemical durability, and leach resistance of
samples cured as bulk pieces at various temperatures.
The tested systems included: Styronal® ND 656, an
emulsion of polystyrene/butadiene used as a general
purpose offset binder; Dow Corning® 84 additive, a
silicon emulsion used as an additive for inks and product
finishes; Paranol® T-6300, a polyacrylic latex; and
natural latex rubber emulsions from Hartex and Killian.
Materials Science and Technology 311