Containment of chemical wastes in near-surface and repository environments is accomplished by designing engineered barriers to fluid flow. Containment barrier technologies such as clay liners, soil/bentonite slurry walls, soil/plastic walls, artificially grouted sediments and soils, and colloidal gelling materials are intended to stop fluid transport and prevent plume migration. However, despite their effectiveness in the short-term, all of these barriers exhibit geochemical or geomechanical instability over the long-term resulting in degradation of the barrier and its ability to contain waste. A new type of containment barrier with a potentially broad range of environmental stability and longevity could result in significant cost-savings.
The objective of this project was to establish the viability of a proposed new type of containment barrier, derived from the in situ precipitation of clays in the pore space of contaminated soils or sediments, with a potentially broad range of environmental stability and longevity. The concept builds upon technologies that exist for colloidal or gel stabilization. Clays have the advantages of being geologically compatible with the near-surface environment and naturally sorptive for a range of contaminants, and further, the precipitation of clays could result in reduced permeability and hydraulic conductivity, and increased mechanical stability through cementation of soil particles.
The research program was conducted in discrete stages that involved a number of different institutions. The first phase of the research effort focused on the laboratory synthesis of clays and clay-like materials from gels at room or ambient temperature. The initial effort focused on reproducing experimental designs from various published studies in which clay products were identified. In addition to these formulations, the synthesis of an anionic clay called a layered double hydroxide that has a naturally occurring mineral counterpart called a hydrotalcite was attempted. A third type of material investigated falls within the new class of mesoporous silica materials that can be formed at room temperature using surfactant templates. Finally, a related study with researchers at the University of Grenoble was conducted to investigate the nucleation of clays on quartz surfaces under ambient conditions.
The second phase of the research effort focused on precipitating two of the studied clay and clay-like materials, anionic clay and mesoporous silica, in natural sediments under laboratory conditions. As originally designed, Phase 2 was to be followed by a third phase that included a pilot scale test to be conducted in a physical model, followed by a full-scale field test.
The Phase 1 effort focused on the laboratory synthesis of clays and clay-like materials from gels at room or ambient temperature, with promising results. The Phase 2 effort focused on precipitation of the two most promising clay or clay-like materials identified in Phase 1, mesoporous silica and layered double hydroxide, in natural sediments under laboratory conditions. Success of the method was to be demonstrated by obtaining a significant reduction in hydraulic conductivity and increase in geomechanical stability, under conditions practical for field application. The experiments needed to demonstrate that practical quantities of clays and/or clay-like solids could be formed within a timeframe appropriate for installment of a barrier, and secondly, that the material properties would be improved over those of existing containment barriers. While some success was achieved in the Phase 2 effort, the laboratory experiments failed to achieve one of the key goals, increased mechanical stability. Further, engineering and environmental considerations were envisioned to complicate field-scale implementation of the process. The ultra-high acidic and basic pH of the precursor chemicals would require non-standard materials to hold, ship, and inject into the ground environment. Moreover, at these same pH levels, the precursor materials might be considered as damaging, if not more so, than some of the contamination plumes they were sought to control. Finally, the mesoporous silicate and layered double hydroxide clay-like materials were not likely to remain stable in the in situ environment for long (due to the near-neutral pH of the natural environment), which again violated one of the central premises for the research.
The combination of the factors described above, combined with the observed level of permeability reduction, indicates that the systems investigated, while scientifically interesting, are not likely to lead to a viable method to mitigate contaminated ground wastes.