Solidification/stabilization (S/S) techniques are akin to locking the contaminants in the soil. It is a process that physically encapsulates the contaminant. This technique can be used alone or combined with other treatment and disposal methods.
The most common form of S/S is a cement process. It simply involves the addition of cement or a cement-based mixture, which thereby limits the solubility or mobility of the waste constituents. These techniques are accomplished either in situ, by injecting a cement based agent into the contaminated materials or ex situ, by excavating the materials, machine-mixing them with a cement-based agent, and depositing the solidified mass in a designated area. The goal of the S/S process is to limit the spread, via leaching, of contaminated material. The end product resulting from the solidification process is a monolithic block of waste with high structural integrity. Types of solidifying/stabilizing agents include the following: Portland; gypsum; modified sulfur cement, consisting of elemental sulfur and hydrocarbon polymers; and grout, consisting of cement and other dry materials, such as acceptable fly ash or blast furnace slag. Processes utilizing modified sulfur cement are typically performed ex situ.
The Department of Energy (DOE) is developing another system called the Polyethylene Encapsulation of Radionuclides and Heavy Metals (PERM) process. This process encapsulates contaminants in polyethylene. It is used for radionuclides (e.g., cesium, strontium, and cobalt) and toxic metals (e.g., chromium, lead, and cadmium). Most S/S products are designed to be left in place, although it is possible that the solid materials could be moved to other locations.
Limitations and Concerns
The depth of contaminants may limit these processes.
Future use of the site and environmental conditions may erode the materials used to stabilize contaminants, thus affecting their capacity to immobilize contaminants. Solidified material may also restrict future use of the site.
Very little data exist to support S/S productsÕ durability over their indefinite disposal life. Long term monitoring is necessary to ensure that contaminants have not been re-mobilized.
Certain waste streams are incompatible with variations of this process, and each application must be carefully tested for long-term compatibility before it is used.
Organic wastes are generally not immobilized, and unless very high temperatures are used to destroy them, they will migrate. However, if a process were designed to destroy organic compounds through heating, the creation of products of incomplete combustion such as dioxin and furan would pose a concern.
Inorganic salts affect the set rate either through acceleration or retardation. Users need to know precisely how different salts individually and collectively affect basic Portland cement stabilization so the proper additive can be used in the dry binder mix.
When radioactive contamination is present, other types of hazardous waste (e.g., organic chemicals) may interfere with solidification. Treatability studies are needed to demonstrate that the S/S process works.
Given the long period of time that radioactive waste will be a hazard, the S/S facility must be particularly careful about the degradation of construction materials. Current research has focused on developing new types of materials to improve liner integrity and to reduce possible Radionuclides migration.
For radioactive waste, there is concern about the likelihood of liner deterioration, liner penetration, and leaching over the long-term, as well as risks associated with the possible excavation, handling, and transportation of radioactive waste.
In situ S/S may not be suitable for some sites because gamma radiation might not be reduced sufficiently.
With in situ S/S, consideration must be given to any debris such as barrels, metal scrap, and wood pieces that may interfere with the solidification process.
Soil characteristics influence whether the technology will contain the waste effectively. These characteristics include void volume, which determines how much grout can be injected into the site; soil pore size, which determines the size of the cement particles that can be injected; and permeability of the surrounding area, which determines whether water will flow preferentially around the solidified mass.
Some cement processes result in significant increases in volume— up to double the original quantity.
In ex-situ applications, cracks extending through the stabilized mass have been observed, the cause of which is suspected to be the high temperature rise during curing.
The target contaminant group for physical S/S is generally inorganics (including radionuclides) in the soil. While it may be effective for some organics, this technology may have limited effectiveness with semi volatile organic compounds (SVOCs) and pesticides. Encapsulation is also used for low-level radioactive mixed waste.
Technology Development Status
Physical stabilization techniques are well documented and commercialized. DOE has demonstrated the PERM process at the bench scale.
Other Resources and Demonstrations
See the descriptions of Chemical Solidification/Stabilization and Thermal Solidification/Stabilization (Vitrification).
See http://www.clu-in.org/download/remed/ss_sfund.pdf ÒSolidification/Stabilization Use at Superfund SitesÓ (EPA 542-R-00-010). September 2000, 28 pages
See http://www.osti.gov/bridge/servlets/purl/206641-sag9ex/webviewable/ for a paper on solidification/stabilization of mixed waste sludge.
See http://www.osti.gov/bridge/servlets/purl/195660-FhAmeV/webviewable/ for a description of when DOEÕs development of polyethylene encapsulation of low-level wastes at INEL.
See http://www.ieer.org/reports/srs/grout.pdf for a report on some of the problems with stabilization technology.