Stabilization is a relatively simple process that does not require highly skilled workers; the mixing materials are not expensive; and it can be accomplished with common construction equipment. But the simplicity of the process can also work against solidification. For example, it may not be effective at sites with multiple pollutants. Also, solidification does not destroy contaminants, and long-term monitoring may be needed at sites to ensure that leaching does not occur.
Solidification is typically linked with a comparable waste treatment process called stabilization. While solidification encapsulates a waste to form a solid material or coats the waste with a low permeability cover, stabilization refers to processes that involve chemical reactions that reduce the leachability of a waste. Some treatment reagents will both solidify and stabilize the contaminant matrix. For example, a treatment reagent such as cement can reduce the mobility of many metal contaminants by forming insoluble hydroxides, carbonates, and silicates (stabilization) and also provide a solid encapsulation matrix to reduce leaching (solidification).
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On- or Off-Site
Depending on the site circumstances, solidification/stabilization (S/S) can be performed either in situ or ex situ. The preferred in situ approach typically involves the use of a large rotary auger 6 to 8 (or more) feet in diameter, which injects binding agents and mixes them into the contaminated soil. After thorough mixing, the auger is withdrawn and advanced to slightly overlap the last plug, and the process is repeated until the contaminated area is completed.
With the ex situ process, the contaminated materials are excavated and staged. Treatment can be performed in drums, a fixed plant, or a mobile plant. The treated materials can then be shipped off-site or returned to the excavation site, provided they have not swelled to a size that prohibits replacement.
Binders and Additives
Perhaps the most complex aspect of S/S involves the selection of the appropriate binders and additives. Binders such as Portland cement have both stabilization and solidification capabilities. While they can often be used as standalone materials, their performance may be improved by using an additive material. For example, phosphate, a stabilization agent, can be mixed with a soil or sludge before mixing with a cementitious binder to improve the stability of some metal contaminants. Challenges often emerge in the treatment of certain hydrophobic organic compounds that do not react with inorganic binders and inhibit the setting of cement. Cementitious-based S/S has been found ineffective with the types of halogenated volatiles and nonhalogenated volatiles common at hazardous waste sites. Better results have been reported when cementitious materials have been used to treat soils/sludges contaminated with semivolatile organic compounds.
Clu-in, a waste-site treatment component of EPA’s Technology Innovation and Field Services Division, lists the following advantages and limitations of S/S.
Advantages
- Many S/S technologies can treat complex mixtures of different wastes.
- S/S can be effective in treating materials contaminated with some types of organic chemicals (e.g., coal tars). Treatment of volatile organics by S/S is problematic and has not found widespread use.
- Most binding agents are relatively inexpensive.
- Some non-aqueous phase liquids (NAPL) have been addressed through S/S.
- The fixed treatment end point can be reached relatively quickly.
- S/S can improve structural properties of soil, waste, and sludge (e.g., strength) to facilitate consideration of land-beneficial reuse.
- The technology is applicable to in situ or ex situ treatment.
- Applications include dry or wet conditions, thus reducing dewatering and waste management issues.
- Simple, readily available equipment and materials are used.
- On-site management of contaminated materials conserves landfill space with no transportation off-site.
- Most S/S techniques require low levels of skill.
- Depending on the site, S/S may be more cost effective than excavation and off-site disposal.
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Limitations
- Many S/S techniques do not decrease contaminant toxicity.
- Contaminants are not destroyed or removed; long-term stewardship may be required.
- Volume increases that occur in the treated mass may require management.
- Implementation requires removal of debris or underground obstructions before treatment.
- With heterogeneous distribution of contaminants in the subsurface, in situ mixing of waste and a binder can result in uneven performance.
- S/S of sensitive areas may inhibit more comprehensive restoration in the future.
- S/S effectiveness for certain contaminants (e.g., some organic species, such as volatile organics or highly mobile species) may require additional measures in testing and design. Cementitious S/S processes alone are generally not effective in treating volatile organics or some metals (e.g., chromium (VI)) that do not form highly insoluble hydroxides.
- Potential changes in physical setting (e.g., groundwater flow, mounding) may need to be assessed.
- Uncertainties are associated with prediction of long-term behavior.
- Options for treatment or posttreatment modifications are limited by time for field performance testing and changed properties of treated material.
Clu-in notes that ex situ S/S was used in 170 projects and in situ S/S, in 41 projects for source control at Superfund sites during 1982 to 2004. An additional 33 ex situ and 15 in situ S/S actions were identified in 2005 to 2008 decision documents. A number of ex situ S/S actions at Superfund national priorities list (NPL) sites were conducted to stabilize contaminated soil before off-site disposal at RCRA Subtitle D facilities.
A short Clu-in paper on S/S is at http://www.clu-in.org/techfocus/default.focus/sec/Solidification/cat/Overview.