Chemical Stabilization/Solidification (S/S) of soils contaminated with hazardous waste is a tried and proven chemical remediation technology. Both the technology and its acceptance has progressed dramatically over a number of years as a simple, cost effective and flexible treatment method for remediation of soils and recycling them back to usable land applications.
Numerous contaminated sites operated by both government and private industry have been returned back to usable land with the in-situ chemical stabilization methods. Successful clean-up projects have been completed in the areas of soils contaminated with metal wastes, chemical wastes, and contaminated dredging.
Soil stabilization/solidification (S/S) is a process that immobilizes contaminants, mitigating the risk of exposure and potential harm to human health and the environment. Cement or lime is mixed with impacted soil and hardens to form a soil-cement matrix that encapsulates the impacted materials. The process is performed on site with soil in-place or on adjacent mixing tables.
Previously, responsible companies would have transferred their contaminated soil off-site to designated landfills, but do to landfill liability and the long-term sustainability of transferring the problem from one location to another regulators have realized the practical approach of in-situ soil stabilization.
In-situ chemical stabilization technology has been evaluated in-depth by the US Environmental Protection Agency (EPA) as part of the SITE program (Superfund Innovative Technology Evaluation) and now is included in their treatment matrix for best available technologies based on cost effectiveness, long-term performance and an environmentally sustainable technology.
Using Lime to Treat Hazardous Wastes
Lime is widely used to treat hazardous wastes both currently generated process wastes and previously disposed or abandoned materials. Lime stabilizes most metals by converting them to more chemical stable forms that are less likely to leach. In addition, lime can react with soils to solidify materials, further reducing the leaching of hazardous wastes. Lime can also be used to neutralize acidic materials.
Under the U.S. EPA's land disposal restrictions regulations, currently generated hazardous wastes that are to be land disposed must be pretreated using the Abest demonstrated available technology. For hazardous wastes containing metals, metals stabilization or metals precipitation is frequently required, and lime is identified by EPA as suitable to treat these wastes (see 40 C.F.R. Part 268.42).
EPA also endorses lime stabilization as a key technology for hazardous waste site cleanups (see, e.g., Handbook for Stabilization/Solidification of Hazardous Wastes (EPA/540/2-86/001, June 1986). In 1997, for example, EPA announced a proposed cleanup plan as part of the Anaconda Regional Water, Waste, and Soils Project for 14,000 acres in Anaconda, Montana. A key element of the plan is to treat arsenic-containing soils with lime and organics. Copper mining created environmental contamination in the 300 square mile area and concern about potential human exposures; EPA recommended in-situ lime treatment over the option of excavating and treating the tailings and contaminated groundwater.
Solidification/Stabilization (S/S) is a treatment technology for contaminated soils, either for clean up/remediation alone or as part of a brownfield redevelopment. Portland cement, often augmented with other materials, such as fly ash, cement kiln dust, and lime, is used as a binding reagent in S/S because of its ability to both solidify - change the physical properties - and stabilize - change the chemical properties - of a wide range of hazardous materials. Solidification increases the compressive strength, decreases the permeability, and encapsulates toxic elements. Stabilization converts hazardous elements into less soluble, mobile or toxic forms. Mixing the right combination of binding reagents into contaminated soils allows them to be both excavated and disposed of in a landfill, or re-used on site to support redevelopment. The solidification treatment has the further benefit of improving the structural properties of the site as well.
S/S is used to remediate or reclaim a contaminated site. Specific treatment effects include:
Solidification/Stabilization is identified as a Best Demonstrated Available Technology for 57 of the 500 specific waste materials listed under RCRA (Resource Conservation and Recovery Act). In addition to "listed" wastes, RCRA identifies "characteristic" wastes, which are less common, so not listed, but have some characteristic (e.g. ignitable, explosive/reactive, corrosive), that is cause for concern and special handling. S/S can often be used to eliminate this characteristic, thus reducing disposal costs or enabling re-use. One of its great strengths as a treatment technology is the ability to handle so many different chemicals. The physical and chemical properties of cement make it particularly suited to solidification and stabilization of hazardous materials because it is multifaceted in the way it reacts with other materials, either binding them - locking up free liquids and organic contaminants; or encapsulating them or chemically transforming them - in the case of heavy metals and other inorganics.
The improved compressive strength of this type of soil treatment versus other treatment methods can serve to improve the site conditions for development in addition to treating the contamination.
While both clean up and reclamation contribute to environmental stewardship, reclaiming contaminated land for redevelopment is particularly key to a sustainable development strategy. Concerns about liability and cost for dealing with contaminated or even potentially contaminated land is a deterrent to in-fill projects and helps drive developers to 'Greenfield' or previously undeveloped land. This contributes to sprawl, where development density is too low and spread out to support the water, sewer and transportation infrastructure needed to service it. Sprawl permanently converts agricultural or forest land on an ever-expanding urban fringe to development. By facilitating the reclamation of previously used sites, S/S helps clean up and rejuvenate 'wasted' urban land. Using 'brownfield' sites may contribute to LEED Credit, as well.