U.S. EPA Contaminated Site Cleanup Information (CLU-IN)

U.S. Environmental Protection Agency
U.S. EPA Technology Innovation and Field Services Division

Dense Nonaqueous Phase Liquids (DNAPLs)

Treatment Technologies


The term "solidification/stabilization" (S/S) refers to a general category of processes used to treat a wide variety of wastes, including solids and liquids. Solidification and stabilization are each distinct technologies. A layman's description of the technology is available in A Citizen's Guide to Solidification/StabilizationAdobe PDF Logo.

  • Solidification refers to processes that encapsulate a waste to form a solid material and to restrict contaminant migration by decreasing the surface area exposed to leaching and/or by coating the waste with low-permeability materials. Solidification can be accomplished by mechanical processes or by a chemical reaction between a waste and binding (solidifying) reagents, such as cement, kiln dust, or lime/fly ash. Solidification of fine waste particles is referred to as microencapsulation, while solidification of a large block or container of waste is referred to as macroencapsulation.
  • Stabilization refers to processes that involve chemical reactions that reduce the leachability of a waste. Stabilization chemically immobilizes hazardous materials or reduces their solubility through a chemical reaction. The physical nature of the waste may or may not be changed by this process (U.S. EPA 2000).

S/S processes can be implemented either in situ or ex situ. In situ stabilization typically involves the addition of binding agents to an area of sludge or soils and addition of water where necessary, followed by repeated in-place mixing with the bucket of a back or track hoe to mix and stabilize the sludges or soils in place. A growing method of in situ S/S involves the use of very large flighted rotary augers, 6 to 8 or more feet in diameter, capable of injecting slurry chemicals and water through the auger flights. The auger bores and mixes a large-diameter "plug" of the contaminated material. During augering, stabilization chemicals and water (if needed) are injected into the soils. After thorough mixing, the auger is removed and the setting slurry is left in place. The auger is advanced to overlap the last plug slightly, and the process is repeated until the contaminated area is completed (USACE 2003).

Ex situ S/S field processes involve excavation and staging of the solids, screening to remove materials too large in diameter to be treated effectively (usually 2 inches in diameter or greater), blending the binding agents and water, when appropriate, with solids (typically in a pug mill), and stockpiling treated solids for testing prior to shipment off site or placement back in the excavation. Ex situ S/S processing can be accomplished in drums, in a fixed plant, or in a mobile plant. A significant consideration in applying the ex situ technology is the "swell factor" in the solid volume created by the binding agent; this factor depends on the amount of reagents that must be added and can approach 50% in some cases. Due to the swell factor, it is possible that the treated material will not fit in the excavation from which it was removed without altering the natural grade (USACE 2003).

S/S treatment can result in monolithic-formed chunks or blocks or in a soil-like matrix. The method is most effective on metals and inorganic contaminants, and less effective with increasing concentrations of organic contaminants. Stabilization of heavy metals is mainly achieved by converting the heavy metals into insoluble precipitates, whereas organics usually are sorbed or encapsulated in the pores, with leachability depending on the solubility of a compound in water and its diffusivity through the waste matrix. Generally, hydrophobic organic compounds do not react with the inorganic binders and may interfere with the hydration reactions of cement or pozzolanic materials and inhibit the setting of cement (Paria and Yuet 2006).

As conventional S/S does not remove or destroy the contaminants present, the selection of binders must address (1) compatibility between the binders and the materials being treated, (2) the presence of chemicals that interfere with the setting and durability of the product, and (3) anticipated ground and groundwater conditions over the long term. Because of the variable nature of contaminated soil encountered, bench-scale testing to evaluate the effectiveness of potential binder systems is an essential prerequisite to S/S in the field. The nature of contaminants may vary across a site requiring remediation, which means that more than one binder formulation may be required for use during an S/S operation. Furthermore, the effects of otherwise unforeseen contaminant/binder interactions can be identified during treatability studies (Bone et al. 2004).

An extensive review of S/S technology was completed in 2004 for the UK's Environment Agency by Bone et al. S/S treatment of inorganic contaminants has been practiced for decades and is supported by many studies, but far less information is available on the use of S/S with organic compounds. Currently, cement-based S/S of organic contaminants can be classified into three categories: direct immobilization of organic contaminants, immobilization of organic contaminants after adsorption, and immobilization of organic contaminants using oxidizing/reducing agents (Paria and Yuet 2006).

In general, organic compounds have a strong effect on the microstructure of the cement paste. The structure and nature of the organic molecules are responsible for the microstructure characteristic. Almost all organic compounds are retarders in cement setting, and many organic acids that strongly chelate calcium also have strong retarding effect. Organic compounds retard the cement setting process by forming a protective layer around the cement grain, thus hindering the formation of calcium hydroxide. Although a variety of fixation technologies have been developed for ex situ S/S treatment (e.g., asphalt batching, polyethylene encapsulation, bituminization), cement-based processes are most commonly used in both ex situ and in situ S/S applications, and many different additives and proprietary blends are being used to improve performance and reduce cost with specific waste streams (Paria and Yuet 2006).

The efficiency of S/S treatment of organic contaminants can be improved by using adsorbents for the organic components. The adsorbents can be incorporated as additives in the cement mix or used as a pretreatment prior to conventional cement-based solidification. Many of these additives are waste products of industrial processes. Additives such as activated carbon, shredded tire particles, and organoclays (sorbents) can increase the chemical containment of the contaminant. Additives such as silica fume and fly ash can improve the physical containment of organic compounds by reducing waste form porosity and permeability.

Activated carbon is commonly used in remediation of organics and trapping many heavy metals. The use of activated carbon as a pretreatment adsorbent in S/S treatment has not been widely reported, perhaps due to high costs, but the use of activated carbon in S/S technology likely will increase if it can be made more cost effective, as might be achieved through the use of regenerated activated carbon. Bates et al. (2002) reported the results of S/S treatment of organic contaminants—creosote, dioxins, and pentachlorophenol (PCP)—using cement formulations containing activated carbon or other proprietary reagents at the American Creosote site in Jackson, TN. The case study results show that the approach successfully reduced the concentration of organics to a target level in the leachate.

Organophilic clays can act as successful adsorbents for organic contaminants and enable them to be treated by cement-based solidification. Organoclay is usually a smectite clay (e.g., bentonite, hectorite) that has been modified to be hydrophobic and to have an affinity for non-soluble organics (Reible 2005). Modifying the clays by exchanging the naturally occurring cations (e.g., Na+, K+) with organic cations greatly improves sorption capacity when compared to unmodified clays. The effectiveness of organophilic clays in immobilizing organic contaminants has been found to be inversely related to the water solubility of the contaminant because organic molecules adsorb on the organophilic clay surface through hydrophobic attraction, which is more favorable when the compound is less water soluble (Paria and Yuet 2006). In a UK cleanup conducted at West Drayton, soil contaminated with high levels of hydrocarbons was treated successfully on a commercial scale using cement containing organophilic clay additives (Al-Tabbaa et al. 2003).

Immobilization of organic compounds in a cement matrix, with or without adsorbent, is mainly a result of physical entrapment. For better long-term effectiveness, a more desirable process would be to transform the organic wastes to less hazardous hydrocarbons. Degradative S/S is a novel remediation technology that combines the immobilization and degradation of contaminants. Cement slurries containing Fe(II) have been tested on PCE, effectively reducing the chlorinated compound to non-chlorinated byproducts. In such a system, the contaminants can be retained in the system until enough time has elapsed for degradation to occur, thereby preventing any environmental releases (Hwang and Batchelor 2000).

Limited data are available on long-term performance of S/S at hazardous waste sites. The long-term environment and conditions to which the solidified waste is exposed can affect the stability of the treated waste. Cement-based stabilized wastes are vulnerable to the same physical and chemical degradation processes as concrete and other cement-based materials; that is, they have the potential to disintegrate over a period of 50 to 100 years (U.S. EPA 2000).

EPA's eleventh status report of treatment technologies used at Superfund sites (2004) stated that S/S was implemented at 24% of Superfund sites to address metals (174 sites) and organics (129 sites) contamination. Eighty-six of the organics-contaminated sites contained volatile or semivolatile halogenated compounds, polycyclic aromatics, and/or pesticides and herbicides. In remedial actions where S/S provided source control treatment, ex situ S/S was used at 157 sites and in situ S/S at 48 sites (EPA 2004).

Al-Tabbaa, A. et al. 2003. Measured and predicted five-year behavior of soil-mixed stabilized/solidified contaminated ground. Grouting and Ground Treatment. ASCE, Geotechnical Special Publication No 120, ISBN: 0-7844-0663-4, p 610-621.

Bates, E.R., E. Sahle-Demessie, and D.W. Grosse. 2000. Solidification/stabilization for remediation of wood preserving sites: treatment for dioxins, PCP, creosote, and metals. Remediation Journal, Vol 10 No 3, p 51-65.

Adobe PDF LogoB.D. Bone, et al. 2004. Review of Scientific Literature on the Use of Stabilisation/Solidification for the Treatment of Contaminated Soil, Solid Waste, and Sludges. Environment Agency, UK, Science Report SC980003/SR2.

Hwang, I. and B. Batchelor. 2000. Reductive dechlorination of tetrachloroethylene by Fe(II) in cement slurries. Environmental Science & Technology, Vol 34 No 23, p 5017-5022.

Adobe PDF LogoParia, S. and P.K. Yuet. 2006. Solidification/stabilization of organic and inorganic contaminants using portland cement: a literature review. Environmental Reviews, Vol 14 No 4, p 217-255.

Adobe PDF LogoReible, D.D. 2005. McCormick and Baxter Creosoting Company Portland, Oregon: Organoclay Laboratory Study. Oregon DEQ.

U.S. Army Corps of Engineers (USACE). 2003. Safety and Health Aspects of HTRW Remediation Technologies. EM 1110-1-4007, p 4-1 - 4-12.

Adobe PDF LogoU.S. EPA. 2000. Solidification/Stabilization Use at Superfund Sites. EPA 542-R-00-010.

Adobe PDF LogoU.S. EPA. 2004. Treatment Technologies for Site Cleanup: Annual Status Report (Eleventh Edition), EPA 542-R-03-009.

General Resources

Development of Performance Specifications for Solidification/Stabilization
Interstate Technology & Regulatory Council (ITRC) Solidification/Stabilization Team.
S/S-1, 162 pp, July 2011

Interstate Technology & Regulatory Council (ITRC) Solidification/Stabilization Team. S/S-1, 162 pp, July 2011

Adobe PDF LogoEngineering and Design: Treatability Studies for Solidification/Stabilization of Contaminated Material
U.S. Army Corps of Engineers. ETL 1110-1-158, 38 pp, 1995

Furnishes information and guidance on scoping a treatability study for S/S of contaminated material.

Adobe PDF LogoEngineering Bulletin: Solidification/Stabilization of Organics and Inorganics
L. Fink and G. Wahl.
EPA 540-S-92-015, 12 pp, 1993

Describes the technology and provides information on S/S applicability, limitations, residuals produced, site requirements, and process performance data. Emphasizes that site-specific treatability studies are the only means of determining the applicability and performance of a particular S/S treatment.

Adobe PDF LogoFlow Injection Determination of Chemical Oxygen Demand in Leaching Liquid
B. Vallejo-Pecharroman, A. Izquierdo-Reina, and M. D. Luque de Castro.
Analyst, Vol 124, p 1261-1264, 1999

In a study of a method for the continuous and rapid determination of the chemical oxygen demand (COD) in leaching liquid by flow injection, metal-containing residues and an oily mud were treated with different stabilizers (lime, plaster, portland cement. and Depocrete OS/3) and then leached. Lime and Depocrete OS/3 (a type of cement that cures very rapidly) acted as effective stabilizers for residues containing high concentrations of organic compounds as they reduced the levels of organic compounds, thus favoring the decrease of COD to levels lower than 148 mg COD/L. Portland cement and plaster were not effective for organic compounds since the COD of the leached liquids was higher than 160 mg/L, and after stabilization, the latter residues were still considered hazardous.

Adobe PDF LogoGuidance on the Use of Stabilisation/Solidification for the Treatment of Contaminated Soil
B.D. Bone, L.H. Barnard, and C.D. Hills.
Environment Agency, UK, Science Report SC980003/SR1, 103 pp, 2004

Examines the use of hydraulic binder systems that are usually designed to provide both leaching and physical improvements to the contaminated material. Strongly recommends treatability studies (bench-scale tests) to evaluate and validate the performance of the mixes for a particular site.

Adobe PDF LogoHandbook for Stabilization/Solidification of Hazardous Waste
M.J. Cullinane, Jr., L.W. Jones, and P.G. Malone.
EPA 540-2-86-001, 166 pp, 1986

Provides guidance in assessing the feasibility of using S/S at a site and describes reagents and ex situ methodologies.

Importance of Microscopy in Durability Studies of Solidified and Stabilized Contaminated Soils
I. Klich, L.P. Wilding, L.R. Drees, and E.R. Landa.
Soil Science Society of America Journal, Vol 63, p 1274-1283, 1999

EPA recognizes S/S as a best demonstrated available technology for the containment of contaminated soils and other hazardous wastes that cannot be destroyed economically by chemical, thermal, or biological means. Unfortunately, published data to verify the performance and durability of landfilled treated wastes over time are rare, although research shows that the same environmental concerns that affect the durability of concrete must be considered when evaluating the durability and permanence of cement-based S/S of contaminated soils. Evaluations should not be based on leaching and chemical analyses alone; the use of all levels of microscopic analyses should be incorporated into studies of the long-term performance of S/S technologies. View longer abstract

Adobe PDF LogoPrinciples and Use of Solidification/Stabilization Treatment for Organic Hazardous Constituents in Soil, Sediment, and Waste
C.M. Wilk.
Waste Management '07 Conference, 25 February-1 March 2007, Tucson, Arizona. 10 pp, 2007

Discusses the chemical and physical mechanisms that can immobilize inorganic and organic hazardous constituents within S/S-treated material and includes six brief examples of full-scale projects where in situ or ex situ S/S has been used successfully to treat soil and sediment contaminated with hazardous substances, such as metals, radionuclides, PCBs, refinery sludge, and creosote.

Adobe PDF LogoReview of Scientific Literature on the Use of Stabilisation/Solidification for the Treatment of Contaminated Soil, Solid Waste, and Sludges
B.D. Bone, L.H. Barnard, D.I. Boardman, P.J. Carey, C.D. Hills, H.M. Jones, C.L. MacLeod, and M. Tyrer.
Environment Agency, UK, Science Report SC980003/SR2, 343 pp, 2004

Provides a reference resource to the "Guidance on the Use of Stabilisation/Solidification for the Treatment of Contaminated Soil" (Environment Agency 2004), which it accompanies.

Adobe PDF LogoSolidification/Stabilization of Organic and Inorganic Contaminants Using Portland Cement: A Literature Review
S. Paria and P.K. Yuet.
Environmental Reviews, Vol 14 No 4, p 217-255, 2006

Surveys the current knowledge in cement S/S, focusing on cement chemistry, the effects of inorganic and organic compounds on cement hydration, and the mechanisms of immobilization of different organic and inorganic compounds. For treating organic contaminants, the use of adsorbents such as organophilic clay and activated carbon, either as a pretreatment or as additives in the cement mix, can improve contaminant immobilization in the waste form. The concept of degradative S/S, which combines chemical degradation with conventional S/S, shows promise, although further study is required to assess its technical and economic feasibility.

Adobe PDF LogoSolidification/Stabilization Use at Superfund Sites
U.S. EPA, Technology Innovation Office.
EPA 542-R-00-002, 23 pp, 2000

Reviews and analyzes trends in use, types of applications, performance, and costs of S/S implementation at Superfund sites.

Adobe PDF LogoStabilization/Solidification of CERCLA and RCRA Wastes: Physical Tests, Chemical Testing Procedures, Technology Screening and Field Activities
M.M. Arozarena, et al.
EPA 625-6-89-022, 79 pp, 1989

Discusses in depth the physical testing methods used to characterize solid and hazardous wastes before and after S/S and contains an overview of leaching mechanisms, leach test methods and applications, factors affecting results, and selection and interpretation of leach tests.

State of Practice Reports, UK Stabilisation/Solidification Treatment and Remediation

STARNET is a network program funded by the UK's Engineering and Physical Sciences Research Council. STARNET sponsored seven S/S state-of-practice reports published between 2002 and 2005. The reports identify knowledge gaps and future research needs in the practice of stabilizing and solidifying hazardous materials. The STARNET site also hosts the presentations from the International Conference on Stabilisation/Solidification Treatment and Remediation, held April 12-13, 2005, at Cambridge University, UK.

Adobe PDF LogoBinders & Technologies — Part I: Basic Principles (2002)
A. Al-Tabbaa and A.S.R. Perera.

Binders & Technologies — Part II: Research (2002)
A. Al-Tabbaa and A.S.R. Perera.

Binders & Technologies — Part III: Applications (2002)
A. Al-Tabbaa and A.S.R. Perera.

Part IV: Testing & Performance Criteria (2004)
A.S.R. Perera, A. Al-Tabbaa, J.M. Reid, and J.A. Stegemann.

Part V: Long-Term Performance and Environmental Impact (2004)
A.S.R. Perera, A. Al-Tabbaa, J.M. Reid, and D. Johnson.

Part VI: Quality Assurance and Quality Control (2004)
A.S.R. Perera, A. Al-Tabbaa, and D. Johnson.

Part VII: Good Practice Guidance Documents (2005)
A.S.R. Perera, A. Al-Tabbaa, and D. Johnson.

Abstracts of Journal Articles

Evaluation of Organics Leaching from Solidified/Stabilized Hazardous Wastes Using a Powder Reactivated Carbon Additive
P. Gong and P.L. Bishop.
Environmental Technology, Vol 24 No 4, p 445-455, 2003

Activated carbon has been found to be the best additive for immobilizing organic contaminants, but it is generally too expensive for routine use. A study of the use of powdered reactivated carbon to prevent organics from leaching from solidified/stabilized waste forms showed that 1% reactivated carbon addition was generally enough to reduce leaching of all of the organic contaminants (phenol, 2-chlorophenol, chlorobenzene, aniline, and methyl ethyl ketone) by more than 70%, while a 2% addition reduced leaching for most of the organics to less than 1%.

Measured and Predicted Five-Year Behavior of Soil-Mixed Stabilized/Solidified Contaminated Ground
A. Al-Tabbaa, B. Chitambira, R. Perera, and N. Boes
Grouting and Ground Treatment. ASCE, ISBN: 0-7844-0663-4, p 610-621, 2003

In 1995 at a site contaminated with metals and organic compounds in West Drayton near Heathrow Airport in the UK, contractors used cement-based grouts and a soil mixing auger to implement in situ S/S. The authors describe the 5-year performance of three soil grout mixes at this site and discuss the development of accelerated ageing methods—elevated temperatures and accelerated carbonation—to simulate longer-term performance.

Stabilization of Chloro-Organics Using Organophilic Bentonite in a Cement-Blast Furnace Slag Matrix
R. Cioffi, L. Maffucci, L. Santoro, and F.P. Glasser.
Waste Management, Vol 21 No 7, p 651-660, 2001

The use of organophilic clays as pre-solidification adsorbents of organic compounds can reduce the adverse effect of organics on cement hydration because of the high adsorption power of the clays and their compatibility with the cementitious matrix. This paper presents an investigation of the effect on hydration kinetics, physico-mechanical properties and leaching behavior of cement-based solidified waste forms containing 2-chlorophenol and 1-chloronaphthalene adsorbed on organophilic bentonites. The effects of the two organic contaminants are differentiated.

Stabilization/Solidification of Wastewater Treatment Sludge
V. Bednarik, M. Vondruska, M. Sild, and M. Koutny.
Journal of Environmental Engineering, Vol 130, p 1527-1533, 2004

A verified procedure for S/S of industrial wastewater treatment sludge consists of sludge solidification by fluidized bed combustion ash with an addition of highly porous carbon black as an adsorption additive and subsequent creation of a coating on the surface of the solidified sludge with an aqueous asphalt emulsion. The carbon black addition simultaneously reduces the binder dosage needed to solidify the sludge and the leachability of organic compounds. The creation of an asphalt coating effectively prevents leaching of all observed pollutants.

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