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.

• Solidification refers to processes that encapsulate a waste to form a solid material and/or coat the waste with low-permeability materials to restrict contaminant migration by decreasing the surface area exposed to leaching. 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 (EPA 2000). The desired changes usually include an increase of the compressive strength, a decrease of permeability, and encapsulation of hazardous constituents (Wilk 2007).
• 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. This process may or may not change the physical nature of the waste (EPA 2000). The desired changes for stabilization include converting contaminants into a less soluble, mobile, or toxic form (Wilk 2007).

Treatment reagents often both solidify and stabilize the contaminant matrix; hence, this treatment technology is frequently referred to as a solidification/stabilization process. For example, a treatment reagent such as cement can reduce the mobility of many metal contaminants by forming insoluble hydroxides, carbonates, and silicates with them (stabilization) as well as providing a solid encapsulation matrix to reduce leaching (solidification) (Wilk 2007). Also, in some S/S applications, a primarily stabilization reagent such as phosphate or organoclay can be used to enhance the ability of the binder to encapsulate the contaminants.

EPA's 2010 Superfund Remedy Report (thirteenth edition) of treatment technologies used at Superfund sites states that, based on project data, ex situ S/S was used in 170 projects and in situ S/S in 41 projects for source control over the period 1982-2004. An additional 33 ex situ and 15 in situ S/S actions were identified in 2005-2008 decision documents. A number of the ex situ S/S actions at National Priorities List (NPL) sites were conducted to stabilize contaminated soil prior to off-site disposal at a RCRA Subtitle D facility.

EPA's 2007 annual status report, Treatment Technologies for Site Cleanup (twelfth edition), breaks down the 207 S/S source treatment projects conducted during the period FY 1982-2005 by contaminant class treated: metals were treated in 180 projects, polycyclic aromatic hydrocarbons and other non-halogenated semivolatile organics in 35 projects, organic pesticides in 16 projects, PCBs in 35 projects, and other organic chemicals in 53 projects. Some cleanups addressed multiple contaminant types and the status report does not indicate whether they were primary or secondary targets of the S/S remedy.

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Contaminant Treatment

S/S contaminant treatment requires careful consideration of binders and additives. Binders, which generally have both stabilization and solidification capabilities, can be divided into inorganic or organic based materials. The inorganic binders are cementitious with the most common being Portland cement and pozzolans. Organic binders can be based on polymer, asphalt, or bitumen materials.

While these binders are often used as stand alone materials their performance can often be improved by using an additive material. For example phosphate, a stabilization agent, can be mixed with a soil or sludge prior to mixing with a cementitious binder to improve the stability of some metal contaminants through the formation of metal phosphate complexes. The low solubility phosphate complexes are then encapsulated when the cementitious binder sets up. Use of organoclays and activated charcoal additives can improve contaminant immobilization of some organic contaminants as they act as stabilization reagents either in a pretreatment step or by direct mixing with the cementitous binder. The organic compounds sorb to the organoclays or activated charcoal and are encapsulated by the binder.

Cementitious 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. Without additives, organics usually are sorbed or encapsulated in the matrix pores, with leachability depending on the solubility of the 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).

However, depending upon the specific chemical properties cementitious based materials have been used to treat soils/sludges contaminated with semivolatile organic contaminants (ITRC 2011, EPRI 2009, Barnett et al. 2009). While this process might involve some stabilization through sorption, it is primarily an encapsulation process and its success is dependent upon the concentration of the target contaminants in the soil sludge matrix and their solubility in water.

Table 1 provides a summary of the effectiveness of cementitious S/S techniques (with and without additives) on various contaminant groups.

• Binders

  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 can vary across a site requiring remediation, which means that more than one binder formulation might 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).

  The S/S process generally is accomplished using either inorganic or organic (polymer, asphalt, bitumen) binders. The most common inorganic binders are Portland cement, pozzolans (siliceous or aluminous materials that can react with calcium hydroxide to form compounds with cementitious properties), and cement/pozzolan mixtures. While these binders are effective for a range of inorganic cations and anions, a treatability study usually is conducted using on-site soil, contaminants, and groundwater, if applicable (EPA 2006). A discussion of inorganic binder chemistry can be found in Al-Tabbaa and Perera (2002), Bone et al. (2004), and Paria and Yuet (2006).

  Polymer binders are thermoplastic or thermosetting. Thermoplastic binders are materials that can be melted repeatedly to a flow state and harden when cooled. Polyethylene, sulfur polymer, and bitumen are examples of theromoplastic binders. Thermosetting binders are materials that require the combination of several liquid ingredients (e.g., monomer, catalyst, promoter) that, when combined, harden to a solid that cannot be reworked (EPA 1997). Thermoplastic binders operate in a temperature range of 120 to 180�C which could be an issue in soil with high moisture content. Thermosetting binders operate at ambient temperatures, but they are not amenable to high moisture content. While polymer binders are effective, they are usually employed ex situ and can be difficult to use in an in situ setting (EPA 2006).

• Additives

  Lime. Lime as used here refers to quicklime (calcium oxide) or hydrated lime (calcium hydroxide). The addition of lime to clayey soils in appropriate quantities raises soil pH to approximately 12.5, which promotes the dissolution of silica and alumina in clay plates. The reaction products are gels that are analogous to those produced during hydration of Portland cement. The result is a tough, water-insoluble gel that cements the soil particles (Bone et al. 2004); however, cyclic wetting and drying of lime-stabilized soil can result in the breakdown of the cementitious bonds.

  The reaction of quicklime and hydrated lime with clays in cohesive soils results in agglomeration and flocculation of clay particles with a consequent reduction in the plasticity and an increase in shear strength of soils, which facilitates easier soil handling. Pretreatment or conditioning of cohesive soils with lime prior to the addition of other binders is a common practice (Bone et al. 2004).

  Lime can also be used with some fly ashes to form compounds possessing cementitious properties. The stabilization effect of fly ash relies on the formation of calcium silicate gels, which gradually harden over a long period of time to form a stable material. Fly ash-lime products containing waste materials possess favorable leaching characteristics, especially for waste containing heavy metals, where the metal ions may be chemically bound to the hydrate complexes (Al-Tabbaa and Perera 2002). More Information

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Evaluation of quicklime mixing for the remediation of petroleum contaminated soils
  Schifano, V., C. MacLeod, N. Hadlow, and R. Dudeney.
  Journal of Hazardous Materials 141(2):395-409(2007)

  After mixing two different soils with quicklime, researchers determined temperature, pH, moisture content, Atterberg limits, and concentrations of petroleum hydrocarbon compounds in samples extracted from the treated soils. Significant decreases in concentrations of petroleum hydrocarbon compounds were measured in soil and leachate were observed, possibly due to volatilization, degradation, and encapsulation of the hydrocarbon compounds promoted by the mixing. The increase in temperature resulting from the exothermic hydration reaction of quicklime when in contact with porewater helps to volatilize the light compounds but may not be entirely responsible for their concentration decreases or for the decrease of heavy aliphatics and aromatics concentrations.

  Immobilization of lead in shooting range soils by means of cement, quicklime, and phosphate amendments (Abstract)
  Cao, X., D. Dermatas, X. Xu, and G. Shen.
  Environmental Science & Pollution Research International 15(2):120-127(2008)

  The treatment additives were applied at rates from 2.5% to 10% (w/w) to two soils (SR1 and SR2) collected from two shooting ranges. Treatment effectiveness was evaluated by Pb leachability, measured by the TCLP. Cement and quicklime treatments were effective in immobilizing Pb in SR1 soil, with reduction of Pb concentration in TCLP leachate (TCLP-Pb) to below the EPA non-hazardous regulatory limit of 5 mg/L at application rates of > or =5% and 28-d incubation. By contrast, cement and quicklime amendments were less effective for Pb stabilization in SR2 soil because the TCLP-Pb levels in the treated soil remained above 5 mg/L at all application rates, although they fell significantly compared with the untreated soil. Phosphate application was most effective in reducing Pb leaching in both soils. Even at an application rate as low as 5% and 1-d incubation, phosphate could reduce TCLP-Pb below 5 mg/L in both soils. Immobilization of Pb in the SR1 soil amended with cement and quicklime was attributed to the formation of pozzolanic minerals (e.g., calcium silicate hydrate and ettringite) that encapsulated soil Pb. The pozzolanic reaction was limited in the SR2 soil upon the application of cement and quicklime. Reduction of the TCLP-Pb might result from complexation of Pb on the surface of the formed calcite. Phosphate-induced Pb immobilization was mainly attributed to formation of less soluble PbHPO4.

  Managing Arsenic Contaminated Soil, Sediment, and Industrial Waste With Solidification/Stabilization Treatment
  Chattopadhyay, S. and P.M. Randall.
  The AIChE 2005 Annual Meeting, 30 October - 4 November, Cincinnati, OH.

  Two types of S/S technologies—commercially available TerraBond® and Portland cement with addition of ferrous sulfate and lime—were evaluated in 3 different soils for their ability to contain the long-term leaching potential of arsenic after disposal. The TerraBond® process involves physical encapsulation and hydrocarbon coating, while immobilization of arsenic by cement and lime results from the formation of calcium arsenic minerals. The arsenic-contaminated materials were a Montana soil spiked with monosodium acid methanearsonate, a composite sample from La Trinidad (California) mine tailings sediment deposits, and chromated copper arsenate wood-treater waste or door-pit residue from Osmose, Inc. Extended X-ray absorption fine-structure spectroscopic analysis was conducted to identify the differences of As coordination between samples before and after treatment. A sequential extraction test was also conducted to obtain a quantifiable idea of the amount of arsenic present in different phases. Though both treatment methods reduced arsenic mobility, the effectiveness of the treatments varied significantly for different types of contaminated matrix. Sequential extraction results indicate that the chemical nature of the arsenic in the contaminated matrix governs the leachability.

  Mechanical and Leaching Behaviour of Slag-Cement and Lime-Activated Slag
  Kogbara, R.B. and A. Al-Tabbaa.
  Science of the Total Environment 409(11):2325-2335(2011)

  This study investigated the potential of ground granulated blast furnace slag (GGBS) activated by cement and lime for S/S treatment of a mixed contaminated soil. A sandy soil spiked with 3000 mg/kg each of a cocktail of heavy metals (Cd, Ni, Zn, Cu and Pb) and 10,000 mg/kg of diesel was treated with binder blends of one part hydrated lime to four parts GGBS (lime-slag), and one part cement to nine parts GGBS (slag-cement). Three binder dosages, 5, 10 and 20% (m/m) were used, and contaminated soil-cement samples were compacted to their optimum water contents. The effectiveness of the treatment was assessed using unconfined compressive strength (UCS), permeability, and acid neutralization capacity tests with determination of contaminant leachability at the different acid additions. UCS values of up to 800 kPa were recorded at 28 d. The lowest coefficient of permeability recorded was 5x10^-9m/s. With up to 20% binder dosage, the leachability of the contaminants was reduced to meet relevant environmental quality standards and landfill waste acceptance criteria. The pH-dependent leachability of the metals decreased over time. The results show that GGBS activated by cement and lime would be effective in reducing the leachability of contaminants in contaminated soils.

  Mechanisms of Lead Immobilization in Treated Soils
  Dermatas, D., N. Menounou, and X.G. Meng.
  Land Contamination & Reclamation 14(1):43-56(2006)

  Bench-scale study of the leachability and immobilization mechanisms of lead in quicklime- and quicklime/fly ash-treated artificial soils and lead-contaminated field soils.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

  Phosphates. Phosphate treatment involves adding phosphate compounds, which form complexes with metal species present in the matrix. The phosphate-metal complexes have low solubility and immobilize the metals over a wide pH range (EPA 1997). Used in this fashion the phosphates are a stabilization technology. They also can be added to cementitious materials to improve the physical characteristics of the treated waste or as a pretreatment step to a solidification process. At the Peak Oil Superfund site, phosphates were added to the soil to immobilize lead before the soil was solidified (Barnett et al. 2009). More Information

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Chemical Stabilization of Lead in Small Arms Firing Range Soils
  Tardy, B.A., S.L. Larson, and R.M. Bricka.
  ERDC/EL-TR-03-20, 66 pp, 2003

  This document reports on a study of the effectiveness of iron and phosphate chemicals as in situ treatments for reducing lead mobility. Two leaching tests were developed: one test evaluated treatment effectiveness during leaching at the natural soil pH, and the other was conducted at a lower, acidic pH to estimate the long-term effects of lead leaching at small arms firing ranges. The study showed phosphate amendments to be more effective than iron amendments in stabilizing lead in soil.

  Chemical Stabilization: Phosphate and Biosolids Treatment
  Mining Waste Treatment Technology Selection. ITRC, MW-1, 2010

  Chemical stabilization using phosphate for treatment of solid mining wastes as a permanent remedy has proven effective at reducing the mobility of divalent heavy metals both ex situ and in situ. Ex situ treatment has been much more widely used than in situ and usually is used in conjunction with off-site disposal. In situ treatment has been used in mines as a coating on exposed ore surfaces. In situ phosphate treatment has been tested and proven effective but not widely implemented at stabilizing Pb-contaminated soil in residential settings. Chemical phosphate treatments have used a variety of phosphate species, but phosphoric acid has been demonstrated to be the most effective. Organic sources of phosphate such as biosolids or composted animal wastes have also been used to stabilize, reclaim, and revegetate barren mine and mill wastes. The primary mode of action of phosphate treatment chemical stabilization is a chemical reaction with phosphorous and divalent metals, including but not be limited to Pb, Zn, Cd, Cu, Fe, and Al, which form various species of metal phosphates. Metal phosphates, especially pyromorphites, are highly insoluble and stable across a wide range of pH. The stable nature of the compound dramatically reduces mobility and bioavailability of the heavy metals.

  Developmental Study of a Low-pH Magnesium Phosphate Cement for Environmental Applications
  Iyengar, S.R. and A. Al-Tabbaa.
  Environmental Technology 28(12):1387-1401(2007)

  This paper describes work toward the development of a cement-based binder with a low enough pH to facilitate biodegradation in combination with in situ S/S processes. Potassium dihydrogen orthophosphate was selected as the phosphate source, dead-burned magnesia as the magnesium source, and boric acid as the retarder. The range of mixes were tested primarily on their pH development, which was found to be in the range of 6-9.5 for a magnesia-to-phosphate ratio range of 1:1 to 1:5. The testing revealed a dense microstructure, high early-age strength development and low volume expansion of the developed cement. Observed fracturing of some of the cured cement samples has been related to the curing conditions and the impurities present in the magnesia. On the basis of microstructural examination, observed white crystalline deposits on cured samples are likely to be a reaction product of magnesia and potassium dihydrogen phosphate.

  Evaluation of Chemically Bonded Phosphate Ceramics for Mercury Stabilization of a Mixed Synthetic Waste
  Chattopadhyay, S.
  EPA 600-R-03-113, 70 pp, 2003

  Leachability tests of the Chemically Bonded Phosphate Ceramics technology (developed by Argonne National Lab) for Hg-contaminated waste materials were carried out by the constant-pH leaching test, the TCLP, and the TCLP "Cage" modification. X-ray diffraction and spectroscopic techniques, using scanning electron microscope, energy-dispersive spectrophotometer, and wave-dispersive spectrophotometer, were used to identify the solid-state mineral phases. Waste stabilization reduced the leachability of Hg considerably. TCLP results showed that leachability of Hg decreased by a minimum of two orders of magnitude and a maximum of five orders of magnitude. The variation in the decrease in leachability was dependent on the amount and state of Hg in the waste. Wastes containing 70 wt% loading of Hg had leachate concentrations exceeding the 0.2 mg/L treatment standard and therefore did not meet RCRA disposal requirements.

  Immobilization of lead in shooting range soils by means of cement, quicklime, and phosphate amendments (Abstract)
  Cao, X., D. Dermatas, X. Xu, and G. Shen.
  Environmental Science & Pollution Research International 15(2):120-127(2008)

  The treatment additives were applied at rates from 2.5% to 10% (w/w) to two soils (SR1 and SR2) collected from two shooting ranges. Treatment effectiveness was evaluated by Pb leachability, measured by the TCLP. Cement and quicklime treatments were effective in immobilizing Pb in SR1 soil, with reduction of Pb concentration in TCLP leachate (TCLP-Pb) to below the EPA non-hazardous regulatory limit of 5 mg/L at application rates of > or =5% and 28-d incubation. By contrast, cement and quicklime amendments were less effective for Pb stabilization in SR2 soil because the TCLP-Pb levels in the treated soil remained above 5 mg/L at all application rates, although they fell significantly compared with the untreated soil. Phosphate application was most effective in reducing Pb leaching in both soils. Even at an application rate as low as 5% and 1-d incubation, phosphate could reduce TCLP-Pb below 5 mg/L in both soils. Immobilization of Pb in the SR1 soil amended with cement and quicklime was attributed to the formation of pozzolanic minerals (e.g., calcium silicate hydrate and ettringite) that encapsulated soil Pb. The pozzolanic reaction was limited in the SR2 soil upon the application of cement and quicklime. Reduction of the TCLP-Pb might result from complexation of Pb on the surface of the formed calcite. Phosphate-induced Pb immobilization was mainly attributed to formation of less soluble PbHPO4.

  Leaching Behaviour of Magnesium Phosphate Cements Containing High Quantities of Heavy Metals
  Buj, I., J. Torras, M. Rovira, and J. de Pablo.
  Journal of Hazardous Materials 175(1-3):789-794(2010)

  Aside from compliance test results, little information is available about metals retention by magnesium potassium phosphate cements matrices. To investigate, several pastes were prepared by reaction between low grade MgO and KH(2)PO(4) in the presence of different heavy metal nitrate solutions (containing Cd(II), Cr(III), Cu(II), Ni(II), Pb(II) or Zn(II)). In all cases, the initial metal content of the dissolution was 25 g dm(-3) and the oxide-phosphate ratio of the pastes was 50:50 in weight. Four different leaching tests were conducted on magnesium potassium phosphate cement pastes: simple batch test, equilibrium leaching test, availability test, and acid neutralization capacity test. The metal leachate concentration was determined by means of ICP-MS. Metals stabilization was successful in all cases, although the immobilizing system showed a better behavior for Pb(II) and Cr(III) under acidic or neutral conditions.

  PIMS using Apatite II™ How it works to remediate soil & water
  Wright, J., J.L. Conca, K.R. Rice, and B. Murphy.
  Sustainable Range Management 2004: Proceedings of the Conference on Sustainable Range Management, January 5-8, 2004, New Orleans. Battelle Press, Columbus, OH. ISBN: 1-57477-144-2, Paper B4-05, 2004

  Phosphate-Induced Metal Stabilization (PIMS?) using Apatite II? stabilizes a wide range of metals, especially Pb, U, Cd, Zn, Cu and Al, in situ or ex situ, by chemically binding them into new phosphate minerals and other low-solubility phases that are stable over geologic time. The excellent stabilization efficiency comes from the extremely low solubility products (Ksp) of the resultant metal-apatites, e.g., for Pb-apatite (pyromorphite) Ksp ~ 10-80 to Ksp ~ 10-167. Combined with this thermodynamic stability, the rapid kinetics of the metal-phosphate precipitation and adsorption ensures immobilization of metals in the face of most transport mechanisms. Depending upon the metal, the concentration of the metal, and the aqueous chemistry of the system, Apatite II works by four general, non mutually exclusive processes. This technology has been successful with contaminated range soils, groundwaters and wastewaters for Pb, U, Cd, Zn, Al and Cu, and has stabilized between 5-50% of its weight in metals depending upon the metal and the environmental conditions. Costs for range soil remediation are $20-$30/yd³ and costs for water remediation are $40 per 1,000,000 gallons of water per mg/L of metal.

  Preparation and Characterization of Chemically Bonded Phosphate Ceramics (CBPC) for Encapsulation of Harmful Waste
  Ibrahim, W.A., H.A. Sibak, and M.F. Abadir.
  Journal of American Science 7(9)543-548(2011)

  A chemically bonded phosphate ceramic material was developed to encapsulate lead battery waste. The optimum conditions for the preparation of magnesium potassium phosphate compacts were determined and the formation of the final product assessed using XRD. Evaluation of the effect of applied pressure and pressing duration as well as the Mg:K molar ratio on the porosity and permeability of compacts showed that a minimum porosity is achieved by using a molar ratio of Mg:K = 1:1 and that a pressing time of 10 min is sufficient to reach compacts of reasonably low permeability. The compressive strength of compacts was found to increase linearly with curing time and to be much more affected by pressing time duration than by the magnitude of the applied pressure.

  Remediating Soil Lead with Fish Bones
  Freeman, K.S.
  Environmental Health Perspectives 120(1):A20-A21(2012)

  Fish bones are made of the phosphate mineral apatite, which readily combines with Pb to form pyromorphite, a stable crystalline mineral that cannot be absorbed by the human digestive system. EPA is overseeing an Agency project using fish bones to clean up Pb-contaminated soils in the South Prescott neighborhood of Oakland, California. The 2-year, $4-million South Prescott project is part of a growing trend to treat soils contaminated with lead and other heavy metals in place.

  Secondary Waste Form Down Selection Data Package: Ceramicrete
  Cantrell, K.J. and J.H. Westsik, Jr.
  PNNL-20681, 99 pp, 2011

  This report contains a discussion of newer solidification waste forms. The Ceramicrete technology is based on chemical reaction between phosphate anions and metal cations to form a strong, dense, durable, low-porosity matrix that immobilizes hazardous and radioactive contaminants as insoluble phosphates and microencapsulates insoluble radioactive components and other constituents that do not form phosphates.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

  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 enhance 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 (Bone et al. 2004). Some fly ashes also contain unburnt carbon that can bind to organic contaminants.

  Organophilic Clays. Organophilic clays are formed by exchanging naturally occurring cations, such as Na⁺, K⁺, Ca²⁺, and Mg²⁺, in bentonite or montmorillonite clays with organic cations, usually from quaternary ammonium salts bearing long alkyl chains (Paria and Yuet 2006). The quaternary alkylammonium ions are substituted between the clay platelets, resulting in increased spacing and enhanced adsorption of organic contaminants (Paria and Yuet 2006). Organophilic clays can be used alone as a stabilization technology as was done at the McCormick and Baxter Creosoting Company, Portland, Oregon (Reible 2005), or the clays can be used as an additive to cementitious materials to improve organic contaminant treatment. Ideally, the organophilic clays are mixed with the waste first, and then allowed to absorb the organic contaminants prior to the addition of an S/S binder, which is used to encapsulate the material within the monolithic mass (Al-Tabbaa and Perera 2006). The clay particles typically are not involved in cementitious setting reactions but are encapsulated into the treated material (ITRC 2011). Note that the effectiveness of organophilic clays in immobilizing organic contaminants is inversely related to the water solubility of the contaminant, due to the fact that organic molecules adsorb on the organophilic clay surface through hydrophobic attraction, which is more favorable when the compound is more hydrophobic (less water soluble) (Paria and Yuet 2006).

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Mixing Processes

S/S processes can be implemented either in situ or ex situ.

In Situ. In situ S/S 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 backhoe or similar excavator to mix and stabilize the sludges or soils in place. The excavator also can be equipped with a mixing head. In addition, in situ mixing can be accomplished using large, flighted, rotary augers, 6 to 8 or more feet in diameter, that are 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, binders 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). Augers generally are used for deep mixing and can be used to treat soils 60 to 100 feet deep (ITRC 2011 EPA 1997). The addition of binders and additives can increase the volume of the treated soils or sludges. The significance of the increase in volume needs to be considered in both shallow and deep mixing.

Ex Situ. 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 with solids (typically in a pug mill) when appropriate, 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 percent 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). Al-Tabbaa and Perera (2006) provide a detailed discussion of mixing technologies with accompanying photographs of the equipment.

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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 NAPLs have been addressed through S/S treatment.
• 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 upon the site, S/S may be more cost effective than excavation and off-site disposal.

*Adapted from AEPI 1998 and ITRC 2011.

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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 prior to treatment.
• With heterogeneous distribution of contaminants in the subsurface, in situ mixing of waste and binder can result in uneven performance.
• S/S of sensitive areas may inhibit future more comprehensive restoration.
• 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 post-treatment modifications are limited by time for field performance testing and changed properties of treated material.

*Adapted from AEPI 1998 and ITRC 2011.

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General Resources

Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

Cement Stabilization and Solidification (STSO): Review of Techniques and Methods
Maijala, A., J. Forsman, P. Lahtinen, M. Leppaenen, A. Helland, A.-O.H. Roger, and M. Konieczny.
Ramboll Norge AS, Oslo, Norway. Rap001-Id01, 57 pp, 2009

This review presents different types of STSO (i.e., S/S) techniques, mixing methods, and usages. The main techniques are column stabilization, mass stabilization, and layer stabilization. The different techniques serve different purposes, e.g., improving the strength of subsoil and/or preventing leaching of contaminants from soil. Different purposes require different stabilizers or mixtures of stabilizers and mixing technology.

Development of Performance Specifications for Solidification/Stabilization
ITRC, 162 pp, 2011

This document specifically focuses on processes and approaches for the treatment of contaminated materials on site, mixed with inorganic cementitious/pozzolanic reagents (the most common application of S/S) and cured in place to create a solid mass with a reduced potential for leaching and typically a lowered hydraulic conductivity. Therefore, the technology as discussed in this document is based on on-site, in situ S/S applications.

OW-5/55R Area In-Situ Geochemical Stabilization Remediation Pilot Test, Former Koppers Company Inc. Site, Nashua, New Hampshire
New Hampshire Department of Environmental Services, 96 pp, 2014

In situ geochemical stabilization (ISGS) technology comprises the injection of an enhanced permanganate-based reagent (RemOx EC) into NAPL-impacted zones to achieve containment or stabilization and solute flux reduction. Silica-based precipitates are deposited around NAPL ganglia and droplets following reagent injection, which leaves a mineral shell that reduces overall permeability in the treated area, thereby reducing the volumetric flux of upgradient groundwater into and through the impacted area. The oxidation of dissolved-phase constituents also "hardens" or chemically weathers the NAPL as it loses its more labile SVOCs. This report describes the results of an ISGS pilot test conducted November 11-13, 2014, to isolate creosote NAPL in a treatment area ~45 ft by 75 ft.

Regional Report: European Practice of Soil Mixing Technology
Massarsch, K.R. and M. Topolnicki.
International Conference on Deep Mixing: Best Practice and Recent Advances, 23-25 May 2005, Stockholm, Sweden. Report 13, Vol 1:3-10(2005)

The evolution and recent developments of deep and shallow soil mixing methods are presented, covering dry and wet mixing methods.

Review of Scientific Literature on the Use of Stabilisation/Solidification for the Treatment of Contaminated Soil, Solid Waste, and Sludges
Bone, B.D., 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

This is a companion document and reference resource to "Guidance on the Use of Stabilisation/Solidification for the Treatment of Contaminated Soil" (Environment Agency 2004).

Solidification/Stabilization of Organic and Inorganic Contaminants Using Portland Cement: A Literature Review
Paria, S. and P.K. Yuet.
Environmental Reviews 14(4):217-255(2006)

This survey focuses on (1) cement chemistry, (2) the effects of inorganic (heavy metals) and organic compounds on cement hydration, and (3) the mechanisms of immobilization of different organic and inorganic compounds. For heavy metals, cement-based S/S technology has been shown to be effective in immobilizing the contaminants, even without any additives. In applying cement-based S/S 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 solidified/stabilized wastes.

Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes
Spence, R.D. and C. Shi.
Lewis Publishers/CRC Press, Boca Raton, FL. ISBN: 1566704448, 392 pp, 2004

This book provides comprehensive information on waste characterization, waste form design approaches, contaminant transport and leachability, testing methods for stabilized waste forms, case studies, and regulatory considerations. It covers all systems used for stabilization and solidification of wastes, discusses the interactions between contaminants and stabilizing components, and provides guidelines for the selection of bonding materials for stabilization. It also demonstrates how to design a stabilized waste form, covers test methods and protocols for treating wastes and evaluating treatment technology, and includes a section on statistical techniques for generating response surface models for large, complicated applications.

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.

STARNET Reports

Technology Performance Review: Selecting and Using Solidification/Stabilization Treatment for Site Remediation
Barnett, F., S. Lynn, and D. Reisman.
EPA 600-R-09-148, 28 pp, 2009

This Technology Performance Review (TPR) includes a discussion on several sites, and addresses important factors to consider in the selection of S/S treatment. Each S/S case study has a brief project description, regulatory status, S/S treatment process that includes binder materials used, and a summary of the performance data. Estimated treatment costs and maintenance activities are also included when available. Estimated costs must be adjusted for inflation and current material price increases.

Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
Bricka, R.M., A. Marwaha, and G. Fabian.
ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

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Inorganic Contaminants

• General

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Factors Affecting Fixation of Heavy Metals in Solidified/Stabilized Matrix: A Review
  Malviya, R. and R. Chaudhary.
  Journal of Environmental Science & Engineering 52(3):269-276(2010)

  This paper examines the factors that affect fixation of heavy metals in a solidified/stabilized matrix and reviews various aspects related to S/S of different heavy metals and metalloids (Ar, Ba, Cu, Cr, Pb, Zn, Hg).

  Immobilisation of Heavy Metal in Cement-Based Solidification/Stabilisation: A Review
  Chen, Q.Y., M. Tyrer, C.D. Hills, X.M. Yang, and P. Carey.
  Waste Management 29(1):390-403(2009)

  This paper discusses interactions of heavy metals and cement phases in the S/S process and clarifies heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulting from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. Molecular models of calcium silicate hydrate gel are presented.

  Solidification-Stabilization Technique for Metal Bearing Solid Waste from Zinc Industry: A Case Study
  Vyas, A.K.
  2011 International Conference on Environmental and Computer Science: IPCBEE 19(30):151-155(2011)

  S/S was performed on Jarosite waste with different combinations of binder/waste ratio in cement concrete mixed in the ratio of 1:2:4. Sand was replaced by Jarosite at 20, 40, 60, 80, and 100% in fine aggregate. Ordinary Portland cement (OPC), sulfate-resistant Portland cement (SRPC), and their combinations were used as binders in cement concrete mixes. The cement concretes thus prepared were tested for compressive strength and TCLP after 28 days. Results showed that concrete with 40% OPC and 60% SRPC and replacement of 60% of sand by Jarosite waste provided the optimum combination. Leaching tests indicated that the S/S technique was effective in terms of restraining the leaching of Zn, Cd, and Pb by 93, 90, and 87%, respectively.

  Study of the Treatment Effectiveness of a Solidification/Stabilization Process for Waste Bearing Heavy Metals
  Malviya, R. and R. Chaudhary.
  Journal of Material Cycles and Waste Management 6(2):147-152(2004)

  Sludge from a steel processing unit bearing Zn, Pb, Fe, and Mn was solidified with ordinary Portland cement and stabilized in the specimens with a waste/binder ratio range of 0.16 to 4.0. Curing up to 28 days improved the performance as shown by less leaching of heavy metals and greater unconfined compressive strength of the specimen. S/S treatment effectiveness for Pb, Zn, Fe, and Mn was assessed as 89, 95, 74, and 90%, respectively, for an optimum ratio of 4.0 after 28 days of curing.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

• Arsenic

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Managing Arsenic Contaminated Soil, Sediment, and Industrial Waste With Solidification/Stabilization Treatment
  Chattopadhyay, S. and P.M. Randall.
  The AIChE 2005 Annual Meeting, 30 October - 4 November, Cincinnati, OH.

  The authors investigated two types of S/S technologies—TerraBond®, a commercially available technology; and Portland cement with addition of ferrous sulfate and lime--to contain the long-term leaching potential of arsenic after disposal. The results obtained from these studies indicated that while both treatment methods reduced arsenic mobility, the effectiveness of these treatments varied significantly for different types of contaminated matrix. TerraBond® is a process involving physical encapsulation and hydrocarbon coating, while immobilization of arsenic by cement and lime is due to the formation of calcium arsenic minerals. Sequential extraction results indicate that the chemical nature of the arsenic in the contaminated matrix governs the leachability.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

• Chromium

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Hexavalent Chromium Analysis, Reduction and Stabilization in Cement and Concrete
  Lokothwayo, Robson B.
  Master's thesis, University of Witwatersrand, Johannesburg, 142 pp, 2007

  Concrete cubes were made with different cement mixtures, and in some cases, reducing agents were added. Bulk and designed surface leaching experiments that simulated different environmental conditions were devised for leaching of Cr(VI). Parameters like addition of MnO(2), aging of concrete, increase of temperature, UV variation, and decrease in pH enhanced the leachability of Cr(VI). Cement type also affected Cr(VI) stabilization. Attempts to stabilize Cr(VI) included addition of humic acid (ineffective), whereas the addition of ferrous sulfate successfully reduced and stabilized Cr(VI) in concrete.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

• Lead

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Mechanisms of Lead Immobilization in Treated Soils
  Dermatas, D., N. Menounou, and X.G. Meng.
  Land Contamination & Reclamation 14(1):43-56(2006)

  Bench-scale study of the leachability and immobilization mechanisms of lead in quicklime- and quicklime/fly ash-treated artificial soils and lead-contaminated field soils.

  Stabilization/Solidification (S/S) of Pb and W Contaminated Soils Using Type I/II Portland Cement, Silica Fume Cement and Cement Kiln Dust
  Grubb, D.G., D.H. Moon, T. Reilly, M. Chrysochoou, and D. Dermatas.
  Global NEST Journal 11(3):267-282(2009)

  S/S processes were used to immobilize lead (Pb) and tungsten (W) in contaminated soils, the inclusion of W motivated by the use of the new W-based ammunition. Spiked soils were prepared by mixing either kaolinite or montmorillonite with 10% Pb and 1% W (all percentages by dry weight). The type I/II Portland cement (PC), silica fume cement (SFC) and cement kiln dust (CKD) used as S/S agents were added at 5, 10, and 15 % for a curing time of 1-, 7-, and 28-days. TCLP results showed that regardless of clay type and stabilizing agents, the Pb concentrations decreased with increasing pozzolan content. PC and SFC exhibited similar performance depending on the particular sample. The most effective stabilizing agent on Pb leachability was PC. TCLP-W was immobilized below 1 mg/L in every case. The most effective stabilizing agent on the synthetic precipitation leaching procedure (SPLP)-Pb leachability was CKD, although PC and SFC exhibited similar performance depending on the particular sample. The W concentrations in SPLP leachate were very low in most samples, indicating that W could be immobilized upon S/S processes even though W solubility is very high at elevated pH conditions (662.9 mg/L at pH~11), and would remain immobilized under SPLP exposure conditions.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

• Mercury

  Advances in Encapsulation Technologies for the Management of Mercury-Contaminated Hazardous Wastes
  Chattopadhyay, S. and W.E. Condit.
  EPA 600-R-02-081, 46 pp, 2002

  This report summarizes the range of encapsulation materials used in bench-, pilot-, and full-scale applications for Hg-containing waste. Several studies have been completed regarding stabilization by the application of sulfur polymer, chemically bonded phosphate ceramic encapsulation, and polyethylene encapsulation. Other S/S technologies or materials reported in the literature or under development include asphalt, polyester resins, synthetic elastomers, polysiloxane, sol-gels (e.g., polycerams), and Dolocrete™. These encapsulation methods primarily immobilize hazardous wastes by physical means to prevent contact with leaching agents, such as water. The methods may include a stabilization step to fix mercury chemically into a highly insoluble form.

  Advances in Encapsulation Technologies for the Management of Mercury-contaminated Hazardous Wastes
  Randall, P. and S. Chattopadhyay.
  Journal of Hazardous Materials 114(1-3):211-223(2004)

  The focus of this article is on the current state of encapsulation technologies used to immobilize elemental mercury, mercury-contaminated debris, and other mercury-contaminated wastes, soils, sediments, or sludges. It was found that several of these encapsulation methods required a pretreatment or stabilization step to chemically fix mercury into a highly insoluble form prior to encapsulation. The article summarizes performance data from the testing and evaluation of various encapsulated, mercury-contaminated wastes.

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Extended Summary on Pre-Treatment Technologies for Metallic Mercury
  European Commission, Brussels. Reference 07.0307/2009/530302, 14 pp, 2010

  This summary provides the conclusions and recommendations from a review of existing literature and practical approaches concerning S/S and encapsulation of elemental Hg.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

  Treatment Technologies for Mercury in Soil, Waste, and Water
  EPA 542-R-07-003, 133 pp, 2007

  This report contains information on the availability, performance, and cost of eight technologies for the treatment of mercury: (1) soil washing, S/S, vitrification, and thermal methods for soil and waste treatment, and (2) precipitation/coprecipitation, adsorption, membrane filtration, and biological methods for water treatment.

• Research

  An Alternative to Portland Cement for Waste Encapsulation: The Calcium Sulfoaluminate Cement System (Abstract)
  Zhou, Q., N.B. Milestone, and M. Hayes.
  Journal of Hazardous Materials 136(1):120-129(2006)

  The high alkalinity of composite Portland cement (PC)-based systems will corrode reactive metals found in some wastes, releasing hydrogen and forming expansive corrosion products. Calcium sulfoaluminate (CSA) cement combines the economy of cost and low emission of CO(2) with rapid strength gain and compatibility with other construction materials. Hydration provides an internal pore solution where the pH is considerably lower than that of PC. The main hydration product, ettringite, can incorporate ions into its crystal structure, making it an ideal candidate for waste immobilization.

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Cement-Based Solidification/Stabilization of Heavy Metal Contaminated Soils with the Objective of Achieving High Compressive Strength for the Final Matrix
  Ganjidoust, H., A. Hassani, and A.R. Ashkiki.
  Transaction A: Civil Engineering 16(2):107-115(2009)

  In an S/S study of heavy metal-contaminated soils using ordinary pontlandite cement (OPC), the soil-binder ratio of 8% by weight was selected by setting a justification constraint on the solidified matrix. Results of compressive strength, TCLP, X-ray diffraction, and scanning electron microscopy on the solidified soil-cement mixtures indicate that despite an intensive decrease in the compressive strength of solidified soil at early curing ages, the justification constraint set on the preparation of specimens enhanced hydration reactions, and the solidified matrix achieved high compressive strength after 7 curing days. The justification constraint was an observation or a visual control of primary hardness.

  An observation or a visual control of primary hardness.

  Evaluation of Leaching Characteristics and Environmental Compatibility of Solidified/Stabilized Industrial Waste
  Malviya, R. and R. Chaudhary.
  Journal of Material Cycles and Waste Management 8(1):78-87(2006)

  This paper examines the environmental compatibility, the governing leaching mechanism, and mobility patterns in a waste-cement S/S matrix after 21 days of curing. The mixed sludge from a steel pickling facility was stabilized by ordinary Portland cement. Iron was the major leachable metal, along with Pb, Zn, and Mn. To characterize the leaching behavior, the mobility and mechanism of leaching were determined by diffusion leaching tests. In the solidified/stabilized matrix, the mobility of all the elements studied was reduced, and average to low mobility was observed.

  Influence of Commercial and Residual Sorbents and Silicates as Additives on the Stabilisation/Solidification of Organic and Inorganic Industrial Waste
  Coz, A., A. Andres, S. Soriano, J.R. Viguri, M.C. Ruiz, and J.A. Irabien.
  Journal of Hazardous Materials 164(2-3):755-761(2009)

  Foundry sludges from gas wet cleaning treatment contain organic and inorganic compounds and a high content of water can be managed using different S/S technologies prior to land disposal. The authors studied S/S formulations to improve the control of pollutant mobility and sample ecotoxicity using different mixtures of cement or lime as binders and additives (foundry sand, silica fume, sodium silicate, silicic acid, activated carbon and black carbon) to compare results for commercial and residual additives. The best results were obtained with sorbents (activated carbon and black carbon) or sodium silicate. Results indicate that foundry sand ash can be used as replacement in cement products; however, silica fume in the samples with lime and siliceous resin sand as additives yields products that do not meet regulatory requirements.

  Investigation of 4-year-old Stabilised/Solidified and Accelerated Carbonated Contaminated Soil
  Antemir, A., C.D. Hills, P.J. Carey, M.C. Magnie, and A. Polettini.
  Journal of Hazardous Materials 181(1-3):543-555(2010)

  Pilot-scale application of two different S/S techniques was carried out at a former fireworks and low explosives manufacturing site in SE England. Cores and granular samples were recovered from uncovered accelerated carbonated (ACT) and cement-treated soils (S/S) after 4 years to evaluate field performance with time. Microstructural examination and leaching test results indicated that the cement-treated soil was progressively carbonated over time, whereas the mineralogy of the carbonated soil remained essentially unchanged. Distinct microstructures were developed in the two soils. Although Pb, Zn, and Cu leached less from the carbonated soil, these metals were immobilized adequately by both treatments. Geochemical modeling of pH-dependent leaching data suggested that the retention of trace metals resulted from different immobilization mechanisms operating in the two soils examined.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

An observation or a visual control of primary hardness.

• Sediment

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  The Importance of Sulphide Binding for Leaching of Heavy Metals from Contaminated Norwegian Marine Sediments Treated by Stabilization/Solidification
  Sparrevik, M., E. Eek, and R.S. Grini.
  Environmental Technology 30(8):831-840(2009)

  Four cementitious binders were tested on sediments from six different locations. Portland cement, Portland cement with fly ash, industry cement, and sulfate resistant cement were tested as binders. This paper focuses on the leaching of Pb and Cu. After 28 days of curing, reduced leaching of Pb was observed for the mixtures, whereas Cu leaching from Hammerfest sediments increased substantially after stabilization for all cementitious additions. Experimental results highlighted the importance of sulfide binding for immobilizing heavy metals in stabilized marine sediments. For practical S/S application, it is important to assess the leachability of the stabilized material.

  Solidification/Stabilization of Soft River Sediments Using Deep Soil Mixing
  Maher, A., H. Najm, and M. Boile.
  U.S. DOT, Federal Highway Administration, FHWA-NJ-2005-028, 44 pp, 2005

  A demonstration project was conducted to investigate the feasibility of using Cement Deep Soil Mixing (CDSM) to treat highly contaminated sediments similar to those found in New York/New Jersey Harbor. The technique amends sediment in situ with a pozzolanic mixture, such as Portland cement. Investigators assessed the utility and viability of using CDSM for in situ S/S of contaminated river sediment and evaluated (1) the efficacy of the technology to stabilize sediments and associated contaminants, (2) the optimum percentage of pozzolanic additive, (3) the potential for dispersion of sediments during treatment, and (4) the impact that highly organic enrichment might have on the pozzolanic treatment. Measurements showed significant improvement in the shear strength of the mixed sediments following stabilization, and moisture content decreased by 40% within the solidified sediments. Turbidity measured during the mixing operation was found to be only within 120 ft of the mixing location.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

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Organic Contaminants

• General

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Principles 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.

  Stabilization/Solidification of Acid Tars
  Leonard, S.A. and J.A. Stegemann.
  Journal of Environmental Science and Health: A 45(8):978-991(2010)

  A systematic treatability study was conducted of S/S treatment of acid tars (AT). Portland cement (CEM I) was combined with high-carbon fly ash (HCFA), an industrial by-product, as a novel sorbent for organic contaminants. A factorial design experiment was adopted to investigate the effects of organic content, HCFA:AT ratio, percentage CEM I addition, and curing time on response variables including unconfined compressive strength (UCS), hydraulic conductivity, porosity, and leachability-related properties of the S/S products. Acid tar reduced UCS, but strength increase was observed with increased curing time. Increased HCFA addition led to an improvement in hydraulic conductivity. The treated acid tars might find application as controlled low-strength materials, landfill liner, and landfill daily cover.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

• Manufactured Gas Plants

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Equipment and Scale-up Considerations for In-situ Solidification of MGP Sites
  Plante, T., A. Gustafson, M. Guay, and K. Corradino.
  Gasworks Europe: Redevelopment, Site Management and Contaminant Issues of former MGPs and other Tar Oil Polluted Sites. Proceedings of MGP 2008 Conference, 4-6 March 2008, Dresden, Germany. Technical Univ. of Dresden, ISBN 978-3-934253-48-3, 191-199(2008)

  Important considerations for using in situ S/S at MGP sites include water content of soils, coal tar saturations and distributions, site heterogeneity of geologic strata, surface and subsurface infrastructure and debris, and proposed site reuse. This paper discusses the factors that affect S/S implementation at MGP sites, as well as scaleup issues commonly encountered when transitioning from lab treatability studies to the field. Also described is construction equipment used to implement S/S for various field conditions, including single-auger mixing, patented rake injectors, high-speed rotating mixing devices, and excavators.

  Evaluation of the Effectiveness of In-Situ Solidification/Stabilization at the Columbus, Georgia Manufactured Gas Plant Site
  Electric Power Research Institute (EPRI), 56 pp, 2003

  This study evaluates the long-term effectiveness of the past use of in situ S/S at a former MGP site. After 10 years, the structural integrity and geochemical nature of the solidified mass continue to exceed the original performance standards established prior to implementation of S/S. These data reveal no evidence that the long-term future integrity of the site would be less stable than current site conditions.

  An Integrated Approach to Evaluating In-Situ Solidification/Stabilization of Coal Tar Impacted Soils
  Electric Power Research Institute (EPRI), 66 pp, 2009

  This report presents an integrated approach to the assessment of the suitability and performance of in situ S/S for coal-tar contaminated soils at former MGP sites.

  Leaching Assessment Methods for the Evaluation of the Effectiveness of In Situ Stabilization of Soil Material at Manufactured Gas Plant Sites
  Electric Power Research Institute (EPRI), 144 pp, 2009

  An overview and status report on an EPRI project to develop an alternative leaching assessment methodology for estimating the release of PAHs from soils treated in situ with S/S.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

• Research

  Amending Soils with Phosphate as Means to Mitigate Soil Lead Hazard: A Critical Review of the State of the Science (Abstract)
  Scheckel , G.L. Diamond , M.F. Burgess , J.M. Klotzbach , M. Maddaloni , B.W. Miller, C.R. Partridge, and S.M. Serda.
  Journal of Toxicology and Environmental Health, Part B: Critical Reviews 16(6):337-380(2013)

  Phosphate amendments have been studied as a means to mitigate risks from exposure to Pb in soil by promoting the formation of highly insoluble Pb species, such as pyromorphite. The formation of insoluble Pb species thereby reduces the risk of Pb leaching through soils into drinking waters and absorption by soil biota, and may make it less bioavailable during physiological transport in the human gastrointestinal tract following incidental ingestion. This paper provides a detailed description of phosphate chemistry and the goal of converting Pb into pyromorphite. Slides by Scheckel et al. (2013)

  Kinetics of Transformation of 1,1,1-trichloroethane by Fe(II) in Cement Slurries
  Jung, B. and B. Batchelor.
  Journal of Hazardous Materials 163(2-3):1315-1321(2009)

  Evaluation of the effects of Fe(II) dose, pH, and initial concentration of 1,1,1-TCA on the kinetics of its degradation showed that degradation of 1,1,1-TCA in iron-containing Portland cement slurries was very rapid and could be described by a pseudo-first-order rate law. When Fe(II) dose ranged from 4.9 to 39.2 mM, the half-lives for 1,1,1-TCA were measured between 0.4 and 5 h. The pseudo-first-order rate constant increased with pH to a maximum near pH 12.5. A saturation rate equation was able to predict degradation kinetics over a wide range of target organic concentrations and at higher Fe(II) doses. The major transformation product of 1,1,1-TCA in mixtures of Fe(II) and cement was 1,1-DCA, which indicates that degradation occurred by a hydrogenolysis pathway. A small amount of ethane was observed. The conversion of 1,1,1-TCA to ethane was better described by a parallel reaction model than by a consecutive reaction model.

  Proposal of a Sequential Treatment Methodology for the Safe Reuse of Oil Sludge-contaminated Soil
  Mater, L., R.M. Sperb, L.A.S. Madureira, A.P. Rosin, A.X.R. Correa, and C.M. Radetski.
  Journal of Hazardous Materials 136(3):967-971(2006)

  Sequential steps—a Fenton-type reaction, stabilization, and solidification—were implemented to treat and immobilize oil constituents of a soil containing oil sludge. An 80-h oxidative treatment period was carried out under three different pH conditions, and then the oxidized contaminated sample was stabilized for 2 h with clay and lime, followed by solidification with sand and Portland cement. The Fenton process achieved only partial degradation of oil contaminants in the soil; residual concentrations were found for the PAH and BTEX compounds. Subsequent clay/lime stabilization followed by Portland cement solidification efficiently immobilized the recalcitrant and hazardous constituents of the contaminated soil. The investigators found that 2-step S/S processes can minimize leachability and render the final product economically profitable in that the treated waste is safe enough to be used in environmental applications, such as roadbed blocks.

  Stabilization/Solidification of Petroleum Drill Cuttings
  Leonard, S.A. and J.A. Stegemann.
  Journal of Hazardous Materials 174(1-3):463-472(2010)

  A systematic treatability study was conducted for the treatment of drill cuttings by S/S with Portland cement (CEM I), with the addition of high-carbon power-plant fly ash (HCFA) as a novel sorbent for organic contaminants. The study investigated the effects of waste-to-binder ratio, binder formulation, and curing time on response variables, including unconfined compressive strength (UCS), hydraulic conductivity, porosity, leachate pH, and acid neutralization capacity (ANC) of the S/S products. All factors had significant effects on the properties of the S/S products. Drill cuttings and HCFA addition both reduced UCS, but HCFA improved hydraulic conductivity, relative to CEM I-only S/S products. Drill cuttings addition had little effect on the ANC of products prepared with CEM I only, and improved that of products containing HCFA. Performance criteria adapted from regulatory and other guidance indicate that the S/S products could find application as controlled low-strength materials, landfill liner, and landfill daily cover.

  Stabilization/Solidification of Petroleum Drill Cuttings: Leaching Studies
  Leonard, S.A. and J.A. Stegemann.
  Journal of Hazardous Materials 174(1-3):484-491(2010)

  This work explores the effectiveness of Portland cement (CEM I) with the addition of high-carbon fly ash (HCFA) as a novel binder for the improvement of leachability-related properties of S/S petroleum drill cuttings. In an investigation of the effects of waste-to-binder ratio, HCFA addition, and curing time on leachate pH; acid neutralization capacity (ANC); and metal, chloride and hydrocarbon leaching, the leachate pH and ANC of all products suggested successful formation of a calcium-silicate-hydrate-based matrix with good resistance to acid attack, and little detrimental effect from drill cuttings addition. Leaching of amphoteric metals was significantly affected by pH. All studied factors also affected leaching of chloride and hydrocarbons. CEM I, without HCFA addition, was more effective in immobilizing chlorides, but the overall chloride immobilization was poor in all runs. HCFA addition significantly reduced hydrocarbon leaching. Comparison of milligram of contaminant leached per kilogram of drill cuttings from the S/S products and untreated drill cuttings clearly demonstrated hydrocarbon and chloride immobilization.

  Treatability Study Report for In Situ Lead Immobilization Using Phosphate-Based Binders
  Bricka, R.M., A. Marwaha, and G. Fabian.
  ATC-9137195, ESTCP Project ER-0111, 195 pp, 2008

  Phosphate-based binders marketed by four vendors were evaluated at Camp Withycombe, OR, for immobilization performance of Pb in small arms firing range soil. Variability in Pb stability was observed in all four soil treatments.

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