The International Information Center for Geotechnical Engineers

Stabilization/Solidification methods have been classified into various categories by different researchers, including US EPA, Jesse Conner, and H.D. Sharma, using differing classification methods.

According to US EPA (1986), most stabilization/solidification systems are proprietary processes involving the addition of absorbents and solidifying agents to a waste. However, the S/S processes can generally be divided into five categories based on the additives used: (1) Sorption, (2) Lime-fly ash pozzolan process, (3)Pozzolan-portland cement systems, (4) Thermoplastic microencapsulation, and (5) Macroencapsulation.

Conner (1998) categorized these processes into three major categories by the mechanism of the involved processes: (1) chemical processes, such as cement-based process, pozzolan-based processes, lime-based processes, etc; (2) physical processes, such as macroencapsulation/containerization and non-chemical microencapsulation; and (3) thermal processes, such as thermoplastic polymer encapsulation and vitrification.

Sharma (1994) divided the S/S methods into two categories by the treatment materials: (1) sorbents involving sorption, and (2) encapsulating agents, involving macro-encapsulation and micro-encapsulation.

Below, we consider the types of S/S based on the classification of USEPA (1986). For each type of S/S method, applicability, advantages, disadvantages and other concerns are discussed.

2.1 Sorption

2.1.1 General background

Sorption is often used to eliminate free water and improve handling of wastes, such as limiting the escape of volatile organic liquids. Sorbents are also used to modify the chemical environment to limit the solubility of the waste. (USEPA, 1986)

The wastes considered for S/S treatment are mainly liquids or sludges (USEPA, 1986). In order to prevent the liquid from draining from the wastes, and to improve the handling properties of the waste, sorbents are added to the wastes. This process is referred to as sorption. Sorption may be realized through a chemical reaction between wastes and sorbents or by sorbent molecules retaining moisture as part of the capillary liquid (USEPA, 1986). Some typically used sorbents are activated carbon, anhydrous sodium silicate, various forms of gypsum, celite, clays, expanded mica, and zeolites (Sharma, 1994).Sorption can be divided into two different processes based on physical mechanisms: absorption and adsorption. Absorption involves "the uptake of a substance into the bulk of the sorbent. This process occurs via the pore structure", while "adsorption refers to the surface phenomenon whereby the liquid or gaseous molecules are attracted to and retained on the sorbent surface. This process occurs either by physical forces or by chemical bonding." (Sharma, 1994; Sell, 1988) Figure 2.1 shows some different mechanisms by which sorbents retain water and ionic materials.

Technical considerations that are important in the selection of a sorbent include the quantity needed to soak up all of the liquid, compatibility of the waste and the sorbent, level and character of contamination likely introduced in the sorbent, and chemical binding properties of sorbent for certain contaminants. (USEPA, 1986) These considerations have been studied by several researchers, and the results are summarized in Table 2.1. However, the quantity of absorbent necessary to soak up all of the liquid varies widely, which needs to be determined through bench scale tests instead of simply accepting the ratios in Table 2.1.

According to USEPA (1986), most large, hazardous waste landfills are using sorption to satisfy requirements regulating burial of liquid wastes. USEPA (1986) cited a case where large-scale sorption was successfully applied; in this case, 5 million gallons of oil sludge from a refinery site were treated with 40,939 tons of kiln dust before placement in a landfill at a cost of approximately $15 per cubic meter.  (a) Chemically bound water (b) Structural water  (c) Surface absorbed water (d) Capillary water or pore water Figure 2.1: Mechanisms retaining water and ionic materials on and in solid phases (USEPA, 1986) Table 2.1: Natural Sorbents and Their Capacity for Removal of Specific Contaminants from Liquid Phases of Neutral, Basic, and Acidic Wastes.  Contaminant Neutral Waste (Calcium Fluoride) Basic Waste (Metal-Finishing Sludge) Acidic Waste (Petroleum Sludge) Ca Zeolite (5054)a Kaolinite (857) Illite (1280) Zeolite (1240) Kaolinite (733) Zeolite (1390) Illite (721) Kaolinite (10.5) Cu Zeolite (8.2) Kaolinite (6.7) Acidic fly ash (2.1) Zeolite (85) Kaolinite (24) Acidic fly ash (13) Zeolite (5.2) Acidic fly ash (2.4) Kaolinite (0) Mg Basic fly ash (155) Zeolite (1328) Illite (1122) Basic fly ash (176) Zeolite (746) Illite (110) Basic fly ash (1.7) Zn Zeolite (10.8) Vermiculite (4.5) Basic fly ash (1.7) Ni Zeolite (13.5) Illite (5.1) Acidic fly ash (3.8) F Illite (175) Kaolinite (132) Acidic fly ash (102) Kaolinite (2.6) Illite (2.2) Illite (9.3) Acidic fly ash (8.7) Kaolinite (3.5) Total CN Illite (12.1) Vermiculite (7.6) Acidic fly ash (2.7) COD Acidic fly ash (690) Illite (108) Illite (1744) Acidic fly ash (1080) Vermiculite (244) Vermiculite (6654) Illite (4807) Acidic fly ash (3818) Source: USEPA, 1986 a Values in parentheses represent sorbent capacity in micrograms of contaminant removed per gram of sorbent used. 2.1.2 Disadvantages In many cases waste constituents may leach from the sorbents, which is an important source of pollution. Thus, sorbents are often used in lined landfills to control the pressure head on the liner, while the leaching is protected mainly by the liner. (USEPA, 1986) 2.2 Cementitious Stabilization/Solidification 2.2.1 General background USEPA (1997) defines cementitious S/S technologies as those that use "inorganic reagents to react with certain waste components; they also react among themselves to form chemically and mechanically stable solids." This is a conventional method used for S/S. Common reagents include Portland cements, fly ash, and lime and kiln dust. These reagents usually react to form a solid matrix, which is often stable and has a rigid, friable structure like many soils and rocks (USEPA, 1997). Cementitious S/S reagents are often called "inorganic polymers". Typically, the S/S process is realized through hydration of Portland cement. In the presence of water, C3S and C2S in cement hydrate to form calcium silicate hydrate gel (C-S-H gel) and Ca(OH)2. C3A hydrates to form calcium trisulfoaluminate hydrate, or calcium monosulfoaluminate hydrate, or tetracalcium aluminate hydrate (Spence and Shi, 2004). These products form hardened paste, mentioned as a solid matrix previously. 2.2.2 Advantages Advantages of cementitious S/S include the wide availability of cementitious reagents, which are inexpensive and can be operated simply. Among them, Portland cement is the most commonly used (Spence and Shi, 2004). 2.2.3 Disadvantages The final pH of the system may not be desirable. There is an optimum pH range to precipitate amphoteric metals is about 10, such as Cd, Cr, Cu, Pb, Ni, and Zn. However, the pH value of a hardened Portland cement is over 12.5. (Spence and Shi, 2004) Many contaminants interfere with the hydration of Portland cement. (Spence and Shi, 2004) Portland cement cannot control the oxidation state of metals. (Spence and Shi, 2004) As recommended by Spence and Shi (2004), these disadvantages can be solved by adding various additives into Portland cement, which includes blast furnace slag, pozzolan and fly ash. These additives not only serve to improve the performance of Portland cement as S/S reagents, but also help to cut down the cost. 2.3 Polymer Stabilization/Solidification 2.3.1 General background As defined by USEPA (1997), "polymer S/S technologies process waste at relatively low temperature by combing or surrounding wastes with liquid polymers. Cooling or curing of the polymer then produces a solidified final waste form product". Polymer S/S is a versatile technology which can be applied for either microencapsulation or macroencapsulation; also can be accomplished ex situ or in situ (USEPA, 1997). USEPA (1997) divided polymers into two categories: thermoplastic and thermosetting. Thermoplastic binders can be melt to a flowable state when heated and harden to a solid when cooled, while thermosetting binders require the combination of several ingredients to polymerize and harden (irreversible) (USEPA, 1997). When the waste particles are small solid particles (< 60 mm) and homogeneously distributed, the organic polymer matrix is known as microencapsulation. In microencapsulation, individual waste particles are fully surrounded and encapsulated by the polymer matrix. When the waste particles are large (> 60 mm), clean polymer can be placed around the waste and this process is usually called macroencapsulation. USEPA has identified macroencapsulation as the best demonstrated available technology (BDAT). (Spence and Shi, 2004) 2.3.2 Advantages Polymer S/S is useful because of its broad application to diverse waste streams. Additionally, polymer S/S results in waste products with improved durability and leachability characteristics compared to cementitious techniques (Weitzman et al., 1997). Polyethylene, one of the most common polymers used for S/S, is relatively inexpensive; costs for 1kg of salt waste are estimated around$1.88, compared to 3.00 for cement (Conner and Hoffner, 1998).

Both thermoplastic polymers and organic polymers are hydrophobic after curing and thus resist leaching, even at very small particle sizes. This property makes polymers ideal for trapping highly toxic metals and organic compounds. Organic polymer microencapsulation, specifically, is a useful method to encapsulate waste because it is well-suited for many types of applications, including liquid waste solidification; it exhibits a high degree of impermeability, and can quickly attain physical strength. Organic polymer-treated wastes typically remain in solid/monolithic form because of their high strength and elasticity properties. Because organic polymers can be synthesized in a wide variety of compositions, they can be tailored to meet different requirements for wastes. This is advantageous for solidifying liquid wastes, which can have highly variable characteristics and remediation requirements (Conner and Hoffner, 1998)

One disadvantage of polymer S/S technologies is sensitivity to particle diameter. Microencapsulation can typically effectively treat particles between 50 µm and 3mm, while macroencapsulation is best for particles >60mm (Weitzman et al., 1997). Particles outside of these size ranges are best treated after processing or with different methods. Polymer S/S treatment also presents the problem of volatilized moisture release, especially VOCs, which can be hazardous. It is best to pre-treat wastes with more than 2% moisture (Weitzman et al., 1997).

One nearly obsolete method of organic polymer microencapsulation, Urea-Formaldehyde S/S, expels free water during the condensation reaction of stabilization. This method require large quantities of resin dispersion for solidification and is expensive for drumming radioactive wastes compared to cementitious processes. Lastly, these systems have generated concern for the environment due to the use of formaldehyde (Weitzman et al., 1997)

Organic Polymer Systems are also relatively unfashionable in recent years. In some cases, waste components may interfere with polymerization by reacting with catalysts or taking up free radicals. Polymer S/S processes often require roughly 25% by weight of polyester resin in the waste mixture, which can significantly increase costs due to chemical requirements (Weitzman et al., 1997)