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Stabilization/Solidification - 1.0 Overview of S/S Technology


1.0 Overview of Stabilization/Solidification Remediation Techniques  

Stabilization and solidification (S/S) is a soil remediation process by which contaminants are rendered immobile through reactions with additives or processes. During this process, also called immobilization, fixation, or encapsulation, contaminants may be chemically bound or encapsulated into a matrix. The EPA describes stabilization and solidification as a process that accomplishes one or more of the following: 1) Improve handling and physical characteristics of waste; 2) decrease surface area of a waste’s mass through which transfer/contaminant leakage can occur; and 3) limit solubility of hazardous constituents in the waste (EPA, 1989). Stabilization is the general term for a process that transforms contaminants into a less mobile or toxic form, while solidification is a more specific process that treats material to increase its solidity and structural integrity. Solidification does not remove nor degrade contaminants, but prevents their transport by eliminating or significantly hindering their mobility.

This process may be performed either ex-situ or in-situ. Ex-situ solidification involves excavation and backfilling, while the in-situ process requires injection of stabilizing compounds into the soil. Typical solidification agents include Type 1 Portland cement, pozzolans, lime, fly ash, and organic binders such as asphalt. A block of solidified soil is often referred to as a "monolith", or a "monofill" if the block encompasses the entire site (EPA, 1989). Stabilization/solidification is best suited for soils contaminated with metals, radionuclides, other inorganic compounds, and non- or semi- volatile organic compounds. Volatile contaminants are typically not good candidates for solidification, as the contaminants may evaporate into the atmosphere during the mixing process. Stabilization/Solidification can be classified into three types of processes: 1) Aqueous stabilization/solidification; 2) Polymer stabilization/solidification, and 3) Vitrification (high temperature S/S). 


 Figure 1.1: Typical S/S process: the binding agents and wasted soil are mixed with an auger (USEPA, 2012)


USEPA has identified S/S as the Best Demonstrated Available Technology (BDAT) for 57 types of hazardous wastes listed in RCRA. (Means, 1995) About 25% of the Superfund remediation sites were treated by S/S technologies (USEPA, 2001).



Figure 1.2: Frequency of application of S/S treatment compared to other technologies at U.S. Superfund sites (USEPA, 2001) 

1.1 Applicability of S/S Processes

Stabilization/Solidification treatment methods are applicable to a wide range of media including soils, sludges, and sediments (Wilk). Stabilization/Solidification is best suited for soils contaminated with metals, radionuclides, other inorganic compounds, and non- or semi- volatile organic compounds. The S/S process is typically able to encapsulate these contaminants in a matrix and impede their mobility. S/S methods are generally most effective on nonvolatile heavy metals (EPA, 1993). Volatile contaminants, however, are typically not good candidates for solidification. These contaminants may evaporate into the atmosphere during the mixing process and are difficult to trap within a solidification matrix. The EPA does not currently recognize stabilization as an acceptable treatment method to treat wastes contaminated with polychlorinated biphenyls.

In order to determine effectiveness of solidification for non- or semi- volatile organics, Superfund policy requires a treatability study on similar waste to ensure that the process would decrease the contaminant concentration by 90-99% (EPA, 1993). For certain constituents, the S/S process performs well at certain concentrations but poorly at others. For example, solidification of copper, lead, and zinc using cementitious materials is effective at low to moderate concentrations. However, these metals interfere with the immobilization process at higher concentrations and render it less effective (EPA, 1993). The specific mix and distribution of contaminants in the soil also affects the effectiveness of the S/S process. Table 1 below summarizes the effectiveness of the S/S procedure on various contaminants in soils and sludges.

Table 1.1: Effectiveness of Stabilization/Solidification on various contaminants (adapted from EPA’s "S&S of Organics and Inorganics", 1993)


Contaminant Group



Reactive Compounds

Demonstrated : Successful tests performed

Halogenated semivolatiles

Nonhalogenated semi- and non- volatiles


Volatile metals Nonvolatile metals


Radioactive materials

Inorganic corrosives

Inorganic cyanides




Potential : Expert opinion that technology will be effective




Organic cyanides

Organic corrosives



Ineffective : Expert opinion that technology will/does not work

Halogenated Volatiles

Nonhalogenated Volatiles




1.2. Advantages and Disadvantages of S/S

Sharma (2004) summarized the advantages and disadvantages of S/S technology.

S/S technology has the following advantages:

  • Low cost because the reagents are widely available and inexpensive
  • Can be used on a large variety of contaminants
  • Can be applied to different types of soils
  • Equipment is widely available and simple
  • High throughput rates

However, it still has the following disadvantages:

  • Contaminants are still in the soil, not destroyed or removed
  • Volume of the treated wastes usually increases significantly
  • Volatile organic compounds and some particulates may come out during treatment process
  • Delivering reagents deep into the wastes and mixing them evenly is difficult
  • In situ S/S site may not be redeveloped
  • Long-term efficiency of S/S is still uncertain

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