Nature Of The Technology

The polysilicate treatment technology is a chemical treatment that uses commercially available soluble silicate solutions and various cementitious materials such as cement, lime, pozzalime, and fly ash [1,2], Relatively small amounts of polysilicates and cementitious materials are used to change the chemical characteristics of heavy metals contained in solid or semisolid matrices. Consequently, this chemical treatment differs significantly from the so-called solidification/stabilization technologies, requiring as much as a 100% addition of reagents. Three principal steps are involved in delivering the treatment: (1) thorough wetting of the material containing the heavy metals with a silicate-water blend, (2) the addition of appropriate cementitious materials, and (3) curing of the mixed material into a friable form suitable for backfilling. The following discussion deals with the chemistry associated with these steps [3],

TVpically, the common metallic compounds found in materials requiring treatment are a mixture of free metallic ions and other metallic ions resulting from metal chlorides, sulfates, carbonates, etc., and metallic oxides and hydroxides as shown below. Free Metallic Ions

Other Metallic Ions

MC10 Mfl+ + a CI" M6(S04)c ^ b Mfl+ + c S042" Mfc(C03)c b Mfl+ + c C032-

Metallic Oxides/Hydroxides zH20 zH20

In field applications, the free metallic ions are usually present in small quantities. The actual concentrations of these ions and of the other metallic ions resulting from salts, oxides, and bases are determined by the equilibrium Gibbs free energy of the constituents of the mixture. These concentrations are also influenced by various kinetic factors related to the presence of catalysts and particle size distributions. On the other hand, the concentrations of the metallic oxides and hydroxides are also typically related to the material generation source and influenced by historical temperature, chemical medium, moisture content, and age. These metallic substances can be contained in a wide variety of substrates such as various soils, clays, soil-clay mixtures, amorphous silica containing a broad spectrum of grain sizes, sludges generated as a by-product of industrial processes, and combinations of these materials. The following lead and zinc reactions are typical of metallic constituents requiring treatment:

As previously mentioned, the first element in the treatment involves wetting with the pol-ysilicate-water blend that is created prior to its introduction into the mixing process. The most effective treatments have used potassium silicates, which are a family of chemicals with a wide range of physical and chemical properties. They are clear, highly viscous liquids having a pH in the range of 11.3-11.7. The viscosity is affected by the Si02/K20 ratio, concentration, and temperature. For example, the lower the ratio and the more alkaline at a given solids content, the lower the viscosity. When the silicate is mixed with water it quickly forms a solution whose viscosity approaches that of water. Also, relatively small temperature increases, on the order of 10°C, can cause a fivefold decrease in viscosity.

The formation of the liquid silicate polymer can be represented by the following equations.

Water

Liquid Silicate

4 n H2O

h3o I

06 I

As shown, the liquid silicates depolymerize when mixed with water, revealing their active negatively charged oxygen sites. Further, the silicon backbones themselves go into smaller, ion-ically charged clusters.

The following shows the nature of the reaction that occurs between the active metallic elements, divalent in this case, in the material and the liquid silicate polymer.1

1 This balance considers the active metals, not the silicates, to be the limiting reactants.

1

■ H3O+-

O 1

o~ 1

-O— Si-1

— +

-0—Si — 1

1

0~ 1

. K.

n—x

. H3O+.

Here, ionically active metallic compounds react with the liquid silicates to form stable metal silicate chelates. The formation of these metal silicates is kinetically related to a number of factors such as the metal ionization state, particle size, reaction temperature, and activation energy. Further, when a combination of heavy metals are present in a material requiring treatment, reaction priority is given to the more electropositive heavy metal. Thermodynamically, the reaction progresses toward a minimum potential energy equilibrium state.

After the material has been thoroughly wetted by the polysilicate-water blend, a cemen-titious material is introduced into the process. The metal polysilicate chelates, and other metallic compounds are incorporated into a crystalline cementitious matrix. The following example illustrates the hydration of cement and lime, which are two commonly used cementitious materials.

Portland Cement (In cement chemistry notation2)

2C3-S + 6H —> 3C-2S-3H + 3(CH) 2C2S + 4H -> 3C-2S-3H + CH 4C-A-F + 10H + 2(CH) -> 6CAF-12H 3C A + 12H + CH —* 3C A CH12H 3C A + 10H + CS-2H-» 3C A-CS-12H

Lime

2CaO + 2H20 Ca0-H20 + Ca(OH)2 + 27.5 kBtu/(Ib-mol) Ca(OH)2 Ca2+ + 2 OH" a Ca2+ + a OH- + Ma+ —» M(OH)a + a Ca2+ CaC03 + 78 kBtu/(lb-mol) ->• CaO + C02 (gas)

2 C = CaO, S = Si02, H = HjO, A = A1203, F = Fe203, S = CaS04.

During the hydration of portland cement, active ionic bonds begin to transform into co-valent bonds of greater stability. As the metal polysilicate chelates are incorporated into the cementitious structure, the size and stability of the usual 7-10-silicon backbone crystalline structure is increased. In addition, the metal poly silicates, along with active metal oxides and hydroxides, tend to chelate the cementitious silicate chains by providing the necessary electronic bonding clouds. Again, the relatively weak ionic bonds are transformed into strong co-valent bonds linking metal to oxide or hydroxide and the oxide or hydroxide to the silicon backbone.

Typically, lime is used in combination with cement. When it is applied separately to multi-metallic materials, highly soluble metal hydroxide chelates can form. However, in combination with cement, it can provide the heat necessary to activate and accelerate metal oxidation and cement hydration reactions, thereby yielding a stronger structure. The resulting configuration is a three-dimensional, low potential energy crystalline cementitious matrix that has orderly passivated metallic elements integrated into its structure. An example of the transformation of this structure from the ionic to the covalent state is shown below.

0-, -O- „ .„ -• O- ^ , -O ^ _-O Ca i, ~'Cai , -/W ' _ JT-Ca^ _ _

Si-o

Si-o

0 0

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