Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 180

100
10
1
0.1
0.01
0.001
0.0001
1E-05
1E-06
1E-07
1E-08
1E-09
1E-10
1E-11
1E-12
1E-13

Pb(OH) 2
Zn(OH) 2

AgO

Cu(OH) 2

Cd(OH) 2

CuS
ZnS
CdS
PbS

AgS

2 3 4 5 6 7 8 9 10 11 12

pH

FIGURE 8.1 Comparing the solubility of metal sulfides and hydroxides.

100,000

10,000

1,000

100

10

1

0 2 4 6 8 10 12 14 16

pH

FIGURE 8.2 Mercuric oxide solubility versus pH from published data in Ref. 5.

8.2.2.2 Soluble Silicates and Rice Hull Ash

Numerous metals can be treated using silicates. Soluble silicates are especially
effective at precipitating the following metals (listed in order of decreasing effec-
tiveness): copper, zinc, manganese, cadmium, lead, nickel, silver, magnesium, and
calcium. Metal silicates exhibit low solubility over a wide pH range of 2 to 11 or
greater. 10 In addition, soluble silicates are also effective at reducing the permeability
of the stabilized grout. By forming precipitates in the matrix that block pores, 11 the
movement of any mobile species through the matrix into the environment is slowed.
If the metals to be treated are already present as insoluble hydroxides, either the
waste must be pretreated by reducing the pH to resolubilize the metal hydroxide or
silicate must be slowly released or produced over a period of time to react with
metal hydroxide that is slowly dissolving and being re-speciated.