Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 166

1.E-06

1.E-07

1.E-08

1.E-09

4 6 8

FIGURE 7.3 Cd, Cu, Ni, Pb, and Zn hydroxide solubility at 25°C in dilute solution, as a
function of pH (based on data in the MINTEQ database 70 ).

precipitation of contaminants in cement-based matrices is dependent on the pore
solution concentration of the metal cation (i.e., not the total metal concentration)
and the concentration of candidate anions with which it could form a precipitate.
Due to the alkaline nature of cement hydration products, the most likely anion to
form a metal precipitate is the hydroxide ion, OH - .
Figure 7.3 shows the pH-dependent solubility of Cd, Cu, Ni, Pb, and Zn hydrox-
ides. The initial pore solution pH of portland and blended cement-based matrices
can range from about 11.9 (for pH control by C-S-H 68 ), to higher than 13 (for high-
alkali products); hydrated calcium aluminate cement also has a pore solution pH
greater than 12. 69 The theoretical concentration ranges of these metal contaminants
over a pH range from 12 to 13.5, calculated on the basis of equilibrium with
hydroxide 70 (without adjustment for ionic strength), are shown in columns 2 and 3
of Table 7.6 (in mg/L). It is notable that Zn and Pb are several orders of magnitude
more soluble than Ni and Cd at the pH values prevailing in the pore solution of
cement-based products, while the solubility of Cu is intermediate. Given a typical
saturated S/S product porosity of 57% vol, 65 the corresponding quantities of metals
that can be dissolved in the pore solution, on the basis of the dry mass of the S/S
product (i.e., in mg/kg dry product), are estimated in columns 4 and 5 of Table 7.6.
In fact, because hydrated cement also has a significant capacity to sorb metals, it
may be possible to add higher quantities of metals to a solidified product without
precipitating hydroxides, but lower quantities than these values will be dissolved in
the pore solution, or immobilized by another mechanism (Section 7.3.4).
It has been demonstrated that the concentrations of Cd, Cu, Ni, Pb, and Zn in
near-equilibrium laboratory leachates from actual cement-based products generally
do not exceed the solubility limits seen in Figure 7.3. However, unexpectedly high
metal solubility can sometimes be observed if complexing agents, such as might be
present in plating wastes, or dissolved organic matter, are present. For instance, the
presence of organic cement admixtures has been shown to enhance the solubility of
radionuclides. 71 Also, metals may not precipitate as their single-metal hydroxides,
but may form other phases such as calcium zincate (CaZn 2 (OH) 6 ), 72 or other mixed
hydroxides, e.g., 73-75 leading to different solubility curves. Hsiao et al. 76 observed