Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 167

TABLE 7.6
Estimated Contaminant Solubility in Cement
Pore Solutions

Pore Solution
Concentration (mg/L)*

Soluble Amount
(mg/kg dry product)**

Minimum
(pH 12)

Maximum
(pH 13.5)

Minimum
(pH 12)

Maximum
(pH 13.5) Metal

Ni 0.005 0.1 0.07 0.1

Cd 0.02 0.09 0.03 0.1

Cu 0.6 9 0.8 12

Zn 10 1400 13 1900

Pb 100 66000 *** ***

* Based on equilibrium with hydroxide in dilute solution (Figure
7.3).
** Assuming 57% saturated porosity.
*** Actual Pb solubility has been shown to be lower than pre-
dicted by hydroxide solubility. 77

respeciation of soluble CuCl 2 to precipitated Cu(OH) 2 , and also oxidation of Cu + to
Cu 2+ in MSWI fly ash solidified with portland cement.
Contaminants that form oxyanions, such as As, Cr(VI), Mo, Sb, and Se, can
form precipitates with calcium. However, Johnson 77 reports that while AsO 4 3- and
MoO 4 2- are known to form calcium salts of low solubility at high pH, the solubility
of calcium salts of AsO 3 3- , CrO 4 2- , SeO 3 2- , and SeO 4 2- is higher, and that of Sb and
V salts is unknown. Both U(VI) and Sn(IV) have been shown to precipitate as
calcium salts, but the solubility of the former is two orders of magnitude greater
than the latter. 78 Phosphates may also precipitate with calcium, to form apatite
[Ca 5 (PO 4 ,CO 3 ) 3 F]. 79

7.3.4 U PTAKE BY C EMENT H YDRATION P RODUCTS

In examining the measured pore solution or leachate concentrations of metal con-
taminants with respect to the solubility of their hydroxides (Figure 7.3), these can
often be observed to lie below their maximum solubilities. 77 Such observations
indicate that, in these cases, the total contaminant concentration in the product is
lower than the capacity of the matrix for uptake of the contaminant, such that the
leachate concentration is controlled by partitioning between the solid and liquid,
rather than metal hydroxide solubility. Thus, the pore solution or leachate concen-
tration does not reach saturation with respect to formation of the metal hydroxide.
Leachate concentrations below the limit of solubility in equilibrium with hydroxide
are particularly noticeable for Pb, whose solubility is an order of magnitude lower
than the solubility in equilibrium with hydroxide at pH 10 to 13.5, and several orders
of magnitude lower at more acidic pH. 77 However, Pb remains by far the most soluble
of these metals in the alkaline environment provided by a cement matrix.