Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 259

10.4.4.1 Leachant Composition

In Section 10.3.2.3, the effect of leachant composition on the leaching process has
been discussed in detail with respect to groundwater contact. Although the same
factors apply to leach testing, the amount of acid added to the extraction solution is
the most often varied leaching test parameter in laboratory testing protocols. The
intent of acid addition is to create release conditions that neutralize the buffering
capacity of the matrix, accelerate dissolution of hydroxide-bearing minerals, and
alter the solubility of pH-dependent species. However, the amount of acid specified
in field-mimicking leaching tests (e.g., TCLP and SPLP) is too small to control the
final pH of the leaching solution. For example, Stegemann et al. report an ANC of
16 meq/g for neat portland cement 65 compared to a maximum of 2 meq/g provided
by the TCLP solution. 12 The limited acidity in the extraction fluid results in a near-
neutral to slightly alkaline pH, where constituent concentrations of many RCRA-
regulated metals are near the minimum of the solubility curve.
Furthermore, the final pH of the extract for several leaching protocols (e.g.,
TCLP) is not expressly required to be measured or reported. Without a final pH
value, little scientific information is provided by the leaching test, as there is no
basis for comparison between treatment recipes, no assurance that steady-state
conditions have been obtained, and no method of quality control evaluation. Thus,
measurement of the final pH and leachate conductivity (as a measure of ionic
strength) of all analytical samples is essential in to order to glean useful information
regarding solution chemistry from leaching protocols.

10.4.4.2 pH Control

Laboratory tests frequently require strong acids to adjust pH in the leachate to a
specified pH. Several tests specify automated pH controllers, which monitor solution
conditions and maintain pH to a set value by delivering small quantities of acid or base
(e.g., EN 12457, pH Static Test). An alternative method to cumbersome and costly pH
controllers is specified in parallel batch extraction tests (e.g., ANC, SR002.1, AV001.1)
using predetermined acid additions based on a preliminary titration curve. However,
simply dumping in the acid may result in localized areas of low pH, possibly resulting
in dissolution of mineral phases and release of some contaminants that otherwise would
not have been released using automated pH control.

10.4.4.3 Particle-Size/Contact Time

Most equilibrium-based leaching tests assume that the extraction process reaches
steady-state conditions within the time frame specified by the protocol. To approach
this end-state, several of these tests require particle-size reduction of samples prior
to testing. Since release from larger particles may be mass transfer-limited over the
abbreviated time frame for which leaching tests are designed, tests with larger
particle-size specifications may require longer contact time to obtain equilibrium or
may not reach steady state within the test duration.
The diffusion-limited release of constituents during equilibrium-based tests may
be studied using the analytical solution for diffusion from a spherical particle into