Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 256

At the end of each leaching interval, the solid phase is separated from the leaching
solution, the leachate is collected, and the solid is placed in fresh leachant for the
subsequent leaching interval. Leachates are analyzed for chemical composition and
physical properties to assess the rates and mechanism of kinetic release. The specifics
of different mass-transport rate tests vary in terms of the physical mode of water
contact (e.g., batches, columns, lysimeters), the duration of leaching intervals, the
specified leachant composition, and the method and frequency of leachate renewal.
The defining characteristic of a mass-transport rate test is the periodic renewal
of leaching solution in order to maintain a maximal driving force for release from
the matrix. The frequency of leachant renewal may be continuous as in a dynamic
leaching test (e.g., a one-time pass through or over the solid) or may follow a
predetermined schedule of intermittent leachant renewals, as in “semi-dynamic”
leaching tests. The ratio of leachant to solid material is much larger than that of the
equilibrium leaching test and often is based on the surface area of the subject material
exposed to leaching. The physical state of the solid material may be monolithic or
granular (e.g., soils, ashes, or particle-size-reduced monoliths), depending on the
specifics of the testing protocol.
Within the category of mass-transport rate tests, four groups may be identified
based on procedural differences:

• Flow-around tests

• Tank leaching tests

• Flow-through tests

• Soxhlet-type tests

10.4.3.2.1 Flow-Around Tests
Flow-around tests are fully dynamic mass-transport tests in which the leachant is
pumped across, or allowed to flow past, the surface of a material and is collected
on a continual basis. This dynamic mass-transport rate test may be conducted on
either monolithic or granular materials as shown in Figure 10.3a. Although flow-
around testing may closely represent the physical contact state for many placement
scenarios, practical application of continuous leachant renewal is limited by analyt-
ical capabilities. For most S/S materials, the slow rates of diffusion through the solid
matrix and high liquid volumes relative to released mass result in leachates that are
well below analytical detection or practical quantifiable limits.

10.4.3.2.2 Tank Leaching Tests
Tank leaching tests are “semi-dynamic” mass-transport rate tests that essentially are
sequential-batch extractions using large volumes of leachant. The leachant is inter-
mittently renewed at intervals designed to maintain a significant diffusive driving
force. In comparison to the dynamic flow-around tests, one advantage of tank
leaching testing is that release concentrations tend to be more consistent with ana-
lytical capabilities. In addition, procedural simplicity is increased, as no pumps or
elaborate gravity-fed dynamic flow systems are required.
Typically, tank leaching tests consist of a series of n leaching periods during
which constituents are released from a sample suspended in a large volume of