Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 255

The leaching test variable (e.g., L/S ratio, leachant composition, contact time)
may be identical or progressive for each sequential extraction. For example, a
published “sequential TCLP” 40 challenges the solid sample to five consecutive batch
extractions using TCLP solution at constant release conditions, while EN 12457-
3 175 consists of two 3-hour DI water batch extractions at progressive L/S ratios (2
ml/g followed by 8 ml/g). Descriptions and uses of two specific variants on the
sequential-batch extraction test approach follow.

10.4.3.1.3.1 L/S Ratio Tests
Concentrations in the leachate may be used to evaluate equilibrium conditions in
the short and long term (e.g., to determine which mineral phases control equilibrium
before and after soluble salts are released). However, serial batch tests do not provide
useful information about the kinetic release rates from granular material during long-
term leaching. Leachate concentrations from serial batch tests often are plotted as
a function of cumulative L/S ratio and compared to the results of flow-through
column tests. The published test, SR003.1, 5 is one example of a sequential extraction
test varying L/S ratio at five levels between 10 ml/g and 0.5 ml/g.

10.4.3.1.3.2 Sequential Chemical Extractions
In sequential chemical extraction tests, a solid sample is carried through a series of
extractions using increasingly aggressive leaching solutions. The purpose of sequen-
tial extraction schemes is to associate fractions of the release constituent (amount
released/total content) to a particular extractant.
The most common example of a sequential chemical extraction procedure is that
of Tessier et al. 191 designed for soil characterization and modified 40 to account for
the high ANC of S/S wastes. Since release of constituents in a sequential extract
test is operationally defined (e.g., peroxide extractable fraction), the speciation of
the extracted fractions may not be well characterized (e.g., contaminant association
with mineral functional groups may not correspond directly to the fractional release)
and caution should be taken upon association of a fraction to a particular binding
mechanism.

10.4.3.1.4 Concentration Buildup Tests
Concentration buildup tests involve extraction of multiple solid samples in the same
leaching solution (Figure 10.2d). These tests are designed to obtain a leachate that
is saturated in terms of all constituent concentrations. Thus, samples are typically
particle-size reduced to maximize surface area and the volume of leaching solution
typically is very small in comparison to other extraction tests. The physical analogy
is that of an elemental volume of leachant percolating through a large column of
waste material.

10.4.3.2 Mass-Transport Rate Tests

Mass-transport rate tests are used to obtain kinetic information about release prior to
steady-state conditions (e.g., release flux, cumulative mass release with time, control-
ling mass-transport mechanisms). In the general procedure, a specified amount of solid
sample is contacted with fresh leachant for a series of n leaching intervals (i.e., a series
of distinct, specified contact times) without reaching steady-state release conditions.