Environmental Engineering Reference
In-Depth Information
10.4.3
L EACHING T ESTS BY P ROCEDURAL G ROUPS
Two broad categories of leaching tests may be defined based on whether or not
equilibrium or steady-state is established during the test duration:
Equilibrium tests
Dynamic tests
Table 10.1 and Table 10.2 present a selection of commonly used equilibrium
and dynamic leaching tests, respectively. In each table, the critical testing parameters
(e.g., leachant composition, contact time, L/S ratio) are shown for comparison and
comments or criticisms from the literature are recorded.
10.4.3.1
Equilibrium Tests
Extraction tests that are designed to reach a steady-state release are termed “equi-
librium tests.” 5 Since mineral dissolution reactions in cementitious media are often
irreversible, the steady state established during an extraction test is not the same as
thermodynamic equilibrium, but rather implies no net transfer of constituents
between the solid and liquid phase within a specific time frame. 12 Equilibrium-based
leaching protocols typically require particle-size reduction of the subject material
in order to reduce the time required to obtain steady-state release via increased
surface area and minimized kinetic transport. In agitated extractions, shaking, stir-
ring, or tumbling further accelerates the rate of extraction and ensures continuous
solid/liquid contact.
Common equilibrium tests may be single-batch extractions at one set of release
conditions (e.g., leachant pH, L/S ratio, contact time), parallel-batch extractions over
a range of conditions, or sequential-batch extractions at constant or progressive
release conditions. A lesser-used approach is to build up concentration in a single
leachate using multiple solid samples. Thus, equilibrium-based leaching protocols
may be further divided into four procedural groups based on the relative number of
samples and resulting leachates:
Single-batch extractions (one sample, one leachate)
Parallel-batch extractions ( n samples, n leachates)
Sequential-batch extractions (one sample, n leachates)
Concentration buildup extraction ( n samples, one leachate)
10.4.3.1.1 Single-Batch Extractions
The goal of single-batch extraction tests is to characterize solubility or release of
constituents at a single set of release conditions. The general procedure (illustrated
in Figure 10.2a) is that a predefined mass of sample is contacted with a volume
leachant for set contact time. At the end of the specified contact interval, the solid
and liquid phases are separated (e.g., filtration, decanting) and the leachate is ana-
lyzed for physical and chemical properties. The solid phase may be discarded or
saved for mineralogical analysis typically supplemental to the leaching protocol.
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