Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 258

Standardized examples of tank leach tests are the protocols such as the Nether-
lands Monolithic Leach Test (NEN 7345) 203 and ANSI/ANS-16.1-2003 204 as well as
a similar published test (MT001.1 and MT002.1) 5 used to evaluate mass transport
for continuously saturated media 187,188,205,206 and intermittently wetted cement mate-
rials. 170,199,207

10.4.3.2.3 Flow-Through Tests
Flow-though tests usually involve passing the leaching solution through a solid
material and collecting the leachate after contact. The resulting leachate concentra-
tions may be used to determine the rates of constituent release during advective
mass transport and to infer primary release mechanisms at low L/S ratios. Usually,
flow-through testing is performed on columns of granular materials (e.g., soils,
incinerator residues, crushed cements) as shown in Figure 10.3c.
In column studies of low-permeability materials, problems with the flow pattern
may be encountered including wall effects, preferential flow pathways (channeling),
and pore plugging. These problems may be minimized by introducing the leachant
in an up-flow direction. Examples of flow-through tests for granular materials include
ASTM 4874, 208 the Netherlands regulatory “up-flow” column test (NEN 7343), 209
and several published field lysimeter tests using cementitious materials. 210,211
Flow-through testing for monolithic materials with low hydraulic conductivity
is limited to permeameter experiments at pressures up to 300 kPa. 212,213 Regressed
diffusion coefficients have been shown to be 7 to 10 orders of magnitude faster in
high-pressure flow-through experiments than in the more standard tank leaching
protocols. Moreover, the largest reported diffusion coefficient was nearly 2 orders
of magnitude higher than molecular ionic diffusion, indicating confusion between
regressed advection and diffusion terms. This flow-through experimental approach
was allegedly applicable to simulate the leaching process of S/S materials either
under the landfill scenario (where the waste is more permeable than the surrounding
materials) or when the S/S waste form has degraded to a state that groundwater may
pass through the matrix. However, the maximum pressures used in these experiments
equate to a pressure head of over 30 m of water at 4°C, which is unrealistic for most
placement scenarios for S/S wastes.

10.4.3.2.4 Soxhlet Tests
In a Soxhlet test, a solid sample is continuously contacted with fresh leachant by
continuously removing constituents from the leachate. Figure 10.3d shows a Soxhlet
test apparatus consisting of a reaction vessel (A), a leachate reservoir (B) where
leachate is boiled to produce water vapor, and a condenser (C) where fresh leachant
condenses before contact with the solid material. Few published Soxhlet procedures
were found in the literature for extraction of inorganic constituents from a synthetic
S/S waste. 214,215

10.4.4 V ARIABLES A FFECTING L EACHING T EST R ESULTS

The testing parameters that most affect the outcome of leaching protocols include
(i) leachant composition, (ii) pH control, (iii) sample particle size, (iv) L/S ratio,
and (v) open or closed testing conditions.