Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 266

In light of these observations and the potential for degradation of the cement
matrices in the long term (see Section 10.3.3), the assumptions of the diffusion model,
namely constant diffusive properties and a nondepleting matrix, may be concluded
invalid in the long term for cementitious matrices. Forcing the cumulative release from
S/S matrices to fit the two parameters of the diffusion model, and then projecting those
parameters forward in time without regard for chemical or physical changes, is likely
to lead to errors in prediction of long-term constituent release, and the simple equations
given above cannot be extrapolated beyond a cumulative release of 20%. The solutions
for finite media for Fick's second law can be used to correct for a nondepleting media
to project forward in time, but do not guarantee that the other assumptions and
boundary conditions hold true; i.e., the math can calculate the fraction released for
long times for a depleting media, but may deviate significantly from the actual behavior
for reasons not obvious from short-term leach testing.
When the diffusion model does not adequately describe contaminant release,
more complex models are required to capture the effect of adsorptive or encapsu-
lating mineral phase dissolution on the leaching process. Two such models, the
Coupled Dissolution-Diffusion (CDD) model 184 and the Shrinking Un-Reacted Core
model, 223 are based on the movement of dissolution fronts within the matrix.

10.6 LEACHING TEST SELECTION

Selection of an appropriate leaching test should be based on the (i) level of release
information detail required, (ii) cost of testing including analytical services, data
handling, and interpretation, (iii) objective(s) of candidate testing procedures, and
(iv) environmental conditions anticipated in the release scenario.
In most cases, more than one leaching test will be required to fully describe
release under a particular scenario. Leach characterization and long-term release
assessment should also consider changes in chemical and physical properties due to
degradation processes via testing of aged and degraded materials. For example,
samples may be aged in the laboratory using accelerated carbonation techniques
prior to testing in parallel to un-aged materials. 170 In addition to analysis of trace
contaminants of the waste material, analytical work on leachates must include (i)
leachate pH in order to put solubility, availability, sorption, and complexation pro-
cesses in perspective, (ii) leachate conductivity as a measure of ionic strength, and
(iii) concentrations of major constituents indicative of the structural durability of
the S/S matrix. The selection process for leaching test protocols may be illustrated
through the two simplified examples for (i) landfill disposal and (ii) near surface
placement of S/S materials.

10.6.1 L EACHATE -C ONTROLLED M ONOFILL E XAMPLE

For an S/S material to be disposed in a monofill scenario with leachate control (e.g.,
hazardous waste landfill with RCRA-approved caps and liners), the following state-
ments may be considered:
•

The infiltration rate will be low due to leachate controls; thus, a low L/S
ratio prevails.