Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 244

10.4.1 L EACHING T EST A PPLICATIONS

Leaching tests, especially regulatory protocols, are designed for use over a wide
range of applications and for a diverse set of purposes. Predictions of constituent
release may be used for regulatory purposes (e.g., waste classification or delisting
applications), development of advanced treatment technologies, preparation of site
assessments, bases for selection of risk-based alternative treatments, or fundamental
research (e.g., regarding release mechanisms, treatment process, materials science).
The objectives for evaluating constituent release from a solid material via leaching
protocols may include one or more of the following:

• Screening of a material as “hazardous” or “non-hazardous”

• Development of an effective S/S treatment recipe

• Compliance to waste treatment acceptance criteria

• Characterization of material-specific intrinsic properties

• Determination of a source term for risk management and assessment

• Comparison of alternative management scenarios

• Prediction of potential release under specified release scenario conditions

The goal of leaching evaluation from an environmental perspective is to answer
the question, “What is the potential for toxic constituent release by leaching from this
waste matrix under this management scenario?” 5 Inherent in the above goal is the
additional question, “How is this potential constituent release affected by alteration of
the release conditions or long-term interactions with the release environment?”
Due to the complexity of the leaching process and the number of factors influ-
encing release, no single leaching test or single set of leaching conditions is appro-
priate for the entire range of leach testing objectives and applications. Therefore,
researchers have developed many different leaching tests, each designed to address
a specific objective or to measure a leaching parameter over a range of leaching
conditions. Since leach test development to date has been largely ad hoc , there are
many slight variations in the parameters of existing leaching protocols, even among
tests designed to provide similar evaluations.
For example, leaching tests designed to determine solid-liquid partitioning usu-
ally are conducted on particle size-reduced material, although a brief scan of the
literature shows that the specific particle-size maximums are observed to range from
125 μm to 9.5 mm. In order to ensure steady-state conditions are obtained, maximum
particle-size guidelines are intertwined with contact times (i.e., longer contact times
are required to obtain steady-state conditions for larger particles). Additional varia-
tions are seen in the number of leaching steps (e.g., a single extraction with one
leaching solution or multiple extractions to a series of solutions) and application of
agitation during the leaching interval. The composition of the leachant varies among
leaching tests; however, the most commonly specified leachants are “reagent grade”
water (i.e., deionized, distilled, or demineralized water) and dilute solutions of
organic or inorganic acids. Site-specific runoff or groundwater is sometimes used
on a more specialized basis. 172