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A two-front leach model for cement-stabilized heavy metal waste
Mohammad Z Islam1, Lionel J J Catalan, Ernest K Yanful
1Department of Chemical Engineering, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1.
Environmental Science & Technology
|March 30, 2004
Summary
Cement-stabilized waste leaching occurs at two fronts, driven by ion diffusion. Metal leaching is linked to the second front, influenced by leachable fractions and remineralization.
Area of Science:
- Geochemistry
- Materials Science
- Environmental Science
Background:
- Cement-stabilized waste is used for immobilizing hazardous materials.
- Understanding leaching behavior is crucial for predicting long-term environmental impact.
- Acid neutralization capacity (ANC) plays a key role in waste stability.
Purpose of the Study:
- To investigate the mechanisms of leaching in cement-stabilized waste.
- To identify the factors controlling the rate and extent of metal release.
- To develop a model for predicting metal leaching behavior.
Main Methods:
- Quantitative scanning electron microscopy (SEM) was used to study waste specimens.
- Specimens were exposed to leaching solutions at controlled pH (4-7).
- Leaching fronts and mineral dissolution were analyzed.
Main Results:
- Two distinct leaching fronts were observed, consuming the waste matrix's ANC.
- The first front involves portlandite dissolution and CSH gel reaction, limited by OH- diffusion.
- The second front involves Ca-Al hydroxy sulfate dissolution, controlled by H+ diffusion, and is associated with metal leaching.
Conclusions:
- Leaching in cement-stabilized waste is a multi-stage process governed by specific ion diffusion.
- Metal leaching is primarily controlled by the advancement of the second leaching front.
- The developed model accounts for leachable metal fractions and remineralization, improving predictions of metal mobility.