Microstructural Investigation of Thermal Stabilization/Solidification of Lead-Contaminated Bentonite Using XRD and TCLP

Articles in Press

Document Type : Article

Authors

1 -Bu-Ali Sina University -University of Tehran

2 Civil Engineering Department,. Faculty of Engineering. Bu-Ali Sina University

3 Civil Eng. Department, Faculty of Eng. Bu-Ali Sina University

4 Civil Eng. Department. Faculty of Eng. Bu-Ali Sina University

10.24200/j30.2026.67837.3462

Abstract

This study evaluates the effectiveness of thermal treatment for the stabilization and solidification (S/S) of lead in contaminated bentonite, with particular focus on phase transformation, leachability, and structural modification across a wide temperature range (25–900 °C). Lead-contaminated bentonite samples with different Pb concentrations were subjected to controlled heating, and the resulting mineralogical and environmental responses were analyzed using X-ray diffraction (XRD), Toxicity Characteristic Leaching Procedure (TCLP), solubility assessments, and pH monitoring.



The results demonstrated a clear sequence of thermally induced reactions that progressively improved the immobilization of lead. At approximately 400 °C, a measurable increase in pH (to around 8.5) and a corresponding reduction in Pb²⁺ solubility indicated the onset of mineral transformations, including the initial formation of stable lead-bearing compounds. With further heating, dihydroxylation reactions became dominant near 700 °C, disrupting the smectite layer structure and facilitating the development of amorphous and crystalline phases such as mullite, cristobalite, and lead silicate. These mineral phases played a crucial role in binding lead within low-solubility matrices, thereby reducing its mobility. At this critical temperature, TCLP results confirmed that the concentration of lead released into the leachate dropped below the U.S. EPA regulatory limit of 5 mg/L, establishing 700 °C as the optimum stabilization threshold.



When the temperature was elevated to 900 °C, leachability of lead was completely suppressed, signifying nearly total immobilization. XRD analyses and weight loss measurements (approximately 30% for high-concentration samples) further confirmed the transformation of bulkier nitrate phases into denser, more stable oxides such as litharge and massicot. These structural changes not only minimized contaminant release but also enhanced the long-term durability of the treated soils for environmental applications.



Overall, the findings highlight that thermal treatment provides an effective pathway for lead immobilization in bentonite without the need for additional chemical stabilizers. The process capitalizes on sequential mineral transformations, with 700 °C identified as the balance point between effective stabilization and preservation of soil properties. Moreover, the synergy between thermal reactions and the alkaline environment was found to be a decisive factor in optimizing retention capacity. This work contributes to the development of sustainable thermal remediation techniques for heavy-metal-contaminated soils.

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Sharif Journal of Civil Engineering

Articles in Press, Accepted Manuscript
Available Online from 09 June 2026

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