Comparison of Enhancement in Cement-Based Stabilization/Solidification of Lead-Contaminated Bentonite Using Calcium Hydroxide and Sodium Hydroxide

Background and Objective:

Soil contamination by heavy metals poses a serious threat to the environment and human health. The U.S. Environmental Protection Agency (EPA) has identified cement-based stabilization and solidification (S/S) as the most effective method to control the mobility of contaminants. The cement hydration process increases the pH of the environment through the production of calcium hydroxide, stabilizing the contaminants. Subsequently, pozzolanic reactions encapsulate the contaminants. However, environmental concerns regarding cement production present a global challenge. This study focuses on comparing the mechanisms controlling the S/S process of lead-contaminated bentonite using cement, along with the alkaline enhancing agents calcium hydroxide and sodium hydroxide, with the goal of reducing cement consumption.

Materials and Methods: This study specifically examines the pH elevation and its impact on the stabilization process. A series of pH adjustment, leachability (TCLP), and X-ray diffraction (XRD) tests were conducted. Bentonite soil was contaminated with 100 cmol/kg-soil of lead and then stabilized/solidified with 10% and 15% by weight of cement. To analyze the stabilization mechanism, the pH of the contaminated soil was adjusted to 10, 11, and 12 using incremental additions of calcium hydroxide and sodium hydroxide, corresponding to the pH range for the precipitation of lead heavy metal. To investigate cement reduction, stabilized soil with calcium hydroxide was solidified with 5%, 7.5%, 10%, and 15% cement.

Results and Discussion:
Due to its significant cation exchange capacity, the presence of calcium carbonate, and large specific surface area, bentonite soil alone can retain about 50% of the applied contamination in the TCLP test, preventing its transfer to the environment. In cement-based S/S processes, if heavy metal contamination exceeds the soil's retention capacity, stabilization is required before encapsulation. Given the amphoteric properties of lead, the soil's pH must be adjusted to the range of 10–12 to stabilize lead ions through oxide-hydroxide phases. Accordingly, the amount of cement used must provide the necessary initial pH increase for stabilization.TCLP and XRD results show that in the S/S process, as pH increases, the retention phase transitions from carbonate to the more stable oxide-hydroxide phase, enhancing contaminant retention in the TCLP test. In soils stabilized with calcium hydroxide, the formation of cementitious products results in greater contaminant retention compared to sodium hydroxide-stabilized soils. However, desorption levels still do not meet the EPA standard limit (<5 mg/L). Achieving this limit requires the presence of cement and solidification mechanisms facilitated by cementitious products. By enhancing contaminated soil with calcium hydroxide and stabilizing free contaminants, cement consumption can be reduced.

Conclusion: Stabilization of heavy metal-contaminated soils with calcium hydroxide and sodium hydroxide and evaluating desorption levels in the TCLP test indicate that an optimal pH of 11 is most effective. The findings suggest that using calcium hydroxide as an enhancer not only activates the oxide-hydroxide precipitation mechanism but also enhances solidification mechanisms. This improved performance makes calcium hydroxide a more effective enhancer in the cement-based S/S process for soils containing heavy metal contaminants like lead. Intensification with calcium hydroxide reduces cement consumption by 33% compared to non-enhanced conditions in the cement-based S/S method.