This study investigates the leaching behavior and medium-term stability of lead (Pb) in phosphate-amended, cement-based solidified/stabilized (S/S) soils under both initial-pH and constant-pH conditions. An advanced automated leaching system was developed, which employs feedback control to maintain constant pH levels throughout the 47-day experiment. Semi-dynamic and modified TCLP leaching tests reveal that sustained acidic or alkaline conditions lead to significant increases in Pb leaching, primarily due to the progressive degradation of the cement matrix, which reduces its ability to immobilize Pb. In contrast, maintaining a pH near 9 resulted in optimal Pb immobilization, with minimal release observed. Sequential extraction demonstrated that Pb was predominantly present in the residual fraction or bound to Fe-Mn oxides. These findings underscore the critical role of pH stability in controlling Pb mobility, highlighting the importance of incorporating constant-pH leaching protocols and chemical speciation analysis for accurate medium-term performance evaluations and effective risk management in S/S systems. From a practical perspective, maintaining near-neutral to slightly alkaline conditions (around pH 9) and co-applying phosphate with cement-based binders are recommended for field-scale stabilization of highly mobile Pb-contaminated soils to ensure enhanced immobilization efficiency and medium-term environmental safety.
Lv et al. (Tue,) studied this question.