Long-Term Leaching Behavior of Lead in Cement-Based Solidified Soils under Constant-pH Conditions

Abstract This study investigates the leaching behavior and long-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 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 long-term performance evaluations and effective risk management in S/S systems.