Effect of acid rain on leaching characteristics of lead, zinc and cadmium- contaminated soils stabilized by phosphate-based binder: semi-dynamic leaching tests

The previous studies show that acid rain pH significantly affects the leachability of stabilized heavy metals-contaminated soils. The parameters of diffusion coefficient (D_e) and retardation factor (R_d) are critical for quantitative assessment of impact of stabilized contaminated soils on the surrounding environment. However, the effective diffusion coefficient and the retardation factor of the stabilized soils have not been well addressed in the previous studies. This study investigates the diffusion of heavy metals leached from a new binder, namely modified KMP, stabilized Pb, Zn and Cd-contaminated soils. A series of tests including desorption tests and semi-dynamic leaching tests are conducted to determine the obvious diffusion coefficient (D^obs) and the retardation factor (R_d), respectively. The desorption test results show that the leached concentrations of Pb, Zn and Cd increase with the increase in their initial concentrations. It is also found that the desorption curves can be fitted using the Freundlich desorption/adsorption model. The semi-dynamic leaching test results show that the leaching of heavy metals released from the stabilized soils is controlled by diffusion process. The obvious diffusion coefficients (D^obs) of Pb, Zn and Cd are estimated as 1.05×10^-16, 7.84×10^-13 and 2.11×10^-12 m²/s. Combined with the desorption and semi-dynamic leaching tests, the retardant factors of Pb, Zn and Cd are estimated as 17155.6, 251.6, and 109.7. The effective diffusion coefficients are therefore determined as 1.80×10^-12, 1.97×10^-10 and 2.32×10^-10 m²/s for Pb, Zn and Cd, respectively. Compared with those of the previous studies, the effective diffusion coefficients of Pb presented in this study is 2 orders of magnitude lower, whereas the effective diffusion coefficients of Zn and Cd are consistent with those reported in the previous studies.