Enhancement of exchangeable Cd and Pb immobilization in contaminated soil using Mg/Al LDH-zeolite as an effective adsorbent

Abstract

In the present study, experiments using zeolite and Mg/Al LDH-zeolite for immobilization of Cd and Pb ions in artificial soil were conducted. The conditions which affect Cd and Pb ion immobilization in soil were evaluated, namely soil pH (5–7), the mass ratio of adsorbents (1%, 3% and 5%), incubation time (15 days, 30 days and 45 days) and soil moisture (30%, 50% and 70%). The results indicated that the optimal soil pH, mass ratio of adsorbents, incubation time and soil moisture for immobilization of Cd and Pb ions by the adsorbent were, respectively, 7.0, 3%, 30 days and 70%. The exchangeable Cd ion content in the contaminated soil dropped from 22.17 mg kg⁻¹ (87.65%) to 11.03 mg kg⁻¹ (43.48%) and 6.47 mg kg⁻¹ (26.36%) on incubation with zeolite and Mg/Al LDH-zeolite, respectively, while the exchangeable Pb content fell from 23.28 mg kg⁻¹ (90.02%) to 14.12 mg kg⁻¹ (54.04%) and 9.47 mg kg⁻¹ (35.24%) using zeolite and Mg/Al LDH-zeolite as absorbents in contaminated soil, respectively. Fe–Mn oxide occluded (F2), carbonate bound (F3) and organically complexed (F4) were the main forms for immobilization of the exchangeable Cd and Pb when the zeolite and Mg/Al LDH-zeolite absorbents were separately cultivated into soil. Precipitation, co-precipitation and electrostatic attraction were the main mechanisms of exchangeable Cd and Pb immobilization onto the Mg/Al LDH-zeolite to form carbonate metals (CdCO₃ and PbCO₃). This was due to the surface functional groups of the adsorbent and the presence of Fe and Al oxyhydroxides, Mn oxides, and Si and O elements in the Mg/Al LDH-zeolite's constituents. The efficiency of Cd and Pb immobilization by the Mg/Al LDH-zeolite was higher than that by zeolite from 1.5 to 1.6 times. The Mg/Al LDH-zeolite showed an enhanced ability of exchangeable Cd and Pb immobilization in contaminated soil.