Adsorption–Desorption Characteristics of Lead in Variable Charge Soils

Abstract

Adsorption–desorption processes of Pb at contaminated levels in two variable charge soils were investigated. The red soil (RAR) developed on the Arenaceous rock (clayey, mixed siliceous thermic typic Dystrochrept) adsorbed more Pb²⁺ than the red soil (REQ) derived from the Quaternary red earths (clayey, kaolinitic thermic plinthite Aquult). The maximum adsorption values (X_m ) that were obtained from the simple Langmuir model were 52.6 mmol Pb²⁺ kg⁻¹ soil and 29.9 mmol Pb²⁺ kg⁻¹ soil, respectively, for the RAR and REQ. Adsorption of Pb²⁺ decreased soil pH by 1.10 unit for the RAR soil and 1.21 unit for the REQ soil at the highest loading. The adsorption equilibrium pH of RAR was higher than that of REQ at the same Pb²⁺ concentration. The distribution coefficient (K _d ) of Pb in the soils decreased exponentially with increasing Pb²⁺ loading. Most of the adsorbed Pb²⁺ in the soils was not desorbed in the 0.01 mol L⁻¹ NaNO₃ solution. After five successive extractions with NaNO₃, only 0–11% of the total adsorbed Pb²⁺ in the RAR soil was desorbed and the corresponding value of the REQ soil was 0–19%, indicating that the RAR soil had a greater affinity for Pb²⁺ than the REQ soil at the same Pb²⁺ loading. Different mechanisms might be involved in Pb²⁺ adsorption/desorption at different levels of Pb²⁺ loading and between the two soils.

Keywords:

• Adsorption
• Desorption
• Lead
• pH
• Variable charge soils

Acknowledgments

This study was, in part, supported by an Outstanding Young Scientist Grant (# 40025104) and a regular grant (# 20177020) from the Natural Science Foundation of China and a grant from the Science and Technology Ministry of China (# 2002CB410800).