Adsorption characteristics of lead onto soils

The metal adsorption characteristics of fifteen Taiwan soils, exemplified by Pb (II), were evaluated using pH as the major variable. The soil samples were thoroughly characterized for their physical chemical properties and composition, particularly organic matter and metal oxides. The adsorption of Pb (II) followed the Langmuir relation and increased with increasing pH between pH 3.0 and 8.5, The greater adsorption was found for soils with a higher organic matter content at constant pH and metal concentration. To better understand the mechanism of adsorption, the experimental results for Pb (II) were tested in a partition coefficient model to relate the adsorption of the Pb (II) by the different soils with soil components: organic matter, iron oxide, aluminum oxide and manganese oxide. This model was not successful when applied to measurements at the natural soil pH because of the competition of protons with lead for available sites. However, partition coefficients obtained from experimental data were highly correlated with those calculated for a partition coefficient between lead and organic matter alone. Normalization of the partition coefficients, K-d, for the organic matter content of the soils, K-om, greatly improved the correlation between the partition coefficient and pH (R-2 increased from 0.602 to 0.855), This suggests that the surficial adsorption sites are principally due to organic matter, For the 24-h equilibration period employed, diffusion of Pb through this superficial organic matter coating to underlying sorptive materials, including metal oxides, is not important in the partitioning of Pb. (C) 1998 Elsevier Science B.V. All rights reserved.