Adsorption characteristics of lead onto soils

The metal adsorption characteristics of fifteen Taiwan soils, exemplified b y Pb (II), were evaluated using pH as the major variable. The soil samples were thoroughly characterized for their physical chemical properties and co mposition, particularly organic matter and metal oxides. The adsorption of Pb (II) followed the Langmuir relation and increased with increasing pH bet ween pH 3.0 and 8.5, The greater adsorption was found for soils with a high er organic matter content at constant pH and metal concentration. To better understand the mechanism of adsorption, the experimental results for Pb (I I) were tested in a partition coefficient model to relate the adsorption of the Pb (II) by the different soils with soil components: organic matter, i ron oxide, aluminum oxide and manganese oxide. This model was not successfu l when applied to measurements at the natural soil pH because of the compet ition of protons with lead for available sites. However, partition coeffici ents obtained from experimental data were highly correlated with those calc ulated for a partition coefficient between lead and organic matter alone. N ormalization of the partition coefficients, K-d, for the organic matter con tent of the soils, K-om, greatly improved the correlation between the parti tion 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 su perficial organic matter coating to underlying sorptive materials, includin g metal oxides, is not important in the partitioning of Pb. (C) 1998 Elsevi er Science B.V. All rights reserved.