Kinetics and mechanisms of lead sorption and desorption on soils and soil materials

Lead is the most common contaminant found at polluted sites. In order to develop more effective risk assessment methods and remediation strategies it is critical that there be a precise understanding of the reaction pathways occurring between Pb and soils. In this study, the effects of time, pH, ionic strength, and soil organic matter on Pb sorption and desorption in soils and soil components were investigated using equilibrium, kinetic, and X-ray absorption fine structure (XAFS) spectroscopy. The use of in situ spectroscopic experiments together with macroscopic experiments resulted in important information about sorption and desorption reaction mechanisms.; Pb adsorption and desorption on Al2O3 was characterized by fast and slow reaction steps. XAFS analyses showed no change in the sorption mechanisms over a period of 1.5 h to 23 days. Based on these experiments, it was proposed that the process responsible for the slow sorption reaction is slow diffusion to sorption sites existing on the interior of the mineral, and that Pb formed an inner-sphere bidentate complex on the edges of the alumina octahedra. Desorption from the Al2O3 was shown to be completely reversible within 3 days.; Lead adsorption on montmorillonite was pH-dependent at high ionic strength, and pH-independent at low ionic strength. This behavior suggested two different adsorption mechanisms were occurring. XAFS results confirmed that at high ionic strength and pH, inner-sphere complexes predominated, and at low ionic strength and pH, outer-sphere complexes predominated. However, at high ionic strength and low pH, or at low ionic strength and high pH, the XAFS data revealed that a mixture of inner- and outer-sphere Pb complexes were occurring.; Sorption and desorption behavior of Pb in soils with varying amounts of soil organic matter were measured using a stirred-flow reactor. The rate of sorption and desorption decreased as the fraction of organic matter in the soils increased. Desorption from the soils was hysteretic within the time frame of the experiments. This behavior is due to the formation of strong complexes between Pb and the ligand sites existing on SOM. Based on a comparison of the Pb desorption behavior from the aluminum oxide, the montmorillonite clay, and the soil organic matter, it was concluded that the bonds formed between Pb and soil organic matter are stronger than the bonds formed between Pb and the soil minerals. Analysis of the extended portion of the XAFS (EXAFS) data from the soil confirms that Pb is predominantly sorbed to the SOM (C and O atoms were the only atoms present in the first and second coordination shells surrounding the Pb atoms).