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In-situ transformations of heavy metal minerals in soils (completed)
The subject of this project is the fate of specific heavy metal phases deposited in soils.
Heavy metal phases such as oxides (e.g. PbO or ZnO), sulfides (e.g. PbS, CdS), or zero valent metals such as Pb enter the soil from a variety of sources (e.g. metal smelting and processing or shooting).
The kinetics of the dissolution and the phase transformation of these initial phases determine the further fate of the metals in the soil.
Results obtained by recent analytical developments (e.g. EXAFS) show that distinct heavy metal phases such as oxides and sulfides are present in soils may be stable for more than 100 years.
Insoluble transformation products such as metal-phosphates have also been identified.
The dissolution and transformation behavior of the initially deposited metal phases has not yet been studied in controlled experiments under environmental conditions.
We propose a new experimental in-situ approach to study these reactions in soils.
We aim at developing a method which enables us to monitor heavy metal phases under in-situ conditions directly in the soil.
The idea is to fix heavy metal phases on a polymer support, so that they are exposed on the surface.
The support is then placed in the soil for various periods of time. After recovery of the support, the fixed metals are examined by electronmicroscopy and X-ray diffraction to study the dissolution and transformation processes.
Acid-digestion of the remaining metals will allow a quantification of the metals on the support and a calculation of the dissolution rate.
The method will be evaluated by conducting batch dissolution experiments in suspension and with the polymer-immobilized metal phases and by placing the slides in saturated and unsaturated columns.
The effect of pH and different soil-born ligands (e.g. oxalate, fulvic acids) will be studied.
The slides will be tested and used in field studies, primarily in a lysimeter experiment under controlled quasi-natural conditions.
The laboratory-based dissolution rates will be compared to the in-situ rates in the soil.
The results will allow us to define the conditions under which the initially deposited heavy metal phase might be stable and to determine how fast the dissolution rates are in the soil.

The supervisor of this project was Dr.B.Nowack. The project was finished in May 2005.

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