To estimate the risks associated with soil contaminants we need to know to what extent they are mobile and accessible to organisms. We can only assess this if we know how the contaminants are bound in the soil. The binding characteristics of different elements can also be utilized in remediation of contaminated areas. In a new doctoral thesis Charlotta Tiberg shows that phosphate greatly affects the sorption of lead, copper, cadmium and arsenate to iron hydroxides. This may be important for the mobility and bioavailability of these contaminants.
With her doctoral thesis, Charlotta Tiberg wants to increase the understanding of how lead, copper, cadmium and arsenate bind to iron hydroxides. The binding of contaminants to iron hydroxides and soils that are rich in iron hydroxides was studied in laboratory experiments. In order to clarify in detail how the elements are bound she used X-ray absorption spectroscopy, XAS. With XAS, the binding can be studied on an atomic level. Based on the results Charlotta developed geochemical models that could simulate the binding of contaminants.
The results reveal that phosphate increased the binding of the investigated metals. This can be explained by the fact that phosphate and the metal together bind more strongly to the iron hydroxide than the elements do separately. For arsenate, the results were rather the opposite. Phosphate ions reduced the amount of arsenate bound to iron hydroxides. The explanation is that they compete with arsenate for the same binding sites. This means that the addition of phosphate to contaminated soils may mobilize arsenic while the binding of metals can increase. Despite the strong effect on pure iron hydroxides, binding of metals to podsolized forest soils did not increase when phosphate was added. This was investigated in laboratory experiments. The reason could be that metals mainly bind to organic matter in the investigated soils, but may also be a result of the quite low additions of phosphate in the experiments.
An important part of the PhD project was to develop geochemical models that can simulate how pollutants are affected when the surrounding conditions change. Such models can be important tools for assessing environmental risks in a long term perspective and optimize remediation solutions to make them as economically and environmentally sustainable as possible.
– I think we could gain a lot by using geochemical models to a greater extent in risk assessments and design of remediation solutions for contaminated soils, says Charlotta Tiberg.
Charlotta Tiberg defends her doctoral thesis on the 26th of May at 9am. Opponent is Dr Juan Antelo from University of Santiago de Compostela in Spain.