Carbon dioxide triggered metal(loid) mobilisation in a mofette
Judith Forberg (08/2012-10/2013)
Support: Britta Planer-Friedrich
Naturally occurring CO2 exhalations like mofettes are well suited to study long-term effects of enhanced CO2 concentrations on soil. CO2-triggered acidication is expected to increase the mobility of potentially toxic trace elements by inducing desorption or mineral dissolution. The hypothesis of a CO2-triggered mobilisation was tested for Fe, As, Pb, Mn, Ni and Cu on a mofette site in the Czech Republic. Depth profiles of redox potential, pH, organic carbon, dissolved methane and CO2 concentrations of two mofettes and two CO2-unaffected references were determined. Total trace element concentrations in soil and pore water were analysed using ICP-MS, As speciation was determined by IC-ICP-MS. Fractionation of the metal(loid)s was specified by sequential extraction. Fe (hydr)oxides, the preferred sorbent in CO2-unaffected soils, were reduced to only 15 % of the original amount in the mofettes, indicating dissolution due to low pH (4.9) and redox potential (300 +/- 40 mV). Besides Fe mobilisation, this resulted in the release of all associated trace elements. Pore water concentrations of As and Pb were increased by a factor of 2.3 and 2.5, respectively, in the mofettes. Profit from desorption of other ions by resorption to organic matter (increased by a factor of 3.2 in mofettes) and other soil constituents caused a net-immobilisation of Mn, Ni and Cu, whose pore water concentrations were 72 %, 53 % and 39 %, respectively in the mofette compared to concentrations in the reference. The mobilisation of toxic elements (As, Pb) and the net-immobilisation of essential micro-nutrients (Mn, Ni, Cu) constitute serious risks of increased CO2 soil concentrations as they can occur from leaking carbon sequestration sites.