Carbon dioxide triggered metal(loid) mobilisation in a mofette

Judith Mehlhorn1, Felix Beulig2, Kirsten Küsel2, Britta Planer-Friedrich1
1 Environmental Geochemistry, University of Bayreuth
2 Aquatic Geomicrobiology, Friedrich Schiller University Jena

O 11.3 in Weathering and chemical processes as keys to ecosystem functioning

15.07.2014, 11:40-12:00, H20


Carbon capture and storage is a frequently discussed option to reduce the CO2 concentration in the atmosphere and its effect on global climate change. Considering the long time-scale of the intended storage, potential CO2 leakage from such underground storage sites to overlying aquifers and soils constitutes a serious threat, among other things since CO2-triggered soil acidification is expected to increase the mobility of potentially toxic trace elements by inducing desorption or mineral dissolution. 

As natural analogues to such leakages, we chose cold volcanic CO2 exhalations, so-called mofettes, in a wetland area in the Czech Republic to study long-term effects of increased CO2 concentrations on soils and their metal(loid) mobilisation potential in aquifers for Fe, As, Pb, Mn, Ni and Cu.

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 inductively coupled plasma mass spectrometry (ICP-MS), As speciation was determined by ion chromatography coupled to ICP-MS. Information about metal(loid) sorption and remobilisation was obtained by sequential extraction of the soil samples as well as from an in situ mobilisation experiment exposing material from CO2-unaffected sites to the mofettes.

Compared to the reference fluvisol at the study site, mofette soils were characterised by decreased pH (by 0.5 to 4.9 ± 0.05) and redox potential (by 80 mV to 300 ± 40 mV), as well as accumulation of organic carbon. Amorphous and crystalline Fe (hydr)oxides were the most important metal(loid) sorbents in the CO2-unaffected soils (7.9 ± 5.9 g kg-1), while in the mofettes reductive dissolution led to strongly decreased solid phase concentrations (1.2 ± 0.4 g kg-1). In turn, this resulted in the release of the adsorbed metal(loid)s As, Pb, Mn, Ni and Cu with up to 2.5 times higher concentrations of Pb (2.0 ± 1.2 µg L-1) and As (58 ± 18 µg L-1) in mofette pore waters. Arsenic methylation (up to 11 % of total As) and thiolation (up to 9 %) contributed to its net-mobilisation. Dissolved Mn (131 ± 53 µg L-1), Ni (9.1 ± 3.1 µg L-1) and especially Cu (2.2 ± 1.0 µg L‑1) remained low, likely due to complexation and/or resorption to organic carbon and remaining Fe (hydr)oxides. The one-month in situ mobilisation experiment showed mobilisation of all considered elements to the aqueous phase suggesting that desorption is the faster and initially dominating process while resorption to newly precipitated minerals is a secondary, slower process.

We conclude that the CO2-induced mobilisation of toxic elements (As, Pb) and net-immobilisation of essential micro-nutrients (Mn, Ni, Cu) constitute serious risks of increased CO2 soil concentrations and should be tested for transferability and relevance at geologic carbon storage sites.

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last modified 2014-06-19