Carbon dioxide triggered metal(loid) mobilisation in a mofette

Carbon capture and geologic storage is a frequently discussed option to reduce atmospheric CO2 concentrations with the long-term risk of leakage from storage sites to overlying aquifers and soils. We chose natural CO2 exhalations, so-called mofettes, in a wetland area in the Czech Republic as analogues to follow the fate of metal(loid)s under CO2-saturated conditions. Compared to the reference fluvisol at the study site, mofette soils exhibited lower pH (4.9 ± 0.05) and redox potential (300 ± 40 mV), as well as higher organic carbon contents. Poorly crystalline and crystalline Fe (hydr)oxides, the most important metal(loid) sorbents in the CO2-unaffected soils (7.9 ± 5.9 g kg− 1), showed significantly lower concentrations under the acidic and reducing conditions in the mofettes (1.2 ± 0.4 g kg− 1). In turn, this increased the mobility of As and resulting concentrations were up to 2.5 times higher than in the CO2-unaffected pore water (58 ± 18 μg L− 1). Methylation (up to 11% of total As) and thiolation (up to 9%) contributed to net As 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) concentrations remained low, likely due to complexation and/or adsorption to organic carbon and the small amount of Fe (hydr)oxides. A one-month-in-situ mobilisation experiment showed mobilisation of all investigated elements to the aqueous phase suggesting that desorption is the faster and initially dominating process while resorption is a secondary, slower process. We conclude that the CO2-induced mobilisation of toxic As and net-immobilisation of essential micro-nutrients (Mn, Ni, Cu) constitute serious risks and must be tested for transferability and relevance at geologic carbon storage sites.