Carbon dioxide triggered metal(loid) mobilisation in a mofette

•We examined the effects of long-term CO2 influence on metal(loid) mobilisation in soil.•Mofettes served as natural analogues to carbon sequestration sites.•Redox potential and pH are decreased while organic matter content is increased on such sites.•Absence of Fe (hydr)oxides induces increased mobility of As and Fe.•Resorption to organic matter causes net-immobilisation of micro-nutrients (Mn, Ni, Cu).

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.