A laboratory study of the initial effects of dissolved carbon dioxide (CO2) on metal release from shallow sediments

Sequestration of carbon dioxide (CO2) into deep geologic reservoirs is a potential approach for controlling the rise of CO2 concentrations in the atmosphere. Proper characterization and permitting of storage sites is expected to include an assessment of the potential impacts of CO2 intrusion into overlying groundwater formations. In most natural settings, the dissolution of CO2 into groundwater will decrease the pH and can also release carbonate ligands into solution. These effects can lead to metals being mobilized from sediments through mechanisms such as ion exchange, desorption, and mineral dissolution. This laboratory-based study evaluates the extent of metal release from a groundwater system in the presence of elevated CO2 concentrations. In particular, the research investigates the geochemical mechanisms involved in metal release, with a focus on distinguishing between pH-driven processes (e.g. carbonate dissolution and ion exchange) and carbonate-driven processes (e.g. reactions enhanced by the formation of metal-carbonate complexes). Measurements from lab-scale leaching experiments and sediment characterizations are compared to data from a concurrent controlled-release field test, where CO2 was injected into a poorly buffered sandy groundwater formation at ∼50 m depth. Results show that the immediate effect of the introduction of CO2 is a pH drop of ∼3 units, which leads to the quick release of some elements (e.g. Ca, Mg, Ba, Sr) by primarily pH-driven processes. The extent of metal release was different across geochemically distinct sediment types from the field site. The results suggest that it would be useful to group constituents by their pH-release trends, and to include pH and concentrations of certain indicator cations (Li, K, Na, Ba, Ca, Mg, Sr, Mn and Si) as parameters in site monitoring plans.