Sequential Extraction Evaluation of Rock-Hosted Elements Using a pH Range Relevant to CO₂ Geo-Sequestration

Detailed geochemical modelling of the potential groundwater impacts of CO2 geo-sequestration requires site-specific knowledge of how mobile elements are hosted within rocks. We present a simple sequential extraction procedure analogous to pH conditions produced by different partial pressures of carbon dioxide (CO2) in contact with water. The procedure consists of three sequential steps: Water at pH 7; acetic acid–ammonium acetate at pH 5 and then at pH 3, with the amounts of specific elements extracted by each step considered with respect to whole-rock total element abundance. Our purpose in developing this procedure is three-fold: 1) identify readily mobilised suites of elements for groundwater baseline and monitor bore studies; 2) provide insights regarding the mode/s of occurrence of easily extracted elements within rock samples; and 3) suggest possible mechanisms for the mobilisation of rock-sourced elements into groundwater under neutral to moderately acidic pH that can inform reactive transport modelling of carbon storage sites. In our case study, the second step extracted most of the main mobile elements of interest.