Rapid CO2 mineralisation into calcite at the CarbFix storage site quantified using calcium isotopes

Publisher's version (útgefin grein).The engineered removal of atmospheric CO 2 is now considered a key component of mitigating climate warming below 1.5 °C. Mineral carbonation is a potential negative emissions technique that, in the case of Iceland’s CarbFix experiment, precipitates dissolved CO 2 as carbonate minerals in basaltic groundwater settings. Here we use calcium (Ca) isotopes in both pre- and post-CO 2 injection waters to quantify the amount of carbonate precipitated, and hence CO 2 stored. Ca isotope ratios rapidly increase with the pH and calcite saturation state, indicating calcite precipitation. Calculations suggest that up to 93% of dissolved Ca is removed into calcite during certain phases of injection. In total, our results suggest that 165 ± 8.3 t CO 2 were precipitated into calcite, an overall carbon storage efficiency of 72 ± 5%. The success of this approach opens the potential for quantification of similar mineral carbonation efforts where drawdown rates cannot be estimated by other means.Analyses and PPvS were funded by NERC Advanced Fellowship NE/I020571/2 and ERC Consolidator grant CONTROLPASTCO2 682760. Chris Coath (Bristol University) is thanked for discussions and formulae on error propagation. The CarbFix pilot infrastructure, injection, sampling, sample distribution and interpretations were funded by the European Commission through the projects CarbFix (EC coordinated action 283148); Min-GRO (MC-RTN-35488); Nordic fund 11029-NORDICCS; the Icelandic GEORG Geothermal Research fund (09-02-001); the U.S. Department of Energy under award number DE-FE0004847) and Reykjavík Energy. CarbFix has further received grants from the European Union’s Horizon 2020 research and innovation program under grant agreements No. 764760 (CarbFix2) and 764810(S4CE). We are indebted to Matin Stute and Jenifer Hall at Columbia University and Juerg Matter at Columbia University and Southampton University; Einar Örn Thrastarson, Trausti Kristinsson, Vordis Eir-iksdottir, Halldor Bergmann and Thorsteinn A. Thorgeirsson at Reykjavıik Energy; Vigdis Harðardottir, Finnbogi Oskarsson, Kristjan Hrafn Sigurðsson and Steindor Nielsson at ISOR; and Thorsteinn Jonsson, Sveinbjörn Steinthorsson, Iwona Galezcka, Eydıs S. Eiriksdottir, Deirdre Clark, Chris Grimm and Flora Brocza at the University of Iceland for helping the injection and sampling campaign.Peer Reviewe