ASSESSING THE CARBON SEQUESTRATION POTENTIAL IN SOUTHWESTERN SWEDEN A comparative study of igneous rock from Greenland, Iceland, and Sweden to evaluate the carbonatization potential of the Billdal area, SW Sweden

Abstract

The main purpose of this study is to investigate the potential of geological Carbon Capture and Storage (CCS) in SW Sweden using an innovative technique which mimics Earth's natural way of regulating the global carbon cycle by turning CO2 gas into carbonate minerals. To achieve this, comparative experiments on CCS potential were conducted using six different igneous rock samples. These encompassed granite, gabbro, and dolerite of Proterozoic age from the Billdal area in Gothenburg, Sweden, 1.3 Ga nepheline syenite and carbonatite from the Grønnedal-Íka alkaline complex in SW Greenland, and a young basalt from the Holuhraun volcanic eruption in Iceland 2014 – 2015. Powdered rock samples of size 45 – 250 μm were subjected to carbonation reactions with carbonated deionized water in closed systems at ambient temperature, 50°C, and 100°C, respectively. The room temperature experiments lasted 63 days, while the 50°C ran for 32 days, and the 100°C were conducted in a manner of three hours. The three experimental sets were monitored daily by recording the pH of the solutions, showing gradual increases in pH with time until reaching stable levels within the basic range. Among the rock samples, gabbro exhibited the highest pH value (8.68) at room temperature, while syenite demonstrated the highest pH (9.01) at 50°C, and carbonatite displayed the highest pH (8.42) at 100°C. After reaching a stable pH ~8, a weak NaHCO3 – Na2CO3 solution of pH 9.2 was added to the solutions to enhance the potential of carbonate precipitation. Pre- and post- treated powdered rock samples were analysed using X-Ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM – EDX), Circular Back Scatter detector (CBS), and Everhart-Thornley Detector (ETD). Results using XRD showed no carbonate precipitate while results of SEM-EDX found carbonate precipitates, most likely calcite, on basalt, carbonatite, and nepheline syenite, but no precipitation on the granite, gabbro, and dolerite. The low carbonatization rate of the latter three are ascribed to the lack of suitable divalent cations for carbonate mineral formation. One surprising result was the degree of mineral alteration in these Swedish rocks, which had turned pyroxenes into amphiboles. Pyroxenes would otherwise have been ideal candidates for CCS. With these results at hand, the highly altered rocks of SW Sweden are deemed unsuitable for CO2 sequestration through carbonate precipitation.