The Formation of Carbonate Minerals and the Mobility of Heavy Metals during Water-CO2-Mafic Rock Interactions

• Main Author: Olsson, Jonas
• Format: Book
• Language: English
• Published: Department of Chemistry, Faculty of Science, University of Copenhagen 2014
• Subjects: Dy; Hvanná; Iceland
• Online Access: https://curis.ku.dk/portal/da/publications/the-formation-of-carbonate-minerals-and-the-mobility-of-heavy-metals-during-waterco2mafic-rock-interactions(0f1157f6-20d5-40ec-8e78-fa260bb53fec).html; https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122112800005763

CarbFix is a pilot project in Iceland created to lock away CO2 through in-situ mineralization in the subsurface. The goal is to dissolve CO2 from the geothermal power plant, Hellisheiði, in water and inject it into basaltic rock formation for permanent storage. The carbonated water dissolves the basaltic host rock and liberates cations. Ideally, the cations react with the dissolved CO2 and form long time stable carbonate minerals. However, dissolution of the basaltic rock can lead to mobility of toxic metals, which is a potential threat to groundwater supplies and surface waters. Besides carbonate minerals, the reaction products are known to be manifold and reflect the complex composition of the basaltic material. Formation of secondary products, such aluminium and iron (hydr)oxides, are considered undesirable, because (1) they consume the cations that could be used to sequester CO2, thus compete with the process of carbonation, and (2) they can form a passivating layer, which inhibit dissolution of the basaltic material and slow down the carbonation process. The purpose of this thesis was to identify formation products, relevant to CarbFix, and assess their ability to immobilize toxic metals released from basaltic material. The work was divided into four projects. In the first project, basaltic material in form of fresh volcanic ash from the 2011 Grímsvötn eruption was classified as glassy tholeiitic basalt with <10 mass% of plagioclase and pyroxene. The ash was exposed water and the effluent analyzed for 74 elements. The effluent was alkaline and high release rates of mainly S, Na, Ca, Mg, F and Cl were observed during the first 10 minutes. After 12 hours, the most abundant element released was Si. Secondary phases of Al and Fe precipitated on the ash surfaces and these were suspected of scavenging As, Ba, Cr, Co, Cu, Ga, Mn, Mo, Ni, P, Te, V and Zn. This study has also other implications: the small particle size of the ash means that they can travel long distances and this can impact air traffic and the chemical balance of surface waters far away from the volcanic eruption. In the second study, I investigated natural calcium carbonate precipitation in the Hvanná River, Iceland. A decrease of the concentrations of Cd, Co, Cu, Mg, Mn and Sr with distance downstream the river correlated with calcium. Partition coefficients derived from the precipitating calcite in the Hvanná River are consistent with values of controlled laboratory experiments from the literature for Ba, Cd, Co, Cu, Mg, Mn, Na, Ni, Sr and Zn. The calcium carbonates also scavenge other elements, including rare earth elements (REE) and the toxic metals As and Pb. This and the next study can be considered natural analogues to the carbonate precipitation in CarbFix project. In the third study, water and solid samples from two alkaline springs in Oman were examined. The elements detected in the spring waters in order of abundance were Na, Cl, Ca, Mg, SO4, K, Br, Si, F, B, Sr, Al, Fe, Mo, Zn, Ni, Cu, Mn, V, Ba, Cr, Co, Ti, Hg and Pb. The carbonate samples were identified as aragonite needles and calcite rosettes with traces of serpentine, dypingite and silica. The average concentration in μmol per gram CaCO3 of the carbonates were: Mg(430) > Na(81) > Si(46) > Sr(11) > K(1) > Al(0.95) > P(0.39) > Fe(0.26) > Ba(0.22) > Mn(0.13) > Zn(0.08) > Ni(0.07) > V(0.005) > Cr, Cu, Ti (0.003) > As, Ce(0.001) > Pb, Nd, La, Mo, Pr, Sm, Gd, Dy, Er, Cd, Yb, Eu, Th, Ho, Tb, Lu, Tm(<0.001). This suggests active scavenging of REE and toxic metals such as As, Ba, Cd and Pb by carbonate minerals but Hg was not removed from solution. In the last project, the basaltic mineral, olivine ((Mg,Fe)2SiO4), was reacted with water and CO2 to form iron and magnesium carbonates both at aerobic and anaerobic conditions. Within days olivine crystals dissolved and secondary minerals formed when exposed to water only. Within 4 days, a red precipitate formed when olivine was reacted at increased temperatures and CO2 partial pressures. The precipitate was identified as goethite, hematite, silica, and clay and carbonate minerals. My experiments suggest that supercritical CO2 is a key factor for the formation of magnesite. Iron did not precipitate as carbonate minerals but rather as iron oxides.