Water-rock interaction during mineral carbonation and volcanic ash weathering

Abstract

The reduction of atmospheric carbon dioxide (CO2) is considered one of the greatest challenges of this century. Carbon capture and storage (CCS) is one of the means proposed to lower the atmospheric CO2 content. The aim of the CarbFix project in Iceland was to design and test a CO2 re-injection system, in which CO2 from the Hellisheidi geothermal power plant was injected, fully dissolved in water, into basaltic rocks. In this way the carbon is mineralized upon basalt dissolution by the precipitation of carbonate minerals. Pre-injection study of the CarbFix site showed that the field consists of primitive basaltic rocks, both glassy and crystalline. The study further showed that the targeted aquifer contains high-pH water, ranging in temperature from 15 to 35°C, and is isolated from the atmosphere. The fluid chemistry indicated equilibrium with secondary minerals such as calcite, Ca-rich zeolites, and clays. Geochemical modelling of the injection schemes predicted that 1-2 moles of basaltic rock would be needed to lower the dissolved carbon in one kg of water back to pre-injection values by precipitation of Ca-Mg-Fe-carbonates. A syringe sampler for CO2-rich fluids and tracers was designed and tested, both in the laboratory and in the field. The sampler was used to monitor the carbonation process and the evolution of the CO2-rich fluid during the subsurface mineral carbonation.The 2010 Eyjafjallajökull eruption provided a unique opportunity to study the impact of eruption mechanism, hydromagmatic versus magmatic, on the environmental chemistry. Plug-flow experiments were conducted on pristine volcanic ash, in order to evaluate the initial leaching from the ash surface. There was a dramatic difference in pH evolution between the effluent waters from the two ash types. Within minutes there was a “chemical divide” by several orders of magnitude in the proton concentration. The effluent from the hydromagmatic ash was alkaline, but the magmatic ash effluent was acidic. The effluent from the hydromagmatic ash thus became highly supersaturated with common secondary minerals formed in volcanic rocks, but nearly all these minerals were undersaturated in the acid effluent from the magmatic ash.The pH of surface waters in the vicinity of Eyjafjallajökull ranged from 4.8 to 8.2, with the low-range pH measured in the smaller Svadbaelisá-flood during the hydromagmatic phase, and in ash-polluted rivers during the magmatic phase. Polluted water from the combined ash layers showed neutral pH and high concentrations of dissolved nutrients and pollutants. The glacial floods in the Markarfljót river were loaded with dissolved magmatic salts, but they also displayed neutralization by ash-dissolution with time. Large amounts of dissolved elements and suspended ash were transported to the North Atlantic Ocean. The flux of dissolved inorganic carbon (DIC) down the Markarfljót river was 7 tonnes/s during the hydromagmatic phase and 2 tonnes/s during the magmatic phase, for an estimated total of 10,300 tonnes. These contrasting environmental impacts of magmatic versus hydromagmatic eruption phases showed that significant iron fertilization of the ocean by readily soluble metal salts was only possible from the magmatic ash. Furthermore, the hydro-magmatic ash protected the environment on land by neutralizing the acidic and polluted waters that had been in contact with the magmatic ash.