CO2-water–basalt interaction. Low temperature experiments and implications for CO2 sequestration into basalts

Abstract

CO2-water–basaltic glass batch experiments were performed in order to study the feasibility of low temperature CO2 sequestration into basalts including the key reactions and chemical mass transfer associated to progressive water–rock interaction. The experiments were carried out at 40°C for up to 260days with initial dissolved CO2 concentrations ranging from 24 to 305mmol/kg. Alteration minerals were identified on the basaltic glass grain surfaces and in the matrix, consisting of poorly crystalline Ca–Mg–Fe carbonates, Fe-hydroxides and/or oxy-hydroxides and Ca–Mg–Fe clays. Other cryptocrystalline phases were identified by variable amounts of Al, Si and Fe reflected by the secondary mineral compositions. The water chemistry was monitored during the experiments and was characterized by an increase in Si, Ca, Mg and Na with time, whereas Al, Fe and CO2 decreased. The dissolution of basaltic glass in CO2-rich waters was observed to be incongruent with the overall water composition and secondary mineralogy depending on reaction progress and pH. The pH was determined primarily by the initial CO2 concentration and its ionization constants, the amount of dissolved basaltic glass and by the mass and composition of secondary minerals formed. Initially, the pH increased rapidly from <4.5 to ∼4.5–6. Under these conditions, Mg and Ca were observed to be mobile and dissolved stoichiometrically relative to Na, whereas Si, Al and Fe were immobile. Upon quantitative CO2 mineralization, the pH increased to >6.5 and the mobility of most elements consequently decreased including Mg and Ca. The experimental results indicate that increased aqueous CO2 concentrations modify considerably the natural water–basalt reaction path. The mineralization of CO2 into carbonates was rapid and controlled by the initial CO2 concentration and rock to water ratio, with the composition of the carbonates depending on the availability of Ca, Mg and Fe and the oxidation state of Fe. At pH <5.5, Fe was in the ferrous oxidation state resulting in the formation of Fe-rich carbonates with the incorporation of Ca and Mg as well as the formation of Ca–(Mg)–Fe smectites. At pH >5.5, the mobility of Fe decreased due to oxidation of Fe(II) to Fe(III) and subsequent formation of ferrihydrite. At pH >6.5, the experimental solutions became progressively supersaturated with calcite, zeolites and Mg-rich clays limiting the mobility of Ca and Mg. The results indicate that reactions between clays (Ca–(Mg)–Fe smectites) and carbonates at pH <6.5, and zeolites, clays (Ca–Mg–Fe smectites) and carbonates at pH >6.5, control together the availability of Ca, Mg and Fe, playing a key role for low temperature CO2 mineralization and sequestration into mafic rocks.