Biogeochemical Controls on Carbon Mineralization in Basalts: Insights from Dissolution Experiments

A durable method for sequestering atmospheric CO2 is in-situ mineralization as carbonates in ultramafic and mafic rocks, a process known as carbonation. Field-scale demonstrations in Iceland have injected carbonic acid solution into basalt, where it reacts with unstable minerals to release cations and precipitates carbonates in as little as two years. Although thermodynamically favored, similar carbonation reactions at the laboratory scale were found to proceed slowly unless reactants were ground to powder and held at elevated pressures and temperatures. Reaction rates are affected by a complex interplay between mineral dissolution, which increases permeability, and the precipitation of solids that decrease the reaction surface area. Biomineralization via microbes that produce reactive enzymes such as carbonic anhydrase can accelerate carbonation. This enzyme catalyzes reversible CO2 hydration and forms metal carbonates that mimic inorganic processes, but little is known about the extremophiles that can tolerate the pressure and temperature conditions required for efficient mineralization. A set of laboratory experiments focused on constraining the optimum parameters for carbonate mineralization, the role of microbial activity, and changes in reactive surface area for the alteration of basaltic rocks. Experiments were conducted in a cold-seal pressure vessel under temperatures of 20-150oC, pressures of 250-900 bars, and a pH range from 4.7 to 8.7. Run durations ranged from 24 hours to 3 weeks using fresh Hawaiian basalts as well as forsterite, diopside, and augite crystals as starting materials. Since the dissolved CO2 in water creates bicarbonate and carbonic acid that leads an acidic environment and precipitation of CO2 as carbonate minerals requires a basic environment, we also tested a thermophilic bacterium, Geobacillus sp. strain WSUCF1, for the pH tolerance between 4.7-6, temperature range of 20⁰C-70⁰C for 27 hours. Further work is in progress to test the optimum conditions using a hyperthermophile, Persephonella marina, and to constrain (or achieve) the effect of both species on rate of carbonate mineralization as a method for sequestering CO2 in quantities that are meaningful on a global scale.