Rates of atmospheric CO2 capture using magnesium oxide powder

Magnesium oxide (MgO) looping is a proposed technology for gigatonne-scale carbon dioxide (CO2) removal from the atmosphere. Here, we determined CO2 removal rates by measuring CO2 fluxes and total inorganic carbon (TIC) using a deposit of MgO powder (10 cm thickness; 76 kg MgO/m(2)). The majority of the MgO powder (-87 wt%) was transformed to brucite [Mg(OH)2] after the first two wetting and drying cycles. CO2 fluxes varied depending on water content and porosity: -3.9 (saturated/wet), -12.5 (optimal), and -2.2 (dry) kg CO2/m(2)/yr. At these rates, only 3-18% of the MgO would react in 1 yr and require areas of 80,000-455,000 km(2) to sequester 1 Gt CO2/yr. Gas-filled porosity of -39-58% and water contents of 7-17 wt% were optimal for CO2 ingress and carbonation. Dypingite [Mg-5(CO3)(4)(OH)(2)center dot similar to 5H(2)O] was the sink of atmospheric CO2 and TIC increased from 0.2% to 7.5% CO2. The delta C-13 values of the solids (avg. - 14.9 parts per thousand; VPDB) were well below those expected for dypingite precipitated in equilibrium with laboratory CO2, demonstrating that CO2 supply was rate-limiting, despite daily tilling of the deposit. A linear extrapolation of the CO2 fluxes shows that 90% carbonation of MgO will require 5-27 yr. CO2 removal rates slowed down with time over 1 yr, in agreement with the expectation that the progressive depletion of reactants (MgO/brucite) is likely to substantially slow CO2 removal rates and greatly extend the time needed to achieve complete carbonation.